QUdpSocket loopback datagram - c++

Question

I'm working with Qt 5.9 and I'm using a bidirectional (send and receive) QUdpSocket.
How can I avoid receiving same message has just sent on the same socket?
Here a snippet of the code
// Socket init
this->UdpSocket->bind( QHostAddress::Any, ARTNET_PROTOCOL_PORT );
connect( this->UdpSocket, SIGNAL(readyRead()), this, SLOT(readPendingDatagrams()), Qt::UniqueConnection );
[...]
void ArtNetManager::readPendingDatagrams()
{
QNetworkDatagram networkDatagram;
qDebug("Udp datagram received");
while( this->UdpSocket->hasPendingDatagrams() )
{
networkDatagram = this->UdpSocket->receiveDatagram();
qDebug("Received datagram from IP address: %s", networkDatagram.senderAddress().toString().toLatin1().data() );
this->receiveDatagram( networkDatagram.data() );
}
}
void ArtNetManager::sendDatagram()
{
QByteArray ArtNet_RawMsg;
ArtNet_RawMsg.append( "Test program" );
// Writes data on the UDP socket
qint64 sentBytes = this->UdpSocket->writeDatagram( ArtNet_RawMsg, QHostAddress::Broadcast, ARTNET_PROTOCOL_PORT );
if( sentBytes == -1 )
{
qDebug("Cannot send data on UPD socket. Error: %d", this->UdpSocket->error() );
}
else if( sentBytes != ArtNet_RawMsg.size() )
{
qDebug("Wrong number of bytes sent. Bytes sent on socket: %d, tx buffer length: %d", sentBytes, ArtNet_RawMsg.size());
}
}

Answer 1

The messages that you receive are due to the ArtNet behavior. UDP protocol normally doesn't reflect the outgoing traffic back. Some other ArtNet device is doing that.
You could ignore these by keeping a list of a few recently sent messages, and looking them up:
class ArtNetManager : public QObject {
Q_OBJECT
int const m_sentFifoLength = 32;
QList<QByteArray> m_sentFifo;
...
}
void ArtNetManager::sendDatagram() {
sendDatagram({"Test program"});
}
void ArtNetManager::sendDatagram(const QByteArray & msg) {
if (m_sent_fifo.size() >= m_sentFifoLength))
fifo.removeLast();
m_sentFifo.prepend(msg);
auto bytes = m_udpSocket->writeDatagram(msg, QHostAddress::Broadcast, ARTNET_PROTOCOL_PORT);
if (bytes != msg.size())
qWarning() << "can't send datagram" << msg.toHex();
}
void ArtNetManager::readPendingDatagrams() {
while (m_udpSocket->hasPendingDatagrams()) {
auto datagram = m_udpSocket->receiveDatagram();
qDebug() << "received datagram from" << datagram.senderAddress().toString();
receiveDatagram(datagram.data());
}
}
void ArtNetManager::receiveDatagram(const QByteArray & msg) {
auto it = std::find(m_sentFifo.begin(), m_sentFifo.end(), msg);
if (it != m_sentFifo.end()) {
m_sentFifo.erase(it);
return;
}
...
}