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Date: Tue, 18 Nov 2025 22:10:47 +0900
To: cygwin AT cygwin DOT com
Subject: Re: [gcc] Bug in emutls?
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From: Takashi Yano via Cygwin <cygwin AT cygwin DOT com>
Reply-To: Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>
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Hi Jeremy,

On Mon, 17 Nov 2025 11:16:41 -0800 (PST)
Jeremy Drake wrote:
> On Sat, 8 Nov 2025, Takashi Yano via Cygwin wrote:
> 
> > I looked into the problem, and found that the executable for
> > the following code registers two pthread_keys with each destructor;
> > one is void emutls_destroy(void *ptr) in libgcc/emutls.c, and the
> > other is void run(void *p) in libstdc++-v3/libsupc++/atexit_thread.cc.
> > emutls_destroy() free's the memory erea of static thread_local X,
> > that is accessed from X::~X() which is called from run(). As a result,
> > if the emutls_destroy() is called before run(), run() referres to
> > the memory erea already free'ed.
> >
> > I think this is a bug of gcc. This issue does not occur in Linux,
> > because Linux does not use emutls.
> 
> 
> There is a similar longstanding issue in mingw-w64.  The problem there is
> that the pthread_key destructors run before the native Windows TLS
> callbacks.  emutls still uses pthread_key to manage static thread_locals,
> but C++ destructors are called from the Windows TLS callbacks (by way of
> __cxa_thread_atexit if memory serves).

Thanks for the information. When I compile my reproducer with mingw
compiler, the issue does not seem to happen. How does mingw handle
this issue?

> Cygwin should have it comparatively easy: it controls all the pieces (it
> doesn't need to care about when Windows TLS callbacks happen because if
> somebody calls ExitThread they get the undefined behavior they deserve).
> Couldn't Cygwin simply provide its own __cxa_thread_atexit and ensure
> destructors registered there run before pthread_key destructors?

It is not difficult to add a workaround for this issue in cygwin side.
However, IIRC, BSD libc does the same with cygwin 3.7.0-dev. I don't
think it is good idea to add workaround to cygwin for a bug of apps
on cygwin.

> Regardless, is it really undefined in what order pthread_key destructors
> run?  I would expect they'd run in reverse order of registration (most
> recently registered first).  Wouldn't that prevent this issue too
> (without mucking about with the Itanium C++ ABI)?

https://pubs.opengroup.org/onlinepubs/9799919799/ says:
"The order of destructor calls is unspecified if more than one destructor
 exists for a thread when it exits."

As you expected, the reverse-order'ed destructor-call hides the issue.
(That is what 3.6.5 does.)

-- 
Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>

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