#[lang = "sync"] pub unsafe trait Sync { }
Types for which it is safe to share references between threads.
This trait is automatically implemented when the compiler determines it's appropriate.
The precise definition is: a type T
is Sync
if &T
is Send
. In other words, if there is no possibility of undefined behavior (including data races) when passing &T
references between threads.
As one would expect, primitive types like u8
and f64
are all Sync
, and so are simple aggregate types containing them, like tuples, structs and enums. More examples of basic Sync
types include "immutable" types like &T
, and those with simple inherited mutability, such as Box<T>
, Vec<T>
and most other collection types. (Generic parameters need to be Sync
for their container to be Sync
.)
A somewhat surprising consequence of the definition is that &mut T
is Sync
(if T
is Sync
) even though it seems like that might provide unsynchronized mutation. The trick is that a mutable reference behind a shared reference (that is, & &mut T
) becomes read-only, as if it were a & &T
. Hence there is no risk of a data race.
Types that are not Sync
are those that have "interior mutability" in a non-thread-safe form, such as cell::Cell
and cell::RefCell
. These types allow for mutation of their contents even through an immutable, shared reference. For example the set
method on Cell<T>
takes &self
, so it requires only a shared reference &Cell<T>
. The method performs no synchronization, thus Cell
cannot be Sync
.
Another example of a non-Sync
type is the reference-counting pointer rc::Rc
. Given any reference &Rc<T>
, you can clone a new Rc<T>
, modifying the reference counts in a non-atomic way.
For cases when one does need thread-safe interior mutability, Rust provides atomic data types, as well as explicit locking via sync::Mutex
and sync::RWLock
. These types ensure that any mutation cannot cause data races, hence the types are Sync
. Likewise, sync::Arc
provides a thread-safe analogue of Rc
.
Any types with interior mutability must also use the cell::UnsafeCell
wrapper around the value(s) which can be mutated through a shared reference. Failing to doing this is undefined behavior. For example, transmute
-ing from &T
to &mut T
is invalid.
See the Nomicon for more details about Sync
.
impl<'a> Sync for std::string::Drain<'a>
impl<T> Sync for IntoIter<T> where
T: Sync,
impl<'a, T> Sync for std::collections::linked_list::IterMut<'a, T> where
T: Sync,
impl<T> Sync for std::sync::Weak<T> where
T: Send + Sync + ?Sized,
impl<T> Sync for LinkedList<T> where
T: Sync,
impl<'a, T> Sync for std::vec::Drain<'a, T> where
T: Sync,
impl<'a, T> Sync for std::collections::linked_list::Iter<'a, T> where
T: Sync,
impl<T> !Sync for Rc<T> where
T: ?Sized,
impl<T> Sync for Arc<T> where
T: Send + Sync + ?Sized,
impl<'a, T> Sync for std::collections::vec_deque::Drain<'a, T> where
T: Sync,
impl<T> !Sync for std::rc::Weak<T> where
T: ?Sized,
impl Sync for AtomicI32
impl<T> !Sync for Shared<T> where
T: ?Sized,
impl Sync for AtomicU16
impl<'a, T> Sync for std::slice::Iter<'a, T> where
T: Sync,
impl<T> Sync for Unique<T> where
T: Sync + ?Sized,
impl Sync for AtomicU32
impl Sync for AtomicU64
impl Sync for AtomicBool
impl Sync for AtomicI16
impl Sync for AtomicIsize
impl<T> !Sync for UnsafeCell<T> where
T: ?Sized,
impl<T> !Sync for Cell<T>
impl<T> !Sync for RefCell<T> where
T: ?Sized,
impl Sync for AtomicUsize
impl<T> Sync for AtomicPtr<T>
impl<T> !Sync for *const T where
T: ?Sized,
impl Sync for AtomicI8
impl Sync for AtomicU8
impl<'a, T> Sync for std::slice::IterMut<'a, T> where
T: Sync,
impl<T> !Sync for *mut T where
T: ?Sized,
impl Sync for AtomicI64
impl<T> !Sync for Receiver<T>
impl<T> !Sync for Sender<T>
impl<T: ?Sized + Send> Sync for Mutex<T>
impl<'a, T: ?Sized + Sync> Sync for MutexGuard<'a, T>
impl Sync for Once
impl<T: ?Sized + Send + Sync> Sync for RwLock<T>
impl<'a, T: ?Sized + Sync> Sync for RwLockReadGuard<'a, T>
impl<'a, T: ?Sized + Sync> Sync for RwLockWriteGuard<'a, T>
© 2010 The Rust Project Developers
Licensed under the Apache License, Version 2.0 or the MIT license, at your option.
https://doc.rust-lang.org/std/marker/trait.Sync.html