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 //! [`Read`]: trait.Read.html
242 //! [`Write`]: trait.Write.html
243 //! [`Seek`]: trait.Seek.html
244 //! [`BufRead`]: trait.BufRead.html
245 //! [`File`]: ../fs/struct.File.html
246 //! [`TcpStream`]: ../net/struct.TcpStream.html
247 //! [`Vec<T>`]: ../vec/struct.Vec.html
248 //! [`BufReader`]: struct.BufReader.html
249 //! [`BufWriter`]: struct.BufWriter.html
250 //! [`Write::write`]: trait.Write.html#tymethod.write
251 //! [`io::stdout`]: fn.stdout.html
252 //! [`println!`]: ../macro.println.html
253 //! [`Lines`]: struct.Lines.html
254 //! [`io::Result`]: type.Result.html
255 //! [`?` operator]: ../../book/appendix-02-operators.html
256 //! [`Read::read`]: trait.Read.html#tymethod.read
257 //! [`Result`]: ../result/enum.Result.html
258 //! [`.unwrap()`]: ../result/enum.Result.html#method.unwrap
259 // ignore-tidy-filelength
261 #![stable(feature = "rust1", since = "1.0.0")]
267 use crate::ops::{Deref, DerefMut};
273 #[stable(feature = "rust1", since = "1.0.0")]
274 pub use self::buffered::IntoInnerError;
275 #[stable(feature = "rust1", since = "1.0.0")]
276 pub use self::buffered::{BufReader, BufWriter, LineWriter};
277 #[stable(feature = "rust1", since = "1.0.0")]
278 pub use self::cursor::Cursor;
279 #[stable(feature = "rust1", since = "1.0.0")]
280 pub use self::error::{Error, ErrorKind, Result};
281 #[stable(feature = "rust1", since = "1.0.0")]
282 pub use self::stdio::{stderr, stdin, stdout, Stderr, Stdin, Stdout};
283 #[stable(feature = "rust1", since = "1.0.0")]
284 pub use self::stdio::{StderrLock, StdinLock, StdoutLock};
285 #[unstable(feature = "print_internals", issue = "none")]
286 pub use self::stdio::{_eprint, _print};
287 #[unstable(feature = "libstd_io_internals", issue = "42788")]
288 #[doc(no_inline, hidden)]
289 pub use self::stdio::{set_panic, set_print};
290 #[stable(feature = "rust1", since = "1.0.0")]
291 pub use self::util::{copy, empty, repeat, sink, Empty, Repeat, Sink};
302 const DEFAULT_BUF_SIZE: usize = crate::sys_common::io::DEFAULT_BUF_SIZE;
305 buf: &'a mut Vec<u8>,
309 impl Drop for Guard<'_> {
312 self.buf.set_len(self.len);
317 // A few methods below (read_to_string, read_line) will append data into a
318 // `String` buffer, but we need to be pretty careful when doing this. The
319 // implementation will just call `.as_mut_vec()` and then delegate to a
320 // byte-oriented reading method, but we must ensure that when returning we never
321 // leave `buf` in a state such that it contains invalid UTF-8 in its bounds.
323 // To this end, we use an RAII guard (to protect against panics) which updates
324 // the length of the string when it is dropped. This guard initially truncates
325 // the string to the prior length and only after we've validated that the
326 // new contents are valid UTF-8 do we allow it to set a longer length.
328 // The unsafety in this function is twofold:
330 // 1. We're looking at the raw bytes of `buf`, so we take on the burden of UTF-8
332 // 2. We're passing a raw buffer to the function `f`, and it is expected that
333 // the function only *appends* bytes to the buffer. We'll get undefined
334 // behavior if existing bytes are overwritten to have non-UTF-8 data.
335 fn append_to_string<F>(buf: &mut String, f: F) -> Result<usize>
337 F: FnOnce(&mut Vec<u8>) -> Result<usize>,
340 let mut g = Guard { len: buf.len(), buf: buf.as_mut_vec() };
342 if str::from_utf8(&g.buf[g.len..]).is_err() {
344 Err(Error::new(ErrorKind::InvalidData, "stream did not contain valid UTF-8"))
353 // This uses an adaptive system to extend the vector when it fills. We want to
354 // avoid paying to allocate and zero a huge chunk of memory if the reader only
355 // has 4 bytes while still making large reads if the reader does have a ton
356 // of data to return. Simply tacking on an extra DEFAULT_BUF_SIZE space every
357 // time is 4,500 times (!) slower than a default reservation size of 32 if the
358 // reader has a very small amount of data to return.
360 // Because we're extending the buffer with uninitialized data for trusted
361 // readers, we need to make sure to truncate that if any of this panics.
362 fn read_to_end<R: Read + ?Sized>(r: &mut R, buf: &mut Vec<u8>) -> Result<usize> {
363 read_to_end_with_reservation(r, buf, |_| 32)
366 fn read_to_end_with_reservation<R, F>(
369 mut reservation_size: F,
373 F: FnMut(&R) -> usize,
375 let start_len = buf.len();
376 let mut g = Guard { len: buf.len(), buf };
379 if g.len == g.buf.len() {
381 // FIXME(danielhenrymantilla): #42788
383 // - This creates a (mut) reference to a slice of
384 // _uninitialized_ integers, which is **undefined behavior**
386 // - Only the standard library gets to soundly "ignore" this,
387 // based on its privileged knowledge of unstable rustc
389 g.buf.reserve(reservation_size(r));
390 let capacity = g.buf.capacity();
391 g.buf.set_len(capacity);
392 r.initializer().initialize(&mut g.buf[g.len..]);
396 match r.read(&mut g.buf[g.len..]) {
398 ret = Ok(g.len - start_len);
402 Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
413 pub(crate) fn default_read_vectored<F>(read: F, bufs: &mut [IoSliceMut<'_>]) -> Result<usize>
415 F: FnOnce(&mut [u8]) -> Result<usize>,
417 let buf = bufs.iter_mut().find(|b| !b.is_empty()).map_or(&mut [][..], |b| &mut **b);
421 pub(crate) fn default_write_vectored<F>(write: F, bufs: &[IoSlice<'_>]) -> Result<usize>
423 F: FnOnce(&[u8]) -> Result<usize>,
425 let buf = bufs.iter().find(|b| !b.is_empty()).map_or(&[][..], |b| &**b);
429 /// The `Read` trait allows for reading bytes from a source.
431 /// Implementors of the `Read` trait are called 'readers'.
433 /// Readers are defined by one required method, [`read()`]. Each call to [`read()`]
434 /// will attempt to pull bytes from this source into a provided buffer. A
435 /// number of other methods are implemented in terms of [`read()`], giving
436 /// implementors a number of ways to read bytes while only needing to implement
439 /// Readers are intended to be composable with one another. Many implementors
440 /// throughout [`std::io`] take and provide types which implement the `Read`
443 /// Please note that each call to [`read()`] may involve a system call, and
444 /// therefore, using something that implements [`BufRead`], such as
445 /// [`BufReader`], will be more efficient.
449 /// [`File`]s implement `Read`:
453 /// use std::io::prelude::*;
454 /// use std::fs::File;
456 /// fn main() -> io::Result<()> {
457 /// let mut f = File::open("foo.txt")?;
458 /// let mut buffer = [0; 10];
460 /// // read up to 10 bytes
461 /// f.read(&mut buffer)?;
463 /// let mut buffer = Vec::new();
464 /// // read the whole file
465 /// f.read_to_end(&mut buffer)?;
467 /// // read into a String, so that you don't need to do the conversion.
468 /// let mut buffer = String::new();
469 /// f.read_to_string(&mut buffer)?;
471 /// // and more! See the other methods for more details.
476 /// Read from [`&str`] because [`&[u8]`][slice] implements `Read`:
480 /// use std::io::prelude::*;
482 /// fn main() -> io::Result<()> {
483 /// let mut b = "This string will be read".as_bytes();
484 /// let mut buffer = [0; 10];
486 /// // read up to 10 bytes
487 /// b.read(&mut buffer)?;
489 /// // etc... it works exactly as a File does!
494 /// [`read()`]: trait.Read.html#tymethod.read
495 /// [`std::io`]: ../../std/io/index.html
496 /// [`File`]: ../fs/struct.File.html
497 /// [`BufRead`]: trait.BufRead.html
498 /// [`BufReader`]: struct.BufReader.html
499 /// [`&str`]: ../../std/primitive.str.html
500 /// [slice]: ../../std/primitive.slice.html
501 #[stable(feature = "rust1", since = "1.0.0")]
504 /// Pull some bytes from this source into the specified buffer, returning
505 /// how many bytes were read.
507 /// This function does not provide any guarantees about whether it blocks
508 /// waiting for data, but if an object needs to block for a read and cannot,
509 /// it will typically signal this via an [`Err`] return value.
511 /// If the return value of this method is [`Ok(n)`], then it must be
512 /// guaranteed that `0 <= n <= buf.len()`. A nonzero `n` value indicates
513 /// that the buffer `buf` has been filled in with `n` bytes of data from this
514 /// source. If `n` is `0`, then it can indicate one of two scenarios:
516 /// 1. This reader has reached its "end of file" and will likely no longer
517 /// be able to produce bytes. Note that this does not mean that the
518 /// reader will *always* no longer be able to produce bytes.
519 /// 2. The buffer specified was 0 bytes in length.
521 /// It is not an error if the returned value `n` is smaller than the buffer size,
522 /// even when the reader is not at the end of the stream yet.
523 /// This may happen for example because fewer bytes are actually available right now
524 /// (e. g. being close to end-of-file) or because read() was interrupted by a signal.
526 /// No guarantees are provided about the contents of `buf` when this
527 /// function is called, implementations cannot rely on any property of the
528 /// contents of `buf` being true. It is recommended that *implementations*
529 /// only write data to `buf` instead of reading its contents.
531 /// Correspondingly, however, *callers* of this method may not assume any guarantees
532 /// about how the implementation uses `buf`. The trait is safe to implement,
533 /// so it is possible that the code that's supposed to write to the buffer might also read
534 /// from it. It is your responsibility to make sure that `buf` is initialized
535 /// before calling `read`. Calling `read` with an uninitialized `buf` (of the kind one
536 /// obtains via [`MaybeUninit<T>`]) is not safe, and can lead to undefined behavior.
538 /// [`MaybeUninit<T>`]: ../mem/union.MaybeUninit.html
542 /// If this function encounters any form of I/O or other error, an error
543 /// variant will be returned. If an error is returned then it must be
544 /// guaranteed that no bytes were read.
546 /// An error of the [`ErrorKind::Interrupted`] kind is non-fatal and the read
547 /// operation should be retried if there is nothing else to do.
551 /// [`File`]s implement `Read`:
553 /// [`Err`]: ../../std/result/enum.Result.html#variant.Err
554 /// [`Ok(n)`]: ../../std/result/enum.Result.html#variant.Ok
555 /// [`ErrorKind::Interrupted`]: ../../std/io/enum.ErrorKind.html#variant.Interrupted
556 /// [`File`]: ../fs/struct.File.html
560 /// use std::io::prelude::*;
561 /// use std::fs::File;
563 /// fn main() -> io::Result<()> {
564 /// let mut f = File::open("foo.txt")?;
565 /// let mut buffer = [0; 10];
567 /// // read up to 10 bytes
568 /// let n = f.read(&mut buffer[..])?;
570 /// println!("The bytes: {:?}", &buffer[..n]);
574 #[stable(feature = "rust1", since = "1.0.0")]
575 fn read(&mut self, buf: &mut [u8]) -> Result<usize>;
577 /// Like `read`, except that it reads into a slice of buffers.
