Qt with ZeroMQ publish subscribe pattern

Question

I would like to use ZeroMQ(4.1.2) with Qt (5.2.1). Idea is to have zmq pub/sub (where server is outside) and sub is qt app. Currently receive in Qt app runs once, could someone drop hint? Should ZeroMQ receiver be implemented in some other way?

Currently my code looks like:

mainwindow.h

namespace Ui {
class MainWindow;
}

class MainWindow : public QMainWindow
{
Q_OBJECT

public:
    explicit MainWindow(QWidget *parent = 0);
    ~MainWindow();

private slots:
    void on_pushButton_clicked();

    void readZMQData();

private:
    Ui::MainWindow *ui;
    QSocketNotifier *qsn;
    void *context;
    void *subscriber;

};

mainwindow.cpp

MainWindow::MainWindow(QWidget *parent) :
    QMainWindow(parent),
    ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    /***** ZMQ *****/

    context = zmq_ctx_new ();
    subscriber = zmq_socket (context, ZMQ_SUB);
    int rc = zmq_connect (subscriber, "tcp://localhost:5556");

    char *filter = "";
    rc = zmq_setsockopt (subscriber, ZMQ_SUBSCRIBE,filter, strlen (filter));
    unsigned int fd=0;
    size_t fd_size = sizeof(fd);
    rc = zmq_getsockopt(subscriber,ZMQ_FD,&fd,&fd_size);

    qsn = new QSocketNotifier(fd, QSocketNotifier::Read, this);
    connect(qsn, SIGNAL(activated(int)), this, SLOT(readZMQData()), Qt::DirectConnection);

}

MainWindow::~MainWindow()
{
    zmq_close (this->subscriber);
    zmq_ctx_destroy (this->context);
    delete ui;
}


void MainWindow::readZMQData()
{
    qsn->setEnabled(false);
    qDebug() << "Got data!";

    int events = 0;
    std::size_t eventsSize = sizeof(events);
    zmq_getsockopt(subscriber,ZMQ_EVENTS, &events, &eventsSize);
    if(events & ZMQ_POLLIN){
        qDebug() << " ======  Data to read ======";

        char *string = s_recv(subscriber);
        qDebug() << "DATA: " << string;
        free(string);
    }

    qsn->setEnabled(true);
}

And server app is (from ZeroMQ examples):

#include "zhelpers.h"

int main (void)
{
    //  Prepare our context and publisher
    void *context = zmq_ctx_new ();
    void *publisher = zmq_socket (context, ZMQ_PUB);
    int rc = zmq_bind (publisher, "tcp://*:5556");
    assert (rc == 0);

    //  Initialize random number generator
    srandom ((unsigned) time (NULL));
    while (1) {
        //  Get values that will fool the boss
        int zipcode, temperature, relhumidity;
        zipcode     = randof (100000);
        temperature = randof (215) - 80;
        relhumidity = randof (50) + 10;

        //  Send message to all subscribers
        char update [20];
        sprintf (update, "%05d %d %d", zipcode, temperature, relhumidity);
        s_send (publisher, update);
    }
    zmq_close (publisher);
    zmq_ctx_destroy (context);
    return 0;
}

Answer 1

First tnx for helping out,

I've found the issue, when ZeroMQ notifies that there is message to read you need to read them all, not just first one.

    void MainWindow::readZMQData(int fd)
{
    qsn->setEnabled(false);

    int events = 0;
    std::size_t eventsSize = sizeof(events);
    zmq_getsockopt(subscriber,ZMQ_EVENTS, &events, &eventsSize);
    if(events & ZMQ_POLLIN){
        qDebug() << " ======  Data to read ======";

        char *string;
        // THIS IS THE TRICK! READ UNTIL THERE IS MSG
        while((string = s_recv_nb(subscriber)) != NULL){
            qDebug() << "DATA: " << string;
            free(string);
        }
    }

    qsn->setEnabled(true);
}

Answer 2

The socket notifier looks like it should work. Have you read the docs on handling it properly? Especially if you are on Windows, it looks like there are special ways to handle it when doing the read... disabling, reading, etc.

http://doc.qt.io/qt-5/qsocketnotifier.html#details

Hope that helps.