The GNU C Library

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This is Edition 0.11, last updated 2007-09-09, of The GNU C Library Reference Manual, for Version 2.7 of the GNU C Library.

Appendices

Indices

--- The Detailed Node Listing ---

Introduction

Standards and Portability

Using the Library

Error Reporting

Memory

Memory Allocation

Unconstrained Allocation

Allocation Debugging

Obstacks

Variable Size Automatic

Locking Pages

Character Handling

String and Array Utilities

Argz and Envz Vectors

Character Set Handling

Restartable multibyte conversion

Non-reentrant Conversion

Generic Charset Conversion

Locales

Locale Information

The Lame Way to Locale Data

Message Translation

Message catalogs a la X/Open

The Uniforum approach

Message catalogs with gettext

Searching and Sorting

Pattern Matching

Globbing

Regular Expressions

Word Expansion

I/O Overview

I/O Concepts

File Names

I/O on Streams

Unreading

Formatted Output

Customizing Printf

Formatted Input

Stream Buffering

Other Kinds of Streams

Custom Streams

Formatted Messages

Low-Level I/O

Stream/Descriptor Precautions

Asynchronous I/O

File Status Flags

File System Interface

Accessing Directories

File Attributes

Pipes and FIFOs

Sockets

Socket Addresses

Local Namespace

Internet Namespace

Host Addresses

Open/Close Sockets

Connections

Transferring Data

Datagrams

Inetd

Socket Options

Low-Level Terminal Interface

Terminal Modes

Special Characters

Pseudo-Terminals

Syslog

Submitting Syslog Messages

Mathematics

Pseudo-Random Numbers

Arithmetic

Floating Point Errors

Arithmetic Functions

Parsing of Numbers

Date and Time

Processor And CPU Time

Calendar Time

Parsing Date and Time

Resource Usage And Limitation

Priority

Traditional Scheduling

Memory Resources

Non-Local Exits

Signal Handling

Concepts of Signals

Standard Signals

Signal Actions

Defining Handlers

Atomic Data Access

Generating Signals

Blocking Signals

Waiting for a Signal

BSD Signal Handling

Program Basics

Program Arguments

Parsing Program Arguments

Environment Variables

Program Termination

Processes

Job Control

Implementing a Shell

Functions for Job Control

Name Service Switch

NSS Configuration File

NSS Module Internals

Extending NSS

Users and Groups

User Accounting Database

User Database

Group Database

Netgroup Database

System Management

Filesystem Handling

Mount Information

System Configuration

Sysconf

Cryptographic Functions

Debugging Support

Language Features

Variadic Functions

How Variadic

Data Type Measurements

Floating Type Macros

Installation

Maintenance

Porting

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1 Introduction

The C language provides no built-in facilities for performing such common operations as input/output, memory management, string manipulation, and the like. Instead, these facilities are defined in a standard library, which you compile and link with your programs. The GNU C library, described in this document, defines all of the library functions that are specified by the ISO C standard, as well as additional features specific to POSIX and other derivatives of the Unix operating system, and extensions specific to the GNU system.

The purpose of this manual is to tell you how to use the facilities of the GNU library. We have mentioned which features belong to which standards to help you identify things that are potentially non-portable to other systems. But the emphasis in this manual is not on strict portability.

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1.1 Getting Started

This manual is written with the assumption that you are at least somewhat familiar with the C programming language and basic programming concepts. Specifically, familiarity with ISO standard C (see ISO C), rather than “traditional” pre-ISO C dialects, is assumed.

The GNU C library includes several header files, each of which provides definitions and declarations for a group of related facilities; this information is used by the C compiler when processing your program. For example, the header file stdio.h declares facilities for performing input and output, and the header file string.h declares string processing utilities. The organization of this manual generally follows the same division as the header files.

If you are reading this manual for the first time, you should read all of the introductory material and skim the remaining chapters. There are a lot of functions in the GNU C library and it's not realistic to expect that you will be able to remember exactly how to use each and every one of them. It's more important to become generally familiar with the kinds of facilities that the library provides, so that when you are writing your programs you can recognize when to make use of library functions, and where in this manual you can find more specific information about them.

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1.2 Standards and Portability

This section discusses the various standards and other sources that the GNU C library is based upon. These sources include the ISO C and POSIX standards, and the System V and Berkeley Unix implementations.

The primary focus of this manual is to tell you how to make effective use of the GNU library facilities. But if you are concerned about making your programs compatible with these standards, or portable to operating systems other than GNU, this can affect how you use the library. This section gives you an overview of these standards, so that you will know what they are when they are mentioned in other parts of the manual.

See Library Summary, for an alphabetical list of the functions and other symbols provided by the library. This list also states which standards each function or symbol comes from.

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1.2.1 ISO C

The GNU C library is compatible with the C standard adopted by the American National Standards Institute (ANSI): American National Standard X3.159-1989—“ANSI C” and later by the International Standardization Organization (ISO): ISO/IEC 9899:1990, “Programming languages—C”. We here refer to the standard as ISO C since this is the more general standard in respect of ratification. The header files and library facilities that make up the GNU library are a superset of those specified by the ISO C standard.

If you are concerned about strict adherence to the ISO C standard, you should use the `-ansi' option when you compile your programs with the GNU C compiler. This tells the compiler to define only ISO standard features from the library header files, unless you explicitly ask for additional features. See Feature Test Macros, for information on how to do this.

Being able to restrict the library to include only ISO C features is important because ISO C puts limitations on what names can be defined by the library implementation, and the GNU extensions don't fit these limitations. See Reserved Names, for more information about these restrictions.

This manual does not attempt to give you complete details on the differences between ISO C and older dialects. It gives advice on how to write programs to work portably under multiple C dialects, but does not aim for completeness.

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1.2.2 POSIX (The Portable Operating System Interface)

The GNU library is also compatible with the ISO POSIX family of standards, known more formally as the Portable Operating System Interface for Computer Environments (ISO/IEC 9945). They were also published as ANSI/IEEE Std 1003. POSIX is derived mostly from various versions of the Unix operating system.

The library facilities specified by the POSIX standards are a superset of those required by ISO C; POSIX specifies additional features for ISO C functions, as well as specifying new additional functions. In general, the additional requirements and functionality defined by the POSIX standards are aimed at providing lower-level support for a particular kind of operating system environment, rather than general programming language support which can run in many diverse operating system environments.

The GNU C library implements all of the functions specified in ISO/IEC 9945-1:1996, the POSIX System Application Program Interface, commonly referred to as POSIX.1. The primary extensions to the ISO C facilities specified by this standard include file system interface primitives (see File System Interface), device-specific terminal control functions (see Low-Level Terminal Interface), and process control functions (see Processes).

Some facilities from ISO/IEC 9945-2:1993, the POSIX Shell and Utilities standard (POSIX.2) are also implemented in the GNU library. These include utilities for dealing with regular expressions and other pattern matching facilities (see Pattern Matching).

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1.2.3 Berkeley Unix

The GNU C library defines facilities from some versions of Unix which are not formally standardized, specifically from the 4.2 BSD, 4.3 BSD, and 4.4 BSD Unix systems (also known as Berkeley Unix) and from SunOS (a popular 4.2 BSD derivative that includes some Unix System V functionality). These systems support most of the ISO C and POSIX facilities, and 4.4 BSD and newer releases of SunOS in fact support them all.

The BSD facilities include symbolic links (see Symbolic Links), the select function (see Waiting for I/O), the BSD signal functions (see BSD Signal Handling), and sockets (see Sockets).

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1.2.4 SVID (The System V Interface Description)

The System V Interface Description (SVID) is a document describing the AT&T Unix System V operating system. It is to some extent a superset of the POSIX standard (see POSIX).

The GNU C library defines most of the facilities required by the SVID that are not also required by the ISO C or POSIX standards, for compatibility with System V Unix and other Unix systems (such as SunOS) which include these facilities. However, many of the more obscure and less generally useful facilities required by the SVID are not included. (In fact, Unix System V itself does not provide them all.)

The supported facilities from System V include the methods for inter-process communication and shared memory, the hsearch and drand48 families of functions, fmtmsg and several of the mathematical functions.

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1.2.5 XPG (The X/Open Portability Guide)

The X/Open Portability Guide, published by the X/Open Company, Ltd., is a more general standard than POSIX. X/Open owns the Unix copyright and the XPG specifies the requirements for systems which are intended to be a Unix system.

The GNU C library complies to the X/Open Portability Guide, Issue 4.2, with all extensions common to XSI (X/Open System Interface) compliant systems and also all X/Open UNIX extensions.

The additions on top of POSIX are mainly derived from functionality available in System V and BSD systems. Some of the really bad mistakes in System V systems were corrected, though. Since fulfilling the XPG standard with the Unix extensions is a precondition for getting the Unix brand chances are good that the functionality is available on commercial systems.

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1.3 Using the Library

This section describes some of the practical issues involved in using the GNU C library.

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1.3.1 Header Files

Libraries for use by C programs really consist of two parts: header files that define types and macros and declare variables and functions; and the actual library or archive that contains the definitions of the variables and functions.

(Recall that in C, a declaration merely provides information that a function or variable exists and gives its type. For a function declaration, information about the types of its arguments might be provided as well. The purpose of declarations is to allow the compiler to correctly process references to the declared variables and functions. A definition, on the other hand, actually allocates storage for a variable or says what a function does.) In order to use the facilities in the GNU C library, you should be sure that your program source files include the appropriate header files. This is so that the compiler has declarations of these facilities available and can correctly process references to them. Once your program has been compiled, the linker resolves these references to the actual definitions provided in the archive file.

Header files are included into a program source file by the `#include' preprocessor directive. The C language supports two forms of this directive; the first, 

     #include "header"

is typically used to include a header file header that you write yourself; this would contain definitions and declarations describing the interfaces between the different parts of your particular application. By contrast, 

     #include <file.h>

is typically used to include a header file file.h that contains definitions and declarations for a standard library. This file would normally be installed in a standard place by your system administrator. You should use this second form for the C library header files.

Typically, `#include' directives are placed at the top of the C source file, before any other code. If you begin your source files with some comments explaining what the code in the file does (a good idea), put the `#include' directives immediately afterwards, following the feature test macro definition (see Feature Test Macros).

For more information about the use of header files and `#include' directives, see Header Files.

The GNU C library provides several header files, each of which contains the type and macro definitions and variable and function declarations for a group of related facilities. This means that your programs may need to include several header files, depending on exactly which facilities you are using.

Some library header files include other library header files automatically. However, as a matter of programming style, you should not rely on this; it is better to explicitly include all the header files required for the library facilities you are using. The GNU C library header files have been written in such a way that it doesn't matter if a header file is accidentally included more than once; including a header file a second time has no effect. Likewise, if your program needs to include multiple header files, the order in which they are included doesn't matter.

Compatibility Note: Inclusion of standard header files in any order and any number of times works in any ISO C implementation. However, this has traditionally not been the case in many older C implementations.

Strictly speaking, you don't have to include a header file to use a function it declares; you could declare the function explicitly yourself, according to the specifications in this manual. But it is usually better to include the header file because it may define types and macros that are not otherwise available and because it may define more efficient macro replacements for some functions. It is also a sure way to have the correct declaration.

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1.3.2 Macro Definitions of Functions

If we describe something as a function in this manual, it may have a macro definition as well. This normally has no effect on how your program runs—the macro definition does the same thing as the function would. In particular, macro equivalents for library functions evaluate arguments exactly once, in the same way that a function call would. The main reason for these macro definitions is that sometimes they can produce an inline expansion that is considerably faster than an actual function call.

Taking the address of a library function works even if it is also defined as a macro. This is because, in this context, the name of the function isn't followed by the left parenthesis that is syntactically necessary to recognize a macro call.

You might occasionally want to avoid using the macro definition of a function—perhaps to make your program easier to debug. There are two ways you can do this:

For example, suppose the header file stdlib.h declares a function named abs with 

     extern int abs (int);

and also provides a macro definition for abs. Then, in: 

     #include <stdlib.h>
     int f (int *i) { return abs (++*i); }

the reference to abs might refer to either a macro or a function. On the other hand, in each of the following examples the reference is to a function and not a macro. 

     #include <stdlib.h>
     int g (int *i) { return (abs) (++*i); }
     
     #undef abs
     int h (int *i) { return abs (++*i); }

Since macro definitions that double for a function behave in exactly the same way as the actual function version, there is usually no need for any of these methods. In fact, removing macro definitions usually just makes your program slower.

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1.3.3 Reserved Names

The names of all library types, macros, variables and functions that come from the ISO C standard are reserved unconditionally; your program may not redefine these names. All other library names are reserved if your program explicitly includes the header file that defines or declares them. There are several reasons for these restrictions:

In addition to the names documented in this manual, reserved names include all external identifiers (global functions and variables) that begin with an underscore (`_') and all identifiers regardless of use that begin with either two underscores or an underscore followed by a capital letter are reserved names. This is so that the library and header files can define functions, variables, and macros for internal purposes without risk of conflict with names in user programs.

Some additional classes of identifier names are reserved for future extensions to the C language or the POSIX.1 environment. While using these names for your own purposes right now might not cause a problem, they do raise the possibility of conflict with future versions of the C or POSIX standards, so you should avoid these names.

In addition, some individual header files reserve names beyond those that they actually define. You only need to worry about these restrictions if your program includes that particular header file.

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1.3.4 Feature Test Macros

The exact set of features available when you compile a source file is controlled by which feature test macros you define.

If you compile your programs using `gcc -ansi', you get only the ISO C library features, unless you explicitly request additional features by defining one or more of the feature macros. See GNU CC Command Options, for more information about GCC options.

You should define these macros by using `#define' preprocessor directives at the top of your source code files. These directives must come before any #include of a system header file. It is best to make them the very first thing in the file, preceded only by comments. You could also use the `-D' option to GCC, but it's better if you make the source files indicate their own meaning in a self-contained way.

This system exists to allow the library to conform to multiple standards. Although the different standards are often described as supersets of each other, they are usually incompatible because larger standards require functions with names that smaller ones reserve to the user program. This is not mere pedantry — it has been a problem in practice. For instance, some non-GNU programs define functions named getline that have nothing to do with this library's getline. They would not be compilable if all features were enabled indiscriminately.

This should not be used to verify that a program conforms to a limited standard. It is insufficient for this purpose, as it will not protect you from including header files outside the standard, or relying on semantics undefined within the standard.

— Macro: _POSIX_SOURCE

If you define this macro, then the functionality from the POSIX.1 standard (IEEE Standard 1003.1) is available, as well as all of the ISO C facilities.

The state of _POSIX_SOURCE is irrelevant if you define the macro _POSIX_C_SOURCE to a positive integer.

— Macro: _POSIX_C_SOURCE

Define this macro to a positive integer to control which POSIX functionality is made available. The greater the value of this macro, the more functionality is made available.

If you define this macro to a value greater than or equal to 1, then the functionality from the 1990 edition of the POSIX.1 standard (IEEE Standard 1003.1-1990) is made available.

If you define this macro to a value greater than or equal to 2, then the functionality from the 1992 edition of the POSIX.2 standard (IEEE Standard 1003.2-1992) is made available.

If you define this macro to a value greater than or equal to 199309L, then the functionality from the 1993 edition of the POSIX.1b standard (IEEE Standard 1003.1b-1993) is made available.

Greater values for _POSIX_C_SOURCE will enable future extensions. The POSIX standards process will define these values as necessary, and the GNU C Library should support them some time after they become standardized. The 1996 edition of POSIX.1 (ISO/IEC 9945-1: 1996) states that if you define _POSIX_C_SOURCE to a value greater than or equal to 199506L, then the functionality from the 1996 edition is made available.

— Macro: _BSD_SOURCE

If you define this macro, functionality derived from 4.3 BSD Unix is included as well as the ISO C, POSIX.1, and POSIX.2 material.

Some of the features derived from 4.3 BSD Unix conflict with the corresponding features specified by the POSIX.1 standard. If this macro is defined, the 4.3 BSD definitions take precedence over the POSIX definitions.

Due to the nature of some of the conflicts between 4.3 BSD and POSIX.1, you need to use a special BSD compatibility library when linking programs compiled for BSD compatibility. This is because some functions must be defined in two different ways, one of them in the normal C library, and one of them in the compatibility library. If your program defines _BSD_SOURCE, you must give the option `-lbsd-compat' to the compiler or linker when linking the program, to tell it to find functions in this special compatibility library before looking for them in the normal C library.

— Macro: _SVID_SOURCE

If you define this macro, functionality derived from SVID is included as well as the ISO C, POSIX.1, POSIX.2, and X/Open material.

— Macro: _XOPEN_SOURCE
— Macro: _XOPEN_SOURCE_EXTENDED

If you define this macro, functionality described in the X/Open Portability Guide is included. This is a superset of the POSIX.1 and POSIX.2 functionality and in fact _POSIX_SOURCE and _POSIX_C_SOURCE are automatically defined.

As the unification of all Unices, functionality only available in BSD and SVID is also included.

If the macro _XOPEN_SOURCE_EXTENDED is also defined, even more functionality is available. The extra functions will make all functions available which are necessary for the X/Open Unix brand.

If the macro _XOPEN_SOURCE has the value 500 this includes all functionality described so far plus some new definitions from the Single Unix Specification, version 2.

— Macro: _LARGEFILE_SOURCE

If this macro is defined some extra functions are available which rectify a few shortcomings in all previous standards. Specifically, the functions fseeko and ftello are available. Without these functions the difference between the ISO C interface (fseek, ftell) and the low-level POSIX interface (lseek) would lead to problems.

This macro was introduced as part of the Large File Support extension (LFS).

— Macro: _LARGEFILE64_SOURCE

If you define this macro an additional set of functions is made available which enables 32 bit systems to use files of sizes beyond the usual limit of 2GB. This interface is not available if the system does not support files that large. On systems where the natural file size limit is greater than 2GB (i.e., on 64 bit systems) the new functions are identical to the replaced functions.

The new functionality is made available by a new set of types and functions which replace the existing ones. The names of these new objects contain 64 to indicate the intention, e.g., off_t vs. off64_t and fseeko vs. fseeko64.

This macro was introduced as part of the Large File Support extension (LFS). It is a transition interface for the period when 64 bit offsets are not generally used (see _FILE_OFFSET_BITS).

— Macro: _FILE_OFFSET_BITS

This macro determines which file system interface shall be used, one replacing the other. Whereas _LARGEFILE64_SOURCE makes the 64 bit interface available as an additional interface, _FILE_OFFSET_BITS allows the 64 bit interface to replace the old interface.

If _FILE_OFFSET_BITS is undefined, or if it is defined to the value 32, nothing changes. The 32 bit interface is used and types like off_t have a size of 32 bits on 32 bit systems.

If the macro is defined to the value 64, the large file interface replaces the old interface. I.e., the functions are not made available under different names (as they are with _LARGEFILE64_SOURCE). Instead the old function names now reference the new functions, e.g., a call to fseeko now indeed calls fseeko64.

This macro should only be selected if the system provides mechanisms for handling large files. On 64 bit systems this macro has no effect since the *64 functions are identical to the normal functions.

This macro was introduced as part of the Large File Support extension (LFS).

— Macro: _ISOC99_SOURCE

Until the revised ISO C standard is widely adopted the new features are not automatically enabled. The GNU libc nevertheless has a complete implementation of the new standard and to enable the new features the macro _ISOC99_SOURCE should be defined.

— Macro: _GNU_SOURCE

If you define this macro, everything is included: ISO C89, ISO C99, POSIX.1, POSIX.2, BSD, SVID, X/Open, LFS, and GNU extensions. In the cases where POSIX.1 conflicts with BSD, the POSIX definitions take precedence.

If you want to get the full effect of _GNU_SOURCE but make the BSD definitions take precedence over the POSIX definitions, use this sequence of definitions: 

          #define _GNU_SOURCE
          #define _BSD_SOURCE
          #define _SVID_SOURCE

Note that if you do this, you must link your program with the BSD compatibility library by passing the `-lbsd-compat' option to the compiler or linker. Note: If you forget to do this, you may get very strange errors at run time.

— Macro: _REENTRANT
— Macro: _THREAD_SAFE

If you define one of these macros, reentrant versions of several functions get declared. Some of the functions are specified in POSIX.1c but many others are only available on a few other systems or are unique to GNU libc. The problem is the delay in the standardization of the thread safe C library interface.

Unlike on some other systems, no special version of the C library must be used for linking. There is only one version but while compiling this it must have been specified to compile as thread safe.

We recommend you use _GNU_SOURCE in new programs. If you don't specify the `-ansi' option to GCC and don't define any of these macros explicitly, the effect is the same as defining _POSIX_C_SOURCE to 2 and _POSIX_SOURCE, _SVID_SOURCE, and _BSD_SOURCE to 1.

When you define a feature test macro to request a larger class of features, it is harmless to define in addition a feature test macro for a subset of those features. For example, if you define _POSIX_C_SOURCE, then defining _POSIX_SOURCE as well has no effect. Likewise, if you define _GNU_SOURCE, then defining either _POSIX_SOURCE or _POSIX_C_SOURCE or _SVID_SOURCE as well has no effect.

Note, however, that the features of _BSD_SOURCE are not a subset of any of the other feature test macros supported. This is because it defines BSD features that take precedence over the POSIX features that are requested by the other macros. For this reason, defining _BSD_SOURCE in addition to the other feature test macros does have an effect: it causes the BSD features to take priority over the conflicting POSIX features.

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1.4 Roadmap to the Manual

Here is an overview of the contents of the remaining chapters of this manual.

If you already know the name of the facility you are interested in, you can look it up in Library Summary. This gives you a summary of its syntax and a pointer to where you can find a more detailed description. This appendix is particularly useful if you just want to verify the order and type of arguments to a function, for example. It also tells you what standard or system each function, variable, or macro is derived from.

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2 Error Reporting

Many functions in the GNU C library detect and report error conditions, and sometimes your programs need to check for these error conditions. For example, when you open an input file, you should verify that the file was actually opened correctly, and print an error message or take other appropriate action if the call to the library function failed.

This chapter describes how the error reporting facility works. Your program should include the header file errno.h to use this facility.

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2.1 Checking for Errors

Most library functions return a special value to indicate that they have failed. The special value is typically -1, a null pointer, or a constant such as EOF that is defined for that purpose. But this return value tells you only that an error has occurred. To find out what kind of error it was, you need to look at the error code stored in the variable errno. This variable is declared in the header file errno.h.

— Variable: volatile int errno

The variable errno contains the system error number. You can change the value of errno.

Since errno is declared volatile, it might be changed asynchronously by a signal handler; see Defining Handlers. However, a properly written signal handler saves and restores the value of errno, so you generally do not need to worry about this possibility except when writing signal handlers.

The initial value of errno at program startup is zero. Many library functions are guaranteed to set it to certain nonzero values when they encounter certain kinds of errors. These error conditions are listed for each function. These functions do not change errno when they succeed; thus, the value of errno after a successful call is not necessarily zero, and you should not use errno to determine whether a call failed. The proper way to do that is documented for each function. If the call failed, you can examine errno.

Many library functions can set errno to a nonzero value as a result of calling other library functions which might fail. You should assume that any library function might alter errno when the function returns an error.

Portability Note: ISO C specifies errno as a “modifiable lvalue” rather than as a variable, permitting it to be implemented as a macro. For example, its expansion might involve a function call, like *_errno (). In fact, that is what it is on the GNU system itself. The GNU library, on non-GNU systems, does whatever is right for the particular system.

There are a few library functions, like sqrt and atan, that return a perfectly legitimate value in case of an error, but also set errno. For these functions, if you want to check to see whether an error occurred, the recommended method is to set errno to zero before calling the function, and then check its value afterward.

All the error codes have symbolic names; they are macros defined in errno.h. The names start with `E' and an upper-case letter or digit; you should consider names of this form to be reserved names. See Reserved Names.

The error code values are all positive integers and are all distinct, with one exception: EWOULDBLOCK and EAGAIN are the same. Since the values are distinct, you can use them as labels in a switch statement; just don't use both EWOULDBLOCK and EAGAIN. Your program should not make any other assumptions about the specific values of these symbolic constants.

The value of errno doesn't necessarily have to correspond to any of these macros, since some library functions might return other error codes of their own for other situations. The only values that are guaranteed to be meaningful for a particular library function are the ones that this manual lists for that function.

On non-GNU systems, almost any system call can return EFAULT if it is given an invalid pointer as an argument. Since this could only happen as a result of a bug in your program, and since it will not happen on the GNU system, we have saved space by not mentioning EFAULT in the descriptions of individual functions.

In some Unix systems, many system calls can also return EFAULT if given as an argument a pointer into the stack, and the kernel for some obscure reason fails in its attempt to extend the stack. If this ever happens, you should probably try using statically or dynamically allocated memory instead of stack memory on that system.

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2.2 Error Codes

The error code macros are defined in the header file errno.h. All of them expand into integer constant values. Some of these error codes can't occur on the GNU system, but they can occur using the GNU library on other systems.

— Macro: int EPERM

Operation not permitted; only the owner of the file (or other resource) or processes with special privileges can perform the operation.

— Macro: int ENOENT

No such file or directory. This is a “file doesn't exist” error for ordinary files that are referenced in contexts where they are expected to already exist.

— Macro: int ESRCH

No process matches the specified process ID.

— Macro: int EINTR

Interrupted function call; an asynchronous signal occurred and prevented completion of the call. When this happens, you should try the call again.

You can choose to have functions resume after a signal that is handled, rather than failing with EINTR; see Interrupted Primitives.

— Macro: int EIO

Input/output error; usually used for physical read or write errors.

— Macro: int ENXIO

No such device or address. The system tried to use the device represented by a file you specified, and it couldn't find the device. This can mean that the device file was installed incorrectly, or that the physical device is missing or not correctly attached to the computer.

— Macro: int E2BIG

Argument list too long; used when the arguments passed to a new program being executed with one of the exec functions (see Executing a File) occupy too much memory space. This condition never arises in the GNU system.

— Macro: int ENOEXEC

Invalid executable file format. This condition is detected by the exec functions; see Executing a File.

— Macro: int EBADF

Bad file descriptor; for example, I/O on a descriptor that has been closed or reading from a descriptor open only for writing (or vice versa).

— Macro: int ECHILD

There are no child processes. This error happens on operations that are supposed to manipulate child processes, when there aren't any processes to manipulate.

— Macro: int EDEADLK

Deadlock avoided; allocating a system resource would have resulted in a deadlock situation. The system does not guarantee that it will notice all such situations. This error means you got lucky and the system noticed; it might just hang. See File Locks, for an example.

— Macro: int ENOMEM

No memory available. The system cannot allocate more virtual memory because its capacity is full.

— Macro: int EACCES

Permission denied; the file permissions do not allow the attempted operation.

— Macro: int EFAULT

Bad address; an invalid pointer was detected. In the GNU system, this error never happens; you get a signal instead.

— Macro: int ENOTBLK

A file that isn't a block special file was given in a situation that requires one. For example, trying to mount an ordinary file as a file system in Unix gives this error.

— Macro: int EBUSY

Resource busy; a system resource that can't be shared is already in use. For example, if you try to delete a file that is the root of a currently mounted filesystem, you get this error.

— Macro: int EEXIST

File exists; an existing file was specified in a context where it only makes sense to specify a new file.

— Macro: int EXDEV

An attempt to make an improper link across file systems was detected. This happens not only when you use link (see Hard Links) but also when you rename a file with rename (see Renaming Files).

— Macro: int ENODEV

The wrong type of device was given to a function that expects a particular sort of device.

— Macro: int ENOTDIR

A file that isn't a directory was specified when a directory is required.

— Macro: int EISDIR

File is a directory; you cannot open a directory for writing, or create or remove hard links to it.

— Macro: int EINVAL

Invalid argument. This is used to indicate various kinds of problems with passing the wrong argument to a library function.

— Macro: int EMFILE

The current process has too many files open and can't open any more. Duplicate descriptors do count toward this limit.

In BSD and GNU, the number of open files is controlled by a resource limit that can usually be increased. If you get this error, you might want to increase the RLIMIT_NOFILE limit or make it unlimited; see Limits on Resources.

— Macro: int ENFILE

There are too many distinct file openings in the entire system. Note that any number of linked channels count as just one file opening; see Linked Channels. This error never occurs in the GNU system.

— Macro: int ENOTTY

Inappropriate I/O control operation, such as trying to set terminal modes on an ordinary file.

— Macro: int ETXTBSY

An attempt to execute a file that is currently open for writing, or write to a file that is currently being executed. Often using a debugger to run a program is considered having it open for writing and will cause this error. (The name stands for “text file busy”.) This is not an error in the GNU system; the text is copied as necessary.

— Macro: int EFBIG

File too big; the size of a file would be larger than allowed by the system.

— Macro: int ENOSPC

No space left on device; write operation on a file failed because the disk is full.

— Macro: int ESPIPE

Invalid seek operation (such as on a pipe).

— Macro: int EROFS

An attempt was made to modify something on a read-only file system.

— Macro: int EMLINK

Too many links; the link count of a single file would become too large. rename can cause this error if the file being renamed already has as many links as it can take (see Renaming Files).

— Macro: int EPIPE

Broken pipe; there is no process reading from the other end of a pipe. Every library function that returns this error code also generates a SIGPIPE signal; this signal terminates the program if not handled or blocked. Thus, your program will never actually see EPIPE unless it has handled or blocked SIGPIPE.

— Macro: int EDOM

Domain error; used by mathematical functions when an argument value does not fall into the domain over which the function is defined.

— Macro: int ERANGE

Range error; used by mathematical functions when the result value is not representable because of overflow or underflow.

— Macro: int EAGAIN

Resource temporarily unavailable; the call might work if you try again later. The macro EWOULDBLOCK is another name for EAGAIN; they are always the same in the GNU C library.

This error can happen in a few different situations:

— Macro: int EWOULDBLOCK

In the GNU C library, this is another name for EAGAIN (above). The values are always the same, on every operating system.

C libraries in many older Unix systems have EWOULDBLOCK as a separate error code.

— Macro: int EINPROGRESS

An operation that cannot complete immediately was initiated on an object that has non-blocking mode selected. Some functions that must always block (such as connect; see Connecting) never return EAGAIN. Instead, they return EINPROGRESS to indicate that the operation has begun and will take some time. Attempts to manipulate the object before the call completes return EALREADY. You can use the select function to find out when the pending operation has completed; see Waiting for I/O.

— Macro: int EALREADY

An operation is already in progress on an object that has non-blocking mode selected.

— Macro: int ENOTSOCK

A file that isn't a socket was specified when a socket is required.

— Macro: int EMSGSIZE

The size of a message sent on a socket was larger than the supported maximum size.

— Macro: int EPROTOTYPE

The socket type does not support the requested communications protocol.

— Macro: int ENOPROTOOPT

You specified a socket option that doesn't make sense for the particular protocol being used by the socket. See Socket Options.

— Macro: int EPROTONOSUPPORT

The socket domain does not support the requested communications protocol (perhaps because the requested protocol is completely invalid). See Creating a Socket.

— Macro: int ESOCKTNOSUPPORT

The socket type is not supported.

— Macro: int EOPNOTSUPP

The operation you requested is not supported. Some socket functions don't make sense for all types of sockets, and others may not be implemented for all communications protocols. In the GNU system, this error can happen for many calls when the object does not support the particular operation; it is a generic indication that the server knows nothing to do for that call.

— Macro: int EPFNOSUPPORT

The socket communications protocol family you requested is not supported.

— Macro: int EAFNOSUPPORT

The address family specified for a socket is not supported; it is inconsistent with the protocol being used on the socket. See Sockets.

— Macro: int EADDRINUSE

The requested socket address is already in use. See Socket Addresses.

— Macro: int EADDRNOTAVAIL

The requested socket address is not available; for example, you tried to give a socket a name that doesn't match the local host name. See Socket Addresses.

— Macro: int ENETDOWN

A socket operation failed because the network was down.

— Macro: int ENETUNREACH

A socket operation failed because the subnet containing the remote host was unreachable.

— Macro: int ENETRESET

A network connection was reset because the remote host crashed.

— Macro: int ECONNABORTED

A network connection was aborted locally.

— Macro: int ECONNRESET

A network connection was closed for reasons outside the control of the local host, such as by the remote machine rebooting or an unrecoverable protocol violation.

— Macro: int ENOBUFS

The kernel's buffers for I/O operations are all in use. In GNU, this error is always synonymous with ENOMEM; you may get one or the other from network operations.

— Macro: int EISCONN

You tried to connect a socket that is already connected. See Connecting.

— Macro: int ENOTCONN

The socket is not connected to anything. You get this error when you try to transmit data over a socket, without first specifying a destination for the data. For a connectionless socket (for datagram protocols, such as UDP), you get EDESTADDRREQ instead.

— Macro: int EDESTADDRREQ

No default destination address was set for the socket. You get this error when you try to transmit data over a connectionless socket, without first specifying a destination for the data with connect.

— Macro: int ESHUTDOWN

The socket has already been shut down.

— Macro: int ETOOMANYREFS

???

— Macro: int ETIMEDOUT

A socket operation with a specified timeout received no response during the timeout period.

— Macro: int ECONNREFUSED

A remote host refused to allow the network connection (typically because it is not running the requested service).

— Macro: int ELOOP

Too many levels of symbolic links were encountered in looking up a file name. This often indicates a cycle of symbolic links.

— Macro: int ENAMETOOLONG

Filename too long (longer than PATH_MAX; see Limits for Files) or host name too long (in gethostname or sethostname; see Host Identification).

— Macro: int EHOSTDOWN

The remote host for a requested network connection is down.

— Macro: int EHOSTUNREACH

The remote host for a requested network connection is not reachable.

— Macro: int ENOTEMPTY

Directory not empty, where an empty directory was expected. Typically, this error occurs when you are trying to delete a directory.

— Macro: int EPROCLIM

This means that the per-user limit on new process would be exceeded by an attempted fork. See Limits on Resources, for details on the RLIMIT_NPROC limit.

— Macro: int EUSERS

The file quota system is confused because there are too many users.

— Macro: int EDQUOT

The user's disk quota was exceeded.

— Macro: int ESTALE

Stale NFS file handle. This indicates an internal confusion in the NFS system which is due to file system rearrangements on the server host. Repairing this condition usually requires unmounting and remounting the NFS file system on the local host.

— Macro: int EREMOTE

An attempt was made to NFS-mount a remote file system with a file name that already specifies an NFS-mounted file. (This is an error on some operating systems, but we expect it to work properly on the GNU system, making this error code impossible.)

— Macro: int EBADRPC

???

— Macro: int ERPCMISMATCH

???

— Macro: int EPROGUNAVAIL

???

— Macro: int EPROGMISMATCH

???

— Macro: int EPROCUNAVAIL

???

— Macro: int ENOLCK

No locks available. This is used by the file locking facilities; see File Locks. This error is never generated by the GNU system, but it can result from an operation to an NFS server running another operating system.

— Macro: int EFTYPE

Inappropriate file type or format. The file was the wrong type for the operation, or a data file had the wrong format.

On some systems chmod returns this error if you try to set the sticky bit on a non-directory file; see Setting Permissions.

— Macro: int EAUTH

???

— Macro: int ENEEDAUTH

???

— Macro: int ENOSYS

Function not implemented. This indicates that the function called is not implemented at all, either in the C library itself or in the operating system. When you get this error, you can be sure that this particular function will always fail with ENOSYS unless you install a new version of the C library or the operating system.

— Macro: int ENOTSUP

Not supported. A function returns this error when certain parameter values are valid, but the functionality they request is not available. This can mean that the function does not implement a particular command or option value or flag bit at all. For functions that operate on some object given in a parameter, such as a file descriptor or a port, it might instead mean that only that specific object (file descriptor, port, etc.) is unable to support the other parameters given; different file descriptors might support different ranges of parameter values.

If the entire function is not available at all in the implementation, it returns ENOSYS instead.

— Macro: int EILSEQ

While decoding a multibyte character the function came along an invalid or an incomplete sequence of bytes or the given wide character is invalid.

— Macro: int EBACKGROUND

In the GNU system, servers supporting the term protocol return this error for certain operations when the caller is not in the foreground process group of the terminal. Users do not usually see this error because functions such as read and write translate it into a SIGTTIN or SIGTTOU signal. See Job Control, for information on process groups and these signals.

— Macro: int EDIED

In the GNU system, opening a file returns this error when the file is translated by a program and the translator program dies while starting up, before it has connected to the file.

— Macro: int ED

The experienced user will know what is wrong.

— Macro: int EGREGIOUS

You did what?

— Macro: int EIEIO

Go home and have a glass of warm, dairy-fresh milk.

— Macro: int EGRATUITOUS

This error code has no purpose.

— Macro: int EBADMSG
— Macro: int EIDRM
— Macro: int EMULTIHOP
— Macro: int ENODATA
— Macro: int ENOLINK
— Macro: int ENOMSG
— Macro: int ENOSR
— Macro: int ENOSTR
— Macro: int EOVERFLOW
— Macro: int EPROTO
— Macro: int ETIME
— Macro: int ECANCELED

Operation canceled; an asynchronous operation was canceled before it completed. See Asynchronous I/O. When you call aio_cancel, the normal result is for the operations affected to complete with this error; see Cancel AIO Operations.

The following error codes are defined by the Linux/i386 kernel. They are not yet documented.

— Macro: int ERESTART
— Macro: int ECHRNG
— Macro: int EL2NSYNC
— Macro: int EL3HLT
— Macro: int EL3RST
— Macro: int ELNRNG
— Macro: int EUNATCH
— Macro: int ENOCSI
— Macro: int EL2HLT
— Macro: int EBADE
— Macro: int EBADR
— Macro: int EXFULL
— Macro: int ENOANO
— Macro: int EBADRQC
— Macro: int EBADSLT
— Macro: int EDEADLOCK
— Macro: int EBFONT
— Macro: int ENONET
— Macro: int ENOPKG
— Macro: int EADV
— Macro: int ESRMNT
— Macro: int ECOMM
— Macro: int EDOTDOT
— Macro: int ENOTUNIQ
— Macro: int EBADFD
— Macro: int EREMCHG
— Macro: int ELIBACC
— Macro: int ELIBBAD
— Macro: int ELIBSCN
— Macro: int ELIBMAX
— Macro: int ELIBEXEC
— Macro: int ESTRPIPE
— Macro: int EUCLEAN
— Macro: int ENOTNAM
— Macro: int ENAVAIL
— Macro: int EISNAM
— Macro: int EREMOTEIO
— Macro: int ENOMEDIUM
— Macro: int EMEDIUMTYPE
— Macro: int ENOKEY
— Macro: int EKEYEXPIRED
— Macro: int EKEYREVOKED
— Macro: int EKEYREJECTED
— Macro: int EOWNERDEAD
— Macro: int ENOTRECOVERABLE

Previous: Error Codes, Up: Error Reporting

2.3 Error Messages

The library has functions and variables designed to make it easy for your program to report informative error messages in the customary format about the failure of a library call. The functions strerror and perror give you the standard error message for a given error code; the variable program_invocation_short_name gives you convenient access to the name of the program that encountered the error.

— Function: char * strerror (int errnum)

The strerror function maps the error code (see Checking for Errors) specified by the errnum argument to a descriptive error message string. The return value is a pointer to this string.

The value errnum normally comes from the variable errno.

You should not modify the string returned by strerror. Also, if you make subsequent calls to strerror, the string might be overwritten. (But it's guaranteed that no library function ever calls strerror behind your back.)

The function strerror is declared in string.h.

— Function: char * strerror_r (int errnum, char *buf, size_t n)

The strerror_r function works like strerror but instead of returning the error message in a statically allocated buffer shared by all threads in the process, it returns a private copy for the thread. This might be either some permanent global data or a message string in the user supplied buffer starting at buf with the length of n bytes.

At most n characters are written (including the NUL byte) so it is up to the user to select the buffer large enough.

This function should always be used in multi-threaded programs since there is no way to guarantee the string returned by strerror really belongs to the last call of the current thread.

This function strerror_r is a GNU extension and it is declared in string.h.

— Function: void perror (const char *message)

This function prints an error message to the stream stderr; see Standard Streams. The orientation of stderr is not changed.

If you call perror with a message that is either a null pointer or an empty string, perror just prints the error message corresponding to errno, adding a trailing newline.

If you supply a non-null message argument, then perror prefixes its output with this string. It adds a colon and a space character to separate the message from the error string corresponding to errno.

The function perror is declared in stdio.h.

strerror and perror produce the exact same message for any given error code; the precise text varies from system to system. On the GNU system, the messages are fairly short; there are no multi-line messages or embedded newlines. Each error message begins with a capital letter and does not include any terminating punctuation.

Compatibility Note: The strerror function was introduced in ISO C89. Many older C systems do not support this function yet.

Many programs that don't read input from the terminal are designed to exit if any system call fails. By convention, the error message from such a program should start with the program's name, sans directories. You can find that name in the variable program_invocation_short_name; the full file name is stored the variable program_invocation_name.

— Variable: char * program_invocation_name

This variable's value is the name that was used to invoke the program running in the current process. It is the same as argv[0]. Note that this is not necessarily a useful file name; often it contains no directory names. See Program Arguments.

— Variable: char * program_invocation_short_name

This variable's value is the name that was used to invoke the program running in the current process, with directory names removed. (That is to say, it is the same as program_invocation_name minus everything up to the last slash, if any.)

The library initialization code sets up both of these variables before calling main.

Portability Note: These two variables are GNU extensions. If you want your program to work with non-GNU libraries, you must save the value of argv[0] in main, and then strip off the directory names yourself. We added these extensions to make it possible to write self-contained error-reporting subroutines that require no explicit cooperation from main.

Here is an example showing how to handle failure to open a file correctly. The function open_sesame tries to open the named file for reading and returns a stream if successful. The fopen library function returns a null pointer if it couldn't open the file for some reason. In that situation, open_sesame constructs an appropriate error message using the strerror function, and terminates the program. If we were going to make some other library calls before passing the error code to strerror, we'd have to save it in a local variable instead, because those other library functions might overwrite errno in the meantime. 

     #include <errno.h>
     #include <stdio.h>
     #include <stdlib.h>
     #include <string.h>
     
     FILE *
     open_sesame (char *name)
     {
       FILE *stream;
     
       errno = 0;
       stream = fopen (name, "r");
       if (stream == NULL)
         {
           fprintf (stderr, "%s: Couldn't open file %s; %s\n",
                    program_invocation_short_name, name, strerror (errno));
           exit (EXIT_FAILURE);
         }
       else
         return stream;
     }

Using perror has the advantage that the function is portable and available on all systems implementing ISO C. But often the text perror generates is not what is wanted and there is no way to extend or change what perror does. The GNU coding standard, for instance, requires error messages to be preceded by the program name and programs which read some input files should should provide information about the input file name and the line number in case an error is encountered while reading the file. For these occasions there are two functions available which are widely used throughout the GNU project. These functions are declared in error.h.

— Function: void error (int status, int errnum, const char *format, ...)

The error function can be used to report general problems during program execution. The format argument is a format string just like those given to the printf family of functions. The arguments required for the format can follow the format parameter. Just like perror, error also can report an error code in textual form. But unlike perror the error value is explicitly passed to the function in the errnum parameter. This eliminates the problem mentioned above that the error reporting function must be called immediately after the function causing the error since otherwise errno might have a different value.

The error prints first the program name. If the application defined a global variable error_print_progname and points it to a function this function will be called to print the program name. Otherwise the string from the global variable program_name is used. The program name is followed by a colon and a space which in turn is followed by the output produced by the format string. If the errnum parameter is non-zero the format string output is followed by a colon and a space, followed by the error message for the error code errnum. In any case is the output terminated with a newline.

The output is directed to the stderr stream. If the stderr wasn't oriented before the call it will be narrow-oriented afterwards.

The function will return unless the status parameter has a non-zero value. In this case the function will call exit with the status value for its parameter and therefore never return. If error returns the global variable error_message_count is incremented by one to keep track of the number of errors reported.

— Function: void error_at_line (int status, int errnum, const char *fname, unsigned int lineno, const char *format, ...)

The error_at_line function is very similar to the error function. The only difference are the additional parameters fname and lineno. The handling of the other parameters is identical to that of error except that between the program name and the string generated by the format string additional text is inserted.

Directly following the program name a colon, followed by the file name pointer to by fname, another colon, and a value of lineno is printed.

This additional output of course is meant to be used to locate an error in an input file (like a programming language source code file etc).

If the global variable error_one_per_line is set to a non-zero value error_at_line will avoid printing consecutive messages for the same file and line. Repetition which are not directly following each other are not caught.

Just like error this function only returned if status is zero. Otherwise exit is called with the non-zero value. If error returns the global variable error_message_count is incremented by one to keep track of the number of errors reported.

As mentioned above the error and error_at_line functions can be customized by defining a variable named error_print_progname.

— Variable: void (*) error_print_progname (void)

If the error_print_progname variable is defined to a non-zero value the function pointed to is called by error or error_at_line. It is expected to print the program name or do something similarly useful.

The function is expected to be print to the stderr stream and must be able to handle whatever orientation the stream has.

The variable is global and shared by all threads.

— Variable: unsigned int error_message_count

The error_message_count variable is incremented whenever one of the functions error or error_at_line returns. The variable is global and shared by all threads.

— Variable: int error_one_per_line

The error_one_per_line variable influences only error_at_line. Normally the error_at_line function creates output for every invocation. If error_one_per_line is set to a non-zero value error_at_line keeps track of the last file name and line number for which an error was reported and avoid directly following messages for the same file and line. This variable is global and shared by all threads.

A program which read some input file and reports errors in it could look like this: 

     {
       char *line = NULL;
       size_t len = 0;
       unsigned int lineno = 0;
     
       error_message_count = 0;
       while (! feof_unlocked (fp))
         {
           ssize_t n = getline (&line, &len, fp);
           if (n <= 0)
             /* End of file or error.  */
             break;
           ++lineno;
     
           /* Process the line.  */
           ...
     
           if (Detect error in line)
             error_at_line (0, errval, filename, lineno,
                            "some error text %s", some_variable);
         }
     
       if (error_message_count != 0)
         error (EXIT_FAILURE, 0, "%u errors found", error_message_count);
     }

error and error_at_line are clearly the functions of choice and enable the programmer to write applications which follow the GNU coding standard. The GNU libc additionally contains functions which are used in BSD for the same purpose. These functions are declared in err.h. It is generally advised to not use these functions. They are included only for compatibility.

— Function: void warn (const char *format, ...)

The warn function is roughly equivalent to a call like 

            error (0, errno, format, the parameters)

except that the global variables error respects and modifies are not used.

— Function: void vwarn (const char *format, va_list)

The vwarn function is just like warn except that the parameters for the handling of the format string format are passed in as an value of type va_list.

— Function: void warnx (const char *format, ...)

The warnx function is roughly equivalent to a call like 

            error (0, 0, format, the parameters)

except that the global variables error respects and modifies are not used. The difference to warn is that no error number string is printed.

— Function: void vwarnx (const char *format, va_list)

The vwarnx function is just like warnx except that the parameters for the handling of the format string format are passed in as an value of type va_list.

— Function: void err (int status, const char *format, ...)

The err function is roughly equivalent to a call like 

            error (status, errno, format, the parameters)

except that the global variables error respects and modifies are not used and that the program is exited even if status is zero.

— Function: void verr (int status, const char *format, va_list)

The verr function is just like err except that the parameters for the handling of the format string format are passed in as an value of type va_list.

— Function: void errx (int status, const char *format, ...)

The errx function is roughly equivalent to a call like 

            error (status, 0, format, the parameters)

except that the global variables error respects and modifies are not used and that the program is exited even if status is zero. The difference to err is that no error number string is printed.

— Function: void verrx (int status, const char *format, va_list)

The verrx function is just like errx except that the parameters for the handling of the format string format are passed in as an value of type va_list.

Next: , Previous: Error Reporting, Up: Top

3 Virtual Memory Allocation And Paging

This chapter describes how processes manage and use memory in a system that uses the GNU C library.

The GNU C Library has several functions for dynamically allocating virtual memory in various ways. They vary in generality and in efficiency. The library also provides functions for controlling paging and allocation of real memory.

Memory mapped I/O is not discussed in this chapter. See Memory-mapped I/O.

Next: , Up: Memory

3.1 Process Memory Concepts

One of the most basic resources a process has available to it is memory. There are a lot of different ways systems organize memory, but in a typical one, each process has one linear virtual address space, with addresses running from zero to some huge maximum. It need not be contiguous; i.e., not all of these addresses actually can be used to store data.

The virtual memory is divided into pages (4 kilobytes is typical). Backing each page of virtual memory is a page of real memory (called a frame) or some secondary storage, usually disk space. The disk space might be swap space or just some ordinary disk file. Actually, a page of all zeroes sometimes has nothing at all backing it – there's just a flag saying it is all zeroes. The same frame of real memory or backing store can back multiple virtual pages belonging to multiple processes. This is normally the case, for example, with virtual memory occupied by GNU C library code. The same real memory frame containing the printf function backs a virtual memory page in each of the existing processes that has a printf call in its program.

In order for a program to access any part of a virtual page, the page must at that moment be backed by (“connected to”) a real frame. But because there is usually a lot more virtual memory than real memory, the pages must move back and forth between real memory and backing store regularly, coming into real memory when a process needs to access them and then retreating to backing store when not needed anymore. This movement is called paging.

When a program attempts to access a page which is not at that moment backed by real memory, this is known as a page fault. When a page fault occurs, the kernel suspends the process, places the page into a real page frame (this is called “paging in” or “faulting in”), then resumes the process so that from the process' point of view, the page was in real memory all along. In fact, to the process, all pages always seem to be in real memory. Except for one thing: the elapsed execution time of an instruction that would normally be a few nanoseconds is suddenly much, much, longer (because the kernel normally has to do I/O to complete the page-in). For programs sensitive to that, the functions described in Locking Pages can control it. Within each virtual address space, a process has to keep track of what is at which addresses, and that process is called memory allocation. Allocation usually brings to mind meting out scarce resources, but in the case of virtual memory, that's not a major goal, because there is generally much more of it than anyone needs. Memory allocation within a process is mainly just a matter of making sure that the same byte of memory isn't used to store two different things.

Processes allocate memory in two major ways: by exec and programmatically. Actually, forking is a third way, but it's not very interesting. See Creating a Process.

Exec is the operation of creating a virtual address space for a process, loading its basic program into it, and executing the program. It is done by the “exec” family of functions (e.g. execl). The operation takes a program file (an executable), it allocates space to load all the data in the executable, loads it, and transfers control to it. That data is most notably the instructions of the program (the text), but also literals and constants in the program and even some variables: C variables with the static storage class (see Memory Allocation and C). Once that program begins to execute, it uses programmatic allocation to gain additional memory. In a C program with the GNU C library, there are two kinds of programmatic allocation: automatic and dynamic. See Memory Allocation and C.

Memory-mapped I/O is another form of dynamic virtual memory allocation. Mapping memory to a file means declaring that the contents of certain range of a process' addresses shall be identical to the contents of a specified regular file. The system makes the virtual memory initially contain the contents of the file, and if you modify the memory, the system writes the same modification to the file. Note that due to the magic of virtual memory and page faults, there is no reason for the system to do I/O to read the file, or allocate real memory for its contents, until the program accesses the virtual memory. See Memory-mapped I/O. Just as it programmatically allocates memory, the program can programmatically deallocate (free) it. You can't free the memory that was allocated by exec. When the program exits or execs, you might say that all its memory gets freed, but since in both cases the address space ceases to exist, the point is really moot. See Program Termination. A process' virtual address space is divided into segments. A segment is a contiguous range of virtual addresses. Three important segments are:

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3.2 Allocating Storage For Program Data

This section covers how ordinary programs manage storage for their data, including the famous malloc function and some fancier facilities special the GNU C library and GNU Compiler.

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3.2.1 Memory Allocation in C Programs

The C language supports two kinds of memory allocation through the variables in C programs:

A third important kind of memory allocation, dynamic allocation, is not supported by C variables but is available via GNU C library functions.

3.2.1.1 Dynamic Memory Allocation

Dynamic memory allocation is a technique in which programs determine as they are running where to store some information. You need dynamic allocation when the amount of memory you need, or how long you continue to need it, depends on factors that are not known before the program runs.

For example, you may need a block to store a line read from an input file; since there is no limit to how long a line can be, you must allocate the memory dynamically and make it dynamically larger as you read more of the line.

Or, you may need a block for each record or each definition in the input data; since you can't know in advance how many there will be, you must allocate a new block for each record or definition as you read it.

When you use dynamic allocation, the allocation of a block of memory is an action that the program requests explicitly. You call a function or macro when you want to allocate space, and specify the size with an argument. If you want to free the space, you do so by calling another function or macro. You can do these things whenever you want, as often as you want.

Dynamic allocation is not supported by C variables; there is no storage class “dynamic”, and there can never be a C variable whose value is stored in dynamically allocated space. The only way to get dynamically allocated memory is via a system call (which is generally via a GNU C library function call), and the only way to refer to dynamically allocated space is through a pointer. Because it is less convenient, and because the actual process of dynamic allocation requires more computation time, programmers generally use dynamic allocation only when neither static nor automatic allocation will serve.

For example, if you want to allocate dynamically some space to hold a struct foobar, you cannot declare a variable of type struct foobar whose contents are the dynamically allocated space. But you can declare a variable of pointer type struct foobar * and assign it the address of the space. Then you can use the operators `*' and `->' on this pointer variable to refer to the contents of the space: 

     {
       struct foobar *ptr
          = (struct foobar *) malloc (sizeof (struct foobar));
       ptr->name = x;
       ptr->next = current_foobar;
       current_foobar = ptr;
     }

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3.2.2 Unconstrained Allocation

The most general dynamic allocation facility is malloc. It allows you to allocate blocks of memory of any size at any time, make them bigger or smaller at any time, and free the blocks individually at any time (or never).

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3.2.2.1 Basic Memory Allocation

To allocate a block of memory, call malloc. The prototype for this function is in stdlib.h.

— Function: void * malloc (size_t size)

This function returns a pointer to a newly allocated block size bytes long, or a null pointer if the block could not be allocated.

The contents of the block are undefined; you must initialize it yourself (or use calloc instead; see Allocating Cleared Space). Normally you would cast the value as a pointer to the kind of object that you want to store in the block. Here we show an example of doing so, and of initializing the space with zeros using the library function memset (see Copying and Concatenation): 

     struct foo *ptr;
     ...
     ptr = (struct foo *) malloc (sizeof (struct foo));
     if (ptr == 0) abort ();
     memset (ptr, 0, sizeof (struct foo));

You can store the result of malloc into any pointer variable without a cast, because ISO C automatically converts the type void * to another type of pointer when necessary. But the cast is necessary in contexts other than assignment operators or if you might want your code to run in traditional C.

Remember that when allocating space for a string, the argument to malloc must be one plus the length of the string. This is because a string is terminated with a null character that doesn't count in the “length” of the string but does need space. For example: 

     char *ptr;
     ...
     ptr = (char *) malloc (length + 1);

See Representation of Strings, for more information about this.

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3.2.2.2 Examples of malloc

If no more space is available, malloc returns a null pointer. You should check the value of every call to malloc. It is useful to write a subroutine that calls malloc and reports an error if the value is a null pointer, returning only if the value is nonzero. This function is conventionally called xmalloc. Here it is: 

     void *
     xmalloc (size_t size)
     {
       register void *value = malloc (size);
       if (value == 0)
         fatal ("virtual memory exhausted");
       return value;
     }

Here is a real example of using malloc (by way of xmalloc). The function savestring will copy a sequence of characters into a newly allocated null-terminated string: 

     char *
     savestring (const char *ptr, size_t len)
     {
       register char *value = (char *) xmalloc (len + 1);
       value[len] = '\0';
       return (char *) memcpy (value, ptr, len);
     }

The block that malloc gives you is guaranteed to be aligned so that it can hold any type of data. In the GNU system, the address is always a multiple of eight on most systems, and a multiple of 16 on 64-bit systems. Only rarely is any higher boundary (such as a page boundary) necessary; for those cases, use memalign, posix_memalign or valloc (see Aligned Memory Blocks).

Note that the memory located after the end of the block is likely to be in use for something else; perhaps a block already allocated by another call to malloc. If you attempt to treat the block as longer than you asked for it to be, you are liable to destroy the data that malloc uses to keep track of its blocks, or you may destroy the contents of another block. If you have already allocated a block and discover you want it to be bigger, use realloc (see Changing Block Size).

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3.2.2.3 Freeing Memory Allocated with malloc

When you no longer need a block that you got with malloc, use the function free to make the block available to be allocated again. The prototype for this function is in stdlib.h.

— Function: void free (void *ptr)

The free function deallocates the block of memory pointed at by ptr.

— Function: void cfree (void *ptr)

This function does the same thing as free. It's provided for backward compatibility with SunOS; you should use free instead.

Freeing a block alters the contents of the block. Do not expect to find any data (such as a pointer to the next block in a chain of blocks) in the block after freeing it. Copy whatever you need out of the block before freeing it! Here is an example of the proper way to free all the blocks in a chain, and the strings that they point to: 

     struct chain
       {
         struct chain *next;
         char *name;
       }
     
     void
     free_chain (struct chain *chain)
     {
       while (chain != 0)
         {
           struct chain *next = chain->next;
           free (chain->name);
           free (chain);
           chain = next;
         }
     }

Occasionally, free can actually return memory to the operating system and make the process smaller. Usually, all it can do is allow a later call to malloc to reuse the space. In the meantime, the space remains in your program as part of a free-list used internally by malloc.

There is no point in freeing blocks at the end of a program, because all of the program's space is given back to the system when the process terminates.

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3.2.2.4 Changing the Size of a Block

Often you do not know for certain how big a block you will ultimately need at the time you must begin to use the block. For example, the block might be a buffer that you use to hold a line being read from a file; no matter how long you make the buffer initially, you may encounter a line that is longer.

You can make the block longer by calling realloc. This function is declared in stdlib.h.

— Function: void * realloc (void *ptr, size_t newsize)

The realloc function changes the size of the block whose address is ptr to be newsize.

Since the space after the end of the block may be in use, realloc may find it necessary to copy the block to a new address where more free space is available. The value of realloc is the new address of the block. If the block needs to be moved, realloc copies the old contents.

If you pass a null pointer for ptr, realloc behaves just like `malloc (newsize)'. This can be convenient, but beware that older implementations (before ISO C) may not support this behavior, and will probably crash when realloc is passed a null pointer.

Like malloc, realloc may return a null pointer if no memory space is available to make the block bigger. When this happens, the original block is untouched; it has not been modified or relocated.

In most cases it makes no difference what happens to the original block when realloc fails, because the application program cannot continue when it is out of memory, and the only thing to do is to give a fatal error message. Often it is convenient to write and use a subroutine, conventionally called xrealloc, that takes care of the error message as xmalloc does for malloc: 

     void *
     xrealloc (void *ptr, size_t size)
     {
       register void *value = realloc (ptr, size);
       if (value == 0)
         fatal ("Virtual memory exhausted");
       return value;
     }

You can also use realloc to make a block smaller. The reason you would do this is to avoid tying up a lot of memory space when only a little is needed. In several allocation implementations, making a block smaller sometimes necessitates copying it, so it can fail if no other space is available.

If the new size you specify is the same as the old size, realloc is guaranteed to change nothing and return the same address that you gave.

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3.2.2.5 Allocating Cleared Space

The function calloc allocates memory and clears it to zero. It is declared in stdlib.h.

— Function: void * calloc (size_t count, size_t eltsize)

This function allocates a block long enough to contain a vector of count elements, each of size eltsize. Its contents are cleared to zero before calloc returns.

You could define calloc as follows: 

     void *
     calloc (size_t count, size_t eltsize)
     {
       size_t size = count * eltsize;
       void *value = malloc (size);
       if (value != 0)
         memset (value, 0, size);
       return value;
     }

But in general, it is not guaranteed that calloc calls malloc internally. Therefore, if an application provides its own malloc/realloc/free outside the C library, it should always define calloc, too.

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3.2.2.6 Efficiency Considerations for malloc

As opposed to other versions, the malloc in the GNU C Library does not round up block sizes to powers of two, neither for large nor for small sizes. Neighboring chunks can be coalesced on a free no matter what their size is. This makes the implementation suitable for all kinds of allocation patterns without generally incurring high memory waste through fragmentation.

Very large blocks (much larger than a page) are allocated with mmap (anonymous or via /dev/zero) by this implementation. This has the great advantage that these chunks are returned to the system immediately when they are freed. Therefore, it cannot happen that a large chunk becomes “locked” in between smaller ones and even after calling free wastes memory. The size threshold for mmap to be used can be adjusted with mallopt. The use of mmap can also be disabled completely.

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3.2.2.7 Allocating Aligned Memory Blocks

The address of a block returned by malloc or realloc in the GNU system is always a multiple of eight (or sixteen on 64-bit systems). If you need a block whose address is a multiple of a higher power of two than that, use memalign, posix_memalign, or valloc. memalign is declared in malloc.h and posix_memalign is declared in stdlib.h.

With the GNU library, you can use free to free the blocks that memalign, posix_memalign, and valloc return. That does not work in BSD, however—BSD does not provide any way to free such blocks.

— Function: void * memalign (size_t boundary, size_t size)

The memalign function allocates a block of size bytes whose address is a multiple of boundary. The boundary must be a power of two! The function memalign works by allocating a somewhat larger block, and then returning an address within the block that is on the specified boundary.

— Function: int posix_memalign (void **memptr, size_t alignment, size_t size)

The posix_memalign function is similar to the memalign function in that it returns a buffer of size bytes aligned to a multiple of alignment. But it adds one requirement to the parameter alignment: the value must be a power of two multiple of sizeof (void *).

If the function succeeds in allocation memory a pointer to the allocated memory is returned in *memptr and the return value is zero. Otherwise the function returns an error value indicating the problem.

This function was introduced in POSIX 1003.1d.

— Function: void * valloc (size_t size)

Using valloc is like using memalign and passing the page size as the value of the second argument. It is implemented like this: 

          void *
          valloc (size_t size)
          {
            return memalign (getpagesize (), size);
          }

Query Memory Parameters for more information about the memory subsystem.

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3.2.2.8 Malloc Tunable Parameters

You can adjust some parameters for dynamic memory allocation with the mallopt function. This function is the general SVID/XPG interface, defined in malloc.h.

— Function: int mallopt (int param, int value)

When calling mallopt, the param argument specifies the parameter to be set, and value the new value to be set. Possible choices for param, as defined in malloc.h, are:

M_TRIM_THRESHOLD
This is the minimum size (in bytes) of the top-most, releasable chunk that will cause sbrk to be called with a negative argument in order to return memory to the system.
M_TOP_PAD
This parameter determines the amount of extra memory to obtain from the system when a call to sbrk is required. It also specifies the number of bytes to retain when shrinking the heap by calling sbrk with a negative argument. This provides the necessary hysteresis in heap size such that excessive amounts of system calls can be avoided.
M_MMAP_THRESHOLD
All chunks larger than this value are allocated outside the normal heap, using the mmap system call. This way it is guaranteed that the memory for these chunks can be returned to the system on free. Note that requests smaller than this threshold might still be allocated via mmap.
M_MMAP_MAX
The maximum number of chunks to allocate with mmap. Setting this to zero disables all use of mmap.

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3.2.2.9 Heap Consistency Checking

You can ask malloc to check the consistency of dynamic memory by using the mcheck function. This function is a GNU extension, declared in mcheck.h.

— Function: int mcheck (void (*abortfn) (enum mcheck_status status))

Calling mcheck tells malloc to perform occasional consistency checks. These will catch things such as writing past the end of a block that was allocated with malloc.

The abortfn argument is the function to call when an inconsistency is found. If you supply a null pointer, then mcheck uses a default function which prints a message and calls abort (see Aborting a Program). The function you supply is called with one argument, which says what sort of inconsistency was detected; its type is described below.

It is too late to begin allocation checking once you have allocated anything with malloc. So mcheck does nothing in that case. The function returns -1 if you call it too late, and 0 otherwise (when it is successful).

The easiest way to arrange to call mcheck early enough is to use the option `-lmcheck' when you link your program; then you don't need to modify your program source at all. Alternatively you might use a debugger to insert a call to mcheck whenever the program is started, for example these gdb commands will automatically call mcheck whenever the program starts: 

          (gdb) break main
          Breakpoint 1, main (argc=2, argv=0xbffff964) at whatever.c:10
          (gdb) command 1
          Type commands for when breakpoint 1 is hit, one per line.
          End with a line saying just "end".
          >call mcheck(0)
          >continue
          >end
          (gdb) ...

This will however only work if no initialization function of any object involved calls any of the malloc functions since mcheck must be called before the first such function.

— Function: enum mcheck_status mprobe (void *pointer)

The mprobe function lets you explicitly check for inconsistencies in a particular allocated block. You must have already called mcheck at the beginning of the program, to do its occasional checks; calling mprobe requests an additional consistency check to be done at the time of the call.

The argument pointer must be a pointer returned by malloc or realloc. mprobe returns a value that says what inconsistency, if any, was found. The values are described below.

— Data Type: enum mcheck_status

This enumerated type describes what kind of inconsistency was detected in an allocated block, if any. Here are the possible values:

MCHECK_DISABLED
mcheck was not called before the first allocation. No consistency checking can be done.
MCHECK_OK
No inconsistency detected.
MCHECK_HEAD
The data immediately before the block was modified. This commonly happens when an array index or pointer is decremented too far.
MCHECK_TAIL
The data immediately after the block was modified. This commonly happens when an array index or pointer is incremented too far.
MCHECK_FREE
The block was already freed.

Another possibility to check for and guard against bugs in the use of malloc, realloc and free is to set the environment variable MALLOC_CHECK_. When MALLOC_CHECK_ is set, a special (less efficient) implementation is used which is designed to be tolerant against simple errors, such as double calls of free with the same argument, or overruns of a single byte (off-by-one bugs). Not all such errors can be protected against, however, and memory leaks can result. If MALLOC_CHECK_ is set to 0, any detected heap corruption is silently ignored; if set to 1, a diagnostic is printed on stderr; if set to 2, abort is called immediately. This can be useful because otherwise a crash may happen much later, and the true cause for the problem is then very hard to track down.

There is one problem with MALLOC_CHECK_: in SUID or SGID binaries it could possibly be exploited since diverging from the normal programs behavior it now writes something to the standard error descriptor. Therefore the use of MALLOC_CHECK_ is disabled by default for SUID and SGID binaries. It can be enabled again by the system administrator by adding a file /etc/suid-debug (the content is not important it could be empty).

So, what's the difference between using MALLOC_CHECK_ and linking with `-lmcheck'? MALLOC_CHECK_ is orthogonal with respect to `-lmcheck'. `-lmcheck' has been added for backward compatibility. Both MALLOC_CHECK_ and `-lmcheck' should uncover the same bugs - but using MALLOC_CHECK_ you don't need to recompile your application.

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3.2.2.10 Memory Allocation Hooks

The GNU C library lets you modify the behavior of malloc, realloc, and free by specifying appropriate hook functions. You can use these hooks to help you debug programs that use dynamic memory allocation, for example.

The hook variables are declared in malloc.h.

— Variable: __malloc_hook

The value of this variable is a pointer to the function that malloc uses whenever it is called. You should define this function to look like malloc; that is, like: 

          void *function (size_t size, const void *caller)

The value of caller is the return address found on the stack when the malloc function was called. This value allows you to trace the memory consumption of the program.

— Variable: __realloc_hook

The value of this variable is a pointer to function that realloc uses whenever it is called. You should define this function to look like realloc; that is, like: 

          void *function (void *ptr, size_t size, const void *caller)

The value of caller is the return address found on the stack when the realloc function was called. This value allows you to trace the memory consumption of the program.

— Variable: __free_hook

The value of this variable is a pointer to function that free uses whenever it is called. You should define this function to look like free; that is, like: 

          void function (void *ptr, const void *caller)

The value of caller is the return address found on the stack when the free function was called. This value allows you to trace the memory consumption of the program.

— Variable: __memalign_hook

The value of this variable is a pointer to function that memalign uses whenever it is called. You should define this function to look like memalign; that is, like: 

          void *function (size_t alignment, size_t size, const void *caller)

The value of caller is the return address found on the stack when the memalign function was called. This value allows you to trace the memory consumption of the program.

You must make sure that the function you install as a hook for one of these functions does not call that function recursively without restoring the old value of the hook first! Otherwise, your program will get stuck in an infinite recursion. Before calling the function recursively, one should make sure to restore all the hooks to their previous value. When coming back from the recursive call, all the hooks should be resaved since a hook might modify itself.

— Variable: __malloc_initialize_hook

The value of this variable is a pointer to a function that is called once when the malloc implementation is initialized. This is a weak variable, so it can be overridden in the application with a definition like the following: 

          void (*__malloc_initialize_hook) (void) = my_init_hook;

An issue to look out for is the time at which the malloc hook functions can be safely installed. If the hook functions call the malloc-related functions recursively, it is necessary that malloc has already properly initialized itself at the time when __malloc_hook etc. is assigned to. On the other hand, if the hook functions provide a complete malloc implementation of their own, it is vital that the hooks are assigned to before the very first malloc call has completed, because otherwise a chunk obtained from the ordinary, un-hooked malloc may later be handed to __free_hook, for example.

In both cases, the problem can be solved by setting up the hooks from within a user-defined function pointed to by __malloc_initialize_hook—then the hooks will be set up safely at the right time.

Here is an example showing how to use __malloc_hook and __free_hook properly. It installs a function that prints out information every time malloc or free is called. We just assume here that realloc and memalign are not used in our program. 

     /* Prototypes for __malloc_hook, __free_hook */
     #include <malloc.h>
     
     /* Prototypes for our hooks.  */
     static void my_init_hook (void);
     static void *my_malloc_hook (size_t, const void *);
     static void my_free_hook (void*, const void *);
     
     /* Override initializing hook from the C library. */
     void (*__malloc_initialize_hook) (void) = my_init_hook;
     
     static void
     my_init_hook (void)
     {
       old_malloc_hook = __malloc_hook;
       old_free_hook = __free_hook;
       __malloc_hook = my_malloc_hook;
       __free_hook = my_free_hook;
     }
     
     static void *
     my_malloc_hook (size_t size, const void *caller)
     {
       void *result;
       /* Restore all old hooks */
       __malloc_hook = old_malloc_hook;
       __free_hook = old_free_hook;
       /* Call recursively */
       result = malloc (size);
       /* Save underlying hooks */
       old_malloc_hook = __malloc_hook;
       old_free_hook = __free_hook;
       /* printf might call malloc, so protect it too. */
       printf ("malloc (%u) returns %p\n", (unsigned int) size, result);
       /* Restore our own hooks */
       __malloc_hook = my_malloc_hook;
       __free_hook = my_free_hook;
       return result;
     }
     
     static void
     my_free_hook (void *ptr, const void *caller)
     {
       /* Restore all old hooks */
       __malloc_hook = old_malloc_hook;
       __free_hook = old_free_hook;
       /* Call recursively */
       free (ptr);
       /* Save underlying hooks */
       old_malloc_hook = __malloc_hook;
       old_free_hook = __free_hook;
       /* printf might call free, so protect it too. */
       printf ("freed pointer %p\n", ptr);
       /* Restore our own hooks */
       __malloc_hook = my_malloc_hook;
       __free_hook = my_free_hook;
     }
     
     main ()
     {
       ...
     }

The mcheck function (see Heap Consistency Checking) works by installing such hooks.

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3.2.2.11 Statistics for Memory Allocation with malloc

You can get information about dynamic memory allocation by calling the mallinfo function. This function and its associated data type are declared in malloc.h; they are an extension of the standard SVID/XPG version.

— Data Type: struct mallinfo

This structure type is used to return information about the dynamic memory allocator. It contains the following members:

int arena
This is the total size of memory allocated with sbrk by malloc, in bytes.
int ordblks
This is the number of chunks not in use. (The memory allocator internally gets chunks of memory from the operating system, and then carves them up to satisfy individual malloc requests; see Efficiency and Malloc.)
int smblks
This field is unused.
int hblks
This is the total number of chunks allocated with mmap.
int hblkhd
This is the total size of memory allocated with mmap, in bytes.
int usmblks
This field is unused.
int fsmblks
This field is unused.
int uordblks
This is the total size of memory occupied by chunks handed out by malloc.
int fordblks
This is the total size of memory occupied by free (not in use) chunks.
int keepcost
This is the size of the top-most releasable chunk that normally borders the end of the heap (i.e., the high end of the virtual address space's data segment).

— Function: struct mallinfo mallinfo (void)

This function returns information about the current dynamic memory usage in a structure of type struct mallinfo.

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3.2.2.12 Summary of malloc-Related Functions

Here is a summary of the functions that work with malloc:

void *malloc (size_t size)
Allocate a block of size bytes. See Basic Allocation.
void free (void *addr)
Free a block previously allocated by malloc. See Freeing after Malloc.
void *realloc (void *addr, size_t size)
Make a block previously allocated by malloc larger or smaller, possibly by copying it to a new location. See Changing Block Size.
void *calloc (size_t count, size_t eltsize)
Allocate a block of count * eltsize bytes using malloc, and set its contents to zero. See Allocating Cleared Space.
void *valloc (size_t size)
Allocate a block of size bytes, starting on a page boundary. See Aligned Memory Blocks.
void *memalign (size_t size, size_t boundary)
Allocate a block of size bytes, starting on an address that is a multiple of boundary. See Aligned Memory Blocks.
int mallopt (int param, int value)
Adjust a tunable parameter. See Malloc Tunable Parameters.
int mcheck (void (*abortfn) (void))
Tell malloc to perform occasional consistency checks on dynamically allocated memory, and to call abortfn when an inconsistency is found. See Heap Consistency Checking.
void *(*__malloc_hook) (size_t size, const void *caller)
A pointer to a function that malloc uses whenever it is called.
void *(*__realloc_hook) (void *ptr, size_t size, const void *caller)
A pointer to a function that realloc uses whenever it is called.
void (*__free_hook) (void *ptr, const void *caller)
A pointer to a function that free uses whenever it is called.
void (*__memalign_hook) (size_t size, size_t alignment, const void *caller)
A pointer to a function that memalign uses whenever it is called.
struct mallinfo mallinfo (void)
Return information about the current dynamic memory usage. See Statistics of Malloc.

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3.2.3 Allocation Debugging

A complicated task when programming with languages which do not use garbage collected dynamic memory allocation is to find memory leaks. Long running programs must assure that dynamically allocated objects are freed at the end of their lifetime. If this does not happen the system runs out of memory, sooner or later.

The malloc implementation in the GNU C library provides some simple means to detect such leaks and obtain some information to find the location. To do this the application must be started in a special mode which is enabled by an environment variable. There are no speed penalties for the program if the debugging mode is not enabled.

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3.2.3.1 How to install the tracing functionality
— Function: void mtrace (void)

When the mtrace function is called it looks for an environment variable named MALLOC_TRACE. This variable is supposed to contain a valid file name. The user must have write access. If the file already exists it is truncated. If the environment variable is not set or it does not name a valid file which can be opened for writing nothing is done. The behavior of malloc etc. is not changed. For obvious reasons this also happens if the application is installed with the SUID or SGID bit set.

If the named file is successfully opened, mtrace installs special handlers for the functions malloc, realloc, and free (see Hooks for Malloc). From then on, all uses of these functions are traced and protocolled into the file. There is now of course a speed penalty for all calls to the traced functions so tracing should not be enabled during normal use.

This function is a GNU extension and generally not available on other systems. The prototype can be found in mcheck.h.

— Function: void muntrace (void)

The muntrace function can be called after mtrace was used to enable tracing the malloc calls. If no (successful) call of mtrace was made muntrace does nothing.

Otherwise it deinstalls the handlers for malloc, realloc, and free and then closes the protocol file. No calls are protocolled anymore and the program runs again at full speed.

This function is a GNU extension and generally not available on other systems. The prototype can be found in mcheck.h.

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3.2.3.2 Example program excerpts

Even though the tracing functionality does not influence the runtime behavior of the program it is not a good idea to call mtrace in all programs. Just imagine that you debug a program using mtrace and all other programs used in the debugging session also trace their malloc calls. The output file would be the same for all programs and thus is unusable. Therefore one should call mtrace only if compiled for debugging. A program could therefore start like this: 

     #include <mcheck.h>
     
     int
     main (int argc, char *argv[])
     {
     #ifdef DEBUGGING
       mtrace ();
     #endif
       ...
     }

This is all what is needed if you want to trace the calls during the whole runtime of the program. Alternatively you can stop the tracing at any time with a call to muntrace. It is even possible to restart the tracing again with a new call to mtrace. But this can cause unreliable results since there may be calls of the functions which are not called. Please note that not only the application uses the traced functions, also libraries (including the C library itself) use these functions.

This last point is also why it is no good idea to call muntrace before the program terminated. The libraries are informed about the termination of the program only after the program returns from main or calls exit and so cannot free the memory they use before this time.

So the best thing one can do is to call mtrace as the very first function in the program and never call muntrace. So the program traces almost all uses of the malloc functions (except those calls which are executed by constructors of the program or used libraries).

Next: , Previous: Using the Memory Debugger, Up: Allocation Debugging
3.2.3.3 Some more or less clever ideas

You know the situation. The program is prepared for debugging and in all debugging sessions it runs well. But once it is started without debugging the error shows up. A typical example is a memory leak that becomes visible only when we turn off the debugging. If you foresee such situations you can still win. Simply use something equivalent to the following little program: 

     #include <mcheck.h>
     #include <signal.h>
     
     static void
     enable (int sig)
     {
       mtrace ();
       signal (SIGUSR1, enable);
     }
     
     static void
     disable (int sig)
     {
       muntrace ();
       signal (SIGUSR2, disable);
     }
     
     int
     main (int argc, char *argv[])
     {
       ...
     
       signal (SIGUSR1, enable);
       signal (SIGUSR2, disable);
     
       ...
     }

I.e., the user can start the memory debugger any time s/he wants if the program was started with MALLOC_TRACE set in the environment. The output will of course not show the allocations which happened before the first signal but if there is a memory leak this will show up nevertheless.

Previous: Tips for the Memory Debugger, Up: Allocation Debugging
3.2.3.4 Interpreting the traces

If you take a look at the output it will look similar to this: 

     = Start
      [0x8048209] - 0x8064cc8
      [0x8048209] - 0x8064ce0
      [0x8048209] - 0x8064cf8
      [0x80481eb] + 0x8064c48 0x14
      [0x80481eb] + 0x8064c60 0x14
      [0x80481eb] + 0x8064c78 0x14
      [0x80481eb] + 0x8064c90 0x14
     = End

What this all means is not really important since the trace file is not meant to be read by a human. Therefore no attention is given to readability. Instead there is a program which comes with the GNU C library which interprets the traces and outputs a summary in an user-friendly way. The program is called mtrace (it is in fact a Perl script) and it takes one or two arguments. In any case the name of the file with the trace output must be specified. If an optional argument precedes the name of the trace file this must be the name of the program which generated the trace. 

     drepper$ mtrace tst-mtrace log
     No memory leaks.

In this case the program tst-mtrace was run and it produced a trace file log. The message printed by mtrace shows there are no problems with the code, all allocated memory was freed afterwards.

If we call mtrace on the example trace given above we would get a different outout: 

     drepper$ mtrace errlog
     - 0x08064cc8 Free 2 was never alloc'd 0x8048209
     - 0x08064ce0 Free 3 was never alloc'd 0x8048209
     - 0x08064cf8 Free 4 was never alloc'd 0x8048209
     
     Memory not freed:
     -----------------
        Address     Size     Caller
     0x08064c48     0x14  at 0x80481eb
     0x08064c60     0x14  at 0x80481eb
     0x08064c78     0x14  at 0x80481eb
     0x08064c90     0x14  at 0x80481eb

We have called mtrace with only one argument and so the script has no chance to find out what is meant with the addresses given in the trace. We can do better: 

     drepper$ mtrace tst errlog
     - 0x08064cc8 Free 2 was never alloc'd /home/drepper/tst.c:39
     - 0x08064ce0 Free 3 was never alloc'd /home/drepper/tst.c:39
     - 0x08064cf8 Free 4 was never alloc'd /home/drepper/tst.c:39
     
     Memory not freed:
     -----------------
        Address     Size     Caller
     0x08064c48     0x14  at /home/drepper/tst.c:33
     0x08064c60     0x14  at /home/drepper/tst.c:33
     0x08064c78     0x14  at /home/drepper/tst.c:33
     0x08064c90     0x14  at /home/drepper/tst.c:33

Suddenly the output makes much more sense and the user can see immediately where the function calls causing the trouble can be found.

Interpreting this output is not complicated. There are at most two different situations being detected. First, free was called for pointers which were never returned by one of the allocation functions. This is usually a very bad problem and what this looks like is shown in the first three lines of the output. Situations like this are quite rare and if they appear they show up very drastically: the program normally crashes.

The other situation which is much harder to detect are memory leaks. As you can see in the output the mtrace function collects all this information and so can say that the program calls an allocation function from line 33 in the source file /home/drepper/tst-mtrace.c four times without freeing this memory before the program terminates. Whether this is a real problem remains to be investigated.

Next: , Previous: Allocation Debugging, Up: Memory Allocation

3.2.4 Obstacks

An obstack is a pool of memory containing a stack of objects. You can create any number of separate obstacks, and then allocate objects in specified obstacks. Within each obstack, the last object allocated must always be the first one freed, but distinct obstacks are independent of each other.

Aside from this one constraint of order of freeing, obstacks are totally general: an obstack can contain any number of objects of any size. They are implemented with macros, so allocation is usually very fast as long as the objects are usually small. And the only space overhead per object is the padding needed to start each object on a suitable boundary.

Next: , Up: Obstacks
3.2.4.1 Creating Obstacks

The utilities for manipulating obstacks are declared in the header file obstack.h.

— Data Type: struct obstack

An obstack is represented by a data structure of type struct obstack. This structure has a small fixed size; it records the status of the obstack and how to find the space in which objects are allocated. It does not contain any of the objects themselves. You should not try to access the contents of the structure directly; use only the functions described in this chapter.

You can declare variables of type struct obstack and use them as obstacks, or you can allocate obstacks dynamically like any other kind of object. Dynamic allocation of obstacks allows your program to have a variable number of different stacks. (You can even allocate an obstack structure in another obstack, but this is rarely useful.)

All the functions that work with obstacks require you to specify which obstack to use. You do this with a pointer of type struct obstack *. In the following, we often say “an obstack” when strictly speaking the object at hand is such a pointer.

The objects in the obstack are packed into large blocks called chunks. The struct obstack structure points to a chain of the chunks currently in use.

The obstack library obtains a new chunk whenever you allocate an object that won't fit in the previous chunk. Since the obstack library manages chunks automatically, you don't need to pay much attention to them, but you do need to supply a function which the obstack library should use to get a chunk. Usually you supply a function which uses malloc directly or indirectly. You must also supply a function to free a chunk. These matters are described in the following section.

Next: , Previous: Creating Obstacks, Up: Obstacks
3.2.4.2 Preparing for Using Obstacks

Each source file in which you plan to use the obstack functions must include the header file obstack.h, like this: 

     #include <obstack.h>

Also, if the source file uses the macro obstack_init, it must declare or define two functions or macros that will be called by the obstack library. One, obstack_chunk_alloc, is used to allocate the chunks of memory into which objects are packed. The other, obstack_chunk_free, is used to return chunks when the objects in them are freed. These macros should appear before any use of obstacks in the source file.

Usually these are defined to use malloc via the intermediary xmalloc (see Unconstrained Allocation). This is done with the following pair of macro definitions: 

     #define obstack_chunk_alloc xmalloc
     #define obstack_chunk_free free

Though the memory you get using obstacks really comes from malloc, using obstacks is faster because malloc is called less often, for larger blocks of memory. See Obstack Chunks, for full details.

At run time, before the program can use a struct obstack object as an obstack, it must initialize the obstack by calling obstack_init.

— Function: int obstack_init (struct obstack *obstack-ptr)

Initialize obstack obstack-ptr for allocation of objects. This function calls the obstack's obstack_chunk_alloc function. If allocation of memory fails, the function pointed to by obstack_alloc_failed_handler is called. The obstack_init function always returns 1 (Compatibility notice: Former versions of obstack returned 0 if allocation failed).

Here are two examples of how to allocate the space for an obstack and initialize it. First, an obstack that is a static variable: 

     static struct obstack myobstack;
     ...
     obstack_init (&myobstack);

Second, an obstack that is itself dynamically allocated: 

     struct obstack *myobstack_ptr
       = (struct obstack *) xmalloc (sizeof (struct obstack));
     
     obstack_init (myobstack_ptr);
— Variable: obstack_alloc_failed_handler

The value of this variable is a pointer to a function that obstack uses when obstack_chunk_alloc fails to allocate memory. The default action is to print a message and abort. You should supply a function that either calls exit (see Program Termination) or longjmp (see Non-Local Exits) and doesn't return. 

          void my_obstack_alloc_failed (void)
          ...
          obstack_alloc_failed_handler = &my_obstack_alloc_failed;

Next: , Previous: Preparing for Obstacks, Up: Obstacks
3.2.4.3 Allocation in an Obstack

The most direct way to allocate an object in an obstack is with obstack_alloc, which is invoked almost like malloc.

— Function: void * obstack_alloc (struct obstack *obstack-ptr, int size)

This allocates an uninitialized block of size bytes in an obstack and returns its address. Here obstack-ptr specifies which obstack to allocate the block in; it is the address of the struct obstack object which represents the obstack. Each obstack function or macro requires you to specify an obstack-ptr as the first argument.

This function calls the obstack's obstack_chunk_alloc function if it needs to allocate a new chunk of memory; it calls obstack_alloc_failed_handler if allocation of memory by obstack_chunk_alloc failed.

For example, here is a function that allocates a copy of a string str in a specific obstack, which is in the variable string_obstack: 

     struct obstack string_obstack;
     
     char *
     copystring (char *string)
     {
       size_t len = strlen (string) + 1;
       char *s = (char *) obstack_alloc (&string_obstack, len);
       memcpy (s, string, len);
       return s;
     }

To allocate a block with specified contents, use the function obstack_copy, declared like this:

— Function: void * obstack_copy (struct obstack *obstack-ptr, void *address, int size)

This allocates a block and initializes it by copying size bytes of data starting at address. It calls obstack_alloc_failed_handler if allocation of memory by obstack_chunk_alloc failed.

— Function: void * obstack_copy0 (struct obstack *obstack-ptr, void *address, int size)

Like obstack_copy, but appends an extra byte containing a null character. This extra byte is not counted in the argument size.

The obstack_copy0 function is convenient for copying a sequence of characters into an obstack as a null-terminated string. Here is an example of its use: 

     char *
     obstack_savestring (char *addr, int size)
     {
       return obstack_copy0 (&myobstack, addr, size);
     }

Contrast this with the previous example of savestring using malloc (see Basic Allocation).

Next: , Previous: Allocation in an Obstack, Up: Obstacks
3.2.4.4 Freeing Objects in an Obstack

To free an object allocated in an obstack, use the function obstack_free. Since the obstack is a stack of objects, freeing one object automatically frees all other objects allocated more recently in the same obstack.

— Function: void obstack_free (struct obstack *obstack-ptr, void *object)

If object is a null pointer, everything allocated in the obstack is freed. Otherwise, object must be the address of an object allocated in the obstack. Then object is freed, along with everything allocated in obstack since object.

Note that if object is a null pointer, the result is an uninitialized obstack. To free all memory in an obstack but leave it valid for further allocation, call obstack_free with the address of the first object allocated on the obstack: 

     obstack_free (obstack_ptr, first_object_allocated_ptr);

Recall that the objects in an obstack are grouped into chunks. When all the objects in a chunk become free, the obstack library automatically frees the chunk (see Preparing for Obstacks). Then other obstacks, or non-obstack allocation, can reuse the space of the chunk.

Next: , Previous: Freeing Obstack Objects, Up: Obstacks
3.2.4.5 Obstack Functions and Macros

The interfaces for using obstacks may be defined either as functions or as macros, depending on the compiler. The obstack facility works with all C compilers, including both ISO C and traditional C, but there are precautions you must take if you plan to use compilers other than GNU C.

If you are using an old-fashioned non-ISO C compiler, all the obstack “functions” are actually defined only as macros. You can call these macros like functions, but you cannot use them in any other way (for example, you cannot take their address).

Calling the macros requires a special precaution: namely, the first operand (the obstack pointer) may not contain any side effects, because it may be computed more than once. For example, if you write this: 

     obstack_alloc (get_obstack (), 4);

you will find that get_obstack may be called several times. If you use *obstack_list_ptr++ as the obstack pointer argument, you will get very strange results since the incrementation may occur several times.

In ISO C, each function has both a macro definition and a function definition. The function definition is used if you take the address of the function without calling it. An ordinary call uses the macro definition by default, but you can request the function definition instead by writing the function name in parentheses, as shown here: 

     char *x;
     void *(*funcp) ();
     /* Use the macro.  */
     x = (char *) obstack_alloc (obptr, size);
     /* Call the function.  */
     x = (char *) (obstack_alloc) (obptr, size);
     /* Take the address of the function.  */
     funcp = obstack_alloc;

This is the same situation that exists in ISO C for the standard library functions. See Macro Definitions.

Warning: When you do use the macros, you must observe the precaution of avoiding side effects in the first operand, even in ISO C.

If you use the GNU C compiler, this precaution is not necessary, because various language extensions in GNU C permit defining the macros so as to compute each argument only once.

Next: , Previous: Obstack Functions, Up: Obstacks
3.2.4.6 Growing Objects

Because memory in obstack chunks is used sequentially, it is possible to build up an object step by step, adding one or more bytes at a time to the end of the object. With this technique, you do not need to know how much data you will put in the object until you come to the end of it. We call this the technique of growing objects. The special functions for adding data to the growing object are described in this section.

You don't need to do anything special when you start to grow an object. Using one of the functions to add data to the object automatically starts it. However, it is necessary to say explicitly when the object is finished. This is done with the function obstack_finish.

The actual address of the object thus built up is not known until the object is finished. Until then, it always remains possible that you will add so much data that the object must be copied into a new chunk.

While the obstack is in use for a growing object, you cannot use it for ordinary allocation of another object. If you try to do so, the space already added to the growing object will become part of the other object.

— Function: void obstack_blank (struct obstack *obstack-ptr, int size)

The most basic function for adding to a growing object is obstack_blank, which adds space without initializing it.

— Function: void obstack_grow (struct obstack *obstack-ptr, void *data, int size)

To add a block of initialized space, use obstack_grow, which is the growing-object analogue of obstack_copy. It adds size bytes of data to the growing object, copying the contents from data.

— Function: void obstack_grow0 (struct obstack *obstack-ptr, void *data, int size)

This is the growing-object analogue of obstack_copy0. It adds size bytes copied from data, followed by an additional null character.

— Function: void obstack_1grow (struct obstack *obstack-ptr, char c)

To add one character at a time, use the function obstack_1grow. It adds a single byte containing c to the growing object.

— Function: void obstack_ptr_grow (struct obstack *obstack-ptr, void *data)

Adding the value of a pointer one can use the function obstack_ptr_grow. It adds sizeof (void *) bytes containing the value of data.

— Function: void obstack_int_grow (struct obstack *obstack-ptr, int data)

A single value of type int can be added by using the obstack_int_grow function. It adds sizeof (int) bytes to the growing object and initializes them with the value of data.

— Function: void * obstack_finish (struct obstack *obstack-ptr)

When you are finished growing the object, use the function obstack_finish to close it off and return its final address.

Once you have finished the object, the obstack is available for ordinary allocation or for growing another object.

This function can return a null pointer under the same conditions as obstack_alloc (see Allocation in an Obstack).

When you build an object by growing it, you will probably need to know afterward how long it became. You need not keep track of this as you grow the object, because you can find out the length from the obstack just before finishing the object with the function obstack_object_size, declared as follows:

— Function: int obstack_object_size (struct obstack *obstack-ptr)

This function returns the current size of the growing object, in bytes. Remember to call this function before finishing the object. After it is finished, obstack_object_size will return zero.

If you have started growing an object and wish to cancel it, you should finish it and then free it, like this: 

     obstack_free (obstack_ptr, obstack_finish (obstack_ptr));

This has no effect if no object was growing.

You can use obstack_blank with a negative size argument to make the current object smaller. Just don't try to shrink it beyond zero length—there's no telling what will happen if you do that.

Next: , Previous: Growing Objects, Up: Obstacks
3.2.4.7 Extra Fast Growing Objects

The usual functions for growing objects incur overhead for checking whether there is room for the new growth in the current chunk. If you are frequently constructing objects in small steps of growth, this overhead can be significant.

You can reduce the overhead by using special “fast growth” functions that grow the object without checking. In order to have a robust program, you must do the checking yourself. If you do this checking in the simplest way each time you are about to add data to the object, you have not saved anything, because that is what the ordinary growth functions do. But if you can arrange to check less often, or check more efficiently, then you make the program faster.

The function obstack_room returns the amount of room available in the current chunk. It is declared as follows:

— Function: int obstack_room (struct obstack *obstack-ptr)

This returns the number of bytes that can be added safely to the current growing object (or to an object about to be started) in obstack obstack using the fast growth functions.

While you know there is room, you can use these fast growth functions for adding data to a growing object:

— Function: void obstack_1grow_fast (struct obstack *obstack-ptr, char c)

The function obstack_1grow_fast adds one byte containing the character c to the growing object in obstack obstack-ptr.

— Function: void obstack_ptr_grow_fast (struct obstack *obstack-ptr, void *data)

The function obstack_ptr_grow_fast adds sizeof (void *) bytes containing the value of data to the growing object in obstack obstack-ptr.

— Function: void obstack_int_grow_fast (struct obstack *obstack-ptr, int data)

The function obstack_int_grow_fast adds sizeof (int) bytes containing the value of data to the growing object in obstack obstack-ptr.

— Function: void obstack_blank_fast (struct obstack *obstack-ptr, int size)

The function obstack_blank_fast adds size bytes to the growing object in obstack obstack-ptr without initializing them.

When you check for space using obstack_room and there is not enough room for what you want to add, the fast growth functions are not safe. In this case, simply use the corresponding ordinary growth function instead. Very soon this will copy the object to a new chunk; then there will be lots of room available again.

So, each time you use an ordinary growth function, check afterward for sufficient space using obstack_room. Once the object is copied to a new chunk, there will be plenty of space again, so the program will start using the fast growth functions again.

Here is an example: 

     void
     add_string (struct obstack *obstack, const char *ptr, int len)
     {
       while (len > 0)
         {
           int room = obstack_room (obstack);
           if (room == 0)
             {
               /* Not enough room. Add one character slowly,
                  which may copy to a new chunk and make room.  */
               obstack_1grow (obstack, *ptr++);
               len--;
             }
           else
             {
               if (room > len)
                 room = len;
               /* Add fast as much as we have room for. */
               len -= room;
               while (room-- > 0)
                 obstack_1grow_fast (obstack, *ptr++);
             }
         }
     }

Next: , Previous: Extra Fast Growing, Up: Obstacks
3.2.4.8 Status of an Obstack

Here are functions that provide information on the current status of allocation in an obstack. You can use them to learn about an object while still growing it.

— Function: void * obstack_base (struct obstack *obstack-ptr)

This function returns the tentative address of the beginning of the currently growing object in obstack-ptr. If you finish the object immediately, it will have that address. If you make it larger first, it may outgrow the current chunk—then its address will change!

If no object is growing, this value says where the next object you allocate will start (once again assuming it fits in the current chunk).

— Function: void * obstack_next_free (struct obstack *obstack-ptr)

This function returns the address of the first free byte in the current chunk of obstack obstack-ptr. This is the end of the currently growing object. If no object is growing, obstack_next_free returns the same value as obstack_base.

— Function: int obstack_object_size (struct obstack *obstack-ptr)

This function returns the size in bytes of the currently growing object. This is equivalent to 

          obstack_next_free (obstack-ptr) - obstack_base (obstack-ptr)

Next: , Previous: Status of an Obstack, Up: Obstacks
3.2.4.9 Alignment of Data in Obstacks

Each obstack has an alignment boundary; each object allocated in the obstack automatically starts on an address that is a multiple of the specified boundary. By default, this boundary is aligned so that the object can hold any type of data.

To access an obstack's alignment boundary, use the macro obstack_alignment_mask, whose function prototype looks like this:

— Macro: int obstack_alignment_mask (struct obstack *obstack-ptr)

The value is a bit mask; a bit that is 1 indicates that the corresponding bit in the address of an object should be 0. The mask value should be one less than a power of 2; the effect is that all object addresses are multiples of that power of 2. The default value of the mask is a value that allows aligned objects to hold any type of data: for example, if its value is 3, any type of data can be stored at locations whose addresses are multiples of 4. A mask value of 0 means an object can start on any multiple of 1 (that is, no alignment is required).

The expansion of the macro obstack_alignment_mask is an lvalue, so you can alter the mask by assignment. For example, this statement: 

          obstack_alignment_mask (obstack_ptr) = 0;

has the effect of turning off alignment processing in the specified obstack.

Note that a change in alignment mask does not take effect until after the next time an object is allocated or finished in the obstack. If you are not growing an object, you can make the new alignment mask take effect immediately by calling obstack_finish. This will finish a zero-length object and then do proper alignment for the next object.

Next: , Previous: Obstacks Data Alignment, Up: Obstacks
3.2.4.10 Obstack Chunks

Obstacks work by allocating space for themselves in large chunks, and then parceling out space in the chunks to satisfy your requests. Chunks are normally 4096 bytes long unless you specify a different chunk size. The chunk size includes 8 bytes of overhead that are not actually used for storing objects. Regardless of the specified size, longer chunks will be allocated when necessary for long objects.

The obstack library allocates chunks by calling the function obstack_chunk_alloc, which you must define. When a chunk is no longer needed because you have freed all the objects in it, the obstack library frees the chunk by calling obstack_chunk_free, which you must also define.

These two must be defined (as macros) or declared (as functions) in each source file that uses obstack_init (see Creating Obstacks). Most often they are defined as macros like this: 

     #define obstack_chunk_alloc malloc
     #define obstack_chunk_free free

Note that these are simple macros (no arguments). Macro definitions with arguments will not work! It is necessary that obstack_chunk_alloc or obstack_chunk_free, alone, expand into a function name if it is not itself a function name.

If you allocate chunks with malloc, the chunk size should be a power of 2. The default chunk size, 4096, was chosen because it is long enough to satisfy many typical requests on the obstack yet short enough not to waste too much memory in the portion of the last chunk not yet used.

— Macro: int obstack_chunk_size (struct obstack *obstack-ptr)

This returns the chunk size of the given obstack.

Since this macro expands to an lvalue, you can specify a new chunk size by assigning it a new value. Doing so does not affect the chunks already allocated, but will change the size of chunks allocated for that particular obstack in the future. It is unlikely to be useful to make the chunk size smaller, but making it larger might improve efficiency if you are allocating many objects whose size is comparable to the chunk size. Here is how to do so cleanly: 

     if (obstack_chunk_size (obstack_ptr) < new-chunk-size)
       obstack_chunk_size (obstack_ptr) = new-chunk-size;

Previous: Obstack Chunks, Up: Obstacks
3.2.4.11 Summary of Obstack Functions

Here is a summary of all the functions associated with obstacks. Each takes the address of an obstack (struct obstack *) as its first argument.

void obstack_init (struct obstack *obstack-ptr)
Initialize use of an obstack. See Creating Obstacks.
void *obstack_alloc (struct obstack *obstack-ptr, int size)
Allocate an object of size uninitialized bytes. See Allocation in an Obstack.
void *obstack_copy (struct obstack *obstack-ptr, void *address, int size)
Allocate an object of size bytes, with contents copied from address. See Allocation in an Obstack.
void *obstack_copy0 (struct obstack *obstack-ptr, void *address, int size)
Allocate an object of size+1 bytes, with size of them copied from address, followed by a null character at the end. See Allocation in an Obstack.
void obstack_free (struct obstack *obstack-ptr, void *object)
Free object (and everything allocated in the specified obstack more recently than object). See Freeing Obstack Objects.
void obstack_blank (struct obstack *obstack-ptr, int size)
Add size uninitialized bytes to a growing object. See Growing Objects.
void obstack_grow (struct obstack *obstack-ptr, void *address, int size)
Add size bytes, copied from address, to a growing object. See Growing Objects.
void obstack_grow0 (struct obstack *obstack-ptr, void *address, int size)
Add size bytes, copied from address, to a growing object, and then add another byte containing a null character. See Growing Objects.
void obstack_1grow (struct obstack *obstack-ptr, char data-char)
Add one byte containing data-char to a growing object. See Growing Objects.
void *obstack_finish (struct obstack *obstack-ptr)
Finalize the object that is growing and return its permanent address. See Growing Objects.
int obstack_object_size (struct obstack *obstack-ptr)
Get the current size of the currently growing object. See Growing Objects.
void obstack_blank_fast (struct obstack *obstack-ptr, int size)
Add size uninitialized bytes to a growing object without checking that there is enough room. See Extra Fast Growing.
void obstack_1grow_fast (struct obstack *obstack-ptr, char data-char)
Add one byte containing data-char to a growing object without checking that there is enough room. See Extra Fast Growing.
int obstack_room (struct obstack *obstack-ptr)
Get the amount of room now available for growing the current object. See Extra Fast Growing.
int obstack_alignment_mask (struct obstack *obstack-ptr)
The mask used for aligning the beginning of an object. This is an lvalue. See Obstacks Data Alignment.
int obstack_chunk_size (struct obstack *obstack-ptr)
The size for allocating chunks. This is an lvalue. See Obstack Chunks.
void *obstack_base (struct obstack *obstack-ptr)
Tentative starting address of the currently growing object. See Status of an Obstack.
void *obstack_next_free (struct obstack *obstack-ptr)
Address just after the end of the currently growing object. See Status of an Obstack.

Previous: Obstacks, Up: Memory Allocation

3.2.5 Automatic Storage with Variable Size

The function alloca supports a kind of half-dynamic allocation in which blocks are allocated dynamically but freed automatically.

Allocating a block with alloca is an explicit action; you can allocate as many blocks as you wish, and compute the size at run time. But all the blocks are freed when you exit the function that alloca was called from, just as if they were automatic variables declared in that function. There is no way to free the space explicitly.

The prototype for alloca is in stdlib.h. This function is a BSD extension.

— Function: void * alloca (size_t size);

The return value of alloca is the address of a block of size bytes of memory, allocated in the stack frame of the calling function.

Do not use alloca inside the arguments of a function call—you will get unpredictable results, because the stack space for the alloca would appear on the stack in the middle of the space for the function arguments. An example of what to avoid is foo (x, alloca (4), y).

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3.2.5.1 alloca Example

As an example of the use of alloca, here is a function that opens a file name made from concatenating two argument strings, and returns a file descriptor or minus one signifying failure: 

     int
     open2 (char *str1, char *str2, int flags, int mode)
     {
       char *name = (char *) alloca (strlen (str1) + strlen (str2) + 1);
       stpcpy (stpcpy (name, str1), str2);
       return open (name, flags, mode);
     }

Here is how you would get the same results with malloc and free: 

     int
     open2 (char *str1, char *str2, int flags, int mode)
     {
       char *name = (char *) malloc (strlen (str1) + strlen (str2) + 1);
       int desc;
       if (name == 0)
         fatal ("virtual memory exceeded");
       stpcpy (stpcpy (name, str1), str2);
       desc = open (name, flags, mode);
       free (name);
       return desc;
     }

As you can see, it is simpler with alloca. But alloca has other, more important advantages, and some disadvantages.

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3.2.5.2 Advantages of alloca

Here are the reasons why alloca may be preferable to malloc:

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3.2.5.3 Disadvantages of alloca

These are the disadvantages of alloca in comparison with malloc:

Previous: Disadvantages of Alloca, Up: Variable Size Automatic
3.2.5.4 GNU C Variable-Size Arrays

In GNU C, you can replace most uses of alloca with an array of variable size. Here is how open2 would look then: 

     int open2 (char *str1, char *str2, int flags, int mode)
     {
       char name[strlen (str1) + strlen (str2) + 1];
       stpcpy (stpcpy (name, str1), str2);
       return open (name, flags, mode);
     }

But alloca is not always equivalent to a variable-sized array, for several reasons:

Note: If you mix use of alloca and variable-sized arrays within one function, exiting a scope in which a variable-sized array was declared frees all blocks allocated with alloca during the execution of that scope.

Previous: Locking Pages, Up: Memory

3.3 Resizing the Data Segment

The symbols in this section are declared in unistd.h.

You will not normally use the functions in this section, because the functions described in Memory Allocation are easier to use. Those are interfaces to a GNU C Library memory allocator that uses the functions below itself. The functions below are simple interfaces to system calls.

— Function: int brk (void *addr)

brk sets the high end of the calling process' data segment to addr.

The address of the end of a segment is defined to be the address of the last byte in the segment plus 1.

The function has no effect if addr is lower than the low end of the data segment. (This is considered success, by the way).

The function fails if it would cause the data segment to overlap another segment or exceed the process' data storage limit (see Limits on Resources).

The function is named for a common historical case where data storage and the stack are in the same segment. Data storage allocation grows upward from the bottom of the segment while the stack grows downward toward it from the top of the segment and the curtain between them is called the break.

The return value is zero on success. On failure, the return value is -1 and errno is set accordingly. The following errno values are specific to this function:

ENOMEM
The request would cause the data segment to overlap another segment or exceed the process' data storage limit.
— Function: void *sbrk (ptrdiff_t delta)

This function is the same as brk except that you specify the new end of the data segment as an offset delta from the current end and on success the return value is the address of the resulting end of the data segment instead of zero.

This means you can use `sbrk(0)' to find out what the current end of the data segment is.

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3.4 Locking Pages

You can tell the system to associate a particular virtual memory page with a real page frame and keep it that way — i.e., cause the page to be paged in if it isn't already and mark it so it will never be paged out and consequently will never cause a page fault. This is called locking a page.

The functions in this chapter lock and unlock the calling process' pages.

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3.4.1 Why Lock Pages

Because page faults cause paged out pages to be paged in transparently, a process rarely needs to be concerned about locking pages. However, there are two reasons people sometimes are:

Be aware that when you lock a page, that's one fewer page frame that can be used to back other virtual memory (by the same or other processes), which can mean more page faults, which means the system runs more slowly. In fact, if you lock enough memory, some programs may not be able to run at all for lack of real memory.

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3.4.2 Locked Memory Details

A memory lock is associated with a virtual page, not a real frame. The paging rule is: If a frame backs at least one locked page, don't page it out.

Memory locks do not stack. I.e., you can't lock a particular page twice so that it has to be unlocked twice before it is truly unlocked. It is either locked or it isn't.

A memory lock persists until the process that owns the memory explicitly unlocks it. (But process termination and exec cause the virtual memory to cease to exist, which you might say means it isn't locked any more).

Memory locks are not inherited by child processes. (But note that on a modern Unix system, immediately after a fork, the parent's and the child's virtual address space are backed by the same real page frames, so the child enjoys the parent's locks). See Creating a Process.

Because of its ability to impact other processes, only the superuser can lock a page. Any process can unlock its own page.

The system sets limits on the amount of memory a process can have locked and the amount of real memory it can have dedicated to it. See Limits on Resources.

In Linux, locked pages aren't as locked as you might think. Two virtual pages that are not shared memory can nonetheless be backed by the same real frame. The kernel does this in the name of efficiency when it knows both virtual pages contain identical data, and does it even if one or both of the virtual pages are locked.

But when a process modifies one of those pages, the kernel must get it a separate frame and fill it with the page's data. This is known as a copy-on-write page fault. It takes a small amount of time and in a pathological case, getting that frame may require I/O. To make sure this doesn't happen to your program, don't just lock the pages. Write to them as well, unless you know you won't write to them ever. And to make sure you have pre-allocated frames for your stack, enter a scope that declares a C automatic variable larger than the maximum stack size you will need, set it to something, then return from its scope.

Previous: Locked Memory Details, Up: Locking Pages

3.4.3 Functions To Lock And Unlock Pages

The symbols in this section are declared in sys/mman.h. These functions are defined by POSIX.1b, but their availability depends on your kernel. If your kernel doesn't allow these functions, they exist but always fail. They are available with a Linux kernel.

Portability Note: POSIX.1b requires that when the mlock and munlock functions are available, the file unistd.h define the macro _POSIX_MEMLOCK_RANGE and the file limits.h define the macro PAGESIZE to be the size of a memory page in bytes. It requires that when the mlockall and munlockall functions are available, the unistd.h file define the macro _POSIX_MEMLOCK. The GNU C library conforms to this requirement.

— Function: int mlock (const void *addr, size_t len)

mlock locks a range of the calling process' virtual pages.

The range of memory starts at address addr and is len bytes long. Actually, since you must lock whole pages, it is the range of pages that include any part of the specified range.

When the function returns successfully, each of those pages is backed by (connected to) a real frame (is resident) and is marked to stay that way. This means the function may cause page-ins and have to wait for them.

When the function fails, it does not affect the lock status of any pages.

The return value is zero if the function succeeds. Otherwise, it is -1 and errno is set accordingly. errno values specific to this function are:

ENOMEM
  • At least some of the specified address range does not exist in the calling process' virtual address space.
  • The locking would cause the process to exceed its locked page limit.

EPERM
The calling process is not superuser.
EINVAL
len is not positive.
ENOSYS
The kernel does not provide mlock capability.

You can lock all a process' memory with mlockall. You unlock memory with munlock or munlockall.

To avoid all page faults in a C program, you have to use mlockall, because some of the memory a program uses is hidden from the C code, e.g. the stack and automatic variables, and you wouldn't know what address to tell mlock.

— Function: int munlock (const void *addr, size_t len)

munlock unlocks a range of the calling process' virtual pages.

munlock is the inverse of mlock and functions completely analogously to mlock, except that there is no EPERM failure.

— Function: int mlockall (int flags)

mlockall locks all the pages in a process' virtual memory address space, and/or any that are added to it in the future. This includes the pages of the code, data and stack segment, as well as shared libraries, user space kernel data, shared memory, and memory mapped files.

flags is a string of single bit flags represented by the following macros. They tell mlockall which of its functions you want. All other bits must be zero.

MCL_CURRENT
Lock all pages which currently exist in the calling process' virtual address space.
MCL_FUTURE
Set a mode such that any pages added to the process' virtual address space in the future will be locked from birth. This mode does not affect future address spaces owned by the same process so exec, which replaces a process' address space, wipes out MCL_FUTURE. See Executing a File.

When the function returns successfully, and you specified MCL_CURRENT, all of the process' pages are backed by (connected to) real frames (they are resident) and are marked to stay that way. This means the function may cause page-ins and have to wait for them.

When the process is in MCL_FUTURE mode because it successfully executed this function and specified MCL_CURRENT, any system call by the process that requires space be added to its virtual address space fails with errno = ENOMEM if locking the additional space would cause the process to exceed its locked page limit. In the case that the address space addition that can't be accommodated is stack expansion, the stack expansion fails and the kernel sends a SIGSEGV signal to the process.

When the function fails, it does not affect the lock status of any pages or the future locking mode.

The return value is zero if the function succeeds. Otherwise, it is -1 and errno is set accordingly. errno values specific to this function are:

ENOMEM
  • At least some of the specified address range does not exist in the calling process' virtual address space.
  • The locking would cause the process to exceed its locked page limit.

EPERM
The calling process is not superuser.
EINVAL
Undefined bits in flags are not zero.
ENOSYS
The kernel does not provide mlockall capability.

You can lock just specific pages with mlock. You unlock pages with munlockall and munlock.

— Function: int munlockall (void)

munlockall unlocks every page in the calling process' virtual address space and turn off MCL_FUTURE future locking mode.

The return value is zero if the function succeeds. Otherwise, it is -1 and errno is set accordingly. The only way this function can fail is for generic reasons that all functions and system calls can fail, so there are no specific errno values.

Next: , Previous: Memory, Up: Top

4 Character Handling

Programs that work with characters and strings often need to classify a character—is it alphabetic, is it a digit, is it whitespace, and so on—and perform case conversion operations on characters. The functions in the header file ctype.h are provided for this purpose. Since the choice of locale and character set can alter the classifications of particular character codes, all of these functions are affected by the current locale. (More precisely, they are affected by the locale currently selected for character classification—the LC_CTYPE category; see Locale Categories.)

The ISO C standard specifies two different sets of functions. The one set works on char type characters, the other one on wchar_t wide characters (see Extended Char Intro).

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4.1 Classification of Characters

This section explains the library functions for classifying characters. For example, isalpha is the function to test for an alphabetic character. It takes one argument, the character to test, and returns a nonzero integer if the character is alphabetic, and zero otherwise. You would use it like this: 

     if (isalpha (c))
       printf ("The character `%c' is alphabetic.\n", c);

Each of the functions in this section tests for membership in a particular class of characters; each has a name starting with `is'. Each of them takes one argument, which is a character to test, and returns an int which is treated as a boolean value. The character argument is passed as an int, and it may be the constant value EOF instead of a real character.

The attributes of any given character can vary between locales. See Locales, for more information on locales.

These functions are declared in the header file ctype.h.

— Function: int islower (int c)

Returns true if c is a lower-case letter. The letter need not be from the Latin alphabet, any alphabet representable is valid.

— Function: int isupper (int c)

Returns true if c is an upper-case letter. The letter need not be from the Latin alphabet, any alphabet representable is valid.

— Function: int isalpha (int c)

Returns true if c is an alphabetic character (a letter). If islower or isupper is true of a character, then isalpha is also true.

In some locales, there may be additional characters for which isalpha is true—letters which are neither upper case nor lower case. But in the standard "C" locale, there are no such additional characters.

— Function: int isdigit (int c)

Returns true if c is a decimal digit (`0' through `9').

— Function: int isalnum (int c)

Returns true if c is an alphanumeric character (a letter or number); in other words, if either isalpha or isdigit is true of a character, then isalnum is also true.

— Function: int isxdigit (int c)

Returns true if c is a hexadecimal digit. Hexadecimal digits include the normal decimal digits `0' through `9' and the letters `A' through `F' and `a' through `f'.

— Function: int ispunct (int c)

Returns true if c is a punctuation character. This means any printing character that is not alphanumeric or a space character.

— Function: int isspace (int c)

Returns true if c is a whitespace character. In the standard "C" locale, isspace returns true for only the standard whitespace characters:

' '
space
'\f'
formfeed
'\n'
newline
'\r'
carriage return
'\t'
horizontal tab
'\v'
vertical tab

— Function: int isblank (int c)

Returns true if c is a blank character; that is, a space or a tab. This function was originally a GNU extension, but was added in ISO C99.

— Function: int isgraph (int c)

Returns true if c is a graphic character; that is, a character that has a glyph associated with it. The whitespace characters are not considered graphic.

— Function: int isprint (int c)

Returns true if c is a printing character. Printing characters include all the graphic characters, plus the space (` ') character.

— Function: int iscntrl (int c)

Returns true if c is a control character (that is, a character that is not a printing character).

— Function: int isascii (int c)

Returns true if c is a 7-bit unsigned char value that fits into the US/UK ASCII character set. This function is a BSD extension and is also an SVID extension.

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4.2 Case Conversion

This section explains the library functions for performing conversions such as case mappings on characters. For example, toupper converts any character to upper case if possible. If the character can't be converted, toupper returns it unchanged.

These functions take one argument of type int, which is the character to convert, and return the converted character as an int. If the conversion is not applicable to the argument given, the argument is returned unchanged.

Compatibility Note: In pre-ISO C dialects, instead of returning the argument unchanged, these functions may fail when the argument is not suitable for the conversion. Thus for portability, you may need to write islower(c) ? toupper(c) : c rather than just toupper(c).

These functions are declared in the header file ctype.h.

— Function: int tolower (int c)

If c is an upper-case letter, tolower returns the corresponding lower-case letter. If c is not an upper-case letter, c is returned unchanged.

— Function: int toupper (int c)

If c is a lower-case letter, toupper returns the corresponding upper-case letter. Otherwise c is returned unchanged.

— Function: int toascii (int c)

This function converts c to a 7-bit unsigned char value that fits into the US/UK ASCII character set, by clearing the high-order bits. This function is a BSD extension and is also an SVID extension.

— Function: int _tolower (int c)

This is identical to tolower, and is provided for compatibility with the SVID. See SVID.

— Function: int _toupper (int c)

This is identical to toupper, and is provided for compatibility with the SVID.

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4.3 Character class determination for wide characters

Amendment 1 to ISO C90 defines functions to classify wide characters. Although the original ISO C90 standard already defined the type wchar_t, no functions operating on them were defined.

The general design of the classification functions for wide characters is more general. It allows extensions to the set of available classifications, beyond those which are always available. The POSIX standard specifies how extensions can be made, and this is already implemented in the GNU C library implementation of the localedef program.

The character class functions are normally implemented with bitsets, with a bitset per character. For a given character, the appropriate bitset is read from a table and a test is performed as to whether a certain bit is set. Which bit is tested for is determined by the class.

For the wide character classification functions this is made visible. There is a type classification type defined, a function to retrieve this value for a given class, and a function to test whether a given character is in this class, using the classification value. On top of this the normal character classification functions as used for char objects can be defined.

— Data type: wctype_t

The wctype_t can hold a value which represents a character class. The only defined way to generate such a value is by using the wctype function.

This type is defined in wctype.h.

— Function: wctype_t wctype (const char *property)

The wctype returns a value representing a class of wide characters which is identified by the string property. Beside some standard properties each locale can define its own ones. In case no property with the given name is known for the current locale selected for the LC_CTYPE category, the function returns zero.

The properties known in every locale are:

"alnum" "alpha" "cntrl" "digit"
"graph" "lower" "print" "punct"
"space" "upper" "xdigit"

This function is declared in wctype.h.

To test the membership of a character to one of the non-standard classes the ISO C standard defines a completely new function.

— Function: int iswctype (wint_t wc, wctype_t desc)

This function returns a nonzero value if wc is in the character class specified by desc. desc must previously be returned by a successful call to wctype.

This function is declared in wctype.h.

To make it easier to use the commonly-used classification functions, they are defined in the C library. There is no need to use wctype if the property string is one of the known character classes. In some situations it is desirable to construct the property strings, and then it is important that wctype can also handle the standard classes.

— Function: int iswalnum (wint_t wc)

This function returns a nonzero value if wc is an alphanumeric character (a letter or number); in other words, if either iswalpha or iswdigit is true of a character, then iswalnum is also true.

This function can be implemented using 

          iswctype (wc, wctype ("alnum"))

It is declared in wctype.h.

— Function: int iswalpha (wint_t wc)

Returns true if wc is an alphabetic character (a letter). If iswlower or iswupper is true of a character, then iswalpha is also true.

In some locales, there may be additional characters for which iswalpha is true—letters which are neither upper case nor lower case. But in the standard "C" locale, there are no such additional characters.

This function can be implemented using 

          iswctype (wc, wctype ("alpha"))

It is declared in wctype.h.

— Function: int iswcntrl (wint_t wc)

Returns true if wc is a control character (that is, a character that is not a printing character).

This function can be implemented using 

          iswctype (wc, wctype ("cntrl"))

It is declared in wctype.h.

— Function: int iswdigit (wint_t wc)

Returns true if wc is a digit (e.g., `0' through `9'). Please note that this function does not only return a nonzero value for decimal digits, but for all kinds of digits. A consequence is that code like the following will not work unconditionally for wide characters: 

          n = 0;
          while (iswdigit (*wc))
            {
              n *= 10;
              n += *wc++ - L'0';
            }

This function can be implemented using 

          iswctype (wc, wctype ("digit"))

It is declared in wctype.h.

— Function: int iswgraph (wint_t wc)

Returns true if wc is a graphic character; that is, a character that has a glyph associated with it. The whitespace characters are not considered graphic.

This function can be implemented using 

          iswctype (wc, wctype ("graph"))

It is declared in wctype.h.

— Function: int iswlower (wint_t wc)

Returns true if wc is a lower-case letter. The letter need not be from the Latin alphabet, any alphabet representable is valid.

This function can be implemented using 

          iswctype (wc, wctype ("lower"))

It is declared in wctype.h.

— Function: int iswprint (wint_t wc)

Returns true if wc is a printing character. Printing characters include all the graphic characters, plus the space (` ') character.

This function can be implemented using 

          iswctype (wc, wctype ("print"))

It is declared in wctype.h.

— Function: int iswpunct (wint_t wc)

Returns true if wc is a punctuation character. This means any printing character that is not alphanumeric or a space character.

This function can be implemented using 

          iswctype (wc, wctype ("punct"))

It is declared in wctype.h.

— Function: int iswspace (wint_t wc)

Returns true if wc is a whitespace character. In the standard "C" locale, iswspace returns true for only the standard whitespace characters:

L' '
space
L'\f'
formfeed
L'\n'
newline
L'\r'
carriage return
L'\t'
horizontal tab
L'\v'
vertical tab

This function can be implemented using 

          iswctype (wc, wctype ("space"))

It is declared in wctype.h.

— Function: int iswupper (wint_t wc)

Returns true if wc is an upper-case letter. The letter need not be from the Latin alphabet, any alphabet representable is valid.

This function can be implemented using 

          iswctype (wc, wctype ("upper"))

It is declared in wctype.h.

— Function: int iswxdigit (wint_t wc)

Returns true if wc is a hexadecimal digit. Hexadecimal digits include the normal decimal digits `0' through `9' and the letters `A' through `F' and `a' through `f'.

This function can be implemented using 

          iswctype (wc, wctype ("xdigit"))

It is declared in wctype.h.

The GNU C library also provides a function which is not defined in the ISO C standard but which is available as a version for single byte characters as well.

— Function: int iswblank (wint_t wc)

Returns true if wc is a blank character; that is, a space or a tab. This function was originally a GNU extension, but was added in ISO C99. It is declared in wchar.h.

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4.4 Notes on using the wide character classes

The first note is probably not astonishing but still occasionally a cause of problems. The iswXXX functions can be implemented using macros and in fact, the GNU C library does this. They are still available as real functions but when the wctype.h header is included the macros will be used. This is the same as the char type versions of these functions.

The second note covers something new. It can be best illustrated by a (real-world) example. The first piece of code is an excerpt from the original code. It is truncated a bit but the intention should be clear. 

     int
     is_in_class (int c, const char *class)
     {
       if (strcmp (class, "alnum") == 0)
         return isalnum (c);
       if (strcmp (class, "alpha") == 0)
         return isalpha (c);
       if (strcmp (class, "cntrl") == 0)
         return iscntrl (c);
       ...
       return 0;
     }

Now, with the wctype and iswctype you can avoid the if cascades, but rewriting the code as follows is wrong: 

     int
     is_in_class (int c, const char *class)
     {
       wctype_t desc = wctype (class);
       return desc ? iswctype ((wint_t) c, desc) : 0;
     }

The problem is that it is not guaranteed that the wide character representation of a single-byte character can be found using casting. In fact, usually this fails miserably. The correct solution to this problem is to write the code as follows: 

     int
     is_in_class (int c, const char *class)
     {
       wctype_t desc = wctype (class);
       return desc ? iswctype (btowc (c), desc) : 0;
     }

See Converting a Character, for more information on btowc. Note that this change probably does not improve the performance of the program a lot since the wctype function still has to make the string comparisons. It gets really interesting if the is_in_class function is called more than once for the same class name. In this case the variable desc could be computed once and reused for all the calls. Therefore the above form of the function is probably not the final one.

Previous: Using Wide Char Classes, Up: Character Handling

4.5 Mapping of wide characters.

The classification functions are also generalized by the ISO C standard. Instead of just allowing the two standard mappings, a locale can contain others. Again, the localedef program already supports generating such locale data files.

— Data Type: wctrans_t

This data type is defined as a scalar type which can hold a value representing the locale-dependent character mapping. There is no way to construct such a value apart from using the return value of the wctrans function.

This type is defined in wctype.h.

— Function: wctrans_t wctrans (const char *property)

The wctrans function has to be used to find out whether a named mapping is defined in the current locale selected for the LC_CTYPE category. If the returned value is non-zero, you can use it afterwards in calls to towctrans. If the return value is zero no such mapping is known in the current locale.

Beside locale-specific mappings there are two mappings which are guaranteed to be available in every locale:

"tolower" "toupper"

These functions are declared in wctype.h.

— Function: wint_t towctrans (wint_t wc, wctrans_t desc)

towctrans maps the input character wc according to the rules of the mapping for which desc is a descriptor, and returns the value it finds. desc must be obtained by a successful call to wctrans.

This function is declared in wctype.h.

For the generally available mappings, the ISO C standard defines convenient shortcuts so that it is not necessary to call wctrans for them.

— Function: wint_t towlower (wint_t wc)

If wc is an upper-case letter, towlower returns the corresponding lower-case letter. If wc is not an upper-case letter, wc is returned unchanged.

towlower can be implemented using 

          towctrans (wc, wctrans ("tolower"))

This function is declared in wctype.h.

— Function: wint_t towupper (wint_t wc)

If wc is a lower-case letter, towupper returns the corresponding upper-case letter. Otherwise wc is returned unchanged.

towupper can be implemented using 

          towctrans (wc, wctrans ("toupper"))

This function is declared in wctype.h.

The same warnings given in the last section for the use of the wide character classification functions apply here. It is not possible to simply cast a char type value to a wint_t and use it as an argument to towctrans calls.

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5 String and Array Utilities

Operations on strings (or arrays of characters) are an important part of many programs. The GNU C library provides an extensive set of string utility functions, including functions for copying, concatenating, comparing, and searching strings. Many of these functions can also operate on arbitrary regions of storage; for example, the memcpy function can be used to copy the contents of any kind of array.

It's fairly common for beginning C programmers to “reinvent the wheel” by duplicating this functionality in their own code, but it pays to become familiar with the library functions and to make use of them, since this offers benefits in maintenance, efficiency, and portability.

For instance, you could easily compare one string to another in two lines of C code, but if you use the built-in strcmp function, you're less likely to make a mistake. And, since these library functions are typically highly optimized, your program may run faster too.

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5.1 Representation of Strings

This section is a quick summary of string concepts for beginning C programmers. It describes how character strings are represented in C and some common pitfalls. If you are already familiar with this material, you can skip this section.

A string is an array of char objects. But string-valued variables are usually declared to be pointers of type char *. Such variables do not include space for the text of a string; that has to be stored somewhere else—in an array variable, a string constant, or dynamically allocated memory (see Memory Allocation). It's up to you to store the address of the chosen memory space into the pointer variable. Alternatively you can store a null pointer in the pointer variable. The null pointer does not point anywhere, so attempting to reference the string it points to gets an error.

“string” normally refers to multibyte character strings as opposed to wide character strings. Wide character strings are arrays of type wchar_t and as for multibyte character strings usually pointers of type wchar_t * are used.

By convention, a null character, '\0', marks the end of a multibyte character string and the null wide character, L'\0', marks the end of a wide character string. For example, in testing to see whether the char * variable p points to a null character marking the end of a string, you can write !*p or *p == '\0'.

A null character is quite different conceptually from a null pointer, although both are represented by the integer 0.

String literals appear in C program source as strings of characters between double-quote characters (`"') where the initial double-quote character is immediately preceded by a capital `L' (ell) character (as in L"foo"). In ISO C, string literals can also be formed by string concatenation: "a" "b" is the same as "ab". For wide character strings one can either use L"a" L"b" or L"a" "b". Modification of string literals is not allowed by the GNU C compiler, because literals are placed in read-only storage.

Character arrays that are declared const cannot be modified either. It's generally good style to declare non-modifiable string pointers to be of type const char *, since this often allows the C compiler to detect accidental modifications as well as providing some amount of documentation about what your program intends to do with the string.

The amount of memory allocated for the character array may extend past the null character that normally marks the end of the string. In this document, the term allocated size is always used to refer to the total amount of memory allocated for the string, while the term length refers to the number of characters up to (but not including) the terminating null character. A notorious source of program bugs is trying to put more characters in a string than fit in its allocated size. When writing code that extends strings or moves characters into a pre-allocated array, you should be very careful to keep track of the length of the text and make explicit checks for overflowing the array. Many of the library functions do not do this for you! Remember also that you need to allocate an extra byte to hold the null character that marks the end of the string.

Originally strings were sequences of bytes where each byte represents a single character. This is still true today if the strings are encoded using a single-byte character encoding. Things are different if the strings are encoded using a multibyte encoding (for more information on encodings see Extended Char Intro). There is no difference in the programming interface for these two kind of strings; the programmer has to be aware of this and interpret the byte sequences accordingly.

But since there is no separate interface taking care of these differences the byte-based string functions are sometimes hard to use. Since the count parameters of these functions specify bytes a call to strncpy could cut a multibyte character in the middle and put an incomplete (and therefore unusable) byte sequence in the target buffer.

To avoid these problems later versions of the ISO C standard introduce a second set of functions which are operating on wide characters (see Extended Char Intro). These functions don't have the problems the single-byte versions have since every wide character is a legal, interpretable value. This does not mean that cutting wide character strings at arbitrary points is without problems. It normally is for alphabet-based languages (except for non-normalized text) but languages based on syllables still have the problem that more than one wide character is necessary to complete a logical unit. This is a higher level problem which the C library functions are not designed to solve. But it is at least good that no invalid byte sequences can be created. Also, the higher level functions can also much easier operate on wide character than on multibyte characters so that a general advise is to use wide characters internally whenever text is more than simply copied.

The remaining of this chapter will discuss the functions for handling wide character strings in parallel with the discussion of the multibyte character strings since there is almost always an exact equivalent available.

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5.2 String and Array Conventions

This chapter describes both functions that work on arbitrary arrays or blocks of memory, and functions that are specific to null-terminated arrays of characters and wide characters.

Functions that operate on arbitrary blocks of memory have names beginning with `mem' and `wmem' (such as memcpy and wmemcpy) and invariably take an argument which specifies the size (in bytes and wide characters respectively) of the block of memory to operate on. The array arguments and return values for these functions have type void * or wchar_t. As a matter of style, the elements of the arrays used with the `mem' functions are referred to as “bytes”. You can pass any kind of pointer to these functions, and the sizeof operator is useful in computing the value for the size argument. Parameters to the `wmem' functions must be of type wchar_t *. These functions are not really usable with anything but arrays of this type.

In contrast, functions that operate specifically on strings and wide character strings have names beginning with `str' and `wcs' respectively (such as strcpy and wcscpy) and look for a null character to terminate the string instead of requiring an explicit size argument to be passed. (Some of these functions accept a specified maximum length, but they also check for premature termination with a null character.) The array arguments and return values for these functions have type char * and wchar_t * respectively, and the array elements are referred to as “characters” and “wide characters”.

In many cases, there are both `mem' and `str'/`wcs' versions of a function. The one that is more appropriate to use depends on the exact situation. When your program is manipulating arbitrary arrays or blocks of storage, then you should always use the `mem' functions. On the other hand, when you are manipulating null-terminated strings it is usually more convenient to use the `str'/`wcs' functions, unless you already know the length of the string in advance. The `wmem' functions should be used for wide character arrays with known size.

Some of the memory and string functions take single characters as arguments. Since a value of type char is automatically promoted into an value of type int when used as a parameter, the functions are declared with int as the type of the parameter in question. In case of the wide character function the situation is similarly: the parameter type for a single wide character is wint_t and not wchar_t. This would for many implementations not be necessary since the wchar_t is large enough to not be automatically promoted, but since the ISO C standard does not require such a choice of types the wint_t type is used.

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5.3 String Length

You can get the length of a string using the strlen function. This function is declared in the header file string.h.

— Function: size_t strlen (const char *s)

The strlen function returns the length of the null-terminated string s in bytes. (In other words, it returns the offset of the terminating null character within the array.)

For example, 

          strlen ("hello, world")
              => 12

When applied to a character array, the strlen function returns the length of the string stored there, not its allocated size. You can get the allocated size of the character array that holds a string using the sizeof operator: 

          char string[32] = "hello, world";
          sizeof (string)
              => 32
          strlen (string)
              => 12

But beware, this will not work unless string is the character array itself, not a pointer to it. For example: 

          char string[32] = "hello, world";
          char *ptr = string;
          sizeof (string)
              => 32
          sizeof (ptr)
              => 4  /* (on a machine with 4 byte pointers) */

This is an easy mistake to make when you are working with functions that take string arguments; those arguments are always pointers, not arrays.

It must also be noted that for multibyte encoded strings the return value does not have to correspond to the number of characters in the string. To get this value the string can be converted to wide characters and wcslen can be used or something like the following code can be used: 

          /* The input is in string.
             The length is expected in n.  */
          {
            mbstate_t t;
            char *scopy = string;
            /* In initial state.  */
            memset (&t, '\0', sizeof (t));
            /* Determine number of characters.  */
            n = mbsrtowcs (NULL, &scopy, strlen (scopy), &t);
          }

This is cumbersome to do so if the number of characters (as opposed to bytes) is needed often it is better to work with wide characters.

The wide character equivalent is declared in wchar.h.

— Function: size_t wcslen (const wchar_t *ws)

The wcslen function is the wide character equivalent to strlen. The return value is the number of wide characters in the wide character string pointed to by ws (this is also the offset of the terminating null wide character of ws).

Since there are no multi wide character sequences making up one character the return value is not only the offset in the array, it is also the number of wide characters.

This function was introduced in Amendment 1 to ISO C90.

— Function: size_t strnlen (const char *s, size_t maxlen)

The strnlen function returns the length of the string s in bytes if this length is smaller than maxlen bytes. Otherwise it returns maxlen. Therefore this function is equivalent to (strlen (s) < n ? strlen (s) : maxlen) but it is more efficient and works even if the string s is not null-terminated. 

          char string[32] = "hello, world";
          strnlen (string, 32)
              => 12
          strnlen (string, 5)
              => 5

This function is a GNU extension and is declared in string.h.

— Function: size_t wcsnlen (const wchar_t *ws, size_t maxlen)

wcsnlen is the wide character equivalent to strnlen. The maxlen parameter specifies the maximum number of wide characters.

This function is a GNU extension and is declared in wchar.h.

Next: , Previous: String Length, Up: String and Array Utilities

5.4 Copying and Concatenation

You can use the functions described in this section to copy the contents of strings and arrays, or to append the contents of one string to another. The `str' and `mem' functions are declared in the header file string.h while the `wstr' and `wmem' functions are declared in the file wchar.h. A helpful way to remember the ordering of the arguments to the functions in this section is that it corresponds to an assignment expression, with the destination array specified to the left of the source array. All of these functions return the address of the destination array.

Most of these functions do not work properly if the source and destination arrays overlap. For example, if the beginning of the destination array overlaps the end of the source array, the original contents of that part of the source array may get overwritten before it is copied. Even worse, in the case of the string functions, the null character marking the end of the string may be lost, and the copy function might get stuck in a loop trashing all the memory allocated to your program.

All functions that have problems copying between overlapping arrays are explicitly identified in this manual. In addition to functions in this section, there are a few others like sprintf (see Formatted Output Functions) and scanf (see Formatted Input Functions).

— Function: void * memcpy (void *restrict to, const void *restrict from, size_t size)

The memcpy function copies size bytes from the object beginning at from into the object beginning at to. The behavior of this function is undefined if the two arrays to and from overlap; use memmove instead if overlapping is possible.

The value returned by memcpy is the value of to.

Here is an example of how you might use memcpy to copy the contents of an array: 

          struct foo *oldarray, *newarray;
          int arraysize;
          ...
          memcpy (new, old, arraysize * sizeof (struct foo));
— Function: wchar_t * wmemcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size)

The wmemcpy function copies size wide characters from the object beginning at wfrom into the object beginning at wto. The behavior of this function is undefined if the two arrays wto and wfrom overlap; use wmemmove instead if overlapping is possible.

The following is a possible implementation of wmemcpy but there are more optimizations possible. 

          wchar_t *
          wmemcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom,
                   size_t size)
          {
            return (wchar_t *) memcpy (wto, wfrom, size * sizeof (wchar_t));
          }

The value returned by wmemcpy is the value of wto.

This function was introduced in Amendment 1 to ISO C90.

— Function: void * mempcpy (void *restrict to, const void *restrict from, size_t size)

The mempcpy function is nearly identical to the memcpy function. It copies size bytes from the object beginning at from into the object pointed to by to. But instead of returning the value of to it returns a pointer to the byte following the last written byte in the object beginning at to. I.e., the value is ((void *) ((char *) to + size)).

This function is useful in situations where a number of objects shall be copied to consecutive memory positions. 

          void *
          combine (void *o1, size_t s1, void *o2, size_t s2)
          {
            void *result = malloc (s1 + s2);
            if (result != NULL)
              mempcpy (mempcpy (result, o1, s1), o2, s2);
            return result;
          }

This function is a GNU extension.

— Function: wchar_t * wmempcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size)

The wmempcpy function is nearly identical to the wmemcpy function. It copies size wide characters from the object beginning at wfrom into the object pointed to by wto. But instead of returning the value of wto it returns a pointer to the wide character following the last written wide character in the object beginning at wto. I.e., the value is wto + size.

This function is useful in situations where a number of objects shall be copied to consecutive memory positions.

The following is a possible implementation of wmemcpy but there are more optimizations possible. 

          wchar_t *
          wmempcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom,
                    size_t size)
          {
            return (wchar_t *) mempcpy (wto, wfrom, size * sizeof (wchar_t));
          }

This function is a GNU extension.

— Function: void * memmove (void *to, const void *from, size_t size)

memmove copies the size bytes at from into the size bytes at to, even if those two blocks of space overlap. In the case of overlap, memmove is careful to copy the original values of the bytes in the block at from, including those bytes which also belong to the block at to.

The value returned by memmove is the value of to.

— Function: wchar_t * wmemmove (wchar *wto, const wchar_t *wfrom, size_t size)

wmemmove copies the size wide characters at wfrom into the size wide characters at wto, even if those two blocks of space overlap. In the case of overlap, memmove is careful to copy the original values of the wide characters in the block at wfrom, including those wide characters which also belong to the block at wto.

The following is a possible implementation of wmemcpy but there are more optimizations possible. 

          wchar_t *
          wmempcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom,
                    size_t size)
          {
            return (wchar_t *) mempcpy (wto, wfrom, size * sizeof (wchar_t));
          }

The value returned by wmemmove is the value of wto.

This function is a GNU extension.

— Function: void * memccpy (void *restrict to, const void *restrict from, int c, size_t size)

This function copies no more than size bytes from from to to, stopping if a byte matching c is found. The return value is a pointer into to one byte past where c was copied, or a null pointer if no byte matching c appeared in the first size bytes of from.

— Function: void * memset (void *block, int c, size_t size)

This function copies the value of c (converted to an unsigned char) into each of the first size bytes of the object beginning at block. It returns the value of block.

— Function: wchar_t * wmemset (wchar_t *block, wchar_t wc, size_t size)

This function copies the value of wc into each of the first size wide characters of the object beginning at block. It returns the value of block.

— Function: char * strcpy (char *restrict to, const char *restrict from)

This copies characters from the string from (up to and including the terminating null character) into the string to. Like memcpy, this function has undefined results if the strings overlap. The return value is the value of to.

— Function: wchar_t * wcscpy (wchar_t *restrict wto, const wchar_t *restrict wfrom)

This copies wide characters from the string wfrom (up to and including the terminating null wide character) into the string wto. Like wmemcpy, this function has undefined results if the strings overlap. The return value is the value of wto.

— Function: char * strncpy (char *restrict to, const char *restrict from, size_t size)

This function is similar to strcpy but always copies exactly size characters into to.

If the length of from is more than size, then strncpy copies just the first size characters. Note that in this case there is no null terminator written into to.

If the length of from is less than size, then strncpy copies all of from, followed by enough null characters to add up to size characters in all. This behavior is rarely useful, but it is specified by the ISO C standard.

The behavior of strncpy is undefined if the strings overlap.

Using strncpy as opposed to strcpy is a way to avoid bugs relating to writing past the end of the allocated space for to. However, it can also make your program much slower in one common case: copying a string which is probably small into a potentially large buffer. In this case, size may be large, and when it is, strncpy will waste a considerable amount of time copying null characters.

— Function: wchar_t * wcsncpy (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size)

This function is similar to wcscpy but always copies exactly size wide characters into wto.

If the length of wfrom is more than size, then wcsncpy copies just the first size wide characters. Note that in this case there is no null terminator written into wto.

If the length of wfrom is less than size, then wcsncpy copies all of wfrom, followed by enough null wide characters to add up to size wide characters in all. This behavior is rarely useful, but it is specified by the ISO C standard.

The behavior of wcsncpy is undefined if the strings overlap.

Using wcsncpy as opposed to wcscpy is a way to avoid bugs relating to writing past the end of the allocated space for wto. However, it can also make your program much slower in one common case: copying a string which is probably small into a potentially large buffer. In this case, size may be large, and when it is, wcsncpy will waste a considerable amount of time copying null wide characters.

— Function: char * strdup (const char *s)

This function copies the null-terminated string s into a newly allocated string. The string is allocated using malloc; see Unconstrained Allocation. If malloc cannot allocate space for the new string, strdup returns a null pointer. Otherwise it returns a pointer to the new string.

— Function: wchar_t * wcsdup (const wchar_t *ws)

This function copies the null-terminated wide character string ws into a newly allocated string. The string is allocated using malloc; see Unconstrained Allocation. If malloc cannot allocate space for the new string, wcsdup returns a null pointer. Otherwise it returns a pointer to the new wide character string.

This function is a GNU extension.

— Function: char * strndup (const char *s, size_t size)

This function is similar to strdup but always copies at most size characters into the newly allocated string.

If the length of s is more than size, then strndup copies just the first size characters and adds a closing null terminator. Otherwise all characters are copied and the string is terminated.

This function is different to strncpy in that it always terminates the destination string.

strndup is a GNU extension.

— Function: char * stpcpy (char *restrict to, const char *restrict from)

This function is like strcpy, except that it returns a pointer to the end of the string to (that is, the address of the terminating null character to + strlen (from)) rather than the beginning.

For example, this program uses stpcpy to concatenate `foo' and `bar' to produce `foobar', which it then prints. 

          #include <string.h>
          #include <stdio.h>
          
          int
          main (void)
          {
            char buffer[10];
            char *to = buffer;
            to = stpcpy (to, "foo");
            to = stpcpy (to, "bar");
            puts (buffer);
            return 0;
          }

This function is not part of the ISO or POSIX standards, and is not customary on Unix systems, but we did not invent it either. Perhaps it comes from MS-DOG.

Its behavior is undefined if the strings overlap. The function is declared in string.h.

— Function: wchar_t * wcpcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom)

This function is like wcscpy, except that it returns a pointer to the end of the string wto (that is, the address of the terminating null character wto + strlen (wfrom)) rather than the beginning.

This function is not part of ISO or POSIX but was found useful while developing the GNU C Library itself.

The behavior of wcpcpy is undefined if the strings overlap.

wcpcpy is a GNU extension and is declared in wchar.h.

— Function: char * stpncpy (char *restrict to, const char *restrict from, size_t size)

This function is similar to stpcpy but copies always exactly size characters into to.

If the length of from is more then size, then stpncpy copies just the first size characters and returns a pointer to the character directly following the one which was copied last. Note that in this case there is no null terminator written into to.

If the length of from is less than size, then stpncpy copies all of from, followed by enough null characters to add up to size characters in all. This behavior is rarely useful, but it is implemented to be useful in contexts where this behavior of the strncpy is used. stpncpy returns a pointer to the first written null character.

This function is not part of ISO or POSIX but was found useful while developing the GNU C Library itself.

Its behavior is undefined if the strings overlap. The function is declared in string.h.

— Function: wchar_t * wcpncpy (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size)

This function is similar to wcpcpy but copies always exactly wsize characters into wto.

If the length of wfrom is more then size, then wcpncpy copies just the first size wide characters and returns a pointer to the wide character directly following the last non-null wide character which was copied last. Note that in this case there is no null terminator written into wto.

If the length of wfrom is less than size, then wcpncpy copies all of wfrom, followed by enough null characters to add up to size characters in all. This behavior is rarely useful, but it is implemented to be useful in contexts where this behavior of the wcsncpy is used. wcpncpy returns a pointer to the first written null character.

This function is not part of ISO or POSIX but was found useful while developing the GNU C Library itself.

Its behavior is undefined if the strings overlap.

wcpncpy is a GNU extension and is declared in wchar.h.

— Macro: char * strdupa (const char *s)

This macro is similar to strdup but allocates the new string using alloca instead of malloc (see Variable Size Automatic). This means of course the returned string has the same limitations as any block of memory allocated using alloca.

For obvious reasons strdupa is implemented only as a macro; you cannot get the address of this function. Despite this limitation it is a useful function. The following code shows a situation where using malloc would be a lot more expensive. 

          #include <paths.h>
          #include <string.h>
          #include <stdio.h>
          
          const char path[] = _PATH_STDPATH;
          
          int
          main (void)
          {
            char *wr_path = strdupa (path);
            char *cp = strtok (wr_path, ":");
          
            while (cp != NULL)
              {
                puts (cp);
                cp = strtok (NULL, ":");
              }
            return 0;
          }

Please note that calling strtok using path directly is invalid. It is also not allowed to call strdupa in the argument list of strtok since strdupa uses alloca (see Variable Size Automatic) can interfere with the parameter passing.

This function is only available if GNU CC is used.

— Macro: char * strndupa (const char *s, size_t size)

This function is similar to strndup but like strdupa it allocates the new string using alloca see Variable Size Automatic. The same advantages and limitations of strdupa are valid for strndupa, too.

This function is implemented only as a macro, just like strdupa. Just as strdupa this macro also must not be used inside the parameter list in a function call.

strndupa is only available if GNU CC is used.

— Function: char * strcat (char *restrict to, const char *restrict from)

The strcat function is similar to strcpy, except that the characters from from are concatenated or appended to the end of to, instead of overwriting it. That is, the first character from from overwrites the null character marking the end of to.

An equivalent definition for strcat would be: 

          char *
          strcat (char *restrict to, const char *restrict from)
          {
            strcpy (to + strlen (to), from);
            return to;
          }

This function has undefined results if the strings overlap.

— Function: wchar_t * wcscat (wchar_t *restrict wto, const wchar_t *restrict wfrom)

The wcscat function is similar to wcscpy, except that the characters from wfrom are concatenated or appended to the end of wto, instead of overwriting it. That is, the first character from wfrom overwrites the null character marking the end of wto.

An equivalent definition for wcscat would be: 

          wchar_t *
          wcscat (wchar_t *wto, const wchar_t *wfrom)
          {
            wcscpy (wto + wcslen (wto), wfrom);
            return wto;
          }

This function has undefined results if the strings overlap.

Programmers using the strcat or wcscat function (or the following strncat or wcsncar functions for that matter) can easily be recognized as lazy and reckless. In almost all situations the lengths of the participating strings are known (it better should be since how can one otherwise ensure the allocated size of the buffer is sufficient?) Or at least, one could know them if one keeps track of the results of the various function calls. But then it is very inefficient to use strcat/wcscat. A lot of time is wasted finding the end of the destination string so that the actual copying can start. This is a common example: 

     /* This function concatenates arbitrarily many strings.  The last
        parameter must be NULL.  */
     char *
     concat (const char *str, ...)
     {
       va_list ap, ap2;
       size_t total = 1;
       const char *s;
       char *result;
     
       va_start (ap, str);
       /* Actually va_copy, but this is the name more gcc versions
          understand.  */
       __va_copy (ap2, ap);
     
       /* Determine how much space we need.  */
       for (s = str; s != NULL; s = va_arg (ap, const char *))
         total += strlen (s);
     
       va_end (ap);
     
       result = (char *) malloc (total);
       if (result != NULL)
         {
           result[0] = '\0';
     
           /* Copy the strings.  */
           for (s = str; s != NULL; s = va_arg (ap2, const char *))
             strcat (result, s);
         }
     
       va_end (ap2);
     
       return result;
     }

This looks quite simple, especially the second loop where the strings are actually copied. But these innocent lines hide a major performance penalty. Just imagine that ten strings of 100 bytes each have to be concatenated. For the second string we search the already stored 100 bytes for the end of the string so that we can append the next string. For all strings in total the comparisons necessary to find the end of the intermediate results sums up to 5500! If we combine the copying with the search for the allocation we can write this function more efficient: 

     char *
     concat (const char *str, ...)
     {
       va_list ap;
       size_t allocated = 100;
       char *result = (char *) malloc (allocated);
     
       if (result != NULL)
         {
           char *newp;
           char *wp;
     
           va_start (ap, str);
     
           wp = result;
           for (s = str; s != NULL; s = va_arg (ap, const char *))
             {
               size_t len = strlen (s);
     
               /* Resize the allocated memory if necessary.  */
               if (wp + len + 1 > result + allocated)
                 {
                   allocated = (allocated + len) * 2;
                   newp = (char *) realloc (result, allocated);
                   if (newp == NULL)
                     {
                       free (result);
                       return NULL;
                     }
                   wp = newp + (wp - result);
                   result = newp;
                 }
     
               wp = mempcpy (wp, s, len);
             }
     
           /* Terminate the result string.  */
           *wp++ = '\0';
     
           /* Resize memory to the optimal size.  */
           newp = realloc (result, wp - result);
           if (newp != NULL)
             result = newp;
     
           va_end (ap);
         }
     
       return result;
     }

With a bit more knowledge about the input strings one could fine-tune the memory allocation. The difference we are pointing to here is that we don't use strcat anymore. We always keep track of the length of the current intermediate result so we can safe us the search for the end of the string and use mempcpy. Please note that we also don't use stpcpy which might seem more natural since we handle with strings. But this is not necessary since we already know the length of the string and therefore can use the faster memory copying function. The example would work for wide characters the same way.

Whenever a programmer feels the need to use strcat she or he should think twice and look through the program whether the code cannot be rewritten to take advantage of already calculated results. Again: it is almost always unnecessary to use strcat.

— Function: char * strncat (char *restrict to, const char *restrict from, size_t size)

This function is like strcat except that not more than size characters from from are appended to the end of to. A single null character is also always appended to to, so the total allocated size of to must be at least size + 1 bytes longer than its initial length.

The strncat function could be implemented like this: 

          char *
          strncat (char *to, const char *from, size_t size)
          {
            to[strlen (to) + size] = '\0';
            strncpy (to + strlen (to), from, size);
            return to;
          }

The behavior of strncat is undefined if the strings overlap.

— Function: wchar_t * wcsncat (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size)

This function is like wcscat except that not more than size characters from from are appended to the end of to. A single null character is also always appended to to, so the total allocated size of to must be at least size + 1 bytes longer than its initial length.

The wcsncat function could be implemented like this: 

          wchar_t *
          wcsncat (wchar_t *restrict wto, const wchar_t *restrict wfrom,
                   size_t size)
          {
            wto[wcslen (to) + size] = L'\0';
            wcsncpy (wto + wcslen (wto), wfrom, size);
            return wto;
          }

The behavior of wcsncat is undefined if the strings overlap.

Here is an example showing the use of strncpy and strncat (the wide character version is equivalent). Notice how, in the call to strncat, the size parameter is computed to avoid overflowing the character array buffer. 

     #include <string.h>
     #include <stdio.h>
     
     #define SIZE 10
     
     static char buffer[SIZE];
     
     main ()
     {
       strncpy (buffer, "hello", SIZE);
       puts (buffer);
       strncat (buffer, ", world", SIZE - strlen (buffer) - 1);
       puts (buffer);
     }

The output produced by this program looks like: 

     hello
     hello, wo
— Function: void bcopy (const void *from, void *to, size_t size)

This is a partially obsolete alternative for memmove, derived from BSD. Note that it is not quite equivalent to memmove, because the arguments are not in the same order and there is no return value.

— Function: void bzero (void *block, size_t size)

This is a partially obsolete alternative for memset, derived from BSD. Note that it is not as general as memset, because the only value it can store is zero.

Next: , Previous: Copying and Concatenation, Up: String and Array Utilities

5.5 String/Array Comparison

You can use the functions in this section to perform comparisons on the contents of strings and arrays. As well as checking for equality, these functions can also be used as the ordering functions for sorting operations. See Searching and Sorting, for an example of this.

Unlike most comparison operations in C, the string comparison functions return a nonzero value if the strings are not equivalent rather than if they are. The sign of the value indicates the relative ordering of the first characters in the strings that are not equivalent: a negative value indicates that the first string is “less” than the second, while a positive value indicates that the first string is “greater”.

The most common use of these functions is to check only for equality. This is canonically done with an expression like `! strcmp (s1, s2)'.

All of these functions are declared in the header file string.h.

— Function: int memcmp (const void *a1, const void *a2, size_t size)

The function memcmp compares the size bytes of memory beginning at a1 against the size bytes of memory beginning at a2. The value returned has the same sign as the difference between the first differing pair of bytes (interpreted as unsigned char objects, then promoted to int).

If the contents of the two blocks are equal, memcmp returns 0.

— Function: int wmemcmp (const wchar_t *a1, const wchar_t *a2, size_t size)

The function wmemcmp compares the size wide characters beginning at a1 against the size wide characters beginning at a2. The value returned is smaller than or larger than zero depending on whether the first differing wide character is a1 is smaller or larger than the corresponding character in a2.

If the contents of the two blocks are equal, wmemcmp returns 0.

On arbitrary arrays, the memcmp function is mostly useful for testing equality. It usually isn't meaningful to do byte-wise ordering comparisons on arrays of things other than bytes. For example, a byte-wise comparison on the bytes that make up floating-point numbers isn't likely to tell you anything about the relationship between the values of the floating-point numbers.

wmemcmp is really only useful to compare arrays of type wchar_t since the function looks at sizeof (wchar_t) bytes at a time and this number of bytes is system dependent.

You should also be careful about using memcmp to compare objects that can contain “holes”, such as the padding inserted into structure objects to enforce alignment requirements, extra space at the end of unions, and extra characters at the ends of strings whose length is less than their allocated size. The contents of these “holes” are indeterminate and may cause strange behavior when performing byte-wise comparisons. For more predictable results, perform an explicit component-wise comparison.

For example, given a structure type definition like: 

     struct foo
       {
         unsigned char tag;
         union
           {
             double f;
             long i;
             char *p;
           } value;
       };

you are better off writing a specialized comparison function to compare struct foo objects instead of comparing them with memcmp.

— Function: int strcmp (const char *s1, const char *s2)

The strcmp function compares the string s1 against s2, returning a value that has the same sign as the difference between the first differing pair of characters (interpreted as unsigned char objects, then promoted to int).

If the two strings are equal, strcmp returns 0.

A consequence of the ordering used by strcmp is that if s1 is an initial substring of s2, then s1 is considered to be “less than” s2.

strcmp does not take sorting conventions of the language the strings are written in into account. To get that one has to use strcoll.

— Function: int wcscmp (const wchar_t *ws1, const wchar_t *ws2)

The wcscmp function compares the wide character string ws1 against ws2. The value returned is smaller than or larger than zero depending on whether the first differing wide character is ws1 is smaller or larger than the corresponding character in ws2.

If the two strings are equal, wcscmp returns 0.

A consequence of the ordering used by wcscmp is that if ws1 is an initial substring of ws2, then ws1 is considered to be “less than” ws2.

wcscmp does not take sorting conventions of the language the strings are written in into account. To get that one has to use wcscoll.

— Function: int strcasecmp (const char *s1, const char *s2)

This function is like strcmp, except that differences in case are ignored. How uppercase and lowercase characters are related is determined by the currently selected locale. In the standard "C" locale the characters Ä and ä do not match but in a locale which regards these characters as parts of the alphabet they do match.

strcasecmp is derived from BSD.

— Function: int wcscasecmp (const wchar_t *ws1, const wchar_T *ws2)

This function is like wcscmp, except that differences in case are ignored. How uppercase and lowercase characters are related is determined by the currently selected locale. In the standard "C" locale the characters Ä and ä do not match but in a locale which regards these characters as parts of the alphabet they do match.

wcscasecmp is a GNU extension.

— Function: int strncmp (const char *s1, const char *s2, size_t size)

This function is the similar to strcmp, except that no more than size characters are compared. In other words, if the two strings are the same in their first size characters, the return value is zero.

— Function: int wcsncmp (const wchar_t *ws1, const wchar_t *ws2, size_t size)

This function is the similar to wcscmp, except that no more than size wide characters are compared. In other words, if the two strings are the same in their first size wide characters, the return value is zero.

— Function: int strncasecmp (const char *s1, const char *s2, size_t n)

This function is like strncmp, except that differences in case are ignored. Like strcasecmp, it is locale dependent how uppercase and lowercase characters are related.

strncasecmp is a GNU extension.

— Function: int wcsncasecmp (const wchar_t *ws1, const wchar_t *s2, size_t n)

This function is like wcsncmp, except that differences in case are ignored. Like wcscasecmp, it is locale dependent how uppercase and lowercase characters are related.

wcsncasecmp is a GNU extension.

Here are some examples showing the use of strcmp and strncmp (equivalent examples can be constructed for the wide character functions). These examples assume the use of the ASCII character set. (If some other character set—say, EBCDIC—is used instead, then the glyphs are associated with different numeric codes, and the return values and ordering may differ.) 

     strcmp ("hello", "hello")
         => 0    /* These two strings are the same. */
     strcmp ("hello", "Hello")
         => 32   /* Comparisons are case-sensitive. */
     strcmp ("hello", "world")
         => -15  /* The character 'h' comes before 'w'. */
     strcmp ("hello", "hello, world")
         => -44  /* Comparing a null character against a comma. */
     strncmp ("hello", "hello, world", 5)
         => 0    /* The initial 5 characters are the same. */
     strncmp ("hello, world", "hello, stupid world!!!", 5)
         => 0    /* The initial 5 characters are the same. */
— Function: int strverscmp (const char *s1, const char *s2)

The strverscmp function compares the string s1 against s2, considering them as holding indices/version numbers. Return value follows the same conventions as found in the strverscmp function. In fact, if s1 and s2 contain no digits, strverscmp behaves like strcmp.

Basically, we compare strings normally (character by character), until we find a digit in each string - then we enter a special comparison mode, where each sequence of digits is taken as a whole. If we reach the end of these two parts without noticing a difference, we return to the standard comparison mode. There are two types of numeric parts: "integral" and "fractional" (those begin with a '0'). The types of the numeric parts affect the way we sort them: 

          strverscmp ("no digit", "no digit")
              => 0    /* same behavior as strcmp. */
          strverscmp ("item#99", "item#100")
              => <0   /* same prefix, but 99 < 100. */
          strverscmp ("alpha1", "alpha001")
              => >0   /* fractional part inferior to integral one. */
          strverscmp ("part1_f012", "part1_f01")
              => >0   /* two fractional parts. */
          strverscmp ("foo.009", "foo.0")
              => <0   /* idem, but with leading zeroes only. */

This function is especially useful when dealing with filename sorting, because filenames frequently hold indices/version numbers.

strverscmp is a GNU extension.

— Function: int bcmp (const void *a1, const void *a2, size_t size)

This is an obsolete alias for memcmp, derived from BSD.

Next: , Previous: String/Array Comparison, Up: String and Array Utilities

5.6 Collation Functions

In some locales, the conventions for lexicographic ordering differ from the strict numeric ordering of character codes. For example, in Spanish most glyphs with diacritical marks such as accents are not considered distinct letters for the purposes of collation. On the other hand, the two-character sequence `ll' is treated as a single letter that is collated immediately after `l'.

You can use the functions strcoll and strxfrm (declared in the headers file string.h) and wcscoll and wcsxfrm (declared in the headers file wchar) to compare strings using a collation ordering appropriate for the current locale. The locale used by these functions in particular can be specified by setting the locale for the LC_COLLATE category; see Locales. In the standard C locale, the collation sequence for strcoll is the same as that for strcmp. Similarly, wcscoll and wcscmp are the same in this situation.

Effectively, the way these functions work is by applying a mapping to transform the characters in a string to a byte sequence that represents the string's position in the collating sequence of the current locale. Comparing two such byte sequences in a simple fashion is equivalent to comparing the strings with the locale's collating sequence.

The functions strcoll and wcscoll perform this translation implicitly, in order to do one comparison. By contrast, strxfrm and wcsxfrm perform the mapping explicitly. If you are making multiple comparisons using the same string or set of strings, it is likely to be more efficient to use strxfrm or wcsxfrm to transform all the strings just once, and subsequently compare the transformed strings with strcmp or wcscmp.

— Function: int strcoll (const char *s1, const char *s2)

The strcoll function is similar to strcmp but uses the collating sequence of the current locale for collation (the LC_COLLATE locale).

— Function: int wcscoll (const wchar_t *ws1, const wchar_t *ws2)

The wcscoll function is similar to wcscmp but uses the collating sequence of the current locale for collation (the LC_COLLATE locale).

Here is an example of sorting an array of strings, using strcoll to compare them. The actual sort algorithm is not written here; it comes from qsort (see Array Sort Function). The job of the code shown here is to say how to compare the strings while sorting them. (Later on in this section, we will show a way to do this more efficiently using strxfrm.) 

     /* This is the comparison function used with qsort. */
     
     int
     compare_elements (char **p1, char **p2)
     {
       return strcoll (*p1, *p2);
     }
     
     /* This is the entry point---the function to sort
        strings using the locale's collating sequence. */
     
     void
     sort_strings (char **array, int nstrings)
     {
       /* Sort temp_array by comparing the strings. */
       qsort (array, nstrings,
              sizeof (char *), compare_elements);
     }

— Function: size_t strxfrm (char *restrict to, const char *restrict from, size_t size)

The function strxfrm transforms the string from using the collation transformation determined by the locale currently selected for collation, and stores the transformed string in the array to. Up to size characters (including a terminating null character) are stored.

The behavior is undefined if the strings to and from overlap; see Copying and Concatenation.

The return value is the length of the entire transformed string. This value is not affected by the value of size, but if it is greater or equal than size, it means that the transformed string did not entirely fit in the array to. In this case, only as much of the string as actually fits was stored. To get the whole transformed string, call strxfrm again with a bigger output array.

The transformed string may be longer than the original string, and it may also be shorter.

If size is zero, no characters are stored in to. In this case, strxfrm simply returns the number of characters that would be the length of the transformed string. This is useful for determining what size the allocated array should be. It does not matter what to is if size is zero; to may even be a null pointer.

— Function: size_t wcsxfrm (wchar_t *restrict wto, const wchar_t *wfrom, size_t size)

The function wcsxfrm transforms wide character string wfrom using the collation transformation determined by the locale currently selected for collation, and stores the transformed string in the array wto. Up to size wide characters (including a terminating null character) are stored.

The behavior is undefined if the strings wto and wfrom overlap; see Copying and Concatenation.

The return value is the length of the entire transformed wide character string. This value is not affected by the value of size, but if it is greater or equal than size, it means that the transformed wide character string did not entirely fit in the array wto. In this case, only as much of the wide character string as actually fits was stored. To get the whole transformed wide character string, call wcsxfrm again with a bigger output array.

The transformed wide character string may be longer than the original wide character string, and it may also be shorter.

If size is zero, no characters are stored in to. In this case, wcsxfrm simply returns the number of wide characters that would be the length of the transformed wide character string. This is useful for determining what size the allocated array should be (remember to multiply with sizeof (wchar_t)). It does not matter what wto is if size is zero; wto may even be a null pointer.

Here is an example of how you can use strxfrm when you plan to do many comparisons. It does the same thing as the previous example, but much faster, because it has to transform each string only once, no matter how many times it is compared with other strings. Even the time needed to allocate and free storage is much less than the time we save, when there are many strings. 

     struct sorter { char *input; char *transformed; };
     
     /* This is the comparison function used with qsort
        to sort an array of struct sorter. */
     
     int
     compare_elements (struct sorter *p1, struct sorter *p2)
     {
       return strcmp (p1->transformed, p2->transformed);
     }
     
     /* This is the entry point---the function to sort
        strings using the locale's collating sequence. */
     
     void
     sort_strings_fast (char **array, int nstrings)
     {
       struct sorter temp_array[nstrings];
       int i;
     
       /* Set up temp_array.  Each element contains
          one input string and its transformed string. */
       for (i = 0; i < nstrings; i++)
         {
           size_t length = strlen (array[i]) * 2;
           char *transformed;
           size_t transformed_length;
     
           temp_array[i].input = array[i];
     
           /* First try a buffer perhaps big enough.  */
           transformed = (char *) xmalloc (length);
     
           /* Transform array[i].  */
           transformed_length = strxfrm (transformed, array[i], length);
     
           /* If the buffer was not large enough, resize it
              and try again.  */
           if (transformed_length >= length)
             {
               /* Allocate the needed space. +1 for terminating
                  NUL character.  */
               transformed = (char *) xrealloc (transformed,
                                                transformed_length + 1);
     
               /* The return value is not interesting because we know
                  how long the transformed string is.  */
               (void) strxfrm (transformed, array[i],
                               transformed_length + 1);
             }
     
           temp_array[i].transformed = transformed;
         }
     
       /* Sort temp_array by comparing transformed strings. */
       qsort (temp_array, sizeof (struct sorter),
              nstrings, compare_elements);
     
       /* Put the elements back in the permanent array
          in their sorted order. */
       for (i = 0; i < nstrings; i++)
         array[i] = temp_array[i].input;
     
       /* Free the strings we allocated. */
       for (i = 0; i < nstrings; i++)
         free (temp_array[i].transformed);
     }

The interesting part of this code for the wide character version would look like this: 

     void
     sort_strings_fast (wchar_t **array, int nstrings)
     {
       ...
           /* Transform array[i].  */
           transformed_length = wcsxfrm (transformed, array[i], length);
     
           /* If the buffer was not large enough, resize it
              and try again.  */
           if (transformed_length >= length)
             {
               /* Allocate the needed space. +1 for terminating
                  NUL character.  */
               transformed = (wchar_t *) xrealloc (transformed,
                                                   (transformed_length + 1)
                                                   * sizeof (wchar_t));
     
               /* The return value is not interesting because we know
                  how long the transformed string is.  */
               (void) wcsxfrm (transformed, array[i],
                               transformed_length + 1);
             }
       ...

Note the additional multiplication with sizeof (wchar_t) in the realloc call.

Compatibility Note: The string collation functions are a new feature of ISO C90. Older C dialects have no equivalent feature. The wide character versions were introduced in Amendment 1 to ISO C90.

Next: , Previous: Collation Functions, Up: String and Array Utilities

5.7 Search Functions

This section describes library functions which perform various kinds of searching operations on strings and arrays. These functions are declared in the header file string.h.

— Function: void * memchr (const void *block, int c, size_t size)

This function finds the first occurrence of the byte c (converted to an unsigned char) in the initial size bytes of the object beginning at block. The return value is a pointer to the located byte, or a null pointer if no match was found.

— Function: wchar_t * wmemchr (const wchar_t *block, wchar_t wc, size_t size)

This function finds the first occurrence of the wide character wc in the initial size wide characters of the object beginning at block. The return value is a pointer to the located wide character, or a null pointer if no match was found.

— Function: void * rawmemchr (const void *block, int c)

Often the memchr function is used with the knowledge that the byte c is available in the memory block specified by the parameters. But this means that the size parameter is not really needed and that the tests performed with it at runtime (to check whether the end of the block is reached) are not needed.

The rawmemchr function exists for just this situation which is surprisingly frequent. The interface is similar to memchr except that the size parameter is missing. The function will look beyond the end of the block pointed to by block in case the programmer made an error in assuming that the byte c is present in the block. In this case the result is unspecified. Otherwise the return value is a pointer to the located byte.

This function is of special interest when looking for the end of a string. Since all strings are terminated by a null byte a call like 

             rawmemchr (str, '\0')

will never go beyond the end of the string.

This function is a GNU extension.

— Function: void * memrchr (const void *block, int c, size_t size)

The function memrchr is like memchr, except that it searches backwards from the end of the block defined by block and size (instead of forwards from the front).

This function is a GNU extension.

— Function: char * strchr (const char *string, int c)

The strchr function finds the first occurrence of the character c (converted to a char) in the null-terminated string beginning at string. The return value is a pointer to the located character, or a null pointer if no match was found.

For example, 

          strchr ("hello, world", 'l')
              => "llo, world"
          strchr ("hello, world", '?')
              => NULL

The terminating null character is considered to be part of the string, so you can use this function get a pointer to the end of a string by specifying a null character as the value of the c argument. It would be better (but less portable) to use strchrnul in this case, though.

— Function: wchar_t * wcschr (const wchar_t *wstring, int wc)

The wcschr function finds the first occurrence of the wide character wc in the null-terminated wide character string beginning at wstring. The return value is a pointer to the located wide character, or a null pointer if no match was found.

The terminating null character is considered to be part of the wide character string, so you can use this function get a pointer to the end of a wide character string by specifying a null wude character as the value of the wc argument. It would be better (but less portable) to use wcschrnul in this case, though.

— Function: char * strchrnul (const char *string, int c)

strchrnul is the same as strchr except that if it does not find the character, it returns a pointer to string's terminating null character rather than a null pointer.

This function is a GNU extension.

— Function: wchar_t * wcschrnul (const wchar_t *wstring, wchar_t wc)

wcschrnul is the same as wcschr except that if it does not find the wide character, it returns a pointer to wide character string's terminating null wide character rather than a null pointer.

This function is a GNU extension.

One useful, but unusual, use of the strchr function is when one wants to have a pointer pointing to the NUL byte terminating a string. This is often written in this way: 

       s += strlen (s);

This is almost optimal but the addition operation duplicated a bit of the work already done in the strlen function. A better solution is this: 

       s = strchr (s, '\0');

There is no restriction on the second parameter of strchr so it could very well also be the NUL character. Those readers thinking very hard about this might now point out that the strchr function is more expensive than the strlen function since we have two abort criteria. This is right. But in the GNU C library the implementation of strchr is optimized in a special way so that strchr actually is faster.

— Function: char * strrchr (const char *string, int c)

The function strrchr is like strchr, except that it searches backwards from the end of the string string (instead of forwards from the front).

For example, 

          strrchr ("hello, world", 'l')
              => "ld"
— Function: wchar_t * wcsrchr (const wchar_t *wstring, wchar_t c)

The function wcsrchr is like wcschr, except that it searches backwards from the end of the string wstring (instead of forwards from the front).

— Function: char * strstr (const char *haystack, const char *needle)

This is like strchr, except that it searches haystack for a substring needle rather than just a single character. It returns a pointer into the string haystack that is the first character of the substring, or a null pointer if no match was found. If needle is an empty string, the function returns haystack.

For example, 

          strstr ("hello, world", "l")
              => "llo, world"
          strstr ("hello, world", "wo")
              => "world"
— Function: wchar_t * wcsstr (const wchar_t *haystack, const wchar_t *needle)

This is like wcschr, except that it searches haystack for a substring needle rather than just a single wide character. It returns a pointer into the string haystack that is the first wide character of the substring, or a null pointer if no match was found. If needle is an empty string, the function returns haystack.

— Function: wchar_t * wcswcs (const wchar_t *haystack, const wchar_t *needle)

wcswcs is an deprecated alias for wcsstr. This is the name originally used in the X/Open Portability Guide before the Amendment 1 to ISO C90 was published.

— Function: char * strcasestr (const char *haystack, const char *needle)

This is like strstr, except that it ignores case in searching for the substring. Like strcasecmp, it is locale dependent how uppercase and lowercase characters are related.

For example, 

          strcasestr ("hello, world", "L")
              => "llo, world"
          strcasestr ("hello, World", "wo")
              => "World"
— Function: void * memmem (const void *haystack, size_t haystack-len,
const void *needle, size_t needle-len)

This is like strstr, but needle and haystack are byte arrays rather than null-terminated strings. needle-len is the length of needle and haystack-len is the length of haystack.

This function is a GNU extension.

— Function: size_t strspn (const char *string, const char *skipset)

The strspn (“string span”) function returns the length of the initial substring of string that consists entirely of characters that are members of the set specified by the string skipset. The order of the characters in skipset is not important.

For example, 

          strspn ("hello, world", "abcdefghijklmnopqrstuvwxyz")
              => 5

Note that “character” is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent.

— Function: size_t wcsspn (const wchar_t *wstring, const wchar_t *skipset)

The wcsspn (“wide character string span”) function returns the length of the initial substring of wstring that consists entirely of wide characters that are members of the set specified by the string skipset. The order of the wide characters in skipset is not important.

— Function: size_t strcspn (const char *string, const char *stopset)

The strcspn (“string complement span”) function returns the length of the initial substring of string that consists entirely of characters that are not members of the set specified by the string stopset. (In other words, it returns the offset of the first character in string that is a member of the set stopset.)

For example, 

          strcspn ("hello, world", " \t\n,.;!?")
              => 5

Note that “character” is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent.

— Function: size_t wcscspn (const wchar_t *wstring, const wchar_t *stopset)

The wcscspn (“wide character string complement span”) function returns the length of the initial substring of wstring that consists entirely of wide characters that are not members of the set specified by the string stopset. (In other words, it returns the offset of the first character in string that is a member of the set stopset.)

— Function: char * strpbrk (const char *string, const char *stopset)

The strpbrk (“string pointer break”) function is related to strcspn, except that it returns a pointer to the first character in string that is a member of the set stopset instead of the length of the initial substring. It returns a null pointer if no such character from stopset is found.

For example, 

          strpbrk ("hello, world", " \t\n,.;!?")
              => ", world"

Note that “character” is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent.

— Function: wchar_t * wcspbrk (const wchar_t *wstring, const wchar_t *stopset)

The wcspbrk (“wide character string pointer break”) function is related to wcscspn, except that it returns a pointer to the first wide character in wstring that is a member of the set stopset instead of the length of the initial substring. It returns a null pointer if no such character from stopset is found.

5.7.1 Compatibility String Search Functions

— Function: char * index (const char *string, int c)

index is another name for strchr; they are exactly the same. New code should always use strchr since this name is defined in ISO C while index is a BSD invention which never was available on System V derived systems.

— Function: char * rindex (const char *string, int c)

rindex is another name for strrchr; they are exactly the same. New code should always use strrchr since this name is defined in ISO C while rindex is a BSD invention which never was available on System V derived systems.

Next: , Previous: Search Functions, Up: String and Array Utilities

5.8 Finding Tokens in a String

It's fairly common for programs to have a need to do some simple kinds of lexical analysis and parsing, such as splitting a command string up into tokens. You can do this with the strtok function, declared in the header file string.h.

— Function: char * strtok (char *restrict newstring, const char *restrict delimiters)

A string can be split into tokens by making a series of calls to the function strtok.

The string to be split up is passed as the newstring argument on the first call only. The strtok function uses this to set up some internal state information. Subsequent calls to get additional tokens from the same string are indicated by passing a null pointer as the newstring argument. Calling strtok with another non-null newstring argument reinitializes the state information. It is guaranteed that no other library function ever calls strtok behind your back (which would mess up this internal state information).

The delimiters argument is a string that specifies a set of delimiters that may surround the token being extracted. All the initial characters that are members of this set are discarded. The first character that is not a member of this set of delimiters marks the beginning of the next token. The end of the token is found by looking for the next character that is a member of the delimiter set. This character in the original string newstring is overwritten by a null character, and the pointer to the beginning of the token in newstring is returned.

On the next call to strtok, the searching begins at the next character beyond the one that marked the end of the previous token. Note that the set of delimiters delimiters do not have to be the same on every call in a series of calls to strtok.

If the end of the string newstring is reached, or if the remainder of string consists only of delimiter characters, strtok returns a null pointer.

Note that “character” is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent.

— Function: wchar_t * wcstok (wchar_t *newstring, const char *delimiters)

A string can be split into tokens by making a series of calls to the function wcstok.

The string to be split up is passed as the newstring argument on the first call only. The wcstok function uses this to set up some internal state information. Subsequent calls to get additional tokens from the same wide character string are indicated by passing a null pointer as the newstring argument. Calling wcstok with another non-null newstring argument reinitializes the state information. It is guaranteed that no other library function ever calls wcstok behind your back (which would mess up this internal state information).

The delimiters argument is a wide character string that specifies a set of delimiters that may surround the token being extracted. All the initial wide characters that are members of this set are discarded. The first wide character that is not a member of this set of delimiters marks the beginning of the next token. The end of the token is found by looking for the next wide character that is a member of the delimiter set. This wide character in the original wide character string newstring is overwritten by a null wide character, and the pointer to the beginning of the token in newstring is returned.

On the next call to wcstok, the searching begins at the next wide character beyond the one that marked the end of the previous token. Note that the set of delimiters delimiters do not have to be the same on every call in a series of calls to wcstok.

If the end of the wide character string newstring is reached, or if the remainder of string consists only of delimiter wide characters, wcstok returns a null pointer.

Note that “character” is here used in the sense of byte. In a string using a multibyte character encoding (abstract) character consisting of more than one byte are not treated as an entity. Each byte is treated separately. The function is not locale-dependent.

Warning: Since strtok and wcstok alter the string they is parsing, you should always copy the string to a temporary buffer before parsing it with strtok/wcstok (see Copying and Concatenation). If you allow strtok or wcstok to modify a string that came from another part of your program, you are asking for trouble; that string might be used for other purposes after strtok or wcstok has modified it, and it would not have the expected value.

The string that you are operating on might even be a constant. Then when strtok or wcstok tries to modify it, your program will get a fatal signal for writing in read-only memory. See Program Error Signals. Even if the operation of strtok or wcstok would not require a modification of the string (e.g., if there is exactly one token) the string can (and in the GNU libc case will) be modified.

This is a special case of a general principle: if a part of a program does not have as its purpose the modification of a certain data structure, then it is error-prone to modify the data structure temporarily.

The functions strtok and wcstok are not reentrant. See Nonreentrancy, for a discussion of where and why reentrancy is important.

Here is a simple example showing the use of strtok. 

     #include <string.h>
     #include <stddef.h>
     
     ...
     
     const char string[] = "words separated by spaces -- and, punctuation!";
     const char delimiters[] = " .,;:!-";
     char *token, *cp;
     
     ...
     
     cp = strdupa (string);                /* Make writable copy.  */
     token = strtok (cp, delimiters);      /* token => "words" */
     token = strtok (NULL, delimiters);    /* token => "separated" */
     token = strtok (NULL, delimiters);    /* token => "by" */
     token = strtok (NULL, delimiters);    /* token => "spaces" */
     token = strtok (NULL, delimiters);    /* token => "and" */
     token = strtok (NULL, delimiters);    /* token => "punctuation" */
     token = strtok (NULL, delimiters);    /* token => NULL */

The GNU C library contains two more functions for tokenizing a string which overcome the limitation of non-reentrancy. They are only available for multibyte character strings.

— Function: char * strtok_r (char *newstring, const char *delimiters, char **save_ptr)

Just like strtok, this function splits the string into several tokens which can be accessed by successive calls to strtok_r. The difference is that the information about the next token is stored in the space pointed to by the third argument, save_ptr, which is a pointer to a string pointer. Calling strtok_r with a null pointer for newstring and leaving save_ptr between the calls unchanged does the job without hindering reentrancy.

This function is defined in POSIX.1 and can be found on many systems which support multi-threading.

— Function: char * strsep (char **string_ptr, const char *delimiter)

This function has a similar functionality as strtok_r with the newstring argument replaced by the save_ptr argument. The initialization of the moving pointer has to be done by the user. Successive calls to strsep move the pointer along the tokens separated by delimiter, returning the address of the next token and updating string_ptr to point to the beginning of the next token.

One difference between strsep and strtok_r is that if the input string contains more than one character from delimiter in a row strsep returns an empty string for each pair of characters from delimiter. This means that a program normally should test for strsep returning an empty string before processing it.

This function was introduced in 4.3BSD and therefore is widely available.

Here is how the above example looks like when strsep is used. 

     #include <string.h>
     #include <stddef.h>
     
     ...
     
     const char string[] = "words separated by spaces -- and, punctuation!";
     const char delimiters[] = " .,;:!-";
     char *running;
     char *token;
     
     ...
     
     running = strdupa (string);
     token = strsep (&running, delimiters);    /* token => "words" */
     token = strsep (&running, delimiters);    /* token => "separated" */
     token = strsep (&running, delimiters);    /* token => "by" */
     token = strsep (&running, delimiters);    /* token => "spaces" */
     token = strsep (&running, delimiters);    /* token => "" */
     token = strsep (&running, delimiters);    /* token => "" */
     token = strsep (&running, delimiters);    /* token => "" */
     token = strsep (&running, delimiters);    /* token => "and" */
     token = strsep (&running, delimiters);    /* token => "" */
     token = strsep (&running, delimiters);    /* token => "punctuation" */
     token = strsep (&running, delimiters);    /* token => "" */
     token = strsep (&running, delimiters);    /* token => NULL */
— Function: char * basename (const char *filename)

The GNU version of the basename function returns the last component of the path in filename. This function is the preferred usage, since it does not modify the argument, filename, and respects trailing slashes. The prototype for basename can be found in string.h. Note, this function is overriden by the XPG version, if libgen.h is included.

Example of using GNU basename: 

          #include <string.h>
          
          int
          main (int argc, char *argv[])
          {
            char *prog = basename (argv[0]);
          
            if (argc < 2)
              {
                fprintf (stderr, "Usage %s <arg>\n", prog);
                exit (1);
              }
          
            ...
          }

Portability Note: This function may produce different results on different systems.

— Function: char * basename (char *path)

This is the standard XPG defined basename. It is similar in spirit to the GNU version, but may modify the path by removing trailing '/' characters. If the path is made up entirely of '/' characters, then "/" will be returned. Also, if path is NULL or an empty string, then "." is returned. The prototype for the XPG version can be found in libgen.h.

Example of using XPG basename: 

          #include <libgen.h>
          
          int
          main (int argc, char *argv[])
          {
            char *prog;
            char *path = strdupa (argv[0]);
          
            prog = basename (path);
          
            if (argc < 2)
              {
                fprintf (stderr, "Usage %s <arg>\n", prog);
                exit (1);
              }
          
            ...
          
          }
— Function: char * dirname (char *path)

The dirname function is the compliment to the XPG version of basename. It returns the parent directory of the file specified by path. If path is NULL, an empty string, or contains no '/' characters, then "." is returned. The prototype for this function can be found in libgen.h.

Next: , Previous: Finding Tokens in a String, Up: String and Array Utilities

5.9 strfry

The function below addresses the perennial programming quandary: “How do I take good data in string form and painlessly turn it into garbage?” This is actually a fairly simple task for C programmers who do not use the GNU C library string functions, but for programs based on the GNU C library, the strfry function is the preferred method for destroying string data.

The prototype for this function is in string.h.

— Function: char * strfry (char *string)

strfry creates a pseudorandom anagram of a string, replacing the input with the anagram in place. For each position in the string, strfry swaps it with a position in the string selected at random (from a uniform distribution). The two positions may be the same.

The return value of strfry is always string.

Portability Note: This function is unique to the GNU C library.

Next: , Previous: strfry, Up: String and Array Utilities

5.10 Trivial Encryption

The memfrob function converts an array of data to something unrecognizable and back again. It is not encryption in its usual sense since it is easy for someone to convert the encrypted data back to clear text. The transformation is analogous to Usenet's “Rot13” encryption method for obscuring offensive jokes from sensitive eyes and such. Unlike Rot13, memfrob works on arbitrary binary data, not just text. For true encryption, See Cryptographic Functions.

This function is declared in string.h.

— Function: void * memfrob (void *mem, size_t length)

memfrob transforms (frobnicates) each byte of the data structure at mem, which is length bytes long, by bitwise exclusive oring it with binary 00101010. It does the transformation in place and its return value is always mem.

Note that memfrob a second time on the same data structure returns it to its original state.

This is a good function for hiding information from someone who doesn't want to see it or doesn't want to see it very much. To really prevent people from retrieving the information, use stronger encryption such as that described in See Cryptographic Functions.

Portability Note: This function is unique to the GNU C library.

Next: , Previous: Trivial Encryption, Up: String and Array Utilities

5.11 Encode Binary Data

To store or transfer binary data in environments which only support text one has to encode the binary data by mapping the input bytes to characters in the range allowed for storing or transfering. SVID systems (and nowadays XPG compliant systems) provide minimal support for this task.

— Function: char * l64a (long int n)

This function encodes a 32-bit input value using characters from the basic character set. It returns a pointer to a 7 character buffer which contains an encoded version of n. To encode a series of bytes the user must copy the returned string to a destination buffer. It returns the empty string if n is zero, which is somewhat bizarre but mandated by the standard.
Warning: Since a static buffer is used this function should not be used in multi-threaded programs. There is no thread-safe alternative to this function in the C library.
Compatibility Note: The XPG standard states that the return value of l64a is undefined if n is negative. In the GNU implementation, l64a treats its argument as unsigned, so it will return a sensible encoding for any nonzero n; however, portable programs should not rely on this.

To encode a large buffer l64a must be called in a loop, once for each 32-bit word of the buffer. For example, one could do something like this: 

          char *
          encode (const void *buf, size_t len)
          {
            /* We know in advance how long the buffer has to be. */
            unsigned char *in = (unsigned char *) buf;
            char *out = malloc (6 + ((len + 3) / 4) * 6 + 1);
            char *cp = out, *p;
          
            /* Encode the length. */
            /* Using `htonl' is necessary so that the data can be
               decoded even on machines with different byte order.
               `l64a' can return a string shorter than 6 bytes, so 
               we pad it with encoding of 0 ('.') at the end by 
               hand. */
          
            p = stpcpy (cp, l64a (htonl (len)));
            cp = mempcpy (p, "......", 6 - (p - cp));
          
            while (len > 3)
              {
                unsigned long int n = *in++;
                n = (n << 8) | *in++;
                n = (n << 8) | *in++;
                n = (n << 8) | *in++;
                len -= 4;
                p = stpcpy (cp, l64a (htonl (n)));
                cp = mempcpy (p, "......", 6 - (p - cp));
              }
            if (len > 0)
              {
                unsigned long int n = *in++;
                if (--len > 0)
                  {
                    n = (n << 8) | *in++;
                    if (--len > 0)
                      n = (n << 8) | *in;
                  }
                cp = stpcpy (cp, l64a (htonl (n)));
              }
            *cp = '\0';
            return out;
          }

It is strange that the library does not provide the complete functionality needed but so be it.

To decode data produced with l64a the following function should be used.

— Function: long int a64l (const char *string)

The parameter string should contain a string which was produced by a call to l64a. The function processes at least 6 characters of this string, and decodes the characters it finds according to the table below. It stops decoding when it finds a character not in the table, rather like atoi; if you have a buffer which has been broken into lines, you must be careful to skip over the end-of-line characters.

The decoded number is returned as a long int value.

The l64a and a64l functions use a base 64 encoding, in which each character of an encoded string represents six bits of an input word. These symbols are used for the base 64 digits:

0 1 2 3 4 5 6 7
0 . / 0 1 2 3 4 5
8 6 7 8 9 A B C D
16 E F G H I J K L
24 M N O P Q R S T
32 U V W X Y Z a b
40 c d e f g h i j
48 k l m n o p q r
56 s t u v w x y z

This encoding scheme is not standard. There are some other encoding methods which are much more widely used (UU encoding, MIME encoding). Generally, it is better to use one of these encodings.

Previous: Encode Binary Data, Up: String and Array Utilities

5.12 Argz and Envz Vectors

argz vectors are vectors of strings in a contiguous block of memory, each element separated from its neighbors by null-characters ('\0').

Envz vectors are an extension of argz vectors where each element is a name-value pair, separated by a '=' character (as in a Unix environment).

Next: , Up: Argz and Envz Vectors

5.12.1 Argz Functions

Each argz vector is represented by a pointer to the first element, of type char *, and a size, of type size_t, both of which can be initialized to 0 to represent an empty argz vector. All argz functions accept either a pointer and a size argument, or pointers to them, if they will be modified.

The argz functions use malloc/realloc to allocate/grow argz vectors, and so any argz vector creating using these functions may be freed by using free; conversely, any argz function that may grow a string expects that string to have been allocated using malloc (those argz functions that only examine their arguments or modify them in place will work on any sort of memory). See Unconstrained Allocation.

All argz functions that do memory allocation have a return type of error_t, and return 0 for success, and ENOMEM if an allocation error occurs.

These functions are declared in the standard include file argz.h.

— Function: error_t argz_create (char *const argv[], char **argz, size_t *argz_len)

The argz_create function converts the Unix-style argument vector argv (a vector of pointers to normal C strings, terminated by (char *)0; see Program Arguments) into an argz vector with the same elements, which is returned in argz and argz_len.

— Function: error_t argz_create_sep (const char *string, int sep, char **argz, size_t *argz_len)

The argz_create_sep function converts the null-terminated string string into an argz vector (returned in argz and argz_len) by splitting it into elements at every occurrence of the character sep.

— Function: size_t argz_count (const char *argz, size_t arg_len)

Returns the number of elements in the argz vector argz and argz_len.

— Function: void argz_extract (char *argz, size_t argz_len, char **argv)

The argz_extract function converts the argz vector argz and argz_len into a Unix-style argument vector stored in argv, by putting pointers to every element in argz into successive positions in argv, followed by a terminator of 0. Argv must be pre-allocated with enough space to hold all the elements in argz plus the terminating (char *)0 ((argz_count (argz, argz_len) + 1) * sizeof (char *) bytes should be enough). Note that the string pointers stored into argv point into argz—they are not copies—and so argz must be copied if it will be changed while argv is still active. This function is useful for passing the elements in argz to an exec function (see Executing a File).

— Function: void argz_stringify (char *argz, size_t len, int sep)

The argz_stringify converts argz into a normal string with the elements separated by the character sep, by replacing each '\0' inside argz (except the last one, which terminates the string) with sep. This is handy for printing argz in a readable manner.

— Function: error_t argz_add (char **argz, size_t *argz_len, const char *str)

The argz_add function adds the string str to the end of the argz vector *argz, and updates *argz and *argz_len accordingly.

— Function: error_t argz_add_sep (char **argz, size_t *argz_len, const char *str, int delim)

The argz_add_sep function is similar to argz_add, but str is split into separate elements in the result at occurrences of the character delim. This is useful, for instance, for adding the components of a Unix search path to an argz vector, by using a value of ':' for delim.

— Function: error_t argz_append (char **argz, size_t *argz_len, const char *buf, size_t buf_len)

The argz_append function appends buf_len bytes starting at buf to the argz vector *argz, reallocating *argz to accommodate it, and adding buf_len to *argz_len.

— Function: void argz_delete (char **argz, size_t *argz_len, char *entry)

If entry points to the beginning of one of the elements in the argz vector *argz, the argz_delete function will remove this entry and reallocate *argz, modifying *argz and *argz_len accordingly. Note that as destructive argz functions usually reallocate their argz argument, pointers into argz vectors such as entry will then become invalid.

— Function: error_t argz_insert (char **argz, size_t *argz_len, char *before, const char *entry)

The argz_insert function inserts the string entry into the argz vector *argz at a point just before the existing element pointed to by before, reallocating *argz and updating *argz and *argz_len. If before is 0, entry is added to the end instead (as if by argz_add). Since the first element is in fact the same as *argz, passing in *argz as the value of before will result in entry being inserted at the beginning.

— Function: char * argz_next (char *argz, size_t argz_len, const char *entry)

The argz_next function provides a convenient way of iterating over the elements in the argz vector argz. It returns a pointer to the next element in argz after the element entry, or 0 if there are no elements following entry. If entry is 0, the first element of argz is returned.

This behavior suggests two styles of iteration: 

              char *entry = 0;
              while ((entry = argz_next (argz, argz_len, entry)))
                action;

(the double parentheses are necessary to make some C compilers shut up about what they consider a questionable while-test) and: 

              char *entry;
              for (entry = argz;
                   entry;
                   entry = argz_next (argz, argz_len, entry))
                action;

Note that the latter depends on argz having a value of 0 if it is empty (rather than a pointer to an empty block of memory); this invariant is maintained for argz vectors created by the functions here.

— Function: error_t argz_replace (char **argz, size_t *argz_len, const char *str, const char *with, unsigned *replace_count)

Replace any occurrences of the string str in argz with with, reallocating argz as necessary. If replace_count is non-zero, *replace_count will be incremented by number of replacements performed.

Previous: Argz Functions, Up: Argz and Envz Vectors

5.12.2 Envz Functions

Envz vectors are just argz vectors with additional constraints on the form of each element; as such, argz functions can also be used on them, where it makes sense.

Each element in an envz vector is a name-value pair, separated by a '=' character; if multiple '=' characters are present in an element, those after the first are considered part of the value, and treated like all other non-'\0' characters.

If no '=' characters are present in an element, that element is considered the name of a “null” entry, as distinct from an entry with an empty value: envz_get will return 0 if given the name of null entry, whereas an entry with an empty value would result in a value of ""; envz_entry will still find such entries, however. Null entries can be removed with envz_strip function.

As with argz functions, envz functions that may allocate memory (and thus fail) have a return type of error_t, and return either 0 or ENOMEM.

These functions are declared in the standard include file envz.h.

— Function: char * envz_entry (const char *envz, size_t envz_len, const char *name)

The envz_entry function finds the entry in envz with the name name, and returns a pointer to the whole entry—that is, the argz element which begins with name followed by a '=' character. If there is no entry with that name, 0 is returned.

— Function: char * envz_get (const char *envz, size_t envz_len, const char *name)

The envz_get function finds the entry in envz with the name name (like envz_entry), and returns a pointer to the value portion of that entry (following the '='). If there is no entry with that name (or only a null entry), 0 is returned.

— Function: error_t envz_add (char **envz, size_t *envz_len, const char *name, const char *value)

The envz_add function adds an entry to *envz (updating *envz and *envz_len) with the name name, and value value. If an entry with the same name already exists in envz, it is removed first. If value is 0, then the new entry will the special null type of entry (mentioned above).

— Function: error_t envz_merge (char **envz, size_t *envz_len, const char *envz2, size_t envz2_len, int override)

The envz_merge function adds each entry in envz2 to envz, as if with envz_add, updating *envz and *envz_len. If override is true, then values in envz2 will supersede those with the same name in envz, otherwise not.

Null entries are treated just like other entries in this respect, so a null entry in envz can prevent an entry of the same name in envz2 from being added to envz, if override is false.

— Function: void envz_strip (char **envz, size_t *envz_len)

The envz_strip function removes any null entries from envz, updating *envz and *envz_len.

Next: , Previous: String and Array Utilities, Up: Top

6 Character Set Handling

Character sets used in the early days of computing had only six, seven, or eight bits for each character: there was never a case where more than eight bits (one byte) were used to represent a single character. The limitations of this approach became more apparent as more people grappled with non-Roman character sets, where not all the characters that make up a language's character set can be represented by 2^8 choices. This chapter shows the functionality that was added to the C library to support multiple character sets.

Next: , Up: Character Set Handling

6.1 Introduction to Extended Characters

A variety of solutions is available to overcome the differences between character sets with a 1:1 relation between bytes and characters and character sets with ratios of 2:1 or 4:1. The remainder of this section gives a few examples to help understand the design decisions made while developing the functionality of the C library.

A distinction we have to make right away is between internal and external representation. Internal representation means the representation used by a program while keeping the text in memory. External representations are used when text is stored or transmitted through some communication channel. Examples of external representations include files waiting in a directory to be read and parsed.

Traditionally there has been no difference between the two representations. It was equally comfortable and useful to use the same single-byte representation internally and externally. This comfort level decreases with more and larger character sets.

One of the problems to overcome with the internal representation is handling text that is externally encoded using different character sets. Assume a program that reads two texts and compares them using some metric. The comparison can be usefully done only if the texts are internally kept in a common format.

For such a common format (= character set) eight bits are certainly no longer enough. So the smallest entity will have to grow: wide characters will now be used. Instead of one byte per character, two or four will be used instead. (Three are not good to address in memory and more than four bytes seem not to be necessary).

As shown in some other part of this manual, a completely new family has been created of functions that can handle wide character texts in memory. The most commonly used character sets for such internal wide character representations are Unicode and ISO 10646 (also known as UCS for Universal Character Set). Unicode was originally planned as a 16-bit character set; whereas, ISO 10646 was designed to be a 31-bit large code space. The two standards are practically identical. They have the same character repertoire and code table, but Unicode specifies added semantics. At the moment, only characters in the first 0x10000 code positions (the so-called Basic Multilingual Plane, BMP) have been assigned, but the assignment of more specialized characters outside this 16-bit space is already in progress. A number of encodings have been defined for Unicode and ISO 10646 characters: UCS-2 is a 16-bit word that can only represent characters from the BMP, UCS-4 is a 32-bit word than can represent any Unicode and ISO 10646 character, UTF-8 is an ASCII compatible encoding where ASCII characters are represented by ASCII bytes and non-ASCII characters by sequences of 2-6 non-ASCII bytes, and finally UTF-16 is an extension of UCS-2 in which pairs of certain UCS-2 words can be used to encode non-BMP characters up to 0x10ffff.

To represent wide characters the char type is not suitable. For this reason the ISO C standard introduces a new type that is designed to keep one character of a wide character string. To maintain the similarity there is also a type corresponding to int for those functions that take a single wide character.

— Data type: wchar_t

This data type is used as the base type for wide character strings. In other words, arrays of objects of this type are the equivalent of char[] for multibyte character strings. The type is defined in stddef.h.

The ISO C90 standard, where wchar_t was introduced, does not say anything specific about the representation. It only requires that this type is capable of storing all elements of the basic character set. Therefore it would be legitimate to define wchar_t as char, which might make sense for embedded systems.

But for GNU systems wchar_t is always 32 bits wide and, therefore, capable of representing all UCS-4 values and, therefore, covering all of ISO 10646. Some Unix systems define wchar_t as a 16-bit type and thereby follow Unicode very strictly. This definition is perfectly fine with the standard, but it also means that to represent all characters from Unicode and ISO 10646 one has to use UTF-16 surrogate characters, which is in fact a multi-wide-character encoding. But resorting to multi-wide-character encoding contradicts the purpose of the wchar_t type.

— Data type: wint_t

wint_t is a data type used for parameters and variables that contain a single wide character. As the name suggests this type is the equivalent of int when using the normal char strings. The types wchar_t and wint_t often have the same representation if their size is 32 bits wide but if wchar_t is defined as char the type wint_t must be defined as int due to the parameter promotion.

This type is defined in wchar.h and was introduced in Amendment 1 to ISO C90.

As there are for the char data type macros are available for specifying the minimum and maximum value representable in an object of type wchar_t.

— Macro: wint_t WCHAR_MIN

The macro WCHAR_MIN evaluates to the minimum value representable by an object of type wint_t.

This macro was introduced in Amendment 1 to ISO C90.

— Macro: wint_t WCHAR_MAX

The macro WCHAR_MAX evaluates to the maximum value representable by an object of type wint_t.

This macro was introduced in Amendment 1 to ISO C90.

Another special wide character value is the equivalent to EOF.

— Macro: wint_t WEOF

The macro WEOF evaluates to a constant expression of type wint_t whose value is different from any member of the extended character set.

WEOF need not be the same value as EOF and unlike EOF it also need not be negative. In other words, sloppy code like 

          {
            int c;
            ...
            while ((c = getc (fp)) < 0)
              ...
          }

has to be rewritten to use WEOF explicitly when wide characters are used: 

          {
            wint_t c;
            ...
            while ((c = wgetc (fp)) != WEOF)
              ...
          }

This macro was introduced in Amendment 1 to ISO C90 and is defined in wchar.h.

These internal representations present problems when it comes to storing and transmittal. Because each single wide character consists of more than one byte, they are effected by byte-ordering. Thus, machines with different endianesses would see different values when accessing the same data. This byte ordering concern also applies for communication protocols that are all byte-based and therefore require that the sender has to decide about splitting the wide character in bytes. A last (but not least important) point is that wide characters often require more storage space than a customized byte-oriented character set.

For all the above reasons, an external encoding that is different from the internal encoding is often used if the latter is UCS-2 or UCS-4. The external encoding is byte-based and can be chosen appropriately for the environment and for the texts to be handled. A variety of different character sets can be used for this external encoding (information that will not be exhaustively presented here–instead, a description of the major groups will suffice). All of the ASCII-based character sets fulfill one requirement: they are "filesystem safe." This means that the character '/' is used in the encoding only to represent itself. Things are a bit different for character sets like EBCDIC (Extended Binary Coded Decimal Interchange Code, a character set family used by IBM), but if the operation system does not understand EBCDIC directly the parameters-to-system calls have to be converted first anyhow.

The question remaining is: how to select the character set or encoding to use. The answer: you cannot decide about it yourself, it is decided by the developers of the system or the majority of the users. Since the goal is interoperability one has to use whatever the other people one works with use. If there are no constraints, the selection is based on the requirements the expected circle of users will have. In other words, if a project is expected to be used in only, say, Russia it is fine to use KOI8-R or a similar character set. But if at the same time people from, say, Greece are participating one should use a character set that allows all people to collaborate.

The most widely useful solution seems to be: go with the most general character set, namely ISO 10646. Use UTF-8 as the external encoding and problems about users not being able to use their own language adequately are a thing of the past.

One final comment about the choice of the wide character representation is necessary at this point. We have said above that the natural choice is using Unicode or ISO 10646. This is not required, but at least encouraged, by the ISO C standard. The standard defines at least a macro __STDC_ISO_10646__ that is only defined on systems where the wchar_t type encodes ISO 10646 characters. If this symbol is not defined one should avoid making assumptions about the wide character representation. If the programmer uses only the functions provided by the C library to handle wide character strings there should be no compatibility problems with other systems.

Next: , Previous: Extended Char Intro, Up: Character Set Handling

6.2 Overview about Character Handling Functions

A Unix C library contains three different sets of functions in two families to handle character set conversion. One of the function families (the most commonly used) is specified in the ISO C90 standard and, therefore, is portable even beyond the Unix world. Unfortunately this family is the least useful one. These functions should be avoided whenever possible, especially when developing libraries (as opposed to applications).

The second family of functions got introduced in the early Unix standards (XPG2) and is still part of the latest and greatest Unix standard: Unix 98. It is also the most powerful and useful set of functions. But we will start with the functions defined in Amendment 1 to ISO C90.

Next: , Previous: Charset Function Overview, Up: Character Set Handling

6.3 Restartable Multibyte Conversion Functions

The ISO C standard defines functions to convert strings from a multibyte representation to wide character strings. There are a number of peculiarities:

Despite these limitations the ISO C functions can be used in many contexts. In graphical user interfaces, for instance, it is not uncommon to have functions that require text to be displayed in a wide character string if the text is not simple ASCII. The text itself might come from a file with translations and the user should decide about the current locale, which determines the translation and therefore also the external encoding used. In such a situation (and many others) the functions described here are perfect. If more freedom while performing the conversion is necessary take a look at the iconv functions (see Generic Charset Conversion).

Next: , Up: Restartable multibyte conversion

6.3.1 Selecting the conversion and its properties

We already said above that the currently selected locale for the LC_CTYPE category decides about the conversion that is performed by the functions we are about to describe. Each locale uses its own character set (given as an argument to localedef) and this is the one assumed as the external multibyte encoding. The wide character character set always is UCS-4, at least on GNU systems.

A characteristic of each multibyte character set is the maximum number of bytes that can be necessary to represent one character. This information is quite important when writing code that uses the conversion functions (as shown in the examples below). The ISO C standard defines two macros that provide this information.

— Macro: int MB_LEN_MAX

MB_LEN_MAX specifies the maximum number of bytes in the multibyte sequence for a single character in any of the supported locales. It is a compile-time constant and is defined in limits.h.

— Macro: int MB_CUR_MAX

MB_CUR_MAX expands into a positive integer expression that is the maximum number of bytes in a multibyte character in the current locale. The value is never greater than MB_LEN_MAX. Unlike MB_LEN_MAX this macro need not be a compile-time constant, and in the GNU C library it is not.

MB_CUR_MAX is defined in stdlib.h.

Two different macros are necessary since strictly ISO C90 compilers do not allow variable length array definitions, but still it is desirable to avoid dynamic allocation. This incomplete piece of code shows the problem: 

     {
       char buf[MB_LEN_MAX];
       ssize_t len = 0;
     
       while (! feof (fp))
         {
           fread (&buf[len], 1, MB_CUR_MAX - len, fp);
           /* ... process buf */
           len -= used;
         }
     }

The code in the inner loop is expected to have always enough bytes in the array buf to convert one multibyte character. The array buf has to be sized statically since many compilers do not allow a variable size. The fread call makes sure that MB_CUR_MAX bytes are always available in buf. Note that it isn't a problem if MB_CUR_MAX is not a compile-time constant.

Next: , Previous: Selecting the Conversion, Up: Restartable multibyte conversion

6.3.2 Representing the state of the conversion

In the introduction of this chapter it was said that certain character sets use a stateful encoding. That is, the encoded values depend in some way on the previous bytes in the text.

Since the conversion functions allow converting a text in more than one step we must have a way to pass this information from one call of the functions to another.

— Data type: mbstate_t

A variable of type mbstate_t can contain all the information about the shift state needed from one call to a conversion function to another.

mbstate_t is defined in wchar.h. It was introduced in Amendment 1 to ISO C90.

To use objects of type mbstate_t the programmer has to define such objects (normally as local variables on the stack) and pass a pointer to the object to the conversion functions. This way the conversion function can update the object if the current multibyte character set is stateful.

There is no specific function or initializer to put the state object in any specific state. The rules are that the object should always represent the initial state before the first use, and this is achieved by clearing the whole variable with code such as follows: 

     {
       mbstate_t state;
       memset (&state, '\0', sizeof (state));
       /* from now on state can be used.  */
       ...
     }

When using the conversion functions to generate output it is often necessary to test whether the current state corresponds to the initial state. This is necessary, for example, to decide whether to emit escape sequences to set the state to the initial state at certain sequence points. Communication protocols often require this.

— Function: int mbsinit (const mbstate_t *ps)

The mbsinit function determines whether the state object pointed to by ps is in the initial state. If ps is a null pointer or the object is in the initial state the return value is nonzero. Otherwise it is zero.

mbsinit was introduced in Amendment 1 to ISO C90 and is declared in wchar.h.

Code using mbsinit often looks similar to this: 

     {
       mbstate_t state;
       memset (&state, '\0', sizeof (state));
       /* Use state.  */
       ...
       if (! mbsinit (&state))
         {
           /* Emit code to return to initial state.  */
           const wchar_t empty[] = L"";
           const wchar_t *srcp = empty;
           wcsrtombs (outbuf, &srcp, outbuflen, &state);
         }
       ...
     }

The code to emit the escape sequence to get back to the initial state is interesting. The wcsrtombs function can be used to determine the necessary output code (see Converting Strings). Please note that on GNU systems it is not necessary to perform this extra action for the conversion from multibyte text to wide character text since the wide character encoding is not stateful. But there is nothing mentioned in any standard that prohibits making wchar_t using a stateful encoding.

Next: , Previous: Keeping the state, Up: Restartable multibyte conversion

6.3.3 Converting Single Characters

The most fundamental of the conversion functions are those dealing with single characters. Please note that this does not always mean single bytes. But since there is very often a subset of the multibyte character set that consists of single byte sequences, there are functions to help with converting bytes. Frequently, ASCII is a subpart of the multibyte character set. In such a scenario, each ASCII character stands for itself, and all other characters have at least a first byte that is beyond the range 0 to 127.

— Function: wint_t btowc (int c)

The btowc function (“byte to wide character”) converts a valid single byte character c in the initial shift state into the wide character equivalent using the conversion rules from the currently selected locale of the LC_CTYPE category.

If (unsigned char) c is no valid single byte multibyte character or if c is EOF, the function returns WEOF.

Please note the restriction of c being tested for validity only in the initial shift state. No mbstate_t object is used from which the state information is taken, and the function also does not use any static state.

The btowc function was introduced in Amendment 1 to ISO C90 and is declared in wchar.h.

Despite the limitation that the single byte value always is interpreted in the initial state this function is actually useful most of the time. Most characters are either entirely single-byte character sets or they are extension to ASCII. But then it is possible to write code like this (not that this specific example is very useful): 

     wchar_t *
     itow (unsigned long int val)
     {
       static wchar_t buf[30];
       wchar_t *wcp = &buf[29];
       *wcp = L'\0';
       while (val != 0)
         {
           *--wcp = btowc ('0' + val % 10);
           val /= 10;
         }
       if (wcp == &buf[29])
         *--wcp = L'0';
       return wcp;
     }

Why is it necessary to use such a complicated implementation and not simply cast '0' + val % 10 to a wide character? The answer is that there is no guarantee that one can perform this kind of arithmetic on the character of the character set used for wchar_t representation. In other situations the bytes are not constant at compile time and so the compiler cannot do the work. In situations like this it is necessary btowc.

There also is a function for the conversion in the other direction.

— Function: int wctob (wint_t c)

The wctob function (“wide character to byte”) takes as the parameter a valid wide character. If the multibyte representation for this character in the initial state is exactly one byte long, the return value of this function is this character. Otherwise the return value is EOF.

wctob was introduced in Amendment 1 to ISO C90 and is declared in wchar.h.

There are more general functions to convert single character from multibyte representation to wide characters and vice versa. These functions pose no limit on the length of the multibyte representation and they also do not require it to be in the initial state.

— Function: size_t mbrtowc (wchar_t *restrict pwc, const char *restrict s, size_t n, mbstate_t *restrict ps)

The mbrtowc function (“multibyte restartable to wide character”) converts the next multibyte character in the string pointed to by s into a wide character and stores it in the wide character string pointed to by pwc. The conversion is performed according to the locale currently selected for the LC_CTYPE category. If the conversion for the character set used in the locale requires a state, the multibyte string is interpreted in the state represented by the object pointed to by ps. If ps is a null pointer, a static, internal state variable used only by the mbrtowc function is used.

If the next multibyte character corresponds to the NUL wide character, the return value of the function is 0 and the state object is afterwards in the initial state. If the next n or fewer bytes form a correct multibyte character, the return value is the number of bytes starting from s that form the multibyte character. The conversion state is updated according to the bytes consumed in the conversion. In both cases the wide character (either the L'\0' or the one found in the conversion) is stored in the string pointed to by pwc if pwc is not null.

If the first n bytes of the multibyte string possibly form a valid multibyte character but there are more than n bytes needed to complete it, the return value of the function is (size_t) -2 and no value is stored. Please note that this can happen even if n has a value greater than or equal to MB_CUR_MAX since the input might contain redundant shift sequences.

If the first n bytes of the multibyte string cannot possibly form a valid multibyte character, no value is stored, the global variable errno is set to the value EILSEQ, and the function returns (size_t) -1. The conversion state is afterwards undefined.

mbrtowc was introduced in Amendment 1 to ISO C90 and is declared in wchar.h.

Use of mbrtowc is straightforward. A function that copies a multibyte string into a wide character string while at the same time converting all lowercase characters into uppercase could look like this (this is not the final version, just an example; it has no error checking, and sometimes leaks memory): 

     wchar_t *
     mbstouwcs (const char *s)
     {
       size_t len = strlen (s);
       wchar_t *result = malloc ((len + 1) * sizeof (wchar_t));
       wchar_t *wcp = result;
       wchar_t tmp[1];
       mbstate_t state;
       size_t nbytes;
     
       memset (&state, '\0', sizeof (state));
       while ((nbytes = mbrtowc (tmp, s, len, &state)) > 0)
         {
           if (nbytes >= (size_t) -2)
             /* Invalid input string.  */
             return NULL;
           *wcp++ = towupper (tmp[0]);
           len -= nbytes;
           s += nbytes;
         }
       return result;
     }

The use of mbrtowc should be clear. A single wide character is stored in tmp[0], and the number of consumed bytes is stored in the variable nbytes. If the conversion is successful, the uppercase variant of the wide character is stored in the result array and the pointer to the input string and the number of available bytes is adjusted.

The only non-obvious thing about mbrtowc might be the way memory is allocated for the result. The above code uses the fact that there can never be more wide characters in the converted results than there are bytes in the multibyte input string. This method yields a pessimistic guess about the size of the result, and if many wide character strings have to be constructed this way or if the strings are long, the extra memory required to be allocated because the input string contains multibyte characters might be significant. The allocated memory block can be resized to the correct size before returning it, but a better solution might be to allocate just the right amount of space for the result right away. Unfortunately there is no function to compute the length of the wide character string directly from the multibyte string. There is, however, a function that does part of the work.

— Function: size_t mbrlen (const char *restrict s, size_t n, mbstate_t *ps)

The mbrlen function (“multibyte restartable length”) computes the number of at most n bytes starting at s, which form the next valid and complete multibyte character.

If the next multibyte character corresponds to the NUL wide character, the return value is 0. If the next n bytes form a valid multibyte character, the number of bytes belonging to this multibyte character byte sequence is returned.

If the first n bytes possibly form a valid multibyte character but the character is incomplete, the return value is (size_t) -2. Otherwise the multibyte character sequence is invalid and the return value is (size_t) -1.

The multibyte sequence is interpreted in the state represented by the object pointed to by ps. If ps is a null pointer, a state object local to mbrlen is used.

mbrlen was introduced in Amendment 1 to ISO C90 and is declared in wchar.h.

The attentive reader now will note that mbrlen can be implemented as 

     mbrtowc (NULL, s, n, ps != NULL ? ps : &internal)

This is true and in fact is mentioned in the official specification. How can this function be used to determine the length of the wide character string created from a multibyte character string? It is not directly usable, but we can define a function mbslen using it: 

     size_t
     mbslen (const char *s)
     {
       mbstate_t state;
       size_t result = 0;
       size_t nbytes;
       memset (&state, '\0', sizeof (state));
       while ((nbytes = mbrlen (s, MB_LEN_MAX, &state)) > 0)
         {
           if (nbytes >= (size_t) -2)
             /* Something is wrong.  */
             return (size_t) -1;
           s += nbytes;
           ++result;
         }
       return result;
     }

This function simply calls mbrlen for each multibyte character in the string and counts the number of function calls. Please note that we here use MB_LEN_MAX as the size argument in the mbrlen call. This is acceptable since a) this value is larger then the length of the longest multibyte character sequence and b) we know that the string s ends with a NUL byte, which cannot be part of any other multibyte character sequence but the one representing the NUL wide character. Therefore, the mbrlen function will never read invalid memory.

Now that this function is available (just to make this clear, this function is not part of the GNU C library) we can compute the number of wide character required to store the converted multibyte character string s using 

     wcs_bytes = (mbslen (s) + 1) * sizeof (wchar_t);

Please note that the mbslen function is quite inefficient. The implementation of mbstouwcs with mbslen would have to perform the conversion of the multibyte character input string twice, and this conversion might be quite expensive. So it is necessary to think about the consequences of using the easier but imprecise method before doing the work twice.

