The usage pattern would be a bit like BSD ptys--the server maintains a few
ports, say named "server0" through "server15" and clients that need a port try
"server0", if it's busy then try "server1", etc..
I suppose the problem here is really that the application running over these
sockets is stateful, the configuration of the nodes (processes) that are
communicating is not known until run time, and nodes may appear and disappear
as the server runs, so the transport needs to be connection-oriented rather
than message-oriented. The semaphore is still a bit of a kludge, since it
doesn't notify a thread blocked on recv() that the remote has hung up, but it
works well enough in conjunction with a message that says "I'm a client that's
newly connected to this server" to reset the state associated with the
connection. I suppose I could implement a sort of TCP handshake on top of the
IDDP socket layer and use that to provide true connection-oriented behavior.
Good to know about in-order delivery; a quick glance at the source code seemed
to suggest that, but I wasn't sure.
Doug Brunner
-----Original Message-----
From: "Philippe Gerum" <[email protected]>
Sent: Tuesday, March 27, 2012 9:26am
To: "Doug Brunner" <[email protected]>
Cc: [email protected]
Subject: Re: [Xenomai-help] Intermixing native and POSIX skins
On 03/26/2012 06:14 PM, Doug Brunner wrote:
> Thanks for the information--the issue is not about picking a free port on the
> server side, but rather about communicating the information on which ports
> are "free" to the clients (server has connected its end, but no other client
> is using the port). The semaphore mechanism isn't that much of a problem,
> though; I've been able to build a satisfactory implementation.
Mm, still scratching my head to understand the issue. Both the client
and server side accept -1 as the port spec, telling the kernel to draw a
free port. I understand the issue is not on the server side, so is it on
the client side?
Could you sketch the usage pattern?
>
> One other question: although I know in-order delivery isn't necessarily a
> feature of datagram based protocols, would I get that with an IDDP socket
> connection between just two processes, and/or an XDDP connection to a
> /dev/rtpN?
Yes, in-order delivery is guaranteed with all RTIPC protocols. This is
written in stone.
>
> Doug Brunner
>
> -----Original Message-----
> From: "Philippe Gerum"<[email protected]>
> Sent: Friday, March 16, 2012 11:29am
> To: "Doug Brunner"<[email protected]>
> Cc: [email protected]
> Subject: Re: [Xenomai-help] Intermixing native and POSIX skins
>
> On 03/15/2012 05:30 PM, Doug Brunner wrote:
>> Thanks Philippe. I hadn't even known about the existence of the RTIPC
>> driver, and I definitely like the idea.
>>
>> I've been experimenting with it a bit today, and found that it seems to be
>> allowed for more than two sockets to connect to the same port. I modified
>> iddp-sendrecv.c to have two client processes, both of which now connect to
>> the same port as the server, then did the same thing with iddp-label.c (two
>> clients both connect()ing to the same label).
>>
>> This would cause havoc with the communications that go on between my
>> processes--they need a one-to-one channel. I could implement semaphores to
>> enforce this, but it would be nice to avoid that complication. Is there a
>> way to make it happen using just the socket interface?
>
> The RTIPC protocols are fundamentally datagram-based, so allowing N:1
> data paths is wanted. If the issue is about picking a different port
> each time you bind a server socket in the AF_RTIPC domain, then I would
> suggest to set sipc_port to -1 when binding the server-side socket: a
> free port will be picked automatically. You could then use getsockname()
> to retrieve the actual port #, and pass it to the clients.
>
--
Philippe.
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