Defined in header <atomic> | ||
---|---|---|
class atomic_flag; | (since C++11) |
std::atomic_flag
is an atomic boolean type. Unlike all specializations of std::atomic
, it is guaranteed to be lock-free. Unlike std::atomic<bool>
, std::atomic_flag
does not provide load or store operations.
constructs an atomic_flag (public member function) |
|
the assignment operator (public member function) |
|
atomically sets flag to false (public member function) |
|
atomically sets the flag to true and obtains its previous value (public member function) |
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 (lock.test_and_set(std::memory_order_acquire)) // acquire lock ; // spin std::cout << "Output from thread " << n << '\n'; lock.clear(std::memory_order_release); // release lock } } 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>...
(C++11)(C++11) | atomically sets the flag to true and returns its previous value (function) |
(C++11)(C++11) | atomically sets the value of the flag to false (function) |
(C++11) | initializes an std::atomic_flag to false (macro constant) |
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