— Function: size_t wcrtomb (char *restrict s, wchar_t wc, mbstate_t *restrict ps)

The wcrtomb function (“wide character restartable to multibyte”) converts a single wide character into a multibyte string corresponding to that wide character.

If s is a null pointer, the function resets the state stored in the objects pointed to by ps (or the internal mbstate_t object) to the initial state. This can also be achieved by a call like this: 

          wcrtombs (temp_buf, L'\0', ps)

since, if s is a null pointer, wcrtomb performs as if it writes into an internal buffer, which is guaranteed to be large enough.

If wc is the NUL wide character, wcrtomb emits, if necessary, a shift sequence to get the state ps into the initial state followed by a single NUL byte, which is stored in the string s.

Otherwise a byte sequence (possibly including shift sequences) is written into the string s. This only happens if wc is a valid wide character (i.e., it has a multibyte representation in the character set selected by locale of the LC_CTYPE category). If wc is no valid wide character, nothing is stored in the strings s, errno is set to EILSEQ, the conversion state in ps is undefined and the return value is (size_t) -1.

If no error occurred the function returns the number of bytes stored in the string s. This includes all bytes representing shift sequences.

One word about the interface of the function: there is no parameter specifying the length of the array s. Instead the function assumes that there are at least MB_CUR_MAX bytes available since this is the maximum length of any byte sequence representing a single character. So the caller has to make sure that there is enough space available, otherwise buffer overruns can occur.

wcrtomb was introduced in Amendment 1 to ISO C90 and is declared in wchar.h.

Using wcrtomb is as easy as using mbrtowc. The following example appends a wide character string to a multibyte character string. Again, the code is not really useful (or correct), it is simply here to demonstrate the use and some problems. 

     char *
     mbscatwcs (char *s, size_t len, const wchar_t *ws)
     {
       mbstate_t state;
       /* Find the end of the existing string.  */
       char *wp = strchr (s, '\0');
       len -= wp - s;
       memset (&state, '\0', sizeof (state));
       do
         {
           size_t nbytes;
           if (len < MB_CUR_LEN)
             {
               /* We cannot guarantee that the next
                  character fits into the buffer, so
                  return an error.  */
               errno = E2BIG;
               return NULL;
             }
           nbytes = wcrtomb (wp, *ws, &state);
           if (nbytes == (size_t) -1)
             /* Error in the conversion.  */
             return NULL;
           len -= nbytes;
           wp += nbytes;
         }
       while (*ws++ != L'\0');
       return s;
     }

First the function has to find the end of the string currently in the array s. The strchr call does this very efficiently since a requirement for multibyte character representations is that the NUL byte is never used except to represent itself (and in this context, the end of the string).

After initializing the state object the loop is entered where the first task is to make sure there is enough room in the array s. We abort if there are not at least MB_CUR_LEN bytes available. This is not always optimal but we have no other choice. We might have less than MB_CUR_LEN bytes available but the next multibyte character might also be only one byte long. At the time the wcrtomb call returns it is too late to decide whether the buffer was large enough. If this solution is unsuitable, there is a very slow but more accurate solution. 

       ...
       if (len < MB_CUR_LEN)
         {
           mbstate_t temp_state;
           memcpy (&temp_state, &state, sizeof (state));
           if (wcrtomb (NULL, *ws, &temp_state) > len)
             {
               /* We cannot guarantee that the next
                  character fits into the buffer, so
                  return an error.  */
               errno = E2BIG;
               return NULL;
             }
         }
       ...

Here we perform the conversion that might overflow the buffer so that we are afterwards in the position to make an exact decision about the buffer size. Please note the NULL argument for the destination buffer in the new wcrtomb call; since we are not interested in the converted text at this point, this is a nice way to express this. The most unusual thing about this piece of code certainly is the duplication of the conversion state object, but if a change of the state is necessary to emit the next multibyte character, we want to have the same shift state change performed in the real conversion. Therefore, we have to preserve the initial shift state information.

There are certainly many more and even better solutions to this problem. This example is only provided for educational purposes.

Next: , Previous: Converting a Character, Up: Restartable multibyte conversion

6.3.4 Converting Multibyte and Wide Character Strings

The functions described in the previous section only convert a single character at a time. Most operations to be performed in real-world programs include strings and therefore the ISO C standard also defines conversions on entire strings. However, the defined set of functions is quite limited; therefore, the GNU C library contains a few extensions that can help in some important situations.

— Function: size_t mbsrtowcs (wchar_t *restrict dst, const char **restrict src, size_t len, mbstate_t *restrict ps)

The mbsrtowcs function (“multibyte string restartable to wide character string”) converts an NUL-terminated multibyte character string at *src into an equivalent wide character string, including the NUL wide character at the end. The conversion is started using the state information from the object pointed to by ps or from an internal object of mbsrtowcs if ps is a null pointer. Before returning, the state object is updated to match the state after the last converted character. The state is the initial state if the terminating NUL byte is reached and converted.

If dst is not a null pointer, the result is stored in the array pointed to by dst; otherwise, the conversion result is not available since it is stored in an internal buffer.

If len wide characters are stored in the array dst before reaching the end of the input string, the conversion stops and len is returned. If dst is a null pointer, len is never checked.

Another reason for a premature return from the function call is if the input string contains an invalid multibyte sequence. In this case the global variable errno is set to EILSEQ and the function returns (size_t) -1.

In all other cases the function returns the number of wide characters converted during this call. If dst is not null, mbsrtowcs stores in the pointer pointed to by src either a null pointer (if the NUL byte in the input string was reached) or the address of the byte following the last converted multibyte character.

mbsrtowcs was introduced in Amendment 1 to ISO C90 and is declared in wchar.h.

The definition of the mbsrtowcs function has one important limitation. The requirement that dst has to be a NUL-terminated string provides problems if one wants to convert buffers with text. A buffer is normally no collection of NUL-terminated strings but instead a continuous collection of lines, separated by newline characters. Now assume that a function to convert one line from a buffer is needed. Since the line is not NUL-terminated, the source pointer cannot directly point into the unmodified text buffer. This means, either one inserts the NUL byte at the appropriate place for the time of the mbsrtowcs function call (which is not doable for a read-only buffer or in a multi-threaded application) or one copies the line in an extra buffer where it can be terminated by a NUL byte. Note that it is not in general possible to limit the number of characters to convert by setting the parameter len to any specific value. Since it is not known how many bytes each multibyte character sequence is in length, one can only guess.

There is still a problem with the method of NUL-terminating a line right after the newline character, which could lead to very strange results. As said in the description of the mbsrtowcs function above the conversion state is guaranteed to be in the initial shift state after processing the NUL byte at the end of the input string. But this NUL byte is not really part of the text (i.e., the conversion state after the newline in the original text could be something different than the initial shift state and therefore the first character of the next line is encoded using this state). But the state in question is never accessible to the user since the conversion stops after the NUL byte (which resets the state). Most stateful character sets in use today require that the shift state after a newline be the initial state–but this is not a strict guarantee. Therefore, simply NUL-terminating a piece of a running text is not always an adequate solution and, therefore, should never be used in generally used code.

The generic conversion interface (see Generic Charset Conversion) does not have this limitation (it simply works on buffers, not strings), and the GNU C library contains a set of functions that take additional parameters specifying the maximal number of bytes that are consumed from the input string. This way the problem of mbsrtowcs's example above could be solved by determining the line length and passing this length to the function.

— Function: size_t wcsrtombs (char *restrict dst, const wchar_t **restrict src, size_t len, mbstate_t *restrict ps)

The wcsrtombs function (“wide character string restartable to multibyte string”) converts the NUL-terminated wide character string at *src into an equivalent multibyte character string and stores the result in the array pointed to by dst. The NUL wide character is also converted. The conversion starts in the state described in the object pointed to by ps or by a state object locally to wcsrtombs in case ps is a null pointer. If dst is a null pointer, the conversion is performed as usual but the result is not available. If all characters of the input string were successfully converted and if dst is not a null pointer, the pointer pointed to by src gets assigned a null pointer.

If one of the wide characters in the input string has no valid multibyte character equivalent, the conversion stops early, sets the global variable errno to EILSEQ, and returns (size_t) -1.

Another reason for a premature stop is if dst is not a null pointer and the next converted character would require more than len bytes in total to the array dst. In this case (and if dest is not a null pointer) the pointer pointed to by src is assigned a value pointing to the wide character right after the last one successfully converted.

Except in the case of an encoding error the return value of the wcsrtombs function is the number of bytes in all the multibyte character sequences stored in dst. Before returning the state in the object pointed to by ps (or the internal object in case ps is a null pointer) is updated to reflect the state after the last conversion. The state is the initial shift state in case the terminating NUL wide character was converted.

The wcsrtombs function was introduced in Amendment 1 to ISO C90 and is declared in wchar.h.

The restriction mentioned above for the mbsrtowcs function applies here also. There is no possibility of directly controlling the number of input characters. One has to place the NUL wide character at the correct place or control the consumed input indirectly via the available output array size (the len parameter).

— Function: size_t mbsnrtowcs (wchar_t *restrict dst, const char **restrict src, size_t nmc, size_t len, mbstate_t *restrict ps)

The mbsnrtowcs function is very similar to the mbsrtowcs function. All the parameters are the same except for nmc, which is new. The return value is the same as for mbsrtowcs.

This new parameter specifies how many bytes at most can be used from the multibyte character string. In other words, the multibyte character string *src need not be NUL-terminated. But if a NUL byte is found within the nmc first bytes of the string, the conversion stops here.

This function is a GNU extension. It is meant to work around the problems mentioned above. Now it is possible to convert a buffer with multibyte character text piece for piece without having to care about inserting NUL bytes and the effect of NUL bytes on the conversion state.

A function to convert a multibyte string into a wide character string and display it could be written like this (this is not a really useful example): 

     void
     showmbs (const char *src, FILE *fp)
     {
       mbstate_t state;
       int cnt = 0;
       memset (&state, '\0', sizeof (state));
       while (1)
         {
           wchar_t linebuf[100];
           const char *endp = strchr (src, '\n');
           size_t n;
     
           /* Exit if there is no more line.  */
           if (endp == NULL)
             break;
     
           n = mbsnrtowcs (linebuf, &src, endp - src, 99, &state);
           linebuf[n] = L'\0';
           fprintf (fp, "line %d: \"%S\"\n", linebuf);
         }
     }

There is no problem with the state after a call to mbsnrtowcs. Since we don't insert characters in the strings that were not in there right from the beginning and we use state only for the conversion of the given buffer, there is no problem with altering the state.

— Function: size_t wcsnrtombs (char *restrict dst, const wchar_t **restrict src, size_t nwc, size_t len, mbstate_t *restrict ps)

The wcsnrtombs function implements the conversion from wide character strings to multibyte character strings. It is similar to wcsrtombs but, just like mbsnrtowcs, it takes an extra parameter, which specifies the length of the input string.

No more than nwc wide characters from the input string *src are converted. If the input string contains a NUL wide character in the first nwc characters, the conversion stops at this place.

The wcsnrtombs function is a GNU extension and just like mbsnrtowcs helps in situations where no NUL-terminated input strings are available.

Previous: Converting Strings, Up: Restartable multibyte conversion

6.3.5 A Complete Multibyte Conversion Example

The example programs given in the last sections are only brief and do not contain all the error checking, etc. Presented here is a complete and documented example. It features the mbrtowc function but it should be easy to derive versions using the other functions. 

     int
     file_mbsrtowcs (int input, int output)
     {
       /* Note the use of MB_LEN_MAX.
          MB_CUR_MAX cannot portably be used here.  */
       char buffer[BUFSIZ + MB_LEN_MAX];
       mbstate_t state;
       int filled = 0;
       int eof = 0;
     
       /* Initialize the state.  */
       memset (&state, '\0', sizeof (state));
     
       while (!eof)
         {
           ssize_t nread;
           ssize_t nwrite;
           char *inp = buffer;
           wchar_t outbuf[BUFSIZ];
           wchar_t *outp = outbuf;
     
           /* Fill up the buffer from the input file.  */
           nread = read (input, buffer + filled, BUFSIZ);
           if (nread < 0)
             {
               perror ("read");
               return 0;
             }
           /* If we reach end of file, make a note to read no more. */
           if (nread == 0)
             eof = 1;
     
           /* filled is now the number of bytes in buffer. */
           filled += nread;
     
           /* Convert those bytes to wide characters--as many as we can. */
           while (1)
             {
               size_t thislen = mbrtowc (outp, inp, filled, &state);
               /* Stop converting at invalid character;
                  this can mean we have read just the first part
                  of a valid character.  */
               if (thislen == (size_t) -1)
                 break;
               /* We want to handle embedded NUL bytes
                  but the return value is 0.  Correct this.  */
               if (thislen == 0)
                 thislen = 1;
               /* Advance past this character. */
               inp += thislen;
               filled -= thislen;
               ++outp;
             }
     
           /* Write the wide characters we just made.  */
           nwrite = write (output, outbuf,
                           (outp - outbuf) * sizeof (wchar_t));
           if (nwrite < 0)
             {
               perror ("write");
               return 0;
             }
     
           /* See if we have a real invalid character. */
           if ((eof && filled > 0) || filled >= MB_CUR_MAX)
             {
               error (0, 0, "invalid multibyte character");
               return 0;
             }
     
           /* If any characters must be carried forward,
              put them at the beginning of buffer. */
           if (filled > 0)
             memmove (inp, buffer, filled);
         }
     
       return 1;
     }

Next: , Previous: Restartable multibyte conversion, Up: Character Set Handling

6.4 Non-reentrant Conversion Function

The functions described in the previous chapter are defined in Amendment 1 to ISO C90, but the original ISO C90 standard also contained functions for character set conversion. The reason that these original functions are not described first is that they are almost entirely useless.

The problem is that all the conversion functions described in the original ISO C90 use a local state. Using a local state implies that multiple conversions at the same time (not only when using threads) cannot be done, and that you cannot first convert single characters and then strings since you cannot tell the conversion functions which state to use.

These original functions are therefore usable only in a very limited set of situations. One must complete converting the entire string before starting a new one, and each string/text must be converted with the same function (there is no problem with the library itself; it is guaranteed that no library function changes the state of any of these functions). For the above reasons it is highly requested that the functions described in the previous section be used in place of non-reentrant conversion functions.

Next: , Up: Non-reentrant Conversion

6.4.1 Non-reentrant Conversion of Single Characters

— Function: int mbtowc (wchar_t *restrict result, const char *restrict string, size_t size)

The mbtowc (“multibyte to wide character”) function when called with non-null string converts the first multibyte character beginning at string to its corresponding wide character code. It stores the result in *result.

mbtowc never examines more than size bytes. (The idea is to supply for size the number of bytes of data you have in hand.)

mbtowc with non-null string distinguishes three possibilities: the first size bytes at string start with valid multibyte characters, they start with an invalid byte sequence or just part of a character, or string points to an empty string (a null character).

For a valid multibyte character, mbtowc converts it to a wide character and stores that in *result, and returns the number of bytes in that character (always at least 1 and never more than size).

For an invalid byte sequence, mbtowc returns -1. For an empty string, it returns 0, also storing '\0' in *result.

If the multibyte character code uses shift characters, then mbtowc maintains and updates a shift state as it scans. If you call mbtowc with a null pointer for string, that initializes the shift state to its standard initial value. It also returns nonzero if the multibyte character code in use actually has a shift state. See Shift State.

— Function: int wctomb (char *string, wchar_t wchar)

The wctomb (“wide character to multibyte”) function converts the wide character code wchar to its corresponding multibyte character sequence, and stores the result in bytes starting at string. At most MB_CUR_MAX characters are stored.

wctomb with non-null string distinguishes three possibilities for wchar: a valid wide character code (one that can be translated to a multibyte character), an invalid code, and L'\0'.

Given a valid code, wctomb converts it to a multibyte character, storing the bytes starting at string. Then it returns the number of bytes in that character (always at least 1 and never more than MB_CUR_MAX).

If wchar is an invalid wide character code, wctomb returns -1. If wchar is L'\0', it returns 0, also storing '\0' in *string.

If the multibyte character code uses shift characters, then wctomb maintains and updates a shift state as it scans. If you call wctomb with a null pointer for string, that initializes the shift state to its standard initial value. It also returns nonzero if the multibyte character code in use actually has a shift state. See Shift State.

Calling this function with a wchar argument of zero when string is not null has the side-effect of reinitializing the stored shift state as well as storing the multibyte character '\0' and returning 0.

Similar to mbrlen there is also a non-reentrant function that computes the length of a multibyte character. It can be defined in terms of mbtowc.

— Function: int mblen (const char *string, size_t size)

The mblen function with a non-null string argument returns the number of bytes that make up the multibyte character beginning at string, never examining more than size bytes. (The idea is to supply for size the number of bytes of data you have in hand.)

The return value of mblen distinguishes three possibilities: the first size bytes at string start with valid multibyte characters, they start with an invalid byte sequence or just part of a character, or string points to an empty string (a null character).

For a valid multibyte character, mblen returns the number of bytes in that character (always at least 1 and never more than size). For an invalid byte sequence, mblen returns -1. For an empty string, it returns 0.

If the multibyte character code uses shift characters, then mblen maintains and updates a shift state as it scans. If you call mblen with a null pointer for string, that initializes the shift state to its standard initial value. It also returns a nonzero value if the multibyte character code in use actually has a shift state. See Shift State.

The function mblen is declared in stdlib.h.

Next: , Previous: Non-reentrant Character Conversion, Up: Non-reentrant Conversion

6.4.2 Non-reentrant Conversion of Strings

For convenience the ISO C90 standard also defines functions to convert entire strings instead of single characters. These functions suffer from the same problems as their reentrant counterparts from Amendment 1 to ISO C90; see Converting Strings.

— Function: size_t mbstowcs (wchar_t *wstring, const char *string, size_t size)

The mbstowcs (“multibyte string to wide character string”) function converts the null-terminated string of multibyte characters string to an array of wide character codes, storing not more than size wide characters into the array beginning at wstring. The terminating null character counts towards the size, so if size is less than the actual number of wide characters resulting from string, no terminating null character is stored.

The conversion of characters from string begins in the initial shift state.

If an invalid multibyte character sequence is found, the mbstowcs function returns a value of -1. Otherwise, it returns the number of wide characters stored in the array wstring. This number does not include the terminating null character, which is present if the number is less than size.

Here is an example showing how to convert a string of multibyte characters, allocating enough space for the result. 

          wchar_t *
          mbstowcs_alloc (const char *string)
          {
            size_t size = strlen (string) + 1;
            wchar_t *buf = xmalloc (size * sizeof (wchar_t));
          
            size = mbstowcs (buf, string, size);
            if (size == (size_t) -1)
              return NULL;
            buf = xrealloc (buf, (size + 1) * sizeof (wchar_t));
            return buf;
          }
— Function: size_t wcstombs (char *string, const wchar_t *wstring, size_t size)

The wcstombs (“wide character string to multibyte string”) function converts the null-terminated wide character array wstring into a string containing multibyte characters, storing not more than size bytes starting at string, followed by a terminating null character if there is room. The conversion of characters begins in the initial shift state.

The terminating null character counts towards the size, so if size is less than or equal to the number of bytes needed in wstring, no terminating null character is stored.

If a code that does not correspond to a valid multibyte character is found, the wcstombs function returns a value of -1. Otherwise, the return value is the number of bytes stored in the array string. This number does not include the terminating null character, which is present if the number is less than size.

Previous: Non-reentrant String Conversion, Up: Non-reentrant Conversion

6.4.3 States in Non-reentrant Functions

In some multibyte character codes, the meaning of any particular byte sequence is not fixed; it depends on what other sequences have come earlier in the same string. Typically there are just a few sequences that can change the meaning of other sequences; these few are called shift sequences and we say that they set the shift state for other sequences that follow.

To illustrate shift state and shift sequences, suppose we decide that the sequence 0200 (just one byte) enters Japanese mode, in which pairs of bytes in the range from 0240 to 0377 are single characters, while 0201 enters Latin-1 mode, in which single bytes in the range from 0240 to 0377 are characters, and interpreted according to the ISO Latin-1 character set. This is a multibyte code that has two alternative shift states (“Japanese mode” and “Latin-1 mode”), and two shift sequences that specify particular shift states.

When the multibyte character code in use has shift states, then mblen, mbtowc, and wctomb must maintain and update the current shift state as they scan the string. To make this work properly, you must follow these rules:

Here is an example of using mblen following these rules: 

     void
     scan_string (char *s)
     {
       int length = strlen (s);
     
       /* Initialize shift state.  */
       mblen (NULL, 0);
     
       while (1)
         {
           int thischar = mblen (s, length);
           /* Deal with end of string and invalid characters.  */
           if (thischar == 0)
             break;
           if (thischar == -1)
             {
               error ("invalid multibyte character");
               break;
             }
           /* Advance past this character.  */
           s += thischar;
           length -= thischar;
         }
     }

The functions mblen, mbtowc and wctomb are not reentrant when using a multibyte code that uses a shift state. However, no other library functions call these functions, so you don't have to worry that the shift state will be changed mysteriously.

Previous: Non-reentrant Conversion, Up: Character Set Handling

6.5 Generic Charset Conversion

The conversion functions mentioned so far in this chapter all had in common that they operate on character sets that are not directly specified by the functions. The multibyte encoding used is specified by the currently selected locale for the LC_CTYPE category. The wide character set is fixed by the implementation (in the case of GNU C library it is always UCS-4 encoded ISO 10646.

This has of course several problems when it comes to general character conversion:

The XPG2 standard defines a completely new set of functions, which has none of these limitations. They are not at all coupled to the selected locales, and they have no constraints on the character sets selected for source and destination. Only the set of available conversions limits them. The standard does not specify that any conversion at all must be available. Such availability is a measure of the quality of the implementation.

In the following text first the interface to iconv and then the conversion function, will be described. Comparisons with other implementations will show what obstacles stand in the way of portable applications. Finally, the implementation is described in so far as might interest the advanced user who wants to extend conversion capabilities.

Next: , Up: Generic Charset Conversion

6.5.1 Generic Character Set Conversion Interface

This set of functions follows the traditional cycle of using a resource: open–use–close. The interface consists of three functions, each of which implements one step.

Before the interfaces are described it is necessary to introduce a data type. Just like other open–use–close interfaces the functions introduced here work using handles and the iconv.h header defines a special type for the handles used.

— Data Type: iconv_t

This data type is an abstract type defined in iconv.h. The user must not assume anything about the definition of this type; it must be completely opaque.

Objects of this type can get assigned handles for the conversions using the iconv functions. The objects themselves need not be freed, but the conversions for which the handles stand for have to.

The first step is the function to create a handle.

— Function: iconv_t iconv_open (const char *tocode, const char *fromcode)

The iconv_open function has to be used before starting a conversion. The two parameters this function takes determine the source and destination character set for the conversion, and if the implementation has the possibility to perform such a conversion, the function returns a handle.

If the wanted conversion is not available, the iconv_open function returns (iconv_t) -1. In this case the global variable errno can have the following values:

EMFILE
The process already has OPEN_MAX file descriptors open.
ENFILE
The system limit of open file is reached.
ENOMEM
Not enough memory to carry out the operation.
EINVAL
The conversion from fromcode to tocode is not supported.

It is not possible to use the same descriptor in different threads to perform independent conversions. The data structures associated with the descriptor include information about the conversion state. This must not be messed up by using it in different conversions.

An iconv descriptor is like a file descriptor as for every use a new descriptor must be created. The descriptor does not stand for all of the conversions from fromset to toset.

The GNU C library implementation of iconv_open has one significant extension to other implementations. To ease the extension of the set of available conversions, the implementation allows storing the necessary files with data and code in an arbitrary number of directories. How this extension must be written will be explained below (see glibc iconv Implementation). Here it is only important to say that all directories mentioned in the GCONV_PATH environment variable are considered only if they contain a file gconv-modules. These directories need not necessarily be created by the system administrator. In fact, this extension is introduced to help users writing and using their own, new conversions. Of course, this does not work for security reasons in SUID binaries; in this case only the system directory is considered and this normally is prefix/lib/gconv. The GCONV_PATH environment variable is examined exactly once at the first call of the iconv_open function. Later modifications of the variable have no effect.

The iconv_open function was introduced early in the X/Open Portability Guide, version 2. It is supported by all commercial Unices as it is required for the Unix branding. However, the quality and completeness of the implementation varies widely. The iconv_open function is declared in iconv.h.

The iconv implementation can associate large data structure with the handle returned by iconv_open. Therefore, it is crucial to free all the resources once all conversions are carried out and the conversion is not needed anymore.

— Function: int iconv_close (iconv_t cd)

The iconv_close function frees all resources associated with the handle cd, which must have been returned by a successful call to the iconv_open function.

If the function call was successful the return value is 0. Otherwise it is -1 and errno is set appropriately. Defined error are:

EBADF
The conversion descriptor is invalid.

The iconv_close function was introduced together with the rest of the iconv functions in XPG2 and is declared in iconv.h.

The standard defines only one actual conversion function. This has, therefore, the most general interface: it allows conversion from one buffer to another. Conversion from a file to a buffer, vice versa, or even file to file can be implemented on top of it.

— Function: size_t iconv (iconv_t cd, char **inbuf, size_t *inbytesleft, char **outbuf, size_t *outbytesleft)

The iconv function converts the text in the input buffer according to the rules associated with the descriptor cd and stores the result in the output buffer. It is possible to call the function for the same text several times in a row since for stateful character sets the necessary state information is kept in the data structures associated with the descriptor.

The input buffer is specified by *inbuf and it contains *inbytesleft bytes. The extra indirection is necessary for communicating the used input back to the caller (see below). It is important to note that the buffer pointer is of type char and the length is measured in bytes even if the input text is encoded in wide characters.

The output buffer is specified in a similar way. *outbuf points to the beginning of the buffer with at least *outbytesleft bytes room for the result. The buffer pointer again is of type char and the length is measured in bytes. If outbuf or *outbuf is a null pointer, the conversion is performed but no output is available.

If inbuf is a null pointer, the iconv function performs the necessary action to put the state of the conversion into the initial state. This is obviously a no-op for non-stateful encodings, but if the encoding has a state, such a function call might put some byte sequences in the output buffer, which perform the necessary state changes. The next call with inbuf not being a null pointer then simply goes on from the initial state. It is important that the programmer never makes any assumption as to whether the conversion has to deal with states. Even if the input and output character sets are not stateful, the implementation might still have to keep states. This is due to the implementation chosen for the GNU C library as it is described below. Therefore an iconv call to reset the state should always be performed if some protocol requires this for the output text.

The conversion stops for one of three reasons. The first is that all characters from the input buffer are converted. This actually can mean two things: either all bytes from the input buffer are consumed or there are some bytes at the end of the buffer that possibly can form a complete character but the input is incomplete. The second reason for a stop is that the output buffer is full. And the third reason is that the input contains invalid characters.

In all of these cases the buffer pointers after the last successful conversion, for input and output buffer, are stored in inbuf and outbuf, and the available room in each buffer is stored in inbytesleft and outbytesleft.

Since the character sets selected in the iconv_open call can be almost arbitrary, there can be situations where the input buffer contains valid characters, which have no identical representation in the output character set. The behavior in this situation is undefined. The current behavior of the GNU C library in this situation is to return with an error immediately. This certainly is not the most desirable solution; therefore, future versions will provide better ones, but they are not yet finished.

If all input from the input buffer is successfully converted and stored in the output buffer, the function returns the number of non-reversible conversions performed. In all other cases the return value is (size_t) -1 and errno is set appropriately. In such cases the value pointed to by inbytesleft is nonzero.

EILSEQ
The conversion stopped because of an invalid byte sequence in the input. After the call, *inbuf points at the first byte of the invalid byte sequence.
E2BIG
The conversion stopped because it ran out of space in the output buffer.
EINVAL
The conversion stopped because of an incomplete byte sequence at the end of the input buffer.
EBADF
The cd argument is invalid.

The iconv function was introduced in the XPG2 standard and is declared in the iconv.h header.

The definition of the iconv function is quite good overall. It provides quite flexible functionality. The only problems lie in the boundary cases, which are incomplete byte sequences at the end of the input buffer and invalid input. A third problem, which is not really a design problem, is the way conversions are selected. The standard does not say anything about the legitimate names, a minimal set of available conversions. We will see how this negatively impacts other implementations, as demonstrated below.

Next: , Previous: Generic Conversion Interface, Up: Generic Charset Conversion

6.5.2 A complete iconv example

The example below features a solution for a common problem. Given that one knows the internal encoding used by the system for wchar_t strings, one often is in the position to read text from a file and store it in wide character buffers. One can do this using mbsrtowcs, but then we run into the problems discussed above. 

     int
     file2wcs (int fd, const char *charset, wchar_t *outbuf, size_t avail)
     {
       char inbuf[BUFSIZ];
       size_t insize = 0;
       char *wrptr = (char *) outbuf;
       int result = 0;
       iconv_t cd;
     
       cd = iconv_open ("WCHAR_T", charset);
       if (cd == (iconv_t) -1)
         {
           /* Something went wrong.  */
           if (errno == EINVAL)
             error (0, 0, "conversion from '%s' to wchar_t not available",
                    charset);
           else
             perror ("iconv_open");
     
           /* Terminate the output string.  */
           *outbuf = L'\0';
     
           return -1;
         }
     
       while (avail > 0)
         {
           size_t nread;
           size_t nconv;
           char *inptr = inbuf;
     
           /* Read more input.  */
           nread = read (fd, inbuf + insize, sizeof (inbuf) - insize);
           if (nread == 0)
             {
               /* When we come here the file is completely read.
                  This still could mean there are some unused
                  characters in the inbuf.  Put them back.  */
               if (lseek (fd, -insize, SEEK_CUR) == -1)
                 result = -1;
     
               /* Now write out the byte sequence to get into the
                  initial state if this is necessary.  */
               iconv (cd, NULL, NULL, &wrptr, &avail);
     
               break;
             }
           insize += nread;
     
           /* Do the conversion.  */
           nconv = iconv (cd, &inptr, &insize, &wrptr, &avail);
           if (nconv == (size_t) -1)
             {
               /* Not everything went right.  It might only be
                  an unfinished byte sequence at the end of the
                  buffer.  Or it is a real problem.  */
               if (errno == EINVAL)
                 /* This is harmless.  Simply move the unused
                    bytes to the beginning of the buffer so that
                    they can be used in the next round.  */
                 memmove (inbuf, inptr, insize);
               else
                 {
                   /* It is a real problem.  Maybe we ran out of
                      space in the output buffer or we have invalid
                      input.  In any case back the file pointer to
                      the position of the last processed byte.  */
                   lseek (fd, -insize, SEEK_CUR);
                   result = -1;
                   break;
                 }
             }
         }
     
       /* Terminate the output string.  */
       if (avail >= sizeof (wchar_t))
         *((wchar_t *) wrptr) = L'\0';
     
       if (iconv_close (cd) != 0)
         perror ("iconv_close");
     
       return (wchar_t *) wrptr - outbuf;
     }

This example shows the most important aspects of using the iconv functions. It shows how successive calls to iconv can be used to convert large amounts of text. The user does not have to care about stateful encodings as the functions take care of everything.

An interesting point is the case where iconv returns an error and errno is set to EINVAL. This is not really an error in the transformation. It can happen whenever the input character set contains byte sequences of more than one byte for some character and texts are not processed in one piece. In this case there is a chance that a multibyte sequence is cut. The caller can then simply read the remainder of the takes and feed the offending bytes together with new character from the input to iconv and continue the work. The internal state kept in the descriptor is not unspecified after such an event as is the case with the conversion functions from the ISO C standard.

The example also shows the problem of using wide character strings with iconv. As explained in the description of the iconv function above, the function always takes a pointer to a char array and the available space is measured in bytes. In the example, the output buffer is a wide character buffer; therefore, we use a local variable wrptr of type char *, which is used in the iconv calls.

This looks rather innocent but can lead to problems on platforms that have tight restriction on alignment. Therefore the caller of iconv has to make sure that the pointers passed are suitable for access of characters from the appropriate character set. Since, in the above case, the input parameter to the function is a wchar_t pointer, this is the case (unless the user violates alignment when computing the parameter). But in other situations, especially when writing generic functions where one does not know what type of character set one uses and, therefore, treats text as a sequence of bytes, it might become tricky.

Next: , Previous: iconv Examples, Up: Generic Charset Conversion

6.5.3 Some Details about other iconv Implementations

This is not really the place to discuss the iconv implementation of other systems but it is necessary to know a bit about them to write portable programs. The above mentioned problems with the specification of the iconv functions can lead to portability issues.

The first thing to notice is that, due to the large number of character sets in use, it is certainly not practical to encode the conversions directly in the C library. Therefore, the conversion information must come from files outside the C library. This is usually done in one or both of the following ways:

Some implementations in commercial Unices implement a mixture of these possibilities; the majority implement only the second solution. Using loadable modules moves the code out of the library itself and keeps the door open for extensions and improvements, but this design is also limiting on some platforms since not many platforms support dynamic loading in statically linked programs. On platforms without this capability it is therefore not possible to use this interface in statically linked programs. The GNU C library has, on ELF platforms, no problems with dynamic loading in these situations; therefore, this point is moot. The danger is that one gets acquainted with this situation and forgets about the restrictions on other systems.

A second thing to know about other iconv implementations is that the number of available conversions is often very limited. Some implementations provide, in the standard release (not special international or developer releases), at most 100 to 200 conversion possibilities. This does not mean 200 different character sets are supported; for example, conversions from one character set to a set of 10 others might count as 10 conversions. Together with the other direction this makes 20 conversion possibilities used up by one character set. One can imagine the thin coverage these platform provide. Some Unix vendors even provide only a handful of conversions, which renders them useless for almost all uses.

This directly leads to a third and probably the most problematic point. The way the iconv conversion functions are implemented on all known Unix systems and the availability of the conversion functions from character set A to B and the conversion from B to C does not imply that the conversion from A to C is available.

This might not seem unreasonable and problematic at first, but it is a quite big problem as one will notice shortly after hitting it. To show the problem we assume to write a program that has to convert from A to C. A call like 

     cd = iconv_open ("C", "A");

fails according to the assumption above. But what does the program do now? The conversion is necessary; therefore, simply giving up is not an option.

This is a nuisance. The iconv function should take care of this. But how should the program proceed from here on? If it tries to convert to character set B, first the two iconv_open calls 

     cd1 = iconv_open ("B", "A");

and 

     cd2 = iconv_open ("C", "B");

will succeed, but how to find B?

Unfortunately, the answer is: there is no general solution. On some systems guessing might help. On those systems most character sets can convert to and from UTF-8 encoded ISO 10646 or Unicode text. Beside this only some very system-specific methods can help. Since the conversion functions come from loadable modules and these modules must be stored somewhere in the filesystem, one could try to find them and determine from the available file which conversions are available and whether there is an indirect route from A to C.

This example shows one of the design errors of iconv mentioned above. It should at least be possible to determine the list of available conversion programmatically so that if iconv_open says there is no such conversion, one could make sure this also is true for indirect routes.

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6.5.4 The iconv Implementation in the GNU C library

After reading about the problems of iconv implementations in the last section it is certainly good to note that the implementation in the GNU C library has none of the problems mentioned above. What follows is a step-by-step analysis of the points raised above. The evaluation is based on the current state of the development (as of January 1999). The development of the iconv functions is not complete, but basic functionality has solidified.

The GNU C library's iconv implementation uses shared loadable modules to implement the conversions. A very small number of conversions are built into the library itself but these are only rather trivial conversions.

All the benefits of loadable modules are available in the GNU C library implementation. This is especially appealing since the interface is well documented (see below), and it, therefore, is easy to write new conversion modules. The drawback of using loadable objects is not a problem in the GNU C library, at least on ELF systems. Since the library is able to load shared objects even in statically linked binaries, static linking need not be forbidden in case one wants to use iconv.

The second mentioned problem is the number of supported conversions. Currently, the GNU C library supports more than 150 character sets. The way the implementation is designed the number of supported conversions is greater than 22350 (150 times 149). If any conversion from or to a character set is missing, it can be added easily.

Particularly impressive as it may be, this high number is due to the fact that the GNU C library implementation of iconv does not have the third problem mentioned above (i.e., whenever there is a conversion from a character set A to B and from B to C it is always possible to convert from A to C directly). If the iconv_open returns an error and sets errno to EINVAL, there is no known way, directly or indirectly, to perform the wanted conversion.

Triangulation is achieved by providing for each character set a conversion from and to UCS-4 encoded ISO 10646. Using ISO 10646 as an intermediate representation it is possible to triangulate (i.e., convert with an intermediate representation).

There is no inherent requirement to provide a conversion to ISO 10646 for a new character set, and it is also possible to provide other conversions where neither source nor destination character set is ISO 10646. The existing set of conversions is simply meant to cover all conversions that might be of interest.

All currently available conversions use the triangulation method above, making conversion run unnecessarily slow. If, for example, somebody often needs the conversion from ISO-2022-JP to EUC-JP, a quicker solution would involve direct conversion between the two character sets, skipping the input to ISO 10646 first. The two character sets of interest are much more similar to each other than to ISO 10646.

In such a situation one easily can write a new conversion and provide it as a better alternative. The GNU C library iconv implementation would automatically use the module implementing the conversion if it is specified to be more efficient.

6.5.4.1 Format of gconv-modules files

All information about the available conversions comes from a file named gconv-modules, which can be found in any of the directories along the GCONV_PATH. The gconv-modules files are line-oriented text files, where each of the lines has one of the following formats:

Returning to the example above where one has written a module to directly convert from ISO-2022-JP to EUC-JP and back. All that has to be done is to put the new module, let its name be ISO2022JP-EUCJP.so, in a directory and add a file gconv-modules with the following content in the same directory: 

     module  ISO-2022-JP//   EUC-JP//        ISO2022JP-EUCJP    1
     module  EUC-JP//        ISO-2022-JP//   ISO2022JP-EUCJP    1

To see why this is sufficient, it is necessary to understand how the conversion used by iconv (and described in the descriptor) is selected. The approach to this problem is quite simple.

At the first call of the iconv_open function the program reads all available gconv-modules files and builds up two tables: one containing all the known aliases and another that contains the information about the conversions and which shared object implements them.

6.5.4.2 Finding the conversion path in iconv

The set of available conversions form a directed graph with weighted edges. The weights on the edges are the costs specified in the gconv-modules files. The iconv_open function uses an algorithm suitable for search for the best path in such a graph and so constructs a list of conversions that must be performed in succession to get the transformation from the source to the destination character set.

Explaining why the above gconv-modules files allows the iconv implementation to resolve the specific ISO-2022-JP to EUC-JP conversion module instead of the conversion coming with the library itself is straightforward. Since the latter conversion takes two steps (from ISO-2022-JP to ISO 10646 and then from ISO 10646 to EUC-JP), the cost is 1+1 = 2. The above gconv-modules file, however, specifies that the new conversion modules can perform this conversion with only the cost of 1.

A mysterious item about the gconv-modules file above (and also the file coming with the GNU C library) are the names of the character sets specified in the module lines. Why do almost all the names end in //? And this is not all: the names can actually be regular expressions. At this point in time this mystery should not be revealed, unless you have the relevant spell-casting materials: ashes from an original DOS 6.2 boot disk burnt in effigy, a crucifix blessed by St. Emacs, assorted herbal roots from Central America, sand from Cebu, etc. Sorry! The part of the implementation where this is used is not yet finished. For now please simply follow the existing examples. It'll become clearer once it is. –drepper

A last remark about the gconv-modules is about the names not ending with //. A character set named INTERNAL is often mentioned. From the discussion above and the chosen name it should have become clear that this is the name for the representation used in the intermediate step of the triangulation. We have said that this is UCS-4 but actually that is not quite right. The UCS-4 specification also includes the specification of the byte ordering used. Since a UCS-4 value consists of four bytes, a stored value is effected by byte ordering. The internal representation is not the same as UCS-4 in case the byte ordering of the processor (or at least the running process) is not the same as the one required for UCS-4. This is done for performance reasons as one does not want to perform unnecessary byte-swapping operations if one is not interested in actually seeing the result in UCS-4. To avoid trouble with endianness, the internal representation consistently is named INTERNAL even on big-endian systems where the representations are identical.

6.5.4.3 iconv module data structures

So far this section has described how modules are located and considered to be used. What remains to be described is the interface of the modules so that one can write new ones. This section describes the interface as it is in use in January 1999. The interface will change a bit in the future but, with luck, only in an upwardly compatible way.

The definitions necessary to write new modules are publicly available in the non-standard header gconv.h. The following text, therefore, describes the definitions from this header file. First, however, it is necessary to get an overview.

From the perspective of the user of iconv the interface is quite simple: the iconv_open function returns a handle that can be used in calls to iconv, and finally the handle is freed with a call to iconv_close. The problem is that the handle has to be able to represent the possibly long sequences of conversion steps and also the state of each conversion since the handle is all that is passed to the iconv function. Therefore, the data structures are really the elements necessary to understanding the implementation.

We need two different kinds of data structures. The first describes the conversion and the second describes the state etc. There are really two type definitions like this in gconv.h.

— Data type: struct __gconv_step

This data structure describes one conversion a module can perform. For each function in a loaded module with conversion functions there is exactly one object of this type. This object is shared by all users of the conversion (i.e., this object does not contain any information corresponding to an actual conversion; it only describes the conversion itself).

struct __gconv_loaded_object *__shlib_handle
const char *__modname
int __counter
All these elements of the structure are used internally in the C library to coordinate loading and unloading the shared. One must not expect any of the other elements to be available or initialized.
const char *__from_name
const char *__to_name
__from_name and __to_name contain the names of the source and destination character sets. They can be used to identify the actual conversion to be carried out since one module might implement conversions for more than one character set and/or direction.
gconv_fct __fct
gconv_init_fct __init_fct
gconv_end_fct __end_fct
These elements contain pointers to the functions in the loadable module. The interface will be explained below.
int __min_needed_from
int __max_needed_from
int __min_needed_to
int __max_needed_to;
These values have to be supplied in the init function of the module. The __min_needed_from value specifies how many bytes a character of the source character set at least needs. The __max_needed_from specifies the maximum value that also includes possible shift sequences.

The __min_needed_to and __max_needed_to values serve the same purpose as __min_needed_from and __max_needed_from but this time for the destination character set.

It is crucial that these values be accurate since otherwise the conversion functions will have problems or not work at all.

int __stateful
This element must also be initialized by the init function. int __stateful is nonzero if the source character set is stateful. Otherwise it is zero.
void *__data
This element can be used freely by the conversion functions in the module. void *__data can be used to communicate extra information from one call to another. void *__data need not be initialized if not needed at all. If void *__data element is assigned a pointer to dynamically allocated memory (presumably in the init function) it has to be made sure that the end function deallocates the memory. Otherwise the application will leak memory.

It is important to be aware that this data structure is shared by all users of this specification conversion and therefore the __data element must not contain data specific to one specific use of the conversion function.

— Data type: struct __gconv_step_data

This is the data structure that contains the information specific to each use of the conversion functions.

char *__outbuf
char *__outbufend
These elements specify the output buffer for the conversion step. The __outbuf element points to the beginning of the buffer, and __outbufend points to the byte following the last byte in the buffer. The conversion function must not assume anything about the size of the buffer but it can be safely assumed the there is room for at least one complete character in the output buffer.

Once the conversion is finished, if the conversion is the last step, the __outbuf element must be modified to point after the last byte written into the buffer to signal how much output is available. If this conversion step is not the last one, the element must not be modified. The __outbufend element must not be modified.

int __is_last
This element is nonzero if this conversion step is the last one. This information is necessary for the recursion. See the description of the conversion function internals below. This element must never be modified.
int __invocation_counter
The conversion function can use this element to see how many calls of the conversion function already happened. Some character sets require a certain prolog when generating output, and by comparing this value with zero, one can find out whether it is the first call and whether, therefore, the prolog should be emitted. This element must never be modified.
int __internal_use
This element is another one rarely used but needed in certain situations. It is assigned a nonzero value in case the conversion functions are used to implement mbsrtowcs et.al. (i.e., the function is not used directly through the iconv interface).

This sometimes makes a difference as it is expected that the iconv functions are used to translate entire texts while the mbsrtowcs functions are normally used only to convert single strings and might be used multiple times to convert entire texts.

But in this situation we would have problem complying with some rules of the character set specification. Some character sets require a prolog, which must appear exactly once for an entire text. If a number of mbsrtowcs calls are used to convert the text, only the first call must add the prolog. However, because there is no communication between the different calls of mbsrtowcs, the conversion functions have no possibility to find this out. The situation is different for sequences of iconv calls since the handle allows access to the needed information.

The int __internal_use element is mostly used together with __invocation_counter as follows: 

               if (!data->__internal_use
                    && data->__invocation_counter == 0)
                 /* Emit prolog.  */
                 ...

This element must never be modified.

mbstate_t *__statep
The __statep element points to an object of type mbstate_t (see Keeping the state). The conversion of a stateful character set must use the object pointed to by __statep to store information about the conversion state. The __statep element itself must never be modified.
mbstate_t __state
This element must never be used directly. It is only part of this structure to have the needed space allocated.

6.5.4.4 iconv module interfaces

With the knowledge about the data structures we now can describe the conversion function itself. To understand the interface a bit of knowledge is necessary about the functionality in the C library that loads the objects with the conversions.

It is often the case that one conversion is used more than once (i.e., there are several iconv_open calls for the same set of character sets during one program run). The mbsrtowcs et.al. functions in the GNU C library also use the iconv functionality, which increases the number of uses of the same functions even more.

Because of this multiple use of conversions, the modules do not get loaded exclusively for one conversion. Instead a module once loaded can be used by an arbitrary number of iconv or mbsrtowcs calls at the same time. The splitting of the information between conversion- function-specific information and conversion data makes this possible. The last section showed the two data structures used to do this.

This is of course also reflected in the interface and semantics of the functions that the modules must provide. There are three functions that must have the following names:

gconv_init
The gconv_init function initializes the conversion function specific data structure. This very same object is shared by all conversions that use this conversion and, therefore, no state information about the conversion itself must be stored in here. If a module implements more than one conversion, the gconv_init function will be called multiple times.
gconv_end
The gconv_end function is responsible for freeing all resources allocated by the gconv_init function. If there is nothing to do, this function can be missing. Special care must be taken if the module implements more than one conversion and the gconv_init function does not allocate the same resources for all conversions.
gconv
This is the actual conversion function. It is called to convert one block of text. It gets passed the conversion step information initialized by gconv_init and the conversion data, specific to this use of the conversion functions.

There are three data types defined for the three module interface functions and these define the interface.

— Data type: int (*__gconv_init_fct) (struct __gconv_step *)

This specifies the interface of the initialization function of the module. It is called exactly once for each conversion the module implements.

As explained in the description of the struct __gconv_step data structure above the initialization function has to initialize parts of it.

__min_needed_from
__max_needed_from
__min_needed_to
__max_needed_to
These elements must be initialized to the exact numbers of the minimum and maximum number of bytes used by one character in the source and destination character sets, respectively. If the characters all have the same size, the minimum and maximum values are the same.
__stateful
This element must be initialized to an nonzero value if the source character set is stateful. Otherwise it must be zero.

If the initialization function needs to communicate some information to the conversion function, this communication can happen using the __data element of the __gconv_step structure. But since this data is shared by all the conversions, it must not be modified by the conversion function. The example below shows how this can be used. 

          #define MIN_NEEDED_FROM         1
          #define MAX_NEEDED_FROM         4
          #define MIN_NEEDED_TO           4
          #define MAX_NEEDED_TO           4
          
          int
          gconv_init (struct __gconv_step *step)
          {
            /* Determine which direction.  */
            struct iso2022jp_data *new_data;
            enum direction dir = illegal_dir;
            enum variant var = illegal_var;
            int result;
          
            if (__strcasecmp (step->__from_name, "ISO-2022-JP//") == 0)
              {
                dir = from_iso2022jp;
                var = iso2022jp;
              }
            else if (__strcasecmp (step->__to_name, "ISO-2022-JP//") == 0)
              {
                dir = to_iso2022jp;
                var = iso2022jp;
              }
            else if (__strcasecmp (step->__from_name, "ISO-2022-JP-2//") == 0)
              {
                dir = from_iso2022jp;
                var = iso2022jp2;
              }
            else if (__strcasecmp (step->__to_name, "ISO-2022-JP-2//") == 0)
              {
                dir = to_iso2022jp;
                var = iso2022jp2;
              }
          
            result = __GCONV_NOCONV;
            if (dir != illegal_dir)
              {
                new_data = (struct iso2022jp_data *)
                  malloc (sizeof (struct iso2022jp_data));
          
                result = __GCONV_NOMEM;
                if (new_data != NULL)
                  {
                    new_data->dir = dir;
                    new_data->var = var;
                    step->__data = new_data;
          
                    if (dir == from_iso2022jp)
                      {
                        step->__min_needed_from = MIN_NEEDED_FROM;
                        step->__max_needed_from = MAX_NEEDED_FROM;
                        step->__min_needed_to = MIN_NEEDED_TO;
                        step->__max_needed_to = MAX_NEEDED_TO;
                      }
                    else
                      {
                        step->__min_needed_from = MIN_NEEDED_TO;
                        step->__max_needed_from = MAX_NEEDED_TO;
                        step->__min_needed_to = MIN_NEEDED_FROM;
                        step->__max_needed_to = MAX_NEEDED_FROM + 2;
                      }
          
                    /* Yes, this is a stateful encoding.  */
                    step->__stateful = 1;
          
                    result = __GCONV_OK;
                  }
              }
          
            return result;
          }

The function first checks which conversion is wanted. The module from which this function is taken implements four different conversions; which one is selected can be determined by comparing the names. The comparison should always be done without paying attention to the case.

Next, a data structure, which contains the necessary information about which conversion is selected, is allocated. The data structure struct iso2022jp_data is locally defined since, outside the module, this data is not used at all. Please note that if all four conversions this modules supports are requested there are four data blocks.

One interesting thing is the initialization of the __min_ and __max_ elements of the step data object. A single ISO-2022-JP character can consist of one to four bytes. Therefore the MIN_NEEDED_FROM and MAX_NEEDED_FROM macros are defined this way. The output is always the INTERNAL character set (aka UCS-4) and therefore each character consists of exactly four bytes. For the conversion from INTERNAL to ISO-2022-JP we have to take into account that escape sequences might be necessary to switch the character sets. Therefore the __max_needed_to element for this direction gets assigned MAX_NEEDED_FROM + 2. This takes into account the two bytes needed for the escape sequences to single the switching. The asymmetry in the maximum values for the two directions can be explained easily: when reading ISO-2022-JP text, escape sequences can be handled alone (i.e., it is not necessary to process a real character since the effect of the escape sequence can be recorded in the state information). The situation is different for the other direction. Since it is in general not known which character comes next, one cannot emit escape sequences to change the state in advance. This means the escape sequences that have to be emitted together with the next character. Therefore one needs more room than only for the character itself.

The possible return values of the initialization function are:

__GCONV_OK
The initialization succeeded
__GCONV_NOCONV
The requested conversion is not supported in the module. This can happen if the gconv-modules file has errors.
__GCONV_NOMEM
Memory required to store additional information could not be allocated.

The function called before the module is unloaded is significantly easier. It often has nothing at all to do; in which case it can be left out completely.

— Data type: void (*__gconv_end_fct) (struct gconv_step *)

The task of this function is to free all resources allocated in the initialization function. Therefore only the __data element of the object pointed to by the argument is of interest. Continuing the example from the initialization function, the finalization function looks like this: 

          void
          gconv_end (struct __gconv_step *data)
          {
            free (data->__data);
          }

The most important function is the conversion function itself, which can get quite complicated for complex character sets. But since this is not of interest here, we will only describe a possible skeleton for the conversion function.

— Data type: int (*__gconv_fct) (struct __gconv_step *, struct __gconv_step_data *, const char **, const char *, size_t *, int)

The conversion function can be called for two basic reason: to convert text or to reset the state. From the description of the iconv function it can be seen why the flushing mode is necessary. What mode is selected is determined by the sixth argument, an integer. This argument being nonzero means that flushing is selected.

Common to both modes is where the output buffer can be found. The information about this buffer is stored in the conversion step data. A pointer to this information is passed as the second argument to this function. The description of the struct __gconv_step_data structure has more information on the conversion step data.

What has to be done for flushing depends on the source character set. If the source character set is not stateful, nothing has to be done. Otherwise the function has to emit a byte sequence to bring the state object into the initial state. Once this all happened the other conversion modules in the chain of conversions have to get the same chance. Whether another step follows can be determined from the __is_last element of the step data structure to which the first parameter points.

The more interesting mode is when actual text has to be converted. The first step in this case is to convert as much text as possible from the input buffer and store the result in the output buffer. The start of the input buffer is determined by the third argument, which is a pointer to a pointer variable referencing the beginning of the buffer. The fourth argument is a pointer to the byte right after the last byte in the buffer.

The conversion has to be performed according to the current state if the character set is stateful. The state is stored in an object pointed to by the __statep element of the step data (second argument). Once either the input buffer is empty or the output buffer is full the conversion stops. At this point, the pointer variable referenced by the third parameter must point to the byte following the last processed byte (i.e., if all of the input is consumed, this pointer and the fourth parameter have the same value).

What now happens depends on whether this step is the last one. If it is the last step, the only thing that has to be done is to update the __outbuf element of the step data structure to point after the last written byte. This update gives the caller the information on how much text is available in the output buffer. In addition, the variable pointed to by the fifth parameter, which is of type size_t, must be incremented by the number of characters (not bytes) that were converted in a non-reversible way. Then, the function can return.

In case the step is not the last one, the later conversion functions have to get a chance to do their work. Therefore, the appropriate conversion function has to be called. The information about the functions is stored in the conversion data structures, passed as the first parameter. This information and the step data are stored in arrays, so the next element in both cases can be found by simple pointer arithmetic: 

          int
          gconv (struct __gconv_step *step, struct __gconv_step_data *data,
                 const char **inbuf, const char *inbufend, size_t *written,
                 int do_flush)
          {
            struct __gconv_step *next_step = step + 1;
            struct __gconv_step_data *next_data = data + 1;
            ...

The next_step pointer references the next step information and next_data the next data record. The call of the next function therefore will look similar to this: 

            next_step->__fct (next_step, next_data, &outerr, outbuf,
                              written, 0)

But this is not yet all. Once the function call returns the conversion function might have some more to do. If the return value of the function is __GCONV_EMPTY_INPUT, more room is available in the output buffer. Unless the input buffer is empty the conversion, functions start all over again and process the rest of the input buffer. If the return value is not __GCONV_EMPTY_INPUT, something went wrong and we have to recover from this.

A requirement for the conversion function is that the input buffer pointer (the third argument) always point to the last character that was put in converted form into the output buffer. This is trivially true after the conversion performed in the current step, but if the conversion functions deeper downstream stop prematurely, not all characters from the output buffer are consumed and, therefore, the input buffer pointers must be backed off to the right position.

Correcting the input buffers is easy to do if the input and output character sets have a fixed width for all characters. In this situation we can compute how many characters are left in the output buffer and, therefore, can correct the input buffer pointer appropriately with a similar computation. Things are getting tricky if either character set has characters represented with variable length byte sequences, and it gets even more complicated if the conversion has to take care of the state. In these cases the conversion has to be performed once again, from the known state before the initial conversion (i.e., if necessary the state of the conversion has to be reset and the conversion loop has to be executed again). The difference now is that it is known how much input must be created, and the conversion can stop before converting the first unused character. Once this is done the input buffer pointers must be updated again and the function can return.

One final thing should be mentioned. If it is necessary for the conversion to know whether it is the first invocation (in case a prolog has to be emitted), the conversion function should increment the __invocation_counter element of the step data structure just before returning to the caller. See the description of the struct __gconv_step_data structure above for more information on how this can be used.

The return value must be one of the following values:

__GCONV_EMPTY_INPUT
All input was consumed and there is room left in the output buffer.
__GCONV_FULL_OUTPUT
No more room in the output buffer. In case this is not the last step this value is propagated down from the call of the next conversion function in the chain.
__GCONV_INCOMPLETE_INPUT
The input buffer is not entirely empty since it contains an incomplete character sequence.

The following example provides a framework for a conversion function. In case a new conversion has to be written the holes in this implementation have to be filled and that is it. 

          int
          gconv (struct __gconv_step *step, struct __gconv_step_data *data,
                 const char **inbuf, const char *inbufend, size_t *written,
                 int do_flush)
          {
            struct __gconv_step *next_step = step + 1;
            struct __gconv_step_data *next_data = data + 1;
            gconv_fct fct = next_step->__fct;
            int status;
          
            /* If the function is called with no input this means we have
               to reset to the initial state.  The possibly partly
               converted input is dropped.  */
            if (do_flush)
              {
                status = __GCONV_OK;
          
                /* Possible emit a byte sequence which put the state object
                   into the initial state.  */
          
                /* Call the steps down the chain if there are any but only
                   if we successfully emitted the escape sequence.  */
                if (status == __GCONV_OK && ! data->__is_last)
                  status = fct (next_step, next_data, NULL, NULL,
                                written, 1);
              }
            else
              {
                /* We preserve the initial values of the pointer variables.  */
                const char *inptr = *inbuf;
                char *outbuf = data->__outbuf;
                char *outend = data->__outbufend;
                char *outptr;
          
                do
                  {
                    /* Remember the start value for this round.  */
                    inptr = *inbuf;
                    /* The outbuf buffer is empty.  */
                    outptr = outbuf;
          
                    /* For stateful encodings the state must be safe here.  */
          
                    /* Run the conversion loop.  status is set
                       appropriately afterwards.  */
          
                    /* If this is the last step, leave the loop. There is
                       nothing we can do.  */
                    if (data->__is_last)
                      {
                        /* Store information about how many bytes are
                           available.  */
                        data->__outbuf = outbuf;
          
                       /* If any non-reversible conversions were performed,
                          add the number to *written.  */
          
                       break;
                     }
          
                    /* Write out all output that was produced.  */
                    if (outbuf > outptr)
                      {
                        const char *outerr = data->__outbuf;
                        int result;
          
                        result = fct (next_step, next_data, &outerr,
                                      outbuf, written, 0);
          
                        if (result != __GCONV_EMPTY_INPUT)
                          {
                            if (outerr != outbuf)
                              {
                                /* Reset the input buffer pointer.  We
                                   document here the complex case.  */
                                size_t nstatus;
          
                                /* Reload the pointers.  */
                                *inbuf = inptr;
                                outbuf = outptr;
          
                                /* Possibly reset the state.  */
          
                                /* Redo the conversion, but this time
                                   the end of the output buffer is at
                                   outerr.  */
                              }
          
                            /* Change the status.  */
                            status = result;
                          }
                        else
                          /* All the output is consumed, we can make
                              another run if everything was ok.  */
                          if (status == __GCONV_FULL_OUTPUT)
                            status = __GCONV_OK;
                     }
                  }
                while (status == __GCONV_OK);
          
                /* We finished one use of this step.  */
                ++data->__invocation_counter;
              }
          
            return status;
          }

This information should be sufficient to write new modules. Anybody doing so should also take a look at the available source code in the GNU C library sources. It contains many examples of working and optimized modules.

Next: , Previous: Character Set Handling, Up: Top

7 Locales and Internationalization

Different countries and cultures have varying conventions for how to communicate. These conventions range from very simple ones, such as the format for representing dates and times, to very complex ones, such as the language spoken.

Internationalization of software means programming it to be able to adapt to the user's favorite conventions. In ISO C, internationalization works by means of locales. Each locale specifies a collection of conventions, one convention for each purpose. The user chooses a set of conventions by specifying a locale (via environment variables).

All programs inherit the chosen locale as part of their environment. Provided the programs are written to obey the choice of locale, they will follow the conventions preferred by the user.

Next: , Up: Locales

7.1 What Effects a Locale Has

Each locale specifies conventions for several purposes, including the following:

Some aspects of adapting to the specified locale are handled automatically by the library subroutines. For example, all your program needs to do in order to use the collating sequence of the chosen locale is to use strcoll or strxfrm to compare strings.

Other aspects of locales are beyond the comprehension of the library. For example, the library can't automatically translate your program's output messages into other languages. The only way you can support output in the user's favorite language is to program this more or less by hand. The C library provides functions to handle translations for multiple languages easily.

This chapter discusses the mechanism by which you can modify the current locale. The effects of the current locale on specific library functions are discussed in more detail in the descriptions of those functions.

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7.2 Choosing a Locale

The simplest way for the user to choose a locale is to set the environment variable LANG. This specifies a single locale to use for all purposes. For example, a user could specify a hypothetical locale named `espana-castellano' to use the standard conventions of most of Spain.

The set of locales supported depends on the operating system you are using, and so do their names. We can't make any promises about what locales will exist, except for one standard locale called `C' or `POSIX'. Later we will describe how to construct locales.

A user also has the option of specifying different locales for different purposes—in effect, choosing a mixture of multiple locales.

For example, the user might specify the locale `espana-castellano' for most purposes, but specify the locale `usa-english' for currency formatting. This might make sense if the user is a Spanish-speaking American, working in Spanish, but representing monetary amounts in US dollars.

Note that both locales `espana-castellano' and `usa-english', like all locales, would include conventions for all of the purposes to which locales apply. However, the user can choose to use each locale for a particular subset of those purposes.

Next: , Previous: Choosing Locale, Up: Locales

7.3 Categories of Activities that Locales Affect

The purposes that locales serve are grouped into categories, so that a user or a program can choose the locale for each category independently. Here is a table of categories; each name is both an environment variable that a user can set, and a macro name that you can use as an argument to setlocale.

LC_COLLATE
This category applies to collation of strings (functions strcoll and strxfrm); see Collation Functions.
LC_CTYPE
This category applies to classification and conversion of characters, and to multibyte and wide characters; see Character Handling, and Character Set Handling.
LC_MONETARY
This category applies to formatting monetary values; see General Numeric.
LC_NUMERIC
This category applies to formatting numeric values that are not monetary; see General Numeric.
LC_TIME
This category applies to formatting date and time values; see Formatting Calendar Time.
LC_MESSAGES
This category applies to selecting the language used in the user interface for message translation (see The Uniforum approach; see Message catalogs a la X/Open) and contains regular expressions for affirmative and negative responses.
LC_ALL
This is not an environment variable; it is only a macro that you can use with setlocale to set a single locale for all purposes. Setting this environment variable overwrites all selections by the other LC_* variables or LANG.
LANG
If this environment variable is defined, its value specifies the locale to use for all purposes except as overridden by the variables above.

When developing the message translation functions it was felt that the functionality provided by the variables above is not sufficient. For example, it should be possible to specify more than one locale name. Take a Swedish user who better speaks German than English, and a program whose messages are output in English by default. It should be possible to specify that the first choice of language is Swedish, the second German, and if this also fails to use English. This is possible with the variable LANGUAGE. For further description of this GNU extension see Using gettextized software.

Next: , Previous: Locale Categories, Up: Locales

7.4 How Programs Set the Locale

A C program inherits its locale environment variables when it starts up. This happens automatically. However, these variables do not automatically control the locale used by the library functions, because ISO C says that all programs start by default in the standard `C' locale. To use the locales specified by the environment, you must call setlocale. Call it as follows: 

     setlocale (LC_ALL, "");

to select a locale based on the user choice of the appropriate environment variables.

You can also use setlocale to specify a particular locale, for general use or for a specific category.

The symbols in this section are defined in the header file locale.h.

— Function: char * setlocale (int category, const char *locale)

The function setlocale sets the current locale for category category to locale. A list of all the locales the system provides can be created by running 

            locale -a

If category is LC_ALL, this specifies the locale for all purposes. The other possible values of category specify an single purpose (see Locale Categories).

You can also use this function to find out the current locale by passing a null pointer as the locale argument. In this case, setlocale returns a string that is the name of the locale currently selected for category category.

The string returned by setlocale can be overwritten by subsequent calls, so you should make a copy of the string (see Copying and Concatenation) if you want to save it past any further calls to setlocale. (The standard library is guaranteed never to call setlocale itself.)

You should not modify the string returned by setlocale. It might be the same string that was passed as an argument in a previous call to setlocale. One requirement is that the category must be the same in the call the string was returned and the one when the string is passed in as locale parameter.

When you read the current locale for category LC_ALL, the value encodes the entire combination of selected locales for all categories. In this case, the value is not just a single locale name. In fact, we don't make any promises about what it looks like. But if you specify the same “locale name” with LC_ALL in a subsequent call to setlocale, it restores the same combination of locale selections.

To be sure you can use the returned string encoding the currently selected locale at a later time, you must make a copy of the string. It is not guaranteed that the returned pointer remains valid over time.

When the locale argument is not a null pointer, the string returned by setlocale reflects the newly-modified locale.

If you specify an empty string for locale, this means to read the appropriate environment variable and use its value to select the locale for category.

If a nonempty string is given for locale, then the locale of that name is used if possible.

If you specify an invalid locale name, setlocale returns a null pointer and leaves the current locale unchanged.

Here is an example showing how you might use setlocale to temporarily switch to a new locale. 

     #include <stddef.h>
     #include <locale.h>
     #include <stdlib.h>
     #include <string.h>
     
     void
     with_other_locale (char *new_locale,
                        void (*subroutine) (int),
                        int argument)
     {
       char *old_locale, *saved_locale;
     
       /* Get the name of the current locale.  */
       old_locale = setlocale (LC_ALL, NULL);
     
       /* Copy the name so it won't be clobbered by setlocale. */
       saved_locale = strdup (old_locale);
       if (saved_locale == NULL)
         fatal ("Out of memory");
     
       /* Now change the locale and do some stuff with it. */
       setlocale (LC_ALL, new_locale);
       (*subroutine) (argument);
     
       /* Restore the original locale. */
       setlocale (LC_ALL, saved_locale);
       free (saved_locale);
     }

Portability Note: Some ISO C systems may define additional locale categories, and future versions of the library will do so. For portability, assume that any symbol beginning with `LC_' might be defined in locale.h.

Next: , Previous: Setting the Locale, Up: Locales

7.5 Standard Locales

The only locale names you can count on finding on all operating systems are these three standard ones:

"C"
This is the standard C locale. The attributes and behavior it provides are specified in the ISO C standard. When your program starts up, it initially uses this locale by default.
"POSIX"
This is the standard POSIX locale. Currently, it is an alias for the standard C locale.
""
The empty name says to select a locale based on environment variables. See Locale Categories.

Defining and installing named locales is normally a responsibility of the system administrator at your site (or the person who installed the GNU C library). It is also possible for the user to create private locales. All this will be discussed later when describing the tool to do so.

If your program needs to use something other than the `C' locale, it will be more portable if you use whatever locale the user specifies with the environment, rather than trying to specify some non-standard locale explicitly by name. Remember, different machines might have different sets of locales installed.

Next: , Previous: Standard Locales, Up: Locales

7.6 Accessing Locale Information

There are several ways to access locale information. The simplest way is to let the C library itself do the work. Several of the functions in this library implicitly access the locale data, and use what information is provided by the currently selected locale. This is how the locale model is meant to work normally.

As an example take the strftime function, which is meant to nicely format date and time information (see Formatting Calendar Time). Part of the standard information contained in the LC_TIME category is the names of the months. Instead of requiring the programmer to take care of providing the translations the strftime function does this all by itself. %A in the format string is replaced by the appropriate weekday name of the locale currently selected by LC_TIME. This is an easy example, and wherever possible functions do things automatically in this way.

But there are quite often situations when there is simply no function to perform the task, or it is simply not possible to do the work automatically. For these cases it is necessary to access the information in the locale directly. To do this the C library provides two functions: localeconv and nl_langinfo. The former is part of ISO C and therefore portable, but has a brain-damaged interface. The second is part of the Unix interface and is portable in as far as the system follows the Unix standards.

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7.6.1 localeconv: It is portable but ...

Together with the setlocale function the ISO C people invented the localeconv function. It is a masterpiece of poor design. It is expensive to use, not extendable, and not generally usable as it provides access to only LC_MONETARY and LC_NUMERIC related information. Nevertheless, if it is applicable to a given situation it should be used since it is very portable. The function strfmon formats monetary amounts according to the selected locale using this information.

— Function: struct lconv * localeconv (void)

The localeconv function returns a pointer to a structure whose components contain information about how numeric and monetary values should be formatted in the current locale.

You should not modify the structure or its contents. The structure might be overwritten by subsequent calls to localeconv, or by calls to setlocale, but no other function in the library overwrites this value.

— Data Type: struct lconv

localeconv's return value is of this data type. Its elements are described in the following subsections.

If a member of the structure struct lconv has type char, and the value is CHAR_MAX, it means that the current locale has no value for that parameter.

Next: , Up: The Lame Way to Locale Data
7.6.1.1 Generic Numeric Formatting Parameters

These are the standard members of struct lconv; there may be others.

char *decimal_point
char *mon_decimal_point
These are the decimal-point separators used in formatting non-monetary and monetary quantities, respectively. In the `C' locale, the value of decimal_point is ".", and the value of mon_decimal_point is "".
char *thousands_sep
char *mon_thousands_sep
These are the separators used to delimit groups of digits to the left of the decimal point in formatting non-monetary and monetary quantities, respectively. In the `C' locale, both members have a value of "" (the empty string).
char *grouping
char *mon_grouping
These are strings that specify how to group the digits to the left of the decimal point. grouping applies to non-monetary quantities and mon_grouping applies to monetary quantities. Use either thousands_sep or mon_thousands_sep to separate the digit groups. Each member of these strings is to be interpreted as an integer value of type char. Successive numbers (from left to right) give the sizes of successive groups (from right to left, starting at the decimal point.) The last member is either 0, in which case the previous member is used over and over again for all the remaining groups, or CHAR_MAX, in which case there is no more grouping—or, put another way, any remaining digits form one large group without separators.

For example, if grouping is "\04\03\02", the correct grouping for the number 123456787654321 is `12', `34', `56', `78', `765', `4321'. This uses a group of 4 digits at the end, preceded by a group of 3 digits, preceded by groups of 2 digits (as many as needed). With a separator of `,', the number would be printed as `12,34,56,78,765,4321'.

A value of "\03" indicates repeated groups of three digits, as normally used in the U.S.

In the standard `C' locale, both grouping and mon_grouping have a value of "". This value specifies no grouping at all.

char int_frac_digits
char frac_digits
These are small integers indicating how many fractional digits (to the right of the decimal point) should be displayed in a monetary value in international and local formats, respectively. (Most often, both members have the same value.)

In the standard `C' locale, both of these members have the value CHAR_MAX, meaning “unspecified”. The ISO standard doesn't say what to do when you find this value; we recommend printing no fractional digits. (This locale also specifies the empty string for mon_decimal_point, so printing any fractional digits would be confusing!)

Next: , Previous: General Numeric, Up: The Lame Way to Locale Data
7.6.1.2 Printing the Currency Symbol

These members of the struct lconv structure specify how to print the symbol to identify a monetary value—the international analog of `$' for US dollars.

Each country has two standard currency symbols. The local currency symbol is used commonly within the country, while the international currency symbol is used internationally to refer to that country's currency when it is necessary to indicate the country unambiguously.

For example, many countries use the dollar as their monetary unit, and when dealing with international currencies it's important to specify that one is dealing with (say) Canadian dollars instead of U.S. dollars or Australian dollars. But when the context is known to be Canada, there is no need to make this explicit—dollar amounts are implicitly assumed to be in Canadian dollars.

char *currency_symbol
The local currency symbol for the selected locale.

In the standard `C' locale, this member has a value of "" (the empty string), meaning “unspecified”. The ISO standard doesn't say what to do when you find this value; we recommend you simply print the empty string as you would print any other string pointed to by this variable.

char *int_curr_symbol
The international currency symbol for the selected locale.

The value of int_curr_symbol should normally consist of a three-letter abbreviation determined by the international standard ISO 4217 Codes for the Representation of Currency and Funds, followed by a one-character separator (often a space).

In the standard `C' locale, this member has a value of "" (the empty string), meaning “unspecified”. We recommend you simply print the empty string as you would print any other string pointed to by this variable.

char p_cs_precedes
char n_cs_precedes
char int_p_cs_precedes
char int_n_cs_precedes
These members are 1 if the currency_symbol or int_curr_symbol strings should precede the value of a monetary amount, or 0 if the strings should follow the value. The p_cs_precedes and int_p_cs_precedes members apply to positive amounts (or zero), and the n_cs_precedes and int_n_cs_precedes members apply to negative amounts.

In the standard `C' locale, all of these members have a value of CHAR_MAX, meaning “unspecified”. The ISO standard doesn't say what to do when you find this value. We recommend printing the currency symbol before the amount, which is right for most countries. In other words, treat all nonzero values alike in these members.

The members with the int_ prefix apply to the int_curr_symbol while the other two apply to currency_symbol.

char p_sep_by_space
char n_sep_by_space
char int_p_sep_by_space
char int_n_sep_by_space
These members are 1 if a space should appear between the currency_symbol or int_curr_symbol strings and the amount, or 0 if no space should appear. The p_sep_by_space and int_p_sep_by_space members apply to positive amounts (or zero), and the n_sep_by_space and int_n_sep_by_space members apply to negative amounts.

In the standard `C' locale, all of these members have a value of CHAR_MAX, meaning “unspecified”. The ISO standard doesn't say what you should do when you find this value; we suggest you treat it as 1 (print a space). In other words, treat all nonzero values alike in these members.

The members with the int_ prefix apply to the int_curr_symbol while the other two apply to currency_symbol. There is one specialty with the int_curr_symbol, though. Since all legal values contain a space at the end the string one either printf this space (if the currency symbol must appear in front and must be separated) or one has to avoid printing this character at all (especially when at the end of the string).

Previous: Currency Symbol, Up: The Lame Way to Locale Data
7.6.1.3 Printing the Sign of a Monetary Amount

These members of the struct lconv structure specify how to print the sign (if any) of a monetary value.

char *positive_sign
char *negative_sign
These are strings used to indicate positive (or zero) and negative monetary quantities, respectively.

In the standard `C' locale, both of these members have a value of "" (the empty string), meaning “unspecified”.

The ISO standard doesn't say what to do when you find this value; we recommend printing positive_sign as you find it, even if it is empty. For a negative value, print negative_sign as you find it unless both it and positive_sign are empty, in which case print `-' instead. (Failing to indicate the sign at all seems rather unreasonable.)

char p_sign_posn
char n_sign_posn
char int_p_sign_posn
char int_n_sign_posn
These members are small integers that indicate how to position the sign for nonnegative and negative monetary quantities, respectively. (The string used by the sign is what was specified with positive_sign or negative_sign.) The possible values are as follows:
0
The currency symbol and quantity should be surrounded by parentheses.
1
Print the sign string before the quantity and currency symbol.
2
Print the sign string after the quantity and currency symbol.
3
Print the sign string right before the currency symbol.
4
Print the sign string right after the currency symbol.
CHAR_MAX
“Unspecified”. Both members have this value in the standard `C' locale.

The ISO standard doesn't say what you should do when the value is CHAR_MAX. We recommend you print the sign after the currency symbol.

The members with the int_ prefix apply to the int_curr_symbol while the other two apply to currency_symbol.

Previous: The Lame Way to Locale Data, Up: Locale Information

7.6.2 Pinpoint Access to Locale Data

When writing the X/Open Portability Guide the authors realized that the localeconv function is not enough to provide reasonable access to locale information. The information which was meant to be available in the locale (as later specified in the POSIX.1 standard) requires more ways to access it. Therefore the nl_langinfo function was introduced.

— Function: char * nl_langinfo (nl_item item)

The nl_langinfo function can be used to access individual elements of the locale categories. Unlike the localeconv function, which returns all the information, nl_langinfo lets the caller select what information it requires. This is very fast and it is not a problem to call this function multiple times.

A second advantage is that in addition to the numeric and monetary formatting information, information from the LC_TIME and LC_MESSAGES categories is available.

The type nl_type is defined in nl_types.h. The argument item is a numeric value defined in the header langinfo.h. The X/Open standard defines the following values:

CODESET
nl_langinfo returns a string with the name of the coded character set used in the selected locale.
ABDAY_1
ABDAY_2
ABDAY_3
ABDAY_4
ABDAY_5
ABDAY_6
ABDAY_7
nl_langinfo returns the abbreviated weekday name. ABDAY_1 corresponds to Sunday.
DAY_1
DAY_2
DAY_3
DAY_4
DAY_5
DAY_6
DAY_7
Similar to ABDAY_1 etc., but here the return value is the unabbreviated weekday name.
ABMON_1
ABMON_2
ABMON_3
ABMON_4
ABMON_5
ABMON_6
ABMON_7
ABMON_8
ABMON_9
ABMON_10
ABMON_11
ABMON_12
The return value is abbreviated name of the month. ABMON_1 corresponds to January.
MON_1
MON_2
MON_3
MON_4
MON_5
MON_6
MON_7
MON_8
MON_9
MON_10
MON_11
MON_12
Similar to ABMON_1 etc., but here the month names are not abbreviated. Here the first value MON_1 also corresponds to January.
AM_STR
PM_STR
The return values are strings which can be used in the representation of time as an hour from 1 to 12 plus an am/pm specifier.

Note that in locales which do not use this time representation these strings might be empty, in which case the am/pm format cannot be used at all.

D_T_FMT
The return value can be used as a format string for strftime to represent time and date in a locale-specific way.
D_FMT
The return value can be used as a format string for strftime to represent a date in a locale-specific way.
T_FMT
The return value can be used as a format string for strftime to represent time in a locale-specific way.
T_FMT_AMPM
The return value can be used as a format string for strftime to represent time in the am/pm format.

Note that if the am/pm format does not make any sense for the selected locale, the return value might be the same as the one for T_FMT.

ERA
The return value represents the era used in the current locale.

Most locales do not define this value. An example of a locale which does define this value is the Japanese one. In Japan, the traditional representation of dates includes the name of the era corresponding to the then-emperor's reign.

Normally it should not be necessary to use this value directly. Specifying the E modifier in their format strings causes the strftime functions to use this information. The format of the returned string is not specified, and therefore you should not assume knowledge of it on different systems.

ERA_YEAR
The return value gives the year in the relevant era of the locale. As for ERA it should not be necessary to use this value directly.
ERA_D_T_FMT
This return value can be used as a format string for strftime to represent dates and times in a locale-specific era-based way.
ERA_D_FMT
This return value can be used as a format string for strftime to represent a date in a locale-specific era-based way.
ERA_T_FMT
This return value can be used as a format string for strftime to represent time in a locale-specific era-based way.
ALT_DIGITS
The return value is a representation of up to 100 values used to represent the values 0 to 99. As for ERA this value is not intended to be used directly, but instead indirectly through the strftime function. When the modifier O is used in a format which would otherwise use numerals to represent hours, minutes, seconds, weekdays, months, or weeks, the appropriate value for the locale is used instead.
INT_CURR_SYMBOL
The same as the value returned by localeconv in the int_curr_symbol element of the struct lconv.
CURRENCY_SYMBOL
CRNCYSTR
The same as the value returned by localeconv in the currency_symbol element of the struct lconv.

CRNCYSTR is a deprecated alias still required by Unix98.

MON_DECIMAL_POINT
The same as the value returned by localeconv in the mon_decimal_point element of the struct lconv.
MON_THOUSANDS_SEP
The same as the value returned by localeconv in the mon_thousands_sep element of the struct lconv.
MON_GROUPING
The same as the value returned by localeconv in the mon_grouping element of the struct lconv.
POSITIVE_SIGN
The same as the value returned by localeconv in the positive_sign element of the struct lconv.
NEGATIVE_SIGN
The same as the value returned by localeconv in the negative_sign element of the struct lconv.
INT_FRAC_DIGITS
The same as the value returned by localeconv in the int_frac_digits element of the struct lconv.
FRAC_DIGITS
The same as the value returned by localeconv in the frac_digits element of the struct lconv.
P_CS_PRECEDES
The same as the value returned by localeconv in the p_cs_precedes element of the struct lconv.
P_SEP_BY_SPACE
The same as the value returned by localeconv in the p_sep_by_space element of the struct lconv.
N_CS_PRECEDES
The same as the value returned by localeconv in the n_cs_precedes element of the struct lconv.
N_SEP_BY_SPACE
The same as the value returned by localeconv in the n_sep_by_space element of the struct lconv.
P_SIGN_POSN
The same as the value returned by localeconv in the p_sign_posn element of the struct lconv.
N_SIGN_POSN
The same as the value returned by localeconv in the n_sign_posn element of the struct lconv.
INT_P_CS_PRECEDES
The same as the value returned by localeconv in the int_p_cs_precedes element of the struct lconv.
INT_P_SEP_BY_SPACE
The same as the value returned by localeconv in the int_p_sep_by_space element of the struct lconv.
INT_N_CS_PRECEDES
The same as the value returned by localeconv in the int_n_cs_precedes element of the struct lconv.
INT_N_SEP_BY_SPACE
The same as the value returned by localeconv in the int_n_sep_by_space element of the struct lconv.
INT_P_SIGN_POSN
The same as the value returned by localeconv in the int_p_sign_posn element of the struct lconv.
INT_N_SIGN_POSN
The same as the value returned by localeconv in the int_n_sign_posn element of the struct lconv.
DECIMAL_POINT
RADIXCHAR
The same as the value returned by localeconv in the decimal_point element of the struct lconv.

The name RADIXCHAR is a deprecated alias still used in Unix98.

THOUSANDS_SEP
THOUSEP
The same as the value returned by localeconv in the thousands_sep element of the struct lconv.

The name THOUSEP is a deprecated alias still used in Unix98.

GROUPING
The same as the value returned by localeconv in the grouping element of the struct lconv.
YESEXPR
The return value is a regular expression which can be used with the regex function to recognize a positive response to a yes/no question. The GNU C library provides the rpmatch function for easier handling in applications.
NOEXPR
The return value is a regular expression which can be used with the regex function to recognize a negative response to a yes/no question.
YESSTR
The return value is a locale-specific translation of the positive response to a yes/no question.

Using this value is deprecated since it is a very special case of message translation, and is better handled by the message translation functions (see Message Translation).

The use of this symbol is deprecated. Instead message translation should be used.

NOSTR
The return value is a locale-specific translation of the negative response to a yes/no question. What is said for YESSTR is also true here.

The use of this symbol is deprecated. Instead message translation should be used.

The file langinfo.h defines a lot more symbols but none of them is official. Using them is not portable, and the format of the return values might change. Therefore we recommended you not use them.

Note that the return value for any valid argument can be used for in all situations (with the possible exception of the am/pm time formatting codes). If the user has not selected any locale for the appropriate category, nl_langinfo returns the information from the "C" locale. It is therefore possible to use this function as shown in the example below.

If the argument item is not valid, a pointer to an empty string is returned.

An example of nl_langinfo usage is a function which has to print a given date and time in a locale-specific way. At first one might think that, since strftime internally uses the locale information, writing something like the following is enough: 

     size_t
     i18n_time_n_data (char *s, size_t len, const struct tm *tp)
     {
       return strftime (s, len, "%X %D", tp);
     }

The format contains no weekday or month names and therefore is internationally usable. Wrong! The output produced is something like "hh:mm:ss MM/DD/YY". This format is only recognizable in the USA. Other countries use different formats. Therefore the function should be rewritten like this: 

     size_t
     i18n_time_n_data (char *s, size_t len, const struct tm *tp)
     {
       return strftime (s, len, nl_langinfo (D_T_FMT), tp);
     }

Now it uses the date and time format of the locale selected when the program runs. If the user selects the locale correctly there should never be a misunderstanding over the time and date format.

Next: , Previous: Locale Information, Up: Locales

7.7 A dedicated function to format numbers

We have seen that the structure returned by localeconv as well as the values given to nl_langinfo allow you to retrieve the various pieces of locale-specific information to format numbers and monetary amounts. We have also seen that the underlying rules are quite complex.

Therefore the X/Open standards introduce a function which uses such locale information, making it easier for the user to format numbers according to these rules.

— Function: ssize_t strfmon (char *s, size_t maxsize, const char *format, ...)

The strfmon function is similar to the strftime function in that it takes a buffer, its size, a format string, and values to write into the buffer as text in a form specified by the format string. Like strftime, the function also returns the number of bytes written into the buffer.

There are two differences: strfmon can take more than one argument, and, of course, the format specification is different. Like strftime, the format string consists of normal text, which is output as is, and format specifiers, which are indicated by a `%'. Immediately after the `%', you can optionally specify various flags and formatting information before the main formatting character, in a similar way to printf:

The next part of a specification is an optional field width. If no width is specified 0 is taken. During output, the function first determines how much space is required. If it requires at least as many characters as given by the field width, it is output using as much space as necessary. Otherwise, it is extended to use the full width by filling with the space character. The presence or absence of the `-' flag determines the side at which such padding occurs. If present, the spaces are added at the right making the output left-justified, and vice versa.

So far the format looks familiar, being similar to the printf and strftime formats. However, the next two optional fields introduce something new. The first one is a `#' character followed by a decimal digit string. The value of the digit string specifies the number of digit positions to the left of the decimal point (or equivalent). This does not include the grouping character when the `^' flag is not given. If the space needed to print the number does not fill the whole width, the field is padded at the left side with the fill character, which can be selected using the `=' flag and by default is a space. For example, if the field width is selected as 6 and the number is 123, the fill character is `*' the result will be `***123'.

The second optional field starts with a `.' (period) and consists of another decimal digit string. Its value describes the number of characters printed after the decimal point. The default is selected from the current locale (frac_digits, int_frac_digits, see see General Numeric). If the exact representation needs more digits than given by the field width, the displayed value is rounded. If the number of fractional digits is selected to be zero, no decimal point is printed.

As a GNU extension, the strfmon implementation in the GNU libc allows an optional `L' next as a format modifier. If this modifier is given, the argument is expected to be a long double instead of a double value.

Finally, the last component is a format specifier. There are three specifiers defined:

`i'
Use the locale's rules for formatting an international currency value.
`n'
Use the locale's rules for formatting a national currency value.
`%'
Place a `%' in the output. There must be no flag, width specifier or modifier given, only `%%' is allowed.

As for printf, the function reads the format string from left to right and uses the values passed to the function following the format string. The values are expected to be either of type double or long double, depending on the presence of the modifier `L'. The result is stored in the buffer pointed to by s. At most maxsize characters are stored.

The return value of the function is the number of characters stored in s, including the terminating NULL byte. If the number of characters stored would exceed maxsize, the function returns -1 and the content of the buffer s is unspecified. In this case errno is set to E2BIG.

A few examples should make clear how the function works. It is assumed that all the following pieces of code are executed in a program which uses the USA locale (en_US). The simplest form of the format is this: 

     strfmon (buf, 100, "@%n@%n@%n@", 123.45, -567.89, 12345.678);

The output produced is 

     "@$123.45@-$567.89@$12,345.68@"

We can notice several things here. First, the widths of the output numbers are different. We have not specified a width in the format string, and so this is no wonder. Second, the third number is printed using thousands separators. The thousands separator for the en_US locale is a comma. The number is also rounded. .678 is rounded to .68 since the format does not specify a precision and the default value in the locale is 2. Finally, note that the national currency symbol is printed since `%n' was used, not `i'. The next example shows how we can align the output. 

     strfmon (buf, 100, "@%=*11n@%=*11n@%=*11n@", 123.45, -567.89, 12345.678);

The output this time is: 

     "@    $123.45@   -$567.89@ $12,345.68@"

Two things stand out. Firstly, all fields have the same width (eleven characters) since this is the width given in the format and since no number required more characters to be printed. The second important point is that the fill character is not used. This is correct since the white space was not used to achieve a precision given by a `#' modifier, but instead to fill to the given width. The difference becomes obvious if we now add a width specification. 

     strfmon (buf, 100, "@%=*11#5n@%=*11#5n@%=*11#5n@",
              123.45, -567.89, 12345.678);

The output is 

     "@ $***123.45@-$***567.89@ $12,456.68@"

Here we can see that all the currency symbols are now aligned, and that the space between the currency sign and the number is filled with the selected fill character. Note that although the width is selected to be 5 and 123.45 has three digits left of the decimal point, the space is filled with three asterisks. This is correct since, as explained above, the width does not include the positions used to store thousands separators. One last example should explain the remaining functionality. 

     strfmon (buf, 100, "@%=0(16#5.3i@%=0(16#5.3i@%=0(16#5.3i@",
              123.45, -567.89, 12345.678);

This rather complex format string produces the following output: 

     "@ USD 000123,450 @(USD 000567.890)@ USD 12,345.678 @"

The most noticeable change is the alternative way of representing negative numbers. In financial circles this is often done using parentheses, and this is what the `(' flag selected. The fill character is now `0'. Note that this `0' character is not regarded as a numeric zero, and therefore the first and second numbers are not printed using a thousands separator. Since we used the format specifier `i' instead of `n', the international form of the currency symbol is used. This is a four letter string, in this case "USD ". The last point is that since the precision right of the decimal point is selected to be three, the first and second numbers are printed with an extra zero at the end and the third number is printed without rounding.

Previous: Formatting Numbers, Up: Locales

7.8 Yes-or-No Questions

Some non GUI programs ask a yes-or-no question. If the messages (especially the questions) are translated into foreign languages, be sure that you localize the answers too. It would be very bad habit to ask a question in one language and request the answer in another, often English.

The GNU C library contains rpmatch to give applications easy access to the corresponding locale definitions.

— Function: int rpmatch (const char *response)

The function rpmatch checks the string in response whether or not it is a correct yes-or-no answer and if yes, which one. The check uses the YESEXPR and NOEXPR data in the LC_MESSAGES category of the currently selected locale. The return value is as follows:

1
The user entered an affirmative answer.
0
The user entered a negative answer.
-1
The answer matched neither the YESEXPR nor the NOEXPR regular expression.

This function is not standardized but available beside in GNU libc at least also in the IBM AIX library.

This function would normally be used like this: 

       ...
       /* Use a safe default.  */
       _Bool doit = false;
     
       fputs (gettext ("Do you really want to do this? "), stdout);
       fflush (stdout);
       /* Prepare the getline call.  */
       line = NULL;
       len = 0;
       while (getline (&line, &len, stdout) >= 0)
         {
           /* Check the response.  */
           int res = rpmatch (line);
           if (res >= 0)
             {
               /* We got a definitive answer.  */
               if (res > 0)
                 doit = true;
               break;
             }
         }
       /* Free what getline allocated.  */
       free (line);

Note that the loop continues until an read error is detected or until a definitive (positive or negative) answer is read.

Next: , Previous: Locales, Up: Top

8 Message Translation

The program's interface with the human should be designed in a way to ease the human the task. One of the possibilities is to use messages in whatever language the user prefers.

Printing messages in different languages can be implemented in different ways. One could add all the different languages in the source code and add among the variants every time a message has to be printed. This is certainly no good solution since extending the set of languages is difficult (the code must be changed) and the code itself can become really big with dozens of message sets.

A better solution is to keep the message sets for each language are kept in separate files which are loaded at runtime depending on the language selection of the user.

The GNU C Library provides two different sets of functions to support message translation. The problem is that neither of the interfaces is officially defined by the POSIX standard. The catgets family of functions is defined in the X/Open standard but this is derived from industry decisions and therefore not necessarily based on reasonable decisions.

As mentioned above the message catalog handling provides easy extendibility by using external data files which contain the message translations. I.e., these files contain for each of the messages used in the program a translation for the appropriate language. So the tasks of the message handling functions are

The two approaches mainly differ in the implementation of this last step. The design decisions made for this influences the whole rest.

Next: , Up: Message Translation

8.1 X/Open Message Catalog Handling

The catgets functions are based on the simple scheme:

Associate every message to translate in the source code with a unique identifier. To retrieve a message from a catalog file solely the identifier is used.

This means for the author of the program that s/he will have to make sure the meaning of the identifier in the program code and in the message catalogs are always the same.

Before a message can be translated the catalog file must be located. The user of the program must be able to guide the responsible function to find whatever catalog the user wants. This is separated from what the programmer had in mind.

All the types, constants and functions for the catgets functions are defined/declared in the nl_types.h header file.

Next: , Up: Message catalogs a la X/Open

8.1.1 The catgets function family

— Function: nl_catd catopen (const char *cat_name, int flag)

The catgets function tries to locate the message data file names cat_name and loads it when found. The return value is of an opaque type and can be used in calls to the other functions to refer to this loaded catalog.

The return value is (nl_catd) -1 in case the function failed and no catalog was loaded. The global variable errno contains a code for the error causing the failure. But even if the function call succeeded this does not mean that all messages can be translated.

Locating the catalog file must happen in a way which lets the user of the program influence the decision. It is up to the user to decide about the language to use and sometimes it is useful to use alternate catalog files. All this can be specified by the user by setting some environment variables.

The first problem is to find out where all the message catalogs are stored. Every program could have its own place to keep all the different files but usually the catalog files are grouped by languages and the catalogs for all programs are kept in the same place.

To tell the catopen function where the catalog for the program can be found the user can set the environment variable NLSPATH to a value which describes her/his choice. Since this value must be usable for different languages and locales it cannot be a simple string. Instead it is a format string (similar to printf's). An example is 

          /usr/share/locale/%L/%N:/usr/share/locale/%L/LC_MESSAGES/%N

First one can see that more than one directory can be specified (with the usual syntax of separating them by colons). The next things to observe are the format string, %L and %N in this case. The catopen function knows about several of them and the replacement for all of them is of course different.

%N
This format element is substituted with the name of the catalog file. This is the value of the cat_name argument given to catgets.
%L
This format element is substituted with the name of the currently selected locale for translating messages. How this is determined is explained below.
%l
(This is the lowercase ell.) This format element is substituted with the language element of the locale name. The string describing the selected locale is expected to have the form lang[_terr[.codeset]] and this format uses the first part lang.
%t
This format element is substituted by the territory part terr of the name of the currently selected locale. See the explanation of the format above.
%c
This format element is substituted by the codeset part codeset of the name of the currently selected locale. See the explanation of the format above.
%%
Since % is used in a meta character there must be a way to express the % character in the result itself. Using %% does this just like it works for printf.

Using NLSPATH allows arbitrary directories to be searched for message catalogs while still allowing different languages to be used. If the NLSPATH environment variable is not set, the default value is 

          prefix/share/locale/%L/%N:prefix/share/locale/%L/LC_MESSAGES/%N

where prefix is given to configure while installing the GNU C Library (this value is in many cases /usr or the empty string).

The remaining problem is to decide which must be used. The value decides about the substitution of the format elements mentioned above. First of all the user can specify a path in the message catalog name (i.e., the name contains a slash character). In this situation the NLSPATH environment variable is not used. The catalog must exist as specified in the program, perhaps relative to the current working directory. This situation in not desirable and catalogs names never should be written this way. Beside this, this behavior is not portable to all other platforms providing the catgets interface.

Otherwise the values of environment variables from the standard environment are examined (see Standard Environment). Which variables are examined is decided by the flag parameter of catopen. If the value is NL_CAT_LOCALE (which is defined in nl_types.h) then the catopen function use the name of the locale currently selected for the LC_MESSAGES category.

If flag is zero the LANG environment variable is examined. This is a left-over from the early days where the concept of the locales had not even reached the level of POSIX locales.

The environment variable and the locale name should have a value of the form lang[_terr[.codeset]] as explained above. If no environment variable is set the "C" locale is used which prevents any translation.

The return value of the function is in any case a valid string. Either it is a translation from a message catalog or it is the same as the string parameter. So a piece of code to decide whether a translation actually happened must look like this: 

          {
            char *trans = catgets (desc, set, msg, input_string);
            if (trans == input_string)
              {
                /* Something went wrong.  */
              }
          }

When an error occurred the global variable errno is set to

EBADF
The catalog does not exist.
ENOMSG
The set/message tuple does not name an existing element in the message catalog.

While it sometimes can be useful to test for errors programs normally will avoid any test. If the translation is not available it is no big problem if the original, untranslated message is printed. Either the user understands this as well or s/he will look for the reason why the messages are not translated.

Please note that the currently selected locale does not depend on a call to the setlocale function. It is not necessary that the locale data files for this locale exist and calling setlocale succeeds. The catopen function directly reads the values of the environment variables.

— Function: char * catgets (nl_catd catalog_desc, int set, int message, const char *string)

The function catgets has to be used to access the massage catalog previously opened using the catopen function. The catalog_desc parameter must be a value previously returned by catopen.

The next two parameters, set and message, reflect the internal organization of the message catalog files. This will be explained in detail below. For now it is interesting to know that a catalog can consists of several set and the messages in each thread are individually numbered using numbers. Neither the set number nor the message number must be consecutive. They can be arbitrarily chosen. But each message (unless equal to another one) must have its own unique pair of set and message number.

Since it is not guaranteed that the message catalog for the language selected by the user exists the last parameter string helps to handle this case gracefully. If no matching string can be found string is returned. This means for the programmer that

It is somewhat uncomfortable to write a program using the catgets functions if no supporting functionality is available. Since each set/message number tuple must be unique the programmer must keep lists of the messages at the same time the code is written. And the work between several people working on the same project must be coordinated. We will see some how these problems can be relaxed a bit (see Common Usage).

— Function: int catclose (nl_catd catalog_desc)

The catclose function can be used to free the resources associated with a message catalog which previously was opened by a call to catopen. If the resources can be successfully freed the function returns 0. Otherwise it return −1 and the global variable errno is set. Errors can occur if the catalog descriptor catalog_desc is not valid in which case errno is set to EBADF.

Next: , Previous: The catgets Functions, Up: Message catalogs a la X/Open

8.1.2 Format of the message catalog files

The only reasonable way the translate all the messages of a function and store the result in a message catalog file which can be read by the catopen function is to write all the message text to the translator and let her/him translate them all. I.e., we must have a file with entries which associate the set/message tuple with a specific translation. This file format is specified in the X/Open standard and is as follows:

Important: The handling of identifiers instead of numbers for the set and messages is a GNU extension. Systems strictly following the X/Open specification do not have this feature. An example for a message catalog file is this: 

     $ This is a leading comment.
     $quote "
     
     $set SetOne
     1 Message with ID 1.
     two "   Message with ID \"two\", which gets the value 2 assigned"
     
     $set SetTwo
     $ Since the last set got the number 1 assigned this set has number 2.
     4000 "The numbers can be arbitrary, they need not start at one."

This small example shows various aspects:

While this file format is pretty easy it is not the best possible for use in a running program. The catopen function would have to parser the file and handle syntactic errors gracefully. This is not so easy and the whole process is pretty slow. Therefore the catgets functions expect the data in another more compact and ready-to-use file format. There is a special program gencat which is explained in detail in the next section.

Files in this other format are not human readable. To be easy to use by programs it is a binary file. But the format is byte order independent so translation files can be shared by systems of arbitrary architecture (as long as they use the GNU C Library).

Details about the binary file format are not important to know since these files are always created by the gencat program. The sources of the GNU C Library also provide the sources for the gencat program and so the interested reader can look through these source files to learn about the file format.

Next: , Previous: The message catalog files, Up: Message catalogs a la X/Open

8.1.3 Generate Message Catalogs files

The gencat program is specified in the X/Open standard and the GNU implementation follows this specification and so processes all correctly formed input files. Additionally some extension are implemented which help to work in a more reasonable way with the catgets functions.

The gencat program can be invoked in two ways: 

     `gencat [Option]... [Output-File [Input-File]...]`

This is the interface defined in the X/Open standard. If no Input-File parameter is given input will be read from standard input. Multiple input files will be read as if they are concatenated. If Output-File is also missing, the output will be written to standard output. To provide the interface one is used to from other programs a second interface is provided. 

     `gencat [Option]... -o Output-File [Input-File]...`

The option `-o' is used to specify the output file and all file arguments are used as input files.

Beside this one can use - or /dev/stdin for Input-File to denote the standard input. Corresponding one can use - and /dev/stdout for Output-File to denote standard output. Using - as a file name is allowed in X/Open while using the device names is a GNU extension.

The gencat program works by concatenating all input files and then merge the resulting collection of message sets with a possibly existing output file. This is done by removing all messages with set/message number tuples matching any of the generated messages from the output file and then adding all the new messages. To regenerate a catalog file while ignoring the old contents therefore requires to remove the output file if it exists. If the output is written to standard output no merging takes place.

The following table shows the options understood by the gencat program. The X/Open standard does not specify any option for the program so all of these are GNU extensions.

`-V'
`--version'
Print the version information and exit.
`-h'
`--help'
Print a usage message listing all available options, then exit successfully.
`--new'
Do never merge the new messages from the input files with the old content of the output files. The old content of the output file is discarded.
`-H'
`--header=name'
This option is used to emit the symbolic names given to sets and messages in the input files for use in the program. Details about how to use this are given in the next section. The name parameter to this option specifies the name of the output file. It will contain a number of C preprocessor #defines to associate a name with a number.

Please note that the generated file only contains the symbols from the input files. If the output is merged with the previous content of the output file the possibly existing symbols from the file(s) which generated the old output files are not in the generated header file.

Previous: The gencat program, Up: Message catalogs a la X/Open

8.1.4 How to use the catgets interface

The catgets functions can be used in two different ways. By following slavishly the X/Open specs and not relying on the extension and by using the GNU extensions. We will take a look at the former method first to understand the benefits of extensions.

8.1.4.1 Not using symbolic names

Since the X/Open format of the message catalog files does not allow symbol names we have to work with numbers all the time. When we start writing a program we have to replace all appearances of translatable strings with something like 

     catgets (catdesc, set, msg, "string")

catgets is retrieved from a call to catopen which is normally done once at the program start. The "string" is the string we want to translate. The problems start with the set and message numbers.

In a bigger program several programmers usually work at the same time on the program and so coordinating the number allocation is crucial. Though no two different strings must be indexed by the same tuple of numbers it is highly desirable to reuse the numbers for equal strings with equal translations (please note that there might be strings which are equal in one language but have different translations due to difference contexts).

The allocation process can be relaxed a bit by different set numbers for different parts of the program. So the number of developers who have to coordinate the allocation can be reduced. But still lists must be keep track of the allocation and errors can easily happen. These errors cannot be discovered by the compiler or the catgets functions. Only the user of the program might see wrong messages printed. In the worst cases the messages are so irritating that they cannot be recognized as wrong. Think about the translations for "true" and "false" being exchanged. This could result in a disaster.

8.1.4.2 Using symbolic names

The problems mentioned in the last section derive from the fact that:

  1. the numbers are allocated once and due to the possibly frequent use of them it is difficult to change a number later.
  2. the numbers do not allow to guess anything about the string and therefore collisions can easily happen.

By constantly using symbolic names and by providing a method which maps the string content to a symbolic name (however this will happen) one can prevent both problems above. The cost of this is that the programmer has to write a complete message catalog file while s/he is writing the program itself.

This is necessary since the symbolic names must be mapped to numbers before the program sources can be compiled. In the last section it was described how to generate a header containing the mapping of the names. E.g., for the example message file given in the last section we could call the gencat program as follow (assume ex.msg contains the sources). 

     gencat -H ex.h -o ex.cat ex.msg

This generates a header file with the following content: 

     #define SetTwoSet 0x2   /* ex.msg:8 */
     
     #define SetOneSet 0x1   /* ex.msg:4 */
     #define SetOnetwo 0x2   /* ex.msg:6 */

As can be seen the various symbols given in the source file are mangled to generate unique identifiers and these identifiers get numbers assigned. Reading the source file and knowing about the rules will allow to predict the content of the header file (it is deterministic) but this is not necessary. The gencat program can take care for everything. All the programmer has to do is to put the generated header file in the dependency list of the source files of her/his project and to add a rules to regenerate the header of any of the input files change.

One word about the symbol mangling. Every symbol consists of two parts: the name of the message set plus the name of the message or the special string Set. So SetOnetwo means this macro can be used to access the translation with identifier two in the message set SetOne.

The other names denote the names of the message sets. The special string Set is used in the place of the message identifier.

If in the code the second string of the set SetOne is used the C code should look like this: 

     catgets (catdesc, SetOneSet, SetOnetwo,
              "   Message with ID \"two\", which gets the value 2 assigned")

Writing the function this way will allow to change the message number and even the set number without requiring any change in the C source code. (The text of the string is normally not the same; this is only for this example.)

8.1.4.3 How does to this allow to develop

To illustrate the usual way to work with the symbolic version numbers here is a little example. Assume we want to write the very complex and famous greeting program. We start by writing the code as usual: 

     #include <stdio.h>
     int
     main (void)
     {
       printf ("Hello, world!\n");
       return 0;
     }

Now we want to internationalize the message and therefore replace the message with whatever the user wants. 

     #include <nl_types.h>
     #include <stdio.h>
     #include "msgnrs.h"
     int
     main (void)
     {
       nl_catd catdesc = catopen ("hello.cat", NL_CAT_LOCALE);
       printf (catgets (catdesc, SetMainSet, SetMainHello,
                        "Hello, world!\n"));
       catclose (catdesc);
       return 0;
     }

We see how the catalog object is opened and the returned descriptor used in the other function calls. It is not really necessary to check for failure of any of the functions since even in these situations the functions will behave reasonable. They simply will be return a translation.

What remains unspecified here are the constants SetMainSet and SetMainHello. These are the symbolic names describing the message. To get the actual definitions which match the information in the catalog file we have to create the message catalog source file and process it using the gencat program. 

     $ Messages for the famous greeting program.
     $quote "
     
     $set Main
     Hello "Hallo, Welt!\n"

Now we can start building the program (assume the message catalog source file is named hello.msg and the program source file hello.c): 

     

     % gencat -H msgnrs.h -o hello.cat hello.msg
     % cat msgnrs.h
     #define MainSet 0x1     /* hello.msg:4 */
     #define MainHello 0x1   /* hello.msg:5 */
     % gcc -o hello hello.c -I.
     % cp hello.cat /usr/share/locale/de/LC_MESSAGES
     % echo $LC_ALL
     de
     % ./hello
     Hallo, Welt!
     %

The call of the gencat program creates the missing header file msgnrs.h as well as the message catalog binary. The former is used in the compilation of hello.c while the later is placed in a directory in which the catopen function will try to locate it. Please check the LC_ALL environment variable and the default path for catopen presented in the description above.

Previous: Message catalogs a la X/Open, Up: Message Translation

8.2 The Uniforum approach to Message Translation

Sun Microsystems tried to standardize a different approach to message translation in the Uniforum group. There never was a real standard defined but still the interface was used in Sun's operation systems. Since this approach fits better in the development process of free software it is also used throughout the GNU project and the GNU gettext package provides support for this outside the GNU C Library.

The code of the libintl from GNU gettext is the same as the code in the GNU C Library. So the documentation in the GNU gettext manual is also valid for the functionality here. The following text will describe the library functions in detail. But the numerous helper programs are not described in this manual. Instead people should read the GNU gettext manual (see GNU gettext utilities). We will only give a short overview.

Though the catgets functions are available by default on more systems the gettext interface is at least as portable as the former. The GNU gettext package can be used wherever the functions are not available.

Next: , Up: The Uniforum approach

8.2.1 The gettext family of functions

The paradigms underlying the gettext approach to message translations is different from that of the catgets functions the basic functionally is equivalent. There are functions of the following categories:

Next: , Up: Message catalogs with gettext
8.2.1.1 What has to be done to translate a message?

The gettext functions have a very simple interface. The most basic function just takes the string which shall be translated as the argument and it returns the translation. This is fundamentally different from the catgets approach where an extra key is necessary and the original string is only used for the error case.

If the string which has to be translated is the only argument this of course means the string itself is the key. I.e., the translation will be selected based on the original string. The message catalogs must therefore contain the original strings plus one translation for any such string. The task of the gettext function is it to compare the argument string with the available strings in the catalog and return the appropriate translation. Of course this process is optimized so that this process is not more expensive than an access using an atomic key like in catgets.

The gettext approach has some advantages but also some disadvantages. Please see the GNU gettext manual for a detailed discussion of the pros and cons.

All the definitions and declarations for gettext can be found in the libintl.h header file. On systems where these functions are not part of the C library they can be found in a separate library named libintl.a (or accordingly different for shared libraries).

— Function: char * gettext (const char *msgid)

The gettext function searches the currently selected message catalogs for a string which is equal to msgid. If there is such a string available it is returned. Otherwise the argument string msgid is returned.

Please note that all though the return value is char * the returned string must not be changed. This broken type results from the history of the function and does not reflect the way the function should be used.

Please note that above we wrote “message catalogs” (plural). This is a specialty of the GNU implementation of these functions and we will say more about this when we talk about the ways message catalogs are selected (see Locating gettext catalog).

The gettext function does not modify the value of the global errno variable. This is necessary to make it possible to write something like 

            printf (gettext ("Operation failed: %m\n"));

Here the errno value is used in the printf function while processing the %m format element and if the gettext function would change this value (it is called before printf is called) we would get a wrong message.

So there is no easy way to detect a missing message catalog beside comparing the argument string with the result. But it is normally the task of the user to react on missing catalogs. The program cannot guess when a message catalog is really necessary since for a user who speaks the language the program was developed in does not need any translation.

The remaining two functions to access the message catalog add some functionality to select a message catalog which is not the default one. This is important if parts of the program are developed independently. Every part can have its own message catalog and all of them can be used at the same time. The C library itself is an example: internally it uses the gettext functions but since it must not depend on a currently selected default message catalog it must specify all ambiguous information.

— Function: char * dgettext (const char *domainname, const char *msgid)

The dgettext functions acts just like the gettext function. It only takes an additional first argument domainname which guides the selection of the message catalogs which are searched for the translation. If the domainname parameter is the null pointer the dgettext function is exactly equivalent to gettext since the default value for the domain name is used.

As for gettext the return value type is char * which is an anachronism. The returned string must never be modified.

— Function: char * dcgettext (const char *domainname, const char *msgid, int category)

The dcgettext adds another argument to those which dgettext takes. This argument category specifies the last piece of information needed to localize the message catalog. I.e., the domain name and the locale category exactly specify which message catalog has to be used (relative to a given directory, see below).

The dgettext function can be expressed in terms of dcgettext by using 

          dcgettext (domain, string, LC_MESSAGES)

instead of 

          dgettext (domain, string)

This also shows which values are expected for the third parameter. One has to use the available selectors for the categories available in locale.h. Normally the available values are LC_CTYPE, LC_COLLATE, LC_MESSAGES, LC_MONETARY, LC_NUMERIC, and LC_TIME. Please note that LC_ALL must not be used and even though the names might suggest this, there is no relation to the environments variables of this name.

The dcgettext function is only implemented for compatibility with other systems which have gettext functions. There is not really any situation where it is necessary (or useful) to use a different value but LC_MESSAGES in for the category parameter. We are dealing with messages here and any other choice can only be irritating.

As for gettext the return value type is char * which is an anachronism. The returned string must never be modified.

When using the three functions above in a program it is a frequent case that the msgid argument is a constant string. So it is worth to optimize this case. Thinking shortly about this one will realize that as long as no new message catalog is loaded the translation of a message will not change. This optimization is actually implemented by the gettext, dgettext and dcgettext functions.

Next: , Previous: Translation with gettext, Up: Message catalogs with gettext
8.2.1.2 How to determine which catalog to be used

The functions to retrieve the translations for a given message have a remarkable simple interface. But to provide the user of the program still the opportunity to select exactly the translation s/he wants and also to provide the programmer the possibility to influence the way to locate the search for catalogs files there is a quite complicated underlying mechanism which controls all this. The code is complicated the use is easy.

Basically we have two different tasks to perform which can also be performed by the catgets functions:

  1. Locate the set of message catalogs. There are a number of files for different languages and which all belong to the package. Usually they are all stored in the filesystem below a certain directory.

    There can be arbitrary many packages installed and they can follow different guidelines for the placement of their files.

  2. Relative to the location specified by the package the actual translation files must be searched, based on the wishes of the user. I.e., for each language the user selects the program should be able to locate the appropriate file.

This is the functionality required by the specifications for gettext and this is also what the catgets functions are able to do. But there are some problems unresolved:

We can divide the configuration actions in two parts: the one is performed by the programmer, the other by the user. We will start with the functions the programmer can use since the user configuration will be based on this.

As the functions described in the last sections already mention separate sets of messages can be selected by a domain name. This is a simple string which should be unique for each program part with uses a separate domain. It is possible to use in one program arbitrary many domains at the same time. E.g., the GNU C Library itself uses a domain named libc while the program using the C Library could use a domain named foo. The important point is that at any time exactly one domain is active. This is controlled with the following function.

— Function: char * textdomain (const char *domainname)

The textdomain function sets the default domain, which is used in all future gettext calls, to domainname. Please note that dgettext and dcgettext calls are not influenced if the domainname parameter of these functions is not the null pointer.

Before the first call to textdomain the default domain is messages. This is the name specified in the specification of the gettext API. This name is as good as any other name. No program should ever really use a domain with this name since this can only lead to problems.

The function returns the value which is from now on taken as the default domain. If the system went out of memory the returned value is NULL and the global variable errno is set to ENOMEM. Despite the return value type being char * the return string must not be changed. It is allocated internally by the textdomain function.

If the domainname parameter is the null pointer no new default domain is set. Instead the currently selected default domain is returned.

If the domainname parameter is the empty string the default domain is reset to its initial value, the domain with the name messages. This possibility is questionable to use since the domain messages really never should be used.

— Function: char * bindtextdomain (const char *domainname, const char *dirname)

The bindtextdomain function can be used to specify the directory which contains the message catalogs for domain domainname for the different languages. To be correct, this is the directory where the hierarchy of directories is expected. Details are explained below.

For the programmer it is important to note that the translations which come with the program have be placed in a directory hierarchy starting at, say, /foo/bar. Then the program should make a bindtextdomain call to bind the domain for the current program to this directory. So it is made sure the catalogs are found. A correctly running program does not depend on the user setting an environment variable.

The bindtextdomain function can be used several times and if the domainname argument is different the previously bound domains will not be overwritten.

If the program which wish to use bindtextdomain at some point of time use the chdir function to change the current working directory it is important that the dirname strings ought to be an absolute pathname. Otherwise the addressed directory might vary with the time.

If the dirname parameter is the null pointer bindtextdomain returns the currently selected directory for the domain with the name domainname.

The bindtextdomain function returns a pointer to a string containing the name of the selected directory name. The string is allocated internally in the function and must not be changed by the user. If the system went out of core during the execution of bindtextdomain the return value is NULL and the global variable errno is set accordingly.

Next: , Previous: Locating gettext catalog, Up: Message catalogs with gettext
8.2.1.3 Additional functions for more complicated situations

The functions of the gettext family described so far (and all the catgets functions as well) have one problem in the real world which have been neglected completely in all existing approaches. What is meant here is the handling of plural forms.

Looking through Unix source code before the time anybody thought about internationalization (and, sadly, even afterwards) one can often find code similar to the following: 

        printf ("%d file%s deleted", n, n == 1 ? "" : "s");

After the first complaints from people internationalizing the code people either completely avoided formulations like this or used strings like "file(s)". Both look unnatural and should be avoided. First tries to solve the problem correctly looked like this: 

        if (n == 1)
          printf ("%d file deleted", n);
        else
          printf ("%d files deleted", n);

But this does not solve the problem. It helps languages where the plural form of a noun is not simply constructed by adding an `s' but that is all. Once again people fell into the trap of believing the rules their language is using are universal. But the handling of plural forms differs widely between the language families. There are two things we can differ between (and even inside language families);

The consequence of this is that application writers should not try to solve the problem in their code. This would be localization since it is only usable for certain, hardcoded language environments. Instead the extended gettext interface should be used.

These extra functions are taking instead of the one key string two strings and an numerical argument. The idea behind this is that using the numerical argument and the first string as a key, the implementation can select using rules specified by the translator the right plural form. The two string arguments then will be used to provide a return value in case no message catalog is found (similar to the normal gettext behavior). In this case the rules for Germanic language is used and it is assumed that the first string argument is the singular form, the second the plural form.

This has the consequence that programs without language catalogs can display the correct strings only if the program itself is written using a Germanic language. This is a limitation but since the GNU C library (as well as the GNU gettext package) are written as part of the GNU package and the coding standards for the GNU project require program being written in English, this solution nevertheless fulfills its purpose.

— Function: char * ngettext (const char *msgid1, const char *msgid2, unsigned long int n)

The ngettext function is similar to the gettext function as it finds the message catalogs in the same way. But it takes two extra arguments. The msgid1 parameter must contain the singular form of the string to be converted. It is also used as the key for the search in the catalog. The msgid2 parameter is the plural form. The parameter n is used to determine the plural form. If no message catalog is found msgid1 is returned if n == 1, otherwise msgid2.

An example for the us of this function is: 

            printf (ngettext ("%d file removed", "%d files removed", n), n);

Please note that the numeric value n has to be passed to the printf function as well. It is not sufficient to pass it only to ngettext.

— Function: char * dngettext (const char *domain, const char *msgid1, const char *msgid2, unsigned long int n)

The dngettext is similar to the dgettext function in the way the message catalog is selected. The difference is that it takes two extra parameter to provide the correct plural form. These two parameters are handled in the same way ngettext handles them.

— Function: char * dcngettext (const char *domain, const char *msgid1, const char *msgid2, unsigned long int n, int category)

The dcngettext is similar to the dcgettext function in the way the message catalog is selected. The difference is that it takes two extra parameter to provide the correct plural form. These two parameters are handled in the same way ngettext handles them.

The problem of plural forms

A description of the problem can be found at the beginning of the last section. Now there is the question how to solve it. Without the input of linguists (which was not available) it was not possible to determine whether there are only a few different forms in which plural forms are formed or whether the number can increase with every new supported language.

Therefore the solution implemented is to allow the translator to specify the rules of how to select the plural form. Since the formula varies with every language this is the only viable solution except for hardcoding the information in the code (which still would require the possibility of extensions to not prevent the use of new languages). The details are explained in the GNU gettext manual. Here only a bit of information is provided.

The information about the plural form selection has to be stored in the header entry (the one with the empty (msgid string). It looks like this: 

     Plural-Forms: nplurals=2; plural=n == 1 ? 0 : 1;

The nplurals value must be a decimal number which specifies how many different plural forms exist for this language. The string following plural is an expression which is using the C language syntax. Exceptions are that no negative number are allowed, numbers must be decimal, and the only variable allowed is n. This expression will be evaluated whenever one of the functions ngettext, dngettext, or dcngettext is called. The numeric value passed to these functions is then substituted for all uses of the variable n in the expression. The resulting value then must be greater or equal to zero and smaller than the value given as the value of nplurals.

The following rules are known at this point. The language with families are listed. But this does not necessarily mean the information can be generalized for the whole family (as can be easily seen in the table below).1

Only one form:
Some languages only require one single form. There is no distinction between the singular and plural form. An appropriate header entry would look like this: 
          Plural-Forms: nplurals=1; plural=0;

Languages with this property include:

Finno-Ugric family
Hungarian
Asian family
Japanese, Korean
Turkic/Altaic family
Turkish

Two forms, singular used for one only
This is the form used in most existing programs since it is what English is using. A header entry would look like this: 
          Plural-Forms: nplurals=2; plural=n != 1;

(Note: this uses the feature of C expressions that boolean expressions have to value zero or one.)

Languages with this property include:

Germanic family
Danish, Dutch, English, German, Norwegian, Swedish
Finno-Ugric family
Estonian, Finnish
Latin/Greek family
Greek
Semitic family
Hebrew
Romance family
Italian, Portuguese, Spanish
Artificial
Esperanto

Two forms, singular used for zero and one
Exceptional case in the language family. The header entry would be: 
          Plural-Forms: nplurals=2; plural=n>1;

Languages with this property include:

Romanic family
French, Brazilian Portuguese

Three forms, special case for zero
The header entry would be: 
          Plural-Forms: nplurals=3; plural=n%10==1 && n%100!=11 ? 0 : n != 0 ? 1 : 2;

Languages with this property include:

Baltic family
Latvian

Three forms, special cases for one and two
The header entry would be: 
          Plural-Forms: nplurals=3; plural=n==1 ? 0 : n==2 ? 1 : 2;

Languages with this property include:

Celtic
Gaeilge (Irish)

Three forms, special case for numbers ending in 1[2-9]
The header entry would look like this: 
          Plural-Forms: nplurals=3; \
              plural=n%10==1 && n%100!=11 ? 0 : \
                     n%10>=2 && (n%100<10 || n%100>=20) ? 1 : 2;

Languages with this property include:

Baltic family
Lithuanian

Three forms, special cases for numbers ending in 1 and 2, 3, 4, except those ending in 1[1-4]
The header entry would look like this: 
          Plural-Forms: nplurals=3; \
              plural=n%100/10==1 ? 2 : n%10==1 ? 0 : (n+9)%10>3 ? 2 : 1;

Languages with this property include:

Slavic family
Croatian, Czech, Russian, Ukrainian

Three forms, special cases for 1 and 2, 3, 4
The header entry would look like this: 
          Plural-Forms: nplurals=3; \
              plural=(n==1) ? 1 : (n>=2 && n<=4) ? 2 : 0;

Languages with this property include:

Slavic family
Slovak

Three forms, special case for one and some numbers ending in 2, 3, or 4
The header entry would look like this: 
          Plural-Forms: nplurals=3; \
              plural=n==1 ? 0 : \
                     n%10>=2 && n%10<=4 && (n%100<10 || n%100>=20) ? 1 : 2;

Languages with this property include:

Slavic family
Polish

Four forms, special case for one and all numbers ending in 02, 03, or 04
The header entry would look like this: 
          Plural-Forms: nplurals=4; \
              plural=n%100==1 ? 0 : n%100==2 ? 1 : n%100==3 || n%100==4 ? 2 : 3;

Languages with this property include:

Slavic family
Slovenian

Next: , Previous: Advanced gettext functions, Up: Message catalogs with gettext
8.2.1.4 How to specify the output character set gettext uses

gettext not only looks up a translation in a message catalog. It also converts the translation on the fly to the desired output character set. This is useful if the user is working in a different character set than the translator who created the message catalog, because it avoids distributing variants of message catalogs which differ only in the character set.

The output character set is, by default, the value of nl_langinfo (CODESET), which depends on the LC_CTYPE part of the current locale. But programs which store strings in a locale independent way (e.g. UTF-8) can request that gettext and related functions return the translations in that encoding, by use of the bind_textdomain_codeset function.

Note that the msgid argument to gettext is not subject to character set conversion. Also, when gettext does not find a translation for msgid, it returns msgid unchanged – independently of the current output character set. It is therefore recommended that all msgids be US-ASCII strings.

— Function: char * bind_textdomain_codeset (const char *domainname, const char *codeset)

The bind_textdomain_codeset function can be used to specify the output character set for message catalogs for domain domainname. The codeset argument must be a valid codeset name which can be used for the iconv_open function, or a null pointer.

If the codeset parameter is the null pointer, bind_textdomain_codeset returns the currently selected codeset for the domain with the name domainname. It returns NULL if no codeset has yet been selected.

The bind_textdomain_codeset function can be used several times. If used multiple times with the same domainname argument, the later call overrides the settings made by the earlier one.

The bind_textdomain_codeset function returns a pointer to a string containing the name of the selected codeset. The string is allocated internally in the function and must not be changed by the user. If the system went out of core during the execution of bind_textdomain_codeset, the return value is NULL and the global variable errno is set accordingly.

Next: , Previous: Charset conversion in gettext, Up: Message catalogs with gettext
8.2.1.5 How to use gettext in GUI programs

One place where the gettext functions, if used normally, have big problems is within programs with graphical user interfaces (GUIs). The problem is that many of the strings which have to be translated are very short. They have to appear in pull-down menus which restricts the length. But strings which are not containing entire sentences or at least large fragments of a sentence may appear in more than one situation in the program but might have different translations. This is especially true for the one-word strings which are frequently used in GUI programs.

As a consequence many people say that the gettext approach is wrong and instead catgets should be used which indeed does not have this problem. But there is a very simple and powerful method to handle these kind of problems with the gettext functions.

As as example consider the following fictional situation. A GUI program has a menu bar with the following entries: 

     +------------+------------+--------------------------------------+
     | File       | Printer    |                                      |
     +------------+------------+--------------------------------------+
     | Open     | | Select   |
     | New      | | Open     |
     +----------+ | Connect  |
                  +----------+

To have the strings File, Printer, Open, New, Select, and Connect translated there has to be at some point in the code a call to a function of the gettext family. But in two places the string passed into the function would be Open. The translations might not be the same and therefore we are in the dilemma described above.

One solution to this problem is to artificially enlengthen the strings to make them unambiguous. But what would the program do if no translation is available? The enlengthened string is not what should be printed. So we should use a little bit modified version of the functions.

To enlengthen the strings a uniform method should be used. E.g., in the example above the strings could be chosen as 

     Menu|File
     Menu|Printer
     Menu|File|Open
     Menu|File|New
     Menu|Printer|Select
     Menu|Printer|Open
     Menu|Printer|Connect

Now all the strings are different and if now instead of gettext the following little wrapper function is used, everything works just fine: 

       char *
       sgettext (const char *msgid)
       {
         char *msgval = gettext (msgid);
         if (msgval == msgid)
           msgval = strrchr (msgid, '|') + 1;
         return msgval;
       }

What this little function does is to recognize the case when no translation is available. This can be done very efficiently by a pointer comparison since the return value is the input value. If there is no translation we know that the input string is in the format we used for the Menu entries and therefore contains a | character. We simply search for the last occurrence of this character and return a pointer to the character following it. That's it!

If one now consistently uses the enlengthened string form and replaces the gettext calls with calls to sgettext (this is normally limited to very few places in the GUI implementation) then it is possible to produce a program which can be internationalized.

With advanced compilers (such as GNU C) one can write the sgettext functions as an inline function or as a macro like this: 

     #define sgettext(msgid) \
       ({ const char *__msgid = (msgid);            \
          char *__msgstr = gettext (__msgid);       \
          if (__msgval == __msgid)                  \
            __msgval = strrchr (__msgid, '|') + 1;  \
          __msgval; })

The other gettext functions (dgettext, dcgettext and the ngettext equivalents) can and should have corresponding functions as well which look almost identical, except for the parameters and the call to the underlying function.

Now there is of course the question why such functions do not exist in the GNU C library? There are two parts of the answer to this question.

There is only one more comment to make left. The wrapper function above require that the translations strings are not enlengthened themselves. This is only logical. There is no need to disambiguate the strings (since they are never used as keys for a search) and one also saves quite some memory and disk space by doing this.

Previous: GUI program problems, Up: Message catalogs with gettext
8.2.1.6 User influence on gettext

The last sections described what the programmer can do to internationalize the messages of the program. But it is finally up to the user to select the message s/he wants to see. S/He must understand them.

The POSIX locale model uses the environment variables LC_COLLATE, LC_CTYPE, LC_MESSAGES, LC_MONETARY, LC_NUMERIC, and LC_TIME to select the locale which is to be used. This way the user can influence lots of functions. As we mentioned above the gettext functions also take advantage of this.

To understand how this happens it is necessary to take a look at the various components of the filename which gets computed to locate a message catalog. It is composed as follows: 

     dir_name/locale/LC_category/domain_name.mo

The default value for dir_name is system specific. It is computed from the value given as the prefix while configuring the C library. This value normally is /usr or /. For the former the complete dir_name is: 

     /usr/share/locale

We can use /usr/share since the .mo files containing the message catalogs are system independent, so all systems can use the same files. If the program executed the bindtextdomain function for the message domain that is currently handled, the dir_name component is exactly the value which was given to the function as the second parameter. I.e., bindtextdomain allows overwriting the only system dependent and fixed value to make it possible to address files anywhere in the filesystem.

The category is the name of the locale category which was selected in the program code. For gettext and dgettext this is always LC_MESSAGES, for dcgettext this is selected by the value of the third parameter. As said above it should be avoided to ever use a category other than LC_MESSAGES.

The locale component is computed based on the category used. Just like for the setlocale function here comes the user selection into the play. Some environment variables are examined in a fixed order and the first environment variable set determines the return value of the lookup process. In detail, for the category LC_xxx the following variables in this order are examined:

LANGUAGE
LC_ALL
LC_xxx
LANG

This looks very familiar. With the exception of the LANGUAGE environment variable this is exactly the lookup order the setlocale function uses. But why introducing the LANGUAGE variable?

The reason is that the syntax of the values these variables can have is different to what is expected by the setlocale function. If we would set LC_ALL to a value following the extended syntax that would mean the setlocale function will never be able to use the value of this variable as well. An additional variable removes this problem plus we can select the language independently of the locale setting which sometimes is useful.

While for the LC_xxx variables the value should consist of exactly one specification of a locale the LANGUAGE variable's value can consist of a colon separated list of locale names. The attentive reader will realize that this is the way we manage to implement one of our additional demands above: we want to be able to specify an ordered list of language.

Back to the constructed filename we have only one component missing. The domain_name part is the name which was either registered using the textdomain function or which was given to dgettext or dcgettext as the first parameter. Now it becomes obvious that a good choice for the domain name in the program code is a string which is closely related to the program/package name. E.g., for the GNU C Library the domain name is libc.

A limit piece of example code should show how the programmer is supposed to work: 

     {
       setlocale (LC_ALL, "");
       textdomain ("test-package");
       bindtextdomain ("test-package", "/usr/local/share/locale");
       puts (gettext ("Hello, world!"));
     }

At the program start the default domain is messages, and the default locale is "C". The setlocale call sets the locale according to the user's environment variables; remember that correct functioning of gettext relies on the correct setting of the LC_MESSAGES locale (for looking up the message catalog) and of the LC_CTYPE locale (for the character set conversion). The textdomain call changes the default domain to test-package. The bindtextdomain call specifies that the message catalogs for the domain test-package can be found below the directory /usr/local/share/locale.

If now the user set in her/his environment the variable LANGUAGE to de the gettext function will try to use the translations from the file 

     /usr/local/share/locale/de/LC_MESSAGES/test-package.mo

From the above descriptions it should be clear which component of this filename is determined by which source.

In the above example we assumed that the LANGUAGE environment variable to de. This might be an appropriate selection but what happens if the user wants to use LC_ALL because of the wider usability and here the required value is de_DE.ISO-8859-1? We already mentioned above that a situation like this is not infrequent. E.g., a person might prefer reading a dialect and if this is not available fall back on the standard language.

The gettext functions know about situations like this and can handle them gracefully. The functions recognize the format of the value of the environment variable. It can split the value is different pieces and by leaving out the only or the other part it can construct new values. This happens of course in a predictable way. To understand this one must know the format of the environment variable value. There is one more or less standardized form, originally from the X/Open specification:

language[_territory[.codeset]][@modifier]

Less specific locale names will be stripped of in the order of the following list:

  1. codeset
  2. normalized codeset
  3. territory
  4. modifier

The language field will never be dropped for obvious reasons.

The only new thing is the normalized codeset entry. This is another goodie which is introduced to help reducing the chaos which derives from the inability of the people to standardize the names of character sets. Instead of ISO-8859-1 one can often see 8859-1, 88591, iso8859-1, or iso_8859-1. The normalized codeset value is generated from the user-provided character set name by applying the following rules:

  1. Remove all characters beside numbers and letters.
  2. Fold letters to lowercase.
  3. If the same only contains digits prepend the string "iso".

So all of the above name will be normalized to iso88591. This allows the program user much more freely choosing the locale name.

Even this extended functionality still does not help to solve the problem that completely different names can be used to denote the same locale (e.g., de and german). To be of help in this situation the locale implementation and also the gettext functions know about aliases.

The file /usr/share/locale/locale.alias (replace /usr with whatever prefix you used for configuring the C library) contains a mapping of alternative names to more regular names. The system manager is free to add new entries to fill her/his own needs. The selected locale from the environment is compared with the entries in the first column of this file ignoring the case. If they match the value of the second column is used instead for the further handling.

In the description of the format of the environment variables we already mentioned the character set as a factor in the selection of the message catalog. In fact, only catalogs which contain text written using the character set of the system/program can be used (directly; there will come a solution for this some day). This means for the user that s/he will always have to take care for this. If in the collection of the message catalogs there are files for the same language but coded using different character sets the user has to be careful.

Previous: Message catalogs with gettext, Up: The Uniforum approach

8.2.2 Programs to handle message catalogs for gettext

The GNU C Library does not contain the source code for the programs to handle message catalogs for the gettext functions. As part of the GNU project the GNU gettext package contains everything the developer needs. The functionality provided by the tools in this package by far exceeds the abilities of the gencat program described above for the catgets functions.

There is a program msgfmt which is the equivalent program to the gencat program. It generates from the human-readable and -editable form of the message catalog a binary file which can be used by the gettext functions. But there are several more programs available.

The xgettext program can be used to automatically extract the translatable messages from a source file. I.e., the programmer need not take care for the translations and the list of messages which have to be translated. S/He will simply wrap the translatable string in calls to gettext et.al and the rest will be done by xgettext. This program has a lot of option which help to customize the output or do help to understand the input better.

Other programs help to manage development cycle when new messages appear in the source files or when a new translation of the messages appear. Here it should only be noted that using all the tools in GNU gettext it is possible to completely automate the handling of message catalog. Beside marking the translatable string in the source code and generating the translations the developers do not have anything to do themselves.

Next: , Previous: Message Translation, Up: Top

9 Searching and Sorting

This chapter describes functions for searching and sorting arrays of arbitrary objects. You pass the appropriate comparison function to be applied as an argument, along with the size of the objects in the array and the total number of elements.

Next: , Up: Searching and Sorting

9.1 Defining the Comparison Function

In order to use the sorted array library functions, you have to describe how to compare the elements of the array.

To do this, you supply a comparison function to compare two elements of the array. The library will call this function, passing as arguments pointers to two array elements to be compared. Your comparison function should return a value the way strcmp (see String/Array Comparison) does: negative if the first argument is “less” than the second, zero if they are “equal”, and positive if the first argument is “greater”.

Here is an example of a comparison function which works with an array of numbers of type double: 

     int
     compare_doubles (const void *a, const void *b)
     {
       const double *da = (const double *) a;
       const double *db = (const double *) b;
     
       return (*da > *db) - (*da < *db);
     }

The header file stdlib.h defines a name for the data type of comparison functions. This type is a GNU extension. 

     int comparison_fn_t (const void *, const void *);

Next: , Previous: Comparison Functions, Up: Searching and Sorting

9.2 Array Search Function

Generally searching for a specific element in an array means that potentially all elements must be checked. The GNU C library contains functions to perform linear search. The prototypes for the following two functions can be found in search.h.

— Function: void * lfind (const void *key, void *base, size_t *nmemb, size_t size, comparison_fn_t compar)

The lfind function searches in the array with *nmemb elements of size bytes pointed to by base for an element which matches the one pointed to by key. The function pointed to by compar is used decide whether two elements match.

The return value is a pointer to the matching element in the array starting at base if it is found. If no matching element is available NULL is returned.

The mean runtime of this function is *nmemb/2. This function should only be used if elements often get added to or deleted from the array in which case it might not be useful to sort the array before searching.

— Function: void * lsearch (const void *key, void *base, size_t *nmemb, size_t size, comparison_fn_t compar)

The lsearch function is similar to the lfind function. It searches the given array for an element and returns it if found. The difference is that if no matching element is found the lsearch function adds the object pointed to by key (with a size of size bytes) at the end of the array and it increments the value of *nmemb to reflect this addition.

This means for the caller that if it is not sure that the array contains the element one is searching for the memory allocated for the array starting at base must have room for at least size more bytes. If one is sure the element is in the array it is better to use lfind so having more room in the array is always necessary when calling lsearch.

To search a sorted array for an element matching the key, use the bsearch function. The prototype for this function is in the header file stdlib.h.

— Function: void * bsearch (const void *key, const void *array, size_t count, size_t size, comparison_fn_t compare)

The bsearch function searches the sorted array array for an object that is equivalent to key. The array contains count elements, each of which is of size size bytes.

The compare function is used to perform the comparison. This function is called with two pointer arguments and should return an integer less than, equal to, or greater than zero corresponding to whether its first argument is considered less than, equal to, or greater than its second argument. The elements of the array must already be sorted in ascending order according to this comparison function.

The return value is a pointer to the matching array element, or a null pointer if no match is found. If the array contains more than one element that matches, the one that is returned is unspecified.

This function derives its name from the fact that it is implemented using the binary search algorithm.

Next: , Previous: Array Search Function, Up: Searching and Sorting

9.3 Array Sort Function

To sort an array using an arbitrary comparison function, use the qsort function. The prototype for this function is in stdlib.h.

— Function: void qsort (void *array, size_t count, size_t size, comparison_fn_t compare)

The qsort function sorts the array array. The array contains count elements, each of which is of size size.

The compare function is used to perform the comparison on the array elements. This function is called with two pointer arguments and should return an integer less than, equal to, or greater than zero corresponding to whether its first argument is considered less than, equal to, or greater than its second argument.

Warning: If two objects compare as equal, their order after sorting is unpredictable. That is to say, the sorting is not stable. This can make a difference when the comparison considers only part of the elements. Two elements with the same sort key may differ in other respects.

If you want the effect of a stable sort, you can get this result by writing the comparison function so that, lacking other reason distinguish between two elements, it compares them by their addresses. Note that doing this may make the sorting algorithm less efficient, so do it only if necessary.

Here is a simple example of sorting an array of doubles in numerical order, using the comparison function defined above (see Comparison Functions): 

          {
            double *array;
            int size;
            ...
            qsort (array, size, sizeof (double), compare_doubles);
          }

The qsort function derives its name from the fact that it was originally implemented using the “quick sort” algorithm.

The implementation of qsort in this library might not be an in-place sort and might thereby use an extra amount of memory to store the array.

Next: , Previous: Array Sort Function, Up: Searching and Sorting

9.4 Searching and Sorting Example

Here is an example showing the use of qsort and bsearch with an array of structures. The objects in the array are sorted by comparing their name fields with the strcmp function. Then, we can look up individual objects based on their names. 

     #include <stdlib.h>
     #include <stdio.h>
     #include <string.h>
     
     /* Define an array of critters to sort. */
     
     struct critter
       {
         const char *name;
         const char *species;
       };
     
     struct critter muppets[] =
       {
         {"Kermit", "frog"},
         {"Piggy", "pig"},
         {"Gonzo", "whatever"},
         {"Fozzie", "bear"},
         {"Sam", "eagle"},
         {"Robin", "frog"},
         {"Animal", "animal"},
         {"Camilla", "chicken"},
         {"Sweetums", "monster"},
         {"Dr. Strangepork", "pig"},
         {"Link Hogthrob", "pig"},
         {"Zoot", "human"},
         {"Dr. Bunsen Honeydew", "human"},
         {"Beaker", "human"},
         {"Swedish Chef", "human"}
       };
     
     int count = sizeof (muppets) / sizeof (struct critter);
     
     
     
     /* This is the comparison function used for sorting and searching. */
     
     int
     critter_cmp (const struct critter *c1, const struct critter *c2)
     {
       return strcmp (c1->name, c2->name);
     }
     
     
     /* Print information about a critter. */
     
     void
     print_critter (const struct critter *c)
     {
       printf ("%s, the %s\n", c->name, c->species);
     }
     
     
     /* Do the lookup into the sorted array. */
     
     void
     find_critter (const char *name)
     {
       struct critter target, *result;
       target.name = name;
       result = bsearch (&target, muppets, count, sizeof (struct critter),
                         critter_cmp);
       if (result)
         print_critter (result);
       else
         printf ("Couldn't find %s.\n", name);
     }
     
     /* Main program. */
     
     int
     main (void)
     {
       int i;
     
       for (i = 0; i < count; i++)
         print_critter (&muppets[i]);
       printf ("\n");
     
       qsort (muppets, count, sizeof (struct critter), critter_cmp);
     
       for (i = 0; i < count; i++)
         print_critter (&muppets[i]);
       printf ("\n");
     
       find_critter ("Kermit");
       find_critter ("Gonzo");
       find_critter ("Janice");
     
       return 0;
     }

The output from this program looks like: 

     Kermit, the frog
     Piggy, the pig
     Gonzo, the whatever
     Fozzie, the bear
     Sam, the eagle
     Robin, the frog
     Animal, the animal
     Camilla, the chicken
     Sweetums, the monster
     Dr. Strangepork, the pig
     Link Hogthrob, the pig
     Zoot, the human
     Dr. Bunsen Honeydew, the human
     Beaker, the human
     Swedish Chef, the human
     
     Animal, the animal
     Beaker, the human
     Camilla, the chicken
     Dr. Bunsen Honeydew, the human
     Dr. Strangepork, the pig
     Fozzie, the bear
     Gonzo, the whatever
     Kermit, the frog
     Link Hogthrob, the pig
     Piggy, the pig
     Robin, the frog
     Sam, the eagle
     Swedish Chef, the human
     Sweetums, the monster
     Zoot, the human
     
     Kermit, the frog
     Gonzo, the whatever
     Couldn't find Janice.

Next: , Previous: Search/Sort Example, Up: Searching and Sorting

9.5 The hsearch function.

The functions mentioned so far in this chapter are for searching in a sorted or unsorted array. There are other methods to organize information which later should be searched. The costs of insert, delete and search differ. One possible implementation is using hashing tables. The following functions are declared in the header file search.h.

— Function: int hcreate (size_t nel)

The hcreate function creates a hashing table which can contain at least nel elements. There is no possibility to grow this table so it is necessary to choose the value for nel wisely. The method used to implement this function might make it necessary to make the number of elements in the hashing table larger than the expected maximal number of elements. Hashing tables usually work inefficiently if they are filled 80% or more. The constant access time guaranteed by hashing can only be achieved if few collisions exist. See Knuth's “The Art of Computer Programming, Part 3: Searching and Sorting” for more information.

The weakest aspect of this function is that there can be at most one hashing table used through the whole program. The table is allocated in local memory out of control of the programmer. As an extension the GNU C library provides an additional set of functions with an reentrant interface which provide a similar interface but which allow to keep arbitrarily many hashing tables.

It is possible to use more than one hashing table in the program run if the former table is first destroyed by a call to hdestroy.

The function returns a non-zero value if successful. If it return zero something went wrong. This could either mean there is already a hashing table in use or the program runs out of memory.

— Function: void hdestroy (void)

The hdestroy function can be used to free all the resources allocated in a previous call of hcreate. After a call to this function it is again possible to call hcreate and allocate a new table with possibly different size.

It is important to remember that the elements contained in the hashing table at the time hdestroy is called are not freed by this function. It is the responsibility of the program code to free those strings (if necessary at all). Freeing all the element memory is not possible without extra, separately kept information since there is no function to iterate through all available elements in the hashing table. If it is really necessary to free a table and all elements the programmer has to keep a list of all table elements and before calling hdestroy s/he has to free all element's data using this list. This is a very unpleasant mechanism and it also shows that this kind of hashing tables is mainly meant for tables which are created once and used until the end of the program run.

Entries of the hashing table and keys for the search are defined using this type:

— Data type: struct ENTRY

Both elements of this structure are pointers to zero-terminated strings. This is a limiting restriction of the functionality of the hsearch functions. They can only be used for data sets which use the NUL character always and solely to terminate the records. It is not possible to handle general binary data.

char *key
Pointer to a zero-terminated string of characters describing the key for the search or the element in the hashing table.
char *data
Pointer to a zero-terminated string of characters describing the data. If the functions will be called only for searching an existing entry this element might stay undefined since it is not used.

— Function: ENTRY * hsearch (ENTRY item, ACTION action)

To search in a hashing table created using hcreate the hsearch function must be used. This function can perform simple search for an element (if action has the FIND) or it can alternatively insert the key element into the hashing table. Entries are never replaced.

The key is denoted by a pointer to an object of type ENTRY. For locating the corresponding position in the hashing table only the key element of the structure is used.

If an entry with matching key is found the action parameter is irrelevant. The found entry is returned. If no matching entry is found and the action parameter has the value FIND the function returns a NULL pointer. If no entry is found and the action parameter has the value ENTER a new entry is added to the hashing table which is initialized with the parameter item. A pointer to the newly added entry is returned.

As mentioned before the hashing table used by the functions described so far is global and there can be at any time at most one hashing table in the program. A solution is to use the following functions which are a GNU extension. All have in common that they operate on a hashing table which is described by the content of an object of the type struct hsearch_data. This type should be treated as opaque, none of its members should be changed directly.

— Function: int hcreate_r (size_t nel, struct hsearch_data *htab)

The hcreate_r function initializes the object pointed to by htab to contain a hashing table with at least nel elements. So this function is equivalent to the hcreate function except that the initialized data structure is controlled by the user.

This allows having more than one hashing table at one time. The memory necessary for the struct hsearch_data object can be allocated dynamically. It must be initialized with zero before calling this function.

The return value is non-zero if the operation was successful. If the return value is zero, something went wrong, which probably means the programs ran out of memory.

— Function: void hdestroy_r (struct hsearch_data *htab)

The hdestroy_r function frees all resources allocated by the hcreate_r function for this very same object htab. As for hdestroy it is the programs responsibility to free the strings for the elements of the table.

— Function: int hsearch_r (ENTRY item, ACTION action, ENTRY **retval, struct hsearch_data *htab)

The hsearch_r function is equivalent to hsearch. The meaning of the first two arguments is identical. But instead of operating on a single global hashing table the function works on the table described by the object pointed to by htab (which is initialized by a call to hcreate_r).

Another difference to hcreate is that the pointer to the found entry in the table is not the return value of the functions. It is returned by storing it in a pointer variables pointed to by the retval parameter. The return value of the function is an integer value indicating success if it is non-zero and failure if it is zero. In the latter case the global variable errno signals the reason for the failure.

ENOMEM
The table is filled and hsearch_r was called with an so far unknown key and action set to ENTER.
ESRCH
The action parameter is FIND and no corresponding element is found in the table.

Previous: Hash Search Function, Up: Searching and Sorting

9.6 The tsearch function.

Another common form to organize data for efficient search is to use trees. The tsearch function family provides a nice interface to functions to organize possibly large amounts of data by providing a mean access time proportional to the logarithm of the number of elements. The GNU C library implementation even guarantees that this bound is never exceeded even for input data which cause problems for simple binary tree implementations.

The functions described in the chapter are all described in the System V and X/Open specifications and are therefore quite portable.

In contrast to the hsearch functions the tsearch functions can be used with arbitrary data and not only zero-terminated strings.

The tsearch functions have the advantage that no function to initialize data structures is necessary. A simple pointer of type void * initialized to NULL is a valid tree and can be extended or searched. The prototypes for these functions can be found in the header file search.h.

— Function: void * tsearch (const void *key, void **rootp, comparison_fn_t compar)

The tsearch function searches in the tree pointed to by *rootp for an element matching key. The function pointed to by compar is used to determine whether two elements match. See Comparison Functions, for a specification of the functions which can be used for the compar parameter.

If the tree does not contain a matching entry the key value will be added to the tree. tsearch does not make a copy of the object pointed to by key (how could it since the size is unknown). Instead it adds a reference to this object which means the object must be available as long as the tree data structure is used.

The tree is represented by a pointer to a pointer since it is sometimes necessary to change the root node of the tree. So it must not be assumed that the variable pointed to by rootp has the same value after the call. This also shows that it is not safe to call the tsearch function more than once at the same time using the same tree. It is no problem to run it more than once at a time on different trees.

The return value is a pointer to the matching element in the tree. If a new element was created the pointer points to the new data (which is in fact key). If an entry had to be created and the program ran out of space NULL is returned.

— Function: void * tfind (const void *key, void *const *rootp, comparison_fn_t compar)

The tfind function is similar to the tsearch function. It locates an element matching the one pointed to by key and returns a pointer to this element. But if no matching element is available no new element is entered (note that the rootp parameter points to a constant pointer). Instead the function returns NULL.

Another advantage of the tsearch function in contrast to the hsearch functions is that there is an easy way to remove elements.

— Function: void * tdelete (const void *key, void **rootp, comparison_fn_t compar)

To remove a specific element matching key from the tree tdelete can be used. It locates the matching element using the same method as tfind. The corresponding element is then removed and a pointer to the parent of the deleted node is returned by the function. If there is no matching entry in the tree nothing can be deleted and the function returns NULL. If the root of the tree is deleted tdelete returns some unspecified value not equal to NULL.

— Function: void tdestroy (void *vroot, __free_fn_t freefct)

If the complete search tree has to be removed one can use tdestroy. It frees all resources allocated by the tsearch function to generate the tree pointed to by vroot.

For the data in each tree node the function freefct is called. The pointer to the data is passed as the argument to the function. If no such work is necessary freefct must point to a function doing nothing. It is called in any case.

This function is a GNU extension and not covered by the System V or X/Open specifications.

In addition to the function to create and destroy the tree data structure, there is another function which allows you to apply a function to all elements of the tree. The function must have this type: 

     void __action_fn_t (const void *nodep, VISIT value, int level);

The nodep is the data value of the current node (once given as the key argument to tsearch). level is a numeric value which corresponds to the depth of the current node in the tree. The root node has the depth 0 and its children have a depth of 1 and so on. The VISIT type is an enumeration type.

— Data Type: VISIT

The VISIT value indicates the status of the current node in the tree and how the function is called. The status of a node is either `leaf' or `internal node'. For each leaf node the function is called exactly once, for each internal node it is called three times: before the first child is processed, after the first child is processed and after both children are processed. This makes it possible to handle all three methods of tree traversal (or even a combination of them).

preorder
The current node is an internal node and the function is called before the first child was processed.
postorder
The current node is an internal node and the function is called after the first child was processed.
endorder
The current node is an internal node and the function is called after the second child was processed.
leaf
The current node is a leaf.

— Function: void twalk (const void *root, __action_fn_t action)

For each node in the tree with a node pointed to by root, the twalk function calls the function provided by the parameter action. For leaf nodes the function is called exactly once with value set to leaf. For internal nodes the function is called three times, setting the value parameter or action to the appropriate value. The level argument for the action function is computed while descending the tree with increasing the value by one for the descend to a child, starting with the value 0 for the root node.

Since the functions used for the action parameter to twalk must not modify the tree data, it is safe to run twalk in more than one thread at the same time, working on the same tree. It is also safe to call tfind in parallel. Functions which modify the tree must not be used, otherwise the behavior is undefined.

Next: , Previous: Searching and Sorting, Up: Top

10 Pattern Matching

The GNU C Library provides pattern matching facilities for two kinds of patterns: regular expressions and file-name wildcards. The library also provides a facility for expanding variable and command references and parsing text into words in the way the shell does.

Next: , Up: Pattern Matching

10.1 Wildcard Matching

This section describes how to match a wildcard pattern against a particular string. The result is a yes or no answer: does the string fit the pattern or not. The symbols described here are all declared in fnmatch.h.

— Function: int fnmatch (const char *pattern, const char *string, int flags)

This function tests whether the string string matches the pattern pattern. It returns 0 if they do match; otherwise, it returns the nonzero value FNM_NOMATCH. The arguments pattern and string are both strings.

The argument flags is a combination of flag bits that alter the details of matching. See below for a list of the defined flags.

In the GNU C Library, fnmatch cannot experience an “error”—it always returns an answer for whether the match succeeds. However, other implementations of fnmatch might sometimes report “errors”. They would do so by returning nonzero values that are not equal to FNM_NOMATCH.

These are the available flags for the flags argument:

FNM_FILE_NAME
Treat the `/' character specially, for matching file names. If this flag is set, wildcard constructs in pattern cannot match `/' in string. Thus, the only way to match `/' is with an explicit `/' in pattern.
FNM_PATHNAME
This is an alias for FNM_FILE_NAME; it comes from POSIX.2. We don't recommend this name because we don't use the term “pathname” for file names.
FNM_PERIOD
Treat the `.' character specially if it appears at the beginning of string. If this flag is set, wildcard constructs in pattern cannot match `.' as the first character of string.

If you set both FNM_PERIOD and FNM_FILE_NAME, then the special treatment applies to `.' following `/' as well as to `.' at the beginning of string. (The shell uses the FNM_PERIOD and FNM_FILE_NAME flags together for matching file names.)

FNM_NOESCAPE
Don't treat the `\' character specially in patterns. Normally, `\' quotes the following character, turning off its special meaning (if any) so that it matches only itself. When quoting is enabled, the pattern `\?' matches only the string `?', because the question mark in the pattern acts like an ordinary character.

If you use FNM_NOESCAPE, then `\' is an ordinary character.

FNM_LEADING_DIR
Ignore a trailing sequence of characters starting with a `/' in string; that is to say, test whether string starts with a directory name that pattern matches.

If this flag is set, either `foo*' or `foobar' as a pattern would match the string `foobar/frobozz'.

FNM_CASEFOLD
Ignore case in comparing string to pattern.
FNM_EXTMATCH
Recognize beside the normal patterns also the extended patterns introduced in ksh. The patterns are written in the form explained in the following table where pattern-list is a | separated list of patterns.
?(pattern-list)
The pattern matches if zero or one occurrences of any of the patterns in the pattern-list allow matching the input string.
*(pattern-list)
The pattern matches if zero or more occurrences of any of the patterns in the pattern-list allow matching the input string.
+(pattern-list)
The pattern matches if one or more occurrences of any of the patterns in the pattern-list allow matching the input string.
@(pattern-list)
The pattern matches if exactly one occurrence of any of the patterns in the pattern-list allows matching the input string.
!(pattern-list)
The pattern matches if the input string cannot be matched with any of the patterns in the pattern-list.

Next: , Previous: Wildcard Matching, Up: Pattern Matching

10.2 Globbing

The archetypal use of wildcards is for matching against the files in a directory, and making a list of all the matches. This is called globbing.

You could do this using fnmatch, by reading the directory entries one by one and testing each one with fnmatch. But that would be slow (and complex, since you would have to handle subdirectories by hand).

The library provides a function glob to make this particular use of wildcards convenient. glob and the other symbols in this section are declared in glob.h.

Next: , Up: Globbing

10.2.1 Calling glob

The result of globbing is a vector of file names (strings). To return this vector, glob uses a special data type, glob_t, which is a structure. You pass glob the address of the structure, and it fills in the structure's fields to tell you about the results.

— Data Type: glob_t

This data type holds a pointer to a word vector. More precisely, it records both the address of the word vector and its size. The GNU implementation contains some more fields which are non-standard extensions.

gl_pathc
The number of elements in the vector, excluding the initial null entries if the GLOB_DOOFFS flag is used (see gl_offs below).
gl_pathv
The address of the vector. This field has type char **.
gl_offs
The offset of the first real element of the vector, from its nominal address in the gl_pathv field. Unlike the other fields, this is always an input to glob, rather than an output from it.

If you use a nonzero offset, then that many elements at the beginning of the vector are left empty. (The glob function fills them with null pointers.)

The gl_offs field is meaningful only if you use the GLOB_DOOFFS flag. Otherwise, the offset is always zero regardless of what is in this field, and the first real element comes at the beginning of the vector.

gl_closedir
The address of an alternative implementation of the closedir function. It is used if the GLOB_ALTDIRFUNC bit is set in the flag parameter. The type of this field is void (*) (void *).

This is a GNU extension.

gl_readdir
The address of an alternative implementation of the readdir function used to read the contents of a directory. It is used if the GLOB_ALTDIRFUNC bit is set in the flag parameter. The type of this field is struct dirent *(*) (void *).

This is a GNU extension.

gl_opendir
The address of an alternative implementation of the opendir function. It is used if the GLOB_ALTDIRFUNC bit is set in the flag parameter. The type of this field is void *(*) (const char *).

This is a GNU extension.

gl_stat
The address of an alternative implementation of the stat function to get information about an object in the filesystem. It is used if the GLOB_ALTDIRFUNC bit is set in the flag parameter. The type of this field is int (*) (const char *, struct stat *).

This is a GNU extension.

gl_lstat
The address of an alternative implementation of the lstat function to get information about an object in the filesystems, not following symbolic links. It is used if the GLOB_ALTDIRFUNC bit is set in the flag parameter. The type of this field is int (*) (const char *, struct stat *).

This is a GNU extension.

For use in the glob64 function glob.h contains another definition for a very similar type. glob64_t differs from glob_t only in the types of the members gl_readdir, gl_stat, and gl_lstat.

— Data Type: glob64_t

This data type holds a pointer to a word vector. More precisely, it records both the address of the word vector and its size. The GNU implementation contains some more fields which are non-standard extensions.

gl_pathc
The number of elements in the vector, excluding the initial null entries if the GLOB_DOOFFS flag is used (see gl_offs below).
gl_pathv
The address of the vector. This field has type char **.
gl_offs
The offset of the first real element of the vector, from its nominal address in the gl_pathv field. Unlike the other fields, this is always an input to glob, rather than an output from it.

If you use a nonzero offset, then that many elements at the beginning of the vector are left empty. (The glob function fills them with null pointers.)

The gl_offs field is meaningful only if you use the GLOB_DOOFFS flag. Otherwise, the offset is always zero regardless of what is in this field, and the first real element comes at the beginning of the vector.

gl_closedir
The address of an alternative implementation of the closedir function. It is used if the GLOB_ALTDIRFUNC bit is set in the flag parameter. The type of this field is void (*) (void *).

This is a GNU extension.

gl_readdir
The address of an alternative implementation of the readdir64 function used to read the contents of a directory. It is used if the GLOB_ALTDIRFUNC bit is set in the flag parameter. The type of this field is struct dirent64 *(*) (void *).

This is a GNU extension.

gl_opendir
The address of an alternative implementation of the opendir function. It is used if the GLOB_ALTDIRFUNC bit is set in the flag parameter. The type of this field is void *(*) (const char *).

This is a GNU extension.

gl_stat
The address of an alternative implementation of the stat64 function to get information about an object in the filesystem. It is used if the GLOB_ALTDIRFUNC bit is set in the flag parameter. The type of this field is int (*) (const char *, struct stat64 *).

This is a GNU extension.

gl_lstat
The address of an alternative implementation of the lstat64 function to get information about an object in the filesystems, not following symbolic links. It is used if the GLOB_ALTDIRFUNC bit is set in the flag parameter. The type of this field is int (*) (const char *, struct stat64 *).

This is a GNU extension.

— Function: int glob (const char *pattern, int flags, int (*errfunc) (const char *filename, int error-code), glob_t *vector-ptr)

The function glob does globbing using the pattern pattern in the current directory. It puts the result in a newly allocated vector, and stores the size and address of this vector into *vector-ptr. The argument flags is a combination of bit flags; see Flags for Globbing, for details of the flags.

The result of globbing is a sequence of file names. The function glob allocates a string for each resulting word, then allocates a vector of type char ** to store the addresses of these strings. The last element of the vector is a null pointer. This vector is called the word vector.

To return this vector, glob stores both its address and its length (number of elements, not counting the terminating null pointer) into *vector-ptr.

Normally, glob sorts the file names alphabetically before returning them. You can turn this off with the flag GLOB_NOSORT if you want to get the information as fast as possible. Usually it's a good idea to let glob sort them—if you process the files in alphabetical order, the users will have a feel for the rate of progress that your application is making.

If glob succeeds, it returns 0. Otherwise, it returns one of these error codes:

GLOB_ABORTED
There was an error opening a directory, and you used the flag GLOB_ERR or your specified errfunc returned a nonzero value. for an explanation of the GLOB_ERR flag and errfunc.
GLOB_NOMATCH
The pattern didn't match any existing files. If you use the GLOB_NOCHECK flag, then you never get this error code, because that flag tells glob to pretend that the pattern matched at least one file.
GLOB_NOSPACE
It was impossible to allocate memory to hold the result.

In the event of an error, glob stores information in *vector-ptr about all the matches it has found so far.

It is important to notice that the glob function will not fail if it encounters directories or files which cannot be handled without the LFS interfaces. The implementation of glob is supposed to use these functions internally. This at least is the assumptions made by the Unix standard. The GNU extension of allowing the user to provide own directory handling and stat functions complicates things a bit. If these callback functions are used and a large file or directory is encountered glob can fail.

— Function: int glob64 (const char *pattern, int flags, int (*errfunc) (const char *filename, int error-code), glob64_t *vector-ptr)

The glob64 function was added as part of the Large File Summit extensions but is not part of the original LFS proposal. The reason for this is simple: it is not necessary. The necessity for a glob64 function is added by the extensions of the GNU glob implementation which allows the user to provide own directory handling and stat functions. The readdir and stat functions do depend on the choice of _FILE_OFFSET_BITS since the definition of the types struct dirent and struct stat will change depending on the choice.

Beside this difference the glob64 works just like glob in all aspects.

This function is a GNU extension.

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10.2.2 Flags for Globbing

This section describes the flags that you can specify in the flags argument to glob. Choose the flags you want, and combine them with the C bitwise OR operator |.

GLOB_APPEND
Append the words from this expansion to the vector of words produced by previous calls to glob. This way you can effectively expand several words as if they were concatenated with spaces between them.

In order for appending to work, you must not modify the contents of the word vector structure between calls to glob. And, if you set GLOB_DOOFFS in the first call to glob, you must also set it when you append to the results.

Note that the pointer stored in gl_pathv may no longer be valid after you call glob the second time, because glob might have relocated the vector. So always fetch gl_pathv from the glob_t structure after each glob call; never save the pointer across calls.

GLOB_DOOFFS
Leave blank slots at the beginning of the vector of words. The gl_offs field says how many slots to leave. The blank slots contain null pointers.
GLOB_ERR
Give up right away and report an error if there is any difficulty reading the directories that must be read in order to expand pattern fully. Such difficulties might include a directory in which you don't have the requisite access. Normally, glob tries its best to keep on going despite any errors, reading whatever directories it can.

You can exercise even more control than this by specifying an error-handler function errfunc when you call glob. If errfunc is not a null pointer, then glob doesn't give up right away when it can't read a directory; instead, it calls errfunc with two arguments, like this: 

          (*errfunc) (filename, error-code)

The argument filename is the name of the directory that glob couldn't open or couldn't read, and error-code is the errno value that was reported to glob.

If the error handler function returns nonzero, then glob gives up right away. Otherwise, it continues.

GLOB_MARK
If the pattern matches the name of a directory, append `/' to the directory's name when returning it.
GLOB_NOCHECK
If the pattern doesn't match any file names, return the pattern itself as if it were a file name that had been matched. (Normally, when the pattern doesn't match anything, glob returns that there were no matches.)
GLOB_NOSORT
Don't sort the file names; return them in no particular order. (In practice, the order will depend on the order of the entries in the directory.) The only reason not to sort is to save time.
GLOB_NOESCAPE
Don't treat the `\' character specially in patterns. Normally, `\' quotes the following character, turning off its special meaning (if any) so that it matches only itself. When quoting is enabled, the pattern `\?' matches only the string `?', because the question mark in the pattern acts like an ordinary character.

If you use GLOB_NOESCAPE, then `\' is an ordinary character.

glob does its work by calling the function fnmatch repeatedly. It handles the flag GLOB_NOESCAPE by turning on the FNM_NOESCAPE flag in calls to fnmatch.

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10.2.3 More Flags for Globbing

Beside the flags described in the last section, the GNU implementation of glob allows a few more flags which are also defined in the glob.h file. Some of the extensions implement functionality which is available in modern shell implementations.

GLOB_PERIOD
The . character (period) is treated special. It cannot be matched by wildcards. See Wildcard Matching, FNM_PERIOD.
GLOB_MAGCHAR
The GLOB_MAGCHAR value is not to be given to glob in the flags parameter. Instead, glob sets this bit in the gl_flags element of the glob_t structure provided as the result if the pattern used for matching contains any wildcard character.
GLOB_ALTDIRFUNC
Instead of the using the using the normal functions for accessing the filesystem the glob implementation uses the user-supplied functions specified in the structure pointed to by pglob parameter. For more information about the functions refer to the sections about directory handling see Accessing Directories, and Reading Attributes.
GLOB_BRACE
If this flag is given the handling of braces in the pattern is changed. It is now required that braces appear correctly grouped. I.e., for each opening brace there must be a closing one. Braces can be used recursively. So it is possible to define one brace expression in another one. It is important to note that the range of each brace expression is completely contained in the outer brace expression (if there is one).

The string between the matching braces is separated into single expressions by splitting at , (comma) characters. The commas themselves are discarded. Please note what we said above about recursive brace expressions. The commas used to separate the subexpressions must be at the same level. Commas in brace subexpressions are not matched. They are used during expansion of the brace expression of the deeper level. The example below shows this 

          glob ("{foo/{,bar,biz},baz}", GLOB_BRACE, NULL, &result)

is equivalent to the sequence 

          glob ("foo/", GLOB_BRACE, NULL, &result)
          glob ("foo/bar", GLOB_BRACE|GLOB_APPEND, NULL, &result)
          glob ("foo/biz", GLOB_BRACE|GLOB_APPEND, NULL, &result)
          glob ("baz", GLOB_BRACE|GLOB_APPEND, NULL, &result)

if we leave aside error handling.

GLOB_NOMAGIC
If the pattern contains no wildcard constructs (it is a literal file name), return it as the sole “matching” word, even if no file exists by that name.
GLOB_TILDE
If this flag is used the character ~ (tilde) is handled special if it appears at the beginning of the pattern. Instead of being taken verbatim it is used to represent the home directory of a known user.

If ~ is the only character in pattern or it is followed by a / (slash), the home directory of the process owner is substituted. Using getlogin and getpwnam the information is read from the system databases. As an example take user bart with his home directory at /home/bart. For him a call like 

          glob ("~/bin/*", GLOB_TILDE, NULL, &result)

would return the contents of the directory /home/bart/bin. Instead of referring to the own home directory it is also possible to name the home directory of other users. To do so one has to append the user name after the tilde character. So the contents of user homer's bin directory can be retrieved by 

          glob ("~homer/bin/*", GLOB_TILDE, NULL, &result)

If the user name is not valid or the home directory cannot be determined for some reason the pattern is left untouched and itself used as the result. I.e., if in the last example home is not available the tilde expansion yields to "~homer/bin/*" and glob is not looking for a directory named ~homer.

This functionality is equivalent to what is available in C-shells if the nonomatch flag is set.

GLOB_TILDE_CHECK
If this flag is used glob behaves like as if GLOB_TILDE is given. The only difference is that if the user name is not available or the home directory cannot be determined for other reasons this leads to an error. glob will return GLOB_NOMATCH instead of using the pattern itself as the name.

This functionality is equivalent to what is available in C-shells if nonomatch flag is not set.

GLOB_ONLYDIR
If this flag is used the globbing function takes this as a hint that the caller is only interested in directories matching the pattern. If the information about the type of the file is easily available non-directories will be rejected but no extra work will be done to determine the information for each file. I.e., the caller must still be able to filter directories out.

This functionality is only available with the GNU glob implementation. It is mainly used internally to increase the performance but might be useful for a user as well and therefore is documented here.

Calling glob will in most cases allocate resources which are used to represent the result of the function call. If the same object of type glob_t is used in multiple call to glob the resources are freed or reused so that no leaks appear. But this does not include the time when all glob calls are done.

— Function: void globfree (glob_t *pglob)

The globfree function frees all resources allocated by previous calls to glob associated with the object pointed to by pglob. This function should be called whenever the currently used glob_t typed object isn't used anymore.

— Function: void globfree64 (glob64_t *pglob)

This function is equivalent to globfree but it frees records of type glob64_t which were allocated by glob64.

Next: , Previous: Globbing, Up: Pattern Matching

10.3 Regular Expression Matching

The GNU C library supports two interfaces for matching regular expressions. One is the standard POSIX.2 interface, and the other is what the GNU system has had for many years.

Both interfaces are declared in the header file regex.h. If you define _POSIX_C_SOURCE, then only the POSIX.2 functions, structures, and constants are declared.

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10.3.1 POSIX Regular Expression Compilation

Before you can actually match a regular expression, you must compile it. This is not true compilation—it produces a special data structure, not machine instructions. But it is like ordinary compilation in that its purpose is to enable you to “execute” the pattern fast. (See Matching POSIX Regexps, for how to use the compiled regular expression for matching.)

There is a special data type for compiled regular expressions:

— Data Type: regex_t

This type of object holds a compiled regular expression. It is actually a structure. It has just one field that your programs should look at:

re_nsub
This field holds the number of parenthetical subexpressions in the regular expression that was compiled.

There are several other fields, but we don't describe them here, because only the functions in the library should use them.

After you create a regex_t object, you can compile a regular expression into it by calling regcomp.

— Function: int regcomp (regex_t *restrict compiled, const char *restrict pattern, int cflags)

The function regcomp “compiles” a regular expression into a data structure that you can use with regexec to match against a string. The compiled regular expression format is designed for efficient matching. regcomp stores it into *compiled.

It's up to you to allocate an object of type regex_t and pass its address to regcomp.

The argument cflags lets you specify various options that control the syntax and semantics of regular expressions. See Flags for POSIX Regexps.

If you use the flag REG_NOSUB, then regcomp omits from the compiled regular expression the information necessary to record how subexpressions actually match. In this case, you might as well pass 0 for the matchptr and nmatch arguments when you call regexec.

If you don't use REG_NOSUB, then the compiled regular expression does have the capacity to record how subexpressions match. Also, regcomp tells you how many subexpressions pattern has, by storing the number in compiled->re_nsub. You can use that value to decide how long an array to allocate to hold information about subexpression matches.

regcomp returns 0 if it succeeds in compiling the regular expression; otherwise, it returns a nonzero error code (see the table below). You can use regerror to produce an error message string describing the reason for a nonzero value; see Regexp Cleanup.

Here are the possible nonzero values that regcomp can return:

REG_BADBR
There was an invalid `\{...\}' construct in the regular expression. A valid `\{...\}' construct must contain either a single number, or two numbers in increasing order separated by a comma.
REG_BADPAT
There was a syntax error in the regular expression.
REG_BADRPT
A repetition operator such as `?' or `*' appeared in a bad position (with no preceding subexpression to act on).
REG_ECOLLATE
The regular expression referred to an invalid collating element (one not defined in the current locale for string collation). See Locale Categories.
REG_ECTYPE
The regular expression referred to an invalid character class name.
REG_EESCAPE
The regular expression ended with `\'.
REG_ESUBREG
There was an invalid number in the `\digit' construct.
REG_EBRACK
There were unbalanced square brackets in the regular expression.
REG_EPAREN
An extended regular expression had unbalanced parentheses, or a basic regular expression had unbalanced `\(' and `\)'.
REG_EBRACE
The regular expression had unbalanced `\{' and `\}'.
REG_ERANGE
One of the endpoints in a range expression was invalid.
REG_ESPACE
regcomp ran out of memory.

Next: , Previous: POSIX Regexp Compilation, Up: Regular Expressions

10.3.2 Flags for POSIX Regular Expressions

These are the bit flags that you can use in the cflags operand when compiling a regular expression with regcomp.

REG_EXTENDED
Treat the pattern as an extended regular expression, rather than as a basic regular expression.
REG_ICASE
Ignore case when matching letters.
REG_NOSUB
Don't bother storing the contents of the matches-ptr array.
REG_NEWLINE
Treat a newline in string as dividing string into multiple lines, so that `$' can match before the newline and `^' can match after. Also, don't permit `.' to match a newline, and don't permit `[^...]' to match a newline.

Otherwise, newline acts like any other ordinary character.

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10.3.3 Matching a Compiled POSIX Regular Expression

Once you have compiled a regular expression, as described in POSIX Regexp Compilation, you can match it against strings using regexec. A match anywhere inside the string counts as success, unless the regular expression contains anchor characters (`^' or `$').

— Function: int regexec (const regex_t *restrict compiled, const char *restrict string, size_t nmatch, regmatch_t matchptr[restrict], int eflags)

This function tries to match the compiled regular expression *compiled against string.

regexec returns 0 if the regular expression matches; otherwise, it returns a nonzero value. See the table below for what nonzero values mean. You can use regerror to produce an error message string describing the reason for a nonzero value; see Regexp Cleanup.

The argument eflags is a word of bit flags that enable various options.

If you want to get information about what part of string actually matched the regular expression or its subexpressions, use the arguments matchptr and nmatch. Otherwise, pass 0 for nmatch, and NULL for matchptr. See Regexp Subexpressions.

You must match the regular expression with the same set of current locales that were in effect when you compiled the regular expression.

The function regexec accepts the following flags in the eflags argument:

REG_NOTBOL
Do not regard the beginning of the specified string as the beginning of a line; more generally, don't make any assumptions about what text might precede it.
REG_NOTEOL
Do not regard the end of the specified string as the end of a line; more generally, don't make any assumptions about what text might follow it.

Here are the possible nonzero values that regexec can return:

REG_NOMATCH
The pattern didn't match the string. This isn't really an error.
REG_ESPACE
regexec ran out of memory.

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10.3.4 Match Results with Subexpressions

When regexec matches parenthetical subexpressions of pattern, it records which parts of string they match. It returns that information by storing the offsets into an array whose elements are structures of type regmatch_t. The first element of the array (index 0) records the part of the string that matched the entire regular expression. Each other element of the array records the beginning and end of the part that matched a single parenthetical subexpression.

— Data Type: regmatch_t

This is the data type of the matcharray array that you pass to regexec. It contains two structure fields, as follows:

rm_so
The offset in string of the beginning of a substring. Add this value to string to get the address of that part.
rm_eo
The offset in string of the end of the substring.

— Data Type: regoff_t

regoff_t is an alias for another signed integer type. The fields of regmatch_t have type regoff_t.

The regmatch_t elements correspond to subexpressions positionally; the first element (index 1) records where the first subexpression matched, the second element records the second subexpression, and so on. The order of the subexpressions is the order in which they begin.

When you call regexec, you specify how long the matchptr array is, with the nmatch argument. This tells regexec how many elements to store. If the actual regular expression has more than nmatch subexpressions, then you won't get offset information about the rest of them. But this doesn't alter whether the pattern matches a particular string or not.

If you don't want regexec to return any information about where the subexpressions matched, you can either supply 0 for nmatch, or use the flag REG_NOSUB when you compile the pattern with regcomp.

Next: , Previous: Regexp Subexpressions, Up: Regular Expressions

10.3.5 Complications in Subexpression Matching

Sometimes a subexpression matches a substring of no characters. This happens when `f\(o*\)' matches the string `fum'. (It really matches just the `f'.) In this case, both of the offsets identify the point in the string where the null substring was found. In this example, the offsets are both 1.

Sometimes the entire regular expression can match without using some of its subexpressions at all—for example, when `ba\(na\)*' matches the string `ba', the parenthetical subexpression is not used. When this happens, regexec stores -1 in both fields of the element for that subexpression.

Sometimes matching the entire regular expression can match a particular subexpression more than once—for example, when `ba\(na\)*' matches the string `bananana', the parenthetical subexpression matches three times. When this happens, regexec usually stores the offsets of the last part of the string that matched the subexpression. In the case of `bananana', these offsets are 6 and 8.

But the last match is not always the one that is chosen. It's more accurate to say that the last opportunity to match is the one that takes precedence. What this means is that when one subexpression appears within another, then the results reported for the inner subexpression reflect whatever happened on the last match of the outer subexpression. For an example, consider `\(ba\(na\)*s \)*' matching the string `bananas bas '. The last time the inner expression actually matches is near the end of the first word. But it is considered again in the second word, and fails to match there. regexec reports nonuse of the “na” subexpression.

Another place where this rule applies is when the regular expression 

     \(ba\(na\)*s \|nefer\(ti\)* \)*

matches `bananas nefertiti'. The “na” subexpression does match in the first word, but it doesn't match in the second word because the other alternative is used there. Once again, the second repetition of the outer subexpression overrides the first, and within that second repetition, the “na” subexpression is not used. So regexec reports nonuse of the “na” subexpression.

Previous: Subexpression Complications, Up: Regular Expressions

10.3.6 POSIX Regexp Matching Cleanup

When you are finished using a compiled regular expression, you can free the storage it uses by calling regfree.

— Function: void regfree (regex_t *compiled)

Calling regfree frees all the storage that *compiled points to. This includes various internal fields of the regex_t structure that aren't documented in this manual.

regfree does not free the object *compiled itself.

You should always free the space in a regex_t structure with regfree before using the structure to compile another regular expression.

When regcomp or regexec reports an error, you can use the function regerror to turn it into an error message string.

— Function: size_t regerror (int errcode, const regex_t *restrict compiled, char *restrict buffer, size_t length)

This function produces an error message string for the error code errcode, and stores the string in length bytes of memory starting at buffer. For the compiled argument, supply the same compiled regular expression structure that regcomp or regexec was working with when it got the error. Alternatively, you can supply NULL for compiled; you will still get a meaningful error message, but it might not be as detailed.

If the error message can't fit in length bytes (including a terminating null character), then regerror truncates it. The string that regerror stores is always null-terminated even if it has been truncated.

The return value of regerror is the minimum length needed to store the entire error message. If this is less than length, then the error message was not truncated, and you can use it. Otherwise, you should call regerror again with a larger buffer.

Here is a function which uses regerror, but always dynamically allocates a buffer for the error message: 

          char *get_regerror (int errcode, regex_t *compiled)
          {
            size_t length = regerror (errcode, compiled, NULL, 0);
            char *buffer = xmalloc (length);
            (void) regerror (errcode, compiled, buffer, length);
            return buffer;
          }

Previous: Regular Expressions, Up: Pattern Matching

10.4 Shell-Style Word Expansion

Word expansion means the process of splitting a string into words and substituting for variables, commands, and wildcards just as the shell does.

For example, when you write `ls -l foo.c', this string is split into three separate words—`ls', `-l' and `foo.c'. This is the most basic function of word expansion.

When you write `ls *.c', this can become many words, because the word `*.c' can be replaced with any number of file names. This is called wildcard expansion, and it is also a part of word expansion.

When you use `echo $PATH' to print your path, you are taking advantage of variable substitution, which is also part of word expansion.

Ordinary programs can perform word expansion just like the shell by calling the library function wordexp.

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10.4.1 The Stages of Word Expansion

When word expansion is applied to a sequence of words, it performs the following transformations in the order shown here:

  1. Tilde expansion: Replacement of `~foo' with the name of the home directory of `foo'.
  2. Next, three different transformations are applied in the same step, from left to right:
  3. Field splitting: subdivision of the text into words.
  4. Wildcard expansion: The replacement of a construct such as `*.c' with a list of `.c' file names. Wildcard expansion applies to an entire word at a time, and replaces that word with 0 or more file names that are themselves words.
  5. Quote removal: The deletion of string-quotes, now that they have done their job by inhibiting the above transformations when appropriate.

For the details of these transformations, and how to write the constructs that use them, see The BASH Manual (to appear).

Next: , Previous: Expansion Stages, Up: Word Expansion

10.4.2 Calling wordexp

All the functions, constants and data types for word expansion are declared in the header file wordexp.h.

Word expansion produces a vector of words (strings). To return this vector, wordexp uses a special data type, wordexp_t, which is a structure. You pass wordexp the address of the structure, and it fills in the structure's fields to tell you about the results.

— Data Type: wordexp_t

This data type holds a pointer to a word vector. More precisely, it records both the address of the word vector and its size.

we_wordc
The number of elements in the vector.
we_wordv
The address of the vector. This field has type char **.
we_offs
The offset of the first real element of the vector, from its nominal address in the we_wordv field. Unlike the other fields, this is always an input to wordexp, rather than an output from it.

If you use a nonzero offset, then that many elements at the beginning of the vector are left empty. (The wordexp function fills them with null pointers.)

The we_offs field is meaningful only if you use the WRDE_DOOFFS flag. Otherwise, the offset is always zero regardless of what is in this field, and the first real element comes at the beginning of the vector.

— Function: int wordexp (const char *words, wordexp_t *word-vector-ptr, int flags)

Perform word expansion on the string words, putting the result in a newly allocated vector, and store the size and address of this vector into *word-vector-ptr. The argument flags is a combination of bit flags; see Flags for Wordexp, for details of the flags.

You shouldn't use any of the characters `|&;<>' in the string words unless they are quoted; likewise for newline. If you use these characters unquoted, you will get the WRDE_BADCHAR error code. Don't use parentheses or braces unless they are quoted or part of a word expansion construct. If you use quotation characters `'"`', they should come in pairs that balance.

The results of word expansion are a sequence of words. The function wordexp allocates a string for each resulting word, then allocates a vector of type char ** to store the addresses of these strings. The last element of the vector is a null pointer. This vector is called the word vector.

To return this vector, wordexp stores both its address and its length (number of elements, not counting the terminating null pointer) into *word-vector-ptr.

If wordexp succeeds, it returns 0. Otherwise, it returns one of these error codes:

WRDE_BADCHAR
The input string words contains an unquoted invalid character such as `|'.
WRDE_BADVAL
The input string refers to an undefined shell variable, and you used the flag WRDE_UNDEF to forbid such references.
WRDE_CMDSUB
The input string uses command substitution, and you used the flag WRDE_NOCMD to forbid command substitution.
WRDE_NOSPACE
It was impossible to allocate memory to hold the result. In this case, wordexp can store part of the results—as much as it could allocate room for.
WRDE_SYNTAX
There was a syntax error in the input string. For example, an unmatched quoting character is a syntax error.

— Function: void wordfree (wordexp_t *word-vector-ptr)

Free the storage used for the word-strings and vector that *word-vector-ptr points to. This does not free the structure *word-vector-ptr itself—only the other data it points to.

Next: , Previous: Calling Wordexp, Up: Word Expansion

10.4.3 Flags for Word Expansion

This section describes the flags that you can specify in the flags argument to wordexp. Choose the flags you want, and combine them with the C operator |.

WRDE_APPEND
Append the words from this expansion to the vector of words produced by previous calls to wordexp. This way you can effectively expand several words as if they were concatenated with spaces between them.

In order for appending to work, you must not modify the contents of the word vector structure between calls to wordexp. And, if you set WRDE_DOOFFS in the first call to wordexp, you must also set it when you append to the results.

WRDE_DOOFFS
Leave blank slots at the beginning of the vector of words. The we_offs field says how many slots to leave. The blank slots contain null pointers.
WRDE_NOCMD
Don't do command substitution; if the input requests command substitution, report an error.
WRDE_REUSE
Reuse a word vector made by a previous call to wordexp. Instead of allocating a new vector of words, this call to wordexp will use the vector that already exists (making it larger if necessary).

Note that the vector may move, so it is not safe to save an old pointer and use it again after calling wordexp. You must fetch we_pathv anew after each call.

WRDE_SHOWERR
Do show any error messages printed by commands run by command substitution. More precisely, allow these commands to inherit the standard error output stream of the current process. By default, wordexp gives these commands a standard error stream that discards all output.
WRDE_UNDEF
If the input refers to a shell variable that is not defined, report an error.

Next: , Previous: Flags for Wordexp, Up: Word Expansion

10.4.4 wordexp Example

Here is an example of using wordexp to expand several strings and use the results to run a shell command. It also shows the use of WRDE_APPEND to concatenate the expansions and of wordfree to free the space allocated by wordexp. 

     int
     expand_and_execute (const char *program, const char **options)
     {
       wordexp_t result;
       pid_t pid
       int status, i;
     
       /* Expand the string for the program to run.  */
       switch (wordexp (program, &result, 0))
         {
         case 0:			/* Successful.  */
           break;
         case WRDE_NOSPACE:
           /* If the error was WRDE_NOSPACE,
              then perhaps part of the result was allocated.  */
           wordfree (&result);
         default:                    /* Some other error.  */
           return -1;
         }
     
       /* Expand the strings specified for the arguments.  */
       for (i = 0; options[i] != NULL; i++)
         {
           if (wordexp (options[i], &result, WRDE_APPEND))
             {
               wordfree (&result);
               return -1;
             }
         }
     
       pid = fork ();
       if (pid == 0)
         {
           /* This is the child process.  Execute the command. */
           execv (result.we_wordv[0], result.we_wordv);
           exit (EXIT_FAILURE);
         }
       else if (pid < 0)
         /* The fork failed.  Report failure.  */
         status = -1;
       else
         /* This is the parent process.  Wait for the child to complete.  */
         if (waitpid (pid, &status, 0) != pid)
           status = -1;
     
       wordfree (&result);
       return status;
     }

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10.4.5 Details of Tilde Expansion

It's a standard part of shell syntax that you can use `~' at the beginning of a file name to stand for your own home directory. You can use `~user' to stand for user's home directory.

Tilde expansion is the process of converting these abbreviations to the directory names that they stand for.

Tilde expansion applies to the `~' plus all following characters up to whitespace or a slash. It takes place only at the beginning of a word, and only if none of the characters to be transformed is quoted in any way.

Plain `~' uses the value of the environment variable HOME as the proper home directory name. `~' followed by a user name uses getpwname to look up that user in the user database, and uses whatever directory is recorded there. Thus, `~' followed by your own name can give different results from plain `~', if the value of HOME is not really your home directory.

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10.4.6 Details of Variable Substitution

Part of ordinary shell syntax is the use of `$variable' to substitute the value of a shell variable into a command. This is called variable substitution, and it is one part of doing word expansion.

There are two basic ways you can write a variable reference for substitution:

${variable}
If you write braces around the variable name, then it is completely unambiguous where the variable name ends. You can concatenate additional letters onto the end of the variable value by writing them immediately after the close brace. For example, `${foo}s' expands into `tractors'.
$variable
If you do not put braces around the variable name, then the variable name consists of all the alphanumeric characters and underscores that follow the `$'. The next punctuation character ends the variable name. Thus, `$foo-bar' refers to the variable foo and expands into `tractor-bar'.

When you use braces, you can also use various constructs to modify the value that is substituted, or test it in various ways.

${variable:-default}
Substitute the value of variable, but if that is empty or undefined, use default instead.
${variable:=default}
Substitute the value of variable, but if that is empty or undefined, use default instead and set the variable to default.
${variable:?message}
If variable is defined and not empty, substitute its value.

Otherwise, print message as an error message on the standard error stream, and consider word expansion a failure.

${variable:+replacement}
Substitute replacement, but only if variable is defined and nonempty. Otherwise, substitute nothing for this construct.
${#variable}
Substitute a numeral which expresses in base ten the number of characters in the value of variable. `${#foo}' stands for `7', because `tractor' is seven characters.

These variants of variable substitution let you remove part of the variable's value before substituting it. The prefix and suffix are not mere strings; they are wildcard patterns, just like the patterns that you use to match multiple file names. But in this context, they match against parts of the variable value rather than against file names.

${variable%%suffix}
Substitute the value of variable, but first discard from that variable any portion at the end that matches the pattern suffix.

If there is more than one alternative for how to match against suffix, this construct uses the longest possible match.

Thus, `${foo%%r*}' substitutes `t', because the largest match for `r*' at the end of `tractor' is `ractor'.

${variable%suffix}
Substitute the value of variable, but first discard from that variable any portion at the end that matches the pattern suffix.

If there is more than one alternative for how to match against suffix, this construct uses the shortest possible alternative.

Thus, `${foo%r*}' substitutes `tracto', because the shortest match for `r*' at the end of `tractor' is just `r'.

${variable##prefix}
Substitute the value of variable, but first discard from that variable any portion at the beginning that matches the pattern prefix.

If there is more than one alternative for how to match against prefix, this construct uses the longest possible match.

Thus, `${foo##*t}' substitutes `or', because the largest match for `*t' at the beginning of `tractor' is `tract'.

${variable#prefix}
Substitute the value of variable, but first discard from that variable any portion at the beginning that matches the pattern prefix.

If there is more than one alternative for how to match against prefix, this construct uses the shortest possible alternative.

Thus, `${foo#*t}' substitutes `ractor', because the shortest match for `*t' at the beginning of `tractor' is just `t'.

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11 Input/Output Overview

Most programs need to do either input (reading data) or output (writing data), or most frequently both, in order to do anything useful. The GNU C library provides such a large selection of input and output functions that the hardest part is often deciding which function is most appropriate!

This chapter introduces concepts and terminology relating to input and output. Other chapters relating to the GNU I/O facilities are:

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11.1 Input/Output Concepts

Before you can read or write the contents of a file, you must establish a connection or communications channel to the file. This process is called opening the file. You can open a file for reading, writing, or both. The connection to an open file is represented either as a stream or as a file descriptor. You pass this as an argument to the functions that do the actual read or write operations, to tell them which file to operate on. Certain functions expect streams, and others are designed to operate on file descriptors.

When you have finished reading to or writing from the file, you can terminate the connection by closing the file. Once you have closed a stream or file descriptor, you cannot do any more input or output operations on it.

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11.1.1 Streams and File Descriptors

When you want to do input or output to a file, you have a choice of two basic mechanisms for representing the connection between your program and the file: file descriptors and streams. File descriptors are represented as objects of type int, while streams are represented as FILE * objects.

File descriptors provide a primitive, low-level interface to input and output operations. Both file descriptors and streams can represent a connection to a device (such as a terminal), or a pipe or socket for communicating with another process, as well as a normal file. But, if you want to do control operations that are specific to a particular kind of device, you must use a file descriptor; there are no facilities to use streams in this way. You must also use file descriptors if your program needs to do input or output in special modes, such as nonblocking (or polled) input (see File Status Flags).

Streams provide a higher-level interface, layered on top of the primitive file descriptor facilities. The stream interface treats all kinds of files pretty much alike—the sole exception being the three styles of buffering that you can choose (see Stream Buffering).

The main advantage of using the stream interface is that the set of functions for performing actual input and output operations (as opposed to control operations) on streams is much richer and more powerful than the corresponding facilities for file descriptors. The file descriptor interface provides only simple functions for transferring blocks of characters, but the stream interface also provides powerful formatted input and output functions (printf and scanf) as well as functions for character- and line-oriented input and output.

Since streams are implemented in terms of file descriptors, you can extract the file descriptor from a stream and perform low-level operations directly on the file descriptor. You can also initially open a connection as a file descriptor and then make a stream associated with that file descriptor.

In general, you should stick with using streams rather than file descriptors, unless there is some specific operation you want to do that can only be done on a file descriptor. If you are a beginning programmer and aren't sure what functions to use, we suggest that you concentrate on the formatted input functions (see Formatted Input) and formatted output functions (see Formatted Output).

If you are concerned about portability of your programs to systems other than GNU, you should also be aware that file descriptors are not as portable as streams. You can expect any system running ISO C to support streams, but non-GNU systems may not support file descriptors at all, or may only implement a subset of the GNU functions that operate on file descriptors. Most of the file descriptor functions in the GNU library are included in the POSIX.1 standard, however.

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11.1.2 File Position

One of the attributes of an open file is its file position that keeps track of where in the file the next character is to be read or written. In the GNU system, and all POSIX.1 systems, the file position is simply an integer representing the number of bytes from the beginning of the file.

The file position is normally set to the beginning of the file when it is opened, and each time a character is read or written, the file position is incremented. In other words, access to the file is normally sequential. Ordinary files permit read or write operations at any position within the file. Some other kinds of files may also permit this. Files which do permit this are sometimes referred to as random-access files. You can change the file position using the fseek function on a stream (see File Positioning) or the lseek function on a file descriptor (see I/O Primitives). If you try to change the file position on a file that doesn't support random access, you get the ESPIPE error. Streams and descriptors that are opened for append access are treated specially for output: output to such files is always appended sequentially to the end of the file, regardless of the file position. However, the file position is still used to control where in the file reading is done. If you think about it, you'll realize that several programs can read a given file at the same time. In order for each program to be able to read the file at its own pace, each program must have its own file pointer, which is not affected by anything the other programs do.

In fact, each opening of a file creates a separate file position. Thus, if you open a file twice even in the same program, you get two streams or descriptors with independent file positions.

By contrast, if you open a descriptor and then duplicate it to get another descriptor, these two descriptors share the same file position: changing the file position of one descriptor will affect the other.

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11.2 File Names

In order to open a connection to a file, or to perform other operations such as deleting a file, you need some way to refer to the file. Nearly all files have names that are strings—even files which are actually devices such as tape drives or terminals. These strings are called file names. You specify the file name to say which file you want to open or operate on.

This section describes the conventions for file names and how the operating system works with them.

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11.2.1 Directories

In order to understand the syntax of file names, you need to understand how the file system is organized into a hierarchy of directories.

A directory is a file that contains information to associate other files with names; these associations are called links or directory entries. Sometimes, people speak of “files in a directory”, but in reality, a directory only contains pointers to files, not the files themselves.

The name of a file contained in a directory entry is called a file name component. In general, a file name consists of a sequence of one or more such components, separated by the slash character (`/'). A file name which is just one component names a file with respect to its directory. A file name with multiple components names a directory, and then a file in that directory, and so on.

Some other documents, such as the POSIX standard, use the term pathname for what we call a file name, and either filename or pathname component for what this manual calls a file name component. We don't use this terminology because a “path” is something completely different (a list of directories to search), and we think that “pathname” used for something else will confuse users. We always use “file name” and “file name component” (or sometimes just “component”, where the context is obvious) in GNU documentation. Some macros use the POSIX terminology in their names, such as PATH_MAX. These macros are defined by the POSIX standard, so we cannot change their names.

You can find more detailed information about operations on directories in File System Interface.

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11.2.2 File Name Resolution

A file name consists of file name components separated by slash (`/') characters. On the systems that the GNU C library supports, multiple successive `/' characters are equivalent to a single `/' character.

The process of determining what file a file name refers to is called file name resolution. This is performed by examining the components that make up a file name in left-to-right order, and locating each successive component in the directory named by the previous component. Of course, each of the files that are referenced as directories must actually exist, be directories instead of regular files, and have the appropriate permissions to be accessible by the process; otherwise the file name resolution fails.

If a file name begins with a `/', the first component in the file name is located in the root directory of the process (usually all processes on the system have the same root directory). Such a file name is called an absolute file name.

Otherwise, the first component in the file name is located in the current working directory (see Working Directory). This kind of file name is called a relative file name.

The file name components . (“dot”) and .. (“dot-dot”) have special meanings. Every directory has entries for these file name components. The file name component . refers to the directory itself, while the file name component .. refers to its parent directory (the directory that contains the link for the directory in question). As a special case, .. in the root directory refers to the root directory itself, since it has no parent; thus /.. is the same as /.

Here are some examples of file names:

/a
The file named a, in the root directory.
/a/b
The file named b, in the directory named a in the root directory.
a
The file named a, in the current working directory.
/a/./b
This is the same as /a/b.
./a
The file named a, in the current working directory.
../a
The file named a, in the parent directory of the current working directory.

A file name that names a directory may optionally end in a `/'. You can specify a file name of / to refer to the root directory, but the empty string is not a meaningful file name. If you want to refer to the current working directory, use a file name of . or ./.

Unlike some other operating systems, the GNU system doesn't have any built-in support for file types (or extensions) or file versions as part of its file name syntax. Many programs and utilities use conventions for file names—for example, files containing C source code usually have names suffixed with `.c'—but there is nothing in the file system itself that enforces this kind of convention.

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11.2.3 File Name Errors

Functions that accept file name arguments usually detect these errno error conditions relating to the file name syntax or trouble finding the named file. These errors are referred to throughout this manual as the usual file name errors.

EACCES
The process does not have search permission for a directory component of the file name.
ENAMETOOLONG
This error is used when either the total length of a file name is greater than PATH_MAX, or when an individual file name component has a length greater than NAME_MAX. See Limits for Files.

In the GNU system, there is no imposed limit on overall file name length, but some file systems may place limits on the length of a component.

ENOENT
This error is reported when a file referenced as a directory component in the file name doesn't exist, or when a component is a symbolic link whose target file does not exist. See Symbolic Links.
ENOTDIR
A file that is referenced as a directory component in the file name exists, but it isn't a directory.
ELOOP
Too many symbolic links were resolved while trying to look up the file name. The system has an arbitrary limit on the number of symbolic links that may be resolved in looking up a single file name, as a primitive way to detect loops. See Symbolic Links.

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11.2.4 Portability of File Names

The rules for the syntax of file names discussed in File Names, are the rules normally used by the GNU system and by other POSIX systems. However, other operating systems may use other conventions.

There are two reasons why it can be important for you to be aware of file name portability issues:

The ISO C standard says very little about file name syntax, only that file names are strings. In addition to varying restrictions on the length of file names and what characters can validly appear in a file name, different operating systems use different conventions and syntax for concepts such as structured directories and file types or extensions. Some concepts such as file versions might be supported in some operating systems and not by others.

The POSIX.1 standard allows implementations to put additional restrictions on file name syntax, concerning what characters are permitted in file names and on the length of file name and file name component strings. However, in the GNU system, you do not need to worry about these restrictions; any character except the null character is permitted in a file name string, and there are no limits on the length of file name strings.

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12 Input/Output on Streams

This chapter describes the functions for creating streams and performing input and output operations on them. As discussed in I/O Overview, a stream is a fairly abstract, high-level concept representing a communications channel to a file, device, or process.

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12.1 Streams

For historical reasons, the type of the C data structure that represents a stream is called FILE rather than “stream”. Since most of the library functions deal with objects of type FILE *, sometimes the term file pointer is also used to mean “stream”. This leads to unfortunate confusion over terminology in many books on C. This manual, however, is careful to use the terms “file” and “stream” only in the technical sense. The FILE type is declared in the header file stdio.h.

— Data Type: FILE

This is the data type used to represent stream objects. A FILE object holds all of the internal state information about the connection to the associated file, including such things as the file position indicator and buffering information. Each stream also has error and end-of-file status indicators that can be tested with the ferror and feof functions; see EOF and Errors.

FILE objects are allocated and managed internally by the input/output library functions. Don't try to create your own objects of type FILE; let the library do it. Your programs should deal only with pointers to these objects (that is, FILE * values) rather than the objects themselves.

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12.2 Standard Streams

When the main function of your program is invoked, it already has three predefined streams open and available for use. These represent the “standard” input and output channels that have been established for the process.

These streams are declared in the header file stdio.h.

— Variable: FILE * stdin

The standard input stream, which is the normal source of input for the program.

— Variable: FILE * stdout

The standard output stream, which is used for normal output from the program.

— Variable: FILE * stderr

The standard error stream, which is used for error messages and diagnostics issued by the program.

In the GNU system, you can specify what files or processes correspond to these streams using the pipe and redirection facilities provided by the shell. (The primitives shells use to implement these facilities are described in File System Interface.) Most other operating systems provide similar mechanisms, but the details of how to use them can vary.

In the GNU C library, stdin, stdout, and stderr are normal variables which you can set just like any others. For example, to redirect the standard output to a file, you could do: 

     fclose (stdout);
     stdout = fopen ("standard-output-file", "w");

Note however, that in other systems stdin, stdout, and stderr are macros that you cannot assign to in the normal way. But you can use freopen to get the effect of closing one and reopening it. See Opening Streams.

The three streams stdin, stdout, and stderr are not unoriented at program start (see Streams and I18N).

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12.3 Opening Streams

Opening a file with the fopen function creates a new stream and establishes a connection between the stream and a file. This may involve creating a new file.

Everything described in this section is declared in the header file stdio.h.

— Function: FILE * fopen (const char *filename, const char *opentype)

The fopen function opens a stream for I/O to the file filename, and returns a pointer to the stream.

The opentype argument is a string that controls how the file is opened and specifies attributes of the resulting stream. It must begin with one of the following sequences of characters:

`r'
Open an existing file for reading only.
`w'
Open the file for writing only. If the file already exists, it is truncated to zero length. Otherwise a new file is created.
`a'
Open a file for append access; that is, writing at the end of file only. If the file already exists, its initial contents are unchanged and output to the stream is appended to the end of the file. Otherwise, a new, empty file is created.
`r+'
Open an existing file for both reading and writing. The initial contents of the file are unchanged and the initial file position is at the beginning of the file.
`w+'
Open a file for both reading and writing. If the file already exists, it is truncated to zero length. Otherwise, a new file is created.
`a+'
Open or create file for both reading and appending. If the file exists, its initial contents are unchanged. Otherwise, a new file is created. The initial file position for reading is at the beginning of the file, but output is always appended to the end of the file.

As you can see, `+' requests a stream that can do both input and output. The ISO standard says that when using such a stream, you must call fflush (see Stream Buffering) or a file positioning function such as fseek (see File Positioning) when switching from reading to writing or vice versa. Otherwise, internal buffers might not be emptied properly. The GNU C library does not have this limitation; you can do arbitrary reading and writing operations on a stream in whatever order.

Additional characters may appear after these to specify flags for the call. Always put the mode (`r', `w+', etc.) first; that is the only part you are guaranteed will be understood by all systems.

The GNU C library defines one additional character for use in opentype: the character `x' insists on creating a new file—if a file filename already exists, fopen fails rather than opening it. If you use `x' you are guaranteed that you will not clobber an existing file. This is equivalent to the O_EXCL option to the open function (see Opening and Closing Files).

The character `b' in opentype has a standard meaning; it requests a binary stream rather than a text stream. But this makes no difference in POSIX systems (including the GNU system). If both `+' and `b' are specified, they can appear in either order. See Binary Streams.

If the opentype string contains the sequence ,ccs=STRING then STRING is taken as the name of a coded character set and fopen will mark the stream as wide-oriented which appropriate conversion functions in place to convert from and to the character set STRING is place. Any other stream is opened initially unoriented and the orientation is decided with the first file operation. If the first operation is a wide character operation, the stream is not only marked as wide-oriented, also the conversion functions to convert to the coded character set used for the current locale are loaded. This will not change anymore from this point on even if the locale selected for the LC_CTYPE category is changed.

Any other characters in opentype are simply ignored. They may be meaningful in other systems.

If the open fails, fopen returns a null pointer.

When the sources are compiling with _FILE_OFFSET_BITS == 64 on a 32 bit machine this function is in fact fopen64 since the LFS interface replaces transparently the old interface.

You can have multiple streams (or file descriptors) pointing to the same file open at the same time. If you do only input, this works straightforwardly, but you must be careful if any output streams are included. See Stream/Descriptor Precautions. This is equally true whether the streams are in one program (not usual) or in several programs (which can easily happen). It may be advantageous to use the file locking facilities to avoid simultaneous access. See File Locks.

— Function: FILE * fopen64 (const char *filename, const char *opentype)

This function is similar to fopen but the stream it returns a pointer for is opened using open64. Therefore this stream can be used even on files larger then 2^31 bytes on 32 bit machines.

Please note that the return type is still FILE *. There is no special FILE type for the LFS interface.

If the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32 bits machine this function is available under the name fopen and so transparently replaces the old interface.

— Macro: int FOPEN_MAX

The value of this macro is an integer constant expression that represents the minimum number of streams that the implementation guarantees can be open simultaneously. You might be able to open more than this many streams, but that is not guaranteed. The value of this constant is at least eight, which includes the three standard streams stdin, stdout, and stderr. In POSIX.1 systems this value is determined by the OPEN_MAX parameter; see General Limits. In BSD and GNU, it is controlled by the RLIMIT_NOFILE resource limit; see Limits on Resources.

— Function: FILE * freopen (const char *filename, const char *opentype, FILE *stream)

This function is like a combination of fclose and fopen. It first closes the stream referred to by stream, ignoring any errors that are detected in the process. (Because errors are ignored, you should not use freopen on an output stream if you have actually done any output using the stream.) Then the file named by filename is opened with mode opentype as for fopen, and associated with the same stream object stream.

If the operation fails, a null pointer is returned; otherwise, freopen returns stream.

freopen has traditionally been used to connect a standard stream such as stdin with a file of your own choice. This is useful in programs in which use of a standard stream for certain purposes is hard-coded. In the GNU C library, you can simply close the standard streams and open new ones with fopen. But other systems lack this ability, so using freopen is more portable.

When the sources are compiling with _FILE_OFFSET_BITS == 64 on a 32 bit machine this function is in fact freopen64 since the LFS interface replaces transparently the old interface.

— Function: FILE * freopen64 (const char *filename, const char *opentype, FILE *stream)

This function is similar to freopen. The only difference is that on 32 bit machine the stream returned is able to read beyond the 2^31 bytes limits imposed by the normal interface. It should be noted that the stream pointed to by stream need not be opened using fopen64 or freopen64 since its mode is not important for this function.

If the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32 bits machine this function is available under the name freopen and so transparently replaces the old interface.

In some situations it is useful to know whether a given stream is available for reading or writing. This information is normally not available and would have to be remembered separately. Solaris introduced a few functions to get this information from the stream descriptor and these functions are also available in the GNU C library.

— Function: int __freadable (FILE *stream)

The __freadable function determines whether the stream stream was opened to allow reading. In this case the return value is nonzero. For write-only streams the function returns zero.

This function is declared in stdio_ext.h.

— Function: int __fwritable (FILE *stream)

The __fwritable function determines whether the stream stream was opened to allow writing. In this case the return value is nonzero. For read-only streams the function returns zero.

This function is declared in stdio_ext.h.

For slightly different kind of problems there are two more functions. They provide even finer-grained information.

— Function: int __freading (FILE *stream)

The __freading function determines whether the stream stream was last read from or whether it is opened read-only. In this case the return value is nonzero, otherwise it is zero. Determining whether a stream opened for reading and writing was last used for writing allows to draw conclusions about the content about the buffer, among other things.

This function is declared in stdio_ext.h.

— Function: int __fwriting (FILE *stream)

The __fwriting function determines whether the stream stream was last written to or whether it is opened write-only. In this case the return value is nonzero, otherwise it is zero.

This function is declared in stdio_ext.h.

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12.4 Closing Streams

When a stream is closed with fclose, the connection between the stream and the file is canceled. After you have closed a stream, you cannot perform any additional operations on it.

— Function: int fclose (FILE *stream)

This function causes stream to be closed and the connection to the corresponding file to be broken. Any buffered output is written and any buffered input is discarded. The fclose function returns a value of 0 if the file was closed successfully, and EOF if an error was detected.

It is important to check for errors when you call fclose to close an output stream, because real, everyday errors can be detected at this time. For example, when fclose writes the remaining buffered output, it might get an error because the disk is full. Even if you know the buffer is empty, errors can still occur when closing a file if you are using NFS.

The function fclose is declared in stdio.h.

To close all streams currently available the GNU C Library provides another function.

— Function: int fcloseall (void)

This function causes all open streams of the process to be closed and the connection to corresponding files to be broken. All buffered data is written and any buffered input is discarded. The fcloseall function returns a value of 0 if all the files were closed successfully, and EOF if an error was detected.

This function should be used only in special situations, e.g., when an error occurred and the program must be aborted. Normally each single stream should be closed separately so that problems with individual streams can be identified. It is also problematic since the standard streams (see Standard Streams) will also be closed.

The function fcloseall is declared in stdio.h.

If the main function to your program returns, or if you call the exit function (see Normal Termination), all open streams are automatically closed properly. If your program terminates in any other manner, such as by calling the abort function (see Aborting a Program) or from a fatal signal (see Signal Handling), open streams might not be closed properly. Buffered output might not be flushed and files may be incomplete. For more information on buffering of streams, see Stream Buffering.

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12.5 Streams and Threads

Streams can be used in multi-threaded applications in the same way they are used in single-threaded applications. But the programmer must be aware of the possible complications. It is important to know about these also if the program one writes never use threads since the design and implementation of many stream functions is heavily influenced by the requirements added by multi-threaded programming.

The POSIX standard requires that by default the stream operations are atomic. I.e., issuing two stream operations for the same stream in two threads at the same time will cause the operations to be executed as if they were issued sequentially. The buffer operations performed while reading or writing are protected from other uses of the same stream. To do this each stream has an internal lock object which has to be (implicitly) acquired before any work can be done.

But there are situations where this is not enough and there are also situations where this is not wanted. The implicit locking is not enough if the program requires more than one stream function call to happen atomically. One example would be if an output line a program wants to generate is created by several function calls. The functions by themselves would ensure only atomicity of their own operation, but not atomicity over all the function calls. For this it is necessary to perform the stream locking in the application code.

— Function: void flockfile (FILE *stream)

The flockfile function acquires the internal locking object associated with the stream stream. This ensures that no other thread can explicitly through flockfile/ftrylockfile or implicit through a call of a stream function lock the stream. The thread will block until the lock is acquired. An explicit call to funlockfile has to be used to release the lock.

— Function: int ftrylockfile (FILE *stream)

The ftrylockfile function tries to acquire the internal locking object associated with the stream stream just like flockfile. But unlike flockfile this function does not block if the lock is not available. ftrylockfile returns zero if the lock was successfully acquired. Otherwise the stream is locked by another thread.

— Function: void funlockfile (FILE *stream)

The funlockfile function releases the internal locking object of the stream stream. The stream must have been locked before by a call to flockfile or a successful call of ftrylockfile. The implicit locking performed by the stream operations do not count. The funlockfile function does not return an error status and the behavior of a call for a stream which is not locked by the current thread is undefined.

The following example shows how the functions above can be used to generate an output line atomically even in multi-threaded applications (yes, the same job could be done with one fprintf call but it is sometimes not possible): 

     FILE *fp;
     {
        ...
        flockfile (fp);
        fputs ("This is test number ", fp);
        fprintf (fp, "%d\n", test);
        funlockfile (fp)
     }

Without the explicit locking it would be possible for another thread to use the stream fp after the fputs call return and before fprintf was called with the result that the number does not follow the word `number'.

From this description it might already be clear that the locking objects in streams are no simple mutexes. Since locking the same stream twice in the same thread is allowed the locking objects must be equivalent to recursive mutexes. These mutexes keep track of the owner and the number of times the lock is acquired. The same number of funlockfile calls by the same threads is necessary to unlock the stream completely. For instance: 

     void
     foo (FILE *fp)
     {
       ftrylockfile (fp);
       fputs ("in foo\n", fp);
       /* This is very wrong!!!  */
       funlockfile (fp);
     }

It is important here that the funlockfile function is only called if the ftrylockfile function succeeded in locking the stream. It is therefore always wrong to ignore the result of ftrylockfile. And it makes no sense since otherwise one would use flockfile. The result of code like that above is that either funlockfile tries to free a stream that hasn't been locked by the current thread or it frees the stream prematurely. The code should look like this: 

     void
     foo (FILE *fp)
     {
       if (ftrylockfile (fp) == 0)
         {
           fputs ("in foo\n", fp);
           funlockfile (fp);
         }
     }

Now that we covered why it is necessary to have these locking it is necessary to talk about situations when locking is unwanted and what can be done. The locking operations (explicit or implicit) don't come for free. Even if a lock is not taken the cost is not zero. The operations which have to be performed require memory operations that are safe in multi-processor environments. With the many local caches involved in such systems this is quite costly. So it is best to avoid the locking completely if it is not needed – because the code in question is never used in a context where two or more threads may use a stream at a time. This can be determined most of the time for application code; for library code which can be used in many contexts one should default to be conservative and use locking.

There are two basic mechanisms to avoid locking. The first is to use the _unlocked variants of the stream operations. The POSIX standard defines quite a few of those and the GNU library adds a few more. These variants of the functions behave just like the functions with the name without the suffix except that they do not lock the stream. Using these functions is very desirable since they are potentially much faster. This is not only because the locking operation itself is avoided. More importantly, functions like putc and getc are very simple and traditionally (before the introduction of threads) were implemented as macros which are very fast if the buffer is not empty. With the addition of locking requirements these functions are no longer implemented as macros since they would would expand to too much code. But these macros are still available with the same functionality under the new names putc_unlocked and getc_unlocked. This possibly huge difference of speed also suggests the use of the _unlocked functions even if locking is required. The difference is that the locking then has to be performed in the program: 

     void
     foo (FILE *fp, char *buf)
     {
       flockfile (fp);
       while (*buf != '/')
         putc_unlocked (*buf++, fp);
       funlockfile (fp);
     }

If in this example the putc function would be used and the explicit locking would be missing the putc function would have to acquire the lock in every call, potentially many times depending on when the loop terminates. Writing it the way illustrated above allows the putc_unlocked macro to be used which means no locking and direct manipulation of the buffer of the stream.

A second way to avoid locking is by using a non-standard function which was introduced in Solaris and is available in the GNU C library as well.

— Function: int __fsetlocking (FILE *stream, int type)

The __fsetlocking function can be used to select whether the stream operations will implicitly acquire the locking object of the stream stream. By default this is done but it can be disabled and reinstated using this function. There are three values defined for the type parameter.

FSETLOCKING_INTERNAL
The stream stream will from now on use the default internal locking. Every stream operation with exception of the _unlocked variants will implicitly lock the stream.
FSETLOCKING_BYCALLER
After the __fsetlocking function returns the user is responsible for locking the stream. None of the stream operations will implicitly do this anymore until the state is set back to FSETLOCKING_INTERNAL.
FSETLOCKING_QUERY
__fsetlocking only queries the current locking state of the stream. The return value will be FSETLOCKING_INTERNAL or FSETLOCKING_BYCALLER depending on the state.

The return value of __fsetlocking is either FSETLOCKING_INTERNAL or FSETLOCKING_BYCALLER depending on the state of the stream before the call.

This function and the values for the type parameter are declared in stdio_ext.h.

This function is especially useful when program code has to be used which is written without knowledge about the _unlocked functions (or if the programmer was too lazy to use them).

Next: , Previous: Streams and Threads, Up: I/O on Streams

12.6 Streams in Internationalized Applications

ISO C90 introduced the new type wchar_t to allow handling larger character sets. What was missing was a possibility to output strings of wchar_t directly. One had to convert them into multibyte strings using mbstowcs (there was no mbsrtowcs yet) and then use the normal stream functions. While this is doable it is very cumbersome since performing the conversions is not trivial and greatly increases program complexity and size.

The Unix standard early on (I think in XPG4.2) introduced two additional format specifiers for the printf and scanf families of functions. Printing and reading of single wide characters was made possible using the %C specifier and wide character strings can be handled with %S. These modifiers behave just like %c and %s only that they expect the corresponding argument to have the wide character type and that the wide character and string are transformed into/from multibyte strings before being used.

This was a beginning but it is still not good enough. Not always is it desirable to use printf and scanf. The other, smaller and faster functions cannot handle wide characters. Second, it is not possible to have a format string for printf and scanf consisting of wide characters. The result is that format strings would have to be generated if they have to contain non-basic characters.

In the Amendment 1 to ISO C90 a whole new set of functions was added to solve the problem. Most of the stream functions got a counterpart which take a wide character or wide character string instead of a character or string respectively. The new functions operate on the same streams (like stdout). This is different from the model of the C++ runtime library where separate streams for wide and normal I/O are used.

Being able to use the same stream for wide and normal operations comes with a restriction: a stream can be used either for wide operations or for normal operations. Once it is decided there is no way back. Only a call to freopen or freopen64 can reset the orientation. The orientation can be decided in three ways:

It is important to never mix the use of wide and not wide operations on a stream. There are no diagnostics issued. The application behavior will simply be strange or the application will simply crash. The fwide function can help avoiding this.

— Function: int fwide (FILE *stream, int mode)

The fwide function can be used to set and query the state of the orientation of the stream stream. If the mode parameter has a positive value the streams get wide oriented, for negative values narrow oriented. It is not possible to overwrite previous orientations with fwide. I.e., if the stream stream was already oriented before the call nothing is done.

If mode is zero the current orientation state is queried and nothing is changed.

