send and recv

Send and receive messages with send and recv.

send and recv

Both operations can report that the other side has closed:

tx.send(v) returns Err(SendError(v)) if the receiver was dropped, giving the value back.
rx.recv() waits until a message arrives; if all senders were dropped and the channel is empty, it returns Err(RecvError).

let (tx, rx) = mpsc::channel::<i32>();
drop(rx);
println!("{}", tx.send(1).is_err());   // true: nobody is listening

These errors are the clean way for one thread to learn that the other has finished.

Handling closure

A worker should stop quietly when the main thread closes:

thread::spawn(move || loop {
    let veri = hesapla();
    if tx.send(veri).is_err() {
        break;   // the receiver is gone, stop working
    }
});

On the recv side, the pattern while let Ok(m) = rx.recv() does the same job.

Exercise Closure errors

Drop the receiver of one channel and print the result of send(1).is_err(). Drop the sender of another channel and print the result of recv() with {:?}.

Show example solution

use std::sync::mpsc;

fn main() {
    let (tx, rx) = mpsc::channel::<i32>();
    drop(rx);
    println!("{}", tx.send(1).is_err());
    let (tx2, rx2) = mpsc::channel::<i32>();
    drop(tx2);
    println!("{:?}", rx2.recv());
}

Test yourself

Why does <code>send</code> give the value back on error?
- Convention
- It doesn't
- So the value isn't lost; it can be sent elsewhere or saved
- To prevent memory leaks
Show answer

So the value isn't lost; it can be sent elsewhere or saved — The value is reachable again via SendError(v).0.

Is the receiver closed?

if tx.send(veri).____() {
    break;
}

Show answer

if tx.send(veri).is_err() {
    break;
}

Case and result

rx dropped + send

Err(SendError(v))
tx dropped + recv (empty)

Err(RecvError)
Message present + recv

Ok(message)
while let Ok(m) = rx.recv()

Ends on close

Next lesson: Multiple producers

We Are Enhancing Your Experience

Cookie Preferences

Strictly Necessary Cookies

Analytical Cookies