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Forum Update on Monday, May 27th 2025

segmentation fault

Scheduled Pinned Locked Moved Unsolved Mobile and Embedded
qt 4.8cross-compilingarmv7mqtt
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  • MilavM Milav

    @jsulm

    Yes i delete the build directory and again i rebuild the whole project but i couldn't run the example. it is not work.

    jsulmJ Offline
    jsulmJ Offline
    jsulm
    Lifetime Qt Champion
    wrote on last edited by
    #15

    @Milav said in segmentation fault:

    it is not work

    What exactly does this mean? Doesn't build? Crashes? Does not do what it should? ...?

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

    MilavM 1 Reply Last reply
    0
    • jsulmJ jsulm

      @Milav said in segmentation fault:

      it is not work

      What exactly does this mean? Doesn't build? Crashes? Does not do what it should? ...?

      MilavM Offline
      MilavM Offline
      Milav
      wrote on last edited by
      #16

      @jsulm

      it means build successful.

      but it is not run, it is crashes application.

      means while running the application, the application are crash.

      with following error.

      "segmentation fault"

      and i debug the application via qDebug(), the application are crash on following line of code.

      mosq->subscribe(NULL, "sensors/temprature", 1); // Segmentation falut
      
      J.HilkJ 1 Reply Last reply
      0
      • MilavM Milav

        @jsulm

        it means build successful.

        but it is not run, it is crashes application.

        means while running the application, the application are crash.

        with following error.

        "segmentation fault"

        and i debug the application via qDebug(), the application are crash on following line of code.

        mosq->subscribe(NULL, "sensors/temprature", 1); // Segmentation falut
        
        J.HilkJ Offline
        J.HilkJ Offline
        J.Hilk
        Moderators
        wrote on last edited by
        #17

        @Milav well, did you press connect before you pressed subscribe?

        If not it will crash...


        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.

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

          @Milav well, did you press connect before you pressed subscribe?

          If not it will crash...

          MilavM Offline
          MilavM Offline
          Milav
          wrote on last edited by
          #18

          @J-Hilk

          I press first Connect button and than after i press subscribed button.

          but it is crash...

          jsulmJ 1 Reply Last reply
          0
          • MilavM Milav

            @jsulm

            when i build the project it give me only warning like follow.

            /home/tdp0009/Teq_Projects/Aug_2019/HMI/Firmware/QT/12_oct_hmi/qt-mosquitto-master/qtmosq.h:14: warning: 'mosqpp::mosquittopp::mosquittopp(const char*, bool)' is deprecated (declared at ../../../../CC_Lib/mosquitto/phytec_wega_build_websockets/usr/local/include/mosquittopp.h:94) [-Wdeprecated-declarations]
            
            /home/tdp0009/Teq_Projects/Aug_2019/HMI/Firmware/QT/12_oct_hmi/qt-mosquitto-master/mainwindow.cpp:57: warning: 'int mosqpp::mosquittopp::connect_async(const char*, int, int)' is deprecated (declared at ../../../../CC_Lib/mosquitto/phytec_wega_build_websockets/usr/local/include/mosquittopp.h:103) [-Wdeprecated-declarations]
            
            /home/tdp0009/Teq_Projects/Aug_2019/HMI/Firmware/QT/12_oct_hmi/qt-mosquitto-master/mainwindow.cpp:80: warning: 'int mosqpp::mosquittopp::subscribe(int*, const char*, int)' is deprecated (declared at ../../../../CC_Lib/mosquitto/phytec_wega_build_websockets/usr/local/include/mosquittopp.h:110) [-Wdeprecated-declarations]
            

            but i face segmentation fault error when i run that project, an i found the line at where i face segmentation fault.

            it is ...

            mosq->subscribe(NULL, "sensors/temprature", 1); // Segmentation falut
            
            M Offline
            M Offline
            mvuori
            wrote on last edited by
            #19

            If temprature is supposed to be temperature, this is a bug:
            mosq->subscribe(NULL, "sensors/temprature", 1);

            MilavM 1 Reply Last reply
            0
            • MilavM Milav

              @J-Hilk

              I press first Connect button and than after i press subscribed button.

              but it is crash...

              jsulmJ Offline
              jsulmJ Offline
              jsulm
              Lifetime Qt Champion
              wrote on last edited by
              #20

              @Milav Did you debug to see whether mosq is not a nullptr? If it's not please provide stack trace after crash.
              Also, in connectPressed it does a return if port is not set - is it set if you press connect?
              You should really run through debugger...

