Re: write to socket problem

[Andreas Höschler]

Hi David,

thanks for your prompt response!

> > call blocks and never returns and thus renders the service 
> > inoperable. I have already spent a couple of hours with the select, 
> > write, send,... documentation but haven't found a solution for this 
> > problem yet. Any idea how I can find out whether the reading end has 
> > closed? I expected select to do this, but it does not. select returns 
> > and FD_ISSET(_sock, &fdset)) gives me TRUE. But then the send blocks!
>
> Unfortunately, there is no good way of doing this.  The connection is 
> being closed due to a timeout (and may be closed by a NAT in the way 
> for the same reason).  The solution that we use for XMPP is to send a 
> space character every 50 seconds of inactivity.  This works because 
> the protocol is XML and cdata in the outer scope is ignored.  You 
> might consider something similar for your protocol.
>
> The problem you are encountering is that this is not a graceful 
> shutdown of the connection.  When the connection times out, there is 
> no tear-down of the TCP session, the other end just gives up.  When 
> you write to the socket, your server does not know that the other end 
> has closed, so it will send the data and wait for the acknowledgement. 
>  It then waits for its own timeout period for this to be received.

Ah, that makes sense!

> On most socket implementations (not sure if this includes Solaris), 
> the a write on a blocking socket will block once the TCP window is 
> full.  The window will start to fill up if acknowledgements are not 
> received for packets.

I am now comparing the time before and afte rthe invocation of the 
method. If more than 70s have passed, i ommit the write of the 
response. What an ugly hack!! :-(  I need make this beast faster...

Thanks a lot,

 Andreas