The preprocessor allows code to be compiled selectively, depending on some condition. For example, if we assume that the macro __unix__ is defined only when compiling for a UNIX system, and that the macro __windows__ is defined only when compiling for a Windows system, then we could provide a single piece of code containing two possible implementations depending on the intended target:

int file_exists(const char *name)
{
#if defined __unix__
  /* Use UNIX system calls to find out if the file exists. */
  . . .
#elif defined __windows__
  /* Use Windows system calls to find out if the file exists. */
  . . .
#else
  /* Don't know what to do - abort compilation. */
#error "No implementation for your platform."
#endif
}

The most common use of conditional compilation, though, is to prevent the declarations in a header file from being made more than once, should the file be inadvertently #included more than once:

/* in the file mydecls.h */
#if !defined(mydecls_header)
#define mydecls_header

typedef int myInteger;

#endif

You should routinely protect all your header files in this way.

Background

Preprocessing