Trait tokio::io::AsyncBufReadExt

pub trait AsyncBufReadExt: AsyncBufRead {
    // Provided methods
    fn read_until<'a>(
        &'a mut self,
        byte: u8,
        buf: &'a mut Vec<u8>
    ) -> ReadUntil<'a, Self>
       where Self: Unpin { ... }
    fn read_line<'a>(&'a mut self, buf: &'a mut String) -> ReadLine<'a, Self>
       where Self: Unpin { ... }
    fn split(self, byte: u8) -> Split<Self>
       where Self: Sized + Unpin { ... }
    fn fill_buf(&mut self) -> FillBuf<'_, Self>
       where Self: Unpin { ... }
    fn consume(&mut self, amt: usize)
       where Self: Unpin { ... }
    fn lines(self) -> Lines<Self>
       where Self: Sized { ... }
}

An extension trait which adds utility methods to AsyncBufRead types.

Provided Methods

fn read_until<'a>( &'a mut self, byte: u8, buf: &'a mut Vec<u8> ) -> ReadUntil<'a, Self>where Self: Unpin,

Reads all bytes into buf until the delimiter byte or EOF is reached.

Equivalent to:

async fn read_until(&mut self, byte: u8, buf: &mut Vec<u8>) -> io::Result<usize>;

This function will read bytes from the underlying stream until the delimiter or EOF is found. Once found, all bytes up to, and including, the delimiter (if found) will be appended to buf.

If successful, this function will return the total number of bytes read.

If this function returns Ok(0), the stream has reached EOF.

Errors

This function will ignore all instances of ErrorKind::Interrupted and will otherwise return any errors returned by fill_buf.

If an I/O error is encountered then all bytes read so far will be present in buf and its length will have been adjusted appropriately.

Cancel safety

If the method is used as the event in a tokio::select! statement and some other branch completes first, then some data may have been partially read. Any partially read bytes are appended to buf, and the method can be called again to continue reading until byte.

This method returns the total number of bytes read. If you cancel the call to read_until and then call it again to continue reading, the counter is reset.

Examples

std::io::Cursor is a type that implements BufRead. In this example, we use Cursor to read all the bytes in a byte slice in hyphen delimited segments:

use tokio::io::AsyncBufReadExt;

use std::io::Cursor;

#[tokio::main]
async fn main() {
    let mut cursor = Cursor::new(b"lorem-ipsum");
    let mut buf = vec![];

    // cursor is at 'l'
    let num_bytes = cursor.read_until(b'-', &mut buf)
        .await
        .expect("reading from cursor won't fail");

    assert_eq!(num_bytes, 6);
    assert_eq!(buf, b"lorem-");
    buf.clear();

    // cursor is at 'i'
    let num_bytes = cursor.read_until(b'-', &mut buf)
        .await
        .expect("reading from cursor won't fail");

    assert_eq!(num_bytes, 5);
    assert_eq!(buf, b"ipsum");
    buf.clear();

    // cursor is at EOF
    let num_bytes = cursor.read_until(b'-', &mut buf)
        .await
        .expect("reading from cursor won't fail");
    assert_eq!(num_bytes, 0);
    assert_eq!(buf, b"");
}

fn read_line<'a>(&'a mut self, buf: &'a mut String) -> ReadLine<'a, Self>where Self: Unpin,

Reads all bytes until a newline (the 0xA byte) is reached, and append them to the provided buffer.

Equivalent to:

async fn read_line(&mut self, buf: &mut String) -> io::Result<usize>;

This function will read bytes from the underlying stream until the newline delimiter (the 0xA byte) or EOF is found. Once found, all bytes up to, and including, the delimiter (if found) will be appended to buf.

If successful, this function will return the total number of bytes read.

If this function returns Ok(0), the stream has reached EOF.

Errors

This function has the same error semantics as read_until and will also return an error if the read bytes are not valid UTF-8. If an I/O error is encountered then buf may contain some bytes already read in the event that all data read so far was valid UTF-8.

Cancel safety

This method is not cancellation safe. If the method is used as the event in a tokio::select! statement and some other branch completes first, then some data may have been partially read, and this data is lost. There are no guarantees regarding the contents of buf when the call is cancelled. The current implementation replaces buf with the empty string, but this may change in the future.

