/in/foo.c:3:1: error: unknown type name 'bool'
 bool isPrime(int x)
 ^~~~
/in/foo.c: In function 'isPrime':
/in/foo.c:8:10: error: 'false' undeclared (first use in this function)
   return false;
          ^~~~~
/in/foo.c:8:10: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:15:9: error: 'true' undeclared (first use in this function)
  return true;
         ^~~~
/in/foo.c: In function 'main':
/in/foo.c:35:6: warning: unused variable 'x' [-Wunused-variable]
  int x;
      ^
/in/foo.c: In function 'isPrime':
/in/foo.c:16:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^