std::atomic_flag - cppreference.com (original) (raw)

| | | | | ------------------- | | ------------- | | 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.

[edit] Member functions

[edit] Example

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

#include #include #include #include #include   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;   static int 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

[edit] See also