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use crate::error;
use crate::fmt;
use crate::io::{
self, ErrorKind, IntoInnerError, IoSlice, Seek, SeekFrom, Write, DEFAULT_BUF_SIZE,
};
use crate::mem;
use crate::ptr;
/// 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`][`TcpStream::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 <code>[Vec]\<u8></code>.
///
/// 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`]:
///
/// ```no_run
/// 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();
/// }
/// ```
///
/// 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>`:
///
/// ```no_run
/// 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();
/// ```
///
/// 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.
///
// HACK(#78696): can't use `crate` for associated items
/// [`TcpStream::write`]: super::super::super::net::TcpStream::write
/// [`TcpStream`]: crate::net::TcpStream
/// [`flush`]: BufWriter::flush
#[stable(feature = "rust1", since = "1.0.0")]
pub struct BufWriter<W: ?Sized + Write> {
// The buffer. Avoid using this like a normal `Vec` in common code paths.
// That is, don't use `buf.push`, `buf.extend_from_slice`, or any other
// methods that require bounds checking or the like. This makes an enormous
// difference to performance (we may want to stop using a `Vec` entirely).
buf: Vec<u8>,
// #30888: If the inner writer panics in a call to write, we don't want to
// write the buffered data a second time in BufWriter's destructor. This
// flag tells the Drop impl if it should skip the flush.
panicked: bool,
inner: W,
}
impl<W: Write> BufWriter<W> {
/// Creates a new `BufWriter<W>` with a default buffer capacity. The default is currently 8 KiB,
/// but may change in the future.
///
/// # Examples
///
/// ```no_run
/// use std::io::BufWriter;
/// use std::net::TcpStream;
///
/// let mut buffer = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub fn new(inner: W) -> BufWriter<W> {
BufWriter::with_capacity(DEFAULT_BUF_SIZE, inner)
}
/// Creates a new `BufWriter<W>` with at least the specified buffer capacity.
///
/// # Examples
///
/// Creating a buffer with a buffer of at least a hundred bytes.
///
/// ```no_run
/// 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);
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub fn with_capacity(capacity: usize, inner: W) -> BufWriter<W> {
BufWriter { inner, buf: Vec::with_capacity(capacity), panicked: false }
}
/// 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
///
/// ```no_run
/// 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();
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub fn into_inner(mut self) -> Result<W, IntoInnerError<BufWriter<W>>> {
match self.flush_buf() {
Err(e) => Err(IntoInnerError::new(self, e)),
Ok(()) => Ok(self.into_parts().0),
}
}
/// 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");
/// ```
#[stable(feature = "bufwriter_into_parts", since = "1.56.0")]
pub fn into_parts(mut self) -> (W, Result<Vec<u8>, WriterPanicked>) {
let buf = mem::take(&mut self.buf);
let buf = if !self.panicked { Ok(buf) } else { Err(WriterPanicked { buf }) };
// SAFETY: forget(self) prevents double dropping inner
let inner = unsafe { ptr::read(&self.inner) };
mem::forget(self);
(inner, buf)
}
}
impl<W: ?Sized + Write> BufWriter<W> {
/// 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.
pub(in crate::io) fn flush_buf(&mut self) -> io::Result<()> {
/// Helper struct to ensure the buffer is updated after all the writes
/// are complete. It tracks the number of written bytes and drains them
/// all from the front of the buffer when dropped.
struct BufGuard<'a> {
buffer: &'a mut Vec<u8>,
written: usize,
}
impl<'a> BufGuard<'a> {
fn new(buffer: &'a mut Vec<u8>) -> Self {
Self { buffer, written: 0 }
}
/// The unwritten part of the buffer
fn remaining(&self) -> &[u8] {
&self.buffer[self.written..]
