Source file inclusion

The most common uses of #include preprocessing directives are as in the following:

#include <stdio.h>
#include "myheader.h"

#include replaces the statement with the contents of the file referred to. Angle brackets (<>) refer to header files installed on the system, while quotation marks (””) are for user-supplied files.

Macros themselves can expand other macros once, as this example illustrates:

#if VERSION == 1
    #define INCFILE  "vers1.h"
#elif VERSION == 2
    #define INCFILE  "vers2.h"
    /*  and so on */
#else
    #define INCFILE  "versN.h"
#endif
/* ... */
#include INCFILE