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 Reader 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);