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Executing QProcess in QThread: memory leak

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  • sitesvS sitesv

    @KroMignon said in Executing QProcess in QThread: memory leak:

    connect(&ping,
    QOverload<int, QProcess::ExitStatus>::of(&QProcess::finished),
    &l, &ping, &success_count, &pingsToDo {
    ...
    }, Qt::QueuedConnection);

    'connect' failed again
    img1.png

    KroMignonK Offline
    KroMignonK Offline
    KroMignon
    wrote on last edited by KroMignon
    #61

    @sitesv Sorry but I was a little bit confused when I suggest you this code.
    This cannot work, because QProcess is killed/destroyed at end of the for loop!
    Should be:

    QStringList ip_list = {"192.168.0.1", "192.168.0.2"};
    int success_count = 0;
    int pingsToDo = ip_list.count();
    QEventLoop l;
    foreach(auto ip, ip_list)
    {
        auto ping = new QProcess();
        ping->setProcessChannelMode(QProcess::MergedChannels);
        connect(ping,
                QOverload<int, QProcess::ExitStatus>::of(&QProcess::finished),
                [&l, ping, &success_count, &pingsToDo]() {
                    --pingsToDo;
                    QString output(ping->readAll());
                    if(output.contains("ttl",Qt::CaseInsensitive))
                        success_count++;
                    // free memory
                    ping->deleteLater();
                    //  exit event loop after all pings done
                    if(!pingsToDo)
                        l.exit();
                });
        ping->start("/bin/ping", QStringList() << ip << "-c" << "1");
    }
    
    // wait all pings done
    l.exec();
    

    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
    0
    • JonBJ Offline
      JonBJ Offline
      JonB
      wrote on last edited by
      #62

      @sitesv , @KroMignon
      I will say one thing.

      You are writing code which will call QEventLoop::exec(), blocking whatever calls it. It relies on hitting the QEventLoop::exit() statement, which you only have in response to the QProcess::finished signal. If for whatever reason that does not get hit, your event loop will never be exited.

      I would not write production (or even development) code like this, and certainly not for distribution. It is asking for an "unseen hang" to occur, one day. I would at minimum hook onto QProcess::errorOccurred, maybe stateChanged() too. And I would put in some sort of timer/timeout, so that if something goes badly wrong you get out of the blocking loop (with perhaps an error flag) instead of waiting for Hell to freeze over....

      KroMignonK 1 Reply Last reply
      3
      • JonBJ JonB

        @sitesv , @KroMignon
        I will say one thing.

        You are writing code which will call QEventLoop::exec(), blocking whatever calls it. It relies on hitting the QEventLoop::exit() statement, which you only have in response to the QProcess::finished signal. If for whatever reason that does not get hit, your event loop will never be exited.

        I would not write production (or even development) code like this, and certainly not for distribution. It is asking for an "unseen hang" to occur, one day. I would at minimum hook onto QProcess::errorOccurred, maybe stateChanged() too. And I would put in some sort of timer/timeout, so that if something goes badly wrong you get out of the blocking loop (with perhaps an error flag) instead of waiting for Hell to freeze over....

        KroMignonK Offline
        KroMignonK Offline
        KroMignon
        wrote on last edited by
        #63

        @JonB said in Executing QProcess in QThread: memory leak:

        I would not write production (or even development) code like this, and certainly not for distribution. It is asking for an "unseen hang" to occur, one day. I would at minimum hook onto QProcess::errorOccurred, maybe stateChanged() too. And I would put in some sort of timer/timeout, so that if something goes badly wrong you get out of the blocking loop (with perhaps an error flag) instead of waiting for Hell to freeze over....

        Yes, this is a good advice.

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

        sitesvS 1 Reply Last reply
        1
        • KroMignonK KroMignon

          @JonB said in Executing QProcess in QThread: memory leak:

          I would not write production (or even development) code like this, and certainly not for distribution. It is asking for an "unseen hang" to occur, one day. I would at minimum hook onto QProcess::errorOccurred, maybe stateChanged() too. And I would put in some sort of timer/timeout, so that if something goes badly wrong you get out of the blocking loop (with perhaps an error flag) instead of waiting for Hell to freeze over....

