std/thread/
mod.rs

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::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(any(target_os = "emscripten", target_os = "wasi"))))]
159mod tests;
160
161use crate::any::Any;
162use crate::cell::UnsafeCell;
163use crate::ffi::CStr;
164use crate::marker::PhantomData;
165use crate::mem::{self, ManuallyDrop, forget};
166use crate::num::NonZero;
167use crate::pin::Pin;
168use crate::sync::Arc;
169use crate::sync::atomic::{Atomic, AtomicUsize, Ordering};
170use crate::sys::sync::Parker;
171use crate::sys::thread as imp;
172use crate::sys_common::{AsInner, IntoInner};
173use crate::time::{Duration, Instant};
174use crate::{env, fmt, io, panic, panicking, str};
175
176#[stable(feature = "scoped_threads", since = "1.63.0")]
177mod scoped;
178
179#[stable(feature = "scoped_threads", since = "1.63.0")]
180pub use scoped::{Scope, ScopedJoinHandle, scope};
181
182mod current;
183
184#[stable(feature = "rust1", since = "1.0.0")]
185pub use current::current;
186#[unstable(feature = "current_thread_id", issue = "147194")]
187pub use current::current_id;
188pub(crate) use current::{current_or_unnamed, current_os_id, drop_current};
189use current::{set_current, try_with_current};
190
191mod spawnhook;
192
193#[unstable(feature = "thread_spawn_hook", issue = "132951")]
194pub use spawnhook::add_spawn_hook;
195
196////////////////////////////////////////////////////////////////////////////////
197// Thread-local storage
198////////////////////////////////////////////////////////////////////////////////
199
200#[macro_use]
201mod local;
202
203#[stable(feature = "rust1", since = "1.0.0")]
204pub use self::local::{AccessError, LocalKey};
205
206// Implementation details used by the thread_local!{} macro.
207#[doc(hidden)]
208#[unstable(feature = "thread_local_internals", issue = "none")]
209pub mod local_impl {
210    pub use super::local::thread_local_process_attrs;
211    pub use crate::sys::thread_local::*;
212}
213
214////////////////////////////////////////////////////////////////////////////////
215// Builder
216////////////////////////////////////////////////////////////////////////////////
217
218/// Thread factory, which can be used in order to configure the properties of
219/// a new thread.
220///
221/// Methods can be chained on it in order to configure it.
222///
223/// The two configurations available are:
224///
225/// - [`name`]: specifies an [associated name for the thread][naming-threads]
226/// - [`stack_size`]: specifies the [desired stack size for the thread][stack-size]
227///
228/// The [`spawn`] method will take ownership of the builder and create an
229/// [`io::Result`] to the thread handle with the given configuration.
230///
231/// The [`thread::spawn`] free function uses a `Builder` with default
232/// configuration and [`unwrap`]s its return value.
233///
234/// You may want to use [`spawn`] instead of [`thread::spawn`], when you want
235/// to recover from a failure to launch a thread, indeed the free function will
236/// panic where the `Builder` method will return a [`io::Result`].
237///
238/// # Examples
239///
240/// ```
241/// use std::thread;
242///
243/// let builder = thread::Builder::new();
244///
245/// let handler = builder.spawn(|| {
246///     // thread code
247/// }).unwrap();
248///
249/// handler.join().unwrap();
250/// ```
251///
252/// [`stack_size`]: Builder::stack_size
253/// [`name`]: Builder::name
254/// [`spawn`]: Builder::spawn
255/// [`thread::spawn`]: spawn
256/// [`io::Result`]: crate::io::Result
257/// [`unwrap`]: crate::result::Result::unwrap
258/// [naming-threads]: ./index.html#naming-threads
259/// [stack-size]: ./index.html#stack-size
260#[must_use = "must eventually spawn the thread"]
261#[stable(feature = "rust1", since = "1.0.0")]
262#[derive(Debug)]
263pub struct Builder {
264    // A name for the thread-to-be, for identification in panic messages
265    name: Option<String>,
266    // The size of the stack for the spawned thread in bytes
267    stack_size: Option<usize>,
268    // Skip running and inheriting the thread spawn hooks
269    no_hooks: bool,
270}
271
272impl Builder {
273    /// Generates the base configuration for spawning a thread, from which
274    /// configuration methods can be chained.
275    ///
276    /// # Examples
277    ///
278    /// ```
279    /// use std::thread;
280    ///
281    /// let builder = thread::Builder::new()
282    ///                               .name("foo".into())
283    ///                               .stack_size(32 * 1024);
284    ///
285    /// let handler = builder.spawn(|| {
286    ///     // thread code
287    /// }).unwrap();
288    ///
289    /// handler.join().unwrap();
290    /// ```
291    #[stable(feature = "rust1", since = "1.0.0")]
292    pub fn new() -> Builder {
293        Builder { name: None, stack_size: None, no_hooks: false }
294    }
295
296    /// Names the thread-to-be. Currently the name is used for identification
297    /// only in panic messages.
298    ///
299    /// The name must not contain null bytes (`\0`).
300    ///
301    /// For more information about named threads, see
302    /// [this module-level documentation][naming-threads].
303    ///
304    /// # Examples
305    ///
306    /// ```
307    /// use std::thread;
308    ///
309    /// let builder = thread::Builder::new()
310    ///     .name("foo".into());
311    ///
312    /// let handler = builder.spawn(|| {
313    ///     assert_eq!(thread::current().name(), Some("foo"))
314    /// }).unwrap();
315    ///
316    /// handler.join().unwrap();
317    /// ```
318    ///
319    /// [naming-threads]: ./index.html#naming-threads
320    #[stable(feature = "rust1", since = "1.0.0")]
321    pub fn name(mut self, name: String) -> Builder {
322        self.name = Some(name);
323        self
324    }
325
326    /// Sets the size of the stack (in bytes) for the new thread.
327    ///
328    /// The actual stack size may be greater than this value if
329    /// the platform specifies a minimal stack size.
330    ///
331    /// For more information about the stack size for threads, see
332    /// [this module-level documentation][stack-size].
333    ///
334    /// # Examples
335    ///
336    /// ```
337    /// use std::thread;
338    ///
339    /// let builder = thread::Builder::new().stack_size(32 * 1024);
340    /// ```
341    ///
342    /// [stack-size]: ./index.html#stack-size
343    #[stable(feature = "rust1", since = "1.0.0")]
344    pub fn stack_size(mut self, size: usize) -> Builder {
345        self.stack_size = Some(size);
346        self
347    }
348
349    /// Disables running and inheriting [spawn hooks](add_spawn_hook).
350    ///
351    /// Use this if the parent thread is in no way relevant for the child thread.
352    /// For example, when lazily spawning threads for a thread pool.
353    #[unstable(feature = "thread_spawn_hook", issue = "132951")]
354    pub fn no_hooks(mut self) -> Builder {
355        self.no_hooks = true;
356        self
357    }
358
359    /// Spawns a new thread by taking ownership of the `Builder`, and returns an
360    /// [`io::Result`] to its [`JoinHandle`].
361    ///
362    /// The spawned thread may outlive the caller (unless the caller thread
363    /// is the main thread; the whole process is terminated when the main
364    /// thread finishes). The join handle can be used to block on
365    /// termination of the spawned thread, including recovering its panics.
366    ///
367    /// For a more complete documentation see [`thread::spawn`][`spawn`].
368    ///
369    /// # Errors
370    ///
371    /// Unlike the [`spawn`] free function, this method yields an
372    /// [`io::Result`] to capture any failure to create the thread at
373    /// the OS level.
374    ///
375    /// [`io::Result`]: crate::io::Result
376    ///
377    /// # Panics
378    ///
379    /// Panics if a thread name was set and it contained null bytes.
380    ///
381    /// # Examples
382    ///
383    /// ```
384    /// use std::thread;
385    ///
386    /// let builder = thread::Builder::new();
387    ///
388    /// let handler = builder.spawn(|| {
389    ///     // thread code
390    /// }).unwrap();
391    ///
392    /// handler.join().unwrap();
393    /// ```
394    #[stable(feature = "rust1", since = "1.0.0")]
395    #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
396    pub fn spawn<F, T>(self, f: F) -> io::Result<JoinHandle<T>>
397    where
398        F: FnOnce() -> T,
399        F: Send + 'static,
400        T: Send + 'static,
401    {
402        unsafe { self.spawn_unchecked(f) }
403    }
404
405    /// Spawns a new thread without any lifetime restrictions by taking ownership
406    /// of the `Builder`, and returns an [`io::Result`] to its [`JoinHandle`].
407    ///
408    /// The spawned thread may outlive the caller (unless the caller thread
409    /// is the main thread; the whole process is terminated when the main
410    /// thread finishes). The join handle can be used to block on
411    /// termination of the spawned thread, including recovering its panics.
412    ///
413    /// This method is identical to [`thread::Builder::spawn`][`Builder::spawn`],
414    /// except for the relaxed lifetime bounds, which render it unsafe.
415    /// For a more complete documentation see [`thread::spawn`][`spawn`].
416    ///
417    /// # Errors
418    ///
419    /// Unlike the [`spawn`] free function, this method yields an
420    /// [`io::Result`] to capture any failure to create the thread at
421    /// the OS level.
422    ///
423    /// # Panics
424    ///
425    /// Panics if a thread name was set and it contained null bytes.
