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.

1. #include<stdio.h>  
2. #ifndef __MATH_H  
3. #error First include then compile  
4. #else  
5. void main(){  
6.     float a;  
7.     a=sqrt(7);  
8.     printf("%f",a);  
9. }  
10. #endif  

Output:

Compile Time Error: First include then compile

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

1. #include<stdio.h>  
2. #include<math.h>  
3. #ifndef __MATH_H  
4. #error First include then compile  
5. #else  
6. void main(){  
7.     float a;  
8.     a=sqrt(7);  
9.     printf("%f",a);  
10. }  
11. #endif  

Output:

2.645751