smol-rs/smol

A small and fast async runtime for Rust

Source code at GitHub https://github.com/smol-rs/smol

{
  "createdAt": "2020-02-04T10:07:52Z",
  "defaultBranch": "master",
  "description": "A small and fast async runtime for Rust",
  "fullName": "smol-rs/smol",
  "homepage": "",
  "language": "Rust",
  "name": "smol",
  "pushedAt": "2025-11-03T13:07:08Z",
  "stargazersCount": 4617,
  "topics": [
    "async",
    "concurrency",
    "futures",
    "networking",
    "runtime",
    "rust"
  ],
  "updatedAt": "2025-11-26T16:58:02Z",
  "url": "https://github.com/smol-rs/smol"
}

smol

[]!( https://github.com/smol-rs/smol/actions) []!( https://github.com/smol-rs/smol) [ Cargo ]!( https://crates.io/crates/smol) []!( https://docs.rs/smol) [ Chat ]!( https://matrix.to/#/#smol-rs:matrix.org)

A small and fast async runtime.

This crate simply re-exports other smaller async crates (see the source).

To use tokio-based libraries with smol, apply the [async-compat] adapter to futures and I/O types.

See the [smol-macros] crate if you want a no proc-macro, fast compiling, easy-to-use async main and/or multi-threaded Executor setup out of the box.

Examples

Connect to an HTTP website, make a GET request, and pipe the response to the standard output:

use smol::{io, net, prelude::*, Unblock};

fn main() -> io::Result<()> {
    smol::block_on(async {
        let mut stream = net::TcpStream::connect("example.com:80").await?;
        let req = b"GET / HTTP/1.1\r\nHost: example.com\r\nConnection: close\r\n\r\n";
        stream.write_all(req).await?;

        let mut stdout = Unblock::new(std::io::stdout());
        io::copy(stream, &mut stdout).await?;
        Ok(())
    })
}

There’s a lot more in the [examples] directory.

[async-compat] !: https://docs.rs/async-compat/latest/async_compat/ [smol-macros] !: https://docs.rs/smol-macros/latest/smol_macros/ [examples] !: https://github.com/smol-rs/smol/tree/master/examples

Subcrates

[async-channel] - Multi-producer multi-consumer channels
[async-executor] - Composable async executors
[async-fs] - Async filesystem primitives
[async-io] - Async adapter for I/O types, also timers
[async-lock] - Async locks (barrier, mutex, reader-writer lock, semaphore)
[async-net] - Async networking primitives (TCP/UDP/Unix)
[async-process] - Async interface for working with processes
[async-task] - Task abstraction for building executors
[blocking] - A thread pool for blocking I/O
[futures-lite] - A lighter fork of [futures]
[polling] - Portable interface to epoll, kqueue, event ports, and wepoll

TLS certificate

Some code examples are using TLS for authentication. The repository contains a self-signed certificate usable for testing, but it should not be used for real-world scenarios. Browsers and tools like curl will show this certificate as insecure.

In browsers, accept the security prompt or use curl -k on the command line to bypass security warnings.

The certificate file was generated using minica and openssl:

minica --domains localhost -ip-addresses 127.0.0.1 -ca-cert certificate.pem
openssl pkcs12 -export -out identity.pfx -inkey localhost/key.pem -in localhost/cert.pem

Another useful tool for making certificates is [mkcert].

[mkcert] !: https://github.com/FiloSottile/mkcert

MSRV Policy

The Minimum Supported Rust Version (MSRV) of this crate is 1.71. As a tentative policy, the MSRV will not advance past the current Rust version provided by Debian Stable. At the time of writing, this version of Rust is 1.85. However, the MSRV may be advanced further in the event of a major ecosystem shift or a security vulnerability.

License

Licensed under either of

Apache License, Version 2.0 ([LICENSE-APACHE]!(LICENSE-APACHE) or http://www.apache.org/licenses/LICENSE-2.0)
MIT license ([LICENSE-MIT]!(LICENSE-MIT) or http://opensource.org/licenses/MIT)

at your option.

Contribution

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.