QT4到QT5的网络问题-UDP

Question

I'm pretty new to Qt. I inherited a QtCreator project built using Qt 4.8.1 and have been attempting to rebuild it with Qt 5.4.2 to add functionality. I have successfully rebuilt the project under windows 7 (Mingw) and linux with Qt5.

Everything works well until I start the connected devices and receive the UDP stream - where the old project (on the same machine as new code) will happily receive and store data, the new project chokes very quickly at some arbitrary amount of data and will not accept new data.

EDIT packet size - all incoming small packets are received. There is typically some control and ack packets of small size ( ~100 bytes ) that work fine. The problem is once data acquisition starts; the incoming data packet payload is 1441 bytes and this will choke rapidly.

#include <QThread>
#include <QUdpSocket>
#ifdef linux
#include <sys/socket.h>
#elif _WIN32
#include <winsock2.h>
#else
#error Platform not supported
#end

// etc

void ReceiveSocket::run()
{
    udpReceiveSocket = new QUdpSocket();
    connect( udpReceiveSocket, SIGNAL( readyRead() ), this, SLOT( readPendingDatagrams() ) );
    udpReceiveSocket->moveToThread( this );
    exec();
}

// etc

void ReceiveSocket::readPendingDatagrams()
{
    QByteArray * data;
    while( udpReceiveSocket->hasPendingDatagrams() ) {
        data = new QByteArray;
        data->resize( udpReceiveSocket->pendingDatagramSize() );
        QHostAddress sender;
        quint16 senderPort;
        udpReceiveSocket->readDatagram( data->data(),
                sdata->size(),&sender, &senderPort );
        emit processDatagrams( data );
    }
}  

Has anyone experience this issue migrating to Qt5? Is there an obvious cause to an arbitrary stoppage? I have added a debug counter in the processDatagrams method to show the running count of datagrams processed and it varies but is around a few hundred before it quits.

EDIT I added a counter to the while loop and printed using qDebug. This would obviously slow things down and the data does acquire at about 1/10th the actual data rate and it will continue indefinitely. This may hint at the problem.

void ReceiveSocket::readPendingDatagrams()
{
    volatile int i = 0;
    QByteArray * data;
    while( udpReceiveSocket->hasPendingDatagrams() ) {
        qDebug()<<i;
        i++;
        data = new QByteArray;
        data->resize( udpReceiveSocket->pendingDatagramSize() );
        QHostAddress sender;
        quint16 senderPort;
        udpReceiveSocket->readDatagram( data->data(),
                sdata->size(),&sender, &senderPort );
        emit processDatagrams( data );
    }
}  

For now, I'm going to use Qt4 libraries until I have a better opportunity to debug behavior with Qt5

Answer 1

First of all, no need for new QByteArray

void ReceiveSocket::readPendingDatagrams()
{
    while( udpReceiveSocket->hasPendingDatagrams() ) {
        int len = udpReceiveSocket->pendingDatagramSize();
        QByteArray data;
        data->resize(len);
        udpReceiveSocket->readDatagram(data.data());
        emit processDatagrams( data );
    }
}  

so this should fix a possible error with data (not initialized) and sdata (whatever this is). I discarded the variables (whatever this is). I discarded the variables sender and senderPort` because they were not needed in your code and you can add them if needed later

Note that UDP datagrams are kind of restricted in size. they will get split up at a certain length by the socket itself when it is longer than the packet size. I usually use packets with size of <512, should be the minimum default length of UDP data packets by routers and such

Answer 2

This bug appeared in Qt 5.4.2. Either downgrade to 5.4.1 or enclose the event handling code in a loop as suggested by Zaiborg.