std::counting_semaphore<LeastMaxValue>::acquire
来自cppreference.com
< cpp | thread | counting semaphore
| void acquire(); |
(C++20 起) | |
若内部计数器大于 0 则尝试将它减少 1 ；否则阻塞直至它大于 0 且能成功减少内部计数器。
前条件
（无）
参数
（无）
异常
可能抛出 std::system_error 。
示例
该示例可视化数个随机化线程在不多于 N 个活跃线程函数，而且其他可能在信号量上等待时的并发作业（ N 为信号量 desired 值）。
运行此代码
#include <array> #include <chrono> #include <cstddef> #include <iomanip> #include <iostream> #include <mutex> #include <random> #include <semaphore> #include <thread> #include <vector> using namespace std::literals; constexpr std::size_t max_threads{10U}; // 更改并查看效果 constexpr std::ptrdiff_t max_sema_threads{3}; // 对二元信号量为 {1} std::counting_semaphore semaphore{max_sema_threads}; constexpr auto time_tick{10ms}; unsigned rnd() { static std::uniform_int_distribution<unsigned> distribution{2U, 9U}; // [延迟] static std::random_device engine; static std::mt19937 noise{engine()}; return distribution(noise); } class alignas( 128 /*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) { n *= x_scale; while (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(); // 模拟获取旗标前的某些工作 semaphore.acquire(); // 等待直至空闲旗标槽可用 guides[id].occupy_sema(); // 模拟获取旗标时的某些工作 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)); } }
可能的输出：
Default case: max_threads{10U}, max_sema_threads{3}
Threads: 10, Throughput: 3 │ Legend: initial delay ░░ │ wait state ▒▒ │ sema occupation ██
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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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──────────────────────────────────────────────────────────────────────────────────────────────────────────────
Binary semaphore case: max_threads{10U}, max_sema_threads{1}
Threads: 10, Throughput: 1 │ Legend: initial delay ░░ │ wait state ▒▒ │ sema occupation ██
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