std::atomic_flag_test_and_set, std::atomic_flag_test_and_set_explicit

Since C++11

Header: <atomic>

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

# Declarations

bool atomic_flag_test_and_set( volatile std::atomic_flag* obj ) noexcept;

(since C++11)

bool atomic_flag_test_and_set( std::atomic_flag* obj ) noexcept;

(since C++11)

bool atomic_flag_test_and_set_explicit( volatile std::atomic_flag* obj,
std::memory_order order ) noexcept;

(since C++11)

bool atomic_flag_test_and_set_explicit( std::atomic_flag* obj,
std::memory_order order ) noexcept;

(since C++11)

# 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();
}

# See also