# 1.0.0[−][src]Struct std::io::BufReader

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

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 `TcpStream` results 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``<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.

# Examples

    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

## Implementations

### `impl<R: Read> BufReader<R>`[src]

#### `pub fn new(inner: R) -> BufReader<R>ⓘ

### Notable traits for BufReader<R>

`impl<R: Read> Read for BufReader<R>`
`[src]

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

# Examples

    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

#### `pub fn with_capacity(capacity: usize, inner: R) -> BufReader<R>ⓘ

### Notable traits for BufReader<R>

`impl<R: Read> Read for BufReader<R>`
`[src]

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

# Examples

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

### `impl<R> BufReader<R>`[src]

#### `pub fn get_ref(&self) -> &Rⓘ

### Notable traits for &'_ mut F

`impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;impl<'_, I> Iterator for &'_ mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<R: Read + ?Sized, '_> Read for &'_ mut Rimpl<W: Write + ?Sized, '_> Write for &'_ mut W`
`[src]

Gets a reference to the underlying reader.

It is inadvisable to directly read from the underlying reader.

# Examples

    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

#### `pub fn get_mut(&mut self) -> &mut Rⓘ

### Notable traits for &'_ mut F

`impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;impl<'_, I> Iterator for &'_ mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<R: Read + ?Sized, '_> Read for &'_ mut Rimpl<W: Write + ?Sized, '_> Write for &'_ mut W`
`[src]

Gets a mutable reference to the underlying reader.

It is inadvisable to directly read from the underlying reader.

# Examples

    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

#### `pub fn buffer(&self) -> &[u8]ⓘ

### Notable traits for &'_ [u8]

`impl<'_> Read for &'_ [u8]impl<'_> Write for &'_ mut [u8]`
`1.37.0[src]

Returns a reference to the internally buffered data.

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

# Examples

    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

#### `pub fn capacity(&self) -> usize` 1.46.0[src]

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

# Examples

    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

#### `pub fn into_inner(self) -> R`[src]

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.

# Examples

    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

### `impl<R: Seek> BufReader<R>`[src]

#### `pub fn seek_relative(&mut self, offset: i64) -> Result<()>`[src]

🔬 This is a nightly-only experimental API. (`bufreader_seek_relative` #31100)

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.

## Trait Implementations

### `impl<R: Read> BufRead for BufReader<R>`[src]

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

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

#### `pub fn consume(&mut self, amt: usize)`[src]

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

#### `pub fn read_until(&mut self, byte: u8, buf: &mut Vec<u8>) -> Result<usize>`[src]

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

#### `pub fn read_line(&mut self, buf: &mut String) -> Result<usize>`[src]

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

#### `pub fn split(self, byte: u8) -> Split<Self>ⓘ

### Notable traits for Split<B>

`impl<B: BufRead> Iterator for Split<B> type Item = Result<Vec<u8>>;`
where
Self: Sized,` [src]

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

#### `pub fn lines(self) -> Lines<Self>ⓘ

### Notable traits for Lines<B>

`impl<B: BufRead> Iterator for Lines<B> type Item = Result<String>;`
where
Self: Sized,` [src]

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

### `impl<R> Debug for BufReader<R> where
R: Debug,` [src]

#### `pub fn fmt(&self, fmt: &mut Formatter<'_>) -> Result`[src]

Formats the value using the given formatter. Read more

### `impl<R: Read> Read for BufReader<R>`[src]

#### `pub fn read(&mut self, buf: &mut [u8]) -> Result<usize>`[src]

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

#### `pub fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> Result<usize>`[src]

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

#### `pub fn is_read_vectored(&self) -> bool`[src]

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

Determines if this `Read`er has an efficient `read_vectored` implementation. Read more

#### `pub unsafe fn initializer(&self) -> Initializer`[src]

🔬 This is a nightly-only experimental API. (`read_initializer` #42788)

Determines if this `Read`er can work with buffers of uninitialized memory. Read more

#### `pub fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize>`[src]

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

#### `pub fn read_to_string(&mut self, buf: &mut String) -> Result<usize>`[src]

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

#### `pub fn read_exact(&mut self, buf: &mut [u8]) -> Result<()>`1.6.0[src]

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

#### `pub fn by_ref(&mut self) -> &mut Selfⓘ

### Notable traits for &'_ mut F

`impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;impl<'_, I> Iterator for &'_ mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<R: Read + ?Sized, '_> Read for &'_ mut Rimpl<W: Write + ?Sized, '_> Write for &'_ mut W`
where
Self: Sized,` [src]

