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Strange crash with QObject hierarchy deletion

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  • Christian EhrlicherC Offline
    Christian EhrlicherC Offline
    Christian Ehrlicher
    Lifetime Qt Champion
    wrote on last edited by
    #7

    Q(Core|Gui)Application is a special object and I would pass it as parent to an object for automatic destruction. The QApplication dtor is cleaning up a lot of stuff internally and then wants to delete it's children. This may work but since all is cleaned up already it may crash as you've noticed. The problem here is that the global 'qApp' instance is already set to nullptr when your widget finally gets destroyed which results in a nullptr access.

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    kshegunovK 1 Reply Last reply
    0
    • Christian EhrlicherC Christian Ehrlicher

      Q(Core|Gui)Application is a special object and I would pass it as parent to an object for automatic destruction. The QApplication dtor is cleaning up a lot of stuff internally and then wants to delete it's children. This may work but since all is cleaned up already it may crash as you've noticed. The problem here is that the global 'qApp' instance is already set to nullptr when your widget finally gets destroyed which results in a nullptr access.

      kshegunovK Offline
      kshegunovK Offline
      kshegunov
      Moderators
      wrote on last edited by
      #8

      I'm not convinced, it should be working in my opinion.

      Read and abide by the Qt Code of Conduct

      1 Reply Last reply
      0
      • Christian EhrlicherC Offline
        Christian EhrlicherC Offline
        Christian Ehrlicher
        Lifetime Qt Champion
        wrote on last edited by Christian Ehrlicher
        #9

        qApp is set to nullptr in the second line of QCoreApplication dtor. Here's the backtrace

        1652            qApp->d_func()->sendSyntheticEnterLeave(this);
        (gdb) bt
        #0  QWidget::~QWidget (this=0x6abef0, __in_chrg=<optimized out>) at /home/chehrlic/kde/qt5/qtbase/src/widgets/kernel/qwidget.cpp:1652
        #1  0x000000000040352d in NotificationManager::~NotificationManager() ()
        #2  0x0000000000403599 in NotificationManager::~NotificationManager() ()
        #3  0x00007ffff6e07eda in QObjectPrivate::deleteChildren (this=this@entry=0x61f1d0) at /home/chehrlic/kde/qt5/qtbase/src/corelib/kernel/qobject.cpp:2004
        #4  0x00007ffff6e09362 in QObject::~QObject (this=<optimized out>, __in_chrg=<optimized out>) at /home/chehrlic/kde/qt5/qtbase/src/corelib/kernel/qobject.cpp:1030
        #5  0x00007ffff6dd29e3 in QCoreApplication::~QCoreApplication (this=0x7fffffffd960, __in_chrg=<optimized out>) at /home/chehrlic/kde/qt5/qtbase/src/corelib/kernel/qcoreapplication.cpp:894
        #6  0x00007ffff7237790 in QGuiApplication::~QGuiApplication (this=0x7fffffffd960, __in_chrg=<optimized out>) at /home/chehrlic/kde/qt5/qtbase/src/gui/kernel/qguiapplication.cpp:641
        

        feel free to create a bug report and ask thiago :)

        Qt Online Installer direct download: https://download.qt.io/official_releases/online_installers/
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        kshegunovK 1 Reply Last reply
        2
        • Christian EhrlicherC Christian Ehrlicher

          qApp is set to nullptr in the second line of QCoreApplication dtor. Here's the backtrace

          1652            qApp->d_func()->sendSyntheticEnterLeave(this);
          (gdb) bt
          #0  QWidget::~QWidget (this=0x6abef0, __in_chrg=<optimized out>) at /home/chehrlic/kde/qt5/qtbase/src/widgets/kernel/qwidget.cpp:1652
          #1  0x000000000040352d in NotificationManager::~NotificationManager() ()
          #2  0x0000000000403599 in NotificationManager::~NotificationManager() ()
          #3  0x00007ffff6e07eda in QObjectPrivate::deleteChildren (this=this@entry=0x61f1d0) at /home/chehrlic/kde/qt5/qtbase/src/corelib/kernel/qobject.cpp:2004
          #4  0x00007ffff6e09362 in QObject::~QObject (this=<optimized out>, __in_chrg=<optimized out>) at /home/chehrlic/kde/qt5/qtbase/src/corelib/kernel/qobject.cpp:1030
          #5  0x00007ffff6dd29e3 in QCoreApplication::~QCoreApplication (this=0x7fffffffd960, __in_chrg=<optimized out>) at /home/chehrlic/kde/qt5/qtbase/src/corelib/kernel/qcoreapplication.cpp:894
          #6  0x00007ffff7237790 in QGuiApplication::~QGuiApplication (this=0x7fffffffd960, __in_chrg=<optimized out>) at /home/chehrlic/kde/qt5/qtbase/src/gui/kernel/qguiapplication.cpp:641
          

          feel free to create a bug report and ask thiago :)

          kshegunovK Offline
          kshegunovK Offline
          kshegunov
          Moderators
          wrote on last edited by kshegunov
          #10

          @Christian-Ehrlicher said in Strange crash with QObject hierarchy deletion:

          feel free to create a bug report and ask thiago

          I just may.

