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std::counting_semaphore<LeastMaxValue>::acquire

From cppreference.com
 
 
Concurrency support library
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void acquire();
(since C++20)

Atomically decrements the internal counter by 1 if it is greater than 0; otherwise blocks until it is greater than 0 and can successfully decrement the internal counter.

Contents

[edit] Preconditions

(none)

[edit] Parameters

(none)

[edit] Exceptions

May throw std::system_error.

[edit] Example

The example visualizes a concurrent work of several randomized threads when no more than N (N is the semaphore desired value) of the thread-functions are active, while the other might wait on the semaphore.

#include <array>
#include <chrono>
#include <cstddef>
#include <iomanip>
#include <iostream>
#include <mutex>
#include <new>
#include <random>
#include <semaphore>
#include <thread>
#include <vector>
 
using namespace std::literals;
 
constexpr std::size_t max_threads{10U}; // change and see the effect
constexpr std::ptrdiff_t max_sema_threads{3}; // {1} for binary semaphore
std::counting_semaphore semaphore{max_sema_threads};
constexpr auto time_tick{10ms};
 
unsigned rnd()
{
    static std::uniform_int_distribution<unsigned> distribution{2U, 9U}; // [delays]
    static std::random_device engine;
    static std::mt19937 noise{engine()};
    return distribution(noise);
}
 
class alignas(std::hardware_destructive_interference_size) Guide
{
    inline static std::mutex cout_mutex;
    inline static std::chrono::time_point<std::chrono::high_resolution_clock> started_at;
    unsigned delay{rnd()}, occupy{rnd()}, wait_on_sema{};
 
public:
    static void start_time() { started_at = std::chrono::high_resolution_clock::now(); }
 
    void initial_delay() { std::this_thread::sleep_for(delay * time_tick); }
 
    void occupy_sema()
    {
        wait_on_sema =
            static_cast<unsigned>(std::chrono::duration_cast<std::chrono::milliseconds>(
                std::chrono::high_resolution_clock::now() - started_at -
                delay * time_tick).count() / time_tick.count());
        std::this_thread::sleep_for(occupy * time_tick);
    }
 
    void visualize(unsigned id, unsigned x_scale = 2) const
    {
        auto cout_n = [=](auto str, unsigned n)
        {
            for (n *= x_scale; n-- > 0; std::cout << str)
                ;
        };
        std::lock_guard lk{cout_mutex};
        std::cout << '#' << std::setw(2) << id << ' ';
        cout_n("░", delay);
        cout_n("▒", wait_on_sema);
        cout_n("█", occupy);
        std::cout << '\n';
    }
 
    static void show_info()
    {
        std::cout << "\nThreads: " << max_threads << ", Throughput: " << max_sema_threads
                  << " │ Legend: initial delay ░░ │ wait state ▒▒ │ sema occupation ██ \n"
                  << std::endl;
    }
};
 
std::array<Guide, max_threads> guides;
 
void workerThread(unsigned id)
{
    guides[id].initial_delay(); // emulate some work before sema acquisition
    semaphore.acquire();        // wait until a free sema slot is available
    guides[id].occupy_sema();   // emulate some work while sema is acquired
    semaphore.release();
    guides[id].visualize(id);
}
 
int main()
{
    std::vector<std::jthread> threads;
    threads.reserve(max_threads);
 
    Guide::show_info();
    Guide::start_time();
 
    for (auto id{0U}; id != max_threads; ++id)
        threads.push_back(std::jthread(workerThread, id));
}

Possible output:

Default case: max_threads{10U}, max_sema_threads{3}
 
Threads: 10, Throughput: 3 │ Legend: initial delay ░░ │ wait state ▒▒ │ sema occupation ██
 
# 1 ░░░░██████
# 2 ░░░░████████
# 5 ░░░░░░██████████
# 8 ░░░░░░░░░░░░████████████
# 9 ░░░░░░░░░░░░██████████████
# 7 ░░░░░░░░░░░░▒▒▒▒████████████████
# 4 ░░░░░░░░░░░░░░▒▒▒▒▒▒▒▒▒▒▒▒████████
# 6 ░░░░░░░░░░░░░░▒▒▒▒▒▒▒▒▒▒██████████████████
# 3 ░░░░░░░░░░░░░░▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒████████████
# 0 ░░░░░░░░░░░░░░░░░░▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒██████████████
 
──────────────────────────────────────────────────────────────────────────────────────────────────────────────
"Enough for everyone" case (no wait states!): max_threads{10U}, max_sema_threads{10}
 
Threads: 10, Throughput: 10 │ Legend: initial delay ░░ │ wait state ▒▒ │ sema occupation ██
 
# 4 ░░░░██████
# 5 ░░░░░░████
# 3 ░░░░██████████
# 1 ░░░░██████████
# 8 ░░░░░░░░████████████
# 6 ░░░░░░░░░░░░░░░░██████
# 7 ░░░░░░░░░░░░░░░░██████
# 9 ░░░░░░░░░░░░░░░░██████████
# 0 ░░░░░░░░░░░░██████████████████
# 2 ░░░░░░░░░░░░░░░░░░████████████
 
──────────────────────────────────────────────────────────────────────────────────────────────────────────────
Binary semaphore case: max_threads{10U}, max_sema_threads{1}
 
Threads: 10, Throughput: 1 │ Legend: initial delay ░░ │ wait state ▒▒ │ sema occupation ██
 
# 6 ░░░░████
# 5 ░░░░▒▒▒▒████
# 4 ░░░░░░░░░░▒▒██████████
# 7 ░░░░░░░░░░▒▒▒▒▒▒▒▒▒▒▒▒████████████████
# 2 ░░░░░░░░░░░░▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒██████
# 3 ░░░░░░░░░░░░░░▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒████████████████
# 0 ░░░░░░░░░░░░░░▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒████████████
# 1 ░░░░░░░░░░░░░░░░▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒████████
# 8 ░░░░░░░░░░░░░░░░▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒██████
# 9 ░░░░░░░░░░░░░░░░░░▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒██████████████

[edit] See also

increments the internal counter and unblocks acquirers
(public member function) [edit]
tries to decrement the internal counter without blocking
(public member function) [edit]
tries to decrement the internal counter, blocking for up to a duration time
(public member function) [edit]
tries to decrement the internal counter, blocking until a point in time
(public member function) [edit]