preprocessor in C Language

                                          Preprocessors
The C  Preprocessor is  not  part  of  the  compiler,  but  is  a  separate  step  in  the
compilation  process.  In  simplistic  terms,  a  C  Preprocessor  is  just  a  text  substitution  tool
and  they  instruct  compiler  to  do  required  pre-processing  before  actual  compilation.  We'll
refer to the C Preprocessor as the CPP.
All  preprocessor  commands  begin  with  a  pound  symbol  (#). It  must  be  the first  nonblank
character,  and  for  readability,  a  preprocessor  directive  should  begin  in  first  column.
Following section lists down all important preprocessor directives:

Directive                          -                                 Description
#define                                     Substitutes a preprocessor macro
#include                                    Inserts a particular header from another file
#undef                                     Undefines a preprocessor macro
#ifdef                                       Returns true if this macro is defined
#ifndef                                    Returns true if this macro is not defined
#if                                           Tests if a compile time condition is true
#else                                     The alternative for #if
#elif                                       #else an #if in one statement
#endif                                    Ends preprocessor conditional
#error                                    Prints error message on stderr
#pragma                                  Issues special commands to the compiler, using a standardized method