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Date:	Thu, 10 Apr 2025 06:04:34 +0000 (UTC)
To:	"cygwin AT cygwin DOT com" <cygwin AT cygwin DOT com>
Message-ID:	<2021152719.99592.1744265074916@mail.yahoo.com>
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Subject:	Re: pthread_atfork vs dlopen/dlclose
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From:	Kevin Schnitzius via Cygwin <cygwin AT cygwin DOT com>
Reply-To:	Kevin Schnitzius <kometes AT yahoo DOT com>
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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.

Kevin

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