Scratch that, in X11 apparently the Display structure is an incomplete type (so sizeof won't work). This means you most probably *have* to use pass it around as an opaque pointer. It's kind of odd because you can still access some of its fields (so it's not totally opaque), but you can't do copies.
So just use pointers and you'll be safe. Hell I've never really done any C programming except what's in K&R, but this seems like a logical conclusion to me..
