Skip to content
  • Categories
  • Recent
  • Tags
  • Popular
  • Users
  • Groups
  • Search
  • Get Qt Extensions
  • Unsolved
Collapse
Brand Logo
  1. Home
  2. Qt Development
  3. General and Desktop
  4. Deleting QGraphicsItem with QGraphicsEffect leads to segfault
Forum Updated to NodeBB v4.3 + New Features

Deleting QGraphicsItem with QGraphicsEffect leads to segfault

Scheduled Pinned Locked Moved Unsolved General and Desktop
c++ qt qgraphicqgraphicseffectqgraphicsitem
26 Posts 8 Posters 11.8k Views 4 Watching
  • Oldest to Newest
  • Newest to Oldest
  • Most Votes
Reply
  • Reply as topic
Log in to reply
This topic has been deleted. Only users with topic management privileges can see it.
  • T Offline
    T Offline
    tfm123
    wrote on last edited by
    #3

    Thanks for your feedback.

    Unfortunately I already had tried that and it does not change anything.

    I'd generally like to know whether or not I am using QGraphicsEffect right.

    kshegunovK 1 Reply Last reply
    0
    • T tfm123

      Thanks for your feedback.

      Unfortunately I already had tried that and it does not change anything.

      I'd generally like to know whether or not I am using QGraphicsEffect right.

      kshegunovK Offline
      kshegunovK Offline
      kshegunov
      Moderators
      wrote on last edited by
      #4

      Okay, can you at least confirm or deny that this is the actual problem - comment out the delete and see if a crash occurs?

      Read and abide by the Qt Code of Conduct

      1 Reply Last reply
      0
      • T Offline
        T Offline
        tfm123
        wrote on last edited by
        #5

        It does not happen when you do one of the following:

        • do not add a child to MyRect
        • do not add a QGraphicsEffect
        • comment out the delete as you suggested

        When commenting out the delete I now noticed something else: The white rectangle of MyRect sometimes remains on the screen although I pressed right mouse button. This is curious because it still is removed from the scene.

        My guess is, these are the cases that lead to segfault when the delete is in place. The scene somehow thinks MyRect is still present. If it is actually still in memory it's okay and just an artefact on the screen. If it is actually deleted it results in segfault.

        kshegunovK 1 Reply Last reply
        0
        • T tfm123

          It does not happen when you do one of the following:

          • do not add a child to MyRect
          • do not add a QGraphicsEffect
          • comment out the delete as you suggested

          When commenting out the delete I now noticed something else: The white rectangle of MyRect sometimes remains on the screen although I pressed right mouse button. This is curious because it still is removed from the scene.

          My guess is, these are the cases that lead to segfault when the delete is in place. The scene somehow thinks MyRect is still present. If it is actually still in memory it's okay and just an artefact on the screen. If it is actually deleted it results in segfault.

          kshegunovK Offline
          kshegunovK Offline
          kshegunov
          Moderators
          wrote on last edited by
          #6

          @tfm123 said in Deleting QGraphicsItem with QGraphicsEffect leads to segfault:

          When commenting out the delete I now noticed something else: The white rectangle of MyRect sometimes remains on the screen although I pressed right mouse button. This is curious because it still is removed from the scene.

          Emit the changed() signal after removing the item to notify the view of the change. I think this should fix it. It may even fix the original problem, although it's a very long shot.

          Read and abide by the Qt Code of Conduct

          1 Reply Last reply
          0
          • T Offline
            T Offline
            tfm123
            wrote on last edited by tfm123
            #7

            I'm not quite sure how to do that. I have not been using signals and slots; I'm only using QGraphics. Is there a method on the view or the scene I can call to do this?

            Edit: I have tried QGraphicsView::update(), QGraphicsView::invalidateScene(), QGraphicsScene::update() and QGraphicsScene::invalidate(). Nothing changes anything.

            kshegunovK 1 Reply Last reply
            0
            • T tfm123

              I'm not quite sure how to do that. I have not been using signals and slots; I'm only using QGraphics. Is there a method on the view or the scene I can call to do this?

