1 //! Traits, helpers, and type definitions for core I/O functionality.
3 //! The `std::io` module contains a number of common things you'll need
4 //! when doing input and output. The most core part of this module is
5 //! the [`Read`] and [`Write`] traits, which provide the
6 //! most general interface for reading and writing input and output.
10 //! Because they are traits, [`Read`] and [`Write`] are implemented by a number
11 //! of other types, and you can implement them for your types too. As such,
12 //! you'll see a few different types of I/O throughout the documentation in
13 //! this module: [`File`]s, [`TcpStream`]s, and sometimes even [`Vec<T>`]s. For
14 //! example, [`Read`] adds a [`read`][`Read::read`] method, which we can use on
19 //! use std::io::prelude::*;
20 //! use std::fs::File;
22 //! fn main() -> io::Result<()> {
23 //! let mut f = File::open("foo.txt")?;
24 //! let mut buffer = [0; 10];
26 //! // read up to 10 bytes
27 //! let n = f.read(&mut buffer)?;
29 //! println!("The bytes: {:?}", &buffer[..n]);
34 //! [`Read`] and [`Write`] are so important, implementors of the two traits have a
35 //! nickname: readers and writers. So you'll sometimes see 'a reader' instead
36 //! of 'a type that implements the [`Read`] trait'. Much easier!
38 //! ## Seek and BufRead
40 //! Beyond that, there are two important traits that are provided: [`Seek`]
41 //! and [`BufRead`]. Both of these build on top of a reader to control
42 //! how the reading happens. [`Seek`] lets you control where the next byte is
47 //! use std::io::prelude::*;
48 //! use std::io::SeekFrom;
49 //! use std::fs::File;
51 //! fn main() -> io::Result<()> {
52 //! let mut f = File::open("foo.txt")?;
53 //! let mut buffer = [0; 10];
55 //! // skip to the last 10 bytes of the file
56 //! f.seek(SeekFrom::End(-10))?;
58 //! // read up to 10 bytes
59 //! let n = f.read(&mut buffer)?;
61 //! println!("The bytes: {:?}", &buffer[..n]);
66 //! [`BufRead`] uses an internal buffer to provide a number of other ways to read, but
67 //! to show it off, we'll need to talk about buffers in general. Keep reading!
69 //! ## BufReader and BufWriter
71 //! Byte-based interfaces are unwieldy and can be inefficient, as we'd need to be
72 //! making near-constant calls to the operating system. To help with this,
73 //! `std::io` comes with two structs, [`BufReader`] and [`BufWriter`], which wrap
74 //! readers and writers. The wrapper uses a buffer, reducing the number of
75 //! calls and providing nicer methods for accessing exactly what you want.
77 //! For example, [`BufReader`] works with the [`BufRead`] trait to add extra
78 //! methods to any reader:
82 //! use std::io::prelude::*;
83 //! use std::io::BufReader;
84 //! use std::fs::File;
86 //! fn main() -> io::Result<()> {
87 //! let f = File::open("foo.txt")?;
88 //! let mut reader = BufReader::new(f);
89 //! let mut buffer = String::new();
91 //! // read a line into buffer
92 //! reader.read_line(&mut buffer)?;
94 //! println!("{}", buffer);
99 //! [`BufWriter`] doesn't add any new ways of writing; it just buffers every call
100 //! to [`write`][`Write::write`]:
104 //! use std::io::prelude::*;
105 //! use std::io::BufWriter;
106 //! use std::fs::File;
108 //! fn main() -> io::Result<()> {
109 //! let f = File::create("foo.txt")?;
111 //! let mut writer = BufWriter::new(f);
113 //! // write a byte to the buffer
114 //! writer.write(&[42])?;
116 //! } // the buffer is flushed once writer goes out of scope
122 //! ## Standard input and output
124 //! A very common source of input is standard input:
129 //! fn main() -> io::Result<()> {
130 //! let mut input = String::new();
132 //! io::stdin().read_line(&mut input)?;
134 //! println!("You typed: {}", input.trim());
139 //! Note that you cannot use the [`?` operator] in functions that do not return
140 //! a [`Result<T, E>`][`Result`]. Instead, you can call [`.unwrap()`]
141 //! or `match` on the return value to catch any possible errors:
146 //! let mut input = String::new();
148 //! io::stdin().read_line(&mut input).unwrap();
151 //! And a very common source of output is standard output:
155 //! use std::io::prelude::*;
157 //! fn main() -> io::Result<()> {
158 //! io::stdout().write(&[42])?;
163 //! Of course, using [`io::stdout`] directly is less common than something like
166 //! ## Iterator types
168 //! A large number of the structures provided by `std::io` are for various
169 //! ways of iterating over I/O. For example, [`Lines`] is used to split over
174 //! use std::io::prelude::*;
175 //! use std::io::BufReader;
176 //! use std::fs::File;
178 //! fn main() -> io::Result<()> {
179 //! let f = File::open("foo.txt")?;
180 //! let reader = BufReader::new(f);
182 //! for line in reader.lines() {
183 //! println!("{}", line?);
191 //! There are a number of [functions][functions-list] that offer access to various
192 //! features. For example, we can use three of these functions to copy everything
193 //! from standard input to standard output:
198 //! fn main() -> io::Result<()> {
199 //! io::copy(&mut io::stdin(), &mut io::stdout())?;
204 //! [functions-list]: #functions-1
208 //! Last, but certainly not least, is [`io::Result`]. This type is used
209 //! as the return type of many `std::io` functions that can cause an error, and
210 //! can be returned from your own functions as well. Many of the examples in this
211 //! module use the [`?` operator]:
216 //! fn read_input() -> io::Result<()> {
217 //! let mut input = String::new();
219 //! io::stdin().read_line(&mut input)?;
221 //! println!("You typed: {}", input.trim());
227 //! The return type of `read_input()`, [`io::Result<()>`][`io::Result`], is a very
228 //! common type for functions which don't have a 'real' return value, but do want to
229 //! return errors if they happen. In this case, the only purpose of this function is
230 //! to read the line and print it, so we use `()`.
232 //! ## Platform-specific behavior
234 //! Many I/O functions throughout the standard library are documented to indicate
235 //! what various library or syscalls they are delegated to. This is done to help
236 //! applications both understand what's happening under the hood as well as investigate
237 //! any possibly unclear semantics. Note, however, that this is informative, not a binding
238 //! contract. The implementation of many of these functions are subject to change over
239 //! time and may call fewer or more syscalls/library functions.
241 //! [`File`]: crate::fs::File
242 //! [`TcpStream`]: crate::net::TcpStream
244 //! [`io::stdout`]: stdout
245 //! [`io::Result`]: self::Result
246 //! [`?` operator]: ../../book/appendix-02-operators.html
247 //! [`Result`]: crate::result::Result
248 //! [`.unwrap()`]: crate::result::Result::unwrap
250 #![stable(feature = "rust1", since = "1.0.0")]
258 use crate::ops
::{Deref, DerefMut}
;
264 #[stable(feature = "rust1", since = "1.0.0")]
265 pub use self::buffered
::IntoInnerError
;
266 #[stable(feature = "rust1", since = "1.0.0")]
267 pub use self::buffered
::{BufReader, BufWriter, LineWriter}
;
268 #[stable(feature = "rust1", since = "1.0.0")]
269 pub use self::cursor
::Cursor
;
270 #[stable(feature = "rust1", since = "1.0.0")]
271 pub use self::error
::{Error, ErrorKind, Result}
;
272 #[stable(feature = "rust1", since = "1.0.0")]
273 pub use self::stdio
::{stderr, stdin, stdout, Stderr, Stdin, Stdout}
;
274 #[stable(feature = "rust1", since = "1.0.0")]
275 pub use self::stdio
::{StderrLock, StdinLock, StdoutLock}
;
276 #[unstable(feature = "print_internals", issue = "none")]
277 pub use self::stdio
::{_eprint, _print}
;
278 #[unstable(feature = "libstd_io_internals", issue = "42788")]
279 #[doc(no_inline, hidden)]
280 pub use self::stdio
::{set_panic, set_print}
;
281 #[stable(feature = "rust1", since = "1.0.0")]
282 pub use self::util
::{copy, empty, repeat, sink, Empty, Repeat, Sink}
;
292 const DEFAULT_BUF_SIZE
: usize = crate::sys_common
::io
::DEFAULT_BUF_SIZE
;
295 buf
: &'a
mut Vec
<u8>,
299 impl Drop
for Guard
<'_
> {
302 self.buf
.set_len(self.len
);
307 // A few methods below (read_to_string, read_line) will append data into a
308 // `String` buffer, but we need to be pretty careful when doing this. The
309 // implementation will just call `.as_mut_vec()` and then delegate to a
310 // byte-oriented reading method, but we must ensure that when returning we never
311 // leave `buf` in a state such that it contains invalid UTF-8 in its bounds.
313 // To this end, we use an RAII guard (to protect against panics) which updates
314 // the length of the string when it is dropped. This guard initially truncates
315 // the string to the prior length and only after we've validated that the
316 // new contents are valid UTF-8 do we allow it to set a longer length.
318 // The unsafety in this function is twofold:
320 // 1. We're looking at the raw bytes of `buf`, so we take on the burden of UTF-8
322 // 2. We're passing a raw buffer to the function `f`, and it is expected that
323 // the function only *appends* bytes to the buffer. We'll get undefined
324 // behavior if existing bytes are overwritten to have non-UTF-8 data.
