typedef enum memory_order {

memory_order_relaxed, // 不對執行順序做保證

memory_order_acquire, // 本線程中,所有后續的讀操作必須在本條原子操作完成后執行

memory_order_release, // 本線程中,所有之前的寫操作完成后才能執行本條原子操作

memory_order_acq_rel, // 同時包含 memory_order_acquire 和 memory_order_release

memory_order_consume, // 本線程中,所有后續的有關本原子類型的操作,必須在本條原子操作完成之后執行

memory_order_seq_cst // 全部存取都按順序執行

} memory_order;

另外rust 相關文檔，寫的好理解；

Relaxed

No ordering constraints, only atomic operations.

Corresponds to memory_order_relaxed in C++20.

Release

When coupled with a store, all previous operations become ordered before any load of this value with Acquire (or stronger) ordering. In particular, all previous writes become visible to all threads that perform an Acquire (or stronger) load of this value.

Notice that using this ordering for an operation that combines loads and stores leads to a Relaxed load operation!

This ordering is only applicable for operations that can perform a store.

Corresponds to memory_order_release in C++20.

Acquire

When coupled with a load, if the loaded value was written by a store operation with Release (or stronger) ordering, then all subsequent operations become ordered after that store. In particular, all subsequent loads will see data written before the store.

Notice that using this ordering for an operation that combines loads and stores leads to a Relaxed store operation!

This ordering is only applicable for operations that can perform a load.

Corresponds to memory_order_acquire in C++20.

AcqRel

Has the effects of both Acquire and Release together: For loads it uses Acquire ordering. For stores it uses the Release ordering.

Notice that in the case of compare_and_swap, it is possible that the operation ends up not performing any store and hence it has just Acquire ordering. However, AcqRel will never perform Relaxed accesses.

This ordering is only applicable for operations that combine both loads and stores.

Corresponds to memory_order_acq_rel in C++20.

SeqCst

Like Acquire/Release/AcqRel (for load, store, and load-with-store operations, respectively) with the additional guarantee that all threads see all sequentially consistent operations in the same order.

Corresponds to memory_order_seq_cst in C++20.

總結

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