totn C Language

C Language: #ifdef Directive

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

Description

In the C Programming Language, the #ifdef directive allows for conditional compilation. The preprocessor determines if the provided macro exists before including the subsequent code in the compilation process.

Syntax

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

#ifdef macro_definition
macro_definition
The macro definition that must 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 #ifdef directive in the C language:

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

#include <stdio.h>

#define YEARS_OLD 10

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;
}

Here is the output of the executable program:

TechOnTheNet is over 10 years old.
TechOnTheNet is a great resource.

A common use for the #ifdef directive is to enable the insertion of platform specific source code into a program.

The following is an example of this:

/* Example using #ifdef directive for inserting platform specific source code by TechOnTheNet.com */

#include <stdio.h>

#define UNIX 1

int main()
{
   #ifdef UNIX
   printf("UNIX specific function calls go here.\n");
   #endif

   printf("TechOnTheNet is over 10 years old.\n");

   return 0;
}

The output of this program is:

UNIX specific function calls go here.
TechOnTheNet is over 10 years old.

In this example, the UNIX source code is enabled. To disable the UNIX source code, change the line #define UNIX 1 to #undef UNIX.

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