1//! Native threads.
2//!
3//! ## The threading model
4//!
5//! An executing Rust program consists of a collection of native OS threads,
6//! each with their own stack and local state. Threads can be named, and
7//! provide some built-in support for low-level synchronization.
8//!
9//! Communication between threads can be done through
10//! [channels], Rust's message-passing types, along with [other forms of thread
11//! synchronization](../../std/sync/index.html) and shared-memory data
12//! structures. In particular, types that are guaranteed to be
13//! threadsafe are easily shared between threads using the
14//! atomically-reference-counted container, [`Arc`].
15//!
16//! Fatal logic errors in Rust cause *thread panic*, during which
17//! a thread will unwind the stack, running destructors and freeing
18//! owned resources. While not meant as a 'try/catch' mechanism, panics
19//! in Rust can nonetheless be caught (unless compiling with `panic=abort`) with
20//! [`catch_unwind`](../../std/panic/fn.catch_unwind.html) and recovered
21//! from, or alternatively be resumed with
22//! [`resume_unwind`](../../std/panic/fn.resume_unwind.html). If the panic
23//! is not caught the thread will exit, but the panic may optionally be
24//! detected from a different thread with [`join`]. If the main thread panics
25//! without the panic being caught, the application will exit with a
26//! non-zero exit code.
27//!
28//! When the main thread of a Rust program terminates, the entire program shuts
29//! down, even if other threads are still running. However, this module provides
30//! convenient facilities for automatically waiting for the termination of a
31//! thread (i.e., join).
32//!
33//! ## Spawning a thread
34//!
35//! A new thread can be spawned using the [`thread::spawn`][`spawn`] function:
36//!
37//! ```rust
38//! use std::thread;
39//!
40//! thread::spawn(move || {
41//! // some work here
42//! });
43//! ```
44//!
45//! In this example, the spawned thread is "detached," which means that there is
46//! no way for the program to learn when the spawned thread completes or otherwise
47//! terminates.
48//!
49//! To learn when a thread completes, it is necessary to capture the [`JoinHandle`]
50//! object that is returned by the call to [`spawn`], which provides
51//! a `join` method that allows the caller to wait for the completion of the
52//! spawned thread:
53//!
54//! ```rust
55//! use std::thread;
56//!
57//! let thread_join_handle = thread::spawn(move || {
58//! // some work here
59//! });
60//! // some work here
61//! let res = thread_join_handle.join();
62//! ```
63//!
64//! The [`join`] method returns a [`thread::Result`] containing [`Ok`] of the final
65//! value produced by the spawned thread, or [`Err`] of the value given to
66//! a call to [`panic!`] if the thread panicked.
67//!
68//! Note that there is no parent/child relationship between a thread that spawns a
69//! new thread and the thread being spawned. In particular, the spawned thread may or
70//! may not outlive the spawning thread, unless the spawning thread is the main thread.
71//!
72//! ## Configuring threads
73//!
74//! A new thread can be configured before it is spawned via the [`Builder`] type,
75//! which currently allows you to set the name and stack size for the thread:
76//!
77//! ```rust
78//! # #![allow(unused_must_use)]
79//! use std::thread;
80//!
81//! thread::Builder::new().name("thread1".to_string()).spawn(move || {
82//! println!("Hello, world!");
83//! });
84//! ```
85//!
86//! ## The `Thread` type
87//!
88//! Threads are represented via the [`Thread`] type, which you can get in one of
89//! two ways:
90//!
91//! * By spawning a new thread, e.g., using the [`thread::spawn`][`spawn`]
92//! function, and calling [`thread`][`JoinHandle::thread`] on the [`JoinHandle`].
93//! * By requesting the current thread, using the [`thread::current`] function.
94//!
95//! The [`thread::current`] function is available even for threads not spawned
96//! by the APIs of this module.
97//!
98//! ## Thread-local storage
99//!
100//! This module also provides an implementation of thread-local storage for Rust
101//! programs. Thread-local storage is a method of storing data into a global
102//! variable that each thread in the program will have its own copy of.
103//! Threads do not share this data, so accesses do not need to be synchronized.
104//!
105//! A thread-local key owns the value it contains and will destroy the value when the
106//! thread exits. It is created with the [`thread_local!`] macro and can contain any
107//! value that is `'static` (no borrowed pointers). It provides an accessor function,
108//! [`with`], that yields a shared reference to the value to the specified
109//! closure. Thread-local keys allow only shared access to values, as there would be no
110//! way to guarantee uniqueness if mutable borrows were allowed. Most values
111//! will want to make use of some form of **interior mutability** through the
112//! [`Cell`] or [`RefCell`] types.
113//!
114//! ## Naming threads
115//!
116//! Threads are able to have associated names for identification purposes. By default, spawned
117//! threads are unnamed. To specify a name for a thread, build the thread with [`Builder`] and pass
118//! the desired thread name to [`Builder::name`]. To retrieve the thread name from within the
119//! thread, use [`Thread::name`]. A couple of examples where the name of a thread gets used:
120//!
121//! * If a panic occurs in a named thread, the thread name will be printed in the panic message.
122//! * The thread name is provided to the OS where applicable (e.g., `pthread_setname_np` in
123//! unix-like platforms).
124//!
125//! ## Stack size
126//!
127//! The default stack size is platform-dependent and subject to change.
128//! Currently, it is 2 MiB on all Tier-1 platforms.
129//!
130//! There are two ways to manually specify the stack size for spawned threads:
131//!
132//! * Build the thread with [`Builder`] and pass the desired stack size to [`Builder::stack_size`].
133//! * Set the `RUST_MIN_STACK` environment variable to an integer representing the desired stack
134//! size (in bytes). Note that setting [`Builder::stack_size`] will override this. Be aware that
135//! changes to `RUST_MIN_STACK` may be ignored after program start.
136//!
137//! Note that the stack size of the main thread is *not* determined by Rust.
138//!
139//! [channels]: crate::sync::mpsc
140//! [`join`]: JoinHandle::join
141//! [`Result`]: crate::result::Result
142//! [`Ok`]: crate::result::Result::Ok
143//! [`Err`]: crate::result::Result::Err
144//! [`thread::current`]: current
145//! [`thread::Result`]: Result
146//! [`unpark`]: Thread::unpark
147//! [`thread::park_timeout`]: park_timeout
148//! [`Cell`]: crate::cell::Cell
149//! [`RefCell`]: crate::cell::RefCell
150//! [`with`]: LocalKey::with
151//! [`thread_local!`]: crate::thread_local
152
153#![stable(feature = "rust1", since = "1.0.0")]
154#![deny(unsafe_op_in_unsafe_fn)]
155// Under `test`, `__FastLocalKeyInner` seems unused.
156#![cfg_attr(test, allow(dead_code))]
157
158#[cfg(all(test, not(target_os = "emscripten")))]
159mod tests;
160
161use crate::any::Any;
162use crate::cell::{OnceCell, UnsafeCell};
163use crate::env;
164use crate::ffi::{CStr, CString};
165use crate::fmt;
166use crate::io;
167use crate::marker::PhantomData;
168use crate::mem::{self, forget};
169use crate::num::NonZero;
170use crate::panic;
171use crate::panicking;
172use crate::pin::Pin;
173use crate::ptr::addr_of_mut;
174use crate::str;
175use crate::sync::atomic::{AtomicUsize, Ordering};
176use crate::sync::Arc;
177use crate::sys::thread as imp;
178use crate::sys_common::thread_parking::Parker;
179use crate::sys_common::{AsInner, IntoInner};
180use crate::time::{Duration, Instant};
181
182#[stable(feature = "scoped_threads", since = "1.63.0")]
183mod scoped;
184
185#[stable(feature = "scoped_threads", since = "1.63.0")]
186pub use scoped::{scope, Scope, ScopedJoinHandle};
187
188////////////////////////////////////////////////////////////////////////////////
189// Thread-local storage
190////////////////////////////////////////////////////////////////////////////////
191
192#[macro_use]
193mod local;
194
195cfg_if::cfg_if! {
196 if #[cfg(test)] {
197 // Avoid duplicating the global state associated with thread-locals between this crate and
198 // realstd. Miri relies on this.
199 pub use realstd::thread::{local_impl, AccessError, LocalKey};
200 } else {
201 #[stable(feature = "rust1", since = "1.0.0")]
202 pub use self::local::{AccessError, LocalKey};
203
204 // Implementation details used by the thread_local!{} macro.
205 #[doc(hidden)]
206 #[unstable(feature = "thread_local_internals", issue = "none")]
207 pub mod local_impl {
208 pub use crate::sys::thread_local::{thread_local_inner, Key, abort_on_dtor_unwind};
209 }
210 }
211}
212
213////////////////////////////////////////////////////////////////////////////////
214// Builder
215////////////////////////////////////////////////////////////////////////////////
216
217/// Thread factory, which can be used in order to configure the properties of
218/// a new thread.
