Re: Warning with clang, error with gcc : attached files

[David Chisnall]

On 03/05/2019 12:39, Bertrand Dekoninck wrote:
>   SEL aSelector = @selector(themeControlState);
>   GSThemeControlState buttonState =  (GSThemeControlState)[self 
> performSelector:aSelector withObject:nil];

This is not only wrong, it is undefined behaviour.  You are calling a 
function that returns an int (well, an enum, but in C that practically 
means an int) with a signature that returns a pointer.  You are then 
converting the result to an int.  If you are lucky, the calling 
convention will return an int and a pointer in the same place (register 
or stack), but this is not guaranteed by the language and, if the 
compiler spots that this is what you're doing then it is free to do 
whatever it likes with your code.

Concretely, it is almost certainly broken on 64-bit big-endian systems, 
where the int will be returned in the first 4 bytes of a register / 
stack location and this will be taking the second 4 bytes.  You may be 
lucky and find that the in-register return works, but it's fragile.

TL;DR: Please don't do this.

David