Mutex in std::sync::poison - Rust
Description: A mutual exclusion primitive useful for protecting shared data

## Mutex

# Struct Mutex

Source

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

🔬This is a nightly-only experimental API. (`sync_poison_mod` #134646)
Expand description

A mutual exclusion primitive useful for protecting shared data

This mutex will block threads waiting for the lock to become available. The mutex can be 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 becomes poisoned if it recognizes that the thread holding it has panicked.

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.

Poisoning is only advisory: 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.

In addition, the panic detection is not ideal, so even unpoisoned mutexes need to be handled with care, since certain panics may have been skipped. Here is a non-exhaustive list of situations where this might occur:

- If a mutex is locked while a panic is underway, e.g. within a `Drop` implementation or a panic hook, panicking for the second time while the lock is held will leave the mutex unpoisoned. Note that while double panic usually aborts the program, `catch_unwind` can prevent this.

- Locking and unlocking the mutex across different panic contexts, e.g. by storing the guard to a `Cell` within `Drop::drop` and accessing it outside, or vice versa, can affect poisoning status in an unexpected way.

- Foreign exceptions do not currently trigger poisoning even in absence of other panics.

While this rarely happens in realistic code, `unsafe` code cannot rely on poisoning for soundness, since the behavior of poisoning can depend on outside context. Here’s an example of incorrect use of poisoning:

    use std::sync::Mutex;

    struct MutexBox<T> {
        data: Mutex<*mut T>,
    }

    impl<T> MutexBox<T> {
        pub fn new(value: T) -> Self {
            Self {
                data: Mutex::new(Box::into_raw(Box::new(value))),
            }
        }

        pub fn replace_with(&self, f: impl FnOnce(T) -> T) {
            let ptr = self.data.lock().expect("poisoned");
            // While `f` is running, the data is moved out of `*ptr`. If `f`
            // panics, `*ptr` keeps pointing at a dropped value. The intention
            // is that this will poison the mutex, so the following calls to
            // `replace_with` will panic without reading `*ptr`. But since
            // poisoning is not guaranteed to occur if this is run from a panic
            // hook, this can lead to use-after-free.
            unsafe {
                (*ptr).write(f((*ptr).read()));
            }
        }
    }

## §Examples

    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..N {
        let (data, tx) = (Arc::clone(&data), 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:

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

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

    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;

To unlock a mutex guard sooner than the end of the enclosing scope, either create an inner scope or drop the guard manually.

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

    const N: usize = 3;

    let data_mutex = Arc::new(Mutex::new(vec![1, 2, 3, 4]));
    let res_mutex = Arc::new(Mutex::new(0));

    let mut threads = Vec::with_capacity(N);
    (0..N).for_each(|_| {
        let data_mutex_clone = Arc::clone(&data_mutex);
        let res_mutex_clone = Arc::clone(&res_mutex);

        threads.push(thread::spawn(move || {
            // Here we use a block to limit the lifetime of the lock guard.
            let result = {
                let mut data = data_mutex_clone.lock().unwrap();
                // This is the result of some important and long-ish work.
                let result = data.iter().fold(0, |acc, x| acc + x * 2);
                data.push(result);
                result
                // The mutex guard gets dropped here, together with any other values
                // created in the critical section.
            };
            // The guard created here is a temporary dropped at the end of the statement, i.e.
            // the lock would not remain being held even if the thread did some additional work.
            *res_mutex_clone.lock().unwrap() += result;
        }));
    });

    let mut data = data_mutex.lock().unwrap();
    // This is the result of some important and long-ish work.
    let result = data.iter().fold(0, |acc, x| acc + x * 2);
    data.push(result);
    // We drop the `data` explicitly because it's not necessary anymore and the
    // thread still has work to do. This allows other threads to start working on
    // the data immediately, without waiting for the rest of the unrelated work
    // to be done here.
    //
    // It's even more important here than in the threads because we `.join` the
    // threads after that. If we had not dropped the mutex guard, a thread could
    // be waiting forever for it, causing a deadlock.
    // As in the threads, a block could have been used instead of calling the
    // `drop` function.
    drop(data);
    // Here the mutex guard is not assigned to a variable and so, even if the
    // scope does not end after this line, the mutex is still released: there is
    // no deadlock.
    *res_mutex.lock().unwrap() += result;

    threads.into_iter().for_each(|thread| {
        thread
            .join()
            .expect("The thread creating or execution failed !")
    });

    assert_eq!(*res_mutex.lock().unwrap(), 800);

## Implementations§

Source§

### impl<T> Mutex<T>

1.0.0 (const: 1.63.0) · Source

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

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

##### §Examples

    use std::sync::Mutex;

    let mutex = Mutex::new(0);

Source

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

🔬This is a nightly-only experimental API. (`lock_value_accessors` #133407)

Returns the contained value by cloning it.

