C #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.

snippet
#include
#ifndef __MATH_H
#error First include then compile
#else
void main(){
    float a;
    a=sqrt(7);
    printf("%f",a);
}
#endif

Output:

Output
Compile Time Error: First include then compile

But, if you include math.h, it does not gives error.

snippet
#include
#include
#ifndef __MATH_H
#error First include then compile
#else
void main(){
    float a;
    a=sqrt(7);
    printf("%f",a);
}
#endif

Output:

Output
2.645751
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