Trait tokio::io::AsyncRead

source · [−]
pub trait AsyncRead {
    fn poll_read(
        self: Pin<&mut Self>, 
        cx: &mut Context<'_>, 
        buf: &mut [u8]
    ) -> Poll<Result<usize>>;

    unsafe fn prepare_uninitialized_buffer(
        &self, 
        buf: &mut [MaybeUninit<u8>]
    ) -> bool { ... }

    fn poll_read_buf<B: BufMut>(
        self: Pin<&mut Self>, 
        cx: &mut Context<'_>, 
        buf: &mut B
    ) -> Poll<Result<usize>>
    where
        Self: Sized,
    { ... }
}
Expand description

Reads bytes from a source.

This trait is analogous to the std::io::Read trait, but integrates with the asynchronous task system. In particular, the poll_read method, unlike Read::read, will automatically queue the current task for wakeup and return if data is not yet available, rather than blocking the calling thread.

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

  • Poll::Ready(Ok(n)) means that n bytes of data was immediately read and placed into the output buffer, where n == 0 implies that EOF has been reached.

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

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

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

Utilities for working with AsyncRead values are provided by AsyncReadExt.

Required methods

Attempts to read from the AsyncRead into buf.

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

If no data is available for reading, the method returns Poll::Pending and arranges for the current task (via cx.waker()) to receive a notification when the object becomes readable or is closed.

Provided methods

Prepares an uninitialized buffer to be safe to pass to read. Returns true if the supplied buffer was zeroed out.

While it would be highly unusual, implementations of io::Read are able to read data from the buffer passed as an argument. Because of this, the buffer passed to io::Read must be initialized memory. In situations where large numbers of buffers are used, constantly having to zero out buffers can be expensive.

This function does any necessary work to prepare an uninitialized buffer to be safe to pass to read. If read guarantees to never attempt to read data out of the supplied buffer, then prepare_uninitialized_buffer doesn’t need to do any work.

If this function returns true, then the memory has been zeroed out. This allows implementations of AsyncRead which are composed of multiple subimplementations to efficiently implement prepare_uninitialized_buffer.

This function isn’t actually unsafe to call but unsafe to implement. The implementer must ensure that either the whole buf has been zeroed or poll_read_buf() overwrites the buffer without reading it and returns correct value.

This function is called from poll_read_buf.

Safety

Implementations that return false must never read from data slices that they did not write to.

Pulls some bytes from this source into the specified BufMut, returning how many bytes were read.

The buf provided will have bytes read into it and the internal cursor will be advanced if any bytes were read. Note that this method typically will not reallocate the buffer provided.

Implementations on Foreign Types

Implementors