Module async_std::task

Types and traits for working with asynchronous tasks.

This module is similar to std::thread, except it uses asynchronous tasks in place of threads.

The task model

An executing asynchronous Rust program consists of a collection of native OS threads, on top of which multiple stackless coroutines are multiplexed. We refer to these as “tasks”. Tasks can be named, and provide some built-in support for synchronization.

Communication between tasks can be done through channels, Rust’s message-passing types, along with other forms of tasks synchronization and shared-memory data structures. In particular, types that are guaranteed to be threadsafe are easily shared between tasks using the atomically-reference-counted container, Arc.

Fatal logic errors in Rust cause thread panic, during which a thread will unwind the stack, running destructors and freeing owned resources. If a panic occurs inside a task, there is no meaningful way of recovering, so the panic will propagate through any thread boundaries all the way to the root task. This is also known as a “panic = abort” model.

Spawning a task

A new task can be spawned using the task::spawn function:

use async_std::task;

task::spawn(async {
    // some work here
});

In this example, the spawned task is “detached” from the current task. This means that it can outlive its parent (the task that spawned it), unless this parent is the root task.

The root task can also wait on the completion of the child task; a call to spawn produces a JoinHandle, which implements Future and can be awaited:

use async_std::task;

let child = task::spawn(async {
    // some work here
});
// some work here
let res = child.await;

The await operator returns the final value produced by the child task.

Configuring tasks

A new task can be configured before it is spawned via the Builder type, which currently allows you to set the name for the child task:

use async_std::task;

task::Builder::new().name("child1".to_string()).spawn(async {
    println!("Hello, world!");
});

The Task type

Tasks are represented via the Task type, which you can get in one of two ways:

• By spawning a new task, e.g., using the task::spawn function, and calling task on the JoinHandle.
• By requesting the current task, using the task::current function.

Task-local storage

This module also provides an implementation of task-local storage for Rust programs. Task-local storage is a method of storing data into a global variable that each task in the program will have its own copy of. Tasks do not share this data, so accesses do not need to be synchronized.

A task-local key owns the value it contains and will destroy the value when the task exits. It is created with the task_local! macro and can contain any value that is 'static (no borrowed pointers). It provides an accessor function, with, that yields a shared reference to the value to the specified closure. Task-local keys allow only shared access to values, as there would be no way to guarantee uniqueness if mutable borrows were allowed.

Naming tasks

Tasks are able to have associated names for identification purposes. By default, spawned tasks are unnamed. To specify a name for a task, build the task with Builder and pass the desired task name to Builder::name. To retrieve the task name from within the task, use Task::name.

Macros

• ready
  Extracts the successful type of a Poll<T>.

Structs

• AccessError
  An error returned by LocalKey::try_with.
• Builder
  Task builder that configures the settings of a new task.
• Context
  The context of an asynchronous task.
• JoinHandle
  A handle that awaits the result of a task.
• LocalKey
  The key for accessing a task-local value.
• Task
  A handle to a task.
• TaskId
  A unique identifier for a task.
• Waker
  A Waker is a handle for waking up a task by notifying its executor that it is ready to be run.

Enums

• Poll
  Indicates whether a value is available or if the current task has been scheduled to receive a wakeup instead.

Functions

• block_on
  Spawns a task and blocks the current thread on its result.
• current
  Returns a handle to the current task.
• sleep
  Sleeps for the specified amount of time.
• spawn
  Spawns a task.
• spawn_blocking
  Spawns a blocking task.
• spawn_localunstable
  Spawns a task onto the thread-local executor.
• try_current
  Returns a handle to the current task if called within the context of a task created by block_on, spawn, or Builder::spawn, otherwise returns None.
• yield_now
  Cooperatively gives up a timeslice to the task scheduler.