buffered.rs.html -- source
Description: Source to the Rust file `src/libstd/io/buffered.rs`.
Keywords: rust, rustlang, rust-lang

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    // Copyright 2013 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    //! Buffering wrappers for I/O traits

    use io::prelude::*;

    use cmp;
    use error;
    use fmt;
    use io::{self, DEFAULT_BUF_SIZE, Error, ErrorKind, SeekFrom};
    use memchr;

    /// The `BufReader` 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` performs large, infrequent reads on the underlying [`Read`]
    /// and maintains an in-memory buffer of the results.
    ///
    /// [`Read`]: ../../std/io/trait.Read.html
    /// [`read`]: ../../std/net/struct.TcpStream.html#method.read
    /// [`TcpStream`]: ../../std/net/struct.TcpStream.html
    ///
    /// # Examples
    ///
    /// ```
    /// use std::io::prelude::*;
    /// use std::io::BufReader;
    /// use std::fs::File;
    ///
    /// # fn foo() -> std::io::Result<()> {
    /// let mut f = try!(File::open("log.txt"));
    /// let mut reader = BufReader::new(f);
    ///
    /// let mut line = String::new();
    /// let len = try!(reader.read_line(&mut line));
    /// println!("First line is {} bytes long", len);
    /// # Ok(())
    /// # }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct BufReader<R> {
        inner: R,
        buf: Box<[u8]>,
        pos: usize,
        cap: usize,
    }

    impl<R: Read> BufReader<R> {
        /// Creates a new `BufReader` with a default buffer capacity.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::io::BufReader;
        /// use std::fs::File;
        ///
        /// # fn foo() -> std::io::Result<()> {
        /// let mut f = try!(File::open("log.txt"));
        /// let mut reader = BufReader::new(f);
        /// # Ok(())
        /// # }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn new(inner: R) -> BufReader<R> {
            BufReader::with_capacity(DEFAULT_BUF_SIZE, inner)
        }

        /// Creates a new `BufReader` with the specified buffer capacity.
        ///
        /// # Examples
        ///
        /// Creating a buffer with ten bytes of capacity:
        ///
        /// ```
        /// use std::io::BufReader;
        /// use std::fs::File;
        ///
        /// # fn foo() -> std::io::Result<()> {
        /// let mut f = try!(File::open("log.txt"));
        /// let mut reader = BufReader::with_capacity(10, f);
        /// # Ok(())
        /// # }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn with_capacity(cap: usize, inner: R) -> BufReader<R> {
            BufReader {
                inner: inner,
                buf: vec![0; cap].into_boxed_slice(),
                pos: 0,
                cap: 0,
            }
        }

        /// 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 foo() -> std::io::Result<()> {
        /// let mut f1 = try!(File::open("log.txt"));
        /// let mut reader = BufReader::new(f1);
        ///
        /// let f2 = reader.get_ref();
        /// # Ok(())
        /// # }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn get_ref(&self) -> &R { &self.inner }

        /// 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 foo() -> std::io::Result<()> {
        /// let mut f1 = try!(File::open("log.txt"));
        /// let mut reader = BufReader::new(f1);
        ///
        /// let f2 = reader.get_mut();
        /// # Ok(())
        /// # }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn get_mut(&mut self) -> &mut R { &mut self.inner }

        /// Unwraps this `BufReader`, returning the underlying reader.
        ///
        /// Note that any leftover data in the internal buffer is lost.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::io::BufReader;
        /// use std::fs::File;
        ///
        /// # fn foo() -> std::io::Result<()> {
        /// let mut f1 = try!(File::open("log.txt"));
        /// let mut reader = BufReader::new(f1);
        ///
        /// let f2 = reader.into_inner();
        /// # Ok(())
        /// # }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn into_inner(self) -> R { self.inner }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<R: Read> Read for BufReader<R> {
        fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
            // If we don't have any buffered data and we're doing a massive read
            // (larger than our internal buffer), bypass our internal buffer
            // entirely.
            if self.pos == self.cap && buf.len() >= self.buf.len() {
                return self.inner.read(buf);
            }
            let nread = {
                let mut rem = self.fill_buf()?;
                rem.read(buf)?
            };
            self.consume(nread);
            Ok(nread)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<R: Read> BufRead for BufReader<R> {
        fn fill_buf(&mut self) -> io::Result<&[u8]> {
            // If we've reached the end of our internal buffer then we need to fetch
            // some more data from the underlying reader.
            // Branch using `>=` instead of the more correct `==`
            // to tell the compiler that the pos..cap slice is always valid.
            if self.pos >= self.cap {
                debug_assert!(self.pos == self.cap);
                self.cap = self.inner.read(&mut self.buf)?;
                self.pos = 0;
            }
            Ok(&self.buf[self.pos..self.cap])
        }

