[PyQt] Object lifetime related segfault on linux

[Kovid Goyal]

I'm afraid that's not the explanation, I still get the crash passing
sys.argv to QApplication. But, thanks for trying :)

Kovid.

On Thu, Oct 09, 2014 at 12:37:11PM +0100, Marcus Ottosson wrote:
> I think you need to pass sys.argv to QApplication
> 
> import sysfrom PyQt5.Qt import QApplication, QMainWindow
> 
> app = QApplication(sys.argv)
> 
> w = QMainWindow()
> w.show()
> app.exec_()
> 
> ​
> 
> On 9 October 2014 12:28, Kovid Goyal <kovid at kovidgoyal.net> wrote:
> 
> > I've recently started getting a seemingly random crash on program
> > termination on linux x64. It can be reproduced with this script:
> >
> > from PyQt5.Qt import QApplication, QMainWindow
> >
> > app = QApplication([])
> >
> > w = QMainWindow()
> > w.show()
> > app.exec_()
> >
> >
> > Running it with python2 causes a segfault about once in 20 executions.
> > Unfortunately, there is stack corruption so the backtrace is not very
> > useful. And the crash does not occur running under gdb.
> >
> > #0  0x00007f0ab58d5069 in ?? ()
> > #1  0x00007f0ab3bb0f10 in ?? ()
> > #2  0x00000000018c8920 in ?? ()
> > #3  0x00007f0ab3bb0f50 in ?? ()
> > #4  0x00000000018c8c80 in ?? ()
> > #5  0x00007f0ab3bb1700 in ?? ()
> > #6  0x00007f0acd8b03be in QThreadPrivate::start (arg=0x18c8920) at
> > thread/qthread_unix.cpp:345
> > Backtrace stopped: previous frame inner to this frame (corrupt stack?)
> >
> > I can however avoid the crash by adding
> >
> > del w
> > del app
> >
> > to the end of the script. In other words explicitly deleting the
> > QMainWindow before the QApplication prevents the crash. As I understand
> > it this should not be necessary as qapplication.sip should be
> > transferring ownership of top level widgets to C++ in the QApplication
> > destructor, and indeed, sip.ispyowned() indicates that ownership is
> > transferred in the QApplication destructor, so there is something else
> > going on here. It might just be a timing issue that the explicit
> > deletion avoids.
> >
> > This is with Qt 5.3.2 and PyQt 5.3.2. In the above back trace there is a
> > reference to the start method of QThreadPrivate. Looking through the Qt
> > source code I can find no reason for a thread being started during
> > application shutdown. The xcb QPA plugin does start a thread to talk to
> > the X server, but that is during startup.
> >
> > Obviously, for this simple script the fix is to delete the QMainWindow
> > before QApplication. However doing that in a more complex application is
> > not always possible (the best I can think of is to save the main window as
> > an
> > attribute of the application object). That should ensure that main
> > window is either deleted before the application or not at all because of
> > a reference cycle. But I would like to have a proper fix for this, since
> > if it is a timing issue then this fix may not be robust.
> >
> > Kovid.
> >
> > --
> > _____________________________________
> >
> > Dr. Kovid Goyal
> > http://www.kovidgoyal.net
> > http://calibre-ebook.com
> > _____________________________________
> >
> > _______________________________________________
> > PyQt mailing list    PyQt at riverbankcomputing.com
> > http://www.riverbankcomputing.com/mailman/listinfo/pyqt
> >
> 
> 
> 
> -- 
> *Marcus Ottosson*
> konstruktion at gmail.com
> 
> 
> !DSPAM:3,5436736918008306969473!

-- 
_____________________________________

Dr. Kovid Goyal 
http://www.kovidgoyal.net
http://calibre-ebook.com
_____________________________________
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