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