[Xcb] How to cleanly terminate an xcb connection?

Uli Schlachter psychon at znc.in
Wed Oct 13 15:20:22 UTC 2021


Am 12.10.21 um 18:00 schrieb Carlo Wood:
> Apparently I am not allowed to just call xcb_disconnect.
> What is the correct way to terminate a connection so that these
> invalid memory accesses won't happen?

short answer: xcb_disconnect() is not thread-safe. You must not be using
the connection from other threads at the same time.

For example, Qt solves this by setting a variable "please exit",
creating a new window, and then sending an event to this window. The
code that waits for events then wakes up and sees that the variable was
set. Look for "CloseConnection" in [0] for details.


(In line 367 you can find the nice comment "A hack to close XCB
connection. Apparently XCB does not have any APIs for this?" :-) )

> PS I also get an error during start up:
> Really strange :/ Why would xcb_flush send uninitialized data over the socket?

No idea, but you could try adding more calls to xcb_flush() in your code
to narrow down which request sending causes this.

A random example on how this can happen: The Qt code above uses
memset(&event, 0, sizeof(event)) on an xcb_client_message_event_t before
sending this event. If you don't do that, you might end up sending
uninitialised data.

I am not saying that libxcb does not cause this, but at least I'd be
surprised about this. Most likely something tells libxcb to send some
uninitialised bytes to the X11 server.

<alanc> I think someone had a Xprint version of glxgears at one point,
    but benchmarking how many GL pages you can print per second
    was deemed too silly to merge

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