This function does not behave like read_until because of the requirement that a string contains only valid utf-8. If you need a cancellation safe read_line, there are three options:

• Call read_until with a newline character and manually perform the utf-8 check.
• The stream returned by lines has a cancellation safe next_line method.
• Use tokio_util::codec::LinesCodec.

Examples

std::io::Cursor is a type that implements AsyncBufRead. In this example, we use Cursor to read all the lines in a byte slice:

use tokio::io::AsyncBufReadExt;

use std::io::Cursor;

#[tokio::main]
async fn main() {
    let mut cursor = Cursor::new(b"foo\nbar");
    let mut buf = String::new();

    // cursor is at 'f'
    let num_bytes = cursor.read_line(&mut buf)
        .await
        .expect("reading from cursor won't fail");

    assert_eq!(num_bytes, 4);
    assert_eq!(buf, "foo\n");
    buf.clear();

    // cursor is at 'b'
    let num_bytes = cursor.read_line(&mut buf)
        .await
        .expect("reading from cursor won't fail");

    assert_eq!(num_bytes, 3);
    assert_eq!(buf, "bar");
    buf.clear();

    // cursor is at EOF
    let num_bytes = cursor.read_line(&mut buf)
        .await
        .expect("reading from cursor won't fail");

    assert_eq!(num_bytes, 0);
    assert_eq!(buf, "");
}

fn split(self, byte: u8) -> Split<Self>where Self: Sized + Unpin,

Returns a stream of the contents of this reader split on the byte byte.

This method is the asynchronous equivalent to BufRead::split.

The stream returned from this function will yield instances of io::Result<Option<Vec<u8>>>. Each vector returned will not have the delimiter byte at the end.

Errors

Each item of the stream has the same error semantics as AsyncBufReadExt::read_until.

Examples

use tokio::io::AsyncBufReadExt;

let mut segments = my_buf_read.split(b'f');

while let Some(segment) = segments.next_segment().await? {
    println!("length = {}", segment.len())
}

fn fill_buf(&mut self) -> FillBuf<'_, Self>where Self: Unpin,

Returns the contents of the internal buffer, filling it with more data from the inner reader if it is empty.

This function is a lower-level call. It needs to be paired with the consume method to function properly. When calling this method, none of the contents will be “read” in the sense that later calling read may return the same contents. As such, consume must be called with the number of bytes that are consumed from this buffer to ensure that the bytes are never returned twice.

An empty buffer returned indicates that the stream has reached EOF.

Equivalent to:

async fn fill_buf(&mut self) -> io::Result<&[u8]>;

Errors

This function will return an I/O error if the underlying reader was read, but returned an error.

fn consume(&mut self, amt: usize)where Self: Unpin,

Tells this buffer that amt bytes have been consumed from the buffer, so they should no longer be returned in calls to read.

This function is a lower-level call. It needs to be paired with the fill_buf method to function properly. This function does not perform any I/O, it simply informs this object that some amount of its buffer, returned from fill_buf, has been consumed and should no longer be returned. As such, this function may do odd things if fill_buf isn’t called before calling it.

The amt must be less than the number of bytes in the buffer returned by fill_buf.

fn lines(self) -> Lines<Self>where Self: Sized,

Returns a stream over the lines of this reader. This method is the async equivalent to BufRead::lines.

The stream returned from this function will yield instances of io::Result<Option<String>>. Each string returned will not have a newline byte (the 0xA byte) or CRLF (0xD, 0xA bytes) at the end.

Errors

Each line of the stream has the same error semantics as AsyncBufReadExt::read_line.

Examples

std::io::Cursor is a type that implements BufRead. In this example, we use Cursor to iterate over all the lines in a byte slice.

use tokio::io::AsyncBufReadExt;

use std::io::Cursor;

#[tokio::main]
async fn main() {
    let cursor = Cursor::new(b"lorem\nipsum\r\ndolor");

    let mut lines = cursor.lines();

    assert_eq!(lines.next_line().await.unwrap(), Some(String::from("lorem")));
    assert_eq!(lines.next_line().await.unwrap(), Some(String::from("ipsum")));
    assert_eq!(lines.next_line().await.unwrap(), Some(String::from("dolor")));
    assert_eq!(lines.next_line().await.unwrap(), None);
}

Object Safety

This trait is not object safe.

Implementors

impl<R: AsyncBufRead + ?Sized> AsyncBufReadExt for R