Shared-State Concurrency

Unlike message passing’s single ownership model, shared-state concurrency allows multiple threads to access the same memory locations simultaneously. Rust provides thread-safe primitives through mutexes and atomic reference counting.

Mutexes for Exclusive Data Access

A mutex (mutual exclusion) guards data with a locking mechanism, ensuring only one thread accesses the data at a time.

use std::sync::Mutex;

fn main() {
    let m = Mutex::new(5);

    {
        let mut num = m.lock().unwrap();
        *num = 6;
    }

    println!("m = {m:?}");
}

Key Mutex<T> characteristics:

lock() returns LockResult<MutexGuard<T>>
MutexGuard<T> implements Deref to access inner data
Drop implementation automatically releases the lock when guard goes out of scope
Lock acquisition blocks until available or panics if poisoned

Attempting to share a mutex across threads fails without proper ownership handling:

use std::sync::Mutex;
use std::thread;

fn main() {
    let counter = Mutex::new(0);
    let mut handles = vec![];

    for _ in 0..10 {
        let handle = thread::spawn(move || {
            let mut num = counter.lock().unwrap();

            *num += 1;
        });
        handles.push(handle);
    }

    for handle in handles {
        handle.join().unwrap();
    }

    println!("Result: {}", *counter.lock().unwrap());
}

Error: cannot move counter into multiple threads:

$ cargo run
   Compiling shared-state v0.1.0 (file:///projects/shared-state)
error[E0382]: borrow of moved value: `counter`
  --> src/main.rs:21:29
   |
5  |     let counter = Mutex::new(0);
   |         ------- move occurs because `counter` has type `Mutex<i32>`, which does not implement the `Copy` trait
...
8  |     for _ in 0..10 {
   |     -------------- inside of this loop
9  |         let handle = thread::spawn(move || {
   |                                    ------- value moved into closure here, in previous iteration of loop
...
21 |     println!("Result: {}", *counter.lock().unwrap());
   |                             ^^^^^^^ value borrowed here after move
   |
help: consider moving the expression out of the loop so it is only moved once
   |
8  ~     let mut value = counter.lock();
9  ~     for _ in 0..10 {
10 |         let handle = thread::spawn(move || {
11 ~             let mut num = value.unwrap();
   |

For more information about this error, try `rustc --explain E0382`.
error: could not compile `shared-state` (bin "shared-state") due to 1 previous error

Multiple Ownership with `Rc<T>` Limitation

Rc<T> provides multiple ownership but is not thread-safe:

use std::rc::Rc;
use std::sync::Mutex;
use std::thread;

fn main() {
    let counter = Rc::new(Mutex::new(0));
    let mut handles = vec![];

    for _ in 0..10 {
        let counter = Rc::clone(&counter);
        let handle = thread::spawn(move || {
            let mut num = counter.lock().unwrap();

            *num += 1;
        });
        handles.push(handle);
    }

    for handle in handles {
        handle.join().unwrap();
    }

    println!("Result: {}", *counter.lock().unwrap());
}

Error: Rc<T> lacks Send trait for thread safety:

$ cargo run
   Compiling shared-state v0.1.0 (file:///projects/shared-state)
error[E0277]: `Rc<Mutex<i32>>` cannot be sent between threads safely
  --> src/main.rs:11:36
   |
11 |           let handle = thread::spawn(move || {
   |                        ------------- ^------
   |                        |             |
   |  ______________________|_____________within this `{closure@src/main.rs:11:36: 11:43}`
   | |                      |
   | |                      required by a bound introduced by this call
12 | |             let mut num = counter.lock().unwrap();
13 | |
14 | |             *num += 1;
15 | |         });
   | |_________^ `Rc<Mutex<i32>>` cannot be sent between threads safely
   |
   = help: within `{closure@src/main.rs:11:36: 11:43}`, the trait `Send` is not implemented for `Rc<Mutex<i32>>`
note: required because it's used within this closure
  --> src/main.rs:11:36
   |
11 |         let handle = thread::spawn(move || {
   |                                    ^^^^^^^
note: required by a bound in `spawn`
  --> /rustc/4eb161250e340c8f48f66e2b929ef4a5bed7c181/library/std/src/thread/mod.rs:728:1

For more information about this error, try `rustc --explain E0277`.
error: could not compile `shared-state` (bin "shared-state") due to 1 previous error

Rc<T> uses non-atomic reference counting, making it unsafe for concurrent access.

Atomic Reference Counting with `Arc<T>`

Arc<T> (atomically reference-counted) provides thread-safe multiple ownership:

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

fn main() {
    let counter = Arc::new(Mutex::new(0));
    let mut handles = vec![];

    for _ in 0..10 {
        let counter = Arc::clone(&counter);
        let handle = thread::spawn(move || {
            let mut num = counter.lock().unwrap();

            *num += 1;
        });
        handles.push(handle);
    }

    for handle in handles {
        handle.join().unwrap();
    }

    println!("Result: {}", *counter.lock().unwrap());
}

Arc<T> uses atomic operations for reference counting, ensuring thread safety. Atomic operations have performance overhead, so use only when needed for concurrent access.

Interior Mutability Pattern

Mutex<T> provides interior mutability similar to RefCell<T>:

RefCell<T>/Rc<T>: Single-threaded shared ownership with runtime borrow checking
Mutex<T>/Arc<T>: Multi-threaded shared ownership with locking

Both patterns allow mutation through immutable references, but Mutex<T> includes deadlock risks when acquiring multiple locks.

The Rust Programming Language