Re: sending data to an asynchronous process

[Eli Zaretskii <eliz@gnu.org>]

> From: Nicolas Graner <nicolas@graner.name>
> Date: Fri, 02 Apr 2021 12:20:26 +0200
> 
> I am writing a program that creates audio samples in an emacs
> buffer, then sends them to an external program (sox) to play in
> the background while I continue working with emacs. Part of the
> code is roughly as follows:
> 
> (setq process
>       (let ((process-connection-type nil))
> 	(start-process "my-process" nil
> 		       "sox" "-r" rate "-c" channels "-b" bits "-e" encoding "-q" "-d")))
> (process-send-region process start end)
> (process-send-eof process)
> 
> The sound plays as expected, but process-send-region does not
> return until about half a second before the sound finishes
> playing. This means that if I send several minutes of audio,
> emacs is stuck during all that time, which completely defeats the
> purpose of an asynchronous process.

Asynchronous subprocesses mean that their processing and reading their
output are asynchronous.  Writing to the subprocess is always
synchronous.  It cannot be any other way, really, unless we start
additional threads to write to the subprocess (which then causes
problems because the Emacs Lisp machine cannot tolerate more than one
thread that has access to Lisp data).  What Emacs does is read output
from subprocesses while it loops sending the data in chunks, so that
at least that part would work -- but that doesn't help in your case.

> The solution I found is to use a synchronous process:
>   (call-process-region start end "sox" nil 0 nil ...
> where the fifth argument 0 means to discard process output and
> not wait for completion.

call-process-region writes the data to a temporary file, and then
invokes the subprocess.  So if you do the same "by hand" and then call
start-process, you will have the best of all worlds.  IOW, the
temporary file solution you didn't like is exactly what
call-process-region does under the hood, and so efficiency is not the
issue here.  As for cleaning up the temporary files, that isn't really
a problem, is it?