}
/// Flag some bytes as removed from the front of the buffer
fn consume(&mut self, amt: usize) {
self.written += amt;
}
/// true if all of the bytes have been written
fn done(&self) -> bool {
self.written >= self.buffer.len()
}
}
impl Drop for BufGuard<'_> {
fn drop(&mut self) {
if self.written > 0 {
self.buffer.drain(..self.written);
}
}
}
let mut guard = BufGuard::new(&mut self.buf);
while !guard.done() {
self.panicked = true;
let r = self.inner.write(guard.remaining());
self.panicked = false;
match r {
Ok(0) => {
return Err(io::const_io_error!(
ErrorKind::WriteZero,
"failed to write the buffered data",
));
}
Ok(n) => guard.consume(n),
Err(ref e) if e.is_interrupted() => {}
Err(e) => return Err(e),
}
}
Ok(())
}
/// 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.
pub(super) fn write_to_buf(&mut self, buf: &[u8]) -> usize {
let available = self.spare_capacity();
let amt_to_buffer = available.min(buf.len());
// SAFETY: `amt_to_buffer` is <= buffer's spare capacity by construction.
unsafe {
self.write_to_buffer_unchecked(&buf[..amt_to_buffer]);
}
amt_to_buffer
}
/// Gets a reference to the underlying writer.
///
/// # Examples
///
/// ```no_run
/// 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();
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub fn get_ref(&self) -> &W {
&self.inner
}
/// Gets a mutable reference to the underlying writer.
///
/// It is inadvisable to directly write to the underlying writer.
///
/// # Examples
///
/// ```no_run
/// 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();
/// ```
#[stable(feature = "rust1", since = "1.0.0")]
pub fn get_mut(&mut self) -> &mut W {
&mut self.inner
}
/// Returns a reference to the internally buffered data.
///
/// # Examples
///
/// ```no_run
/// 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();
/// ```
#[stable(feature = "bufreader_buffer", since = "1.37.0")]
pub fn buffer(&self) -> &[u8] {
&self.buf
}
/// 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.
pub(in crate::io) fn buffer_mut(&mut self) -> &mut Vec<u8> {
&mut self.buf
}
/// Returns the number of bytes the internal buffer can hold without flushing.
///
/// # Examples
///
/// ```no_run
/// 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();
/// ```
#[stable(feature = "buffered_io_capacity", since = "1.46.0")]
pub fn capacity(&self) -> usize {
self.buf.capacity()
}
// Ensure this function does not get inlined into `write`, so that it
// remains inlineable and its common path remains as short as possible.
// If this function ends up being called frequently relative to `write`,
// it's likely a sign that the client is using an improperly sized buffer
// or their write patterns are somewhat pathological.
#[cold]
#[inline(never)]
fn write_cold(&mut self, buf: &[u8]) -> io::Result<usize> {
if buf.len() > self.spare_capacity() {
self.flush_buf()?;
}
// Why not len > capacity? To avoid a needless trip through the buffer when the input
// exactly fills it. We'd just need to flush it to the underlying writer anyway.
if buf.len() >= self.buf.capacity() {
self.panicked = true;
let r = self.get_mut().write(buf);
self.panicked = false;
r
} else {
// Write to the buffer. In this case, we write to the buffer even if it fills it
// exactly. Doing otherwise would mean flushing the buffer, then writing this
// input to the inner writer, which in many cases would be a worse strategy.
// SAFETY: There was either enough spare capacity already, or there wasn't and we
// flushed the buffer to ensure that there is. In the latter case, we know that there
// is because flushing ensured that our entire buffer is spare capacity, and we entered
// this block because the input buffer length is less than that capacity. In either
// case, it's safe to write the input buffer to our buffer.
unsafe {
self.write_to_buffer_unchecked(buf);
}
Ok(buf.len())
}
}
// Ensure this function does not get inlined into `write_all`, so that it
// remains inlineable and its common path remains as short as possible.
// If this function ends up being called frequently relative to `write_all`,
// it's likely a sign that the client is using an improperly sized buffer
// or their write patterns are somewhat pathological.
#[cold]
#[inline(never)]
fn write_all_cold(&mut self, buf: &[u8]) -> io::Result<()> {
// Normally, `write_all` just calls `write` in a loop. We can do better
// by calling `self.get_mut().write_all()` directly, which avoids
// round trips through the buffer in the event of a series of partial
// writes in some circumstances.
if buf.len() > self.spare_capacity() {
self.flush_buf()?;
}
// Why not len > capacity? To avoid a needless trip through the buffer when the input
// exactly fills it. We'd just need to flush it to the underlying writer anyway.
if buf.len() >= self.buf.capacity() {
self.panicked = true;
let r = self.get_mut().write_all(buf);
self.panicked = false;
r
} else {
// Write to the buffer. In this case, we write to the buffer even if it fills it
// exactly. Doing otherwise would mean flushing the buffer, then writing this
// input to the inner writer, which in many cases would be a worse strategy.
