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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

    Thank you for a reply, I am familiar with gdb stack trace and was hoping to see it, but at the time of the crash, textual log doesn't contain anything that resembles it, and even stack in QT is empty. But it might not be enabled by default, and I can't find a way to enable it.

    I look into getting a stack trace from gdb

    JonBJ Offline
    JonBJ Offline
    JonB
    wrote on last edited by
    #4

    @vedranMv
    I am not used to seeing anything like the textual output you show when running/debugging inside Qt Creator. Is that what you are doing? Normally then the Creator debugger windows handle all of this, you don't need to look at "gdb output".

    and even stack in QT is empty

    Show screenshot of what you mean here?

    V 1 Reply Last reply
    0
    • JonBJ JonB

      @vedranMv
      I am not used to seeing anything like the textual output you show when running/debugging inside Qt Creator. Is that what you are doing? Normally then the Creator debugger windows handle all of this, you don't need to look at "gdb output".

      and even stack in QT is empty

      Show screenshot of what you mean here?

      V Offline
      V Offline
      vedranMv
      wrote on last edited by
      #5

      @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 '??'

      JonBJ 1 Reply Last reply
      0
      • V vedranMv

        @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 '??'

        JonBJ Offline
        JonBJ Offline
        JonB
        wrote on 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
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        • JonBJ JonB

          @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 Offline
          V Offline
          vedranMv
          wrote on 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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          • SGaistS Offline
            SGaistS Offline
            SGaist
            Lifetime Qt Champion
            wrote on 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
            0
            • SGaistS SGaist

              Hi,

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

              V Offline
              V Offline
              vedranMv
              wrote on 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

              JonBJ 1 Reply Last reply
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              • V vedranMv

                @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

                JonBJ Offline
                JonBJ Offline
                JonB
                wrote on 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.

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                • JonBJ JonB

                  @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 Offline
                  V Offline
                  vedranMv
                  wrote on 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 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

                    1 Reply Last reply
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                    • JohanSoloJ Offline
                      JohanSoloJ Offline
                      JohanSolo
                      wrote on 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
                      2
                      • JohanSoloJ JohanSolo

                        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.

                        V Offline
                        V Offline
                        vedranMv
                        wrote on 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.

                        JohanSoloJ 1 Reply Last reply
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                        • V vedranMv

                          @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.

                          JohanSoloJ Offline
                          JohanSoloJ Offline
                          JohanSolo
                          wrote on 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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