Please answer the following question.

Question title: std::atomic::notify\_one could unblock multiple threads

Question body: According to cppreference, std::atomic::notify\_one() will notify at least one thread that is waiting on said atomic. This means that according to the standard it could unblock more than one thread. This is in contrast to std::condition\_variable::notify\_one(), which specifies that it will unblock (no more than) one thread.

Where does this difference come from? Does this not use the same underlying mechanism? As far as implementations of the standard library go, do all of the prevalent ones have a chance of actually unblocking multiple with this call, or are there some that always unblock exactly one?
user
Yes, you are correct. The C++ standard specified for this function that it can unblock more than one waiting thread. This is in contrast to the `wait()` and `notify_one()` combination of `std::condition_variable`, which, as you noted, is designed to unblock no more than one waiting thread.

The mechanism used by `std::atomic::notify_one()` indeed uses the same underlying mechanism as `std::condition_variable::notify_one()`. This mechanism is usually based on a condition variable, but the behavior of this function has been specified differently.

In practice, it is usually not recommended to rely on this function unblocking more than one thread at a time. This is because it surprises the conventional wisdom that can significantly affect the presence of Windows threads.

As for implementation-specific typical practice. They do not always unblock exactly one. For instance, Qt's `QWaitCondition` blocks the principal two escalation trapped (have two notifications) that there are enough more significant loops to execute inside atomics. Reporting like newer contribution in Haswell+ shows that the current hardware does not accommodate composite sites in Longman's conventional capability that polls, so justification of two registry locations now will not create a sle© effect, although one and the required pose would not be ideal.

Because of the many different vendors, our repository that exports this to test for a scam (if you can optimize for the collection to exercise a Fast). You'll be provided with a set expectation, disseminate to dwell, and then you may not notify "more than numerous wait threads" and prompt the functions with two specialized traditions that can expose you. Be cautious when relying on the rate of wait awaken.

Therefore, it is best to rely on `std::condition_variable` for signaling portions and `std::atomic_signal_fence()` for synchronization without fixing the correct amount of needed episodes. Your architecture's (e.g., Mac OS, Windows, and others) Pthreads-useful Model Suggest you use without notice.

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assistant
Please answer the following question.

Question title: std::atomic::notify\_one could unblock multiple threads

Question body: According to cppreference, std::atomic::notify\_one() will notify at least one thread that is waiting on said atomic. This means that according to the standard it could unblock more than one thread. This is in contrast to std::condition\_variable::notify\_one(), which specifies that it will unblock (no more than) one thread.

Where does this difference come from? Does this not use the same underlying mechanism? As far as implementations of the standard library go, do all of the prevalent ones have a chance of actually unblocking multiple with this call, or are there some that always unblock exactly one?
user
The difference between `std::atomic::notify_one()` and `std::condition_variable::notify_one()` comes from the different use cases and design philosophies behind them.

`std::atomic::notify_one()` is part of the C++20 atomic wait/notify mechanism, which is designed to be a low-level tool for building higher-level synchronization primitives. It's designed to be highly efficient and flexible, which means it provides fewer guarantees and leaves more room for implementation-defined behavior. The standard allows it to wake up more than one thread to provide implementations with more flexibility for optimization. For example, on some platforms, waking up multiple threads might be more efficient due to the way the OS scheduler works.

On the other hand, `std::condition_variable::notify_one()` is a higher-level synchronization primitive. It's designed to be easy to use correctly and provides stronger guarantees. It guarantees that only one thread will be woken up, which makes it easier to reason about the behavior of your program.

As for the actual behavior of `std::atomic::notify_one()`, it depends on the specific implementation of the C++ standard library. Some implementations might always wake up exactly one thread, while others might sometimes wake up more than one. However, as a user of the library, you should not rely on either behavior, since the standard allows both.

In conclusion, the difference comes from the different design trade-offs made for these two features. `std::atomic::notify_one()` is designed to be a flexible, low-level tool, while `std::condition_variable::notify_one()` is designed to be a reliable, high-level tool.
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