std::atomic_flag_test_and_set, std::atomic_flag_test_and_set_explicit
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atomic_flag_test_and_setatomic_flag_test_and_set_explicit
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Defined in header
<atomic>
bool atomic_flag_test_and_set( volatile std::atomic_flag * obj ) noexcept;
(1)
(since C++11)
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,
std::memory_order order ) noexcept;
(3)
(since C++11)
std::memory_order order ) noexcept;
bool atomic_flag_test_and_set_explicit( std::atomic_flag * obj,
std::memory_order order ) noexcept;
(4)
(since C++11)
std::memory_order order ) 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.
Contents
[edit] Parameters
obj
-
pointer to std::atomic_flag to access
order
-
the memory synchronization order
[edit] Return value
The value previously held by the flag pointed to by obj.
[edit] 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.
[edit] 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>...
[edit] See also
C documentation for atomic_flag_test_and_set, atomic_flag_test_and_set_explicit