[][src]Function tokio::task::spawn_blocking

pub fn spawn_blocking<F, R>(f: F) -> JoinHandle<R>

Important traits for JoinHandle<T>

impl<T> Future for JoinHandle<T> type Output = Result<T, JoinError>;
    F: FnOnce() -> R + Send + 'static,
    R: Send + 'static, 
This is supported on feature="blocking" only.

Runs the provided closure on a thread where blocking is acceptable.

In general, issuing a blocking call or performing a lot of compute in a future without yielding is not okay, as it may prevent the executor from driving other futures forward. This function runs the provided closure on a thread dedicated to blocking operations. See the CPU-bound tasks and blocking code section for more information.

Tokio will spawn more blocking threads when they are requested through this function until the upper limit configured on the Builder is reached. This limit is very large by default, because spawn_blocking is often used for various kinds of IO operations that cannot be performed asynchronously. When you run CPU-bound code using spawn_blocking, you should keep this large upper limit in mind; to run your CPU-bound computations on only a few threads, you should use a separate thread pool such as rayon rather than configuring the number of blocking threads.

This function is intended for non-async operations that eventually finish on their own. If you want to spawn an ordinary thread, you should use thread::spawn instead.

Closures spawned using spawn_blocking cannot be cancelled. When you shut down the executor, it will wait indefinitely for all blocking operations to finish. You can use shutdown_timeout to stop waiting for them after a certain timeout. Be aware that this will still not cancel the tasks — they are simply allowed to keep running after the method returns.

Note that if you are using the basic scheduler, this function will still spawn additional threads for blocking operations. The basic scheduler's single thread is only used for asynchronous code.


use tokio::task;

let res = task::spawn_blocking(move || {
    // do some compute-heavy work or call synchronous code
    "done computing"

assert_eq!(res, "done computing");