              Edit: I have tried QGraphicsView::update(), QGraphicsView::invalidateScene(), QGraphicsScene::update() and QGraphicsScene::invalidate(). Nothing changes anything.

              kshegunovK Offline
              kshegunovK Offline
              kshegunov
              Moderators
              wrote on last edited by kshegunov
              #8
              class MyScene: public QGraphicsScene
              {
                  Q_OBJECT //< You need also to add this
              
              public:
                  void mouseReleaseEvent(QGraphicsSceneMouseEvent* me) override
                  {
                      if (me->button()==Qt::LeftButton)  {
                          createRect(this);
                          emit changed(); //< Emit changed()
                      }
                      else if (me->button()==Qt::RightButton)  {
                          destroyRect(this);
                          emit changed();
                      }
                      // ...
                  }
              };
              

              PS.
              Sorry, the example won't work like this, one needs to pass a number of regions to the signal, but I'm not exactly sure how to obtain them in this case.

              Perhaps something like this:

              class MyScene: public QGraphicsScene
              {
                  Q_OBJECT
              
              public:
                  void mouseReleaseEvent(QGraphicsSceneMouseEvent* me) override
                  {
                      // ...
                      else if (me->button()==Qt::RightButton)  {
                          if (global_rect)  {
                              QRectF rect = global_rect->rect();
                              scene->removeItem(global_rect);
              
                              emit changed(QList<QRectF>() << rect);
              
                              delete global_rect; //< Still might crash, it's a long shot
                              global_rect=nullptr;
                          }
                      }
                      // ...
                  }
              };
              

              Read and abide by the Qt Code of Conduct

              1 Reply Last reply
              0
              • T Offline
                T Offline
                tfm123
                wrote on last edited by
                #9

                Unfortunately it changes nothing. Without delete there are still the artifects, With delete there still is segfault.

                Thanks for your help though.

                But I'd like to come back to a more basic question: Do you think that my code is correct this way? Or do I use QT in a wrong way?

                kshegunovK 1 Reply Last reply
                0
                • T tfm123

                  Unfortunately it changes nothing. Without delete there are still the artifects, With delete there still is segfault.

                  Thanks for your help though.

                  But I'd like to come back to a more basic question: Do you think that my code is correct this way? Or do I use QT in a wrong way?

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

                  I have no way to answer that, as I neither have any idea what you're trying to accomplish, nor can I tell if this code is a stripped down example that should reproduce the problem, or is an actual snippet from your project ... or something in between.

                  Read and abide by the Qt Code of Conduct

                  1 Reply Last reply
                  0
                  • T Offline
                    T Offline
                    tfm123
                    wrote on last edited by tfm123
                    #11

                    It is a minimal example written to reproduce the error. It is fully compilable, I even provided the compiler call.

                    I was hoping someone with experience in QT would be able to tell me whether I am using QT the right way.

                    Or, to put it in other words, is this kind of error expected behaviour for the example program?

                    kshegunovK 1 Reply Last reply
                    0
                    • T tfm123

                      It is a minimal example written to reproduce the error. It is fully compilable, I even provided the compiler call.

                      I was hoping someone with experience in QT would be able to tell me whether I am using QT the right way.

                      Or, to put it in other words, is this kind of error expected behaviour for the example program?

                      kshegunovK Offline
                      kshegunovK Offline
                      kshegunov
                      Moderators
                      wrote on last edited by kshegunov
                      #12

                      Well I had to run it through the debugger to figure out what's going on. The problem is you add child items in the constructor of the parent item. At that point the scene the item holds a reference to isn't initialized (apparently), so you get a weird event-race of some kind. Anyway, moving new QGraphicsRectItem{this}; away from the constructor of MyRect and into createRect() should fix the issue, at least it did for me.

                      EDIT: Well it didn't as it appears. Okay back to the debugger.

                      As for your code (the one posted), there's a few things I don't like:

                      1. You use global functions to do internal bookkeeping
                      2. You use global variables for no good reason
                      3. You use a heap allocation (i.e. in main()) when that's not really warranted, declaring the view and scene on the stack works just as well.

                      I don't know if that was the feedback you were looking for, but I hope it helps.

