tokio::sync

Learn async synchronization primitives from tokio::sync.

tokio::sync

tokio::sync offers synchronization tools between tasks: Mutex, RwLock (they don't block the thread while waiting for the lock), mpsc, broadcast, watch and oneshot for a one-time reply:

let (tx, rx) = tokio::sync::oneshot::channel();
tokio::spawn(async move {
    tx.send(6 * 7).unwrap();
});
println!("{}", rx.await.unwrap());   // 42

oneshot is ideal for the "give a task some work, get its result in a single message" pattern.

std counterpart: a one-message channel

The standard library has no separate oneshot, but an mpsc channel used for a single message does the same job:

use std::sync::mpsc;
use std::thread;

let (tx, rx) = mpsc::channel();
thread::spawn(move || {
    tx.send(6 * 7).unwrap();
});
println!("{}", rx.recv().unwrap());

recv waits until a message arrives; if the sender is dropped without sending, it returns Err.

Exercise One-time reply

Open a channel with mpsc::channel. Let a separate thread send the result of 6 * 7; let the main thread receive it with recv and print it.

Show example solution

use std::sync::mpsc;
use std::thread;

fn main() {
    let (tx, rx) = mpsc::channel();
    thread::spawn(move || {
        tx.send(6 * 7).unwrap();
    });
    println!("{}", rx.recv().unwrap());
}

Test yourself

When is <code>std::sync::Mutex</code> acceptable in async code?
- Never
- Always
- Only on a single thread
- When the lock is held briefly and not across an .await
Show answer

When the lock is held briefly and not across an .await — Holding a std mutex across .await can block and also makes the future non-Send; in that case tokio::sync::Mutex is needed.

Wait for the message

let (tx, rx) = mpsc::channel();
thread::spawn(move || tx.send(42).unwrap());
let n = rx.____().unwrap();

Show answer

let (tx, rx) = mpsc::channel();
thread::spawn(move || tx.send(42).unwrap());
let n = rx.recv().unwrap();

Tool and use

oneshot

Single reply
mpsc

Many producers, one consumer
broadcast

Broadcast to all
watch

Watch the latest value

Next lesson: tokio::fs

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