std::atomic_flag (3) - Linux Manuals

std::atomic_flag: std::atomic_flag


std::atomic_flag - std::atomic_flag


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
constructor (public member function)
              the assignment operator
operator= (public member function)
              atomically sets flag to false
clear (public member function)
              atomically sets the flag to true and obtains its previous value
test_and_set (public member function)


A spinlock mutex can be implemented in userspace using an atomic_flag
// Run this code

  #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

atomic_flag_test_and_set_explicit atomically sets the flag to true and returns its previous value

atomic_flag_clear_explicit atomically sets the value of the flag to false

ATOMIC_FLAG_INIT initializes an std::atomic_flag to false
                                  (macro constant)