426    ///
427    /// # Safety
428    ///
429    /// The caller has to ensure that the spawned thread does not outlive any
430    /// references in the supplied thread closure and its return type.
431    /// This can be guaranteed in two ways:
432    ///
433    /// - ensure that [`join`][`JoinHandle::join`] is called before any referenced
434    /// data is dropped
435    /// - use only types with `'static` lifetime bounds, i.e., those with no or only
436    /// `'static` references (both [`thread::Builder::spawn`][`Builder::spawn`]
437    /// and [`thread::spawn`][`spawn`] enforce this property statically)
438    ///
439    /// # Examples
440    ///
441    /// ```
442    /// use std::thread;
443    ///
444    /// let builder = thread::Builder::new();
445    ///
446    /// let x = 1;
447    /// let thread_x = &x;
448    ///
449    /// let handler = unsafe {
450    ///     builder.spawn_unchecked(move || {
451    ///         println!("x = {}", *thread_x);
452    ///     }).unwrap()
453    /// };
454    ///
455    /// // caller has to ensure `join()` is called, otherwise
456    /// // it is possible to access freed memory if `x` gets
457    /// // dropped before the thread closure is executed!
458    /// handler.join().unwrap();
459    /// ```
460    ///
461    /// [`io::Result`]: crate::io::Result
462    #[stable(feature = "thread_spawn_unchecked", since = "1.82.0")]
463    #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
464    pub unsafe fn spawn_unchecked<F, T>(self, f: F) -> io::Result<JoinHandle<T>>
465    where
466        F: FnOnce() -> T,
467        F: Send,
468        T: Send,
469    {
470        Ok(JoinHandle(unsafe { self.spawn_unchecked_(f, None) }?))
471    }
472
473    #[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
474    unsafe fn spawn_unchecked_<'scope, F, T>(
475        self,
476        f: F,
477        scope_data: Option<Arc<scoped::ScopeData>>,
478    ) -> io::Result<JoinInner<'scope, T>>
479    where
480        F: FnOnce() -> T,
481        F: Send,
482        T: Send,
483    {
484        let Builder { name, stack_size, no_hooks } = self;
485
486        let stack_size = stack_size.unwrap_or_else(|| {
487            static MIN: Atomic<usize> = AtomicUsize::new(0);
488
489            match MIN.load(Ordering::Relaxed) {
490                0 => {}
491                n => return n - 1,
492            }
493
494            let amt = env::var_os("RUST_MIN_STACK")
495                .and_then(|s| s.to_str().and_then(|s| s.parse().ok()))
496                .unwrap_or(imp::DEFAULT_MIN_STACK_SIZE);
497
498            // 0 is our sentinel value, so ensure that we'll never see 0 after
499            // initialization has run
500            MIN.store(amt + 1, Ordering::Relaxed);
501            amt
502        });
503
504        let id = ThreadId::new();
505        let my_thread = Thread::new(id, name);
506
507        let hooks = if no_hooks {
508            spawnhook::ChildSpawnHooks::default()
509        } else {
510            spawnhook::run_spawn_hooks(&my_thread)
511        };
512
513        let their_thread = my_thread.clone();
514
515        let my_packet: Arc<Packet<'scope, T>> = Arc::new(Packet {
516            scope: scope_data,
517            result: UnsafeCell::new(None),
518            _marker: PhantomData,
519        });
520        let their_packet = my_packet.clone();
521
522        // Pass `f` in `MaybeUninit` because actually that closure might *run longer than the lifetime of `F`*.
523        // See <https://github.com/rust-lang/rust/issues/101983> for more details.
524        // To prevent leaks we use a wrapper that drops its contents.
525        #[repr(transparent)]
526        struct MaybeDangling<T>(mem::MaybeUninit<T>);
527        impl<T> MaybeDangling<T> {
528            fn new(x: T) -> Self {
529                MaybeDangling(mem::MaybeUninit::new(x))
530            }
531            fn into_inner(self) -> T {
532                // Make sure we don't drop.
533                let this = ManuallyDrop::new(self);
534                // SAFETY: we are always initialized.
535                unsafe { this.0.assume_init_read() }
536            }
537        }
538        impl<T> Drop for MaybeDangling<T> {
539            fn drop(&mut self) {
540                // SAFETY: we are always initialized.
541                unsafe { self.0.assume_init_drop() };
542            }
543        }
544
545        let f = MaybeDangling::new(f);
546        let main = move || {
547            if let Err(_thread) = set_current(their_thread.clone()) {
548                // Both the current thread handle and the ID should not be
549                // initialized yet. Since only the C runtime and some of our
550                // platform code run before this, this point shouldn't be
551                // reachable. Use an abort to save binary size (see #123356).
552                rtabort!("something here is badly broken!");
553            }
554
555            if let Some(name) = their_thread.cname() {
556                imp::set_name(name);
557            }
558
559            let f = f.into_inner();
560            let try_result = panic::catch_unwind(panic::AssertUnwindSafe(|| {
561                crate::sys::backtrace::__rust_begin_short_backtrace(|| hooks.run());
562                crate::sys::backtrace::__rust_begin_short_backtrace(f)
563            }));
564            // SAFETY: `their_packet` as been built just above and moved by the
565            // closure (it is an Arc<...>) and `my_packet` will be stored in the
566            // same `JoinInner` as this closure meaning the mutation will be
567            // safe (not modify it and affect a value far away).
568            unsafe { *their_packet.result.get() = Some(try_result) };
569            // Here `their_packet` gets dropped, and if this is the last `Arc` for that packet that
570            // will call `decrement_num_running_threads` and therefore signal that this thread is
571            // done.
572            drop(their_packet);
573            // Here, the lifetime `'scope` can end. `main` keeps running for a bit
574            // after that before returning itself.
575        };
576
577        if let Some(scope_data) = &my_packet.scope {
578            scope_data.increment_num_running_threads();
579        }
580
581        let main = Box::new(main);
582        // SAFETY: dynamic size and alignment of the Box remain the same. See below for why the
583        // lifetime change is justified.
584        let main =
585            unsafe { Box::from_raw(Box::into_raw(main) as *mut (dyn FnOnce() + Send + 'static)) };
586
587        Ok(JoinInner {
588            // SAFETY:
589            //
590            // `imp::Thread::new` takes a closure with a `'static` lifetime, since it's passed
591            // through FFI or otherwise used with low-level threading primitives that have no
592            // notion of or way to enforce lifetimes.
593            //
594            // As mentioned in the `Safety` section of this function's documentation, the caller of
595            // this function needs to guarantee that the passed-in lifetime is sufficiently long
596            // for the lifetime of the thread.
597            //
598            // Similarly, the `sys` implementation must guarantee that no references to the closure
599            // exist after the thread has terminated, which is signaled by `Thread::join`
600            // returning.
601            native: unsafe { imp::Thread::new(stack_size, my_thread.name(), main)? },
602            thread: my_thread,
603            packet: my_packet,
604        })
605    }
606}
607
608////////////////////////////////////////////////////////////////////////////////
609// Free functions
610////////////////////////////////////////////////////////////////////////////////
611
612/// Spawns a new thread, returning a [`JoinHandle`] for it.
613///
614/// The join handle provides a [`join`] method that can be used to join the spawned
615/// thread. If the spawned thread panics, [`join`] will return an [`Err`] containing
616/// the argument given to [`panic!`].
617///
618/// If the join handle is dropped, the spawned thread will implicitly be *detached*.
619/// In this case, the spawned thread may no longer be joined.
620/// (It is the responsibility of the program to either eventually join threads it
621/// creates or detach them; otherwise, a resource leak will result.)
622///
623/// This call will create a thread using default parameters of [`Builder`], if you
624/// want to specify the stack size or the name of the thread, use this API
625/// instead.
626///
627/// As you can see in the signature of `spawn` there are two constraints on
628/// both the closure given to `spawn` and its return value, let's explain them:
629///
630/// - The `'static` constraint means that the closure and its return value
631///   must have a lifetime of the whole program execution. The reason for this
632///   is that threads can outlive the lifetime they have been created in.
633///
634///   Indeed if the thread, and by extension its return value, can outlive their
635///   caller, we need to make sure that they will be valid afterwards, and since
636///   we *can't* know when it will return we need to have them valid as long as
637///   possible, that is until the end of the program, hence the `'static`
638///   lifetime.
639/// - The [`Send`] constraint is because the closure will need to be passed
640///   *by value* from the thread where it is spawned to the new thread. Its
641///   return value will need to be passed from the new thread to the thread
642///   where it is `join`ed.
643///   As a reminder, the [`Send`] marker trait expresses that it is safe to be
644///   passed from thread to thread. [`Sync`] expresses that it is safe to have a
645///   reference be passed from thread to thread.
646///
647/// # Panics
648///
649/// Panics if the OS fails to create a thread; use [`Builder::spawn`]
650/// to recover from such errors.
651///
652/// # Examples
653///
654/// Creating a thread.
655///
656/// ```
657/// use std::thread;
658///
659/// let handler = thread::spawn(|| {
660///     // thread code
661/// });
662///
663/// handler.join().unwrap();
664/// ```
665///
666/// As mentioned in the module documentation, threads are usually made to
667/// communicate using [`channels`], here is how it usually looks.
668///
669/// This example also shows how to use `move`, in order to give ownership
670/// of values to a thread.