                      EDIT 2:

                      Well, apparently it some kind of double-deletion problem. Moving all things as members with auto-storage fixes it. Here's the code I tested with:
                      main.cpp

                      #include<QApplication>
                      #include<QGraphicsView>
                      #include<QScreen>
                      
                      #include "myscene.h"
                      
                      int main(int argc, char ** argv)
                      {
                          QApplication qapp(argc, argv);
                      
                          QGraphicsView view;
                          MyScene scene;
                      
                          QRect rect = QGuiApplication::primaryScreen()->geometry();
                          scene.setSceneRect(0.0f, 0.0f, rect.width(), rect.height());
                          view.setVerticalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
                          view.setHorizontalScrollBarPolicy(Qt::ScrollBarAlwaysOff);
                          view.setFrameShape(QFrame::NoFrame);
                          view.setBackgroundBrush(QBrush(Qt::black, Qt::SolidPattern));
                          view.setScene(&scene);
                          view.showFullScreen();
                      
                          return qapp.exec();
                      }
                      

                      myscene.h

                      #ifndef MYSCENE_H
                      #define MYSCENE_H
                      
                      #include <QGraphicsScene>
                      #include <QGraphicsSceneMouseEvent>
                      #include <QGraphicsRectItem>
                      #include <QGraphicsDropShadowEffect>
                      
                      class MyRect : public QGraphicsRectItem
                      {
                      public:
                          MyRect(QGraphicsItem * parent = nullptr)
                              : QGraphicsRectItem(QRectF{0.0f, 0.0f, 100.0f, 100.0f}, parent)
                          {
                              setPen(QPen(Qt::white));
                              setBrush(QBrush(Qt::white, Qt::SolidPattern));
                          }
                      };
                      
                      class MyScene: public QGraphicsScene
                      {
                          Q_OBJECT
                      
                      public:
                          MyScene(QObject * parent = Q_NULLPTR)
                              : QGraphicsScene(parent), background(0.0f, 0.0f, 500.0f, 500.0f), global_rect(Q_NULLPTR)
                          {
                              background.setPen(QPen(Qt::white));
                              background.setBrush(QBrush(Qt::red, Qt::SolidPattern));
                              addItem(&background);
                      
                              QGraphicsDropShadowEffect * shadow=new QGraphicsDropShadowEffect();
                              shadow->setBlurRadius(15.0f);
                      
                              global_rect.setGraphicsEffect(shadow);
                      
                      
                              addItem(new QGraphicsRectItem(&global_rect));
                      
                          }
                      
                          void mouseReleaseEvent(QGraphicsSceneMouseEvent* me) override
                          {
                              if (me->button()==Qt::LeftButton)
                              {
                                  addItem(&global_rect);
                                  global_rect.setPos(me->scenePos());
                              }
                              else if (me->button()==Qt::RightButton)
                              {
                                  removeItem(&global_rect);
                              }
                      
                              QGraphicsScene::mouseReleaseEvent(me);
                          }
                      
                          QGraphicsRectItem background;
                          MyRect global_rect;
                      };
                      
                      #endif // MYSCENE_H
                      

                      Kind regards.

                      Read and abide by the Qt Code of Conduct

                      1 Reply Last reply
                      0
                      • Chris KawaC Offline
                        Chris KawaC Offline
                        Chris Kawa
                        Lifetime Qt Champion
                        wrote on last edited by
                        #13

                        I've seen this problem recently in my colleague's code, and a (curious enough) workaround we found was calling prepareGeometryChange() before the item was removed from the scene. Unfortunately it's protected so you need to expose it e.g.

                        class MyRect: public QGraphicsRectItem {
                        public:
                            void ouch() { prepareGeometryChange(); }
                            ...
                        };
                        
                        //and then
                            if (global_rect!=nullptr)
                            {
                                global_rect->ouch(); //call it
                                scene->removeItem(global_rect);
                                delete global_rect;
                                global_rect=nullptr;
                            }
                        

                        Unfortunately I never got to investigate what's going on in depth but it looks like a Qt bug.
                        A working theory based on the callstack we got was that the crash occurs when the area of the effect crosses the bounds of the internal BSP structure, but the item itself does not. The idea was that the item is removed and that part of the tree it occupied is invalidated but the part that only displayed the effect is not and somehow still somewhere holds a pointer to now deleted object. Calling prepareGeometryChange() would invalidate the whole area (including the area of the effect) and thus prevented the crash.
                        As I said - it's just a shot in the dark and can be totally off track, so take it with a grain of salt, but it's definitely not an expected behavior and I'd consider reporting it.

