| defined

#if

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.