自定义QDialog在exec（）上导致段错误

Question

I have a program that essentially consists of a central QMainWindow and a member QToolbar pointer.

One action on this toolbar has its triggered signal connected to a ShowNodeEditBox() slot, which instantiates and attempts to exec a custom QDialog -derived widget:

 void
 Toolbar::ShowNodeEditBox(){
   ...
   //Side note: The custom box stores a pointer to a custom QGLWidget on the main window
   NodeEditBox nodeEdit(this, m_mainWindow->GetGLScene());
   nodeEdit.exec(); 
 }

For some reason, calling exec causes a segfault, even though just constructing the window without it works just fine. The stack trace is as follows:

#0  0x0000000000000000 in ?? ()
#1  0x00000030922e64ff in ?? () from /usr/lib64/libQtGui.so.4
#2  0x00000030922e795a in QPainter::QPainter(QPaintDevice*) () from /usr/lib64/libQtGui.so.4
#3  0x00000030921fd7af in QWidgetPrivate::drawWidget(QPaintDevice*, QRegion const&, QPoint const&, int, QPainter*, QWidgetBackingStore*) ()
   from /usr/lib64/libQtGui.so.4
#4  0x00000030923a0675 in ?? () from /usr/lib64/libQtGui.so.4
#5  0x00000030923a09c9 in ?? () from /usr/lib64/libQtGui.so.4
#6  0x00000030922162da in ?? () from /usr/lib64/libQtGui.so.4
#7  0x00000030922213e7 in QApplication::x11ProcessEvent(_XEvent*) () from /usr/lib64/libQtGui.so.4
#8  0x0000003092249da2 in ?? () from /usr/lib64/libQtGui.so.4
#9  0x00007ffff7410f0e in g_main_context_dispatch () from /lib64/libglib-2.0.so.0
#10 0x00007ffff7414938 in ?? () from /lib64/libglib-2.0.so.0
#11 0x00007ffff7414a3a in g_main_context_iteration () from /lib64/libglib-2.0.so.0
#12 0x0000003091b7d5f3 in QEventDispatcherGlib::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib64/libQtCore.so.4
#13 0x0000003092249a6e in ?? () from /usr/lib64/libQtGui.so.4
#14 0x0000003091b56722 in QEventLoop::processEvents(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib64/libQtCore.so.4
#15 0x0000003091b569ec in QEventLoop::exec(QFlags<QEventLoop::ProcessEventsFlag>) () from /usr/lib64/libQtCore.so.4
#16 0x000000309262aaae in QDialog::exec() () from /usr/lib64/libQtGui.so.4
#17 0x000000000045d9e8 in RoadmapOptions::ShowNodeEditBox (this=0xce5da0) at GUI/RoadmapOptions.cpp:562
#18 0x0000000000486f32 in RoadmapOptions::qt_metacall (this=0xce5da0, _c=QMetaObject::InvokeMetaMethod, _id=12, _a=0x7fffffffc740)
    at GUI/MOC/moc_RoadmapOptions.cpp:114

Something is definitely up here, since I see an 0x0, but I haven't been able to figure it out. The construction of the NodeEditBox itself is quite intricate (lots of sliders, labels, etc. all over the place), so I tried clearing it all off and just calling exec on an empty custom box, constructed like this:

NodeEditBox::NodeEditBox(QWidget* _parent, GLWidget* _scene){ /* nothing! */ }

Lo and behold (and perhaps thankfully, since it's a really complicated widget), this causes the exact same segfault! I also tried making a pointer to a NodeEditBox with new rather than just instantiating it, as well as making the NodeEditBox a pointer member of the toolbar class and constructing it earlier (thus, only doing exec in ShowNodeEditBox() . And the parent and GLWidget scene pointers are already created, accounted for, and robust, as far as I know. But I still get the same problem every time.

What am I missing here?

**Other info/edit: Using a regular, empty QDialog and popping it up instead of the custom version works just fine. So maybe it's a parent-related thing.

Even more interestingly, deriving NodeEditBox from QWidget instead of QDialog works just fine and does not crash! I wanted to use QDialog , though, so that I could call exec. show doesn't seem to work unless the NodeEditBox is a member of the toolbar class, which I didn't want because I need multiple, 'disposable' node edit boxes to pop up at different times...

Answer 1

Should be

NodeEditBox::NodeEditBox(QWidget* _parent, GLWidget* _scene)
: QDialog(parent)
{ /* nothing! */ }

instead of

NodeEditBox::NodeEditBox(QWidget* _parent, GLWidget* _scene){ /* nothing! */ }

• you need to pass through input arguments, otehrwise default ctor is called for unbderlying widget, with 0 parent. Not 100% sure if this causes segfault, but it is definitely error prone.

Answer 2

I don't think there is enough information here to solve this, so here is what I would do to narrow it down:

1. Make sure you can reproduce this crash from a squeaky clean build (ie make clean , etc). Sometimes "mysterious" crashes can be caused by an unclean build (outdated objects/moc files, etc).

2. Start with a trivial version of NodeEditBox and work towards the crash. You already stubbed out the constructor - do the same with all other methods and members. Comment them all out, and then put them back in piece by piece to find the minimal version that makes it crash. Maybe post that minimal version, if it still does not make sense to you.

3. Don't get side-tracked with alternative implementations - the code you presented looks ok - you should be able to get it to work. The cause of the crash must be in the code you didn't show here.