The fwide function returns a negative value, zero, or a positive value if the stream is narrow, not at all, or wide oriented respectively.

This function was introduced in Amendment 1 to ISO C90 and is declared in wchar.h.

It is generally a good idea to orient a stream as early as possible. This can prevent surprise especially for the standard streams stdin, stdout, and stderr. If some library function in some situations uses one of these streams and this use orients the stream in a different way the rest of the application expects it one might end up with hard to reproduce errors. Remember that no errors are signal if the streams are used incorrectly. Leaving a stream unoriented after creation is normally only necessary for library functions which create streams which can be used in different contexts.

When writing code which uses streams and which can be used in different contexts it is important to query the orientation of the stream before using it (unless the rules of the library interface demand a specific orientation). The following little, silly function illustrates this. 

     void
     print_f (FILE *fp)
     {
       if (fwide (fp, 0) > 0)
         /* Positive return value means wide orientation.  */
         fputwc (L'f', fp);
       else
         fputc ('f', fp);
     }

Note that in this case the function print_f decides about the orientation of the stream if it was unoriented before (will not happen if the advise above is followed).

The encoding used for the wchar_t values is unspecified and the user must not make any assumptions about it. For I/O of wchar_t values this means that it is impossible to write these values directly to the stream. This is not what follows from the ISO C locale model either. What happens instead is that the bytes read from or written to the underlying media are first converted into the internal encoding chosen by the implementation for wchar_t. The external encoding is determined by the LC_CTYPE category of the current locale or by the `ccs' part of the mode specification given to fopen, fopen64, freopen, or freopen64. How and when the conversion happens is unspecified and it happens invisible to the user.

Since a stream is created in the unoriented state it has at that point no conversion associated with it. The conversion which will be used is determined by the LC_CTYPE category selected at the time the stream is oriented. If the locales are changed at the runtime this might produce surprising results unless one pays attention. This is just another good reason to orient the stream explicitly as soon as possible, perhaps with a call to fwide.

Next: , Previous: Streams and I18N, Up: I/O on Streams

12.7 Simple Output by Characters or Lines

This section describes functions for performing character- and line-oriented output.

These narrow streams functions are declared in the header file stdio.h and the wide stream functions in wchar.h.

— Function: int fputc (int c, FILE *stream)

The fputc function converts the character c to type unsigned char, and writes it to the stream stream. EOF is returned if a write error occurs; otherwise the character c is returned.

— Function: wint_t fputwc (wchar_t wc, FILE *stream)

The fputwc function writes the wide character wc to the stream stream. WEOF is returned if a write error occurs; otherwise the character wc is returned.

— Function: int fputc_unlocked (int c, FILE *stream)

The fputc_unlocked function is equivalent to the fputc function except that it does not implicitly lock the stream.

— Function: wint_t fputwc_unlocked (wint_t wc, FILE *stream)

The fputwc_unlocked function is equivalent to the fputwc function except that it does not implicitly lock the stream.

This function is a GNU extension.

— Function: int putc (int c, FILE *stream)

This is just like fputc, except that most systems implement it as a macro, making it faster. One consequence is that it may evaluate the stream argument more than once, which is an exception to the general rule for macros. putc is usually the best function to use for writing a single character.

— Function: wint_t putwc (wchar_t wc, FILE *stream)

This is just like fputwc, except that it can be implement as a macro, making it faster. One consequence is that it may evaluate the stream argument more than once, which is an exception to the general rule for macros. putwc is usually the best function to use for writing a single wide character.

— Function: int putc_unlocked (int c, FILE *stream)

The putc_unlocked function is equivalent to the putc function except that it does not implicitly lock the stream.

— Function: wint_t putwc_unlocked (wchar_t wc, FILE *stream)

The putwc_unlocked function is equivalent to the putwc function except that it does not implicitly lock the stream.

This function is a GNU extension.

— Function: int putchar (int c)

The putchar function is equivalent to putc with stdout as the value of the stream argument.

— Function: wint_t putwchar (wchar_t wc)

The putwchar function is equivalent to putwc with stdout as the value of the stream argument.

— Function: int putchar_unlocked (int c)

The putchar_unlocked function is equivalent to the putchar function except that it does not implicitly lock the stream.

— Function: wint_t putwchar_unlocked (wchar_t wc)

The putwchar_unlocked function is equivalent to the putwchar function except that it does not implicitly lock the stream.

This function is a GNU extension.

— Function: int fputs (const char *s, FILE *stream)

The function fputs writes the string s to the stream stream. The terminating null character is not written. This function does not add a newline character, either. It outputs only the characters in the string.

This function returns EOF if a write error occurs, and otherwise a non-negative value.

For example: 

          fputs ("Are ", stdout);
          fputs ("you ", stdout);
          fputs ("hungry?\n", stdout);

outputs the text `Are you hungry?' followed by a newline.

— Function: int fputws (const wchar_t *ws, FILE *stream)

The function fputws writes the wide character string ws to the stream stream. The terminating null character is not written. This function does not add a newline character, either. It outputs only the characters in the string.

This function returns WEOF if a write error occurs, and otherwise a non-negative value.

— Function: int fputs_unlocked (const char *s, FILE *stream)

The fputs_unlocked function is equivalent to the fputs function except that it does not implicitly lock the stream.

This function is a GNU extension.

— Function: int fputws_unlocked (const wchar_t *ws, FILE *stream)

The fputws_unlocked function is equivalent to the fputws function except that it does not implicitly lock the stream.

This function is a GNU extension.

— Function: int puts (const char *s)

The puts function writes the string s to the stream stdout followed by a newline. The terminating null character of the string is not written. (Note that fputs does not write a newline as this function does.)

puts is the most convenient function for printing simple messages. For example: 

          puts ("This is a message.");

outputs the text `This is a message.' followed by a newline.

— Function: int putw (int w, FILE *stream)

This function writes the word w (that is, an int) to stream. It is provided for compatibility with SVID, but we recommend you use fwrite instead (see Block Input/Output).

Next: , Previous: Simple Output, Up: I/O on Streams

12.8 Character Input

This section describes functions for performing character-oriented input. These narrow streams functions are declared in the header file stdio.h and the wide character functions are declared in wchar.h. These functions return an int or wint_t value (for narrow and wide stream functions respectively) that is either a character of input, or the special value EOF/WEOF (usually -1). For the narrow stream functions it is important to store the result of these functions in a variable of type int instead of char, even when you plan to use it only as a character. Storing EOF in a char variable truncates its value to the size of a character, so that it is no longer distinguishable from the valid character `(char) -1'. So always use an int for the result of getc and friends, and check for EOF after the call; once you've verified that the result is not EOF, you can be sure that it will fit in a `char' variable without loss of information.

— Function: int fgetc (FILE *stream)

This function reads the next character as an unsigned char from the stream stream and returns its value, converted to an int. If an end-of-file condition or read error occurs, EOF is returned instead.

— Function: wint_t fgetwc (FILE *stream)

This function reads the next wide character from the stream stream and returns its value. If an end-of-file condition or read error occurs, WEOF is returned instead.

— Function: int fgetc_unlocked (FILE *stream)

The fgetc_unlocked function is equivalent to the fgetc function except that it does not implicitly lock the stream.

— Function: wint_t fgetwc_unlocked (FILE *stream)

The fgetwc_unlocked function is equivalent to the fgetwc function except that it does not implicitly lock the stream.

This function is a GNU extension.

— Function: int getc (FILE *stream)

This is just like fgetc, except that it is permissible (and typical) for it to be implemented as a macro that evaluates the stream argument more than once. getc is often highly optimized, so it is usually the best function to use to read a single character.

— Function: wint_t getwc (FILE *stream)

This is just like fgetwc, except that it is permissible for it to be implemented as a macro that evaluates the stream argument more than once. getwc can be highly optimized, so it is usually the best function to use to read a single wide character.

— Function: int getc_unlocked (FILE *stream)

The getc_unlocked function is equivalent to the getc function except that it does not implicitly lock the stream.

— Function: wint_t getwc_unlocked (FILE *stream)

The getwc_unlocked function is equivalent to the getwc function except that it does not implicitly lock the stream.

This function is a GNU extension.

— Function: int getchar (void)

The getchar function is equivalent to getc with stdin as the value of the stream argument.

— Function: wint_t getwchar (void)

The getwchar function is equivalent to getwc with stdin as the value of the stream argument.

— Function: int getchar_unlocked (void)

The getchar_unlocked function is equivalent to the getchar function except that it does not implicitly lock the stream.

— Function: wint_t getwchar_unlocked (void)

The getwchar_unlocked function is equivalent to the getwchar function except that it does not implicitly lock the stream.

This function is a GNU extension.

Here is an example of a function that does input using fgetc. It would work just as well using getc instead, or using getchar () instead of fgetc (stdin). The code would also work the same for the wide character stream functions. 

     int
     y_or_n_p (const char *question)
     {
       fputs (question, stdout);
       while (1)
         {
           int c, answer;
           /* Write a space to separate answer from question. */
           fputc (' ', stdout);
           /* Read the first character of the line.
              This should be the answer character, but might not be. */
           c = tolower (fgetc (stdin));
           answer = c;
           /* Discard rest of input line. */
           while (c != '\n' && c != EOF)
             c = fgetc (stdin);
           /* Obey the answer if it was valid. */
           if (answer == 'y')
             return 1;
           if (answer == 'n')
             return 0;
           /* Answer was invalid: ask for valid answer. */
           fputs ("Please answer y or n:", stdout);
         }
     }
— Function: int getw (FILE *stream)

This function reads a word (that is, an int) from stream. It's provided for compatibility with SVID. We recommend you use fread instead (see Block Input/Output). Unlike getc, any int value could be a valid result. getw returns EOF when it encounters end-of-file or an error, but there is no way to distinguish this from an input word with value -1.

Next: , Previous: Character Input, Up: I/O on Streams

12.9 Line-Oriented Input

Since many programs interpret input on the basis of lines, it is convenient to have functions to read a line of text from a stream.

Standard C has functions to do this, but they aren't very safe: null characters and even (for gets) long lines can confuse them. So the GNU library provides the nonstandard getline function that makes it easy to read lines reliably.

Another GNU extension, getdelim, generalizes getline. It reads a delimited record, defined as everything through the next occurrence of a specified delimiter character.

All these functions are declared in stdio.h.

— Function: ssize_t getline (char **lineptr, size_t *n, FILE *stream)

This function reads an entire line from stream, storing the text (including the newline and a terminating null character) in a buffer and storing the buffer address in *lineptr.

Before calling getline, you should place in *lineptr the address of a buffer *n bytes long, allocated with malloc. If this buffer is long enough to hold the line, getline stores the line in this buffer. Otherwise, getline makes the buffer bigger using realloc, storing the new buffer address back in *lineptr and the increased size back in *n. See Unconstrained Allocation.

If you set *lineptr to a null pointer, and *n to zero, before the call, then getline allocates the initial buffer for you by calling malloc.

In either case, when getline returns, *lineptr is a char * which points to the text of the line.

When getline is successful, it returns the number of characters read (including the newline, but not including the terminating null). This value enables you to distinguish null characters that are part of the line from the null character inserted as a terminator.

This function is a GNU extension, but it is the recommended way to read lines from a stream. The alternative standard functions are unreliable.

If an error occurs or end of file is reached without any bytes read, getline returns -1.

— Function: ssize_t getdelim (char **lineptr, size_t *n, int delimiter, FILE *stream)

This function is like getline except that the character which tells it to stop reading is not necessarily newline. The argument delimiter specifies the delimiter character; getdelim keeps reading until it sees that character (or end of file).

The text is stored in lineptr, including the delimiter character and a terminating null. Like getline, getdelim makes lineptr bigger if it isn't big enough.

getline is in fact implemented in terms of getdelim, just like this: 

          ssize_t
          getline (char **lineptr, size_t *n, FILE *stream)
          {
            return getdelim (lineptr, n, '\n', stream);
          }
— Function: char * fgets (char *s, int count, FILE *stream)

The fgets function reads characters from the stream stream up to and including a newline character and stores them in the string s, adding a null character to mark the end of the string. You must supply count characters worth of space in s, but the number of characters read is at most count − 1. The extra character space is used to hold the null character at the end of the string.

If the system is already at end of file when you call fgets, then the contents of the array s are unchanged and a null pointer is returned. A null pointer is also returned if a read error occurs. Otherwise, the return value is the pointer s.

Warning: If the input data has a null character, you can't tell. So don't use fgets unless you know the data cannot contain a null. Don't use it to read files edited by the user because, if the user inserts a null character, you should either handle it properly or print a clear error message. We recommend using getline instead of fgets.

— Function: wchar_t * fgetws (wchar_t *ws, int count, FILE *stream)

The fgetws function reads wide characters from the stream stream up to and including a newline character and stores them in the string ws, adding a null wide character to mark the end of the string. You must supply count wide characters worth of space in ws, but the number of characters read is at most count − 1. The extra character space is used to hold the null wide character at the end of the string.

If the system is already at end of file when you call fgetws, then the contents of the array ws are unchanged and a null pointer is returned. A null pointer is also returned if a read error occurs. Otherwise, the return value is the pointer ws.

Warning: If the input data has a null wide character (which are null bytes in the input stream), you can't tell. So don't use fgetws unless you know the data cannot contain a null. Don't use it to read files edited by the user because, if the user inserts a null character, you should either handle it properly or print a clear error message.

— Function: char * fgets_unlocked (char *s, int count, FILE *stream)

The fgets_unlocked function is equivalent to the fgets function except that it does not implicitly lock the stream.

This function is a GNU extension.

— Function: wchar_t * fgetws_unlocked (wchar_t *ws, int count, FILE *stream)

The fgetws_unlocked function is equivalent to the fgetws function except that it does not implicitly lock the stream.

This function is a GNU extension.

— Deprecated function: char * gets (char *s)

The function gets reads characters from the stream stdin up to the next newline character, and stores them in the string s. The newline character is discarded (note that this differs from the behavior of fgets, which copies the newline character into the string). If gets encounters a read error or end-of-file, it returns a null pointer; otherwise it returns s.

Warning: The gets function is very dangerous because it provides no protection against overflowing the string s. The GNU library includes it for compatibility only. You should always use fgets or getline instead. To remind you of this, the linker (if using GNU ld) will issue a warning whenever you use gets.

Next: , Previous: Line Input, Up: I/O on Streams

12.10 Unreading

In parser programs it is often useful to examine the next character in the input stream without removing it from the stream. This is called “peeking ahead” at the input because your program gets a glimpse of the input it will read next.

Using stream I/O, you can peek ahead at input by first reading it and then unreading it (also called pushing it back on the stream). Unreading a character makes it available to be input again from the stream, by the next call to fgetc or other input function on that stream.

Next: , Up: Unreading

12.10.1 What Unreading Means

Here is a pictorial explanation of unreading. Suppose you have a stream reading a file that contains just six characters, the letters `foobar'. Suppose you have read three characters so far. The situation looks like this: 

     f  o  o  b  a  r
              ^

so the next input character will be `b'.

If instead of reading `b' you unread the letter `o', you get a situation like this: 

     f  o  o  b  a  r
              |
           o--
           ^

so that the next input characters will be `o' and `b'.

If you unread `9' instead of `o', you get this situation: 

     f  o  o  b  a  r
              |
           9--
           ^

so that the next input characters will be `9' and `b'.

Previous: Unreading Idea, Up: Unreading

12.10.2 Using ungetc To Do Unreading

The function to unread a character is called ungetc, because it reverses the action of getc.

— Function: int ungetc (int c, FILE *stream)

The ungetc function pushes back the character c onto the input stream stream. So the next input from stream will read c before anything else.

If c is EOF, ungetc does nothing and just returns EOF. This lets you call ungetc with the return value of getc without needing to check for an error from getc.

The character that you push back doesn't have to be the same as the last character that was actually read from the stream. In fact, it isn't necessary to actually read any characters from the stream before unreading them with ungetc! But that is a strange way to write a program; usually ungetc is used only to unread a character that was just read from the same stream. The GNU C library supports this even on files opened in binary mode, but other systems might not.

The GNU C library only supports one character of pushback—in other words, it does not work to call ungetc twice without doing input in between. Other systems might let you push back multiple characters; then reading from the stream retrieves the characters in the reverse order that they were pushed.

Pushing back characters doesn't alter the file; only the internal buffering for the stream is affected. If a file positioning function (such as fseek, fseeko or rewind; see File Positioning) is called, any pending pushed-back characters are discarded.

Unreading a character on a stream that is at end of file clears the end-of-file indicator for the stream, because it makes the character of input available. After you read that character, trying to read again will encounter end of file.

— Function: wint_t ungetwc (wint_t wc, FILE *stream)

The ungetwc function behaves just like ungetc just that it pushes back a wide character.

Here is an example showing the use of getc and ungetc to skip over whitespace characters. When this function reaches a non-whitespace character, it unreads that character to be seen again on the next read operation on the stream. 

     #include <stdio.h>
     #include <ctype.h>
     
     void
     skip_whitespace (FILE *stream)
     {
       int c;
       do
         /* No need to check for EOF because it is not
            isspace, and ungetc ignores EOF.  */
         c = getc (stream);
       while (isspace (c));
       ungetc (c, stream);
     }

Next: , Previous: Unreading, Up: I/O on Streams

12.11 Block Input/Output

This section describes how to do input and output operations on blocks of data. You can use these functions to read and write binary data, as well as to read and write text in fixed-size blocks instead of by characters or lines. Binary files are typically used to read and write blocks of data in the same format as is used to represent the data in a running program. In other words, arbitrary blocks of memory—not just character or string objects—can be written to a binary file, and meaningfully read in again by the same program.

Storing data in binary form is often considerably more efficient than using the formatted I/O functions. Also, for floating-point numbers, the binary form avoids possible loss of precision in the conversion process. On the other hand, binary files can't be examined or modified easily using many standard file utilities (such as text editors), and are not portable between different implementations of the language, or different kinds of computers.

These functions are declared in stdio.h.

— Function: size_t fread (void *data, size_t size, size_t count, FILE *stream)

This function reads up to count objects of size size into the array data, from the stream stream. It returns the number of objects actually read, which might be less than count if a read error occurs or the end of the file is reached. This function returns a value of zero (and doesn't read anything) if either size or count is zero.

If fread encounters end of file in the middle of an object, it returns the number of complete objects read, and discards the partial object. Therefore, the stream remains at the actual end of the file.

— Function: size_t fread_unlocked (void *data, size_t size, size_t count, FILE *stream)

The fread_unlocked function is equivalent to the fread function except that it does not implicitly lock the stream.

This function is a GNU extension.

— Function: size_t fwrite (const void *data, size_t size, size_t count, FILE *stream)

This function writes up to count objects of size size from the array data, to the stream stream. The return value is normally count, if the call succeeds. Any other value indicates some sort of error, such as running out of space.

— Function: size_t fwrite_unlocked (const void *data, size_t size, size_t count, FILE *stream)

The fwrite_unlocked function is equivalent to the fwrite function except that it does not implicitly lock the stream.

This function is a GNU extension.

Next: , Previous: Block Input/Output, Up: I/O on Streams

12.12 Formatted Output

The functions described in this section (printf and related functions) provide a convenient way to perform formatted output. You call printf with a format string or template string that specifies how to format the values of the remaining arguments.

Unless your program is a filter that specifically performs line- or character-oriented processing, using printf or one of the other related functions described in this section is usually the easiest and most concise way to perform output. These functions are especially useful for printing error messages, tables of data, and the like.

Next: , Up: Formatted Output

12.12.1 Formatted Output Basics

The printf function can be used to print any number of arguments. The template string argument you supply in a call provides information not only about the number of additional arguments, but also about their types and what style should be used for printing them.

Ordinary characters in the template string are simply written to the output stream as-is, while conversion specifications introduced by a `%' character in the template cause subsequent arguments to be formatted and written to the output stream. For example, 

     int pct = 37;
     char filename[] = "foo.txt";
     printf ("Processing of `%s' is %d%% finished.\nPlease be patient.\n",
             filename, pct);

produces output like 

     Processing of `foo.txt' is 37% finished.
     Please be patient.

This example shows the use of the `%d' conversion to specify that an int argument should be printed in decimal notation, the `%s' conversion to specify printing of a string argument, and the `%%' conversion to print a literal `%' character.

There are also conversions for printing an integer argument as an unsigned value in octal, decimal, or hexadecimal radix (`%o', `%u', or `%x', respectively); or as a character value (`%c').

Floating-point numbers can be printed in normal, fixed-point notation using the `%f' conversion or in exponential notation using the `%e' conversion. The `%g' conversion uses either `%e' or `%f' format, depending on what is more appropriate for the magnitude of the particular number.

You can control formatting more precisely by writing modifiers between the `%' and the character that indicates which conversion to apply. These slightly alter the ordinary behavior of the conversion. For example, most conversion specifications permit you to specify a minimum field width and a flag indicating whether you want the result left- or right-justified within the field.

The specific flags and modifiers that are permitted and their interpretation vary depending on the particular conversion. They're all described in more detail in the following sections. Don't worry if this all seems excessively complicated at first; you can almost always get reasonable free-format output without using any of the modifiers at all. The modifiers are mostly used to make the output look “prettier” in tables.

Next: , Previous: Formatted Output Basics, Up: Formatted Output

12.12.2 Output Conversion Syntax

This section provides details about the precise syntax of conversion specifications that can appear in a printf template string.

Characters in the template string that are not part of a conversion specification are printed as-is to the output stream. Multibyte character sequences (see Character Set Handling) are permitted in a template string.

The conversion specifications in a printf template string have the general form: 

     % [ param-no $] flags width [ . precision ] type conversion

or 

     % [ param-no $] flags width . * [ param-no $] type conversion

For example, in the conversion specifier `%-10.8ld', the `-' is a flag, `10' specifies the field width, the precision is `8', the letter `l' is a type modifier, and `d' specifies the conversion style. (This particular type specifier says to print a long int argument in decimal notation, with a minimum of 8 digits left-justified in a field at least 10 characters wide.)

In more detail, output conversion specifications consist of an initial `%' character followed in sequence by:

The exact options that are permitted and how they are interpreted vary between the different conversion specifiers. See the descriptions of the individual conversions for information about the particular options that they use.

With the `-Wformat' option, the GNU C compiler checks calls to printf and related functions. It examines the format string and verifies that the correct number and types of arguments are supplied. There is also a GNU C syntax to tell the compiler that a function you write uses a printf-style format string. See Declaring Attributes of Functions, for more information.

Next: , Previous: Output Conversion Syntax, Up: Formatted Output

12.12.3 Table of Output Conversions

Here is a table summarizing what all the different conversions do:

`%d', `%i'
Print an integer as a signed decimal number. See Integer Conversions, for details. `%d' and `%i' are synonymous for output, but are different when used with scanf for input (see Table of Input Conversions).
`%o'
Print an integer as an unsigned octal number. See Integer Conversions, for details.
`%u'
Print an integer as an unsigned decimal number. See Integer Conversions, for details.
`%x', `%X'
Print an integer as an unsigned hexadecimal number. `%x' uses lower-case letters and `%X' uses upper-case. See Integer Conversions, for details.
`%f'
Print a floating-point number in normal (fixed-point) notation. See Floating-Point Conversions, for details.
`%e', `%E'
Print a floating-point number in exponential notation. `%e' uses lower-case letters and `%E' uses upper-case. See Floating-Point Conversions, for details.
`%g', `%G'
Print a floating-point number in either normal or exponential notation, whichever is more appropriate for its magnitude. `%g' uses lower-case letters and `%G' uses upper-case. See Floating-Point Conversions, for details.
`%a', `%A'
Print a floating-point number in a hexadecimal fractional notation which the exponent to base 2 represented in decimal digits. `%a' uses lower-case letters and `%A' uses upper-case. See Floating-Point Conversions, for details.
`%c'
Print a single character. See Other Output Conversions.
`%C'
This is an alias for `%lc' which is supported for compatibility with the Unix standard.
`%s'
Print a string. See Other Output Conversions.
`%S'
This is an alias for `%ls' which is supported for compatibility with the Unix standard.
`%p'
Print the value of a pointer. See Other Output Conversions.
`%n'
Get the number of characters printed so far. See Other Output Conversions. Note that this conversion specification never produces any output.
`%m'
Print the string corresponding to the value of errno. (This is a GNU extension.) See Other Output Conversions.
`%%'
Print a literal `%' character. See Other Output Conversions.

If the syntax of a conversion specification is invalid, unpredictable things will happen, so don't do this. If there aren't enough function arguments provided to supply values for all the conversion specifications in the template string, or if the arguments are not of the correct types, the results are unpredictable. If you supply more arguments than conversion specifications, the extra argument values are simply ignored; this is sometimes useful.

Next: , Previous: Table of Output Conversions, Up: Formatted Output

12.12.4 Integer Conversions

This section describes the options for the `%d', `%i', `%o', `%u', `%x', and `%X' conversion specifications. These conversions print integers in various formats.

The `%d' and `%i' conversion specifications both print an int argument as a signed decimal number; while `%o', `%u', and `%x' print the argument as an unsigned octal, decimal, or hexadecimal number (respectively). The `%X' conversion specification is just like `%x' except that it uses the characters `ABCDEF' as digits instead of `abcdef'.

The following flags are meaningful:

`-'
Left-justify the result in the field (instead of the normal right-justification).
`+'
For the signed `%d' and `%i' conversions, print a plus sign if the value is positive.
` '
For the signed `%d' and `%i' conversions, if the result doesn't start with a plus or minus sign, prefix it with a space character instead. Since the `+' flag ensures that the result includes a sign, this flag is ignored if you supply both of them.
`#'
For the `%o' conversion, this forces the leading digit to be `0', as if by increasing the precision. For `%x' or `%X', this prefixes a leading `0x' or `0X' (respectively) to the result. This doesn't do anything useful for the `%d', `%i', or `%u' conversions. Using this flag produces output which can be parsed by the strtoul function (see Parsing of Integers) and scanf with the `%i' conversion (see Numeric Input Conversions).
`''
Separate the digits into groups as specified by the locale specified for the LC_NUMERIC category; see General Numeric. This flag is a GNU extension.
`0'
Pad the field with zeros instead of spaces. The zeros are placed after any indication of sign or base. This flag is ignored if the `-' flag is also specified, or if a precision is specified.

If a precision is supplied, it specifies the minimum number of digits to appear; leading zeros are produced if necessary. If you don't specify a precision, the number is printed with as many digits as it needs. If you convert a value of zero with an explicit precision of zero, then no characters at all are produced.

Without a type modifier, the corresponding argument is treated as an int (for the signed conversions `%i' and `%d') or unsigned int (for the unsigned conversions `%o', `%u', `%x', and `%X'). Recall that since printf and friends are variadic, any char and short arguments are automatically converted to int by the default argument promotions. For arguments of other integer types, you can use these modifiers:

`hh'
Specifies that the argument is a signed char or unsigned char, as appropriate. A char argument is converted to an int or unsigned int by the default argument promotions anyway, but the `h' modifier says to convert it back to a char again.

This modifier was introduced in ISO C99.

`h'
Specifies that the argument is a short int or unsigned short int, as appropriate. A short argument is converted to an int or unsigned int by the default argument promotions anyway, but the `h' modifier says to convert it back to a short again.
`j'
Specifies that the argument is a intmax_t or uintmax_t, as appropriate.

This modifier was introduced in ISO C99.

`l'
Specifies that the argument is a long int or unsigned long int, as appropriate. Two `l' characters is like the `L' modifier, below.

If used with `%c' or `%s' the corresponding parameter is considered as a wide character or wide character string respectively. This use of `l' was introduced in Amendment 1 to ISO C90.

`L'
`ll'
`q'
Specifies that the argument is a long long int. (This type is an extension supported by the GNU C compiler. On systems that don't support extra-long integers, this is the same as long int.)

The `q' modifier is another name for the same thing, which comes from 4.4 BSD; a long long int is sometimes called a “quad” int.

`t'
Specifies that the argument is a ptrdiff_t.

This modifier was introduced in ISO C99.

`z'
`Z'
Specifies that the argument is a size_t.

`z' was introduced in ISO C99. `Z' is a GNU extension predating this addition and should not be used in new code.

Here is an example. Using the template string: 

     "|%5d|%-5d|%+5d|%+-5d|% 5d|%05d|%5.0d|%5.2d|%d|\n"

to print numbers using the different options for the `%d' conversion gives results like: 

     |    0|0    |   +0|+0   |    0|00000|     |   00|0|
     |    1|1    |   +1|+1   |    1|00001|    1|   01|1|
     |   -1|-1   |   -1|-1   |   -1|-0001|   -1|  -01|-1|
     |100000|100000|+100000|+100000| 100000|100000|100000|100000|100000|

In particular, notice what happens in the last case where the number is too large to fit in the minimum field width specified.

Here are some more examples showing how unsigned integers print under various format options, using the template string: 

     "|%5u|%5o|%5x|%5X|%#5o|%#5x|%#5X|%#10.8x|\n"

     |    0|    0|    0|    0|    0|    0|    0|  00000000|
     |    1|    1|    1|    1|   01|  0x1|  0X1|0x00000001|
     |100000|303240|186a0|186A0|0303240|0x186a0|0X186A0|0x000186a0|

Next: , Previous: Integer Conversions, Up: Formatted Output

12.12.5 Floating-Point Conversions

This section discusses the conversion specifications for floating-point numbers: the `%f', `%e', `%E', `%g', and `%G' conversions.

The `%f' conversion prints its argument in fixed-point notation, producing output of the form [-]ddd.ddd, where the number of digits following the decimal point is controlled by the precision you specify.

The `%e' conversion prints its argument in exponential notation, producing output of the form [-]d.ddde[+|-]dd. Again, the number of digits following the decimal point is controlled by the precision. The exponent always contains at least two digits. The `%E' conversion is similar but the exponent is marked with the letter `E' instead of `e'.

The `%g' and `%G' conversions print the argument in the style of `%e' or `%E' (respectively) if the exponent would be less than -4 or greater than or equal to the precision; otherwise they use the `%f' style. A precision of 0, is taken as 1. Trailing zeros are removed from the fractional portion of the result and a decimal-point character appears only if it is followed by a digit.

The `%a' and `%A' conversions are meant for representing floating-point numbers exactly in textual form so that they can be exchanged as texts between different programs and/or machines. The numbers are represented is the form [-]0xh.hhhp[+|-]dd. At the left of the decimal-point character exactly one digit is print. This character is only 0 if the number is denormalized. Otherwise the value is unspecified; it is implementation dependent how many bits are used. The number of hexadecimal digits on the right side of the decimal-point character is equal to the precision. If the precision is zero it is determined to be large enough to provide an exact representation of the number (or it is large enough to distinguish two adjacent values if the FLT_RADIX is not a power of 2, see Floating Point Parameters). For the `%a' conversion lower-case characters are used to represent the hexadecimal number and the prefix and exponent sign are printed as 0x and p respectively. Otherwise upper-case characters are used and 0X and P are used for the representation of prefix and exponent string. The exponent to the base of two is printed as a decimal number using at least one digit but at most as many digits as necessary to represent the value exactly.

If the value to be printed represents infinity or a NaN, the output is [-]inf or nan respectively if the conversion specifier is `%a', `%e', `%f', or `%g' and it is [-]INF or NAN respectively if the conversion is `%A', `%E', or `%G'.

The following flags can be used to modify the behavior:

`-'
Left-justify the result in the field. Normally the result is right-justified.
`+'
Always include a plus or minus sign in the result.
` '
If the result doesn't start with a plus or minus sign, prefix it with a space instead. Since the `+' flag ensures that the result includes a sign, this flag is ignored if you supply both of them.
`#'
Specifies that the result should always include a decimal point, even if no digits follow it. For the `%g' and `%G' conversions, this also forces trailing zeros after the decimal point to be left in place where they would otherwise be removed.
`''
Separate the digits of the integer part of the result into groups as specified by the locale specified for the LC_NUMERIC category; see General Numeric. This flag is a GNU extension.
`0'
Pad the field with zeros instead of spaces; the zeros are placed after any sign. This flag is ignored if the `-' flag is also specified.

The precision specifies how many digits follow the decimal-point character for the `%f', `%e', and `%E' conversions. For these conversions, the default precision is 6. If the precision is explicitly 0, this suppresses the decimal point character entirely. For the `%g' and `%G' conversions, the precision specifies how many significant digits to print. Significant digits are the first digit before the decimal point, and all the digits after it. If the precision is 0 or not specified for `%g' or `%G', it is treated like a value of 1. If the value being printed cannot be expressed accurately in the specified number of digits, the value is rounded to the nearest number that fits.

Without a type modifier, the floating-point conversions use an argument of type double. (By the default argument promotions, any float arguments are automatically converted to double.) The following type modifier is supported:

`L'
An uppercase `L' specifies that the argument is a long double.

Here are some examples showing how numbers print using the various floating-point conversions. All of the numbers were printed using this template string: 

     "|%13.4a|%13.4f|%13.4e|%13.4g|\n"

Here is the output: 

     |  0x0.0000p+0|       0.0000|   0.0000e+00|            0|
     |  0x1.0000p-1|       0.5000|   5.0000e-01|          0.5|
     |  0x1.0000p+0|       1.0000|   1.0000e+00|            1|
     | -0x1.0000p+0|      -1.0000|  -1.0000e+00|           -1|
     |  0x1.9000p+6|     100.0000|   1.0000e+02|          100|
     |  0x1.f400p+9|    1000.0000|   1.0000e+03|         1000|
     | 0x1.3880p+13|   10000.0000|   1.0000e+04|        1e+04|
     | 0x1.81c8p+13|   12345.0000|   1.2345e+04|    1.234e+04|
     | 0x1.86a0p+16|  100000.0000|   1.0000e+05|        1e+05|
     | 0x1.e240p+16|  123456.0000|   1.2346e+05|    1.235e+05|

Notice how the `%g' conversion drops trailing zeros.

Next: , Previous: Floating-Point Conversions, Up: Formatted Output

12.12.6 Other Output Conversions

This section describes miscellaneous conversions for printf.

The `%c' conversion prints a single character. In case there is no `l' modifier the int argument is first converted to an unsigned char. Then, if used in a wide stream function, the character is converted into the corresponding wide character. The `-' flag can be used to specify left-justification in the field, but no other flags are defined, and no precision or type modifier can be given. For example: 

     printf ("%c%c%c%c%c", 'h', 'e', 'l', 'l', 'o');

prints `hello'.

If there is a `l' modifier present the argument is expected to be of type wint_t. If used in a multibyte function the wide character is converted into a multibyte character before being added to the output. In this case more than one output byte can be produced.

The `%s' conversion prints a string. If no `l' modifier is present the corresponding argument must be of type char * (or const char *). If used in a wide stream function the string is first converted in a wide character string. A precision can be specified to indicate the maximum number of characters to write; otherwise characters in the string up to but not including the terminating null character are written to the output stream. The `-' flag can be used to specify left-justification in the field, but no other flags or type modifiers are defined for this conversion. For example: 

     printf ("%3s%-6s", "no", "where");

prints ` nowhere '.

If there is a `l' modifier present the argument is expected to be of type wchar_t (or const wchar_t *).

If you accidentally pass a null pointer as the argument for a `%s' conversion, the GNU library prints it as `(null)'. We think this is more useful than crashing. But it's not good practice to pass a null argument intentionally.

The `%m' conversion prints the string corresponding to the error code in errno. See Error Messages. Thus: 

     fprintf (stderr, "can't open `%s': %m\n", filename);

is equivalent to: 

     fprintf (stderr, "can't open `%s': %s\n", filename, strerror (errno));

The `%m' conversion is a GNU C library extension.

The `%p' conversion prints a pointer value. The corresponding argument must be of type void *. In practice, you can use any type of pointer.

In the GNU system, non-null pointers are printed as unsigned integers, as if a `%#x' conversion were used. Null pointers print as `(nil)'. (Pointers might print differently in other systems.)

For example: 

     printf ("%p", "testing");

prints `0x' followed by a hexadecimal number—the address of the string constant "testing". It does not print the word `testing'.

You can supply the `-' flag with the `%p' conversion to specify left-justification, but no other flags, precision, or type modifiers are defined.

The `%n' conversion is unlike any of the other output conversions. It uses an argument which must be a pointer to an int, but instead of printing anything it stores the number of characters printed so far by this call at that location. The `h' and `l' type modifiers are permitted to specify that the argument is of type short int * or long int * instead of int *, but no flags, field width, or precision are permitted.

For example, 

     int nchar;
     printf ("%d %s%n\n", 3, "bears", &nchar);

prints: 

     3 bears

and sets nchar to 7, because `3 bears' is seven characters.

The `%%' conversion prints a literal `%' character. This conversion doesn't use an argument, and no flags, field width, precision, or type modifiers are permitted.

Next: , Previous: Other Output Conversions, Up: Formatted Output

12.12.7 Formatted Output Functions

This section describes how to call printf and related functions. Prototypes for these functions are in the header file stdio.h. Because these functions take a variable number of arguments, you must declare prototypes for them before using them. Of course, the easiest way to make sure you have all the right prototypes is to just include stdio.h.

— Function: int printf (const char *template, ...)

The printf function prints the optional arguments under the control of the template string template to the stream stdout. It returns the number of characters printed, or a negative value if there was an output error.

— Function: int wprintf (const wchar_t *template, ...)

The wprintf function prints the optional arguments under the control of the wide template string template to the stream stdout. It returns the number of wide characters printed, or a negative value if there was an output error.

— Function: int fprintf (FILE *stream, const char *template, ...)

This function is just like printf, except that the output is written to the stream stream instead of stdout.

— Function: int fwprintf (FILE *stream, const wchar_t *template, ...)

This function is just like wprintf, except that the output is written to the stream stream instead of stdout.

— Function: int sprintf (char *s, const char *template, ...)

This is like printf, except that the output is stored in the character array s instead of written to a stream. A null character is written to mark the end of the string.

The sprintf function returns the number of characters stored in the array s, not including the terminating null character.

The behavior of this function is undefined if copying takes place between objects that overlap—for example, if s is also given as an argument to be printed under control of the `%s' conversion. See Copying and Concatenation.

Warning: The sprintf function can be dangerous because it can potentially output more characters than can fit in the allocation size of the string s. Remember that the field width given in a conversion specification is only a minimum value.

To avoid this problem, you can use snprintf or asprintf, described below.

— Function: int swprintf (wchar_t *s, size_t size, const wchar_t *template, ...)

This is like wprintf, except that the output is stored in the wide character array ws instead of written to a stream. A null wide character is written to mark the end of the string. The size argument specifies the maximum number of characters to produce. The trailing null character is counted towards this limit, so you should allocate at least size wide characters for the string ws.

The return value is the number of characters generated for the given input, excluding the trailing null. If not all output fits into the provided buffer a negative value is returned. You should try again with a bigger output string. Note: this is different from how snprintf handles this situation.

Note that the corresponding narrow stream function takes fewer parameters. swprintf in fact corresponds to the snprintf function. Since the sprintf function can be dangerous and should be avoided the ISO C committee refused to make the same mistake again and decided to not define an function exactly corresponding to sprintf.

— Function: int snprintf (char *s, size_t size, const char *template, ...)

The snprintf function is similar to sprintf, except that the size argument specifies the maximum number of characters to produce. The trailing null character is counted towards this limit, so you should allocate at least size characters for the string s.

The return value is the number of characters which would be generated for the given input, excluding the trailing null. If this value is greater or equal to size, not all characters from the result have been stored in s. You should try again with a bigger output string. Here is an example of doing this: 

          /* Construct a message describing the value of a variable
             whose name is name and whose value is value. */
          char *
          make_message (char *name, char *value)
          {
            /* Guess we need no more than 100 chars of space. */
            int size = 100;
            char *buffer = (char *) xmalloc (size);
            int nchars;
            if (buffer == NULL)
              return NULL;
          
           /* Try to print in the allocated space. */
            nchars = snprintf (buffer, size, "value of %s is %s",
                               name, value);
            if (nchars >= size)
              {
                /* Reallocate buffer now that we know
                   how much space is needed. */
                size = nchars + 1;
                buffer = (char *) xrealloc (buffer, size);
          
                if (buffer != NULL)
                  /* Try again. */
                  snprintf (buffer, size, "value of %s is %s",
                            name, value);
              }
            /* The last call worked, return the string. */
            return buffer;
          }

In practice, it is often easier just to use asprintf, below.

Attention: In versions of the GNU C library prior to 2.1 the return value is the number of characters stored, not including the terminating null; unless there was not enough space in s to store the result in which case -1 is returned. This was changed in order to comply with the ISO C99 standard.

Next: , Previous: Formatted Output Functions, Up: Formatted Output

12.12.8 Dynamically Allocating Formatted Output

The functions in this section do formatted output and place the results in dynamically allocated memory.

— Function: int asprintf (char **ptr, const char *template, ...)

This function is similar to sprintf, except that it dynamically allocates a string (as with malloc; see Unconstrained Allocation) to hold the output, instead of putting the output in a buffer you allocate in advance. The ptr argument should be the address of a char * object, and a successful call to asprintf stores a pointer to the newly allocated string at that location.

The return value is the number of characters allocated for the buffer, or less than zero if an error occurred. Usually this means that the buffer could not be allocated.

Here is how to use asprintf to get the same result as the snprintf example, but more easily: 

          /* Construct a message describing the value of a variable
             whose name is name and whose value is value. */
          char *
          make_message (char *name, char *value)
          {
            char *result;
            if (asprintf (&result, "value of %s is %s", name, value) < 0)
              return NULL;
            return result;
          }
— Function: int obstack_printf (struct obstack *obstack, const char *template, ...)

This function is similar to asprintf, except that it uses the obstack obstack to allocate the space. See Obstacks.

The characters are written onto the end of the current object. To get at them, you must finish the object with obstack_finish (see Growing Objects).

Next: , Previous: Dynamic Output, Up: Formatted Output

12.12.9 Variable Arguments Output Functions

The functions vprintf and friends are provided so that you can define your own variadic printf-like functions that make use of the same internals as the built-in formatted output functions.

The most natural way to define such functions would be to use a language construct to say, “Call printf and pass this template plus all of my arguments after the first five.” But there is no way to do this in C, and it would be hard to provide a way, since at the C language level there is no way to tell how many arguments your function received.

Since that method is impossible, we provide alternative functions, the vprintf series, which lets you pass a va_list to describe “all of my arguments after the first five.”

When it is sufficient to define a macro rather than a real function, the GNU C compiler provides a way to do this much more easily with macros. For example: 

     #define myprintf(a, b, c, d, e, rest...) \
                 printf (mytemplate , ## rest)

See Variadic Macros, for details. But this is limited to macros, and does not apply to real functions at all.

Before calling vprintf or the other functions listed in this section, you must call va_start (see Variadic Functions) to initialize a pointer to the variable arguments. Then you can call va_arg to fetch the arguments that you want to handle yourself. This advances the pointer past those arguments.

Once your va_list pointer is pointing at the argument of your choice, you are ready to call vprintf. That argument and all subsequent arguments that were passed to your function are used by vprintf along with the template that you specified separately.

In some other systems, the va_list pointer may become invalid after the call to vprintf, so you must not use va_arg after you call vprintf. Instead, you should call va_end to retire the pointer from service. However, you can safely call va_start on another pointer variable and begin fetching the arguments again through that pointer. Calling vprintf does not destroy the argument list of your function, merely the particular pointer that you passed to it.

GNU C does not have such restrictions. You can safely continue to fetch arguments from a va_list pointer after passing it to vprintf, and va_end is a no-op. (Note, however, that subsequent va_arg calls will fetch the same arguments which vprintf previously used.)

Prototypes for these functions are declared in stdio.h.

— Function: int vprintf (const char *template, va_list ap)

This function is similar to printf except that, instead of taking a variable number of arguments directly, it takes an argument list pointer ap.

— Function: int vwprintf (const wchar_t *template, va_list ap)

This function is similar to wprintf except that, instead of taking a variable number of arguments directly, it takes an argument list pointer ap.

— Function: int vfprintf (FILE *stream, const char *template, va_list ap)

This is the equivalent of fprintf with the variable argument list specified directly as for vprintf.

— Function: int vfwprintf (FILE *stream, const wchar_t *template, va_list ap)

This is the equivalent of fwprintf with the variable argument list specified directly as for vwprintf.

— Function: int vsprintf (char *s, const char *template, va_list ap)

This is the equivalent of sprintf with the variable argument list specified directly as for vprintf.

— Function: int vswprintf (wchar_t *s, size_t size, const wchar_t *template, va_list ap)

This is the equivalent of swprintf with the variable argument list specified directly as for vwprintf.

— Function: int vsnprintf (char *s, size_t size, const char *template, va_list ap)

This is the equivalent of snprintf with the variable argument list specified directly as for vprintf.

— Function: int vasprintf (char **ptr, const char *template, va_list ap)

The vasprintf function is the equivalent of asprintf with the variable argument list specified directly as for vprintf.

— Function: int obstack_vprintf (struct obstack *obstack, const char *template, va_list ap)

The obstack_vprintf function is the equivalent of obstack_printf with the variable argument list specified directly as for vprintf.

Here's an example showing how you might use vfprintf. This is a function that prints error messages to the stream stderr, along with a prefix indicating the name of the program (see Error Messages, for a description of program_invocation_short_name). 

     #include <stdio.h>
     #include <stdarg.h>
     
     void
     eprintf (const char *template, ...)
     {
       va_list ap;
       extern char *program_invocation_short_name;
     
       fprintf (stderr, "%s: ", program_invocation_short_name);
       va_start (ap, template);
       vfprintf (stderr, template, ap);
       va_end (ap);
     }

You could call eprintf like this: 

     eprintf ("file `%s' does not exist\n", filename);

In GNU C, there is a special construct you can use to let the compiler know that a function uses a printf-style format string. Then it can check the number and types of arguments in each call to the function, and warn you when they do not match the format string. For example, take this declaration of eprintf: 

     void eprintf (const char *template, ...)
             __attribute__ ((format (printf, 1, 2)));

This tells the compiler that eprintf uses a format string like printf (as opposed to scanf; see Formatted Input); the format string appears as the first argument; and the arguments to satisfy the format begin with the second. See Declaring Attributes of Functions, for more information.

Next: , Previous: Variable Arguments Output, Up: Formatted Output

12.12.10 Parsing a Template String

You can use the function parse_printf_format to obtain information about the number and types of arguments that are expected by a given template string. This function permits interpreters that provide interfaces to printf to avoid passing along invalid arguments from the user's program, which could cause a crash.

All the symbols described in this section are declared in the header file printf.h.

— Function: size_t parse_printf_format (const char *template, size_t n, int *argtypes)

This function returns information about the number and types of arguments expected by the printf template string template. The information is stored in the array argtypes; each element of this array describes one argument. This information is encoded using the various `PA_' macros, listed below.

The argument n specifies the number of elements in the array argtypes. This is the maximum number of elements that parse_printf_format will try to write.

parse_printf_format returns the total number of arguments required by template. If this number is greater than n, then the information returned describes only the first n arguments. If you want information about additional arguments, allocate a bigger array and call parse_printf_format again.

The argument types are encoded as a combination of a basic type and modifier flag bits.

— Macro: int PA_FLAG_MASK

This macro is a bitmask for the type modifier flag bits. You can write the expression (argtypes[i] & PA_FLAG_MASK) to extract just the flag bits for an argument, or (argtypes[i] & ~PA_FLAG_MASK) to extract just the basic type code.

Here are symbolic constants that represent the basic types; they stand for integer values.

PA_INT
This specifies that the base type is int.
PA_CHAR
This specifies that the base type is int, cast to char.
PA_STRING
This specifies that the base type is char *, a null-terminated string.
PA_POINTER
This specifies that the base type is void *, an arbitrary pointer.
PA_FLOAT
This specifies that the base type is float.
PA_DOUBLE
This specifies that the base type is double.
PA_LAST
You can define additional base types for your own programs as offsets from PA_LAST. For example, if you have data types `foo' and `bar' with their own specialized printf conversions, you could define encodings for these types as: 
          #define PA_FOO  PA_LAST
          #define PA_BAR  (PA_LAST + 1)

Here are the flag bits that modify a basic type. They are combined with the code for the basic type using inclusive-or.

PA_FLAG_PTR
If this bit is set, it indicates that the encoded type is a pointer to the base type, rather than an immediate value. For example, `PA_INT|PA_FLAG_PTR' represents the type `int *'.
PA_FLAG_SHORT
If this bit is set, it indicates that the base type is modified with short. (This corresponds to the `h' type modifier.)
PA_FLAG_LONG
If this bit is set, it indicates that the base type is modified with long. (This corresponds to the `l' type modifier.)
PA_FLAG_LONG_LONG
If this bit is set, it indicates that the base type is modified with long long. (This corresponds to the `L' type modifier.)
PA_FLAG_LONG_DOUBLE
This is a synonym for PA_FLAG_LONG_LONG, used by convention with a base type of PA_DOUBLE to indicate a type of long double.

Previous: Parsing a Template String, Up: Formatted Output

12.12.11 Example of Parsing a Template String

Here is an example of decoding argument types for a format string. We assume this is part of an interpreter which contains arguments of type NUMBER, CHAR, STRING and STRUCTURE (and perhaps others which are not valid here). 

     /* Test whether the nargs specified objects
        in the vector args are valid
        for the format string format:
        if so, return 1.
        If not, return 0 after printing an error message.  */
     
     int
     validate_args (char *format, int nargs, OBJECT *args)
     {
       int *argtypes;
       int nwanted;
     
       /* Get the information about the arguments.
          Each conversion specification must be at least two characters
          long, so there cannot be more specifications than half the
          length of the string.  */
     
       argtypes = (int *) alloca (strlen (format) / 2 * sizeof (int));
       nwanted = parse_printf_format (string, nelts, argtypes);
     
       /* Check the number of arguments.  */
       if (nwanted > nargs)
         {
           error ("too few arguments (at least %d required)", nwanted);
           return 0;
         }
     
       /* Check the C type wanted for each argument
          and see if the object given is suitable.  */
       for (i = 0; i < nwanted; i++)
         {
           int wanted;
     
           if (argtypes[i] & PA_FLAG_PTR)
             wanted = STRUCTURE;
           else
             switch (argtypes[i] & ~PA_FLAG_MASK)
               {
               case PA_INT:
               case PA_FLOAT:
               case PA_DOUBLE:
                 wanted = NUMBER;
                 break;
               case PA_CHAR:
                 wanted = CHAR;
                 break;
               case PA_STRING:
                 wanted = STRING;
                 break;
               case PA_POINTER:
                 wanted = STRUCTURE;
                 break;
               }
           if (TYPE (args[i]) != wanted)
             {
               error ("type mismatch for arg number %d", i);
               return 0;
             }
         }
       return 1;
     }

Next: , Previous: Formatted Output, Up: I/O on Streams

12.13 Customizing printf

The GNU C library lets you define your own custom conversion specifiers for printf template strings, to teach printf clever ways to print the important data structures of your program.

The way you do this is by registering the conversion with the function register_printf_function; see Registering New Conversions. One of the arguments you pass to this function is a pointer to a handler function that produces the actual output; see Defining the Output Handler, for information on how to write this function.

You can also install a function that just returns information about the number and type of arguments expected by the conversion specifier. See Parsing a Template String, for information about this.

The facilities of this section are declared in the header file printf.h.

Portability Note: The ability to extend the syntax of printf template strings is a GNU extension. ISO standard C has nothing similar.

Next: , Up: Customizing Printf

12.13.1 Registering New Conversions

The function to register a new output conversion is register_printf_function, declared in printf.h.

— Function: int register_printf_function (int spec, printf_function handler-function, printf_arginfo_function arginfo-function)

This function defines the conversion specifier character spec. Thus, if spec is 'Y', it defines the conversion `%Y'. You can redefine the built-in conversions like `%s', but flag characters like `#' and type modifiers like `l' can never be used as conversions; calling register_printf_function for those characters has no effect. It is advisable not to use lowercase letters, since the ISO C standard warns that additional lowercase letters may be standardized in future editions of the standard.

The handler-function is the function called by printf and friends when this conversion appears in a template string. See Defining the Output Handler, for information about how to define a function to pass as this argument. If you specify a null pointer, any existing handler function for spec is removed.

The arginfo-function is the function called by parse_printf_format when this conversion appears in a template string. See Parsing a Template String, for information about this.

Attention: In the GNU C library versions before 2.0 the arginfo-function function did not need to be installed unless the user used the parse_printf_format function. This has changed. Now a call to any of the printf functions will call this function when this format specifier appears in the format string.

The return value is 0 on success, and -1 on failure (which occurs if spec is out of range).

You can redefine the standard output conversions, but this is probably not a good idea because of the potential for confusion. Library routines written by other people could break if you do this.

Next: , Previous: Registering New Conversions, Up: Customizing Printf

12.13.2 Conversion Specifier Options

If you define a meaning for `%A', what if the template contains `%+23A' or `%-#A'? To implement a sensible meaning for these, the handler when called needs to be able to get the options specified in the template.

Both the handler-function and arginfo-function accept an argument that points to a struct printf_info, which contains information about the options appearing in an instance of the conversion specifier. This data type is declared in the header file printf.h.

— Type: struct printf_info

This structure is used to pass information about the options appearing in an instance of a conversion specifier in a printf template string to the handler and arginfo functions for that specifier. It contains the following members:

int prec
This is the precision specified. The value is -1 if no precision was specified. If the precision was given as `*', the printf_info structure passed to the handler function contains the actual value retrieved from the argument list. But the structure passed to the arginfo function contains a value of INT_MIN, since the actual value is not known.
int width
This is the minimum field width specified. The value is 0 if no width was specified. If the field width was given as `*', the printf_info structure passed to the handler function contains the actual value retrieved from the argument list. But the structure passed to the arginfo function contains a value of INT_MIN, since the actual value is not known.
wchar_t spec
This is the conversion specifier character specified. It's stored in the structure so that you can register the same handler function for multiple characters, but still have a way to tell them apart when the handler function is called.
unsigned int is_long_double
This is a boolean that is true if the `L', `ll', or `q' type modifier was specified. For integer conversions, this indicates long long int, as opposed to long double for floating point conversions.
unsigned int is_char
This is a boolean that is true if the `hh' type modifier was specified.
unsigned int is_short
This is a boolean that is true if the `h' type modifier was specified.
unsigned int is_long
This is a boolean that is true if the `l' type modifier was specified.
unsigned int alt
This is a boolean that is true if the `#' flag was specified.
unsigned int space
This is a boolean that is true if the ` ' flag was specified.
unsigned int left
This is a boolean that is true if the `-' flag was specified.
unsigned int showsign
This is a boolean that is true if the `+' flag was specified.
unsigned int group
This is a boolean that is true if the `'' flag was specified.
unsigned int extra
This flag has a special meaning depending on the context. It could be used freely by the user-defined handlers but when called from the printf function this variable always contains the value 0.
unsigned int wide
This flag is set if the stream is wide oriented.
wchar_t pad
This is the character to use for padding the output to the minimum field width. The value is '0' if the `0' flag was specified, and ' ' otherwise.

Next: , Previous: Conversion Specifier Options, Up: Customizing Printf

12.13.3 Defining the Output Handler

Now let's look at how to define the handler and arginfo functions which are passed as arguments to register_printf_function.

Compatibility Note: The interface changed in GNU libc version 2.0. Previously the third argument was of type va_list *.

You should define your handler functions with a prototype like: 

     int function (FILE *stream, const struct printf_info *info,
                         const void *const *args)

The stream argument passed to the handler function is the stream to which it should write output.

The info argument is a pointer to a structure that contains information about the various options that were included with the conversion in the template string. You should not modify this structure inside your handler function. See Conversion Specifier Options, for a description of this data structure.

The args is a vector of pointers to the arguments data. The number of arguments was determined by calling the argument information function provided by the user.

Your handler function should return a value just like printf does: it should return the number of characters it has written, or a negative value to indicate an error.

— Data Type: printf_function

This is the data type that a handler function should have.

If you are going to use parse_printf_format in your application, you must also define a function to pass as the arginfo-function argument for each new conversion you install with register_printf_function.

You have to define these functions with a prototype like: 

     int function (const struct printf_info *info,
                         size_t n, int *argtypes)

The return value from the function should be the number of arguments the conversion expects. The function should also fill in no more than n elements of the argtypes array with information about the types of each of these arguments. This information is encoded using the various `PA_' macros. (You will notice that this is the same calling convention parse_printf_format itself uses.)

— Data Type: printf_arginfo_function

This type is used to describe functions that return information about the number and type of arguments used by a conversion specifier.

Next: , Previous: Defining the Output Handler, Up: Customizing Printf

12.13.4 printf Extension Example

Here is an example showing how to define a printf handler function. This program defines a data structure called a Widget and defines the `%W' conversion to print information about Widget * arguments, including the pointer value and the name stored in the data structure. The `%W' conversion supports the minimum field width and left-justification options, but ignores everything else. 

     #include <stdio.h>
     #include <stdlib.h>
     #include <printf.h>
     
     typedef struct
     {
       char *name;
     }
     Widget;
     
     int
     print_widget (FILE *stream,
                   const struct printf_info *info,
                   const void *const *args)
     {
       const Widget *w;
       char *buffer;
       int len;
     
       /* Format the output into a string. */
       w = *((const Widget **) (args[0]));
       len = asprintf (&buffer, "<Widget %p: %s>", w, w->name);
       if (len == -1)
         return -1;
     
       /* Pad to the minimum field width and print to the stream. */
       len = fprintf (stream, "%*s",
                      (info->left ? -info->width : info->width),
                      buffer);
     
       /* Clean up and return. */
       free (buffer);
       return len;
     }
     
     
     int
     print_widget_arginfo (const struct printf_info *info, size_t n,
                           int *argtypes)
     {
       /* We always take exactly one argument and this is a pointer to the
          structure.. */
       if (n > 0)
         argtypes[0] = PA_POINTER;
       return 1;
     }
     
     
     int
     main (void)
     {
       /* Make a widget to print. */
       Widget mywidget;
       mywidget.name = "mywidget";
     
       /* Register the print function for widgets. */
       register_printf_function ('W', print_widget, print_widget_arginfo);
     
       /* Now print the widget. */
       printf ("|%W|\n", &mywidget);
       printf ("|%35W|\n", &mywidget);
       printf ("|%-35W|\n", &mywidget);
     
       return 0;
     }

The output produced by this program looks like: 

     |<Widget 0xffeffb7c: mywidget>|
     |      <Widget 0xffeffb7c: mywidget>|
     |<Widget 0xffeffb7c: mywidget>      |

Previous: Printf Extension Example, Up: Customizing Printf

12.13.5 Predefined printf Handlers

The GNU libc also contains a concrete and useful application of the printf handler extension. There are two functions available which implement a special way to print floating-point numbers.

— Function: int printf_size (FILE *fp, const struct printf_info *info, const void *const *args)

Print a given floating point number as for the format %f except that there is a postfix character indicating the divisor for the number to make this less than 1000. There are two possible divisors: powers of 1024 or powers of 1000. Which one is used depends on the format character specified while registered this handler. If the character is of lower case, 1024 is used. For upper case characters, 1000 is used.

The postfix tag corresponds to bytes, kilobytes, megabytes, gigabytes, etc. The full table is:

The default precision is 3, i.e., 1024 is printed with a lower-case format character as if it were %.3fk and will yield 1.000k.

Due to the requirements of register_printf_function we must also provide the function which returns information about the arguments.

— Function: int printf_size_info (const struct printf_info *info, size_t n, int *argtypes)

This function will return in argtypes the information about the used parameters in the way the vfprintf implementation expects it. The format always takes one argument.

To use these functions both functions must be registered with a call like 

     register_printf_function ('B', printf_size, printf_size_info);

Here we register the functions to print numbers as powers of 1000 since the format character 'B' is an upper-case character. If we would additionally use 'b' in a line like 

     register_printf_function ('b', printf_size, printf_size_info);

we could also print using a power of 1024. Please note that all that is different in these two lines is the format specifier. The printf_size function knows about the difference between lower and upper case format specifiers.

The use of 'B' and 'b' is no coincidence. Rather it is the preferred way to use this functionality since it is available on some other systems which also use format specifiers.

Next: , Previous: Customizing Printf, Up: I/O on Streams

12.14 Formatted Input

The functions described in this section (scanf and related functions) provide facilities for formatted input analogous to the formatted output facilities. These functions provide a mechanism for reading arbitrary values under the control of a format string or template string.

Next: , Up: Formatted Input

12.14.1 Formatted Input Basics

Calls to scanf are superficially similar to calls to printf in that arbitrary arguments are read under the control of a template string. While the syntax of the conversion specifications in the template is very similar to that for printf, the interpretation of the template is oriented more towards free-format input and simple pattern matching, rather than fixed-field formatting. For example, most scanf conversions skip over any amount of “white space” (including spaces, tabs, and newlines) in the input file, and there is no concept of precision for the numeric input conversions as there is for the corresponding output conversions. Ordinarily, non-whitespace characters in the template are expected to match characters in the input stream exactly, but a matching failure is distinct from an input error on the stream. Another area of difference between scanf and printf is that you must remember to supply pointers rather than immediate values as the optional arguments to scanf; the values that are read are stored in the objects that the pointers point to. Even experienced programmers tend to forget this occasionally, so if your program is getting strange errors that seem to be related to scanf, you might want to double-check this.

When a matching failure occurs, scanf returns immediately, leaving the first non-matching character as the next character to be read from the stream. The normal return value from scanf is the number of values that were assigned, so you can use this to determine if a matching error happened before all the expected values were read. The scanf function is typically used for things like reading in the contents of tables. For example, here is a function that uses scanf to initialize an array of double: 

     void
     readarray (double *array, int n)
     {
       int i;
       for (i=0; i<n; i++)
         if (scanf (" %lf", &(array[i])) != 1)
           invalid_input_error ();
     }

The formatted input functions are not used as frequently as the formatted output functions. Partly, this is because it takes some care to use them properly. Another reason is that it is difficult to recover from a matching error.

If you are trying to read input that doesn't match a single, fixed pattern, you may be better off using a tool such as Flex to generate a lexical scanner, or Bison to generate a parser, rather than using scanf. For more information about these tools, see Top, and Top.

Next: , Previous: Formatted Input Basics, Up: Formatted Input

12.14.2 Input Conversion Syntax

A scanf template string is a string that contains ordinary multibyte characters interspersed with conversion specifications that start with `%'.

Any whitespace character (as defined by the isspace function; see Classification of Characters) in the template causes any number of whitespace characters in the input stream to be read and discarded. The whitespace characters that are matched need not be exactly the same whitespace characters that appear in the template string. For example, write ` , ' in the template to recognize a comma with optional whitespace before and after.

Other characters in the template string that are not part of conversion specifications must match characters in the input stream exactly; if this is not the case, a matching failure occurs.

The conversion specifications in a scanf template string have the general form: 

     % flags width type conversion

In more detail, an input conversion specification consists of an initial `%' character followed in sequence by:

The exact options that are permitted and how they are interpreted vary between the different conversion specifiers. See the descriptions of the individual conversions for information about the particular options that they allow.

With the `-Wformat' option, the GNU C compiler checks calls to scanf and related functions. It examines the format string and verifies that the correct number and types of arguments are supplied. There is also a GNU C syntax to tell the compiler that a function you write uses a scanf-style format string. See Declaring Attributes of Functions, for more information.

Next: , Previous: Input Conversion Syntax, Up: Formatted Input

12.14.3 Table of Input Conversions

Here is a table that summarizes the various conversion specifications:

`%d'
Matches an optionally signed integer written in decimal. See Numeric Input Conversions.
`%i'
Matches an optionally signed integer in any of the formats that the C language defines for specifying an integer constant. See Numeric Input Conversions.
`%o'
Matches an unsigned integer written in octal radix. See Numeric Input Conversions.
`%u'
Matches an unsigned integer written in decimal radix. See Numeric Input Conversions.
`%x', `%X'
Matches an unsigned integer written in hexadecimal radix. See Numeric Input Conversions.
`%e', `%f', `%g', `%E', `%G'
Matches an optionally signed floating-point number. See Numeric Input Conversions.
`%s'
Matches a string containing only non-whitespace characters. See String Input Conversions. The presence of the `l' modifier determines whether the output is stored as a wide character string or a multibyte string. If `%s' is used in a wide character function the string is converted as with multiple calls to wcrtomb into a multibyte string. This means that the buffer must provide room for MB_CUR_MAX bytes for each wide character read. In case `%ls' is used in a multibyte function the result is converted into wide characters as with multiple calls of mbrtowc before being stored in the user provided buffer.
`%S'
This is an alias for `%ls' which is supported for compatibility with the Unix standard.
`%['
Matches a string of characters that belong to a specified set. See String Input Conversions. The presence of the `l' modifier determines whether the output is stored as a wide character string or a multibyte string. If `%[' is used in a wide character function the string is converted as with multiple calls to wcrtomb into a multibyte string. This means that the buffer must provide room for MB_CUR_MAX bytes for each wide character read. In case `%l[' is used in a multibyte function the result is converted into wide characters as with multiple calls of mbrtowc before being stored in the user provided buffer.
`%c'
Matches a string of one or more characters; the number of characters read is controlled by the maximum field width given for the conversion. See String Input Conversions.

If the `%c' is used in a wide stream function the read value is converted from a wide character to the corresponding multibyte character before storing it. Note that this conversion can produce more than one byte of output and therefore the provided buffer be large enough for up to MB_CUR_MAX bytes for each character. If `%lc' is used in a multibyte function the input is treated as a multibyte sequence (and not bytes) and the result is converted as with calls to mbrtowc.

`%C'
This is an alias for `%lc' which is supported for compatibility with the Unix standard.
`%p'
Matches a pointer value in the same implementation-defined format used by the `%p' output conversion for printf. See Other Input Conversions.
`%n'
This conversion doesn't read any characters; it records the number of characters read so far by this call. See Other Input Conversions.
`%%'
This matches a literal `%' character in the input stream. No corresponding argument is used. See Other Input Conversions.

If the syntax of a conversion specification is invalid, the behavior is undefined. If there aren't enough function arguments provided to supply addresses for all the conversion specifications in the template strings that perform assignments, or if the arguments are not of the correct types, the behavior is also undefined. On the other hand, extra arguments are simply ignored.

Next: , Previous: Table of Input Conversions, Up: Formatted Input

12.14.4 Numeric Input Conversions

This section describes the scanf conversions for reading numeric values.

The `%d' conversion matches an optionally signed integer in decimal radix. The syntax that is recognized is the same as that for the strtol function (see Parsing of Integers) with the value 10 for the base argument.

The `%i' conversion matches an optionally signed integer in any of the formats that the C language defines for specifying an integer constant. The syntax that is recognized is the same as that for the strtol function (see Parsing of Integers) with the value 0 for the base argument. (You can print integers in this syntax with printf by using the `#' flag character with the `%x', `%o', or `%d' conversion. See Integer Conversions.)

For example, any of the strings `10', `0xa', or `012' could be read in as integers under the `%i' conversion. Each of these specifies a number with decimal value 10.

The `%o', `%u', and `%x' conversions match unsigned integers in octal, decimal, and hexadecimal radices, respectively. The syntax that is recognized is the same as that for the strtoul function (see Parsing of Integers) with the appropriate value (8, 10, or 16) for the base argument.

The `%X' conversion is identical to the `%x' conversion. They both permit either uppercase or lowercase letters to be used as digits.

The default type of the corresponding argument for the %d and %i conversions is int *, and unsigned int * for the other integer conversions. You can use the following type modifiers to specify other sizes of integer:

`hh'
Specifies that the argument is a signed char * or unsigned char *.

This modifier was introduced in ISO C99.

`h'
Specifies that the argument is a short int * or unsigned short int *.
`j'
Specifies that the argument is a intmax_t * or uintmax_t *.

This modifier was introduced in ISO C99.

`l'
Specifies that the argument is a long int * or unsigned long int *. Two `l' characters is like the `L' modifier, below.

If used with `%c' or `%s' the corresponding parameter is considered as a pointer to a wide character or wide character string respectively. This use of `l' was introduced in Amendment 1 to ISO C90.

`ll'
`L'
`q'
Specifies that the argument is a long long int * or unsigned long long int *. (The long long type is an extension supported by the GNU C compiler. For systems that don't provide extra-long integers, this is the same as long int.)

The `q' modifier is another name for the same thing, which comes from 4.4 BSD; a long long int is sometimes called a “quad” int.

`t'
Specifies that the argument is a ptrdiff_t *.

This modifier was introduced in ISO C99.

`z'
Specifies that the argument is a size_t *.

This modifier was introduced in ISO C99.

All of the `%e', `%f', `%g', `%E', and `%G' input conversions are interchangeable. They all match an optionally signed floating point number, in the same syntax as for the strtod function (see Parsing of Floats).

For the floating-point input conversions, the default argument type is float *. (This is different from the corresponding output conversions, where the default type is double; remember that float arguments to printf are converted to double by the default argument promotions, but float * arguments are not promoted to double *.) You can specify other sizes of float using these type modifiers:

`l'
Specifies that the argument is of type double *.
`L'
Specifies that the argument is of type long double *.

For all the above number parsing formats there is an additional optional flag `''. When this flag is given the scanf function expects the number represented in the input string to be formatted according to the grouping rules of the currently selected locale (see General Numeric).

If the "C" or "POSIX" locale is selected there is no difference. But for a locale which specifies values for the appropriate fields in the locale the input must have the correct form in the input. Otherwise the longest prefix with a correct form is processed.

Next: , Previous: Numeric Input Conversions, Up: Formatted Input

12.14.5 String Input Conversions

This section describes the scanf input conversions for reading string and character values: `%s', `%S', `%[', `%c', and `%C'.

You have two options for how to receive the input from these conversions:

The `%c' conversion is the simplest: it matches a fixed number of characters, always. The maximum field width says how many characters to read; if you don't specify the maximum, the default is 1. This conversion doesn't append a null character to the end of the text it reads. It also does not skip over initial whitespace characters. It reads precisely the next n characters, and fails if it cannot get that many. Since there is always a maximum field width with `%c' (whether specified, or 1 by default), you can always prevent overflow by making the buffer long enough.

If the format is `%lc' or `%C' the function stores wide characters which are converted using the conversion determined at the time the stream was opened from the external byte stream. The number of bytes read from the medium is limited by MB_CUR_LEN * n but at most n wide character get stored in the output string.

The `%s' conversion matches a string of non-whitespace characters. It skips and discards initial whitespace, but stops when it encounters more whitespace after having read something. It stores a null character at the end of the text that it reads.

For example, reading the input: 

      hello, world

with the conversion `%10c' produces " hello, wo", but reading the same input with the conversion `%10s' produces "hello,".

Warning: If you do not specify a field width for `%s', then the number of characters read is limited only by where the next whitespace character appears. This almost certainly means that invalid input can make your program crash—which is a bug.

The `%ls' and `%S' format are handled just like `%s' except that the external byte sequence is converted using the conversion associated with the stream to wide characters with their own encoding. A width or precision specified with the format do not directly determine how many bytes are read from the stream since they measure wide characters. But an upper limit can be computed by multiplying the value of the width or precision by MB_CUR_MAX.

To read in characters that belong to an arbitrary set of your choice, use the `%[' conversion. You specify the set between the `[' character and a following `]' character, using the same syntax used in regular expressions. As special cases:

The `%[' conversion does not skip over initial whitespace characters.

Here are some examples of `%[' conversions and what they mean:

`%25[1234567890]'
Matches a string of up to 25 digits.
`%25[][]'
Matches a string of up to 25 square brackets.
`%25[^ \f\n\r\t\v]'
Matches a string up to 25 characters long that doesn't contain any of the standard whitespace characters. This is slightly different from `%s', because if the input begins with a whitespace character, `%[' reports a matching failure while `%s' simply discards the initial whitespace.
`%25[a-z]'
Matches up to 25 lowercase characters.

As for `%c' and `%s' the `%[' format is also modified to produce wide characters if the `l' modifier is present. All what is said about `%ls' above is true for `%l['.

One more reminder: the `%s' and `%[' conversions are dangerous if you don't specify a maximum width or use the `a' flag, because input too long would overflow whatever buffer you have provided for it. No matter how long your buffer is, a user could supply input that is longer. A well-written program reports invalid input with a comprehensible error message, not with a crash.

Next: , Previous: String Input Conversions, Up: Formatted Input

12.14.6 Dynamically Allocating String Conversions

A GNU extension to formatted input lets you safely read a string with no maximum size. Using this feature, you don't supply a buffer; instead, scanf allocates a buffer big enough to hold the data and gives you its address. To use this feature, write `a' as a flag character, as in `%as' or `%a[0-9a-z]'.

The pointer argument you supply for where to store the input should have type char **. The scanf function allocates a buffer and stores its address in the word that the argument points to. You should free the buffer with free when you no longer need it.

Here is an example of using the `a' flag with the `%[...]' conversion specification to read a “variable assignment” of the form `variable = value'. 

     {
       char *variable, *value;
     
       if (2 > scanf ("%a[a-zA-Z0-9] = %a[^\n]\n",
                      &variable, &value))
         {
           invalid_input_error ();
           return 0;
         }
     
       ...
     }

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12.14.7 Other Input Conversions

This section describes the miscellaneous input conversions.

The `%p' conversion is used to read a pointer value. It recognizes the same syntax used by the `%p' output conversion for printf (see Other Output Conversions); that is, a hexadecimal number just as the `%x' conversion accepts. The corresponding argument should be of type void **; that is, the address of a place to store a pointer.

The resulting pointer value is not guaranteed to be valid if it was not originally written during the same program execution that reads it in.

The `%n' conversion produces the number of characters read so far by this call. The corresponding argument should be of type int *. This conversion works in the same way as the `%n' conversion for printf; see Other Output Conversions, for an example.

The `%n' conversion is the only mechanism for determining the success of literal matches or conversions with suppressed assignments. If the `%n' follows the locus of a matching failure, then no value is stored for it since scanf returns before processing the `%n'. If you store -1 in that argument slot before calling scanf, the presence of -1 after scanf indicates an error occurred before the `%n' was reached.

Finally, the `%%' conversion matches a literal `%' character in the input stream, without using an argument. This conversion does not permit any flags, field width, or type modifier to be specified.

Next: , Previous: Other Input Conversions, Up: Formatted Input

12.14.8 Formatted Input Functions

Here are the descriptions of the functions for performing formatted input. Prototypes for these functions are in the header file stdio.h.

— Function: int scanf (const char *template, ...)

The scanf function reads formatted input from the stream stdin under the control of the template string template. The optional arguments are pointers to the places which receive the resulting values.

The return value is normally the number of successful assignments. If an end-of-file condition is detected before any matches are performed, including matches against whitespace and literal characters in the template, then EOF is returned.

— Function: int wscanf (const wchar_t *template, ...)

The wscanf function reads formatted input from the stream stdin under the control of the template string template. The optional arguments are pointers to the places which receive the resulting values.

The return value is normally the number of successful assignments. If an end-of-file condition is detected before any matches are performed, including matches against whitespace and literal characters in the template, then WEOF is returned.

— Function: int fscanf (FILE *stream, const char *template, ...)

This function is just like scanf, except that the input is read from the stream stream instead of stdin.

— Function: int fwscanf (FILE *stream, const wchar_t *template, ...)

This function is just like wscanf, except that the input is read from the stream stream instead of stdin.

— Function: int sscanf (const char *s, const char *template, ...)

This is like scanf, except that the characters are taken from the null-terminated string s instead of from a stream. Reaching the end of the string is treated as an end-of-file condition.

The behavior of this function is undefined if copying takes place between objects that overlap—for example, if s is also given as an argument to receive a string read under control of the `%s', `%S', or `%[' conversion.

— Function: int swscanf (const wchar_t *ws, const char *template, ...)

This is like wscanf, except that the characters are taken from the null-terminated string ws instead of from a stream. Reaching the end of the string is treated as an end-of-file condition.

The behavior of this function is undefined if copying takes place between objects that overlap—for example, if ws is also given as an argument to receive a string read under control of the `%s', `%S', or `%[' conversion.

Previous: Formatted Input Functions, Up: Formatted Input

12.14.9 Variable Arguments Input Functions

The functions vscanf and friends are provided so that you can define your own variadic scanf-like functions that make use of the same internals as the built-in formatted output functions. These functions are analogous to the vprintf series of output functions. See Variable Arguments Output, for important information on how to use them.

Portability Note: The functions listed in this section were introduced in ISO C99 and were before available as GNU extensions.

— Function: int vscanf (const char *template, va_list ap)

This function is similar to scanf, but instead of taking a variable number of arguments directly, it takes an argument list pointer ap of type va_list (see Variadic Functions).

— Function: int vwscanf (const wchar_t *template, va_list ap)

This function is similar to wscanf, but instead of taking a variable number of arguments directly, it takes an argument list pointer ap of type va_list (see Variadic Functions).

— Function: int vfscanf (FILE *stream, const char *template, va_list ap)

This is the equivalent of fscanf with the variable argument list specified directly as for vscanf.

— Function: int vfwscanf (FILE *stream, const wchar_t *template, va_list ap)

This is the equivalent of fwscanf with the variable argument list specified directly as for vwscanf.

— Function: int vsscanf (const char *s, const char *template, va_list ap)

This is the equivalent of sscanf with the variable argument list specified directly as for vscanf.

— Function: int vswscanf (const wchar_t *s, const wchar_t *template, va_list ap)

This is the equivalent of swscanf with the variable argument list specified directly as for vwscanf.

In GNU C, there is a special construct you can use to let the compiler know that a function uses a scanf-style format string. Then it can check the number and types of arguments in each call to the function, and warn you when they do not match the format string. For details, See Declaring Attributes of Functions.

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12.15 End-Of-File and Errors

Many of the functions described in this chapter return the value of the macro EOF to indicate unsuccessful completion of the operation. Since EOF is used to report both end of file and random errors, it's often better to use the feof function to check explicitly for end of file and ferror to check for errors. These functions check indicators that are part of the internal state of the stream object, indicators set if the appropriate condition was detected by a previous I/O operation on that stream.

— Macro: int EOF

This macro is an integer value that is returned by a number of narrow stream functions to indicate an end-of-file condition, or some other error situation. With the GNU library, EOF is -1. In other libraries, its value may be some other negative number.

This symbol is declared in stdio.h.

— Macro: int WEOF

This macro is an integer value that is returned by a number of wide stream functions to indicate an end-of-file condition, or some other error situation. With the GNU library, WEOF is -1. In other libraries, its value may be some other negative number.

This symbol is declared in wchar.h.

— Function: int feof (FILE *stream)

The feof function returns nonzero if and only if the end-of-file indicator for the stream stream is set.

This symbol is declared in stdio.h.

— Function: int feof_unlocked (FILE *stream)

The feof_unlocked function is equivalent to the feof function except that it does not implicitly lock the stream.

This function is a GNU extension.

This symbol is declared in stdio.h.

— Function: int ferror (FILE *stream)

The ferror function returns nonzero if and only if the error indicator for the stream stream is set, indicating that an error has occurred on a previous operation on the stream.

This symbol is declared in stdio.h.

— Function: int ferror_unlocked (FILE *stream)

The ferror_unlocked function is equivalent to the ferror function except that it does not implicitly lock the stream.

This function is a GNU extension.

This symbol is declared in stdio.h.

In addition to setting the error indicator associated with the stream, the functions that operate on streams also set errno in the same way as the corresponding low-level functions that operate on file descriptors. For example, all of the functions that perform output to a stream—such as fputc, printf, and fflush—are implemented in terms of write, and all of the errno error conditions defined for write are meaningful for these functions. For more information about the descriptor-level I/O functions, see Low-Level I/O.

Next: , Previous: EOF and Errors, Up: I/O on Streams

12.16 Recovering from errors

You may explicitly clear the error and EOF flags with the clearerr function.

— Function: void clearerr (FILE *stream)

This function clears the end-of-file and error indicators for the stream stream.

The file positioning functions (see File Positioning) also clear the end-of-file indicator for the stream.

— Function: void clearerr_unlocked (FILE *stream)

The clearerr_unlocked function is equivalent to the clearerr function except that it does not implicitly lock the stream.

This function is a GNU extension.

Note that it is not correct to just clear the error flag and retry a failed stream operation. After a failed write, any number of characters since the last buffer flush may have been committed to the file, while some buffered data may have been discarded. Merely retrying can thus cause lost or repeated data.

A failed read may leave the file pointer in an inappropriate position for a second try. In both cases, you should seek to a known position before retrying.

Most errors that can happen are not recoverable — a second try will always fail again in the same way. So usually it is best to give up and report the error to the user, rather than install complicated recovery logic.

One important exception is EINTR (see Interrupted Primitives). Many stream I/O implementations will treat it as an ordinary error, which can be quite inconvenient. You can avoid this hassle by installing all signals with the SA_RESTART flag.

For similar reasons, setting nonblocking I/O on a stream's file descriptor is not usually advisable.

Next: , Previous: Error Recovery, Up: I/O on Streams

12.17 Text and Binary Streams

The GNU system and other POSIX-compatible operating systems organize all files as uniform sequences of characters. However, some other systems make a distinction between files containing text and files containing binary data, and the input and output facilities of ISO C provide for this distinction. This section tells you how to write programs portable to such systems.

When you open a stream, you can specify either a text stream or a binary stream. You indicate that you want a binary stream by specifying the `b' modifier in the opentype argument to fopen; see Opening Streams. Without this option, fopen opens the file as a text stream.

Text and binary streams differ in several ways:

Since a binary stream is always more capable and more predictable than a text stream, you might wonder what purpose text streams serve. Why not simply always use binary streams? The answer is that on these operating systems, text and binary streams use different file formats, and the only way to read or write “an ordinary file of text” that can work with other text-oriented programs is through a text stream.

In the GNU library, and on all POSIX systems, there is no difference between text streams and binary streams. When you open a stream, you get the same kind of stream regardless of whether you ask for binary. This stream can handle any file content, and has none of the restrictions that text streams sometimes have.

Next: , Previous: Binary Streams, Up: I/O on Streams

12.18 File Positioning

The file position of a stream describes where in the file the stream is currently reading or writing. I/O on the stream advances the file position through the file. In the GNU system, the file position is represented as an integer, which counts the number of bytes from the beginning of the file. See File Position.

During I/O to an ordinary disk file, you can change the file position whenever you wish, so as to read or write any portion of the file. Some other kinds of files may also permit this. Files which support changing the file position are sometimes referred to as random-access files.

You can use the functions in this section to examine or modify the file position indicator associated with a stream. The symbols listed below are declared in the header file stdio.h.

— Function: long int ftell (FILE *stream)

This function returns the current file position of the stream stream.

This function can fail if the stream doesn't support file positioning, or if the file position can't be represented in a long int, and possibly for other reasons as well. If a failure occurs, a value of -1 is returned.

— Function: off_t ftello (FILE *stream)

The ftello function is similar to ftell, except that it returns a value of type off_t. Systems which support this type use it to describe all file positions, unlike the POSIX specification which uses a long int. The two are not necessarily the same size. Therefore, using ftell can lead to problems if the implementation is written on top of a POSIX compliant low-level I/O implementation, and using ftello is preferable whenever it is available.

If this function fails it returns (off_t) -1. This can happen due to missing support for file positioning or internal errors. Otherwise the return value is the current file position.

The function is an extension defined in the Unix Single Specification version 2.

When the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32 bit system this function is in fact ftello64. I.e., the LFS interface transparently replaces the old interface.

— Function: off64_t ftello64 (FILE *stream)

This function is similar to ftello with the only difference that the return value is of type off64_t. This also requires that the stream stream was opened using either fopen64, freopen64, or tmpfile64 since otherwise the underlying file operations to position the file pointer beyond the 2^31 bytes limit might fail.

If the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32 bits machine this function is available under the name ftello and so transparently replaces the old interface.

— Function: int fseek (FILE *stream, long int offset, int whence)

The fseek function is used to change the file position of the stream stream. The value of whence must be one of the constants SEEK_SET, SEEK_CUR, or SEEK_END, to indicate whether the offset is relative to the beginning of the file, the current file position, or the end of the file, respectively.

This function returns a value of zero if the operation was successful, and a nonzero value to indicate failure. A successful call also clears the end-of-file indicator of stream and discards any characters that were “pushed back” by the use of ungetc.

fseek either flushes any buffered output before setting the file position or else remembers it so it will be written later in its proper place in the file.

— Function: int fseeko (FILE *stream, off_t offset, int whence)

This function is similar to fseek but it corrects a problem with fseek in a system with POSIX types. Using a value of type long int for the offset is not compatible with POSIX. fseeko uses the correct type off_t for the offset parameter.

For this reason it is a good idea to prefer ftello whenever it is available since its functionality is (if different at all) closer the underlying definition.

The functionality and return value is the same as for fseek.

The function is an extension defined in the Unix Single Specification version 2.

When the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32 bit system this function is in fact fseeko64. I.e., the LFS interface transparently replaces the old interface.

— Function: int fseeko64 (FILE *stream, off64_t offset, int whence)

This function is similar to fseeko with the only difference that the offset parameter is of type off64_t. This also requires that the stream stream was opened using either fopen64, freopen64, or tmpfile64 since otherwise the underlying file operations to position the file pointer beyond the 2^31 bytes limit might fail.

If the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32 bits machine this function is available under the name fseeko and so transparently replaces the old interface.

Portability Note: In non-POSIX systems, ftell, ftello, fseek and fseeko might work reliably only on binary streams. See Binary Streams.

The following symbolic constants are defined for use as the whence argument to fseek. They are also used with the lseek function (see I/O Primitives) and to specify offsets for file locks (see Control Operations).

— Macro: int SEEK_SET

This is an integer constant which, when used as the whence argument to the fseek or fseeko function, specifies that the offset provided is relative to the beginning of the file.

— Macro: int SEEK_CUR

This is an integer constant which, when used as the whence argument to the fseek or fseeko function, specifies that the offset provided is relative to the current file position.

— Macro: int SEEK_END

This is an integer constant which, when used as the whence argument to the fseek or fseeko function, specifies that the offset provided is relative to the end of the file.

— Function: void rewind (FILE *stream)

The rewind function positions the stream stream at the beginning of the file. It is equivalent to calling fseek or fseeko on the stream with an offset argument of 0L and a whence argument of SEEK_SET, except that the return value is discarded and the error indicator for the stream is reset.

These three aliases for the `SEEK_...' constants exist for the sake of compatibility with older BSD systems. They are defined in two different header files: fcntl.h and sys/file.h.

L_SET
An alias for SEEK_SET.
L_INCR
An alias for SEEK_CUR.
L_XTND
An alias for SEEK_END.

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12.19 Portable File-Position Functions

On the GNU system, the file position is truly a character count. You can specify any character count value as an argument to fseek or fseeko and get reliable results for any random access file. However, some ISO C systems do not represent file positions in this way.

On some systems where text streams truly differ from binary streams, it is impossible to represent the file position of a text stream as a count of characters from the beginning of the file. For example, the file position on some systems must encode both a record offset within the file, and a character offset within the record.

As a consequence, if you want your programs to be portable to these systems, you must observe certain rules:

But even if you observe these rules, you may still have trouble for long files, because ftell and fseek use a long int value to represent the file position. This type may not have room to encode all the file positions in a large file. Using the ftello and fseeko functions might help here since the off_t type is expected to be able to hold all file position values but this still does not help to handle additional information which must be associated with a file position.

So if you do want to support systems with peculiar encodings for the file positions, it is better to use the functions fgetpos and fsetpos instead. These functions represent the file position using the data type fpos_t, whose internal representation varies from system to system.

These symbols are declared in the header file stdio.h.

— Data Type: fpos_t

This is the type of an object that can encode information about the file position of a stream, for use by the functions fgetpos and fsetpos.

In the GNU system, fpos_t is an opaque data structure that contains internal data to represent file offset and conversion state information. In other systems, it might have a different internal representation.

When compiling with _FILE_OFFSET_BITS == 64 on a 32 bit machine this type is in fact equivalent to fpos64_t since the LFS interface transparently replaces the old interface.

— Data Type: fpos64_t

This is the type of an object that can encode information about the file position of a stream, for use by the functions fgetpos64 and fsetpos64.

In the GNU system, fpos64_t is an opaque data structure that contains internal data to represent file offset and conversion state information. In other systems, it might have a different internal representation.

— Function: int fgetpos (FILE *stream, fpos_t *position)

This function stores the value of the file position indicator for the stream stream in the fpos_t object pointed to by position. If successful, fgetpos returns zero; otherwise it returns a nonzero value and stores an implementation-defined positive value in errno.

When the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32 bit system the function is in fact fgetpos64. I.e., the LFS interface transparently replaces the old interface.

— Function: int fgetpos64 (FILE *stream, fpos64_t *position)

This function is similar to fgetpos but the file position is returned in a variable of type fpos64_t to which position points.

If the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32 bits machine this function is available under the name fgetpos and so transparently replaces the old interface.

— Function: int fsetpos (FILE *stream, const fpos_t *position)

This function sets the file position indicator for the stream stream to the position position, which must have been set by a previous call to fgetpos on the same stream. If successful, fsetpos clears the end-of-file indicator on the stream, discards any characters that were “pushed back” by the use of ungetc, and returns a value of zero. Otherwise, fsetpos returns a nonzero value and stores an implementation-defined positive value in errno.

When the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32 bit system the function is in fact fsetpos64. I.e., the LFS interface transparently replaces the old interface.

— Function: int fsetpos64 (FILE *stream, const fpos64_t *position)

This function is similar to fsetpos but the file position used for positioning is provided in a variable of type fpos64_t to which position points.

If the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32 bits machine this function is available under the name fsetpos and so transparently replaces the old interface.

Next: , Previous: Portable Positioning, Up: I/O on Streams

12.20 Stream Buffering

Characters that are written to a stream are normally accumulated and transmitted asynchronously to the file in a block, instead of appearing as soon as they are output by the application program. Similarly, streams often retrieve input from the host environment in blocks rather than on a character-by-character basis. This is called buffering.

If you are writing programs that do interactive input and output using streams, you need to understand how buffering works when you design the user interface to your program. Otherwise, you might find that output (such as progress or prompt messages) doesn't appear when you intended it to, or displays some other unexpected behavior.

This section deals only with controlling when characters are transmitted between the stream and the file or device, and not with how things like echoing, flow control, and the like are handled on specific classes of devices. For information on common control operations on terminal devices, see Low-Level Terminal Interface.

You can bypass the stream buffering facilities altogether by using the low-level input and output functions that operate on file descriptors instead. See Low-Level I/O.

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12.20.1 Buffering Concepts

There are three different kinds of buffering strategies:

Newly opened streams are normally fully buffered, with one exception: a stream connected to an interactive device such as a terminal is initially line buffered. See Controlling Buffering, for information on how to select a different kind of buffering. Usually the automatic selection gives you the most convenient kind of buffering for the file or device you open.

The use of line buffering for interactive devices implies that output messages ending in a newline will appear immediately—which is usually what you want. Output that doesn't end in a newline might or might not show up immediately, so if you want them to appear immediately, you should flush buffered output explicitly with fflush, as described in Flushing Buffers.

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12.20.2 Flushing Buffers

Flushing output on a buffered stream means transmitting all accumulated characters to the file. There are many circumstances when buffered output on a stream is flushed automatically:

If you want to flush the buffered output at another time, call fflush, which is declared in the header file stdio.h.

— Function: int fflush (FILE *stream)

This function causes any buffered output on stream to be delivered to the file. If stream is a null pointer, then fflush causes buffered output on all open output streams to be flushed.

This function returns EOF if a write error occurs, or zero otherwise.

— Function: int fflush_unlocked (FILE *stream)

The fflush_unlocked function is equivalent to the fflush function except that it does not implicitly lock the stream.

The fflush function can be used to flush all streams currently opened. While this is useful in some situations it does often more than necessary since it might be done in situations when terminal input is required and the program wants to be sure that all output is visible on the terminal. But this means that only line buffered streams have to be flushed. Solaris introduced a function especially for this. It was always available in the GNU C library in some form but never officially exported.

— Function: void _flushlbf (void)

The _flushlbf function flushes all line buffered streams currently opened.

This function is declared in the stdio_ext.h header.

Compatibility Note: Some brain-damaged operating systems have been known to be so thoroughly fixated on line-oriented input and output that flushing a line buffered stream causes a newline to be written! Fortunately, this “feature” seems to be becoming less common. You do not need to worry about this in the GNU system.

In some situations it might be useful to not flush the output pending for a stream but instead simply forget it. If transmission is costly and the output is not needed anymore this is valid reasoning. In this situation a non-standard function introduced in Solaris and available in the GNU C library can be used.

— Function: void __fpurge (FILE *stream)

The __fpurge function causes the buffer of the stream stream to be emptied. If the stream is currently in read mode all input in the buffer is lost. If the stream is in output mode the buffered output is not written to the device (or whatever other underlying storage) and the buffer the cleared.

This function is declared in stdio_ext.h.

Previous: Flushing Buffers, Up: Stream Buffering

12.20.3 Controlling Which Kind of Buffering

After opening a stream (but before any other operations have been performed on it), you can explicitly specify what kind of buffering you want it to have using the setvbuf function. The facilities listed in this section are declared in the header file stdio.h.

— Function: int setvbuf (FILE *stream, char *buf, int mode, size_t size)

This function is used to specify that the stream stream should have the buffering mode mode, which can be either _IOFBF (for full buffering), _IOLBF (for line buffering), or _IONBF (for unbuffered input/output).

If you specify a null pointer as the buf argument, then setvbuf allocates a buffer itself using malloc. This buffer will be freed when you close the stream.

Otherwise, buf should be a character array that can hold at least size characters. You should not free the space for this array as long as the stream remains open and this array remains its buffer. You should usually either allocate it statically, or malloc (see Unconstrained Allocation) the buffer. Using an automatic array is not a good idea unless you close the file before exiting the block that declares the array.

While the array remains a stream buffer, the stream I/O functions will use the buffer for their internal purposes. You shouldn't try to access the values in the array directly while the stream is using it for buffering.

The setvbuf function returns zero on success, or a nonzero value if the value of mode is not valid or if the request could not be honored.

— Macro: int _IOFBF

The value of this macro is an integer constant expression that can be used as the mode argument to the setvbuf function to specify that the stream should be fully buffered.

— Macro: int _IOLBF

The value of this macro is an integer constant expression that can be used as the mode argument to the setvbuf function to specify that the stream should be line buffered.

— Macro: int _IONBF

The value of this macro is an integer constant expression that can be used as the mode argument to the setvbuf function to specify that the stream should be unbuffered.

— Macro: int BUFSIZ

The value of this macro is an integer constant expression that is good to use for the size argument to setvbuf. This value is guaranteed to be at least 256.

The value of BUFSIZ is chosen on each system so as to make stream I/O efficient. So it is a good idea to use BUFSIZ as the size for the buffer when you call setvbuf.

Actually, you can get an even better value to use for the buffer size by means of the fstat system call: it is found in the st_blksize field of the file attributes. See Attribute Meanings.

Sometimes people also use BUFSIZ as the allocation size of buffers used for related purposes, such as strings used to receive a line of input with fgets (see Character Input). There is no particular reason to use BUFSIZ for this instead of any other integer, except that it might lead to doing I/O in chunks of an efficient size.

— Function: void setbuf (FILE *stream, char *buf)

If buf is a null pointer, the effect of this function is equivalent to calling setvbuf with a mode argument of _IONBF. Otherwise, it is equivalent to calling setvbuf with buf, and a mode of _IOFBF and a size argument of BUFSIZ.

The setbuf function is provided for compatibility with old code; use setvbuf in all new programs.

— Function: void setbuffer (FILE *stream, char *buf, size_t size)

If buf is a null pointer, this function makes stream unbuffered. Otherwise, it makes stream fully buffered using buf as the buffer. The size argument specifies the length of buf.

This function is provided for compatibility with old BSD code. Use setvbuf instead.

— Function: void setlinebuf (FILE *stream)

This function makes stream be line buffered, and allocates the buffer for you.

This function is provided for compatibility with old BSD code. Use setvbuf instead.

It is possible to query whether a given stream is line buffered or not using a non-standard function introduced in Solaris and available in the GNU C library.

— Function: int __flbf (FILE *stream)

The __flbf function will return a nonzero value in case the stream stream is line buffered. Otherwise the return value is zero.

This function is declared in the stdio_ext.h header.

Two more extensions allow to determine the size of the buffer and how much of it is used. These functions were also introduced in Solaris.

— Function: size_t __fbufsize (FILE *stream)

The __fbufsize function return the size of the buffer in the stream stream. This value can be used to optimize the use of the stream.

This function is declared in the stdio_ext.h header.

— Function: size_t __fpending (FILE *stream) The __fpending

function returns the number of bytes currently in the output buffer. For wide-oriented stream the measuring unit is wide characters. This function should not be used on buffers in read mode or opened read-only.

This function is declared in the stdio_ext.h header.

Next: , Previous: Stream Buffering, Up: I/O on Streams

12.21 Other Kinds of Streams

The GNU library provides ways for you to define additional kinds of streams that do not necessarily correspond to an open file.

One such type of stream takes input from or writes output to a string. These kinds of streams are used internally to implement the sprintf and sscanf functions. You can also create such a stream explicitly, using the functions described in String Streams.

More generally, you can define streams that do input/output to arbitrary objects using functions supplied by your program. This protocol is discussed in Custom Streams.

Portability Note: The facilities described in this section are specific to GNU. Other systems or C implementations might or might not provide equivalent functionality.

Next: , Up: Other Kinds of Streams

12.21.1 String Streams

The fmemopen and open_memstream functions allow you to do I/O to a string or memory buffer. These facilities are declared in stdio.h.

— Function: FILE * fmemopen (void *buf, size_t size, const char *opentype)

This function opens a stream that allows the access specified by the opentype argument, that reads from or writes to the buffer specified by the argument buf. This array must be at least size bytes long.

If you specify a null pointer as the buf argument, fmemopen dynamically allocates an array size bytes long (as with malloc; see Unconstrained Allocation). This is really only useful if you are going to write things to the buffer and then read them back in again, because you have no way of actually getting a pointer to the buffer (for this, try open_memstream, below). The buffer is freed when the stream is closed.

The argument opentype is the same as in fopen (see Opening Streams). If the opentype specifies append mode, then the initial file position is set to the first null character in the buffer. Otherwise the initial file position is at the beginning of the buffer.

When a stream open for writing is flushed or closed, a null character (zero byte) is written at the end of the buffer if it fits. You should add an extra byte to the size argument to account for this. Attempts to write more than size bytes to the buffer result in an error.

For a stream open for reading, null characters (zero bytes) in the buffer do not count as “end of file”. Read operations indicate end of file only when the file position advances past size bytes. So, if you want to read characters from a null-terminated string, you should supply the length of the string as the size argument.

Here is an example of using fmemopen to create a stream for reading from a string: 

     #include <stdio.h>
     
     static char buffer[] = "foobar";
     
     int
     main (void)
     {
       int ch;
       FILE *stream;
     
       stream = fmemopen (buffer, strlen (buffer), "r");
       while ((ch = fgetc (stream)) != EOF)
         printf ("Got %c\n", ch);
       fclose (stream);
     
       return 0;
     }

This program produces the following output: 

     Got f
     Got o
     Got o
     Got b
     Got a
     Got r
— Function: FILE * open_memstream (char **ptr, size_t *sizeloc)

This function opens a stream for writing to a buffer. The buffer is allocated dynamically and grown as necessary, using malloc. After you've closed the stream, this buffer is your responsibility to clean up using free or realloc. See Unconstrained Allocation.

When the stream is closed with fclose or flushed with fflush, the locations ptr and sizeloc are updated to contain the pointer to the buffer and its size. The values thus stored remain valid only as long as no further output on the stream takes place. If you do more output, you must flush the stream again to store new values before you use them again.

A null character is written at the end of the buffer. This null character is not included in the size value stored at sizeloc.

You can move the stream's file position with fseek or fseeko (see File Positioning). Moving the file position past the end of the data already written fills the intervening space with zeroes.

Here is an example of using open_memstream: 

     #include <stdio.h>
     
     int
     main (void)
     {
       char *bp;
       size_t size;
       FILE *stream;
     
       stream = open_memstream (&bp, &size);
       fprintf (stream, "hello");
       fflush (stream);
       printf ("buf = `%s', size = %d\n", bp, size);
       fprintf (stream, ", world");
       fclose (stream);
       printf ("buf = `%s', size = %d\n", bp, size);
     
       return 0;
     }

This program produces the following output: 

     buf = `hello', size = 5
     buf = `hello, world', size = 12

Next: , Previous: String Streams, Up: Other Kinds of Streams

12.21.2 Obstack Streams

You can open an output stream that puts it data in an obstack. See Obstacks.

— Function: FILE * open_obstack_stream (struct obstack *obstack)

This function opens a stream for writing data into the obstack obstack. This starts an object in the obstack and makes it grow as data is written (see Growing Objects).

Calling fflush on this stream updates the current size of the object to match the amount of data that has been written. After a call to fflush, you can examine the object temporarily.

You can move the file position of an obstack stream with fseek or fseeko (see File Positioning). Moving the file position past the end of the data written fills the intervening space with zeros.

To make the object permanent, update the obstack with fflush, and then use obstack_finish to finalize the object and get its address. The following write to the stream starts a new object in the obstack, and later writes add to that object until you do another fflush and obstack_finish.

But how do you find out how long the object is? You can get the length in bytes by calling obstack_object_size (see Status of an Obstack), or you can null-terminate the object like this: 

          obstack_1grow (obstack, 0);

Whichever one you do, you must do it before calling obstack_finish. (You can do both if you wish.)

Here is a sample function that uses open_obstack_stream: 

     char *
     make_message_string (const char *a, int b)
     {
       FILE *stream = open_obstack_stream (&message_obstack);
       output_task (stream);
       fprintf (stream, ": ");
       fprintf (stream, a, b);
       fprintf (stream, "\n");
       fclose (stream);
       obstack_1grow (&message_obstack, 0);
       return obstack_finish (&message_obstack);
     }

Previous: Obstack Streams, Up: Other Kinds of Streams

12.21.3 Programming Your Own Custom Streams

This section describes how you can make a stream that gets input from an arbitrary data source or writes output to an arbitrary data sink programmed by you. We call these custom streams. The functions and types described here are all GNU extensions.

Next: , Up: Custom Streams
12.21.3.1 Custom Streams and Cookies

Inside every custom stream is a special object called the cookie. This is an object supplied by you which records where to fetch or store the data read or written. It is up to you to define a data type to use for the cookie. The stream functions in the library never refer directly to its contents, and they don't even know what the type is; they record its address with type void *.

To implement a custom stream, you must specify how to fetch or store the data in the specified place. You do this by defining hook functions to read, write, change “file position”, and close the stream. All four of these functions will be passed the stream's cookie so they can tell where to fetch or store the data. The library functions don't know what's inside the cookie, but your functions will know.

When you create a custom stream, you must specify the cookie pointer, and also the four hook functions stored in a structure of type cookie_io_functions_t.

These facilities are declared in stdio.h.

— Data Type: cookie_io_functions_t

This is a structure type that holds the functions that define the communications protocol between the stream and its cookie. It has the following members:

cookie_read_function_t *read
This is the function that reads data from the cookie. If the value is a null pointer instead of a function, then read operations on this stream always return EOF.
cookie_write_function_t *write
This is the function that writes data to the cookie. If the value is a null pointer instead of a function, then data written to the stream is discarded.
cookie_seek_function_t *seek
This is the function that performs the equivalent of file positioning on the cookie. If the value is a null pointer instead of a function, calls to fseek or fseeko on this stream can only seek to locations within the buffer; any attempt to seek outside the buffer will return an ESPIPE error.
cookie_close_function_t *close
This function performs any appropriate cleanup on the cookie when closing the stream. If the value is a null pointer instead of a function, nothing special is done to close the cookie when the stream is closed.

— Function: FILE * fopencookie (void *cookie, const char *opentype, cookie_io_functions_t io-functions)

This function actually creates the stream for communicating with the cookie using the functions in the io-functions argument. The opentype argument is interpreted as for fopen; see Opening Streams. (But note that the “truncate on open” option is ignored.) The new stream is fully buffered.

The fopencookie function returns the newly created stream, or a null pointer in case of an error.

Previous: Streams and Cookies, Up: Custom Streams
12.21.3.2 Custom Stream Hook Functions

Here are more details on how you should define the four hook functions that a custom stream needs.

You should define the function to read data from the cookie as: 

     ssize_t reader (void *cookie, char *buffer, size_t size)

This is very similar to the read function; see I/O Primitives. Your function should transfer up to size bytes into the buffer, and return the number of bytes read, or zero to indicate end-of-file. You can return a value of -1 to indicate an error.

You should define the function to write data to the cookie as: 

     ssize_t writer (void *cookie, const char *buffer, size_t size)

This is very similar to the write function; see I/O Primitives. Your function should transfer up to size bytes from the buffer, and return the number of bytes written. You can return a value of -1 to indicate an error.

You should define the function to perform seek operations on the cookie as: 

     int seeker (void *cookie, off64_t *position, int whence)

For this function, the position and whence arguments are interpreted as for fgetpos; see Portable Positioning.

After doing the seek operation, your function should store the resulting file position relative to the beginning of the file in position. Your function should return a value of 0 on success and -1 to indicate an error.

You should define the function to do cleanup operations on the cookie appropriate for closing the stream as: 

     int cleaner (void *cookie)

Your function should return -1 to indicate an error, and 0 otherwise.

— Data Type: cookie_read_function

This is the data type that the read function for a custom stream should have. If you declare the function as shown above, this is the type it will have.

— Data Type: cookie_write_function

The data type of the write function for a custom stream.

— Data Type: cookie_seek_function

The data type of the seek function for a custom stream.

— Data Type: cookie_close_function

The data type of the close function for a custom stream.

Previous: Other Kinds of Streams, Up: I/O on Streams

12.22 Formatted Messages

On systems which are based on System V messages of programs (especially the system tools) are printed in a strict form using the fmtmsg function. The uniformity sometimes helps the user to interpret messages and the strictness tests of the fmtmsg function ensure that the programmer follows some minimal requirements.

Next: , Up: Formatted Messages

12.22.1 Printing Formatted Messages

Messages can be printed to standard error and/or to the console. To select the destination the programmer can use the following two values, bitwise OR combined if wanted, for the classification parameter of fmtmsg:

MM_PRINT
Display the message in standard error.
MM_CONSOLE
Display the message on the system console.

The erroneous piece of the system can be signalled by exactly one of the following values which also is bitwise ORed with the classification parameter to fmtmsg:

MM_HARD
The source of the condition is some hardware.
MM_SOFT
The source of the condition is some software.
MM_FIRM
The source of the condition is some firmware.

A third component of the classification parameter to fmtmsg can describe the part of the system which detects the problem. This is done by using exactly one of the following values:

MM_APPL
The erroneous condition is detected by the application.
MM_UTIL
The erroneous condition is detected by a utility.
MM_OPSYS
The erroneous condition is detected by the operating system.

A last component of classification can signal the results of this message. Exactly one of the following values can be used:

MM_RECOVER
It is a recoverable error.
MM_NRECOV
It is a non-recoverable error.
— Function: int fmtmsg (long int classification, const char *label, int severity, const char *text, const char *action, const char *tag)

Display a message described by its parameters on the device(s) specified in the classification parameter. The label parameter identifies the source of the message. The string should consist of two colon separated parts where the first part has not more than 10 and the second part not more than 14 characters. The text parameter describes the condition of the error, the action parameter possible steps to recover from the error and the tag parameter is a reference to the online documentation where more information can be found. It should contain the label value and a unique identification number.

Each of the parameters can be a special value which means this value is to be omitted. The symbolic names for these values are:

MM_NULLLBL
Ignore label parameter.
MM_NULLSEV
Ignore severity parameter.
MM_NULLMC
Ignore classification parameter. This implies that nothing is actually printed.
MM_NULLTXT
Ignore text parameter.
MM_NULLACT
Ignore action parameter.
MM_NULLTAG
Ignore tag parameter.

There is another way certain fields can be omitted from the output to standard error. This is described below in the description of environment variables influencing the behavior.

The severity parameter can have one of the values in the following table:

MM_NOSEV
Nothing is printed, this value is the same as MM_NULLSEV.
MM_HALT
This value is printed as HALT.
MM_ERROR
This value is printed as ERROR.
MM_WARNING
This value is printed as WARNING.
MM_INFO
This value is printed as INFO.

The numeric value of these five macros are between 0 and 4. Using the environment variable SEV_LEVEL or using the addseverity function one can add more severity levels with their corresponding string to print. This is described below (see Adding Severity Classes).

If no parameter is ignored the output looks like this: 

          label: severity-string: text
          TO FIX: action tag

The colons, new line characters and the TO FIX string are inserted if necessary, i.e., if the corresponding parameter is not ignored.

This function is specified in the X/Open Portability Guide. It is also available on all systems derived from System V.

The function returns the value MM_OK if no error occurred. If only the printing to standard error failed, it returns MM_NOMSG. If printing to the console fails, it returns MM_NOCON. If nothing is printed MM_NOTOK is returned. Among situations where all outputs fail this last value is also returned if a parameter value is incorrect.

There are two environment variables which influence the behavior of fmtmsg. The first is MSGVERB. It is used to control the output actually happening on standard error (not the console output). Each of the five fields can explicitly be enabled. To do this the user has to put the MSGVERB variable with a format like the following in the environment before calling the fmtmsg function the first time: 

     MSGVERB=keyword[:keyword[:...]]

Valid keywords are label, severity, text, action, and tag. If the environment variable is not given or is the empty string, a not supported keyword is given or the value is somehow else invalid, no part of the message is masked out.

The second environment variable which influences the behavior of fmtmsg is SEV_LEVEL. This variable and the change in the behavior of fmtmsg is not specified in the X/Open Portability Guide. It is available in System V systems, though. It can be used to introduce new severity levels. By default, only the five severity levels described above are available. Any other numeric value would make fmtmsg print nothing.

If the user puts SEV_LEVEL with a format like 

     SEV_LEVEL=[description[:description[:...]]]

in the environment of the process before the first call to fmtmsg, where description has a value of the form 

     severity-keyword,level,printstring

The severity-keyword part is not used by fmtmsg but it has to be present. The level part is a string representation of a number. The numeric value must be a number greater than 4. This value must be used in the severity parameter of fmtmsg to select this class. It is not possible to overwrite any of the predefined classes. The printstring is the string printed when a message of this class is processed by fmtmsg (see above, fmtsmg does not print the numeric value but instead the string representation).

Next: , Previous: Printing Formatted Messages, Up: Formatted Messages

12.22.2 Adding Severity Classes

There is another possibility to introduce severity classes besides using the environment variable SEV_LEVEL. This simplifies the task of introducing new classes in a running program. One could use the setenv or putenv function to set the environment variable, but this is toilsome.

— Function: int addseverity (int severity, const char *string)

This function allows the introduction of new severity classes which can be addressed by the severity parameter of the fmtmsg function. The severity parameter of addseverity must match the value for the parameter with the same name of fmtmsg, and string is the string printed in the actual messages instead of the numeric value.

If string is NULL the severity class with the numeric value according to severity is removed.

It is not possible to overwrite or remove one of the default severity classes. All calls to addseverity with severity set to one of the values for the default classes will fail.

The return value is MM_OK if the task was successfully performed. If the return value is MM_NOTOK something went wrong. This could mean that no more memory is available or a class is not available when it has to be removed.

This function is not specified in the X/Open Portability Guide although the fmtsmg function is. It is available on System V systems.

Previous: Adding Severity Classes, Up: Formatted Messages

12.22.3 How to use fmtmsg and addseverity

Here is a simple example program to illustrate the use of the both functions described in this section. 

     #include <fmtmsg.h>
     
     int
     main (void)
     {
       addseverity (5, "NOTE2");
       fmtmsg (MM_PRINT, "only1field", MM_INFO, "text2", "action2", "tag2");
       fmtmsg (MM_PRINT, "UX:cat", 5, "invalid syntax", "refer to manual",
               "UX:cat:001");
       fmtmsg (MM_PRINT, "label:foo", 6, "text", "action", "tag");
       return 0;
     }

The second call to fmtmsg illustrates a use of this function as it usually occurs on System V systems, which heavily use this function. It seems worthwhile to give a short explanation here of how this system works on System V. The value of the label field (UX:cat) says that the error occurred in the Unix program cat. The explanation of the error follows and the value for the action parameter is "refer to manual". One could be more specific here, if necessary. The tag field contains, as proposed above, the value of the string given for the label parameter, and additionally a unique ID (001 in this case). For a GNU environment this string could contain a reference to the corresponding node in the Info page for the program.

Running this program without specifying the MSGVERB and SEV_LEVEL function produces the following output: 

     UX:cat: NOTE2: invalid syntax
     TO FIX: refer to manual UX:cat:001

We see the different fields of the message and how the extra glue (the colons and the TO FIX string) are printed. But only one of the three calls to fmtmsg produced output. The first call does not print anything because the label parameter is not in the correct form. The string must contain two fields, separated by a colon (see Printing Formatted Messages). The third fmtmsg call produced no output since the class with the numeric value 6 is not defined. Although a class with numeric value 5 is also not defined by default, the call to addseverity introduces it and the second call to fmtmsg produces the above output.

When we change the environment of the program to contain SEV_LEVEL=XXX,6,NOTE when running it we get a different result: 

     UX:cat: NOTE2: invalid syntax
     TO FIX: refer to manual UX:cat:001
     label:foo: NOTE: text
     TO FIX: action tag

Now the third call to fmtmsg produced some output and we see how the string NOTE from the environment variable appears in the message.

Now we can reduce the output by specifying which fields we are interested in. If we additionally set the environment variable MSGVERB to the value severity:label:action we get the following output: 

     UX:cat: NOTE2
     TO FIX: refer to manual
     label:foo: NOTE
     TO FIX: action

I.e., the output produced by the text and the tag parameters to fmtmsg vanished. Please also note that now there is no colon after the NOTE and NOTE2 strings in the output. This is not necessary since there is no more output on this line because the text is missing.

Next: , Previous: I/O on Streams, Up: Top

13 Low-Level Input/Output

This chapter describes functions for performing low-level input/output operations on file descriptors. These functions include the primitives for the higher-level I/O functions described in I/O on Streams, as well as functions for performing low-level control operations for which there are no equivalents on streams.

Stream-level I/O is more flexible and usually more convenient; therefore, programmers generally use the descriptor-level functions only when necessary. These are some of the usual reasons:

Next: , Up: Low-Level I/O

13.1 Opening and Closing Files

This section describes the primitives for opening and closing files using file descriptors. The open and creat functions are declared in the header file fcntl.h, while close is declared in unistd.h.

— Function: int open (const char *filename, int flags[, mode_t mode])

The open function creates and returns a new file descriptor for the file named by filename. Initially, the file position indicator for the file is at the beginning of the file. The argument mode is used only when a file is created, but it doesn't hurt to supply the argument in any case.

The flags argument controls how the file is to be opened. This is a bit mask; you create the value by the bitwise OR of the appropriate parameters (using the `|' operator in C). See File Status Flags, for the parameters available.

The normal return value from open is a non-negative integer file descriptor. In the case of an error, a value of -1 is returned instead. In addition to the usual file name errors (see File Name Errors), the following errno error conditions are defined for this function:

EACCES
The file exists but is not readable/writable as requested by the flags argument, the file does not exist and the directory is unwritable so it cannot be created.
EEXIST
Both O_CREAT and O_EXCL are set, and the named file already exists.
EINTR
The open operation was interrupted by a signal. See Interrupted Primitives.
EISDIR
The flags argument specified write access, and the file is a directory.
EMFILE
The process has too many files open. The maximum number of file descriptors is controlled by the RLIMIT_NOFILE resource limit; see Limits on Resources.
ENFILE
The entire system, or perhaps the file system which contains the directory, cannot support any additional open files at the moment. (This problem cannot happen on the GNU system.)
ENOENT
The named file does not exist, and O_CREAT is not specified.
ENOSPC
The directory or file system that would contain the new file cannot be extended, because there is no disk space left.
ENXIO
O_NONBLOCK and O_WRONLY are both set in the flags argument, the file named by filename is a FIFO (see Pipes and FIFOs), and no process has the file open for reading.
EROFS
The file resides on a read-only file system and any of O_WRONLY, O_RDWR, and O_TRUNC are set in the flags argument, or O_CREAT is set and the file does not already exist.

If on a 32 bit machine the sources are translated with _FILE_OFFSET_BITS == 64 the function open returns a file descriptor opened in the large file mode which enables the file handling functions to use files up to 2^63 bytes in size and offset from -2^63 to 2^63. This happens transparently for the user since all of the lowlevel file handling functions are equally replaced.

This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time open is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this calls to open should be protected using cancellation handlers.

The open function is the underlying primitive for the fopen and freopen functions, that create streams.

— Function: int open64 (const char *filename, int flags[, mode_t mode])

This function is similar to open. It returns a file descriptor which can be used to access the file named by filename. The only difference is that on 32 bit systems the file is opened in the large file mode. I.e., file length and file offsets can exceed 31 bits.

When the sources are translated with _FILE_OFFSET_BITS == 64 this function is actually available under the name open. I.e., the new, extended API using 64 bit file sizes and offsets transparently replaces the old API.

— Obsolete function: int creat (const char *filename, mode_t mode)

This function is obsolete. The call: 

          creat (filename, mode)

is equivalent to: 

          open (filename, O_WRONLY | O_CREAT | O_TRUNC, mode)

If on a 32 bit machine the sources are translated with _FILE_OFFSET_BITS == 64 the function creat returns a file descriptor opened in the large file mode which enables the file handling functions to use files up to 2^63 in size and offset from -2^63 to 2^63. This happens transparently for the user since all of the lowlevel file handling functions are equally replaced.

— Obsolete function: int creat64 (const char *filename, mode_t mode)

This function is similar to creat. It returns a file descriptor which can be used to access the file named by filename. The only the difference is that on 32 bit systems the file is opened in the large file mode. I.e., file length and file offsets can exceed 31 bits.

To use this file descriptor one must not use the normal operations but instead the counterparts named *64, e.g., read64.

When the sources are translated with _FILE_OFFSET_BITS == 64 this function is actually available under the name open. I.e., the new, extended API using 64 bit file sizes and offsets transparently replaces the old API.

— Function: int close (int filedes)

The function close closes the file descriptor filedes. Closing a file has the following consequences:

This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time close is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this, calls to close should be protected using cancellation handlers.

The normal return value from close is 0; a value of -1 is returned in case of failure. The following errno error conditions are defined for this function:

EBADF
The filedes argument is not a valid file descriptor.
EINTR
The close call was interrupted by a signal. See Interrupted Primitives. Here is an example of how to handle EINTR properly: 
               TEMP_FAILURE_RETRY (close (desc));

ENOSPC
EIO
EDQUOT
When the file is accessed by NFS, these errors from write can sometimes not be detected until close. See I/O Primitives, for details on their meaning.

Please note that there is no separate close64 function. This is not necessary since this function does not determine nor depend on the mode of the file. The kernel which performs the close operation knows which mode the descriptor is used for and can handle this situation.

To close a stream, call fclose (see Closing Streams) instead of trying to close its underlying file descriptor with close. This flushes any buffered output and updates the stream object to indicate that it is closed.

Next: , Previous: Opening and Closing Files, Up: Low-Level I/O

13.2 Input and Output Primitives

This section describes the functions for performing primitive input and output operations on file descriptors: read, write, and lseek. These functions are declared in the header file unistd.h.

— Data Type: ssize_t

This data type is used to represent the sizes of blocks that can be read or written in a single operation. It is similar to size_t, but must be a signed type.

— Function: ssize_t read (int filedes, void *buffer, size_t size)

The read function reads up to size bytes from the file with descriptor filedes, storing the results in the buffer. (This is not necessarily a character string, and no terminating null character is added.)

The return value is the number of bytes actually read. This might be less than size; for example, if there aren't that many bytes left in the file or if there aren't that many bytes immediately available. The exact behavior depends on what kind of file it is. Note that reading less than size bytes is not an error.

A value of zero indicates end-of-file (except if the value of the size argument is also zero). This is not considered an error. If you keep calling read while at end-of-file, it will keep returning zero and doing nothing else.

If read returns at least one character, there is no way you can tell whether end-of-file was reached. But if you did reach the end, the next read will return zero.

In case of an error, read returns -1. The following errno error conditions are defined for this function:

EAGAIN
Normally, when no input is immediately available, read waits for some input. But if the O_NONBLOCK flag is set for the file (see File Status Flags), read returns immediately without reading any data, and reports this error.

Compatibility Note: Most versions of BSD Unix use a different error code for this: EWOULDBLOCK. In the GNU library, EWOULDBLOCK is an alias for EAGAIN, so it doesn't matter which name you use.

On some systems, reading a large amount of data from a character special file can also fail with EAGAIN if the kernel cannot find enough physical memory to lock down the user's pages. This is limited to devices that transfer with direct memory access into the user's memory, which means it does not include terminals, since they always use separate buffers inside the kernel. This problem never happens in the GNU system.

Any condition that could result in EAGAIN can instead result in a successful read which returns fewer bytes than requested. Calling read again immediately would result in EAGAIN.

EBADF
The filedes argument is not a valid file descriptor, or is not open for reading.
EINTR
read was interrupted by a signal while it was waiting for input. See Interrupted Primitives. A signal will not necessary cause read to return EINTR; it may instead result in a successful read which returns fewer bytes than requested.
EIO
For many devices, and for disk files, this error code indicates a hardware error.

EIO also occurs when a background process tries to read from the controlling terminal, and the normal action of stopping the process by sending it a SIGTTIN signal isn't working. This might happen if the signal is being blocked or ignored, or because the process group is orphaned. See Job Control, for more information about job control, and Signal Handling, for information about signals.

EINVAL
In some systems, when reading from a character or block device, position and size offsets must be aligned to a particular block size. This error indicates that the offsets were not properly aligned.

Please note that there is no function named read64. This is not necessary since this function does not directly modify or handle the possibly wide file offset. Since the kernel handles this state internally, the read function can be used for all cases.

This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time read is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this, calls to read should be protected using cancellation handlers.

The read function is the underlying primitive for all of the functions that read from streams, such as fgetc.

— Function: ssize_t pread (int filedes, void *buffer, size_t size, off_t offset)

The pread function is similar to the read function. The first three arguments are identical, and the return values and error codes also correspond.

The difference is the fourth argument and its handling. The data block is not read from the current position of the file descriptor filedes. Instead the data is read from the file starting at position offset. The position of the file descriptor itself is not affected by the operation. The value is the same as before the call.

When the source file is compiled with _FILE_OFFSET_BITS == 64 the pread function is in fact pread64 and the type off_t has 64 bits, which makes it possible to handle files up to 2^63 bytes in length.

The return value of pread describes the number of bytes read. In the error case it returns -1 like read does and the error codes are also the same, with these additions:

EINVAL
The value given for offset is negative and therefore illegal.
ESPIPE
The file descriptor filedes is associate with a pipe or a FIFO and this device does not allow positioning of the file pointer.

The function is an extension defined in the Unix Single Specification version 2.

— Function: ssize_t pread64 (int filedes, void *buffer, size_t size, off64_t offset)

This function is similar to the pread function. The difference is that the offset parameter is of type off64_t instead of off_t which makes it possible on 32 bit machines to address files larger than 2^31 bytes and up to 2^63 bytes. The file descriptor filedes must be opened using open64 since otherwise the large offsets possible with off64_t will lead to errors with a descriptor in small file mode.

When the source file is compiled with _FILE_OFFSET_BITS == 64 on a 32 bit machine this function is actually available under the name pread and so transparently replaces the 32 bit interface.

— Function: ssize_t write (int filedes, const void *buffer, size_t size)

The write function writes up to size bytes from buffer to the file with descriptor filedes. The data in buffer is not necessarily a character string and a null character is output like any other character.

The return value is the number of bytes actually written. This may be size, but can always be smaller. Your program should always call write in a loop, iterating until all the data is written.

Once write returns, the data is enqueued to be written and can be read back right away, but it is not necessarily written out to permanent storage immediately. You can use fsync when you need to be sure your data has been permanently stored before continuing. (It is more efficient for the system to batch up consecutive writes and do them all at once when convenient. Normally they will always be written to disk within a minute or less.) Modern systems provide another function fdatasync which guarantees integrity only for the file data and is therefore faster. You can use the O_FSYNC open mode to make write always store the data to disk before returning; see Operating Modes.

In the case of an error, write returns -1. The following errno error conditions are defined for this function:

EAGAIN
Normally, write blocks until the write operation is complete. But if the O_NONBLOCK flag is set for the file (see Control Operations), it returns immediately without writing any data and reports this error. An example of a situation that might cause the process to block on output is writing to a terminal device that supports flow control, where output has been suspended by receipt of a STOP character.

Compatibility Note: Most versions of BSD Unix use a different error code for this: EWOULDBLOCK. In the GNU library, EWOULDBLOCK is an alias for EAGAIN, so it doesn't matter which name you use.

On some systems, writing a large amount of data from a character special file can also fail with EAGAIN if the kernel cannot find enough physical memory to lock down the user's pages. This is limited to devices that transfer with direct memory access into the user's memory, which means it does not include terminals, since they always use separate buffers inside the kernel. This problem does not arise in the GNU system.

EBADF
The filedes argument is not a valid file descriptor, or is not open for writing.
EFBIG
The size of the file would become larger than the implementation can support.
EINTR
The write operation was interrupted by a signal while it was blocked waiting for completion. A signal will not necessarily cause write to return EINTR; it may instead result in a successful write which writes fewer bytes than requested. See Interrupted Primitives.
EIO
For many devices, and for disk files, this error code indicates a hardware error.
ENOSPC
The device containing the file is full.
EPIPE
This error is returned when you try to write to a pipe or FIFO that isn't open for reading by any process. When this happens, a SIGPIPE signal is also sent to the process; see Signal Handling.
EINVAL
In some systems, when writing to a character or block device, position and size offsets must be aligned to a particular block size. This error indicates that the offsets were not properly aligned.

Unless you have arranged to prevent EINTR failures, you should check errno after each failing call to write, and if the error was EINTR, you should simply repeat the call. See Interrupted Primitives. The easy way to do this is with the macro TEMP_FAILURE_RETRY, as follows: 

          nbytes = TEMP_FAILURE_RETRY (write (desc, buffer, count));

Please note that there is no function named write64. This is not necessary since this function does not directly modify or handle the possibly wide file offset. Since the kernel handles this state internally the write function can be used for all cases.

This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time write is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this, calls to write should be protected using cancellation handlers.

The write function is the underlying primitive for all of the functions that write to streams, such as fputc.

— Function: ssize_t pwrite (int filedes, const void *buffer, size_t size, off_t offset)

The pwrite function is similar to the write function. The first three arguments are identical, and the return values and error codes also correspond.

The difference is the fourth argument and its handling. The data block is not written to the current position of the file descriptor filedes. Instead the data is written to the file starting at position offset. The position of the file descriptor itself is not affected by the operation. The value is the same as before the call.

When the source file is compiled with _FILE_OFFSET_BITS == 64 the pwrite function is in fact pwrite64 and the type off_t has 64 bits, which makes it possible to handle files up to 2^63 bytes in length.

The return value of pwrite describes the number of written bytes. In the error case it returns -1 like write does and the error codes are also the same, with these additions:

EINVAL
The value given for offset is negative and therefore illegal.
ESPIPE
The file descriptor filedes is associated with a pipe or a FIFO and this device does not allow positioning of the file pointer.

The function is an extension defined in the Unix Single Specification version 2.

— Function: ssize_t pwrite64 (int filedes, const void *buffer, size_t size, off64_t offset)

This function is similar to the pwrite function. The difference is that the offset parameter is of type off64_t instead of off_t which makes it possible on 32 bit machines to address files larger than 2^31 bytes and up to 2^63 bytes. The file descriptor filedes must be opened using open64 since otherwise the large offsets possible with off64_t will lead to errors with a descriptor in small file mode.

When the source file is compiled using _FILE_OFFSET_BITS == 64 on a 32 bit machine this function is actually available under the name pwrite and so transparently replaces the 32 bit interface.

Next: , Previous: I/O Primitives, Up: Low-Level I/O

13.3 Setting the File Position of a Descriptor

Just as you can set the file position of a stream with fseek, you can set the file position of a descriptor with lseek. This specifies the position in the file for the next read or write operation. See File Positioning, for more information on the file position and what it means.

To read the current file position value from a descriptor, use lseek (desc, 0, SEEK_CUR).

— Function: off_t lseek (int filedes, off_t offset, int whence)

The lseek function is used to change the file position of the file with descriptor filedes.

The whence argument specifies how the offset should be interpreted, in the same way as for the fseek function, and it must be one of the symbolic constants SEEK_SET, SEEK_CUR, or SEEK_END.

SEEK_SET
Specifies that whence is a count of characters from the beginning of the file.
SEEK_CUR
Specifies that whence is a count of characters from the current file position. This count may be positive or negative.
SEEK_END
Specifies that whence is a count of characters from the end of the file. A negative count specifies a position within the current extent of the file; a positive count specifies a position past the current end. If you set the position past the current end, and actually write data, you will extend the file with zeros up to that position.

The return value from lseek is normally the resulting file position, measured in bytes from the beginning of the file. You can use this feature together with SEEK_CUR to read the current file position.

If you want to append to the file, setting the file position to the current end of file with SEEK_END is not sufficient. Another process may write more data after you seek but before you write, extending the file so the position you write onto clobbers their data. Instead, use the O_APPEND operating mode; see Operating Modes.

You can set the file position past the current end of the file. This does not by itself make the file longer; lseek never changes the file. But subsequent output at that position will extend the file. Characters between the previous end of file and the new position are filled with zeros. Extending the file in this way can create a “hole”: the blocks of zeros are not actually allocated on disk, so the file takes up less space than it appears to; it is then called a “sparse file”. If the file position cannot be changed, or the operation is in some way invalid, lseek returns a value of -1. The following errno error conditions are defined for this function:

EBADF
The filedes is not a valid file descriptor.
EINVAL
The whence argument value is not valid, or the resulting file offset is not valid. A file offset is invalid.
ESPIPE
The filedes corresponds to an object that cannot be positioned, such as a pipe, FIFO or terminal device. (POSIX.1 specifies this error only for pipes and FIFOs, but in the GNU system, you always get ESPIPE if the object is not seekable.)

When the source file is compiled with _FILE_OFFSET_BITS == 64 the lseek function is in fact lseek64 and the type off_t has 64 bits which makes it possible to handle files up to 2^63 bytes in length.

This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time lseek is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this calls to lseek should be protected using cancellation handlers.

The lseek function is the underlying primitive for the fseek, fseeko, ftell, ftello and rewind functions, which operate on streams instead of file descriptors.

— Function: off64_t lseek64 (int filedes, off64_t offset, int whence)

This function is similar to the lseek function. The difference is that the offset parameter is of type off64_t instead of off_t which makes it possible on 32 bit machines to address files larger than 2^31 bytes and up to 2^63 bytes. The file descriptor filedes must be opened using open64 since otherwise the large offsets possible with off64_t will lead to errors with a descriptor in small file mode.

When the source file is compiled with _FILE_OFFSET_BITS == 64 on a 32 bits machine this function is actually available under the name lseek and so transparently replaces the 32 bit interface.

You can have multiple descriptors for the same file if you open the file more than once, or if you duplicate a descriptor with dup. Descriptors that come from separate calls to open have independent file positions; using lseek on one descriptor has no effect on the other. For example, 

     {
       int d1, d2;
       char buf[4];
       d1 = open ("foo", O_RDONLY);
       d2 = open ("foo", O_RDONLY);
       lseek (d1, 1024, SEEK_SET);
       read (d2, buf, 4);
     }

will read the first four characters of the file foo. (The error-checking code necessary for a real program has been omitted here for brevity.)

By contrast, descriptors made by duplication share a common file position with the original descriptor that was duplicated. Anything which alters the file position of one of the duplicates, including reading or writing data, affects all of them alike. Thus, for example, 

     {
       int d1, d2, d3;
       char buf1[4], buf2[4];
       d1 = open ("foo", O_RDONLY);
       d2 = dup (d1);
       d3 = dup (d2);
       lseek (d3, 1024, SEEK_SET);
       read (d1, buf1, 4);
       read (d2, buf2, 4);
     }

will read four characters starting with the 1024'th character of foo, and then four more characters starting with the 1028'th character.

— Data Type: off_t

This is an arithmetic data type used to represent file sizes. In the GNU system, this is equivalent to fpos_t or long int.

If the source is compiled with _FILE_OFFSET_BITS == 64 this type is transparently replaced by off64_t.

— Data Type: off64_t

This type is used similar to off_t. The difference is that even on 32 bit machines, where the off_t type would have 32 bits, off64_t has 64 bits and so is able to address files up to 2^63 bytes in length.

When compiling with _FILE_OFFSET_BITS == 64 this type is available under the name off_t.

These aliases for the `SEEK_...' constants exist for the sake of compatibility with older BSD systems. They are defined in two different header files: fcntl.h and sys/file.h.

L_SET
An alias for SEEK_SET.
L_INCR
An alias for SEEK_CUR.
L_XTND
An alias for SEEK_END.

Next: , Previous: File Position Primitive, Up: Low-Level I/O

13.4 Descriptors and Streams

Given an open file descriptor, you can create a stream for it with the fdopen function. You can get the underlying file descriptor for an existing stream with the fileno function. These functions are declared in the header file stdio.h.

— Function: FILE * fdopen (int filedes, const char *opentype)

The fdopen function returns a new stream for the file descriptor filedes.

The opentype argument is interpreted in the same way as for the fopen function (see Opening Streams), except that the `b' option is not permitted; this is because GNU makes no distinction between text and binary files. Also, "w" and "w+" do not cause truncation of the file; these have an effect only when opening a file, and in this case the file has already been opened. You must make sure that the opentype argument matches the actual mode of the open file descriptor.

The return value is the new stream. If the stream cannot be created (for example, if the modes for the file indicated by the file descriptor do not permit the access specified by the opentype argument), a null pointer is returned instead.

In some other systems, fdopen may fail to detect that the modes for file descriptor do not permit the access specified by opentype. The GNU C library always checks for this.

For an example showing the use of the fdopen function, see Creating a Pipe.

— Function: int fileno (FILE *stream)

This function returns the file descriptor associated with the stream stream. If an error is detected (for example, if the stream is not valid) or if stream does not do I/O to a file, fileno returns -1.

— Function: int fileno_unlocked (FILE *stream)

The fileno_unlocked function is equivalent to the fileno function except that it does not implicitly lock the stream if the state is FSETLOCKING_INTERNAL.

This function is a GNU extension.

There are also symbolic constants defined in unistd.h for the file descriptors belonging to the standard streams stdin, stdout, and stderr; see Standard Streams.

STDIN_FILENO
This macro has value 0, which is the file descriptor for standard input.
STDOUT_FILENO
This macro has value 1, which is the file descriptor for standard output.
STDERR_FILENO
This macro has value 2, which is the file descriptor for standard error output.

Next: , Previous: Descriptors and Streams, Up: Low-Level I/O

13.5 Dangers of Mixing Streams and Descriptors

You can have multiple file descriptors and streams (let's call both streams and descriptors “channels” for short) connected to the same file, but you must take care to avoid confusion between channels. There are two cases to consider: linked channels that share a single file position value, and independent channels that have their own file positions.

It's best to use just one channel in your program for actual data transfer to any given file, except when all the access is for input. For example, if you open a pipe (something you can only do at the file descriptor level), either do all I/O with the descriptor, or construct a stream from the descriptor with fdopen and then do all I/O with the stream.

Next: , Up: Stream/Descriptor Precautions

13.5.1 Linked Channels

Channels that come from a single opening share the same file position; we call them linked channels. Linked channels result when you make a stream from a descriptor using fdopen, when you get a descriptor from a stream with fileno, when you copy a descriptor with dup or dup2, and when descriptors are inherited during fork. For files that don't support random access, such as terminals and pipes, all channels are effectively linked. On random-access files, all append-type output streams are effectively linked to each other.

If you have been using a stream for I/O (or have just opened the stream), and you want to do I/O using another channel (either a stream or a descriptor) that is linked to it, you must first clean up the stream that you have been using. See Cleaning Streams.

Terminating a process, or executing a new program in the process, destroys all the streams in the process. If descriptors linked to these streams persist in other processes, their file positions become undefined as a result. To prevent this, you must clean up the streams before destroying them.

Next: , Previous: Linked Channels, Up: Stream/Descriptor Precautions

13.5.2 Independent Channels

When you open channels (streams or descriptors) separately on a seekable file, each channel has its own file position. These are called independent channels.

The system handles each channel independently. Most of the time, this is quite predictable and natural (especially for input): each channel can read or write sequentially at its own place in the file. However, if some of the channels are streams, you must take these precautions:

If you do output to one channel at the end of the file, this will certainly leave the other independent channels positioned somewhere before the new end. You cannot reliably set their file positions to the new end of file before writing, because the file can always be extended by another process between when you set the file position and when you write the data. Instead, use an append-type descriptor or stream; they always output at the current end of the file. In order to make the end-of-file position accurate, you must clean the output channel you were using, if it is a stream.

It's impossible for two channels to have separate file pointers for a file that doesn't support random access. Thus, channels for reading or writing such files are always linked, never independent. Append-type channels are also always linked. For these channels, follow the rules for linked channels; see Linked Channels.

Previous: Independent Channels, Up: Stream/Descriptor Precautions

13.5.3 Cleaning Streams

On the GNU system, you can clean up any stream with fclean:

— Function: int fclean (FILE *stream)

Clean up the stream stream so that its buffer is empty. If stream is doing output, force it out. If stream is doing input, give the data in the buffer back to the system, arranging to reread it.

On other systems, you can use fflush to clean a stream in most cases.

You can skip the fclean or fflush if you know the stream is already clean. A stream is clean whenever its buffer is empty. For example, an unbuffered stream is always clean. An input stream that is at end-of-file is clean. A line-buffered stream is clean when the last character output was a newline. However, a just-opened input stream might not be clean, as its input buffer might not be empty.

There is one case in which cleaning a stream is impossible on most systems. This is when the stream is doing input from a file that is not random-access. Such streams typically read ahead, and when the file is not random access, there is no way to give back the excess data already read. When an input stream reads from a random-access file, fflush does clean the stream, but leaves the file pointer at an unpredictable place; you must set the file pointer before doing any further I/O. On the GNU system, using fclean avoids both of these problems.

Closing an output-only stream also does fflush, so this is a valid way of cleaning an output stream. On the GNU system, closing an input stream does fclean.

You need not clean a stream before using its descriptor for control operations such as setting terminal modes; these operations don't affect the file position and are not affected by it. You can use any descriptor for these operations, and all channels are affected simultaneously. However, text already “output” to a stream but still buffered by the stream will be subject to the new terminal modes when subsequently flushed. To make sure “past” output is covered by the terminal settings that were in effect at the time, flush the output streams for that terminal before setting the modes. See Terminal Modes.

Next: , Previous: Stream/Descriptor Precautions, Up: Low-Level I/O

13.6 Fast Scatter-Gather I/O

Some applications may need to read or write data to multiple buffers, which are separated in memory. Although this can be done easily enough with multiple calls to read and write, it is inefficient because there is overhead associated with each kernel call.

Instead, many platforms provide special high-speed primitives to perform these scatter-gather operations in a single kernel call. The GNU C library will provide an emulation on any system that lacks these primitives, so they are not a portability threat. They are defined in sys/uio.h.

These functions are controlled with arrays of iovec structures, which describe the location and size of each buffer.

— Data Type: struct iovec

The iovec structure describes a buffer. It contains two fields:

void *iov_base
Contains the address of a buffer.
size_t iov_len
Contains the length of the buffer.

— Function: ssize_t readv (int filedes, const struct iovec *vector, int count)

The readv function reads data from filedes and scatters it into the buffers described in vector, which is taken to be count structures long. As each buffer is filled, data is sent to the next.

Note that readv is not guaranteed to fill all the buffers. It may stop at any point, for the same reasons read would.

The return value is a count of bytes (not buffers) read, 0 indicating end-of-file, or -1 indicating an error. The possible errors are the same as in read.

— Function: ssize_t writev (int filedes, const struct iovec *vector, int count)

The writev function gathers data from the buffers described in vector, which is taken to be count structures long, and writes them to filedes. As each buffer is written, it moves on to the next.

Like readv, writev may stop midstream under the same conditions write would.

The return value is a count of bytes written, or -1 indicating an error. The possible errors are the same as in write.

Note that if the buffers are small (under about 1kB), high-level streams may be easier to use than these functions. However, readv and writev are more efficient when the individual buffers themselves (as opposed to the total output), are large. In that case, a high-level stream would not be able to cache the data effectively.

Next: , Previous: Scatter-Gather, Up: Low-Level I/O

13.7 Memory-mapped I/O

On modern operating systems, it is possible to mmap (pronounced “em-map”) a file to a region of memory. When this is done, the file can be accessed just like an array in the program.

This is more efficient than read or write, as only the regions of the file that a program actually accesses are loaded. Accesses to not-yet-loaded parts of the mmapped region are handled in the same way as swapped out pages.

Since mmapped pages can be stored back to their file when physical memory is low, it is possible to mmap files orders of magnitude larger than both the physical memory and swap space. The only limit is address space. The theoretical limit is 4GB on a 32-bit machine - however, the actual limit will be smaller since some areas will be reserved for other purposes. If the LFS interface is used the file size on 32-bit systems is not limited to 2GB (offsets are signed which reduces the addressable area of 4GB by half); the full 64-bit are available.

Memory mapping only works on entire pages of memory. Thus, addresses for mapping must be page-aligned, and length values will be rounded up. To determine the size of a page the machine uses one should use 

     size_t page_size = (size_t) sysconf (_SC_PAGESIZE);

These functions are declared in sys/mman.h.

— Function: void * mmap (void *address, size_t length,int protect, int flags, int filedes, off_t offset)

The mmap function creates a new mapping, connected to bytes (offset) to (offset + length - 1) in the file open on filedes. A new reference for the file specified by filedes is created, which is not removed by closing the file.

address gives a preferred starting address for the mapping. NULL expresses no preference. Any previous mapping at that address is automatically removed. The address you give may still be changed, unless you use the MAP_FIXED flag.

protect contains flags that control what kind of access is permitted. They include PROT_READ, PROT_WRITE, and PROT_EXEC, which permit reading, writing, and execution, respectively. Inappropriate access will cause a segfault (see Program Error Signals).

Note that most hardware designs cannot support write permission without read permission, and many do not distinguish read and execute permission. Thus, you may receive wider permissions than you ask for, and mappings of write-only files may be denied even if you do not use PROT_READ.

flags contains flags that control the nature of the map. One of MAP_SHARED or MAP_PRIVATE must be specified.

They include:

MAP_PRIVATE
This specifies that writes to the region should never be written back to the attached file. Instead, a copy is made for the process, and the region will be swapped normally if memory runs low. No other process will see the changes.

Since private mappings effectively revert to ordinary memory when written to, you must have enough virtual memory for a copy of the entire mmapped region if you use this mode with PROT_WRITE.

MAP_SHARED
This specifies that writes to the region will be written back to the file. Changes made will be shared immediately with other processes mmaping the same file.

Note that actual writing may take place at any time. You need to use msync, described below, if it is important that other processes using conventional I/O get a consistent view of the file.

MAP_FIXED
This forces the system to use the exact mapping address specified in address and fail if it can't.
MAP_ANONYMOUS
MAP_ANON
This flag tells the system to create an anonymous mapping, not connected to a file. filedes and off are ignored, and the region is initialized with zeros.

Anonymous maps are used as the basic primitive to extend the heap on some systems. They are also useful to share data between multiple tasks without creating a file.

On some systems using private anonymous mmaps is more efficient than using malloc for large blocks. This is not an issue with the GNU C library, as the included malloc automatically uses mmap where appropriate.

mmap returns the address of the new mapping, or -1 for an error.

Possible errors include:

EINVAL
Either address was unusable, or inconsistent flags were given.
EACCES
filedes was not open for the type of access specified in protect.
ENOMEM
Either there is not enough memory for the operation, or the process is out of address space.
ENODEV
This file is of a type that doesn't support mapping.
ENOEXEC
The file is on a filesystem that doesn't support mapping.
— Function: void * mmap64 (void *address, size_t length,int protect, int flags, int filedes, off64_t offset)

The mmap64 function is equivalent to the mmap function but the offset parameter is of type off64_t. On 32-bit systems this allows the file associated with the filedes descriptor to be larger than 2GB. filedes must be a descriptor returned from a call to open64 or fopen64 and freopen64 where the descriptor is retrieved with fileno.

When the sources are translated with _FILE_OFFSET_BITS == 64 this function is actually available under the name mmap. I.e., the new, extended API using 64 bit file sizes and offsets transparently replaces the old API.

— Function: int munmap (void *addr, size_t length)

munmap removes any memory maps from (addr) to (addr + length). length should be the length of the mapping.

It is safe to unmap multiple mappings in one command, or include unmapped space in the range. It is also possible to unmap only part of an existing mapping. However, only entire pages can be removed. If length is not an even number of pages, it will be rounded up.

It returns 0 for success and -1 for an error.

One error is possible:

EINVAL
The memory range given was outside the user mmap range or wasn't page aligned.
— Function: int msync (void *address, size_t length, int flags)

When using shared mappings, the kernel can write the file at any time before the mapping is removed. To be certain data has actually been written to the file and will be accessible to non-memory-mapped I/O, it is necessary to use this function.

It operates on the region address to (address + length). It may be used on part of a mapping or multiple mappings, however the region given should not contain any unmapped space.

flags can contain some options:

MS_SYNC
This flag makes sure the data is actually written to disk. Normally msync only makes sure that accesses to a file with conventional I/O reflect the recent changes.
MS_ASYNC
This tells msync to begin the synchronization, but not to wait for it to complete.

msync returns 0 for success and -1 for error. Errors include:

EINVAL
An invalid region was given, or the flags were invalid.
EFAULT
There is no existing mapping in at least part of the given region.
— Function: void * mremap (void *address, size_t length, size_t new_length, int flag)

This function can be used to change the size of an existing memory area. address and length must cover a region entirely mapped in the same mmap statement. A new mapping with the same characteristics will be returned with the length new_length.

One option is possible, MREMAP_MAYMOVE. If it is given in flags, the system may remove the existing mapping and create a new one of the desired length in another location.

The address of the resulting mapping is returned, or -1. Possible error codes include:

EFAULT
There is no existing mapping in at least part of the original region, or the region covers two or more distinct mappings.
EINVAL
The address given is misaligned or inappropriate.
EAGAIN
The region has pages locked, and if extended it would exceed the process's resource limit for locked pages. See Limits on Resources.
ENOMEM
The region is private writable, and insufficient virtual memory is available to extend it. Also, this error will occur if MREMAP_MAYMOVE is not given and the extension would collide with another mapped region.

This function is only available on a few systems. Except for performing optional optimizations one should not rely on this function.

Not all file descriptors may be mapped. Sockets, pipes, and most devices only allow sequential access and do not fit into the mapping abstraction. In addition, some regular files may not be mmapable, and older kernels may not support mapping at all. Thus, programs using mmap should have a fallback method to use should it fail. See Mmap.

— Function: int madvise (void *addr, size_t length, int advice)

This function can be used to provide the system with advice about the intended usage patterns of the memory region starting at addr and extending length bytes.

The valid BSD values for advice are:

MADV_NORMAL
The region should receive no further special treatment.
MADV_RANDOM
The region will be accessed via random page references. The kernel should page-in the minimal number of pages for each page fault.
MADV_SEQUENTIAL
The region will be accessed via sequential page references. This may cause the kernel to aggressively read-ahead, expecting further sequential references after any page fault within this region.
MADV_WILLNEED
The region will be needed. The pages within this region may be pre-faulted in by the kernel.
MADV_DONTNEED
The region is no longer needed. The kernel may free these pages, causing any changes to the pages to be lost, as well as swapped out pages to be discarded.

The POSIX names are slightly different, but with the same meanings:

POSIX_MADV_NORMAL
This corresponds with BSD's MADV_NORMAL.
POSIX_MADV_RANDOM
This corresponds with BSD's MADV_RANDOM.
POSIX_MADV_SEQUENTIAL
This corresponds with BSD's MADV_SEQUENTIAL.
POSIX_MADV_WILLNEED
This corresponds with BSD's MADV_WILLNEED.
POSIX_MADV_DONTNEED
This corresponds with BSD's MADV_DONTNEED.

msync returns 0 for success and -1 for error. Errors include:

EINVAL
An invalid region was given, or the advice was invalid.
EFAULT
There is no existing mapping in at least part of the given region.

Next: , Previous: Memory-mapped I/O, Up: Low-Level I/O

13.8 Waiting for Input or Output

Sometimes a program needs to accept input on multiple input channels whenever input arrives. For example, some workstations may have devices such as a digitizing tablet, function button box, or dial box that are connected via normal asynchronous serial interfaces; good user interface style requires responding immediately to input on any device. Another example is a program that acts as a server to several other processes via pipes or sockets.

You cannot normally use read for this purpose, because this blocks the program until input is available on one particular file descriptor; input on other channels won't wake it up. You could set nonblocking mode and poll each file descriptor in turn, but this is very inefficient.

A better solution is to use the select function. This blocks the program until input or output is ready on a specified set of file descriptors, or until a timer expires, whichever comes first. This facility is declared in the header file sys/types.h. In the case of a server socket (see Listening), we say that “input” is available when there are pending connections that could be accepted (see Accepting Connections). accept for server sockets blocks and interacts with select just as read does for normal input.

The file descriptor sets for the select function are specified as fd_set objects. Here is the description of the data type and some macros for manipulating these objects.

— Data Type: fd_set

The fd_set data type represents file descriptor sets for the select function. It is actually a bit array.

— Macro: int FD_SETSIZE

The value of this macro is the maximum number of file descriptors that a fd_set object can hold information about. On systems with a fixed maximum number, FD_SETSIZE is at least that number. On some systems, including GNU, there is no absolute limit on the number of descriptors open, but this macro still has a constant value which controls the number of bits in an fd_set; if you get a file descriptor with a value as high as FD_SETSIZE, you cannot put that descriptor into an fd_set.

— Macro: void FD_ZERO (fd_set *set)

This macro initializes the file descriptor set set to be the empty set.

— Macro: void FD_SET (int filedes, fd_set *set)

This macro adds filedes to the file descriptor set set.

The filedes parameter must not have side effects since it is evaluated more than once.

— Macro: void FD_CLR (int filedes, fd_set *set)

This macro removes filedes from the file descriptor set set.

The filedes parameter must not have side effects since it is evaluated more than once.

— Macro: int FD_ISSET (int filedes, const fd_set *set)

This macro returns a nonzero value (true) if filedes is a member of the file descriptor set set, and zero (false) otherwise.

The filedes parameter must not have side effects since it is evaluated more than once.

Next, here is the description of the select function itself.

— Function: int select (int nfds, fd_set *read-fds, fd_set *write-fds, fd_set *except-fds, struct timeval *timeout)

The select function blocks the calling process until there is activity on any of the specified sets of file descriptors, or until the timeout period has expired.

The file descriptors specified by the read-fds argument are checked to see if they are ready for reading; the write-fds file descriptors are checked to see if they are ready for writing; and the except-fds file descriptors are checked for exceptional conditions. You can pass a null pointer for any of these arguments if you are not interested in checking for that kind of condition.

A file descriptor is considered ready for reading if a read call will not block. This usually includes the read offset being at the end of the file or there is an error to report. A server socket is considered ready for reading if there is a pending connection which can be accepted with accept; see Accepting Connections. A client socket is ready for writing when its connection is fully established; see Connecting.

“Exceptional conditions” does not mean errors—errors are reported immediately when an erroneous system call is executed, and do not constitute a state of the descriptor. Rather, they include conditions such as the presence of an urgent message on a socket. (See Sockets, for information on urgent messages.)

The select function checks only the first nfds file descriptors. The usual thing is to pass FD_SETSIZE as the value of this argument.

The timeout specifies the maximum time to wait. If you pass a null pointer for this argument, it means to block indefinitely until one of the file descriptors is ready. Otherwise, you should provide the time in struct timeval format; see High-Resolution Calendar. Specify zero as the time (a struct timeval containing all zeros) if you want to find out which descriptors are ready without waiting if none are ready.

The normal return value from select is the total number of ready file descriptors in all of the sets. Each of the argument sets is overwritten with information about the descriptors that are ready for the corresponding operation. Thus, to see if a particular descriptor desc has input, use FD_ISSET (desc, read-fds) after select returns.

If select returns because the timeout period expires, it returns a value of zero.

Any signal will cause select to return immediately. So if your program uses signals, you can't rely on select to keep waiting for the full time specified. If you want to be sure of waiting for a particular amount of time, you must check for EINTR and repeat the select with a newly calculated timeout based on the current time. See the example below. See also Interrupted Primitives.

If an error occurs, select returns -1 and does not modify the argument file descriptor sets. The following errno error conditions are defined for this function:

EBADF
One of the file descriptor sets specified an invalid file descriptor.
EINTR
The operation was interrupted by a signal. See Interrupted Primitives.
EINVAL
The timeout argument is invalid; one of the components is negative or too large.

Portability Note: The select function is a BSD Unix feature.

Here is an example showing how you can use select to establish a timeout period for reading from a file descriptor. The input_timeout function blocks the calling process until input is available on the file descriptor, or until the timeout period expires. 

     #include <errno.h>
     #include <stdio.h>
     #include <unistd.h>
     #include <sys/types.h>
     #include <sys/time.h>
     
     int
     input_timeout (int filedes, unsigned int seconds)
     {
       fd_set set;
       struct timeval timeout;
     
       /* Initialize the file descriptor set. */
       FD_ZERO (&set);
       FD_SET (filedes, &set);
     
       /* Initialize the timeout data structure. */
       timeout.tv_sec = seconds;
       timeout.tv_usec = 0;
     
       /* select returns 0 if timeout, 1 if input available, -1 if error. */
       return TEMP_FAILURE_RETRY (select (FD_SETSIZE,
                                          &set, NULL, NULL,
                                          &timeout));
     }
     
     int
     main (void)
     {
       fprintf (stderr, "select returned %d.\n",
                input_timeout (STDIN_FILENO, 5));
       return 0;
     }

There is another example showing the use of select to multiplex input from multiple sockets in Server Example.

Next: , Previous: Waiting for I/O, Up: Low-Level I/O

13.9 Synchronizing I/O operations

In most modern operating systems, the normal I/O operations are not executed synchronously. I.e., even if a write system call returns, this does not mean the data is actually written to the media, e.g., the disk.

In situations where synchronization points are necessary, you can use special functions which ensure that all operations finish before they return.

— Function: int sync (void)

A call to this function will not return as long as there is data which has not been written to the device. All dirty buffers in the kernel will be written and so an overall consistent system can be achieved (if no other process in parallel writes data).

A prototype for sync can be found in unistd.h.

The return value is zero to indicate no error.

Programs more often want to ensure that data written to a given file is committed, rather than all data in the system. For this, sync is overkill.

— Function: int fsync (int fildes)

The fsync function can be used to make sure all data associated with the open file fildes is written to the device associated with the descriptor. The function call does not return unless all actions have finished.

A prototype for fsync can be found in unistd.h.

This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time fsync is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this, calls to fsync should be protected using cancellation handlers.

The return value of the function is zero if no error occurred. Otherwise it is -1 and the global variable errno is set to the following values:

EBADF
The descriptor fildes is not valid.
EINVAL
No synchronization is possible since the system does not implement this.

Sometimes it is not even necessary to write all data associated with a file descriptor. E.g., in database files which do not change in size it is enough to write all the file content data to the device. Meta-information, like the modification time etc., are not that important and leaving such information uncommitted does not prevent a successful recovering of the file in case of a problem.

— Function: int fdatasync (int fildes)

When a call to the fdatasync function returns, it is ensured that all of the file data is written to the device. For all pending I/O operations, the parts guaranteeing data integrity finished.

Not all systems implement the fdatasync operation. On systems missing this functionality fdatasync is emulated by a call to fsync since the performed actions are a superset of those required by fdatasync.

The prototype for fdatasync is in unistd.h.

The return value of the function is zero if no error occurred. Otherwise it is -1 and the global variable errno is set to the following values:

EBADF
The descriptor fildes is not valid.
EINVAL
No synchronization is possible since the system does not implement this.

Next: , Previous: Synchronizing I/O, Up: Low-Level I/O

13.10 Perform I/O Operations in Parallel

The POSIX.1b standard defines a new set of I/O operations which can significantly reduce the time an application spends waiting at I/O. The new functions allow a program to initiate one or more I/O operations and then immediately resume normal work while the I/O operations are executed in parallel. This functionality is available if the unistd.h file defines the symbol _POSIX_ASYNCHRONOUS_IO.

These functions are part of the library with realtime functions named librt. They are not actually part of the libc binary. The implementation of these functions can be done using support in the kernel (if available) or using an implementation based on threads at userlevel. In the latter case it might be necessary to link applications with the thread library libpthread in addition to librt.

All AIO operations operate on files which were opened previously. There might be arbitrarily many operations running for one file. The asynchronous I/O operations are controlled using a data structure named struct aiocb (AIO control block). It is defined in aio.h as follows.

— Data Type: struct aiocb

The POSIX.1b standard mandates that the struct aiocb structure contains at least the members described in the following table. There might be more elements which are used by the implementation, but depending upon these elements is not portable and is highly deprecated.

int aio_fildes
This element specifies the file descriptor to be used for the operation. It must be a legal descriptor, otherwise the operation will fail.

The device on which the file is opened must allow the seek operation. I.e., it is not possible to use any of the AIO operations on devices like terminals where an lseek call would lead to an error.

off_t aio_offset
This element specifies the offset in the file at which the operation (input or output) is performed. Since the operations are carried out in arbitrary order and more than one operation for one file descriptor can be started, one cannot expect a current read/write position of the file descriptor.
volatile void *aio_buf
This is a pointer to the buffer with the data to be written or the place where the read data is stored.
size_t aio_nbytes
This element specifies the length of the buffer pointed to by aio_buf.
int aio_reqprio
If the platform has defined _POSIX_PRIORITIZED_IO and _POSIX_PRIORITY_SCHEDULING, the AIO requests are processed based on the current scheduling priority. The aio_reqprio element can then be used to lower the priority of the AIO operation.
struct sigevent aio_sigevent
This element specifies how the calling process is notified once the operation terminates. If the sigev_notify element is SIGEV_NONE, no notification is sent. If it is SIGEV_SIGNAL, the signal determined by sigev_signo is sent. Otherwise, sigev_notify must be SIGEV_THREAD. In this case, a thread is created which starts executing the function pointed to by sigev_notify_function.
int aio_lio_opcode
This element is only used by the lio_listio and lio_listio64 functions. Since these functions allow an arbitrary number of operations to start at once, and each operation can be input or output (or nothing), the information must be stored in the control block. The possible values are:
LIO_READ
Start a read operation. Read from the file at position aio_offset and store the next aio_nbytes bytes in the buffer pointed to by aio_buf.
LIO_WRITE
Start a write operation. Write aio_nbytes bytes starting at aio_buf into the file starting at position aio_offset.
LIO_NOP
Do nothing for this control block. This value is useful sometimes when an array of struct aiocb values contains holes, i.e., some of the values must not be handled although the whole array is presented to the lio_listio function.

When the sources are compiled using _FILE_OFFSET_BITS == 64 on a 32 bit machine, this type is in fact struct aiocb64, since the LFS interface transparently replaces the struct aiocb definition.

For use with the AIO functions defined in the LFS, there is a similar type defined which replaces the types of the appropriate members with larger types but otherwise is equivalent to struct aiocb. Particularly, all member names are the same.

— Data Type: struct aiocb64
int aio_fildes
This element specifies the file descriptor which is used for the operation. It must be a legal descriptor since otherwise the operation fails for obvious reasons.

The device on which the file is opened must allow the seek operation. I.e., it is not possible to use any of the AIO operations on devices like terminals where an lseek call would lead to an error.

off64_t aio_offset
This element specifies at which offset in the file the operation (input or output) is performed. Since the operation are carried in arbitrary order and more than one operation for one file descriptor can be started, one cannot expect a current read/write position of the file descriptor.
volatile void *aio_buf
This is a pointer to the buffer with the data to be written or the place where the read data is stored.
size_t aio_nbytes
This element specifies the length of the buffer pointed to by aio_buf.
int aio_reqprio
If for the platform _POSIX_PRIORITIZED_IO and _POSIX_PRIORITY_SCHEDULING are defined the AIO requests are processed based on the current scheduling priority. The aio_reqprio element can then be used to lower the priority of the AIO operation.
struct sigevent aio_sigevent
This element specifies how the calling process is notified once the operation terminates. If the sigev_notify, element is SIGEV_NONE no notification is sent. If it is SIGEV_SIGNAL, the signal determined by sigev_signo is sent. Otherwise, sigev_notify must be SIGEV_THREAD in which case a thread which starts executing the function pointed to by sigev_notify_function.
int aio_lio_opcode
This element is only used by the lio_listio and [lio_listio64 functions. Since these functions allow an arbitrary number of operations to start at once, and since each operation can be input or output (or nothing), the information must be stored in the control block. See the description of struct aiocb for a description of the possible values.

When the sources are compiled using _FILE_OFFSET_BITS == 64 on a 32 bit machine, this type is available under the name struct aiocb64, since the LFS transparently replaces the old interface.

Next: , Up: Asynchronous I/O

13.10.1 Asynchronous Read and Write Operations

— Function: int aio_read (struct aiocb *aiocbp)

This function initiates an asynchronous read operation. It immediately returns after the operation was enqueued or when an error was encountered.

The first aiocbp->aio_nbytes bytes of the file for which aiocbp->aio_fildes is a descriptor are written to the buffer starting at aiocbp->aio_buf. Reading starts at the absolute position aiocbp->aio_offset in the file.

If prioritized I/O is supported by the platform the aiocbp->aio_reqprio value is used to adjust the priority before the request is actually enqueued.

The calling process is notified about the termination of the read request according to the aiocbp->aio_sigevent value.

When aio_read returns, the return value is zero if no error occurred that can be found before the process is enqueued. If such an early error is found, the function returns -1 and sets errno to one of the following values:

EAGAIN
The request was not enqueued due to (temporarily) exceeded resource limitations.
ENOSYS
The aio_read function is not implemented.
EBADF
The aiocbp->aio_fildes descriptor is not valid. This condition need not be recognized before enqueueing the request and so this error might also be signaled asynchronously.
EINVAL
The aiocbp->aio_offset or aiocbp->aio_reqpiro value is invalid. This condition need not be recognized before enqueueing the request and so this error might also be signaled asynchronously.

If aio_read returns zero, the current status of the request can be queried using aio_error and aio_return functions. As long as the value returned by aio_error is EINPROGRESS the operation has not yet completed. If aio_error returns zero, the operation successfully terminated, otherwise the value is to be interpreted as an error code. If the function terminated, the result of the operation can be obtained using a call to aio_return. The returned value is the same as an equivalent call to read would have returned. Possible error codes returned by aio_error are:

EBADF
The aiocbp->aio_fildes descriptor is not valid.
ECANCELED
The operation was canceled before the operation was finished (see Cancel AIO Operations)
EINVAL
The aiocbp->aio_offset value is invalid.

When the sources are compiled with _FILE_OFFSET_BITS == 64 this function is in fact aio_read64 since the LFS interface transparently replaces the normal implementation.

— Function: int aio_read64 (struct aiocb *aiocbp)

This function is similar to the aio_read function. The only difference is that on 32 bit machines, the file descriptor should be opened in the large file mode. Internally, aio_read64 uses functionality equivalent to lseek64 (see File Position Primitive) to position the file descriptor correctly for the reading, as opposed to lseek functionality used in aio_read.

When the sources are compiled with _FILE_OFFSET_BITS == 64, this function is available under the name aio_read and so transparently replaces the interface for small files on 32 bit machines.

To write data asynchronously to a file, there exists an equivalent pair of functions with a very similar interface.

— Function: int aio_write (struct aiocb *aiocbp)

This function initiates an asynchronous write operation. The function call immediately returns after the operation was enqueued or if before this happens an error was encountered.

The first aiocbp->aio_nbytes bytes from the buffer starting at aiocbp->aio_buf are written to the file for which aiocbp->aio_fildes is an descriptor, starting at the absolute position aiocbp->aio_offset in the file.

If prioritized I/O is supported by the platform, the aiocbp->aio_reqprio value is used to adjust the priority before the request is actually enqueued.

The calling process is notified about the termination of the read request according to the aiocbp->aio_sigevent value.

When aio_write returns, the return value is zero if no error occurred that can be found before the process is enqueued. If such an early error is found the function returns -1 and sets errno to one of the following values.

EAGAIN
The request was not enqueued due to (temporarily) exceeded resource limitations.
ENOSYS
The aio_write function is not implemented.
EBADF
The aiocbp->aio_fildes descriptor is not valid. This condition may not be recognized before enqueueing the request, and so this error might also be signaled asynchronously.
EINVAL
The aiocbp->aio_offset or aiocbp->aio_reqprio value is invalid. This condition may not be recognized before enqueueing the request and so this error might also be signaled asynchronously.

In the case aio_write returns zero, the current status of the request can be queried using aio_error and aio_return functions. As long as the value returned by aio_error is EINPROGRESS the operation has not yet completed. If aio_error returns zero, the operation successfully terminated, otherwise the value is to be interpreted as an error code. If the function terminated, the result of the operation can be get using a call to aio_return. The returned value is the same as an equivalent call to read would have returned. Possible error codes returned by aio_error are:

EBADF
The aiocbp->aio_fildes descriptor is not valid.
ECANCELED
The operation was canceled before the operation was finished. (see Cancel AIO Operations)
EINVAL
The aiocbp->aio_offset value is invalid.

When the sources are compiled with _FILE_OFFSET_BITS == 64, this function is in fact aio_write64 since the LFS interface transparently replaces the normal implementation.

— Function: int aio_write64 (struct aiocb *aiocbp)

This function is similar to the aio_write function. The only difference is that on 32 bit machines the file descriptor should be opened in the large file mode. Internally aio_write64 uses functionality equivalent to lseek64 (see File Position Primitive) to position the file descriptor correctly for the writing, as opposed to lseek functionality used in aio_write.

When the sources are compiled with _FILE_OFFSET_BITS == 64, this function is available under the name aio_write and so transparently replaces the interface for small files on 32 bit machines.

Besides these functions with the more or less traditional interface, POSIX.1b also defines a function which can initiate more than one operation at a time, and which can handle freely mixed read and write operations. It is therefore similar to a combination of readv and writev.

— Function: int lio_listio (int mode, struct aiocb *const list[], int nent, struct sigevent *sig)

The lio_listio function can be used to enqueue an arbitrary number of read and write requests at one time. The requests can all be meant for the same file, all for different files or every solution in between.

lio_listio gets the nent requests from the array pointed to by list. The operation to be performed is determined by the aio_lio_opcode member in each element of list. If this field is LIO_READ a read operation is enqueued, similar to a call of aio_read for this element of the array (except that the way the termination is signalled is different, as we will see below). If the aio_lio_opcode member is LIO_WRITE a write operation is enqueued. Otherwise the aio_lio_opcode must be LIO_NOP in which case this element of list is simply ignored. This “operation” is useful in situations where one has a fixed array of struct aiocb elements from which only a few need to be handled at a time. Another situation is where the lio_listio call was canceled before all requests are processed (see Cancel AIO Operations) and the remaining requests have to be reissued.

The other members of each element of the array pointed to by list must have values suitable for the operation as described in the documentation for aio_read and aio_write above.

The mode argument determines how lio_listio behaves after having enqueued all the requests. If mode is LIO_WAIT it waits until all requests terminated. Otherwise mode must be LIO_NOWAIT and in this case the function returns immediately after having enqueued all the requests. In this case the caller gets a notification of the termination of all requests according to the sig parameter. If sig is NULL no notification is send. Otherwise a signal is sent or a thread is started, just as described in the description for aio_read or aio_write.

If mode is LIO_WAIT, the return value of lio_listio is 0 when all requests completed successfully. Otherwise the function return -1 and errno is set accordingly. To find out which request or requests failed one has to use the aio_error function on all the elements of the array list.

In case mode is LIO_NOWAIT, the function returns 0 if all requests were enqueued correctly. The current state of the requests can be found using aio_error and aio_return as described above. If lio_listio returns -1 in this mode, the global variable errno is set accordingly. If a request did not yet terminate, a call to aio_error returns EINPROGRESS. If the value is different, the request is finished and the error value (or 0) is returned and the result of the operation can be retrieved using aio_return.

Possible values for errno are:

EAGAIN
The resources necessary to queue all the requests are not available at the moment. The error status for each element of list must be checked to determine which request failed.

Another reason could be that the system wide limit of AIO requests is exceeded. This cannot be the case for the implementation on GNU systems since no arbitrary limits exist.

EINVAL
The mode parameter is invalid or nent is larger than AIO_LISTIO_MAX.
EIO
One or more of the request's I/O operations failed. The error status of each request should be checked to determine which one failed.
ENOSYS
The lio_listio function is not supported.

If the mode parameter is LIO_NOWAIT and the caller cancels a request, the error status for this request returned by aio_error is ECANCELED.

When the sources are compiled with _FILE_OFFSET_BITS == 64, this function is in fact lio_listio64 since the LFS interface transparently replaces the normal implementation.

— Function: int lio_listio64 (int mode, struct aiocb *const list, int nent, struct sigevent *sig)

This function is similar to the lio_listio function. The only difference is that on 32 bit machines, the file descriptor should be opened in the large file mode. Internally, lio_listio64 uses functionality equivalent to lseek64 (see File Position Primitive) to position the file descriptor correctly for the reading or writing, as opposed to lseek functionality used in lio_listio.

When the sources are compiled with _FILE_OFFSET_BITS == 64, this function is available under the name lio_listio and so transparently replaces the interface for small files on 32 bit machines.

Next: , Previous: Asynchronous Reads/Writes, Up: Asynchronous I/O

13.10.2 Getting the Status of AIO Operations

As already described in the documentation of the functions in the last section, it must be possible to get information about the status of an I/O request. When the operation is performed truly asynchronously (as with aio_read and aio_write and with lio_listio when the mode is LIO_NOWAIT), one sometimes needs to know whether a specific request already terminated and if so, what the result was. The following two functions allow you to get this kind of information.

— Function: int aio_error (const struct aiocb *aiocbp)

This function determines the error state of the request described by the struct aiocb variable pointed to by aiocbp. If the request has not yet terminated the value returned is always EINPROGRESS. Once the request has terminated the value aio_error returns is either 0 if the request completed successfully or it returns the value which would be stored in the errno variable if the request would have been done using read, write, or fsync.

The function can return ENOSYS if it is not implemented. It could also return EINVAL if the aiocbp parameter does not refer to an asynchronous operation whose return status is not yet known.

When the sources are compiled with _FILE_OFFSET_BITS == 64 this function is in fact aio_error64 since the LFS interface transparently replaces the normal implementation.

— Function: int aio_error64 (const struct aiocb64 *aiocbp)

This function is similar to aio_error with the only difference that the argument is a reference to a variable of type struct aiocb64.

When the sources are compiled with _FILE_OFFSET_BITS == 64 this function is available under the name aio_error and so transparently replaces the interface for small files on 32 bit machines.

— Function: ssize_t aio_return (const struct aiocb *aiocbp)

This function can be used to retrieve the return status of the operation carried out by the request described in the variable pointed to by aiocbp. As long as the error status of this request as returned by aio_error is EINPROGRESS the return of this function is undefined.

Once the request is finished this function can be used exactly once to retrieve the return value. Following calls might lead to undefined behavior. The return value itself is the value which would have been returned by the read, write, or fsync call.

The function can return ENOSYS if it is not implemented. It could also return EINVAL if the aiocbp parameter does not refer to an asynchronous operation whose return status is not yet known.

When the sources are compiled with _FILE_OFFSET_BITS == 64 this function is in fact aio_return64 since the LFS interface transparently replaces the normal implementation.

— Function: int aio_return64 (const struct aiocb64 *aiocbp)

This function is similar to aio_return with the only difference that the argument is a reference to a variable of type struct aiocb64.

When the sources are compiled with _FILE_OFFSET_BITS == 64 this function is available under the name aio_return and so transparently replaces the interface for small files on 32 bit machines.

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13.10.3 Getting into a Consistent State

When dealing with asynchronous operations it is sometimes necessary to get into a consistent state. This would mean for AIO that one wants to know whether a certain request or a group of request were processed. This could be done by waiting for the notification sent by the system after the operation terminated, but this sometimes would mean wasting resources (mainly computation time). Instead POSIX.1b defines two functions which will help with most kinds of consistency.

The aio_fsync and aio_fsync64 functions are only available if the symbol _POSIX_SYNCHRONIZED_IO is defined in unistd.h.

— Function: int aio_fsync (int op, struct aiocb *aiocbp)

Calling this function forces all I/O operations operating queued at the time of the function call operating on the file descriptor aiocbp->aio_fildes into the synchronized I/O completion state (see Synchronizing I/O). The aio_fsync function returns immediately but the notification through the method described in aiocbp->aio_sigevent will happen only after all requests for this file descriptor have terminated and the file is synchronized. This also means that requests for this very same file descriptor which are queued after the synchronization request are not affected.

If op is O_DSYNC the synchronization happens as with a call to fdatasync. Otherwise op should be O_SYNC and the synchronization happens as with fsync.

As long as the synchronization has not happened, a call to aio_error with the reference to the object pointed to by aiocbp returns EINPROGRESS. Once the synchronization is done aio_error return 0 if the synchronization was not successful. Otherwise the value returned is the value to which the fsync or fdatasync function would have set the errno variable. In this case nothing can be assumed about the consistency for the data written to this file descriptor.

The return value of this function is 0 if the request was successfully enqueued. Otherwise the return value is -1 and errno is set to one of the following values:

EAGAIN
The request could not be enqueued due to temporary lack of resources.
EBADF
The file descriptor aiocbp->aio_fildes is not valid or not open for writing.
EINVAL
The implementation does not support I/O synchronization or the op parameter is other than O_DSYNC and O_SYNC.
ENOSYS
This function is not implemented.

When the sources are compiled with _FILE_OFFSET_BITS == 64 this function is in fact aio_fsync64 since the LFS interface transparently replaces the normal implementation.

— Function: int aio_fsync64 (int op, struct aiocb64 *aiocbp)

This function is similar to aio_fsync with the only difference that the argument is a reference to a variable of type struct aiocb64.

When the sources are compiled with _FILE_OFFSET_BITS == 64 this function is available under the name aio_fsync and so transparently replaces the interface for small files on 32 bit machines.

Another method of synchronization is to wait until one or more requests of a specific set terminated. This could be achieved by the aio_* functions to notify the initiating process about the termination but in some situations this is not the ideal solution. In a program which constantly updates clients somehow connected to the server it is not always the best solution to go round robin since some connections might be slow. On the other hand letting the aio_* function notify the caller might also be not the best solution since whenever the process works on preparing data for on client it makes no sense to be interrupted by a notification since the new client will not be handled before the current client is served. For situations like this aio_suspend should be used.

— Function: int aio_suspend (const struct aiocb *const list[], int nent, const struct timespec *timeout)

When calling this function, the calling thread is suspended until at least one of the requests pointed to by the nent elements of the array list has completed. If any of the requests has already completed at the time aio_suspend is called, the function returns immediately. Whether a request has terminated or not is determined by comparing the error status of the request with EINPROGRESS. If an element of list is NULL, the entry is simply ignored.

If no request has finished, the calling process is suspended. If timeout is NULL, the process is not woken until a request has finished. If timeout is not NULL, the process remains suspended at least as long as specified in timeout. In this case, aio_suspend returns with an error.

The return value of the function is 0 if one or more requests from the list have terminated. Otherwise the function returns -1 and errno is set to one of the following values:

EAGAIN
None of the requests from the list completed in the time specified by timeout.
EINTR
A signal interrupted the aio_suspend function. This signal might also be sent by the AIO implementation while signalling the termination of one of the requests.
ENOSYS
The aio_suspend function is not implemented.

When the sources are compiled with _FILE_OFFSET_BITS == 64 this function is in fact aio_suspend64 since the LFS interface transparently replaces the normal implementation.

— Function: int aio_suspend64 (const struct aiocb64 *const list[], int nent, const struct timespec *timeout)

This function is similar to aio_suspend with the only difference that the argument is a reference to a variable of type struct aiocb64.

When the sources are compiled with _FILE_OFFSET_BITS == 64 this function is available under the name aio_suspend and so transparently replaces the interface for small files on 32 bit machines.

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13.10.4 Cancellation of AIO Operations

When one or more requests are asynchronously processed, it might be useful in some situations to cancel a selected operation, e.g., if it becomes obvious that the written data is no longer accurate and would have to be overwritten soon. As an example, assume an application, which writes data in files in a situation where new incoming data would have to be written in a file which will be updated by an enqueued request. The POSIX AIO implementation provides such a function, but this function is not capable of forcing the cancellation of the request. It is up to the implementation to decide whether it is possible to cancel the operation or not. Therefore using this function is merely a hint.

— Function: int aio_cancel (int fildes, struct aiocb *aiocbp)

The aio_cancel function can be used to cancel one or more outstanding requests. If the aiocbp parameter is NULL, the function tries to cancel all of the outstanding requests which would process the file descriptor fildes (i.e., whose aio_fildes member is fildes). If aiocbp is not NULL, aio_cancel attempts to cancel the specific request pointed to by aiocbp.

For requests which were successfully canceled, the normal notification about the termination of the request should take place. I.e., depending on the struct sigevent object which controls this, nothing happens, a signal is sent or a thread is started. If the request cannot be canceled, it terminates the usual way after performing the operation.

After a request is successfully canceled, a call to aio_error with a reference to this request as the parameter will return ECANCELED and a call to aio_return will return -1. If the request wasn't canceled and is still running the error status is still EINPROGRESS.

The return value of the function is AIO_CANCELED if there were requests which haven't terminated and which were successfully canceled. If there is one or more requests left which couldn't be canceled, the return value is AIO_NOTCANCELED. In this case aio_error must be used to find out which of the, perhaps multiple, requests (in aiocbp is NULL) weren't successfully canceled. If all requests already terminated at the time aio_cancel is called the return value is AIO_ALLDONE.

If an error occurred during the execution of aio_cancel the function returns -1 and sets errno to one of the following values.

EBADF
The file descriptor fildes is not valid.
ENOSYS
aio_cancel is not implemented.

When the sources are compiled with _FILE_OFFSET_BITS == 64, this function is in fact aio_cancel64 since the LFS interface transparently replaces the normal implementation.

— Function: int aio_cancel64 (int fildes, struct aiocb64 *aiocbp)

This function is similar to aio_cancel with the only difference that the argument is a reference to a variable of type struct aiocb64.

When the sources are compiled with _FILE_OFFSET_BITS == 64, this function is available under the name aio_cancel and so transparently replaces the interface for small files on 32 bit machines.

Previous: Cancel AIO Operations, Up: Asynchronous I/O

13.10.5 How to optimize the AIO implementation

The POSIX standard does not specify how the AIO functions are implemented. They could be system calls, but it is also possible to emulate them at userlevel.

At the point of this writing, the available implementation is a userlevel implementation which uses threads for handling the enqueued requests. While this implementation requires making some decisions about limitations, hard limitations are something which is best avoided in the GNU C library. Therefore, the GNU C library provides a means for tuning the AIO implementation according to the individual use.

— Data Type: struct aioinit

This data type is used to pass the configuration or tunable parameters to the implementation. The program has to initialize the members of this struct and pass it to the implementation using the aio_init function.

int aio_threads
This member specifies the maximal number of threads which may be used at any one time.
int aio_num
This number provides an estimate on the maximal number of simultaneously enqueued requests.
int aio_locks
Unused.
int aio_usedba
Unused.
int aio_debug
Unused.
int aio_numusers
Unused.
int aio_reserved[2]
Unused.

— Function: void aio_init (const struct aioinit *init)

This function must be called before any other AIO function. Calling it is completely voluntary, as it is only meant to help the AIO implementation perform better.

Before calling the aio_init, function the members of a variable of type struct aioinit must be initialized. Then a reference to this variable is passed as the parameter to aio_init which itself may or may not pay attention to the hints.

The function has no return value and no error cases are defined. It is a extension which follows a proposal from the SGI implementation in Irix 6. It is not covered by POSIX.1b or Unix98.

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13.11 Control Operations on Files

This section describes how you can perform various other operations on file descriptors, such as inquiring about or setting flags describing the status of the file descriptor, manipulating record locks, and the like. All of these operations are performed by the function fcntl.

The second argument to the fcntl function is a command that specifies which operation to perform. The function and macros that name various flags that are used with it are declared in the header file fcntl.h. Many of these flags are also used by the open function; see Opening and Closing Files.

— Function: int fcntl (int filedes, int command, ...)

The fcntl function performs the operation specified by command on the file descriptor filedes. Some commands require additional arguments to be supplied. These additional arguments and the return value and error conditions are given in the detailed descriptions of the individual commands.

Briefly, here is a list of what the various commands are.

F_DUPFD
Duplicate the file descriptor (return another file descriptor pointing to the same open file). See Duplicating Descriptors.
F_GETFD
Get flags associated with the file descriptor. See Descriptor Flags.
F_SETFD
Set flags associated with the file descriptor. See Descriptor Flags.
F_GETFL
Get flags associated with the open file. See File Status Flags.
F_SETFL
Set flags associated with the open file. See File Status Flags.
F_GETLK
Get a file lock. See File Locks.
F_SETLK
Set or clear a file lock. See File Locks.
F_SETLKW
Like F_SETLK, but wait for completion. See File Locks.
F_GETOWN
Get process or process group ID to receive SIGIO signals. See Interrupt Input.
F_SETOWN
Set process or process group ID to receive SIGIO signals. See Interrupt Input.

This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time fcntl is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this calls to fcntl should be protected using cancellation handlers.

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13.12 Duplicating Descriptors

You can duplicate a file descriptor, or allocate another file descriptor that refers to the same open file as the original. Duplicate descriptors share one file position and one set of file status flags (see File Status Flags), but each has its own set of file descriptor flags (see Descriptor Flags).

The major use of duplicating a file descriptor is to implement redirection of input or output: that is, to change the file or pipe that a particular file descriptor corresponds to.

You can perform this operation using the fcntl function with the F_DUPFD command, but there are also convenient functions dup and dup2 for duplicating descriptors.

The fcntl function and flags are declared in fcntl.h, while prototypes for dup and dup2 are in the header file unistd.h.

— Function: int dup (int old)

This function copies descriptor old to the first available descriptor number (the first number not currently open). It is equivalent to fcntl (old, F_DUPFD, 0).

— Function: int dup2 (int old, int new)

This function copies the descriptor old to descriptor number new.

If old is an invalid descriptor, then dup2 does nothing; it does not close new. Otherwise, the new duplicate of old replaces any previous meaning of descriptor new, as if new were closed first.

If old and new are different numbers, and old is a valid descriptor number, then dup2 is equivalent to: 

          close (new);
          fcntl (old, F_DUPFD, new)

However, dup2 does this atomically; there is no instant in the middle of calling dup2 at which new is closed and not yet a duplicate of old.

— Macro: int F_DUPFD

This macro is used as the command argument to fcntl, to copy the file descriptor given as the first argument.

The form of the call in this case is: 

          fcntl (old, F_DUPFD, next-filedes)

The next-filedes argument is of type int and specifies that the file descriptor returned should be the next available one greater than or equal to this value.

The return value from fcntl with this command is normally the value of the new file descriptor. A return value of -1 indicates an error. The following errno error conditions are defined for this command:

EBADF
The old argument is invalid.
EINVAL
The next-filedes argument is invalid.
EMFILE
There are no more file descriptors available—your program is already using the maximum. In BSD and GNU, the maximum is controlled by a resource limit that can be changed; see Limits on Resources, for more information about the RLIMIT_NOFILE limit.

ENFILE is not a possible error code for dup2 because dup2 does not create a new opening of a file; duplicate descriptors do not count toward the limit which ENFILE indicates. EMFILE is possible because it refers to the limit on distinct descriptor numbers in use in one process.

Here is an example showing how to use dup2 to do redirection. Typically, redirection of the standard streams (like stdin) is done by a shell or shell-like program before calling one of the exec functions (see Executing a File) to execute a new program in a child process. When the new program is executed, it creates and initializes the standard streams to point to the corresponding file descriptors, before its main function is invoked.

So, to redirect standard input to a file, the shell could do something like: 

     pid = fork ();
     if (pid == 0)
       {
         char *filename;
         char *program;
         int file;
         ...
         file = TEMP_FAILURE_RETRY (open (filename, O_RDONLY));
         dup2 (file, STDIN_FILENO);
         TEMP_FAILURE_RETRY (close (file));
         execv (program, NULL);
       }

There is also a more detailed example showing how to implement redirection in the context of a pipeline of processes in Launching Jobs.

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13.13 File Descriptor Flags

File descriptor flags are miscellaneous attributes of a file descriptor. These flags are associated with particular file descriptors, so that if you have created duplicate file descriptors from a single opening of a file, each descriptor has its own set of flags.

Currently there is just one file descriptor flag: FD_CLOEXEC, which causes the descriptor to be closed if you use any of the exec... functions (see Executing a File).

The symbols in this section are defined in the header file fcntl.h.

— Macro: int F_GETFD

This macro is used as the command argument to fcntl, to specify that it should return the file descriptor flags associated with the filedes argument.

The normal return value from fcntl with this command is a nonnegative number which can be interpreted as the bitwise OR of the individual flags (except that currently there is only one flag to use).

In case of an error, fcntl returns -1. The following errno error conditions are defined for this command:

EBADF
The filedes argument is invalid.

— Macro: int F_SETFD

This macro is used as the command argument to fcntl, to specify that it should set the file descriptor flags associated with the filedes argument. This requires a third int argument to specify the new flags, so the form of the call is: 

          fcntl (filedes, F_SETFD, new-flags)

The normal return value from fcntl with this command is an unspecified value other than -1, which indicates an error. The flags and error conditions are the same as for the F_GETFD command.

The following macro is defined for use as a file descriptor flag with the fcntl function. The value is an integer constant usable as a bit mask value.

— Macro: int FD_CLOEXEC

This flag specifies that the file descriptor should be closed when an exec function is invoked; see Executing a File. When a file descriptor is allocated (as with open or dup), this bit is initially cleared on the new file descriptor, meaning that descriptor will survive into the new program after exec.

If you want to modify the file descriptor flags, you should get the current flags with F_GETFD and modify the value. Don't assume that the flags listed here are the only ones that are implemented; your program may be run years from now and more flags may exist then. For example, here is a function to set or clear the flag FD_CLOEXEC without altering any other flags: 

     /* Set the FD_CLOEXEC flag of desc if value is nonzero,
        or clear the flag if value is 0.
        Return 0 on success, or -1 on error with errno set. */
     
     int
     set_cloexec_flag (int desc, int value)
     {
       int oldflags = fcntl (desc, F_GETFD, 0);
       /* If reading the flags failed, return error indication now. */
       if (oldflags < 0)
         return oldflags;
       /* Set just the flag we want to set. */
       if (value != 0)
         oldflags |= FD_CLOEXEC;
       else
         oldflags &= ~FD_CLOEXEC;
       /* Store modified flag word in the descriptor. */
       return fcntl (desc, F_SETFD, oldflags);
     }

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13.14 File Status Flags

File status flags are used to specify attributes of the opening of a file. Unlike the file descriptor flags discussed in Descriptor Flags, the file status flags are shared by duplicated file descriptors resulting from a single opening of the file. The file status flags are specified with the flags argument to open; see Opening and Closing Files.

File status flags fall into three categories, which are described in the following sections.

The symbols in this section are defined in the header file fcntl.h.

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13.14.1 File Access Modes

The file access modes allow a file descriptor to be used for reading, writing, or both. (In the GNU system, they can also allow none of these, and allow execution of the file as a program.) The access modes are chosen when the file is opened, and never change.

— Macro: int O_RDONLY

Open the file for read access.

— Macro: int O_WRONLY

Open the file for write access.

— Macro: int O_RDWR

Open the file for both reading and writing.

In the GNU system (and not in other systems), O_RDONLY and O_WRONLY are independent bits that can be bitwise-ORed together, and it is valid for either bit to be set or clear. This means that O_RDWR is the same as O_RDONLY|O_WRONLY. A file access mode of zero is permissible; it allows no operations that do input or output to the file, but does allow other operations such as fchmod. On the GNU system, since “read-only” or “write-only” is a misnomer, fcntl.h defines additional names for the file access modes. These names are preferred when writing GNU-specific code. But most programs will want to be portable to other POSIX.1 systems and should use the POSIX.1 names above instead.

— Macro: int O_READ

Open the file for reading. Same as O_RDONLY; only defined on GNU.

— Macro: int O_WRITE

Open the file for writing. Same as O_WRONLY; only defined on GNU.

— Macro: int O_EXEC

Open the file for executing. Only defined on GNU.

To determine the file access mode with fcntl, you must extract the access mode bits from the retrieved file status flags. In the GNU system, you can just test the O_READ and O_WRITE bits in the flags word. But in other POSIX.1 systems, reading and writing access modes are not stored as distinct bit flags. The portable way to extract the file access mode bits is with O_ACCMODE.

— Macro: int O_ACCMODE

This macro stands for a mask that can be bitwise-ANDed with the file status flag value to produce a value representing the file access mode. The mode will be O_RDONLY, O_WRONLY, or O_RDWR. (In the GNU system it could also be zero, and it never includes the O_EXEC bit.)

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13.14.2 Open-time Flags

The open-time flags specify options affecting how open will behave. These options are not preserved once the file is open. The exception to this is O_NONBLOCK, which is also an I/O operating mode and so it is saved. See Opening and Closing Files, for how to call open.

There are two sorts of options specified by open-time flags.

Here are the file name translation flags.

— Macro: int O_CREAT

If set, the file will be created if it doesn't already exist.

— Macro: int O_EXCL

If both O_CREAT and O_EXCL are set, then open fails if the specified file already exists. This is guaranteed to never clobber an existing file.

— Macro: int O_NONBLOCK

This prevents open from blocking for a “long time” to open the file. This is only meaningful for some kinds of files, usually devices such as serial ports; when it is not meaningful, it is harmless and ignored. Often opening a port to a modem blocks until the modem reports carrier detection; if O_NONBLOCK is specified, open will return immediately without a carrier.

Note that the O_NONBLOCK flag is overloaded as both an I/O operating mode and a file name translation flag. This means that specifying O_NONBLOCK in open also sets nonblocking I/O mode; see Operating Modes. To open the file without blocking but do normal I/O that blocks, you must call open with O_NONBLOCK set and then call fcntl to turn the bit off.

— Macro: int O_NOCTTY

If the named file is a terminal device, don't make it the controlling terminal for the process. See Job Control, for information about what it means to be the controlling terminal.

In the GNU system and 4.4 BSD, opening a file never makes it the controlling terminal and O_NOCTTY is zero. However, other systems may use a nonzero value for O_NOCTTY and set the controlling terminal when you open a file that is a terminal device; so to be portable, use O_NOCTTY when it is important to avoid this.

The following three file name translation flags exist only in the GNU system.

— Macro: int O_IGNORE_CTTY

Do not recognize the named file as the controlling terminal, even if it refers to the process's existing controlling terminal device. Operations on the new file descriptor will never induce job control signals. See Job Control.

— Macro: int O_NOLINK

If the named file is a symbolic link, open the link itself instead of the file it refers to. (fstat on the new file descriptor will return the information returned by lstat on the link's name.)

— Macro: int O_NOTRANS

If the named file is specially translated, do not invoke the translator. Open the bare file the translator itself sees.

The open-time action flags tell open to do additional operations which are not really related to opening the file. The reason to do them as part of open instead of in separate calls is that open can do them atomically.

— Macro: int O_TRUNC

Truncate the file to zero length. This option is only useful for regular files, not special files such as directories or FIFOs. POSIX.1 requires that you open the file for writing to use O_TRUNC. In BSD and GNU you must have permission to write the file to truncate it, but you need not open for write access.

This is the only open-time action flag specified by POSIX.1. There is no good reason for truncation to be done by open, instead of by calling ftruncate afterwards. The O_TRUNC flag existed in Unix before ftruncate was invented, and is retained for backward compatibility.

The remaining operating modes are BSD extensions. They exist only on some systems. On other systems, these macros are not defined.

— Macro: int O_SHLOCK

Acquire a shared lock on the file, as with flock. See File Locks.

If O_CREAT is specified, the locking is done atomically when creating the file. You are guaranteed that no other process will get the lock on the new file first.

— Macro: int O_EXLOCK

Acquire an exclusive lock on the file, as with flock. See File Locks. This is atomic like O_SHLOCK.

Next: , Previous: Open-time Flags, Up: File Status Flags

13.14.3 I/O Operating Modes

The operating modes affect how input and output operations using a file descriptor work. These flags are set by open and can be fetched and changed with fcntl.

— Macro: int O_APPEND

The bit that enables append mode for the file. If set, then all write operations write the data at the end of the file, extending it, regardless of the current file position. This is the only reliable way to append to a file. In append mode, you are guaranteed that the data you write will always go to the current end of the file, regardless of other processes writing to the file. Conversely, if you simply set the file position to the end of file and write, then another process can extend the file after you set the file position but before you write, resulting in your data appearing someplace before the real end of file.

— Macro: int O_NONBLOCK

The bit that enables nonblocking mode for the file. If this bit is set, read requests on the file can return immediately with a failure status if there is no input immediately available, instead of blocking. Likewise, write requests can also return immediately with a failure status if the output can't be written immediately.

Note that the O_NONBLOCK flag is overloaded as both an I/O operating mode and a file name translation flag; see Open-time Flags.

— Macro: int O_NDELAY

This is an obsolete name for O_NONBLOCK, provided for compatibility with BSD. It is not defined by the POSIX.1 standard.

The remaining operating modes are BSD and GNU extensions. They exist only on some systems. On other systems, these macros are not defined.

— Macro: int O_ASYNC

The bit that enables asynchronous input mode. If set, then SIGIO signals will be generated when input is available. See Interrupt Input.

Asynchronous input mode is a BSD feature.

— Macro: int O_FSYNC

The bit that enables synchronous writing for the file. If set, each write call will make sure the data is reliably stored on disk before returning.

Synchronous writing is a BSD feature.

— Macro: int O_SYNC

This is another name for O_FSYNC. They have the same value.

— Macro: int O_NOATIME

If this bit is set, read will not update the access time of the file. See File Times. This is used by programs that do backups, so that backing a file up does not count as reading it. Only the owner of the file or the superuser may use this bit.

This is a GNU extension.

Previous: Operating Modes, Up: File Status Flags

13.14.4 Getting and Setting File Status Flags

The fcntl function can fetch or change file status flags.

— Macro: int F_GETFL

This macro is used as the command argument to fcntl, to read the file status flags for the open file with descriptor filedes.

The normal return value from fcntl with this command is a nonnegative number which can be interpreted as the bitwise OR of the individual flags. Since the file access modes are not single-bit values, you can mask off other bits in the returned flags with O_ACCMODE to compare them.

In case of an error, fcntl returns -1. The following errno error conditions are defined for this command:

EBADF
The filedes argument is invalid.

— Macro: int F_SETFL

This macro is used as the command argument to fcntl, to set the file status flags for the open file corresponding to the filedes argument. This command requires a third int argument to specify the new flags, so the call looks like this: 

          fcntl (filedes, F_SETFL, new-flags)

You can't change the access mode for the file in this way; that is, whether the file descriptor was opened for reading or writing.

The normal return value from fcntl with this command is an unspecified value other than -1, which indicates an error. The error conditions are the same as for the F_GETFL command.

If you want to modify the file status flags, you should get the current flags with F_GETFL and modify the value. Don't assume that the flags listed here are the only ones that are implemented; your program may be run years from now and more flags may exist then. For example, here is a function to set or clear the flag O_NONBLOCK without altering any other flags: 

     /* Set the O_NONBLOCK flag of desc if value is nonzero,
        or clear the flag if value is 0.
        Return 0 on success, or -1 on error with errno set. */
     
     int
     set_nonblock_flag (int desc, int value)
     {
       int oldflags = fcntl (desc, F_GETFL, 0);
       /* If reading the flags failed, return error indication now. */
       if (oldflags == -1)
         return -1;
       /* Set just the flag we want to set. */
       if (value != 0)
         oldflags |= O_NONBLOCK;
       else
         oldflags &= ~O_NONBLOCK;
       /* Store modified flag word in the descriptor. */
       return fcntl (desc, F_SETFL, oldflags);
     }

Next: , Previous: File Status Flags, Up: Low-Level I/O

13.15 File Locks

The remaining fcntl commands are used to support record locking, which permits multiple cooperating programs to prevent each other from simultaneously accessing parts of a file in error-prone ways.

An exclusive or write lock gives a process exclusive access for writing to the specified part of the file. While a write lock is in place, no other process can lock that part of the file.

A shared or read lock prohibits any other process from requesting a write lock on the specified part of the file. However, other processes can request read locks.

The read and write functions do not actually check to see whether there are any locks in place. If you want to implement a locking protocol for a file shared by multiple processes, your application must do explicit fcntl calls to request and clear locks at the appropriate points.

Locks are associated with processes. A process can only have one kind of lock set for each byte of a given file. When any file descriptor for that file is closed by the process, all of the locks that process holds on that file are released, even if the locks were made using other descriptors that remain open. Likewise, locks are released when a process exits, and are not inherited by child processes created using fork (see Creating a Process).

When making a lock, use a struct flock to specify what kind of lock and where. This data type and the associated macros for the fcntl function are declared in the header file fcntl.h.

— Data Type: struct flock

This structure is used with the fcntl function to describe a file lock. It has these members:

short int l_type
Specifies the type of the lock; one of F_RDLCK, F_WRLCK, or F_UNLCK.
short int l_whence
This corresponds to the whence argument to fseek or lseek, and specifies what the offset is relative to. Its value can be one of SEEK_SET, SEEK_CUR, or SEEK_END.
off_t l_start
This specifies the offset of the start of the region to which the lock applies, and is given in bytes relative to the point specified by l_whence member.
off_t l_len
This specifies the length of the region to be locked. A value of 0 is treated specially; it means the region extends to the end of the file.
pid_t l_pid
This field is the process ID (see Process Creation Concepts) of the process holding the lock. It is filled in by calling fcntl with the F_GETLK command, but is ignored when making a lock.

— Macro: int F_GETLK

This macro is used as the command argument to fcntl, to specify that it should get information about a lock. This command requires a third argument of type struct flock * to be passed to fcntl, so that the form of the call is: 

          fcntl (filedes, F_GETLK, lockp)

If there is a lock already in place that would block the lock described by the lockp argument, information about that lock overwrites *lockp. Existing locks are not reported if they are compatible with making a new lock as specified. Thus, you should specify a lock type of F_WRLCK if you want to find out about both read and write locks, or F_RDLCK if you want to find out about write locks only.

There might be more than one lock affecting the region specified by the lockp argument, but fcntl only returns information about one of them. The l_whence member of the lockp structure is set to SEEK_SET and the l_start and l_len fields set to identify the locked region.

If no lock applies, the only change to the lockp structure is to update the l_type to a value of F_UNLCK.

The normal return value from fcntl with this command is an unspecified value other than -1, which is reserved to indicate an error. The following errno error conditions are defined for this command:

EBADF
The filedes argument is invalid.
EINVAL
Either the lockp argument doesn't specify valid lock information, or the file associated with filedes doesn't support locks.

— Macro: int F_SETLK

This macro is used as the command argument to fcntl, to specify that it should set or clear a lock. This command requires a third argument of type struct flock * to be passed to fcntl, so that the form of the call is: 

          fcntl (filedes, F_SETLK, lockp)

If the process already has a lock on any part of the region, the old lock on that part is replaced with the new lock. You can remove a lock by specifying a lock type of F_UNLCK.

If the lock cannot be set, fcntl returns immediately with a value of -1. This function does not block waiting for other processes to release locks. If fcntl succeeds, it return a value other than -1.

The following errno error conditions are defined for this function:

EAGAIN
EACCES
The lock cannot be set because it is blocked by an existing lock on the file. Some systems use EAGAIN in this case, and other systems use EACCES; your program should treat them alike, after F_SETLK. (The GNU system always uses EAGAIN.)
EBADF
Either: the filedes argument is invalid; you requested a read lock but the filedes is not open for read access; or, you requested a write lock but the filedes is not open for write access.
EINVAL
Either the lockp argument doesn't specify valid lock information, or the file associated with filedes doesn't support locks.
ENOLCK
The system has run out of file lock resources; there are already too many file locks in place.

Well-designed file systems never report this error, because they have no limitation on the number of locks. However, you must still take account of the possibility of this error, as it could result from network access to a file system on another machine.

— Macro: int F_SETLKW

This macro is used as the command argument to fcntl, to specify that it should set or clear a lock. It is just like the F_SETLK command, but causes the process to block (or wait) until the request can be specified.

This command requires a third argument of type struct flock *, as for the F_SETLK command.

The fcntl return values and errors are the same as for the F_SETLK command, but these additional errno error conditions are defined for this command:

EINTR
The function was interrupted by a signal while it was waiting. See Interrupted Primitives.
EDEADLK
The specified region is being locked by another process. But that process is waiting to lock a region which the current process has locked, so waiting for the lock would result in deadlock. The system does not guarantee that it will detect all such conditions, but it lets you know if it notices one.

The following macros are defined for use as values for the l_type member of the flock structure. The values are integer constants.

F_RDLCK
This macro is used to specify a read (or shared) lock.


F_WRLCK
This macro is used to specify a write (or exclusive) lock.


F_UNLCK
This macro is used to specify that the region is unlocked.

As an example of a situation where file locking is useful, consider a program that can be run simultaneously by several different users, that logs status information to a common file. One example of such a program might be a game that uses a file to keep track of high scores. Another example might be a program that records usage or accounting information for billing purposes.

Having multiple copies of the program simultaneously writing to the file could cause the contents of the file to become mixed up. But you can prevent this kind of problem by setting a write lock on the file before actually writing to the file.

If the program also needs to read the file and wants to make sure that the contents of the file are in a consistent state, then it can also use a read lock. While the read lock is set, no other process can lock that part of the file for writing.

Remember that file locks are only a voluntary protocol for controlling access to a file. There is still potential for access to the file by programs that don't use the lock protocol.

Next: , Previous: File Locks, Up: Low-Level I/O

13.16 Interrupt-Driven Input

If you set the O_ASYNC status flag on a file descriptor (see File Status Flags), a SIGIO signal is sent whenever input or output becomes possible on that file descriptor. The process or process group to receive the signal can be selected by using the F_SETOWN command to the fcntl function. If the file descriptor is a socket, this also selects the recipient of SIGURG signals that are delivered when out-of-band data arrives on that socket; see Out-of-Band Data. (SIGURG is sent in any situation where select would report the socket as having an “exceptional condition”. See Waiting for I/O.)

If the file descriptor corresponds to a terminal device, then SIGIO signals are sent to the foreground process group of the terminal. See Job Control.

The symbols in this section are defined in the header file fcntl.h.

— Macro: int F_GETOWN

This macro is used as the command argument to fcntl, to specify that it should get information about the process or process group to which SIGIO signals are sent. (For a terminal, this is actually the foreground process group ID, which you can get using tcgetpgrp; see Terminal Access Functions.)

The return value is interpreted as a process ID; if negative, its absolute value is the process group ID.

The following errno error condition is defined for this command:

EBADF
The filedes argument is invalid.

— Macro: int F_SETOWN

This macro is used as the command argument to fcntl, to specify that it should set the process or process group to which SIGIO signals are sent. This command requires a third argument of type pid_t to be passed to fcntl, so that the form of the call is: 

          fcntl (filedes, F_SETOWN, pid)

The pid argument should be a process ID. You can also pass a negative number whose absolute value is a process group ID.

The return value from fcntl with this command is -1 in case of error and some other value if successful. The following errno error conditions are defined for this command:

EBADF
The filedes argument is invalid.
ESRCH
There is no process or process group corresponding to pid.

Previous: Interrupt Input, Up: Low-Level I/O

13.17 Generic I/O Control operations

The GNU system can handle most input/output operations on many different devices and objects in terms of a few file primitives - read, write and lseek. However, most devices also have a few peculiar operations which do not fit into this model. Such as:

Although some such objects such as sockets and terminals 2 have special functions of their own, it would not be practical to create functions for all these cases.

Instead these minor operations, known as IOCTLs, are assigned code numbers and multiplexed through the ioctl function, defined in sys/ioctl.h. The code numbers themselves are defined in many different headers.

— Function: int ioctl (int filedes, int command, ...)

The ioctl function performs the generic I/O operation command on filedes.

A third argument is usually present, either a single number or a pointer to a structure. The meaning of this argument, the returned value, and any error codes depends upon the command used. Often -1 is returned for a failure.

On some systems, IOCTLs used by different devices share the same numbers. Thus, although use of an inappropriate IOCTL usually only produces an error, you should not attempt to use device-specific IOCTLs on an unknown device.

Most IOCTLs are OS-specific and/or only used in special system utilities, and are thus beyond the scope of this document. For an example of the use of an IOCTL, see Out-of-Band Data.

Next: , Previous: Low-Level I/O, Up: Top

14 File System Interface

This chapter describes the GNU C library's functions for manipulating files. Unlike the input and output functions (see I/O on Streams; see Low-Level I/O), these functions are concerned with operating on the files themselves rather than on their contents.

Among the facilities described in this chapter are functions for examining or modifying directories, functions for renaming and deleting files, and functions for examining and setting file attributes such as access permissions and modification times.

Next: , Up: File System Interface

14.1 Working Directory

Each process has associated with it a directory, called its current working directory or simply working directory, that is used in the resolution of relative file names (see File Name Resolution).

When you log in and begin a new session, your working directory is initially set to the home directory associated with your login account in the system user database. You can find any user's home directory using the getpwuid or getpwnam functions; see User Database.

Users can change the working directory using shell commands like cd. The functions described in this section are the primitives used by those commands and by other programs for examining and changing the working directory. Prototypes for these functions are declared in the header file unistd.h.

— Function: char * getcwd (char *buffer, size_t size)

The getcwd function returns an absolute file name representing the current working directory, storing it in the character array buffer that you provide. The size argument is how you tell the system the allocation size of buffer.

The GNU library version of this function also permits you to specify a null pointer for the buffer argument. Then getcwd allocates a buffer automatically, as with malloc (see Unconstrained Allocation). If the size is greater than zero, then the buffer is that large; otherwise, the buffer is as large as necessary to hold the result.

The return value is buffer on success and a null pointer on failure. The following errno error conditions are defined for this function:

EINVAL
The size argument is zero and buffer is not a null pointer.
ERANGE
The size argument is less than the length of the working directory name. You need to allocate a bigger array and try again.
EACCES
Permission to read or search a component of the file name was denied.

You could implement the behavior of GNU's getcwd (NULL, 0) using only the standard behavior of getcwd: 

     char *
     gnu_getcwd ()
     {
       size_t size = 100;
     
       while (1)
         {
           char *buffer = (char *) xmalloc (size);
           if (getcwd (buffer, size) == buffer)
             return buffer;
           free (buffer);
           if (errno != ERANGE)
             return 0;
           size *= 2;
         }
     }

See Malloc Examples, for information about xmalloc, which is not a library function but is a customary name used in most GNU software.

— Deprecated Function: char * getwd (char *buffer)

This is similar to getcwd, but has no way to specify the size of the buffer. The GNU library provides getwd only for backwards compatibility with BSD.

The buffer argument should be a pointer to an array at least PATH_MAX bytes long (see Limits for Files). In the GNU system there is no limit to the size of a file name, so this is not necessarily enough space to contain the directory name. That is why this function is deprecated.

— Function: char * get_current_dir_name (void)

This get_current_dir_name function is basically equivalent to getcwd (NULL, 0). The only difference is that the value of the PWD variable is returned if this value is correct. This is a subtle difference which is visible if the path described by the PWD value is using one or more symbol links in which case the value returned by getcwd can resolve the symbol links and therefore yield a different result.

This function is a GNU extension.

— Function: int chdir (const char *filename)

This function is used to set the process's working directory to filename.

The normal, successful return value from chdir is 0. A value of -1 is returned to indicate an error. The errno error conditions defined for this function are the usual file name syntax errors (see File Name Errors), plus ENOTDIR if the file filename is not a directory.

— Function: int fchdir (int filedes)

This function is used to set the process's working directory to directory associated with the file descriptor filedes.

The normal, successful return value from fchdir is 0. A value of -1 is returned to indicate an error. The following errno error conditions are defined for this function:

EACCES
Read permission is denied for the directory named by dirname.
EBADF
The filedes argument is not a valid file descriptor.
ENOTDIR
The file descriptor filedes is not associated with a directory.
EINTR
The function call was interrupt by a signal.
EIO
An I/O error occurred.

Next: , Previous: Working Directory, Up: File System Interface

14.2 Accessing Directories

The facilities described in this section let you read the contents of a directory file. This is useful if you want your program to list all the files in a directory, perhaps as part of a menu.

The opendir function opens a directory stream whose elements are directory entries. Alternatively fdopendir can be used which can have advantages if the program needs to have more control over the way the directory is opened for reading. This allows, for instance, to pass the O_NOATIME flag to open.

You use the readdir function on the directory stream to retrieve these entries, represented as struct dirent objects. The name of the file for each entry is stored in the d_name member of this structure. There are obvious parallels here to the stream facilities for ordinary files, described in I/O on Streams.

Next: , Up: Accessing Directories

14.2.1 Format of a Directory Entry

This section describes what you find in a single directory entry, as you might obtain it from a directory stream. All the symbols are declared in the header file dirent.h.

— Data Type: struct dirent

This is a structure type used to return information about directory entries. It contains the following fields:

char d_name[]
This is the null-terminated file name component. This is the only field you can count on in all POSIX systems.
ino_t d_fileno
This is the file serial number. For BSD compatibility, you can also refer to this member as d_ino. In the GNU system and most POSIX systems, for most files this the same as the st_ino member that stat will return for the file. See File Attributes.
unsigned char d_namlen
This is the length of the file name, not including the terminating null character. Its type is unsigned char because that is the integer type of the appropriate size
unsigned char d_type
This is the type of the file, possibly unknown. The following constants are defined for its value:
DT_UNKNOWN
The type is unknown. On some systems this is the only value returned.
DT_REG
A regular file.
DT_DIR
A directory.
DT_FIFO
A named pipe, or FIFO. See FIFO Special Files.
DT_SOCK
A local-domain socket.
DT_CHR
A character device.
DT_BLK
A block device.

This member is a BSD extension. The symbol _DIRENT_HAVE_D_TYPE is defined if this member is available. On systems where it is used, it corresponds to the file type bits in the st_mode member of struct statbuf. If the value cannot be determine the member value is DT_UNKNOWN. These two macros convert between d_type values and st_mode values:

— Function: int IFTODT (mode_t mode)

This returns the d_type value corresponding to mode.

— Function: mode_t DTTOIF (int dtype)

This returns the st_mode value corresponding to dtype.

This structure may contain additional members in the future. Their availability is always announced in the compilation environment by a macro names _DIRENT_HAVE_D_xxx where xxx is replaced by the name of the new member. For instance, the member d_reclen available on some systems is announced through the macro _DIRENT_HAVE_D_RECLEN.

When a file has multiple names, each name has its own directory entry. The only way you can tell that the directory entries belong to a single file is that they have the same value for the d_fileno field.

File attributes such as size, modification times etc., are part of the file itself, not of any particular directory entry. See File Attributes.

Next: , Previous: Directory Entries, Up: Accessing Directories

14.2.2 Opening a Directory Stream

This section describes how to open a directory stream. All the symbols are declared in the header file dirent.h.

— Data Type: DIR

The DIR data type represents a directory stream.

You shouldn't ever allocate objects of the struct dirent or DIR data types, since the directory access functions do that for you. Instead, you refer to these objects using the pointers returned by the following functions.

— Function: DIR * opendir (const char *dirname)

The opendir function opens and returns a directory stream for reading the directory whose file name is dirname. The stream has type DIR *.

If unsuccessful, opendir returns a null pointer. In addition to the usual file name errors (see File Name Errors), the following errno error conditions are defined for this function:

EACCES
Read permission is denied for the directory named by dirname.
EMFILE
The process has too many files open.
ENFILE
The entire system, or perhaps the file system which contains the directory, cannot support any additional open files at the moment. (This problem cannot happen on the GNU system.)
ENOMEM
Not enough memory available.

The DIR type is typically implemented using a file descriptor, and the opendir function in terms of the open function. See Low-Level I/O. Directory streams and the underlying file descriptors are closed on exec (see Executing a File).

The directory which is opened for reading by opendir is identified by the name. In some situations this is not sufficient. Or the way opendir implicitly creates a file descriptor for the directory is not the way a program might want it. In these cases an alternative interface can be used.

— Function: DIR * fdopendir (int fd)

The fdopendir function works just like opendir but instead of taking a file name and opening a file descriptor for the directory the caller is required to provide a file descriptor. This file descriptor is then used in subsequent uses of the returned directory stream object.

The caller must make sure the file descriptor is associated with a directory and it allows reading.

If the fdopendir call returns successfully the file descriptor is now under the control of the system. It can be used in the same way the descriptor implicitly created by opendir can be used but the program must not close the descriptor.

In case the function is unsuccessful it returns a null pointer and the file descriptor remains to be usable by the program. The following errno error conditions are defined for this function:

EBADF
The file descriptor is not valid.
ENOTDIR
The file descriptor is not associated with a directory.
EINVAL
The descriptor does not allow reading the directory content.
ENOMEM
Not enough memory available.

In some situations it can be desirable to get hold of the file descriptor which is created by the opendir call. For instance, to switch the current working directory to the directory just read the fchdir function could be used. Historically the DIR type was exposed and programs could access the fields. This does not happen in the GNU C library. Instead a separate function is provided to allow access.

— Function: int dirfd (DIR *dirstream)

The function dirfd returns the file descriptor associated with the directory stream dirstream. This descriptor can be used until the directory is closed with closedir. If the directory stream implementation is not using file descriptors the return value is -1.

Next: , Previous: Opening a Directory, Up: Accessing Directories

14.2.3 Reading and Closing a Directory Stream

This section describes how to read directory entries from a directory stream, and how to close the stream when you are done with it. All the symbols are declared in the header file dirent.h.

