pthread_mutex_trylock() should never block
mtm at identd.net
Thu Jan 29 01:43:42 PST 2004
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
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
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.
Mike Makonnen | GPG-KEY: http://www.identd.net/~mtm/mtm.asc
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