std:: atomic_flag_test_and_set, std:: atomic_flag_test_and_set_explicit

Atomically changes the state of a std::atomic_flag pointed to by obj to set ( true ) and returns the value it held before.

Parameters

Return value

The value previously held by the flag pointed to by obj .

Notes

std::atomic_flag_test_and_set and std::atomic_flag_test_and_set_explicit can be implemented as obj - > test_and_set ( ) and obj - > test_and_set ( order ) respectively.

Example

#include <atomic>
#include <iostream>
#include <thread>
#include <vector>
 
std::atomic_flag lock = ATOMIC_FLAG_INIT;
 
void f(int n)
{
    for (int cnt = 0; cnt < 100; ++cnt)
    {
        while (std::atomic_flag_test_and_set_explicit(&lock, std::memory_order_acquire))
            ; // spin until the lock is acquired
        std::cout << "Output from thread " << n << '\n';
        std::atomic_flag_clear_explicit(&lock, std::memory_order_release);
    }
}
 
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 from thread 2
Output from thread 6
Output from thread 7
...<exactly 1000 lines>...

See also