Struct std::io::buffered::bufwriter::BufWriter1.0.0[][src]

pub struct BufWriter<W: Write> {
    inner: W,
    buf: Vec<u8>,
    panicked: bool,
}
Expand description

Wraps a writer and buffers its output.

It can be excessively inefficient to work directly with something that implements Write. For example, every call to write on TcpStream results in a system call. A BufWriter<W> keeps an in-memory buffer of data and writes it to an underlying writer in large, infrequent batches.

BufWriter<W> can improve the speed of programs that make small and repeated write calls to the same file or network socket. It does not help when writing very large amounts at once, or writing just one or a few times. It also provides no advantage when writing to a destination that is in memory, like a Vec<u8>.

It is critical to call flush before BufWriter<W> is dropped. Though dropping will attempt to flush the contents of the buffer, any errors that happen in the process of dropping will be ignored. Calling flush ensures that the buffer is empty and thus dropping will not even attempt file operations.

Examples

Let’s write the numbers one through ten to a TcpStream:

use std::io::prelude::*;
use std::net::TcpStream;

let mut stream = TcpStream::connect("127.0.0.1:34254").unwrap();

for i in 0..10 {
    stream.write(&[i+1]).unwrap();
}
Run

Because we’re not buffering, we write each one in turn, incurring the overhead of a system call per byte written. We can fix this with a BufWriter<W>:

use std::io::prelude::*;
use std::io::BufWriter;
use std::net::TcpStream;

let mut stream = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());

for i in 0..10 {
    stream.write(&[i+1]).unwrap();
}
stream.flush().unwrap();
Run

By wrapping the stream with a BufWriter<W>, these ten writes are all grouped together by the buffer and will all be written out in one system call when the stream is flushed.

Fields

inner: Wbuf: Vec<u8>panicked: bool

Implementations

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

Examples
use std::io::BufWriter;
use std::net::TcpStream;

let mut buffer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());
Run

Creates a new BufWriter<W> with the specified buffer capacity.

Examples

Creating a buffer with a buffer of a hundred bytes.

use std::io::BufWriter;
use std::net::TcpStream;

let stream = TcpStream::connect("127.0.0.1:34254").unwrap();
let mut buffer = BufWriter::with_capacity(100, stream);
Run

Send data in our local buffer into the inner writer, looping as necessary until either it’s all been sent or an error occurs.

Because all the data in the buffer has been reported to our owner as “successfully written” (by returning nonzero success values from write), any 0-length writes from inner must be reported as i/o errors from this method.

Buffer some data without flushing it, regardless of the size of the data. Writes as much as possible without exceeding capacity. Returns the number of bytes written.

Gets a reference to the underlying writer.

Examples
use std::io::BufWriter;
use std::net::TcpStream;

let mut buffer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());

// we can use reference just like buffer
let reference = buffer.get_ref();
Run

Gets a mutable reference to the underlying writer.

It is inadvisable to directly write to the underlying writer.

Examples
use std::io::BufWriter;
use std::net::TcpStream;

let mut buffer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());

// we can use reference just like buffer
let reference = buffer.get_mut();
Run

Returns a reference to the internally buffered data.

Examples
use std::io::BufWriter;
use std::net::TcpStream;

let buf_writer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());

// See how many bytes are currently buffered
let bytes_buffered = buf_writer.buffer().len();
Run

Returns a mutable reference to the internal buffer.

This can be used to write data directly into the buffer without triggering writers to the underlying writer.

That the buffer is a Vec is an implementation detail. Callers should not modify the capacity as there currently is no public API to do so and thus any capacity changes would be unexpected by the user.

Returns the number of bytes the internal buffer can hold without flushing.

Examples
use std::io::BufWriter;
use std::net::TcpStream;

let buf_writer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());

// Check the capacity of the inner buffer
let capacity = buf_writer.capacity();
// Calculate how many bytes can be written without flushing
let without_flush = capacity - buf_writer.buffer().len();
Run

Unwraps this BufWriter<W>, returning the underlying writer.

The buffer is written out before returning the writer.

Errors

An Err will be returned if an error occurs while flushing the buffer.

Examples
use std::io::BufWriter;
use std::net::TcpStream;

let mut buffer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());

// unwrap the TcpStream and flush the buffer
let stream = buffer.into_inner().unwrap();
Run

Disassembles this BufWriter<W>, returning the underlying writer, and any buffered but unwritten data.

If the underlying writer panicked, it is not known what portion of the data was written. In this case, we return WriterPanicked for the buffered data (from which the buffer contents can still be recovered).

into_parts makes no attempt to flush data and cannot fail.

Examples
use std::io::{BufWriter, Write};

let mut buffer = [0u8; 10];
let mut stream = BufWriter::new(buffer.as_mut());
write!(stream, "too much data").unwrap();
stream.flush().expect_err("it doesn't fit");
let (recovered_writer, buffered_data) = stream.into_parts();
assert_eq!(recovered_writer.len(), 0);
assert_eq!(&buffered_data.unwrap(), b"ata");
Run

Trait Implementations

This is supported on Linux or Android only.

Extracts the file descriptor and hints/metadata, delegating through wrappers if necessary.

Formats the value using the given formatter. Read more

Executes the destructor for this type. Read more

Seek to the offset, in bytes, in the underlying writer.

Seeking always writes out the internal buffer before seeking.

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

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

Write a buffer into this writer, returning how many bytes were written. Read more

Attempts to write an entire buffer into this writer. Read more

Like write, except that it writes from a slice of buffers. Read more

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

Determines if this Writer has an efficient write_vectored implementation. Read more

Flush this output stream, ensuring that all intermediately buffered contents reach their destination. Read more

🔬 This is a nightly-only experimental API. (write_all_vectored #70436)

Attempts to write multiple buffers into this writer. Read more

Writes a formatted string into this writer, returning any error encountered. Read more

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

Auto Trait Implementations

Blanket Implementations

Gets the TypeId of self. Read more

Immutably borrows from an owned value. Read more

Mutably borrows from an owned value. Read more

Performs the conversion.

Performs the conversion.

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.