        fn consume(&mut self, amt: usize) {
            self.pos = cmp::min(self.pos + amt, self.cap);
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<R> fmt::Debug for BufReader<R> where R: fmt::Debug {
        fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
            fmt.debug_struct("BufReader")
                .field("reader", &self.inner)
                .field("buffer", &format_args!("{}/{}", self.cap - self.pos, self.buf.len()))
                .finish()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<R: Seek> Seek for BufReader<R> {
        /// 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` had no
        /// internal buffer.
        ///
        /// Seeking always discards the internal buffer, even if the seek position
        /// would otherwise fall within it. This guarantees that calling
        /// `.into_inner()` immediately after a seek yields the underlying reader
        /// at the same position.
        ///
        /// 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 underflows 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 seeked to `SeekFrom::Current(0)`.
        fn seek(&mut self, pos: SeekFrom) -> io::Result<u64> {
            let result: u64;
            if let SeekFrom::Current(n) = pos {
                let remainder = (self.cap - self.pos) as i64;
                // it should be safe to assume that remainder fits within an i64 as the alternative
                // means we managed to allocate 8 exbibytes and that's absurd.
                // But it's not out of the realm of possibility for some weird underlying reader to
                // support seeking by i64::min_value() so we need to handle underflow when subtracting
                // remainder.
                if let Some(offset) = n.checked_sub(remainder) {
                    result = self.inner.seek(SeekFrom::Current(offset))?;
                } else {
                    // seek backwards by our remainder, and then by the offset
                    self.inner.seek(SeekFrom::Current(-remainder))?;
                    self.pos = self.cap; // empty the buffer
                    result = self.inner.seek(SeekFrom::Current(n))?;
                }
            } else {
                // Seeking with Start/End doesn't care about our buffer length.
                result = self.inner.seek(pos)?;
            }
            self.pos = self.cap; // empty the buffer
            Ok(result)
        }
    }

    /// 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` keeps an in-memory buffer of data
    /// and writes it to an underlying writer in large, infrequent batches.
    ///
    /// The buffer will be written out when the writer is dropped.
    ///
    /// # 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 1..10 {
    ///     stream.write(&[i]).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`:
    ///
    /// ```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 1..10 {
    ///     stream.write(&[i]).unwrap();
    /// }
    /// ```
    ///
    /// By wrapping the stream with a `BufWriter`, these ten writes are all grouped
    /// together by the buffer, and will all be written out in one system call when
    /// the `stream` is dropped.
    ///
    /// [`Write`]: ../../std/io/trait.Write.html
    /// [`write`]: ../../std/net/struct.TcpStream.html#method.write
    /// [`TcpStream`]: ../../std/net/struct.TcpStream.html
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct BufWriter<W: Write> {
        inner: Option<W>,
        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,
    }

