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

[Thorsten Jolitz <tjolitz@gmail.com>]

"Pascal J. Bourguignon" <pjb@informatimago.com> writes:

> Thorsten Jolitz <tjolitz@gmail.com> writes:
>
>> "Pascal J. Bourguignon" <pjb@informatimago.com> writes:
>>
>>> Thorsten Jolitz <tjolitz@gmail.com> 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":
> http://www.gnu.org/software/emacs/manual/html_node/elisp/Network-Processes.html
> http://www.gnu.org/software/emacs/manual/html_node/elisp/Processes.html

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.

-- 
cheers,
 Thorsten