[erlang-questions] Difference in 'close' for gen_tcp recv vs. active

Mike Oxford <>
Wed Apr 9 03:34:26 CEST 2014


http://en.wikipedia.org/wiki/Transmission_Control_Protocol#Out-of-band_data?



On Tue, Apr 8, 2014 at 5:49 PM, Fred Hebert <> wrote:

> The difference here is that we have a peer that actively closes the
> connection and the packets are readable and detected as far as tcpdumps
> go. Things like FINs or RSTs. They're making it to our host, they're
> just entirely ignored unless we're willing to read undefined amounts of
> data from the buffer for the stack to apparently get access to them.
> 99.99% of the time that's gonna be 0 bytes because of HTTP clients' rare
> tendency to pipeline requests.
>
> I'm just extremely annoyed having to figure out a way to add in a buffer
> to carry possibly unlimited amounts of data to read from a socket so it
> can figure out if a FIN or RST was sent recently, but I guess that's the
> way we'll have to do it.
>
> Oh well.
>
> On 04/08, Garret Smith wrote:
> > Basically, this has everything to do with TCP sockets (and the OS
> > implementation) and very little to do with Erlang.  I do get {error,
> > closed} from gen_tcp:send when the client disconnects.  I'm running
> > R16B03-1 on FreeBSD 10.
> >
> > Here are some decent descriptions of the problem:
> >
> >
> http://www.linuxquestions.org/questions/programming-9/how-could-server-detect-closed-client-socket-using-tcp-and-c-824615/
> >
> > The second answer in the page above has some good links too, pulled out
> for
> > easy reference:
> >
> >
> http://stackoverflow.com/questions/722240/instantly-detect-client-disconnection-from-server-socket
> >
> http://www.softlab.ntua.gr/facilities/documentation/unix/unix-socket-faq/unix-socket-faq-2.html#ss2.8
> >
> > -Garret
> >
> >
> > On Tue, Apr 8, 2014 at 1:40 PM, Fred Hebert <> wrote:
> >
> > > Hi there,
> > >
> > > Happy fun case we're hitting on nodes using Erlang right now. We have
> an
> > > HTTP proxy that does direct data streaming from a server to a client.
> > > This is done through a series of recv from the server, and of sends to
> > > the client. The TCP sockets involved are all in passive mode.
> > >
> > > The problem with this approach is that because this is a purely
> > > unidirectional stream until the server says so or the client quits,
> we'd
> > > like to detect both of these events.
> > >
> > > The server quitting or being done is easy enough, but the client
> > > quitting cannot be detected, apparently.
> > >
> > > It turns out that gen_tcp:send/2 always returns 'ok' even if the
> > > connection has been closed by the peer before. Erlang/OTP just won't
> > > acknowledge the fact unless someone tries to read from the socket,
> > > either through gen_tcp:recv or by using inet:setopts(Port, [{active,
> > > once}]), at which point {error, closed} starts being returned by
> > > gen_tcp:send/2. This happens, even if `{exit_on_close, true}` is
> > > specified as an option.
> > >
> > > The question I have here is why is this behavior different for send
> than
> > > recv. It seems that `gen_tcp:send` will happily wait for hours
> > > pretending to send data (no matter the timeout values used are), even
> > > once the connection has been closed by the other peer.
> > >
> > > Is there any way for me to detect that a connection has been closed
> > > without possibly having to poll `recv` on each packet I try to send
> (and
> > > then may need to buffer all that data, which I'd prefer to avoid) or
> > > changing the entire app's workflow to be active?
> > >
> > > In case, here are the socket options we use:
> > >
> > >     [{active,false},
> > >      {broadcast,false},
> > >      {buffer,1460},
> > >      {delay_send,false},
> > >      {dontroute,false},
> > >      {exit_on_close,true},
> > >      {header,0},
> > >      {high_watermark,8192},
> > >      {keepalive,false},
> > >      {linger,{false,0}},
> > >      {low_watermark,4096},
> > >      {mode,binary},
> > >      {nodelay,true},
> > >      {packet,0},
> > >      {packet_size,0},
> > >      {priority,0},
> > >      {recbuf,87380},
> > >      {reuseaddr,true},
> > >      {send_timeout,infinity},
> > >      {sndbuf,65536}]
> > >
> > > Regards,
> > > Fred.
> > > _______________________________________________
> > > erlang-questions mailing list
> > > 
> > > http://erlang.org/mailman/listinfo/erlang-questions
> > >
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