[solved]Condvar makes Mutex lock twice?

help
1

Here is the code:

#![allow(unused)]
fn main() {
use std::sync::{Arc, Mutex, Condvar};
use std::thread;
use std::time::Duration;
use std::thread::sleep;

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;
    sleep(Duration::from_secs(1));
    println!("t try lock");
    let mut started = lock.lock().unwrap();
    println!("t locked");
    *started = true;
    // We notify the condvar that the value has changed.
    cvar.notify_one();
    println!("t unlocking");
});

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

println!("main unlocking");
}

Here is the output:

main locked
t try lock
t locked
t unlocking
main unlocking
2

From the docs:

This function will atomically unlock the mutex specified

So cvar.wait(started) unlocks the mutex, allowing the other thread to lock. Once it's notified, it locks again:

When this function call returns, the lock specified will have been re-acquired.

3

Thank you!:blush: