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My program start to crash because of MessageBox

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  • EngelardE Engelard

    Hi everyone! Today i added function:

    void MainWindow::err(QString text)
    {
        QMessageBox::information(this, "ERROR", text);
    }
    

    It worked well, but in some point, my program start to crash when i try to use it, and i don't know what changed it and no info about reason of crash from Qt...

    Please help, i need such error handler back!

    J.HilkJ Offline
    J.HilkJ Offline
    J.Hilk
    Moderators
    wrote on last edited by
    #11

    @Engelard what os and qt version is this?

    I had some issues like this under ios and 5.12.0 went away with one of the later updates

    also do you call processEvents() somewhre inside your code?
    the static call to QMessageBox spins its own QEventLoop. That on combination with processEvent calls can cause very strange behaviour and crashes.


    Be aware of the Qt Code of Conduct, when posting : https://forum.qt.io/topic/113070/qt-code-of-conduct


    Q: What's that?
    A: It's blue light.
    Q: What does it do?
    A: It turns blue.

    JonBJ 1 Reply Last reply
    2
    • J.HilkJ J.Hilk

      @Engelard what os and qt version is this?

      I had some issues like this under ios and 5.12.0 went away with one of the later updates

      also do you call processEvents() somewhre inside your code?
      the static call to QMessageBox spins its own QEventLoop. That on combination with processEvent calls can cause very strange behaviour and crashes.

      JonBJ Offline
      JonBJ Offline
      JonB
      wrote on last edited by
      #12

      @J.Hilk

      QMessageBox::information();
      

      just as that statement, blocks, spins its own loop, and does not return till the user has pressed a button, correct or not? So you can't be calling processEvents() elsewhere in your own code, unless you're doing something really odd, is that not right?

      J.HilkJ 1 Reply Last reply
      0
      • Christian EhrlicherC Offline
        Christian EhrlicherC Offline
        Christian Ehrlicher
        Lifetime Qt Champion
        wrote on last edited by
        #13

        @JonB said in My program start to crash because of MessageBox:

        is that not right

        When there is a timer which is executed while the message box is shown and in the slot you call processEvents() you will get likely into trouble.

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        JonBJ 1 Reply Last reply
        4
        • JonBJ JonB

          @J.Hilk

          QMessageBox::information();
          

          just as that statement, blocks, spins its own loop, and does not return till the user has pressed a button, correct or not? So you can't be calling processEvents() elsewhere in your own code, unless you're doing something really odd, is that not right?

          J.HilkJ Offline
          J.HilkJ Offline
          J.Hilk
          Moderators
          wrote on last edited by
          #14

          @JonB Just because you call start on an other event loop doesn't mean your original one simply stops.

          You get a general setDisbaled on your MainApplication Ui, everything else is still going on normally.

          Take this example, based on what @Christian-Ehrlicher said:

          int main(int argc, char *argv[])
          {
              QApplication a(argc, argv);
          
              QTimer alive;
              QObject::connect(&alive, &QTimer::timeout, []()->void{qDebug("Alive");});
              alive.start(500);
          
              QPushButton btn;
              btn.show();
              QObject::connect(&btn, &QPushButton::clicked, []()->void{
                                   QMessageBox::information(nullptr, QString("Error"), QString("Some Error Text"));
                               });
          
              return a.exec();
          }
          

          Be aware of the Qt Code of Conduct, when posting : https://forum.qt.io/topic/113070/qt-code-of-conduct


          Q: What's that?
          A: It's blue light.
          Q: What does it do?
          A: It turns blue.

          1 Reply Last reply
          3
          • Christian EhrlicherC Christian Ehrlicher

            @JonB said in My program start to crash because of MessageBox:

            is that not right

            When there is a timer which is executed while the message box is shown and in the slot you call processEvents() you will get likely into trouble.

            JonBJ Offline
            JonBJ Offline
            JonB
            wrote on last edited by
            #15

            @Christian-Ehrlicher , @J-Hilk
            Yep, fair enough, thank you. I was not thinking about a timer. And nor about a timer calling processEvents(), which I would think would not be advisable!

            We shall see whether the OP's problem stems from this.

            1 Reply Last reply
            0
            • mrjjM mrjj

              Hi
              Just start the app in debug mode. And then make it crash.
              It should show in call stack what it was doing before it died.

