# tokio::sync::Semaphore - Rust
API documentation for the Rust `Semaphore` struct in crate `tokio`.
Keywords: rust, rustlang, rust-lang, Semaphore

# Struct tokio::sync::Semaphore [−] [src]

    pub struct Semaphore { /* fields omitted */ }

Counting semaphore performing asynchronous permit acquisition.

A semaphore maintains a set of permits. Permits are used to synchronize access to a shared resource. A semaphore differs from a mutex in that it can allow more than one concurrent caller to access the shared resource at a time.

When `acquire` is called and the semaphore has remaining permits, the function immediately returns a permit. However, if no remaining permits are available, `acquire` (asynchronously) waits until an outstanding permit is dropped. At this point, the freed permit is assigned to the caller.

To use the `Semaphore` in a poll function, you can use the `PollSemaphore` utility.

## Implementations

### `impl Semaphore` [src]

#### `pub fn new(permits: usize) -> Self` [src]

Creates a new semaphore with the initial number of permits.

#### `pub const fn const_new(permits: usize) -> Self` [src]

Creates a new semaphore with the initial number of permits.

#### `pub fn available_permits(&self) -> usize` [src]

Returns the current number of available permits.

#### `pub fn add_permits(&self, n: usize)` [src]

Adds `n` new permits to the semaphore.

The maximum number of permits is `usize::MAX >> 3`, and this function will panic if the limit is exceeded.

#### `pub async fn acquire(&self) -> Result<SemaphorePermit<'_>, AcquireError>` [src]

Acquires a permit from the semaphore.

If the semaphore has been closed, this returns an `AcquireError`. Otherwise, this returns a `SemaphorePermit` representing the acquired permit.

#### `pub async fn acquire_many(
&self,
n: u32
) -> Result<SemaphorePermit<'_>, AcquireError>` [src]

Acquires `n` permits from the semaphore.

If the semaphore has been closed, this returns an `AcquireError`. Otherwise, this returns a `SemaphorePermit` representing the acquired permits.

#### `pub fn try_acquire(&self) -> Result<SemaphorePermit<'_>, TryAcquireError>` [src]

Tries to acquire a permit from the semaphore.

If the semaphore has been closed, this returns a `TryAcquireError::Closed` and a `TryAcquireError::NoPermits` if there are no permits left. Otherwise, this returns a `SemaphorePermit` representing the acquired permits.

#### `pub fn try_acquire_many(
&self,
n: u32
) -> Result<SemaphorePermit<'_>, TryAcquireError>` [src]

Tries to acquire `n` permits from the semaphore.

If the semaphore has been closed, this returns a `TryAcquireError::Closed` and a `TryAcquireError::NoPermits` if there are no permits left. Otherwise, this returns a `SemaphorePermit` representing the acquired permits.

#### `pub async fn acquire_owned(
self: Arc<Self>
) -> Result<OwnedSemaphorePermit, AcquireError>` [src]

Acquires a permit from the semaphore.

The semaphore must be wrapped in an `Arc` to call this method. If the semaphore has been closed, this returns an `AcquireError`. Otherwise, this returns a `OwnedSemaphorePermit` representing the acquired permit.

#### `pub async fn acquire_many_owned(
self: Arc<Self>,
n: u32
) -> Result<OwnedSemaphorePermit, AcquireError>` [src]

Acquires `n` permits from the semaphore.

The semaphore must be wrapped in an `Arc` to call this method. If the semaphore has been closed, this returns an `AcquireError`. Otherwise, this returns a `OwnedSemaphorePermit` representing the acquired permit.

#### `pub fn try_acquire_owned(
self: Arc<Self>
) -> Result<OwnedSemaphorePermit, TryAcquireError>` [src]

Tries to acquire a permit from the semaphore.

The semaphore must be wrapped in an `Arc` to call this method. If the semaphore has been closed, this returns a `TryAcquireError::Closed` and a `TryAcquireError::NoPermits` if there are no permits left. Otherwise, this returns a `OwnedSemaphorePermit` representing the acquired permit.

#### `pub fn try_acquire_many_owned(
self: Arc<Self>,
n: u32
) -> Result<OwnedSemaphorePermit, TryAcquireError>` [src]

Tries to acquire `n` permits from the semaphore.

The semaphore must be wrapped in an `Arc` to call this method. If the semaphore has been closed, this returns a `TryAcquireError::Closed` and a `TryAcquireError::NoPermits` if there are no permits left. Otherwise, this returns a `OwnedSemaphorePermit` representing the acquired permit.

#### `pub fn close(&self)` [src]

Closes the semaphore.

This prevents the semaphore from issuing new permits and notifies all pending waiters.

# Examples

    use tokio::sync::Semaphore;
    use std::sync::Arc;
    use tokio::sync::TryAcquireError;

    #[tokio::main]
    async fn main() {
        let semaphore = Arc::new(Semaphore::new(1));
        let semaphore2 = semaphore.clone();

        tokio::spawn(async move {
            let permit = semaphore.acquire_many(2).await;
            assert!(permit.is_err());
            println!("waiter received error");
        });

        println!("closing semaphore");
        semaphore2.close();

        // Cannot obtain more permits
        assert_eq!(semaphore2.try_acquire().err(), Some(TryAcquireError::Closed))
    }

## Trait Implementations

### `impl Debug for Semaphore` [src]

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

Formats the value using the given formatter. Read more

## Auto Trait Implementations

### `impl !RefUnwindSafe for Semaphore` [src]

### `impl Send for Semaphore` [src]

### `impl Sync for Semaphore` [src]

### `impl Unpin for Semaphore` [src]

### `impl !UnwindSafe for Semaphore` [src]

## Blanket Implementations

### `impl<T> Any for T where
T: 'static + ?Sized,` [src]

#### `pub fn type_id(&self) -> TypeId` [src]

Gets the `TypeId` of `self`. Read more

### `impl<T> Borrow<T> for T where
T: ?Sized,` [src]

#### `pub fn borrow(&self) -> &Tⓘ

### Notable traits for &'_ mut F

`impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;`
` [src]

Immutably borrows from an owned value. Read more

### `impl<T> BorrowMut<T> for T where
T: ?Sized,` [src]

#### `pub fn borrow_mut(&mut self) -> &mut Tⓘ

### Notable traits for &'_ mut F

`impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;`
` [src]

Mutably borrows from an owned value. Read more

### `impl<T> From<T> for T` [src]

#### `pub fn from(t: T) -> T` [src]

Performs the conversion.

### `impl<T, U> Into<U> for T where
U: From<T>,` [src]

#### `pub fn into(self) -> U` [src]

Performs the conversion.

### `impl<T, U> TryFrom<U> for T where
U: Into<T>,` [src]

#### `type Error = Infallible`

The type returned in the event of a conversion error.

#### `pub fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>` [src]

Performs the conversion.

### `impl<T, U> TryInto<U> for T where
U: TryFrom<T>,` [src]

#### `type Error = <U as TryFrom<T>>::Error`

The type returned in the event of a conversion error.

#### `pub fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>` [src]

Performs the conversion.