C Language: #warning Directive

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

Description

In the C Programming Language, the #warning directive is similar to an #error directive, but does not result in the cancellation of preprocessing. Information following the #warning directive is output as a message prior to preprocessing continuing.

Syntax

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

#warning message

message

The message to output prior to continuing preprocessing.

Example

Let's look at how to use #warning directives in your C program.

The following example shows the output of the #warning directive:

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

#include <stdio.h>

int main()
{
   /* The age of TechOnTheNet in seconds */
   int age;

   #warning The variable age may exceed the size of a 32 bit integer

   /* 12 years, 365 days/year, 24 hours/day, 60 minutes/hour, 60 seconds/min */
   age = 12 * 365 * 24 * 60 * 60;

   printf("TechOnTheNet is %d seconds old\n", age);

   return 0;
}

When compiling this program, the preprocessor will output the following warning:

warning: The variable age may exceed the size of a 32 bit integer

Since this is a #warning directive, the program compilation continues and we can execute the program to see its output.

Here is the output of the executable program:

TechOnTheNet is 378432000 seconds old