219///
220/// Methods can be chained on it in order to configure it.
221///
222/// The two configurations available are:
223///
224/// - [`name`]: specifies an [associated name for the thread][naming-threads]
225/// - [`stack_size`]: specifies the [desired stack size for the thread][stack-size]
226///
227/// The [`spawn`] method will take ownership of the builder and create an
228/// [`io::Result`] to the thread handle with the given configuration.
229///
230/// The [`thread::spawn`] free function uses a `Builder` with default
231/// configuration and [`unwrap`]s its return value.
232///
233/// You may want to use [`spawn`] instead of [`thread::spawn`], when you want
234/// to recover from a failure to launch a thread, indeed the free function will
235/// panic where the `Builder` method will return a [`io::Result`].
236///
237/// # Examples
238///
239/// ```
240/// use std::thread;
241///
242/// let builder = thread::Builder::new();
243///
244/// let handler = builder.spawn(|| {
245/// // thread code
246/// }).unwrap();
247///
248/// handler.join().unwrap();
249/// ```
250///
251/// [`stack_size`]: Builder::stack_size
252/// [`name`]: Builder::name
253/// [`spawn`]: Builder::spawn
254/// [`thread::spawn`]: spawn
255/// [`io::Result`]: crate::io::Result
256/// [`unwrap`]: crate::result::Result::unwrap
257/// [naming-threads]: ./index.html#naming-threads
258/// [stack-size]: ./index.html#stack-size
259#[must_use = "must eventually spawn the thread"]
260#[stable(feature = "rust1", since = "1.0.0")]
261#[derive(Debug)]
262pub struct Builder {
263 // A name for the thread-to-be, for identification in panic messages
264 name: Option<String>,
265 // The size of the stack for the spawned thread in bytes
266 stack_size: Option<usize>,
267}
268
269impl Builder {
270 /// Generates the base configuration for spawning a thread, from which
271 /// configuration methods can be chained.
272 ///
273 /// # Examples
274 ///
275 /// ```
276 /// use std::thread;
277 ///
278 /// let builder = thread::Builder::new()
279 /// .name("foo".into())
280 /// .stack_size(32 * 1024);
281 ///
282 /// let handler = builder.spawn(|| {
283 /// // thread code
284 /// }).unwrap();
285 ///
286 /// handler.join().unwrap();
287 /// ```
288 #[stable(feature = "rust1", since = "1.0.0")]
289 pub fn new() -> Builder {
290 Builder { name: None, stack_size: None }
291 }
292
293 /// Names the thread-to-be. Currently the name is used for identification
294 /// only in panic messages.
295 ///
296 /// The name must not contain null bytes (`\0`).
297 ///
298 /// For more information about named threads, see
299 /// [this module-level documentation][naming-threads].
300 ///
301 /// # Examples
302 ///
303 /// ```
304 /// use std::thread;
305 ///
306 /// let builder = thread::Builder::new()
307 /// .name("foo".into());
308 ///
309 /// let handler = builder.spawn(|| {
310 /// assert_eq!(thread::current().name(), Some("foo"))
311 /// }).unwrap();
312 ///
313 /// handler.join().unwrap();
314 /// ```
315 ///
316 /// [naming-threads]: ./index.html#naming-threads
317 #[stable(feature = "rust1", since = "1.0.0")]
318 pub fn name(mut self, name: String) -> Builder {
319 self.name = Some(name);
320 self
321 }
322
323 /// Sets the size of the stack (in bytes) for the new thread.
324 ///
325 /// The actual stack size may be greater than this value if
326 /// the platform specifies a minimal stack size.
327 ///
328 /// For more information about the stack size for threads, see
329 /// [this module-level documentation][stack-size].
330 ///
331 /// # Examples
332 ///
333 /// ```
334 /// use std::thread;
335 ///
336 /// let builder = thread::Builder::new().stack_size(32 * 1024);
337 /// ```
338 ///
339 /// [stack-size]: ./index.html#stack-size
340 #[stable(feature = "rust1", since = "1.0.0")]
341 pub fn stack_size(mut self, size: usize) -> Builder {
342 self.stack_size = Some(size);
343 self
344 }
345
346 /// Spawns a new thread by taking ownership of the `Builder`, and returns an
347 /// [`io::Result`] to its [`JoinHandle`].
348 ///
349 /// The spawned thread may outlive the caller (unless the caller thread
350 /// is the main thread; the whole process is terminated when the main
351 /// thread finishes). The join handle can be used to block on
352 /// termination of the spawned thread, including recovering its panics.
353 ///
354 /// For a more complete documentation see [`thread::spawn`][`spawn`].
355 ///
356 /// # Errors
357 ///
358 /// Unlike the [`spawn`] free function, this method yields an
359 /// [`io::Result`] to capture any failure to create the thread at
360 /// the OS level.
361 ///
362 /// [`io::Result`]: crate::io::Result
363 ///
364 /// # Panics
365 ///
366 /// Panics if a thread name was set and it contained null bytes.
367 ///
368 /// # Examples
369 ///
370 /// ```
371 /// use std::thread;
372 ///
373 /// let builder = thread::Builder::new();
374 ///
375 /// let handler = builder.spawn(|| {
376 /// // thread code
377 /// }).unwrap();
378 ///
379 /// handler.join().unwrap();
380 /// ```
381 #[stable(feature = "rust1", since = "1.0.0")]
382 pub fn spawn<F, T>(self, f: F) -> io::Result<JoinHandle<T>>
383 where
384 F: FnOnce() -> T,
385 F: Send + 'static,
386 T: Send + 'static,
387 {
388 unsafe { self.spawn_unchecked(f) }
389 }
390
391 /// Spawns a new thread without any lifetime restrictions by taking ownership
392 /// of the `Builder`, and returns an [`io::Result`] to its [`JoinHandle`].
393 ///
394 /// The spawned thread may outlive the caller (unless the caller thread
395 /// is the main thread; the whole process is terminated when the main
396 /// thread finishes). The join handle can be used to block on
397 /// termination of the spawned thread, including recovering its panics.
398 ///
399 /// This method is identical to [`thread::Builder::spawn`][`Builder::spawn`],
400 /// except for the relaxed lifetime bounds, which render it unsafe.
401 /// For a more complete documentation see [`thread::spawn`][`spawn`].
402 ///
403 /// # Errors
404 ///
405 /// Unlike the [`spawn`] free function, this method yields an
406 /// [`io::Result`] to capture any failure to create the thread at
407 /// the OS level.
408 ///
409 /// # Panics
410 ///
411 /// Panics if a thread name was set and it contained null bytes.
412 ///
413 /// # Safety
414 ///
415 /// The caller has to ensure that the spawned thread does not outlive any
416 /// references in the supplied thread closure and its return type.
417 /// This can be guaranteed in two ways:
418 ///
419 /// - ensure that [`join`][`JoinHandle::join`] is called before any referenced
420 /// data is dropped
421 /// - use only types with `'static` lifetime bounds, i.e., those with no or only
422 /// `'static` references (both [`thread::Builder::spawn`][`Builder::spawn`]
423 /// and [`thread::spawn`][`spawn`] enforce this property statically)
424 ///
425 /// # Examples
426 ///
427 /// ```
428 /// #![feature(thread_spawn_unchecked)]
429 /// use std::thread;
430 ///
431 /// let builder = thread::Builder::new();
432 ///
433 /// let x = 1;
434 /// let thread_x = &x;
435 ///
436 /// let handler = unsafe {
437 /// builder.spawn_unchecked(move || {
438 /// println!("x = {}", *thread_x);
439 /// }).unwrap()
440 /// };
441 ///
442 /// // caller has to ensure `join()` is called, otherwise
443 /// // it is possible to access freed memory if `x` gets
444 /// // dropped before the thread closure is executed!
