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From: Ben Bacarisse <ben@bsb.me.uk>
Newsgroups: comp.lang.c++
Subject: Re: counting_semaphore question
Date: Thu, 12 Sep 2024 11:25:07 +0100
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Muttley@dastardlyhq.com writes:

> On Wed, 11 Sep 2024 14:10:05 +0200
> Bonita Montero <Bonita.Montero@gmail.com> boringly babbled:
>>Am 11.09.2024 um 14:03 schrieb Muttley@dastardlyhq.com:
>>
>>> What upper limit? The max number of threads allowed in the protected section
>>> above is 2, not 10. Or do you mean it'll only count up to 10 threads waiting
>>> and ignore any beyond that? What happens if thread 11 comes along?
>>
>>The static parmeter is the maximum counter and the the two is the
>>initial counter. So two threads can currently acquire that semaphore.
>
> Sorry, I still don't get it. What do you mean by maximum counter?

You might want to consider who you are talking to.

Let's take std::counting_semaphore<M> sem(N); as an example.  The N is
the important number but it is not a 'maximum' in any hard sense -- it
is simply the initial value of the counter.  If it were some sort of
hard maximum, std::counting_semaphore sem(0); would be useless but it
isn't -- it simply means that at least one release() is needed before an
acquire() won't block.  Of course, if often /does/ represent the
maximum number of threads that can access some resource because the
usual pattern is to use std::counting_semaphore sem(N) with the standard
pattern of every thread calling acquire() and then release().

The template parameter M is very different and rather unusual.  It is,
first and foremost, just a hint to the implementation about how much
space will be needed for the counter, so the most useful value for M is
1 because some systems can implement binary semaphores more efficiently
than counting ones.

But this "limit" is not enforced other than by making the behaviour
undefined when it is exceeded.  Pre-conditions on the operations state
that the counter must always be >= 0 and <= M, but you won't be able to
check this unless the implementation decides to enforce these
pre-conditions with some sort of error report.

-- 
Ben.