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C preprocessor directive

define

Also called macro subtitution. Create symbolic constants. Its scope start when declared and stop at the end of the file. #undef can be used to remove the macro.

#define NAME REPLACEMENT
// ..
#undef NAME

// example
#define PI 3.14159265


main()
{
  int a = PI * 2;
}

// more complex example

#define forever for(;;)

#define max(A, B) ((A) > (B) ? (A) : (B))

int x = max(1, 2)

When compiled, NAME is replace with REPLACEMENT.

include

// < > mostly for system files
#include <stdio.h>

// " " mostly for user's programs or files
#include "foo.h"

condional inclusion

Use #if, #elif, #else

#ifdef and #ifndef test if a name is defined or not.

the keyword defined(name) can also be used: #if defined(NAME)

// make sure that boo.h is included only once
#ifndef HDR

#define HDR

// hdr.h content

#endif

// decide which file to include
#if SYSTEM == SYSV
  #define HDR "sysv.h"
#elif SYSTEM == BSD
  #define HDR "bsd.h"
#else
  #define HDR "defaut.h"
#endif

#include HDR