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Re: POSIX semaphore for GNU/Hurd

From: Neal H. Walfield
Subject: Re: POSIX semaphore for GNU/Hurd
Date: 02 Nov 2002 19:45:51 -0500
User-agent: Gnus/5.0808 (Gnus v5.8.8) Emacs/21.2

> > int 
> > sem_init (sem_t *sem, int pshared, unsigned int value)
> > {
> >   if (pshared)
> >     return -1;
>  If you wanted to be consistant with LinuxThreads this should be
> if (pshared)
>   {
>      errno = ENOSYS;
>      return -1;
>   }
> However, I think it would be better to actually implement this
> feature.

It may be better but it is non-trivial.  We do not support the process
shared attribute in pthread mutexes et al either.

>  would be shorter.  The real question is, are either of these implementations
> cancellable?  As far as I can tell both are because pthread_cond_wait is
> cancellable.

pthread_cond_wait is a cancelation point as is sem_wait.  Thus, there
should be a pthread_cleanup_push and pthread_cleanup_pop to unlock the
mutex if need be.  E.g.

  pthread_cleanup_push (pthread_mutex, (void *) &mutex);


  pthread_cleanup_pop (1);

>  To be paranoid, this should also contain
>  if (sem->count == SEM_VALUE_MAX)
>    {
>      pthread_mutex_unlock (&sem->count_lock);
>      return -1;
>    }

Only if SEM_VALUE_MAX is defined: POSIX does not say that it has to

> > /* Initialize the semaphore and set the initial value - as in LinuxThreads
> >    pshared must be zero right now. */
>  How about for a comment:
> /* Initialize the semaphore and set the initial value.  PSHARED must be set
> to 0 processes sharing semaphores is not implemented yet. */

No.  This is wrong.  Any comment belongs in the documentation, not

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