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

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    // Copyright 2015 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.

    //! Traits, helpers, and type definitions for core I/O functionality.
    //!
    //! The `std::io` module contains a number of common things you'll need
    //! when doing input and output. The most core part of this module is
    //! the [`Read`] and [`Write`] traits, which provide the
    //! most general interface for reading and writing input and output.
    //!
    //! # Read and Write
    //!
    //! Because they are traits, [`Read`] and [`Write`] are implemented by a number
    //! of other types, and you can implement them for your types too. As such,
    //! you'll see a few different types of I/O throughout the documentation in
    //! this module: [`File`]s, [`TcpStream`]s, and sometimes even [`Vec<T>`]s. For
    //! example, `Read` adds a `read()` method, which we can use on `File`s:
    //!
    //! ```
    //! use std::io;
    //! use std::io::prelude::*;
    //! use std::fs::File;
    //!
    //! # fn foo() -> io::Result<()> {
    //! let mut f = try!(File::open("foo.txt"));
    //! let mut buffer = [0; 10];
    //!
    //! // read up to 10 bytes
    //! try!(f.read(&mut buffer));
    //!
    //! println!("The bytes: {:?}", buffer);
    //! # Ok(())
    //! # }
    //! ```
    //!
    //! [`Read`] and [`Write`] are so important, implementors of the two traits have a
    //! nickname: readers and writers. So you'll sometimes see 'a reader' instead
    //! of 'a type that implements the [`Read`] trait'. Much easier!
    //!
    //! ## Seek and BufRead
    //!
    //! Beyond that, there are two important traits that are provided: [`Seek`]
    //! and [`BufRead`]. Both of these build on top of a reader to control
    //! how the reading happens. [`Seek`] lets you control where the next byte is
    //! coming from:
    //!
    //! ```
    //! use std::io;
    //! use std::io::prelude::*;
    //! use std::io::SeekFrom;
    //! use std::fs::File;
    //!
    //! # fn foo() -> io::Result<()> {
    //! let mut f = try!(File::open("foo.txt"));
    //! let mut buffer = [0; 10];
    //!
    //! // skip to the last 10 bytes of the file
    //! try!(f.seek(SeekFrom::End(-10)));
    //!
    //! // read up to 10 bytes
    //! try!(f.read(&mut buffer));
    //!
    //! println!("The bytes: {:?}", buffer);
    //! # Ok(())
    //! # }
    //! ```
    //!
    //! [`BufRead`] uses an internal buffer to provide a number of other ways to read, but
    //! to show it off, we'll need to talk about buffers in general. Keep reading!
    //!
    //! ## BufReader and BufWriter
    //!
    //! Byte-based interfaces are unwieldy and can be inefficient, as we'd need to be
    //! making near-constant calls to the operating system. To help with this,
    //! `std::io` comes with two structs, [`BufReader`] and [`BufWriter`], which wrap
    //! readers and writers. The wrapper uses a buffer, reducing the number of
    //! calls and providing nicer methods for accessing exactly what you want.
    //!
    //! For example, [`BufReader`] works with the [`BufRead`] trait to add extra
    //! methods to any reader:
    //!
    //! ```
    //! use std::io;
    //! use std::io::prelude::*;
    //! use std::io::BufReader;
    //! use std::fs::File;
    //!
    //! # fn foo() -> io::Result<()> {
    //! let f = try!(File::open("foo.txt"));
    //! let mut reader = BufReader::new(f);
    //! let mut buffer = String::new();
    //!
    //! // read a line into buffer
    //! try!(reader.read_line(&mut buffer));
    //!
    //! println!("{}", buffer);
    //! # Ok(())
    //! # }
    //! ```
    //!
    //! [`BufWriter`] doesn't add any new ways of writing; it just buffers every call
    //! to [`write()`]:
    //!
    //! ```
    //! use std::io;
    //! use std::io::prelude::*;
    //! use std::io::BufWriter;
    //! use std::fs::File;
    //!
    //! # fn foo() -> io::Result<()> {
    //! let f = try!(File::create("foo.txt"));
    //! {
    //!     let mut writer = BufWriter::new(f);
    //!
    //!     // write a byte to the buffer
    //!     try!(writer.write(&[42]));
    //!
    //! } // the buffer is flushed once writer goes out of scope
    //!
    //! # Ok(())
    //! # }
    //! ```
    //!
    //! ## Standard input and output
    //!
    //! A very common source of input is standard input:
    //!
    //! ```
    //! use std::io;
    //!
    //! # fn foo() -> io::Result<()> {
    //! let mut input = String::new();
    //!
    //! try!(io::stdin().read_line(&mut input));
    //!
    //! println!("You typed: {}", input.trim());
    //! # Ok(())
    //! # }
    //! ```
    //!
    //! And a very common source of output is standard output:
    //!
    //! ```
    //! use std::io;
    //! use std::io::prelude::*;
    //!
    //! # fn foo() -> io::Result<()> {
    //! try!(io::stdout().write(&[42]));
    //! # Ok(())
    //! # }
    //! ```
    //!
    //! Of course, using [`io::stdout()`] directly is less common than something like
    //! [`println!`].
    //!
    //! ## Iterator types
    //!
    //! A large number of the structures provided by `std::io` are for various
    //! ways of iterating over I/O. For example, [`Lines`] is used to split over
    //! lines:
    //!
    //! ```
    //! use std::io;
    //! use std::io::prelude::*;
    //! use std::io::BufReader;
    //! use std::fs::File;
    //!
    //! # fn foo() -> io::Result<()> {
    //! let f = try!(File::open("foo.txt"));
    //! let reader = BufReader::new(f);
    //!
    //! for line in reader.lines() {
    //!     println!("{}", try!(line));
    //! }
    //!
    //! # Ok(())
    //! # }
    //! ```
    //!
    //! ## Functions
    //!
    //! There are a number of [functions][functions-list] that offer access to various
    //! features. For example, we can use three of these functions to copy everything
    //! from standard input to standard output:
    //!
    //! ```
    //! use std::io;
    //!
    //! # fn foo() -> io::Result<()> {
    //! try!(io::copy(&mut io::stdin(), &mut io::stdout()));
    //! # Ok(())
    //! # }
    //! ```
    //!
    //! [functions-list]: #functions-1
    //!
    //! ## io::Result
    //!
    //! Last, but certainly not least, is [`io::Result`]. This type is used
    //! as the return type of many `std::io` functions that can cause an error, and
    //! can be returned from your own functions as well. Many of the examples in this
    //! module use the [`try!`] macro:
    //!
    //! ```
    //! use std::io;
    //!
    //! fn read_input() -> io::Result<()> {
    //!     let mut input = String::new();
    //!
    //!     try!(io::stdin().read_line(&mut input));
    //!
    //!     println!("You typed: {}", input.trim());
    //!
    //!     Ok(())
    //! }
    //! ```
    //!
    //! The return type of `read_input()`, [`io::Result<()>`][`io::Result`], is a very
    //! common type for functions which don't have a 'real' return value, but do want to
    //! return errors if they happen. In this case, the only purpose of this function is
    //! to read the line and print it, so we use `()`.
    //!
    //! ## Platform-specific behavior
    //!
    //! Many I/O functions throughout the standard library are documented to indicate
    //! what various library or syscalls they are delegated to. This is done to help
    //! applications both understand what's happening under the hood as well as investigate
    //! any possibly unclear semantics. Note, however, that this is informative, not a binding
    //! contract. The implementation of many of these functions are subject to change over
    //! time and may call fewer or more syscalls/library functions.
    //!
    //! [`Read`]: trait.Read.html
    //! [`Write`]: trait.Write.html
    //! [`Seek`]: trait.Seek.html
    //! [`BufRead`]: trait.BufRead.html
    //! [`File`]: ../fs/struct.File.html
    //! [`TcpStream`]: ../net/struct.TcpStream.html
    //! [`Vec<T>`]: ../vec/struct.Vec.html
    //! [`BufReader`]: struct.BufReader.html
    //! [`BufWriter`]: struct.BufWriter.html
    //! [`write()`]: trait.Write.html#tymethod.write
    //! [`io::stdout()`]: fn.stdout.html
    //! [`println!`]: ../macro.println.html
    //! [`Lines`]: struct.Lines.html
    //! [`io::Result`]: type.Result.html
    //! [`try!`]: ../macro.try.html

