# Function std::thread::park

1.0.0 · source ·

    pub fn park()

Expand description

Blocks unless or until the current thread’s token is made available.

A call to `park` does not guarantee that the thread will remain parked forever, and callers should be prepared for this possibility. However, it is guaranteed that this function will not panic (it may abort the process if the implementation encounters some rare errors).

## `park` and `unpark`

Every thread is equipped with some basic low-level blocking support, via the `thread::park` function and `thread::Thread::unpark` method. `park` blocks the current thread, which can then be resumed from another thread by calling the `unpark` method on the blocked thread’s handle.

Conceptually, each `Thread` handle has an associated token, which is initially not present:

- The `thread::park` function blocks the current thread unless or until the token is available for its thread handle, at which point it atomically consumes the token. It may also return spuriously, without consuming the token. `thread::park_timeout` does the same, but allows specifying a maximum time to block the thread for.

- The `unpark` method on a `Thread` atomically makes the token available if it wasn’t already. Because the token is initially absent, `unpark` followed by `park` will result in the second call returning immediately.

The API is typically used by acquiring a handle to the current thread, placing that handle in a shared data structure so that other threads can find it, and then `park`ing in a loop. When some desired condition is met, another thread calls `unpark` on the handle.

The motivation for this design is twofold:

- It avoids the need to allocate mutexes and condvars when building new synchronization primitives; the threads already provide basic blocking/signaling.

- It can be implemented very efficiently on many platforms.

## Memory Ordering

Calls to `park` synchronize-with calls to `unpark`, meaning that memory operations performed before a call to `unpark` are made visible to the thread that consumes the token and returns from `park`. Note that all `park` and `unpark` operations for a given thread form a total order and `park` synchronizes-with all prior `unpark` operations.

In atomic ordering terms, `unpark` performs a `Release` operation and `park` performs the corresponding `Acquire` operation. Calls to `unpark` for the same thread form a release sequence.

Note that being unblocked does not imply a call was made to `unpark`, because wakeups can also be spurious. For example, a valid, but inefficient, implementation could have `park` and `unpark` return immediately without doing anything, making all wakeups spurious.

## Examples

    use std::thread;
    use std::sync::{Arc, atomic::{Ordering, AtomicBool}};
    use std::time::Duration;

    let flag = Arc::new(AtomicBool::new(false));
    let flag2 = Arc::clone(&flag);

    let parked_thread = thread::spawn(move || {
        // We want to wait until the flag is set. We *could* just spin, but using
        // park/unpark is more efficient.
        while !flag2.load(Ordering::Relaxed) {
            println!("Parking thread");
            thread::park();
            // We *could* get here spuriously, i.e., way before the 10ms below are over!
            // But that is no problem, we are in a loop until the flag is set anyway.
            println!("Thread unparked");
        }
        println!("Flag received");
    });

    // Let some time pass for the thread to be spawned.
    thread::sleep(Duration::from_millis(10));

    // Set the flag, and let the thread wake up.
    // There is no race condition here, if `unpark`
    // happens first, `park` will return immediately.
    // Hence there is no risk of a deadlock.
    flag.store(true, Ordering::Relaxed);
    println!("Unpark the thread");
    parked_thread.thread().unpark();

    parked_thread.join().unwrap();

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