#ifdef，#if defined，#if使用

#if defined BUFSIZE && BUFSIZE >= 1024

 can generally be simplified to just #if BUFSIZE >= 1024, since if BUFSIZE is not defined, it will be interpreted as having the value zero.

 

#if defined (__vax__) || defined (__ns16000__) would succeed if either of the names __vax__ or __ns16000__ is defined as a macro.