579 /// Data is copied to fill each buffer in order, with the final buffer
580 /// written to possibly being only partially filled. This method must
581 /// behave equivalently to a single call to `read` with concatenated
584 /// The default implementation calls `read` with either the first nonempty
585 /// buffer provided, or an empty one if none exists.
586 #[stable(feature = "iovec", since = "1.36.0")]
587 fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> Result<usize> {
588 default_read_vectored(|b| self.read(b), bufs)
591 /// Determines if this `Read`er has an efficient `read_vectored`
594 /// If a `Read`er does not override the default `read_vectored`
595 /// implementation, code using it may want to avoid the method all together
596 /// and coalesce writes into a single buffer for higher performance.
598 /// The default implementation returns `false`.
599 #[unstable(feature = "can_vector", issue = "69941")]
600 fn is_read_vectored(&self) -> bool {
604 /// Determines if this `Read`er can work with buffers of uninitialized
607 /// The default implementation returns an initializer which will zero
610 /// If a `Read`er guarantees that it can work properly with uninitialized
611 /// memory, it should call [`Initializer::nop()`]. See the documentation for
612 /// [`Initializer`] for details.
614 /// The behavior of this method must be independent of the state of the
615 /// `Read`er - the method only takes `&self` so that it can be used through
620 /// This method is unsafe because a `Read`er could otherwise return a
621 /// non-zeroing `Initializer` from another `Read` type without an `unsafe`
624 /// [`Initializer::nop()`]: ../../std/io/struct.Initializer.html#method.nop
625 /// [`Initializer`]: ../../std/io/struct.Initializer.html
626 #[unstable(feature = "read_initializer", issue = "42788")]
628 unsafe fn initializer(&self) -> Initializer {
629 Initializer::zeroing()
632 /// Read all bytes until EOF in this source, placing them into `buf`.
634 /// All bytes read from this source will be appended to the specified buffer
635 /// `buf`. This function will continuously call [`read()`] to append more data to
636 /// `buf` until [`read()`] returns either [`Ok(0)`] or an error of
637 /// non-[`ErrorKind::Interrupted`] kind.
639 /// If successful, this function will return the total number of bytes read.
643 /// If this function encounters an error of the kind
644 /// [`ErrorKind::Interrupted`] then the error is ignored and the operation
647 /// If any other read error is encountered then this function immediately
648 /// returns. Any bytes which have already been read will be appended to
653 /// [`File`]s implement `Read`:
655 /// [`read()`]: trait.Read.html#tymethod.read
656 /// [`Ok(0)`]: ../../std/result/enum.Result.html#variant.Ok
657 /// [`ErrorKind::Interrupted`]: ../../std/io/enum.ErrorKind.html#variant.Interrupted
658 /// [`File`]: ../fs/struct.File.html
662 /// use std::io::prelude::*;
663 /// use std::fs::File;
665 /// fn main() -> io::Result<()> {
666 /// let mut f = File::open("foo.txt")?;
667 /// let mut buffer = Vec::new();
669 /// // read the whole file
670 /// f.read_to_end(&mut buffer)?;
675 /// (See also the [`std::fs::read`] convenience function for reading from a
678 /// [`std::fs::read`]: ../fs/fn.read.html
679 #[stable(feature = "rust1", since = "1.0.0")]
680 fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize> {
681 read_to_end(self, buf)
684 /// Read all bytes until EOF in this source, appending them to `buf`.
686 /// If successful, this function returns the number of bytes which were read
687 /// and appended to `buf`.
691 /// If the data in this stream is *not* valid UTF-8 then an error is
692 /// returned and `buf` is unchanged.
694 /// See [`read_to_end`][readtoend] for other error semantics.
696 /// [readtoend]: #method.read_to_end
700 /// [`File`][file]s implement `Read`:
702 /// [file]: ../fs/struct.File.html
706 /// use std::io::prelude::*;
707 /// use std::fs::File;
709 /// fn main() -> io::Result<()> {
710 /// let mut f = File::open("foo.txt")?;
711 /// let mut buffer = String::new();
713 /// f.read_to_string(&mut buffer)?;
718 /// (See also the [`std::fs::read_to_string`] convenience function for
719 /// reading from a file.)
721 /// [`std::fs::read_to_string`]: ../fs/fn.read_to_string.html
722 #[stable(feature = "rust1", since = "1.0.0")]
723 fn read_to_string(&mut self, buf: &mut String) -> Result<usize> {
724 // Note that we do *not* call `.read_to_end()` here. We are passing
725 // `&mut Vec<u8>` (the raw contents of `buf`) into the `read_to_end`
726 // method to fill it up. An arbitrary implementation could overwrite the
727 // entire contents of the vector, not just append to it (which is what
728 // we are expecting).
730 // To prevent extraneously checking the UTF-8-ness of the entire buffer
731 // we pass it to our hardcoded `read_to_end` implementation which we
732 // know is guaranteed to only read data into the end of the buffer.
733 append_to_string(buf, |b| read_to_end(self, b))
736 /// Read the exact number of bytes required to fill `buf`.
738 /// This function reads as many bytes as necessary to completely fill the
739 /// specified buffer `buf`.
741 /// No guarantees are provided about the contents of `buf` when this
742 /// function is called, implementations cannot rely on any property of the
743 /// contents of `buf` being true. It is recommended that implementations
744 /// only write data to `buf` instead of reading its contents. The
745 /// documentation on [`read`] has a more detailed explanation on this
750 /// If this function encounters an error of the kind
751 /// [`ErrorKind::Interrupted`] then the error is ignored and the operation
754 /// If this function encounters an "end of file" before completely filling
755 /// the buffer, it returns an error of the kind [`ErrorKind::UnexpectedEof`].
756 /// The contents of `buf` are unspecified in this case.
758 /// If any other read error is encountered then this function immediately
759 /// returns. The contents of `buf` are unspecified in this case.
761 /// If this function returns an error, it is unspecified how many bytes it
762 /// has read, but it will never read more than would be necessary to
763 /// completely fill the buffer.
767 /// [`File`]s implement `Read`:
769 /// [`read`]: Read::read
770 /// [`File`]: ../fs/struct.File.html
771 /// [`ErrorKind::Interrupted`]: ../../std/io/enum.ErrorKind.html#variant.Interrupted
772 /// [`ErrorKind::UnexpectedEof`]: ../../std/io/enum.ErrorKind.html#variant.UnexpectedEof
776 /// use std::io::prelude::*;
777 /// use std::fs::File;
779 /// fn main() -> io::Result<()> {
780 /// let mut f = File::open("foo.txt")?;
781 /// let mut buffer = [0; 10];
783 /// // read exactly 10 bytes
784 /// f.read_exact(&mut buffer)?;
788 #[stable(feature = "read_exact", since = "1.6.0")]
789 fn read_exact(&mut self, mut buf: &mut [u8]) -> Result<()> {
790 while !buf.is_empty() {
791 match self.read(buf) {
797 Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
798 Err(e) => return Err(e),
802 Err(Error::new(ErrorKind::UnexpectedEof, "failed to fill whole buffer"))
808 /// Creates a "by reference" adaptor for this instance of `Read`.
810 /// The returned adaptor also implements `Read` and will simply borrow this
815 /// [`File`][file]s implement `Read`:
817 /// [file]: ../fs/struct.File.html
821 /// use std::io::Read;
822 /// use std::fs::File;
824 /// fn main() -> io::Result<()> {
825 /// let mut f = File::open("foo.txt")?;
826 /// let mut buffer = Vec::new();
827 /// let mut other_buffer = Vec::new();
830 /// let reference = f.by_ref();
832 /// // read at most 5 bytes
833 /// reference.take(5).read_to_end(&mut buffer)?;
835 /// } // drop our &mut reference so we can use f again
837 /// // original file still usable, read the rest
838 /// f.read_to_end(&mut other_buffer)?;
842 #[stable(feature = "rust1", since = "1.0.0")]
843 fn by_ref(&mut self) -> &mut Self
850 /// Transforms this `Read` instance to an [`Iterator`] over its bytes.
852 /// The returned type implements [`Iterator`] where the `Item` is
853 /// [`Result`]`<`[`u8`]`, `[`io::Error`]`>`.
854 /// The yielded item is [`Ok`] if a byte was successfully read and [`Err`]
855 /// otherwise. EOF is mapped to returning [`None`] from this iterator.
859 /// [`File`][file]s implement `Read`:
861 /// [file]: ../fs/struct.File.html
862 /// [`Iterator`]: ../../std/iter/trait.Iterator.html
863 /// [`Result`]: ../../std/result/enum.Result.html
864 /// [`io::Error`]: ../../std/io/struct.Error.html
865 /// [`u8`]: ../../std/primitive.u8.html
866 /// [`Ok`]: ../../std/result/enum.Result.html#variant.Ok
867 /// [`Err`]: ../../std/result/enum.Result.html#variant.Err
868 /// [`None`]: ../../std/option/enum.Option.html#variant.None
872 /// use std::io::prelude::*;
873 /// use std::fs::File;
875 /// fn main() -> io::Result<()> {
876 /// let mut f = File::open("foo.txt")?;
878 /// for byte in f.bytes() {
879 /// println!("{}", byte.unwrap());
884 #[stable(feature = "rust1", since = "1.0.0")]
885 fn bytes(self) -> Bytes<Self>
889 Bytes { inner: self }
892 /// Creates an adaptor which will chain this stream with another.
894 /// The returned `Read` instance will first read all bytes from this object
895 /// until EOF is encountered. Afterwards the output is equivalent to the
896 /// output of `next`.
900 /// [`File`][file]s implement `Read`:
902 /// [file]: ../fs/struct.File.html
906 /// use std::io::prelude::*;
907 /// use std::fs::File;
909 /// fn main() -> io::Result<()> {
910 /// let mut f1 = File::open("foo.txt")?;
911 /// let mut f2 = File::open("bar.txt")?;
913 /// let mut handle = f1.chain(f2);
914 /// let mut buffer = String::new();
916 /// // read the value into a String. We could use any Read method here,
917 /// // this is just one example.
918 /// handle.read_to_string(&mut buffer)?;
922 #[stable(feature = "rust1", since = "1.0.0")]
923 fn chain<R: Read>(self, next: R) -> Chain<Self, R>
927 Chain { first: self, second: next, done_first: false }
930 /// Creates an adaptor which will read at most `limit` bytes from it.
932 /// This function returns a new instance of `Read` which will read at most
933 /// `limit` bytes, after which it will always return EOF ([`Ok(0)`]). Any
934 /// read errors will not count towards the number of bytes read and future
935 /// calls to [`read()`] may succeed.
