6 releases

0.2.16 Jan 30, 2020
0.2.15 Jan 23, 2020

#1327 in Asynchronous

MIT license

23K SLoC

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A runtime for writing reliable, asynchronous, and slim applications.

Tokio is an event-driven, non-blocking I/O platform for writing asynchronous applications with the Rust programming language. At a high level, it provides a few major components:

Guide level documentation is found on the website.

A Tour of Tokio

Tokio consists of a number of modules that provide a range of functionality essential for implementing asynchronous applications in Rust. In this section, we will take a brief tour of Tokio, summarizing the major APIs and their uses.

The easiest way to get started is to enable all features. Do this by enabling the full feature flag:

tokio = { version = "0.2", features = ["full"] }

Feature flags

Tokio uses a set of feature flags to reduce the amount of compiled code. It is possible to just enable certain features over others. By default, Tokio does not enable any features but allows one to enable a subset for their use case. Below is a list of the available feature flags. You may also notice above each function, struct and trait there is a set of feature flags that are required for that item to be enabled. If you are new to Tokio it is recommended that you use the full feature flag which will enable everything. Beware though that this will pull in many extra dependencies that you may not need.

  • full: Enables all Tokio features and every API will be available.
  • rt-core: Enables tokio::spawn and the basic (single-threaded) scheduler.
  • rt-threaded: Enables the heavier, multi-threaded, work-stealing scheduler.
  • rt-util: Enables non-scheduler utilities.
  • io-driver: Enables the mio based IO driver.
  • io-util: Enables the IO based Ext traits.
  • io-std: Enable Stdout, Stdin and Stderr types.
  • net: Enables tokio::net types such as TcpStream, UnixStream and UdpSocket.
  • tcp: Enables all tokio::net::tcp types.
  • udp: Enables all tokio::net::udp types.
  • uds: Enables all tokio::net::unix types.
  • time: Enables tokio::time types and allows the schedulers to enable the built in timer.
  • process: Enables tokio::process types.
  • macros: Enables #[tokio::main] and #[tokio::test] macros.
  • sync: Enables all tokio::sync types.
  • stream: Enables optional Stream implementations for types within Tokio.
  • signal: Enables all tokio::signal types.
  • fs: Enables tokio::fs types.
  • dns: Enables async tokio::net::ToSocketAddrs.
  • test-util: Enables testing based infrastructure for the Tokio runtime.
  • blocking: Enables block_in_place and spawn_blocking.

Note: AsyncRead and AsyncWrite do not require any features and are enabled by default.

Authoring applications

Tokio is great for writing applications and most users in this case shouldn't worry to much about what features they should pick. If you're unsure, we suggest going with full to ensure that you don't run into any road blocks while you're building your application.


This example shows the quickest way to get started with Tokio.

tokio = { version = "0.2", features = ["full"] }

Authoring libraries

As a library author your goal should be to provide the lighest weight crate that is based on Tokio. To achieve this you should ensure that you only enable the features you need. This allows users to pick up your crate without having to enable unnecessary features.


This example shows how you may want to import features for a library that just needs to tokio::spawn and use a TcpStream.

tokio = { version = "0.2", features = ["rt-core", "tcp"] }

Working With Tasks

Asynchronous programs in Rust are based around lightweight, non-blocking units of execution called tasks. The tokio::task module provides important tools for working with tasks:

  • The spawn function and JoinHandle type, for scheduling a new task on the Tokio runtime and awaiting the output of a spawned task, respectively,
  • Functions for running blocking operations in an asynchronous task context.

The tokio::task module is present only when the "rt-core" feature flag is enabled.

The tokio::sync module contains synchronization primitives to use when needing to communicate or share data. These include:

  • channels (oneshot, mpsc, and watch), for sending values between tasks,
  • a non-blocking Mutex, for controlling access to a shared, mutable value,
  • an asynchronous Barrier type, for multiple tasks to synchronize before beginning a computation.

The tokio::sync module is present only when the "sync" feature flag is enabled.

The tokio::time module provides utilities for tracking time and scheduling work. This includes functions for setting timeouts for tasks, delaying work to run in the future, or repeating an operation at an interval.

In order to use tokio::time, the "time" feature flag must be enabled.

Finally, Tokio provides a runtime for executing asynchronous tasks. Most applications can use the #[tokio::main] macro to run their code on the Tokio runtime. In use-cases where manual control over the runtime is required, the tokio::runtime module provides APIs for configuring and managing runtimes.

Using the runtime requires the "rt-core" or "rt-threaded" feature flags, to enable the basic single-threaded scheduler and the thread-pool scheduler, respectively. See the runtime module documentation for details. In addition, the "macros" feature flag enables the #[tokio::main] and #[tokio::test] attributes.

Asynchronous IO

As well as scheduling and running tasks, Tokio provides everything you need to perform input and output asynchronously.

The tokio::io module provides Tokio's asynchronous core I/O primitives, the AsyncRead, AsyncWrite, and AsyncBufRead traits. In addition, when the "io-util" feature flag is enabled, it also provides combinators and functions for working with these traits, forming as an asynchronous counterpart to std::io. When the "io-driver" feature flag is enabled, it also provides utilities for library authors implementing I/O resources.

Tokio also includes APIs for performing various kinds of I/O and interacting with the operating system asynchronously. These include:

  • tokio::net, which contains non-blocking versions of TCP, UDP, and Unix Domain Sockets (enabled by the "net" feature flag),
  • tokio::fs, similar to std::fs but for performing filesystem I/O asynchronously (enabled by the "fs" feature flag),
  • tokio::signal, for asynchronously handling Unix and Windows OS signals (enabled by the "signal" feature flag),
  • tokio::process, for spawning and managing child processes (enabled by the "process" feature flag).


A simple TCP echo server:

use tokio::net::TcpListener;
use tokio::prelude::*;

async fn main() -> Result<(), Box<dyn std::error::Error>> {
    let mut listener = TcpListener::bind("").await?;

    loop {
        let (mut socket, _) = listener.accept().await?;

        tokio::spawn(async move {
            let mut buf = [0; 1024];

            // In a loop, read data from the socket and write the data back.
            loop {
                let n = match socket.read(&mut buf).await {
                    // socket closed
                    Ok(n) if n == 0 => return,
                    Ok(n) => n,
                    Err(e) => {
                        eprintln!("failed to read from socket; err = {:?}", e);

                // Write the data back
                if let Err(e) = socket.write_all(&buf[0..n]).await {
                    eprintln!("failed to write to socket; err = {:?}", e);


~16K SLoC