# Struct std::sync::Mutex 1.0.0 [−] [src]

    pub struct Mutex<T: ?Sized> {
        // some fields omitted
    }

A mutual exclusion primitive useful for protecting shared data

This mutex will block threads waiting for the lock to become available. The mutex can also be statically initialized or created via a `new` constructor. Each mutex has a type parameter which represents the data that it is protecting. The data can only be accessed through the RAII guards returned from `lock` and `try_lock`, which guarantees that the data is only ever accessed when the mutex is locked.

# Poisoning

The mutexes in this module implement a strategy called "poisoning" where a mutex is considered poisoned whenever a thread panics while holding the lock. Once a mutex is poisoned, all other threads are unable to access the data by default as it is likely tainted (some invariant is not being upheld).

For a mutex, this means that the `lock` and `try_lock` methods return a `Result` which indicates whether a mutex has been poisoned or not. Most usage of a mutex will simply `unwrap()` these results, propagating panics among threads to ensure that a possibly invalid invariant is not witnessed.

A poisoned mutex, however, does not prevent all access to the underlying data. The `PoisonError` type has an `into_inner` method which will return the guard that would have otherwise been returned on a successful lock. This allows access to the data, despite the lock being poisoned.

# Examples

fn main() { use std::sync::{Arc, Mutex}; use std::thread; use std::sync::mpsc::channel; const N: usize = 10; // Spawn a few threads to increment a shared variable (non-atomically), and // let the main thread know once all increments are done. // // Here we're using an Arc to share memory among threads, and the data inside // the Arc is protected with a mutex. let data = Arc::new(Mutex::new(0)); let (tx, rx) = channel(); for _ in 0..10 { let (data, tx) = (data.clone(), tx.clone()); thread::spawn(move || { // The shared state can only be accessed once the lock is held. // Our non-atomic increment is safe because we're the only thread // which can access the shared state when the lock is held. // // We unwrap() the return value to assert that we are not expecting // threads to ever fail while holding the lock. let mut data = data.lock().unwrap(); *data += 1; if *data == N { tx.send(()).unwrap(); } // the lock is unlocked here when `data` goes out of scope. }); } rx.recv().unwrap(); }

    use std::sync::{Arc, Mutex};
    use std::thread;
    use std::sync::mpsc::channel;

    const N: usize = 10;

    // Spawn a few threads to increment a shared variable (non-atomically), and
    // let the main thread know once all increments are done.
    //
    // Here we're using an Arc to share memory among threads, and the data inside
    // the Arc is protected with a mutex.
    let data = Arc::new(Mutex::new(0));

    let (tx, rx) = channel();
    for _ in 0..10 {
        let (data, tx) = (data.clone(), tx.clone());
        thread::spawn(move || {
            // The shared state can only be accessed once the lock is held.
            // Our non-atomic increment is safe because we're the only thread
            // which can access the shared state when the lock is held.
            //
            // We unwrap() the return value to assert that we are not expecting
            // threads to ever fail while holding the lock.
            let mut data = data.lock().unwrap();
            *data += 1;
            if *data == N {
                tx.send(()).unwrap();
            }
            // the lock is unlocked here when `data` goes out of scope.
        });
    }

    rx.recv().unwrap();

To recover from a poisoned mutex:

fn main() { use std::sync::{Arc, Mutex}; use std::thread; let lock = Arc::new(Mutex::new(0_u32)); let lock2 = lock.clone(); let _ = thread::spawn(move || -> () { // This thread will acquire the mutex first, unwrapping the result of // `lock` because the lock has not been poisoned. let _guard = lock2.lock().unwrap(); // This panic while holding the lock (`_guard` is in scope) will poison // the mutex. panic!(); }).join(); // The lock is poisoned by this point, but the returned result can be // pattern matched on to return the underlying guard on both branches. let mut guard = match lock.lock() { Ok(guard) => guard, Err(poisoned) => poisoned.into_inner(), }; *guard += 1; }

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

    let lock = Arc::new(Mutex::new(0_u32));
    let lock2 = lock.clone();

    let _ = thread::spawn(move || -> () {
        // This thread will acquire the mutex first, unwrapping the result of
        // `lock` because the lock has not been poisoned.
        let _guard = lock2.lock().unwrap();

        // This panic while holding the lock (`_guard` is in scope) will poison
        // the mutex.
        panic!();
    }).join();

    // The lock is poisoned by this point, but the returned result can be
    // pattern matched on to return the underlying guard on both branches.
    let mut guard = match lock.lock() {
        Ok(guard) => guard,
        Err(poisoned) => poisoned.into_inner(),
    };

    *guard += 1;

## Methods

### `impl<T> Mutex<T>`
[src]

#### `fn new(t: T) -> Mutex<T>`

Creates a new mutex in an unlocked state ready for use.

### `impl<T: ?Sized> Mutex<T>`
[src]

#### `fn lock(&self) -> LockResult<MutexGuard<T>>`

Acquires a mutex, blocking the current thread until it is able to do so.

This function will block the local thread until it is available to acquire the mutex. Upon returning, the thread is the only thread with the mutex held. An RAII guard is returned to allow scoped unlock of the lock. When the guard goes out of scope, the mutex will be unlocked.

The exact behavior on locking a mutex in the thread which already holds the lock is left unspecified. However, this function will not return on the second call (it might panic or deadlock, for example).

# Errors

If another user of this mutex panicked while holding the mutex, then this call will return an error once the mutex is acquired.

# Panics

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

#### `fn try_lock(&self) -> TryLockResult<MutexGuard<T>>`

Attempts to acquire this lock.

If the lock could not be acquired at this time, then `Err` is returned. Otherwise, an RAII guard is returned. The lock will be unlocked when the guard is dropped.

This function does not block.

# Errors

If another user of this mutex panicked while holding the mutex, then this call will return failure if the mutex would otherwise be acquired.

#### `fn is_poisoned(&self) -> bool`
1.2.0

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.

#### `fn into_inner(self) -> LockResult<T> where T: Sized`
1.6.0

Consumes this mutex, returning the underlying data.

# Errors

If another user of this mutex panicked while holding the mutex, then this call will return an error instead.

#### `fn get_mut(&mut self) -> LockResult<&mut T>`
1.6.0

Returns a mutable reference to the underlying data.

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

# Errors

If another user of this mutex panicked while holding the mutex, then this call will return an error instead.

## Trait Implementations

### `impl<T: ?Sized> UnwindSafe for Mutex<T>`
1.9.0
[src]

### `impl<T: ?Sized> RefUnwindSafe for Mutex<T>`
1.12.0
[src]

### `impl<T: ?Sized + Send> Send for Mutex<T>`
[src]

### `impl<T: ?Sized + Send> Sync for Mutex<T>`
[src]

### `impl<T: ?Sized> Drop for Mutex<T>`
[src]

#### `fn drop(&mut self)`

A method called when the value goes out of scope. Read more

### `impl<T: ?Sized + Default> Default for Mutex<T>`
1.9.0
[src]

#### `fn default() -> Mutex<T>`

Returns the "default value" for a type. Read more

### `impl<T: ?Sized + Debug> Debug for Mutex<T>`
[src]

#### `fn fmt(&self, f: &mut Formatter) -> Result`

Formats the value using the given formatter.

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