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*
p ) noexcept;
bool atomic_flag_test_and_set( std::atomic_flag* p )
noexcept;
bool atomic_flag_test_and_set_explicit( volatile (1) (since
std::atomic_flag* p, C++11)
std::memory_order order ) noexcept; (2) (since
bool atomic_flag_test_and_set_explicit( std::atomic_flag* C++11)
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 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)