Re: Once again, I need a binary semaphore

[Grant Edwards <grant dot b dot edwards at gmail dot com>]

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).

-- 
Grant Edwards               grant.b.edwards        Yow! World War III?
                                  at               No thanks!
                              gmail.com            


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