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

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

    @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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