mod.rs.html -- source
Description: Source to the Rust file `src/libstd/thread/mod.rs`.
Keywords: rust, rustlang, rust-lang

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    //! Native threads.
    //!
    //! ## The threading model
    //!
    //! An executing Rust program consists of a collection of native OS threads,
    //! each with their own stack and local state. Threads can be named, and
    //! provide some built-in support for low-level synchronization.
    //!
    //! Communication between threads can be done through
    //! [channels], Rust's message-passing types, along with [other forms of thread
    //! synchronization](../../std/sync/index.html) and shared-memory data
    //! structures. In particular, types that are guaranteed to be
    //! threadsafe are easily shared between threads using the
    //! atomically-reference-counted container, [`Arc`].
    //!
    //! Fatal logic errors in Rust cause *thread panic*, during which
    //! a thread will unwind the stack, running destructors and freeing
    //! owned resources. While not meant as a 'try/catch' mechanism, panics
    //! in Rust can nonetheless be caught (unless compiling with `panic=abort`) with
    //! [`catch_unwind`](../../std/panic/fn.catch_unwind.html) and recovered
    //! from, or alternatively be resumed with
    //! [`resume_unwind`](../../std/panic/fn.resume_unwind.html). If the panic
    //! is not caught the thread will exit, but the panic may optionally be
    //! detected from a different thread with [`join`]. If the main thread panics
    //! without the panic being caught, the application will exit with a
    //! non-zero exit code.
    //!
    //! When the main thread of a Rust program terminates, the entire program shuts
    //! down, even if other threads are still running. However, this module provides
    //! convenient facilities for automatically waiting for the termination of a
    //! child thread (i.e., join).
    //!
    //! ## Spawning a thread
    //!
    //! A new thread can be spawned using the [`thread::spawn`][`spawn`] function:
    //!
    //! ```rust
    //! use std::thread;
    //!
    //! thread::spawn(move || {
    //!     // some work here
    //! });
    //! ```
    //!
    //! In this example, the spawned thread is "detached" from the current
    //! thread. This means that it can outlive its parent (the thread that spawned
    //! it), unless this parent is the main thread.
    //!
    //! The parent thread can also wait on the completion of the child
    //! thread; a call to [`spawn`] produces a [`JoinHandle`], which provides
    //! a `join` method for waiting:
    //!
    //! ```rust
    //! use std::thread;
    //!
    //! let child = thread::spawn(move || {
    //!     // some work here
    //! });
    //! // some work here
    //! let res = child.join();
    //! ```
    //!
    //! The [`join`] method returns a [`thread::Result`] containing [`Ok`] of the final
    //! value produced by the child thread, or [`Err`] of the value given to
    //! a call to [`panic!`] if the child panicked.
    //!
    //! ## Configuring threads
    //!
    //! A new thread can be configured before it is spawned via the [`Builder`] type,
    //! which currently allows you to set the name and stack size for the child thread:
    //!
    //! ```rust
    //! # #![allow(unused_must_use)]
    //! use std::thread;
    //!
    //! thread::Builder::new().name("child1".to_string()).spawn(move || {
    //!     println!("Hello, world!");
    //! });
    //! ```
    //!
    //! ## The `Thread` type
    //!
    //! Threads are represented via the [`Thread`] type, which you can get in one of
    //! two ways:
    //!
    //! * By spawning a new thread, e.g., using the [`thread::spawn`][`spawn`]
    //!   function, and calling [`thread`][`JoinHandle::thread`] on the [`JoinHandle`].
    //! * By requesting the current thread, using the [`thread::current`] function.
    //!
    //! The [`thread::current`] function is available even for threads not spawned
    //! by the APIs of this module.
    //!
    //! ## Thread-local storage
    //!
    //! This module also provides an implementation of thread-local storage for Rust
    //! programs. Thread-local storage is a method of storing data into a global
    //! variable that each thread in the program will have its own copy of.
    //! Threads do not share this data, so accesses do not need to be synchronized.
    //!
    //! A thread-local key owns the value it contains and will destroy the value when the
    //! thread exits. It is created with the [`thread_local!`] macro and can contain any
    //! value that is `'static` (no borrowed pointers). It provides an accessor function,
    //! [`with`], that yields a shared reference to the value to the specified
    //! closure. Thread-local keys allow only shared access to values, as there would be no
    //! way to guarantee uniqueness if mutable borrows were allowed. Most values
    //! will want to make use of some form of **interior mutability** through the
    //! [`Cell`] or [`RefCell`] types.
    //!
    //! ## Naming threads
    //!
    //! Threads are able to have associated names for identification purposes. By default, spawned
    //! threads are unnamed. To specify a name for a thread, build the thread with [`Builder`] and pass
    //! the desired thread name to [`Builder::name`]. To retrieve the thread name from within the
    //! thread, use [`Thread::name`]. A couple examples of where the name of a thread gets used:
    //!
    //! * If a panic occurs in a named thread, the thread name will be printed in the panic message.
    //! * The thread name is provided to the OS where applicable (e.g., `pthread_setname_np` in
    //!   unix-like platforms).
    //!
    //! ## Stack size
    //!
    //! The default stack size for spawned threads is 2 MiB, though this particular stack size is
    //! subject to change in the future. There are two ways to manually specify the stack size for
    //! spawned threads:
    //!
    //! * Build the thread with [`Builder`] and pass the desired stack size to [`Builder::stack_size`].
    //! * Set the `RUST_MIN_STACK` environment variable to an integer representing the desired stack
    //!   size (in bytes). Note that setting [`Builder::stack_size`] will override this.
    //!
    //! Note that the stack size of the main thread is *not* determined by Rust.
    //!
    //! [channels]: ../../std/sync/mpsc/index.html
    //! [`Arc`]: ../../std/sync/struct.Arc.html
    //! [`spawn`]: ../../std/thread/fn.spawn.html
    //! [`JoinHandle`]: ../../std/thread/struct.JoinHandle.html
    //! [`JoinHandle::thread`]: ../../std/thread/struct.JoinHandle.html#method.thread
    //! [`join`]: ../../std/thread/struct.JoinHandle.html#method.join
    //! [`Result`]: ../../std/result/enum.Result.html
    //! [`Ok`]: ../../std/result/enum.Result.html#variant.Ok
    //! [`Err`]: ../../std/result/enum.Result.html#variant.Err
    //! [`panic!`]: ../../std/macro.panic.html
    //! [`Builder`]: ../../std/thread/struct.Builder.html
    //! [`Builder::stack_size`]: ../../std/thread/struct.Builder.html#method.stack_size
    //! [`Builder::name`]: ../../std/thread/struct.Builder.html#method.name
    //! [`thread::current`]: ../../std/thread/fn.current.html
    //! [`thread::Result`]: ../../std/thread/type.Result.html
    //! [`Thread`]: ../../std/thread/struct.Thread.html
    //! [`park`]: ../../std/thread/fn.park.html
    //! [`unpark`]: ../../std/thread/struct.Thread.html#method.unpark
    //! [`Thread::name`]: ../../std/thread/struct.Thread.html#method.name
    //! [`thread::park_timeout`]: ../../std/thread/fn.park_timeout.html
    //! [`Cell`]: ../cell/struct.Cell.html
    //! [`RefCell`]: ../cell/struct.RefCell.html
    //! [`thread_local!`]: ../macro.thread_local.html
    //! [`with`]: struct.LocalKey.html#method.with

