C ++ atomic_flag查询状态 [英] C++ atomic_flag query state

问题描述

我使用C ++ std :: atomic_flag 作为原子布尔标志。将标志设置为true或false不是一个问题，但是如何查询标志的当前状态，而不将其设置为某个值？我知道有方法' atomic_flag_clear '和' atomic_flag_set '。它们返回先前状态，但也修改当前状态。有没有办法查询标志状态而不修改它，或者我必须使用完全成熟的' std :: atomic< bool> '。

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>'.

推荐答案

无法读取 std :: atomic_flag 的值将其设置为 true 。这是设计。它不是一个布尔变量（我们有 std :: atomic< bool> ），但是一个最小标志被保证在支持C ++ 11。

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.

在某些平台上，唯一的原子指令是交换指令。在这些平台上， std :: atomic_flag :: test_and_set（）可以用 exchange var，1 和 clear（）与交换var，0 ，但没有读取该值的原子指令。

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.

因此，如果你想读取值而不改变它，那么你需要 std :: atomic< bool> 。

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

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