C Language: #undef Directive

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

Description

In the C Programming Language, the #undef directive tells the preprocessor to remove all definitions for the specified macro. A macro can be redefined after it has been removed by the #undef directive.

Once a macro is undefined, an #ifdef directive on that macro will evaluate to false.

Syntax

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

#undef macro_definition

macro_definition

The name of the macro which will be removed by the preprocessor.

Example

The following example shows how to use the #undef directive:

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

#include <stdio.h>

#define YEARS_OLD 12

#undef YEARS_OLD

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

   printf("TechOnTheNet is a great resource.\n");

   return 0;
}

In this example, the YEARS_OLD macro is first defined with a value of 12 and then undefined using the #undef directive. Since the macro no longer exists, the statement #ifdef YEARS_OLD evaluates to false. This causes the subsequent printf function to be skipped.

Here is the output of the executable program:

TechOnTheNet is a great resource.