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};
284 pub(crate) use self::stdio::clone_io;
294 const DEFAULT_BUF_SIZE: usize = crate::sys_common::io::DEFAULT_BUF_SIZE;
297 buf: &'a mut Vec<u8>,
301 impl Drop for Guard<'_> {
304 self.buf.set_len(self.len);
309 // A few methods below (read_to_string, read_line) will append data into a
310 // `String` buffer, but we need to be pretty careful when doing this. The
311 // implementation will just call `.as_mut_vec()` and then delegate to a
312 // byte-oriented reading method, but we must ensure that when returning we never
313 // leave `buf` in a state such that it contains invalid UTF-8 in its bounds.
315 // To this end, we use an RAII guard (to protect against panics) which updates
316 // the length of the string when it is dropped. This guard initially truncates
317 // the string to the prior length and only after we've validated that the
318 // new contents are valid UTF-8 do we allow it to set a longer length.
320 // The unsafety in this function is twofold:
322 // 1. We're looking at the raw bytes of `buf`, so we take on the burden of UTF-8
324 // 2. We're passing a raw buffer to the function `f`, and it is expected that
325 // the function only *appends* bytes to the buffer. We'll get undefined
326 // behavior if existing bytes are overwritten to have non-UTF-8 data.
327 fn append_to_string<F>(buf: &mut String, f: F) -> Result<usize>
329 F: FnOnce(&mut Vec<u8>) -> Result<usize>,
332 let mut g = Guard { len: buf.len(), buf: buf.as_mut_vec() };
334 if str::from_utf8(&g.buf[g.len..]).is_err() {
336 Err(Error::new(ErrorKind::InvalidData, "stream did not contain valid UTF-8"))
345 // This uses an adaptive system to extend the vector when it fills. We want to
346 // avoid paying to allocate and zero a huge chunk of memory if the reader only
347 // has 4 bytes while still making large reads if the reader does have a ton
348 // of data to return. Simply tacking on an extra DEFAULT_BUF_SIZE space every
349 // time is 4,500 times (!) slower than a default reservation size of 32 if the
350 // reader has a very small amount of data to return.
352 // Because we're extending the buffer with uninitialized data for trusted
353 // readers, we need to make sure to truncate that if any of this panics.
354 fn read_to_end<R: Read + ?Sized>(r: &mut R, buf: &mut Vec<u8>) -> Result<usize> {
355 read_to_end_with_reservation(r, buf, |_| 32)
358 fn read_to_end_with_reservation<R, F>(
361 mut reservation_size: F,
365 F: FnMut(&R) -> usize,
367 let start_len = buf.len();
368 let mut g = Guard { len: buf.len(), buf };
371 if g.len == g.buf.len() {
373 // FIXME(danielhenrymantilla): #42788
375 // - This creates a (mut) reference to a slice of
376 // _uninitialized_ integers, which is **undefined behavior**
378 // - Only the standard library gets to soundly "ignore" this,
379 // based on its privileged knowledge of unstable rustc
381 g.buf.reserve(reservation_size(r));
382 let capacity = g.buf.capacity();
383 g.buf.set_len(capacity);
384 r.initializer().initialize(&mut g.buf[g.len..]);
388 match r.read(&mut g.buf[g.len..]) {
390 ret = Ok(g.len - start_len);
394 Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
405 pub(crate) fn default_read_vectored<F>(read: F, bufs: &mut [IoSliceMut<'_>]) -> Result<usize>
407 F: FnOnce(&mut [u8]) -> Result<usize>,
409 let buf = bufs.iter_mut().find(|b| !b.is_empty()).map_or(&mut [][..], |b| &mut **b);
413 pub(crate) fn default_write_vectored<F>(write: F, bufs: &[IoSlice<'_>]) -> Result<usize>
415 F: FnOnce(&[u8]) -> Result<usize>,
417 let buf = bufs.iter().find(|b| !b.is_empty()).map_or(&[][..], |b| &**b);
421 /// The `Read` trait allows for reading bytes from a source.
423 /// Implementors of the `Read` trait are called 'readers'.