    #![stable(feature = "rust1", since = "1.0.0")]

    use cmp;
    use rustc_unicode::str as core_str;
    use error as std_error;
    use fmt;
    use result;
    use str;
    use memchr;

    #[stable(feature = "rust1", since = "1.0.0")]
    pub use self::buffered::{BufReader, BufWriter, LineWriter};
    #[stable(feature = "rust1", since = "1.0.0")]
    pub use self::buffered::IntoInnerError;
    #[stable(feature = "rust1", since = "1.0.0")]
    pub use self::cursor::Cursor;
    #[stable(feature = "rust1", since = "1.0.0")]
    pub use self::error::{Result, Error, ErrorKind};
    #[stable(feature = "rust1", since = "1.0.0")]
    pub use self::util::{copy, sink, Sink, empty, Empty, repeat, Repeat};
    #[stable(feature = "rust1", since = "1.0.0")]
    pub use self::stdio::{stdin, stdout, stderr, _print, Stdin, Stdout, Stderr};
    #[stable(feature = "rust1", since = "1.0.0")]
    pub use self::stdio::{StdoutLock, StderrLock, StdinLock};
    #[unstable(feature = "libstd_io_internals", issue = "0")]
    #[doc(no_inline, hidden)]
    pub use self::stdio::{set_panic, set_print};

    pub mod prelude;
    mod buffered;
    mod cursor;
    mod error;
    mod impls;
    mod lazy;
    mod util;
    mod stdio;

    const DEFAULT_BUF_SIZE: usize = ::sys_common::io::DEFAULT_BUF_SIZE;

    // A few methods below (read_to_string, read_line) will append data into a
    // `String` buffer, but we need to be pretty careful when doing this. The
    // implementation will just call `.as_mut_vec()` and then delegate to a
    // byte-oriented reading method, but we must ensure that when returning we never
    // leave `buf` in a state such that it contains invalid UTF-8 in its bounds.
    //
    // To this end, we use an RAII guard (to protect against panics) which updates
    // the length of the string when it is dropped. This guard initially truncates
    // the string to the prior length and only after we've validated that the
    // new contents are valid UTF-8 do we allow it to set a longer length.
    //
    // The unsafety in this function is twofold:
    //
    // 1. We're looking at the raw bytes of `buf`, so we take on the burden of UTF-8
    //    checks.
    // 2. We're passing a raw buffer to the function `f`, and it is expected that
    //    the function only *appends* bytes to the buffer. We'll get undefined
    //    behavior if existing bytes are overwritten to have non-UTF-8 data.
    fn append_to_string<F>(buf: &mut String, f: F) -> Result<usize>
        where F: FnOnce(&mut Vec<u8>) -> Result<usize>
    {
        struct Guard<'a> { s: &'a mut Vec<u8>, len: usize }
            impl<'a> Drop for Guard<'a> {
            fn drop(&mut self) {
                unsafe { self.s.set_len(self.len); }
            }
        }

        unsafe {
            let mut g = Guard { len: buf.len(), s: buf.as_mut_vec() };
            let ret = f(g.s);
            if str::from_utf8(&g.s[g.len..]).is_err() {
                ret.and_then(|_| {
                    Err(Error::new(ErrorKind::InvalidData,
                                   "stream did not contain valid UTF-8"))
                })
            } else {
                g.len = g.s.len();
                ret
            }
        }
    }

    // This uses an adaptive system to extend the vector when it fills. We want to
    // avoid paying to allocate and zero a huge chunk of memory if the reader only
    // has 4 bytes while still making large reads if the reader does have a ton
    // of data to return. Simply tacking on an extra DEFAULT_BUF_SIZE space every
    // time is 4,500 times (!) slower than this if the reader has a very small
    // amount of data to return.
    fn read_to_end<R: Read + ?Sized>(r: &mut R, buf: &mut Vec<u8>) -> Result<usize> {
        let start_len = buf.len();
        let mut len = start_len;
        let mut new_write_size = 16;
        let ret;
        loop {
            if len == buf.len() {
                if new_write_size < DEFAULT_BUF_SIZE {
                    new_write_size *= 2;
                }
                buf.resize(len + new_write_size, 0);
            }

            match r.read(&mut buf[len..]) {
                Ok(0) => {
                    ret = Ok(len - start_len);
                    break;
                }
                Ok(n) => len += n,
                Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
                Err(e) => {
                    ret = Err(e);
                    break;
                }
            }
        }

        buf.truncate(len);
        ret
    }

    /// The `Read` trait allows for reading bytes from a source.
    ///
    /// Implementors of the `Read` trait are sometimes called 'readers'.
    ///
    /// Readers are defined by one required method, `read()`. Each call to `read`
    /// will attempt to pull bytes from this source into a provided buffer. A
    /// number of other methods are implemented in terms of `read()`, giving
    /// implementors a number of ways to read bytes while only needing to implement
    /// a single method.
    ///
    /// Readers are intended to be composable with one another. Many implementors
    /// throughout `std::io` take and provide types which implement the `Read`
    /// trait.
    ///
    /// Please note that each call to `read` may involve a system call, and
    /// therefore, using something that implements [`BufRead`][bufread], such as
    /// [`BufReader`][bufreader], will be more efficient.
    ///
    /// [bufread]: trait.BufRead.html
    /// [bufreader]: struct.BufReader.html
    ///
    /// # Examples
    ///
    /// [`File`][file]s implement `Read`:
    ///
    /// [file]: ../fs/struct.File.html
    ///
    /// ```
    /// use std::io;
    /// use std::io::prelude::*;
    /// use std::fs::File;
    ///
    /// # fn foo() -> io::Result<()> {
    /// let mut f = try!(File::open("foo.txt"));
    /// let mut buffer = [0; 10];
    ///
    /// // read up to 10 bytes
    /// try!(f.read(&mut buffer));
    ///
    /// let mut buffer = vec![0; 10];
    /// // read the whole file
    /// try!(f.read_to_end(&mut buffer));
    ///
    /// // read into a String, so that you don't need to do the conversion.
    /// let mut buffer = String::new();
    /// try!(f.read_to_string(&mut buffer));
    ///
    /// // and more! See the other methods for more details.
    /// # Ok(())
    /// # }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait Read {
        /// Pull some bytes from this source into the specified buffer, returning
        /// how many bytes were read.
        ///
        /// This function does not provide any guarantees about whether it blocks
        /// waiting for data, but if an object needs to block for a read but cannot
        /// it will typically signal this via an `Err` return value.
        ///
        /// If the return value of this method is `Ok(n)`, then it must be
        /// guaranteed that `0 <= n <= buf.len()`. A nonzero `n` value indicates
        /// that the buffer `buf` has been filled in with `n` bytes of data from this
        /// source. If `n` is `0`, then it can indicate one of two scenarios:
        ///
        /// 1. This reader has reached its "end of file" and will likely no longer
        ///    be able to produce bytes. Note that this does not mean that the
        ///    reader will *always* no longer be able to produce bytes.
        /// 2. The buffer specified was 0 bytes in length.
        ///
        /// No guarantees are provided about the contents of `buf` when this
        /// function is called, implementations cannot rely on any property of the
        /// contents of `buf` being true. It is recommended that implementations
        /// only write data to `buf` instead of reading its contents.
        ///
        /// # Errors
        ///
        /// If this function encounters any form of I/O or other error, an error
        /// variant will be returned. If an error is returned then it must be
        /// guaranteed that no bytes were read.
        ///
        /// # Examples
        ///
        /// [`File`][file]s implement `Read`:
        ///
        /// [file]: ../fs/struct.File.html
        ///
        /// ```
        /// use std::io;
        /// use std::io::prelude::*;
        /// use std::fs::File;
        ///
        /// # fn foo() -> io::Result<()> {
        /// let mut f = try!(File::open("foo.txt"));
        /// let mut buffer = [0; 10];
        ///
        /// // read 10 bytes
        /// try!(f.read(&mut buffer[..]));
        /// # Ok(())
        /// # }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn read(&mut self, buf: &mut [u8]) -> Result<usize>;