445 /// handler.join().unwrap();
446 /// ```
447 ///
448 /// [`io::Result`]: crate::io::Result
449 #[unstable(feature = "thread_spawn_unchecked", issue = "55132")]
450 pub unsafe fn spawn_unchecked<'a, F, T>(self, f: F) -> io::Result<JoinHandle<T>>
451 where
452 F: FnOnce() -> T,
453 F: Send + 'a,
454 T: Send + 'a,
455 {
456 Ok(JoinHandle(unsafe { self.spawn_unchecked_(f, None) }?))
457 }
458
459 unsafe fn spawn_unchecked_<'a, 'scope, F, T>(
460 self,
461 f: F,
462 scope_data: Option<Arc<scoped::ScopeData>>,
463 ) -> io::Result<JoinInner<'scope, T>>
464 where
465 F: FnOnce() -> T,
466 F: Send + 'a,
467 T: Send + 'a,
468 'scope: 'a,
469 {
470 let Builder { name, stack_size } = self;
471
472 let stack_size = stack_size.unwrap_or_else(|| {
473 static MIN: AtomicUsize = AtomicUsize::new(0);
474
475 match MIN.load(Ordering::Relaxed) {
476 0 => {}
477 n => return n - 1,
478 }
479
480 let amt = env::var_os("RUST_MIN_STACK")
481 .and_then(|s| s.to_str().and_then(|s| s.parse().ok()))
482 .unwrap_or(imp::DEFAULT_MIN_STACK_SIZE);
483
484 // 0 is our sentinel value, so ensure that we'll never see 0 after
485 // initialization has run
486 MIN.store(amt + 1, Ordering::Relaxed);
487 amt
488 });
489
490 let my_thread = name.map_or_else(Thread::new_unnamed, |name| unsafe {
491 Thread::new(
492 CString::new(name).expect("thread name may not contain interior null bytes"),
493 )
494 });
495 let their_thread = my_thread.clone();
496
497 let my_packet: Arc<Packet<'scope, T>> = Arc::new(Packet {
498 scope: scope_data,
499 result: UnsafeCell::new(None),
500 _marker: PhantomData,
501 });
502 let their_packet = my_packet.clone();
503
504 let output_capture = crate::io::set_output_capture(None);
505 crate::io::set_output_capture(output_capture.clone());
506
507 // Pass `f` in `MaybeUninit` because actually that closure might *run longer than the lifetime of `F`*.
508 // See <https://github.com/rust-lang/rust/issues/101983> for more details.
509 // To prevent leaks we use a wrapper that drops its contents.
510 #[repr(transparent)]
511 struct MaybeDangling<T>(mem::MaybeUninit<T>);
512 impl<T> MaybeDangling<T> {
513 fn new(x: T) -> Self {
514 MaybeDangling(mem::MaybeUninit::new(x))
515 }
516 fn into_inner(self) -> T {
517 // SAFETY: we are always initialized.
518 let ret = unsafe { self.0.assume_init_read() };
519 // Make sure we don't drop.
520 mem::forget(self);
521 ret
522 }
523 }
524 impl<T> Drop for MaybeDangling<T> {
525 fn drop(&mut self) {
526 // SAFETY: we are always initialized.
527 unsafe { self.0.assume_init_drop() };
528 }
529 }
530
531 let f = MaybeDangling::new(f);
532 let main = move || {
533 if let Some(name) = their_thread.cname() {
534 imp::Thread::set_name(name);
535 }
536
537 crate::io::set_output_capture(output_capture);
538
539 let f = f.into_inner();
540 set_current(their_thread);
541 let try_result = panic::catch_unwind(panic::AssertUnwindSafe(|| {
542 crate::sys_common::backtrace::__rust_begin_short_backtrace(f)
543 }));
544 // SAFETY: `their_packet` as been built just above and moved by the
545 // closure (it is an Arc<...>) and `my_packet` will be stored in the
546 // same `JoinInner` as this closure meaning the mutation will be
547 // safe (not modify it and affect a value far away).
548 unsafe { *their_packet.result.get() = Some(try_result) };
549 // Here `their_packet` gets dropped, and if this is the last `Arc` for that packet that
550 // will call `decrement_num_running_threads` and therefore signal that this thread is
551 // done.
552 drop(their_packet);
553 // Here, the lifetime `'a` and even `'scope` can end. `main` keeps running for a bit
554 // after that before returning itself.
555 };
556
557 if let Some(scope_data) = &my_packet.scope {
558 scope_data.increment_num_running_threads();
559 }
560
561 let main = Box::new(main);
562 // SAFETY: dynamic size and alignment of the Box remain the same. See below for why the
563 // lifetime change is justified.
564 let main = unsafe { Box::from_raw(Box::into_raw(main) as *mut (dyn FnOnce() + 'static)) };
565
566 Ok(JoinInner {
567 // SAFETY:
568 //
569 // `imp::Thread::new` takes a closure with a `'static` lifetime, since it's passed
570 // through FFI or otherwise used with low-level threading primitives that have no
571 // notion of or way to enforce lifetimes.
572 //
573 // As mentioned in the `Safety` section of this function's documentation, the caller of
574 // this function needs to guarantee that the passed-in lifetime is sufficiently long
575 // for the lifetime of the thread.
576 //
577 // Similarly, the `sys` implementation must guarantee that no references to the closure
578 // exist after the thread has terminated, which is signaled by `Thread::join`
579 // returning.
580 native: unsafe { imp::Thread::new(stack_size, main)? },
581 thread: my_thread,
582 packet: my_packet,
583 })
584 }
585}
586
587////////////////////////////////////////////////////////////////////////////////
588// Free functions
589////////////////////////////////////////////////////////////////////////////////
590
591/// Spawns a new thread, returning a [`JoinHandle`] for it.
592///
593/// The join handle provides a [`join`] method that can be used to join the spawned
594/// thread. If the spawned thread panics, [`join`] will return an [`Err`] containing
595/// the argument given to [`panic!`].
596///
597/// If the join handle is dropped, the spawned thread will implicitly be *detached*.
598/// In this case, the spawned thread may no longer be joined.
599/// (It is the responsibility of the program to either eventually join threads it
600/// creates or detach them; otherwise, a resource leak will result.)
601///
602/// This call will create a thread using default parameters of [`Builder`], if you
603/// want to specify the stack size or the name of the thread, use this API
604/// instead.
605///
606/// As you can see in the signature of `spawn` there are two constraints on
607/// both the closure given to `spawn` and its return value, let's explain them:
608///
609/// - The `'static` constraint means that the closure and its return value
610/// must have a lifetime of the whole program execution. The reason for this
611/// is that threads can outlive the lifetime they have been created in.
612///
613/// Indeed if the thread, and by extension its return value, can outlive their
614/// caller, we need to make sure that they will be valid afterwards, and since
615/// we *can't* know when it will return we need to have them valid as long as
616/// possible, that is until the end of the program, hence the `'static`
617/// lifetime.
618/// - The [`Send`] constraint is because the closure will need to be passed
619/// *by value* from the thread where it is spawned to the new thread. Its
620/// return value will need to be passed from the new thread to the thread
621/// where it is `join`ed.
622/// As a reminder, the [`Send`] marker trait expresses that it is safe to be
623/// passed from thread to thread. [`Sync`] expresses that it is safe to have a
624/// reference be passed from thread to thread.
625///
626/// # Panics
627///
628/// Panics if the OS fails to create a thread; use [`Builder::spawn`]
629/// to recover from such errors.
630///
631/// # Examples
632///
633/// Creating a thread.
634///
635/// ```
636/// use std::thread;
637///
638/// let handler = thread::spawn(|| {
639/// // thread code
640/// });
641///
642/// handler.join().unwrap();
643/// ```
644///
645/// As mentioned in the module documentation, threads are usually made to
646/// communicate using [`channels`], here is how it usually looks.
647///
648/// This example also shows how to use `move`, in order to give ownership
649/// of values to a thread.
650///
651/// ```
652/// use std::thread;
653/// use std::sync::mpsc::channel;
654///
655/// let (tx, rx) = channel();
656///
657/// let sender = thread::spawn(move || {
658/// tx.send("Hello, thread".to_owned())
659/// .expect("Unable to send on channel");
660/// });
661///
662/// let receiver = thread::spawn(move || {
663/// let value = rx.recv().expect("Unable to receive from channel");
664/// println!("{value}");
665/// });
666///
667/// sender.join().expect("The sender thread has panicked");
668/// receiver.join().expect("The receiver thread has panicked");
669/// ```
670///
671/// A thread can also return a value through its [`JoinHandle`], you can use
672/// this to make asynchronous computations (futures might be more appropriate
673/// though).
674///
675/// ```
676/// use std::thread;
677///
678/// let computation = thread::spawn(|| {
679/// // Some expensive computation.
680/// 42
681/// });
682///
683/// let result = computation.join().unwrap();
684/// println!("{result}");
685/// ```
686///
687/// [`channels`]: crate::sync::mpsc
688/// [`join`]: JoinHandle::join
689/// [`Err`]: crate::result::Result::Err
690#[stable(feature = "rust1", since = "1.0.0")]
691pub fn spawn<F, T>(f: F) -> JoinHandle<T>
692where
693 F: FnOnce() -> T,
694 F: Send + 'static,
695 T: Send + 'static,
696{
697 Builder::new().spawn(f).expect(msg:"failed to spawn thread")
698}
699
700thread_local! {
701 static CURRENT: OnceCell<Thread> = const { OnceCell::new() };
702}
703
704/// Sets the thread handle for the current thread.