— Function: struct dirent * readdir (DIR *dirstream)

This function reads the next entry from the directory. It normally returns a pointer to a structure containing information about the file. This structure is statically allocated and can be rewritten by a subsequent call.

Portability Note: On some systems readdir may not return entries for . and .., even though these are always valid file names in any directory. See File Name Resolution.

If there are no more entries in the directory or an error is detected, readdir returns a null pointer. The following errno error conditions are defined for this function:

EBADF
The dirstream argument is not valid.

readdir is not thread safe. Multiple threads using readdir on the same dirstream may overwrite the return value. Use readdir_r when this is critical.

— Function: int readdir_r (DIR *dirstream, struct dirent *entry, struct dirent **result)

This function is the reentrant version of readdir. Like readdir it returns the next entry from the directory. But to prevent conflicts between simultaneously running threads the result is not stored in statically allocated memory. Instead the argument entry points to a place to store the result.

Normally readdir_r returns zero and sets *result to entry. If there are no more entries in the directory or an error is detected, readdir_r sets *result to a null pointer and returns a nonzero error code, also stored in errno, as described for readdir.

Portability Note: On some systems readdir_r may not return a NUL terminated string for the file name, even when there is no d_reclen field in struct dirent and the file name is the maximum allowed size. Modern systems all have the d_reclen field, and on old systems multi-threading is not critical. In any case there is no such problem with the readdir function, so that even on systems without the d_reclen member one could use multiple threads by using external locking.

It is also important to look at the definition of the struct dirent type. Simply passing a pointer to an object of this type for the second parameter of readdir_r might not be enough. Some systems don't define the d_name element sufficiently long. In this case the user has to provide additional space. There must be room for at least NAME_MAX + 1 characters in the d_name array. Code to call readdir_r could look like this: 

            union
            {
              struct dirent d;
              char b[offsetof (struct dirent, d_name) + NAME_MAX + 1];
            } u;
          
            if (readdir_r (dir, &u.d, &res) == 0)
              ...

To support large filesystems on 32-bit machines there are LFS variants of the last two functions.

— Function: struct dirent64 * readdir64 (DIR *dirstream)

The readdir64 function is just like the readdir function except that it returns a pointer to a record of type struct dirent64. Some of the members of this data type (notably d_ino) might have a different size to allow large filesystems.

In all other aspects this function is equivalent to readdir.

— Function: int readdir64_r (DIR *dirstream, struct dirent64 *entry, struct dirent64 **result)

The readdir64_r function is equivalent to the readdir_r function except that it takes parameters of base type struct dirent64 instead of struct dirent in the second and third position. The same precautions mentioned in the documentation of readdir_r also apply here.

— Function: int closedir (DIR *dirstream)

This function closes the directory stream dirstream. It returns 0 on success and -1 on failure.

The following errno error conditions are defined for this function:

EBADF
The dirstream argument is not valid.

Next: , Previous: Reading/Closing Directory, Up: Accessing Directories

14.2.4 Simple Program to List a Directory

Here's a simple program that prints the names of the files in the current working directory: 

     #include <stdio.h>
     #include <sys/types.h>
     #include <dirent.h>
     
     int
     main (void)
     {
       DIR *dp;
       struct dirent *ep;
     
       dp = opendir ("./");
       if (dp != NULL)
         {
           while (ep = readdir (dp))
             puts (ep->d_name);
           (void) closedir (dp);
         }
       else
         perror ("Couldn't open the directory");
     
       return 0;
     }

The order in which files appear in a directory tends to be fairly random. A more useful program would sort the entries (perhaps by alphabetizing them) before printing them; see Scanning Directory Content, and Array Sort Function.

Next: , Previous: Simple Directory Lister, Up: Accessing Directories

14.2.5 Random Access in a Directory Stream

This section describes how to reread parts of a directory that you have already read from an open directory stream. All the symbols are declared in the header file dirent.h.

— Function: void rewinddir (DIR *dirstream)

The rewinddir function is used to reinitialize the directory stream dirstream, so that if you call readdir it returns information about the first entry in the directory again. This function also notices if files have been added or removed to the directory since it was opened with opendir. (Entries for these files might or might not be returned by readdir if they were added or removed since you last called opendir or rewinddir.)

— Function: long int telldir (DIR *dirstream)

The telldir function returns the file position of the directory stream dirstream. You can use this value with seekdir to restore the directory stream to that position.

— Function: void seekdir (DIR *dirstream, long int pos)

The seekdir function sets the file position of the directory stream dirstream to pos. The value pos must be the result of a previous call to telldir on this particular stream; closing and reopening the directory can invalidate values returned by telldir.

Next: , Previous: Random Access Directory, Up: Accessing Directories

14.2.6 Scanning the Content of a Directory

A higher-level interface to the directory handling functions is the scandir function. With its help one can select a subset of the entries in a directory, possibly sort them and get a list of names as the result.

— Function: int scandir (const char *dir, struct dirent ***namelist, int (*selector) (const struct dirent *), int (*cmp) (const void *, const void *))

The scandir function scans the contents of the directory selected by dir. The result in *namelist is an array of pointers to structure of type struct dirent which describe all selected directory entries and which is allocated using malloc. Instead of always getting all directory entries returned, the user supplied function selector can be used to decide which entries are in the result. Only the entries for which selector returns a non-zero value are selected.

Finally the entries in *namelist are sorted using the user-supplied function cmp. The arguments passed to the cmp function are of type struct dirent **, therefore one cannot directly use the strcmp or strcoll functions; instead see the functions alphasort and versionsort below.

The return value of the function is the number of entries placed in *namelist. If it is -1 an error occurred (either the directory could not be opened for reading or the malloc call failed) and the global variable errno contains more information on the error.

As described above the fourth argument to the scandir function must be a pointer to a sorting function. For the convenience of the programmer the GNU C library contains implementations of functions which are very helpful for this purpose.

— Function: int alphasort (const void *a, const void *b)

The alphasort function behaves like the strcoll function (see String/Array Comparison). The difference is that the arguments are not string pointers but instead they are of type struct dirent **.

The return value of alphasort is less than, equal to, or greater than zero depending on the order of the two entries a and b.

— Function: int versionsort (const void *a, const void *b)

The versionsort function is like alphasort except that it uses the strverscmp function internally.

If the filesystem supports large files we cannot use the scandir anymore since the dirent structure might not able to contain all the information. The LFS provides the new type struct dirent64. To use this we need a new function.

— Function: int scandir64 (const char *dir, struct dirent64 ***namelist, int (*selector) (const struct dirent64 *), int (*cmp) (const void *, const void *))

The scandir64 function works like the scandir function except that the directory entries it returns are described by elements of type struct dirent64. The function pointed to by selector is again used to select the desired entries, except that selector now must point to a function which takes a struct dirent64 * parameter.

Similarly the cmp function should expect its two arguments to be of type struct dirent64 **.

As cmp is now a function of a different type, the functions alphasort and versionsort cannot be supplied for that argument. Instead we provide the two replacement functions below.

— Function: int alphasort64 (const void *a, const void *b)

The alphasort64 function behaves like the strcoll function (see String/Array Comparison). The difference is that the arguments are not string pointers but instead they are of type struct dirent64 **.

Return value of alphasort64 is less than, equal to, or greater than zero depending on the order of the two entries a and b.

— Function: int versionsort64 (const void *a, const void *b)

The versionsort64 function is like alphasort64, excepted that it uses the strverscmp function internally.

It is important not to mix the use of scandir and the 64-bit comparison functions or vice versa. There are systems on which this works but on others it will fail miserably.

Previous: Scanning Directory Content, Up: Accessing Directories

14.2.7 Simple Program to List a Directory, Mark II

Here is a revised version of the directory lister found above (see Simple Directory Lister). Using the scandir function we can avoid the functions which work directly with the directory contents. After the call the returned entries are available for direct use. 

     #include <stdio.h>
     #include <dirent.h>
     
     static int
     one (const struct dirent *unused)
     {
       return 1;
     }
     
     int
     main (void)
     {
       struct dirent **eps;
       int n;
     
       n = scandir ("./", &eps, one, alphasort);
       if (n >= 0)
         {
           int cnt;
           for (cnt = 0; cnt < n; ++cnt)
             puts (eps[cnt]->d_name);
         }
       else
         perror ("Couldn't open the directory");
     
       return 0;
     }

Note the simple selector function in this example. Since we want to see all directory entries we always return 1.

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14.3 Working with Directory Trees

The functions described so far for handling the files in a directory have allowed you to either retrieve the information bit by bit, or to process all the files as a group (see scandir). Sometimes it is useful to process whole hierarchies of directories and their contained files. The X/Open specification defines two functions to do this. The simpler form is derived from an early definition in System V systems and therefore this function is available on SVID-derived systems. The prototypes and required definitions can be found in the ftw.h header.

There are four functions in this family: ftw, nftw and their 64-bit counterparts ftw64 and nftw64. These functions take as one of their arguments a pointer to a callback function of the appropriate type.

— Data Type: __ftw_func_t
          int (*) (const char *, const struct stat *, int)

The type of callback functions given to the ftw function. The first parameter points to the file name, the second parameter to an object of type struct stat which is filled in for the file named in the first parameter.

The last parameter is a flag giving more information about the current file. It can have the following values:

FTW_F
The item is either a normal file or a file which does not fit into one of the following categories. This could be special files, sockets etc.
FTW_D
The item is a directory.
FTW_NS
The stat call failed and so the information pointed to by the second paramater is invalid.
FTW_DNR
The item is a directory which cannot be read.
FTW_SL
The item is a symbolic link. Since symbolic links are normally followed seeing this value in a ftw callback function means the referenced file does not exist. The situation for nftw is different.

This value is only available if the program is compiled with _BSD_SOURCE or _XOPEN_EXTENDED defined before including the first header. The original SVID systems do not have symbolic links.

If the sources are compiled with _FILE_OFFSET_BITS == 64 this type is in fact __ftw64_func_t since this mode changes struct stat to be struct stat64.

For the LFS interface and for use in the function ftw64, the header ftw.h defines another function type.

— Data Type: __ftw64_func_t
          int (*) (const char *, const struct stat64 *, int)

This type is used just like __ftw_func_t for the callback function, but this time is called from ftw64. The second parameter to the function is a pointer to a variable of type struct stat64 which is able to represent the larger values.

— Data Type: __nftw_func_t
          int (*) (const char *, const struct stat *, int, struct FTW *)

The first three arguments are the same as for the __ftw_func_t type. However for the third argument some additional values are defined to allow finer differentiation:

FTW_DP
The current item is a directory and all subdirectories have already been visited and reported. This flag is returned instead of FTW_D if the FTW_DEPTH flag is passed to nftw (see below).
FTW_SLN
The current item is a stale symbolic link. The file it points to does not exist.

The last parameter of the callback function is a pointer to a structure with some extra information as described below.

If the sources are compiled with _FILE_OFFSET_BITS == 64 this type is in fact __nftw64_func_t since this mode changes struct stat to be struct stat64.

For the LFS interface there is also a variant of this data type available which has to be used with the nftw64 function.

— Data Type: __nftw64_func_t
          int (*) (const char *, const struct stat64 *, int, struct FTW *)

This type is used just like __nftw_func_t for the callback function, but this time is called from nftw64. The second parameter to the function is this time a pointer to a variable of type struct stat64 which is able to represent the larger values.

— Data Type: struct FTW

The information contained in this structure helps in interpreting the name parameter and gives some information about the current state of the traversal of the directory hierarchy.

int base
The value is the offset into the string passed in the first parameter to the callback function of the beginning of the file name. The rest of the string is the path of the file. This information is especially important if the FTW_CHDIR flag was set in calling nftw since then the current directory is the one the current item is found in.
int level
Whilst processing, the code tracks how many directories down it has gone to find the current file. This nesting level starts at 0 for files in the initial directory (or is zero for the initial file if a file was passed).

— Function: int ftw (const char *filename, __ftw_func_t func, int descriptors)

The ftw function calls the callback function given in the parameter func for every item which is found in the directory specified by filename and all directories below. The function follows symbolic links if necessary but does not process an item twice. If filename is not a directory then it itself is the only object returned to the callback function.

The file name passed to the callback function is constructed by taking the filename parameter and appending the names of all passed directories and then the local file name. So the callback function can use this parameter to access the file. ftw also calls stat for the file and passes that information on to the callback function. If this stat call was not successful the failure is indicated by setting the third argument of the callback function to FTW_NS. Otherwise it is set according to the description given in the account of __ftw_func_t above.

The callback function is expected to return 0 to indicate that no error occurred and that processing should continue. If an error occurred in the callback function or it wants ftw to return immediately, the callback function can return a value other than 0. This is the only correct way to stop the function. The program must not use setjmp or similar techniques to continue from another place. This would leave resources allocated by the ftw function unfreed.

The descriptors parameter to ftw specifies how many file descriptors it is allowed to consume. The function runs faster the more descriptors it can use. For each level in the directory hierarchy at most one descriptor is used, but for very deep ones any limit on open file descriptors for the process or the system may be exceeded. Moreover, file descriptor limits in a multi-threaded program apply to all the threads as a group, and therefore it is a good idea to supply a reasonable limit to the number of open descriptors.

The return value of the ftw function is 0 if all callback function calls returned 0 and all actions performed by the ftw succeeded. If a function call failed (other than calling stat on an item) the function returns -1. If a callback function returns a value other than 0 this value is returned as the return value of ftw.

When the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32-bit system this function is in fact ftw64, i.e., the LFS interface transparently replaces the old interface.

— Function: int ftw64 (const char *filename, __ftw64_func_t func, int descriptors)

This function is similar to ftw but it can work on filesystems with large files. File information is reported using a variable of type struct stat64 which is passed by reference to the callback function.

When the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32-bit system this function is available under the name ftw and transparently replaces the old implementation.

— Function: int nftw (const char *filename, __nftw_func_t func, int descriptors, int flag)

The nftw function works like the ftw functions. They call the callback function func for all items found in the directory filename and below. At most descriptors file descriptors are consumed during the nftw call.

One difference is that the callback function is of a different type. It is of type struct FTW * and provides the callback function with the extra information described above.

A second difference is that nftw takes a fourth argument, which is 0 or a bitwise-OR combination of any of the following values.

FTW_PHYS
While traversing the directory symbolic links are not followed. Instead symbolic links are reported using the FTW_SL value for the type parameter to the callback function. If the file referenced by a symbolic link does not exist FTW_SLN is returned instead.
FTW_MOUNT
The callback function is only called for items which are on the same mounted filesystem as the directory given by the filename parameter to nftw.
FTW_CHDIR
If this flag is given the current working directory is changed to the directory of the reported object before the callback function is called. When ntfw finally returns the current directory is restored to its original value.
FTW_DEPTH
If this option is specified then all subdirectories and files within them are processed before processing the top directory itself (depth-first processing). This also means the type flag given to the callback function is FTW_DP and not FTW_D.
FTW_ACTIONRETVAL
If this option is specified then return values from callbacks are handled differently. If the callback returns FTW_CONTINUE, walking continues normally. FTW_STOP means walking stops and FTW_STOP is returned to the caller. If FTW_SKIP_SUBTREE is returned by the callback with FTW_D argument, the subtree is skipped and walking continues with next sibling of the directory. If FTW_SKIP_SIBLINGS is returned by the callback, all siblings of the current entry are skipped and walking continues in its parent. No other return values should be returned from the callbacks if this option is set. This option is a GNU extension.

The return value is computed in the same way as for ftw. nftw returns 0 if no failures occurred and all callback functions returned 0. In case of internal errors, such as memory problems, the return value is -1 and errno is set accordingly. If the return value of a callback invocation was non-zero then that value is returned.

When the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32-bit system this function is in fact nftw64, i.e., the LFS interface transparently replaces the old interface.

— Function: int nftw64 (const char *filename, __nftw64_func_t func, int descriptors, int flag)

This function is similar to nftw but it can work on filesystems with large files. File information is reported using a variable of type struct stat64 which is passed by reference to the callback function.

When the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32-bit system this function is available under the name nftw and transparently replaces the old implementation.

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14.4 Hard Links

In POSIX systems, one file can have many names at the same time. All of the names are equally real, and no one of them is preferred to the others.

To add a name to a file, use the link function. (The new name is also called a hard link to the file.) Creating a new link to a file does not copy the contents of the file; it simply makes a new name by which the file can be known, in addition to the file's existing name or names.

One file can have names in several directories, so the organization of the file system is not a strict hierarchy or tree.

In most implementations, it is not possible to have hard links to the same file in multiple file systems. link reports an error if you try to make a hard link to the file from another file system when this cannot be done.

The prototype for the link function is declared in the header file unistd.h.

— Function: int link (const char *oldname, const char *newname)

The link function makes a new link to the existing file named by oldname, under the new name newname.

This function returns a value of 0 if it is successful and -1 on failure. In addition to the usual file name errors (see File Name Errors) for both oldname and newname, the following errno error conditions are defined for this function:

EACCES
You are not allowed to write to the directory in which the new link is to be written.
EEXIST
There is already a file named newname. If you want to replace this link with a new link, you must remove the old link explicitly first.
EMLINK
There are already too many links to the file named by oldname. (The maximum number of links to a file is LINK_MAX; see Limits for Files.)
ENOENT
The file named by oldname doesn't exist. You can't make a link to a file that doesn't exist.
ENOSPC
The directory or file system that would contain the new link is full and cannot be extended.
EPERM
In the GNU system and some others, you cannot make links to directories. Many systems allow only privileged users to do so. This error is used to report the problem.
EROFS
The directory containing the new link can't be modified because it's on a read-only file system.
EXDEV
The directory specified in newname is on a different file system than the existing file.
EIO
A hardware error occurred while trying to read or write the to filesystem.

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14.5 Symbolic Links

The GNU system supports soft links or symbolic links. This is a kind of “file” that is essentially a pointer to another file name. Unlike hard links, symbolic links can be made to directories or across file systems with no restrictions. You can also make a symbolic link to a name which is not the name of any file. (Opening this link will fail until a file by that name is created.) Likewise, if the symbolic link points to an existing file which is later deleted, the symbolic link continues to point to the same file name even though the name no longer names any file.

The reason symbolic links work the way they do is that special things happen when you try to open the link. The open function realizes you have specified the name of a link, reads the file name contained in the link, and opens that file name instead. The stat function likewise operates on the file that the symbolic link points to, instead of on the link itself.

By contrast, other operations such as deleting or renaming the file operate on the link itself. The functions readlink and lstat also refrain from following symbolic links, because their purpose is to obtain information about the link. link, the function that makes a hard link, does too. It makes a hard link to the symbolic link, which one rarely wants.

Some systems have for some functions operating on files have a limit on how many symbolic links are followed when resolving a path name. The limit if it exists is published in the sys/param.h header file.

— Macro: int MAXSYMLINKS

The macro MAXSYMLINKS specifies how many symlinks some function will follow before returning ELOOP. Not all functions behave the same and this value is not the same a that returned for _SC_SYMLOOP by sysconf. In fact, the sysconf result can indicate that there is no fixed limit although MAXSYMLINKS exists and has a finite value.

Prototypes for most of the functions listed in this section are in unistd.h.

— Function: int symlink (const char *oldname, const char *newname)

The symlink function makes a symbolic link to oldname named newname.

The normal return value from symlink is 0. A return value of -1 indicates an error. In addition to the usual file name syntax errors (see File Name Errors), the following errno error conditions are defined for this function:

EEXIST
There is already an existing file named newname.
EROFS
The file newname would exist on a read-only file system.
ENOSPC
The directory or file system cannot be extended to make the new link.
EIO
A hardware error occurred while reading or writing data on the disk.

— Function: int readlink (const char *filename, char *buffer, size_t size)

The readlink function gets the value of the symbolic link filename. The file name that the link points to is copied into buffer. This file name string is not null-terminated; readlink normally returns the number of characters copied. The size argument specifies the maximum number of characters to copy, usually the allocation size of buffer.

If the return value equals size, you cannot tell whether or not there was room to return the entire name. So make a bigger buffer and call readlink again. Here is an example: 

          char *
          readlink_malloc (const char *filename)
          {
            int size = 100;
            char *buffer = NULL;
          
            while (1)
              {
                buffer = (char *) xrealloc (buffer, size);
                int nchars = readlink (filename, buffer, size);
                if (nchars < 0)
                  {
                    free (buffer);
                    return NULL;
                  }
                if (nchars < size)
                  return buffer;
                size *= 2;
              }
          }

A value of -1 is returned in case of error. In addition to the usual file name errors (see File Name Errors), the following errno error conditions are defined for this function:

EINVAL
The named file is not a symbolic link.
EIO
A hardware error occurred while reading or writing data on the disk.

In some situations it is desirable to resolve all the symbolic links to get the real name of a file where no prefix names a symbolic link which is followed and no filename in the path is . or ... This is for instance desirable if files have to be compare in which case different names can refer to the same inode.

— Function: char * canonicalize_file_name (const char *name)

The canonicalize_file_name function returns the absolute name of the file named by name which contains no ., .. components nor any repeated path separators (/) or symlinks. The result is passed back as the return value of the function in a block of memory allocated with malloc. If the result is not used anymore the memory should be freed with a call to free.

If any of the path components is missing the function returns a NULL pointer. This is also what is returned if the length of the path reaches or exceeds PATH_MAX characters. In any case errno is set accordingly.

ENAMETOOLONG
The resulting path is too long. This error only occurs on systems which have a limit on the file name length.
EACCES
At least one of the path components is not readable.
ENOENT
The input file name is empty.
ENOENT
At least one of the path components does not exist.
ELOOP
More than MAXSYMLINKS many symlinks have been followed.

This function is a GNU extension and is declared in stdlib.h.

The Unix standard includes a similar function which differs from canonicalize_file_name in that the user has to provide the buffer where the result is placed in.

— Function: char * realpath (const char *restrict name, char *restrict resolved)

A call to realpath where the resolved parameter is NULL behaves exactly like canonicalize_file_name. The function allocates a buffer for the file name and returns a pointer to it. If resolved is not NULL it points to a buffer into which the result is copied. It is the callers responsibility to allocate a buffer which is large enough. On systems which define PATH_MAX this means the buffer must be large enough for a pathname of this size. For systems without limitations on the pathname length the requirement cannot be met and programs should not call realpath with anything but NULL for the second parameter.

One other difference is that the buffer resolved (if nonzero) will contain the part of the path component which does not exist or is not readable if the function returns NULL and errno is set to EACCES or ENOENT.

This function is declared in stdlib.h.

The advantage of using this function is that it is more widely available. The drawback is that it reports failures for long path on systems which have no limits on the file name length.

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14.6 Deleting Files

You can delete a file with unlink or remove.

Deletion actually deletes a file name. If this is the file's only name, then the file is deleted as well. If the file has other remaining names (see Hard Links), it remains accessible under those names.

— Function: int unlink (const char *filename)

The unlink function deletes the file name filename. If this is a file's sole name, the file itself is also deleted. (Actually, if any process has the file open when this happens, deletion is postponed until all processes have closed the file.)

The function unlink is declared in the header file unistd.h.

This function returns 0 on successful completion, and -1 on error. In addition to the usual file name errors (see File Name Errors), the following errno error conditions are defined for this function:

EACCES
Write permission is denied for the directory from which the file is to be removed, or the directory has the sticky bit set and you do not own the file.
EBUSY
This error indicates that the file is being used by the system in such a way that it can't be unlinked. For example, you might see this error if the file name specifies the root directory or a mount point for a file system.
ENOENT
The file name to be deleted doesn't exist.
EPERM
On some systems unlink cannot be used to delete the name of a directory, or at least can only be used this way by a privileged user. To avoid such problems, use rmdir to delete directories. (In the GNU system unlink can never delete the name of a directory.)
EROFS
The directory containing the file name to be deleted is on a read-only file system and can't be modified.

— Function: int rmdir (const char *filename)

The rmdir function deletes a directory. The directory must be empty before it can be removed; in other words, it can only contain entries for . and ...

In most other respects, rmdir behaves like unlink. There are two additional errno error conditions defined for rmdir:

ENOTEMPTY
EEXIST
The directory to be deleted is not empty.

These two error codes are synonymous; some systems use one, and some use the other. The GNU system always uses ENOTEMPTY.

The prototype for this function is declared in the header file unistd.h.

— Function: int remove (const char *filename)

This is the ISO C function to remove a file. It works like unlink for files and like rmdir for directories. remove is declared in stdio.h.

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14.7 Renaming Files

The rename function is used to change a file's name.

— Function: int rename (const char *oldname, const char *newname)

The rename function renames the file oldname to newname. The file formerly accessible under the name oldname is afterwards accessible as newname instead. (If the file had any other names aside from oldname, it continues to have those names.)

The directory containing the name newname must be on the same file system as the directory containing the name oldname.

One special case for rename is when oldname and newname are two names for the same file. The consistent way to handle this case is to delete oldname. However, in this case POSIX requires that rename do nothing and report success—which is inconsistent. We don't know what your operating system will do.

If oldname is not a directory, then any existing file named newname is removed during the renaming operation. However, if newname is the name of a directory, rename fails in this case.

If oldname is a directory, then either newname must not exist or it must name a directory that is empty. In the latter case, the existing directory named newname is deleted first. The name newname must not specify a subdirectory of the directory oldname which is being renamed.

One useful feature of rename is that the meaning of newname changes “atomically” from any previously existing file by that name to its new meaning (i.e., the file that was called oldname). There is no instant at which newname is non-existent “in between” the old meaning and the new meaning. If there is a system crash during the operation, it is possible for both names to still exist; but newname will always be intact if it exists at all.

If rename fails, it returns -1. In addition to the usual file name errors (see File Name Errors), the following errno error conditions are defined for this function:

EACCES
One of the directories containing newname or oldname refuses write permission; or newname and oldname are directories and write permission is refused for one of them.
EBUSY
A directory named by oldname or newname is being used by the system in a way that prevents the renaming from working. This includes directories that are mount points for filesystems, and directories that are the current working directories of processes.
ENOTEMPTY
EEXIST
The directory newname isn't empty. The GNU system always returns ENOTEMPTY for this, but some other systems return EEXIST.
EINVAL
oldname is a directory that contains newname.
EISDIR
newname is a directory but the oldname isn't.
EMLINK
The parent directory of newname would have too many links (entries).
ENOENT
The file oldname doesn't exist.
ENOSPC
The directory that would contain newname has no room for another entry, and there is no space left in the file system to expand it.
EROFS
The operation would involve writing to a directory on a read-only file system.
EXDEV
The two file names newname and oldname are on different file systems.

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14.8 Creating Directories

Directories are created with the mkdir function. (There is also a shell command mkdir which does the same thing.)

— Function: int mkdir (const char *filename, mode_t mode)

The mkdir function creates a new, empty directory with name filename.

The argument mode specifies the file permissions for the new directory file. See Permission Bits, for more information about this.

A return value of 0 indicates successful completion, and -1 indicates failure. In addition to the usual file name syntax errors (see File Name Errors), the following errno error conditions are defined for this function:

EACCES
Write permission is denied for the parent directory in which the new directory is to be added.
EEXIST
A file named filename already exists.
EMLINK
The parent directory has too many links (entries).

Well-designed file systems never report this error, because they permit more links than your disk could possibly hold. However, you must still take account of the possibility of this error, as it could result from network access to a file system on another machine.

ENOSPC
The file system doesn't have enough room to create the new directory.
EROFS
The parent directory of the directory being created is on a read-only file system and cannot be modified.

To use this function, your program should include the header file sys/stat.h.

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14.9 File Attributes

When you issue an `ls -l' shell command on a file, it gives you information about the size of the file, who owns it, when it was last modified, etc. These are called the file attributes, and are associated with the file itself and not a particular one of its names.

This section contains information about how you can inquire about and modify the attributes of a file.

Next: , Up: File Attributes

14.9.1 The meaning of the File Attributes

When you read the attributes of a file, they come back in a structure called struct stat. This section describes the names of the attributes, their data types, and what they mean. For the functions to read the attributes of a file, see Reading Attributes.

The header file sys/stat.h declares all the symbols defined in this section.

— Data Type: struct stat

The stat structure type is used to return information about the attributes of a file. It contains at least the following members:

mode_t st_mode
Specifies the mode of the file. This includes file type information (see Testing File Type) and the file permission bits (see Permission Bits).
ino_t st_ino
The file serial number, which distinguishes this file from all other files on the same device.
dev_t st_dev
Identifies the device containing the file. The st_ino and st_dev, taken together, uniquely identify the file. The st_dev value is not necessarily consistent across reboots or system crashes, however.
nlink_t st_nlink
The number of hard links to the file. This count keeps track of how many directories have entries for this file. If the count is ever decremented to zero, then the file itself is discarded as soon as no process still holds it open. Symbolic links are not counted in the total.
uid_t st_uid
The user ID of the file's owner. See File Owner.
gid_t st_gid
The group ID of the file. See File Owner.
off_t st_size
This specifies the size of a regular file in bytes. For files that are really devices this field isn't usually meaningful. For symbolic links this specifies the length of the file name the link refers to.
time_t st_atime
This is the last access time for the file. See File Times.
unsigned long int st_atime_usec
This is the fractional part of the last access time for the file. See File Times.
time_t st_mtime
This is the time of the last modification to the contents of the file. See File Times.
unsigned long int st_mtime_usec
This is the fractional part of the time of the last modification to the contents of the file. See File Times.
time_t st_ctime
This is the time of the last modification to the attributes of the file. See File Times.
unsigned long int st_ctime_usec
This is the fractional part of the time of the last modification to the attributes of the file. See File Times.
blkcnt_t st_blocks
This is the amount of disk space that the file occupies, measured in units of 512-byte blocks.

The number of disk blocks is not strictly proportional to the size of the file, for two reasons: the file system may use some blocks for internal record keeping; and the file may be sparse—it may have “holes” which contain zeros but do not actually take up space on the disk.

You can tell (approximately) whether a file is sparse by comparing this value with st_size, like this: 

               (st.st_blocks * 512 < st.st_size)

This test is not perfect because a file that is just slightly sparse might not be detected as sparse at all. For practical applications, this is not a problem.

unsigned int st_blksize
The optimal block size for reading of writing this file, in bytes. You might use this size for allocating the buffer space for reading of writing the file. (This is unrelated to st_blocks.)

The extensions for the Large File Support (LFS) require, even on 32-bit machines, types which can handle file sizes up to 2^63. Therefore a new definition of struct stat is necessary.

— Data Type: struct stat64

The members of this type are the same and have the same names as those in struct stat. The only difference is that the members st_ino, st_size, and st_blocks have a different type to support larger values.

mode_t st_mode
Specifies the mode of the file. This includes file type information (see Testing File Type) and the file permission bits (see Permission Bits).
ino64_t st_ino
The file serial number, which distinguishes this file from all other files on the same device.
dev_t st_dev
Identifies the device containing the file. The st_ino and st_dev, taken together, uniquely identify the file. The st_dev value is not necessarily consistent across reboots or system crashes, however.
nlink_t st_nlink
The number of hard links to the file. This count keeps track of how many directories have entries for this file. If the count is ever decremented to zero, then the file itself is discarded as soon as no process still holds it open. Symbolic links are not counted in the total.
uid_t st_uid
The user ID of the file's owner. See File Owner.
gid_t st_gid
The group ID of the file. See File Owner.
off64_t st_size
This specifies the size of a regular file in bytes. For files that are really devices this field isn't usually meaningful. For symbolic links this specifies the length of the file name the link refers to.
time_t st_atime
This is the last access time for the file. See File Times.
unsigned long int st_atime_usec
This is the fractional part of the last access time for the file. See File Times.
time_t st_mtime
This is the time of the last modification to the contents of the file. See File Times.
unsigned long int st_mtime_usec
This is the fractional part of the time of the last modification to the contents of the file. See File Times.
time_t st_ctime
This is the time of the last modification to the attributes of the file. See File Times.
unsigned long int st_ctime_usec
This is the fractional part of the time of the last modification to the attributes of the file. See File Times.
blkcnt64_t st_blocks
This is the amount of disk space that the file occupies, measured in units of 512-byte blocks.
unsigned int st_blksize
The optimal block size for reading of writing this file, in bytes. You might use this size for allocating the buffer space for reading of writing the file. (This is unrelated to st_blocks.)

Some of the file attributes have special data type names which exist specifically for those attributes. (They are all aliases for well-known integer types that you know and love.) These typedef names are defined in the header file sys/types.h as well as in sys/stat.h. Here is a list of them.

— Data Type: mode_t

This is an integer data type used to represent file modes. In the GNU system, this is equivalent to unsigned int.

— Data Type: ino_t

This is an arithmetic data type used to represent file serial numbers. (In Unix jargon, these are sometimes called inode numbers.) In the GNU system, this type is equivalent to unsigned long int.

If the source is compiled with _FILE_OFFSET_BITS == 64 this type is transparently replaced by ino64_t.

— Data Type: ino64_t

This is an arithmetic data type used to represent file serial numbers for the use in LFS. In the GNU system, this type is equivalent to unsigned long long int.

When compiling with _FILE_OFFSET_BITS == 64 this type is available under the name ino_t.

— Data Type: dev_t

This is an arithmetic data type used to represent file device numbers. In the GNU system, this is equivalent to int.

— Data Type: nlink_t

This is an arithmetic data type used to represent file link counts. In the GNU system, this is equivalent to unsigned short int.

— Data Type: blkcnt_t

This is an arithmetic data type used to represent block counts. In the GNU system, this is equivalent to unsigned long int.

If the source is compiled with _FILE_OFFSET_BITS == 64 this type is transparently replaced by blkcnt64_t.

— Data Type: blkcnt64_t

This is an arithmetic data type used to represent block counts for the use in LFS. In the GNU system, this is equivalent to unsigned long long int.

When compiling with _FILE_OFFSET_BITS == 64 this type is available under the name blkcnt_t.

Next: , Previous: Attribute Meanings, Up: File Attributes

14.9.2 Reading the Attributes of a File

To examine the attributes of files, use the functions stat, fstat and lstat. They return the attribute information in a struct stat object. All three functions are declared in the header file sys/stat.h.

— Function: int stat (const char *filename, struct stat *buf)

The stat function returns information about the attributes of the file named by filename in the structure pointed to by buf.

If filename is the name of a symbolic link, the attributes you get describe the file that the link points to. If the link points to a nonexistent file name, then stat fails reporting a nonexistent file.

The return value is 0 if the operation is successful, or -1 on failure. In addition to the usual file name errors (see File Name Errors, the following errno error conditions are defined for this function:

ENOENT
The file named by filename doesn't exist.

When the sources are compiled with _FILE_OFFSET_BITS == 64 this function is in fact stat64 since the LFS interface transparently replaces the normal implementation.

— Function: int stat64 (const char *filename, struct stat64 *buf)

This function is similar to stat but it is also able to work on files larger then 2^31 bytes on 32-bit systems. To be able to do this the result is stored in a variable of type struct stat64 to which buf must point.

When the sources are compiled with _FILE_OFFSET_BITS == 64 this function is available under the name stat and so transparently replaces the interface for small files on 32-bit machines.

— Function: int fstat (int filedes, struct stat *buf)

The fstat function is like stat, except that it takes an open file descriptor as an argument instead of a file name. See Low-Level I/O.

Like stat, fstat returns 0 on success and -1 on failure. The following errno error conditions are defined for fstat:

EBADF
The filedes argument is not a valid file descriptor.

When the sources are compiled with _FILE_OFFSET_BITS == 64 this function is in fact fstat64 since the LFS interface transparently replaces the normal implementation.

— Function: int fstat64 (int filedes, struct stat64 *buf)

This function is similar to fstat but is able to work on large files on 32-bit platforms. For large files the file descriptor filedes should be obtained by open64 or creat64. The buf pointer points to a variable of type struct stat64 which is able to represent the larger values.

When the sources are compiled with _FILE_OFFSET_BITS == 64 this function is available under the name fstat and so transparently replaces the interface for small files on 32-bit machines.

— Function: int lstat (const char *filename, struct stat *buf)

The lstat function is like stat, except that it does not follow symbolic links. If filename is the name of a symbolic link, lstat returns information about the link itself; otherwise lstat works like stat. See Symbolic Links.

When the sources are compiled with _FILE_OFFSET_BITS == 64 this function is in fact lstat64 since the LFS interface transparently replaces the normal implementation.

— Function: int lstat64 (const char *filename, struct stat64 *buf)

This function is similar to lstat but it is also able to work on files larger then 2^31 bytes on 32-bit systems. To be able to do this the result is stored in a variable of type struct stat64 to which buf must point.

When the sources are compiled with _FILE_OFFSET_BITS == 64 this function is available under the name lstat and so transparently replaces the interface for small files on 32-bit machines.

Next: , Previous: Reading Attributes, Up: File Attributes

14.9.3 Testing the Type of a File

The file mode, stored in the st_mode field of the file attributes, contains two kinds of information: the file type code, and the access permission bits. This section discusses only the type code, which you can use to tell whether the file is a directory, socket, symbolic link, and so on. For details about access permissions see Permission Bits.

There are two ways you can access the file type information in a file mode. Firstly, for each file type there is a predicate macro which examines a given file mode and returns whether it is of that type or not. Secondly, you can mask out the rest of the file mode to leave just the file type code, and compare this against constants for each of the supported file types.

All of the symbols listed in this section are defined in the header file sys/stat.h. The following predicate macros test the type of a file, given the value m which is the st_mode field returned by stat on that file:

— Macro: int S_ISDIR (mode_t m)

This macro returns non-zero if the file is a directory.

— Macro: int S_ISCHR (mode_t m)

This macro returns non-zero if the file is a character special file (a device like a terminal).

— Macro: int S_ISBLK (mode_t m)

This macro returns non-zero if the file is a block special file (a device like a disk).

— Macro: int S_ISREG (mode_t m)

This macro returns non-zero if the file is a regular file.

— Macro: int S_ISFIFO (mode_t m)

This macro returns non-zero if the file is a FIFO special file, or a pipe. See Pipes and FIFOs.

— Macro: int S_ISLNK (mode_t m)

This macro returns non-zero if the file is a symbolic link. See Symbolic Links.

— Macro: int S_ISSOCK (mode_t m)

This macro returns non-zero if the file is a socket. See Sockets.

An alternate non-POSIX method of testing the file type is supported for compatibility with BSD. The mode can be bitwise AND-ed with S_IFMT to extract the file type code, and compared to the appropriate constant. For example, 

     S_ISCHR (mode)

is equivalent to: 

     ((mode & S_IFMT) == S_IFCHR)
— Macro: int S_IFMT

This is a bit mask used to extract the file type code from a mode value.

These are the symbolic names for the different file type codes:

S_IFDIR
This is the file type constant of a directory file.
S_IFCHR
This is the file type constant of a character-oriented device file.
S_IFBLK
This is the file type constant of a block-oriented device file.
S_IFREG
This is the file type constant of a regular file.
S_IFLNK
This is the file type constant of a symbolic link.
S_IFSOCK
This is the file type constant of a socket.
S_IFIFO
This is the file type constant of a FIFO or pipe.

The POSIX.1b standard introduced a few more objects which possibly can be implemented as object in the filesystem. These are message queues, semaphores, and shared memory objects. To allow differentiating these objects from other files the POSIX standard introduces three new test macros. But unlike the other macros it does not take the value of the st_mode field as the parameter. Instead they expect a pointer to the whole struct stat structure.

— Macro: int S_TYPEISMQ (struct stat *s)

If the system implement POSIX message queues as distinct objects and the file is a message queue object, this macro returns a non-zero value. In all other cases the result is zero.

— Macro: int S_TYPEISSEM (struct stat *s)

If the system implement POSIX semaphores as distinct objects and the file is a semaphore object, this macro returns a non-zero value. In all other cases the result is zero.

— Macro: int S_TYPEISSHM (struct stat *s)

If the system implement POSIX shared memory objects as distinct objects and the file is an shared memory object, this macro returns a non-zero value. In all other cases the result is zero.

Next: , Previous: Testing File Type, Up: File Attributes

14.9.4 File Owner

Every file has an owner which is one of the registered user names defined on the system. Each file also has a group which is one of the defined groups. The file owner can often be useful for showing you who edited the file (especially when you edit with GNU Emacs), but its main purpose is for access control.

The file owner and group play a role in determining access because the file has one set of access permission bits for the owner, another set that applies to users who belong to the file's group, and a third set of bits that applies to everyone else. See Access Permission, for the details of how access is decided based on this data.

When a file is created, its owner is set to the effective user ID of the process that creates it (see Process Persona). The file's group ID may be set to either the effective group ID of the process, or the group ID of the directory that contains the file, depending on the system where the file is stored. When you access a remote file system, it behaves according to its own rules, not according to the system your program is running on. Thus, your program must be prepared to encounter either kind of behavior no matter what kind of system you run it on.

You can change the owner and/or group owner of an existing file using the chown function. This is the primitive for the chown and chgrp shell commands.

The prototype for this function is declared in unistd.h.

— Function: int chown (const char *filename, uid_t owner, gid_t group)

The chown function changes the owner of the file filename to owner, and its group owner to group.

Changing the owner of the file on certain systems clears the set-user-ID and set-group-ID permission bits. (This is because those bits may not be appropriate for the new owner.) Other file permission bits are not changed.

The return value is 0 on success and -1 on failure. In addition to the usual file name errors (see File Name Errors), the following errno error conditions are defined for this function:

EPERM
This process lacks permission to make the requested change.

Only privileged users or the file's owner can change the file's group. On most file systems, only privileged users can change the file owner; some file systems allow you to change the owner if you are currently the owner. When you access a remote file system, the behavior you encounter is determined by the system that actually holds the file, not by the system your program is running on.

See Options for Files, for information about the _POSIX_CHOWN_RESTRICTED macro.

EROFS
The file is on a read-only file system.

— Function: int fchown (int filedes, int owner, int group)

This is like chown, except that it changes the owner of the open file with descriptor filedes.

The return value from fchown is 0 on success and -1 on failure. The following errno error codes are defined for this function:

EBADF
The filedes argument is not a valid file descriptor.
EINVAL
The filedes argument corresponds to a pipe or socket, not an ordinary file.
EPERM
This process lacks permission to make the requested change. For details see chmod above.
EROFS
The file resides on a read-only file system.

Next: , Previous: File Owner, Up: File Attributes

14.9.5 The Mode Bits for Access Permission

The file mode, stored in the st_mode field of the file attributes, contains two kinds of information: the file type code, and the access permission bits. This section discusses only the access permission bits, which control who can read or write the file. See Testing File Type, for information about the file type code.

All of the symbols listed in this section are defined in the header file sys/stat.h. These symbolic constants are defined for the file mode bits that control access permission for the file:

S_IRUSR
S_IREAD
Read permission bit for the owner of the file. On many systems this bit is 0400. S_IREAD is an obsolete synonym provided for BSD compatibility.
S_IWUSR
S_IWRITE
Write permission bit for the owner of the file. Usually 0200. S_IWRITE is an obsolete synonym provided for BSD compatibility.
S_IXUSR
S_IEXEC
Execute (for ordinary files) or search (for directories) permission bit for the owner of the file. Usually 0100. S_IEXEC is an obsolete synonym provided for BSD compatibility.
S_IRWXU
This is equivalent to `(S_IRUSR | S_IWUSR | S_IXUSR)'.
S_IRGRP
Read permission bit for the group owner of the file. Usually 040.
S_IWGRP
Write permission bit for the group owner of the file. Usually 020.
S_IXGRP
Execute or search permission bit for the group owner of the file. Usually 010.
S_IRWXG
This is equivalent to `(S_IRGRP | S_IWGRP | S_IXGRP)'.
S_IROTH
Read permission bit for other users. Usually 04.
S_IWOTH
Write permission bit for other users. Usually 02.
S_IXOTH
Execute or search permission bit for other users. Usually 01.
S_IRWXO
This is equivalent to `(S_IROTH | S_IWOTH | S_IXOTH)'.
S_ISUID
This is the set-user-ID on execute bit, usually 04000. See How Change Persona.
S_ISGID
This is the set-group-ID on execute bit, usually 02000. See How Change Persona.


S_ISVTX
This is the sticky bit, usually 01000.

For a directory it gives permission to delete a file in that directory only if you own that file. Ordinarily, a user can either delete all the files in a directory or cannot delete any of them (based on whether the user has write permission for the directory). The same restriction applies—you must have both write permission for the directory and own the file you want to delete. The one exception is that the owner of the directory can delete any file in the directory, no matter who owns it (provided the owner has given himself write permission for the directory). This is commonly used for the /tmp directory, where anyone may create files but not delete files created by other users.

Originally the sticky bit on an executable file modified the swapping policies of the system. Normally, when a program terminated, its pages in core were immediately freed and reused. If the sticky bit was set on the executable file, the system kept the pages in core for a while as if the program were still running. This was advantageous for a program likely to be run many times in succession. This usage is obsolete in modern systems. When a program terminates, its pages always remain in core as long as there is no shortage of memory in the system. When the program is next run, its pages will still be in core if no shortage arose since the last run.

On some modern systems where the sticky bit has no useful meaning for an executable file, you cannot set the bit at all for a non-directory. If you try, chmod fails with EFTYPE; see Setting Permissions.

Some systems (particularly SunOS) have yet another use for the sticky bit. If the sticky bit is set on a file that is not executable, it means the opposite: never cache the pages of this file at all. The main use of this is for the files on an NFS server machine which are used as the swap area of diskless client machines. The idea is that the pages of the file will be cached in the client's memory, so it is a waste of the server's memory to cache them a second time. With this usage the sticky bit also implies that the filesystem may fail to record the file's modification time onto disk reliably (the idea being that no-one cares for a swap file).

This bit is only available on BSD systems (and those derived from them). Therefore one has to use the _BSD_SOURCE feature select macro to get the definition (see Feature Test Macros).

The actual bit values of the symbols are listed in the table above so you can decode file mode values when debugging your programs. These bit values are correct for most systems, but they are not guaranteed.

Warning: Writing explicit numbers for file permissions is bad practice. Not only is it not portable, it also requires everyone who reads your program to remember what the bits mean. To make your program clean use the symbolic names.

Next: , Previous: Permission Bits, Up: File Attributes

14.9.6 How Your Access to a File is Decided

Recall that the operating system normally decides access permission for a file based on the effective user and group IDs of the process and its supplementary group IDs, together with the file's owner, group and permission bits. These concepts are discussed in detail in Process Persona.

If the effective user ID of the process matches the owner user ID of the file, then permissions for read, write, and execute/search are controlled by the corresponding “user” (or “owner”) bits. Likewise, if any of the effective group ID or supplementary group IDs of the process matches the group owner ID of the file, then permissions are controlled by the “group” bits. Otherwise, permissions are controlled by the “other” bits.

Privileged users, like `root', can access any file regardless of its permission bits. As a special case, for a file to be executable even by a privileged user, at least one of its execute bits must be set.

Next: , Previous: Access Permission, Up: File Attributes

14.9.7 Assigning File Permissions

The primitive functions for creating files (for example, open or mkdir) take a mode argument, which specifies the file permissions to give the newly created file. This mode is modified by the process's file creation mask, or umask, before it is used.

The bits that are set in the file creation mask identify permissions that are always to be disabled for newly created files. For example, if you set all the “other” access bits in the mask, then newly created files are not accessible at all to processes in the “other” category, even if the mode argument passed to the create function would permit such access. In other words, the file creation mask is the complement of the ordinary access permissions you want to grant.

Programs that create files typically specify a mode argument that includes all the permissions that make sense for the particular file. For an ordinary file, this is typically read and write permission for all classes of users. These permissions are then restricted as specified by the individual user's own file creation mask.

To change the permission of an existing file given its name, call chmod. This function uses the specified permission bits and ignores the file creation mask.

In normal use, the file creation mask is initialized by the user's login shell (using the umask shell command), and inherited by all subprocesses. Application programs normally don't need to worry about the file creation mask. It will automatically do what it is supposed to do.

When your program needs to create a file and bypass the umask for its access permissions, the easiest way to do this is to use fchmod after opening the file, rather than changing the umask. In fact, changing the umask is usually done only by shells. They use the umask function.

The functions in this section are declared in sys/stat.h.

— Function: mode_t umask (mode_t mask)

The umask function sets the file creation mask of the current process to mask, and returns the previous value of the file creation mask.

Here is an example showing how to read the mask with umask without changing it permanently: 

          mode_t
          read_umask (void)
          {
            mode_t mask = umask (0);
            umask (mask);
            return mask;
          }

However, it is better to use getumask if you just want to read the mask value, because it is reentrant (at least if you use the GNU operating system).

— Function: mode_t getumask (void)

Return the current value of the file creation mask for the current process. This function is a GNU extension.

— Function: int chmod (const char *filename, mode_t mode)

The chmod function sets the access permission bits for the file named by filename to mode.

If filename is a symbolic link, chmod changes the permissions of the file pointed to by the link, not those of the link itself.

This function returns 0 if successful and -1 if not. In addition to the usual file name errors (see File Name Errors), the following errno error conditions are defined for this function:

ENOENT
The named file doesn't exist.
EPERM
This process does not have permission to change the access permissions of this file. Only the file's owner (as judged by the effective user ID of the process) or a privileged user can change them.
EROFS
The file resides on a read-only file system.
EFTYPE
mode has the S_ISVTX bit (the “sticky bit”) set, and the named file is not a directory. Some systems do not allow setting the sticky bit on non-directory files, and some do (and only some of those assign a useful meaning to the bit for non-directory files).

You only get EFTYPE on systems where the sticky bit has no useful meaning for non-directory files, so it is always safe to just clear the bit in mode and call chmod again. See Permission Bits, for full details on the sticky bit.

— Function: int fchmod (int filedes, int mode)

This is like chmod, except that it changes the permissions of the currently open file given by filedes.

The return value from fchmod is 0 on success and -1 on failure. The following errno error codes are defined for this function:

EBADF
The filedes argument is not a valid file descriptor.
EINVAL
The filedes argument corresponds to a pipe or socket, or something else that doesn't really have access permissions.
EPERM
This process does not have permission to change the access permissions of this file. Only the file's owner (as judged by the effective user ID of the process) or a privileged user can change them.
EROFS
The file resides on a read-only file system.

Next: , Previous: Setting Permissions, Up: File Attributes

14.9.8 Testing Permission to Access a File

In some situations it is desirable to allow programs to access files or devices even if this is not possible with the permissions granted to the user. One possible solution is to set the setuid-bit of the program file. If such a program is started the effective user ID of the process is changed to that of the owner of the program file. So to allow write access to files like /etc/passwd, which normally can be written only by the super-user, the modifying program will have to be owned by root and the setuid-bit must be set.

But beside the files the program is intended to change the user should not be allowed to access any file to which s/he would not have access anyway. The program therefore must explicitly check whether the user would have the necessary access to a file, before it reads or writes the file.

To do this, use the function access, which checks for access permission based on the process's real user ID rather than the effective user ID. (The setuid feature does not alter the real user ID, so it reflects the user who actually ran the program.)

There is another way you could check this access, which is easy to describe, but very hard to use. This is to examine the file mode bits and mimic the system's own access computation. This method is undesirable because many systems have additional access control features; your program cannot portably mimic them, and you would not want to try to keep track of the diverse features that different systems have. Using access is simple and automatically does whatever is appropriate for the system you are using.

access is only only appropriate to use in setuid programs. A non-setuid program will always use the effective ID rather than the real ID.

The symbols in this section are declared in unistd.h.

— Function: int access (const char *filename, int how)

The access function checks to see whether the file named by filename can be accessed in the way specified by the how argument. The how argument either can be the bitwise OR of the flags R_OK, W_OK, X_OK, or the existence test F_OK.

This function uses the real user and group IDs of the calling process, rather than the effective IDs, to check for access permission. As a result, if you use the function from a setuid or setgid program (see How Change Persona), it gives information relative to the user who actually ran the program.

The return value is 0 if the access is permitted, and -1 otherwise. (In other words, treated as a predicate function, access returns true if the requested access is denied.)

In addition to the usual file name errors (see File Name Errors), the following errno error conditions are defined for this function:

EACCES
The access specified by how is denied.
ENOENT
The file doesn't exist.
EROFS
Write permission was requested for a file on a read-only file system.

These macros are defined in the header file unistd.h for use as the how argument to the access function. The values are integer constants.

— Macro: int R_OK

Flag meaning test for read permission.

— Macro: int W_OK

Flag meaning test for write permission.

— Macro: int X_OK

Flag meaning test for execute/search permission.

— Macro: int F_OK

Flag meaning test for existence of the file.

Next: , Previous: Testing File Access, Up: File Attributes

14.9.9 File Times

Each file has three time stamps associated with it: its access time, its modification time, and its attribute modification time. These correspond to the st_atime, st_mtime, and st_ctime members of the stat structure; see File Attributes.

All of these times are represented in calendar time format, as time_t objects. This data type is defined in time.h. For more information about representation and manipulation of time values, see Calendar Time. Reading from a file updates its access time attribute, and writing updates its modification time. When a file is created, all three time stamps for that file are set to the current time. In addition, the attribute change time and modification time fields of the directory that contains the new entry are updated.

Adding a new name for a file with the link function updates the attribute change time field of the file being linked, and both the attribute change time and modification time fields of the directory containing the new name. These same fields are affected if a file name is deleted with unlink, remove or rmdir. Renaming a file with rename affects only the attribute change time and modification time fields of the two parent directories involved, and not the times for the file being renamed.

Changing the attributes of a file (for example, with chmod) updates its attribute change time field.

You can also change some of the time stamps of a file explicitly using the utime function—all except the attribute change time. You need to include the header file utime.h to use this facility.

— Data Type: struct utimbuf

The utimbuf structure is used with the utime function to specify new access and modification times for a file. It contains the following members:

time_t actime
This is the access time for the file.
time_t modtime
This is the modification time for the file.

— Function: int utime (const char *filename, const struct utimbuf *times)

This function is used to modify the file times associated with the file named filename.

If times is a null pointer, then the access and modification times of the file are set to the current time. Otherwise, they are set to the values from the actime and modtime members (respectively) of the utimbuf structure pointed to by times.

The attribute modification time for the file is set to the current time in either case (since changing the time stamps is itself a modification of the file attributes).

The utime function returns 0 if successful and -1 on failure. In addition to the usual file name errors (see File Name Errors), the following errno error conditions are defined for this function:

EACCES
There is a permission problem in the case where a null pointer was passed as the times argument. In order to update the time stamp on the file, you must either be the owner of the file, have write permission for the file, or be a privileged user.
ENOENT
The file doesn't exist.
EPERM
If the times argument is not a null pointer, you must either be the owner of the file or be a privileged user.
EROFS
The file lives on a read-only file system.

Each of the three time stamps has a corresponding microsecond part, which extends its resolution. These fields are called st_atime_usec, st_mtime_usec, and st_ctime_usec; each has a value between 0 and 999,999, which indicates the time in microseconds. They correspond to the tv_usec field of a timeval structure; see High-Resolution Calendar.

The utimes function is like utime, but also lets you specify the fractional part of the file times. The prototype for this function is in the header file sys/time.h.

— Function: int utimes (const char *filename, struct timeval tvp[2])

This function sets the file access and modification times of the file filename. The new file access time is specified by tvp[0], and the new modification time by tvp[1]. Similar to utime, if tvp is a null pointer then the access and modification times of the file are set to the current time. This function comes from BSD.

The return values and error conditions are the same as for the utime function.

— Function: int lutimes (const char *filename, struct timeval tvp[2])

This function is like utimes, except that it does not follow symbolic links. If filename is the name of a symbolic link, lutimes sets the file access and modification times of the symbolic link special file itself (as seen by lstat; see Symbolic Links) while utimes sets the file access and modification times of the file the symbolic link refers to. This function comes from FreeBSD, and is not available on all platforms (if not available, it will fail with ENOSYS).

The return values and error conditions are the same as for the utime function.

— Function: int futimes (int fd, struct timeval tvp[2])

This function is like utimes, except that it takes an open file descriptor as an argument instead of a file name. See Low-Level I/O. This function comes from FreeBSD, and is not available on all platforms (if not available, it will fail with ENOSYS).

Like utimes, futimes returns 0 on success and -1 on failure. The following errno error conditions are defined for futimes:

EACCES
There is a permission problem in the case where a null pointer was passed as the times argument. In order to update the time stamp on the file, you must either be the owner of the file, have write permission for the file, or be a privileged user.
EBADF
The filedes argument is not a valid file descriptor.
EPERM
If the times argument is not a null pointer, you must either be the owner of the file or be a privileged user.
EROFS
The file lives on a read-only file system.

Previous: File Times, Up: File Attributes

14.9.10 File Size

Normally file sizes are maintained automatically. A file begins with a size of 0 and is automatically extended when data is written past its end. It is also possible to empty a file completely by an open or fopen call.

However, sometimes it is necessary to reduce the size of a file. This can be done with the truncate and ftruncate functions. They were introduced in BSD Unix. ftruncate was later added to POSIX.1.

Some systems allow you to extend a file (creating holes) with these functions. This is useful when using memory-mapped I/O (see Memory-mapped I/O), where files are not automatically extended. However, it is not portable but must be implemented if mmap allows mapping of files (i.e., _POSIX_MAPPED_FILES is defined).

Using these functions on anything other than a regular file gives undefined results. On many systems, such a call will appear to succeed, without actually accomplishing anything.

— Function: int truncate (const char *filename, off_t length)

The truncate function changes the size of filename to length. If length is shorter than the previous length, data at the end will be lost. The file must be writable by the user to perform this operation.

If length is longer, holes will be added to the end. However, some systems do not support this feature and will leave the file unchanged.

When the source file is compiled with _FILE_OFFSET_BITS == 64 the truncate function is in fact truncate64 and the type off_t has 64 bits which makes it possible to handle files up to 2^63 bytes in length.

The return value is 0 for success, or -1 for an error. In addition to the usual file name errors, the following errors may occur:

EACCES
The file is a directory or not writable.
EINVAL
length is negative.
EFBIG
The operation would extend the file beyond the limits of the operating system.
EIO
A hardware I/O error occurred.
EPERM
The file is "append-only" or "immutable".
EINTR
The operation was interrupted by a signal.
— Function: int truncate64 (const char *name, off64_t length)

This function is similar to the truncate function. The difference is that the length argument is 64 bits wide even on 32 bits machines, which allows the handling of files with sizes up to 2^63 bytes.

When the source file is compiled with _FILE_OFFSET_BITS == 64 on a 32 bits machine this function is actually available under the name truncate and so transparently replaces the 32 bits interface.

— Function: int ftruncate (int fd, off_t length)

This is like truncate, but it works on a file descriptor fd for an opened file instead of a file name to identify the object. The file must be opened for writing to successfully carry out the operation.

The POSIX standard leaves it implementation defined what happens if the specified new length of the file is bigger than the original size. The ftruncate function might simply leave the file alone and do nothing or it can increase the size to the desired size. In this later case the extended area should be zero-filled. So using ftruncate is no reliable way to increase the file size but if it is possible it is probably the fastest way. The function also operates on POSIX shared memory segments if these are implemented by the system.

ftruncate is especially useful in combination with mmap. Since the mapped region must have a fixed size one cannot enlarge the file by writing something beyond the last mapped page. Instead one has to enlarge the file itself and then remap the file with the new size. The example below shows how this works.

When the source file is compiled with _FILE_OFFSET_BITS == 64 the ftruncate function is in fact ftruncate64 and the type off_t has 64 bits which makes it possible to handle files up to 2^63 bytes in length.

The return value is 0 for success, or -1 for an error. The following errors may occur:

EBADF
fd does not correspond to an open file.
EACCES
fd is a directory or not open for writing.
EINVAL
length is negative.
EFBIG
The operation would extend the file beyond the limits of the operating system.
EIO
A hardware I/O error occurred.
EPERM
The file is "append-only" or "immutable".
EINTR
The operation was interrupted by a signal.
— Function: int ftruncate64 (int id, off64_t length)

This function is similar to the ftruncate function. The difference is that the length argument is 64 bits wide even on 32 bits machines which allows the handling of files with sizes up to 2^63 bytes.

When the source file is compiled with _FILE_OFFSET_BITS == 64 on a 32 bits machine this function is actually available under the name ftruncate and so transparently replaces the 32 bits interface.

As announced here is a little example of how to use ftruncate in combination with mmap: 

     int fd;
     void *start;
     size_t len;
     
     int
     add (off_t at, void *block, size_t size)
     {
       if (at + size > len)
         {
           /* Resize the file and remap.  */
           size_t ps = sysconf (_SC_PAGESIZE);
           size_t ns = (at + size + ps - 1) & ~(ps - 1);
           void *np;
           if (ftruncate (fd, ns) < 0)
             return -1;
           np = mmap (NULL, ns, PROT_READ|PROT_WRITE, MAP_SHARED, fd, 0);
           if (np == MAP_FAILED)
             return -1;
           start = np;
           len = ns;
         }
       memcpy ((char *) start + at, block, size);
       return 0;
     }

The function add writes a block of memory at an arbitrary position in the file. If the current size of the file is too small it is extended. Note the it is extended by a round number of pages. This is a requirement of mmap. The program has to keep track of the real size, and when it has finished a final ftruncate call should set the real size of the file.

Next: , Previous: File Attributes, Up: File System Interface

14.10 Making Special Files

The mknod function is the primitive for making special files, such as files that correspond to devices. The GNU library includes this function for compatibility with BSD.

The prototype for mknod is declared in sys/stat.h.

— Function: int mknod (const char *filename, int mode, int dev)

The mknod function makes a special file with name filename. The mode specifies the mode of the file, and may include the various special file bits, such as S_IFCHR (for a character special file) or S_IFBLK (for a block special file). See Testing File Type.

The dev argument specifies which device the special file refers to. Its exact interpretation depends on the kind of special file being created.

The return value is 0 on success and -1 on error. In addition to the usual file name errors (see File Name Errors), the following errno error conditions are defined for this function:

EPERM
The calling process is not privileged. Only the superuser can create special files.
ENOSPC
The directory or file system that would contain the new file is full and cannot be extended.
EROFS
The directory containing the new file can't be modified because it's on a read-only file system.
EEXIST
There is already a file named filename. If you want to replace this file, you must remove the old file explicitly first.

Previous: Making Special Files, Up: File System Interface

14.11 Temporary Files

If you need to use a temporary file in your program, you can use the tmpfile function to open it. Or you can use the tmpnam (better: tmpnam_r) function to provide a name for a temporary file and then you can open it in the usual way with fopen.

The tempnam function is like tmpnam but lets you choose what directory temporary files will go in, and something about what their file names will look like. Important for multi-threaded programs is that tempnam is reentrant, while tmpnam is not since it returns a pointer to a static buffer.

These facilities are declared in the header file stdio.h.

— Function: FILE * tmpfile (void)

This function creates a temporary binary file for update mode, as if by calling fopen with mode "wb+". The file is deleted automatically when it is closed or when the program terminates. (On some other ISO C systems the file may fail to be deleted if the program terminates abnormally).

This function is reentrant.

When the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32-bit system this function is in fact tmpfile64, i.e., the LFS interface transparently replaces the old interface.

— Function: FILE * tmpfile64 (void)

This function is similar to tmpfile, but the stream it returns a pointer to was opened using tmpfile64. Therefore this stream can be used for files larger then 2^31 bytes on 32-bit machines.

Please note that the return type is still FILE *. There is no special FILE type for the LFS interface.

If the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32 bits machine this function is available under the name tmpfile and so transparently replaces the old interface.

— Function: char * tmpnam (char *result)

This function constructs and returns a valid file name that does not refer to any existing file. If the result argument is a null pointer, the return value is a pointer to an internal static string, which might be modified by subsequent calls and therefore makes this function non-reentrant. Otherwise, the result argument should be a pointer to an array of at least L_tmpnam characters, and the result is written into that array.

It is possible for tmpnam to fail if you call it too many times without removing previously-created files. This is because the limited length of the temporary file names gives room for only a finite number of different names. If tmpnam fails it returns a null pointer.

Warning: Between the time the pathname is constructed and the file is created another process might have created a file with the same name using tmpnam, leading to a possible security hole. The implementation generates names which can hardly be predicted, but when opening the file you should use the O_EXCL flag. Using tmpfile or mkstemp is a safe way to avoid this problem.

— Function: char * tmpnam_r (char *result)

This function is nearly identical to the tmpnam function, except that if result is a null pointer it returns a null pointer.

This guarantees reentrancy because the non-reentrant situation of tmpnam cannot happen here.

Warning: This function has the same security problems as tmpnam.

— Macro: int L_tmpnam

The value of this macro is an integer constant expression that represents the minimum size of a string large enough to hold a file name generated by the tmpnam function.

— Macro: int TMP_MAX

The macro TMP_MAX is a lower bound for how many temporary names you can create with tmpnam. You can rely on being able to call tmpnam at least this many times before it might fail saying you have made too many temporary file names.

With the GNU library, you can create a very large number of temporary file names. If you actually created the files, you would probably run out of disk space before you ran out of names. Some other systems have a fixed, small limit on the number of temporary files. The limit is never less than 25.

— Function: char * tempnam (const char *dir, const char *prefix)

This function generates a unique temporary file name. If prefix is not a null pointer, up to five characters of this string are used as a prefix for the file name. The return value is a string newly allocated with malloc, so you should release its storage with free when it is no longer needed.

Because the string is dynamically allocated this function is reentrant.

The directory prefix for the temporary file name is determined by testing each of the following in sequence. The directory must exist and be writable.

This function is defined for SVID compatibility.

Warning: Between the time the pathname is constructed and the file is created another process might have created a file with the same name using tempnam, leading to a possible security hole. The implementation generates names which can hardly be predicted, but when opening the file you should use the O_EXCL flag. Using tmpfile or mkstemp is a safe way to avoid this problem.

— SVID Macro: char * P_tmpdir

This macro is the name of the default directory for temporary files.

Older Unix systems did not have the functions just described. Instead they used mktemp and mkstemp. Both of these functions work by modifying a file name template string you pass. The last six characters of this string must be `XXXXXX'. These six `X's are replaced with six characters which make the whole string a unique file name. Usually the template string is something like `/tmp/prefixXXXXXX', and each program uses a unique prefix.

Note: Because mktemp and mkstemp modify the template string, you must not pass string constants to them. String constants are normally in read-only storage, so your program would crash when mktemp or mkstemp tried to modify the string. These functions are declared in the header file stdlib.h.

— Function: char * mktemp (char *template)

The mktemp function generates a unique file name by modifying template as described above. If successful, it returns template as modified. If mktemp cannot find a unique file name, it makes template an empty string and returns that. If template does not end with `XXXXXX', mktemp returns a null pointer.

Warning: Between the time the pathname is constructed and the file is created another process might have created a file with the same name using mktemp, leading to a possible security hole. The implementation generates names which can hardly be predicted, but when opening the file you should use the O_EXCL flag. Using mkstemp is a safe way to avoid this problem.

— Function: int mkstemp (char *template)

The mkstemp function generates a unique file name just as mktemp does, but it also opens the file for you with open (see Opening and Closing Files). If successful, it modifies template in place and returns a file descriptor for that file open for reading and writing. If mkstemp cannot create a uniquely-named file, it returns -1. If template does not end with `XXXXXX', mkstemp returns -1 and does not modify template.

The file is opened using mode 0600. If the file is meant to be used by other users this mode must be changed explicitly.

Unlike mktemp, mkstemp is actually guaranteed to create a unique file that cannot possibly clash with any other program trying to create a temporary file. This is because it works by calling open with the O_EXCL flag, which says you want to create a new file and get an error if the file already exists.

— Function: char * mkdtemp (char *template)

The mkdtemp function creates a directory with a unique name. If it succeeds, it overwrites template with the name of the directory, and returns template. As with mktemp and mkstemp, template should be a string ending with `XXXXXX'.

If mkdtemp cannot create an uniquely named directory, it returns NULL and sets errno appropriately. If template does not end with `XXXXXX', mkdtemp returns NULL and does not modify template. errno will be set to EINVAL in this case.

The directory is created using mode 0700.

The directory created by mkdtemp cannot clash with temporary files or directories created by other users. This is because directory creation always works like open with O_EXCL. See Creating Directories.

The mkdtemp function comes from OpenBSD.

Next: , Previous: File System Interface, Up: Top

15 Pipes and FIFOs

A pipe is a mechanism for interprocess communication; data written to the pipe by one process can be read by another process. The data is handled in a first-in, first-out (FIFO) order. The pipe has no name; it is created for one use and both ends must be inherited from the single process which created the pipe.

A FIFO special file is similar to a pipe, but instead of being an anonymous, temporary connection, a FIFO has a name or names like any other file. Processes open the FIFO by name in order to communicate through it.

A pipe or FIFO has to be open at both ends simultaneously. If you read from a pipe or FIFO file that doesn't have any processes writing to it (perhaps because they have all closed the file, or exited), the read returns end-of-file. Writing to a pipe or FIFO that doesn't have a reading process is treated as an error condition; it generates a SIGPIPE signal, and fails with error code EPIPE if the signal is handled or blocked.

Neither pipes nor FIFO special files allow file positioning. Both reading and writing operations happen sequentially; reading from the beginning of the file and writing at the end.

Next: , Up: Pipes and FIFOs

15.1 Creating a Pipe

The primitive for creating a pipe is the pipe function. This creates both the reading and writing ends of the pipe. It is not very useful for a single process to use a pipe to talk to itself. In typical use, a process creates a pipe just before it forks one or more child processes (see Creating a Process). The pipe is then used for communication either between the parent or child processes, or between two sibling processes.

The pipe function is declared in the header file unistd.h.

— Function: int pipe (int filedes[2])

The pipe function creates a pipe and puts the file descriptors for the reading and writing ends of the pipe (respectively) into filedes[0] and filedes[1].

An easy way to remember that the input end comes first is that file descriptor 0 is standard input, and file descriptor 1 is standard output.

If successful, pipe returns a value of 0. On failure, -1 is returned. The following errno error conditions are defined for this function:

EMFILE
The process has too many files open.
ENFILE
There are too many open files in the entire system. See Error Codes, for more information about ENFILE. This error never occurs in the GNU system.

Here is an example of a simple program that creates a pipe. This program uses the fork function (see Creating a Process) to create a child process. The parent process writes data to the pipe, which is read by the child process. 

     #include <sys/types.h>
     #include <unistd.h>
     #include <stdio.h>
     #include <stdlib.h>
     
     /* Read characters from the pipe and echo them to stdout. */
     
     void
     read_from_pipe (int file)
     {
       FILE *stream;
       int c;
       stream = fdopen (file, "r");
       while ((c = fgetc (stream)) != EOF)
         putchar (c);
       fclose (stream);
     }
     
     /* Write some random text to the pipe. */
     
     void
     write_to_pipe (int file)
     {
       FILE *stream;
       stream = fdopen (file, "w");
       fprintf (stream, "hello, world!\n");
       fprintf (stream, "goodbye, world!\n");
       fclose (stream);
     }
     
     int
     main (void)
     {
       pid_t pid;
       int mypipe[2];
     
       /* Create the pipe. */
       if (pipe (mypipe))
         {
           fprintf (stderr, "Pipe failed.\n");
           return EXIT_FAILURE;
         }
     
       /* Create the child process. */
       pid = fork ();
       if (pid == (pid_t) 0)
         {
           /* This is the child process.
              Close other end first. */
           close (mypipe[1]);
           read_from_pipe (mypipe[0]);
           return EXIT_SUCCESS;
         }
       else if (pid < (pid_t) 0)
         {
           /* The fork failed. */
           fprintf (stderr, "Fork failed.\n");
           return EXIT_FAILURE;
         }
       else
         {
           /* This is the parent process.
              Close other end first. */
           close (mypipe[0]);
           write_to_pipe (mypipe[1]);
           return EXIT_SUCCESS;
         }
     }

Next: , Previous: Creating a Pipe, Up: Pipes and FIFOs

15.2 Pipe to a Subprocess

A common use of pipes is to send data to or receive data from a program being run as a subprocess. One way of doing this is by using a combination of pipe (to create the pipe), fork (to create the subprocess), dup2 (to force the subprocess to use the pipe as its standard input or output channel), and exec (to execute the new program). Or, you can use popen and pclose.

The advantage of using popen and pclose is that the interface is much simpler and easier to use. But it doesn't offer as much flexibility as using the low-level functions directly.

— Function: FILE * popen (const char *command, const char *mode)

The popen function is closely related to the system function; see Running a Command. It executes the shell command command as a subprocess. However, instead of waiting for the command to complete, it creates a pipe to the subprocess and returns a stream that corresponds to that pipe.

If you specify a mode argument of "r", you can read from the stream to retrieve data from the standard output channel of the subprocess. The subprocess inherits its standard input channel from the parent process.

Similarly, if you specify a mode argument of "w", you can write to the stream to send data to the standard input channel of the subprocess. The subprocess inherits its standard output channel from the parent process.

In the event of an error popen returns a null pointer. This might happen if the pipe or stream cannot be created, if the subprocess cannot be forked, or if the program cannot be executed.

— Function: int pclose (FILE *stream)

The pclose function is used to close a stream created by popen. It waits for the child process to terminate and returns its status value, as for the system function.

Here is an example showing how to use popen and pclose to filter output through another program, in this case the paging program more. 

     #include <stdio.h>
     #include <stdlib.h>
     
     void
     write_data (FILE * stream)
     {
       int i;
       for (i = 0; i < 100; i++)
         fprintf (stream, "%d\n", i);
       if (ferror (stream))
         {
           fprintf (stderr, "Output to stream failed.\n");
           exit (EXIT_FAILURE);
         }
     }
     
     int
     main (void)
     {
       FILE *output;
     
       output = popen ("more", "w");
       if (!output)
         {
           fprintf (stderr,
                    "incorrect parameters or too many files.\n");
           return EXIT_FAILURE;
         }
       write_data (output);
       if (pclose (output) != 0)
         {
           fprintf (stderr,
                    "Could not run more or other error.\n");
         }
       return EXIT_SUCCESS;
     }

Next: , Previous: Pipe to a Subprocess, Up: Pipes and FIFOs

15.3 FIFO Special Files

A FIFO special file is similar to a pipe, except that it is created in a different way. Instead of being an anonymous communications channel, a FIFO special file is entered into the file system by calling mkfifo.

Once you have created a FIFO special file in this way, any process can open it for reading or writing, in the same way as an ordinary file. However, it has to be open at both ends simultaneously before you can proceed to do any input or output operations on it. Opening a FIFO for reading normally blocks until some other process opens the same FIFO for writing, and vice versa.

The mkfifo function is declared in the header file sys/stat.h.

— Function: int mkfifo (const char *filename, mode_t mode)

The mkfifo function makes a FIFO special file with name filename. The mode argument is used to set the file's permissions; see Setting Permissions.

The normal, successful return value from mkfifo is 0. In the case of an error, -1 is returned. In addition to the usual file name errors (see File Name Errors), the following errno error conditions are defined for this function:

EEXIST
The named file already exists.
ENOSPC
The directory or file system cannot be extended.
EROFS
The directory that would contain the file resides on a read-only file system.

Previous: FIFO Special Files, Up: Pipes and FIFOs

15.4 Atomicity of Pipe I/O

Reading or writing pipe data is atomic if the size of data written is not greater than PIPE_BUF. This means that the data transfer seems to be an instantaneous unit, in that nothing else in the system can observe a state in which it is partially complete. Atomic I/O may not begin right away (it may need to wait for buffer space or for data), but once it does begin it finishes immediately.

Reading or writing a larger amount of data may not be atomic; for example, output data from other processes sharing the descriptor may be interspersed. Also, once PIPE_BUF characters have been written, further writes will block until some characters are read.

See Limits for Files, for information about the PIPE_BUF parameter.

Next: , Previous: Pipes and FIFOs, Up: Top

16 Sockets

This chapter describes the GNU facilities for interprocess communication using sockets.

A socket is a generalized interprocess communication channel. Like a pipe, a socket is represented as a file descriptor. Unlike pipes sockets support communication between unrelated processes, and even between processes running on different machines that communicate over a network. Sockets are the primary means of communicating with other machines; telnet, rlogin, ftp, talk and the other familiar network programs use sockets.

Not all operating systems support sockets. In the GNU library, the header file sys/socket.h exists regardless of the operating system, and the socket functions always exist, but if the system does not really support sockets these functions always fail.

Incomplete: We do not currently document the facilities for broadcast messages or for configuring Internet interfaces. The reentrant functions and some newer functions that are related to IPv6 aren't documented either so far.

Next: , Up: Sockets

16.1 Socket Concepts

When you create a socket, you must specify the style of communication you want to use and the type of protocol that should implement it. The communication style of a socket defines the user-level semantics of sending and receiving data on the socket. Choosing a communication style specifies the answers to questions such as these:

You must also choose a namespace for naming the socket. A socket name (“address”) is meaningful only in the context of a particular namespace. In fact, even the data type to use for a socket name may depend on the namespace. Namespaces are also called “domains”, but we avoid that word as it can be confused with other usage of the same term. Each namespace has a symbolic name that starts with `PF_'. A corresponding symbolic name starting with `AF_' designates the address format for that namespace.

Finally you must choose the protocol to carry out the communication. The protocol determines what low-level mechanism is used to transmit and receive data. Each protocol is valid for a particular namespace and communication style; a namespace is sometimes called a protocol family because of this, which is why the namespace names start with `PF_'.

The rules of a protocol apply to the data passing between two programs, perhaps on different computers; most of these rules are handled by the operating system and you need not know about them. What you do need to know about protocols is this:

Throughout the following description at various places variables/parameters to denote sizes are required. And here the trouble starts. In the first implementations the type of these variables was simply int. On most machines at that time an int was 32 bits wide, which created a de facto standard requiring 32-bit variables. This is important since references to variables of this type are passed to the kernel.

Then the POSIX people came and unified the interface with the words "all size values are of type size_t". On 64-bit machines size_t is 64 bits wide, so pointers to variables were no longer possible.

The Unix98 specification provides a solution by introducing a type socklen_t. This type is used in all of the cases that POSIX changed to use size_t. The only requirement of this type is that it be an unsigned type of at least 32 bits. Therefore, implementations which require that references to 32-bit variables be passed can be as happy as implementations which use 64-bit values.

Next: , Previous: Socket Concepts, Up: Sockets

16.2 Communication Styles

The GNU library includes support for several different kinds of sockets, each with different characteristics. This section describes the supported socket types. The symbolic constants listed here are defined in sys/socket.h.

— Macro: int SOCK_STREAM

The SOCK_STREAM style is like a pipe (see Pipes and FIFOs). It operates over a connection with a particular remote socket and transmits data reliably as a stream of bytes.

Use of this style is covered in detail in Connections.

— Macro: int SOCK_DGRAM

The SOCK_DGRAM style is used for sending individually-addressed packets unreliably. It is the diametrical opposite of SOCK_STREAM.

Each time you write data to a socket of this kind, that data becomes one packet. Since SOCK_DGRAM sockets do not have connections, you must specify the recipient address with each packet.

The only guarantee that the system makes about your requests to transmit data is that it will try its best to deliver each packet you send. It may succeed with the sixth packet after failing with the fourth and fifth packets; the seventh packet may arrive before the sixth, and may arrive a second time after the sixth.

The typical use for SOCK_DGRAM is in situations where it is acceptable to simply re-send a packet if no response is seen in a reasonable amount of time.

See Datagrams, for detailed information about how to use datagram sockets.

— Macro: int SOCK_RAW

This style provides access to low-level network protocols and interfaces. Ordinary user programs usually have no need to use this style.

Next: , Previous: Communication Styles, Up: Sockets

16.3 Socket Addresses

The name of a socket is normally called an address. The functions and symbols for dealing with socket addresses were named inconsistently, sometimes using the term “name” and sometimes using “address”. You can regard these terms as synonymous where sockets are concerned.

A socket newly created with the socket function has no address. Other processes can find it for communication only if you give it an address. We call this binding the address to the socket, and the way to do it is with the bind function.

You need be concerned with the address of a socket if other processes are to find it and start communicating with it. You can specify an address for other sockets, but this is usually pointless; the first time you send data from a socket, or use it to initiate a connection, the system assigns an address automatically if you have not specified one.

Occasionally a client needs to specify an address because the server discriminates based on address; for example, the rsh and rlogin protocols look at the client's socket address and only bypass password checking if it is less than IPPORT_RESERVED (see Ports).

The details of socket addresses vary depending on what namespace you are using. See Local Namespace, or Internet Namespace, for specific information.

Regardless of the namespace, you use the same functions bind and getsockname to set and examine a socket's address. These functions use a phony data type, struct sockaddr *, to accept the address. In practice, the address lives in a structure of some other data type appropriate to the address format you are using, but you cast its address to struct sockaddr * when you pass it to bind.

Next: , Up: Socket Addresses

16.3.1 Address Formats

The functions bind and getsockname use the generic data type struct sockaddr * to represent a pointer to a socket address. You can't use this data type effectively to interpret an address or construct one; for that, you must use the proper data type for the socket's namespace.

Thus, the usual practice is to construct an address of the proper namespace-specific type, then cast a pointer to struct sockaddr * when you call bind or getsockname.

The one piece of information that you can get from the struct sockaddr data type is the address format designator. This tells you which data type to use to understand the address fully.

The symbols in this section are defined in the header file sys/socket.h.

— Data Type: struct sockaddr

The struct sockaddr type itself has the following members:

short int sa_family
This is the code for the address format of this address. It identifies the format of the data which follows.
char sa_data[14]
This is the actual socket address data, which is format-dependent. Its length also depends on the format, and may well be more than 14. The length 14 of sa_data is essentially arbitrary.

Each address format has a symbolic name which starts with `AF_'. Each of them corresponds to a `PF_' symbol which designates the corresponding namespace. Here is a list of address format names:

AF_LOCAL
This designates the address format that goes with the local namespace. (PF_LOCAL is the name of that namespace.) See Local Namespace Details, for information about this address format.
AF_UNIX
This is a synonym for AF_LOCAL. Although AF_LOCAL is mandated by POSIX.1g, AF_UNIX is portable to more systems. AF_UNIX was the traditional name stemming from BSD, so even most POSIX systems support it. It is also the name of choice in the Unix98 specification. (The same is true for PF_UNIX vs. PF_LOCAL).
AF_FILE
This is another synonym for AF_LOCAL, for compatibility. (PF_FILE is likewise a synonym for PF_LOCAL.)
AF_INET
This designates the address format that goes with the Internet namespace. (PF_INET is the name of that namespace.) See Internet Address Formats.
AF_INET6
This is similar to AF_INET, but refers to the IPv6 protocol. (PF_INET6 is the name of the corresponding namespace.)
AF_UNSPEC
This designates no particular address format. It is used only in rare cases, such as to clear out the default destination address of a “connected” datagram socket. See Sending Datagrams.

The corresponding namespace designator symbol PF_UNSPEC exists for completeness, but there is no reason to use it in a program.

sys/socket.h defines symbols starting with `AF_' for many different kinds of networks, most or all of which are not actually implemented. We will document those that really work as we receive information about how to use them.

Next: , Previous: Address Formats, Up: Socket Addresses

16.3.2 Setting the Address of a Socket

Use the bind function to assign an address to a socket. The prototype for bind is in the header file sys/socket.h. For examples of use, see Local Socket Example, or see Inet Example.

— Function: int bind (int socket, struct sockaddr *addr, socklen_t length)

The bind function assigns an address to the socket socket. The addr and length arguments specify the address; the detailed format of the address depends on the namespace. The first part of the address is always the format designator, which specifies a namespace, and says that the address is in the format of that namespace.

The return value is 0 on success and -1 on failure. The following errno error conditions are defined for this function:

EBADF
The socket argument is not a valid file descriptor.
ENOTSOCK
The descriptor socket is not a socket.
EADDRNOTAVAIL
The specified address is not available on this machine.
EADDRINUSE
Some other socket is already using the specified address.
EINVAL
The socket socket already has an address.
EACCES
You do not have permission to access the requested address. (In the Internet domain, only the super-user is allowed to specify a port number in the range 0 through IPPORT_RESERVED minus one; see Ports.)

Additional conditions may be possible depending on the particular namespace of the socket.

Previous: Setting Address, Up: Socket Addresses

16.3.3 Reading the Address of a Socket

Use the function getsockname to examine the address of an Internet socket. The prototype for this function is in the header file sys/socket.h.

— Function: int getsockname (int socket, struct sockaddr *addr, socklen_t *length-ptr)

The getsockname function returns information about the address of the socket socket in the locations specified by the addr and length-ptr arguments. Note that the length-ptr is a pointer; you should initialize it to be the allocation size of addr, and on return it contains the actual size of the address data.

The format of the address data depends on the socket namespace. The length of the information is usually fixed for a given namespace, so normally you can know exactly how much space is needed and can provide that much. The usual practice is to allocate a place for the value using the proper data type for the socket's namespace, then cast its address to struct sockaddr * to pass it to getsockname.

The return value is 0 on success and -1 on error. The following errno error conditions are defined for this function:

EBADF
The socket argument is not a valid file descriptor.
ENOTSOCK
The descriptor socket is not a socket.
ENOBUFS
There are not enough internal buffers available for the operation.

You can't read the address of a socket in the file namespace. This is consistent with the rest of the system; in general, there's no way to find a file's name from a descriptor for that file.

Next: , Previous: Socket Addresses, Up: Sockets

16.4 Interface Naming

Each network interface has a name. This usually consists of a few letters that relate to the type of interface, which may be followed by a number if there is more than one interface of that type. Examples might be lo (the loopback interface) and eth0 (the first Ethernet interface).

Although such names are convenient for humans, it would be clumsy to have to use them whenever a program needs to refer to an interface. In such situations an interface is referred to by its index, which is an arbitrarily-assigned small positive integer.

The following functions, constants and data types are declared in the header file net/if.h.

— Constant: size_t IFNAMSIZ

This constant defines the maximum buffer size needed to hold an interface name, including its terminating zero byte.

— Function: unsigned int if_nametoindex (const char *ifname)

This function yields the interface index corresponding to a particular name. If no interface exists with the name given, it returns 0.

— Function: char * if_indextoname (unsigned int ifindex, char *ifname)

This function maps an interface index to its corresponding name. The returned name is placed in the buffer pointed to by ifname, which must be at least IFNAMSIZ bytes in length. If the index was invalid, the function's return value is a null pointer, otherwise it is ifname.

— Data Type: struct if_nameindex

This data type is used to hold the information about a single interface. It has the following members:

unsigned int if_index;
This is the interface index.
char *if_name
This is the null-terminated index name.

— Function: struct if_nameindex * if_nameindex (void)

This function returns an array of if_nameindex structures, one for every interface that is present. The end of the list is indicated by a structure with an interface of 0 and a null name pointer. If an error occurs, this function returns a null pointer.

The returned structure must be freed with if_freenameindex after use.

— Function: void if_freenameindex (struct if_nameindex *ptr)

This function frees the structure returned by an earlier call to if_nameindex.

Next: , Previous: Interface Naming, Up: Sockets

16.5 The Local Namespace

This section describes the details of the local namespace, whose symbolic name (required when you create a socket) is PF_LOCAL. The local namespace is also known as “Unix domain sockets”. Another name is file namespace since socket addresses are normally implemented as file names.

Next: , Up: Local Namespace

16.5.1 Local Namespace Concepts

In the local namespace socket addresses are file names. You can specify any file name you want as the address of the socket, but you must have write permission on the directory containing it. It's common to put these files in the /tmp directory.

One peculiarity of the local namespace is that the name is only used when opening the connection; once open the address is not meaningful and may not exist.

Another peculiarity is that you cannot connect to such a socket from another machine–not even if the other machine shares the file system which contains the name of the socket. You can see the socket in a directory listing, but connecting to it never succeeds. Some programs take advantage of this, such as by asking the client to send its own process ID, and using the process IDs to distinguish between clients. However, we recommend you not use this method in protocols you design, as we might someday permit connections from other machines that mount the same file systems. Instead, send each new client an identifying number if you want it to have one.

After you close a socket in the local namespace, you should delete the file name from the file system. Use unlink or remove to do this; see Deleting Files.

The local namespace supports just one protocol for any communication style; it is protocol number 0.

Next: , Previous: Local Namespace Concepts, Up: Local Namespace

16.5.2 Details of Local Namespace

To create a socket in the local namespace, use the constant PF_LOCAL as the namespace argument to socket or socketpair. This constant is defined in sys/socket.h.

— Macro: int PF_LOCAL

This designates the local namespace, in which socket addresses are local names, and its associated family of protocols. PF_Local is the macro used by Posix.1g.

— Macro: int PF_UNIX

This is a synonym for PF_LOCAL, for compatibility's sake.

— Macro: int PF_FILE

This is a synonym for PF_LOCAL, for compatibility's sake.

The structure for specifying socket names in the local namespace is defined in the header file sys/un.h:

— Data Type: struct sockaddr_un

This structure is used to specify local namespace socket addresses. It has the following members:

short int sun_family
This identifies the address family or format of the socket address. You should store the value AF_LOCAL to designate the local namespace. See Socket Addresses.
char sun_path[108]
This is the file name to use.

Incomplete: Why is 108 a magic number? RMS suggests making this a zero-length array and tweaking the following example to use alloca to allocate an appropriate amount of storage based on the length of the filename.

You should compute the length parameter for a socket address in the local namespace as the sum of the size of the sun_family component and the string length (not the allocation size!) of the file name string. This can be done using the macro SUN_LEN:

— Macro: int SUN_LEN (struct sockaddr_un * ptr)

The macro computes the length of socket address in the local namespace.

Previous: Local Namespace Details, Up: Local Namespace

16.5.3 Example of Local-Namespace Sockets

Here is an example showing how to create and name a socket in the local namespace. 

     #include <stddef.h>
     #include <stdio.h>
     #include <errno.h>
     #include <stdlib.h>
     #include <string.h>
     #include <sys/socket.h>
     #include <sys/un.h>
     
     int
     make_named_socket (const char *filename)
     {
       struct sockaddr_un name;
       int sock;
       size_t size;
     
       /* Create the socket. */
       sock = socket (PF_LOCAL, SOCK_DGRAM, 0);
       if (sock < 0)
         {
           perror ("socket");
           exit (EXIT_FAILURE);
         }
     
       /* Bind a name to the socket. */
       name.sun_family = AF_LOCAL;
       strncpy (name.sun_path, filename, sizeof (name.sun_path));
       name.sun_path[sizeof (name.sun_path) - 1] = '\0';
     
       /* The size of the address is
          the offset of the start of the filename,
          plus its length,
          plus one for the terminating null byte.
          Alternatively you can just do:
          size = SUN_LEN (&name);
      */
       size = (offsetof (struct sockaddr_un, sun_path)
               + strlen (name.sun_path) + 1);
     
       if (bind (sock, (struct sockaddr *) &name, size) < 0)
         {
           perror ("bind");
           exit (EXIT_FAILURE);
         }
     
       return sock;
     }

Next: , Previous: Local Namespace, Up: Sockets

16.6 The Internet Namespace

This section describes the details of the protocols and socket naming conventions used in the Internet namespace.

Originally the Internet namespace used only IP version 4 (IPv4). With the growing number of hosts on the Internet, a new protocol with a larger address space was necessary: IP version 6 (IPv6). IPv6 introduces 128-bit addresses (IPv4 has 32-bit addresses) and other features, and will eventually replace IPv4.

To create a socket in the IPv4 Internet namespace, use the symbolic name PF_INET of this namespace as the namespace argument to socket or socketpair. For IPv6 addresses you need the macro PF_INET6. These macros are defined in sys/socket.h.

— Macro: int PF_INET

This designates the IPv4 Internet namespace and associated family of protocols.

— Macro: int PF_INET6

This designates the IPv6 Internet namespace and associated family of protocols.

A socket address for the Internet namespace includes the following components:

You must ensure that the address and port number are represented in a canonical format called network byte order. See Byte Order, for information about this.

Next: , Up: Internet Namespace

16.6.1 Internet Socket Address Formats

In the Internet namespace, for both IPv4 (AF_INET) and IPv6 (AF_INET6), a socket address consists of a host address and a port on that host. In addition, the protocol you choose serves effectively as a part of the address because local port numbers are meaningful only within a particular protocol.

The data types for representing socket addresses in the Internet namespace are defined in the header file netinet/in.h.

— Data Type: struct sockaddr_in

This is the data type used to represent socket addresses in the Internet namespace. It has the following members:

sa_family_t sin_family
This identifies the address family or format of the socket address. You should store the value AF_INET in this member. See Socket Addresses.
struct in_addr sin_addr
This is the Internet address of the host machine. See Host Addresses, and Host Names, for how to get a value to store here.
unsigned short int sin_port
This is the port number. See Ports.

When you call bind or getsockname, you should specify sizeof (struct sockaddr_in) as the length parameter if you are using an IPv4 Internet namespace socket address.

— Data Type: struct sockaddr_in6

This is the data type used to represent socket addresses in the IPv6 namespace. It has the following members:

sa_family_t sin6_family
This identifies the address family or format of the socket address. You should store the value of AF_INET6 in this member. See Socket Addresses.
struct in6_addr sin6_addr
This is the IPv6 address of the host machine. See Host Addresses, and Host Names, for how to get a value to store here.
uint32_t sin6_flowinfo
This is a currently unimplemented field.
uint16_t sin6_port
This is the port number. See Ports.

Next: , Previous: Internet Address Formats, Up: Internet Namespace

16.6.2 Host Addresses

Each computer on the Internet has one or more Internet addresses, numbers which identify that computer among all those on the Internet. Users typically write IPv4 numeric host addresses as sequences of four numbers, separated by periods, as in `128.52.46.32', and IPv6 numeric host addresses as sequences of up to eight numbers separated by colons, as in `5f03:1200:836f:c100::1'.

Each computer also has one or more host names, which are strings of words separated by periods, as in `mescaline.gnu.org'.

Programs that let the user specify a host typically accept both numeric addresses and host names. To open a connection a program needs a numeric address, and so must convert a host name to the numeric address it stands for.

Next: , Up: Host Addresses
16.6.2.1 Internet Host Addresses

An IPv4 Internet host address is a number containing four bytes of data. Historically these are divided into two parts, a network number and a local network address number within that network. In the mid-1990s classless addresses were introduced which changed this behavior. Since some functions implicitly expect the old definitions, we first describe the class-based network and will then describe classless addresses. IPv6 uses only classless addresses and therefore the following paragraphs don't apply.

The class-based IPv4 network number consists of the first one, two or three bytes; the rest of the bytes are the local address.

IPv4 network numbers are registered with the Network Information Center (NIC), and are divided into three classes—A, B and C. The local network address numbers of individual machines are registered with the administrator of the particular network.

Class A networks have single-byte numbers in the range 0 to 127. There are only a small number of Class A networks, but they can each support a very large number of hosts. Medium-sized Class B networks have two-byte network numbers, with the first byte in the range 128 to 191. Class C networks are the smallest; they have three-byte network numbers, with the first byte in the range 192-255. Thus, the first 1, 2, or 3 bytes of an Internet address specify a network. The remaining bytes of the Internet address specify the address within that network.

The Class A network 0 is reserved for broadcast to all networks. In addition, the host number 0 within each network is reserved for broadcast to all hosts in that network. These uses are obsolete now but for compatibility reasons you shouldn't use network 0 and host number 0.

The Class A network 127 is reserved for loopback; you can always use the Internet address `127.0.0.1' to refer to the host machine.

Since a single machine can be a member of multiple networks, it can have multiple Internet host addresses. However, there is never supposed to be more than one machine with the same host address.

There are four forms of the standard numbers-and-dots notation for Internet addresses:

a.b.c.d
This specifies all four bytes of the address individually and is the commonly used representation.
a.b.c
The last part of the address, c, is interpreted as a 2-byte quantity. This is useful for specifying host addresses in a Class B network with network address number a.b.
a.b
The last part of the address, b, is interpreted as a 3-byte quantity. This is useful for specifying host addresses in a Class A network with network address number a.
a
If only one part is given, this corresponds directly to the host address number.

Within each part of the address, the usual C conventions for specifying the radix apply. In other words, a leading `0x' or `0X' implies hexadecimal radix; a leading `0' implies octal; and otherwise decimal radix is assumed.

Classless Addresses

IPv4 addresses (and IPv6 addresses also) are now considered classless; the distinction between classes A, B and C can be ignored. Instead an IPv4 host address consists of a 32-bit address and a 32-bit mask. The mask contains set bits for the network part and cleared bits for the host part. The network part is contiguous from the left, with the remaining bits representing the host. As a consequence, the netmask can simply be specified as the number of set bits. Classes A, B and C are just special cases of this general rule. For example, class A addresses have a netmask of `255.0.0.0' or a prefix length of 8.

Classless IPv4 network addresses are written in numbers-and-dots notation with the prefix length appended and a slash as separator. For example the class A network 10 is written as `10.0.0.0/8'.

IPv6 Addresses

IPv6 addresses contain 128 bits (IPv4 has 32 bits) of data. A host address is usually written as eight 16-bit hexadecimal numbers that are separated by colons. Two colons are used to abbreviate strings of consecutive zeros. For example, the IPv6 loopback address `0:0:0:0:0:0:0:1' can just be written as `::1'.

Next: , Previous: Abstract Host Addresses, Up: Host Addresses
16.6.2.2 Host Address Data Type

IPv4 Internet host addresses are represented in some contexts as integers (type uint32_t). In other contexts, the integer is packaged inside a structure of type struct in_addr. It would be better if the usage were made consistent, but it is not hard to extract the integer from the structure or put the integer into a structure.

You will find older code that uses unsigned long int for IPv4 Internet host addresses instead of uint32_t or struct in_addr. Historically unsigned long int was a 32-bit number but with 64-bit machines this has changed. Using unsigned long int might break the code if it is used on machines where this type doesn't have 32 bits. uint32_t is specified by Unix98 and guaranteed to have 32 bits.

IPv6 Internet host addresses have 128 bits and are packaged inside a structure of type struct in6_addr.

The following basic definitions for Internet addresses are declared in the header file netinet/in.h:

— Data Type: struct in_addr

This data type is used in certain contexts to contain an IPv4 Internet host address. It has just one field, named s_addr, which records the host address number as an uint32_t.

— Macro: uint32_t INADDR_LOOPBACK

You can use this constant to stand for “the address of this machine,” instead of finding its actual address. It is the IPv4 Internet address `127.0.0.1', which is usually called `localhost'. This special constant saves you the trouble of looking up the address of your own machine. Also, the system usually implements INADDR_LOOPBACK specially, avoiding any network traffic for the case of one machine talking to itself.

— Macro: uint32_t INADDR_ANY

You can use this constant to stand for “any incoming address” when binding to an address. See Setting Address. This is the usual address to give in the sin_addr member of struct sockaddr_in when you want to accept Internet connections.

— Macro: uint32_t INADDR_BROADCAST

This constant is the address you use to send a broadcast message.

— Macro: uint32_t INADDR_NONE

This constant is returned by some functions to indicate an error.

— Data Type: struct in6_addr

This data type is used to store an IPv6 address. It stores 128 bits of data, which can be accessed (via a union) in a variety of ways.

— Constant: struct in6_addr in6addr_loopback

This constant is the IPv6 address `::1', the loopback address. See above for a description of what this means. The macro IN6ADDR_LOOPBACK_INIT is provided to allow you to initialize your own variables to this value.

— Constant: struct in6_addr in6addr_any

This constant is the IPv6 address `::', the unspecified address. See above for a description of what this means. The macro IN6ADDR_ANY_INIT is provided to allow you to initialize your own variables to this value.

Next: , Previous: Host Address Data Type, Up: Host Addresses
16.6.2.3 Host Address Functions

These additional functions for manipulating Internet addresses are declared in the header file arpa/inet.h. They represent Internet addresses in network byte order, and network numbers and local-address-within-network numbers in host byte order. See Byte Order, for an explanation of network and host byte order.

— Function: int inet_aton (const char *name, struct in_addr *addr)

This function converts the IPv4 Internet host address name from the standard numbers-and-dots notation into binary data and stores it in the struct in_addr that addr points to. inet_aton returns nonzero if the address is valid, zero if not.

— Function: uint32_t inet_addr (const char *name)

This function converts the IPv4 Internet host address name from the standard numbers-and-dots notation into binary data. If the input is not valid, inet_addr returns INADDR_NONE. This is an obsolete interface to inet_aton, described immediately above. It is obsolete because INADDR_NONE is a valid address (255.255.255.255), and inet_aton provides a cleaner way to indicate error return.

— Function: uint32_t inet_network (const char *name)

This function extracts the network number from the address name, given in the standard numbers-and-dots notation. The returned address is in host order. If the input is not valid, inet_network returns -1.

The function works only with traditional IPv4 class A, B and C network types. It doesn't work with classless addresses and shouldn't be used anymore.

— Function: char * inet_ntoa (struct in_addr addr)

This function converts the IPv4 Internet host address addr to a string in the standard numbers-and-dots notation. The return value is a pointer into a statically-allocated buffer. Subsequent calls will overwrite the same buffer, so you should copy the string if you need to save it.

In multi-threaded programs each thread has an own statically-allocated buffer. But still subsequent calls of inet_ntoa in the same thread will overwrite the result of the last call.

Instead of inet_ntoa the newer function inet_ntop which is described below should be used since it handles both IPv4 and IPv6 addresses.

— Function: struct in_addr inet_makeaddr (uint32_t net, uint32_t local)

This function makes an IPv4 Internet host address by combining the network number net with the local-address-within-network number local.

— Function: uint32_t inet_lnaof (struct in_addr addr)

This function returns the local-address-within-network part of the Internet host address addr.

The function works only with traditional IPv4 class A, B and C network types. It doesn't work with classless addresses and shouldn't be used anymore.

— Function: uint32_t inet_netof (struct in_addr addr)

This function returns the network number part of the Internet host address addr.

The function works only with traditional IPv4 class A, B and C network types. It doesn't work with classless addresses and shouldn't be used anymore.

— Function: int inet_pton (int af, const char *cp, void *buf)

This function converts an Internet address (either IPv4 or IPv6) from presentation (textual) to network (binary) format. af should be either AF_INET or AF_INET6, as appropriate for the type of address being converted. cp is a pointer to the input string, and buf is a pointer to a buffer for the result. It is the caller's responsibility to make sure the buffer is large enough.

— Function: const char * inet_ntop (int af, const void *cp, char *buf, size_t len)

This function converts an Internet address (either IPv4 or IPv6) from network (binary) to presentation (textual) form. af should be either AF_INET or AF_INET6, as appropriate. cp is a pointer to the address to be converted. buf should be a pointer to a buffer to hold the result, and len is the length of this buffer. The return value from the function will be this buffer address.

Previous: Host Address Functions, Up: Host Addresses
16.6.2.4 Host Names

Besides the standard numbers-and-dots notation for Internet addresses, you can also refer to a host by a symbolic name. The advantage of a symbolic name is that it is usually easier to remember. For example, the machine with Internet address `158.121.106.19' is also known as `alpha.gnu.org'; and other machines in the `gnu.org' domain can refer to it simply as `alpha'.

Internally, the system uses a database to keep track of the mapping between host names and host numbers. This database is usually either the file /etc/hosts or an equivalent provided by a name server. The functions and other symbols for accessing this database are declared in netdb.h. They are BSD features, defined unconditionally if you include netdb.h.

— Data Type: struct hostent

This data type is used to represent an entry in the hosts database. It has the following members:

char *h_name
This is the “official” name of the host.
char **h_aliases
These are alternative names for the host, represented as a null-terminated vector of strings.
int h_addrtype
This is the host address type; in practice, its value is always either AF_INET or AF_INET6, with the latter being used for IPv6 hosts. In principle other kinds of addresses could be represented in the database as well as Internet addresses; if this were done, you might find a value in this field other than AF_INET or AF_INET6. See Socket Addresses.
int h_length
This is the length, in bytes, of each address.
char **h_addr_list
This is the vector of addresses for the host. (Recall that the host might be connected to multiple networks and have different addresses on each one.) The vector is terminated by a null pointer.
char *h_addr
This is a synonym for h_addr_list[0]; in other words, it is the first host address.

As far as the host database is concerned, each address is just a block of memory h_length bytes long. But in other contexts there is an implicit assumption that you can convert IPv4 addresses to a struct in_addr or an uint32_t. Host addresses in a struct hostent structure are always given in network byte order; see Byte Order.

You can use gethostbyname, gethostbyname2 or gethostbyaddr to search the hosts database for information about a particular host. The information is returned in a statically-allocated structure; you must copy the information if you need to save it across calls. You can also use getaddrinfo and getnameinfo to obtain this information.

— Function: struct hostent * gethostbyname (const char *name)

The gethostbyname function returns information about the host named name. If the lookup fails, it returns a null pointer.

— Function: struct hostent * gethostbyname2 (const char *name, int af)

The gethostbyname2 function is like gethostbyname, but allows the caller to specify the desired address family (e.g. AF_INET or AF_INET6) of the result.

— Function: struct hostent * gethostbyaddr (const char *addr, size_t length, int format)

The gethostbyaddr function returns information about the host with Internet address addr. The parameter addr is not really a pointer to char - it can be a pointer to an IPv4 or an IPv6 address. The length argument is the size (in bytes) of the address at addr. format specifies the address format; for an IPv4 Internet address, specify a value of AF_INET; for an IPv6 Internet address, use AF_INET6.

If the lookup fails, gethostbyaddr returns a null pointer.

If the name lookup by gethostbyname or gethostbyaddr fails, you can find out the reason by looking at the value of the variable h_errno. (It would be cleaner design for these functions to set errno, but use of h_errno is compatible with other systems.)

Here are the error codes that you may find in h_errno:

HOST_NOT_FOUND
No such host is known in the database.
TRY_AGAIN
This condition happens when the name server could not be contacted. If you try again later, you may succeed then.
NO_RECOVERY
A non-recoverable error occurred.
NO_ADDRESS
The host database contains an entry for the name, but it doesn't have an associated Internet address.

The lookup functions above all have one in common: they are not reentrant and therefore unusable in multi-threaded applications. Therefore provides the GNU C library a new set of functions which can be used in this context.

— Function: int gethostbyname_r (const char *restrict name, struct hostent *restrict result_buf, char *restrict buf, size_t buflen, struct hostent **restrict result, int *restrict h_errnop)

The gethostbyname_r function returns information about the host named name. The caller must pass a pointer to an object of type struct hostent in the result_buf parameter. In addition the function may need extra buffer space and the caller must pass an pointer and the size of the buffer in the buf and buflen parameters.

A pointer to the buffer, in which the result is stored, is available in *result after the function call successfully returned. If an error occurs or if no entry is found, the pointer *result is a null pointer. Success is signalled by a zero return value. If the function failed the return value is an error number. In addition to the errors defined for gethostbyname it can also be ERANGE. In this case the call should be repeated with a larger buffer. Additional error information is not stored in the global variable h_errno but instead in the object pointed to by h_errnop.

Here's a small example: 

          struct hostent *
          gethostname (char *host)
          {
            struct hostent hostbuf, *hp;
            size_t hstbuflen;
            char *tmphstbuf;
            int res;
            int herr;
          
            hstbuflen = 1024;
            /* Allocate buffer, remember to free it to avoid memory leakage.  */
            tmphstbuf = malloc (hstbuflen);
          
            while ((res = gethostbyname_r (host, &hostbuf, tmphstbuf, hstbuflen,
                                           &hp, &herr)) == ERANGE)
              {
                /* Enlarge the buffer.  */
                hstbuflen *= 2;
                tmphstbuf = realloc (tmphstbuf, hstbuflen);
              }
            /*  Check for errors.  */
            if (res || hp == NULL)
              return NULL;
            return hp;
          }
— Function: int gethostbyname2_r (const char *name, int af, struct hostent *restrict result_buf, char *restrict buf, size_t buflen, struct hostent **restrict result, int *restrict h_errnop)

The gethostbyname2_r function is like gethostbyname_r, but allows the caller to specify the desired address family (e.g. AF_INET or AF_INET6) for the result.

— Function: int gethostbyaddr_r (const char *addr, size_t length, int format, struct hostent *restrict result_buf, char *restrict buf, size_t buflen, struct hostent **restrict result, int *restrict h_errnop)

The gethostbyaddr_r function returns information about the host with Internet address addr. The parameter addr is not really a pointer to char - it can be a pointer to an IPv4 or an IPv6 address. The length argument is the size (in bytes) of the address at addr. format specifies the address format; for an IPv4 Internet address, specify a value of AF_INET; for an IPv6 Internet address, use AF_INET6.

Similar to the gethostbyname_r function, the caller must provide buffers for the result and memory used internally. In case of success the function returns zero. Otherwise the value is an error number where ERANGE has the special meaning that the caller-provided buffer is too small.

You can also scan the entire hosts database one entry at a time using sethostent, gethostent and endhostent. Be careful when using these functions because they are not reentrant.

— Function: void sethostent (int stayopen)

This function opens the hosts database to begin scanning it. You can then call gethostent to read the entries.

If the stayopen argument is nonzero, this sets a flag so that subsequent calls to gethostbyname or gethostbyaddr will not close the database (as they usually would). This makes for more efficiency if you call those functions several times, by avoiding reopening the database for each call.

— Function: struct hostent * gethostent (void)

This function returns the next entry in the hosts database. It returns a null pointer if there are no more entries.

— Function: void endhostent (void)

This function closes the hosts database.

Next: , Previous: Protocols Database, Up: Internet Namespace

16.6.3 Internet Ports

A socket address in the Internet namespace consists of a machine's Internet address plus a port number which distinguishes the sockets on a given machine (for a given protocol). Port numbers range from 0 to 65,535.

Port numbers less than IPPORT_RESERVED are reserved for standard servers, such as finger and telnet. There is a database that keeps track of these, and you can use the getservbyname function to map a service name onto a port number; see Services Database.

If you write a server that is not one of the standard ones defined in the database, you must choose a port number for it. Use a number greater than IPPORT_USERRESERVED; such numbers are reserved for servers and won't ever be generated automatically by the system. Avoiding conflicts with servers being run by other users is up to you.

When you use a socket without specifying its address, the system generates a port number for it. This number is between IPPORT_RESERVED and IPPORT_USERRESERVED.

On the Internet, it is actually legitimate to have two different sockets with the same port number, as long as they never both try to communicate with the same socket address (host address plus port number). You shouldn't duplicate a port number except in special circumstances where a higher-level protocol requires it. Normally, the system won't let you do it; bind normally insists on distinct port numbers. To reuse a port number, you must set the socket option SO_REUSEADDR. See Socket-Level Options.

These macros are defined in the header file netinet/in.h.

— Macro: int IPPORT_RESERVED

Port numbers less than IPPORT_RESERVED are reserved for superuser use.

— Macro: int IPPORT_USERRESERVED

Port numbers greater than or equal to IPPORT_USERRESERVED are reserved for explicit use; they will never be allocated automatically.

Next: , Previous: Ports, Up: Internet Namespace

16.6.4 The Services Database

The database that keeps track of “well-known” services is usually either the file /etc/services or an equivalent from a name server. You can use these utilities, declared in netdb.h, to access the services database.

— Data Type: struct servent

This data type holds information about entries from the services database. It has the following members:

char *s_name
This is the “official” name of the service.
char **s_aliases
These are alternate names for the service, represented as an array of strings. A null pointer terminates the array.
int s_port
This is the port number for the service. Port numbers are given in network byte order; see Byte Order.
char *s_proto
This is the name of the protocol to use with this service. See Protocols Database.

To get information about a particular service, use the getservbyname or getservbyport functions. The information is returned in a statically-allocated structure; you must copy the information if you need to save it across calls.

— Function: struct servent * getservbyname (const char *name, const char *proto)

The getservbyname function returns information about the service named name using protocol proto. If it can't find such a service, it returns a null pointer.

This function is useful for servers as well as for clients; servers use it to determine which port they should listen on (see Listening).

— Function: struct servent * getservbyport (int port, const char *proto)

The getservbyport function returns information about the service at port port using protocol proto. If it can't find such a service, it returns a null pointer.

You can also scan the services database using setservent, getservent and endservent. Be careful when using these functions because they are not reentrant.

— Function: void setservent (int stayopen)

This function opens the services database to begin scanning it.

If the stayopen argument is nonzero, this sets a flag so that subsequent calls to getservbyname or getservbyport will not close the database (as they usually would). This makes for more efficiency if you call those functions several times, by avoiding reopening the database for each call.

— Function: struct servent * getservent (void)

This function returns the next entry in the services database. If there are no more entries, it returns a null pointer.

— Function: void endservent (void)

This function closes the services database.

Next: , Previous: Services Database, Up: Internet Namespace

16.6.5 Byte Order Conversion

Different kinds of computers use different conventions for the ordering of bytes within a word. Some computers put the most significant byte within a word first (this is called “big-endian” order), and others put it last (“little-endian” order).

So that machines with different byte order conventions can communicate, the Internet protocols specify a canonical byte order convention for data transmitted over the network. This is known as network byte order.

When establishing an Internet socket connection, you must make sure that the data in the sin_port and sin_addr members of the sockaddr_in structure are represented in network byte order. If you are encoding integer data in the messages sent through the socket, you should convert this to network byte order too. If you don't do this, your program may fail when running on or talking to other kinds of machines.

If you use getservbyname and gethostbyname or inet_addr to get the port number and host address, the values are already in network byte order, and you can copy them directly into the sockaddr_in structure.

Otherwise, you have to convert the values explicitly. Use htons and ntohs to convert values for the sin_port member. Use htonl and ntohl to convert IPv4 addresses for the sin_addr member. (Remember, struct in_addr is equivalent to uint32_t.) These functions are declared in netinet/in.h.

— Function: uint16_t htons (uint16_t hostshort)

This function converts the uint16_t integer hostshort from host byte order to network byte order.

— Function: uint16_t ntohs (uint16_t netshort)

This function converts the uint16_t integer netshort from network byte order to host byte order.

— Function: uint32_t htonl (uint32_t hostlong)

This function converts the uint32_t integer hostlong from host byte order to network byte order.

This is used for IPv4 Internet addresses.

— Function: uint32_t ntohl (uint32_t netlong)

This function converts the uint32_t integer netlong from network byte order to host byte order.

This is used for IPv4 Internet addresses.

Next: , Previous: Host Addresses, Up: Internet Namespace

16.6.6 Protocols Database

The communications protocol used with a socket controls low-level details of how data are exchanged. For example, the protocol implements things like checksums to detect errors in transmissions, and routing instructions for messages. Normal user programs have little reason to mess with these details directly.

The default communications protocol for the Internet namespace depends on the communication style. For stream communication, the default is TCP (“transmission control protocol”). For datagram communication, the default is UDP (“user datagram protocol”). For reliable datagram communication, the default is RDP (“reliable datagram protocol”). You should nearly always use the default.

Internet protocols are generally specified by a name instead of a number. The network protocols that a host knows about are stored in a database. This is usually either derived from the file /etc/protocols, or it may be an equivalent provided by a name server. You look up the protocol number associated with a named protocol in the database using the getprotobyname function.

Here are detailed descriptions of the utilities for accessing the protocols database. These are declared in netdb.h.

— Data Type: struct protoent

This data type is used to represent entries in the network protocols database. It has the following members:

char *p_name
This is the official name of the protocol.
char **p_aliases
These are alternate names for the protocol, specified as an array of strings. The last element of the array is a null pointer.
int p_proto
This is the protocol number (in host byte order); use this member as the protocol argument to socket.

You can use getprotobyname and getprotobynumber to search the protocols database for a specific protocol. The information is returned in a statically-allocated structure; you must copy the information if you need to save it across calls.

— Function: struct protoent * getprotobyname (const char *name)

The getprotobyname function returns information about the network protocol named name. If there is no such protocol, it returns a null pointer.

— Function: struct protoent * getprotobynumber (int protocol)

The getprotobynumber function returns information about the network protocol with number protocol. If there is no such protocol, it returns a null pointer.

You can also scan the whole protocols database one protocol at a time by using setprotoent, getprotoent and endprotoent. Be careful when using these functions because they are not reentrant.

— Function: void setprotoent (int stayopen)

This function opens the protocols database to begin scanning it.

If the stayopen argument is nonzero, this sets a flag so that subsequent calls to getprotobyname or getprotobynumber will not close the database (as they usually would). This makes for more efficiency if you call those functions several times, by avoiding reopening the database for each call.

— Function: struct protoent * getprotoent (void)

This function returns the next entry in the protocols database. It returns a null pointer if there are no more entries.

— Function: void endprotoent (void)

This function closes the protocols database.

Previous: Byte Order, Up: Internet Namespace

16.6.7 Internet Socket Example

Here is an example showing how to create and name a socket in the Internet namespace. The newly created socket exists on the machine that the program is running on. Rather than finding and using the machine's Internet address, this example specifies INADDR_ANY as the host address; the system replaces that with the machine's actual address. 

     #include <stdio.h>
     #include <stdlib.h>
     #include <sys/socket.h>
     #include <netinet/in.h>
     
     int
     make_socket (uint16_t port)
     {
       int sock;
       struct sockaddr_in name;
     
       /* Create the socket. */
       sock = socket (PF_INET, SOCK_STREAM, 0);
       if (sock < 0)
         {
           perror ("socket");
           exit (EXIT_FAILURE);
         }
     
       /* Give the socket a name. */
       name.sin_family = AF_INET;
       name.sin_port = htons (port);
       name.sin_addr.s_addr = htonl (INADDR_ANY);
       if (bind (sock, (struct sockaddr *) &name, sizeof (name)) < 0)
         {
           perror ("bind");
           exit (EXIT_FAILURE);
         }
     
       return sock;
     }

Here is another example, showing how you can fill in a sockaddr_in structure, given a host name string and a port number: 

     #include <stdio.h>
     #include <stdlib.h>
     #include <sys/socket.h>
     #include <netinet/in.h>
     #include <netdb.h>
     
     void
     init_sockaddr (struct sockaddr_in *name,
                    const char *hostname,
                    uint16_t port)
     {
       struct hostent *hostinfo;
     
       name->sin_family = AF_INET;
       name->sin_port = htons (port);
       hostinfo = gethostbyname (hostname);
       if (hostinfo == NULL)
         {
           fprintf (stderr, "Unknown host %s.\n", hostname);
           exit (EXIT_FAILURE);
         }
       name->sin_addr = *(struct in_addr *) hostinfo->h_addr;
     }

Next: , Previous: Internet Namespace, Up: Sockets

16.7 Other Namespaces

Certain other namespaces and associated protocol families are supported but not documented yet because they are not often used. PF_NS refers to the Xerox Network Software protocols. PF_ISO stands for Open Systems Interconnect. PF_CCITT refers to protocols from CCITT. socket.h defines these symbols and others naming protocols not actually implemented.

PF_IMPLINK is used for communicating between hosts and Internet Message Processors. For information on this and PF_ROUTE, an occasionally-used local area routing protocol, see the GNU Hurd Manual (to appear in the future).

Next: , Previous: Misc Namespaces, Up: Sockets

16.8 Opening and Closing Sockets

This section describes the actual library functions for opening and closing sockets. The same functions work for all namespaces and connection styles.

Next: , Up: Open/Close Sockets

16.8.1 Creating a Socket

The primitive for creating a socket is the socket function, declared in sys/socket.h.

— Function: int socket (int namespace, int style, int protocol)

This function creates a socket and specifies communication style style, which should be one of the socket styles listed in Communication Styles. The namespace argument specifies the namespace; it must be PF_LOCAL (see Local Namespace) or PF_INET (see Internet Namespace). protocol designates the specific protocol (see Socket Concepts); zero is usually right for protocol.

The return value from socket is the file descriptor for the new socket, or -1 in case of error. The following errno error conditions are defined for this function:

EPROTONOSUPPORT
The protocol or style is not supported by the namespace specified.
EMFILE
The process already has too many file descriptors open.
ENFILE
The system already has too many file descriptors open.
EACCES
The process does not have the privilege to create a socket of the specified style or protocol.
ENOBUFS
The system ran out of internal buffer space.

The file descriptor returned by the socket function supports both read and write operations. However, like pipes, sockets do not support file positioning operations.

For examples of how to call the socket function, see Local Socket Example, or Inet Example.

Next: , Previous: Creating a Socket, Up: Open/Close Sockets

16.8.2 Closing a Socket

When you have finished using a socket, you can simply close its file descriptor with close; see Opening and Closing Files. If there is still data waiting to be transmitted over the connection, normally close tries to complete this transmission. You can control this behavior using the SO_LINGER socket option to specify a timeout period; see Socket Options.

You can also shut down only reception or transmission on a connection by calling shutdown, which is declared in sys/socket.h.

— Function: int shutdown (int socket, int how)

The shutdown function shuts down the connection of socket socket. The argument how specifies what action to perform:

0
Stop receiving data for this socket. If further data arrives, reject it.
1
Stop trying to transmit data from this socket. Discard any data waiting to be sent. Stop looking for acknowledgement of data already sent; don't retransmit it if it is lost.
2
Stop both reception and transmission.

The return value is 0 on success and -1 on failure. The following errno error conditions are defined for this function:

EBADF
socket is not a valid file descriptor.
ENOTSOCK
socket is not a socket.
ENOTCONN
socket is not connected.

Previous: Closing a Socket, Up: Open/Close Sockets

16.8.3 Socket Pairs

A socket pair consists of a pair of connected (but unnamed) sockets. It is very similar to a pipe and is used in much the same way. Socket pairs are created with the socketpair function, declared in sys/socket.h. A socket pair is much like a pipe; the main difference is that the socket pair is bidirectional, whereas the pipe has one input-only end and one output-only end (see Pipes and FIFOs).

— Function: int socketpair (int namespace, int style, int protocol, int filedes[2])

This function creates a socket pair, returning the file descriptors in filedes[0] and filedes[1]. The socket pair is a full-duplex communications channel, so that both reading and writing may be performed at either end.

The namespace, style and protocol arguments are interpreted as for the socket function. style should be one of the communication styles listed in Communication Styles. The namespace argument specifies the namespace, which must be AF_LOCAL (see Local Namespace); protocol specifies the communications protocol, but zero is the only meaningful value.

If style specifies a connectionless communication style, then the two sockets you get are not connected, strictly speaking, but each of them knows the other as the default destination address, so they can send packets to each other.

The socketpair function returns 0 on success and -1 on failure. The following errno error conditions are defined for this function:

EMFILE
The process has too many file descriptors open.
EAFNOSUPPORT
The specified namespace is not supported.
EPROTONOSUPPORT
The specified protocol is not supported.
EOPNOTSUPP
The specified protocol does not support the creation of socket pairs.

Next: , Previous: Open/Close Sockets, Up: Sockets

16.9 Using Sockets with Connections

The most common communication styles involve making a connection to a particular other socket, and then exchanging data with that socket over and over. Making a connection is asymmetric; one side (the client) acts to request a connection, while the other side (the server) makes a socket and waits for the connection request.

Next: , Up: Connections

16.9.1 Making a Connection

In making a connection, the client makes a connection while the server waits for and accepts the connection. Here we discuss what the client program must do with the connect function, which is declared in sys/socket.h.

— Function: int connect (int socket, struct sockaddr *addr, socklen_t length)

The connect function initiates a connection from the socket with file descriptor socket to the socket whose address is specified by the addr and length arguments. (This socket is typically on another machine, and it must be already set up as a server.) See Socket Addresses, for information about how these arguments are interpreted.

Normally, connect waits until the server responds to the request before it returns. You can set nonblocking mode on the socket socket to make connect return immediately without waiting for the response. See File Status Flags, for information about nonblocking mode.

The normal return value from connect is 0. If an error occurs, connect returns -1. The following errno error conditions are defined for this function:

EBADF
The socket socket is not a valid file descriptor.
ENOTSOCK
File descriptor socket is not a socket.
EADDRNOTAVAIL
The specified address is not available on the remote machine.
EAFNOSUPPORT
The namespace of the addr is not supported by this socket.
EISCONN
The socket socket is already connected.
ETIMEDOUT
The attempt to establish the connection timed out.
ECONNREFUSED
The server has actively refused to establish the connection.
ENETUNREACH
The network of the given addr isn't reachable from this host.
EADDRINUSE
The socket address of the given addr is already in use.
EINPROGRESS
The socket socket is non-blocking and the connection could not be established immediately. You can determine when the connection is completely established with select; see Waiting for I/O. Another connect call on the same socket, before the connection is completely established, will fail with EALREADY.
EALREADY
The socket socket is non-blocking and already has a pending connection in progress (see EINPROGRESS above).

This function is defined as a cancellation point in multi-threaded programs, so one has to be prepared for this and make sure that allocated resources (like memory, files descriptors, semaphores or whatever) are freed even if the thread is canceled.

Next: , Previous: Connecting, Up: Connections

16.9.2 Listening for Connections

Now let us consider what the server process must do to accept connections on a socket. First it must use the listen function to enable connection requests on the socket, and then accept each incoming connection with a call to accept (see Accepting Connections). Once connection requests are enabled on a server socket, the select function reports when the socket has a connection ready to be accepted (see Waiting for I/O).

The listen function is not allowed for sockets using connectionless communication styles.

You can write a network server that does not even start running until a connection to it is requested. See Inetd Servers.

In the Internet namespace, there are no special protection mechanisms for controlling access to a port; any process on any machine can make a connection to your server. If you want to restrict access to your server, make it examine the addresses associated with connection requests or implement some other handshaking or identification protocol.

In the local namespace, the ordinary file protection bits control who has access to connect to the socket.

— Function: int listen (int socket, unsigned int n)

The listen function enables the socket socket to accept connections, thus making it a server socket.

The argument n specifies the length of the queue for pending connections. When the queue fills, new clients attempting to connect fail with ECONNREFUSED until the server calls accept to accept a connection from the queue.

The listen function returns 0 on success and -1 on failure. The following errno error conditions are defined for this function:

EBADF
The argument socket is not a valid file descriptor.
ENOTSOCK
The argument socket is not a socket.
EOPNOTSUPP
The socket socket does not support this operation.

Next: , Previous: Listening, Up: Connections

16.9.3 Accepting Connections

When a server receives a connection request, it can complete the connection by accepting the request. Use the function accept to do this.

A socket that has been established as a server can accept connection requests from multiple clients. The server's original socket does not become part of the connection; instead, accept makes a new socket which participates in the connection. accept returns the descriptor for this socket. The server's original socket remains available for listening for further connection requests.

The number of pending connection requests on a server socket is finite. If connection requests arrive from clients faster than the server can act upon them, the queue can fill up and additional requests are refused with an ECONNREFUSED error. You can specify the maximum length of this queue as an argument to the listen function, although the system may also impose its own internal limit on the length of this queue.

— Function: int accept (int socket, struct sockaddr *addr, socklen_t *length_ptr)

This function is used to accept a connection request on the server socket socket.

The accept function waits if there are no connections pending, unless the socket socket has nonblocking mode set. (You can use select to wait for a pending connection, with a nonblocking socket.) See File Status Flags, for information about nonblocking mode.

The addr and length-ptr arguments are used to return information about the name of the client socket that initiated the connection. See Socket Addresses, for information about the format of the information.

Accepting a connection does not make socket part of the connection. Instead, it creates a new socket which becomes connected. The normal return value of accept is the file descriptor for the new socket.

After accept, the original socket socket remains open and unconnected, and continues listening until you close it. You can accept further connections with socket by calling accept again.

If an error occurs, accept returns -1. The following errno error conditions are defined for this function:

EBADF
The socket argument is not a valid file descriptor.
ENOTSOCK
The descriptor socket argument is not a socket.
EOPNOTSUPP
The descriptor socket does not support this operation.
EWOULDBLOCK
socket has nonblocking mode set, and there are no pending connections immediately available.

This function is defined as a cancellation point in multi-threaded programs, so one has to be prepared for this and make sure that allocated resources (like memory, files descriptors, semaphores or whatever) are freed even if the thread is canceled.

The accept function is not allowed for sockets using connectionless communication styles.

Next: , Previous: Accepting Connections, Up: Connections

16.9.4 Who is Connected to Me?

— Function: int getpeername (int socket, struct sockaddr *addr, socklen_t *length-ptr)

The getpeername function returns the address of the socket that socket is connected to; it stores the address in the memory space specified by addr and length-ptr. It stores the length of the address in *length-ptr.

See Socket Addresses, for information about the format of the address. In some operating systems, getpeername works only for sockets in the Internet domain.

The return value is 0 on success and -1 on error. The following errno error conditions are defined for this function:

EBADF
The argument socket is not a valid file descriptor.
ENOTSOCK
The descriptor socket is not a socket.
ENOTCONN
The socket socket is not connected.
ENOBUFS
There are not enough internal buffers available.

Next: , Previous: Who is Connected, Up: Connections

16.9.5 Transferring Data

Once a socket has been connected to a peer, you can use the ordinary read and write operations (see I/O Primitives) to transfer data. A socket is a two-way communications channel, so read and write operations can be performed at either end.

There are also some I/O modes that are specific to socket operations. In order to specify these modes, you must use the recv and send functions instead of the more generic read and write functions. The recv and send functions take an additional argument which you can use to specify various flags to control special I/O modes. For example, you can specify the MSG_OOB flag to read or write out-of-band data, the MSG_PEEK flag to peek at input, or the MSG_DONTROUTE flag to control inclusion of routing information on output.

Next: , Up: Transferring Data
16.9.5.1 Sending Data

The send function is declared in the header file sys/socket.h. If your flags argument is zero, you can just as well use write instead of send; see I/O Primitives. If the socket was connected but the connection has broken, you get a SIGPIPE signal for any use of send or write (see Miscellaneous Signals).

— Function: int send (int socket, void *buffer, size_t size, int flags)

The send function is like write, but with the additional flags flags. The possible values of flags are described in Socket Data Options.

This function returns the number of bytes transmitted, or -1 on failure. If the socket is nonblocking, then send (like write) can return after sending just part of the data. See File Status Flags, for information about nonblocking mode.

Note, however, that a successful return value merely indicates that the message has been sent without error, not necessarily that it has been received without error.

The following errno error conditions are defined for this function:

EBADF
The socket argument is not a valid file descriptor.
EINTR
The operation was interrupted by a signal before any data was sent. See Interrupted Primitives.
ENOTSOCK
The descriptor socket is not a socket.
EMSGSIZE
The socket type requires that the message be sent atomically, but the message is too large for this to be possible.
EWOULDBLOCK
Nonblocking mode has been set on the socket, and the write operation would block. (Normally send blocks until the operation can be completed.)
ENOBUFS
There is not enough internal buffer space available.
ENOTCONN
You never connected this socket.
EPIPE
This socket was connected but the connection is now broken. In this case, send generates a SIGPIPE signal first; if that signal is ignored or blocked, or if its handler returns, then send fails with EPIPE.

This function is defined as a cancellation point in multi-threaded programs, so one has to be prepared for this and make sure that allocated resources (like memory, files descriptors, semaphores or whatever) are freed even if the thread is canceled.

Next: , Previous: Sending Data, Up: Transferring Data
16.9.5.2 Receiving Data

The recv function is declared in the header file sys/socket.h. If your flags argument is zero, you can just as well use read instead of recv; see I/O Primitives.

— Function: int recv (int socket, void *buffer, size_t size, int flags)

The recv function is like read, but with the additional flags flags. The possible values of flags are described in Socket Data Options.

If nonblocking mode is set for socket, and no data are available to be read, recv fails immediately rather than waiting. See File Status Flags, for information about nonblocking mode.

This function returns the number of bytes received, or -1 on failure. The following errno error conditions are defined for this function:

EBADF
The socket argument is not a valid file descriptor.
ENOTSOCK
The descriptor socket is not a socket.
EWOULDBLOCK
Nonblocking mode has been set on the socket, and the read operation would block. (Normally, recv blocks until there is input available to be read.)
EINTR
The operation was interrupted by a signal before any data was read. See Interrupted Primitives.
ENOTCONN
You never connected this socket.

This function is defined as a cancellation point in multi-threaded programs, so one has to be prepared for this and make sure that allocated resources (like memory, files descriptors, semaphores or whatever) are freed even if the thread is canceled.

Previous: Receiving Data, Up: Transferring Data
16.9.5.3 Socket Data Options

The flags argument to send and recv is a bit mask. You can bitwise-OR the values of the following macros together to obtain a value for this argument. All are defined in the header file sys/socket.h.

— Macro: int MSG_OOB

Send or receive out-of-band data. See Out-of-Band Data.

— Macro: int MSG_PEEK

Look at the data but don't remove it from the input queue. This is only meaningful with input functions such as recv, not with send.

— Macro: int MSG_DONTROUTE

Don't include routing information in the message. This is only meaningful with output operations, and is usually only of interest for diagnostic or routing programs. We don't try to explain it here.

Next: , Previous: Transferring Data, Up: Connections

16.9.6 Byte Stream Socket Example

Here is an example client program that makes a connection for a byte stream socket in the Internet namespace. It doesn't do anything particularly interesting once it has connected to the server; it just sends a text string to the server and exits.

This program uses init_sockaddr to set up the socket address; see Inet Example. 

     #include <stdio.h>
     #include <errno.h>
     #include <stdlib.h>
     #include <unistd.h>
     #include <sys/types.h>
     #include <sys/socket.h>
     #include <netinet/in.h>
     #include <netdb.h>
     
     #define PORT            5555
     #define MESSAGE         "Yow!!! Are we having fun yet?!?"
     #define SERVERHOST      "mescaline.gnu.org"
     
     void
     write_to_server (int filedes)
     {
       int nbytes;
     
       nbytes = write (filedes, MESSAGE, strlen (MESSAGE) + 1);
       if (nbytes < 0)
         {
           perror ("write");
           exit (EXIT_FAILURE);
         }
     }
     
     
     int
     main (void)
     {
       extern void init_sockaddr (struct sockaddr_in *name,
                                  const char *hostname,
                                  uint16_t port);
       int sock;
       struct sockaddr_in servername;
     
       /* Create the socket. */
       sock = socket (PF_INET, SOCK_STREAM, 0);
       if (sock < 0)
         {
           perror ("socket (client)");
           exit (EXIT_FAILURE);
         }
     
       /* Connect to the server. */
       init_sockaddr (&servername, SERVERHOST, PORT);
       if (0 > connect (sock,
                        (struct sockaddr *) &servername,
                        sizeof (servername)))
         {
           perror ("connect (client)");
           exit (EXIT_FAILURE);
         }
     
       /* Send data to the server. */
       write_to_server (sock);
       close (sock);
       exit (EXIT_SUCCESS);
     }

Next: , Previous: Byte Stream Example, Up: Connections

16.9.7 Byte Stream Connection Server Example

The server end is much more complicated. Since we want to allow multiple clients to be connected to the server at the same time, it would be incorrect to wait for input from a single client by simply calling read or recv. Instead, the right thing to do is to use select (see Waiting for I/O) to wait for input on all of the open sockets. This also allows the server to deal with additional connection requests.

This particular server doesn't do anything interesting once it has gotten a message from a client. It does close the socket for that client when it detects an end-of-file condition (resulting from the client shutting down its end of the connection).

This program uses make_socket to set up the socket address; see Inet Example. 

     #include <stdio.h>
     #include <errno.h>
     #include <stdlib.h>
     #include <unistd.h>
     #include <sys/types.h>
     #include <sys/socket.h>
     #include <netinet/in.h>
     #include <netdb.h>
     
     #define PORT    5555
     #define MAXMSG  512
     
     int
     read_from_client (int filedes)
     {
       char buffer[MAXMSG];
       int nbytes;
     
       nbytes = read (filedes, buffer, MAXMSG);
       if (nbytes < 0)
         {
           /* Read error. */
           perror ("read");
           exit (EXIT_FAILURE);
         }
       else if (nbytes == 0)
         /* End-of-file. */
         return -1;
       else
         {
           /* Data read. */
           fprintf (stderr, "Server: got message: `%s'\n", buffer);
           return 0;
         }
     }
     
     int
     main (void)
     {
       extern int make_socket (uint16_t port);
       int sock;
       fd_set active_fd_set, read_fd_set;
       int i;
       struct sockaddr_in clientname;
       size_t size;
     
       /* Create the socket and set it up to accept connections. */
       sock = make_socket (PORT);
       if (listen (sock, 1) < 0)
         {
           perror ("listen");
           exit (EXIT_FAILURE);
         }
     
       /* Initialize the set of active sockets. */
       FD_ZERO (&active_fd_set);
       FD_SET (sock, &active_fd_set);
     
       while (1)
         {
           /* Block until input arrives on one or more active sockets. */
           read_fd_set = active_fd_set;
           if (select (FD_SETSIZE, &read_fd_set, NULL, NULL, NULL) < 0)
             {
               perror ("select");
               exit (EXIT_FAILURE);
             }
     
           /* Service all the sockets with input pending. */
           for (i = 0; i < FD_SETSIZE; ++i)
             if (FD_ISSET (i, &read_fd_set))
               {
                 if (i == sock)
                   {
                     /* Connection request on original socket. */
                     int new;
                     size = sizeof (clientname);
                     new = accept (sock,
                                   (struct sockaddr *) &clientname,
                                   &size);
                     if (new < 0)
                       {
                         perror ("accept");
                         exit (EXIT_FAILURE);
                       }
                     fprintf (stderr,
                              "Server: connect from host %s, port %hd.\n",
                              inet_ntoa (clientname.sin_addr),
                              ntohs (clientname.sin_port));
                     FD_SET (new, &active_fd_set);
                   }
                 else
                   {
                     /* Data arriving on an already-connected socket. */
                     if (read_from_client (i) < 0)
                       {
                         close (i);
                         FD_CLR (i, &active_fd_set);
                       }
                   }
               }
         }
     }

Previous: Server Example, Up: Connections

16.9.8 Out-of-Band Data

Streams with connections permit out-of-band data that is delivered with higher priority than ordinary data. Typically the reason for sending out-of-band data is to send notice of an exceptional condition. To send out-of-band data use send, specifying the flag MSG_OOB (see Sending Data).

Out-of-band data are received with higher priority because the receiving process need not read it in sequence; to read the next available out-of-band data, use recv with the MSG_OOB flag (see Receiving Data). Ordinary read operations do not read out-of-band data; they read only ordinary data.

When a socket finds that out-of-band data are on their way, it sends a SIGURG signal to the owner process or process group of the socket. You can specify the owner using the F_SETOWN command to the fcntl function; see Interrupt Input. You must also establish a handler for this signal, as described in Signal Handling, in order to take appropriate action such as reading the out-of-band data.

Alternatively, you can test for pending out-of-band data, or wait until there is out-of-band data, using the select function; it can wait for an exceptional condition on the socket. See Waiting for I/O, for more information about select.

Notification of out-of-band data (whether with SIGURG or with select) indicates that out-of-band data are on the way; the data may not actually arrive until later. If you try to read the out-of-band data before it arrives, recv fails with an EWOULDBLOCK error.

Sending out-of-band data automatically places a “mark” in the stream of ordinary data, showing where in the sequence the out-of-band data “would have been”. This is useful when the meaning of out-of-band data is “cancel everything sent so far”. Here is how you can test, in the receiving process, whether any ordinary data was sent before the mark: 

     success = ioctl (socket, SIOCATMARK, &atmark);

The integer variable atmark is set to a nonzero value if the socket's read pointer has reached the “mark”.

Here's a function to discard any ordinary data preceding the out-of-band mark: 

     int
     discard_until_mark (int socket)
     {
       while (1)
         {
           /* This is not an arbitrary limit; any size will do.  */
           char buffer[1024];
           int atmark, success;
     
           /* If we have reached the mark, return.  */
           success = ioctl (socket, SIOCATMARK, &atmark);
           if (success < 0)
             perror ("ioctl");
           if (result)
             return;
     
           /* Otherwise, read a bunch of ordinary data and discard it.
              This is guaranteed not to read past the mark
              if it starts before the mark.  */
           success = read (socket, buffer, sizeof buffer);
           if (success < 0)
             perror ("read");
         }
     }

If you don't want to discard the ordinary data preceding the mark, you may need to read some of it anyway, to make room in internal system buffers for the out-of-band data. If you try to read out-of-band data and get an EWOULDBLOCK error, try reading some ordinary data (saving it so that you can use it when you want it) and see if that makes room. Here is an example: 

     struct buffer
     {
       char *buf;
       int size;
       struct buffer *next;
     };
     
     /* Read the out-of-band data from SOCKET and return it
        as a `struct buffer', which records the address of the data
        and its size.
     
        It may be necessary to read some ordinary data
        in order to make room for the out-of-band data.
        If so, the ordinary data are saved as a chain of buffers
        found in the `next' field of the value.  */
     
     struct buffer *
     read_oob (int socket)
     {
       struct buffer *tail = 0;
       struct buffer *list = 0;
     
       while (1)
         {
           /* This is an arbitrary limit.
              Does anyone know how to do this without a limit?  */
     #define BUF_SZ 1024
           char *buf = (char *) xmalloc (BUF_SZ);
           int success;
           int atmark;
     
           /* Try again to read the out-of-band data.  */
           success = recv (socket, buf, BUF_SZ, MSG_OOB);
           if (success >= 0)
             {
               /* We got it, so return it.  */
               struct buffer *link
                 = (struct buffer *) xmalloc (sizeof (struct buffer));
               link->buf = buf;
               link->size = success;
               link->next = list;
               return link;
             }
     
           /* If we fail, see if we are at the mark.  */
           success = ioctl (socket, SIOCATMARK, &atmark);
           if (success < 0)
             perror ("ioctl");
           if (atmark)
             {
               /* At the mark; skipping past more ordinary data cannot help.
                  So just wait a while.  */
               sleep (1);
               continue;
             }
     
           /* Otherwise, read a bunch of ordinary data and save it.
              This is guaranteed not to read past the mark
              if it starts before the mark.  */
           success = read (socket, buf, BUF_SZ);
           if (success < 0)
             perror ("read");
     
           /* Save this data in the buffer list.  */
           {
             struct buffer *link
               = (struct buffer *) xmalloc (sizeof (struct buffer));
             link->buf = buf;
             link->size = success;
     
             /* Add the new link to the end of the list.  */
             if (tail)
               tail->next = link;
             else
               list = link;
             tail = link;
           }
         }
     }

Next: , Previous: Connections, Up: Sockets

16.10 Datagram Socket Operations

This section describes how to use communication styles that don't use connections (styles SOCK_DGRAM and SOCK_RDM). Using these styles, you group data into packets and each packet is an independent communication. You specify the destination for each packet individually.

Datagram packets are like letters: you send each one independently with its own destination address, and they may arrive in the wrong order or not at all.

The listen and accept functions are not allowed for sockets using connectionless communication styles.

Next: , Up: Datagrams

16.10.1 Sending Datagrams

The normal way of sending data on a datagram socket is by using the sendto function, declared in sys/socket.h.

You can call connect on a datagram socket, but this only specifies a default destination for further data transmission on the socket. When a socket has a default destination you can use send (see Sending Data) or even write (see I/O Primitives) to send a packet there. You can cancel the default destination by calling connect using an address format of AF_UNSPEC in the addr argument. See Connecting, for more information about the connect function.

— Function: int sendto (int socket, void *buffer. size_t size, int flags, struct sockaddr *addr, socklen_t length)

The sendto function transmits the data in the buffer through the socket socket to the destination address specified by the addr and length arguments. The size argument specifies the number of bytes to be transmitted.

The flags are interpreted the same way as for send; see Socket Data Options.

The return value and error conditions are also the same as for send, but you cannot rely on the system to detect errors and report them; the most common error is that the packet is lost or there is no-one at the specified address to receive it, and the operating system on your machine usually does not know this.

It is also possible for one call to sendto to report an error owing to a problem related to a previous call.

This function is defined as a cancellation point in multi-threaded programs, so one has to be prepared for this and make sure that allocated resources (like memory, files descriptors, semaphores or whatever) are freed even if the thread is canceled.

Next: , Previous: Sending Datagrams, Up: Datagrams

16.10.2 Receiving Datagrams

The recvfrom function reads a packet from a datagram socket and also tells you where it was sent from. This function is declared in sys/socket.h.

— Function: int recvfrom (int socket, void *buffer, size_t size, int flags, struct sockaddr *addr, socklen_t *length-ptr)

The recvfrom function reads one packet from the socket socket into the buffer buffer. The size argument specifies the maximum number of bytes to be read.

If the packet is longer than size bytes, then you get the first size bytes of the packet and the rest of the packet is lost. There's no way to read the rest of the packet. Thus, when you use a packet protocol, you must always know how long a packet to expect.

The addr and length-ptr arguments are used to return the address where the packet came from. See Socket Addresses. For a socket in the local domain the address information won't be meaningful, since you can't read the address of such a socket (see Local Namespace). You can specify a null pointer as the addr argument if you are not interested in this information.

The flags are interpreted the same way as for recv (see Socket Data Options). The return value and error conditions are also the same as for recv.

This function is defined as a cancellation point in multi-threaded programs, so one has to be prepared for this and make sure that allocated resources (like memory, files descriptors, semaphores or whatever) are freed even if the thread is canceled.

You can use plain recv (see Receiving Data) instead of recvfrom if you don't need to find out who sent the packet (either because you know where it should come from or because you treat all possible senders alike). Even read can be used if you don't want to specify flags (see I/O Primitives).

Next: , Previous: Receiving Datagrams, Up: Datagrams

16.10.3 Datagram Socket Example

Here is a set of example programs that send messages over a datagram stream in the local namespace. Both the client and server programs use the make_named_socket function that was presented in Local Socket Example, to create and name their sockets.

First, here is the server program. It sits in a loop waiting for messages to arrive, bouncing each message back to the sender. Obviously this isn't a particularly useful program, but it does show the general ideas involved. 

     #include <stdio.h>
     #include <errno.h>
     #include <stdlib.h>
     #include <sys/socket.h>
     #include <sys/un.h>
     
     #define SERVER  "/tmp/serversocket"
     #define MAXMSG  512
     
     int
     main (void)
     {
       int sock;
       char message[MAXMSG];
       struct sockaddr_un name;
       size_t size;
       int nbytes;
     
       /* Remove the filename first, it's ok if the call fails */
       unlink (SERVER);
     
       /* Make the socket, then loop endlessly. */
       sock = make_named_socket (SERVER);
       while (1)
         {
           /* Wait for a datagram. */
           size = sizeof (name);
           nbytes = recvfrom (sock, message, MAXMSG, 0,
                              (struct sockaddr *) & name, &size);
           if (nbytes < 0)
             {
               perror ("recfrom (server)");
               exit (EXIT_FAILURE);
             }
     
           /* Give a diagnostic message. */
           fprintf (stderr, "Server: got message: %s\n", message);
     
           /* Bounce the message back to the sender. */
           nbytes = sendto (sock, message, nbytes, 0,
                            (struct sockaddr *) & name, size);
           if (nbytes < 0)
             {
               perror ("sendto (server)");
               exit (EXIT_FAILURE);
             }
         }
     }

Previous: Datagram Example, Up: Datagrams

16.10.4 Example of Reading Datagrams

Here is the client program corresponding to the server above.

It sends a datagram to the server and then waits for a reply. Notice that the socket for the client (as well as for the server) in this example has to be given a name. This is so that the server can direct a message back to the client. Since the socket has no associated connection state, the only way the server can do this is by referencing the name of the client. 

     #include <stdio.h>
     #include <errno.h>
     #include <unistd.h>
     #include <stdlib.h>
     #include <sys/socket.h>
     #include <sys/un.h>
     
     #define SERVER  "/tmp/serversocket"
     #define CLIENT  "/tmp/mysocket"
     #define MAXMSG  512
     #define MESSAGE "Yow!!! Are we having fun yet?!?"
     
     int
     main (void)
     {
       extern int make_named_socket (const char *name);
       int sock;
       char message[MAXMSG];
       struct sockaddr_un name;
       size_t size;
       int nbytes;
     
       /* Make the socket. */
       sock = make_named_socket (CLIENT);
     
       /* Initialize the server socket address. */
       name.sun_family = AF_LOCAL;
       strcpy (name.sun_path, SERVER);
       size = strlen (name.sun_path) + sizeof (name.sun_family);
     
       /* Send the datagram. */
       nbytes = sendto (sock, MESSAGE, strlen (MESSAGE) + 1, 0,
                        (struct sockaddr *) & name, size);
       if (nbytes < 0)
         {
           perror ("sendto (client)");
           exit (EXIT_FAILURE);
         }
     
       /* Wait for a reply. */
       nbytes = recvfrom (sock, message, MAXMSG, 0, NULL, 0);
       if (nbytes < 0)
         {
           perror ("recfrom (client)");
           exit (EXIT_FAILURE);
         }
     
       /* Print a diagnostic message. */
       fprintf (stderr, "Client: got message: %s\n", message);
     
       /* Clean up. */
       remove (CLIENT);
       close (sock);
     }

Keep in mind that datagram socket communications are unreliable. In this example, the client program waits indefinitely if the message never reaches the server or if the server's response never comes back. It's up to the user running the program to kill and restart it if desired. A more automatic solution could be to use select (see Waiting for I/O) to establish a timeout period for the reply, and in case of timeout either re-send the message or shut down the socket and exit.

Next: , Previous: Datagrams, Up: Sockets

16.11 The inetd Daemon

We've explained above how to write a server program that does its own listening. Such a server must already be running in order for anyone to connect to it.

Another way to provide a service on an Internet port is to let the daemon program inetd do the listening. inetd is a program that runs all the time and waits (using select) for messages on a specified set of ports. When it receives a message, it accepts the connection (if the socket style calls for connections) and then forks a child process to run the corresponding server program. You specify the ports and their programs in the file /etc/inetd.conf.

Next: , Up: Inetd

16.11.1 inetd Servers

Writing a server program to be run by inetd is very simple. Each time someone requests a connection to the appropriate port, a new server process starts. The connection already exists at this time; the socket is available as the standard input descriptor and as the standard output descriptor (descriptors 0 and 1) in the server process. Thus the server program can begin reading and writing data right away. Often the program needs only the ordinary I/O facilities; in fact, a general-purpose filter program that knows nothing about sockets can work as a byte stream server run by inetd.

You can also use inetd for servers that use connectionless communication styles. For these servers, inetd does not try to accept a connection since no connection is possible. It just starts the server program, which can read the incoming datagram packet from descriptor 0. The server program can handle one request and then exit, or you can choose to write it to keep reading more requests until no more arrive, and then exit. You must specify which of these two techniques the server uses when you configure inetd.

Previous: Inetd Servers, Up: Inetd

16.11.2 Configuring inetd

The file /etc/inetd.conf tells inetd which ports to listen to and what server programs to run for them. Normally each entry in the file is one line, but you can split it onto multiple lines provided all but the first line of the entry start with whitespace. Lines that start with `#' are comments.

Here are two standard entries in /etc/inetd.conf: 

     ftp	stream	tcp	nowait	root	/libexec/ftpd	ftpd
     talk	dgram	udp	wait	root	/libexec/talkd	talkd

An entry has this format: 

     service style protocol wait username program arguments

The service field says which service this program provides. It should be the name of a service defined in /etc/services. inetd uses service to decide which port to listen on for this entry.

The fields style and protocol specify the communication style and the protocol to use for the listening socket. The style should be the name of a communication style, converted to lower case and with `SOCK_' deleted—for example, `stream' or `dgram'. protocol should be one of the protocols listed in /etc/protocols. The typical protocol names are `tcp' for byte stream connections and `udp' for unreliable datagrams.

The wait field should be either `wait' or `nowait'. Use `wait' if style is a connectionless style and the server, once started, handles multiple requests as they come in. Use `nowait' if inetd should start a new process for each message or request that comes in. If style uses connections, then wait must be `nowait'.

user is the user name that the server should run as. inetd runs as root, so it can set the user ID of its children arbitrarily. It's best to avoid using `root' for user if you can; but some servers, such as Telnet and FTP, read a username and password themselves. These servers need to be root initially so they can log in as commanded by the data coming over the network.

program together with arguments specifies the command to run to start the server. program should be an absolute file name specifying the executable file to run. arguments consists of any number of whitespace-separated words, which become the command-line arguments of program. The first word in arguments is argument zero, which should by convention be the program name itself (sans directories).

If you edit /etc/inetd.conf, you can tell inetd to reread the file and obey its new contents by sending the inetd process the SIGHUP signal. You'll have to use ps to determine the process ID of the inetd process as it is not fixed.

Next: , Previous: Inetd, Up: Sockets

16.12 Socket Options

This section describes how to read or set various options that modify the behavior of sockets and their underlying communications protocols.

When you are manipulating a socket option, you must specify which level the option pertains to. This describes whether the option applies to the socket interface, or to a lower-level communications protocol interface.

Next: , Up: Socket Options

16.12.1 Socket Option Functions

Here are the functions for examining and modifying socket options. They are declared in sys/socket.h.

— Function: int getsockopt (int socket, int level, int optname, void *optval, socklen_t *optlen-ptr)

The getsockopt function gets information about the value of option optname at level level for socket socket.

The option value is stored in a buffer that optval points to. Before the call, you should supply in *optlen-ptr the size of this buffer; on return, it contains the number of bytes of information actually stored in the buffer.

Most options interpret the optval buffer as a single int value.

The actual return value of getsockopt is 0 on success and -1 on failure. The following errno error conditions are defined:

EBADF
The socket argument is not a valid file descriptor.
ENOTSOCK
The descriptor socket is not a socket.
ENOPROTOOPT
The optname doesn't make sense for the given level.

— Function: int setsockopt (int socket, int level, int optname, void *optval, socklen_t optlen)

This function is used to set the socket option optname at level level for socket socket. The value of the option is passed in the buffer optval of size optlen.

Previous: Socket Option Functions, Up: Socket Options

16.12.2 Socket-Level Options

— Constant: int SOL_SOCKET

Use this constant as the level argument to getsockopt or setsockopt to manipulate the socket-level options described in this section.

Here is a table of socket-level option names; all are defined in the header file sys/socket.h.

SO_DEBUG

This option toggles recording of debugging information in the underlying protocol modules. The value has type int; a nonzero value means “yes”.

SO_REUSEADDR
This option controls whether bind (see Setting Address) should permit reuse of local addresses for this socket. If you enable this option, you can actually have two sockets with the same Internet port number; but the system won't allow you to use the two identically-named sockets in a way that would confuse the Internet. The reason for this option is that some higher-level Internet protocols, including FTP, require you to keep reusing the same port number.

The value has type int; a nonzero value means “yes”.

SO_KEEPALIVE
This option controls whether the underlying protocol should periodically transmit messages on a connected socket. If the peer fails to respond to these messages, the connection is considered broken. The value has type int; a nonzero value means “yes”.
SO_DONTROUTE
This option controls whether outgoing messages bypass the normal message routing facilities. If set, messages are sent directly to the network interface instead. The value has type int; a nonzero value means “yes”.
SO_LINGER
This option specifies what should happen when the socket of a type that promises reliable delivery still has untransmitted messages when it is closed; see Closing a Socket. The value has type struct linger.
— Data Type: struct linger

This structure type has the following members:

int l_onoff
This field is interpreted as a boolean. If nonzero, close blocks until the data are transmitted or the timeout period has expired.
int l_linger
This specifies the timeout period, in seconds.


SO_BROADCAST
This option controls whether datagrams may be broadcast from the socket. The value has type int; a nonzero value means “yes”.
SO_OOBINLINE
If this option is set, out-of-band data received on the socket is placed in the normal input queue. This permits it to be read using read or recv without specifying the MSG_OOB flag. See Out-of-Band Data. The value has type int; a nonzero value means “yes”.
SO_SNDBUF
This option gets or sets the size of the output buffer. The value is a size_t, which is the size in bytes.
SO_RCVBUF
This option gets or sets the size of the input buffer. The value is a size_t, which is the size in bytes.
SO_STYLE
SO_TYPE
This option can be used with getsockopt only. It is used to get the socket's communication style. SO_TYPE is the historical name, and SO_STYLE is the preferred name in GNU. The value has type int and its value designates a communication style; see Communication Styles.
SO_ERROR

This option can be used with getsockopt only. It is used to reset the error status of the socket. The value is an int, which represents the previous error status.

Previous: Socket Options, Up: Sockets

16.13 Networks Database

Many systems come with a database that records a list of networks known to the system developer. This is usually kept either in the file /etc/networks or in an equivalent from a name server. This data base is useful for routing programs such as route, but it is not useful for programs that simply communicate over the network. We provide functions to access this database, which are declared in netdb.h.

— Data Type: struct netent

This data type is used to represent information about entries in the networks database. It has the following members:

char *n_name
This is the “official” name of the network.
char **n_aliases
These are alternative names for the network, represented as a vector of strings. A null pointer terminates the array.
int n_addrtype
This is the type of the network number; this is always equal to AF_INET for Internet networks.
unsigned long int n_net
This is the network number. Network numbers are returned in host byte order; see Byte Order.

Use the getnetbyname or getnetbyaddr functions to search the networks database for information about a specific network. The information is returned in a statically-allocated structure; you must copy the information if you need to save it.

— Function: struct netent * getnetbyname (const char *name)

The getnetbyname function returns information about the network named name. It returns a null pointer if there is no such network.

— Function: struct netent * getnetbyaddr (unsigned long int net, int type)

The getnetbyaddr function returns information about the network of type type with number net. You should specify a value of AF_INET for the type argument for Internet networks.

getnetbyaddr returns a null pointer if there is no such network.

You can also scan the networks database using setnetent, getnetent and endnetent. Be careful when using these functions because they are not reentrant.

— Function: void setnetent (int stayopen)

This function opens and rewinds the networks database.

If the stayopen argument is nonzero, this sets a flag so that subsequent calls to getnetbyname or getnetbyaddr will not close the database (as they usually would). This makes for more efficiency if you call those functions several times, by avoiding reopening the database for each call.

— Function: struct netent * getnetent (void)

This function returns the next entry in the networks database. It returns a null pointer if there are no more entries.

— Function: void endnetent (void)

This function closes the networks database.

Next: , Previous: Sockets, Up: Top

17 Low-Level Terminal Interface

This chapter describes functions that are specific to terminal devices. You can use these functions to do things like turn off input echoing; set serial line characteristics such as line speed and flow control; and change which characters are used for end-of-file, command-line editing, sending signals, and similar control functions.

Most of the functions in this chapter operate on file descriptors. See Low-Level I/O, for more information about what a file descriptor is and how to open a file descriptor for a terminal device.

Next: , Up: Low-Level Terminal Interface

17.1 Identifying Terminals

The functions described in this chapter only work on files that correspond to terminal devices. You can find out whether a file descriptor is associated with a terminal by using the isatty function.

Prototypes for the functions in this section are declared in the header file unistd.h.

— Function: int isatty (int filedes)

This function returns 1 if filedes is a file descriptor associated with an open terminal device, and 0 otherwise.

If a file descriptor is associated with a terminal, you can get its associated file name using the ttyname function. See also the ctermid function, described in Identifying the Terminal.

— Function: char * ttyname (int filedes)

If the file descriptor filedes is associated with a terminal device, the ttyname function returns a pointer to a statically-allocated, null-terminated string containing the file name of the terminal file. The value is a null pointer if the file descriptor isn't associated with a terminal, or the file name cannot be determined.

— Function: int ttyname_r (int filedes, char *buf, size_t len)

The ttyname_r function is similar to the ttyname function except that it places its result into the user-specified buffer starting at buf with length len.

The normal return value from ttyname_r is 0. Otherwise an error number is returned to indicate the error. The following errno error conditions are defined for this function:

EBADF
The filedes argument is not a valid file descriptor.
ENOTTY
The filedes is not associated with a terminal.
ERANGE
The buffer length len is too small to store the string to be returned.

Next: , Previous: Is It a Terminal, Up: Low-Level Terminal Interface

17.2 I/O Queues

Many of the remaining functions in this section refer to the input and output queues of a terminal device. These queues implement a form of buffering within the kernel independent of the buffering implemented by I/O streams (see I/O on Streams).

The terminal input queue is also sometimes referred to as its typeahead buffer. It holds the characters that have been received from the terminal but not yet read by any process.

The size of the input queue is described by the MAX_INPUT and _POSIX_MAX_INPUT parameters; see Limits for Files. You are guaranteed a queue size of at least MAX_INPUT, but the queue might be larger, and might even dynamically change size. If input flow control is enabled by setting the IXOFF input mode bit (see Input Modes), the terminal driver transmits STOP and START characters to the terminal when necessary to prevent the queue from overflowing. Otherwise, input may be lost if it comes in too fast from the terminal. In canonical mode, all input stays in the queue until a newline character is received, so the terminal input queue can fill up when you type a very long line. See Canonical or Not.

The terminal output queue is like the input queue, but for output; it contains characters that have been written by processes, but not yet transmitted to the terminal. If output flow control is enabled by setting the IXON input mode bit (see Input Modes), the terminal driver obeys START and STOP characters sent by the terminal to stop and restart transmission of output.

Clearing the terminal input queue means discarding any characters that have been received but not yet read. Similarly, clearing the terminal output queue means discarding any characters that have been written but not yet transmitted.

Next: , Previous: I/O Queues, Up: Low-Level Terminal Interface

17.3 Two Styles of Input: Canonical or Not

POSIX systems support two basic modes of input: canonical and noncanonical.

In canonical input processing mode, terminal input is processed in lines terminated by newline ('\n'), EOF, or EOL characters. No input can be read until an entire line has been typed by the user, and the read function (see I/O Primitives) returns at most a single line of input, no matter how many bytes are requested.

In canonical input mode, the operating system provides input editing facilities: some characters are interpreted specially to perform editing operations within the current line of text, such as ERASE and KILL. See Editing Characters.

The constants _POSIX_MAX_CANON and MAX_CANON parameterize the maximum number of bytes which may appear in a single line of canonical input. See Limits for Files. You are guaranteed a maximum line length of at least MAX_CANON bytes, but the maximum might be larger, and might even dynamically change size.

In noncanonical input processing mode, characters are not grouped into lines, and ERASE and KILL processing is not performed. The granularity with which bytes are read in noncanonical input mode is controlled by the MIN and TIME settings. See Noncanonical Input.

Most programs use canonical input mode, because this gives the user a way to edit input line by line. The usual reason to use noncanonical mode is when the program accepts single-character commands or provides its own editing facilities.

The choice of canonical or noncanonical input is controlled by the ICANON flag in the c_lflag member of struct termios. See Local Modes.

Next: , Previous: Canonical or Not, Up: Low-Level Terminal Interface

17.4 Terminal Modes

This section describes the various terminal attributes that control how input and output are done. The functions, data structures, and symbolic constants are all declared in the header file termios.h.

Don't confuse terminal attributes with file attributes. A device special file which is associated with a terminal has file attributes as described in File Attributes. These are unrelated to the attributes of the terminal device itself, which are discussed in this section.

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17.4.1 Terminal Mode Data Types

The entire collection of attributes of a terminal is stored in a structure of type struct termios. This structure is used with the functions tcgetattr and tcsetattr to read and set the attributes.

— Data Type: struct termios

Structure that records all the I/O attributes of a terminal. The structure includes at least the following members:

tcflag_t c_iflag
A bit mask specifying flags for input modes; see Input Modes.
tcflag_t c_oflag
A bit mask specifying flags for output modes; see Output Modes.
tcflag_t c_cflag
A bit mask specifying flags for control modes; see Control Modes.
tcflag_t c_lflag
A bit mask specifying flags for local modes; see Local Modes.
cc_t c_cc[NCCS]
An array specifying which characters are associated with various control functions; see Special Characters.

The struct termios structure also contains members which encode input and output transmission speeds, but the representation is not specified. See Line Speed, for how to examine and store the speed values.

The following sections describe the details of the members of the struct termios structure.

— Data Type: tcflag_t

This is an unsigned integer type used to represent the various bit masks for terminal flags.

— Data Type: cc_t

This is an unsigned integer type used to represent characters associated with various terminal control functions.

— Macro: int NCCS

The value of this macro is the number of elements in the c_cc array.

Next: , Previous: Mode Data Types, Up: Terminal Modes

17.4.2 Terminal Mode Functions

— Function: int tcgetattr (int filedes, struct termios *termios-p)

This function is used to examine the attributes of the terminal device with file descriptor filedes. The attributes are returned in the structure that termios-p points to.

If successful, tcgetattr returns 0. A return value of -1 indicates an error. The following errno error conditions are defined for this function:

EBADF
The filedes argument is not a valid file descriptor.
ENOTTY
The filedes is not associated with a terminal.

— Function: int tcsetattr (int filedes, int when, const struct termios *termios-p)

This function sets the attributes of the terminal device with file descriptor filedes. The new attributes are taken from the structure that termios-p points to.

The when argument specifies how to deal with input and output already queued. It can be one of the following values:

TCSANOW
Make the change immediately.
TCSADRAIN
Make the change after waiting until all queued output has been written. You should usually use this option when changing parameters that affect output.
TCSAFLUSH
This is like TCSADRAIN, but also discards any queued input.
TCSASOFT
This is a flag bit that you can add to any of the above alternatives. Its meaning is to inhibit alteration of the state of the terminal hardware. It is a BSD extension; it is only supported on BSD systems and the GNU system.

Using TCSASOFT is exactly the same as setting the CIGNORE bit in the c_cflag member of the structure termios-p points to. See Control Modes, for a description of CIGNORE.

If this function is called from a background process on its controlling terminal, normally all processes in the process group are sent a SIGTTOU signal, in the same way as if the process were trying to write to the terminal. The exception is if the calling process itself is ignoring or blocking SIGTTOU signals, in which case the operation is performed and no signal is sent. See Job Control.

If successful, tcsetattr returns 0. A return value of -1 indicates an error. The following errno error conditions are defined for this function:

EBADF
The filedes argument is not a valid file descriptor.
ENOTTY
The filedes is not associated with a terminal.
EINVAL
Either the value of the when argument is not valid, or there is something wrong with the data in the termios-p argument.

Although tcgetattr and tcsetattr specify the terminal device with a file descriptor, the attributes are those of the terminal device itself and not of the file descriptor. This means that the effects of changing terminal attributes are persistent; if another process opens the terminal file later on, it will see the changed attributes even though it doesn't have anything to do with the open file descriptor you originally specified in changing the attributes.

Similarly, if a single process has multiple or duplicated file descriptors for the same terminal device, changing the terminal attributes affects input and output to all of these file descriptors. This means, for example, that you can't open one file descriptor or stream to read from a terminal in the normal line-buffered, echoed mode; and simultaneously have another file descriptor for the same terminal that you use to read from it in single-character, non-echoed mode. Instead, you have to explicitly switch the terminal back and forth between the two modes.

Next: , Previous: Mode Functions, Up: Terminal Modes

17.4.3 Setting Terminal Modes Properly

When you set terminal modes, you should call tcgetattr first to get the current modes of the particular terminal device, modify only those modes that you are really interested in, and store the result with tcsetattr.

It's a bad idea to simply initialize a struct termios structure to a chosen set of attributes and pass it directly to tcsetattr. Your program may be run years from now, on systems that support members not documented in this manual. The way to avoid setting these members to unreasonable values is to avoid changing them.

What's more, different terminal devices may require different mode settings in order to function properly. So you should avoid blindly copying attributes from one terminal device to another.

When a member contains a collection of independent flags, as the c_iflag, c_oflag and c_cflag members do, even setting the entire member is a bad idea, because particular operating systems have their own flags. Instead, you should start with the current value of the member and alter only the flags whose values matter in your program, leaving any other flags unchanged.

Here is an example of how to set one flag (ISTRIP) in the struct termios structure while properly preserving all the other data in the structure: 

     int
     set_istrip (int desc, int value)
     {
       struct termios settings;
       int result;
     
       result = tcgetattr (desc, &settings);
       if (result < 0)
         {
           perror ("error in tcgetattr");
           return 0;
         }
       settings.c_iflag &= ~ISTRIP;
       if (value)
         settings.c_iflag |= ISTRIP;
       result = tcsetattr (desc, TCSANOW, &settings);
       if (result < 0)
         {
           perror ("error in tcsetattr");
           return 0;
        }
       return 1;
     }

Next: , Previous: Setting Modes, Up: Terminal Modes

17.4.4 Input Modes

This section describes the terminal attribute flags that control fairly low-level aspects of input processing: handling of parity errors, break signals, flow control, and <RET> and <LFD> characters.

All of these flags are bits in the c_iflag member of the struct termios structure. The member is an integer, and you change flags using the operators &, | and ^. Don't try to specify the entire value for c_iflag—instead, change only specific flags and leave the rest untouched (see Setting Modes).

— Macro: tcflag_t INPCK

If this bit is set, input parity checking is enabled. If it is not set, no checking at all is done for parity errors on input; the characters are simply passed through to the application.

Parity checking on input processing is independent of whether parity detection and generation on the underlying terminal hardware is enabled; see Control Modes. For example, you could clear the INPCK input mode flag and set the PARENB control mode flag to ignore parity errors on input, but still generate parity on output.

If this bit is set, what happens when a parity error is detected depends on whether the IGNPAR or PARMRK bits are set. If neither of these bits are set, a byte with a parity error is passed to the application as a '\0' character.

— Macro: tcflag_t IGNPAR

If this bit is set, any byte with a framing or parity error is ignored. This is only useful if INPCK is also set.

— Macro: tcflag_t PARMRK

If this bit is set, input bytes with parity or framing errors are marked when passed to the program. This bit is meaningful only when INPCK is set and IGNPAR is not set.

The way erroneous bytes are marked is with two preceding bytes, 377 and 0. Thus, the program actually reads three bytes for one erroneous byte received from the terminal.

If a valid byte has the value 0377, and ISTRIP (see below) is not set, the program might confuse it with the prefix that marks a parity error. So a valid byte 0377 is passed to the program as two bytes, 0377 0377, in this case.

— Macro: tcflag_t ISTRIP

If this bit is set, valid input bytes are stripped to seven bits; otherwise, all eight bits are available for programs to read.

— Macro: tcflag_t IGNBRK

If this bit is set, break conditions are ignored.

A break condition is defined in the context of asynchronous serial data transmission as a series of zero-value bits longer than a single byte.

— Macro: tcflag_t BRKINT

If this bit is set and IGNBRK is not set, a break condition clears the terminal input and output queues and raises a SIGINT signal for the foreground process group associated with the terminal.

If neither BRKINT nor IGNBRK are set, a break condition is passed to the application as a single '\0' character if PARMRK is not set, or otherwise as a three-character sequence '\377', '\0', '\0'.

— Macro: tcflag_t IGNCR

If this bit is set, carriage return characters ('\r') are discarded on input. Discarding carriage return may be useful on terminals that send both carriage return and linefeed when you type the <RET> key.

— Macro: tcflag_t ICRNL

If this bit is set and IGNCR is not set, carriage return characters ('\r') received as input are passed to the application as newline characters ('\n').

— Macro: tcflag_t INLCR

If this bit is set, newline characters ('\n') received as input are passed to the application as carriage return characters ('\r').

— Macro: tcflag_t IXOFF

If this bit is set, start/stop control on input is enabled. In other words, the computer sends STOP and START characters as necessary to prevent input from coming in faster than programs are reading it. The idea is that the actual terminal hardware that is generating the input data responds to a STOP character by suspending transmission, and to a START character by resuming transmission. See Start/Stop Characters.

— Macro: tcflag_t IXON

If this bit is set, start/stop control on output is enabled. In other words, if the computer receives a STOP character, it suspends output until a START character is received. In this case, the STOP and START characters are never passed to the application program. If this bit is not set, then START and STOP can be read as ordinary characters. See Start/Stop Characters.

— Macro: tcflag_t IXANY

If this bit is set, any input character restarts output when output has been suspended with the STOP character. Otherwise, only the START character restarts output.

This is a BSD extension; it exists only on BSD systems and the GNU system.

— Macro: tcflag_t IMAXBEL

If this bit is set, then filling up the terminal input buffer sends a BEL character (code 007) to the terminal to ring the bell.

This is a BSD extension.

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17.4.5 Output Modes

This section describes the terminal flags and fields that control how output characters are translated and padded for display. All of these are contained in the c_oflag member of the struct termios structure.

The c_oflag member itself is an integer, and you change the flags and fields using the operators &, |, and ^. Don't try to specify the entire value for c_oflag—instead, change only specific flags and leave the rest untouched (see Setting Modes).

— Macro: tcflag_t OPOST

If this bit is set, output data is processed in some unspecified way so that it is displayed appropriately on the terminal device. This typically includes mapping newline characters ('\n') onto carriage return and linefeed pairs.

If this bit isn't set, the characters are transmitted as-is.

The following three bits are BSD features, and they exist only BSD systems and the GNU system. They are effective only if OPOST is set.

— Macro: tcflag_t ONLCR

If this bit is set, convert the newline character on output into a pair of characters, carriage return followed by linefeed.

— Macro: tcflag_t OXTABS

If this bit is set, convert tab characters on output into the appropriate number of spaces to emulate a tab stop every eight columns.

— Macro: tcflag_t ONOEOT

If this bit is set, discard C-d characters (code 004) on output. These characters cause many dial-up terminals to disconnect.

Next: , Previous: Output Modes, Up: Terminal Modes

17.4.6 Control Modes

This section describes the terminal flags and fields that control parameters usually associated with asynchronous serial data transmission. These flags may not make sense for other kinds of terminal ports (such as a network connection pseudo-terminal). All of these are contained in the c_cflag member of the struct termios structure.

The c_cflag member itself is an integer, and you change the flags and fields using the operators &, |, and ^. Don't try to specify the entire value for c_cflag—instead, change only specific flags and leave the rest untouched (see Setting Modes).

— Macro: tcflag_t CLOCAL

If this bit is set, it indicates that the terminal is connected “locally” and that the modem status lines (such as carrier detect) should be ignored. On many systems if this bit is not set and you call open without the O_NONBLOCK flag set, open blocks until a modem connection is established.

If this bit is not set and a modem disconnect is detected, a SIGHUP signal is sent to the controlling process group for the terminal (if it has one). Normally, this causes the process to exit; see Signal Handling. Reading from the terminal after a disconnect causes an end-of-file condition, and writing causes an EIO error to be returned. The terminal device must be closed and reopened to clear the condition.

— Macro: tcflag_t HUPCL

If this bit is set, a modem disconnect is generated when all processes that have the terminal device open have either closed the file or exited.

— Macro: tcflag_t CREAD

If this bit is set, input can be read from the terminal. Otherwise, input is discarded when it arrives.

— Macro: tcflag_t CSTOPB

If this bit is set, two stop bits are used. Otherwise, only one stop bit is used.

— Macro: tcflag_t PARENB

If this bit is set, generation and detection of a parity bit are enabled. See Input Modes, for information on how input parity errors are handled.

If this bit is not set, no parity bit is added to output characters, and input characters are not checked for correct parity.

— Macro: tcflag_t PARODD

This bit is only useful if PARENB is set. If PARODD is set, odd parity is used, otherwise even parity is used.

The control mode flags also includes a field for the number of bits per character. You can use the CSIZE macro as a mask to extract the value, like this: settings.c_cflag & CSIZE.

— Macro: tcflag_t CSIZE

This is a mask for the number of bits per character.

— Macro: tcflag_t CS5

This specifies five bits per byte.

— Macro: tcflag_t CS6

This specifies six bits per byte.

— Macro: tcflag_t CS7

This specifies seven bits per byte.

— Macro: tcflag_t CS8

This specifies eight bits per byte.

The following four bits are BSD extensions; this exist only on BSD systems and the GNU system.

— Macro: tcflag_t CCTS_OFLOW

If this bit is set, enable flow control of output based on the CTS wire (RS232 protocol).

— Macro: tcflag_t CRTS_IFLOW

If this bit is set, enable flow control of input based on the RTS wire (RS232 protocol).

— Macro: tcflag_t MDMBUF

If this bit is set, enable carrier-based flow control of output.

— Macro: tcflag_t CIGNORE

If this bit is set, it says to ignore the control modes and line speed values entirely. This is only meaningful in a call to tcsetattr.

The c_cflag member and the line speed values returned by cfgetispeed and cfgetospeed will be unaffected by the call. CIGNORE is useful if you want to set all the software modes in the other members, but leave the hardware details in c_cflag unchanged. (This is how the TCSASOFT flag to tcsettattr works.)

This bit is never set in the structure filled in by tcgetattr.

Next: , Previous: Control Modes, Up: Terminal Modes

17.4.7 Local Modes

This section describes the flags for the c_lflag member of the struct termios structure. These flags generally control higher-level aspects of input processing than the input modes flags described in Input Modes, such as echoing, signals, and the choice of canonical or noncanonical input.

The c_lflag member itself is an integer, and you change the flags and fields using the operators &, |, and ^. Don't try to specify the entire value for c_lflag—instead, change only specific flags and leave the rest untouched (see Setting Modes).

— Macro: tcflag_t ICANON

This bit, if set, enables canonical input processing mode. Otherwise, input is processed in noncanonical mode. See Canonical or Not.

— Macro: tcflag_t ECHO

If this bit is set, echoing of input characters back to the terminal is enabled.

— Macro: tcflag_t ECHOE

If this bit is set, echoing indicates erasure of input with the ERASE character by erasing the last character in the current line from the screen. Otherwise, the character erased is re-echoed to show what has happened (suitable for a printing terminal).

This bit only controls the display behavior; the ICANON bit by itself controls actual recognition of the ERASE character and erasure of input, without which ECHOE is simply irrelevant.

— Macro: tcflag_t ECHOPRT

This bit is like ECHOE, enables display of the ERASE character in a way that is geared to a hardcopy terminal. When you type the ERASE character, a `\' character is printed followed by the first character erased. Typing the ERASE character again just prints the next character erased. Then, the next time you type a normal character, a `/' character is printed before the character echoes.

This is a BSD extension, and exists only in BSD systems and the GNU system.

— Macro: tcflag_t ECHOK

This bit enables special display of the KILL character by moving to a new line after echoing the KILL character normally. The behavior of ECHOKE (below) is nicer to look at.

If this bit is not set, the KILL character echoes just as it would if it were not the KILL character. Then it is up to the user to remember that the KILL character has erased the preceding input; there is no indication of this on the screen.

This bit only controls the display behavior; the ICANON bit by itself controls actual recognition of the KILL character and erasure of input, without which ECHOK is simply irrelevant.

— Macro: tcflag_t ECHOKE

This bit is similar to ECHOK. It enables special display of the KILL character by erasing on the screen the entire line that has been killed. This is a BSD extension, and exists only in BSD systems and the GNU system.

— Macro: tcflag_t ECHONL

If this bit is set and the ICANON bit is also set, then the newline ('\n') character is echoed even if the ECHO bit is not set.

— Macro: tcflag_t ECHOCTL

If this bit is set and the ECHO bit is also set, echo control characters with `^' followed by the corresponding text character. Thus, control-A echoes as `^A'. This is usually the preferred mode for interactive input, because echoing a control character back to the terminal could have some undesired effect on the terminal.

This is a BSD extension, and exists only in BSD systems and the GNU system.

— Macro: tcflag_t ISIG

This bit controls whether the INTR, QUIT, and SUSP characters are recognized. The functions associated with these characters are performed if and only if this bit is set. Being in canonical or noncanonical input mode has no affect on the interpretation of these characters.

You should use caution when disabling recognition of these characters. Programs that cannot be interrupted interactively are very user-unfriendly. If you clear this bit, your program should provide some alternate interface that allows the user to interactively send the signals associated with these characters, or to escape from the program. See Signal Characters.

— Macro: tcflag_t IEXTEN

POSIX.1 gives IEXTEN implementation-defined meaning, so you cannot rely on this interpretation on all systems.

On BSD systems and the GNU system, it enables the LNEXT and DISCARD characters. See Other Special.

— Macro: tcflag_t NOFLSH

Normally, the INTR, QUIT, and SUSP characters cause input and output queues for the terminal to be cleared. If this bit is set, the queues are not cleared.

— Macro: tcflag_t TOSTOP

If this bit is set and the system supports job control, then SIGTTOU signals are generated by background processes that attempt to write to the terminal. See Access to the Terminal.

The following bits are BSD extensions; they exist only in BSD systems and the GNU system.

— Macro: tcflag_t ALTWERASE

This bit determines how far the WERASE character should erase. The WERASE character erases back to the beginning of a word; the question is, where do words begin?

If this bit is clear, then the beginning of a word is a nonwhitespace character following a whitespace character. If the bit is set, then the beginning of a word is an alphanumeric character or underscore following a character which is none of those.

See Editing Characters, for more information about the WERASE character.

— Macro: tcflag_t FLUSHO

This is the bit that toggles when the user types the DISCARD character. While this bit is set, all output is discarded. See Other Special.

— Macro: tcflag_t NOKERNINFO

Setting this bit disables handling of the STATUS character. See Other Special.

— Macro: tcflag_t PENDIN

If this bit is set, it indicates that there is a line of input that needs to be reprinted. Typing the REPRINT character sets this bit; the bit remains set until reprinting is finished. See Editing Characters.

Next: , Previous: Local Modes, Up: Terminal Modes

17.4.8 Line Speed

The terminal line speed tells the computer how fast to read and write data on the terminal.

If the terminal is connected to a real serial line, the terminal speed you specify actually controls the line—if it doesn't match the terminal's own idea of the speed, communication does not work. Real serial ports accept only certain standard speeds. Also, particular hardware may not support even all the standard speeds. Specifying a speed of zero hangs up a dialup connection and turns off modem control signals.

If the terminal is not a real serial line (for example, if it is a network connection), then the line speed won't really affect data transmission speed, but some programs will use it to determine the amount of padding needed. It's best to specify a line speed value that matches the actual speed of the actual terminal, but you can safely experiment with different values to vary the amount of padding.

There are actually two line speeds for each terminal, one for input and one for output. You can set them independently, but most often terminals use the same speed for both directions.

The speed values are stored in the struct termios structure, but don't try to access them in the struct termios structure directly. Instead, you should use the following functions to read and store them:

— Function: speed_t cfgetospeed (const struct termios *termios-p)

This function returns the output line speed stored in the structure *termios-p.

— Function: speed_t cfgetispeed (const struct termios *termios-p)

This function returns the input line speed stored in the structure *termios-p.

— Function: int cfsetospeed (struct termios *termios-p, speed_t speed)

This function stores speed in *termios-p as the output speed. The normal return value is 0; a value of -1 indicates an error. If speed is not a speed, cfsetospeed returns -1.

— Function: int cfsetispeed (struct termios *termios-p, speed_t speed)

This function stores speed in *termios-p as the input speed. The normal return value is 0; a value of -1 indicates an error. If speed is not a speed, cfsetospeed returns -1.

— Function: int cfsetspeed (struct termios *termios-p, speed_t speed)

This function stores speed in *termios-p as both the input and output speeds. The normal return value is 0; a value of -1 indicates an error. If speed is not a speed, cfsetspeed returns -1. This function is an extension in 4.4 BSD.

— Data Type: speed_t

The speed_t type is an unsigned integer data type used to represent line speeds.

The functions cfsetospeed and cfsetispeed report errors only for speed values that the system simply cannot handle. If you specify a speed value that is basically acceptable, then those functions will succeed. But they do not check that a particular hardware device can actually support the specified speeds—in fact, they don't know which device you plan to set the speed for. If you use tcsetattr to set the speed of a particular device to a value that it cannot handle, tcsetattr returns -1.

Portability note: In the GNU library, the functions above accept speeds measured in bits per second as input, and return speed values measured in bits per second. Other libraries require speeds to be indicated by special codes. For POSIX.1 portability, you must use one of the following symbols to represent the speed; their precise numeric values are system-dependent, but each name has a fixed meaning: B110 stands for 110 bps, B300 for 300 bps, and so on. There is no portable way to represent any speed but these, but these are the only speeds that typical serial lines can support. 

     B0  B50  B75  B110  B134  B150  B200
     B300  B600  B1200  B1800  B2400  B4800
     B9600  B19200  B38400  B57600  B115200
     B230400  B460800

BSD defines two additional speed symbols as aliases: EXTA is an alias for B19200 and EXTB is an alias for B38400. These aliases are obsolete.

Next: , Previous: Line Speed, Up: Terminal Modes

17.4.9 Special Characters

In canonical input, the terminal driver recognizes a number of special characters which perform various control functions. These include the ERASE character (usually <DEL>) for editing input, and other editing characters. The INTR character (normally C-c) for sending a SIGINT signal, and other signal-raising characters, may be available in either canonical or noncanonical input mode. All these characters are described in this section.

The particular characters used are specified in the c_cc member of the struct termios structure. This member is an array; each element specifies the character for a particular role. Each element has a symbolic constant that stands for the index of that element—for example, VINTR is the index of the element that specifies the INTR character, so storing '=' in termios.c_cc[VINTR] specifies `=' as the INTR character.

On some systems, you can disable a particular special character function by specifying the value _POSIX_VDISABLE for that role. This value is unequal to any possible character code. See Options for Files, for more information about how to tell whether the operating system you are using supports _POSIX_VDISABLE.

Next: , Up: Special Characters
17.4.9.1 Characters for Input Editing

These special characters are active only in canonical input mode. See Canonical or Not.

— Macro: int VEOF

This is the subscript for the EOF character in the special control character array. termios.c_cc[VEOF] holds the character itself.

The EOF character is recognized only in canonical input mode. It acts as a line terminator in the same way as a newline character, but if the EOF character is typed at the beginning of a line it causes read to return a byte count of zero, indicating end-of-file. The EOF character itself is discarded.

Usually, the EOF character is C-d.

— Macro: int VEOL

This is the subscript for the EOL character in the special control character array. termios.c_cc[VEOL] holds the character itself.

The EOL character is recognized only in canonical input mode. It acts as a line terminator, just like a newline character. The EOL character is not discarded; it is read as the last character in the input line.

You don't need to use the EOL character to make <RET> end a line. Just set the ICRNL flag. In fact, this is the default state of affairs.

— Macro: int VEOL2

This is the subscript for the EOL2 character in the special control character array. termios.c_cc[VEOL2] holds the character itself.

The EOL2 character works just like the EOL character (see above), but it can be a different character. Thus, you can specify two characters to terminate an input line, by setting EOL to one of them and EOL2 to the other.

The EOL2 character is a BSD extension; it exists only on BSD systems and the GNU system.

— Macro: int VERASE

This is the subscript for the ERASE character in the special control character array. termios.c_cc[VERASE] holds the character itself.

The ERASE character is recognized only in canonical input mode. When the user types the erase character, the previous character typed is discarded. (If the terminal generates multibyte character sequences, this may cause more than one byte of input to be discarded.) This cannot be used to erase past the beginning of the current line of text. The ERASE character itself is discarded.

Usually, the ERASE character is <DEL>.

— Macro: int VWERASE

This is the subscript for the WERASE character in the special control character array. termios.c_cc[VWERASE] holds the character itself.

The WERASE character is recognized only in canonical mode. It erases an entire word of prior input, and any whitespace after it; whitespace characters before the word are not erased.

The definition of a “word” depends on the setting of the ALTWERASE mode; see Local Modes.

If the ALTWERASE mode is not set, a word is defined as a sequence of any characters except space or tab.

If the ALTWERASE mode is set, a word is defined as a sequence of characters containing only letters, numbers, and underscores, optionally followed by one character that is not a letter, number, or underscore.

The WERASE character is usually C-w.

This is a BSD extension.

— Macro: int VKILL

This is the subscript for the KILL character in the special control character array. termios.c_cc[VKILL] holds the character itself.

The KILL character is recognized only in canonical input mode. When the user types the kill character, the entire contents of the current line of input are discarded. The kill character itself is discarded too.

The KILL character is usually C-u.

— Macro: int VREPRINT

This is the subscript for the REPRINT character in the special control character array. termios.c_cc[VREPRINT] holds the character itself.

The REPRINT character is recognized only in canonical mode. It reprints the current input line. If some asynchronous output has come while you are typing, this lets you see the line you are typing clearly again.

The REPRINT character is usually C-r.

This is a BSD extension.

Next: , Previous: Editing Characters, Up: Special Characters
17.4.9.2 Characters that Cause Signals

These special characters may be active in either canonical or noncanonical input mode, but only when the ISIG flag is set (see Local Modes).

— Macro: int VINTR

This is the subscript for the INTR character in the special control character array. termios.c_cc[VINTR] holds the character itself.

The INTR (interrupt) character raises a SIGINT signal for all processes in the foreground job associated with the terminal. The INTR character itself is then discarded. See Signal Handling, for more information about signals.

Typically, the INTR character is C-c.

— Macro: int VQUIT

This is the subscript for the QUIT character in the special control character array. termios.c_cc[VQUIT] holds the character itself.

The QUIT character raises a SIGQUIT signal for all processes in the foreground job associated with the terminal. The QUIT character itself is then discarded. See Signal Handling, for more information about signals.

Typically, the QUIT character is C-\.

— Macro: int VSUSP

This is the subscript for the SUSP character in the special control character array. termios.c_cc[VSUSP] holds the character itself.

The SUSP (suspend) character is recognized only if the implementation supports job control (see Job Control). It causes a SIGTSTP signal to be sent to all processes in the foreground job associated with the terminal. The SUSP character itself is then discarded. See Signal Handling, for more information about signals.

Typically, the SUSP character is C-z.

Few applications disable the normal interpretation of the SUSP character. If your program does this, it should provide some other mechanism for the user to stop the job. When the user invokes this mechanism, the program should send a SIGTSTP signal to the process group of the process, not just to the process itself. See Signaling Another Process.

— Macro: int VDSUSP

This is the subscript for the DSUSP character in the special control character array. termios.c_cc[VDSUSP] holds the character itself.

The DSUSP (suspend) character is recognized only if the implementation supports job control (see Job Control). It sends a SIGTSTP signal, like the SUSP character, but not right away—only when the program tries to read it as input. Not all systems with job control support DSUSP; only BSD-compatible systems (including the GNU system).

See Signal Handling, for more information about signals.

Typically, the DSUSP character is C-y.

Next: , Previous: Signal Characters, Up: Special Characters
17.4.9.3 Special Characters for Flow Control

These special characters may be active in either canonical or noncanonical input mode, but their use is controlled by the flags IXON and IXOFF (see Input Modes).

— Macro: int VSTART

This is the subscript for the START character in the special control character array. termios.c_cc[VSTART] holds the character itself.

The START character is used to support the IXON and IXOFF input modes. If IXON is set, receiving a START character resumes suspended output; the START character itself is discarded. If IXANY is set, receiving any character at all resumes suspended output; the resuming character is not discarded unless it is the START character. IXOFF is set, the system may also transmit START characters to the terminal.

The usual value for the START character is C-q. You may not be able to change this value—the hardware may insist on using C-q regardless of what you specify.

— Macro: int VSTOP

This is the subscript for the STOP character in the special control character array. termios.c_cc[VSTOP] holds the character itself.

The STOP character is used to support the IXON and IXOFF input modes. If IXON is set, receiving a STOP character causes output to be suspended; the STOP character itself is discarded. If IXOFF is set, the system may also transmit STOP characters to the terminal, to prevent the input queue from overflowing.

The usual value for the STOP character is C-s. You may not be able to change this value—the hardware may insist on using C-s regardless of what you specify.

Previous: Start/Stop Characters, Up: Special Characters
17.4.9.4 Other Special Characters

These special characters exist only in BSD systems and the GNU system.

— Macro: int VLNEXT

This is the subscript for the LNEXT character in the special control character array. termios.c_cc[VLNEXT] holds the character itself.

The LNEXT character is recognized only when IEXTEN is set, but in both canonical and noncanonical mode. It disables any special significance of the next character the user types. Even if the character would normally perform some editing function or generate a signal, it is read as a plain character. This is the analogue of the C-q command in Emacs. “LNEXT” stands for “literal next.”

The LNEXT character is usually C-v.

— Macro: int VDISCARD

This is the subscript for the DISCARD character in the special control character array. termios.c_cc[VDISCARD] holds the character itself.

The DISCARD character is recognized only when IEXTEN is set, but in both canonical and noncanonical mode. Its effect is to toggle the discard-output flag. When this flag is set, all program output is discarded. Setting the flag also discards all output currently in the output buffer. Typing any other character resets the flag.

— Macro: int VSTATUS

This is the subscript for the STATUS character in the special control character array. termios.c_cc[VSTATUS] holds the character itself.

The STATUS character's effect is to print out a status message about how the current process is running.

The STATUS character is recognized only in canonical mode, and only if NOKERNINFO is not set.

Previous: Special Characters, Up: Terminal Modes

17.4.10 Noncanonical Input

In noncanonical input mode, the special editing characters such as ERASE and KILL are ignored. The system facilities for the user to edit input are disabled in noncanonical mode, so that all input characters (unless they are special for signal or flow-control purposes) are passed to the application program exactly as typed. It is up to the application program to give the user ways to edit the input, if appropriate.

Noncanonical mode offers special parameters called MIN and TIME for controlling whether and how long to wait for input to be available. You can even use them to avoid ever waiting—to return immediately with whatever input is available, or with no input.

The MIN and TIME are stored in elements of the c_cc array, which is a member of the struct termios structure. Each element of this array has a particular role, and each element has a symbolic constant that stands for the index of that element. VMIN and VMAX are the names for the indices in the array of the MIN and TIME slots.

— Macro: int VMIN

This is the subscript for the MIN slot in the c_cc array. Thus, termios.c_cc[VMIN] is the value itself.

The MIN slot is only meaningful in noncanonical input mode; it specifies the minimum number of bytes that must be available in the input queue in order for read to return.

— Macro: int VTIME

This is the subscript for the TIME slot in the c_cc array. Thus, termios.c_cc[VTIME] is the value itself.

The TIME slot is only meaningful in noncanonical input mode; it specifies how long to wait for input before returning, in units of 0.1 seconds.

The MIN and TIME values interact to determine the criterion for when read should return; their precise meanings depend on which of them are nonzero. There are four possible cases:

What happens if MIN is 50 and you ask to read just 10 bytes? Normally, read waits until there are 50 bytes in the buffer (or, more generally, the wait condition described above is satisfied), and then reads 10 of them, leaving the other 40 buffered in the operating system for a subsequent call to read.

Portability note: On some systems, the MIN and TIME slots are actually the same as the EOF and EOL slots. This causes no serious problem because the MIN and TIME slots are used only in noncanonical input and the EOF and EOL slots are used only in canonical input, but it isn't very clean. The GNU library allocates separate slots for these uses.

— Function: void cfmakeraw (struct termios *termios-p)

This function provides an easy way to set up *termios-p for what has traditionally been called “raw mode” in BSD. This uses noncanonical input, and turns off most processing to give an unmodified channel to the terminal.

It does exactly this: 

            termios-p->c_iflag &= ~(IGNBRK|BRKINT|PARMRK|ISTRIP
                                          |INLCR|IGNCR|ICRNL|IXON);
            termios-p->c_oflag &= ~OPOST;
            termios-p->c_lflag &= ~(ECHO|ECHONL|ICANON|ISIG|IEXTEN);
            termios-p->c_cflag &= ~(CSIZE|PARENB);
            termios-p->c_cflag |= CS8;

Next: , Previous: Terminal Modes, Up: Low-Level Terminal Interface

17.5 BSD Terminal Modes

The usual way to get and set terminal modes is with the functions described in Terminal Modes. However, on some systems you can use the BSD-derived functions in this section to do some of the same thing. On many systems, these functions do not exist. Even with the GNU C library, the functions simply fail with errno = ENOSYS with many kernels, including Linux.

The symbols used in this section are declared in sgtty.h.

— Data Type: struct sgttyb

This structure is an input or output parameter list for gtty and stty.

char sg_ispeed
Line speed for input
char sg_ospeed
Line speed for output
char sg_erase
Erase character
char sg_kill
Kill character
int sg_flags
Various flags

— Function: int gtty (int filedes, struct sgttyb *attributes)

This function gets the attributes of a terminal.

gtty sets *attributes to describe the terminal attributes of the terminal which is open with file descriptor filedes.

— Function: int stty (int filedes, struct sgttyb * attributes)

This function sets the attributes of a terminal.

stty sets the terminal attributes of the terminal which is open with file descriptor filedes to those described by *filedes.

Next: , Previous: BSD Terminal Modes, Up: Low-Level Terminal Interface

17.6 Line Control Functions

These functions perform miscellaneous control actions on terminal devices. As regards terminal access, they are treated like doing output: if any of these functions is used by a background process on its controlling terminal, normally all processes in the process group are sent a SIGTTOU signal. The exception is if the calling process itself is ignoring or blocking SIGTTOU signals, in which case the operation is performed and no signal is sent. See Job Control.

— Function: int tcsendbreak (int filedes, int duration)

This function generates a break condition by transmitting a stream of zero bits on the terminal associated with the file descriptor filedes. The duration of the break is controlled by the duration argument. If zero, the duration is between 0.25 and 0.5 seconds. The meaning of a nonzero value depends on the operating system.

This function does nothing if the terminal is not an asynchronous serial data port.

The return value is normally zero. In the event of an error, a value of -1 is returned. The following errno error conditions are defined for this function:

EBADF
The filedes is not a valid file descriptor.
ENOTTY
The filedes is not associated with a terminal device.

— Function: int tcdrain (int filedes)

The tcdrain function waits until all queued output to the terminal filedes has been transmitted.

This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time tcdrain is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this calls to tcdrain should be protected using cancellation handlers.

The return value is normally zero. In the event of an error, a value of -1 is returned. The following errno error conditions are defined for this function:

EBADF
The filedes is not a valid file descriptor.
ENOTTY
The filedes is not associated with a terminal device.
EINTR
The operation was interrupted by delivery of a signal. See Interrupted Primitives.

— Function: int tcflush (int filedes, int queue)

The tcflush function is used to clear the input and/or output queues associated with the terminal file filedes. The queue argument specifies which queue(s) to clear, and can be one of the following values:

TCIFLUSH
Clear any input data received, but not yet read.


TCOFLUSH
Clear any output data written, but not yet transmitted.


TCIOFLUSH
Clear both queued input and output.

The return value is normally zero. In the event of an error, a value of -1 is returned. The following errno error conditions are defined for this function:

EBADF
The filedes is not a valid file descriptor.
ENOTTY
The filedes is not associated with a terminal device.
EINVAL
A bad value was supplied as the queue argument.

It is unfortunate that this function is named tcflush, because the term “flush” is normally used for quite another operation—waiting until all output is transmitted—and using it for discarding input or output would be confusing. Unfortunately, the name tcflush comes from POSIX and we cannot change it.

— Function: int tcflow (int filedes, int action)

The tcflow function is used to perform operations relating to XON/XOFF flow control on the terminal file specified by filedes.

The action argument specifies what operation to perform, and can be one of the following values:

TCOOFF
Suspend transmission of output.


TCOON
Restart transmission of output.


TCIOFF
Transmit a STOP character.


TCION
Transmit a START character.

For more information about the STOP and START characters, see Special Characters.

The return value is normally zero. In the event of an error, a value of -1 is returned. The following errno error conditions are defined for this function:

EBADF
The filedes is not a valid file descriptor.


ENOTTY
The filedes is not associated with a terminal device.


EINVAL
A bad value was supplied as the action argument.

Next: , Previous: Line Control, Up: Low-Level Terminal Interface

17.7 Noncanonical Mode Example

Here is an example program that shows how you can set up a terminal device to read single characters in noncanonical input mode, without echo. 

     #include <unistd.h>
     #include <stdio.h>
     #include <stdlib.h>
     #include <termios.h>
     
     /* Use this variable to remember original terminal attributes. */
     
     struct termios saved_attributes;
     
     void
     reset_input_mode (void)
     {
       tcsetattr (STDIN_FILENO, TCSANOW, &saved_attributes);
     }
     
     void
     set_input_mode (void)
     {
       struct termios tattr;
       char *name;
     
       /* Make sure stdin is a terminal. */
       if (!isatty (STDIN_FILENO))
         {
           fprintf (stderr, "Not a terminal.\n");
           exit (EXIT_FAILURE);
         }
     
       /* Save the terminal attributes so we can restore them later. */
       tcgetattr (STDIN_FILENO, &saved_attributes);
       atexit (reset_input_mode);
     
       /* Set the funny terminal modes. */
       tcgetattr (STDIN_FILENO, &tattr);
       tattr.c_lflag &= ~(ICANON|ECHO); /* Clear ICANON and ECHO. */
       tattr.c_cc[VMIN] = 1;
       tattr.c_cc[VTIME] = 0;
       tcsetattr (STDIN_FILENO, TCSAFLUSH, &tattr);
     }
     
     int
     main (void)
     {
       char c;
     
       set_input_mode ();
     
       while (1)
         {
           read (STDIN_FILENO, &c, 1);
           if (c == '\004')          /* C-d */
             break;
           else
             putchar (c);
         }
     
       return EXIT_SUCCESS;
     }

This program is careful to restore the original terminal modes before exiting or terminating with a signal. It uses the atexit function (see Cleanups on Exit) to make sure this is done by exit.

The shell is supposed to take care of resetting the terminal modes when a process is stopped or continued; see Job Control. But some existing shells do not actually do this, so you may wish to establish handlers for job control signals that reset terminal modes. The above example does so.

Previous: Noncanon Example, Up: Low-Level Terminal Interface

17.8 Pseudo-Terminals

A pseudo-terminal is a special interprocess communication channel that acts like a terminal. One end of the channel is called the master side or master pseudo-terminal device, the other side is called the slave side. Data written to the master side is received by the slave side as if it was the result of a user typing at an ordinary terminal, and data written to the slave side is sent to the master side as if it was written on an ordinary terminal.

Pseudo terminals are the way programs like xterm and emacs implement their terminal emulation functionality.

Next: , Up: Pseudo-Terminals

17.8.1 Allocating Pseudo-Terminals

This subsection describes functions for allocating a pseudo-terminal, and for making this pseudo-terminal available for actual use. These functions are declared in the header file stdlib.h.

— Function: int getpt (void)

The getpt function returns a new file descriptor for the next available master pseudo-terminal. The normal return value from getpt is a non-negative integer file descriptor. In the case of an error, a value of -1 is returned instead. The following errno conditions are defined for this function:

ENOENT
There are no free master pseudo-terminals available.

This function is a GNU extension.

— Function: int grantpt (int filedes)

The grantpt function changes the ownership and access permission of the slave pseudo-terminal device corresponding to the master pseudo-terminal device associated with the file descriptor filedes. The owner is set from the real user ID of the calling process (see Process Persona), and the group is set to a special group (typically tty) or from the real group ID of the calling process. The access permission is set such that the file is both readable and writable by the owner and only writable by the group.

On some systems this function is implemented by invoking a special setuid root program (see How Change Persona). As a consequence, installing a signal handler for the SIGCHLD signal (see Job Control Signals) may interfere with a call to grantpt.

The normal return value from grantpt is 0; a value of -1 is returned in case of failure. The following errno error conditions are defined for this function:

EBADF
The filedes argument is not a valid file descriptor.
EINVAL
The filedes argument is not associated with a master pseudo-terminal device.
EACCES
The slave pseudo-terminal device corresponding to the master associated with filedes could not be accessed.
— Function: int unlockpt (int filedes)

The unlockpt function unlocks the slave pseudo-terminal device corresponding to the master pseudo-terminal device associated with the file descriptor filedes. On many systems, the slave can only be opened after unlocking, so portable applications should always call unlockpt before trying to open the slave.

The normal return value from unlockpt is 0; a value of -1 is returned in case of failure. The following errno error conditions are defined for this function:

EBADF
The filedes argument is not a valid file descriptor.
EINVAL
The filedes argument is not associated with a master pseudo-terminal device.

— Function: char * ptsname (int filedes)

If the file descriptor filedes is associated with a master pseudo-terminal device, the ptsname function returns a pointer to a statically-allocated, null-terminated string containing the file name of the associated slave pseudo-terminal file. This string might be overwritten by subsequent calls to ptsname.

— Function: int ptsname_r (int filedes, char *buf, size_t len)

The ptsname_r function is similar to the ptsname function except that it places its result into the user-specified buffer starting at buf with length len.

This function is a GNU extension.

Portability Note: On System V derived systems, the file returned by the ptsname and ptsname_r functions may be STREAMS-based, and therefore require additional processing after opening before it actually behaves as a pseudo terminal.

Typical usage of these functions is illustrated by the following example: 

     int
     open_pty_pair (int *amaster, int *aslave)
     {
       int master, slave;
       char *name;
     
       master = getpt ();
       if (master < 0)
         return 0;
     
       if (grantpt (master) < 0 || unlockpt (master) < 0)
         goto close_master;
       name = ptsname (master);
       if (name == NULL)
         goto close_master;
     
       slave = open (name, O_RDWR);
       if (slave == -1)
         goto close_master;
     
       if (isastream (slave))
         {
           if (ioctl (slave, I_PUSH, "ptem") < 0
               || ioctl (slave, I_PUSH, "ldterm") < 0)
             goto close_slave;
         }
     
       *amaster = master;
       *aslave = slave;
       return 1;
     
     close_slave:
       close (slave);
     
     close_master:
       close (master);
       return 0;
     }

Previous: Allocation, Up: Pseudo-Terminals

17.8.2 Opening a Pseudo-Terminal Pair

These functions, derived from BSD, are available in the separate libutil library, and declared in pty.h.

— Function: int openpty (int *amaster, int *aslave, char *name, struct termios *termp, struct winsize *winp)

This function allocates and opens a pseudo-terminal pair, returning the file descriptor for the master in *amaster, and the file descriptor for the slave in *aslave. If the argument name is not a null pointer, the file name of the slave pseudo-terminal device is stored in *name. If termp is not a null pointer, the terminal attributes of the slave are set to the ones specified in the structure that termp points to (see Terminal Modes). Likewise, if the winp is not a null pointer, the screen size of the slave is set to the values specified in the structure that winp points to.

The normal return value from openpty is 0; a value of -1 is returned in case of failure. The following errno conditions are defined for this function:

ENOENT
There are no free pseudo-terminal pairs available.

Warning: Using the openpty function with name not set to NULL is very dangerous because it provides no protection against overflowing the string name. You should use the ttyname function on the file descriptor returned in *slave to find out the file name of the slave pseudo-terminal device instead.

— Function: int forkpty (int *amaster, char *name, struct termios *termp, struct winsize *winp)

This function is similar to the openpty function, but in addition, forks a new process (see Creating a Process) and makes the newly opened slave pseudo-terminal device the controlling terminal (see Controlling Terminal) for the child process.

If the operation is successful, there are then both parent and child processes and both see forkpty return, but with different values: it returns a value of 0 in the child process and returns the child's process ID in the parent process.

If the allocation of a pseudo-terminal pair or the process creation failed, forkpty returns a value of -1 in the parent process.

Warning: The forkpty function has the same problems with respect to the name argument as openpty.

Next: , Previous: Low-Level Terminal Interface, Up: Top

18 Syslog

This chapter describes facilities for issuing and logging messages of system administration interest. This chapter has nothing to do with programs issuing messages to their own users or keeping private logs (One would typically do that with the facilities described in I/O on Streams).

Most systems have a facility called “Syslog” that allows programs to submit messages of interest to system administrators and can be configured to pass these messages on in various ways, such as printing on the console, mailing to a particular person, or recording in a log file for future reference.

A program uses the facilities in this chapter to submit such messages.

Next: , Up: Syslog

18.1 Overview of Syslog

System administrators have to deal with lots of different kinds of messages from a plethora of subsystems within each system, and usually lots of systems as well. For example, an FTP server might report every connection it gets. The kernel might report hardware failures on a disk drive. A DNS server might report usage statistics at regular intervals.

Some of these messages need to be brought to a system administrator's attention immediately. And it may not be just any system administrator – there may be a particular system administrator who deals with a particular kind of message. Other messages just need to be recorded for future reference if there is a problem. Still others may need to have information extracted from them by an automated process that generates monthly reports.

To deal with these messages, most Unix systems have a facility called "Syslog." It is generally based on a daemon called “Syslogd” Syslogd listens for messages on a Unix domain socket named /dev/log. Based on classification information in the messages and its configuration file (usually /etc/syslog.conf), Syslogd routes them in various ways. Some of the popular routings are:

Syslogd can also handle messages from other systems. It listens on the syslog UDP port as well as the local socket for messages.

Syslog can handle messages from the kernel itself. But the kernel doesn't write to /dev/log; rather, another daemon (sometimes called “Klogd”) extracts messages from the kernel and passes them on to Syslog as any other process would (and it properly identifies them as messages from the kernel).

Syslog can even handle messages that the kernel issued before Syslogd or Klogd was running. A Linux kernel, for example, stores startup messages in a kernel message ring and they are normally still there when Klogd later starts up. Assuming Syslogd is running by the time Klogd starts, Klogd then passes everything in the message ring to it.

In order to classify messages for disposition, Syslog requires any process that submits a message to it to provide two pieces of classification information with it:

facility
This identifies who submitted the message. There are a small number of facilities defined. The kernel, the mail subsystem, and an FTP server are examples of recognized facilities. For the complete list, See syslog; vsyslog. Keep in mind that these are essentially arbitrary classifications. "Mail subsystem" doesn't have any more meaning than the system administrator gives to it.
priority
This tells how important the content of the message is. Examples of defined priority values are: debug, informational, warning, critical. For the complete list, See syslog; vsyslog. Except for the fact that the priorities have a defined order, the meaning of each of these priorities is entirely determined by the system administrator.

A “facility/priority” is a number that indicates both the facility and the priority.

Warning: This terminology is not universal. Some people use “level” to refer to the priority and “priority” to refer to the combination of facility and priority. A Linux kernel has a concept of a message “level,” which corresponds both to a Syslog priority and to a Syslog facility/priority (It can be both because the facility code for the kernel is zero, and that makes priority and facility/priority the same value).

The GNU C library provides functions to submit messages to Syslog. They do it by writing to the /dev/log socket. See Submitting Syslog Messages.

The GNU C library functions only work to submit messages to the Syslog facility on the same system. To submit a message to the Syslog facility on another system, use the socket I/O functions to write a UDP datagram to the syslog UDP port on that system. See Sockets.

Previous: Overview of Syslog, Up: Syslog

18.2 Submitting Syslog Messages

The GNU C library provides functions to submit messages to the Syslog facility:

These functions only work to submit messages to the Syslog facility on the same system. To submit a message to the Syslog facility on another system, use the socket I/O functions to write a UDP datagram to the syslog UDP port on that system. See Sockets.

Next: , Up: Submitting Syslog Messages

18.2.1 openlog

The symbols referred to in this section are declared in the file syslog.h.

— Function: void openlog (const char *ident, int option, int facility)

openlog opens or reopens a connection to Syslog in preparation for submitting messages.

ident is an arbitrary identification string which future syslog invocations will prefix to each message. This is intended to identify the source of the message, and people conventionally set it to the name of the program that will submit the messages.

If ident is NULL, or if openlog is not called, the default identification string used in Syslog messages will be the program name, taken from argv[0].

Please note that the string pointer ident will be retained internally by the Syslog routines. You must not free the memory that ident points to. It is also dangerous to pass a reference to an automatic variable since leaving the scope would mean ending the lifetime of the variable. If you want to change the ident string, you must call openlog again; overwriting the string pointed to by ident is not thread-safe.

You can cause the Syslog routines to drop the reference to ident and go back to the default string (the program name taken from argv[0]), by calling closelog: See closelog.

In particular, if you are writing code for a shared library that might get loaded and then unloaded (e.g. a PAM module), and you use openlog, you must call closelog before any point where your library might get unloaded, as in this example: 

          #include <syslog.h>
          
          void
          shared_library_function (void)
          {
            openlog ("mylibrary", option, priority);
          
            syslog (LOG_INFO, "shared library has been invoked");
          
            closelog ();
          }

Without the call to closelog, future invocations of syslog by the program using the shared library may crash, if the library gets unloaded and the memory containing the string "mylibrary" becomes unmapped. This is a limitation of the BSD syslog interface.

openlog may or may not open the /dev/log socket, depending on option. If it does, it tries to open it and connect it as a stream socket. If that doesn't work, it tries to open it and connect it as a datagram socket. The socket has the “Close on Exec” attribute, so the kernel will close it if the process performs an exec.

You don't have to use openlog. If you call syslog without having called openlog, syslog just opens the connection implicitly and uses defaults for the information in ident and options.

options is a bit string, with the bits as defined by the following single bit masks:

LOG_PERROR
If on, openlog sets up the connection so that any syslog on this connection writes its message to the calling process' Standard Error stream in addition to submitting it to Syslog. If off, syslog does not write the message to Standard Error.
LOG_CONS
If on, openlog sets up the connection so that a syslog on this connection that fails to submit a message to Syslog writes the message instead to system console. If off, syslog does not write to the system console (but of course Syslog may write messages it receives to the console).
LOG_PID
When on, openlog sets up the connection so that a syslog on this connection inserts the calling process' Process ID (PID) into the message. When off, openlog does not insert the PID.
LOG_NDELAY
When on, openlog opens and connects the /dev/log socket. When off, a future syslog call must open and connect the socket.

Portability note: In early systems, the sense of this bit was exactly the opposite.

LOG_ODELAY
This bit does nothing. It exists for backward compatibility.

If any other bit in options is on, the result is undefined.

facility is the default facility code for this connection. A syslog on this connection that specifies default facility causes this facility to be associated with the message. See syslog for possible values. A value of zero means the default default, which is LOG_USER.

If a Syslog connection is already open when you call openlog, openlog “reopens” the connection. Reopening is like opening except that if you specify zero for the default facility code, the default facility code simply remains unchanged and if you specify LOG_NDELAY and the socket is already open and connected, openlog just leaves it that way.

Next: , Previous: openlog, Up: Submitting Syslog Messages

18.2.2 syslog, vsyslog

The symbols referred to in this section are declared in the file syslog.h.

— Function: void syslog (int facility_priority, char *format, ...)

syslog submits a message to the Syslog facility. It does this by writing to the Unix domain socket /dev/log.

syslog submits the message with the facility and priority indicated by facility_priority. The macro LOG_MAKEPRI generates a facility/priority from a facility and a priority, as in the following example: 

          LOG_MAKEPRI(LOG_USER, LOG_WARNING)

The possible values for the facility code are (macros):

LOG_USER
A miscellaneous user process
LOG_MAIL
Mail
LOG_DAEMON
A miscellaneous system daemon
LOG_AUTH
Security (authorization)
LOG_SYSLOG
Syslog
LOG_LPR
Central printer
LOG_NEWS
Network news (e.g. Usenet)
LOG_UUCP
UUCP
LOG_CRON
Cron and At
LOG_AUTHPRIV
Private security (authorization)
LOG_FTP
Ftp server
LOG_LOCAL0
Locally defined
LOG_LOCAL1
Locally defined
LOG_LOCAL2
Locally defined
LOG_LOCAL3
Locally defined
LOG_LOCAL4
Locally defined
LOG_LOCAL5
Locally defined
LOG_LOCAL6
Locally defined
LOG_LOCAL7
Locally defined

Results are undefined if the facility code is anything else.

note: syslog recognizes one other facility code: that of the kernel. But you can't specify that facility code with these functions. If you try, it looks the same to syslog as if you are requesting the default facility. But you wouldn't want to anyway, because any program that uses the GNU C library is not the kernel.

