Weird crash with QGesture

achard.ju

Hi every one !

First of all, i am using Qt 4.8.6 64 bit under debian 8.

I have a weird crash while deleting some of my widgets, in the Qt Code. This is a gdb print of the generated core :

	qDeleteAll<QSet<QGesture*>::const_iterator>		322	0x7f34a91da4cd	
	qDeleteAll<QSet<QGesture*> >		330	0x7f34a91da4cd	
	QGestureManager::filterEventThroughContexts		389	0x7f34a91da4cd	
	QGestureManager::filterEvent		496	0x7f34a91dd205	
	QApplication::notify		3916	0x7f34a917f4ea	
	QCoreApplication::notifyInternal		955	0x7f34a8c4871d	
	sendEvent		231	0x7f34a91cb1ee	
	QWidget::~QWidget		1699	0x7f34a91cb1ee	
	QWidget::~QWidget		1703	0x7f34a91cb489	
	QObjectPrivate::deleteChildren		1935	0x7f34a8c60248	
	QWidget::~QWidget		1679	0x7f34a91cb173

I am not using QGesture at all in the project by the way.

The code called is in QGestureManager.cpp line 389 :

    //Clean up the Gestures
    qDeleteAll(m_gesturesToDelete);

Does anyone understand this ?

SGaist

Hi,

How are you deleting your widget ?

achard.ju

Hi,

function close() is called in a slot. Then an event (deferredDelete) actually call delete on the widget.
This widget is embedded in a MDIArea.

It's not happening at every close of the widget.

achard.ju

I switched from the Qt 4.8.6 from debian package to a 4.8.6 builded from source with the define QT_NO_GESTURES.
The crash seems to be avoided.

Is there a way to disable all the treatment that Qt do for Gesture management, as we do not use them in our project, without compiling from source ?

SGaist

Something's not clear, you are calling close and then deleteLater on your widget ?

achard.ju

No, i think that the close() call implicate a DeferedDelete event (Qt mechanism).

SGaist

No it doesn't unless you have set the theQt::WA_DeleteOnClose attribute. Otherwise it hides the widget.

achard.ju

Yes, the attribute is actually set .

By the way, i have found another way to reproduce the problem and this time i initiate the delete of the widget manually, without close().

The idea is :
i have a QSplitter, the QSplitter is used as a container for 2 widgets. The two widgets are called "upper" and "bottom" area. The upper part in this case contains a list of items. When user click an item, we display a "bottom" part with details on the item selected in the "upper" part.
The crash happens (sometimes) when we switch the "bottom" part.

In this code, m_pBottomArea is the current bottom, pNewBottomArea is the one we want to set.

    if (m_pBottomArea != NULL)
    {
        //Already a bottom: delete it, this will remove it from splitter
        delete m_pBottomArea;
        //m_pBottomArea->deleteLater;
    }

    m_pBottomArea = pNewBottomArea;

    if (m_pBottomArea != NULL)
    {
        //Add the bottom
        m_pSplitter->insertWidget(1, m_pBottomArea);
    }

Either if i use a delete, or a deleteLater, there is sometimes a crash. Always with the same call stack.

Did i do something wrong ?
Do you know if there is a way to disable the GestureManager without compiling Qt from source ?

SGaist

AFAIK, no.

One thing you can try is to set m_pBottomArea's parent to nullptr before deleting it.

achard.ju

Already tried that :(

SGaist

Can you share the complete implementation of the function where you manipulate m_pBottomArea ? The handling pNewBottomArea is absent and the look of the code looks like it might be more complicated that necessary.