705///
706/// Panics if the handle has been set already or when called from a TLS destructor.
707pub(crate) fn set_current(thread: Thread) {
708 CURRENT.with(|current: &OnceCell| current.set(thread).unwrap());
709}
710
711/// Gets a handle to the thread that invokes it.
712///
713/// In contrast to the public `current` function, this will not panic if called
714/// from inside a TLS destructor.
715pub(crate) fn try_current() -> Option<Thread> {
716 CURRENT.try_with(|current: &OnceCell| current.get_or_init(|| Thread::new_unnamed()).clone()).ok()
717}
718
719/// Gets a handle to the thread that invokes it.
720///
721/// # Examples
722///
723/// Getting a handle to the current thread with `thread::current()`:
724///
725/// ```
726/// use std::thread;
727///
728/// let handler = thread::Builder::new()
729/// .name("named thread".into())
730/// .spawn(|| {
731/// let handle = thread::current();
732/// assert_eq!(handle.name(), Some("named thread"));
733/// })
734/// .unwrap();
735///
736/// handler.join().unwrap();
737/// ```
738#[must_use]
739#[stable(feature = "rust1", since = "1.0.0")]
740pub fn current() -> Thread {
741 try_current().expect(
742 msg:"use of std::thread::current() is not possible \
743msg: after the thread's local data has been destroyed",
744 )
745}
746
747/// Cooperatively gives up a timeslice to the OS scheduler.
748///
749/// This calls the underlying OS scheduler's yield primitive, signaling
750/// that the calling thread is willing to give up its remaining timeslice
751/// so that the OS may schedule other threads on the CPU.
752///
753/// A drawback of yielding in a loop is that if the OS does not have any
754/// other ready threads to run on the current CPU, the thread will effectively
755/// busy-wait, which wastes CPU time and energy.
756///
757/// Therefore, when waiting for events of interest, a programmer's first
758/// choice should be to use synchronization devices such as [`channel`]s,
759/// [`Condvar`]s, [`Mutex`]es or [`join`] since these primitives are
760/// implemented in a blocking manner, giving up the CPU until the event
761/// of interest has occurred which avoids repeated yielding.
762///
763/// `yield_now` should thus be used only rarely, mostly in situations where
764/// repeated polling is required because there is no other suitable way to
765/// learn when an event of interest has occurred.
766///
767/// # Examples
768///
769/// ```
770/// use std::thread;
771///
772/// thread::yield_now();
773/// ```
774///
775/// [`channel`]: crate::sync::mpsc
776/// [`join`]: JoinHandle::join
777/// [`Condvar`]: crate::sync::Condvar
778/// [`Mutex`]: crate::sync::Mutex
779#[stable(feature = "rust1", since = "1.0.0")]
780pub fn yield_now() {
781 imp::Thread::yield_now()
782}
783
784/// Determines whether the current thread is unwinding because of panic.
785///
786/// A common use of this feature is to poison shared resources when writing
787/// unsafe code, by checking `panicking` when the `drop` is called.
788///
789/// This is usually not needed when writing safe code, as [`Mutex`es][Mutex]
790/// already poison themselves when a thread panics while holding the lock.
791///
792/// This can also be used in multithreaded applications, in order to send a
793/// message to other threads warning that a thread has panicked (e.g., for
794/// monitoring purposes).
795///
796/// # Examples
797///
798/// ```should_panic
799/// use std::thread;
800///
801/// struct SomeStruct;
802///
803/// impl Drop for SomeStruct {
804/// fn drop(&mut self) {
805/// if thread::panicking() {
806/// println!("dropped while unwinding");
807/// } else {
808/// println!("dropped while not unwinding");
809/// }
810/// }
811/// }
812///
813/// {
814/// print!("a: ");
815/// let a = SomeStruct;
816/// }
817///
818/// {
819/// print!("b: ");
820/// let b = SomeStruct;
821/// panic!()
822/// }
823/// ```
824///
825/// [Mutex]: crate::sync::Mutex
826#[inline]
827#[must_use]
828#[stable(feature = "rust1", since = "1.0.0")]
829pub fn panicking() -> bool {
830 panicking::panicking()
831}
832
833/// Use [`sleep`].
834///
835/// Puts the current thread to sleep for at least the specified amount of time.
836///
837/// The thread may sleep longer than the duration specified due to scheduling
838/// specifics or platform-dependent functionality. It will never sleep less.
839///
840/// This function is blocking, and should not be used in `async` functions.
841///
842/// # Platform-specific behavior
843///
844/// On Unix platforms, the underlying syscall may be interrupted by a
845/// spurious wakeup or signal handler. To ensure the sleep occurs for at least
846/// the specified duration, this function may invoke that system call multiple
847/// times.
848///
849/// # Examples
850///
851/// ```no_run
852/// use std::thread;
853///
854/// // Let's sleep for 2 seconds:
855/// thread::sleep_ms(2000);
856/// ```
857#[stable(feature = "rust1", since = "1.0.0")]
858#[deprecated(since = "1.6.0", note = "replaced by `std::thread::sleep`")]
859pub fn sleep_ms(ms: u32) {
860 sleep(dur:Duration::from_millis(ms as u64))
861}
862
863/// Puts the current thread to sleep for at least the specified amount of time.
864///
865/// The thread may sleep longer than the duration specified due to scheduling
866/// specifics or platform-dependent functionality. It will never sleep less.
867///
868/// This function is blocking, and should not be used in `async` functions.
869///
870/// # Platform-specific behavior
871///
872/// On Unix platforms, the underlying syscall may be interrupted by a
873/// spurious wakeup or signal handler. To ensure the sleep occurs for at least
874/// the specified duration, this function may invoke that system call multiple
875/// times.
876/// Platforms which do not support nanosecond precision for sleeping will
877/// have `dur` rounded up to the nearest granularity of time they can sleep for.
878///
879/// Currently, specifying a zero duration on Unix platforms returns immediately
880/// without invoking the underlying [`nanosleep`] syscall, whereas on Windows
881/// platforms the underlying [`Sleep`] syscall is always invoked.
882/// If the intention is to yield the current time-slice you may want to use
883/// [`yield_now`] instead.
884///
885/// [`nanosleep`]: https://linux.die.net/man/2/nanosleep
886/// [`Sleep`]: https://docs.microsoft.com/en-us/windows/win32/api/synchapi/nf-synchapi-sleep
887///
888/// # Examples
889///
890/// ```no_run
891/// use std::{thread, time};
892///
893/// let ten_millis = time::Duration::from_millis(10);
894/// let now = time::Instant::now();
895///
896/// thread::sleep(ten_millis);
897///
898/// assert!(now.elapsed() >= ten_millis);
899/// ```
900#[stable(feature = "thread_sleep", since = "1.4.0")]
901pub fn sleep(dur: Duration) {
902 imp::Thread::sleep(dur)
903}
904
905/// Puts the current thread to sleep until the specified deadline has passed.
906///
907/// The thread may still be asleep after the deadline specified due to
908/// scheduling specifics or platform-dependent functionality. It will never
909/// wake before.
910///
911/// This function is blocking, and should not be used in `async` functions.
912///
913/// # Platform-specific behavior
914///
915/// This function uses [`sleep`] internally, see its platform-specific behaviour.
916///
917///
918/// # Examples
919///
920/// A simple game loop that limits the game to 60 frames per second.
921///
922/// ```no_run
923/// #![feature(thread_sleep_until)]
924/// # use std::time::{Duration, Instant};
925/// # use std::thread;
926/// #
927/// # fn update() {}
928/// # fn render() {}
929/// #
930/// let max_fps = 60.0;
931/// let frame_time = Duration::from_secs_f32(1.0/max_fps);
932/// let mut next_frame = Instant::now();
933/// loop {
934/// thread::sleep_until(next_frame);
935/// next_frame += frame_time;
936/// update();
937/// render();
938/// }
939/// ```
940///
941/// A slow api we must not call too fast and which takes a few
942/// tries before succeeding. By using `sleep_until` the time the
943/// api call takes does not influence when we retry or when we give up
944///
945/// ```no_run
946/// #![feature(thread_sleep_until)]
947/// # use std::time::{Duration, Instant};
948/// # use std::thread;
949/// #
950/// # enum Status {
951/// # Ready(usize),
952/// # Waiting,
953/// # }
954/// # fn slow_web_api_call() -> Status { Status::Ready(42) }
955/// #
956/// # const MAX_DURATION: Duration = Duration::from_secs(10);
957/// #
958/// # fn try_api_call() -> Result<usize, ()> {
959/// let deadline = Instant::now() + MAX_DURATION;
960/// let delay = Duration::from_millis(250);
961/// let mut next_attempt = Instant::now();
962/// loop {
963/// if Instant::now() > deadline {
964/// break Err(());
965/// }
966/// if let Status::Ready(data) = slow_web_api_call() {
967/// break Ok(data);
968/// }
969///
970/// next_attempt = deadline.min(next_attempt + delay);
971/// thread::sleep_until(next_attempt);
972/// }
973/// # }
974/// # let _data = try_api_call();
975/// ```
976#[unstable(feature = "thread_sleep_until", issue = "113752")]
977pub fn sleep_until(deadline: Instant) {
978 let now: Instant = Instant::now();
979
980 if let Some(delay: Duration) = deadline.checked_duration_since(earlier:now) {
981 sleep(dur:delay);
982 }
983}
984
985/// Used to ensure that `park` and `park_timeout` do not unwind, as that can
986/// cause undefined behaviour if not handled correctly (see #102398 for context).
