# Struct std::sync::Condvar

1.0.0 · source ·

    pub struct Condvar { /* private fields */ }

Expand description

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();
    }

Run

## Implementations§

source§

### impl Condvar

const: 1.63.0 · source

#### 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();

Run
source

#### pub fn wait<'a, T>( &self, guard: MutexGuard<'a, T> ) -> LockResult<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();
    }

Run
1.42.0 · source

#### pub fn wait_while<'a, T, F>( &self, guard: MutexGuard<'a, T>, condition: F ) -> LockResult<MutexGuard<'a, T>>where F: FnMut(&mut T) -> bool,

Blocks the current thread until this condition variable receives a notification and the provided condition is false.

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();

Run
source

#### pub fn wait_timeout_ms<'a, T>( &self, guard: MutexGuard<'a, T>, ms: u32 ) -> LockResult<(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
        }
    }

Run
1.5.0 · source

#### pub fn wait_timeout<'a, T>( &self, guard: MutexGuard<'a, T>, dur: Duration ) -> LockResult<(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
        }
    }

Run
1.42.0 · source

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

Run
source

#### 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();
    }

Run
source

#### 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();
    }

Run

## Trait Implementations§

1.16.0 · source§

### impl Debug for Condvar

source§

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

Formats the value using the given formatter. Read more
1.10.0 · source§

### impl Default for Condvar

source§

#### fn default() -> Condvar

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

## Auto Trait Implementations§

§

### impl RefUnwindSafe for Condvar

§

### impl Send for Condvar

§

### impl Sync for Condvar

§

### impl Unpin for Condvar

§

### impl UnwindSafe for Condvar

## Blanket Implementations§

source§

### impl<T> Any for Twhere T: 'static + ?Sized,

source§

#### fn type_id(&self) -> TypeId

Gets the `TypeId` of `self`. Read more
source§

### impl<T> Borrow<T> for Twhere T: ?Sized,

source§

#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
source§

### impl<T> BorrowMut<T> for Twhere T: ?Sized,

source§

#### fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
source§

### impl<T> From<T> for T

source§

#### fn from(t: T) -> T

Returns the argument unchanged.

source§

### impl<T, U> Into<U> for Twhere U: From<T>,

source§

#### fn into(self) -> U

Calls `U::from(self)`.

That is, this conversion is whatever the implementation of `From<T> for U` chooses to do.

source§

### impl<T, U> TryFrom<U> for Twhere U: Into<T>,

§

#### type Error = Infallible

The type returned in the event of a conversion error.
source§

#### fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
source§

### impl<T, U> TryInto<U> for Twhere U: TryFrom<T>,

§

#### type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
source§

#### fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.