Mutex and MutexGuard

In earlier examples, RAII was used to manage concrete resources like file descriptors. With a Mutex, the “resource” is mutable access to a value. You access the value by calling lock, which then returns a MutexGuard which will unlock the Mutex automatically when dropped.

use std::sync::Mutex;

fn main() {
    let m = Mutex::new(vec![1, 2, 3]);

    let mut guard = m.lock().unwrap();
    guard.push(4);
    guard.push(5);
    println!("{guard:?}");
}

  • A Mutex controls exclusive access to a value. Unlike earlier RAII examples, the resource here is logical: temporary exclusive access to the data inside.

  • This right is represented by a MutexGuard. Only one guard for this mutex can exist at a time. While it lives, it provides &mut T access.

  • Although lock() takes &self, it returns a MutexGuard with mutable access. This works through interior mutability, where a type manages its own borrowing rules internally to allow mutation through &self.

  • MutexGuard implements Deref and DerefMut, making access ergonomic. You lock the mutex and use the guard like a &mut T.

  • The mutex is released by MutexGuard::drop(). You never call an explicit unlock function.