# Trait std::io::BufRead

1.0.0 · source ·

    pub trait BufRead: Read {
        // Required methods
        fn fill_buf(&mut self) -> Result<&[u8]>;
        fn consume(&mut self, amt: usize);

        // Provided methods
        fn has_data_left(&mut self) -> Result<bool> { ... }
        fn read_until(&mut self, byte: u8, buf: &mut Vec<u8>) -> Result<usize> { ... }
        fn read_line(&mut self, buf: &mut String) -> Result<usize> { ... }
        fn split(self, byte: u8) -> Split<Self> ⓘ
           where Self: Sized { ... }
        fn lines(self) -> Lines<Self> ⓘ
           where Self: Sized { ... }
    }

Expand description

A `BufRead` is a type of `Read`er which has an internal buffer, allowing it to perform extra ways of reading.

For example, reading line-by-line is inefficient without using a buffer, so if you want to read by line, you’ll need `BufRead`, which includes a `read_line` method as well as a `lines` iterator.

## Examples

A locked standard input implements `BufRead`:

    use std::io;
    use std::io::prelude::*;

    let stdin = io::stdin();
    for line in stdin.lock().lines() {
        println!("{}", line.unwrap());
    }

Run

If you have something that implements `Read`, you can use the `BufReader` type to turn it into a `BufRead`.

For example, `File` implements `Read`, but not `BufRead`. `BufReader` to the rescue!

    use std::io::{self, BufReader};
    use std::io::prelude::*;
    use std::fs::File;

    fn main() -> io::Result<()> {
        let f = File::open("foo.txt")?;
        let f = BufReader::new(f);

        for line in f.lines() {
            println!("{}", line.unwrap());
        }

        Ok(())
    }

Run

## Required Methods§

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#### fn fill_buf(&mut self) -> Result<&[u8]>

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.

##### Errors

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

##### Examples

A locked standard input implements `BufRead`:

    use std::io;
    use std::io::prelude::*;

    let stdin = io::stdin();
    let mut stdin = stdin.lock();

    let buffer = stdin.fill_buf().unwrap();

    // work with buffer
    println!("{buffer:?}");

    // ensure the bytes we worked with aren't returned again later
    let length = buffer.len();
    stdin.consume(length);

Run
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#### fn consume(&mut self, amt: usize)

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 `<=` the number of bytes in the buffer returned by `fill_buf`.

##### Examples

Since `consume()` is meant to be used with `fill_buf`, that method’s example includes an example of `consume()`.

## Provided Methods§

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#### fn has_data_left(&mut self) -> Result<bool>

🔬This is a nightly-only experimental API. (`buf_read_has_data_left` #86423)

Check if the underlying `Read` has any data left to be read.

This function may fill the buffer to check for data, so this functions returns `Result<bool>`, not `bool`.

Default implementation calls `fill_buf` and checks that returned slice is empty (which means that there is no data left, since EOF is reached).

Examples

    #![feature(buf_read_has_data_left)]
    use std::io;
    use std::io::prelude::*;

    let stdin = io::stdin();
    let mut stdin = stdin.lock();

    while stdin.has_data_left().unwrap() {
        let mut line = String::new();
        stdin.read_line(&mut line).unwrap();
        // work with line
        println!("{line:?}");
    }

Run
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#### fn read_until(&mut self, byte: u8, buf: &mut Vec<u8>) -> Result<usize>

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

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.

This function is blocking and should be used carefully: it is possible for an attacker to continuously send bytes without ever sending the delimiter or 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.

##### 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 std::io::{self, BufRead};

    let mut cursor = io::Cursor::new(b"lorem-ipsum");
    let mut buf = vec![];

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

Run
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#### fn read_line(&mut self, buf: &mut String) -> Result<usize>

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

Previous content of the buffer will be preserved. To avoid appending to the buffer, you need to `clear` it first.

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.

This function is blocking and should be used carefully: it is possible for an attacker to continuously send bytes without ever sending a newline or EOF. You can use `take` to limit the maximum number of bytes read.

##### 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.

##### Examples

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

    use std::io::{self, BufRead};

    let mut cursor = io::Cursor::new(b"foo\nbar");
    let mut buf = String::new();

    // cursor is at 'f'
    let num_bytes = cursor.read_line(&mut buf)
        .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)
        .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)
        .expect("reading from cursor won't fail");
    assert_eq!(num_bytes, 0);
    assert_eq!(buf, "");

Run
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#### fn split(self, byte: u8) -> Split<Self> ⓘwhere Self: Sized,

Returns an iterator over the contents of this reader split on the byte `byte`.

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

This function will yield errors whenever `read_until` would have also yielded an error.

##### Examples

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

    use std::io::{self, BufRead};

    let cursor = io::Cursor::new(b"lorem-ipsum-dolor");

    let mut split_iter = cursor.split(b'-').map(|l| l.unwrap());
    assert_eq!(split_iter.next(), Some(b"lorem".to_vec()));
    assert_eq!(split_iter.next(), Some(b"ipsum".to_vec()));
    assert_eq!(split_iter.next(), Some(b"dolor".to_vec()));
    assert_eq!(split_iter.next(), None);

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

Returns an iterator over the lines of this reader.

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

##### 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 std::io::{self, BufRead};

    let cursor = io::Cursor::new(b"lorem\nipsum\r\ndolor");

    let mut lines_iter = cursor.lines().map(|l| l.unwrap());
    assert_eq!(lines_iter.next(), Some(String::from("lorem")));
    assert_eq!(lines_iter.next(), Some(String::from("ipsum")));
    assert_eq!(lines_iter.next(), Some(String::from("dolor")));
    assert_eq!(lines_iter.next(), None);

Run

##### Errors

Each line of the iterator has the same error semantics as `BufRead::read_line`.

## Implementors§

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### impl BufRead for &[u8]

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### impl BufRead for Empty

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### impl BufRead for StdinLock<'_>

1.75.0 · source§

### impl<A: Allocator> BufRead for VecDeque<u8, A>

BufRead is implemented for `VecDeque<u8>` by reading bytes from the front of the `VecDeque`.

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### impl<B: BufRead + ?Sized> BufRead for &mut B

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### impl<B: BufRead + ?Sized> BufRead for Box<B>

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### impl<R: ?Sized + Read> BufRead for BufReader<R>

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### impl<T> BufRead for Cursor<T>where T: AsRef<[u8]>,

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### impl<T: BufRead> BufRead for Take<T>

1.9.0 · source§

### impl<T: BufRead, U: BufRead> BufRead for Chain<T, U>