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| Member functions | ||||
(C++20) | ||||
(C++20) | ||||
(C++20) | ||||
(C++20) |
Defined in header <atomic> | ||
class atomic_flag; | (since C++11) | |
std::atomic_flag is an atomic boolean type. Unlike all specializations ofstd::atomic, it is guaranteed to be lock-free. Unlikestd::atomic<bool>,std::atomic_flag does not provide load or store operations.
| constructs an atomic_flag (public member function) | |
[deleted] | the assignment operator (deleted) (public member function) |
| atomically sets flag tofalse (public member function)[edit] | |
| atomically sets the flag totrue and obtains its previous value (public member function)[edit] | |
(C++20) | atomically returns the value of the flag (public member function)[edit] |
(C++20) | blocks the thread until notified and the atomic value changes (public member function)[edit] |
(C++20) | notifies at least one thread waiting on the atomic object (public member function)[edit] |
(C++20) | notifies all threads blocked waiting on the atomic object (public member function)[edit] |
Aspinlock mutex demo can be implemented in userspace using anatomic_flag. Do note that spinlock mutexes areextremely dubious in practice.
#include <atomic>#include <iostream>#include <mutex>#include <thread>#include <vector> class mutex{ std::atomic_flag m_{}; public:void lock()noexcept{while(m_.test_and_set(std::memory_order_acquire))#if defined(__cpp_lib_atomic_wait) && __cpp_lib_atomic_wait >= 201907L// Since C++20, locks can be acquired only after notification in the unlock,// avoiding any unnecessary spinning.// Note that even though wait guarantees it returns only after the value has// changed, the lock is acquired after the next condition check. m_.wait(true,std::memory_order_relaxed)#endif;}bool try_lock()noexcept{return!m_.test_and_set(std::memory_order_acquire);}void unlock()noexcept{ m_.clear(std::memory_order_release);#if defined(__cpp_lib_atomic_wait) && __cpp_lib_atomic_wait >= 201907L m_.notify_one();#endif}}; static mutex m; staticint out{}; void f(std::size_t n){for(std::size_t cnt{}; cnt<40;++cnt){std::lock_guard lock{m};std::cout<< n<<((++out%40)==0?'\n':' ');}} int main(){std::vector<std::thread> v;for(std::size_t n{}; n<10;++n) v.emplace_back(f, n);for(auto&t: v) t.join();}
Possible output:
0 1 1 2 0 1 3 2 3 2 0 1 2 3 2 3 0 1 3 2 0 1 2 3 2 3 0 3 2 3 2 3 2 3 1 2 3 0 1 32 3 2 0 1 2 3 0 1 2 3 2 0 1 2 3 0 1 2 3 2 3 2 3 2 0 1 2 3 2 3 0 1 3 2 3 0 2 1 11 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 3 2 0 2 3 2 3 2 3 2 3 2 3 0 32 3 0 3 0 3 2 3 0 3 2 3 2 3 0 2 3 0 3 2 0 2 2 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 04 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 4 45 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 5 56 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 6 67 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 7 78 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 8 89 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9 9
| atomically sets the flag totrue and returns its previous value (function)[edit] | |
(C++11)(C++11) | atomically sets the value of the flag tofalse (function)[edit] |
(C++20)(C++20) | blocks the thread until notified and the flag changes (function)[edit] |
(C++20) | notifies a thread blocked in atomic_flag_wait (function)[edit] |
(C++20) | notifies all threads blocked in atomic_flag_wait (function)[edit] |
(C++11) | initializes anstd::atomic_flag tofalse (macro constant)[edit] |
C documentation foratomic_flag | |