939 /// [`File`]s implement `Read`:
941 /// [`File`]: ../fs/struct.File.html
942 /// [`Ok(0)`]: ../../std/result/enum.Result.html#variant.Ok
943 /// [`read()`]: trait.Read.html#tymethod.read
947 /// use std::io::prelude::*;
948 /// use std::fs::File;
950 /// fn main() -> io::Result<()> {
951 /// let mut f = File::open("foo.txt")?;
952 /// let mut buffer = [0; 5];
954 /// // read at most five bytes
955 /// let mut handle = f.take(5);
957 /// handle.read(&mut buffer)?;
961 #[stable(feature = "rust1", since = "1.0.0")]
962 fn take(self, limit: u64) -> Take<Self>
966 Take { inner: self, limit }
970 /// A buffer type used with `Read::read_vectored`.
972 /// It is semantically a wrapper around an `&mut [u8]`, but is guaranteed to be
973 /// ABI compatible with the `iovec` type on Unix platforms and `WSABUF` on
975 #[stable(feature = "iovec", since = "1.36.0")]
977 pub struct IoSliceMut<'a>(sys::io::IoSliceMut<'a>);
979 #[stable(feature = "iovec-send-sync", since = "1.44.0")]
980 unsafe impl<'a> Send for IoSliceMut<'a> {}
982 #[stable(feature = "iovec-send-sync", since = "1.44.0")]
983 unsafe impl<'a> Sync for IoSliceMut<'a> {}
985 #[stable(feature = "iovec", since = "1.36.0")]
986 impl<'a> fmt::Debug for IoSliceMut<'a> {
987 fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
988 fmt::Debug::fmt(self.0.as_slice(), fmt)
992 impl<'a> IoSliceMut<'a> {
993 /// Creates a new `IoSliceMut` wrapping a byte slice.
997 /// Panics on Windows if the slice is larger than 4GB.
998 #[stable(feature = "iovec", since = "1.36.0")]
1000 pub fn new(buf: &'a mut [u8]) -> IoSliceMut<'a> {
1001 IoSliceMut(sys::io::IoSliceMut::new(buf))
1004 /// Advance the internal cursor of the slice.
1008 /// Elements in the slice may be modified if the cursor is not advanced to
1009 /// the end of the slice. For example if we have a slice of buffers with 2
1010 /// `IoSliceMut`s, both of length 8, and we advance the cursor by 10 bytes
1011 /// the first `IoSliceMut` will be untouched however the second will be
1012 /// modified to remove the first 2 bytes (10 - 8).
1017 /// #![feature(io_slice_advance)]
1019 /// use std::io::IoSliceMut;
1020 /// use std::ops::Deref;
1022 /// let mut buf1 = [1; 8];
1023 /// let mut buf2 = [2; 16];
1024 /// let mut buf3 = [3; 8];
1025 /// let mut bufs = &mut [
1026 /// IoSliceMut::new(&mut buf1),
1027 /// IoSliceMut::new(&mut buf2),
1028 /// IoSliceMut::new(&mut buf3),
1031 /// // Mark 10 bytes as read.
1032 /// bufs = IoSliceMut::advance(bufs, 10);
1033 /// assert_eq!(bufs[0].deref(), [2; 14].as_ref());
1034 /// assert_eq!(bufs[1].deref(), [3; 8].as_ref());
1036 #[unstable(feature = "io_slice_advance", issue = "62726")]
1038 pub fn advance<'b>(bufs: &'b mut [IoSliceMut<'a>], n: usize) -> &'b mut [IoSliceMut<'a>] {
1039 // Number of buffers to remove.
1041 // Total length of all the to be removed buffers.
1042 let mut accumulated_len = 0;
1043 for buf in bufs.iter() {
1044 if accumulated_len + buf.len() > n {
1047 accumulated_len += buf.len();
1052 let bufs = &mut bufs[remove..];
1053 if !bufs.is_empty() {
1054 bufs[0].0.advance(n - accumulated_len)
1060 #[stable(feature = "iovec", since = "1.36.0")]
1061 impl<'a> Deref for IoSliceMut<'a> {
1065 fn deref(&self) -> &[u8] {
1070 #[stable(feature = "iovec", since = "1.36.0")]
1071 impl<'a> DerefMut for IoSliceMut<'a> {
1073 fn deref_mut(&mut self) -> &mut [u8] {
1074 self.0.as_mut_slice()
1078 /// A buffer type used with `Write::write_vectored`.
1080 /// It is semantically a wrapper around an `&[u8]`, but is guaranteed to be
1081 /// ABI compatible with the `iovec` type on Unix platforms and `WSABUF` on
1083 #[stable(feature = "iovec", since = "1.36.0")]
1084 #[derive(Copy, Clone)]
1085 #[repr(transparent)]
1086 pub struct IoSlice<'a>(sys::io::IoSlice<'a>);
1088 #[stable(feature = "iovec-send-sync", since = "1.44.0")]
1089 unsafe impl<'a> Send for IoSlice<'a> {}
1091 #[stable(feature = "iovec-send-sync", since = "1.44.0")]
1092 unsafe impl<'a> Sync for IoSlice<'a> {}
1094 #[stable(feature = "iovec", since = "1.36.0")]
1095 impl<'a> fmt::Debug for IoSlice<'a> {
1096 fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
1097 fmt::Debug::fmt(self.0.as_slice(), fmt)
1101 impl<'a> IoSlice<'a> {
1102 /// Creates a new `IoSlice` wrapping a byte slice.
1106 /// Panics on Windows if the slice is larger than 4GB.
1107 #[stable(feature = "iovec", since = "1.36.0")]
1109 pub fn new(buf: &'a [u8]) -> IoSlice<'a> {
1110 IoSlice(sys::io::IoSlice::new(buf))
1113 /// Advance the internal cursor of the slice.
1117 /// Elements in the slice may be modified if the cursor is not advanced to
1118 /// the end of the slice. For example if we have a slice of buffers with 2
1119 /// `IoSlice`s, both of length 8, and we advance the cursor by 10 bytes the
1120 /// first `IoSlice` will be untouched however the second will be modified to
1121 /// remove the first 2 bytes (10 - 8).
1126 /// #![feature(io_slice_advance)]
1128 /// use std::io::IoSlice;
1129 /// use std::ops::Deref;
1131 /// let buf1 = [1; 8];
1132 /// let buf2 = [2; 16];
1133 /// let buf3 = [3; 8];
1134 /// let mut bufs = &mut [
1135 /// IoSlice::new(&buf1),
1136 /// IoSlice::new(&buf2),
1137 /// IoSlice::new(&buf3),
1140 /// // Mark 10 bytes as written.
1141 /// bufs = IoSlice::advance(bufs, 10);
1142 /// assert_eq!(bufs[0].deref(), [2; 14].as_ref());
1143 /// assert_eq!(bufs[1].deref(), [3; 8].as_ref());
1144 #[unstable(feature = "io_slice_advance", issue = "62726")]
1146 pub fn advance<'b>(bufs: &'b mut [IoSlice<'a>], n: usize) -> &'b mut [IoSlice<'a>] {
1147 // Number of buffers to remove.
1149 // Total length of all the to be removed buffers.
1150 let mut accumulated_len = 0;
1151 for buf in bufs.iter() {
1152 if accumulated_len + buf.len() > n {
1155 accumulated_len += buf.len();
1160 let bufs = &mut bufs[remove..];
1161 if !bufs.is_empty() {
1162 bufs[0].0.advance(n - accumulated_len)
1168 #[stable(feature = "iovec", since = "1.36.0")]
1169 impl<'a> Deref for IoSlice<'a> {
1173 fn deref(&self) -> &[u8] {
1178 /// A type used to conditionally initialize buffers passed to `Read` methods.
1179 #[unstable(feature = "read_initializer", issue = "42788")]
1181 pub struct Initializer(bool);
1184 /// Returns a new `Initializer` which will zero out buffers.
1185 #[unstable(feature = "read_initializer", issue = "42788")]
1187 pub fn zeroing() -> Initializer {
1191 /// Returns a new `Initializer` which will not zero out buffers.
1195 /// This may only be called by `Read`ers which guarantee that they will not
1196 /// read from buffers passed to `Read` methods, and that the return value of
1197 /// the method accurately reflects the number of bytes that have been
1198 /// written to the head of the buffer.
1199 #[unstable(feature = "read_initializer", issue = "42788")]
1201 pub unsafe fn nop() -> Initializer {
1205 /// Indicates if a buffer should be initialized.
1206 #[unstable(feature = "read_initializer", issue = "42788")]
1208 pub fn should_initialize(&self) -> bool {
1212 /// Initializes a buffer if necessary.
1213 #[unstable(feature = "read_initializer", issue = "42788")]
1215 pub fn initialize(&self, buf: &mut [u8]) {
1216 if self.should_initialize() {
1217 unsafe { ptr::write_bytes(buf.as_mut_ptr(), 0, buf.len()) }
1222 /// A trait for objects which are byte-oriented sinks.
1224 /// Implementors of the `Write` trait are sometimes called 'writers'.
1226 /// Writers are defined by two required methods, [`write`] and [`flush`]:
1228 /// * The [`write`] method will attempt to write some data into the object,
1229 /// returning how many bytes were successfully written.
1231 /// * The [`flush`] method is useful for adaptors and explicit buffers
1232 /// themselves for ensuring that all buffered data has been pushed out to the
1235 /// Writers are intended to be composable with one another. Many implementors
1236 /// throughout [`std::io`] take and provide types which implement the `Write`
1239 /// [`write`]: #tymethod.write
1240 /// [`flush`]: #tymethod.flush
1241 /// [`std::io`]: index.html
1246 /// use std::io::prelude::*;
1247 /// use std::fs::File;
1249 /// fn main() -> std::io::Result<()> {
1250 /// let data = b"some bytes";
1252 /// let mut pos = 0;
1253 /// let mut buffer = File::create("foo.txt")?;
1255 /// while pos < data.len() {
1256 /// let bytes_written = buffer.write(&data[pos..])?;
1257 /// pos += bytes_written;
1263 /// The trait also provides convenience methods like [`write_all`], which calls
1264 /// `write` in a loop until its entire input has been written.
1266 /// [`write_all`]: #method.write_all
1267 #[stable(feature = "rust1", since = "1.0.0")]
1270 /// Write a buffer into this writer, returning how many bytes were written.
1272 /// This function will attempt to write the entire contents of `buf`, but
1273 /// the entire write may not succeed, or the write may also generate an
1274 /// error. A call to `write` represents *at most one* attempt to write to
1275 /// any wrapped object.
1277 /// Calls to `write` are not guaranteed to block waiting for data to be
1278 /// written, and a write which would otherwise block can be indicated through
1279 /// an [`Err`] variant.
1281 /// If the return value is [`Ok(n)`] then it must be guaranteed that
1282 /// `n <= buf.len()`. A return value of `0` typically means that the
1283 /// underlying object is no longer able to accept bytes and will likely not
1284 /// be able to in the future as well, or that the buffer provided is empty.
1288 /// Each call to `write` may generate an I/O error indicating that the
1289 /// operation could not be completed. If an error is returned then no bytes
1290 /// in the buffer were written to this writer.
1292 /// It is **not** considered an error if the entire buffer could not be
1293 /// written to this writer.
1295 /// An error of the [`ErrorKind::Interrupted`] kind is non-fatal and the
1296 /// write operation should be retried if there is nothing else to do.
