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<!DOCTYPE html><html lang="en"><head><meta charset="utf-8"><meta name="viewport" content="width=device-width, initial-scale=1.0"><meta name="generator" content="rustdoc"><meta name="description" content="Source of the Rust file `/root/.cargo/git/checkouts/tokio-377c595163f99a10/dfe252d/tokio/src/sync/mod.rs`."><meta name="keywords" content="rust, rustlang, rust-lang"><title>mod.rs - source</title><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../SourceSerif4-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../FiraSans-Regular.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../FiraSans-Medium.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../SourceCodePro-Regular.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../SourceSerif4-Bold.ttf.woff2"><link rel="preload" as="font" type="font/woff2" crossorigin href="../../../SourceCodePro-Semibold.ttf.woff2"><link rel="stylesheet" href="../../../normalize.css"><link rel="stylesheet" href="../../../rustdoc.css" id="mainThemeStyle"><link rel="stylesheet" href="../../../ayu.css" disabled><link rel="stylesheet" href="../../../dark.css" disabled><link rel="stylesheet" href="../../../light.css" id="themeStyle"><script id="default-settings" ></script><script src="../../../storage.js"></script><script defer src="../../../source-script.js"></script><script defer src="../../../source-files.js"></script><script defer src="../../../main.js"></script><noscript><link rel="stylesheet" href="../../../noscript.css"></noscript><link rel="alternate icon" type="image/png" href="../../../favicon-16x16.png"><link rel="alternate icon" type="image/png" href="../../../favicon-32x32.png"><link rel="icon" type="image/svg+xml" href="../../../favicon.svg"></head><body class="rustdoc source"><!--[if lte IE 11]><div class="warning">This old browser is unsupported and will most likely display funky things.</div><![endif]--><nav class="sidebar"><a class="sidebar-logo" href="../../../tokio/index.html"><div class="logo-container"><img class="rust-logo" src="../../../rust-logo.svg" alt="logo"></div></a></nav><main><div class="width-limiter"><nav class="sub"><a class="sub-logo-container" href="../../../tokio/index.html"><img class="rust-logo" src="../../../rust-logo.svg" alt="logo"></a><form class="search-form"><div class="search-container"><span></span><input class="search-input" name="search" autocomplete="off" spellcheck="false" placeholder="Click or press ‘S’ to search, ‘?’ for more options…" type="search"><div id="help-button" title="help" tabindex="-1"><a href="../../../help.html">?</a></div><div id="settings-menu" tabindex="-1"><a href="../../../settings.html" title="settings"><img width="22" height="22" alt="Change settings" src="../../../wheel.svg"></a></div></div></form></nav><section id="main-content" class="content"><div class="example-wrap"><pre class="src-line-numbers"><span id="1">1</span>
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</pre><pre class="rust"><code><span class="attribute">#![cfg_attr(loom, allow(dead_code, unreachable_pub, unused_imports))]
</span><span class="doccomment">//! Synchronization primitives for use in asynchronous contexts.
//!
//! Tokio programs tend to be organized as a set of [tasks] where each task
//! operates independently and may be executed on separate physical threads. The
//! synchronization primitives provided in this module permit these independent
//! tasks to communicate together.
//!
//! [tasks]: crate::task
//!
//! # Message passing
//!
//! The most common form of synchronization in a Tokio program is message
//! passing. Two tasks operate independently and send messages to each other to
//! synchronize. Doing so has the advantage of avoiding shared state.
//!
//! Message passing is implemented using channels. A channel supports sending a
//! message from one producer task to one or more consumer tasks. There are a
//! few flavors of channels provided by Tokio. Each channel flavor supports
//! different message passing patterns. When a channel supports multiple
//! producers, many separate tasks may **send** messages. When a channel
//! supports multiple consumers, many different separate tasks may **receive**
//! messages.
//!
//! Tokio provides many different channel flavors as different message passing
//! patterns are best handled with different implementations.
//!
//! ## `oneshot` channel
//!
//! The [`oneshot` channel][oneshot] supports sending a **single** value from a
//! single producer to a single consumer. This channel is usually used to send
//! the result of a computation to a waiter.
//!
