# Trait BufRead

1.0.0 · Source

    pub trait BufRead: Read {
        // Required methods
        fn fill_buf(&mut self) -> Result<&[u8]>;
        fn consume(&mut self, amount: 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 skip_until(&mut self, byte: 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?);
    }

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() {
            let line = line?;
            println!("{line}");
        }

        Ok(())
    }

## Required Methods§

1.0.0 · Source

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

Returns the contents of the internal buffer, filling it with more data, via `Read` methods, if empty.

This is a lower-level method and is meant to be used together with `consume`, which can be used to mark bytes that should not be returned by subsequent calls to `read`.

Returns an empty buffer when the stream has reached EOF.

##### §Errors

This function will return an I/O error if a `Read` method was called, 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()?;

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

    // mark the bytes we worked with as read
    let length = buffer.len();
    stdin.consume(length);

1.0.0 · Source

#### fn consume(&mut self, amount: usize)

Marks the given `amount` of additional bytes from the internal buffer as having been read. Subsequent calls to `read` only return bytes that have not been marked as read.

This is a lower-level method and is meant to be used together with `fill_buf`, which can be used to fill the internal buffer via `Read` methods.

It is a logic error if `amount` exceeds the number of unread bytes in the internal buffer, which is 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§

Source

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

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

Checks if there is any data left to be `read`.

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

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

##### §Errors

This function will return an I/O error if a `Read` method was called, but returned an error.

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

1.0.0 · Source

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

Reads 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"");

1.83.0 · Source

#### fn skip_until(&mut self, byte: u8) -> Result<usize>

Skips all bytes until the delimiter `byte` or EOF is reached.

This function will read (and discard) bytes from the underlying stream until the delimiter or EOF is found.

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

This is useful for efficiently skipping data such as NUL-terminated strings in binary file formats without buffering.

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 some NUL-terminated information about Ferris from a binary string, skipping the fun fact:

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

    let mut cursor = io::Cursor::new(b"Ferris\0Likes long walks on the beach\0Crustacean\0");

    // read name
    let mut name = Vec::new();
    let num_bytes = cursor.read_until(b'\0', &mut name)
        .expect("reading from cursor won't fail");
    assert_eq!(num_bytes, 7);
    assert_eq!(name, b"Ferris\0");

    // skip fun fact
    let num_bytes = cursor.skip_until(b'\0')
        .expect("reading from cursor won't fail");
    assert_eq!(num_bytes, 30);

    // read animal type
    let mut animal = Vec::new();
    let num_bytes = cursor.read_until(b'\0', &mut animal)
        .expect("reading from cursor won't fail");
    assert_eq!(num_bytes, 11);
    assert_eq!(animal, b"Crustacean\0");

1.0.0 · Source

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

Reads 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, "");

1.0.0 · Source

#### 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);

1.0.0 · Source

#### 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);

##### §Errors

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

## Implementors§

1.0.0 · Source§

### impl BufRead for &[u8]

1.0.0 · Source§

### impl BufRead for Empty

1.0.0 · Source§

### 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`.

1.0.0 · Source§

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

1.0.0 · Source§

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

1.0.0 · Source§

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

1.0.0 · Source§

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

1.0.0 · Source§

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

1.9.0 · Source§

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