Crate crossbeam::channel

Multi-producer multi-consumer channels for message passing.

This crate is an alternative to std::sync::mpsc with more features and better performance.

Hello, world!

use crossbeam_channel::unbounded;

// Create a channel of unbounded capacity.
let (s, r) = unbounded();

// Send a message into the channel.
s.send("Hello, world!").unwrap();

// Receive the message from the channel.
assert_eq!(r.recv(), Ok("Hello, world!"));

Channel types

Channels can be created using two functions:

• bounded creates a channel of bounded capacity, i.e. there is a limit to how many messages it can hold at a time.

• unbounded creates a channel of unbounded capacity, i.e. it can hold any number of messages at a time.

Both functions return a Sender and a Receiver, which represent the two opposite sides of a channel.

Creating a bounded channel:

use crossbeam_channel::bounded;

// Create a channel that can hold at most 5 messages at a time.
let (s, r) = bounded(5);

// Can send only 5 messages without blocking.
for i in 0..5 {
    s.send(i).unwrap();
}

// Another call to `send` would block because the channel is full.
// s.send(5).unwrap();

Creating an unbounded channel:

use crossbeam_channel::unbounded;

// Create an unbounded channel.
let (s, r) = unbounded();

// Can send any number of messages into the channel without blocking.
for i in 0..1000 {
    s.send(i).unwrap();
}

A special case is zero-capacity channel, which cannot hold any messages. Instead, send and receive operations must appear at the same time in order to pair up and pass the message over:

use std::thread;
use crossbeam_channel::bounded;

// Create a zero-capacity channel.
let (s, r) = bounded(0);

// Sending blocks until a receive operation appears on the other side.
thread::spawn(move || s.send("Hi!").unwrap());

// Receiving blocks until a send operation appears on the other side.
assert_eq!(r.recv(), Ok("Hi!"));

Sharing channels

Senders and receivers can be cloned and sent to other threads:

use std::thread;
use crossbeam_channel::bounded;

let (s1, r1) = bounded(0);
let (s2, r2) = (s1.clone(), r1.clone());

// Spawn a thread that receives a message and then sends one.
thread::spawn(move || {
    r2.recv().unwrap();
    s2.send(2).unwrap();
});

// Send a message and then receive one.
s1.send(1).unwrap();
r1.recv().unwrap();

Note that cloning only creates a new handle to the same sending or receiving side. It does not create a separate stream of messages in any way:

use crossbeam_channel::unbounded;

let (s1, r1) = unbounded();
let (s2, r2) = (s1.clone(), r1.clone());
let (s3, r3) = (s2.clone(), r2.clone());

s1.send(10).unwrap();
s2.send(20).unwrap();
s3.send(30).unwrap();

assert_eq!(r3.recv(), Ok(10));
assert_eq!(r1.recv(), Ok(20));
assert_eq!(r2.recv(), Ok(30));

It’s also possible to share senders and receivers by reference:

use crossbeam_channel::bounded;
use crossbeam_utils::thread::scope;

let (s, r) = bounded(0);

scope(|scope| {
    // Spawn a thread that receives a message and then sends one.
    scope.spawn(|_| {
        r.recv().unwrap();
        s.send(2).unwrap();
    });

    // Send a message and then receive one.
    s.send(1).unwrap();
    r.recv().unwrap();
}).unwrap();

Disconnection

When all senders or all receivers associated with a channel get dropped, the channel becomes disconnected. No more messages can be sent, but any remaining messages can still be received. Send and receive operations on a disconnected channel never block.

use crossbeam_channel::{unbounded, RecvError};

let (s, r) = unbounded();
s.send(1).unwrap();
s.send(2).unwrap();
s.send(3).unwrap();

// The only sender is dropped, disconnecting the channel.
drop(s);

// The remaining messages can be received.
assert_eq!(r.recv(), Ok(1));
assert_eq!(r.recv(), Ok(2));
assert_eq!(r.recv(), Ok(3));

// There are no more messages in the channel.
assert!(r.is_empty());

// Note that calling `r.recv()` does not block.
// Instead, `Err(RecvError)` is returned immediately.
assert_eq!(r.recv(), Err(RecvError));

Blocking operations

Send and receive operations come in three flavors:

• Non-blocking (returns immediately with success or failure).
• Blocking (waits until the operation succeeds or the channel becomes disconnected).
• Blocking with a timeout (blocks only for a certain duration of time).