        /// Read all bytes until EOF in this source, placing them into `buf`.
        ///
        /// All bytes read from this source will be appended to the specified buffer
        /// `buf`. This function will continuously call `read` to append more data to
        /// `buf` until `read` returns either `Ok(0)` or an error of
        /// non-`ErrorKind::Interrupted` kind.
        ///
        /// If successful, this function will return the total number of bytes read.
        ///
        /// # Errors
        ///
        /// If this function encounters an error of the kind
        /// `ErrorKind::Interrupted` then the error is ignored and the operation
        /// will continue.
        ///
        /// If any other read error is encountered then this function immediately
        /// returns. Any bytes which have already been read will be appended to
        /// `buf`.
        ///
        /// # Examples
        ///
        /// [`File`][file]s implement `Read`:
        ///
        /// [file]: ../fs/struct.File.html
        ///
        /// ```
        /// use std::io;
        /// use std::io::prelude::*;
        /// use std::fs::File;
        ///
        /// # fn foo() -> io::Result<()> {
        /// let mut f = try!(File::open("foo.txt"));
        /// let mut buffer = Vec::new();
        ///
        /// // read the whole file
        /// try!(f.read_to_end(&mut buffer));
        /// # Ok(())
        /// # }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize> {
            read_to_end(self, buf)
        }

        /// Read all bytes until EOF in this source, placing them into `buf`.
        ///
        /// If successful, this function returns the number of bytes which were read
        /// and appended to `buf`.
        ///
        /// # Errors
        ///
        /// If the data in this stream is *not* valid UTF-8 then an error is
        /// returned and `buf` is unchanged.
        ///
        /// See [`read_to_end()`][readtoend] for other error semantics.
        ///
        /// [readtoend]: #method.read_to_end
        ///
        /// # Examples
        ///
        /// [`File`][file]s implement `Read`:
        ///
        /// [file]: ../fs/struct.File.html
        ///
        /// ```
        /// use std::io;
        /// use std::io::prelude::*;
        /// use std::fs::File;
        ///
        /// # fn foo() -> io::Result<()> {
        /// let mut f = try!(File::open("foo.txt"));
        /// let mut buffer = String::new();
        ///
        /// try!(f.read_to_string(&mut buffer));
        /// # Ok(())
        /// # }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn read_to_string(&mut self, buf: &mut String) -> Result<usize> {
            // Note that we do *not* call `.read_to_end()` here. We are passing
            // `&mut Vec<u8>` (the raw contents of `buf`) into the `read_to_end`
            // method to fill it up. An arbitrary implementation could overwrite the
            // entire contents of the vector, not just append to it (which is what
            // we are expecting).
            //
            // To prevent extraneously checking the UTF-8-ness of the entire buffer
            // we pass it to our hardcoded `read_to_end` implementation which we
            // know is guaranteed to only read data into the end of the buffer.
            append_to_string(buf, |b| read_to_end(self, b))
        }

        /// Read the exact number of bytes required to fill `buf`.
        ///
        /// This function reads as many bytes as necessary to completely fill the
        /// specified buffer `buf`.
        ///
        /// No guarantees are provided about the contents of `buf` when this
        /// function is called, implementations cannot rely on any property of the
        /// contents of `buf` being true. It is recommended that implementations
        /// only write data to `buf` instead of reading its contents.
        ///
        /// # Errors
        ///
        /// If this function encounters an error of the kind
        /// `ErrorKind::Interrupted` then the error is ignored and the operation
        /// will continue.
        ///
        /// If this function encounters an "end of file" before completely filling
        /// the buffer, it returns an error of the kind `ErrorKind::UnexpectedEof`.
        /// The contents of `buf` are unspecified in this case.
        ///
        /// If any other read error is encountered then this function immediately
        /// returns. The contents of `buf` are unspecified in this case.
        ///
        /// If this function returns an error, it is unspecified how many bytes it
        /// has read, but it will never read more than would be necessary to
        /// completely fill the buffer.
        ///
        /// # Examples
        ///
        /// [`File`][file]s implement `Read`:
        ///
        /// [file]: ../fs/struct.File.html
        ///
        /// ```
        /// use std::io;
        /// use std::io::prelude::*;
        /// use std::fs::File;
        ///
        /// # fn foo() -> io::Result<()> {
        /// let mut f = try!(File::open("foo.txt"));
        /// let mut buffer = [0; 10];
        ///
        /// // read exactly 10 bytes
        /// try!(f.read_exact(&mut buffer));
        /// # Ok(())
        /// # }
        /// ```
        #[stable(feature = "read_exact", since = "1.6.0")]
        fn read_exact(&mut self, mut buf: &mut [u8]) -> Result<()> {
            while !buf.is_empty() {
                match self.read(buf) {
                    Ok(0) => break,
                    Ok(n) => { let tmp = buf; buf = &mut tmp[n..]; }
                    Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
                    Err(e) => return Err(e),
                }
            }
            if !buf.is_empty() {
                Err(Error::new(ErrorKind::UnexpectedEof,
                               "failed to fill whole buffer"))
            } else {
                Ok(())
            }
        }

        /// Creates a "by reference" adaptor for this instance of `Read`.
        ///
        /// The returned adaptor also implements `Read` and will simply borrow this
        /// current reader.
        ///
        /// # Examples
        ///
        /// [`File`][file]s implement `Read`:
        ///
        /// [file]: ../fs/struct.File.html
        ///
        /// ```
        /// use std::io;
        /// use std::io::Read;
        /// use std::fs::File;
        ///
        /// # fn foo() -> io::Result<()> {
        /// let mut f = try!(File::open("foo.txt"));
        /// let mut buffer = Vec::new();
        /// let mut other_buffer = Vec::new();
        ///
        /// {
        ///     let reference = f.by_ref();
        ///
        ///     // read at most 5 bytes
        ///     try!(reference.take(5).read_to_end(&mut buffer));
        ///
        /// } // drop our &mut reference so we can use f again
        ///
        /// // original file still usable, read the rest
        /// try!(f.read_to_end(&mut other_buffer));
        /// # Ok(())
        /// # }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn by_ref(&mut self) -> &mut Self where Self: Sized { self }

        /// Transforms this `Read` instance to an `Iterator` over its bytes.
        ///
        /// The returned type implements `Iterator` where the `Item` is `Result<u8,
        /// R::Err>`.  The yielded item is `Ok` if a byte was successfully read and
        /// `Err` otherwise for I/O errors. EOF is mapped to returning `None` from
        /// this iterator.
        ///
        /// # Examples
        ///
        /// [`File`][file]s implement `Read`:
        ///
        /// [file]: ../fs/struct.File.html
        ///
        /// ```
        /// use std::io;
        /// use std::io::prelude::*;
        /// use std::fs::File;
        ///
        /// # fn foo() -> io::Result<()> {
        /// let mut f = try!(File::open("foo.txt"));
        ///
        /// for byte in f.bytes() {
        ///     println!("{}", byte.unwrap());
        /// }
        /// # Ok(())
        /// # }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn bytes(self) -> Bytes<Self> where Self: Sized {
            Bytes { inner: self }
        }

        /// Transforms this `Read` instance to an `Iterator` over `char`s.
        ///
        /// This adaptor will attempt to interpret this reader as a UTF-8 encoded
        /// sequence of characters. The returned iterator will return `None` once
        /// EOF is reached for this reader. Otherwise each element yielded will be a
        /// `Result<char, E>` where `E` may contain information about what I/O error
        /// occurred or where decoding failed.
        ///
        /// Currently this adaptor will discard intermediate data read, and should
        /// be avoided if this is not desired.
        ///
        /// # Examples
        ///
        /// [`File`][file]s implement `Read`:
        ///
        /// [file]: ../fs/struct.File.html
        ///
        /// ```
        /// #![feature(io)]
        /// use std::io;
        /// use std::io::prelude::*;
        /// use std::fs::File;
        ///
        /// # fn foo() -> io::Result<()> {
        /// let mut f = try!(File::open("foo.txt"));
        ///
        /// for c in f.chars() {
        ///     println!("{}", c.unwrap());
        /// }
        /// # Ok(())
        /// # }
        /// ```
        #[unstable(feature = "io", reason = "the semantics of a partial read/write \
                                             of where errors happen is currently \
                                             unclear and may change",
                   issue = "27802")]
        fn chars(self) -> Chars<Self> where Self: Sized {
            Chars { inner: self }
        }

