How to properly create QUdpSocket on non-gui thread ? Readyread not emitted

Question

I'm writing a network library that wraps the QUdpSocket:

QAbstractSocket *UdpNetworkStreamer::addConnection()
{
    QUdpSocket *udpSocket = new QUdpSocket(this);
    udpSocket->bind(connection.port, QUdpSocket::ShareAddress);
    bool ret = udpSocket->joinMulticastGroup(QHostAddress(connection.ip));
    connect(udpSocket, SIGNAL(readyRead()), this, SLOT(readyRead()), Qt::QueuedConnection);

    return udpSocket;
}

create a new QUdpSocket.
connect to its readyRead signal.
call readDatagram when readyRead is raised.

All is working fine when I use the library from a Qt GUI application.

The problem starts when another user includes the library used outside of a Qt GUI application.

He calls the addConnection (which creates the socket and calls connect on the readyRead)

The thread on which the addConnection is called is non-Qt.

The addConnection seems to end successfully but the readyRead is never emitted.

Calling read (even though no readyRead was emitted) leads to a successful datagram read.

Fixes that did not work :

moving the the UDP socket thread to the this->thread

QUdpSocket *udpSocket = new QUdpSocket();
udpSocket->moveToThread(this->thread());
udpSocket->setParent(this);

I tried to simulate the problem by calling:void

MainWindow::on__btnOpenMulticastReceiver_clicked()
{
   QFuture<void> future = QtConcurrent::run(this, 
   &MainWindow::CreateMulticastConnection, testHandle);
}

This also led to same symptoms as the one the user had with my library, meaning the readyRead wasn't emitted.

QSignalSpy - I've activated a spy on the readyRead signal; the counter kept on being zero although I could read data directly from the socket. The spy gave valid results (i.e. progressed) when used the socket was initialized on the main thread.

My Questions:

What am I missing and doing wrong ?
What is the simplest way of having the readyRead emitted even though it is not created on the main GUI thread - I couldn't find any sample that works with no GUI or outside Qt threads.

Does the thread on which the user creates the `QUdpSocket` have an active event loop? What happens in you alter the connection type to `Qt::DirectConnection` (just by way of a quick test)? — G.M., Apr 05 '17 at 08:20
@G.M. tried to use direct connetion it didn't help. the thread that creates the udpsocket does not have an event loop, its not a qt thread. — Adiel Yaacov, Apr 05 '17 at 08:38
If the thread on which the `UdpNetworkStreamer` exists doesn't have an event loop then queued signals can never be delivered to it. I suspect the `readyRead` signal *is* being emitted but the resulting events are sitting in an event queue and will never be processed. — G.M., Apr 05 '17 at 08:46
i understand what you are saying, but wouldn't the signal spy counter be updated at least once ? it remains zero ... — Adiel Yaacov, Apr 05 '17 at 08:47
I'm pretty sure you need an event loop for `readyRead` to be emitted. What would emit it, if there wasn't an event that would trigger it? — thuga, Apr 05 '17 at 09:12
@thuga - i'd be happy to have an event loop that will process it ? how can you do it, if the calling thread does not have one ? create a new qthread and move the socket to that thread using moveToThread ? — Adiel Yaacov, Apr 05 '17 at 09:17
You should see [this question](http://stackoverflow.com/questions/27801572/is-it-possible-to-create-local-event-loops-without-calling-qapplicationexec). — thuga, Apr 05 '17 at 09:32
Are you creating a [`QCoreApplication`](http://doc.qt.io/qt-5/qapplication.html) and calling `exec()` on it? The message loop that Qt dispatches on won't run otherwise. — Paul Belanger, Oct 09 '18 at 17:42

score 2 · Accepted Answer · edited Oct 09 '18 at 15:50

I ended up solving the problem this way :

void MainWindow::OpenConnection()
{
  QThread *t = new QThread();
  t->start();

  SocketWrapper *w= new SocketWrapper();

  w->moveToThread(t);

  w->metaObject()->invokeMethod(w, "CreateSocket", Qt::QueuedConnection);
}

You must call invokeMethod() with the thread the socket wrapper was movedTo() upon creation of the socket, so that the thread that creates the socket will have a running event loop.

In addition to that, the CreateSocket() needs to be a slot in the SocketWrapper, something like that :

class SocketWrapper : public QObject
{
  Q_OBJECT
public:
  explicit SocketWrapper(QObject *parent = 0);


signals:

public slots:
  void readyRead();
  void CreateSocket();
private:
  QUdpSocket *_socket;
};

You should be able to simply connect the thread's `started()` to your `CreateSocket()` - obviously don't start the thread until after the `moveToThread()` and `connect()`. — Toby Speight, Oct 09 '18 at 15:51

score 0 · Answer 2 · answered Apr 05 '17 at 07:31

0

My guess is that you need to add Q_OBJECT macro to the beginning of the class from where you need to emit the signal. Unless you do this, the signal-slot mechanism will not function properly.

answered Apr 05 '17 at 07:31

Doğukan Tunç

337
2
16

thanks but, i have that macro, Obviously since it is working perfectly when i use a qt gui applicationto use the library. – Adiel Yaacov Apr 05 '17 at 07:52

How to properly create QUdpSocket on non-gui thread ? Readyread not emitted

My Questions:

2 Answers2