Struct Condvar

pub struct Condvar { /* private fields */ }

🔬This is a nightly-only experimental API. (sync_poison_mod)

Available on crate feature std only.

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 a thread must block.

Functions in this module will block the current thread of execution. Note that any attempt to use multiple mutexes on the same condition variable may result in a runtime panic.

Examples

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

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

// Inside of our lock, spawn a new thread, and then wait for it to start.
thread::spawn(move || {
    let (lock, cvar) = &*pair2;
    let mut started = lock.lock().unwrap();
    *started = true;
    // We notify the condvar that the value has changed.
    cvar.notify_one();
});

// Wait for the thread to start up.
let (lock, cvar) = &*pair;
let mut started = lock.lock().unwrap();
while !*started {
    started = cvar.wait(started).unwrap();
}

Implementations

impl Condvar

pub const fn new() -> Condvar

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

Examples

use std::sync::Condvar;

let condvar = Condvar::new();

pub fn wait<'a, T>( &self, guard: MutexGuard<'a, T>, ) -> Result<MutexGuard<'a, T>, PoisonError<MutexGuard<'a, T>>>

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

This function will atomically unlock the mutex specified (represented by guard) and block the current thread. This means that any calls to notify_one or notify_all 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.

Note that this function is susceptible to spurious wakeups. Condition variables normally have a boolean predicate associated with them, and the predicate must always be checked each time this function returns to protect against spurious wakeups.

Errors

This function will return an error if the mutex being waited on is poisoned when this thread re-acquires the lock. For more information, see information about poisoning on the Mutex type.

Panics

This function may panic! if it is used with more than one mutex over time.

Examples

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

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

thread::spawn(move || {
    let (lock, cvar) = &*pair2;
    let mut started = lock.lock().unwrap();
    *started = true;
    // We notify the condvar that the value has changed.
    cvar.notify_one();
});

// Wait for the thread to start up.
let (lock, cvar) = &*pair;
let mut started = lock.lock().unwrap();
// As long as the value inside the `Mutex<bool>` is `false`, we wait.
while !*started {
    started = cvar.wait(started).unwrap();
}

pub fn wait_while<'a, T, F>( &self, guard: MutexGuard<'a, T>, condition: F, ) -> Result<MutexGuard<'a, T>, PoisonError<MutexGuard<'a, T>>>

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

Blocks the current thread until the provided condition becomes false.

condition is checked immediately; if not met (returns true), this will wait for the next notification then check again. This repeats until condition returns false, in which case this function returns.

This function will atomically unlock the mutex specified (represented by guard) and block the current thread. This means that any calls to notify_one or notify_all 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.

Errors

This function will return an error if the mutex being waited on is poisoned when this thread re-acquires the lock. For more information, see information about poisoning on the Mutex type.

Examples

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

let pair = Arc::new((Mutex::new(true), Condvar::new()));
let pair2 = Arc::clone(&pair);

thread::spawn(move || {
    let (lock, cvar) = &*pair2;
    let mut pending = lock.lock().unwrap();
    *pending = false;
    // We notify the condvar that the value has changed.
    cvar.notify_one();
});

// Wait for the thread to start up.
let (lock, cvar) = &*pair;
// As long as the value inside the `Mutex<bool>` is `true`, we wait.
let _guard = cvar.wait_while(lock.lock().unwrap(), |pending| { *pending }).unwrap();

pub fn wait_timeout_ms<'a, T>( &self, guard: MutexGuard<'a, T>, ms: u32, ) -> Result<(MutexGuard<'a, T>, bool), PoisonError<(MutexGuard<'a, T>, bool)>>

👎Deprecated since 1.6.0: replaced by std::sync::Condvar::wait_timeout

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 ms milliseconds. This method should not be used for precise timing due to anomalies such as preemption or platform differences that might not cause the maximum amount of time waited to be precisely ms.

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 boolean is false only 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.

