< cpp‎ | atomic
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

constructs an atomic_flag
(public member function)
the assignment operator
(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)


A spinlock mutex can be implemented in userspace using an atomic_flag

#include <thread>
#include <vector>
#include <iostream>
#include <atomic>
std::atomic_flag lock = ATOMIC_FLAG_INIT;
void f(int n)
    for (int cnt = 0; cnt < 100; ++cnt) {
        while (lock.test_and_set(std::memory_order_acquire))  // acquire lock
             ; // spin
        std::cout << "Output from thread " << n << '\n';
        lock.clear(std::memory_order_release);               // release lock
int main()
    std::vector<std::thread> v;
    for (int n = 0; n < 10; ++n) {
        v.emplace_back(f, n);
    for (auto& t : v) {


Output from thread 2
Output from thread 6
Output from thread 7
...<exactly 1000 lines>...

See also

atomically sets the flag to true and returns its previous value
atomically sets the value of the flag to false
initializes an std::atomic_flag to false
(macro constant)
C documentation for atomic_flag