What is #error directives ?

The Error Message Given by c Compiler, so no see the details information of an error message..

Syntax : #error
If compiler compiles this line then it shows a compiler fatal error i.e it only issue an error message and this error message includes . i.e it only issue an error message and this error message includes .
e.g :

#ifndef __MATH_H
#error First include then compile
#else
void main()
{
float a,b=25;
a=sqrt(b);
printf(“%f”,a);
}
#endif

Output: compiler error --> Error directive :First include then compile