without Qt::QueuedConnection will crash

[hashb]

Hi, All

I do a testing which use a work thread to access a variable (here is str) which belong to main thread. I found if I add QueuedConnection in connect function ,it will not crash,but without it will crash .

Does anybody know why?

Thanks advance for your help.

#ifndef AAA_
#define AAA_
#include <QThread>
#include <QString>
#include <QDebug>
      class myClass : public QThread
      {
        Q_OBJECT
      public:
          myClass ()
          {
              connect(this, 
                          SIGNAL(startSignal()),
                           this, SLOT(onDoComm())); //will crash
                           //,Qt::QueuedConnection); //add this parameter will not
          }
          void run() { exec(); }
          void startComm() {emit startSignal();}
      signals:
        void startSignal();
      public slots:
        void onDoComm()
        {
            static long i=1;
            qDebug()<<str;
            str=QString("%1%2").arg(i++);
            emit startSignal();
        };
      private:
        QString str;
      };
 
 
#endif
 
 
 
#include <iostream>
#include <QApplication>
#include "aaa.h"
 
using namespace std;
int main(int argc ,char *argv[])
{
    QApplication app(argc, argv);
 
    myClass a;
    a.start();
    a.startComm();
    app.exec();
}

#ifndef AAA_
#define AAA_
#include <QThread>
#include <QString>
#include <QDebug>
      class myClass : public QThread
      {
        Q_OBJECT
      public:
          myClass ()
          {
              connect(this, 
                          SIGNAL(startSignal()),
                           this, SLOT(onDoComm())); //will crash
                           //,Qt::QueuedConnection); //add this parameter will not
          }
          void run() { exec(); }
          void startComm() {emit startSignal();}
      signals:
        void startSignal();
      public slots:
        void onDoComm()
        {
            static long i=1;
            qDebug()<<str;
            str=QString("%1%2").arg(i++);
            emit startSignal();
        };
      private:
        QString str;
      };


#endif



#include <iostream>
#include <QApplication>
#include "aaa.h"

using namespace std;
int main(int argc ,char *argv[])
{
    QApplication app(argc, argv);

    myClass a;
    a.start();
    a.startComm();
    app.exec();
}

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[tbscope]

Read this:
http://doc.qt.nokia.com/4.7/threads.html
and this:
http://doc.qt.nokia.com/4.7/signalsandslots.html
and this:
http://labs.qt.nokia.com/2010/06/17/...oing-it-wrong/

[Lesiok]

Because without Qt::QueuedConnection emmiting signal is like a calling slot function. So You call onDoComm() BEFORE ending this method that is to say "classic stack overflow".
With Qt::QueuedConnection slot function is calling by signal dispatcher in idle time.

Question is : why You emit StartSignal() in onDoComm() ?

[tbscope]

Here the big problem is that you're dealing with two distinct event loops.
Even with a queued connection, the logic is still wrong as the slot of the thread is not executed in the thread itself but in the main thread resulting in possible crashes.

Edit: never mind, the connection is made in the main thread. Just ignore this post.

[hashb]

Hi tbscope,you mean the slot of the thread executed in the main thread ?

Here the big problem is that you're dealing with two distinct event loops.
Even with a queued connection, the logic is still wrong as the slot of the thread is not executed in the thread itself but in the main thread resulting in possible crashes.

Edit: never mind, the connection is made in the main thread. Just ignore this post.

Thanks a lot for you explanation,
it was just a test in order to produce a loop

Because without Qt::QueuedConnection emmiting signal is like a calling slot function. So You call onDoComm() BEFORE ending this method that is to say "classic stack overflow".
With Qt::QueuedConnection slot function is calling by signal dispatcher in idle time.

Question is : why You emit StartSignal() in onDoComm() ?

[Lesiok]

Hi tbscope,you mean the slot of the thread executed in the main thread ?




Thanks a lot for you explanation,
it was just a test in order to produce a loop

Do this with QTimer. Something like this :

#ifndef AAA_
#define AAA_
#include <QThread>
#include <QString>
#include <QDebug>
#include <QTimer>
 
      class myClass : public QThread
      {
        Q_OBJECT
      public:
          myClass ()
          {
              connect(&timer, 
                          SIGNAL(timeout()),
                           this, SLOT(onDoComm()));
          }
          void run() { exec(); }
          void startComm() {timer.start(10);}
      signals:
        void startSignal();
      public slots:
        void onDoComm()
        {
            static long i=1;
            qDebug()<<str;
            str=QString("%1%2").arg(i++);
        };
      private:
        QString str;
        QTimer timer;
      };
#endif

#ifndef AAA_
#define AAA_
#include <QThread>
#include <QString>
#include <QDebug>
#include <QTimer>

      class myClass : public QThread
      {
        Q_OBJECT
      public:
          myClass ()
          {
              connect(&timer, 
                          SIGNAL(timeout()),
                           this, SLOT(onDoComm()));
          }
          void run() { exec(); }
          void startComm() {timer.start(10);}
      signals:
        void startSignal();
      public slots:
        void onDoComm()
        {
            static long i=1;
            qDebug()<<str;
            str=QString("%1%2").arg(i++);
        };
      private:
        QString str;
        QTimer timer;
      };
#endif

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In this example onDoComm will be executed one time per 10 ms