[PyQt] Object lifetime related segfault on linux

[Marcus Ottosson]

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
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>



-- 
*Marcus Ottosson*
konstruktion at gmail.com
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