I think the best way to handle this problem is to put the socket into non-blocking I/O mode, so that the thread will never block inside recv() (or in send(), for that matter). The thread should only ever block inside select() (or WaitMultipleObjects()). That way the select() (or WaitMultipleObjects()) call will return if data arrives for the socket (in which case you can then call recv() to get the new data without blocking), but you can also have select()/WaitMultipleObjects() return when something else happens; e.g. when it gets a prompt from the main thread. If you are using select(), that prompt can be the main thread sending a byte on a different socket-pair (with the main thread holding one end of the socket-pair, and the I/O thread holding the other end); if you are using WaitMultipleObjects() then I believe you can use any of the standard Windows event/signaling methods that would cause WaitMultipleObjects() to return.
