[][src]Crate async_std

Async version of the Rust standard library

async-std is a foundation of portable Rust software, a set of minimal and battle-tested shared abstractions for the broader Rust ecosystem. It offers std types, like Future and Stream, library-defined operations on language primitives, standard macros, I/O and multithreading, among many other things.

async-std is available from crates.io. Once included, async-std can be accessed in use statements through the path async_std, as in use async_std::future.

How to read this documentation

If you already know the name of what you are looking for, the fastest way to find it is to use the search bar at the top of the page.

Otherwise, you may want to jump to one of these useful sections:

If this is your first time, the documentation for async-std is written to be casually perused. Clicking on interesting things should generally lead you to interesting places. Still, there are important bits you don't want to miss, so read on for a tour of the async-std and its documentation!

Once you are familiar with the contents of async-std you may begin to find the verbosity of the prose distracting. At this stage in your development you may want to press the [-] button near the top of the page to collapse it into a more skimmable view.

While you are looking at that [-] button also notice the [src] button. Rust's API documentation comes with the source code and you are encouraged to read it. The async-std source is generally high quality and a peek behind the curtains is often enlightening.

Modules in this crate are organized in the same way as in std, except blocking functions have been replaced with async functions and threads have been replaced with lightweight tasks.

You can find more information, reading materials, and other resources here:

What is in the async-std documentation?

First, async-std is divided into a number of focused modules, all listed further down this page. These modules are the bedrock upon which async Rust is forged, and they have mighty names like async_std::os and async_std::task. Modules' documentation typically includes an overview of the module along with examples, and are a smart place to start familiarizing yourself with the library.

Second, async-std defines The Async Prelude, a small collection of items - mostly traits - that should be imported into every module of every async crate. The traits in the prelude are pervasive, making the prelude documentation a good entry point to learning about the library.

And finally, async-std exports a number of async macros, and lists them on this page.

Contributing changes to the documentation

Check out async-std's contribution guidelines here. The source for this documentation can be found on GitHub. To contribute changes, make sure you read the guidelines first, then submit pull requests for your suggested changes.

Contributions are appreciated! If you see a part of the docs that can be improved, submit a PR, or chat with us first on Discord.

A tour of async-std

The rest of this crate documentation is dedicated to pointing out notable features of async-std.

Platform abstractions and I/O

Besides basic data types, async-std is largely concerned with abstracting over differences in common platforms, most notably Windows and Unix derivatives.

Common types of I/O, including files, TCP, UDP, are defined in the io, fs, and net modules.

The task module contains async-std's task abstractions. sync contains further primitive shared memory types. channel contains the channel types for message passing.

Timeouts, intervals, and delays

async-std provides several methods to manipulate time:


All examples require the "attributes" feature to be enabled. This feature is not enabled by default because it significantly impacts compile times. See task::block_on for an alternative way to start executing tasks.

Call an async function from the main function:

async fn say_hello() {
    println!("Hello, world!");

async fn main() {

Await two futures concurrently, and return a tuple of their output:

use async_std::prelude::*;

async fn main() {
    let a = async { 1u8 };
    let b = async { 2u8 };
    assert_eq!(a.join(b).await, (1u8, 2u8))

Create a UDP server that echoes back each received message to the sender:

use async_std::net::UdpSocket;

async fn main() -> std::io::Result<()> {
    let socket = UdpSocket::bind("").await?;
    println!("Listening on {}", socket.local_addr()?);

    let mut buf = vec![0u8; 1024];

    loop {
        let (recv, peer) = socket.recv_from(&mut buf).await?;
        let sent = socket.send_to(&buf[..recv], &peer).await?;
        println!("Sent {} out of {} bytes to {}", sent, recv, peer);


Items marked with unstable are available only when the unstable Cargo feature is enabled:

version = "1.7.0"
features = ["unstable"]

Items marked with attributes are available only when the attributes Cargo feature is enabled:

version = "1.7.0"
features = ["attributes"]

Compatibility with the tokio 1.0 runtime is also simultaneously possible using the tokio1 Cargo feature:

version = "1.7.0"
features = ["tokio1"]

Compatibility with the tokio 0.2 runtime is possible using the tokio02 Cargo feature:

version = "1.7.0"
features = ["tokio02"]

Compatibility with the tokio 0.3 runtime is also simultaneously possible using the tokio03 Cargo feature:

version = "1.7.0"
features = ["tokio03"]

Additionally it's possible to only use the core traits and combinators by only enabling the std Cargo feature:

version = "1.7.0"
default-features = false
features = ["std"]

And to use async-std on no_std targets that only support alloc only enable the alloc Cargo feature:

version = "1.7.0"
default-features = false
features = ["alloc"]

Runtime configuration

Several environment variables are available to tune the async-std runtime:

  • ASYNC_STD_THREAD_COUNT: The number of threads that the async-std runtime will start. By default, this is one per logical cpu as reported by the num_cpus crate, which may be different than the number of physical cpus. Async-std will panic if this is set to any value other than a positive integer.
  • ASYNC_STD_THREAD_NAME: The name that async-std's runtime threads report to the operating system. The default value is "async-std/runtime".





Filesystem manipulation operations.


Asynchronous values.


Traits, helpers, and type definitions for core I/O functionality.


Networking primitives for TCP/UDP communication.


OS-specific extensions.


Cross-platform path manipulation.


Types that pin data to its location in memory.


The async prelude.


A module for working with processes.


Composable asynchronous iteration.


Synchronization primitives.


Types and traits for working with asynchronous tasks.



Prints to the standard error.


Prints to the standard error, with a newline.


Prints to the standard output.


Prints to the standard output, with a newline.


Declares task-local values.


Writes formatted data into a buffer.


Write formatted data into a buffer, with a newline appended.

Attribute Macros


Enables an async main function.


Enables an async test function.