425 /// Readers are defined by one required method, [`read()`]. Each call to [`read()`]
426 /// will attempt to pull bytes from this source into a provided buffer. A
427 /// number of other methods are implemented in terms of [`read()`], giving
428 /// implementors a number of ways to read bytes while only needing to implement
431 /// Readers are intended to be composable with one another. Many implementors
432 /// throughout [`std::io`] take and provide types which implement the `Read`
435 /// Please note that each call to [`read()`] may involve a system call, and
436 /// therefore, using something that implements [`BufRead`], such as
437 /// [`BufReader`], will be more efficient.
441 /// [`File`]s implement `Read`:
445 /// use std::io::prelude::*;
446 /// use std::fs::File;
448 /// fn main() -> io::Result<()> {
449 /// let mut f = File::open("foo.txt")?;
450 /// let mut buffer = [0; 10];
452 /// // read up to 10 bytes
453 /// f.read(&mut buffer)?;
455 /// let mut buffer = Vec::new();
456 /// // read the whole file
457 /// f.read_to_end(&mut buffer)?;
459 /// // read into a String, so that you don't need to do the conversion.
460 /// let mut buffer = String::new();
461 /// f.read_to_string(&mut buffer)?;
463 /// // and more! See the other methods for more details.
468 /// Read from [`&str`] because [`&[u8]`][slice] implements `Read`:
472 /// use std::io::prelude::*;
474 /// fn main() -> io::Result<()> {
475 /// let mut b = "This string will be read".as_bytes();
476 /// let mut buffer = [0; 10];
478 /// // read up to 10 bytes
479 /// b.read(&mut buffer)?;
481 /// // etc... it works exactly as a File does!
486 /// [`read()`]: Read::read
487 /// [`&str`]: prim@str
488 /// [`std::io`]: self
489 /// [`File`]: crate::fs::File
490 /// [slice]: ../../std/primitive.slice.html
491 #[stable(feature = "rust1", since = "1.0.0")]
494 /// Pull some bytes from this source into the specified buffer, returning
495 /// how many bytes were read.
497 /// This function does not provide any guarantees about whether it blocks
498 /// waiting for data, but if an object needs to block for a read and cannot,
499 /// it will typically signal this via an [`Err`] return value.
501 /// If the return value of this method is [`Ok(n)`], then it must be
502 /// guaranteed that `0 <= n <= buf.len()`. A nonzero `n` value indicates
503 /// that the buffer `buf` has been filled in with `n` bytes of data from this
504 /// source. If `n` is `0`, then it can indicate one of two scenarios:
506 /// 1. This reader has reached its "end of file" and will likely no longer
507 /// be able to produce bytes. Note that this does not mean that the
508 /// reader will *always* no longer be able to produce bytes.
509 /// 2. The buffer specified was 0 bytes in length.
511 /// It is not an error if the returned value `n` is smaller than the buffer size,
512 /// even when the reader is not at the end of the stream yet.
513 /// This may happen for example because fewer bytes are actually available right now
514 /// (e. g. being close to end-of-file) or because read() was interrupted by a signal.
516 /// No guarantees are provided about the contents of `buf` when this
517 /// function is called, implementations cannot rely on any property of the
518 /// contents of `buf` being true. It is recommended that *implementations*
519 /// only write data to `buf` instead of reading its contents.
521 /// Correspondingly, however, *callers* of this method may not assume any guarantees
522 /// about how the implementation uses `buf`. The trait is safe to implement,
523 /// so it is possible that the code that's supposed to write to the buffer might also read
524 /// from it. It is your responsibility to make sure that `buf` is initialized
525 /// before calling `read`. Calling `read` with an uninitialized `buf` (of the kind one
526 /// obtains via [`MaybeUninit<T>`]) is not safe, and can lead to undefined behavior.
528 /// [`MaybeUninit<T>`]: crate::mem::MaybeUninit
532 /// If this function encounters any form of I/O or other error, an error
533 /// variant will be returned. If an error is returned then it must be
534 /// guaranteed that no bytes were read.
536 /// An error of the [`ErrorKind::Interrupted`] kind is non-fatal and the read
537 /// operation should be retried if there is nothing else to do.
541 /// [`File`]s implement `Read`:
544 /// [`File`]: crate::fs::File
548 /// use std::io::prelude::*;
549 /// use std::fs::File;
551 /// fn main() -> io::Result<()> {
552 /// let mut f = File::open("foo.txt")?;
553 /// let mut buffer = [0; 10];
555 /// // read up to 10 bytes
556 /// let n = f.read(&mut buffer[..])?;
558 /// println!("The bytes: {:?}", &buffer[..n]);
562 #[stable(feature = "rust1", since = "1.0.0")]
563 fn read(&mut self, buf: &mut [u8]) -> Result<usize>;
565 /// Like `read`, except that it reads into a slice of buffers.
