Re: [Boost-users] Boost ASIO - SO_LINGER Socket option is not working

Subject: Re: [Boost-users] Boost ASIO - SO_LINGER Socket option is not working
From: Zachary Turner (divisortheory_at_[hidden])
Date: 2009-06-02 10:41:36

On Thu, May 28, 2009 at 9:42 PM, Nick Aschberger <
nick.aschberger_at_[hidden]> wrote:

>
> I experimented with m_socket->shutdown() instead of close, and it made
> little difference. In fact, from the documentation it seems shutdown() is
> the one I definitely don't want to call, as close() is supposed to wait for
> data to be sent.
>

This is a common point of confusion with sockets programming.  Here is how
it works according to my understanding.  Note that I know nothing of how
boost sockets are implemented, the following applies specifically to the BSD
socket interface, which means that by extension it should apply to boost
sockets although I can't guarantee that.

1) shutdown() initiates a graceful termination sequence.  It never blocks,
regardless of the value of SO_LINGER.  shutdown() specifically tells the
kernel "I'm done, start flushing out any pending data that's left over".
But it never frees any underlying O/S resources or handles.

2) close() terminates the connection and frees O/S handles.  On windows, the
function is called closesocket() and first initiates a shutdown (according
to the definition above), if one has not been initiated already.  This
behavior is not guaranteed as far as I know (someone correct me if I'm
wrong) and probably does not happen on O/Ses other than Windows.  Even the
windows documentation recommends not relying on it.   *IF* a graceful
shutdown has been initiated *AND* the SO_LINGER option is set, then close()
blocks until either a) the linger timeout has expired, or b) all pending
data has been sent, whichever comes first.

Therefore, I believe the correct sequence is to first set the SO_LINGER
option, then call shutdown, then call close.

Have you tried that?