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.
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 aThread
atomically makes the token available if it wasn’t already. Because the token is initially absent,unpark
followed bypark
will result in the second call returning immediately.
In other words, each Thread
acts a bit like a spinlock that can be
locked and unlocked using park
and unpark
.
Notice that being unblocked does not imply any synchronization with someone
that unparked this thread, it could also be spurious.
For example, it would be a valid, but inefficient, implementation to make both park
and
unpark
return immediately without doing anything.
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.
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::Acquire) {
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::Release);
println!("Unpark the thread");
parked_thread.thread().unpark();
parked_thread.join().unwrap();
Run