    #![stable(feature = "rust1", since = "1.0.0")]

    use any::Any;
    use boxed::FnBox;
    use cell::UnsafeCell;
    use ffi::{CStr, CString};
    use fmt;
    use io;
    use mem;
    use panic;
    use panicking;
    use str;
    use sync::{Mutex, Condvar, Arc};
    use sync::atomic::AtomicUsize;
    use sync::atomic::Ordering::SeqCst;
    use sys::thread as imp;
    use sys_common::mutex;
    use sys_common::thread_info;
    use sys_common::thread;
    use sys_common::{AsInner, IntoInner};
    use time::Duration;

    ////////////////////////////////////////////////////////////////////////////////
    // Thread-local storage
    ////////////////////////////////////////////////////////////////////////////////

    #[macro_use] mod local;

    #[stable(feature = "rust1", since = "1.0.0")]
    pub use self::local::{LocalKey, AccessError};

    // The types used by the thread_local! macro to access TLS keys. Note that there
    // are two types, the "OS" type and the "fast" type. The OS thread local key
    // type is accessed via platform-specific API calls and is slow, while the fast
    // key type is accessed via code generated via LLVM, where TLS keys are set up
    // by the elf linker. Note that the OS TLS type is always available: on macOS
    // the standard library is compiled with support for older platform versions
    // where fast TLS was not available; end-user code is compiled with fast TLS
    // where available, but both are needed.

    #[unstable(feature = "libstd_thread_internals", issue = "0")]
    #[cfg(all(target_arch = "wasm32", not(target_feature = "atomics")))]
    #[doc(hidden)] pub use self::local::statik::Key as __StaticLocalKeyInner;
    #[unstable(feature = "libstd_thread_internals", issue = "0")]
    #[cfg(target_thread_local)]
    #[doc(hidden)] pub use self::local::fast::Key as __FastLocalKeyInner;
    #[unstable(feature = "libstd_thread_internals", issue = "0")]
    #[doc(hidden)] pub use self::local::os::Key as __OsLocalKeyInner;

    ////////////////////////////////////////////////////////////////////////////////
    // Builder
    ////////////////////////////////////////////////////////////////////////////////

    /// Thread factory, which can be used in order to configure the properties of
    /// a new thread.
    ///
    /// Methods can be chained on it in order to configure it.
    ///
    /// The two configurations available are:
    ///
    /// - [`name`]: specifies an [associated name for the thread][naming-threads]
    /// - [`stack_size`]: specifies the [desired stack size for the thread][stack-size]
    ///
    /// The [`spawn`] method will take ownership of the builder and create an
    /// [`io::Result`] to the thread handle with the given configuration.
    ///
    /// The [`thread::spawn`] free function uses a `Builder` with default
    /// configuration and [`unwrap`]s its return value.
    ///
    /// You may want to use [`spawn`] instead of [`thread::spawn`], when you want
    /// to recover from a failure to launch a thread, indeed the free function will
    /// panic where the `Builder` method will return a [`io::Result`].
    ///
    /// # Examples
    ///
    /// ```
    /// use std::thread;
    ///
    /// let builder = thread::Builder::new();
    ///
    /// let handler = builder.spawn(|| {
    ///     // thread code
    /// }).unwrap();
    ///
    /// handler.join().unwrap();
    /// ```
    ///
    /// [`thread::spawn`]: ../../std/thread/fn.spawn.html
    /// [`stack_size`]: ../../std/thread/struct.Builder.html#method.stack_size
    /// [`name`]: ../../std/thread/struct.Builder.html#method.name
    /// [`spawn`]: ../../std/thread/struct.Builder.html#method.spawn
    /// [`io::Result`]: ../../std/io/type.Result.html
    /// [`unwrap`]: ../../std/result/enum.Result.html#method.unwrap
    /// [naming-threads]: ./index.html#naming-threads
    /// [stack-size]: ./index.html#stack-size
    #[stable(feature = "rust1", since = "1.0.0")]
    #[derive(Debug)]
    pub struct Builder {
        // A name for the thread-to-be, for identification in panic messages
        name: Option<String>,
        // The size of the stack for the spawned thread in bytes
        stack_size: Option<usize>,
    }

    impl Builder {
        /// Generates the base configuration for spawning a thread, from which
        /// configuration methods can be chained.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::thread;
        ///
        /// let builder = thread::Builder::new()
        ///                               .name("foo".into())
        ///                               .stack_size(10);
        ///
        /// let handler = builder.spawn(|| {
        ///     // thread code
        /// }).unwrap();
        ///
        /// handler.join().unwrap();
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn new() -> Builder {
            Builder {
                name: None,
                stack_size: None,
            }
        }

        /// Names the thread-to-be. Currently the name is used for identification
        /// only in panic messages.
        ///
        /// The name must not contain null bytes (`\0`).
        ///
        /// For more information about named threads, see
        /// [this module-level documentation][naming-threads].
        ///
        /// # Examples
        ///
        /// ```
        /// use std::thread;
        ///
        /// let builder = thread::Builder::new()
        ///     .name("foo".into());
        ///
        /// let handler = builder.spawn(|| {
        ///     assert_eq!(thread::current().name(), Some("foo"))
        /// }).unwrap();
        ///
        /// handler.join().unwrap();
        /// ```
        ///
        /// [naming-threads]: ./index.html#naming-threads
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn name(mut self, name: String) -> Builder {
            self.name = Some(name);
            self
        }

        /// Sets the size of the stack (in bytes) for the new thread.
        ///
        /// The actual stack size may be greater than this value if
        /// the platform specifies a minimal stack size.
        ///
        /// For more information about the stack size for threads, see
        /// [this module-level documentation][stack-size].
        ///
        /// # Examples
        ///
        /// ```
        /// use std::thread;
        ///
        /// let builder = thread::Builder::new().stack_size(32 * 1024);
        /// ```
        ///
        /// [stack-size]: ./index.html#stack-size
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn stack_size(mut self, size: usize) -> Builder {
            self.stack_size = Some(size);
            self
        }

        /// Spawns a new thread by taking ownership of the `Builder`, and returns an
        /// [`io::Result`] to its [`JoinHandle`].
        ///
        /// The spawned thread may outlive the caller (unless the caller thread
        /// is the main thread; the whole process is terminated when the main
        /// thread finishes). The join handle can be used to block on
        /// termination of the child thread, including recovering its panics.
        ///
        /// For a more complete documentation see [`thread::spawn`][`spawn`].
        ///
        /// # Errors
        ///
        /// Unlike the [`spawn`] free function, this method yields an
        /// [`io::Result`] to capture any failure to create the thread at
        /// the OS level.
        ///
        /// [`spawn`]: ../../std/thread/fn.spawn.html
        /// [`io::Result`]: ../../std/io/type.Result.html
        /// [`JoinHandle`]: ../../std/thread/struct.JoinHandle.html
        ///
        /// # Panics
        ///
        /// Panics if a thread name was set and it contained null bytes.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::thread;
        ///
        /// let builder = thread::Builder::new();
        ///
        /// let handler = builder.spawn(|| {
        ///     // thread code
        /// }).unwrap();
        ///
        /// handler.join().unwrap();
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn spawn<F, T>(self, f: F) -> io::Result<JoinHandle<T>> where
            F: FnOnce() -> T, F: Send + 'static, T: Send + 'static
        {
            unsafe { self.spawn_unchecked(f) }
        }

