[Xcb] xlib

Jeremy Kolb jkolb at brandeis.edu
Fri Mar 4 17:28:15 PST 2005

Jamey Sharp wrote:
> On Wed, Mar 02, 2005 at 06:44:28PM -0500, Jeremy Kolb wrote:
>>Any word on that xlib bug?  I notice that it's still there, but does 
>>anyone know where to start looking for it (assuming it's only one)? 
>>I'll go hunting tomorrow.
> Oops. I forgot I didn't already reply to this. I hope Josh will help me
> fill in the details: I'm trying to do this from memory.
> I assume you're referring to the occasional assertion failures from
> instances of the _xcb_assert_valid_sequence macro that is declared in
> xcbint.h and used by both XCB and Xlib.
> I think that the assertions are placed correctly, though it could be
> that they're firing wrongly. I suspect that some XCB function is being
> called while Xlib "owns" XCB's buffers. (See X11/src/xcl/xcblock.c,
> especially _X{Put,Get}XCBBuffer.) That could mean an X request is being
> sent by XCB while Xlib is responsible for tracking outgoing requests. If
> the asserts weren't there in that case, Xlib would reset XCB's state as
> though the request hadn't been sent, and sometime later everything would
> get confused.
> One non-obvious way this might happen is when XCB upcalls back into Xlib
> through the "unexpected reply handler". That path is very hard to reason
> about. I'd love to design it away. Maybe we can get away with a queue of
> unexpected replies that Xlib can poll at some appropriate point, I
> dunno. Even better if we could make all replies expected; the async
> reply handler mechanism of Xlib makes that tricky, but maybe not as
> impossible as I thought the last time I looked at it.
> I don't expect that's enough to help find the bug, but I don't know what
> else to say. Please don't hesitate to ask more questions.
> --Jamey
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Thanks Jamey, how do I find out when this is happening?  I can't make 
any sense of ethereal for the wire stuff, it's just too confusing. 
Also, gdb doesn't give me any function names on a backtrace even though 
I load the shared-library files and have everything compiled with -g.


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