Answer an UDP packet - c++

Question

I have a UDP server using the following code:
void initialize()
{
connect(&_udpSocket, SIGNAL(readyRead()), this, SLOT(onUdpDatagram()));
_udpSocket.bind(QHostAddress::Any, 28283);
}
void onUdpDatagram()
{
qDebug() << "udp packet received!";
_udpSocket.write("Hello");
}
Unfortunately when a UDP packet is received, I have the following error in the log:
QIODevice::write: device not open
How can I make the UDP socket writable? I tried to create another socket for the answer that connect to the sender address and port but the sending won't use the 28283 port anymore...
Any idea?
For info: I'm using Qt 5.2.1 on MacOS 10.9

Answer 1

UDP is not a connection-based protocol. You don't get a separate socket for each peer, instead there's one socket for all communication on a single port.
Therefore, there's some extra effort needed to reply to an incoming UDP packet. You need to retrieve the sender address from the datagram you received, and send back to that same address. In the sockets API this is done by using recvfrom and sendto functions instead of recv (or read) and send (or write) -- the latter are designed for connected sockets like you use with TCP.
You didn't show the declaration (really, the type) for your _udpSocket variable, so I'm assuming that you are using a QUdpSocket. In that case, it looks like you will want to use the readDatagram and writeDatagram functions, which like recvfrom and sendto, have an additional parameter for the peer address (actually, it's a pair, one for the IP address, one for the port).
Here's what the Qt documentation says about that:
The most common way to use this class is to bind to an address and port using bind(), then call writeDatagram() and readDatagram() to transfer data. If you want to use the standard QIODevice functions read(), readLine(), write(), etc., you must first connect the socket directly to a peer by calling connectToHost().

Answer 2

Coincidentally, this warning was introduced by me in Qt upstream:
QIODevice::write: device not open
It should be pretty clear unlike before the introduction of this, namely: you have forgotten to connect to the host with your udp socket. You cannot expect it to write and/or read if it is not even open and/or connected. See the documentation for details:
If you want to use the standard QIODevice functions read(), readLine(), write(), etc., you must first connect the socket directly to a peer by calling connectToHost().
You have to do something like this somewhere in your code:
_udpSocket.connectToHost(myHostAddress, 28283, ReadWrite, AnyIPProtocol);
The last two parameters can be skipped as they are the default. As you can read from the documentation, this method call will open the socket for you, too, which is necessary to get done for QIODevice read and write operations.
That being said, you really should not neglect error checking in your code as it currently seems to stand. It will be difficult to find the issues this way.
Also, it is ice on the cake, but I would encourage you to start using the "new" signal-slot syntax, which is not so new, but much more modern and handier:
void initialize()
{
connect(&_udpSocket, &QUdpSocket::connected, [&_udpSocket]() {
connect(&_udpSocket, &QUdpSocket::readyRead, [&_udpSocket]() {
qDebug() << "udp packet received!";
if (_udpSocket.write("Hello") != 6)
qDebug() << "Failed to write:" << _udpSocket.errorString();
});
});
connect(&_udpSocket, &QUdpSocket::error, [&_udpSocket]() {
qDebug() << "Error occured:" << _udpSocket.errorString();
});
_udpSocket.connectToHost(myHostAddress, 28283, ReadWrite, AnyIPProtocol);
}