From: Grant Edwards <grante@visi.com>
To: Nick Garnett <nickg@cygnus.co.uk>
Cc: ecos-discuss@sourceware.cygnus.com
Subject: Re: [ECOS] No binary semaphore in C API?
Date: Mon, 26 Mar 2001 11:37:00 -0000 [thread overview]
Message-ID: <20010326133727.A3008@visi.com> (raw)
In-Reply-To: <wwghf0glpyc.fsf@balti.cambridge.redhat.com>
On Mon, Mar 26, 2001 at 12:39:07PM +0100, Nick Garnett wrote:
> > > Is there a technical reason that there isn't a C API for binary
> > > semaphores, or is it something that just hasn't been done yet?
> >
> > Certainly no technical reason. Nick may not have done it out of
> > principle. But right now he's on vacation, so it's best to wait
> > till he returns.
>
> It was never intended that the KAPI be a complete reflection of
> the kernel implementation. It is meant to be a consistent,
> self-contained, small API that can be used by C applications.
> Like the uITRON and POSIX APIs it only exposes a subset. It was
> considered unnecessary to export binary semaphores,
Why impliment binary semaphores if there is no intent to
allow their use?
> since a counting semaphore initialized to 1 is functionally
> equivalent.
Not quite. There are sequences of wait()/post() that will not
give the same results for an boolean and integer semaphore.
Task 1 Task 2
------ ------
[semaphore value == 1]
wait()
post()
post()
wait() wait()
At this point, only one of the tasks is runnable with binary
semaphores, but both are runnable with a counting semaphore.
Admittedly, multiple posts is probably a bug if you intend the
semaphore to be used for mutual exclusion, but using a binary
semaphore protects you from such bugs.
--
Grant Edwards
grante@visi.com
next prev parent reply other threads:[~2001-03-26 11:37 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2001-03-16 12:01 Grant Edwards
2001-03-19 13:42 ` Jonathan Larmour
2001-03-26 3:53 ` Nick Garnett
2001-03-26 7:12 ` Fabrice Gautier
2001-03-26 11:37 ` Grant Edwards [this message]
2001-03-27 2:00 ` Nick Garnett
2001-03-27 5:32 ` Grant Edwards
2001-03-28 5:29 ` Bart Veer
2001-03-28 6:45 ` Sergei Organov
2001-03-28 7:38 ` Grant Edwards
2001-03-28 7:39 ` Bart Veer
2001-03-28 10:01 ` Jonathan Larmour
2001-03-28 10:01 ` Sergei Organov
2001-03-28 11:05 ` Grant Edwards
2001-03-29 11:51 ` Sergei Organov
2001-03-29 11:49 ` Grant Edwards
2001-03-29 11:55 ` Chris Gray
2001-03-29 19:33 ` Bart Veer
2001-03-29 20:50 ` Grant Edwards
2001-03-30 2:22 ` Sergei Organov
2001-03-30 6:54 ` Grant Edwards
2001-03-30 7:42 ` Sergei Organov
2001-03-30 5:39 Rosimildo daSilva
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