stddef.h: #if defined (_STDDEF_H) || defined (__need_NULL) #undef NULL /* in case <stdio.h> has defined it. */ #ifdef __GNUG__ #define NULL __null #else /* G++ */ #ifndef __cplusplus #define NULL ((void *)0) #else /* C++ */ #define NULL 0 #endif /* C++ */ #endif /* G++ */ #endif /* NULL not defined and <stddef.h> or need NULL. */ #undef __need_NULL