Difference between revisions of "cpp/atomic/atomic flag"
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Revision as of 08:20, 8 February 2020
| 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.
Member functions
| 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) | |
| (C++20) |
blocks the thread until notified and the atomic value changes (public member function) |
| (C++20) |
notifies at least one thread waiting on the atomic object (public member function) |
| (C++20) |
notifies all threads blocked waiting on the atomic object (public member function) |
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 (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>...
See also
| 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++20)(C++20) |
blocks the thread until notified and the flag changes (function) |
| (C++20) |
notifies a thread blocked in atomic_flag_wait (function) |
| (C++20) |
notifies all threads blocked in atomic_flag_wait (function) |
| (C++11) |
initializes an std::atomic_flag to false (macro constant) |
| C documentation for atomic_flag
| |