    /// An error returned by `into_inner` which combines an error that
    /// happened while writing out the buffer, and the buffered writer object
    /// which may be used to recover from the condition.
    ///
    /// # Examples
    ///
    /// ```no_run
    /// use std::io::BufWriter;
    /// use std::net::TcpStream;
    ///
    /// let mut stream = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());
    ///
    /// // do stuff with the stream
    ///
    /// // we want to get our `TcpStream` back, so let's try:
    ///
    /// let stream = match stream.into_inner() {
    ///     Ok(s) => s,
    ///     Err(e) => {
    ///         // Here, e is an IntoInnerError
    ///         panic!("An error occurred");
    ///     }
    /// };
    /// ```
    #[derive(Debug)]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct IntoInnerError<W>(W, Error);

    impl<W: Write> BufWriter<W> {
        /// Creates a new `BufWriter` with a default buffer capacity.
        ///
        /// # 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` with the specified buffer capacity.
        ///
        /// # Examples
        ///
        /// Creating a buffer with a buffer of 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(cap: usize, inner: W) -> BufWriter<W> {
            BufWriter {
                inner: Some(inner),
                buf: Vec::with_capacity(cap),
                panicked: false,
            }
        }

        fn flush_buf(&mut self) -> io::Result<()> {
            let mut written = 0;
            let len = self.buf.len();
            let mut ret = Ok(());
            while written < len {
                self.panicked = true;
                let r = self.inner.as_mut().unwrap().write(&self.buf[written..]);
                self.panicked = false;

                match r {
                    Ok(0) => {
                        ret = Err(Error::new(ErrorKind::WriteZero,
                                             "failed to write the buffered data"));
                        break;
                    }
                    Ok(n) => written += n,
                    Err(ref e) if e.kind() == io::ErrorKind::Interrupted => {}
                    Err(e) => { ret = Err(e); break }

                }
            }
            if written > 0 {
                self.buf.drain(..written);
            }
            ret
        }

        /// 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.as_ref().unwrap() }

        /// 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 { self.inner.as_mut().unwrap() }

        /// Unwraps this `BufWriter`, returning the underlying writer.
        ///
        /// The buffer is written out before returning the 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());
        ///
        /// // 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(self, e)),
                Ok(()) => Ok(self.inner.take().unwrap())
            }
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<W: Write> Write for BufWriter<W> {
        fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
            if self.buf.len() + buf.len() > self.buf.capacity() {
                self.flush_buf()?;
            }
            if buf.len() >= self.buf.capacity() {
                self.panicked = true;
                let r = self.inner.as_mut().unwrap().write(buf);
                self.panicked = false;
                r
            } else {
                Write::write(&mut self.buf, buf)
            }
        }
        fn flush(&mut self) -> io::Result<()> {
            self.flush_buf().and_then(|()| self.get_mut().flush())
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<W: 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.as_ref().unwrap())
                .field("buffer", &format_args!("{}/{}", self.buf.len(), self.buf.capacity()))
                .finish()
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<W: 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().and_then(|_| self.get_mut().seek(pos))
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<W: Write> Drop for BufWriter<W> {
        fn drop(&mut self) {
            if self.inner.is_some() && !self.panicked {
                // dtors should not panic, so we ignore a failed flush
                let _r = self.flush_buf();
            }
        }
    }

    impl<W> IntoInnerError<W> {
        /// Returns the error which caused the call to `into_inner()` to fail.
        ///
        /// This error was returned when attempting to write the internal buffer.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use std::io::BufWriter;
        /// use std::net::TcpStream;
        ///
        /// let mut stream = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());
        ///
        /// // do stuff with the stream
        ///
        /// // we want to get our `TcpStream` back, so let's try:
        ///
        /// let stream = match stream.into_inner() {
        ///     Ok(s) => s,
        ///     Err(e) => {
        ///         // Here, e is an IntoInnerError, let's log the inner error.
        ///         //
        ///         // We'll just 'log' to stdout for this example.
        ///         println!("{}", e.error());
        ///
        ///         panic!("An unexpected error occurred.");
        ///     }
        /// };
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn error(&self) -> &Error { &self.1 }

