Re: [ECOS] Re: Once again, I need a binary semaphore

[Frank Pagliughi <fpagliughi@mindspring.com>]

I'm in agreement, for what it's worth. I've had to make use of the 
binary semaphore through the C++ API, and it always made me wonder why 
it wasn't brought out to the public/C API. Why code up the exact same 
thing with a condition variable when there's a perfectly good binary 
semaphore in the OS?

Frank

On 03/17/2015 10:47 AM, Grant Edwards wrote:
> On 2015-03-17, Grant Edwards <grant.b.edwards@gmail.com> wrote:
>> On 2015-03-17, lesc <lesc@zhaw.ch> wrote:
>>>
>>> On 16.03.2015 18:17, Grant Edwards wrote:
>>>> Once again, I find I need a binary semaphore for a C application I'm
>>>> porting from another OS.
>>> And just using a mutex is not a option? (Sorry if you allready ruled
>>> that out, but you didn't metion why youd need that specific
>>> sync-mechanism).
>> The Semaphore is used so that one thread can wait for completion of a
>> task that was farmed out to different thread: Thread A waits on the
>> semaphore until thread B posts.  It's an inter-thread signalling
>> mechanism, not a mutual-exclusion mechansim.
> Perhaps I should be a bit more detailed: a Mutex is owned by the
> thread that calls cyg_mytex_lock(), and it can't be unlocked by a
> different thread.  This enforcement of ownership prevents a mutex from
> being used in place of a binary semaphore for inter-thread signalling.
>
> I've run across situations in the past where I needed to use a
> semaphore for mutual exclusion, but a counting semaphore initialized
> to 1 works fine for that as long as you only release the resource once
> after acquiring it (not usually a problem).
>


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