std::atomic_flag

 

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Atomic types atomic (C++11) atomic_ref (C++20) atomic_flag (C++11) Initialization of atomic types atomic_init (C++11)(deprecated in C++20) ATOMIC_VAR_INIT (C++11)(deprecated in C++20) ATOMIC_FLAG_INIT (C++11) Memory ordering memory_order (C++11) kill_dependency (C++11)(deprecated in C++26) atomic_thread_fence (C++11) atomic_signal_fence (C++11) Free functions for atomic operations atomic_storeatomic_store_explicit (C++11)(C++11) atomic_loadatomic_load_explicit (C++11)(C++11) atomic_exchangeatomic_exchange_explicit (C++11)(C++11) atomic_compare_exchange_weakatomic_compare_exchange_weak_explicitatomic_compare_exchange_strongatomic_compare_exchange_strong_explicit (C++11)(C++11)(C++11)(C++11) atomic_fetch_addatomic_fetch_add_explicit (C++11)(C++11) atomic_fetch_subatomic_fetch_sub_explicit (C++11)(C++11) atomic_fetch_andatomic_fetch_and_explicit (C++11)(C++11) atomic_fetch_oratomic_fetch_or_explicit (C++11)(C++11) atomic_fetch_xoratomic_fetch_xor_explicit (C++11)(C++11) atomic_fetch_maxatomic_fetch_max_explicit (C++26)(C++26) atomic_fetch_minatomic_fetch_min_explicit (C++26)(C++26) atomic_is_lock_free (C++11) atomic_waitatomic_wait_explicit (C++20)(C++20) atomic_notify_one (C++20) atomic_notify_all (C++20) Free functions for atomic flags atomic_flag_test_and_setatomic_flag_test_and_set_explicit (C++11)(C++11) atomic_flag_clearatomic_flag_clear_explicit (C++11)(C++11) atomic_flag_testatomic_flag_test_explicit (C++20)(C++20) atomic_flag_waitatomic_flag_wait_explicit (C++20)(C++20) atomic_flag_notify_one (C++20) atomic_flag_notify_all (C++20)

 

std::atomic_flag

Member functions

atomic_flag::atomic_flag

atomic_flag::operator=

atomic_flag::clear

atomic_flag::test_and_set

atomic_flag::test (C++20)

atomic_flag::wait (C++20)

atomic_flag::notify_one (C++20)

atomic_flag::notify_all (C++20)

 

Defined in header <atomic>
class atomic_flag;		(since C++11)

std::atomic_flag is an atomic boolean type. Unlike all specializations of std::atomic, it is guaranteed to be lock-free. Unlike std::atomic<bool>, std::atomic_flag does not provide load or store operations.

Member functions

(constructor)	constructs an atomic_flag (public member function)
operator= [deleted]	the assignment operator (deleted) (public member function)
clear	atomically sets flag to false (public member function)
test_and_set	atomically sets the flag to true and obtains its previous value (public member function)
test (C++20)	atomically returns the value of the flag (public member function)
wait (C++20)	blocks the thread until notified and the atomic value changes (public member function)
notify_one (C++20)	notifies at least one thread waiting on the atomic object (public member function)
notify_all (C++20)	notifies all threads blocked waiting on the atomic object (public member function)

Example

A spinlock mutex demo can be implemented in userspace using an atomic_flag. Do note that spinlock mutexes are extremely dubious in practice.

Run this code

#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 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)
atomic_flag_notify_one (C++20)	notifies a thread blocked in atomic_flag_wait (function)
atomic_flag_notify_all (C++20)	notifies all threads blocked in atomic_flag_wait (function)
ATOMIC_FLAG_INIT (C++11)	initializes an std::atomic_flag to false (macro constant)
C documentation for atomic_flag