        /// Returns the buffered writer instance which generated the error.
        ///
        /// The returned object can be used for error recovery, such as
        /// re-inspecting the buffer.
        ///
        /// # Examples
        ///
        /// ```no_run
        /// use std::io::BufWriter;
        /// use std::net::TcpStream;
        ///
        /// let mut stream = BufWriter::new(TcpStream::connect("127.0.0.1:34254").unwrap());
        ///
        /// // do stuff with the stream
        ///
        /// // we want to get our `TcpStream` back, so let's try:
        ///
        /// let stream = match stream.into_inner() {
        ///     Ok(s) => s,
        ///     Err(e) => {
        ///         // Here, e is an IntoInnerError, let's re-examine the buffer:
        ///         let buffer = e.into_inner();
        ///
        ///         // do stuff to try to recover
        ///
        ///         // afterwards, let's just return the stream
        ///         buffer.into_inner().unwrap()
        ///     }
        /// };
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn into_inner(self) -> W { self.0 }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<W> From<IntoInnerError<W>> for Error {
        fn from(iie: IntoInnerError<W>) -> Error { iie.1 }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<W: Send + fmt::Debug> error::Error for IntoInnerError<W> {
        fn description(&self) -> &str {
            error::Error::description(self.error())
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<W> fmt::Display for IntoInnerError<W> {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            self.error().fmt(f)
        }
    }

    /// Wraps a writer and buffers output to it, flushing whenever a newline
    /// (`0x0a`, `'\n'`) is detected.
    ///
    /// The [`BufWriter`][bufwriter] struct wraps a writer and buffers its output.
    /// But it only does this batched write when it goes out of scope, or when the
    /// internal buffer is full. Sometimes, you'd prefer to write each line as it's
    /// completed, rather than the entire buffer at once. Enter `LineWriter`. It
    /// does exactly that.
    ///
    /// [bufwriter]: struct.BufWriter.html
    ///
    /// If there's still a partial line in the buffer when the `LineWriter` is
    /// dropped, it will flush those contents.
    ///
    /// # Examples
    ///
    /// We can use `LineWriter` to write one line at a time, significantly
    /// reducing the number of actual writes to the file.
    ///
    /// ```
    /// use std::fs::File;
    /// use std::io::prelude::*;
    /// use std::io::LineWriter;
    ///
    /// # fn foo() -> std::io::Result<()> {
    /// let road_not_taken = b"I shall be telling this with a sigh
    /// Somewhere ages and ages hence:
    /// Two roads diverged in a wood, and I -
    /// I took the one less traveled by,
    /// And that has made all the difference.";
    ///
    /// let file = try!(File::create("poem.txt"));
    /// let mut file = LineWriter::new(file);
    ///
    /// for &byte in road_not_taken.iter() {
    ///    file.write(&[byte]).unwrap();
    /// }
    ///
    /// // let's check we did the right thing.
    /// let mut file = try!(File::open("poem.txt"));
    /// let mut contents = String::new();
    ///
    /// try!(file.read_to_string(&mut contents));
    ///
    /// assert_eq!(contents.as_bytes(), &road_not_taken[..]);
    /// # Ok(())
    /// # }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct LineWriter<W: Write> {
        inner: BufWriter<W>,
    }

    impl<W: Write> LineWriter<W> {
        /// Creates a new `LineWriter`.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::fs::File;
        /// use std::io::LineWriter;
        ///
        /// # fn foo() -> std::io::Result<()> {
        /// let file = try!(File::create("poem.txt"));
        /// let file = LineWriter::new(file);
        /// # Ok(())
        /// # }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn new(inner: W) -> LineWriter<W> {
            // Lines typically aren't that long, don't use a giant buffer
            LineWriter::with_capacity(1024, inner)
        }

        /// Creates a new `LineWriter` with a specified capacity for the internal
        /// buffer.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::fs::File;
        /// use std::io::LineWriter;
        ///
        /// # fn foo() -> std::io::Result<()> {
        /// let file = try!(File::create("poem.txt"));
        /// let file = LineWriter::with_capacity(100, file);
        /// # Ok(())
        /// # }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn with_capacity(cap: usize, inner: W) -> LineWriter<W> {
            LineWriter { inner: BufWriter::with_capacity(cap, inner) }
        }

        /// Gets a reference to the underlying writer.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::fs::File;
        /// use std::io::LineWriter;
        ///
        /// # fn foo() -> std::io::Result<()> {
        /// let file = try!(File::create("poem.txt"));
        /// let file = LineWriter::new(file);
        ///
        /// let reference = file.get_ref();
        /// # Ok(())
        /// # }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn get_ref(&self) -> &W { self.inner.get_ref() }