        /// Spawns a new thread without any lifetime restrictions by taking ownership
        /// of the `Builder`, and returns an [`io::Result`] to its [`JoinHandle`].
        ///
        /// The spawned thread may outlive the caller (unless the caller thread
        /// is the main thread; the whole process is terminated when the main
        /// thread finishes). The join handle can be used to block on
        /// termination of the child thread, including recovering its panics.
        ///
        /// This method is identical to [`thread::Builder::spawn`][`Builder::spawn`],
        /// except for the relaxed lifetime bounds, which render it unsafe.
        /// For a more complete documentation see [`thread::spawn`][`spawn`].
        ///
        /// # Errors
        ///
        /// Unlike the [`spawn`] free function, this method yields an
        /// [`io::Result`] to capture any failure to create the thread at
        /// the OS level.
        ///
        /// # Panics
        ///
        /// Panics if a thread name was set and it contained null bytes.
        ///
        /// # Safety
        ///
        /// The caller has to ensure that no references in the supplied thread closure
        /// or its return type can outlive the spawned thread's lifetime. This can be
        /// guaranteed in two ways:
        ///
        /// - ensure that [`join`][`JoinHandle::join`] is called before any referenced
        /// data is dropped
        /// - use only types with `'static` lifetime bounds, i.e., those with no or only
        /// `'static` references (both [`thread::Builder::spawn`][`Builder::spawn`]
        /// and [`thread::spawn`][`spawn`] enforce this property statically)
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(thread_spawn_unchecked)]
        /// use std::thread;
        ///
        /// let builder = thread::Builder::new();
        ///
        /// let x = 1;
        /// let thread_x = &x;
        ///
        /// let handler = unsafe {
        ///     builder.spawn_unchecked(move || {
        ///         println!("x = {}", *thread_x);
        ///     }).unwrap()
        /// };
        ///
        /// // caller has to ensure `join()` is called, otherwise
        /// // it is possible to access freed memory if `x` gets
        /// // dropped before the thread closure is executed!
        /// handler.join().unwrap();
        /// ```
        ///
        /// [`spawn`]: ../../std/thread/fn.spawn.html
        /// [`Builder::spawn`]: ../../std/thread/struct.Builder.html#method.spawn
        /// [`io::Result`]: ../../std/io/type.Result.html
        /// [`JoinHandle`]: ../../std/thread/struct.JoinHandle.html
        #[unstable(feature = "thread_spawn_unchecked", issue = "55132")]
        pub unsafe fn spawn_unchecked<'a, F, T>(self, f: F) -> io::Result<JoinHandle<T>> where
            F: FnOnce() -> T, F: Send + 'a, T: Send + 'a
        {
            let Builder { name, stack_size } = self;

            let stack_size = stack_size.unwrap_or_else(thread::min_stack);

            let my_thread = Thread::new(name);
            let their_thread = my_thread.clone();

            let my_packet : Arc<UnsafeCell<Option<Result<T>>>>
                = Arc::new(UnsafeCell::new(None));
            let their_packet = my_packet.clone();

            let main = move || {
                if let Some(name) = their_thread.cname() {
                    imp::Thread::set_name(name);
                }

                thread_info::set(imp::guard::current(), their_thread);
                #[cfg(feature = "backtrace")]
                let try_result = panic::catch_unwind(panic::AssertUnwindSafe(|| {
                    ::sys_common::backtrace::__rust_begin_short_backtrace(f)
                }));
                #[cfg(not(feature = "backtrace"))]
                let try_result = panic::catch_unwind(panic::AssertUnwindSafe(f));
                *their_packet.get() = Some(try_result);
            };

            Ok(JoinHandle(JoinInner {
                // `imp::Thread::new` takes a closure with a `'static` lifetime, since it's passed
                // through FFI or otherwise used with low-level threading primitives that have no
                // notion of or way to enforce lifetimes.
                //
                // As mentioned in the `Safety` section of this function's documentation, the caller of
                // this function needs to guarantee that the passed-in lifetime is sufficiently long
                // for the lifetime of the thread.
                //
                // Similarly, the `sys` implementation must guarantee that no references to the closure
                // exist after the thread has terminated, which is signaled by `Thread::join`
                // returning.
                native: Some(imp::Thread::new(
                    stack_size,
                    mem::transmute::<Box<dyn FnBox() + 'a>, Box<dyn FnBox() + 'static>>(Box::new(main))
                )?),
                thread: my_thread,
                packet: Packet(my_packet),
            }))
        }
    }

    ////////////////////////////////////////////////////////////////////////////////
    // Free functions
    ////////////////////////////////////////////////////////////////////////////////

    /// Spawns a new thread, returning a [`JoinHandle`] for it.
    ///
    /// The join handle will implicitly *detach* the child thread upon being
    /// dropped. In this case, the child thread may outlive the parent (unless
    /// the parent thread is the main thread; the whole process is terminated when
    /// the main thread finishes). Additionally, the join handle provides a [`join`]
    /// method that can be used to join the child thread. If the child thread
    /// panics, [`join`] will return an [`Err`] containing the argument given to
    /// [`panic`].
    ///
    /// This will create a thread using default parameters of [`Builder`], if you
    /// want to specify the stack size or the name of the thread, use this API
    /// instead.
    ///
    /// As you can see in the signature of `spawn` there are two constraints on
    /// both the closure given to `spawn` and its return value, let's explain them:
    ///
    /// - The `'static` constraint means that the closure and its return value
    ///   must have a lifetime of the whole program execution. The reason for this
    ///   is that threads can `detach` and outlive the lifetime they have been
    ///   created in.
    ///   Indeed if the thread, and by extension its return value, can outlive their
    ///   caller, we need to make sure that they will be valid afterwards, and since
    ///   we *can't* know when it will return we need to have them valid as long as
    ///   possible, that is until the end of the program, hence the `'static`
    ///   lifetime.
    /// - The [`Send`] constraint is because the closure will need to be passed
    ///   *by value* from the thread where it is spawned to the new thread. Its
    ///   return value will need to be passed from the new thread to the thread
    ///   where it is `join`ed.
    ///   As a reminder, the [`Send`] marker trait expresses that it is safe to be
    ///   passed from thread to thread. [`Sync`] expresses that it is safe to have a
    ///   reference be passed from thread to thread.
    ///
    /// # Panics
    ///
    /// Panics if the OS fails to create a thread; use [`Builder::spawn`]
    /// to recover from such errors.
    ///
    /// # Examples
    ///
    /// Creating a thread.
    ///
    /// ```
    /// use std::thread;
    ///
    /// let handler = thread::spawn(|| {
    ///     // thread code
    /// });
    ///
    /// handler.join().unwrap();
    /// ```
    ///
    /// As mentioned in the module documentation, threads are usually made to
    /// communicate using [`channels`], here is how it usually looks.
    ///
    /// This example also shows how to use `move`, in order to give ownership
    /// of values to a thread.
    ///
    /// ```
    /// use std::thread;
    /// use std::sync::mpsc::channel;
    ///
    /// let (tx, rx) = channel();
    ///
    /// let sender = thread::spawn(move || {
    ///     tx.send("Hello, thread".to_owned())
    ///         .expect("Unable to send on channel");
    /// });
    ///
    /// let receiver = thread::spawn(move || {
    ///     let value = rx.recv().expect("Unable to receive from channel");
    ///     println!("{}", value);
    /// });
    ///
    /// sender.join().expect("The sender thread has panicked");
    /// receiver.join().expect("The receiver thread has panicked");
    /// ```
    ///
    /// A thread can also return a value through its [`JoinHandle`], you can use
    /// this to make asynchronous computations (futures might be more appropriate
    /// though).
    ///
    /// ```
    /// use std::thread;
    ///
    /// let computation = thread::spawn(|| {
    ///     // Some expensive computation.
    ///     42
    /// });
    ///
    /// let result = computation.join().unwrap();
    /// println!("{}", result);
    /// ```
    ///
    /// [`channels`]: ../../std/sync/mpsc/index.html
    /// [`JoinHandle`]: ../../std/thread/struct.JoinHandle.html
    /// [`join`]: ../../std/thread/struct.JoinHandle.html#method.join
    /// [`Err`]: ../../std/result/enum.Result.html#variant.Err
    /// [`panic`]: ../../std/macro.panic.html
    /// [`Builder::spawn`]: ../../std/thread/struct.Builder.html#method.spawn
    /// [`Builder`]: ../../std/thread/struct.Builder.html
    /// [`Send`]: ../../std/marker/trait.Send.html
    /// [`Sync`]: ../../std/marker/trait.Sync.html
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn spawn<F, T>(f: F) -> JoinHandle<T> where
        F: FnOnce() -> T, F: Send + 'static, T: Send + 'static
    {
        Builder::new().spawn(f).expect("failed to spawn thread")
    }