671///
672/// ```
673/// use std::thread;
674/// use std::sync::mpsc::channel;
675///
676/// let (tx, rx) = channel();
677///
678/// let sender = thread::spawn(move || {
679///     tx.send("Hello, thread".to_owned())
680///         .expect("Unable to send on channel");
681/// });
682///
683/// let receiver = thread::spawn(move || {
684///     let value = rx.recv().expect("Unable to receive from channel");
685///     println!("{value}");
686/// });
687///
688/// sender.join().expect("The sender thread has panicked");
689/// receiver.join().expect("The receiver thread has panicked");
690/// ```
691///
692/// A thread can also return a value through its [`JoinHandle`], you can use
693/// this to make asynchronous computations (futures might be more appropriate
694/// though).
695///
696/// ```
697/// use std::thread;
698///
699/// let computation = thread::spawn(|| {
700///     // Some expensive computation.
701///     42
702/// });
703///
704/// let result = computation.join().unwrap();
705/// println!("{result}");
706/// ```
707///
708/// # Notes
709///
710/// This function has the same minimal guarantee regarding "foreign" unwinding operations (e.g.
711/// an exception thrown from C++ code, or a `panic!` in Rust code compiled or linked with a
712/// different runtime) as [`catch_unwind`]; namely, if the thread created with `thread::spawn`
713/// unwinds all the way to the root with such an exception, one of two behaviors are possible,
714/// and it is unspecified which will occur:
715///
716/// * The process aborts.
717/// * The process does not abort, and [`join`] will return a `Result::Err`
718///   containing an opaque type.
719///
720/// [`catch_unwind`]: ../../std/panic/fn.catch_unwind.html
721/// [`channels`]: crate::sync::mpsc
722/// [`join`]: JoinHandle::join
723/// [`Err`]: crate::result::Result::Err
724#[stable(feature = "rust1", since = "1.0.0")]
725#[cfg_attr(miri, track_caller)] // even without panics, this helps for Miri backtraces
726pub fn spawn<F, T>(f: F) -> JoinHandle<T>
727where
728    F: FnOnce() -> T,
729    F: Send + 'static,
730    T: Send + 'static,
731{
732    Builder::new().spawn(f).expect("failed to spawn thread")
733}
734
735/// Cooperatively gives up a timeslice to the OS scheduler.
736///
737/// This calls the underlying OS scheduler's yield primitive, signaling
738/// that the calling thread is willing to give up its remaining timeslice
739/// so that the OS may schedule other threads on the CPU.
740///
741/// A drawback of yielding in a loop is that if the OS does not have any
742/// other ready threads to run on the current CPU, the thread will effectively
743/// busy-wait, which wastes CPU time and energy.
744///
745/// Therefore, when waiting for events of interest, a programmer's first
746/// choice should be to use synchronization devices such as [`channel`]s,
747/// [`Condvar`]s, [`Mutex`]es or [`join`] since these primitives are
748/// implemented in a blocking manner, giving up the CPU until the event
749/// of interest has occurred which avoids repeated yielding.
750///
751/// `yield_now` should thus be used only rarely, mostly in situations where
752/// repeated polling is required because there is no other suitable way to
753/// learn when an event of interest has occurred.
754///
755/// # Examples
756///
757/// ```
758/// use std::thread;
759///
760/// thread::yield_now();
761/// ```
762///
763/// [`channel`]: crate::sync::mpsc
764/// [`join`]: JoinHandle::join
765/// [`Condvar`]: crate::sync::Condvar
766/// [`Mutex`]: crate::sync::Mutex
767#[stable(feature = "rust1", since = "1.0.0")]
768pub fn yield_now() {
769    imp::yield_now()
770}
771
772/// Determines whether the current thread is unwinding because of panic.
773///
774/// A common use of this feature is to poison shared resources when writing
775/// unsafe code, by checking `panicking` when the `drop` is called.
776///
777/// This is usually not needed when writing safe code, as [`Mutex`es][Mutex]
778/// already poison themselves when a thread panics while holding the lock.
779///
780/// This can also be used in multithreaded applications, in order to send a
781/// message to other threads warning that a thread has panicked (e.g., for
782/// monitoring purposes).
783///
784/// # Examples
785///
786/// ```should_panic
787/// use std::thread;
788///
789/// struct SomeStruct;
790///
791/// impl Drop for SomeStruct {
792///     fn drop(&mut self) {
793///         if thread::panicking() {
794///             println!("dropped while unwinding");
795///         } else {
796///             println!("dropped while not unwinding");
797///         }
798///     }
799/// }
800///
801/// {
802///     print!("a: ");
803///     let a = SomeStruct;
804/// }
805///
806/// {
807///     print!("b: ");
808///     let b = SomeStruct;
809///     panic!()
810/// }
811/// ```
812///
813/// [Mutex]: crate::sync::Mutex
814#[inline]
815#[must_use]
816#[stable(feature = "rust1", since = "1.0.0")]
817pub fn panicking() -> bool {
818    panicking::panicking()
819}
820
821/// Uses [`sleep`].
822///
823/// Puts the current thread to sleep for at least the specified amount of time.
824///
825/// The thread may sleep longer than the duration specified due to scheduling
826/// specifics or platform-dependent functionality. It will never sleep less.
827///
828/// This function is blocking, and should not be used in `async` functions.
829///
830/// # Platform-specific behavior
831///
832/// On Unix platforms, the underlying syscall may be interrupted by a
833/// spurious wakeup or signal handler. To ensure the sleep occurs for at least
834/// the specified duration, this function may invoke that system call multiple
835/// times.
836///
837/// # Examples
838///
839/// ```no_run
840/// use std::thread;
841///
842/// // Let's sleep for 2 seconds:
843/// thread::sleep_ms(2000);
844/// ```
845#[stable(feature = "rust1", since = "1.0.0")]
846#[deprecated(since = "1.6.0", note = "replaced by `std::thread::sleep`")]
847pub fn sleep_ms(ms: u32) {
848    sleep(Duration::from_millis(ms as u64))
849}
850
851/// Puts the current thread to sleep for at least the specified amount of time.
852///
853/// The thread may sleep longer than the duration specified due to scheduling
854/// specifics or platform-dependent functionality. It will never sleep less.
855///
856/// This function is blocking, and should not be used in `async` functions.
857///
858/// # Platform-specific behavior
859///
860/// On Unix platforms, the underlying syscall may be interrupted by a
861/// spurious wakeup or signal handler. To ensure the sleep occurs for at least
862/// the specified duration, this function may invoke that system call multiple
863/// times.
864/// Platforms which do not support nanosecond precision for sleeping will
865/// have `dur` rounded up to the nearest granularity of time they can sleep for.
866///
867/// Currently, specifying a zero duration on Unix platforms returns immediately
868/// without invoking the underlying [`nanosleep`] syscall, whereas on Windows
869/// platforms the underlying [`Sleep`] syscall is always invoked.
870/// If the intention is to yield the current time-slice you may want to use
871/// [`yield_now`] instead.
872///
873/// [`nanosleep`]: https://linux.die.net/man/2/nanosleep
874/// [`Sleep`]: https://docs.microsoft.com/en-us/windows/win32/api/synchapi/nf-synchapi-sleep
875///
876/// # Examples
877///
878/// ```no_run
879/// use std::{thread, time};
880///
881/// let ten_millis = time::Duration::from_millis(10);
882/// let now = time::Instant::now();
883///
884/// thread::sleep(ten_millis);
885///
886/// assert!(now.elapsed() >= ten_millis);
887/// ```
888#[stable(feature = "thread_sleep", since = "1.4.0")]
889pub fn sleep(dur: Duration) {
890    imp::sleep(dur)
891}
892
893/// Puts the current thread to sleep until the specified deadline has passed.
894///
895/// The thread may still be asleep after the deadline specified due to
896/// scheduling specifics or platform-dependent functionality. It will never
897/// wake before.
898///
899/// This function is blocking, and should not be used in `async` functions.
900///
901/// # Platform-specific behavior
902///
903/// In most cases this function will call an OS specific function. Where that
904/// is not supported [`sleep`] is used. Those platforms are referred to as other
905/// in the table below.
906///
907/// # Underlying System calls
908///
909/// The following system calls are [currently] being used:
910///
911/// |  Platform |               System call                                            |
912/// |-----------|----------------------------------------------------------------------|
913/// | Linux     | [clock_nanosleep] (Monotonic clock)                                  |
914/// | BSD except OpenBSD | [clock_nanosleep] (Monotonic Clock)]                        |
915/// | Android   | [clock_nanosleep] (Monotonic Clock)]                                 |
916/// | Solaris   | [clock_nanosleep] (Monotonic Clock)]                                 |
917/// | Illumos   | [clock_nanosleep] (Monotonic Clock)]                                 |
918/// | Dragonfly | [clock_nanosleep] (Monotonic Clock)]                                 |
919/// | Hurd      | [clock_nanosleep] (Monotonic Clock)]                                 |
920/// | Fuchsia   | [clock_nanosleep] (Monotonic Clock)]                                 |
921/// | Vxworks   | [clock_nanosleep] (Monotonic Clock)]                                 |
922/// | Other     | `sleep_until` uses [`sleep`] and does not issue a syscall itself     |
923///
924/// [currently]: crate::io#platform-specific-behavior
925/// [clock_nanosleep]: https://linux.die.net/man/3/clock_nanosleep
926///
927/// **Disclaimer:** These system calls might change over time.