        /// Gets a mutable reference to the underlying writer.
        ///
        /// Caution must be taken when calling methods on the mutable reference
        /// returned as extra writes could corrupt the output stream.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::fs::File;
        /// use std::io::LineWriter;
        ///
        /// # fn foo() -> std::io::Result<()> {
        /// let file = try!(File::create("poem.txt"));
        /// let mut file = LineWriter::new(file);
        ///
        /// // we can use reference just like file
        /// let reference = file.get_mut();
        /// # Ok(())
        /// # }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn get_mut(&mut self) -> &mut W { self.inner.get_mut() }

        /// Unwraps this `LineWriter`, returning the underlying writer.
        ///
        /// The internal buffer is written out before returning the writer.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::fs::File;
        /// use std::io::LineWriter;
        ///
        /// # fn foo() -> std::io::Result<()> {
        /// let file = try!(File::create("poem.txt"));
        ///
        /// let writer: LineWriter<File> = LineWriter::new(file);
        ///
        /// let file: File = try!(writer.into_inner());
        /// # Ok(())
        /// # }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn into_inner(self) -> Result<W, IntoInnerError<LineWriter<W>>> {
            self.inner.into_inner().map_err(|IntoInnerError(buf, e)| {
                IntoInnerError(LineWriter { inner: buf }, e)
            })
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<W: Write> Write for LineWriter<W> {
        fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
            match memchr::memrchr(b'\n', buf) {
                Some(i) => {
                    let n = self.inner.write(&buf[..i + 1])?;
                    if n != i + 1 || self.inner.flush().is_err() {
                        // Do not return errors on partial writes.
                        return Ok(n);
                    }
                    self.inner.write(&buf[i + 1..]).map(|i| n + i)
                }
                None => self.inner.write(buf),
            }
        }

        fn flush(&mut self) -> io::Result<()> { self.inner.flush() }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<W: Write> fmt::Debug for LineWriter<W> where W: fmt::Debug {
        fn fmt(&self, fmt: &mut fmt::Formatter) -> fmt::Result {
            fmt.debug_struct("LineWriter")
                .field("writer", &self.inner.inner)
                .field("buffer",
                       &format_args!("{}/{}", self.inner.buf.len(), self.inner.buf.capacity()))
                .finish()
        }
    }

    #[cfg(test)]
    mod tests {
        use io::prelude::*;
        use io::{self, BufReader, BufWriter, LineWriter, SeekFrom};
        use sync::atomic::{AtomicUsize, Ordering};
        use thread;
        use test;

        /// A dummy reader intended at testing short-reads propagation.
        pub struct ShortReader {
            lengths: Vec<usize>,
        }

        impl Read for ShortReader {
            fn read(&mut self, _: &mut [u8]) -> io::Result<usize> {
                if self.lengths.is_empty() {
                    Ok(0)
                } else {
                    Ok(self.lengths.remove(0))
                }
            }
        }

        #[test]
        fn test_buffered_reader() {
            let inner: &[u8] = &[5, 6, 7, 0, 1, 2, 3, 4];
            let mut reader = BufReader::with_capacity(2, inner);

            let mut buf = [0, 0, 0];
            let nread = reader.read(&mut buf);
            assert_eq!(nread.unwrap(), 3);
            let b: &[_] = &[5, 6, 7];
            assert_eq!(buf, b);

            let mut buf = [0, 0];
            let nread = reader.read(&mut buf);
            assert_eq!(nread.unwrap(), 2);
            let b: &[_] = &[0, 1];
            assert_eq!(buf, b);

            let mut buf = [0];
            let nread = reader.read(&mut buf);
            assert_eq!(nread.unwrap(), 1);
            let b: &[_] = &[2];
            assert_eq!(buf, b);

            let mut buf = [0, 0, 0];
            let nread = reader.read(&mut buf);
            assert_eq!(nread.unwrap(), 1);
            let b: &[_] = &[3, 0, 0];
            assert_eq!(buf, b);

