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struct.RwLock

pub struct RwLock<T> where
    T: ?Sized,  { /* fields omitted */ }

An asynchronous reader-writer lock.

This type of lock allows a number of readers or at most one writer at any point in time. The write portion of this lock typically allows modification of the underlying data (exclusive access) and the read portion of this lock typically allows for read-only access (shared access).

In comparison, a Mutex does not distinguish between readers or writers that acquire the lock, therefore causing any tasks waiting for the lock to become available to yield. An RwLock will allow any number of readers to acquire the lock as long as a writer is not holding the lock.

The priority policy of Tokio’s read-write lock is fair (or write-preferring), in order to ensure that readers cannot starve writers. Fairness is ensured using a first-in, first-out queue for the tasks awaiting the lock; if a task that wishes to acquire the write lock is at the head of the queue, read locks will not be given out until the write lock has been released. This is in contrast to the Rust standard library’s std::sync::RwLock, where the priority policy is dependent on the operating system’s implementation.

The type parameter T represents the data that this lock protects. It is required that T satisfies Send to be shared across threads. The RAII guards returned from the locking methods implement Deref (and DerefMut for the write methods) to allow access to the content of the lock.

Examples#

use tokio::sync::RwLock;

#[tokio::main]
async fn main() {
    let lock = RwLock::new(5);

    // many reader locks can be held at once
    {
        let r1 = lock.read().await;
        let r2 = lock.read().await;
        assert_eq!(*r1, 5);
        assert_eq!(*r2, 5);
    } // read locks are dropped at this point

    // only one write lock may be held, however
    {
        let mut w = lock.write().await;
        *w += 1;
        assert_eq!(*w, 6);
    } // write lock is dropped here
}

Implementations#

impl<T> RwLock<T> where T: ?Sized,#

pub fn new(value: T) -> RwLock<T>#

Creates a new instance of an RwLock<T> which is unlocked.

Examples#

use tokio::sync::RwLock;

let lock = RwLock::new(5);

pub fn with_max_readers(value: T, max_reads: u32) -> RwLock<T>#

Creates a new instance of an RwLock<T> which is unlocked and allows a maximum of max_reads concurrent readers.

Examples#

use tokio::sync::RwLock;

let lock = RwLock::with_max_readers(5, 1024);

Panics#

Panics if max_reads is more than u32::MAX >> 3.

pub async fn read(&'_ self) -> RwLockReadGuard<'_, T>#

Locks this RwLock with shared read access, causing the current task to yield until the lock has been acquired.

The calling task will yield until there are no writers which hold the lock. There may be other readers inside the lock when the task resumes.

Note that under the priority policy of RwLock, read locks are not granted until prior write locks, to prevent starvation. Therefore deadlock may occur if a read lock is held by the current task, a write lock attempt is made, and then a subsequent read lock attempt is made by the current task.

Returns an RAII guard which will drop this read access of the RwLock when dropped.

Examples#

use std::sync::Arc;
use tokio::sync::RwLock;

#[tokio::main]
async fn main() {
    let lock = Arc::new(RwLock::new(1));
    let c_lock = lock.clone();

    let n = lock.read().await;
    assert_eq!(*n, 1);

    tokio::spawn(async move {
        // While main has an active read lock, we acquire one too.
        let r = c_lock.read().await;
        assert_eq!(*r, 1);
    }).await.expect("The spawned task has panicked");

    // Drop the guard after the spawned task finishes.
    drop(n);
}

pub async fn read_owned(self: Arc<RwLock<T>>) -> OwnedRwLockReadGuard<T, T>#

Locks this RwLock with shared read access, causing the current task to yield until the lock has been acquired.

The calling task will yield until there are no writers which hold the lock. There may be other readers inside the lock when the task resumes.

This method is identical to RwLock::read, except that the returned guard references the RwLock with an Arc rather than by borrowing it. Therefore, the RwLock must be wrapped in an Arc to call this method, and the guard will live for the 'static lifetime, as it keeps the RwLock alive by holding an Arc.

Note that under the priority policy of RwLock, read locks are not granted until prior write locks, to prevent starvation. Therefore deadlock may occur if a read lock is held by the current task, a write lock attempt is made, and then a subsequent read lock attempt is made by the current task.

