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| </pre><pre class="rust"><code><span class="kw">use </span><span class="kw">crate</span>::task::JoinHandle; |
| |
| <span class="macro">cfg_rt_multi_thread! </span>{ |
| <span class="doccomment">/// 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 |
| </span><span class="attribute">#[track_caller] |
| </span><span class="kw">pub fn </span>block_in_place<F, R>(f: F) -> R |
| <span class="kw">where |
| </span>F: FnOnce() -> R, |
| { |
| <span class="kw">crate</span>::runtime::scheduler::multi_thread::block_in_place(f) |
| } |
| } |
| |
| <span class="macro">cfg_rt! </span>{ |
| <span class="doccomment">/// 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 abruptly; there |
| /// is no standard low level API to cause a thread to stop running. However, |
| /// a useful pattern is to pass some form of "cancellation token" into |
| /// the thread. This could be an [`AtomicBool`] that the task checks periodically. |
| /// Another approach is to have the thread primarily read or write from a channel, |
| /// and to exit when the channel closes; assuming the other side of the channel is dropped |
| /// when cancellation occurs, this will cause the blocking task thread to exit |
| /// soon after as well. |
| /// |
| /// 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. It is possible |
| /// for a blocking task to be cancelled if it has not yet started running, but this |
| /// is not guaranteed. |
| /// |
| /// Note that if you are using the single threaded runtime, this function will |
| /// still spawn additional threads for blocking operations. The current-thread |
| /// 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 |
| /// [`AtomicBool`]: struct@std::sync::atomic::AtomicBool |
| /// |
| /// # Examples |
| /// |
| /// Pass an input value and receive result of computation: |
| /// |
| /// ``` |
| /// use tokio::task; |
| /// |
| /// # async fn docs() -> Result<(), Box<dyn std::error::Error>>{ |
| /// // 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(); |
| /// # } |
| /// ``` |
| </span><span class="attribute">#[track_caller] |
| </span><span class="kw">pub fn </span>spawn_blocking<F, R>(f: F) -> JoinHandle<R> |
| <span class="kw">where |
| </span>F: FnOnce() -> R + Send + <span class="lifetime">'static</span>, |
| R: Send + <span class="lifetime">'static</span>, |
| { |
| <span class="kw">crate</span>::runtime::spawn_blocking(f) |
| } |
| } |
| </code></pre></div> |
| </section></div></main><div id="rustdoc-vars" data-root-path="../../../" data-current-crate="tokio" data-themes="ayu,dark,light" data-resource-suffix="" data-rustdoc-version="1.66.0-nightly (5c8bff74b 2022-10-21)" ></div></body></html> |