            let nread = reader.read(&mut buf);
            assert_eq!(nread.unwrap(), 1);
            let b: &[_] = &[4, 0, 0];
            assert_eq!(buf, b);

            assert_eq!(reader.read(&mut buf).unwrap(), 0);
        }

        #[test]
        fn test_buffered_reader_seek() {
            let inner: &[u8] = &[5, 6, 7, 0, 1, 2, 3, 4];
            let mut reader = BufReader::with_capacity(2, io::Cursor::new(inner));

            assert_eq!(reader.seek(SeekFrom::Start(3)).ok(), Some(3));
            assert_eq!(reader.fill_buf().ok(), Some(&[0, 1][..]));
            assert_eq!(reader.seek(SeekFrom::Current(0)).ok(), Some(3));
            assert_eq!(reader.fill_buf().ok(), Some(&[0, 1][..]));
            assert_eq!(reader.seek(SeekFrom::Current(1)).ok(), Some(4));
            assert_eq!(reader.fill_buf().ok(), Some(&[1, 2][..]));
            reader.consume(1);
            assert_eq!(reader.seek(SeekFrom::Current(-2)).ok(), Some(3));
        }

        #[test]
        fn test_buffered_reader_seek_underflow() {
            // gimmick reader that yields its position modulo 256 for each byte
            struct PositionReader {
                pos: u64
            }
            impl Read for PositionReader {
                fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
                    let len = buf.len();
                    for x in buf {
                        *x = self.pos as u8;
                        self.pos = self.pos.wrapping_add(1);
                    }
                    Ok(len)
                }
            }
            impl Seek for PositionReader {
                fn seek(&mut self, pos: SeekFrom) -> io::Result<u64> {
                    match pos {
                        SeekFrom::Start(n) => {
                            self.pos = n;
                        }
                        SeekFrom::Current(n) => {
                            self.pos = self.pos.wrapping_add(n as u64);
                        }
                        SeekFrom::End(n) => {
                            self.pos = u64::max_value().wrapping_add(n as u64);
                        }
                    }
                    Ok(self.pos)
                }
            }

            let mut reader = BufReader::with_capacity(5, PositionReader { pos: 0 });
            assert_eq!(reader.fill_buf().ok(), Some(&[0, 1, 2, 3, 4][..]));
            assert_eq!(reader.seek(SeekFrom::End(-5)).ok(), Some(u64::max_value()-5));
            assert_eq!(reader.fill_buf().ok().map(|s| s.len()), Some(5));
            // the following seek will require two underlying seeks
            let expected = 9223372036854775802;
            assert_eq!(reader.seek(SeekFrom::Current(i64::min_value())).ok(), Some(expected));
            assert_eq!(reader.fill_buf().ok().map(|s| s.len()), Some(5));
            // seeking to 0 should empty the buffer.
            assert_eq!(reader.seek(SeekFrom::Current(0)).ok(), Some(expected));
            assert_eq!(reader.get_ref().pos, expected);
        }

        #[test]
        fn test_buffered_writer() {
            let inner = Vec::new();
            let mut writer = BufWriter::with_capacity(2, inner);

            writer.write(&[0, 1]).unwrap();
            assert_eq!(*writer.get_ref(), [0, 1]);

            writer.write(&[2]).unwrap();
            assert_eq!(*writer.get_ref(), [0, 1]);

            writer.write(&[3]).unwrap();
            assert_eq!(*writer.get_ref(), [0, 1]);

            writer.flush().unwrap();
            assert_eq!(*writer.get_ref(), [0, 1, 2, 3]);

            writer.write(&[4]).unwrap();
            writer.write(&[5]).unwrap();
            assert_eq!(*writer.get_ref(), [0, 1, 2, 3]);

            writer.write(&[6]).unwrap();
            assert_eq!(*writer.get_ref(), [0, 1, 2, 3, 4, 5]);

            writer.write(&[7, 8]).unwrap();
            assert_eq!(*writer.get_ref(), [0, 1, 2, 3, 4, 5, 6, 7, 8]);

            writer.write(&[9, 10, 11]).unwrap();
            assert_eq!(*writer.get_ref(), [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]);

            writer.flush().unwrap();
            assert_eq!(*writer.get_ref(), [0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]);
        }

