The special operator
is used in ‘
’ and ‘
’ 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 MACRO
is precisely equivalent to
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
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
is not defined, it will be interpreted as having the value zero.
operator appears as a result of a macro expansion, the C standard says the behavior is undefined. GNU cpp treats it as a genuine
operator and evaluates it normally. It will warn wherever your code uses this feature if you use the command-line option
, since other compilers may handle it differently. The warning is also enabled by
, and can also be enabled individually with