                        T 1 Reply Last reply
                        3
                        • Chris KawaC Chris Kawa

                          I've seen this problem recently in my colleague's code, and a (curious enough) workaround we found was calling prepareGeometryChange() before the item was removed from the scene. Unfortunately it's protected so you need to expose it e.g.

                          class MyRect: public QGraphicsRectItem {
                          public:
                              void ouch() { prepareGeometryChange(); }
                              ...
                          };
                          
                          //and then
                              if (global_rect!=nullptr)
                              {
                                  global_rect->ouch(); //call it
                                  scene->removeItem(global_rect);
                                  delete global_rect;
                                  global_rect=nullptr;
                              }
                          

                          Unfortunately I never got to investigate what's going on in depth but it looks like a Qt bug.
                          A working theory based on the callstack we got was that the crash occurs when the area of the effect crosses the bounds of the internal BSP structure, but the item itself does not. The idea was that the item is removed and that part of the tree it occupied is invalidated but the part that only displayed the effect is not and somehow still somewhere holds a pointer to now deleted object. Calling prepareGeometryChange() would invalidate the whole area (including the area of the effect) and thus prevented the crash.
                          As I said - it's just a shot in the dark and can be totally off track, so take it with a grain of salt, but it's definitely not an expected behavior and I'd consider reporting it.

                          T Offline
                          T Offline
                          tfm123
                          wrote on last edited by
                          #14

                          @Chris-Kawa Thank you a lot for your assessment. Also your suggestion fixes the problem.

                          I have put the prepareGeometryChange() into MyRect's destructor. I now just delete the item and do not explicitly remove it from the scene. ~QGraphicsItem() does remove itself from the scene and it is called after ~MyRect(). So this is basicly equivalent to what you wrote, but saves the exposure of prepareGeometryChange().

                          The only thing you cannot do now, is to remove the item from the scene without deleting it. I'm going to file a bug report when i have some time.

                          Thanks again!

                          O jsulmJ 3 Replies Last reply
                          0
                          • T tfm123

                            @Chris-Kawa Thank you a lot for your assessment. Also your suggestion fixes the problem.

                            I have put the prepareGeometryChange() into MyRect's destructor. I now just delete the item and do not explicitly remove it from the scene. ~QGraphicsItem() does remove itself from the scene and it is called after ~MyRect(). So this is basicly equivalent to what you wrote, but saves the exposure of prepareGeometryChange().

                            The only thing you cannot do now, is to remove the item from the scene without deleting it. I'm going to file a bug report when i have some time.

                            Thanks again!

                            O Offline
                            O Offline
                            Oleksandr Malyushytskyy
                            wrote on last edited by
                            #15

                            Check if item bounding rectangle increases if QGraphicsDropShadowEffect is on.
                            If not this is a problem.
                            This would be also a reason why calling prepareGeometryChange() helped.
                            I had similar problem for my custom items if bounding rectangle was not set properly.

                            T 1 Reply Last reply
                            0
                            • T tfm123

                              @Chris-Kawa Thank you a lot for your assessment. Also your suggestion fixes the problem.

                              I have put the prepareGeometryChange() into MyRect's destructor. I now just delete the item and do not explicitly remove it from the scene. ~QGraphicsItem() does remove itself from the scene and it is called after ~MyRect(). So this is basicly equivalent to what you wrote, but saves the exposure of prepareGeometryChange().

                              The only thing you cannot do now, is to remove the item from the scene without deleting it. I'm going to file a bug report when i have some time.

                              Thanks again!

