gen_tcp:recv - last segement when closed port

Per Hedeland <>
Sat Mar 11 22:15:01 CET 2006


"Valentin Micic" <> wrote:
>
>It's been a long time since I've been thinking about this, but as much as I 
>could remember the remote close request is treated as OUT-OF-BAND data, 
>which means that it will bypass anything that exist in/on protocol stack's 
>buffer. So, if there is, say, 1200 octets in buffer, and you read 348, and 
>before you managed to read remaining octets, TCP_CLOSE request arrives, you 
>will lose unread data. This has nothing to do with ERLANG, but with the way 
>tcp works.
>But then again, it's been a long time, and my memory might not serve me 
>correctly.

I'm afraid I have to agree with that - your memory is not serving you
correctly. TCP promises to deliver a reliable byte stream, and with the
socket interface you can read it however you want, and still get all the
bytes that the remote wrote before closing. Many protocols rely on this,
notably "plain" HTTP/1.0 as in Johan's case. If there is a problem here,
it is either in Erlang's inet_drv driver or in the TCP/socket
implementation that Johan is using (but the latter is extremely
unlikely, even on that particular OS).

Maybe you're thinking of TCP reset (RST), which will indeed in common
implementations throw away any unread data. And come to think of it,
it's not uncommon that "that particular OS" seems to think that RST is
the proper way to close a TCP session... Johan, you're not seeing an RST
from the server in the traces, are you?

--Per



More information about the erlang-questions mailing list