C Preprocessors

Preprocessors process directives before compilation.

1. Include Directive

#include <stdio.h>    // system header
#include "myheader.h"  // user header

2. Define Directive

#define PI 3.14159
#define MAX(a,b) ((a) > (b) ? (a) : (b))

3. Conditional Compilation

#ifdef DEBUG
    printf("Debug mode\n");
#endif

#if VERSION == 1
    // version 1 code
#elif VERSION == 2
    // version 2 code
#endif

4. Undefine

#undef PI