567 /// Data is copied to fill each buffer in order, with the final buffer
568 /// written to possibly being only partially filled. This method must
569 /// behave equivalently to a single call to `read` with concatenated
572 /// The default implementation calls `read` with either the first nonempty
573 /// buffer provided, or an empty one if none exists.
574 #[stable(feature = "iovec", since = "1.36.0")]
575 fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> Result<usize> {
576 default_read_vectored(|b| self.read(b), bufs)
579 /// Determines if this `Read`er has an efficient `read_vectored`
582 /// If a `Read`er does not override the default `read_vectored`
583 /// implementation, code using it may want to avoid the method all together
584 /// and coalesce writes into a single buffer for higher performance.
586 /// The default implementation returns `false`.
587 #[unstable(feature = "can_vector", issue = "69941")]
588 fn is_read_vectored(&self) -> bool {
592 /// Determines if this `Read`er can work with buffers of uninitialized
595 /// The default implementation returns an initializer which will zero
598 /// If a `Read`er guarantees that it can work properly with uninitialized
599 /// memory, it should call [`Initializer::nop()`]. See the documentation for
600 /// [`Initializer`] for details.
602 /// The behavior of this method must be independent of the state of the
603 /// `Read`er - the method only takes `&self` so that it can be used through
608 /// This method is unsafe because a `Read`er could otherwise return a
609 /// non-zeroing `Initializer` from another `Read` type without an `unsafe`
611 #[unstable(feature = "read_initializer", issue = "42788")]
613 unsafe fn initializer(&self) -> Initializer {
614 Initializer::zeroing()
617 /// Read all bytes until EOF in this source, placing them into `buf`.
619 /// All bytes read from this source will be appended to the specified buffer
620 /// `buf`. This function will continuously call [`read()`] to append more data to
621 /// `buf` until [`read()`] returns either [`Ok(0)`] or an error of
622 /// non-[`ErrorKind::Interrupted`] kind.
624 /// If successful, this function will return the total number of bytes read.
628 /// If this function encounters an error of the kind
629 /// [`ErrorKind::Interrupted`] then the error is ignored and the operation
632 /// If any other read error is encountered then this function immediately
633 /// returns. Any bytes which have already been read will be appended to
638 /// [`File`]s implement `Read`:
640 /// [`read()`]: Read::read
642 /// [`File`]: crate::fs::File
646 /// use std::io::prelude::*;
647 /// use std::fs::File;
649 /// fn main() -> io::Result<()> {
650 /// let mut f = File::open("foo.txt")?;
651 /// let mut buffer = Vec::new();
653 /// // read the whole file
654 /// f.read_to_end(&mut buffer)?;
659 /// (See also the [`std::fs::read`] convenience function for reading from a
662 /// [`std::fs::read`]: crate::fs::read
663 #[stable(feature = "rust1", since = "1.0.0")]
664 fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize> {
665 read_to_end(self, buf)
668 /// Read all bytes until EOF in this source, appending them to `buf`.
670 /// If successful, this function returns the number of bytes which were read
671 /// and appended to `buf`.
675 /// If the data in this stream is *not* valid UTF-8 then an error is
676 /// returned and `buf` is unchanged.
678 /// See [`read_to_end`] for other error semantics.
680 /// [`read_to_end`]: Read::read_to_end
684 /// [`File`]s implement `Read`:
686 /// [`File`]: crate::fs::File
690 /// use std::io::prelude::*;
691 /// use std::fs::File;
693 /// fn main() -> io::Result<()> {
694 /// let mut f = File::open("foo.txt")?;
695 /// let mut buffer = String::new();
697 /// f.read_to_string(&mut buffer)?;
702 /// (See also the [`std::fs::read_to_string`] convenience function for
703 /// reading from a file.)
