/in/foo.c:3:1: error: unknown type name 'bool'
 bool isPrime(int x)
 ^~~~
/in/foo.c: In function 'isPrime':
/in/foo.c:6:10: error: 'false' undeclared (first use in this function)
   return false;
          ^~~~~
/in/foo.c:6:10: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:10:9: error: 'true' undeclared (first use in this function)
  return true;
         ^~~~
/in/foo.c: In function 'main':
/in/foo.c:20:19: error: 'true' undeclared (first use in this function)
   if(isPrime(x1)==true && isPrime(x2)==true)
                   ^~~~
/in/foo.c: In function 'isPrime':
/in/foo.c:11:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^