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Important Rust Topics

Rust Arc and Mutex

Let several threads change one value safely: Arc shares it, Mutex guards it.

  • Advanced
  • 3 min read
  • 1 runnable example
What you'll learn
  • Mutex and lock
  • Arc::clone
  • Why the count is always right
Run code →
4 threads x 1000 increments
  1. 1Arc::cloneeach thread gets a handle
  2. 2lock()one at a time
  3. 3*value += 1safe update
  4. 44000always

One lock, many owners

When threads must change the same value, a Mutex makes sure only one of them touches it at a time. Arc lets every thread own a handle to that Mutex.

Arc::clone gives each thread its own handle to the same counter. lock waits until no other thread holds the lock and returns a guard; the lock is released when the guard goes out of scope. Four threads each add 1000, so the result is always 4000.

Key points

  1. lock gives one thread access at a time.
  2. The lock is released when the guard goes out of scope.
  3. Arc lets every thread own a handle.

Runnable Rust example

Rust
Run code →
main.rs
use std::sync::{Arc, Mutex};
use std::thread;

fn main() {
    let counter = Arc::new(Mutex::new(0));
    let mut handles = Vec::new();

    for _ in 0..4 {
        let counter = Arc::clone(&counter);
        handles.push(thread::spawn(move || {
            for _ in 0..1000 {
                let mut value = counter.lock().unwrap();
                *value += 1;
            }
        }));
    }

    for handle in handles {
        handle.join().unwrap();
    }
    println!("final count: {}", *counter.lock().unwrap());
}
Get a Run button for this example Result of running the code
final count: 4000

Practice

Open the example in the online compiler, change a value or add a line, and predict the output before you run it. Then break the code on purpose and read the compiler's message.

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