##### §Errors

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

##### §Examples

    #![feature(lock_value_accessors)]

    use std::sync::Mutex;

    let mut mutex = Mutex::new(7);

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

Source

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

🔬This is a nightly-only experimental API. (`lock_value_accessors` #133407)

Sets the contained value.

##### §Errors

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

##### §Examples

    #![feature(lock_value_accessors)]

    use std::sync::Mutex;

    let mut mutex = Mutex::new(7);

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

Source

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

🔬This is a nightly-only experimental API. (`lock_value_accessors` #133407)

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

##### §Errors

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

##### §Examples

    #![feature(lock_value_accessors)]

    use std::sync::Mutex;

    let mut mutex = Mutex::new(7);

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

Source§

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

1.0.0 · Source

#### pub 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 lock 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. The acquired mutex 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.

##### §Examples

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

    let mutex = Arc::new(Mutex::new(0));
    let c_mutex = Arc::clone(&mutex);

    thread::spawn(move || {
        *c_mutex.lock().unwrap() = 10;
    }).join().expect("thread::spawn failed");
    assert_eq!(*mutex.lock().unwrap(), 10);

1.0.0 · Source

#### pub 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 the `Poisoned` error if the mutex would otherwise be acquired. An acquired lock guard will be contained in the returned error.

If the mutex could not be acquired because it is already locked, then this call will return the `WouldBlock` error.

##### §Examples

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

    let mutex = Arc::new(Mutex::new(0));
    let c_mutex = Arc::clone(&mutex);

    thread::spawn(move || {
        let mut lock = c_mutex.try_lock();
        if let Ok(ref mut mutex) = lock {
            **mutex = 10;
        } else {
            println!("try_lock failed");
        }
    }).join().expect("thread::spawn failed");
    assert_eq!(*mutex.lock().unwrap(), 10);

1.2.0 · Source

#### pub fn is_poisoned(&self) -> bool

Determines whether the mutex is poisoned.

If another thread is active, the mutex 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, Mutex};
    use std::thread;

    let mutex = Arc::new(Mutex::new(0));
    let c_mutex = Arc::clone(&mutex);

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

1.77.0 · Source

#### pub fn clear_poison(&self)

Clear the poisoned state from a mutex.

If the mutex 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 mutex 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, Mutex};
    use std::thread;

    let mutex = Arc::new(Mutex::new(0));
    let c_mutex = Arc::clone(&mutex);

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

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

1.6.0 · Source

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

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 containing the underlying data instead.

##### §Examples

    use std::sync::Mutex;

    let mutex = Mutex::new(0);
    assert_eq!(mutex.into_inner().unwrap(), 0);

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 `Mutex` 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 previous abandoned locks (e.g., via `forget()` on a `MutexGuard`).

##### §Errors

If another user of this mutex panicked while holding the mutex, then this call will return an error containing a mutable reference to the underlying data instead.

##### §Examples

    use std::sync::Mutex;

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

Source

#### pub const fn data_ptr(&self) -> *mut T

🔬This is a nightly-only experimental API. (`mutex_data_ptr` #140368)

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 mutex is dropped.

## Trait Implementations§

1.0.0 · Source§

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

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

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

1.24.0 · Source§

### impl<T> From<T> for Mutex<T>

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

Creates a new mutex in an unlocked state ready for use. This is equivalent to `Mutex::new`.

1.12.0 · Source§

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

1.0.0 · Source§

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

`T` must be `Send` for a `Mutex` to be `Send` because it is possible to acquire the owned `T` from the `Mutex` via `into_inner`.

1.0.0 · Source§

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

`T` must be `Send` for `Mutex` to be `Sync`. This ensures that the protected data can be accessed safely from multiple threads without causing data races or other unsafe behavior.

`Mutex<T>` provides mutable access to `T` to one thread at a time. However, it’s essential for `T` to be `Send` because it’s not safe for non-`Send` structures to be accessed in this manner. For instance, consider `Rc`, a non-atomic reference counted smart pointer, which is not `Send`. With `Rc`, we can have multiple copies pointing to the same heap allocation with a non-atomic reference count. If we were to use `Mutex<Rc<_>>`, it would only protect one instance of `Rc` from shared access, leaving other copies vulnerable to potential data races.

Also note that it is not necessary for `T` to be `Sync` as `&T` is only made available to one thread at a time if `T` is not `Sync`.

1.9.0 · Source§

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

## Auto Trait Implementations§

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### impl<T> !Freeze for Mutex<T>

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

## Blanket Implementations§

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### impl<T> Any for T
where 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 T
where 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 T
where 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 T
where 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 T
where 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 T
where 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.