TCP Socket keepalive

I have a problem with the TCP socket, where SWI Prolog is client,
disappearing after some time (presumably some inactivity timer expired). I
have set my server side to keepalive, but that does not seem to help.

The message I get from trying to use the socket again after a long idle
time:
ERROR: format/3: I/O error in write on stream <stream>(000000000834B260)
(Socket operation on non-socket)

Any idea on how keep the socket alive?

Thanks,
Haitao
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