928///
929/// # Examples
930///
931/// A simple game loop that limits the game to 60 frames per second.
932///
933/// ```no_run
934/// #![feature(thread_sleep_until)]
935/// # use std::time::{Duration, Instant};
936/// # use std::thread;
937/// #
938/// # fn update() {}
939/// # fn render() {}
940/// #
941/// let max_fps = 60.0;
942/// let frame_time = Duration::from_secs_f32(1.0/max_fps);
943/// let mut next_frame = Instant::now();
944/// loop {
945///     thread::sleep_until(next_frame);
946///     next_frame += frame_time;
947///     update();
948///     render();
949/// }
950/// ```
951///
952/// A slow API we must not call too fast and which takes a few
953/// tries before succeeding. By using `sleep_until` the time the
954/// API call takes does not influence when we retry or when we give up
955///
956/// ```no_run
957/// #![feature(thread_sleep_until)]
958/// # use std::time::{Duration, Instant};
959/// # use std::thread;
960/// #
961/// # enum Status {
962/// #     Ready(usize),
963/// #     Waiting,
964/// # }
965/// # fn slow_web_api_call() -> Status { Status::Ready(42) }
966/// #
967/// # const MAX_DURATION: Duration = Duration::from_secs(10);
968/// #
969/// # fn try_api_call() -> Result<usize, ()> {
970/// let deadline = Instant::now() + MAX_DURATION;
971/// let delay = Duration::from_millis(250);
972/// let mut next_attempt = Instant::now();
973/// loop {
974///     if Instant::now() > deadline {
975///         break Err(());
976///     }
977///     if let Status::Ready(data) = slow_web_api_call() {
978///         break Ok(data);
979///     }
980///
981///     next_attempt = deadline.min(next_attempt + delay);
982///     thread::sleep_until(next_attempt);
983/// }
984/// # }
985/// # let _data = try_api_call();
986/// ```
987#[unstable(feature = "thread_sleep_until", issue = "113752")]
988pub fn sleep_until(deadline: Instant) {
989    imp::sleep_until(deadline)
990}
991
992/// Used to ensure that `park` and `park_timeout` do not unwind, as that can
993/// cause undefined behavior if not handled correctly (see #102398 for context).
994struct PanicGuard;
995
996impl Drop for PanicGuard {
997    fn drop(&mut self) {
998        rtabort!("an irrecoverable error occurred while synchronizing threads")
999    }
1000}
1001
1002/// Blocks unless or until the current thread's token is made available.
1003///
1004/// A call to `park` does not guarantee that the thread will remain parked
1005/// forever, and callers should be prepared for this possibility. However,
1006/// it is guaranteed that this function will not panic (it may abort the
1007/// process if the implementation encounters some rare errors).
1008///
1009/// # `park` and `unpark`
1010///
1011/// Every thread is equipped with some basic low-level blocking support, via the
1012/// [`thread::park`][`park`] function and [`thread::Thread::unpark`][`unpark`]
1013/// method. [`park`] blocks the current thread, which can then be resumed from
1014/// another thread by calling the [`unpark`] method on the blocked thread's
1015/// handle.
1016///
1017/// Conceptually, each [`Thread`] handle has an associated token, which is
1018/// initially not present:
1019///
1020/// * The [`thread::park`][`park`] function blocks the current thread unless or
1021///   until the token is available for its thread handle, at which point it
1022///   atomically consumes the token. It may also return *spuriously*, without
1023///   consuming the token. [`thread::park_timeout`] does the same, but allows
1024///   specifying a maximum time to block the thread for.
1025///
1026/// * The [`unpark`] method on a [`Thread`] atomically makes the token available
1027///   if it wasn't already. Because the token can be held by a thread even if it is currently not
1028///   parked, [`unpark`] followed by [`park`] will result in the second call returning immediately.
1029///   However, note that to rely on this guarantee, you need to make sure that your `unpark` happens
1030///   after all `park` that may be done by other data structures!
1031///
1032/// The API is typically used by acquiring a handle to the current thread, placing that handle in a
1033/// shared data structure so that other threads can find it, and then `park`ing in a loop. When some
1034/// desired condition is met, another thread calls [`unpark`] on the handle. The last bullet point
1035/// above guarantees that even if the `unpark` occurs before the thread is finished `park`ing, it
1036/// will be woken up properly.
1037///
1038/// Note that the coordination via the shared data structure is crucial: If you `unpark` a thread
1039/// without first establishing that it is about to be `park`ing within your code, that `unpark` may
1040/// get consumed by a *different* `park` in the same thread, leading to a deadlock. This also means
1041/// you must not call unknown code between setting up for parking and calling `park`; for instance,
1042/// if you invoke `println!`, that may itself call `park` and thus consume your `unpark` and cause a
1043/// deadlock.
1044///
1045/// The motivation for this design is twofold:
1046///
1047/// * It avoids the need to allocate mutexes and condvars when building new
1048///   synchronization primitives; the threads already provide basic
1049///   blocking/signaling.
1050///
1051/// * It can be implemented very efficiently on many platforms.
1052///
1053/// # Memory Ordering
1054///
1055/// Calls to `unpark` _synchronize-with_ calls to `park`, meaning that memory
1056/// operations performed before a call to `unpark` are made visible to the thread that
1057/// consumes the token and returns from `park`. Note that all `park` and `unpark`
1058/// operations for a given thread form a total order and _all_ prior `unpark` operations
1059/// synchronize-with `park`.
1060///
1061/// In atomic ordering terms, `unpark` performs a `Release` operation and `park`
1062/// performs the corresponding `Acquire` operation. Calls to `unpark` for the same
1063/// thread form a [release sequence].
1064///
1065/// Note that being unblocked does not imply a call was made to `unpark`, because
1066/// wakeups can also be spurious. For example, a valid, but inefficient,
1067/// implementation could have `park` and `unpark` return immediately without doing anything,
1068/// making *all* wakeups spurious.
1069///
1070/// # Examples
1071///
1072/// ```
1073/// use std::thread;
1074/// use std::sync::atomic::{Ordering, AtomicBool};
1075/// use std::time::Duration;
1076///
1077/// static QUEUED: AtomicBool = AtomicBool::new(false);
1078/// static FLAG: AtomicBool = AtomicBool::new(false);
1079///
1080/// let parked_thread = thread::spawn(move || {
1081///     println!("Thread spawned");
1082///     // Signal that we are going to `park`. Between this store and our `park`, there may
1083///     // be no other `park`, or else that `park` could consume our `unpark` token!
1084///     QUEUED.store(true, Ordering::Release);
1085///     // We want to wait until the flag is set. We *could* just spin, but using
1086///     // park/unpark is more efficient.
1087///     while !FLAG.load(Ordering::Acquire) {
1088///         // We can *not* use `println!` here since that could use thread parking internally.
1089///         thread::park();
1090///         // We *could* get here spuriously, i.e., way before the 10ms below are over!
1091///         // But that is no problem, we are in a loop until the flag is set anyway.
1092///     }
1093///     println!("Flag received");
1094/// });
1095///
1096/// // Let some time pass for the thread to be spawned.
1097/// thread::sleep(Duration::from_millis(10));
1098///
1099/// // Ensure the thread is about to park.
1100/// // This is crucial! It guarantees that the `unpark` below is not consumed
1101/// // by some other code in the parked thread (e.g. inside `println!`).
1102/// while !QUEUED.load(Ordering::Acquire) {
1103///     // Spinning is of course inefficient; in practice, this would more likely be
1104///     // a dequeue where we have no work to do if there's nobody queued.
1105///     std::hint::spin_loop();
1106/// }
1107///
1108/// // Set the flag, and let the thread wake up.
1109/// // There is no race condition here: if `unpark`
1110/// // happens first, `park` will return immediately.
1111/// // There is also no other `park` that could consume this token,
1112/// // since we waited until the other thread got queued.
1113/// // Hence there is no risk of a deadlock.
1114/// FLAG.store(true, Ordering::Release);
1115/// println!("Unpark the thread");
1116/// parked_thread.thread().unpark();
1117///
1118/// parked_thread.join().unwrap();
1119/// ```
1120///
1121/// [`unpark`]: Thread::unpark
1122/// [`thread::park_timeout`]: park_timeout
1123/// [release sequence]: https://en.cppreference.com/w/cpp/atomic/memory_order#Release_sequence
1124#[stable(feature = "rust1", since = "1.0.0")]
1125pub fn park() {
1126    let guard = PanicGuard;
1127    // SAFETY: park_timeout is called on the parker owned by this thread.
1128    unsafe {
1129        current().park();
1130    }
1131    // No panic occurred, do not abort.
1132    forget(guard);
1133}
1134
1135/// Uses [`park_timeout`].
1136///
1137/// Blocks unless or until the current thread's token is made available or
1138/// the specified duration has been reached (may wake spuriously).
1139///
1140/// The semantics of this function are equivalent to [`park`] except
1141/// that the thread will be blocked for roughly no longer than `dur`. This
1142/// method should not be used for precise timing due to anomalies such as
1143/// preemption or platform differences that might not cause the maximum
1144/// amount of time waited to be precisely `ms` long.
1145///
1146/// See the [park documentation][`park`] for more detail.
