C++ multithreading: QSocketNotifier:Socket notifiers cannot be enabled or disabled from another thread

Question

I have 3 threads. The main thread (for my GUI) -> worker thread A -> worker thread B

In worker thread B (in my code called recognitionThread) I am trying to start a process and write data to that process. The process is supposed to start my python virtual environment (this part works) and execute some commands in that environment (that doesn t work).

When I try to write data to that process I get: Socket notifiers cannot be enabled or disabled from another thread

I think I kind of understand what the problem is, but not totally and have no clue about how to solve my issue.

Those are the relevant parts of my code:

recognitionClass.cpp:

  QProcess recognitionClass::p;

 recognitionClass::recognitionClass(QObject *parent) : QObject(parent)
{
     startPythonVirtualEnv();
}

void recognitionClass::startPythonVirtualEnv()
{
    qRegisterMetaType<QProcess::ProcessError>("QProcess::ProcessError");
params<<"-f"<<"-c"<<"python2"<< "/home/John/Desktop/python.log";
    qDebug()<<"parameters: "<<params;

    if(p.state()==2)
    {qDebug()<<"python virtualenv already running";}
    else
    {
        p.start("script", params);
        qDebug()<<"started process";
        while(!p.waitForStarted())
        {qDebug()<<"waiting for virtualenv to be ready";}
    } 
}

recognitionClass::person recognitionClass::runRecognitionScript()
{
      person persStrObj;

      qRegisterMetaType<QProcess::ProcessError>("QProcess::ProcessError");
      connect(&p, &QProcess::errorOccurred, qApp, &QApplication::aboutQt );

       qint64 successFailWrite;
       delay();
       qDebug()<<"before p.write()";
       successFailWrite = p.write("import imp; foo = imp.load_source('myTest', '/home/John/openface/demos/recognitionClass.py'); from myTest import recognitionClass; myClassObj = recognitionClass(); print myClassObj.recognize('/home/John/Desktop/camera.jpg');");
       qDebug()<<"after p.write()";
       delay();

       qDebug()<<"started process, result write op: "<< successFailWrite;

}

recognitionClass.h:

     class recognitionClass : public QObject
     {
         Q_OBJECT
     private:
         struct person{
             QString persName;
             double persCertitude;
         };

         static QProcess p;
         QStringList params;

     public:
         explicit recognitionClass(QObject *parent = nullptr);
         struct person runRecognitionScript();
         void startPythonVirtualEnv();
         void stopPythonVirtualEnv();
         void delay();
     signals:

     public slots:
         double recognizePerson();
     };

webcamclass.cpp:

     webcamClass::webcamClass(QObject *parent) : QObject(parent)
     {
         recognizePerson=false;

         //setup recognition thread
         recognitionThread = new QThread(this);
         recognitionClObj = new recognitionClass();

         connect( recognitionThread, SIGNAL(started()), recognitionClObj, SLOT(recognizePerson()) );
         recognitionClObj->moveToThread(recognitionThread);
     }


     void webcamClass:: getVideoFrame()
     {
         static cv::VideoCapture cap(CV_CAP_ANY);
         cv::Mat imgFrame;

         if( !cap.isOpened() )
         {
             qDebug()<< "Could not initialize capturing...\n";
         }

         while(1)
         {
             cap >> imgFrame;
             cv::cvtColor(imgFrame, imgFrame, CV_BGR2RGB);
             QImage img;
             img = QImage((uchar*)imgFrame.data, imgFrame.cols, imgFrame.rows, QImage::Format_RGB888);
             QPixmap pixmap = QPixmap::fromImage(img);

             emit gottenVideoFrame(pixmap);
             if(recognizePerson==true)
             {
                 //write image to recognition buffer
                 recognitionThread->start();

                 if(myGlobalVariable==0)
                 {
                      frameForRecognition = imgFrame;
                      myGlobalVariable=55;
                 }

             }

             cv::waitKey(100);
         }
     }

webcamclass (AKA worker thread A) is a worker thread that creates a new worker thread implementing recognitionclass. (recognitionThread, AKA worker thread B) Inside the recognitionThread I try to create a process to start my python virtual environment in my constructor. This works. The error occurs at runtime at line containing successFailWrite = p.write("import imp...."); in the recognitionClass.cpp file on line 37.

My guess is that I am tryong to program this is in a very bad way which is why I am bumping from one problem into another. I already read other posts related to similar issues, but I was unable to find my way out.

Could someone please provide some explanation please?

Thanks!

C++ multithreading: QSocketNotifier:Socket notifiers cannot be enabled or disabled from another thread

0 Answers0