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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 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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