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  4. Why cross-thread invoke will report “QObject: Cannot create children for a parent that is in a different thread.”?
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Why cross-thread invoke will report “QObject: Cannot create children for a parent that is in a different thread.”?

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  • Crawl.WC Offline
    Crawl.WC Offline
    Crawl.W
    wrote on last edited by Crawl.W
    #1

    Such as the title,confusing warning!Such as a member funcation of QSerialPort's instance invoked by child thread. I do not wanna get a solution,rahter than know why for the warning is given.

    //main.cpp
    int main(int argc, char *argv[])
    {
        //...
        TestClass *testclass2 = TestClass::GetInstance();
    
        TempMyThread *mythread = new TempMyThread;
        mythread->start();
        //...
    }
    
    //tempthread.cpp
    void TempMyThread::run(){
        TestClass *testclass2 = TestClass::GetInstance();
        qDebug() << "temp thread " << testclass2;
    
        testclass2->myserialport->write("1");
    }
    

    myserialport is TestClass's member var.

    J.HilkJ 1 Reply Last reply
    0
    • Crawl.WC Crawl.W

      Such as the title,confusing warning!Such as a member funcation of QSerialPort's instance invoked by child thread. I do not wanna get a solution,rahter than know why for the warning is given.

      //main.cpp
      int main(int argc, char *argv[])
      {
          //...
          TestClass *testclass2 = TestClass::GetInstance();
      
          TempMyThread *mythread = new TempMyThread;
          mythread->start();
          //...
      }
      
      //tempthread.cpp
      void TempMyThread::run(){
          TestClass *testclass2 = TestClass::GetInstance();
          qDebug() << "temp thread " << testclass2;
      
          testclass2->myserialport->write("1");
      }
      

      myserialport is TestClass's member var.

      J.HilkJ Offline
      J.HilkJ Offline
      J.Hilk
      Moderators
      wrote on last edited by
      #2

      hi @Crawl.W

      it's not a warning, its an error and not really confusing.

      What is the issue exactly ?

      Chances are, the the function is executed in the thread of the function caller. If you inside the function create a new QObject with for example this as parent, then you're trying to parent a Object to a parent that lives in a different thread compared to the creating thread.


      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.

      Crawl.WC 1 Reply Last reply
      3
      • Crawl.WC Offline
        Crawl.WC Offline
        Crawl.W
        wrote on last edited by
        #3
        This post is deleted!
        1 Reply Last reply
        0
        • J.HilkJ J.Hilk

          hi @Crawl.W

          it's not a warning, its an error and not really confusing.

          What is the issue exactly ?

          Chances are, the the function is executed in the thread of the function caller. If you inside the function create a new QObject with for example this as parent, then you're trying to parent a Object to a parent that lives in a different thread compared to the creating thread.

          Crawl.WC Offline
          Crawl.WC Offline
          Crawl.W
          wrote on last edited by Crawl.W
          #4

          @J.Hilk But If I just call a funcation like QSerialPort::write(), there is no qobject created in the whole process.That's your a assumption, and sometimes it's not tenable.

          J.HilkJ 1 Reply Last reply
          0
          • Crawl.WC Crawl.W

            @J.Hilk But If I just call a funcation like QSerialPort::write(), there is no qobject created in the whole process.That's your a assumption, and sometimes it's not tenable.

            J.HilkJ Offline
            J.HilkJ Offline
            J.Hilk
            Moderators
            wrote on last edited by
            #5

            @Crawl-W
            of course, that's my assumption, you haven't shown any code. What else can I do but assume ?


            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.

            Crawl.WC 1 Reply Last reply
            2
            • J.HilkJ J.Hilk

              @Crawl-W
              of course, that's my assumption, you haven't shown any code. What else can I do but assume ?

              Crawl.WC Offline
              Crawl.WC Offline
              Crawl.W
              wrote on last edited by
              #6

              @J.Hilk About QSerialPort::write() or QSerialPort::open()? I had tried to seach the source, but no object created seemingly.

              jsulmJ 1 Reply Last reply
              0
              • Crawl.WC Crawl.W

                @J.Hilk About QSerialPort::write() or QSerialPort::open()? I had tried to seach the source, but no object created seemingly.

                jsulmJ Offline
                jsulmJ Offline
                jsulm
                Lifetime Qt Champion
                wrote on last edited by
                #7

                @Crawl.W What else do you do in that other thread?

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

                Crawl.WC 1 Reply Last reply
                0
                • jsulmJ jsulm

                  @Crawl.W What else do you do in that other thread?

                  Crawl.WC Offline
                  Crawl.WC Offline
                  Crawl.W
                  wrote on last edited by
                  #8

                  @jsulm noting.

                  jsulmJ KroMignonK 2 Replies Last reply
                  0
                  • Crawl.WC Crawl.W

                    @jsulm noting.

                    jsulmJ Offline
                    jsulmJ Offline
                    jsulm
                    Lifetime Qt Champion
                    wrote on last edited by
                    #9

                    @Crawl.W Well, without source code others can only guess...

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

                    1 Reply Last reply
                    3
                    • Crawl.WC Crawl.W

                      @jsulm noting.

                      KroMignonK Offline
                      KroMignonK Offline
                      KroMignon
                      wrote on last edited by
                      #10

                      @Crawl.W said in Why cross-thread invoke will report “QObject: Cannot create children for a parent that is in a different thread.”?:

                      noting.

