C++ atomic_flag 查询状态

Question

I am using C++ std::atomic_flag as an atomic Boolean flag. Setting the flag to true or false is not a problem but how to query the current state of flag without setting it to some value? I know that there are methods ' atomic_flag_clear ' and ' atomic_flag_set '. They do give back the previous state but also modify the current state. Is there any way to query flag state without modifying it or do I have to use full fledged ' std::atomic<bool> '.

Answer 1

You cannot read the value of a std::atomic_flag without setting it to true . This is by design. It is not a boolean variable (we have std::atomic<bool> for that), but a minimal flag that is guaranteed lock free on all architectures that support C++11.

On some platforms the only atomic instructions are exchange instructions. On such platforms, std::atomic_flag::test_and_set() can be implemented with exchange var,1 and clear() with exchange var,0 , but there is no atomic instruction for reading the value.

So, if you want to read the value without changing it, then you need std::atomic<bool> .

Answer 2

If you want to use atomic_flag to determine whether a thread should exit, you can do it like this:

Initialization:

std::atomic_flag keep_running = ATOMIC_FLAG_INIT;
keep_running.test_and_set();

Thread loop:

while (keep_running.test_and_set()) {
    // do thread stuff
}

When you want the thread to exit:

keep_running.clear();

Answer 3

使用 C++20，我们得到了test()方法，它完全符合 OP 的要求。