std::atomic_flag

#include <atomic>
#include <iostream>
#include <thread>
#include <vector>
 
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();
}