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

              MilavM 1 Reply Last reply
              0
              • jsulmJ jsulm

                @Milav Did you debug to see whether mosq is not a nullptr? If it's not please provide stack trace after crash.
                Also, in connectPressed it does a return if port is not set - is it set if you press connect?
                You should really run through debugger...

                MilavM Offline
                MilavM Offline
                Milav
                wrote on last edited by
                #21

                @jsulm

                i debug the size of mosq pointer through following code.

                 int size = sizeof(*mosq);
                    qDebug() << size;
                

                and output is 20;

                and in connectpressed button slot are run succesfully.

                port value are shown correctly.

                when i run debugger it shows following error.

                The inferior stopped because it received a signal from the Operating System.
                
                Signal name : 
                SIGILL
                Signal meaning : 
                Illegal instruction
                
                

                hope you got my point.

                jsulmJ 1 Reply Last reply
                0
                • M mvuori

                  If temprature is supposed to be temperature, this is a bug:
                  mosq->subscribe(NULL, "sensors/temprature", 1);

                  MilavM Offline
                  MilavM Offline
                  Milav
                  wrote on last edited by
                  #22

                  @mvuori

                  no it is supposed to be sensors/temprature.

                  1 Reply Last reply
                  0
                  • MilavM Milav

                    @jsulm

                    i debug the size of mosq pointer through following code.

                     int size = sizeof(*mosq);
                        qDebug() << size;
                    

                    and output is 20;

                    and in connectpressed button slot are run succesfully.

                    port value are shown correctly.

                    when i run debugger it shows following error.

                    The inferior stopped because it received a signal from the Operating System.
                    
                    Signal name : 
                    SIGILL
                    Signal meaning : 
                    Illegal instruction
                    
                    

                    hope you got my point.

                    jsulmJ Offline
                    jsulmJ Offline
                    jsulm
                    Lifetime Qt Champion
                    wrote on last edited by
                    #23

                    @Milav said in segmentation fault:

                    i debug the size of mosq pointer through following code.

                    The size of a pointer is fixed, it doesn't matter whether it contains nullptr or something else.
                    Do

                    qDebug() << mosq;
                    

                    instead to see the value of the pointer.
                    Without stack trace I can't tell you what is going on...

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

                    MilavM JonBJ 2 Replies Last reply
                    0
                    • jsulmJ jsulm

                      @Milav said in segmentation fault:

                      i debug the size of mosq pointer through following code.

                      The size of a pointer is fixed, it doesn't matter whether it contains nullptr or something else.
                      Do

                      qDebug() << mosq;
                      

                      instead to see the value of the pointer.
                      Without stack trace I can't tell you what is going on...

                      MilavM Offline
                      MilavM Offline
                      Milav
                      wrote on last edited by
                      #24

                      @jsulm

                      when i execute "qDebug() << mosq;" it gives following output.

                      qtmosq(0xac50b0)
                      

                      how i see stack trace in qt?

                      can you please explore me more about stack trace?

                      JonBJ 1 Reply Last reply
                      0
                      • jsulmJ jsulm

                        @Milav said in segmentation fault:

                        i debug the size of mosq pointer through following code.

                        The size of a pointer is fixed, it doesn't matter whether it contains nullptr or something else.
                        Do

                        qDebug() << mosq;
                        

                        instead to see the value of the pointer.
                        Without stack trace I can't tell you what is going on...

                        JonBJ Offline
                        JonBJ Offline
                        JonB
                        wrote on last edited by
                        #25

                        @Milav

                        int size = sizeof(*mosq);

                        @jsulm

                        The size of a pointer is fixed,

                        A slight slip of the tongue there. I think what my colleague @jsulm meant to clarify is: sizeof(...) is a compile-time calculation, it does not matter what the value of mosq is or what it points at run-time, rather this statement just assigns sizeof(qtmosq) regardless. As he says, show us what is in the mosq.

                        MilavM 1 Reply Last reply
                        0
                        • JonBJ JonB

                          @Milav

                          int size = sizeof(*mosq);

                          @jsulm

                          The size of a pointer is fixed,

                          A slight slip of the tongue there. I think what my colleague @jsulm meant to clarify is: sizeof(...) is a compile-time calculation, it does not matter what the value of mosq is or what it points at run-time, rather this statement just assigns sizeof(qtmosq) regardless. As he says, show us what is in the mosq.

