#include <stdio.h>
int  that_character(char test_character);

int main(void) {

    char this_character= 'a';

    if ( that_character) this_character)  ) {
    printf("This is lowercase\n");

    }
    return 0;
}

int that_character(char test_character) {

    if( test_character & 0x20) {
//checks if lowercase by returning a non-zero if true
        return 1;

        }

    return 0;
}

In function 'main':
Line 8: warning: the address of 'that_character', will always evaluate as 'true'
Line 8: error: expected ';' before ')' token
Line 8: error: expected statement before ')' token
Line 8: error: expected statement before ')' token