cpp.info: Defined

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   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
   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.