                              O Offline
                              O Offline
                              Oleksandr Malyushytskyy
                              wrote on last edited by Oleksandr Malyushytskyy
                              #16

                              @tfm123 As a workaround you could call setGraphicsEffect(0). before removing item., but this does not guarantee to fix all problems, the same as approach you used.
                              Problem may still occur in other usage cases which were not found by you yet. Drawing of the item should be always inside the bounding box it reports.

                              T 1 Reply Last reply
                              0
                              • T tfm123

                                @Chris-Kawa Thank you a lot for your assessment. Also your suggestion fixes the problem.

                                I have put the prepareGeometryChange() into MyRect's destructor. I now just delete the item and do not explicitly remove it from the scene. ~QGraphicsItem() does remove itself from the scene and it is called after ~MyRect(). So this is basicly equivalent to what you wrote, but saves the exposure of prepareGeometryChange().

                                The only thing you cannot do now, is to remove the item from the scene without deleting it. I'm going to file a bug report when i have some time.

                                Thanks again!

                                jsulmJ Offline
                                jsulmJ Offline
                                jsulm
                                Lifetime Qt Champion
                                wrote on last edited by
                                #17

                                @tfm123 Why do you create a QGraphicsRectItem instance in the body of the constructor?

                                MyRect(QGraphicsItem* parent = nullptr):
                                		QGraphicsRectItem{QRectF{0.0f, 0.0f, 100.0f, 100.0f}, parent}
                                {
                                    setPen(QPen{Qt::white});
                                    setBrush(QBrush{Qt::white, Qt::SolidPattern});
                                    new QGraphicsRectItem{this}; // <-- why this?
                                }
                                

                                https://forum.qt.io/topic/113070/qt-code-of-conduct

                                T 1 Reply Last reply
                                0
                                • O Oleksandr Malyushytskyy

                                  Check if item bounding rectangle increases if QGraphicsDropShadowEffect is on.
                                  If not this is a problem.
                                  This would be also a reason why calling prepareGeometryChange() helped.
                                  I had similar problem for my custom items if bounding rectangle was not set properly.

                                  T Offline
                                  T Offline
                                  tfm123
                                  wrote on last edited by
                                  #18

                                  @Oleksandr-Malyushytskyy The boundingRect() of the rectangle does not change when the effect is applied to it. But the effect itself has a boundingRect() which changes when the effect is applied. I don't know if this is how it is supposed to be.

                                  1 Reply Last reply
                                  0
                                  • O Oleksandr Malyushytskyy

                                    @tfm123 As a workaround you could call setGraphicsEffect(0). before removing item., but this does not guarantee to fix all problems, the same as approach you used.
                                    Problem may still occur in other usage cases which were not found by you yet. Drawing of the item should be always inside the bounding box it reports.

                                    T Offline
                                    T Offline
                                    tfm123
                                    wrote on last edited by
                                    #19

                                    @Oleksandr-Malyushytskyy This is one of the first things I tried and unfortunately it does not help.

                                    1 Reply Last reply
                                    0
                                    • jsulmJ jsulm

                                      @tfm123 Why do you create a QGraphicsRectItem instance in the body of the constructor?

                                      MyRect(QGraphicsItem* parent = nullptr):
                                      		QGraphicsRectItem{QRectF{0.0f, 0.0f, 100.0f, 100.0f}, parent}
                                      {
                                          setPen(QPen{Qt::white});
                                          setBrush(QBrush{Qt::white, Qt::SolidPattern});
                                          new QGraphicsRectItem{this}; // <-- why this?
                                      }
                                      
                                      T Offline
                                      T Offline
                                      tfm123
                                      wrote on last edited by
                                      #20

                                      @jsulm Like I already wrote, the issue only triggers when MyRect has a child. This is an example. The only reason for things to be like they are is that they are required to be this way to trigger the issue.

                                      1 Reply Last reply
                                      0
                                      • A Offline
                                        A Offline
                                        aziesemer
                                        wrote on last edited by aziesemer
                                        #21

                                        @tfm123

                                        I think this bug is described here:
                                        https://bugreports.qt.io/browse/QTBUG-18021

                                        My workaround was to setVisible(false) before removing the item.

