[Xcb] Why is an explicit xcb_flush needed?

Nicholas Allen nick.allen at onlinehome.de
Tue Mar 30 04:19:59 PDT 2010

Josh Triplett wrote:
> I can think of only a few things that could allow
> xcb_wait_for_reply to block forever:
> - A bug in XCB.
> - A bug in the X server.
> - Someone grabbing the X server.
> - Some kind of corruption or severe misuse caused by the calling code
>   (for instance, forking and trying to use the connection from both
>   processes).
> Any of those seems possible.  A symbol-enabled backtrace with gdb would
> probably nail the first or last, and the xtrace plus the backtrace
> should nail the second.  As for the third, hopefully you can rule out
> the possibility of an XGrabServer somehow? :)
I've attached the xtrace to my bug report so maybe we can already rule
out a bug in the x server from that.

I don't do any XGrabServer or forking so I think we can rule them out
unless Cairo XCB surface does this behind the scenes (but I highly doubt

So if we can rule out the bug in the x server then that leaves only a
bug in XCB. I'm still reading up on how to build the libraries so I can
try to debug it a bit myself and provide the complete stack trace...


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