          Read and abide by the Qt Code of Conduct

          1 Reply Last reply
          0
          • P pozdnyakov

            I have an app without main window - tray-only application which shows some notifications from time to time. This app has a main QApplication-derived class, which stores QObject-derived "manager" component as a data member. Manager component is created dynamically in application constructor, and is initialized with application object as a QObject parent. This manager component has a QWidget-derived data member (simple notification window) - stored by value.
            This window has no parent (since there is no main window or other suitable widget).

            I have a simple tray menu with two actions - to show a notification window and to quit the application. The problem is, when I quit the app while the notification window is still on screen, I get a crash with the following info and call stack:

            Exception thrown: read access violation.
            this was 0x8.

            Qt5Widgetsd.dll!QScopedPointer<QObjectData,QScopedPointerDeleter<QObjectData> >::data() Line 140 C++
            Qt5Widgetsd.dll!qGetPtrHelper<QScopedPointer<QObjectData,QScopedPointerDeleter<QObjectData> > >(const QScopedPointer<QObjectData,QScopedPointerDeleter<QObjectData> > & p) Line 1038 C++
            Qt5Widgetsd.dll!QApplication::d_func() Line 209 C++
            Qt5Widgetsd.dll!QWidget::~QWidget() Line 1652 C++
            QtTest.exe!NotificationWindow::~NotificationWindow() Line 15 C++
            QtTest.exe!NotificationManager::~NotificationManager() Line 13 C++

            The crash is in Qt code, with internal QObjectData pointer for QApplication being corrupted for some reason.
            If I create manager component without passing QApplication-derived parent in constructor (and then delete manager manually in application destructor), everything works fine.
            The same is true if I just store the manager component by value (instead of a pointer).
            So, the workaround is simple, but I really don't understand - what is wrong with the original code?
            It should work just fine.

            I use Qt 5.11.2 (msvc2017_64). I've narrowed the code to the minimum example which reproduces the problem (shown below). I would really appreciate some help with understanding the problem.

            //main.cpp
            #include "Application.h"
            
            int main(int argc, char *argv[])
            {
            	Application app(argc, argv);
            	return app.exec();
            }
            // end of main.cpp
            
            
            // Application.h
            #pragma once
            
            #include <QApplication>
            #include <QPointer>
            
            class NotificationManager;
            
            class Application : 
                public QApplication
            {
                Q_OBJECT
            
            public:
            	Application(int& argc, char** argv);
            	virtual ~Application();
            	
            private:
            	QPointer<NotificationManager> m_notificationManager;
            };
            // end of Application.h
            
            
            // Application.cpp
            #include <QTimer>
            
            #include "Application.h"
            #include "NotificationManager.h"
            
            Application::Application(int& argc, char** argv) :
                QApplication(argc, argv)
            {
            	setQuitOnLastWindowClosed(false);
            
            	//m_notificationManager = new NotificationManager();
            	m_notificationManager = new NotificationManager(this);
            
            	m_notificationManager->showNotification();
            	QTimer::singleShot(5000, this, SLOT(quit()));
            }
            
            Application::~Application()
            {
            	//delete m_notificationManager;
            }
            // end of Application.cpp
            
            
            // NotificationManager.h
            #pragma once
            
            #include <QObject>
            
            #include "NotificationWindow.h"
            
            class NotificationManager : public QObject
            {
            	Q_OBJECT
            
            public:
            	NotificationManager(QObject* parent = 0);
            	virtual ~NotificationManager();
            
            public slots:
            	void showNotification();
            
            private:
            	NotificationWindow m_notification;
            };
            // end of NotificationManager.h
            
            
            // NotificationManager.cpp
            #include "NotificationManager.h"
            
            NotificationManager::NotificationManager(QObject *parent) :
            	QObject(parent)
            {
            }
            
            NotificationManager::~NotificationManager()
            {
            }
            
            void NotificationManager::showNotification()
            {
            	m_notification.show();
            }
            // end of NotificationManager.cpp
            
            
            // NotificationWindow.h
            #pragma once
            
            #include <QWidget>
            
            #include "ui_NotificationWindow.h"
            
            class NotificationWindow : public QWidget
            {
            	Q_OBJECT
            
            public:
            	NotificationWindow(QWidget* parent = 0);
            	virtual ~NotificationWindow();
            
            private:
            	Ui::NotificationWindow m_ui;
            };
            // end of NotificationWindow.h
            
