pthread_mutex_trylock() should never block

[Mike Makonnen]

On Wed, Jan 28, 2004 at 12:01:27PM -0500, Daniel Eischen wrote:
> On Wed, 28 Jan 2004, Mike Makonnen wrote:
> 
> > On Sun, Dec 07, 2003 at 01:34:31AM -0500, Daniel Eischen wrote:
> > > 
> > > The man page may not mention it, and that may be a bug, but I
> > > think a pthread_mutex_trylock() on a non-recursive mutex is allowed
> > > to return EDEADLK.
> > > 
> > >   http://www.opengroup.org/onlinepubs/007904975/functions/pthread_mutex_trylock.html
> > > 
> > >   If the mutex type is PTHREAD_MUTEX_ERRORCHECK, then error
> > >   checking shall be provided. If a thread attempts to relock a
> > >   mutex that it has already locked, an error shall be returned.
> > >   If a thread attempts to unlock a mutex that it has not locked
> > >   or a mutex which is unlocked, an error shall be returned.
> > > 
> > >   ...
> > 
> > If you look further down in the spec you will see:
> > 
> >    The pthread_mutex_trylock() function shall be equivalent to
> >    pthread_mutex_lock(), except that if the mutex object referenced
> >    by mutex is currently locked (by any thread, including
> >    the current thread), the call shall return immediately...
> 
> Yes, but the implementation has to use internal locks to
> keep the mutex in a consistent state.  There is no problem
> here.

Apologies, I was commenting on the EDEADLK vs. EBUSY issue, not the "is an
internal lock allowed to block in pthread_mutex_trylock" issue. I should
have also explained better instead of just throwing out the quote.

EDEADLK is returned by pthread_mutex_lock() only on error
checking mutexes that would otherwise deadlock if they were not error
checking. Since pthread_mutex_trylock() must return immediately whether
the mutex is error-checking or not, I think for consistency sake it should
return EBUSY regardless of what type of mutex it is operating on. Also,
the 'ERRORS' section lists EDEADLK as a _possible_ return value only for
pthread_mutex_lock().

I'm not a standards expert but I think it is permissable for an
implementation to return EDEADLK from pthread_mutex_trylock(). However,
I think it's better to always return EBUSY, rather than EDEADLK in
some cases and EBUSY in others. 

Cheers.
-- 
Mike Makonnen  | GPG-KEY: http://www.identd.net/~mtm/mtm.asc
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