Returns an RAII guard which will drop this read access of the RwLock when dropped.

Examples#

use std::sync::Arc;
use tokio::sync::RwLock;

#[tokio::main]
async fn main() {
    let lock = Arc::new(RwLock::new(1));
    let c_lock = lock.clone();

    let n = lock.read_owned().await;
    assert_eq!(*n, 1);

    tokio::spawn(async move {
        // While main has an active read lock, we acquire one too.
        let r = c_lock.read_owned().await;
        assert_eq!(*r, 1);
    }).await.expect("The spawned task has panicked");

    // Drop the guard after the spawned task finishes.
    drop(n);
}

pub fn try_read(&self) -> Result<RwLockReadGuard<'_, T>, TryLockError>#

Attempts to acquire this RwLock with shared read access.

If the access couldn’t be acquired immediately, returns TryLockError. Otherwise, an RAII guard is returned which will release read access when dropped.

Examples#

use std::sync::Arc;
use tokio::sync::RwLock;

#[tokio::main]
async fn main() {
    let lock = Arc::new(RwLock::new(1));
    let c_lock = lock.clone();

    let v = lock.try_read().unwrap();
    assert_eq!(*v, 1);

    tokio::spawn(async move {
        // While main has an active read lock, we acquire one too.
        let n = c_lock.read().await;
        assert_eq!(*n, 1);
    }).await.expect("The spawned task has panicked");

    // Drop the guard when spawned task finishes.
    drop(v);
}

pub fn try_read_owned( self: Arc<RwLock<T>> ) -> Result<OwnedRwLockReadGuard<T, T>, TryLockError>#

Attempts to acquire this RwLock with shared read access.

If the access couldn’t be acquired immediately, returns TryLockError. Otherwise, an RAII guard is returned which will release read access when dropped.

This method is identical to RwLock::try_read, except that the returned guard references the RwLock with an Arc rather than by borrowing it. Therefore, the RwLock must be wrapped in an Arc to call this method, and the guard will live for the 'static lifetime, as it keeps the RwLock alive by holding an Arc.

Examples#

use std::sync::Arc;
use tokio::sync::RwLock;

#[tokio::main]
async fn main() {
    let lock = Arc::new(RwLock::new(1));
    let c_lock = lock.clone();

    let v = lock.try_read_owned().unwrap();
    assert_eq!(*v, 1);

    tokio::spawn(async move {
        // While main has an active read lock, we acquire one too.
        let n = c_lock.read_owned().await;
        assert_eq!(*n, 1);
    }).await.expect("The spawned task has panicked");

    // Drop the guard when spawned task finishes.
    drop(v);
}

pub async fn write(&'_ self) -> RwLockWriteGuard<'_, T>#

Locks this RwLock with exclusive write access, causing the current task to yield until the lock has been acquired.

The calling task will yield while other writers or readers currently have access to the lock.

Returns an RAII guard which will drop the write access of this RwLock when dropped.

Examples#

use tokio::sync::RwLock;

#[tokio::main]
async fn main() {
  let lock = RwLock::new(1);

  let mut n = lock.write().await;
  *n = 2;
}

pub async fn write_owned(self: Arc<RwLock<T>>) -> OwnedRwLockWriteGuard<T>#

Locks this RwLock with exclusive write access, causing the current task to yield until the lock has been acquired.

The calling task will yield while other writers or readers currently have access to the lock.

This method is identical to RwLock::write, except that the returned guard references the RwLock with an Arc rather than by borrowing it. Therefore, the RwLock must be wrapped in an Arc to call this method, and the guard will live for the 'static lifetime, as it keeps the RwLock alive by holding an Arc.

Returns an RAII guard which will drop the write access of this RwLock when dropped.

Examples#

use std::sync::Arc;
use tokio::sync::RwLock;

#[tokio::main]
async fn main() {
  let lock = Arc::new(RwLock::new(1));

  let mut n = lock.write_owned().await;
  *n = 2;
}

pub fn try_write(&self) -> Result<RwLockWriteGuard<'_, T>, TryLockError>#

Attempts to acquire this RwLock with exclusive write access.

If the access couldn’t be acquired immediately, returns TryLockError. Otherwise, an RAII guard is returned which will release write access when dropped.

