Trait tokio::io::AsyncWrite[][src]

pub trait AsyncWrite {
    fn poll_write(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        buf: &[u8]
    ) -> Poll<Result<usize, Error>>;
fn poll_flush(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>
    ) -> Poll<Result<(), Error>>;
fn poll_shutdown(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>
    ) -> Poll<Result<(), Error>>; fn poll_write_vectored(
        self: Pin<&mut Self>,
        cx: &mut Context<'_>,
        bufs: &[IoSlice<'_>]
    ) -> Poll<Result<usize, Error>> { ... }
fn is_write_vectored(&self) -> bool { ... } }
Expand description

Writes bytes asynchronously.

The trait inherits from std::io::Write and indicates that an I/O object is nonblocking. All non-blocking I/O objects must return an error when bytes cannot be written instead of blocking the current thread.

Specifically, this means that the poll_write function will return one of the following:

  • Poll::Ready(Ok(n)) means that n bytes of data was immediately written.

  • Poll::Pending means that no data was written from the buffer provided. The I/O object is not currently writable but may become writable in the future. Most importantly, the current future’s task is scheduled to get unparked when the object is writable. This means that like Future::poll you’ll receive a notification when the I/O object is writable again.

  • Poll::Ready(Err(e)) for other errors are standard I/O errors coming from the underlying object.

This trait importantly means that the write method only works in the context of a future’s task. The object may panic if used outside of a task.

Note that this trait also represents that the Write::flush method works very similarly to the write method, notably that Ok(()) means that the writer has successfully been flushed, a “would block” error means that the current task is ready to receive a notification when flushing can make more progress, and otherwise normal errors can happen as well.

Utilities for working with AsyncWrite values are provided by AsyncWriteExt.

Required methods

Attempt to write bytes from buf into the object.

On success, returns Poll::Ready(Ok(num_bytes_written)). If successful, then it must be guaranteed that n <= buf.len(). A return value of 0 typically means that the underlying object is no longer able to accept bytes and will likely not be able to in the future as well, or that the buffer provided is empty.

If the object is not ready for writing, the method returns Poll::Pending and arranges for the current task (via cx.waker()) to receive a notification when the object becomes writable or is closed.

Attempts to flush the object, ensuring that any buffered data reach their destination.

On success, returns Poll::Ready(Ok(())).

If flushing cannot immediately complete, this method returns Poll::Pending and arranges for the current task (via cx.waker()) to receive a notification when the object can make progress towards flushing.

Initiates or attempts to shut down this writer, returning success when the I/O connection has completely shut down.

This method is intended to be used for asynchronous shutdown of I/O connections. For example this is suitable for implementing shutdown of a TLS connection or calling TcpStream::shutdown on a proxied connection. Protocols sometimes need to flush out final pieces of data or otherwise perform a graceful shutdown handshake, reading/writing more data as appropriate. This method is the hook for such protocols to implement the graceful shutdown logic.

This shutdown method is required by implementers of the AsyncWrite trait. Wrappers typically just want to proxy this call through to the wrapped type, and base types will typically implement shutdown logic here or just return Ok(().into()). Note that if you’re wrapping an underlying AsyncWrite a call to shutdown implies that transitively the entire stream has been shut down. After your wrapper’s shutdown logic has been executed you should shut down the underlying stream.

Invocation of a shutdown implies an invocation of flush. Once this method returns Ready it implies that a flush successfully happened before the shutdown happened. That is, callers don’t need to call flush before calling shutdown. They can rely that by calling shutdown any pending buffered data will be written out.

Return value

This function returns a Poll<io::Result<()>> classified as such:

  • Poll::Ready(Ok(())) - indicates that the connection was successfully shut down and is now safe to deallocate/drop/close resources associated with it. This method means that the current task will no longer receive any notifications due to this method and the I/O object itself is likely no longer usable.

  • Poll::Pending - indicates that shutdown is initiated but could not complete just yet. This may mean that more I/O needs to happen to continue this shutdown operation. The current task is scheduled to receive a notification when it’s otherwise ready to continue the shutdown operation. When woken up this method should be called again.

  • Poll::Ready(Err(e)) - indicates a fatal error has happened with shutdown, indicating that the shutdown operation did not complete successfully. This typically means that the I/O object is no longer usable.


This function can return normal I/O errors through Err, described above. Additionally this method may also render the underlying Write::write method no longer usable (e.g. will return errors in the future). It’s recommended that once shutdown is called the write method is no longer called.


This function will panic if not called within the context of a future’s task.

Provided methods

Like poll_write, except that it writes from a slice of buffers.

Data is copied from each buffer in order, with the final buffer read from possibly being only partially consumed. This method must behave as a call to write with the buffers concatenated would.

The default implementation calls poll_write with either the first nonempty buffer provided, or an empty one if none exists.

On success, returns Poll::Ready(Ok(num_bytes_written)).

If the object is not ready for writing, the method returns Poll::Pending and arranges for the current task (via cx.waker()) to receive a notification when the object becomes writable or is closed.


This should be implemented as a single “atomic” write action. If any data has been partially written, it is wrong to return an error or pending.

Determines if this writer has an efficient poll_write_vectored implementation.

If a writer does not override the default poll_write_vectored implementation, code using it may want to avoid the method all together and coalesce writes into a single buffer for higher performance.

The default implementation returns false.

Implementations on Foreign Types