std::counting_semaphore<LeastMaxValue>::acquire
From cppreference.com
< cpp | thread | counting semaphore
| 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.
Preconditions
(none)
Parameters
(none)
Exceptions
May throw std::system_error.
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.
Run this code
#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 ░░░░██████
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──────────────────────────────────────────────────────────────────────────────────────────────────────────────
"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 ██
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# 0 ░░░░░░░░░░░░██████████████████
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──────────────────────────────────────────────────────────────────────────────────────────────────────────────
Binary semaphore case: max_threads{10U}, max_sema_threads{1}
Threads: 10, Throughput: 1 │ Legend: initial delay ░░ │ wait state ▒▒ │ sema occupation ██
# 6 ░░░░████
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| increments the internal counter and unblocks acquirers (public member function) | |
| tries to decrement the internal counter without blocking (public member function) | |
| tries to decrement the internal counter, blocking for up to a duration time (public member function) | |
| tries to decrement the internal counter, blocking until a point in time (public member function) |