Trait frame_support::dispatch::marker::Send

1.0.0 · source · 
pub unsafe auto trait Send { }
Expand description

Types that can be transferred across thread boundaries.

This trait is automatically implemented when the compiler determines it’s appropriate.

An example of a non-Send type is the reference-counting pointer rc::Rc. If two threads attempt to clone Rcs that point to the same reference-counted value, they might try to update the reference count at the same time, which is undefined behavior because Rc doesn’t use atomic operations. Its cousin sync::Arc does use atomic operations (incurring some overhead) and thus is Send.

See the Nomicon for more details.

Implementors§

Dir implements Send but not Sync, because we use readdir which is not guaranteed to be thread-safe. Users can wrap this in a Mutex if they need Sync, which is effectively what’d need to do to implement Sync ourselves.

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NonNull pointers are not Send because the data they reference may be aliased.

ThinBox<T> is Send if T is Send because the data is owned.

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Bit-Slice Thread Safety

This allows bit-slice references to be moved across thread boundaries only when the underlying T element can tolerate concurrency.

All BitSlice references, shared or exclusive, are only threadsafe if the T element type is Send, because any given bit-slice reference may only have partial control of a memory element that is also being shared by a bit-slice reference on another thread. As such, this is never implemented for Cell<U>, but always implemented for AtomicU and U for a given unsigned integer type U.

Atomic integers safely handle concurrent writes, cells do not allow concurrency at all, so the only missing piece is &mut BitSlice<_, U: Unsigned>. This is handled by the aliasing system that the mutable splitters employ: a mutable reference to an unsynchronized bit-slice can only cross threads when no other handle is able to exist to the elements it governs. Splitting a mutable bit-slice causes the split halves to change over to either atomics or cells, so concurrency is either safe or impossible.

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Auto implementors§