          Yes, this is a good advice.

          sitesvS Offline
          sitesvS Offline
          sitesv
          wrote on last edited by sitesv
          #64

          @JonB said in Executing QProcess in QThread: memory leak:

          You are writing code which will call QEventLoop::exec(), blocking whatever calls it. It relies on hitting the QEventLoop::exit() statement, which you only have in response to the QProcess::finished signal. If for whatever reason that does not get hit, your event loop will never be exited.

          Maybe my first variant was good (QThread + QProcess)?

          if(!myProcess) myProcess = new QProcess(this);
          myProcess->start(exe_path, arguments);
          myProcess->waitForFinished(500);
          output = myProcess->readAll();
          output_str = codec->toUnicode(output);
          output_strlst = output_str.split("\r\n");
          myProcess->close();
          ...
          

          There is QProcess "waitForFinished" with timeout.
          The app wasn't freezing with this approach.

          KroMignonK jsulmJ 2 Replies Last reply
          0
          • sitesvS sitesv

            @JonB said in Executing QProcess in QThread: memory leak:

            You are writing code which will call QEventLoop::exec(), blocking whatever calls it. It relies on hitting the QEventLoop::exit() statement, which you only have in response to the QProcess::finished signal. If for whatever reason that does not get hit, your event loop will never be exited.

            Maybe my first variant was good (QThread + QProcess)?

            if(!myProcess) myProcess = new QProcess(this);
            myProcess->start(exe_path, arguments);
            myProcess->waitForFinished(500);
            output = myProcess->readAll();
            output_str = codec->toUnicode(output);
            output_strlst = output_str.split("\r\n");
            myProcess->close();
            ...
            

            There is QProcess "waitForFinished" with timeout.
            The app wasn't freezing with this approach.

            KroMignonK Offline
            KroMignonK Offline
            KroMignon
            wrote on last edited by
            #65

            @sitesv said in Executing QProcess in QThread: memory leak:

            There is QProcess "waitForFinished" with timeout.
            The app wasn't freezing with this approach.

            Why not, but you have to wait ping finished before starting next.
            If it is what you want, then go with it.

            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
            0
            • sitesvS sitesv

              @JonB said in Executing QProcess in QThread: memory leak:

              You are writing code which will call QEventLoop::exec(), blocking whatever calls it. It relies on hitting the QEventLoop::exit() statement, which you only have in response to the QProcess::finished signal. If for whatever reason that does not get hit, your event loop will never be exited.

              Maybe my first variant was good (QThread + QProcess)?

              if(!myProcess) myProcess = new QProcess(this);
              myProcess->start(exe_path, arguments);
              myProcess->waitForFinished(500);
              output = myProcess->readAll();
              output_str = codec->toUnicode(output);
              output_strlst = output_str.split("\r\n");
              myProcess->close();
              ...
              

              There is QProcess "waitForFinished" with timeout.
              The app wasn't freezing with this approach.

              jsulmJ Offline
              jsulmJ Offline
              jsulm
              Lifetime Qt Champion
              wrote on last edited by
              #66

              @sitesv I'm still wondering why you think you need a local event loop...

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

              sitesvS 1 Reply Last reply
              1
              • jsulmJ jsulm

                @sitesv I'm still wondering why you think you need a local event loop...

                sitesvS Offline
                sitesvS Offline
                sitesv
                wrote on last edited by
                #67

                @jsulm said in Executing QProcess in QThread: memory leak:

                @sitesv I'm still wondering why you think you need a local event loop...

