BufReader in std::io - Rust (original) (raw)

pub struct BufReader<R> { /* private fields */ }

Expand description

The BufReader<R> struct adds buffering to any reader.

It can be excessively inefficient to work directly with a Read instance. For example, every call to read on TcpStreamresults in a system call. A BufReader<R> performs large, infrequent reads on the underlying Read and maintains an in-memory buffer of the results.

BufReader<R> can improve the speed of programs that make small and_repeated_ read calls to the same file or network socket. It does not help when reading very large amounts at once, or reading just one or a few times. It also provides no advantage when reading from a source that is already in memory, like a [Vec](../vec/struct.Vec.html "Vec")<u8>.

When the BufReader<R> is dropped, the contents of its buffer will be discarded. Creating multiple instances of a BufReader<R> on the same stream can cause data loss. Reading from the underlying reader after unwrapping the BufReader<R> with BufReader::into_inner can also cause data loss.

use std::io::prelude::*;
use std::io::BufReader;
use std::fs::File;

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

    let mut line = String::new();
    let len = reader.read_line(&mut line)?;
    println!("First line is {} bytes long", len);
    Ok(())
}

Run

Creates a new BufReader<R> with a default buffer capacity. The default is currently 8 KB, but may change in the future.

use std::io::BufReader;
use std::fs::File;

fn main() -> std::io::Result<()> {
    let f = File::open("log.txt")?;
    let reader = BufReader::new(f);
    Ok(())
}

Run

Creates a new BufReader<R> with the specified buffer capacity.

Creating a buffer with ten bytes of capacity:

use std::io::BufReader;
use std::fs::File;

fn main() -> std::io::Result<()> {
    let f = File::open("log.txt")?;
    let reader = BufReader::with_capacity(10, f);
    Ok(())
}

Run

Gets a reference to the underlying reader.

It is inadvisable to directly read from the underlying reader.

use std::io::BufReader;
use std::fs::File;

fn main() -> std::io::Result<()> {
    let f1 = File::open("log.txt")?;
    let reader = BufReader::new(f1);

    let f2 = reader.get_ref();
    Ok(())
}

Run

Gets a mutable reference to the underlying reader.

It is inadvisable to directly read from the underlying reader.

use std::io::BufReader;
use std::fs::File;

fn main() -> std::io::Result<()> {
    let f1 = File::open("log.txt")?;
    let mut reader = BufReader::new(f1);

    let f2 = reader.get_mut();
    Ok(())
}

Run

Returns a reference to the internally buffered data.

Unlike fill_buf, this will not attempt to fill the buffer if it is empty.

use std::io::{BufReader, BufRead};
use std::fs::File;

fn main() -> std::io::Result<()> {
    let f = File::open("log.txt")?;
    let mut reader = BufReader::new(f);
    assert!(reader.buffer().is_empty());

    if reader.fill_buf()?.len() > 0 {
        assert!(!reader.buffer().is_empty());
    }
    Ok(())
}

Run

Returns the number of bytes the internal buffer can hold at once.

use std::io::{BufReader, BufRead};
use std::fs::File;

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

    let capacity = reader.capacity();
    let buffer = reader.fill_buf()?;
    assert!(buffer.len() <= capacity);
    Ok(())
}

Run

Unwraps this BufReader<R>, returning the underlying reader.

Note that any leftover data in the internal buffer is lost. Therefore, a following read from the underlying reader may lead to data loss.

use std::io::BufReader;
use std::fs::File;

fn main() -> std::io::Result<()> {
    let f1 = File::open("log.txt")?;
    let reader = BufReader::new(f1);

    let f2 = reader.into_inner();
    Ok(())
}

Run

Seeks relative to the current position. If the new position lies within the buffer, the buffer will not be flushed, allowing for more efficient seeks. This method does not return the location of the underlying reader, so the caller must track this information themselves if it is required.

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

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

🔬 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. Read more

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

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

Returns an iterator over the contents of this reader split on the bytebyte. Read more

Returns an iterator over the lines of this reader. Read more

Formats the value using the given formatter. Read more

Pull some bytes from this source into the specified buffer, returning how many bytes were read. Read more

🔬 This is a nightly-only experimental API. (read_buf #78485)

Pull some bytes from this source into the specified buffer. Read more

Read the exact number of bytes required to fill buf. Read more

Like read, except that it reads into a slice of buffers. Read more

🔬 This is a nightly-only experimental API. (can_vector #69941)

Determines if this Reader has an efficient read_vectoredimplementation. Read more

Read all bytes until EOF in this source, placing them into buf. Read more

Read all bytes until EOF in this source, appending them to buf. Read more

🔬 This is a nightly-only experimental API. (read_buf #78485)

Read the exact number of bytes required to fill buf. Read more

Creates a “by reference” adaptor for this instance of Read. Read more

Transforms this Read instance to an Iterator over its bytes. Read more

Creates an adapter which will chain this stream with another. Read more

Creates an adapter which will read at most limit bytes from it. Read more

Seek to an offset, in bytes, in the underlying reader.

The position used for seeking with [SeekFrom::Current](enum.SeekFrom.html#variant.Current "SeekFrom::Current")(_) is the position the underlying reader would be at if the BufReader<R> had no internal buffer.

Seeking always discards the internal buffer, even if the seek position would otherwise fall within it. This guarantees that callingBufReader::into_inner() immediately after a seek yields the underlying reader at the same position.

To seek without discarding the internal buffer, use BufReader::seek_relative.

See std::io::Seek for more details.

Note: In the edge case where you’re seeking with [SeekFrom::Current](enum.SeekFrom.html#variant.Current "SeekFrom::Current")(n)where n minus the internal buffer length overflows an i64, two seeks will be performed instead of one. If the second seek returnsErr, the underlying reader will be left at the same position it would have if you called seek with [SeekFrom::Current](enum.SeekFrom.html#variant.Current "SeekFrom::Current")(0).

Returns the current seek position from the start of the stream.

The value returned is equivalent to self.seek(SeekFrom::Current(0))but does not flush the internal buffer. Due to this optimization the function does not guarantee that calling .into_inner() immediately afterwards will yield the underlying reader at the same position. UseBufReader::seek instead if you require that guarantee.

This function will panic if the position of the inner reader is smaller than the amount of buffered data. That can happen if the inner reader has an incorrect implementation of Seek::stream_position, or if the position has gone out of sync due to calling Seek::seek directly on the underlying reader.

use std::{
    io::{self, BufRead, BufReader, Seek},
    fs::File,
};

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

    let before = f.stream_position()?;
    f.read_line(&mut String::new())?;
    let after = f.stream_position()?;

    println!("The first line was {} bytes long", after - before);
    Ok(())
}

Run

Rewind to the beginning of a stream. Read more

🔬 This is a nightly-only experimental API. (seek_stream_len #59359)

Returns the length of this stream (in bytes). Read more

impl Any for T where

T: 'static + ?Sized,

Immutably borrows from an owned value. Read more

Mutably borrows from an owned value. Read more

impl From for T

impl<T, U> Into for T where

U: From,

The type returned in the event of a conversion error.

Performs the conversion.

The type returned in the event of a conversion error.

Performs the conversion.