/in/foo.c:3:1: error: unknown type name 'bool'
 bool isPrime(int x)
 ^~~~
/in/foo.c: In function 'isPrime':
/in/foo.c:6:23: error: 'false' undeclared (first use in this function)
     if(x < 2)  return false;
                       ^~~~~
/in/foo.c:6:23: note: each undeclared identifier is reported only once for each function it appears in
/in/foo.c:7:5: warning: this 'for' clause does not guard... [-Wmisleading-indentation]
     for(i = 2; i * i <= x; i++) 
     ^~~
/in/foo.c:10:2: note: ...this statement, but the latter is misleadingly indented as if it is guarded by the 'for'
  return true;
  ^~~~~~
/in/foo.c:10:9: error: 'true' undeclared (first use in this function)
  return true;
         ^~~~
/in/foo.c:11:1: warning: control reaches end of non-void function [-Wreturn-type]
 }
 ^