QScrollbar causing crash on moved

Plou

Hello, first post here, because I'm in a bind :(

I'm trying to do a parser/viewer that takes some XML files and display useful data in QTableWidgets.

So far so good, I've got a QMainWindow with a central widget containing some buttons, that open other forms containing QTableWidget themselves, and some groupboxes with buttons. Foresaid forms are attributes of the central widgets, and not meant to be destroyed before the closing of the app, they are juste shown and hidden (I think).

The tables contain some data, between thousands and tens of thousands cells for some. They are initialized at the creation of the central widget, within seconds.

On the first opening (first click on "Open Thistable" button, leading to a slot "Thistable.show", the scrollbars on the side of the tableWidget work fine, no lag or freeze, but if you close the form, and reopen it, suddenly the moving the slider cause a crash.
The sliderPressed and sliderReleased signals work fine though, but just after the sliderMoved is called (got myself a custom slot saying "hey I'm moving") the app crashes, and gives me a "read access violation at 0x0000000000000008".

Well, I'm in a pinch, cause I don't ever handle the ScrollBars in my app, the forms are created in Qt Designer, and just filled in my code.
Any idea on what could cause this, or some tests I could run ?

Plou <thanks in advance>

SGaist

Hi and welcome to devnet,

One thing you can do to find out more information is to run a debug build of your application through the debugger and so get the stack trace associated with your crash.

By the way:

What version of Qt are you using ?
What compiler are you using ?
What OS are you running ?

Plou

Hello, thanks for answering so quickly :)

I'm using 5.12.4, compiling with MSVC2017, on Windows 10.

So I ran the app through the debugger, and got this stack trace :

1  QToolBar::setMovable                      Qt5Widgetsd         0x7ffc5eddeb9a 
2  QToolBar::setMovable                      Qt5Widgetsd         0x7ffc5edd3e23 
3  QToolBar::setMovable                      Qt5Widgetsd         0x7ffc5ede3e7a 
4  QToolBar::setMovable                      Qt5Widgetsd         0x7ffc5ee17a61 
5  QToolBar::setMovable                      Qt5Widgetsd         0x7ffc5ee1248f 
6  QToolBar::setMovable                      Qt5Widgetsd         0x7ffc5ee69dca 
7  QToolBar::setMovable                      Qt5Widgetsd         0x7ffc5ee59da6 
8  QToolBar::setMovable                      Qt5Widgetsd         0x7ffc5f275deb 
9  QToolBar::setMovable                      Qt5Widgetsd         0x7ffc5f275f2f 
10 QToolBar::setMovable                      Qt5Widgetsd         0x7ffc5f27ea99 
11 QToolBar::setMovable                      Qt5Widgetsd         0x7ffc5f2edbf4 
12 QToolBar::setMovable                      Qt5Widgetsd         0x7ffc5efb7f8a 
13 QToolBar::setMovable                      Qt5Widgetsd         0x7ffc5efb371c 
14 QFSFileEngine::copy                       Qt5Cored            0x7ffc5fdb6598 
15 QFSFileEngine::copy                       Qt5Cored            0x7ffc5fdb5ce8 
16 QToolBar::setMovable                      Qt5Widgetsd         0x7ffc5efc5dfa 
17 QToolBar::setMovable                      Qt5Widgetsd         0x7ffc5efc5c03 
18 QToolBar::setMovable                      Qt5Widgetsd         0x7ffc5efc5a89 
19 QToolBar::setMovable                      Qt5Widgetsd         0x7ffc5efc577d 
20 QToolBar::setMovable                      Qt5Widgetsd         0x7ffc5f0f1dfe 
21 QToolBar::setMovable                      Qt5Widgetsd         0x7ffc5ee5b6c1 
22 QToolBar::setMovable                      Qt5Widgetsd         0x7ffc5efc5fad 
23 QToolBar::setMovable                      Qt5Widgetsd         0x7ffc5f0f11fe 
24 QToolBar::setMovable                      Qt5Widgetsd         0x7ffc5edfc188 
25 QToolBar::setMovable                      Qt5Widgetsd         0x7ffc5edf7418 
26 QFSFileEngine::copy                       Qt5Cored            0x7ffc5fd5ef96 
27 QFSFileEngine::copy                       Qt5Cored            0x7ffc5fd5ee3b 
28 QToolBar::setMovable                      Qt5Widgetsd         0x7ffc5edfe8e4 
29 QToolBar::setMovable                      Qt5Widgetsd         0x7ffc5eea6f53 
30 QToolBar::setMovable                      Qt5Widgetsd         0x7ffc5eea5596 
31 QToolBar::setMovable                      Qt5Widgetsd         0x7ffc5edfc188 
32 QToolBar::setMovable                      Qt5Widgetsd         0x7ffc5edf6c0f 
33 QFSFileEngine::copy                       Qt5Cored            0x7ffc5fd5ef96 
34 QFSFileEngine::copy                       Qt5Cored            0x7ffc5fd5ee3b 
35 QOpenGLExtraFunctions::glUseProgramStages Qt5Guid             0x7ffc5d18249e 
36 QOpenGLExtraFunctions::glUseProgramStages Qt5Guid             0x7ffc5d185213 
37 QOpenGLExtraFunctions::glUseProgramStages Qt5Guid             0x7ffc5d1531a5 
38 FT_Property_Set                           qwindowsd           0x7ffc5e865c82 
39 QFSFileEngine::copy                       Qt5Cored            0x7ffc5fe20148 
40 CallWindowProcW                           USER32              0x7ffc9ffdb85d 
41 DispatchMessageW                          USER32              0x7ffc9ffdb1ef 
42 QFSFileEngine::copy                       Qt5Cored            0x7ffc5fe20a4a 
43 FT_Property_Set                           qwindowsd           0x7ffc5e865c34 
44 QFSFileEngine::copy                       Qt5Cored            0x7ffc5fd59173 
45 QFSFileEngine::copy                       Qt5Cored            0x7ffc5fd593ae 
46 QFSFileEngine::copy                       Qt5Cored            0x7ffc5fd5cadf 
47 QOpenGLExtraFunctions::glUseProgramStages Qt5Guid             0x7ffc5d17f838 
48 QToolBar::setMovable                      Qt5Widgetsd         0x7ffc5edf65ba 
49 main                                      main.cpp        10  0x7ff6857d2cb9 
50 WinMain                                   qtmain_win.cpp  97  0x7ff6857da265 
51 invoke_main                               exe_common.inl  107 0x7ff6857d7d5d 
52 __scrt_common_main_seh                    exe_common.inl  283 0x7ff6857d7c14 
53 __scrt_common_main                        exe_common.inl  326 0x7ff6857d7ade 
54 WinMainCRTStartup                         exe_winmain.cpp 17  0x7ff6857d7de9 
55 BaseThreadInitThunk                       KERNEL32            0x7ffc9f421fe4 
56 RtlUserThreadStart                        ntdll               0x7ffca025cb81

