Microsoft say this about Winsock error 10057 - WSAENOTCONN:

A request to send or receive data was disallowed because the socket is not connected and (when sending on a datagram socket using sendto) no address was supplied. Any other type of operation might also return this error—for example, setsockopt setting SO_KEEPALIVE if the connection has been reset.

Full details of Winsock TCP error 10057 - WSAENOTCONN:

Berkeley description: A request to send or receive data was disallowed because the socket is not connected and (when sending on a datagram socket) no address was supplied.

WinSock description: Same as Berkeley, and then some. An application attempted an input/output network function call before establishing an association with a remote socket (i.e. before calling connect() or accept()). It also has a specific meaning for setsockopt().

Detailed description:

setsockopt(): WinSock generates this error if you try to set SO_KEEPALIVE but the connection has already been aborted (e.g. a TCP reset received from remote host).