#if defined in c

The special operator defined is used in `#if' and `#elif' expressions to test whether a certain name is defined as a macro. defined name and defined (name) are both expressions whose value is 1 if name is defined as a macro at the current point in the program, and 0 otherwise. Thus, #if defined MACRO is precisely equivalent to #ifdef MACRO.
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.


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