C Programming Tutorial
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.
The #error preprocessor directive indicates error. The compiler gives fatal error if #error directive is found and skips further compilation process.
C #error example
Let's see a simple example to use #error preprocessor directive.
#include<stdio.h>
#ifndef __MATH_H
#error First include then compile
#else
void main()
{
float a;
a=sqrt(7);
printf("%f",a);
}
#endif
Output:
Compile Time Error: First include then compile
But, if you include math.h, it does not gives error.
#include<stdio.h>
#include<math.h>
#ifndef __MATH_H
#error First include then compile
#else
void main(){
float a;
a=sqrt(7);
printf("%f",a);
}
#endif
Output:
2.645751
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 c programming */
#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 C programming 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 c programming 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.)
