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Mio – Metal I/O

Mio is a fast, low-level I/O library for Rust focusing on non-blocking APIs and event notification for building high performance I/O apps with as little overhead as possible over the OS abstractions. MIT licensed Build Status Build Status

API documentation

This is a low level library, if you are looking for something easier to get started with, see Tokio.


To use mio, first add this to your Cargo.toml:

mio = "1"

Next we can start using Mio. The following is quick introduction using TcpListener and TcpStream. Note that features = ["os-poll", "net"] must be specified for this example.

use std::error::Error;

use mio::net::{TcpListener, TcpStream};
use mio::{Events, Interest, Poll, Token};

// Some tokens to allow us to identify which event is for which socket.
const SERVER: Token = Token(0);
const CLIENT: Token = Token(1);

fn main() -> Result<(), Box<dyn Error>> {
    // Create a poll instance.
    let mut poll = Poll::new()?;
    // Create storage for events.
    let mut events = Events::with_capacity(128);

    // Setup the server socket.
    let addr = "".parse()?;
    let mut server = TcpListener::bind(addr)?;
    // Start listening for incoming connections.
        .register(&mut server, SERVER, Interest::READABLE)?;

    // Setup the client socket.
    let mut client = TcpStream::connect(addr)?;
    // Register the socket.
        .register(&mut client, CLIENT, Interest::READABLE | Interest::WRITABLE)?;

    // Start an event loop.
    loop {
        // Poll Mio for events, blocking until we get an event.
        poll.poll(&mut events, None)?;

        // Process each event.
        for event in events.iter() {
            // We can use the token we previously provided to `register` to
            // determine for which socket the event is.
            match event.token() {
                SERVER => {
                    // If this is an event for the server, it means a connection
                    // is ready to be accepted.
                    // Accept the connection and drop it immediately. This will
                    // close the socket and notify the client of the EOF.
                    let connection = server.accept();
                CLIENT => {
                    if event.is_writable() {
                        // We can (likely) write to the socket without blocking.

                    if event.is_readable() {
                        // We can (likely) read from the socket without blocking.

                    // Since the server just shuts down the connection, let's
                    // just exit from our event loop.
                    return Ok(());
                // We don't expect any events with tokens other than those we provided.
                _ => unreachable!(),


  • Non-blocking TCP, UDP, UDS
  • I/O event queue backed by epoll, kqueue, and IOCP
  • Zero allocations at runtime
  • Platform specific extensions


The following are specifically omitted from Mio and are left to the user or higher-level libraries.

  • File operations
  • Thread pools / multi-threaded event loop
  • Timers


Currently supported platforms:

  • Android (API level 21)
  • DragonFly BSD
  • FreeBSD
  • Linux
  • NetBSD
  • OpenBSD
  • Windows
  • iOS
  • macOS

Mio can handle interfacing with each of the event systems of the aforementioned platforms. The details of their implementation are further discussed in the Poll type of the API documentation (see above).

Mio generally supports the same versions of the above mentioned platforms as Rust the language (rustc) does, unless otherwise noted.

The Windows implementation for polling sockets is using the wepoll strategy. This uses the Windows AFD system to access socket readiness events.


MSRV Policy

The MSRV (Minimum Supported Rust Version) is fixed for a given minor (1.x) version. However it can be increased when bumping minor versions, i.e. going from 1.0 to 1.1 allows us to increase the MSRV. Users unable to increase their Rust version can an older minor version instead. Below is a list of Mio versions and their MSRV:

  • v0.8: Rust 1.46.
  • v1.0: Rust 1.70.

Note however that Mio also has dependencies, which might have different MSRV policies. We try to stick to the above policy when updating dependencies, but this is not always possible.

Unsupported flags

Mio uses different implementations to support the same functionality depending on the platform. Mio generally uses the "best" implementation possible, where "best" usually means most efficient for Mio's use case. However this means that the implementation is often specific to a limited number of platforms, meaning we often have multiple implementations for the same functionality. In some cases it might be required to not use the "best" implementation, but another implementation Mio supports (on other platforms). Mio does not officially support secondary implementations on platforms, however we do have various cfg flags to force another implementation for these situations.

Current flags:

  • mio_unsupported_force_poll_poll, uses an implementation based on poll(2) for mio::Poll.
  • mio_unsupported_force_waker_pipe, uses an implementation based on pipe(2) for mio::Waker.

Again, Mio does not officially supports this. Furthermore these flags may disappear in the future.


A group of Mio users hang out on Discord, this can be a good place to go for questions. It's also possible to open a new issue on GitHub to ask questions, report bugs or suggest new features.


Interested in getting involved? We would love to help you! For simple bug fixes, just submit a PR with the fix and we can discuss the fix directly in the PR. If the fix is more complex, start with an issue.

If you want to propose an API change, create an issue to start a discussion with the community. Also, feel free to talk with us in Discord.

Finally, be kind. We support the Rust Code of Conduct.