Struct RwLock

struct RwLock<T: ?Sized> { ... }

A 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 threads 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. In particular, a writer which is waiting to acquire the lock in write might or might not block concurrent calls to read, e.g.:

Potential deadlock example 
// Thread 1              |  // Thread 2
let _rg1 = lock.read();  |
                         |  // will block
                         |  let _wg = lock.write();
// may deadlock          |
let _rg2 = lock.read();  |

The type parameter T represents the data that this lock protects. It is required that T satisfies Send to be shared across threads and Sync to allow concurrent access through readers. 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.

Poisoning

An RwLock, like Mutex, will usually become poisoned on a panic. Note, however, that an RwLock may only be poisoned if a panic occurs while it is locked exclusively (write mode). If a panic occurs in any reader, then the lock will not be poisoned.

Examples

use std::sync::{Arc, RwLock};
use std::thread;
use std::time::Duration;

let data = Arc::new(RwLock::new(5));

// Multiple readers can access in parallel.
for i in 0..3 {
    let lock_clone = Arc::clone(&data);

    thread::spawn(move || {
        let value = lock_clone.read().unwrap();

        println!("Reader {}: Read value {}, now holding lock...", i, *value);

        // Simulating a long read operation
        thread::sleep(Duration::from_secs(1));

        println!("Reader {}: Dropping lock.", i);
        // Read lock unlocked when going out of scope.
    });
}

thread::sleep(Duration::from_millis(100));  // Wait for readers to start

// While all readers can proceed, a call to .write() has to wait for
let mut writable_data = data.write().unwrap();
println!("Writer proceeds...");
*writable_data += 1;

Implementations

impl<T> RwLock<T>

const fn new(t: T) -> RwLock<T>

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

Examples

use std::sync::RwLock;

let lock = RwLock::new(5);

fn get_cloned(self: &Self) -> Result<T, PoisonError<()>>
where
    T: Clone

Returns the contained value by cloning it.

Errors

This function will return an error if the RwLock is poisoned. An RwLock is poisoned whenever a writer panics while holding an exclusive lock.

Examples

#![feature(lock_value_accessors)]

use std::sync::RwLock;

let mut lock = RwLock::new(7);

assert_eq!(lock.get_cloned().unwrap(), 7);

fn set(self: &Self, value: T) -> Result<(), PoisonError<T>>

Sets the contained value.

Errors

This function will return an error containing the provided value if the RwLock is poisoned. An RwLock is poisoned whenever a writer panics while holding an exclusive lock.

Examples

#![feature(lock_value_accessors)]

use std::sync::RwLock;

let mut lock = RwLock::new(7);

assert_eq!(lock.get_cloned().unwrap(), 7);
lock.set(11).unwrap();
assert_eq!(lock.get_cloned().unwrap(), 11);

fn replace(self: &Self, value: T) -> LockResult<T>

Replaces the contained value with value, and returns the old contained value.

Errors

This function will return an error containing the provided value if the RwLock is poisoned. An RwLock is poisoned whenever a writer panics while holding an exclusive lock.

Examples

#![feature(lock_value_accessors)]

use std::sync::RwLock;

let mut lock = RwLock::new(7);

assert_eq!(lock.replace(11).unwrap(), 7);
assert_eq!(lock.get_cloned().unwrap(), 11);

impl<T: ?Sized> RwLock<T>

fn read(self: &Self) -> LockResult<RwLockReadGuard<'_, T>>

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

The calling thread 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. This method does not provide any guarantees with respect to the ordering of whether contentious readers or writers will acquire the lock first.

Returns an RAII guard which will release this thread's shared access once it is dropped.

Errors

This function will return an error if the RwLock is poisoned. An RwLock is poisoned whenever a writer panics while holding an exclusive lock. The failure will occur immediately after the lock has been acquired. The acquired lock guard will be contained in the returned error.

Panics

This function might panic when called if the lock is already held by the current thread in read or write mode.

Examples

use std::sync::{Arc, RwLock};
use std::thread;

let lock = Arc::new(RwLock::new(1));
let c_lock = Arc::clone(&lock);

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

thread::spawn(move || {
    let r = c_lock.read();
    assert!(r.is_ok());
}).join().unwrap();

fn try_read(self: &Self) -> TryLockResult<RwLockReadGuard<'_, T>>

Attempts to acquire this RwLock with shared read access.

If the access could not be granted at this time, then Err is returned. Otherwise, an RAII guard is returned which will release the shared access when it is dropped.

This function does not block.

This function does not provide any guarantees with respect to the ordering of whether contentious readers or writers will acquire the lock first.

Errors

This function will return the Poisoned error if the RwLock is poisoned. An RwLock is poisoned whenever a writer panics while holding an exclusive lock. Poisoned will only be returned if the lock would have otherwise been acquired. An acquired lock guard will be contained in the returned error.

This function will return the WouldBlock error if the RwLock could not be acquired because it was already locked exclusively.

Examples

use std::sync::RwLock;

let lock = RwLock::new(1);

match lock.try_read() {
    Ok(n) => assert_eq!(*n, 1),
    Err(_) => unreachable!(),
};

fn write(self: &Self) -> LockResult<RwLockWriteGuard<'_, T>>

Locks this RwLock with exclusive write access, blocking the current thread 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.

Errors

This function will return an error if the RwLock is poisoned. An RwLock is poisoned whenever a writer panics while holding an exclusive lock. An error will be returned when the lock is acquired. The acquired lock guard will be contained in the returned error.