1147#[stable(feature = "rust1", since = "1.0.0")]
1148#[deprecated(since = "1.6.0", note = "replaced by `std::thread::park_timeout`")]
1149pub fn park_timeout_ms(ms: u32) {
1150    park_timeout(Duration::from_millis(ms as u64))
1151}
1152
1153/// Blocks unless or until the current thread's token is made available or
1154/// the specified duration has been reached (may wake spuriously).
1155///
1156/// The semantics of this function are equivalent to [`park`][park] except
1157/// that the thread will be blocked for roughly no longer than `dur`. This
1158/// method should not be used for precise timing due to anomalies such as
1159/// preemption or platform differences that might not cause the maximum
1160/// amount of time waited to be precisely `dur` long.
1161///
1162/// See the [park documentation][park] for more details.
1163///
1164/// # Platform-specific behavior
1165///
1166/// Platforms which do not support nanosecond precision for sleeping will have
1167/// `dur` rounded up to the nearest granularity of time they can sleep for.
1168///
1169/// # Examples
1170///
1171/// Waiting for the complete expiration of the timeout:
1172///
1173/// ```rust,no_run
1174/// use std::thread::park_timeout;
1175/// use std::time::{Instant, Duration};
1176///
1177/// let timeout = Duration::from_secs(2);
1178/// let beginning_park = Instant::now();
1179///
1180/// let mut timeout_remaining = timeout;
1181/// loop {
1182///     park_timeout(timeout_remaining);
1183///     let elapsed = beginning_park.elapsed();
1184///     if elapsed >= timeout {
1185///         break;
1186///     }
1187///     println!("restarting park_timeout after {elapsed:?}");
1188///     timeout_remaining = timeout - elapsed;
1189/// }
1190/// ```
1191#[stable(feature = "park_timeout", since = "1.4.0")]
1192pub fn park_timeout(dur: Duration) {
1193    let guard = PanicGuard;
1194    // SAFETY: park_timeout is called on a handle owned by this thread.
1195    unsafe {
1196        current().park_timeout(dur);
1197    }
1198    // No panic occurred, do not abort.
1199    forget(guard);
1200}
1201
1202////////////////////////////////////////////////////////////////////////////////
1203// ThreadId
1204////////////////////////////////////////////////////////////////////////////////
1205
1206/// A unique identifier for a running thread.
1207///
1208/// A `ThreadId` is an opaque object that uniquely identifies each thread
1209/// created during the lifetime of a process. `ThreadId`s are guaranteed not to
1210/// be reused, even when a thread terminates. `ThreadId`s are under the control
1211/// of Rust's standard library and there may not be any relationship between
1212/// `ThreadId` and the underlying platform's notion of a thread identifier --
1213/// the two concepts cannot, therefore, be used interchangeably. A `ThreadId`
1214/// can be retrieved from the [`id`] method on a [`Thread`].
1215///
1216/// # Examples
1217///
1218/// ```
1219/// use std::thread;
1220///
1221/// let other_thread = thread::spawn(|| {
1222///     thread::current().id()
1223/// });
1224///
1225/// let other_thread_id = other_thread.join().unwrap();
1226/// assert!(thread::current().id() != other_thread_id);
1227/// ```
1228///
1229/// [`id`]: Thread::id
1230#[stable(feature = "thread_id", since = "1.19.0")]
1231#[derive(Eq, PartialEq, Clone, Copy, Hash, Debug)]
1232pub struct ThreadId(NonZero<u64>);
1233
1234impl ThreadId {
1235    // Generate a new unique thread ID.
1236    pub(crate) fn new() -> ThreadId {
1237        #[cold]
1238        fn exhausted() -> ! {
1239            panic!("failed to generate unique thread ID: bitspace exhausted")
1240        }
1241
1242        cfg_select! {
1243            target_has_atomic = "64" => {
1244                use crate::sync::atomic::{Atomic, AtomicU64};
1245
1246                static COUNTER: Atomic<u64> = AtomicU64::new(0);
1247
1248                let mut last = COUNTER.load(Ordering::Relaxed);
1249                loop {
1250                    let Some(id) = last.checked_add(1) else {
1251                        exhausted();
1252                    };
1253
1254                    match COUNTER.compare_exchange_weak(last, id, Ordering::Relaxed, Ordering::Relaxed) {
1255                        Ok(_) => return ThreadId(NonZero::new(id).unwrap()),
1256                        Err(id) => last = id,
1257                    }
1258                }
1259            }
1260            _ => {
1261                use crate::sync::{Mutex, PoisonError};
1262
1263                static COUNTER: Mutex<u64> = Mutex::new(0);
1264
1265                let mut counter = COUNTER.lock().unwrap_or_else(PoisonError::into_inner);
1266                let Some(id) = counter.checked_add(1) else {
1267                    // in case the panic handler ends up calling `ThreadId::new()`,
1268                    // avoid reentrant lock acquire.
1269                    drop(counter);
1270                    exhausted();
1271                };
1272
1273                *counter = id;
1274                drop(counter);
1275                ThreadId(NonZero::new(id).unwrap())
1276            }
1277        }
1278    }
1279
1280    #[cfg(any(not(target_thread_local), target_has_atomic = "64"))]
1281    fn from_u64(v: u64) -> Option<ThreadId> {
1282        NonZero::new(v).map(ThreadId)
1283    }
1284
1285    /// This returns a numeric identifier for the thread identified by this
1286    /// `ThreadId`.
1287    ///
1288    /// As noted in the documentation for the type itself, it is essentially an
1289    /// opaque ID, but is guaranteed to be unique for each thread. The returned
1290    /// value is entirely opaque -- only equality testing is stable. Note that
1291    /// it is not guaranteed which values new threads will return, and this may
1292    /// change across Rust versions.
1293    #[must_use]
1294    #[unstable(feature = "thread_id_value", issue = "67939")]
1295    pub fn as_u64(&self) -> NonZero<u64> {
1296        self.0
1297    }
1298}
1299
1300////////////////////////////////////////////////////////////////////////////////
1301// Thread
1302////////////////////////////////////////////////////////////////////////////////
1303
1304// This module ensures private fields are kept private, which is necessary to enforce the safety requirements.
1305mod thread_name_string {
1306    use crate::ffi::{CStr, CString};
1307    use crate::str;
1308
1309    /// Like a `String` it's guaranteed UTF-8 and like a `CString` it's null terminated.
1310    pub(crate) struct ThreadNameString {
1311        inner: CString,
1312    }
1313
1314    impl From<String> for ThreadNameString {
1315        fn from(s: String) -> Self {
1316            Self {
1317                inner: CString::new(s).expect("thread name may not contain interior null bytes"),
1318            }
1319        }
1320    }
1321
1322    impl ThreadNameString {
1323        pub fn as_cstr(&self) -> &CStr {
1324            &self.inner
1325        }
1326
1327        pub fn as_str(&self) -> &str {
1328            // SAFETY: `ThreadNameString` is guaranteed to be UTF-8.
1329            unsafe { str::from_utf8_unchecked(self.inner.to_bytes()) }
1330        }
1331    }
1332}
1333
1334use thread_name_string::ThreadNameString;
1335
1336/// Store the ID of the main thread.
1337///
1338/// The thread handle for the main thread is created lazily, and this might even
1339/// happen pre-main. Since not every platform has a way to identify the main
1340/// thread when that happens – macOS's `pthread_main_np` function being a notable
1341/// exception – we cannot assign it the right name right then. Instead, in our
1342/// runtime startup code, we remember the thread ID of the main thread (through
1343/// this modules `set` function) and use it to identify the main thread from then
1344/// on. This works reliably and has the additional advantage that we can report
1345/// the right thread name on main even after the thread handle has been destroyed.
1346/// Note however that this also means that the name reported in pre-main functions
1347/// will be incorrect, but that's just something we have to live with.
1348pub(crate) mod main_thread {
1349    cfg_select! {
1350        target_has_atomic = "64" => {
1351            use super::ThreadId;
1352            use crate::sync::atomic::{Atomic, AtomicU64};
1353            use crate::sync::atomic::Ordering::Relaxed;
1354
1355            static MAIN: Atomic<u64> = AtomicU64::new(0);
1356
1357            pub(super) fn get() -> Option<ThreadId> {
1358                ThreadId::from_u64(MAIN.load(Relaxed))
1359            }
1360
1361            /// # Safety
1362            /// May only be called once.
1363            pub(crate) unsafe fn set(id: ThreadId) {
1364                MAIN.store(id.as_u64().get(), Relaxed)
1365            }
1366        }
1367        _ => {
1368            use super::ThreadId;
1369            use crate::mem::MaybeUninit;
1370            use crate::sync::atomic::{Atomic, AtomicBool};
1371            use crate::sync::atomic::Ordering::{Acquire, Release};
1372
1373            static INIT: Atomic<bool> = AtomicBool::new(false);
1374            static mut MAIN: MaybeUninit<ThreadId> = MaybeUninit::uninit();
1375
1376            pub(super) fn get() -> Option<ThreadId> {
1377                if INIT.load(Acquire) {
1378                    Some(unsafe { MAIN.assume_init() })
1379                } else {
1380                    None
1381                }
1382            }
1383
1384            /// # Safety
1385            /// May only be called once.
1386            pub(crate) unsafe fn set(id: ThreadId) {
1387                unsafe { MAIN = MaybeUninit::new(id) };
1388                INIT.store(true, Release);
1389            }
1390        }
1391    }
1392}
1393
1394/// Run a function with the current thread's name.