1298 /// [`Err`]: ../../std/result/enum.Result.html#variant.Err
1299 /// [`Ok(n)`]: ../../std/result/enum.Result.html#variant.Ok
1300 /// [`ErrorKind::Interrupted`]: ../../std/io/enum.ErrorKind.html#variant.Interrupted
1305 /// use std::io::prelude::*;
1306 /// use std::fs::File;
1308 /// fn main() -> std::io::Result<()> {
1309 /// let mut buffer = File::create("foo.txt")?;
1311 /// // Writes some prefix of the byte string, not necessarily all of it.
1312 /// buffer.write(b"some bytes")?;
1316 #[stable(feature = "rust1", since = "1.0.0")]
1317 fn write(&mut self, buf: &[u8]) -> Result<usize>;
1319 /// Like `write`, except that it writes from a slice of buffers.
1321 /// Data is copied from each buffer in order, with the final buffer
1322 /// read from possibly being only partially consumed. This method must
1323 /// behave as a call to `write` with the buffers concatenated would.
1325 /// The default implementation calls `write` with either the first nonempty
1326 /// buffer provided, or an empty one if none exists.
1327 #[stable(feature = "iovec", since = "1.36.0")]
1328 fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize> {
1329 default_write_vectored(|b| self.write(b), bufs)
1332 /// Determines if this `Write`er has an efficient `write_vectored`
1335 /// If a `Write`er does not override the default `write_vectored`
1336 /// implementation, code using it may want to avoid the method all together
1337 /// and coalesce writes into a single buffer for higher performance.
1339 /// The default implementation returns `false`.
1340 #[unstable(feature = "can_vector", issue = "69941")]
1341 fn is_write_vectored(&self) -> bool {
1345 /// Flush this output stream, ensuring that all intermediately buffered
1346 /// contents reach their destination.
1350 /// It is considered an error if not all bytes could be written due to
1351 /// I/O errors or EOF being reached.
1356 /// use std::io::prelude::*;
1357 /// use std::io::BufWriter;
1358 /// use std::fs::File;
1360 /// fn main() -> std::io::Result<()> {
1361 /// let mut buffer = BufWriter::new(File::create("foo.txt")?);
1363 /// buffer.write_all(b"some bytes")?;
1364 /// buffer.flush()?;
1368 #[stable(feature = "rust1", since = "1.0.0")]
1369 fn flush(&mut self) -> Result<()>;
1371 /// Attempts to write an entire buffer into this writer.
1373 /// This method will continuously call [`write`] until there is no more data
1374 /// to be written or an error of non-[`ErrorKind::Interrupted`] kind is
1375 /// returned. This method will not return until the entire buffer has been
1376 /// successfully written or such an error occurs. The first error that is
1377 /// not of [`ErrorKind::Interrupted`] kind generated from this method will be
1380 /// If the buffer contains no data, this will never call [`write`].
1384 /// This function will return the first error of
1385 /// non-[`ErrorKind::Interrupted`] kind that [`write`] returns.
1387 /// [`ErrorKind::Interrupted`]: ../../std/io/enum.ErrorKind.html#variant.Interrupted
1388 /// [`write`]: #tymethod.write
1393 /// use std::io::prelude::*;
1394 /// use std::fs::File;
1396 /// fn main() -> std::io::Result<()> {
1397 /// let mut buffer = File::create("foo.txt")?;
1399 /// buffer.write_all(b"some bytes")?;
1403 #[stable(feature = "rust1", since = "1.0.0")]
1404 fn write_all(&mut self, mut buf: &[u8]) -> Result<()> {
1405 while !buf.is_empty() {
1406 match self.write(buf) {
1408 return Err(Error::new(ErrorKind::WriteZero, "failed to write whole buffer"));
1410 Ok(n) => buf = &buf[n..],
1411 Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
1412 Err(e) => return Err(e),
1418 /// Attempts to write multiple buffers into this writer.
1420 /// This method will continuously call [`write_vectored`] until there is no
1421 /// more data to be written or an error of non-[`ErrorKind::Interrupted`]
1422 /// kind is returned. This method will not return until all buffers have
1423 /// been successfully written or such an error occurs. The first error that
1424 /// is not of [`ErrorKind::Interrupted`] kind generated from this method
1425 /// will be returned.
1427 /// If the buffer contains no data, this will never call [`write_vectored`].
1429 /// [`write_vectored`]: #method.write_vectored
1430 /// [`ErrorKind::Interrupted`]: ../../std/io/enum.ErrorKind.html#variant.Interrupted
1435 /// Unlike `io::Write::write_vectored`, this takes a *mutable* reference to
1436 /// a slice of `IoSlice`s, not an immutable one. That's because we need to
1437 /// modify the slice to keep track of the bytes already written.
1439 /// Once this function returns, the contents of `bufs` are unspecified, as
1440 /// this depends on how many calls to `write_vectored` were necessary. It is
1441 /// best to understand this function as taking ownership of `bufs` and to
1442 /// not use `bufs` afterwards. The underlying buffers, to which the
1443 /// `IoSlice`s point (but not the `IoSlice`s themselves), are unchanged and
1449 /// #![feature(write_all_vectored)]
1450 /// # fn main() -> std::io::Result<()> {
1452 /// use std::io::{Write, IoSlice};
1454 /// let mut writer = Vec::new();
1455 /// let bufs = &mut [
1456 /// IoSlice::new(&[1]),
1457 /// IoSlice::new(&[2, 3]),
1458 /// IoSlice::new(&[4, 5, 6]),
1461 /// writer.write_all_vectored(bufs)?;
1462 /// // Note: the contents of `bufs` is now undefined, see the Notes section.
1464 /// assert_eq!(writer, &[1, 2, 3, 4, 5, 6]);
1467 #[unstable(feature = "write_all_vectored", issue = "70436")]
1468 fn write_all_vectored(&mut self, mut bufs: &mut [IoSlice<'_>]) -> Result<()> {
1469 while !bufs.is_empty() {
1470 match self.write_vectored(bufs) {
1472 return Err(Error::new(ErrorKind::WriteZero, "failed to write whole buffer"));
1474 Ok(n) => bufs = IoSlice::advance(mem::take(&mut bufs), n),
1475 Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
1476 Err(e) => return Err(e),
1482 /// Writes a formatted string into this writer, returning any error
1485 /// This method is primarily used to interface with the
1486 /// [`format_args!`][formatargs] macro, but it is rare that this should
1487 /// explicitly be called. The [`write!`][write] macro should be favored to
1488 /// invoke this method instead.
1490 /// [formatargs]: ../macro.format_args.html
1491 /// [write]: ../macro.write.html
1493 /// This function internally uses the [`write_all`][writeall] method on
1494 /// this trait and hence will continuously write data so long as no errors
1495 /// are received. This also means that partial writes are not indicated in
1498 /// [writeall]: #method.write_all
1502 /// This function will return any I/O error reported while formatting.
1507 /// use std::io::prelude::*;
1508 /// use std::fs::File;
1510 /// fn main() -> std::io::Result<()> {
1511 /// let mut buffer = File::create("foo.txt")?;
1514 /// write!(buffer, "{:.*}", 2, 1.234567)?;
1515 /// // turns into this:
1516 /// buffer.write_fmt(format_args!("{:.*}", 2, 1.234567))?;
1520 #[stable(feature = "rust1", since = "1.0.0")]
1521 fn write_fmt(&mut self, fmt: fmt::Arguments<'_>) -> Result<()> {
1522 // Create a shim which translates a Write to a fmt::Write and saves
1523 // off I/O errors. instead of discarding them
1524 struct Adaptor<'a, T: ?Sized + 'a> {
1529 impl<T: Write + ?Sized> fmt::Write for Adaptor<'_, T> {
1530 fn write_str(&mut self, s: &str) -> fmt::Result {
1531 match self.inner.write_all(s.as_bytes()) {
1534 self.error = Err(e);
1541 let mut output = Adaptor { inner: self, error: Ok(()) };
1542 match fmt::write(&mut output, fmt) {
1545 // check if the error came from the underlying `Write` or not
1546 if output.error.is_err() {
1549 Err(Error::new(ErrorKind::Other, "formatter error"))
1555 /// Creates a "by reference" adaptor for this instance of `Write`.
1557 /// The returned adaptor also implements `Write` and will simply borrow this
1563 /// use std::io::Write;
1564 /// use std::fs::File;
1566 /// fn main() -> std::io::Result<()> {
1567 /// let mut buffer = File::create("foo.txt")?;
1569 /// let reference = buffer.by_ref();
1571 /// // we can use reference just like our original buffer
1572 /// reference.write_all(b"some bytes")?;
1576 #[stable(feature = "rust1", since = "1.0.0")]
1577 fn by_ref(&mut self) -> &mut Self
1585 /// The `Seek` trait provides a cursor which can be moved within a stream of
1588 /// The stream typically has a fixed size, allowing seeking relative to either
1589 /// end or the current offset.
1593 /// [`File`][file]s implement `Seek`:
1595 /// [file]: ../fs/struct.File.html
1599 /// use std::io::prelude::*;
1600 /// use std::fs::File;
1601 /// use std::io::SeekFrom;
1603 /// fn main() -> io::Result<()> {
1604 /// let mut f = File::open("foo.txt")?;
1606 /// // move the cursor 42 bytes from the start of the file
1607 /// f.seek(SeekFrom::Start(42))?;
1611 #[stable(feature = "rust1", since = "1.0.0")]
1613 /// Seek to an offset, in bytes, in a stream.
1615 /// A seek beyond the end of a stream is allowed, but behavior is defined
1616 /// by the implementation.
1618 /// If the seek operation completed successfully,
1619 /// this method returns the new position from the start of the stream.
1620 /// That position can be used later with [`SeekFrom::Start`].
1624 /// Seeking to a negative offset is considered an error.
1626 /// [`SeekFrom::Start`]: enum.SeekFrom.html#variant.Start
1627 #[stable(feature = "rust1", since = "1.0.0")]
1628 fn seek(&mut self, pos: SeekFrom) -> Result<u64>;
1630 /// Returns the length of this stream (in bytes).
1632 /// This method is implemented using up to three seek operations. If this
1633 /// method returns successfully, the seek position is unchanged (i.e. the
1634 /// position before calling this method is the same as afterwards).
1635 /// However, if this method returns an error, the seek position is
1638 /// If you need to obtain the length of *many* streams and you don't care
1639 /// about the seek position afterwards, you can reduce the number of seek
1640 /// operations by simply calling `seek(SeekFrom::End(0))` and using its
1641 /// return value (it is also the stream length).
1643 /// Note that length of a stream can change over time (for example, when
1644 /// data is appended to a file). So calling this method multiple times does
1645 /// not necessarily return the same length each time.
1651 /// #![feature(seek_convenience)]
1653 /// io::{self, Seek},
1657 /// fn main() -> io::Result<()> {
1658 /// let mut f = File::open("foo.txt")?;
1660 /// let len = f.stream_len()?;
1661 /// println!("The file is currently {} bytes long", len);
1665 #[unstable(feature = "seek_convenience", issue = "59359")]
1666 fn stream_len(&mut self) -> Result<u64> {
1667 let old_pos = self.stream_position()?;
1668 let len = self.seek(SeekFrom::End(0))?;
1670 // Avoid seeking a third time when we were already at the end of the
1671 // stream. The branch is usually way cheaper than a seek operation.
