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Date: Wed, 9 Apr 2025 23:13:53 -0700 (PDT)
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Subject: Re: pthread_atfork vs dlopen/dlclose
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From: Jeremy Drake via Cygwin <cygwin AT cygwin DOT com>
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On Thu, 10 Apr 2025, Kevin Schnitzius via Cygwin wrote:

> On Wednesday, April 9, 2025 at 06:54:34 PM EDT, Jeremy Drake via Cygwin <cygwin AT cygwin DOT com> wrote:
>
> > The recent issue with pthread_atfork handlers reminded me of a scenario
> > that I know glibc handles, but it seems that Cygwin does not.  Test case:
>
> <... code that loads a shared lib, registers some functions in shared lib with pthread_atfork(), unloads the shared lib, and crashes on fork...>
>
> Calling functions in an unloaded library should result in undefined behavior.
>
> However, further investigation reveals that the Linux pthread_atfork() registered functions are not being called and POSIX does not proved a mechanism for un-registering these functions.   Note: pthread_atfork() is not bumping the ref count on the shared lib--those functions are definitely unavailable after the dlclose()
>
> In the Cygwin version, calling the functions in the unloaded library when the fork happens causes the crash.
>
> This seems to be a bug with fork(), if it is a bug at all.

I did a quick search, and found a write-up of *my* bug report :D

https://developers.redhat.com/articles/2022/12/14/how-we-addressed-unforeseen-use-case-pthreadatfork

"Now, dlclose()'ing a module means that any fork handlers registered by it
should not be executed after the dlclose and should therefore implicitly
be deregistered."

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