gcc is running in -fno-permissive by default, it will give you a warning if you implicitly try to converted between pointers of different types. (turn on warning as errors -Werror so that you have to deal with these messages)

Now what I don't understand is that for C you are not allowed to disable this, -fpermissive is not allowed for C, whereas for C++ you can. Anyone knows the reason why? The compiler error says "command line option "-fpermissive" is valid for C++/ObjC++ but not for C" - why is the compiler having this distinction?