[Qt-interest] Segfault when opening a QMessageBox::about

[Andre Somers]

Bastiaan Naber wrote:
> Hi,
>
> I'm making an experimental application to see if I can use Qt with our 
> research software. This software doesn't allow me to make threads or let 
> someone hijack the main event loop. I have been able to intergrate Qt by 
> creating a timer and calling:
>
> processEvents(); of QApplication 
>
> every x mil seconds. This works fine and I have been able to create a window 
> with some buttons on it. Even the Qt OpenGL example can be intergrate in this 
> way.
>
> However I can't seem to open and QMessageBox-es by calling one of their static 
> methods. When I try this my application segfaults somewhere in 
> kernel_qwidget_x11.cpp:
>
> XSetWMProperties(dpy, id, 0, 0,
>                          qApp->d_func()->argv, qApp->d_func()->argc,
>                          &size_hints, &wm_hints, &class_hint);
>
> This is the statement that results in the crash:
>
> QMessageBox::about( this, "Example",
>                         "This example demonstrates simple use of Qt");
>
> Nothing spectacular I would say. This works offcourse if execute the Qt 
> eventloop exec() but I really can't do this.
>
> Does anyone have any ideas about what is going on? I'm using Qt 4.6.2 on 
> CentOS. I do have core dump if this helps.
I think what happens is that the message box *does* "hijack" the 
eventloop. A modal messagebox, as will be created using the static 
methods or using the ::exec method, creates it's own local eventloop. 
That is probably now biting you...

André