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Forum Update on Tuesday, May 27th 2025

Seg fault when deleting Q3DScatter object

Scheduled Pinned Locked Moved Unsolved General and Desktop
q3dscattersegmentationsegfault
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  • V vedranMv
    25 Oct 2020, 09:25

    @JonB Yes, I simply press on the debug button inside QT creator. The view looks something like:
    ac6271f3-314a-45fa-a7e3-873d191764d4-image.png
    I might've pressed something to add the two windows with textual output on top. If I remember correctly they were not there by default.
    Blue arrow shows what I refer to as "QT stack being empty". I'm used to seeing a call stack in here, but when the issue happens, it shows '??'

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    JonB
    wrote on 25 Oct 2020, 11:24 last edited by JonB
    #6

    @vedranMv
    Why are you in the Disassembler pane? I think use the up/down arrows there to pick the "stack trace" pane? Find & show that, even if it turns out the stack is corrupted (which can happen), at least let's see the appropriate pane. I don't recognise your layout/the top pane, but then I am Qt Creator/gdb/gcc under Linux.

    V 1 Reply Last reply 25 Oct 2020, 13:10
    0
    • J JonB
      25 Oct 2020, 11:24

      @vedranMv
      Why are you in the Disassembler pane? I think use the up/down arrows there to pick the "stack trace" pane? Find & show that, even if it turns out the stack is corrupted (which can happen), at least let's see the appropriate pane. I don't recognise your layout/the top pane, but then I am Qt Creator/gdb/gcc under Linux.

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      vedranMv
      wrote on 25 Oct 2020, 13:10 last edited by
      #7

      @JonB Thanks for suggestion. That's where I'm taken when fault happens. I guess it means that fault is thrown in some of the libraries, which can't be tracked? If I use the left/right arrows, I get taken back to another dissasembly screen when pressing left arrow for the first time, then back to whatever document I edited the last, when pressed for the second time.

      I have been playing with destructor of ScatterPlot class. If I leave it empty or make it like this, I get seg fault:

      ScatterPlot ::~ScatterPlot ()
      {
            m_graph->seriesList().clear();
            m_graph->close();
            m_graph->deleteLater();
      }
      

      If I instead directly call m_graph's destructor, seg fault is not happening.

      ScatterPlot ::~ScatterPlot ()
      {
            m_graph->seriesList().clear();
            m_graph->close();
            m_graph->~Q3DScatter();
      }
      
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      • S Offline
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        SGaist
        Lifetime Qt Champion
        wrote on 25 Oct 2020, 16:27 last edited by
        #8

        Hi,

        Might be a silly question but are you sure you are using a debug build ?

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

        V 1 Reply Last reply 25 Oct 2020, 19:05
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        • S SGaist
          25 Oct 2020, 16:27

          Hi,

          Might be a silly question but are you sure you are using a debug build ?

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          vedranMv
          wrote on 25 Oct 2020, 19:05 last edited by
          #9

          @SGaist Thanks, I don't mind revisiting the basics :) I am running a debug build and if I force a fault in my part of the code, I get output as expected with a fault point in code and stack trace underneath

          45e5d7df-ea90-4831-8ef3-dfd0f875ea6b-image.png

          J 1 Reply Last reply 26 Oct 2020, 09:18
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          • V vedranMv
            25 Oct 2020, 19:05

            @SGaist Thanks, I don't mind revisiting the basics :) I am running a debug build and if I force a fault in my part of the code, I get output as expected with a fault point in code and stack trace underneath

            45e5d7df-ea90-4831-8ef3-dfd0f875ea6b-image.png

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            JonB
            wrote on 26 Oct 2020, 09:18 last edited by
            #10

            @vedranMv
            So now you do have the stack trace pane which you said you couldn't get at!

            It's dumping inside QComboBox::currentText(). For SIGSEGV I would check in debugger what your *X is, taken from your inCh, for validity.

            V 1 Reply Last reply 26 Oct 2020, 15:51
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            • J JonB
              26 Oct 2020, 09:18

              @vedranMv
              So now you do have the stack trace pane which you said you couldn't get at!

