MULTICAST with QT 4.1 and above.

[wysota]

Qt uses BSD sockets to do its job. You can even set a "pure" BSD socket (using QAbstractSocket::setSocketDescriptor), so you can create a bsd socket, set the options you need and make a Q*Socket out of it. I have never used multicasting but if it only needs setting a proper multicast address, then Qt should handle it out of the box.

[jlarsj]

Hi Wysota,

Well, I have been programming those small Multicast handlers both under Linux and Windows. Actually Winsock is based also on the BSD "Like" socket lib ( Not MFC, just Winsock ), and they use the same system calls.

Well I tried using QUdpSocket but it doesn't catch any Multicasted packet. Here a simple example of code provided by Trolltech:

QUdpSocket udpSocket = new QUdpSocket(this);
udpSocket->bind(QHostAddress::LocalHost, 24001);
 
while (udpSocket->hasPendingDatagrams()) 
{
            QByteArray datagram;
            datagram.resize(udpSocket->pendingDatagramSize());
            QHostAddress sender;
            quint16 senderPort;
 
            udpSocket->readDatagram(datagram.data(), datagram.size(),
                                    &sender, &senderPort);
 
            processTheDatagram(datagram);
}

QUdpSocket udpSocket = new QUdpSocket(this);
udpSocket->bind(QHostAddress::LocalHost, 24001);

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

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

            processTheDatagram(datagram);
}

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The problem here is pretty obvious : where do you join the multicast group ????

So I tried to put in the bind() function a QHostAddress containing the multicast address, but the bind function fails each time.

So, I don't know I do believe that Qt doesn't handle multicast by itself ?? Sad isn't it

[yuriy]

I had the same issue a while ago and used setsockopt to set multicast option. The code looks like this:

udpSendingSocket = new QUdpSocket;
if (!udpSendingSocket->bind(sendPort, QUdpSocket::ShareAddress))
std::cout <<"send bind failed\n";
sendingFd = udpSendingSocket->socketDescriptor();

if (sendingFd != -1)
{
/* set up destination address */
// struct sockaddr_in sendAddr;
memset(&sendAddr,0,sizeof(sendAddr));
sendAddr.sin_family=AF_INET;
sendAddr.sin_addr.s_addr=inet_addr(HELLO_GROUP);
sendAddr.sin_port=htons(sendPort);

// set TTL
unsigned int ttl = 1; // restricted to the same subnet
if (setsockopt(sendingFd, IPPROTO_IP, IP_MULTICAST_TTL, (unsigned int*)&ttl, sizeof(ttl) ) < 0)
{
std::cout << "TTL failed\n";
}
}

Can Qt add a multicast option in QUdpSocket class?