Date: Wed, 18 Sep 2002 22:16:07 -0400 (EDT) From: Daniel Eischen <eischen@pcnet1.pcnet.com> To: Alfred Perlstein <alfred@FreeBSD.org> Cc: hackers@FreeBSD.org, deischen@FreeBSD.org, Garrett Wollman <wollman@lcs.mit.edu> Subject: Re: sem_init help? Message-ID: <Pine.GSO.4.10.10209182206210.3071-100000@pcnet1.pcnet.com> In-Reply-To: <20020919015911.GQ86737@elvis.mu.org>
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On Wed, 18 Sep 2002, Alfred Perlstein wrote:
> * Daniel Eischen <eischen@pcnet1.pcnet.com> [020918 18:40] wrote:
> > On Wed, 18 Sep 2002, Alfred Perlstein wrote:
> >
> > >
> > > Ok, any of you guys have a copy of the standards documents that
> > > describe the sem_* API?
> > >
> > > I have a question...
> > >
> > > What are the semantics of the sem_init when pshared is set to true?
> > >
> > > Like, if the process that created the semaphore exits, then what?
> > > Is it only valid for the life of the process that did the sem_init?
> > > Or am I going to have to do some nastyness to record the memory
> > > location where the semaphore is and track that page's allocation?
> >
> > My take on the spec is that as long as the semaphore can be
> > accessed by other processes, then it remains perfectly valid
> > regardless of whether the originating process exited or not.
> >
> > Here's a sem_init() blurb from Austin Draft 6:
> >
> > "If the pshared argument has a non-zero value, then the
> > semaphore is shared between processes; in this case, any
> > process that can access the semaphore sem can use sem for
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> > performing sem_wait(), sem_trywait(), sem_post(), and
> > sem_destroy() operations."
^^^^^^^^^^^
> >
> > The sem_close() and sem_unlink() man pages make it even clearer
> > that it has to remain around.
>
> Wait, see, you're confusing named and un-named semaphores.
Yes, but it doesn't change what the spec says above. It's
implying that any process can perform sem_destroy() on it and
the spec also says for sem_init() that "This semaphore shall
remain usable until the semaphore is destroyed.".
So the sem_init()'ing process can exit and leave the destroying
to one of the other processes that have access to it.
> I've got the named ones all set, it's the un-named ones I'm having
> trouble with.
>
> The only valid operations on un-named semaphores are:
>
> sem_init - to create it
> sem_wait/sem_trywait - to wait on it
> sem_destroy - to destroy it
>
> now here's the problem, one can basically stick a sem_t into a shared
> memory location, then one can share it amongst processes (provided
> pshared was set). However there's no requirement that these other
> processes do any form of "open" on the sem_t, hence I really have no
> way of tracking its usage other than by:
>
> a) auto-terminating it when the process that did sem_init exits.
> b) tracking the page where the actual sem_t was created until either
> it is reclaimed via sem_destroy or the page is free'd back to the
> OS.
>
> Unless I'm missing something here.
The semaphore remains active until it is destroyed. If you don't
want to track its page, can you hook it into ipcrm(1)?
--
Dan Eischen
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