C Header Files and The C Preprocessor

Header files contain information to be shared among several C source files. The mechanism used for sharing header files is the C preprocessor. The header files for the standard C library are a good example of header files. We use a header file by having the C preprocessor include it in our source files. We very often begin a C source (program) file with:


#include <stdio.h>

The above line of code is an instruction to the preprocessor telling it to include the stdio.h standard library header file with the source file it sends to the C compiler.
Most headers contain function prototypes, type definitions, and/or macro definitions.
As C programmers we can make our own header files for use in our programs. For example, a header file named boolean.h might contain the following lines:

#define boolean int #define FALSE 0 #define TRUE !FALSE

Other files can access these macro definitions by using the directive:

#include "boolean.h"

Putting macro definitions in header files has several advantages:

We don't have to copy the definitions into each source file.
Changing the definition of a macro requires that we edit only a single header file.
We don't have to worry about the possibility that different source files might contain different versions of the macro.

Another common kind of header file contains declarations of the functions in a particular source file. Suppose we have a file demo.c which contains:

int f1(int a, int b) {...} /* for external use */ void f2(float x, int y) {...} /* for external use */ float f3(float x) {...} /* for internal use */ void f4(char c) {...} /* for internal use */

Declarations of f1 and f2 should be put in a header file named demo.h:

int f1(int a, int b); void f2(float x, int y);

Then demo.h should be included in every file in which f1 or f2 is called.