#if defined in c
The special operator
definedis used in `#if' and `#elif' expressions to test whether a certain name is defined as a macro.
)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 MACROis precisely equivalent to
definedis 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 namesConditionals written like this:
__ns16000__is defined as a macro.
#if defined BUFSIZE && BUFSIZE >= 1024
can generally be simplified to justIf the
#if BUFSIZE >= 1024, since if
BUFSIZEis not defined, it will be interpreted as having the value zero.
definedoperator appears as a result of a macro expansion, the C standard says the behavior is undefined. GNU cpp treats it as a genuine
definedoperator and evaluates it normally.