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#9 in Asynchronous

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Used in 760 crates (38 directly)

Apache-2.0 OR MIT



Build License Cargo Documentation

Async I/O and timers.

This crate provides two tools:

  • Async, an adapter for standard networking types (and many other types) to use in async programs.
  • Timer, a future that expires at a point in time.

For concrete async networking types built on top of this crate, see async-net.


The first time Async or Timer is used, a thread named "async-io" will be spawned. The purpose of this thread is to wait for I/O events reported by the operating system, and then wake appropriate futures blocked on I/O or timers when they can be resumed.

To wait for the next I/O event, the "async-io" thread uses epoll on Linux/Android/illumos, kqueue on macOS/iOS/BSD, event ports on illumos/Solaris, and wepoll on Windows. That functionality is provided by the polling crate.

However, note that you can also process I/O events and wake futures on any thread using the block_on() function. The "async-io" thread is therefore just a fallback mechanism processing I/O events in case no other threads are.


Connect to example.com:80, or time out after 10 seconds.

use async_io::{Async, Timer};
use futures_lite::{future::FutureExt, io};

use std::net::{TcpStream, ToSocketAddrs};
use std::time::Duration;

let addr = "example.com:80".to_socket_addrs()?.next().unwrap();

let stream = Async::<TcpStream>::connect(addr).or(async {


Licensed under either of

at your option.


Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.


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