Date: Thu, 29 Jan 2004 11:21:22 +0300 From: Mike Makonnen <mtm@identd.net> To: Daniel Eischen <eischen@vigrid.com> Cc: Peter Kostouros <kpeter@melbpc.org.au> Subject: Re: pthread_mutex_trylock() should never block Message-ID: <20040129082122.GA1439@mobile.acsolutions.com> In-Reply-To: <Pine.GSO.4.10.10401281158460.7302-100000@pcnet5.pcnet.com> References: <20040128165031.GA3461@mobile.acsolutions.com> <Pine.GSO.4.10.10401281158460.7302-100000@pcnet5.pcnet.com>
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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 mtm@identd.net | Fingerprint: 00E8 61BC 0D75 7FFB E4D3 6BF1 B239 D010 3215 D418 mtm@FreeBSD.Org| FreeBSD - Unleash the Daemon !
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