// SAFETY: There was either enough spare capacity already, or there wasn't and we
// flushed the buffer to ensure that there is. In the latter case, we know that there
// is because flushing ensured that our entire buffer is spare capacity, and we entered
// this block because the input buffer length is less than that capacity. In either
// case, it's safe to write the input buffer to our buffer.
unsafe {
self.write_to_buffer_unchecked(buf);
}
Ok(())
}
}
// SAFETY: Requires `buf.len() <= self.buf.capacity() - self.buf.len()`,
// i.e., that input buffer length is less than or equal to spare capacity.
#[inline]
unsafe fn write_to_buffer_unchecked(&mut self, buf: &[u8]) {
debug_assert!(buf.len() <= self.spare_capacity());
let old_len = self.buf.len();
let buf_len = buf.len();
let src = buf.as_ptr();
let dst = self.buf.as_mut_ptr().add(old_len);
ptr::copy_nonoverlapping(src, dst, buf_len);
self.buf.set_len(old_len + buf_len);
}
#[inline]
fn spare_capacity(&self) -> usize {
self.buf.capacity() - self.buf.len()
}
}
#[stable(feature = "bufwriter_into_parts", since = "1.56.0")]
/// Error returned for the buffered data from `BufWriter::into_parts`, when the underlying
/// writer has previously panicked. Contains the (possibly partly written) buffered data.
///
/// # Example
///
/// ```
/// use std::io::{self, BufWriter, Write};
/// use std::panic::{catch_unwind, AssertUnwindSafe};
///
/// struct PanickingWriter;
/// impl Write for PanickingWriter {
/// fn write(&mut self, buf: &[u8]) -> io::Result<usize> { panic!() }
/// fn flush(&mut self) -> io::Result<()> { panic!() }
/// }
///
/// let mut stream = BufWriter::new(PanickingWriter);
/// write!(stream, "some data").unwrap();
/// let result = catch_unwind(AssertUnwindSafe(|| {
/// stream.flush().unwrap()
/// }));
/// assert!(result.is_err());
/// let (recovered_writer, buffered_data) = stream.into_parts();
/// assert!(matches!(recovered_writer, PanickingWriter));
/// assert_eq!(buffered_data.unwrap_err().into_inner(), b"some data");
/// ```
pub struct WriterPanicked {
buf: Vec<u8>,
}
impl WriterPanicked {
/// Returns the perhaps-unwritten data. Some of this data may have been written by the
/// panicking call(s) to the underlying writer, so simply writing it again is not a good idea.
#[must_use = "`self` will be dropped if the result is not used"]
#[stable(feature = "bufwriter_into_parts", since = "1.56.0")]
pub fn into_inner(self) -> Vec<u8> {
self.buf
}
const DESCRIPTION: &'static str =
"BufWriter inner writer panicked, what data remains unwritten is not known";
}
#[stable(feature = "bufwriter_into_parts", since = "1.56.0")]
impl error::Error for WriterPanicked {
#[allow(deprecated, deprecated_in_future)]
fn description(&self) -> &str {
Self::DESCRIPTION
}
}
#[stable(feature = "bufwriter_into_parts", since = "1.56.0")]
impl fmt::Display for WriterPanicked {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
write!(f, "{}", Self::DESCRIPTION)
}
}
#[stable(feature = "bufwriter_into_parts", since = "1.56.0")]
impl fmt::Debug for WriterPanicked {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.debug_struct("WriterPanicked")
.field("buffer", &format_args!("{}/{}", self.buf.len(), self.buf.capacity()))
.finish()
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<W: ?Sized + Write> Write for BufWriter<W> {
#[inline]
fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
// Use < instead of <= to avoid a needless trip through the buffer in some cases.
// See `write_cold` for details.
if buf.len() < self.spare_capacity() {
// SAFETY: safe by above conditional.
unsafe {
self.write_to_buffer_unchecked(buf);
}
Ok(buf.len())
} else {
self.write_cold(buf)
}
}
#[inline]
fn write_all(&mut self, buf: &[u8]) -> io::Result<()> {
// Use < instead of <= to avoid a needless trip through the buffer in some cases.
