Recognizing wl_display pointer for EGL (Re: Smart comparing of wl_display_interfaces)

[Bill Spitzak]

On 10/16/2014 11:16 PM, Pekka Paalanen wrote:

> ...adding a function int/bool wl_is_wl_display_pointer(void *p) into
> libwayland-client.so. As that function would be built into
> libwayland-client.so, it naturally uses the client.so version of
> wl_display_interface symbol (unless dynamic linking miraculously
> manages to mess that up?).
>
> 'p' would be the tentative 'struct wl_display *', so that the
> implementation details of struct wl_display would be contained in
> libwayland-client.so, and not leaked elsewhere like e.g. in Mesa.

Might be better if the function was more like a cast. It returns either 
(wl_display*)p or NULL depending on the test. The reason is so the user 
can just assign it to a correct pointer without doing the cast 
themselves. This might be important if more of these casts are added in 
the future, it makes it harder to accidentally run the wrong test for 
the cast you want to do:

   struct wl_display* wl_display_dynamic_cast (void* p) {
     if (test(p)) return (struct wl_display*)p;
     else return 0;
   }

> I am unsure about the exact definition of wl_is_wl_display_pointer():
> can it do the dereferencability check itself, or should we require the
> caller to guarantee that dereferencing cannot crash. The latter would
> leave the portability burden on the callers rather than libwayland.

Probably it should handle NULL.

Maybe it should check alignment if wl_display has stricter alignment 
restrictions than some other plausable arguments.

It does not seem to me that a check for pointing to valid memory is 
necessary.