1673 self.seek(SeekFrom::Start(old_pos))?;
1679 /// Returns the current seek position from the start of the stream.
1681 /// This is equivalent to `self.seek(SeekFrom::Current(0))`.
1687 /// #![feature(seek_convenience)]
1689 /// io::{self, BufRead, BufReader, Seek},
1693 /// fn main() -> io::Result<()> {
1694 /// let mut f = BufReader::new(File::open("foo.txt")?);
1696 /// let before = f.stream_position()?;
1697 /// f.read_line(&mut String::new())?;
1698 /// let after = f.stream_position()?;
1700 /// println!("The first line was {} bytes long", after - before);
1704 #[unstable(feature = "seek_convenience", issue = "59359")]
1705 fn stream_position(&mut self) -> Result<u64> {
1706 self.seek(SeekFrom::Current(0))
1710 /// Enumeration of possible methods to seek within an I/O object.
1712 /// It is used by the [`Seek`] trait.
1714 /// [`Seek`]: trait.Seek.html
1715 #[derive(Copy, PartialEq, Eq, Clone, Debug)]
1716 #[stable(feature = "rust1", since = "1.0.0")]
1718 /// Sets the offset to the provided number of bytes.
1719 #[stable(feature = "rust1", since = "1.0.0")]
1720 Start(#[stable(feature = "rust1", since = "1.0.0")] u64),
1722 /// Sets the offset to the size of this object plus the specified number of
1725 /// It is possible to seek beyond the end of an object, but it's an error to
1726 /// seek before byte 0.
1727 #[stable(feature = "rust1", since = "1.0.0")]
1728 End(#[stable(feature = "rust1", since = "1.0.0")] i64),
1730 /// Sets the offset to the current position plus the specified number of
1733 /// It is possible to seek beyond the end of an object, but it's an error to
1734 /// seek before byte 0.
1735 #[stable(feature = "rust1", since = "1.0.0")]
1736 Current(#[stable(feature = "rust1", since = "1.0.0")] i64),
1739 fn read_until<R: BufRead + ?Sized>(r: &mut R, delim: u8, buf: &mut Vec<u8>) -> Result<usize> {
1742 let (done, used) = {
1743 let available = match r.fill_buf() {
1745 Err(ref e) if e.kind() == ErrorKind::Interrupted => continue,
1746 Err(e) => return Err(e),
1748 match memchr::memchr(delim, available) {
1750 buf.extend_from_slice(&available[..=i]);
1754 buf.extend_from_slice(available);
1755 (false, available.len())
1761 if done || used == 0 {
1767 /// A `BufRead` is a type of `Read`er which has an internal buffer, allowing it
1768 /// to perform extra ways of reading.
1770 /// For example, reading line-by-line is inefficient without using a buffer, so
1771 /// if you want to read by line, you'll need `BufRead`, which includes a
1772 /// [`read_line`] method as well as a [`lines`] iterator.
1776 /// A locked standard input implements `BufRead`:
1780 /// use std::io::prelude::*;
1782 /// let stdin = io::stdin();
1783 /// for line in stdin.lock().lines() {
1784 /// println!("{}", line.unwrap());
1788 /// If you have something that implements [`Read`], you can use the [`BufReader`
1789 /// type][`BufReader`] to turn it into a `BufRead`.
1791 /// For example, [`File`] implements [`Read`], but not `BufRead`.
1792 /// [`BufReader`] to the rescue!
1794 /// [`BufReader`]: struct.BufReader.html
1795 /// [`File`]: ../fs/struct.File.html
1796 /// [`read_line`]: #method.read_line
1797 /// [`lines`]: #method.lines
1798 /// [`Read`]: trait.Read.html
1801 /// use std::io::{self, BufReader};
1802 /// use std::io::prelude::*;
1803 /// use std::fs::File;
1805 /// fn main() -> io::Result<()> {
1806 /// let f = File::open("foo.txt")?;
1807 /// let f = BufReader::new(f);
1809 /// for line in f.lines() {
1810 /// println!("{}", line.unwrap());
1817 #[stable(feature = "rust1", since = "1.0.0")]
1818 pub trait BufRead: Read {
1819 /// Returns the contents of the internal buffer, filling it with more data
1820 /// from the inner reader if it is empty.
1822 /// This function is a lower-level call. It needs to be paired with the
1823 /// [`consume`] method to function properly. When calling this
1824 /// method, none of the contents will be "read" in the sense that later
1825 /// calling `read` may return the same contents. As such, [`consume`] must
1826 /// be called with the number of bytes that are consumed from this buffer to
1827 /// ensure that the bytes are never returned twice.
1829 /// [`consume`]: #tymethod.consume
1831 /// An empty buffer returned indicates that the stream has reached EOF.
1835 /// This function will return an I/O error if the underlying reader was
1836 /// read, but returned an error.
1840 /// A locked standard input implements `BufRead`:
1844 /// use std::io::prelude::*;
1846 /// let stdin = io::stdin();
1847 /// let mut stdin = stdin.lock();
1849 /// let buffer = stdin.fill_buf().unwrap();
1851 /// // work with buffer
1852 /// println!("{:?}", buffer);
1854 /// // ensure the bytes we worked with aren't returned again later
1855 /// let length = buffer.len();
1856 /// stdin.consume(length);
1858 #[stable(feature = "rust1", since = "1.0.0")]
1859 fn fill_buf(&mut self) -> Result<&[u8]>;
1861 /// Tells this buffer that `amt` bytes have been consumed from the buffer,
1862 /// so they should no longer be returned in calls to `read`.
1864 /// This function is a lower-level call. It needs to be paired with the
1865 /// [`fill_buf`] method to function properly. This function does
1866 /// not perform any I/O, it simply informs this object that some amount of
1867 /// its buffer, returned from [`fill_buf`], has been consumed and should
1868 /// no longer be returned. As such, this function may do odd things if
1869 /// [`fill_buf`] isn't called before calling it.
1871 /// The `amt` must be `<=` the number of bytes in the buffer returned by
1876 /// Since `consume()` is meant to be used with [`fill_buf`],
1877 /// that method's example includes an example of `consume()`.
1879 /// [`fill_buf`]: #tymethod.fill_buf
1880 #[stable(feature = "rust1", since = "1.0.0")]
1881 fn consume(&mut self, amt: usize);
1883 /// Read all bytes into `buf` until the delimiter `byte` or EOF is reached.
1885 /// This function will read bytes from the underlying stream until the
1886 /// delimiter or EOF is found. Once found, all bytes up to, and including,
1887 /// the delimiter (if found) will be appended to `buf`.
1889 /// If successful, this function will return the total number of bytes read.
1891 /// This function is blocking and should be used carefully: it is possible for
1892 /// an attacker to continuously send bytes without ever sending the delimiter
1897 /// This function will ignore all instances of [`ErrorKind::Interrupted`] and
1898 /// will otherwise return any errors returned by [`fill_buf`].
1900 /// If an I/O error is encountered then all bytes read so far will be
1901 /// present in `buf` and its length will have been adjusted appropriately.
1903 /// [`fill_buf`]: #tymethod.fill_buf
1904 /// [`ErrorKind::Interrupted`]: enum.ErrorKind.html#variant.Interrupted
1908 /// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
1909 /// this example, we use [`Cursor`] to read all the bytes in a byte slice
1910 /// in hyphen delimited segments:
1912 /// [`Cursor`]: struct.Cursor.html
1915 /// use std::io::{self, BufRead};
1917 /// let mut cursor = io::Cursor::new(b"lorem-ipsum");
1918 /// let mut buf = vec![];
1920 /// // cursor is at 'l'
1921 /// let num_bytes = cursor.read_until(b'-', &mut buf)
1922 /// .expect("reading from cursor won't fail");
1923 /// assert_eq!(num_bytes, 6);
1924 /// assert_eq!(buf, b"lorem-");
1927 /// // cursor is at 'i'
1928 /// let num_bytes = cursor.read_until(b'-', &mut buf)
1929 /// .expect("reading from cursor won't fail");
1930 /// assert_eq!(num_bytes, 5);
1931 /// assert_eq!(buf, b"ipsum");
1934 /// // cursor is at EOF
1935 /// let num_bytes = cursor.read_until(b'-', &mut buf)
1936 /// .expect("reading from cursor won't fail");
1937 /// assert_eq!(num_bytes, 0);
1938 /// assert_eq!(buf, b"");
1940 #[stable(feature = "rust1", since = "1.0.0")]
1941 fn read_until(&mut self, byte: u8, buf: &mut Vec<u8>) -> Result<usize> {
1942 read_until(self, byte, buf)
1945 /// Read all bytes until a newline (the 0xA byte) is reached, and append
1946 /// them to the provided buffer.
1948 /// This function will read bytes from the underlying stream until the
1949 /// newline delimiter (the 0xA byte) or EOF is found. Once found, all bytes
1950 /// up to, and including, the delimiter (if found) will be appended to
1953 /// If successful, this function will return the total number of bytes read.
1955 /// If this function returns `Ok(0)`, the stream has reached EOF.
1957 /// This function is blocking and should be used carefully: it is possible for
1958 /// an attacker to continuously send bytes without ever sending a newline
1963 /// This function has the same error semantics as [`read_until`] and will
1964 /// also return an error if the read bytes are not valid UTF-8. If an I/O
1965 /// error is encountered then `buf` may contain some bytes already read in
1966 /// the event that all data read so far was valid UTF-8.
1968 /// [`read_until`]: #method.read_until
1972 /// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
1973 /// this example, we use [`Cursor`] to read all the lines in a byte slice:
1975 /// [`Cursor`]: struct.Cursor.html
1978 /// use std::io::{self, BufRead};
1980 /// let mut cursor = io::Cursor::new(b"foo\nbar");
1981 /// let mut buf = String::new();
1983 /// // cursor is at 'f'
1984 /// let num_bytes = cursor.read_line(&mut buf)
1985 /// .expect("reading from cursor won't fail");
1986 /// assert_eq!(num_bytes, 4);
1987 /// assert_eq!(buf, "foo\n");
1990 /// // cursor is at 'b'
1991 /// let num_bytes = cursor.read_line(&mut buf)
1992 /// .expect("reading from cursor won't fail");
1993 /// assert_eq!(num_bytes, 3);
1994 /// assert_eq!(buf, "bar");
1997 /// // cursor is at EOF
1998 /// let num_bytes = cursor.read_line(&mut buf)
1999 /// .expect("reading from cursor won't fail");
2000 /// assert_eq!(num_bytes, 0);
2001 /// assert_eq!(buf, "");
2003 #[stable(feature = "rust1", since = "1.0.0")]
2004 fn read_line(&mut self, buf: &mut String) -> Result<usize> {
2005 // Note that we are not calling the `.read_until` method here, but
2006 // rather our hardcoded implementation. For more details as to why, see
2007 // the comments in `read_to_end`.
2008 append_to_string(buf, |b| read_until(self, b'\n', b))
2011 /// Returns an iterator over the contents of this reader split on the byte
2014 /// The iterator returned from this function will return instances of
2015 /// [`io::Result`]`<`[`Vec<u8>`]`>`. Each vector returned will *not* have
2016 /// the delimiter byte at the end.
2018 /// This function will yield errors whenever [`read_until`] would have
2019 /// also yielded an error.
