#error in C

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