< 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.

[edit] Member functions

constructs an atomic_flag
(public member function)
the assignment operator
(public member function)
atomically sets flag to false
(public member function) [edit]
atomically sets the flag to true and obtains its previous value
(public member function) [edit]

[edit] Example

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>...

[edit] See also

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