                Hi! I have done variant as you recommend. It's working!

                void PingTester::doPing(){
                    QStringList ip_list = {"192.168.0.1", "192.168.0.2"};
                    int success_count = 0;
                    int pingsToDo = ip_list.count();
                    int ipCnt = pingsToDo;
                    foreach(auto ip, ip_list)
                    {
                        QProcess ping;
                        ping.setProcessChannelMode(QProcess::MergedChannels);
                        connect(&ping,
                                QOverload<int, QProcess::ExitStatus>::of(&QProcess::finished),
                                [this, &ping, &success_count, &pingsToDo, ipCnt]() {
                                    --pingsToDo;
                                    QString output(ping.readAll());
                                    if(output.contains("ttl",Qt::CaseInsensitive)){
                                        success_count++;
                                    }
                                    if(!pingsToDo){
                                        if(success_count == ipCnt) emit setStatus(true);
                                        else                       emit setStatus(false);
                                        m_timer->start();
                                    }
                                });
                        ping.start("/bin/ping", QStringList() << ip << "-c" << "1" << "-w" << "1");
                    }
                }
                

                I made another experiment:

                1. Created a QThread
                2. In QThread::run() method::
                • made a QTimer* object,
                • set him as "one shot kind",
                • ran QTimer, and in final
                  * ran QThread::exec() (for run local EventLoop)...
                1. There is timer->start() in timeout slot, It executed, but QTimer doesn't start again.

                Any ideas?

                JonBJ KroMignonK 2 Replies Last reply
                0
                • sitesvS sitesv

                  @jsulm said in Executing QProcess in QThread: memory leak:

                  @sitesv I'm still wondering why you think you need a local event loop...

                  Hi! I have done variant as you recommend. It's working!

                  void PingTester::doPing(){
                      QStringList ip_list = {"192.168.0.1", "192.168.0.2"};
                      int success_count = 0;
                      int pingsToDo = ip_list.count();
                      int ipCnt = pingsToDo;
                      foreach(auto ip, ip_list)
                      {
                          QProcess ping;
                          ping.setProcessChannelMode(QProcess::MergedChannels);
                          connect(&ping,
                                  QOverload<int, QProcess::ExitStatus>::of(&QProcess::finished),
                                  [this, &ping, &success_count, &pingsToDo, ipCnt]() {
                                      --pingsToDo;
                                      QString output(ping.readAll());
                                      if(output.contains("ttl",Qt::CaseInsensitive)){
                                          success_count++;
                                      }
                                      if(!pingsToDo){
                                          if(success_count == ipCnt) emit setStatus(true);
                                          else                       emit setStatus(false);
                                          m_timer->start();
                                      }
                                  });
                          ping.start("/bin/ping", QStringList() << ip << "-c" << "1" << "-w" << "1");
                      }
                  }
                  

                  I made another experiment:

                  1. Created a QThread
                  2. In QThread::run() method::
                  • made a QTimer* object,
                  • set him as "one shot kind",
                  • ran QTimer, and in final
                    * ran QThread::exec() (for run local EventLoop)...
                  1. There is timer->start() in timeout slot, It executed, but QTimer doesn't start again.

                  Any ideas?

                  JonBJ Offline
                  JonBJ Offline
                  JonB
                  wrote on last edited by JonB
                  #68

                  @sitesv
                  I don't know how/whether your issue relates this, but in your code: you set off ping.start(), but QProcess ping; is a local variable in your foreach loop and so immediately goes out of scope (not to mention, you also re-use the same local variable for each time round the loop, overwriting/destroying the previous one). Nothing should work (it might actually "crash"), I don't understand how you say it does.

                  On top of all of this: I think we've said it already here above, but goodness only knows why you are using a thread, with all the complications that involves? If you want to run a QProcess and the main thread to know when it's finished, it's asynchronous anyway, it would be a whole lot simpler not to have any thread. I think I said this earlier, but up to you.

                  And finally, while I'm on a roll: I think you are just using a /bin/ping in order to read the textual output, parse it, and see whether something is there/alive. In which case I'd be tempted to just write it myself in Qt instead of running some external command, I would have thought it's only a few lines of code. Your "parsing" of the output is beyond hokey, not even sure what you think it tells you....

                  1 Reply Last reply
                  0
                  • sitesvS sitesv

                    @jsulm said in Executing QProcess in QThread: memory leak:

                    @sitesv I'm still wondering why you think you need a local event loop...

