totn C Language

C Language: #error Directive

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

Description

In the C Programming Language, the #error directive causes preprocessing to stop at the location where the directive is encountered. Information following the #error directive is output as a message prior to stopping preprocessing.

Syntax

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

#error message
message
Message to output prior to stopping preprocessing.

Example

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

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

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

#include <stdio.h>
#include <limits.h>

/* 
 * Calculate the number of milliseconds for the provided age in years
 * Milliseconds = age in years * 365 days/year * 24 hours/day, 60 minutes/hour, 60 seconds/min, 1000 millisconds/sec
 */
#define MILLISECONDS(age) (age * 365 * 24 * 60 * 60 * 1000)

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

   #if INT_MAX < MILLISECONDS(12)
   #error Integer size cannot hold our age in milliseconds
   #endif

   /* Calculate the number of milliseconds in 12 years */
   age = MILLISECONDS(12);

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

   return 0;
}

In this example, we are using the int data type to hold the age of TechOnTheNet in milliseconds. The int data type has a maximum value of INT_MAX which is defined in the limits.h header file and holds a value of 2^31 - 1 on both 32 and 64 bit systems. (See here for other variable types and their sizes.)

The statement #if INT_MAX < MILLISECONDS(12) evaluates whether or not the int data type is large enough to store the age in milliseconds. If it is not, the processor will output the following error message:

error: Integer size cannot hold our age in milliseconds

Since an error occurred, the program compilation will not complete and therefore no executable program will be created.

A typical use of the #error directive is to prevent compilation if a known condition exists that would cause the program to not function properly if the compilation completed. (For example, the C source was designed for a specific platform and was being compiled on an incompatible platform.)

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