use crate::task::JoinHandle; cfg_rt_multi_thread! { /// Runs the provided blocking function on the current thread without /// blocking the executor. /// /// In general, issuing a blocking call or performing a lot of compute in a /// future without yielding is problematic, as it may prevent the executor /// from driving other tasks forward. Calling this function informs the /// executor that the currently executing task is about to block the thread, /// so the executor is able to hand off any other tasks it has to a new /// worker thread before that happens. See the [CPU-bound tasks and blocking /// code][blocking] section for more information. /// /// Be aware that although this function avoids starving other independently /// spawned tasks, any other code running concurrently in the same task will /// be suspended during the call to `block_in_place`. This can happen e.g. /// when using the [`join!`] macro. To avoid this issue, use /// [`spawn_blocking`] instead of `block_in_place`. /// /// Note that this function cannot be used within a [`current_thread`] runtime /// because in this case there are no other worker threads to hand off tasks /// to. On the other hand, calling the function outside a runtime is /// allowed. In this case, `block_in_place` just calls the provided closure /// normally. /// /// Code running behind `block_in_place` 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. /// /// [blocking]: ../index.html#cpu-bound-tasks-and-blocking-code /// [`spawn_blocking`]: fn@crate::task::spawn_blocking /// [`join!`]: macro@join /// [`thread::spawn`]: fn@std::thread::spawn /// [`shutdown_timeout`]: fn@crate::runtime::Runtime::shutdown_timeout /// /// # Examples /// /// ``` /// use tokio::task; /// /// # async fn docs() { /// task::block_in_place(move || { /// // do some compute-heavy work or call synchronous code /// }); /// # } /// ``` /// /// Code running inside `block_in_place` may use `block_on` to reenter the /// async context. /// /// ``` /// use tokio::task; /// use tokio::runtime::Handle; /// /// # async fn docs() { /// task::block_in_place(move || { /// Handle::current().block_on(async move { /// // do something async /// }); /// }); /// # } /// ``` /// /// # Panics /// /// This function panics if called from a [`current_thread`] runtime. /// /// [`current_thread`]: fn@crate::runtime::Builder::new_current_thread pub fn block_in_place(f: F) -> R where F: FnOnce() -> R, { crate::runtime::thread_pool::block_in_place(f) } } cfg_rt! { /// 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 problematic, 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][blocking] 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. /// After reaching the upper limit, the tasks are put in a queue. /// The thread 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. When running many CPU-bound /// computations, a semaphore or some other synchronization primitive should be /// used to limit the number of computation executed in parallel. Specialized /// CPU-bound executors, such as [rayon], may also be a good fit. /// /// 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 single threaded runtime, this function will /// still spawn additional threads for blocking operations. The basic /// scheduler's single thread is only used for asynchronous code. /// /// # Related APIs and patterns for bridging asynchronous and blocking code /// /// In simple cases, it is sufficient to have the closure accept input /// parameters at creation time and return a single value (or struct/tuple, etc.). /// /// For more complex situations in which it is desirable to stream data to or from /// the synchronous context, the [`mpsc channel`] has `blocking_send` and /// `blocking_recv` methods for use in non-async code such as the thread created /// by `spawn_blocking`. /// /// Another option is [`SyncIoBridge`] for cases where the synchronous context /// is operating on byte streams. For example, you might use an asynchronous /// HTTP client such as [hyper] to fetch data, but perform complex parsing /// of the payload body using a library written for synchronous I/O. /// /// Finally, see also [Bridging with sync code][bridgesync] for discussions /// around the opposite case of using Tokio as part of a larger synchronous /// codebase. /// /// [`Builder`]: struct@crate::runtime::Builder /// [blocking]: ../index.html#cpu-bound-tasks-and-blocking-code /// [rayon]: https://docs.rs/rayon /// [`mpsc channel`]: crate::sync::mpsc /// [`SyncIoBridge`]: https://docs.rs/tokio-util/0.6/tokio_util/io/struct.SyncIoBridge.html /// [hyper]: https://docs.rs/hyper /// [`thread::spawn`]: fn@std::thread::spawn /// [`shutdown_timeout`]: fn@crate::runtime::Runtime::shutdown_timeout /// [bridgesync]: https://tokio.rs/tokio/topics/bridging /// /// # Examples /// /// Pass an input value and receive result of computation: /// /// ``` /// use tokio::task; /// /// # async fn docs() -> Result<(), Box>{ /// // Initial input /// let mut v = "Hello, ".to_string(); /// let res = task::spawn_blocking(move || { /// // Stand-in for compute-heavy work or using synchronous APIs /// v.push_str("world"); /// // Pass ownership of the value back to the asynchronous context /// v /// }).await?; /// /// // `res` is the value returned from the thread /// assert_eq!(res.as_str(), "Hello, world"); /// # Ok(()) /// # } /// ``` /// /// Use a channel: /// /// ``` /// use tokio::task; /// use tokio::sync::mpsc; /// /// # async fn docs() { /// let (tx, mut rx) = mpsc::channel(2); /// let start = 5; /// let worker = task::spawn_blocking(move || { /// for x in 0..10 { /// // Stand in for complex computation /// tx.blocking_send(start + x).unwrap(); /// } /// }); /// /// let mut acc = 0; /// while let Some(v) = rx.recv().await { /// acc += v; /// } /// assert_eq!(acc, 95); /// worker.await.unwrap(); /// # } /// ``` #[track_caller] pub fn spawn_blocking(f: F) -> JoinHandle where F: FnOnce() -> R + Send + 'static, R: Send + 'static, { crate::runtime::spawn_blocking(f) } }