| 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) | |
| operator=
| 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) |
Example
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>...
See also
| (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) |
C documentation for atomic_flag | |
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