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.

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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.

Original

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.

Original

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