        #[test]
        fn test_buffered_writer_inner_flushes() {
            let mut w = BufWriter::with_capacity(3, Vec::new());
            w.write(&[0, 1]).unwrap();
            assert_eq!(*w.get_ref(), []);
            let w = w.into_inner().unwrap();
            assert_eq!(w, [0, 1]);
        }

        #[test]
        fn test_buffered_writer_seek() {
            let mut w = BufWriter::with_capacity(3, io::Cursor::new(Vec::new()));
            w.write_all(&[0, 1, 2, 3, 4, 5]).unwrap();
            w.write_all(&[6, 7]).unwrap();
            assert_eq!(w.seek(SeekFrom::Current(0)).ok(), Some(8));
            assert_eq!(&w.get_ref().get_ref()[..], &[0, 1, 2, 3, 4, 5, 6, 7][..]);
            assert_eq!(w.seek(SeekFrom::Start(2)).ok(), Some(2));
            w.write_all(&[8, 9]).unwrap();
            assert_eq!(&w.into_inner().unwrap().into_inner()[..], &[0, 1, 8, 9, 4, 5, 6, 7]);
        }

        #[test]
        fn test_read_until() {
            let inner: &[u8] = &[0, 1, 2, 1, 0];
            let mut reader = BufReader::with_capacity(2, inner);
            let mut v = Vec::new();
            reader.read_until(0, &mut v).unwrap();
            assert_eq!(v, [0]);
            v.truncate(0);
            reader.read_until(2, &mut v).unwrap();
            assert_eq!(v, [1, 2]);
            v.truncate(0);
            reader.read_until(1, &mut v).unwrap();
            assert_eq!(v, [1]);
            v.truncate(0);
            reader.read_until(8, &mut v).unwrap();
            assert_eq!(v, [0]);
            v.truncate(0);
            reader.read_until(9, &mut v).unwrap();
            assert_eq!(v, []);
        }

        #[test]
        fn test_line_buffer_fail_flush() {
            // Issue #32085
            struct FailFlushWriter<'a>(&'a mut Vec<u8>);

            impl<'a> Write for FailFlushWriter<'a> {
                fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
                    self.0.extend_from_slice(buf);
                    Ok(buf.len())
                }
                fn flush(&mut self) -> io::Result<()> {
                    Err(io::Error::new(io::ErrorKind::Other, "flush failed"))
                }
            }

            let mut buf = Vec::new();
            {
                let mut writer = LineWriter::new(FailFlushWriter(&mut buf));
                let to_write = b"abc\ndef";
                if let Ok(written) = writer.write(to_write) {
                    assert!(written < to_write.len(), "didn't flush on new line");
                    // PASS
                    return;
                }
            }
            assert!(buf.is_empty(), "write returned an error but wrote data");
        }

        #[test]
        fn test_line_buffer() {
            let mut writer = LineWriter::new(Vec::new());
            writer.write(&[0]).unwrap();
            assert_eq!(*writer.get_ref(), []);
            writer.write(&[1]).unwrap();
            assert_eq!(*writer.get_ref(), []);
            writer.flush().unwrap();
            assert_eq!(*writer.get_ref(), [0, 1]);
            writer.write(&[0, b'\n', 1, b'\n', 2]).unwrap();
            assert_eq!(*writer.get_ref(), [0, 1, 0, b'\n', 1, b'\n']);
            writer.flush().unwrap();
            assert_eq!(*writer.get_ref(), [0, 1, 0, b'\n', 1, b'\n', 2]);
            writer.write(&[3, b'\n']).unwrap();
            assert_eq!(*writer.get_ref(), [0, 1, 0, b'\n', 1, b'\n', 2, 3, b'\n']);
        }

