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

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    // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    //! 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](../../std/sync/mpsc/index.html), 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`](../../std/sync/struct.Arc.html).
    //!
    //! Fatal logic errors in Rust cause *thread panic*, during which
    //! a thread will unwind the stack, running destructors and freeing
    //! owned resources. Thread panic is unrecoverable from within
    //! the panicking thread (i.e. there is no 'try/catch' in Rust), but
    //! the panic may optionally be detected from a different thread. If
    //! the main thread panics, the application will exit with a non-zero
    //! exit code.
    //!
    //! When the main thread of a Rust program terminates, the entire program shuts
    //! down, even if other threads are still running. However, this module provides
    //! convenient facilities for automatically waiting for the termination of a
    //! child thread (i.e., join).
    //!
    //! ## Spawning a thread
    //!
    //! A new thread can be spawned using the `thread::spawn` function:
    //!
    //! ```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 `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` function, and
    //!   calling `thread()` on the `JoinHandle`.
    //! * By requesting the current thread, using the `thread::current` function.
    //!
    //! The `thread::current()` function is available even for threads not spawned
    //! by the APIs of this module.
    //!
    //! ## Blocking support: park and unpark
    //!
    //! Every thread is equipped with some basic low-level blocking support, via the
    //! `thread::park()` function and `thread::Thread::unpark()` method. `park()`
    //! blocks the current thread, which can then be resumed from another thread by
    //! calling the `unpark()` method on the blocked thread's handle.
    //!
    //! Conceptually, each `Thread` handle has an associated token, which is
    //! initially not present:
    //!
    //! * The `thread::park()` function blocks the current thread unless or until
    //!   the token is available for its thread handle, at which point it atomically
    //!   consumes the token. It may also return *spuriously*, without consuming the
    //!   token. `thread::park_timeout()` does the same, but allows specifying a
    //!   maximum time to block the thread for.
    //!
    //! * The `unpark()` method on a `Thread` atomically makes the token available
    //!   if it wasn't already.
    //!
    //! In other words, each `Thread` acts a bit like a semaphore with initial count
    //! 0, except that the semaphore is *saturating* (the count cannot go above 1),
    //! and can return spuriously.
    //!
    //! The API is typically used by acquiring a handle to the current thread,
    //! placing that handle in a shared data structure so that other threads can
    //! find it, and then `park`ing. When some desired condition is met, another
    //! thread calls `unpark` on the handle.
    //!
    //! The motivation for this design is twofold:
    //!
    //! * It avoids the need to allocate mutexes and condvars when building new
    //!   synchronization primitives; the threads already provide basic blocking/signaling.
    //!
    //! * It can be implemented very efficiently on many platforms.
    //!
    //! ## Thread-local storage
    //!
    //! This module also provides an implementation of thread-local storage for Rust
    //! programs. Thread-local storage is a method of storing data into a global
    //! variable that each thread in the program will have its own copy of.
    //! Threads do not share this data, so accesses do not need to be synchronized.
    //!
    //! A thread-local key owns the value it contains and will destroy the value when the
    //! thread exits. It is created with the [`thread_local!`] macro and can contain any
    //! value that is `'static` (no borrowed pointers). It provides an accessor function,
    //! [`with`], that yields a shared reference to the value to the specified
    //! closure. Thread-local keys allow only shared access to values, as there would be no
    //! way to guarantee uniqueness if mutable borrows were allowed. Most values
    //! will want to make use of some form of **interior mutability** through the
    //! [`Cell`] or [`RefCell`] types.
    //!
    //! [`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 cell::UnsafeCell;
    use ffi::{CStr, CString};
    use fmt;
    use io;
    use panic;
    use panicking;
    use str;
    use sync::{Mutex, Condvar, Arc};
    use sys::thread as imp;
    use sys_common::mutex;
    use sys_common::thread_info;
    use sys_common::util;
    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, LocalKeyState};

