Waker in std::task - Rust
Description: A `Waker` is a handle for waking up a task by notifying its executor that it is ready to be run.

## Waker

# Struct Waker

1.36.0 · Source

    pub struct Waker { /* private fields */ }

Expand description

A `Waker` is a handle for waking up a task by notifying its executor that it is ready to be run.

This handle encapsulates a `RawWaker` instance, which defines the executor-specific wakeup behavior.

The typical life of a `Waker` is that it is constructed by an executor, wrapped in a `Context`, then passed to `Future::poll()`. Then, if the future chooses to return `Poll::Pending`, it must also store the waker somehow and call `Waker::wake()` when the future should be polled again.

Implements `Clone`, `Send`, and `Sync`; therefore, a waker may be invoked from any thread, including ones not in any way managed by the executor. For example, this might be done to wake a future when a blocking function call completes on another thread.

Note that it is preferable to use `waker.clone_from(&new_waker)` instead of `*waker = new_waker.clone()`, as the former will avoid cloning the waker unnecessarily if the two wakers wake the same task.

Constructing a `Waker` from a `RawWaker` is unsafe. Implementing the `Wake` trait is a safe alternative that requires memory allocation.

## Implementations§

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### impl Waker

1.36.0 · Source

#### pub fn wake(self)

Wakes up the task associated with this `Waker`.

As long as the executor keeps running and the task is not finished, it is guaranteed that each invocation of `wake()` (or `wake_by_ref()`) will be followed by at least one `poll()` of the task to which this `Waker` belongs. This makes it possible to temporarily yield to other tasks while running potentially unbounded processing loops.

Note that the above implies that multiple wake-ups may be coalesced into a single `poll()` invocation by the runtime.

Also note that yielding to competing tasks is not guaranteed: it is the executor’s choice which task to run and the executor may choose to run the current task again.

1.36.0 · Source

#### pub fn wake_by_ref(&self)

Wakes up the task associated with this `Waker` without consuming the `Waker`.

This is similar to `wake()`, but may be slightly less efficient in the case where an owned `Waker` is available. This method should be preferred to calling `waker.clone().wake()`.

1.36.0 · Source

#### pub fn will_wake(&self, other: &Waker) -> bool

Returns `true` if this `Waker` and another `Waker` would awake the same task.

This function works on a best-effort basis, and may return false even when the `Waker`s would awaken the same task. However, if this function returns `true`, it is guaranteed that the `Waker`s will awaken the same task.

This function is primarily used for optimization purposes — for example, this type’s `clone_from` implementation uses it to avoid cloning the waker when they would wake the same task anyway.

1.83.0 (const: 1.83.0) · Source

#### pub const unsafe fn new( data: *const (), vtable: &'static RawWakerVTable, ) -> Waker

Creates a new `Waker` from the provided `data` pointer and `vtable`.

The `data` pointer can be used to store arbitrary data as required by the executor. This could be e.g. a type-erased pointer to an `Arc` that is associated with the task. The value of this pointer will get passed to all functions that are part of the `vtable` as the first parameter.

It is important to consider that the `data` pointer must point to a thread safe type such as an `Arc`.

The `vtable` customizes the behavior of a `Waker`. For each operation on the `Waker`, the associated function in the `vtable` will be called.

##### §Safety

The behavior of the returned `Waker` is undefined if the contract defined in `RawWakerVTable`’s documentation is not upheld.

(Authors wishing to avoid unsafe code may implement the `Wake` trait instead, at the cost of a required heap allocation.)

1.36.0 (const: 1.82.0) · Source

#### pub const unsafe fn from_raw(waker: RawWaker) -> Waker

Creates a new `Waker` from `RawWaker`.

##### §Safety

The behavior of the returned `Waker` is undefined if the contract defined in `RawWaker`’s and `RawWakerVTable`’s documentation is not upheld.

(Authors wishing to avoid unsafe code may implement the `Wake` trait instead, at the cost of a required heap allocation.)

1.85.0 (const: 1.85.0) · Source

#### pub const fn noop() -> &'static Waker

Returns a reference to a `Waker` that does nothing when used.

This is mostly useful for writing tests that need a `Context` to poll some futures, but are not expecting those futures to wake the waker or do not need to do anything specific if it happens.

More generally, using `Waker::noop()` to poll a future means discarding the notification of when the future should be polled again. So it should only be used when such a notification will not be needed to make progress.

