Question

I’m trying to write a program that reads broadcasted UDP datagrams in linux.I’m a beginner in socket programming .
My code is :

#include <QUdpSocket>
#include <iostream>


int main ()
{
    QUdpSocket *udpSocket ;
    udpSocket= new QUdpSocket(0);
    udpSocket->bind(QHostAddress::LocalHost, 3838);


    udpSocket->connect(udpSocket, SIGNAL(readyRead()),
             this, SLOT(readPendingDatagrams()));

    while (1)
    {
        if (udpSocket->hasPendingDatagrams())
        {
             QByteArray datagram;
             datagram.resize(udpSocket->pendingDatagramSize());
             QHostAddress sender;
             quint16 senderPort;

             udpSocket->readDatagram(datagram.data(), datagram.size(),
                                     &sender, &senderPort);
        }
    }
}

but it returns error in this.

main.cpp:13:18: error: invalid use of ‘this’ in non-member function

what should I do ?

Answer 1

You need an event loop to use signals and slots (with a QCoreApplication, QApplication, or QEventLoop) and a QObject derived class to host the slots.

But you can use the sockets synchronously without signal/slot or an event loop, by using the functions QUdpSocket::waitForReadyRead, waitForBytesWritten… :

#include <QUdpSocket>
#include <QTextStream>

int main()
{
    QTextStream qout(stdout);

    QUdpSocket *udpSocket = new QUdpSocket(0);
    udpSocket->bind(3838, QUdpSocket::ShareAddress);

    while (udpSocket->waitForReadyRead(-1)) {
        while(udpSocket->hasPendingDatagrams()) {
            QByteArray datagram;
            datagram.resize(udpSocket->pendingDatagramSize());
            QHostAddress sender;
            quint16 senderPort;

            udpSocket->readDatagram(datagram.data(), datagram.size(),
                                    &sender, &senderPort);
            qout << "datagram received from " << sender.toString() << endl;
        }
    }
}

Edit: To listen to broadcast UDP datagrams, you also should not listen to QHostAddress::LocalHost but to QHostAddress::Any (or at least to an IP address attached to an external interface).