325 fn append_to_string
<F
>(buf
: &mut String
, f
: F
) -> Result
<usize>
327 F
: FnOnce(&mut Vec
<u8>) -> Result
<usize>,
330 let mut g
= Guard { len: buf.len(), buf: buf.as_mut_vec() }
;
332 if str::from_utf8(&g
.buf
[g
.len
..]).is_err() {
334 Err(Error
::new(ErrorKind
::InvalidData
, "stream did not contain valid UTF-8"))
343 // This uses an adaptive system to extend the vector when it fills. We want to
344 // avoid paying to allocate and zero a huge chunk of memory if the reader only
345 // has 4 bytes while still making large reads if the reader does have a ton
346 // of data to return. Simply tacking on an extra DEFAULT_BUF_SIZE space every
347 // time is 4,500 times (!) slower than a default reservation size of 32 if the
348 // reader has a very small amount of data to return.
350 // Because we're extending the buffer with uninitialized data for trusted
351 // readers, we need to make sure to truncate that if any of this panics.
352 fn read_to_end
<R
: Read
+ ?Sized
>(r
: &mut R
, buf
: &mut Vec
<u8>) -> Result
<usize> {
353 read_to_end_with_reservation(r
, buf
, |_
| 32)
356 fn read_to_end_with_reservation
<R
, F
>(
359 mut reservation_size
: F
,
363 F
: FnMut(&R
) -> usize,
365 let start_len
= buf
.len();
366 let mut g
= Guard { len: buf.len(), buf }
;
369 if g
.len
== g
.buf
.len() {
371 // FIXME(danielhenrymantilla): #42788
373 // - This creates a (mut) reference to a slice of
374 // _uninitialized_ integers, which is **undefined behavior**
376 // - Only the standard library gets to soundly "ignore" this,
377 // based on its privileged knowledge of unstable rustc
379 g
.buf
.reserve(reservation_size(r
));
380 let capacity
= g
.buf
.capacity();
381 g
.buf
.set_len(capacity
);
382 r
.initializer().initialize(&mut g
.buf
[g
.len
..]);
386 match r
.read(&mut g
.buf
[g
.len
..]) {
388 ret
= Ok(g
.len
- start_len
);
392 Err(ref e
) if e
.kind() == ErrorKind
::Interrupted
=> {}
403 pub(crate) fn default_read_vectored
<F
>(read
: F
, bufs
: &mut [IoSliceMut
<'_
>]) -> Result
<usize>
405 F
: FnOnce(&mut [u8]) -> Result
<usize>,
407 let buf
= bufs
.iter_mut().find(|b
| !b
.is_empty()).map_or(&mut [][..], |b
| &mut **b
);
411 pub(crate) fn default_write_vectored
<F
>(write
: F
, bufs
: &[IoSlice
<'_
>]) -> Result
<usize>
413 F
: FnOnce(&[u8]) -> Result
<usize>,
415 let buf
= bufs
.iter().find(|b
| !b
.is_empty()).map_or(&[][..], |b
| &**b
);
419 /// The `Read` trait allows for reading bytes from a source.
421 /// Implementors of the `Read` trait are called 'readers'.
423 /// Readers are defined by one required method, [`read()`]. Each call to [`read()`]
424 /// will attempt to pull bytes from this source into a provided buffer. A
425 /// number of other methods are implemented in terms of [`read()`], giving
426 /// implementors a number of ways to read bytes while only needing to implement
429 /// Readers are intended to be composable with one another. Many implementors
430 /// throughout [`std::io`] take and provide types which implement the `Read`
433 /// Please note that each call to [`read()`] may involve a system call, and
434 /// therefore, using something that implements [`BufRead`], such as
435 /// [`BufReader`], will be more efficient.
439 /// [`File`]s implement `Read`:
443 /// use std::io::prelude::*;
444 /// use std::fs::File;
446 /// fn main() -> io::Result<()> {
447 /// let mut f = File::open("foo.txt")?;
448 /// let mut buffer = [0; 10];
450 /// // read up to 10 bytes
451 /// f.read(&mut buffer)?;
453 /// let mut buffer = Vec::new();
454 /// // read the whole file
455 /// f.read_to_end(&mut buffer)?;
457 /// // read into a String, so that you don't need to do the conversion.
458 /// let mut buffer = String::new();
459 /// f.read_to_string(&mut buffer)?;
461 /// // and more! See the other methods for more details.
466 /// Read from [`&str`] because [`&[u8]`][slice] implements `Read`:
470 /// use std::io::prelude::*;
472 /// fn main() -> io::Result<()> {
473 /// let mut b = "This string will be read".as_bytes();
474 /// let mut buffer = [0; 10];
476 /// // read up to 10 bytes
477 /// b.read(&mut buffer)?;
479 /// // etc... it works exactly as a File does!
484 /// [`read()`]: Read::read
485 /// [`&str`]: prim@str
486 /// [`std::io`]: self
487 /// [`File`]: crate::fs::File
488 /// [slice]: ../../std/primitive.slice.html
489 #[stable(feature = "rust1", since = "1.0.0")]
492 /// Pull some bytes from this source into the specified buffer, returning
493 /// how many bytes were read.
495 /// This function does not provide any guarantees about whether it blocks
496 /// waiting for data, but if an object needs to block for a read and cannot,
497 /// it will typically signal this via an [`Err`] return value.
499 /// If the return value of this method is [`Ok(n)`], then it must be
500 /// guaranteed that `0 <= n <= buf.len()`. A nonzero `n` value indicates
501 /// that the buffer `buf` has been filled in with `n` bytes of data from this
502 /// source. If `n` is `0`, then it can indicate one of two scenarios:
504 /// 1. This reader has reached its "end of file" and will likely no longer
505 /// be able to produce bytes. Note that this does not mean that the
506 /// reader will *always* no longer be able to produce bytes.
507 /// 2. The buffer specified was 0 bytes in length.
509 /// It is not an error if the returned value `n` is smaller than the buffer size,
510 /// even when the reader is not at the end of the stream yet.
511 /// This may happen for example because fewer bytes are actually available right now
512 /// (e. g. being close to end-of-file) or because read() was interrupted by a signal.
514 /// No guarantees are provided about the contents of `buf` when this
515 /// function is called, implementations cannot rely on any property of the
516 /// contents of `buf` being true. It is recommended that *implementations*
517 /// only write data to `buf` instead of reading its contents.
519 /// Correspondingly, however, *callers* of this method may not assume any guarantees
520 /// about how the implementation uses `buf`. The trait is safe to implement,
521 /// so it is possible that the code that's supposed to write to the buffer might also read
522 /// from it. It is your responsibility to make sure that `buf` is initialized
523 /// before calling `read`. Calling `read` with an uninitialized `buf` (of the kind one
524 /// obtains via [`MaybeUninit<T>`]) is not safe, and can lead to undefined behavior.
526 /// [`MaybeUninit<T>`]: crate::mem::MaybeUninit
530 /// If this function encounters any form of I/O or other error, an error
531 /// variant will be returned. If an error is returned then it must be
532 /// guaranteed that no bytes were read.
534 /// An error of the [`ErrorKind::Interrupted`] kind is non-fatal and the read
535 /// operation should be retried if there is nothing else to do.
539 /// [`File`]s implement `Read`:
542 /// [`File`]: crate::fs::File
546 /// use std::io::prelude::*;
547 /// use std::fs::File;
549 /// fn main() -> io::Result<()> {
550 /// let mut f = File::open("foo.txt")?;
551 /// let mut buffer = [0; 10];
553 /// // read up to 10 bytes
554 /// let n = f.read(&mut buffer[..])?;
556 /// println!("The bytes: {:?}", &buffer[..n]);
560 #[stable(feature = "rust1", since = "1.0.0")]
561 fn read(&mut self, buf
: &mut [u8]) -> Result
<usize>;
563 /// Like `read`, except that it reads into a slice of buffers.
565 /// Data is copied to fill each buffer in order, with the final buffer
566 /// written to possibly being only partially filled. This method must
567 /// behave equivalently to a single call to `read` with concatenated
570 /// The default implementation calls `read` with either the first nonempty
571 /// buffer provided, or an empty one if none exists.
572 #[stable(feature = "iovec", since = "1.36.0")]
573 fn read_vectored(&mut self, bufs
: &mut [IoSliceMut
<'_
>]) -> Result
<usize> {
574 default_read_vectored(|b
| self.read(b
), bufs
)
577 /// Determines if this `Read`er has an efficient `read_vectored`
580 /// If a `Read`er does not override the default `read_vectored`
581 /// implementation, code using it may want to avoid the method all together
582 /// and coalesce writes into a single buffer for higher performance.
584 /// The default implementation returns `false`.
585 #[unstable(feature = "can_vector", issue = "69941")]
586 fn is_read_vectored(&self) -> bool
{
590 /// Determines if this `Read`er can work with buffers of uninitialized
593 /// The default implementation returns an initializer which will zero
596 /// If a `Read`er guarantees that it can work properly with uninitialized
597 /// memory, it should call [`Initializer::nop()`]. See the documentation for
598 /// [`Initializer`] for details.
600 /// The behavior of this method must be independent of the state of the
601 /// `Read`er - the method only takes `&self` so that it can be used through
606 /// This method is unsafe because a `Read`er could otherwise return a
607 /// non-zeroing `Initializer` from another `Read` type without an `unsafe`
609 #[unstable(feature = "read_initializer", issue = "42788")]
611 unsafe fn initializer(&self) -> Initializer
{
612 Initializer
::zeroing()
615 /// Read all bytes until EOF in this source, placing them into `buf`.
