[Xcb] Feature Request: Function "xcb_test_for_event"

[Uli Schlachter]

Hi,

On 09.04.2013 06:14, Josh Triplett wrote:
> On Mon, Apr 08, 2013 at 07:52:19PM -0400, Micah Nordland wrote:
>> I believe this is what you are looking for:
>>
>> int xcb_get_file_descriptor (xcb_connection_t *c)
>>   Access the file descriptor of the connection. ]
>>
>> Awesome WM currently uses this with glib, and in the past it used it with
>> libev.
>> bspwm uses this to do something similar to what you are doing.
>> https://github.com/baskerville/bspwm
> 
> That does get a file descriptor referring to the X connection, but you
> can't use that to wait for X responses without quite a bit of care,
> because XCB might have some already read off the wire and stored in
> internal buffers; thus, if you block on the X connection having data,
> you might wait forever while XCB already has data to give you.

The following doesn't really help with Micah's issue directly, but it might make
it possible to poll() the XCB fd for readability.

There is currently a patch[0] for Wayland "in the pipe" which makes something
similar possible. The (simplified) idea there is that there are two functions
wl_display_acquire_fd() and wl_display_release_fd(). When a thread has acquired
the fd, no other thread may read from it.

This makes it relatively easy to integrate wayland into another main loop. In
XCB-speak, before calling e.g. poll(), you would have to acquire the fd (and
then check if there are any pending events?), call poll() and release the fd
again afterwards.

I am not 100% sure that the above description would really work for toolkits
(can they really handle new events coming in while preparing for poll()?), but
if would be great if something like this were implemented. Currently, Qt5 spawns
a thread which just sits in xcb_wait_for_event() so that X11 events can get
handled properly.

Cheers,
Uli

[0]: http://lists.freedesktop.org/archives/wayland-devel/2013-April/008569.html
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