NAME¶

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¶

constructor constructs an atomic_flag
(public member function)
operator= the assignment operator
(public member function)
clear atomically sets flag to false
(public member function)
test_and_set atomically sets the flag to true and obtains its previous value
(public member function)
test atomically returns the value of the flag
(C++20) (public member function)
wait blocks the thread until notified and the atomic value changes
(C++20) (public member function)
notify_one notifies at least one thread waiting on the atomic object
(C++20) (public member function)
notify_all notifies all threads blocked waiting on the atomic object
(C++20) (public member function)

Example¶

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

// 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 < 40; ++cnt) {
while (lock.test_and_set(std::memory_order_acquire)) { // acquire lock
// Since C++20, it is possible to update atomic_flag's
// value only when there is a chance to acquire the lock.
// See also: https://stackoverflow.com/questions/62318642
#if defined(__cpp_lib_atomic_flag_test)
while (lock.test(std::memory_order_relaxed)) // test lock
#endif
; // spin
}
static int out{};
std::cout << n << ((++out % 40) == 0 ? '\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();
}
}

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

See also¶

atomic_flag_test_and_set atomically sets the flag to true and returns its
atomic_flag_test_and_set_explicit previous value
(C++11) (function)
(C++11)
atomic_flag_clear
atomic_flag_clear_explicit atomically sets the value of the flag to false
(C++11) (function)
(C++11)
atomic_flag_wait blocks the thread until notified and the flag
atomic_flag_wait_explicit changes
(C++20) (function)
(C++20)
atomic_flag_notify_one notifies a thread blocked in atomic_flag_wait
(C++20) (function)
atomic_flag_notify_all notifies all threads blocked in atomic_flag_wait
(C++20) (function)
ATOMIC_FLAG_INIT initializes an std::atomic_flag to false
(C++11) (macro constant)