Conditional Inclusion

4.11.3 ConditionalInclusion

It is possible to control preprocessing itself with conditionalstatements that are evaluated during preprocessing. This provides away to include code selectively, depending on the value ofconditions evaluated during compilation.

The #if line evaluates a constant integer expression(which may not include sizeof, casts, or enumconstants). If the expression is non-zero, subsequent lines untilan #endif or #elif or #else areincluded. (The preprocessor statement #elif is likeelse-if.) The expression defined(name)in a #if is 1 if the name has been defined, and 0otherwise.

For example, to make sure that the contents of a filehdr.h are included only once, the contents of the file aresurrounded with a conditional like this:

   #if !defined(HDR)
   #define HDR

   

   #endif

The first inclusion of hdr.h defines the nameHDR; subsequent inclusions will find the name defined andskip down to the #endif. A similar style can be used toavoid including files multiple times. If this style is usedconsistently, then each header can itself include any other headerson which it depends, without the user of the header having to dealwith the interdependence.

This sequence tests the name SYSTEM to decide whichversion of a header to include:

   #if SYSTEM == SYSV
       #define HDR "sysv.h"
   #elif SYSTEM == BSD
       #define HDR "bsd.h"
   #elif SYSTEM == MSDOS
       #define HDR "msdos.h"
   #else
       #define HDR "default.h"
   #endif
   #include HDR

The #ifdef and #ifndef lines are specializedforms that test whether a name is defined. The first example of#if above could have been written

   #ifndef HDR
   #define HDR

   

   #endif