2021 /// [`io::Result`]: type.Result.html
2022 /// [`Vec<u8>`]: ../vec/struct.Vec.html
2023 /// [`read_until`]: #method.read_until
2027 /// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
2028 /// this example, we use [`Cursor`] to iterate over all hyphen delimited
2029 /// segments in a byte slice
2031 /// [`Cursor`]: struct.Cursor.html
2034 /// use std::io::{self, BufRead};
2036 /// let cursor = io::Cursor::new(b"lorem-ipsum-dolor");
2038 /// let mut split_iter = cursor.split(b'-').map(|l| l.unwrap());
2039 /// assert_eq!(split_iter.next(), Some(b"lorem".to_vec()));
2040 /// assert_eq!(split_iter.next(), Some(b"ipsum".to_vec()));
2041 /// assert_eq!(split_iter.next(), Some(b"dolor".to_vec()));
2042 /// assert_eq!(split_iter.next(), None);
2044 #[stable(feature = "rust1", since = "1.0.0")]
2045 fn split(self, byte: u8) -> Split<Self>
2049 Split { buf: self, delim: byte }
2052 /// Returns an iterator over the lines of this reader.
2054 /// The iterator returned from this function will yield instances of
2055 /// [`io::Result`]`<`[`String`]`>`. Each string returned will *not* have a newline
2056 /// byte (the 0xA byte) or CRLF (0xD, 0xA bytes) at the end.
2058 /// [`io::Result`]: type.Result.html
2059 /// [`String`]: ../string/struct.String.html
2063 /// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
2064 /// this example, we use [`Cursor`] to iterate over all the lines in a byte
2067 /// [`Cursor`]: struct.Cursor.html
2070 /// use std::io::{self, BufRead};
2072 /// let cursor = io::Cursor::new(b"lorem\nipsum\r\ndolor");
2074 /// let mut lines_iter = cursor.lines().map(|l| l.unwrap());
2075 /// assert_eq!(lines_iter.next(), Some(String::from("lorem")));
2076 /// assert_eq!(lines_iter.next(), Some(String::from("ipsum")));
2077 /// assert_eq!(lines_iter.next(), Some(String::from("dolor")));
2078 /// assert_eq!(lines_iter.next(), None);
2083 /// Each line of the iterator has the same error semantics as [`BufRead::read_line`].
2085 /// [`BufRead::read_line`]: trait.BufRead.html#method.read_line
2086 #[stable(feature = "rust1", since = "1.0.0")]
2087 fn lines(self) -> Lines<Self>
2095 /// Adaptor to chain together two readers.
2097 /// This struct is generally created by calling [`chain`] on a reader.
2098 /// Please see the documentation of [`chain`] for more details.
2100 /// [`chain`]: trait.Read.html#method.chain
2101 #[stable(feature = "rust1", since = "1.0.0")]
2102 pub struct Chain<T, U> {
2108 impl<T, U> Chain<T, U> {
2109 /// Consumes the `Chain`, returning the wrapped readers.
2115 /// use std::io::prelude::*;
2116 /// use std::fs::File;
2118 /// fn main() -> io::Result<()> {
2119 /// let mut foo_file = File::open("foo.txt")?;
2120 /// let mut bar_file = File::open("bar.txt")?;
2122 /// let chain = foo_file.chain(bar_file);
2123 /// let (foo_file, bar_file) = chain.into_inner();
2127 #[stable(feature = "more_io_inner_methods", since = "1.20.0")]
2128 pub fn into_inner(self) -> (T, U) {
2129 (self.first, self.second)
2132 /// Gets references to the underlying readers in this `Chain`.
2138 /// use std::io::prelude::*;
2139 /// use std::fs::File;
2141 /// fn main() -> io::Result<()> {
2142 /// let mut foo_file = File::open("foo.txt")?;
2143 /// let mut bar_file = File::open("bar.txt")?;
2145 /// let chain = foo_file.chain(bar_file);
2146 /// let (foo_file, bar_file) = chain.get_ref();
2150 #[stable(feature = "more_io_inner_methods", since = "1.20.0")]
2151 pub fn get_ref(&self) -> (&T, &U) {
2152 (&self.first, &self.second)
2155 /// Gets mutable references to the underlying readers in this `Chain`.
2157 /// Care should be taken to avoid modifying the internal I/O state of the
2158 /// underlying readers as doing so may corrupt the internal state of this
2165 /// use std::io::prelude::*;
2166 /// use std::fs::File;
2168 /// fn main() -> io::Result<()> {
2169 /// let mut foo_file = File::open("foo.txt")?;
2170 /// let mut bar_file = File::open("bar.txt")?;
2172 /// let mut chain = foo_file.chain(bar_file);
2173 /// let (foo_file, bar_file) = chain.get_mut();
2177 #[stable(feature = "more_io_inner_methods", since = "1.20.0")]
2178 pub fn get_mut(&mut self) -> (&mut T, &mut U) {
2179 (&mut self.first, &mut self.second)
2183 #[stable(feature = "std_debug", since = "1.16.0")]
2184 impl<T: fmt::Debug, U: fmt::Debug> fmt::Debug for Chain<T, U> {
2185 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2186 f.debug_struct("Chain").field("t", &self.first).field("u", &self.second).finish()
2190 #[stable(feature = "rust1", since = "1.0.0")]
2191 impl<T: Read, U: Read> Read for Chain<T, U> {
2192 fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
2193 if !self.done_first {
2194 match self.first.read(buf)? {
2195 0 if !buf.is_empty() => self.done_first = true,
2199 self.second.read(buf)
2202 fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> Result<usize> {
2203 if !self.done_first {
2204 match self.first.read_vectored(bufs)? {
2205 0 if bufs.iter().any(|b| !b.is_empty()) => self.done_first = true,
2209 self.second.read_vectored(bufs)
2212 unsafe fn initializer(&self) -> Initializer {
2213 let initializer = self.first.initializer();
2214 if initializer.should_initialize() { initializer } else { self.second.initializer() }
2218 #[stable(feature = "chain_bufread", since = "1.9.0")]
2219 impl<T: BufRead, U: BufRead> BufRead for Chain<T, U> {
2220 fn fill_buf(&mut self) -> Result<&[u8]> {
2221 if !self.done_first {
2222 match self.first.fill_buf()? {
2223 buf if buf.is_empty() => {
2224 self.done_first = true;
2226 buf => return Ok(buf),
2229 self.second.fill_buf()
2232 fn consume(&mut self, amt: usize) {
2233 if !self.done_first { self.first.consume(amt) } else { self.second.consume(amt) }
2237 /// Reader adaptor which limits the bytes read from an underlying reader.
2239 /// This struct is generally created by calling [`take`] on a reader.
2240 /// Please see the documentation of [`take`] for more details.
2242 /// [`take`]: trait.Read.html#method.take
2243 #[stable(feature = "rust1", since = "1.0.0")]
2245 pub struct Take<T> {
2251 /// Returns the number of bytes that can be read before this instance will
2256 /// This instance may reach `EOF` after reading fewer bytes than indicated by
2257 /// this method if the underlying [`Read`] instance reaches EOF.
2259 /// [`Read`]: ../../std/io/trait.Read.html
2265 /// use std::io::prelude::*;
2266 /// use std::fs::File;
2268 /// fn main() -> io::Result<()> {
2269 /// let f = File::open("foo.txt")?;
2271 /// // read at most five bytes
2272 /// let handle = f.take(5);
2274 /// println!("limit: {}", handle.limit());
2278 #[stable(feature = "rust1", since = "1.0.0")]
2279 pub fn limit(&self) -> u64 {
2283 /// Sets the number of bytes that can be read before this instance will
2284 /// return EOF. This is the same as constructing a new `Take` instance, so
2285 /// the amount of bytes read and the previous limit value don't matter when
2286 /// calling this method.
2292 /// use std::io::prelude::*;
2293 /// use std::fs::File;
2295 /// fn main() -> io::Result<()> {
2296 /// let f = File::open("foo.txt")?;
2298 /// // read at most five bytes
2299 /// let mut handle = f.take(5);
2300 /// handle.set_limit(10);
2302 /// assert_eq!(handle.limit(), 10);
2306 #[stable(feature = "take_set_limit", since = "1.27.0")]
2307 pub fn set_limit(&mut self, limit: u64) {
2311 /// Consumes the `Take`, returning the wrapped reader.
2317 /// use std::io::prelude::*;
2318 /// use std::fs::File;
2320 /// fn main() -> io::Result<()> {
2321 /// let mut file = File::open("foo.txt")?;
2323 /// let mut buffer = [0; 5];
2324 /// let mut handle = file.take(5);
2325 /// handle.read(&mut buffer)?;
2327 /// let file = handle.into_inner();
2331 #[stable(feature = "io_take_into_inner", since = "1.15.0")]
2332 pub fn into_inner(self) -> T {
2336 /// Gets a reference to the underlying reader.
2342 /// use std::io::prelude::*;
2343 /// use std::fs::File;
2345 /// fn main() -> io::Result<()> {
2346 /// let mut file = File::open("foo.txt")?;
2348 /// let mut buffer = [0; 5];
2349 /// let mut handle = file.take(5);
2350 /// handle.read(&mut buffer)?;
2352 /// let file = handle.get_ref();
2356 #[stable(feature = "more_io_inner_methods", since = "1.20.0")]
2357 pub fn get_ref(&self) -> &T {
2361 /// Gets a mutable reference to the underlying reader.
2363 /// Care should be taken to avoid modifying the internal I/O state of the
2364 /// underlying reader as doing so may corrupt the internal limit of this
2371 /// use std::io::prelude::*;
2372 /// use std::fs::File;
2374 /// fn main() -> io::Result<()> {
2375 /// let mut file = File::open("foo.txt")?;
2377 /// let mut buffer = [0; 5];
2378 /// let mut handle = file.take(5);
2379 /// handle.read(&mut buffer)?;
2381 /// let file = handle.get_mut();
2385 #[stable(feature = "more_io_inner_methods", since = "1.20.0")]
2386 pub fn get_mut(&mut self) -> &mut T {
2391 #[stable(feature = "rust1", since = "1.0.0")]
2392 impl<T: Read> Read for Take<T> {
2393 fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
2394 // Don't call into inner reader at all at EOF because it may still block
2395 if self.limit == 0 {
2399 let max = cmp::min(buf.len() as u64, self.limit) as usize;
2400 let n = self.inner.read(&mut buf[..max])?;
2401 self.limit -= n as u64;
2405 unsafe fn initializer(&self) -> Initializer {
2406 self.inner.initializer()
2409 fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize> {
2410 // Pass in a reservation_size closure that respects the current value
2411 // of limit for each read. If we hit the read limit, this prevents the
2412 // final zero-byte read from allocating again.
2413 read_to_end_with_reservation(self, buf, |self_| cmp::min(self_.limit, 32) as usize)
2417 #[stable(feature = "rust1", since = "1.0.0")]
2418 impl<T: BufRead> BufRead for Take<T> {
2419 fn fill_buf(&mut self) -> Result<&[u8]> {
2420 // Don't call into inner reader at all at EOF because it may still block
2421 if self.limit == 0 {
2425 let buf = self.inner.fill_buf()?;
2426 let cap = cmp::min(buf.len() as u64, self.limit) as usize;
2430 fn consume(&mut self, amt: usize) {
2431 // Don't let callers reset the limit by passing an overlarge value
2432 let amt = cmp::min(amt as u64, self.limit) as usize;
2433 self.limit -= amt as u64;
2434 self.inner.consume(amt);
2438 /// An iterator over `u8` values of a reader.