    /// Gets a handle to the thread that invokes it.
    ///
    /// # Examples
    ///
    /// Getting a handle to the current thread with `thread::current()`:
    ///
    /// ```
    /// use std::thread;
    ///
    /// let handler = thread::Builder::new()
    ///     .name("named thread".into())
    ///     .spawn(|| {
    ///         let handle = thread::current();
    ///         assert_eq!(handle.name(), Some("named thread"));
    ///     })
    ///     .unwrap();
    ///
    /// handler.join().unwrap();
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn current() -> Thread {
        thread_info::current_thread().expect("use of std::thread::current() is not \
                                              possible after the thread's local \
                                              data has been destroyed")
    }

    /// Cooperatively gives up a timeslice to the OS scheduler.
    ///
    /// This is used when the programmer knows that the thread will have nothing
    /// to do for some time, and thus avoid wasting computing time.
    ///
    /// For example when polling on a resource, it is common to check that it is
    /// available, and if not to yield in order to avoid busy waiting.
    ///
    /// Thus the pattern of `yield`ing after a failed poll is rather common when
    /// implementing low-level shared resources or synchronization primitives.
    ///
    /// However programmers will usually prefer to use [`channel`]s, [`Condvar`]s,
    /// [`Mutex`]es or [`join`] for their synchronization routines, as they avoid
    /// thinking about thread scheduling.
    ///
    /// Note that [`channel`]s for example are implemented using this primitive.
    /// Indeed when you call `send` or `recv`, which are blocking, they will yield
    /// if the channel is not available.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::thread;
    ///
    /// thread::yield_now();
    /// ```
    ///
    /// [`channel`]: ../../std/sync/mpsc/index.html
    /// [`spawn`]: ../../std/thread/fn.spawn.html
    /// [`join`]: ../../std/thread/struct.JoinHandle.html#method.join
    /// [`Mutex`]: ../../std/sync/struct.Mutex.html
    /// [`Condvar`]: ../../std/sync/struct.Condvar.html
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn yield_now() {
        imp::Thread::yield_now()
    }

    /// Determines whether the current thread is unwinding because of panic.
    ///
    /// A common use of this feature is to poison shared resources when writing
    /// unsafe code, by checking `panicking` when the `drop` is called.
    ///
    /// This is usually not needed when writing safe code, as [`Mutex`es][Mutex]
    /// already poison themselves when a thread panics while holding the lock.
    ///
    /// This can also be used in multithreaded applications, in order to send a
    /// message to other threads warning that a thread has panicked (e.g., for
    /// monitoring purposes).
    ///
    /// # Examples
    ///
    /// ```should_panic
    /// use std::thread;
    ///
    /// struct SomeStruct;
    ///
    /// impl Drop for SomeStruct {
    ///     fn drop(&mut self) {
    ///         if thread::panicking() {
    ///             println!("dropped while unwinding");
    ///         } else {
    ///             println!("dropped while not unwinding");
    ///         }
    ///     }
    /// }
    ///
    /// {
    ///     print!("a: ");
    ///     let a = SomeStruct;
    /// }
    ///
    /// {
    ///     print!("b: ");
    ///     let b = SomeStruct;
    ///     panic!()
    /// }
    /// ```
    ///
    /// [Mutex]: ../../std/sync/struct.Mutex.html
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn panicking() -> bool {
        panicking::panicking()
    }

    /// Puts the current thread to sleep for at least the specified amount of time.
    ///
    /// The thread may sleep longer than the duration specified due to scheduling
    /// specifics or platform-dependent functionality. It will never sleep less.
    ///
    /// # Platform-specific behavior
    ///
    /// On Unix platforms, the underlying syscall may be interrupted by a
    /// spurious wakeup or signal handler. To ensure the sleep occurs for at least
    /// the specified duration, this function may invoke that system call multiple
    /// times.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use std::thread;
    ///
    /// // Let's sleep for 2 seconds:
    /// thread::sleep_ms(2000);
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_deprecated(since = "1.6.0", reason = "replaced by `std::thread::sleep`")]
    pub fn sleep_ms(ms: u32) {
        sleep(Duration::from_millis(ms as u64))
    }

    /// Puts the current thread to sleep for at least the specified amount of time.
    ///
    /// The thread may sleep longer than the duration specified due to scheduling
    /// specifics or platform-dependent functionality. It will never sleep less.
    ///
    /// # Platform-specific behavior
    ///
    /// On Unix platforms, the underlying syscall may be interrupted by a
    /// spurious wakeup or signal handler. To ensure the sleep occurs for at least
    /// the specified duration, this function may invoke that system call multiple
    /// times.
    /// Platforms which do not support nanosecond precision for sleeping will
    /// have `dur` rounded up to the nearest granularity of time they can sleep for.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use std::{thread, time};
    ///
    /// let ten_millis = time::Duration::from_millis(10);
    /// let now = time::Instant::now();
    ///
    /// thread::sleep(ten_millis);
    ///
    /// assert!(now.elapsed() >= ten_millis);
    /// ```
    #[stable(feature = "thread_sleep", since = "1.4.0")]
    pub fn sleep(dur: Duration) {
        imp::Thread::sleep(dur)
    }

    // constants for park/unpark
    const EMPTY: usize = 0;
    const PARKED: usize = 1;
    const NOTIFIED: usize = 2;