Examples

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

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

thread::spawn(move || {
    let (lock, cvar) = &*pair2;
    let mut started = lock.lock().unwrap();
    *started = true;
    // We notify the condvar that the value has changed.
    cvar.notify_one();
});

// Wait for the thread to start up.
let (lock, cvar) = &*pair;
let mut started = lock.lock().unwrap();
// As long as the value inside the `Mutex<bool>` is `false`, we wait.
loop {
    let result = cvar.wait_timeout_ms(started, 10).unwrap();
    // 10 milliseconds have passed, or maybe the value changed!
    started = result.0;
    if *started == true {
        // We received the notification and the value has been updated, we can leave.
        break
    }
}

pub fn wait_timeout<'a, T>( &self, guard: MutexGuard<'a, T>, dur: Duration, ) -> Result<(MutexGuard<'a, T>, WaitTimeoutResult), PoisonError<(MutexGuard<'a, T>, 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 dur. This method should not be used for precise timing due to anomalies such as preemption or platform differences that might not cause the maximum amount of time waited to be precisely dur.

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. This function is susceptible to spurious wakeups. Condition variables normally have a boolean predicate associated with them, and the predicate must always be checked each time this function returns to protect against spurious wakeups. Additionally, it is typically desirable for the timeout to not exceed some duration in spite of spurious wakes, thus the sleep-duration is decremented by the amount slept. Alternatively, use the wait_timeout_while method to wait with a timeout while a predicate is true.

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.

Examples

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

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

thread::spawn(move || {
    let (lock, cvar) = &*pair2;
    let mut started = lock.lock().unwrap();
    *started = true;
    // We notify the condvar that the value has changed.
    cvar.notify_one();
});

// wait for the thread to start up
let (lock, cvar) = &*pair;
let mut started = lock.lock().unwrap();
// as long as the value inside the `Mutex<bool>` is `false`, we wait
loop {
    let result = cvar.wait_timeout(started, Duration::from_millis(10)).unwrap();
    // 10 milliseconds have passed, or maybe the value changed!
    started = result.0;
    if *started == true {
        // We received the notification and the value has been updated, we can leave.
        break
    }
}

pub fn wait_timeout_while<'a, T, F>( &self, guard: MutexGuard<'a, T>, dur: Duration, condition: F, ) -> Result<(MutexGuard<'a, T>, WaitTimeoutResult), PoisonError<(MutexGuard<'a, T>, WaitTimeoutResult)>>

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

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

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

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 without the condition being met.

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

Examples

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

let pair = Arc::new((Mutex::new(true), Condvar::new()));
let pair2 = Arc::clone(&pair);

thread::spawn(move || {
    let (lock, cvar) = &*pair2;
    let mut pending = lock.lock().unwrap();
    *pending = false;
    // We notify the condvar that the value has changed.
    cvar.notify_one();
});

// wait for the thread to start up
let (lock, cvar) = &*pair;
let result = cvar.wait_timeout_while(
    lock.lock().unwrap(),
    Duration::from_millis(100),
    |&mut pending| pending,
).unwrap();
if result.1.timed_out() {
    // timed-out without the condition ever evaluating to false.
}
// access the locked mutex via result.0

pub fn notify_one(&self)

Wakes up one blocked thread on this condvar.

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

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

thread::spawn(move || {
    let (lock, cvar) = &*pair2;
    let mut started = lock.lock().unwrap();
    *started = true;
    // We notify the condvar that the value has changed.
    cvar.notify_one();
});

// Wait for the thread to start up.
let (lock, cvar) = &*pair;
let mut started = lock.lock().unwrap();
// As long as the value inside the `Mutex<bool>` is `false`, we wait.
while !*started {
    started = cvar.wait(started).unwrap();
}

pub fn notify_all(&self)

Wakes up all blocked threads on this condvar.

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.

Examples

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

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

thread::spawn(move || {
    let (lock, cvar) = &*pair2;
    let mut started = lock.lock().unwrap();
    *started = true;
    // We notify the condvar that the value has changed.
    cvar.notify_all();
});

// Wait for the thread to start up.
let (lock, cvar) = &*pair;
let mut started = lock.lock().unwrap();
// As long as the value inside the `Mutex<bool>` is `false`, we wait.
while !*started {
    started = cvar.wait(started).unwrap();
}

Trait Implementations

impl ConstDefault for Condvar

const DEFAULT: Self

Returns the compile-time “default value” for a type.

impl Debug for Condvar

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

Formats the value using the given formatter.

impl Default for Condvar

fn default() -> Condvar

Creates a Condvar which is ready to be waited on and notified.

impl RefUnwindSafe for Condvar

impl UnwindSafe for Condvar