    // 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(target_thread_local)]
    #[doc(hidden)] pub use sys::fast_thread_local::Key as __FastLocalKeyInner;
    #[unstable(feature = "libstd_thread_internals", issue = "0")]
    #[doc(hidden)] pub use self::local::os::Key as __OsLocalKeyInner;

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

    /// Thread configuration. Provides detailed control over the properties
    /// and behavior of new threads.
    #[stable(feature = "rust1", since = "1.0.0")]
    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
        stack_size: Option<usize>,
    }

    impl Builder {
        /// Generates the base configuration for spawning a thread, from which
        /// configuration methods can be chained.
        #[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.
        ///
        /// # Examples
        ///
        /// ```rust
        /// 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();
        /// ```
        #[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 for the new thread.
        #[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, and returns a join handle for it.
        ///
        /// 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). The join handle can be used to block on
        /// termination of the child thread, including recovering its panics.
        ///
        /// # Errors
        ///
        /// Unlike the `spawn` free function, this method yields an
        /// `io::Result` to capture any failure to create the thread at
        /// the OS level.
        #[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
        {
            let Builder { name, stack_size } = self;

            let stack_size = stack_size.unwrap_or(util::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);
                }
                unsafe {
                    thread_info::set(imp::guard::current(), their_thread);
                    let try_result = panic::catch_unwind(panic::AssertUnwindSafe(f));
                    *their_packet.get() = Some(try_result);
                }
            };

            Ok(JoinHandle(JoinInner {
                native: unsafe {
                    Some(imp::Thread::new(stack_size, 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`.
    ///
    /// # Panics
    ///
    /// Panics if the OS fails to create a thread; use `Builder::spawn`
    /// to recover from such errors.
    #[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).unwrap()
    }

    /// 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.
    #[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.
    ///
    /// # Examples
    ///
    /// ```rust,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!()
    /// }
    /// ```
    #[inline]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub fn panicking() -> bool {
        panicking::panicking()
    }

    /// Puts the current thread to sleep for the specified amount of time.
    ///
    /// The thread may sleep longer than the duration specified due to scheduling
    /// specifics or platform-dependent functionality. Note that on unix platforms
    /// this function will not return early due to a signal being received or a
    /// spurious wakeup.
    #[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 the specified amount of time.
    ///
    /// The thread may sleep longer than the duration specified due to scheduling
    /// specifics or platform-dependent functionality.
    ///
    /// # Platform behavior
    ///
    /// On Unix platforms this function will not return early due to a
    /// signal being received or a spurious wakeup. 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
    ///
    /// ```rust,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)
    }

    /// Blocks unless or until the current thread's token is made available.
    ///
    /// Every thread is equipped with some basic low-level blocking support, via
    /// the `park()` function and the [`unpark()`][unpark] method. These can be
    /// used as a more CPU-efficient implementation of a spinlock.
    ///
    /// [unpark]: struct.Thread.html#method.unpark
    ///
    /// The API is typically used by acquiring a handle to the current thread,
    /// placing that handle in a shared data structure so that other threads can
    /// find it, and then parking (in a loop with a check for the token actually
    /// being acquired).
    ///
    /// A call to `park` does not guarantee that the thread will remain parked
    /// forever, and callers should be prepared for this possibility.
    ///
    /// See the [module documentation][thread] for more detail.
    ///
    /// [thread]: index.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();
        let mut guard = thread.inner.lock.lock().unwrap();
        while !*guard {
            guard = thread.inner.cvar.wait(guard).unwrap();
        }
        *guard = false;
    }

    /// 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 `ms`. 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 [module documentation][thread] for more detail.
    ///
    /// [thread]: index.html
    /// [park_timeout]: fn.park_timeout.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()` 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 module doc for more detail.
    ///
    /// # Platform 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.
    ///
    /// # Example
    ///
    /// 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();
    /// park_timeout(timeout);
    ///
    /// while beginning_park.elapsed() < timeout {
    ///     println!("restarting park_timeout after {:?}", beginning_park.elapsed());
    ///     let timeout = timeout - beginning_park.elapsed();
    ///     park_timeout(timeout);
    /// }
    /// ```
    #[stable(feature = "park_timeout", since = "1.4.0")]
    pub fn park_timeout(dur: Duration) {
        let thread = current();
        let mut guard = thread.inner.lock.lock().unwrap();
        if !*guard {
            let (g, _) = thread.inner.cvar.wait_timeout(guard, dur).unwrap();
            guard = g;
        }
        *guard = false;
    }

