#define IDENTIFIER without a token

Question

What does the following statement mean:

#define FAHAD

I am familiar with the statements like:

#define FAHAD 1

Answer 1

it means that FAHAD is defined, you can later check if it's defined or not with:

#ifdef FAHAD
  //do something
#else
  //something else
#endif

Or:

#ifndef FAHAD //if not defined
//do something
#endif

A real life example use is to check if a function or a header is available for your platform, usually a build system will define macros to indicate that some functions or headers exist before actually compiling, for example this checks if signal.h is available:

#ifdef HAVE_SIGNAL_H
#   include 
#endif/*HAVE_SIGNAL_H*/

This checks if some function is available

#ifdef HAVE_SOME_FUNCTION
//use this function
#else
//else use another one
#endif