#include<stdio.h>

int lowercase(char check_character);

int main(void) {

char check = 'a';

    if (lowercase(check)){
    printf("it is lowercase");
    }

                int lowercase(char check_character){
                 If(check_character & 0x20);{
                return 1;
                            }
    return 0;
    }
return 0;
}

In function `main':
undefined reference to `lowercase'