617 /// All bytes read from this source will be appended to the specified buffer
618 /// `buf`. This function will continuously call [`read()`] to append more data to
619 /// `buf` until [`read()`] returns either [`Ok(0)`] or an error of
620 /// non-[`ErrorKind::Interrupted`] kind.
622 /// If successful, this function will return the total number of bytes read.
626 /// If this function encounters an error of the kind
627 /// [`ErrorKind::Interrupted`] then the error is ignored and the operation
630 /// If any other read error is encountered then this function immediately
631 /// returns. Any bytes which have already been read will be appended to
636 /// [`File`]s implement `Read`:
638 /// [`read()`]: Read::read
640 /// [`File`]: crate::fs::File
644 /// use std::io::prelude::*;
645 /// use std::fs::File;
647 /// fn main() -> io::Result<()> {
648 /// let mut f = File::open("foo.txt")?;
649 /// let mut buffer = Vec::new();
651 /// // read the whole file
652 /// f.read_to_end(&mut buffer)?;
657 /// (See also the [`std::fs::read`] convenience function for reading from a
660 /// [`std::fs::read`]: crate::fs::read
661 #[stable(feature = "rust1", since = "1.0.0")]
662 fn read_to_end(&mut self, buf
: &mut Vec
<u8>) -> Result
<usize> {
663 read_to_end(self, buf
)
666 /// Read all bytes until EOF in this source, appending them to `buf`.
668 /// If successful, this function returns the number of bytes which were read
669 /// and appended to `buf`.
673 /// If the data in this stream is *not* valid UTF-8 then an error is
674 /// returned and `buf` is unchanged.
676 /// See [`read_to_end`] for other error semantics.
678 /// [`read_to_end`]: Read::read_to_end
682 /// [`File`]s implement `Read`:
684 /// [`File`]: crate::fs::File
688 /// use std::io::prelude::*;
689 /// use std::fs::File;
691 /// fn main() -> io::Result<()> {
692 /// let mut f = File::open("foo.txt")?;
693 /// let mut buffer = String::new();
695 /// f.read_to_string(&mut buffer)?;
700 /// (See also the [`std::fs::read_to_string`] convenience function for
701 /// reading from a file.)
703 /// [`std::fs::read_to_string`]: crate::fs::read_to_string
704 #[stable(feature = "rust1", since = "1.0.0")]
705 fn read_to_string(&mut self, buf
: &mut String
) -> Result
<usize> {
706 // Note that we do *not* call `.read_to_end()` here. We are passing
707 // `&mut Vec<u8>` (the raw contents of `buf`) into the `read_to_end`
708 // method to fill it up. An arbitrary implementation could overwrite the
709 // entire contents of the vector, not just append to it (which is what
710 // we are expecting).
712 // To prevent extraneously checking the UTF-8-ness of the entire buffer
713 // we pass it to our hardcoded `read_to_end` implementation which we
714 // know is guaranteed to only read data into the end of the buffer.
715 append_to_string(buf
, |b
| read_to_end(self, b
))
718 /// Read the exact number of bytes required to fill `buf`.
720 /// This function reads as many bytes as necessary to completely fill the
721 /// specified buffer `buf`.
723 /// No guarantees are provided about the contents of `buf` when this
724 /// function is called, implementations cannot rely on any property of the
725 /// contents of `buf` being true. It is recommended that implementations
726 /// only write data to `buf` instead of reading its contents. The
727 /// documentation on [`read`] has a more detailed explanation on this
732 /// If this function encounters an error of the kind
733 /// [`ErrorKind::Interrupted`] then the error is ignored and the operation
736 /// If this function encounters an "end of file" before completely filling
737 /// the buffer, it returns an error of the kind [`ErrorKind::UnexpectedEof`].
738 /// The contents of `buf` are unspecified in this case.
740 /// If any other read error is encountered then this function immediately
741 /// returns. The contents of `buf` are unspecified in this case.
743 /// If this function returns an error, it is unspecified how many bytes it
744 /// has read, but it will never read more than would be necessary to
745 /// completely fill the buffer.
749 /// [`File`]s implement `Read`:
751 /// [`read`]: Read::read
752 /// [`File`]: crate::fs::File
756 /// use std::io::prelude::*;
757 /// use std::fs::File;
759 /// fn main() -> io::Result<()> {
760 /// let mut f = File::open("foo.txt")?;
761 /// let mut buffer = [0; 10];
763 /// // read exactly 10 bytes
764 /// f.read_exact(&mut buffer)?;
768 #[stable(feature = "read_exact", since = "1.6.0")]
769 fn read_exact(&mut self, mut buf
: &mut [u8]) -> Result
<()> {
770 while !buf
.is_empty() {
771 match self.read(buf
) {
777 Err(ref e
) if e
.kind() == ErrorKind
::Interrupted
=> {}
778 Err(e
) => return Err(e
),
782 Err(Error
::new(ErrorKind
::UnexpectedEof
, "failed to fill whole buffer"))
788 /// Creates a "by reference" adaptor for this instance of `Read`.
790 /// The returned adaptor also implements `Read` and will simply borrow this
795 /// [`File`]s implement `Read`:
797 /// [`File`]: crate::fs::File
801 /// use std::io::Read;
802 /// use std::fs::File;
804 /// fn main() -> io::Result<()> {
805 /// let mut f = File::open("foo.txt")?;
806 /// let mut buffer = Vec::new();
807 /// let mut other_buffer = Vec::new();
810 /// let reference = f.by_ref();
812 /// // read at most 5 bytes
813 /// reference.take(5).read_to_end(&mut buffer)?;
815 /// } // drop our &mut reference so we can use f again
817 /// // original file still usable, read the rest
818 /// f.read_to_end(&mut other_buffer)?;
822 #[stable(feature = "rust1", since = "1.0.0")]
823 fn by_ref(&mut self) -> &mut Self
830 /// Transforms this `Read` instance to an [`Iterator`] over its bytes.
832 /// The returned type implements [`Iterator`] where the `Item` is
833 /// [`Result`]`<`[`u8`]`, `[`io::Error`]`>`.
834 /// The yielded item is [`Ok`] if a byte was successfully read and [`Err`]
835 /// otherwise. EOF is mapped to returning [`None`] from this iterator.
839 /// [`File`]s implement `Read`:
841 /// [`File`]: crate::fs::File
842 /// [`Result`]: crate::result::Result
843 /// [`io::Error`]: self::Error
847 /// use std::io::prelude::*;
848 /// use std::fs::File;
850 /// fn main() -> io::Result<()> {
851 /// let mut f = File::open("foo.txt")?;
853 /// for byte in f.bytes() {
854 /// println!("{}", byte.unwrap());
859 #[stable(feature = "rust1", since = "1.0.0")]
860 fn bytes(self) -> Bytes
<Self>
864 Bytes { inner: self }
867 /// Creates an adaptor which will chain this stream with another.
869 /// The returned `Read` instance will first read all bytes from this object
870 /// until EOF is encountered. Afterwards the output is equivalent to the
871 /// output of `next`.
875 /// [`File`]s implement `Read`:
877 /// [`File`]: crate::fs::File
881 /// use std::io::prelude::*;
882 /// use std::fs::File;
884 /// fn main() -> io::Result<()> {
885 /// let mut f1 = File::open("foo.txt")?;
886 /// let mut f2 = File::open("bar.txt")?;
888 /// let mut handle = f1.chain(f2);
889 /// let mut buffer = String::new();
891 /// // read the value into a String. We could use any Read method here,
892 /// // this is just one example.
893 /// handle.read_to_string(&mut buffer)?;
897 #[stable(feature = "rust1", since = "1.0.0")]
898 fn chain
<R
: Read
>(self, next
: R
) -> Chain
<Self, R
>
902 Chain { first: self, second: next, done_first: false }
905 /// Creates an adaptor which will read at most `limit` bytes from it.
907 /// This function returns a new instance of `Read` which will read at most
908 /// `limit` bytes, after which it will always return EOF ([`Ok(0)`]). Any
909 /// read errors will not count towards the number of bytes read and future
910 /// calls to [`read()`] may succeed.
914 /// [`File`]s implement `Read`:
916 /// [`File`]: crate::fs::File
918 /// [`read()`]: Read::read
922 /// use std::io::prelude::*;
923 /// use std::fs::File;
925 /// fn main() -> io::Result<()> {
926 /// let mut f = File::open("foo.txt")?;
927 /// let mut buffer = [0; 5];
929 /// // read at most five bytes
930 /// let mut handle = f.take(5);
932 /// handle.read(&mut buffer)?;
936 #[stable(feature = "rust1", since = "1.0.0")]
937 fn take(self, limit
: u64) -> Take
<Self>
941 Take { inner: self, limit }
945 /// A buffer type used with `Read::read_vectored`.
947 /// It is semantically a wrapper around an `&mut [u8]`, but is guaranteed to be
948 /// ABI compatible with the `iovec` type on Unix platforms and `WSABUF` on
950 #[stable(feature = "iovec", since = "1.36.0")]
952 pub struct IoSliceMut
<'a
>(sys
::io
::IoSliceMut
<'a
>);
954 #[stable(feature = "iovec-send-sync", since = "1.44.0")]
955 unsafe impl<'a
> Send
for IoSliceMut
<'a
> {}
957 #[stable(feature = "iovec-send-sync", since = "1.44.0")]
958 unsafe impl<'a
> Sync
for IoSliceMut
<'a
> {}
960 #[stable(feature = "iovec", since = "1.36.0")]
961 impl<'a
> fmt
::Debug
for IoSliceMut
<'a
> {
962 fn fmt(&self, fmt
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
963 fmt
::Debug
::fmt(self.0.as_slice(), fmt
)
967 impl<'a
> IoSliceMut
<'a
> {
968 /// Creates a new `IoSliceMut` wrapping a byte slice.