        /// Creates an adaptor which will chain this stream with another.
        ///
        /// The returned `Read` instance will first read all bytes from this object
        /// until EOF is encountered. Afterwards the output is equivalent to the
        /// output of `next`.
        ///
        /// # Examples
        ///
        /// [`File`][file]s implement `Read`:
        ///
        /// [file]: ../fs/struct.File.html
        ///
        /// ```
        /// use std::io;
        /// use std::io::prelude::*;
        /// use std::fs::File;
        ///
        /// # fn foo() -> io::Result<()> {
        /// let mut f1 = try!(File::open("foo.txt"));
        /// let mut f2 = try!(File::open("bar.txt"));
        ///
        /// let mut handle = f1.chain(f2);
        /// let mut buffer = String::new();
        ///
        /// // read the value into a String. We could use any Read method here,
        /// // this is just one example.
        /// try!(handle.read_to_string(&mut buffer));
        /// # Ok(())
        /// # }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn chain<R: Read>(self, next: R) -> Chain<Self, R> where Self: Sized {
            Chain { first: self, second: next, done_first: false }
        }

        /// Creates an adaptor which will read at most `limit` bytes from it.
        ///
        /// This function returns a new instance of `Read` which will read at most
        /// `limit` bytes, after which it will always return EOF (`Ok(0)`). Any
        /// read errors will not count towards the number of bytes read and future
        /// calls to `read` may succeed.
        ///
        /// # Examples
        ///
        /// [`File`][file]s implement `Read`:
        ///
        /// [file]: ../fs/struct.File.html
        ///
        /// ```
        /// use std::io;
        /// use std::io::prelude::*;
        /// use std::fs::File;
        ///
        /// # fn foo() -> io::Result<()> {
        /// let mut f = try!(File::open("foo.txt"));
        /// let mut buffer = [0; 5];
        ///
        /// // read at most five bytes
        /// let mut handle = f.take(5);
        ///
        /// try!(handle.read(&mut buffer));
        /// # Ok(())
        /// # }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn take(self, limit: u64) -> Take<Self> where Self: Sized {
            Take { inner: self, limit: limit }
        }
    }

    /// A trait for objects which are byte-oriented sinks.
    ///
    /// Implementors of the `Write` trait are sometimes called 'writers'.
    ///
    /// Writers are defined by two required methods, `write()` and `flush()`:
    ///
    /// * The `write()` method will attempt to write some data into the object,
    ///   returning how many bytes were successfully written.
    ///
    /// * The `flush()` method is useful for adaptors and explicit buffers
    ///   themselves for ensuring that all buffered data has been pushed out to the
    ///   'true sink'.
    ///
    /// Writers are intended to be composable with one another. Many implementors
    /// throughout `std::io` take and provide types which implement the `Write`
    /// trait.
    ///
    /// # Examples
    ///
    /// ```
    /// use std::io::prelude::*;
    /// use std::fs::File;
    ///
    /// # fn foo() -> std::io::Result<()> {
    /// let mut buffer = try!(File::create("foo.txt"));
    ///
    /// try!(buffer.write(b"some bytes"));
    /// # Ok(())
    /// # }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait Write {
        /// Write a buffer into this object, returning how many bytes were written.
        ///
        /// This function will attempt to write the entire contents of `buf`, but
        /// the entire write may not succeed, or the write may also generate an
        /// error. A call to `write` represents *at most one* attempt to write to
        /// any wrapped object.
        ///
        /// Calls to `write` are not guaranteed to block waiting for data to be
        /// written, and a write which would otherwise block can be indicated through
        /// an `Err` variant.
        ///
        /// If the return value is `Ok(n)` then it must be guaranteed that
        /// `0 <= n <= buf.len()`. A return value of `0` typically means that the
        /// underlying object is no longer able to accept bytes and will likely not
        /// be able to in the future as well, or that the buffer provided is empty.
        ///
        /// # Errors
        ///
        /// Each call to `write` may generate an I/O error indicating that the
        /// operation could not be completed. If an error is returned then no bytes
        /// in the buffer were written to this writer.
        ///
        /// It is **not** considered an error if the entire buffer could not be
        /// written to this writer.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::io::prelude::*;
        /// use std::fs::File;
        ///
        /// # fn foo() -> std::io::Result<()> {
        /// let mut buffer = try!(File::create("foo.txt"));
        ///
        /// try!(buffer.write(b"some bytes"));
        /// # Ok(())
        /// # }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn write(&mut self, buf: &[u8]) -> Result<usize>;

        /// Flush this output stream, ensuring that all intermediately buffered
        /// contents reach their destination.
        ///
        /// # Errors
        ///
        /// It is considered an error if not all bytes could be written due to
        /// I/O errors or EOF being reached.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::io::prelude::*;
        /// use std::io::BufWriter;
        /// use std::fs::File;
        ///
        /// # fn foo() -> std::io::Result<()> {
        /// let mut buffer = BufWriter::new(try!(File::create("foo.txt")));
        ///
        /// try!(buffer.write(b"some bytes"));
        /// try!(buffer.flush());
        /// # Ok(())
        /// # }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn flush(&mut self) -> Result<()>;

        /// Attempts to write an entire buffer into this write.
        ///
        /// This method will continuously call `write` while there is more data to
        /// write. This method will not return until the entire buffer has been
        /// successfully written or an error occurs. The first error generated from
        /// this method will be returned.
        ///
        /// # Errors
        ///
        /// This function will return the first error that `write` returns.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::io::prelude::*;
        /// use std::fs::File;
        ///
        /// # fn foo() -> std::io::Result<()> {
        /// let mut buffer = try!(File::create("foo.txt"));
        ///
        /// try!(buffer.write_all(b"some bytes"));
        /// # Ok(())
        /// # }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn write_all(&mut self, mut buf: &[u8]) -> Result<()> {
            while !buf.is_empty() {
                match self.write(buf) {
                    Ok(0) => return Err(Error::new(ErrorKind::WriteZero,
                                                   "failed to write whole buffer")),
                    Ok(n) => buf = &buf[n..],
                    Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
                    Err(e) => return Err(e),
                }
            }
            Ok(())
        }

        /// Writes a formatted string into this writer, returning any error
        /// encountered.
        ///
        /// This method is primarily used to interface with the
        /// [`format_args!`][formatargs] macro, but it is rare that this should
        /// explicitly be called. The [`write!`][write] macro should be favored to
        /// invoke this method instead.
        ///
        /// [formatargs]: ../macro.format_args.html
        /// [write]: ../macro.write.html
        ///
        /// This function internally uses the [`write_all`][writeall] method on
        /// this trait and hence will continuously write data so long as no errors
        /// are received. This also means that partial writes are not indicated in
        /// this signature.
        ///
        /// [writeall]: #method.write_all
        ///
        /// # Errors
        ///
        /// This function will return any I/O error reported while formatting.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::io::prelude::*;
        /// use std::fs::File;
        ///
        /// # fn foo() -> std::io::Result<()> {
        /// let mut buffer = try!(File::create("foo.txt"));
        ///
        /// // this call
        /// try!(write!(buffer, "{:.*}", 2, 1.234567));
        /// // turns into this:
        /// try!(buffer.write_fmt(format_args!("{:.*}", 2, 1.234567)));
        /// # Ok(())
        /// # }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn write_fmt(&mut self, fmt: fmt::Arguments) -> Result<()> {
            // Create a shim which translates a Write to a fmt::Write and saves
            // off I/O errors. instead of discarding them
            struct Adaptor<'a, T: ?Sized + 'a> {
                inner: &'a mut T,
                error: Result<()>,
            }

            impl<'a, T: Write + ?Sized> fmt::Write for Adaptor<'a, T> {
                fn write_str(&mut self, s: &str) -> fmt::Result {
                    match self.inner.write_all(s.as_bytes()) {
                        Ok(()) => Ok(()),
                        Err(e) => {
                            self.error = Err(e);
                            Err(fmt::Error)
                        }
                    }
                }
            }

            let mut output = Adaptor { inner: self, error: Ok(()) };
            match fmt::write(&mut output, fmt) {
                Ok(()) => Ok(()),
                Err(..) => {
                    // check if the error came from the underlying `Write` or not
                    if output.error.is_err() {
                        output.error
                    } else {
                        Err(Error::new(ErrorKind::Other, "formatter error"))
                    }
                }
            }
        }