987struct PanicGuard;
988
989impl Drop for PanicGuard {
990 fn drop(&mut self) {
991 rtabort!("an irrecoverable error occurred while synchronizing threads")
992 }
993}
994
995/// Blocks unless or until the current thread's token is made available.
996///
997/// A call to `park` does not guarantee that the thread will remain parked
998/// forever, and callers should be prepared for this possibility. However,
999/// it is guaranteed that this function will not panic (it may abort the
1000/// process if the implementation encounters some rare errors).
1001///
1002/// # `park` and `unpark`
1003///
1004/// Every thread is equipped with some basic low-level blocking support, via the
1005/// [`thread::park`][`park`] function and [`thread::Thread::unpark`][`unpark`]
1006/// method. [`park`] blocks the current thread, which can then be resumed from
1007/// another thread by calling the [`unpark`] method on the blocked thread's
1008/// handle.
1009///
1010/// Conceptually, each [`Thread`] handle has an associated token, which is
1011/// initially not present:
1012///
1013/// * The [`thread::park`][`park`] function blocks the current thread unless or
1014/// until the token is available for its thread handle, at which point it
1015/// atomically consumes the token. It may also return *spuriously*, without
1016/// consuming the token. [`thread::park_timeout`] does the same, but allows
1017/// specifying a maximum time to block the thread for.
1018///
1019/// * The [`unpark`] method on a [`Thread`] atomically makes the token available
1020/// if it wasn't already. Because the token is initially absent, [`unpark`]
1021/// followed by [`park`] will result in the second call returning immediately.
1022///
1023/// The API is typically used by acquiring a handle to the current thread,
1024/// placing that handle in a shared data structure so that other threads can
1025/// find it, and then `park`ing in a loop. When some desired condition is met, another
1026/// thread calls [`unpark`] on the handle.
1027///
1028/// The motivation for this design is twofold:
1029///
1030/// * It avoids the need to allocate mutexes and condvars when building new
1031/// synchronization primitives; the threads already provide basic
1032/// blocking/signaling.
1033///
1034/// * It can be implemented very efficiently on many platforms.
1035///
1036/// # Memory Ordering
1037///
1038/// Calls to `park` _synchronize-with_ calls to `unpark`, meaning that memory
1039/// operations performed before a call to `unpark` are made visible to the thread that
1040/// consumes the token and returns from `park`. Note that all `park` and `unpark`
1041/// operations for a given thread form a total order and `park` synchronizes-with
1042/// _all_ prior `unpark` operations.
1043///
1044/// In atomic ordering terms, `unpark` performs a `Release` operation and `park`
1045/// performs the corresponding `Acquire` operation. Calls to `unpark` for the same
1046/// thread form a [release sequence].
1047///
1048/// Note that being unblocked does not imply a call was made to `unpark`, because
1049/// wakeups can also be spurious. For example, a valid, but inefficient,
1050/// implementation could have `park` and `unpark` return immediately without doing anything,
1051/// making *all* wakeups spurious.
1052///
1053/// # Examples
1054///
1055/// ```
1056/// use std::thread;
1057/// use std::sync::{Arc, atomic::{Ordering, AtomicBool}};
1058/// use std::time::Duration;
1059///
1060/// let flag = Arc::new(AtomicBool::new(false));
1061/// let flag2 = Arc::clone(&flag);
1062///
1063/// let parked_thread = thread::spawn(move || {
1064/// // We want to wait until the flag is set. We *could* just spin, but using
1065/// // park/unpark is more efficient.
1066/// while !flag2.load(Ordering::Relaxed) {
1067/// println!("Parking thread");
1068/// thread::park();
1069/// // We *could* get here spuriously, i.e., way before the 10ms below are over!
1070/// // But that is no problem, we are in a loop until the flag is set anyway.
1071/// println!("Thread unparked");
1072/// }
1073/// println!("Flag received");
1074/// });
1075///
1076/// // Let some time pass for the thread to be spawned.
1077/// thread::sleep(Duration::from_millis(10));
1078///
1079/// // Set the flag, and let the thread wake up.
1080/// // There is no race condition here, if `unpark`
1081/// // happens first, `park` will return immediately.
1082/// // Hence there is no risk of a deadlock.
1083/// flag.store(true, Ordering::Relaxed);
1084/// println!("Unpark the thread");
1085/// parked_thread.thread().unpark();
1086///
1087/// parked_thread.join().unwrap();
1088/// ```
1089///
1090/// [`unpark`]: Thread::unpark
1091/// [`thread::park_timeout`]: park_timeout
1092/// [release sequence]: https://en.cppreference.com/w/cpp/atomic/memory_order#Release_sequence
1093#[stable(feature = "rust1", since = "1.0.0")]
1094pub fn park() {
1095 let guard: PanicGuard = PanicGuard;
1096 // SAFETY: park_timeout is called on the parker owned by this thread.
1097 unsafe {
1098 current().park();
1099 }
1100 // No panic occurred, do not abort.
1101 forget(guard);
1102}
1103
1104/// Use [`park_timeout`].
1105///
1106/// Blocks unless or until the current thread's token is made available or
1107/// the specified duration has been reached (may wake spuriously).
1108///
1109/// The semantics of this function are equivalent to [`park`] except
1110/// that the thread will be blocked for roughly no longer than `dur`. This
1111/// method should not be used for precise timing due to anomalies such as
1112/// preemption or platform differences that might not cause the maximum
1113/// amount of time waited to be precisely `ms` long.
1114///
1115/// See the [park documentation][`park`] for more detail.
1116#[stable(feature = "rust1", since = "1.0.0")]
1117#[deprecated(since = "1.6.0", note = "replaced by `std::thread::park_timeout`")]
1118pub fn park_timeout_ms(ms: u32) {
1119 park_timeout(dur:Duration::from_millis(ms as u64))
1120}
1121
1122/// Blocks unless or until the current thread's token is made available or
1123/// the specified duration has been reached (may wake spuriously).
1124///
1125/// The semantics of this function are equivalent to [`park`][park] except
1126/// that the thread will be blocked for roughly no longer than `dur`. This
1127/// method should not be used for precise timing due to anomalies such as
1128/// preemption or platform differences that might not cause the maximum
1129/// amount of time waited to be precisely `dur` long.
1130///
1131/// See the [park documentation][park] for more details.
1132///
1133/// # Platform-specific behavior
1134///
1135/// Platforms which do not support nanosecond precision for sleeping will have
1136/// `dur` rounded up to the nearest granularity of time they can sleep for.
1137///
1138/// # Examples
1139///
1140/// Waiting for the complete expiration of the timeout:
1141///
1142/// ```rust,no_run
1143/// use std::thread::park_timeout;
1144/// use std::time::{Instant, Duration};
1145///
1146/// let timeout = Duration::from_secs(2);
1147/// let beginning_park = Instant::now();
1148///
1149/// let mut timeout_remaining = timeout;
1150/// loop {
1151/// park_timeout(timeout_remaining);
1152/// let elapsed = beginning_park.elapsed();
1153/// if elapsed >= timeout {
1154/// break;
1155/// }
1156/// println!("restarting park_timeout after {elapsed:?}");
1157/// timeout_remaining = timeout - elapsed;
1158/// }
1159/// ```
1160#[stable(feature = "park_timeout", since = "1.4.0")]
1161pub fn park_timeout(dur: Duration) {
1162 let guard: PanicGuard = PanicGuard;
1163 // SAFETY: park_timeout is called on the parker owned by this thread.
1164 unsafe {
1165 current().inner.as_ref().parker().park_timeout(dur);
1166 }
1167 // No panic occurred, do not abort.
1168 forget(guard);
1169}
1170
1171////////////////////////////////////////////////////////////////////////////////
1172// ThreadId
1173////////////////////////////////////////////////////////////////////////////////
1174
1175/// A unique identifier for a running thread.
