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Re: GTK build crashes under X

From: Jan Djärv
Subject: Re: GTK build crashes under X
Date: Tue, 16 Jan 2007 08:10:32 +0100
User-agent: Thunderbird (X11/20070102)

I think you should file a bug report on libXft or possibly fontconfig.

        Jan D.

Benjamin Riefenstahl skrev:
Hi Stephen, all,

Stephen Berman writes:
Program received signal SIGSEGV, Segmentation fault.
0xb74b88fa in strcmp () from /lib/libc.so.6
(gdb) bt
#0  0xb74b88fa in strcmp () from /lib/libc.so.6
#1  0xb79c1b45 in FcObjectToPtr () from /usr/lib/libfontconfig.so.1
#2  0xb79c5741 in FcPatternAddWithBinding () from /usr/lib/libfontconfig.so.1
#41 0xb7df2c9c in gtk_widget_size_request ()
   from /opt/gnome/lib/libgtk-x11-2.0.so.0
#42 0x080f181c in xg_update_frame_menubar (f=0x8644250)
    at /home/steve/emacs-22.0.90/src/gtkutil.c:2924
#43 0x0808bb95 in set_frame_menubar (f=0x8644250, first_time=1, deep_p=1)
    at /home/steve/emacs-22.0.90/src/xmenu.c:2098
#44 0x0808bd90 in initialize_frame_menubar (f=0x8644250)
    at /home/steve/emacs-22.0.90/src/xmenu.c:2495
#45 0x080d6735 in Fx_create_frame (parms=139409981)
    at /home/steve/emacs-22.0.90/src/xfns.c:3368
#46 0x08159461 in Ffuncall (nargs=2, args=0xbfe1dfa8)
    at /home/steve/emacs-22.0.90/src/eval.c:2997

I got a crash in the same spot with the latest pretest and I found
this thread in the mail archive.

I analysed it like this:

- The crash occurs because Fontconfig's (libfontconfig.so) data
  structures are corrupted, more specifically this involves a linked
  list in Fontconfig's fcname.c.

- That linked list is built from data that is passed-in through a
  Fontconfig API and used unchecked.

- The caller that registered this particular piece of data is Xft
  (libXft.so), called through the QT library linked in by
  gtk-qt-engine.  gtk-qt-engine seems to be a Gnome theme, probably
  used to coordinate settings of Gnome clients with KDE (my main

- gtk-qt-engine is loaded during Emacs' call to
  gtk_settings_set_string_property() in gtkutil.c:xg_initialize().

- When the crash occurs, gtk-qt-engine is not loaded any more.  It
  seems to get unloaded after the settings have been determined.  Xft
  is loaded (through Pango), but it is in a different place now than
  it used to be before, because Pango has re-loaded it on-demand long
  after it was already unloaded together with gtk-qt-engine.

The root cause seems to be that the Xft shared library is not
unloadable, it doesn't cleanup and unregister the data that it has
passed to fontconfig.

Work-arounds that fix it for me:

- Uninstall gtk-qt-engine. - Preload Xft using LD_PRELOAD.

Possible work-around in Emacs:

- Link to Xft and call XftInit(0) in gtkutil.c:xg_initialize() or even
  before that.

I'm not sure where exactly the problem *should* be fixed.

- Fontconfig could copy the data that comes in.

- Xft could allocate the data on the heap instead of using a static

- Xft could prevent unloading of itself.
- Xft could provide a cleanup routine for QT and/or gtk-qt-engine to

- gtk-qt-engine could prevent unloading of Xft.  It makes things
  unusually complicated by combining the two toolkits in one process.


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