Struct Condvar

pub struct Condvar { /* private fields */ }

A Condition Variable

Condition variables represent the ability to block a thread such that it consumes no CPU time while waiting for an event to occur. Condition variables are typically associated with a boolean predicate (a condition) and a mutex. The predicate is always verified inside of the mutex before determining that thread must block.

Note that this module places one additional restriction over the system condition variables: each condvar can be used with only one mutex at a time. Any attempt to use multiple mutexes on the same condition variable simultaneously will result in a runtime panic. However it is possible to switch to a different mutex if there are no threads currently waiting on the condition variable.

Differences from the standard library Condvar

• No spurious wakeups: A wait will only return a non-timeout result if it was woken up by notify_one or notify_all.
• Condvar::notify_all will only wake up a single thread, the rest are requeued to wait for the Mutex to be unlocked by the thread that was woken up.
• Only requires 1 word of space, whereas the standard library boxes the Condvar due to platform limitations.
• Can be statically constructed.
• Does not require any drop glue when dropped.
• Inline fast path for the uncontended case.

Examples

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

let pair = Arc::new((Mutex::new(false), Condvar::new()));
let pair2 = pair.clone();

// Inside of our lock, spawn a new thread, and then wait for it to start
thread::spawn(move|| {
    let &(ref lock, ref cvar) = &*pair2;
    let mut started = lock.lock();
    *started = true;
    cvar.notify_one();
});

// wait for the thread to start up
let &(ref lock, ref cvar) = &*pair;
let mut started = lock.lock();
if !*started {
    cvar.wait(&mut started);
}
// Note that we used an if instead of a while loop above. This is only
// possible because parking_lot's Condvar will never spuriously wake up.
// This means that wait() will only return after notify_one or notify_all is
// called.

Implementations

impl Condvar

pub const fn new() -> Condvar

Creates a new condition variable which is ready to be waited on and notified.

pub fn notify_one(&self) -> bool

Wakes up one blocked thread on this condvar.

Returns whether a thread was woken up.

If there is a blocked thread on this condition variable, then it will be woken up from its call to wait or wait_timeout. Calls to notify_one are not buffered in any way.

To wake up all threads, see notify_all().

Examples

use parking_lot::Condvar;

let condvar = Condvar::new();

// do something with condvar, share it with other threads

if !condvar.notify_one() {
    println!("Nobody was listening for this.");
}

pub fn notify_all(&self) -> usize

Wakes up all blocked threads on this condvar.

Returns the number of threads woken up.

This method will ensure that any current waiters on the condition variable are awoken. Calls to notify_all() are not buffered in any way.

To wake up only one thread, see notify_one().

pub fn wait<T: ?Sized>(&self, mutex_guard: &mut MutexGuard<'_, T>)

Blocks the current thread until this condition variable receives a notification.

This function will atomically unlock the mutex specified (represented by mutex_guard) and block the current thread. This means that any calls to notify_*() which happen logically after the mutex is unlocked are candidates to wake this thread up. When this function call returns, the lock specified will have been re-acquired.

Panics

This function will panic if another thread is waiting on the Condvar with a different Mutex object.

pub fn wait_until<T: ?Sized>( &self, mutex_guard: &mut MutexGuard<'_, T>, timeout: Instant, ) -> WaitTimeoutResult

Waits on this condition variable for a notification, timing out after the specified time instant.

The semantics of this function are equivalent to wait() except that the thread will be blocked roughly until timeout is reached. This method should not be used for precise timing due to anomalies such as preemption or platform differences that may not cause the maximum amount of time waited to be precisely timeout.

Note that the best effort is made to ensure that the time waited is measured with a monotonic clock, and not affected by the changes made to the system time.

The returned WaitTimeoutResult value indicates if the timeout is known to have elapsed.

Like wait, the lock specified will be re-acquired when this function returns, regardless of whether the timeout elapsed or not.

Panics

This function will panic if another thread is waiting on the Condvar with a different Mutex object.

pub fn wait_for<T: ?Sized>( &self, mutex_guard: &mut MutexGuard<'_, T>, timeout: Duration, ) -> WaitTimeoutResult

Waits on this condition variable for a notification, timing out after a specified duration.

The semantics of this function are equivalent to wait() except that the thread will be blocked for roughly no longer than timeout. This method should not be used for precise timing due to anomalies such as preemption or platform differences that may not cause the maximum amount of time waited to be precisely timeout.

Note that the best effort is made to ensure that the time waited is measured with a monotonic clock, and not affected by the changes made to the system time.

The returned WaitTimeoutResult value indicates if the timeout is known to have elapsed.

Like wait, the lock specified will be re-acquired when this function returns, regardless of whether the timeout elapsed or not.

pub fn wait_while<T, F>( &self, mutex_guard: &mut MutexGuard<'_, T>, condition: F, )

where T: ?Sized, F: FnMut(&mut T) -> bool,

Blocks the current thread until this condition variable receives a notification. If the provided condition evaluates to false, then the thread is no longer blocked and the operation is completed. If the condition evaluates to true, then the thread is blocked again and waits for another notification before repeating this process.

This function will atomically unlock the mutex specified (represented by mutex_guard) and block the current thread. This means that any calls to notify_*() which happen logically after the mutex is unlocked are candidates to wake this thread up. When this function call returns, the lock specified will have been re-acquired.

Panics

This function will panic if another thread is waiting on the Condvar with a different Mutex object.

pub fn wait_while_until<T, F>( &self, mutex_guard: &mut MutexGuard<'_, T>, condition: F, timeout: Instant, ) -> WaitTimeoutResult

where T: ?Sized, F: FnMut(&mut T) -> bool,

Waits on this condition variable for a notification, timing out after the specified time instant. If the provided condition evaluates to false, then the thread is no longer blocked and the operation is completed. If the condition evaluates to true, then the thread is blocked again and waits for another notification before repeating this process.

The semantics of this function are equivalent to wait() except that the thread will be blocked roughly until timeout is reached. This method should not be used for precise timing due to anomalies such as preemption or platform differences that may not cause the maximum amount of time waited to be precisely timeout.

Note that the best effort is made to ensure that the time waited is measured with a monotonic clock, and not affected by the changes made to the system time.

The returned WaitTimeoutResult value indicates if the timeout is known to have elapsed.

Like wait, the lock specified will be re-acquired when this function returns, regardless of whether the timeout elapsed or not.

Panics

This function will panic if another thread is waiting on the Condvar with a different Mutex object.

pub fn wait_while_for<T: ?Sized, F>( &self, mutex_guard: &mut MutexGuard<'_, T>, condition: F, timeout: Duration, ) -> WaitTimeoutResult

where F: FnMut(&mut T) -> bool,

Waits on this condition variable for a notification, timing out after a specified duration. If the provided condition evaluates to false, then the thread is no longer blocked and the operation is completed. If the condition evaluates to true, then the thread is blocked again and waits for another notification before repeating this process.

The semantics of this function are equivalent to wait() except that the thread will be blocked for roughly no longer than timeout. This method should not be used for precise timing due to anomalies such as preemption or platform differences that may not cause the maximum amount of time waited to be precisely timeout.

Note that the best effort is made to ensure that the time waited is measured with a monotonic clock, and not affected by the changes made to the system time.

The returned WaitTimeoutResult value indicates if the timeout is known to have elapsed.

Like wait, the lock specified will be re-acquired when this function returns, regardless of whether the timeout elapsed or not.

Trait Implementations

impl Debug for Condvar

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

Formats the value using the given formatter.

impl Default for Condvar

fn default() -> Condvar

Returns the “default value” for a type.