When a large C program is split over several modules, code in one module might need to make references to named code in another, or two modules might need to refer to the same type declaration consistently. The usual way to achieve these is to precede the reference with a declaration that shows what the name means. Some example declarations:

/* This declares the type struct point. */
struct point {
  int x, y;
};

/* This declares the global variable errno. */
extern int errno;

/* this declares the function getchar. */
int getchar(void);

It would be tedious to repeat such declarations in each source file that requires them, particularly if they need to be modified as the program develops. Instead, these could be placed in a separate file (usually with a .h extension), and inserted automatically by the preprocessor when it encounters an #include directive embedded in the source code, for example:

#include "mydecls.h"

These header files are also preprocessed, and so may contain further #include (or other) directives.

Header files containing declarations for the standard library are also available to the preprocessor. These are normally accessed with a variant of the #include directive:

/* Include declarations for input/output routines. */
#include <stdio.h>

You should normally use the "" form for your own headers rather than <>.

Do not put definitions of functions or variables in header files — it may result in multiple definitions of the same name within one program, so linking will fail. Header files should normally only contain types, function prototypes, variable declarations, and macro definitions. Note that inline functions are exceptional.