Mutex lock twice

help
1

I try lock twice in the mutex, but why there no dead lock in it?

use std::sync::Mutex;
fn main() {
    let l = Mutex::<i32>::new(32);

    println!("ttt1");
    *l.lock().unwrap() = 10;
    println!("ttt2");
    *l.lock().unwrap() = 11;
    println!("ttt3");
}
1 Like
2

l.lock().unwrap(), when not assigned to any binding (i.e. not stored in a variable), is a temporary which is dropped at the end of statement, therefore unlocking the lock. If you store the guard, code deadlocks as expected:

use std::sync::Mutex;
fn main() {
    let l = Mutex::<i32>::new(32);

    println!("ttt1");
    let lock1 = l.lock().unwrap();
    println!("ttt2");
    let lock2 = l.lock().unwrap(); // locks forever
    println!("ttt3"); // never runs
}
3 Likes
3

In case you have to troubleshoot this situation, panic is also a possible outcome.