                    Hi! I have done variant as you recommend. It's working!

                    void PingTester::doPing(){
                        QStringList ip_list = {"192.168.0.1", "192.168.0.2"};
                        int success_count = 0;
                        int pingsToDo = ip_list.count();
                        int ipCnt = pingsToDo;
                        foreach(auto ip, ip_list)
                        {
                            QProcess ping;
                            ping.setProcessChannelMode(QProcess::MergedChannels);
                            connect(&ping,
                                    QOverload<int, QProcess::ExitStatus>::of(&QProcess::finished),
                                    [this, &ping, &success_count, &pingsToDo, ipCnt]() {
                                        --pingsToDo;
                                        QString output(ping.readAll());
                                        if(output.contains("ttl",Qt::CaseInsensitive)){
                                            success_count++;
                                        }
                                        if(!pingsToDo){
                                            if(success_count == ipCnt) emit setStatus(true);
                                            else                       emit setStatus(false);
                                            m_timer->start();
                                        }
                                    });
                            ping.start("/bin/ping", QStringList() << ip << "-c" << "1" << "-w" << "1");
                        }
                    }
                    

                    I made another experiment:

                    1. Created a QThread
                    2. In QThread::run() method::
                    • made a QTimer* object,
                    • set him as "one shot kind",
                    • ran QTimer, and in final
                      * ran QThread::exec() (for run local EventLoop)...
                    1. There is timer->start() in timeout slot, It executed, but QTimer doesn't start again.

                    Any ideas?

                    KroMignonK Offline
                    KroMignonK Offline
                    KroMignon
                    wrote on last edited by
                    #69

                    @sitesv said in Executing QProcess in QThread: memory leak:

                    Hi! I have done variant as you recommend. It's working!

                    I am not sure this is really working!
                    I think you have to (re)learn C++ object life cycle.
                    You create a QProcess local instance in the for loop, this object will be destroyed at loop end.

                    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
                    1
                    • sitesvS Offline
                      sitesvS Offline
                      sitesv
                      wrote on last edited by sitesv
                      #70

                      @JonB @KroMignon
                      Agree with you, guys.
                      But it works...
                      I can reimplement to QProcess pointers of PingTester class...

                      KroMignonK 1 Reply Last reply
                      0
                      • sitesvS sitesv

                        @JonB @KroMignon
                        Agree with you, guys.
                        But it works...
                        I can reimplement to QProcess pointers of PingTester class...

                        KroMignonK Offline
                        KroMignonK Offline
                        KroMignon
                        wrote on last edited by
                        #71

                        @sitesv said in Executing QProcess in QThread: memory leak:

                        But it works...

                        It don't, "it seems to work" would be the correct answer ;)

                        If you try to ping a not valid/accessible IP address, I am pretty sure it will not work.

                        If you want to do it sequentially, you have to be consistent in your choice:

                        foreach(auto ip, ip_list)
                        {
                            QProcess ping;
                            ping.setProcessChannelMode(QProcess::MergedChannels);
                            ping.start("/bin/ping", QStringList() << ip << "-c" << "1" << "-w" << "1");
                            ping.waitForFinished(5000); // wait up to 5 seconds
                            QString output(ping.readAll());
                            if(output.contains("ttl",Qt::CaseInsensitive)){
                                success_count++;
                            }
                        }
                        

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

                        sitesvS JonBJ 2 Replies Last reply
                        0
                        • KroMignonK KroMignon

                          @sitesv said in Executing QProcess in QThread: memory leak:

                          But it works...

                          It don't, "it seems to work" would be the correct answer ;)

                          If you try to ping a not valid/accessible IP address, I am pretty sure it will not work.

                          If you want to do it sequentially, you have to be consistent in your choice:

                          foreach(auto ip, ip_list)
                          {
                              QProcess ping;
                              ping.setProcessChannelMode(QProcess::MergedChannels);
                              ping.start("/bin/ping", QStringList() << ip << "-c" << "1" << "-w" << "1");
                              ping.waitForFinished(5000); // wait up to 5 seconds
                              QString output(ping.readAll());
                              if(output.contains("ttl",Qt::CaseInsensitive)){
                                  success_count++;
                              }
                          }
                          
                          sitesvS Offline
                          sitesvS Offline
                          sitesv
                          wrote on last edited by
                          #72

                          @KroMignon said in Executing QProcess in QThread: memory leak:

                          If you try to ping a not valid/accessible IP address, I am pretty sure it will not work.

