#if, #elif, #ifndef, #ifdef in C (Conditional Inclusion)

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It is possible to control preprocessing itself with conditional statements that are evaluated during preprocessing. This provides a way to include code selectively, depending on the value of conditions evaluated during compilation.

The #if line evaluates a constant integer expression (which may not include sizeof, casts, or enum constants). If the expression is non-zero, subsequent lines until an #endif or #elif or #else are included. (The preprocessor statement #elif is like else-if.) The expression defined(name) in a #if is 1 if the name has been defined, and 0 otherwise.

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

   #if !defined(HDR)
   #define HDR

   /* contents of hdr.h go here */

   #endif
The first inclusion of hdr.h defines the name HDR; subsequent inclusions will find the name defined and skip down to the #endif. A similar style can be used to avoid including files multiple times. If this style is used consistently, then each header can itself include any other headers on which it depends, without the user of the header having to deal with the interdependence.

This sequence tests the name SYSTEM to decide which version 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 specialized forms that test whether a name is defined. The first example of #if above could have been written
   #ifndef HDR
   #define HDR

   /* contents of hdr.h go here */

   #endif

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