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Date: Tue, 17 Mar 2026 15:08:19 +0100
To: Noel Grandin <noelgrandin AT gmail DOT com>
Subject: Re: std::mutex bug: Windows handle growth
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From: Corinna Vinschen via Cygwin <cygwin AT cygwin DOT com>
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Cc: Corinna Vinschen <corinna-cygwin AT cygwin DOT com>,
        Frank Eskesen <feskesen AT hvc DOT rr DOT com>,
        Cygin Mailing List <cygwin AT cygwin DOT com>
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On Mar 17 09:06, Noel Grandin via Cygwin wrote:
> Hi
> 
> Hazarding a guess here:
> 
> On 3/16/2026 11:39 PM, Frank Eskesen via Cygwin wrote:
> > I've run into a problem that occurs on Cygwin that doesn't occur on Linux
> > systems: When a std::mutex is used, it doesn't clean up a Windows handle that
> > it uses. This sample program demonstrates the problem, failing in under 60
> This code looks suspicious:
> 
>     pthread_mutex::destroy ()
>     {
>       if (condwaits || trylock ())
>         // Do not destroy a condwaited or locked mutex
>         return EBUSY;
> 
> I think it should be
> 
>       if (condwaits || !trylock ())

No, checking trylock() is correct.  pthread_mutex_destroy() must not
destroy a busy mutex.  trylock() returns != 0 if the lock is busy.

Given the implementation of std::mutex is based on pthread_mutex,
Frank's code is wrong, or at least somewhat borderline.  The mutex
should first be unlocked before leaving do_something().

The reason this problem doesn't occur on Linux is the fact that mutexes
are implemented without allocating a system object, so the
pthread_mutex_destroy() function is basically a no-op, after checking if
the lock is busy and returning 0 or EBUSY, whatever is right.


Corinna

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