Re: [PATCH] At-exit hook

[Mikael Djurfeldt <mikael@djurfeldt.com>]

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OK, so people have brought up two issues:

1. It is for various reasons not recommended to call atexit() from a
dynamically linked library (which Guile already does before my suggested
change, n.b.).

2. It is not async signal safe.

A suggested remedy would then be:

Instead of calling the at-exit-hook from really_cleanup_for_exit, we could
call it (still within an scm_with_guile) from the end of scm_boot_guile(),
just before exit(), with the disadvantage that it wouldn't be called if
main_func() calls exit on its own. It's kind of a pity that we didn't early
on introduce some kind of scm_finalize_guile() which the user would have to
call when done with the library...

And, well, perhaps we should block asyncs, but I don't know about signals
with this new setup.

Best regards,
Mikael

On Thu, Nov 7, 2024 at 1:26 PM Mailer <vine24683579@gmail.com> wrote:

> On Thu, 7 Nov 2024 12:09:25 +0000
> Mailer <vine24683579@gmail.com> wrote:
> > On Thu, 7 Nov 2024 12:23:08 +0100
> > Maxime Devos <maximedevos@telenet.be> wrote:
> > > ‘atexit’ functions are run at ‘exit’. ‘exit’ can be run from signal
> > > handlers (*). Since the hook runs Scheme code, it could do a lot of
> > > AC-unsafe things, resulting in problems.
> > >
> > > (*) glibc documentation says ‘exit’ is AC-unsafe, but this is
> > > unsupported by POSIX AFAICT. OTOH the same applies to even ‘malloc’,
> > > so likely I’m looking in the wrong places.
> >
> > I think you meant async-signal-safe (AS-safe).  'exit' is not a-s-s and
> > cannot be called in a signal handler (for example it can flush buffers)
> > whereas '_exit' is a-s-s.  Furthermore a registered handler cannot
> > itself safely call 'exit'.
> >
> > I believe the main reason that use of 'atexit' or 'on_exit' is
> > discouraged is that it does not handle abnormal process termination.
> > (Registered handlers also don't run on termination by '_exit', but that
> > is usually what you want.)
>
> I believe also that use of 'atexit' is discouraged in dynamically linked
> libraries because of the uncertain timing of the unloading of the
> library, but I think in fact glibc is OK with this, so I guess it may
> depend on your libc.
>
> Chris
>
>

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