//! **Example:** using a [`oneshot` channel][oneshot] to receive the result of a
//! computation.
//!
//! ```
//! use tokio::sync::oneshot;
//!
//! async fn some_computation() -&gt; String {
//! &quot;represents the result of the computation&quot;.to_string()
//! }
//!
//! #[tokio::main]
//! async fn main() {
//! let (tx, rx) = oneshot::channel();
//!
//! tokio::spawn(async move {
//! let res = some_computation().await;
//! tx.send(res).unwrap();
//! });
//!
//! // Do other work while the computation is happening in the background
//!
//! // Wait for the computation result
//! let res = rx.await.unwrap();
//! }
//! ```
//!
//! Note, if the task produces a computation result as its final
//! action before terminating, the [`JoinHandle`] can be used to
//! receive that value instead of allocating resources for the
//! `oneshot` channel. Awaiting on [`JoinHandle`] returns `Result`. If
//! the task panics, the `Joinhandle` yields `Err` with the panic
//! cause.
//!
//! **Example:**
//!
//! ```
//! async fn some_computation() -&gt; String {
//! &quot;the result of the computation&quot;.to_string()
//! }
//!
//! #[tokio::main]
//! async fn main() {
//! let join_handle = tokio::spawn(async move {
//! some_computation().await
//! });
//!
//! // Do other work while the computation is happening in the background
//!
//! // Wait for the computation result
//! let res = join_handle.await.unwrap();
//! }
//! ```
//!
//! [oneshot]: oneshot
//! [`JoinHandle`]: crate::task::JoinHandle
//!
//! ## `mpsc` channel
//!
//! The [`mpsc` channel][mpsc] supports sending **many** values from **many**
//! producers to a single consumer. This channel is often used to send work to a
//! task or to receive the result of many computations.
//!
//! This is also the channel you should use if you want to send many messages
//! from a single producer to a single consumer. There is no dedicated spsc
//! channel.
//!
//! **Example:** using an mpsc to incrementally stream the results of a series
//! of computations.
//!
//! ```
//! use tokio::sync::mpsc;
//!
//! async fn some_computation(input: u32) -&gt; String {
//! format!(&quot;the result of computation {}&quot;, input)
//! }
//!
//! #[tokio::main]
//! async fn main() {
//! let (tx, mut rx) = mpsc::channel(100);
//!
//! tokio::spawn(async move {
//! for i in 0..10 {
//! let res = some_computation(i).await;
//! tx.send(res).await.unwrap();
//! }
//! });
//!
//! while let Some(res) = rx.recv().await {
//! println!(&quot;got = {}&quot;, res);
//! }
//! }
//! ```
//!
//! The argument to `mpsc::channel` is the channel capacity. This is the maximum
//! number of values that can be stored in the channel pending receipt at any
//! given time. Properly setting this value is key in implementing robust
//! programs as the channel capacity plays a critical part in handling back
//! pressure.
//!
//! A common concurrency pattern for resource management is to spawn a task
//! dedicated to managing that resource and using message passing between other
//! tasks to interact with the resource. The resource may be anything that may
//! not be concurrently used. Some examples include a socket and program state.
//! For example, if multiple tasks need to send data over a single socket, spawn
//! a task to manage the socket and use a channel to synchronize.
//!
//! **Example:** sending data from many tasks over a single socket using message
//! passing.
//!
//! ```no_run
//! use tokio::io::{self, AsyncWriteExt};
//! use tokio::net::TcpStream;
//! use tokio::sync::mpsc;
//!
//! #[tokio::main]
//! async fn main() -&gt; io::Result&lt;()&gt; {
//! let mut socket = TcpStream::connect(&quot;www.example.com:1234&quot;).await?;
//! let (tx, mut rx) = mpsc::channel(100);
//!
//! for _ in 0..10 {
//! // Each task needs its own `tx` handle. This is done by cloning the
//! // original handle.
//! let tx = tx.clone();
//!
//! tokio::spawn(async move {
//! tx.send(&amp;b&quot;data to write&quot;[..]).await.unwrap();
//! });
//! }
//!
