Slot on main thread not called when signal is emitted from another thread

[Paul Offord <Paul.Offord () advance7 com>]

I am trying to write a plugin for Wireshark that will run a TCP server on a service thread.  The TCP server will 
receive a new frame number from another application and then use the Plugin IF function plugin_if_goto_frame to cause 
Wireshark to jump to the new frame number.  I have found through various experiments that plugin_if_goto_frame must run 
on the main thread.  I'm writing the plugin in Qt and C++, exposing the callbacks using extern "C" declarations. The 
callbacks are being called correctly and that all looks fine.

I'm using a Qt cross-thread (QueuedConnection) signal and slot to communicate between my service thread and the main 
thread.  The problem I am having is that although the service thread emits the signal (well it executes the 
instruction) the slot in the main thread doesn't get called.  I've removed all of the TCP server code to simplify 
things.  The relevant bits of the code looks like this:

syncro.h

#ifndef SYNCRO_H
#define SYNCRO_H
#include <QtCore>

#include "mythread.h"

class Syncro : public QObject
{
    Q_OBJECT

public:
    void Initialise(MyThread *);

public slots:
    void jumpToFrame(int);
};

#endif // SYNCRO_H


syncro.cpp

#include <plugin_if.h>

#include "syncro.h"
#include <QtCore>
#include <QDebug>

void Syncro::Initialise(MyThread *serviceThread)
{
    // connect(serviceThread, SIGNAL(syncroGoFrame(int)), this, SLOT(jumpToFrame(int)), Qt::DirectConnection);
    connect(serviceThread, SIGNAL(syncroGoFrame(int)), this, SLOT(jumpToFrame(int)), Qt::QueuedConnection);
}

void Syncro::jumpToFrame(int new_frame)
{
    plugin_if_goto_frame(new_frame);
    return;
}


mythread.h

#ifndef MYTHREAD_H
#define MYTHREAD_H
#include <QtCore>

class MyThread : public QThread
{
    Q_OBJECT

public:
    MyThread();
    void run();

signals:
    void syncroGoFrame(int);
};

#endif // MYTHREAD_H


mythread.cpp

#include "mythread.h"
#include <QtCore>
#include <QDebug>

MyThread::MyThread()
{

}

void MyThread::run()
{
    while (true)
    {
        qDebug() << "Running";
        sleep(3);
        qDebug() << "Still Running";
        emit syncroGoFrame(22);
    }
}

packet-syncro.cpp

static int proto_syncro = -1;
static MyThread *serviceThread;
static Syncro *mainObject;

void
proto_register_syncro(void)
{
    module_t *syncro_module;

     proto_syncro = proto_register_protocol("Syncro Service",
        "Syncro",
        "syncro");

    syncro_module = prefs_register_protocol(proto_syncro, proto_reg_handoff_syncro);

    if (!serviceThread)
    {
        serviceThread = new MyThread;
        mainObject = new Syncro;
        mainObject->Initialise(serviceThread);

        serviceThread->start();
    }

    return;
}


When I run the code the extra thread starts OK and it starts to cycle around the loop with the 3 second sleep. However, 
the connected slot function Syncro::jumpToFrame(int new_frame) does not get called. If I change the connect from 
QueuedConnection to DirectConnection the slot function gets called but, as expected, in the context of my 
serviceThread. I need the slot function to run on the main thread.

I guess it is because my jumpToFrame slot function is not on the main thread event loop, but I don't understand how to 
get it on the loop.

Any advice would be much appreciated.

Thanks and regards...Paul


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