So a bunch of tries with setMovable then boom.

I tried to reproduce my crash on a mini app, and it crashes too. The app only got a MainWindow with a button that opens a form, with the same behavior :

button clicked
opens form, QTableWidget behaves normally (scrolling H and V works)
close the form
open it again, with the same button -> crash when trying to scroll

To create the MainWindow and the form, I just went with Qt Designer as follow :

New Qt App, with MainWindow class
Add a QPushButton to the MainWindow, set a Vertical layout
New Qt Designer Form class, add a QTableWidget, set a vertical layout
Add the TableForm to MainWindow as an attribute
Connect the button to the show slot in the MainWindow constructor

mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow),
    _table(new TableForm)
{
    ui->setupUi(this);
    QObject::connect(ui->butOpen,&QPushButton::clicked,
                     _table, &TableForm::show);
}

MainWindow::~MainWindow()
{
    delete _table;
    delete ui;
}

tableform.cpp

#include "tableform.h"
#include "ui_tableform.h"

TableForm::TableForm(QWidget *parent) :
    QWidget(parent),
    ui(new Ui::TableForm)
{
    ui->setupUi(this);

    QStringList head;
    head.append("A"); head.append("A"); head.append("A");
    head.append("A"); head.append("A"); head.append("A");
    head.append("A"); head.append("A"); head.append("A");
    ui->tableTest->setHorizontalHeaderLabels(head);
    ui->tableTest->setColumnCount(head.size());
    ui->tableTest->setRowCount(20);
}

TableForm::~TableForm()
{
    delete ui;
}

main.cpp

#include "mainwindow.h"
#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();

    return a.exec();
}

I'm probably doing something wrong, maybe lacking some signal handling, but I can't pinpoint it.

SGaist

Can you post the .ui and .pro files ? This will allow to have the same code and build instructions.

