|
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| Member functions | ||||
| Operations | ||||
counting_semaphore::acquire | ||||
| Constants | ||||
void acquire(); | (since C++20) | |
Atomically decrements the internal counter by1 if it is greater than0; otherwise blocks until it is greater than0 and can successfully decrement the internal counter.
Contents |
(none)
(none)
May throwstd::system_error.
The example visualizes a concurrent work of several randomized threads when no more than N (N is the semaphoredesired 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> usingnamespace std::literals; constexprstd::size_t max_threads{10U};// change and see the effectconstexprstd::ptrdiff_t max_sema_threads{3};// {1} for binary semaphorestd::counting_semaphore semaphore{max_sema_threads};constexprauto time_tick{10ms}; unsigned rnd(){staticstd::uniform_int_distribution<unsigned> distribution{2U, 9U};// [delays]staticstd::random_device engine;staticstd::mt19937 noise{engine()};return distribution(noise);} class alignas(std::hardware_destructive_interference_size) Guide{inlinestaticstd::mutex cout_mutex;inlinestaticstd::chrono::time_point<std::chrono::high_resolution_clock> started_at;unsigned delay{rnd()}, occupy{rnd()}, wait_on_sema{}; public:staticvoid 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';} staticvoid 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 ░░░░░░░░░░░░░░░░░░▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒▒██████████████| 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] |