1176///
1177/// A `ThreadId` is an opaque object that uniquely identifies each thread
1178/// created during the lifetime of a process. `ThreadId`s are guaranteed not to
1179/// be reused, even when a thread terminates. `ThreadId`s are under the control
1180/// of Rust's standard library and there may not be any relationship between
1181/// `ThreadId` and the underlying platform's notion of a thread identifier --
1182/// the two concepts cannot, therefore, be used interchangeably. A `ThreadId`
1183/// can be retrieved from the [`id`] method on a [`Thread`].
1184///
1185/// # Examples
1186///
1187/// ```
1188/// use std::thread;
1189///
1190/// let other_thread = thread::spawn(|| {
1191/// thread::current().id()
1192/// });
1193///
1194/// let other_thread_id = other_thread.join().unwrap();
1195/// assert!(thread::current().id() != other_thread_id);
1196/// ```
1197///
1198/// [`id`]: Thread::id
1199#[stable(feature = "thread_id", since = "1.19.0")]
1200#[derive(Eq, PartialEq, Clone, Copy, Hash, Debug)]
1201pub struct ThreadId(NonZero<u64>);
1202
1203impl ThreadId {
1204 // Generate a new unique thread ID.
1205 fn new() -> ThreadId {
1206 #[cold]
1207 fn exhausted() -> ! {
1208 panic!("failed to generate unique thread ID: bitspace exhausted")
1209 }
1210
1211 cfg_if::cfg_if! {
1212 if #[cfg(target_has_atomic = "64")] {
1213 use crate::sync::atomic::AtomicU64;
1214
1215 static COUNTER: AtomicU64 = AtomicU64::new(0);
1216
1217 let mut last = COUNTER.load(Ordering::Relaxed);
1218 loop {
1219 let Some(id) = last.checked_add(1) else {
1220 exhausted();
1221 };
1222
1223 match COUNTER.compare_exchange_weak(last, id, Ordering::Relaxed, Ordering::Relaxed) {
1224 Ok(_) => return ThreadId(NonZero::new(id).unwrap()),
1225 Err(id) => last = id,
1226 }
1227 }
1228 } else {
1229 use crate::sync::{Mutex, PoisonError};
1230
1231 static COUNTER: Mutex<u64> = Mutex::new(0);
1232
1233 let mut counter = COUNTER.lock().unwrap_or_else(PoisonError::into_inner);
1234 let Some(id) = counter.checked_add(1) else {
1235 // in case the panic handler ends up calling `ThreadId::new()`,
1236 // avoid reentrant lock acquire.
1237 drop(counter);
1238 exhausted();
1239 };
1240
1241 *counter = id;
1242 drop(counter);
1243 ThreadId(NonZero::new(id).unwrap())
1244 }
1245 }
1246 }
1247
1248 /// This returns a numeric identifier for the thread identified by this
1249 /// `ThreadId`.
1250 ///
1251 /// As noted in the documentation for the type itself, it is essentially an
1252 /// opaque ID, but is guaranteed to be unique for each thread. The returned
1253 /// value is entirely opaque -- only equality testing is stable. Note that
1254 /// it is not guaranteed which values new threads will return, and this may
1255 /// change across Rust versions.
1256 #[must_use]
1257 #[unstable(feature = "thread_id_value", issue = "67939")]
1258 pub fn as_u64(&self) -> NonZero<u64> {
1259 self.0
1260 }
1261}
1262
1263////////////////////////////////////////////////////////////////////////////////
1264// Thread
1265////////////////////////////////////////////////////////////////////////////////
1266
1267/// The internal representation of a `Thread`'s name.
1268enum ThreadName {
1269 Main,
1270 Other(CString),
1271 Unnamed,
1272}
1273
1274/// The internal representation of a `Thread` handle
1275struct Inner {
1276 name: ThreadName, // Guaranteed to be UTF-8
1277 id: ThreadId,
1278 parker: Parker,
1279}
1280
1281impl Inner {
1282 fn parker(self: Pin<&Self>) -> Pin<&Parker> {
1283 unsafe { Pin::map_unchecked(self, |inner: &Inner| &inner.parker) }
1284 }
1285}
1286
1287#[derive(Clone)]
1288#[stable(feature = "rust1", since = "1.0.0")]
1289/// A handle to a thread.
1290///
1291/// Threads are represented via the `Thread` type, which you can get in one of
1292/// two ways:
1293///
1294/// * By spawning a new thread, e.g., using the [`thread::spawn`][`spawn`]
1295/// function, and calling [`thread`][`JoinHandle::thread`] on the
1296/// [`JoinHandle`].
1297/// * By requesting the current thread, using the [`thread::current`] function.
1298///
1299/// The [`thread::current`] function is available even for threads not spawned
1300/// by the APIs of this module.
1301///
1302/// There is usually no need to create a `Thread` struct yourself, one
1303/// should instead use a function like `spawn` to create new threads, see the
1304/// docs of [`Builder`] and [`spawn`] for more details.
1305///
1306/// [`thread::current`]: current
1307pub struct Thread {
1308 inner: Pin<Arc<Inner>>,
1309}
1310
1311impl Thread {
1312 /// Used only internally to construct a thread object without spawning.
1313 ///
1314 /// # Safety
1315 /// `name` must be valid UTF-8.
1316 pub(crate) unsafe fn new(name: CString) -> Thread {
1317 unsafe { Self::new_inner(ThreadName::Other(name)) }
1318 }
1319
1320 pub(crate) fn new_unnamed() -> Thread {
1321 unsafe { Self::new_inner(ThreadName::Unnamed) }
1322 }
1323
1324 // Used in runtime to construct main thread
1325 pub(crate) fn new_main() -> Thread {
1326 unsafe { Self::new_inner(ThreadName::Main) }
1327 }
1328
1329 /// # Safety
1330 /// If `name` is `ThreadName::Other(_)`, the contained string must be valid UTF-8.
1331 unsafe fn new_inner(name: ThreadName) -> Thread {
1332 // We have to use `unsafe` here to construct the `Parker` in-place,
1333 // which is required for the UNIX implementation.
1334 //
1335 // SAFETY: We pin the Arc immediately after creation, so its address never
1336 // changes.
1337 let inner = unsafe {
1338 let mut arc = Arc::<Inner>::new_uninit();
1339 let ptr = Arc::get_mut_unchecked(&mut arc).as_mut_ptr();
1340 addr_of_mut!((*ptr).name).write(name);
1341 addr_of_mut!((*ptr).id).write(ThreadId::new());
1342 Parker::new_in_place(addr_of_mut!((*ptr).parker));
1343 Pin::new_unchecked(arc.assume_init())
1344 };
1345
1346 Thread { inner }
1347 }
1348
1349 /// Like the public [`park`], but callable on any handle. This is used to
1350 /// allow parking in TLS destructors.
1351 ///
1352 /// # Safety
1353 /// May only be called from the thread to which this handle belongs.
1354 pub(crate) unsafe fn park(&self) {
1355 unsafe { self.inner.as_ref().parker().park() }
1356 }
1357
1358 /// Atomically makes the handle's token available if it is not already.
1359 ///
1360 /// Every thread is equipped with some basic low-level blocking support, via
1361 /// the [`park`][park] function and the `unpark()` method. These can be
1362 /// used as a more CPU-efficient implementation of a spinlock.
1363 ///
1364 /// See the [park documentation][park] for more details.
1365 ///
1366 /// # Examples
1367 ///
1368 /// ```
1369 /// use std::thread;
1370 /// use std::time::Duration;
1371 ///
1372 /// let parked_thread = thread::Builder::new()
1373 /// .spawn(|| {
1374 /// println!("Parking thread");
1375 /// thread::park();
1376 /// println!("Thread unparked");
1377 /// })
1378 /// .unwrap();
1379 ///
1380 /// // Let some time pass for the thread to be spawned.
1381 /// thread::sleep(Duration::from_millis(10));
1382 ///
1383 /// println!("Unpark the thread");
1384 /// parked_thread.thread().unpark();
1385 ///
1386 /// parked_thread.join().unwrap();
1387 /// ```
1388 #[stable(feature = "rust1", since = "1.0.0")]
1389 #[inline]
1390 pub fn unpark(&self) {
1391 self.inner.as_ref().parker().unpark();
1392 }
1393
1394 /// Gets the thread's unique identifier.
1395 ///
1396 /// # Examples
1397 ///
1398 /// ```
1399 /// use std::thread;
1400 ///
1401 /// let other_thread = thread::spawn(|| {
1402 /// thread::current().id()
1403 /// });
1404 ///
1405 /// let other_thread_id = other_thread.join().unwrap();
1406 /// assert!(thread::current().id() != other_thread_id);
1407 /// ```
1408 #[stable(feature = "thread_id", since = "1.19.0")]
1409 #[must_use]
1410 pub fn id(&self) -> ThreadId {
1411 self.inner.id
1412 }
1413
1414 /// Gets the thread's name.
