# Struct std::io::BufReader

1.0.0 · source ·

    pub struct BufReader<R: ?Sized> { /* 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 `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 {len} bytes long");
        Ok(())
    }

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## Implementations§

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### impl<R: Read> BufReader<R>

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

Creates a new `BufReader<R>` with a default buffer capacity. The default is currently 8 KiB, 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(())
    }

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

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(())
    }

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### impl<R: ?Sized> BufReader<R>

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#### pub fn get_ref(&self) -> &R

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(())
    }

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

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(())
    }

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1.37.0 · source

#### pub fn buffer(&self) -> &[u8] ⓘ

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(())
    }

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1.46.0 · source

#### pub fn capacity(&self) -> usize

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(())
    }

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#### pub fn into_inner(self) -> Rwhere R: Sized,

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(())
    }

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### impl<R: ?Sized + Seek> BufReader<R>

1.53.0 · source

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

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§

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

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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. Read more
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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`. Read more
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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. Read more
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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. Read more
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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. Read more
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### impl<R> Debug for BufReader<R>where R: ?Sized + Debug,

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#### fn fmt(&self, fmt: &mut Formatter<'_>) -> Result

Formats the value using the given formatter. Read more
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### impl<R: ?Sized + Read> Read for BufReader<R>

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

Pull some bytes from this source into the specified buffer, returning how many bytes were read. Read more
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#### fn read_buf(&mut self, cursor: BorrowedCursor<'_>) -> Result<()>

🔬This is a nightly-only experimental API. (`read_buf` #78485)
Pull some bytes from this source into the specified buffer. Read more
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#### fn read_exact(&mut self, buf: &mut [u8]) -> Result<()>

Read the exact number of bytes required to fill `buf`. Read more
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#### fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> Result<usize>

Like `read`, except that it reads into a slice of buffers. Read more
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#### fn is_read_vectored(&self) -> bool

🔬This is a nightly-only experimental API. (`can_vector` #69941)
Determines if this `Read`er has an efficient `read_vectored` implementation. Read more
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#### fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize>

Read all bytes until EOF in this source, placing them into `buf`. Read more
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#### fn read_to_string(&mut self, buf: &mut String) -> Result<usize>

Read all bytes until EOF in this source, appending them to `buf`. Read more
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#### fn read_buf_exact(&mut self, cursor: BorrowedCursor<'_>) -> Result<()>

🔬This is a nightly-only experimental API. (`read_buf` #78485)
Read the exact number of bytes required to fill `cursor`. Read more
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### impl<R: ?Sized + Seek> Seek for BufReader<R>

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#### fn seek(&mut self, pos: SeekFrom) -> Result<u64>

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)`.

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#### fn stream_position(&mut self) -> Result<u64>

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

    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(())
    }

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1.55.0 · source§

#### fn rewind(&mut self) -> Result<()>

Rewind to the beginning of a stream. Read more
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#### fn stream_len(&mut self) -> Result<u64>

🔬This is a nightly-only experimental API. (`seek_stream_len` #59359)
Returns the length of this stream (in bytes). Read more

## Auto Trait Implementations§

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### impl<R: ?Sized> RefUnwindSafe for BufReader<R>where R: RefUnwindSafe,

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### impl<R: ?Sized> Send for BufReader<R>where R: Send,

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### impl<R: ?Sized> Sync for BufReader<R>where R: Sync,

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### impl<R: ?Sized> Unpin for BufReader<R>where R: Unpin,

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### impl<R: ?Sized> UnwindSafe for BufReader<R>where R: UnwindSafe,

## Blanket Implementations§

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### impl<T> Any for Twhere T: 'static + ?Sized,

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#### fn type_id(&self) -> TypeId

Gets the `TypeId` of `self`. Read more
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### impl<T> Borrow<T> for Twhere T: ?Sized,

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#### fn borrow(&self) -> &T

Immutably borrows from an owned value. Read more
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### impl<T> BorrowMut<T> for Twhere T: ?Sized,

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#### fn borrow_mut(&mut self) -> &mut T

Mutably borrows from an owned value. Read more
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### impl<T> From<T> for T

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#### fn from(t: T) -> T

Returns the argument unchanged.

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### impl<T, U> Into<U> for Twhere U: From<T>,

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#### fn into(self) -> U

Calls `U::from(self)`.

That is, this conversion is whatever the implementation of `From<T> for U` chooses to do.

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### impl<T, U> TryFrom<U> for Twhere U: Into<T>,

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#### type Error = Infallible

The type returned in the event of a conversion error.
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#### fn try_from(value: U) -> Result<T, <T as TryFrom<U>>::Error>

Performs the conversion.
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### impl<T, U> TryInto<U> for Twhere U: TryFrom<T>,

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

The type returned in the event of a conversion error.
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#### fn try_into(self) -> Result<U, <U as TryFrom<T>>::Error>

Performs the conversion.