#ifdef -- Preprocessing Directive

Include code conditionally

The  preprocessing  directive #ifdef  checks  whether  identifier has  been
defined as a  macro name.  If identifier has been  defined as a macro, then
the preprocessor  includes group within the program, up  to the next #elif,
#else, or  #endif directive.  If identifier has  not been defined, however,
then group is skipped.

An #ifdef directive  can be followed by any number  of #elif directives, by
one #else directive, and must be followed by an #endif directive.

Example

For an example of using this directive in a program, see assert.

See Also

#elif,
#else,
#endif,
#if,
#ifndef,
C preprocessor,
defined
ANSI Standard, §6.8.1

© 2012 by Stephen A. Ness