Module std::thread

Native threads.

The threading model

An executing Rust program consists of a collection of native OS threads, each with their own stack and local state. Threads can be named, and provide some built-in support for low-level synchronization.

Communication between threads can be done through channels, Rust's message-passing types, along with other forms of thread synchronization and shared-memory data structures. In particular, types that are guaranteed to be threadsafe are easily shared between threads 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. Thread panic is unrecoverable from within the panicking thread (i.e. there is no 'try/catch' in Rust), but the panic may optionally be detected from a different thread. If the main thread panics, the application will exit with a non-zero exit code.

When the main thread of a Rust program terminates, the entire program shuts down, even if other threads are still running. However, this module provides convenient facilities for automatically waiting for the termination of a child thread (i.e., join).

Spawning a thread

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

use std::thread;

thread::spawn(move || {
    // some work here
});

In this example, the spawned thread is "detached" from the current thread. This means that it can outlive its parent (the thread that spawned it), unless this parent is the main thread.

The parent thread can also wait on the completion of the child thread; a call to spawn produces a JoinHandle, which provides a join method for waiting:

use std::thread;

let child = thread::spawn(move || {
    // some work here
});
// some work here
let res = child.join();

The join method returns a Result containing Ok of the final value produced by the child thread, or Err of the value given to a call to panic! if the child panicked.

Configuring threads

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

use std::thread;

thread::Builder::new().name("child1".to_string()).spawn(move || {
    println!("Hello, world!");
});

The Thread type

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

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

The thread::current() function is available even for threads not spawned by the APIs of this module.

Blocking support: park and unpark

Every thread is equipped with some basic low-level blocking support, via the thread::park() function and thread::Thread::unpark() method. park() blocks the current thread, which can then be resumed from another thread by calling the unpark() method on the blocked thread's handle.

Conceptually, each Thread handle has an associated token, which is initially not present:

• The thread::park() function blocks the current thread unless or until the token is available for its thread handle, at which point it atomically consumes the token. It may also return spuriously, without consuming the token. thread::park_timeout() does the same, but allows specifying a maximum time to block the thread for.

• The unpark() method on a Thread atomically makes the token available if it wasn't already.

In other words, each Thread acts a bit like a semaphore with initial count 0, except that the semaphore is saturating (the count cannot go above 1), and can return spuriously.

The API is typically used by acquiring a handle to the current thread, placing that handle in a shared data structure so that other threads can find it, and then parking. When some desired condition is met, another thread calls unpark() on the handle.

The motivation for this design is twofold:

• It avoids the need to allocate mutexes and condvars when building new synchronization primitives; the threads already provide basic blocking/signaling.

• It can be implemented very efficiently on many platforms.

Thread-local storage

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

A thread-local key owns the value it contains and will destroy the value when the thread exits. It is created with the thread_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. Thread-local keys allow only shared access to values, as there would be no way to guarantee uniqueness if mutable borrows were allowed. Most values will want to make use of some form of interior mutability through the Cell or RefCell types.

Structs

Builder	Thread configuration. Provides detailed control over the properties and behavior of new threads.
JoinHandle	An owned permission to join on a thread (block on its termination).
LocalKey	A thread local storage key which owns its contents.
Thread	A handle to a thread.
ThreadId	[ Experimental ] A unique identifier for a running thread.

Enums

LocalKeyState

[ Experimental ] Indicator of the state of a thread local storage key.

Functions

current	Gets a handle to the thread that invokes it.
panicking	Determines whether the current thread is unwinding because of panic.
park	Blocks unless or until the current thread's token is made available.
park_timeout	Blocks unless or until the current thread's token is made available or the specified duration has been reached (may wake spuriously).
park_timeout_ms	[ Deprecated ] Use park_timeout.
sleep	Puts the current thread to sleep for the specified amount of time.
sleep_ms	[ Deprecated ] Puts the current thread to sleep for the specified amount of time.
spawn	Spawns a new thread, returning a JoinHandle for it.
yield_now	Cooperatively gives up a timeslice to the OS scheduler.

Type Definitions

Result

Indicates the manner in which a thread exited.