2440 /// This struct is generally created by calling [`bytes`] on a reader.
2441 /// Please see the documentation of [`bytes`] for more details.
2443 /// [`bytes`]: trait.Read.html#method.bytes
2444 #[stable(feature = "rust1", since = "1.0.0")]
2446 pub struct Bytes<R> {
2450 #[stable(feature = "rust1", since = "1.0.0")]
2451 impl<R: Read> Iterator for Bytes<R> {
2452 type Item = Result<u8>;
2454 fn next(&mut self) -> Option<Result<u8>> {
2457 return match self.inner.read(slice::from_mut(&mut byte)) {
2459 Ok(..) => Some(Ok(byte)),
2460 Err(ref e) if e.kind() == ErrorKind::Interrupted => continue,
2461 Err(e) => Some(Err(e)),
2467 /// An iterator over the contents of an instance of `BufRead` split on a
2468 /// particular byte.
2470 /// This struct is generally created by calling [`split`] on a `BufRead`.
2471 /// Please see the documentation of [`split`] for more details.
2473 /// [`split`]: trait.BufRead.html#method.split
2474 #[stable(feature = "rust1", since = "1.0.0")]
2476 pub struct Split<B> {
2481 #[stable(feature = "rust1", since = "1.0.0")]
2482 impl<B: BufRead> Iterator for Split<B> {
2483 type Item = Result<Vec<u8>>;
2485 fn next(&mut self) -> Option<Result<Vec<u8>>> {
2486 let mut buf = Vec::new();
2487 match self.buf.read_until(self.delim, &mut buf) {
2490 if buf[buf.len() - 1] == self.delim {
2495 Err(e) => Some(Err(e)),
2500 /// An iterator over the lines of an instance of `BufRead`.
2502 /// This struct is generally created by calling [`lines`] on a `BufRead`.
2503 /// Please see the documentation of [`lines`] for more details.
2505 /// [`lines`]: trait.BufRead.html#method.lines
2506 #[stable(feature = "rust1", since = "1.0.0")]
2508 pub struct Lines<B> {
2512 #[stable(feature = "rust1", since = "1.0.0")]
2513 impl<B: BufRead> Iterator for Lines<B> {
2514 type Item = Result<String>;
2516 fn next(&mut self) -> Option<Result<String>> {
2517 let mut buf = String::new();
2518 match self.buf.read_line(&mut buf) {
2521 if buf.ends_with('\n') {
2523 if buf.ends_with('\r') {
2529 Err(e) => Some(Err(e)),
2536 use super::{repeat, Cursor, SeekFrom};
2537 use crate::cmp::{self, min};
2538 use crate::io::prelude::*;
2539 use crate::io::{self, IoSlice, IoSliceMut};
2540 use crate::ops::Deref;
2543 #[cfg_attr(target_os = "emscripten", ignore)]
2545 let mut buf = Cursor::new(&b"12"[..]);
2546 let mut v = Vec::new();
2547 assert_eq!(buf.read_until(b'3', &mut v).unwrap(), 2);
2548 assert_eq!(v, b"12");
2550 let mut buf = Cursor::new(&b"1233"[..]);
2551 let mut v = Vec::new();
2552 assert_eq!(buf.read_until(b'3', &mut v).unwrap(), 3);
2553 assert_eq!(v, b"123");
2555 assert_eq!(buf.read_until(b'3', &mut v).unwrap(), 1);
2556 assert_eq!(v, b"3");
2558 assert_eq!(buf.read_until(b'3', &mut v).unwrap(), 0);
2564 let buf = Cursor::new(&b"12"[..]);
2565 let mut s = buf.split(b'3');
2566 assert_eq!(s.next().unwrap().unwrap(), vec![b'1', b'2']);
2567 assert!(s.next().is_none());
2569 let buf = Cursor::new(&b"1233"[..]);
2570 let mut s = buf.split(b'3');
2571 assert_eq!(s.next().unwrap().unwrap(), vec![b'1', b'2']);
2572 assert_eq!(s.next().unwrap().unwrap(), vec![]);
2573 assert!(s.next().is_none());
2578 let mut buf = Cursor::new(&b"12"[..]);
2579 let mut v = String::new();
2580 assert_eq!(buf.read_line(&mut v).unwrap(), 2);
2581 assert_eq!(v, "12");
2583 let mut buf = Cursor::new(&b"12\n\n"[..]);
2584 let mut v = String::new();
2585 assert_eq!(buf.read_line(&mut v).unwrap(), 3);
2586 assert_eq!(v, "12\n");
2588 assert_eq!(buf.read_line(&mut v).unwrap(), 1);
2589 assert_eq!(v, "\n");
2591 assert_eq!(buf.read_line(&mut v).unwrap(), 0);
2597 let buf = Cursor::new(&b"12\r"[..]);
2598 let mut s = buf.lines();
2599 assert_eq!(s.next().unwrap().unwrap(), "12\r".to_string());
2600 assert!(s.next().is_none());
2602 let buf = Cursor::new(&b"12\r\n\n"[..]);
2603 let mut s = buf.lines();
2604 assert_eq!(s.next().unwrap().unwrap(), "12".to_string());
2605 assert_eq!(s.next().unwrap().unwrap(), "".to_string());
2606 assert!(s.next().is_none());
2611 let mut c = Cursor::new(&b""[..]);
2612 let mut v = Vec::new();
2613 assert_eq!(c.read_to_end(&mut v).unwrap(), 0);
2616 let mut c = Cursor::new(&b"1"[..]);
2617 let mut v = Vec::new();
2618 assert_eq!(c.read_to_end(&mut v).unwrap(), 1);
2619 assert_eq!(v, b"1");
2621 let cap = 1024 * 1024;
2622 let data = (0..cap).map(|i| (i / 3) as u8).collect::<Vec<_>>();
2623 let mut v = Vec::new();
2624 let (a, b) = data.split_at(data.len() / 2);
2625 assert_eq!(Cursor::new(a).read_to_end(&mut v).unwrap(), a.len());
2626 assert_eq!(Cursor::new(b).read_to_end(&mut v).unwrap(), b.len());
2627 assert_eq!(v, data);
2631 fn read_to_string() {
2632 let mut c = Cursor::new(&b""[..]);
2633 let mut v = String::new();
2634 assert_eq!(c.read_to_string(&mut v).unwrap(), 0);
2637 let mut c = Cursor::new(&b"1"[..]);
2638 let mut v = String::new();
2639 assert_eq!(c.read_to_string(&mut v).unwrap(), 1);
2642 let mut c = Cursor::new(&b"\xff"[..]);
2643 let mut v = String::new();
2644 assert!(c.read_to_string(&mut v).is_err());
2649 let mut buf = [0; 4];
2651 let mut c = Cursor::new(&b""[..]);
2652 assert_eq!(c.read_exact(&mut buf).unwrap_err().kind(), io::ErrorKind::UnexpectedEof);
2654 let mut c = Cursor::new(&b"123"[..]).chain(Cursor::new(&b"456789"[..]));
2655 c.read_exact(&mut buf).unwrap();
2656 assert_eq!(&buf, b"1234");
2657 c.read_exact(&mut buf).unwrap();
2658 assert_eq!(&buf, b"5678");
2659 assert_eq!(c.read_exact(&mut buf).unwrap_err().kind(), io::ErrorKind::UnexpectedEof);
2663 fn read_exact_slice() {
2664 let mut buf = [0; 4];
2666 let mut c = &b""[..];
2667 assert_eq!(c.read_exact(&mut buf).unwrap_err().kind(), io::ErrorKind::UnexpectedEof);
2669 let mut c = &b"123"[..];
2670 assert_eq!(c.read_exact(&mut buf).unwrap_err().kind(), io::ErrorKind::UnexpectedEof);
2671 // make sure the optimized (early returning) method is being used
2672 assert_eq!(&buf, &[0; 4]);
2674 let mut c = &b"1234"[..];
2675 c.read_exact(&mut buf).unwrap();
2676 assert_eq!(&buf, b"1234");
2678 let mut c = &b"56789"[..];
2679 c.read_exact(&mut buf).unwrap();
2680 assert_eq!(&buf, b"5678");
2681 assert_eq!(c, b"9");
2689 fn read(&mut self, _: &mut [u8]) -> io::Result<usize> {
2690 Err(io::Error::new(io::ErrorKind::Other, ""))
2693 impl BufRead for R {
2694 fn fill_buf(&mut self) -> io::Result<&[u8]> {
2695 Err(io::Error::new(io::ErrorKind::Other, ""))
2697 fn consume(&mut self, _amt: usize) {}
2700 let mut buf = [0; 1];
2701 assert_eq!(0, R.take(0).read(&mut buf).unwrap());
2702 assert_eq!(b"", R.take(0).fill_buf().unwrap());
2705 fn cmp_bufread<Br1: BufRead, Br2: BufRead>(mut br1: Br1, mut br2: Br2, exp: &[u8]) {
2706 let mut cat = Vec::new();
2709 let buf1 = br1.fill_buf().unwrap();
2710 let buf2 = br2.fill_buf().unwrap();
2711 let minlen = if buf1.len() < buf2.len() { buf1.len() } else { buf2.len() };
2712 assert_eq!(buf1[..minlen], buf2[..minlen]);
2713 cat.extend_from_slice(&buf1[..minlen]);
2719 br1.consume(consume);
2720 br2.consume(consume);
2722 assert_eq!(br1.fill_buf().unwrap().len(), 0);
2723 assert_eq!(br2.fill_buf().unwrap().len(), 0);
2724 assert_eq!(&cat[..], &exp[..])
2728 fn chain_bufread() {
2729 let testdata = b"ABCDEFGHIJKL";
2731 (&testdata[..3]).chain(&testdata[3..6]).chain(&testdata[6..9]).chain(&testdata[9..]);
2732 let chain2 = (&testdata[..4]).chain(&testdata[4..8]).chain(&testdata[8..]);
2733 cmp_bufread(chain1, chain2, &testdata[..]);
2737 fn chain_zero_length_read_is_not_eof() {
2740 let mut s = String::new();
2741 let mut chain = (&a[..]).chain(&b[..]);
2742 chain.read(&mut []).unwrap();
2743 chain.read_to_string(&mut s).unwrap();
2744 assert_eq!("AB", s);
2748 #[cfg_attr(target_os = "emscripten", ignore)]
2749 fn bench_read_to_end(b: &mut test::Bencher) {
2751 let mut lr = repeat(1).take(10000000);
2752 let mut vec = Vec::with_capacity(1024);
2753 super::read_to_end(&mut lr, &mut vec)
2758 fn seek_len() -> io::Result<()> {
2759 let mut c = Cursor::new(vec![0; 15]);
2760 assert_eq!(c.stream_len()?, 15);
2762 c.seek(SeekFrom::End(0))?;
2763 let old_pos = c.stream_position()?;
2764 assert_eq!(c.stream_len()?, 15);
2765 assert_eq!(c.stream_position()?, old_pos);
2767 c.seek(SeekFrom::Start(7))?;
2768 c.seek(SeekFrom::Current(2))?;
2769 let old_pos = c.stream_position()?;
2770 assert_eq!(c.stream_len()?, 15);
2771 assert_eq!(c.stream_position()?, old_pos);
2777 fn seek_position() -> io::Result<()> {
2778 // All `asserts` are duplicated here to make sure the method does not
2779 // change anything about the seek state.
