How to detect "port unreachable" condition from QUdpSocket? - c++

Question

I'm trying to use QUdpSocket to send some data to a well-defined host:port destination. When the destination port is not being listened on (e.g. the client is not running), ICMP messages appear that say "port unreachable".
I'd like to detect this condition in my program and act when it happens. I read somewhere that if I directly "connect" to the host instead of just doing "fire and forget" with my datagrams, then the application should receive these ICMP notifications. But the following code doesn't appear to get any errors.
#include <QTimer>
#include <QString>
#include <QUdpSocket>
#include <QCoreApplication>
class DataSender : public QObject
{
public:
DataSender(const QString& host, quint16 port);
void send(const char* data, unsigned size);
private:
void onSocketError(QUdpSocket::SocketError error);
private:
QUdpSocket socket;
};
DataSender::DataSender(const QString& host, const quint16 port)
{
connect(&socket, &QUdpSocket::connected, this, []{qDebug() << "connected";});
connect(&socket, &QUdpSocket::disconnected, this, []{qDebug() << "disconnected";});
connect(&socket, &QUdpSocket::hostFound, this, []{qDebug() << "hostFound";});
connect(&socket, qOverload<QUdpSocket::SocketError>(&QUdpSocket::error), this, &DataSender::onSocketError);
socket.connectToHost(host, port, QIODevice::WriteOnly);
qDebug() << "UDP server started";
}
void DataSender::send(const char*const data, const unsigned size)
{
socket.write(data, size);
}
void DataSender::onSocketError(const QUdpSocket::SocketError)
{
qDebug() << "UDP socket error:" << socket.errorString();
}
int main(int argc, char** argv)
{
QCoreApplication app(argc, argv);
DataSender sender("127.0.0.1", 33333);
QTimer timer;
QObject::connect(&timer, &QTimer::timeout, [&sender]{ sender.send("Test", 4); });
timer.start(1000);
return app.exec();
}
In the output I only get
hostFound
connected
UDP server started
At the same time, when running this test, I do see the "Port unreachable" ICMP messages in Wireshark. How can I actually receive and handle them?
I've found that in the Linux socket API the write(2) syscall returns -1 with errno=ECONNREFUSED in this case. But with QUdpSocket I'd like to get a cross-platform solution. I've also found that syscall-set errno does appear to be preserved after QUdpSocket::write() returns, so I might be able to use this, but this is specific to Linux, while the list of my target platforms also includes Windows.