Re: about async process

[Eli Zaretskii <eliz@gnu.org>]

> From: akrl <akrl@sdf.org>
> Date: Sat, 25 Aug 2018 18:15:50 +0000
> 
> the question is: when is the process buffer updated after an async
> process finish to execute?

When Emacs is idle.  From the ELisp manual:

     Output from a subprocess can arrive only while Emacs is waiting: when
  reading terminal input (see the function ‘waiting-for-user-input-p’), in
  ‘sit-for’ and ‘sleep-for’ (*note Waiting::), and in
  ‘accept-process-output’ (*note Accepting Output::).  This minimizes the
  problem of timing errors that usually plague parallel programming.  For
  example, you can safely create a process and only then specify its
  buffer or filter function; no output can arrive before you finish, if
  the code in between does not call any primitive that waits.

> (make-thread (lambda ()
> 	       (let ((prc
> 		      (start-process-shell-command "test"
> 						   "out-buff"
> 						   "echo foo")))
> 		 (while (not (equal (process-status prc)
> 				    'exit))
> 		   (thread-yield))
> 		 (with-current-buffer "out-buff"
> 		   (print (buffer-string))
> 		   (sleep-for 0.1)
> 		   (print (buffer-string))))))
> 
> I'm trying to execut an async process in a thread yielding till this has
> finished and then reading the output.
> When I execute this code the first print is printing "" and just the
> second print after the sleep il latching the output I would expect.
> So my question is when is the output buffer updated and if ther's a way to
> ensure this has happend or to request for it?

With this code, the first instance of Emacs becoming idle is when you
call sleep-for.  Calls to thread-yield don't count as idling.