From cppreference.com
structatomic_flag;(since C++11)std::atomic_flag is an atomic boolean type. Unlike all specializations of
, it is guaranteed to be lock-free. Unlike std::atomic<bool>, std::atomic_flag does not provide load or store operations.
Member functions
constructs an atomic_flag
(public member function)
[deleted]
the assignment operator (deleted)
(public member function)
atomically sets flag to false
(public member function)
atomically sets the flag to true and obtains its previous value
(public member function)
(C++20)
atomically returns the value of the flag
(public member function)
(C++20)
blocks the thread until notified and the atomic value changes
(public member function)
(C++20)
notifies at least one thread waiting on the atomic object
(public member function)
(C++20)
notifies all threads blocked waiting on the atomic object
(public member function)
Example
A
mutex demo can be implemented in userspace using an atomic_flag. Do note that spinlock mutexes are
in practice.
Run this code
#include<atomic>#include<iostream>#include<mutex>#include<thread>#include<vector>classmutex{std::atomic_flagm_{};public:voidlock()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;}booltry_lock()noexcept{return!m_.test_and_set(std::memory_order_acquire);}voidunlock()noexcept{m_.clear(std::memory_order_release);#if defined(__cpp_lib_atomic_wait) && __cpp_lib_atomic_wait >= 201907Lm_.notify_one();#endif}};staticmutexm;staticintout{};voidf(std::size_tn){for(std::size_tcnt{};cnt<40;++cnt){std::lock_guardlock{m};std::cout<<n<<((++out%40)==0?'\n':' ');}}intmain(){std::vector<std::thread>v;for(std::size_tn{};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 3 2 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 1 1 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 3 2 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 0 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 4 4 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 5 5 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 6 6 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 7 7 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 8 8 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 9 See also
atomic_flag_test_and_setatomic_flag_test_and_set_explicit
(C++11)(C++11)
atomically sets the flag to true and returns its previous value
(function)
atomic_flag_clearatomic_flag_clear_explicit
(C++11)(C++11)
atomically sets the value of the flag to false
(function)
atomic_flag_waitatomic_flag_wait_explicit
(C++20)(C++20)
blocks the thread until notified and the flag changes
(function)
(C++20)
notifies a thread blocked in atomic_flag_wait
(function)
(C++20)
notifies all threads blocked in atomic_flag_wait
(function)
(C++11)
initializes an
to false
(macro constant)
for atomic_flag