1415 ///
1416 /// For more information about named threads, see
1417 /// [this module-level documentation][naming-threads].
1418 ///
1419 /// # Examples
1420 ///
1421 /// Threads by default have no name specified:
1422 ///
1423 /// ```
1424 /// use std::thread;
1425 ///
1426 /// let builder = thread::Builder::new();
1427 ///
1428 /// let handler = builder.spawn(|| {
1429 /// assert!(thread::current().name().is_none());
1430 /// }).unwrap();
1431 ///
1432 /// handler.join().unwrap();
1433 /// ```
1434 ///
1435 /// Thread with a specified name:
1436 ///
1437 /// ```
1438 /// use std::thread;
1439 ///
1440 /// let builder = thread::Builder::new()
1441 /// .name("foo".into());
1442 ///
1443 /// let handler = builder.spawn(|| {
1444 /// assert_eq!(thread::current().name(), Some("foo"))
1445 /// }).unwrap();
1446 ///
1447 /// handler.join().unwrap();
1448 /// ```
1449 ///
1450 /// [naming-threads]: ./index.html#naming-threads
1451 #[stable(feature = "rust1", since = "1.0.0")]
1452 #[must_use]
1453 pub fn name(&self) -> Option<&str> {
1454 self.cname().map(|s| unsafe { str::from_utf8_unchecked(s.to_bytes()) })
1455 }
1456
1457 fn cname(&self) -> Option<&CStr> {
1458 match &self.inner.name {
1459 ThreadName::Main => Some(c"main"),
1460 ThreadName::Other(other) => Some(&other),
1461 ThreadName::Unnamed => None,
1462 }
1463 }
1464}
1465
1466#[stable(feature = "rust1", since = "1.0.0")]
1467impl fmt::Debug for Thread {
1468 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1469 f&mut DebugStruct<'_, '_>.debug_struct("Thread")
1470 .field("id", &self.id())
1471 .field(name:"name", &self.name())
1472 .finish_non_exhaustive()
1473 }
1474}
1475
1476////////////////////////////////////////////////////////////////////////////////
1477// JoinHandle
1478////////////////////////////////////////////////////////////////////////////////
1479
1480/// A specialized [`Result`] type for threads.
1481///
1482/// Indicates the manner in which a thread exited.
1483///
1484/// The value contained in the `Result::Err` variant
1485/// is the value the thread panicked with;
1486/// that is, the argument the `panic!` macro was called with.
1487/// Unlike with normal errors, this value doesn't implement
1488/// the [`Error`](crate::error::Error) trait.
1489///
1490/// Thus, a sensible way to handle a thread panic is to either:
1491///
1492/// 1. propagate the panic with [`std::panic::resume_unwind`]
1493/// 2. or in case the thread is intended to be a subsystem boundary
1494/// that is supposed to isolate system-level failures,
1495/// match on the `Err` variant and handle the panic in an appropriate way
1496///
1497/// A thread that completes without panicking is considered to exit successfully.
1498///
1499/// # Examples
1500///
1501/// Matching on the result of a joined thread:
1502///
1503/// ```no_run
1504/// use std::{fs, thread, panic};
1505///
1506/// fn copy_in_thread() -> thread::Result<()> {
1507/// thread::spawn(|| {
1508/// fs::copy("foo.txt", "bar.txt").unwrap();
1509/// }).join()
1510/// }
1511///
1512/// fn main() {
1513/// match copy_in_thread() {
1514/// Ok(_) => println!("copy succeeded"),
1515/// Err(e) => panic::resume_unwind(e),
1516/// }
1517/// }
1518/// ```
1519///
1520/// [`Result`]: crate::result::Result
1521/// [`std::panic::resume_unwind`]: crate::panic::resume_unwind
1522#[stable(feature = "rust1", since = "1.0.0")]
1523pub type Result<T> = crate::result::Result<T, Box<dyn Any + Send + 'static>>;
1524
1525// This packet is used to communicate the return value between the spawned
1526// thread and the rest of the program. It is shared through an `Arc` and
1527// there's no need for a mutex here because synchronization happens with `join()`
1528// (the caller will never read this packet until the thread has exited).
1529//
1530// An Arc to the packet is stored into a `JoinInner` which in turns is placed
1531// in `JoinHandle`.
1532struct Packet<'scope, T> {
1533 scope: Option<Arc<scoped::ScopeData>>,
1534 result: UnsafeCell<Option<Result<T>>>,
1535 _marker: PhantomData<Option<&'scope scoped::ScopeData>>,
1536}
1537
1538// Due to the usage of `UnsafeCell` we need to manually implement Sync.
1539// The type `T` should already always be Send (otherwise the thread could not
1540// have been created) and the Packet is Sync because all access to the
1541// `UnsafeCell` synchronized (by the `join()` boundary), and `ScopeData` is Sync.
1542unsafe impl<'scope, T: Sync> Sync for Packet<'scope, T> {}
1543
1544impl<'scope, T> Drop for Packet<'scope, T> {
1545 fn drop(&mut self) {
1546 // If this packet was for a thread that ran in a scope, the thread
1547 // panicked, and nobody consumed the panic payload, we make sure
1548 // the scope function will panic.
1549 let unhandled_panic = matches!(self.result.get_mut(), Some(Err(_)));
1550 // Drop the result without causing unwinding.
1551 // This is only relevant for threads that aren't join()ed, as
1552 // join() will take the `result` and set it to None, such that
1553 // there is nothing left to drop here.
1554 // If this panics, we should handle that, because we're outside the
1555 // outermost `catch_unwind` of our thread.
1556 // We just abort in that case, since there's nothing else we can do.
1557 // (And even if we tried to handle it somehow, we'd also need to handle
1558 // the case where the panic payload we get out of it also panics on
1559 // drop, and so on. See issue #86027.)
1560 if let Err(_) = panic::catch_unwind(panic::AssertUnwindSafe(|| {
1561 *self.result.get_mut() = None;
1562 })) {
1563 rtabort!("thread result panicked on drop");
1564 }
1565 // Book-keeping so the scope knows when it's done.
1566 if let Some(scope) = &self.scope {
1567 // Now that there will be no more user code running on this thread
1568 // that can use 'scope, mark the thread as 'finished'.
1569 // It's important we only do this after the `result` has been dropped,
1570 // since dropping it might still use things it borrowed from 'scope.
1571 scope.decrement_num_running_threads(unhandled_panic);
1572 }
1573 }
1574}
1575
1576/// Inner representation for JoinHandle
1577struct JoinInner<'scope, T> {
1578 native: imp::Thread,
1579 thread: Thread,
1580 packet: Arc<Packet<'scope, T>>,
1581}
1582
1583impl<'scope, T> JoinInner<'scope, T> {
1584 fn join(mut self) -> Result<T> {
1585 self.native.join();
1586 Arc::get_mut(&mut self.packet).unwrap().result.get_mut().take().unwrap()
1587 }
1588}
1589
1590/// An owned permission to join on a thread (block on its termination).
1591///
1592/// A `JoinHandle` *detaches* the associated thread when it is dropped, which
1593/// means that there is no longer any handle to the thread and no way to `join`
1594/// on it.
1595///
1596/// Due to platform restrictions, it is not possible to [`Clone`] this
1597/// handle: the ability to join a thread is a uniquely-owned permission.
1598///
1599/// This `struct` is created by the [`thread::spawn`] function and the
1600/// [`thread::Builder::spawn`] method.
1601///
1602/// # Examples
1603///
1604/// Creation from [`thread::spawn`]:
1605///
1606/// ```
1607/// use std::thread;
1608///
1609/// let join_handle: thread::JoinHandle<_> = thread::spawn(|| {
1610/// // some work here
1611/// });
1612/// ```
1613///
1614/// Creation from [`thread::Builder::spawn`]:
1615///
1616/// ```
1617/// use std::thread;
1618///
1619/// let builder = thread::Builder::new();
1620///
1621/// let join_handle: thread::JoinHandle<_> = builder.spawn(|| {
1622/// // some work here
1623/// }).unwrap();
1624/// ```
1625///
1626/// A thread being detached and outliving the thread that spawned it:
1627///
1628/// ```no_run
1629/// use std::thread;
1630/// use std::time::Duration;
1631///
1632/// let original_thread = thread::spawn(|| {
1633/// let _detached_thread = thread::spawn(|| {
1634/// // Here we sleep to make sure that the first thread returns before.
1635/// thread::sleep(Duration::from_millis(10));
1636/// // This will be called, even though the JoinHandle is dropped.
1637/// println!("♫ Still alive ♫");
1638/// });
1639/// });
1640///
1641/// original_thread.join().expect("The thread being joined has panicked");
1642/// println!("Original thread is joined.");
1643///
1644/// // We make sure that the new thread has time to run, before the main
1645/// // thread returns.
