Module std::sync::mpsc

Multi-producer, single-consumer FIFO queue communication primitives.

This module provides message-based communication over channels, concretely defined among three types:

• Sender
• SyncSender
• Receiver

A Sender or SyncSender is used to send data to a Receiver. Both senders are clone-able (multi-producer) such that many threads can send simultaneously to one receiver (single-consumer).

These channels come in two flavors:

1. An asynchronous, infinitely buffered channel. The channel function will return a (Sender, Receiver) tuple where all sends will be asynchronous (they never block). The channel conceptually has an infinite buffer.

2. A synchronous, bounded channel. The sync_channel function will return a (SyncSender, Receiver) tuple where the storage for pending messages is a pre-allocated buffer of a fixed size. All sends will be synchronous by blocking until there is buffer space available. Note that a bound of 0 is allowed, causing the channel to become a “rendezvous” channel where each sender atomically hands off a message to a receiver.

Disconnection

The send and receive operations on channels will all return a Result indicating whether the operation succeeded or not. An unsuccessful operation is normally indicative of the other half of a channel having “hung up” by being dropped in its corresponding thread.

Once half of a channel has been deallocated, most operations can no longer continue to make progress, so Err will be returned. Many applications will continue to unwrap the results returned from this module, instigating a propagation of failure among threads if one unexpectedly dies.

Examples

Simple usage:

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

// Create a simple streaming channel
let (tx, rx) = channel();
thread::spawn(move|| {
    tx.send(10).unwrap();
});
assert_eq!(rx.recv().unwrap(), 10);

Shared usage:

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

// Create a shared channel that can be sent along from many threads
// where tx is the sending half (tx for transmission), and rx is the receiving
// half (rx for receiving).
let (tx, rx) = channel();
for i in 0..10 {
    let tx = tx.clone();
    thread::spawn(move|| {
        tx.send(i).unwrap();
    });
}

for _ in 0..10 {
    let j = rx.recv().unwrap();
    assert!(0 <= j && j < 10);
}

Propagating panics:

use std::sync::mpsc::channel;

// The call to recv() will return an error because the channel has already
// hung up (or been deallocated)
let (tx, rx) = channel::<i32>();
drop(tx);
assert!(rx.recv().is_err());

Synchronous channels:

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

let (tx, rx) = sync_channel::<i32>(0);
thread::spawn(move|| {
    // This will wait for the parent thread to start receiving
    tx.send(53).unwrap();
});
rx.recv().unwrap();

Unbounded receive loop:

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

let (tx, rx) = sync_channel(3);

for _ in 0..3 {
    // It would be the same without thread and clone here
    // since there will still be one `tx` left.
    let tx = tx.clone();
    // cloned tx dropped within thread
    thread::spawn(move || tx.send("ok").unwrap());
}

// Drop the last sender to stop `rx` waiting for message.
// The program will not complete if we comment this out.
// **All** `tx` needs to be dropped for `rx` to have `Err`.
drop(tx);

// Unbounded receiver waiting for all senders to complete.
while let Ok(msg) = rx.recv() {
    println!("{msg}");
}

println!("completed");

Structs

• IntoIter
  An owning iterator over messages on a Receiver, created by into_iter.
• Iter
  An iterator over messages on a Receiver, created by iter.
• Receiver
  The receiving half of Rust’s channel (or sync_channel) type. This half can only be owned by one thread.
• RecvError
  An error returned from the recv function on a Receiver.
• SendError
  An error returned from the Sender::send or SyncSender::send function on channels.
• Sender
  The sending-half of Rust’s asynchronous channel type.
• SyncSender
  The sending-half of Rust’s synchronous sync_channel type.
• TryIter
  An iterator that attempts to yield all pending values for a Receiver, created by try_iter.

Enums

• RecvTimeoutError
  This enumeration is the list of possible errors that made recv_timeout unable to return data when called. This can occur with both a channel and a sync_channel.
• TryRecvError
  This enumeration is the list of the possible reasons that try_recv could not return data when called. This can occur with both a channel and a sync_channel.
• TrySendError
  This enumeration is the list of the possible error outcomes for the try_send method.

Functions

• channel
  Creates a new asynchronous channel, returning the sender/receiver halves. All data sent on the Sender will become available on the Receiver in the same order as it was sent, and no send will block the calling thread (this channel has an “infinite buffer”, unlike sync_channel, which will block after its buffer limit is reached). recv will block until a message is available while there is at least one Sender alive (including clones).
• sync_channel
  Creates a new synchronous, bounded channel. All data sent on the SyncSender will become available on the Receiver in the same order as it was sent. Like asynchronous channels, the Receiver will block until a message becomes available. sync_channel differs greatly in the semantics of the sender, however.