Berkeley description: An operation was attempted on
something that is not a socket. The specified socket parameter refers to
a file, not a socket.
WinSock description: Same as Berkeley. The socket input
parameter is not a valid socket handle (either it never was valid, it's
a file handle (not a socket handle), or if it was a socket handle, it
has been closed).
select(): fails with
WSAENOTSOCK if any socket in an fd_set is an invalid socket handle.