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Re: counting_semaphore question

From "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com>
Newsgroups comp.lang.c++
Subject Re: counting_semaphore question
Date 2024-09-17 11:56 -0700
Organization A noiseless patient Spider
Message-ID <vccjdk$3lb34$2@dont-email.me> (permalink)
References (3 earlier) <vbs1al$3jgnp$1@raubtier-asyl.eternal-september.org> <vbs2fm$3jp23$1@dont-email.me> <vbs95s$3l7ms$1@dont-email.me> <vca7hs$32c6u$1@dont-email.me> <vcbant$3cto1$1@dont-email.me>

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On 9/17/2024 12:22 AM, David Brown wrote:
> On 16/09/2024 23:22, Chris M. Thomasson wrote:
>> On 9/11/2024 7:23 AM, David Brown wrote:
>>> On 11/09/2024 14:29, Muttley@dastardlyhq.com wrote:
>>>> 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?
>>>>
>>>> Maximum threads that can wait on the semaphore and 11 gets an error?
>>>> Maximum threads that can ever access the protected section and after 
>>>> 10 have
>>>> accessed it the section is blocked to all threads?
>>>>
>>>
>>> You are talking about a counting semaphore here - the number will be 
>>> the maximum value of the count.  (The actual maximum value could be 
>>> bigger than the template parameter, but not smaller than it.)  If 
>>> this maximum value is 10, then you can acquire the semaphore 10 times 
>>> without blocking - any future acquires will block until there are 
>>> releases.
>>>
>>> Semaphores can be acquired and released by any threads - one thread 
>>> can acquire often, another thread can release them, or you can have 
>>> any combination.  They are not used for protection sections or data - 
>>> they are used as signalling mechanisms.  One or more "producer" 
>>> thread might make objects, and one or more "consumer" threads used 
>>> them, in which case your 10 here is the maximum number of these 
>>> objects that will be buffered up at a time.
>>
>> [...]
>>
>> Think of creating a queue with 100,000 elements in it. The semaphore 
>> to protect this queue should have an initial state of 100,000. The 
>> least min value, should be high because the queue might have 1.9 
>> million elements in there. So, for this queue:
>>
>> std::counting_semaphore<PTRDIFF_MAX> sem(100000);
>>
>> right? There are 100000 elements in the queue because its initialized 
>> that way. A single thread can pop and work on 100000 elements before 
>> it blocks on a queue empty condition.
> 
> Roughly yes - assuming you are setting up the queue with the first 
> 100,000 elements and then creating the semaphore, then allowing the 
> queue to grow or shrink unhindered.  It's more common for queues to 
> start with a semaphore of count 0 (an empty queue) and then add 
> elements, but if you have 100,000 elements in advance then this would be 
> a more efficient initialisation.

I sure think so. :^)


> You can also omit the template parameter, in which case an 
> implementation-defined default is used.  A quick test on godbolt's gcc 
> shows that this gives a max() of 0x7fff'ffff, and that this is the 
> largest value supported by the C++ library used by that implementation - 
> it keeps the count as a signed 32-bit integer.  Attempting to use 
> PTRDIFF_MAX triggers a static assertion:
> 
>      static_assert(__least_max_value <= __semaphore_impl::_S_max);

Oh shit! I assumed setting it to PTRDIFF_MAX is okay. Shit! Well, damn. 
thanks for the info David. Sometimes I think the template parameter is 
there for specialization purposes ala binary semaphore with its template 
param as 1.


> I don't know if this is a conformance flaw in the C++ standard library, 
> or if I just haven't read the documentation details well enough.  And I 
> couldn't see any specification for what the default value should be, but 
> I can't think of any other reasonable choice than the biggest acceptable 
> value.

