#error

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