972 /// Panics on Windows if the slice is larger than 4GB.
973 #[stable(feature = "iovec", since = "1.36.0")]
975 pub fn new(buf
: &'a
mut [u8]) -> IoSliceMut
<'a
> {
976 IoSliceMut(sys
::io
::IoSliceMut
::new(buf
))
979 /// Advance the internal cursor of the slice.
983 /// Elements in the slice may be modified if the cursor is not advanced to
984 /// the end of the slice. For example if we have a slice of buffers with 2
985 /// `IoSliceMut`s, both of length 8, and we advance the cursor by 10 bytes
986 /// the first `IoSliceMut` will be untouched however the second will be
987 /// modified to remove the first 2 bytes (10 - 8).
992 /// #![feature(io_slice_advance)]
994 /// use std::io::IoSliceMut;
995 /// use std::ops::Deref;
997 /// let mut buf1 = [1; 8];
998 /// let mut buf2 = [2; 16];
999 /// let mut buf3 = [3; 8];
1000 /// let mut bufs = &mut [
1001 /// IoSliceMut::new(&mut buf1),
1002 /// IoSliceMut::new(&mut buf2),
1003 /// IoSliceMut::new(&mut buf3),
1006 /// // Mark 10 bytes as read.
1007 /// bufs = IoSliceMut::advance(bufs, 10);
1008 /// assert_eq!(bufs[0].deref(), [2; 14].as_ref());
1009 /// assert_eq!(bufs[1].deref(), [3; 8].as_ref());
1011 #[unstable(feature = "io_slice_advance", issue = "62726")]
1013 pub fn advance
<'b
>(bufs
: &'b
mut [IoSliceMut
<'a
>], n
: usize) -> &'b
mut [IoSliceMut
<'a
>] {
1014 // Number of buffers to remove.
1016 // Total length of all the to be removed buffers.
1017 let mut accumulated_len
= 0;
1018 for buf
in bufs
.iter() {
1019 if accumulated_len
+ buf
.len() > n
{
1022 accumulated_len
+= buf
.len();
1027 let bufs
= &mut bufs
[remove
..];
1028 if !bufs
.is_empty() {
1029 bufs
[0].0.advance(n
- accumulated_len
)
1035 #[stable(feature = "iovec", since = "1.36.0")]
1036 impl<'a
> Deref
for IoSliceMut
<'a
> {
1040 fn deref(&self) -> &[u8] {
1045 #[stable(feature = "iovec", since = "1.36.0")]
1046 impl<'a
> DerefMut
for IoSliceMut
<'a
> {
1048 fn deref_mut(&mut self) -> &mut [u8] {
1049 self.0.as_mut_slice()
1053 /// A buffer type used with `Write::write_vectored`.
1055 /// It is semantically a wrapper around an `&[u8]`, but is guaranteed to be
1056 /// ABI compatible with the `iovec` type on Unix platforms and `WSABUF` on
1058 #[stable(feature = "iovec", since = "1.36.0")]
1059 #[derive(Copy, Clone)]
1060 #[repr(transparent)]
1061 pub struct IoSlice
<'a
>(sys
::io
::IoSlice
<'a
>);
1063 #[stable(feature = "iovec-send-sync", since = "1.44.0")]
1064 unsafe impl<'a
> Send
for IoSlice
<'a
> {}
1066 #[stable(feature = "iovec-send-sync", since = "1.44.0")]
1067 unsafe impl<'a
> Sync
for IoSlice
<'a
> {}
1069 #[stable(feature = "iovec", since = "1.36.0")]
1070 impl<'a
> fmt
::Debug
for IoSlice
<'a
> {
1071 fn fmt(&self, fmt
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
1072 fmt
::Debug
::fmt(self.0.as_slice(), fmt
)
1076 impl<'a
> IoSlice
<'a
> {
1077 /// Creates a new `IoSlice` wrapping a byte slice.
1081 /// Panics on Windows if the slice is larger than 4GB.
1082 #[stable(feature = "iovec", since = "1.36.0")]
1084 pub fn new(buf
: &'a
[u8]) -> IoSlice
<'a
> {
1085 IoSlice(sys
::io
::IoSlice
::new(buf
))
1088 /// Advance the internal cursor of the slice.
1092 /// Elements in the slice may be modified if the cursor is not advanced to
1093 /// the end of the slice. For example if we have a slice of buffers with 2
1094 /// `IoSlice`s, both of length 8, and we advance the cursor by 10 bytes the
1095 /// first `IoSlice` will be untouched however the second will be modified to
1096 /// remove the first 2 bytes (10 - 8).
1101 /// #![feature(io_slice_advance)]
1103 /// use std::io::IoSlice;
1104 /// use std::ops::Deref;
1106 /// let buf1 = [1; 8];
1107 /// let buf2 = [2; 16];
1108 /// let buf3 = [3; 8];
1109 /// let mut bufs = &mut [
1110 /// IoSlice::new(&buf1),
1111 /// IoSlice::new(&buf2),
1112 /// IoSlice::new(&buf3),
1115 /// // Mark 10 bytes as written.
1116 /// bufs = IoSlice::advance(bufs, 10);
1117 /// assert_eq!(bufs[0].deref(), [2; 14].as_ref());
1118 /// assert_eq!(bufs[1].deref(), [3; 8].as_ref());
1119 #[unstable(feature = "io_slice_advance", issue = "62726")]
1121 pub fn advance
<'b
>(bufs
: &'b
mut [IoSlice
<'a
>], n
: usize) -> &'b
mut [IoSlice
<'a
>] {
1122 // Number of buffers to remove.
1124 // Total length of all the to be removed buffers.
1125 let mut accumulated_len
= 0;
1126 for buf
in bufs
.iter() {
1127 if accumulated_len
+ buf
.len() > n
{
1130 accumulated_len
+= buf
.len();
1135 let bufs
= &mut bufs
[remove
..];
1136 if !bufs
.is_empty() {
1137 bufs
[0].0.advance(n
- accumulated_len
)
1143 #[stable(feature = "iovec", since = "1.36.0")]
1144 impl<'a
> Deref
for IoSlice
<'a
> {
1148 fn deref(&self) -> &[u8] {
1153 /// A type used to conditionally initialize buffers passed to `Read` methods.
1154 #[unstable(feature = "read_initializer", issue = "42788")]
1156 pub struct Initializer(bool
);
1159 /// Returns a new `Initializer` which will zero out buffers.
1160 #[unstable(feature = "read_initializer", issue = "42788")]
1162 pub fn zeroing() -> Initializer
{
1166 /// Returns a new `Initializer` which will not zero out buffers.
1170 /// This may only be called by `Read`ers which guarantee that they will not
1171 /// read from buffers passed to `Read` methods, and that the return value of
1172 /// the method accurately reflects the number of bytes that have been
1173 /// written to the head of the buffer.
1174 #[unstable(feature = "read_initializer", issue = "42788")]
1176 pub unsafe fn nop() -> Initializer
{
1180 /// Indicates if a buffer should be initialized.
1181 #[unstable(feature = "read_initializer", issue = "42788")]
1183 pub fn should_initialize(&self) -> bool
{
1187 /// Initializes a buffer if necessary.
1188 #[unstable(feature = "read_initializer", issue = "42788")]
1190 pub fn initialize(&self, buf
: &mut [u8]) {
1191 if self.should_initialize() {
1192 unsafe { ptr::write_bytes(buf.as_mut_ptr(), 0, buf.len()) }
1197 /// A trait for objects which are byte-oriented sinks.
1199 /// Implementors of the `Write` trait are sometimes called 'writers'.
1201 /// Writers are defined by two required methods, [`write`] and [`flush`]:
1203 /// * The [`write`] method will attempt to write some data into the object,
1204 /// returning how many bytes were successfully written.
1206 /// * The [`flush`] method is useful for adaptors and explicit buffers
1207 /// themselves for ensuring that all buffered data has been pushed out to the
1210 /// Writers are intended to be composable with one another. Many implementors
1211 /// throughout [`std::io`] take and provide types which implement the `Write`
1214 /// [`write`]: Write::write
1215 /// [`flush`]: Write::flush
1216 /// [`std::io`]: self
1221 /// use std::io::prelude::*;
1222 /// use std::fs::File;
1224 /// fn main() -> std::io::Result<()> {
1225 /// let data = b"some bytes";
1227 /// let mut pos = 0;
1228 /// let mut buffer = File::create("foo.txt")?;
1230 /// while pos < data.len() {
1231 /// let bytes_written = buffer.write(&data[pos..])?;
1232 /// pos += bytes_written;
1238 /// The trait also provides convenience methods like [`write_all`], which calls
1239 /// `write` in a loop until its entire input has been written.
1241 /// [`write_all`]: Write::write_all
1242 #[stable(feature = "rust1", since = "1.0.0")]
1245 /// Write a buffer into this writer, returning how many bytes were written.
1247 /// This function will attempt to write the entire contents of `buf`, but
1248 /// the entire write may not succeed, or the write may also generate an
1249 /// error. A call to `write` represents *at most one* attempt to write to
1250 /// any wrapped object.
