/in/foo.c:5:1: error: unknown type name 'bool'
 bool isPrime(int x)
 ^~~~
/in/foo.c: In function 'isPrime':
/in/foo.c:7:23: error: 'false' undeclared (first use in this function)
     if(x < 2)  return false;
                       ^~~~~
/in/foo.c:7:23: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:8:5: warning: this 'for' clause does not guard... [-Wmisleading-indentation]
     for(int i = 2; i * i <= x; i++)
     ^~~
/in/foo.c:11:2: note: ...this statement, but the latter is misleadingly indented as if it is guarded by the 'for'
  return true;
  ^~~~~~
/in/foo.c:11:9: 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]
 }
 ^