Panics

This function might panic when called if the lock is already held by the current thread in read or write mode.

Examples

use std::sync::RwLock;

let lock = RwLock::new(1);

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

assert!(lock.try_read().is_err());

fn try_write(self: &Self) -> TryLockResult<RwLockWriteGuard<'_, T>>

Attempts to lock this RwLock with exclusive write access.

If the lock could not be acquired at this time, then Err is returned. Otherwise, an RAII guard is returned which will release the lock when it is dropped.

This function does not block.

This function does not provide any guarantees with respect to the ordering of whether contentious readers or writers will acquire the lock first.

Errors

This function will return the Poisoned error if the RwLock is poisoned. An RwLock is poisoned whenever a writer panics while holding an exclusive lock. Poisoned will only be returned if the lock would have otherwise been acquired. An acquired lock guard will be contained in the returned error.

This function will return the WouldBlock error if the RwLock could not be acquired because it was already locked.

Examples

use std::sync::RwLock;

let lock = RwLock::new(1);

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

assert!(lock.try_write().is_err());

fn is_poisoned(self: &Self) -> bool

Determines whether the lock is poisoned.

If another thread is active, the lock can still become poisoned at any time. You should not trust a false value for program correctness without additional synchronization.

Examples

use std::sync::{Arc, RwLock};
use std::thread;

let lock = Arc::new(RwLock::new(0));
let c_lock = Arc::clone(&lock);

let _ = thread::spawn(move || {
    let _lock = c_lock.write().unwrap();
    panic!(); // the lock gets poisoned
}).join();
assert_eq!(lock.is_poisoned(), true);

fn clear_poison(self: &Self)

Clear the poisoned state from a lock.

If the lock is poisoned, it will remain poisoned until this function is called. This allows recovering from a poisoned state and marking that it has recovered. For example, if the value is overwritten by a known-good value, then the lock can be marked as un-poisoned. Or possibly, the value could be inspected to determine if it is in a consistent state, and if so the poison is removed.

Examples

use std::sync::{Arc, RwLock};
use std::thread;

let lock = Arc::new(RwLock::new(0));
let c_lock = Arc::clone(&lock);

let _ = thread::spawn(move || {
    let _lock = c_lock.write().unwrap();
    panic!(); // the lock gets poisoned
}).join();

assert_eq!(lock.is_poisoned(), true);
let guard = lock.write().unwrap_or_else(|mut e| {
    **e.get_mut() = 1;
    lock.clear_poison();
    e.into_inner()
});
assert_eq!(lock.is_poisoned(), false);
assert_eq!(*guard, 1);

fn into_inner(self: Self) -> LockResult<T>
where
    T: Sized

Consumes this RwLock, returning the underlying data.

Errors

This function will return an error containing the underlying data if the RwLock is poisoned. An RwLock is poisoned whenever a writer panics while holding an exclusive lock. An error will only be returned if the lock would have otherwise been acquired.

Examples

use std::sync::RwLock;

let lock = RwLock::new(String::new());
{
    let mut s = lock.write().unwrap();
    *s = "modified".to_owned();
}
assert_eq!(lock.into_inner().unwrap(), "modified");

fn get_mut(self: &mut Self) -> LockResult<&mut T>

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 new locks can be acquired while this reference exists. Note that this method does not clear any previously abandoned locks (e.g., via [forget()] on a RwLockReadGuard or RwLockWriteGuard).

Errors

This function will return an error containing a mutable reference to the underlying data if the RwLock is poisoned. An RwLock is poisoned whenever a writer panics while holding an exclusive lock. An error will only be returned if the lock would have otherwise been acquired.

Examples

use std::sync::RwLock;

let mut lock = RwLock::new(0);
*lock.get_mut().unwrap() = 10;
assert_eq!(*lock.read().unwrap(), 10);

const fn data_ptr(self: &Self) -> *mut T

Returns a raw pointer to the underlying data.

The returned pointer is always non-null and properly aligned, but it is the user's responsibility to ensure that any reads and writes through it are properly synchronized to avoid data races, and that it is not read or written through after the lock is dropped.

impl<T> Any for RwLock<T>

fn type_id(self: &Self) -> TypeId

impl<T> Borrow for RwLock<T>

fn borrow(self: &Self) -> &T

impl<T> BorrowMut for RwLock<T>

fn borrow_mut(self: &mut Self) -> &mut T

impl<T> Freeze for RwLock<T>

impl<T> From for RwLock<T>

fn from(t: T) -> T

Returns the argument unchanged.

impl<T> From for RwLock<T>

fn from(t: T) -> Self

Creates a new instance of an RwLock<T> which is unlocked. This is equivalent to RwLock::new.

impl<T> From for RwLock<T>

fn from(t: never) -> T

impl<T> Unpin for RwLock<T>

impl<T> UnsafeUnpin for RwLock<T>

impl<T, U> Into for RwLock<T>

fn into(self: Self) -> U

Calls U::from(self).

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

impl<T, U> TryFrom for RwLock<T>

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

impl<T, U> TryInto for RwLock<T>

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

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

impl<T: ?Sized + Send> Send for RwLock<T>

impl<T: ?Sized + fmt::Debug> Debug for RwLock<T>

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

impl<T: ?Sized> RefUnwindSafe for RwLock<T>

impl<T: ?Sized> UnwindSafe for RwLock<T>

impl<T: Default> Default for RwLock<T>

fn default() -> RwLock<T>

Creates a new RwLock<T>, with the Default value for T.