Agreed. Again, thanks for the info. :^)

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counting_semaphore question Muttley@dastardlyhq.com - 2024-09-11 10:15 +0000
  Re: counting_semaphore question Bonita Montero <Bonita.Montero@gmail.com> - 2024-09-11 13:48 +0200
    Re: counting_semaphore question Muttley@dastardlyhq.com - 2024-09-11 12:03 +0000
      Re: counting_semaphore question Bonita Montero <Bonita.Montero@gmail.com> - 2024-09-11 14:10 +0200
        Re: counting_semaphore question Muttley@dastardlyhq.com - 2024-09-11 12:29 +0000
          Re: counting_semaphore question Bonita Montero <Bonita.Montero@gmail.com> - 2024-09-11 15:54 +0200
            Re: counting_semaphore question Muttley@dastardlyhq.com - 2024-09-11 14:12 +0000
          Re: counting_semaphore question David Brown <david.brown@hesbynett.no> - 2024-09-11 16:23 +0200
            Re: counting_semaphore question "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-09-16 14:22 -0700
              Re: counting_semaphore question David Brown <david.brown@hesbynett.no> - 2024-09-17 09:22 +0200
                Re: counting_semaphore question "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-09-17 11:56 -0700
                Re: counting_semaphore question David Brown <david.brown@hesbynett.no> - 2024-09-18 09:32 +0200
                Re: counting_semaphore question Bonita Montero <Bonita.Montero@gmail.com> - 2024-09-19 12:57 +0200
                Re: counting_semaphore question Bonita Montero <Bonita.Montero@gmail.com> - 2024-09-19 13:01 +0200
                Re: counting_semaphore question "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-09-19 12:09 -0700
                Re: counting_semaphore question "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-09-19 13:27 -0700
                Re: counting_semaphore question "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-09-19 13:30 -0700
          Re: counting_semaphore question Ben Bacarisse <ben@bsb.me.uk> - 2024-09-12 11:25 +0100
            Re: counting_semaphore question Muttley@dastardlyhq.com - 2024-09-12 11:00 +0000
              Re: counting_semaphore question Andrey Tarasevich <andreytarasevich@hotmail.com> - 2024-09-12 07:34 -0700
                Re: counting_semaphore question Muttley@dastardlyhq.com - 2024-09-12 14:53 +0000
                Re: counting_semaphore question David Brown <david.brown@hesbynett.no> - 2024-09-13 08:39 +0200
                Re: counting_semaphore question Muttley@dastardlyhq.com - 2024-09-13 07:26 +0000
                Re: counting_semaphore question David Brown <david.brown@hesbynett.no> - 2024-09-13 12:54 +0200
                Re: counting_semaphore question Muttley@dastardlyhq.com - 2024-09-13 13:21 +0000
                Re: counting_semaphore question Bonita Montero <Bonita.Montero@gmail.com> - 2024-09-13 15:29 +0200
                Re: counting_semaphore question Muttley@dastardlyhq.com - 2024-09-13 13:41 +0000
                Re: counting_semaphore question David Brown <david.brown@hesbynett.no> - 2024-09-15 17:33 +0200
                Re: counting_semaphore question David Brown <david.brown@hesbynett.no> - 2024-09-13 16:46 +0200
                Re: counting_semaphore question Muttley@dastardlyhq.com - 2024-09-13 14:59 +0000
                Re: counting_semaphore question Andrey Tarasevich <andreytarasevich@hotmail.com> - 2024-09-13 07:43 -0700
        Re: counting_semaphore question "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-09-11 13:51 -0700
          Re: counting_semaphore question Bonita Montero <Bonita.Montero@gmail.com> - 2024-09-12 04:21 +0200
            Re: counting_semaphore question "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-09-12 11:57 -0700
              Re: counting_semaphore question Bonita Montero <Bonita.Montero@gmail.com> - 2024-09-13 14:29 +0200
                Re: counting_semaphore question "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-09-13 12:44 -0700
                Re: counting_semaphore question Bonita Montero <Bonita.Montero@gmail.com> - 2024-09-14 01:43 +0200
                Re: counting_semaphore question "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-09-13 22:21 -0700
                Re: counting_semaphore question "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-09-14 12:07 -0700
                Re: counting_semaphore question Muttley@dastardlyhq.com - 2024-09-14 09:18 +0000
