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I want to call a command asynchronously, and define a function that handles output from that command. I don't want to simply write output to a buffer.

According to the Elisp info (36.9.2 Process Filter Functions) I need to set a process filter function. I tried this:

(defun my-echo-filter (proc string)
  (message "proc: %s string: %s" proc string))

(let ((my-proc (start-file-process "foo" nil "date")))
  (set-process-filter my-proc #'my-echo-filter)) ; writes the time to *Messages*

However, this makes me nervous. I'm starting the process first, and only afterwards am I attaching the process filter.

If I do anything between creating the process and attaching the process filter, I can miss output.

(let ((my-proc (start-file-process "foo" nil "date")))
  (sleep-for 2)
  (set-process-filter my-proc #'my-echo-filter)) ; Does not write the time!

How do I create a process and ensure that my process filter doesn't miss anything?

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  • 2
    I'm not an expert, but I think this is a non-issue. Emacs is single-threaded, and only processes output from subprocesses when idle. There's no time in between calling start-file-process and set-process-filter when process output could arrive behind your back (as it were). You would have to call sit-for or accept-process-output for that to happen.
    – user1968
    Nov 15, 2014 at 13:42
  • 1
    Hm, interesting. Presumably sleep-for counts as idle. What about if garbage collection occurs? Nov 15, 2014 at 14:13
  • It seems very unlikely to me that GC would trigger anything related to subprocesses -- AFAIK there's very little that can safely be handled during GC -- but you would have to ask someone more expert for a definitive answer.
    – user1968
    Nov 15, 2014 at 14:22
  • 1
    BTW, the relevant line is in (elisp) Accepting Output: "Output from asynchronous subprocesses normally arrives only while Emacs is waiting for some sort of external event, such as elapsed time or terminal input."
    – user1968
    Nov 15, 2014 at 14:23
  • @JonO. would you like to make your comments an answer? Nov 15, 2014 at 16:53

1 Answer 1

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As I understand it, this is a non-issue. Emacs is single-threaded, and only processes output from subprocesses when idle. Therefore, there is no time between calling start-file-process and set-process-filter when process output could arrive without being handled.

The info page (elisp) Accepting Output states:

Output from asynchronous subprocesses normally arrives only while Emacs is waiting for some sort of external event, such as elapsed time or terminal input.

For this to become a problem, you would have to separate creating the process and setting the filter function with a call to sit-for, accept-process-output, or some other command that involves user interaction via the event loop.

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