# std::atomic_flag_test_and_set, std::atomic_flag_test_and_set_explicit

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< cpp‎ | atomic

Defined in header `<atomic>` |  |
| (1) | (since C++11)
bool atomic_flag_test_and_set( volatile std::atomic_flag* p ) noexcept; |  |
bool atomic_flag_test_and_set( std::atomic_flag* p ) noexcept; |  |
| (2) | (since C++11)
bool atomic_flag_test_and_set_explicit( volatile std::atomic_flag* p, std::memory_order order ) noexcept; |  |
bool atomic_flag_test_and_set_explicit( std::atomic_flag* p, std::memory_order order ) noexcept; |  |

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

### Parameters

p | - | pointer to std::atomic_flag to access
order | - | the memory synchronization order for this operation

### Return value

The value previously held by the flag pointed to by `p`

### Possible implementation

First version

    bool atomic_flag_test_and_set(volatile std::atomic_flag* p)
    {
        return p->test_and_set();
    }

Second version

    bool atomic_flag_test_and_set(std::atomic_flag* p)
    {
        return p->test_and_set();
    }

Third version

    bool atomic_flag_test_and_set_explicit(volatile std::atomic_flag* p,
                                           std::memory_order order)
    {
        return p->test_and_set(order);
    }

Fourth version

    bool atomic_flag_test_and_set_explicit(std::atomic_flag* p,
                                           std::memory_order order)
    {
        return p->test_and_set(order);
    }

### Example

A spinlock mutex can be implemented in userspace using an atomic_flag

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 < 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:

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

### See also

atomic_flag (C++11) | the lock-free boolean atomic type (class)
atomic_flag_clear atomic_flag_clear_explicit (C++11)(C++11) | atomically sets the value of the flag to false (function)
memory_order (C++11) | defines memory ordering constraints for the given atomic operation (enum)

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