JoinHandle in std::thread - Rust
Description: An owned permission to join on a thread (block on its termination).

## JoinHandle

# Struct JoinHandle

1.0.0 · Source

    pub struct JoinHandle<T>(/* private fields */);

Expand description

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 the 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();

A thread being detached and outliving the thread that spawned it:

    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));

## Implementations§

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### impl<T> JoinHandle<T>

1.0.0 · Source

#### pub fn thread(&self) -> &Thread

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());

1.0.0 · Source

#### pub fn join(self) -> Result<T>

Waits for the associated thread to finish.

This function will return immediately if the associated thread has already finished.

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 happen before all operations that happen after `join` returns.

If the associated thread panics, `Err` is returned with the parameter given to `panic!` (though see the Notes below).

##### §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");

##### §Notes

If a “foreign” unwinding operation (e.g. an exception thrown from C++ code, or a `panic!` in Rust code compiled or linked with a different runtime) unwinds all the way to the thread root, the process may be aborted; see the Notes on `thread::spawn`. If the process is not aborted, this function will return a `Result::Err` containing an opaque type.

1.61.0 · Source

#### pub fn is_finished(&self) -> bool

Checks if the associated thread has finished running its main function.

`is_finished` supports implementing a non-blocking join operation, by checking `is_finished`, and calling `join` if it returns `true`. This function does not block. To block while waiting on the thread to finish, use `join`.

This might return `true` for a brief moment after the thread’s main function has returned, but before the thread itself has stopped running. However, once this returns `true`, `join` can be expected to return quickly, without blocking for any significant amount of time.

## Trait Implementations§

1.16.0 · Source§

### impl<T> Debug for JoinHandle<T>

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#### fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter. Read more
1.29.0 · Source§

### impl<T> Send for JoinHandle<T>

1.29.0 · Source§

### impl<T> Sync for JoinHandle<T>

## Auto Trait Implementations§

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### impl<T> Freeze for JoinHandle<T>

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### impl<T> !RefUnwindSafe for JoinHandle<T>

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### impl<T> Unpin for JoinHandle<T>

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### impl<T> !UnwindSafe for JoinHandle<T>

## Blanket Implementations§

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### impl<T> Any for T
where T: 'static + ?Sized,

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#### fn type_id(&self) -> TypeId

Gets the `TypeId` of `self`. Read more
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### impl<T> Borrow<T> for T
where T: ?Sized,

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#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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### impl<T> BorrowMut<T> for T
where T: ?Sized,

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#### fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
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### impl<T> From<T> for T

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#### fn from(t: T) -> T

Returns the argument unchanged.

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### impl<T, U> Into<U> for T
where U: From<T>,

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#### fn into(self) -> U

Calls `U::from(self)`.

That is, this conversion is whatever the implementation of `From<T> for U` chooses to do.

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### impl<T, U> TryFrom<U> for T
where U: Into<T>,

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#### type Error = Infallible

The type returned in the event of a conversion error.
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#### fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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### impl<T, U> TryInto<U> for T
where U: TryFrom<T>,

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#### type Error = <U as TryFrom<T>>::Error

The type returned in the event of a conversion error.
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#### fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.