    ////////////////////////////////////////////////////////////////////////////////
    // 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 do not correspond to a thread's system-
    /// designated identifier.
    #[unstable(feature = "thread_id", issue = "21507")]
    #[derive(Eq, PartialEq, Copy, Clone)]
    pub struct ThreadId(u64);

    impl ThreadId {
        // Generate a new unique thread ID.
        fn new() -> ThreadId {
            static GUARD: mutex::Mutex = mutex::Mutex::new();
            static mut COUNTER: u64 = 0;

            unsafe {
                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 {
                    GUARD.unlock();
                    panic!("failed to generate unique thread ID: bitspace exhausted");
                }

                let id = COUNTER;
                COUNTER += 1;

                GUARD.unlock();

                ThreadId(id)
            }
        }
    }

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

    /// The internal representation of a `Thread` handle
    struct Inner {
        name: Option<CString>,      // Guaranteed to be UTF-8
        id: ThreadId,
        lock: Mutex<bool>,          // true when there is a buffered unpark
        cvar: Condvar,
    }

    #[derive(Clone)]
    #[stable(feature = "rust1", since = "1.0.0")]
    /// A handle to a thread.
    pub struct Thread {
        inner: Arc<Inner>,
    }

    impl Thread {
        // Used only internally to construct a thread object without spawning
        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(),
                    lock: Mutex::new(false),
                    cvar: Condvar::new(),
                })
            }
        }

        /// Atomically makes the handle's token available if it is not already.
        ///
        /// See the module doc for more detail.
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn unpark(&self) {
            let mut guard = self.inner.lock.lock().unwrap();
            if !*guard {
                *guard = true;
                self.inner.cvar.notify_one();
            }
        }

        /// Gets the thread's unique identifier.
        #[unstable(feature = "thread_id", issue = "21507")]
        pub fn id(&self) -> ThreadId {
            self.inner.id
        }

        /// Gets the thread's name.
        ///
        /// # 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();
        /// ```
        #[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)
        }
    }

    // a hack to get around privacy restrictions
    impl thread_info::NewThread for Thread {
        fn new(name: Option<String>) -> Thread { Thread::new(name) }
    }

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

    /// Indicates the manner in which a thread exited.
    ///
    /// A thread that completes without panicking is considered to exit successfully.
    #[stable(feature = "rust1", since = "1.0.0")]
    pub type Result<T> = ::result::Result<T, Box<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 child thread when it is dropped.
    ///
    /// Due to platform restrictions, it is not possible to `Clone` this
    /// handle: the ability to join a child 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`]:
    ///
    /// ```rust
    /// use std::thread;
    ///
    /// let join_handle: thread::JoinHandle<_> = thread::spawn(|| {
    ///     // some work here
    /// });
    /// ```
    ///
    /// Creation from [`thread::Builder::spawn`]:
    ///
    /// ```rust
    /// use std::thread;
    ///
    /// let builder = thread::Builder::new();
    ///
    /// let join_handle: thread::JoinHandle<_> = builder.spawn(|| {
    ///     // some work here
    /// }).unwrap();
    /// ```
    ///
    /// [`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>);

    impl<T> JoinHandle<T> {
        /// Extracts a handle to the underlying thread
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn thread(&self) -> &Thread {
            &self.0.thread
        }

        /// Waits for the associated thread to finish.
        ///
        /// If the child thread panics, `Err` is returned with the parameter given
        /// to `panic`.
        #[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() }
    }

    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<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<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<Any + Send>);
            }).join() {
                Err(e) => {
                    type T = Box<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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