                Re: counting_semaphore question Bonita Montero <Bonita.Montero@gmail.com> - 2024-09-14 11:40 +0200
                Re: counting_semaphore question Muttley@dastardlyhq.com - 2024-09-14 10:58 +0000
                Re: counting_semaphore question Bonita Montero <Bonita.Montero@gmail.com> - 2024-09-14 13:15 +0200
                Re: counting_semaphore question Muttley@dastardlyhq.com - 2024-09-14 15:13 +0000
                Re: counting_semaphore question Bonita Montero <Bonita.Montero@gmail.com> - 2024-09-14 17:26 +0200
                Re: counting_semaphore question Muttley@dastardlyhq.com - 2024-09-14 15:48 +0000
                Re: counting_semaphore question Bonita Montero <Bonita.Montero@gmail.com> - 2024-09-14 17:50 +0200
                Re: counting_semaphore question Muttley@dastardlyhq.com - 2024-09-14 15:55 +0000
                Re: counting_semaphore question Bonita Montero <Bonita.Montero@gmail.com> - 2024-09-14 17:57 +0200
                Re: counting_semaphore question Muttley@dastardlyhq.com - 2024-09-15 09:05 +0000
                Re: counting_semaphore question Bonita Montero <Bonita.Montero@gmail.com> - 2024-09-15 11:48 +0200
                Re: counting_semaphore question Muttley@dastardlyhq.com - 2024-09-15 09:52 +0000
                Re: counting_semaphore question Bonita Montero <Bonita.Montero@gmail.com> - 2024-09-15 14:50 +0200
                Re: counting_semaphore question Muttley@dastardlyhq.com - 2024-09-15 15:28 +0000
                Re: counting_semaphore question Bonita Montero <Bonita.Montero@gmail.com> - 2024-09-16 05:02 +0200
                Re: counting_semaphore question scott@slp53.sl.home (Scott Lurndal) - 2024-09-15 16:27 +0000
                Re: counting_semaphore question scott@slp53.sl.home (Scott Lurndal) - 2024-09-15 16:26 +0000
                Re: counting_semaphore question "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-09-14 12:12 -0700
                Re: counting_semaphore question "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-09-14 17:21 -0700
                Re: counting_semaphore question Bonita Montero <Bonita.Montero@gmail.com> - 2024-09-15 07:11 +0200
                Re: counting_semaphore question "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-09-14 22:20 -0700
                Re: counting_semaphore question Bonita Montero <Bonita.Montero@gmail.com> - 2024-09-15 09:35 +0200
                Re: counting_semaphore question "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-09-15 12:59 -0700
                Re: counting_semaphore question Bonita Montero <Bonita.Montero@gmail.com> - 2024-09-16 05:04 +0200
                Re: counting_semaphore question "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-09-16 13:30 -0700
                Re: counting_semaphore question "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-09-14 12:09 -0700
  Re: counting_semaphore question Andrey Tarasevich <andreytarasevich@hotmail.com> - 2024-09-11 07:37 -0700
    Re: counting_semaphore question Muttley@dastardlyhq.com - 2024-09-11 15:03 +0000
      Re: counting_semaphore question Bonita Montero <Bonita.Montero@gmail.com> - 2024-09-11 17:27 +0200
        Re: counting_semaphore question Muttley@dastardlyhq.com - 2024-09-11 16:04 +0000
          Re: counting_semaphore question Bonita Montero <Bonita.Montero@gmail.com> - 2024-09-11 18:16 +0200
            Re: counting_semaphore question Muttley@dastardlyhq.com - 2024-09-11 18:16 +0000
              Re: counting_semaphore question Bonita Montero <Bonita.Montero@gmail.com> - 2024-09-12 04:20 +0200
          Re: counting_semaphore question "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com> - 2024-09-12 13:00 -0700
      Re: counting_semaphore question Andrey Tarasevich <andreytarasevich@hotmail.com> - 2024-09-11 20:56 -0700
        Re: counting_semaphore question Muttley@dastardlyhq.com - 2024-09-12 07:43 +0000
          Re: counting_semaphore question David Brown <david.brown@hesbynett.no> - 2024-09-12 13:30 +0200
            Re: counting_semaphore question Muttley@dastardlyhq.com - 2024-09-12 14:48 +0000
          Re: counting_semaphore question Bonita Montero <Bonita.Montero@gmail.com> - 2024-09-12 15:38 +0200
          Re: counting_semaphore question Andrey Tarasevich <andreytarasevich@hotmail.com> - 2024-09-12 07:05 -0700

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