RE: [PATCH] At-exit hook

[Maxime Devos <maximedevos@telenet.be>]

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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'. […]

No, I did mean exactly what I wrote. Read the glibc documentation of ‘exit’ and you’ll see. (Likewise for the POSIX page for ‘exit’ – POSIX does not seem to restrict things to _outside_ signal handlers.)

Also, when two authorative sources (POSIX and glibc in this case) have contrary claims, then simply repeating one of those claim does not help at all, you would need to explain the cause of the discrepancy instead.

That ‘exit’ flushes buffers does not imply that ‘exit’ is async-unsafe, alternatives include buffer flushing being safe, ‘exit’ having its own implementation of flushing that is AC-safe, or ‘you may call ‘exit’ but only if no files (as in FILE*) are open’.

Best regards,
Maxime Devos

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