QUdpSocket.readyread is not emitting when QCoreApplication not created from main thread.

Diren

Re: Why the main thread is needed for QCoreApplication?

I wrote this code to create QCoreApplication from different thread. It is ok for many situations except udp send and QUdpSocket.readyread not working. Is it a bug or known issue?

#include <QCoreApplication>
#include <QNetworkDatagram>
#include <QObject>
#include <QUdpSocket>
#include <thread>

class UDPListener : public QObject
{
public:
    UDPListener(){}
    void startListening(quint16 port)
    {
        if (udpSocket.bind(port))
            connect(&udpSocket, &QUdpSocket::readyRead, this, &UDPListener::processPendingDatagrams);
    }    
    
private:
    QUdpSocket udpSocket;
    
public Q_SLOTS:
    void processPendingDatagrams()
    {
        while (udpSocket.state() == QUdpSocket::BoundState && udpSocket.hasPendingDatagrams())
        {
            QNetworkDatagram datagram = udpSocket.receiveDatagram();
            qDebug() << datagram.data();
        }
    }
};

void runsInOtherThread(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    a.exec();
}

void udpSend()
{
    quint16 port = 5000;    
    QHostAddress pTargetAddress;
    pTargetAddress.setAddress("127.0.0.1");
    
    QUdpSocket test;
    test.writeDatagram("hello",5,pTargetAddress,port);
}

#define OTHERTHREAD

int main(int argc, char *argv[])
{
#ifdef OTHERTHREAD
    std::thread t(runsInOtherThread, argc, argv);
#else
    QCoreApplication a(argc, argv);
#endif
    
    UDPListener u;
    u.startListening(5000);
    
    udpSend(); 
    std::thread t2(udpSend);
    
#ifdef OTHERTHREAD
    t.join();
#else
    a.exec();
#endif
    return 0;
}

JonB

@Diren
I don't know if this is relevant/the cause, but as we have said to another similar-ish post elsewhere: really think you should do thread work via QThread not std::thread if you want Qt things to work smoothly...?

VRonin

It shouldn't matter what thread the QCoreApplication is but you still have to run an event loop in the thread that owns the UDPListener. The thread that should take care of executing the processPendingDatagrams is blocked by t.join(); in your example

Diren

@VRonin
When i move 2 lines to
UDPListener u;
u.startListening(5000);

between
QCoreApplication a(argc, argv);
and
a.exec();

it works. I have to think about it. Thank you.

Pablo J. Rogina

@Diren said in QUdpSocket.readyread is not emitting when QCoreApplication not created from main thread.:

it works.

time to mark your post as solved then? thanks.