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bug#17036: Continuation for Emacs: invoking a process on exit?


From: Reuben Thomas
Subject: bug#17036: Continuation for Emacs: invoking a process on exit?
Date: Thu, 20 Mar 2014 12:02:49 +0000

On 20 March 2014 03:45, Eli Zaretskii <address@hidden> wrote:
> Date: Wed, 19 Mar 2014 21:14:22 +0000
> From: Reuben Thomas <address@hidden>
> Cc: Stefan Monnier <address@hidden>, Andreas Schwab <address@hidden>,
>       address@hidden
>
> > Don't believe the sales people.  MS's execvp is buggy, and even if we
> > forget about those bugs, it won't do what is expected here: it won't
> > keep the file descriptors open in the original process still open in
> > the overlaid process.  That's because there's no 'exec' system call on
> > Windows, so execvp is _emulated_: the original process simply invokes
> > the new one as its child process, and then immediately exits.
> >
>
> That's good enough for restart-emacs.

Maybe so, it's hard to say, since you never described what that should
do.

I didn't discuss the command (it was Glenn Morris who suggested the name), but in my original bug report I said: "This would be useful for restarting having updated my configuration…as it would save having manually to issue a new 'emacs' command…" For this, a simple "exec emacs" is enough, but why not throw in command-line arguments too.
 
I very much doubt that this limitation would not render the whole
issue moot on Windows.  E.g., how will restart-emacs then be different
from a simple call-process?

Because Emacs does not continue running after it exits. As I said in my second email to this bug: "…to reexec Emacs, it needs to be a proper exec [so that] Emacs has[…] finished shutting down when it runs."

If you simply use CallProcess (or fork/exec on POSIX systems), then the newly-started emacs will be in contention with the old one, even if the old one has nearly finished exiting.
 
 But again, since you didn't say what the
feature is supposed to do, ...

A tail-call, but for processes. (BTW, sorry to have mentioned call/cc earlier, that was a bad analogy.)

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