C - ifdef


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C - ifdef - #ifdef - Definition and Usage

  • If the value of expression is true, then the code that immediately follows the command will be compiled.
  • “#ifdef” directive checks whether the particular macro is defined or not.
C Preprocessors #ifdef

C Syntax

(#ifdef identifier newline );

Syntax explanation:

  • #ifdef directive checks whether the identifier is currently defined.
  • Identifiers can be defined by a #define directive or on the command line.
  • If such identifiers have not been subsequently undefined, they are considered currently defined.


  • If #ifdef directive is defined, “If” clause statements are included in source file. Otherwise, “else” clause statements are included in source file for compilation and execution..

C Syntax

#endif newline;


  • #endif always matches the nearest #ifdef (or #ifndef, or #if).
  • Also, we cannot start a conditional group in one file and end it in another.
  • The number of necessary #endif directives changes according to whether the elseif or #else directive is used.

C Syntax

(#ifdef identifier newline );

Sample - C Code

#include <stdio.h>
#define wikitechy 16
void main()
    #ifdef  wikitechy
    printf("Hello! welcome to wikitechy\n");

C Code - Explanation

  1. In C programming #include<stdio.h> specifies a statement which tells the compiler to insert the contents of stdio at that particular place. A header file is a C file, that typically ends in ".h".
  2. In this example, #define WIKITECHY 16 defines the constant name (WIKITECHY) with its value (16).
  3. In this example, #ifdef WIKITECHY specifies this directive checks whether the identifier(WIKITECHY) is currently defined, there by executing the statement “Hello! welcome to wikitechy”.
  4. The command #elseif simply truncates the if statement and prints the else statement www.wikitechy.com.
  5. In this example, #endif specifies the end of the program.

Sample Output - Programming Examples

  1. Here in this output the “Hello! welcome to wikitechy” is printed in the console window since the ifdef directive defines that the identifier(WIKITECHY) is currently defined.
  1. Here in this output , the ifdef directive doesn’t define the identifier(WIKITECHY) so the else statement prints the statement “www.wikitechy.com”.

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