Module 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. While not meant as a ‘try/catch’ mechanism, panics in Rust can nonetheless be caught (unless compiling with panic=abort) with catch_unwind and recovered from, or alternatively be resumed with resume_unwind. If the panic is not caught the thread will exit, but the panic may optionally be detected from a different thread with join. If the main thread panics without the panic being caught, 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 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,” which means that there is no way for the program to learn when the spawned thread completes or otherwise terminates.

To learn when a thread completes, it is necessary to capture the JoinHandle object that is returned by the call to spawn, which provides a join method that allows the caller to wait for the completion of the spawned thread:

use std::thread;

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

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

Note that there is no parent/child relationship between a thread that spawns a new thread and the thread being spawned. In particular, the spawned thread may or may not outlive the spawning thread, unless the spawning thread is the main thread.

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 thread:

use std::thread;

thread::Builder::new().name("thread1".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.

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.

Naming threads

Threads are able to have associated names for identification purposes. By default, spawned threads are unnamed. To specify a name for a thread, build the thread with Builder and pass the desired thread name to Builder::name. To retrieve the thread name from within the thread, use Thread::name. A couple of examples where the name of a thread gets used:

• If a panic occurs in a named thread, the thread name will be printed in the panic message.
• The thread name is provided to the OS where applicable (e.g., pthread_setname_np in unix-like platforms).

Stack size

The default stack size is platform-dependent and subject to change. Currently, it is 2 MiB on all Tier-1 platforms.

There are two ways to manually specify the stack size for spawned threads:

• Build the thread with Builder and pass the desired stack size to Builder::stack_size.
• Set the RUST_MIN_STACK environment variable to an integer representing the desired stack size (in bytes). Note that setting Builder::stack_size will override this. Be aware that changes to RUST_MIN_STACK may be ignored after program start.

Note that the stack size of the main thread is not determined by Rust.

Structs

• AccessError
  An error returned by LocalKey::try_with.
• Builder
  Thread factory, which can be used in order to configure the properties of a new thread.
• JoinHandle
  An owned permission to join on a thread (block on its termination).
• LocalKey
  A thread local storage key which owns its contents.
• Scope
  A scope to spawn scoped threads in.
• ScopedJoinHandle
  An owned permission to join on a scoped thread (block on its termination).
• Thread
  A handle to a thread.
• ThreadId
  A unique identifier for a running thread.

Functions

• available_parallelism
  Returns an estimate of the default amount of parallelism a program should use.
• 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_msDeprecated
  Uses park_timeout.
• scope
  Creates a scope for spawning scoped threads.
• sleep
  Puts the current thread to sleep for at least the specified amount of time.
• sleep_msDeprecated
  Uses sleep.
• spawn
  Spawns a new thread, returning a JoinHandle for it.
• yield_now
  Cooperatively gives up a timeslice to the OS scheduler.
• sleep_untilExperimental
  Puts the current thread to sleep until the specified deadline has passed.

Type Aliases

• Result
  A specialized Result type for threads.