//! // The `rx` half of the channel returns `None` once **all** `tx` clones
//! // drop. To ensure `None` is returned, drop the handle owned by the
//! // current task. If this `tx` handle is not dropped, there will always
//! // be a single outstanding `tx` handle.
//! drop(tx);
//!
//! while let Some(res) = rx.recv().await {
//! socket.write_all(res).await?;
//! }
//!
//! Ok(())
//! }
//! ```
//!
//! The [`mpsc`][mpsc] and [`oneshot`][oneshot] channels can be combined to
//! provide a request / response type synchronization pattern with a shared
//! resource. A task is spawned to synchronize a resource and waits on commands
//! received on a [`mpsc`][mpsc] channel. Each command includes a
//! [`oneshot`][oneshot] `Sender` on which the result of the command is sent.
//!
//! **Example:** use a task to synchronize a `u64` counter. Each task sends an
//! &quot;fetch and increment&quot; command. The counter value **before** the increment is
//! sent over the provided `oneshot` channel.
//!
//! ```
//! use tokio::sync::{oneshot, mpsc};
//! use Command::Increment;
//!
//! enum Command {
//! Increment,
//! // Other commands can be added here
//! }
//!
//! #[tokio::main]
//! async fn main() {
//! let (cmd_tx, mut cmd_rx) = mpsc::channel::&lt;(Command, oneshot::Sender&lt;u64&gt;)&gt;(100);
//!
//! // Spawn a task to manage the counter
//! tokio::spawn(async move {
//! let mut counter: u64 = 0;
//!
//! while let Some((cmd, response)) = cmd_rx.recv().await {
//! match cmd {
//! Increment =&gt; {
//! let prev = counter;
//! counter += 1;
//! response.send(prev).unwrap();
//! }
//! }
//! }
//! });
//!
//! let mut join_handles = vec![];
//!
//! // Spawn tasks that will send the increment command.
//! for _ in 0..10 {
//! let cmd_tx = cmd_tx.clone();
//!
//! join_handles.push(tokio::spawn(async move {
//! let (resp_tx, resp_rx) = oneshot::channel();
//!
//! cmd_tx.send((Increment, resp_tx)).await.ok().unwrap();
//! let res = resp_rx.await.unwrap();
//!
//! println!(&quot;previous value = {}&quot;, res);
//! }));
//! }
//!
//! // Wait for all tasks to complete
//! for join_handle in join_handles.drain(..) {
//! join_handle.await.unwrap();
//! }
//! }
//! ```
//!
//! [mpsc]: mpsc
//!
//! ## `broadcast` channel
//!
//! The [`broadcast` channel] supports sending **many** values from
//! **many** producers to **many** consumers. Each consumer will receive
//! **each** value. This channel can be used to implement &quot;fan out&quot; style
//! patterns common with pub / sub or &quot;chat&quot; systems.
//!
//! This channel tends to be used less often than `oneshot` and `mpsc` but still
//! has its use cases.
//!
//! This is also the channel you should use if you want to broadcast values from
//! a single producer to many consumers. There is no dedicated spmc broadcast
//! channel.
//!
//! Basic usage
//!
//! ```
//! use tokio::sync::broadcast;
//!
//! #[tokio::main]
//! async fn main() {
//! let (tx, mut rx1) = broadcast::channel(16);
//! let mut rx2 = tx.subscribe();
//!
//! tokio::spawn(async move {
//! assert_eq!(rx1.recv().await.unwrap(), 10);
//! assert_eq!(rx1.recv().await.unwrap(), 20);
//! });
//!
//! tokio::spawn(async move {
//! assert_eq!(rx2.recv().await.unwrap(), 10);
//! assert_eq!(rx2.recv().await.unwrap(), 20);
//! });
//!
//! tx.send(10).unwrap();
//! tx.send(20).unwrap();
//! }
//! ```
//!
//! [`broadcast` channel]: crate::sync::broadcast
//!
//! ## `watch` channel
//!
//! The [`watch` channel] supports sending **many** values from a **single**
//! producer to **many** consumers. However, only the **most recent** value is
//! stored in the channel. Consumers are notified when a new value is sent, but
//! there is no guarantee that consumers will see **all** values.
//!
