Sharing data

Other than sending data into threads one way, many programs operate on a shared state where multiple execution streams have to access and change one or more shared variables. Typically, this warrants a mutex (short for mutual exclusion), so that any time something is accessed within this locked mutex, it is guaranteed to be a single thread.

This is an old concept and implemented in the Rust standard library. How does that facilitate accessing a variable? Wrapping a variable into a Mutex type will provide for the locking mechanism, thereby making it accessible from multiple concurrent writers. However, they don't have ownership of that memory area yet.

In order to provide that ownership across threads—similar to what Rc does within a single thread—Rust provides the concept of an Arc, an atomic reference counter. Using this Mutex on top, it's the thread-safe equivalent of an Rc wrapping a RefCell, a reference counter that wraps a mutable container. To provide an example, this works nicely:

use std::thread;
use std::sync::{Mutex, Arc};

fn shared_state() {
    let v = Arc::new(Mutex::new(vec![]));
    let handles = (0..10).map(|i| {
        let numbers = Arc::clone(&v);
        thread::spawn(move || {
            let mut vector = numbers
                .lock()
                .unwrap();
            (*vector).push(i);
        })
    });

    for handle in handles {
        handle.join().unwrap();
    }
    println!("{:?}", *v.lock().unwrap());
}

When running this example, the output is this:

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9] 

While the preferred way of doing concurrent programming is still to use immutable variables as often as possible, safe Rust provides the tools for working with shared data without side effects.