Re: problem using pthread_cancel and pthread_mutex_lock

["vc" <vcotirlea1 at hotmail dot com>]

Thanks to all of you for your answers.
Viv

----- Original Message -----
From: "Simon Gerblich" <sgerblich@daronmont.com.au>
To: "vc" <vcotirlea1@hotmail.com>
Cc: <pthreads-win32@sources.redhat.com>
Sent: Tuesday, February 24, 2004 5:23 AM
Subject: RE: problem using pthread_cancel and pthread_mutex_lock


> Hi Viv,
>
> >So, my question is: how can a thread cleanly cancel another thread
> >which is waiting in a 'pthread_mutex_lock' call, so that this mutex is
> >available again ?
> I can see a few problems in what you are doing.
>
> A mutex is not really designed to be locked in a thread for a long time
and
> unlocked by another thread.  When I use a mutex in a thread I lock it - do
> what I have to do  - and then unlock it ASAP.  I don't use mutexes to
> synchronise threads.  I use them to protect data.  Pthread mutexes are not
> cancellation points.
>
> I use deferred cancellation instead of asynchronous cancellation because
of
> recommendations in "Programming with POSIX Threads" by David Butenhof.
> Page 150 "Avoid asynchronous cancellation.  It is difficult to use
correctly
> and is rarely useful."
> Page 151 "Asynchronous cancellation can occur at any hardware instruction.
> On some computers it may even be possible to interrupt some instructions
in
> the middle.  That makes it really difficult to determine what the canceled
> thread was doing."
> Page 151 "Call no code with asynchronous cancellation enabled unless you
> wrote it to be async-cancel safe - and even then, think twice!"
>
> I write my applications with deferred cancellation and the threads
blocking
> on message queues, which are cancellation points.  When a cancel is
> requested the threads exit cleanly when they next go to read the message
> queue.  I have minimal cancellation points in my code so that I know
exactly
> where my threads will cancel.
>
> Without knowing exactly what you are trying to do I think you should have
a
> look at using a condition variable instead of a mutex for your thread
> syncing.  Waiting on a condition variable is a cancellation point.
>
> Be aware that the Sleep() function on windows is not a cancellation point.
> I have written my own sleep functions to use with pthreads-win32 that
waits
> on condition variables with timeouts.
>
> If your not lurking in the google groups comp.programming.threads I'd
> recommend it.  I've learnt a lot following threads on pthreads, etc.
>
> Cheers,
> Simon
>
>