defined is useful when you wish to test more than one macro for existence at once. For example,

     #if defined (__vax__) || defined (__ns16000__)

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

Conditionals written like this:

     #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 the defined operator appears as a result of a macro expansion, the C standard says the behavior is undefined. GNU cpp treats it as a genuine defined operator and evaluates it normally. It will warn wherever your code uses this feature if you use the command-line option -pedantic , since other compilers may handle it differently.