On 03/27/2012 11:35 PM, Doug Brunner wrote:
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..
An option to mimic this partially might be to define a fixed port number
clients would use to ask the server for a free port to connect to later
on? The server would let the RTIPC layer pick a free port when binding,
then return it to the client via a converse message to the fixed port.
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.
Yep, RTIPC won't help that much by default in that area, it only
exhibits connectionless semantics.
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"<r...@xenomai.org>
Sent: Tuesday, March 27, 2012 9:26am
To: "Doug Brunner"<dbrun...@ebus.com>
Cc: xenomai-help@gna.org
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"<r...@xenomai.org>
Sent: Friday, March 16, 2012 11:29am
To: "Doug Brunner"<dbrun...@ebus.com>
Cc: xenomai-help@gna.org
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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