text: /**
   * @file 4_pre_processors.c
   * @author Bharath G
   * @brief  Preprocessor directives in C
   * @version 0.1
   * @date 2021-03-16
   * 
   * @copyright Copyright (c) 2021
   * 
  # Checklist
  * Role of preprocessor 
  * Special constants &directives
  * Macro examples,side effects 
  * Conditional inclusion directives
      #include
      #define
      #undef
      #ifdef
      #ifndef
      #if
      #else
      #elif
      #endif
      #error
      #pragma
      defined 
  
  * Header guard technique - include once only : #ifndef vs #pragma once 
  * Special constants: __FILE__,__LINE__, __FUNCTION__, __DATE__,__TIME__ 
  * Special directives : #line 
  * Special operators :##, #
  */