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Date: Thu, 10 Apr 2025 11:03:59 -0700 (PDT)
To: Corinna Vinschen via Cygwin <cygwin@cygwin.com>
Subject: Re: pthread_atfork vs dlopen/dlclose
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From: Jeremy Drake via Cygwin <cygwin@cygwin.com>
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On Thu, 10 Apr 2025, Corinna Vinschen via Cygwin wrote:

> > It seems glibc takes care of this implicit deregistration in
> > __cxa_finalize, after calling __cxa_atexit functions, it unregisters any
> > at_quick_exit or pthread_atfork callbacks from the DSO being unloaded.
> >
> > https://sourceware.org/git/?p=glibc.git;a=blob;f=stdlib/cxa_finalize.c;h=2bb35602edc6bf842e5d2c93ad03454d7b57ee65;hb=HEAD
> >
> > It doesn't look like newlib deals with either at_quick_exit or
> > pthread_atfork handlers in its __cxa_finalize implementation.
>
> Cygwin is doing this stuff mostly in its own code, see thread.cc.
> It keeps lists of the callbacks in a global structure which is
> called MT_INTERFACE throughout thread.cc.
>
> The functions pthread::atforkprepare(), pthread::atforkparent() and
> pthread::atforkchild () are called from different spots during fork().
>
> Here's what we're missing:
>
> - Either keep track of the DLL a callback function is coming from by
>   calling dlls.find (handle, true) and store the struct dll pointer.
>   In dlclose(), if the DLL is of type DLL_LOAD or DLL_NATIVE, and
>   the refcounter indicates that this is the last FreeLibrary, check the
>   three callback lists in MT_INTERFACE and remove all functions
>   with the same struct dll pointer.

I'm inclined to take this approach.  What about at_quick_exit though?
That's entirely within newlib, isn't it?

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