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. It will warn
wherever your code uses this feature if you use the command-line option
-pedantic, since other compilers may handle it differently.