Question 5.2

How do I get a null pointer in my programs?

According to the language definition, a constant 0 in a pointer context is converted into a null pointer at compile time. That is, in an initialization, assignment, or comparison when one side is a variable or expression of pointer type, the compiler can tell that a constant 0 on the other side requests a null pointer, and generate the correctly-typed null pointer value. Therefore, the following fragments are perfectly legal:

	char *p = 0;
	if(p != 0)

(See also question 5.3.)

However, an argument being passed to a function is not necessarily recognizable as a pointer context, and the compiler may not be able to tell that an unadorned 0 ``means'' a null pointer. To generate a null pointer in a function call context, an explicit cast may be required, to force the 0 to be recognized as a pointer. For example, the Unix system call execl takes a variable-length, null-pointer-terminated list of character pointer arguments, and is correctly called like this:

	execl("/bin/sh", "sh", "-c", "date", (char *)0);

If the (char *) cast on the last argument were omitted, the compiler would not know to pass a null pointer, and would pass an integer 0 instead. (Note that many Unix manuals get this example wrong .)

When function prototypes are in scope, argument passing becomes an ``assignment context,'' and most casts may safely be omitted, since the prototype tells the compiler that a pointer is required, and of which type, enabling it to correctly convert an unadorned 0. Function prototypes cannot provide the types for variable arguments in variable-length argument lists however, so explicit casts are still required for those arguments. (See also question 15.3.) It is safest to properly cast all null pointer constants in function calls: to guard against varargs functions or those without prototypes, to allow interim use of non-ANSI compilers, and to demonstrate that you know what you are doing. (Incidentally, it's also a simpler rule to remember.)

Summary:

	Unadorned 0 okay:	Explicit cast required:

	initialization		function call,
				no prototype in scope
	assignment
				variable argument in
	comparison		varargs function call

	function call,
	prototype in scope,
	fixed argument

References: K&R1 Sec. A7.7 p. 190, Sec. A7.14 p. 192
K&R2 Sec. A7.10 p. 207, Sec. A7.17 p. 209
ANSI Sec. 3.2.2.3
ISO Sec. 6.2.2.3
H&S Sec. 4.6.3 p. 95, Sec. 6.2.7 p. 171

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