std::atomic_flag (3) Linux Manual Page

std::atomic_flag – std::atomic_flag

Synopsis

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)

Example

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) {
        t.join();
    }
}

Output:

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

See also

atomic_flag_test_and_set
atomic_flag_test_and_set_explicit atomically sets the flag to true and returns its previous value
                                  (function)
(C++11)
(C++11)
atomic_flag_clear
atomic_flag_clear_explicit atomically sets the value of the flag to false
                                  (function)
(C++11)
(C++11)
ATOMIC_FLAG_INIT initializes an std::atomic_flag to false
                                  (macro constant)
(C++11)