1395///
1396/// Modulo thread local accesses, this function is safe to call from signal
1397/// handlers and in similar circumstances where allocations are not possible.
1398pub(crate) fn with_current_name<F, R>(f: F) -> R
1399where
1400    F: FnOnce(Option<&str>) -> R,
1401{
1402    try_with_current(|thread| {
1403        if let Some(thread) = thread {
1404            // If there is a current thread handle, try to use the name stored
1405            // there.
1406            if let Some(name) = &thread.inner.name {
1407                return f(Some(name.as_str()));
1408            } else if Some(thread.inner.id) == main_thread::get() {
1409                // The main thread doesn't store its name in the handle, we must
1410                // identify it through its ID. Since we already have the `Thread`,
1411                // we can retrieve the ID from it instead of going through another
1412                // thread local.
1413                return f(Some("main"));
1414            }
1415        } else if let Some(main) = main_thread::get()
1416            && let Some(id) = current::id::get()
1417            && id == main
1418        {
1419            // The main thread doesn't always have a thread handle, we must
1420            // identify it through its ID instead. The checks are ordered so
1421            // that the current ID is only loaded if it is actually needed,
1422            // since loading it from TLS might need multiple expensive accesses.
1423            return f(Some("main"));
1424        }
1425
1426        f(None)
1427    })
1428}
1429
1430/// The internal representation of a `Thread` handle
1431///
1432/// We explicitly set the alignment for our guarantee in Thread::into_raw. This
1433/// allows applications to stuff extra metadata bits into the alignment, which
1434/// can be rather useful when working with atomics.
1435#[repr(align(8))]
1436struct Inner {
1437    name: Option<ThreadNameString>,
1438    id: ThreadId,
1439    parker: Parker,
1440}
1441
1442impl Inner {
1443    fn parker(self: Pin<&Self>) -> Pin<&Parker> {
1444        unsafe { Pin::map_unchecked(self, |inner| &inner.parker) }
1445    }
1446}
1447
1448#[derive(Clone)]
1449#[stable(feature = "rust1", since = "1.0.0")]
1450/// A handle to a thread.
1451///
1452/// Threads are represented via the `Thread` type, which you can get in one of
1453/// two ways:
1454///
1455/// * By spawning a new thread, e.g., using the [`thread::spawn`][`spawn`]
1456///   function, and calling [`thread`][`JoinHandle::thread`] on the
1457///   [`JoinHandle`].
1458/// * By requesting the current thread, using the [`thread::current`] function.
1459///
1460/// The [`thread::current`] function is available even for threads not spawned
1461/// by the APIs of this module.
1462///
1463/// There is usually no need to create a `Thread` struct yourself, one
1464/// should instead use a function like `spawn` to create new threads, see the
1465/// docs of [`Builder`] and [`spawn`] for more details.
1466///
1467/// [`thread::current`]: current::current
1468pub struct Thread {
1469    inner: Pin<Arc<Inner>>,
1470}
1471
1472impl Thread {
1473    pub(crate) fn new(id: ThreadId, name: Option<String>) -> Thread {
1474        let name = name.map(ThreadNameString::from);
1475
1476        // We have to use `unsafe` here to construct the `Parker` in-place,
1477        // which is required for the UNIX implementation.
1478        //
1479        // SAFETY: We pin the Arc immediately after creation, so its address never
1480        // changes.
1481        let inner = unsafe {
1482            let mut arc = Arc::<Inner>::new_uninit();
1483            let ptr = Arc::get_mut_unchecked(&mut arc).as_mut_ptr();
1484            (&raw mut (*ptr).name).write(name);
1485            (&raw mut (*ptr).id).write(id);
1486            Parker::new_in_place(&raw mut (*ptr).parker);
1487            Pin::new_unchecked(arc.assume_init())
1488        };
1489
1490        Thread { inner }
1491    }
1492
1493    /// Like the public [`park`], but callable on any handle. This is used to
1494    /// allow parking in TLS destructors.
1495    ///
1496    /// # Safety
1497    /// May only be called from the thread to which this handle belongs.
1498    pub(crate) unsafe fn park(&self) {
1499        unsafe { self.inner.as_ref().parker().park() }
1500    }
1501
1502    /// Like the public [`park_timeout`], but callable on any handle. This is
1503    /// used to allow parking in TLS destructors.
1504    ///
1505    /// # Safety
1506    /// May only be called from the thread to which this handle belongs.
1507    pub(crate) unsafe fn park_timeout(&self, dur: Duration) {
1508        unsafe { self.inner.as_ref().parker().park_timeout(dur) }
1509    }
1510
1511    /// Atomically makes the handle's token available if it is not already.
1512    ///
1513    /// Every thread is equipped with some basic low-level blocking support, via
1514    /// the [`park`][park] function and the `unpark()` method. These can be
1515    /// used as a more CPU-efficient implementation of a spinlock.
1516    ///
1517    /// See the [park documentation][park] for more details.
1518    ///
1519    /// # Examples
1520    ///
1521    /// ```
1522    /// use std::thread;
1523    /// use std::time::Duration;
1524    /// use std::sync::atomic::{AtomicBool, Ordering};
1525    ///
1526    /// static QUEUED: AtomicBool = AtomicBool::new(false);
1527    ///
1528    /// let parked_thread = thread::Builder::new()
1529    ///     .spawn(|| {
1530    ///         println!("Parking thread");
1531    ///         QUEUED.store(true, Ordering::Release);
1532    ///         thread::park();
1533    ///         println!("Thread unparked");
1534    ///     })
1535    ///     .unwrap();
1536    ///
1537    /// // Let some time pass for the thread to be spawned.
1538    /// thread::sleep(Duration::from_millis(10));
1539    ///
1540    /// // Wait until the other thread is queued.
1541    /// // This is crucial! It guarantees that the `unpark` below is not consumed
1542    /// // by some other code in the parked thread (e.g. inside `println!`).
1543    /// while !QUEUED.load(Ordering::Acquire) {
1544    ///     // Spinning is of course inefficient; in practice, this would more likely be
1545    ///     // a dequeue where we have no work to do if there's nobody queued.
1546    ///     std::hint::spin_loop();
1547    /// }
1548    ///
1549    /// println!("Unpark the thread");
1550    /// parked_thread.thread().unpark();
1551    ///
1552    /// parked_thread.join().unwrap();
1553    /// ```
1554    #[stable(feature = "rust1", since = "1.0.0")]
1555    #[inline]
1556    pub fn unpark(&self) {
1557        self.inner.as_ref().parker().unpark();
1558    }
1559
1560    /// Gets the thread's unique identifier.
1561    ///
1562    /// # Examples
1563    ///
1564    /// ```
1565    /// use std::thread;
1566    ///
1567    /// let other_thread = thread::spawn(|| {
1568    ///     thread::current().id()
1569    /// });
1570    ///
1571    /// let other_thread_id = other_thread.join().unwrap();
1572    /// assert!(thread::current().id() != other_thread_id);
1573    /// ```
1574    #[stable(feature = "thread_id", since = "1.19.0")]
1575    #[must_use]
1576    pub fn id(&self) -> ThreadId {
1577        self.inner.id
1578    }
1579
1580    /// Gets the thread's name.
1581    ///
1582    /// For more information about named threads, see
1583    /// [this module-level documentation][naming-threads].
1584    ///
1585    /// # Examples
1586    ///
1587    /// Threads by default have no name specified:
1588    ///
1589    /// ```
1590    /// use std::thread;
1591    ///
1592    /// let builder = thread::Builder::new();
1593    ///
1594    /// let handler = builder.spawn(|| {
1595    ///     assert!(thread::current().name().is_none());
1596    /// }).unwrap();
1597    ///
1598    /// handler.join().unwrap();
1599    /// ```
1600    ///
1601    /// Thread with a specified name:
1602    ///
1603    /// ```
1604    /// use std::thread;
1605    ///
1606    /// let builder = thread::Builder::new()
1607    ///     .name("foo".into());
1608    ///
1609    /// let handler = builder.spawn(|| {
1610    ///     assert_eq!(thread::current().name(), Some("foo"))
1611    /// }).unwrap();
1612    ///
1613    /// handler.join().unwrap();
1614    /// ```
1615    ///
1616    /// [naming-threads]: ./index.html#naming-threads
1617    #[stable(feature = "rust1", since = "1.0.0")]
1618    #[must_use]
1619    pub fn name(&self) -> Option<&str> {
1620        if let Some(name) = &self.inner.name {
1621            Some(name.as_str())
1622        } else if main_thread::get() == Some(self.inner.id) {
1623            Some("main")
1624        } else {
1625            None
1626        }
1627    }
1628
1629    /// Consumes the `Thread`, returning a raw pointer.
1630    ///
1631    /// To avoid a memory leak the pointer must be converted
1632    /// back into a `Thread` using [`Thread::from_raw`]. The pointer is
1633    /// guaranteed to be aligned to at least 8 bytes.
1634    ///
1635    /// # Examples
1636    ///
1637    /// ```
1638    /// #![feature(thread_raw)]
1639    ///
1640    /// use std::thread::{self, Thread};
1641    ///
1642    /// let thread = thread::current();
1643    /// let id = thread.id();
1644    /// let ptr = Thread::into_raw(thread);
1645    /// unsafe {
1646    ///     assert_eq!(Thread::from_raw(ptr).id(), id);
1647    /// }
1648    /// ```
1649    #[unstable(feature = "thread_raw", issue = "97523")]
1650    pub fn into_raw(self) -> *const () {
1651        // Safety: We only expose an opaque pointer, which maintains the `Pin` invariant.
