Struct tokio::sync::Semaphore

pub struct Semaphore { /* fields omitted */ }

This is supported on crate feature sync only.

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.

This Semaphore is fair, which means that permits are given out in the order they were requested. This fairness is also applied when acquire_many gets involved, so if a call to acquire_many at the front of the queue requests more permits than currently available, this can prevent a call to acquire from completing, even if the semaphore has enough permits complete the call to acquire.

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

Implementations

impl Semaphore

pub fn new(permits: usize) -> Self

Creates a new semaphore with the initial number of permits.

pub const fn const_new(permits: usize) -> Self

This is supported on crate feature parking_lot only.

Creates a new semaphore with the initial number of permits.

pub fn available_permits(&self) -> usize

Returns the current number of available permits.

pub fn add_permits(&self, n: usize)

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>

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>

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>

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>

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>

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>

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>

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>

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)

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))
}

pub fn is_closed(&self) -> bool

Returns true if the semaphore is closed

Trait Implementations

impl Debug for Semaphore

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

Formats the value using the given formatter.