RE: What is the correct behaviour for local socket(AF_UNIX) in the following scenario?

Hi Robert
>From this I understand  that if p1 closes the socket,
then all the data written by it is discarded since
the buffers associated with this socket no longer exists.
	If that is the case, then this is not at all true
that p1 can write data on a socket and close it and exit 
and p2 will still be able to 	retrieve the data written
by p1. Does FreeBSD differ with solaris or other unixes
in this implementation. I believe that Solaris does support
the above scenario although I am not sure. Do you know
How other unixes behave here. Because it makes a lot 
of sense for localsockets to facilitate IPC on a system
by supporting the above scenario between two processes.

Regards
Mayank

-----Original Message-----
From: Robert Watson [mailto:rwatson_at_FreeBSD.org] 
Sent: Friday, November 25, 2005 5:54 AM
To: Mayank Kumar
Cc: freebsd-current_at_freebsd.org
Subject: Re: What is the correct behaviour for local socket(AF_UNIX) in
the following scenario?


On Fri, 25 Nov 2005, Mayank Kumar wrote:

> I am trying to understand the behavior of localsockets in the 
> following scenario.
>
> A process p1 writes a huge amoount of data to a AF_UNIX,DGRAM socket 
> and exits. Now if there is no process p2 to read the data written by 
> process
> p1 from the same localsocket, then this has resulted in a huge memory 
> leak on a FreeBSD system.
>
> I want to understand, if there is a mechanism in FreeBsd to take care 
> of this leak or this is the expected behaviour and application writers

> should take care of this situation. Also what should be the behaviour 
> on such a socket if shutdown or close is issued on such a socket. Any 
> help on the behaviour on other unixes in the same scenario would also 
> help a lot.

Mayank,

The key to understanding how this is handled is to understand that UNIX
domain sockets aren't file system objects -- the file system simply
provides a name space by which to reach the socket.  The buffers
associated with UNIX domain sockets belong to the sockets, not to the
name.  You can think of this in the same way as you might think of port
numbers and IP addresses, although there are some subtle differences.

There are two common operational modes for UNIX domain sockets: stream
mode, and datagram mode.  In stream mode, a listen socket is bound to
the name, and then new socket pairs are generated when that name is
connected. 
In datagram mode, a single socket exists on the "server" end, and then a
series of other sockets may send to it using sendto and send.  The
buffers are associated with the active communication sockets in both
case, so if all endpoints are closed, the name persists, but has no
persisting buffers.  So a name can be leaked (i.e., not be unlinked when
a process is done with it), which is similar to leaking a temporary file
that isn't unlinked.

Hope this helps,

Robert N M Watson