Once again, I need a binary semaphore

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Once again, I need a binary semaphore

Grant Edwards-6
Once again, I find I need a binary semaphore for a C application I'm
porting from another OS.

The eCos kernel has binary semaphores, but there is only a C++ API and
no C API.  I and others have asked why there is no C API for binary
semaphores, and all I've ever really understood of the answer was that
certain developers didn't think binary semaphores were cool enough.

I'm implementing a C API for binary semaphores.  It'c completely
trivial and consits of only a few dozen lines of code.  Any chance
such a patch would be accepted, or are binary semaphores still too
uncool to sit at the same lunch table as mutexes and counting
semaphores?

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Re: Once again, I need a binary semaphore

Leschke Serafin (lesc)


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).

Serafin

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Re: Once again, I need a binary semaphore

Grant Edwards-6
On 2015-03-17, lesc <[hidden email]> 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.

I'm not sure why a counting semaphore wasn't used by the applications
author.  I haven't analyzed all of the possible execution paths, but
the original author of the application very specifically chose to use
a binary semaphore instead of a counting semaphore, and it seemed
wisest not to change things without a good reason.  It took a
half-hour to implement a C API for binary semaphores, it could take
weeks to analyze tens of thousands lines of third-party code to see if
a counting semaphore would work the same as a binary semaphore in all
the corner cases.

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Re: Once again, I need a binary semaphore

Grant Edwards-6
On 2015-03-17, Grant Edwards <[hidden email]> wrote:

> On 2015-03-17, lesc <[hidden email]> 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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Re: Once again, I need a binary semaphore

Frank Pagliughi
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 <[hidden email]> wrote:
>> On 2015-03-17, lesc <[hidden email]> 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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Re: Once again, I need a binary semaphore

Grant Edwards-6
On Tue, Mar 17, 2015 at 10:57:06AM -0400, Frank Pagliughi wrote:

> 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.

I've been asking that for 15 years.  OK, I exaggerate.  I just looked
itup and the first thread I started on this top was March 2001, so
it's only been 14 years.  The reason given for lack of a binary
semaphore C API was:

   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, since a
   counting semaphore initialized to 1 is functionally equivalent.

   If I had had my way the KAPI would have been even more minimal that
   it currently is.

   --
   Nick Garnett, eCos Kernel Architect
   Red Hat, Cambridge, UK

Except a counting semaphore initialized to 1 is _not_ functionally
equivalent to a binary semaphore in all cases.  I pointed this out and
provided examples of sequences of wait/post where the behavior
differs, but was told those sequences were "bugs" and (jokingly, I
hope) if I didn't shut up and stop asking questions then the C++
binary semaphores would be removed copletely.

It was quite clear that Nick was adamantly opposed to providing a C
API for binary semaphores and that the needs of eCos users'
application code were to be overruled by his idea of what an comprised
a minimal, complete, elegent set of C APIs: people who want to
write/port C apps that use binary semaphores should just and rewrite
the application code.

IOW, all you really need are two-input NAND gates to model any Turing
complete machine so stop bitching and get to work.

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Grant


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