26.6.2.3 The defined test

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 (__arm__) || defined (__PPC__)

would succeed if either of the names __arm__ or __PPC__ is defined as a macro—in other words, when compiling for ARM processors or PowerPC processors.

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.

In GCC, you can include defined as part of another macro definition, like this:

#define MACRO_DEFINED(X) defined X

#if MACRO_DEFINED(BUFSIZE)

which would expand the #if expression to:

#if defined BUFSIZE

Generating defined in this way is a GNU C extension.