                                        JoeCFDJ 1 Reply Last reply
                                        0
                                        • A aziesemer

                                          @tfm123

                                          I think this bug is described here:
                                          https://bugreports.qt.io/browse/QTBUG-18021

                                          My workaround was to setVisible(false) before removing the item.

                                          JoeCFDJ Offline
                                          JoeCFDJ Offline
                                          JoeCFD
                                          wrote on last edited by JoeCFD
                                          #22

                                          @aziesemer Got random crashes of QLabel with PixMap + QGraphicsEffect on Ubuntu 18.04 + Qt 5.15.2.
                                          Backtrace:
                                          #0 0x00007ffff1510fb7 in __GI_raise (sig=sig@entry=6) at ../sysdeps/unix/sysv/linux/raise.c:51
                                          #1 0x00007ffff1512921 in __GI_abort () at abort.c:79
                                          #2 0x00007ffff155b967 in __libc_message (action=action@entry=do_abort, fmt=fmt@entry=0x7ffff1688b0d "%s\n") at ../sysdeps/posix/libc_fatal.c:181
                                          #3 0x00007ffff15629da in malloc_printerr (str=str@entry=0x7ffff1686c3a "corrupted double-linked list") at malloc.c:5342
                                          #4 0x00007ffff1562b94 in malloc_consolidate (av=av@entry=0x7ffff18bdc40 <main_arena>) at malloc.c:4486
                                          #5 0x00007ffff156a12b in _int_free (have_lock=0, p=<optimized out>, av=0x7ffff18bdc40 <main_arena>) at malloc.c:4392
                                          #6 0x00007ffff156a12b in __GI___libc_free (mem=0x5555600bf810) at malloc.c:3134
                                          #7 0x00007ffff3f512ba in QImageData::~QImageData() () at /opt/thirdParties/Qt/5.15.2/gcc_64/lib/libQt5Gui.so.5
                                          #8 0x00007ffff3f517a7 in QImage::~QImage() () at /opt/thirdParties/Qt/5.15.2/gcc_64/lib/libQt5Gui.so.5
                                          #9 0x00007ffff3f8b4dc in QRasterPlatformPixmap::~QRasterPlatformPixmap() () at /opt/thirdParties/Qt/5.15.2/gcc_64/lib/libQt5Gui.so.5
                                          #10 0x00007ffff3f8b4f9 in QRasterPlatformPixmap::~QRasterPlatformPixmap() () at /opt/thirdParties/Qt/5.15.2/gcc_64/lib/libQt5Gui.so.5
                                          #11 0x00007ffff3f81091 in QPixmap::~QPixmap() () at /opt/thirdParties/Qt/5.15.2/gcc_64/lib/libQt5Gui.so.5
                                          #12 0x00007ffff3f858b1 in QPixmapCache::remove(QPixmapCache::Key const&) () at /opt/thirdParties/Qt/5.15.2/gcc_64/lib/libQt5Gui.so.5
                                          #13 0x00007ffff4b66165 in QWidgetPrivate::invalidateGraphicsEffectsRecursively() () at /opt/thirdParties/Qt/5.15.2/gcc_64/lib/libQt5Widgets.so.5
                                          #14 0x00007ffff4b661b2 in QWidgetPrivate::setDirtyOpaqueRegion() () at /opt/thirdParties/Qt/5.15.2/gcc_64/lib/libQt5Widgets.so.5
                                          #15 0x00007ffff4b79180 in QWidget::~QWidget() () at /opt/thirdParties/Qt/5.15.2/gcc_64/lib/libQt5Widgets.so.5
                                          #16 0x00007ffff4c6baa9 in QLabel::~QLabel() () at /opt/thirdParties/Qt/5.15.2/gcc_64/lib/libQt5Widgets.so.5

                                          Anyone had the same issue?

                                          JoeCFDJ 1 Reply Last reply
                                          0

                                          • Login

                                          • Login or register to search.
                                          • First post
                                            Last post
                                          0
                                          • Categories
                                          • Recent
                                          • Tags
                                          • Popular
                                          • Users
                                          • Groups
                                          • Search
                                          • Get Qt Extensions
                                          • Unsolved