                          There is "-w 1" key. It helps with unaccessible IP.

                          KroMignonK 1 Reply Last reply
                          0
                          • KroMignonK KroMignon

                            @sitesv said in Executing QProcess in QThread: memory leak:

                            But it works...

                            It don't, "it seems to work" would be the correct answer ;)

                            If you try to ping a not valid/accessible IP address, I am pretty sure it will not work.

                            If you want to do it sequentially, you have to be consistent in your choice:

                            foreach(auto ip, ip_list)
                            {
                                QProcess ping;
                                ping.setProcessChannelMode(QProcess::MergedChannels);
                                ping.start("/bin/ping", QStringList() << ip << "-c" << "1" << "-w" << "1");
                                ping.waitForFinished(5000); // wait up to 5 seconds
                                QString output(ping.readAll());
                                if(output.contains("ttl",Qt::CaseInsensitive)){
                                    success_count++;
                                }
                            }
                            
                            JonBJ Offline
                            JonBJ Offline
                            JonB
                            wrote on last edited by JonB
                            #73

                            @KroMignon
                            I would think doing it sequentially, with waitForFinished(), for his multiple IP addresses would be a poor way to do things. He has a number of IP addresses to check, these should be done in parallel....

                            1 Reply Last reply
                            0
                            • sitesvS sitesv

                              @KroMignon said in Executing QProcess in QThread: memory leak:

                              If you try to ping a not valid/accessible IP address, I am pretty sure it will not work.

                              There is "-w 1" key. It helps with unaccessible IP.

                              KroMignonK Offline
                              KroMignonK Offline
                              KroMignon
                              wrote on last edited by
                              #74

                              @sitesv said in Executing QProcess in QThread: memory leak:

                              There is "-w 1" key. It helps with unaccessible IP.

                              AFAIK "-w 1" will wait up to 1 second.
                              I am sure you will got QProcess warnings about killing a process which is still running.

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

                              sitesvS 1 Reply Last reply
                              0
                              • KroMignonK KroMignon

                                @sitesv said in Executing QProcess in QThread: memory leak:

                                There is "-w 1" key. It helps with unaccessible IP.

                                AFAIK "-w 1" will wait up to 1 second.
                                I am sure you will got QProcess warnings about killing a process which is still running.

                                sitesvS Offline
                                sitesvS Offline
                                sitesv
                                wrote on last edited by sitesv
                                #75

                                @KroMignon said in Executing QProcess in QThread: memory leak:

                                AFAIK "-w 1" will wait up to 1 second.

                                ...and QProcess will emit a "finished" signal. Why you are writing about "killing" process?

                                KroMignonK 1 Reply Last reply
                                0
                                • sitesvS sitesv

                                  @KroMignon said in Executing QProcess in QThread: memory leak:

                                  AFAIK "-w 1" will wait up to 1 second.

                                  ...and QProcess will emit a "finished" signal. Why you are writing about "killing" process?

                                  KroMignonK Offline
                                  KroMignonK Offline
                                  KroMignon
                                  wrote on last edited by KroMignon
                                  #76

                                  @sitesv said in Executing QProcess in QThread: memory leak:

                                  ...and QProcess will emit a "finished" signal. Why you are writing about "killing" process?

                                  This is the way I would implement multiple pings in parallel:

                                  int PingTester::doPing(const QStringList &ip_list)
                                  {
                                      // to avoid issues ;)
                                      if(ip_list.isEmpty())
                                          return 0;
                                  