705 /// [`std::fs::read_to_string`]: crate::fs::read_to_string
706 #[stable(feature = "rust1", since = "1.0.0")]
707 fn read_to_string(&mut self, buf: &mut String) -> Result<usize> {
708 // Note that we do *not* call `.read_to_end()` here. We are passing
709 // `&mut Vec<u8>` (the raw contents of `buf`) into the `read_to_end`
710 // method to fill it up. An arbitrary implementation could overwrite the
711 // entire contents of the vector, not just append to it (which is what
712 // we are expecting).
714 // To prevent extraneously checking the UTF-8-ness of the entire buffer
715 // we pass it to our hardcoded `read_to_end` implementation which we
716 // know is guaranteed to only read data into the end of the buffer.
717 append_to_string(buf, |b| read_to_end(self, b))
720 /// Read the exact number of bytes required to fill `buf`.
722 /// This function reads as many bytes as necessary to completely fill the
723 /// specified buffer `buf`.
725 /// No guarantees are provided about the contents of `buf` when this
726 /// function is called, implementations cannot rely on any property of the
727 /// contents of `buf` being true. It is recommended that implementations
728 /// only write data to `buf` instead of reading its contents. The
729 /// documentation on [`read`] has a more detailed explanation on this
734 /// If this function encounters an error of the kind
735 /// [`ErrorKind::Interrupted`] then the error is ignored and the operation
738 /// If this function encounters an "end of file" before completely filling
739 /// the buffer, it returns an error of the kind [`ErrorKind::UnexpectedEof`].
740 /// The contents of `buf` are unspecified in this case.
742 /// If any other read error is encountered then this function immediately
743 /// returns. The contents of `buf` are unspecified in this case.
745 /// If this function returns an error, it is unspecified how many bytes it
746 /// has read, but it will never read more than would be necessary to
747 /// completely fill the buffer.
751 /// [`File`]s implement `Read`:
753 /// [`read`]: Read::read
754 /// [`File`]: crate::fs::File
758 /// use std::io::prelude::*;
759 /// use std::fs::File;
761 /// fn main() -> io::Result<()> {
762 /// let mut f = File::open("foo.txt")?;
763 /// let mut buffer = [0; 10];
765 /// // read exactly 10 bytes
766 /// f.read_exact(&mut buffer)?;
770 #[stable(feature = "read_exact", since = "1.6.0")]
771 fn read_exact(&mut self, mut buf: &mut [u8]) -> Result<()> {
772 while !buf.is_empty() {
773 match self.read(buf) {
779 Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
780 Err(e) => return Err(e),
784 Err(Error::new(ErrorKind::UnexpectedEof, "failed to fill whole buffer"))
790 /// Creates a "by reference" adaptor for this instance of `Read`.
792 /// The returned adaptor also implements `Read` and will simply borrow this
797 /// [`File`]s implement `Read`:
799 /// [`File`]: crate::fs::File
803 /// use std::io::Read;
804 /// use std::fs::File;
806 /// fn main() -> io::Result<()> {
807 /// let mut f = File::open("foo.txt")?;
808 /// let mut buffer = Vec::new();
809 /// let mut other_buffer = Vec::new();
812 /// let reference = f.by_ref();
814 /// // read at most 5 bytes
815 /// reference.take(5).read_to_end(&mut buffer)?;
817 /// } // drop our &mut reference so we can use f again
819 /// // original file still usable, read the rest
820 /// f.read_to_end(&mut other_buffer)?;
824 #[stable(feature = "rust1", since = "1.0.0")]
825 fn by_ref(&mut self) -> &mut Self
832 /// Transforms this `Read` instance to an [`Iterator`] over its bytes.
834 /// The returned type implements [`Iterator`] where the `Item` is
835 /// [`Result`]`<`[`u8`]`, `[`io::Error`]`>`.
836 /// The yielded item is [`Ok`] if a byte was successfully read and [`Err`]
837 /// otherwise. EOF is mapped to returning [`None`] from this iterator.
841 /// [`File`]s implement `Read`:
843 /// [`File`]: crate::fs::File
844 /// [`Result`]: crate::result::Result
845 /// [`io::Error`]: self::Error
849 /// use std::io::prelude::*;
850 /// use std::fs::File;
852 /// fn main() -> io::Result<()> {
853 /// let mut f = File::open("foo.txt")?;
855 /// for byte in f.bytes() {
856 /// println!("{}", byte.unwrap());
861 #[stable(feature = "rust1", since = "1.0.0")]
862 fn bytes(self) -> Bytes<Self>
866 Bytes { inner: self }
869 /// Creates an adaptor which will chain this stream with another.