You can use just a priority code as facility_priority. In that case, syslog assumes the default facility established when the Syslog connection was opened. See Syslog Example.

The possible values for the priority code are (macros):

LOG_EMERG
The message says the system is unusable.
LOG_ALERT
Action on the message must be taken immediately.
LOG_CRIT
The message states a critical condition.
LOG_ERR
The message describes an error.
LOG_WARNING
The message is a warning.
LOG_NOTICE
The message describes a normal but important event.
LOG_INFO
The message is purely informational.
LOG_DEBUG
The message is only for debugging purposes.

Results are undefined if the priority code is anything else.

If the process does not presently have a Syslog connection open (i.e., it did not call openlog), syslog implicitly opens the connection the same as openlog would, with the following defaults for information that would otherwise be included in an openlog call: The default identification string is the program name. The default default facility is LOG_USER. The default for all the connection options in options is as if those bits were off. syslog leaves the Syslog connection open.

If the dev/log socket is not open and connected, syslog opens and connects it, the same as openlog with the LOG_NDELAY option would.

syslog leaves /dev/log open and connected unless its attempt to send the message failed, in which case syslog closes it (with the hope that a future implicit open will restore the Syslog connection to a usable state).

Example: 

          
          #include <syslog.h>
          syslog (LOG_MAKEPRI(LOG_LOCAL1, LOG_ERROR),
                  "Unable to make network connection to %s.  Error=%m", host);
— Function: void vsyslog (int facility_priority, char *format, va_list arglist)

This is functionally identical to syslog, with the BSD style variable length argument.

Next: , Previous: syslog; vsyslog, Up: Submitting Syslog Messages

18.2.3 closelog

The symbols referred to in this section are declared in the file syslog.h.

— Function: void closelog (void)

closelog closes the current Syslog connection, if there is one. This includes closing the dev/log socket, if it is open. closelog also sets the identification string for Syslog messages back to the default, if openlog was called with a non-NULL argument to ident. The default identification string is the program name taken from argv[0].

If you are writing shared library code that uses openlog to generate custom syslog output, you should use closelog to drop the GNU C library's internal reference to the ident pointer when you are done. Please read the section on openlog for more information: See openlog.

closelog does not flush any buffers. You do not have to call closelog before re-opening a Syslog connection with initlog. Syslog connections are automatically closed on exec or exit.

Next: , Previous: closelog, Up: Submitting Syslog Messages

18.2.4 setlogmask

The symbols referred to in this section are declared in the file syslog.h.

— Function: int setlogmask (int mask)

setlogmask sets a mask (the “logmask”) that determines which future syslog calls shall be ignored. If a program has not called setlogmask, syslog doesn't ignore any calls. You can use setlogmask to specify that messages of particular priorities shall be ignored in the future.

A setlogmask call overrides any previous setlogmask call.

Note that the logmask exists entirely independently of opening and closing of Syslog connections.

Setting the logmask has a similar effect to, but is not the same as, configuring Syslog. The Syslog configuration may cause Syslog to discard certain messages it receives, but the logmask causes certain messages never to get submitted to Syslog in the first place.

mask is a bit string with one bit corresponding to each of the possible message priorities. If the bit is on, syslog handles messages of that priority normally. If it is off, syslog discards messages of that priority. Use the message priority macros described in syslog; vsyslog and the LOG_MASK to construct an appropriate mask value, as in this example: 

          LOG_MASK(LOG_EMERG) | LOG_MASK(LOG_ERROR)

or 

          ~(LOG_MASK(LOG_INFO))

There is also a LOG_UPTO macro, which generates a mask with the bits on for a certain priority and all priorities above it: 

          LOG_UPTO(LOG_ERROR)

The unfortunate naming of the macro is due to the fact that internally, higher numbers are used for lower message priorities.

Previous: setlogmask, Up: Submitting Syslog Messages

18.2.5 Syslog Example

Here is an example of openlog, syslog, and closelog:

This example sets the logmask so that debug and informational messages get discarded without ever reaching Syslog. So the second syslog in the example does nothing. 

     #include <syslog.h>
     
     setlogmask (LOG_UPTO (LOG_NOTICE));
     
     openlog ("exampleprog", LOG_CONS | LOG_PID | LOG_NDELAY, LOG_LOCAL1);
     
     syslog (LOG_NOTICE, "Program started by User %d", getuid ());
     syslog (LOG_INFO, "A tree falls in a forest");
     
     closelog ();

Next: , Previous: Syslog, Up: Top

19 Mathematics

This chapter contains information about functions for performing mathematical computations, such as trigonometric functions. Most of these functions have prototypes declared in the header file math.h. The complex-valued functions are defined in complex.h. All mathematical functions which take a floating-point argument have three variants, one each for double, float, and long double arguments. The double versions are mostly defined in ISO C89. The float and long double versions are from the numeric extensions to C included in ISO C99.

Which of the three versions of a function should be used depends on the situation. For most calculations, the float functions are the fastest. On the other hand, the long double functions have the highest precision. double is somewhere in between. It is usually wise to pick the narrowest type that can accommodate your data. Not all machines have a distinct long double type; it may be the same as double.

Next: , Up: Mathematics

19.1 Predefined Mathematical Constants

The header math.h defines several useful mathematical constants. All values are defined as preprocessor macros starting with M_. The values provided are:

M_E
The base of natural logarithms.
M_LOG2E
The logarithm to base 2 of M_E.
M_LOG10E
The logarithm to base 10 of M_E.
M_LN2
The natural logarithm of 2.
M_LN10
The natural logarithm of 10.
M_PI
Pi, the ratio of a circle's circumference to its diameter.
M_PI_2
Pi divided by two.
M_PI_4
Pi divided by four.
M_1_PI
The reciprocal of pi (1/pi)
M_2_PI
Two times the reciprocal of pi.
M_2_SQRTPI
Two times the reciprocal of the square root of pi.
M_SQRT2
The square root of two.
M_SQRT1_2
The reciprocal of the square root of two (also the square root of 1/2).

These constants come from the Unix98 standard and were also available in 4.4BSD; therefore they are only defined if _BSD_SOURCE or _XOPEN_SOURCE=500, or a more general feature select macro, is defined. The default set of features includes these constants. See Feature Test Macros.

All values are of type double. As an extension, the GNU C library also defines these constants with type long double. The long double macros have a lowercase `l' appended to their names: M_El, M_PIl, and so forth. These are only available if _GNU_SOURCE is defined.

Note: Some programs use a constant named PI which has the same value as M_PI. This constant is not standard; it may have appeared in some old AT&T headers, and is mentioned in Stroustrup's book on C++. It infringes on the user's name space, so the GNU C library does not define it. Fixing programs written to expect it is simple: replace PI with M_PI throughout, or put `-DPI=M_PI' on the compiler command line.

Next: , Previous: Mathematical Constants, Up: Mathematics

19.2 Trigonometric Functions

These are the familiar sin, cos, and tan functions. The arguments to all of these functions are in units of radians; recall that pi radians equals 180 degrees.

The math library normally defines M_PI to a double approximation of pi. If strict ISO and/or POSIX compliance are requested this constant is not defined, but you can easily define it yourself: 

     #define M_PI 3.14159265358979323846264338327

You can also compute the value of pi with the expression acos (-1.0).

— Function: double sin (double x)
— Function: float sinf (float x)
— Function: long double sinl (long double x)

These functions return the sine of x, where x is given in radians. The return value is in the range -1 to 1.

— Function: double cos (double x)
— Function: float cosf (float x)
— Function: long double cosl (long double x)

These functions return the cosine of x, where x is given in radians. The return value is in the range -1 to 1.

— Function: double tan (double x)
— Function: float tanf (float x)
— Function: long double tanl (long double x)

These functions return the tangent of x, where x is given in radians.

Mathematically, the tangent function has singularities at odd multiples of pi/2. If the argument x is too close to one of these singularities, tan will signal overflow.

In many applications where sin and cos are used, the sine and cosine of the same angle are needed at the same time. It is more efficient to compute them simultaneously, so the library provides a function to do that.

— Function: void sincos (double x, double *sinx, double *cosx)
— Function: void sincosf (float x, float *sinx, float *cosx)
— Function: void sincosl (long double x, long double *sinx, long double *cosx)

These functions return the sine of x in *sinx and the cosine of x in *cos, where x is given in radians. Both values, *sinx and *cosx, are in the range of -1 to 1.

This function is a GNU extension. Portable programs should be prepared to cope with its absence.

ISO C99 defines variants of the trig functions which work on complex numbers. The GNU C library provides these functions, but they are only useful if your compiler supports the new complex types defined by the standard. (As of this writing GCC supports complex numbers, but there are bugs in the implementation.)

— Function: complex double csin (complex double z)
— Function: complex float csinf (complex float z)
— Function: complex long double csinl (complex long double z)

These functions return the complex sine of z. The mathematical definition of the complex sine is

sin (z) = 1/(2*i) * (exp (z*i) - exp (-z*i)).

— Function: complex double ccos (complex double z)
— Function: complex float ccosf (complex float z)
— Function: complex long double ccosl (complex long double z)

These functions return the complex cosine of z. The mathematical definition of the complex cosine is

cos (z) = 1/2 * (exp (z*i) + exp (-z*i))

— Function: complex double ctan (complex double z)
— Function: complex float ctanf (complex float z)
— Function: complex long double ctanl (complex long double z)

These functions return the complex tangent of z. The mathematical definition of the complex tangent is

tan (z) = -i * (exp (z*i) - exp (-z*i)) / (exp (z*i) + exp (-z*i))

The complex tangent has poles at pi/2 + 2n, where n is an integer. ctan may signal overflow if z is too close to a pole.

Next: , Previous: Trig Functions, Up: Mathematics

19.3 Inverse Trigonometric Functions

These are the usual arc sine, arc cosine and arc tangent functions, which are the inverses of the sine, cosine and tangent functions respectively.

— Function: double asin (double x)
— Function: float asinf (float x)
— Function: long double asinl (long double x)

These functions compute the arc sine of x—that is, the value whose sine is x. The value is in units of radians. Mathematically, there are infinitely many such values; the one actually returned is the one between -pi/2 and pi/2 (inclusive).

The arc sine function is defined mathematically only over the domain -1 to 1. If x is outside the domain, asin signals a domain error.

— Function: double acos (double x)
— Function: float acosf (float x)
— Function: long double acosl (long double x)

These functions compute the arc cosine of x—that is, the value whose cosine is x. The value is in units of radians. Mathematically, there are infinitely many such values; the one actually returned is the one between 0 and pi (inclusive).

The arc cosine function is defined mathematically only over the domain -1 to 1. If x is outside the domain, acos signals a domain error.

— Function: double atan (double x)
— Function: float atanf (float x)
— Function: long double atanl (long double x)

These functions compute the arc tangent of x—that is, the value whose tangent is x. The value is in units of radians. Mathematically, there are infinitely many such values; the one actually returned is the one between -pi/2 and pi/2 (inclusive).

— Function: double atan2 (double y, double x)
— Function: float atan2f (float y, float x)
— Function: long double atan2l (long double y, long double x)

This function computes the arc tangent of y/x, but the signs of both arguments are used to determine the quadrant of the result, and x is permitted to be zero. The return value is given in radians and is in the range -pi to pi, inclusive.

If x and y are coordinates of a point in the plane, atan2 returns the signed angle between the line from the origin to that point and the x-axis. Thus, atan2 is useful for converting Cartesian coordinates to polar coordinates. (To compute the radial coordinate, use hypot; see Exponents and Logarithms.)

If both x and y are zero, atan2 returns zero.

ISO C99 defines complex versions of the inverse trig functions.

— Function: complex double casin (complex double z)
— Function: complex float casinf (complex float z)
— Function: complex long double casinl (complex long double z)

These functions compute the complex arc sine of z—that is, the value whose sine is z. The value returned is in radians.

Unlike the real-valued functions, casin is defined for all values of z.

— Function: complex double cacos (complex double z)
— Function: complex float cacosf (complex float z)
— Function: complex long double cacosl (complex long double z)

These functions compute the complex arc cosine of z—that is, the value whose cosine is z. The value returned is in radians.

Unlike the real-valued functions, cacos is defined for all values of z.

— Function: complex double catan (complex double z)
— Function: complex float catanf (complex float z)
— Function: complex long double catanl (complex long double z)

These functions compute the complex arc tangent of z—that is, the value whose tangent is z. The value is in units of radians.

Next: , Previous: Inverse Trig Functions, Up: Mathematics

19.4 Exponentiation and Logarithms

— Function: double exp (double x)
— Function: float expf (float x)
— Function: long double expl (long double x)

These functions compute e (the base of natural logarithms) raised to the power x.

If the magnitude of the result is too large to be representable, exp signals overflow.

— Function: double exp2 (double x)
— Function: float exp2f (float x)
— Function: long double exp2l (long double x)

These functions compute 2 raised to the power x. Mathematically, exp2 (x) is the same as exp (x * log (2)).

— Function: double exp10 (double x)
— Function: float exp10f (float x)
— Function: long double exp10l (long double x)
— Function: double pow10 (double x)
— Function: float pow10f (float x)
— Function: long double pow10l (long double x)

These functions compute 10 raised to the power x. Mathematically, exp10 (x) is the same as exp (x * log (10)).

These functions are GNU extensions. The name exp10 is preferred, since it is analogous to exp and exp2.

— Function: double log (double x)
— Function: float logf (float x)
— Function: long double logl (long double x)

These functions compute the natural logarithm of x. exp (log (x)) equals x, exactly in mathematics and approximately in C.

If x is negative, log signals a domain error. If x is zero, it returns negative infinity; if x is too close to zero, it may signal overflow.

— Function: double log10 (double x)
— Function: float log10f (float x)
— Function: long double log10l (long double x)

These functions return the base-10 logarithm of x. log10 (x) equals log (x) / log (10).

— Function: double log2 (double x)
— Function: float log2f (float x)
— Function: long double log2l (long double x)

These functions return the base-2 logarithm of x. log2 (x) equals log (x) / log (2).

— Function: double logb (double x)
— Function: float logbf (float x)
— Function: long double logbl (long double x)

These functions extract the exponent of x and return it as a floating-point value. If FLT_RADIX is two, logb is equal to floor (log2 (x)), except it's probably faster.

If x is de-normalized, logb returns the exponent x would have if it were normalized. If x is infinity (positive or negative), logb returns &infin;. If x is zero, logb returns &infin;. It does not signal.

— Function: int ilogb (double x)
— Function: int ilogbf (float x)
— Function: int ilogbl (long double x)

These functions are equivalent to the corresponding logb functions except that they return signed integer values.

Since integers cannot represent infinity and NaN, ilogb instead returns an integer that can't be the exponent of a normal floating-point number. math.h defines constants so you can check for this.

— Macro: int FP_ILOGB0

ilogb returns this value if its argument is 0. The numeric value is either INT_MIN or -INT_MAX.

This macro is defined in ISO C99.

— Macro: int FP_ILOGBNAN

ilogb returns this value if its argument is NaN. The numeric value is either INT_MIN or INT_MAX.

This macro is defined in ISO C99.

These values are system specific. They might even be the same. The proper way to test the result of ilogb is as follows: 

     i = ilogb (f);
     if (i == FP_ILOGB0 || i == FP_ILOGBNAN)
       {
         if (isnan (f))
           {
             /* Handle NaN.  */
           }
         else if (f  == 0.0)
           {
             /* Handle 0.0.  */
           }
         else
           {
             /* Some other value with large exponent,
                perhaps +Inf.  */
           }
       }
— Function: double pow (double base, double power)
— Function: float powf (float base, float power)
— Function: long double powl (long double base, long double power)

These are general exponentiation functions, returning base raised to power.

Mathematically, pow would return a complex number when base is negative and power is not an integral value. pow can't do that, so instead it signals a domain error. pow may also underflow or overflow the destination type.

— Function: double sqrt (double x)
— Function: float sqrtf (float x)
— Function: long double sqrtl (long double x)

These functions return the nonnegative square root of x.

If x is negative, sqrt signals a domain error. Mathematically, it should return a complex number.

— Function: double cbrt (double x)
— Function: float cbrtf (float x)
— Function: long double cbrtl (long double x)

These functions return the cube root of x. They cannot fail; every representable real value has a representable real cube root.

— Function: double hypot (double x, double y)
— Function: float hypotf (float x, float y)
— Function: long double hypotl (long double x, long double y)

These functions return sqrt (x*x + y*y). This is the length of the hypotenuse of a right triangle with sides of length x and y, or the distance of the point (x, y) from the origin. Using this function instead of the direct formula is wise, since the error is much smaller. See also the function cabs in Absolute Value.

— Function: double expm1 (double x)
— Function: float expm1f (float x)
— Function: long double expm1l (long double x)

These functions return a value equivalent to exp (x) - 1. They are computed in a way that is accurate even if x is near zero—a case where exp (x) - 1 would be inaccurate owing to subtraction of two numbers that are nearly equal.

— Function: double log1p (double x)
— Function: float log1pf (float x)
— Function: long double log1pl (long double x)

These functions returns a value equivalent to log (1 + x). They are computed in a way that is accurate even if x is near zero.

ISO C99 defines complex variants of some of the exponentiation and logarithm functions.

— Function: complex double cexp (complex double z)
— Function: complex float cexpf (complex float z)
— Function: complex long double cexpl (complex long double z)

These functions return e (the base of natural logarithms) raised to the power of z. Mathematically, this corresponds to the value

exp (z) = exp (creal (z)) * (cos (cimag (z)) + I * sin (cimag (z)))

— Function: complex double clog (complex double z)
— Function: complex float clogf (complex float z)
— Function: complex long double clogl (complex long double z)

These functions return the natural logarithm of z. Mathematically, this corresponds to the value

log (z) = log (cabs (z)) + I * carg (z)

clog has a pole at 0, and will signal overflow if z equals or is very close to 0. It is well-defined for all other values of z.

— Function: complex double clog10 (complex double z)
— Function: complex float clog10f (complex float z)
— Function: complex long double clog10l (complex long double z)

These functions return the base 10 logarithm of the complex value z. Mathematically, this corresponds to the value

log (z) = log10 (cabs (z)) + I * carg (z)

These functions are GNU extensions.

— Function: complex double csqrt (complex double z)
— Function: complex float csqrtf (complex float z)
— Function: complex long double csqrtl (complex long double z)

These functions return the complex square root of the argument z. Unlike the real-valued functions, they are defined for all values of z.

— Function: complex double cpow (complex double base, complex double power)
— Function: complex float cpowf (complex float base, complex float power)
— Function: complex long double cpowl (complex long double base, complex long double power)

These functions return base raised to the power of power. This is equivalent to cexp (y * clog (x))

Next: , Previous: Exponents and Logarithms, Up: Mathematics

19.5 Hyperbolic Functions

The functions in this section are related to the exponential functions; see Exponents and Logarithms.

— Function: double sinh (double x)
— Function: float sinhf (float x)
— Function: long double sinhl (long double x)

These functions return the hyperbolic sine of x, defined mathematically as (exp (x) - exp (-x)) / 2. They may signal overflow if x is too large.

— Function: double cosh (double x)
— Function: float coshf (float x)
— Function: long double coshl (long double x)

These function return the hyperbolic cosine of x, defined mathematically as (exp (x) + exp (-x)) / 2. They may signal overflow if x is too large.

— Function: double tanh (double x)
— Function: float tanhf (float x)
— Function: long double tanhl (long double x)

These functions return the hyperbolic tangent of x, defined mathematically as sinh (x) / cosh (x). They may signal overflow if x is too large.

There are counterparts for the hyperbolic functions which take complex arguments.

— Function: complex double csinh (complex double z)
— Function: complex float csinhf (complex float z)
— Function: complex long double csinhl (complex long double z)

These functions return the complex hyperbolic sine of z, defined mathematically as (exp (z) - exp (-z)) / 2.

— Function: complex double ccosh (complex double z)
— Function: complex float ccoshf (complex float z)
— Function: complex long double ccoshl (complex long double z)

These functions return the complex hyperbolic cosine of z, defined mathematically as (exp (z) + exp (-z)) / 2.

— Function: complex double ctanh (complex double z)
— Function: complex float ctanhf (complex float z)
— Function: complex long double ctanhl (complex long double z)

These functions return the complex hyperbolic tangent of z, defined mathematically as csinh (z) / ccosh (z).

— Function: double asinh (double x)
— Function: float asinhf (float x)
— Function: long double asinhl (long double x)

These functions return the inverse hyperbolic sine of x—the value whose hyperbolic sine is x.

— Function: double acosh (double x)
— Function: float acoshf (float x)
— Function: long double acoshl (long double x)

These functions return the inverse hyperbolic cosine of x—the value whose hyperbolic cosine is x. If x is less than 1, acosh signals a domain error.

— Function: double atanh (double x)
— Function: float atanhf (float x)
— Function: long double atanhl (long double x)

These functions return the inverse hyperbolic tangent of x—the value whose hyperbolic tangent is x. If the absolute value of x is greater than 1, atanh signals a domain error; if it is equal to 1, atanh returns infinity.

— Function: complex double casinh (complex double z)
— Function: complex float casinhf (complex float z)
— Function: complex long double casinhl (complex long double z)

These functions return the inverse complex hyperbolic sine of z—the value whose complex hyperbolic sine is z.

— Function: complex double cacosh (complex double z)
— Function: complex float cacoshf (complex float z)
— Function: complex long double cacoshl (complex long double z)

These functions return the inverse complex hyperbolic cosine of z—the value whose complex hyperbolic cosine is z. Unlike the real-valued functions, there are no restrictions on the value of z.

— Function: complex double catanh (complex double z)
— Function: complex float catanhf (complex float z)
— Function: complex long double catanhl (complex long double z)

These functions return the inverse complex hyperbolic tangent of z—the value whose complex hyperbolic tangent is z. Unlike the real-valued functions, there are no restrictions on the value of z.

Next: , Previous: Hyperbolic Functions, Up: Mathematics

19.6 Special Functions

These are some more exotic mathematical functions which are sometimes useful. Currently they only have real-valued versions.

— Function: double erf (double x)
— Function: float erff (float x)
— Function: long double erfl (long double x)

erf returns the error function of x. The error function is defined as 

          erf (x) = 2/sqrt(pi) * integral from 0 to x of exp(-t^2) dt
— Function: double erfc (double x)
— Function: float erfcf (float x)
— Function: long double erfcl (long double x)

erfc returns 1.0 - erf(x), but computed in a fashion that avoids round-off error when x is large.

— Function: double lgamma (double x)
— Function: float lgammaf (float x)
— Function: long double lgammal (long double x)

lgamma returns the natural logarithm of the absolute value of the gamma function of x. The gamma function is defined as 

          gamma (x) = integral from 0 to &infin; of t^(x-1) e^-t dt

The sign of the gamma function is stored in the global variable signgam, which is declared in math.h. It is 1 if the intermediate result was positive or zero, or -1 if it was negative.

To compute the real gamma function you can use the tgamma function or you can compute the values as follows: 

          lgam = lgamma(x);
          gam  = signgam*exp(lgam);

The gamma function has singularities at the non-positive integers. lgamma will raise the zero divide exception if evaluated at a singularity.

— Function: double lgamma_r (double x, int *signp)
— Function: float lgammaf_r (float x, int *signp)
— Function: long double lgammal_r (long double x, int *signp)

lgamma_r is just like lgamma, but it stores the sign of the intermediate result in the variable pointed to by signp instead of in the signgam global. This means it is reentrant.

— Function: double gamma (double x)
— Function: float gammaf (float x)
— Function: long double gammal (long double x)

These functions exist for compatibility reasons. They are equivalent to lgamma etc. It is better to use lgamma since for one the name reflects better the actual computation, moreover lgamma is standardized in ISO C99 while gamma is not.

— Function: double tgamma (double x)
— Function: float tgammaf (float x)
— Function: long double tgammal (long double x)

tgamma applies the gamma function to x. The gamma function is defined as 

          gamma (x) = integral from 0 to &infin; of t^(x-1) e^-t dt

This function was introduced in ISO C99.

— Function: double j0 (double x)
— Function: float j0f (float x)
— Function: long double j0l (long double x)

j0 returns the Bessel function of the first kind of order 0 of x. It may signal underflow if x is too large.

— Function: double j1 (double x)
— Function: float j1f (float x)
— Function: long double j1l (long double x)

j1 returns the Bessel function of the first kind of order 1 of x. It may signal underflow if x is too large.

— Function: double jn (int n, double x)
— Function: float jnf (int n, float x)
— Function: long double jnl (int n, long double x)

jn returns the Bessel function of the first kind of order n of x. It may signal underflow if x is too large.

— Function: double y0 (double x)
— Function: float y0f (float x)
— Function: long double y0l (long double x)

y0 returns the Bessel function of the second kind of order 0 of x. It may signal underflow if x is too large. If x is negative, y0 signals a domain error; if it is zero, y0 signals overflow and returns -&infin;.

— Function: double y1 (double x)
— Function: float y1f (float x)
— Function: long double y1l (long double x)

y1 returns the Bessel function of the second kind of order 1 of x. It may signal underflow if x is too large. If x is negative, y1 signals a domain error; if it is zero, y1 signals overflow and returns -&infin;.

— Function: double yn (int n, double x)
— Function: float ynf (int n, float x)
— Function: long double ynl (int n, long double x)

yn returns the Bessel function of the second kind of order n of x. It may signal underflow if x is too large. If x is negative, yn signals a domain error; if it is zero, yn signals overflow and returns -&infin;.

Next: , Previous: Special Functions, Up: Mathematics

19.7 Known Maximum Errors in Math Functions

This section lists the known errors of the functions in the math library. Errors are measured in “units of the last place”. This is a measure for the relative error. For a number z with the representation d.d...d&middot;2^e (we assume IEEE floating-point numbers with base 2) the ULP is represented by 

     |d.d...d - (z / 2^e)| / 2^(p - 1)

where p is the number of bits in the mantissa of the floating-point number representation. Ideally the error for all functions is always less than 0.5ulps. Using rounding bits this is also possible and normally implemented for the basic operations. To achieve the same for the complex math functions requires a lot more work and this has not yet been done.

Therefore many of the functions in the math library have errors. The table lists the maximum error for each function which is exposed by one of the existing tests in the test suite. The table tries to cover as much as possible and list the actual maximum error (or at least a ballpark figure) but this is often not achieved due to the large search space.

The table lists the ULP values for different architectures. Different architectures have different results since their hardware support for floating-point operations varies and also the existing hardware support is different.

Function Alpha Generic ix86 IA64 PowerPC
acosf - - - - -
acos - - - - -
acosl - - 622 - 1
acoshf - - - - -
acosh - - - - -
acoshl - - - - 1
asinf - - - - -
asin - - - - -
asinl - - 1 - 2
asinhf - - - - -
asinh - - - - -
asinhl - - - - 1
atanf - - - - -
atan - - - - -
atanl - - - - -
atanhf 1 - - - 1
atanh - - - - -
atanhl - - 1 - -
atan2f 1 - - - 1
atan2 - - - - -
atan2l 1 - - - 1
cabsf - - - - -
cabs - - - - -
cabsl - - - - 1
cacosf - - 0 + i 1 0 + i 1 -
cacos - - - - -
cacosl 0 + i 1 - 0 + i 2 0 + i 2 1 + i 1
cacoshf 0 + i 1 - 9 + i 4 7 + i 0 7 + i 3
cacosh - - 1 + i 1 1 + i 1 1 + i 1
cacoshl 0 + i 1 - 6 + i 1 7 + i 1 1 + i 0
cargf - - - - -
carg - - - - -
cargl - - - - -
casinf 1 + i 0 - 1 + i 1 1 + i 1 1 + i 0
casin 1 + i 0 - 1 + i 0 1 + i 0 1 + i 0
casinl 0 + i 1 - 2 + i 2 2 + i 2 1 + i 1
casinhf 1 + i 6 - 1 + i 6 1 + i 6 1 + i 6
casinh 5 + i 3 - 5 + i 3 5 + i 3 5 + i 3
casinhl 4 + i 2 - 5 + i 5 5 + i 5 4 + i 1
catanf 0 + i 1 - 0 + i 1 0 + i 1 4 + i 1
catan 0 + i 1 - 0 + i 1 0 + i 1 0 + i 1
catanl 0 + i 1 - - - 1 + i 1
catanhf - - 1 + i 0 - 0 + i 6
catanh 4 + i 0 - 2 + i 0 4 + i 0 4 + i 0
catanhl 1 + i 1 - 1 + i 0 1 + i 0 -
cbrtf - - - - -
cbrt 1 - - - 1
cbrtl 1 - 1 - 1
ccosf 1 + i 1 - 0 + i 1 0 + i 1 1 + i 1
ccos 1 + i 0 - 1 + i 0 1 + i 0 1 + i 0
ccosl 1 + i 1 - 1 + i 1 1 + i 1 1 + i 1
ccoshf 1 + i 1 - 1 + i 1 1 + i 1 1 + i 1
ccosh 1 + i 0 - 1 + i 1 1 + i 1 1 + i 0
ccoshl 1 + i 1 - 0 + i 1 0 + i 1 1 + i 2
ceilf - - - - -
ceil - - - - -
ceill - - - - -
cexpf 1 + i 1 - - 1 + i 1 1 + i 1
cexp - - - - -
cexpl 1 + i 1 - 1 + i 1 0 + i 1 2 + i 1
cimagf - - - - -
cimag - - - - -
cimagl - - - - -
clogf 1 + i 0 - 1 + i 0 1 + i 0 1 + i 3
clog - - - - -
clogl 1 + i 0 - 1 + i 0 1 + i 0 2 + i 1
clog10f 1 + i 1 - 1 + i 1 1 + i 1 1 + i 5
clog10 0 + i 1 - 1 + i 1 1 + i 1 0 + i 1
clog10l 1 + i 1 - 1 + i 1 1 + i 1 3 + i 1
conjf - - - - -
conj - - - - -
conjl - - - - -
copysignf - - - - -
copysign - - - - -
copysignl - - - - -
cosf 1 - 1 1 1
cos 2 - 2 2 2
cosl 1 - 1 1 1
coshf - - - - -
cosh - - - - -
coshl - - - - 1
cpowf 4 + i 2 - 4 + i 3 5 + i 3 5 + i 2
cpow 2 + i 2 - 1 + i 2 2 + i 2 2 + i 2
cpowl 10 + i 1 - 763 + i 2 6 + i 4 2 + i 2
cprojf - - - - -
cproj - - - - -
cprojl - - - - 0 + i 1
crealf - - - - -
creal - - - - -
creall - - - - -
csinf - - 1 + i 1 1 + i 1 -
csin - - - - -
csinl 1 + i 1 - 1 + i 0 1 + i 0 1 + i 0
csinhf 1 + i 1 - 1 + i 1 1 + i 1 1 + i 1
csinh 0 + i 1 - 1 + i 1 1 + i 1 0 + i 1
csinhl 1 + i 0 - 1 + i 2 1 + i 2 1 + i 1
csqrtf 1 + i 0 - - 1 + i 0 1 + i 0
csqrt - - - - -
csqrtl 1 + i 1 - - - 1 + i 1
ctanf - - 0 + i 1 0 + i 1 -
ctan 0 + i 1 - 1 + i 1 1 + i 1 1 + i 1
ctanl 1 + i 2 - 439 + i 3 2 + i 1 1 + i 1
ctanhf 2 + i 1 - 1 + i 1 0 + i 1 2 + i 1
ctanh 1 + i 0 - 1 + i 1 1 + i 1 1 + i 0
ctanhl 1 + i 1 - 5 + i 25 1 + i 24 1 + i 1
erff - - - - -
erf 1 - 1 1 1
erfl - - - - 1
erfcf - - 1 1 1
erfc 1 - 1 1 1
erfcl 1 - 1 1 1
expf - - - - -
exp - - - - -
expl - - - - 1
exp10f 2 - - 2 2
exp10 6 - - 6 6
exp10l 1 - 8 3 8
exp2f - - - - -
exp2 - - - - -
exp2l 2 - - - 2
expm1f 1 - - - 1
expm1 1 - - - 1
expm1l 1 - - 1 -
fabsf - - - - -
fabs - - - - -
fabsl - - - - -
fdimf - - - - -
fdim - - - - -
fdiml - - - - -
floorf - - - - -
floor - - - - -
floorl - - - - -
fmaf - - - - -
fma - - - - -
fmal - - - - -
fmaxf - - - - -
fmax - - - - -
fmaxl - - - - -
fminf - - - - -
fmin - - - - -
fminl - - - - -
fmodf - - - - -
fmod - - - - -
fmodl - - - - -
frexpf - - - - -
frexp - - - - -
frexpl - - - - -
gammaf - - - - -
gamma - - 1 - -
gammal 1 - 1 1 1
hypotf 1 - 1 1 1
hypot - - - - -
hypotl - - - - 1
ilogbf - - - - -
ilogb - - - - -
ilogbl - - - - -
j0f 2 - 2 2 2
j0 2 - 3 3 3
j0l 2 - 1 2 1
j1f 2 - 1 2 2
j1 1 - 1 1 1
j1l 4 - 1 1 1
jnf 4 - 2 4 4
jn 4 - 5 3 3
jnl 4 - 2 2 4
lgammaf 2 - 2 2 2
lgamma 1 - 1 1 1
lgammal 1 - 1 1 3
lrintf - - - - -
lrint - - - - -
lrintl - - - - -
llrintf - - - - -
llrint - - - - -
llrintl - - - - -
logf - - 1 1 -
log - - - - -
logl - - - - 1
log10f 2 - 1 1 2
log10 1 - - - 1
log10l 1 - 1 1 1
log1pf 1 - - - 1
log1p - - - - -
log1pl 1 - - - 1
log2f - - - - -
log2 - - - - -
log2l 1 - - - 1
logbf - - - - -
logb - - - - -
logbl - - - - -
lroundf - - - - -
lround - - - - -
lroundl - - - - -
llroundf - - - - -
llround - - - - -
llroundl - - - - -
modff - - - - -
modf - - - - -
modfl - - - - -
nearbyintf - - - - -
nearbyint - - - - -
nearbyintl - - - - -
nextafterf - - - - -
nextafter - - - - -
nextafterl - - - - -
nexttowardf - - - - -
nexttoward - - - - -
nexttowardl - - - - -
powf - - - - -
pow - - - - -
powl - - - - 1
remainderf - - - - -
remainder - - - - -
remainderl - - - - -
remquof - - - - -
remquo - - - - -
remquol - - - - -
rintf - - - - -
rint - - - - -
rintl - - - - -
roundf - - - - -
round - - - - -
roundl - - - - -
scalbf - - - - -
scalb - - - - -
scalbl - - - - -
scalbnf - - - - -
scalbn - - - - -
scalbnl - - - - -
scalblnf - - - - -
scalbln - - - - -
scalblnl - - - - -
sinf - - - - -
sin - - - - -
sinl - - - - 1
sincosf 1 - 1 1 1
sincos 1 - 1 1 1
sincosl 1 - 1 1 1
sinhf - - - - -
sinh - - 1 - -
sinhl - - - - 1
sqrtf - - - - -
sqrt - - - - -
sqrtl 1 - - - -
tanf - - - - -
tan 1 - 1 1 1
tanl - - - - 1
tanhf - - - - -
tanh - - - - -
tanhl 1 - - - 1
tgammaf 1 - 1 1 1
tgamma 1 - 2 1 1
tgammal 1 - 1 1 1
truncf - - - - -
trunc - - - - -
truncl - - - - -
y0f 1 - 1 1 1
y0 2 - 2 2 2
y0l 3 - 1 1 1
y1f 2 - 2 2 2
y1 3 - 2 3 3
y1l 1 - 1 1 2
ynf 2 - 3 2 2
yn 3 - 2 3 3
ynl 5 - 4 2 2

Function S/390 SH4 Sparc 32-bit Sparc 64-bit x86_64/fpu
acosf - - - - -
acos - - - - -
acosl - - - - 1
acoshf - - - - -
acosh - - - - -
acoshl - - - - -
asinf - 2 - - -
asin - 1 - - -
asinl - - - - 1
asinhf - - - - -
asinh - - - - -
asinhl - - - - -
atanf - - - - -
atan - - - - -
atanl - - - - -
atanhf 1 - 1 1 1
atanh - 1 - - -
atanhl - - - - 1
atan2f 1 4 6 6 1
atan2 - - - - -
atan2l 1 - 1 1 -
cabsf - 1 - - -
cabs - 1 - - -
cabsl - - - - -
cacosf - 1 + i 1 - - 0 + i 1
cacos - 1 + i 0 - - -
cacosl 0 + i 1 - 0 + i 1 0 + i 1 0 + i 2
cacoshf 7 + i 3 7 + i 3 7 + i 3 7 + i 3 7 + i 3
cacosh 1 + i 1 1 + i 1 1 + i 1 1 + i 1 1 + i 1
cacoshl 0 + i 1 - 5 + i 1 5 + i 1 6 + i 1
cargf - - - - -
carg - - - - -
cargl - - - - -
casinf 1 + i 0 2 + i 1 1 + i 0 1 + i 0 1 + i 1
casin 1 + i 0 3 + i 0 1 + i 0 1 + i 0 1 + i 0
casinl 0 + i 1 - 0 + i 1 0 + i 1 2 + i 2
casinhf 1 + i 6 1 + i 6 1 + i 6 1 + i 6 1 + i 6
casinh 5 + i 3 5 + i 3 5 + i 3 5 + i 3 5 + i 3
casinhl 4 + i 2 - 4 + i 2 4 + i 2 5 + i 5
catanf 4 + i 1 4 + i 1 4 + i 1 4 + i 1 4 + i 1
catan 0 + i 1 0 + i 1 0 + i 1 0 + i 1 0 + i 1
catanl 0 + i 1 - 0 + i 1 0 + i 1 -
catanhf 0 + i 6 1 + i 6 0 + i 6 0 + i 6 0 + i 6
catanh 4 + i 0 4 + i 1 4 + i 0 4 + i 0 4 + i 0
catanhl 1 + i 1 - 1 + i 1 1 + i 1 1 + i 0
cbrtf - - - - -
cbrt 1 1 1 1 1
cbrtl 1 - 1 1 1
ccosf 1 + i 1 0 + i 1 1 + i 1 1 + i 1 1 + i 1
ccos 1 + i 0 1 + i 1 1 + i 0 1 + i 0 1 + i 0
ccosl 1 + i 1 - 1 + i 1 1 + i 1 1 + i 1
ccoshf 1 + i 1 1 + i 1 1 + i 1 1 + i 1 1 + i 1
ccosh 1 + i 0 1 + i 1 1 + i 0 1 + i 0 1 + i 1
ccoshl 1 + i 1 - 1 + i 1 1 + i 1 0 + i 1
ceilf - - - - -
ceil - - - - -
ceill - - - - -
cexpf 1 + i 1 1 + i 1 1 + i 1 1 + i 1 1 + i 1
cexp - 1 + i 0 - - -
cexpl 1 + i 1 - 1 + i 1 1 + i 1 0 + i 1
cimagf - - - - -
cimag - - - - -
cimagl - - - - -
clogf 1 + i 3 0 + i 3 1 + i 3 1 + i 3 1 + i 3
clog - 0 + i 1 - - -
clogl 1 + i 0 - 1 + i 0 1 + i 0 1 + i 0
clog10f 1 + i 5 1 + i 5 1 + i 5 1 + i 5 1 + i 5
clog10 0 + i 1 1 + i 1 0 + i 1 0 + i 1 1 + i 1
clog10l 1 + i 1 - 1 + i 1 1 + i 1 1 + i 1
conjf - - - - -
conj - - - - -
conjl - - - - -
copysignf - - - - -
copysign - - - - -
copysignl - - - - -
cosf 1 1 1 1 1
cos 2 2 2 2 2
cosl 1 - 1 1 1
coshf - - - - -
cosh - - - - -
coshl - - - - -
cpowf 4 + i 2 4 + i 2 4 + i 2 4 + i 2 5 + i 2
cpow 2 + i 2 1 + i 1.1031 2 + i 2 2 + i 2 2 + i 2
cpowl 10 + i 1 - 10 + i 1 10 + i 1 5 + i 2
cprojf - - - - -
cproj - - - - -
cprojl - - - - -
crealf - - - - -
creal - - - - -
creall - - - - -
csinf - 0 + i 1 - - 0 + i 1
csin - - - - 0 + i 1
csinl 1 + i 1 - 1 + i 1 1 + i 1 1 + i 0
csinhf 1 + i 1 1 + i 1 1 + i 1 1 + i 1 1 + i 1
csinh 0 + i 1 0 + i 1 0 + i 1 0 + i 1 1 + i 1
csinhl 1 + i 0 - 1 + i 0 1 + i 0 1 + i 2
csqrtf 1 + i 0 1 + i 1 1 + i 0 1 + i 0 1 + i 0
csqrt - 1 + i 0 - - -
csqrtl 1 + i 1 - 1 + i 1 1 + i 1 -
ctanf - 1 + i 1 - - 0 + i 1
ctan 1 + i 1 1 + i 1 1 + i 1 1 + i 1 1 + i 1
ctanl 1 + i 2 - 1 + i 2 1 + i 2 439 + i 3
ctanhf 2 + i 1 2 + i 1 2 + i 1 2 + i 1 2 + i 1
ctanh 1 + i 0 2 + i 2 1 + i 0 1 + i 0 1 + i 1
ctanhl 1 + i 1 - 1 + i 1 1 + i 1 5 + i 25
erff - - - - -
erf 1 - 1 1 1
erfl - - - - -
erfcf 1 12 - - -
erfc 1 24 1 1 1
erfcl 1 - 1 1 1
expf - - - - -
exp - - - - -
expl - - - - -
exp10f 2 2 2 2 2
exp10 6 6 6 6 6
exp10l 1 - 1 1 8
exp2f - - - - -
exp2 - - - - -
exp2l 2 - 2 2 -
expm1f 1 1 1 1 1
expm1 1 - 1 1 1
expm1l 1 - 1 1 -
fabsf - - - - -
fabs - - - - -
fabsl - - - - -
fdimf - - - - -
fdim - - - - -
fdiml - - - - -
floorf - - - - -
floor - - - - -
floorl - - - - -
fmaf - - - - -
fma - - - - -
fmal - - - - -
fmaxf - - - - -
fmax - - - - -
fmaxl - - - - -
fminf - - - - -
fmin - - - - -
fminl - - - - -
fmodf - 1 - - -
fmod - 2 - - -
fmodl - - - - -
frexpf - - - - -
frexp - - - - -
frexpl - - - - -
gammaf - - - - -
gamma - - - - -
gammal 1 - 1 1 1
hypotf 1 1 1 1 1
hypot - 1 - - -
hypotl - - - - -
ilogbf - - - - -
ilogb - - - - -
ilogbl - - - - -
j0f 2 2 2 2 2
j0 3 2 2 2 2
j0l 2 - 2 2 1
j1f 2 2 2 2 2
j1 1 1 1 1 1
j1l 4 - 4 4 1
jnf 4 4 4 4 4
jn 4 6 4 4 4
jnl 4 - 4 4 2
lgammaf 2 2 2 2 2
lgamma 1 1 1 1 1
lgammal 1 - 1 1 1
lrintf - - - - -
lrint - - - - -
lrintl - - - - -
llrintf - - - - -
llrint - - - - -
llrintl - - - - -
logf - 1 - - -
log - 1 - - -
logl - - - - -
log10f 2 1 2 2 2
log10 1 1 1 1 1
log10l 1 - 1 1 1
log1pf 1 1 1 1 1
log1p - 1 - - -
log1pl 1 - 1 1 -
log2f - 1 - - -
log2 - 1 - - -
log2l 1 - 1 1 -
logbf - - - - -
logb - - - - -
logbl - - - - -
lroundf - - - - -
lround - - - - -
lroundl - - - - -
llroundf - - - - -
llround - - - - -
llroundl - - - - -
modff - - - - -
modf - - - - -
modfl - - - - -
nearbyintf - - - - -
nearbyint - - - - -
nearbyintl - - - - -
nextafterf - - - - -
nextafter - - - - -
nextafterl - - - - -
nexttowardf - - - - -
nexttoward - - - - -
nexttowardl - - - - -
powf - - - - -
pow - - - - -
powl - - - - -
remainderf - - - - -
remainder - - - - -
remainderl - - - - -
remquof - - - - -
remquo - - - - -
remquol - - - - -
rintf - - - - -
rint - - - - -
rintl - - - - -
roundf - - - - -
round - - - - -
roundl - - - - -
scalbf - - - - -
scalb - - - - -
scalbl - - - - -
scalbnf - - - - -
scalbn - - - - -
scalbnl - - - - -
scalblnf - - - - -
scalbln - - - - -
scalblnl - - - - -
sinf - - - - -
sin - - - - -
sinl - - - - -
sincosf 1 1 1 1 1
sincos 1 1 1 1 1
sincosl 1 - 1 1 1
sinhf - 1 - - -
sinh - 1 - - -
sinhl - - - - -
sqrtf - - - - -
sqrt - - - - -
sqrtl 1 - 1 1 -
tanf - - - - -
tan 1 0.5 1 1 1
tanl - - - - -
tanhf - 1 - - -
tanh - 1 - - -
tanhl 1 - 1 1 -
tgammaf 1 1 1 1 1
tgamma 1 1 1 1 1
tgammal 1 - 1 1 1
truncf - - - - -
trunc - - - - -
truncl - - - - -
y0f 1 1 1 1 1
y0 2 2 2 2 2
y0l 3 - 3 3 1
y1f 2 2 2 2 2
y1 3 3 3 3 3
y1l 1 - 1 1 1
ynf 2 2 2 2 2
yn 3 3 3 3 3
ynl 5 - 5 5 4

Next: , Previous: Errors in Math Functions, Up: Mathematics

19.8 Pseudo-Random Numbers

This section describes the GNU facilities for generating a series of pseudo-random numbers. The numbers generated are not truly random; typically, they form a sequence that repeats periodically, with a period so large that you can ignore it for ordinary purposes. The random number generator works by remembering a seed value which it uses to compute the next random number and also to compute a new seed.

Although the generated numbers look unpredictable within one run of a program, the sequence of numbers is exactly the same from one run to the next. This is because the initial seed is always the same. This is convenient when you are debugging a program, but it is unhelpful if you want the program to behave unpredictably. If you want a different pseudo-random series each time your program runs, you must specify a different seed each time. For ordinary purposes, basing the seed on the current time works well.

You can obtain repeatable sequences of numbers on a particular machine type by specifying the same initial seed value for the random number generator. There is no standard meaning for a particular seed value; the same seed, used in different C libraries or on different CPU types, will give you different random numbers.

The GNU library supports the standard ISO C random number functions plus two other sets derived from BSD and SVID. The BSD and ISO C functions provide identical, somewhat limited functionality. If only a small number of random bits are required, we recommend you use the ISO C interface, rand and srand. The SVID functions provide a more flexible interface, which allows better random number generator algorithms, provides more random bits (up to 48) per call, and can provide random floating-point numbers. These functions are required by the XPG standard and therefore will be present in all modern Unix systems.

Next: , Up: Pseudo-Random Numbers

19.8.1 ISO C Random Number Functions

This section describes the random number functions that are part of the ISO C standard.

To use these facilities, you should include the header file stdlib.h in your program.

— Macro: int RAND_MAX

The value of this macro is an integer constant representing the largest value the rand function can return. In the GNU library, it is 2147483647, which is the largest signed integer representable in 32 bits. In other libraries, it may be as low as 32767.

— Function: int rand (void)

The rand function returns the next pseudo-random number in the series. The value ranges from 0 to RAND_MAX.

— Function: void srand (unsigned int seed)

This function establishes seed as the seed for a new series of pseudo-random numbers. If you call rand before a seed has been established with srand, it uses the value 1 as a default seed.

To produce a different pseudo-random series each time your program is run, do srand (time (0)).

POSIX.1 extended the C standard functions to support reproducible random numbers in multi-threaded programs. However, the extension is badly designed and unsuitable for serious work.

— Function: int rand_r (unsigned int *seed)

This function returns a random number in the range 0 to RAND_MAX just as rand does. However, all its state is stored in the seed argument. This means the RNG's state can only have as many bits as the type unsigned int has. This is far too few to provide a good RNG.

If your program requires a reentrant RNG, we recommend you use the reentrant GNU extensions to the SVID random number generator. The POSIX.1 interface should only be used when the GNU extensions are not available.

Next: , Previous: ISO Random, Up: Pseudo-Random Numbers

19.8.2 BSD Random Number Functions

This section describes a set of random number generation functions that are derived from BSD. There is no advantage to using these functions with the GNU C library; we support them for BSD compatibility only.

The prototypes for these functions are in stdlib.h.

— Function: long int random (void)

This function returns the next pseudo-random number in the sequence. The value returned ranges from 0 to RAND_MAX.

Note: Temporarily this function was defined to return a int32_t value to indicate that the return value always contains 32 bits even if long int is wider. The standard demands it differently. Users must always be aware of the 32-bit limitation, though.

— Function: void srandom (unsigned int seed)

The srandom function sets the state of the random number generator based on the integer seed. If you supply a seed value of 1, this will cause random to reproduce the default set of random numbers.

To produce a different set of pseudo-random numbers each time your program runs, do srandom (time (0)).

— Function: void * initstate (unsigned int seed, void *state, size_t size)

The initstate function is used to initialize the random number generator state. The argument state is an array of size bytes, used to hold the state information. It is initialized based on seed. The size must be between 8 and 256 bytes, and should be a power of two. The bigger the state array, the better.

The return value is the previous value of the state information array. You can use this value later as an argument to setstate to restore that state.

— Function: void * setstate (void *state)

The setstate function restores the random number state information state. The argument must have been the result of a previous call to initstate or setstate.

The return value is the previous value of the state information array. You can use this value later as an argument to setstate to restore that state.

If the function fails the return value is NULL.

The four functions described so far in this section all work on a state which is shared by all threads. The state is not directly accessible to the user and can only be modified by these functions. This makes it hard to deal with situations where each thread should have its own pseudo-random number generator.

The GNU C library contains four additional functions which contain the state as an explicit parameter and therefore make it possible to handle thread-local PRNGs. Beside this there are no difference. In fact, the four functions already discussed are implemented internally using the following interfaces.

The stdlib.h header contains a definition of the following type:

— Data Type: struct random_data

Objects of type struct random_data contain the information necessary to represent the state of the PRNG. Although a complete definition of the type is present the type should be treated as opaque.

The functions modifying the state follow exactly the already described functions.

— Function: int random_r (struct random_data *restrict buf, int32_t *restrict result)

The random_r function behaves exactly like the random function except that it uses and modifies the state in the object pointed to by the first parameter instead of the global state.

— Function: int srandom_r (unsigned int seed, struct random_data *buf)

The srandom_r function behaves exactly like the srandom function except that it uses and modifies the state in the object pointed to by the second parameter instead of the global state.

— Function: int initstate_r (unsigned int seed, char *restrict statebuf, size_t statelen, struct random_data *restrict buf)

The initstate_r function behaves exactly like the initstate function except that it uses and modifies the state in the object pointed to by the fourth parameter instead of the global state.

— Function: int setstate_r (char *restrict statebuf, struct random_data *restrict buf)

The setstate_r function behaves exactly like the setstate function except that it uses and modifies the state in the object pointed to by the first parameter instead of the global state.

Previous: BSD Random, Up: Pseudo-Random Numbers

19.8.3 SVID Random Number Function

The C library on SVID systems contains yet another kind of random number generator functions. They use a state of 48 bits of data. The user can choose among a collection of functions which return the random bits in different forms.

Generally there are two kinds of function. The first uses a state of the random number generator which is shared among several functions and by all threads of the process. The second requires the user to handle the state.

All functions have in common that they use the same congruential formula with the same constants. The formula is 

     Y = (a * X + c) mod m

where X is the state of the generator at the beginning and Y the state at the end. a and c are constants determining the way the generator works. By default they are 

     a = 0x5DEECE66D = 25214903917
     c = 0xb = 11

but they can also be changed by the user. m is of course 2^48 since the state consists of a 48-bit array.

The prototypes for these functions are in stdlib.h.

— Function: double drand48 (void)

This function returns a double value in the range of 0.0 to 1.0 (exclusive). The random bits are determined by the global state of the random number generator in the C library.

Since the double type according to IEEE 754 has a 52-bit mantissa this means 4 bits are not initialized by the random number generator. These are (of course) chosen to be the least significant bits and they are initialized to 0.

— Function: double erand48 (unsigned short int xsubi[3])

This function returns a double value in the range of 0.0 to 1.0 (exclusive), similarly to drand48. The argument is an array describing the state of the random number generator.

This function can be called subsequently since it updates the array to guarantee random numbers. The array should have been initialized before initial use to obtain reproducible results.

— Function: long int lrand48 (void)

The lrand48 function returns an integer value in the range of 0 to 2^31 (exclusive). Even if the size of the long int type can take more than 32 bits, no higher numbers are returned. The random bits are determined by the global state of the random number generator in the C library.

— Function: long int nrand48 (unsigned short int xsubi[3])

This function is similar to the lrand48 function in that it returns a number in the range of 0 to 2^31 (exclusive) but the state of the random number generator used to produce the random bits is determined by the array provided as the parameter to the function.

The numbers in the array are updated afterwards so that subsequent calls to this function yield different results (as is expected of a random number generator). The array should have been initialized before the first call to obtain reproducible results.

— Function: long int mrand48 (void)

The mrand48 function is similar to lrand48. The only difference is that the numbers returned are in the range -2^31 to 2^31 (exclusive).

— Function: long int jrand48 (unsigned short int xsubi[3])

The jrand48 function is similar to nrand48. The only difference is that the numbers returned are in the range -2^31 to 2^31 (exclusive). For the xsubi parameter the same requirements are necessary.

The internal state of the random number generator can be initialized in several ways. The methods differ in the completeness of the information provided.

— Function: void srand48 (long int seedval)

The srand48 function sets the most significant 32 bits of the internal state of the random number generator to the least significant 32 bits of the seedval parameter. The lower 16 bits are initialized to the value 0x330E. Even if the long int type contains more than 32 bits only the lower 32 bits are used.

Owing to this limitation, initialization of the state of this function is not very useful. But it makes it easy to use a construct like srand48 (time (0)).

A side-effect of this function is that the values a and c from the internal state, which are used in the congruential formula, are reset to the default values given above. This is of importance once the user has called the lcong48 function (see below).

— Function: unsigned short int * seed48 (unsigned short int seed16v[3])

The seed48 function initializes all 48 bits of the state of the internal random number generator from the contents of the parameter seed16v. Here the lower 16 bits of the first element of see16v initialize the least significant 16 bits of the internal state, the lower 16 bits of seed16v[1] initialize the mid-order 16 bits of the state and the 16 lower bits of seed16v[2] initialize the most significant 16 bits of the state.

Unlike srand48 this function lets the user initialize all 48 bits of the state.

The value returned by seed48 is a pointer to an array containing the values of the internal state before the change. This might be useful to restart the random number generator at a certain state. Otherwise the value can simply be ignored.

As for srand48, the values a and c from the congruential formula are reset to the default values.

There is one more function to initialize the random number generator which enables you to specify even more information by allowing you to change the parameters in the congruential formula.

— Function: void lcong48 (unsigned short int param[7])

The lcong48 function allows the user to change the complete state of the random number generator. Unlike srand48 and seed48, this function also changes the constants in the congruential formula.

From the seven elements in the array param the least significant 16 bits of the entries param[0] to param[2] determine the initial state, the least significant 16 bits of param[3] to param[5] determine the 48 bit constant a and param[6] determines the 16-bit value c.

All the above functions have in common that they use the global parameters for the congruential formula. In multi-threaded programs it might sometimes be useful to have different parameters in different threads. For this reason all the above functions have a counterpart which works on a description of the random number generator in the user-supplied buffer instead of the global state.

Please note that it is no problem if several threads use the global state if all threads use the functions which take a pointer to an array containing the state. The random numbers are computed following the same loop but if the state in the array is different all threads will obtain an individual random number generator.

The user-supplied buffer must be of type struct drand48_data. This type should be regarded as opaque and not manipulated directly.

— Function: int drand48_r (struct drand48_data *buffer, double *result)

This function is equivalent to the drand48 function with the difference that it does not modify the global random number generator parameters but instead the parameters in the buffer supplied through the pointer buffer. The random number is returned in the variable pointed to by result.

The return value of the function indicates whether the call succeeded. If the value is less than 0 an error occurred and errno is set to indicate the problem.

This function is a GNU extension and should not be used in portable programs.

— Function: int erand48_r (unsigned short int xsubi[3], struct drand48_data *buffer, double *result)

The erand48_r function works like erand48, but in addition it takes an argument buffer which describes the random number generator. The state of the random number generator is taken from the xsubi array, the parameters for the congruential formula from the global random number generator data. The random number is returned in the variable pointed to by result.

The return value is non-negative if the call succeeded.

This function is a GNU extension and should not be used in portable programs.

— Function: int lrand48_r (struct drand48_data *buffer, double *result)

This function is similar to lrand48, but in addition it takes a pointer to a buffer describing the state of the random number generator just like drand48.

If the return value of the function is non-negative the variable pointed to by result contains the result. Otherwise an error occurred.

This function is a GNU extension and should not be used in portable programs.

— Function: int nrand48_r (unsigned short int xsubi[3], struct drand48_data *buffer, long int *result)

The nrand48_r function works like nrand48 in that it produces a random number in the range 0 to 2^31. But instead of using the global parameters for the congruential formula it uses the information from the buffer pointed to by buffer. The state is described by the values in xsubi.

If the return value is non-negative the variable pointed to by result contains the result.

This function is a GNU extension and should not be used in portable programs.

— Function: int mrand48_r (struct drand48_data *buffer, double *result)

This function is similar to mrand48 but like the other reentrant functions it uses the random number generator described by the value in the buffer pointed to by buffer.

If the return value is non-negative the variable pointed to by result contains the result.

This function is a GNU extension and should not be used in portable programs.

— Function: int jrand48_r (unsigned short int xsubi[3], struct drand48_data *buffer, long int *result)

The jrand48_r function is similar to jrand48. Like the other reentrant functions of this function family it uses the congruential formula parameters from the buffer pointed to by buffer.

If the return value is non-negative the variable pointed to by result contains the result.

This function is a GNU extension and should not be used in portable programs.

Before any of the above functions are used the buffer of type struct drand48_data should be initialized. The easiest way to do this is to fill the whole buffer with null bytes, e.g. by 

     memset (buffer, '\0', sizeof (struct drand48_data));

Using any of the reentrant functions of this family now will automatically initialize the random number generator to the default values for the state and the parameters of the congruential formula.

The other possibility is to use any of the functions which explicitly initialize the buffer. Though it might be obvious how to initialize the buffer from looking at the parameter to the function, it is highly recommended to use these functions since the result might not always be what you expect.

— Function: int srand48_r (long int seedval, struct drand48_data *buffer)

The description of the random number generator represented by the information in buffer is initialized similarly to what the function srand48 does. The state is initialized from the parameter seedval and the parameters for the congruential formula are initialized to their default values.

If the return value is non-negative the function call succeeded.

This function is a GNU extension and should not be used in portable programs.

— Function: int seed48_r (unsigned short int seed16v[3], struct drand48_data *buffer)

This function is similar to srand48_r but like seed48 it initializes all 48 bits of the state from the parameter seed16v.

If the return value is non-negative the function call succeeded. It does not return a pointer to the previous state of the random number generator like the seed48 function does. If the user wants to preserve the state for a later re-run s/he can copy the whole buffer pointed to by buffer.

This function is a GNU extension and should not be used in portable programs.

— Function: int lcong48_r (unsigned short int param[7], struct drand48_data *buffer)

This function initializes all aspects of the random number generator described in buffer with the data in param. Here it is especially true that the function does more than just copying the contents of param and buffer. More work is required and therefore it is important to use this function rather than initializing the random number generator directly.

If the return value is non-negative the function call succeeded.

This function is a GNU extension and should not be used in portable programs.

Previous: Pseudo-Random Numbers, Up: Mathematics

19.9 Is Fast Code or Small Code preferred?

If an application uses many floating point functions it is often the case that the cost of the function calls themselves is not negligible. Modern processors can often execute the operations themselves very fast, but the function call disrupts the instruction pipeline.

For this reason the GNU C Library provides optimizations for many of the frequently-used math functions. When GNU CC is used and the user activates the optimizer, several new inline functions and macros are defined. These new functions and macros have the same names as the library functions and so are used instead of the latter. In the case of inline functions the compiler will decide whether it is reasonable to use them, and this decision is usually correct.

This means that no calls to the library functions may be necessary, and can increase the speed of generated code significantly. The drawback is that code size will increase, and the increase is not always negligible.

There are two kind of inline functions: Those that give the same result as the library functions and others that might not set errno and might have a reduced precision and/or argument range in comparison with the library functions. The latter inline functions are only available if the flag -ffast-math is given to GNU CC.

In cases where the inline functions and macros are not wanted the symbol __NO_MATH_INLINES should be defined before any system header is included. This will ensure that only library functions are used. Of course, it can be determined for each file in the project whether giving this option is preferable or not.

Not all hardware implements the entire IEEE 754 standard, and even if it does there may be a substantial performance penalty for using some of its features. For example, enabling traps on some processors forces the FPU to run un-pipelined, which can more than double calculation time.

Next: , Previous: Mathematics, Up: Top

20 Arithmetic Functions

This chapter contains information about functions for doing basic arithmetic operations, such as splitting a float into its integer and fractional parts or retrieving the imaginary part of a complex value. These functions are declared in the header files math.h and complex.h.

Next: , Up: Arithmetic

20.1 Integers

The C language defines several integer data types: integer, short integer, long integer, and character, all in both signed and unsigned varieties. The GNU C compiler extends the language to contain long long integers as well. The C integer types were intended to allow code to be portable among machines with different inherent data sizes (word sizes), so each type may have different ranges on different machines. The problem with this is that a program often needs to be written for a particular range of integers, and sometimes must be written for a particular size of storage, regardless of what machine the program runs on.

To address this problem, the GNU C library contains C type definitions you can use to declare integers that meet your exact needs. Because the GNU C library header files are customized to a specific machine, your program source code doesn't have to be.

These typedefs are in stdint.h. If you require that an integer be represented in exactly N bits, use one of the following types, with the obvious mapping to bit size and signedness:

If your C compiler and target machine do not allow integers of a certain size, the corresponding above type does not exist.

If you don't need a specific storage size, but want the smallest data structure with at least N bits, use one of these:

If you don't need a specific storage size, but want the data structure that allows the fastest access while having at least N bits (and among data structures with the same access speed, the smallest one), use one of these:

If you want an integer with the widest range possible on the platform on which it is being used, use one of the following. If you use these, you should write code that takes into account the variable size and range of the integer.

The GNU C library also provides macros that tell you the maximum and minimum possible values for each integer data type. The macro names follow these examples: INT32_MAX, UINT8_MAX, INT_FAST32_MIN, INT_LEAST64_MIN, UINTMAX_MAX, INTMAX_MAX, INTMAX_MIN. Note that there are no macros for unsigned integer minima. These are always zero. There are similar macros for use with C's built in integer types which should come with your C compiler. These are described in Data Type Measurements.

Don't forget you can use the C sizeof function with any of these data types to get the number of bytes of storage each uses.

Next: , Previous: Integers, Up: Arithmetic

20.2 Integer Division

This section describes functions for performing integer division. These functions are redundant when GNU CC is used, because in GNU C the `/' operator always rounds towards zero. But in other C implementations, `/' may round differently with negative arguments. div and ldiv are useful because they specify how to round the quotient: towards zero. The remainder has the same sign as the numerator.

These functions are specified to return a result r such that the value r.quot*denominator + r.rem equals numerator.

To use these facilities, you should include the header file stdlib.h in your program.

— Data Type: div_t

This is a structure type used to hold the result returned by the div function. It has the following members:

int quot
The quotient from the division.
int rem
The remainder from the division.

— Function: div_t div (int numerator, int denominator)

This function div computes the quotient and remainder from the division of numerator by denominator, returning the result in a structure of type div_t.

If the result cannot be represented (as in a division by zero), the behavior is undefined.

Here is an example, albeit not a very useful one. 

          div_t result;
          result = div (20, -6);

Now result.quot is -3 and result.rem is 2.

— Data Type: ldiv_t

This is a structure type used to hold the result returned by the ldiv function. It has the following members:

long int quot
The quotient from the division.
long int rem
The remainder from the division.

(This is identical to div_t except that the components are of type long int rather than int.)

— Function: ldiv_t ldiv (long int numerator, long int denominator)

The ldiv function is similar to div, except that the arguments are of type long int and the result is returned as a structure of type ldiv_t.

— Data Type: lldiv_t

This is a structure type used to hold the result returned by the lldiv function. It has the following members:

long long int quot
The quotient from the division.
long long int rem
The remainder from the division.

(This is identical to div_t except that the components are of type long long int rather than int.)

— Function: lldiv_t lldiv (long long int numerator, long long int denominator)

The lldiv function is like the div function, but the arguments are of type long long int and the result is returned as a structure of type lldiv_t.

The lldiv function was added in ISO C99.

— Data Type: imaxdiv_t

This is a structure type used to hold the result returned by the imaxdiv function. It has the following members:

intmax_t quot
The quotient from the division.
intmax_t rem
The remainder from the division.

(This is identical to div_t except that the components are of type intmax_t rather than int.)

See Integers for a description of the intmax_t type.

— Function: imaxdiv_t imaxdiv (intmax_t numerator, intmax_t denominator)

The imaxdiv function is like the div function, but the arguments are of type intmax_t and the result is returned as a structure of type imaxdiv_t.

See Integers for a description of the intmax_t type.

The imaxdiv function was added in ISO C99.

Next: , Previous: Integer Division, Up: Arithmetic

20.3 Floating Point Numbers

Most computer hardware has support for two different kinds of numbers: integers (...-3, -2, -1, 0, 1, 2, 3...) and floating-point numbers. Floating-point numbers have three parts: the mantissa, the exponent, and the sign bit. The real number represented by a floating-point value is given by (s ? -1 : 1) &middot; 2^e &middot; M where s is the sign bit, e the exponent, and M the mantissa. See Floating Point Concepts, for details. (It is possible to have a different base for the exponent, but all modern hardware uses 2.)

Floating-point numbers can represent a finite subset of the real numbers. While this subset is large enough for most purposes, it is important to remember that the only reals that can be represented exactly are rational numbers that have a terminating binary expansion shorter than the width of the mantissa. Even simple fractions such as 1/5 can only be approximated by floating point.

Mathematical operations and functions frequently need to produce values that are not representable. Often these values can be approximated closely enough for practical purposes, but sometimes they can't. Historically there was no way to tell when the results of a calculation were inaccurate. Modern computers implement the IEEE 754 standard for numerical computations, which defines a framework for indicating to the program when the results of calculation are not trustworthy. This framework consists of a set of exceptions that indicate why a result could not be represented, and the special values infinity and not a number (NaN).

Next: , Previous: Floating Point Numbers, Up: Arithmetic

20.4 Floating-Point Number Classification Functions

ISO C99 defines macros that let you determine what sort of floating-point number a variable holds.

— Macro: int fpclassify (float-type x)

This is a generic macro which works on all floating-point types and which returns a value of type int. The possible values are:

FP_NAN
The floating-point number x is “Not a Number” (see Infinity and NaN)
FP_INFINITE
The value of x is either plus or minus infinity (see Infinity and NaN)
FP_ZERO
The value of x is zero. In floating-point formats like IEEE 754, where zero can be signed, this value is also returned if x is negative zero.
FP_SUBNORMAL
Numbers whose absolute value is too small to be represented in the normal format are represented in an alternate, denormalized format (see Floating Point Concepts). This format is less precise but can represent values closer to zero. fpclassify returns this value for values of x in this alternate format.
FP_NORMAL
This value is returned for all other values of x. It indicates that there is nothing special about the number.

fpclassify is most useful if more than one property of a number must be tested. There are more specific macros which only test one property at a time. Generally these macros execute faster than fpclassify, since there is special hardware support for them. You should therefore use the specific macros whenever possible.

— Macro: int isfinite (float-type x)

This macro returns a nonzero value if x is finite: not plus or minus infinity, and not NaN. It is equivalent to 

          (fpclassify (x) != FP_NAN && fpclassify (x) != FP_INFINITE)

isfinite is implemented as a macro which accepts any floating-point type.

— Macro: int isnormal (float-type x)

This macro returns a nonzero value if x is finite and normalized. It is equivalent to 

          (fpclassify (x) == FP_NORMAL)
— Macro: int isnan (float-type x)

This macro returns a nonzero value if x is NaN. It is equivalent to 

          (fpclassify (x) == FP_NAN)

Another set of floating-point classification functions was provided by BSD. The GNU C library also supports these functions; however, we recommend that you use the ISO C99 macros in new code. Those are standard and will be available more widely. Also, since they are macros, you do not have to worry about the type of their argument.

— Function: int isinf (double x)
— Function: int isinff (float x)
— Function: int isinfl (long double x)

This function returns -1 if x represents negative infinity, 1 if x represents positive infinity, and 0 otherwise.

— Function: int isnan (double x)
— Function: int isnanf (float x)
— Function: int isnanl (long double x)

This function returns a nonzero value if x is a “not a number” value, and zero otherwise.

Note: The isnan macro defined by ISO C99 overrides the BSD function. This is normally not a problem, because the two routines behave identically. However, if you really need to get the BSD function for some reason, you can write 

          (isnan) (x)
— Function: int finite (double x)
— Function: int finitef (float x)
— Function: int finitel (long double x)

This function returns a nonzero value if x is finite or a “not a number” value, and zero otherwise.

Portability Note: The functions listed in this section are BSD extensions.

Next: , Previous: Floating Point Classes, Up: Arithmetic

20.5 Errors in Floating-Point Calculations

Next: , Up: Floating Point Errors

20.5.1 FP Exceptions

The IEEE 754 standard defines five exceptions that can occur during a calculation. Each corresponds to a particular sort of error, such as overflow.

When exceptions occur (when exceptions are raised, in the language of the standard), one of two things can happen. By default the exception is simply noted in the floating-point status word, and the program continues as if nothing had happened. The operation produces a default value, which depends on the exception (see the table below). Your program can check the status word to find out which exceptions happened.

Alternatively, you can enable traps for exceptions. In that case, when an exception is raised, your program will receive the SIGFPE signal. The default action for this signal is to terminate the program. See Signal Handling, for how you can change the effect of the signal.

In the System V math library, the user-defined function matherr is called when certain exceptions occur inside math library functions. However, the Unix98 standard deprecates this interface. We support it for historical compatibility, but recommend that you do not use it in new programs.

The exceptions defined in IEEE 754 are:

`Invalid Operation'
This exception is raised if the given operands are invalid for the operation to be performed. Examples are (see IEEE 754, section 7):
  1. Addition or subtraction: &infin; - &infin;. (But &infin; + &infin; = &infin;).
  2. Multiplication: 0 &middot; &infin;.
  3. Division: 0/0 or &infin;/&infin;.
  4. Remainder: x REM y, where y is zero or x is infinite.
  5. Square root if the operand is less then zero. More generally, any mathematical function evaluated outside its domain produces this exception.
  6. Conversion of a floating-point number to an integer or decimal string, when the number cannot be represented in the target format (due to overflow, infinity, or NaN).
  7. Conversion of an unrecognizable input string.
  8. Comparison via predicates involving < or >, when one or other of the operands is NaN. You can prevent this exception by using the unordered comparison functions instead; see FP Comparison Functions.

If the exception does not trap, the result of the operation is NaN.

`Division by Zero'
This exception is raised when a finite nonzero number is divided by zero. If no trap occurs the result is either +&infin; or -&infin;, depending on the signs of the operands.
`Overflow'
This exception is raised whenever the result cannot be represented as a finite value in the precision format of the destination. If no trap occurs the result depends on the sign of the intermediate result and the current rounding mode (IEEE 754, section 7.3):
  1. Round to nearest carries all overflows to &infin; with the sign of the intermediate result.
  2. Round toward 0 carries all overflows to the largest representable finite number with the sign of the intermediate result.
  3. Round toward -&infin; carries positive overflows to the largest representable finite number and negative overflows to -&infin;.
  4. Round toward &infin; carries negative overflows to the most negative representable finite number and positive overflows to &infin;.

Whenever the overflow exception is raised, the inexact exception is also raised.

`Underflow'
The underflow exception is raised when an intermediate result is too small to be calculated accurately, or if the operation's result rounded to the destination precision is too small to be normalized.

When no trap is installed for the underflow exception, underflow is signaled (via the underflow flag) only when both tininess and loss of accuracy have been detected. If no trap handler is installed the operation continues with an imprecise small value, or zero if the destination precision cannot hold the small exact result.

`Inexact'
This exception is signalled if a rounded result is not exact (such as when calculating the square root of two) or a result overflows without an overflow trap.

Next: , Previous: FP Exceptions, Up: Floating Point Errors

20.5.2 Infinity and NaN

IEEE 754 floating point numbers can represent positive or negative infinity, and NaN (not a number). These three values arise from calculations whose result is undefined or cannot be represented accurately. You can also deliberately set a floating-point variable to any of them, which is sometimes useful. Some examples of calculations that produce infinity or NaN: 

     1/0 = &infin;
     log (0) = -&infin;
     sqrt (-1) = NaN

When a calculation produces any of these values, an exception also occurs; see FP Exceptions.

The basic operations and math functions all accept infinity and NaN and produce sensible output. Infinities propagate through calculations as one would expect: for example, 2 + &infin; = &infin;, 4/&infin; = 0, atan (&infin;) = &pi;/2. NaN, on the other hand, infects any calculation that involves it. Unless the calculation would produce the same result no matter what real value replaced NaN, the result is NaN.

In comparison operations, positive infinity is larger than all values except itself and NaN, and negative infinity is smaller than all values except itself and NaN. NaN is unordered: it is not equal to, greater than, or less than anything, including itself. x == x is false if the value of x is NaN. You can use this to test whether a value is NaN or not, but the recommended way to test for NaN is with the isnan function (see Floating Point Classes). In addition, <, >, <=, and >= will raise an exception when applied to NaNs.

math.h defines macros that allow you to explicitly set a variable to infinity or NaN.

— Macro: float INFINITY

An expression representing positive infinity. It is equal to the value produced by mathematical operations like 1.0 / 0.0. -INFINITY represents negative infinity.

You can test whether a floating-point value is infinite by comparing it to this macro. However, this is not recommended; you should use the isfinite macro instead. See Floating Point Classes.

This macro was introduced in the ISO C99 standard.

— Macro: float NAN

An expression representing a value which is “not a number”. This macro is a GNU extension, available only on machines that support the “not a number” value—that is to say, on all machines that support IEEE floating point.

You can use `#ifdef NAN' to test whether the machine supports NaN. (Of course, you must arrange for GNU extensions to be visible, such as by defining _GNU_SOURCE, and then you must include math.h.)

IEEE 754 also allows for another unusual value: negative zero. This value is produced when you divide a positive number by negative infinity, or when a negative result is smaller than the limits of representation. Negative zero behaves identically to zero in all calculations, unless you explicitly test the sign bit with signbit or copysign.

Next: , Previous: Infinity and NaN, Up: Floating Point Errors

20.5.3 Examining the FPU status word

ISO C99 defines functions to query and manipulate the floating-point status word. You can use these functions to check for untrapped exceptions when it's convenient, rather than worrying about them in the middle of a calculation.

These constants represent the various IEEE 754 exceptions. Not all FPUs report all the different exceptions. Each constant is defined if and only if the FPU you are compiling for supports that exception, so you can test for FPU support with `#ifdef'. They are defined in fenv.h.

FE_INEXACT
The inexact exception.
FE_DIVBYZERO
The divide by zero exception.
FE_UNDERFLOW
The underflow exception.
FE_OVERFLOW
The overflow exception.
FE_INVALID
The invalid exception.

The macro FE_ALL_EXCEPT is the bitwise OR of all exception macros which are supported by the FP implementation.

These functions allow you to clear exception flags, test for exceptions, and save and restore the set of exceptions flagged.

— Function: int feclearexcept (int excepts)

This function clears all of the supported exception flags indicated by excepts.

The function returns zero in case the operation was successful, a non-zero value otherwise.

— Function: int feraiseexcept (int excepts)

This function raises the supported exceptions indicated by excepts. If more than one exception bit in excepts is set the order in which the exceptions are raised is undefined except that overflow (FE_OVERFLOW) or underflow (FE_UNDERFLOW) are raised before inexact (FE_INEXACT). Whether for overflow or underflow the inexact exception is also raised is also implementation dependent.

The function returns zero in case the operation was successful, a non-zero value otherwise.

— Function: int fetestexcept (int excepts)

Test whether the exception flags indicated by the parameter except are currently set. If any of them are, a nonzero value is returned which specifies which exceptions are set. Otherwise the result is zero.

To understand these functions, imagine that the status word is an integer variable named status. feclearexcept is then equivalent to `status &= ~excepts' and fetestexcept is equivalent to `(status & excepts)'. The actual implementation may be very different, of course.

Exception flags are only cleared when the program explicitly requests it, by calling feclearexcept. If you want to check for exceptions from a set of calculations, you should clear all the flags first. Here is a simple example of the way to use fetestexcept: 

     {
       double f;
       int raised;
       feclearexcept (FE_ALL_EXCEPT);
       f = compute ();
       raised = fetestexcept (FE_OVERFLOW | FE_INVALID);
       if (raised & FE_OVERFLOW) { /* ... */ }
       if (raised & FE_INVALID) { /* ... */ }
       /* ... */
     }

You cannot explicitly set bits in the status word. You can, however, save the entire status word and restore it later. This is done with the following functions:

— Function: int fegetexceptflag (fexcept_t *flagp, int excepts)

This function stores in the variable pointed to by flagp an implementation-defined value representing the current setting of the exception flags indicated by excepts.

The function returns zero in case the operation was successful, a non-zero value otherwise.

— Function: int fesetexceptflag (const fexcept_t *flagp, int excepts)

This function restores the flags for the exceptions indicated by excepts to the values stored in the variable pointed to by flagp.

The function returns zero in case the operation was successful, a non-zero value otherwise.

Note that the value stored in fexcept_t bears no resemblance to the bit mask returned by fetestexcept. The type may not even be an integer. Do not attempt to modify an fexcept_t variable.

Previous: Status bit operations, Up: Floating Point Errors

20.5.4 Error Reporting by Mathematical Functions

Many of the math functions are defined only over a subset of the real or complex numbers. Even if they are mathematically defined, their result may be larger or smaller than the range representable by their return type. These are known as domain errors, overflows, and underflows, respectively. Math functions do several things when one of these errors occurs. In this manual we will refer to the complete response as signalling a domain error, overflow, or underflow.

When a math function suffers a domain error, it raises the invalid exception and returns NaN. It also sets errno to EDOM; this is for compatibility with old systems that do not support IEEE 754 exception handling. Likewise, when overflow occurs, math functions raise the overflow exception and return &infin; or -&infin; as appropriate. They also set errno to ERANGE. When underflow occurs, the underflow exception is raised, and zero (appropriately signed) is returned. errno may be set to ERANGE, but this is not guaranteed.

Some of the math functions are defined mathematically to result in a complex value over parts of their domains. The most familiar example of this is taking the square root of a negative number. The complex math functions, such as csqrt, will return the appropriate complex value in this case. The real-valued functions, such as sqrt, will signal a domain error.

Some older hardware does not support infinities. On that hardware, overflows instead return a particular very large number (usually the largest representable number). math.h defines macros you can use to test for overflow on both old and new hardware.

— Macro: double HUGE_VAL
— Macro: float HUGE_VALF
— Macro: long double HUGE_VALL

An expression representing a particular very large number. On machines that use IEEE 754 floating point format, HUGE_VAL is infinity. On other machines, it's typically the largest positive number that can be represented.

Mathematical functions return the appropriately typed version of HUGE_VAL or −HUGE_VAL when the result is too large to be represented.

Next: , Previous: Floating Point Errors, Up: Arithmetic

20.6 Rounding Modes

Floating-point calculations are carried out internally with extra precision, and then rounded to fit into the destination type. This ensures that results are as precise as the input data. IEEE 754 defines four possible rounding modes:

Round to nearest.
This is the default mode. It should be used unless there is a specific need for one of the others. In this mode results are rounded to the nearest representable value. If the result is midway between two representable values, the even representable is chosen. Even here means the lowest-order bit is zero. This rounding mode prevents statistical bias and guarantees numeric stability: round-off errors in a lengthy calculation will remain smaller than half of FLT_EPSILON.
Round toward plus Infinity.
All results are rounded to the smallest representable value which is greater than the result.
Round toward minus Infinity.
All results are rounded to the largest representable value which is less than the result.
Round toward zero.
All results are rounded to the largest representable value whose magnitude is less than that of the result. In other words, if the result is negative it is rounded up; if it is positive, it is rounded down.

fenv.h defines constants which you can use to refer to the various rounding modes. Each one will be defined if and only if the FPU supports the corresponding rounding mode.

FE_TONEAREST
Round to nearest.


FE_UPWARD
Round toward +&infin;.


FE_DOWNWARD
Round toward -&infin;.


FE_TOWARDZERO
Round toward zero.

Underflow is an unusual case. Normally, IEEE 754 floating point numbers are always normalized (see Floating Point Concepts). Numbers smaller than 2^r (where r is the minimum exponent, FLT_MIN_RADIX-1 for float) cannot be represented as normalized numbers. Rounding all such numbers to zero or 2^r would cause some algorithms to fail at 0. Therefore, they are left in denormalized form. That produces loss of precision, since some bits of the mantissa are stolen to indicate the decimal point.

If a result is too small to be represented as a denormalized number, it is rounded to zero. However, the sign of the result is preserved; if the calculation was negative, the result is negative zero. Negative zero can also result from some operations on infinity, such as 4/-&infin;. Negative zero behaves identically to zero except when the copysign or signbit functions are used to check the sign bit directly.

At any time one of the above four rounding modes is selected. You can find out which one with this function:

— Function: int fegetround (void)

Returns the currently selected rounding mode, represented by one of the values of the defined rounding mode macros.

To change the rounding mode, use this function:

— Function: int fesetround (int round)

Changes the currently selected rounding mode to round. If round does not correspond to one of the supported rounding modes nothing is changed. fesetround returns zero if it changed the rounding mode, a nonzero value if the mode is not supported.

You should avoid changing the rounding mode if possible. It can be an expensive operation; also, some hardware requires you to compile your program differently for it to work. The resulting code may run slower. See your compiler documentation for details.

Next: , Previous: Rounding, Up: Arithmetic

20.7 Floating-Point Control Functions

IEEE 754 floating-point implementations allow the programmer to decide whether traps will occur for each of the exceptions, by setting bits in the control word. In C, traps result in the program receiving the SIGFPE signal; see Signal Handling.

Note: IEEE 754 says that trap handlers are given details of the exceptional situation, and can set the result value. C signals do not provide any mechanism to pass this information back and forth. Trapping exceptions in C is therefore not very useful.

It is sometimes necessary to save the state of the floating-point unit while you perform some calculation. The library provides functions which save and restore the exception flags, the set of exceptions that generate traps, and the rounding mode. This information is known as the floating-point environment.

The functions to save and restore the floating-point environment all use a variable of type fenv_t to store information. This type is defined in fenv.h. Its size and contents are implementation-defined. You should not attempt to manipulate a variable of this type directly.

To save the state of the FPU, use one of these functions:

— Function: int fegetenv (fenv_t *envp)

Store the floating-point environment in the variable pointed to by envp.

The function returns zero in case the operation was successful, a non-zero value otherwise.

— Function: int feholdexcept (fenv_t *envp)

Store the current floating-point environment in the object pointed to by envp. Then clear all exception flags, and set the FPU to trap no exceptions. Not all FPUs support trapping no exceptions; if feholdexcept cannot set this mode, it returns nonzero value. If it succeeds, it returns zero.

The functions which restore the floating-point environment can take these kinds of arguments:

To set the floating-point environment, you can use either of these functions:

— Function: int fesetenv (const fenv_t *envp)

Set the floating-point environment to that described by envp.

The function returns zero in case the operation was successful, a non-zero value otherwise.

— Function: int feupdateenv (const fenv_t *envp)

Like fesetenv, this function sets the floating-point environment to that described by envp. However, if any exceptions were flagged in the status word before feupdateenv was called, they remain flagged after the call. In other words, after feupdateenv is called, the status word is the bitwise OR of the previous status word and the one saved in envp.

The function returns zero in case the operation was successful, a non-zero value otherwise.

To control for individual exceptions if raising them causes a trap to occur, you can use the following two functions.

Portability Note: These functions are all GNU extensions.

— Function: int feenableexcept (int excepts)

This functions enables traps for each of the exceptions as indicated by the parameter except. The individual excepetions are described in Status bit operations. Only the specified exceptions are enabled, the status of the other exceptions is not changed.

The function returns the previous enabled exceptions in case the operation was successful, -1 otherwise.

— Function: int fedisableexcept (int excepts)

This functions disables traps for each of the exceptions as indicated by the parameter except. The individual excepetions are described in Status bit operations. Only the specified exceptions are disabled, the status of the other exceptions is not changed.

The function returns the previous enabled exceptions in case the operation was successful, -1 otherwise.

— Function: int fegetexcept (int excepts)

The function returns a bitmask of all currently enabled exceptions. It returns -1 in case of failure.

Next: , Previous: Control Functions, Up: Arithmetic

20.8 Arithmetic Functions

The C library provides functions to do basic operations on floating-point numbers. These include absolute value, maximum and minimum, normalization, bit twiddling, rounding, and a few others.

Next: , Up: Arithmetic Functions

20.8.1 Absolute Value

These functions are provided for obtaining the absolute value (or magnitude) of a number. The absolute value of a real number x is x if x is positive, −x if x is negative. For a complex number z, whose real part is x and whose imaginary part is y, the absolute value is sqrt (x*x + y*y).

Prototypes for abs, labs and llabs are in stdlib.h; imaxabs is declared in inttypes.h; fabs, fabsf and fabsl are declared in math.h. cabs, cabsf and cabsl are declared in complex.h.

— Function: int abs (int number)
— Function: long int labs (long int number)
— Function: long long int llabs (long long int number)
— Function: intmax_t imaxabs (intmax_t number)

These functions return the absolute value of number.

Most computers use a two's complement integer representation, in which the absolute value of INT_MIN (the smallest possible int) cannot be represented; thus, abs (INT_MIN) is not defined.

llabs and imaxdiv are new to ISO C99.

See Integers for a description of the intmax_t type.

— Function: double fabs (double number)
— Function: float fabsf (float number)
— Function: long double fabsl (long double number)

This function returns the absolute value of the floating-point number number.

— Function: double cabs (complex double z)
— Function: float cabsf (complex float z)
— Function: long double cabsl (complex long double z)

These functions return the absolute value of the complex number z (see Complex Numbers). The absolute value of a complex number is: 

          sqrt (creal (z) * creal (z) + cimag (z) * cimag (z))

This function should always be used instead of the direct formula because it takes special care to avoid losing precision. It may also take advantage of hardware support for this operation. See hypot in Exponents and Logarithms.

Next: , Previous: Absolute Value, Up: Arithmetic Functions

20.8.2 Normalization Functions

The functions described in this section are primarily provided as a way to efficiently perform certain low-level manipulations on floating point numbers that are represented internally using a binary radix; see Floating Point Concepts. These functions are required to have equivalent behavior even if the representation does not use a radix of 2, but of course they are unlikely to be particularly efficient in those cases.

All these functions are declared in math.h.

— Function: double frexp (double value, int *exponent)
— Function: float frexpf (float value, int *exponent)
— Function: long double frexpl (long double value, int *exponent)

These functions are used to split the number value into a normalized fraction and an exponent.

If the argument value is not zero, the return value is value times a power of two, and is always in the range 1/2 (inclusive) to 1 (exclusive). The corresponding exponent is stored in *exponent; the return value multiplied by 2 raised to this exponent equals the original number value.

For example, frexp (12.8, &exponent) returns 0.8 and stores 4 in exponent.

If value is zero, then the return value is zero and zero is stored in *exponent.

— Function: double ldexp (double value, int exponent)
— Function: float ldexpf (float value, int exponent)
— Function: long double ldexpl (long double value, int exponent)

These functions return the result of multiplying the floating-point number value by 2 raised to the power exponent. (It can be used to reassemble floating-point numbers that were taken apart by frexp.)

For example, ldexp (0.8, 4) returns 12.8.

The following functions, which come from BSD, provide facilities equivalent to those of ldexp and frexp. See also the ISO C function logb which originally also appeared in BSD.

— Function: double scalb (double value, int exponent)
— Function: float scalbf (float value, int exponent)
— Function: long double scalbl (long double value, int exponent)

The scalb function is the BSD name for ldexp.

— Function: long long int scalbn (double x, int n)
— Function: long long int scalbnf (float x, int n)
— Function: long long int scalbnl (long double x, int n)

scalbn is identical to scalb, except that the exponent n is an int instead of a floating-point number.

— Function: long long int scalbln (double x, long int n)
— Function: long long int scalblnf (float x, long int n)
— Function: long long int scalblnl (long double x, long int n)

scalbln is identical to scalb, except that the exponent n is a long int instead of a floating-point number.

— Function: long long int significand (double x)
— Function: long long int significandf (float x)
— Function: long long int significandl (long double x)

significand returns the mantissa of x scaled to the range [1, 2). It is equivalent to scalb (x, (double) -ilogb (x)).

This function exists mainly for use in certain standardized tests of IEEE 754 conformance.

Next: , Previous: Normalization Functions, Up: Arithmetic Functions

20.8.3 Rounding Functions

The functions listed here perform operations such as rounding and truncation of floating-point values. Some of these functions convert floating point numbers to integer values. They are all declared in math.h.

You can also convert floating-point numbers to integers simply by casting them to int. This discards the fractional part, effectively rounding towards zero. However, this only works if the result can actually be represented as an int—for very large numbers, this is impossible. The functions listed here return the result as a double instead to get around this problem.

— Function: double ceil (double x)
— Function: float ceilf (float x)
— Function: long double ceill (long double x)

These functions round x upwards to the nearest integer, returning that value as a double. Thus, ceil (1.5) is 2.0.

— Function: double floor (double x)
— Function: float floorf (float x)
— Function: long double floorl (long double x)

These functions round x downwards to the nearest integer, returning that value as a double. Thus, floor (1.5) is 1.0 and floor (-1.5) is -2.0.

— Function: double trunc (double x)
— Function: float truncf (float x)
— Function: long double truncl (long double x)

The trunc functions round x towards zero to the nearest integer (returned in floating-point format). Thus, trunc (1.5) is 1.0 and trunc (-1.5) is -1.0.

— Function: double rint (double x)
— Function: float rintf (float x)
— Function: long double rintl (long double x)

These functions round x to an integer value according to the current rounding mode. See Floating Point Parameters, for information about the various rounding modes. The default rounding mode is to round to the nearest integer; some machines support other modes, but round-to-nearest is always used unless you explicitly select another.

If x was not initially an integer, these functions raise the inexact exception.

— Function: double nearbyint (double x)
— Function: float nearbyintf (float x)
— Function: long double nearbyintl (long double x)

These functions return the same value as the rint functions, but do not raise the inexact exception if x is not an integer.

— Function: double round (double x)
— Function: float roundf (float x)
— Function: long double roundl (long double x)

These functions are similar to rint, but they round halfway cases away from zero instead of to the nearest even integer.

— Function: long int lrint (double x)
— Function: long int lrintf (float x)
— Function: long int lrintl (long double x)

These functions are just like rint, but they return a long int instead of a floating-point number.

— Function: long long int llrint (double x)
— Function: long long int llrintf (float x)
— Function: long long int llrintl (long double x)

These functions are just like rint, but they return a long long int instead of a floating-point number.

— Function: long int lround (double x)
— Function: long int lroundf (float x)
— Function: long int lroundl (long double x)

These functions are just like round, but they return a long int instead of a floating-point number.

— Function: long long int llround (double x)
— Function: long long int llroundf (float x)
— Function: long long int llroundl (long double x)

These functions are just like round, but they return a long long int instead of a floating-point number.

— Function: double modf (double value, double *integer-part)
— Function: float modff (float value, float *integer-part)
— Function: long double modfl (long double value, long double *integer-part)

These functions break the argument value into an integer part and a fractional part (between -1 and 1, exclusive). Their sum equals value. Each of the parts has the same sign as value, and the integer part is always rounded toward zero.

modf stores the integer part in *integer-part, and returns the fractional part. For example, modf (2.5, &intpart) returns 0.5 and stores 2.0 into intpart.

Next: , Previous: Rounding Functions, Up: Arithmetic Functions

20.8.4 Remainder Functions

The functions in this section compute the remainder on division of two floating-point numbers. Each is a little different; pick the one that suits your problem.

— Function: double fmod (double numerator, double denominator)
— Function: float fmodf (float numerator, float denominator)
— Function: long double fmodl (long double numerator, long double denominator)

These functions compute the remainder from the division of numerator by denominator. Specifically, the return value is numerator - n * denominator, where n is the quotient of numerator divided by denominator, rounded towards zero to an integer. Thus, fmod (6.5, 2.3) returns 1.9, which is 6.5 minus 4.6.

The result has the same sign as the numerator and has magnitude less than the magnitude of the denominator.

If denominator is zero, fmod signals a domain error.

— Function: double drem (double numerator, double denominator)
— Function: float dremf (float numerator, float denominator)
— Function: long double dreml (long double numerator, long double denominator)

These functions are like fmod except that they rounds the internal quotient n to the nearest integer instead of towards zero to an integer. For example, drem (6.5, 2.3) returns -0.4, which is 6.5 minus 6.9.

The absolute value of the result is less than or equal to half the absolute value of the denominator. The difference between fmod (numerator, denominator) and drem (numerator, denominator) is always either denominator, minus denominator, or zero.

If denominator is zero, drem signals a domain error.

— Function: double remainder (double numerator, double denominator)
— Function: float remainderf (float numerator, float denominator)
— Function: long double remainderl (long double numerator, long double denominator)

This function is another name for drem.

Next: , Previous: Remainder Functions, Up: Arithmetic Functions

20.8.5 Setting and modifying single bits of FP values

There are some operations that are too complicated or expensive to perform by hand on floating-point numbers. ISO C99 defines functions to do these operations, which mostly involve changing single bits.

— Function: double copysign (double x, double y)
— Function: float copysignf (float x, float y)
— Function: long double copysignl (long double x, long double y)

These functions return x but with the sign of y. They work even if x or y are NaN or zero. Both of these can carry a sign (although not all implementations support it) and this is one of the few operations that can tell the difference.

copysign never raises an exception.

This function is defined in IEC 559 (and the appendix with recommended functions in IEEE 754/IEEE 854).

— Function: int signbit (float-type x)

signbit is a generic macro which can work on all floating-point types. It returns a nonzero value if the value of x has its sign bit set.

This is not the same as x < 0.0, because IEEE 754 floating point allows zero to be signed. The comparison -0.0 < 0.0 is false, but signbit (-0.0) will return a nonzero value.

— Function: double nextafter (double x, double y)
— Function: float nextafterf (float x, float y)
— Function: long double nextafterl (long double x, long double y)

The nextafter function returns the next representable neighbor of x in the direction towards y. The size of the step between x and the result depends on the type of the result. If x = y the function simply returns y. If either value is NaN, NaN is returned. Otherwise a value corresponding to the value of the least significant bit in the mantissa is added or subtracted, depending on the direction. nextafter will signal overflow or underflow if the result goes outside of the range of normalized numbers.

This function is defined in IEC 559 (and the appendix with recommended functions in IEEE 754/IEEE 854).

— Function: double nexttoward (double x, long double y)
— Function: float nexttowardf (float x, long double y)
— Function: long double nexttowardl (long double x, long double y)

These functions are identical to the corresponding versions of nextafter except that their second argument is a long double.

— Function: double nan (const char *tagp)
— Function: float nanf (const char *tagp)
— Function: long double nanl (const char *tagp)

The nan function returns a representation of NaN, provided that NaN is supported by the target platform. nan ("n-char-sequence") is equivalent to strtod ("NAN(n-char-sequence)").

The argument tagp is used in an unspecified manner. On IEEE 754 systems, there are many representations of NaN, and tagp selects one. On other systems it may do nothing.

Next: , Previous: FP Bit Twiddling, Up: Arithmetic Functions

20.8.6 Floating-Point Comparison Functions

The standard C comparison operators provoke exceptions when one or other of the operands is NaN. For example, 

     int v = a < 1.0;

will raise an exception if a is NaN. (This does not happen with == and !=; those merely return false and true, respectively, when NaN is examined.) Frequently this exception is undesirable. ISO C99 therefore defines comparison functions that do not raise exceptions when NaN is examined. All of the functions are implemented as macros which allow their arguments to be of any floating-point type. The macros are guaranteed to evaluate their arguments only once.

— Macro: int isgreater (real-floating x, real-floating y)

This macro determines whether the argument x is greater than y. It is equivalent to (x) > (y), but no exception is raised if x or y are NaN.

— Macro: int isgreaterequal (real-floating x, real-floating y)

This macro determines whether the argument x is greater than or equal to y. It is equivalent to (x) >= (y), but no exception is raised if x or y are NaN.

— Macro: int isless (real-floating x, real-floating y)

This macro determines whether the argument x is less than y. It is equivalent to (x) < (y), but no exception is raised if x or y are NaN.

— Macro: int islessequal (real-floating x, real-floating y)

This macro determines whether the argument x is less than or equal to y. It is equivalent to (x) <= (y), but no exception is raised if x or y are NaN.

— Macro: int islessgreater (real-floating x, real-floating y)

This macro determines whether the argument x is less or greater than y. It is equivalent to (x) < (y) || (x) > (y) (although it only evaluates x and y once), but no exception is raised if x or y are NaN.

This macro is not equivalent to x != y, because that expression is true if x or y are NaN.

— Macro: int isunordered (real-floating x, real-floating y)

This macro determines whether its arguments are unordered. In other words, it is true if x or y are NaN, and false otherwise.

Not all machines provide hardware support for these operations. On machines that don't, the macros can be very slow. Therefore, you should not use these functions when NaN is not a concern.

Note: There are no macros isequal or isunequal. They are unnecessary, because the == and != operators do not throw an exception if one or both of the operands are NaN.

Previous: FP Comparison Functions, Up: Arithmetic Functions

20.8.7 Miscellaneous FP arithmetic functions

The functions in this section perform miscellaneous but common operations that are awkward to express with C operators. On some processors these functions can use special machine instructions to perform these operations faster than the equivalent C code.

— Function: double fmin (double x, double y)
— Function: float fminf (float x, float y)
— Function: long double fminl (long double x, long double y)

The fmin function returns the lesser of the two values x and y. It is similar to the expression 

          ((x) < (y) ? (x) : (y))

except that x and y are only evaluated once.

If an argument is NaN, the other argument is returned. If both arguments are NaN, NaN is returned.

— Function: double fmax (double x, double y)
— Function: float fmaxf (float x, float y)
— Function: long double fmaxl (long double x, long double y)

The fmax function returns the greater of the two values x and y.

If an argument is NaN, the other argument is returned. If both arguments are NaN, NaN is returned.

— Function: double fdim (double x, double y)
— Function: float fdimf (float x, float y)
— Function: long double fdiml (long double x, long double y)

The fdim function returns the positive difference between x and y. The positive difference is x - y if x is greater than y, and 0 otherwise.

If x, y, or both are NaN, NaN is returned.

— Function: double fma (double x, double y, double z)
— Function: float fmaf (float x, float y, float z)
— Function: long double fmal (long double x, long double y, long double z)

The fma function performs floating-point multiply-add. This is the operation (x &middot; y) + z, but the intermediate result is not rounded to the destination type. This can sometimes improve the precision of a calculation.

This function was introduced because some processors have a special instruction to perform multiply-add. The C compiler cannot use it directly, because the expression `x*y + z' is defined to round the intermediate result. fma lets you choose when you want to round only once.

On processors which do not implement multiply-add in hardware, fma can be very slow since it must avoid intermediate rounding. math.h defines the symbols FP_FAST_FMA, FP_FAST_FMAF, and FP_FAST_FMAL when the corresponding version of fma is no slower than the expression `x*y + z'. In the GNU C library, this always means the operation is implemented in hardware.

Next: , Previous: Arithmetic Functions, Up: Arithmetic

20.9 Complex Numbers

ISO C99 introduces support for complex numbers in C. This is done with a new type qualifier, complex. It is a keyword if and only if complex.h has been included. There are three complex types, corresponding to the three real types: float complex, double complex, and long double complex.

To construct complex numbers you need a way to indicate the imaginary part of a number. There is no standard notation for an imaginary floating point constant. Instead, complex.h defines two macros that can be used to create complex numbers.

— Macro: const float complex _Complex_I

This macro is a representation of the complex number “0+1i”. Multiplying a real floating-point value by _Complex_I gives a complex number whose value is purely imaginary. You can use this to construct complex constants: 

          3.0 + 4.0i = 3.0 + 4.0 * _Complex_I

Note that _Complex_I * _Complex_I has the value -1, but the type of that value is complex.

_Complex_I is a bit of a mouthful. complex.h also defines a shorter name for the same constant.

— Macro: const float complex I

This macro has exactly the same value as _Complex_I. Most of the time it is preferable. However, it causes problems if you want to use the identifier I for something else. You can safely write 

          #include <complex.h>
          #undef I

if you need I for your own purposes. (In that case we recommend you also define some other short name for _Complex_I, such as J.)

Next: , Previous: Complex Numbers, Up: Arithmetic

20.10 Projections, Conjugates, and Decomposing of Complex Numbers

ISO C99 also defines functions that perform basic operations on complex numbers, such as decomposition and conjugation. The prototypes for all these functions are in complex.h. All functions are available in three variants, one for each of the three complex types.

— Function: double creal (complex double z)
— Function: float crealf (complex float z)
— Function: long double creall (complex long double z)

These functions return the real part of the complex number z.

— Function: double cimag (complex double z)
— Function: float cimagf (complex float z)
— Function: long double cimagl (complex long double z)

These functions return the imaginary part of the complex number z.

— Function: complex double conj (complex double z)
— Function: complex float conjf (complex float z)
— Function: complex long double conjl (complex long double z)

These functions return the conjugate value of the complex number z. The conjugate of a complex number has the same real part and a negated imaginary part. In other words, `conj(a + bi) = a + -bi'.

— Function: double carg (complex double z)
— Function: float cargf (complex float z)
— Function: long double cargl (complex long double z)

These functions return the argument of the complex number z. The argument of a complex number is the angle in the complex plane between the positive real axis and a line passing through zero and the number. This angle is measured in the usual fashion and ranges from 0 to 2&pi;.

carg has a branch cut along the positive real axis.

— Function: complex double cproj (complex double z)
— Function: complex float cprojf (complex float z)
— Function: complex long double cprojl (complex long double z)

These functions return the projection of the complex value z onto the Riemann sphere. Values with a infinite imaginary part are projected to positive infinity on the real axis, even if the real part is NaN. If the real part is infinite, the result is equivalent to 

          INFINITY + I * copysign (0.0, cimag (z))

Next: , Previous: Operations on Complex, Up: Arithmetic

20.11 Parsing of Numbers

This section describes functions for “reading” integer and floating-point numbers from a string. It may be more convenient in some cases to use sscanf or one of the related functions; see Formatted Input. But often you can make a program more robust by finding the tokens in the string by hand, then converting the numbers one by one.

Next: , Up: Parsing of Numbers

20.11.1 Parsing of Integers

The `str' functions are declared in stdlib.h and those beginning with `wcs' are declared in wchar.h. One might wonder about the use of restrict in the prototypes of the functions in this section. It is seemingly useless but the ISO C standard uses it (for the functions defined there) so we have to do it as well.

— Function: long int strtol (const char *restrict string, char **restrict tailptr, int base)

The strtol (“string-to-long”) function converts the initial part of string to a signed integer, which is returned as a value of type long int.

This function attempts to decompose string as follows:

If the string is empty, contains only whitespace, or does not contain an initial substring that has the expected syntax for an integer in the specified base, no conversion is performed. In this case, strtol returns a value of zero and the value stored in *tailptr is the value of string.

In a locale other than the standard "C" locale, this function may recognize additional implementation-dependent syntax.

If the string has valid syntax for an integer but the value is not representable because of overflow, strtol returns either LONG_MAX or LONG_MIN (see Range of Type), as appropriate for the sign of the value. It also sets errno to ERANGE to indicate there was overflow.

You should not check for errors by examining the return value of strtol, because the string might be a valid representation of 0l, LONG_MAX, or LONG_MIN. Instead, check whether tailptr points to what you expect after the number (e.g. '\0' if the string should end after the number). You also need to clear errno before the call and check it afterward, in case there was overflow.

There is an example at the end of this section.

— Function: long int wcstol (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)

The wcstol function is equivalent to the strtol function in nearly all aspects but handles wide character strings.

The wcstol function was introduced in Amendment 1 of ISO C90.

— Function: unsigned long int strtoul (const char *retrict string, char **restrict tailptr, int base)

The strtoul (“string-to-unsigned-long”) function is like strtol except it converts to an unsigned long int value. The syntax is the same as described above for strtol. The value returned on overflow is ULONG_MAX (see Range of Type).

If string depicts a negative number, strtoul acts the same as strtol but casts the result to an unsigned integer. That means for example that strtoul on "-1" returns ULONG_MAX and an input more negative than LONG_MIN returns (ULONG_MAX + 1) / 2.

strtoul sets errno to EINVAL if base is out of range, or ERANGE on overflow.

— Function: unsigned long int wcstoul (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)

The wcstoul function is equivalent to the strtoul function in nearly all aspects but handles wide character strings.

The wcstoul function was introduced in Amendment 1 of ISO C90.

— Function: long long int strtoll (const char *restrict string, char **restrict tailptr, int base)

The strtoll function is like strtol except that it returns a long long int value, and accepts numbers with a correspondingly larger range.

If the string has valid syntax for an integer but the value is not representable because of overflow, strtoll returns either LONG_LONG_MAX or LONG_LONG_MIN (see Range of Type), as appropriate for the sign of the value. It also sets errno to ERANGE to indicate there was overflow.

The strtoll function was introduced in ISO C99.

— Function: long long int wcstoll (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)

The wcstoll function is equivalent to the strtoll function in nearly all aspects but handles wide character strings.

The wcstoll function was introduced in Amendment 1 of ISO C90.

— Function: long long int strtoq (const char *restrict string, char **restrict tailptr, int base)

strtoq (“string-to-quad-word”) is the BSD name for strtoll.

— Function: long long int wcstoq (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)

The wcstoq function is equivalent to the strtoq function in nearly all aspects but handles wide character strings.

The wcstoq function is a GNU extension.

— Function: unsigned long long int strtoull (const char *restrict string, char **restrict tailptr, int base)

The strtoull function is related to strtoll the same way strtoul is related to strtol.

The strtoull function was introduced in ISO C99.

— Function: unsigned long long int wcstoull (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)

The wcstoull function is equivalent to the strtoull function in nearly all aspects but handles wide character strings.

The wcstoull function was introduced in Amendment 1 of ISO C90.

— Function: unsigned long long int strtouq (const char *restrict string, char **restrict tailptr, int base)

strtouq is the BSD name for strtoull.

— Function: unsigned long long int wcstouq (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)

The wcstouq function is equivalent to the strtouq function in nearly all aspects but handles wide character strings.

The wcstouq function is a GNU extension.

— Function: intmax_t strtoimax (const char *restrict string, char **restrict tailptr, int base)

The strtoimax function is like strtol except that it returns a intmax_t value, and accepts numbers of a corresponding range.

If the string has valid syntax for an integer but the value is not representable because of overflow, strtoimax returns either INTMAX_MAX or INTMAX_MIN (see Integers), as appropriate for the sign of the value. It also sets errno to ERANGE to indicate there was overflow.

See Integers for a description of the intmax_t type. The strtoimax function was introduced in ISO C99.

— Function: intmax_t wcstoimax (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)

The wcstoimax function is equivalent to the strtoimax function in nearly all aspects but handles wide character strings.

The wcstoimax function was introduced in ISO C99.

— Function: uintmax_t strtoumax (const char *restrict string, char **restrict tailptr, int base)

The strtoumax function is related to strtoimax the same way that strtoul is related to strtol.

See Integers for a description of the intmax_t type. The strtoumax function was introduced in ISO C99.

— Function: uintmax_t wcstoumax (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)

The wcstoumax function is equivalent to the strtoumax function in nearly all aspects but handles wide character strings.

The wcstoumax function was introduced in ISO C99.

— Function: long int atol (const char *string)

This function is similar to the strtol function with a base argument of 10, except that it need not detect overflow errors. The atol function is provided mostly for compatibility with existing code; using strtol is more robust.

— Function: int atoi (const char *string)

This function is like atol, except that it returns an int. The atoi function is also considered obsolete; use strtol instead.

— Function: long long int atoll (const char *string)

This function is similar to atol, except it returns a long long int.

The atoll function was introduced in ISO C99. It too is obsolete (despite having just been added); use strtoll instead.

All the functions mentioned in this section so far do not handle alternative representations of characters as described in the locale data. Some locales specify thousands separator and the way they have to be used which can help to make large numbers more readable. To read such numbers one has to use the scanf functions with the `'' flag.

Here is a function which parses a string as a sequence of integers and returns the sum of them: 

     int
     sum_ints_from_string (char *string)
     {
       int sum = 0;
     
       while (1) {
         char *tail;
         int next;
     
         /* Skip whitespace by hand, to detect the end.  */
         while (isspace (*string)) string++;
         if (*string == 0)
           break;
     
         /* There is more nonwhitespace,  */
         /* so it ought to be another number.  */
         errno = 0;
         /* Parse it.  */
         next = strtol (string, &tail, 0);
         /* Add it in, if not overflow.  */
         if (errno)
           printf ("Overflow\n");
         else
           sum += next;
         /* Advance past it.  */
         string = tail;
       }
     
       return sum;
     }

Previous: Parsing of Integers, Up: Parsing of Numbers

20.11.2 Parsing of Floats

The `str' functions are declared in stdlib.h and those beginning with `wcs' are declared in wchar.h. One might wonder about the use of restrict in the prototypes of the functions in this section. It is seemingly useless but the ISO C standard uses it (for the functions defined there) so we have to do it as well.

— Function: double strtod (const char *restrict string, char **restrict tailptr)

The strtod (“string-to-double”) function converts the initial part of string to a floating-point number, which is returned as a value of type double.

This function attempts to decompose string as follows:

If the string is empty, contains only whitespace, or does not contain an initial substring that has the expected syntax for a floating-point number, no conversion is performed. In this case, strtod returns a value of zero and the value returned in *tailptr is the value of string.

In a locale other than the standard "C" or "POSIX" locales, this function may recognize additional locale-dependent syntax.

If the string has valid syntax for a floating-point number but the value is outside the range of a double, strtod will signal overflow or underflow as described in Math Error Reporting.

strtod recognizes four special input strings. The strings "inf" and "infinity" are converted to &infin;, or to the largest representable value if the floating-point format doesn't support infinities. You can prepend a "+" or "-" to specify the sign. Case is ignored when scanning these strings.

The strings "nan" and "nan(chars...)" are converted to NaN. Again, case is ignored. If chars... are provided, they are used in some unspecified fashion to select a particular representation of NaN (there can be several).

Since zero is a valid result as well as the value returned on error, you should check for errors in the same way as for strtol, by examining errno and tailptr.

— Function: float strtof (const char *string, char **tailptr)
— Function: long double strtold (const char *string, char **tailptr)

These functions are analogous to strtod, but return float and long double values respectively. They report errors in the same way as strtod. strtof can be substantially faster than strtod, but has less precision; conversely, strtold can be much slower but has more precision (on systems where long double is a separate type).

These functions have been GNU extensions and are new to ISO C99.

— Function: double wcstod (const wchar_t *restrict string, wchar_t **restrict tailptr)
— Function: float wcstof (const wchar_t *string, wchar_t **tailptr)
— Function: long double wcstold (const wchar_t *string, wchar_t **tailptr)

The wcstod, wcstof, and wcstol functions are equivalent in nearly all aspect to the strtod, strtof, and strtold functions but it handles wide character string.

The wcstod function was introduced in Amendment 1 of ISO C90. The wcstof and wcstold functions were introduced in ISO C99.

— Function: double atof (const char *string)

This function is similar to the strtod function, except that it need not detect overflow and underflow errors. The atof function is provided mostly for compatibility with existing code; using strtod is more robust.

The GNU C library also provides `_l' versions of these functions, which take an additional argument, the locale to use in conversion. See Parsing of Integers.

Previous: Parsing of Numbers, Up: Arithmetic

20.12 Old-fashioned System V number-to-string functions

The old System V C library provided three functions to convert numbers to strings, with unusual and hard-to-use semantics. The GNU C library also provides these functions and some natural extensions.

These functions are only available in glibc and on systems descended from AT&T Unix. Therefore, unless these functions do precisely what you need, it is better to use sprintf, which is standard.

All these functions are defined in stdlib.h.

— Function: char * ecvt (double value, int ndigit, int *decpt, int *neg)

The function ecvt converts the floating-point number value to a string with at most ndigit decimal digits. The returned string contains no decimal point or sign. The first digit of the string is non-zero (unless value is actually zero) and the last digit is rounded to nearest. *decpt is set to the index in the string of the first digit after the decimal point. *neg is set to a nonzero value if value is negative, zero otherwise.

If ndigit decimal digits would exceed the precision of a double it is reduced to a system-specific value.

The returned string is statically allocated and overwritten by each call to ecvt.

If value is zero, it is implementation defined whether *decpt is 0 or 1.

For example: ecvt (12.3, 5, &d, &n) returns "12300" and sets d to 2 and n to 0.

— Function: char * fcvt (double value, int ndigit, int *decpt, int *neg)

The function fcvt is like ecvt, but ndigit specifies the number of digits after the decimal point. If ndigit is less than zero, value is rounded to the ndigit+1'th place to the left of the decimal point. For example, if ndigit is -1, value will be rounded to the nearest 10. If ndigit is negative and larger than the number of digits to the left of the decimal point in value, value will be rounded to one significant digit.

If ndigit decimal digits would exceed the precision of a double it is reduced to a system-specific value.

The returned string is statically allocated and overwritten by each call to fcvt.

— Function: char * gcvt (double value, int ndigit, char *buf)

gcvt is functionally equivalent to `sprintf(buf, "%*g", ndigit, value'. It is provided only for compatibility's sake. It returns buf.

If ndigit decimal digits would exceed the precision of a double it is reduced to a system-specific value.

As extensions, the GNU C library provides versions of these three functions that take long double arguments.

— Function: char * qecvt (long double value, int ndigit, int *decpt, int *neg)

This function is equivalent to ecvt except that it takes a long double for the first parameter and that ndigit is restricted by the precision of a long double.

— Function: char * qfcvt (long double value, int ndigit, int *decpt, int *neg)

This function is equivalent to fcvt except that it takes a long double for the first parameter and that ndigit is restricted by the precision of a long double.

— Function: char * qgcvt (long double value, int ndigit, char *buf)

This function is equivalent to gcvt except that it takes a long double for the first parameter and that ndigit is restricted by the precision of a long double.

The ecvt and fcvt functions, and their long double equivalents, all return a string located in a static buffer which is overwritten by the next call to the function. The GNU C library provides another set of extended functions which write the converted string into a user-supplied buffer. These have the conventional _r suffix.

gcvt_r is not necessary, because gcvt already uses a user-supplied buffer.

— Function: int ecvt_r (double value, int ndigit, int *decpt, int *neg, char *buf, size_t len)

The ecvt_r function is the same as ecvt, except that it places its result into the user-specified buffer pointed to by buf, with length len. The return value is -1 in case of an error and zero otherwise.

This function is a GNU extension.

— Function: int fcvt_r (double value, int ndigit, int *decpt, int *neg, char *buf, size_t len)

The fcvt_r function is the same as fcvt, except that it places its result into the user-specified buffer pointed to by buf, with length len. The return value is -1 in case of an error and zero otherwise.

This function is a GNU extension.

— Function: int qecvt_r (long double value, int ndigit, int *decpt, int *neg, char *buf, size_t len)

The qecvt_r function is the same as qecvt, except that it places its result into the user-specified buffer pointed to by buf, with length len. The return value is -1 in case of an error and zero otherwise.

This function is a GNU extension.

— Function: int qfcvt_r (long double value, int ndigit, int *decpt, int *neg, char *buf, size_t len)

The qfcvt_r function is the same as qfcvt, except that it places its result into the user-specified buffer pointed to by buf, with length len. The return value is -1 in case of an error and zero otherwise.

This function is a GNU extension.

Next: , Previous: Arithmetic, Up: Top

21 Date and Time

This chapter describes functions for manipulating dates and times, including functions for determining what time it is and conversion between different time representations.

Next: , Up: Date and Time

21.1 Time Basics

Discussing time in a technical manual can be difficult because the word “time” in English refers to lots of different things. In this manual, we use a rigorous terminology to avoid confusion, and the only thing we use the simple word “time” for is to talk about the abstract concept.

A calendar time is a point in the time continuum, for example November 4, 1990 at 18:02.5 UTC. Sometimes this is called “absolute time”. We don't speak of a “date”, because that is inherent in a calendar time. An interval is a contiguous part of the time continuum between two calendar times, for example the hour between 9:00 and 10:00 on July 4, 1980. An elapsed time is the length of an interval, for example, 35 minutes. People sometimes sloppily use the word “interval” to refer to the elapsed time of some interval. An amount of time is a sum of elapsed times, which need not be of any specific intervals. For example, the amount of time it takes to read a book might be 9 hours, independently of when and in how many sittings it is read.

A period is the elapsed time of an interval between two events, especially when they are part of a sequence of regularly repeating events. CPU time is like calendar time, except that it is based on the subset of the time continuum when a particular process is actively using a CPU. CPU time is, therefore, relative to a process. Processor time is an amount of time that a CPU is in use. In fact, it's a basic system resource, since there's a limit to how much can exist in any given interval (that limit is the elapsed time of the interval times the number of CPUs in the processor). People often call this CPU time, but we reserve the latter term in this manual for the definition above.

Next: , Previous: Time Basics, Up: Date and Time

21.2 Elapsed Time

One way to represent an elapsed time is with a simple arithmetic data type, as with the following function to compute the elapsed time between two calendar times. This function is declared in time.h.

— Function: double difftime (time_t time1, time_t time0)

The difftime function returns the number of seconds of elapsed time between calendar time time1 and calendar time time0, as a value of type double. The difference ignores leap seconds unless leap second support is enabled.

In the GNU system, you can simply subtract time_t values. But on other systems, the time_t data type might use some other encoding where subtraction doesn't work directly.

The GNU C library provides two data types specifically for representing an elapsed time. They are used by various GNU C library functions, and you can use them for your own purposes too. They're exactly the same except that one has a resolution in microseconds, and the other, newer one, is in nanoseconds.

— Data Type: struct timeval

The struct timeval structure represents an elapsed time. It is declared in sys/time.h and has the following members:

long int tv_sec
This represents the number of whole seconds of elapsed time.
long int tv_usec
This is the rest of the elapsed time (a fraction of a second), represented as the number of microseconds. It is always less than one million.

— Data Type: struct timespec

The struct timespec structure represents an elapsed time. It is declared in time.h and has the following members:

long int tv_sec
This represents the number of whole seconds of elapsed time.
long int tv_nsec
This is the rest of the elapsed time (a fraction of a second), represented as the number of nanoseconds. It is always less than one billion.

It is often necessary to subtract two values of type struct timeval or struct timespec. Here is the best way to do this. It works even on some peculiar operating systems where the tv_sec member has an unsigned type. 

     /* Subtract the `struct timeval' values X and Y,
        storing the result in RESULT.
        Return 1 if the difference is negative, otherwise 0.  */
     
     int
     timeval_subtract (result, x, y)
          struct timeval *result, *x, *y;
     {
       /* Perform the carry for the later subtraction by updating y. */
       if (x->tv_usec < y->tv_usec) {
         int nsec = (y->tv_usec - x->tv_usec) / 1000000 + 1;
         y->tv_usec -= 1000000 * nsec;
         y->tv_sec += nsec;
       }
       if (x->tv_usec - y->tv_usec > 1000000) {
         int nsec = (x->tv_usec - y->tv_usec) / 1000000;
         y->tv_usec += 1000000 * nsec;
         y->tv_sec -= nsec;
       }
     
       /* Compute the time remaining to wait.
          tv_usec is certainly positive. */
       result->tv_sec = x->tv_sec - y->tv_sec;
       result->tv_usec = x->tv_usec - y->tv_usec;
     
       /* Return 1 if result is negative. */
       return x->tv_sec < y->tv_sec;
     }

Common functions that use struct timeval are gettimeofday and settimeofday.

There are no GNU C library functions specifically oriented toward dealing with elapsed times, but the calendar time, processor time, and alarm and sleeping functions have a lot to do with them.

Next: , Previous: Elapsed Time, Up: Date and Time

21.3 Processor And CPU Time

If you're trying to optimize your program or measure its efficiency, it's very useful to know how much processor time it uses. For that, calendar time and elapsed times are useless because a process may spend time waiting for I/O or for other processes to use the CPU. However, you can get the information with the functions in this section.

CPU time (see Time Basics) is represented by the data type clock_t, which is a number of clock ticks. It gives the total amount of time a process has actively used a CPU since some arbitrary event. On the GNU system, that event is the creation of the process. While arbitrary in general, the event is always the same event for any particular process, so you can always measure how much time on the CPU a particular computation takes by examining the process' CPU time before and after the computation. In the GNU system, clock_t is equivalent to long int and CLOCKS_PER_SEC is an integer value. But in other systems, both clock_t and the macro CLOCKS_PER_SEC can be either integer or floating-point types. Casting CPU time values to double, as in the example above, makes sure that operations such as arithmetic and printing work properly and consistently no matter what the underlying representation is.

Note that the clock can wrap around. On a 32bit system with CLOCKS_PER_SEC set to one million this function will return the same value approximately every 72 minutes.

For additional functions to examine a process' use of processor time, and to control it, See Resource Usage And Limitation.

Next: , Up: Processor And CPU Time

21.3.1 CPU Time Inquiry

To get a process' CPU time, you can use the clock function. This facility is declared in the header file time.h. In typical usage, you call the clock function at the beginning and end of the interval you want to time, subtract the values, and then divide by CLOCKS_PER_SEC (the number of clock ticks per second) to get processor time, like this: 

     #include <time.h>
     
     clock_t start, end;
     double cpu_time_used;
     
     start = clock();
     ... /* Do the work. */
     end = clock();
     cpu_time_used = ((double) (end - start)) / CLOCKS_PER_SEC;

Do not use a single CPU time as an amount of time; it doesn't work that way. Either do a subtraction as shown above or query processor time directly. See Processor Time.

Different computers and operating systems vary wildly in how they keep track of CPU time. It's common for the internal processor clock to have a resolution somewhere between a hundredth and millionth of a second.

— Macro: int CLOCKS_PER_SEC

The value of this macro is the number of clock ticks per second measured by the clock function. POSIX requires that this value be one million independent of the actual resolution.

— Macro: int CLK_TCK

This is an obsolete name for CLOCKS_PER_SEC.

— Data Type: clock_t

This is the type of the value returned by the clock function. Values of type clock_t are numbers of clock ticks.

— Function: clock_t clock (void)

This function returns the calling process' current CPU time. If the CPU time is not available or cannot be represented, clock returns the value (clock_t)(-1).

Previous: CPU Time, Up: Processor And CPU Time

21.3.2 Processor Time Inquiry

The times function returns information about a process' consumption of processor time in a struct tms object, in addition to the process' CPU time. See Time Basics. You should include the header file sys/times.h to use this facility.

— Data Type: struct tms

The tms structure is used to return information about process times. It contains at least the following members:

clock_t tms_utime
This is the total processor time the calling process has used in executing the instructions of its program.
clock_t tms_stime
This is the processor time the system has used on behalf of the calling process.
clock_t tms_cutime
This is the sum of the tms_utime values and the tms_cutime values of all terminated child processes of the calling process, whose status has been reported to the parent process by wait or waitpid; see Process Completion. In other words, it represents the total processor time used in executing the instructions of all the terminated child processes of the calling process, excluding child processes which have not yet been reported by wait or waitpid.
clock_t tms_cstime
This is similar to tms_cutime, but represents the total processor time system has used on behalf of all the terminated child processes of the calling process.

All of the times are given in numbers of clock ticks. Unlike CPU time, these are the actual amounts of time; not relative to any event. See Creating a Process.

— Function: clock_t times (struct tms *buffer)

The times function stores the processor time information for the calling process in buffer.

The return value is the calling process' CPU time (the same value you get from clock(). times returns (clock_t)(-1) to indicate failure.

Portability Note: The clock function described in CPU Time is specified by the ISO C standard. The times function is a feature of POSIX.1. In the GNU system, the CPU time is defined to be equivalent to the sum of the tms_utime and tms_stime fields returned by times.

Next: , Previous: Processor And CPU Time, Up: Date and Time

21.4 Calendar Time

This section describes facilities for keeping track of calendar time. See Time Basics.

The GNU C library represents calendar time three ways:

Next: , Up: Calendar Time

21.4.1 Simple Calendar Time

This section describes the time_t data type for representing calendar time as simple time, and the functions which operate on simple time objects. These facilities are declared in the header file time.h.

— Data Type: time_t

This is the data type used to represent simple time. Sometimes, it also represents an elapsed time. When interpreted as a calendar time value, it represents the number of seconds elapsed since 00:00:00 on January 1, 1970, Coordinated Universal Time. (This calendar time is sometimes referred to as the epoch.) POSIX requires that this count not include leap seconds, but on some systems this count includes leap seconds if you set TZ to certain values (see TZ Variable).

Note that a simple time has no concept of local time zone. Calendar Time T is the same instant in time regardless of where on the globe the computer is.

In the GNU C library, time_t is equivalent to long int. In other systems, time_t might be either an integer or floating-point type.

The function difftime tells you the elapsed time between two simple calendar times, which is not always as easy to compute as just subtracting. See Elapsed Time.

— Function: time_t time (time_t *result)

The time function returns the current calendar time as a value of type time_t. If the argument result is not a null pointer, the calendar time value is also stored in *result. If the current calendar time is not available, the value (time_t)(-1) is returned.

— Function: int stime (time_t *newtime)

stime sets the system clock, i.e., it tells the system that the current calendar time is newtime, where newtime is interpreted as described in the above definition of time_t.

settimeofday is a newer function which sets the system clock to better than one second precision. settimeofday is generally a better choice than stime. See High-Resolution Calendar.

Only the superuser can set the system clock.

If the function succeeds, the return value is zero. Otherwise, it is -1 and errno is set accordingly:

EPERM
The process is not superuser.

Next: , Previous: Simple Calendar Time, Up: Calendar Time

21.4.2 High-Resolution Calendar

The time_t data type used to represent simple times has a resolution of only one second. Some applications need more precision.

So, the GNU C library also contains functions which are capable of representing calendar times to a higher resolution than one second. The functions and the associated data types described in this section are declared in sys/time.h.

— Data Type: struct timezone

The struct timezone structure is used to hold minimal information about the local time zone. It has the following members:

int tz_minuteswest
This is the number of minutes west of UTC.
int tz_dsttime
If nonzero, Daylight Saving Time applies during some part of the year.

The struct timezone type is obsolete and should never be used. Instead, use the facilities described in Time Zone Functions.

— Function: int gettimeofday (struct timeval *tp, struct timezone *tzp)

The gettimeofday function returns the current calendar time as the elapsed time since the epoch in the struct timeval structure indicated by tp. (see Elapsed Time for a description of struct timeval). Information about the time zone is returned in the structure pointed at tzp. If the tzp argument is a null pointer, time zone information is ignored.

The return value is 0 on success and -1 on failure. The following errno error condition is defined for this function:

ENOSYS
The operating system does not support getting time zone information, and tzp is not a null pointer. The GNU operating system does not support using struct timezone to represent time zone information; that is an obsolete feature of 4.3 BSD. Instead, use the facilities described in Time Zone Functions.

— Function: int settimeofday (const struct timeval *tp, const struct timezone *tzp)

The settimeofday function sets the current calendar time in the system clock according to the arguments. As for gettimeofday, the calendar time is represented as the elapsed time since the epoch. As for gettimeofday, time zone information is ignored if tzp is a null pointer.

You must be a privileged user in order to use settimeofday.

Some kernels automatically set the system clock from some source such as a hardware clock when they start up. Others, including Linux, place the system clock in an “invalid” state (in which attempts to read the clock fail). A call of stime removes the system clock from an invalid state, and system startup scripts typically run a program that calls stime.

settimeofday causes a sudden jump forwards or backwards, which can cause a variety of problems in a system. Use adjtime (below) to make a smooth transition from one time to another by temporarily speeding up or slowing down the clock.

With a Linux kernel, adjtimex does the same thing and can also make permanent changes to the speed of the system clock so it doesn't need to be corrected as often.

The return value is 0 on success and -1 on failure. The following errno error conditions are defined for this function:

EPERM
This process cannot set the clock because it is not privileged.
ENOSYS
The operating system does not support setting time zone information, and tzp is not a null pointer.

— Function: int adjtime (const struct timeval *delta, struct timeval *olddelta)

This function speeds up or slows down the system clock in order to make a gradual adjustment. This ensures that the calendar time reported by the system clock is always monotonically increasing, which might not happen if you simply set the clock.

The delta argument specifies a relative adjustment to be made to the clock time. If negative, the system clock is slowed down for a while until it has lost this much elapsed time. If positive, the system clock is speeded up for a while.

If the olddelta argument is not a null pointer, the adjtime function returns information about any previous time adjustment that has not yet completed.

This function is typically used to synchronize the clocks of computers in a local network. You must be a privileged user to use it.

With a Linux kernel, you can use the adjtimex function to permanently change the clock speed.

The return value is 0 on success and -1 on failure. The following errno error condition is defined for this function:

EPERM
You do not have privilege to set the time.

Portability Note: The gettimeofday, settimeofday, and adjtime functions are derived from BSD.

Symbols for the following function are declared in sys/timex.h.

— Function: int adjtimex (struct timex *timex)

adjtimex is functionally identical to ntp_adjtime. See High Accuracy Clock.

This function is present only with a Linux kernel.

Next: , Previous: High-Resolution Calendar, Up: Calendar Time

21.4.3 Broken-down Time

Calendar time is represented by the usual GNU C library functions as an elapsed time since a fixed base calendar time. This is convenient for computation, but has no relation to the way people normally think of calendar time. By contrast, broken-down time is a binary representation of calendar time separated into year, month, day, and so on. Broken-down time values are not useful for calculations, but they are useful for printing human readable time information.

A broken-down time value is always relative to a choice of time zone, and it also indicates which time zone that is.

The symbols in this section are declared in the header file time.h.

— Data Type: struct tm

This is the data type used to represent a broken-down time. The structure contains at least the following members, which can appear in any order.

int tm_sec
This is the number of full seconds since the top of the minute (normally in the range 0 through 59, but the actual upper limit is 60, to allow for leap seconds if leap second support is available).
int tm_min
This is the number of full minutes since the top of the hour (in the range 0 through 59).
int tm_hour
This is the number of full hours past midnight (in the range 0 through 23).
int tm_mday
This is the ordinal day of the month (in the range 1 through 31). Watch out for this one! As the only ordinal number in the structure, it is inconsistent with the rest of the structure.
int tm_mon
This is the number of full calendar months since the beginning of the year (in the range 0 through 11). Watch out for this one! People usually use ordinal numbers for month-of-year (where January = 1).
int tm_year
This is the number of full calendar years since 1900.
int tm_wday
This is the number of full days since Sunday (in the range 0 through 6).
int tm_yday
This is the number of full days since the beginning of the year (in the range 0 through 365).
int tm_isdst
This is a flag that indicates whether Daylight Saving Time is (or was, or will be) in effect at the time described. The value is positive if Daylight Saving Time is in effect, zero if it is not, and negative if the information is not available.
long int tm_gmtoff
This field describes the time zone that was used to compute this broken-down time value, including any adjustment for daylight saving; it is the number of seconds that you must add to UTC to get local time. You can also think of this as the number of seconds east of UTC. For example, for U.S. Eastern Standard Time, the value is -5*60*60. The tm_gmtoff field is derived from BSD and is a GNU library extension; it is not visible in a strict ISO C environment.
const char *tm_zone
This field is the name for the time zone that was used to compute this broken-down time value. Like tm_gmtoff, this field is a BSD and GNU extension, and is not visible in a strict ISO C environment.

— Function: struct tm * localtime (const time_t *time)

The localtime function converts the simple time pointed to by time to broken-down time representation, expressed relative to the user's specified time zone.

The return value is a pointer to a static broken-down time structure, which might be overwritten by subsequent calls to ctime, gmtime, or localtime. (But no other library function overwrites the contents of this object.)

The return value is the null pointer if time cannot be represented as a broken-down time; typically this is because the year cannot fit into an int.

Calling localtime has one other effect: it sets the variable tzname with information about the current time zone. See Time Zone Functions.

Using the localtime function is a big problem in multi-threaded programs. The result is returned in a static buffer and this is used in all threads. POSIX.1c introduced a variant of this function.

— Function: struct tm * localtime_r (const time_t *time, struct tm *resultp)

The localtime_r function works just like the localtime function. It takes a pointer to a variable containing a simple time and converts it to the broken-down time format.

But the result is not placed in a static buffer. Instead it is placed in the object of type struct tm to which the parameter resultp points.

If the conversion is successful the function returns a pointer to the object the result was written into, i.e., it returns resultp.

— Function: struct tm * gmtime (const time_t *time)

This function is similar to localtime, except that the broken-down time is expressed as Coordinated Universal Time (UTC) (formerly called Greenwich Mean Time (GMT)) rather than relative to a local time zone.

As for the localtime function we have the problem that the result is placed in a static variable. POSIX.1c also provides a replacement for gmtime.

— Function: struct tm * gmtime_r (const time_t *time, struct tm *resultp)

This function is similar to localtime_r, except that it converts just like gmtime the given time as Coordinated Universal Time.

If the conversion is successful the function returns a pointer to the object the result was written into, i.e., it returns resultp.

— Function: time_t mktime (struct tm *brokentime)

The mktime function is used to convert a broken-down time structure to a simple time representation. It also “normalizes” the contents of the broken-down time structure, by filling in the day of week and day of year based on the other date and time components.

The mktime function ignores the specified contents of the tm_wday and tm_yday members of the broken-down time structure. It uses the values of the other components to determine the calendar time; it's permissible for these components to have unnormalized values outside their normal ranges. The last thing that mktime does is adjust the components of the brokentime structure (including the tm_wday and tm_yday).

If the specified broken-down time cannot be represented as a simple time, mktime returns a value of (time_t)(-1) and does not modify the contents of brokentime.

Calling mktime also sets the variable tzname with information about the current time zone. See Time Zone Functions.

— Function: time_t timelocal (struct tm *brokentime)

timelocal is functionally identical to mktime, but more mnemonically named. Note that it is the inverse of the localtime function.

Portability note: mktime is essentially universally available. timelocal is rather rare.

— Function: time_t timegm (struct tm *brokentime)

timegm is functionally identical to mktime except it always takes the input values to be Coordinated Universal Time (UTC) regardless of any local time zone setting.

Note that timegm is the inverse of gmtime.

Portability note: mktime is essentially universally available. timegm is rather rare. For the most portable conversion from a UTC broken-down time to a simple time, set the TZ environment variable to UTC, call mktime, then set TZ back.

Next: , Previous: Broken-down Time, Up: Calendar Time

21.4.4 High Accuracy Clock

The ntp_gettime and ntp_adjtime functions provide an interface to monitor and manipulate the system clock to maintain high accuracy time. For example, you can fine tune the speed of the clock or synchronize it with another time source.

A typical use of these functions is by a server implementing the Network Time Protocol to synchronize the clocks of multiple systems and high precision clocks.

These functions are declared in sys/timex.h.

— Data Type: struct ntptimeval

This structure is used for information about the system clock. It contains the following members:

struct timeval time
This is the current calendar time, expressed as the elapsed time since the epoch. The struct timeval data type is described in Elapsed Time.
long int maxerror
This is the maximum error, measured in microseconds. Unless updated via ntp_adjtime periodically, this value will reach some platform-specific maximum value.
long int esterror
This is the estimated error, measured in microseconds. This value can be set by ntp_adjtime to indicate the estimated offset of the system clock from the true calendar time.

— Function: int ntp_gettime (struct ntptimeval *tptr)

The ntp_gettime function sets the structure pointed to by tptr to current values. The elements of the structure afterwards contain the values the timer implementation in the kernel assumes. They might or might not be correct. If they are not a ntp_adjtime call is necessary.

The return value is 0 on success and other values on failure. The following errno error conditions are defined for this function:

TIME_ERROR
The precision clock model is not properly set up at the moment, thus the clock must be considered unsynchronized, and the values should be treated with care.

— Data Type: struct timex

This structure is used to control and monitor the system clock. It contains the following members:

unsigned int modes
This variable controls whether and which values are set. Several symbolic constants have to be combined with binary or to specify the effective mode. These constants start with MOD_.
long int offset
This value indicates the current offset of the system clock from the true calendar time. The value is given in microseconds. If bit MOD_OFFSET is set in modes, the offset (and possibly other dependent values) can be set. The offset's absolute value must not exceed MAXPHASE.
long int frequency
This value indicates the difference in frequency between the true calendar time and the system clock. The value is expressed as scaled PPM (parts per million, 0.0001%). The scaling is 1 << SHIFT_USEC. The value can be set with bit MOD_FREQUENCY, but the absolute value must not exceed MAXFREQ.
long int maxerror
This is the maximum error, measured in microseconds. A new value can be set using bit MOD_MAXERROR. Unless updated via ntp_adjtime periodically, this value will increase steadily and reach some platform-specific maximum value.
long int esterror
This is the estimated error, measured in microseconds. This value can be set using bit MOD_ESTERROR.
int status
This variable reflects the various states of the clock machinery. There are symbolic constants for the significant bits, starting with STA_. Some of these flags can be updated using the MOD_STATUS bit.
long int constant
This value represents the bandwidth or stiffness of the PLL (phase locked loop) implemented in the kernel. The value can be changed using bit MOD_TIMECONST.
long int precision
This value represents the accuracy or the maximum error when reading the system clock. The value is expressed in microseconds.
long int tolerance
This value represents the maximum frequency error of the system clock in scaled PPM. This value is used to increase the maxerror every second.
struct timeval time
The current calendar time.
long int tick
The elapsed time between clock ticks in microseconds. A clock tick is a periodic timer interrupt on which the system clock is based.
long int ppsfreq
This is the first of a few optional variables that are present only if the system clock can use a PPS (pulse per second) signal to discipline the system clock. The value is expressed in scaled PPM and it denotes the difference in frequency between the system clock and the PPS signal.
long int jitter
This value expresses a median filtered average of the PPS signal's dispersion in microseconds.
int shift
This value is a binary exponent for the duration of the PPS calibration interval, ranging from PPS_SHIFT to PPS_SHIFTMAX.
long int stabil
This value represents the median filtered dispersion of the PPS frequency in scaled PPM.
long int jitcnt
This counter represents the number of pulses where the jitter exceeded the allowed maximum MAXTIME.
long int calcnt
This counter reflects the number of successful calibration intervals.
long int errcnt
This counter represents the number of calibration errors (caused by large offsets or jitter).
long int stbcnt
This counter denotes the number of of calibrations where the stability exceeded the threshold.

— Function: int ntp_adjtime (struct timex *tptr)

The ntp_adjtime function sets the structure specified by tptr to current values.

In addition, ntp_adjtime updates some settings to match what you pass to it in *tptr. Use the modes element of *tptr to select what settings to update. You can set offset, freq, maxerror, esterror, status, constant, and tick.

modes = zero means set nothing.

Only the superuser can update settings.

The return value is 0 on success and other values on failure. The following errno error conditions are defined for this function:

TIME_ERROR
The high accuracy clock model is not properly set up at the moment, thus the clock must be considered unsynchronized, and the values should be treated with care. Another reason could be that the specified new values are not allowed.
EPERM
The process specified a settings update, but is not superuser.

For more details see RFC1305 (Network Time Protocol, Version 3) and related documents.

Portability note: Early versions of the GNU C library did not have this function but did have the synonymous adjtimex.

Next: , Previous: High Accuracy Clock, Up: Calendar Time

21.4.5 Formatting Calendar Time

The functions described in this section format calendar time values as strings. These functions are declared in the header file time.h.

— Function: char * asctime (const struct tm *brokentime)

The asctime function converts the broken-down time value that brokentime points to into a string in a standard format: 

          "Tue May 21 13:46:22 1991\n"

The abbreviations for the days of week are: `Sun', `Mon', `Tue', `Wed', `Thu', `Fri', and `Sat'.

The abbreviations for the months are: `Jan', `Feb', `Mar', `Apr', `May', `Jun', `Jul', `Aug', `Sep', `Oct', `Nov', and `Dec'.

The return value points to a statically allocated string, which might be overwritten by subsequent calls to asctime or ctime. (But no other library function overwrites the contents of this string.)

— Function: char * asctime_r (const struct tm *brokentime, char *buffer)

This function is similar to asctime but instead of placing the result in a static buffer it writes the string in the buffer pointed to by the parameter buffer. This buffer should have room for at least 26 bytes, including the terminating null.

If no error occurred the function returns a pointer to the string the result was written into, i.e., it returns buffer. Otherwise return NULL.

— Function: char * ctime (const time_t *time)

The ctime function is similar to asctime, except that you specify the calendar time argument as a time_t simple time value rather than in broken-down local time format. It is equivalent to 

          asctime (localtime (time))

ctime sets the variable tzname, because localtime does so. See Time Zone Functions.

— Function: char * ctime_r (const time_t *time, char *buffer)

This function is similar to ctime, but places the result in the string pointed to by buffer. It is equivalent to (written using gcc extensions, see Statement Exprs): 

          ({ struct tm tm; asctime_r (localtime_r (time, &tm), buf); })

If no error occurred the function returns a pointer to the string the result was written into, i.e., it returns buffer. Otherwise return NULL.

— Function: size_t strftime (char *s, size_t size, const char *template, const struct tm *brokentime)

This function is similar to the sprintf function (see Formatted Input), but the conversion specifications that can appear in the format template template are specialized for printing components of the date and time brokentime according to the locale currently specified for time conversion (see Locales).

Ordinary characters appearing in the template are copied to the output string s; this can include multibyte character sequences. Conversion specifiers are introduced by a `%' character, followed by an optional flag which can be one of the following. These flags are all GNU extensions. The first three affect only the output of numbers:

_
The number is padded with spaces.
-
The number is not padded at all.
0
The number is padded with zeros even if the format specifies padding with spaces.
^
The output uses uppercase characters, but only if this is possible (see Case Conversion).

The default action is to pad the number with zeros to keep it a constant width. Numbers that do not have a range indicated below are never padded, since there is no natural width for them.

Following the flag an optional specification of the width is possible. This is specified in decimal notation. If the natural size of the output is of the field has less than the specified number of characters, the result is written right adjusted and space padded to the given size.

An optional modifier can follow the optional flag and width specification. The modifiers, which were first standardized by POSIX.2-1992 and by ISO C99, are:

E
Use the locale's alternate representation for date and time. This modifier applies to the %c, %C, %x, %X, %y and %Y format specifiers. In a Japanese locale, for example, %Ex might yield a date format based on the Japanese Emperors' reigns.
O
Use the locale's alternate numeric symbols for numbers. This modifier applies only to numeric format specifiers.

If the format supports the modifier but no alternate representation is available, it is ignored.

The conversion specifier ends with a format specifier taken from the following list. The whole `%' sequence is replaced in the output string as follows:

%a
The abbreviated weekday name according to the current locale.
%A
The full weekday name according to the current locale.
%b
The abbreviated month name according to the current locale.
%B
The full month name according to the current locale.

Using %B together with %d produces grammatically incorrect results for some locales.

%c
The preferred calendar time representation for the current locale.
%C
The century of the year. This is equivalent to the greatest integer not greater than the year divided by 100.

This format was first standardized by POSIX.2-1992 and by ISO C99.

%d
The day of the month as a decimal number (range 01 through 31).
%D
The date using the format %m/%d/%y.

This format was first standardized by POSIX.2-1992 and by ISO C99.

%e
The day of the month like with %d, but padded with blank (range 1 through 31).

This format was first standardized by POSIX.2-1992 and by ISO C99.

%F
The date using the format %Y-%m-%d. This is the form specified in the ISO 8601 standard and is the preferred form for all uses.

This format was first standardized by ISO C99 and by POSIX.1-2001.

%g
The year corresponding to the ISO week number, but without the century (range 00 through 99). This has the same format and value as %y, except that if the ISO week number (see %V) belongs to the previous or next year, that year is used instead.

This format was first standardized by ISO C99 and by POSIX.1-2001.

%G
The year corresponding to the ISO week number. This has the same format and value as %Y, except that if the ISO week number (see %V) belongs to the previous or next year, that year is used instead.

This format was first standardized by ISO C99 and by POSIX.1-2001 but was previously available as a GNU extension.

%h
The abbreviated month name according to the current locale. The action is the same as for %b.

This format was first standardized by POSIX.2-1992 and by ISO C99.

%H
The hour as a decimal number, using a 24-hour clock (range 00 through 23).
%I
The hour as a decimal number, using a 12-hour clock (range 01 through 12).
%j
The day of the year as a decimal number (range 001 through 366).
%k
The hour as a decimal number, using a 24-hour clock like %H, but padded with blank (range 0 through 23).

This format is a GNU extension.

%l
The hour as a decimal number, using a 12-hour clock like %I, but padded with blank (range 1 through 12).

This format is a GNU extension.

%m
The month as a decimal number (range 01 through 12).
%M
The minute as a decimal number (range 00 through 59).
%n
A single `\n' (newline) character.

This format was first standardized by POSIX.2-1992 and by ISO C99.

%p
Either `AM' or `PM', according to the given time value; or the corresponding strings for the current locale. Noon is treated as `PM' and midnight as `AM'. In most locales `AM'/`PM' format is not supported, in such cases "%p" yields an empty string.
%P
Either `am' or `pm', according to the given time value; or the corresponding strings for the current locale, printed in lowercase characters. Noon is treated as `pm' and midnight as `am'. In most locales `AM'/`PM' format is not supported, in such cases "%P" yields an empty string.

This format is a GNU extension.

%r
The complete calendar time using the AM/PM format of the current locale.

This format was first standardized by POSIX.2-1992 and by ISO C99. In the POSIX locale, this format is equivalent to %I:%M:%S %p.

%R
The hour and minute in decimal numbers using the format %H:%M.

This format was first standardized by ISO C99 and by POSIX.1-2001 but was previously available as a GNU extension.

%s
The number of seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC. Leap seconds are not counted unless leap second support is available.

This format is a GNU extension.

%S
The seconds as a decimal number (range 00 through 60).
%t
A single `\t' (tabulator) character.

This format was first standardized by POSIX.2-1992 and by ISO C99.

%T
The time of day using decimal numbers using the format %H:%M:%S.

This format was first standardized by POSIX.2-1992 and by ISO C99.

%u
The day of the week as a decimal number (range 1 through 7), Monday being 1.

This format was first standardized by POSIX.2-1992 and by ISO C99.

%U
The week number of the current year as a decimal number (range 00 through 53), starting with the first Sunday as the first day of the first week. Days preceding the first Sunday in the year are considered to be in week 00.
%V
The ISO 8601:1988 week number as a decimal number (range 01 through 53). ISO weeks start with Monday and end with Sunday. Week 01 of a year is the first week which has the majority of its days in that year; this is equivalent to the week containing the year's first Thursday, and it is also equivalent to the week containing January 4. Week 01 of a year can contain days from the previous year. The week before week 01 of a year is the last week (52 or 53) of the previous year even if it contains days from the new year.

This format was first standardized by POSIX.2-1992 and by ISO C99.

%w
The day of the week as a decimal number (range 0 through 6), Sunday being 0.
%W
The week number of the current year as a decimal number (range 00 through 53), starting with the first Monday as the first day of the first week. All days preceding the first Monday in the year are considered to be in week 00.
%x
The preferred date representation for the current locale.
%X
The preferred time of day representation for the current locale.
%y
The year without a century as a decimal number (range 00 through 99). This is equivalent to the year modulo 100.
%Y
The year as a decimal number, using the Gregorian calendar. Years before the year 1 are numbered 0, -1, and so on.
%z
RFC 822/ISO 8601:1988 style numeric time zone (e.g., -0600 or +0100), or nothing if no time zone is determinable.

This format was first standardized by ISO C99 and by POSIX.1-2001 but was previously available as a GNU extension.

In the POSIX locale, a full RFC 822 timestamp is generated by the format `"%a, %d %b %Y %H:%M:%S %z"' (or the equivalent `"%a, %d %b %Y %T %z"').

%Z
The time zone abbreviation (empty if the time zone can't be determined).
%%
A literal `%' character.

The size parameter can be used to specify the maximum number of characters to be stored in the array s, including the terminating null character. If the formatted time requires more than size characters, strftime returns zero and the contents of the array s are undefined. Otherwise the return value indicates the number of characters placed in the array s, not including the terminating null character.

Warning: This convention for the return value which is prescribed in ISO C can lead to problems in some situations. For certain format strings and certain locales the output really can be the empty string and this cannot be discovered by testing the return value only. E.g., in most locales the AM/PM time format is not supported (most of the world uses the 24 hour time representation). In such locales "%p" will return the empty string, i.e., the return value is zero. To detect situations like this something similar to the following code should be used: 

          buf[0] = '\1';
          len = strftime (buf, bufsize, format, tp);
          if (len == 0 && buf[0] != '\0')
            {
              /* Something went wrong in the strftime call.  */
              ...
            }

If s is a null pointer, strftime does not actually write anything, but instead returns the number of characters it would have written.

According to POSIX.1 every call to strftime implies a call to tzset. So the contents of the environment variable TZ is examined before any output is produced.

For an example of strftime, see Time Functions Example.

— Function: size_t wcsftime (wchar_t *s, size_t size, const wchar_t *template, const struct tm *brokentime)

The wcsftime function is equivalent to the strftime function with the difference that it operates on wide character strings. The buffer where the result is stored, pointed to by s, must be an array of wide characters. The parameter size which specifies the size of the output buffer gives the number of wide character, not the number of bytes.

Also the format string template is a wide character string. Since all characters needed to specify the format string are in the basic character set it is portably possible to write format strings in the C source code using the L"..." notation. The parameter brokentime has the same meaning as in the strftime call.

The wcsftime function supports the same flags, modifiers, and format specifiers as the strftime function.

The return value of wcsftime is the number of wide characters stored in s. When more characters would have to be written than can be placed in the buffer s the return value is zero, with the same problems indicated in the strftime documentation.

Next: , Previous: Formatting Calendar Time, Up: Calendar Time

21.4.6 Convert textual time and date information back

The ISO C standard does not specify any functions which can convert the output of the strftime function back into a binary format. This led to a variety of more-or-less successful implementations with different interfaces over the years. Then the Unix standard was extended by the addition of two functions: strptime and getdate. Both have strange interfaces but at least they are widely available.

Next: , Up: Parsing Date and Time
21.4.6.1 Interpret string according to given format

The first function is rather low-level. It is nevertheless frequently used in software since it is better known. Its interface and implementation are heavily influenced by the getdate function, which is defined and implemented in terms of calls to strptime.

— Function: char * strptime (const char *s, const char *fmt, struct tm *tp)

The strptime function parses the input string s according to the format string fmt and stores its results in the structure tp.

The input string could be generated by a strftime call or obtained any other way. It does not need to be in a human-recognizable format; e.g. a date passed as "02:1999:9" is acceptable, even though it is ambiguous without context. As long as the format string fmt matches the input string the function will succeed.

The user has to make sure, though, that the input can be parsed in a unambiguous way. The string "1999112" can be parsed using the format "%Y%m%d" as 1999-1-12, 1999-11-2, or even 19991-1-2. It is necessary to add appropriate separators to reliably get results.

The format string consists of the same components as the format string of the strftime function. The only difference is that the flags _, -, 0, and ^ are not allowed. Several of the distinct formats of strftime do the same work in strptime since differences like case of the input do not matter. For reasons of symmetry all formats are supported, though.

The modifiers E and O are also allowed everywhere the strftime function allows them.

The formats are:

%a
%A
The weekday name according to the current locale, in abbreviated form or the full name.
%b
%B
%h
The month name according to the current locale, in abbreviated form or the full name.
%c
The date and time representation for the current locale.
%Ec
Like %c but the locale's alternative date and time format is used.
%C
The century of the year.

It makes sense to use this format only if the format string also contains the %y format.

%EC
The locale's representation of the period.

Unlike %C it sometimes makes sense to use this format since some cultures represent years relative to the beginning of eras instead of using the Gregorian years.

%d
%e
The day of the month as a decimal number (range 1 through 31). Leading zeroes are permitted but not required.
%Od
%Oe
Same as %d but using the locale's alternative numeric symbols.

Leading zeroes are permitted but not required.

%D
Equivalent to %m/%d/%y.
%F
Equivalent to %Y-%m-%d, which is the ISO 8601 date format.

This is a GNU extension following an ISO C99 extension to strftime.

%g
The year corresponding to the ISO week number, but without the century (range 00 through 99).

Note: Currently, this is not fully implemented. The format is recognized, input is consumed but no field in tm is set.

This format is a GNU extension following a GNU extension of strftime.

%G
The year corresponding to the ISO week number.

Note: Currently, this is not fully implemented. The format is recognized, input is consumed but no field in tm is set.

This format is a GNU extension following a GNU extension of strftime.

%H
%k
The hour as a decimal number, using a 24-hour clock (range 00 through 23).

%k is a GNU extension following a GNU extension of strftime.

%OH
Same as %H but using the locale's alternative numeric symbols.
%I
%l
The hour as a decimal number, using a 12-hour clock (range 01 through 12).

%l is a GNU extension following a GNU extension of strftime.

%OI
Same as %I but using the locale's alternative numeric symbols.
%j
The day of the year as a decimal number (range 1 through 366).

Leading zeroes are permitted but not required.

%m
The month as a decimal number (range 1 through 12).

Leading zeroes are permitted but not required.

%Om
Same as %m but using the locale's alternative numeric symbols.
%M
The minute as a decimal number (range 0 through 59).

Leading zeroes are permitted but not required.

%OM
Same as %M but using the locale's alternative numeric symbols.
%n
%t
Matches any white space.
%p
%P
The locale-dependent equivalent to `AM' or `PM'.

This format is not useful unless %I or %l is also used. Another complication is that the locale might not define these values at all and therefore the conversion fails.

%P is a GNU extension following a GNU extension to strftime.

%r
The complete time using the AM/PM format of the current locale.

A complication is that the locale might not define this format at all and therefore the conversion fails.

%R
The hour and minute in decimal numbers using the format %H:%M.

%R is a GNU extension following a GNU extension to strftime.

%s
The number of seconds since the epoch, i.e., since 1970-01-01 00:00:00 UTC. Leap seconds are not counted unless leap second support is available.

%s is a GNU extension following a GNU extension to strftime.

%S
The seconds as a decimal number (range 0 through 60).

Leading zeroes are permitted but not required.

Note: The Unix specification says the upper bound on this value is 61, a result of a decision to allow double leap seconds. You will not see the value 61 because no minute has more than one leap second, but the myth persists.

%OS
Same as %S but using the locale's alternative numeric symbols.
%T
Equivalent to the use of %H:%M:%S in this place.
%u
The day of the week as a decimal number (range 1 through 7), Monday being 1.

Leading zeroes are permitted but not required.

Note: Currently, this is not fully implemented. The format is recognized, input is consumed but no field in tm is set.

%U
The week number of the current year as a decimal number (range 0 through 53).

Leading zeroes are permitted but not required.

%OU
Same as %U but using the locale's alternative numeric symbols.
%V
The ISO 8601:1988 week number as a decimal number (range 1 through 53).

Leading zeroes are permitted but not required.

Note: Currently, this is not fully implemented. The format is recognized, input is consumed but no field in tm is set.

%w
The day of the week as a decimal number (range 0 through 6), Sunday being 0.

Leading zeroes are permitted but not required.

Note: Currently, this is not fully implemented. The format is recognized, input is consumed but no field in tm is set.

%Ow
Same as %w but using the locale's alternative numeric symbols.
%W
The week number of the current year as a decimal number (range 0 through 53).

Leading zeroes are permitted but not required.

Note: Currently, this is not fully implemented. The format is recognized, input is consumed but no field in tm is set.

%OW
Same as %W but using the locale's alternative numeric symbols.
%x
The date using the locale's date format.
%Ex
Like %x but the locale's alternative data representation is used.
%X
The time using the locale's time format.
%EX
Like %X but the locale's alternative time representation is used.
%y
The year without a century as a decimal number (range 0 through 99).

Leading zeroes are permitted but not required.

Note that it is questionable to use this format without the %C format. The strptime function does regard input values in the range 68 to 99 as the years 1969 to 1999 and the values 0 to 68 as the years 2000 to 2068. But maybe this heuristic fails for some input data.

Therefore it is best to avoid %y completely and use %Y instead.

%Ey
The offset from %EC in the locale's alternative representation.
%Oy
The offset of the year (from %C) using the locale's alternative numeric symbols.
%Y
The year as a decimal number, using the Gregorian calendar.
%EY
The full alternative year representation.
%z
The offset from GMT in ISO 8601/RFC822 format.
%Z
The timezone name.

Note: Currently, this is not fully implemented. The format is recognized, input is consumed but no field in tm is set.

%%
A literal `%' character.

All other characters in the format string must have a matching character in the input string. Exceptions are white spaces in the input string which can match zero or more whitespace characters in the format string.

Portability Note: The XPG standard advises applications to use at least one whitespace character (as specified by isspace) or other non-alphanumeric characters between any two conversion specifications. The GNU C Library does not have this limitation but other libraries might have trouble parsing formats like "%d%m%Y%H%M%S".

The strptime function processes the input string from right to left. Each of the three possible input elements (white space, literal, or format) are handled one after the other. If the input cannot be matched to the format string the function stops. The remainder of the format and input strings are not processed.

The function returns a pointer to the first character it was unable to process. If the input string contains more characters than required by the format string the return value points right after the last consumed input character. If the whole input string is consumed the return value points to the NULL byte at the end of the string. If an error occurs, i.e., strptime fails to match all of the format string, the function returns NULL.

The specification of the function in the XPG standard is rather vague, leaving out a few important pieces of information. Most importantly, it does not specify what happens to those elements of tm which are not directly initialized by the different formats. The implementations on different Unix systems vary here.

The GNU libc implementation does not touch those fields which are not directly initialized. Exceptions are the tm_wday and tm_yday elements, which are recomputed if any of the year, month, or date elements changed. This has two implications:

The following example shows a function which parses a string which is contains the date information in either US style or ISO 8601 form: 

     const char *
     parse_date (const char *input, struct tm *tm)
     {
       const char *cp;
     
       /* First clear the result structure.  */
       memset (tm, '\0', sizeof (*tm));
     
       /* Try the ISO format first.  */
       cp = strptime (input, "%F", tm);
       if (cp == NULL)
         {
           /* Does not match.  Try the US form.  */
           cp = strptime (input, "%D", tm);
         }
     
       return cp;
     }

Previous: Low-Level Time String Parsing, Up: Parsing Date and Time
21.4.6.2 A More User-friendly Way to Parse Times and Dates

The Unix standard defines another function for parsing date strings. The interface is weird, but if the function happens to suit your application it is just fine. It is problematic to use this function in multi-threaded programs or libraries, since it returns a pointer to a static variable, and uses a global variable and global state (an environment variable).

— Variable: getdate_err

This variable of type int contains the error code of the last unsuccessful call to getdate. Defined values are:

1
The environment variable DATEMSK is not defined or null.
2
The template file denoted by the DATEMSK environment variable cannot be opened.
3
Information about the template file cannot retrieved.
4
The template file is not a regular file.
5
An I/O error occurred while reading the template file.
6
Not enough memory available to execute the function.
7
The template file contains no matching template.
8
The input date is invalid, but would match a template otherwise. This includes dates like February 31st, and dates which cannot be represented in a time_t variable.

— Function: struct tm * getdate (const char *string)

The interface to getdate is the simplest possible for a function to parse a string and return the value. string is the input string and the result is returned in a statically-allocated variable.

The details about how the string is processed are hidden from the user. In fact, they can be outside the control of the program. Which formats are recognized is controlled by the file named by the environment variable DATEMSK. This file should contain lines of valid format strings which could be passed to strptime.

The getdate function reads these format strings one after the other and tries to match the input string. The first line which completely matches the input string is used.

Elements not initialized through the format string retain the values present at the time of the getdate function call.

The formats recognized by getdate are the same as for strptime. See above for an explanation. There are only a few extensions to the strptime behavior:

It should be noted that the format in the template file need not only contain format elements. The following is a list of possible format strings (taken from the Unix standard): 

          %m
          %A %B %d, %Y %H:%M:%S
          %A
          %B
          %m/%d/%y %I %p
          %d,%m,%Y %H:%M
          at %A the %dst of %B in %Y
          run job at %I %p,%B %dnd
          %A den %d. %B %Y %H.%M Uhr

As you can see, the template list can contain very specific strings like run job at %I %p,%B %dnd. Using the above list of templates and assuming the current time is Mon Sep 22 12:19:47 EDT 1986 we can obtain the following results for the given input.

Input Match Result
Mon %a Mon Sep 22 12:19:47 EDT 1986
Sun %a Sun Sep 28 12:19:47 EDT 1986
Fri %a Fri Sep 26 12:19:47 EDT 1986
September %B Mon Sep 1 12:19:47 EDT 1986
January %B Thu Jan 1 12:19:47 EST 1987
December %B Mon Dec 1 12:19:47 EST 1986
Sep Mon %b %a Mon Sep 1 12:19:47 EDT 1986
Jan Fri %b %a Fri Jan 2 12:19:47 EST 1987
Dec Mon %b %a Mon Dec 1 12:19:47 EST 1986
Jan Wed 1989 %b %a %Y Wed Jan 4 12:19:47 EST 1989
Fri 9 %a %H Fri Sep 26 09:00:00 EDT 1986
Feb 10:30 %b %H:%S Sun Feb 1 10:00:30 EST 1987
10:30 %H:%M Tue Sep 23 10:30:00 EDT 1986
13:30 %H:%M Mon Sep 22 13:30:00 EDT 1986

The return value of the function is a pointer to a static variable of type struct tm, or a null pointer if an error occurred. The result is only valid until the next getdate call, making this function unusable in multi-threaded applications.

The errno variable is not changed. Error conditions are stored in the global variable getdate_err. See the description above for a list of the possible error values.

Warning: The getdate function should never be used in SUID-programs. The reason is obvious: using the DATEMSK environment variable you can get the function to open any arbitrary file and chances are high that with some bogus input (such as a binary file) the program will crash.

— Function: int getdate_r (const char *string, struct tm *tp)

The getdate_r function is the reentrant counterpart of getdate. It does not use the global variable getdate_err to signal an error, but instead returns an error code. The same error codes as described in the getdate_err documentation above are used, with 0 meaning success.

Moreover, getdate_r stores the broken-down time in the variable of type struct tm pointed to by the second argument, rather than in a static variable.

This function is not defined in the Unix standard. Nevertheless it is available on some other Unix systems as well.

The warning against using getdate in SUID-programs applies to getdate_r as well.

Next: , Previous: Parsing Date and Time, Up: Calendar Time

21.4.7 Specifying the Time Zone with TZ

In POSIX systems, a user can specify the time zone by means of the TZ environment variable. For information about how to set environment variables, see Environment Variables. The functions for accessing the time zone are declared in time.h. You should not normally need to set TZ. If the system is configured properly, the default time zone will be correct. You might set TZ if you are using a computer over a network from a different time zone, and would like times reported to you in the time zone local to you, rather than what is local to the computer.

In POSIX.1 systems the value of the TZ variable can be in one of three formats. With the GNU C library, the most common format is the last one, which can specify a selection from a large database of time zone information for many regions of the world. The first two formats are used to describe the time zone information directly, which is both more cumbersome and less precise. But the POSIX.1 standard only specifies the details of the first two formats, so it is good to be familiar with them in case you come across a POSIX.1 system that doesn't support a time zone information database.

The first format is used when there is no Daylight Saving Time (or summer time) in the local time zone: 

     std offset

The std string specifies the name of the time zone. It must be three or more characters long and must not contain a leading colon, embedded digits, commas, nor plus and minus signs. There is no space character separating the time zone name from the offset, so these restrictions are necessary to parse the specification correctly.

The offset specifies the time value you must add to the local time to get a Coordinated Universal Time value. It has syntax like [+|-]hh[:mm[:ss]]. This is positive if the local time zone is west of the Prime Meridian and negative if it is east. The hour must be between 0 and 23, and the minute and seconds between 0 and 59.

For example, here is how we would specify Eastern Standard Time, but without any Daylight Saving Time alternative: 

     EST+5

The second format is used when there is Daylight Saving Time: 

     std offset dst [offset],start[/time],end[/time]

The initial std and offset specify the standard time zone, as described above. The dst string and offset specify the name and offset for the corresponding Daylight Saving Time zone; if the offset is omitted, it defaults to one hour ahead of standard time.

The remainder of the specification describes when Daylight Saving Time is in effect. The start field is when Daylight Saving Time goes into effect and the end field is when the change is made back to standard time. The following formats are recognized for these fields:

Jn
This specifies the Julian day, with n between 1 and 365. February 29 is never counted, even in leap years.
n
This specifies the Julian day, with n between 0 and 365. February 29 is counted in leap years.
Mm.w.d
This specifies day d of week w of month m. The day d must be between 0 (Sunday) and 6. The week w must be between 1 and 5; week 1 is the first week in which day d occurs, and week 5 specifies the last d day in the month. The month m should be between 1 and 12.

The time fields specify when, in the local time currently in effect, the change to the other time occurs. If omitted, the default is 02:00:00.

For example, here is how you would specify the Eastern time zone in the United States, including the appropriate Daylight Saving Time and its dates of applicability. The normal offset from UTC is 5 hours; since this is west of the prime meridian, the sign is positive. Summer time begins on the first Sunday in April at 2:00am, and ends on the last Sunday in October at 2:00am. 

     EST+5EDT,M4.1.0/2,M10.5.0/2

The schedule of Daylight Saving Time in any particular jurisdiction has changed over the years. To be strictly correct, the conversion of dates and times in the past should be based on the schedule that was in effect then. However, this format has no facilities to let you specify how the schedule has changed from year to year. The most you can do is specify one particular schedule—usually the present day schedule—and this is used to convert any date, no matter when. For precise time zone specifications, it is best to use the time zone information database (see below).

The third format looks like this: 

     :characters

Each operating system interprets this format differently; in the GNU C library, characters is the name of a file which describes the time zone.

If the TZ environment variable does not have a value, the operation chooses a time zone by default. In the GNU C library, the default time zone is like the specification `TZ=:/etc/localtime' (or `TZ=:/usr/local/etc/localtime', depending on how GNU C library was configured; see Installation). Other C libraries use their own rule for choosing the default time zone, so there is little we can say about them.

If characters begins with a slash, it is an absolute file name; otherwise the library looks for the file /share/lib/zoneinfo/characters. The zoneinfo directory contains data files describing local time zones in many different parts of the world. The names represent major cities, with subdirectories for geographical areas; for example, America/New_York, Europe/London, Asia/Hong_Kong. These data files are installed by the system administrator, who also sets /etc/localtime to point to the data file for the local time zone. The GNU C library comes with a large database of time zone information for most regions of the world, which is maintained by a community of volunteers and put in the public domain.

Next: , Previous: TZ Variable, Up: Calendar Time

21.4.8 Functions and Variables for Time Zones

— Variable: char * tzname [2]

The array tzname contains two strings, which are the standard names of the pair of time zones (standard and Daylight Saving) that the user has selected. tzname[0] is the name of the standard time zone (for example, "EST"), and tzname[1] is the name for the time zone when Daylight Saving Time is in use (for example, "EDT"). These correspond to the std and dst strings (respectively) from the TZ environment variable. If Daylight Saving Time is never used, tzname[1] is the empty string.

The tzname array is initialized from the TZ environment variable whenever tzset, ctime, strftime, mktime, or localtime is called. If multiple abbreviations have been used (e.g. "EWT" and "EDT" for U.S. Eastern War Time and Eastern Daylight Time), the array contains the most recent abbreviation.

The tzname array is required for POSIX.1 compatibility, but in GNU programs it is better to use the tm_zone member of the broken-down time structure, since tm_zone reports the correct abbreviation even when it is not the latest one.

Though the strings are declared as char * the user must refrain from modifying these strings. Modifying the strings will almost certainly lead to trouble.

— Function: void tzset (void)

The tzset function initializes the tzname variable from the value of the TZ environment variable. It is not usually necessary for your program to call this function, because it is called automatically when you use the other time conversion functions that depend on the time zone.

The following variables are defined for compatibility with System V Unix. Like tzname, these variables are set by calling tzset or the other time conversion functions.

— Variable: long int timezone

This contains the difference between UTC and the latest local standard time, in seconds west of UTC. For example, in the U.S. Eastern time zone, the value is 5*60*60. Unlike the tm_gmtoff member of the broken-down time structure, this value is not adjusted for daylight saving, and its sign is reversed. In GNU programs it is better to use tm_gmtoff, since it contains the correct offset even when it is not the latest one.

— Variable: int daylight

This variable has a nonzero value if Daylight Saving Time rules apply. A nonzero value does not necessarily mean that Daylight Saving Time is now in effect; it means only that Daylight Saving Time is sometimes in effect.

Previous: Time Zone Functions, Up: Calendar Time

21.4.9 Time Functions Example

Here is an example program showing the use of some of the calendar time functions. 

     #include <time.h>
     #include <stdio.h>
     
     #define SIZE 256
     
     int
     main (void)
     {
       char buffer[SIZE];
       time_t curtime;
       struct tm *loctime;
     
       /* Get the current time. */
       curtime = time (NULL);
     
       /* Convert it to local time representation. */
       loctime = localtime (&curtime);
     
       /* Print out the date and time in the standard format. */
       fputs (asctime (loctime), stdout);
     
       /* Print it out in a nice format. */
       strftime (buffer, SIZE, "Today is %A, %B %d.\n", loctime);
       fputs (buffer, stdout);
       strftime (buffer, SIZE, "The time is %I:%M %p.\n", loctime);
       fputs (buffer, stdout);
     
       return 0;
     }

It produces output like this: 

     Wed Jul 31 13:02:36 1991
     Today is Wednesday, July 31.
     The time is 01:02 PM.

Next: , Previous: Calendar Time, Up: Date and Time

21.5 Setting an Alarm

The alarm and setitimer functions provide a mechanism for a process to interrupt itself in the future. They do this by setting a timer; when the timer expires, the process receives a signal.

Each process has three independent interval timers available:

You can only have one timer of each kind set at any given time. If you set a timer that has not yet expired, that timer is simply reset to the new value.

You should establish a handler for the appropriate alarm signal using signal or sigaction before issuing a call to setitimer or alarm. Otherwise, an unusual chain of events could cause the timer to expire before your program establishes the handler. In this case it would be terminated, since termination is the default action for the alarm signals. See Signal Handling.

To be able to use the alarm function to interrupt a system call which might block otherwise indefinitely it is important to not set the SA_RESTART flag when registering the signal handler using sigaction. When not using sigaction things get even uglier: the signal function has to fixed semantics with respect to restarts. The BSD semantics for this function is to set the flag. Therefore, if sigaction for whatever reason cannot be used, it is necessary to use sysv_signal and not signal.

The setitimer function is the primary means for setting an alarm. This facility is declared in the header file sys/time.h. The alarm function, declared in unistd.h, provides a somewhat simpler interface for setting the real-time timer.

— Data Type: struct itimerval

This structure is used to specify when a timer should expire. It contains the following members:

struct timeval it_interval
This is the period between successive timer interrupts. If zero, the alarm will only be sent once.
struct timeval it_value
This is the period between now and the first timer interrupt. If zero, the alarm is disabled.

The struct timeval data type is described in Elapsed Time.

— Function: int setitimer (int which, struct itimerval *new, struct itimerval *old)

The setitimer function sets the timer specified by which according to new. The which argument can have a value of ITIMER_REAL, ITIMER_VIRTUAL, or ITIMER_PROF.

If old is not a null pointer, setitimer returns information about any previous unexpired timer of the same kind in the structure it points to.

The return value is 0 on success and -1 on failure. The following errno error conditions are defined for this function:

EINVAL
The timer period is too large.

— Function: int getitimer (int which, struct itimerval *old)

The getitimer function stores information about the timer specified by which in the structure pointed at by old.

The return value and error conditions are the same as for setitimer.

ITIMER_REAL
This constant can be used as the which argument to the setitimer and getitimer functions to specify the real-time timer.
ITIMER_VIRTUAL
This constant can be used as the which argument to the setitimer and getitimer functions to specify the virtual timer.
ITIMER_PROF
This constant can be used as the which argument to the setitimer and getitimer functions to specify the profiling timer.
— Function: unsigned int alarm (unsigned int seconds)

The alarm function sets the real-time timer to expire in seconds seconds. If you want to cancel any existing alarm, you can do this by calling alarm with a seconds argument of zero.

The return value indicates how many seconds remain before the previous alarm would have been sent. If there is no previous alarm, alarm returns zero.

The alarm function could be defined in terms of setitimer like this: 

     unsigned int
     alarm (unsigned int seconds)
     {
       struct itimerval old, new;
       new.it_interval.tv_usec = 0;
       new.it_interval.tv_sec = 0;
       new.it_value.tv_usec = 0;
       new.it_value.tv_sec = (long int) seconds;
       if (setitimer (ITIMER_REAL, &new, &old) < 0)
         return 0;
       else
         return old.it_value.tv_sec;
     }

There is an example showing the use of the alarm function in Handler Returns.

If you simply want your process to wait for a given number of seconds, you should use the sleep function. See Sleeping.

You shouldn't count on the signal arriving precisely when the timer expires. In a multiprocessing environment there is typically some amount of delay involved.

Portability Note: The setitimer and getitimer functions are derived from BSD Unix, while the alarm function is specified by the POSIX.1 standard. setitimer is more powerful than alarm, but alarm is more widely used.

Previous: Setting an Alarm, Up: Date and Time

21.6 Sleeping

The function sleep gives a simple way to make the program wait for a short interval. If your program doesn't use signals (except to terminate), then you can expect sleep to wait reliably throughout the specified interval. Otherwise, sleep can return sooner if a signal arrives; if you want to wait for a given interval regardless of signals, use select (see Waiting for I/O) and don't specify any descriptors to wait for.

— Function: unsigned int sleep (unsigned int seconds)

The sleep function waits for seconds or until a signal is delivered, whichever happens first.

If sleep function returns because the requested interval is over, it returns a value of zero. If it returns because of delivery of a signal, its return value is the remaining time in the sleep interval.

The sleep function is declared in unistd.h.

Resist the temptation to implement a sleep for a fixed amount of time by using the return value of sleep, when nonzero, to call sleep again. This will work with a certain amount of accuracy as long as signals arrive infrequently. But each signal can cause the eventual wakeup time to be off by an additional second or so. Suppose a few signals happen to arrive in rapid succession by bad luck—there is no limit on how much this could shorten or lengthen the wait.

Instead, compute the calendar time at which the program should stop waiting, and keep trying to wait until that calendar time. This won't be off by more than a second. With just a little more work, you can use select and make the waiting period quite accurate. (Of course, heavy system load can cause additional unavoidable delays—unless the machine is dedicated to one application, there is no way you can avoid this.)

On some systems, sleep can do strange things if your program uses SIGALRM explicitly. Even if SIGALRM signals are being ignored or blocked when sleep is called, sleep might return prematurely on delivery of a SIGALRM signal. If you have established a handler for SIGALRM signals and a SIGALRM signal is delivered while the process is sleeping, the action taken might be just to cause sleep to return instead of invoking your handler. And, if sleep is interrupted by delivery of a signal whose handler requests an alarm or alters the handling of SIGALRM, this handler and sleep will interfere.

On the GNU system, it is safe to use sleep and SIGALRM in the same program, because sleep does not work by means of SIGALRM.

— Function: int nanosleep (const struct timespec *requested_time, struct timespec *remaining)

If resolution to seconds is not enough the nanosleep function can be used. As the name suggests the sleep interval can be specified in nanoseconds. The actual elapsed time of the sleep interval might be longer since the system rounds the elapsed time you request up to the next integer multiple of the actual resolution the system can deliver.

*requested_time is the elapsed time of the interval you want to sleep.

The function returns as *remaining the elapsed time left in the interval for which you requested to sleep. If the interval completed without getting interrupted by a signal, this is zero.

struct timespec is described in See Elapsed Time.

If the function returns because the interval is over the return value is zero. If the function returns -1 the global variable errno is set to the following values:

EINTR
The call was interrupted because a signal was delivered to the thread. If the remaining parameter is not the null pointer the structure pointed to by remaining is updated to contain the remaining elapsed time.
EINVAL
The nanosecond value in the requested_time parameter contains an illegal value. Either the value is negative or greater than or equal to 1000 million.

This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time nanosleep is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this calls to nanosleep should be protected using cancellation handlers.

The nanosleep function is declared in time.h.

Next: , Previous: Date and Time, Up: Top

22 Resource Usage And Limitation

This chapter describes functions for examining how much of various kinds of resources (CPU time, memory, etc.) a process has used and getting and setting limits on future usage.

Next: , Up: Resource Usage And Limitation

22.1 Resource Usage

The function getrusage and the data type struct rusage are used to examine the resource usage of a process. They are declared in sys/resource.h.

— Function: int getrusage (int processes, struct rusage *rusage)

This function reports resource usage totals for processes specified by processes, storing the information in *rusage.

In most systems, processes has only two valid values:

RUSAGE_SELF
Just the current process.
RUSAGE_CHILDREN
All child processes (direct and indirect) that have already terminated.

In the GNU system, you can also inquire about a particular child process by specifying its process ID.

The return value of getrusage is zero for success, and -1 for failure.

EINVAL
The argument processes is not valid.

One way of getting resource usage for a particular child process is with the function wait4, which returns totals for a child when it terminates. See BSD Wait Functions.

— Data Type: struct rusage

This data type stores various resource usage statistics. It has the following members, and possibly others:

struct timeval ru_utime
Time spent executing user instructions.
struct timeval ru_stime
Time spent in operating system code on behalf of processes.
long int ru_maxrss
The maximum resident set size used, in kilobytes. That is, the maximum number of kilobytes of physical memory that processes used simultaneously.
long int ru_ixrss
An integral value expressed in kilobytes times ticks of execution, which indicates the amount of memory used by text that was shared with other processes.
long int ru_idrss
An integral value expressed the same way, which is the amount of unshared memory used for data.
long int ru_isrss
An integral value expressed the same way, which is the amount of unshared memory used for stack space.
long int ru_minflt
The number of page faults which were serviced without requiring any I/O.
long int ru_majflt
The number of page faults which were serviced by doing I/O.
long int ru_nswap
The number of times processes was swapped entirely out of main memory.
long int ru_inblock
The number of times the file system had to read from the disk on behalf of processes.
long int ru_oublock
The number of times the file system had to write to the disk on behalf of processes.
long int ru_msgsnd
Number of IPC messages sent.
long int ru_msgrcv
Number of IPC messages received.
long int ru_nsignals
Number of signals received.
long int ru_nvcsw
The number of times processes voluntarily invoked a context switch (usually to wait for some service).
long int ru_nivcsw
The number of times an involuntary context switch took place (because a time slice expired, or another process of higher priority was scheduled).

vtimes is a historical function that does some of what getrusage does. getrusage is a better choice.

vtimes and its vtimes data structure are declared in sys/vtimes.h.

— Function: int vtimes (struct vtimes current, struct vtimes child)

vtimes reports resource usage totals for a process.

If current is non-null, vtimes stores resource usage totals for the invoking process alone in the structure to which it points. If child is non-null, vtimes stores resource usage totals for all past children (which have terminated) of the invoking process in the structure to which it points.

— Data Type: struct vtimes

This data type contains information about the resource usage of a process. Each member corresponds to a member of the struct rusage data type described above.

vm_utime
User CPU time. Analogous to ru_utime in struct rusage
vm_stime
System CPU time. Analogous to ru_stime in struct rusage
vm_idsrss
Data and stack memory. The sum of the values that would be reported as ru_idrss and ru_isrss in struct rusage
vm_ixrss
Shared memory. Analogous to ru_ixrss in struct rusage
vm_maxrss
Maximent resident set size. Analogous to ru_maxrss in struct rusage
vm_majflt
Major page faults. Analogous to ru_majflt in struct rusage
vm_minflt
Minor page faults. Analogous to ru_minflt in struct rusage
vm_nswap
Swap count. Analogous to ru_nswap in struct rusage
vm_inblk
Disk reads. Analogous to ru_inblk in struct rusage
vm_oublk
Disk writes. Analogous to ru_oublk in struct rusage

The return value is zero if the function succeeds; -1 otherwise.

An additional historical function for examining resource usage, vtimes, is supported but not documented here. It is declared in sys/vtimes.h.

Next: , Previous: Resource Usage, Up: Resource Usage And Limitation

22.2 Limiting Resource Usage

You can specify limits for the resource usage of a process. When the process tries to exceed a limit, it may get a signal, or the system call by which it tried to do so may fail, depending on the resource. Each process initially inherits its limit values from its parent, but it can subsequently change them.

There are two per-process limits associated with a resource:

current limit
The current limit is the value the system will not allow usage to exceed. It is also called the “soft limit” because the process being limited can generally raise the current limit at will.
maximum limit
The maximum limit is the maximum value to which a process is allowed to set its current limit. It is also called the “hard limit” because there is no way for a process to get around it. A process may lower its own maximum limit, but only the superuser may increase a maximum limit.

The symbols for use with getrlimit, setrlimit, getrlimit64, and setrlimit64 are defined in sys/resource.h.

— Function: int getrlimit (int resource, struct rlimit *rlp)

Read the current and maximum limits for the resource resource and store them in *rlp.

The return value is 0 on success and -1 on failure. The only possible errno error condition is EFAULT.

When the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32-bit system this function is in fact getrlimit64. Thus, the LFS interface transparently replaces the old interface.

— Function: int getrlimit64 (int resource, struct rlimit64 *rlp)

This function is similar to getrlimit but its second parameter is a pointer to a variable of type struct rlimit64, which allows it to read values which wouldn't fit in the member of a struct rlimit.

If the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32-bit machine, this function is available under the name getrlimit and so transparently replaces the old interface.

— Function: int setrlimit (int resource, const struct rlimit *rlp)

Store the current and maximum limits for the resource resource in *rlp.

The return value is 0 on success and -1 on failure. The following errno error condition is possible:

EPERM
  • The process tried to raise a current limit beyond the maximum limit.
  • The process tried to raise a maximum limit, but is not superuser.

When the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32-bit system this function is in fact setrlimit64. Thus, the LFS interface transparently replaces the old interface.

— Function: int setrlimit64 (int resource, const struct rlimit64 *rlp)

This function is similar to setrlimit but its second parameter is a pointer to a variable of type struct rlimit64 which allows it to set values which wouldn't fit in the member of a struct rlimit.

If the sources are compiled with _FILE_OFFSET_BITS == 64 on a 32-bit machine this function is available under the name setrlimit and so transparently replaces the old interface.

— Data Type: struct rlimit

This structure is used with getrlimit to receive limit values, and with setrlimit to specify limit values for a particular process and resource. It has two fields:

rlim_t rlim_cur
The current limit
rlim_t rlim_max
The maximum limit.

For getrlimit, the structure is an output; it receives the current values. For setrlimit, it specifies the new values.

For the LFS functions a similar type is defined in sys/resource.h.

— Data Type: struct rlimit64

This structure is analogous to the rlimit structure above, but its components have wider ranges. It has two fields:

rlim64_t rlim_cur
This is analogous to rlimit.rlim_cur, but with a different type.
rlim64_t rlim_max
This is analogous to rlimit.rlim_max, but with a different type.

Here is a list of resources for which you can specify a limit. Memory and file sizes are measured in bytes.

RLIMIT_CPU
The maximum amount of CPU time the process can use. If it runs for longer than this, it gets a signal: SIGXCPU. The value is measured in seconds. See Operation Error Signals.
RLIMIT_FSIZE
The maximum size of file the process can create. Trying to write a larger file causes a signal: SIGXFSZ. See Operation Error Signals.
RLIMIT_DATA
The maximum size of data memory for the process. If the process tries to allocate data memory beyond this amount, the allocation function fails.
RLIMIT_STACK
The maximum stack size for the process. If the process tries to extend its stack past this size, it gets a SIGSEGV signal. See Program Error Signals.
RLIMIT_CORE
The maximum size core file that this process can create. If the process terminates and would dump a core file larger than this, then no core file is created. So setting this limit to zero prevents core files from ever being created.
RLIMIT_RSS
The maximum amount of physical memory that this process should get. This parameter is a guide for the system's scheduler and memory allocator; the system may give the process more memory when there is a surplus.
RLIMIT_MEMLOCK
The maximum amount of memory that can be locked into physical memory (so it will never be paged out).
RLIMIT_NPROC
The maximum number of processes that can be created with the same user ID. If you have reached the limit for your user ID, fork will fail with EAGAIN. See Creating a Process.
RLIMIT_NOFILE
RLIMIT_OFILE
The maximum number of files that the process can open. If it tries to open more files than this, its open attempt fails with errno EMFILE. See Error Codes. Not all systems support this limit; GNU does, and 4.4 BSD does.
RLIMIT_AS
The maximum size of total memory that this process should get. If the process tries to allocate more memory beyond this amount with, for example, brk, malloc, mmap or sbrk, the allocation function fails.
RLIM_NLIMITS
The number of different resource limits. Any valid resource operand must be less than RLIM_NLIMITS.
— Constant: int RLIM_INFINITY

This constant stands for a value of “infinity” when supplied as the limit value in setrlimit.

The following are historical functions to do some of what the functions above do. The functions above are better choices.

ulimit and the command symbols are declared in ulimit.h.

— Function: int ulimit (int cmd, ...)

ulimit gets the current limit or sets the current and maximum limit for a particular resource for the calling process according to the command cmd.a

If you are getting a limit, the command argument is the only argument. If you are setting a limit, there is a second argument: long int limit which is the value to which you are setting the limit.

The cmd values and the operations they specify are:

GETFSIZE
Get the current limit on the size of a file, in units of 512 bytes.
SETFSIZE
Set the current and maximum limit on the size of a file to limit * 512 bytes.

There are also some other cmd values that may do things on some systems, but they are not supported.

Only the superuser may increase a maximum limit.

When you successfully get a limit, the return value of ulimit is that limit, which is never negative. When you successfully set a limit, the return value is zero. When the function fails, the return value is -1 and errno is set according to the reason:

EPERM
A process tried to increase a maximum limit, but is not superuser.

vlimit and its resource symbols are declared in sys/vlimit.h.

— Function: int vlimit (int resource, int limit)

vlimit sets the current limit for a resource for a process.

resource identifies the resource:

LIM_CPU
Maximum CPU time. Same as RLIMIT_CPU for setrlimit.
LIM_FSIZE
Maximum file size. Same as RLIMIT_FSIZE for setrlimit.
LIM_DATA
Maximum data memory. Same as RLIMIT_DATA for setrlimit.
LIM_STACK
Maximum stack size. Same as RLIMIT_STACK for setrlimit.
LIM_CORE
Maximum core file size. Same as RLIMIT_COR for setrlimit.
LIM_MAXRSS
Maximum physical memory. Same as RLIMIT_RSS for setrlimit.

The return value is zero for success, and -1 with errno set accordingly for failure:

EPERM
The process tried to set its current limit beyond its maximum limit.

Next: , Previous: Limits on Resources, Up: Resource Usage And Limitation

22.3 Process CPU Priority And Scheduling

When multiple processes simultaneously require CPU time, the system's scheduling policy and process CPU priorities determine which processes get it. This section describes how that determination is made and GNU C library functions to control it.

It is common to refer to CPU scheduling simply as scheduling and a process' CPU priority simply as the process' priority, with the CPU resource being implied. Bear in mind, though, that CPU time is not the only resource a process uses or that processes contend for. In some cases, it is not even particularly important. Giving a process a high “priority” may have very little effect on how fast a process runs with respect to other processes. The priorities discussed in this section apply only to CPU time.

CPU scheduling is a complex issue and different systems do it in wildly different ways. New ideas continually develop and find their way into the intricacies of the various systems' scheduling algorithms. This section discusses the general concepts, some specifics of systems that commonly use the GNU C library, and some standards.

For simplicity, we talk about CPU contention as if there is only one CPU in the system. But all the same principles apply when a processor has multiple CPUs, and knowing that the number of processes that can run at any one time is equal to the number of CPUs, you can easily extrapolate the information.

The functions described in this section are all defined by the POSIX.1 and POSIX.1b standards (the sched... functions are POSIX.1b). However, POSIX does not define any semantics for the values that these functions get and set. In this chapter, the semantics are based on the Linux kernel's implementation of the POSIX standard. As you will see, the Linux implementation is quite the inverse of what the authors of the POSIX syntax had in mind.

Next: , Up: Priority

22.3.1 Absolute Priority

Every process has an absolute priority, and it is represented by a number. The higher the number, the higher the absolute priority.

On systems of the past, and most systems today, all processes have absolute priority 0 and this section is irrelevant. In that case, See Traditional Scheduling. Absolute priorities were invented to accommodate realtime systems, in which it is vital that certain processes be able to respond to external events happening in real time, which means they cannot wait around while some other process that wants to, but doesn't need to run occupies the CPU.

When two processes are in contention to use the CPU at any instant, the one with the higher absolute priority always gets it. This is true even if the process with the lower priority is already using the CPU (i.e., the scheduling is preemptive). Of course, we're only talking about processes that are running or “ready to run,” which means they are ready to execute instructions right now. When a process blocks to wait for something like I/O, its absolute priority is irrelevant.

Note: The term “runnable” is a synonym for “ready to run.”

When two processes are running or ready to run and both have the same absolute priority, it's more interesting. In that case, who gets the CPU is determined by the scheduling policy. If the processes have absolute priority 0, the traditional scheduling policy described in Traditional Scheduling applies. Otherwise, the policies described in Realtime Scheduling apply.

You normally give an absolute priority above 0 only to a process that can be trusted not to hog the CPU. Such processes are designed to block (or terminate) after relatively short CPU runs.

A process begins life with the same absolute priority as its parent process. Functions described in Basic Scheduling Functions can change it.

Only a privileged process can change a process' absolute priority to something other than 0. Only a privileged process or the target process' owner can change its absolute priority at all.

POSIX requires absolute priority values used with the realtime scheduling policies to be consecutive with a range of at least 32. On Linux, they are 1 through 99. The functions sched_get_priority_max and sched_set_priority_min portably tell you what the range is on a particular system.

22.3.1.1 Using Absolute Priority

One thing you must keep in mind when designing real time applications is that having higher absolute priority than any other process doesn't guarantee the process can run continuously. Two things that can wreck a good CPU run are interrupts and page faults.

Interrupt handlers live in that limbo between processes. The CPU is executing instructions, but they aren't part of any process. An interrupt will stop even the highest priority process. So you must allow for slight delays and make sure that no device in the system has an interrupt handler that could cause too long a delay between instructions for your process.

Similarly, a page fault causes what looks like a straightforward sequence of instructions to take a long time. The fact that other processes get to run while the page faults in is of no consequence, because as soon as the I/O is complete, the high priority process will kick them out and run again, but the wait for the I/O itself could be a problem. To neutralize this threat, use mlock or mlockall.

There are a few ramifications of the absoluteness of this priority on a single-CPU system that you need to keep in mind when you choose to set a priority and also when you're working on a program that runs with high absolute priority. Consider a process that has higher absolute priority than any other process in the system and due to a bug in its program, it gets into an infinite loop. It will never cede the CPU. You can't run a command to kill it because your command would need to get the CPU in order to run. The errant program is in complete control. It controls the vertical, it controls the horizontal.

There are two ways to avoid this: 1) keep a shell running somewhere with a higher absolute priority. 2) keep a controlling terminal attached to the high priority process group. All the priority in the world won't stop an interrupt handler from running and delivering a signal to the process if you hit Control-C.

Some systems use absolute priority as a means of allocating a fixed percentage of CPU time to a process. To do this, a super high priority privileged process constantly monitors the process' CPU usage and raises its absolute priority when the process isn't getting its entitled share and lowers it when the process is exceeding it.

Note: The absolute priority is sometimes called the “static priority.” We don't use that term in this manual because it misses the most important feature of the absolute priority: its absoluteness.

Next: , Previous: Absolute Priority, Up: Priority

22.3.2 Realtime Scheduling

Whenever two processes with the same absolute priority are ready to run, the kernel has a decision to make, because only one can run at a time. If the processes have absolute priority 0, the kernel makes this decision as described in Traditional Scheduling. Otherwise, the decision is as described in this section.

If two processes are ready to run but have different absolute priorities, the decision is much simpler, and is described in Absolute Priority.

Each process has a scheduling policy. For processes with absolute priority other than zero, there are two available:

  1. First Come First Served
  2. Round Robin

The most sensible case is where all the processes with a certain absolute priority have the same scheduling policy. We'll discuss that first.

In Round Robin, processes share the CPU, each one running for a small quantum of time (“time slice”) and then yielding to another in a circular fashion. Of course, only processes that are ready to run and have the same absolute priority are in this circle.

In First Come First Served, the process that has been waiting the longest to run gets the CPU, and it keeps it until it voluntarily relinquishes the CPU, runs out of things to do (blocks), or gets preempted by a higher priority process.

First Come First Served, along with maximal absolute priority and careful control of interrupts and page faults, is the one to use when a process absolutely, positively has to run at full CPU speed or not at all.

Judicious use of sched_yield function invocations by processes with First Come First Served scheduling policy forms a good compromise between Round Robin and First Come First Served.

To understand how scheduling works when processes of different scheduling policies occupy the same absolute priority, you have to know the nitty gritty details of how processes enter and exit the ready to run list:

In both cases, the ready to run list is organized as a true queue, where a process gets pushed onto the tail when it becomes ready to run and is popped off the head when the scheduler decides to run it. Note that ready to run and running are two mutually exclusive states. When the scheduler runs a process, that process is no longer ready to run and no longer in the ready to run list. When the process stops running, it may go back to being ready to run again.

The only difference between a process that is assigned the Round Robin scheduling policy and a process that is assigned First Come First Serve is that in the former case, the process is automatically booted off the CPU after a certain amount of time. When that happens, the process goes back to being ready to run, which means it enters the queue at the tail. The time quantum we're talking about is small. Really small. This is not your father's timesharing. For example, with the Linux kernel, the round robin time slice is a thousand times shorter than its typical time slice for traditional scheduling.

A process begins life with the same scheduling policy as its parent process. Functions described in Basic Scheduling Functions can change it.

Only a privileged process can set the scheduling policy of a process that has absolute priority higher than 0.

Next: , Previous: Realtime Scheduling, Up: Priority

22.3.3 Basic Scheduling Functions

This section describes functions in the GNU C library for setting the absolute priority and scheduling policy of a process.

Portability Note: On systems that have the functions in this section, the macro _POSIX_PRIORITY_SCHEDULING is defined in <unistd.h>.

For the case that the scheduling policy is traditional scheduling, more functions to fine tune the scheduling are in Traditional Scheduling.

Don't try to make too much out of the naming and structure of these functions. They don't match the concepts described in this manual because the functions are as defined by POSIX.1b, but the implementation on systems that use the GNU C library is the inverse of what the POSIX structure contemplates. The POSIX scheme assumes that the primary scheduling parameter is the scheduling policy and that the priority value, if any, is a parameter of the scheduling policy. In the implementation, though, the priority value is king and the scheduling policy, if anything, only fine tunes the effect of that priority.

The symbols in this section are declared by including file sched.h.

— Data Type: struct sched_param

This structure describes an absolute priority.

int sched_priority
absolute priority value

— Function: int sched_setscheduler (pid_t pid, int policy, const struct sched_param *param)

This function sets both the absolute priority and the scheduling policy for a process.

It assigns the absolute priority value given by param and the scheduling policy policy to the process with Process ID pid, or the calling process if pid is zero. If policy is negative, sched_setscheduler keeps the existing scheduling policy.

The following macros represent the valid values for policy:

SCHED_OTHER
Traditional Scheduling
SCHED_FIFO
First In First Out
SCHED_RR
Round Robin

On success, the return value is 0. Otherwise, it is -1 and ERRNO is set accordingly. The errno values specific to this function are:

EPERM
  • The calling process does not have CAP_SYS_NICE permission and policy is not SCHED_OTHER (or it's negative and the existing policy is not SCHED_OTHER.
  • The calling process does not have CAP_SYS_NICE permission and its owner is not the target process' owner. I.e., the effective uid of the calling process is neither the effective nor the real uid of process pid.

ESRCH
There is no process with pid pid and pid is not zero.
EINVAL
  • policy does not identify an existing scheduling policy.
  • The absolute priority value identified by *param is outside the valid range for the scheduling policy policy (or the existing scheduling policy if policy is negative) or param is null. sched_get_priority_max and sched_get_priority_min tell you what the valid range is.
  • pid is negative.
— Function: int sched_getscheduler (pid_t pid)

This function returns the scheduling policy assigned to the process with Process ID (pid) pid, or the calling process if pid is zero.

The return value is the scheduling policy. See sched_setscheduler for the possible values.

If the function fails, the return value is instead -1 and errno is set accordingly.

The errno values specific to this function are:

ESRCH
There is no process with pid pid and it is not zero.
EINVAL
pid is negative.

Note that this function is not an exact mate to sched_setscheduler because while that function sets the scheduling policy and the absolute priority, this function gets only the scheduling policy. To get the absolute priority, use sched_getparam.

— Function: int sched_setparam (pid_t pid, const struct sched_param *param)

This function sets a process' absolute priority.

It is functionally identical to sched_setscheduler with policy = -1.

— Function: int sched_getparam (pid_t pid, const struct sched_param *param)

This function returns a process' absolute priority.

pid is the Process ID (pid) of the process whose absolute priority you want to know.

param is a pointer to a structure in which the function stores the absolute priority of the process.

On success, the return value is 0. Otherwise, it is -1 and ERRNO is set accordingly. The errno values specific to this function are:

ESRCH
There is no process with pid pid and it is not zero.
EINVAL
pid is negative.
— Function: int sched_get_priority_min (int *policy);

This function returns the lowest absolute priority value that is allowable for a process with scheduling policy policy.

On Linux, it is 0 for SCHED_OTHER and 1 for everything else.

On success, the return value is 0. Otherwise, it is -1 and ERRNO is set accordingly. The errno values specific to this function are:

EINVAL
policy does not identify an existing scheduling policy.
— Function: int sched_get_priority_max (int *policy);

This function returns the highest absolute priority value that is allowable for a process that with scheduling policy policy.

On Linux, it is 0 for SCHED_OTHER and 99 for everything else.

On success, the return value is 0. Otherwise, it is -1 and ERRNO is set accordingly. The errno values specific to this function are:

EINVAL
policy does not identify an existing scheduling policy.
— Function: int sched_rr_get_interval (pid_t pid, struct timespec *interval)

This function returns the length of the quantum (time slice) used with the Round Robin scheduling policy, if it is used, for the process with Process ID pid.

It returns the length of time as interval.

With a Linux kernel, the round robin time slice is always 150 microseconds, and pid need not even be a real pid.

The return value is 0 on success and in the pathological case that it fails, the return value is -1 and errno is set accordingly. There is nothing specific that can go wrong with this function, so there are no specific errno values.

— Function: int sched_yield (void)

This function voluntarily gives up the process' claim on the CPU.

Technically, sched_yield causes the calling process to be made immediately ready to run (as opposed to running, which is what it was before). This means that if it has absolute priority higher than 0, it gets pushed onto the tail of the queue of processes that share its absolute priority and are ready to run, and it will run again when its turn next arrives. If its absolute priority is 0, it is more complicated, but still has the effect of yielding the CPU to other processes.

If there are no other processes that share the calling process' absolute priority, this function doesn't have any effect.

To the extent that the containing program is oblivious to what other processes in the system are doing and how fast it executes, this function appears as a no-op.

The return value is 0 on success and in the pathological case that it fails, the return value is -1 and errno is set accordingly. There is nothing specific that can go wrong with this function, so there are no specific errno values.

Next: , Previous: Basic Scheduling Functions, Up: Priority

22.3.4 Traditional Scheduling

This section is about the scheduling among processes whose absolute priority is 0. When the system hands out the scraps of CPU time that are left over after the processes with higher absolute priority have taken all they want, the scheduling described herein determines who among the great unwashed processes gets them.

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22.3.4.1 Introduction To Traditional Scheduling

Long before there was absolute priority (See Absolute Priority), Unix systems were scheduling the CPU using this system. When Posix came in like the Romans and imposed absolute priorities to accommodate the needs of realtime processing, it left the indigenous Absolute Priority Zero processes to govern themselves by their own familiar scheduling policy.

Indeed, absolute priorities higher than zero are not available on many systems today and are not typically used when they are, being intended mainly for computers that do realtime processing. So this section describes the only scheduling many programmers need to be concerned about.

But just to be clear about the scope of this scheduling: Any time a process with a absolute priority of 0 and a process with an absolute priority higher than 0 are ready to run at the same time, the one with absolute priority 0 does not run. If it's already running when the higher priority ready-to-run process comes into existence, it stops immediately.

In addition to its absolute priority of zero, every process has another priority, which we will refer to as "dynamic priority" because it changes over time. The dynamic priority is meaningless for processes with an absolute priority higher than zero.

The dynamic priority sometimes determines who gets the next turn on the CPU. Sometimes it determines how long turns last. Sometimes it determines whether a process can kick another off the CPU.

In Linux, the value is a combination of these things, but mostly it is just determines the length of the time slice. The higher a process' dynamic priority, the longer a shot it gets on the CPU when it gets one. If it doesn't use up its time slice before giving up the CPU to do something like wait for I/O, it is favored for getting the CPU back when it's ready for it, to finish out its time slice. Other than that, selection of processes for new time slices is basically round robin. But the scheduler does throw a bone to the low priority processes: A process' dynamic priority rises every time it is snubbed in the scheduling process. In Linux, even the fat kid gets to play.

The fluctuation of a process' dynamic priority is regulated by another value: The “nice” value. The nice value is an integer, usually in the range -20 to 20, and represents an upper limit on a process' dynamic priority. The higher the nice number, the lower that limit.

On a typical Linux system, for example, a process with a nice value of 20 can get only 10 milliseconds on the CPU at a time, whereas a process with a nice value of -20 can achieve a high enough priority to get 400 milliseconds.

The idea of the nice value is deferential courtesy. In the beginning, in the Unix garden of Eden, all processes shared equally in the bounty of the computer system. But not all processes really need the same share of CPU time, so the nice value gave a courteous process the ability to refuse its equal share of CPU time that others might prosper. Hence, the higher a process' nice value, the nicer the process is. (Then a snake came along and offered some process a negative nice value and the system became the crass resource allocation system we know today).

Dynamic priorities tend upward and downward with an objective of smoothing out allocation of CPU time and giving quick response time to infrequent requests. But they never exceed their nice limits, so on a heavily loaded CPU, the nice value effectively determines how fast a process runs.

In keeping with the socialistic heritage of Unix process priority, a process begins life with the same nice value as its parent process and can raise it at will. A process can also raise the nice value of any other process owned by the same user (or effective user). But only a privileged process can lower its nice value. A privileged process can also raise or lower another process' nice value.

GNU C Library functions for getting and setting nice values are described in See Traditional Scheduling Functions.

Previous: Traditional Scheduling Intro, Up: Traditional Scheduling
22.3.4.2 Functions For Traditional Scheduling

This section describes how you can read and set the nice value of a process. All these symbols are declared in sys/resource.h.

The function and macro names are defined by POSIX, and refer to "priority," but the functions actually have to do with nice values, as the terms are used both in the manual and POSIX.

The range of valid nice values depends on the kernel, but typically it runs from -20 to 20. A lower nice value corresponds to higher priority for the process. These constants describe the range of priority values:

PRIO_MIN
The lowest valid nice value.
PRIO_MAX
The highest valid nice value.
— Function: int getpriority (int class, int id)

Return the nice value of a set of processes; class and id specify which ones (see below). If the processes specified do not all have the same nice value, this returns the lowest value that any of them has.

On success, the return value is 0. Otherwise, it is -1 and ERRNO is set accordingly. The errno values specific to this function are:

ESRCH
The combination of class and id does not match any existing process.
EINVAL
The value of class is not valid.

If the return value is -1, it could indicate failure, or it could be the nice value. The only way to make certain is to set errno = 0 before calling getpriority, then use errno != 0 afterward as the criterion for failure.

— Function: int setpriority (int class, int id, int niceval)

Set the nice value of a set of processes to niceval; class and id specify which ones (see below).

The return value is 0 on success, and -1 on failure. The following errno error condition are possible for this function:

ESRCH
The combination of class and id does not match any existing process.
EINVAL
The value of class is not valid.
EPERM
The call would set the nice value of a process which is owned by a different user than the calling process (i.e., the target process' real or effective uid does not match the calling process' effective uid) and the calling process does not have CAP_SYS_NICE permission.
EACCES
The call would lower the process' nice value and the process does not have CAP_SYS_NICE permission.

The arguments class and id together specify a set of processes in which you are interested. These are the possible values of class:

PRIO_PROCESS
One particular process. The argument id is a process ID (pid).
PRIO_PGRP
All the processes in a particular process group. The argument id is a process group ID (pgid).
PRIO_USER
All the processes owned by a particular user (i.e., whose real uid indicates the user). The argument id is a user ID (uid).

If the argument id is 0, it stands for the calling process, its process group, or its owner (real uid), according to class.

— Function: int nice (int increment)

Increment the nice value of the calling process by increment. The return value is the new nice value on success, and -1 on failure. In the case of failure, errno will be set to the same values as for setpriority.

Here is an equivalent definition of nice: 

          int
          nice (int increment)
          {
            int result, old = getpriority (PRIO_PROCESS, 0);
            result = setpriority (PRIO_PROCESS, 0, old + increment);
            if (result != -1)
                return old + increment;
            else
                return -1;
          }

Previous: Traditional Scheduling, Up: Priority

22.3.5 Limiting execution to certain CPUs

On a multi-processor system the operating system usually distributes the different processes which are runnable on all available CPUs in a way which allows the system to work most efficiently. Which processes and threads run can be to some extend be control with the scheduling functionality described in the last sections. But which CPU finally executes which process or thread is not covered.

There are a number of reasons why a program might want to have control over this aspect of the system as well:

The POSIX standard up to this date is of not much help to solve this problem. The Linux kernel provides a set of interfaces to allow specifying affinity sets for a process. The scheduler will schedule the thread or process on on CPUs specified by the affinity masks. The interfaces which the GNU C library define follow to some extend the Linux kernel interface.

— Data Type: cpu_set_t

This data set is a bitset where each bit represents a CPU. How the system's CPUs are mapped to bits in the bitset is system dependent. The data type has a fixed size; in the unlikely case that the number of bits are not sufficient to describe the CPUs of the system a different interface has to be used.

This type is a GNU extension and is defined in sched.h.

To manipulate the bitset, to set and reset bits, a number of macros is defined. Some of the macros take a CPU number as a parameter. Here it is important to never exceed the size of the bitset. The following macro specifies the number of bits in the cpu_set_t bitset.

— Macro: int CPU_SETSIZE

The value of this macro is the maximum number of CPUs which can be handled with a cpu_set_t object.

The type cpu_set_t should be considered opaque; all manipulation should happen via the next four macros.

— Macro: void CPU_ZERO (cpu_set_t *set)

This macro initializes the CPU set set to be the empty set.

This macro is a GNU extension and is defined in sched.h.

— Macro: void CPU_SET (int cpu, cpu_set_t *set)

This macro adds cpu to the CPU set set.

The cpu parameter must not have side effects since it is evaluated more than once.

This macro is a GNU extension and is defined in sched.h.

— Macro: void CPU_CLR (int cpu, cpu_set_t *set)

This macro removes cpu from the CPU set set.

The cpu parameter must not have side effects since it is evaluated more than once.

This macro is a GNU extension and is defined in sched.h.

— Macro: int CPU_ISSET (int cpu, const cpu_set_t *set)

This macro returns a nonzero value (true) if cpu is a member of the CPU set set, and zero (false) otherwise.

The cpu parameter must not have side effects since it is evaluated more than once.

This macro is a GNU extension and is defined in sched.h.

CPU bitsets can be constructed from scratch or the currently installed affinity mask can be retrieved from the system.

— Function: int sched_getaffinity (pid_t pid, size_t cpusetsize, cpu_set_t *cpuset)

This functions stores the CPU affinity mask for the process or thread with the ID pid in the cpusetsize bytes long bitmap pointed to by cpuset. If successful, the function always initializes all bits in the cpu_set_t object and returns zero.

If pid does not correspond to a process or thread on the system the or the function fails for some other reason, it returns -1 and errno is set to represent the error condition.

ESRCH
No process or thread with the given ID found.
EFAULT
The pointer cpuset is does not point to a valid object.

This function is a GNU extension and is declared in sched.h.

Note that it is not portably possible to use this information to retrieve the information for different POSIX threads. A separate interface must be provided for that.

— Function: int sched_setaffinity (pid_t pid, size_t cpusetsize, const cpu_set_t *cpuset)

This function installs the cpusetsize bytes long affinity mask pointed to by cpuset for the process or thread with the ID pid. If successful the function returns zero and the scheduler will in future take the affinity information into account.

If the function fails it will return -1 and errno is set to the error code:

ESRCH
No process or thread with the given ID found.
EFAULT
The pointer cpuset is does not point to a valid object.
EINVAL
The bitset is not valid. This might mean that the affinity set might not leave a processor for the process or thread to run on.

This function is a GNU extension and is declared in sched.h.

Next: , Previous: Priority, Up: Resource Usage And Limitation

22.4 Querying memory available resources

The amount of memory available in the system and the way it is organized determines oftentimes the way programs can and have to work. For functions like mmap it is necessary to know about the size of individual memory pages and knowing how much memory is available enables a program to select appropriate sizes for, say, caches. Before we get into these details a few words about memory subsystems in traditional Unix systems will be given.

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22.4.1 Overview about traditional Unix memory handling

Unix systems normally provide processes virtual address spaces. This means that the addresses of the memory regions do not have to correspond directly to the addresses of the actual physical memory which stores the data. An extra level of indirection is introduced which translates virtual addresses into physical addresses. This is normally done by the hardware of the processor.

Using a virtual address space has several advantage. The most important is process isolation. The different processes running on the system cannot interfere directly with each other. No process can write into the address space of another process (except when shared memory is used but then it is wanted and controlled).

Another advantage of virtual memory is that the address space the processes see can actually be larger than the physical memory available. The physical memory can be extended by storage on an external media where the content of currently unused memory regions is stored. The address translation can then intercept accesses to these memory regions and make memory content available again by loading the data back into memory. This concept makes it necessary that programs which have to use lots of memory know the difference between available virtual address space and available physical memory. If the working set of virtual memory of all the processes is larger than the available physical memory the system will slow down dramatically due to constant swapping of memory content from the memory to the storage media and back. This is called “thrashing”. A final aspect of virtual memory which is important and follows from what is said in the last paragraph is the granularity of the virtual address space handling. When we said that the virtual address handling stores memory content externally it cannot do this on a byte-by-byte basis. The administrative overhead does not allow this (leaving alone the processor hardware). Instead several thousand bytes are handled together and form a page. The size of each page is always a power of two byte. The smallest page size in use today is 4096, with 8192, 16384, and 65536 being other popular sizes.

Previous: Memory Subsystem, Up: Memory Resources

22.4.2 How to get information about the memory subsystem?

The page size of the virtual memory the process sees is essential to know in several situations. Some programming interface (e.g., mmap, see Memory-mapped I/O) require the user to provide information adjusted to the page size. In the case of mmap is it necessary to provide a length argument which is a multiple of the page size. Another place where the knowledge about the page size is useful is in memory allocation. If one allocates pieces of memory in larger chunks which are then subdivided by the application code it is useful to adjust the size of the larger blocks to the page size. If the total memory requirement for the block is close (but not larger) to a multiple of the page size the kernel's memory handling can work more effectively since it only has to allocate memory pages which are fully used. (To do this optimization it is necessary to know a bit about the memory allocator which will require a bit of memory itself for each block and this overhead must not push the total size over the page size multiple.

The page size traditionally was a compile time constant. But recent development of processors changed this. Processors now support different page sizes and they can possibly even vary among different processes on the same system. Therefore the system should be queried at runtime about the current page size and no assumptions (except about it being a power of two) should be made.

The correct interface to query about the page size is sysconf (see Sysconf Definition) with the parameter _SC_PAGESIZE. There is a much older interface available, too.

— Function: int getpagesize (void)

The getpagesize function returns the page size of the process. This value is fixed for the runtime of the process but can vary in different runs of the application.

The function is declared in unistd.h.

Widely available on System V derived systems is a method to get information about the physical memory the system has. The call 

       sysconf (_SC_PHYS_PAGES)

returns the total number of pages of physical the system has. This does not mean all this memory is available. This information can be found using 

       sysconf (_SC_AVPHYS_PAGES)

These two values help to optimize applications. The value returned for _SC_AVPHYS_PAGES is the amount of memory the application can use without hindering any other process (given that no other process increases its memory usage). The value returned for _SC_PHYS_PAGES is more or less a hard limit for the working set. If all applications together constantly use more than that amount of memory the system is in trouble.

The GNU C library provides in addition to these already described way to get this information two functions. They are declared in the file sys/sysinfo.h. Programmers should prefer to use the sysconf method described above.

— Function: long int get_phys_pages (void)

The get_phys_pages function returns the total number of pages of physical the system has. To get the amount of memory this number has to be multiplied by the page size.

This function is a GNU extension.

— Function: long int get_avphys_pages (void)

The get_phys_pages function returns the number of available pages of physical the system has. To get the amount of memory this number has to be multiplied by the page size.

This function is a GNU extension.

