std::atomic_flag

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Defined in header `<atomic>`

class atomic_flag;		(C++11 feature)

An std::atomic_flag is an atomic boolean type. Unlike all specializations of std::atomic, it is guaranteed to be lock-free.

[edit] 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)

[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) {
        t.join();
    }
}

Output:

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

[edit] See also

atomic_flag_test_and_set atomic_flag_test_and_set_explicit (C++11)
(C++11)

atomically sets the flag to true and returns its previous value
(function)

atomic_flag_clear atomic_flag_clear_explicit (C++11)
(C++11)

atomically sets the value of the flag to false
(function)

ATOMIC_FLAG_INIT (C++11)

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