        /// Creates a "by reference" adaptor for this instance of `Write`.
        ///
        /// The returned adaptor also implements `Write` and will simply borrow this
        /// current writer.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::io::Write;
        /// use std::fs::File;
        ///
        /// # fn foo() -> std::io::Result<()> {
        /// let mut buffer = try!(File::create("foo.txt"));
        ///
        /// let reference = buffer.by_ref();
        ///
        /// // we can use reference just like our original buffer
        /// try!(reference.write_all(b"some bytes"));
        /// # Ok(())
        /// # }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn by_ref(&mut self) -> &mut Self where Self: Sized { self }
    }

    /// The `Seek` trait provides a cursor which can be moved within a stream of
    /// bytes.
    ///
    /// The stream typically has a fixed size, allowing seeking relative to either
    /// end or the current offset.
    ///
    /// # Examples
    ///
    /// [`File`][file]s implement `Seek`:
    ///
    /// [file]: ../fs/struct.File.html
    ///
    /// ```
    /// use std::io;
    /// use std::io::prelude::*;
    /// use std::fs::File;
    /// use std::io::SeekFrom;
    ///
    /// # fn foo() -> io::Result<()> {
    /// let mut f = try!(File::open("foo.txt"));
    ///
    /// // move the cursor 42 bytes from the start of the file
    /// try!(f.seek(SeekFrom::Start(42)));
    /// # Ok(())
    /// # }
    /// ```
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait Seek {
        /// Seek to an offset, in bytes, in a stream.
        ///
        /// A seek beyond the end of a stream is allowed, but implementation
        /// defined.
        ///
        /// If the seek operation completed successfully,
        /// this method returns the new position from the start of the stream.
        /// That position can be used later with [`SeekFrom::Start`].
        ///
        /// # Errors
        ///
        /// Seeking to a negative offset is considered an error.
        ///
        /// [`SeekFrom::Start`]: enum.SeekFrom.html#variant.Start
        #[stable(feature = "rust1", since = "1.0.0")]
        fn seek(&mut self, pos: SeekFrom) -> Result<u64>;
    }

    /// Enumeration of possible methods to seek within an I/O object.
    ///
    /// It is used by the [`Seek`] trait.
    ///
    /// [`Seek`]: trait.Seek.html
    #[derive(Copy, PartialEq, Eq, Clone, Debug)]
    #[stable(feature = "rust1", since = "1.0.0")]
    pub enum SeekFrom {
        /// Set the offset to the provided number of bytes.
        #[stable(feature = "rust1", since = "1.0.0")]
        Start(#[stable(feature = "rust1", since = "1.0.0")] u64),

        /// Set the offset to the size of this object plus the specified number of
        /// bytes.
        ///
        /// It is possible to seek beyond the end of an object, but it's an error to
        /// seek before byte 0.
        #[stable(feature = "rust1", since = "1.0.0")]
        End(#[stable(feature = "rust1", since = "1.0.0")] i64),

        /// Set the offset to the current position plus the specified number of
        /// bytes.
        ///
        /// It is possible to seek beyond the end of an object, but it's an error to
        /// seek before byte 0.
        #[stable(feature = "rust1", since = "1.0.0")]
        Current(#[stable(feature = "rust1", since = "1.0.0")] i64),
    }

    fn read_until<R: BufRead + ?Sized>(r: &mut R, delim: u8, buf: &mut Vec<u8>)
                                       -> Result<usize> {
        let mut read = 0;
        loop {
            let (done, used) = {
                let available = match r.fill_buf() {
                    Ok(n) => n,
                    Err(ref e) if e.kind() == ErrorKind::Interrupted => continue,
                    Err(e) => return Err(e)
                };
                match memchr::memchr(delim, available) {
                    Some(i) => {
                        buf.extend_from_slice(&available[..i + 1]);
                        (true, i + 1)
                    }
                    None => {
                        buf.extend_from_slice(available);
                        (false, available.len())
                    }
                }
            };
            r.consume(used);
            read += used;
            if done || used == 0 {
                return Ok(read);
            }
        }
    }

    /// A `BufRead` is a type of `Read`er which has an internal buffer, allowing it
    /// to perform extra ways of reading.
    ///
    /// For example, reading line-by-line is inefficient without using a buffer, so
    /// if you want to read by line, you'll need `BufRead`, which includes a
    /// [`read_line()`] method as well as a [`lines()`] iterator.
    ///
    /// # Examples
    ///
    /// A locked standard input implements `BufRead`:
    ///
    /// ```
    /// use std::io;
    /// use std::io::prelude::*;
    ///
    /// let stdin = io::stdin();
    /// for line in stdin.lock().lines() {
    ///     println!("{}", line.unwrap());
    /// }
    /// ```
    ///
    /// If you have something that implements [`Read`], you can use the [`BufReader`
    /// type][`BufReader`] to turn it into a `BufRead`.
    ///
    /// For example, [`File`] implements [`Read`], but not `BufRead`.
    /// [`BufReader`] to the rescue!
    ///
    /// [`BufReader`]: struct.BufReader.html
    /// [`File`]: ../fs/struct.File.html
    /// [`read_line()`]: #method.read_line
    /// [`lines()`]: #method.lines
    /// [`Read`]: trait.Read.html
    ///
    /// ```
    /// use std::io::{self, BufReader};
    /// use std::io::prelude::*;
    /// use std::fs::File;
    ///
    /// # fn foo() -> io::Result<()> {
    /// let f = try!(File::open("foo.txt"));
    /// let f = BufReader::new(f);
    ///
    /// for line in f.lines() {
    ///     println!("{}", line.unwrap());
    /// }
    ///
    /// # Ok(())
    /// # }
    /// ```
    ///
    #[stable(feature = "rust1", since = "1.0.0")]
    pub trait BufRead: Read {
        /// Fills the internal buffer of this object, returning the buffer contents.
        ///
        /// This function is a lower-level call. It needs to be paired with the
        /// [`consume()`] method to function properly. When calling this
        /// method, none of the contents will be "read" in the sense that later
        /// calling `read` may return the same contents. As such, [`consume()`] must
        /// be called with the number of bytes that are consumed from this buffer to
        /// ensure that the bytes are never returned twice.
        ///
        /// [`consume()`]: #tymethod.consume
        ///
        /// An empty buffer returned indicates that the stream has reached EOF.
        ///
        /// # Errors
        ///
        /// This function will return an I/O error if the underlying reader was
        /// read, but returned an error.
        ///
        /// # Examples
        ///
        /// A locked standard input implements `BufRead`:
        ///
        /// ```
        /// use std::io;
        /// use std::io::prelude::*;
        ///
        /// let stdin = io::stdin();
        /// let mut stdin = stdin.lock();
        ///
        /// // we can't have two `&mut` references to `stdin`, so use a block
        /// // to end the borrow early.
        /// let length = {
        ///     let buffer = stdin.fill_buf().unwrap();
        ///
        ///     // work with buffer
        ///     println!("{:?}", buffer);
        ///
        ///     buffer.len()
        /// };
        ///
        /// // ensure the bytes we worked with aren't returned again later
        /// stdin.consume(length);
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn fill_buf(&mut self) -> Result<&[u8]>;

