text: 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. We'll refer to the C Preprocessor as the CPP.
  All preprocessor lines begin with #
  The unconditional directives are:
  #include -
  Analyze following examples to understand various directives.
  #define MAX_ARRAY_LENGTH 20
  Tells the CPP to replace instances of MAX_ARRAY_LENGTH with 20. Use #define for constants to increase readability.
  #include <stdio.h>
  #include "myheader.h"
  Tells the CPP to get stdio.h from System Libraries and add the text to this file. The next line tells CPP to get myheader.h from the local directory and add the text to the file.
  #undef FILE_SIZE
  #define FILE_SIZE 42
  Tells the CPP to undefine FILE_SIZE and define it for 42.
  #ifndef MESSAGE
  #define MESSAGE "You wish!"
  #endif
  Tells the CPP to define MESSAGE only if MESSAGE isn't defined already.
  #ifdef DEBUG
  /* Your debugging statements here */
  #endif
  Tells the CPP to do the following statements if DEBUG is defined. This is useful if you pass the -DDEBUG flag to gcc. This will define DEBUG, so you can turn debugging on and off on the fly!).