std::atomic_flag_test_and_set

Defined in header <atomic>

(1) (since C++11)

1	`bool` `atomic_flag_test_and_set(` `volatile` `std::atomic_flag* p );`

1	`bool` `atomic_flag_test_and_set( std::atomic_flag* p );`

(2) (since C++11)

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

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

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.

Exceptions

noexcept specification:

noexcept

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.

#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>...

atomic_flag (C++11)	the lock-free boolean atomic type (class)
atomic_flag_clearatomic_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 (typedef)
C documentation for `atomic_flag_test_and_set, atomic_flag_test_and_set_explicit`

Links:

http://en.cppreference.com/w/cpp/atomic/atomic_flag_test_and_set

doc_CPP

2025-01-10 15:47:30

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