Condition Variables in Rust

A Condition Variable is used to notify one or more threads of an event when the shared data is protected by a Mutex.

A condition variable takes the lock guard, unlocks the mutex, and puts the thread to sleep. When a notification is received, it re-acquires the mutex lock and returns the guard.

std::sync::Condvar

Condvar can be used to notify one or more threads of an event if the data is protected by a mutex.

use std::collections::VecDeque;
use std::sync::{Condvar, Mutex};
use std::thread;
use std::time::Duration;

fn main() {
	let queue = Mutex::new(VecDeque::new());
	let not_empty = Condvar::new();
	
	thread::scope(|s| {
		s.spawn(|| {
			loop {
				let mut q = queue.lock().unwrap();
				let item = loop {
					if let Some(item) = q.pop_front() {
						break item;
					} else {
						q = not_empty.wait(q).unwrap();
					}
				};
				drop(q);
				dbg!(item);
			}
		});
		for i in 0.. {
			let mut q = queue.lock().unwrap();
			q.push_back(i);
			not_empty.notify_one();
			thread::sleep(Duration::from_secs(1));
		}
	});
}

Condvar takes the MutexGuard, unlocks it, and goes to sleep, and when a notification is received, it locks the Mutex and returns the guard. We can use notify_one or notify_all to notify a single thread or all threads.