        #[test]
        fn test_read_line() {
            let in_buf: &[u8] = b"a\nb\nc";
            let mut reader = BufReader::with_capacity(2, in_buf);
            let mut s = String::new();
            reader.read_line(&mut s).unwrap();
            assert_eq!(s, "a\n");
            s.truncate(0);
            reader.read_line(&mut s).unwrap();
            assert_eq!(s, "b\n");
            s.truncate(0);
            reader.read_line(&mut s).unwrap();
            assert_eq!(s, "c");
            s.truncate(0);
            reader.read_line(&mut s).unwrap();
            assert_eq!(s, "");
        }

        #[test]
        fn test_lines() {
            let in_buf: &[u8] = b"a\nb\nc";
            let reader = BufReader::with_capacity(2, in_buf);
            let mut it = reader.lines();
            assert_eq!(it.next().unwrap().unwrap(), "a".to_string());
            assert_eq!(it.next().unwrap().unwrap(), "b".to_string());
            assert_eq!(it.next().unwrap().unwrap(), "c".to_string());
            assert!(it.next().is_none());
        }

        #[test]
        fn test_short_reads() {
            let inner = ShortReader{lengths: vec![0, 1, 2, 0, 1, 0]};
            let mut reader = BufReader::new(inner);
            let mut buf = [0, 0];
            assert_eq!(reader.read(&mut buf).unwrap(), 0);
            assert_eq!(reader.read(&mut buf).unwrap(), 1);
            assert_eq!(reader.read(&mut buf).unwrap(), 2);
            assert_eq!(reader.read(&mut buf).unwrap(), 0);
            assert_eq!(reader.read(&mut buf).unwrap(), 1);
            assert_eq!(reader.read(&mut buf).unwrap(), 0);
            assert_eq!(reader.read(&mut buf).unwrap(), 0);
        }

        #[test]
        fn read_char_buffered() {
            let buf = [195, 159];
            let reader = BufReader::with_capacity(1, &buf[..]);
            assert_eq!(reader.chars().next().unwrap().unwrap(), 'ß');
        }

        #[test]
        fn test_chars() {
            let buf = [195, 159, b'a'];
            let reader = BufReader::with_capacity(1, &buf[..]);
            let mut it = reader.chars();
            assert_eq!(it.next().unwrap().unwrap(), 'ß');
            assert_eq!(it.next().unwrap().unwrap(), 'a');
            assert!(it.next().is_none());
        }

        #[test]
        #[should_panic]
        fn dont_panic_in_drop_on_panicked_flush() {
            struct FailFlushWriter;

            impl Write for FailFlushWriter {
                fn write(&mut self, buf: &[u8]) -> io::Result<usize> { Ok(buf.len()) }
                fn flush(&mut self) -> io::Result<()> {
                    Err(io::Error::last_os_error())
                }
            }

            let writer = FailFlushWriter;
            let _writer = BufWriter::new(writer);

            // If writer panics *again* due to the flush error then the process will
            // abort.
            panic!();
        }

        #[test]
        #[cfg_attr(target_os = "emscripten", ignore)]
        fn panic_in_write_doesnt_flush_in_drop() {
            static WRITES: AtomicUsize = AtomicUsize::new(0);

            struct PanicWriter;

            impl Write for PanicWriter {
                fn write(&mut self, _: &[u8]) -> io::Result<usize> {
                    WRITES.fetch_add(1, Ordering::SeqCst);
                    panic!();
                }
                fn flush(&mut self) -> io::Result<()> { Ok(()) }
            }

            thread::spawn(|| {
                let mut writer = BufWriter::new(PanicWriter);
                let _ = writer.write(b"hello world");
                let _ = writer.flush();
            }).join().unwrap_err();

            assert_eq!(WRITES.load(Ordering::SeqCst), 1);
        }

        #[bench]
        fn bench_buffered_reader(b: &mut test::Bencher) {
            b.iter(|| {
                BufReader::new(io::empty())
            });
        }

        #[bench]
        fn bench_buffered_writer(b: &mut test::Bencher) {
            b.iter(|| {
                BufWriter::new(io::sink())
            });
        }
    }

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