Re: Starting a subprocess in stopped state

[Philipp Stephani <p.stephani2@gmail.com>]

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Eli Zaretskii <eliz@gnu.org> schrieb am Sa., 6. Mai 2017 um 09:59 Uhr:

> > From: Thien-Thi Nguyen <ttn@gnu.org>
> > Date: Sat, 06 May 2017 07:42:18 +0200
> >
> > I see the docstring for ‘make-process’ says:
> >
> >  :stop BOOL -- Start process in the ‘stopped’ state if BOOL
> >  non-nil.  In the stopped state, a process does not accept
> >  incoming data, but you can send outgoing data.  The stopped
> >  state is cleared by ‘continue-process’ and set by
> >  ‘stop-process’.
> >
> > My understanding is that "stopped state", given that "you can
> > send outgoing data", is an Emacs-internal data routing concern
> > rather than an OS-level concern (SIGTSTP).  The latter would
> > completely prevent the asymmetric data feature (documented by
> > "you can send outgoing data"), i'd think.
>
> However, stop-process does send SIGTSTP to the process, and
> continue-process sends SIGCONT.  The above means that the process
> started with :stop non-nil will indeed run, unlike a process which was
> stopped with stop-process immediately after starting it.  So there are
> two flavors of "stopped" state here, one with SIGTSTP delivered to the
> process, the other without.
>
> Also, not reading the Emacs end of the pipe, while the process runs is
> probably going to get is to undefined behavior in some situations,
> especially with multithreaded processes.
>
> For network or serial or pipe "process", not reading from the
> descriptor is all that counts.  But for a local subprocess, this is
> not so, IMO.
>
> Is there a way to create a process on Unix and GNU systems, but
> prevent it from running until it gets SIGCONT or something similar?


It's at least theoretically possible on macOS using posix_spawn(3) and
POSIX_SPAWN_START_SUSPENDED (
https://developer.apple.com/legacy/library/documentation/Darwin/Reference/ManPages/man3/posix_spawnattr_setflags.3.html).
I don't think it's possible using fork/exec (but then, switching to
posix_spawn might be a good idea anyway).

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