Is boolean return type allowed in C?

#include<stdio.h>
#include<stdbool.h>
void main(){
    bool x = true;
    if(x)
        printf("Boolean works in 'C'. \n");
    else
        printf("Boolean doesn't work in 'C'. \n");
}

bool does not exist as a keyword pre-C99.

In C99, it should work, but as @pmg points out below, it's still not a keyword. It's a macro declared in <stdbool.h>.

try to include:

#include <stdbool.h>