Re: UDP: Send and receive on same port?

[Robert Pluim]

>>>>> On Tue, 30 Mar 2021 23:03:58 +0200, Mario Lang <mlang@blind.guru> said:

    Mario> Robert Pluim <rpluim@gmail.com> writes:
    >>>>>>> On Tue, 30 Mar 2021 13:11:47 +0200, Mario Lang <mlang@blind.guru> 
said:
    >> 
    Mario> Replying to self
    Mario> Nevermind, it works by creating a datagram server first, and then 
using
    Mario> set-process-datagram-address to set the destination.  The server 
process
    Mario> can now be used to send UDP as well.  However, I am seeing lost 
incoming
    Mario> packets.  According to tcpdump, the reply is clearly there, but the
    Mario> process filter only seems to pick it up 1 out of roughly 4 times.
    Mario> Weird.
    >> 
    >> Hmm, I have a patch somewhere to allow setting the sending port
    >> directly, Iʼll see if I can find it and resurrect it.

    Mario> OK. However, we'd also need a way to get the listening port of a 
server
    Mario> process with :service t.  At least I haven't seen a way to query the
    Mario> listening port yet.

(aref (process-contact server :local) 4)

    >> Having said that, this sounds like it might be a bug (although I
    >> haven't checked if the emacs code supports sending from a datagram
    >> server like this).

    Mario> It does works, and also flawlessly.  The weirdness I saw was due to a
    Mario> coding-system issue, `osc-make-server` did not set :coding 'binary,
    Mario> which is now fixed.

    >> Could you open a bug with M-x report-emacs-bug with your emacs
    >> version, and sample code?

    Mario> I'd rather not, since this behaviour is actually wanted.
    Mario> It is the only way I have to communicate with a certain external 
program
    Mario> directly from elisp.  scsynth always replies to the sender port...

No need, since it appears thereʼs no bug :-)

    Mario> To reproduce, all you need is create a datagram server, and set the
    Mario> datagram-address to the desired destination.  You can now receive
    Mario> datagrams, and also use process-send-string to send to the desired
    Mario> destination.

Based on my (vague) recollection, for a server process you should be
able to pass in :service <port> anyway, so then thereʼs no need to
query for the port.

Robert
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