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
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
__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.
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 -Wpedantic, since other compilers may handle it differently. The warning is also enabled by -Wextra, and can also be enabled individually with -Wexpansion-to-defined.
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