A simple example showing the difference between non-blocking and blocking operations:

use crossbeam_channel::{bounded, RecvError, TryRecvError};

let (s, r) = bounded(1);

// Send a message into the channel.
s.send("foo").unwrap();

// This call would block because the channel is full.
// s.send("bar").unwrap();

// Receive the message.
assert_eq!(r.recv(), Ok("foo"));

// This call would block because the channel is empty.
// r.recv();

// Try receiving a message without blocking.
assert_eq!(r.try_recv(), Err(TryRecvError::Empty));

// Disconnect the channel.
drop(s);

// This call doesn't block because the channel is now disconnected.
assert_eq!(r.recv(), Err(RecvError));

Iteration

Receivers can be used as iterators. For example, method iter creates an iterator that receives messages until the channel becomes empty and disconnected. Note that iteration may block waiting for next message to arrive.

use std::thread;
use crossbeam_channel::unbounded;

let (s, r) = unbounded();

thread::spawn(move || {
    s.send(1).unwrap();
    s.send(2).unwrap();
    s.send(3).unwrap();
    drop(s); // Disconnect the channel.
});

// Collect all messages from the channel.
// Note that the call to `collect` blocks until the sender is dropped.
let v: Vec<_> = r.iter().collect();

assert_eq!(v, [1, 2, 3]);

A non-blocking iterator can be created using try_iter, which receives all available messages without blocking:

use crossbeam_channel::unbounded;

let (s, r) = unbounded();
s.send(1).unwrap();
s.send(2).unwrap();
s.send(3).unwrap();
// No need to drop the sender.

// Receive all messages currently in the channel.
let v: Vec<_> = r.try_iter().collect();

assert_eq!(v, [1, 2, 3]);

Selection

The select! macro allows you to define a set of channel operations, wait until any one of them becomes ready, and finally execute it. If multiple operations are ready at the same time, a random one among them is selected.

It is also possible to define a default case that gets executed if none of the operations are ready, either right away or for a certain duration of time.

An operation is considered to be ready if it doesn’t have to block. Note that it is ready even when it will simply return an error because the channel is disconnected.

An example of receiving a message from two channels:

use std::thread;
use std::time::Duration;
use crossbeam_channel::{select, unbounded};

let (s1, r1) = unbounded();
let (s2, r2) = unbounded();

thread::spawn(move || s1.send(10).unwrap());
thread::spawn(move || s2.send(20).unwrap());

// At most one of these two receive operations will be executed.
select! {
    recv(r1) -> msg => assert_eq!(msg, Ok(10)),
    recv(r2) -> msg => assert_eq!(msg, Ok(20)),
    default(Duration::from_secs(1)) => println!("timed out"),
}

If you need to select over a dynamically created list of channel operations, use Select instead. The select! macro is just a convenience wrapper around Select.

Extra channels

Three functions can create special kinds of channels, all of which return just a Receiver handle:

• after creates a channel that delivers a single message after a certain duration of time.
• tick creates a channel that delivers messages periodically.
• never creates a channel that never delivers messages.

These channels are very efficient because messages get lazily generated on receive operations.

An example that prints elapsed time every 50 milliseconds for the duration of 1 second:

use std::time::{Duration, Instant};
use crossbeam_channel::{after, select, tick};

let start = Instant::now();
let ticker = tick(Duration::from_millis(50));
let timeout = after(Duration::from_secs(1));

loop {
    select! {
        recv(ticker) -> _ => println!("elapsed: {:?}", start.elapsed()),
        recv(timeout) -> _ => break,
    }
}

Macros

• select
  Selects from a set of channel operations.

Structs

• IntoIter
  A blocking iterator over messages in a channel.
• Iter
  A blocking iterator over messages in a channel.
• ReadyTimeoutError
  An error returned from the ready_timeout method.
• Receiver
  The receiving side of a channel.
• RecvError
  An error returned from the recv method.
• Select
  Selects from a set of channel operations.
• SelectTimeoutError
  An error returned from the select_timeout method.
• SelectedOperation
  A selected operation that needs to be completed.
• SendError
  An error returned from the send method.
• Sender
  The sending side of a channel.
• TryIter
  A non-blocking iterator over messages in a channel.
• TryReadyError
  An error returned from the try_ready method.
• TrySelectError
  An error returned from the try_select method.

Enums

• RecvTimeoutError
  An error returned from the recv_timeout method.
• SendTimeoutError
  An error returned from the send_timeout method.
• TryRecvError
  An error returned from the try_recv method.
• TrySendError
  An error returned from the try_send method.

Functions

• after
  Creates a receiver that delivers a message after a certain duration of time.
• at
  Creates a receiver that delivers a message at a certain instant in time.
• bounded
  Creates a channel of bounded capacity.
• never
  Creates a receiver that never delivers messages.
• tick
  Creates a receiver that delivers messages periodically.
• unbounded
  Creates a channel of unbounded capacity.