                                      int success_count = 0;
                                      int pingsToDo = ip_list.count();
                                      QEventLoop l;
                                      QTimer timer;
                                      foreach(auto ip, ip_list)
                                      {
                                          auto ping = new QProcess();
                                          ping->setProcessChannelMode(QProcess::MergedChannels);
                                          connect(timer, &QTimer::timeout, ping, &QProcess::kill);
                                          connect(ping, &QProcess::stateChanged,
                                                  [&l, ping, &success_count, &pingsToDo](QProcess::ProcessState newState) {
                                                      if(newState != QProcess::NotRunning)
                                                          return;
                                                      --pingsToDo;
                                                      QString output(ping->readAll());
                                                      if(output.contains("ttl",Qt::CaseInsensitive))
                                                          success_count++;
                                                      // free memory
                                                      ping->deleteLater();
                                                      // exit loop when done.
                                                      if(!pingsToDo)
                                                          l.exit();
                                                  });
                                          ping->start("/bin/ping", QStringList() << ip << "-c" << "1" << "-w" << "1");
                                      }
                                      timer.start(1500); // kill process after 1.5 seconds if still running
                                      // wait all pings done
                                      l.exec();
                                      return success_count;
                                  }
                                  

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

                                  sitesvS 1 Reply Last reply
                                  1
                                  • sitesvS Offline
                                    sitesvS Offline
                                    sitesv
                                    wrote on last edited by sitesv
                                    #77

                                    @jsulm There is a local QEventLoop...
                                    @KroMignon I really don't understand why you are checking state changing of QProcess...
                                    QProcess "Finished" method is not suitable?
                                    And why kill a process?

                                    KroMignonK 1 Reply Last reply
                                    0
                                    • sitesvS sitesv

                                      @jsulm There is a local QEventLoop...
                                      @KroMignon I really don't understand why you are checking state changing of QProcess...
                                      QProcess "Finished" method is not suitable?
                                      And why kill a process?

                                      KroMignonK Offline
                                      KroMignonK Offline
                                      KroMignon
                                      wrote on last edited by KroMignon
                                      #78

                                      @sitesv said in Executing QProcess in QThread: memory leak:

                                      QProcess "Finished" method is not suitable?

                                      I found it easier to use QProcess::stateChanged(), because there is no overload of this slot, and I am not sure if QProcess::finished(int, QProcess::ExitStatus) is triggered if process is killed.

                                      And why kill a process?

                                      This is a security, when calling an external application, many things can happen: execution right issues, application not exist, network issues and so on.
                                      99.9% of time it will not be useful, but it will ensure QEventLoop will exit at the end!

                                      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
                                      0
                                      • KroMignonK KroMignon

                                        @sitesv said in Executing QProcess in QThread: memory leak:

                                        ...and QProcess will emit a "finished" signal. Why you are writing about "killing" process?

                                        This is the way I would implement multiple pings in parallel:

                                        int PingTester::doPing(const QStringList &ip_list)
                                        {
                                            // to avoid issues ;)
                                            if(ip_list.isEmpty())
                                                return 0;
                                        
                                            int success_count = 0;
                                            int pingsToDo = ip_list.count();
                                            QEventLoop l;
                                            QTimer timer;
                                            foreach(auto ip, ip_list)
                                            {
                                                auto ping = new QProcess();
                                                ping->setProcessChannelMode(QProcess::MergedChannels);
                                                connect(timer, &QTimer::timeout, ping, &QProcess::kill);
                                                connect(ping, &QProcess::stateChanged,
                                                        [&l, ping, &success_count, &pingsToDo](QProcess::ProcessState newState) {
                                                            if(newState != QProcess::NotRunning)
                                                                return;
                                                            --pingsToDo;
                                                            QString output(ping->readAll());
                                                            if(output.contains("ttl",Qt::CaseInsensitive))
                                                                success_count++;
                                                            // free memory
                                                            ping->deleteLater();
                                                            // exit loop when done.
                                                            if(!pingsToDo)
                                                                l.exit();
                                                        });
                                                ping->start("/bin/ping", QStringList() << ip << "-c" << "1" << "-w" << "1");
                                            }
                                            timer.start(1500); // kill process after 1.5 seconds if still running
                                            // wait all pings done
                                            l.exec();
                                            return success_count;
                                        }
                                        
                                        sitesvS Offline
                                        sitesvS Offline
                                        sitesv
                                        wrote on last edited by sitesv
                                        #79

                                        @KroMignon said in Executing QProcess in QThread: memory leak:

                                        This is the way I would implement multiple pings in parallel:

                                        This code doesn't work. Only one iteration.

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