Struct std::sync::RwLock 1.0.0[−][src]
pub struct RwLock<T: ?Sized> { /* fields omitted */ }Expand description
A 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 blocking any threads waiting for the lock to
become available. 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 the lock is dependent on the underlying operating
system’s implementation, and this type does not guarantee that any
particular policy will be used. In particular, a writer which is waiting to
acquire the lock in write might or might not block concurrent calls to
read, e.g.:
Potential deadlock example
// Thread 1 | // Thread 2
let _rg = lock.read(); |
| // will block
| let _wg = lock.write();
// may deadlock |
let _rg = lock.read(); |The type parameter T represents the data that this lock protects. It is
required that T satisfies Send to be shared across threads and
Sync to allow concurrent access through readers. 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.
Poisoning
An RwLock, like Mutex, will become poisoned on a panic. Note, however,
that an RwLock may only be poisoned if a panic occurs while it is locked
exclusively (write mode). If a panic occurs in any reader, then the lock
will not be poisoned.
Examples
use std::sync::RwLock;
let lock = RwLock::new(5);
// many reader locks can be held at once
{
let r1 = lock.read().unwrap();
let r2 = lock.read().unwrap();
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().unwrap();
*w += 1;
assert_eq!(*w, 6);
} // write lock is dropped hereImplementations
Locks this rwlock with shared read access, blocking the current thread until it can be acquired.
The calling thread will be blocked until there are no more writers which hold the lock. There may be other readers currently inside the lock when this method returns. This method does not provide any guarantees with respect to the ordering of whether contentious readers or writers will acquire the lock first.
Returns an RAII guard which will release this thread’s shared access once it is dropped.
Errors
This function will return an error if the RwLock is poisoned. An RwLock is poisoned whenever a writer panics while holding an exclusive lock. The failure will occur immediately after the lock has been acquired.
Panics
This function might panic when called if the lock is already held by the current thread.
Examples
use std::sync::{Arc, RwLock};
use std::thread;
let lock = Arc::new(RwLock::new(1));
let c_lock = Arc::clone(&lock);
let n = lock.read().unwrap();
assert_eq!(*n, 1);
thread::spawn(move || {
let r = c_lock.read();
assert!(r.is_ok());
}).join().unwrap();Attempts to acquire this rwlock with shared read access.
If the access could not be granted at this time, then Err is returned.
Otherwise, an RAII guard is returned which will release the shared access
when it is dropped.
This function does not block.
This function does not provide any guarantees with respect to the ordering of whether contentious readers or writers will acquire the lock first.
Errors
This function will return the Poisoned error if the RwLock is poisoned.
An RwLock is poisoned whenever a writer panics while holding an exclusive
lock. Poisoned will only be returned if the lock would have otherwise been
acquired.
This function will return the WouldBlock error if the RwLock could not
be acquired because it was already locked exclusively.
Examples
use std::sync::RwLock;
let lock = RwLock::new(1);
match lock.try_read() {
Ok(n) => assert_eq!(*n, 1),
Err(_) => unreachable!(),
};Locks this rwlock with exclusive write access, blocking the current thread until it can be acquired.
This function will not return while other writers or other readers currently have access to the lock.
Returns an RAII guard which will drop the write access of this rwlock when dropped.
Errors
This function will return an error if the RwLock is poisoned. An RwLock is poisoned whenever a writer panics while holding an exclusive lock. An error will be returned when the lock is acquired.
Panics
This function might panic when called if the lock is already held by the current thread.
Examples
use std::sync::RwLock;
let lock = RwLock::new(1);
let mut n = lock.write().unwrap();
*n = 2;
assert!(lock.try_read().is_err());Attempts to lock this rwlock with exclusive write access.
If the lock could not be acquired at this time, then Err is returned.
Otherwise, an RAII guard is returned which will release the lock when
it is dropped.
This function does not block.
This function does not provide any guarantees with respect to the ordering of whether contentious readers or writers will acquire the lock first.
Errors
This function will return the Poisoned error if the RwLock is
poisoned. An RwLock is poisoned whenever a writer panics while holding
an exclusive lock. Poisoned will only be returned if the lock would have
otherwise been acquired.
This function will return the WouldBlock error if the RwLock could not
be acquired because it was already locked exclusively.
Examples
use std::sync::RwLock;
let lock = RwLock::new(1);
let n = lock.read().unwrap();
assert_eq!(*n, 1);
assert!(lock.try_write().is_err());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.
Examples
use std::sync::{Arc, RwLock};
use std::thread;
let lock = Arc::new(RwLock::new(0));
let c_lock = Arc::clone(&lock);
let _ = thread::spawn(move || {
let _lock = c_lock.write().unwrap();
panic!(); // the lock gets poisoned
}).join();
assert_eq!(lock.is_poisoned(), true);Consumes this RwLock, returning the underlying data.
Errors
This function will return an error if the RwLock is poisoned. An RwLock is poisoned whenever a writer panics while holding an exclusive lock. An error will only be returned if the lock would have otherwise been acquired.
Examples
use std::sync::RwLock;
let lock = RwLock::new(String::new());
{
let mut s = lock.write().unwrap();
*s = "modified".to_owned();
}
assert_eq!(lock.into_inner().unwrap(), "modified");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.
Errors
This function will return an error if the RwLock is poisoned. An RwLock is poisoned whenever a writer panics while holding an exclusive lock. An error will only be returned if the lock would have otherwise been acquired.
Examples
use std::sync::RwLock;
let mut lock = RwLock::new(0);
*lock.get_mut().unwrap() = 10;
assert_eq!(*lock.read().unwrap(), 10);