[maemo-developers] [maemo-developers] weird crashing... (X server badmatch)

[From: Eero Tamminen]

Hi,

> The program 'testing-xul' received an X Window Systemerror.
> This probably reflects a bug in the program. The error was 'BadMatch'.
> (Details:
> serial 2527
> error_code 8
> request_code 56
> minor_code 0)

Request code 56 is X_ChangeGC.

(Request codes are listed in X books like
"Introduction to X Window System" by Oliver Jones.)


> (Note to programmers: normally, X errors are reported asynchronously;
> that is, you will receive the error a while after causing it.   To debug
> your program, run it with the --sync command line option to change this
> behavior. You can then get ameaningful backtrace from your debugger if
> you break on thegdk_x_error() function.)

This error means that your application tried to access an X server
resource that doesn't exist (anymore), either directly or through
the libraries it used.  For example you removed a window as a response
to a button click, but you had an event callback for the window that
handled some event for the window only after you had removed the window.

In this case the resource was (down below) an X server graphics context,
not a window.


> It starts up, but after some time, it just crash, printing as above.
> The '--sync' suggested option has no effect.

Your stacktrace in GDB is not more readable?

Normally --sync just forces the error to manifest immediately when
it's done instead of later.


If --sync option helps to fix the problem, in the above example it
would mean that the two operations were not properly synchronized
(X server works asynchronously to your program and events & requests
are buffered) and you can fix the problem with correctly placed XSync().
Or preferably, with the corresponding Gdk alternative.


> Any special reason for that ?

The code is broken.


	- Eero

PS. In some very rare cases (if you're writing a window manager),
application needs to access X server resources which it doesn't
control itself.  In these cases it can trap the X error while
accessing such a resource.