QUdpSocket::readyRead() doesn't emit after thread event loop restart

usamytch

Good day, colleagues!

I have one Q_OBJECT class (A) with second Q_OBJECT class (B) nested in A. B contains QUdpSocket that listens specified port and some code that process incoming UDP packets. In ctor of A I create a QThread and move B with QUdpSocket to the new thread. Then I call thread->start() and all works well - B happily receives UDP packets and process it.

After some time, I stop listening thread calling thread->quit() from A. Then, again after some time, I restart thread calling thread->start(). Thread starts successfully but readyRead() is never emitted, although UDP packets arrive to target port.

I have a full control over incoming UDP packets, so I sure that I read all UDP packets before calling thread->quit(), my problem is not caused by presence of pending datagrams at QUdpSocket.

If I don't stop and start the thread, all is OK and readyRead() signal is emitted as usual. Here is pseudocode:

@class B : public QObject {
Q_OBJECT

public:
B (int port, QMutex * mutex);
QUdpSocket socket;
void Move_To_Thread (); // Safely move B and UDP socket to target thread

private:
QMutex * mutex_;
void Process ();
};

class A : public QObject {
Q_OBJECT

public:
A ();
void Run();

private:
B b;
QThread * thread;
QMutex mutex;
};

A::A() {
thread = new QThread(this);
b.Move_To_Thread (thread);
}

void A::Run () {

thread->start();
Sleep (SOME_TIME);
thread->quit();
Sleep (OTHER_TIME);
thread->start();

}@

dangelog

Try to simplify the problem (I won't ask why you're doing networking in a separate thread).

How are you putting the socket in listening mode? Without/with threads involved, if packets arrive before the event loop is started (but after the socket is listening), is readyRead emitted?

usamytch

Here are the pseudocode for putting socket in a listening mode:

@B::B (int port, QMutex * mutex) : mutex_ (mutex) {
socket.bind(port);
connect (&socket, SIGNAL(readyRead()), this, SIGNAL(Process()));
}@

UDP packets start arriving only after I start event loop. So:

@void A::Run () {

thread->start();
// UDP packets start arriving, for this piece of code readyRead() emits as usual
Sleep (SOME_TIME);
// UDP packets stop arriving
Sleep (WAIT_PACKETS_TIME);
thread->quit();

Sleep (OTHER_TIME);
thread->start();
// UDP packets start arriving again, but readyRead() not emitted

}@

And how can I check if the readyRead() was emitted without starting event loop?