# Struct std::sync::Condvar 1.0.0 [−] [src]

    pub struct Condvar { /* fields omitted */ }

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 and are bindings to system-provided condition variables where possible. Note that this module places one additional restriction over the system condition variables: each condvar can be used with precisely one mutex at runtime. Any attempt to use multiple mutexes on the same condition variable will result in a runtime panic. If this is not desired, then the unsafe primitives in `sys` do not have this restriction but may result in undefined behavior.

# Examples

    use std::sync::{Arc, Mutex, Condvar};
    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().unwrap();
        *started = true;
        // We notify the condvar that the value has changed.
        cvar.notify_one();
    });

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

## Methods

### `impl Condvar`
[src]

#### `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

#### `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 will `panic!` if it is used with more than one mutex over time. Each condition variable is dynamically bound to exactly one mutex to ensure defined behavior across platforms. If this functionality is not desired, then unsafe primitives in `sys` are provided.

# Examples

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

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

    thread::spawn(move|| {
        let &(ref lock, ref 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 &(ref lock, ref cvar) = &*pair;
    let mut started = lock.lock().unwrap();
    // As long as the value inside the `Mutex` is false, we wait.
    while !*started {
        started = cvar.wait(started).unwrap();
    }Run

#### `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 may 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 = pair.clone();

    thread::spawn(move|| {
        let &(ref lock, ref 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 &(ref lock, ref cvar) = &*pair;
    let mut started = lock.lock().unwrap();
    // As long as the value inside the `Mutex` 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

#### `fn wait_timeout<'a, T>(&self,
guard: MutexGuard<'a, T>,
dur: Duration)
-> LockResult<(MutexGuard<'a, T>, WaitTimeoutResult)>`
1.5.0

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 may 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.

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 = pair.clone();

    thread::spawn(move|| {
        let &(ref lock, ref 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 &(ref lock, ref cvar) = &*pair;
    let mut started = lock.lock().unwrap();
    // as long as the value inside the `Mutex` 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

#### `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 = pair.clone();

    thread::spawn(move|| {
        let &(ref lock, ref 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 &(ref lock, ref cvar) = &*pair;
    let mut started = lock.lock().unwrap();
    // As long as the value inside the `Mutex` is false, we wait.
    while !*started {
        started = cvar.wait(started).unwrap();
    }Run

#### `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 = pair.clone();

    thread::spawn(move|| {
        let &(ref lock, ref 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 &(ref lock, ref cvar) = &*pair;
    let mut started = lock.lock().unwrap();
    // As long as the value inside the `Mutex` is false, we wait.
    while !*started {
        started = cvar.wait(started).unwrap();
    }Run

## Trait Implementations

### `impl Debug for Condvar`
1.16.0
[src]

#### `fn fmt(&self, f: &mut Formatter) -> Result`

Formats the value using the given formatter.

### `impl Default for Condvar`
1.9.0
[src]

#### `fn default() -> Condvar`

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

### `impl Drop for Condvar`
[src]

#### `fn drop(&mut self)`

A method called when the value goes out of scope. Read more

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