        /// Tells this buffer that `amt` bytes have been consumed from the buffer,
        /// so they should no longer be returned in calls to `read`.
        ///
        /// This function is a lower-level call. It needs to be paired with the
        /// [`fill_buf()`] method to function properly. This function does
        /// not perform any I/O, it simply informs this object that some amount of
        /// its buffer, returned from [`fill_buf()`], has been consumed and should
        /// no longer be returned. As such, this function may do odd things if
        /// [`fill_buf()`] isn't called before calling it.
        ///
        /// The `amt` must be `<=` the number of bytes in the buffer returned by
        /// [`fill_buf()`].
        ///
        /// # Examples
        ///
        /// Since `consume()` is meant to be used with [`fill_buf()`],
        /// that method's example includes an example of `consume()`.
        ///
        /// [`fill_buf()`]: #tymethod.fill_buf
        #[stable(feature = "rust1", since = "1.0.0")]
        fn consume(&mut self, amt: usize);

        /// Read all bytes into `buf` until the delimiter `byte` or EOF is reached.
        ///
        /// This function will read bytes from the underlying stream until the
        /// delimiter or EOF is found. Once found, all bytes up to, and including,
        /// the delimiter (if found) will be appended to `buf`.
        ///
        /// If successful, this function will return the total number of bytes read.
        ///
        /// # Errors
        ///
        /// This function will ignore all instances of [`ErrorKind::Interrupted`] and
        /// will otherwise return any errors returned by [`fill_buf()`].
        ///
        /// If an I/O error is encountered then all bytes read so far will be
        /// present in `buf` and its length will have been adjusted appropriately.
        ///
        /// # Examples
        ///
        /// A locked standard input implements `BufRead`. In this example, we'll
        /// read from standard input until we see an `a` byte.
        ///
        /// [`fill_buf()`]: #tymethod.fill_buf
        /// [`ErrorKind::Interrupted`]: enum.ErrorKind.html#variant.Interrupted
        ///
        /// ```
        /// use std::io;
        /// use std::io::prelude::*;
        ///
        /// fn foo() -> io::Result<()> {
        /// let stdin = io::stdin();
        /// let mut stdin = stdin.lock();
        /// let mut buffer = Vec::new();
        ///
        /// try!(stdin.read_until(b'a', &mut buffer));
        ///
        /// println!("{:?}", buffer);
        /// # Ok(())
        /// # }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn read_until(&mut self, byte: u8, buf: &mut Vec<u8>) -> Result<usize> {
            read_until(self, byte, buf)
        }

        /// Read all bytes until a newline (the 0xA byte) is reached, and append
        /// them to the provided buffer.
        ///
        /// This function will read bytes from the underlying stream until the
        /// newline delimiter (the 0xA byte) or EOF is found. Once found, all bytes
        /// up to, and including, the delimiter (if found) will be appended to
        /// `buf`.
        ///
        /// If successful, this function will return the total number of bytes read.
        ///
        /// # Errors
        ///
        /// This function has the same error semantics as [`read_until()`] and will
        /// also return an error if the read bytes are not valid UTF-8. If an I/O
        /// error is encountered then `buf` may contain some bytes already read in
        /// the event that all data read so far was valid UTF-8.
        ///
        /// # Examples
        ///
        /// A locked standard input implements `BufRead`. In this example, we'll
        /// read all of the lines from standard input. If we were to do this in
        /// an actual project, the [`lines()`] method would be easier, of
        /// course.
        ///
        /// [`lines()`]: #method.lines
        /// [`read_until()`]: #method.read_until
        ///
        /// ```
        /// use std::io;
        /// use std::io::prelude::*;
        ///
        /// let stdin = io::stdin();
        /// let mut stdin = stdin.lock();
        /// let mut buffer = String::new();
        ///
        /// while stdin.read_line(&mut buffer).unwrap() > 0 {
        ///     // work with buffer
        ///     println!("{:?}", buffer);
        ///
        ///     buffer.clear();
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn read_line(&mut self, buf: &mut String) -> Result<usize> {
            // Note that we are not calling the `.read_until` method here, but
            // rather our hardcoded implementation. For more details as to why, see
            // the comments in `read_to_end`.
            append_to_string(buf, |b| read_until(self, b'\n', b))
        }

        /// Returns an iterator over the contents of this reader split on the byte
        /// `byte`.
        ///
        /// The iterator returned from this function will return instances of
        /// [`io::Result`]`<`[`Vec<u8>`]`>`. Each vector returned will *not* have
        /// the delimiter byte at the end.
        ///
        /// This function will yield errors whenever [`read_until()`] would have
        /// also yielded an error.
        ///
        /// # Examples
        ///
        /// A locked standard input implements `BufRead`. In this example, we'll
        /// read some input from standard input, splitting on commas.
        ///
        /// [`io::Result`]: type.Result.html
        /// [`Vec<u8>`]: ../vec/struct.Vec.html
        /// [`read_until()`]: #method.read_until
        ///
        /// ```
        /// use std::io;
        /// use std::io::prelude::*;
        ///
        /// let stdin = io::stdin();
        ///
        /// for content in stdin.lock().split(b',') {
        ///     println!("{:?}", content.unwrap());
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn split(self, byte: u8) -> Split<Self> where Self: Sized {
            Split { buf: self, delim: byte }
        }

        /// Returns an iterator over the lines of this reader.
        ///
        /// The iterator returned from this function will yield instances of
        /// [`io::Result`]`<`[`String`]`>`. Each string returned will *not* have a newline
        /// byte (the 0xA byte) or CRLF (0xD, 0xA bytes) at the end.
        ///
        /// [`io::Result`]: type.Result.html
        /// [`String`]: ../string/struct.String.html
        ///
        /// # Examples
        ///
        /// A locked standard input implements `BufRead`:
        ///
        /// ```
        /// use std::io;
        /// use std::io::prelude::*;
        ///
        /// let stdin = io::stdin();
        ///
        /// for line in stdin.lock().lines() {
        ///     println!("{}", line.unwrap());
        /// }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        fn lines(self) -> Lines<Self> where Self: Sized {
            Lines { buf: self }
        }
    }

    /// Adaptor to chain together two readers.
    ///
    /// This struct is generally created by calling [`chain()`] on a reader.
    /// Please see the documentation of [`chain()`] for more details.
    ///
    /// [`chain()`]: trait.Read.html#method.chain
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Chain<T, U> {
        first: T,
        second: U,
        done_first: bool,
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: Read, U: Read> Read for Chain<T, U> {
        fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
            if !self.done_first {
                match self.first.read(buf)? {
                    0 if buf.len() != 0 => { self.done_first = true; }
                    n => return Ok(n),
                }
            }
            self.second.read(buf)
        }
    }

    #[stable(feature = "chain_bufread", since = "1.9.0")]
    impl<T: BufRead, U: BufRead> BufRead for Chain<T, U> {
        fn fill_buf(&mut self) -> Result<&[u8]> {
            if !self.done_first {
                match self.first.fill_buf()? {
                    buf if buf.len() == 0 => { self.done_first = true; }
                    buf => return Ok(buf),
                }
            }
            self.second.fill_buf()
        }

        fn consume(&mut self, amt: usize) {
            if !self.done_first {
                self.first.consume(amt)
            } else {
                self.second.consume(amt)
            }
        }
    }

    /// Reader adaptor which limits the bytes read from an underlying reader.
    ///
    /// This struct is generally created by calling [`take()`][take] on a reader.
    /// Please see the documentation of `take()` for more details.
    ///
    /// [take]: trait.Read.html#method.take
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Take<T> {
        inner: T,
        limit: u64,
    }

    impl<T> Take<T> {
        /// Returns the number of bytes that can be read before this instance will
        /// return EOF.
        ///
        /// # Note
        ///
        /// This instance may reach EOF after reading fewer bytes than indicated by
        /// this method if the underlying `Read` instance reaches EOF.
        ///
        /// # Examples
        ///
        /// ```
        /// use std::io;
        /// use std::io::prelude::*;
        /// use std::fs::File;
        ///
        /// # fn foo() -> io::Result<()> {
        /// let f = try!(File::open("foo.txt"));
        ///
        /// // read at most five bytes
        /// let handle = f.take(5);
        ///
        /// println!("limit: {}", handle.limit());
        /// # Ok(())
        /// # }
        /// ```
        #[stable(feature = "rust1", since = "1.0.0")]
        pub fn limit(&self) -> u64 { self.limit }

