Re: TCP Socket keepalive

Hi Haitao:

Supposing that your client looks something like this:

client_stream(IPAddress, In, Out) :-
  tcp_socket(S),
  tcp_connect(S, IPAddress, In, Out).

If client_stream/3 succeeds then In and Out will be unified with the appropriate Prolog streams. If it succeeds, then safety requires that you eventually close both streams using "close(Stream, [force(true)])," and you must not release the socket via tcp_close_socket/1 since it is no longer valid.

BUT, it is perfectly reasonable to expect that occasionally the above will throw an exception due the failure to connect. In that case you *must not* close the streams since they are unbound and you must release the socket using tcp_close_socket/1. If you don't then the socket is leaked.

Your client_stream/3 needs to look something like:

client_stream(IPAddress, In, Out) :-
        setup_call_catcher_cleanup(tcp_socket(Socket),
                                   tcp_connect(Socket, IPAddress, In, Out),
                                   exception(_),
                                   tcp_close_socket(Socket)).

(Code not tested) 

Please see Section 6.2 of the clib docs to see how the socket should be managed at this critical time.

Regards,
Jeff R.


-----Original Message-----
From: Haitao Zhang <[email protected]>
To: Jan Wielemaker <[email protected]>
Cc: swi-prolog <[email protected]>
Sent: Wed, Dec 11, 2013 3:08 am
Subject: Re: [SWIPL] TCP Socket keepalive


Hi Jan,
Thanks for the response. I will take a look at tcp_setopt/setsockopt.
On a related note, after the send error (as the socket closed unexpectedly)
f I call close/1 on the stream, I get the same error. Now that is a little
onfusing. Shouldn't close/1 just silently ignore the error?
cp_close_socket on the other hand seems to gracefully handle the
ituation. When I close down a channel, should I
A) just call tcp_close_socket on the socket, and don't call close on the
tream?
or
B) do both, but wrap the close on the stream to trap the error condition?
t seems weird that one needs to wrap the close/1 since it should be in the
leanup clause of an exception handler.
Haitao

n Tue, Dec 10, 2013 at 12:57 AM, Jan Wielemaker <[email protected]> wrote:
> On 12/10/2013 08:26 AM, Haitao Zhang wrote:

> 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?
>

 There is tcp_setopt/2, which binds to setsockopt().  It doesn't support
 the SO_KEEPALIVE option yet.  Should be easy enough to add.  If you use
 a source version, see packages/clib.  Try it and submit a patch with
 additional options you see fit.

         Hope this helps

                 --- Jan



> Thanks,
> Haitao
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