Previous: Memory Resources, Up: Resource Usage And Limitation

22.5 Learn about the processors available

The use of threads or processes with shared memory allows an application to take advantage of all the processing power a system can provide. If the task can be parallelized the optimal way to write an application is to have at any time as many processes running as there are processors. To determine the number of processors available to the system one can run 

       sysconf (_SC_NPROCESSORS_CONF)

which returns the number of processors the operating system configured. But it might be possible for the operating system to disable individual processors and so the call 

       sysconf (_SC_NPROCESSORS_ONLN)

returns the number of processors which are currently inline (i.e., available).

For these two pieces of information the GNU C library also provides functions to get the information directly. The functions are declared in sys/sysinfo.h.

— Function: int get_nprocs_conf (void)

The get_nprocs_conf function returns the number of processors the operating system configured.

This function is a GNU extension.

— Function: int get_nprocs (void)

The get_nprocs function returns the number of available processors.

This function is a GNU extension.

Before starting more threads it should be checked whether the processors are not already overused. Unix systems calculate something called the load average. This is a number indicating how many processes were running. This number is average over different periods of times (normally 1, 5, and 15 minutes).

— Function: int getloadavg (double loadavg[], int nelem)

This function gets the 1, 5 and 15 minute load averages of the system. The values are placed in loadavg. getloadavg will place at most nelem elements into the array but never more than three elements. The return value is the number of elements written to loadavg, or -1 on error.

This function is declared in stdlib.h.

Next: , Previous: Resource Usage And Limitation, Up: Top

23 Non-Local Exits

Sometimes when your program detects an unusual situation inside a deeply nested set of function calls, you would like to be able to immediately return to an outer level of control. This section describes how you can do such non-local exits using the setjmp and longjmp functions.

Next: , Up: Non-Local Exits

23.1 Introduction to Non-Local Exits

As an example of a situation where a non-local exit can be useful, suppose you have an interactive program that has a “main loop” that prompts for and executes commands. Suppose the “read” command reads input from a file, doing some lexical analysis and parsing of the input while processing it. If a low-level input error is detected, it would be useful to be able to return immediately to the “main loop” instead of having to make each of the lexical analysis, parsing, and processing phases all have to explicitly deal with error situations initially detected by nested calls.

(On the other hand, if each of these phases has to do a substantial amount of cleanup when it exits—such as closing files, deallocating buffers or other data structures, and the like—then it can be more appropriate to do a normal return and have each phase do its own cleanup, because a non-local exit would bypass the intervening phases and their associated cleanup code entirely. Alternatively, you could use a non-local exit but do the cleanup explicitly either before or after returning to the “main loop”.)

In some ways, a non-local exit is similar to using the `return' statement to return from a function. But while `return' abandons only a single function call, transferring control back to the point at which it was called, a non-local exit can potentially abandon many levels of nested function calls.

You identify return points for non-local exits by calling the function setjmp. This function saves information about the execution environment in which the call to setjmp appears in an object of type jmp_buf. Execution of the program continues normally after the call to setjmp, but if an exit is later made to this return point by calling longjmp with the corresponding jmp_buf object, control is transferred back to the point where setjmp was called. The return value from setjmp is used to distinguish between an ordinary return and a return made by a call to longjmp, so calls to setjmp usually appear in an `if' statement.

Here is how the example program described above might be set up: 

     #include <setjmp.h>
     #include <stdlib.h>
     #include <stdio.h>
     
     jmp_buf main_loop;
     
     void
     abort_to_main_loop (int status)
     {
       longjmp (main_loop, status);
     }
     
     int
     main (void)
     {
       while (1)
         if (setjmp (main_loop))
           puts ("Back at main loop....");
         else
           do_command ();
     }
     
     
     void
     do_command (void)
     {
       char buffer[128];
       if (fgets (buffer, 128, stdin) == NULL)
         abort_to_main_loop (-1);
       else
         exit (EXIT_SUCCESS);
     }

The function abort_to_main_loop causes an immediate transfer of control back to the main loop of the program, no matter where it is called from.

The flow of control inside the main function may appear a little mysterious at first, but it is actually a common idiom with setjmp. A normal call to setjmp returns zero, so the “else” clause of the conditional is executed. If abort_to_main_loop is called somewhere within the execution of do_command, then it actually appears as if the same call to setjmp in main were returning a second time with a value of -1.

So, the general pattern for using setjmp looks something like: 

     if (setjmp (buffer))
       /* Code to clean up after premature return. */
       ...
     else
       /* Code to be executed normally after setting up the return point. */
       ...

Next: , Previous: Non-Local Intro, Up: Non-Local Exits

23.2 Details of Non-Local Exits

Here are the details on the functions and data structures used for performing non-local exits. These facilities are declared in setjmp.h.

— Data Type: jmp_buf

Objects of type jmp_buf hold the state information to be restored by a non-local exit. The contents of a jmp_buf identify a specific place to return to.

— Macro: int setjmp (jmp_buf state)

When called normally, setjmp stores information about the execution state of the program in state and returns zero. If longjmp is later used to perform a non-local exit to this state, setjmp returns a nonzero value.

— Function: void longjmp (jmp_buf state, int value)

This function restores current execution to the state saved in state, and continues execution from the call to setjmp that established that return point. Returning from setjmp by means of longjmp returns the value argument that was passed to longjmp, rather than 0. (But if value is given as 0, setjmp returns 1).

There are a lot of obscure but important restrictions on the use of setjmp and longjmp. Most of these restrictions are present because non-local exits require a fair amount of magic on the part of the C compiler and can interact with other parts of the language in strange ways.

The setjmp function is actually a macro without an actual function definition, so you shouldn't try to `#undef' it or take its address. In addition, calls to setjmp are safe in only the following contexts:

Return points are valid only during the dynamic extent of the function that called setjmp to establish them. If you longjmp to a return point that was established in a function that has already returned, unpredictable and disastrous things are likely to happen.

You should use a nonzero value argument to longjmp. While longjmp refuses to pass back a zero argument as the return value from setjmp, this is intended as a safety net against accidental misuse and is not really good programming style.

When you perform a non-local exit, accessible objects generally retain whatever values they had at the time longjmp was called. The exception is that the values of automatic variables local to the function containing the setjmp call that have been changed since the call to setjmp are indeterminate, unless you have declared them volatile.

Next: , Previous: Non-Local Details, Up: Non-Local Exits

23.3 Non-Local Exits and Signals

In BSD Unix systems, setjmp and longjmp also save and restore the set of blocked signals; see Blocking Signals. However, the POSIX.1 standard requires setjmp and longjmp not to change the set of blocked signals, and provides an additional pair of functions (sigsetjmp and siglongjmp) to get the BSD behavior.

The behavior of setjmp and longjmp in the GNU library is controlled by feature test macros; see Feature Test Macros. The default in the GNU system is the POSIX.1 behavior rather than the BSD behavior.

The facilities in this section are declared in the header file setjmp.h.

— Data Type: sigjmp_buf

This is similar to jmp_buf, except that it can also store state information about the set of blocked signals.

— Function: int sigsetjmp (sigjmp_buf state, int savesigs)

This is similar to setjmp. If savesigs is nonzero, the set of blocked signals is saved in state and will be restored if a siglongjmp is later performed with this state.

— Function: void siglongjmp (sigjmp_buf state, int value)

This is similar to longjmp except for the type of its state argument. If the sigsetjmp call that set this state used a nonzero savesigs flag, siglongjmp also restores the set of blocked signals.

Previous: Non-Local Exits and Signals, Up: Non-Local Exits

23.4 Complete Context Control

The Unix standard one more set of function to control the execution path and these functions are more powerful than those discussed in this chapter so far. These function were part of the original System V API and by this route were added to the Unix API. Beside on branded Unix implementations these interfaces are not widely available. Not all platforms and/or architectures the GNU C Library is available on provide this interface. Use configure to detect the availability.

Similar to the jmp_buf and sigjmp_buf types used for the variables to contain the state of the longjmp functions the interfaces of interest here have an appropriate type as well. Objects of this type are normally much larger since more information is contained. The type is also used in a few more places as we will see. The types and functions described in this section are all defined and declared respectively in the ucontext.h header file.

— Data Type: ucontext_t

The ucontext_t type is defined as a structure with as least the following elements:

ucontext_t *uc_link
This is a pointer to the next context structure which is used if the context described in the current structure returns.
sigset_t uc_sigmask
Set of signals which are blocked when this context is used.
stack_t uc_stack
Stack used for this context. The value need not be (and normally is not) the stack pointer. See Signal Stack.
mcontext_t uc_mcontext
This element contains the actual state of the process. The mcontext_t type is also defined in this header but the definition should be treated as opaque. Any use of knowledge of the type makes applications less portable.

Objects of this type have to be created by the user. The initialization and modification happens through one of the following functions:

— Function: int getcontext (ucontext_t *ucp)

The getcontext function initializes the variable pointed to by ucp with the context of the calling thread. The context contains the content of the registers, the signal mask, and the current stack. Executing the contents would start at the point where the getcontext call just returned.

The function returns 0 if successful. Otherwise it returns -1 and sets errno accordingly.

The getcontext function is similar to setjmp but it does not provide an indication of whether the function returns for the first time or whether the initialized context was used and the execution is resumed at just that point. If this is necessary the user has to take determine this herself. This must be done carefully since the context contains registers which might contain register variables. This is a good situation to define variables with volatile.

Once the context variable is initialized it can be used as is or it can be modified. The latter is normally done to implement co-routines or similar constructs. The makecontext function is what has to be used to do that.

— Function: void makecontext (ucontext_t *ucp, void (*func) (void), int argc, ...)

The ucp parameter passed to the makecontext shall be initialized by a call to getcontext. The context will be modified to in a way so that if the context is resumed it will start by calling the function func which gets argc integer arguments passed. The integer arguments which are to be passed should follow the argc parameter in the call to makecontext.

Before the call to this function the uc_stack and uc_link element of the ucp structure should be initialized. The uc_stack element describes the stack which is used for this context. No two contexts which are used at the same time should use the same memory region for a stack.

The uc_link element of the object pointed to by ucp should be a pointer to the context to be executed when the function func returns or it should be a null pointer. See setcontext for more information about the exact use.

While allocating the memory for the stack one has to be careful. Most modern processors keep track of whether a certain memory region is allowed to contain code which is executed or not. Data segments and heap memory is normally not tagged to allow this. The result is that programs would fail. Examples for such code include the calling sequences the GNU C compiler generates for calls to nested functions. Safe ways to allocate stacks correctly include using memory on the original threads stack or explicitly allocate memory tagged for execution using (see Memory-mapped I/O).

Compatibility note: The current Unix standard is very imprecise about the way the stack is allocated. All implementations seem to agree that the uc_stack element must be used but the values stored in the elements of the stack_t value are unclear. The GNU C library and most other Unix implementations require the ss_sp value of the uc_stack element to point to the base of the memory region allocated for the stack and the size of the memory region is stored in ss_size. There are implements out there which require ss_sp to be set to the value the stack pointer will have (which can depending on the direction the stack grows be different). This difference makes the makecontext function hard to use and it requires detection of the platform at compile time.

— Function: int setcontext (const ucontext_t *ucp)

The setcontext function restores the context described by ucp. The context is not modified and can be reused as often as wanted.

If the context was created by getcontext execution resumes with the registers filled with the same values and the same stack as if the getcontext call just returned.

If the context was modified with a call to makecontext execution continues with the function passed to makecontext which gets the specified parameters passed. If this function returns execution is resumed in the context which was referenced by the uc_link element of the context structure passed to makecontext at the time of the call. If uc_link was a null pointer the application terminates in this case.

Since the context contains information about the stack no two threads should use the same context at the same time. The result in most cases would be disastrous.

The setcontext function does not return unless an error occurred in which case it returns -1.

The setcontext function simply replaces the current context with the one described by the ucp parameter. This is often useful but there are situations where the current context has to be preserved.

— Function: int swapcontext (ucontext_t *restrict oucp, const ucontext_t *restrict ucp)

The swapcontext function is similar to setcontext but instead of just replacing the current context the latter is first saved in the object pointed to by oucp as if this was a call to getcontext. The saved context would resume after the call to swapcontext.

Once the current context is saved the context described in ucp is installed and execution continues as described in this context.

If swapcontext succeeds the function does not return unless the context oucp is used without prior modification by makecontext. The return value in this case is 0. If the function fails it returns -1 and set errno accordingly.

Example for SVID Context Handling

The easiest way to use the context handling functions is as a replacement for setjmp and longjmp. The context contains on most platforms more information which might lead to less surprises but this also means using these functions is more expensive (beside being less portable). 

     int
     random_search (int n, int (*fp) (int, ucontext_t *))
     {
       volatile int cnt = 0;
       ucontext_t uc;
     
       /* Safe current context.  */
       if (getcontext (&uc) < 0)
         return -1;
     
       /* If we have not tried n times try again.  */
       if (cnt++ < n)
         /* Call the function with a new random number
            and the context.  */
         if (fp (rand (), &uc) != 0)
           /* We found what we were looking for.  */
           return 1;
     
       /* Not found.  */
       return 0;
     }

Using contexts in such a way enables emulating exception handling. The search functions passed in the fp parameter could be very large, nested, and complex which would make it complicated (or at least would require a lot of code) to leave the function with an error value which has to be passed down to the caller. By using the context it is possible to leave the search function in one step and allow restarting the search which also has the nice side effect that it can be significantly faster.

Something which is harder to implement with setjmp and longjmp is to switch temporarily to a different execution path and then resume where execution was stopped. 

     #include <signal.h>
     #include <stdio.h>
     #include <stdlib.h>
     #include <ucontext.h>
     #include <sys/time.h>
     
     /* Set by the signal handler. */
     static volatile int expired;
     
     /* The contexts. */
     static ucontext_t uc[3];
     
     /* We do only a certain number of switches. */
     static int switches;
     
     
     /* This is the function doing the work.  It is just a
        skeleton, real code has to be filled in. */
     static void
     f (int n)
     {
       int m = 0;
       while (1)
         {
           /* This is where the work would be done. */
           if (++m % 100 == 0)
             {
               putchar ('.');
               fflush (stdout);
             }
     
           /* Regularly the expire variable must be checked. */
           if (expired)
             {
               /* We do not want the program to run forever. */
               if (++switches == 20)
                 return;
     
               printf ("\nswitching from %d to %d\n", n, 3 - n);
               expired = 0;
               /* Switch to the other context, saving the current one. */
               swapcontext (&uc[n], &uc[3 - n]);
             }
         }
     }
     
     /* This is the signal handler which simply set the variable. */
     void
     handler (int signal)
     {
       expired = 1;
     }
     
     
     int
     main (void)
     {
       struct sigaction sa;
       struct itimerval it;
       char st1[8192];
       char st2[8192];
     
       /* Initialize the data structures for the interval timer. */
       sa.sa_flags = SA_RESTART;
       sigfillset (&sa.sa_mask);
       sa.sa_handler = handler;
       it.it_interval.tv_sec = 0;
       it.it_interval.tv_usec = 1;
       it.it_value = it.it_interval;
     
       /* Install the timer and get the context we can manipulate. */
       if (sigaction (SIGPROF, &sa, NULL) < 0
           || setitimer (ITIMER_PROF, &it, NULL) < 0
           || getcontext (&uc[1]) == -1
           || getcontext (&uc[2]) == -1)
         abort ();
     
       /* Create a context with a separate stack which causes the
          function f to be call with the parameter 1.
          Note that the uc_link points to the main context
          which will cause the program to terminate once the function
          return. */
       uc[1].uc_link = &uc[0];
       uc[1].uc_stack.ss_sp = st1;
       uc[1].uc_stack.ss_size = sizeof st1;
       makecontext (&uc[1], (void (*) (void)) f, 1, 1);
     
       /* Similarly, but 2 is passed as the parameter to f. */
       uc[2].uc_link = &uc[0];
       uc[2].uc_stack.ss_sp = st2;
       uc[2].uc_stack.ss_size = sizeof st2;
       makecontext (&uc[2], (void (*) (void)) f, 1, 2);
     
       /* Start running. */
       swapcontext (&uc[0], &uc[1]);
       putchar ('\n');
     
       return 0;
     }

This an example how the context functions can be used to implement co-routines or cooperative multi-threading. All that has to be done is to call every once in a while swapcontext to continue running a different context. It is not allowed to do the context switching from the signal handler directly since neither setcontext nor swapcontext are functions which can be called from a signal handler. But setting a variable in the signal handler and checking it in the body of the functions which are executed. Since swapcontext is saving the current context it is possible to have multiple different scheduling points in the code. Execution will always resume where it was left.

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24 Signal Handling

A signal is a software interrupt delivered to a process. The operating system uses signals to report exceptional situations to an executing program. Some signals report errors such as references to invalid memory addresses; others report asynchronous events, such as disconnection of a phone line.

The GNU C library defines a variety of signal types, each for a particular kind of event. Some kinds of events make it inadvisable or impossible for the program to proceed as usual, and the corresponding signals normally abort the program. Other kinds of signals that report harmless events are ignored by default.

If you anticipate an event that causes signals, you can define a handler function and tell the operating system to run it when that particular type of signal arrives.

Finally, one process can send a signal to another process; this allows a parent process to abort a child, or two related processes to communicate and synchronize.

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24.1 Basic Concepts of Signals

This section explains basic concepts of how signals are generated, what happens after a signal is delivered, and how programs can handle signals.

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24.1.1 Some Kinds of Signals

A signal reports the occurrence of an exceptional event. These are some of the events that can cause (or generate, or raise) a signal:

Each of these kinds of events (excepting explicit calls to kill and raise) generates its own particular kind of signal. The various kinds of signals are listed and described in detail in Standard Signals.

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24.1.2 Concepts of Signal Generation

In general, the events that generate signals fall into three major categories: errors, external events, and explicit requests.

An error means that a program has done something invalid and cannot continue execution. But not all kinds of errors generate signals—in fact, most do not. For example, opening a nonexistent file is an error, but it does not raise a signal; instead, open returns -1. In general, errors that are necessarily associated with certain library functions are reported by returning a value that indicates an error. The errors which raise signals are those which can happen anywhere in the program, not just in library calls. These include division by zero and invalid memory addresses.

An external event generally has to do with I/O or other processes. These include the arrival of input, the expiration of a timer, and the termination of a child process.

An explicit request means the use of a library function such as kill whose purpose is specifically to generate a signal.

Signals may be generated synchronously or asynchronously. A synchronous signal pertains to a specific action in the program, and is delivered (unless blocked) during that action. Most errors generate signals synchronously, and so do explicit requests by a process to generate a signal for that same process. On some machines, certain kinds of hardware errors (usually floating-point exceptions) are not reported completely synchronously, but may arrive a few instructions later.

Asynchronous signals are generated by events outside the control of the process that receives them. These signals arrive at unpredictable times during execution. External events generate signals asynchronously, and so do explicit requests that apply to some other process.

A given type of signal is either typically synchronous or typically asynchronous. For example, signals for errors are typically synchronous because errors generate signals synchronously. But any type of signal can be generated synchronously or asynchronously with an explicit request.

Previous: Signal Generation, Up: Concepts of Signals

24.1.3 How Signals Are Delivered

When a signal is generated, it becomes pending. Normally it remains pending for just a short period of time and then is delivered to the process that was signaled. However, if that kind of signal is currently blocked, it may remain pending indefinitely—until signals of that kind are unblocked. Once unblocked, it will be delivered immediately. See Blocking Signals.

When the signal is delivered, whether right away or after a long delay, the specified action for that signal is taken. For certain signals, such as SIGKILL and SIGSTOP, the action is fixed, but for most signals, the program has a choice: ignore the signal, specify a handler function, or accept the default action for that kind of signal. The program specifies its choice using functions such as signal or sigaction (see Signal Actions). We sometimes say that a handler catches the signal. While the handler is running, that particular signal is normally blocked.

If the specified action for a kind of signal is to ignore it, then any such signal which is generated is discarded immediately. This happens even if the signal is also blocked at the time. A signal discarded in this way will never be delivered, not even if the program subsequently specifies a different action for that kind of signal and then unblocks it.

If a signal arrives which the program has neither handled nor ignored, its default action takes place. Each kind of signal has its own default action, documented below (see Standard Signals). For most kinds of signals, the default action is to terminate the process. For certain kinds of signals that represent “harmless” events, the default action is to do nothing.

When a signal terminates a process, its parent process can determine the cause of termination by examining the termination status code reported by the wait or waitpid functions. (This is discussed in more detail in Process Completion.) The information it can get includes the fact that termination was due to a signal and the kind of signal involved. If a program you run from a shell is terminated by a signal, the shell typically prints some kind of error message.

The signals that normally represent program errors have a special property: when one of these signals terminates the process, it also writes a core dump file which records the state of the process at the time of termination. You can examine the core dump with a debugger to investigate what caused the error.

If you raise a “program error” signal by explicit request, and this terminates the process, it makes a core dump file just as if the signal had been due directly to an error.

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24.2 Standard Signals

This section lists the names for various standard kinds of signals and describes what kind of event they mean. Each signal name is a macro which stands for a positive integer—the signal number for that kind of signal. Your programs should never make assumptions about the numeric code for a particular kind of signal, but rather refer to them always by the names defined here. This is because the number for a given kind of signal can vary from system to system, but the meanings of the names are standardized and fairly uniform.

The signal names are defined in the header file signal.h.

— Macro: int NSIG

The value of this symbolic constant is the total number of signals defined. Since the signal numbers are allocated consecutively, NSIG is also one greater than the largest defined signal number.

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24.2.1 Program Error Signals

The following signals are generated when a serious program error is detected by the operating system or the computer itself. In general, all of these signals are indications that your program is seriously broken in some way, and there's usually no way to continue the computation which encountered the error.

Some programs handle program error signals in order to tidy up before terminating; for example, programs that turn off echoing of terminal input should handle program error signals in order to turn echoing back on. The handler should end by specifying the default action for the signal that happened and then reraising it; this will cause the program to terminate with that signal, as if it had not had a handler. (See Termination in Handler.)

Termination is the sensible ultimate outcome from a program error in most programs. However, programming systems such as Lisp that can load compiled user programs might need to keep executing even if a user program incurs an error. These programs have handlers which use longjmp to return control to the command level.

The default action for all of these signals is to cause the process to terminate. If you block or ignore these signals or establish handlers for them that return normally, your program will probably break horribly when such signals happen, unless they are generated by raise or kill instead of a real error.

When one of these program error signals terminates a process, it also writes a core dump file which records the state of the process at the time of termination. The core dump file is named core and is written in whichever directory is current in the process at the time. (On the GNU system, you can specify the file name for core dumps with the environment variable COREFILE.) The purpose of core dump files is so that you can examine them with a debugger to investigate what caused the error.

— Macro: int SIGFPE

The SIGFPE signal reports a fatal arithmetic error. Although the name is derived from “floating-point exception”, this signal actually covers all arithmetic errors, including division by zero and overflow. If a program stores integer data in a location which is then used in a floating-point operation, this often causes an “invalid operation” exception, because the processor cannot recognize the data as a floating-point number. Actual floating-point exceptions are a complicated subject because there are many types of exceptions with subtly different meanings, and the SIGFPE signal doesn't distinguish between them. The IEEE Standard for Binary Floating-Point Arithmetic (ANSI/IEEE Std 754-1985 and ANSI/IEEE Std 854-1987) defines various floating-point exceptions and requires conforming computer systems to report their occurrences. However, this standard does not specify how the exceptions are reported, or what kinds of handling and control the operating system can offer to the programmer.

BSD systems provide the SIGFPE handler with an extra argument that distinguishes various causes of the exception. In order to access this argument, you must define the handler to accept two arguments, which means you must cast it to a one-argument function type in order to establish the handler. The GNU library does provide this extra argument, but the value is meaningful only on operating systems that provide the information (BSD systems and GNU systems).

FPE_INTOVF_TRAP
Integer overflow (impossible in a C program unless you enable overflow trapping in a hardware-specific fashion).
FPE_INTDIV_TRAP
Integer division by zero.
FPE_SUBRNG_TRAP
Subscript-range (something that C programs never check for).
FPE_FLTOVF_TRAP
Floating overflow trap.
FPE_FLTDIV_TRAP
Floating/decimal division by zero.
FPE_FLTUND_TRAP
Floating underflow trap. (Trapping on floating underflow is not normally enabled.)
FPE_DECOVF_TRAP
Decimal overflow trap. (Only a few machines have decimal arithmetic and C never uses it.)
— Macro: int SIGILL

The name of this signal is derived from “illegal instruction”; it usually means your program is trying to execute garbage or a privileged instruction. Since the C compiler generates only valid instructions, SIGILL typically indicates that the executable file is corrupted, or that you are trying to execute data. Some common ways of getting into the latter situation are by passing an invalid object where a pointer to a function was expected, or by writing past the end of an automatic array (or similar problems with pointers to automatic variables) and corrupting other data on the stack such as the return address of a stack frame.

SIGILL can also be generated when the stack overflows, or when the system has trouble running the handler for a signal.

— Macro: int SIGSEGV

This signal is generated when a program tries to read or write outside the memory that is allocated for it, or to write memory that can only be read. (Actually, the signals only occur when the program goes far enough outside to be detected by the system's memory protection mechanism.) The name is an abbreviation for “segmentation violation”.

Common ways of getting a SIGSEGV condition include dereferencing a null or uninitialized pointer, or when you use a pointer to step through an array, but fail to check for the end of the array. It varies among systems whether dereferencing a null pointer generates SIGSEGV or SIGBUS.

— Macro: int SIGBUS

This signal is generated when an invalid pointer is dereferenced. Like SIGSEGV, this signal is typically the result of dereferencing an uninitialized pointer. The difference between the two is that SIGSEGV indicates an invalid access to valid memory, while SIGBUS indicates an access to an invalid address. In particular, SIGBUS signals often result from dereferencing a misaligned pointer, such as referring to a four-word integer at an address not divisible by four. (Each kind of computer has its own requirements for address alignment.)

The name of this signal is an abbreviation for “bus error”.

— Macro: int SIGABRT

This signal indicates an error detected by the program itself and reported by calling abort. See Aborting a Program.

— Macro: int SIGIOT

Generated by the PDP-11 “iot” instruction. On most machines, this is just another name for SIGABRT.

— Macro: int SIGTRAP

Generated by the machine's breakpoint instruction, and possibly other trap instructions. This signal is used by debuggers. Your program will probably only see SIGTRAP if it is somehow executing bad instructions.

— Macro: int SIGEMT

Emulator trap; this results from certain unimplemented instructions which might be emulated in software, or the operating system's failure to properly emulate them.

— Macro: int SIGSYS

Bad system call; that is to say, the instruction to trap to the operating system was executed, but the code number for the system call to perform was invalid.

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24.2.2 Termination Signals

These signals are all used to tell a process to terminate, in one way or another. They have different names because they're used for slightly different purposes, and programs might want to handle them differently.

The reason for handling these signals is usually so your program can tidy up as appropriate before actually terminating. For example, you might want to save state information, delete temporary files, or restore the previous terminal modes. Such a handler should end by specifying the default action for the signal that happened and then reraising it; this will cause the program to terminate with that signal, as if it had not had a handler. (See Termination in Handler.)

The (obvious) default action for all of these signals is to cause the process to terminate.

— Macro: int SIGTERM

The SIGTERM signal is a generic signal used to cause program termination. Unlike SIGKILL, this signal can be blocked, handled, and ignored. It is the normal way to politely ask a program to terminate.

The shell command kill generates SIGTERM by default.

— Macro: int SIGINT

The SIGINT (“program interrupt”) signal is sent when the user types the INTR character (normally C-c). See Special Characters, for information about terminal driver support for C-c.

— Macro: int SIGQUIT

The SIGQUIT signal is similar to SIGINT, except that it's controlled by a different key—the QUIT character, usually C-\—and produces a core dump when it terminates the process, just like a program error signal. You can think of this as a program error condition “detected” by the user.

See Program Error Signals, for information about core dumps. See Special Characters, for information about terminal driver support.

Certain kinds of cleanups are best omitted in handling SIGQUIT. For example, if the program creates temporary files, it should handle the other termination requests by deleting the temporary files. But it is better for SIGQUIT not to delete them, so that the user can examine them in conjunction with the core dump.

— Macro: int SIGKILL

The SIGKILL signal is used to cause immediate program termination. It cannot be handled or ignored, and is therefore always fatal. It is also not possible to block this signal.

This signal is usually generated only by explicit request. Since it cannot be handled, you should generate it only as a last resort, after first trying a less drastic method such as C-c or SIGTERM. If a process does not respond to any other termination signals, sending it a SIGKILL signal will almost always cause it to go away.

In fact, if SIGKILL fails to terminate a process, that by itself constitutes an operating system bug which you should report.

The system will generate SIGKILL for a process itself under some unusual conditions where the program cannot possibly continue to run (even to run a signal handler).

— Macro: int SIGHUP

The SIGHUP (“hang-up”) signal is used to report that the user's terminal is disconnected, perhaps because a network or telephone connection was broken. For more information about this, see Control Modes.

This signal is also used to report the termination of the controlling process on a terminal to jobs associated with that session; this termination effectively disconnects all processes in the session from the controlling terminal. For more information, see Termination Internals.

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24.2.3 Alarm Signals

These signals are used to indicate the expiration of timers. See Setting an Alarm, for information about functions that cause these signals to be sent.

The default behavior for these signals is to cause program termination. This default is rarely useful, but no other default would be useful; most of the ways of using these signals would require handler functions in any case.

— Macro: int SIGALRM

This signal typically indicates expiration of a timer that measures real or clock time. It is used by the alarm function, for example.

— Macro: int SIGVTALRM

This signal typically indicates expiration of a timer that measures CPU time used by the current process. The name is an abbreviation for “virtual time alarm”.

— Macro: int SIGPROF

This signal typically indicates expiration of a timer that measures both CPU time used by the current process, and CPU time expended on behalf of the process by the system. Such a timer is used to implement code profiling facilities, hence the name of this signal.

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24.2.4 Asynchronous I/O Signals

The signals listed in this section are used in conjunction with asynchronous I/O facilities. You have to take explicit action by calling fcntl to enable a particular file descriptor to generate these signals (see Interrupt Input). The default action for these signals is to ignore them.

— Macro: int SIGIO

This signal is sent when a file descriptor is ready to perform input or output.

On most operating systems, terminals and sockets are the only kinds of files that can generate SIGIO; other kinds, including ordinary files, never generate SIGIO even if you ask them to.

In the GNU system SIGIO will always be generated properly if you successfully set asynchronous mode with fcntl.

— Macro: int SIGURG

This signal is sent when “urgent” or out-of-band data arrives on a socket. See Out-of-Band Data.

— Macro: int SIGPOLL

This is a System V signal name, more or less similar to SIGIO. It is defined only for compatibility.

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24.2.5 Job Control Signals

These signals are used to support job control. If your system doesn't support job control, then these macros are defined but the signals themselves can't be raised or handled.

You should generally leave these signals alone unless you really understand how job control works. See Job Control.

— Macro: int SIGCHLD

This signal is sent to a parent process whenever one of its child processes terminates or stops.

The default action for this signal is to ignore it. If you establish a handler for this signal while there are child processes that have terminated but not reported their status via wait or waitpid (see Process Completion), whether your new handler applies to those processes or not depends on the particular operating system.

— Macro: int SIGCLD

This is an obsolete name for SIGCHLD.

— Macro: int SIGCONT

You can send a SIGCONT signal to a process to make it continue. This signal is special—it always makes the process continue if it is stopped, before the signal is delivered. The default behavior is to do nothing else. You cannot block this signal. You can set a handler, but SIGCONT always makes the process continue regardless.

Most programs have no reason to handle SIGCONT; they simply resume execution without realizing they were ever stopped. You can use a handler for SIGCONT to make a program do something special when it is stopped and continued—for example, to reprint a prompt when it is suspended while waiting for input.

— Macro: int SIGSTOP

The SIGSTOP signal stops the process. It cannot be handled, ignored, or blocked.

— Macro: int SIGTSTP

The SIGTSTP signal is an interactive stop signal. Unlike SIGSTOP, this signal can be handled and ignored.

Your program should handle this signal if you have a special need to leave files or system tables in a secure state when a process is stopped. For example, programs that turn off echoing should handle SIGTSTP so they can turn echoing back on before stopping.

This signal is generated when the user types the SUSP character (normally C-z). For more information about terminal driver support, see Special Characters.

— Macro: int SIGTTIN

A process cannot read from the user's terminal while it is running as a background job. When any process in a background job tries to read from the terminal, all of the processes in the job are sent a SIGTTIN signal. The default action for this signal is to stop the process. For more information about how this interacts with the terminal driver, see Access to the Terminal.

— Macro: int SIGTTOU

This is similar to SIGTTIN, but is generated when a process in a background job attempts to write to the terminal or set its modes. Again, the default action is to stop the process. SIGTTOU is only generated for an attempt to write to the terminal if the TOSTOP output mode is set; see Output Modes.

While a process is stopped, no more signals can be delivered to it until it is continued, except SIGKILL signals and (obviously) SIGCONT signals. The signals are marked as pending, but not delivered until the process is continued. The SIGKILL signal always causes termination of the process and can't be blocked, handled or ignored. You can ignore SIGCONT, but it always causes the process to be continued anyway if it is stopped. Sending a SIGCONT signal to a process causes any pending stop signals for that process to be discarded. Likewise, any pending SIGCONT signals for a process are discarded when it receives a stop signal.

When a process in an orphaned process group (see Orphaned Process Groups) receives a SIGTSTP, SIGTTIN, or SIGTTOU signal and does not handle it, the process does not stop. Stopping the process would probably not be very useful, since there is no shell program that will notice it stop and allow the user to continue it. What happens instead depends on the operating system you are using. Some systems may do nothing; others may deliver another signal instead, such as SIGKILL or SIGHUP. In the GNU system, the process dies with SIGKILL; this avoids the problem of many stopped, orphaned processes lying around the system.

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24.2.6 Operation Error Signals

These signals are used to report various errors generated by an operation done by the program. They do not necessarily indicate a programming error in the program, but an error that prevents an operating system call from completing. The default action for all of them is to cause the process to terminate.

— Macro: int SIGPIPE

Broken pipe. If you use pipes or FIFOs, you have to design your application so that one process opens the pipe for reading before another starts writing. If the reading process never starts, or terminates unexpectedly, writing to the pipe or FIFO raises a SIGPIPE signal. If SIGPIPE is blocked, handled or ignored, the offending call fails with EPIPE instead.

Pipes and FIFO special files are discussed in more detail in Pipes and FIFOs.

Another cause of SIGPIPE is when you try to output to a socket that isn't connected. See Sending Data.

— Macro: int SIGLOST

Resource lost. This signal is generated when you have an advisory lock on an NFS file, and the NFS server reboots and forgets about your lock.

In the GNU system, SIGLOST is generated when any server program dies unexpectedly. It is usually fine to ignore the signal; whatever call was made to the server that died just returns an error.

— Macro: int SIGXCPU

CPU time limit exceeded. This signal is generated when the process exceeds its soft resource limit on CPU time. See Limits on Resources.

— Macro: int SIGXFSZ

File size limit exceeded. This signal is generated when the process attempts to extend a file so it exceeds the process's soft resource limit on file size. See Limits on Resources.

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24.2.7 Miscellaneous Signals

These signals are used for various other purposes. In general, they will not affect your program unless it explicitly uses them for something.

— Macro: int SIGUSR1
— Macro: int SIGUSR2

The SIGUSR1 and SIGUSR2 signals are set aside for you to use any way you want. They're useful for simple interprocess communication, if you write a signal handler for them in the program that receives the signal.

There is an example showing the use of SIGUSR1 and SIGUSR2 in Signaling Another Process.

The default action is to terminate the process.

— Macro: int SIGWINCH

Window size change. This is generated on some systems (including GNU) when the terminal driver's record of the number of rows and columns on the screen is changed. The default action is to ignore it.

If a program does full-screen display, it should handle SIGWINCH. When the signal arrives, it should fetch the new screen size and reformat its display accordingly.

— Macro: int SIGINFO

Information request. In 4.4 BSD and the GNU system, this signal is sent to all the processes in the foreground process group of the controlling terminal when the user types the STATUS character in canonical mode; see Signal Characters.

If the process is the leader of the process group, the default action is to print some status information about the system and what the process is doing. Otherwise the default is to do nothing.

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24.2.8 Signal Messages

We mentioned above that the shell prints a message describing the signal that terminated a child process. The clean way to print a message describing a signal is to use the functions strsignal and psignal. These functions use a signal number to specify which kind of signal to describe. The signal number may come from the termination status of a child process (see Process Completion) or it may come from a signal handler in the same process.

— Function: char * strsignal (int signum)

This function returns a pointer to a statically-allocated string containing a message describing the signal signum. You should not modify the contents of this string; and, since it can be rewritten on subsequent calls, you should save a copy of it if you need to reference it later.

This function is a GNU extension, declared in the header file string.h.

— Function: void psignal (int signum, const char *message)

This function prints a message describing the signal signum to the standard error output stream stderr; see Standard Streams.

If you call psignal with a message that is either a null pointer or an empty string, psignal just prints the message corresponding to signum, adding a trailing newline.

If you supply a non-null message argument, then psignal prefixes its output with this string. It adds a colon and a space character to separate the message from the string corresponding to signum.

This function is a BSD feature, declared in the header file signal.h.

There is also an array sys_siglist which contains the messages for the various signal codes. This array exists on BSD systems, unlike strsignal.

Next: , Previous: Standard Signals, Up: Signal Handling

24.3 Specifying Signal Actions

The simplest way to change the action for a signal is to use the signal function. You can specify a built-in action (such as to ignore the signal), or you can establish a handler.

The GNU library also implements the more versatile sigaction facility. This section describes both facilities and gives suggestions on which to use when.

Next: , Up: Signal Actions

24.3.1 Basic Signal Handling

The signal function provides a simple interface for establishing an action for a particular signal. The function and associated macros are declared in the header file signal.h.

— Data Type: sighandler_t

This is the type of signal handler functions. Signal handlers take one integer argument specifying the signal number, and have return type void. So, you should define handler functions like this: 

          void handler (int signum) { ... }

The name sighandler_t for this data type is a GNU extension.

— Function: sighandler_t signal (int signum, sighandler_t action)

The signal function establishes action as the action for the signal signum.

The first argument, signum, identifies the signal whose behavior you want to control, and should be a signal number. The proper way to specify a signal number is with one of the symbolic signal names (see Standard Signals)—don't use an explicit number, because the numerical code for a given kind of signal may vary from operating system to operating system.

The second argument, action, specifies the action to use for the signal signum. This can be one of the following:

SIG_DFL
SIG_DFL specifies the default action for the particular signal. The default actions for various kinds of signals are stated in Standard Signals.
SIG_IGN
SIG_IGN specifies that the signal should be ignored.

Your program generally should not ignore signals that represent serious events or that are normally used to request termination. You cannot ignore the SIGKILL or SIGSTOP signals at all. You can ignore program error signals like SIGSEGV, but ignoring the error won't enable the program to continue executing meaningfully. Ignoring user requests such as SIGINT, SIGQUIT, and SIGTSTP is unfriendly.

When you do not wish signals to be delivered during a certain part of the program, the thing to do is to block them, not ignore them. See Blocking Signals.

handler
Supply the address of a handler function in your program, to specify running this handler as the way to deliver the signal.

For more information about defining signal handler functions, see Defining Handlers.

If you set the action for a signal to SIG_IGN, or if you set it to SIG_DFL and the default action is to ignore that signal, then any pending signals of that type are discarded (even if they are blocked). Discarding the pending signals means that they will never be delivered, not even if you subsequently specify another action and unblock this kind of signal.

The signal function returns the action that was previously in effect for the specified signum. You can save this value and restore it later by calling signal again.

If signal can't honor the request, it returns SIG_ERR instead. The following errno error conditions are defined for this function:

EINVAL
You specified an invalid signum; or you tried to ignore or provide a handler for SIGKILL or SIGSTOP.

Compatibility Note: A problem encountered when working with the signal function is that it has different semantics on BSD and SVID systems. The difference is that on SVID systems the signal handler is deinstalled after signal delivery. On BSD systems the handler must be explicitly deinstalled. In the GNU C Library we use the BSD version by default. To use the SVID version you can either use the function sysv_signal (see below) or use the _XOPEN_SOURCE feature select macro (see Feature Test Macros). In general, use of these functions should be avoided because of compatibility problems. It is better to use sigaction if it is available since the results are much more reliable.

Here is a simple example of setting up a handler to delete temporary files when certain fatal signals happen: 

     #include <signal.h>
     
     void
     termination_handler (int signum)
     {
       struct temp_file *p;
     
       for (p = temp_file_list; p; p = p->next)
         unlink (p->name);
     }
     
     int
     main (void)
     {
       ...
       if (signal (SIGINT, termination_handler) == SIG_IGN)
         signal (SIGINT, SIG_IGN);
       if (signal (SIGHUP, termination_handler) == SIG_IGN)
         signal (SIGHUP, SIG_IGN);
       if (signal (SIGTERM, termination_handler) == SIG_IGN)
         signal (SIGTERM, SIG_IGN);
       ...
     }

Note that if a given signal was previously set to be ignored, this code avoids altering that setting. This is because non-job-control shells often ignore certain signals when starting children, and it is important for the children to respect this.

We do not handle SIGQUIT or the program error signals in this example because these are designed to provide information for debugging (a core dump), and the temporary files may give useful information.

— Function: sighandler_t sysv_signal (int signum, sighandler_t action)

The sysv_signal implements the behavior of the standard signal function as found on SVID systems. The difference to BSD systems is that the handler is deinstalled after a delivery of a signal.

Compatibility Note: As said above for signal, this function should be avoided when possible. sigaction is the preferred method.

— Function: sighandler_t ssignal (int signum, sighandler_t action)

The ssignal function does the same thing as signal; it is provided only for compatibility with SVID.

— Macro: sighandler_t SIG_ERR

The value of this macro is used as the return value from signal to indicate an error.

Next: , Previous: Basic Signal Handling, Up: Signal Actions

24.3.2 Advanced Signal Handling

The sigaction function has the same basic effect as signal: to specify how a signal should be handled by the process. However, sigaction offers more control, at the expense of more complexity. In particular, sigaction allows you to specify additional flags to control when the signal is generated and how the handler is invoked.

The sigaction function is declared in signal.h.

— Data Type: struct sigaction

Structures of type struct sigaction are used in the sigaction function to specify all the information about how to handle a particular signal. This structure contains at least the following members:

sighandler_t sa_handler
This is used in the same way as the action argument to the signal function. The value can be SIG_DFL, SIG_IGN, or a function pointer. See Basic Signal Handling.
sigset_t sa_mask
This specifies a set of signals to be blocked while the handler runs. Blocking is explained in Blocking for Handler. Note that the signal that was delivered is automatically blocked by default before its handler is started; this is true regardless of the value in sa_mask. If you want that signal not to be blocked within its handler, you must write code in the handler to unblock it.
int sa_flags
This specifies various flags which can affect the behavior of the signal. These are described in more detail in Flags for Sigaction.

— Function: int sigaction (int signum, const struct sigaction *restrict action, struct sigaction *restrict old-action)

The action argument is used to set up a new action for the signal signum, while the old-action argument is used to return information about the action previously associated with this symbol. (In other words, old-action has the same purpose as the signal function's return value—you can check to see what the old action in effect for the signal was, and restore it later if you want.)

Either action or old-action can be a null pointer. If old-action is a null pointer, this simply suppresses the return of information about the old action. If action is a null pointer, the action associated with the signal signum is unchanged; this allows you to inquire about how a signal is being handled without changing that handling.

The return value from sigaction is zero if it succeeds, and -1 on failure. The following errno error conditions are defined for this function:

EINVAL
The signum argument is not valid, or you are trying to trap or ignore SIGKILL or SIGSTOP.

Next: , Previous: Advanced Signal Handling, Up: Signal Actions

24.3.3 Interaction of signal and sigaction

It's possible to use both the signal and sigaction functions within a single program, but you have to be careful because they can interact in slightly strange ways.

The sigaction function specifies more information than the signal function, so the return value from signal cannot express the full range of sigaction possibilities. Therefore, if you use signal to save and later reestablish an action, it may not be able to reestablish properly a handler that was established with sigaction.

To avoid having problems as a result, always use sigaction to save and restore a handler if your program uses sigaction at all. Since sigaction is more general, it can properly save and reestablish any action, regardless of whether it was established originally with signal or sigaction.

On some systems if you establish an action with signal and then examine it with sigaction, the handler address that you get may not be the same as what you specified with signal. It may not even be suitable for use as an action argument with signal. But you can rely on using it as an argument to sigaction. This problem never happens on the GNU system.

So, you're better off using one or the other of the mechanisms consistently within a single program.

Portability Note: The basic signal function is a feature of ISO C, while sigaction is part of the POSIX.1 standard. If you are concerned about portability to non-POSIX systems, then you should use the signal function instead.

Next: , Previous: Signal and Sigaction, Up: Signal Actions

24.3.4 sigaction Function Example

In Basic Signal Handling, we gave an example of establishing a simple handler for termination signals using signal. Here is an equivalent example using sigaction: 

     #include <signal.h>
     
     void
     termination_handler (int signum)
     {
       struct temp_file *p;
     
       for (p = temp_file_list; p; p = p->next)
         unlink (p->name);
     }
     
     int
     main (void)
     {
       ...
       struct sigaction new_action, old_action;
     
       /* Set up the structure to specify the new action. */
       new_action.sa_handler = termination_handler;
       sigemptyset (&new_action.sa_mask);
       new_action.sa_flags = 0;
     
       sigaction (SIGINT, NULL, &old_action);
       if (old_action.sa_handler != SIG_IGN)
         sigaction (SIGINT, &new_action, NULL);
       sigaction (SIGHUP, NULL, &old_action);
       if (old_action.sa_handler != SIG_IGN)
         sigaction (SIGHUP, &new_action, NULL);
       sigaction (SIGTERM, NULL, &old_action);
       if (old_action.sa_handler != SIG_IGN)
         sigaction (SIGTERM, &new_action, NULL);
       ...
     }

The program just loads the new_action structure with the desired parameters and passes it in the sigaction call. The usage of sigemptyset is described later; see Blocking Signals.

As in the example using signal, we avoid handling signals previously set to be ignored. Here we can avoid altering the signal handler even momentarily, by using the feature of sigaction that lets us examine the current action without specifying a new one.

Here is another example. It retrieves information about the current action for SIGINT without changing that action. 

     struct sigaction query_action;
     
     if (sigaction (SIGINT, NULL, &query_action) < 0)
       /* sigaction returns -1 in case of error. */
     else if (query_action.sa_handler == SIG_DFL)
       /* SIGINT is handled in the default, fatal manner. */
     else if (query_action.sa_handler == SIG_IGN)
       /* SIGINT is ignored. */
     else
       /* A programmer-defined signal handler is in effect. */

Next: , Previous: Sigaction Function Example, Up: Signal Actions

24.3.5 Flags for sigaction

The sa_flags member of the sigaction structure is a catch-all for special features. Most of the time, SA_RESTART is a good value to use for this field.

The value of sa_flags is interpreted as a bit mask. Thus, you should choose the flags you want to set, or those flags together, and store the result in the sa_flags member of your sigaction structure.

Each signal number has its own set of flags. Each call to sigaction affects one particular signal number, and the flags that you specify apply only to that particular signal.

In the GNU C library, establishing a handler with signal sets all the flags to zero except for SA_RESTART, whose value depends on the settings you have made with siginterrupt. See Interrupted Primitives, to see what this is about.

These macros are defined in the header file signal.h.

— Macro: int SA_NOCLDSTOP

This flag is meaningful only for the SIGCHLD signal. When the flag is set, the system delivers the signal for a terminated child process but not for one that is stopped. By default, SIGCHLD is delivered for both terminated children and stopped children.

Setting this flag for a signal other than SIGCHLD has no effect.

— Macro: int SA_ONSTACK

If this flag is set for a particular signal number, the system uses the signal stack when delivering that kind of signal. See Signal Stack. If a signal with this flag arrives and you have not set a signal stack, the system terminates the program with SIGILL.

— Macro: int SA_RESTART

This flag controls what happens when a signal is delivered during certain primitives (such as open, read or write), and the signal handler returns normally. There are two alternatives: the library function can resume, or it can return failure with error code EINTR.

The choice is controlled by the SA_RESTART flag for the particular kind of signal that was delivered. If the flag is set, returning from a handler resumes the library function. If the flag is clear, returning from a handler makes the function fail. See Interrupted Primitives.

Previous: Flags for Sigaction, Up: Signal Actions

24.3.6 Initial Signal Actions

When a new process is created (see Creating a Process), it inherits handling of signals from its parent process. However, when you load a new process image using the exec function (see Executing a File), any signals that you've defined your own handlers for revert to their SIG_DFL handling. (If you think about it a little, this makes sense; the handler functions from the old program are specific to that program, and aren't even present in the address space of the new program image.) Of course, the new program can establish its own handlers.

When a program is run by a shell, the shell normally sets the initial actions for the child process to SIG_DFL or SIG_IGN, as appropriate. It's a good idea to check to make sure that the shell has not set up an initial action of SIG_IGN before you establish your own signal handlers.

Here is an example of how to establish a handler for SIGHUP, but not if SIGHUP is currently ignored: 

     ...
     struct sigaction temp;
     
     sigaction (SIGHUP, NULL, &temp);
     
     if (temp.sa_handler != SIG_IGN)
       {
         temp.sa_handler = handle_sighup;
         sigemptyset (&temp.sa_mask);
         sigaction (SIGHUP, &temp, NULL);
       }

Next: , Previous: Signal Actions, Up: Signal Handling

24.4 Defining Signal Handlers

This section describes how to write a signal handler function that can be established with the signal or sigaction functions.

A signal handler is just a function that you compile together with the rest of the program. Instead of directly invoking the function, you use signal or sigaction to tell the operating system to call it when a signal arrives. This is known as establishing the handler. See Signal Actions.

There are two basic strategies you can use in signal handler functions:

You need to take special care in writing handler functions because they can be called asynchronously. That is, a handler might be called at any point in the program, unpredictably. If two signals arrive during a very short interval, one handler can run within another. This section describes what your handler should do, and what you should avoid.

Next: , Up: Defining Handlers

24.4.1 Signal Handlers that Return

Handlers which return normally are usually used for signals such as SIGALRM and the I/O and interprocess communication signals. But a handler for SIGINT might also return normally after setting a flag that tells the program to exit at a convenient time.

It is not safe to return normally from the handler for a program error signal, because the behavior of the program when the handler function returns is not defined after a program error. See Program Error Signals.

Handlers that return normally must modify some global variable in order to have any effect. Typically, the variable is one that is examined periodically by the program during normal operation. Its data type should be sig_atomic_t for reasons described in Atomic Data Access.

Here is a simple example of such a program. It executes the body of the loop until it has noticed that a SIGALRM signal has arrived. This technique is useful because it allows the iteration in progress when the signal arrives to complete before the loop exits. 

     #include <signal.h>
     #include <stdio.h>
     #include <stdlib.h>
     
     /* This flag controls termination of the main loop. */
     volatile sig_atomic_t keep_going = 1;
     
     /* The signal handler just clears the flag and re-enables itself. */
     void
     catch_alarm (int sig)
     {
       keep_going = 0;
       signal (sig, catch_alarm);
     }
     
     void
     do_stuff (void)
     {
       puts ("Doing stuff while waiting for alarm....");
     }
     
     int
     main (void)
     {
       /* Establish a handler for SIGALRM signals. */
       signal (SIGALRM, catch_alarm);
     
       /* Set an alarm to go off in a little while. */
       alarm (2);
     
       /* Check the flag once in a while to see when to quit. */
       while (keep_going)
         do_stuff ();
     
       return EXIT_SUCCESS;
     }

Next: , Previous: Handler Returns, Up: Defining Handlers

24.4.2 Handlers That Terminate the Process

Handler functions that terminate the program are typically used to cause orderly cleanup or recovery from program error signals and interactive interrupts.

The cleanest way for a handler to terminate the process is to raise the same signal that ran the handler in the first place. Here is how to do this: 

     volatile sig_atomic_t fatal_error_in_progress = 0;
     
     void
     fatal_error_signal (int sig)
     {
       /* Since this handler is established for more than one kind of signal, 
          it might still get invoked recursively by delivery of some other kind
          of signal.  Use a static variable to keep track of that. */
       if (fatal_error_in_progress)
         raise (sig);
       fatal_error_in_progress = 1;
     
       /* Now do the clean up actions:
          - reset terminal modes
          - kill child processes
          - remove lock files */
       ...
     
       /* Now reraise the signal.  We reactivate the signal's
          default handling, which is to terminate the process.
          We could just call exit or abort,
          but reraising the signal sets the return status
          from the process correctly. */
       signal (sig, SIG_DFL);
       raise (sig);
     }

Next: , Previous: Termination in Handler, Up: Defining Handlers

24.4.3 Nonlocal Control Transfer in Handlers

You can do a nonlocal transfer of control out of a signal handler using the setjmp and longjmp facilities (see Non-Local Exits).

When the handler does a nonlocal control transfer, the part of the program that was running will not continue. If this part of the program was in the middle of updating an important data structure, the data structure will remain inconsistent. Since the program does not terminate, the inconsistency is likely to be noticed later on.

There are two ways to avoid this problem. One is to block the signal for the parts of the program that update important data structures. Blocking the signal delays its delivery until it is unblocked, once the critical updating is finished. See Blocking Signals.

The other way to re-initialize the crucial data structures in the signal handler, or make their values consistent.

Here is a rather schematic example showing the reinitialization of one global variable. 

     #include <signal.h>
     #include <setjmp.h>
     
     jmp_buf return_to_top_level;
     
     volatile sig_atomic_t waiting_for_input;
     
     void
     handle_sigint (int signum)
     {
       /* We may have been waiting for input when the signal arrived,
          but we are no longer waiting once we transfer control. */
       waiting_for_input = 0;
       longjmp (return_to_top_level, 1);
     }
     
     int
     main (void)
     {
       ...
       signal (SIGINT, sigint_handler);
       ...
       while (1) {
         prepare_for_command ();
         if (setjmp (return_to_top_level) == 0)
           read_and_execute_command ();
       }
     }
     
     /* Imagine this is a subroutine used by various commands. */
     char *
     read_data ()
     {
       if (input_from_terminal) {
         waiting_for_input = 1;
         ...
         waiting_for_input = 0;
       } else {
         ...
       }
     }

Next: , Previous: Longjmp in Handler, Up: Defining Handlers

24.4.4 Signals Arriving While a Handler Runs

What happens if another signal arrives while your signal handler function is running?

When the handler for a particular signal is invoked, that signal is automatically blocked until the handler returns. That means that if two signals of the same kind arrive close together, the second one will be held until the first has been handled. (The handler can explicitly unblock the signal using sigprocmask, if you want to allow more signals of this type to arrive; see Process Signal Mask.)

However, your handler can still be interrupted by delivery of another kind of signal. To avoid this, you can use the sa_mask member of the action structure passed to sigaction to explicitly specify which signals should be blocked while the signal handler runs. These signals are in addition to the signal for which the handler was invoked, and any other signals that are normally blocked by the process. See Blocking for Handler.

When the handler returns, the set of blocked signals is restored to the value it had before the handler ran. So using sigprocmask inside the handler only affects what signals can arrive during the execution of the handler itself, not what signals can arrive once the handler returns.

Portability Note: Always use sigaction to establish a handler for a signal that you expect to receive asynchronously, if you want your program to work properly on System V Unix. On this system, the handling of a signal whose handler was established with signal automatically sets the signal's action back to SIG_DFL, and the handler must re-establish itself each time it runs. This practice, while inconvenient, does work when signals cannot arrive in succession. However, if another signal can arrive right away, it may arrive before the handler can re-establish itself. Then the second signal would receive the default handling, which could terminate the process.

Next: , Previous: Signals in Handler, Up: Defining Handlers

24.4.5 Signals Close Together Merge into One

If multiple signals of the same type are delivered to your process before your signal handler has a chance to be invoked at all, the handler may only be invoked once, as if only a single signal had arrived. In effect, the signals merge into one. This situation can arise when the signal is blocked, or in a multiprocessing environment where the system is busy running some other processes while the signals are delivered. This means, for example, that you cannot reliably use a signal handler to count signals. The only distinction you can reliably make is whether at least one signal has arrived since a given time in the past.

Here is an example of a handler for SIGCHLD that compensates for the fact that the number of signals received may not equal the number of child processes that generate them. It assumes that the program keeps track of all the child processes with a chain of structures as follows: 

     struct process
     {
       struct process *next;
       /* The process ID of this child.  */
       int pid;
       /* The descriptor of the pipe or pseudo terminal
          on which output comes from this child.  */
       int input_descriptor;
       /* Nonzero if this process has stopped or terminated.  */
       sig_atomic_t have_status;
       /* The status of this child; 0 if running,
          otherwise a status value from waitpid.  */
       int status;
     };
     
     struct process *process_list;

This example also uses a flag to indicate whether signals have arrived since some time in the past—whenever the program last cleared it to zero. 

     /* Nonzero means some child's status has changed
        so look at process_list for the details.  */
     int process_status_change;

Here is the handler itself: 

     void
     sigchld_handler (int signo)
     {
       int old_errno = errno;
     
       while (1) {
         register int pid;
         int w;
         struct process *p;
     
         /* Keep asking for a status until we get a definitive result.  */
         do
           {
             errno = 0;
             pid = waitpid (WAIT_ANY, &w, WNOHANG | WUNTRACED);
           }
         while (pid <= 0 && errno == EINTR);
     
         if (pid <= 0) {
           /* A real failure means there are no more
              stopped or terminated child processes, so return.  */
           errno = old_errno;
           return;
         }
     
         /* Find the process that signaled us, and record its status.  */
     
         for (p = process_list; p; p = p->next)
           if (p->pid == pid) {
             p->status = w;
             /* Indicate that the status field
                has data to look at.  We do this only after storing it.  */
             p->have_status = 1;
     
             /* If process has terminated, stop waiting for its output.  */
             if (WIFSIGNALED (w) || WIFEXITED (w))
               if (p->input_descriptor)
                 FD_CLR (p->input_descriptor, &input_wait_mask);
     
             /* The program should check this flag from time to time
                to see if there is any news in process_list.  */
             ++process_status_change;
           }
     
         /* Loop around to handle all the processes
            that have something to tell us.  */
       }
     }

Here is the proper way to check the flag process_status_change: 

     if (process_status_change) {
       struct process *p;
       process_status_change = 0;
       for (p = process_list; p; p = p->next)
         if (p->have_status) {
           ... Examine p->status ...
         }
     }

It is vital to clear the flag before examining the list; otherwise, if a signal were delivered just before the clearing of the flag, and after the appropriate element of the process list had been checked, the status change would go unnoticed until the next signal arrived to set the flag again. You could, of course, avoid this problem by blocking the signal while scanning the list, but it is much more elegant to guarantee correctness by doing things in the right order.

The loop which checks process status avoids examining p->status until it sees that status has been validly stored. This is to make sure that the status cannot change in the middle of accessing it. Once p->have_status is set, it means that the child process is stopped or terminated, and in either case, it cannot stop or terminate again until the program has taken notice. See Atomic Usage, for more information about coping with interruptions during accesses of a variable.

Here is another way you can test whether the handler has run since the last time you checked. This technique uses a counter which is never changed outside the handler. Instead of clearing the count, the program remembers the previous value and sees whether it has changed since the previous check. The advantage of this method is that different parts of the program can check independently, each part checking whether there has been a signal since that part last checked. 

     sig_atomic_t process_status_change;
     
     sig_atomic_t last_process_status_change;
     
     ...
     {
       sig_atomic_t prev = last_process_status_change;
       last_process_status_change = process_status_change;
       if (last_process_status_change != prev) {
         struct process *p;
         for (p = process_list; p; p = p->next)
           if (p->have_status) {
             ... Examine p->status ...
           }
       }
     }

Next: , Previous: Merged Signals, Up: Defining Handlers

24.4.6 Signal Handling and Nonreentrant Functions

Handler functions usually don't do very much. The best practice is to write a handler that does nothing but set an external variable that the program checks regularly, and leave all serious work to the program. This is best because the handler can be called asynchronously, at unpredictable times—perhaps in the middle of a primitive function, or even between the beginning and the end of a C operator that requires multiple instructions. The data structures being manipulated might therefore be in an inconsistent state when the handler function is invoked. Even copying one int variable into another can take two instructions on most machines.

This means you have to be very careful about what you do in a signal handler.

A function can be non-reentrant if it uses memory that is not on the stack.

Previous: Nonreentrancy, Up: Defining Handlers

24.4.7 Atomic Data Access and Signal Handling

Whether the data in your application concerns atoms, or mere text, you have to be careful about the fact that access to a single datum is not necessarily atomic. This means that it can take more than one instruction to read or write a single object. In such cases, a signal handler might be invoked in the middle of reading or writing the object.

There are three ways you can cope with this problem. You can use data types that are always accessed atomically; you can carefully arrange that nothing untoward happens if an access is interrupted, or you can block all signals around any access that had better not be interrupted (see Blocking Signals).

Next: , Up: Atomic Data Access
24.4.7.1 Problems with Non-Atomic Access

Here is an example which shows what can happen if a signal handler runs in the middle of modifying a variable. (Interrupting the reading of a variable can also lead to paradoxical results, but here we only show writing.) 

     #include <signal.h>
     #include <stdio.h>
     
     volatile struct two_words { int a, b; } memory;
     
     void
     handler(int signum)
     {
        printf ("%d,%d\n", memory.a, memory.b);
        alarm (1);
     }
     
     int
     main (void)
     {
        static struct two_words zeros = { 0, 0 }, ones = { 1, 1 };
        signal (SIGALRM, handler);
        memory = zeros;
        alarm (1);
        while (1)
          {
            memory = zeros;
            memory = ones;
          }
     }

This program fills memory with zeros, ones, zeros, ones, alternating forever; meanwhile, once per second, the alarm signal handler prints the current contents. (Calling printf in the handler is safe in this program because it is certainly not being called outside the handler when the signal happens.)

Clearly, this program can print a pair of zeros or a pair of ones. But that's not all it can do! On most machines, it takes several instructions to store a new value in memory, and the value is stored one word at a time. If the signal is delivered in between these instructions, the handler might find that memory.a is zero and memory.b is one (or vice versa).

On some machines it may be possible to store a new value in memory with just one instruction that cannot be interrupted. On these machines, the handler will always print two zeros or two ones.

Next: , Previous: Non-atomic Example, Up: Atomic Data Access
24.4.7.2 Atomic Types

To avoid uncertainty about interrupting access to a variable, you can use a particular data type for which access is always atomic: sig_atomic_t. Reading and writing this data type is guaranteed to happen in a single instruction, so there's no way for a handler to run “in the middle” of an access.

The type sig_atomic_t is always an integer data type, but which one it is, and how many bits it contains, may vary from machine to machine.

— Data Type: sig_atomic_t

This is an integer data type. Objects of this type are always accessed atomically.

In practice, you can assume that int is atomic. You can also assume that pointer types are atomic; that is very convenient. Both of these assumptions are true on all of the machines that the GNU C library supports and on all POSIX systems we know of.

Previous: Atomic Types, Up: Atomic Data Access
24.4.7.3 Atomic Usage Patterns

Certain patterns of access avoid any problem even if an access is interrupted. For example, a flag which is set by the handler, and tested and cleared by the main program from time to time, is always safe even if access actually requires two instructions. To show that this is so, we must consider each access that could be interrupted, and show that there is no problem if it is interrupted.

An interrupt in the middle of testing the flag is safe because either it's recognized to be nonzero, in which case the precise value doesn't matter, or it will be seen to be nonzero the next time it's tested.

An interrupt in the middle of clearing the flag is no problem because either the value ends up zero, which is what happens if a signal comes in just before the flag is cleared, or the value ends up nonzero, and subsequent events occur as if the signal had come in just after the flag was cleared. As long as the code handles both of these cases properly, it can also handle a signal in the middle of clearing the flag. (This is an example of the sort of reasoning you need to do to figure out whether non-atomic usage is safe.)

Sometimes you can insure uninterrupted access to one object by protecting its use with another object, perhaps one whose type guarantees atomicity. See Merged Signals, for an example.

Next: , Previous: Defining Handlers, Up: Signal Handling

24.5 Primitives Interrupted by Signals

A signal can arrive and be handled while an I/O primitive such as open or read is waiting for an I/O device. If the signal handler returns, the system faces the question: what should happen next?

POSIX specifies one approach: make the primitive fail right away. The error code for this kind of failure is EINTR. This is flexible, but usually inconvenient. Typically, POSIX applications that use signal handlers must check for EINTR after each library function that can return it, in order to try the call again. Often programmers forget to check, which is a common source of error.

The GNU library provides a convenient way to retry a call after a temporary failure, with the macro TEMP_FAILURE_RETRY:

— Macro: TEMP_FAILURE_RETRY (expression)

This macro evaluates expression once, and examines its value as type long int. If the value equals -1, that indicates a failure and errno should be set to show what kind of failure. If it fails and reports error code EINTR, TEMP_FAILURE_RETRY evaluates it again, and over and over until the result is not a temporary failure.

The value returned by TEMP_FAILURE_RETRY is whatever value expression produced.

BSD avoids EINTR entirely and provides a more convenient approach: to restart the interrupted primitive, instead of making it fail. If you choose this approach, you need not be concerned with EINTR.

You can choose either approach with the GNU library. If you use sigaction to establish a signal handler, you can specify how that handler should behave. If you specify the SA_RESTART flag, return from that handler will resume a primitive; otherwise, return from that handler will cause EINTR. See Flags for Sigaction.

Another way to specify the choice is with the siginterrupt function. See BSD Handler.

When you don't specify with sigaction or siginterrupt what a particular handler should do, it uses a default choice. The default choice in the GNU library depends on the feature test macros you have defined. If you define _BSD_SOURCE or _GNU_SOURCE before calling signal, the default is to resume primitives; otherwise, the default is to make them fail with EINTR. (The library contains alternate versions of the signal function, and the feature test macros determine which one you really call.) See Feature Test Macros.

The description of each primitive affected by this issue lists EINTR among the error codes it can return.

There is one situation where resumption never happens no matter which choice you make: when a data-transfer function such as read or write is interrupted by a signal after transferring part of the data. In this case, the function returns the number of bytes already transferred, indicating partial success.

This might at first appear to cause unreliable behavior on record-oriented devices (including datagram sockets; see Datagrams), where splitting one read or write into two would read or write two records. Actually, there is no problem, because interruption after a partial transfer cannot happen on such devices; they always transfer an entire record in one burst, with no waiting once data transfer has started.

Next: , Previous: Interrupted Primitives, Up: Signal Handling

24.6 Generating Signals

Besides signals that are generated as a result of a hardware trap or interrupt, your program can explicitly send signals to itself or to another process.

Next: , Up: Generating Signals

24.6.1 Signaling Yourself

A process can send itself a signal with the raise function. This function is declared in signal.h.

— Function: int raise (int signum)

The raise function sends the signal signum to the calling process. It returns zero if successful and a nonzero value if it fails. About the only reason for failure would be if the value of signum is invalid.

— Function: int gsignal (int signum)

The gsignal function does the same thing as raise; it is provided only for compatibility with SVID.

One convenient use for raise is to reproduce the default behavior of a signal that you have trapped. For instance, suppose a user of your program types the SUSP character (usually C-z; see Special Characters) to send it an interactive stop signal (SIGTSTP), and you want to clean up some internal data buffers before stopping. You might set this up like this: 

     #include <signal.h>
     
     /* When a stop signal arrives, set the action back to the default
        and then resend the signal after doing cleanup actions. */
     
     void
     tstp_handler (int sig)
     {
       signal (SIGTSTP, SIG_DFL);
       /* Do cleanup actions here. */
       ...
       raise (SIGTSTP);
     }
     
     /* When the process is continued again, restore the signal handler. */
     
     void
     cont_handler (int sig)
     {
       signal (SIGCONT, cont_handler);
       signal (SIGTSTP, tstp_handler);
     }
     
     /* Enable both handlers during program initialization. */
     
     int
     main (void)
     {
       signal (SIGCONT, cont_handler);
       signal (SIGTSTP, tstp_handler);
       ...
     }

Portability note: raise was invented by the ISO C committee. Older systems may not support it, so using kill may be more portable. See Signaling Another Process.

Next: , Previous: Signaling Yourself, Up: Generating Signals

24.6.2 Signaling Another Process

The kill function can be used to send a signal to another process. In spite of its name, it can be used for a lot of things other than causing a process to terminate. Some examples of situations where you might want to send signals between processes are:

This section assumes that you know a little bit about how processes work. For more information on this subject, see Processes.

The kill function is declared in signal.h.

— Function: int kill (pid_t pid, int signum)

The kill function sends the signal signum to the process or process group specified by pid. Besides the signals listed in Standard Signals, signum can also have a value of zero to check the validity of the pid.

The pid specifies the process or process group to receive the signal:

pid > 0
The process whose identifier is pid.
pid == 0
All processes in the same process group as the sender.
pid < -1
The process group whose identifier is −pid.
pid == -1
If the process is privileged, send the signal to all processes except for some special system processes. Otherwise, send the signal to all processes with the same effective user ID.

A process can send a signal to itself with a call like kill (getpid(), signum). If kill is used by a process to send a signal to itself, and the signal is not blocked, then kill delivers at least one signal (which might be some other pending unblocked signal instead of the signal signum) to that process before it returns.

The return value from kill is zero if the signal can be sent successfully. Otherwise, no signal is sent, and a value of -1 is returned. If pid specifies sending a signal to several processes, kill succeeds if it can send the signal to at least one of them. There's no way you can tell which of the processes got the signal or whether all of them did.

The following errno error conditions are defined for this function:

EINVAL
The signum argument is an invalid or unsupported number.
EPERM
You do not have the privilege to send a signal to the process or any of the processes in the process group named by pid.
ESRCH
The pid argument does not refer to an existing process or group.

— Function: int killpg (int pgid, int signum)

This is similar to kill, but sends signal signum to the process group pgid. This function is provided for compatibility with BSD; using kill to do this is more portable.

As a simple example of kill, the call kill (getpid (), sig) has the same effect as raise (sig).

Next: , Previous: Signaling Another Process, Up: Generating Signals

24.6.3 Permission for using kill

There are restrictions that prevent you from using kill to send signals to any random process. These are intended to prevent antisocial behavior such as arbitrarily killing off processes belonging to another user. In typical use, kill is used to pass signals between parent, child, and sibling processes, and in these situations you normally do have permission to send signals. The only common exception is when you run a setuid program in a child process; if the program changes its real UID as well as its effective UID, you may not have permission to send a signal. The su program does this.

Whether a process has permission to send a signal to another process is determined by the user IDs of the two processes. This concept is discussed in detail in Process Persona.

Generally, for a process to be able to send a signal to another process, either the sending process must belong to a privileged user (like `root'), or the real or effective user ID of the sending process must match the real or effective user ID of the receiving process. If the receiving process has changed its effective user ID from the set-user-ID mode bit on its process image file, then the owner of the process image file is used in place of its current effective user ID. In some implementations, a parent process might be able to send signals to a child process even if the user ID's don't match, and other implementations might enforce other restrictions.

The SIGCONT signal is a special case. It can be sent if the sender is part of the same session as the receiver, regardless of user IDs.

Previous: Permission for kill, Up: Generating Signals

24.6.4 Using kill for Communication

Here is a longer example showing how signals can be used for interprocess communication. This is what the SIGUSR1 and SIGUSR2 signals are provided for. Since these signals are fatal by default, the process that is supposed to receive them must trap them through signal or sigaction.

In this example, a parent process forks a child process and then waits for the child to complete its initialization. The child process tells the parent when it is ready by sending it a SIGUSR1 signal, using the kill function. 

     #include <signal.h>
     #include <stdio.h>
     #include <sys/types.h>
     #include <unistd.h>
     
     /* When a SIGUSR1 signal arrives, set this variable. */
     volatile sig_atomic_t usr_interrupt = 0;
     
     void
     synch_signal (int sig)
     {
       usr_interrupt = 1;
     }
     
     /* The child process executes this function. */
     void
     child_function (void)
     {
       /* Perform initialization. */
       printf ("I'm here!!!  My pid is %d.\n", (int) getpid ());
     
       /* Let parent know you're done. */
       kill (getppid (), SIGUSR1);
     
       /* Continue with execution. */
       puts ("Bye, now....");
       exit (0);
     }
     
     int
     main (void)
     {
       struct sigaction usr_action;
       sigset_t block_mask;
       pid_t child_id;
     
       /* Establish the signal handler. */
       sigfillset (&block_mask);
       usr_action.sa_handler = synch_signal;
       usr_action.sa_mask = block_mask;
       usr_action.sa_flags = 0;
       sigaction (SIGUSR1, &usr_action, NULL);
     
       /* Create the child process. */
       child_id = fork ();
       if (child_id == 0)
         child_function ();          /* Does not return. */
     
       /* Busy wait for the child to send a signal. */
       while (!usr_interrupt)
         ;
     
       /* Now continue execution. */
       puts ("That's all, folks!");
     
       return 0;
     }

This example uses a busy wait, which is bad, because it wastes CPU cycles that other programs could otherwise use. It is better to ask the system to wait until the signal arrives. See the example in Waiting for a Signal.

Next: , Previous: Generating Signals, Up: Signal Handling

24.7 Blocking Signals

Blocking a signal means telling the operating system to hold it and deliver it later. Generally, a program does not block signals indefinitely—it might as well ignore them by setting their actions to SIG_IGN. But it is useful to block signals briefly, to prevent them from interrupting sensitive operations. For instance:

Next: , Up: Blocking Signals

24.7.1 Why Blocking Signals is Useful

Temporary blocking of signals with sigprocmask gives you a way to prevent interrupts during critical parts of your code. If signals arrive in that part of the program, they are delivered later, after you unblock them.

One example where this is useful is for sharing data between a signal handler and the rest of the program. If the type of the data is not sig_atomic_t (see Atomic Data Access), then the signal handler could run when the rest of the program has only half finished reading or writing the data. This would lead to confusing consequences.

To make the program reliable, you can prevent the signal handler from running while the rest of the program is examining or modifying that data—by blocking the appropriate signal around the parts of the program that touch the data.

Blocking signals is also necessary when you want to perform a certain action only if a signal has not arrived. Suppose that the handler for the signal sets a flag of type sig_atomic_t; you would like to test the flag and perform the action if the flag is not set. This is unreliable. Suppose the signal is delivered immediately after you test the flag, but before the consequent action: then the program will perform the action even though the signal has arrived.

The only way to test reliably for whether a signal has yet arrived is to test while the signal is blocked.

Next: , Previous: Why Block, Up: Blocking Signals

24.7.2 Signal Sets

All of the signal blocking functions use a data structure called a signal set to specify what signals are affected. Thus, every activity involves two stages: creating the signal set, and then passing it as an argument to a library function. These facilities are declared in the header file signal.h.

— Data Type: sigset_t

The sigset_t data type is used to represent a signal set. Internally, it may be implemented as either an integer or structure type.

For portability, use only the functions described in this section to initialize, change, and retrieve information from sigset_t objects—don't try to manipulate them directly.

There are two ways to initialize a signal set. You can initially specify it to be empty with sigemptyset and then add specified signals individually. Or you can specify it to be full with sigfillset and then delete specified signals individually.

You must always initialize the signal set with one of these two functions before using it in any other way. Don't try to set all the signals explicitly because the sigset_t object might include some other information (like a version field) that needs to be initialized as well. (In addition, it's not wise to put into your program an assumption that the system has no signals aside from the ones you know about.)

— Function: int sigemptyset (sigset_t *set)

This function initializes the signal set set to exclude all of the defined signals. It always returns 0.

— Function: int sigfillset (sigset_t *set)

This function initializes the signal set set to include all of the defined signals. Again, the return value is 0.

— Function: int sigaddset (sigset_t *set, int signum)

This function adds the signal signum to the signal set set. All sigaddset does is modify set; it does not block or unblock any signals.

The return value is 0 on success and -1 on failure. The following errno error condition is defined for this function:

EINVAL
The signum argument doesn't specify a valid signal.

— Function: int sigdelset (sigset_t *set, int signum)

This function removes the signal signum from the signal set set. All sigdelset does is modify set; it does not block or unblock any signals. The return value and error conditions are the same as for sigaddset.

Finally, there is a function to test what signals are in a signal set:

— Function: int sigismember (const sigset_t *set, int signum)

The sigismember function tests whether the signal signum is a member of the signal set set. It returns 1 if the signal is in the set, 0 if not, and -1 if there is an error.

The following errno error condition is defined for this function:

EINVAL
The signum argument doesn't specify a valid signal.

Next: , Previous: Signal Sets, Up: Blocking Signals

24.7.3 Process Signal Mask

The collection of signals that are currently blocked is called the signal mask. Each process has its own signal mask. When you create a new process (see Creating a Process), it inherits its parent's mask. You can block or unblock signals with total flexibility by modifying the signal mask.

The prototype for the sigprocmask function is in signal.h. Note that you must not use sigprocmask in multi-threaded processes, because each thread has its own signal mask and there is no single process signal mask. According to POSIX, the behavior of sigprocmask in a multi-threaded process is “unspecified”. Instead, use pthread_sigmask.

— Function: int sigprocmask (int how, const sigset_t *restrict set, sigset_t *restrict oldset)

The sigprocmask function is used to examine or change the calling process's signal mask. The how argument determines how the signal mask is changed, and must be one of the following values:

SIG_BLOCK
Block the signals in set—add them to the existing mask. In other words, the new mask is the union of the existing mask and set.


SIG_UNBLOCK
Unblock the signals in set—remove them from the existing mask.


SIG_SETMASK
Use set for the mask; ignore the previous value of the mask.

The last argument, oldset, is used to return information about the old process signal mask. If you just want to change the mask without looking at it, pass a null pointer as the oldset argument. Similarly, if you want to know what's in the mask without changing it, pass a null pointer for set (in this case the how argument is not significant). The oldset argument is often used to remember the previous signal mask in order to restore it later. (Since the signal mask is inherited over fork and exec calls, you can't predict what its contents are when your program starts running.)

If invoking sigprocmask causes any pending signals to be unblocked, at least one of those signals is delivered to the process before sigprocmask returns. The order in which pending signals are delivered is not specified, but you can control the order explicitly by making multiple sigprocmask calls to unblock various signals one at a time.

The sigprocmask function returns 0 if successful, and -1 to indicate an error. The following errno error conditions are defined for this function:

EINVAL
The how argument is invalid.

You can't block the SIGKILL and SIGSTOP signals, but if the signal set includes these, sigprocmask just ignores them instead of returning an error status.

Remember, too, that blocking program error signals such as SIGFPE leads to undesirable results for signals generated by an actual program error (as opposed to signals sent with raise or kill). This is because your program may be too broken to be able to continue executing to a point where the signal is unblocked again. See Program Error Signals.

Next: , Previous: Process Signal Mask, Up: Blocking Signals

24.7.4 Blocking to Test for Delivery of a Signal

Now for a simple example. Suppose you establish a handler for SIGALRM signals that sets a flag whenever a signal arrives, and your main program checks this flag from time to time and then resets it. You can prevent additional SIGALRM signals from arriving in the meantime by wrapping the critical part of the code with calls to sigprocmask, like this: 

     /* This variable is set by the SIGALRM signal handler. */
     volatile sig_atomic_t flag = 0;
     
     int
     main (void)
     {
       sigset_t block_alarm;
     
       ...
     
       /* Initialize the signal mask. */
       sigemptyset (&block_alarm);
       sigaddset (&block_alarm, SIGALRM);
     
       while (1)
         {
           /* Check if a signal has arrived; if so, reset the flag. */
           sigprocmask (SIG_BLOCK, &block_alarm, NULL);
           if (flag)
             {
               actions-if-not-arrived
               flag = 0;
             }
           sigprocmask (SIG_UNBLOCK, &block_alarm, NULL);
     
           ...
         }
     }

Next: , Previous: Testing for Delivery, Up: Blocking Signals

24.7.5 Blocking Signals for a Handler

When a signal handler is invoked, you usually want it to be able to finish without being interrupted by another signal. From the moment the handler starts until the moment it finishes, you must block signals that might confuse it or corrupt its data.

When a handler function is invoked on a signal, that signal is automatically blocked (in addition to any other signals that are already in the process's signal mask) during the time the handler is running. If you set up a handler for SIGTSTP, for instance, then the arrival of that signal forces further SIGTSTP signals to wait during the execution of the handler.

However, by default, other kinds of signals are not blocked; they can arrive during handler execution.

The reliable way to block other kinds of signals during the execution of the handler is to use the sa_mask member of the sigaction structure.

Here is an example: 

     #include <signal.h>
     #include <stddef.h>
     
     void catch_stop ();
     
     void
     install_handler (void)
     {
       struct sigaction setup_action;
       sigset_t block_mask;
     
       sigemptyset (&block_mask);
       /* Block other terminal-generated signals while handler runs. */
       sigaddset (&block_mask, SIGINT);
       sigaddset (&block_mask, SIGQUIT);
       setup_action.sa_handler = catch_stop;
       setup_action.sa_mask = block_mask;
       setup_action.sa_flags = 0;
       sigaction (SIGTSTP, &setup_action, NULL);
     }

This is more reliable than blocking the other signals explicitly in the code for the handler. If you block signals explicitly in the handler, you can't avoid at least a short interval at the beginning of the handler where they are not yet blocked.

You cannot remove signals from the process's current mask using this mechanism. However, you can make calls to sigprocmask within your handler to block or unblock signals as you wish.

In any case, when the handler returns, the system restores the mask that was in place before the handler was entered. If any signals that become unblocked by this restoration are pending, the process will receive those signals immediately, before returning to the code that was interrupted.

Next: , Previous: Blocking for Handler, Up: Blocking Signals

24.7.6 Checking for Pending Signals

You can find out which signals are pending at any time by calling sigpending. This function is declared in signal.h.

— Function: int sigpending (sigset_t *set)

The sigpending function stores information about pending signals in set. If there is a pending signal that is blocked from delivery, then that signal is a member of the returned set. (You can test whether a particular signal is a member of this set using sigismember; see Signal Sets.)

The return value is 0 if successful, and -1 on failure.

Testing whether a signal is pending is not often useful. Testing when that signal is not blocked is almost certainly bad design.

Here is an example. 

     #include <signal.h>
     #include <stddef.h>
     
     sigset_t base_mask, waiting_mask;
     
     sigemptyset (&base_mask);
     sigaddset (&base_mask, SIGINT);
     sigaddset (&base_mask, SIGTSTP);
     
     /* Block user interrupts while doing other processing. */
     sigprocmask (SIG_SETMASK, &base_mask, NULL);
     ...
     
     /* After a while, check to see whether any signals are pending. */
     sigpending (&waiting_mask);
     if (sigismember (&waiting_mask, SIGINT)) {
       /* User has tried to kill the process. */
     }
     else if (sigismember (&waiting_mask, SIGTSTP)) {
       /* User has tried to stop the process. */
     }

Remember that if there is a particular signal pending for your process, additional signals of that same type that arrive in the meantime might be discarded. For example, if a SIGINT signal is pending when another SIGINT signal arrives, your program will probably only see one of them when you unblock this signal.

Portability Note: The sigpending function is new in POSIX.1. Older systems have no equivalent facility.

Previous: Checking for Pending Signals, Up: Blocking Signals

24.7.7 Remembering a Signal to Act On Later

Instead of blocking a signal using the library facilities, you can get almost the same results by making the handler set a flag to be tested later, when you “unblock”. Here is an example: 

     /* If this flag is nonzero, don't handle the signal right away. */
     volatile sig_atomic_t signal_pending;
     
     /* This is nonzero if a signal arrived and was not handled. */
     volatile sig_atomic_t defer_signal;
     
     void
     handler (int signum)
     {
       if (defer_signal)
         signal_pending = signum;
       else
         ... /* ``Really'' handle the signal. */
     }
     
     ...
     
     void
     update_mumble (int frob)
     {
       /* Prevent signals from having immediate effect. */
       defer_signal++;
       /* Now update mumble, without worrying about interruption. */
       mumble.a = 1;
       mumble.b = hack ();
       mumble.c = frob;
       /* We have updated mumble.  Handle any signal that came in. */
       defer_signal--;
       if (defer_signal == 0 && signal_pending != 0)
         raise (signal_pending);
     }

Note how the particular signal that arrives is stored in signal_pending. That way, we can handle several types of inconvenient signals with the same mechanism.

We increment and decrement defer_signal so that nested critical sections will work properly; thus, if update_mumble were called with signal_pending already nonzero, signals would be deferred not only within update_mumble, but also within the caller. This is also why we do not check signal_pending if defer_signal is still nonzero.

The incrementing and decrementing of defer_signal each require more than one instruction; it is possible for a signal to happen in the middle. But that does not cause any problem. If the signal happens early enough to see the value from before the increment or decrement, that is equivalent to a signal which came before the beginning of the increment or decrement, which is a case that works properly.

It is absolutely vital to decrement defer_signal before testing signal_pending, because this avoids a subtle bug. If we did these things in the other order, like this, 

       if (defer_signal == 1 && signal_pending != 0)
         raise (signal_pending);
       defer_signal--;

then a signal arriving in between the if statement and the decrement would be effectively “lost” for an indefinite amount of time. The handler would merely set defer_signal, but the program having already tested this variable, it would not test the variable again.

Bugs like these are called timing errors. They are especially bad because they happen only rarely and are nearly impossible to reproduce. You can't expect to find them with a debugger as you would find a reproducible bug. So it is worth being especially careful to avoid them.

(You would not be tempted to write the code in this order, given the use of defer_signal as a counter which must be tested along with signal_pending. After all, testing for zero is cleaner than testing for one. But if you did not use defer_signal as a counter, and gave it values of zero and one only, then either order might seem equally simple. This is a further advantage of using a counter for defer_signal: it will reduce the chance you will write the code in the wrong order and create a subtle bug.)

Next: , Previous: Blocking Signals, Up: Signal Handling

24.8 Waiting for a Signal

If your program is driven by external events, or uses signals for synchronization, then when it has nothing to do it should probably wait until a signal arrives.

Next: , Up: Waiting for a Signal

24.8.1 Using pause

The simple way to wait until a signal arrives is to call pause. Please read about its disadvantages, in the following section, before you use it.

— Function: int pause ()

The pause function suspends program execution until a signal arrives whose action is either to execute a handler function, or to terminate the process.

If the signal causes a handler function to be executed, then pause returns. This is considered an unsuccessful return (since “successful” behavior would be to suspend the program forever), so the return value is -1. Even if you specify that other primitives should resume when a system handler returns (see Interrupted Primitives), this has no effect on pause; it always fails when a signal is handled.

The following errno error conditions are defined for this function:

EINTR
The function was interrupted by delivery of a signal.

If the signal causes program termination, pause doesn't return (obviously).

This function is a cancellation point in multithreaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time pause is called. If the thread gets cancelled these resources stay allocated until the program ends. To avoid this calls to pause should be protected using cancellation handlers.

The pause function is declared in unistd.h.

Next: , Previous: Using Pause, Up: Waiting for a Signal

24.8.2 Problems with pause

The simplicity of pause can conceal serious timing errors that can make a program hang mysteriously.

It is safe to use pause if the real work of your program is done by the signal handlers themselves, and the “main program” does nothing but call pause. Each time a signal is delivered, the handler will do the next batch of work that is to be done, and then return, so that the main loop of the program can call pause again.

You can't safely use pause to wait until one more signal arrives, and then resume real work. Even if you arrange for the signal handler to cooperate by setting a flag, you still can't use pause reliably. Here is an example of this problem: 

     /* usr_interrupt is set by the signal handler.  */
     if (!usr_interrupt)
       pause ();
     
     /* Do work once the signal arrives.  */
     ...

This has a bug: the signal could arrive after the variable usr_interrupt is checked, but before the call to pause. If no further signals arrive, the process would never wake up again.

You can put an upper limit on the excess waiting by using sleep in a loop, instead of using pause. (See Sleeping, for more about sleep.) Here is what this looks like: 

     /* usr_interrupt is set by the signal handler.
     while (!usr_interrupt)
       sleep (1);
     
     /* Do work once the signal arrives.  */
     ...

For some purposes, that is good enough. But with a little more complexity, you can wait reliably until a particular signal handler is run, using sigsuspend.

Previous: Pause Problems, Up: Waiting for a Signal

24.8.3 Using sigsuspend

The clean and reliable way to wait for a signal to arrive is to block it and then use sigsuspend. By using sigsuspend in a loop, you can wait for certain kinds of signals, while letting other kinds of signals be handled by their handlers.

— Function: int sigsuspend (const sigset_t *set)

This function replaces the process's signal mask with set and then suspends the process until a signal is delivered whose action is either to terminate the process or invoke a signal handling function. In other words, the program is effectively suspended until one of the signals that is not a member of set arrives.

If the process is woken up by delivery of a signal that invokes a handler function, and the handler function returns, then sigsuspend also returns.

The mask remains set only as long as sigsuspend is waiting. The function sigsuspend always restores the previous signal mask when it returns.

The return value and error conditions are the same as for pause.

With sigsuspend, you can replace the pause or sleep loop in the previous section with something completely reliable: 

     sigset_t mask, oldmask;
     
     ...
     
     /* Set up the mask of signals to temporarily block. */
     sigemptyset (&mask);
     sigaddset (&mask, SIGUSR1);
     
     ...
     
     /* Wait for a signal to arrive. */
     sigprocmask (SIG_BLOCK, &mask, &oldmask);
     while (!usr_interrupt)
       sigsuspend (&oldmask);
     sigprocmask (SIG_UNBLOCK, &mask, NULL);

This last piece of code is a little tricky. The key point to remember here is that when sigsuspend returns, it resets the process's signal mask to the original value, the value from before the call to sigsuspend—in this case, the SIGUSR1 signal is once again blocked. The second call to sigprocmask is necessary to explicitly unblock this signal.

One other point: you may be wondering why the while loop is necessary at all, since the program is apparently only waiting for one SIGUSR1 signal. The answer is that the mask passed to sigsuspend permits the process to be woken up by the delivery of other kinds of signals, as well—for example, job control signals. If the process is woken up by a signal that doesn't set usr_interrupt, it just suspends itself again until the “right” kind of signal eventually arrives.

This technique takes a few more lines of preparation, but that is needed just once for each kind of wait criterion you want to use. The code that actually waits is just four lines.

Next: , Previous: Waiting for a Signal, Up: Signal Handling

24.9 Using a Separate Signal Stack

A signal stack is a special area of memory to be used as the execution stack during signal handlers. It should be fairly large, to avoid any danger that it will overflow in turn; the macro SIGSTKSZ is defined to a canonical size for signal stacks. You can use malloc to allocate the space for the stack. Then call sigaltstack or sigstack to tell the system to use that space for the signal stack.

You don't need to write signal handlers differently in order to use a signal stack. Switching from one stack to the other happens automatically. (Some non-GNU debuggers on some machines may get confused if you examine a stack trace while a handler that uses the signal stack is running.)

There are two interfaces for telling the system to use a separate signal stack. sigstack is the older interface, which comes from 4.2 BSD. sigaltstack is the newer interface, and comes from 4.4 BSD. The sigaltstack interface has the advantage that it does not require your program to know which direction the stack grows, which depends on the specific machine and operating system.

— Data Type: stack_t

This structure describes a signal stack. It contains the following members:

void *ss_sp
This points to the base of the signal stack.
size_t ss_size
This is the size (in bytes) of the signal stack which `ss_sp' points to. You should set this to however much space you allocated for the stack.

There are two macros defined in signal.h that you should use in calculating this size:

SIGSTKSZ
This is the canonical size for a signal stack. It is judged to be sufficient for normal uses.
MINSIGSTKSZ
This is the amount of signal stack space the operating system needs just to implement signal delivery. The size of a signal stack must be greater than this.

For most cases, just using SIGSTKSZ for ss_size is sufficient. But if you know how much stack space your program's signal handlers will need, you may want to use a different size. In this case, you should allocate MINSIGSTKSZ additional bytes for the signal stack and increase ss_size accordingly.


int ss_flags
This field contains the bitwise or of these flags:
SS_DISABLE
This tells the system that it should not use the signal stack.
SS_ONSTACK
This is set by the system, and indicates that the signal stack is currently in use. If this bit is not set, then signals will be delivered on the normal user stack.

— Function: int sigaltstack (const stack_t *restrict stack, stack_t *restrict oldstack)

The sigaltstack function specifies an alternate stack for use during signal handling. When a signal is received by the process and its action indicates that the signal stack is used, the system arranges a switch to the currently installed signal stack while the handler for that signal is executed.

If oldstack is not a null pointer, information about the currently installed signal stack is returned in the location it points to. If stack is not a null pointer, then this is installed as the new stack for use by signal handlers.

The return value is 0 on success and -1 on failure. If sigaltstack fails, it sets errno to one of these values:

EINVAL
You tried to disable a stack that was in fact currently in use.
ENOMEM
The size of the alternate stack was too small. It must be greater than MINSIGSTKSZ.

Here is the older sigstack interface. You should use sigaltstack instead on systems that have it.

— Data Type: struct sigstack

This structure describes a signal stack. It contains the following members:

void *ss_sp
This is the stack pointer. If the stack grows downwards on your machine, this should point to the top of the area you allocated. If the stack grows upwards, it should point to the bottom.
int ss_onstack
This field is true if the process is currently using this stack.

— Function: int sigstack (const struct sigstack *stack, struct sigstack *oldstack)

The sigstack function specifies an alternate stack for use during signal handling. When a signal is received by the process and its action indicates that the signal stack is used, the system arranges a switch to the currently installed signal stack while the handler for that signal is executed.

If oldstack is not a null pointer, information about the currently installed signal stack is returned in the location it points to. If stack is not a null pointer, then this is installed as the new stack for use by signal handlers.

The return value is 0 on success and -1 on failure.

Previous: Signal Stack, Up: Signal Handling

24.10 BSD Signal Handling

This section describes alternative signal handling functions derived from BSD Unix. These facilities were an advance, in their time; today, they are mostly obsolete, and supported mainly for compatibility with BSD Unix.

There are many similarities between the BSD and POSIX signal handling facilities, because the POSIX facilities were inspired by the BSD facilities. Besides having different names for all the functions to avoid conflicts, the main differences between the two are:

The BSD facilities are declared in signal.h.

Next: , Up: BSD Signal Handling

24.10.1 BSD Function to Establish a Handler

— Data Type: struct sigvec

This data type is the BSD equivalent of struct sigaction (see Advanced Signal Handling); it is used to specify signal actions to the sigvec function. It contains the following members:

sighandler_t sv_handler
This is the handler function.
int sv_mask
This is the mask of additional signals to be blocked while the handler function is being called.
int sv_flags
This is a bit mask used to specify various flags which affect the behavior of the signal. You can also refer to this field as sv_onstack.

These symbolic constants can be used to provide values for the sv_flags field of a sigvec structure. This field is a bit mask value, so you bitwise-OR the flags of interest to you together.

— Macro: int SV_ONSTACK

If this bit is set in the sv_flags field of a sigvec structure, it means to use the signal stack when delivering the signal.

— Macro: int SV_INTERRUPT

If this bit is set in the sv_flags field of a sigvec structure, it means that system calls interrupted by this kind of signal should not be restarted if the handler returns; instead, the system calls should return with a EINTR error status. See Interrupted Primitives.

— Macro: int SV_RESETHAND

If this bit is set in the sv_flags field of a sigvec structure, it means to reset the action for the signal back to SIG_DFL when the signal is received.

— Function: int sigvec (int signum, const struct sigvec *action,struct sigvec *old-action)

This function is the equivalent of sigaction (see Advanced Signal Handling); it installs the action action for the signal signum, returning information about the previous action in effect for that signal in old-action.

— Function: int siginterrupt (int signum, int failflag)

This function specifies which approach to use when certain primitives are interrupted by handling signal signum. If failflag is false, signal signum restarts primitives. If failflag is true, handling signum causes these primitives to fail with error code EINTR. See Interrupted Primitives.

Previous: BSD Handler, Up: BSD Signal Handling

24.10.2 BSD Functions for Blocking Signals

— Macro: int sigmask (int signum)

This macro returns a signal mask that has the bit for signal signum set. You can bitwise-OR the results of several calls to sigmask together to specify more than one signal. For example, 

          (sigmask (SIGTSTP) | sigmask (SIGSTOP)
           | sigmask (SIGTTIN) | sigmask (SIGTTOU))

specifies a mask that includes all the job-control stop signals.

— Function: int sigblock (int mask)

This function is equivalent to sigprocmask (see Process Signal Mask) with a how argument of SIG_BLOCK: it adds the signals specified by mask to the calling process's set of blocked signals. The return value is the previous set of blocked signals.

— Function: int sigsetmask (int mask)

This function equivalent to sigprocmask (see Process Signal Mask) with a how argument of SIG_SETMASK: it sets the calling process's signal mask to mask. The return value is the previous set of blocked signals.

— Function: int sigpause (int mask)

This function is the equivalent of sigsuspend (see Waiting for a Signal): it sets the calling process's signal mask to mask, and waits for a signal to arrive. On return the previous set of blocked signals is restored.

Next: , Previous: Signal Handling, Up: Top

25 The Basic Program/System Interface

Processes are the primitive units for allocation of system resources. Each process has its own address space and (usually) one thread of control. A process executes a program; you can have multiple processes executing the same program, but each process has its own copy of the program within its own address space and executes it independently of the other copies. Though it may have multiple threads of control within the same program and a program may be composed of multiple logically separate modules, a process always executes exactly one program.

Note that we are using a specific definition of “program” for the purposes of this manual, which corresponds to a common definition in the context of Unix system. In popular usage, “program” enjoys a much broader definition; it can refer for example to a system's kernel, an editor macro, a complex package of software, or a discrete section of code executing within a process.

Writing the program is what this manual is all about. This chapter explains the most basic interface between your program and the system that runs, or calls, it. This includes passing of parameters (arguments and environment) from the system, requesting basic services from the system, and telling the system the program is done.

A program starts another program with the exec family of system calls. This chapter looks at program startup from the execee's point of view. To see the event from the execor's point of view, See Executing a File.

Next: , Up: Program Basics

25.1 Program Arguments

The system starts a C program by calling the function main. It is up to you to write a function named main—otherwise, you won't even be able to link your program without errors.

In ISO C you can define main either to take no arguments, or to take two arguments that represent the command line arguments to the program, like this: 

     int main (int argc, char *argv[])

The command line arguments are the whitespace-separated tokens given in the shell command used to invoke the program; thus, in `cat foo bar', the arguments are `foo' and `bar'. The only way a program can look at its command line arguments is via the arguments of main. If main doesn't take arguments, then you cannot get at the command line.

The value of the argc argument is the number of command line arguments. The argv argument is a vector of C strings; its elements are the individual command line argument strings. The file name of the program being run is also included in the vector as the first element; the value of argc counts this element. A null pointer always follows the last element: argv[argc] is this null pointer.

For the command `cat foo bar', argc is 3 and argv has three elements, "cat", "foo" and "bar".

In Unix systems you can define main a third way, using three arguments: 

     int main (int argc, char *argv[], char *envp[])

The first two arguments are just the same. The third argument envp gives the program's environment; it is the same as the value of environ. See Environment Variables. POSIX.1 does not allow this three-argument form, so to be portable it is best to write main to take two arguments, and use the value of environ.

Next: , Up: Program Arguments

25.1.1 Program Argument Syntax Conventions

POSIX recommends these conventions for command line arguments. getopt (see Getopt) and argp_parse (see Argp) make it easy to implement them.

GNU adds long options to these conventions. Long options consist of `--' followed by a name made of alphanumeric characters and dashes. Option names are typically one to three words long, with hyphens to separate words. Users can abbreviate the option names as long as the abbreviations are unique.

To specify an argument for a long option, write `--name=value'. This syntax enables a long option to accept an argument that is itself optional.

Eventually, the GNU system will provide completion for long option names in the shell.

Previous: Argument Syntax, Up: Program Arguments

25.1.2 Parsing Program Arguments

If the syntax for the command line arguments to your program is simple enough, you can simply pick the arguments off from argv by hand. But unless your program takes a fixed number of arguments, or all of the arguments are interpreted in the same way (as file names, for example), you are usually better off using getopt (see Getopt) or argp_parse (see Argp) to do the parsing.

getopt is more standard (the short-option only version of it is a part of the POSIX standard), but using argp_parse is often easier, both for very simple and very complex option structures, because it does more of the dirty work for you.

Next: , Up: Parsing Program Arguments

25.2 Parsing program options using getopt

The getopt and getopt_long functions automate some of the chore involved in parsing typical unix command line options.

Next: , Up: Getopt

25.2.1 Using the getopt function

Here are the details about how to call the getopt function. To use this facility, your program must include the header file unistd.h.

— Variable: int opterr

If the value of this variable is nonzero, then getopt prints an error message to the standard error stream if it encounters an unknown option character or an option with a missing required argument. This is the default behavior. If you set this variable to zero, getopt does not print any messages, but it still returns the character ? to indicate an error.

— Variable: int optopt

When getopt encounters an unknown option character or an option with a missing required argument, it stores that option character in this variable. You can use this for providing your own diagnostic messages.

— Variable: int optind

This variable is set by getopt to the index of the next element of the argv array to be processed. Once getopt has found all of the option arguments, you can use this variable to determine where the remaining non-option arguments begin. The initial value of this variable is 1.

— Variable: char * optarg

This variable is set by getopt to point at the value of the option argument, for those options that accept arguments.

— Function: int getopt (int argc, char **argv, const char *options)

The getopt function gets the next option argument from the argument list specified by the argv and argc arguments. Normally these values come directly from the arguments received by main.

The options argument is a string that specifies the option characters that are valid for this program. An option character in this string can be followed by a colon (`:') to indicate that it takes a required argument. If an option character is followed by two colons (`::'), its argument is optional; this is a GNU extension.

getopt has three ways to deal with options that follow non-options argv elements. The special argument `--' forces in all cases the end of option scanning.

The getopt function returns the option character for the next command line option. When no more option arguments are available, it returns -1. There may still be more non-option arguments; you must compare the external variable optind against the argc parameter to check this.

If the option has an argument, getopt returns the argument by storing it in the variable optarg. You don't ordinarily need to copy the optarg string, since it is a pointer into the original argv array, not into a static area that might be overwritten.

If getopt finds an option character in argv that was not included in options, or a missing option argument, it returns `?' and sets the external variable optopt to the actual option character. If the first character of options is a colon (`:'), then getopt returns `:' instead of `?' to indicate a missing option argument. In addition, if the external variable opterr is nonzero (which is the default), getopt prints an error message.

Next: , Previous: Using Getopt, Up: Getopt

25.2.2 Example of Parsing Arguments with getopt

Here is an example showing how getopt is typically used. The key points to notice are: 

     #include <ctype.h>
     #include <stdio.h>
     #include <stdlib.h>
     #include <unistd.h>
     
     int
     main (int argc, char **argv)
     {
       int aflag = 0;
       int bflag = 0;
       char *cvalue = NULL;
       int index;
       int c;
     
       opterr = 0;
     
       while ((c = getopt (argc, argv, "abc:")) != -1)
         switch (c)
           {
           case 'a':
             aflag = 1;
             break;
           case 'b':
             bflag = 1;
             break;
           case 'c':
             cvalue = optarg;
             break;
           case '?':
             if (optopt == 'c')
               fprintf (stderr, "Option -%c requires an argument.\n", optopt);
             else if (isprint (optopt))
               fprintf (stderr, "Unknown option `-%c'.\n", optopt);
             else
               fprintf (stderr,
                        "Unknown option character `\\x%x'.\n",
                        optopt);
             return 1;
           default:
             abort ();
           }
     
       printf ("aflag = %d, bflag = %d, cvalue = %s\n",
               aflag, bflag, cvalue);
     
       for (index = optind; index < argc; index++)
         printf ("Non-option argument %s\n", argv[index]);
       return 0;
     }

Here are some examples showing what this program prints with different combinations of arguments: 

     % testopt
     aflag = 0, bflag = 0, cvalue = (null)
     
     % testopt -a -b
     aflag = 1, bflag = 1, cvalue = (null)
     
     % testopt -ab
     aflag = 1, bflag = 1, cvalue = (null)
     
     % testopt -c foo
     aflag = 0, bflag = 0, cvalue = foo
     
     % testopt -cfoo
     aflag = 0, bflag = 0, cvalue = foo
     
     % testopt arg1
     aflag = 0, bflag = 0, cvalue = (null)
     Non-option argument arg1
     
     % testopt -a arg1
     aflag = 1, bflag = 0, cvalue = (null)
     Non-option argument arg1
     
     % testopt -c foo arg1
     aflag = 0, bflag = 0, cvalue = foo
     Non-option argument arg1
     
     % testopt -a -- -b
     aflag = 1, bflag = 0, cvalue = (null)
     Non-option argument -b
     
     % testopt -a -
     aflag = 1, bflag = 0, cvalue = (null)
     Non-option argument -

Next: , Previous: Example of Getopt, Up: Getopt

25.2.3 Parsing Long Options with getopt_long

To accept GNU-style long options as well as single-character options, use getopt_long instead of getopt. This function is declared in getopt.h, not unistd.h. You should make every program accept long options if it uses any options, for this takes little extra work and helps beginners remember how to use the program.

— Data Type: struct option

This structure describes a single long option name for the sake of getopt_long. The argument longopts must be an array of these structures, one for each long option. Terminate the array with an element containing all zeros.

The struct option structure has these fields:

const char *name
This field is the name of the option. It is a string.
int has_arg
This field says whether the option takes an argument. It is an integer, and there are three legitimate values: no_argument, required_argument and optional_argument.
int *flag
int val
These fields control how to report or act on the option when it occurs.

If flag is a null pointer, then the val is a value which identifies this option. Often these values are chosen to uniquely identify particular long options.

If flag is not a null pointer, it should be the address of an int variable which is the flag for this option. The value in val is the value to store in the flag to indicate that the option was seen.

— Function: int getopt_long (int argc, char *const *argv, const char *shortopts, const struct option *longopts, int *indexptr)

Decode options from the vector argv (whose length is argc). The argument shortopts describes the short options to accept, just as it does in getopt. The argument longopts describes the long options to accept (see above).

When getopt_long encounters a short option, it does the same thing that getopt would do: it returns the character code for the option, and stores the options argument (if it has one) in optarg.

When getopt_long encounters a long option, it takes actions based on the flag and val fields of the definition of that option.

If flag is a null pointer, then getopt_long returns the contents of val to indicate which option it found. You should arrange distinct values in the val field for options with different meanings, so you can decode these values after getopt_long returns. If the long option is equivalent to a short option, you can use the short option's character code in val.

If flag is not a null pointer, that means this option should just set a flag in the program. The flag is a variable of type int that you define. Put the address of the flag in the flag field. Put in the val field the value you would like this option to store in the flag. In this case, getopt_long returns 0.

For any long option, getopt_long tells you the index in the array longopts of the options definition, by storing it into *indexptr. You can get the name of the option with longopts[*indexptr].name. So you can distinguish among long options either by the values in their val fields or by their indices. You can also distinguish in this way among long options that set flags.

When a long option has an argument, getopt_long puts the argument value in the variable optarg before returning. When the option has no argument, the value in optarg is a null pointer. This is how you can tell whether an optional argument was supplied.

When getopt_long has no more options to handle, it returns -1, and leaves in the variable optind the index in argv of the next remaining argument.

Since long option names were used before before the getopt_long options was invented there are program interfaces which require programs to recognize options like `-option value' instead of `--option value'. To enable these programs to use the GNU getopt functionality there is one more function available.

— Function: int getopt_long_only (int argc, char *const *argv, const char *shortopts, const struct option *longopts, int *indexptr)

The getopt_long_only function is equivalent to the getopt_long function but it allows to specify the user of the application to pass long options with only `-' instead of `--'. The `--' prefix is still recognized but instead of looking through the short options if a `-' is seen it is first tried whether this parameter names a long option. If not, it is parsed as a short option.

Assuming getopt_long_only is used starting an application with 

            app -foo

the getopt_long_only will first look for a long option named `foo'. If this is not found, the short options `f', `o', and again `o' are recognized.

Previous: Getopt Long Options, Up: Getopt

25.2.4 Example of Parsing Long Options with getopt_long

     #include <stdio.h>
     #include <stdlib.h>
     #include <getopt.h>
     
     /* Flag set by `--verbose'. */
     static int verbose_flag;
     
     int
     main (argc, argv)
          int argc;
          char **argv;
     {
       int c;
     
       while (1)
         {
           static struct option long_options[] =
             {
               /* These options set a flag. */
               {"verbose", no_argument,       &verbose_flag, 1},
               {"brief",   no_argument,       &verbose_flag, 0},
               /* These options don't set a flag.
                  We distinguish them by their indices. */
               {"add",     no_argument,       0, 'a'},
               {"append",  no_argument,       0, 'b'},
               {"delete",  required_argument, 0, 'd'},
               {"create",  required_argument, 0, 'c'},
               {"file",    required_argument, 0, 'f'},
               {0, 0, 0, 0}
             };
           /* getopt_long stores the option index here. */
           int option_index = 0;
     
           c = getopt_long (argc, argv, "abc:d:f:",
                            long_options, &option_index);
     
           /* Detect the end of the options. */
           if (c == -1)
             break;
     
           switch (c)
             {
             case 0:
               /* If this option set a flag, do nothing else now. */
               if (long_options[option_index].flag != 0)
                 break;
               printf ("option %s", long_options[option_index].name);
               if (optarg)
                 printf (" with arg %s", optarg);
               printf ("\n");
               break;
     
             case 'a':
               puts ("option -a\n");
               break;
     
             case 'b':
               puts ("option -b\n");
               break;
     
             case 'c':
               printf ("option -c with value `%s'\n", optarg);
               break;
     
             case 'd':
               printf ("option -d with value `%s'\n", optarg);
               break;
     
             case 'f':
               printf ("option -f with value `%s'\n", optarg);
               break;
     
             case '?':
               /* getopt_long already printed an error message. */
               break;
     
             default:
               abort ();
             }
         }
     
       /* Instead of reporting `--verbose'
          and `--brief' as they are encountered,
          we report the final status resulting from them. */
       if (verbose_flag)
         puts ("verbose flag is set");
     
       /* Print any remaining command line arguments (not options). */
       if (optind < argc)
         {
           printf ("non-option ARGV-elements: ");
           while (optind < argc)
             printf ("%s ", argv[optind++]);
           putchar ('\n');
         }
     
       exit (0);
     }

Next: , Previous: Getopt, Up: Parsing Program Arguments

25.3 Parsing Program Options with Argp

Argp is an interface for parsing unix-style argument vectors. See Program Arguments.

Argp provides features unavailable in the more commonly used getopt interface. These features include automatically producing output in response to the `--help' and `--version' options, as described in the GNU coding standards. Using argp makes it less likely that programmers will neglect to implement these additional options or keep them up to date.

Argp also provides the ability to merge several independently defined option parsers into one, mediating conflicts between them and making the result appear seamless. A library can export an argp option parser that user programs might employ in conjunction with their own option parsers, resulting in less work for the user programs. Some programs may use only argument parsers exported by libraries, thereby achieving consistent and efficient option-parsing for abstractions implemented by the libraries.

The header file <argp.h> should be included to use argp.

25.3.1 The argp_parse Function

The main interface to argp is the argp_parse function. In many cases, calling argp_parse is the only argument-parsing code needed in main. See Program Arguments.

— Function: error_t argp_parse (const struct argp *argp, int argc, char **argv, unsigned flags, int *arg_index, void *input)

The argp_parse function parses the arguments in argv, of length argc, using the argp parser argp. See Argp Parsers.

A value of zero is the same as a struct argpcontaining all zeros. flags is a set of flag bits that modify the parsing behavior. See Argp Flags. input is passed through to the argp parser argp, and has meaning defined by argp. A typical usage is to pass a pointer to a structure which is used for specifying parameters to the parser and passing back the results.

Unless the ARGP_NO_EXIT or ARGP_NO_HELP flags are included in flags, calling argp_parse may result in the program exiting. This behavior is true if an error is detected, or when an unknown option is encountered. See Program Termination.

If arg_index is non-null, the index of the first unparsed option in argv is returned as a value.

The return value is zero for successful parsing, or an error code (see Error Codes) if an error is detected. Different argp parsers may return arbitrary error codes, but the standard error codes are: ENOMEM if a memory allocation error occurred, or EINVAL if an unknown option or option argument is encountered.

Next: , Up: Argp

25.3.2 Argp Global Variables

These variables make it easy for user programs to implement the `--version' option and provide a bug-reporting address in the `--help' output. These are implemented in argp by default.

— Variable: const char * argp_program_version

If defined or set by the user program to a non-zero value, then a `--version' option is added when parsing with argp_parse, which will print the `--version' string followed by a newline and exit. The exception to this is if the ARGP_NO_EXIT flag is used.

— Variable: const char * argp_program_bug_address

If defined or set by the user program to a non-zero value, argp_program_bug_address should point to a string that will be printed at the end of the standard output for the `--help' option, embedded in a sentence that says `Report bugs to address.'.

— Variable: argp_program_version_hook

If defined or set by the user program to a non-zero value, a `--version' option is added when parsing with arg_parse, which prints the program version and exits with a status of zero. This is not the case if the ARGP_NO_HELP flag is used. If the ARGP_NO_EXIT flag is set, the exit behavior of the program is suppressed or modified, as when the argp parser is going to be used by other programs.

It should point to a function with this type of signature: 

          void print-version (FILE *stream, struct argp_state *state)

See Argp Parsing State, for an explanation of state.

This variable takes precedence over argp_program_version, and is useful if a program has version information not easily expressed in a simple string.

— Variable: error_t argp_err_exit_status

This is the exit status used when argp exits due to a parsing error. If not defined or set by the user program, this defaults to: EX_USAGE from <sysexits.h>.

Next: , Previous: Argp Global Variables, Up: Argp

25.3.3 Specifying Argp Parsers

The first argument to the argp_parse function is a pointer to a struct argp, which is known as an argp parser:

— Data Type: struct argp

This structure specifies how to parse a given set of options and arguments, perhaps in conjunction with other argp parsers. It has the following fields:

const struct argp_option *options
A pointer to a vector of argp_option structures specifying which options this argp parser understands; it may be zero if there are no options at all. See Argp Option Vectors.
argp_parser_t parser
A pointer to a function that defines actions for this parser; it is called for each option parsed, and at other well-defined points in the parsing process. A value of zero is the same as a pointer to a function that always returns ARGP_ERR_UNKNOWN. See Argp Parser Functions.
const char *args_doc
If non-zero, a string describing what non-option arguments are called by this parser. This is only used to print the `Usage:' message. If it contains newlines, the strings separated by them are considered alternative usage patterns and printed on separate lines. Lines after the first are prefixed by ` or: ' instead of `Usage:'.
const char *doc
If non-zero, a string containing extra text to be printed before and after the options in a long help message, with the two sections separated by a vertical tab ('\v', '\013') character. By convention, the documentation before the options is just a short string explaining what the program does. Documentation printed after the options describe behavior in more detail.
const struct argp_child *children
A pointer to a vector of argp_children structures. This pointer specifies which additional argp parsers should be combined with this one. See Argp Children.
char *(*help_filter)(int key, const char *text, void *input)
If non-zero, a pointer to a function that filters the output of help messages. See Argp Help Filtering.
const char *argp_domain
If non-zero, the strings used in the argp library are translated using the domain described by this string. If zero, the current default domain is used.

Of the above group, options, parser, args_doc, and the doc fields are usually all that are needed. If an argp parser is defined as an initialized C variable, only the fields used need be specified in the initializer. The rest will default to zero due to the way C structure initialization works. This design is exploited in most argp structures; the most-used fields are grouped near the beginning, the unused fields left unspecified.

Next: , Previous: Argp Parsers, Up: Argp Parsers

25.3.4 Specifying Options in an Argp Parser

The options field in a struct argp points to a vector of struct argp_option structures, each of which specifies an option that the argp parser supports. Multiple entries may be used for a single option provided it has multiple names. This should be terminated by an entry with zero in all fields. Note that when using an initialized C array for options, writing { 0 } is enough to achieve this.

— Data Type: struct argp_option

This structure specifies a single option that an argp parser understands, as well as how to parse and document that option. It has the following fields:

const char *name
The long name for this option, corresponding to the long option `--name'; this field may be zero if this option only has a short name. To specify multiple names for an option, additional entries may follow this one, with the OPTION_ALIAS flag set. See Argp Option Flags.
int key
The integer key provided by the current option to the option parser. If key has a value that is a printable ascii character (i.e., isascii (key) is true), it also specifies a short option `-char', where char is the ascii character with the code key.
const char *arg
If non-zero, this is the name of an argument associated with this option, which must be provided (e.g., with the `--name=value' or `-char value' syntaxes), unless the OPTION_ARG_OPTIONAL flag (see Argp Option Flags) is set, in which case it may be provided.
int flags
Flags associated with this option, some of which are referred to above. See Argp Option Flags.
const char *doc
A documentation string for this option, for printing in help messages.

If both the name and key fields are zero, this string will be printed tabbed left from the normal option column, making it useful as a group header. This will be the first thing printed in its group. In this usage, it's conventional to end the string with a `:' character.

int group
Group identity for this option.

In a long help message, options are sorted alphabetically within each group, and the groups presented in the order 0, 1, 2, ..., n, −m, ..., −2, −1.

Every entry in an options array with this field 0 will inherit the group number of the previous entry, or zero if it's the first one. If it's a group header with name and key fields both zero, the previous entry + 1 is the default. Automagic options such as `--help' are put into group −1.

Note that because of C structure initialization rules, this field often need not be specified, because 0 is the correct value.

Up: Argp Option Vectors
25.3.4.1 Flags for Argp Options

The following flags may be or'd together in the flags field of a struct argp_option. These flags control various aspects of how that option is parsed or displayed in help messages:

OPTION_ARG_OPTIONAL
The argument associated with this option is optional.
OPTION_HIDDEN
This option isn't displayed in any help messages.
OPTION_ALIAS
This option is an alias for the closest previous non-alias option. This means that it will be displayed in the same help entry, and will inherit fields other than name and key from the option being aliased.
OPTION_DOC
This option isn't actually an option and should be ignored by the actual option parser. It is an arbitrary section of documentation that should be displayed in much the same manner as the options. This is known as a documentation option.

If this flag is set, then the option name field is displayed unmodified (e.g., no `--' prefix is added) at the left-margin where a short option would normally be displayed, and this documentation string is left in it's usual place. For purposes of sorting, any leading whitespace and punctuation is ignored, unless the first non-whitespace character is `-'. This entry is displayed after all options, after OPTION_DOC entries with a leading `-', in the same group.

OPTION_NO_USAGE
This option shouldn't be included in `long' usage messages, but should still be included in other help messages. This is intended for options that are completely documented in an argp's args_doc field. See Argp Parsers. Including this option in the generic usage list would be redundant, and should be avoided.

For instance, if args_doc is "FOO BAR\n-x BLAH", and the `-x' option's purpose is to distinguish these two cases, `-x' should probably be marked OPTION_NO_USAGE.

Next: , Previous: Argp Option Vectors, Up: Argp Parsers

25.3.5 Argp Parser Functions

The function pointed to by the parser field in a struct argp (see Argp Parsers) defines what actions take place in response to each option or argument parsed. It is also used as a hook, allowing a parser to perform tasks at certain other points during parsing.

Argp parser functions have the following type signature: 

     error_t parser (int key, char *arg, struct argp_state *state)

where the arguments are as follows:

key
For each option that is parsed, parser is called with a value of key from that option's key field in the option vector. See Argp Option Vectors. parser is also called at other times with special reserved keys, such as ARGP_KEY_ARG for non-option arguments. See Argp Special Keys.
arg
If key is an option, arg is its given value. This defaults to zero if no value is specified. Only options that have a non-zero arg field can ever have a value. These must always have a value unless the OPTION_ARG_OPTIONAL flag is specified. If the input being parsed specifies a value for an option that doesn't allow one, an error results before parser ever gets called.

If key is ARGP_KEY_ARG, arg is a non-option argument. Other special keys always have a zero arg.

state
state points to a struct argp_state, containing useful information about the current parsing state for use by parser. See Argp Parsing State.

When parser is called, it should perform whatever action is appropriate for key, and return 0 for success, ARGP_ERR_UNKNOWN if the value of key is not handled by this parser function, or a unix error code if a real error occurred. See Error Codes.

— Macro: int ARGP_ERR_UNKNOWN

Argp parser functions should return ARGP_ERR_UNKNOWN for any key value they do not recognize, or for non-option arguments (key == ARGP_KEY_ARG) that they are not equipped to handle.

A typical parser function uses a switch statement on key: 

     error_t
     parse_opt (int key, char *arg, struct argp_state *state)
     {
       switch (key)
         {
         case option_key:
           action
           break;
         ...
         default:
           return ARGP_ERR_UNKNOWN;
         }
       return 0;
     }

Next: , Up: Argp Parser Functions
25.3.5.1 Special Keys for Argp Parser Functions

In addition to key values corresponding to user options, the key argument to argp parser functions may have a number of other special values. In the following example arg and state refer to parser function arguments. See Argp Parser Functions.

ARGP_KEY_ARG
This is not an option at all, but rather a command line argument, whose value is pointed to by arg.

When there are multiple parser functions in play due to argp parsers being combined, it's impossible to know which one will handle a specific argument. Each is called until one returns 0 or an error other than ARGP_ERR_UNKNOWN; if an argument is not handled, argp_parse immediately returns success, without parsing any more arguments.

Once a parser function returns success for this key, that fact is recorded, and the ARGP_KEY_NO_ARGS case won't be used. However, if while processing the argument a parser function decrements the next field of its state argument, the option won't be considered processed; this is to allow you to actually modify the argument, perhaps into an option, and have it processed again.

ARGP_KEY_ARGS
If a parser function returns ARGP_ERR_UNKNOWN for ARGP_KEY_ARG, it is immediately called again with the key ARGP_KEY_ARGS, which has a similar meaning, but is slightly more convenient for consuming all remaining arguments. arg is 0, and the tail of the argument vector may be found at state->argv + state->next. If success is returned for this key, and state->next is unchanged, all remaining arguments are considered to have been consumed. Otherwise, the amount by which state->next has been adjusted indicates how many were used. Here's an example that uses both, for different args: 
          ...
          case ARGP_KEY_ARG:
            if (state->arg_num == 0)
              /* First argument */
              first_arg = arg;
            else
              /* Let the next case parse it.  */
              return ARGP_KEY_UNKNOWN;
            break;
          case ARGP_KEY_ARGS:
            remaining_args = state->argv + state->next;
            num_remaining_args = state->argc - state->next;
            break;

ARGP_KEY_END
This indicates that there are no more command line arguments. Parser functions are called in a different order, children first. This allows each parser to clean up its state for the parent.
ARGP_KEY_NO_ARGS
Because it's common to do some special processing if there aren't any non-option args, parser functions are called with this key if they didn't successfully process any non-option arguments. This is called just before ARGP_KEY_END, where more general validity checks on previously parsed arguments take place.
ARGP_KEY_INIT
This is passed in before any parsing is done. Afterwards, the values of each element of the child_input field of state, if any, are copied to each child's state to be the initial value of the input when their parsers are called.
ARGP_KEY_SUCCESS
Passed in when parsing has successfully been completed, even if arguments remain.
ARGP_KEY_ERROR
Passed in if an error has occurred and parsing is terminated. In this case a call with a key of ARGP_KEY_SUCCESS is never made.
ARGP_KEY_FINI
The final key ever seen by any parser, even after ARGP_KEY_SUCCESS and ARGP_KEY_ERROR. Any resources allocated by ARGP_KEY_INIT may be freed here. At times, certain resources allocated are to be returned to the caller after a successful parse. In that case, those particular resources can be freed in the ARGP_KEY_ERROR case.

In all cases, ARGP_KEY_INIT is the first key seen by parser functions, and ARGP_KEY_FINI the last, unless an error was returned by the parser for ARGP_KEY_INIT. Other keys can occur in one the following orders. opt refers to an arbitrary option key:

opt... ARGP_KEY_NO_ARGS ARGP_KEY_END ARGP_KEY_SUCCESS
The arguments being parsed did not contain any non-option arguments.
( opt | ARGP_KEY_ARG )... ARGP_KEY_END ARGP_KEY_SUCCESS
All non-option arguments were successfully handled by a parser function. There may be multiple parser functions if multiple argp parsers were combined.
( opt | ARGP_KEY_ARG )... ARGP_KEY_SUCCESS
Some non-option argument went unrecognized.

This occurs when every parser function returns ARGP_KEY_UNKNOWN for an argument, in which case parsing stops at that argument if arg_index is a null pointer. Otherwise an error occurs.

In all cases, if a non-null value for arg_index gets passed to argp_parse, the index of the first unparsed command-line argument is passed back in that value.

If an error occurs and is either detected by argp or because a parser function returned an error value, each parser is called with ARGP_KEY_ERROR. No further calls are made, except the final call with ARGP_KEY_FINI.

Previous: Argp Parsing State, Up: Argp Parser Functions
25.3.5.2 Functions For Use in Argp Parsers

Argp provides a number of functions available to the user of argp (see Argp Parser Functions), mostly for producing error messages. These take as their first argument the state argument to the parser function. See Argp Parsing State.

— Function: void argp_usage (const struct argp_state *state)

Outputs the standard usage message for the argp parser referred to by state to state->err_stream and terminate the program with exit (argp_err_exit_status). See Argp Global Variables.

— Function: void argp_error (const struct argp_state *state, const char *fmt, ...)

Prints the printf format string fmt and following args, preceded by the program name and `:', and followed by a `Try ... --help' message, and terminates the program with an exit status of argp_err_exit_status. See Argp Global Variables.

— Function: void argp_failure (const struct argp_state *state, int status, int errnum, const char *fmt, ...)

Similar to the standard gnu error-reporting function error, this prints the program name and `:', the printf format string fmt, and the appropriate following args. If it is non-zero, the standard unix error text for errnum is printed. If status is non-zero, it terminates the program with that value as its exit status.

The difference between argp_failure and argp_error is that argp_error is for parsing errors, whereas argp_failure is for other problems that occur during parsing but don't reflect a syntactic problem with the input, such as illegal values for options, bad phase of the moon, etc.

— Function: void argp_state_help (const struct argp_state *state, FILE *stream, unsigned flags)

Outputs a help message for the argp parser referred to by state, to stream. The flags argument determines what sort of help message is produced. See Argp Help Flags.

Error output is sent to state->err_stream, and the program name printed is state->name.

The output or program termination behavior of these functions may be suppressed if the ARGP_NO_EXIT or ARGP_NO_ERRS flags are passed to argp_parse. See Argp Flags.

This behavior is useful if an argp parser is exported for use by other programs (e.g., by a library), and may be used in a context where it is not desirable to terminate the program in response to parsing errors. In argp parsers intended for such general use, and for the case where the program doesn't terminate, calls to any of these functions should be followed by code that returns the appropriate error code: 

     if (bad argument syntax)
       {
          argp_usage (state);
          return EINVAL;
       }

If a parser function will only be used when ARGP_NO_EXIT is not set, the return may be omitted.

Next: , Previous: Argp Special Keys, Up: Argp Parser Functions
25.3.5.3 Argp Parsing State

The third argument to argp parser functions (see Argp Parser Functions) is a pointer to a struct argp_state, which contains information about the state of the option parsing.

— Data Type: struct argp_state

This structure has the following fields, which may be modified as noted:

const struct argp *const root_argp
The top level argp parser being parsed. Note that this is often not the same struct argp passed into argp_parse by the invoking program. See Argp. It is an internal argp parser that contains options implemented by argp_parse itself, such as `--help'.
int argc
char **argv
The argument vector being parsed. This may be modified.
int next
The index in argv of the next argument to be parsed. This may be modified.

One way to consume all remaining arguments in the input is to set state->next = state->argc, perhaps after recording the value of the next field to find the consumed arguments. The current option can be re-parsed immediately by decrementing this field, then modifying state->argv[state->next] to reflect the option that should be reexamined.

unsigned flags
The flags supplied to argp_parse. These may be modified, although some flags may only take effect when argp_parse is first invoked. See Argp Flags.
unsigned arg_num
While calling a parsing function with the key argument ARGP_KEY_ARG, this represents the number of the current arg, starting at 0. It is incremented after each ARGP_KEY_ARG call returns. At all other times, this is the number of ARGP_KEY_ARG arguments that have been processed.
int quoted
If non-zero, the index in argv of the first argument following a special `--' argument. This prevents anything that follows from being interpreted as an option. It is only set after argument parsing has proceeded past this point.
void *input
An arbitrary pointer passed in from the caller of argp_parse, in the input argument.
void **child_inputs
These are values that will be passed to child parsers. This vector will be the same length as the number of children in the current parser. Each child parser will be given the value of state->child_inputs[i] as its state->input field, where i is the index of the child in the this parser's children field. See Argp Children.
void *hook
For the parser function's use. Initialized to 0, but otherwise ignored by argp.
char *name
The name used when printing messages. This is initialized to argv[0], or program_invocation_name if argv[0] is unavailable.
FILE *err_stream
FILE *out_stream
The stdio streams used when argp prints. Error messages are printed to err_stream, all other output, such as `--help' output) to out_stream. These are initialized to stderr and stdout respectively. See Standard Streams.
void *pstate
Private, for use by the argp implementation.

Next: , Previous: Argp Parser Functions, Up: Argp Parsers

25.3.6 Combining Multiple Argp Parsers

The children field in a struct argp enables other argp parsers to be combined with the referencing one for the parsing of a single set of arguments. This field should point to a vector of struct argp_child, which is terminated by an entry having a value of zero in the argp field.

Where conflicts between combined parsers arise, as when two specify an option with the same name, the parser conflicts are resolved in favor of the parent argp parser(s), or the earlier of the argp parsers in the list of children.

— Data Type: struct argp_child

An entry in the list of subsidiary argp parsers pointed to by the children field in a struct argp. The fields are as follows:

const struct argp *argp
The child argp parser, or zero to end of the list.
int flags
Flags for this child.
const char *header
If non-zero, this is an optional header to be printed within help output before the child options. As a side-effect, a non-zero value forces the child options to be grouped together. To achieve this effect without actually printing a header string, use a value of "". As with header strings specified in an option entry, the conventional value of the last character is `:'. See Argp Option Vectors.
int group
This is where the child options are grouped relative to the other `consolidated' options in the parent argp parser. The values are the same as the group field in struct argp_option. See Argp Option Vectors. All child-groupings follow parent options at a particular group level. If both this field and header are zero, then the child's options aren't grouped together, they are merged with parent options at the parent option group level.

Next: , Previous: Argp Parsers, Up: Argp

25.3.7 Flags for argp_parse

The default behavior of argp_parse is designed to be convenient for the most common case of parsing program command line argument. To modify these defaults, the following flags may be or'd together in the flags argument to argp_parse:

ARGP_PARSE_ARGV0
Don't ignore the first element of the argv argument to argp_parse. Unless ARGP_NO_ERRS is set, the first element of the argument vector is skipped for option parsing purposes, as it corresponds to the program name in a command line.
ARGP_NO_ERRS
Don't print error messages for unknown options to stderr; unless this flag is set, ARGP_PARSE_ARGV0 is ignored, as argv[0] is used as the program name in the error messages. This flag implies ARGP_NO_EXIT. This is based on the assumption that silent exiting upon errors is bad behavior.
ARGP_NO_ARGS
Don't parse any non-option args. Normally these are parsed by calling the parse functions with a key of ARGP_KEY_ARG, the actual argument being the value. This flag needn't normally be set, as the default behavior is to stop parsing as soon as an argument fails to be parsed. See Argp Parser Functions.
ARGP_IN_ORDER
Parse options and arguments in the same order they occur on the command line. Normally they're rearranged so that all options come first.
ARGP_NO_HELP
Don't provide the standard long option `--help', which ordinarily causes usage and option help information to be output to stdout and exit (0).
ARGP_NO_EXIT
Don't exit on errors, although they may still result in error messages.
ARGP_LONG_ONLY
Use the gnu getopt `long-only' rules for parsing arguments. This allows long-options to be recognized with only a single `-' (i.e., `-help'). This results in a less useful interface, and its use is discouraged as it conflicts with the way most GNU programs work as well as the GNU coding standards.
ARGP_SILENT
Turns off any message-printing/exiting options, specifically ARGP_NO_EXIT, ARGP_NO_ERRS, and ARGP_NO_HELP.

Previous: Argp Children, Up: Argp Parsers

25.3.8 Customizing Argp Help Output

The help_filter field in a struct argp is a pointer to a function that filters the text of help messages before displaying them. They have a function signature like: 

     char *help-filter (int key, const char *text, void *input)

Where key is either a key from an option, in which case text is that option's help text. See Argp Option Vectors. Alternately, one of the special keys with names beginning with `ARGP_KEY_HELP_' might be used, describing which other help text text will contain. See Argp Help Filter Keys.

The function should return either text if it remains as-is, or a replacement string allocated using malloc. This will be either be freed by argp or zero, which prints nothing. The value of text is supplied after any translation has been done, so if any of the replacement text needs translation, it will be done by the filter function. input is either the input supplied to argp_parse or it is zero, if argp_help was called directly by the user.

Up: Argp Help Filtering
25.3.8.1 Special Keys for Argp Help Filter Functions

The following special values may be passed to an argp help filter function as the first argument in addition to key values for user options. They specify which help text the text argument contains:

ARGP_KEY_HELP_PRE_DOC
The help text preceding options.
ARGP_KEY_HELP_POST_DOC
The help text following options.
ARGP_KEY_HELP_HEADER
The option header string.
ARGP_KEY_HELP_EXTRA
This is used after all other documentation; text is zero for this key.
ARGP_KEY_HELP_DUP_ARGS_NOTE
The explanatory note printed when duplicate option arguments have been suppressed.
ARGP_KEY_HELP_ARGS_DOC
The argument doc string; formally the args_doc field from the argp parser. See Argp Parsers.

Next: , Previous: Argp Flags, Up: Argp

25.3.9 The argp_help Function

Normally programs using argp need not be written with particular printing argument-usage-type help messages in mind as the standard `--help' option is handled automatically by argp. Typical error cases can be handled using argp_usage and argp_error. See Argp Helper Functions. However, if it's desirable to print a help message in some context other than parsing the program options, argp offers the argp_help interface.

— Function: void argp_help (const struct argp *argp, FILE *stream, unsigned flags, char *name)

This outputs a help message for the argp parser argp to stream. The type of messages printed will be determined by flags.

Any options such as `--help' that are implemented automatically by argp itself will not be present in the help output; for this reason it is best to use argp_state_help if calling from within an argp parser function. See Argp Helper Functions.

Up: Argp Help

25.3.10 Flags for the argp_help Function

When calling argp_help (see Argp Help) or argp_state_help (see Argp Helper Functions) the exact output is determined by the flags argument. This should consist of any of the following flags, or'd together:

ARGP_HELP_USAGE
A unix `Usage:' message that explicitly lists all options.
ARGP_HELP_SHORT_USAGE
A unix `Usage:' message that displays an appropriate placeholder to indicate where the options go; useful for showing the non-option argument syntax.
ARGP_HELP_SEE
A `Try ... for more help' message; `...' contains the program name and `--help'.
ARGP_HELP_LONG
A verbose option help message that gives each option available along with its documentation string.
ARGP_HELP_PRE_DOC
The part of the argp parser doc string preceding the verbose option help.
ARGP_HELP_POST_DOC
The part of the argp parser doc string that following the verbose option help.
ARGP_HELP_DOC
(ARGP_HELP_PRE_DOC | ARGP_HELP_POST_DOC)
ARGP_HELP_BUG_ADDR
A message that prints where to report bugs for this program, if the argp_program_bug_address variable contains this information.
ARGP_HELP_LONG_ONLY
This will modify any output to reflect the ARGP_LONG_ONLY mode.

The following flags are only understood when used with argp_state_help. They control whether the function returns after printing its output, or terminates the program:

ARGP_HELP_EXIT_ERR
This will terminate the program with exit (argp_err_exit_status).
ARGP_HELP_EXIT_OK
This will terminate the program with exit (0).

The following flags are combinations of the basic flags for printing standard messages:

ARGP_HELP_STD_ERR
Assuming that an error message for a parsing error has printed, this prints a message on how to get help, and terminates the program with an error.
ARGP_HELP_STD_USAGE
This prints a standard usage message and terminates the program with an error. This is used when no other specific error messages are appropriate or available.
ARGP_HELP_STD_HELP
This prints the standard response for a `--help' option, and terminates the program successfully.

Next: , Previous: Argp Help, Up: Argp

25.3.11 Argp Examples

These example programs demonstrate the basic usage of argp.

Next: , Up: Argp Examples
25.3.11.1 A Minimal Program Using Argp

This is perhaps the smallest program possible that uses argp. It won't do much except give an error messages and exit when there are any arguments, and prints a rather pointless message for `--help'. 

     /* Argp example #1 -- a minimal program using argp */
     
     /* This is (probably) the smallest possible program that
        uses argp.  It won't do much except give an error
        messages and exit when there are any arguments, and print
        a (rather pointless) messages for --help. */
     
     #include <argp.h>
     
     int main (int argc, char **argv)
     {
       argp_parse (0, argc, argv, 0, 0, 0);
       exit (0);
     }

Next: , Previous: Argp Example 1, Up: Argp Examples
25.3.11.2 A Program Using Argp with Only Default Options

This program doesn't use any options or arguments, it uses argp to be compliant with the GNU standard command line format.

In addition to giving no arguments and implementing a `--help' option, this example has a `--version' option, which will put the given documentation string and bug address in the `--help' output, as per GNU standards.

The variable argp contains the argument parser specification. Adding fields to this structure is the way most parameters are passed to argp_parse. The first three fields are normally used, but they are not in this small program. There are also two global variables that argp can use defined here, argp_program_version and argp_program_bug_address. They are considered global variables because they will almost always be constant for a given program, even if they use different argument parsers for various tasks. 

     /* Argp example #2 -- a pretty minimal program using argp */
     
     /* This program doesn't use any options or arguments, but uses
        argp to be compliant with the GNU standard command line
        format.
     
        In addition to making sure no arguments are given, and
        implementing a --help option, this example will have a
        --version option, and will put the given documentation string
        and bug address in the --help output, as per GNU standards.
     
        The variable ARGP contains the argument parser specification;
        adding fields to this structure is the way most parameters are
        passed to argp_parse (the first three fields are usually used,
        but not in this small program).  There are also two global
        variables that argp knows about defined here,
        ARGP_PROGRAM_VERSION and ARGP_PROGRAM_BUG_ADDRESS (they are
        global variables because they will almost always be constant
        for a given program, even if it uses different argument
        parsers for various tasks). */
     
     #include <argp.h>
     
     const char *argp_program_version =
       "argp-ex2 1.0";
     const char *argp_program_bug_address =
       "<bug-gnu-utils@gnu.org>";
     
     /* Program documentation. */
     static char doc[] =
       "Argp example #2 -- a pretty minimal program using argp";
     
     /* Our argument parser.  The options, parser, and
        args_doc fields are zero because we have neither options or
        arguments; doc and argp_program_bug_address will be
        used in the output for `--help', and the `--version'
        option will print out argp_program_version. */
     static struct argp argp = { 0, 0, 0, doc };
     
     int main (int argc, char **argv)
     {
       argp_parse (&argp, argc, argv, 0, 0, 0);
       exit (0);
     }

Next: , Previous: Argp Example 2, Up: Argp Examples
25.3.11.3 A Program Using Argp with User Options

This program uses the same features as example 2, adding user options and arguments.

We now use the first four fields in argp (see Argp Parsers) and specify parse_opt as the parser function. See Argp Parser Functions.

Note that in this example, main uses a structure to communicate with the parse_opt function, a pointer to which it passes in the input argument to argp_parse. See Argp. It is retrieved by parse_opt through the input field in its state argument. See Argp Parsing State. Of course, it's also possible to use global variables instead, but using a structure like this is somewhat more flexible and clean. 

     /* Argp example #3 -- a program with options and arguments using argp */
     
     /* This program uses the same features as example 2, and uses options and
        arguments.
     
        We now use the first four fields in ARGP, so here's a description of them:
          OPTIONS  -- A pointer to a vector of struct argp_option (see below)
          PARSER   -- A function to parse a single option, called by argp
          ARGS_DOC -- A string describing how the non-option arguments should look
          DOC      -- A descriptive string about this program; if it contains a
                      vertical tab character (\v), the part after it will be
                      printed *following* the options
     
        The function PARSER takes the following arguments:
          KEY  -- An integer specifying which option this is (taken
                  from the KEY field in each struct argp_option), or
                  a special key specifying something else; the only
                  special keys we use here are ARGP_KEY_ARG, meaning
                  a non-option argument, and ARGP_KEY_END, meaning
                  that all arguments have been parsed
          ARG  -- For an option KEY, the string value of its
                  argument, or NULL if it has none
          STATE-- A pointer to a struct argp_state, containing
                  various useful information about the parsing state; used here
                  are the INPUT field, which reflects the INPUT argument to
                  argp_parse, and the ARG_NUM field, which is the number of the
                  current non-option argument being parsed
        It should return either 0, meaning success, ARGP_ERR_UNKNOWN, meaning the
        given KEY wasn't recognized, or an errno value indicating some other
        error.
     
        Note that in this example, main uses a structure to communicate with the
        parse_opt function, a pointer to which it passes in the INPUT argument to
        argp_parse.  Of course, it's also possible to use global variables
        instead, but this is somewhat more flexible.
     
        The OPTIONS field contains a pointer to a vector of struct argp_option's;
        that structure has the following fields (if you assign your option
        structures using array initialization like this example, unspecified
        fields will be defaulted to 0, and need not be specified):
          NAME   -- The name of this option's long option (may be zero)
          KEY    -- The KEY to pass to the PARSER function when parsing this option,
                    *and* the name of this option's short option, if it is a
                    printable ascii character
          ARG    -- The name of this option's argument, if any
          FLAGS  -- Flags describing this option; some of them are:
                      OPTION_ARG_OPTIONAL -- The argument to this option is optional
                      OPTION_ALIAS        -- This option is an alias for the
                                             previous option
                      OPTION_HIDDEN       -- Don't show this option in --help output
          DOC    -- A documentation string for this option, shown in --help output
     
        An options vector should be terminated by an option with all fields zero. */
     
     #include <argp.h>
     
     const char *argp_program_version =
       "argp-ex3 1.0";
     const char *argp_program_bug_address =
       "<bug-gnu-utils@gnu.org>";
     
     /* Program documentation. */
     static char doc[] =
       "Argp example #3 -- a program with options and arguments using argp";
     
     /* A description of the arguments we accept. */
     static char args_doc[] = "ARG1 ARG2";
     
     /* The options we understand. */
     static struct argp_option options[] = {
       {"verbose",  'v', 0,      0,  "Produce verbose output" },
       {"quiet",    'q', 0,      0,  "Don't produce any output" },
       {"silent",   's', 0,      OPTION_ALIAS },
       {"output",   'o', "FILE", 0,
        "Output to FILE instead of standard output" },
       { 0 }
     };
     
     /* Used by main to communicate with parse_opt. */
     struct arguments
     {
       char *args[2];                /* arg1 & arg2 */
       int silent, verbose;
       char *output_file;
     };
     
     /* Parse a single option. */
     static error_t
     parse_opt (int key, char *arg, struct argp_state *state)
     {
       /* Get the input argument from argp_parse, which we
          know is a pointer to our arguments structure. */
       struct arguments *arguments = state->input;
     
       switch (key)
         {
         case 'q': case 's':
           arguments->silent = 1;
           break;
         case 'v':
           arguments->verbose = 1;
           break;
         case 'o':
           arguments->output_file = arg;
           break;
     
         case ARGP_KEY_ARG:
           if (state->arg_num >= 2)
             /* Too many arguments. */
             argp_usage (state);
     
           arguments->args[state->arg_num] = arg;
     
           break;
     
         case ARGP_KEY_END:
           if (state->arg_num < 2)
             /* Not enough arguments. */
             argp_usage (state);
           break;
     
         default:
           return ARGP_ERR_UNKNOWN;
         }
       return 0;
     }
     
     /* Our argp parser. */
     static struct argp argp = { options, parse_opt, args_doc, doc };
     
     int main (int argc, char **argv)
     {
       struct arguments arguments;
     
       /* Default values. */
       arguments.silent = 0;
       arguments.verbose = 0;
       arguments.output_file = "-";
     
       /* Parse our arguments; every option seen by parse_opt will
          be reflected in arguments. */
       argp_parse (&argp, argc, argv, 0, 0, &arguments);
     
       printf ("ARG1 = %s\nARG2 = %s\nOUTPUT_FILE = %s\n"
               "VERBOSE = %s\nSILENT = %s\n",
               arguments.args[0], arguments.args[1],
               arguments.output_file,
               arguments.verbose ? "yes" : "no",
               arguments.silent ? "yes" : "no");
     
       exit (0);
     }

Previous: Argp Example 3, Up: Argp Examples
25.3.11.4 A Program Using Multiple Combined Argp Parsers

This program uses the same features as example 3, but has more options, and presents more structure in the `--help' output. It also illustrates how you can `steal' the remainder of the input arguments past a certain point for programs that accept a list of items. It also illustrates the key value ARGP_KEY_NO_ARGS, which is only given if no non-option arguments were supplied to the program. See Argp Special Keys.

For structuring help output, two features are used: headers and a two part option string. The headers are entries in the options vector. See Argp Option Vectors. The first four fields are zero. The two part documentation string are in the variable doc, which allows documentation both before and after the options. See Argp Parsers, the two parts of doc are separated by a vertical-tab character ('\v', or '\013'). By convention, the documentation before the options is a short string stating what the program does, and after any options it is longer, describing the behavior in more detail. All documentation strings are automatically filled for output, although newlines may be included to force a line break at a particular point. In addition, documentation strings are passed to the gettext function, for possible translation into the current locale. 

     /* Argp example #4 -- a program with somewhat more complicated options */
     
     /* This program uses the same features as example 3, but has more
        options, and somewhat more structure in the -help output.  It
        also shows how you can `steal' the remainder of the input
        arguments past a certain point, for programs that accept a
        list of items.  It also shows the special argp KEY value
        ARGP_KEY_NO_ARGS, which is only given if no non-option
        arguments were supplied to the program.
     
        For structuring the help output, two features are used,
        *headers* which are entries in the options vector with the
        first four fields being zero, and a two part documentation
        string (in the variable DOC), which allows documentation both
        before and after the options; the two parts of DOC are
        separated by a vertical-tab character ('\v', or '\013').  By
        convention, the documentation before the options is just a
        short string saying what the program does, and that afterwards
        is longer, describing the behavior in more detail.  All
        documentation strings are automatically filled for output,
        although newlines may be included to force a line break at a
        particular point.  All documentation strings are also passed to
        the `gettext' function, for possible translation into the
        current locale. */
     
     #include <stdlib.h>
     #include <error.h>
     #include <argp.h>
     
     const char *argp_program_version =
       "argp-ex4 1.0";
     const char *argp_program_bug_address =
       "<bug-gnu-utils@prep.ai.mit.edu>";
     
     /* Program documentation. */
     static char doc[] =
       "Argp example #4 -- a program with somewhat more complicated\
     options\
     \vThis part of the documentation comes *after* the options;\
      note that the text is automatically filled, but it's possible\
      to force a line-break, e.g.\n<-- here.";
     
     /* A description of the arguments we accept. */
     static char args_doc[] = "ARG1 [STRING...]";
     
     /* Keys for options without short-options. */
     #define OPT_ABORT  1            /* --abort */
     
     /* The options we understand. */
     static struct argp_option options[] = {
       {"verbose",  'v', 0,       0, "Produce verbose output" },
       {"quiet",    'q', 0,       0, "Don't produce any output" },
       {"silent",   's', 0,       OPTION_ALIAS },
       {"output",   'o', "FILE",  0,
        "Output to FILE instead of standard output" },
     
       {0,0,0,0, "The following options should be grouped together:" },
       {"repeat",   'r', "COUNT", OPTION_ARG_OPTIONAL,
        "Repeat the output COUNT (default 10) times"},
       {"abort",    OPT_ABORT, 0, 0, "Abort before showing any output"},
     
       { 0 }
     };
     
     /* Used by main to communicate with parse_opt. */
     struct arguments
     {
       char *arg1;                   /* arg1 */
       char **strings;               /* [string...] */
       int silent, verbose, abort;   /* `-s', `-v', `--abort' */
       char *output_file;            /* file arg to `--output' */
       int repeat_count;             /* count arg to `--repeat' */
     };
     
     /* Parse a single option. */
     static error_t
     parse_opt (int key, char *arg, struct argp_state *state)
     {
       /* Get the input argument from argp_parse, which we
          know is a pointer to our arguments structure. */
       struct arguments *arguments = state->input;
     
       switch (key)
         {
         case 'q': case 's':
           arguments->silent = 1;
           break;
         case 'v':
           arguments->verbose = 1;
           break;
         case 'o':
           arguments->output_file = arg;
           break;
         case 'r':
           arguments->repeat_count = arg ? atoi (arg) : 10;
           break;
         case OPT_ABORT:
           arguments->abort = 1;
           break;
     
         case ARGP_KEY_NO_ARGS:
           argp_usage (state);
     
         case ARGP_KEY_ARG:
           /* Here we know that state->arg_num == 0, since we
              force argument parsing to end before any more arguments can
              get here. */
           arguments->arg1 = arg;
     
           /* Now we consume all the rest of the arguments.
              state->next is the index in state->argv of the
              next argument to be parsed, which is the first string
              we're interested in, so we can just use
              &state->argv[state->next] as the value for
              arguments->strings.
     
              In addition, by setting state->next to the end
              of the arguments, we can force argp to stop parsing here and
              return. */
           arguments->strings = &state->argv[state->next];
           state->next = state->argc;
     
           break;
     
         default:
           return ARGP_ERR_UNKNOWN;
         }
       return 0;
     }
     
     /* Our argp parser. */
     static struct argp argp = { options, parse_opt, args_doc, doc };
     
     int main (int argc, char **argv)
     {
       int i, j;
       struct arguments arguments;
     
       /* Default values. */
       arguments.silent = 0;
       arguments.verbose = 0;
       arguments.output_file = "-";
       arguments.repeat_count = 1;
       arguments.abort = 0;
     
       /* Parse our arguments; every option seen by parse_opt will be
          reflected in arguments. */
       argp_parse (&argp, argc, argv, 0, 0, &arguments);
     
       if (arguments.abort)
         error (10, 0, "ABORTED");
     
       for (i = 0; i < arguments.repeat_count; i++)
         {
           printf ("ARG1 = %s\n", arguments.arg1);
           printf ("STRINGS = ");
           for (j = 0; arguments.strings[j]; j++)
             printf (j == 0 ? "%s" : ", %s", arguments.strings[j]);
           printf ("\n");
           printf ("OUTPUT_FILE = %s\nVERBOSE = %s\nSILENT = %s\n",
                   arguments.output_file,
                   arguments.verbose ? "yes" : "no",
                   arguments.silent ? "yes" : "no");
         }
     
       exit (0);
     }

Previous: Argp Examples, Up: Argp

25.3.12 Argp User Customization

The formatting of argp `--help' output may be controlled to some extent by a program's users, by setting the ARGP_HELP_FMT environment variable to a comma-separated list of tokens. Whitespace is ignored:

`dup-args'
`no-dup-args'
These turn duplicate-argument-mode on or off. In duplicate argument mode, if an option that accepts an argument has multiple names, the argument is shown for each name. Otherwise, it is only shown for the first long option. A note is subsequently printed so the user knows that it applies to other names as well. The default is `no-dup-args', which is less consistent, but prettier.
`dup-args-note'
`no-dup-args-note'
These will enable or disable the note informing the user of suppressed option argument duplication. The default is `dup-args-note'.
`short-opt-col=n'
This prints the first short option in column n. The default is 2.
`long-opt-col=n'
This prints the first long option in column n. The default is 6.
`doc-opt-col=n'
This prints `documentation options' (see Argp Option Flags) in column n. The default is 2.
`opt-doc-col=n'
This prints the documentation for options starting in column n. The default is 29.
`header-col=n'
This will indent the group headers that document groups of options to column n. The default is 1.
`usage-indent=n'
This will indent continuation lines in `Usage:' messages to column n. The default is 12.
`rmargin=n'
This will word wrap help output at or before column n. The default is 79.

Next: , Previous: Argp, Up: Parsing Program Arguments
25.3.12.1 Parsing of Suboptions

Having a single level of options is sometimes not enough. There might be too many options which have to be available or a set of options is closely related.

For this case some programs use suboptions. One of the most prominent programs is certainly mount(8). The -o option take one argument which itself is a comma separated list of options. To ease the programming of code like this the function getsubopt is available.

— Function: int getsubopt (char **optionp, const char* const *tokens, char **valuep)

The optionp parameter must be a pointer to a variable containing the address of the string to process. When the function returns the reference is updated to point to the next suboption or to the terminating `\0' character if there is no more suboption available.

The tokens parameter references an array of strings containing the known suboptions. All strings must be `\0' terminated and to mark the end a null pointer must be stored. When getsubopt finds a possible legal suboption it compares it with all strings available in the tokens array and returns the index in the string as the indicator.

In case the suboption has an associated value introduced by a `=' character, a pointer to the value is returned in valuep. The string is `\0' terminated. If no argument is available valuep is set to the null pointer. By doing this the caller can check whether a necessary value is given or whether no unexpected value is present.

In case the next suboption in the string is not mentioned in the tokens array the starting address of the suboption including a possible value is returned in valuep and the return value of the function is `-1'.

Previous: Suboptions, Up: Parsing Program Arguments

25.3.13 Parsing of Suboptions Example

The code which might appear in the mount(8) program is a perfect example of the use of getsubopt: 

     #include <stdio.h>
     #include <stdlib.h>
     #include <unistd.h>
     
     int do_all;
     const char *type;
     int read_size;
     int write_size;
     int read_only;
     
     enum
     {
       RO_OPTION = 0,
       RW_OPTION,
       READ_SIZE_OPTION,
       WRITE_SIZE_OPTION,
       THE_END
     };
     
     const char *mount_opts[] =
     {
       [RO_OPTION] = "ro",
       [RW_OPTION] = "rw",
       [READ_SIZE_OPTION] = "rsize",
       [WRITE_SIZE_OPTION] = "wsize",
       [THE_END] = NULL
     };
     
     int
     main (int argc, char *argv[])
     {
       char *subopts, *value;
       int opt;
     
       while ((opt = getopt (argc, argv, "at:o:")) != -1)
         switch (opt)
           {
           case 'a':
             do_all = 1;
             break;
           case 't':
             type = optarg;
             break;
           case 'o':
             subopts = optarg;
             while (*subopts != '\0')
               switch (getsubopt (&subopts, mount_opts, &value))
                 {
                 case RO_OPTION:
                   read_only = 1;
                   break;
                 case RW_OPTION:
                   read_only = 0;
                   break;
                 case READ_SIZE_OPTION:
                   if (value == NULL)
                     abort ();
                   read_size = atoi (value);
                   break;
                 case WRITE_SIZE_OPTION:
                   if (value == NULL)
                     abort ();
                   write_size = atoi (value);
                   break;
                 default:
                   /* Unknown suboption. */
                   printf ("Unknown suboption `%s'\n", value);
                   break;
                 }
             break;
           default:
             abort ();
           }
     
       /* Do the real work. */
     
       return 0;
     }

Next: , Previous: Program Arguments, Up: Program Basics

25.4 Environment Variables

When a program is executed, it receives information about the context in which it was invoked in two ways. The first mechanism uses the argv and argc arguments to its main function, and is discussed in Program Arguments. The second mechanism uses environment variables and is discussed in this section.

The argv mechanism is typically used to pass command-line arguments specific to the particular program being invoked. The environment, on the other hand, keeps track of information that is shared by many programs, changes infrequently, and that is less frequently used.

The environment variables discussed in this section are the same environment variables that you set using assignments and the export command in the shell. Programs executed from the shell inherit all of the environment variables from the shell.

Standard environment variables are used for information about the user's home directory, terminal type, current locale, and so on; you can define additional variables for other purposes. The set of all environment variables that have values is collectively known as the environment.

Names of environment variables are case-sensitive and must not contain the character `='. System-defined environment variables are invariably uppercase.

The values of environment variables can be anything that can be represented as a string. A value must not contain an embedded null character, since this is assumed to terminate the string.

Next: , Up: Environment Variables

25.4.1 Environment Access

The value of an environment variable can be accessed with the getenv function. This is declared in the header file stdlib.h. All of the following functions can be safely used in multi-threaded programs. It is made sure that concurrent modifications to the environment do not lead to errors.

— Function: char * getenv (const char *name)

This function returns a string that is the value of the environment variable name. You must not modify this string. In some non-Unix systems not using the GNU library, it might be overwritten by subsequent calls to getenv (but not by any other library function). If the environment variable name is not defined, the value is a null pointer.

— Function: int putenv (char *string)

The putenv function adds or removes definitions from the environment. If the string is of the form `name=value', the definition is added to the environment. Otherwise, the string is interpreted as the name of an environment variable, and any definition for this variable in the environment is removed.

The difference to the setenv function is that the exact string given as the parameter string is put into the environment. If the user should change the string after the putenv call this will reflect in automatically in the environment. This also requires that string is no automatic variable which scope is left before the variable is removed from the environment. The same applies of course to dynamically allocated variables which are freed later.

This function is part of the extended Unix interface. Since it was also available in old SVID libraries you should define either _XOPEN_SOURCE or _SVID_SOURCE before including any header.

— Function: int setenv (const char *name, const char *value, int replace)

The setenv function can be used to add a new definition to the environment. The entry with the name name is replaced by the value `name=value'. Please note that this is also true if value is the empty string. To do this a new string is created and the strings name and value are copied. A null pointer for the value parameter is illegal. If the environment already contains an entry with key name the replace parameter controls the action. If replace is zero, nothing happens. Otherwise the old entry is replaced by the new one.

Please note that you cannot remove an entry completely using this function.

This function was originally part of the BSD library but is now part of the Unix standard.

— Function: int unsetenv (const char *name)

Using this function one can remove an entry completely from the environment. If the environment contains an entry with the key name this whole entry is removed. A call to this function is equivalent to a call to putenv when the value part of the string is empty.

The function return -1 if name is a null pointer, points to an empty string, or points to a string containing a = character. It returns 0 if the call succeeded.

This function was originally part of the BSD library but is now part of the Unix standard. The BSD version had no return value, though.

There is one more function to modify the whole environment. This function is said to be used in the POSIX.9 (POSIX bindings for Fortran 77) and so one should expect it did made it into POSIX.1. But this never happened. But we still provide this function as a GNU extension to enable writing standard compliant Fortran environments.

— Function: int clearenv (void)

The clearenv function removes all entries from the environment. Using putenv and setenv new entries can be added again later.

If the function is successful it returns 0. Otherwise the return value is nonzero.

You can deal directly with the underlying representation of environment objects to add more variables to the environment (for example, to communicate with another program you are about to execute; see Executing a File).

— Variable: char ** environ

The environment is represented as an array of strings. Each string is of the format `name=value'. The order in which strings appear in the environment is not significant, but the same name must not appear more than once. The last element of the array is a null pointer.

This variable is declared in the header file unistd.h.

If you just want to get the value of an environment variable, use getenv.

Unix systems, and the GNU system, pass the initial value of environ as the third argument to main. See Program Arguments.

Previous: Environment Access, Up: Environment Variables

25.4.2 Standard Environment Variables

These environment variables have standard meanings. This doesn't mean that they are always present in the environment; but if these variables are present, they have these meanings. You shouldn't try to use these environment variable names for some other purpose.

HOME
This is a string representing the user's home directory, or initial default working directory.

The user can set HOME to any value. If you need to make sure to obtain the proper home directory for a particular user, you should not use HOME; instead, look up the user's name in the user database (see User Database).

For most purposes, it is better to use HOME, precisely because this lets the user specify the value.

LOGNAME
This is the name that the user used to log in. Since the value in the environment can be tweaked arbitrarily, this is not a reliable way to identify the user who is running a program; a function like getlogin (see Who Logged In) is better for that purpose.

For most purposes, it is better to use LOGNAME, precisely because this lets the user specify the value.

PATH
A path is a sequence of directory names which is used for searching for a file. The variable PATH holds a path used for searching for programs to be run.

The execlp and execvp functions (see Executing a File) use this environment variable, as do many shells and other utilities which are implemented in terms of those functions.

The syntax of a path is a sequence of directory names separated by colons. An empty string instead of a directory name stands for the current directory (see Working Directory).

A typical value for this environment variable might be a string like: 

          :/bin:/etc:/usr/bin:/usr/new/X11:/usr/new:/usr/local/bin

This means that if the user tries to execute a program named foo, the system will look for files named foo, /bin/foo, /etc/foo, and so on. The first of these files that exists is the one that is executed.

TERM
This specifies the kind of terminal that is receiving program output. Some programs can make use of this information to take advantage of special escape sequences or terminal modes supported by particular kinds of terminals. Many programs which use the termcap library (see Find) use the TERM environment variable, for example.
TZ
This specifies the time zone. See TZ Variable, for information about the format of this string and how it is used.
LANG
This specifies the default locale to use for attribute categories where neither LC_ALL nor the specific environment variable for that category is set. See Locales, for more information about locales.
LC_ALL
If this environment variable is set it overrides the selection for all the locales done using the other LC_* environment variables. The value of the other LC_* environment variables is simply ignored in this case.
LC_COLLATE
This specifies what locale to use for string sorting.
LC_CTYPE
This specifies what locale to use for character sets and character classification.
LC_MESSAGES
This specifies what locale to use for printing messages and to parse responses.
LC_MONETARY
This specifies what locale to use for formatting monetary values.
LC_NUMERIC
This specifies what locale to use for formatting numbers.
LC_TIME
This specifies what locale to use for formatting date/time values.
NLSPATH
This specifies the directories in which the catopen function looks for message translation catalogs.
_POSIX_OPTION_ORDER
If this environment variable is defined, it suppresses the usual reordering of command line arguments by getopt and argp_parse. See Argument Syntax.

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25.5 System Calls

A system call is a request for service that a program makes of the kernel. The service is generally something that only the kernel has the privilege to do, such as doing I/O. Programmers don't normally need to be concerned with system calls because there are functions in the GNU C library to do virtually everything that system calls do. These functions work by making system calls themselves. For example, there is a system call that changes the permissions of a file, but you don't need to know about it because you can just use the GNU C library's chmod function.

System calls are sometimes called kernel calls.

However, there are times when you want to make a system call explicitly, and for that, the GNU C library provides the syscall function. syscall is harder to use and less portable than functions like chmod, but easier and more portable than coding the system call in assembler instructions.

syscall is most useful when you are working with a system call which is special to your system or is newer than the GNU C library you are using. syscall is implemented in an entirely generic way; the function does not know anything about what a particular system call does or even if it is valid.

The description of syscall in this section assumes a certain protocol for system calls on the various platforms on which the GNU C library runs. That protocol is not defined by any strong authority, but we won't describe it here either because anyone who is coding syscall probably won't accept anything less than kernel and C library source code as a specification of the interface between them anyway.

syscall is declared in unistd.h.

— Function: long int syscall (long int sysno, ...)

syscall performs a generic system call.

sysno is the system call number. Each kind of system call is identified by a number. Macros for all the possible system call numbers are defined in sys/syscall.h

The remaining arguments are the arguments for the system call, in order, and their meanings depend on the kind of system call. Each kind of system call has a definite number of arguments, from zero to five. If you code more arguments than the system call takes, the extra ones to the right are ignored.

The return value is the return value from the system call, unless the system call failed. In that case, syscall returns -1 and sets errno to an error code that the system call returned. Note that system calls do not return -1 when they succeed. If you specify an invalid sysno, syscall returns -1 with errno = ENOSYS.

Example: 

          
          #include <unistd.h>
          #include <sys/syscall.h>
          #include <errno.h>
          
          ...
          
          int rc;
          
          rc = syscall(SYS_chmod, "/etc/passwd", 0444);
          
          if (rc == -1)
             fprintf(stderr, "chmod failed, errno = %d\n", errno);

This, if all the compatibility stars are aligned, is equivalent to the following preferable code: 

          
          #include <sys/types.h>
          #include <sys/stat.h>
          #include <errno.h>
          
          ...
          
          int rc;
          
          rc = chmod("/etc/passwd", 0444);
          if (rc == -1)
             fprintf(stderr, "chmod failed, errno = %d\n", errno);

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25.6 Program Termination

The usual way for a program to terminate is simply for its main function to return. The exit status value returned from the main function is used to report information back to the process's parent process or shell.

A program can also terminate normally by calling the exit function.

In addition, programs can be terminated by signals; this is discussed in more detail in Signal Handling. The abort function causes a signal that kills the program.

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25.6.1 Normal Termination

A process terminates normally when its program signals it is done by calling exit. Returning from main is equivalent to calling exit, and the value that main returns is used as the argument to exit.

— Function: void exit (int status)

The exit function tells the system that the program is done, which causes it to terminate the process.

status is the program's exit status, which becomes part of the process' termination status. This function does not return.

Normal termination causes the following actions:

  1. Functions that were registered with the atexit or on_exit functions are called in the reverse order of their registration. This mechanism allows your application to specify its own “cleanup” actions to be performed at program termination. Typically, this is used to do things like saving program state information in a file, or unlocking locks in shared data bases.
  2. All open streams are closed, writing out any buffered output data. See Closing Streams. In addition, temporary files opened with the tmpfile function are removed; see Temporary Files.
  3. _exit is called, terminating the program. See Termination Internals.

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25.6.2 Exit Status

When a program exits, it can return to the parent process a small amount of information about the cause of termination, using the exit status. This is a value between 0 and 255 that the exiting process passes as an argument to exit.

Normally you should use the exit status to report very broad information about success or failure. You can't provide a lot of detail about the reasons for the failure, and most parent processes would not want much detail anyway.

There are conventions for what sorts of status values certain programs should return. The most common convention is simply 0 for success and 1 for failure. Programs that perform comparison use a different convention: they use status 1 to indicate a mismatch, and status 2 to indicate an inability to compare. Your program should follow an existing convention if an existing convention makes sense for it.

A general convention reserves status values 128 and up for special purposes. In particular, the value 128 is used to indicate failure to execute another program in a subprocess. This convention is not universally obeyed, but it is a good idea to follow it in your programs.

Warning: Don't try to use the number of errors as the exit status. This is actually not very useful; a parent process would generally not care how many errors occurred. Worse than that, it does not work, because the status value is truncated to eight bits. Thus, if the program tried to report 256 errors, the parent would receive a report of 0 errors—that is, success.

For the same reason, it does not work to use the value of errno as the exit status—these can exceed 255.

Portability note: Some non-POSIX systems use different conventions for exit status values. For greater portability, you can use the macros EXIT_SUCCESS and EXIT_FAILURE for the conventional status value for success and failure, respectively. They are declared in the file stdlib.h.

— Macro: int EXIT_SUCCESS

This macro can be used with the exit function to indicate successful program completion.

On POSIX systems, the value of this macro is 0. On other systems, the value might be some other (possibly non-constant) integer expression.

— Macro: int EXIT_FAILURE

This macro can be used with the exit function to indicate unsuccessful program completion in a general sense.

On POSIX systems, the value of this macro is 1. On other systems, the value might be some other (possibly non-constant) integer expression. Other nonzero status values also indicate failures. Certain programs use different nonzero status values to indicate particular kinds of "non-success". For example, diff uses status value 1 to mean that the files are different, and 2 or more to mean that there was difficulty in opening the files.

Don't confuse a program's exit status with a process' termination status. There are lots of ways a process can terminate besides having it's program finish. In the event that the process termination is caused by program termination (i.e., exit), though, the program's exit status becomes part of the process' termination status.

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25.6.3 Cleanups on Exit

Your program can arrange to run its own cleanup functions if normal termination happens. If you are writing a library for use in various application programs, then it is unreliable to insist that all applications call the library's cleanup functions explicitly before exiting. It is much more robust to make the cleanup invisible to the application, by setting up a cleanup function in the library itself using atexit or on_exit.

— Function: int atexit (void (*function) (void))

The atexit function registers the function function to be called at normal program termination. The function is called with no arguments.

The return value from atexit is zero on success and nonzero if the function cannot be registered.

— Function: int on_exit (void (*function)(int status, void *arg), void *arg)

This function is a somewhat more powerful variant of atexit. It accepts two arguments, a function function and an arbitrary pointer arg. At normal program termination, the function is called with two arguments: the status value passed to exit, and the arg.

This function is included in the GNU C library only for compatibility for SunOS, and may not be supported by other implementations.

Here's a trivial program that illustrates the use of exit and atexit: 

     #include <stdio.h>
     #include <stdlib.h>
     
     void
     bye (void)
     {
       puts ("Goodbye, cruel world....");
     }
     
     int
     main (void)
     {
       atexit (bye);
       exit (EXIT_SUCCESS);
     }

When this program is executed, it just prints the message and exits.

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25.6.4 Aborting a Program

You can abort your program using the abort function. The prototype for this function is in stdlib.h.

— Function: void abort (void)

The abort function causes abnormal program termination. This does not execute cleanup functions registered with atexit or on_exit.

This function actually terminates the process by raising a SIGABRT signal, and your program can include a handler to intercept this signal; see Signal Handling.

Future Change Warning: Proposed Federal censorship regulations may prohibit us from giving you information about the possibility of calling this function. We would be required to say that this is not an acceptable way of terminating a program.

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25.6.5 Termination Internals

The _exit function is the primitive used for process termination by exit. It is declared in the header file unistd.h.

— Function: void _exit (int status)

The _exit function is the primitive for causing a process to terminate with status status. Calling this function does not execute cleanup functions registered with atexit or on_exit.

— Function: void _Exit (int status)

The _Exit function is the ISO C equivalent to _exit. The ISO C committee members were not sure whether the definitions of _exit and _Exit were compatible so they have not used the POSIX name.

This function was introduced in ISO C99 and is declared in stdlib.h.

When a process terminates for any reason—either because the program terminates, or as a result of a signal—the following things happen:

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26 Processes

Processes are the primitive units for allocation of system resources. Each process has its own address space and (usually) one thread of control. A process executes a program; you can have multiple processes executing the same program, but each process has its own copy of the program within its own address space and executes it independently of the other copies.

Processes are organized hierarchically. Each process has a parent process which explicitly arranged to create it. The processes created by a given parent are called its child processes. A child inherits many of its attributes from the parent process.

This chapter describes how a program can create, terminate, and control child processes. Actually, there are three distinct operations involved: creating a new child process, causing the new process to execute a program, and coordinating the completion of the child process with the original program.

The system function provides a simple, portable mechanism for running another program; it does all three steps automatically. If you need more control over the details of how this is done, you can use the primitive functions to do each step individually instead.

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26.1 Running a Command

The easy way to run another program is to use the system function. This function does all the work of running a subprogram, but it doesn't give you much control over the details: you have to wait until the subprogram terminates before you can do anything else.

— Function: int system (const char *command)

This function executes command as a shell command. In the GNU C library, it always uses the default shell sh to run the command. In particular, it searches the directories in PATH to find programs to execute. The return value is -1 if it wasn't possible to create the shell process, and otherwise is the status of the shell process. See Process Completion, for details on how this status code can be interpreted.

If the command argument is a null pointer, a return value of zero indicates that no command processor is available.

This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time system is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this calls to system should be protected using cancellation handlers.

The system function is declared in the header file stdlib.h.

Portability Note: Some C implementations may not have any notion of a command processor that can execute other programs. You can determine whether a command processor exists by executing system (NULL); if the return value is nonzero, a command processor is available.

The popen and pclose functions (see Pipe to a Subprocess) are closely related to the system function. They allow the parent process to communicate with the standard input and output channels of the command being executed.

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26.2 Process Creation Concepts

This section gives an overview of processes and of the steps involved in creating a process and making it run another program.

Each process is named by a process ID number. A unique process ID is allocated to each process when it is created. The lifetime of a process ends when its termination is reported to its parent process; at that time, all of the process resources, including its process ID, are freed.

Processes are created with the fork system call (so the operation of creating a new process is sometimes called forking a process). The child process created by fork is a copy of the original parent process, except that it has its own process ID.

After forking a child process, both the parent and child processes continue to execute normally. If you want your program to wait for a child process to finish executing before continuing, you must do this explicitly after the fork operation, by calling wait or waitpid (see Process Completion). These functions give you limited information about why the child terminated—for example, its exit status code.

A newly forked child process continues to execute the same program as its parent process, at the point where the fork call returns. You can use the return value from fork to tell whether the program is running in the parent process or the child.

Having several processes run the same program is only occasionally useful. But the child can execute another program using one of the exec functions; see Executing a File. The program that the process is executing is called its process image. Starting execution of a new program causes the process to forget all about its previous process image; when the new program exits, the process exits too, instead of returning to the previous process image.

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26.3 Process Identification

The pid_t data type represents process IDs. You can get the process ID of a process by calling getpid. The function getppid returns the process ID of the parent of the current process (this is also known as the parent process ID). Your program should include the header files unistd.h and sys/types.h to use these functions.

— Data Type: pid_t

The pid_t data type is a signed integer type which is capable of representing a process ID. In the GNU library, this is an int.

— Function: pid_t getpid (void)

The getpid function returns the process ID of the current process.

— Function: pid_t getppid (void)

The getppid function returns the process ID of the parent of the current process.

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26.4 Creating a Process

The fork function is the primitive for creating a process. It is declared in the header file unistd.h.

— Function: pid_t fork (void)

The fork function creates a new process.

If the operation is successful, there are then both parent and child processes and both see fork return, but with different values: it returns a value of 0 in the child process and returns the child's process ID in the parent process.

If process creation failed, fork returns a value of -1 in the parent process. The following errno error conditions are defined for fork:

EAGAIN
There aren't enough system resources to create another process, or the user already has too many processes running. This means exceeding the RLIMIT_NPROC resource limit, which can usually be increased; see Limits on Resources.
ENOMEM
The process requires more space than the system can supply.

The specific attributes of the child process that differ from the parent process are:

— Function: pid_t vfork (void)

The vfork function is similar to fork but on some systems it is more efficient; however, there are restrictions you must follow to use it safely.

While fork makes a complete copy of the calling process's address space and allows both the parent and child to execute independently, vfork does not make this copy. Instead, the child process created with vfork shares its parent's address space until it calls _exit or one of the exec functions. In the meantime, the parent process suspends execution.

You must be very careful not to allow the child process created with vfork to modify any global data or even local variables shared with the parent. Furthermore, the child process cannot return from (or do a long jump out of) the function that called vfork! This would leave the parent process's control information very confused. If in doubt, use fork instead.

Some operating systems don't really implement vfork. The GNU C library permits you to use vfork on all systems, but actually executes fork if vfork isn't available. If you follow the proper precautions for using vfork, your program will still work even if the system uses fork instead.

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26.5 Executing a File

This section describes the exec family of functions, for executing a file as a process image. You can use these functions to make a child process execute a new program after it has been forked.

To see the effects of exec from the point of view of the called program, See Program Basics.

The functions in this family differ in how you specify the arguments, but otherwise they all do the same thing. They are declared in the header file unistd.h.

— Function: int execv (const char *filename, char *const argv[])

The execv function executes the file named by filename as a new process image.

The argv argument is an array of null-terminated strings that is used to provide a value for the argv argument to the main function of the program to be executed. The last element of this array must be a null pointer. By convention, the first element of this array is the file name of the program sans directory names. See Program Arguments, for full details on how programs can access these arguments.

The environment for the new process image is taken from the environ variable of the current process image; see Environment Variables, for information about environments.

— Function: int execl (const char *filename, const char *arg0, ...)

This is similar to execv, but the argv strings are specified individually instead of as an array. A null pointer must be passed as the last such argument.

— Function: int execve (const char *filename, char *const argv[], char *const env[])

This is similar to execv, but permits you to specify the environment for the new program explicitly as the env argument. This should be an array of strings in the same format as for the environ variable; see Environment Access.

— Function: int execle (const char *filename, const char *arg0, char *const env[], ...)

This is similar to execl, but permits you to specify the environment for the new program explicitly. The environment argument is passed following the null pointer that marks the last argv argument, and should be an array of strings in the same format as for the environ variable.

— Function: int execvp (const char *filename, char *const argv[])

The execvp function is similar to execv, except that it searches the directories listed in the PATH environment variable (see Standard Environment) to find the full file name of a file from filename if filename does not contain a slash.

This function is useful for executing system utility programs, because it looks for them in the places that the user has chosen. Shells use it to run the commands that users type.

— Function: int execlp (const char *filename, const char *arg0, ...)

This function is like execl, except that it performs the same file name searching as the execvp function.

The size of the argument list and environment list taken together must not be greater than ARG_MAX bytes. See General Limits. In the GNU system, the size (which compares against ARG_MAX) includes, for each string, the number of characters in the string, plus the size of a char *, plus one, rounded up to a multiple of the size of a char *. Other systems may have somewhat different rules for counting.

These functions normally don't return, since execution of a new program causes the currently executing program to go away completely. A value of -1 is returned in the event of a failure. In addition to the usual file name errors (see File Name Errors), the following errno error conditions are defined for these functions:

E2BIG
The combined size of the new program's argument list and environment list is larger than ARG_MAX bytes. The GNU system has no specific limit on the argument list size, so this error code cannot result, but you may get ENOMEM instead if the arguments are too big for available memory.
ENOEXEC
The specified file can't be executed because it isn't in the right format.
ENOMEM
Executing the specified file requires more storage than is available.

If execution of the new file succeeds, it updates the access time field of the file as if the file had been read. See File Times, for more details about access times of files.

The point at which the file is closed again is not specified, but is at some point before the process exits or before another process image is executed.

Executing a new process image completely changes the contents of memory, copying only the argument and environment strings to new locations. But many other attributes of the process are unchanged:

If the set-user-ID and set-group-ID mode bits of the process image file are set, this affects the effective user ID and effective group ID (respectively) of the process. These concepts are discussed in detail in Process Persona.

Signals that are set to be ignored in the existing process image are also set to be ignored in the new process image. All other signals are set to the default action in the new process image. For more information about signals, see Signal Handling.

File descriptors open in the existing process image remain open in the new process image, unless they have the FD_CLOEXEC (close-on-exec) flag set. The files that remain open inherit all attributes of the open file description from the existing process image, including file locks. File descriptors are discussed in Low-Level I/O.

Streams, by contrast, cannot survive through exec functions, because they are located in the memory of the process itself. The new process image has no streams except those it creates afresh. Each of the streams in the pre-exec process image has a descriptor inside it, and these descriptors do survive through exec (provided that they do not have FD_CLOEXEC set). The new process image can reconnect these to new streams using fdopen (see Descriptors and Streams).

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26.6 Process Completion

The functions described in this section are used to wait for a child process to terminate or stop, and determine its status. These functions are declared in the header file sys/wait.h.

— Function: pid_t waitpid (pid_t pid, int *status-ptr, int options)

The waitpid function is used to request status information from a child process whose process ID is pid. Normally, the calling process is suspended until the child process makes status information available by terminating.

Other values for the pid argument have special interpretations. A value of -1 or WAIT_ANY requests status information for any child process; a value of 0 or WAIT_MYPGRP requests information for any child process in the same process group as the calling process; and any other negative value − pgid requests information for any child process whose process group ID is pgid.

If status information for a child process is available immediately, this function returns immediately without waiting. If more than one eligible child process has status information available, one of them is chosen randomly, and its status is returned immediately. To get the status from the other eligible child processes, you need to call waitpid again.

The options argument is a bit mask. Its value should be the bitwise OR (that is, the `|' operator) of zero or more of the WNOHANG and WUNTRACED flags. You can use the WNOHANG flag to indicate that the parent process shouldn't wait; and the WUNTRACED flag to request status information from stopped processes as well as processes that have terminated.

The status information from the child process is stored in the object that status-ptr points to, unless status-ptr is a null pointer.

This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time waitpid is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this calls to waitpid should be protected using cancellation handlers.

The return value is normally the process ID of the child process whose status is reported. If there are child processes but none of them is waiting to be noticed, waitpid will block until one is. However, if the WNOHANG option was specified, waitpid will return zero instead of blocking.

If a specific PID to wait for was given to waitpid, it will ignore all other children (if any). Therefore if there are children waiting to be noticed but the child whose PID was specified is not one of them, waitpid will block or return zero as described above.

A value of -1 is returned in case of error. The following errno error conditions are defined for this function:

EINTR
The function was interrupted by delivery of a signal to the calling process. See Interrupted Primitives.
ECHILD
There are no child processes to wait for, or the specified pid is not a child of the calling process.
EINVAL
An invalid value was provided for the options argument.

These symbolic constants are defined as values for the pid argument to the waitpid function.

WAIT_ANY
This constant macro (whose value is -1) specifies that waitpid should return status information about any child process.
WAIT_MYPGRP
This constant (with value 0) specifies that waitpid should return status information about any child process in the same process group as the calling process.

These symbolic constants are defined as flags for the options argument to the waitpid function. You can bitwise-OR the flags together to obtain a value to use as the argument.

WNOHANG
This flag specifies that waitpid should return immediately instead of waiting, if there is no child process ready to be noticed.
WUNTRACED
This flag specifies that waitpid should report the status of any child processes that have been stopped as well as those that have terminated.
— Function: pid_t wait (int *status-ptr)

This is a simplified version of waitpid, and is used to wait until any one child process terminates. The call: 

          wait (&status)

is exactly equivalent to: 

          waitpid (-1, &status, 0)

This function is a cancellation point in multi-threaded programs. This is a problem if the thread allocates some resources (like memory, file descriptors, semaphores or whatever) at the time wait is called. If the thread gets canceled these resources stay allocated until the program ends. To avoid this calls to wait should be protected using cancellation handlers.

— Function: pid_t wait4 (pid_t pid, int *status-ptr, int options, struct rusage *usage)

If usage is a null pointer, wait4 is equivalent to waitpid (pid, status-ptr, options).

If usage is not null, wait4 stores usage figures for the child process in *rusage (but only if the child has terminated, not if it has stopped). See Resource Usage.

This function is a BSD extension.

Here's an example of how to use waitpid to get the status from all child processes that have terminated, without ever waiting. This function is designed to be a handler for SIGCHLD, the signal that indicates that at least one child process has terminated. 

     void
     sigchld_handler (int signum)
     {
       int pid, status, serrno;
       serrno = errno;
       while (1)
         {
           pid = waitpid (WAIT_ANY, &status, WNOHANG);
           if (pid < 0)
             {
               perror ("waitpid");
               break;
             }
           if (pid == 0)
             break;
           notice_termination (pid, status);
         }
       errno = serrno;
     }

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26.7 Process Completion Status

If the exit status value (see Program Termination) of the child process is zero, then the status value reported by waitpid or wait is also zero. You can test for other kinds of information encoded in the returned status value using the following macros. These macros are defined in the header file sys/wait.h.

— Macro: int WIFEXITED (int status)

This macro returns a nonzero value if the child process terminated normally with exit or _exit.

— Macro: int WEXITSTATUS (int status)

If WIFEXITED is true of status, this macro returns the low-order 8 bits of the exit status value from the child process. See Exit Status.

— Macro: int WIFSIGNALED (int status)

This macro returns a nonzero value if the child process terminated because it received a signal that was not handled. See Signal Handling.

— Macro: int WTERMSIG (int status)

If WIFSIGNALED is true of status, this macro returns the signal number of the signal that terminated the child process.

— Macro: int WCOREDUMP (int status)

This macro returns a nonzero value if the child process terminated and produced a core dump.

— Macro: int WIFSTOPPED (int status)

This macro returns a nonzero value if the child process is stopped.

— Macro: int WSTOPSIG (int status)

If WIFSTOPPED is true of status, this macro returns the signal number of the signal that caused the child process to stop.

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26.8 BSD Process Wait Functions

The GNU library also provides these related facilities for compatibility with BSD Unix. BSD uses the union wait data type to represent status values rather than an int. The two representations are actually interchangeable; they describe the same bit patterns. The GNU C Library defines macros such as WEXITSTATUS so that they will work on either kind of object, and the wait function is defined to accept either type of pointer as its status-ptr argument.

These functions are declared in sys/wait.h.

— Data Type: union wait

This data type represents program termination status values. It has the following members:

int w_termsig
The value of this member is the same as that of the WTERMSIG macro.
int w_coredump
The value of this member is the same as that of the WCOREDUMP macro.
int w_retcode
The value of this member is the same as that of the WEXITSTATUS macro.
int w_stopsig
The value of this member is the same as that of the WSTOPSIG macro.

Instead of accessing these members directly, you should use the equivalent macros.

The wait3 function is the predecessor to wait4, which is more flexible. wait3 is now obsolete.

— Function: pid_t wait3 (union wait *status-ptr, int options, struct rusage *usage)

If usage is a null pointer, wait3 is equivalent to waitpid (-1, status-ptr, options).

If usage is not null, wait3 stores usage figures for the child process in *rusage (but only if the child has terminated, not if it has stopped). See Resource Usage.

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26.9 Process Creation Example

Here is an example program showing how you might write a function similar to the built-in system. It executes its command argument using the equivalent of `sh -c command'. 

     #include <stddef.h>
     #include <stdlib.h>
     #include <unistd.h>
     #include <sys/types.h>
     #include <sys/wait.h>
     
     /* Execute the command using this shell program.  */
     #define SHELL "/bin/sh"
     
     int
     my_system (const char *command)
     {
       int status;
       pid_t pid;
     
       pid = fork ();
       if (pid == 0)
         {
           /* This is the child process.  Execute the shell command. */
           execl (SHELL, SHELL, "-c", command, NULL);
           _exit (EXIT_FAILURE);
         }
       else if (pid < 0)
         /* The fork failed.  Report failure.  */
         status = -1;
       else
         /* This is the parent process.  Wait for the child to complete.  */
         if (waitpid (pid, &status, 0) != pid)
           status = -1;
       return status;
     }

There are a couple of things you should pay attention to in this example.

Remember that the first argv argument supplied to the program represents the name of the program being executed. That is why, in the call to execl, SHELL is supplied once to name the program to execute and a second time to supply a value for argv[0].

The execl call in the child process doesn't return if it is successful. If it fails, you must do something to make the child process terminate. Just returning a bad status code with return would leave two processes running the original program. Instead, the right behavior is for the child process to report failure to its parent process.

Call _exit to accomplish this. The reason for using _exit instead of exit is to avoid flushing fully buffered streams such as stdout. The buffers of these streams probably contain data that was copied from the parent process by the fork, data that will be output eventually by the parent process. Calling exit in the child would output the data twice. See Termination Internals.

Next: , Previous: Processes, Up: Top

27 Job Control

Job control refers to the protocol for allowing a user to move between multiple process groups (or jobs) within a single login session. The job control facilities are set up so that appropriate behavior for most programs happens automatically and they need not do anything special about job control. So you can probably ignore the material in this chapter unless you are writing a shell or login program.

You need to be familiar with concepts relating to process creation (see Process Creation Concepts) and signal handling (see Signal Handling) in order to understand this material presented in this chapter.

Next: , Up: Job Control

27.1 Concepts of Job Control

The fundamental purpose of an interactive shell is to read commands from the user's terminal and create processes to execute the programs specified by those commands. It can do this using the fork (see Creating a Process) and exec (see Executing a File) functions.

A single command may run just one process—but often one command uses several processes. If you use the `|' operator in a shell command, you explicitly request several programs in their own processes. But even if you run just one program, it can use multiple processes internally. For example, a single compilation command such as `cc -c foo.c' typically uses four processes (though normally only two at any given time). If you run make, its job is to run other programs in separate processes.

The processes belonging to a single command are called a process group or job. This is so that you can operate on all of them at once. For example, typing C-c sends the signal SIGINT to terminate all the processes in the foreground process group.

A session is a larger group of processes. Normally all the processes that stem from a single login belong to the same session.

Every process belongs to a process group. When a process is created, it becomes a member of the same process group and session as its parent process. You can put it in another process group using the setpgid function, provided the process group belongs to the same session.

The only way to put a process in a different session is to make it the initial process of a new session, or a session leader, using the setsid function. This also puts the session leader into a new process group, and you can't move it out of that process group again.

Usually, new sessions are created by the system login program, and the session leader is the process running the user's login shell.

A shell that supports job control must arrange to control which job can use the terminal at any time. Otherwise there might be multiple jobs trying to read from the terminal at once, and confusion about which process should receive the input typed by the user. To prevent this, the shell must cooperate with the terminal driver using the protocol described in this chapter.

The shell can give unlimited access to the controlling terminal to only one process group at a time. This is called the foreground job on that controlling terminal. Other process groups managed by the shell that are executing without such access to the terminal are called background jobs.

If a background job needs to read from its controlling terminal, it is stopped by the terminal driver; if the TOSTOP mode is set, likewise for writing. The user can stop a foreground job by typing the SUSP character (see Special Characters) and a program can stop any job by sending it a SIGSTOP signal. It's the responsibility of the shell to notice when jobs stop, to notify the user about them, and to provide mechanisms for allowing the user to interactively continue stopped jobs and switch jobs between foreground and background.

See Access to the Terminal, for more information about I/O to the controlling terminal,

Next: , Previous: Concepts of Job Control, Up: Job Control

27.2 Job Control is Optional

Not all operating systems support job control. The GNU system does support job control, but if you are using the GNU library on some other system, that system may not support job control itself.

You can use the _POSIX_JOB_CONTROL macro to test at compile-time whether the system supports job control. See System Options.

If job control is not supported, then there can be only one process group per session, which behaves as if it were always in the foreground. The functions for creating additional process groups simply fail with the error code ENOSYS.

The macros naming the various job control signals (see Job Control Signals) are defined even if job control is not supported. However, the system never generates these signals, and attempts to send a job control signal or examine or specify their actions report errors or do nothing.

Next: , Previous: Job Control is Optional, Up: Job Control

27.3 Controlling Terminal of a Process

One of the attributes of a process is its controlling terminal. Child processes created with fork inherit the controlling terminal from their parent process. In this way, all the processes in a session inherit the controlling terminal from the session leader. A session leader that has control of a terminal is called the controlling process of that terminal.

You generally do not need to worry about the exact mechanism used to allocate a controlling terminal to a session, since it is done for you by the system when you log in.

An individual process disconnects from its controlling terminal when it calls setsid to become the leader of a new session. See Process Group Functions.

Next: , Previous: Controlling Terminal, Up: Job Control

27.4 Access to the Controlling Terminal

Processes in the foreground job of a controlling terminal have unrestricted access to that terminal; background processes do not. This section describes in more detail what happens when a process in a background job tries to access its controlling terminal.

When a process in a background job tries to read from its controlling terminal, the process group is usually sent a SIGTTIN signal. This normally causes all of the processes in that group to stop (unless they handle the signal and don't stop themselves). However, if the reading process is ignoring or blocking this signal, then read fails with an EIO error instead.

Similarly, when a process in a background job tries to write to its controlling terminal, the default behavior is to send a SIGTTOU signal to the process group. However, the behavior is modified by the TOSTOP bit of the local modes flags (see Local Modes). If this bit is not set (which is the default), then writing to the controlling terminal is always permitted without sending a signal. Writing is also permitted if the SIGTTOU signal is being ignored or blocked by the writing process.

Most other terminal operations that a program can do are treated as reading or as writing. (The description of each operation should say which.)

For more information about the primitive read and write functions, see I/O Primitives.

Next: , Previous: Access to the Terminal, Up: Job Control

27.5 Orphaned Process Groups

When a controlling process terminates, its terminal becomes free and a new session can be established on it. (In fact, another user could log in on the terminal.) This could cause a problem if any processes from the old session are still trying to use that terminal.

To prevent problems, process groups that continue running even after the session leader has terminated are marked as orphaned process groups.

When a process group becomes an orphan, its processes are sent a SIGHUP signal. Ordinarily, this causes the processes to terminate. However, if a program ignores this signal or establishes a handler for it (see Signal Handling), it can continue running as in the orphan process group even after its controlling process terminates; but it still cannot access the terminal any more.

Next: , Previous: Orphaned Process Groups, Up: Job Control

27.6 Implementing a Job Control Shell

This section describes what a shell must do to implement job control, by presenting an extensive sample program to illustrate the concepts involved.

Next: , Up: Implementing a Shell

27.6.1 Data Structures for the Shell

All of the program examples included in this chapter are part of a simple shell program. This section presents data structures and utility functions which are used throughout the example.

The sample shell deals mainly with two data structures. The job type contains information about a job, which is a set of subprocesses linked together with pipes. The process type holds information about a single subprocess. Here are the relevant data structure declarations: 

     /* A process is a single process.  */
     typedef struct process
     {
       struct process *next;       /* next process in pipeline */
       char **argv;                /* for exec */
       pid_t pid;                  /* process ID */
       char completed;             /* true if process has completed */
       char stopped;               /* true if process has stopped */
       int status;                 /* reported status value */
     } process;
     
     /* A job is a pipeline of processes.  */
     typedef struct job
     {
       struct job *next;           /* next active job */
       char *command;              /* command line, used for messages */
       process *first_process;     /* list of processes in this job */
       pid_t pgid;                 /* process group ID */
       char notified;              /* true if user told about stopped job */
       struct termios tmodes;      /* saved terminal modes */
       int stdin, stdout, stderr;  /* standard i/o channels */
     } job;
     
     /* The active jobs are linked into a list.  This is its head.   */
     job *first_job = NULL;

Here are some utility functions that are used for operating on job objects. 

     /* Find the active job with the indicated pgid.  */
     job *
     find_job (pid_t pgid)
     {
       job *j;
     
       for (j = first_job; j; j = j->next)
         if (j->pgid == pgid)
           return j;
       return NULL;
     }
     
     /* Return true if all processes in the job have stopped or completed.  */
     int
     job_is_stopped (job *j)
     {
       process *p;
     
       for (p = j->first_process; p; p = p->next)
         if (!p->completed && !p->stopped)
           return 0;
       return 1;
     }
     
     /* Return true if all processes in the job have completed.  */
     int
     job_is_completed (job *j)
     {
       process *p;
     
       for (p = j->first_process; p; p = p->next)
         if (!p->completed)
           return 0;
       return 1;
     }

Next: , Previous: Data Structures, Up: Implementing a Shell

27.6.2 Initializing the Shell

When a shell program that normally performs job control is started, it has to be careful in case it has been invoked from another shell that is already doing its own job control.

A subshell that runs interactively has to ensure that it has been placed in the foreground by its parent shell before it can enable job control itself. It does this by getting its initial process group ID with the getpgrp function, and comparing it to the process group ID of the current foreground job associated with its controlling terminal (which can be retrieved using the tcgetpgrp function).

If the subshell is not running as a foreground job, it must stop itself by sending a SIGTTIN signal to its own process group. It may not arbitrarily put itself into the foreground; it must wait for the user to tell the parent shell to do this. If the subshell is continued again, it should repeat the check and stop itself again if it is still not in the foreground.

Once the subshell has been placed into the foreground by its parent shell, it can enable its own job control. It does this by calling setpgid to put itself into its own process group, and then calling tcsetpgrp to place this process group into the foreground.

When a shell enables job control, it should set itself to ignore all the job control stop signals so that it doesn't accidentally stop itself. You can do this by setting the action for all the stop signals to SIG_IGN.

A subshell that runs non-interactively cannot and should not support job control. It must leave all processes it creates in the same process group as the shell itself; this allows the non-interactive shell and its child processes to be treated as a single job by the parent shell. This is easy to do—just don't use any of the job control primitives—but you must remember to make the shell do it.

Here is the initialization code for the sample shell that shows how to do all of this. 

     /* Keep track of attributes of the shell.  */
     
     #include <sys/types.h>
     #include <termios.h>
     #include <unistd.h>
     
     pid_t shell_pgid;
     struct termios shell_tmodes;
     int shell_terminal;
     int shell_is_interactive;
     
     
     /* Make sure the shell is running interactively as the foreground job
        before proceeding. */
     
     void
     init_shell ()
     {
     
       /* See if we are running interactively.  */
       shell_terminal = STDIN_FILENO;
       shell_is_interactive = isatty (shell_terminal);
     
       if (shell_is_interactive)
         {
           /* Loop until we are in the foreground.  */
           while (tcgetpgrp (shell_terminal) != (shell_pgid = getpgrp ()))
             kill (- shell_pgid, SIGTTIN);
     
           /* Ignore interactive and job-control signals.  */
           signal (SIGINT, SIG_IGN);
           signal (SIGQUIT, SIG_IGN);
           signal (SIGTSTP, SIG_IGN);
           signal (SIGTTIN, SIG_IGN);
           signal (SIGTTOU, SIG_IGN);
           signal (SIGCHLD, SIG_IGN);
     
           /* Put ourselves in our own process group.  */
           shell_pgid = getpid ();
           if (setpgid (shell_pgid, shell_pgid) < 0)
             {
               perror ("Couldn't put the shell in its own process group");
               exit (1);
             }
     
           /* Grab control of the terminal.  */
           tcsetpgrp (shell_terminal, shell_pgid);
     
           /* Save default terminal attributes for shell.  */
           tcgetattr (shell_terminal, &shell_tmodes);
         }
     }

Next: , Previous: Initializing the Shell, Up: Implementing a Shell

27.6.3 Launching Jobs

Once the shell has taken responsibility for performing job control on its controlling terminal, it can launch jobs in response to commands typed by the user.

To create the processes in a process group, you use the same fork and exec functions described in Process Creation Concepts. Since there are multiple child processes involved, though, things are a little more complicated and you must be careful to do things in the right order. Otherwise, nasty race conditions can result.

You have two choices for how to structure the tree of parent-child relationships among the processes. You can either make all the processes in the process group be children of the shell process, or you can make one process in group be the ancestor of all the other processes in that group. The sample shell program presented in this chapter uses the first approach because it makes bookkeeping somewhat simpler.

As each process is forked, it should put itself in the new process group by calling setpgid; see Process Group Functions. The first process in the new group becomes its process group leader, and its process ID becomes the process group ID for the group.

The shell should also call setpgid to put each of its child processes into the new process group. This is because there is a potential timing problem: each child process must be put in the process group before it begins executing a new program, and the shell depends on having all the child processes in the group before it continues executing. If both the child processes and the shell call setpgid, this ensures that the right things happen no matter which process gets to it first.

If the job is being launched as a foreground job, the new process group also needs to be put into the foreground on the controlling terminal using tcsetpgrp. Again, this should be done by the shell as well as by each of its child processes, to avoid race conditions.

The next thing each child process should do is to reset its signal actions.

During initialization, the shell process set itself to ignore job control signals; see Initializing the Shell. As a result, any child processes it creates also ignore these signals by inheritance. This is definitely undesirable, so each child process should explicitly set the actions for these signals back to SIG_DFL just after it is forked.

Since shells follow this convention, applications can assume that they inherit the correct handling of these signals from the parent process. But every application has a responsibility not to mess up the handling of stop signals. Applications that disable the normal interpretation of the SUSP character should provide some other mechanism for the user to stop the job. When the user invokes this mechanism, the program should send a SIGTSTP signal to the process group of the process, not just to the process itself. See Signaling Another Process.

Finally, each child process should call exec in the normal way. This is also the point at which redirection of the standard input and output channels should be handled. See Duplicating Descriptors, for an explanation of how to do this.

Here is the function from the sample shell program that is responsible for launching a program. The function is executed by each child process immediately after it has been forked by the shell, and never returns. 

     void
     launch_process (process *p, pid_t pgid,
                     int infile, int outfile, int errfile,
                     int foreground)
     {
       pid_t pid;
     
       if (shell_is_interactive)
         {
           /* Put the process into the process group and give the process group
              the terminal, if appropriate.
              This has to be done both by the shell and in the individual
              child processes because of potential race conditions.  */
           pid = getpid ();
           if (pgid == 0) pgid = pid;
           setpgid (pid, pgid);
           if (foreground)
             tcsetpgrp (shell_terminal, pgid);
     
           /* Set the handling for job control signals back to the default.  */
           signal (SIGINT, SIG_DFL);
           signal (SIGQUIT, SIG_DFL);
           signal (SIGTSTP, SIG_DFL);
           signal (SIGTTIN, SIG_DFL);
           signal (SIGTTOU, SIG_DFL);
           signal (SIGCHLD, SIG_DFL);
         }
     
       /* Set the standard input/output channels of the new process.  */
       if (infile != STDIN_FILENO)
         {
           dup2 (infile, STDIN_FILENO);
           close (infile);
         }
       if (outfile != STDOUT_FILENO)
         {
           dup2 (outfile, STDOUT_FILENO);
           close (outfile);
         }
       if (errfile != STDERR_FILENO)
         {
           dup2 (errfile, STDERR_FILENO);
           close (errfile);
         }
     
       /* Exec the new process.  Make sure we exit.  */
       execvp (p->argv[0], p->argv);
       perror ("execvp");
       exit (1);
     }

If the shell is not running interactively, this function does not do anything with process groups or signals. Remember that a shell not performing job control must keep all of its subprocesses in the same process group as the shell itself.

Next, here is the function that actually launches a complete job. After creating the child processes, this function calls some other functions to put the newly created job into the foreground or background; these are discussed in Foreground and Background. 

     void
     launch_job (job *j, int foreground)
     {
       process *p;
       pid_t pid;
       int mypipe[2], infile, outfile;
     
       infile = j->stdin;
       for (p = j->first_process; p; p = p->next)
         {
           /* Set up pipes, if necessary.  */
           if (p->next)
             {
               if (pipe (mypipe) < 0)
                 {
                   perror ("pipe");
                   exit (1);
                 }
               outfile = mypipe[1];
             }
           else
             outfile = j->stdout;
     
           /* Fork the child processes.  */
           pid = fork ();
           if (pid == 0)
             /* This is the child process.  */
             launch_process (p, j->pgid, infile,
                             outfile, j->stderr, foreground);
           else if (pid < 0)
             {
               /* The fork failed.  */
               perror ("fork");
               exit (1);
             }
           else
             {
               /* This is the parent process.  */
               p->pid = pid;
               if (shell_is_interactive)
                 {
                   if (!j->pgid)
                     j->pgid = pid;
                   setpgid (pid, j->pgid);
                 }
             }
     
           /* Clean up after pipes.  */
           if (infile != j->stdin)
             close (infile);
           if (outfile != j->stdout)
             close (outfile);
           infile = mypipe[0];
         }
     
       format_job_info (j, "launched");
     
       if (!shell_is_interactive)
         wait_for_job (j);
       else if (foreground)
         put_job_in_foreground (j, 0);
       else
         put_job_in_background (j, 0);
     }

Next: , Previous: Launching Jobs, Up: Implementing a Shell

27.6.4 Foreground and Background

Now let's consider what actions must be taken by the shell when it launches a job into the foreground, and how this differs from what must be done when a background job is launched.

When a foreground job is launched, the shell must first give it access to the controlling terminal by calling tcsetpgrp. Then, the shell should wait for processes in that process group to terminate or stop. This is discussed in more detail in Stopped and Terminated Jobs.

When all of the processes in the group have either completed or stopped, the shell should regain control of the terminal for its own process group by calling tcsetpgrp again. Since stop signals caused by I/O from a background process or a SUSP character typed by the user are sent to the process group, normally all the processes in the job stop together.

The foreground job may have left the terminal in a strange state, so the shell should restore its own saved terminal modes before continuing. In case the job is merely stopped, the shell should first save the current terminal modes so that it can restore them later if the job is continued. The functions for dealing with terminal modes are tcgetattr and tcsetattr; these are described in Terminal Modes.

Here is the sample shell's function for doing all of this. 

     /* Put job j in the foreground.  If cont is nonzero,
        restore the saved terminal modes and send the process group a
        SIGCONT signal to wake it up before we block.  */
     
     void
     put_job_in_foreground (job *j, int cont)
     {
       /* Put the job into the foreground.  */
       tcsetpgrp (shell_terminal, j->pgid);
     
       /* Send the job a continue signal, if necessary.  */
       if (cont)
         {
           tcsetattr (shell_terminal, TCSADRAIN, &j->tmodes);
           if (kill (- j->pgid, SIGCONT) < 0)
             perror ("kill (SIGCONT)");
         }
     
       /* Wait for it to report.  */
       wait_for_job (j);
     
       /* Put the shell back in the foreground.  */
       tcsetpgrp (shell_terminal, shell_pgid);
     
       /* Restore the shell's terminal modes.  */
       tcgetattr (shell_terminal, &j->tmodes);
       tcsetattr (shell_terminal, TCSADRAIN, &shell_tmodes);
     }

If the process group is launched as a background job, the shell should remain in the foreground itself and continue to read commands from the terminal.

In the sample shell, there is not much that needs to be done to put a job into the background. Here is the function it uses: 

     /* Put a job in the background.  If the cont argument is true, send
        the process group a SIGCONT signal to wake it up.  */
     
     void
     put_job_in_background (job *j, int cont)
     {
       /* Send the job a continue signal, if necessary.  */
       if (cont)
         if (kill (-j->pgid, SIGCONT) < 0)
           perror ("kill (SIGCONT)");
     }

Next: , Previous: Foreground and Background, Up: Implementing a Shell

27.6.5 Stopped and Terminated Jobs

When a foreground process is launched, the shell must block until all of the processes in that job have either terminated or stopped. It can do this by calling the waitpid function; see Process Completion. Use the WUNTRACED option so that status is reported for processes that stop as well as processes that terminate.

The shell must also check on the status of background jobs so that it can report terminated and stopped jobs to the user; this can be done by calling waitpid with the WNOHANG option. A good place to put a such a check for terminated and stopped jobs is just before prompting for a new command.

The shell can also receive asynchronous notification that there is status information available for a child process by establishing a handler for SIGCHLD signals. See Signal Handling.

In the sample shell program, the SIGCHLD signal is normally ignored. This is to avoid reentrancy problems involving the global data structures the shell manipulates. But at specific times when the shell is not using these data structures—such as when it is waiting for input on the terminal—it makes sense to enable a handler for SIGCHLD. The same function that is used to do the synchronous status checks (do_job_notification, in this case) can also be called from within this handler.

Here are the parts of the sample shell program that deal with checking the status of jobs and reporting the information to the user. 

     /* Store the status of the process pid that was returned by waitpid.
        Return 0 if all went well, nonzero otherwise.  */
     
     int
     mark_process_status (pid_t pid, int status)
     {
       job *j;
       process *p;
     
       if (pid > 0)
         {
           /* Update the record for the process.  */
           for (j = first_job; j; j = j->next)
             for (p = j->first_process; p; p = p->next)
               if (p->pid == pid)
                 {
                   p->status = status;
                   if (WIFSTOPPED (status))
                     p->stopped = 1;
                   else
                     {
                       p->completed = 1;
                       if (WIFSIGNALED (status))
                         fprintf (stderr, "%d: Terminated by signal %d.\n",
                                  (int) pid, WTERMSIG (p->status));
                     }
                   return 0;
                  }
           fprintf (stderr, "No child process %d.\n", pid);
           return -1;
         }
       else if (pid == 0 || errno == ECHILD)
         /* No processes ready to report.  */
         return -1;
       else {
         /* Other weird errors.  */
         perror ("waitpid");
         return -1;
       }
     }
     
     /* Check for processes that have status information available,
        without blocking.  */
     
     void
     update_status (void)
     {
       int status;
       pid_t pid;
     
       do
         pid = waitpid (WAIT_ANY, &status, WUNTRACED|WNOHANG);
       while (!mark_process_status (pid, status));
     }
     
     /* Check for processes that have status information available,
        blocking until all processes in the given job have reported.  */
     
     void
     wait_for_job (job *j)
     {
       int status;
       pid_t pid;
     
       do
         pid = waitpid (WAIT_ANY, &status, WUNTRACED);
       while (!mark_process_status (pid, status)
              && !job_is_stopped (j)
              && !job_is_completed (j));
     }
     
     /* Format information about job status for the user to look at.  */
     
     void
     format_job_info (job *j, const char *status)
     {
       fprintf (stderr, "%ld (%s): %s\n", (long)j->pgid, status, j->command);
     }
     
     /* Notify the user about stopped or terminated jobs.
        Delete terminated jobs from the active job list.  */
     
     void
     do_job_notification (void)
     {
       job *j, *jlast, *jnext;
       process *p;
     
       /* Update status information for child processes.  */
       update_status ();
     
       jlast = NULL;
       for (j = first_job; j; j = jnext)
         {
           jnext = j->next;
     
           /* If all processes have completed, tell the user the job has
              completed and delete it from the list of active jobs.  */
           if (job_is_completed (j)) {
             format_job_info (j, "completed");
             if (jlast)
               jlast->next = jnext;
             else
               first_job = jnext;
             free_job (j);
           }
     
           /* Notify the user about stopped jobs,
              marking them so that we won't do this more than once.  */
           else if (job_is_stopped (j) && !j->notified) {
             format_job_info (j, "stopped");
             j->notified = 1;
             jlast = j;
           }
     
           /* Don't say anything about jobs that are still running.  */
           else
             jlast = j;
         }
     }

Next: , Previous: Stopped and Terminated Jobs, Up: Implementing a Shell

27.6.6 Continuing Stopped Jobs

The shell can continue a stopped job by sending a SIGCONT signal to its process group. If the job is being continued in the foreground, the shell should first invoke tcsetpgrp to give the job access to the terminal, and restore the saved terminal settings. After continuing a job in the foreground, the shell should wait for the job to stop or complete, as if the job had just been launched in the foreground.

The sample shell program handles both newly created and continued jobs with the same pair of functions, put_job_in_foreground and put_job_in_background. The definitions of these functions were given in Foreground and Background. When continuing a stopped job, a nonzero value is passed as the cont argument to ensure that the SIGCONT signal is sent and the terminal modes reset, as appropriate.

This leaves only a function for updating the shell's internal bookkeeping about the job being continued: 

     /* Mark a stopped job J as being running again.  */
     
     void
     mark_job_as_running (job *j)
     {
       Process *p;
     
       for (p = j->first_process; p; p = p->next)
         p->stopped = 0;
       j->notified = 0;
     }
     
     /* Continue the job J.  */
     
     void
     continue_job (job *j, int foreground)
     {
       mark_job_as_running (j);
       if (foreground)
         put_job_in_foreground (j, 1);
       else
         put_job_in_background (j, 1);
     }

Previous: Continuing Stopped Jobs, Up: Implementing a Shell

27.6.7 The Missing Pieces

The code extracts for the sample shell included in this chapter are only a part of the entire shell program. In particular, nothing at all has been said about how job and program data structures are allocated and initialized.

Most real shells provide a complex user interface that has support for a command language; variables; abbreviations, substitutions, and pattern matching on file names; and the like. All of this is far too complicated to explain here! Instead, we have concentrated on showing how to implement the core process creation and job control functions that can be called from such a shell.

Here is a table summarizing the major entry points we have presented:

void init_shell (void)
Initialize the shell's internal state. See Initializing the Shell.
void launch_job (job *j, int foreground)
Launch the job j as either a foreground or background job. See Launching Jobs.
void do_job_notification (void)
Check for and report any jobs that have terminated or stopped. Can be called synchronously or within a handler for SIGCHLD signals. See Stopped and Terminated Jobs.
void continue_job (job *j, int foreground)
Continue the job j. See Continuing Stopped Jobs.

Of course, a real shell would also want to provide other functions for managing jobs. For example, it would be useful to have commands to list all active jobs or to send a signal (such as SIGKILL) to a job.

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27.7 Functions for Job Control

This section contains detailed descriptions of the functions relating to job control.

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27.7.1 Identifying the Controlling Terminal

You can use the ctermid function to get a file name that you can use to open the controlling terminal. In the GNU library, it returns the same string all the time: "/dev/tty". That is a special “magic” file name that refers to the controlling terminal of the current process (if it has one). To find the name of the specific terminal device, use ttyname; see Is It a Terminal.

The function ctermid is declared in the header file stdio.h.

— Function: char * ctermid (char *string)

The ctermid function returns a string containing the file name of the controlling terminal for the current process. If string is not a null pointer, it should be an array that can hold at least L_ctermid characters; the string is returned in this array. Otherwise, a pointer to a string in a static area is returned, which might get overwritten on subsequent calls to this function.

An empty string is returned if the file name cannot be determined for any reason. Even if a file name is returned, access to the file it represents is not guaranteed.

— Macro: int L_ctermid

The value of this macro is an integer constant expression that represents the size of a string large enough to hold the file name returned by ctermid.

See also the isatty and ttyname functions, in Is It a Terminal.

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27.7.2 Process Group Functions

Here are descriptions of the functions for manipulating process groups. Your program should include the header files sys/types.h and unistd.h to use these functions.

— Function: pid_t setsid (void)

The setsid function creates a new session. The calling process becomes the session leader, and is put in a new process group whose process group ID is the same as the process ID of that process. There are initially no other processes in the new process group, and no other process groups in the new session.

This function also makes the calling process have no controlling terminal.

The setsid function returns the new process group ID of the calling process if successful. A return value of -1 indicates an error. The following errno error conditions are defined for this function:

EPERM
The calling process is already a process group leader, or there is already another process group around that has the same process group ID.

— Function: pid_t getsid (pid_t pid)

The getsid function returns the process group ID of the session leader of the specified process. If a pid is 0, the process group ID of the session leader of the current process is returned.

In case of error -1 is returned and errno is set. The following errno error conditions are defined for this function:

ESRCH
There is no process with the given process ID pid.
EPERM
The calling process and the process specified by pid are in different sessions, and the implementation doesn't allow to access the process group ID of the session leader of the process with ID pid from the calling process.

The getpgrp function has two definitions: one derived from BSD Unix, and one from the POSIX.1 standard. The feature test macros you have selected (see Feature Test Macros) determine which definition you get. Specifically, you get the BSD version if you define _BSD_SOURCE; otherwise, you get the POSIX version if you define _POSIX_SOURCE or _GNU_SOURCE. Programs written for old BSD systems will not include unistd.h, which defines getpgrp specially under _BSD_SOURCE. You must link such programs with the -lbsd-compat option to get the BSD definition.

— POSIX.1 Function: pid_t getpgrp (void)

The POSIX.1 definition of getpgrp returns the process group ID of the calling process.

— BSD Function: pid_t getpgrp (pid_t pid)

The BSD definition of getpgrp returns the process group ID of the process pid. You can supply a value of 0 for the pid argument to get information about the calling process.

— System V Function: int getpgid (pid_t pid)

getpgid is the same as the BSD function getpgrp. It returns the process group ID of the process pid. You can supply a value of 0 for the pid argument to get information about the calling process.