                          MilavM Offline
                          MilavM Offline
                          Milav
                          wrote on last edited by
                          #26

                          @JonB

                          when i execute "qDebug() << mosq;" it gives following output.

                          qtmosq(0xac50b0)

                          1 Reply Last reply
                          0
                          • MilavM Milav

                            @jsulm

                            when i execute "qDebug() << mosq;" it gives following output.

                            qtmosq(0xac50b0)
                            

                            how i see stack trace in qt?

                            can you please explore me more about stack trace?

                            JonBJ Offline
                            JonBJ Offline
                            JonB
                            wrote on last edited by JonB
                            #27

                            @Milav

                            when i execute "qDebug() << mosq;" it gives following output.
                            qtmosq(0xac50b0)

                            I'm not sure this is actually trying to deference the pointer, it may just print its value. I would try something likeqDebug() << mosq->something;, where something is some field you know is supposed to exist & be valid in a qtmosq. Or try qDebug() << *mosq (note the *)?

                            how i see stack trace in qt?
                            can you please explore me more about stack trace?

                            I don't know how you run in your "Embedded Custom Board", but can you run the app from inside a debugger? then when it seg faults you will see a "stack trace" in the debugger showing the call stack when the fault occurred.

                            jsulmJ MilavM 2 Replies Last reply
                            1
                            • JonBJ JonB

                              @Milav

                              when i execute "qDebug() << mosq;" it gives following output.
                              qtmosq(0xac50b0)

                              I'm not sure this is actually trying to deference the pointer, it may just print its value. I would try something likeqDebug() << mosq->something;, where something is some field you know is supposed to exist & be valid in a qtmosq. Or try qDebug() << *mosq (note the *)?

                              how i see stack trace in qt?
                              can you please explore me more about stack trace?

                              I don't know how you run in your "Embedded Custom Board", but can you run the app from inside a debugger? then when it seg faults you will see a "stack trace" in the debugger showing the call stack when the fault occurred.

                              jsulmJ Offline
                              jsulmJ Offline
                              jsulm
                              Lifetime Qt Champion
                              wrote on last edited by jsulm
                              #28

                              @JonB said in segmentation fault:

                              I'm not sure this is actually trying to deference the pointer

                              It does not, but I wanted to see its value :-)
                              Looks like it's not NULL.

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

                              JonBJ 1 Reply Last reply
                              1
                              • jsulmJ jsulm

                                @JonB said in segmentation fault:

                                I'm not sure this is actually trying to deference the pointer

                                It does not, but I wanted to see its value :-)
                                Looks like it's not NULL.

                                JonBJ Offline
                                JonBJ Offline
                                JonB
                                wrote on last edited by JonB
                                #29

                                @jsulm
                                Indeedy :) So now let's see how qDebug() << *mosq fares?

                                MilavM 1 Reply Last reply
                                0
                                • JonBJ JonB

                                  @Milav

                                  when i execute "qDebug() << mosq;" it gives following output.
                                  qtmosq(0xac50b0)

                                  I'm not sure this is actually trying to deference the pointer, it may just print its value. I would try something likeqDebug() << mosq->something;, where something is some field you know is supposed to exist & be valid in a qtmosq. Or try qDebug() << *mosq (note the *)?

                                  how i see stack trace in qt?
                                  can you please explore me more about stack trace?

                                  I don't know how you run in your "Embedded Custom Board", but can you run the app from inside a debugger? then when it seg faults you will see a "stack trace" in the debugger showing the call stack when the fault occurred.

                                  MilavM Offline
                                  MilavM Offline
                                  Milav
                                  wrote on last edited by
                                  #30

                                  @JonB said in segmentation fault:

                                  qDebug() << *mosq

                                  gives error :-

                                  /home/tdp0009/Teq_Projects/Aug_2019/HMI/Firmware/QT/12_oct_hmi/qt-mosquitto-master/mainwindow.cpp:54: note:   'qtmosq' is not derived from 'const QFlags<T>'
                                  

                                  @JonB said in segmentation fault:

                                  qDebug() << mosq->something

                                  i tried following things and it output are '0'.

                                  1. qDebug() << mosq->connect_async(host.data(), port);
                                  2. qDebug() << mosq->loop_start();
                                  3. qDebug() << mosq->subscribe(NULL, "sensors/temprature", 1);

                                  the output of all above command are zero (0).

                                  @JonB said in segmentation fault:

                                  app from inside a debugge

                                  i am not able to run the debugger because it gives following error.