1252 /// Calls to `write` are not guaranteed to block waiting for data to be
1253 /// written, and a write which would otherwise block can be indicated through
1254 /// an [`Err`] variant.
1256 /// If the return value is [`Ok(n)`] then it must be guaranteed that
1257 /// `n <= buf.len()`. A return value of `0` typically means that the
1258 /// underlying object is no longer able to accept bytes and will likely not
1259 /// be able to in the future as well, or that the buffer provided is empty.
1263 /// Each call to `write` may generate an I/O error indicating that the
1264 /// operation could not be completed. If an error is returned then no bytes
1265 /// in the buffer were written to this writer.
1267 /// It is **not** considered an error if the entire buffer could not be
1268 /// written to this writer.
1270 /// An error of the [`ErrorKind::Interrupted`] kind is non-fatal and the
1271 /// write operation should be retried if there is nothing else to do.
1276 /// use std::io::prelude::*;
1277 /// use std::fs::File;
1279 /// fn main() -> std::io::Result<()> {
1280 /// let mut buffer = File::create("foo.txt")?;
1282 /// // Writes some prefix of the byte string, not necessarily all of it.
1283 /// buffer.write(b"some bytes")?;
1289 #[stable(feature = "rust1", since = "1.0.0")]
1290 fn write(&mut self, buf
: &[u8]) -> Result
<usize>;
1292 /// Like [`write`], except that it writes from a slice of buffers.
1294 /// Data is copied from each buffer in order, with the final buffer
1295 /// read from possibly being only partially consumed. This method must
1296 /// behave as a call to [`write`] with the buffers concatenated would.
1298 /// The default implementation calls [`write`] with either the first nonempty
1299 /// buffer provided, or an empty one if none exists.
1301 /// [`write`]: Write::write
1302 #[stable(feature = "iovec", since = "1.36.0")]
1303 fn write_vectored(&mut self, bufs
: &[IoSlice
<'_
>]) -> Result
<usize> {
1304 default_write_vectored(|b
| self.write(b
), bufs
)
1307 /// Determines if this `Write`er has an efficient [`write_vectored`]
1310 /// If a `Write`er does not override the default [`write_vectored`]
1311 /// implementation, code using it may want to avoid the method all together
1312 /// and coalesce writes into a single buffer for higher performance.
1314 /// The default implementation returns `false`.
1316 /// [`write_vectored`]: Write::write_vectored
1317 #[unstable(feature = "can_vector", issue = "69941")]
1318 fn is_write_vectored(&self) -> bool
{
1322 /// Flush this output stream, ensuring that all intermediately buffered
1323 /// contents reach their destination.
1327 /// It is considered an error if not all bytes could be written due to
1328 /// I/O errors or EOF being reached.
1333 /// use std::io::prelude::*;
1334 /// use std::io::BufWriter;
1335 /// use std::fs::File;
1337 /// fn main() -> std::io::Result<()> {
1338 /// let mut buffer = BufWriter::new(File::create("foo.txt")?);
1340 /// buffer.write_all(b"some bytes")?;
1341 /// buffer.flush()?;
1345 #[stable(feature = "rust1", since = "1.0.0")]
1346 fn flush(&mut self) -> Result
<()>;
1348 /// Attempts to write an entire buffer into this writer.
1350 /// This method will continuously call [`write`] until there is no more data
1351 /// to be written or an error of non-[`ErrorKind::Interrupted`] kind is
1352 /// returned. This method will not return until the entire buffer has been
1353 /// successfully written or such an error occurs. The first error that is
1354 /// not of [`ErrorKind::Interrupted`] kind generated from this method will be
1357 /// If the buffer contains no data, this will never call [`write`].
1361 /// This function will return the first error of
1362 /// non-[`ErrorKind::Interrupted`] kind that [`write`] returns.
1364 /// [`write`]: Write::write
1369 /// use std::io::prelude::*;
1370 /// use std::fs::File;
1372 /// fn main() -> std::io::Result<()> {
1373 /// let mut buffer = File::create("foo.txt")?;
1375 /// buffer.write_all(b"some bytes")?;
1379 #[stable(feature = "rust1", since = "1.0.0")]
1380 fn write_all(&mut self, mut buf
: &[u8]) -> Result
<()> {
1381 while !buf
.is_empty() {
1382 match self.write(buf
) {
1384 return Err(Error
::new(ErrorKind
::WriteZero
, "failed to write whole buffer"));
1386 Ok(n
) => buf
= &buf
[n
..],
1387 Err(ref e
) if e
.kind() == ErrorKind
::Interrupted
=> {}
1388 Err(e
) => return Err(e
),
1394 /// Attempts to write multiple buffers into this writer.
1396 /// This method will continuously call [`write_vectored`] until there is no
1397 /// more data to be written or an error of non-[`ErrorKind::Interrupted`]
1398 /// kind is returned. This method will not return until all buffers have
1399 /// been successfully written or such an error occurs. The first error that
1400 /// is not of [`ErrorKind::Interrupted`] kind generated from this method
1401 /// will be returned.
1403 /// If the buffer contains no data, this will never call [`write_vectored`].
1407 /// Unlike [`write_vectored`], this takes a *mutable* reference to
1408 /// a slice of [`IoSlice`]s, not an immutable one. That's because we need to
1409 /// modify the slice to keep track of the bytes already written.
1411 /// Once this function returns, the contents of `bufs` are unspecified, as
1412 /// this depends on how many calls to [`write_vectored`] were necessary. It is
1413 /// best to understand this function as taking ownership of `bufs` and to
1414 /// not use `bufs` afterwards. The underlying buffers, to which the
1415 /// [`IoSlice`]s point (but not the [`IoSlice`]s themselves), are unchanged and
1418 /// [`write_vectored`]: Write::write_vectored
1423 /// #![feature(write_all_vectored)]
1424 /// # fn main() -> std::io::Result<()> {
1426 /// use std::io::{Write, IoSlice};
1428 /// let mut writer = Vec::new();
1429 /// let bufs = &mut [
1430 /// IoSlice::new(&[1]),
1431 /// IoSlice::new(&[2, 3]),
1432 /// IoSlice::new(&[4, 5, 6]),
1435 /// writer.write_all_vectored(bufs)?;
1436 /// // Note: the contents of `bufs` is now undefined, see the Notes section.
1438 /// assert_eq!(writer, &[1, 2, 3, 4, 5, 6]);
1441 #[unstable(feature = "write_all_vectored", issue = "70436")]
1442 fn write_all_vectored(&mut self, mut bufs
: &mut [IoSlice
<'_
>]) -> Result
<()> {
1443 // Guarantee that bufs is empty if it contains no data,
1444 // to avoid calling write_vectored if there is no data to be written.
1445 bufs
= IoSlice
::advance(bufs
, 0);
1446 while !bufs
.is_empty() {
1447 match self.write_vectored(bufs
) {
1449 return Err(Error
::new(ErrorKind
::WriteZero
, "failed to write whole buffer"));
1451 Ok(n
) => bufs
= IoSlice
::advance(bufs
, n
),
1452 Err(ref e
) if e
.kind() == ErrorKind
::Interrupted
=> {}
1453 Err(e
) => return Err(e
),
1459 /// Writes a formatted string into this writer, returning any error
1462 /// This method is primarily used to interface with the
1463 /// [`format_args!()`] macro, but it is rare that this should
1464 /// explicitly be called. The [`write!()`] macro should be favored to
1465 /// invoke this method instead.
1467 /// This function internally uses the [`write_all`] method on
1468 /// this trait and hence will continuously write data so long as no errors
1469 /// are received. This also means that partial writes are not indicated in
1472 /// [`write_all`]: Write::write_all
1476 /// This function will return any I/O error reported while formatting.
1481 /// use std::io::prelude::*;
1482 /// use std::fs::File;
1484 /// fn main() -> std::io::Result<()> {
1485 /// let mut buffer = File::create("foo.txt")?;
1488 /// write!(buffer, "{:.*}", 2, 1.234567)?;
1489 /// // turns into this:
1490 /// buffer.write_fmt(format_args!("{:.*}", 2, 1.234567))?;
1494 #[stable(feature = "rust1", since = "1.0.0")]
1495 fn write_fmt(&mut self, fmt
: fmt
::Arguments
<'_
>) -> Result
<()> {
1496 // Create a shim which translates a Write to a fmt::Write and saves
1497 // off I/O errors. instead of discarding them
1498 struct Adaptor
<'a
, T
: ?Sized
+ 'a
> {
1503 impl<T
: Write
+ ?Sized
> fmt
::Write
for Adaptor
<'_
, T
> {
1504 fn write_str(&mut self, s
: &str) -> fmt
::Result
{
1505 match self.inner
.write_all(s
.as_bytes()) {
1508 self.error
= Err(e
);
1515 let mut output
= Adaptor { inner: self, error: Ok(()) }
;
1516 match fmt
::write(&mut output
, fmt
) {
1519 // check if the error came from the underlying `Write` or not
1520 if output
.error
.is_err() {
1523 Err(Error
::new(ErrorKind
::Other
, "formatter error"))
1529 /// Creates a "by reference" adaptor for this instance of `Write`.