In case of error -1 is returned and errno is set. The following errno error conditions are defined for this function:

ESRCH
There is no process with the given process ID pid. The calling process and the process specified by pid are in different sessions, and the implementation doesn't allow to access the process group ID of the process with ID pid from the calling process.

— Function: int setpgid (pid_t pid, pid_t pgid)

The setpgid function puts the process pid into the process group pgid. As a special case, either pid or pgid can be zero to indicate the process ID of the calling process.

This function fails on a system that does not support job control. See Job Control is Optional, for more information.

If the operation is successful, setpgid returns zero. Otherwise it returns -1. The following errno error conditions are defined for this function:

EACCES
The child process named by pid has executed an exec function since it was forked.
EINVAL
The value of the pgid is not valid.
ENOSYS
The system doesn't support job control.
EPERM
The process indicated by the pid argument is a session leader, or is not in the same session as the calling process, or the value of the pgid argument doesn't match a process group ID in the same session as the calling process.
ESRCH
The process indicated by the pid argument is not the calling process or a child of the calling process.

— Function: int setpgrp (pid_t pid, pid_t pgid)

This is the BSD Unix name for setpgid. Both functions do exactly the same thing.

Previous: Process Group Functions, Up: Functions for Job Control

27.7.3 Functions for Controlling Terminal Access

These are the functions for reading or setting the foreground process group of a terminal. You should include the header files sys/types.h and unistd.h in your application to use these functions. Although these functions take a file descriptor argument to specify the terminal device, the foreground job is associated with the terminal file itself and not a particular open file descriptor.

— Function: pid_t tcgetpgrp (int filedes)

This function returns the process group ID of the foreground process group associated with the terminal open on descriptor filedes.

If there is no foreground process group, the return value is a number greater than 1 that does not match the process group ID of any existing process group. This can happen if all of the processes in the job that was formerly the foreground job have terminated, and no other job has yet been moved into the foreground.

In case of an error, a value of -1 is returned. The following errno error conditions are defined for this function:

EBADF
The filedes argument is not a valid file descriptor.
ENOSYS
The system doesn't support job control.
ENOTTY
The terminal file associated with the filedes argument isn't the controlling terminal of the calling process.

— Function: int tcsetpgrp (int filedes, pid_t pgid)

This function is used to set a terminal's foreground process group ID. The argument filedes is a descriptor which specifies the terminal; pgid specifies the process group. The calling process must be a member of the same session as pgid and must have the same controlling terminal.

For terminal access purposes, this function is treated as output. If it is called from a background process on its controlling terminal, normally all processes in the process group are sent a SIGTTOU signal. The exception is if the calling process itself is ignoring or blocking SIGTTOU signals, in which case the operation is performed and no signal is sent.

If successful, tcsetpgrp returns 0. A return value of -1 indicates an error. The following errno error conditions are defined for this function:

EBADF
The filedes argument is not a valid file descriptor.
EINVAL
The pgid argument is not valid.
ENOSYS
The system doesn't support job control.
ENOTTY
The filedes isn't the controlling terminal of the calling process.
EPERM
The pgid isn't a process group in the same session as the calling process.

— Function: pid_t tcgetsid (int fildes)

This function is used to obtain the process group ID of the session for which the terminal specified by fildes is the controlling terminal. If the call is successful the group ID is returned. Otherwise the return value is (pid_t) -1 and the global variable errno is set to the following value:

EBADF
The filedes argument is not a valid file descriptor.
ENOTTY
The calling process does not have a controlling terminal, or the file is not the controlling terminal.

Next: , Previous: Job Control, Up: Top

28 System Databases and Name Service Switch

Various functions in the C Library need to be configured to work correctly in the local environment. Traditionally, this was done by using files (e.g., /etc/passwd), but other nameservices (like the Network Information Service (NIS) and the Domain Name Service (DNS)) became popular, and were hacked into the C library, usually with a fixed search order (see frobnicate).

The GNU C Library contains a cleaner solution of this problem. It is designed after a method used by Sun Microsystems in the C library of Solaris 2. GNU C Library follows their name and calls this scheme Name Service Switch (NSS).

Though the interface might be similar to Sun's version there is no common code. We never saw any source code of Sun's implementation and so the internal interface is incompatible. This also manifests in the file names we use as we will see later.

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28.1 NSS Basics

The basic idea is to put the implementation of the different services offered to access the databases in separate modules. This has some advantages:

  1. Contributors can add new services without adding them to GNU C Library.
  2. The modules can be updated separately.
  3. The C library image is smaller.

To fulfill the first goal above the ABI of the modules will be described below. For getting the implementation of a new service right it is important to understand how the functions in the modules get called. They are in no way designed to be used by the programmer directly. Instead the programmer should only use the documented and standardized functions to access the databases.

The databases available in the NSS are

aliases
Mail aliases
ethers
Ethernet numbers,
group
Groups of users, see Group Database.
hosts
Host names and numbers, see Host Names.
netgroup
Network wide list of host and users, see Netgroup Database.
networks
Network names and numbers, see Networks Database.
protocols
Network protocols, see Protocols Database.
passwd
User passwords, see User Database.
rpc
Remote procedure call names and numbers,
services
Network services, see Services Database.
shadow
Shadow user passwords,

There will be some more added later (automount, bootparams, netmasks, and publickey).

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28.2 The NSS Configuration File

Somehow the NSS code must be told about the wishes of the user. For this reason there is the file /etc/nsswitch.conf. For each database this file contain a specification how the lookup process should work. The file could look like this: 

     # /etc/nsswitch.conf
     #
     # Name Service Switch configuration file.
     #
     
     passwd:     db files nis
     shadow:     files
     group:      db files nis
     
     hosts:      files nisplus nis dns
     networks:   nisplus [NOTFOUND=return] files
     
     ethers:     nisplus [NOTFOUND=return] db files
     protocols:  nisplus [NOTFOUND=return] db files
     rpc:        nisplus [NOTFOUND=return] db files
     services:   nisplus [NOTFOUND=return] db files

The first column is the database as you can guess from the table above. The rest of the line specifies how the lookup process works. Please note that you specify the way it works for each database individually. This cannot be done with the old way of a monolithic implementation.

The configuration specification for each database can contain two different items:

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28.2.1 Services in the NSS configuration File

The above example file mentions four different services: files, db, nis, and nisplus. This does not mean these services are available on all sites and it does also not mean these are all the services which will ever be available.

In fact, these names are simply strings which the NSS code uses to find the implicitly addressed functions. The internal interface will be described later. Visible to the user are the modules which implement an individual service.

Assume the service name shall be used for a lookup. The code for this service is implemented in a module called libnss_name. On a system supporting shared libraries this is in fact a shared library with the name (for example) libnss_name.so.2. The number at the end is the currently used version of the interface which will not change frequently. Normally the user should not have to be cognizant of these files since they should be placed in a directory where they are found automatically. Only the names of all available services are important.

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28.2.2 Actions in the NSS configuration

The second item in the specification gives the user much finer control on the lookup process. Action items are placed between two service names and are written within brackets. The general form is 

     [ ( !? status = action )+ ]

where 

     status => success | notfound | unavail | tryagain
     action => return | continue

The case of the keywords is insignificant. The status values are the results of a call to a lookup function of a specific service. They mean

`success'
No error occurred and the wanted entry is returned. The default action for this is return.
`notfound'
The lookup process works ok but the needed value was not found. The default action is continue.
`unavail'
The service is permanently unavailable. This can either mean the needed file is not available, or, for DNS, the server is not available or does not allow queries. The default action is continue.
`tryagain'
The service is temporarily unavailable. This could mean a file is locked or a server currently cannot accept more connections. The default action is continue.

If we have a line like 

     ethers: nisplus [NOTFOUND=return] db files

this is equivalent to 

     ethers: nisplus [SUCCESS=return NOTFOUND=return UNAVAIL=continue
                      TRYAGAIN=continue]
             db      [SUCCESS=return NOTFOUND=continue UNAVAIL=continue
                      TRYAGAIN=continue]
             files

(except that it would have to be written on one line). The default value for the actions are normally what you want, and only need to be changed in exceptional cases.

If the optional ! is placed before the status this means the following action is used for all statuses but status itself. I.e., ! is negation as in the C language (and others).

Before we explain the exception which makes this action item necessary one more remark: obviously it makes no sense to add another action item after the files service. Since there is no other service following the action always is return.

Now, why is this [NOTFOUND=return] action useful? To understand this we should know that the nisplus service is often complete; i.e., if an entry is not available in the NIS+ tables it is not available anywhere else. This is what is expressed by this action item: it is useless to examine further services since they will not give us a result.

The situation would be different if the NIS+ service is not available because the machine is booting. In this case the return value of the lookup function is not notfound but instead unavail. And as you can see in the complete form above: in this situation the db and files services are used. Neat, isn't it? The system administrator need not pay special care for the time the system is not completely ready to work (while booting or shutdown or network problems).

Previous: Actions in the NSS configuration, Up: NSS Configuration File

28.2.3 Notes on the NSS Configuration File

Finally a few more hints. The NSS implementation is not completely helpless if /etc/nsswitch.conf does not exist. For all supported databases there is a default value so it should normally be possible to get the system running even if the file is corrupted or missing.

For the hosts and networks databases the default value is dns [!UNAVAIL=return] files. I.e., the system is prepared for the DNS service not to be available but if it is available the answer it returns is definitive.

The passwd, group, and shadow databases are traditionally handled in a special way. The appropriate files in the /etc directory are read but if an entry with a name starting with a + character is found NIS is used. This kind of lookup remains possible by using the special lookup service compat and the default value for the three databases above is compat [NOTFOUND=return] files.

For all other databases the default value is nis [NOTFOUND=return] files. This solution give the best chance to be correct since NIS and file based lookup is used.

A second point is that the user should try to optimize the lookup process. The different service have different response times. A simple file look up on a local file could be fast, but if the file is long and the needed entry is near the end of the file this may take quite some time. In this case it might be better to use the db service which allows fast local access to large data sets.

Often the situation is that some global information like NIS must be used. So it is unavoidable to use service entries like nis etc. But one should avoid slow services like this if possible.

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28.3 NSS Module Internals

Now it is time to describe what the modules look like. The functions contained in a module are identified by their names. I.e., there is no jump table or the like. How this is done is of no interest here; those interested in this topic should read about Dynamic Linking.

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28.3.1 The Naming Scheme of the NSS Modules

The name of each function consist of various parts:

_nss_service_function

service of course corresponds to the name of the module this function is found in.3 The function part is derived from the interface function in the C library itself. If the user calls the function gethostbyname and the service used is files the function 

            _nss_files_gethostbyname_r

in the module 

            libnss_files.so.2

is used. You see, what is explained above in not the whole truth. In fact the NSS modules only contain reentrant versions of the lookup functions. I.e., if the user would call the gethostbyname_r function this also would end in the above function. For all user interface functions the C library maps this call to a call to the reentrant function. For reentrant functions this is trivial since the interface is (nearly) the same. For the non-reentrant version The library keeps internal buffers which are used to replace the user supplied buffer.

I.e., the reentrant functions can have counterparts. No service module is forced to have functions for all databases and all kinds to access them. If a function is not available it is simply treated as if the function would return unavail (see Actions in the NSS configuration).

The file name libnss_files.so.2 would be on a Solaris 2 system nss_files.so.2. This is the difference mentioned above. Sun's NSS modules are usable as modules which get indirectly loaded only.

The NSS modules in the GNU C Library are prepared to be used as normal libraries themselves. This is not true at the moment, though. However, the organization of the name space in the modules does not make it impossible like it is for Solaris. Now you can see why the modules are still libraries.4

Previous: NSS Module Names, Up: NSS Module Internals

28.3.2 The Interface of the Function in NSS Modules

Now we know about the functions contained in the modules. It is now time to describe the types. When we mentioned the reentrant versions of the functions above, this means there are some additional arguments (compared with the standard, non-reentrant version). The prototypes for the non-reentrant and reentrant versions of our function above are: 

     struct hostent *gethostbyname (const char *name)
     
     int gethostbyname_r (const char *name, struct hostent *result_buf,
                          char *buf, size_t buflen, struct hostent **result,
                          int *h_errnop)

The actual prototype of the function in the NSS modules in this case is 

     enum nss_status _nss_files_gethostbyname_r (const char *name,
                                                 struct hostent *result_buf,
                                                 char *buf, size_t buflen,
                                                 int *errnop, int *h_errnop)

I.e., the interface function is in fact the reentrant function with the change of the return value and the omission of the result parameter. While the user-level function returns a pointer to the result the reentrant function return an enum nss_status value:

NSS_STATUS_TRYAGAIN
numeric value -2
NSS_STATUS_UNAVAIL
numeric value -1
NSS_STATUS_NOTFOUND
numeric value 0
NSS_STATUS_SUCCESS
numeric value 1

Now you see where the action items of the /etc/nsswitch.conf file are used.

If you study the source code you will find there is a fifth value: NSS_STATUS_RETURN. This is an internal use only value, used by a few functions in places where none of the above value can be used. If necessary the source code should be examined to learn about the details.

In case the interface function has to return an error it is important that the correct error code is stored in *errnop. Some return status value have only one associated error code, others have more.

NSS_STATUS_TRYAGAIN EAGAIN One of the functions used ran temporarily out of resources or a service is currently not available.
ERANGE The provided buffer is not large enough. The function should be called again with a larger buffer.
NSS_STATUS_UNAVAIL ENOENT A necessary input file cannot be found.
NSS_STATUS_NOTFOUND ENOENT The requested entry is not available.

These are proposed values. There can be other error codes and the described error codes can have different meaning. With one exception: when returning NSS_STATUS_TRYAGAIN the error code ERANGE must mean that the user provided buffer is too small. Everything is non-critical.

The above function has something special which is missing for almost all the other module functions. There is an argument h_errnop. This points to a variable which will be filled with the error code in case the execution of the function fails for some reason. The reentrant function cannot use the global variable h_errno; gethostbyname calls gethostbyname_r with the last argument set to &h_errno.

The getXXXbyYYY functions are the most important functions in the NSS modules. But there are others which implement the other ways to access system databases (say for the password database, there are setpwent, getpwent, and endpwent). These will be described in more detail later. Here we give a general way to determine the signature of the module function:

This table is correct for all functions but the set...ent and end...ent functions.

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28.4 Extending NSS

One of the advantages of NSS mentioned above is that it can be extended quite easily. There are two ways in which the extension can happen: adding another database or adding another service. The former is normally done only by the C library developers. It is here only important to remember that adding another database is independent from adding another service because a service need not support all databases or lookup functions.

A designer/implementor of a new service is therefore free to choose the databases s/he is interested in and leave the rest for later (or completely aside).

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28.4.1 Adding another Service to NSS

The sources for a new service need not (and should not) be part of the GNU C Library itself. The developer retains complete control over the sources and its development. The links between the C library and the new service module consists solely of the interface functions.

Each module is designed following a specific interface specification. For now the version is 2 (the interface in version 1 was not adequate) and this manifests in the version number of the shared library object of the NSS modules: they have the extension .2. If the interface changes again in an incompatible way, this number will be increased. Modules using the old interface will still be usable.

Developers of a new service will have to make sure that their module is created using the correct interface number. This means the file itself must have the correct name and on ELF systems the soname (Shared Object Name) must also have this number. Building a module from a bunch of object files on an ELF system using GNU CC could be done like this: 

     gcc -shared -o libnss_NAME.so.2 -Wl,-soname,libnss_NAME.so.2 OBJECTS

Options for Linking, to learn more about this command line.

To use the new module the library must be able to find it. This can be achieved by using options for the dynamic linker so that it will search the directory where the binary is placed. For an ELF system this could be done by adding the wanted directory to the value of LD_LIBRARY_PATH.

But this is not always possible since some programs (those which run under IDs which do not belong to the user) ignore this variable. Therefore the stable version of the module should be placed into a directory which is searched by the dynamic linker. Normally this should be the directory $prefix/lib, where $prefix corresponds to the value given to configure using the --prefix option. But be careful: this should only be done if it is clear the module does not cause any harm. System administrators should be careful.

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28.4.2 Internals of the NSS Module Functions

Until now we only provided the syntactic interface for the functions in the NSS module. In fact there is not much more we can say since the implementation obviously is different for each function. But a few general rules must be followed by all functions.

In fact there are four kinds of different functions which may appear in the interface. All derive from the traditional ones for system databases. db in the following table is normally an abbreviation for the database (e.g., it is pw for the password database).

enum nss_status _nss_database_setdbent (void)
This function prepares the service for following operations. For a simple file based lookup this means files could be opened, for other services this function simply is a noop.

One special case for this function is that it takes an additional argument for some databases (i.e., the interface is int setdbent (int)). Host Names, which describes the sethostent function.

The return value should be NSS_STATUS_SUCCESS or according to the table above in case of an error (see NSS Modules Interface).

enum nss_status _nss_database_enddbent (void)
This function simply closes all files which are still open or removes buffer caches. If there are no files or buffers to remove this is again a simple noop.

There normally is no return value different to NSS_STATUS_SUCCESS.

enum nss_status _nss_database_getdbent_r (STRUCTURE *result, char *buffer, size_t buflen, int *errnop)
Since this function will be called several times in a row to retrieve one entry after the other it must keep some kind of state. But this also means the functions are not really reentrant. They are reentrant only in that simultaneous calls to this function will not try to write the retrieved data in the same place (as it would be the case for the non-reentrant functions); instead, it writes to the structure pointed to by the result parameter. But the calls share a common state and in the case of a file access this means they return neighboring entries in the file.

The buffer of length buflen pointed to by buffer can be used for storing some additional data for the result. It is not guaranteed that the same buffer will be passed for the next call of this function. Therefore one must not misuse this buffer to save some state information from one call to another.

Before the function returns the implementation should store the value of the local errno variable in the variable pointed to be errnop. This is important to guarantee the module working in statically linked programs.

As explained above this function could also have an additional last argument. This depends on the database used; it happens only for host and networks.

The function shall return NSS_STATUS_SUCCESS as long as there are more entries. When the last entry was read it should return NSS_STATUS_NOTFOUND. When the buffer given as an argument is too small for the data to be returned NSS_STATUS_TRYAGAIN should be returned. When the service was not formerly initialized by a call to _nss_DATABASE_setdbent all return value allowed for this function can also be returned here.

enum nss_status _nss_DATABASE_getdbbyXX_r (PARAMS, STRUCTURE *result, char *buffer, size_t buflen, int *errnop)
This function shall return the entry from the database which is addressed by the PARAMS. The type and number of these arguments vary. It must be individually determined by looking to the user-level interface functions. All arguments given to the non-reentrant version are here described by PARAMS.

The result must be stored in the structure pointed to by result. If there is additional data to return (say strings, where the result structure only contains pointers) the function must use the buffer or length buflen. There must not be any references to non-constant global data.

The implementation of this function should honor the stayopen flag set by the setDBent function whenever this makes sense.

Before the function returns the implementation should store the value of the local errno variable in the variable pointed to be errnop. This is important to guarantee the module working in statically linked programs.

Again, this function takes an additional last argument for the host and networks database.

The return value should as always follow the rules given above (see NSS Modules Interface).

Next: , Previous: Name Service Switch, Up: Top

29 Users and Groups

Every user who can log in on the system is identified by a unique number called the user ID. Each process has an effective user ID which says which user's access permissions it has.

Users are classified into groups for access control purposes. Each process has one or more group ID values which say which groups the process can use for access to files.

The effective user and group IDs of a process collectively form its persona. This determines which files the process can access. Normally, a process inherits its persona from the parent process, but under special circumstances a process can change its persona and thus change its access permissions.

Each file in the system also has a user ID and a group ID. Access control works by comparing the user and group IDs of the file with those of the running process.

The system keeps a database of all the registered users, and another database of all the defined groups. There are library functions you can use to examine these databases.

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29.1 User and Group IDs

Each user account on a computer system is identified by a user name (or login name) and user ID. Normally, each user name has a unique user ID, but it is possible for several login names to have the same user ID. The user names and corresponding user IDs are stored in a data base which you can access as described in User Database.

Users are classified in groups. Each user name belongs to one default group and may also belong to any number of supplementary groups. Users who are members of the same group can share resources (such as files) that are not accessible to users who are not a member of that group. Each group has a group name and group ID. See Group Database, for how to find information about a group ID or group name.

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29.2 The Persona of a Process

At any time, each process has an effective user ID, a effective group ID, and a set of supplementary group IDs. These IDs determine the privileges of the process. They are collectively called the persona of the process, because they determine “who it is” for purposes of access control.

Your login shell starts out with a persona which consists of your user ID, your default group ID, and your supplementary group IDs (if you are in more than one group). In normal circumstances, all your other processes inherit these values.

A process also has a real user ID which identifies the user who created the process, and a real group ID which identifies that user's default group. These values do not play a role in access control, so we do not consider them part of the persona. But they are also important.

Both the real and effective user ID can be changed during the lifetime of a process. See Why Change Persona.

For details on how a process's effective user ID and group IDs affect its permission to access files, see Access Permission.

The effective user ID of a process also controls permissions for sending signals using the kill function. See Signaling Another Process.

Finally, there are many operations which can only be performed by a process whose effective user ID is zero. A process with this user ID is a privileged process. Commonly the user name root is associated with user ID 0, but there may be other user names with this ID.

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29.3 Why Change the Persona of a Process?

The most obvious situation where it is necessary for a process to change its user and/or group IDs is the login program. When login starts running, its user ID is root. Its job is to start a shell whose user and group IDs are those of the user who is logging in. (To accomplish this fully, login must set the real user and group IDs as well as its persona. But this is a special case.)

The more common case of changing persona is when an ordinary user program needs access to a resource that wouldn't ordinarily be accessible to the user actually running it.

For example, you may have a file that is controlled by your program but that shouldn't be read or modified directly by other users, either because it implements some kind of locking protocol, or because you want to preserve the integrity or privacy of the information it contains. This kind of restricted access can be implemented by having the program change its effective user or group ID to match that of the resource.

Thus, imagine a game program that saves scores in a file. The game program itself needs to be able to update this file no matter who is running it, but if users can write the file without going through the game, they can give themselves any scores they like. Some people consider this undesirable, or even reprehensible. It can be prevented by creating a new user ID and login name (say, games) to own the scores file, and make the file writable only by this user. Then, when the game program wants to update this file, it can change its effective user ID to be that for games. In effect, the program must adopt the persona of games so it can write the scores file.

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29.4 How an Application Can Change Persona

The ability to change the persona of a process can be a source of unintentional privacy violations, or even intentional abuse. Because of the potential for problems, changing persona is restricted to special circumstances.

You can't arbitrarily set your user ID or group ID to anything you want; only privileged processes can do that. Instead, the normal way for a program to change its persona is that it has been set up in advance to change to a particular user or group. This is the function of the setuid and setgid bits of a file's access mode. See Permission Bits.

When the setuid bit of an executable file is on, executing that file gives the process a third user ID: the file user ID. This ID is set to the owner ID of the file. The system then changes the effective user ID to the file user ID. The real user ID remains as it was. Likewise, if the setgid bit is on, the process is given a file group ID equal to the group ID of the file, and its effective group ID is changed to the file group ID.

If a process has a file ID (user or group), then it can at any time change its effective ID to its real ID and back to its file ID. Programs use this feature to relinquish their special privileges except when they actually need them. This makes it less likely that they can be tricked into doing something inappropriate with their privileges.

Portability Note: Older systems do not have file IDs. To determine if a system has this feature, you can test the compiler define _POSIX_SAVED_IDS. (In the POSIX standard, file IDs are known as saved IDs.)

See File Attributes, for a more general discussion of file modes and accessibility.

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29.5 Reading the Persona of a Process

Here are detailed descriptions of the functions for reading the user and group IDs of a process, both real and effective. To use these facilities, you must include the header files sys/types.h and unistd.h.

— Data Type: uid_t

This is an integer data type used to represent user IDs. In the GNU library, this is an alias for unsigned int.

— Data Type: gid_t

This is an integer data type used to represent group IDs. In the GNU library, this is an alias for unsigned int.

— Function: uid_t getuid (void)

The getuid function returns the real user ID of the process.

— Function: gid_t getgid (void)

The getgid function returns the real group ID of the process.

— Function: uid_t geteuid (void)

The geteuid function returns the effective user ID of the process.

— Function: gid_t getegid (void)

The getegid function returns the effective group ID of the process.

— Function: int getgroups (int count, gid_t *groups)

The getgroups function is used to inquire about the supplementary group IDs of the process. Up to count of these group IDs are stored in the array groups; the return value from the function is the number of group IDs actually stored. If count is smaller than the total number of supplementary group IDs, then getgroups returns a value of -1 and errno is set to EINVAL.

If count is zero, then getgroups just returns the total number of supplementary group IDs. On systems that do not support supplementary groups, this will always be zero.

Here's how to use getgroups to read all the supplementary group IDs: 

          gid_t *
          read_all_groups (void)
          {
            int ngroups = getgroups (0, NULL);
            gid_t *groups
              = (gid_t *) xmalloc (ngroups * sizeof (gid_t));
            int val = getgroups (ngroups, groups);
            if (val < 0)
              {
                free (groups);
                return NULL;
              }
            return groups;
          }

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29.6 Setting the User ID

This section describes the functions for altering the user ID (real and/or effective) of a process. To use these facilities, you must include the header files sys/types.h and unistd.h.

— Function: int seteuid (uid_t neweuid)

This function sets the effective user ID of a process to newuid, provided that the process is allowed to change its effective user ID. A privileged process (effective user ID zero) can change its effective user ID to any legal value. An unprivileged process with a file user ID can change its effective user ID to its real user ID or to its file user ID. Otherwise, a process may not change its effective user ID at all.

The seteuid function returns a value of 0 to indicate successful completion, and a value of -1 to indicate an error. The following errno error conditions are defined for this function:

EINVAL
The value of the newuid argument is invalid.
EPERM
The process may not change to the specified ID.

Older systems (those without the _POSIX_SAVED_IDS feature) do not have this function.

— Function: int setuid (uid_t newuid)

If the calling process is privileged, this function sets both the real and effective user ID of the process to newuid. It also deletes the file user ID of the process, if any. newuid may be any legal value. (Once this has been done, there is no way to recover the old effective user ID.)

If the process is not privileged, and the system supports the _POSIX_SAVED_IDS feature, then this function behaves like seteuid.

The return values and error conditions are the same as for seteuid.

— Function: int setreuid (uid_t ruid, uid_t euid)

This function sets the real user ID of the process to ruid and the effective user ID to euid. If ruid is -1, it means not to change the real user ID; likewise if euid is -1, it means not to change the effective user ID.

The setreuid function exists for compatibility with 4.3 BSD Unix, which does not support file IDs. You can use this function to swap the effective and real user IDs of the process. (Privileged processes are not limited to this particular usage.) If file IDs are supported, you should use that feature instead of this function. See Enable/Disable Setuid.

The return value is 0 on success and -1 on failure. The following errno error conditions are defined for this function:

EPERM
The process does not have the appropriate privileges; you do not have permission to change to the specified ID.

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29.7 Setting the Group IDs

This section describes the functions for altering the group IDs (real and effective) of a process. To use these facilities, you must include the header files sys/types.h and unistd.h.

— Function: int setegid (gid_t newgid)

This function sets the effective group ID of the process to newgid, provided that the process is allowed to change its group ID. Just as with seteuid, if the process is privileged it may change its effective group ID to any value; if it isn't, but it has a file group ID, then it may change to its real group ID or file group ID; otherwise it may not change its effective group ID.

Note that a process is only privileged if its effective user ID is zero. The effective group ID only affects access permissions.

The return values and error conditions for setegid are the same as those for seteuid.

This function is only present if _POSIX_SAVED_IDS is defined.

— Function: int setgid (gid_t newgid)

This function sets both the real and effective group ID of the process to newgid, provided that the process is privileged. It also deletes the file group ID, if any.

If the process is not privileged, then setgid behaves like setegid.

The return values and error conditions for setgid are the same as those for seteuid.

— Function: int setregid (gid_t rgid, gid_t egid)

This function sets the real group ID of the process to rgid and the effective group ID to egid. If rgid is -1, it means not to change the real group ID; likewise if egid is -1, it means not to change the effective group ID.

The setregid function is provided for compatibility with 4.3 BSD Unix, which does not support file IDs. You can use this function to swap the effective and real group IDs of the process. (Privileged processes are not limited to this usage.) If file IDs are supported, you should use that feature instead of using this function. See Enable/Disable Setuid.

The return values and error conditions for setregid are the same as those for setreuid.

setuid and setgid behave differently depending on whether the effective user ID at the time is zero. If it is not zero, they behave like seteuid and setegid. If it is, they change both effective and real IDs and delete the file ID. To avoid confusion, we recommend you always use seteuid and setegid except when you know the effective user ID is zero and your intent is to change the persona permanently. This case is rare—most of the programs that need it, such as login and su, have already been written.

Note that if your program is setuid to some user other than root, there is no way to drop privileges permanently.

The system also lets privileged processes change their supplementary group IDs. To use setgroups or initgroups, your programs should include the header file grp.h.

— Function: int setgroups (size_t count, gid_t *groups)

This function sets the process's supplementary group IDs. It can only be called from privileged processes. The count argument specifies the number of group IDs in the array groups.

This function returns 0 if successful and -1 on error. The following errno error conditions are defined for this function:

EPERM
The calling process is not privileged.

— Function: int initgroups (const char *user, gid_t group)

The initgroups function sets the process's supplementary group IDs to be the normal default for the user name user. The group group is automatically included.

This function works by scanning the group database for all the groups user belongs to. It then calls setgroups with the list it has constructed.

The return values and error conditions are the same as for setgroups.

If you are interested in the groups a particular user belongs to, but do not want to change the process's supplementary group IDs, you can use getgrouplist. To use getgrouplist, your programs should include the header file grp.h.

— Function: int getgrouplist (const char *user, gid_t group, gid_t *groups, int *ngroups)

The getgrouplist function scans the group database for all the groups user belongs to. Up to *ngroups group IDs corresponding to these groups are stored in the array groups; the return value from the function is the number of group IDs actually stored. If *ngroups is smaller than the total number of groups found, then getgrouplist returns a value of -1 and stores the actual number of groups in *ngroups. The group group is automatically included in the list of groups returned by getgrouplist.

Here's how to use getgrouplist to read all supplementary groups for user: 

          gid_t *
          supplementary_groups (char *user)
          {
            int ngroups = 16;
            gid_t *groups
              = (gid_t *) xmalloc (ngroups * sizeof (gid_t));
            struct passwd *pw = getpwnam (user);
          
            if (pw == NULL)
              return NULL;
          
            if (getgrouplist (pw->pw_name, pw->pw_gid, groups, &ngroups) < 0)
              {
                groups = xrealloc (ngroups * sizeof (gid_t));
                getgrouplist (pw->pw_name, pw->pw_gid, groups, &ngroups);
              }
            return groups;
          }

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29.8 Enabling and Disabling Setuid Access

A typical setuid program does not need its special access all of the time. It's a good idea to turn off this access when it isn't needed, so it can't possibly give unintended access.

If the system supports the _POSIX_SAVED_IDS feature, you can accomplish this with seteuid. When the game program starts, its real user ID is jdoe, its effective user ID is games, and its saved user ID is also games. The program should record both user ID values once at the beginning, like this: 

     user_user_id = getuid ();
     game_user_id = geteuid ();

Then it can turn off game file access with 

     seteuid (user_user_id);

and turn it on with 

     seteuid (game_user_id);

Throughout this process, the real user ID remains jdoe and the file user ID remains games, so the program can always set its effective user ID to either one.

On other systems that don't support file user IDs, you can turn setuid access on and off by using setreuid to swap the real and effective user IDs of the process, as follows: 

     setreuid (geteuid (), getuid ());

This special case is always allowed—it cannot fail.

Why does this have the effect of toggling the setuid access? Suppose a game program has just started, and its real user ID is jdoe while its effective user ID is games. In this state, the game can write the scores file. If it swaps the two uids, the real becomes games and the effective becomes jdoe; now the program has only jdoe access. Another swap brings games back to the effective user ID and restores access to the scores file.

In order to handle both kinds of systems, test for the saved user ID feature with a preprocessor conditional, like this: 

     #ifdef _POSIX_SAVED_IDS
       seteuid (user_user_id);
     #else
       setreuid (geteuid (), getuid ());
     #endif

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29.9 Setuid Program Example

Here's an example showing how to set up a program that changes its effective user ID.

This is part of a game program called caber-toss that manipulates a file scores that should be writable only by the game program itself. The program assumes that its executable file will be installed with the setuid bit set and owned by the same user as the scores file. Typically, a system administrator will set up an account like games for this purpose.

The executable file is given mode 4755, so that doing an `ls -l' on it produces output like: 

     -rwsr-xr-x   1 games    184422 Jul 30 15:17 caber-toss

The setuid bit shows up in the file modes as the `s'.

The scores file is given mode 644, and doing an `ls -l' on it shows: 

     -rw-r--r--  1 games           0 Jul 31 15:33 scores

Here are the parts of the program that show how to set up the changed user ID. This program is conditionalized so that it makes use of the file IDs feature if it is supported, and otherwise uses setreuid to swap the effective and real user IDs. 

     #include <stdio.h>
     #include <sys/types.h>
     #include <unistd.h>
     #include <stdlib.h>
     
     
     /* Remember the effective and real UIDs. */
     
     static uid_t euid, ruid;
     
     
     /* Restore the effective UID to its original value. */
     
     void
     do_setuid (void)
     {
       int status;
     
     #ifdef _POSIX_SAVED_IDS
       status = seteuid (euid);
     #else
       status = setreuid (ruid, euid);
     #endif
       if (status < 0) {
         fprintf (stderr, "Couldn't set uid.\n");
         exit (status);
         }
     }
     
     
     /* Set the effective UID to the real UID. */
     
     void
     undo_setuid (void)
     {
       int status;
     
     #ifdef _POSIX_SAVED_IDS
       status = seteuid (ruid);
     #else
       status = setreuid (euid, ruid);
     #endif
       if (status < 0) {
         fprintf (stderr, "Couldn't set uid.\n");
         exit (status);
         }
     }
     
     /* Main program. */
     
     int
     main (void)
     {
       /* Remember the real and effective user IDs.  */
       ruid = getuid ();
       euid = geteuid ();
       undo_setuid ();
     
       /* Do the game and record the score.  */
       ...
     }

Notice how the first thing the main function does is to set the effective user ID back to the real user ID. This is so that any other file accesses that are performed while the user is playing the game use the real user ID for determining permissions. Only when the program needs to open the scores file does it switch back to the file user ID, like this: 

     /* Record the score. */
     
     int
     record_score (int score)
     {
       FILE *stream;
       char *myname;
     
       /* Open the scores file. */
       do_setuid ();
       stream = fopen (SCORES_FILE, "a");
       undo_setuid ();
     
       /* Write the score to the file. */
       if (stream)
         {
           myname = cuserid (NULL);
           if (score < 0)
             fprintf (stream, "%10s: Couldn't lift the caber.\n", myname);
           else
             fprintf (stream, "%10s: %d feet.\n", myname, score);
           fclose (stream);
           return 0;
         }
       else
         return -1;
     }

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29.10 Tips for Writing Setuid Programs

It is easy for setuid programs to give the user access that isn't intended—in fact, if you want to avoid this, you need to be careful. Here are some guidelines for preventing unintended access and minimizing its consequences when it does occur:

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29.11 Identifying Who Logged In

You can use the functions listed in this section to determine the login name of the user who is running a process, and the name of the user who logged in the current session. See also the function getuid and friends (see Reading Persona). How this information is collected by the system and how to control/add/remove information from the background storage is described in User Accounting Database.

The getlogin function is declared in unistd.h, while cuserid and L_cuserid are declared in stdio.h.

— Function: char * getlogin (void)

The getlogin function returns a pointer to a string containing the name of the user logged in on the controlling terminal of the process, or a null pointer if this information cannot be determined. The string is statically allocated and might be overwritten on subsequent calls to this function or to cuserid.

— Function: char * cuserid (char *string)

The cuserid function returns a pointer to a string containing a user name associated with the effective ID of the process. If string is not a null pointer, it should be an array that can hold at least L_cuserid characters; the string is returned in this array. Otherwise, a pointer to a string in a static area is returned. This string is statically allocated and might be overwritten on subsequent calls to this function or to getlogin.

The use of this function is deprecated since it is marked to be withdrawn in XPG4.2 and has already been removed from newer revisions of POSIX.1.

— Macro: int L_cuserid

An integer constant that indicates how long an array you might need to store a user name.

These functions let your program identify positively the user who is running or the user who logged in this session. (These can differ when setuid programs are involved; see Process Persona.) The user cannot do anything to fool these functions.

For most purposes, it is more useful to use the environment variable LOGNAME to find out who the user is. This is more flexible precisely because the user can set LOGNAME arbitrarily. See Standard Environment.

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29.12 The User Accounting Database

Most Unix-like operating systems keep track of logged in users by maintaining a user accounting database. This user accounting database stores for each terminal, who has logged on, at what time, the process ID of the user's login shell, etc., etc., but also stores information about the run level of the system, the time of the last system reboot, and possibly more.

The user accounting database typically lives in /etc/utmp, /var/adm/utmp or /var/run/utmp. However, these files should never be accessed directly. For reading information from and writing information to the user accounting database, the functions described in this section should be used.

Next: , Up: User Accounting Database

29.12.1 Manipulating the User Accounting Database

These functions and the corresponding data structures are declared in the header file utmp.h.

— Data Type: struct exit_status

The exit_status data structure is used to hold information about the exit status of processes marked as DEAD_PROCESS in the user accounting database.

short int e_termination
The exit status of the process.
short int e_exit
The exit status of the process.

— Data Type: struct utmp

The utmp data structure is used to hold information about entries in the user accounting database. On the GNU system it has the following members:

short int ut_type
Specifies the type of login; one of EMPTY, RUN_LVL, BOOT_TIME, OLD_TIME, NEW_TIME, INIT_PROCESS, LOGIN_PROCESS, USER_PROCESS, DEAD_PROCESS or ACCOUNTING.
pid_t ut_pid
The process ID number of the login process.
char ut_line[]
The device name of the tty (without /dev/).
char ut_id[]
The inittab ID of the process.
char ut_user[]
The user's login name.
char ut_host[]
The name of the host from which the user logged in.
struct exit_status ut_exit
The exit status of a process marked as DEAD_PROCESS.
long ut_session
The Session ID, used for windowing.
struct timeval ut_tv
Time the entry was made. For entries of type OLD_TIME this is the time when the system clock changed, and for entries of type NEW_TIME this is the time the system clock was set to.
int32_t ut_addr_v6[4]
The Internet address of a remote host.

The ut_type, ut_pid, ut_id, ut_tv, and ut_host fields are not available on all systems. Portable applications therefore should be prepared for these situations. To help doing this the utmp.h header provides macros _HAVE_UT_TYPE, _HAVE_UT_PID, _HAVE_UT_ID, _HAVE_UT_TV, and _HAVE_UT_HOST if the respective field is available. The programmer can handle the situations by using #ifdef in the program code.

The following macros are defined for use as values for the ut_type member of the utmp structure. The values are integer constants.

EMPTY
This macro is used to indicate that the entry contains no valid user accounting information.


RUN_LVL
This macro is used to identify the systems runlevel.


BOOT_TIME
This macro is used to identify the time of system boot.


OLD_TIME
This macro is used to identify the time when the system clock changed.


NEW_TIME
This macro is used to identify the time after the system changed.


INIT_PROCESS
This macro is used to identify a process spawned by the init process.


LOGIN_PROCESS
This macro is used to identify the session leader of a logged in user.


USER_PROCESS
This macro is used to identify a user process.


DEAD_PROCESS
This macro is used to identify a terminated process.


ACCOUNTING
???

The size of the ut_line, ut_id, ut_user and ut_host arrays can be found using the sizeof operator.

Many older systems have, instead of an ut_tv member, an ut_time member, usually of type time_t, for representing the time associated with the entry. Therefore, for backwards compatibility only, utmp.h defines ut_time as an alias for ut_tv.tv_sec.

— Function: void setutent (void)

This function opens the user accounting database to begin scanning it. You can then call getutent, getutid or getutline to read entries and pututline to write entries.

If the database is already open, it resets the input to the beginning of the database.

— Function: struct utmp * getutent (void)

The getutent function reads the next entry from the user accounting database. It returns a pointer to the entry, which is statically allocated and may be overwritten by subsequent calls to getutent. You must copy the contents of the structure if you wish to save the information or you can use the getutent_r function which stores the data in a user-provided buffer.

A null pointer is returned in case no further entry is available.

— Function: void endutent (void)

This function closes the user accounting database.

— Function: struct utmp * getutid (const struct utmp *id)

This function searches forward from the current point in the database for an entry that matches id. If the ut_type member of the id structure is one of RUN_LVL, BOOT_TIME, OLD_TIME or NEW_TIME the entries match if the ut_type members are identical. If the ut_type member of the id structure is INIT_PROCESS, LOGIN_PROCESS, USER_PROCESS or DEAD_PROCESS, the entries match if the ut_type member of the entry read from the database is one of these four, and the ut_id members match. However if the ut_id member of either the id structure or the entry read from the database is empty it checks if the ut_line members match instead. If a matching entry is found, getutid returns a pointer to the entry, which is statically allocated, and may be overwritten by a subsequent call to getutent, getutid or getutline. You must copy the contents of the structure if you wish to save the information.

A null pointer is returned in case the end of the database is reached without a match.

The getutid function may cache the last read entry. Therefore, if you are using getutid to search for multiple occurrences, it is necessary to zero out the static data after each call. Otherwise getutid could just return a pointer to the same entry over and over again.

— Function: struct utmp * getutline (const struct utmp *line)

This function searches forward from the current point in the database until it finds an entry whose ut_type value is LOGIN_PROCESS or USER_PROCESS, and whose ut_line member matches the ut_line member of the line structure. If it finds such an entry, it returns a pointer to the entry which is statically allocated, and may be overwritten by a subsequent call to getutent, getutid or getutline. You must copy the contents of the structure if you wish to save the information.

A null pointer is returned in case the end of the database is reached without a match.

The getutline function may cache the last read entry. Therefore if you are using getutline to search for multiple occurrences, it is necessary to zero out the static data after each call. Otherwise getutline could just return a pointer to the same entry over and over again.

— Function: struct utmp * pututline (const struct utmp *utmp)

The pututline function inserts the entry *utmp at the appropriate place in the user accounting database. If it finds that it is not already at the correct place in the database, it uses getutid to search for the position to insert the entry, however this will not modify the static structure returned by getutent, getutid and getutline. If this search fails, the entry is appended to the database.

The pututline function returns a pointer to a copy of the entry inserted in the user accounting database, or a null pointer if the entry could not be added. The following errno error conditions are defined for this function:

EPERM
The process does not have the appropriate privileges; you cannot modify the user accounting database.

All the get* functions mentioned before store the information they return in a static buffer. This can be a problem in multi-threaded programs since the data returned for the request is overwritten by the return value data in another thread. Therefore the GNU C Library provides as extensions three more functions which return the data in a user-provided buffer.

— Function: int getutent_r (struct utmp *buffer, struct utmp **result)

The getutent_r is equivalent to the getutent function. It returns the next entry from the database. But instead of storing the information in a static buffer it stores it in the buffer pointed to by the parameter buffer.

If the call was successful, the function returns 0 and the pointer variable pointed to by the parameter result contains a pointer to the buffer which contains the result (this is most probably the same value as buffer). If something went wrong during the execution of getutent_r the function returns -1.

This function is a GNU extension.

— Function: int getutid_r (const struct utmp *id, struct utmp *buffer, struct utmp **result)

This function retrieves just like getutid the next entry matching the information stored in id. But the result is stored in the buffer pointed to by the parameter buffer.

If successful the function returns 0 and the pointer variable pointed to by the parameter result contains a pointer to the buffer with the result (probably the same as result. If not successful the function return -1.

This function is a GNU extension.

— Function: int getutline_r (const struct utmp *line, struct utmp *buffer, struct utmp **result)

This function retrieves just like getutline the next entry matching the information stored in line. But the result is stored in the buffer pointed to by the parameter buffer.

If successful the function returns 0 and the pointer variable pointed to by the parameter result contains a pointer to the buffer with the result (probably the same as result. If not successful the function return -1.

This function is a GNU extension.

In addition to the user accounting database, most systems keep a number of similar databases. For example most systems keep a log file with all previous logins (usually in /etc/wtmp or /var/log/wtmp).

For specifying which database to examine, the following function should be used.

— Function: int utmpname (const char *file)

The utmpname function changes the name of the database to be examined to file, and closes any previously opened database. By default getutent, getutid, getutline and pututline read from and write to the user accounting database.

The following macros are defined for use as the file argument:

— Macro: char * _PATH_UTMP

This macro is used to specify the user accounting database.

— Macro: char * _PATH_WTMP

This macro is used to specify the user accounting log file.

The utmpname function returns a value of 0 if the new name was successfully stored, and a value of -1 to indicate an error. Note that utmpname does not try to open the database, and that therefore the return value does not say anything about whether the database can be successfully opened.

Specially for maintaining log-like databases the GNU C Library provides the following function:

— Function: void updwtmp (const char *wtmp_file, const struct utmp *utmp)

The updwtmp function appends the entry *utmp to the database specified by wtmp_file. For possible values for the wtmp_file argument see the utmpname function.

Portability Note: Although many operating systems provide a subset of these functions, they are not standardized. There are often subtle differences in the return types, and there are considerable differences between the various definitions of struct utmp. When programming for the GNU system, it is probably best to stick with the functions described in this section. If however, you want your program to be portable, consider using the XPG functions described in XPG Functions, or take a look at the BSD compatible functions in Logging In and Out.

Next: , Previous: Manipulating the Database, Up: User Accounting Database

29.12.2 XPG User Accounting Database Functions

These functions, described in the X/Open Portability Guide, are declared in the header file utmpx.h.

— Data Type: struct utmpx

The utmpx data structure contains at least the following members:

short int ut_type
Specifies the type of login; one of EMPTY, RUN_LVL, BOOT_TIME, OLD_TIME, NEW_TIME, INIT_PROCESS, LOGIN_PROCESS, USER_PROCESS or DEAD_PROCESS.
pid_t ut_pid
The process ID number of the login process.
char ut_line[]
The device name of the tty (without /dev/).
char ut_id[]
The inittab ID of the process.
char ut_user[]
The user's login name.
struct timeval ut_tv
Time the entry was made. For entries of type OLD_TIME this is the time when the system clock changed, and for entries of type NEW_TIME this is the time the system clock was set to.
On the GNU system, struct utmpx is identical to struct utmp except for the fact that including utmpx.h does not make visible the declaration of struct exit_status.

The following macros are defined for use as values for the ut_type member of the utmpx structure. The values are integer constants and are, on the GNU system, identical to the definitions in utmp.h.

EMPTY
This macro is used to indicate that the entry contains no valid user accounting information.


RUN_LVL
This macro is used to identify the systems runlevel.


BOOT_TIME
This macro is used to identify the time of system boot.


OLD_TIME
This macro is used to identify the time when the system clock changed.


NEW_TIME
This macro is used to identify the time after the system changed.


INIT_PROCESS
This macro is used to identify a process spawned by the init process.


LOGIN_PROCESS
This macro is used to identify the session leader of a logged in user.


USER_PROCESS
This macro is used to identify a user process.


DEAD_PROCESS
This macro is used to identify a terminated process.

The size of the ut_line, ut_id and ut_user arrays can be found using the sizeof operator.

— Function: void setutxent (void)

This function is similar to setutent. On the GNU system it is simply an alias for setutent.

— Function: struct utmpx * getutxent (void)

The getutxent function is similar to getutent, but returns a pointer to a struct utmpx instead of struct utmp. On the GNU system it simply is an alias for getutent.

— Function: void endutxent (void)

This function is similar to endutent. On the GNU system it is simply an alias for endutent.

— Function: struct utmpx * getutxid (const struct utmpx *id)

This function is similar to getutid, but uses struct utmpx instead of struct utmp. On the GNU system it is simply an alias for getutid.

— Function: struct utmpx * getutxline (const struct utmpx *line)

This function is similar to getutid, but uses struct utmpx instead of struct utmp. On the GNU system it is simply an alias for getutline.

— Function: struct utmpx * pututxline (const struct utmpx *utmp)

The pututxline function is functionally identical to pututline, but uses struct utmpx instead of struct utmp. On the GNU system, pututxline is simply an alias for pututline.

— Function: int utmpxname (const char *file)

The utmpxname function is functionally identical to utmpname. On the GNU system, utmpxname is simply an alias for utmpname.

You can translate between a traditional struct utmp and an XPG struct utmpx with the following functions. On the GNU system, these functions are merely copies, since the two structures are identical.

— Function: int getutmp (const struct utmpx *utmpx, struct utmp *utmp)

getutmp copies the information, insofar as the structures are compatible, from utmpx to utmp.

— Function: int getutmpx (const struct utmp *utmp, struct utmpx *utmpx)

getutmpx copies the information, insofar as the structures are compatible, from utmp to utmpx.

Previous: XPG Functions, Up: User Accounting Database

29.12.3 Logging In and Out

These functions, derived from BSD, are available in the separate libutil library, and declared in utmp.h. Note that the ut_user member of struct utmp is called ut_name in BSD. Therefore, ut_name is defined as an alias for ut_user in utmp.h.

— Function: int login_tty (int filedes)

This function makes filedes the controlling terminal of the current process, redirects standard input, standard output and standard error output to this terminal, and closes filedes.

This function returns 0 on successful completion, and -1 on error.

— Function: void login (const struct utmp *entry)

The login functions inserts an entry into the user accounting database. The ut_line member is set to the name of the terminal on standard input. If standard input is not a terminal login uses standard output or standard error output to determine the name of the terminal. If struct utmp has a ut_type member, login sets it to USER_PROCESS, and if there is an ut_pid member, it will be set to the process ID of the current process. The remaining entries are copied from entry.

A copy of the entry is written to the user accounting log file.

— Function: int logout (const char *ut_line)

This function modifies the user accounting database to indicate that the user on ut_line has logged out.

The logout function returns 1 if the entry was successfully written to the database, or 0 on error.

— Function: void logwtmp (const char *ut_line, const char *ut_name, const char *ut_host)

The logwtmp function appends an entry to the user accounting log file, for the current time and the information provided in the ut_line, ut_name and ut_host arguments.

Portability Note: The BSD struct utmp only has the ut_line, ut_name, ut_host and ut_time members. Older systems do not even have the ut_host member.

Next: , Previous: User Accounting Database, Up: Users and Groups

29.13 User Database

This section describes how to search and scan the database of registered users. The database itself is kept in the file /etc/passwd on most systems, but on some systems a special network server gives access to it.

Next: , Up: User Database

29.13.1 The Data Structure that Describes a User

The functions and data structures for accessing the system user database are declared in the header file pwd.h.

— Data Type: struct passwd

The passwd data structure is used to hold information about entries in the system user data base. It has at least the following members:

char *pw_name
The user's login name.
char *pw_passwd.
The encrypted password string.
uid_t pw_uid
The user ID number.
gid_t pw_gid
The user's default group ID number.
char *pw_gecos
A string typically containing the user's real name, and possibly other information such as a phone number.
char *pw_dir
The user's home directory, or initial working directory. This might be a null pointer, in which case the interpretation is system-dependent.
char *pw_shell
The user's default shell, or the initial program run when the user logs in. This might be a null pointer, indicating that the system default should be used.

Next: , Previous: User Data Structure, Up: User Database

29.13.2 Looking Up One User

You can search the system user database for information about a specific user using getpwuid or getpwnam. These functions are declared in pwd.h.

— Function: struct passwd * getpwuid (uid_t uid)

This function returns a pointer to a statically-allocated structure containing information about the user whose user ID is uid. This structure may be overwritten on subsequent calls to getpwuid.

A null pointer value indicates there is no user in the data base with user ID uid.

— Function: int getpwuid_r (uid_t uid, struct passwd *result_buf, char *buffer, size_t buflen, struct passwd **result)

This function is similar to getpwuid in that it returns information about the user whose user ID is uid. However, it fills the user supplied structure pointed to by result_buf with the information instead of using a static buffer. The first buflen bytes of the additional buffer pointed to by buffer are used to contain additional information, normally strings which are pointed to by the elements of the result structure.

If a user with ID uid is found, the pointer returned in result points to the record which contains the wanted data (i.e., result contains the value result_buf). If no user is found or if an error occurred, the pointer returned in result is a null pointer. The function returns zero or an error code. If the buffer buffer is too small to contain all the needed information, the error code ERANGE is returned and errno is set to ERANGE.

— Function: struct passwd * getpwnam (const char *name)

This function returns a pointer to a statically-allocated structure containing information about the user whose user name is name. This structure may be overwritten on subsequent calls to getpwnam.

A null pointer return indicates there is no user named name.

— Function: int getpwnam_r (const char *name, struct passwd *result_buf, char *buffer, size_t buflen, struct passwd **result)

This function is similar to getpwnam in that is returns information about the user whose user name is name. However, like getpwuid_r, it fills the user supplied buffers in result_buf and buffer with the information instead of using a static buffer.

The return values are the same as for getpwuid_r.

Next: , Previous: Lookup User, Up: User Database

29.13.3 Scanning the List of All Users

This section explains how a program can read the list of all users in the system, one user at a time. The functions described here are declared in pwd.h.

You can use the fgetpwent function to read user entries from a particular file.

— Function: struct passwd * fgetpwent (FILE *stream)

This function reads the next user entry from stream and returns a pointer to the entry. The structure is statically allocated and is rewritten on subsequent calls to fgetpwent. You must copy the contents of the structure if you wish to save the information.

The stream must correspond to a file in the same format as the standard password database file.

— Function: int fgetpwent_r (FILE *stream, struct passwd *result_buf, char *buffer, size_t buflen, struct passwd **result)

This function is similar to fgetpwent in that it reads the next user entry from stream. But the result is returned in the structure pointed to by result_buf. The first buflen bytes of the additional buffer pointed to by buffer are used to contain additional information, normally strings which are pointed to by the elements of the result structure.

The stream must correspond to a file in the same format as the standard password database file.

If the function returns zero result points to the structure with the wanted data (normally this is in result_buf). If errors occurred the return value is nonzero and result contains a null pointer.

The way to scan all the entries in the user database is with setpwent, getpwent, and endpwent.

— Function: void setpwent (void)

This function initializes a stream which getpwent and getpwent_r use to read the user database.

— Function: struct passwd * getpwent (void)

The getpwent function reads the next entry from the stream initialized by setpwent. It returns a pointer to the entry. The structure is statically allocated and is rewritten on subsequent calls to getpwent. You must copy the contents of the structure if you wish to save the information.

A null pointer is returned when no more entries are available.

— Function: int getpwent_r (struct passwd *result_buf, char *buffer, int buflen, struct passwd **result)

This function is similar to getpwent in that it returns the next entry from the stream initialized by setpwent. Like fgetpwent_r, it uses the user-supplied buffers in result_buf and buffer to return the information requested.

The return values are the same as for fgetpwent_r.

— Function: void endpwent (void)

This function closes the internal stream used by getpwent or getpwent_r.

Previous: Scanning All Users, Up: User Database

29.13.4 Writing a User Entry

— Function: int putpwent (const struct passwd *p, FILE *stream)

This function writes the user entry *p to the stream stream, in the format used for the standard user database file. The return value is zero on success and nonzero on failure.

This function exists for compatibility with SVID. We recommend that you avoid using it, because it makes sense only on the assumption that the struct passwd structure has no members except the standard ones; on a system which merges the traditional Unix data base with other extended information about users, adding an entry using this function would inevitably leave out much of the important information.

The group and user ID fields are left empty if the group or user name starts with a - or +.

The function putpwent is declared in pwd.h.

Next: , Previous: User Database, Up: Users and Groups

29.14 Group Database

This section describes how to search and scan the database of registered groups. The database itself is kept in the file /etc/group on most systems, but on some systems a special network service provides access to it.

Next: , Up: Group Database

29.14.1 The Data Structure for a Group

The functions and data structures for accessing the system group database are declared in the header file grp.h.

— Data Type: struct group

The group structure is used to hold information about an entry in the system group database. It has at least the following members:

char *gr_name
The name of the group.
gid_t gr_gid
The group ID of the group.
char **gr_mem
A vector of pointers to the names of users in the group. Each user name is a null-terminated string, and the vector itself is terminated by a null pointer.

Next: , Previous: Group Data Structure, Up: Group Database

29.14.2 Looking Up One Group

You can search the group database for information about a specific group using getgrgid or getgrnam. These functions are declared in grp.h.

— Function: struct group * getgrgid (gid_t gid)

This function returns a pointer to a statically-allocated structure containing information about the group whose group ID is gid. This structure may be overwritten by subsequent calls to getgrgid.

A null pointer indicates there is no group with ID gid.

— Function: int getgrgid_r (gid_t gid, struct group *result_buf, char *buffer, size_t buflen, struct group **result)

This function is similar to getgrgid in that it returns information about the group whose group ID is gid. However, it fills the user supplied structure pointed to by result_buf with the information instead of using a static buffer. The first buflen bytes of the additional buffer pointed to by buffer are used to contain additional information, normally strings which are pointed to by the elements of the result structure.

If a group with ID gid is found, the pointer returned in result points to the record which contains the wanted data (i.e., result contains the value result_buf). If no group is found or if an error occurred, the pointer returned in result is a null pointer. The function returns zero or an error code. If the buffer buffer is too small to contain all the needed information, the error code ERANGE is returned and errno is set to ERANGE.

— Function: struct group * getgrnam (const char *name)

This function returns a pointer to a statically-allocated structure containing information about the group whose group name is name. This structure may be overwritten by subsequent calls to getgrnam.

A null pointer indicates there is no group named name.

— Function: int getgrnam_r (const char *name, struct group *result_buf, char *buffer, size_t buflen, struct group **result)

This function is similar to getgrnam in that is returns information about the group whose group name is name. Like getgrgid_r, it uses the user supplied buffers in result_buf and buffer, not a static buffer.

The return values are the same as for getgrgid_r ERANGE.

Previous: Lookup Group, Up: Group Database

29.14.3 Scanning the List of All Groups

This section explains how a program can read the list of all groups in the system, one group at a time. The functions described here are declared in grp.h.

You can use the fgetgrent function to read group entries from a particular file.

— Function: struct group * fgetgrent (FILE *stream)

The fgetgrent function reads the next entry from stream. It returns a pointer to the entry. The structure is statically allocated and is overwritten on subsequent calls to fgetgrent. You must copy the contents of the structure if you wish to save the information.

The stream must correspond to a file in the same format as the standard group database file.

— Function: int fgetgrent_r (FILE *stream, struct group *result_buf, char *buffer, size_t buflen, struct group **result)

This function is similar to fgetgrent in that it reads the next user entry from stream. But the result is returned in the structure pointed to by result_buf. The first buflen bytes of the additional buffer pointed to by buffer are used to contain additional information, normally strings which are pointed to by the elements of the result structure.

This stream must correspond to a file in the same format as the standard group database file.

If the function returns zero result points to the structure with the wanted data (normally this is in result_buf). If errors occurred the return value is non-zero and result contains a null pointer.

The way to scan all the entries in the group database is with setgrent, getgrent, and endgrent.

— Function: void setgrent (void)

This function initializes a stream for reading from the group data base. You use this stream by calling getgrent or getgrent_r.

— Function: struct group * getgrent (void)

The getgrent function reads the next entry from the stream initialized by setgrent. It returns a pointer to the entry. The structure is statically allocated and is overwritten on subsequent calls to getgrent. You must copy the contents of the structure if you wish to save the information.

— Function: int getgrent_r (struct group *result_buf, char *buffer, size_t buflen, struct group **result)

This function is similar to getgrent in that it returns the next entry from the stream initialized by setgrent. Like fgetgrent_r, it places the result in user-supplied buffers pointed to result_buf and buffer.

If the function returns zero result contains a pointer to the data (normally equal to result_buf). If errors occurred the return value is non-zero and result contains a null pointer.

— Function: void endgrent (void)

This function closes the internal stream used by getgrent or getgrent_r.

Next: , Previous: Group Database, Up: Users and Groups

29.15 User and Group Database Example

Here is an example program showing the use of the system database inquiry functions. The program prints some information about the user running the program. 

     #include <grp.h>
     #include <pwd.h>
     #include <sys/types.h>
     #include <unistd.h>
     #include <stdlib.h>
     
     int
     main (void)
     {
       uid_t me;
       struct passwd *my_passwd;
       struct group *my_group;
       char **members;
     
       /* Get information about the user ID. */
       me = getuid ();
       my_passwd = getpwuid (me);
       if (!my_passwd)
         {
           printf ("Couldn't find out about user %d.\n", (int) me);
           exit (EXIT_FAILURE);
         }
     
       /* Print the information. */
       printf ("I am %s.\n", my_passwd->pw_gecos);
       printf ("My login name is %s.\n", my_passwd->pw_name);
       printf ("My uid is %d.\n", (int) (my_passwd->pw_uid));
       printf ("My home directory is %s.\n", my_passwd->pw_dir);
       printf ("My default shell is %s.\n", my_passwd->pw_shell);
     
       /* Get information about the default group ID. */
       my_group = getgrgid (my_passwd->pw_gid);
       if (!my_group)
         {
           printf ("Couldn't find out about group %d.\n",
                   (int) my_passwd->pw_gid);
           exit (EXIT_FAILURE);
         }
     
       /* Print the information. */
       printf ("My default group is %s (%d).\n",
               my_group->gr_name, (int) (my_passwd->pw_gid));
       printf ("The members of this group are:\n");
       members = my_group->gr_mem;
       while (*members)
         {
           printf ("  %s\n", *(members));
           members++;
         }
     
       return EXIT_SUCCESS;
     }

Here is some output from this program: 

     I am Throckmorton Snurd.
     My login name is snurd.
     My uid is 31093.
     My home directory is /home/fsg/snurd.
     My default shell is /bin/sh.
     My default group is guest (12).
     The members of this group are:
       friedman
       tami

Previous: Database Example, Up: Users and Groups

29.16 Netgroup Database

Next: , Up: Netgroup Database

29.16.1 Netgroup Data

Sometimes it is useful to group users according to other criteria (see Group Database). E.g., it is useful to associate a certain group of users with a certain machine. On the other hand grouping of host names is not supported so far.

In Sun Microsystems SunOS appeared a new kind of database, the netgroup database. It allows grouping hosts, users, and domains freely, giving them individual names. To be more concrete, a netgroup is a list of triples consisting of a host name, a user name, and a domain name where any of the entries can be a wildcard entry matching all inputs. A last possibility is that names of other netgroups can also be given in the list specifying a netgroup. So one can construct arbitrary hierarchies without loops.

Sun's implementation allows netgroups only for the nis or nisplus service, see Services in the NSS configuration. The implementation in the GNU C library has no such restriction. An entry in either of the input services must have the following form: 

     groupname ( groupname | (hostname,username,domainname) )+

Any of the fields in the triple can be empty which means anything matches. While describing the functions we will see that the opposite case is useful as well. I.e., there may be entries which will not match any input. For entries like this, a name consisting of the single character - shall be used.

Next: , Previous: Netgroup Data, Up: Netgroup Database

29.16.2 Looking up one Netgroup

The lookup functions for netgroups are a bit different to all other system database handling functions. Since a single netgroup can contain many entries a two-step process is needed. First a single netgroup is selected and then one can iterate over all entries in this netgroup. These functions are declared in netdb.h.

— Function: int setnetgrent (const char *netgroup)

A call to this function initializes the internal state of the library to allow following calls of the getnetgrent to iterate over all entries in the netgroup with name netgroup.

When the call is successful (i.e., when a netgroup with this name exists) the return value is 1. When the return value is 0 no netgroup of this name is known or some other error occurred.

It is important to remember that there is only one single state for iterating the netgroups. Even if the programmer uses the getnetgrent_r function the result is not really reentrant since always only one single netgroup at a time can be processed. If the program needs to process more than one netgroup simultaneously she must protect this by using external locking. This problem was introduced in the original netgroups implementation in SunOS and since we must stay compatible it is not possible to change this.

Some other functions also use the netgroups state. Currently these are the innetgr function and parts of the implementation of the compat service part of the NSS implementation.

— Function: int getnetgrent (char **hostp, char **userp, char **domainp)

This function returns the next unprocessed entry of the currently selected netgroup. The string pointers, in which addresses are passed in the arguments hostp, userp, and domainp, will contain after a successful call pointers to appropriate strings. If the string in the next entry is empty the pointer has the value NULL. The returned string pointers are only valid if none of the netgroup related functions are called.

The return value is 1 if the next entry was successfully read. A value of 0 means no further entries exist or internal errors occurred.

— Function: int getnetgrent_r (char **hostp, char **userp, char **domainp, char *buffer, int buflen)

This function is similar to getnetgrent with only one exception: the strings the three string pointers hostp, userp, and domainp point to, are placed in the buffer of buflen bytes starting at buffer. This means the returned values are valid even after other netgroup related functions are called.

The return value is 1 if the next entry was successfully read and the buffer contains enough room to place the strings in it. 0 is returned in case no more entries are found, the buffer is too small, or internal errors occurred.

This function is a GNU extension. The original implementation in the SunOS libc does not provide this function.

— Function: void endnetgrent (void)

This function frees all buffers which were allocated to process the last selected netgroup. As a result all string pointers returned by calls to getnetgrent are invalid afterwards.

Previous: Lookup Netgroup, Up: Netgroup Database

29.16.3 Testing for Netgroup Membership

It is often not necessary to scan the whole netgroup since often the only interesting question is whether a given entry is part of the selected netgroup.

— Function: int innetgr (const char *netgroup, const char *host, const char *user, const char *domain)

This function tests whether the triple specified by the parameters hostp, userp, and domainp is part of the netgroup netgroup. Using this function has the advantage that

  1. no other netgroup function can use the global netgroup state since internal locking is used and
  2. the function is implemented more efficiently than successive calls to the other set/get/endnetgrent functions.

Any of the pointers hostp, userp, and domainp can be NULL which means any value is accepted in this position. This is also true for the name - which should not match any other string otherwise.

The return value is 1 if an entry matching the given triple is found in the netgroup. The return value is 0 if the netgroup itself is not found, the netgroup does not contain the triple or internal errors occurred.

Next: , Previous: Users and Groups, Up: Top

30 System Management

This chapter describes facilities for controlling the system that underlies a process (including the operating system and hardware) and for getting information about it. Anyone can generally use the informational facilities, but usually only a properly privileged process can make changes.

To get information on parameters of the system that are built into the system, such as the maximum length of a filename, System Configuration.

Next: , Up: System Management

30.1 Host Identification

This section explains how to identify the particular system on which your program is running. First, let's review the various ways computer systems are named, which is a little complicated because of the history of the development of the Internet.

Every Unix system (also known as a host) has a host name, whether it's connected to a network or not. In its simplest form, as used before computer networks were an issue, it's just a word like `chicken'. But any system attached to the Internet or any network like it conforms to a more rigorous naming convention as part of the Domain Name System (DNS). In DNS, every host name is composed of two parts:

  1. hostname
  2. domain name

You will note that “hostname” looks a lot like “host name”, but is not the same thing, and that people often incorrectly refer to entire host names as “domain names.”

In DNS, the full host name is properly called the FQDN (Fully Qualified Domain Name) and consists of the hostname, then a period, then the domain name. The domain name itself usually has multiple components separated by periods. So for example, a system's hostname may be `chicken' and its domain name might be `ai.mit.edu', so its FQDN (which is its host name) is `chicken.ai.mit.edu'. Adding to the confusion, though, is that DNS is not the only name space in which a computer needs to be known. Another name space is the NIS (aka YP) name space. For NIS purposes, there is another domain name, which is called the NIS domain name or the YP domain name. It need not have anything to do with the DNS domain name. Confusing things even more is the fact that in DNS, it is possible for multiple FQDNs to refer to the same system. However, there is always exactly one of them that is the true host name, and it is called the canonical FQDN.

In some contexts, the host name is called a “node name.”

For more information on DNS host naming, See Host Names.

Prototypes for these functions appear in unistd.h.

The programs hostname, hostid, and domainname work by calling these functions.

— Function: int gethostname (char *name, size_t size)

This function returns the host name of the system on which it is called, in the array name. The size argument specifies the size of this array, in bytes. Note that this is not the DNS hostname. If the system participates in DNS, this is the FQDN (see above).

The return value is 0 on success and -1 on failure. In the GNU C library, gethostname fails if size is not large enough; then you can try again with a larger array. The following errno error condition is defined for this function:

ENAMETOOLONG
The size argument is less than the size of the host name plus one.

On some systems, there is a symbol for the maximum possible host name length: MAXHOSTNAMELEN. It is defined in sys/param.h. But you can't count on this to exist, so it is cleaner to handle failure and try again.

gethostname stores the beginning of the host name in name even if the host name won't entirely fit. For some purposes, a truncated host name is good enough. If it is, you can ignore the error code.

— Function: int sethostname (const char *name, size_t length)

The sethostname function sets the host name of the system that calls it to name, a string with length length. Only privileged processes are permitted to do this.

Usually sethostname gets called just once, at system boot time. Often, the program that calls it sets it to the value it finds in the file /etc/hostname. Be sure to set the host name to the full host name, not just the DNS hostname (see above).

The return value is 0 on success and -1 on failure. The following errno error condition is defined for this function:

EPERM
This process cannot set the host name because it is not privileged.

— Function: int getdomainnname (char *name, size_t length)

getdomainname returns the NIS (aka YP) domain name of the system on which it is called. Note that this is not the more popular DNS domain name. Get that with gethostname.

The specifics of this function are analogous to gethostname, above.

— Function: int setdomainname (const char *name, size_t length)

getdomainname sets the NIS (aka YP) domain name of the system on which it is called. Note that this is not the more popular DNS domain name. Set that with sethostname.

The specifics of this function are analogous to sethostname, above.

— Function: long int gethostid (void)

This function returns the “host ID” of the machine the program is running on. By convention, this is usually the primary Internet IP address of that machine, converted to a long int. However, on some systems it is a meaningless but unique number which is hard-coded for each machine.

This is not widely used. It arose in BSD 4.2, but was dropped in BSD 4.4. It is not required by POSIX.

The proper way to query the IP address is to use gethostbyname on the results of gethostname. For more information on IP addresses, See Host Addresses.

— Function: int sethostid (long int id)

The sethostid function sets the “host ID” of the host machine to id. Only privileged processes are permitted to do this. Usually it happens just once, at system boot time.

The proper way to establish the primary IP address of a system is to configure the IP address resolver to associate that IP address with the system's host name as returned by gethostname. For example, put a record for the system in /etc/hosts.

See gethostid above for more information on host ids.

The return value is 0 on success and -1 on failure. The following errno error conditions are defined for this function:

EPERM
This process cannot set the host name because it is not privileged.
ENOSYS
The operating system does not support setting the host ID. On some systems, the host ID is a meaningless but unique number hard-coded for each machine.

Next: , Previous: Host Identification, Up: System Management

30.2 Platform Type Identification

You can use the uname function to find out some information about the type of computer your program is running on. This function and the associated data type are declared in the header file sys/utsname.h. As a bonus, uname also gives some information identifying the particular system your program is running on. This is the same information which you can get with functions targetted to this purpose described in Host Identification.

— Data Type: struct utsname

The utsname structure is used to hold information returned by the uname function. It has the following members:

char sysname[]
This is the name of the operating system in use.
char release[]
This is the current release level of the operating system implementation.
char version[]
This is the current version level within the release of the operating system.
char machine[]
This is a description of the type of hardware that is in use.

Some systems provide a mechanism to interrogate the kernel directly for this information. On systems without such a mechanism, the GNU C library fills in this field based on the configuration name that was specified when building and installing the library.

GNU uses a three-part name to describe a system configuration; the three parts are cpu, manufacturer and system-type, and they are separated with dashes. Any possible combination of three names is potentially meaningful, but most such combinations are meaningless in practice and even the meaningful ones are not necessarily supported by any particular GNU program.

Since the value in machine is supposed to describe just the hardware, it consists of the first two parts of the configuration name: `cpu-manufacturer'. For example, it might be one of these:

"sparc-sun", "i386-anything", "m68k-hp", "m68k-sony", "m68k-sun", "mips-dec"

char nodename[]
This is the host name of this particular computer. In the GNU C library, the value is the same as that returned by gethostname; see Host Identification.

gethostname() is implemented with a call to uname().

char domainname[]
This is the NIS or YP domain name. It is the same value returned by getdomainname; see Host Identification. This element is a relatively recent invention and use of it is not as portable as use of the rest of the structure.

— Function: int uname (struct utsname *info)

The uname function fills in the structure pointed to by info with information about the operating system and host machine. A non-negative value indicates that the data was successfully stored.

-1 as the value indicates an error. The only error possible is EFAULT, which we normally don't mention as it is always a possibility.

Next: , Previous: Platform Type, Up: System Management

30.3 Controlling and Querying Mounts

All files are in filesystems, and before you can access any file, its filesystem must be mounted. Because of Unix's concept of Everything is a file, mounting of filesystems is central to doing almost anything. This section explains how to find out what filesystems are currently mounted and what filesystems are available for mounting, and how to change what is mounted.

The classic filesystem is the contents of a disk drive. The concept is considerably more abstract, though, and lots of things other than disk drives can be mounted.

Some block devices don't correspond to traditional devices like disk drives. For example, a loop device is a block device whose driver uses a regular file in another filesystem as its medium. So if that regular file contains appropriate data for a filesystem, you can by mounting the loop device essentially mount a regular file.

Some filesystems aren't based on a device of any kind. The “proc” filesystem, for example, contains files whose data is made up by the filesystem driver on the fly whenever you ask for it. And when you write to it, the data you write causes changes in the system. No data gets stored.

Next: , Up: Filesystem Handling

30.3.1 Mount Information

For some programs it is desirable and necessary to access information about whether a certain filesystem is mounted and, if it is, where, or simply to get lists of all the available filesystems. The GNU libc provides some functions to retrieve this information portably.

Traditionally Unix systems have a file named /etc/fstab which describes all possibly mounted filesystems. The mount program uses this file to mount at startup time of the system all the necessary filesystems. The information about all the filesystems actually mounted is normally kept in a file named either /var/run/mtab or /etc/mtab. Both files share the same syntax and it is crucial that this syntax is followed all the time. Therefore it is best to never directly write the files. The functions described in this section can do this and they also provide the functionality to convert the external textual representation to the internal representation.

Note that the fstab and mtab files are maintained on a system by convention. It is possible for the files not to exist or not to be consistent with what is really mounted or available to mount, if the system's administration policy allows it. But programs that mount and unmount filesystems typically maintain and use these files as described herein.

The filenames given above should never be used directly. The portable way to handle these file is to use the macro _PATH_FSTAB, defined in fstab.h, or _PATH_MNTTAB, defined in mntent.h and paths.h, for fstab; and the macro _PATH_MOUNTED, also defined in mntent.h and paths.h, for mtab. There are also two alternate macro names FSTAB, MNTTAB, and MOUNTED defined but these names are deprecated and kept only for backward compatibility. The names _PATH_MNTTAB and _PATH_MOUNTED should always be used.

Next: , Up: Mount Information
30.3.1.1 The fstab file

The internal representation for entries of the file is struct fstab, defined in fstab.h.

— Data Type: struct fstab

This structure is used with the getfsent, getfsspec, and getfsfile functions.

char *fs_spec
This element describes the device from which the filesystem is mounted. Normally this is the name of a special device, such as a hard disk partition, but it could also be a more or less generic string. For NFS it would be a hostname and directory name combination.

Even though the element is not declared const it shouldn't be modified. The missing const has historic reasons, since this function predates ISO C. The same is true for the other string elements of this structure.

char *fs_file
This describes the mount point on the local system. I.e., accessing any file in this filesystem has implicitly or explicitly this string as a prefix.
char *fs_vfstype
This is the type of the filesystem. Depending on what the underlying kernel understands it can be any string.
char *fs_mntops
This is a string containing options passed to the kernel with the mount call. Again, this can be almost anything. There can be more than one option, separated from the others by a comma. Each option consists of a name and an optional value part, introduced by an = character.

If the value of this element must be processed it should ideally be done using the getsubopt function; see Suboptions.

const char *fs_type
This name is poorly chosen. This element points to a string (possibly in the fs_mntops string) which describes the modes with which the filesystem is mounted. fstab defines five macros to describe the possible values:
FSTAB_RW
The filesystems gets mounted with read and write enabled.
FSTAB_RQ
The filesystems gets mounted with read and write enabled. Write access is restricted by quotas.
FSTAB_RO
The filesystem gets mounted read-only.
FSTAB_SW
This is not a real filesystem, it is a swap device.
FSTAB_XX
This entry from the fstab file is totally ignored.

Testing for equality with these value must happen using strcmp since these are all strings. Comparing the pointer will probably always fail.

int fs_freq
This element describes the dump frequency in days.
int fs_passno
This element describes the pass number on parallel dumps. It is closely related to the dump utility used on Unix systems.

To read the entire content of the of the fstab file the GNU libc contains a set of three functions which are designed in the usual way.

— Function: int setfsent (void)

This function makes sure that the internal read pointer for the fstab file is at the beginning of the file. This is done by either opening the file or resetting the read pointer.

Since the file handle is internal to the libc this function is not thread-safe.

This function returns a non-zero value if the operation was successful and the getfs* functions can be used to read the entries of the file.

— Function: void endfsent (void)

This function makes sure that all resources acquired by a prior call to setfsent (explicitly or implicitly by calling getfsent) are freed.

— Function: struct fstab * getfsent (void)

This function returns the next entry of the fstab file. If this is the first call to any of the functions handling fstab since program start or the last call of endfsent, the file will be opened.

The function returns a pointer to a variable of type struct fstab. This variable is shared by all threads and therefore this function is not thread-safe. If an error occurred getfsent returns a NULL pointer.

— Function: struct fstab * getfsspec (const char *name)

This function returns the next entry of the fstab file which has a string equal to name pointed to by the fs_spec element. Since there is normally exactly one entry for each special device it makes no sense to call this function more than once for the same argument. If this is the first call to any of the functions handling fstab since program start or the last call of endfsent, the file will be opened.

The function returns a pointer to a variable of type struct fstab. This variable is shared by all threads and therefore this function is not thread-safe. If an error occurred getfsent returns a NULL pointer.

— Function: struct fstab * getfsfile (const char *name)

This function returns the next entry of the fstab file which has a string equal to name pointed to by the fs_file element. Since there is normally exactly one entry for each mount point it makes no sense to call this function more than once for the same argument. If this is the first call to any of the functions handling fstab since program start or the last call of endfsent, the file will be opened.

The function returns a pointer to a variable of type struct fstab. This variable is shared by all threads and therefore this function is not thread-safe. If an error occurred getfsent returns a NULL pointer.

Next: , Previous: fstab, Up: Mount Information
30.3.1.2 The mtab file

The following functions and data structure access the mtab file.

— Data Type: struct mntent

This structure is used with the getmntent, getmntent_t, addmntent, and hasmntopt functions.

char *mnt_fsname
This element contains a pointer to a string describing the name of the special device from which the filesystem is mounted. It corresponds to the fs_spec element in struct fstab.
char *mnt_dir
This element points to a string describing the mount point of the filesystem. It corresponds to the fs_file element in struct fstab.
char *mnt_type
mnt_type describes the filesystem type and is therefore equivalent to fs_vfstype in struct fstab. mntent.h defines a few symbolic names for some of the values this string can have. But since the kernel can support arbitrary filesystems it does not make much sense to give them symbolic names. If one knows the symbol name one also knows the filesystem name. Nevertheless here follows the list of the symbols provided in mntent.h.
MNTTYPE_IGNORE
This symbol expands to "ignore". The value is sometime used in fstab files to make sure entries are not used without removing them.
MNTTYPE_NFS
Expands to "nfs". Using this macro sometimes could make sense since it names the default NFS implementation, in case both version 2 and 3 are supported.
MNTTYPE_SWAP
This symbol expands to "swap". It names the special fstab entry which names one of the possibly multiple swap partitions.

char *mnt_opts
The element contains a string describing the options used while mounting the filesystem. As for the equivalent element fs_mntops of struct fstab it is best to use the function getsubopt (see Suboptions) to access the parts of this string.

The mntent.h file defines a number of macros with string values which correspond to some of the options understood by the kernel. There might be many more options which are possible so it doesn't make much sense to rely on these macros but to be consistent here is the list:

MNTOPT_DEFAULTS
Expands to "defaults". This option should be used alone since it indicates all values for the customizable values are chosen to be the default.
MNTOPT_RO
Expands to "ro". See the FSTAB_RO value, it means the filesystem is mounted read-only.
MNTOPT_RW
Expand to "rw". See the FSTAB_RW value, it means the filesystem is mounted with read and write permissions.
MNTOPT_SUID
Expands to "suid". This means that the SUID bit (see How Change Persona) is respected when a program from the filesystem is started.
MNTOPT_NOSUID
Expands to "nosuid". This is the opposite of MNTOPT_SUID, the SUID bit for all files from the filesystem is ignored.
MNTOPT_NOAUTO
Expands to "noauto". At startup time the mount program will ignore this entry if it is started with the -a option to mount all filesystems mentioned in the fstab file.

As for the FSTAB_* entries introduced above it is important to use strcmp to check for equality.

mnt_freq
This elements corresponds to fs_freq and also specifies the frequency in days in which dumps are made.
mnt_passno
This element is equivalent to fs_passno with the same meaning which is uninteresting for all programs beside dump.

For accessing the mtab file there is again a set of three functions to access all entries in a row. Unlike the functions to handle fstab these functions do not access a fixed file and there is even a thread safe variant of the get function. Beside this the GNU libc contains functions to alter the file and test for specific options.

— Function: FILE * setmntent (const char *file, const char *mode)

The setmntent function prepares the file named FILE which must be in the format of a fstab and mtab file for the upcoming processing through the other functions of the family. The mode parameter can be chosen in the way the opentype parameter for fopen (see Opening Streams) can be chosen. If the file is opened for writing the file is also allowed to be empty.

If the file was successfully opened setmntent returns a file descriptor for future use. Otherwise the return value is NULL and errno is set accordingly.

— Function: int endmntent (FILE *stream)

This function takes for the stream parameter a file handle which previously was returned from the setmntent call. endmntent closes the stream and frees all resources.

The return value is 1 unless an error occurred in which case it is 0.

— Function: struct mntent * getmntent (FILE *stream)

The getmntent function takes as the parameter a file handle previously returned by successful call to setmntent. It returns a pointer to a static variable of type struct mntent which is filled with the information from the next entry from the file currently read.

The file format used prescribes the use of spaces or tab characters to separate the fields. This makes it harder to use name containing one of these characters (e.g., mount points using spaces). Therefore these characters are encoded in the files and the getmntent function takes care of the decoding while reading the entries back in. '\040' is used to encode a space character, '\011' to encode a tab character, '\012' to encode a newline character, and '\\' to encode a backslash.

If there was an error or the end of the file is reached the return value is NULL.

This function is not thread-safe since all calls to this function return a pointer to the same static variable. getmntent_r should be used in situations where multiple threads access the file.

— Function: struct mntent * getmntent_r (FILE *stream, struct mentent *result, char *buffer, int bufsize)

The getmntent_r function is the reentrant variant of getmntent. It also returns the next entry from the file and returns a pointer. The actual variable the values are stored in is not static, though. Instead the function stores the values in the variable pointed to by the result parameter. Additional information (e.g., the strings pointed to by the elements of the result) are kept in the buffer of size bufsize pointed to by buffer.

Escaped characters (space, tab, backslash) are converted back in the same way as it happens for getmentent.

The function returns a NULL pointer in error cases. Errors could be:

— Function: int addmntent (FILE *stream, const struct mntent *mnt)

The addmntent function allows adding a new entry to the file previously opened with setmntent. The new entries are always appended. I.e., even if the position of the file descriptor is not at the end of the file this function does not overwrite an existing entry following the current position.

The implication of this is that to remove an entry from a file one has to create a new file while leaving out the entry to be removed and after closing the file remove the old one and rename the new file to the chosen name.

This function takes care of spaces and tab characters in the names to be written to the file. It converts them and the backslash character into the format describe in the getmntent description above.

This function returns 0 in case the operation was successful. Otherwise the return value is 1 and errno is set appropriately.

— Function: char * hasmntopt (const struct mntent *mnt, const char *opt)

This function can be used to check whether the string pointed to by the mnt_opts element of the variable pointed to by mnt contains the option opt. If this is true a pointer to the beginning of the option in the mnt_opts element is returned. If no such option exists the function returns NULL.

This function is useful to test whether a specific option is present but when all options have to be processed one is better off with using the getsubopt function to iterate over all options in the string.

Previous: mtab, Up: Mount Information
30.3.1.3 Other (Non-libc) Sources of Mount Information

On a system with a Linux kernel and the proc filesystem, you can get information on currently mounted filesystems from the file mounts in the proc filesystem. Its format is similar to that of the mtab file, but represents what is truly mounted without relying on facilities outside the kernel to keep mtab up to date.

Previous: Mount Information, Up: Filesystem Handling

30.3.2 Mount, Unmount, Remount

This section describes the functions for mounting, unmounting, and remounting filesystems.

Only the superuser can mount, unmount, or remount a filesystem.

These functions do not access the fstab and mtab files. You should maintain and use these separately. See Mount Information.

The symbols in this section are declared in sys/mount.h.

— Function: int mount (const char *special_file, const char *dir, const char *fstype, unsigned long int options, const void *data)

mount mounts or remounts a filesystem. The two operations are quite different and are merged rather unnaturally into this one function. The MS_REMOUNT option, explained below, determines whether mount mounts or remounts.

For a mount, the filesystem on the block device represented by the device special file named special_file gets mounted over the mount point dir. This means that the directory dir (along with any files in it) is no longer visible; in its place (and still with the name dir) is the root directory of the filesystem on the device.

As an exception, if the filesystem type (see below) is one which is not based on a device (e.g. “proc”), mount instantiates a filesystem and mounts it over dir and ignores special_file.

For a remount, dir specifies the mount point where the filesystem to be remounted is (and remains) mounted and special_file is ignored. Remounting a filesystem means changing the options that control operations on the filesystem while it is mounted. It does not mean unmounting and mounting again.

For a mount, you must identify the type of the filesystem as fstype. This type tells the kernel how to access the filesystem and can be thought of as the name of a filesystem driver. The acceptable values are system dependent. On a system with a Linux kernel and the proc filesystem, the list of possible values is in the file filesystems in the proc filesystem (e.g. type cat /proc/filesystems to see the list). With a Linux kernel, the types of filesystems that mount can mount, and their type names, depends on what filesystem drivers are configured into the kernel or loaded as loadable kernel modules. An example of a common value for fstype is ext2.

For a remount, mount ignores fstype.

options specifies a variety of options that apply until the filesystem is unmounted or remounted. The precise meaning of an option depends on the filesystem and with some filesystems, an option may have no effect at all. Furthermore, for some filesystems, some of these options (but never MS_RDONLY) can be overridden for individual file accesses via ioctl.

options is a bit string with bit fields defined using the following mask and masked value macros:

MS_MGC_MASK
This multibit field contains a magic number. If it does not have the value MS_MGC_VAL, mount assumes all the following bits are zero and the data argument is a null string, regardless of their actual values.
MS_REMOUNT
This bit on means to remount the filesystem. Off means to mount it.
MS_RDONLY
This bit on specifies that no writing to the filesystem shall be allowed while it is mounted. This cannot be overridden by ioctl. This option is available on nearly all filesystems.
S_IMMUTABLE
This bit on specifies that no writing to the files in the filesystem shall be allowed while it is mounted. This can be overridden for a particular file access by a properly privileged call to ioctl. This option is a relatively new invention and is not available on many filesystems.
S_APPEND
This bit on specifies that the only file writing that shall be allowed while the filesystem is mounted is appending. Some filesystems allow this to be overridden for a particular process by a properly privileged call to ioctl. This is a relatively new invention and is not available on many filesystems.
MS_NOSUID
This bit on specifies that Setuid and Setgid permissions on files in the filesystem shall be ignored while it is mounted.
MS_NOEXEC
This bit on specifies that no files in the filesystem shall be executed while the filesystem is mounted.
MS_NODEV
This bit on specifies that no device special files in the filesystem shall be accessible while the filesystem is mounted.
MS_SYNCHRONOUS
This bit on specifies that all writes to the filesystem while it is mounted shall be synchronous; i.e., data shall be synced before each write completes rather than held in the buffer cache.
MS_MANDLOCK
This bit on specifies that mandatory locks on files shall be permitted while the filesystem is mounted.
MS_NOATIME
This bit on specifies that access times of files shall not be updated when the files are accessed while the filesystem is mounted.
MS_NODIRATIME
This bit on specifies that access times of directories shall not be updated when the directories are accessed while the filesystem in mounted.

Any bits not covered by the above masks should be set off; otherwise, results are undefined.

The meaning of data depends on the filesystem type and is controlled entirely by the filesystem driver in the kernel.

Example: 

          #include <sys/mount.h>
          
          mount("/dev/hdb", "/cdrom", MS_MGC_VAL | MS_RDONLY | MS_NOSUID, "");
          
          mount("/dev/hda2", "/mnt", MS_MGC_VAL | MS_REMOUNT, "");

Appropriate arguments for mount are conventionally recorded in the fstab table. See Mount Information.

The return value is zero if the mount or remount is successful. Otherwise, it is -1 and errno is set appropriately. The values of errno are filesystem dependent, but here is a general list:

EPERM
The process is not superuser.
ENODEV
The file system type fstype is not known to the kernel.
ENOTBLK
The file dev is not a block device special file.
EBUSY
  • The device is already mounted.
  • The mount point is busy. (E.g. it is some process' working directory or has a filesystem mounted on it already).
  • The request is to remount read-only, but there are files open for write.

EINVAL
  • A remount was attempted, but there is no filesystem mounted over the specified mount point.
  • The supposed filesystem has an invalid superblock.

EACCES
  • The filesystem is inherently read-only (possibly due to a switch on the device) and the process attempted to mount it read/write (by setting the MS_RDONLY bit off).
  • special_file or dir is not accessible due to file permissions.
  • special_file is not accessible because it is in a filesystem that is mounted with the MS_NODEV option.

EM_FILE
The table of dummy devices is full. mount needs to create a dummy device (aka “unnamed” device) if the filesystem being mounted is not one that uses a device.
— Function: int umount2 (const char *file, int flags)

umount2 unmounts a filesystem.

You can identify the filesystem to unmount either by the device special file that contains the filesystem or by the mount point. The effect is the same. Specify either as the string file.

flags contains the one-bit field identified by the following mask macro:

MNT_FORCE
This bit on means to force the unmounting even if the filesystem is busy, by making it unbusy first. If the bit is off and the filesystem is busy, umount2 fails with errno = EBUSY. Depending on the filesystem, this may override all, some, or no busy conditions.

All other bits in flags should be set to zero; otherwise, the result is undefined.

Example: 

          #include <sys/mount.h>
          
          umount2("/mnt", MNT_FORCE);
          
          umount2("/dev/hdd1", 0);

After the filesystem is unmounted, the directory that was the mount point is visible, as are any files in it.

As part of unmounting, umount2 syncs the filesystem.

If the unmounting is successful, the return value is zero. Otherwise, it is -1 and errno is set accordingly:

EPERM
The process is not superuser.
EBUSY
The filesystem cannot be unmounted because it is busy. E.g. it contains a directory that is some process's working directory or a file that some process has open. With some filesystems in some cases, you can avoid this failure with the MNT_FORCE option.
EINVAL
file validly refers to a file, but that file is neither a mount point nor a device special file of a currently mounted filesystem.

This function is not available on all systems.

— Function: int umount (const char *file)

umount does the same thing as umount2 with flags set to zeroes. It is more widely available than umount2 but since it lacks the possibility to forcefully unmount a filesystem is deprecated when umount2 is also available.

Previous: Filesystem Handling, Up: System Management

30.4 System Parameters

This section describes the sysctl function, which gets and sets a variety of system parameters.

The symbols used in this section are declared in the file sysctl.h.

— Function: int sysctl (int *names, int nlen, void *oldval, size_t *oldlenp, void *newval, size_t newlen)

sysctl gets or sets a specified system parameter. There are so many of these parameters that it is not practical to list them all here, but here are some examples:

The set of available parameters depends on the kernel configuration and can change while the system is running, particularly when you load and unload loadable kernel modules.

The system parameters with which syslog is concerned are arranged in a hierarchical structure like a hierarchical filesystem. To identify a particular parameter, you specify a path through the structure in a way analogous to specifying the pathname of a file. Each component of the path is specified by an integer and each of these integers has a macro defined for it by sysctl.h. names is the path, in the form of an array of integers. Each component of the path is one element of the array, in order. nlen is the number of components in the path.

For example, the first component of the path for all the paging parameters is the value CTL_VM. For the free page thresholds, the second component of the path is VM_FREEPG. So to get the free page threshold values, make names an array containing the two elements CTL_VM and VM_FREEPG and make nlen = 2.

The format of the value of a parameter depends on the parameter. Sometimes it is an integer; sometimes it is an ASCII string; sometimes it is an elaborate structure. In the case of the free page thresholds used in the example above, the parameter value is a structure containing several integers.

In any case, you identify a place to return the parameter's value with oldval and specify the amount of storage available at that location as *oldlenp. *oldlenp does double duty because it is also the output location that contains the actual length of the returned value.

If you don't want the parameter value returned, specify a null pointer for oldval.

To set the parameter, specify the address and length of the new value as newval and newlen. If you don't want to set the parameter, specify a null pointer as newval.

If you get and set a parameter in the same sysctl call, the value returned is the value of the parameter before it was set.

Each system parameter has a set of permissions similar to the permissions for a file (including the permissions on directories in its path) that determine whether you may get or set it. For the purposes of these permissions, every parameter is considered to be owned by the superuser and Group 0 so processes with that effective uid or gid may have more access to system parameters. Unlike with files, the superuser does not invariably have full permission to all system parameters, because some of them are designed not to be changed ever.

sysctl returns a zero return value if it succeeds. Otherwise, it returns -1 and sets errno appropriately. Besides the failures that apply to all system calls, the following are the errno codes for all possible failures:

EPERM
The process is not permitted to access one of the components of the path of the system parameter or is not permitted to access the system parameter itself in the way (read or write) that it requested.
ENOTDIR
There is no system parameter corresponding to name.
EFAULT
oldval is not null, which means the process wanted to read the parameter, but *oldlenp is zero, so there is no place to return it.
EINVAL
  • The process attempted to set a system parameter to a value that is not valid for that parameter.
  • The space provided for the return of the system parameter is not the right size for that parameter.

ENOMEM
This value may be returned instead of the more correct EINVAL in some cases where the space provided for the return of the system parameter is too small.

If you have a Linux kernel with the proc filesystem, you can get and set most of the same parameters by reading and writing to files in the sys directory of the proc filesystem. In the sys directory, the directory structure represents the hierarchical structure of the parameters. E.g. you can display the free page thresholds with 

     cat /proc/sys/vm/freepages

Some more traditional and more widely available, though less general, GNU C library functions for getting and setting some of the same system parameters are:

Next: , Previous: System Management, Up: Top

31 System Configuration Parameters

The functions and macros listed in this chapter give information about configuration parameters of the operating system—for example, capacity limits, presence of optional POSIX features, and the default path for executable files (see String Parameters).

Next: , Up: System Configuration

31.1 General Capacity Limits

The POSIX.1 and POSIX.2 standards specify a number of parameters that describe capacity limitations of the system. These limits can be fixed constants for a given operating system, or they can vary from machine to machine. For example, some limit values may be configurable by the system administrator, either at run time or by rebuilding the kernel, and this should not require recompiling application programs.

Each of the following limit parameters has a macro that is defined in limits.h only if the system has a fixed, uniform limit for the parameter in question. If the system allows different file systems or files to have different limits, then the macro is undefined; use sysconf to find out the limit that applies at a particular time on a particular machine. See Sysconf.

Each of these parameters also has another macro, with a name starting with `_POSIX', which gives the lowest value that the limit is allowed to have on any POSIX system. See Minimums.

— Macro: int ARG_MAX

If defined, the unvarying maximum combined length of the argv and environ arguments that can be passed to the exec functions.

— Macro: int CHILD_MAX

If defined, the unvarying maximum number of processes that can exist with the same real user ID at any one time. In BSD and GNU, this is controlled by the RLIMIT_NPROC resource limit; see Limits on Resources.

— Macro: int OPEN_MAX

If defined, the unvarying maximum number of files that a single process can have open simultaneously. In BSD and GNU, this is controlled by the RLIMIT_NOFILE resource limit; see Limits on Resources.

— Macro: int STREAM_MAX

If defined, the unvarying maximum number of streams that a single process can have open simultaneously. See Opening Streams.

— Macro: int TZNAME_MAX

If defined, the unvarying maximum length of a time zone name. See Time Zone Functions.

These limit macros are always defined in limits.h.

— Macro: int NGROUPS_MAX

The maximum number of supplementary group IDs that one process can have.

The value of this macro is actually a lower bound for the maximum. That is, you can count on being able to have that many supplementary group IDs, but a particular machine might let you have even more. You can use sysconf to see whether a particular machine will let you have more (see Sysconf).

— Macro: int SSIZE_MAX

The largest value that can fit in an object of type ssize_t. Effectively, this is the limit on the number of bytes that can be read or written in a single operation.

This macro is defined in all POSIX systems because this limit is never configurable.

— Macro: int RE_DUP_MAX

The largest number of repetitions you are guaranteed is allowed in the construct `\{min,max\}' in a regular expression.

The value of this macro is actually a lower bound for the maximum. That is, you can count on being able to have that many repetitions, but a particular machine might let you have even more. You can use sysconf to see whether a particular machine will let you have more (see Sysconf). And even the value that sysconf tells you is just a lower bound—larger values might work.

This macro is defined in all POSIX.2 systems, because POSIX.2 says it should always be defined even if there is no specific imposed limit.

Next: , Previous: General Limits, Up: System Configuration

31.2 Overall System Options

POSIX defines certain system-specific options that not all POSIX systems support. Since these options are provided in the kernel, not in the library, simply using the GNU C library does not guarantee any of these features is supported; it depends on the system you are using.

You can test for the availability of a given option using the macros in this section, together with the function sysconf. The macros are defined only if you include unistd.h.

For the following macros, if the macro is defined in unistd.h, then the option is supported. Otherwise, the option may or may not be supported; use sysconf to find out. See Sysconf.

— Macro: int _POSIX_JOB_CONTROL

If this symbol is defined, it indicates that the system supports job control. Otherwise, the implementation behaves as if all processes within a session belong to a single process group. See Job Control.

— Macro: int _POSIX_SAVED_IDS

If this symbol is defined, it indicates that the system remembers the effective user and group IDs of a process before it executes an executable file with the set-user-ID or set-group-ID bits set, and that explicitly changing the effective user or group IDs back to these values is permitted. If this option is not defined, then if a nonprivileged process changes its effective user or group ID to the real user or group ID of the process, it can't change it back again. See Enable/Disable Setuid.

For the following macros, if the macro is defined in unistd.h, then its value indicates whether the option is supported. A value of -1 means no, and any other value means yes. If the macro is not defined, then the option may or may not be supported; use sysconf to find out. See Sysconf.

— Macro: int _POSIX2_C_DEV

If this symbol is defined, it indicates that the system has the POSIX.2 C compiler command, c89. The GNU C library always defines this as 1, on the assumption that you would not have installed it if you didn't have a C compiler.

— Macro: int _POSIX2_FORT_DEV

If this symbol is defined, it indicates that the system has the POSIX.2 Fortran compiler command, fort77. The GNU C library never defines this, because we don't know what the system has.

— Macro: int _POSIX2_FORT_RUN

If this symbol is defined, it indicates that the system has the POSIX.2 asa command to interpret Fortran carriage control. The GNU C library never defines this, because we don't know what the system has.

— Macro: int _POSIX2_LOCALEDEF

If this symbol is defined, it indicates that the system has the POSIX.2 localedef command. The GNU C library never defines this, because we don't know what the system has.

— Macro: int _POSIX2_SW_DEV

If this symbol is defined, it indicates that the system has the POSIX.2 commands ar, make, and strip. The GNU C library always defines this as 1, on the assumption that you had to have ar and make to install the library, and it's unlikely that strip would be absent when those are present.

Next: , Previous: System Options, Up: System Configuration

31.3 Which Version of POSIX is Supported

— Macro: long int _POSIX_VERSION

This constant represents the version of the POSIX.1 standard to which the implementation conforms. For an implementation conforming to the 1995 POSIX.1 standard, the value is the integer 199506L.

_POSIX_VERSION is always defined (in unistd.h) in any POSIX system.

Usage Note: Don't try to test whether the system supports POSIX by including unistd.h and then checking whether _POSIX_VERSION is defined. On a non-POSIX system, this will probably fail because there is no unistd.h. We do not know of any way you can reliably test at compilation time whether your target system supports POSIX or whether unistd.h exists.

The GNU C compiler predefines the symbol __POSIX__ if the target system is a POSIX system. Provided you do not use any other compilers on POSIX systems, testing defined (__POSIX__) will reliably detect such systems.

— Macro: long int _POSIX2_C_VERSION

This constant represents the version of the POSIX.2 standard which the library and system kernel support. We don't know what value this will be for the first version of the POSIX.2 standard, because the value is based on the year and month in which the standard is officially adopted.

The value of this symbol says nothing about the utilities installed on the system.

Usage Note: You can use this macro to tell whether a POSIX.1 system library supports POSIX.2 as well. Any POSIX.1 system contains unistd.h, so include that file and then test defined (_POSIX2_C_VERSION).

Next: , Previous: Version Supported, Up: System Configuration

31.4 Using sysconf

When your system has configurable system limits, you can use the sysconf function to find out the value that applies to any particular machine. The function and the associated parameter constants are declared in the header file unistd.h.

Next: , Up: Sysconf

31.4.1 Definition of sysconf

— Function: long int sysconf (int parameter)

This function is used to inquire about runtime system parameters. The parameter argument should be one of the `_SC_' symbols listed below.

The normal return value from sysconf is the value you requested. A value of -1 is returned both if the implementation does not impose a limit, and in case of an error.

The following errno error conditions are defined for this function:

EINVAL
The value of the parameter is invalid.

Next: , Previous: Sysconf Definition, Up: Sysconf

31.4.2 Constants for sysconf Parameters

Here are the symbolic constants for use as the parameter argument to sysconf. The values are all integer constants (more specifically, enumeration type values).

_SC_ARG_MAX
Inquire about the parameter corresponding to ARG_MAX.
_SC_CHILD_MAX
Inquire about the parameter corresponding to CHILD_MAX.
_SC_OPEN_MAX
Inquire about the parameter corresponding to OPEN_MAX.
_SC_STREAM_MAX
Inquire about the parameter corresponding to STREAM_MAX.
_SC_TZNAME_MAX
Inquire about the parameter corresponding to TZNAME_MAX.
_SC_NGROUPS_MAX
Inquire about the parameter corresponding to NGROUPS_MAX.
_SC_JOB_CONTROL
Inquire about the parameter corresponding to _POSIX_JOB_CONTROL.
_SC_SAVED_IDS
Inquire about the parameter corresponding to _POSIX_SAVED_IDS.
_SC_VERSION
Inquire about the parameter corresponding to _POSIX_VERSION.
_SC_CLK_TCK
Inquire about the parameter corresponding to CLOCKS_PER_SEC; see CPU Time.
_SC_CHARCLASS_NAME_MAX
Inquire about the parameter corresponding to maximal length allowed for a character class name in an extended locale specification. These extensions are not yet standardized and so this option is not standardized as well.
_SC_REALTIME_SIGNALS
Inquire about the parameter corresponding to _POSIX_REALTIME_SIGNALS.
_SC_PRIORITY_SCHEDULING
Inquire about the parameter corresponding to _POSIX_PRIORITY_SCHEDULING.
_SC_TIMERS
Inquire about the parameter corresponding to _POSIX_TIMERS.
_SC_ASYNCHRONOUS_IO
Inquire about the parameter corresponding to _POSIX_ASYNCHRONOUS_IO.
_SC_PRIORITIZED_IO
Inquire about the parameter corresponding to _POSIX_PRIORITIZED_IO.
_SC_SYNCHRONIZED_IO
Inquire about the parameter corresponding to _POSIX_SYNCHRONIZED_IO.
_SC_FSYNC
Inquire about the parameter corresponding to _POSIX_FSYNC.
_SC_MAPPED_FILES
Inquire about the parameter corresponding to _POSIX_MAPPED_FILES.
_SC_MEMLOCK
Inquire about the parameter corresponding to _POSIX_MEMLOCK.
_SC_MEMLOCK_RANGE
Inquire about the parameter corresponding to _POSIX_MEMLOCK_RANGE.
_SC_MEMORY_PROTECTION
Inquire about the parameter corresponding to _POSIX_MEMORY_PROTECTION.
_SC_MESSAGE_PASSING
Inquire about the parameter corresponding to _POSIX_MESSAGE_PASSING.
_SC_SEMAPHORES
Inquire about the parameter corresponding to _POSIX_SEMAPHORES.
_SC_SHARED_MEMORY_OBJECTS
Inquire about the parameter corresponding to
_POSIX_SHARED_MEMORY_OBJECTS.
_SC_AIO_LISTIO_MAX
Inquire about the parameter corresponding to _POSIX_AIO_LISTIO_MAX.
_SC_AIO_MAX
Inquire about the parameter corresponding to _POSIX_AIO_MAX.
_SC_AIO_PRIO_DELTA_MAX
Inquire the value by which a process can decrease its asynchronous I/O priority level from its own scheduling priority. This corresponds to the run-time invariant value AIO_PRIO_DELTA_MAX.
_SC_DELAYTIMER_MAX
Inquire about the parameter corresponding to _POSIX_DELAYTIMER_MAX.
_SC_MQ_OPEN_MAX
Inquire about the parameter corresponding to _POSIX_MQ_OPEN_MAX.
_SC_MQ_PRIO_MAX
Inquire about the parameter corresponding to _POSIX_MQ_PRIO_MAX.
_SC_RTSIG_MAX
Inquire about the parameter corresponding to _POSIX_RTSIG_MAX.
_SC_SEM_NSEMS_MAX
Inquire about the parameter corresponding to _POSIX_SEM_NSEMS_MAX.
_SC_SEM_VALUE_MAX
Inquire about the parameter corresponding to _POSIX_SEM_VALUE_MAX.
_SC_SIGQUEUE_MAX
Inquire about the parameter corresponding to _POSIX_SIGQUEUE_MAX.
_SC_TIMER_MAX
Inquire about the parameter corresponding to _POSIX_TIMER_MAX.
_SC_PII
Inquire about the parameter corresponding to _POSIX_PII.
_SC_PII_XTI
Inquire about the parameter corresponding to _POSIX_PII_XTI.
_SC_PII_SOCKET
Inquire about the parameter corresponding to _POSIX_PII_SOCKET.
_SC_PII_INTERNET
Inquire about the parameter corresponding to _POSIX_PII_INTERNET.
_SC_PII_OSI
Inquire about the parameter corresponding to _POSIX_PII_OSI.
_SC_SELECT
Inquire about the parameter corresponding to _POSIX_SELECT.
_SC_UIO_MAXIOV
Inquire about the parameter corresponding to _POSIX_UIO_MAXIOV.
_SC_PII_INTERNET_STREAM
Inquire about the parameter corresponding to _POSIX_PII_INTERNET_STREAM.
_SC_PII_INTERNET_DGRAM
Inquire about the parameter corresponding to _POSIX_PII_INTERNET_DGRAM.
_SC_PII_OSI_COTS
Inquire about the parameter corresponding to _POSIX_PII_OSI_COTS.
_SC_PII_OSI_CLTS
Inquire about the parameter corresponding to _POSIX_PII_OSI_CLTS.
_SC_PII_OSI_M
Inquire about the parameter corresponding to _POSIX_PII_OSI_M.
_SC_T_IOV_MAX
Inquire the value of the value associated with the T_IOV_MAX variable.
_SC_THREADS
Inquire about the parameter corresponding to _POSIX_THREADS.
_SC_THREAD_SAFE_FUNCTIONS
Inquire about the parameter corresponding to
_POSIX_THREAD_SAFE_FUNCTIONS.
_SC_GETGR_R_SIZE_MAX
Inquire about the parameter corresponding to _POSIX_GETGR_R_SIZE_MAX.
_SC_GETPW_R_SIZE_MAX
Inquire about the parameter corresponding to _POSIX_GETPW_R_SIZE_MAX.
_SC_LOGIN_NAME_MAX
Inquire about the parameter corresponding to _POSIX_LOGIN_NAME_MAX.
_SC_TTY_NAME_MAX
Inquire about the parameter corresponding to _POSIX_TTY_NAME_MAX.
_SC_THREAD_DESTRUCTOR_ITERATIONS
Inquire about the parameter corresponding to _POSIX_THREAD_DESTRUCTOR_ITERATIONS.
_SC_THREAD_KEYS_MAX
Inquire about the parameter corresponding to _POSIX_THREAD_KEYS_MAX.
_SC_THREAD_STACK_MIN
Inquire about the parameter corresponding to _POSIX_THREAD_STACK_MIN.
_SC_THREAD_THREADS_MAX
Inquire about the parameter corresponding to _POSIX_THREAD_THREADS_MAX.
_SC_THREAD_ATTR_STACKADDR
Inquire about the parameter corresponding to
a _POSIX_THREAD_ATTR_STACKADDR.
_SC_THREAD_ATTR_STACKSIZE
Inquire about the parameter corresponding to
_POSIX_THREAD_ATTR_STACKSIZE.
_SC_THREAD_PRIORITY_SCHEDULING
Inquire about the parameter corresponding to _POSIX_THREAD_PRIORITY_SCHEDULING.
_SC_THREAD_PRIO_INHERIT
Inquire about the parameter corresponding to _POSIX_THREAD_PRIO_INHERIT.
_SC_THREAD_PRIO_PROTECT
Inquire about the parameter corresponding to _POSIX_THREAD_PRIO_PROTECT.
_SC_THREAD_PROCESS_SHARED
Inquire about the parameter corresponding to _POSIX_THREAD_PROCESS_SHARED.
_SC_2_C_DEV
Inquire about whether the system has the POSIX.2 C compiler command, c89.
_SC_2_FORT_DEV
Inquire about whether the system has the POSIX.2 Fortran compiler command, fort77.
_SC_2_FORT_RUN
Inquire about whether the system has the POSIX.2 asa command to interpret Fortran carriage control.
_SC_2_LOCALEDEF
Inquire about whether the system has the POSIX.2 localedef command.
_SC_2_SW_DEV
Inquire about whether the system has the POSIX.2 commands ar, make, and strip.
_SC_BC_BASE_MAX
Inquire about the maximum value of obase in the bc utility.
_SC_BC_DIM_MAX
Inquire about the maximum size of an array in the bc utility.
_SC_BC_SCALE_MAX
Inquire about the maximum value of scale in the bc utility.
_SC_BC_STRING_MAX
Inquire about the maximum size of a string constant in the bc utility.
_SC_COLL_WEIGHTS_MAX
Inquire about the maximum number of weights that can necessarily be used in defining the collating sequence for a locale.
_SC_EXPR_NEST_MAX
Inquire about the maximum number of expressions nested within parentheses when using the expr utility.
_SC_LINE_MAX
Inquire about the maximum size of a text line that the POSIX.2 text utilities can handle.
_SC_EQUIV_CLASS_MAX
Inquire about the maximum number of weights that can be assigned to an entry of the LC_COLLATE category `order' keyword in a locale definition. The GNU C library does not presently support locale definitions.
_SC_VERSION
Inquire about the version number of POSIX.1 that the library and kernel support.
_SC_2_VERSION
Inquire about the version number of POSIX.2 that the system utilities support.
_SC_PAGESIZE
Inquire about the virtual memory page size of the machine. getpagesize returns the same value (see Query Memory Parameters).
_SC_NPROCESSORS_CONF
Inquire about the number of configured processors.
_SC_NPROCESSORS_ONLN
Inquire about the number of processors online.
_SC_PHYS_PAGES
Inquire about the number of physical pages in the system.
_SC_AVPHYS_PAGES
Inquire about the number of available physical pages in the system.
_SC_ATEXIT_MAX
Inquire about the number of functions which can be registered as termination functions for atexit; see Cleanups on Exit.
_SC_XOPEN_VERSION
Inquire about the parameter corresponding to _XOPEN_VERSION.
_SC_XOPEN_XCU_VERSION
Inquire about the parameter corresponding to _XOPEN_XCU_VERSION.
_SC_XOPEN_UNIX
Inquire about the parameter corresponding to _XOPEN_UNIX.
_SC_XOPEN_REALTIME
Inquire about the parameter corresponding to _XOPEN_REALTIME.
_SC_XOPEN_REALTIME_THREADS
Inquire about the parameter corresponding to _XOPEN_REALTIME_THREADS.
_SC_XOPEN_LEGACY
Inquire about the parameter corresponding to _XOPEN_LEGACY.
_SC_XOPEN_CRYPT
Inquire about the parameter corresponding to _XOPEN_CRYPT.
_SC_XOPEN_ENH_I18N
Inquire about the parameter corresponding to _XOPEN_ENH_I18N.
_SC_XOPEN_SHM
Inquire about the parameter corresponding to _XOPEN_SHM.
_SC_XOPEN_XPG2
Inquire about the parameter corresponding to _XOPEN_XPG2.
_SC_XOPEN_XPG3
Inquire about the parameter corresponding to _XOPEN_XPG3.
_SC_XOPEN_XPG4
Inquire about the parameter corresponding to _XOPEN_XPG4.
_SC_CHAR_BIT
Inquire about the number of bits in a variable of type char.
_SC_CHAR_MAX
Inquire about the maximum value which can be stored in a variable of type char.
_SC_CHAR_MIN
Inquire about the minimum value which can be stored in a variable of type char.
_SC_INT_MAX
Inquire about the maximum value which can be stored in a variable of type int.
_SC_INT_MIN
Inquire about the minimum value which can be stored in a variable of type int.
_SC_LONG_BIT
Inquire about the number of bits in a variable of type long int.
_SC_WORD_BIT
Inquire about the number of bits in a variable of a register word.
_SC_MB_LEN_MAX
Inquire the maximum length of a multi-byte representation of a wide character value.
_SC_NZERO
Inquire about the value used to internally represent the zero priority level for the process execution.
SC_SSIZE_MAX
Inquire about the maximum value which can be stored in a variable of type ssize_t.
_SC_SCHAR_MAX
Inquire about the maximum value which can be stored in a variable of type signed char.
_SC_SCHAR_MIN
Inquire about the minimum value which can be stored in a variable of type signed char.
_SC_SHRT_MAX
Inquire about the maximum value which can be stored in a variable of type short int.
_SC_SHRT_MIN
Inquire about the minimum value which can be stored in a variable of type short int.
_SC_UCHAR_MAX
Inquire about the maximum value which can be stored in a variable of type unsigned char.
_SC_UINT_MAX
Inquire about the maximum value which can be stored in a variable of type unsigned int.
_SC_ULONG_MAX
Inquire about the maximum value which can be stored in a variable of type unsigned long int.
_SC_USHRT_MAX
Inquire about the maximum value which can be stored in a variable of type unsigned short int.
_SC_NL_ARGMAX
Inquire about the parameter corresponding to NL_ARGMAX.
_SC_NL_LANGMAX
Inquire about the parameter corresponding to NL_LANGMAX.
_SC_NL_MSGMAX
Inquire about the parameter corresponding to NL_MSGMAX.
_SC_NL_NMAX
Inquire about the parameter corresponding to NL_NMAX.
_SC_NL_SETMAX
Inquire about the parameter corresponding to NL_SETMAX.
_SC_NL_TEXTMAX
Inquire about the parameter corresponding to NL_TEXTMAX.

Previous: Constants for Sysconf, Up: Sysconf

31.4.3 Examples of sysconf

We recommend that you first test for a macro definition for the parameter you are interested in, and call sysconf only if the macro is not defined. For example, here is how to test whether job control is supported: 

     int
     have_job_control (void)
     {
     #ifdef _POSIX_JOB_CONTROL
       return 1;
     #else
       int value = sysconf (_SC_JOB_CONTROL);
       if (value < 0)
         /* If the system is that badly wedged,
            there's no use trying to go on.  */
         fatal (strerror (errno));
       return value;
     #endif
     }

Here is how to get the value of a numeric limit: 

     int
     get_child_max ()
     {
     #ifdef CHILD_MAX
       return CHILD_MAX;
     #else
       int value = sysconf (_SC_CHILD_MAX);
       if (value < 0)
         fatal (strerror (errno));
       return value;
     #endif
     }

Next: , Previous: Sysconf, Up: System Configuration

31.5 Minimum Values for General Capacity Limits

Here are the names for the POSIX minimum upper bounds for the system limit parameters. The significance of these values is that you can safely push to these limits without checking whether the particular system you are using can go that far.

_POSIX_AIO_LISTIO_MAX
The most restrictive limit permitted by POSIX for the maximum number of I/O operations that can be specified in a list I/O call. The value of this constant is 2; thus you can add up to two new entries of the list of outstanding operations.
_POSIX_AIO_MAX
The most restrictive limit permitted by POSIX for the maximum number of outstanding asynchronous I/O operations. The value of this constant is 1. So you cannot expect that you can issue more than one operation and immediately continue with the normal work, receiving the notifications asynchronously.
_POSIX_ARG_MAX
The value of this macro is the most restrictive limit permitted by POSIX for the maximum combined length of the argv and environ arguments that can be passed to the exec functions. Its value is 4096.
_POSIX_CHILD_MAX
The value of this macro is the most restrictive limit permitted by POSIX for the maximum number of simultaneous processes per real user ID. Its value is 6.
_POSIX_NGROUPS_MAX
The value of this macro is the most restrictive limit permitted by POSIX for the maximum number of supplementary group IDs per process. Its value is 0.
_POSIX_OPEN_MAX
The value of this macro is the most restrictive limit permitted by POSIX for the maximum number of files that a single process can have open simultaneously. Its value is 16.
_POSIX_SSIZE_MAX
The value of this macro is the most restrictive limit permitted by POSIX for the maximum value that can be stored in an object of type ssize_t. Its value is 32767.
_POSIX_STREAM_MAX
The value of this macro is the most restrictive limit permitted by POSIX for the maximum number of streams that a single process can have open simultaneously. Its value is 8.
_POSIX_TZNAME_MAX
The value of this macro is the most restrictive limit permitted by POSIX for the maximum length of a time zone name. Its value is 3.
_POSIX2_RE_DUP_MAX
The value of this macro is the most restrictive limit permitted by POSIX for the numbers used in the `\{min,max\}' construct in a regular expression. Its value is 255.

Next: , Previous: Minimums, Up: System Configuration

31.6 Limits on File System Capacity

The POSIX.1 standard specifies a number of parameters that describe the limitations of the file system. It's possible for the system to have a fixed, uniform limit for a parameter, but this isn't the usual case. On most systems, it's possible for different file systems (and, for some parameters, even different files) to have different maximum limits. For example, this is very likely if you use NFS to mount some of the file systems from other machines.

Each of the following macros is defined in limits.h only if the system has a fixed, uniform limit for the parameter in question. If the system allows different file systems or files to have different limits, then the macro is undefined; use pathconf or fpathconf to find out the limit that applies to a particular file. See Pathconf.

Each parameter also has another macro, with a name starting with `_POSIX', which gives the lowest value that the limit is allowed to have on any POSIX system. See File Minimums.

— Macro: int LINK_MAX

The uniform system limit (if any) for the number of names for a given file. See Hard Links.

— Macro: int MAX_CANON

The uniform system limit (if any) for the amount of text in a line of input when input editing is enabled. See Canonical or Not.

— Macro: int MAX_INPUT

The uniform system limit (if any) for the total number of characters typed ahead as input. See I/O Queues.

— Macro: int NAME_MAX

The uniform system limit (if any) for the length of a file name component.

— Macro: int PATH_MAX

The uniform system limit (if any) for the length of an entire file name (that is, the argument given to system calls such as open).

— Macro: int PIPE_BUF

The uniform system limit (if any) for the number of bytes that can be written atomically to a pipe. If multiple processes are writing to the same pipe simultaneously, output from different processes might be interleaved in chunks of this size. See Pipes and FIFOs.

These are alternative macro names for some of the same information.

— Macro: int MAXNAMLEN

This is the BSD name for NAME_MAX. It is defined in dirent.h.

— Macro: int FILENAME_MAX

The value of this macro is an integer constant expression that represents the maximum length of a file name string. It is defined in stdio.h.

Unlike PATH_MAX, this macro is defined even if there is no actual limit imposed. In such a case, its value is typically a very large number. This is always the case on the GNU system.

Usage Note: Don't use FILENAME_MAX as the size of an array in which to store a file name! You can't possibly make an array that big! Use dynamic allocation (see Memory Allocation) instead.

Next: , Previous: Limits for Files, Up: System Configuration

31.7 Optional Features in File Support

POSIX defines certain system-specific options in the system calls for operating on files. Some systems support these options and others do not. Since these options are provided in the kernel, not in the library, simply using the GNU C library does not guarantee that any of these features is supported; it depends on the system you are using. They can also vary between file systems on a single machine.

This section describes the macros you can test to determine whether a particular option is supported on your machine. If a given macro is defined in unistd.h, then its value says whether the corresponding feature is supported. (A value of -1 indicates no; any other value indicates yes.) If the macro is undefined, it means particular files may or may not support the feature.

Since all the machines that support the GNU C library also support NFS, one can never make a general statement about whether all file systems support the _POSIX_CHOWN_RESTRICTED and _POSIX_NO_TRUNC features. So these names are never defined as macros in the GNU C library.

— Macro: int _POSIX_CHOWN_RESTRICTED

If this option is in effect, the chown function is restricted so that the only changes permitted to nonprivileged processes is to change the group owner of a file to either be the effective group ID of the process, or one of its supplementary group IDs. See File Owner.

— Macro: int _POSIX_NO_TRUNC

If this option is in effect, file name components longer than NAME_MAX generate an ENAMETOOLONG error. Otherwise, file name components that are too long are silently truncated.

— Macro: unsigned char _POSIX_VDISABLE

This option is only meaningful for files that are terminal devices. If it is enabled, then handling for special control characters can be disabled individually. See Special Characters.

If one of these macros is undefined, that means that the option might be in effect for some files and not for others. To inquire about a particular file, call pathconf or fpathconf. See Pathconf.

Next: , Previous: Options for Files, Up: System Configuration

31.8 Minimum Values for File System Limits

Here are the names for the POSIX minimum upper bounds for some of the above parameters. The significance of these values is that you can safely push to these limits without checking whether the particular system you are using can go that far. In most cases GNU systems do not have these strict limitations. The actual limit should be requested if necessary.

_POSIX_LINK_MAX
The most restrictive limit permitted by POSIX for the maximum value of a file's link count. The value of this constant is 8; thus, you can always make up to eight names for a file without running into a system limit.
_POSIX_MAX_CANON
The most restrictive limit permitted by POSIX for the maximum number of bytes in a canonical input line from a terminal device. The value of this constant is 255.
_POSIX_MAX_INPUT
The most restrictive limit permitted by POSIX for the maximum number of bytes in a terminal device input queue (or typeahead buffer). See Input Modes. The value of this constant is 255.
_POSIX_NAME_MAX
The most restrictive limit permitted by POSIX for the maximum number of bytes in a file name component. The value of this constant is 14.
_POSIX_PATH_MAX
The most restrictive limit permitted by POSIX for the maximum number of bytes in a file name. The value of this constant is 256.
_POSIX_PIPE_BUF
The most restrictive limit permitted by POSIX for the maximum number of bytes that can be written atomically to a pipe. The value of this constant is 512.
SYMLINK_MAX
Maximum number of bytes in a symbolic link.
POSIX_REC_INCR_XFER_SIZE
Recommended increment for file transfer sizes between the POSIX_REC_MIN_XFER_SIZE and POSIX_REC_MAX_XFER_SIZE values.
POSIX_REC_MAX_XFER_SIZE
Maximum recommended file transfer size.
POSIX_REC_MIN_XFER_SIZE
Minimum recommended file transfer size.
POSIX_REC_XFER_ALIGN
Recommended file transfer buffer alignment.

Next: , Previous: File Minimums, Up: System Configuration

31.9 Using pathconf

When your machine allows different files to have different values for a file system parameter, you can use the functions in this section to find out the value that applies to any particular file.

These functions and the associated constants for the parameter argument are declared in the header file unistd.h.

— Function: long int pathconf (const char *filename, int parameter)

This function is used to inquire about the limits that apply to the file named filename.

The parameter argument should be one of the `_PC_' constants listed below.

The normal return value from pathconf is the value you requested. A value of -1 is returned both if the implementation does not impose a limit, and in case of an error. In the former case, errno is not set, while in the latter case, errno is set to indicate the cause of the problem. So the only way to use this function robustly is to store 0 into errno just before calling it.

Besides the usual file name errors (see File Name Errors), the following error condition is defined for this function:

EINVAL
The value of parameter is invalid, or the implementation doesn't support the parameter for the specific file.

— Function: long int fpathconf (int filedes, int parameter)

This is just like pathconf except that an open file descriptor is used to specify the file for which information is requested, instead of a file name.

The following errno error conditions are defined for this function:

EBADF
The filedes argument is not a valid file descriptor.
EINVAL
The value of parameter is invalid, or the implementation doesn't support the parameter for the specific file.

Here are the symbolic constants that you can use as the parameter argument to pathconf and fpathconf. The values are all integer constants.

_PC_LINK_MAX
Inquire about the value of LINK_MAX.
_PC_MAX_CANON
Inquire about the value of MAX_CANON.
_PC_MAX_INPUT
Inquire about the value of MAX_INPUT.
_PC_NAME_MAX
Inquire about the value of NAME_MAX.
_PC_PATH_MAX
Inquire about the value of PATH_MAX.
_PC_PIPE_BUF
Inquire about the value of PIPE_BUF.
_PC_CHOWN_RESTRICTED
Inquire about the value of _POSIX_CHOWN_RESTRICTED.
_PC_NO_TRUNC
Inquire about the value of _POSIX_NO_TRUNC.
_PC_VDISABLE
Inquire about the value of _POSIX_VDISABLE.
_PC_SYNC_IO
Inquire about the value of _POSIX_SYNC_IO.
_PC_ASYNC_IO
Inquire about the value of _POSIX_ASYNC_IO.
_PC_PRIO_IO
Inquire about the value of _POSIX_PRIO_IO.
_PC_FILESIZEBITS
Inquire about the availability of large files on the filesystem.
_PC_REC_INCR_XFER_SIZE
Inquire about the value of POSIX_REC_INCR_XFER_SIZE.
_PC_REC_MAX_XFER_SIZE
Inquire about the value of POSIX_REC_MAX_XFER_SIZE.
_PC_REC_MIN_XFER_SIZE
Inquire about the value of POSIX_REC_MIN_XFER_SIZE.
_PC_REC_XFER_ALIGN
Inquire about the value of POSIX_REC_XFER_ALIGN.

Next: , Previous: Pathconf, Up: System Configuration

31.10 Utility Program Capacity Limits

The POSIX.2 standard specifies certain system limits that you can access through sysconf that apply to utility behavior rather than the behavior of the library or the operating system.

The GNU C library defines macros for these limits, and sysconf returns values for them if you ask; but these values convey no meaningful information. They are simply the smallest values that POSIX.2 permits.

— Macro: int BC_BASE_MAX

The largest value of obase that the bc utility is guaranteed to support.

— Macro: int BC_DIM_MAX

The largest number of elements in one array that the bc utility is guaranteed to support.

— Macro: int BC_SCALE_MAX

The largest value of scale that the bc utility is guaranteed to support.

— Macro: int BC_STRING_MAX

The largest number of characters in one string constant that the bc utility is guaranteed to support.

— Macro: int COLL_WEIGHTS_MAX

The largest number of weights that can necessarily be used in defining the collating sequence for a locale.

— Macro: int EXPR_NEST_MAX

The maximum number of expressions that can be nested within parenthesis by the expr utility.

— Macro: int LINE_MAX

The largest text line that the text-oriented POSIX.2 utilities can support. (If you are using the GNU versions of these utilities, then there is no actual limit except that imposed by the available virtual memory, but there is no way that the library can tell you this.)

— Macro: int EQUIV_CLASS_MAX

The maximum number of weights that can be assigned to an entry of the LC_COLLATE category `order' keyword in a locale definition. The GNU C library does not presently support locale definitions.

Next: , Previous: Utility Limits, Up: System Configuration

31.11 Minimum Values for Utility Limits

_POSIX2_BC_BASE_MAX
The most restrictive limit permitted by POSIX.2 for the maximum value of obase in the bc utility. Its value is 99.
_POSIX2_BC_DIM_MAX
The most restrictive limit permitted by POSIX.2 for the maximum size of an array in the bc utility. Its value is 2048.
_POSIX2_BC_SCALE_MAX
The most restrictive limit permitted by POSIX.2 for the maximum value of scale in the bc utility. Its value is 99.
_POSIX2_BC_STRING_MAX
The most restrictive limit permitted by POSIX.2 for the maximum size of a string constant in the bc utility. Its value is 1000.
_POSIX2_COLL_WEIGHTS_MAX
The most restrictive limit permitted by POSIX.2 for the maximum number of weights that can necessarily be used in defining the collating sequence for a locale. Its value is 2.
_POSIX2_EXPR_NEST_MAX
The most restrictive limit permitted by POSIX.2 for the maximum number of expressions nested within parenthesis when using the expr utility. Its value is 32.
_POSIX2_LINE_MAX
The most restrictive limit permitted by POSIX.2 for the maximum size of a text line that the text utilities can handle. Its value is 2048.
_POSIX2_EQUIV_CLASS_MAX
The most restrictive limit permitted by POSIX.2 for the maximum number of weights that can be assigned to an entry of the LC_COLLATE category `order' keyword in a locale definition. Its value is 2. The GNU C library does not presently support locale definitions.

Previous: Utility Minimums, Up: System Configuration

31.12 String-Valued Parameters

POSIX.2 defines a way to get string-valued parameters from the operating system with the function confstr:

— Function: size_t confstr (int parameter, char *buf, size_t len)

This function reads the value of a string-valued system parameter, storing the string into len bytes of memory space starting at buf. The parameter argument should be one of the `_CS_' symbols listed below.

The normal return value from confstr is the length of the string value that you asked for. If you supply a null pointer for buf, then confstr does not try to store the string; it just returns its length. A value of 0 indicates an error.

If the string you asked for is too long for the buffer (that is, longer than len - 1), then confstr stores just that much (leaving room for the terminating null character). You can tell that this has happened because confstr returns a value greater than or equal to len.

The following errno error conditions are defined for this function:

EINVAL
The value of the parameter is invalid.

Currently there is just one parameter you can read with confstr:

_CS_PATH
This parameter's value is the recommended default path for searching for executable files. This is the path that a user has by default just after logging in.
_CS_LFS_CFLAGS
The returned string specifies which additional flags must be given to the C compiler if a source is compiled using the _LARGEFILE_SOURCE feature select macro; see Feature Test Macros.
_CS_LFS_LDFLAGS
The returned string specifies which additional flags must be given to the linker if a source is compiled using the _LARGEFILE_SOURCE feature select macro; see Feature Test Macros.
_CS_LFS_LIBS
The returned string specifies which additional libraries must be linked to the application if a source is compiled using the _LARGEFILE_SOURCE feature select macro; see Feature Test Macros.
_CS_LFS_LINTFLAGS
The returned string specifies which additional flags must be given to the lint tool if a source is compiled using the _LARGEFILE_SOURCE feature select macro; see Feature Test Macros.
_CS_LFS64_CFLAGS
The returned string specifies which additional flags must be given to the C compiler if a source is compiled using the _LARGEFILE64_SOURCE feature select macro; see Feature Test Macros.
_CS_LFS64_LDFLAGS
The returned string specifies which additional flags must be given to the linker if a source is compiled using the _LARGEFILE64_SOURCE feature select macro; see Feature Test Macros.
_CS_LFS64_LIBS
The returned string specifies which additional libraries must be linked to the application if a source is compiled using the _LARGEFILE64_SOURCE feature select macro; see Feature Test Macros.
_CS_LFS64_LINTFLAGS
The returned string specifies which additional flags must be given to the lint tool if a source is compiled using the _LARGEFILE64_SOURCE feature select macro; see Feature Test Macros.

The way to use confstr without any arbitrary limit on string size is to call it twice: first call it to get the length, allocate the buffer accordingly, and then call confstr again to fill the buffer, like this: 

     char *
     get_default_path (void)
     {
       size_t len = confstr (_CS_PATH, NULL, 0);
       char *buffer = (char *) xmalloc (len);
     
       if (confstr (_CS_PATH, buf, len + 1) == 0)
         {
           free (buffer);
           return NULL;
         }
     
       return buffer;
     }

Next: , Previous: System Configuration, Up: Top

32 DES Encryption and Password Handling

On many systems, it is unnecessary to have any kind of user authentication; for instance, a workstation which is not connected to a network probably does not need any user authentication, because to use the machine an intruder must have physical access.

Sometimes, however, it is necessary to be sure that a user is authorized to use some service a machine provides—for instance, to log in as a particular user id (see Users and Groups). One traditional way of doing this is for each user to choose a secret password; then, the system can ask someone claiming to be a user what the user's password is, and if the person gives the correct password then the system can grant the appropriate privileges.

If all the passwords are just stored in a file somewhere, then this file has to be very carefully protected. To avoid this, passwords are run through a one-way function, a function which makes it difficult to work out what its input was by looking at its output, before storing in the file.

The GNU C library provides a one-way function that is compatible with the behavior of the crypt function introduced in FreeBSD 2.0. It supports two one-way algorithms: one based on the MD5 message-digest algorithm that is compatible with modern BSD systems, and the other based on the Data Encryption Standard (DES) that is compatible with Unix systems.

It also provides support for Secure RPC, and some library functions that can be used to perform normal DES encryption.

Next: , Up: Cryptographic Functions

32.1 Legal Problems

Because of the continuously changing state of the law, it's not possible to provide a definitive survey of the laws affecting cryptography. Instead, this section warns you of some of the known trouble spots; this may help you when you try to find out what the laws of your country are.

Some countries require that you have a licence to use, possess, or import cryptography. These countries are believed to include Byelorussia, Burma, India, Indonesia, Israel, Kazakhstan, Pakistan, Russia, and Saudi Arabia.

Some countries restrict the transmission of encrypted messages by radio; some telecommunications carriers restrict the transmission of encrypted messages over their network.

Many countries have some form of export control for encryption software. The Wassenaar Arrangement is a multilateral agreement between 33 countries (Argentina, Australia, Austria, Belgium, Bulgaria, Canada, the Czech Republic, Denmark, Finland, France, Germany, Greece, Hungary, Ireland, Italy, Japan, Luxembourg, the Netherlands, New Zealand, Norway, Poland, Portugal, the Republic of Korea, Romania, the Russian Federation, the Slovak Republic, Spain, Sweden, Switzerland, Turkey, Ukraine, the United Kingdom and the United States) which restricts some kinds of encryption exports. Different countries apply the arrangement in different ways; some do not allow the exception for certain kinds of “public domain” software (which would include this library), some only restrict the export of software in tangible form, and others impose significant additional restrictions.

The United States has additional rules. This software would generally be exportable under 15 CFR 740.13(e), which permits exports of “encryption source code” which is “publicly available” and which is “not subject to an express agreement for the payment of a licensing fee or royalty for commercial production or sale of any product developed with the source code” to most countries.

The rules in this area are continuously changing. If you know of any information in this manual that is out-of-date, please report it to the bug database. See Reporting Bugs.

Next: , Previous: Legal Problems, Up: Cryptographic Functions

32.2 Reading Passwords

When reading in a password, it is desirable to avoid displaying it on the screen, to help keep it secret. The following function handles this in a convenient way.

— Function: char * getpass (const char *prompt)

getpass outputs prompt, then reads a string in from the terminal without echoing it. It tries to connect to the real terminal, /dev/tty, if possible, to encourage users not to put plaintext passwords in files; otherwise, it uses stdin and stderr. getpass also disables the INTR, QUIT, and SUSP characters on the terminal using the ISIG terminal attribute (see Local Modes). The terminal is flushed before and after getpass, so that characters of a mistyped password are not accidentally visible.

In other C libraries, getpass may only return the first PASS_MAX bytes of a password. The GNU C library has no limit, so PASS_MAX is undefined.

The prototype for this function is in unistd.h. PASS_MAX would be defined in limits.h.

This precise set of operations may not suit all possible situations. In this case, it is recommended that users write their own getpass substitute. For instance, a very simple substitute is as follows: 

     #include <termios.h>
     #include <stdio.h>
     
     ssize_t
     my_getpass (char **lineptr, size_t *n, FILE *stream)
     {
       struct termios old, new;
       int nread;
     
       /* Turn echoing off and fail if we can't. */
       if (tcgetattr (fileno (stream), &old) != 0)
         return -1;
       new = old;
       new.c_lflag &= ~ECHO;
       if (tcsetattr (fileno (stream), TCSAFLUSH, &new) != 0)
         return -1;
     
       /* Read the password. */
       nread = getline (lineptr, n, stream);
     
       /* Restore terminal. */
       (void) tcsetattr (fileno (stream), TCSAFLUSH, &old);
     
       return nread;
     }

The substitute takes the same parameters as getline (see Line Input); the user must print any prompt desired.

Next: , Previous: getpass, Up: Cryptographic Functions

32.3 Encrypting Passwords

— Function: char * crypt (const char *key, const char *salt)

The crypt function takes a password, key, as a string, and a salt character array which is described below, and returns a printable ASCII string which starts with another salt. It is believed that, given the output of the function, the best way to find a key that will produce that output is to guess values of key until the original value of key is found.

The salt parameter does two things. Firstly, it selects which algorithm is used, the MD5-based one or the DES-based one. Secondly, it makes life harder for someone trying to guess passwords against a file containing many passwords; without a salt, an intruder can make a guess, run crypt on it once, and compare the result with all the passwords. With a salt, the intruder must run crypt once for each different salt.

For the MD5-based algorithm, the salt should consist of the string $1$, followed by up to 8 characters, terminated by either another $ or the end of the string. The result of crypt will be the salt, followed by a $ if the salt didn't end with one, followed by 22 characters from the alphabet ./0-9A-Za-z, up to 34 characters total. Every character in the key is significant.

For the DES-based algorithm, the salt should consist of two characters from the alphabet ./0-9A-Za-z, and the result of crypt will be those two characters followed by 11 more from the same alphabet, 13 in total. Only the first 8 characters in the key are significant.

The MD5-based algorithm has no limit on the useful length of the password used, and is slightly more secure. It is therefore preferred over the DES-based algorithm.

When the user enters their password for the first time, the salt should be set to a new string which is reasonably random. To verify a password against the result of a previous call to crypt, pass the result of the previous call as the salt.

The following short program is an example of how to use crypt the first time a password is entered. Note that the salt generation is just barely acceptable; in particular, it is not unique between machines, and in many applications it would not be acceptable to let an attacker know what time the user's password was last set. 

     #include <stdio.h>
     #include <time.h>
     #include <unistd.h>
     #include <crypt.h>
     
     int
     main(void)
     {
       unsigned long seed[2];
       char salt[] = "$1$........";
       const char *const seedchars =
         "./0123456789ABCDEFGHIJKLMNOPQRST"
         "UVWXYZabcdefghijklmnopqrstuvwxyz";
       char *password;
       int i;
     
       /* Generate a (not very) random seed.
          You should do it better than this... */
       seed[0] = time(NULL);
       seed[1] = getpid() ^ (seed[0] >> 14 & 0x30000);
     
       /* Turn it into printable characters from `seedchars'. */
       for (i = 0; i < 8; i++)
         salt[3+i] = seedchars[(seed[i/5] >> (i%5)*6) & 0x3f];
     
       /* Read in the user's password and encrypt it. */
       password = crypt(getpass("Password:"), salt);
     
       /* Print the results. */
       puts(password);
       return 0;
     }

The next program shows how to verify a password. It prompts the user for a password and prints “Access granted.” if the user types GNU libc manual. 

     #include <stdio.h>
     #include <string.h>
     #include <unistd.h>
     #include <crypt.h>
     
     int
     main(void)
     {
       /* Hashed form of "GNU libc manual". */
       const char *const pass = "$1$/iSaq7rB$EoUw5jJPPvAPECNaaWzMK/";
     
       char *result;
       int ok;
     
       /* Read in the user's password and encrypt it,
          passing the expected password in as the salt. */
       result = crypt(getpass("Password:"), pass);
     
       /* Test the result. */
       ok = strcmp (result, pass) == 0;
     
       puts(ok ? "Access granted." : "Access denied.");
       return ok ? 0 : 1;
     }
— Function: char * crypt_r (const char *key, const char *salt, struct crypt_data * data)

The crypt_r function does the same thing as crypt, but takes an extra parameter which includes space for its result (among other things), so it can be reentrant. data->initialized must be cleared to zero before the first time crypt_r is called.

The crypt_r function is a GNU extension.

The crypt and crypt_r functions are prototyped in the header crypt.h.

Previous: crypt, Up: Cryptographic Functions

32.4 DES Encryption

The Data Encryption Standard is described in the US Government Federal Information Processing Standards (FIPS) 46-3 published by the National Institute of Standards and Technology. The DES has been very thoroughly analyzed since it was developed in the late 1970s, and no new significant flaws have been found.

However, the DES uses only a 56-bit key (plus 8 parity bits), and a machine has been built in 1998 which can search through all possible keys in about 6 days, which cost about US$200000; faster searches would be possible with more money. This makes simple DES insecure for most purposes, and NIST no longer permits new US government systems to use simple DES.

For serious encryption functionality, it is recommended that one of the many free encryption libraries be used instead of these routines.

The DES is a reversible operation which takes a 64-bit block and a 64-bit key, and produces another 64-bit block. Usually the bits are numbered so that the most-significant bit, the first bit, of each block is numbered 1.

Under that numbering, every 8th bit of the key (the 8th, 16th, and so on) is not used by the encryption algorithm itself. But the key must have odd parity; that is, out of bits 1 through 8, and 9 through 16, and so on, there must be an odd number of `1' bits, and this completely specifies the unused bits.

— Function: void setkey (const char *key)

The setkey function sets an internal data structure to be an expanded form of key. key is specified as an array of 64 bits each stored in a char, the first bit is key[0] and the 64th bit is key[63]. The key should have the correct parity.

— Function: void encrypt (char *block, int edflag)

The encrypt function encrypts block if edflag is 0, otherwise it decrypts block, using a key previously set by setkey. The result is placed in block.

Like setkey, block is specified as an array of 64 bits each stored in a char, but there are no parity bits in block.

— Function: void setkey_r (const char *key, struct crypt_data * data)
— Function: void encrypt_r (char *block, int edflag, struct crypt_data * data)

These are reentrant versions of setkey and encrypt. The only difference is the extra parameter, which stores the expanded version of key. Before calling setkey_r the first time, data->initialized must be cleared to zero.

The setkey_r and encrypt_r functions are GNU extensions. setkey, encrypt, setkey_r, and encrypt_r are defined in crypt.h.

— Function: int ecb_crypt (char *key, char *blocks, unsigned len, unsigned mode)

The function ecb_crypt encrypts or decrypts one or more blocks using DES. Each block is encrypted independently.

The blocks and the key are stored packed in 8-bit bytes, so that the first bit of the key is the most-significant bit of key[0] and the 63rd bit of the key is stored as the least-significant bit of key[7]. The key should have the correct parity.

len is the number of bytes in blocks. It should be a multiple of 8 (so that there is a whole number of blocks to encrypt). len is limited to a maximum of DES_MAXDATA bytes.

The result of the encryption replaces the input in blocks.

The mode parameter is the bitwise OR of two of the following:

DES_ENCRYPT
This constant, used in the mode parameter, specifies that blocks is to be encrypted.
DES_DECRYPT
This constant, used in the mode parameter, specifies that blocks is to be decrypted.
DES_HW
This constant, used in the mode parameter, asks to use a hardware device. If no hardware device is available, encryption happens anyway, but in software.
DES_SW
This constant, used in the mode parameter, specifies that no hardware device is to be used.

The result of the function will be one of these values:

DESERR_NONE
The encryption succeeded.
DESERR_NOHWDEVICE
The encryption succeeded, but there was no hardware device available.
DESERR_HWERROR
The encryption failed because of a hardware problem.
DESERR_BADPARAM
The encryption failed because of a bad parameter, for instance len is not a multiple of 8 or len is larger than DES_MAXDATA.

— Function: int DES_FAILED (int err)

This macro returns 1 if err is a `success' result code from ecb_crypt or cbc_crypt, and 0 otherwise.

— Function: int cbc_crypt (char *key, char *blocks, unsigned len, unsigned mode, char *ivec)

The function cbc_crypt encrypts or decrypts one or more blocks using DES in Cipher Block Chaining mode.

For encryption in CBC mode, each block is exclusive-ored with ivec before being encrypted, then ivec is replaced with the result of the encryption, then the next block is processed. Decryption is the reverse of this process.

This has the advantage that blocks which are the same before being encrypted are very unlikely to be the same after being encrypted, making it much harder to detect patterns in the data.

Usually, ivec is set to 8 random bytes before encryption starts. Then the 8 random bytes are transmitted along with the encrypted data (without themselves being encrypted), and passed back in as ivec for decryption. Another possibility is to set ivec to 8 zeroes initially, and have the first the block encrypted consist of 8 random bytes.

Otherwise, all the parameters are similar to those for ecb_crypt.

— Function: void des_setparity (char *key)

The function des_setparity changes the 64-bit key, stored packed in 8-bit bytes, to have odd parity by altering the low bits of each byte.

The ecb_crypt, cbc_crypt, and des_setparity functions and their accompanying macros are all defined in the header rpc/des_crypt.h.

Next: , Previous: Cryptographic Functions, Up: Top

33 Debugging support

Applications are usually debugged using dedicated debugger programs. But sometimes this is not possible and, in any case, it is useful to provide the developer with as much information as possible at the time the problems are experienced. For this reason a few functions are provided which a program can use to help the developer more easily locate the problem.

Up: Debugging Support

33.1 Backtraces

A backtrace is a list of the function calls that are currently active in a thread. The usual way to inspect a backtrace of a program is to use an external debugger such as gdb. However, sometimes it is useful to obtain a backtrace programmatically from within a program, e.g., for the purposes of logging or diagnostics.

The header file execinfo.h declares three functions that obtain and manipulate backtraces of the current thread.

— Function: int backtrace (void **buffer, int size)

The backtrace function obtains a backtrace for the current thread, as a list of pointers, and places the information into buffer. The argument size should be the number of void * elements that will fit into buffer. The return value is the actual number of entries of buffer that are obtained, and is at most size.

The pointers placed in buffer are actually return addresses obtained by inspecting the stack, one return address per stack frame.

Note that certain compiler optimizations may interfere with obtaining a valid backtrace. Function inlining causes the inlined function to not have a stack frame; tail call optimization replaces one stack frame with another; frame pointer elimination will stop backtrace from interpreting the stack contents correctly.

— Function: char ** backtrace_symbols (void *const *buffer, int size)

The backtrace_symbols function translates the information obtained from the backtrace function into an array of strings. The argument buffer should be a pointer to an array of addresses obtained via the backtrace function, and size is the number of entries in that array (the return value of backtrace).

The return value is a pointer to an array of strings, which has size entries just like the array buffer. Each string contains a printable representation of the corresponding element of buffer. It includes the function name (if this can be determined), an offset into the function, and the actual return address (in hexadecimal).

Currently, the function name and offset only be obtained on systems that use the ELF binary format for programs and libraries. On other systems, only the hexadecimal return address will be present. Also, you may need to pass additional flags to the linker to make the function names available to the program. (For example, on systems using GNU ld, you must pass (-rdynamic.)

The return value of backtrace_symbols is a pointer obtained via the malloc function, and it is the responsibility of the caller to free that pointer. Note that only the return value need be freed, not the individual strings.

The return value is NULL if sufficient memory for the strings cannot be obtained.

— Function: void backtrace_symbols_fd (void *const *buffer, int size, int fd)

The backtrace_symbols_fd function performs the same translation as the function backtrace_symbols function. Instead of returning the strings to the caller, it writes the strings to the file descriptor fd, one per line. It does not use the malloc function, and can therefore be used in situations where that function might fail.

The following program illustrates the use of these functions. Note that the array to contain the return addresses returned by backtrace is allocated on the stack. Therefore code like this can be used in situations where the memory handling via malloc does not work anymore (in which case the backtrace_symbols has to be replaced by a backtrace_symbols_fd call as well). The number of return addresses is normally not very large. Even complicated programs rather seldom have a nesting level of more than, say, 50 and with 200 possible entries probably all programs should be covered. 

     #include <execinfo.h>
     #include <stdio.h>
     #include <stdlib.h>
     
     /* Obtain a backtrace and print it to stdout. */
     void
     print_trace (void)
     {
       void *array[10];
       size_t size;
       char **strings;
       size_t i;
     
       size = backtrace (array, 10);
       strings = backtrace_symbols (array, size);
     
       printf ("Obtained %zd stack frames.\n", size);
     
       for (i = 0; i < size; i++)
          printf ("%s\n", strings[i]);
     
       free (strings);
     }
     
     /* A dummy function to make the backtrace more interesting. */
     void
     dummy_function (void)
     {
       print_trace ();
     }
     
     int
     main (void)
     {
       dummy_function ();
       return 0;
     }

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Appendix A C Language Facilities in the Library

Some of the facilities implemented by the C library really should be thought of as parts of the C language itself. These facilities ought to be documented in the C Language Manual, not in the library manual; but since we don't have the language manual yet, and documentation for these features has been written, we are publishing it here.

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A.1 Explicitly Checking Internal Consistency

When you're writing a program, it's often a good idea to put in checks at strategic places for “impossible” errors or violations of basic assumptions. These kinds of checks are helpful in debugging problems with the interfaces between different parts of the program, for example.

The assert macro, defined in the header file assert.h, provides a convenient way to abort the program while printing a message about where in the program the error was detected.

Once you think your program is debugged, you can disable the error checks performed by the assert macro by recompiling with the macro NDEBUG defined. This means you don't actually have to change the program source code to disable these checks.

But disabling these consistency checks is undesirable unless they make the program significantly slower. All else being equal, more error checking is good no matter who is running the program. A wise user would rather have a program crash, visibly, than have it return nonsense without indicating anything might be wrong.

— Macro: void assert (int expression)

Verify the programmer's belief that expression is nonzero at this point in the program.

If NDEBUG is not defined, assert tests the value of expression. If it is false (zero), assert aborts the program (see Aborting a Program) after printing a message of the form: 

          file:linenum: function: Assertion `expression' failed.

on the standard error stream stderr (see Standard Streams). The filename and line number are taken from the C preprocessor macros __FILE__ and __LINE__ and specify where the call to assert was made. When using the GNU C compiler, the name of the function which calls assert is taken from the built-in variable __PRETTY_FUNCTION__; with older compilers, the function name and following colon are omitted.

If the preprocessor macro NDEBUG is defined before assert.h is included, the assert macro is defined to do absolutely nothing.

Warning: Even the argument expression expression is not evaluated if NDEBUG is in effect. So never use assert with arguments that involve side effects. For example, assert (++i > 0); is a bad idea, because i will not be incremented if NDEBUG is defined.

Sometimes the “impossible” condition you want to check for is an error return from an operating system function. Then it is useful to display not only where the program crashes, but also what error was returned. The assert_perror macro makes this easy.

— Macro: void assert_perror (int errnum)

Similar to assert, but verifies that errnum is zero.

If NDEBUG is not defined, assert_perror tests the value of errnum. If it is nonzero, assert_perror aborts the program after printing a message of the form: 

          file:linenum: function: error text

on the standard error stream. The file name, line number, and function name are as for assert. The error text is the result of strerror (errnum). See Error Messages.

Like assert, if NDEBUG is defined before assert.h is included, the assert_perror macro does absolutely nothing. It does not evaluate the argument, so errnum should not have any side effects. It is best for errnum to be just a simple variable reference; often it will be errno.

This macro is a GNU extension.

Usage note: The assert facility is designed for detecting internal inconsistency; it is not suitable for reporting invalid input or improper usage by the user of the program.

The information in the diagnostic messages printed by the assert and assert_perror macro is intended to help you, the programmer, track down the cause of a bug, but is not really useful for telling a user of your program why his or her input was invalid or why a command could not be carried out. What's more, your program should not abort when given invalid input, as assert would do—it should exit with nonzero status (see Exit Status) after printing its error messages, or perhaps read another command or move on to the next input file.

See Error Messages, for information on printing error messages for problems that do not represent bugs in the program.

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A.2 Variadic Functions

ISO C defines a syntax for declaring a function to take a variable number or type of arguments. (Such functions are referred to as varargs functions or variadic functions.) However, the language itself provides no mechanism for such functions to access their non-required arguments; instead, you use the variable arguments macros defined in stdarg.h.

This section describes how to declare variadic functions, how to write them, and how to call them properly.

Compatibility Note: Many older C dialects provide a similar, but incompatible, mechanism for defining functions with variable numbers of arguments, using varargs.h.

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A.2.1 Why Variadic Functions are Used

Ordinary C functions take a fixed number of arguments. When you define a function, you specify the data type for each argument. Every call to the function should supply the expected number of arguments, with types that can be converted to the specified ones. Thus, if the function `foo' is declared with int foo (int, char *); then you must call it with two arguments, a number (any kind will do) and a string pointer.

But some functions perform operations that can meaningfully accept an unlimited number of arguments.

In some cases a function can handle any number of values by operating on all of them as a block. For example, consider a function that allocates a one-dimensional array with malloc to hold a specified set of values. This operation makes sense for any number of values, as long as the length of the array corresponds to that number. Without facilities for variable arguments, you would have to define a separate function for each possible array size.

The library function printf (see Formatted Output) is an example of another class of function where variable arguments are useful. This function prints its arguments (which can vary in type as well as number) under the control of a format template string.

These are good reasons to define a variadic function which can handle as many arguments as the caller chooses to pass.

Some functions such as open take a fixed set of arguments, but occasionally ignore the last few. Strict adherence to ISO C requires these functions to be defined as variadic; in practice, however, the GNU C compiler and most other C compilers let you define such a function to take a fixed set of arguments—the most it can ever use—and then only declare the function as variadic (or not declare its arguments at all!).

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A.2.2 How Variadic Functions are Defined and Used

Defining and using a variadic function involves three steps:

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A.2.2.1 Syntax for Variable Arguments

A function that accepts a variable number of arguments must be declared with a prototype that says so. You write the fixed arguments as usual, and then tack on `...' to indicate the possibility of additional arguments. The syntax of ISO C requires at least one fixed argument before the `...'. For example, 

     int
     func (const char *a, int b, ...)
     {
       ...
     }

defines a function func which returns an int and takes two required arguments, a const char * and an int. These are followed by any number of anonymous arguments.

Portability note: For some C compilers, the last required argument must not be declared register in the function definition. Furthermore, this argument's type must be self-promoting: that is, the default promotions must not change its type. This rules out array and function types, as well as float, char (whether signed or not) and short int (whether signed or not). This is actually an ISO C requirement.

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A.2.2.2 Receiving the Argument Values

Ordinary fixed arguments have individual names, and you can use these names to access their values. But optional arguments have no names—nothing but `...'. How can you access them?

The only way to access them is sequentially, in the order they were written, and you must use special macros from stdarg.h in the following three step process:

  1. You initialize an argument pointer variable of type va_list using va_start. The argument pointer when initialized points to the first optional argument.
  2. You access the optional arguments by successive calls to va_arg. The first call to va_arg gives you the first optional argument, the next call gives you the second, and so on.

    You can stop at any time if you wish to ignore any remaining optional arguments. It is perfectly all right for a function to access fewer arguments than were supplied in the call, but you will get garbage values if you try to access too many arguments.

  3. You indicate that you are finished with the argument pointer variable by calling va_end.

    (In practice, with most C compilers, calling va_end does nothing. This is always true in the GNU C compiler. But you might as well call va_end just in case your program is someday compiled with a peculiar compiler.)

See Argument Macros, for the full definitions of va_start, va_arg and va_end.

Steps 1 and 3 must be performed in the function that accepts the optional arguments. However, you can pass the va_list variable as an argument to another function and perform all or part of step 2 there.

You can perform the entire sequence of three steps multiple times within a single function invocation. If you want to ignore the optional arguments, you can do these steps zero times.

You can have more than one argument pointer variable if you like. You can initialize each variable with va_start when you wish, and then you can fetch arguments with each argument pointer as you wish. Each argument pointer variable will sequence through the same set of argument values, but at its own pace.

Portability note: With some compilers, once you pass an argument pointer value to a subroutine, you must not keep using the same argument pointer value after that subroutine returns. For full portability, you should just pass it to va_end. This is actually an ISO C requirement, but most ANSI C compilers work happily regardless.

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A.2.2.3 How Many Arguments Were Supplied

There is no general way for a function to determine the number and type of the optional arguments it was called with. So whoever designs the function typically designs a convention for the caller to specify the number and type of arguments. It is up to you to define an appropriate calling convention for each variadic function, and write all calls accordingly.

One kind of calling convention is to pass the number of optional arguments as one of the fixed arguments. This convention works provided all of the optional arguments are of the same type.

A similar alternative is to have one of the required arguments be a bit mask, with a bit for each possible purpose for which an optional argument might be supplied. You would test the bits in a predefined sequence; if the bit is set, fetch the value of the next argument, otherwise use a default value.

A required argument can be used as a pattern to specify both the number and types of the optional arguments. The format string argument to printf is one example of this (see Formatted Output Functions).

Another possibility is to pass an “end marker” value as the last optional argument. For example, for a function that manipulates an arbitrary number of pointer arguments, a null pointer might indicate the end of the argument list. (This assumes that a null pointer isn't otherwise meaningful to the function.) The execl function works in just this way; see Executing a File.

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A.2.2.4 Calling Variadic Functions

You don't have to do anything special to call a variadic function. Just put the arguments (required arguments, followed by optional ones) inside parentheses, separated by commas, as usual. But you must declare the function with a prototype and know how the argument values are converted.

In principle, functions that are defined to be variadic must also be declared to be variadic using a function prototype whenever you call them. (See Variadic Prototypes, for how.) This is because some C compilers use a different calling convention to pass the same set of argument values to a function depending on whether that function takes variable arguments or fixed arguments.

In practice, the GNU C compiler always passes a given set of argument types in the same way regardless of whether they are optional or required. So, as long as the argument types are self-promoting, you can safely omit declaring them. Usually it is a good idea to declare the argument types for variadic functions, and indeed for all functions. But there are a few functions which it is extremely convenient not to have to declare as variadic—for example, open and printf.

Since the prototype doesn't specify types for optional arguments, in a call to a variadic function the default argument promotions are performed on the optional argument values. This means the objects of type char or short int (whether signed or not) are promoted to either int or unsigned int, as appropriate; and that objects of type float are promoted to type double. So, if the caller passes a char as an optional argument, it is promoted to an int, and the function can access it with va_arg (ap, int).

Conversion of the required arguments is controlled by the function prototype in the usual way: the argument expression is converted to the declared argument type as if it were being assigned to a variable of that type.

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A.2.2.5 Argument Access Macros

Here are descriptions of the macros used to retrieve variable arguments. These macros are defined in the header file stdarg.h.

— Data Type: va_list

The type va_list is used for argument pointer variables.

— Macro: void va_start (va_list ap, last-required)

This macro initializes the argument pointer variable ap to point to the first of the optional arguments of the current function; last-required must be the last required argument to the function.

See Old Varargs, for an alternate definition of va_start found in the header file varargs.h.

— Macro: type va_arg (va_list ap, type)

The va_arg macro returns the value of the next optional argument, and modifies the value of ap to point to the subsequent argument. Thus, successive uses of va_arg return successive optional arguments.

The type of the value returned by va_arg is type as specified in the call. type must be a self-promoting type (not char or short int or float) that matches the type of the actual argument.

— Macro: void va_end (va_list ap)

This ends the use of ap. After a va_end call, further va_arg calls with the same ap may not work. You should invoke va_end before returning from the function in which va_start was invoked with the same ap argument.

In the GNU C library, va_end does nothing, and you need not ever use it except for reasons of portability.

Sometimes it is necessary to parse the list of parameters more than once or one wants to remember a certain position in the parameter list. To do this, one will have to make a copy of the current value of the argument. But va_list is an opaque type and one cannot necessarily assign the value of one variable of type va_list to another variable of the same type.

— Macro: void __va_copy (va_list dest, va_list src)

The __va_copy macro allows copying of objects of type va_list even if this is not an integral type. The argument pointer in dest is initialized to point to the same argument as the pointer in src.

This macro is a GNU extension but it will hopefully also be available in the next update of the ISO C standard.

If you want to use __va_copy you should always be prepared for the possibility that this macro will not be available. On architectures where a simple assignment is invalid, hopefully __va_copy will be available, so one should always write something like this: 

     {
       va_list ap, save;
       ...
     #ifdef __va_copy
       __va_copy (save, ap);
     #else
       save = ap;
     #endif
       ...
     }

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A.2.3 Example of a Variadic Function

Here is a complete sample function that accepts a variable number of arguments. The first argument to the function is the count of remaining arguments, which are added up and the result returned. While trivial, this function is sufficient to illustrate how to use the variable arguments facility. 

     #include <stdarg.h>
     #include <stdio.h>
     
     int
     add_em_up (int count,...)
     {
       va_list ap;
       int i, sum;
     
       va_start (ap, count);         /* Initialize the argument list. */
     
       sum = 0;
       for (i = 0; i < count; i++)
         sum += va_arg (ap, int);    /* Get the next argument value. */
     
       va_end (ap);                  /* Clean up. */
       return sum;
     }
     
     int
     main (void)
     {
       /* This call prints 16. */
       printf ("%d\n", add_em_up (3, 5, 5, 6));
     
       /* This call prints 55. */
       printf ("%d\n", add_em_up (10, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10));
     
       return 0;
     }

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A.2.3.1 Old-Style Variadic Functions

Before ISO C, programmers used a slightly different facility for writing variadic functions. The GNU C compiler still supports it; currently, it is more portable than the ISO C facility, since support for ISO C is still not universal. The header file which defines the old-fashioned variadic facility is called varargs.h.

Using varargs.h is almost the same as using stdarg.h. There is no difference in how you call a variadic function; see Calling Variadics. The only difference is in how you define them. First of all, you must use old-style non-prototype syntax, like this: 

     tree
     build (va_alist)
          va_dcl
     {

Secondly, you must give va_start only one argument, like this: 

       va_list p;
       va_start (p);

These are the special macros used for defining old-style variadic functions:

— Macro: va_alist

This macro stands for the argument name list required in a variadic function.

— Macro: va_dcl

This macro declares the implicit argument or arguments for a variadic function.

— Macro: void va_start (va_list ap)

This macro, as defined in varargs.h, initializes the argument pointer variable ap to point to the first argument of the current function.

The other argument macros, va_arg and va_end, are the same in varargs.h as in stdarg.h; see Argument Macros, for details.

It does not work to include both varargs.h and stdarg.h in the same compilation; they define va_start in conflicting ways.

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A.3 Null Pointer Constant

The null pointer constant is guaranteed not to point to any real object. You can assign it to any pointer variable since it has type void *. The preferred way to write a null pointer constant is with NULL.

— Macro: void * NULL

This is a null pointer constant.

You can also use 0 or (void *)0 as a null pointer constant, but using NULL is cleaner because it makes the purpose of the constant more evident.

If you use the null pointer constant as a function argument, then for complete portability you should make sure that the function has a prototype declaration. Otherwise, if the target machine has two different pointer representations, the compiler won't know which representation to use for that argument. You can avoid the problem by explicitly casting the constant to the proper pointer type, but we recommend instead adding a prototype for the function you are calling.

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A.4 Important Data Types

The result of subtracting two pointers in C is always an integer, but the precise data type varies from C compiler to C compiler. Likewise, the data type of the result of sizeof also varies between compilers. ISO defines standard aliases for these two types, so you can refer to them in a portable fashion. They are defined in the header file stddef.h.

— Data Type: ptrdiff_t

This is the signed integer type of the result of subtracting two pointers. For example, with the declaration char *p1, *p2;, the expression p2 - p1 is of type ptrdiff_t. This will probably be one of the standard signed integer types (short int, int or long int), but might be a nonstandard type that exists only for this purpose.

— Data Type: size_t

This is an unsigned integer type used to represent the sizes of objects. The result of the sizeof operator is of this type, and functions such as malloc (see Unconstrained Allocation) and memcpy (see Copying and Concatenation) accept arguments of this type to specify object sizes.

Usage Note: size_t is the preferred way to declare any arguments or variables that hold the size of an object.

In the GNU system size_t is equivalent to either unsigned int or unsigned long int. These types have identical properties on the GNU system and, for most purposes, you can use them interchangeably. However, they are distinct as data types, which makes a difference in certain contexts.

For example, when you specify the type of a function argument in a function prototype, it makes a difference which one you use. If the system header files declare malloc with an argument of type size_t and you declare malloc with an argument of type unsigned int, you will get a compilation error if size_t happens to be unsigned long int on your system. To avoid any possibility of error, when a function argument or value is supposed to have type size_t, never declare its type in any other way.

Compatibility Note: Implementations of C before the advent of ISO C generally used unsigned int for representing object sizes and int for pointer subtraction results. They did not necessarily define either size_t or ptrdiff_t. Unix systems did define size_t, in sys/types.h, but the definition was usually a signed type.

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A.5 Data Type Measurements

Most of the time, if you choose the proper C data type for each object in your program, you need not be concerned with just how it is represented or how many bits it uses. When you do need such information, the C language itself does not provide a way to get it. The header files limits.h and float.h contain macros which give you this information in full detail.

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A.5.1 Computing the Width of an Integer Data Type

The most common reason that a program needs to know how many bits are in an integer type is for using an array of long int as a bit vector. You can access the bit at index n with 

     vector[n / LONGBITS] & (1 << (n % LONGBITS))

provided you define LONGBITS as the number of bits in a long int.

There is no operator in the C language that can give you the number of bits in an integer data type. But you can compute it from the macro CHAR_BIT, defined in the header file limits.h.

CHAR_BIT
This is the number of bits in a char—eight, on most systems. The value has type int.

You can compute the number of bits in any data type type like this: 

          sizeof (type) * CHAR_BIT

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A.5.2 Range of an Integer Type

Suppose you need to store an integer value which can range from zero to one million. Which is the smallest type you can use? There is no general rule; it depends on the C compiler and target machine. You can use the `MIN' and `MAX' macros in limits.h to determine which type will work.

Each signed integer type has a pair of macros which give the smallest and largest values that it can hold. Each unsigned integer type has one such macro, for the maximum value; the minimum value is, of course, zero.

The values of these macros are all integer constant expressions. The `MAX' and `MIN' macros for char and short int types have values of type int. The `MAX' and `MIN' macros for the other types have values of the same type described by the macro—thus, ULONG_MAX has type unsigned long int.

SCHAR_MIN
This is the minimum value that can be represented by a signed char.
SCHAR_MAX
UCHAR_MAX
These are the maximum values that can be represented by a signed char and unsigned char, respectively.
CHAR_MIN
This is the minimum value that can be represented by a char. It's equal to SCHAR_MIN if char is signed, or zero otherwise.
CHAR_MAX
This is the maximum value that can be represented by a char. It's equal to SCHAR_MAX if char is signed, or UCHAR_MAX otherwise.
SHRT_MIN
This is the minimum value that can be represented by a signed short int. On most machines that the GNU C library runs on, short integers are 16-bit quantities.
SHRT_MAX
USHRT_MAX
These are the maximum values that can be represented by a signed short int and unsigned short int, respectively.
INT_MIN
This is the minimum value that can be represented by a signed int. On most machines that the GNU C system runs on, an int is a 32-bit quantity.
INT_MAX
UINT_MAX
These are the maximum values that can be represented by, respectively, the type signed int and the type unsigned int.
LONG_MIN
This is the minimum value that can be represented by a signed long int. On most machines that the GNU C system runs on, long integers are 32-bit quantities, the same size as int.
LONG_MAX
ULONG_MAX
These are the maximum values that can be represented by a signed long int and unsigned long int, respectively.
LONG_LONG_MIN
This is the minimum value that can be represented by a signed long long int. On most machines that the GNU C system runs on, long long integers are 64-bit quantities.
LONG_LONG_MAX
ULONG_LONG_MAX
These are the maximum values that can be represented by a signed long long int and unsigned long long int, respectively.
WCHAR_MAX
This is the maximum value that can be represented by a wchar_t. See Extended Char Intro.

The header file limits.h also defines some additional constants that parameterize various operating system and file system limits. These constants are described in System Configuration.

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A.5.3 Floating Type Macros

The specific representation of floating point numbers varies from machine to machine. Because floating point numbers are represented internally as approximate quantities, algorithms for manipulating floating point data often need to take account of the precise details of the machine's floating point representation.

Some of the functions in the C library itself need this information; for example, the algorithms for printing and reading floating point numbers (see I/O on Streams) and for calculating trigonometric and irrational functions (see Mathematics) use it to avoid round-off error and loss of accuracy. User programs that implement numerical analysis techniques also often need this information in order to minimize or compute error bounds.

The header file float.h describes the format used by your machine.

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A.5.3.1 Floating Point Representation Concepts

This section introduces the terminology for describing floating point representations.

You are probably already familiar with most of these concepts in terms of scientific or exponential notation for floating point numbers. For example, the number 123456.0 could be expressed in exponential notation as 1.23456e+05, a shorthand notation indicating that the mantissa 1.23456 is multiplied by the base 10 raised to power 5.

More formally, the internal representation of a floating point number can be characterized in terms of the following parameters:

The mantissa of a floating point number represents an implicit fraction whose denominator is the base raised to the power of the precision. Since the largest representable mantissa is one less than this denominator, the value of the fraction is always strictly less than 1. The mathematical value of a floating point number is then the product of this fraction, the sign, and the base raised to the exponent.

We say that the floating point number is normalized if the fraction is at least 1/b, where b is the base. In other words, the mantissa would be too large to fit if it were multiplied by the base. Non-normalized numbers are sometimes called denormal; they contain less precision than the representation normally can hold.

If the number is not normalized, then you can subtract 1 from the exponent while multiplying the mantissa by the base, and get another floating point number with the same value. Normalization consists of doing this repeatedly until the number is normalized. Two distinct normalized floating point numbers cannot be equal in value.

(There is an exception to this rule: if the mantissa is zero, it is considered normalized. Another exception happens on certain machines where the exponent is as small as the representation can hold. Then it is impossible to subtract 1 from the exponent, so a number may be normalized even if its fraction is less than 1/b.)

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A.5.3.2 Floating Point Parameters

These macro definitions can be accessed by including the header file float.h in your program.

Macro names starting with `FLT_' refer to the float type, while names beginning with `DBL_' refer to the double type and names beginning with `LDBL_' refer to the long double type. (If GCC does not support long double as a distinct data type on a target machine then the values for the `LDBL_' constants are equal to the corresponding constants for the double type.)

Of these macros, only FLT_RADIX is guaranteed to be a constant expression. The other macros listed here cannot be reliably used in places that require constant expressions, such as `#if' preprocessing directives or in the dimensions of static arrays.

Although the ISO C standard specifies minimum and maximum values for most of these parameters, the GNU C implementation uses whatever values describe the floating point representation of the target machine. So in principle GNU C actually satisfies the ISO C requirements only if the target machine is suitable. In practice, all the machines currently supported are suitable.

FLT_ROUNDS
This value characterizes the rounding mode for floating point addition. The following values indicate standard rounding modes:
-1
The mode is indeterminable.
0
Rounding is towards zero.
1
Rounding is to the nearest number.
2
Rounding is towards positive infinity.
3
Rounding is towards negative infinity.

Any other value represents a machine-dependent nonstandard rounding mode.

On most machines, the value is 1, in accordance with the IEEE standard for floating point.

Here is a table showing how certain values round for each possible value of FLT_ROUNDS, if the other aspects of the representation match the IEEE single-precision standard. 

                          0      1             2             3
           1.00000003    1.0    1.0           1.00000012    1.0
           1.00000007    1.0    1.00000012    1.00000012    1.0
          -1.00000003   -1.0   -1.0          -1.0          -1.00000012
          -1.00000007   -1.0   -1.00000012   -1.0          -1.00000012

FLT_RADIX
This is the value of the base, or radix, of the exponent representation. This is guaranteed to be a constant expression, unlike the other macros described in this section. The value is 2 on all machines we know of except the IBM 360 and derivatives.
FLT_MANT_DIG
This is the number of base-FLT_RADIX digits in the floating point mantissa for the float data type. The following expression yields 1.0 (even though mathematically it should not) due to the limited number of mantissa digits: 
          float radix = FLT_RADIX;
          
          1.0f + 1.0f / radix / radix / ... / radix

where radix appears FLT_MANT_DIG times.

DBL_MANT_DIG
LDBL_MANT_DIG
This is the number of base-FLT_RADIX digits in the floating point mantissa for the data types double and long double, respectively.
FLT_DIG
This is the number of decimal digits of precision for the float data type. Technically, if p and b are the precision and base (respectively) for the representation, then the decimal precision q is the maximum number of decimal digits such that any floating point number with q base 10 digits can be rounded to a floating point number with p base b digits and back again, without change to the q decimal digits.

The value of this macro is supposed to be at least 6, to satisfy ISO C.

DBL_DIG
LDBL_DIG
These are similar to FLT_DIG, but for the data types double and long double, respectively. The values of these macros are supposed to be at least 10.
FLT_MIN_EXP
This is the smallest possible exponent value for type float. More precisely, is the minimum negative integer such that the value FLT_RADIX raised to this power minus 1 can be represented as a normalized floating point number of type float.
DBL_MIN_EXP
LDBL_MIN_EXP
These are similar to FLT_MIN_EXP, but for the data types double and long double, respectively.
FLT_MIN_10_EXP
This is the minimum negative integer such that 10 raised to this power minus 1 can be represented as a normalized floating point number of type float. This is supposed to be -37 or even less.
DBL_MIN_10_EXP
LDBL_MIN_10_EXP
These are similar to FLT_MIN_10_EXP, but for the data types double and long double, respectively.
FLT_MAX_EXP
This is the largest possible exponent value for type float. More precisely, this is the maximum positive integer such that value FLT_RADIX raised to this power minus 1 can be represented as a floating point number of type float.
DBL_MAX_EXP
LDBL_MAX_EXP
These are similar to FLT_MAX_EXP, but for the data types double and long double, respectively.
FLT_MAX_10_EXP
This is the maximum positive integer such that 10 raised to this power minus 1 can be represented as a normalized floating point number of type float. This is supposed to be at least 37.
DBL_MAX_10_EXP
LDBL_MAX_10_EXP
These are similar to FLT_MAX_10_EXP, but for the data types double and long double, respectively.
FLT_MAX
The value of this macro is the maximum number representable in type float. It is supposed to be at least 1E+37. The value has type float.

The smallest representable number is - FLT_MAX.

DBL_MAX
LDBL_MAX
These are similar to FLT_MAX, but for the data types double and long double, respectively. The type of the macro's value is the same as the type it describes.
FLT_MIN
The value of this macro is the minimum normalized positive floating point number that is representable in type float. It is supposed to be no more than 1E-37.
DBL_MIN
LDBL_MIN
These are similar to FLT_MIN, but for the data types double and long double, respectively. The type of the macro's value is the same as the type it describes.
FLT_EPSILON
This is the minimum positive floating point number of type float such that 1.0 + FLT_EPSILON != 1.0 is true. It's supposed to be no greater than 1E-5.
DBL_EPSILON
LDBL_EPSILON
These are similar to FLT_EPSILON, but for the data types double and long double, respectively. The type of the macro's value is the same as the type it describes. The values are not supposed to be greater than 1E-9.

Previous: Floating Point Parameters, Up: Floating Type Macros
A.5.3.3 IEEE Floating Point

Here is an example showing how the floating type measurements come out for the most common floating point representation, specified by the IEEE Standard for Binary Floating Point Arithmetic (ANSI/IEEE Std 754-1985). Nearly all computers designed since the 1980s use this format.

The IEEE single-precision float representation uses a base of 2. There is a sign bit, a mantissa with 23 bits plus one hidden bit (so the total precision is 24 base-2 digits), and an 8-bit exponent that can represent values in the range -125 to 128, inclusive.

So, for an implementation that uses this representation for the float data type, appropriate values for the corresponding parameters are: 

     FLT_RADIX                             2
     FLT_MANT_DIG                         24
     FLT_DIG                               6
     FLT_MIN_EXP                        -125
     FLT_MIN_10_EXP                      -37
     FLT_MAX_EXP                         128
     FLT_MAX_10_EXP                      +38
     FLT_MIN                 1.17549435E-38F
     FLT_MAX                 3.40282347E+38F
     FLT_EPSILON             1.19209290E-07F

Here are the values for the double data type: 

     DBL_MANT_DIG                         53
     DBL_DIG                              15
     DBL_MIN_EXP                       -1021
     DBL_MIN_10_EXP                     -307
     DBL_MAX_EXP                        1024
     DBL_MAX_10_EXP                      308
     DBL_MAX         1.7976931348623157E+308
     DBL_MIN         2.2250738585072014E-308
     DBL_EPSILON     2.2204460492503131E-016

Previous: Floating Type Macros, Up: Data Type Measurements

A.5.4 Structure Field Offset Measurement

You can use offsetof to measure the location within a structure type of a particular structure member.

— Macro: size_t offsetof (type, member)

This expands to a integer constant expression that is the offset of the structure member named member in the structure type type. For example, offsetof (struct s, elem) is the offset, in bytes, of the member elem in a struct s.

This macro won't work if member is a bit field; you get an error from the C compiler in that case.

Next: , Previous: Language Features, Up: Top

Appendix B Summary of Library Facilities

This appendix is a complete list of the facilities declared within the header files supplied with the GNU C library. Each entry also lists the standard or other source from which each facility is derived, and tells you where in the manual you can find more information about how to use it.

