Question 10.6

I'm splitting up a program into multiple source files for the first time, and I'm wondering what to put in .c files and what to put in .h files. (What does ``.h'' mean, anyway?)

As a general rule, you should put these things in header (.h) files:

macro definitions (preprocessor #defines)
structure, union, and enumeration declarations
typedef declarations
external function declarations (see also question 1.11)
global variable declarations

It's especially important to put a declaration or definition in a header file when it will be shared between several other files. (In particular, never put external function prototypes in .c files. See also question 1.7.)

On the other hand, when a definition or declaration should remain private to one source file, it's fine to leave it there.