1652        let inner = unsafe { Pin::into_inner_unchecked(self.inner) };
1653        Arc::into_raw(inner) as *const ()
1654    }
1655
1656    /// Constructs a `Thread` from a raw pointer.
1657    ///
1658    /// The raw pointer must have been previously returned
1659    /// by a call to [`Thread::into_raw`].
1660    ///
1661    /// # Safety
1662    ///
1663    /// This function is unsafe because improper use may lead
1664    /// to memory unsafety, even if the returned `Thread` is never
1665    /// accessed.
1666    ///
1667    /// Creating a `Thread` from a pointer other than one returned
1668    /// from [`Thread::into_raw`] is **undefined behavior**.
1669    ///
1670    /// Calling this function twice on the same raw pointer can lead
1671    /// to a double-free if both `Thread` instances are dropped.
1672    #[unstable(feature = "thread_raw", issue = "97523")]
1673    pub unsafe fn from_raw(ptr: *const ()) -> Thread {
1674        // Safety: Upheld by caller.
1675        unsafe { Thread { inner: Pin::new_unchecked(Arc::from_raw(ptr as *const Inner)) } }
1676    }
1677
1678    fn cname(&self) -> Option<&CStr> {
1679        if let Some(name) = &self.inner.name {
1680            Some(name.as_cstr())
1681        } else if main_thread::get() == Some(self.inner.id) {
1682            Some(c"main")
1683        } else {
1684            None
1685        }
1686    }
1687}
1688
1689#[stable(feature = "rust1", since = "1.0.0")]
1690impl fmt::Debug for Thread {
1691    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1692        f.debug_struct("Thread")
1693            .field("id", &self.id())
1694            .field("name", &self.name())
1695            .finish_non_exhaustive()
1696    }
1697}
1698
1699////////////////////////////////////////////////////////////////////////////////
1700// JoinHandle
1701////////////////////////////////////////////////////////////////////////////////
1702
1703/// A specialized [`Result`] type for threads.
1704///
1705/// Indicates the manner in which a thread exited.
1706///
1707/// The value contained in the `Result::Err` variant
1708/// is the value the thread panicked with;
1709/// that is, the argument the `panic!` macro was called with.
1710/// Unlike with normal errors, this value doesn't implement
1711/// the [`Error`](crate::error::Error) trait.
1712///
1713/// Thus, a sensible way to handle a thread panic is to either:
1714///
1715/// 1. propagate the panic with [`std::panic::resume_unwind`]
1716/// 2. or in case the thread is intended to be a subsystem boundary
1717/// that is supposed to isolate system-level failures,
1718/// match on the `Err` variant and handle the panic in an appropriate way
1719///
1720/// A thread that completes without panicking is considered to exit successfully.
1721///
1722/// # Examples
1723///
1724/// Matching on the result of a joined thread:
1725///
1726/// ```no_run
1727/// use std::{fs, thread, panic};
1728///
1729/// fn copy_in_thread() -> thread::Result<()> {
1730///     thread::spawn(|| {
1731///         fs::copy("foo.txt", "bar.txt").unwrap();
1732///     }).join()
1733/// }
1734///
1735/// fn main() {
1736///     match copy_in_thread() {
1737///         Ok(_) => println!("copy succeeded"),
1738///         Err(e) => panic::resume_unwind(e),
1739///     }
1740/// }
1741/// ```
1742///
1743/// [`Result`]: crate::result::Result
1744/// [`std::panic::resume_unwind`]: crate::panic::resume_unwind
1745#[stable(feature = "rust1", since = "1.0.0")]
1746#[doc(search_unbox)]
1747pub type Result<T> = crate::result::Result<T, Box<dyn Any + Send + 'static>>;
1748
1749// This packet is used to communicate the return value between the spawned
1750// thread and the rest of the program. It is shared through an `Arc` and
1751// there's no need for a mutex here because synchronization happens with `join()`
1752// (the caller will never read this packet until the thread has exited).
1753//
1754// An Arc to the packet is stored into a `JoinInner` which in turns is placed
1755// in `JoinHandle`.
1756struct Packet<'scope, T> {
1757    scope: Option<Arc<scoped::ScopeData>>,
1758    result: UnsafeCell<Option<Result<T>>>,
1759    _marker: PhantomData<Option<&'scope scoped::ScopeData>>,
1760}
1761
1762// Due to the usage of `UnsafeCell` we need to manually implement Sync.
1763// The type `T` should already always be Send (otherwise the thread could not
1764// have been created) and the Packet is Sync because all access to the
1765// `UnsafeCell` synchronized (by the `join()` boundary), and `ScopeData` is Sync.
1766unsafe impl<'scope, T: Send> Sync for Packet<'scope, T> {}
1767
1768impl<'scope, T> Drop for Packet<'scope, T> {
1769    fn drop(&mut self) {
1770        // If this packet was for a thread that ran in a scope, the thread
1771        // panicked, and nobody consumed the panic payload, we make sure
1772        // the scope function will panic.
1773        let unhandled_panic = matches!(self.result.get_mut(), Some(Err(_)));
1774        // Drop the result without causing unwinding.
1775        // This is only relevant for threads that aren't join()ed, as
1776        // join() will take the `result` and set it to None, such that
1777        // there is nothing left to drop here.
1778        // If this panics, we should handle that, because we're outside the
1779        // outermost `catch_unwind` of our thread.
1780        // We just abort in that case, since there's nothing else we can do.
1781        // (And even if we tried to handle it somehow, we'd also need to handle
1782        // the case where the panic payload we get out of it also panics on
1783        // drop, and so on. See issue #86027.)
1784        if let Err(_) = panic::catch_unwind(panic::AssertUnwindSafe(|| {
1785            *self.result.get_mut() = None;
1786        })) {
1787            rtabort!("thread result panicked on drop");
1788        }
1789        // Book-keeping so the scope knows when it's done.
1790        if let Some(scope) = &self.scope {
1791            // Now that there will be no more user code running on this thread
1792            // that can use 'scope, mark the thread as 'finished'.
1793            // It's important we only do this after the `result` has been dropped,
1794            // since dropping it might still use things it borrowed from 'scope.
1795            scope.decrement_num_running_threads(unhandled_panic);
1796        }
1797    }
1798}
1799
1800/// Inner representation for JoinHandle
1801struct JoinInner<'scope, T> {
1802    native: imp::Thread,
1803    thread: Thread,
1804    packet: Arc<Packet<'scope, T>>,
1805}
1806
1807impl<'scope, T> JoinInner<'scope, T> {
1808    fn join(mut self) -> Result<T> {
1809        self.native.join();
1810        Arc::get_mut(&mut self.packet)
1811            // FIXME(fuzzypixelz): returning an error instead of panicking here
1812            // would require updating the documentation of
1813            // `std::thread::Result`; currently we can return `Err` if and only
1814            // if the thread had panicked.
1815            .expect("threads should not terminate unexpectedly")
1816            .result
1817            .get_mut()
1818            .take()
1819            .unwrap()
1820    }
1821}
1822
1823/// An owned permission to join on a thread (block on its termination).
1824///
1825/// A `JoinHandle` *detaches* the associated thread when it is dropped, which
1826/// means that there is no longer any handle to the thread and no way to `join`
1827/// on it.
1828///
1829/// Due to platform restrictions, it is not possible to [`Clone`] this
1830/// handle: the ability to join a thread is a uniquely-owned permission.
1831///
1832/// This `struct` is created by the [`thread::spawn`] function and the
1833/// [`thread::Builder::spawn`] method.
1834///
1835/// # Examples
1836///
1837/// Creation from [`thread::spawn`]:
1838///
1839/// ```
1840/// use std::thread;
1841///
1842/// let join_handle: thread::JoinHandle<_> = thread::spawn(|| {
1843///     // some work here
1844/// });
1845/// ```
1846///
1847/// Creation from [`thread::Builder::spawn`]:
1848///
1849/// ```
1850/// use std::thread;
1851///
1852/// let builder = thread::Builder::new();
1853///
1854/// let join_handle: thread::JoinHandle<_> = builder.spawn(|| {
1855///     // some work here
1856/// }).unwrap();
1857/// ```
1858///
1859/// A thread being detached and outliving the thread that spawned it:
1860///
1861/// ```no_run
1862/// use std::thread;
1863/// use std::time::Duration;
1864///
1865/// let original_thread = thread::spawn(|| {
1866///     let _detached_thread = thread::spawn(|| {
1867///         // Here we sleep to make sure that the first thread returns before.
1868///         thread::sleep(Duration::from_millis(10));
1869///         // This will be called, even though the JoinHandle is dropped.
1870///         println!("♫ Still alive ♫");
1871///     });
1872/// });
1873///
1874/// original_thread.join().expect("The thread being joined has panicked");
1875/// println!("Original thread is joined.");
1876///
1877/// // We make sure that the new thread has time to run, before the main
1878/// // thread returns.