              Also, just for test add a new button and call
              err("TEST");
              to see if that triggers the crash.
              If not, it must have something to do with the surrounding code from where you
              use err() . maybe a dangling pointer.

              EngelardE Offline
              EngelardE Offline
              Engelard
              wrote on last edited by Engelard
              #16

              @mrjj said in My program start to crash because of MessageBox:

              Just start the app in debug mode. And then make it crash.
              It should show in call stack what it was doing before it died.

              It shows numerous windows like this:

              0_1553493942688_1111.jpg

              What is all mean?

              0_1553493971315_mmm.jpg

              @J.Hilk said in My program start to crash because of MessageBox:

              also do you call processEvents() somewhre inside your code?

              No

              aha_1980A 1 Reply Last reply
              0
              • EngelardE Engelard

                @mrjj said in My program start to crash because of MessageBox:

                Just start the app in debug mode. And then make it crash.
                It should show in call stack what it was doing before it died.

                It shows numerous windows like this:

                0_1553493942688_1111.jpg

                What is all mean?

                0_1553493971315_mmm.jpg

                @J.Hilk said in My program start to crash because of MessageBox:

                also do you call processEvents() somewhre inside your code?

                No

                aha_1980A Offline
                aha_1980A Offline
                aha_1980
                Lifetime Qt Champion
                wrote on last edited by
                #17

                @Engelard said in My program start to crash because of MessageBox:

                It shows numerous windows like this:

                These are uninteresting.

                We need the call stack (also called stack trace, or backtrace)

                Qt has to stay free or it will die.

                1 Reply Last reply
                1
                • mrjjM Offline
                  mrjjM Offline
                  mrjj
                  Lifetime Qt Champion
                  wrote on last edited by mrjj
                  #18

                  Hi
                  the call stack / trace it looks like this
                  alt text

                  1 Reply Last reply
                  1
                  • EngelardE Offline
                    EngelardE Offline
                    Engelard
                    wrote on last edited by Engelard
                    #19

                    0_1553503901234_111.jpg
                    0_1553503907073_222.jpg
                    0_1553503911733_333.jpg

                    mrjjM 1 Reply Last reply
                    0
                    • EngelardE Engelard

                      0_1553503901234_111.jpg
                      0_1553503907073_222.jpg
                      0_1553503911733_333.jpg

                      mrjjM Offline
                      mrjjM Offline
                      mrjj
                      Lifetime Qt Champion
                      wrote on last edited by
                      #20

                      @Engelard
                      and that is from it crashes ?

                      EngelardE 1 Reply Last reply
                      0
                      • mrjjM mrjj

                        @Engelard
                        and that is from it crashes ?

                        EngelardE Offline
                        EngelardE Offline
                        Engelard
                        wrote on last edited by Engelard
                        #21

                        @mrjj If i comment out my line

                        QMessageBox::information(nullptr, "ERROR", "trtrtr");
                        

                        Debugger will be absolutely empty. When i use this static function of messBox, it give me such output.

                        1 Reply Last reply
                        0
                        • EngelardE Offline
                          EngelardE Offline
                          Engelard
                          wrote on last edited by Engelard
                          #22

                          Hey i just found that it work well if i put it in the constructor, so it crashes only when it in my other functions, strange.

                          UPDATE:

                          Here is info for everyone. Yesterday, i noticed that the app not just crashed, it crashed like.. twice, and all my app mostly about the use of "pressed" and "released" SLOTs of the buttons. I removed usage of release slot, and now app do just single crash. Here is how "release button" chain looks like:

                          void MainWindow::on_button_Q_released()
                          {
                              //lightRestore(currentButton);
                          }
                          

                          And here is that only function which it use:

                          void MainWindow::lightRestore(QPushButton *lamp)
                          {
                              lamp->setStyleSheet("QPushButton{color: white;background-color: rgb(35,35,35);border-style: outset;border-width: 2px;border-radius: 15px;border-color:  black;font: bold 12px;}");
                          }
                          
                          J.HilkJ 1 Reply Last reply
                          0
                          • hskoglundH Online
                            hskoglundH Online
                            hskoglund
                            wrote on last edited by
                            #23

                            Hi, if I read the trace correctly, the user presses the button_G, that functions calls mainJourney() which issues the QMessageBox() call.
                            Could you show the code for mainJourney()?

