The preprocessor allows macros to be defined which serve a number of purposes:

• Some macros are used to hold constants or expressions:

  #define PI 3.14159
  
  double pi_twice = PI * 2;
  

  PI will be replaced by the numeric value wherever it is used.

• Some macros take arguments:

  #define MAX(A,B) ((A) > (B) ? (A) : (B))
  

  …that provide a convenient way to emulate functions without the overhead of a real function call. (See a good book on C for the limitations of this.)

• Some macros are merely defined to exist:

  #define JOB_DONE
  

  …and are used in conditional compilation.