/in/foo.c:2:1: error: unknown type name 'bool'
 bool isPrime(int x)
 ^~~~
/in/foo.c: In function 'isPrime':
/in/foo.c:5:10: error: 'false' undeclared (first use in this function)
   return false;
          ^~~~~
/in/foo.c:5:10: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c: In function 'main':
/in/foo.c:16:18: error: 'true' undeclared (first use in this function)
  if(isPrime(x1)==true && isPrime(x2)==true)
                  ^~~~
/in/foo.c: In function 'isPrime':
/in/foo.c:9:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^