        /// Consumes the `Take`, returning the wrapped reader.
        ///
        /// # Examples
        ///
        /// ```
        /// #![feature(io_take_into_inner)]
        ///
        /// use std::io;
        /// use std::io::prelude::*;
        /// use std::fs::File;
        ///
        /// # fn foo() -> io::Result<()> {
        /// let mut file = try!(File::open("foo.txt"));
        ///
        /// let mut buffer = [0; 5];
        /// let mut handle = file.take(5);
        /// try!(handle.read(&mut buffer));
        ///
        /// let file = handle.into_inner();
        /// # Ok(())
        /// # }
        /// ```
        #[unstable(feature = "io_take_into_inner", issue = "23755")]
        pub fn into_inner(self) -> T {
            self.inner
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: Read> Read for Take<T> {
        fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
            // Don't call into inner reader at all at EOF because it may still block
            if self.limit == 0 {
                return Ok(0);
            }

            let max = cmp::min(buf.len() as u64, self.limit) as usize;
            let n = self.inner.read(&mut buf[..max])?;
            self.limit -= n as u64;
            Ok(n)
        }
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<T: BufRead> BufRead for Take<T> {
        fn fill_buf(&mut self) -> Result<&[u8]> {
            // Don't call into inner reader at all at EOF because it may still block
            if self.limit == 0 {
                return Ok(&[]);
            }

            let buf = self.inner.fill_buf()?;
            let cap = cmp::min(buf.len() as u64, self.limit) as usize;
            Ok(&buf[..cap])
        }

        fn consume(&mut self, amt: usize) {
            // Don't let callers reset the limit by passing an overlarge value
            let amt = cmp::min(amt as u64, self.limit) as usize;
            self.limit -= amt as u64;
            self.inner.consume(amt);
        }
    }

    fn read_one_byte(reader: &mut Read) -> Option<Result<u8>> {
        let mut buf = [0];
        loop {
            return match reader.read(&mut buf) {
                Ok(0) => None,
                Ok(..) => Some(Ok(buf[0])),
                Err(ref e) if e.kind() == ErrorKind::Interrupted => continue,
                Err(e) => Some(Err(e)),
            };
        }
    }

    /// An iterator over `u8` values of a reader.
    ///
    /// This struct is generally created by calling [`bytes()`] on a reader.
    /// Please see the documentation of [`bytes()`] for more details.
    ///
    /// [`bytes()`]: trait.Read.html#method.bytes
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Bytes<R> {
        inner: R,
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<R: Read> Iterator for Bytes<R> {
        type Item = Result<u8>;

        fn next(&mut self) -> Option<Result<u8>> {
            read_one_byte(&mut self.inner)
        }
    }

    /// An iterator over the `char`s of a reader.
    ///
    /// This struct is generally created by calling [`chars()`][chars] on a reader.
    /// Please see the documentation of `chars()` for more details.
    ///
    /// [chars]: trait.Read.html#method.chars
    #[unstable(feature = "io", reason = "awaiting stability of Read::chars",
               issue = "27802")]
    pub struct Chars<R> {
        inner: R,
    }

    /// An enumeration of possible errors that can be generated from the `Chars`
    /// adapter.
    #[derive(Debug)]
    #[unstable(feature = "io", reason = "awaiting stability of Read::chars",
               issue = "27802")]
    pub enum CharsError {
        /// Variant representing that the underlying stream was read successfully
        /// but it did not contain valid utf8 data.
        NotUtf8,

        /// Variant representing that an I/O error occurred.
        Other(Error),
    }

    #[unstable(feature = "io", reason = "awaiting stability of Read::chars",
               issue = "27802")]
    impl<R: Read> Iterator for Chars<R> {
        type Item = result::Result<char, CharsError>;

        fn next(&mut self) -> Option<result::Result<char, CharsError>> {
            let first_byte = match read_one_byte(&mut self.inner) {
                None => return None,
                Some(Ok(b)) => b,
                Some(Err(e)) => return Some(Err(CharsError::Other(e))),
            };
            let width = core_str::utf8_char_width(first_byte);
            if width == 1 { return Some(Ok(first_byte as char)) }
            if width == 0 { return Some(Err(CharsError::NotUtf8)) }
            let mut buf = [first_byte, 0, 0, 0];
            {
                let mut start = 1;
                while start < width {
                    match self.inner.read(&mut buf[start..width]) {
                        Ok(0) => return Some(Err(CharsError::NotUtf8)),
                        Ok(n) => start += n,
                        Err(ref e) if e.kind() == ErrorKind::Interrupted => continue,
                        Err(e) => return Some(Err(CharsError::Other(e))),
                    }
                }
            }
            Some(match str::from_utf8(&buf[..width]).ok() {
                Some(s) => Ok(s.chars().next().unwrap()),
                None => Err(CharsError::NotUtf8),
            })
        }
    }

    #[unstable(feature = "io", reason = "awaiting stability of Read::chars",
               issue = "27802")]
    impl std_error::Error for CharsError {
        fn description(&self) -> &str {
            match *self {
                CharsError::NotUtf8 => "invalid utf8 encoding",
                CharsError::Other(ref e) => std_error::Error::description(e),
            }
        }
        fn cause(&self) -> Option<&std_error::Error> {
            match *self {
                CharsError::NotUtf8 => None,
                CharsError::Other(ref e) => e.cause(),
            }
        }
    }

    #[unstable(feature = "io", reason = "awaiting stability of Read::chars",
               issue = "27802")]
    impl fmt::Display for CharsError {
        fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
            match *self {
                CharsError::NotUtf8 => {
                    "byte stream did not contain valid utf8".fmt(f)
                }
                CharsError::Other(ref e) => e.fmt(f),
            }
        }
    }

    /// An iterator over the contents of an instance of `BufRead` split on a
    /// particular byte.
    ///
    /// This struct is generally created by calling [`split()`][split] on a
    /// `BufRead`. Please see the documentation of `split()` for more details.
    ///
    /// [split]: trait.BufRead.html#method.split
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Split<B> {
        buf: B,
        delim: u8,
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<B: BufRead> Iterator for Split<B> {
        type Item = Result<Vec<u8>>;

        fn next(&mut self) -> Option<Result<Vec<u8>>> {
            let mut buf = Vec::new();
            match self.buf.read_until(self.delim, &mut buf) {
                Ok(0) => None,
                Ok(_n) => {
                    if buf[buf.len() - 1] == self.delim {
                        buf.pop();
                    }
                    Some(Ok(buf))
                }
                Err(e) => Some(Err(e))
            }
        }
    }

    /// An iterator over the lines of an instance of `BufRead`.
    ///
    /// This struct is generally created by calling [`lines()`][lines] on a
    /// `BufRead`. Please see the documentation of `lines()` for more details.
    ///
    /// [lines]: trait.BufRead.html#method.lines
    #[stable(feature = "rust1", since = "1.0.0")]
    pub struct Lines<B> {
        buf: B,
    }

    #[stable(feature = "rust1", since = "1.0.0")]
    impl<B: BufRead> Iterator for Lines<B> {
        type Item = Result<String>;

        fn next(&mut self) -> Option<Result<String>> {
            let mut buf = String::new();
            match self.buf.read_line(&mut buf) {
                Ok(0) => None,
                Ok(_n) => {
                    if buf.ends_with("\n") {
                        buf.pop();
                        if buf.ends_with("\r") {
                            buf.pop();
                        }
                    }
                    Some(Ok(buf))
                }
                Err(e) => Some(Err(e))
            }
        }
    }

    #[cfg(test)]
    mod tests {
        use io::prelude::*;
        use io;
        use super::Cursor;
        use test;
        use super::repeat;