1531 /// The returned adaptor also implements `Write` and will simply borrow this
1537 /// use std::io::Write;
1538 /// use std::fs::File;
1540 /// fn main() -> std::io::Result<()> {
1541 /// let mut buffer = File::create("foo.txt")?;
1543 /// let reference = buffer.by_ref();
1545 /// // we can use reference just like our original buffer
1546 /// reference.write_all(b"some bytes")?;
1550 #[stable(feature = "rust1", since = "1.0.0")]
1551 fn by_ref(&mut self) -> &mut Self
1559 /// The `Seek` trait provides a cursor which can be moved within a stream of
1562 /// The stream typically has a fixed size, allowing seeking relative to either
1563 /// end or the current offset.
1567 /// [`File`]s implement `Seek`:
1569 /// [`File`]: crate::fs::File
1573 /// use std::io::prelude::*;
1574 /// use std::fs::File;
1575 /// use std::io::SeekFrom;
1577 /// fn main() -> io::Result<()> {
1578 /// let mut f = File::open("foo.txt")?;
1580 /// // move the cursor 42 bytes from the start of the file
1581 /// f.seek(SeekFrom::Start(42))?;
1585 #[stable(feature = "rust1", since = "1.0.0")]
1587 /// Seek to an offset, in bytes, in a stream.
1589 /// A seek beyond the end of a stream is allowed, but behavior is defined
1590 /// by the implementation.
1592 /// If the seek operation completed successfully,
1593 /// this method returns the new position from the start of the stream.
1594 /// That position can be used later with [`SeekFrom::Start`].
1598 /// Seeking to a negative offset is considered an error.
1599 #[stable(feature = "rust1", since = "1.0.0")]
1600 fn seek(&mut self, pos
: SeekFrom
) -> Result
<u64>;
1602 /// Returns the length of this stream (in bytes).
1604 /// This method is implemented using up to three seek operations. If this
1605 /// method returns successfully, the seek position is unchanged (i.e. the
1606 /// position before calling this method is the same as afterwards).
1607 /// However, if this method returns an error, the seek position is
1610 /// If you need to obtain the length of *many* streams and you don't care
1611 /// about the seek position afterwards, you can reduce the number of seek
1612 /// operations by simply calling `seek(SeekFrom::End(0))` and using its
1613 /// return value (it is also the stream length).
1615 /// Note that length of a stream can change over time (for example, when
1616 /// data is appended to a file). So calling this method multiple times does
1617 /// not necessarily return the same length each time.
1622 /// #![feature(seek_convenience)]
1624 /// io::{self, Seek},
1628 /// fn main() -> io::Result<()> {
1629 /// let mut f = File::open("foo.txt")?;
1631 /// let len = f.stream_len()?;
1632 /// println!("The file is currently {} bytes long", len);
1636 #[unstable(feature = "seek_convenience", issue = "59359")]
1637 fn stream_len(&mut self) -> Result
<u64> {
1638 let old_pos
= self.stream_position()?
;
1639 let len
= self.seek(SeekFrom
::End(0))?
;
1641 // Avoid seeking a third time when we were already at the end of the
1642 // stream. The branch is usually way cheaper than a seek operation.
1644 self.seek(SeekFrom
::Start(old_pos
))?
;
1650 /// Returns the current seek position from the start of the stream.
1652 /// This is equivalent to `self.seek(SeekFrom::Current(0))`.
1657 /// #![feature(seek_convenience)]
1659 /// io::{self, BufRead, BufReader, Seek},
1663 /// fn main() -> io::Result<()> {
1664 /// let mut f = BufReader::new(File::open("foo.txt")?);
1666 /// let before = f.stream_position()?;
1667 /// f.read_line(&mut String::new())?;
1668 /// let after = f.stream_position()?;
1670 /// println!("The first line was {} bytes long", after - before);
1674 #[unstable(feature = "seek_convenience", issue = "59359")]
1675 fn stream_position(&mut self) -> Result
<u64> {
1676 self.seek(SeekFrom
::Current(0))
1680 /// Enumeration of possible methods to seek within an I/O object.
1682 /// It is used by the [`Seek`] trait.
1683 #[derive(Copy, PartialEq, Eq, Clone, Debug)]
1684 #[stable(feature = "rust1", since = "1.0.0")]
1686 /// Sets the offset to the provided number of bytes.
1687 #[stable(feature = "rust1", since = "1.0.0")]
1688 Start(#[stable(feature = "rust1", since = "1.0.0")] u64),
1690 /// Sets the offset to the size of this object plus the specified number of
1693 /// It is possible to seek beyond the end of an object, but it's an error to
1694 /// seek before byte 0.
1695 #[stable(feature = "rust1", since = "1.0.0")]
1696 End(#[stable(feature = "rust1", since = "1.0.0")] i64),
1698 /// Sets the offset to the current position plus the specified number of
1701 /// It is possible to seek beyond the end of an object, but it's an error to
1702 /// seek before byte 0.
1703 #[stable(feature = "rust1", since = "1.0.0")]
1704 Current(#[stable(feature = "rust1", since = "1.0.0")] i64),
1707 fn read_until
<R
: BufRead
+ ?Sized
>(r
: &mut R
, delim
: u8, buf
: &mut Vec
<u8>) -> Result
<usize> {
1710 let (done
, used
) = {
1711 let available
= match r
.fill_buf() {
1713 Err(ref e
) if e
.kind() == ErrorKind
::Interrupted
=> continue,
1714 Err(e
) => return Err(e
),
1716 match memchr
::memchr(delim
, available
) {
1718 buf
.extend_from_slice(&available
[..=i
]);
1722 buf
.extend_from_slice(available
);
1723 (false, available
.len())
1729 if done
|| used
== 0 {
1735 /// A `BufRead` is a type of `Read`er which has an internal buffer, allowing it
1736 /// to perform extra ways of reading.
1738 /// For example, reading line-by-line is inefficient without using a buffer, so
1739 /// if you want to read by line, you'll need `BufRead`, which includes a
1740 /// [`read_line`] method as well as a [`lines`] iterator.
1744 /// A locked standard input implements `BufRead`:
1748 /// use std::io::prelude::*;
1750 /// let stdin = io::stdin();
1751 /// for line in stdin.lock().lines() {
1752 /// println!("{}", line.unwrap());
1756 /// If you have something that implements [`Read`], you can use the [`BufReader`
1757 /// type][`BufReader`] to turn it into a `BufRead`.
1759 /// For example, [`File`] implements [`Read`], but not `BufRead`.
1760 /// [`BufReader`] to the rescue!
1762 /// [`File`]: crate::fs::File
1763 /// [`read_line`]: BufRead::read_line
1764 /// [`lines`]: BufRead::lines
1767 /// use std::io::{self, BufReader};
1768 /// use std::io::prelude::*;
1769 /// use std::fs::File;
1771 /// fn main() -> io::Result<()> {
1772 /// let f = File::open("foo.txt")?;
1773 /// let f = BufReader::new(f);
1775 /// for line in f.lines() {
1776 /// println!("{}", line.unwrap());
1782 #[stable(feature = "rust1", since = "1.0.0")]
1783 pub trait BufRead
: Read
{
1784 /// Returns the contents of the internal buffer, filling it with more data
1785 /// from the inner reader if it is empty.
1787 /// This function is a lower-level call. It needs to be paired with the
1788 /// [`consume`] method to function properly. When calling this
1789 /// method, none of the contents will be "read" in the sense that later
1790 /// calling `read` may return the same contents. As such, [`consume`] must
1791 /// be called with the number of bytes that are consumed from this buffer to
1792 /// ensure that the bytes are never returned twice.
1794 /// [`consume`]: BufRead::consume
1796 /// An empty buffer returned indicates that the stream has reached EOF.
1800 /// This function will return an I/O error if the underlying reader was
1801 /// read, but returned an error.
1805 /// A locked standard input implements `BufRead`:
1809 /// use std::io::prelude::*;
1811 /// let stdin = io::stdin();
1812 /// let mut stdin = stdin.lock();
1814 /// let buffer = stdin.fill_buf().unwrap();
1816 /// // work with buffer
1817 /// println!("{:?}", buffer);
1819 /// // ensure the bytes we worked with aren't returned again later
1820 /// let length = buffer.len();
1821 /// stdin.consume(length);
1823 #[stable(feature = "rust1", since = "1.0.0")]
1824 fn fill_buf(&mut self) -> Result
<&[u8]>;
1826 /// Tells this buffer that `amt` bytes have been consumed from the buffer,
1827 /// so they should no longer be returned in calls to `read`.
1829 /// This function is a lower-level call. It needs to be paired with the
1830 /// [`fill_buf`] method to function properly. This function does
1831 /// not perform any I/O, it simply informs this object that some amount of
1832 /// its buffer, returned from [`fill_buf`], has been consumed and should
1833 /// no longer be returned. As such, this function may do odd things if
1834 /// [`fill_buf`] isn't called before calling it.
1836 /// The `amt` must be `<=` the number of bytes in the buffer returned by
1841 /// Since `consume()` is meant to be used with [`fill_buf`],
1842 /// that method's example includes an example of `consume()`.
1844 /// [`fill_buf`]: BufRead::fill_buf
1845 #[stable(feature = "rust1", since = "1.0.0")]
1846 fn consume(&mut self, amt
: usize);
1848 /// Read all bytes into `buf` until the delimiter `byte` or EOF is reached.
1850 /// This function will read bytes from the underlying stream until the
1851 /// delimiter or EOF is found. Once found, all bytes up to, and including,
1852 /// the delimiter (if found) will be appended to `buf`.
1854 /// If successful, this function will return the total number of bytes read.
1856 /// This function is blocking and should be used carefully: it is possible for
1857 /// an attacker to continuously send bytes without ever sending the delimiter
1862 /// This function will ignore all instances of [`ErrorKind::Interrupted`] and
1863 /// will otherwise return any errors returned by [`fill_buf`].