    /// Blocks unless or until the current thread's token is made available.
    ///
    /// A call to `park` does not guarantee that the thread will remain parked
    /// forever, and callers should be prepared for this possibility.
    ///
    /// # park and unpark
    ///
    /// Every thread is equipped with some basic low-level blocking support, via the
    /// [`thread::park`][`park`] function and [`thread::Thread::unpark`][`unpark`]
    /// method. [`park`] blocks the current thread, which can then be resumed from
    /// another thread by calling the [`unpark`] method on the blocked thread's
    /// handle.
    ///
    /// Conceptually, each [`Thread`] handle has an associated token, which is
    /// initially not present:
    ///
    /// * The [`thread::park`][`park`] function blocks the current thread unless or
    ///   until the token is available for its thread handle, at which point it
    ///   atomically consumes the token. It may also return *spuriously*, without
    ///   consuming the token. [`thread::park_timeout`] does the same, but allows
    ///   specifying a maximum time to block the thread for.
    ///
    /// * The [`unpark`] method on a [`Thread`] atomically makes the token available
    ///   if it wasn't already. Because the token is initially absent, [`unpark`]
    ///   followed by [`park`] will result in the second call returning immediately.
    ///
    /// In other words, each [`Thread`] acts a bit like a spinlock that can be
    /// locked and unlocked using `park` and `unpark`.
    ///
    /// Notice that being unblocked does not imply any synchronization with someone
    /// that unparked this thread, it could also be spurious.
    /// For example, it would be a valid, but inefficient, implementation to make both [`park`] and
    /// [`unpark`] return immediately without doing anything.
    ///
    /// The API is typically used by acquiring a handle to the current thread,
    /// placing that handle in a shared data structure so that other threads can
    /// find it, and then `park`ing in a loop. When some desired condition is met, another
    /// thread calls [`unpark`] on the handle.
    ///
    /// The motivation for this design is twofold:
    ///
    /// * It avoids the need to allocate mutexes and condvars when building new
    ///   synchronization primitives; the threads already provide basic
    ///   blocking/signaling.
    ///
    /// * It can be implemented very efficiently on many platforms.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::thread;
    /// use std::sync::{Arc, atomic::{Ordering, AtomicBool}};
    /// use std::time::Duration;
    ///
    /// let flag = Arc::new(AtomicBool::new(false));
    /// let flag2 = Arc::clone(&flag);
    ///
    /// let parked_thread = thread::spawn(move || {
    ///     // We want to wait until the flag is set. We *could* just spin, but using
    ///     // park/unpark is more efficient.
    ///     while !flag2.load(Ordering::Acquire) {
    ///         println!("Parking thread");
    ///         thread::park();
    ///         // We *could* get here spuriously, i.e., way before the 10ms below are over!
    ///         // But that is no problem, we are in a loop until the flag is set anyway.
    ///         println!("Thread unparked");
    ///     }
    ///     println!("Flag received");
    /// });
    ///
    /// // Let some time pass for the thread to be spawned.
    /// thread::sleep(Duration::from_millis(10));
    ///
    /// // Set the flag, and let the thread wake up.
    /// // There is no race condition here, if `unpark`
    /// // happens first, `park` will return immediately.
    /// // Hence there is no risk of a deadlock.
    /// flag.store(true, Ordering::Release);
    /// println!("Unpark the thread");
    /// parked_thread.thread().unpark();
    ///
    /// parked_thread.join().unwrap();
    /// ```
    ///
    /// [`Thread`]: ../../std/thread/struct.Thread.html
    /// [`park`]: ../../std/thread/fn.park.html
    /// [`unpark`]: ../../std/thread/struct.Thread.html#method.unpark
    /// [`thread::park_timeout`]: ../../std/thread/fn.park_timeout.html
    //
    // The implementation currently uses the trivial strategy of a Mutex+Condvar
    // with wakeup flag, which does not actually allow spurious wakeups. In the
    // future, this will be implemented in a more efficient way, perhaps along the lines of
    //   http://cr.openjdk.java.net/~stefank/6989984.1/raw_files/new/src/os/linux/vm/os_linux.cpp
    // or futuxes, and in either case may allow spurious wakeups.
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn park() {
        let thread = current();

        // If we were previously notified then we consume this notification and
        // return quickly.
        if thread.inner.state.compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst).is_ok() {
            return
        }

        // Otherwise we need to coordinate going to sleep
        let mut m = thread.inner.lock.lock().unwrap();
        match thread.inner.state.compare_exchange(EMPTY, PARKED, SeqCst, SeqCst) {
            Ok(_) => {}
            Err(NOTIFIED) => {
                // We must read here, even though we know it will be `NOTIFIED`.
                // This is because `unpark` may have been called again since we read
                // `NOTIFIED` in the `compare_exchange` above. We must perform an
                // acquire operation that synchronizes with that `unpark` to observe
                // any writes it made before the call to unpark. To do that we must
                // read from the write it made to `state`.
                let old = thread.inner.state.swap(EMPTY, SeqCst);
                assert_eq!(old, NOTIFIED, "park state changed unexpectedly");
                return;
            } // should consume this notification, so prohibit spurious wakeups in next park.
            Err(_) => panic!("inconsistent park state"),
        }
        loop {
            m = thread.inner.cvar.wait(m).unwrap();
            match thread.inner.state.compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst) {
                Ok(_) => return, // got a notification
                Err(_) => {} // spurious wakeup, go back to sleep
            }
        }
    }

    /// Use [`park_timeout`].
    ///
    /// Blocks unless or until the current thread's token is made available or
    /// the specified duration has been reached (may wake spuriously).
    ///
    /// The semantics of this function are equivalent to [`park`] except
    /// that the thread will be blocked for roughly no longer than `dur`. This
    /// method should not be used for precise timing due to anomalies such as
    /// preemption or platform differences that may not cause the maximum
    /// amount of time waited to be precisely `ms` long.
    ///
    /// See the [park documentation][`park`] for more detail.
    ///
    /// [`park_timeout`]: fn.park_timeout.html
    /// [`park`]: ../../std/thread/fn.park.html
    #[stable(feature = "rust1", since = "1.0.0")]
    #[rustc_deprecated(since = "1.6.0", reason = "replaced by `std::thread::park_timeout`")]
    pub fn park_timeout_ms(ms: u32) {
        park_timeout(Duration::from_millis(ms as u64))
    }

    /// Blocks unless or until the current thread's token is made available or
    /// the specified duration has been reached (may wake spuriously).
    ///
    /// The semantics of this function are equivalent to [`park`][park] except
    /// that the thread will be blocked for roughly no longer than `dur`. This
    /// method should not be used for precise timing due to anomalies such as
    /// preemption or platform differences that may not cause the maximum
    /// amount of time waited to be precisely `dur` long.
    ///
    /// See the [park documentation][park] for more details.
    ///
    /// # Platform-specific behavior
    ///
    /// Platforms which do not support nanosecond precision for sleeping will have
    /// `dur` rounded up to the nearest granularity of time they can sleep for.
    ///
    /// # Examples
    ///
    /// Waiting for the complete expiration of the timeout:
    ///
    /// ```rust,no_run
    /// use std::thread::park_timeout;
    /// use std::time::{Instant, Duration};
    ///
    /// let timeout = Duration::from_secs(2);
    /// let beginning_park = Instant::now();
    ///
    /// let mut timeout_remaining = timeout;
    /// loop {
    ///     park_timeout(timeout_remaining);
    ///     let elapsed = beginning_park.elapsed();
    ///     if elapsed >= timeout {
    ///         break;
    ///     }
    ///     println!("restarting park_timeout after {:?}", elapsed);
    ///     timeout_remaining = timeout - elapsed;
    /// }
    /// ```
    ///
    /// [park]: fn.park.html
    #[stable(feature = "park_timeout", since = "1.4.0")]
    pub fn park_timeout(dur: Duration) {
        let thread = current();

        // Like `park` above we have a fast path for an already-notified thread, and
        // afterwards we start coordinating for a sleep.
        // return quickly.
        if thread.inner.state.compare_exchange(NOTIFIED, EMPTY, SeqCst, SeqCst).is_ok() {
            return
        }
        let m = thread.inner.lock.lock().unwrap();
        match thread.inner.state.compare_exchange(EMPTY, PARKED, SeqCst, SeqCst) {
            Ok(_) => {}
            Err(NOTIFIED) => {
                // We must read again here, see `park`.
                let old = thread.inner.state.swap(EMPTY, SeqCst);
                assert_eq!(old, NOTIFIED, "park state changed unexpectedly");
                return;
            } // should consume this notification, so prohibit spurious wakeups in next park.
            Err(_) => panic!("inconsistent park_timeout state"),
        }

