GLib-WARNING **: poll(2) failed due to: Invalid argument.

[wysota]

So how can I detect this problem is occurring and have my program take steps to prevent this crash?

As for Linux, you can query /proc or /sys for the number of allowed file descriptors per process. As for other OSes, I have no idea. Do you really need such a large number of sockets?

QTcpServer does not seem to have any onError signals :-(

You can access the error from the regular socket (See QAbstractSocket::error ()).