Associates readiness events with event::Sources.

Token is a wrapper around usize and is used as an argument to Registry::register and Registry::reregister.

See Poll for more documentation on polling.

Example

Using Token to track which socket generated the event. In this example, HashMap is used, but usually something like slab is better.

# use std::error::Error;
# fn main() -> Result<(), Box<dyn Error>> {
# // WASI does not yet support multithreading:
# if cfg!(target_os = "wasi") { return Ok(()) }
use mio::{Events, Interest, Poll, Token};
use mio::net::TcpListener;

use std::thread;
use std::io::{self, Read};
use std::collections::HashMap;

// After this number of sockets is accepted, the server will shutdown.
const MAX_SOCKETS: usize = 32;

// Pick a token that will not be used by any other socket and use that one
// for the listener.
const LISTENER: Token = Token(1024);

// Used to store the sockets.
let mut sockets = HashMap::new();

// This is used to generate a unique token for a socket
let mut next_socket_index = 0;

// The `Poll` instance
let mut poll = Poll::new()?;

// Tcp listener
let mut listener = TcpListener::bind("127.0.0.1:0".parse()?)?;

// Register the listener
poll.registry().register(&mut listener, LISTENER, Interest::READABLE)?;

// Spawn a thread that will connect a bunch of sockets then close them
let addr = listener.local_addr()?;
thread::spawn(move || {
    use std::net::TcpStream;

    // +1 here is to connect an extra socket to signal the socket to close
    for _ in 0..(MAX_SOCKETS+1) {
        // Connect then drop the socket
        let _ = TcpStream::connect(addr).unwrap();
    }
});

// Event storage
let mut events = Events::with_capacity(1024);

// Read buffer, this will never actually get filled
let mut buf = [0; 256];

// The main event loop
loop {
    // Wait for events
    poll.poll(&mut events, None)?;

    for event in &events {
        match event.token() {
            LISTENER => {
                // Perform operations in a loop until `WouldBlock` is
                // encountered.
                loop {
                    match listener.accept() {
                        Ok((mut socket, _)) => {
                            // Shutdown the server
                            if next_socket_index == MAX_SOCKETS {
                                return Ok(());
                            }

                            // Get the token for the socket
                            let token = Token(next_socket_index);
                            next_socket_index += 1;

                            // Register the new socket w/ poll
                            poll.registry().register(&mut socket, token, Interest::READABLE)?;

                            // Store the socket
                            sockets.insert(token, socket);
                        }
                        Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
                            // Socket is not ready anymore, stop accepting
                            break;
                        }
                        e => panic!("err={:?}", e), // Unexpected error
                    }
                }
            }
            token => {
                // Always operate in a loop
                loop {
                    match sockets.get_mut(&token).unwrap().read(&mut buf) {
                        Ok(0) => {
                            // Socket is closed, remove it from the map
                            sockets.remove(&token);
                            break;
                        }
                        // Data is not actually sent in this example
                        Ok(_) => unreachable!(),
                        Err(ref e) if e.kind() == io::ErrorKind::WouldBlock => {
                            // Socket is not ready anymore, stop reading
                            break;
                        }
                        e => panic!("err={:?}", e), // Unexpected error
                    }
                }
            }
        }
    }
}
# }