                            EngelardE 1 Reply Last reply
                            0
                            • EngelardE Engelard

                              Hey i just found that it work well if i put it in the constructor, so it crashes only when it in my other functions, strange.

                              UPDATE:

                              Here is info for everyone. Yesterday, i noticed that the app not just crashed, it crashed like.. twice, and all my app mostly about the use of "pressed" and "released" SLOTs of the buttons. I removed usage of release slot, and now app do just single crash. Here is how "release button" chain looks like:

                              void MainWindow::on_button_Q_released()
                              {
                                  //lightRestore(currentButton);
                              }
                              

                              And here is that only function which it use:

                              void MainWindow::lightRestore(QPushButton *lamp)
                              {
                                  lamp->setStyleSheet("QPushButton{color: white;background-color: rgb(35,35,35);border-style: outset;border-width: 2px;border-radius: 15px;border-color:  black;font: bold 12px;}");
                              }
                              
                              J.HilkJ Offline
                              J.HilkJ Offline
                              J.Hilk
                              Moderators
                              wrote on last edited by
                              #24

                              @Engelard said in My program start to crash because of MessageBox:

                              currentButton

                              seems like currentButton is no longer a valid QPushButton-Pointer,

                              If you have a pointer member variable, you should make sure that you always set it to a nullptr when the pointed to object is destroyed/invalid.
                              And always check against that before accessing it:

                              void MainWindow::lightRestore(QPushButton *lamp)
                              {
                                  if(lamp)
                                      lamp->setStyleSheet(...);
                              }
                              

                              Be aware of the Qt Code of Conduct, when posting : https://forum.qt.io/topic/113070/qt-code-of-conduct


                              Q: What's that?
                              A: It's blue light.
                              Q: What does it do?
                              A: It turns blue.

                              EngelardE 1 Reply Last reply
                              3
                              • hskoglundH hskoglund

                                Hi, if I read the trace correctly, the user presses the button_G, that functions calls mainJourney() which issues the QMessageBox() call.
                                Could you show the code for mainJourney()?

                                EngelardE Offline
                                EngelardE Offline
                                Engelard
                                wrote on last edited by Engelard
                                #25

                                @hskoglund said in My program start to crash because of MessageBox:

                                Could you show the code for mainJourney()?

                                No, that stuff is too big, it called from pressButton SLOT, above much simplier and compact example of release, it does the same crash.

                                1 Reply Last reply
                                0
                                • J.HilkJ J.Hilk

                                  @Engelard said in My program start to crash because of MessageBox:

                                  currentButton

                                  seems like currentButton is no longer a valid QPushButton-Pointer,

                                  If you have a pointer member variable, you should make sure that you always set it to a nullptr when the pointed to object is destroyed/invalid.
                                  And always check against that before accessing it:

                                  void MainWindow::lightRestore(QPushButton *lamp)
                                  {
                                      if(lamp)
                                          lamp->setStyleSheet(...);
                                  }
                                  
                                  EngelardE Offline
                                  EngelardE Offline
                                  Engelard
                                  wrote on last edited by Engelard
                                  #26

                                  @J.Hilk nice advice, tnx. But it valid(without that messageBox stuff everything works perfectly), i added if-check line. Nothing it changed(2nd crash exist).

                                  UPDATE:

                                  If i put messageBox AFTER assignment of that button - will be no crashes, but also it is not possible to hold the button(because this messageBox take over priority and it is now main waindow).

                                  void MainWindow::on_button_Q_pressed()
                                  {
                                      currentButton = mainJourney('Q');
                                      lightSet(currentButton);
                                      QMessageBox::information(nullptr, "ERROR", "trtrtr");
                                  }
                                  
                                  1 Reply Last reply
                                  1
                                  • EngelardE Offline
                                    EngelardE Offline
                                    Engelard
                                    wrote on last edited by Engelard
                                    #27

                                    Oh, finally! With the help of @J-Hilk i realised what was wrong.

                                    1. add if-cheks in "lights" functions.
                                    2. assign currentButton to nullptr in the constructor. (i mistakenly thought that created pointers of the objects without assignment by default nullptr)

                                    Now no crashes. Tnx @J-Hilk !

                                    1 Reply Last reply
                                    0

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