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Re: How to communicate with a running external process with given PID?

From: Thorsten Jolitz
Subject: Re: How to communicate with a running external process with given PID?
Date: Mon, 22 Jul 2013 10:06:26 +0200
User-agent: Gnus/5.130002 (Ma Gnus v0.2) Emacs/24.3 (gnu/linux)

"Pascal J. Bourguignon" <> writes:

> Thorsten Jolitz <> writes:
>> "Pascal J. Bourguignon" <> writes:
>>> Thorsten Jolitz <> writes:
>>>> Hi List,
>>>> say I want to call another program (more exactly, another Lisp
>>>> program that is not Emacs Lisp) from an Emacs Lisp program.
>> [..snip..]
>>>> I could run a server in the external lisp program and use a
>>>> network-connection-object to send http-requests (e.g. with the help
>>>> of emacs-request.el) via TCP - but that seems to be total overkill
>>>> for my requirements.
>>> Ok, I'll bit the bullet: the only communication channel available to
>>> you, a-priori, knowing only the process ID, is the kill(2) syscall.
>>> And you're lucky, you can send more than one bit of information with
>>> each kill call!
>> Yes, I should read UNP, but actually I read quite a lot of stuff
>> about this topic, but reading about it and starting to use it is not
>> the same thing. And I did not know that 'kill is so useful and
>> universal, only used it for its core task so far.
>> But in the light of your answer I think I go for the HTTP
>> client/server solution, expecially since the server stuff is all
>> there anyway.
>> Thanks for the tips.
> Ok, that's a starting point.
> The thing is that you cannot talk to a process that's not ready to
> talk, with some protocol.
> The summary of unp, is that there are various IPC (Inter Process
> Communication) mechanisms:
> - signals (kill(2), signal(2)), - semaphores (semctl, semget, semop,
> semtimedop) - messages (msgctl, msgget, msgrct, msgsnd) - shared
> memory (shmat, shmctl, shmdt, shmget) - sockets (listen, accept, read,
> write, close) - pipes (pipe, fork, read, write, close) - named pipes
> (open, read, write close) - files (open, read, write close)
> and the only mechanism that does something by default in the receiver
> is signals (cf. signal(2) and signal(7)). Now, of course, most signals
> are ignored, and a few others only kill the receiving process by
> default (hence the name of kill(2)), which is not much of a
> communication, but that's something, contrarily to the other
> mechanisms, when the receiving process doesn't do anything about them.
> But since you're saying that the process includes a HTTP server, that
> means you can open a TCP socket on the port it's listening to, and you
> can communicate with the HTTP protocol.
> In emacs, you can do that with "network processes":

Not that I have not heard about this UNP stuff, I probably hoped that
Emacs has some kind of 'magic' functionality for arranging things behind
the scene ...

But HTTP is a good and actually not that difficult solution in this
case. Thanks for the links.


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