Creates a "by reference" adaptor for this instance of `Read`. Read more

#### `pub fn bytes(self) -> Bytes<Self>ⓘ

### Notable traits for Bytes<R>

`impl<R: Read> Iterator for Bytes<R> type Item = Result<u8>;`
where
Self: Sized,` [src]

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

#### `pub fn chain<R: Read>(self, next: R) -> Chain<Self, R>ⓘ

### Notable traits for Chain<T, U>

`impl<T: Read, U: Read> Read for Chain<T, U>`
where
Self: Sized,` [src]

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

#### `pub fn take(self, limit: u64) -> Take<Self>ⓘ

### Notable traits for Take<T>

`impl<T: Read> Read for Take<T>`
where
Self: Sized,` [src]

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

### `impl<R: Seek> Seek for BufReader<R>`[src]

#### `pub fn seek(&mut self, pos: SeekFrom) -> Result<u64>`[src]

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

The position used for seeking with `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 calling `BufReader::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``(n)` where `n` minus the internal buffer length overflows an `i64`, two seeks will be performed instead of one. If the second seek returns `Err`, the underlying reader will be left at the same position it would have if you called `seek` with `SeekFrom::Current``(0)`.

#### `pub fn stream_position(&mut self) -> Result<u64>`[src]

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

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. Use `BufReader::seek` instead if you require that guarantee.

# Panics

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.

# Example

    #![feature(seek_convenience)]
    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

#### `pub fn stream_len(&mut self) -> Result<u64>`[src]

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

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

## Auto Trait Implementations

### `impl<R> RefUnwindSafe for BufReader<R> where
R: RefUnwindSafe,`

### `impl<R> Send for BufReader<R> where
R: Send,`

### `impl<R> Sync for BufReader<R> where
R: Sync,`

### `impl<R> Unpin for BufReader<R> where
R: Unpin,`

### `impl<R> UnwindSafe for BufReader<R> where
R: UnwindSafe,`

## Blanket Implementations

### `impl<T> Any for T where
T: 'static + ?Sized,` [src]

#### `pub fn type_id(&self) -> TypeId`[src]

Gets the `TypeId` of `self`. Read more

### `impl<T> Borrow<T> for T where
T: ?Sized,` [src]

#### `pub fn borrow(&self) -> &Tⓘ

### Notable traits for &'_ mut F

`impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;impl<'_, I> Iterator for &'_ mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<R: Read + ?Sized, '_> Read for &'_ mut Rimpl<W: Write + ?Sized, '_> Write for &'_ mut W`
`[src]

Immutably borrows from an owned value. Read more

### `impl<T> BorrowMut<T> for T where
T: ?Sized,` [src]

#### `pub fn borrow_mut(&mut self) -> &mut Tⓘ

### Notable traits for &'_ mut F

`impl<'_, F> Future for &'_ mut F where
F: Unpin + Future + ?Sized, type Output = <F as Future>::Output;impl<'_, I> Iterator for &'_ mut I where
I: Iterator + ?Sized, type Item = <I as Iterator>::Item;impl<R: Read + ?Sized, '_> Read for &'_ mut Rimpl<W: Write + ?Sized, '_> Write for &'_ mut W`
`[src]

Mutably borrows from an owned value. Read more

### `impl<T> From<T> for T`[src]

#### `pub fn from(t: T) -> T`[src]

Performs the conversion.

### `impl<T, U> Into<U> for T where
U: From<T>,` [src]

#### `pub fn into(self) -> U`[src]

Performs the conversion.

### `impl<T, U> TryFrom<U> for T where
U: Into<T>,` [src]

#### `type Error = Infallible`

The type returned in the event of a conversion error.

#### `pub fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>`[src]

Performs the conversion.

### `impl<T, U> TryInto<U> for T where
U: TryFrom<T>,` [src]

#### `type Error = <U as TryFrom<T>>::Error`

The type returned in the event of a conversion error.

#### `pub fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>`[src]

Performs the conversion.