totn C Language

C Language: #ifndef Directive

This C tutorial explains how to use the #ifndef preprocessor directive in the C language.

Description

In the C Programming Language, the #ifndef directive allows for conditional compilation. The preprocessor determines if the provided macro does not exist before including the subsequent code in the compilation process.

Syntax

The syntax for the #ifndef directive in the C language is:

#ifndef macro_definition
macro_definition
The macro definition that must not be defined for the preprocessor to include the C source code into the compiled application.

Note

Example

The following example shows how to use the #ifndef directive in the C language:

/* Example using #ifndef directive by TechOnTheNet.com */

#include <stdio.h>

#define YEARS_OLD 12
#ifndef YEARS_OLD
#define YEARS_OLD 10
#endif

int main()
{
   printf("TechOnTheNet is over %d years old.\n", YEARS_OLD);

   return 0;
}

In this example, if the macro YEARS_OLD is not defined before the #ifndef directive is encountered, it will be defined with a value of 10.

Here is the output of the executable program:

TechOnTheNet is over 12 years old.

If you remove the line #define YEARS_OLD 12, you will see the following output from the executable program:

TechOnTheNet is over 10 years old.
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