NAME¶

std::atomic_flag_test_and_set,std::atomic_flag_test_and_set_explicit - std::atomic_flag_test_and_set,std::atomic_flag_test_and_set_explicit

Synopsis¶

Defined in header <atomic>
bool atomic_flag_test_and_set( volatile std::atomic_flag* obj ) (1) (since C++11)
noexcept;
bool atomic_flag_test_and_set( std::atomic_flag* obj ) noexcept; (2) (since C++11)
bool atomic_flag_test_and_set_explicit( volatile std::atomic_flag*
obj, (3) (since C++11)
std::memory_order order )
noexcept;
bool atomic_flag_test_and_set_explicit( std::atomic_flag* obj,
std::memory_order order ) (4) (since C++11)
noexcept;

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

1,2) The memory synchronization order is std::memory_order_seq_cst.
3,4) The memory synchronization order is order.

Parameters¶

obj - pointer to std::atomic_flag to access
order - the memory synchronization order

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¶

A spinlock mutex can be implemented in userspace using an std::atomic_flag.

// Run this code

#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:¶

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

See also¶

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