Re: [ECOS] No binary semaphore in C API?

[Grant Edwards <grante at visi dot 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