// See `write_all_cold` for details.
if buf.len() < self.spare_capacity() {
// SAFETY: safe by above conditional.
unsafe {
self.write_to_buffer_unchecked(buf);
}
Ok(())
} else {
self.write_all_cold(buf)
}
}
fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
// FIXME: Consider applying `#[inline]` / `#[inline(never)]` optimizations already applied
// to `write` and `write_all`. The performance benefits can be significant. See #79930.
if self.get_ref().is_write_vectored() {
// We have to handle the possibility that the total length of the buffers overflows
// `usize` (even though this can only happen if multiple `IoSlice`s reference the
// same underlying buffer, as otherwise the buffers wouldn't fit in memory). If the
// computation overflows, then surely the input cannot fit in our buffer, so we forward
// to the inner writer's `write_vectored` method to let it handle it appropriately.
let saturated_total_len =
bufs.iter().fold(0usize, |acc, b| acc.saturating_add(b.len()));
if saturated_total_len > self.spare_capacity() {
// Flush if the total length of the input exceeds our buffer's spare capacity.
// If we would have overflowed, this condition also holds, and we need to flush.
self.flush_buf()?;
}
if saturated_total_len >= self.buf.capacity() {
// Forward to our inner writer if the total length of the input is greater than or
// equal to our buffer capacity. If we would have overflowed, this condition also
// holds, and we punt to the inner writer.
self.panicked = true;
let r = self.get_mut().write_vectored(bufs);
self.panicked = false;
r
} else {
// `saturated_total_len < self.buf.capacity()` implies that we did not saturate.
// SAFETY: We checked whether or not the spare capacity was large enough above. If
// it was, then we're safe already. If it wasn't, we flushed, making sufficient
// room for any input <= the buffer size, which includes this input.
unsafe {
bufs.iter().for_each(|b| self.write_to_buffer_unchecked(b));
};
Ok(saturated_total_len)
}
} else {
let mut iter = bufs.iter();
let mut total_written = if let Some(buf) = iter.by_ref().find(|&buf| !buf.is_empty()) {
// This is the first non-empty slice to write, so if it does
// not fit in the buffer, we still get to flush and proceed.
if buf.len() > self.spare_capacity() {
self.flush_buf()?;
}
if buf.len() >= self.buf.capacity() {
// The slice is at least as large as the buffering capacity,
// so it's better to write it directly, bypassing the buffer.
self.panicked = true;
let r = self.get_mut().write(buf);
self.panicked = false;
return r;
} else {
// SAFETY: We checked whether or not the spare capacity was large enough above.
// If it was, then we're safe already. If it wasn't, we flushed, making
// sufficient room for any input <= the buffer size, which includes this input.
unsafe {
self.write_to_buffer_unchecked(buf);
}
buf.len()
}
} else {
return Ok(0);
};
debug_assert!(total_written != 0);
for buf in iter {
if buf.len() <= self.spare_capacity() {
// SAFETY: safe by above conditional.
unsafe {
self.write_to_buffer_unchecked(buf);
}
// This cannot overflow `usize`. If we are here, we've written all of the bytes
// so far to our buffer, and we've ensured that we never exceed the buffer's
// capacity. Therefore, `total_written` <= `self.buf.capacity()` <= `usize::MAX`.
total_written += buf.len();
} else {
break;
}
}
Ok(total_written)
}
}
fn is_write_vectored(&self) -> bool {
true
}
fn flush(&mut self) -> io::Result<()> {
self.flush_buf().and_then(|()| self.get_mut().flush())
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<W: ?Sized + Write> fmt::Debug for BufWriter<W>
where
W: fmt::Debug,
{
fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
fmt.debug_struct("BufWriter")
.field("writer", &&self.inner)
.field("buffer", &format_args!("{}/{}", self.buf.len(), self.buf.capacity()))
.finish()
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<W: ?Sized + Write + Seek> Seek for BufWriter<W> {
/// Seek to the offset, in bytes, in the underlying writer.
///
/// Seeking always writes out the internal buffer before seeking.
fn seek(&mut self, pos: SeekFrom) -> io::Result<u64> {
self.flush_buf()?;
self.get_mut().seek(pos)
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<W: ?Sized + Write> Drop for BufWriter<W> {
fn drop(&mut self) {
if !self.panicked {
// dtors should not panic, so we ignore a failed flush
let _r = self.flush_buf();
}
}
}