                      You are doing something or you would not have this error message.
                      If you don't want to show your code, then you will have to find yourself where you create a QObject instance with a parent in a different thread.

                      It is an old maxim of mine that when you have excluded the impossible, whatever remains, however improbable, must be the truth. (Sherlock Holmes)

                      Crawl.WC 1 Reply Last reply
                      3
                      • KroMignonK KroMignon

                        @Crawl.W said in Why cross-thread invoke will report “QObject: Cannot create children for a parent that is in a different thread.”?:

                        noting.

                        You are doing something or you would not have this error message.
                        If you don't want to show your code, then you will have to find yourself where you create a QObject instance with a parent in a different thread.

                        Crawl.WC Offline
                        Crawl.WC Offline
                        Crawl.W
                        wrote on last edited by Crawl.W
                        #11

                        I had assumed that the question only need simple description.Code has supplymented.

                        J.HilkJ 1 Reply Last reply
                        1
                        • Crawl.WC Crawl.W

                          I had assumed that the question only need simple description.Code has supplymented.

                          J.HilkJ Offline
                          J.HilkJ Offline
                          J.Hilk
                          Moderators
                          wrote on last edited by
                          #12

                          @Crawl.W

                          now it's obvious.

                          Your singleton is created in the main thread. you write to it inside the worker thread. during write process, a QTimer is created and set, to monitor timeouts. That is created from the wrong thread of course. -> Therefore the error


                          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.

                          Crawl.WC 2 Replies Last reply
                          5
                          • J.HilkJ J.Hilk

                            @Crawl.W

                            now it's obvious.

                            Your singleton is created in the main thread. you write to it inside the worker thread. during write process, a QTimer is created and set, to monitor timeouts. That is created from the wrong thread of course. -> Therefore the error

                            Crawl.WC Offline
                            Crawl.WC Offline
                            Crawl.W
                            wrote on last edited by Crawl.W
                            #13

                            @J.Hilk So it is,You rocks!But I did not find where QTimer created, can you post those source?

                            K 1 Reply Last reply
                            1
                            • Crawl.WC Crawl.W

                              @J.Hilk So it is,You rocks!But I did not find where QTimer created, can you post those source?

                              K Offline
                              K Offline
                              kuzulis
                              Qt Champions 2020
                              wrote on last edited by
                              #14

                              https://github.com/qt/qtserialport/blob/5.12/src/serialport/qserialport_win.cpp#L625

                              Crawl.WC 1 Reply Last reply
                              6
                              • K kuzulis

                                https://github.com/qt/qtserialport/blob/5.12/src/serialport/qserialport_win.cpp#L625

                                Crawl.WC Offline
                                Crawl.WC Offline
                                Crawl.W
                                wrote on last edited by Crawl.W
                                #15

                                @kuzulis @J-Hilk Thanks for everyone,I saw.

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

                                  @Crawl.W

                                  now it's obvious.

                                  Your singleton is created in the main thread. you write to it inside the worker thread. during write process, a QTimer is created and set, to monitor timeouts. That is created from the wrong thread of course. -> Therefore the error

                                  Crawl.WC Offline
                                  Crawl.WC Offline
                                  Crawl.W
                                  wrote on last edited by
                                  #16

                                  @J.Hilk As show in source,the QTimer is very strange, which is single shot and no interval set.Why?Why do not call his timeout's slot function directly?

                                  jsulmJ KroMignonK 2 Replies Last reply
                                  0
                                  • Crawl.WC Crawl.W

                                    @J.Hilk As show in source,the QTimer is very strange, which is single shot and no interval set.Why?Why do not call his timeout's slot function directly?

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

                                    @Crawl.W said in Why cross-thread invoke will report “QObject: Cannot create children for a parent that is in a different thread.”?:

                                    Why do not call his timeout's slot function directly?

                                    Because then it would be a synchronous call - write() would block until it's finished. But since it is an asynchronous API it should not block the caller.

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

                                    Crawl.WC 1 Reply Last reply
                                    5
                                    • Crawl.WC Crawl.W

                                      @J.Hilk As show in source,the QTimer is very strange, which is single shot and no interval set.Why?Why do not call his timeout's slot function directly?

                                      KroMignonK Offline
                                      KroMignonK Offline
                                      KroMignon
                                      wrote on last edited by
                                      #18

                                      @Crawl.W said in Why cross-thread invoke will report “QObject: Cannot create children for a parent that is in a different thread.”?:

                                      Why do not call his timeout's slot function directly?

                                      This is done to call the timeout function after all events in thread queue have been processed.
                                      Here the extract for QTimer::interval:

                                      The default value for this property is 0. A QTimer with a timeout interval of 0 will time out as soon as all the events in the window system's event queue have been processed

                                      It is an old maxim of mine that when you have excluded the impossible, whatever remains, however improbable, must be the truth. (Sherlock Holmes)

                                      1 Reply Last reply
                                      5
                                      • jsulmJ jsulm

                                        @Crawl.W said in Why cross-thread invoke will report “QObject: Cannot create children for a parent that is in a different thread.”?:

                                        Why do not call his timeout's slot function directly?

                                        Because then it would be a synchronous call - write() would block until it's finished. But since it is an asynchronous API it should not block the caller.

                                        Crawl.WC Offline
                                        Crawl.WC Offline
                                        Crawl.W
                                        wrote on last edited by
                                        #19

                                        @jsulm Great,I got it!Thanks.

                                        1 Reply Last reply
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