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#### 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

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.