                                  The inferior stopped because it received a signal from the Operating System.
                                  
                                  Signal name : 
                                  SIGILL
                                  Signal meaning : 
                                  Illegal instruction
                                  
                                  

                                  hope you got your answer.

                                  JonBJ 1 Reply Last reply
                                  0
                                  • JonBJ JonB

                                    @jsulm
                                    Indeedy :) So now let's see how qDebug() << *mosq fares?

                                    MilavM Offline
                                    MilavM Offline
                                    Milav
                                    wrote on last edited by
                                    #31

                                    @JonB said in segmentation fault:

                                    @jsulm
                                    Indeedy :) So now let's see how qDebug() << *mosq fares?

                                    sorry the error is as following not previous one.

                                    /home/tdp0009/Teq_Projects/Aug_2019/HMI/Firmware/QT/12_oct_hmi/qt-mosquitto-master/mainwindow.cpp:54: error: no match for 'operator<<' in 'qDebug()() << *((MainWindow*)this)->MainWindow::mosq'
                                    
                                    MilavM 1 Reply Last reply
                                    0
                                    • MilavM Milav

                                      @JonB said in segmentation fault:

                                      qDebug() << *mosq

                                      gives error :-

                                      /home/tdp0009/Teq_Projects/Aug_2019/HMI/Firmware/QT/12_oct_hmi/qt-mosquitto-master/mainwindow.cpp:54: note:   'qtmosq' is not derived from 'const QFlags<T>'
                                      

                                      @JonB said in segmentation fault:

                                      qDebug() << mosq->something

                                      i tried following things and it output are '0'.

                                      1. qDebug() << mosq->connect_async(host.data(), port);
                                      2. qDebug() << mosq->loop_start();
                                      3. qDebug() << mosq->subscribe(NULL, "sensors/temprature", 1);

                                      the output of all above command are zero (0).

                                      @JonB said in segmentation fault:

                                      app from inside a debugge

                                      i am not able to run the debugger because it gives following error.

                                      The inferior stopped because it received a signal from the Operating System.
                                      
                                      Signal name : 
                                      SIGILL
                                      Signal meaning : 
                                      Illegal instruction
                                      
                                      

                                      hope you got your answer.

                                      JonBJ Offline
                                      JonBJ Offline
                                      JonB
                                      wrote on last edited by
                                      #32

                                      @Milav
                                      First you wrote

                                      and i debug the application via qDebug(), the application are crash on following line of code.
                                      mosq->subscribe(NULL, "sensors/temprature", 1); // Segmentation falut

                                      Now you write

                                      qDebug() << mosq->subscribe(NULL, "sensors/temprature", 1);
                                      the output of all above command are zero (0).

                                      This is a problem if you say sometimes it seg faults and sometimes it returns 0? Is that the case? You need to be very specific in your answers!

                                      i am not able to run the debugger because it gives following error.

                                      That is indeed a big problem. There is little for us to answer from what you have posted. Is a debugger supposed to work for your "Embedded Custom Board" (I don't know about this)? You really need one. Otherwise you are likely to have to try printing out all sorts of information in your code to try to diagnose what is going on....

                                      1 Reply Last reply
                                      0
                                      • MilavM Milav

                                        @JonB said in segmentation fault:

                                        @jsulm
                                        Indeedy :) So now let's see how qDebug() << *mosq fares?

                                        sorry the error is as following not previous one.

                                        /home/tdp0009/Teq_Projects/Aug_2019/HMI/Firmware/QT/12_oct_hmi/qt-mosquitto-master/mainwindow.cpp:54: error: no match for 'operator<<' in 'qDebug()() << *((MainWindow*)this)->MainWindow::mosq'
                                        
                                        MilavM Offline
                                        MilavM Offline
                                        Milav
                                        wrote on last edited by
                                        #33

                                        Screenshot from 2019-10-15 13-03-18.png

                                        jsulmJ 1 Reply Last reply
                                        0
                                        • MilavM Milav

                                          Screenshot from 2019-10-15 13-03-18.png

                                          jsulmJ Offline
                                          jsulmJ Offline
                                          jsulm
                                          Lifetime Qt Champion
                                          wrote on last edited by
                                          #34

                                          @Milav Forget about

                                          qDebug() << *mosq;
                                          

                                          it's only going to work if operator<< was overloaded for that class, apparently it isn't, but this is not an issue.
                                          Can you try to run the app directly with gdb?

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

                                          JonBJ MilavM 2 Replies Last reply
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