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Subject: pshared mutex's and cond variables - does anyone use them?
From: Robert Collins <robert DOT collins AT itdomain DOT com DOT au>
To: Frederic Devernay <Frederic DOT Devernay AT sophia DOT inria DOT fr>
Cc: cygwin AT cygwin DOT com, Olivier Bantiche <Olivier DOT Bantiche AT sophia DOT inria DOT fr>
In-Reply-To: <20010830094600.A9594@perf.inria.fr>
References: <20010830094600 DOT A9594 AT perf DOT inria DOT fr>
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Date: 30 Aug 2001 19:40:19 +1000
Message-Id: <999164420.1059.3.camel@lifelesswks>
Mime-Version: 1.0

On 30 Aug 2001 09:46:00 +0200, Frederic Devernay wrote:
> Hello,
> 
> there are officially two ways to initialize a pthread mutex:
> pthread_mutex_t mut = PTHREAD_MUTEX_INITIALIZER;
> or
> pthread_mutex_t mut;
> pthread_mutex_init(&mut,NULL);
> 
> the problem is that the pthread_mutex_init in cygwin reads the of-course
> uninitialized memory:
> 
> as seen in cygwin-1.3.2-1/winsup/cygwin/thread.cc:
> 
> __pthread_mutex_init (pthread_mutex_t * mutex,
>                       const pthread_mutexattr_t * attr)
> {
>   if ((((pshared_mutex *)(mutex))->flags & SYS_BASE == SYS_BASE))
>     // a pshared mutex
>     return EBUSY;
> ...
> 
> If found this bug not by declaring a local variable as above, but by allocating
> the mutex. the only workaround I found was to memset the memory area before
> calling pthread_mutex_init.
> 
> I don't know what a pshared_mutex is, but pthread_mutex_init should really
> not assume that anything is valid in the given mutex!

Yes, this is a known bug, last discussed right here about a week ago!.

I'm seriously considering pulling the pshared code out of pthreads
again.
They cannot be implemented cleanly pending some significant work within
cygwin1.dll, which has no timeframe (by this I mean a daemon for
'kernel-style' activity). 

They add significantly to the internal complexity in the meantime...

Also note, that it is always possible to confuse pthread_mutex_init, on
most platforms, but it shouldn't be possible to cause a segfault or
anything nasty like that :].

Rob


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