            
            // NotificationWindow.cpp
            #include "NotificationWindow.h"
            
            NotificationWindow::NotificationWindow(QWidget *parent)	:
            	QWidget(parent)
            {
            	m_ui.setupUi(this);
            }
            
            NotificationWindow::~NotificationWindow()
            {
            }
            // end of NotificationWindow.cpp
            
            
            kshegunovK Offline
            kshegunovK Offline
            kshegunov
            Moderators
            wrote on last edited by kshegunov
            #11

            Workarounds:

            1. Move the object to the stack:
            class Application : public QApplication
            {
                Q_OBJECT
            
            public:
                Application(int& argc, char** argv);
                virtual ~Application();
            
            private:
                NotificationManager m_notificationManager;
            };
            
            Application::Application(int& argc, char** argv) :
                QApplication(argc, argv), m_notificationManager(this)
            {
            }
            
            1. Delete manually in Application::~Application.

            Read and abide by the Qt Code of Conduct

            kshegunovK JKSHJ P 3 Replies Last reply
            0
            • kshegunovK kshegunov

              Workarounds:

              1. Move the object to the stack:
              class Application : public QApplication
              {
                  Q_OBJECT
              
              public:
                  Application(int& argc, char** argv);
                  virtual ~Application();
              
              private:
                  NotificationManager m_notificationManager;
              };
              
              Application::Application(int& argc, char** argv) :
                  QApplication(argc, argv), m_notificationManager(this)
              {
              }
              
              1. Delete manually in Application::~Application.
              kshegunovK Offline
              kshegunovK Offline
              kshegunov
              Moderators
              wrote on last edited by
              #12

              Reported as QTBUG-71545.

              Read and abide by the Qt Code of Conduct

              1 Reply Last reply
              0
              • SGaistS Offline
                SGaistS Offline
                SGaist
                Lifetime Qt Champion
                wrote on last edited by
                #13

                Hi,

                QScopedPointer might also be of interest.

                Interested in AI ? www.idiap.ch
                Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

                1 Reply Last reply
                1
                • kshegunovK kshegunov

                  Workarounds:

                  1. Move the object to the stack:
                  class Application : public QApplication
                  {
                      Q_OBJECT
                  
                  public:
                      Application(int& argc, char** argv);
                      virtual ~Application();
                  
                  private:
                      NotificationManager m_notificationManager;
                  };
                  
                  Application::Application(int& argc, char** argv) :
                      QApplication(argc, argv), m_notificationManager(this)
                  {
                  }
                  
                  1. Delete manually in Application::~Application.
                  JKSHJ Offline
                  JKSHJ Offline
                  JKSH
                  Moderators
                  wrote on last edited by
                  #14

                  @kshegunov said in Strange crash with QObject hierarchy deletion:

                  Workarounds:

                  1. Move the object to the stack:

                  ...

                  1. Delete manually in Application::~Application.
                  1. (For programs that don't subclass QApplication) Delete manually in a lambda connected to QCoreApplication::aboutToQuit()

                  Qt Doc Search for browsers: forum.qt.io/topic/35616/web-browser-extension-for-improved-doc-searches

                  1 Reply Last reply
                  3
                  • kshegunovK kshegunov

                    Workarounds:

                    1. Move the object to the stack:
                    class Application : public QApplication
                    {
                        Q_OBJECT
                    
                    public:
                        Application(int& argc, char** argv);
                        virtual ~Application();
                    
                    private:
                        NotificationManager m_notificationManager;
                    };
                    
                    Application::Application(int& argc, char** argv) :
                        QApplication(argc, argv), m_notificationManager(this)
                    {
                    }
                    
                    1. Delete manually in Application::~Application.
                    P Offline
                    P Offline
                    pozdnyakov
                    wrote on last edited by
                    #15

                    @kshegunov Thank you for taking your time to investigate this. I've already tried these workarounds, as stated in the original post. But it's nice to know that I've found a bug and it's not just some misunderstanding on my side. It's been making me crazy for the last day or so :)

                    kshegunovK 1 Reply Last reply
                    0
                    • P pozdnyakov

                      @kshegunov Thank you for taking your time to investigate this. I've already tried these workarounds, as stated in the original post. But it's nice to know that I've found a bug and it's not just some misunderstanding on my side. It's been making me crazy for the last day or so :)

                      kshegunovK Offline
                      kshegunovK Offline
                      kshegunov
                      Moderators
                      wrote on last edited by
                      #16

                      @pozdnyakov said in Strange crash with QObject hierarchy deletion:

                      But it's nice to know that I've found a bug and it's not just some misunderstanding on my side. It's been making me crazy for the last day or so :)

                      Thiago says it won't be fixed. So act accordingly.

                      Read and abide by the Qt Code of Conduct

                      1 Reply Last reply
                      0

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