Struct tokio::sync::RwLock [−][src]
pub struct RwLock<T: ?Sized> { /* fields omitted */ }Expand description
An asynchronous 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 causing any tasks waiting for the lock to
become available to yield. 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 Tokio’s read-write lock is fair (or
write-preferring), in order to ensure that readers cannot starve
writers. Fairness is ensured using a first-in, first-out queue for the tasks
awaiting the lock; if a task that wishes to acquire the write lock is at the
head of the queue, read locks will not be given out until the write lock has
been released. This is in contrast to the Rust standard library’s
std::sync::RwLock, where the priority policy is dependent on the
operating system’s implementation.
The type parameter T represents the data that this lock protects. It is
required that T satisfies Send to be shared across threads. 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.
Examples
use tokio::sync::RwLock;
#[tokio::main]
async fn main() {
let lock = RwLock::new(5);
// many reader locks can be held at once
{
let r1 = lock.read().await;
let r2 = lock.read().await;
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().await;
*w += 1;
assert_eq!(*w, 6);
} // write lock is dropped here
}Implementations
Creates a new instance of an RwLock<T> which is unlocked.
Examples
use tokio::sync::RwLock;
let lock = RwLock::new(5);Creates a new instance of an RwLock<T> which is unlocked.
Examples
use tokio::sync::RwLock;
static LOCK: RwLock<i32> = RwLock::const_new(5);Creates a new instance of an RwLock<T> which is unlocked
and allows a maximum of max_reads concurrent readers.
Examples
use tokio::sync::RwLock;
static LOCK: RwLock<i32> = RwLock::const_with_max_readers(5, 1024);Locks this RwLock with shared read access, causing the current task
to yield until the lock has been acquired.
The calling task will yield until there are no writers which hold the lock. There may be other readers inside the lock when the task resumes.
Note that under the priority policy of RwLock, read locks are not
granted until prior write locks, to prevent starvation. Therefore
deadlock may occur if a read lock is held by the current task, a write
lock attempt is made, and then a subsequent read lock attempt is made
by the current task.
Returns an RAII guard which will drop this read access of the RwLock
when dropped.
Cancel safety
This method uses a queue to fairly distribute locks in the order they
were requested. Cancelling a call to read makes you lose your place in
the queue.
Examples
use std::sync::Arc;
use tokio::sync::RwLock;
#[tokio::main]
async fn main() {
let lock = Arc::new(RwLock::new(1));
let c_lock = lock.clone();
let n = lock.read().await;
assert_eq!(*n, 1);
tokio::spawn(async move {
// While main has an active read lock, we acquire one too.
let r = c_lock.read().await;
assert_eq!(*r, 1);
}).await.expect("The spawned task has panicked");
// Drop the guard after the spawned task finishes.
drop(n);
}Locks this RwLock with shared read access, causing the current task
to yield until the lock has been acquired.
The calling task will yield until there are no writers which hold the lock. There may be other readers inside the lock when the task resumes.
This method is identical to RwLock::read, except that the returned
guard references the RwLock with an Arc rather than by borrowing
it. Therefore, the RwLock must be wrapped in an Arc to call this
method, and the guard will live for the 'static lifetime, as it keeps
the RwLock alive by holding an Arc.
Note that under the priority policy of RwLock, read locks are not
granted until prior write locks, to prevent starvation. Therefore
deadlock may occur if a read lock is held by the current task, a write
lock attempt is made, and then a subsequent read lock attempt is made
by the current task.
Returns an RAII guard which will drop this read access of the RwLock
when dropped.
Cancel safety
This method uses a queue to fairly distribute locks in the order they
were requested. Cancelling a call to read_owned makes you lose your
place in the queue.
Examples
use std::sync::Arc;
use tokio::sync::RwLock;
#[tokio::main]
async fn main() {
let lock = Arc::new(RwLock::new(1));
let c_lock = lock.clone();
let n = lock.read_owned().await;
assert_eq!(*n, 1);
tokio::spawn(async move {
// While main has an active read lock, we acquire one too.
let r = c_lock.read_owned().await;
assert_eq!(*r, 1);
}).await.expect("The spawned task has panicked");
// Drop the guard after the spawned task finishes.
drop(n);
}Attempts to acquire this RwLock with shared read access.
If the access couldn’t be acquired immediately, returns TryLockError.