        // Wait with a timeout, and if we spuriously wake up or otherwise wake up
        // from a notification we just want to unconditionally set the state back to
        // empty, either consuming a notification or un-flagging ourselves as
        // parked.
        let (_m, _result) = thread.inner.cvar.wait_timeout(m, dur).unwrap();
        match thread.inner.state.swap(EMPTY, SeqCst) {
            NOTIFIED => {} // got a notification, hurray!
            PARKED => {} // no notification, alas
            n => panic!("inconsistent park_timeout state: {}", n),
        }
    }

    ////////////////////////////////////////////////////////////////////////////////
    // ThreadId
    ////////////////////////////////////////////////////////////////////////////////

    /// A unique identifier for a running thread.
    ///
    /// A `ThreadId` is an opaque object that has a unique value for each thread
    /// that creates one. `ThreadId`s are not guaranteed to correspond to a thread's
    /// system-designated identifier. A `ThreadId` can be retrieved from the [`id`]
    /// method on a [`Thread`].
    ///
    /// # Examples
    ///
    /// ```
    /// use std::thread;
    ///
    /// let other_thread = thread::spawn(|| {
    ///     thread::current().id()
    /// });
    ///
    /// let other_thread_id = other_thread.join().unwrap();
    /// assert!(thread::current().id() != other_thread_id);
    /// ```
    ///
    /// [`id`]: ../../std/thread/struct.Thread.html#method.id
    /// [`Thread`]: ../../std/thread/struct.Thread.html
    #[stable(feature = "thread_id", since = "1.19.0")]
    #[derive(Eq, PartialEq, Clone, Copy, Hash, Debug)]
    pub struct ThreadId(u64);

    impl ThreadId {
        // Generate a new unique thread ID.
        fn new() -> ThreadId {
            // We never call `GUARD.init()`, so it is UB to attempt to
            // acquire this mutex reentrantly!
            static GUARD: mutex::Mutex = mutex::Mutex::new();
            static mut COUNTER: u64 = 0;

            unsafe {
                let _guard = GUARD.lock();

                // If we somehow use up all our bits, panic so that we're not
                // covering up subtle bugs of IDs being reused.
                if COUNTER == ::u64::MAX {
                    panic!("failed to generate unique thread ID: bitspace exhausted");
                }

                let id = COUNTER;
                COUNTER += 1;

                ThreadId(id)
            }
        }
    }

    ////////////////////////////////////////////////////////////////////////////////
    // Thread
    ////////////////////////////////////////////////////////////////////////////////

    /// The internal representation of a `Thread` handle
    struct Inner {
        name: Option<CString>,      // Guaranteed to be UTF-8
        id: ThreadId,

        // state for thread park/unpark
        state: AtomicUsize,
        lock: Mutex<()>,
        cvar: Condvar,
    }

    #[derive(Clone)]
    #[stable(feature = "rust1", since = "1.0.0")]
    /// A handle to a thread.
    ///
    /// Threads are represented via the `Thread` type, which you can get in one of
    /// two ways:
    ///
    /// * By spawning a new thread, e.g., using the [`thread::spawn`][`spawn`]
    ///   function, and calling [`thread`][`JoinHandle::thread`] on the
    ///   [`JoinHandle`].
    /// * By requesting the current thread, using the [`thread::current`] function.
    ///
    /// The [`thread::current`] function is available even for threads not spawned
    /// by the APIs of this module.
    ///
    /// There is usually no need to create a `Thread` struct yourself, one
    /// should instead use a function like `spawn` to create new threads, see the
    /// docs of [`Builder`] and [`spawn`] for more details.
    ///
    /// [`Builder`]: ../../std/thread/struct.Builder.html
    /// [`JoinHandle::thread`]: ../../std/thread/struct.JoinHandle.html#method.thread
    /// [`JoinHandle`]: ../../std/thread/struct.JoinHandle.html
    /// [`thread::current`]: ../../std/thread/fn.current.html
    /// [`spawn`]: ../../std/thread/fn.spawn.html

    pub struct Thread {
        inner: Arc<Inner>,
    }

    impl Thread {
        // Used only internally to construct a thread object without spawning
        // Panics if the name contains nuls.
        pub(crate) fn new(name: Option<String>) -> Thread {
            let cname = name.map(|n| {
                CString::new(n).expect("thread name may not contain interior null bytes")
            });
            Thread {
                inner: Arc::new(Inner {
                    name: cname,
                    id: ThreadId::new(),
                    state: AtomicUsize::new(EMPTY),
                    lock: Mutex::new(()),
                    cvar: Condvar::new(),
                })
            }
        }

        /// Atomically makes the handle's token available if it is not already.
        ///
        /// Every thread is equipped with some basic low-level blocking support, via
        /// the [`park`][park] function and the `unpark()` method. These can be
        /// used as a more CPU-efficient implementation of a spinlock.
        ///
        /// See the [park documentation][park] for more details.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::thread;
        /// use std::time::Duration;
        ///
        /// let parked_thread = thread::Builder::new()
        ///     .spawn(|| {
        ///         println!("Parking thread");
        ///         thread::park();
        ///         println!("Thread unparked");
        ///     })
        ///     .unwrap();
        ///
        /// // Let some time pass for the thread to be spawned.
        /// thread::sleep(Duration::from_millis(10));
        ///
        /// println!("Unpark the thread");
        /// parked_thread.thread().unpark();
        ///
        /// parked_thread.join().unwrap();
        /// ```
        ///
        /// [park]: fn.park.html
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn unpark(&self) {
            // To ensure the unparked thread will observe any writes we made
            // before this call, we must perform a release operation that `park`
            // can synchronize with. To do that we must write `NOTIFIED` even if
            // `state` is already `NOTIFIED`. That is why this must be a swap
            // rather than a compare-and-swap that returns if it reads `NOTIFIED`
            // on failure.
            match self.inner.state.swap(NOTIFIED, SeqCst) {
                EMPTY => return, // no one was waiting
                NOTIFIED => return, // already unparked
                PARKED => {} // gotta go wake someone up
                _ => panic!("inconsistent state in unpark"),
            }

            // There is a period between when the parked thread sets `state` to
            // `PARKED` (or last checked `state` in the case of a spurious wake
            // up) and when it actually waits on `cvar`. If we were to notify
            // during this period it would be ignored and then when the parked
            // thread went to sleep it would never wake up. Fortunately, it has
            // `lock` locked at this stage so we can acquire `lock` to wait until
            // it is ready to receive the notification.
            //
            // Releasing `lock` before the call to `notify_one` means that when the
            // parked thread wakes it doesn't get woken only to have to wait for us
            // to release `lock`.
            drop(self.inner.lock.lock().unwrap());
            self.inner.cvar.notify_one()
        }

        /// Gets the thread's unique identifier.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::thread;
        ///
        /// let other_thread = thread::spawn(|| {
        ///     thread::current().id()
        /// });
        ///
        /// let other_thread_id = other_thread.join().unwrap();
        /// assert!(thread::current().id() != other_thread_id);
        /// ```
        #[stable(feature = "thread_id", since = "1.19.0")]
        pub fn id(&self) -> ThreadId {
            self.inner.id
        }

