Help with proxy queues please

Pekka Paalanen ppaalanen at
Mon Sep 11 14:51:49 UTC 2023

On Mon, 11 Sep 2023 12:53:15 +0100
John Cox <jc at> wrote:

> Hi
> I am clearly deeply confused by event queues and proxies - I have this
> snippet of code (from some VLC work I'm doing):
> sys->eventq = wl_display_create_queue(video_display(sys));
> if (sys->eventq == NULL) {
>     msg_Err(vd, "Failed to create event Q");
>     goto error;
> }
> if ((sys->wrapped_display =
> wl_proxy_create_wrapper(video_display(sys))) == NULL)
> {
>     msg_Err(vd, "Failed to create wrapper");
>     goto error;
> }
> wl_proxy_set_queue((struct wl_proxy *)sys->wrapped_display, sys->eventq);
> struct wl_callback * cb;
> struct wl_registry *const registry =
> wl_display_get_registry(sys->wrapped_display);
> if (registry == NULL) {
>     msg_Err(vd, "Cannot get registry for display");
>     goto error;
> }
> wl_registry_add_listener(registry, &registry_cbs, vd);
> cb = wl_display_sync(sys->wrapped_display);
> wl_callback_add_listener(cb, &reg_done_sync_listener, vd);
> msg_Info(vd, "Roundtrip start");
> wl_display_roundtrip_queue(sys->wrapped_display, sys->eventq);

The problem is probably here.

wrapped_display is just a shell of a wl_proxy, but
wl_display_roundtrip_queue() needs the original wl_display, just like
wl_display_dispatch_queue() it calls.

Thanks to elinor in IRC for pointing this out, when I first went on a
complete wild goose chase wondering if a wl_display can even be
wrapped, but of course it must be wrappable.

> msg_Info(vd, "Roundtrip done");
> wl_registry_destroy(registry);
> video_display() returns the display we are rendering to. My
> expectation was that the registry & sync listeners would be called
> somewhere between the roundtrip start/end debug lines, but what seems
> to happen is that the roundtrip just returns immediately.
> I have clearly misunderstood something at a fundamental level. I
> thought that the wrapped display would set the Q that the listeners
> would send events on and th eroundtrip would wait for that. The
> wl_proxy_create_wrapper docn on
> also seems to imply this.
> Any hints would be gratefully received - thanks

Yeah, it's a bit awkward to have wl_display both as a proxy and a
Wayland connection. Some APIs need only a wl_proxy, others need the

That's something to add in the docs somehow.

As a rule of thumb, I'd say that anything that explicitly takes a
'struct wl_display *' as an argument in wayland-client-core.h needs the
original wl_display and does not work with a proxy-wrapper.

Another exception is that you cannot change the queue of the original
wl_display (or at least it's buggy), while a proxy-wrapped wl_display
you can.

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