/in/foo.c:3:1: error: unknown type name 'bool'
 bool is_prime(int n)
 ^~~~
/in/foo.c: In function 'is_prime':
/in/foo.c:5:23: error: 'false' undeclared (first use in this function)
     if(n < 2)  return false;
                       ^~~~~
/in/foo.c:5:23: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:9:9: error: 'true' undeclared (first use in this function)
  return true;
         ^~~~
/in/foo.c:10:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^