1. Predefined Macros (examples)
GCC: __GNUC__
__STRICT_ANSI__ (When strict ANSI applied)
__linux__ (On linux systems)
__FreeBSD__ (Another variant of unix)
__hp9000s700__ (HP workstations)
CL (VC++): WIN32,
_WINDOWS,
_MSC_VER (Version number)
NDEBUG (Release version)
DEBUG, _DEBUG (Debug version)

2. Malloc / Free Macros #ifdef _DEBUG
VoidPtr XXXXMalloc(unsigned Size, char *ObjType,
char *FileName, int LineNum);
#define XXXXMalloc(x) XXXXMalloc((x), #x, \
__FILE__, __LINE__)
void XXXXFree(VoidPtr p);
#define XXXXFree(x) { XXXXFree(x); x = NULL; }
#else
#define XXXXMalloc(Size) malloc(Size)
#define XXXXFree(Ptr) free(Ptr)
#endif /* _DEBUG */

3. MACRO examples
# if defined(OSF1DEC) || defined(sgi) || defined(SUN4) || \
defined(OS2GCC) || defined(__FreeBSD__)
# ifdef OSF1DEC
# ifdef _XOPEN_SOURCE
# undef _XOPEN_SOURCE /* For usleep */
# endif /* _XOPEN_SOURCE */
# endif /* OSF1DEC */
# include <unistd.h>
# endif /* OSF1DEC || sgi || SUN4 || OS2GCC || FreeBSD */
# ifdef __WINNT__
# include <direct.h>
# if _MSC_VER >= /* Visual 8, 2005 */
# define mkdir(Dir, Permit) _mkdir(Dir)
# else
# define mkdir(Dir, Permit) mkdir(Dir)
# endif /* _MSC_VER >= 1400 */
# endif /* __WINNT__ */

4. memcpy C function
void *memcpy(void *dest, const void *src, int count);

5. qsort C function void qsort(void *base_array, int num_elem, size_elem,
int (*compare )(const void *, const void *));