#if -- Preprocessing Directive

Include code conditionally

The preprocessing  directive #if tells  the preprocessor that  if constant-
expression is  true (i.e., that  it evalutes to  a value other  than zero),
then include the following lines of  code within the program until it reads
the next #elif, #else, or #endif directive.

The  constant-expression must  be  an integral  expression,  and it  cannot
include a  sizeof operator, a cast, or an  enumeration constant.  All macro
substitutions  are  performed upon  the  constant-expression  before it  is
evaluated.  All integer constants are treated as long objects, and are then
evaluated.  If constant-expression includes character constants, all escape
sequences are converted into characters before evaluation.

If constant-expression  is an undefined symbol,  the preprocessor treats it
the same as it would a false statement.

See Also

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

© 2012 by Stephen A. Ness