        /// Gets the thread's name.
        ///
        /// For more information about named threads, see
        /// [this module-level documentation][naming-threads].
        ///
        /// # Examples
        ///
        /// Threads by default have no name specified:
        ///
        /// ```
        /// use std::thread;
        ///
        /// let builder = thread::Builder::new();
        ///
        /// let handler = builder.spawn(|| {
        ///     assert!(thread::current().name().is_none());
        /// }).unwrap();
        ///
        /// handler.join().unwrap();
        /// ```
        ///
        /// Thread with a specified name:
        ///
        /// ```
        /// use std::thread;
        ///
        /// let builder = thread::Builder::new()
        ///     .name("foo".into());
        ///
        /// let handler = builder.spawn(|| {
        ///     assert_eq!(thread::current().name(), Some("foo"))
        /// }).unwrap();
        ///
        /// handler.join().unwrap();
        /// ```
        ///
        /// [naming-threads]: ./index.html#naming-threads
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn name(&self) -> Option<&str> {
            self.cname().map(|s| unsafe { str::from_utf8_unchecked(s.to_bytes()) } )
        }

        fn cname(&self) -> Option<&CStr> {
            self.inner.name.as_ref().map(|s| &**s)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl fmt::Debug for Thread {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            fmt::Debug::fmt(&self.name(), f)
        }
    }

    ////////////////////////////////////////////////////////////////////////////////
    // JoinHandle
    ////////////////////////////////////////////////////////////////////////////////

    /// A specialized [`Result`] type for threads.
    ///
    /// Indicates the manner in which a thread exited.
    ///
    /// A thread that completes without panicking is considered to exit successfully.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use std::thread;
    /// use std::fs;
    ///
    /// fn copy_in_thread() -> thread::Result<()> {
    ///     thread::spawn(move || { fs::copy("foo.txt", "bar.txt").unwrap(); }).join()
    /// }
    ///
    /// fn main() {
    ///     match copy_in_thread() {
    ///         Ok(_) => println!("this is fine"),
    ///         Err(_) => println!("thread panicked"),
    ///     }
    /// }
    /// ```
    ///
    /// [`Result`]: ../../std/result/enum.Result.html
    #[stable(feature = "rust1", since = "1.0.0")]
    pub type Result<T> = ::result::Result<T, Box<dyn Any + Send + 'static>>;

    // This packet is used to communicate the return value between the child thread
    // and the parent thread. Memory is shared through the `Arc` within and there's
    // no need for a mutex here because synchronization happens with `join()` (the
    // parent thread never reads this packet until the child has exited).
    //
    // This packet itself is then stored into a `JoinInner` which in turns is placed
    // in `JoinHandle` and `JoinGuard`. Due to the usage of `UnsafeCell` we need to
    // manually worry about impls like Send and Sync. The type `T` should
    // already always be Send (otherwise the thread could not have been created) and
    // this type is inherently Sync because no methods take &self. Regardless,
    // however, we add inheriting impls for Send/Sync to this type to ensure it's
    // Send/Sync and that future modifications will still appropriately classify it.
    struct Packet<T>(Arc<UnsafeCell<Option<Result<T>>>>);

    unsafe impl<T: Send> Send for Packet<T> {}
    unsafe impl<T: Sync> Sync for Packet<T> {}

    /// Inner representation for JoinHandle
    struct JoinInner<T> {
        native: Option<imp::Thread>,
        thread: Thread,
        packet: Packet<T>,
    }

    impl<T> JoinInner<T> {
        fn join(&mut self) -> Result<T> {
            self.native.take().unwrap().join();
            unsafe {
                (*self.packet.0.get()).take().unwrap()
            }
        }
    }

    /// An owned permission to join on a thread (block on its termination).
    ///
    /// A `JoinHandle` *detaches* the associated thread when it is dropped, which
    /// means that there is no longer any handle to thread and no way to `join`
    /// on it.
    ///
    /// Due to platform restrictions, it is not possible to [`Clone`] this
    /// handle: the ability to join a thread is a uniquely-owned permission.
    ///
    /// This `struct` is created by the [`thread::spawn`] function and the
    /// [`thread::Builder::spawn`] method.
    ///
    /// # Examples
    ///
    /// Creation from [`thread::spawn`]:
    ///
    /// ```
    /// use std::thread;
    ///
    /// let join_handle: thread::JoinHandle<_> = thread::spawn(|| {
    ///     // some work here
    /// });
    /// ```
    ///
    /// Creation from [`thread::Builder::spawn`]:
    ///
    /// ```
    /// use std::thread;
    ///
    /// let builder = thread::Builder::new();
    ///
    /// let join_handle: thread::JoinHandle<_> = builder.spawn(|| {
    ///     // some work here
    /// }).unwrap();
    /// ```
    ///
    /// Child being detached and outliving its parent:
    ///
    /// ```no_run
    /// use std::thread;
    /// use std::time::Duration;
    ///
    /// let original_thread = thread::spawn(|| {
    ///     let _detached_thread = thread::spawn(|| {
    ///         // Here we sleep to make sure that the first thread returns before.
    ///         thread::sleep(Duration::from_millis(10));
    ///         // This will be called, even though the JoinHandle is dropped.
    ///         println!("♫ Still alive ♫");
    ///     });
    /// });
    ///
    /// original_thread.join().expect("The thread being joined has panicked");
    /// println!("Original thread is joined.");
    ///
    /// // We make sure that the new thread has time to run, before the main
    /// // thread returns.
    ///
    /// thread::sleep(Duration::from_millis(1000));
    /// ```
    ///
    /// [`Clone`]: ../../std/clone/trait.Clone.html
    /// [`thread::spawn`]: fn.spawn.html
    /// [`thread::Builder::spawn`]: struct.Builder.html#method.spawn
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct JoinHandle<T>(JoinInner<T>);

    #[stable(feature = "joinhandle_impl_send_sync", since = "1.29.0")]
    unsafe impl<T> Send for JoinHandle<T> {}
    #[stable(feature = "joinhandle_impl_send_sync", since = "1.29.0")]
    unsafe impl<T> Sync for JoinHandle<T> {}

    impl<T> JoinHandle<T> {
        /// Extracts a handle to the underlying thread.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::thread;
        ///
        /// let builder = thread::Builder::new();
        ///
        /// let join_handle: thread::JoinHandle<_> = builder.spawn(|| {
        ///     // some work here
        /// }).unwrap();
        ///
        /// let thread = join_handle.thread();
        /// println!("thread id: {:?}", thread.id());
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn thread(&self) -> &Thread {
            &self.0.thread
        }

        /// Waits for the associated thread to finish.
        ///
        /// In terms of [atomic memory orderings],  the completion of the associated
        /// thread synchronizes with this function returning. In other words, all
        /// operations performed by that thread are ordered before all
        /// operations that happen after `join` returns.
        ///
        /// If the child thread panics, [`Err`] is returned with the parameter given
        /// to [`panic`].
        ///
        /// [`Err`]: ../../std/result/enum.Result.html#variant.Err
        /// [`panic`]: ../../std/macro.panic.html
        /// [atomic memory orderings]: ../../std/sync/atomic/index.html
        ///
        /// # Panics
        ///
        /// This function may panic on some platforms if a thread attempts to join
        /// itself or otherwise may create a deadlock with joining threads.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::thread;
        ///
        /// let builder = thread::Builder::new();
        ///
        /// let join_handle: thread::JoinHandle<_> = builder.spawn(|| {
        ///     // some work here
        /// }).unwrap();
        /// join_handle.join().expect("Couldn't join on the associated thread");
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn join(mut self) -> Result<T> {
            self.0.join()
        }
    }

    impl<T> AsInner<imp::Thread> for JoinHandle<T> {
        fn as_inner(&self) -> &imp::Thread { self.0.native.as_ref().unwrap() }
    }

    impl<T> IntoInner<imp::Thread> for JoinHandle<T> {
        fn into_inner(self) -> imp::Thread { self.0.native.unwrap() }
    }