1646///
1647/// thread::sleep(Duration::from_millis(1000));
1648/// ```
1649///
1650/// [`thread::Builder::spawn`]: Builder::spawn
1651/// [`thread::spawn`]: spawn
1652#[stable(feature = "rust1", since = "1.0.0")]
1653#[cfg_attr(target_os = "teeos", must_use)]
1654pub struct JoinHandle<T>(JoinInner<'static, T>);
1655
1656#[stable(feature = "joinhandle_impl_send_sync", since = "1.29.0")]
1657unsafe impl<T> Send for JoinHandle<T> {}
1658#[stable(feature = "joinhandle_impl_send_sync", since = "1.29.0")]
1659unsafe impl<T> Sync for JoinHandle<T> {}
1660
1661impl<T> JoinHandle<T> {
1662 /// Extracts a handle to the underlying thread.
1663 ///
1664 /// # Examples
1665 ///
1666 /// ```
1667 /// use std::thread;
1668 ///
1669 /// let builder = thread::Builder::new();
1670 ///
1671 /// let join_handle: thread::JoinHandle<_> = builder.spawn(|| {
1672 /// // some work here
1673 /// }).unwrap();
1674 ///
1675 /// let thread = join_handle.thread();
1676 /// println!("thread id: {:?}", thread.id());
1677 /// ```
1678 #[stable(feature = "rust1", since = "1.0.0")]
1679 #[must_use]
1680 pub fn thread(&self) -> &Thread {
1681 &self.0.thread
1682 }
1683
1684 /// Waits for the associated thread to finish.
1685 ///
1686 /// This function will return immediately if the associated thread has already finished.
1687 ///
1688 /// In terms of [atomic memory orderings], the completion of the associated
1689 /// thread synchronizes with this function returning. In other words, all
1690 /// operations performed by that thread [happen
1691 /// before](https://doc.rust-lang.org/nomicon/atomics.html#data-accesses) all
1692 /// operations that happen after `join` returns.
1693 ///
1694 /// If the associated thread panics, [`Err`] is returned with the parameter given
1695 /// to [`panic!`].
1696 ///
1697 /// [`Err`]: crate::result::Result::Err
1698 /// [atomic memory orderings]: crate::sync::atomic
1699 ///
1700 /// # Panics
1701 ///
1702 /// This function may panic on some platforms if a thread attempts to join
1703 /// itself or otherwise may create a deadlock with joining threads.
1704 ///
1705 /// # Examples
1706 ///
1707 /// ```
1708 /// use std::thread;
1709 ///
1710 /// let builder = thread::Builder::new();
1711 ///
1712 /// let join_handle: thread::JoinHandle<_> = builder.spawn(|| {
1713 /// // some work here
1714 /// }).unwrap();
1715 /// join_handle.join().expect("Couldn't join on the associated thread");
1716 /// ```
1717 #[stable(feature = "rust1", since = "1.0.0")]
1718 pub fn join(self) -> Result<T> {
1719 self.0.join()
1720 }
1721
1722 /// Checks if the associated thread has finished running its main function.
1723 ///
1724 /// `is_finished` supports implementing a non-blocking join operation, by checking
1725 /// `is_finished`, and calling `join` if it returns `true`. This function does not block. To
1726 /// block while waiting on the thread to finish, use [`join`][Self::join].
1727 ///
1728 /// This might return `true` for a brief moment after the thread's main
1729 /// function has returned, but before the thread itself has stopped running.
1730 /// However, once this returns `true`, [`join`][Self::join] can be expected
1731 /// to return quickly, without blocking for any significant amount of time.
1732 #[stable(feature = "thread_is_running", since = "1.61.0")]
1733 pub fn is_finished(&self) -> bool {
1734 Arc::strong_count(&self.0.packet) == 1
1735 }
1736}
1737
1738impl<T> AsInner<imp::Thread> for JoinHandle<T> {
1739 fn as_inner(&self) -> &imp::Thread {
1740 &self.0.native
1741 }
1742}
1743
1744impl<T> IntoInner<imp::Thread> for JoinHandle<T> {
1745 fn into_inner(self) -> imp::Thread {
1746 self.0.native
1747 }
1748}
1749
1750#[stable(feature = "std_debug", since = "1.16.0")]
1751impl<T> fmt::Debug for JoinHandle<T> {
1752 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1753 f.debug_struct(name:"JoinHandle").finish_non_exhaustive()
1754 }
1755}
1756
1757fn _assert_sync_and_send() {
1758 fn _assert_both<T: Send + Sync>() {}
1759 _assert_both::<JoinHandle<()>>();
1760 _assert_both::<Thread>();
1761}
1762
1763/// Returns an estimate of the default amount of parallelism a program should use.
1764///
1765/// Parallelism is a resource. A given machine provides a certain capacity for
1766/// parallelism, i.e., a bound on the number of computations it can perform
1767/// simultaneously. This number often corresponds to the amount of CPUs a
1768/// computer has, but it may diverge in various cases.
1769///
1770/// Host environments such as VMs or container orchestrators may want to
1771/// restrict the amount of parallelism made available to programs in them. This
1772/// is often done to limit the potential impact of (unintentionally)
1773/// resource-intensive programs on other programs running on the same machine.
1774///
1775/// # Limitations
1776///
1777/// The purpose of this API is to provide an easy and portable way to query
1778/// the default amount of parallelism the program should use. Among other things it
1779/// does not expose information on NUMA regions, does not account for
1780/// differences in (co)processor capabilities or current system load,
1781/// and will not modify the program's global state in order to more accurately
1782/// query the amount of available parallelism.
1783///
1784/// Where both fixed steady-state and burst limits are available the steady-state
1785/// capacity will be used to ensure more predictable latencies.
1786///
1787/// Resource limits can be changed during the runtime of a program, therefore the value is
1788/// not cached and instead recomputed every time this function is called. It should not be
1789/// called from hot code.
1790///
1791/// The value returned by this function should be considered a simplified
1792/// approximation of the actual amount of parallelism available at any given
1793/// time. To get a more detailed or precise overview of the amount of
1794/// parallelism available to the program, you may wish to use
1795/// platform-specific APIs as well. The following platform limitations currently
1796/// apply to `available_parallelism`:
1797///
1798/// On Windows:
1799/// - It may undercount the amount of parallelism available on systems with more
1800/// than 64 logical CPUs. However, programs typically need specific support to
1801/// take advantage of more than 64 logical CPUs, and in the absence of such
1802/// support, the number returned by this function accurately reflects the
1803/// number of logical CPUs the program can use by default.
1804/// - It may overcount the amount of parallelism available on systems limited by
1805/// process-wide affinity masks, or job object limitations.
1806///
1807/// On Linux:
1808/// - It may overcount the amount of parallelism available when limited by a
1809/// process-wide affinity mask or cgroup quotas and `sched_getaffinity()` or cgroup fs can't be
1810/// queried, e.g. due to sandboxing.
1811/// - It may undercount the amount of parallelism if the current thread's affinity mask
1812/// does not reflect the process' cpuset, e.g. due to pinned threads.
1813/// - If the process is in a cgroup v1 cpu controller, this may need to
1814/// scan mountpoints to find the corresponding cgroup v1 controller,
1815/// which may take time on systems with large numbers of mountpoints.
1816/// (This does not apply to cgroup v2, or to processes not in a
1817/// cgroup.)
1818///
1819/// On all targets:
1820/// - It may overcount the amount of parallelism available when running in a VM
1821/// with CPU usage limits (e.g. an overcommitted host).
1822///
1823/// # Errors
1824///
1825/// This function will, but is not limited to, return errors in the following
1826/// cases:
1827///
1828/// - If the amount of parallelism is not known for the target platform.
1829/// - If the program lacks permission to query the amount of parallelism made
1830/// available to it.
1831///
1832/// # Examples
1833///
1834/// ```
1835/// # #![allow(dead_code)]
1836/// use std::{io, thread};
1837///
1838/// fn main() -> io::Result<()> {
1839/// let count = thread::available_parallelism()?.get();
1840/// assert!(count >= 1_usize);
1841/// Ok(())
1842/// }
1843/// ```
1844#[doc(alias = "available_concurrency")] // Alias for a previous name we gave this API on unstable.
1845#[doc(alias = "hardware_concurrency")] // Alias for C++ `std::thread::hardware_concurrency`.
1846#[doc(alias = "num_cpus")] // Alias for a popular ecosystem crate which provides similar functionality.
1847#[stable(feature = "available_parallelism", since = "1.59.0")]
1848pub fn available_parallelism() -> io::Result<NonZero<usize>> {
1849 imp::available_parallelism()
1850}
1851