define 的用法小结——摘自《THE PROGRAMMING LANGUAGE C》

1.make sure that the contents of a file are included only once.

#if !defined(HDR)
#define HDR

/* contents of hdr.h go here */

#endif

also we can write this way

#ifndef HDR
#define HDR

/* contents of hdr.h go here */

#endif

2.test the name SYSTEM to decide which version of a header to include

#if SYSTEM == SYSV
    #define HDR "sysv.h"
#elif SYSTEM == BSD
    #define HDR "bsd.h"
#elif SYSTEM == MSDOS
    #define HDR "msdos.h"
#else
    #define HDR "default.h"
#endif
#include HDR