std::atomic_flag_test_and_set, std::atomic_flag_test_and_set_explicit
Defined in header <atomic> |
||
|---|---|---|
|
(1) | (since C++11) |
|
(2) | (since C++11) |
|
(3) | (since C++11) |
|
(4) | (since C++11) |
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.
#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
|
(C++11)
|
the lock-free boolean atomic type (class) |
|
(C++11)(C++11)
|
atomically sets the value of the flag to false (function) |
|
(C++11)
|
defines memory ordering constraints for the given atomic operation (enum) |
C documentation for atomic_flag_test_and_set, atomic_flag_test_and_set_explicit |
|
© cppreference.com
Licensed under the Creative Commons Attribution-ShareAlike Unported License v3.0.
https://en.cppreference.com/w/cpp/atomic/atomic_flag_test_and_set