Function spawn

pub fn spawn<F>(future: F) -> JoinHandle<F::Output> 
where
    F: Future + Send + 'static,
    F::Output: Send + 'static,

Spawns a new asynchronous task, returning a JoinHandle for it.

The provided future will start running in the background immediately when spawn is called, even if you don’t await the returned JoinHandle.

Spawning a task enables the task to execute concurrently to other tasks. The spawned task may execute on the current thread, or it may be sent to a different thread to be executed. The specifics depend on the current Runtime configuration.

It is guaranteed that spawn will not synchronously poll the task being spawned. This means that calling spawn while holding a lock does not pose a risk of deadlocking with the spawned task.

There is no guarantee that a spawned task will execute to completion. When a runtime is shutdown, all outstanding tasks are dropped, regardless of the lifecycle of that task.

This function must be called from the context of a Tokio runtime. Tasks running on the Tokio runtime are always inside its context, but you can also enter the context using the Runtime::enter method.

Examples

In this example, a server is started and spawn is used to start a new task that processes each received connection.

use tokio::net::{TcpListener, TcpStream};

use std::io;

async fn process(socket: TcpStream) {
    // ...
}

#[tokio::main]
async fn main() -> io::Result<()> {
    let listener = TcpListener::bind("127.0.0.1:8080").await?;

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

        tokio::spawn(async move {
            // Process each socket concurrently.
            process(socket).await
        });
    }
}

To run multiple tasks in parallel and receive their results, join handles can be stored in a vector.

async fn my_background_op(id: i32) -> String {
    let s = format!("Starting background task {}.", id);
    println!("{}", s);
    s
}

let ops = vec![1, 2, 3];
let mut tasks = Vec::with_capacity(ops.len());
for op in ops {
    // This call will make them start running in the background
    // immediately.
    tasks.push(tokio::spawn(my_background_op(op)));
}

let mut outputs = Vec::with_capacity(tasks.len());
for task in tasks {
    outputs.push(task.await.unwrap());
}
println!("{:?}", outputs);

This example pushes the tasks to outputs in the order they were started in. If you do not care about the ordering of the outputs, then you can also use a JoinSet.

Panics

Panics if called from outside of the Tokio runtime.

Using !Send values from a task

The task supplied to spawn must implement Send. However, it is possible to use !Send values from the task as long as they only exist between calls to .await.

For example, this will work:

use tokio::task;

use std::rc::Rc;

fn use_rc(rc: Rc<()>) {
    // Do stuff w/ rc
}

#[tokio::main]
async fn main() {
    tokio::spawn(async {
        // Force the `Rc` to stay in a scope with no `.await`
        {
            let rc = Rc::new(());
            use_rc(rc.clone());
        }

        task::yield_now().await;
    }).await.unwrap();
}

This will not work:

use tokio::task;

use std::rc::Rc;

fn use_rc(rc: Rc<()>) {
    // Do stuff w/ rc
}

#[tokio::main]
async fn main() {
    tokio::spawn(async {
        let rc = Rc::new(());

        task::yield_now().await;

        use_rc(rc.clone());
    }).await.unwrap();
}

Holding on to a !Send value across calls to .await will result in an unfriendly compile error message similar to:

`[... some type ...]` cannot be sent between threads safely

or:

error[E0391]: cycle detected when processing `main`