              It's dumping inside QComboBox::currentText(). For SIGSEGV I would check in debugger what your *X is, taken from your inCh, for validity.

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              vedranMv
              wrote on 26 Oct 2020, 15:51 last edited by
              #11

              @JonB No, this is an example to show that normally I am able to see the stack trace. This was provoked on purpose. But thanks :)

              When the original segfault happens, this window remains empty as shown above

              1 Reply Last reply
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              • V Offline
                V Offline
                vedranMv
                wrote on 26 Oct 2020, 21:30 last edited by vedranMv
                #12

                I have uploaded the code to github, in case anybody is interested in trying it out: https://github.com/vedranMv/dataDashboard/

                There's a branch called segfault which in combination with the description above creates the fault. To recap, steps are:

                1. Run the app
                2. In the Dashboard tab press "Add 3D" button
                3. In the Dashboard tab press "Add scatter" button
                4. Press 'X' to close scatter window
                5. Enjoy the segfault

                There's also what I believe is a fix, but I'd like to understand why it works. Documentation and general recommendation is to use deleteLater() instead, but that also results in a segfault :)
                156ef785-bcd3-48e5-a037-c864f4afda61-image.png

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                • J Offline
                  J Offline
                  JohanSolo
                  wrote on 27 Oct 2020, 06:59 last edited by JohanSolo
                  #13

                  Ouch, that hurts! Why do you explicitly call the destructor?
                  If you do want to free the memory (and do you?) use delete _graph. Did you set a parent to your Q3DScatter instance? If you did, then it will be deleted for you.

                  Edit:
                  @vedranMv said in Seg fault when deleting Q3DScatter object:

                  There's also what I believe is a fix, but I'd like to understand why it works. Documentation and general recommendation is to use deleteLater() instead, but that also results in a segfault :)

                  Then it means you're most probably deleting twice the instance: the first time with your dicey call to the destructor / or deleteLater, and the second when the parent object gets deleted.

                  `They did not know it was impossible, so they did it.'
                  -- Mark Twain

                  V 1 Reply Last reply 27 Oct 2020, 08:29
                  2
                  • J JohanSolo
                    27 Oct 2020, 06:59

                    Ouch, that hurts! Why do you explicitly call the destructor?
                    If you do want to free the memory (and do you?) use delete _graph. Did you set a parent to your Q3DScatter instance? If you did, then it will be deleted for you.

                    Edit:
                    @vedranMv said in Seg fault when deleting Q3DScatter object:

                    There's also what I believe is a fix, but I'd like to understand why it works. Documentation and general recommendation is to use deleteLater() instead, but that also results in a segfault :)

                    Then it means you're most probably deleting twice the instance: the first time with your dicey call to the destructor / or deleteLater, and the second when the parent object gets deleted.

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                    vedranMv
                    wrote on 27 Oct 2020, 08:29 last edited by
                    #14

                    @JohanSolo Thanks, I for some reason did not think of using delete here at all :D That works just as good!

                    Regarding your edit, what you're writing makes perfect sense, but as you can see in the code, it's the other way around. If I don't call delete manually, I get a segfault. And yes, object gets a parent because I encapsulate it into a container widget, which is then added into the UI window.

                    J 1 Reply Last reply 27 Oct 2020, 08:32
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                    • V vedranMv
                      27 Oct 2020, 08:29

                      @JohanSolo Thanks, I for some reason did not think of using delete here at all :D That works just as good!

                      Regarding your edit, what you're writing makes perfect sense, but as you can see in the code, it's the other way around. If I don't call delete manually, I get a segfault. And yes, object gets a parent because I encapsulate it into a container widget, which is then added into the UI window.

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                      JohanSolo
                      wrote on 27 Oct 2020, 08:32 last edited by
                      #15

                      @vedranMv said in Seg fault when deleting Q3DScatter object:

                      Regarding your edit, what you're writing makes perfect sense, but as you can see in the code, it's the other way around. If I don't call delete manually, I get a segfault.

                      Then it seems to me there must be something else wrong in your code.

                      `They did not know it was impossible, so they did it.'
                      -- Mark Twain

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                      15/15

                      27 Oct 2020, 08:32

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