//! The [`watch` channel] is similar to a [`broadcast` channel] with capacity 1.
//!
//! Use cases for the [`watch` channel] include broadcasting configuration
//! changes or signalling program state changes, such as transitioning to
//! shutdown.
//!
//! **Example:** use a [`watch` channel] to notify tasks of configuration
//! changes. In this example, a configuration file is checked periodically. When
//! the file changes, the configuration changes are signalled to consumers.
//!
//! ```
//! use tokio::sync::watch;
//! use tokio::time::{self, Duration, Instant};
//!
//! use std::io;
//!
//! #[derive(Debug, Clone, Eq, PartialEq)]
//! struct Config {
//! timeout: Duration,
//! }
//!
//! impl Config {
//! async fn load_from_file() -&gt; io::Result&lt;Config&gt; {
//! // file loading and deserialization logic here
//! # Ok(Config { timeout: Duration::from_secs(1) })
//! }
//! }
//!
//! async fn my_async_operation() {
//! // Do something here
//! }
//!
//! #[tokio::main]
//! async fn main() {
//! // Load initial configuration value
//! let mut config = Config::load_from_file().await.unwrap();
//!
//! // Create the watch channel, initialized with the loaded configuration
//! let (tx, rx) = watch::channel(config.clone());
//!
//! // Spawn a task to monitor the file.
//! tokio::spawn(async move {
//! loop {
//! // Wait 10 seconds between checks
//! time::sleep(Duration::from_secs(10)).await;
//!
//! // Load the configuration file
//! let new_config = Config::load_from_file().await.unwrap();
//!
//! // If the configuration changed, send the new config value
//! // on the watch channel.
//! if new_config != config {
//! tx.send(new_config.clone()).unwrap();
//! config = new_config;
//! }
//! }
//! });
//!
//! let mut handles = vec![];
//!
//! // Spawn tasks that runs the async operation for at most `timeout`. If
//! // the timeout elapses, restart the operation.
//! //
//! // The task simultaneously watches the `Config` for changes. When the
//! // timeout duration changes, the timeout is updated without restarting
//! // the in-flight operation.
//! for _ in 0..5 {
//! // Clone a config watch handle for use in this task
//! let mut rx = rx.clone();
//!
//! let handle = tokio::spawn(async move {
//! // Start the initial operation and pin the future to the stack.
//! // Pinning to the stack is required to resume the operation
//! // across multiple calls to `select!`
//! let op = my_async_operation();
//! tokio::pin!(op);
//!
//! // Get the initial config value
//! let mut conf = rx.borrow().clone();
//!
//! let mut op_start = Instant::now();
//! let sleep = time::sleep_until(op_start + conf.timeout);
//! tokio::pin!(sleep);
//!
//! loop {
//! tokio::select! {
//! _ = &amp;mut sleep =&gt; {
//! // The operation elapsed. Restart it
//! op.set(my_async_operation());
//!
//! // Track the new start time
//! op_start = Instant::now();
//!
//! // Restart the timeout
//! sleep.set(time::sleep_until(op_start + conf.timeout));
//! }
//! _ = rx.changed() =&gt; {
//! conf = rx.borrow().clone();
//!
//! // The configuration has been updated. Update the
//! // `sleep` using the new `timeout` value.
//! sleep.as_mut().reset(op_start + conf.timeout);
//! }
//! _ = &amp;mut op =&gt; {
//! // The operation completed!
//! return
//! }
//! }
//! }
//! });
//!
//! handles.push(handle);
//! }
//!
//! for handle in handles.drain(..) {
//! handle.await.unwrap();
//! }
//! }
//! ```
//!
//! [`watch` channel]: mod@crate::sync::watch
//! [`broadcast` channel]: mod@crate::sync::broadcast
//!
//! # State synchronization
//!
//! The remaining synchronization primitives focus on synchronizing state.
//! These are asynchronous equivalents to versions provided by `std`. They
//! operate in a similar way as their `std` counterparts but will wait
//! asynchronously instead of blocking the thread.
//!
//! * [`Barrier`](Barrier) Ensures multiple tasks will wait for each other to
//! reach a point in the program, before continuing execution all together.
//!
