Function park

pub fn park()

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 parking 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();