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From: Bonita Montero <Bonita.Montero@gmail.com>
Newsgroups: comp.lang.c++
Subject: Re: counting_semaphore question
Date: Wed, 11 Sep 2024 13:48:04 +0200
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Am 11.09.2024 um 12:15 schrieb Muttley@dastardlyhq.com:
> I've been looking at counting_semaphore and it looks useful but something
> that doesn't seem to be properly explained anywhere is the template
> parameter value. eg you can do:
> 
> std::counting_semaphore sem(2)
> 
> which will let a max of 2 threads into the protected block at a time or:
> 
> std::counting_semaphore<some number> sem(2)
> 
> such as
> 
> std::counting_semaphore<10> sem(2)
> 
> I don't understand what the '10' will do. ..

The ten gives an upper limit beyond the semaphore wont't increment.

Usually you won't need a C++20 semaphore yourself. For most purpose
the mutex and the condition_variable is sufficient.
But if you will build your own synchronization-primitives you inter-
nally would also need a semaphore. The semaphore class itself is
usually built on top of the system-provided futex. I implemented
a reader's writer lock with configurable priority for either rea-
ders or writers (std::shared_mutex only supports reader-priority)
and therefore I needed C++20's counting_semaphore and binary_sema-
phore (which is an template-alias for counting_senaphore<1>).