Rafi Cohen wrote:
Hi, as a pproject for a company I'm writing a tcp/ip application on
linux using C language.
ah welcome, welcome to the pleasure dome...
My application has 2 connections as client to remote servers and is by
itself a server "accepting" remote client connections.
I'm using select() mechanism to manage all those connections. Everyting
works nicely until any of the remote sideds disconnects.
In some of such cases, I'm not succeeding to reconnect to this remote as
client or re-accept connection from tjhe remote client.
you're not describing things properly here, since later you say you are
not managing to disconnect (the problem is not with re-connecting -
which, if happened, would imply a different problem altogether)
Reading some documentation on tcp/ip programming, I had the impression
that the select mechanism should detect such remote disconnect event,
thus enabling me to make a further "read" from this socket which should
end in reading 0 bytes. Reading 0 bytes should indicate disconnection
and let me disconnect propperly from my side and try to reconnect.
However, it seems that select does not detect all those disconnect
events and even worse, I can not see any rule behind when it does detect
this and when it does not.
select does not notice "disconnections". it only notices if the socket
was closed by the remote side. that's a completely different issue, and
that's also the only time when you get a 0 return value from the read()
system call.
So, I have a couple of questions and I'll most apreciate any assistance.
1. Would you confirm that select, indeed, does not detect each and every
remote disconnection and do you know if there is a rule behind those cases?
TCP/IP stacks on linux, by default, will only notice a network
disconnection (i.e. the network went down) "reliably" after 2 hours.
that's how TCP/IP's internal keep alive mechanism is set. 2 hours is a
completely impractical value for any sane system you might develop.
you can tweak this parameter of the TCP/IP stack for specific sockets,
on current linux kernels, using a socket option. (man 7 tcp - and look
for 'keepalive' - there are 3 parameters for this). i never used this
mechanism, since it was only possible to make this change globally when
i needed that.
2. If I need to detect those disconnections to react propperly in my
application and I can not rely on select, what would you suggest me to
do? should I use a kind of "ping" mechanism to check every some seconds
if the connection is still "alive"? or may be use multithread instead of
select, where each thread is responsible for each connection source and
instead of select I loop on read from this source and so, can detect
when I read 0 bytes, which is the disconnect indication and react
accordingly?
read would fail just like select does, and because of the same reason.
you could implement the keepalive in your application, in case the
keepalive parameters tweaking of the TCP stack does not work, for some
reason.
--guy
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