If an owned `Waker` is needed, `clone()` this one.

##### §Examples

    use std::future::Future;
    use std::task;

    let mut cx = task::Context::from_waker(task::Waker::noop());

    let mut future = Box::pin(async { 10 });
    assert_eq!(future.as_mut().poll(&mut cx), task::Poll::Ready(10));

1.83.0 · Source

#### pub fn data(&self) -> *const ()

Gets the `data` pointer used to create this `Waker`.

1.83.0 · Source

#### pub fn vtable(&self) -> &'static RawWakerVTable

Gets the `vtable` pointer used to create this `Waker`.

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#### pub const fn from_fn_ptr(f: fn()) -> Waker

🔬This is a nightly-only experimental API. (`waker_from_fn_ptr` #148457)

Constructs a `Waker` from a function pointer.

## Trait Implementations§

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### impl AsRef<LocalWaker> for Waker

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#### fn as_ref(&self) -> &LocalWaker

Converts this type into a shared reference of the (usually inferred) input type.
1.36.0 · Source§

### impl Clone for Waker

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#### fn clone_from(&mut self, source: &Waker)

Assigns a clone of `source` to `self`, unless `self.will_wake(source)` anyway.

This method is preferred over simply assigning `source.clone()` to `self`, as it avoids cloning the waker if `self` is already the same waker.

##### §Examples

    use std::future::Future;
    use std::pin::Pin;
    use std::sync::{Arc, Mutex};
    use std::task::{Context, Poll, Waker};

    struct Waiter {
        shared: Arc<Mutex<Shared>>,
    }

    struct Shared {
        waker: Waker,
        // ...
    }

    impl Future for Waiter {
        type Output = ();
        fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<()> {
            let mut shared = self.shared.lock().unwrap();

            // update the waker
            shared.waker.clone_from(cx.waker());

            // readiness logic ...
        }
    }

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#### fn clone(&self) -> Waker

Returns a duplicate of the value. Read more
1.36.0 · Source§

### impl Debug for Waker

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#### fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter. Read more
1.36.0 · Source§

### impl Drop for Waker

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#### fn drop(&mut self)

Executes the destructor for this type. Read more
1.51.0 · Source§

### impl<W> From<Arc<W>> for Waker
where W: Wake + Send + Sync + 'static,

Available on `target_has_atomic=ptr` only.
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#### fn from(waker: Arc<W>) -> Waker

Use a `Wake`-able type as a `Waker`.

No heap allocations or atomic operations are used for this conversion.

1.36.0 · Source§

### impl Send for Waker

1.36.0 · Source§

### impl Sync for Waker

1.36.0 · Source§

### impl Unpin for Waker

## Auto Trait Implementations§

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### impl Freeze for Waker

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### impl RefUnwindSafe for Waker

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### impl UnwindSafe for Waker

## Blanket Implementations§

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### impl<T> Any for T
where T: 'static + ?Sized,

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#### fn type_id(&self) -> TypeId

Gets the `TypeId` of `self`. Read more
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### impl<T> Borrow<T> for T
where T: ?Sized,

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#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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### impl<T> BorrowMut<T> for T
where T: ?Sized,

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#### fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
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### impl<T> CloneToUninit for T
where T: Clone,

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#### unsafe fn clone_to_uninit(&self, dest: *mut u8)

🔬This is a nightly-only experimental API. (`clone_to_uninit` #126799)
Performs copy-assignment from `self` to `dest`. Read more
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### impl<T> From<T> for T

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#### fn from(t: T) -> T

Returns the argument unchanged.

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### impl<T, U> Into<U> for T
where U: From<T>,

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#### fn into(self) -> U

Calls `U::from(self)`.

That is, this conversion is whatever the implementation of `From<T> for U` chooses to do.

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### impl<T> ToOwned for T
where T: Clone,

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#### type Owned = T

The resulting type after obtaining ownership.
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#### fn to_owned(&self) -> T

Creates owned data from borrowed data, usually by cloning. Read more
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#### fn clone_into(&self, target: &mut T)

Uses borrowed data to replace owned data, usually by cloning. Read more
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### impl<T, U> TryFrom<U> for T
where U: Into<T>,

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#### type Error = Infallible

The type returned in the event of a conversion error.
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#### fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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### impl<T, U> TryInto<U> for T
where U: TryFrom<T>,

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#### type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
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#### fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.