pub enum Ordering {
Relaxed,
Release,
Acquire,
AcqRel,
SeqCst,
}Expand description
Atomic memory orderings
Memory orderings specify the way atomic operations synchronize memory.
In its weakest Ordering::Relaxed, only the memory directly touched by the
operation is synchronized. On the other hand, a store-load pair of Ordering::SeqCst
operations synchronize other memory while additionally preserving a total order of such
operations across all threads.
Rust’s memory orderings are the same as those of C++20.
For more information see the nomicon.
Variants (Non-exhaustive)§
This enum is marked as non-exhaustive
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.