1879///
1880/// thread::sleep(Duration::from_millis(1000));
1881/// ```
1882///
1883/// [`thread::Builder::spawn`]: Builder::spawn
1884/// [`thread::spawn`]: spawn
1885#[stable(feature = "rust1", since = "1.0.0")]
1886#[cfg_attr(target_os = "teeos", must_use)]
1887pub struct JoinHandle<T>(JoinInner<'static, T>);
1888
1889#[stable(feature = "joinhandle_impl_send_sync", since = "1.29.0")]
1890unsafe impl<T> Send for JoinHandle<T> {}
1891#[stable(feature = "joinhandle_impl_send_sync", since = "1.29.0")]
1892unsafe impl<T> Sync for JoinHandle<T> {}
1893
1894impl<T> JoinHandle<T> {
1895    /// Extracts a handle to the underlying thread.
1896    ///
1897    /// # Examples
1898    ///
1899    /// ```
1900    /// use std::thread;
1901    ///
1902    /// let builder = thread::Builder::new();
1903    ///
1904    /// let join_handle: thread::JoinHandle<_> = builder.spawn(|| {
1905    ///     // some work here
1906    /// }).unwrap();
1907    ///
1908    /// let thread = join_handle.thread();
1909    /// println!("thread id: {:?}", thread.id());
1910    /// ```
1911    #[stable(feature = "rust1", since = "1.0.0")]
1912    #[must_use]
1913    pub fn thread(&self) -> &Thread {
1914        &self.0.thread
1915    }
1916
1917    /// Waits for the associated thread to finish.
1918    ///
1919    /// This function will return immediately if the associated thread has already finished.
1920    ///
1921    /// In terms of [atomic memory orderings],  the completion of the associated
1922    /// thread synchronizes with this function returning. In other words, all
1923    /// operations performed by that thread [happen
1924    /// before](https://doc.rust-lang.org/nomicon/atomics.html#data-accesses) all
1925    /// operations that happen after `join` returns.
1926    ///
1927    /// If the associated thread panics, [`Err`] is returned with the parameter given
1928    /// to [`panic!`] (though see the Notes below).
1929    ///
1930    /// [`Err`]: crate::result::Result::Err
1931    /// [atomic memory orderings]: crate::sync::atomic
1932    ///
1933    /// # Panics
1934    ///
1935    /// This function may panic on some platforms if a thread attempts to join
1936    /// itself or otherwise may create a deadlock with joining threads.
1937    ///
1938    /// # Examples
1939    ///
1940    /// ```
1941    /// use std::thread;
1942    ///
1943    /// let builder = thread::Builder::new();
1944    ///
1945    /// let join_handle: thread::JoinHandle<_> = builder.spawn(|| {
1946    ///     // some work here
1947    /// }).unwrap();
1948    /// join_handle.join().expect("Couldn't join on the associated thread");
1949    /// ```
1950    ///
1951    /// # Notes
1952    ///
1953    /// If a "foreign" unwinding operation (e.g. an exception thrown from C++
1954    /// code, or a `panic!` in Rust code compiled or linked with a different
1955    /// runtime) unwinds all the way to the thread root, the process may be
1956    /// aborted; see the Notes on [`thread::spawn`]. If the process is not
1957    /// aborted, this function will return a `Result::Err` containing an opaque
1958    /// type.
1959    ///
1960    /// [`catch_unwind`]: ../../std/panic/fn.catch_unwind.html
1961    /// [`thread::spawn`]: spawn
1962    #[stable(feature = "rust1", since = "1.0.0")]
1963    pub fn join(self) -> Result<T> {
1964        self.0.join()
1965    }
1966
1967    /// Checks if the associated thread has finished running its main function.
1968    ///
1969    /// `is_finished` supports implementing a non-blocking join operation, by checking
1970    /// `is_finished`, and calling `join` if it returns `true`. This function does not block. To
1971    /// block while waiting on the thread to finish, use [`join`][Self::join].
1972    ///
1973    /// This might return `true` for a brief moment after the thread's main
1974    /// function has returned, but before the thread itself has stopped running.
1975    /// However, once this returns `true`, [`join`][Self::join] can be expected
1976    /// to return quickly, without blocking for any significant amount of time.
1977    #[stable(feature = "thread_is_running", since = "1.61.0")]
1978    pub fn is_finished(&self) -> bool {
1979        Arc::strong_count(&self.0.packet) == 1
1980    }
1981}
1982
1983impl<T> AsInner<imp::Thread> for JoinHandle<T> {
1984    fn as_inner(&self) -> &imp::Thread {
1985        &self.0.native
1986    }
1987}
1988
1989impl<T> IntoInner<imp::Thread> for JoinHandle<T> {
1990    fn into_inner(self) -> imp::Thread {
1991        self.0.native
1992    }
1993}
1994
1995#[stable(feature = "std_debug", since = "1.16.0")]
1996impl<T> fmt::Debug for JoinHandle<T> {
1997    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
1998        f.debug_struct("JoinHandle").finish_non_exhaustive()
1999    }
2000}
2001
2002fn _assert_sync_and_send() {
2003    fn _assert_both<T: Send + Sync>() {}
2004    _assert_both::<JoinHandle<()>>();
2005    _assert_both::<Thread>();
2006}
2007
2008/// Returns an estimate of the default amount of parallelism a program should use.
2009///
2010/// Parallelism is a resource. A given machine provides a certain capacity for
2011/// parallelism, i.e., a bound on the number of computations it can perform
2012/// simultaneously. This number often corresponds to the amount of CPUs a
2013/// computer has, but it may diverge in various cases.
2014///
2015/// Host environments such as VMs or container orchestrators may want to
2016/// restrict the amount of parallelism made available to programs in them. This
2017/// is often done to limit the potential impact of (unintentionally)
2018/// resource-intensive programs on other programs running on the same machine.
2019///
2020/// # Limitations
2021///
2022/// The purpose of this API is to provide an easy and portable way to query
2023/// the default amount of parallelism the program should use. Among other things it
2024/// does not expose information on NUMA regions, does not account for
2025/// differences in (co)processor capabilities or current system load,
2026/// and will not modify the program's global state in order to more accurately
2027/// query the amount of available parallelism.
2028///
2029/// Where both fixed steady-state and burst limits are available the steady-state
2030/// capacity will be used to ensure more predictable latencies.
2031///
2032/// Resource limits can be changed during the runtime of a program, therefore the value is
2033/// not cached and instead recomputed every time this function is called. It should not be
2034/// called from hot code.
2035///
2036/// The value returned by this function should be considered a simplified
2037/// approximation of the actual amount of parallelism available at any given
2038/// time. To get a more detailed or precise overview of the amount of
2039/// parallelism available to the program, you may wish to use
2040/// platform-specific APIs as well. The following platform limitations currently
2041/// apply to `available_parallelism`:
2042///
2043/// On Windows:
2044/// - It may undercount the amount of parallelism available on systems with more
2045///   than 64 logical CPUs. However, programs typically need specific support to
2046///   take advantage of more than 64 logical CPUs, and in the absence of such
2047///   support, the number returned by this function accurately reflects the
2048///   number of logical CPUs the program can use by default.
2049/// - It may overcount the amount of parallelism available on systems limited by
2050///   process-wide affinity masks, or job object limitations.
2051///
2052/// On Linux:
2053/// - It may overcount the amount of parallelism available when limited by a
2054///   process-wide affinity mask or cgroup quotas and `sched_getaffinity()` or cgroup fs can't be
2055///   queried, e.g. due to sandboxing.
2056/// - It may undercount the amount of parallelism if the current thread's affinity mask
2057///   does not reflect the process' cpuset, e.g. due to pinned threads.
2058/// - If the process is in a cgroup v1 cpu controller, this may need to
2059///   scan mountpoints to find the corresponding cgroup v1 controller,
2060///   which may take time on systems with large numbers of mountpoints.
2061///   (This does not apply to cgroup v2, or to processes not in a
2062///   cgroup.)
2063/// - It does not attempt to take `ulimit` into account. If there is a limit set on the number of
2064///   threads, `available_parallelism` cannot know how much of that limit a Rust program should
2065///   take, or know in a reliable and race-free way how much of that limit is already taken.
2066///
2067/// On all targets:
2068/// - It may overcount the amount of parallelism available when running in a VM
2069/// with CPU usage limits (e.g. an overcommitted host).
2070///
2071/// # Errors
2072///
2073/// This function will, but is not limited to, return errors in the following
2074/// cases:
2075///
2076/// - If the amount of parallelism is not known for the target platform.
2077/// - If the program lacks permission to query the amount of parallelism made
2078///   available to it.
2079///
2080/// # Examples
2081///
2082/// ```
2083/// # #![allow(dead_code)]
2084/// use std::{io, thread};
2085///
2086/// fn main() -> io::Result<()> {
2087///     let count = thread::available_parallelism()?.get();
2088///     assert!(count >= 1_usize);
2089///     Ok(())
2090/// }
2091/// ```
2092#[doc(alias = "available_concurrency")] // Alias for a previous name we gave this API on unstable.
2093#[doc(alias = "hardware_concurrency")] // Alias for C++ `std::thread::hardware_concurrency`.
2094#[doc(alias = "num_cpus")] // Alias for a popular ecosystem crate which provides similar functionality.
2095#[stable(feature = "available_parallelism", since = "1.59.0")]
2096pub fn available_parallelism() -> io::Result<NonZero<usize>> {
2097    imp::available_parallelism()
2098}