Struct RwLock

pub struct RwLock<T> { /* private fields */ }

Available on crate feature sync only.

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 blocking any tasks waiting for the lock to become available. 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 the lock is dependent on the underlying operating system’s implementation, and this type does not guarantee that any particular policy will be used.

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>

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

Locks this rwlock with shared read access, blocking the current task until it can be acquired.

The calling task will be blocked until there are no more writers which hold the lock. There may be other readers currently inside the lock when this method returns.

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 {
        let r = c_lock.read().await;
        assert_eq!(*r, 1);
    });
}

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

Locks this rwlock with exclusive write access, blocking the current task until it can be acquired.

This function will not return while other writers or other 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;
}

Trait Implementations

impl<T: Debug> Debug for RwLock<T>

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

Formats the value using the given formatter.

impl<T> Default for RwLock<T>

where T: Default,

fn default() -> Self

Returns the “default value” for a type.

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

fn from(s: T) -> Self

Converts to this type from the input type.

impl<T> Send for RwLock<T>

where T: Send,

impl<T> Sync for RwLock<T>

where T: Send + Sync,