/in/foo.c:2:1: error: unknown type name 'bool'
 bool isPrime(int num) {
 ^~~~
/in/foo.c: In function 'isPrime':
/in/foo.c:4:16: error: 'false' undeclared (first use in this function)
         return false;
                ^~~~~
/in/foo.c:4:16: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:11:12: error: 'true' undeclared (first use in this function)
     return true;
            ^~~~
/in/foo.c:12:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^