Struct Signal

pub struct Signal { /* private fields */ }

An listener for receiving a particular type of OS signal.

The listener can be turned into a Stream using SignalStream.

In general signal handling on Unix is a pretty tricky topic, and this structure is no exception! There are some important limitations to keep in mind when using Signal streams:

• Signals handling in Unix already necessitates coalescing signals together sometimes. This Signal stream is also no exception here in that it will also coalesce signals. That is, even if the signal handler for this process runs multiple times, the Signal stream may only return one signal notification. Specifically, before poll is called, all signal notifications are coalesced into one item returned from poll. Once poll has been called, however, a further signal is guaranteed to be yielded as an item.

  Put another way, any element pulled off the returned listener corresponds to at least one signal, but possibly more.

• Signal handling in general is relatively inefficient. Although some improvements are possible in this crate, it’s recommended to not plan on having millions of signal channels open.

If you’ve got any questions about this feel free to open an issue on the repo! New approaches to alleviate some of these limitations are always appreciated!

Caveats

The first time that a Signal instance is registered for a particular signal kind, an OS signal-handler is installed which replaces the default platform behavior when that signal is received, for the duration of the entire process.

For example, Unix systems will terminate a process by default when it receives SIGINT. But, when a Signal instance is created to listen for this signal, the next SIGINT that arrives will be translated to a stream event, and the process will continue to execute. Even if this Signal instance is dropped, subsequent SIGINT deliveries will end up captured by Tokio, and the default platform behavior will NOT be reset.

Thus, applications should take care to ensure the expected signal behavior occurs as expected after listening for specific signals.

Examples

Wait for SIGHUP

use tokio::signal::unix::{signal, SignalKind};

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    // An infinite stream of hangup signals.
    let mut sig = signal(SignalKind::hangup())?;

    // Print whenever a HUP signal is received
    loop {
        sig.recv().await;
        println!("got signal HUP");
    }
}

Implementations

impl Signal

pub async fn recv(&mut self) -> Option<()>

Receives the next signal notification event.

None is returned if no more events can be received by this stream.

Cancel safety

This method is cancel safe. If you use it as the event in a tokio::select! statement and some other branch completes first, then it is guaranteed that no signal is lost.

Examples

Wait for SIGHUP

use tokio::signal::unix::{signal, SignalKind};

#[tokio::main]
async fn main() -> Result<(), Box<dyn std::error::Error>> {
    // An infinite stream of hangup signals.
    let mut stream = signal(SignalKind::hangup())?;

    // Print whenever a HUP signal is received
    loop {
        stream.recv().await;
        println!("got signal HUP");
    }
}

pub fn poll_recv(&mut self, cx: &mut Context<'_>) -> Poll<Option<()>>

Polls to receive the next signal notification event, outside of an async context.

This method returns:

• Poll::Pending if no signals are available but the channel is not closed.
• Poll::Ready(Some(())) if a signal is available.
• Poll::Ready(None) if the channel has been closed and all signals sent before it was closed have been received.

Examples

Polling from a manually implemented future

use std::pin::Pin;
use std::future::Future;
use std::task::{Context, Poll};
use tokio::signal::unix::Signal;

struct MyFuture {
    signal: Signal,
}

impl Future for MyFuture {
    type Output = Option<()>;

    fn poll(mut self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<Self::Output> {
        println!("polling MyFuture");
        self.signal.poll_recv(cx)
    }
}

Trait Implementations

impl Debug for Signal

fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter.