How to force a thread waiting for signal

Christian Ehrlicher

There is no difference between std::thread and QThread in your case at all. You need a running event loop to process Qt signals and slots with std::thread and with QThread.

SimonSchroeder

@Christian-Ehrlicher Actually, there is a slight difference between std::thread and QThread: If you take a plain QThread object and call start() on it, the default implementation of run() will launch an event loop. std::thread does not know about Qt and thus you have to start the event loop yourself.

I suggest using QThread to get an event loop.

dual

@SimonSchroeder thank you for answering. I tried to use QThread but it closes before receiving readyread signal. This is my implementation:

/* Main thread: */
    int sd = tcpServer->nextPendingConnection()->socketDescriptor();
    std::cout<<"Socket descriptor: "<<sd<<std::endl;
    thread = QThread::create([sd]{

        sharing s(sd);

    });
    thread->start();

/* class sharing: */
sharing::sharing(int socketDescriptor){
    clientConnection = new QTcpSocket;
    if (!clientConnection->setSocketDescriptor(socketDescriptor)) {
        std::cout<<"Error on setting Socket Descriptor..."<<std::endl;
        return;
    }

    in.setDevice(clientConnection);
    in.setVersion(QDataStream::Qt_5_5);

    connect(clientConnection, &QIODevice::readyRead, this, &sharing::receive);
}

sharing is the class in charge of creating the QTcpSocket and of managing the file receiving. After the construction of sharing, I'd expect the thread to wait until readyread is emitted but it dosn't.

/edit: Edited by moderator: Please use the code - tags for better readability

Christian Ehrlicher

As we already told you you need a running event loop in your thread - no matter if it's a QThread or std::thread or whatever. So please start one.

dual

@Christian-Ehrlicher What about this sentence?

@SimonSchroeder said in How to force a thread waiting for signal:

If you take a plain QThread object and call start() on it, the default implementation of run() will launch an event loop

Also online i've found that Qthread has a eventLoop on the default run() method. This point is not clear to me.
By the way, do you suggest to place a eventLoop in the sharing constructor?

Christian Ehrlicher

@dual said in How to force a thread waiting for signal:

This point is not clear to me.

What is not clear? QThread's run method simply executes an eventloop.

By the way, do you suggest to place a eventLoop in the sharing constructor?

No, it should go into the thread's main function.

SimonSchroeder

What your last code does is placing only the constructor call into a separate thread. It will not continue running. What you should do is more like this:

QThread *thread = new QThread();
thread->start();

sharing *s = new sharing(sd);
s->moveToThread(thread);

I haven't check with the exact syntax, so you need to check if this compiles. Notice that your sharing object also needs to be a pointer to outlive the current scope. It also should inherit from QObject so that you can move it to the other thread. Only then will your new thread's event loop handle its slots.

You should also think about the lifetime of the thread and sharing objects. You need to delete both when the connection handled by this thread is closed.

dual

@SimonSchroeder I did some tries but still it doesn't work as expected.

In server.h (that is the main thread object) I defined

class server : public QObject
{
    Q_OBJECT
    QThread workerThread;
private:
   sharing *worker;
   void newClient();
}

and in server.cpp

void server::newClient()
{
    int sd = tcpServer->nextPendingConnection()->socketDescriptor();
    std::cout<<"Socket descriptor: "<<sd<<std::endl;
    if(workerThread.isRunning()){
        std::cout<<"thread is running.. my thread id " << std::this_thread::get_id()<<std::endl;
    }
    else
        std::cout<<"thread is not running.."<<std::endl;


    worker = new sharing(sd);
    connect(&workerThread, &QThread::started, worker, &sharing::doWork);
    connect(&workerThread, &QThread::finished, worker, &QObject::deleteLater);
    worker->moveToThread(&workerThread);

    workerThread.start();
    //workerThread.wait();
}

where the if-else part is used as debug in order to check if the thread is still running when a new client tries to connect.

While in sharing.cpp I have

sharing::sharing(int socketDescriptor):socketDescriptor(socketDescriptor){
}

void sharing::doWork(){
    std::cout<<"Connecting... "<<std::endl;
    clientConnection = new QTcpSocket;
    if (!clientConnection->setSocketDescriptor(socketDescriptor)) {
        std::cout<<"Error on setting Socket Descriptor..."<<std::endl;
        return;
    }

    in.setDevice(clientConnection);
    in.setVersion(QDataStream::Qt_5_5);

    connect(clientConnection, &QIODevice::readyRead, this, &sharing::receive);
    std::cout<<"Connected "<<std::endl;
}

and in sharing::receive I have a switch to read json received by the client. Sharing class inherits from QObject.

At the moment the code works only if the workerThread.wait(); is not commented, while sharing will not receive any other readyRead when the newClient function ends. But the thread is still running when I create a new client, so I think that the eventLoop is correctly working.
Of course this code is not good to manage multiple clients, it is just a preliminary version because I want to reach a working version of multi-thread communication between a client and the server.

Christian Ehrlicher

@dual said in How to force a thread waiting for signal:

tcpServer->nextPendingConnection()->socketDescriptor();

This is wrong since this already creates a QTcpSocket which is repsonsible for your socket and receives the readyRead signals.
You want to override QTcpServer::incomingConnection() as described in the threaded fortune server example.

dual

@Christian-Ehrlicher your answer totally solved my problem. Thank you so much.