# Struct std::sync::RwLock

1.0.0 · source ·

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

Expand description

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 _rg = lock.read();  |
                            |  // will block
                            |  let _wg = lock.write();
    // may deadlock         |
    let _rg = 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 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::RwLock;

    let lock = RwLock::new(5);

    // many reader locks can be held at once
    {
        let r1 = lock.read().unwrap();
        let r2 = lock.read().unwrap();
        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().unwrap();
        *w += 1;
        assert_eq!(*w, 6);
    } // write lock is dropped here

Run

## Implementations§

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### impl<T> RwLock<T>

const: 1.63.0 · source

#### pub 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);

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

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#### pub fn read(&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.

##### Panics

This function might panic when called if the lock is already held by the current thread.

##### 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();

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#### pub fn try_read(&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.

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!(),
    };

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#### pub fn write(&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.

##### Panics

This function might panic when called if the lock is already held by the current thread.

##### Examples

    use std::sync::RwLock;

    let lock = RwLock::new(1);

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

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

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#### pub fn try_write(&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.

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);

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

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

Run
1.2.0 · source

#### pub fn is_poisoned(&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);

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#### pub fn clear_poison(&self)

🔬This is a nightly-only experimental API. (`mutex_unpoison` #96469)

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

    #![feature(mutex_unpoison)]

    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);

Run
1.6.0 · source

#### pub fn into_inner(self) -> LockResult<T>where T: Sized,

Consumes this `RwLock`, returning the underlying data.

##### 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 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");

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1.6.0 · source

#### pub fn get_mut(&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 locks exist.

##### 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 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);

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## Trait Implementations§

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

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

Formats the value using the given formatter. Read more
1.10.0 · source§

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

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

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

1.24.0 · source§

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

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

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

1.12.0 · source§

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

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

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

1.9.0 · source§

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

## Auto Trait Implementations§

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

## Blanket Implementations§

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### impl<T> Any for Twhere 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 Twhere 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 Twhere 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> From<!> for T

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

Converts to this type from the input type.
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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 Twhere 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, U> TryFrom<U> for Twhere 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 Twhere 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.