std::對象可以是兩種狀態之一:設置或清除,且須用ATOMIC_FLAG_INIT初始化,這會將該標誌初始化爲清除狀態(此標誌總是以清除狀態初始化的),如下:
std::atomic_flag flag=ATOMIC_FLAG_INIT;
只支持兩種原子操作:
test_and_set函數:如果atomic_flag對象已經被設置,則返回true; 如果atomic_flag對象未被設置,則設置之,返回false。
std::atomic_flag可用於多線程之間的同步操作,使用atomic_flag可實現mutex
example1:用atomic_flag可實現自旋鎖:
example2:
https://en.cppreference.com/w/cpp/atomic/atomic_flag
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();
}
}