//! * [`Mutex`](Mutex) Mutual Exclusion mechanism, which ensures that at most
//! one thread at a time is able to access some data.
//!
//! * [`Notify`](Notify) Basic task notification. `Notify` supports notifying a
//! receiving task without sending data. In this case, the task wakes up and
//! resumes processing.
//!
//! * [`RwLock`](RwLock) Provides a mutual exclusion mechanism which allows
//! multiple readers at the same time, while allowing only one writer at a
//! time. In some cases, this can be more efficient than a mutex.
//!
//! * [`Semaphore`](Semaphore) Limits the amount of concurrency. A semaphore
//! holds a number of permits, which tasks may request in order to enter a
//! critical section. Semaphores are useful for implementing limiting or
//! bounding of any kind.
</span><span class="macro">cfg_sync! </span>{
<span class="doccomment">/// Named future types.
</span><span class="kw">pub mod </span>futures {
<span class="kw">pub use </span><span class="kw">super</span>::notify::Notified;
}
<span class="kw">mod </span>barrier;
<span class="kw">pub use </span>barrier::{Barrier, BarrierWaitResult};
<span class="kw">pub mod </span>broadcast;
<span class="kw">pub mod </span>mpsc;
<span class="kw">mod </span>mutex;
<span class="kw">pub use </span>mutex::{Mutex, MutexGuard, TryLockError, OwnedMutexGuard, MappedMutexGuard};
<span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">mod </span>notify;
<span class="kw">pub use </span>notify::Notify;
<span class="kw">pub mod </span>oneshot;
<span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">mod </span>batch_semaphore;
<span class="kw">pub use </span>batch_semaphore::{AcquireError, TryAcquireError};
<span class="kw">mod </span>semaphore;
<span class="kw">pub use </span>semaphore::{Semaphore, SemaphorePermit, OwnedSemaphorePermit};
<span class="kw">mod </span>rwlock;
<span class="kw">pub use </span>rwlock::RwLock;
<span class="kw">pub use </span>rwlock::owned_read_guard::OwnedRwLockReadGuard;
<span class="kw">pub use </span>rwlock::owned_write_guard::OwnedRwLockWriteGuard;
<span class="kw">pub use </span>rwlock::owned_write_guard_mapped::OwnedRwLockMappedWriteGuard;
<span class="kw">pub use </span>rwlock::read_guard::RwLockReadGuard;
<span class="kw">pub use </span>rwlock::write_guard::RwLockWriteGuard;
<span class="kw">pub use </span>rwlock::write_guard_mapped::RwLockMappedWriteGuard;
<span class="kw">mod </span>task;
<span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">use </span>task::AtomicWaker;
<span class="kw">mod </span>once_cell;
<span class="kw">pub use </span><span class="self">self</span>::once_cell::{OnceCell, SetError};
<span class="kw">pub mod </span>watch;
}
<span class="macro">cfg_not_sync! </span>{
<span class="macro">cfg_fs! </span>{
<span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">mod </span>batch_semaphore;
<span class="kw">mod </span>mutex;
<span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">use </span>mutex::Mutex;
}
<span class="attribute">#[cfg(any(feature = <span class="string">&quot;rt&quot;</span>, feature = <span class="string">&quot;signal&quot;</span>, all(unix, feature = <span class="string">&quot;process&quot;</span>)))]
</span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">mod </span>notify;
<span class="attribute">#[cfg(any(feature = <span class="string">&quot;rt&quot;</span>, all(windows, feature = <span class="string">&quot;process&quot;</span>)))]
</span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">mod </span>oneshot;
<span class="macro">cfg_atomic_waker_impl! </span>{
<span class="kw">mod </span>task;
<span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">use </span>task::AtomicWaker;
}
<span class="attribute">#[cfg(any(feature = <span class="string">&quot;signal&quot;</span>, all(unix, feature = <span class="string">&quot;process&quot;</span>)))]
</span><span class="kw">pub</span>(<span class="kw">crate</span>) <span class="kw">mod </span>watch;
}
<span class="doccomment">/// Unit tests
</span><span class="attribute">#[cfg(test)]
</span><span class="kw">mod </span>tests;
</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>