Otherwise, an RAII guard is returned which will release read access
when dropped.
Examples
use std::sync::Arc;
use tokio::sync::RwLock;
#[tokio::main]
async fn main() {
let lock = Arc::new(RwLock::new(1));
let c_lock = lock.clone();
let v = lock.try_read().unwrap();
assert_eq!(*v, 1);
tokio::spawn(async move {
// While main has an active read lock, we acquire one too.
let n = c_lock.read().await;
assert_eq!(*n, 1);
}).await.expect("The spawned task has panicked");
// Drop the guard when spawned task finishes.
drop(v);
}Attempts to acquire this RwLock with shared read access.
If the access couldn’t be acquired immediately, returns TryLockError.
Otherwise, an RAII guard is returned which will release read access
when dropped.
This method is identical to RwLock::try_read, except that the
returned guard references the RwLock with an Arc rather than by
borrowing it. Therefore, the RwLock must be wrapped in an Arc to
call this method, and the guard will live for the 'static lifetime,
as it keeps the RwLock alive by holding an Arc.
Examples
use std::sync::Arc;
use tokio::sync::RwLock;
#[tokio::main]
async fn main() {
let lock = Arc::new(RwLock::new(1));
let c_lock = lock.clone();
let v = lock.try_read_owned().unwrap();
assert_eq!(*v, 1);
tokio::spawn(async move {
// While main has an active read lock, we acquire one too.
let n = c_lock.read_owned().await;
assert_eq!(*n, 1);
}).await.expect("The spawned task has panicked");
// Drop the guard when spawned task finishes.
drop(v);
}Locks this RwLock with exclusive write access, causing the current
task to yield until the lock has been acquired.
The calling task will yield while other writers or readers currently have access to the lock.
Returns an RAII guard which will drop the write access of this RwLock
when dropped.
Cancel safety
This method uses a queue to fairly distribute locks in the order they
were requested. Cancelling a call to write makes you lose your place
in the queue.
Examples
use tokio::sync::RwLock;
#[tokio::main]
async fn main() {
let lock = RwLock::new(1);
let mut n = lock.write().await;
*n = 2;
}Locks this RwLock with exclusive write access, causing the current
task to yield until the lock has been acquired.
The calling task will yield while other writers or readers currently have access to the lock.
This method is identical to RwLock::write, except that the returned
guard references the RwLock with an Arc rather than by borrowing
it. Therefore, the RwLock must be wrapped in an Arc to call this
method, and the guard will live for the 'static lifetime, as it keeps
the RwLock alive by holding an Arc.
Returns an RAII guard which will drop the write access of this RwLock
when dropped.
Cancel safety
This method uses a queue to fairly distribute locks in the order they
were requested. Cancelling a call to write_owned makes you lose your
place in the queue.
Examples
use std::sync::Arc;
use tokio::sync::RwLock;
#[tokio::main]
async fn main() {
let lock = Arc::new(RwLock::new(1));
let mut n = lock.write_owned().await;
*n = 2;
}Attempts to acquire this RwLock with exclusive write access.
If the access couldn’t be acquired immediately, returns TryLockError.
Otherwise, an RAII guard is returned which will release write access
when dropped.
Examples
use tokio::sync::RwLock;
#[tokio::main]
async fn main() {
let rw = RwLock::new(1);
let v = rw.read().await;
assert_eq!(*v, 1);
assert!(rw.try_write().is_err());
}Attempts to acquire this RwLock with exclusive write access.
If the access couldn’t be acquired immediately, returns TryLockError.
Otherwise, an RAII guard is returned which will release write access
when dropped.
This method is identical to RwLock::try_write, except that the
returned guard references the RwLock with an Arc rather than by
borrowing it. Therefore, the RwLock must be wrapped in an Arc to
call this method, and the guard will live for the 'static lifetime,
as it keeps the RwLock alive by holding an Arc.
Examples
use std::sync::Arc;
use tokio::sync::RwLock;
#[tokio::main]
async fn main() {
let rw = Arc::new(RwLock::new(1));
let v = Arc::clone(&rw).read_owned().await;
assert_eq!(*v, 1);
assert!(rw.try_write_owned().is_err());
}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.
Examples
use tokio::sync::RwLock;
fn main() {
let mut lock = RwLock::new(1);
let n = lock.get_mut();
*n = 2;
}Consumes the lock, returning the underlying data.