1865 /// If an I/O error is encountered then all bytes read so far will be
1866 /// present in `buf` and its length will have been adjusted appropriately.
1868 /// [`fill_buf`]: BufRead::fill_buf
1872 /// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
1873 /// this example, we use [`Cursor`] to read all the bytes in a byte slice
1874 /// in hyphen delimited segments:
1877 /// use std::io::{self, BufRead};
1879 /// let mut cursor = io::Cursor::new(b"lorem-ipsum");
1880 /// let mut buf = vec![];
1882 /// // cursor is at 'l'
1883 /// let num_bytes = cursor.read_until(b'-', &mut buf)
1884 /// .expect("reading from cursor won't fail");
1885 /// assert_eq!(num_bytes, 6);
1886 /// assert_eq!(buf, b"lorem-");
1889 /// // cursor is at 'i'
1890 /// let num_bytes = cursor.read_until(b'-', &mut buf)
1891 /// .expect("reading from cursor won't fail");
1892 /// assert_eq!(num_bytes, 5);
1893 /// assert_eq!(buf, b"ipsum");
1896 /// // cursor is at EOF
1897 /// let num_bytes = cursor.read_until(b'-', &mut buf)
1898 /// .expect("reading from cursor won't fail");
1899 /// assert_eq!(num_bytes, 0);
1900 /// assert_eq!(buf, b"");
1902 #[stable(feature = "rust1", since = "1.0.0")]
1903 fn read_until(&mut self, byte
: u8, buf
: &mut Vec
<u8>) -> Result
<usize> {
1904 read_until(self, byte
, buf
)
1907 /// Read all bytes until a newline (the `0xA` byte) is reached, and append
1908 /// them to the provided buffer.
1910 /// This function will read bytes from the underlying stream until the
1911 /// newline delimiter (the `0xA` byte) or EOF is found. Once found, all bytes
1912 /// up to, and including, the delimiter (if found) will be appended to
1915 /// If successful, this function will return the total number of bytes read.
1917 /// If this function returns [`Ok(0)`], the stream has reached EOF.
1919 /// This function is blocking and should be used carefully: it is possible for
1920 /// an attacker to continuously send bytes without ever sending a newline
1927 /// This function has the same error semantics as [`read_until`] and will
1928 /// also return an error if the read bytes are not valid UTF-8. If an I/O
1929 /// error is encountered then `buf` may contain some bytes already read in
1930 /// the event that all data read so far was valid UTF-8.
1932 /// [`read_until`]: BufRead::read_until
1936 /// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
1937 /// this example, we use [`Cursor`] to read all the lines in a byte slice:
1940 /// use std::io::{self, BufRead};
1942 /// let mut cursor = io::Cursor::new(b"foo\nbar");
1943 /// let mut buf = String::new();
1945 /// // cursor is at 'f'
1946 /// let num_bytes = cursor.read_line(&mut buf)
1947 /// .expect("reading from cursor won't fail");
1948 /// assert_eq!(num_bytes, 4);
1949 /// assert_eq!(buf, "foo\n");
1952 /// // cursor is at 'b'
1953 /// let num_bytes = cursor.read_line(&mut buf)
1954 /// .expect("reading from cursor won't fail");
1955 /// assert_eq!(num_bytes, 3);
1956 /// assert_eq!(buf, "bar");
1959 /// // cursor is at EOF
1960 /// let num_bytes = cursor.read_line(&mut buf)
1961 /// .expect("reading from cursor won't fail");
1962 /// assert_eq!(num_bytes, 0);
1963 /// assert_eq!(buf, "");
1965 #[stable(feature = "rust1", since = "1.0.0")]
1966 fn read_line(&mut self, buf
: &mut String
) -> Result
<usize> {
1967 // Note that we are not calling the `.read_until` method here, but
1968 // rather our hardcoded implementation. For more details as to why, see
1969 // the comments in `read_to_end`.
1970 append_to_string(buf
, |b
| read_until(self, b'
\n'
, b
))
1973 /// Returns an iterator over the contents of this reader split on the byte
1976 /// The iterator returned from this function will return instances of
1977 /// [`io::Result`]`<`[`Vec<u8>`]`>`. Each vector returned will *not* have
1978 /// the delimiter byte at the end.
1980 /// This function will yield errors whenever [`read_until`] would have
1981 /// also yielded an error.
1983 /// [`io::Result`]: self::Result
1984 /// [`Vec<u8>`]: Vec
1985 /// [`read_until`]: BufRead::read_until
1989 /// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
1990 /// this example, we use [`Cursor`] to iterate over all hyphen delimited
1991 /// segments in a byte slice
1994 /// use std::io::{self, BufRead};
1996 /// let cursor = io::Cursor::new(b"lorem-ipsum-dolor");
1998 /// let mut split_iter = cursor.split(b'-').map(|l| l.unwrap());
1999 /// assert_eq!(split_iter.next(), Some(b"lorem".to_vec()));
2000 /// assert_eq!(split_iter.next(), Some(b"ipsum".to_vec()));
2001 /// assert_eq!(split_iter.next(), Some(b"dolor".to_vec()));
2002 /// assert_eq!(split_iter.next(), None);
2004 #[stable(feature = "rust1", since = "1.0.0")]
2005 fn split(self, byte
: u8) -> Split
<Self>
2009 Split { buf: self, delim: byte }
2012 /// Returns an iterator over the lines of this reader.
2014 /// The iterator returned from this function will yield instances of
2015 /// [`io::Result`]`<`[`String`]`>`. Each string returned will *not* have a newline
2016 /// byte (the `0xA` byte) or `CRLF` (`0xD`, `0xA` bytes) at the end.
2018 /// [`io::Result`]: self::Result
2022 /// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
2023 /// this example, we use [`Cursor`] to iterate over all the lines in a byte
2027 /// use std::io::{self, BufRead};
2029 /// let cursor = io::Cursor::new(b"lorem\nipsum\r\ndolor");
2031 /// let mut lines_iter = cursor.lines().map(|l| l.unwrap());
2032 /// assert_eq!(lines_iter.next(), Some(String::from("lorem")));
2033 /// assert_eq!(lines_iter.next(), Some(String::from("ipsum")));
2034 /// assert_eq!(lines_iter.next(), Some(String::from("dolor")));
2035 /// assert_eq!(lines_iter.next(), None);
2040 /// Each line of the iterator has the same error semantics as [`BufRead::read_line`].
2041 #[stable(feature = "rust1", since = "1.0.0")]
2042 fn lines(self) -> Lines
<Self>
2050 /// Adaptor to chain together two readers.
2052 /// This struct is generally created by calling [`chain`] on a reader.
2053 /// Please see the documentation of [`chain`] for more details.
2055 /// [`chain`]: Read::chain
2056 #[stable(feature = "rust1", since = "1.0.0")]
2057 pub struct Chain
<T
, U
> {
2063 impl<T
, U
> Chain
<T
, U
> {
2064 /// Consumes the `Chain`, returning the wrapped readers.
2070 /// use std::io::prelude::*;
2071 /// use std::fs::File;
2073 /// fn main() -> io::Result<()> {
2074 /// let mut foo_file = File::open("foo.txt")?;
2075 /// let mut bar_file = File::open("bar.txt")?;
2077 /// let chain = foo_file.chain(bar_file);
2078 /// let (foo_file, bar_file) = chain.into_inner();
2082 #[stable(feature = "more_io_inner_methods", since = "1.20.0")]
2083 pub fn into_inner(self) -> (T
, U
) {
2084 (self.first
, self.second
)
2087 /// Gets references to the underlying readers in this `Chain`.
2093 /// use std::io::prelude::*;
2094 /// use std::fs::File;
2096 /// fn main() -> io::Result<()> {
2097 /// let mut foo_file = File::open("foo.txt")?;
2098 /// let mut bar_file = File::open("bar.txt")?;
2100 /// let chain = foo_file.chain(bar_file);
2101 /// let (foo_file, bar_file) = chain.get_ref();
2105 #[stable(feature = "more_io_inner_methods", since = "1.20.0")]
2106 pub fn get_ref(&self) -> (&T
, &U
) {
2107 (&self.first
, &self.second
)
2110 /// Gets mutable references to the underlying readers in this `Chain`.
2112 /// Care should be taken to avoid modifying the internal I/O state of the
2113 /// underlying readers as doing so may corrupt the internal state of this
2120 /// use std::io::prelude::*;
2121 /// use std::fs::File;
2123 /// fn main() -> io::Result<()> {
2124 /// let mut foo_file = File::open("foo.txt")?;
2125 /// let mut bar_file = File::open("bar.txt")?;
2127 /// let mut chain = foo_file.chain(bar_file);
2128 /// let (foo_file, bar_file) = chain.get_mut();
2132 #[stable(feature = "more_io_inner_methods", since = "1.20.0")]
2133 pub fn get_mut(&mut self) -> (&mut T
, &mut U
) {
2134 (&mut self.first
, &mut self.second
)
2138 #[stable(feature = "std_debug", since = "1.16.0")]
2139 impl<T
: fmt
::Debug
, U
: fmt
::Debug
> fmt
::Debug
for Chain
<T
, U
> {
2140 fn fmt(&self, f
: &mut fmt
::Formatter
<'_
>) -> fmt
::Result
{
2141 f
.debug_struct("Chain").field("t", &self.first
).field("u", &self.second
).finish()
2145 #[stable(feature = "rust1", since = "1.0.0")]
2146 impl<T
: Read
, U
: Read
> Read
for Chain
<T
, U
> {
2147 fn read(&mut self, buf
: &mut [u8]) -> Result
<usize> {
2148 if !self.done_first
{
2149 match self.first
.read(buf
)?