Plou

mainwindow.ui

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>MainWindow</class>
 <widget class="QMainWindow" name="MainWindow">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>400</width>
    <height>300</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>MainWindow</string>
  </property>
  <widget class="QWidget" name="centralWidget">
   <layout class="QVBoxLayout" name="verticalLayout">
    <item>
     <widget class="QPushButton" name="butOpen">
      <property name="text">
       <string>PushButton</string>
      </property>
     </widget>
    </item>
   </layout>
  </widget>
  <widget class="QMenuBar" name="menuBar">
   <property name="geometry">
    <rect>
     <x>0</x>
     <y>0</y>
     <width>400</width>
     <height>20</height>
    </rect>
   </property>
  </widget>
  <widget class="QToolBar" name="mainToolBar">
   <attribute name="toolBarArea">
    <enum>TopToolBarArea</enum>
   </attribute>
   <attribute name="toolBarBreak">
    <bool>false</bool>
   </attribute>
  </widget>
  <widget class="QStatusBar" name="statusBar"/>
 </widget>
 <layoutdefault spacing="6" margin="11"/>
 <resources/>
 <connections/>
</ui>

tableform.ui

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>TableForm</class>
 <widget class="QWidget" name="TableForm">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>400</width>
    <height>300</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>Form</string>
  </property>
  <layout class="QVBoxLayout" name="verticalLayout">
   <item>
    <widget class="QTableWidget" name="tableTest"/>
   </item>
  </layout>
 </widget>
 <resources/>
 <connections/>
</ui>

.pro

QT       += core gui

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

TARGET = TestSimpleTableWidget
TEMPLATE = app
DEFINES += QT_DEPRECATED_WARNINGS

CONFIG += c++11

SOURCES += \
        main.cpp \
        mainwindow.cpp \
        tableform.cpp

HEADERS += \
        mainwindow.h \
        tableform.h

FORMS += \
        mainwindow.ui \
        tableform.ui

# Default rules for deployment.
qnx: target.path = /tmp/$${TARGET}/bin
else: unix:!android: target.path = /opt/$${TARGET}/bin
!isEmpty(target.path): INSTALLS += target

Plou

I've done a little more testing, showing interesting results (I think ?).

Firstly, I confirmed that the crash happens on a simple implementation without using the designer. (code at the bottom)

Then, I used testAttribute(Qt::WA_DeleteOnClose) to test and see if the widgets containing the QTableWidget would call their destructors on close, and the flag was not set, neither on the designer test nor on the handcrafted app.

I tried to tweak the closeEvent, and... it worked ? I did this :

void TestTableForm::closeEvent(QCloseEvent *event)
{
    qDebug() << "Closing... Not ! But Hiding.";
    event->ignore();
    this->hide();
}

And it works, but I don't really see the difference between this, and the closeEvent without the delete-on-close flag.

Could you explain me, or tell me if this is kind of bad practice and there is a better solution ?

/// No designer implementation ///
Edit : code to pastebin

JonB

@Plou
I'm lost as to where you're at with all your long code and delete-on-close. But I will say: if you do put Qt::WA_DeleteOnClose on any widget then as soon as it gets closed (not hidden) any attempt to access it from wherever may well lead to an access violation, because it has been deallocated.

If it helps, I think you can see when that happens by slotting onto the destroyed signal.

Plou

@JonB
After re-reading it was not so clear, my bad. I was trying to see if the flag could cause the issue, but it is not set, so I assume it is not.

I tried some slotting with the destroyed signal, and it figures that the closeEvent does not destroy neither the Widget containing the table, nor the table itself, nor the QScrollBars.

I tried to reimplement the moveEvent on a custom QScrollBar, and to affect this scrollbar to the view.
The moveEvent was supposed to display the address of the custom scrollbar, and did it.
On the first opening of the widget, it works. On the second, it crashes, but before displaying the address, so I think it might not be the moveEvent.

I cannot pinpoint where the issue comes from. Any idea ?

Edit : I think it's somewhere after the closeEvent. If I don't ignore the QCloseEvent in the reimplementation of closeEvent in the widget containing my table, it crashes as usual.

I still don't have any other solution than the previous workaround, ignoring the QCloseEvent.

What does the QCloseEvent actually do ?

Haseeb Ahmad

@JonB @Plou I have exactly the same problem in the exact same scenario. I am using PyQt5 UI code generator 5.14.1 and Qt Designer 5.12.8. I tried to figure it out but had no success till now. My code crashes without throwing any error, but when I put these lines in my code.
"""
scroller = QScroller.scroller(self.listWidget)
gesture = QScroller.TouchGesture
scroller.grabGesture(self.listWidget, gesture)
"""
When this mainWindow was open for the second or third time not on the first attempt.
I have custom Rasbian running on RaspberryPI.