Checking a port if it's in use/open

[squidge]

Under Windows there are several freeware programs such as Net Monitor which will show you when ports are connected to.

If you want to do it yourself, then your looking at monitoring Winsock, which normally uses a proxy DLL or injecting your own DLL into running processes to monitor calls.

If you just want to know when someone has connected to a port (and don't care about forwarding it onto another program), then things become much easier and you can do it all in Qt.