{
2150 0 if !buf
.is_empty() => self.done_first
= true,
2154 self.second
.read(buf
)
2157 fn read_vectored(&mut self, bufs
: &mut [IoSliceMut
<'_
>]) -> Result
<usize> {
2158 if !self.done_first
{
2159 match self.first
.read_vectored(bufs
)?
{
2160 0 if bufs
.iter().any(|b
| !b
.is_empty()) => self.done_first
= true,
2164 self.second
.read_vectored(bufs
)
2167 unsafe fn initializer(&self) -> Initializer
{
2168 let initializer
= self.first
.initializer();
2169 if initializer
.should_initialize() { initializer }
else { self.second.initializer() }
2173 #[stable(feature = "chain_bufread", since = "1.9.0")]
2174 impl<T
: BufRead
, U
: BufRead
> BufRead
for Chain
<T
, U
> {
2175 fn fill_buf(&mut self) -> Result
<&[u8]> {
2176 if !self.done_first
{
2177 match self.first
.fill_buf()?
{
2178 buf
if buf
.is_empty() => {
2179 self.done_first
= true;
2181 buf
=> return Ok(buf
),
2184 self.second
.fill_buf()
2187 fn consume(&mut self, amt
: usize) {
2188 if !self.done_first { self.first.consume(amt) }
else { self.second.consume(amt) }
2192 /// Reader adaptor which limits the bytes read from an underlying reader.
2194 /// This struct is generally created by calling [`take`] on a reader.
2195 /// Please see the documentation of [`take`] for more details.
2197 /// [`take`]: Read::take
2198 #[stable(feature = "rust1", since = "1.0.0")]
2200 pub struct Take
<T
> {
2206 /// Returns the number of bytes that can be read before this instance will
2211 /// This instance may reach `EOF` after reading fewer bytes than indicated by
2212 /// this method if the underlying [`Read`] instance reaches EOF.
2218 /// use std::io::prelude::*;
2219 /// use std::fs::File;
2221 /// fn main() -> io::Result<()> {
2222 /// let f = File::open("foo.txt")?;
2224 /// // read at most five bytes
2225 /// let handle = f.take(5);
2227 /// println!("limit: {}", handle.limit());
2231 #[stable(feature = "rust1", since = "1.0.0")]
2232 pub fn limit(&self) -> u64 {
2236 /// Sets the number of bytes that can be read before this instance will
2237 /// return EOF. This is the same as constructing a new `Take` instance, so
2238 /// the amount of bytes read and the previous limit value don't matter when
2239 /// calling this method.
2245 /// use std::io::prelude::*;
2246 /// use std::fs::File;
2248 /// fn main() -> io::Result<()> {
2249 /// let f = File::open("foo.txt")?;
2251 /// // read at most five bytes
2252 /// let mut handle = f.take(5);
2253 /// handle.set_limit(10);
2255 /// assert_eq!(handle.limit(), 10);
2259 #[stable(feature = "take_set_limit", since = "1.27.0")]
2260 pub fn set_limit(&mut self, limit
: u64) {
2264 /// Consumes the `Take`, returning the wrapped reader.
2270 /// use std::io::prelude::*;
2271 /// use std::fs::File;
2273 /// fn main() -> io::Result<()> {
2274 /// let mut file = File::open("foo.txt")?;
2276 /// let mut buffer = [0; 5];
2277 /// let mut handle = file.take(5);
2278 /// handle.read(&mut buffer)?;
2280 /// let file = handle.into_inner();
2284 #[stable(feature = "io_take_into_inner", since = "1.15.0")]
2285 pub fn into_inner(self) -> T
{
2289 /// Gets a reference to the underlying reader.
2295 /// use std::io::prelude::*;
2296 /// use std::fs::File;
2298 /// fn main() -> io::Result<()> {
2299 /// let mut file = File::open("foo.txt")?;
2301 /// let mut buffer = [0; 5];
2302 /// let mut handle = file.take(5);
2303 /// handle.read(&mut buffer)?;
2305 /// let file = handle.get_ref();
2309 #[stable(feature = "more_io_inner_methods", since = "1.20.0")]
2310 pub fn get_ref(&self) -> &T
{
2314 /// Gets a mutable reference to the underlying reader.
2316 /// Care should be taken to avoid modifying the internal I/O state of the
2317 /// underlying reader as doing so may corrupt the internal limit of this
2324 /// use std::io::prelude::*;
2325 /// use std::fs::File;
2327 /// fn main() -> io::Result<()> {
2328 /// let mut file = File::open("foo.txt")?;
2330 /// let mut buffer = [0; 5];
2331 /// let mut handle = file.take(5);
2332 /// handle.read(&mut buffer)?;
2334 /// let file = handle.get_mut();
2338 #[stable(feature = "more_io_inner_methods", since = "1.20.0")]
2339 pub fn get_mut(&mut self) -> &mut T
{
2344 #[stable(feature = "rust1", since = "1.0.0")]
2345 impl<T
: Read
> Read
for Take
<T
> {
2346 fn read(&mut self, buf
: &mut [u8]) -> Result
<usize> {
2347 // Don't call into inner reader at all at EOF because it may still block
2348 if self.limit
== 0 {
2352 let max
= cmp
::min(buf
.len() as u64, self.limit
) as usize;
2353 let n
= self.inner
.read(&mut buf
[..max
])?
;
2354 self.limit
-= n
as u64;
2358 unsafe fn initializer(&self) -> Initializer
{
2359 self.inner
.initializer()
2362 fn read_to_end(&mut self, buf
: &mut Vec
<u8>) -> Result
<usize> {
2363 // Pass in a reservation_size closure that respects the current value
2364 // of limit for each read. If we hit the read limit, this prevents the
2365 // final zero-byte read from allocating again.
2366 read_to_end_with_reservation(self, buf
, |self_
| cmp
::min(self_
.limit
, 32) as usize)
2370 #[stable(feature = "rust1", since = "1.0.0")]
2371 impl<T
: BufRead
> BufRead
for Take
<T
> {
2372 fn fill_buf(&mut self) -> Result
<&[u8]> {
2373 // Don't call into inner reader at all at EOF because it may still block
2374 if self.limit
== 0 {
2378 let buf
= self.inner
.fill_buf()?
;
2379 let cap
= cmp
::min(buf
.len() as u64, self.limit
) as usize;
2383 fn consume(&mut self, amt
: usize) {
2384 // Don't let callers reset the limit by passing an overlarge value
2385 let amt
= cmp
::min(amt
as u64, self.limit
) as usize;
2386 self.limit
-= amt
as u64;
2387 self.inner
.consume(amt
);
2391 /// An iterator over `u8` values of a reader.
2393 /// This struct is generally created by calling [`bytes`] on a reader.
2394 /// Please see the documentation of [`bytes`] for more details.
2396 /// [`bytes`]: Read::bytes
2397 #[stable(feature = "rust1", since = "1.0.0")]
2399 pub struct Bytes
<R
> {
2403 #[stable(feature = "rust1", since = "1.0.0")]
2404 impl<R
: Read
> Iterator
for Bytes
<R
> {
2405 type Item
= Result
<u8>;
2407 fn next(&mut self) -> Option
<Result
<u8>> {
2410 return match self.inner
.read(slice
::from_mut(&mut byte
)) {
2412 Ok(..) => Some(Ok(byte
)),
2413 Err(ref e
) if e
.kind() == ErrorKind
::Interrupted
=> continue,
2414 Err(e
) => Some(Err(e
)),
2420 /// An iterator over the contents of an instance of `BufRead` split on a
2421 /// particular byte.
2423 /// This struct is generally created by calling [`split`] on a `BufRead`.
2424 /// Please see the documentation of [`split`] for more details.
2426 /// [`split`]: BufRead::split
2427 #[stable(feature = "rust1", since = "1.0.0")]
2429 pub struct Split
<B
> {
2434 #[stable(feature = "rust1", since = "1.0.0")]
2435 impl<B
: BufRead
> Iterator
for Split
<B
> {
2436 type Item
= Result
<Vec
<u8>>;
2438 fn next(&mut self) -> Option
<Result
<Vec
<u8>>> {
2439 let mut buf
= Vec
::new();
2440 match self.buf
.read_until(self.delim
, &mut buf
) {
2443 if buf
[buf
.len() - 1] == self.delim
{
2448 Err(e
) => Some(Err(e
)),
2453 /// An iterator over the lines of an instance of `BufRead`.
2455 /// This struct is generally created by calling [`lines`] on a `BufRead`.
2456 /// Please see the documentation of [`lines`] for more details.
2458 /// [`lines`]: BufRead::lines
2459 #[stable(feature = "rust1", since = "1.0.0")]
2461 pub struct Lines
<B
> {
2465 #[stable(feature = "rust1", since = "1.0.0")]
2466 impl<B
: BufRead
> Iterator
for Lines
<B
> {
2467 type Item
= Result
<String
>;
2469 fn next(&mut self) -> Option
<Result
<String
>> {
2470 let mut buf
= String
::new();
2471 match self.buf
.read_line(&mut buf
) {
2474 if buf
.ends_with('
\n'
) {
2476 if buf
.ends_with('
\r'
) {
2482 Err(e
) => Some(Err(e
)),