Examples#

use tokio::sync::RwLock;

#[tokio::main]
async fn main() {
    let rw = RwLock::new(1);

    let v = rw.read().await;
    assert_eq!(*v, 1);

    assert!(rw.try_write().is_err());
}

pub fn try_write_owned( self: Arc<RwLock<T>> ) -> Result<OwnedRwLockWriteGuard<T>, TryLockError>#

Attempts to acquire this RwLock with exclusive write access.

If the access couldn’t be acquired immediately, returns TryLockError. Otherwise, an RAII guard is returned which will release write access when dropped.

This method is identical to RwLock::try_write, except that the returned guard references the RwLock with an Arc rather than by borrowing it. Therefore, the RwLock must be wrapped in an Arc to call this method, and the guard will live for the 'static lifetime, as it keeps the RwLock alive by holding an Arc.

Examples#

use std::sync::Arc;
use tokio::sync::RwLock;

#[tokio::main]
async fn main() {
    let rw = Arc::new(RwLock::new(1));

    let v = Arc::clone(&rw).read_owned().await;
    assert_eq!(*v, 1);

    assert!(rw.try_write_owned().is_err());
}

pub fn get_mut(&mut self) -> &mutT#

Returns a mutable reference to the underlying data.

Since this call borrows the RwLock mutably, no actual locking needs to take place – the mutable borrow statically guarantees no locks exist.

Examples#

use tokio::sync::RwLock;

fn main() {
    let mut lock = RwLock::new(1);

    let n = lock.get_mut();
    *n = 2;
}

pub fn into_inner(self) -> T#

Consumes the lock, returning the underlying data.

Trait Implementations#

impl<T> Debug for RwLock<T> where T: Debug + ?Sized,#

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

Formats the value using the given formatter. Read more

impl<T> Default for RwLock<T> where T: Default + ?Sized,#

pub fn default() -> RwLock<T>#

Returns the “default value” for a type. Read more

impl<T> From<T> for RwLock<T>#

pub fn from(s: T) -> RwLock<T>#

Performs the conversion.

impl<T> Send for RwLock<T> where T: Send + ?Sized,#

impl<T> Sync for RwLock<T> where T: Send + Sync + ?Sized,#

Auto Trait Implementations#

impl<T> !RefUnwindSafe for RwLock<T>#

impl<T: ?Sized> Unpin for RwLock<T> where T: Unpin,#

impl<T: ?Sized> UnwindSafe for RwLock<T> where T: UnwindSafe,#

Blanket Implementations#

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

pub fn type_id(&self) -> TypeId#

Gets the TypeId of self. Read more

impl<T> Borrow<T> for T where T: ?Sized,#

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

Immutably borrows from an owned value. Read more

impl<T> BorrowMut<T> for T where T: ?Sized,#

pub fn borrow_mut(&mut self) -> &mutT#

Mutably borrows from an owned value. Read more

impl<T> From<!> for T#

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

Performs the conversion.

impl<T> From<T> for T#

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

Performs the conversion.

impl<T> Instrument for T#

pub fn instrument(self, span: Span) -> Instrumented<Self>#

Instruments this type with the provided Span, returning an Instrumented wrapper. Read more

pub fn in_current_span(self) -> Instrumented<Self>#

Instruments this type with the current Span, returning an Instrumented wrapper. Read more

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

pub fn into(self) -> U#

Performs the conversion.

impl<T> Pointable for T#

pub const ALIGN: usize#

The alignment of pointer.

type Init = T#

The type for initializers.

pub unsafe fn init(init: <T as Pointable>::Init) -> usize#

Initializes a with the given initializer. Read more

pub unsafe fn deref<'a>(ptr: usize) -> &'aT#

Dereferences the given pointer. Read more

pub unsafe fn deref_mut<'a>(ptr: usize) -> &'a mutT#

Mutably dereferences the given pointer. Read more

pub unsafe fn drop(ptr: usize)#

Drops the object pointed to by the given pointer. Read more

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

type Error = Infallible#

The type returned in the event of a conversion error.

pub fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>#

Performs the conversion.

impl<T, U> TryInto<U> for T where U: TryFrom<T>,#

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

The type returned in the event of a conversion error.

pub fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>#

Performs the conversion.

impl<V, T> VZip<V> for T where V: MultiLane<T>,#

pub fn vzip(self) -> V#