2780 let mut c = Cursor::new(vec![0; 15]);
2781 assert_eq!(c.stream_position()?, 0);
2782 assert_eq!(c.stream_position()?, 0);
2784 c.seek(SeekFrom::End(0))?;
2785 assert_eq!(c.stream_position()?, 15);
2786 assert_eq!(c.stream_position()?, 15);
2788 c.seek(SeekFrom::Start(7))?;
2789 c.seek(SeekFrom::Current(2))?;
2790 assert_eq!(c.stream_position()?, 9);
2791 assert_eq!(c.stream_position()?, 9);
2793 c.seek(SeekFrom::End(-3))?;
2794 c.seek(SeekFrom::Current(1))?;
2795 c.seek(SeekFrom::Current(-5))?;
2796 assert_eq!(c.stream_position()?, 8);
2797 assert_eq!(c.stream_position()?, 8);
2802 // A simple example reader which uses the default implementation of
2804 struct ExampleSliceReader<'a> {
2808 impl<'a> Read for ExampleSliceReader<'a> {
2809 fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
2810 let len = cmp::min(self.slice.len(), buf.len());
2811 buf[..len].copy_from_slice(&self.slice[..len]);
2812 self.slice = &self.slice[len..];
2818 fn test_read_to_end_capacity() -> io::Result<()> {
2819 let input = &b"foo"[..];
2821 // read_to_end() generally needs to over-allocate, both for efficiency
2822 // and so that it can distinguish EOF. Assert that this is the case
2823 // with this simple ExampleSliceReader struct, which uses the default
2824 // implementation of read_to_end. Even though vec1 is allocated with
2825 // exactly enough capacity for the read, read_to_end will allocate more
2827 let mut vec1 = Vec::with_capacity(input.len());
2828 ExampleSliceReader { slice: input }.read_to_end(&mut vec1)?;
2829 assert_eq!(vec1.len(), input.len());
2830 assert!(vec1.capacity() > input.len(), "allocated more");
2832 // However, std::io::Take includes an implementation of read_to_end
2833 // that will not allocate when the limit has already been reached. In
2834 // this case, vec2 never grows.
2835 let mut vec2 = Vec::with_capacity(input.len());
2836 ExampleSliceReader { slice: input }.take(input.len() as u64).read_to_end(&mut vec2)?;
2837 assert_eq!(vec2.len(), input.len());
2838 assert_eq!(vec2.capacity(), input.len(), "did not allocate more");
2844 fn io_slice_mut_advance() {
2845 let mut buf1 = [1; 8];
2846 let mut buf2 = [2; 16];
2847 let mut buf3 = [3; 8];
2848 let mut bufs = &mut [
2849 IoSliceMut::new(&mut buf1),
2850 IoSliceMut::new(&mut buf2),
2851 IoSliceMut::new(&mut buf3),
2854 // Only in a single buffer..
2855 bufs = IoSliceMut::advance(bufs, 1);
2856 assert_eq!(bufs[0].deref(), [1; 7].as_ref());
2857 assert_eq!(bufs[1].deref(), [2; 16].as_ref());
2858 assert_eq!(bufs[2].deref(), [3; 8].as_ref());
2860 // Removing a buffer, leaving others as is.
2861 bufs = IoSliceMut::advance(bufs, 7);
2862 assert_eq!(bufs[0].deref(), [2; 16].as_ref());
2863 assert_eq!(bufs[1].deref(), [3; 8].as_ref());
2865 // Removing a buffer and removing from the next buffer.
2866 bufs = IoSliceMut::advance(bufs, 18);
2867 assert_eq!(bufs[0].deref(), [3; 6].as_ref());
2871 fn io_slice_mut_advance_empty_slice() {
2872 let empty_bufs = &mut [][..];
2874 IoSliceMut::advance(empty_bufs, 1);
2878 fn io_slice_mut_advance_beyond_total_length() {
2879 let mut buf1 = [1; 8];
2880 let mut bufs = &mut [IoSliceMut::new(&mut buf1)][..];
2882 // Going beyond the total length should be ok.
2883 bufs = IoSliceMut::advance(bufs, 9);
2884 assert!(bufs.is_empty());
2888 fn io_slice_advance() {
2892 let mut bufs = &mut [IoSlice::new(&buf1), IoSlice::new(&buf2), IoSlice::new(&buf3)][..];
2894 // Only in a single buffer..
2895 bufs = IoSlice::advance(bufs, 1);
2896 assert_eq!(bufs[0].deref(), [1; 7].as_ref());
2897 assert_eq!(bufs[1].deref(), [2; 16].as_ref());
2898 assert_eq!(bufs[2].deref(), [3; 8].as_ref());
2900 // Removing a buffer, leaving others as is.
2901 bufs = IoSlice::advance(bufs, 7);
2902 assert_eq!(bufs[0].deref(), [2; 16].as_ref());
2903 assert_eq!(bufs[1].deref(), [3; 8].as_ref());
2905 // Removing a buffer and removing from the next buffer.
2906 bufs = IoSlice::advance(bufs, 18);
2907 assert_eq!(bufs[0].deref(), [3; 6].as_ref());
2911 fn io_slice_advance_empty_slice() {
2912 let empty_bufs = &mut [][..];
2914 IoSlice::advance(empty_bufs, 1);
2918 fn io_slice_advance_beyond_total_length() {
2920 let mut bufs = &mut [IoSlice::new(&buf1)][..];
2922 // Going beyond the total length should be ok.
2923 bufs = IoSlice::advance(bufs, 9);
2924 assert!(bufs.is_empty());
2927 /// Create a new writer that reads from at most `n_bufs` and reads
2928 /// `per_call` bytes (in total) per call to write.
2929 fn test_writer(n_bufs: usize, per_call: usize) -> TestWriter {
2930 TestWriter { n_bufs, per_call, written: Vec::new() }
2939 impl Write for TestWriter {
2940 fn write(&mut self, buf: &[u8]) -> io::Result<usize> {
2941 self.write_vectored(&[IoSlice::new(buf)])
2944 fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> io::Result<usize> {
2945 let mut left = self.per_call;
2946 let mut written = 0;
2947 for buf in bufs.iter().take(self.n_bufs) {
2948 let n = min(left, buf.len());
2949 self.written.extend_from_slice(&buf[0..n]);
2956 fn flush(&mut self) -> io::Result<()> {
2962 fn test_writer_read_from_one_buf() {
2963 let mut writer = test_writer(1, 2);
2965 assert_eq!(writer.write(&[]).unwrap(), 0);
2966 assert_eq!(writer.write_vectored(&[]).unwrap(), 0);
2968 // Read at most 2 bytes.
2969 assert_eq!(writer.write(&[1, 1, 1]).unwrap(), 2);
2970 let bufs = &[IoSlice::new(&[2, 2, 2])];
2971 assert_eq!(writer.write_vectored(bufs).unwrap(), 2);
2973 // Only read from first buf.
2974 let bufs = &[IoSlice::new(&[3]), IoSlice::new(&[4, 4])];
2975 assert_eq!(writer.write_vectored(bufs).unwrap(), 1);
2977 assert_eq!(writer.written, &[1, 1, 2, 2, 3]);
2981 fn test_writer_read_from_multiple_bufs() {
2982 let mut writer = test_writer(3, 3);
2984 // Read at most 3 bytes from two buffers.
2985 let bufs = &[IoSlice::new(&[1]), IoSlice::new(&[2, 2, 2])];
2986 assert_eq!(writer.write_vectored(bufs).unwrap(), 3);
2988 // Read at most 3 bytes from three buffers.
2989 let bufs = &[IoSlice::new(&[3]), IoSlice::new(&[4]), IoSlice::new(&[5, 5])];
2990 assert_eq!(writer.write_vectored(bufs).unwrap(), 3);
2992 assert_eq!(writer.written, &[1, 2, 2, 3, 4, 5]);
2996 fn test_write_all_vectored() {
2997 #[rustfmt::skip] // Becomes unreadable otherwise.
2998 let tests: Vec<(_, &'static [u8])> = vec![
3000 (vec![IoSlice::new(&[1])], &[1]),
3001 (vec![IoSlice::new(&[1, 2])], &[1, 2]),
3002 (vec![IoSlice::new(&[1, 2, 3])], &[1, 2, 3]),
3003 (vec![IoSlice::new(&[1, 2, 3, 4])], &[1, 2, 3, 4]),
3004 (vec![IoSlice::new(&[1, 2, 3, 4, 5])], &[1, 2, 3, 4, 5]),
3005 (vec![IoSlice::new(&[1]), IoSlice::new(&[2])], &[1, 2]),
3006 (vec![IoSlice::new(&[1]), IoSlice::new(&[2, 2])], &[1, 2, 2]),
3007 (vec![IoSlice::new(&[1, 1]), IoSlice::new(&[2, 2])], &[1, 1, 2, 2]),
3008 (vec![IoSlice::new(&[1, 1]), IoSlice::new(&[2, 2, 2])], &[1, 1, 2, 2, 2]),
3009 (vec![IoSlice::new(&[1, 1]), IoSlice::new(&[2, 2, 2])], &[1, 1, 2, 2, 2]),
3010 (vec![IoSlice::new(&[1, 1, 1]), IoSlice::new(&[2, 2, 2])], &[1, 1, 1, 2, 2, 2]),
3011 (vec![IoSlice::new(&[1, 1, 1]), IoSlice::new(&[2, 2, 2, 2])], &[1, 1, 1, 2, 2, 2, 2]),
3012 (vec![IoSlice::new(&[1, 1, 1, 1]), IoSlice::new(&[2, 2, 2, 2])], &[1, 1, 1, 1, 2, 2, 2, 2]),
3013 (vec![IoSlice::new(&[1]), IoSlice::new(&[2]), IoSlice::new(&[3])], &[1, 2, 3]),
3014 (vec![IoSlice::new(&[1, 1]), IoSlice::new(&[2, 2]), IoSlice::new(&[3, 3])], &[1, 1, 2, 2, 3, 3]),
3015 (vec![IoSlice::new(&[1]), IoSlice::new(&[2, 2]), IoSlice::new(&[3, 3, 3])], &[1, 2, 2, 3, 3, 3]),
3016 (vec![IoSlice::new(&[1, 1, 1]), IoSlice::new(&[2, 2, 2]), IoSlice::new(&[3, 3, 3])], &[1, 1, 1, 2, 2, 2, 3, 3, 3]),
3019 let writer_configs = &[(1, 1), (1, 2), (1, 3), (2, 2), (2, 3), (3, 3)];
3021 for (n_bufs, per_call) in writer_configs.iter().copied() {
3022 for (mut input, wanted) in tests.clone().into_iter() {
3023 let mut writer = test_writer(n_bufs, per_call);
3024 assert!(writer.write_all_vectored(&mut *input).is_ok());
3025 assert_eq!(&*writer.written, &*wanted);