        #[test]
        #[cfg_attr(target_os = "emscripten", ignore)]
        fn read_until() {
            let mut buf = Cursor::new(&b"12"[..]);
            let mut v = Vec::new();
            assert_eq!(buf.read_until(b'3', &mut v).unwrap(), 2);
            assert_eq!(v, b"12");

            let mut buf = Cursor::new(&b"1233"[..]);
            let mut v = Vec::new();
            assert_eq!(buf.read_until(b'3', &mut v).unwrap(), 3);
            assert_eq!(v, b"123");
            v.truncate(0);
            assert_eq!(buf.read_until(b'3', &mut v).unwrap(), 1);
            assert_eq!(v, b"3");
            v.truncate(0);
            assert_eq!(buf.read_until(b'3', &mut v).unwrap(), 0);
            assert_eq!(v, []);
        }

        #[test]
        fn split() {
            let buf = Cursor::new(&b"12"[..]);
            let mut s = buf.split(b'3');
            assert_eq!(s.next().unwrap().unwrap(), vec![b'1', b'2']);
            assert!(s.next().is_none());

            let buf = Cursor::new(&b"1233"[..]);
            let mut s = buf.split(b'3');
            assert_eq!(s.next().unwrap().unwrap(), vec![b'1', b'2']);
            assert_eq!(s.next().unwrap().unwrap(), vec![]);
            assert!(s.next().is_none());
        }

        #[test]
        fn read_line() {
            let mut buf = Cursor::new(&b"12"[..]);
            let mut v = String::new();
            assert_eq!(buf.read_line(&mut v).unwrap(), 2);
            assert_eq!(v, "12");

            let mut buf = Cursor::new(&b"12\n\n"[..]);
            let mut v = String::new();
            assert_eq!(buf.read_line(&mut v).unwrap(), 3);
            assert_eq!(v, "12\n");
            v.truncate(0);
            assert_eq!(buf.read_line(&mut v).unwrap(), 1);
            assert_eq!(v, "\n");
            v.truncate(0);
            assert_eq!(buf.read_line(&mut v).unwrap(), 0);
            assert_eq!(v, "");
        }

        #[test]
        fn lines() {
            let buf = Cursor::new(&b"12\r"[..]);
            let mut s = buf.lines();
            assert_eq!(s.next().unwrap().unwrap(), "12\r".to_string());
            assert!(s.next().is_none());

            let buf = Cursor::new(&b"12\r\n\n"[..]);
            let mut s = buf.lines();
            assert_eq!(s.next().unwrap().unwrap(), "12".to_string());
            assert_eq!(s.next().unwrap().unwrap(), "".to_string());
            assert!(s.next().is_none());
        }

        #[test]
        fn read_to_end() {
            let mut c = Cursor::new(&b""[..]);
            let mut v = Vec::new();
            assert_eq!(c.read_to_end(&mut v).unwrap(), 0);
            assert_eq!(v, []);

            let mut c = Cursor::new(&b"1"[..]);
            let mut v = Vec::new();
            assert_eq!(c.read_to_end(&mut v).unwrap(), 1);
            assert_eq!(v, b"1");

            let cap = 1024 * 1024;
            let data = (0..cap).map(|i| (i / 3) as u8).collect::<Vec<_>>();
            let mut v = Vec::new();
            let (a, b) = data.split_at(data.len() / 2);
            assert_eq!(Cursor::new(a).read_to_end(&mut v).unwrap(), a.len());
            assert_eq!(Cursor::new(b).read_to_end(&mut v).unwrap(), b.len());
            assert_eq!(v, data);
        }

        #[test]
        fn read_to_string() {
            let mut c = Cursor::new(&b""[..]);
            let mut v = String::new();
            assert_eq!(c.read_to_string(&mut v).unwrap(), 0);
            assert_eq!(v, "");

            let mut c = Cursor::new(&b"1"[..]);
            let mut v = String::new();
            assert_eq!(c.read_to_string(&mut v).unwrap(), 1);
            assert_eq!(v, "1");

            let mut c = Cursor::new(&b"\xff"[..]);
            let mut v = String::new();
            assert!(c.read_to_string(&mut v).is_err());
        }

        #[test]
        fn read_exact() {
            let mut buf = [0; 4];

            let mut c = Cursor::new(&b""[..]);
            assert_eq!(c.read_exact(&mut buf).unwrap_err().kind(),
                       io::ErrorKind::UnexpectedEof);

            let mut c = Cursor::new(&b"123"[..]).chain(Cursor::new(&b"456789"[..]));
            c.read_exact(&mut buf).unwrap();
            assert_eq!(&buf, b"1234");
            c.read_exact(&mut buf).unwrap();
            assert_eq!(&buf, b"5678");
            assert_eq!(c.read_exact(&mut buf).unwrap_err().kind(),
                       io::ErrorKind::UnexpectedEof);
        }

        #[test]
        fn read_exact_slice() {
            let mut buf = [0; 4];

            let mut c = &b""[..];
            assert_eq!(c.read_exact(&mut buf).unwrap_err().kind(),
                       io::ErrorKind::UnexpectedEof);

            let mut c = &b"123"[..];
            assert_eq!(c.read_exact(&mut buf).unwrap_err().kind(),
                       io::ErrorKind::UnexpectedEof);
            // make sure the optimized (early returning) method is being used
            assert_eq!(&buf, &[0; 4]);

            let mut c = &b"1234"[..];
            c.read_exact(&mut buf).unwrap();
            assert_eq!(&buf, b"1234");

            let mut c = &b"56789"[..];
            c.read_exact(&mut buf).unwrap();
            assert_eq!(&buf, b"5678");
            assert_eq!(c, b"9");
        }

        #[test]
        fn take_eof() {
            struct R;

            impl Read for R {
                fn read(&mut self, _: &mut [u8]) -> io::Result<usize> {
                    Err(io::Error::new(io::ErrorKind::Other, ""))
                }
            }
            impl BufRead for R {
                fn fill_buf(&mut self) -> io::Result<&[u8]> {
                    Err(io::Error::new(io::ErrorKind::Other, ""))
                }
                fn consume(&mut self, _amt: usize) { }
            }

            let mut buf = [0; 1];
            assert_eq!(0, R.take(0).read(&mut buf).unwrap());
            assert_eq!(b"", R.take(0).fill_buf().unwrap());
        }

        fn cmp_bufread<Br1: BufRead, Br2: BufRead>(mut br1: Br1, mut br2: Br2, exp: &[u8]) {
            let mut cat = Vec::new();
            loop {
                let consume = {
                    let buf1 = br1.fill_buf().unwrap();
                    let buf2 = br2.fill_buf().unwrap();
                    let minlen = if buf1.len() < buf2.len() { buf1.len() } else { buf2.len() };
                    assert_eq!(buf1[..minlen], buf2[..minlen]);
                    cat.extend_from_slice(&buf1[..minlen]);
                    minlen
                };
                if consume == 0 {
                    break;
                }
                br1.consume(consume);
                br2.consume(consume);
            }
            assert_eq!(br1.fill_buf().unwrap().len(), 0);
            assert_eq!(br2.fill_buf().unwrap().len(), 0);
            assert_eq!(&cat[..], &exp[..])
        }

        #[test]
        fn chain_bufread() {
            let testdata = b"ABCDEFGHIJKL";
            let chain1 = (&testdata[..3]).chain(&testdata[3..6])
                                         .chain(&testdata[6..9])
                                         .chain(&testdata[9..]);
            let chain2 = (&testdata[..4]).chain(&testdata[4..8])
                                         .chain(&testdata[8..]);
            cmp_bufread(chain1, chain2, &testdata[..]);
        }

        #[test]
        fn chain_zero_length_read_is_not_eof() {
            let a = b"A";
            let b = b"B";
            let mut s = String::new();
            let mut chain = (&a[..]).chain(&b[..]);
            chain.read(&mut []).unwrap();
            chain.read_to_string(&mut s).unwrap();
            assert_eq!("AB", s);
        }

        #[bench]
        #[cfg_attr(target_os = "emscripten", ignore)]
        fn bench_read_to_end(b: &mut test::Bencher) {
            b.iter(|| {
                let mut lr = repeat(1).take(10000000);
                let mut vec = Vec::with_capacity(1024);
                super::read_to_end(&mut lr, &mut vec)
            });
        }
    }

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