    #[stable(feature = "std_debug", since = "1.16.0")]
    impl<T> fmt::Debug for JoinHandle<T> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            f.pad("JoinHandle { .. }")
        }
    }

    fn _assert_sync_and_send() {
        fn _assert_both<T: Send + Sync>() {}
        _assert_both::<JoinHandle<()>>();
        _assert_both::<Thread>();
    }

    ////////////////////////////////////////////////////////////////////////////////
    // Tests
    ////////////////////////////////////////////////////////////////////////////////

    #[cfg(all(test, not(target_os = "emscripten")))]
    mod tests {
        use any::Any;
        use sync::mpsc::{channel, Sender};
        use result;
        use super::{Builder};
        use thread;
        use time::Duration;
        use u32;

        // !!! These tests are dangerous. If something is buggy, they will hang, !!!
        // !!! instead of exiting cleanly. This might wedge the buildbots.       !!!

        #[test]
        fn test_unnamed_thread() {
            thread::spawn(move|| {
                assert!(thread::current().name().is_none());
            }).join().ok().unwrap();
        }

        #[test]
        fn test_named_thread() {
            Builder::new().name("ada lovelace".to_string()).spawn(move|| {
                assert!(thread::current().name().unwrap() == "ada lovelace".to_string());
            }).unwrap().join().unwrap();
        }

        #[test]
        #[should_panic]
        fn test_invalid_named_thread() {
            let _ = Builder::new().name("ada l\0velace".to_string()).spawn(|| {});
        }

        #[test]
        fn test_run_basic() {
            let (tx, rx) = channel();
            thread::spawn(move|| {
                tx.send(()).unwrap();
            });
            rx.recv().unwrap();
        }

        #[test]
        fn test_join_panic() {
            match thread::spawn(move|| {
                panic!()
            }).join() {
                result::Result::Err(_) => (),
                result::Result::Ok(()) => panic!()
            }
        }

        #[test]
        fn test_spawn_sched() {
            let (tx, rx) = channel();

            fn f(i: i32, tx: Sender<()>) {
                let tx = tx.clone();
                thread::spawn(move|| {
                    if i == 0 {
                        tx.send(()).unwrap();
                    } else {
                        f(i - 1, tx);
                    }
                });

            }
            f(10, tx);
            rx.recv().unwrap();
        }

        #[test]
        fn test_spawn_sched_childs_on_default_sched() {
            let (tx, rx) = channel();

            thread::spawn(move|| {
                thread::spawn(move|| {
                    tx.send(()).unwrap();
                });
            });

            rx.recv().unwrap();
        }

        fn avoid_copying_the_body<F>(spawnfn: F) where F: FnOnce(Box<dyn Fn() + Send>) {
            let (tx, rx) = channel();

            let x: Box<_> = box 1;
            let x_in_parent = (&*x) as *const i32 as usize;

            spawnfn(Box::new(move|| {
                let x_in_child = (&*x) as *const i32 as usize;
                tx.send(x_in_child).unwrap();
            }));

            let x_in_child = rx.recv().unwrap();
            assert_eq!(x_in_parent, x_in_child);
        }

        #[test]
        fn test_avoid_copying_the_body_spawn() {
            avoid_copying_the_body(|v| {
                thread::spawn(move || v());
            });
        }

        #[test]
        fn test_avoid_copying_the_body_thread_spawn() {
            avoid_copying_the_body(|f| {
                thread::spawn(move|| {
                    f();
                });
            })
        }

        #[test]
        fn test_avoid_copying_the_body_join() {
            avoid_copying_the_body(|f| {
                let _ = thread::spawn(move|| {
                    f()
                }).join();
            })
        }

        #[test]
        fn test_child_doesnt_ref_parent() {
            // If the child refcounts the parent thread, this will stack overflow when
            // climbing the thread tree to dereference each ancestor. (See #1789)
            // (well, it would if the constant were 8000+ - I lowered it to be more
            // valgrind-friendly. try this at home, instead..!)
            const GENERATIONS: u32 = 16;
            fn child_no(x: u32) -> Box<dyn Fn() + Send> {
                return Box::new(move|| {
                    if x < GENERATIONS {
                        thread::spawn(move|| child_no(x+1)());
                    }
                });
            }
            thread::spawn(|| child_no(0)());
        }

        #[test]
        fn test_simple_newsched_spawn() {
            thread::spawn(move || {});
        }

        #[test]
        fn test_try_panic_message_static_str() {
            match thread::spawn(move|| {
                panic!("static string");
            }).join() {
                Err(e) => {
                    type T = &'static str;
                    assert!(e.is::<T>());
                    assert_eq!(*e.downcast::<T>().unwrap(), "static string");
                }
                Ok(()) => panic!()
            }
        }

        #[test]
        fn test_try_panic_message_owned_str() {
            match thread::spawn(move|| {
                panic!("owned string".to_string());
            }).join() {
                Err(e) => {
                    type T = String;
                    assert!(e.is::<T>());
                    assert_eq!(*e.downcast::<T>().unwrap(), "owned string".to_string());
                }
                Ok(()) => panic!()
            }
        }

        #[test]
        fn test_try_panic_message_any() {
            match thread::spawn(move|| {
                panic!(box 413u16 as Box<dyn Any + Send>);
            }).join() {
                Err(e) => {
                    type T = Box<dyn Any + Send>;
                    assert!(e.is::<T>());
                    let any = e.downcast::<T>().unwrap();
                    assert!(any.is::<u16>());
                    assert_eq!(*any.downcast::<u16>().unwrap(), 413);
                }
                Ok(()) => panic!()
            }
        }

        #[test]
        fn test_try_panic_message_unit_struct() {
            struct Juju;

            match thread::spawn(move|| {
                panic!(Juju)
            }).join() {
                Err(ref e) if e.is::<Juju>() => {}
                Err(_) | Ok(()) => panic!()
            }
        }

        #[test]
        fn test_park_timeout_unpark_before() {
            for _ in 0..10 {
                thread::current().unpark();
                thread::park_timeout(Duration::from_millis(u32::MAX as u64));
            }
        }

        #[test]
        fn test_park_timeout_unpark_not_called() {
            for _ in 0..10 {
                thread::park_timeout(Duration::from_millis(10));
            }
        }

        #[test]
        fn test_park_timeout_unpark_called_other_thread() {
            for _ in 0..10 {
                let th = thread::current();

                let _guard = thread::spawn(move || {
                    super::sleep(Duration::from_millis(50));
                    th.unpark();
                });

                thread::park_timeout(Duration::from_millis(u32::MAX as u64));
            }
        }

        #[test]
        fn sleep_ms_smoke() {
            thread::sleep(Duration::from_millis(2));
        }

        #[test]
        fn test_thread_id_equal() {
            assert!(thread::current().id() == thread::current().id());
        }

        #[test]
        fn test_thread_id_not_equal() {
            let spawned_id = thread::spawn(|| thread::current().id()).join().unwrap();
            assert!(thread::current().id() != spawned_id);
        }

        // NOTE: the corresponding test for stderr is in run-pass/thread-stderr, due
        // to the test harness apparently interfering with stderr configuration.
    }

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