4.2.3. Defined
4.2.3 Defined
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
