crash from error from xcb_io.c:249

Jim Ham jimham at
Mon Apr 12 08:21:42 PDT 2010

Julien Cristau wrote:
> [cc:ing the xcb list]
> On Sun, Apr 11, 2010 at 14:27:38 -0700, Jim Ham wrote:
>> I have an application that uses direct calls to X. This application
>> worked with a previous incarnation of X (before but crashes
>> with the following message on newer versions of X. I've tried 7.4+20 and
>> 7.5+5, both Debian on a x86 machine.
>> xcb_io.c:249: process_responses: Assertion `(((long)
>> (dpy->last_request_read) - (long) (dpy->request))<= 0)' failed
>> Without going into a lot of detail, here's what happens:
>> The main thread blocks with a the call XNextEvent(...). A secondary
>> thread sends a user event to X with XSentEvent(...). The application
>> immediately crashes on the this first call to XSendEvent with the above
>> error message. A normal event such as a mouse click generated by the
>> Xserver does not cause a crash.
>> XInitThreads() is called before any other X calls. All calls to X are
>> bracketed with XLockDisplay() and XUnlockDisplay() except for the
>> XNextEvent call in the main thread. As I mentioned, the app has worked
>> for several years.
>> If this is a new problem I can certainly provide more detail. The entire
>> project may be available via SVN if necessary. There is some custom
>> hardware, but a couple of patches could fix that.
> Do you have a backtrace from the assertion, or an X protocol trace of
> what happens before the crash?
I don't know how to generate either of these. Can you tell me where they 
are located or how to turn the logs on? Meanwhile I'll go back to the 
documentation to see if I can figure it out.



> Cheers,
> Julien
> _______________________________________________
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