long int a64l (const char *string)
stdlib.h (XPG): Encode Binary Data.
void abort (void)
stdlib.h (ISO): Aborting a Program.
int abs (int number)
stdlib.h (ISO): Absolute Value.
int accept (int socket, struct sockaddr *addr, socklen_t *length_ptr)
sys/socket.h (BSD): Accepting Connections.
int access (const char *filename, int how)
unistd.h (POSIX.1): Testing File Access.
ACCOUNTING
utmp.h (SVID): Manipulating the Database.
double acos (double x)
math.h (ISO): Inverse Trig Functions.
float acosf (float x)
math.h (ISO): Inverse Trig Functions.
double acosh (double x)
math.h (ISO): Hyperbolic Functions.
float acoshf (float x)
math.h (ISO): Hyperbolic Functions.
long double acoshl (long double x)
math.h (ISO): Hyperbolic Functions.
long double acosl (long double x)
math.h (ISO): Inverse Trig Functions.
int addmntent (FILE *stream, const struct mntent *mnt)
mntent.h (BSD): mtab.
int adjtime (const struct timeval *delta, struct timeval *olddelta)
sys/time.h (BSD): High-Resolution Calendar.
int adjtimex (struct timex *timex)
sys/timex.h (GNU): High-Resolution Calendar.
AF_FILE
sys/socket.h (GNU): Address Formats.
AF_INET
sys/socket.h (BSD): Address Formats.
AF_INET6
sys/socket.h (IPv6 Basic API): Address Formats.
AF_LOCAL
sys/socket.h (POSIX): Address Formats.
AF_UNIX
sys/socket.h (BSD, Unix98): Address Formats.
AF_UNSPEC
sys/socket.h (BSD): Address Formats.
int aio_cancel (int fildes, struct aiocb *aiocbp)
aio.h (POSIX.1b): Cancel AIO Operations.
int aio_cancel64 (int fildes, struct aiocb64 *aiocbp)
aio.h (Unix98): Cancel AIO Operations.
int aio_error (const struct aiocb *aiocbp)
aio.h (POSIX.1b): Status of AIO Operations.
int aio_error64 (const struct aiocb64 *aiocbp)
aio.h (Unix98): Status of AIO Operations.
int aio_fsync (int op, struct aiocb *aiocbp)
aio.h (POSIX.1b): Synchronizing AIO Operations.
int aio_fsync64 (int op, struct aiocb64 *aiocbp)
aio.h (Unix98): Synchronizing AIO Operations.
void aio_init (const struct aioinit *init)
aio.h (GNU): Configuration of AIO.
int aio_read (struct aiocb *aiocbp)
aio.h (POSIX.1b): Asynchronous Reads/Writes.
int aio_read64 (struct aiocb *aiocbp)
aio.h (Unix98): Asynchronous Reads/Writes.
ssize_t aio_return (const struct aiocb *aiocbp)
aio.h (POSIX.1b): Status of AIO Operations.
int aio_return64 (const struct aiocb64 *aiocbp)
aio.h (Unix98): Status of AIO Operations.
int aio_suspend (const struct aiocb *const list[], int nent, const struct timespec *timeout)
aio.h (POSIX.1b): Synchronizing AIO Operations.
int aio_suspend64 (const struct aiocb64 *const list[], int nent, const struct timespec *timeout)
aio.h (Unix98): Synchronizing AIO Operations.
int aio_write (struct aiocb *aiocbp)
aio.h (POSIX.1b): Asynchronous Reads/Writes.
int aio_write64 (struct aiocb *aiocbp)
aio.h (Unix98): Asynchronous Reads/Writes.
unsigned int alarm (unsigned int seconds)
unistd.h (POSIX.1): Setting an Alarm.
void * alloca (size_t size);
stdlib.h (GNU, BSD): Variable Size Automatic.
int alphasort (const void *a, const void *b)
dirent.h (BSD/SVID): Scanning Directory Content.
int alphasort64 (const void *a, const void *b)
dirent.h (GNU): Scanning Directory Content.
tcflag_t ALTWERASE
termios.h (BSD): Local Modes.
int ARG_MAX
limits.h (POSIX.1): General Limits.
error_t argp_err_exit_status
argp.h (GNU): Argp Global Variables.
void argp_error (const struct argp_state *state, const char *fmt, ...)
argp.h (GNU): Argp Helper Functions.
int ARGP_ERR_UNKNOWN
argp.h (GNU): Argp Parser Functions.
void argp_failure (const struct argp_state *state, int status, int errnum, const char *fmt, ...)
argp.h (GNU): Argp Helper Functions.
void argp_help (const struct argp *argp, FILE *stream, unsigned flags, char *name)
argp.h (GNU): Argp Help.
ARGP_IN_ORDER
argp.h (GNU): Argp Flags.
ARGP_KEY_ARG
argp.h (GNU): Argp Special Keys.
ARGP_KEY_ARGS
argp.h (GNU): Argp Special Keys.
ARGP_KEY_END
argp.h (GNU): Argp Special Keys.
ARGP_KEY_ERROR
argp.h (GNU): Argp Special Keys.
ARGP_KEY_FINI
argp.h (GNU): Argp Special Keys.
ARGP_KEY_HELP_ARGS_DOC
argp.h (GNU): Argp Help Filter Keys.
ARGP_KEY_HELP_DUP_ARGS_NOTE
argp.h (GNU): Argp Help Filter Keys.
ARGP_KEY_HELP_EXTRA
argp.h (GNU): Argp Help Filter Keys.
ARGP_KEY_HELP_HEADER
argp.h (GNU): Argp Help Filter Keys.
ARGP_KEY_HELP_POST_DOC
argp.h (GNU): Argp Help Filter Keys.
ARGP_KEY_HELP_PRE_DOC
argp.h (GNU): Argp Help Filter Keys.
ARGP_KEY_INIT
argp.h (GNU): Argp Special Keys.
ARGP_KEY_NO_ARGS
argp.h (GNU): Argp Special Keys.
ARGP_KEY_SUCCESS
argp.h (GNU): Argp Special Keys.
ARGP_LONG_ONLY
argp.h (GNU): Argp Flags.
ARGP_NO_ARGS
argp.h (GNU): Argp Flags.
ARGP_NO_ERRS
argp.h (GNU): Argp Flags.
ARGP_NO_EXIT
argp.h (GNU): Argp Flags.
ARGP_NO_HELP
argp.h (GNU): Argp Flags.
error_t argp_parse (const struct argp *argp, int argc, char **argv, unsigned flags, int *arg_index, void *input)
argp.h (GNU): Suboptions.
ARGP_PARSE_ARGV0
argp.h (GNU): Argp Flags.
const char * argp_program_bug_address
argp.h (GNU): Argp Global Variables.
const char * argp_program_version
argp.h (GNU): Argp Global Variables.
argp_program_version_hook
argp.h (GNU): Argp Global Variables.
ARGP_SILENT
argp.h (GNU): Argp Flags.
void argp_state_help (const struct argp_state *state, FILE *stream, unsigned flags)
argp.h (GNU): Argp Helper Functions.
void argp_usage (const struct argp_state *state)
argp.h (GNU): Argp Helper Functions.
error_t argz_add (char **argz, size_t *argz_len, const char *str)
argz.h (GNU): Argz Functions.
error_t argz_add_sep (char **argz, size_t *argz_len, const char *str, int delim)
argz.h (GNU): Argz Functions.
error_t argz_append (char **argz, size_t *argz_len, const char *buf, size_t buf_len)
argz.h (GNU): Argz Functions.
size_t argz_count (const char *argz, size_t arg_len)
argz.h (GNU): Argz Functions.
error_t argz_create (char *const argv[], char **argz, size_t *argz_len)
argz.h (GNU): Argz Functions.
error_t argz_create_sep (const char *string, int sep, char **argz, size_t *argz_len)
argz.h (GNU): Argz Functions.
void argz_delete (char **argz, size_t *argz_len, char *entry)
argz.h (GNU): Argz Functions.
void argz_extract (char *argz, size_t argz_len, char **argv)
argz.h (GNU): Argz Functions.
error_t argz_insert (char **argz, size_t *argz_len, char *before, const char *entry)
argz.h (GNU): Argz Functions.
char * argz_next (char *argz, size_t argz_len, const char *entry)
argz.h (GNU): Argz Functions.
error_t argz_replace (char **argz, size_t *argz_len, const char *str, const char *with, unsigned *replace_count)
argz.h (GNU): Argz Functions.
void argz_stringify (char *argz, size_t len, int sep)
argz.h (GNU): Argz Functions.
char * asctime (const struct tm *brokentime)
time.h (ISO): Formatting Calendar Time.
char * asctime_r (const struct tm *brokentime, char *buffer)
time.h (POSIX.1c): Formatting Calendar Time.
double asin (double x)
math.h (ISO): Inverse Trig Functions.
float asinf (float x)
math.h (ISO): Inverse Trig Functions.
double asinh (double x)
math.h (ISO): Hyperbolic Functions.
float asinhf (float x)
math.h (ISO): Hyperbolic Functions.
long double asinhl (long double x)
math.h (ISO): Hyperbolic Functions.
long double asinl (long double x)
math.h (ISO): Inverse Trig Functions.
int asprintf (char **ptr, const char *template, ...)
stdio.h (GNU): Dynamic Output.
void assert (int expression)
assert.h (ISO): Consistency Checking.
void assert_perror (int errnum)
assert.h (GNU): Consistency Checking.
double atan (double x)
math.h (ISO): Inverse Trig Functions.
double atan2 (double y, double x)
math.h (ISO): Inverse Trig Functions.
float atan2f (float y, float x)
math.h (ISO): Inverse Trig Functions.
long double atan2l (long double y, long double x)
math.h (ISO): Inverse Trig Functions.
float atanf (float x)
math.h (ISO): Inverse Trig Functions.
double atanh (double x)
math.h (ISO): Hyperbolic Functions.
float atanhf (float x)
math.h (ISO): Hyperbolic Functions.
long double atanhl (long double x)
math.h (ISO): Hyperbolic Functions.
long double atanl (long double x)
math.h (ISO): Inverse Trig Functions.
int atexit (void (*function) (void))
stdlib.h (ISO): Cleanups on Exit.
double atof (const char *string)
stdlib.h (ISO): Parsing of Floats.
int atoi (const char *string)
stdlib.h (ISO): Parsing of Integers.
long int atol (const char *string)
stdlib.h (ISO): Parsing of Integers.
long long int atoll (const char *string)
stdlib.h (ISO): Parsing of Integers.
B0
termios.h (POSIX.1): Line Speed.
B110
termios.h (POSIX.1): Line Speed.
B115200
termios.h (GNU): Line Speed.
B1200
termios.h (POSIX.1): Line Speed.
B134
termios.h (POSIX.1): Line Speed.
B150
termios.h (POSIX.1): Line Speed.
B1800
termios.h (POSIX.1): Line Speed.
B19200
termios.h (POSIX.1): Line Speed.
B200
termios.h (POSIX.1): Line Speed.
B230400
termios.h (GNU): Line Speed.
B2400
termios.h (POSIX.1): Line Speed.
B300
termios.h (POSIX.1): Line Speed.
B38400
termios.h (POSIX.1): Line Speed.
B460800
termios.h (GNU): Line Speed.
B4800
termios.h (POSIX.1): Line Speed.
B50
termios.h (POSIX.1): Line Speed.
B57600
termios.h (GNU): Line Speed.
B600
termios.h (POSIX.1): Line Speed.
B75
termios.h (POSIX.1): Line Speed.
B9600
termios.h (POSIX.1): Line Speed.
int backtrace (void **buffer, int size)
execinfo.h (GNU): Backtraces.
char ** backtrace_symbols (void *const *buffer, int size)
execinfo.h (GNU): Backtraces.
void backtrace_symbols_fd (void *const *buffer, int size, int fd)
execinfo.h (GNU): Backtraces.
char * basename (char *path)
libgen.h (XPG): Finding Tokens in a String.
char * basename (const char *filename)
string.h (GNU): Finding Tokens in a String.
int BC_BASE_MAX
limits.h (POSIX.2): Utility Limits.
int BC_DIM_MAX
limits.h (POSIX.2): Utility Limits.
int bcmp (const void *a1, const void *a2, size_t size)
string.h (BSD): String/Array Comparison.
void bcopy (const void *from, void *to, size_t size)
string.h (BSD): Copying and Concatenation.
int BC_SCALE_MAX
limits.h (POSIX.2): Utility Limits.
int BC_STRING_MAX
limits.h (POSIX.2): Utility Limits.
int bind (int socket, struct sockaddr *addr, socklen_t length)
sys/socket.h (BSD): Setting Address.
char * bindtextdomain (const char *domainname, const char *dirname)
libintl.h (GNU): Locating gettext catalog.
char * bind_textdomain_codeset (const char *domainname, const char *codeset)
libintl.h (GNU): Charset conversion in gettext.
blkcnt64_t
sys/types.h (Unix98): Attribute Meanings.
blkcnt_t
sys/types.h (Unix98): Attribute Meanings.
BOOT_TIME
utmp.h (SVID): Manipulating the Database.
BOOT_TIME
utmpx.h (XPG4.2): XPG Functions.
int brk (void *addr)
unistd.h (BSD): Resizing the Data Segment.
tcflag_t BRKINT
termios.h (POSIX.1): Input Modes.
_BSD_SOURCE
(GNU): Feature Test Macros.
void * bsearch (const void *key, const void *array, size_t count, size_t size, comparison_fn_t compare)
stdlib.h (ISO): Array Search Function.
wint_t btowc (int c)
wchar.h (ISO): Converting a Character.
int BUFSIZ
stdio.h (ISO): Controlling Buffering.
void bzero (void *block, size_t size)
string.h (BSD): Copying and Concatenation.
double cabs (complex double z)
complex.h (ISO): Absolute Value.
float cabsf (complex float z)
complex.h (ISO): Absolute Value.
long double cabsl (complex long double z)
complex.h (ISO): Absolute Value.
complex double cacos (complex double z)
complex.h (ISO): Inverse Trig Functions.
complex float cacosf (complex float z)
complex.h (ISO): Inverse Trig Functions.
complex double cacosh (complex double z)
complex.h (ISO): Hyperbolic Functions.
complex float cacoshf (complex float z)
complex.h (ISO): Hyperbolic Functions.
complex long double cacoshl (complex long double z)
complex.h (ISO): Hyperbolic Functions.
complex long double cacosl (complex long double z)
complex.h (ISO): Inverse Trig Functions.
void * calloc (size_t count, size_t eltsize)
malloc.h, stdlib.h (ISO): Allocating Cleared Space.
char * canonicalize_file_name (const char *name)
stdlib.h (GNU): Symbolic Links.
double carg (complex double z)
complex.h (ISO): Operations on Complex.
float cargf (complex float z)
complex.h (ISO): Operations on Complex.
long double cargl (complex long double z)
complex.h (ISO): Operations on Complex.
complex double casin (complex double z)
complex.h (ISO): Inverse Trig Functions.
complex float casinf (complex float z)
complex.h (ISO): Inverse Trig Functions.
complex double casinh (complex double z)
complex.h (ISO): Hyperbolic Functions.
complex float casinhf (complex float z)
complex.h (ISO): Hyperbolic Functions.
complex long double casinhl (complex long double z)
complex.h (ISO): Hyperbolic Functions.
complex long double casinl (complex long double z)
complex.h (ISO): Inverse Trig Functions.
complex double catan (complex double z)
complex.h (ISO): Inverse Trig Functions.
complex float catanf (complex float z)
complex.h (ISO): Inverse Trig Functions.
complex double catanh (complex double z)
complex.h (ISO): Hyperbolic Functions.
complex float catanhf (complex float z)
complex.h (ISO): Hyperbolic Functions.
complex long double catanhl (complex long double z)
complex.h (ISO): Hyperbolic Functions.
complex long double catanl (complex long double z)
complex.h (ISO): Inverse Trig Functions.
nl_catd catopen (const char *cat_name, int flag)
nl_types.h (X/Open): The catgets Functions.
int cbc_crypt (char *key, char *blocks, unsigned len, unsigned mode, char *ivec)
rpc/des_crypt.h (SUNRPC): DES Encryption.
double cbrt (double x)
math.h (BSD): Exponents and Logarithms.
float cbrtf (float x)
math.h (BSD): Exponents and Logarithms.
long double cbrtl (long double x)
math.h (BSD): Exponents and Logarithms.
complex double ccos (complex double z)
complex.h (ISO): Trig Functions.
complex float ccosf (complex float z)
complex.h (ISO): Trig Functions.
complex double ccosh (complex double z)
complex.h (ISO): Hyperbolic Functions.
complex float ccoshf (complex float z)
complex.h (ISO): Hyperbolic Functions.
complex long double ccoshl (complex long double z)
complex.h (ISO): Hyperbolic Functions.
complex long double ccosl (complex long double z)
complex.h (ISO): Trig Functions.
cc_t
termios.h (POSIX.1): Mode Data Types.
tcflag_t CCTS_OFLOW
termios.h (BSD): Control Modes.
double ceil (double x)
math.h (ISO): Rounding Functions.
float ceilf (float x)
math.h (ISO): Rounding Functions.
long double ceill (long double x)
math.h (ISO): Rounding Functions.
complex double cexp (complex double z)
complex.h (ISO): Exponents and Logarithms.
complex float cexpf (complex float z)
complex.h (ISO): Exponents and Logarithms.
complex long double cexpl (complex long double z)
complex.h (ISO): Exponents and Logarithms.
speed_t cfgetispeed (const struct termios *termios-p)
termios.h (POSIX.1): Line Speed.
speed_t cfgetospeed (const struct termios *termios-p)
termios.h (POSIX.1): Line Speed.
void cfmakeraw (struct termios *termios-p)
termios.h (BSD): Noncanonical Input.
void cfree (void *ptr)
stdlib.h (Sun): Freeing after Malloc.
int cfsetispeed (struct termios *termios-p, speed_t speed)
termios.h (POSIX.1): Line Speed.
int cfsetospeed (struct termios *termios-p, speed_t speed)
termios.h (POSIX.1): Line Speed.
int cfsetspeed (struct termios *termios-p, speed_t speed)
termios.h (BSD): Line Speed.
CHAR_BIT
limits.h (ISO): Width of Type.
CHAR_MAX
limits.h (ISO): Range of Type.
CHAR_MIN
limits.h (ISO): Range of Type.
int chdir (const char *filename)
unistd.h (POSIX.1): Working Directory.
int CHILD_MAX
limits.h (POSIX.1): General Limits.
int chmod (const char *filename, mode_t mode)
sys/stat.h (POSIX.1): Setting Permissions.
int chown (const char *filename, uid_t owner, gid_t group)
unistd.h (POSIX.1): File Owner.
tcflag_t CIGNORE
termios.h (BSD): Control Modes.
double cimag (complex double z)
complex.h (ISO): Operations on Complex.
float cimagf (complex float z)
complex.h (ISO): Operations on Complex.
long double cimagl (complex long double z)
complex.h (ISO): Operations on Complex.
int clearenv (void)
stdlib.h (GNU): Environment Access.
void clearerr (FILE *stream)
stdio.h (ISO): Error Recovery.
void clearerr_unlocked (FILE *stream)
stdio.h (GNU): Error Recovery.
int CLK_TCK
time.h (POSIX.1): CPU Time.
tcflag_t CLOCAL
termios.h (POSIX.1): Control Modes.
clock_t clock (void)
time.h (ISO): CPU Time.
int CLOCKS_PER_SEC
time.h (ISO): CPU Time.
clock_t
time.h (ISO): CPU Time.
complex double clog (complex double z)
complex.h (ISO): Exponents and Logarithms.
complex double clog10 (complex double z)
complex.h (GNU): Exponents and Logarithms.
complex float clog10f (complex float z)
complex.h (GNU): Exponents and Logarithms.
complex long double clog10l (complex long double z)
complex.h (GNU): Exponents and Logarithms.
complex float clogf (complex float z)
complex.h (ISO): Exponents and Logarithms.
complex long double clogl (complex long double z)
complex.h (ISO): Exponents and Logarithms.
int close (int filedes)
unistd.h (POSIX.1): Opening and Closing Files.
int closedir (DIR *dirstream)
dirent.h (POSIX.1): Reading/Closing Directory.
void closelog (void)
syslog.h (BSD): closelog.
int COLL_WEIGHTS_MAX
limits.h (POSIX.2): Utility Limits.
size_t confstr (int parameter, char *buf, size_t len)
unistd.h (POSIX.2): String Parameters.
complex double conj (complex double z)
complex.h (ISO): Operations on Complex.
complex float conjf (complex float z)
complex.h (ISO): Operations on Complex.
complex long double conjl (complex long double z)
complex.h (ISO): Operations on Complex.
int connect (int socket, struct sockaddr *addr, socklen_t length)
sys/socket.h (BSD): Connecting.
cookie_close_function
stdio.h (GNU): Hook Functions.
cookie_io_functions_t
stdio.h (GNU): Streams and Cookies.
cookie_read_function
stdio.h (GNU): Hook Functions.
cookie_seek_function
stdio.h (GNU): Hook Functions.
cookie_write_function
stdio.h (GNU): Hook Functions.
double copysign (double x, double y)
math.h (ISO): FP Bit Twiddling.
float copysignf (float x, float y)
math.h (ISO): FP Bit Twiddling.
long double copysignl (long double x, long double y)
math.h (ISO): FP Bit Twiddling.
double cos (double x)
math.h (ISO): Trig Functions.
float cosf (float x)
math.h (ISO): Trig Functions.
double cosh (double x)
math.h (ISO): Hyperbolic Functions.
float coshf (float x)
math.h (ISO): Hyperbolic Functions.
long double coshl (long double x)
math.h (ISO): Hyperbolic Functions.
long double cosl (long double x)
math.h (ISO): Trig Functions.
complex double cpow (complex double base, complex double power)
complex.h (ISO): Exponents and Logarithms.
complex float cpowf (complex float base, complex float power)
complex.h (ISO): Exponents and Logarithms.
complex long double cpowl (complex long double base, complex long double power)
complex.h (ISO): Exponents and Logarithms.
complex double cproj (complex double z)
complex.h (ISO): Operations on Complex.
complex float cprojf (complex float z)
complex.h (ISO): Operations on Complex.
complex long double cprojl (complex long double z)
complex.h (ISO): Operations on Complex.
void CPU_CLR (int cpu, cpu_set_t *set)
sched.h (GNU): CPU Affinity.
int CPU_ISSET (int cpu, const cpu_set_t *set)
sched.h (GNU): CPU Affinity.
void CPU_SET (int cpu, cpu_set_t *set)
sched.h (GNU): CPU Affinity.
int CPU_SETSIZE
sched.h (GNU): CPU Affinity.
cpu_set_t
sched.h (GNU): CPU Affinity.
void CPU_ZERO (cpu_set_t *set)
sched.h (GNU): CPU Affinity.
tcflag_t CREAD
termios.h (POSIX.1): Control Modes.
double creal (complex double z)
complex.h (ISO): Operations on Complex.
float crealf (complex float z)
complex.h (ISO): Operations on Complex.
long double creall (complex long double z)
complex.h (ISO): Operations on Complex.
int creat (const char *filename, mode_t mode)
fcntl.h (POSIX.1): Opening and Closing Files.
int creat64 (const char *filename, mode_t mode)
fcntl.h (Unix98): Opening and Closing Files.
tcflag_t CRTS_IFLOW
termios.h (BSD): Control Modes.
char * crypt (const char *key, const char *salt)
crypt.h (BSD, SVID): crypt.
char * crypt_r (const char *key, const char *salt, struct crypt_data * data)
crypt.h (GNU): crypt.
tcflag_t CS5
termios.h (POSIX.1): Control Modes.
tcflag_t CS6
termios.h (POSIX.1): Control Modes.
tcflag_t CS7
termios.h (POSIX.1): Control Modes.
tcflag_t CS8
termios.h (POSIX.1): Control Modes.
complex double csin (complex double z)
complex.h (ISO): Trig Functions.
complex float csinf (complex float z)
complex.h (ISO): Trig Functions.
complex double csinh (complex double z)
complex.h (ISO): Hyperbolic Functions.
complex float csinhf (complex float z)
complex.h (ISO): Hyperbolic Functions.
complex long double csinhl (complex long double z)
complex.h (ISO): Hyperbolic Functions.
complex long double csinl (complex long double z)
complex.h (ISO): Trig Functions.
tcflag_t CSIZE
termios.h (POSIX.1): Control Modes.
_CS_LFS64_CFLAGS
unistd.h (Unix98): String Parameters.
_CS_LFS64_LDFLAGS
unistd.h (Unix98): String Parameters.
_CS_LFS64_LIBS
unistd.h (Unix98): String Parameters.
_CS_LFS64_LINTFLAGS
unistd.h (Unix98): String Parameters.
_CS_LFS_CFLAGS
unistd.h (Unix98): String Parameters.
_CS_LFS_LDFLAGS
unistd.h (Unix98): String Parameters.
_CS_LFS_LIBS
unistd.h (Unix98): String Parameters.
_CS_LFS_LINTFLAGS
unistd.h (Unix98): String Parameters.
_CS_PATH
unistd.h (POSIX.2): String Parameters.
complex double csqrt (complex double z)
complex.h (ISO): Exponents and Logarithms.
complex float csqrtf (complex float z)
complex.h (ISO): Exponents and Logarithms.
complex long double csqrtl (complex long double z)
complex.h (ISO): Exponents and Logarithms.
tcflag_t CSTOPB
termios.h (POSIX.1): Control Modes.
complex double ctan (complex double z)
complex.h (ISO): Trig Functions.
complex float ctanf (complex float z)
complex.h (ISO): Trig Functions.
complex double ctanh (complex double z)
complex.h (ISO): Hyperbolic Functions.
complex float ctanhf (complex float z)
complex.h (ISO): Hyperbolic Functions.
complex long double ctanhl (complex long double z)
complex.h (ISO): Hyperbolic Functions.
complex long double ctanl (complex long double z)
complex.h (ISO): Trig Functions.
char * ctermid (char *string)
stdio.h (POSIX.1): Identifying the Terminal.
char * ctime (const time_t *time)
time.h (ISO): Formatting Calendar Time.
char * ctime_r (const time_t *time, char *buffer)
time.h (POSIX.1c): Formatting Calendar Time.
char * cuserid (char *string)
stdio.h (POSIX.1): Who Logged In.
int daylight
time.h (SVID): Time Zone Functions.
DBL_DIG
float.h (ISO): Floating Point Parameters.
DBL_EPSILON
float.h (ISO): Floating Point Parameters.
DBL_MANT_DIG
float.h (ISO): Floating Point Parameters.
DBL_MAX
float.h (ISO): Floating Point Parameters.
DBL_MAX_10_EXP
float.h (ISO): Floating Point Parameters.
DBL_MAX_EXP
float.h (ISO): Floating Point Parameters.
DBL_MIN
float.h (ISO): Floating Point Parameters.
DBL_MIN_10_EXP
float.h (ISO): Floating Point Parameters.
DBL_MIN_EXP
float.h (ISO): Floating Point Parameters.
char * dcgettext (const char *domainname, const char *msgid, int category)
libintl.h (GNU): Translation with gettext.
char * dcngettext (const char *domain, const char *msgid1, const char *msgid2, unsigned long int n, int category)
libintl.h (GNU): Advanced gettext functions.
DEAD_PROCESS
utmp.h (SVID): Manipulating the Database.
DEAD_PROCESS
utmpx.h (XPG4.2): XPG Functions.
DES_DECRYPT
rpc/des_crypt.h (SUNRPC): DES Encryption.
DES_ENCRYPT
rpc/des_crypt.h (SUNRPC): DES Encryption.
DESERR_BADPARAM
rpc/des_crypt.h (SUNRPC): DES Encryption.
DESERR_HWERROR
rpc/des_crypt.h (SUNRPC): DES Encryption.
DESERR_NOHWDEVICE
rpc/des_crypt.h (SUNRPC): DES Encryption.
DESERR_NONE
rpc/des_crypt.h (SUNRPC): DES Encryption.
int DES_FAILED (int err)
rpc/des_crypt.h (SUNRPC): DES Encryption.
DES_HW
rpc/des_crypt.h (SUNRPC): DES Encryption.
void des_setparity (char *key)
rpc/des_crypt.h (SUNRPC): DES Encryption.
DES_SW
rpc/des_crypt.h (SUNRPC): DES Encryption.
dev_t
sys/types.h (POSIX.1): Attribute Meanings.
char * dgettext (const char *domainname, const char *msgid)
libintl.h (GNU): Translation with gettext.
double difftime (time_t time1, time_t time0)
time.h (ISO): Elapsed Time.
DIR
dirent.h (POSIX.1): Opening a Directory.
int dirfd (DIR *dirstream)
dirent.h (GNU): Opening a Directory.
char * dirname (char *path)
libgen.h (XPG): Finding Tokens in a String.
div_t div (int numerator, int denominator)
stdlib.h (ISO): Integer Division.
div_t
stdlib.h (ISO): Integer Division.
char * dngettext (const char *domain, const char *msgid1, const char *msgid2, unsigned long int n)
libintl.h (GNU): Advanced gettext functions.
double drand48 (void)
stdlib.h (SVID): SVID Random.
int drand48_r (struct drand48_data *buffer, double *result)
stdlib.h (GNU): SVID Random.
double drem (double numerator, double denominator)
math.h (BSD): Remainder Functions.
float dremf (float numerator, float denominator)
math.h (BSD): Remainder Functions.
long double dreml (long double numerator, long double denominator)
math.h (BSD): Remainder Functions.
mode_t DTTOIF (int dtype)
dirent.h (BSD): Directory Entries.
int dup (int old)
unistd.h (POSIX.1): Duplicating Descriptors.
int dup2 (int old, int new)
unistd.h (POSIX.1): Duplicating Descriptors.
int E2BIG
errno.h (POSIX.1: Argument list too long): Error Codes.
int EACCES
errno.h (POSIX.1: Permission denied): Error Codes.
int EADDRINUSE
errno.h (BSD: Address already in use): Error Codes.
int EADDRNOTAVAIL
errno.h (BSD: Cannot assign requested address): Error Codes.
int EADV
errno.h (Linux???: Advertise error): Error Codes.
int EAFNOSUPPORT
errno.h (BSD: Address family not supported by protocol): Error Codes.
int EAGAIN
errno.h (POSIX.1: Resource temporarily unavailable): Error Codes.
int EALREADY
errno.h (BSD: Operation already in progress): Error Codes.
int EAUTH
errno.h (BSD: Authentication error): Error Codes.
int EBACKGROUND
errno.h (GNU: Inappropriate operation for background process): Error Codes.
int EBADE
errno.h (Linux???: Invalid exchange): Error Codes.
int EBADF
errno.h (POSIX.1: Bad file descriptor): Error Codes.
int EBADFD
errno.h (Linux???: File descriptor in bad state): Error Codes.
int EBADMSG
errno.h (XOPEN: Bad message): Error Codes.
int EBADR
errno.h (Linux???: Invalid request descriptor): Error Codes.
int EBADRPC
errno.h (BSD: RPC struct is bad): Error Codes.
int EBADRQC
errno.h (Linux???: Invalid request code): Error Codes.
int EBADSLT
errno.h (Linux???: Invalid slot): Error Codes.
int EBFONT
errno.h (Linux???: Bad font file format): Error Codes.
int EBUSY
errno.h (POSIX.1: Device or resource busy): Error Codes.
int ECANCELED
errno.h (POSIX.1: Operation canceled): Error Codes.
int ecb_crypt (char *key, char *blocks, unsigned len, unsigned mode)
rpc/des_crypt.h (SUNRPC): DES Encryption.
int ECHILD
errno.h (POSIX.1: No child processes): Error Codes.
tcflag_t ECHO
termios.h (POSIX.1): Local Modes.
tcflag_t ECHOCTL
termios.h (BSD): Local Modes.
tcflag_t ECHOE
termios.h (POSIX.1): Local Modes.
tcflag_t ECHOK
termios.h (POSIX.1): Local Modes.
tcflag_t ECHOKE
termios.h (BSD): Local Modes.
tcflag_t ECHONL
termios.h (POSIX.1): Local Modes.
tcflag_t ECHOPRT
termios.h (BSD): Local Modes.
int ECHRNG
errno.h (Linux???: Channel number out of range): Error Codes.
int ECOMM
errno.h (Linux???: Communication error on send): Error Codes.
int ECONNABORTED
errno.h (BSD: Software caused connection abort): Error Codes.
int ECONNREFUSED
errno.h (BSD: Connection refused): Error Codes.
int ECONNRESET
errno.h (BSD: Connection reset by peer): Error Codes.
char * ecvt (double value, int ndigit, int *decpt, int *neg)
stdlib.h (SVID, Unix98): System V Number Conversion.
int ecvt_r (double value, int ndigit, int *decpt, int *neg, char *buf, size_t len)
stdlib.h (GNU): System V Number Conversion.
int ED
errno.h (GNU: ?): Error Codes.
int EDEADLK
errno.h (POSIX.1: Resource deadlock avoided): Error Codes.
int EDEADLOCK
errno.h (Linux???: File locking deadlock error): Error Codes.
int EDESTADDRREQ
errno.h (BSD: Destination address required): Error Codes.
int EDIED
errno.h (GNU: Translator died): Error Codes.
int EDOM
errno.h (ISO: Numerical argument out of domain): Error Codes.
int EDOTDOT
errno.h (Linux???: RFS specific error): Error Codes.
int EDQUOT
errno.h (BSD: Disk quota exceeded): Error Codes.
int EEXIST
errno.h (POSIX.1: File exists): Error Codes.
int EFAULT
errno.h (POSIX.1: Bad address): Error Codes.
int EFBIG
errno.h (POSIX.1: File too large): Error Codes.
int EFTYPE
errno.h (BSD: Inappropriate file type or format): Error Codes.
int EGRATUITOUS
errno.h (GNU: Gratuitous error): Error Codes.
int EGREGIOUS
errno.h (GNU: You really blew it this time): Error Codes.
int EHOSTDOWN
errno.h (BSD: Host is down): Error Codes.
int EHOSTUNREACH
errno.h (BSD: No route to host): Error Codes.
int EIDRM
errno.h (XOPEN: Identifier removed): Error Codes.
int EIEIO
errno.h (GNU: Computer bought the farm): Error Codes.
int EILSEQ
errno.h (ISO: Invalid or incomplete multibyte or wide character): Error Codes.
int EINPROGRESS
errno.h (BSD: Operation now in progress): Error Codes.
int EINTR
errno.h (POSIX.1: Interrupted system call): Error Codes.
int EINVAL
errno.h (POSIX.1: Invalid argument): Error Codes.
int EIO
errno.h (POSIX.1: Input/output error): Error Codes.
int EISCONN
errno.h (BSD: Transport endpoint is already connected): Error Codes.
int EISDIR
errno.h (POSIX.1: Is a directory): Error Codes.
int EISNAM
errno.h (Linux???: Is a named type file): Error Codes.
int EKEYEXPIRED
errno.h (Linux: Key has expired): Error Codes.
int EKEYREJECTED
errno.h (Linux: Key was rejected by service): Error Codes.
int EKEYREVOKED
errno.h (Linux: Key has been revoked): Error Codes.
int EL2HLT
errno.h (Obsolete: Level 2 halted): Error Codes.
int EL2NSYNC
errno.h (Obsolete: Level 2 not synchronized): Error Codes.
int EL3HLT
errno.h (Obsolete: Level 3 halted): Error Codes.
int EL3RST
errno.h (Obsolete: Level 3 reset): Error Codes.
int ELIBACC
errno.h (Linux???: Can not access a needed shared library): Error Codes.
int ELIBBAD
errno.h (Linux???: Accessing a corrupted shared library): Error Codes.
int ELIBEXEC
errno.h (Linux???: Cannot exec a shared library directly): Error Codes.
int ELIBMAX
errno.h (Linux???: Attempting to link in too many shared libraries): Error Codes.
int ELIBSCN
errno.h (Linux???: .lib section in a.out corrupted): Error Codes.
int ELNRNG
errno.h (Linux???: Link number out of range): Error Codes.
int ELOOP
errno.h (BSD: Too many levels of symbolic links): Error Codes.
int EMEDIUMTYPE
errno.h (Linux???: Wrong medium type): Error Codes.
int EMFILE
errno.h (POSIX.1: Too many open files): Error Codes.
int EMLINK
errno.h (POSIX.1: Too many links): Error Codes.
EMPTY
utmp.h (SVID): Manipulating the Database.
EMPTY
utmpx.h (XPG4.2): XPG Functions.
int EMSGSIZE
errno.h (BSD: Message too long): Error Codes.
int EMULTIHOP
errno.h (XOPEN: Multihop attempted): Error Codes.
int ENAMETOOLONG
errno.h (POSIX.1: File name too long): Error Codes.
int ENAVAIL
errno.h (Linux???: No XENIX semaphores available): Error Codes.
void encrypt (char *block, int edflag)
crypt.h (BSD, SVID): DES Encryption.
void encrypt_r (char *block, int edflag, struct crypt_data * data)
crypt.h (GNU): DES Encryption.
void endfsent (void)
fstab.h (BSD): fstab.
void endgrent (void)
grp.h (SVID, BSD): Scanning All Groups.
void endhostent (void)
netdb.h (BSD): Host Names.
int endmntent (FILE *stream)
mntent.h (BSD): mtab.
void endnetent (void)
netdb.h (BSD): Networks Database.
void endnetgrent (void)
netdb.h (BSD): Lookup Netgroup.
void endprotoent (void)
netdb.h (BSD): Protocols Database.
void endpwent (void)
pwd.h (SVID, BSD): Scanning All Users.
void endservent (void)
netdb.h (BSD): Services Database.
void endutent (void)
utmp.h (SVID): Manipulating the Database.
void endutxent (void)
utmpx.h (XPG4.2): XPG Functions.
int ENEEDAUTH
errno.h (BSD: Need authenticator): Error Codes.
int ENETDOWN
errno.h (BSD: Network is down): Error Codes.
int ENETRESET
errno.h (BSD: Network dropped connection on reset): Error Codes.
int ENETUNREACH
errno.h (BSD: Network is unreachable): Error Codes.
int ENFILE
errno.h (POSIX.1: Too many open files in system): Error Codes.
int ENOANO
errno.h (Linux???: No anode): Error Codes.
int ENOBUFS
errno.h (BSD: No buffer space available): Error Codes.
int ENOCSI
errno.h (Linux???: No CSI structure available): Error Codes.
int ENODATA
errno.h (XOPEN: No data available): Error Codes.
int ENODEV
errno.h (POSIX.1: No such device): Error Codes.
int ENOENT
errno.h (POSIX.1: No such file or directory): Error Codes.
int ENOEXEC
errno.h (POSIX.1: Exec format error): Error Codes.
int ENOKEY
errno.h (Linux: Required key not available): Error Codes.
int ENOLCK
errno.h (POSIX.1: No locks available): Error Codes.
int ENOLINK
errno.h (XOPEN: Link has been severed): Error Codes.
int ENOMEDIUM
errno.h (Linux???: No medium found): Error Codes.
int ENOMEM
errno.h (POSIX.1: Cannot allocate memory): Error Codes.
int ENOMSG
errno.h (XOPEN: No message of desired type): Error Codes.
int ENONET
errno.h (Linux???: Machine is not on the network): Error Codes.
int ENOPKG
errno.h (Linux???: Package not installed): Error Codes.
int ENOPROTOOPT
errno.h (BSD: Protocol not available): Error Codes.
int ENOSPC
errno.h (POSIX.1: No space left on device): Error Codes.
int ENOSR
errno.h (XOPEN: Out of streams resources): Error Codes.
int ENOSTR
errno.h (XOPEN: Device not a stream): Error Codes.
int ENOSYS
errno.h (POSIX.1: Function not implemented): Error Codes.
int ENOTBLK
errno.h (BSD: Block device required): Error Codes.
int ENOTCONN
errno.h (BSD: Transport endpoint is not connected): Error Codes.
int ENOTDIR
errno.h (POSIX.1: Not a directory): Error Codes.
int ENOTEMPTY
errno.h (POSIX.1: Directory not empty): Error Codes.
int ENOTNAM
errno.h (Linux???: Not a XENIX named type file): Error Codes.
int ENOTRECOVERABLE
errno.h (Linux: State not recoverable): Error Codes.
int ENOTSOCK
errno.h (BSD: Socket operation on non-socket): Error Codes.
int ENOTSUP
errno.h (POSIX.1: Not supported): Error Codes.
int ENOTTY
errno.h (POSIX.1: Inappropriate ioctl for device): Error Codes.
int ENOTUNIQ
errno.h (Linux???: Name not unique on network): Error Codes.
char ** environ
unistd.h (POSIX.1): Environment Access.
error_t envz_add (char **envz, size_t *envz_len, const char *name, const char *value)
envz.h (GNU): Envz Functions.
char * envz_entry (const char *envz, size_t envz_len, const char *name)
envz.h (GNU): Envz Functions.
char * envz_get (const char *envz, size_t envz_len, const char *name)
envz.h (GNU): Envz Functions.
error_t envz_merge (char **envz, size_t *envz_len, const char *envz2, size_t envz2_len, int override)
envz.h (GNU): Envz Functions.
void envz_strip (char **envz, size_t *envz_len)
envz.h (GNU): Envz Functions.
int ENXIO
errno.h (POSIX.1: No such device or address): Error Codes.
int EOF
stdio.h (ISO): EOF and Errors.
int EOPNOTSUPP
errno.h (BSD: Operation not supported): Error Codes.
int EOVERFLOW
errno.h (XOPEN: Value too large for defined data type): Error Codes.
int EOWNERDEAD
errno.h (Linux: Owner died): Error Codes.
int EPERM
errno.h (POSIX.1: Operation not permitted): Error Codes.
int EPFNOSUPPORT
errno.h (BSD: Protocol family not supported): Error Codes.
int EPIPE
errno.h (POSIX.1: Broken pipe): Error Codes.
int EPROCLIM
errno.h (BSD: Too many processes): Error Codes.
int EPROCUNAVAIL
errno.h (BSD: RPC bad procedure for program): Error Codes.
int EPROGMISMATCH
errno.h (BSD: RPC program version wrong): Error Codes.
int EPROGUNAVAIL
errno.h (BSD: RPC program not available): Error Codes.
int EPROTO
errno.h (XOPEN: Protocol error): Error Codes.
int EPROTONOSUPPORT
errno.h (BSD: Protocol not supported): Error Codes.
int EPROTOTYPE
errno.h (BSD: Protocol wrong type for socket): Error Codes.
int EQUIV_CLASS_MAX
limits.h (POSIX.2): Utility Limits.
double erand48 (unsigned short int xsubi[3])
stdlib.h (SVID): SVID Random.
int erand48_r (unsigned short int xsubi[3], struct drand48_data *buffer, double *result)
stdlib.h (GNU): SVID Random.
int ERANGE
errno.h (ISO: Numerical result out of range): Error Codes.
int EREMCHG
errno.h (Linux???: Remote address changed): Error Codes.
int EREMOTE
errno.h (BSD: Object is remote): Error Codes.
int EREMOTEIO
errno.h (Linux???: Remote I/O error): Error Codes.
int ERESTART
errno.h (Linux???: Interrupted system call should be restarted): Error Codes.
double erf (double x)
math.h (SVID): Special Functions.
double erfc (double x)
math.h (SVID): Special Functions.
float erfcf (float x)
math.h (SVID): Special Functions.
long double erfcl (long double x)
math.h (SVID): Special Functions.
float erff (float x)
math.h (SVID): Special Functions.
long double erfl (long double x)
math.h (SVID): Special Functions.
int EROFS
errno.h (POSIX.1: Read-only file system): Error Codes.
int ERPCMISMATCH
errno.h (BSD: RPC version wrong): Error Codes.
void err (int status, const char *format, ...)
err.h (BSD): Error Messages.
volatile int errno
errno.h (ISO): Checking for Errors.
void error (int status, int errnum, const char *format, ...)
error.h (GNU): Error Messages.
void error_at_line (int status, int errnum, const char *fname, unsigned int lineno, const char *format, ...)
error.h (GNU): Error Messages.
unsigned int error_message_count
error.h (GNU): Error Messages.
int error_one_per_line
error.h (GNU): Error Messages.
void errx (int status, const char *format, ...)
err.h (BSD): Error Messages.
int ESHUTDOWN
errno.h (BSD: Cannot send after transport endpoint shutdown): Error Codes.
int ESOCKTNOSUPPORT
errno.h (BSD: Socket type not supported): Error Codes.
int ESPIPE
errno.h (POSIX.1: Illegal seek): Error Codes.
int ESRCH
errno.h (POSIX.1: No such process): Error Codes.
int ESRMNT
errno.h (Linux???: Srmount error): Error Codes.
int ESTALE
errno.h (BSD: Stale NFS file handle): Error Codes.
int ESTRPIPE
errno.h (Linux???: Streams pipe error): Error Codes.
int ETIME
errno.h (XOPEN: Timer expired): Error Codes.
int ETIMEDOUT
errno.h (BSD: Connection timed out): Error Codes.
int ETOOMANYREFS
errno.h (BSD: Too many references: cannot splice): Error Codes.
int ETXTBSY
errno.h (BSD: Text file busy): Error Codes.
int EUCLEAN
errno.h (Linux???: Structure needs cleaning): Error Codes.
int EUNATCH
errno.h (Linux???: Protocol driver not attached): Error Codes.
int EUSERS
errno.h (BSD: Too many users): Error Codes.
int EWOULDBLOCK
errno.h (BSD: Operation would block): Error Codes.
int EXDEV
errno.h (POSIX.1: Invalid cross-device link): Error Codes.
int execl (const char *filename, const char *arg0, ...)
unistd.h (POSIX.1): Executing a File.
int execle (const char *filename, const char *arg0, char *const env[], ...)
unistd.h (POSIX.1): Executing a File.
int execlp (const char *filename, const char *arg0, ...)
unistd.h (POSIX.1): Executing a File.
int execv (const char *filename, char *const argv[])
unistd.h (POSIX.1): Executing a File.
int execve (const char *filename, char *const argv[], char *const env[])
unistd.h (POSIX.1): Executing a File.
int execvp (const char *filename, char *const argv[])
unistd.h (POSIX.1): Executing a File.
int EXFULL
errno.h (Linux???: Exchange full): Error Codes.
void exit (int status)
stdlib.h (ISO): Normal Termination.
void _Exit (int status)
stdlib.h (ISO): Termination Internals.
void _exit (int status)
unistd.h (POSIX.1): Termination Internals.
int EXIT_FAILURE
stdlib.h (ISO): Exit Status.
int EXIT_SUCCESS
stdlib.h (ISO): Exit Status.
double exp (double x)
math.h (ISO): Exponents and Logarithms.
double exp10 (double x)
math.h (GNU): Exponents and Logarithms.
float exp10f (float x)
math.h (GNU): Exponents and Logarithms.
long double exp10l (long double x)
math.h (GNU): Exponents and Logarithms.
double exp2 (double x)
math.h (ISO): Exponents and Logarithms.
float exp2f (float x)
math.h (ISO): Exponents and Logarithms.
long double exp2l (long double x)
math.h (ISO): Exponents and Logarithms.
float expf (float x)
math.h (ISO): Exponents and Logarithms.
long double expl (long double x)
math.h (ISO): Exponents and Logarithms.
double expm1 (double x)
math.h (ISO): Exponents and Logarithms.
float expm1f (float x)
math.h (ISO): Exponents and Logarithms.
long double expm1l (long double x)
math.h (ISO): Exponents and Logarithms.
int EXPR_NEST_MAX
limits.h (POSIX.2): Utility Limits.
double fabs (double number)
math.h (ISO): Absolute Value.
float fabsf (float number)
math.h (ISO): Absolute Value.
long double fabsl (long double number)
math.h (ISO): Absolute Value.
size_t __fbufsize (FILE *stream)
stdio_ext.h (GNU): Controlling Buffering.
int fchdir (int filedes)
unistd.h (XPG): Working Directory.
int fchmod (int filedes, int mode)
sys/stat.h (BSD): Setting Permissions.
int fchown (int filedes, int owner, int group)
unistd.h (BSD): File Owner.
int fclean (FILE *stream)
stdio.h (GNU): Cleaning Streams.
int fclose (FILE *stream)
stdio.h (ISO): Closing Streams.
int fcloseall (void)
stdio.h (GNU): Closing Streams.
int fcntl (int filedes, int command, ...)
fcntl.h (POSIX.1): Control Operations.
char * fcvt (double value, int ndigit, int *decpt, int *neg)
stdlib.h (SVID, Unix98): System V Number Conversion.
int fcvt_r (double value, int ndigit, int *decpt, int *neg, char *buf, size_t len)
stdlib.h (SVID, Unix98): System V Number Conversion.
int fdatasync (int fildes)
unistd.h (POSIX): Synchronizing I/O.
int FD_CLOEXEC
fcntl.h (POSIX.1): Descriptor Flags.
void FD_CLR (int filedes, fd_set *set)
sys/types.h (BSD): Waiting for I/O.
double fdim (double x, double y)
math.h (ISO): Misc FP Arithmetic.
float fdimf (float x, float y)
math.h (ISO): Misc FP Arithmetic.
long double fdiml (long double x, long double y)
math.h (ISO): Misc FP Arithmetic.
int FD_ISSET (int filedes, const fd_set *set)
sys/types.h (BSD): Waiting for I/O.
FILE * fdopen (int filedes, const char *opentype)
stdio.h (POSIX.1): Descriptors and Streams.
DIR * fdopendir (int fd)
dirent.h (GNU): Opening a Directory.
fd_set
sys/types.h (BSD): Waiting for I/O.
void FD_SET (int filedes, fd_set *set)
sys/types.h (BSD): Waiting for I/O.
int FD_SETSIZE
sys/types.h (BSD): Waiting for I/O.
int F_DUPFD
fcntl.h (POSIX.1): Duplicating Descriptors.
void FD_ZERO (fd_set *set)
sys/types.h (BSD): Waiting for I/O.
int feclearexcept (int excepts)
fenv.h (ISO): Status bit operations.
int fedisableexcept (int excepts)
fenv.h (GNU): Control Functions.
FE_DIVBYZERO
fenv.h (ISO): Status bit operations.
FE_DOWNWARD
fenv.h (ISO): Rounding.
int feenableexcept (int excepts)
fenv.h (GNU): Control Functions.
int fegetenv (fenv_t *envp)
fenv.h (ISO): Control Functions.
int fegetexcept (int excepts)
fenv.h (GNU): Control Functions.
int fegetexceptflag (fexcept_t *flagp, int excepts)
fenv.h (ISO): Status bit operations.
int fegetround (void)
fenv.h (ISO): Rounding.
int feholdexcept (fenv_t *envp)
fenv.h (ISO): Control Functions.
FE_INEXACT
fenv.h (ISO): Status bit operations.
FE_INVALID
fenv.h (ISO): Status bit operations.
int feof (FILE *stream)
stdio.h (ISO): EOF and Errors.
int feof_unlocked (FILE *stream)
stdio.h (GNU): EOF and Errors.
FE_OVERFLOW
fenv.h (ISO): Status bit operations.
int feraiseexcept (int excepts)
fenv.h (ISO): Status bit operations.
int ferror (FILE *stream)
stdio.h (ISO): EOF and Errors.
int ferror_unlocked (FILE *stream)
stdio.h (GNU): EOF and Errors.
int fesetenv (const fenv_t *envp)
fenv.h (ISO): Control Functions.
int fesetexceptflag (const fexcept_t *flagp, int excepts)
fenv.h (ISO): Status bit operations.
int fesetround (int round)
fenv.h (ISO): Rounding.
int fetestexcept (int excepts)
fenv.h (ISO): Status bit operations.
FE_TONEAREST
fenv.h (ISO): Rounding.
FE_TOWARDZERO
fenv.h (ISO): Rounding.
FE_UNDERFLOW
fenv.h (ISO): Status bit operations.
int feupdateenv (const fenv_t *envp)
fenv.h (ISO): Control Functions.
FE_UPWARD
fenv.h (ISO): Rounding.
int fflush (FILE *stream)
stdio.h (ISO): Flushing Buffers.
int fflush_unlocked (FILE *stream)
stdio.h (POSIX): Flushing Buffers.
int fgetc (FILE *stream)
stdio.h (ISO): Character Input.
int fgetc_unlocked (FILE *stream)
stdio.h (POSIX): Character Input.
int F_GETFD
fcntl.h (POSIX.1): Descriptor Flags.
int F_GETFL
fcntl.h (POSIX.1): Getting File Status Flags.
struct group * fgetgrent (FILE *stream)
grp.h (SVID): Scanning All Groups.
int fgetgrent_r (FILE *stream, struct group *result_buf, char *buffer, size_t buflen, struct group **result)
grp.h (GNU): Scanning All Groups.
int F_GETLK
fcntl.h (POSIX.1): File Locks.
int F_GETOWN
fcntl.h (BSD): Interrupt Input.
int fgetpos (FILE *stream, fpos_t *position)
stdio.h (ISO): Portable Positioning.
int fgetpos64 (FILE *stream, fpos64_t *position)
stdio.h (Unix98): Portable Positioning.
struct passwd * fgetpwent (FILE *stream)
pwd.h (SVID): Scanning All Users.
int fgetpwent_r (FILE *stream, struct passwd *result_buf, char *buffer, size_t buflen, struct passwd **result)
pwd.h (GNU): Scanning All Users.
char * fgets (char *s, int count, FILE *stream)
stdio.h (ISO): Line Input.
char * fgets_unlocked (char *s, int count, FILE *stream)
stdio.h (GNU): Line Input.
wint_t fgetwc (FILE *stream)
wchar.h (ISO): Character Input.
wint_t fgetwc_unlocked (FILE *stream)
wchar.h (GNU): Character Input.
wchar_t * fgetws (wchar_t *ws, int count, FILE *stream)
wchar.h (ISO): Line Input.
wchar_t * fgetws_unlocked (wchar_t *ws, int count, FILE *stream)
wchar.h (GNU): Line Input.
FILE
stdio.h (ISO): Streams.
int FILENAME_MAX
stdio.h (ISO): Limits for Files.
int fileno (FILE *stream)
stdio.h (POSIX.1): Descriptors and Streams.
int fileno_unlocked (FILE *stream)
stdio.h (GNU): Descriptors and Streams.
int finite (double x)
math.h (BSD): Floating Point Classes.
int finitef (float x)
math.h (BSD): Floating Point Classes.
int finitel (long double x)
math.h (BSD): Floating Point Classes.
int __flbf (FILE *stream)
stdio_ext.h (GNU): Controlling Buffering.
void flockfile (FILE *stream)
stdio.h (POSIX): Streams and Threads.
double floor (double x)
math.h (ISO): Rounding Functions.
float floorf (float x)
math.h (ISO): Rounding Functions.
long double floorl (long double x)
math.h (ISO): Rounding Functions.
FLT_DIG
float.h (ISO): Floating Point Parameters.
FLT_EPSILON
float.h (ISO): Floating Point Parameters.
FLT_MANT_DIG
float.h (ISO): Floating Point Parameters.
FLT_MAX
float.h (ISO): Floating Point Parameters.
FLT_MAX_10_EXP
float.h (ISO): Floating Point Parameters.
FLT_MAX_EXP
float.h (ISO): Floating Point Parameters.
FLT_MIN
float.h (ISO): Floating Point Parameters.
FLT_MIN_10_EXP
float.h (ISO): Floating Point Parameters.
FLT_MIN_EXP
float.h (ISO): Floating Point Parameters.
FLT_RADIX
float.h (ISO): Floating Point Parameters.
FLT_ROUNDS
float.h (ISO): Floating Point Parameters.
void _flushlbf (void)
stdio_ext.h (GNU): Flushing Buffers.
tcflag_t FLUSHO
termios.h (BSD): Local Modes.
double fma (double x, double y, double z)
math.h (ISO): Misc FP Arithmetic.
float fmaf (float x, float y, float z)
math.h (ISO): Misc FP Arithmetic.
long double fmal (long double x, long double y, long double z)
math.h (ISO): Misc FP Arithmetic.
double fmax (double x, double y)
math.h (ISO): Misc FP Arithmetic.
float fmaxf (float x, float y)
math.h (ISO): Misc FP Arithmetic.
long double fmaxl (long double x, long double y)
math.h (ISO): Misc FP Arithmetic.
FILE * fmemopen (void *buf, size_t size, const char *opentype)
stdio.h (GNU): String Streams.
double fmin (double x, double y)
math.h (ISO): Misc FP Arithmetic.
float fminf (float x, float y)
math.h (ISO): Misc FP Arithmetic.
long double fminl (long double x, long double y)
math.h (ISO): Misc FP Arithmetic.
double fmod (double numerator, double denominator)
math.h (ISO): Remainder Functions.
float fmodf (float numerator, float denominator)
math.h (ISO): Remainder Functions.
long double fmodl (long double numerator, long double denominator)
math.h (ISO): Remainder Functions.
int fmtmsg (long int classification, const char *label, int severity, const char *text, const char *action, const char *tag)
fmtmsg.h (XPG): Printing Formatted Messages.
int fnmatch (const char *pattern, const char *string, int flags)
fnmatch.h (POSIX.2): Wildcard Matching.
FNM_CASEFOLD
fnmatch.h (GNU): Wildcard Matching.
FNM_EXTMATCH
fnmatch.h (GNU): Wildcard Matching.
FNM_FILE_NAME
fnmatch.h (GNU): Wildcard Matching.
FNM_LEADING_DIR
fnmatch.h (GNU): Wildcard Matching.
FNM_NOESCAPE
fnmatch.h (POSIX.2): Wildcard Matching.
FNM_PATHNAME
fnmatch.h (POSIX.2): Wildcard Matching.
FNM_PERIOD
fnmatch.h (POSIX.2): Wildcard Matching.
int F_OK
unistd.h (POSIX.1): Testing File Access.
FILE * fopen (const char *filename, const char *opentype)
stdio.h (ISO): Opening Streams.
FILE * fopen64 (const char *filename, const char *opentype)
stdio.h (Unix98): Opening Streams.
FILE * fopencookie (void *cookie, const char *opentype, cookie_io_functions_t io-functions)
stdio.h (GNU): Streams and Cookies.
int FOPEN_MAX
stdio.h (ISO): Opening Streams.
pid_t fork (void)
unistd.h (POSIX.1): Creating a Process.
int forkpty (int *amaster, char *name, struct termios *termp, struct winsize *winp)
pty.h (BSD): Pseudo-Terminal Pairs.
long int fpathconf (int filedes, int parameter)
unistd.h (POSIX.1): Pathconf.
int fpclassify (float-type x)
math.h (ISO): Floating Point Classes.
FPE_DECOVF_TRAP
signal.h (BSD): Program Error Signals.
FPE_FLTDIV_FAULT
signal.h (BSD): Program Error Signals.
FPE_FLTDIV_TRAP
signal.h (BSD): Program Error Signals.
FPE_FLTOVF_FAULT
signal.h (BSD): Program Error Signals.
FPE_FLTOVF_TRAP
signal.h (BSD): Program Error Signals.
FPE_FLTUND_FAULT
signal.h (BSD): Program Error Signals.
FPE_FLTUND_TRAP
signal.h (BSD): Program Error Signals.
FPE_INTDIV_TRAP
signal.h (BSD): Program Error Signals.
FPE_INTOVF_TRAP
signal.h (BSD): Program Error Signals.
size_t __fpending (FILE *stream) The __fpending
stdio_ext.h (GNU): Controlling Buffering.
FPE_SUBRNG_TRAP
signal.h (BSD): Program Error Signals.
int FP_ILOGB0
math.h (ISO): Exponents and Logarithms.
int FP_ILOGBNAN
math.h (ISO): Exponents and Logarithms.
fpos64_t
stdio.h (Unix98): Portable Positioning.
fpos_t
stdio.h (ISO): Portable Positioning.
int fprintf (FILE *stream, const char *template, ...)
stdio.h (ISO): Formatted Output Functions.
void __fpurge (FILE *stream)
stdio_ext.h (GNU): Flushing Buffers.
int fputc (int c, FILE *stream)
stdio.h (ISO): Simple Output.
int fputc_unlocked (int c, FILE *stream)
stdio.h (POSIX): Simple Output.
int fputs (const char *s, FILE *stream)
stdio.h (ISO): Simple Output.
int fputs_unlocked (const char *s, FILE *stream)
stdio.h (GNU): Simple Output.
wint_t fputwc (wchar_t wc, FILE *stream)
wchar.h (ISO): Simple Output.
wint_t fputwc_unlocked (wint_t wc, FILE *stream)
wchar.h (POSIX): Simple Output.
int fputws (const wchar_t *ws, FILE *stream)
wchar.h (ISO): Simple Output.
int fputws_unlocked (const wchar_t *ws, FILE *stream)
wchar.h (GNU): Simple Output.
F_RDLCK
fcntl.h (POSIX.1): File Locks.
size_t fread (void *data, size_t size, size_t count, FILE *stream)
stdio.h (ISO): Block Input/Output.
int __freadable (FILE *stream)
stdio_ext.h (GNU): Opening Streams.
int __freading (FILE *stream)
stdio_ext.h (GNU): Opening Streams.
size_t fread_unlocked (void *data, size_t size, size_t count, FILE *stream)
stdio.h (GNU): Block Input/Output.
void free (void *ptr)
malloc.h, stdlib.h (ISO): Freeing after Malloc.
__free_hook
malloc.h (GNU): Hooks for Malloc.
FILE * freopen (const char *filename, const char *opentype, FILE *stream)
stdio.h (ISO): Opening Streams.
FILE * freopen64 (const char *filename, const char *opentype, FILE *stream)
stdio.h (Unix98): Opening Streams.
double frexp (double value, int *exponent)
math.h (ISO): Normalization Functions.
float frexpf (float value, int *exponent)
math.h (ISO): Normalization Functions.
long double frexpl (long double value, int *exponent)
math.h (ISO): Normalization Functions.
int fscanf (FILE *stream, const char *template, ...)
stdio.h (ISO): Formatted Input Functions.
int fseek (FILE *stream, long int offset, int whence)
stdio.h (ISO): File Positioning.
int fseeko (FILE *stream, off_t offset, int whence)
stdio.h (Unix98): File Positioning.
int fseeko64 (FILE *stream, off64_t offset, int whence)
stdio.h (Unix98): File Positioning.
int F_SETFD
fcntl.h (POSIX.1): Descriptor Flags.
int F_SETFL
fcntl.h (POSIX.1): Getting File Status Flags.
int F_SETLK
fcntl.h (POSIX.1): File Locks.
int F_SETLKW
fcntl.h (POSIX.1): File Locks.
int __fsetlocking (FILE *stream, int type)
stdio_ext.h (GNU): Streams and Threads.
int F_SETOWN
fcntl.h (BSD): Interrupt Input.
int fsetpos (FILE *stream, const fpos_t *position)
stdio.h (ISO): Portable Positioning.
int fsetpos64 (FILE *stream, const fpos64_t *position)
stdio.h (Unix98): Portable Positioning.
int fstat (int filedes, struct stat *buf)
sys/stat.h (POSIX.1): Reading Attributes.
int fstat64 (int filedes, struct stat64 *buf)
sys/stat.h (Unix98): Reading Attributes.
int fsync (int fildes)
unistd.h (POSIX): Synchronizing I/O.
long int ftell (FILE *stream)
stdio.h (ISO): File Positioning.
off_t ftello (FILE *stream)
stdio.h (Unix98): File Positioning.
off64_t ftello64 (FILE *stream)
stdio.h (Unix98): File Positioning.
int ftruncate (int fd, off_t length)
unistd.h (POSIX): File Size.
int ftruncate64 (int id, off64_t length)
unistd.h (Unix98): File Size.
int ftrylockfile (FILE *stream)
stdio.h (POSIX): Streams and Threads.
int ftw (const char *filename, __ftw_func_t func, int descriptors)
ftw.h (SVID): Working with Directory Trees.
int ftw64 (const char *filename, __ftw64_func_t func, int descriptors)
ftw.h (Unix98): Working with Directory Trees.
__ftw64_func_t
ftw.h (GNU): Working with Directory Trees.
__ftw_func_t
ftw.h (GNU): Working with Directory Trees.
F_UNLCK
fcntl.h (POSIX.1): File Locks.
void funlockfile (FILE *stream)
stdio.h (POSIX): Streams and Threads.
int futimes (int fd, struct timeval tvp[2])
sys/time.h (BSD): File Times.
int fwide (FILE *stream, int mode)
wchar.h (ISO): Streams and I18N.
int fwprintf (FILE *stream, const wchar_t *template, ...)
wchar.h (ISO): Formatted Output Functions.
int __fwritable (FILE *stream)
stdio_ext.h (GNU): Opening Streams.
size_t fwrite (const void *data, size_t size, size_t count, FILE *stream)
stdio.h (ISO): Block Input/Output.
size_t fwrite_unlocked (const void *data, size_t size, size_t count, FILE *stream)
stdio.h (GNU): Block Input/Output.
int __fwriting (FILE *stream)
stdio_ext.h (GNU): Opening Streams.
F_WRLCK
fcntl.h (POSIX.1): File Locks.
int fwscanf (FILE *stream, const wchar_t *template, ...)
wchar.h (ISO): Formatted Input Functions.
double gamma (double x)
math.h (SVID): Special Functions.
float gammaf (float x)
math.h (SVID): Special Functions.
long double gammal (long double x)
math.h (SVID): Special Functions.
void (*__gconv_end_fct) (struct gconv_step *)
gconv.h (GNU): glibc iconv Implementation.
int (*__gconv_fct) (struct __gconv_step *, struct __gconv_step_data *, const char **, const char *, size_t *, int)
gconv.h (GNU): glibc iconv Implementation.
int (*__gconv_init_fct) (struct __gconv_step *)
gconv.h (GNU): glibc iconv Implementation.
char * gcvt (double value, int ndigit, char *buf)
stdlib.h (SVID, Unix98): System V Number Conversion.
long int get_avphys_pages (void)
sys/sysinfo.h (GNU): Query Memory Parameters.
int getc (FILE *stream)
stdio.h (ISO): Character Input.
int getchar (void)
stdio.h (ISO): Character Input.
int getchar_unlocked (void)
stdio.h (POSIX): Character Input.
int getcontext (ucontext_t *ucp)
ucontext.h (SVID): System V contexts.
int getc_unlocked (FILE *stream)
stdio.h (POSIX): Character Input.
char * get_current_dir_name (void)
unistd.h (GNU): Working Directory.
char * getcwd (char *buffer, size_t size)
unistd.h (POSIX.1): Working Directory.
struct tm * getdate (const char *string)
time.h (Unix98): General Time String Parsing.
getdate_err
time.h (Unix98): General Time String Parsing.
int getdate_r (const char *string, struct tm *tp)
time.h (GNU): General Time String Parsing.
ssize_t getdelim (char **lineptr, size_t *n, int delimiter, FILE *stream)
stdio.h (GNU): Line Input.
int getdomainnname (char *name, size_t length)
unistd.h (???): Host Identification.
gid_t getegid (void)
unistd.h (POSIX.1): Reading Persona.
char * getenv (const char *name)
stdlib.h (ISO): Environment Access.
uid_t geteuid (void)
unistd.h (POSIX.1): Reading Persona.
struct fstab * getfsent (void)
fstab.h (BSD): fstab.
struct fstab * getfsfile (const char *name)
fstab.h (BSD): fstab.
struct fstab * getfsspec (const char *name)
fstab.h (BSD): fstab.
gid_t getgid (void)
unistd.h (POSIX.1): Reading Persona.
struct group * getgrent (void)
grp.h (SVID, BSD): Scanning All Groups.
int getgrent_r (struct group *result_buf, char *buffer, size_t buflen, struct group **result)
grp.h (GNU): Scanning All Groups.
struct group * getgrgid (gid_t gid)
grp.h (POSIX.1): Lookup Group.
int getgrgid_r (gid_t gid, struct group *result_buf, char *buffer, size_t buflen, struct group **result)
grp.h (POSIX.1c): Lookup Group.
struct group * getgrnam (const char *name)
grp.h (SVID, BSD): Lookup Group.
int getgrnam_r (const char *name, struct group *result_buf, char *buffer, size_t buflen, struct group **result)
grp.h (POSIX.1c): Lookup Group.
int getgrouplist (const char *user, gid_t group, gid_t *groups, int *ngroups)
grp.h (BSD): Setting Groups.
int getgroups (int count, gid_t *groups)
unistd.h (POSIX.1): Reading Persona.
struct hostent * gethostbyaddr (const char *addr, size_t length, int format)
netdb.h (BSD): Host Names.
int gethostbyaddr_r (const char *addr, size_t length, int format, struct hostent *restrict result_buf, char *restrict buf, size_t buflen, struct hostent **restrict result, int *restrict h_errnop)
netdb.h (GNU): Host Names.
struct hostent * gethostbyname (const char *name)
netdb.h (BSD): Host Names.
struct hostent * gethostbyname2 (const char *name, int af)
netdb.h (IPv6 Basic API): Host Names.
int gethostbyname2_r (const char *name, int af, struct hostent *restrict result_buf, char *restrict buf, size_t buflen, struct hostent **restrict result, int *restrict h_errnop)
netdb.h (GNU): Host Names.
int gethostbyname_r (const char *restrict name, struct hostent *restrict result_buf, char *restrict buf, size_t buflen, struct hostent **restrict result, int *restrict h_errnop)
netdb.h (GNU): Host Names.
struct hostent * gethostent (void)
netdb.h (BSD): Host Names.
long int gethostid (void)
unistd.h (BSD): Host Identification.
int gethostname (char *name, size_t size)
unistd.h (BSD): Host Identification.
int getitimer (int which, struct itimerval *old)
sys/time.h (BSD): Setting an Alarm.
ssize_t getline (char **lineptr, size_t *n, FILE *stream)
stdio.h (GNU): Line Input.
int getloadavg (double loadavg[], int nelem)
stdlib.h (BSD): Processor Resources.
char * getlogin (void)
unistd.h (POSIX.1): Who Logged In.
struct mntent * getmntent (FILE *stream)
mntent.h (BSD): mtab.
struct mntent * getmntent_r (FILE *stream, struct mentent *result, char *buffer, int bufsize)
mntent.h (BSD): mtab.
struct netent * getnetbyaddr (unsigned long int net, int type)
netdb.h (BSD): Networks Database.
struct netent * getnetbyname (const char *name)
netdb.h (BSD): Networks Database.
struct netent * getnetent (void)
netdb.h (BSD): Networks Database.
int getnetgrent (char **hostp, char **userp, char **domainp)
netdb.h (BSD): Lookup Netgroup.
int getnetgrent_r (char **hostp, char **userp, char **domainp, char *buffer, int buflen)
netdb.h (GNU): Lookup Netgroup.
int get_nprocs (void)
sys/sysinfo.h (GNU): Processor Resources.
int get_nprocs_conf (void)
sys/sysinfo.h (GNU): Processor Resources.
int getopt (int argc, char **argv, const char *options)
unistd.h (POSIX.2): Using Getopt.
int getopt_long (int argc, char *const *argv, const char *shortopts, const struct option *longopts, int *indexptr)
getopt.h (GNU): Getopt Long Options.
int getopt_long_only (int argc, char *const *argv, const char *shortopts, const struct option *longopts, int *indexptr)
getopt.h (GNU): Getopt Long Options.
int getpagesize (void)
unistd.h (BSD): Query Memory Parameters.
char * getpass (const char *prompt)
unistd.h (BSD): getpass.
int getpeername (int socket, struct sockaddr *addr, socklen_t *length-ptr)
sys/socket.h (BSD): Who is Connected.
int getpgid (pid_t pid)
unistd.h (SVID): Process Group Functions.
pid_t getpgrp (pid_t pid)
unistd.h (BSD): Process Group Functions.
pid_t getpgrp (void)
unistd.h (POSIX.1): Process Group Functions.
long int get_phys_pages (void)
sys/sysinfo.h (GNU): Query Memory Parameters.
pid_t getpid (void)
unistd.h (POSIX.1): Process Identification.
pid_t getppid (void)
unistd.h (POSIX.1): Process Identification.
int getpriority (int class, int id)
sys/resource.h (BSD,POSIX): Traditional Scheduling Functions.
struct protoent * getprotobyname (const char *name)
netdb.h (BSD): Protocols Database.
struct protoent * getprotobynumber (int protocol)
netdb.h (BSD): Protocols Database.
struct protoent * getprotoent (void)
netdb.h (BSD): Protocols Database.
int getpt (void)
stdlib.h (GNU): Allocation.
struct passwd * getpwent (void)
pwd.h (POSIX.1): Scanning All Users.
int getpwent_r (struct passwd *result_buf, char *buffer, int buflen, struct passwd **result)
pwd.h (GNU): Scanning All Users.
struct passwd * getpwnam (const char *name)
pwd.h (POSIX.1): Lookup User.
int getpwnam_r (const char *name, struct passwd *result_buf, char *buffer, size_t buflen, struct passwd **result)
pwd.h (POSIX.1c): Lookup User.
struct passwd * getpwuid (uid_t uid)
pwd.h (POSIX.1): Lookup User.
int getpwuid_r (uid_t uid, struct passwd *result_buf, char *buffer, size_t buflen, struct passwd **result)
pwd.h (POSIX.1c): Lookup User.
int getrlimit (int resource, struct rlimit *rlp)
sys/resource.h (BSD): Limits on Resources.
int getrlimit64 (int resource, struct rlimit64 *rlp)
sys/resource.h (Unix98): Limits on Resources.
int getrusage (int processes, struct rusage *rusage)
sys/resource.h (BSD): Resource Usage.
char * gets (char *s)
stdio.h (ISO): Line Input.
struct servent * getservbyname (const char *name, const char *proto)
netdb.h (BSD): Services Database.
struct servent * getservbyport (int port, const char *proto)
netdb.h (BSD): Services Database.
struct servent * getservent (void)
netdb.h (BSD): Services Database.
pid_t getsid (pid_t pid)
unistd.h (SVID): Process Group Functions.
int getsockname (int socket, struct sockaddr *addr, socklen_t *length-ptr)
sys/socket.h (BSD): Reading Address.
int getsockopt (int socket, int level, int optname, void *optval, socklen_t *optlen-ptr)
sys/socket.h (BSD): Socket Option Functions.
int getsubopt (char **optionp, const char* const *tokens, char **valuep)
stdlib.h (stdlib.h): Suboptions Example.
char * gettext (const char *msgid)
libintl.h (GNU): Translation with gettext.
int gettimeofday (struct timeval *tp, struct timezone *tzp)
sys/time.h (BSD): High-Resolution Calendar.
uid_t getuid (void)
unistd.h (POSIX.1): Reading Persona.
mode_t getumask (void)
sys/stat.h (GNU): Setting Permissions.
struct utmp * getutent (void)
utmp.h (SVID): Manipulating the Database.
int getutent_r (struct utmp *buffer, struct utmp **result)
utmp.h (GNU): Manipulating the Database.
struct utmp * getutid (const struct utmp *id)
utmp.h (SVID): Manipulating the Database.
int getutid_r (const struct utmp *id, struct utmp *buffer, struct utmp **result)
utmp.h (GNU): Manipulating the Database.
struct utmp * getutline (const struct utmp *line)
utmp.h (SVID): Manipulating the Database.
int getutline_r (const struct utmp *line, struct utmp *buffer, struct utmp **result)
utmp.h (GNU): Manipulating the Database.
int getutmp (const struct utmpx *utmpx, struct utmp *utmp)
utmp.h (GNU): XPG Functions.
int getutmpx (const struct utmp *utmp, struct utmpx *utmpx)
utmp.h (GNU): XPG Functions.
struct utmpx * getutxent (void)
utmpx.h (XPG4.2): XPG Functions.
struct utmpx * getutxid (const struct utmpx *id)
utmpx.h (XPG4.2): XPG Functions.
struct utmpx * getutxline (const struct utmpx *line)
utmpx.h (XPG4.2): XPG Functions.
int getw (FILE *stream)
stdio.h (SVID): Character Input.
wint_t getwc (FILE *stream)
wchar.h (ISO): Character Input.
wint_t getwchar (void)
wchar.h (ISO): Character Input.
wint_t getwchar_unlocked (void)
wchar.h (GNU): Character Input.
wint_t getwc_unlocked (FILE *stream)
wchar.h (GNU): Character Input.
char * getwd (char *buffer)
unistd.h (BSD): Working Directory.
gid_t
sys/types.h (POSIX.1): Reading Persona.
int glob (const char *pattern, int flags, int (*errfunc) (const char *filename, int error-code), glob_t *vector-ptr)
glob.h (POSIX.2): Calling Glob.
int glob64 (const char *pattern, int flags, int (*errfunc) (const char *filename, int error-code), glob64_t *vector-ptr)
glob.h (GNU): Calling Glob.
glob64_t
glob.h (GNU): Calling Glob.
GLOB_ABORTED
glob.h (POSIX.2): Calling Glob.
GLOB_ALTDIRFUNC
glob.h (GNU): More Flags for Globbing.
GLOB_APPEND
glob.h (POSIX.2): Flags for Globbing.
GLOB_BRACE
glob.h (GNU): More Flags for Globbing.
GLOB_DOOFFS
glob.h (POSIX.2): Flags for Globbing.
GLOB_ERR
glob.h (POSIX.2): Flags for Globbing.
void globfree (glob_t *pglob)
glob.h (POSIX.2): More Flags for Globbing.
void globfree64 (glob64_t *pglob)
glob.h (GNU): More Flags for Globbing.
GLOB_MAGCHAR
glob.h (GNU): More Flags for Globbing.
GLOB_MARK
glob.h (POSIX.2): Flags for Globbing.
GLOB_NOCHECK
glob.h (POSIX.2): Flags for Globbing.
GLOB_NOESCAPE
glob.h (POSIX.2): Flags for Globbing.
GLOB_NOMAGIC
glob.h (GNU): More Flags for Globbing.
GLOB_NOMATCH
glob.h (POSIX.2): Calling Glob.
GLOB_NOSORT
glob.h (POSIX.2): Flags for Globbing.
GLOB_NOSPACE
glob.h (POSIX.2): Calling Glob.
GLOB_ONLYDIR
glob.h (GNU): More Flags for Globbing.
GLOB_PERIOD
glob.h (GNU): More Flags for Globbing.
glob_t
glob.h (POSIX.2): Calling Glob.
GLOB_TILDE
glob.h (GNU): More Flags for Globbing.
GLOB_TILDE_CHECK
glob.h (GNU): More Flags for Globbing.
struct tm * gmtime (const time_t *time)
time.h (ISO): Broken-down Time.
struct tm * gmtime_r (const time_t *time, struct tm *resultp)
time.h (POSIX.1c): Broken-down Time.
_GNU_SOURCE
(GNU): Feature Test Macros.
int grantpt (int filedes)
stdlib.h (SVID, XPG4.2): Allocation.
int gsignal (int signum)
signal.h (SVID): Signaling Yourself.
int gtty (int filedes, struct sgttyb *attributes)
sgtty.h (BSD): BSD Terminal Modes.
char * hasmntopt (const struct mntent *mnt, const char *opt)
mntent.h (BSD): mtab.
int hcreate (size_t nel)
search.h (SVID): Hash Search Function.
int hcreate_r (size_t nel, struct hsearch_data *htab)
search.h (GNU): Hash Search Function.
void hdestroy (void)
search.h (SVID): Hash Search Function.
void hdestroy_r (struct hsearch_data *htab)
search.h (GNU): Hash Search Function.
HOST_NOT_FOUND
netdb.h (BSD): Host Names.
ENTRY * hsearch (ENTRY item, ACTION action)
search.h (SVID): Hash Search Function.
int hsearch_r (ENTRY item, ACTION action, ENTRY **retval, struct hsearch_data *htab)
search.h (GNU): Hash Search Function.
uint32_t htonl (uint32_t hostlong)
netinet/in.h (BSD): Byte Order.
uint16_t htons (uint16_t hostshort)
netinet/in.h (BSD): Byte Order.
double HUGE_VAL
math.h (ISO): Math Error Reporting.
float HUGE_VALF
math.h (ISO): Math Error Reporting.
long double HUGE_VALL
math.h (ISO): Math Error Reporting.
tcflag_t HUPCL
termios.h (POSIX.1): Control Modes.
double hypot (double x, double y)
math.h (ISO): Exponents and Logarithms.
float hypotf (float x, float y)
math.h (ISO): Exponents and Logarithms.
long double hypotl (long double x, long double y)
math.h (ISO): Exponents and Logarithms.
tcflag_t ICANON
termios.h (POSIX.1): Local Modes.
size_t iconv (iconv_t cd, char **inbuf, size_t *inbytesleft, char **outbuf, size_t *outbytesleft)
iconv.h (XPG2): Generic Conversion Interface.
int iconv_close (iconv_t cd)
iconv.h (XPG2): Generic Conversion Interface.
iconv_t iconv_open (const char *tocode, const char *fromcode)
iconv.h (XPG2): Generic Conversion Interface.
iconv_t
iconv.h (XPG2): Generic Conversion Interface.
tcflag_t ICRNL
termios.h (POSIX.1): Input Modes.
tcflag_t IEXTEN
termios.h (POSIX.1): Local Modes.
void if_freenameindex (struct if_nameindex *ptr)
net/if.h (IPv6 basic API): Interface Naming.
char * if_indextoname (unsigned int ifindex, char *ifname)
net/if.h (IPv6 basic API): Interface Naming.
struct if_nameindex * if_nameindex (void)
net/if.h (IPv6 basic API): Interface Naming.
unsigned int if_nametoindex (const char *ifname)
net/if.h (IPv6 basic API): Interface Naming.
size_t IFNAMSIZ
net/if.h (net/if.h): Interface Naming.
int IFTODT (mode_t mode)
dirent.h (BSD): Directory Entries.
tcflag_t IGNBRK
termios.h (POSIX.1): Input Modes.
tcflag_t IGNCR
termios.h (POSIX.1): Input Modes.
tcflag_t IGNPAR
termios.h (POSIX.1): Input Modes.
int ilogb (double x)
math.h (ISO): Exponents and Logarithms.
int ilogbf (float x)
math.h (ISO): Exponents and Logarithms.
int ilogbl (long double x)
math.h (ISO): Exponents and Logarithms.
intmax_t imaxabs (intmax_t number)
inttypes.h (ISO): Absolute Value.
tcflag_t IMAXBEL
termios.h (BSD): Input Modes.
imaxdiv_t imaxdiv (intmax_t numerator, intmax_t denominator)
inttypes.h (ISO): Integer Division.
imaxdiv_t
inttypes.h (ISO): Integer Division.
struct in6_addr in6addr_any
netinet/in.h (IPv6 basic API): Host Address Data Type.
struct in6_addr in6addr_loopback
netinet/in.h (IPv6 basic API): Host Address Data Type.
uint32_t INADDR_ANY
netinet/in.h (BSD): Host Address Data Type.
uint32_t INADDR_BROADCAST
netinet/in.h (BSD): Host Address Data Type.
uint32_t INADDR_LOOPBACK
netinet/in.h (BSD): Host Address Data Type.
uint32_t INADDR_NONE
netinet/in.h (BSD): Host Address Data Type.
char * index (const char *string, int c)
string.h (BSD): Search Functions.
uint32_t inet_addr (const char *name)
arpa/inet.h (BSD): Host Address Functions.
int inet_aton (const char *name, struct in_addr *addr)
arpa/inet.h (BSD): Host Address Functions.
uint32_t inet_lnaof (struct in_addr addr)
arpa/inet.h (BSD): Host Address Functions.
struct in_addr inet_makeaddr (uint32_t net, uint32_t local)
arpa/inet.h (BSD): Host Address Functions.
uint32_t inet_netof (struct in_addr addr)
arpa/inet.h (BSD): Host Address Functions.
uint32_t inet_network (const char *name)
arpa/inet.h (BSD): Host Address Functions.
char * inet_ntoa (struct in_addr addr)
arpa/inet.h (BSD): Host Address Functions.
const char * inet_ntop (int af, const void *cp, char *buf, size_t len)
arpa/inet.h (IPv6 basic API): Host Address Functions.
int inet_pton (int af, const char *cp, void *buf)
arpa/inet.h (IPv6 basic API): Host Address Functions.
float INFINITY
math.h (ISO): Infinity and NaN.
int initgroups (const char *user, gid_t group)
grp.h (BSD): Setting Groups.
INIT_PROCESS
utmp.h (SVID): Manipulating the Database.
INIT_PROCESS
utmpx.h (XPG4.2): XPG Functions.
void * initstate (unsigned int seed, void *state, size_t size)
stdlib.h (BSD): BSD Random.
int initstate_r (unsigned int seed, char *restrict statebuf, size_t statelen, struct random_data *restrict buf)
stdlib.h (GNU): BSD Random.
tcflag_t INLCR
termios.h (POSIX.1): Input Modes.
int innetgr (const char *netgroup, const char *host, const char *user, const char *domain)
netdb.h (BSD): Netgroup Membership.
ino64_t
sys/types.h (Unix98): Attribute Meanings.
ino_t
sys/types.h (POSIX.1): Attribute Meanings.
tcflag_t INPCK
termios.h (POSIX.1): Input Modes.
long int telldir (DIR *dirstream)
dirent.h (BSD): Random Access Directory.
INT_MAX
limits.h (ISO): Range of Type.
INT_MIN
limits.h (ISO): Range of Type.
int ioctl (int filedes, int command, ...)
sys/ioctl.h (BSD): IOCTLs.
int _IOFBF
stdio.h (ISO): Controlling Buffering.
int _IOLBF
stdio.h (ISO): Controlling Buffering.
int _IONBF
stdio.h (ISO): Controlling Buffering.
int IPPORT_RESERVED
netinet/in.h (BSD): Ports.
int IPPORT_USERRESERVED
netinet/in.h (BSD): Ports.
int isalnum (int c)
ctype.h (ISO): Classification of Characters.
int isalpha (int c)
ctype.h (ISO): Classification of Characters.
int isascii (int c)
ctype.h (SVID, BSD): Classification of Characters.
int isatty (int filedes)
unistd.h (POSIX.1): Is It a Terminal.
int isblank (int c)
ctype.h (ISO): Classification of Characters.
int iscntrl (int c)
ctype.h (ISO): Classification of Characters.
int isdigit (int c)
ctype.h (ISO): Classification of Characters.
int isfinite (float-type x)
math.h (ISO): Floating Point Classes.
int isgraph (int c)
ctype.h (ISO): Classification of Characters.
int isgreater (real-floating x, real-floating y)
math.h (ISO): FP Comparison Functions.
int isgreaterequal (real-floating x, real-floating y)
math.h (ISO): FP Comparison Functions.
tcflag_t ISIG
termios.h (POSIX.1): Local Modes.
int isinf (double x)
math.h (BSD): Floating Point Classes.
int isinff (float x)
math.h (BSD): Floating Point Classes.
int isinfl (long double x)
math.h (BSD): Floating Point Classes.
int isless (real-floating x, real-floating y)
math.h (ISO): FP Comparison Functions.
int islessequal (real-floating x, real-floating y)
math.h (ISO): FP Comparison Functions.
int islessgreater (real-floating x, real-floating y)
math.h (ISO): FP Comparison Functions.
int islower (int c)
ctype.h (ISO): Classification of Characters.
int isnan (double x)
math.h (BSD): Floating Point Classes.
int isnan (float-type x)
math.h (ISO): Floating Point Classes.
int isnanf (float x)
math.h (BSD): Floating Point Classes.
int isnanl (long double x)
math.h (BSD): Floating Point Classes.
int isnormal (float-type x)
math.h (ISO): Floating Point Classes.
_ISOC99_SOURCE
(GNU): Feature Test Macros.
int isprint (int c)
ctype.h (ISO): Classification of Characters.
int ispunct (int c)
ctype.h (ISO): Classification of Characters.
int isspace (int c)
ctype.h (ISO): Classification of Characters.
tcflag_t ISTRIP
termios.h (POSIX.1): Input Modes.
int isunordered (real-floating x, real-floating y)
math.h (ISO): FP Comparison Functions.
int isupper (int c)
ctype.h (ISO): Classification of Characters.
int iswalnum (wint_t wc)
wctype.h (ISO): Classification of Wide Characters.
int iswalpha (wint_t wc)
wctype.h (ISO): Classification of Wide Characters.
int iswblank (wint_t wc)
wctype.h (ISO): Classification of Wide Characters.
int iswcntrl (wint_t wc)
wctype.h (ISO): Classification of Wide Characters.
int iswctype (wint_t wc, wctype_t desc)
wctype.h (ISO): Classification of Wide Characters.
int iswdigit (wint_t wc)
wctype.h (ISO): Classification of Wide Characters.
int iswgraph (wint_t wc)
wctype.h (ISO): Classification of Wide Characters.
int iswlower (wint_t wc)
ctype.h (ISO): Classification of Wide Characters.
int iswprint (wint_t wc)
wctype.h (ISO): Classification of Wide Characters.
int iswpunct (wint_t wc)
wctype.h (ISO): Classification of Wide Characters.
int iswspace (wint_t wc)
wctype.h (ISO): Classification of Wide Characters.
int iswupper (wint_t wc)
wctype.h (ISO): Classification of Wide Characters.
int iswxdigit (wint_t wc)
wctype.h (ISO): Classification of Wide Characters.
int isxdigit (int c)
ctype.h (ISO): Classification of Characters.
ITIMER_PROF
sys/time.h (BSD): Setting an Alarm.
ITIMER_REAL
sys/time.h (BSD): Setting an Alarm.
ITIMER_VIRTUAL
sys/time.h (BSD): Setting an Alarm.
tcflag_t IXANY
termios.h (BSD): Input Modes.
tcflag_t IXOFF
termios.h (POSIX.1): Input Modes.
tcflag_t IXON
termios.h (POSIX.1): Input Modes.
double j0 (double x)
math.h (SVID): Special Functions.
float j0f (float x)
math.h (SVID): Special Functions.
long double j0l (long double x)
math.h (SVID): Special Functions.
double j1 (double x)
math.h (SVID): Special Functions.
float j1f (float x)
math.h (SVID): Special Functions.
long double j1l (long double x)
math.h (SVID): Special Functions.
jmp_buf
setjmp.h (ISO): Non-Local Details.
double jn (int n, double x)
math.h (SVID): Special Functions.
float jnf (int n, float x)
math.h (SVID): Special Functions.
long double jnl (int n, long double x)
math.h (SVID): Special Functions.
long int jrand48 (unsigned short int xsubi[3])
stdlib.h (SVID): SVID Random.
int jrand48_r (unsigned short int xsubi[3], struct drand48_data *buffer, long int *result)
stdlib.h (GNU): SVID Random.
int kill (pid_t pid, int signum)
signal.h (POSIX.1): Signaling Another Process.
int killpg (int pgid, int signum)
signal.h (BSD): Signaling Another Process.
char * l64a (long int n)
stdlib.h (XPG): Encode Binary Data.
long int labs (long int number)
stdlib.h (ISO): Absolute Value.
LANG
locale.h (ISO): Locale Categories.
LC_ALL
locale.h (ISO): Locale Categories.
LC_COLLATE
locale.h (ISO): Locale Categories.
LC_CTYPE
locale.h (ISO): Locale Categories.
LC_MESSAGES
locale.h (XOPEN): Locale Categories.
LC_MONETARY
locale.h (ISO): Locale Categories.
LC_NUMERIC
locale.h (ISO): Locale Categories.
void lcong48 (unsigned short int param[7])
stdlib.h (SVID): SVID Random.
int lcong48_r (unsigned short int param[7], struct drand48_data *buffer)
stdlib.h (GNU): SVID Random.
int L_ctermid
stdio.h (POSIX.1): Identifying the Terminal.
LC_TIME
locale.h (ISO): Locale Categories.
int L_cuserid
stdio.h (POSIX.1): Who Logged In.
double ldexp (double value, int exponent)
math.h (ISO): Normalization Functions.
float ldexpf (float value, int exponent)
math.h (ISO): Normalization Functions.
long double ldexpl (long double value, int exponent)
math.h (ISO): Normalization Functions.
ldiv_t ldiv (long int numerator, long int denominator)
stdlib.h (ISO): Integer Division.
ldiv_t
stdlib.h (ISO): Integer Division.
void * lfind (const void *key, void *base, size_t *nmemb, size_t size, comparison_fn_t compar)
search.h (SVID): Array Search Function.
double lgamma (double x)
math.h (SVID): Special Functions.
float lgammaf (float x)
math.h (SVID): Special Functions.
float lgammaf_r (float x, int *signp)
math.h (XPG): Special Functions.
long double lgammal (long double x)
math.h (SVID): Special Functions.
long double lgammal_r (long double x, int *signp)
math.h (XPG): Special Functions.
double lgamma_r (double x, int *signp)
math.h (XPG): Special Functions.
L_INCR
sys/file.h (BSD): File Positioning.
int LINE_MAX
limits.h (POSIX.2): Utility Limits.
int link (const char *oldname, const char *newname)
unistd.h (POSIX.1): Hard Links.
int LINK_MAX
limits.h (POSIX.1): Limits for Files.
int lio_listio (int mode, struct aiocb *const list[], int nent, struct sigevent *sig)
aio.h (POSIX.1b): Asynchronous Reads/Writes.
int lio_listio64 (int mode, struct aiocb *const list, int nent, struct sigevent *sig)
aio.h (Unix98): Asynchronous Reads/Writes.
int listen (int socket, unsigned int n)
sys/socket.h (BSD): Listening.
long long int llabs (long long int number)
stdlib.h (ISO): Absolute Value.
lldiv_t lldiv (long long int numerator, long long int denominator)
stdlib.h (ISO): Integer Division.
lldiv_t
stdlib.h (ISO): Integer Division.
long long int llrint (double x)
math.h (ISO): Rounding Functions.
long long int llrintf (float x)
math.h (ISO): Rounding Functions.
long long int llrintl (long double x)
math.h (ISO): Rounding Functions.
long long int llround (double x)
math.h (ISO): Rounding Functions.
long long int llroundf (float x)
math.h (ISO): Rounding Functions.
long long int llroundl (long double x)
math.h (ISO): Rounding Functions.
struct lconv * localeconv (void)
locale.h (ISO): The Lame Way to Locale Data.
struct tm * localtime (const time_t *time)
time.h (ISO): Broken-down Time.
struct tm * localtime_r (const time_t *time, struct tm *resultp)
time.h (POSIX.1c): Broken-down Time.
double log (double x)
math.h (ISO): Exponents and Logarithms.
double log10 (double x)
math.h (ISO): Exponents and Logarithms.
float log10f (float x)
math.h (ISO): Exponents and Logarithms.
long double log10l (long double x)
math.h (ISO): Exponents and Logarithms.
double log1p (double x)
math.h (ISO): Exponents and Logarithms.
float log1pf (float x)
math.h (ISO): Exponents and Logarithms.
long double log1pl (long double x)
math.h (ISO): Exponents and Logarithms.
double log2 (double x)
math.h (ISO): Exponents and Logarithms.
float log2f (float x)
math.h (ISO): Exponents and Logarithms.
long double log2l (long double x)
math.h (ISO): Exponents and Logarithms.
double logb (double x)
math.h (ISO): Exponents and Logarithms.
float logbf (float x)
math.h (ISO): Exponents and Logarithms.
long double logbl (long double x)
math.h (ISO): Exponents and Logarithms.
float logf (float x)
math.h (ISO): Exponents and Logarithms.
void login (const struct utmp *entry)
utmp.h (BSD): Logging In and Out.
LOGIN_PROCESS
utmp.h (SVID): Manipulating the Database.
LOGIN_PROCESS
utmpx.h (XPG4.2): XPG Functions.
int login_tty (int filedes)
utmp.h (BSD): Logging In and Out.
long double logl (long double x)
math.h (ISO): Exponents and Logarithms.
int logout (const char *ut_line)
utmp.h (BSD): Logging In and Out.
void logwtmp (const char *ut_line, const char *ut_name, const char *ut_host)
utmp.h (BSD): Logging In and Out.
void longjmp (jmp_buf state, int value)
setjmp.h (ISO): Non-Local Details.
LONG_LONG_MAX
limits.h (GNU): Range of Type.
LONG_LONG_MIN
limits.h (GNU): Range of Type.
LONG_MAX
limits.h (ISO): Range of Type.
LONG_MIN
limits.h (ISO): Range of Type.
long int lrand48 (void)
stdlib.h (SVID): SVID Random.
int lrand48_r (struct drand48_data *buffer, double *result)
stdlib.h (GNU): SVID Random.
long int lrint (double x)
math.h (ISO): Rounding Functions.
long int lrintf (float x)
math.h (ISO): Rounding Functions.
long int lrintl (long double x)
math.h (ISO): Rounding Functions.
long int lround (double x)
math.h (ISO): Rounding Functions.
long int lroundf (float x)
math.h (ISO): Rounding Functions.
long int lroundl (long double x)
math.h (ISO): Rounding Functions.
void * lsearch (const void *key, void *base, size_t *nmemb, size_t size, comparison_fn_t compar)
search.h (SVID): Array Search Function.
off_t lseek (int filedes, off_t offset, int whence)
unistd.h (POSIX.1): File Position Primitive.
off64_t lseek64 (int filedes, off64_t offset, int whence)
unistd.h (Unix98): File Position Primitive.
L_SET
sys/file.h (BSD): File Positioning.
int lstat (const char *filename, struct stat *buf)
sys/stat.h (BSD): Reading Attributes.
int lstat64 (const char *filename, struct stat64 *buf)
sys/stat.h (Unix98): Reading Attributes.
int L_tmpnam
stdio.h (ISO): Temporary Files.
int lutimes (const char *filename, struct timeval tvp[2])
sys/time.h (BSD): File Times.
L_XTND
sys/file.h (BSD): File Positioning.
int madvise (void *addr, size_t length, int advice)
sys/mman.h (POSIX): Memory-mapped I/O.
void makecontext (ucontext_t *ucp, void (*func) (void), int argc, ...)
ucontext.h (SVID): System V contexts.
struct mallinfo mallinfo (void)
malloc.h (SVID): Statistics of Malloc.
void * malloc (size_t size)
malloc.h, stdlib.h (ISO): Basic Allocation.
__malloc_hook
malloc.h (GNU): Hooks for Malloc.
__malloc_initialize_hook
malloc.h (GNU): Hooks for Malloc.
int MAX_CANON
limits.h (POSIX.1): Limits for Files.
int MAX_INPUT
limits.h (POSIX.1): Limits for Files.
int MAXNAMLEN
dirent.h (BSD): Limits for Files.
int MAXSYMLINKS
sys/param.h (BSD): Symbolic Links.
int MB_CUR_MAX
stdlib.h (ISO): Selecting the Conversion.
int mblen (const char *string, size_t size)
stdlib.h (ISO): Non-reentrant Character Conversion.
int MB_LEN_MAX
limits.h (ISO): Selecting the Conversion.
size_t mbrlen (const char *restrict s, size_t n, mbstate_t *ps)
wchar.h (ISO): Converting a Character.
size_t mbrtowc (wchar_t *restrict pwc, const char *restrict s, size_t n, mbstate_t *restrict ps)
wchar.h (ISO): Converting a Character.
int mbsinit (const mbstate_t *ps)
wchar.h (ISO): Keeping the state.
size_t mbsnrtowcs (wchar_t *restrict dst, const char **restrict src, size_t nmc, size_t len, mbstate_t *restrict ps)
wchar.h (GNU): Converting Strings.
size_t mbsrtowcs (wchar_t *restrict dst, const char **restrict src, size_t len, mbstate_t *restrict ps)
wchar.h (ISO): Converting Strings.
mbstate_t
wchar.h (ISO): Keeping the state.
size_t mbstowcs (wchar_t *wstring, const char *string, size_t size)
stdlib.h (ISO): Non-reentrant String Conversion.
int mbtowc (wchar_t *restrict result, const char *restrict string, size_t size)
stdlib.h (ISO): Non-reentrant Character Conversion.
int mcheck (void (*abortfn) (enum mcheck_status status))
mcheck.h (GNU): Heap Consistency Checking.
tcflag_t MDMBUF
termios.h (BSD): Control Modes.
void * memalign (size_t boundary, size_t size)
malloc.h (BSD): Aligned Memory Blocks.
__memalign_hook
malloc.h (GNU): Hooks for Malloc.
void * memccpy (void *restrict to, const void *restrict from, int c, size_t size)
string.h (SVID): Copying and Concatenation.
void * memchr (const void *block, int c, size_t size)
string.h (ISO): Search Functions.
int memcmp (const void *a1, const void *a2, size_t size)
string.h (ISO): String/Array Comparison.
void * memcpy (void *restrict to, const void *restrict from, size_t size)
string.h (ISO): Copying and Concatenation.
void * memfrob (void *mem, size_t length)
string.h (GNU): Trivial Encryption.
void * memmem (const void *haystack, size_t haystack-len,
const void *needle, size_t needle-len)
string.h (GNU): Search Functions.
void * memmove (void *to, const void *from, size_t size)
string.h (ISO): Copying and Concatenation.
void * mempcpy (void *restrict to, const void *restrict from, size_t size)
string.h (GNU): Copying and Concatenation.
void * memrchr (const void *block, int c, size_t size)
string.h (GNU): Search Functions.
void * memset (void *block, int c, size_t size)
string.h (ISO): Copying and Concatenation.
int mkdir (const char *filename, mode_t mode)
sys/stat.h (POSIX.1): Creating Directories.
char * mkdtemp (char *template)
stdlib.h (BSD): Temporary Files.
int mkfifo (const char *filename, mode_t mode)
sys/stat.h (POSIX.1): FIFO Special Files.
int mknod (const char *filename, int mode, int dev)
sys/stat.h (BSD): Making Special Files.
int mkstemp (char *template)
stdlib.h (BSD): Temporary Files.
char * mktemp (char *template)
stdlib.h (Unix): Temporary Files.
time_t mktime (struct tm *brokentime)
time.h (ISO): Broken-down Time.
int mlock (const void *addr, size_t len)
sys/mman.h (POSIX.1b): Page Lock Functions.
int mlockall (int flags)
sys/mman.h (POSIX.1b): Page Lock Functions.
void * mmap (void *address, size_t length,int protect, int flags, int filedes, off_t offset)
sys/mman.h (POSIX): Memory-mapped I/O.
void * mmap64 (void *address, size_t length,int protect, int flags, int filedes, off64_t offset)
sys/mman.h (LFS): Memory-mapped I/O.
mode_t
sys/types.h (POSIX.1): Attribute Meanings.
double modf (double value, double *integer-part)
math.h (ISO): Rounding Functions.
float modff (float value, float *integer-part)
math.h (ISO): Rounding Functions.
long double modfl (long double value, long double *integer-part)
math.h (ISO): Rounding Functions.
int mount (const char *special_file, const char *dir, const char *fstype, unsigned long int options, const void *data)
sys/mount.h (SVID, BSD): Mount-Unmount-Remount.
long int mrand48 (void)
stdlib.h (SVID): SVID Random.
int mrand48_r (struct drand48_data *buffer, double *result)
stdlib.h (GNU): SVID Random.
void * mremap (void *address, size_t length, size_t new_length, int flag)
sys/mman.h (GNU): Memory-mapped I/O.
int MSG_DONTROUTE
sys/socket.h (BSD): Socket Data Options.
int MSG_OOB
sys/socket.h (BSD): Socket Data Options.
int MSG_PEEK
sys/socket.h (BSD): Socket Data Options.
int msync (void *address, size_t length, int flags)
sys/mman.h (POSIX): Memory-mapped I/O.
void mtrace (void)
mcheck.h (GNU): Tracing malloc.
int munlock (const void *addr, size_t len)
sys/mman.h (POSIX.1b): Page Lock Functions.
int munlockall (void)
sys/mman.h (POSIX.1b): Page Lock Functions.
int munmap (void *addr, size_t length)
sys/mman.h (POSIX): Memory-mapped I/O.
void muntrace (void)
mcheck.h (GNU): Tracing malloc.
int NAME_MAX
limits.h (POSIX.1): Limits for Files.
double nan (const char *tagp)
math.h (ISO): FP Bit Twiddling.
float NAN
math.h (GNU): Infinity and NaN.
float nanf (const char *tagp)
math.h (ISO): FP Bit Twiddling.
long double nanl (const char *tagp)
math.h (ISO): FP Bit Twiddling.
int nanosleep (const struct timespec *requested_time, struct timespec *remaining)
time.h (POSIX.1): Sleeping.
int NCCS
termios.h (POSIX.1): Mode Data Types.
double nearbyint (double x)
math.h (ISO): Rounding Functions.
float nearbyintf (float x)
math.h (ISO): Rounding Functions.
long double nearbyintl (long double x)
math.h (ISO): Rounding Functions.
NEW_TIME
utmp.h (SVID): Manipulating the Database.
NEW_TIME
utmpx.h (XPG4.2): XPG Functions.
double nextafter (double x, double y)
math.h (ISO): FP Bit Twiddling.
float nextafterf (float x, float y)
math.h (ISO): FP Bit Twiddling.
long double nextafterl (long double x, long double y)
math.h (ISO): FP Bit Twiddling.
double nexttoward (double x, long double y)
math.h (ISO): FP Bit Twiddling.
float nexttowardf (float x, long double y)
math.h (ISO): FP Bit Twiddling.
long double nexttowardl (long double x, long double y)
math.h (ISO): FP Bit Twiddling.
int nftw (const char *filename, __nftw_func_t func, int descriptors, int flag)
ftw.h (XPG4.2): Working with Directory Trees.
int nftw64 (const char *filename, __nftw64_func_t func, int descriptors, int flag)
ftw.h (Unix98): Working with Directory Trees.
__nftw64_func_t
ftw.h (GNU): Working with Directory Trees.
__nftw_func_t
ftw.h (GNU): Working with Directory Trees.
char * ngettext (const char *msgid1, const char *msgid2, unsigned long int n)
libintl.h (GNU): Advanced gettext functions.
int NGROUPS_MAX
limits.h (POSIX.1): General Limits.
int nice (int increment)
unistd.h (BSD): Traditional Scheduling Functions.
nlink_t
sys/types.h (POSIX.1): Attribute Meanings.
char * nl_langinfo (nl_item item)
langinfo.h (XOPEN): The Elegant and Fast Way.
NO_ADDRESS
netdb.h (BSD): Host Names.
tcflag_t NOFLSH
termios.h (POSIX.1): Local Modes.
tcflag_t NOKERNINFO
termios.h (BSD): Local Modes.
NO_RECOVERY
netdb.h (BSD): Host Names.
long int nrand48 (unsigned short int xsubi[3])
stdlib.h (SVID): SVID Random.
int nrand48_r (unsigned short int xsubi[3], struct drand48_data *buffer, long int *result)
stdlib.h (GNU): SVID Random.
int NSIG
signal.h (BSD): Standard Signals.
uint32_t ntohl (uint32_t netlong)
netinet/in.h (BSD): Byte Order.
uint16_t ntohs (uint16_t netshort)
netinet/in.h (BSD): Byte Order.
int ntp_adjtime (struct timex *tptr)
sys/timex.h (GNU): High Accuracy Clock.
int ntp_gettime (struct ntptimeval *tptr)
sys/timex.h (GNU): High Accuracy Clock.
void * NULL
stddef.h (ISO): Null Pointer Constant.
int O_ACCMODE
fcntl.h (POSIX.1): Access Modes.
int O_APPEND
fcntl.h (POSIX.1): Operating Modes.
int O_ASYNC
fcntl.h (BSD): Operating Modes.
void obstack_1grow (struct obstack *obstack-ptr, char c)
obstack.h (GNU): Growing Objects.
void obstack_1grow_fast (struct obstack *obstack-ptr, char c)
obstack.h (GNU): Extra Fast Growing.
int obstack_alignment_mask (struct obstack *obstack-ptr)
obstack.h (GNU): Obstacks Data Alignment.
void * obstack_alloc (struct obstack *obstack-ptr, int size)
obstack.h (GNU): Allocation in an Obstack.
obstack_alloc_failed_handler
obstack.h (GNU): Preparing for Obstacks.
void * obstack_base (struct obstack *obstack-ptr)
obstack.h (GNU): Status of an Obstack.
void obstack_blank (struct obstack *obstack-ptr, int size)
obstack.h (GNU): Growing Objects.
void obstack_blank_fast (struct obstack *obstack-ptr, int size)
obstack.h (GNU): Extra Fast Growing.
int obstack_chunk_size (struct obstack *obstack-ptr)
obstack.h (GNU): Obstack Chunks.
void * obstack_copy (struct obstack *obstack-ptr, void *address, int size)
obstack.h (GNU): Allocation in an Obstack.
void * obstack_copy0 (struct obstack *obstack-ptr, void *address, int size)
obstack.h (GNU): Allocation in an Obstack.
void * obstack_finish (struct obstack *obstack-ptr)
obstack.h (GNU): Growing Objects.
void obstack_free (struct obstack *obstack-ptr, void *object)
obstack.h (GNU): Freeing Obstack Objects.
void obstack_grow (struct obstack *obstack-ptr, void *data, int size)
obstack.h (GNU): Growing Objects.
void obstack_grow0 (struct obstack *obstack-ptr, void *data, int size)
obstack.h (GNU): Growing Objects.
int obstack_init (struct obstack *obstack-ptr)
obstack.h (GNU): Preparing for Obstacks.
void obstack_int_grow (struct obstack *obstack-ptr, int data)
obstack.h (GNU): Growing Objects.
void obstack_int_grow_fast (struct obstack *obstack-ptr, int data)
obstack.h (GNU): Extra Fast Growing.
void * obstack_next_free (struct obstack *obstack-ptr)
obstack.h (GNU): Status of an Obstack.
int obstack_object_size (struct obstack *obstack-ptr)
obstack.h (GNU): Growing Objects.
int obstack_object_size (struct obstack *obstack-ptr)
obstack.h (GNU): Status of an Obstack.
int obstack_printf (struct obstack *obstack, const char *template, ...)
stdio.h (GNU): Dynamic Output.
void obstack_ptr_grow (struct obstack *obstack-ptr, void *data)
obstack.h (GNU): Growing Objects.
void obstack_ptr_grow_fast (struct obstack *obstack-ptr, void *data)
obstack.h (GNU): Extra Fast Growing.
int obstack_room (struct obstack *obstack-ptr)
obstack.h (GNU): Extra Fast Growing.
int obstack_vprintf (struct obstack *obstack, const char *template, va_list ap)
stdio.h (GNU): Variable Arguments Output.
int O_CREAT
fcntl.h (POSIX.1): Open-time Flags.
int O_EXCL
fcntl.h (POSIX.1): Open-time Flags.
int O_EXEC
fcntl.h (GNU): Access Modes.
int O_EXLOCK
fcntl.h (BSD): Open-time Flags.
off64_t
sys/types.h (Unix98): File Position Primitive.
size_t offsetof (type, member)
stddef.h (ISO): Structure Measurement.
off_t
sys/types.h (POSIX.1): File Position Primitive.
int O_FSYNC
fcntl.h (BSD): Operating Modes.
int O_IGNORE_CTTY
fcntl.h (GNU): Open-time Flags.
OLD_TIME
utmp.h (SVID): Manipulating the Database.
OLD_TIME
utmpx.h (XPG4.2): XPG Functions.
int O_NDELAY
fcntl.h (BSD): Operating Modes.
int on_exit (void (*function)(int status, void *arg), void *arg)
stdlib.h (SunOS): Cleanups on Exit.
tcflag_t ONLCR
termios.h (BSD): Output Modes.
int O_NOATIME
fcntl.h (GNU): Operating Modes.
int O_NOCTTY
fcntl.h (POSIX.1): Open-time Flags.
tcflag_t ONOEOT
termios.h (BSD): Output Modes.
int O_NOLINK
fcntl.h (GNU): Open-time Flags.
int O_NONBLOCK
fcntl.h (POSIX.1): Open-time Flags.
int O_NONBLOCK
fcntl.h (POSIX.1): Operating Modes.
int O_NOTRANS
fcntl.h (GNU): Open-time Flags.
int open (const char *filename, int flags[, mode_t mode])
fcntl.h (POSIX.1): Opening and Closing Files.
int open64 (const char *filename, int flags[, mode_t mode])
fcntl.h (Unix98): Opening and Closing Files.
DIR * opendir (const char *dirname)
dirent.h (POSIX.1): Opening a Directory.
void openlog (const char *ident, int option, int facility)
syslog.h (BSD): openlog.
int OPEN_MAX
limits.h (POSIX.1): General Limits.
FILE * open_memstream (char **ptr, size_t *sizeloc)
stdio.h (GNU): String Streams.
FILE * open_obstack_stream (struct obstack *obstack)
stdio.h (GNU): Obstack Streams.
int openpty (int *amaster, int *aslave, char *name, struct termios *termp, struct winsize *winp)
pty.h (BSD): Pseudo-Terminal Pairs.
tcflag_t OPOST
termios.h (POSIX.1): Output Modes.
char * optarg
unistd.h (POSIX.2): Using Getopt.
int opterr
unistd.h (POSIX.2): Using Getopt.
int optind
unistd.h (POSIX.2): Using Getopt.
OPTION_ALIAS
argp.h (GNU): Argp Option Flags.
OPTION_ARG_OPTIONAL
argp.h (GNU): Argp Option Flags.
OPTION_DOC
argp.h (GNU): Argp Option Flags.
OPTION_HIDDEN
argp.h (GNU): Argp Option Flags.
OPTION_NO_USAGE
argp.h (GNU): Argp Option Flags.
int optopt
unistd.h (POSIX.2): Using Getopt.
int O_RDONLY
fcntl.h (POSIX.1): Access Modes.
int O_RDWR
fcntl.h (POSIX.1): Access Modes.
int O_READ
fcntl.h (GNU): Access Modes.
int O_SHLOCK
fcntl.h (BSD): Open-time Flags.
int O_SYNC
fcntl.h (BSD): Operating Modes.
int O_TRUNC
fcntl.h (POSIX.1): Open-time Flags.
int O_WRITE
fcntl.h (GNU): Access Modes.
int O_WRONLY
fcntl.h (POSIX.1): Access Modes.
tcflag_t OXTABS
termios.h (BSD): Output Modes.
PA_CHAR
printf.h (GNU): Parsing a Template String.
PA_DOUBLE
printf.h (GNU): Parsing a Template String.
PA_FLAG_LONG
printf.h (GNU): Parsing a Template String.
PA_FLAG_LONG_DOUBLE
printf.h (GNU): Parsing a Template String.
PA_FLAG_LONG_LONG
printf.h (GNU): Parsing a Template String.
int PA_FLAG_MASK
printf.h (GNU): Parsing a Template String.
PA_FLAG_PTR
printf.h (GNU): Parsing a Template String.
PA_FLAG_SHORT
printf.h (GNU): Parsing a Template String.
PA_FLOAT
printf.h (GNU): Parsing a Template String.
PA_INT
printf.h (GNU): Parsing a Template String.
PA_LAST
printf.h (GNU): Parsing a Template String.
PA_POINTER
printf.h (GNU): Parsing a Template String.
tcflag_t PARENB
termios.h (POSIX.1): Control Modes.
tcflag_t PARMRK
termios.h (POSIX.1): Input Modes.
tcflag_t PARODD
termios.h (POSIX.1): Control Modes.
size_t parse_printf_format (const char *template, size_t n, int *argtypes)
printf.h (GNU): Parsing a Template String.
PA_STRING
printf.h (GNU): Parsing a Template String.
long int pathconf (const char *filename, int parameter)
unistd.h (POSIX.1): Pathconf.
int PATH_MAX
limits.h (POSIX.1): Limits for Files.
int pause ()
unistd.h (POSIX.1): Using Pause.
_PC_ASYNC_IO
unistd.h (POSIX.1): Pathconf.
_PC_CHOWN_RESTRICTED
unistd.h (POSIX.1): Pathconf.
_PC_FILESIZEBITS
unistd.h (LFS): Pathconf.
_PC_LINK_MAX
unistd.h (POSIX.1): Pathconf.
int pclose (FILE *stream)
stdio.h (POSIX.2, SVID, BSD): Pipe to a Subprocess.
_PC_MAX_CANON
unistd.h (POSIX.1): Pathconf.
_PC_MAX_INPUT
unistd.h (POSIX.1): Pathconf.
_PC_NAME_MAX
unistd.h (POSIX.1): Pathconf.
_PC_NO_TRUNC
unistd.h (POSIX.1): Pathconf.
_PC_PATH_MAX
unistd.h (POSIX.1): Pathconf.
_PC_PIPE_BUF
unistd.h (POSIX.1): Pathconf.
_PC_PRIO_IO
unistd.h (POSIX.1): Pathconf.
_PC_REC_INCR_XFER_SIZE
unistd.h (POSIX.1): Pathconf.
_PC_REC_MAX_XFER_SIZE
unistd.h (POSIX.1): Pathconf.
_PC_REC_MIN_XFER_SIZE
unistd.h (POSIX.1): Pathconf.
_PC_REC_XFER_ALIGN
unistd.h (POSIX.1): Pathconf.
_PC_SYNC_IO
unistd.h (POSIX.1): Pathconf.
_PC_VDISABLE
unistd.h (POSIX.1): Pathconf.
tcflag_t PENDIN
termios.h (BSD): Local Modes.
void perror (const char *message)
stdio.h (ISO): Error Messages.
int PF_FILE
sys/socket.h (GNU): Local Namespace Details.
int PF_INET
sys/socket.h (BSD): Internet Namespace.
int PF_INET6
sys/socket.h (X/Open): Internet Namespace.
int PF_LOCAL
sys/socket.h (POSIX): Local Namespace Details.
int PF_UNIX
sys/socket.h (BSD): Local Namespace Details.
pid_t
sys/types.h (POSIX.1): Process Identification.
int pipe (int filedes[2])
unistd.h (POSIX.1): Creating a Pipe.
int PIPE_BUF
limits.h (POSIX.1): Limits for Files.
FILE * popen (const char *command, const char *mode)
stdio.h (POSIX.2, SVID, BSD): Pipe to a Subprocess.
_POSIX2_BC_BASE_MAX
limits.h (POSIX.2): Utility Minimums.
_POSIX2_BC_DIM_MAX
limits.h (POSIX.2): Utility Minimums.
_POSIX2_BC_SCALE_MAX
limits.h (POSIX.2): Utility Minimums.
_POSIX2_BC_STRING_MAX
limits.h (POSIX.2): Utility Minimums.
int _POSIX2_C_DEV
unistd.h (POSIX.2): System Options.
_POSIX2_COLL_WEIGHTS_MAX
limits.h (POSIX.2): Utility Minimums.
long int _POSIX2_C_VERSION
unistd.h (POSIX.2): Version Supported.
_POSIX2_EQUIV_CLASS_MAX
limits.h (POSIX.2): Utility Minimums.
_POSIX2_EXPR_NEST_MAX
limits.h (POSIX.2): Utility Minimums.
int _POSIX2_FORT_DEV
unistd.h (POSIX.2): System Options.
int _POSIX2_FORT_RUN
unistd.h (POSIX.2): System Options.
_POSIX2_LINE_MAX
limits.h (POSIX.2): Utility Minimums.
int _POSIX2_LOCALEDEF
unistd.h (POSIX.2): System Options.
_POSIX2_RE_DUP_MAX
limits.h (POSIX.2): Minimums.
int _POSIX2_SW_DEV
unistd.h (POSIX.2): System Options.
_POSIX_AIO_LISTIO_MAX
limits.h (POSIX.1): Minimums.
_POSIX_AIO_MAX
limits.h (POSIX.1): Minimums.
_POSIX_ARG_MAX
limits.h (POSIX.1): Minimums.
_POSIX_CHILD_MAX
limits.h (POSIX.1): Minimums.
int _POSIX_CHOWN_RESTRICTED
unistd.h (POSIX.1): Options for Files.
_POSIX_C_SOURCE
(POSIX.2): Feature Test Macros.
int _POSIX_JOB_CONTROL
unistd.h (POSIX.1): System Options.
_POSIX_LINK_MAX
limits.h (POSIX.1): File Minimums.
_POSIX_MAX_CANON
limits.h (POSIX.1): File Minimums.
_POSIX_MAX_INPUT
limits.h (POSIX.1): File Minimums.
int posix_memalign (void **memptr, size_t alignment, size_t size)
stdlib.h (POSIX): Aligned Memory Blocks.
_POSIX_NAME_MAX
limits.h (POSIX.1): File Minimums.
_POSIX_NGROUPS_MAX
limits.h (POSIX.1): Minimums.
int _POSIX_NO_TRUNC
unistd.h (POSIX.1): Options for Files.
_POSIX_OPEN_MAX
limits.h (POSIX.1): Minimums.
_POSIX_PATH_MAX
limits.h (POSIX.1): File Minimums.
_POSIX_PIPE_BUF
limits.h (POSIX.1): File Minimums.
POSIX_REC_INCR_XFER_SIZE
limits.h (POSIX.1): File Minimums.
POSIX_REC_MAX_XFER_SIZE
limits.h (POSIX.1): File Minimums.
POSIX_REC_MIN_XFER_SIZE
limits.h (POSIX.1): File Minimums.
POSIX_REC_XFER_ALIGN
limits.h (POSIX.1): File Minimums.
int _POSIX_SAVED_IDS
unistd.h (POSIX.1): System Options.
_POSIX_SOURCE
(POSIX.1): Feature Test Macros.
_POSIX_SSIZE_MAX
limits.h (POSIX.1): Minimums.
_POSIX_STREAM_MAX
limits.h (POSIX.1): Minimums.
_POSIX_TZNAME_MAX
limits.h (POSIX.1): Minimums.
unsigned char _POSIX_VDISABLE
unistd.h (POSIX.1): Options for Files.
long int _POSIX_VERSION
unistd.h (POSIX.1): Version Supported.
double pow (double base, double power)
math.h (ISO): Exponents and Logarithms.
double pow10 (double x)
math.h (GNU): Exponents and Logarithms.
float pow10f (float x)
math.h (GNU): Exponents and Logarithms.
long double pow10l (long double x)
math.h (GNU): Exponents and Logarithms.
float powf (float base, float power)
math.h (ISO): Exponents and Logarithms.
long double powl (long double base, long double power)
math.h (ISO): Exponents and Logarithms.
ssize_t pread (int filedes, void *buffer, size_t size, off_t offset)
unistd.h (Unix98): I/O Primitives.
ssize_t pread64 (int filedes, void *buffer, size_t size, off64_t offset)
unistd.h (Unix98): I/O Primitives.
int printf (const char *template, ...)
stdio.h (ISO): Formatted Output Functions.
printf_arginfo_function
printf.h (GNU): Defining the Output Handler.
printf_function
printf.h (GNU): Defining the Output Handler.
int printf_size (FILE *fp, const struct printf_info *info, const void *const *args)
printf.h (GNU): Predefined Printf Handlers.
int printf_size_info (const struct printf_info *info, size_t n, int *argtypes)
printf.h (GNU): Predefined Printf Handlers.
PRIO_MAX
sys/resource.h (BSD): Traditional Scheduling Functions.
PRIO_MIN
sys/resource.h (BSD): Traditional Scheduling Functions.
PRIO_PGRP
sys/resource.h (BSD): Traditional Scheduling Functions.
PRIO_PROCESS
sys/resource.h (BSD): Traditional Scheduling Functions.
PRIO_USER
sys/resource.h (BSD): Traditional Scheduling Functions.
char * program_invocation_name
errno.h (GNU): Error Messages.
char * program_invocation_short_name
errno.h (GNU): Error Messages.
void psignal (int signum, const char *message)
signal.h (BSD): Signal Messages.
char * P_tmpdir
stdio.h (SVID): Temporary Files.
ptrdiff_t
stddef.h (ISO): Important Data Types.
char * ptsname (int filedes)
stdlib.h (SVID, XPG4.2): Allocation.
int ptsname_r (int filedes, char *buf, size_t len)
stdlib.h (GNU): Allocation.
int putc (int c, FILE *stream)
stdio.h (ISO): Simple Output.
int putchar (int c)
stdio.h (ISO): Simple Output.
int putchar_unlocked (int c)
stdio.h (POSIX): Simple Output.
int putc_unlocked (int c, FILE *stream)
stdio.h (POSIX): Simple Output.
int putenv (char *string)
stdlib.h (SVID): Environment Access.
int putpwent (const struct passwd *p, FILE *stream)
pwd.h (SVID): Writing a User Entry.
int puts (const char *s)
stdio.h (ISO): Simple Output.
struct utmp * pututline (const struct utmp *utmp)
utmp.h (SVID): Manipulating the Database.
struct utmpx * pututxline (const struct utmpx *utmp)
utmpx.h (XPG4.2): XPG Functions.
int putw (int w, FILE *stream)
stdio.h (SVID): Simple Output.
wint_t putwc (wchar_t wc, FILE *stream)
wchar.h (ISO): Simple Output.
wint_t putwchar (wchar_t wc)
wchar.h (ISO): Simple Output.
wint_t putwchar_unlocked (wchar_t wc)
wchar.h (GNU): Simple Output.
wint_t putwc_unlocked (wchar_t wc, FILE *stream)
wchar.h (GNU): Simple Output.
ssize_t pwrite (int filedes, const void *buffer, size_t size, off_t offset)
unistd.h (Unix98): I/O Primitives.
ssize_t pwrite64 (int filedes, const void *buffer, size_t size, off64_t offset)
unistd.h (Unix98): I/O Primitives.
char * qecvt (long double value, int ndigit, int *decpt, int *neg)
stdlib.h (GNU): System V Number Conversion.
int qecvt_r (long double value, int ndigit, int *decpt, int *neg, char *buf, size_t len)
stdlib.h (GNU): System V Number Conversion.
char * qfcvt (long double value, int ndigit, int *decpt, int *neg)
stdlib.h (GNU): System V Number Conversion.
int qfcvt_r (long double value, int ndigit, int *decpt, int *neg, char *buf, size_t len)
stdlib.h (GNU): System V Number Conversion.
char * qgcvt (long double value, int ndigit, char *buf)
stdlib.h (GNU): System V Number Conversion.
void qsort (void *array, size_t count, size_t size, comparison_fn_t compare)
stdlib.h (ISO): Array Sort Function.
int raise (int signum)
signal.h (ISO): Signaling Yourself.
int rand (void)
stdlib.h (ISO): ISO Random.
int RAND_MAX
stdlib.h (ISO): ISO Random.
long int random (void)
stdlib.h (BSD): BSD Random.
int random_r (struct random_data *restrict buf, int32_t *restrict result)
stdlib.h (GNU): BSD Random.
int rand_r (unsigned int *seed)
stdlib.h (POSIX.1): ISO Random.
void * rawmemchr (const void *block, int c)
string.h (GNU): Search Functions.
ssize_t read (int filedes, void *buffer, size_t size)
unistd.h (POSIX.1): I/O Primitives.
struct dirent * readdir (DIR *dirstream)
dirent.h (POSIX.1): Reading/Closing Directory.
struct dirent64 * readdir64 (DIR *dirstream)
dirent.h (LFS): Reading/Closing Directory.
int readdir64_r (DIR *dirstream, struct dirent64 *entry, struct dirent64 **result)
dirent.h (LFS): Reading/Closing Directory.
int readdir_r (DIR *dirstream, struct dirent *entry, struct dirent **result)
dirent.h (GNU): Reading/Closing Directory.
int readlink (const char *filename, char *buffer, size_t size)
unistd.h (BSD): Symbolic Links.
ssize_t readv (int filedes, const struct iovec *vector, int count)
sys/uio.h (BSD): Scatter-Gather.
void * realloc (void *ptr, size_t newsize)
malloc.h, stdlib.h (ISO): Changing Block Size.
__realloc_hook
malloc.h (GNU): Hooks for Malloc.
char * realpath (const char *restrict name, char *restrict resolved)
stdlib.h (XPG): Symbolic Links.
int recv (int socket, void *buffer, size_t size, int flags)
sys/socket.h (BSD): Receiving Data.
int recvfrom (int socket, void *buffer, size_t size, int flags, struct sockaddr *addr, socklen_t *length-ptr)
sys/socket.h (BSD): Receiving Datagrams.
int recvmsg (int socket, struct msghdr *message, int flags)
sys/socket.h (BSD): Receiving Datagrams.
int RE_DUP_MAX
limits.h (POSIX.2): General Limits.
_REENTRANT
(GNU): Feature Test Macros.
REG_BADBR
regex.h (POSIX.2): POSIX Regexp Compilation.
REG_BADPAT
regex.h (POSIX.2): POSIX Regexp Compilation.
REG_BADRPT
regex.h (POSIX.2): POSIX Regexp Compilation.
int regcomp (regex_t *restrict compiled, const char *restrict pattern, int cflags)
regex.h (POSIX.2): POSIX Regexp Compilation.
REG_EBRACE
regex.h (POSIX.2): POSIX Regexp Compilation.
REG_EBRACK
regex.h (POSIX.2): POSIX Regexp Compilation.
REG_ECOLLATE
regex.h (POSIX.2): POSIX Regexp Compilation.
REG_ECTYPE
regex.h (POSIX.2): POSIX Regexp Compilation.
REG_EESCAPE
regex.h (POSIX.2): POSIX Regexp Compilation.
REG_EPAREN
regex.h (POSIX.2): POSIX Regexp Compilation.
REG_ERANGE
regex.h (POSIX.2): POSIX Regexp Compilation.
size_t regerror (int errcode, const regex_t *restrict compiled, char *restrict buffer, size_t length)
regex.h (POSIX.2): Regexp Cleanup.
REG_ESPACE
regex.h (POSIX.2): Matching POSIX Regexps.
REG_ESPACE
regex.h (POSIX.2): POSIX Regexp Compilation.
REG_ESUBREG
regex.h (POSIX.2): POSIX Regexp Compilation.
int regexec (const regex_t *restrict compiled, const char *restrict string, size_t nmatch, regmatch_t matchptr[restrict], int eflags)
regex.h (POSIX.2): Matching POSIX Regexps.
regex_t
regex.h (POSIX.2): POSIX Regexp Compilation.
REG_EXTENDED
regex.h (POSIX.2): Flags for POSIX Regexps.
void regfree (regex_t *compiled)
regex.h (POSIX.2): Regexp Cleanup.
REG_ICASE
regex.h (POSIX.2): Flags for POSIX Regexps.
int register_printf_function (int spec, printf_function handler-function, printf_arginfo_function arginfo-function)
printf.h (GNU): Registering New Conversions.
regmatch_t
regex.h (POSIX.2): Regexp Subexpressions.
REG_NEWLINE
regex.h (POSIX.2): Flags for POSIX Regexps.
REG_NOMATCH
regex.h (POSIX.2): Matching POSIX Regexps.
REG_NOSUB
regex.h (POSIX.2): Flags for POSIX Regexps.
REG_NOTBOL
regex.h (POSIX.2): Matching POSIX Regexps.
REG_NOTEOL
regex.h (POSIX.2): Matching POSIX Regexps.
regoff_t
regex.h (POSIX.2): Regexp Subexpressions.
double remainder (double numerator, double denominator)
math.h (BSD): Remainder Functions.
float remainderf (float numerator, float denominator)
math.h (BSD): Remainder Functions.
long double remainderl (long double numerator, long double denominator)
math.h (BSD): Remainder Functions.
int remove (const char *filename)
stdio.h (ISO): Deleting Files.
int rename (const char *oldname, const char *newname)
stdio.h (ISO): Renaming Files.
void rewind (FILE *stream)
stdio.h (ISO): File Positioning.
void rewinddir (DIR *dirstream)
dirent.h (POSIX.1): Random Access Directory.
char * rindex (const char *string, int c)
string.h (BSD): Search Functions.
double rint (double x)
math.h (ISO): Rounding Functions.
float rintf (float x)
math.h (ISO): Rounding Functions.
long double rintl (long double x)
math.h (ISO): Rounding Functions.
int RLIM_INFINITY
sys/resource.h (BSD): Limits on Resources.
RLIMIT_AS
sys/resource.h (Unix98): Limits on Resources.
RLIMIT_CORE
sys/resource.h (BSD): Limits on Resources.
RLIMIT_CPU
sys/resource.h (BSD): Limits on Resources.
RLIMIT_DATA
sys/resource.h (BSD): Limits on Resources.
RLIMIT_FSIZE
sys/resource.h (BSD): Limits on Resources.
RLIMIT_MEMLOCK
sys/resource.h (BSD): Limits on Resources.
RLIMIT_NOFILE
sys/resource.h (BSD): Limits on Resources.
RLIMIT_NPROC
sys/resource.h (BSD): Limits on Resources.
RLIMIT_RSS
sys/resource.h (BSD): Limits on Resources.
RLIMIT_STACK
sys/resource.h (BSD): Limits on Resources.
RLIM_NLIMITS
sys/resource.h (BSD): Limits on Resources.
int rmdir (const char *filename)
unistd.h (POSIX.1): Deleting Files.
int R_OK
unistd.h (POSIX.1): Testing File Access.
double round (double x)
math.h (ISO): Rounding Functions.
float roundf (float x)
math.h (ISO): Rounding Functions.
long double roundl (long double x)
math.h (ISO): Rounding Functions.
int rpmatch (const char *response)
stdlib.h (stdlib.h): Yes-or-No Questions.
RUN_LVL
utmp.h (SVID): Manipulating the Database.
RUN_LVL
utmpx.h (XPG4.2): XPG Functions.
RUSAGE_CHILDREN
sys/resource.h (BSD): Resource Usage.
RUSAGE_SELF
sys/resource.h (BSD): Resource Usage.
int SA_NOCLDSTOP
signal.h (POSIX.1): Flags for Sigaction.
int SA_ONSTACK
signal.h (BSD): Flags for Sigaction.
int SA_RESTART
signal.h (BSD): Flags for Sigaction.
void *sbrk (ptrdiff_t delta)
unistd.h (BSD): Resizing the Data Segment.
_SC_2_C_DEV
unistd.h (POSIX.2): Constants for Sysconf.
_SC_2_FORT_DEV
unistd.h (POSIX.2): Constants for Sysconf.
_SC_2_FORT_RUN
unistd.h (POSIX.2): Constants for Sysconf.
_SC_2_LOCALEDEF
unistd.h (POSIX.2): Constants for Sysconf.
_SC_2_SW_DEV
unistd.h (POSIX.2): Constants for Sysconf.
_SC_2_VERSION
unistd.h (POSIX.2): Constants for Sysconf.
_SC_AIO_LISTIO_MAX
unistd.h (POSIX.1): Constants for Sysconf.
_SC_AIO_MAX
unistd.h (POSIX.1): Constants for Sysconf.
_SC_AIO_PRIO_DELTA_MAX
unistd.h (POSIX.1): Constants for Sysconf.
double scalb (double value, int exponent)
math.h (BSD): Normalization Functions.
float scalbf (float value, int exponent)
math.h (BSD): Normalization Functions.
long double scalbl (long double value, int exponent)
math.h (BSD): Normalization Functions.
long long int scalbln (double x, long int n)
math.h (BSD): Normalization Functions.
long long int scalblnf (float x, long int n)
math.h (BSD): Normalization Functions.
long long int scalblnl (long double x, long int n)
math.h (BSD): Normalization Functions.
long long int scalbn (double x, int n)
math.h (BSD): Normalization Functions.
long long int scalbnf (float x, int n)
math.h (BSD): Normalization Functions.
long long int scalbnl (long double x, int n)
math.h (BSD): Normalization Functions.
int scandir (const char *dir, struct dirent ***namelist, int (*selector) (const struct dirent *), int (*cmp) (const void *, const void *))
dirent.h (BSD/SVID): Scanning Directory Content.
int scandir64 (const char *dir, struct dirent64 ***namelist, int (*selector) (const struct dirent64 *), int (*cmp) (const void *, const void *))
dirent.h (GNU): Scanning Directory Content.
int scanf (const char *template, ...)
stdio.h (ISO): Formatted Input Functions.
_SC_ARG_MAX
unistd.h (POSIX.1): Constants for Sysconf.
_SC_ASYNCHRONOUS_IO
unistd.h (POSIX.1): Constants for Sysconf.
_SC_ATEXIT_MAX
unistd.h (GNU): Constants for Sysconf.
_SC_AVPHYS_PAGES
unistd.h (GNU): Constants for Sysconf.
_SC_BC_BASE_MAX
unistd.h (POSIX.2): Constants for Sysconf.
_SC_BC_DIM_MAX
unistd.h (POSIX.2): Constants for Sysconf.
_SC_BC_SCALE_MAX
unistd.h (POSIX.2): Constants for Sysconf.
_SC_BC_STRING_MAX
unistd.h (POSIX.2): Constants for Sysconf.
_SC_CHAR_BIT
unistd.h (X/Open): Constants for Sysconf.
_SC_CHARCLASS_NAME_MAX
unistd.h (GNU): Constants for Sysconf.
_SC_CHAR_MAX
unistd.h (X/Open): Constants for Sysconf.
_SC_CHAR_MIN
unistd.h (X/Open): Constants for Sysconf.
_SC_CHILD_MAX
unistd.h (POSIX.1): Constants for Sysconf.
_SC_CLK_TCK
unistd.h (POSIX.1): Constants for Sysconf.
_SC_COLL_WEIGHTS_MAX
unistd.h (POSIX.2): Constants for Sysconf.
_SC_DELAYTIMER_MAX
unistd.h (POSIX.1): Constants for Sysconf.
_SC_EQUIV_CLASS_MAX
unistd.h (POSIX.2): Constants for Sysconf.
_SC_EXPR_NEST_MAX
unistd.h (POSIX.2): Constants for Sysconf.
_SC_FSYNC
unistd.h (POSIX.1): Constants for Sysconf.
_SC_GETGR_R_SIZE_MAX
unistd.h (POSIX.1): Constants for Sysconf.
_SC_GETPW_R_SIZE_MAX
unistd.h (POSIX.1): Constants for Sysconf.
SCHAR_MAX
limits.h (ISO): Range of Type.
SCHAR_MIN
limits.h (ISO): Range of Type.
int sched_getaffinity (pid_t pid, size_t cpusetsize, cpu_set_t *cpuset)
sched.h (GNU): CPU Affinity.
int sched_getparam (pid_t pid, const struct sched_param *param)
sched.h (POSIX): Basic Scheduling Functions.
int sched_get_priority_max (int *policy);
sched.h (POSIX): Basic Scheduling Functions.
int sched_get_priority_min (int *policy);
sched.h (POSIX): Basic Scheduling Functions.
int sched_getscheduler (pid_t pid)
sched.h (POSIX): Basic Scheduling Functions.
int sched_rr_get_interval (pid_t pid, struct timespec *interval)
sched.h (POSIX): Basic Scheduling Functions.
int sched_setaffinity (pid_t pid, size_t cpusetsize, const cpu_set_t *cpuset)
sched.h (GNU): CPU Affinity.
int sched_setparam (pid_t pid, const struct sched_param *param)
sched.h (POSIX): Basic Scheduling Functions.
int sched_setscheduler (pid_t pid, int policy, const struct sched_param *param)
sched.h (POSIX): Basic Scheduling Functions.
int sched_yield (void)
sched.h (POSIX): Basic Scheduling Functions.
_SC_INT_MAX
unistd.h (X/Open): Constants for Sysconf.
_SC_INT_MIN
unistd.h (X/Open): Constants for Sysconf.
_SC_JOB_CONTROL
unistd.h (POSIX.1): Constants for Sysconf.
_SC_LINE_MAX
unistd.h (POSIX.2): Constants for Sysconf.
_SC_LOGIN_NAME_MAX
unistd.h (POSIX.1): Constants for Sysconf.
_SC_LONG_BIT
unistd.h (X/Open): Constants for Sysconf.
_SC_MAPPED_FILES
unistd.h (POSIX.1): Constants for Sysconf.
_SC_MB_LEN_MAX
unistd.h (X/Open): Constants for Sysconf.
_SC_MEMLOCK
unistd.h (POSIX.1): Constants for Sysconf.
_SC_MEMLOCK_RANGE
unistd.h (POSIX.1): Constants for Sysconf.
_SC_MEMORY_PROTECTION
unistd.h (POSIX.1): Constants for Sysconf.
_SC_MESSAGE_PASSING
unistd.h (POSIX.1): Constants for Sysconf.
_SC_MQ_OPEN_MAX
unistd.h (POSIX.1): Constants for Sysconf.
_SC_MQ_PRIO_MAX
unistd.h (POSIX.1): Constants for Sysconf.
_SC_NGROUPS_MAX
unistd.h (POSIX.1): Constants for Sysconf.
_SC_NL_ARGMAX
unistd.h (X/Open): Constants for Sysconf.
_SC_NL_LANGMAX
unistd.h (X/Open): Constants for Sysconf.
_SC_NL_MSGMAX
unistd.h (X/Open): Constants for Sysconf.
_SC_NL_NMAX
unistd.h (X/Open): Constants for Sysconf.
_SC_NL_SETMAX
unistd.h (X/Open): Constants for Sysconf.
_SC_NL_TEXTMAX
unistd.h (X/Open): Constants for Sysconf.
_SC_NPROCESSORS_CONF
unistd.h (GNU): Constants for Sysconf.
_SC_NPROCESSORS_ONLN
unistd.h (GNU): Constants for Sysconf.
_SC_NZERO
unistd.h (X/Open): Constants for Sysconf.
_SC_OPEN_MAX
unistd.h (POSIX.1): Constants for Sysconf.
_SC_PAGESIZE
unistd.h (GNU): Constants for Sysconf.
_SC_PHYS_PAGES
unistd.h (GNU): Constants for Sysconf.
_SC_PII
unistd.h (POSIX.1g): Constants for Sysconf.
_SC_PII_INTERNET
unistd.h (POSIX.1g): Constants for Sysconf.
_SC_PII_INTERNET_DGRAM
unistd.h (POSIX.1g): Constants for Sysconf.
_SC_PII_INTERNET_STREAM
unistd.h (POSIX.1g): Constants for Sysconf.
_SC_PII_OSI
unistd.h (POSIX.1g): Constants for Sysconf.
_SC_PII_OSI_CLTS
unistd.h (POSIX.1g): Constants for Sysconf.
_SC_PII_OSI_COTS
unistd.h (POSIX.1g): Constants for Sysconf.
_SC_PII_OSI_M
unistd.h (POSIX.1g): Constants for Sysconf.
_SC_PII_SOCKET
unistd.h (POSIX.1g): Constants for Sysconf.
_SC_PII_XTI
unistd.h (POSIX.1g): Constants for Sysconf.
_SC_PRIORITIZED_IO
unistd.h (POSIX.1): Constants for Sysconf.
_SC_PRIORITY_SCHEDULING
unistd.h (POSIX.1): Constants for Sysconf.
_SC_REALTIME_SIGNALS
unistdh.h (POSIX.1): Constants for Sysconf.
_SC_RTSIG_MAX
unistd.h (POSIX.1): Constants for Sysconf.
_SC_SAVED_IDS
unistd.h (POSIX.1): Constants for Sysconf.
_SC_SCHAR_MAX
unistd.h (X/Open): Constants for Sysconf.
_SC_SCHAR_MIN
unistd.h (X/Open): Constants for Sysconf.
_SC_SELECT
unistd.h (POSIX.1g): Constants for Sysconf.
_SC_SEMAPHORES
unistd.h (POSIX.1): Constants for Sysconf.
_SC_SEM_NSEMS_MAX
unistd.h (POSIX.1): Constants for Sysconf.
_SC_SEM_VALUE_MAX
unistd.h (POSIX.1): Constants for Sysconf.
_SC_SHARED_MEMORY_OBJECTS
unistd.h (POSIX.1): Constants for Sysconf.
_SC_SHRT_MAX
unistd.h (X/Open): Constants for Sysconf.
_SC_SHRT_MIN
unistd.h (X/Open): Constants for Sysconf.
_SC_SIGQUEUE_MAX
unistd.h (POSIX.1): Constants for Sysconf.
SC_SSIZE_MAX
unistd.h (X/Open): Constants for Sysconf.
_SC_STREAM_MAX
unistd.h (POSIX.1): Constants for Sysconf.
_SC_SYNCHRONIZED_IO
unistd.h (POSIX.1): Constants for Sysconf.
_SC_THREAD_ATTR_STACKADDR
unistd.h (POSIX.1): Constants for Sysconf.
_SC_THREAD_ATTR_STACKSIZE
unistd.h (POSIX.1): Constants for Sysconf.
_SC_THREAD_DESTRUCTOR_ITERATIONS
unistd.h (POSIX.1): Constants for Sysconf.
_SC_THREAD_KEYS_MAX
unistd.h (POSIX.1): Constants for Sysconf.
_SC_THREAD_PRIO_INHERIT
unistd.h (POSIX.1): Constants for Sysconf.
_SC_THREAD_PRIO_PROTECT
unistd.h (POSIX.1): Constants for Sysconf.
_SC_THREAD_PRIORITY_SCHEDULING
unistd.h (POSIX.1): Constants for Sysconf.
_SC_THREAD_PROCESS_SHARED
unistd.h (POSIX.1): Constants for Sysconf.
_SC_THREADS
unistd.h (POSIX.1): Constants for Sysconf.
_SC_THREAD_SAFE_FUNCTIONS
unistd.h (POSIX.1): Constants for Sysconf.
_SC_THREAD_STACK_MIN
unistd.h (POSIX.1): Constants for Sysconf.
_SC_THREAD_THREADS_MAX
unistd.h (POSIX.1): Constants for Sysconf.
_SC_TIMER_MAX
unistd.h (POSIX.1): Constants for Sysconf.
_SC_TIMERS
unistd.h (POSIX.1): Constants for Sysconf.
_SC_T_IOV_MAX
unistd.h (POSIX.1g): Constants for Sysconf.
_SC_TTY_NAME_MAX
unistd.h (POSIX.1): Constants for Sysconf.
_SC_TZNAME_MAX
unistd.h (POSIX.1): Constants for Sysconf.
_SC_UCHAR_MAX
unistd.h (X/Open): Constants for Sysconf.
_SC_UINT_MAX
unistd.h (X/Open): Constants for Sysconf.
_SC_UIO_MAXIOV
unistd.h (POSIX.1g): Constants for Sysconf.
_SC_ULONG_MAX
unistd.h (X/Open): Constants for Sysconf.
_SC_USHRT_MAX
unistd.h (X/Open): Constants for Sysconf.
_SC_VERSION
unistd.h (POSIX.1): Constants for Sysconf.
_SC_VERSION
unistd.h (POSIX.2): Constants for Sysconf.
_SC_WORD_BIT
unistd.h (X/Open): Constants for Sysconf.
_SC_XOPEN_CRYPT
unistd.h (X/Open): Constants for Sysconf.
_SC_XOPEN_ENH_I18N
unistd.h (X/Open): Constants for Sysconf.
_SC_XOPEN_LEGACY
unistd.h (X/Open): Constants for Sysconf.
_SC_XOPEN_REALTIME
unistd.h (X/Open): Constants for Sysconf.
_SC_XOPEN_REALTIME_THREADS
unistd.h (X/Open): Constants for Sysconf.
_SC_XOPEN_SHM
unistd.h (X/Open): Constants for Sysconf.
_SC_XOPEN_UNIX
unistd.h (X/Open): Constants for Sysconf.
_SC_XOPEN_VERSION
unistd.h (X/Open): Constants for Sysconf.
_SC_XOPEN_XCU_VERSION
unistd.h (X/Open): Constants for Sysconf.
_SC_XOPEN_XPG2
unistd.h (X/Open): Constants for Sysconf.
_SC_XOPEN_XPG3
unistd.h (X/Open): Constants for Sysconf.
_SC_XOPEN_XPG4
unistd.h (X/Open): Constants for Sysconf.
unsigned short int * seed48 (unsigned short int seed16v[3])
stdlib.h (SVID): SVID Random.
int seed48_r (unsigned short int seed16v[3], struct drand48_data *buffer)
stdlib.h (GNU): SVID Random.
int SEEK_CUR
stdio.h (ISO): File Positioning.
void seekdir (DIR *dirstream, long int pos)
dirent.h (BSD): Random Access Directory.
int SEEK_END
stdio.h (ISO): File Positioning.
int SEEK_SET
stdio.h (ISO): File Positioning.
int select (int nfds, fd_set *read-fds, fd_set *write-fds, fd_set *except-fds, struct timeval *timeout)
sys/types.h (BSD): Waiting for I/O.
int send (int socket, void *buffer, size_t size, int flags)
sys/socket.h (BSD): Sending Data.
int sendmsg (int socket, const struct msghdr *message, int flags)
sys/socket.h (BSD): Receiving Datagrams.
int sendto (int socket, void *buffer. size_t size, int flags, struct sockaddr *addr, socklen_t length)
sys/socket.h (BSD): Sending Datagrams.
void setbuf (FILE *stream, char *buf)
stdio.h (ISO): Controlling Buffering.
void setbuffer (FILE *stream, char *buf, size_t size)
stdio.h (BSD): Controlling Buffering.
int setcontext (const ucontext_t *ucp)
ucontext.h (SVID): System V contexts.
int setdomainname (const char *name, size_t length)
unistd.h (???): Host Identification.
int setegid (gid_t newgid)
unistd.h (POSIX.1): Setting Groups.
int setenv (const char *name, const char *value, int replace)
stdlib.h (BSD): Environment Access.
int seteuid (uid_t neweuid)
unistd.h (POSIX.1): Setting User ID.
int setfsent (void)
fstab.h (BSD): fstab.
int setgid (gid_t newgid)
unistd.h (POSIX.1): Setting Groups.
void setgrent (void)
grp.h (SVID, BSD): Scanning All Groups.
int setgroups (size_t count, gid_t *groups)
grp.h (BSD): Setting Groups.
void sethostent (int stayopen)
netdb.h (BSD): Host Names.
int sethostid (long int id)
unistd.h (BSD): Host Identification.
int sethostname (const char *name, size_t length)
unistd.h (BSD): Host Identification.
int setitimer (int which, struct itimerval *new, struct itimerval *old)
sys/time.h (BSD): Setting an Alarm.
int setjmp (jmp_buf state)
setjmp.h (ISO): Non-Local Details.
void setkey (const char *key)
crypt.h (BSD, SVID): DES Encryption.
void setkey_r (const char *key, struct crypt_data * data)
crypt.h (GNU): DES Encryption.
void setlinebuf (FILE *stream)
stdio.h (BSD): Controlling Buffering.
char * setlocale (int category, const char *locale)
locale.h (ISO): Setting the Locale.
int setlogmask (int mask)
syslog.h (BSD): setlogmask.
FILE * setmntent (const char *file, const char *mode)
mntent.h (BSD): mtab.
void setnetent (int stayopen)
netdb.h (BSD): Networks Database.
int setnetgrent (const char *netgroup)
netdb.h (BSD): Lookup Netgroup.
int setpgid (pid_t pid, pid_t pgid)
unistd.h (POSIX.1): Process Group Functions.
int setpgrp (pid_t pid, pid_t pgid)
unistd.h (BSD): Process Group Functions.
int setpriority (int class, int id, int niceval)
sys/resource.h (BSD,POSIX): Traditional Scheduling Functions.
void setprotoent (int stayopen)
netdb.h (BSD): Protocols Database.
void setpwent (void)
pwd.h (SVID, BSD): Scanning All Users.
int setregid (gid_t rgid, gid_t egid)
unistd.h (BSD): Setting Groups.
int setreuid (uid_t ruid, uid_t euid)
unistd.h (BSD): Setting User ID.
int setrlimit (int resource, const struct rlimit *rlp)
sys/resource.h (BSD): Limits on Resources.
int setrlimit64 (int resource, const struct rlimit64 *rlp)
sys/resource.h (Unix98): Limits on Resources.
void setservent (int stayopen)
netdb.h (BSD): Services Database.
pid_t setsid (void)
unistd.h (POSIX.1): Process Group Functions.
int setsockopt (int socket, int level, int optname, void *optval, socklen_t optlen)
sys/socket.h (BSD): Socket Option Functions.
void * setstate (void *state)
stdlib.h (BSD): BSD Random.
int setstate_r (char *restrict statebuf, struct random_data *restrict buf)
stdlib.h (GNU): BSD Random.
int settimeofday (const struct timeval *tp, const struct timezone *tzp)
sys/time.h (BSD): High-Resolution Calendar.
int setuid (uid_t newuid)
unistd.h (POSIX.1): Setting User ID.
void setutent (void)
utmp.h (SVID): Manipulating the Database.
void setutxent (void)
utmpx.h (XPG4.2): XPG Functions.
int setvbuf (FILE *stream, char *buf, int mode, size_t size)
stdio.h (ISO): Controlling Buffering.
SHRT_MAX
limits.h (ISO): Range of Type.
SHRT_MIN
limits.h (ISO): Range of Type.
int shutdown (int socket, int how)
sys/socket.h (BSD): Closing a Socket.
S_IEXEC
sys/stat.h (BSD): Permission Bits.
S_IFBLK
sys/stat.h (BSD): Testing File Type.
S_IFCHR
sys/stat.h (BSD): Testing File Type.
S_IFDIR
sys/stat.h (BSD): Testing File Type.
S_IFIFO
sys/stat.h (BSD): Testing File Type.
S_IFLNK
sys/stat.h (BSD): Testing File Type.
int S_IFMT
sys/stat.h (BSD): Testing File Type.
S_IFREG
sys/stat.h (BSD): Testing File Type.
S_IFSOCK
sys/stat.h (BSD): Testing File Type.
int SIGABRT
signal.h (ISO): Program Error Signals.
int sigaction (int signum, const struct sigaction *restrict action, struct sigaction *restrict old-action)
signal.h (POSIX.1): Advanced Signal Handling.
int sigaddset (sigset_t *set, int signum)
signal.h (POSIX.1): Signal Sets.
int SIGALRM
signal.h (POSIX.1): Alarm Signals.
int sigaltstack (const stack_t *restrict stack, stack_t *restrict oldstack)
signal.h (XPG): Signal Stack.
sig_atomic_t
signal.h (ISO): Atomic Types.
int sigblock (int mask)
signal.h (BSD): Blocking in BSD.
SIG_BLOCK
signal.h (POSIX.1): Process Signal Mask.
int SIGBUS
signal.h (BSD): Program Error Signals.
int SIGCHLD
signal.h (POSIX.1): Job Control Signals.
int SIGCLD
signal.h (SVID): Job Control Signals.
int SIGCONT
signal.h (POSIX.1): Job Control Signals.
int sigdelset (sigset_t *set, int signum)
signal.h (POSIX.1): Signal Sets.
int sigemptyset (sigset_t *set)
signal.h (POSIX.1): Signal Sets.
int SIGEMT
signal.h (BSD): Program Error Signals.
sighandler_t SIG_ERR
signal.h (ISO): Basic Signal Handling.
int sigfillset (sigset_t *set)
signal.h (POSIX.1): Signal Sets.
int SIGFPE
signal.h (ISO): Program Error Signals.
sighandler_t
signal.h (GNU): Basic Signal Handling.
int SIGHUP
signal.h (POSIX.1): Termination Signals.
int SIGILL
signal.h (ISO): Program Error Signals.
int SIGINFO
signal.h (BSD): Miscellaneous Signals.
int SIGINT
signal.h (ISO): Termination Signals.
int siginterrupt (int signum, int failflag)
signal.h (BSD): BSD Handler.
int SIGIO
signal.h (BSD): Asynchronous I/O Signals.
int SIGIOT
signal.h (Unix): Program Error Signals.
int sigismember (const sigset_t *set, int signum)
signal.h (POSIX.1): Signal Sets.
sigjmp_buf
setjmp.h (POSIX.1): Non-Local Exits and Signals.
int SIGKILL
signal.h (POSIX.1): Termination Signals.
void siglongjmp (sigjmp_buf state, int value)
setjmp.h (POSIX.1): Non-Local Exits and Signals.
int SIGLOST
signal.h (GNU): Operation Error Signals.
int sigmask (int signum)
signal.h (BSD): Blocking in BSD.
sighandler_t signal (int signum, sighandler_t action)
signal.h (ISO): Basic Signal Handling.
int signbit (float-type x)
math.h (ISO): FP Bit Twiddling.
long long int significand (double x)
math.h (BSD): Normalization Functions.
long long int significandf (float x)
math.h (BSD): Normalization Functions.
long long int significandl (long double x)
math.h (BSD): Normalization Functions.
int sigpause (int mask)
signal.h (BSD): Blocking in BSD.
int sigpending (sigset_t *set)
signal.h (POSIX.1): Checking for Pending Signals.
int SIGPIPE
signal.h (POSIX.1): Operation Error Signals.
int SIGPOLL
signal.h (SVID): Asynchronous I/O Signals.
int sigprocmask (int how, const sigset_t *restrict set, sigset_t *restrict oldset)
signal.h (POSIX.1): Process Signal Mask.
int SIGPROF
signal.h (BSD): Alarm Signals.
int SIGQUIT
signal.h (POSIX.1): Termination Signals.
int SIGSEGV
signal.h (ISO): Program Error Signals.
int sigsetjmp (sigjmp_buf state, int savesigs)
setjmp.h (POSIX.1): Non-Local Exits and Signals.
int sigsetmask (int mask)
signal.h (BSD): Blocking in BSD.
SIG_SETMASK
signal.h (POSIX.1): Process Signal Mask.
sigset_t
signal.h (POSIX.1): Signal Sets.
int sigstack (const struct sigstack *stack, struct sigstack *oldstack)
signal.h (BSD): Signal Stack.
int SIGSTOP
signal.h (POSIX.1): Job Control Signals.
int sigsuspend (const sigset_t *set)
signal.h (POSIX.1): Sigsuspend.
int SIGSYS
signal.h (Unix): Program Error Signals.
int SIGTERM
signal.h (ISO): Termination Signals.
int SIGTRAP
signal.h (BSD): Program Error Signals.
int SIGTSTP
signal.h (POSIX.1): Job Control Signals.
int SIGTTIN
signal.h (POSIX.1): Job Control Signals.
int SIGTTOU
signal.h (POSIX.1): Job Control Signals.
SIG_UNBLOCK
signal.h (POSIX.1): Process Signal Mask.
int SIGURG
signal.h (BSD): Asynchronous I/O Signals.
int SIGUSR1
signal.h (POSIX.1): Miscellaneous Signals.
int SIGUSR2
signal.h (POSIX.1): Miscellaneous Signals.
int sigvec (int signum, const struct sigvec *action,struct sigvec *old-action)
signal.h (BSD): BSD Handler.
int SIGVTALRM
signal.h (BSD): Alarm Signals.
int SIGWINCH
signal.h (BSD): Miscellaneous Signals.
int SIGXCPU
signal.h (BSD): Operation Error Signals.
int SIGXFSZ
signal.h (BSD): Operation Error Signals.
double sin (double x)
math.h (ISO): Trig Functions.
void sincos (double x, double *sinx, double *cosx)
math.h (GNU): Trig Functions.
void sincosf (float x, float *sinx, float *cosx)
math.h (GNU): Trig Functions.
void sincosl (long double x, long double *sinx, long double *cosx)
math.h (GNU): Trig Functions.
float sinf (float x)
math.h (ISO): Trig Functions.
double sinh (double x)
math.h (ISO): Hyperbolic Functions.
float sinhf (float x)
math.h (ISO): Hyperbolic Functions.
long double sinhl (long double x)
math.h (ISO): Hyperbolic Functions.
long double sinl (long double x)
math.h (ISO): Trig Functions.
S_IREAD
sys/stat.h (BSD): Permission Bits.
S_IRGRP
sys/stat.h (POSIX.1): Permission Bits.
S_IROTH
sys/stat.h (POSIX.1): Permission Bits.
S_IRUSR
sys/stat.h (POSIX.1): Permission Bits.
S_IRWXG
sys/stat.h (POSIX.1): Permission Bits.
S_IRWXO
sys/stat.h (POSIX.1): Permission Bits.
S_IRWXU
sys/stat.h (POSIX.1): Permission Bits.
int S_ISBLK (mode_t m)
sys/stat.h (POSIX): Testing File Type.
int S_ISCHR (mode_t m)
sys/stat.h (POSIX): Testing File Type.
int S_ISDIR (mode_t m)
sys/stat.h (POSIX): Testing File Type.
int S_ISFIFO (mode_t m)
sys/stat.h (POSIX): Testing File Type.
S_ISGID
sys/stat.h (POSIX): Permission Bits.
int S_ISLNK (mode_t m)
sys/stat.h (GNU): Testing File Type.
int S_ISREG (mode_t m)
sys/stat.h (POSIX): Testing File Type.
int S_ISSOCK (mode_t m)
sys/stat.h (GNU): Testing File Type.
S_ISUID
sys/stat.h (POSIX): Permission Bits.
S_ISVTX
sys/stat.h (BSD): Permission Bits.
S_IWGRP
sys/stat.h (POSIX.1): Permission Bits.
S_IWOTH
sys/stat.h (POSIX.1): Permission Bits.
S_IWRITE
sys/stat.h (BSD): Permission Bits.
S_IWUSR
sys/stat.h (POSIX.1): Permission Bits.
S_IXGRP
sys/stat.h (POSIX.1): Permission Bits.
S_IXOTH
sys/stat.h (POSIX.1): Permission Bits.
S_IXUSR
sys/stat.h (POSIX.1): Permission Bits.
size_t
stddef.h (ISO): Important Data Types.
unsigned int sleep (unsigned int seconds)
unistd.h (POSIX.1): Sleeping.
int snprintf (char *s, size_t size, const char *template, ...)
stdio.h (GNU): Formatted Output Functions.
SO_BROADCAST
sys/socket.h (BSD): Socket-Level Options.
int SOCK_DGRAM
sys/socket.h (BSD): Communication Styles.
int socket (int namespace, int style, int protocol)
sys/socket.h (BSD): Creating a Socket.
int socketpair (int namespace, int style, int protocol, int filedes[2])
sys/socket.h (BSD): Socket Pairs.
int SOCK_RAW
sys/socket.h (BSD): Communication Styles.
int SOCK_RDM
sys/socket.h (BSD): Communication Styles.
int SOCK_SEQPACKET
sys/socket.h (BSD): Communication Styles.
int SOCK_STREAM
sys/socket.h (BSD): Communication Styles.
SO_DEBUG
sys/socket.h (BSD): Socket-Level Options.
SO_DONTROUTE
sys/socket.h (BSD): Socket-Level Options.
SO_ERROR
sys/socket.h (BSD): Socket-Level Options.
SO_KEEPALIVE
sys/socket.h (BSD): Socket-Level Options.
SO_LINGER
sys/socket.h (BSD): Socket-Level Options.
int SOL_SOCKET
sys/socket.h (BSD): Socket-Level Options.
SO_OOBINLINE
sys/socket.h (BSD): Socket-Level Options.
SO_RCVBUF
sys/socket.h (BSD): Socket-Level Options.
SO_REUSEADDR
sys/socket.h (BSD): Socket-Level Options.
SO_SNDBUF
sys/socket.h (BSD): Socket-Level Options.
SO_STYLE
sys/socket.h (GNU): Socket-Level Options.
SO_TYPE
sys/socket.h (BSD): Socket-Level Options.
speed_t
termios.h (POSIX.1): Line Speed.
int sprintf (char *s, const char *template, ...)
stdio.h (ISO): Formatted Output Functions.
double sqrt (double x)
math.h (ISO): Exponents and Logarithms.
float sqrtf (float x)
math.h (ISO): Exponents and Logarithms.
long double sqrtl (long double x)
math.h (ISO): Exponents and Logarithms.
void srand (unsigned int seed)
stdlib.h (ISO): ISO Random.
void srand48 (long int seedval)
stdlib.h (SVID): SVID Random.
int srand48_r (long int seedval, struct drand48_data *buffer)
stdlib.h (GNU): SVID Random.
void srandom (unsigned int seed)
stdlib.h (BSD): BSD Random.
int srandom_r (unsigned int seed, struct random_data *buf)
stdlib.h (GNU): BSD Random.
int sscanf (const char *s, const char *template, ...)
stdio.h (ISO): Formatted Input Functions.
sighandler_t ssignal (int signum, sighandler_t action)
signal.h (SVID): Basic Signal Handling.
int SSIZE_MAX
limits.h (POSIX.1): General Limits.
ssize_t
unistd.h (POSIX.1): I/O Primitives.
stack_t
signal.h (XPG): Signal Stack.
int stat (const char *filename, struct stat *buf)
sys/stat.h (POSIX.1): Reading Attributes.
int stat64 (const char *filename, struct stat64 *buf)
sys/stat.h (Unix98): Reading Attributes.
FILE * stderr
stdio.h (ISO): Standard Streams.
STDERR_FILENO
unistd.h (POSIX.1): Descriptors and Streams.
FILE * stdin
stdio.h (ISO): Standard Streams.
STDIN_FILENO
unistd.h (POSIX.1): Descriptors and Streams.
FILE * stdout
stdio.h (ISO): Standard Streams.
STDOUT_FILENO
unistd.h (POSIX.1): Descriptors and Streams.
int stime (time_t *newtime)
time.h (SVID, XPG): Simple Calendar Time.
char * stpcpy (char *restrict to, const char *restrict from)
string.h (Unknown origin): Copying and Concatenation.
char * stpncpy (char *restrict to, const char *restrict from, size_t size)
string.h (GNU): Copying and Concatenation.
int strcasecmp (const char *s1, const char *s2)
string.h (BSD): String/Array Comparison.
char * strcasestr (const char *haystack, const char *needle)
string.h (GNU): Search Functions.
char * strcat (char *restrict to, const char *restrict from)
string.h (ISO): Copying and Concatenation.
char * strchr (const char *string, int c)
string.h (ISO): Search Functions.
char * strchrnul (const char *string, int c)
string.h (GNU): Search Functions.
int strcmp (const char *s1, const char *s2)
string.h (ISO): String/Array Comparison.
int strcoll (const char *s1, const char *s2)
string.h (ISO): Collation Functions.
char * strcpy (char *restrict to, const char *restrict from)
string.h (ISO): Copying and Concatenation.
size_t strcspn (const char *string, const char *stopset)
string.h (ISO): Search Functions.
char * strdup (const char *s)
string.h (SVID): Copying and Concatenation.
char * strdupa (const char *s)
string.h (GNU): Copying and Concatenation.
int STREAM_MAX
limits.h (POSIX.1): General Limits.
char * strerror (int errnum)
string.h (ISO): Error Messages.
char * strerror_r (int errnum, char *buf, size_t n)
string.h (GNU): Error Messages.
char * strfry (char *string)
string.h (GNU): strfry.
size_t strftime (char *s, size_t size, const char *template, const struct tm *brokentime)
time.h (ISO): Formatting Calendar Time.
size_t strlen (const char *s)
string.h (ISO): String Length.
int strncasecmp (const char *s1, const char *s2, size_t n)
string.h (BSD): String/Array Comparison.
char * strncat (char *restrict to, const char *restrict from, size_t size)
string.h (ISO): Copying and Concatenation.
int strncmp (const char *s1, const char *s2, size_t size)
string.h (ISO): String/Array Comparison.
char * strncpy (char *restrict to, const char *restrict from, size_t size)
string.h (ISO): Copying and Concatenation.
char * strndup (const char *s, size_t size)
string.h (GNU): Copying and Concatenation.
char * strndupa (const char *s, size_t size)
string.h (GNU): Copying and Concatenation.
size_t strnlen (const char *s, size_t maxlen)
string.h (GNU): String Length.
char * strpbrk (const char *string, const char *stopset)
string.h (ISO): Search Functions.
char * strptime (const char *s, const char *fmt, struct tm *tp)
time.h (XPG4): Low-Level Time String Parsing.
char * strrchr (const char *string, int c)
string.h (ISO): Search Functions.
char * strsep (char **string_ptr, const char *delimiter)
string.h (BSD): Finding Tokens in a String.
char * strsignal (int signum)
string.h (GNU): Signal Messages.
size_t strspn (const char *string, const char *skipset)
string.h (ISO): Search Functions.
char * strstr (const char *haystack, const char *needle)
string.h (ISO): Search Functions.
double strtod (const char *restrict string, char **restrict tailptr)
stdlib.h (ISO): Parsing of Floats.
float strtof (const char *string, char **tailptr)
stdlib.h (ISO): Parsing of Floats.
intmax_t strtoimax (const char *restrict string, char **restrict tailptr, int base)
inttypes.h (ISO): Parsing of Integers.
char * strtok (char *restrict newstring, const char *restrict delimiters)
string.h (ISO): Finding Tokens in a String.
char * strtok_r (char *newstring, const char *delimiters, char **save_ptr)
string.h (POSIX): Finding Tokens in a String.
long int strtol (const char *restrict string, char **restrict tailptr, int base)
stdlib.h (ISO): Parsing of Integers.
long double strtold (const char *string, char **tailptr)
stdlib.h (ISO): Parsing of Floats.
long long int strtoll (const char *restrict string, char **restrict tailptr, int base)
stdlib.h (ISO): Parsing of Integers.
long long int strtoq (const char *restrict string, char **restrict tailptr, int base)
stdlib.h (BSD): Parsing of Integers.
unsigned long int strtoul (const char *retrict string, char **restrict tailptr, int base)
stdlib.h (ISO): Parsing of Integers.
unsigned long long int strtoull (const char *restrict string, char **restrict tailptr, int base)
stdlib.h (ISO): Parsing of Integers.
uintmax_t strtoumax (const char *restrict string, char **restrict tailptr, int base)
inttypes.h (ISO): Parsing of Integers.
unsigned long long int strtouq (const char *restrict string, char **restrict tailptr, int base)
stdlib.h (BSD): Parsing of Integers.
struct aiocb
aio.h (POSIX.1b): Asynchronous I/O.
struct aiocb64
aio.h (POSIX.1b): Asynchronous I/O.
struct aioinit
aio.h (GNU): Configuration of AIO.
struct argp
argp.h (GNU): Argp Parsers.
struct argp_child
argp.h (GNU): Argp Children.
struct argp_option
argp.h (GNU): Argp Option Vectors.
struct argp_state
argp.h (GNU): Argp Parsing State.
struct dirent
dirent.h (POSIX.1): Directory Entries.
struct exit_status
utmp.h (SVID): Manipulating the Database.
struct flock
fcntl.h (POSIX.1): File Locks.
struct fstab
fstab.h (BSD): fstab.
struct FTW
ftw.h (XPG4.2): Working with Directory Trees.
struct __gconv_step
gconv.h (GNU): glibc iconv Implementation.
struct __gconv_step_data
gconv.h (GNU): glibc iconv Implementation.
struct group
grp.h (POSIX.1): Group Data Structure.
struct hostent
netdb.h (BSD): Host Names.
struct if_nameindex
net/if.h (IPv6 basic API): Interface Naming.
struct in6_addr
netinet/in.h (IPv6 basic API): Host Address Data Type.
struct in_addr
netinet/in.h (BSD): Host Address Data Type.
struct iovec
sys/uio.h (BSD): Scatter-Gather.
struct itimerval
sys/time.h (BSD): Setting an Alarm.
struct lconv
locale.h (ISO): The Lame Way to Locale Data.
struct linger
sys/socket.h (BSD): Socket-Level Options.
struct mallinfo
malloc.h (GNU): Statistics of Malloc.
struct mntent
mntent.h (BSD): mtab.
struct msghdr
sys/socket.h (BSD): Receiving Datagrams.
struct netent
netdb.h (BSD): Networks Database.
struct obstack
obstack.h (GNU): Creating Obstacks.
struct option
getopt.h (GNU): Getopt Long Options.
struct passwd
pwd.h (POSIX.1): User Data Structure.
struct printf_info
printf.h (GNU): Conversion Specifier Options.
struct protoent
netdb.h (BSD): Protocols Database.
struct random_data
stdlib.h (GNU): BSD Random.
struct rlimit
sys/resource.h (BSD): Limits on Resources.
struct rlimit64
sys/resource.h (Unix98): Limits on Resources.
struct rusage
sys/resource.h (BSD): Resource Usage.
struct sched_param
sched.h (POSIX): Basic Scheduling Functions.
struct servent
netdb.h (BSD): Services Database.
struct sgttyb
termios.h (BSD): BSD Terminal Modes.
struct sigaction
signal.h (POSIX.1): Advanced Signal Handling.
struct sigstack
signal.h (BSD): Signal Stack.
struct sigvec
signal.h (BSD): BSD Handler.
struct sockaddr
sys/socket.h (BSD): Address Formats.
struct sockaddr_in
netinet/in.h (BSD): Internet Address Formats.
struct sockaddr_un
sys/un.h (BSD): Local Namespace Details.
struct stat
sys/stat.h (POSIX.1): Attribute Meanings.
struct stat64
sys/stat.h (LFS): Attribute Meanings.
struct termios
termios.h (POSIX.1): Mode Data Types.
struct timespec
sys/time.h (POSIX.1): Elapsed Time.
struct timeval
sys/time.h (BSD): Elapsed Time.
struct timezone
sys/time.h (BSD): High-Resolution Calendar.
struct tm
time.h (ISO): Broken-down Time.
struct tms
sys/times.h (POSIX.1): Processor Time.
struct utimbuf
time.h (POSIX.1): File Times.
struct utsname
sys/utsname.h (POSIX.1): Platform Type.
int strverscmp (const char *s1, const char *s2)
string.h (GNU): String/Array Comparison.
size_t strxfrm (char *restrict to, const char *restrict from, size_t size)
string.h (ISO): Collation Functions.
int stty (int filedes, struct sgttyb * attributes)
sgtty.h (BSD): BSD Terminal Modes.
int S_TYPEISMQ (struct stat *s)
sys/stat.h (POSIX): Testing File Type.
int S_TYPEISSEM (struct stat *s)
sys/stat.h (POSIX): Testing File Type.
int S_TYPEISSHM (struct stat *s)
sys/stat.h (POSIX): Testing File Type.
int SUN_LEN (struct sockaddr_un * ptr)
sys/un.h (BSD): Local Namespace Details.
_SVID_SOURCE
(GNU): Feature Test Macros.
int SV_INTERRUPT
signal.h (BSD): BSD Handler.
int SV_ONSTACK
signal.h (BSD): BSD Handler.
int SV_RESETHAND
signal.h (Sun): BSD Handler.
int swapcontext (ucontext_t *restrict oucp, const ucontext_t *restrict ucp)
ucontext.h (SVID): System V contexts.
int swprintf (wchar_t *s, size_t size, const wchar_t *template, ...)
wchar.h (GNU): Formatted Output Functions.
int swscanf (const wchar_t *ws, const char *template, ...)
wchar.h (ISO): Formatted Input Functions.
int symlink (const char *oldname, const char *newname)
unistd.h (BSD): Symbolic Links.
SYMLINK_MAX
limits.h (POSIX.1): File Minimums.
int sync (void)
unistd.h (X/Open): Synchronizing I/O.
long int syscall (long int sysno, ...)
unistd.h (???): System Calls.
long int sysconf (int parameter)
unistd.h (POSIX.1): Sysconf Definition.
int sysctl (int *names, int nlen, void *oldval, size_t *oldlenp, void *newval, size_t newlen)
sysctl.h (BSD): System Parameters.
void syslog (int facility_priority, char *format, ...)
syslog.h (BSD): syslog; vsyslog.
int system (const char *command)
stdlib.h (ISO): Running a Command.
sighandler_t sysv_signal (int signum, sighandler_t action)
signal.h (GNU): Basic Signal Handling.
double tan (double x)
math.h (ISO): Trig Functions.
float tanf (float x)
math.h (ISO): Trig Functions.
double tanh (double x)
math.h (ISO): Hyperbolic Functions.
float tanhf (float x)
math.h (ISO): Hyperbolic Functions.
long double tanhl (long double x)
math.h (ISO): Hyperbolic Functions.
long double tanl (long double x)
math.h (ISO): Trig Functions.
int tcdrain (int filedes)
termios.h (POSIX.1): Line Control.
tcflag_t
termios.h (POSIX.1): Mode Data Types.
int tcflow (int filedes, int action)
termios.h (POSIX.1): Line Control.
int tcflush (int filedes, int queue)
termios.h (POSIX.1): Line Control.
int tcgetattr (int filedes, struct termios *termios-p)
termios.h (POSIX.1): Mode Functions.
pid_t tcgetpgrp (int filedes)
unistd.h (POSIX.1): Terminal Access Functions.
pid_t tcgetsid (int fildes)
termios.h (Unix98): Terminal Access Functions.
TCSADRAIN
termios.h (POSIX.1): Mode Functions.
TCSAFLUSH
termios.h (POSIX.1): Mode Functions.
TCSANOW
termios.h (POSIX.1): Mode Functions.
TCSASOFT
termios.h (BSD): Mode Functions.
int tcsendbreak (int filedes, int duration)
termios.h (POSIX.1): Line Control.
int tcsetattr (int filedes, int when, const struct termios *termios-p)
termios.h (POSIX.1): Mode Functions.
int tcsetpgrp (int filedes, pid_t pgid)
unistd.h (POSIX.1): Terminal Access Functions.
void * tdelete (const void *key, void **rootp, comparison_fn_t compar)
search.h (SVID): Tree Search Function.
void tdestroy (void *vroot, __free_fn_t freefct)
search.h (GNU): Tree Search Function.
TEMP_FAILURE_RETRY (expression)
unistd.h (GNU): Interrupted Primitives.
char * tempnam (const char *dir, const char *prefix)
stdio.h (SVID): Temporary Files.
char * textdomain (const char *domainname)
libintl.h (GNU): Locating gettext catalog.
void * tfind (const void *key, void *const *rootp, comparison_fn_t compar)
search.h (SVID): Tree Search Function.
double tgamma (double x)
math.h (XPG, ISO): Special Functions.
float tgammaf (float x)
math.h (XPG, ISO): Special Functions.
long double tgammal (long double x)
math.h (XPG, ISO): Special Functions.
time_t time (time_t *result)
time.h (ISO): Simple Calendar Time.
time_t timegm (struct tm *brokentime)
time.h (???): Broken-down Time.
time_t timelocal (struct tm *brokentime)
time.h (???): Broken-down Time.
clock_t times (struct tms *buffer)
sys/times.h (POSIX.1): Processor Time.
time_t
time.h (ISO): Simple Calendar Time.
long int timezone
time.h (SVID): Time Zone Functions.
FILE * tmpfile (void)
stdio.h (ISO): Temporary Files.
FILE * tmpfile64 (void)
stdio.h (Unix98): Temporary Files.
int TMP_MAX
stdio.h (ISO): Temporary Files.
char * tmpnam (char *result)
stdio.h (ISO): Temporary Files.
char * tmpnam_r (char *result)
stdio.h (GNU): Temporary Files.
int toascii (int c)
ctype.h (SVID, BSD): Case Conversion.
int tolower (int c)
ctype.h (ISO): Case Conversion.
int _tolower (int c)
ctype.h (SVID): Case Conversion.
tcflag_t TOSTOP
termios.h (POSIX.1): Local Modes.
int toupper (int c)
ctype.h (ISO): Case Conversion.
int _toupper (int c)
ctype.h (SVID): Case Conversion.
wint_t towctrans (wint_t wc, wctrans_t desc)
wctype.h (ISO): Wide Character Case Conversion.
wint_t towlower (wint_t wc)
wctype.h (ISO): Wide Character Case Conversion.
wint_t towupper (wint_t wc)
wctype.h (ISO): Wide Character Case Conversion.
double trunc (double x)
math.h (ISO): Rounding Functions.
int truncate (const char *filename, off_t length)
unistd.h (X/Open): File Size.
int truncate64 (const char *name, off64_t length)
unistd.h (Unix98): File Size.
float truncf (float x)
math.h (ISO): Rounding Functions.
long double truncl (long double x)
math.h (ISO): Rounding Functions.
TRY_AGAIN
netdb.h (BSD): Host Names.
void * tsearch (const void *key, void **rootp, comparison_fn_t compar)
search.h (SVID): Tree Search Function.
char * ttyname (int filedes)
unistd.h (POSIX.1): Is It a Terminal.
int ttyname_r (int filedes, char *buf, size_t len)
unistd.h (POSIX.1): Is It a Terminal.
void twalk (const void *root, __action_fn_t action)
search.h (SVID): Tree Search Function.
char * tzname [2]
time.h (POSIX.1): Time Zone Functions.
int TZNAME_MAX
limits.h (POSIX.1): General Limits.
void tzset (void)
time.h (POSIX.1): Time Zone Functions.
UCHAR_MAX
limits.h (ISO): Range of Type.
ucontext_t
ucontext.h (SVID): System V contexts.
uid_t
sys/types.h (POSIX.1): Reading Persona.
UINT_MAX
limits.h (ISO): Range of Type.
int ulimit (int cmd, ...)
ulimit.h (BSD): Limits on Resources.
ULONG_LONG_MAX
limits.h (ISO): Range of Type.
ULONG_MAX
limits.h (ISO): Range of Type.
mode_t umask (mode_t mask)
sys/stat.h (POSIX.1): Setting Permissions.
int umount (const char *file)
sys/mount.h (SVID, GNU): Mount-Unmount-Remount.
int umount2 (const char *file, int flags)
sys/mount.h (GNU): Mount-Unmount-Remount.
int uname (struct utsname *info)
sys/utsname.h (POSIX.1): Platform Type.
int ungetc (int c, FILE *stream)
stdio.h (ISO): How Unread.
wint_t ungetwc (wint_t wc, FILE *stream)
wchar.h (ISO): How Unread.
union wait
sys/wait.h (BSD): BSD Wait Functions.
int unlink (const char *filename)
unistd.h (POSIX.1): Deleting Files.
int unlockpt (int filedes)
stdlib.h (SVID, XPG4.2): Allocation.
int unsetenv (const char *name)
stdlib.h (BSD): Environment Access.
void updwtmp (const char *wtmp_file, const struct utmp *utmp)
utmp.h (SVID): Manipulating the Database.
USER_PROCESS
utmp.h (SVID): Manipulating the Database.
USER_PROCESS
utmpx.h (XPG4.2): XPG Functions.
USHRT_MAX
limits.h (ISO): Range of Type.
int utime (const char *filename, const struct utimbuf *times)
time.h (POSIX.1): File Times.
int utimes (const char *filename, struct timeval tvp[2])
sys/time.h (BSD): File Times.
int utmpname (const char *file)
utmp.h (SVID): Manipulating the Database.
int utmpxname (const char *file)
utmpx.h (XPG4.2): XPG Functions.
va_alist
varargs.h (Unix): Old Varargs.
type va_arg (va_list ap, type)
stdarg.h (ISO): Argument Macros.
void __va_copy (va_list dest, va_list src)
stdarg.h (GNU): Argument Macros.
va_dcl
varargs.h (Unix): Old Varargs.
void va_end (va_list ap)
stdarg.h (ISO): Argument Macros.
va_list
stdarg.h (ISO): Argument Macros.
void * valloc (size_t size)
malloc.h, stdlib.h (BSD): Aligned Memory Blocks.
int vasprintf (char **ptr, const char *template, va_list ap)
stdio.h (GNU): Variable Arguments Output.
void va_start (va_list ap)
varargs.h (Unix): Old Varargs.
void va_start (va_list ap, last-required)
stdarg.h (ISO): Argument Macros.
int VDISCARD
termios.h (BSD): Other Special.
int VDSUSP
termios.h (BSD): Signal Characters.
int VEOF
termios.h (POSIX.1): Editing Characters.
int VEOL
termios.h (POSIX.1): Editing Characters.
int VEOL2
termios.h (BSD): Editing Characters.
int VERASE
termios.h (POSIX.1): Editing Characters.
void verr (int status, const char *format, va_list)
err.h (BSD): Error Messages.
void verrx (int status, const char *format, va_list)
err.h (BSD): Error Messages.
int versionsort (const void *a, const void *b)
dirent.h (GNU): Scanning Directory Content.
int versionsort64 (const void *a, const void *b)
dirent.h (GNU): Scanning Directory Content.
pid_t vfork (void)
unistd.h (BSD): Creating a Process.
int vfprintf (FILE *stream, const char *template, va_list ap)
stdio.h (ISO): Variable Arguments Output.
int vfscanf (FILE *stream, const char *template, va_list ap)
stdio.h (ISO): Variable Arguments Input.
int vfwprintf (FILE *stream, const wchar_t *template, va_list ap)
wchar.h (ISO): Variable Arguments Output.
int vfwscanf (FILE *stream, const wchar_t *template, va_list ap)
wchar.h (ISO): Variable Arguments Input.
int VINTR
termios.h (POSIX.1): Signal Characters.
int VKILL
termios.h (POSIX.1): Editing Characters.
int vlimit (int resource, int limit)
sys/vlimit.h (BSD): Limits on Resources.
int VLNEXT
termios.h (BSD): Other Special.
int VMIN
termios.h (POSIX.1): Noncanonical Input.
void (*) error_print_progname (void)
error.h (GNU): Error Messages.
int vprintf (const char *template, va_list ap)
stdio.h (ISO): Variable Arguments Output.
int VQUIT
termios.h (POSIX.1): Signal Characters.
int VREPRINT
termios.h (BSD): Editing Characters.
int vscanf (const char *template, va_list ap)
stdio.h (ISO): Variable Arguments Input.
int vsnprintf (char *s, size_t size, const char *template, va_list ap)
stdio.h (GNU): Variable Arguments Output.
int vsprintf (char *s, const char *template, va_list ap)
stdio.h (ISO): Variable Arguments Output.
int vsscanf (const char *s, const char *template, va_list ap)
stdio.h (ISO): Variable Arguments Input.
int VSTART
termios.h (POSIX.1): Start/Stop Characters.
int VSTATUS
termios.h (BSD): Other Special.
int VSTOP
termios.h (POSIX.1): Start/Stop Characters.
int VSUSP
termios.h (POSIX.1): Signal Characters.
int vswprintf (wchar_t *s, size_t size, const wchar_t *template, va_list ap)
wchar.h (GNU): Variable Arguments Output.
int vswscanf (const wchar_t *s, const wchar_t *template, va_list ap)
wchar.h (ISO): Variable Arguments Input.
void vsyslog (int facility_priority, char *format, va_list arglist)
syslog.h (BSD): syslog; vsyslog.
int VTIME
termios.h (POSIX.1): Noncanonical Input.
int vtimes (struct vtimes current, struct vtimes child)
vtimes.h (vtimes.h): Resource Usage.
void vwarn (const char *format, va_list)
err.h (BSD): Error Messages.
void vwarnx (const char *format, va_list)
err.h (BSD): Error Messages.
int VWERASE
termios.h (BSD): Editing Characters.
int vwprintf (const wchar_t *template, va_list ap)
wchar.h (ISO): Variable Arguments Output.
int vwscanf (const wchar_t *template, va_list ap)
wchar.h (ISO): Variable Arguments Input.
pid_t wait (int *status-ptr)
sys/wait.h (POSIX.1): Process Completion.
pid_t wait3 (union wait *status-ptr, int options, struct rusage *usage)
sys/wait.h (BSD): BSD Wait Functions.
pid_t wait4 (pid_t pid, int *status-ptr, int options, struct rusage *usage)
sys/wait.h (BSD): Process Completion.
pid_t waitpid (pid_t pid, int *status-ptr, int options)
sys/wait.h (POSIX.1): Process Completion.
void warn (const char *format, ...)
err.h (BSD): Error Messages.
void warnx (const char *format, ...)
err.h (BSD): Error Messages.
WCHAR_MAX
limits.h (GNU): Range of Type.
wint_t WCHAR_MAX
wchar.h (ISO): Extended Char Intro.
wint_t WCHAR_MIN
wchar.h (ISO): Extended Char Intro.
wchar_t
stddef.h (ISO): Extended Char Intro.
int WCOREDUMP (int status)
sys/wait.h (BSD): Process Completion Status.
wchar_t * wcpcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom)
wchar.h (GNU): Copying and Concatenation.
wchar_t * wcpncpy (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size)
wchar.h (GNU): Copying and Concatenation.
size_t wcrtomb (char *restrict s, wchar_t wc, mbstate_t *restrict ps)
wchar.h (ISO): Converting a Character.
int wcscasecmp (const wchar_t *ws1, const wchar_T *ws2)
wchar.h (GNU): String/Array Comparison.
wchar_t * wcscat (wchar_t *restrict wto, const wchar_t *restrict wfrom)
wchar.h (ISO): Copying and Concatenation.
wchar_t * wcschr (const wchar_t *wstring, int wc)
wchar.h (ISO): Search Functions.
wchar_t * wcschrnul (const wchar_t *wstring, wchar_t wc)
wchar.h (GNU): Search Functions.
int wcscmp (const wchar_t *ws1, const wchar_t *ws2)
wchar.h (ISO): String/Array Comparison.
int wcscoll (const wchar_t *ws1, const wchar_t *ws2)
wchar.h (ISO): Collation Functions.
wchar_t * wcscpy (wchar_t *restrict wto, const wchar_t *restrict wfrom)
wchar.h (ISO): Copying and Concatenation.
size_t wcscspn (const wchar_t *wstring, const wchar_t *stopset)
wchar.h (ISO): Search Functions.
wchar_t * wcsdup (const wchar_t *ws)
wchar.h (GNU): Copying and Concatenation.
size_t wcsftime (wchar_t *s, size_t size, const wchar_t *template, const struct tm *brokentime)
time.h (ISO/Amend1): Formatting Calendar Time.
size_t wcslen (const wchar_t *ws)
wchar.h (ISO): String Length.
int wcsncasecmp (const wchar_t *ws1, const wchar_t *s2, size_t n)
wchar.h (GNU): String/Array Comparison.
wchar_t * wcsncat (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size)
wchar.h (ISO): Copying and Concatenation.
int wcsncmp (const wchar_t *ws1, const wchar_t *ws2, size_t size)
wchar.h (ISO): String/Array Comparison.
wchar_t * wcsncpy (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size)
wchar.h (ISO): Copying and Concatenation.
size_t wcsnlen (const wchar_t *ws, size_t maxlen)
wchar.h (GNU): String Length.
size_t wcsnrtombs (char *restrict dst, const wchar_t **restrict src, size_t nwc, size_t len, mbstate_t *restrict ps)
wchar.h (GNU): Converting Strings.
wchar_t * wcspbrk (const wchar_t *wstring, const wchar_t *stopset)
wchar.h (ISO): Search Functions.
wchar_t * wcsrchr (const wchar_t *wstring, wchar_t c)
wchar.h (ISO): Search Functions.
size_t wcsrtombs (char *restrict dst, const wchar_t **restrict src, size_t len, mbstate_t *restrict ps)
wchar.h (ISO): Converting Strings.
size_t wcsspn (const wchar_t *wstring, const wchar_t *skipset)
wchar.h (ISO): Search Functions.
wchar_t * wcsstr (const wchar_t *haystack, const wchar_t *needle)
wchar.h (ISO): Search Functions.
double wcstod (const wchar_t *restrict string, wchar_t **restrict tailptr)
wchar.h (ISO): Parsing of Floats.
float wcstof (const wchar_t *string, wchar_t **tailptr)
stdlib.h (ISO): Parsing of Floats.
intmax_t wcstoimax (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)
wchar.h (ISO): Parsing of Integers.
wchar_t * wcstok (wchar_t *newstring, const char *delimiters)
wchar.h (ISO): Finding Tokens in a String.
long int wcstol (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)
wchar.h (ISO): Parsing of Integers.
long double wcstold (const wchar_t *string, wchar_t **tailptr)
stdlib.h (ISO): Parsing of Floats.
long long int wcstoll (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)
wchar.h (ISO): Parsing of Integers.
size_t wcstombs (char *string, const wchar_t *wstring, size_t size)
stdlib.h (ISO): Non-reentrant String Conversion.
long long int wcstoq (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)
wchar.h (GNU): Parsing of Integers.
unsigned long int wcstoul (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)
wchar.h (ISO): Parsing of Integers.
unsigned long long int wcstoull (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)
wchar.h (ISO): Parsing of Integers.
uintmax_t wcstoumax (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)
wchar.h (ISO): Parsing of Integers.
unsigned long long int wcstouq (const wchar_t *restrict string, wchar_t **restrict tailptr, int base)
wchar.h (GNU): Parsing of Integers.
wchar_t * wcswcs (const wchar_t *haystack, const wchar_t *needle)
wchar.h (XPG): Search Functions.
size_t wcsxfrm (wchar_t *restrict wto, const wchar_t *wfrom, size_t size)
wchar.h (ISO): Collation Functions.
int wctob (wint_t c)
wchar.h (ISO): Converting a Character.
int wctomb (char *string, wchar_t wchar)
stdlib.h (ISO): Non-reentrant Character Conversion.
wctrans_t wctrans (const char *property)
wctype.h (ISO): Wide Character Case Conversion.
wctrans_t
wctype.h (ISO): Wide Character Case Conversion.
wctype_t wctype (const char *property)
wctype.h (ISO): Classification of Wide Characters.
wctype_t
wctype.h (ISO): Classification of Wide Characters.
int WEOF
wchar.h (ISO): EOF and Errors.
wint_t WEOF
wchar.h (ISO): Extended Char Intro.
int WEXITSTATUS (int status)
sys/wait.h (POSIX.1): Process Completion Status.
int WIFEXITED (int status)
sys/wait.h (POSIX.1): Process Completion Status.
int WIFSIGNALED (int status)
sys/wait.h (POSIX.1): Process Completion Status.
int WIFSTOPPED (int status)
sys/wait.h (POSIX.1): Process Completion Status.
wint_t
wchar.h (ISO): Extended Char Intro.
wchar_t * wmemchr (const wchar_t *block, wchar_t wc, size_t size)
wchar.h (ISO): Search Functions.
int wmemcmp (const wchar_t *a1, const wchar_t *a2, size_t size)
wcjar.h (ISO): String/Array Comparison.
wchar_t * wmemcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size)
wchar.h (ISO): Copying and Concatenation.
wchar_t * wmemmove (wchar *wto, const wchar_t *wfrom, size_t size)
wchar.h (ISO): Copying and Concatenation.
wchar_t * wmempcpy (wchar_t *restrict wto, const wchar_t *restrict wfrom, size_t size)
wchar.h (GNU): Copying and Concatenation.
wchar_t * wmemset (wchar_t *block, wchar_t wc, size_t size)
wchar.h (ISO): Copying and Concatenation.
int W_OK
unistd.h (POSIX.1): Testing File Access.
int wordexp (const char *words, wordexp_t *word-vector-ptr, int flags)
wordexp.h (POSIX.2): Calling Wordexp.
wordexp_t
wordexp.h (POSIX.2): Calling Wordexp.
void wordfree (wordexp_t *word-vector-ptr)
wordexp.h (POSIX.2): Calling Wordexp.
int wprintf (const wchar_t *template, ...)
wchar.h (ISO): Formatted Output Functions.
WRDE_APPEND
wordexp.h (POSIX.2): Flags for Wordexp.
WRDE_BADCHAR
wordexp.h (POSIX.2): Calling Wordexp.
WRDE_BADVAL
wordexp.h (POSIX.2): Calling Wordexp.
WRDE_CMDSUB
wordexp.h (POSIX.2): Calling Wordexp.
WRDE_DOOFFS
wordexp.h (POSIX.2): Flags for Wordexp.
WRDE_NOCMD
wordexp.h (POSIX.2): Flags for Wordexp.
WRDE_NOSPACE
wordexp.h (POSIX.2): Calling Wordexp.
WRDE_REUSE
wordexp.h (POSIX.2): Flags for Wordexp.
WRDE_SHOWERR
wordexp.h (POSIX.2): Flags for Wordexp.
WRDE_SYNTAX
wordexp.h (POSIX.2): Calling Wordexp.
WRDE_UNDEF
wordexp.h (POSIX.2): Flags for Wordexp.
ssize_t write (int filedes, const void *buffer, size_t size)
unistd.h (POSIX.1): I/O Primitives.
ssize_t writev (int filedes, const struct iovec *vector, int count)
sys/uio.h (BSD): Scatter-Gather.
int wscanf (const wchar_t *template, ...)
wchar.h (ISO): Formatted Input Functions.
int WSTOPSIG (int status)
sys/wait.h (POSIX.1): Process Completion Status.
int WTERMSIG (int status)
sys/wait.h (POSIX.1): Process Completion Status.
int X_OK
unistd.h (POSIX.1): Testing File Access.
_XOPEN_SOURCE
(X/Open): Feature Test Macros.
_XOPEN_SOURCE_EXTENDED
(X/Open): Feature Test Macros.
double y0 (double x)
math.h (SVID): Special Functions.
float y0f (float x)
math.h (SVID): Special Functions.
long double y0l (long double x)
math.h (SVID): Special Functions.
double y1 (double x)
math.h (SVID): Special Functions.
float y1f (float x)
math.h (SVID): Special Functions.
long double y1l (long double x)
math.h (SVID): Special Functions.
double yn (int n, double x)
math.h (SVID): Special Functions.
float ynf (int n, float x)
math.h (SVID): Special Functions.
long double ynl (int n, long double x)
math.h (SVID): Special Functions.

Next: , Previous: Library Summary, Up: Top

Appendix C Installing the GNU C Library

Before you do anything else, you should read the file FAQ located at the top level of the source tree. This file answers common questions and describes problems you may experience with compilation and installation. It is updated more frequently than this manual.

Features can be added to GNU Libc via add-on bundles. These are separate tar files, which you unpack into the top level of the source tree. Then you give configure the `--enable-add-ons' option to activate them, and they will be compiled into the library.

You will need recent versions of several GNU tools: definitely GCC and GNU Make, and possibly others. See Tools for Compilation, below.

Next: , Up: Installation

C.1 Configuring and compiling GNU Libc

GNU libc cannot be compiled in the source directory. You must build it in a separate build directory. For example, if you have unpacked the glibc sources in /src/gnu/glibc-2.4, create a directory /src/gnu/glibc-build to put the object files in. This allows removing the whole build directory in case an error occurs, which is the safest way to get a fresh start and should always be done.

From your object directory, run the shell script configure located at the top level of the source tree. In the scenario above, you'd type 

     $ ../glibc-2.4/configure args...

Please note that even though you're building in a separate build directory, the compilation needs to modify a few files in the source directory, especially some files in the manual subdirectory.

configure takes many options, but the only one that is usually mandatory is `--prefix'. This option tells configure where you want glibc installed. This defaults to /usr/local, but the normal setting to install as the standard system library is `--prefix=/usr' for GNU/Linux systems and `--prefix=' (an empty prefix) for GNU/Hurd systems.

It may also be useful to set the CC and CFLAGS variables in the environment when running configure. CC selects the C compiler that will be used, and CFLAGS sets optimization options for the compiler.

The following list describes all of the available options for configure:

`--prefix=directory'
Install machine-independent data files in subdirectories of directory. The default is to install in /usr/local.
`--exec-prefix=directory'
Install the library and other machine-dependent files in subdirectories of directory. The default is to the `--prefix' directory if that option is specified, or /usr/local otherwise.
`--with-headers=directory'
Look for kernel header files in directory, not /usr/include. Glibc needs information from the kernel's private header files. Glibc will normally look in /usr/include for them, but if you specify this option, it will look in DIRECTORY instead.

This option is primarily of use on a system where the headers in /usr/include come from an older version of glibc. Conflicts can occasionally happen in this case. Note that Linux libc5 qualifies as an older version of glibc. You can also use this option if you want to compile glibc with a newer set of kernel headers than the ones found in /usr/include.

`--enable-add-ons[=list]'
Specify add-on packages to include in the build. If this option is specified with no list, it enables all the add-on packages it finds in the main source directory; this is the default behavior. You may specify an explicit list of add-ons to use in list, separated by spaces or commas (if you use spaces, remember to quote them from the shell). Each add-on in list can be an absolute directory name or can be a directory name relative to the main source directory, or relative to the build directory (that is, the current working directory). For example, `--enable-add-ons=nptl,../glibc-libidn-2.4'.
`--enable-kernel=version'
This option is currently only useful on GNU/Linux systems. The version parameter should have the form X.Y.Z and describes the smallest version of the Linux kernel the generated library is expected to support. The higher the version number is, the less compatibility code is added, and the faster the code gets.
`--with-binutils=directory'
Use the binutils (assembler and linker) in directory, not the ones the C compiler would default to. You can use this option if the default binutils on your system cannot deal with all the constructs in the GNU C library. In that case, configure will detect the problem and suppress these constructs, so that the library will still be usable, but functionality may be lost—for example, you can't build a shared libc with old binutils.
`--without-fp'
Use this option if your computer lacks hardware floating-point support and your operating system does not emulate an FPU.

these

`--disable-shared'
Don't build shared libraries even if it is possible. Not all systems support shared libraries; you need ELF support and (currently) the GNU linker.
`--disable-profile'
Don't build libraries with profiling information. You may want to use this option if you don't plan to do profiling.
`--enable-omitfp'
Use maximum optimization for the normal (static and shared) libraries, and compile separate static libraries with debugging information and no optimization. We recommend not doing this. The extra optimization doesn't gain you much, it may provoke compiler bugs, and you won't be able to trace bugs through the C library.
`--disable-versioning'
Don't compile the shared libraries with symbol version information. Doing this will make the resulting library incompatible with old binaries, so it's not recommended.
`--enable-static-nss'
Compile static versions of the NSS (Name Service Switch) libraries. This is not recommended because it defeats the purpose of NSS; a program linked statically with the NSS libraries cannot be dynamically reconfigured to use a different name database.
`--without-tls'
By default the C library is built with support for thread-local storage if the used tools support it. By using `--without-tls' this can be prevented though there generally is no reason since it creates compatibility problems.
`--build=build-system'
`--host=host-system'
These options are for cross-compiling. If you specify both options and build-system is different from host-system, configure will prepare to cross-compile glibc from build-system to be used on host-system. You'll probably need the `--with-headers' option too, and you may have to override configure's selection of the compiler and/or binutils.

If you only specify `--host', configure will prepare for a native compile but use what you specify instead of guessing what your system is. This is most useful to change the CPU submodel. For example, if configure guesses your machine as i586-pc-linux-gnu but you want to compile a library for 386es, give `--host=i386-pc-linux-gnu' or just `--host=i386-linux' and add the appropriate compiler flags (`-mcpu=i386' will do the trick) to CFLAGS.

If you specify just `--build', configure will get confused.

To build the library and related programs, type make. This will produce a lot of output, some of which may look like errors from make but isn't. Look for error messages from make containing `***'. Those indicate that something is seriously wrong.

The compilation process can take a long time, depending on the configuration and the speed of your machine. Some complex modules may take a very long time to compile, as much as several minutes on slower machines. Do not panic if the compiler appears to hang.

If you want to run a parallel make, simply pass the `-j' option with an appropriate numeric parameter to make. You need a recent GNU make version, though.

To build and run test programs which exercise some of the library facilities, type make check. If it does not complete successfully, do not use the built library, and report a bug after verifying that the problem is not already known. See Reporting Bugs, for instructions on reporting bugs. Note that some of the tests assume they are not being run by root. We recommend you compile and test glibc as an unprivileged user.

Before reporting bugs make sure there is no problem with your system. The tests (and later installation) use some pre-existing files of the system such as /etc/passwd, /etc/nsswitch.conf and others. These files must all contain correct and sensible content.

To format the GNU C Library Reference Manual for printing, type make dvi. You need a working TeX installation to do this. The distribution already includes the on-line formatted version of the manual, as Info files. You can regenerate those with make info, but it shouldn't be necessary.

The library has a number of special-purpose configuration parameters which you can find in Makeconfig. These can be overwritten with the file configparms. To change them, create a configparms in your build directory and add values as appropriate for your system. The file is included and parsed by make and has to follow the conventions for makefiles.

It is easy to configure the GNU C library for cross-compilation by setting a few variables in configparms. Set CC to the cross-compiler for the target you configured the library for; it is important to use this same CC value when running configure, like this: `CC=target-gcc configure target'. Set BUILD_CC to the compiler to use for programs run on the build system as part of compiling the library. You may need to set AR and RANLIB to cross-compiling versions of ar and ranlib if the native tools are not configured to work with object files for the target you configured for.

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C.2 Installing the C Library

To install the library and its header files, and the Info files of the manual, type env LANGUAGE=C LC_ALL=C make install. This will build things, if necessary, before installing them; however, you should still compile everything first. If you are installing glibc as your primary C library, we recommend that you shut the system down to single-user mode first, and reboot afterward. This minimizes the risk of breaking things when the library changes out from underneath.

If you're upgrading from Linux libc5 or some other C library, you need to replace the /usr/include with a fresh directory before installing it. The new /usr/include should contain the Linux headers, but nothing else.

You must first build the library (`make'), optionally check it (`make check'), switch the include directories and then install (`make install'). The steps must be done in this order. Not moving the directory before install will result in an unusable mixture of header files from both libraries, but configuring, building, and checking the library requires the ability to compile and run programs against the old library.

If you are upgrading from a previous installation of glibc 2.0 or 2.1, `make install' will do the entire job. You do not need to remove the old includes – if you want to do so anyway you must then follow the order given above.

You may also need to reconfigure GCC to work with the new library. The easiest way to do that is to figure out the compiler switches to make it work again (`-Wl,--dynamic-linker=/lib/ld-linux.so.2' should work on GNU/Linux systems) and use them to recompile gcc. You can also edit the specs file (/usr/lib/gcc-lib/TARGET/VERSION/specs), but that is a bit of a black art.

You can install glibc somewhere other than where you configured it to go by setting the install_root variable on the command line for `make install'. The value of this variable is prepended to all the paths for installation. This is useful when setting up a chroot environment or preparing a binary distribution. The directory should be specified with an absolute file name.

Glibc 2.2 includes a daemon called nscd, which you may or may not want to run. nscd caches name service lookups; it can dramatically improve performance with NIS+, and may help with DNS as well.

One auxiliary program, /usr/libexec/pt_chown, is installed setuid root. This program is invoked by the grantpt function; it sets the permissions on a pseudoterminal so it can be used by the calling process. This means programs like xterm and screen do not have to be setuid to get a pty. (There may be other reasons why they need privileges.) If you are using a 2.1 or newer Linux kernel with the devptsfs or devfs filesystems providing pty slaves, you don't need this program; otherwise you do. The source for pt_chown is in login/programs/pt_chown.c.

After installation you might want to configure the timezone and locale installation of your system. The GNU C library comes with a locale database which gets configured with localedef. For example, to set up a German locale with name de_DE, simply issue the command `localedef -i de_DE -f ISO-8859-1 de_DE'. To configure all locales that are supported by glibc, you can issue from your build directory the command `make localedata/install-locales'.

To configure the locally used timezone, set the TZ environment variable. The script tzselect helps you to select the right value. As an example, for Germany, tzselect would tell you to use `TZ='Europe/Berlin''. For a system wide installation (the given paths are for an installation with `--prefix=/usr'), link the timezone file which is in /usr/share/zoneinfo to the file /etc/localtime. For Germany, you might execute `ln -s /usr/share/zoneinfo/Europe/Berlin /etc/localtime'.

Next: , Previous: Running make install, Up: Installation

C.3 Recommended Tools for Compilation

We recommend installing the following GNU tools before attempting to build the GNU C library:

If you change any of the configure.in files you will also need

and if you change any of the message translation files you will need

You may also need these packages if you upgrade your source tree using patches, although we try to avoid this.

Next: , Previous: Tools for Compilation, Up: Installation

C.4 Specific advice for GNU/Linux systems

If you are installing GNU libc on a GNU/Linux system, you need to have the header files from a 2.2 or newer kernel around for reference. For some architectures, like ia64, sh and hppa, you need at least headers from kernel 2.3.99 (sh and hppa) or 2.4.0 (ia64). You do not need to use that kernel, just have its headers where glibc can access at them. The easiest way to do this is to unpack it in a directory such as /usr/src/linux-2.2.1. In that directory, run `make config' and accept all the defaults. Then run `make include/linux/version.h'. Finally, configure glibc with the option `--with-headers=/usr/src/linux-2.2.1/include'. Use the most recent kernel you can get your hands on.

An alternate tactic is to unpack the 2.2 kernel and run `make config' as above; then, rename or delete /usr/include, create a new /usr/include, and make symbolic links of /usr/include/linux and /usr/include/asm into the kernel sources. You can then configure glibc with no special options. This tactic is recommended if you are upgrading from libc5, since you need to get rid of the old header files anyway.

After installing GNU libc, you may need to remove or rename /usr/include/linux and /usr/include/asm, and replace them with copies of include/linux and include/asm-$ARCHITECTURE taken from the Linux source package which supplied kernel headers for building the library. ARCHITECTURE will be the machine architecture for which the library was built, such as `i386' or `alpha'. You do not need to do this if you did not specify an alternate kernel header source using `--with-headers'. The intent here is that these directories should be copies of, not symlinks to, the kernel headers used to build the library.

Note that /usr/include/net and /usr/include/scsi should not be symlinks into the kernel sources. GNU libc provides its own versions of these files.

GNU/Linux expects some components of the libc installation to be in /lib and some in /usr/lib. This is handled automatically if you configure glibc with `--prefix=/usr'. If you set some other prefix or allow it to default to /usr/local, then all the components are installed there.

If you are upgrading from libc5, you need to recompile every shared library on your system against the new library for the sake of new code, but keep the old libraries around for old binaries to use. This is complicated and difficult. Consult the Glibc2 HOWTO at http://www.imaxx.net/~thrytis/glibc for details.

You cannot use nscd with 2.0 kernels, due to bugs in the kernel-side thread support. nscd happens to hit these bugs particularly hard, but you might have problems with any threaded program.

Previous: Linux, Up: Installation

C.5 Reporting Bugs

There are probably bugs in the GNU C library. There are certainly errors and omissions in this manual. If you report them, they will get fixed. If you don't, no one will ever know about them and they will remain unfixed for all eternity, if not longer.

It is a good idea to verify that the problem has not already been reported. Bugs are documented in two places: The file BUGS describes a number of well known bugs and the bug tracking system has a WWW interface at http://sources.redhat.com/bugzilla/. The WWW interface gives you access to open and closed reports. A closed report normally includes a patch or a hint on solving the problem.

To report a bug, first you must find it. With any luck, this will be the hard part. Once you've found a bug, make sure it's really a bug. A good way to do this is to see if the GNU C library behaves the same way some other C library does. If so, probably you are wrong and the libraries are right (but not necessarily). If not, one of the libraries is probably wrong. It might not be the GNU library. Many historical Unix C libraries permit things that we don't, such as closing a file twice.

If you think you have found some way in which the GNU C library does not conform to the ISO and POSIX standards (see Standards and Portability), that is definitely a bug. Report it!

Once you're sure you've found a bug, try to narrow it down to the smallest test case that reproduces the problem. In the case of a C library, you really only need to narrow it down to one library function call, if possible. This should not be too difficult.

The final step when you have a simple test case is to report the bug. Do this using the WWW interface to the bug database.

If you are not sure how a function should behave, and this manual doesn't tell you, that's a bug in the manual. Report that too! If the function's behavior disagrees with the manual, then either the library or the manual has a bug, so report the disagreement. If you find any errors or omissions in this manual, please report them to the bug database. If you refer to specific sections of the manual, please include the section names for easier identification.

Next: , Previous: Installation, Up: Top

Appendix D Library Maintenance

Next: , Up: Maintenance

D.1 Adding New Functions

The process of building the library is driven by the makefiles, which make heavy use of special features of GNU make. The makefiles are very complex, and you probably don't want to try to understand them. But what they do is fairly straightforward, and only requires that you define a few variables in the right places.

The library sources are divided into subdirectories, grouped by topic.

The string subdirectory has all the string-manipulation functions, math has all the mathematical functions, etc.

Each subdirectory contains a simple makefile, called Makefile, which defines a few make variables and then includes the global makefile Rules with a line like: 

     include ../Rules

The basic variables that a subdirectory makefile defines are:

subdir
The name of the subdirectory, for example stdio. This variable must be defined.
headers
The names of the header files in this section of the library, such as stdio.h.
routines
aux
The names of the modules (source files) in this section of the library. These should be simple names, such as `strlen' (rather than complete file names, such as strlen.c). Use routines for modules that define functions in the library, and aux for auxiliary modules containing things like data definitions. But the values of routines and aux are just concatenated, so there really is no practical difference.
tests
The names of test programs for this section of the library. These should be simple names, such as `tester' (rather than complete file names, such as tester.c). `make tests' will build and run all the test programs. If a test program needs input, put the test data in a file called test-program.input; it will be given to the test program on its standard input. If a test program wants to be run with arguments, put the arguments (all on a single line) in a file called test-program.args. Test programs should exit with zero status when the test passes, and nonzero status when the test indicates a bug in the library or error in building.
others
The names of “other” programs associated with this section of the library. These are programs which are not tests per se, but are other small programs included with the library. They are built by `make others'.
install-lib
install-data
install
Files to be installed by `make install'. Files listed in `install-lib' are installed in the directory specified by `libdir' in configparms or Makeconfig (see Installation). Files listed in install-data are installed in the directory specified by `datadir' in configparms or Makeconfig. Files listed in install are installed in the directory specified by `bindir' in configparms or Makeconfig.
distribute
Other files from this subdirectory which should be put into a distribution tar file. You need not list here the makefile itself or the source and header files listed in the other standard variables. Only define distribute if there are files used in an unusual way that should go into the distribution.
generated
Files which are generated by Makefile in this subdirectory. These files will be removed by `make clean', and they will never go into a distribution.
extra-objs
Extra object files which are built by Makefile in this subdirectory. This should be a list of file names like foo.o; the files will actually be found in whatever directory object files are being built in. These files will be removed by `make clean'. This variable is used for secondary object files needed to build others or tests.

Previous: Source Layout, Up: Maintenance

D.2 Porting the GNU C Library

The GNU C library is written to be easily portable to a variety of machines and operating systems. Machine- and operating system-dependent functions are well separated to make it easy to add implementations for new machines or operating systems. This section describes the layout of the library source tree and explains the mechanisms used to select machine-dependent code to use.

All the machine-dependent and operating system-dependent files in the library are in the subdirectory sysdeps under the top-level library source directory. This directory contains a hierarchy of subdirectories (see Hierarchy Conventions).

Each subdirectory of sysdeps contains source files for a particular machine or operating system, or for a class of machine or operating system (for example, systems by a particular vendor, or all machines that use IEEE 754 floating-point format). A configuration specifies an ordered list of these subdirectories. Each subdirectory implicitly appends its parent directory to the list. For example, specifying the list unix/bsd/vax is equivalent to specifying the list unix/bsd/vax unix/bsd unix. A subdirectory can also specify that it implies other subdirectories which are not directly above it in the directory hierarchy. If the file Implies exists in a subdirectory, it lists other subdirectories of sysdeps which are appended to the list, appearing after the subdirectory containing the Implies file. Lines in an Implies file that begin with a `#' character are ignored as comments. For example, unix/bsd/Implies contains: 

     # BSD has Internet-related things.
     unix/inet

and unix/Implies contains: 

     posix

So the final list is unix/bsd/vax unix/bsd unix/inet unix posix.

sysdeps has a “special” subdirectory called generic. It is always implicitly appended to the list of subdirectories, so you needn't put it in an Implies file, and you should not create any subdirectories under it intended to be new specific categories. generic serves two purposes. First, the makefiles do not bother to look for a system-dependent version of a file that's not in generic. This means that any system-dependent source file must have an analogue in generic, even if the routines defined by that file are not implemented on other platforms. Second, the generic version of a system-dependent file is used if the makefiles do not find a version specific to the system you're compiling for.

If it is possible to implement the routines in a generic file in machine-independent C, using only other machine-independent functions in the C library, then you should do so. Otherwise, make them stubs. A stub function is a function which cannot be implemented on a particular machine or operating system. Stub functions always return an error, and set errno to ENOSYS (Function not implemented). See Error Reporting. If you define a stub function, you must place the statement stub_warning(function), where function is the name of your function, after its definition; also, you must include the file <stub-tag.h> into your file. This causes the function to be listed in the installed <gnu/stubs.h>, and makes GNU ld warn when the function is used.

Some rare functions are only useful on specific systems and aren't defined at all on others; these do not appear anywhere in the system-independent source code or makefiles (including the generic directory), only in the system-dependent Makefile in the specific system's subdirectory.

If you come across a file that is in one of the main source directories (string, stdio, etc.), and you want to write a machine- or operating system-dependent version of it, move the file into sysdeps/generic and write your new implementation in the appropriate system-specific subdirectory. Note that if a file is to be system-dependent, it must not appear in one of the main source directories.

There are a few special files that may exist in each subdirectory of sysdeps:

Makefile
A makefile for this machine or operating system, or class of machine or operating system. This file is included by the library makefile Makerules, which is used by the top-level makefile and the subdirectory makefiles. It can change the variables set in the including makefile or add new rules. It can use GNU make conditional directives based on the variable `subdir' (see above) to select different sets of variables and rules for different sections of the library. It can also set the make variable `sysdep-routines', to specify extra modules to be included in the library. You should use `sysdep-routines' rather than adding modules to `routines' because the latter is used in determining what to distribute for each subdirectory of the main source tree.

Each makefile in a subdirectory in the ordered list of subdirectories to be searched is included in order. Since several system-dependent makefiles may be included, each should append to `sysdep-routines' rather than simply setting it: 

          sysdep-routines := $(sysdep-routines) foo bar

Subdirs
This file contains the names of new whole subdirectories under the top-level library source tree that should be included for this system. These subdirectories are treated just like the system-independent subdirectories in the library source tree, such as stdio and math.

Use this when there are completely new sets of functions and header files that should go into the library for the system this subdirectory of sysdeps implements. For example, sysdeps/unix/inet/Subdirs contains inet; the inet directory contains various network-oriented operations which only make sense to put in the library on systems that support the Internet.

configure
This file is a shell script fragment to be run at configuration time. The top-level configure script uses the shell . command to read the configure file in each system-dependent directory chosen, in order. The configure files are often generated from configure.in files using Autoconf.

A system-dependent configure script will usually add things to the shell variables `DEFS' and `config_vars'; see the top-level configure script for details. The script can check for `--with-package' options that were passed to the top-level configure. For an option `--with-package=value' configure sets the shell variable `with_package' (with any dashes in package converted to underscores) to value; if the option is just `--with-package' (no argument), then it sets `with_package' to `yes'.

configure.in
This file is an Autoconf input fragment to be processed into the file configure in this subdirectory. See Introduction, for a description of Autoconf. You should write either configure or configure.in, but not both. The first line of configure.in should invoke the m4 macro `GLIBC_PROVIDES'. This macro does several AC_PROVIDE calls for Autoconf macros which are used by the top-level configure script; without this, those macros might be invoked again unnecessarily by Autoconf.

That is the general system for how system-dependencies are isolated.

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D.2.1 Layout of the sysdeps Directory Hierarchy

A GNU configuration name has three parts: the CPU type, the manufacturer's name, and the operating system. configure uses these to pick the list of system-dependent directories to look for. If the `--nfp' option is not passed to configure, the directory machine/fpu is also used. The operating system often has a base operating system; for example, if the operating system is `Linux', the base operating system is `unix/sysv'. The algorithm used to pick the list of directories is simple: configure makes a list of the base operating system, manufacturer, CPU type, and operating system, in that order. It then concatenates all these together with slashes in between, to produce a directory name; for example, the configuration `i686-linux-gnu' results in unix/sysv/linux/i386/i686. configure then tries removing each element of the list in turn, so unix/sysv/linux and unix/sysv are also tried, among others. Since the precise version number of the operating system is often not important, and it would be very inconvenient, for example, to have identical irix6.2 and irix6.3 directories, configure tries successively less specific operating system names by removing trailing suffixes starting with a period.

As an example, here is the complete list of directories that would be tried for the configuration `i686-linux-gnu' (with the crypt and linuxthreads add-on): 

     sysdeps/i386/elf
     crypt/sysdeps/unix
     linuxthreads/sysdeps/unix/sysv/linux
     linuxthreads/sysdeps/pthread
     linuxthreads/sysdeps/unix/sysv
     linuxthreads/sysdeps/unix
     linuxthreads/sysdeps/i386/i686
     linuxthreads/sysdeps/i386
     linuxthreads/sysdeps/pthread/no-cmpxchg
     sysdeps/unix/sysv/linux/i386
     sysdeps/unix/sysv/linux
     sysdeps/gnu
     sysdeps/unix/common
     sysdeps/unix/mman
     sysdeps/unix/inet
     sysdeps/unix/sysv/i386/i686
     sysdeps/unix/sysv/i386
     sysdeps/unix/sysv
     sysdeps/unix/i386
     sysdeps/unix
     sysdeps/posix
     sysdeps/i386/i686
     sysdeps/i386/i486
     sysdeps/libm-i387/i686
     sysdeps/i386/fpu
     sysdeps/libm-i387
     sysdeps/i386
     sysdeps/wordsize-32
     sysdeps/ieee754
     sysdeps/libm-ieee754
     sysdeps/generic

Different machine architectures are conventionally subdirectories at the top level of the sysdeps directory tree. For example, sysdeps/sparc and sysdeps/m68k. These contain files specific to those machine architectures, but not specific to any particular operating system. There might be subdirectories for specializations of those architectures, such as sysdeps/m68k/68020. Code which is specific to the floating-point coprocessor used with a particular machine should go in sysdeps/machine/fpu.

There are a few directories at the top level of the sysdeps hierarchy that are not for particular machine architectures.

generic
As described above (see Porting), this is the subdirectory that every configuration implicitly uses after all others.
ieee754
This directory is for code using the IEEE 754 floating-point format, where the C type float is IEEE 754 single-precision format, and double is IEEE 754 double-precision format. Usually this directory is referred to in the Implies file in a machine architecture-specific directory, such as m68k/Implies.
libm-ieee754
This directory contains an implementation of a mathematical library usable on platforms which use IEEE 754 conformant floating-point arithmetic.
libm-i387
This is a special case. Ideally the code should be in sysdeps/i386/fpu but for various reasons it is kept aside.
posix
This directory contains implementations of things in the library in terms of POSIX.1 functions. This includes some of the POSIX.1 functions themselves. Of course, POSIX.1 cannot be completely implemented in terms of itself, so a configuration using just posix cannot be complete.
unix
This is the directory for Unix-like things. See Porting to Unix. unix implies posix. There are some special-purpose subdirectories of unix:
unix/common
This directory is for things common to both BSD and System V release 4. Both unix/bsd and unix/sysv/sysv4 imply unix/common.
unix/inet
This directory is for socket and related functions on Unix systems. unix/inet/Subdirs enables the inet top-level subdirectory. unix/common implies unix/inet.

mach
This is the directory for things based on the Mach microkernel from CMU (including the GNU operating system). Other basic operating systems (VMS, for example) would have their own directories at the top level of the sysdeps hierarchy, parallel to unix and mach.

Previous: Hierarchy Conventions, Up: Porting

D.2.2 Porting the GNU C Library to Unix Systems

Most Unix systems are fundamentally very similar. There are variations between different machines, and variations in what facilities are provided by the kernel. But the interface to the operating system facilities is, for the most part, pretty uniform and simple.

The code for Unix systems is in the directory unix, at the top level of the sysdeps hierarchy. This directory contains subdirectories (and subdirectory trees) for various Unix variants.

The functions which are system calls in most Unix systems are implemented in assembly code, which is generated automatically from specifications in files named syscalls.list. There are several such files, one in sysdeps/unix and others in its subdirectories. Some special system calls are implemented in files that are named with a suffix of `.S'; for example, _exit.S. Files ending in `.S' are run through the C preprocessor before being fed to the assembler.

These files all use a set of macros that should be defined in sysdep.h. The sysdep.h file in sysdeps/unix partially defines them; a sysdep.h file in another directory must finish defining them for the particular machine and operating system variant. See sysdeps/unix/sysdep.h and the machine-specific sysdep.h implementations to see what these macros are and what they should do.

The system-specific makefile for the unix directory (sysdeps/unix/Makefile) gives rules to generate several files from the Unix system you are building the library on (which is assumed to be the target system you are building the library for). All the generated files are put in the directory where the object files are kept; they should not affect the source tree itself. The files generated are ioctls.h, errnos.h, sys/param.h, and errlist.c (for the stdio section of the library).

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Appendix E Contributors to the GNU C Library

The GNU C library was written originally by Roland McGrath, and is currently maintained by Ulrich Drepper. Some parts of the library were contributed or worked on by other people.

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Appendix F Free Software Needs Free Documentation

The biggest deficiency in the free software community today is not in the software—it is the lack of good free documentation that we can include with the free software. Many of our most important programs do not come with free reference manuals and free introductory texts. Documentation is an essential part of any software package; when an important free software package does not come with a free manual and a free tutorial, that is a major gap. We have many such gaps today.

Consider Perl, for instance. The tutorial manuals that people normally use are non-free. How did this come about? Because the authors of those manuals published them with restrictive terms—no copying, no modification, source files not available—which exclude them from the free software world.

That wasn't the first time this sort of thing happened, and it was far from the last. Many times we have heard a GNU user eagerly describe a manual that he is writing, his intended contribution to the community, only to learn that he had ruined everything by signing a publication contract to make it non-free.

Free documentation, like free software, is a matter of freedom, not price. The problem with the non-free manual is not that publishers charge a price for printed copies—that in itself is fine. (The Free Software Foundation sells printed copies of manuals, too.) The problem is the restrictions on the use of the manual. Free manuals are available in source code form, and give you permission to copy and modify. Non-free manuals do not allow this.

The criteria of freedom for a free manual are roughly the same as for free software. Redistribution (including the normal kinds of commercial redistribution) must be permitted, so that the manual can accompany every copy of the program, both on-line and on paper.

Permission for modification of the technical content is crucial too. When people modify the software, adding or changing features, if they are conscientious they will change the manual too—so they can provide accurate and clear documentation for the modified program. A manual that leaves you no choice but to write a new manual to document a changed version of the program is not really available to our community.

Some kinds of limits on the way modification is handled are acceptable. For example, requirements to preserve the original author's copyright notice, the distribution terms, or the list of authors, are ok. It is also no problem to require modified versions to include notice that they were modified. Even entire sections that may not be deleted or changed are acceptable, as long as they deal with nontechnical topics (like this one). These kinds of restrictions are acceptable because they don't obstruct the community's normal use of the manual.

However, it must be possible to modify all the technical content of the manual, and then distribute the result in all the usual media, through all the usual channels. Otherwise, the restrictions obstruct the use of the manual, it is not free, and we need another manual to replace it.

Please spread the word about this issue. Our community continues to lose manuals to proprietary publishing. If we spread the word that free software needs free reference manuals and free tutorials, perhaps the next person who wants to contribute by writing documentation will realize, before it is too late, that only free manuals contribute to the free software community.

If you are writing documentation, please insist on publishing it under the GNU Free Documentation License or another free documentation license. Remember that this decision requires your approval—you don't have to let the publisher decide. Some commercial publishers will use a free license if you insist, but they will not propose the option; it is up to you to raise the issue and say firmly that this is what you want. If the publisher you are dealing with refuses, please try other publishers. If you're not sure whether a proposed license is free, write to licensing@gnu.org.

You can encourage commercial publishers to sell more free, copylefted manuals and tutorials by buying them, and particularly by buying copies from the publishers that paid for their writing or for major improvements. Meanwhile, try to avoid buying non-free documentation at all. Check the distribution terms of a manual before you buy it, and insist that whoever seeks your business must respect your freedom. Check the history of the book, and try reward the publishers that have paid or pay the authors to work on it.

The Free Software Foundation maintains a list of free documentation published by other publishers, at http://www.fsf.org/doc/other-free-books.html.

Next: , Previous: Free Manuals, Up: Top

Appendix G GNU Lesser General Public License

Version 2.1, February 1999
     Copyright © 1991, 1999 Free Software Foundation, Inc.
     59 Temple Place – Suite 330, Boston, MA 02111-1307, USA
     
     Everyone is permitted to copy and distribute verbatim copies
     of this license document, but changing it is not allowed.
     
     [This is the first released version of the Lesser GPL.  It also counts
     as the successor of the GNU Library Public License, version 2, hence the
     version number 2.1.]

G.0.1 Preamble

The licenses for most software are designed to take away your freedom to share and change it. By contrast, the GNU General Public Licenses are intended to guarantee your freedom to share and change free software—to make sure the software is free for all its users.

This license, the Lesser General Public License, applies to some specially designated software—typically libraries—of the Free Software Foundation and other authors who decide to use it. You can use it too, but we suggest you first think carefully about whether this license or the ordinary General Public License is the better strategy to use in any particular case, based on the explanations below.

When we speak of free software, we are referring to freedom of use, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for this service if you wish); that you receive source code or can get it if you want it; that you can change the software and use pieces of it in new free programs; and that you are informed that you can do these things.

To protect your rights, we need to make restrictions that forbid distributors to deny you these rights or to ask you to surrender these rights. These restrictions translate to certain responsibilities for you if you distribute copies of the library or if you modify it.

For example, if you distribute copies of the library, whether gratis or for a fee, you must give the recipients all the rights that we gave you. You must make sure that they, too, receive or can get the source code. If you link other code with the library, you must provide complete object files to the recipients, so that they can relink them with the library after making changes to the library and recompiling it. And you must show them these terms so they know their rights.

We protect your rights with a two-step method: (1) we copyright the library, and (2) we offer you this license, which gives you legal permission to copy, distribute and/or modify the library.

To protect each distributor, we want to make it very clear that there is no warranty for the free library. Also, if the library is modified by someone else and passed on, the recipients should know that what they have is not the original version, so that the original author's reputation will not be affected by problems that might be introduced by others.

Finally, software patents pose a constant threat to the existence of any free program. We wish to make sure that a company cannot effectively restrict the users of a free program by obtaining a restrictive license from a patent holder. Therefore, we insist that any patent license obtained for a version of the library must be consistent with the full freedom of use specified in this license.

Most GNU software, including some libraries, is covered by the ordinary GNU General Public License. This license, the GNU Lesser General Public License, applies to certain designated libraries, and is quite different from the ordinary General Public License. We use this license for certain libraries in order to permit linking those libraries into non-free programs.

When a program is linked with a library, whether statically or using a shared library, the combination of the two is legally speaking a combined work, a derivative of the original library. The ordinary General Public License therefore permits such linking only if the entire combination fits its criteria of freedom. The Lesser General Public License permits more lax criteria for linking other code with the library.

We call this license the Lesser General Public License because it does Less to protect the user's freedom than the ordinary General Public License. It also provides other free software developers Less of an advantage over competing non-free programs. These disadvantages are the reason we use the ordinary General Public License for many libraries. However, the Lesser license provides advantages in certain special circumstances.

For example, on rare occasions, there may be a special need to encourage the widest possible use of a certain library, so that it becomes a de-facto standard. To achieve this, non-free programs must be allowed to use the library. A more frequent case is that a free library does the same job as widely used non-free libraries. In this case, there is little to gain by limiting the free library to free software only, so we use the Lesser General Public License.

In other cases, permission to use a particular library in non-free programs enables a greater number of people to use a large body of free software. For example, permission to use the GNU C Library in non-free programs enables many more people to use the whole GNU operating system, as well as its variant, the GNU/Linux operating system.

Although the Lesser General Public License is Less protective of the users' freedom, it does ensure that the user of a program that is linked with the Library has the freedom and the wherewithal to run that program using a modified version of the Library.

The precise terms and conditions for copying, distribution and modification follow. Pay close attention to the difference between a “work based on the library” and a “work that uses the library”. The former contains code derived from the library, whereas the latter must be combined with the library in order to run.

  1. This License Agreement applies to any software library or other program which contains a notice placed by the copyright holder or other authorized party saying it may be distributed under the terms of this Lesser General Public License (also called “this License”). Each licensee is addressed as “you”.

    A “library” means a collection of software functions and/or data prepared so as to be conveniently linked with application programs (which use some of those functions and data) to form executables.

    The “Library”, below, refers to any such software library or work which has been distributed under these terms. A “work based on the Library” means either the Library or any derivative work under copyright law: that is to say, a work containing the Library or a portion of it, either verbatim or with modifications and/or translated straightforwardly into another language. (Hereinafter, translation is included without limitation in the term “modification”.)

    “Source code” for a work means the preferred form of the work for making modifications to it. For a library, complete source code means all the source code for all modules it contains, plus any associated interface definition files, plus the scripts used to control compilation and installation of the library.

    Activities other than copying, distribution and modification are not covered by this License; they are outside its scope. The act of running a program using the Library is not restricted, and output from such a program is covered only if its contents constitute a work based on the Library (independent of the use of the Library in a tool for writing it). Whether that is true depends on what the Library does and what the program that uses the Library does.

  2. You may copy and distribute verbatim copies of the Library's complete source code as you receive it, in any medium, provided that you conspicuously and appropriately publish on each copy an appropriate copyright notice and disclaimer of warranty; keep intact all the notices that refer to this License and to the absence of any warranty; and distribute a copy of this License along with the Library.

    You may charge a fee for the physical act of transferring a copy, and you may at your option offer warranty protection in exchange for a fee.

  3. You may modify your copy or copies of the Library or any portion of it, thus forming a work based on the Library, and copy and distribute such modifications or work under the terms of Section 1 above, provided that you also meet all of these conditions:
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    2. You must cause the files modified to carry prominent notices stating that you changed the files and the date of any change.
    3. You must cause the whole of the work to be licensed at no charge to all third parties under the terms of this License.
    4. If a facility in the modified Library refers to a function or a table of data to be supplied by an application program that uses the facility, other than as an argument passed when the facility is invoked, then you must make a good faith effort to ensure that, in the event an application does not supply such function or table, the facility still operates, and performs whatever part of its purpose remains meaningful.

      (For example, a function in a library to compute square roots has a purpose that is entirely well-defined independent of the application. Therefore, Subsection 2d requires that any application-supplied function or table used by this function must be optional: if the application does not supply it, the square root function must still compute square roots.)

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    Thus, it is not the intent of this section to claim rights or contest your rights to work written entirely by you; rather, the intent is to exercise the right to control the distribution of derivative or collective works based on the Library.

    In addition, mere aggregation of another work not based on the Library with the Library (or with a work based on the Library) on a volume of a storage or distribution medium does not bring the other work under the scope of this License.

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    Once this change is made in a given copy, it is irreversible for that copy, so the ordinary GNU General Public License applies to all subsequent copies and derivative works made from that copy.

    This option is useful when you wish to copy part of the code of the Library into a program that is not a library.

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  8. You may place library facilities that are a work based on the Library side-by-side in a single library together with other library facilities not covered by this License, and distribute such a combined library, provided that the separate distribution of the work based on the Library and of the other library facilities is otherwise permitted, and provided that you do these two things:
    1. Accompany the combined library with a copy of the same work based on the Library, uncombined with any other library facilities. This must be distributed under the terms of the Sections above.
    2. Give prominent notice with the combined library of the fact that part of it is a work based on the Library, and explaining where to find the accompanying uncombined form of the same work.
  9. You may not copy, modify, sublicense, link with, or distribute the Library except as expressly provided under this License. Any attempt otherwise to copy, modify, sublicense, link with, or distribute the Library is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance.
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  11. Each time you redistribute the Library (or any work based on the Library), the recipient automatically receives a license from the original licensor to copy, distribute, link with or modify the Library subject to these terms and conditions. You may not impose any further restrictions on the recipients' exercise of the rights granted herein. You are not responsible for enforcing compliance by third parties with this License.
  12. If, as a consequence of a court judgment or allegation of patent infringement or for any other reason (not limited to patent issues), conditions are imposed on you (whether by court order, agreement or otherwise) that contradict the conditions of this License, they do not excuse you from the conditions of this License. If you cannot distribute so as to satisfy simultaneously your obligations under this License and any other pertinent obligations, then as a consequence you may not distribute the Library at all. For example, if a patent license would not permit royalty-free redistribution of the Library by all those who receive copies directly or indirectly through you, then the only way you could satisfy both it and this License would be to refrain entirely from distribution of the Library.

    If any portion of this section is held invalid or unenforceable under any particular circumstance, the balance of the section is intended to apply, and the section as a whole is intended to apply in other circumstances.

    It is not the purpose of this section to induce you to infringe any patents or other property right claims or to contest validity of any such claims; this section has the sole purpose of protecting the integrity of the free software distribution system which is implemented by public license practices. Many people have made generous contributions to the wide range of software distributed through that system in reliance on consistent application of that system; it is up to the author/donor to decide if he or she is willing to distribute software through any other system and a licensee cannot impose that choice.

    This section is intended to make thoroughly clear what is believed to be a consequence of the rest of this License.

  13. If the distribution and/or use of the Library is restricted in certain countries either by patents or by copyrighted interfaces, the original copyright holder who places the Library under this License may add an explicit geographical distribution limitation excluding those countries, so that distribution is permitted only in or among countries not thus excluded. In such case, this License incorporates the limitation as if written in the body of this License.
  14. The Free Software Foundation may publish revised and/or new versions of the Lesser General Public License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns.

    Each version is given a distinguishing version number. If the Library specifies a version number of this License which applies to it and “any later version”, you have the option of following the terms and conditions either of that version or of any later version published by the Free Software Foundation. If the Library does not specify a license version number, you may choose any version ever published by the Free Software Foundation.

  15. If you wish to incorporate parts of the Library into other free programs whose distribution conditions are incompatible with these, write to the author to ask for permission. For software which is copyrighted by the Free Software Foundation, write to the Free Software Foundation; we sometimes make exceptions for this. Our decision will be guided by the two goals of preserving the free status of all derivatives of our free software and of promoting the sharing and reuse of software generally.
  16. BECAUSE THE LIBRARY IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY FOR THE LIBRARY, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES PROVIDE THE LIBRARY “AS IS” WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE LIBRARY IS WITH YOU. SHOULD THE LIBRARY PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, REPAIR OR CORRECTION.
  17. IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR REDISTRIBUTE THE LIBRARY AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING OUT OF THE USE OR INABILITY TO USE THE LIBRARY (INCLUDING BUT NOT LIMITED TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY YOU OR THIRD PARTIES OR A FAILURE OF THE LIBRARY TO OPERATE WITH ANY OTHER SOFTWARE), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.

G.0.2 How to Apply These Terms to Your New Libraries

If you develop a new library, and you want it to be of the greatest possible use to the public, we recommend making it free software that everyone can redistribute and change. You can do so by permitting redistribution under these terms (or, alternatively, under the terms of the ordinary General Public License).

To apply these terms, attach the following notices to the library. It is safest to attach them to the start of each source file to most effectively convey the exclusion of warranty; and each file should have at least the “copyright” line and a pointer to where the full notice is found. 

     one line to give the library's name and an idea of what it does.
     Copyright (C) year  name of author
     
     This library is free software; you can redistribute it and/or modify it
     under the terms of the GNU Lesser General Public License as published by
     the Free Software Foundation; either version 2.1 of the License, or (at
     your option) any later version.
     
     This library is distributed in the hope that it will be useful, but
     WITHOUT ANY WARRANTY; without even the implied warranty of
     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
     Lesser General Public License for more details.
     
     You should have received a copy of the GNU Lesser General Public
     License along with this library; if not, write to the Free Software
     Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307,
     USA.

Also add information on how to contact you by electronic and paper mail.

You should also get your employer (if you work as a programmer) or your school, if any, to sign a “copyright disclaimer” for the library, if necessary. Here is a sample; alter the names: 

     Yoyodyne, Inc., hereby disclaims all copyright interest in the library
     `Frob' (a library for tweaking knobs) written by James Random Hacker.
     
     signature of Ty Coon, 1 April 1990
     Ty Coon, President of Vice

That's all there is to it!

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Appendix H GNU Free Documentation License

Version 1.1, March 2000
     Copyright © 2000 Free Software Foundation, Inc.
     59 Temple Place, Suite 330, Boston, MA  02111-1307, USA
     
     Everyone is permitted to copy and distribute verbatim copies
     of this license document, but changing it is not allowed.
  1. PREAMBLE

    The purpose of this License is to make a manual, textbook, or other written document free in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.

    This License is a kind of “copyleft”, which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.

    We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.

  2. APPLICABILITY AND DEFINITIONS

    This License applies to any manual or other work that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. The “Document”, below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as “you”.

    A “Modified Version” of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/or translated into another language.

    A “Secondary Section” is a named appendix or a front-matter section of the Document that deals exclusively with the relationship of the publishers or authors of the Document to the Document's overall subject (or to related matters) and contains nothing that could fall directly within that overall subject. (For example, if the Document is in part a textbook of mathematics, a Secondary Section may not explain any mathematics.) The relationship could be a matter of historical connection with the subject or with related matters, or of legal, commercial, philosophical, ethical or political position regarding them.

    The “Invariant Sections” are certain Secondary Sections whose titles are designated, as being those of Invariant Sections, in the notice that says that the Document is released under this License.

    The “Cover Texts” are certain short passages of text that are listed, as Front-Cover Texts or Back-Cover Texts, in the notice that says that the Document is released under this License.

    A “Transparent” copy of the Document means a machine-readable copy, represented in a format whose specification is available to the general public, whose contents can be viewed and edited directly and straightforwardly with generic text editors or (for images composed of pixels) generic paint programs or (for drawings) some widely available drawing editor, and that is suitable for input to text formatters or for automatic translation to a variety of formats suitable for input to text formatters. A copy made in an otherwise Transparent file format whose markup has been designed to thwart or discourage subsequent modification by readers is not Transparent. A copy that is not “Transparent” is called “Opaque”.

    Examples of suitable formats for Transparent copies include plain ascii without markup, Texinfo input format, LaTeX input format, SGML or XML using a publicly available DTD, and standard-conforming simple HTML designed for human modification. Opaque formats include PostScript, PDF, proprietary formats that can be read and edited only by proprietary word processors, SGML or XML for which the DTD and/or processing tools are not generally available, and the machine-generated HTML produced by some word processors for output purposes only.

    The “Title Page” means, for a printed book, the title page itself, plus such following pages as are needed to hold, legibly, the material this License requires to appear in the title page. For works in formats which do not have any title page as such, “Title Page” means the text near the most prominent appearance of the work's title, preceding the beginning of the body of the text.

  3. VERBATIM COPYING

    You may copy and distribute the Document in any medium, either commercially or noncommercially, provided that this License, the copyright notices, and the license notice saying this License applies to the Document are reproduced in all copies, and that you add no other conditions whatsoever to those of this License. You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute. However, you may accept compensation in exchange for copies. If you distribute a large enough number of copies you must also follow the conditions in section 3.

    You may also lend copies, under the same conditions stated above, and you may publicly display copies.

  4. COPYING IN QUANTITY

    If you publish printed copies of the Document numbering more than 100, and the Document's license notice requires Cover Texts, you must enclose the copies in covers that carry, clearly and legibly, all these Cover Texts: Front-Cover Texts on the front cover, and Back-Cover Texts on the back cover. Both covers must also clearly and legibly identify you as the publisher of these copies. The front cover must present the full title with all words of the title equally prominent and visible. You may add other material on the covers in addition. Copying with changes limited to the covers, as long as they preserve the title of the Document and satisfy these conditions, can be treated as verbatim copying in other respects.

    If the required texts for either cover are too voluminous to fit legibly, you should put the first ones listed (as many as fit reasonably) on the actual cover, and continue the rest onto adjacent pages.

    If you publish or distribute Opaque copies of the Document numbering more than 100, you must either include a machine-readable Transparent copy along with each Opaque copy, or state in or with each Opaque copy a publicly-accessible computer-network location containing a complete Transparent copy of the Document, free of added material, which the general network-using public has access to download anonymously at no charge using public-standard network protocols. If you use the latter option, you must take reasonably prudent steps, when you begin distribution of Opaque copies in quantity, to ensure that this Transparent copy will remain thus accessible at the stated location until at least one year after the last time you distribute an Opaque copy (directly or through your agents or retailers) of that edition to the public.

    It is requested, but not required, that you contact the authors of the Document well before redistributing any large number of copies, to give them a chance to provide you with an updated version of the Document.

  5. MODIFICATIONS

    You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above, provided that you release the Modified Version under precisely this License, with the Modified Version filling the role of the Document, thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it. In addition, you must do these things in the Modified Version:

    1. Use in the Title Page (and on the covers, if any) a title distinct from that of the Document, and from those of previous versions (which should, if there were any, be listed in the History section of the Document). You may use the same title as a previous version if the original publisher of that version gives permission.
    2. List on the Title Page, as authors, one or more persons or entities responsible for authorship of the modifications in the Modified Version, together with at least five of the principal authors of the Document (all of its principal authors, if it has less than five).
    3. State on the Title page the name of the publisher of the Modified Version, as the publisher.
    4. Preserve all the copyright notices of the Document.
    5. Add an appropriate copyright notice for your modifications adjacent to the other copyright notices.
    6. Include, immediately after the copyright notices, a license notice giving the public permission to use the Modified Version under the terms of this License, in the form shown in the Addendum below.
    7. Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the Document's license notice.
    8. Include an unaltered copy of this License.
    9. Preserve the section entitled “History”, and its title, and add to it an item stating at least the title, year, new authors, and publisher of the Modified Version as given on the Title Page. If there is no section entitled “History” in the Document, create one stating the title, year, authors, and publisher of the Document as given on its Title Page, then add an item describing the Modified Version as stated in the previous sentence.
    10. Preserve the network location, if any, given in the Document for public access to a Transparent copy of the Document, and likewise the network locations given in the Document for previous versions it was based on. These may be placed in the “History” section. You may omit a network location for a work that was published at least four years before the Document itself, or if the original publisher of the version it refers to gives permission.
    11. In any section entitled “Acknowledgments” or “Dedications”, preserve the section's title, and preserve in the section all the substance and tone of each of the contributor acknowledgments and/or dedications given therein.
    12. Preserve all the Invariant Sections of the Document, unaltered in their text and in their titles. Section numbers or the equivalent are not considered part of the section titles.
    13. Delete any section entitled “Endorsements”. Such a section may not be included in the Modified Version.
    14. Do not retitle any existing section as “Endorsements” or to conflict in title with any Invariant Section.

    If the Modified Version includes new front-matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document, you may at your option designate some or all of these sections as invariant. To do this, add their titles to the list of Invariant Sections in the Modified Version's license notice. These titles must be distinct from any other section titles.

    You may add a section entitled “Endorsements”, provided it contains nothing but endorsements of your Modified Version by various parties—for example, statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard.

    You may add a passage of up to five words as a Front-Cover Text, and a passage of up to 25 words as a Back-Cover Text, to the end of the list of Cover Texts in the Modified Version. Only one passage of Front-Cover Text and one of Back-Cover Text may be added by (or through arrangements made by) any one entity. If the Document already includes a cover text for the same cover, previously added by you or by arrangement made by the same entity you are acting on behalf of, you may not add another; but you may replace the old one, on explicit permission from the previous publisher that added the old one.

    The author(s) and publisher(s) of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version.

  6. COMBINING DOCUMENTS

    You may combine the Document with other documents released under this License, under the terms defined in section 4 above for modified versions, provided that you include in the combination all of the Invariant Sections of all of the original documents, unmodified, and list them all as Invariant Sections of your combined work in its license notice.

    The combined work need only contain one copy of this License, and multiple identical Invariant Sections may be replaced with a single copy. If there are multiple Invariant Sections with the same name but different contents, make the title of each such section unique by adding at the end of it, in parentheses, the name of the original author or publisher of that section if known, or else a unique number. Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work.

    In the combination, you must combine any sections entitled “History” in the various original documents, forming one section entitled “History”; likewise combine any sections entitled “Acknowledgments”, and any sections entitled “Dedications”. You must delete all sections entitled “Endorsements.”

  7. COLLECTIONS OF DOCUMENTS

    You may make a collection consisting of the Document and other documents released under this License, and replace the individual copies of this License in the various documents with a single copy that is included in the collection, provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects.

    You may extract a single document from such a collection, and distribute it individually under this License, provided you insert a copy of this License into the extracted document, and follow this License in all other respects regarding verbatim copying of that document.

  8. AGGREGATION WITH INDEPENDENT WORKS

    A compilation of the Document or its derivatives with other separate and independent documents or works, in or on a volume of a storage or distribution medium, does not as a whole count as a Modified Version of the Document, provided no compilation copyright is claimed for the compilation. Such a compilation is called an “aggregate”, and this License does not apply to the other self-contained works thus compiled with the Document, on account of their being thus compiled, if they are not themselves derivative works of the Document.

    If the Cover Text requirement of section 3 is applicable to these copies of the Document, then if the Document is less than one quarter of the entire aggregate, the Document's Cover Texts may be placed on covers that surround only the Document within the aggregate. Otherwise they must appear on covers around the whole aggregate.

  9. TRANSLATION

    Translation is considered a kind of modification, so you may distribute translations of the Document under the terms of section 4. Replacing Invariant Sections with translations requires special permission from their copyright holders, but you may include translations of some or all Invariant Sections in addition to the original versions of these Invariant Sections. You may include a translation of this License provided that you also include the original English version of this License. In case of a disagreement between the translation and the original English version of this License, the original English version will prevail.

  10. TERMINATION

    You may not copy, modify, sublicense, or distribute the Document except as expressly provided for under this License. Any other attempt to copy, modify, sublicense or distribute the Document is void, and will automatically terminate your rights under this License. However, parties who have received copies, or rights, from you under this License will not have their licenses terminated so long as such parties remain in full compliance.

  11. FUTURE REVISIONS OF THIS LICENSE

    The Free Software Foundation may publish new, revised versions of the GNU Free Documentation License from time to time. Such new versions will be similar in spirit to the present version, but may differ in detail to address new problems or concerns. See http://www.gnu.org/copyleft/.

    Each version of the License is given a distinguishing version number. If the Document specifies that a particular numbered version of this License “or any later version” applies to it, you have the option of following the terms and conditions either of that specified version or of any later version that has been published (not as a draft) by the Free Software Foundation. If the Document does not specify a version number of this License, you may choose any version ever published (not as a draft) by the Free Software Foundation.

H.0.1 ADDENDUM: How to use this License for your documents

To use this License in a document you have written, include a copy of the License in the document and put the following copyright and license notices just after the title page: 

       Copyright (C)  year  your name.
       Permission is granted to copy, distribute and/or modify this document
       under the terms of the GNU Free Documentation License, Version 1.1
       or any later version published by the Free Software Foundation;
       with the Invariant Sections being list their titles, with the
       Front-Cover Texts being list, and with the Back-Cover Texts being list.
       A copy of the license is included in the section entitled ``GNU
       Free Documentation License''.

If you have no Invariant Sections, write “with no Invariant Sections” instead of saying which ones are invariant. If you have no Front-Cover Texts, write “no Front-Cover Texts” instead of “Front-Cover Texts being list”; likewise for Back-Cover Texts.

If your document contains nontrivial examples of program code, we recommend releasing these examples in parallel under your choice of free software license, such as the GNU General Public License, to permit their use in free software.

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Footnotes

[1] Additions are welcome. Send appropriate information to bug-glibc-manual@gnu.org.

[2] Actually, the terminal-specific functions are implemented with IOCTLs on many platforms.

[3] Now you might ask why this information is duplicated. The answer is that we want to make it possible to link directly with these shared objects.

[4] There is a second explanation: we were too lazy to change the Makefiles to allow the generation of shared objects not starting with lib but don't tell this to anybody.