Execution Hangs if Mutex Lock Created in Main Thread Before Joins

[benoy]

Hi, I'm trying to understand why this code hangs

fn main(){
    let num = Arc::new(Mutex::new(0_u32));
    let num2 = num.clone();

    let h = thread::spawn(move || {
        thread::sleep(Duration::from_millis(1000));
        let mut num2_lock = num2.lock().unwrap();
        *num2_lock = 10_u32;
    } );

    let num_lock = num.lock().unwrap(); // <-- hangs if uncommented

    h.join().unwrap();

    //let num_lock = num.lock().unwrap(); // <-- does not hang 

    println!("{}", *num_lock);
}

If I create num_lock before the join() then it hangs. There is no compile time error. Just that run hangs as far as I can tell.

But if I create num_lock after the join(), the output is the value that I had set in the thread. Which is what I expected.

Can anyone please help me understand why the first version would hang?
Thanks,

[alice]

It is because the Mutex stays locked while you call join, but the other thread is waiting for the main thread to release the lock.