What is the use of the #error directive in C?

It's a preprocessor directive that is used (for example) when you expect one of several possible -D symbols to be defined, but none is.

#if defined(BUILD_TYPE_NORMAL)
# define DEBUG(x) do {;} while (0) /* paranoid-style null code */
#elif defined(BUILD_TYPE_DEBUG)
# define DEBUG(x) _debug_trace x /* e.g. DEBUG((_debug_trace args)) */
#else
# error "Please specify build type in the Makefile"
#endif

When the preprocessor hits the #error directive, it will report the string as an error message and halt compilation; what exactly the error message looks like depends on the compiler.

If compiler compiles this line then it shows a compiler fatal error: and stop further compilation of program:

#include<stdio.h>
#ifndef __MATH_H
#error First include then compile
#else
int main(){
    float a,b=25;
    a=sqrt(b);
    printf("%f",a);
    return 0;
}
#endif

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

I may have invalid code but its something like...

#if defined USING_SQLITE && defined USING_MYSQL
#error You cannot use both sqlite and mysql at the same time
#endif

#if !(defined USING_SQLITE && defined USING_MYSQL)
#error You must use either sqlite or mysql
#endif


#ifdef USING_SQLITE
//...
#endif

#ifdef USING_MYSQL
//...
#endif