871 /// The returned `Read` instance will first read all bytes from this object
872 /// until EOF is encountered. Afterwards the output is equivalent to the
873 /// output of `next`.
877 /// [`File`]s implement `Read`:
879 /// [`File`]: crate::fs::File
883 /// use std::io::prelude::*;
884 /// use std::fs::File;
886 /// fn main() -> io::Result<()> {
887 /// let mut f1 = File::open("foo.txt")?;
888 /// let mut f2 = File::open("bar.txt")?;
890 /// let mut handle = f1.chain(f2);
891 /// let mut buffer = String::new();
893 /// // read the value into a String. We could use any Read method here,
894 /// // this is just one example.
895 /// handle.read_to_string(&mut buffer)?;
899 #[stable(feature = "rust1", since = "1.0.0")]
900 fn chain<R: Read>(self, next: R) -> Chain<Self, R>
904 Chain { first: self, second: next, done_first: false }
907 /// Creates an adaptor which will read at most `limit` bytes from it.
909 /// This function returns a new instance of `Read` which will read at most
910 /// `limit` bytes, after which it will always return EOF ([`Ok(0)`]). Any
911 /// read errors will not count towards the number of bytes read and future
912 /// calls to [`read()`] may succeed.
916 /// [`File`]s implement `Read`:
918 /// [`File`]: crate::fs::File
920 /// [`read()`]: Read::read
924 /// use std::io::prelude::*;
925 /// use std::fs::File;
927 /// fn main() -> io::Result<()> {
928 /// let mut f = File::open("foo.txt")?;
929 /// let mut buffer = [0; 5];
931 /// // read at most five bytes
932 /// let mut handle = f.take(5);
934 /// handle.read(&mut buffer)?;
938 #[stable(feature = "rust1", since = "1.0.0")]
939 fn take(self, limit: u64) -> Take<Self>
943 Take { inner: self, limit }
947 /// A buffer type used with `Read::read_vectored`.
949 /// It is semantically a wrapper around an `&mut [u8]`, but is guaranteed to be
950 /// ABI compatible with the `iovec` type on Unix platforms and `WSABUF` on
952 #[stable(feature = "iovec", since = "1.36.0")]
954 pub struct IoSliceMut<'a>(sys::io::IoSliceMut<'a>);
956 #[stable(feature = "iovec-send-sync", since = "1.44.0")]
957 unsafe impl<'a> Send for IoSliceMut<'a> {}
959 #[stable(feature = "iovec-send-sync", since = "1.44.0")]
960 unsafe impl<'a> Sync for IoSliceMut<'a> {}
962 #[stable(feature = "iovec", since = "1.36.0")]
963 impl<'a> fmt::Debug for IoSliceMut<'a> {
964 fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
965 fmt::Debug::fmt(self.0.as_slice(), fmt)
969 impl<'a> IoSliceMut<'a> {
970 /// Creates a new `IoSliceMut` wrapping a byte slice.
974 /// Panics on Windows if the slice is larger than 4GB.
975 #[stable(feature = "iovec", since = "1.36.0")]
977 pub fn new(buf: &'a mut [u8]) -> IoSliceMut<'a> {
978 IoSliceMut(sys::io::IoSliceMut::new(buf))
981 /// Advance the internal cursor of the slice.
985 /// Elements in the slice may be modified if the cursor is not advanced to
986 /// the end of the slice. For example if we have a slice of buffers with 2
987 /// `IoSliceMut`s, both of length 8, and we advance the cursor by 10 bytes
988 /// the first `IoSliceMut` will be untouched however the second will be
989 /// modified to remove the first 2 bytes (10 - 8).
994 /// #![feature(io_slice_advance)]
996 /// use std::io::IoSliceMut;
997 /// use std::ops::Deref;
999 /// let mut buf1 = [1; 8];
1000 /// let mut buf2 = [2; 16];
1001 /// let mut buf3 = [3; 8];
1002 /// let mut bufs = &mut [
1003 /// IoSliceMut::new(&mut buf1),
1004 /// IoSliceMut::new(&mut buf2),
1005 /// IoSliceMut::new(&mut buf3),
1008 /// // Mark 10 bytes as read.
1009 /// bufs = IoSliceMut::advance(bufs, 10);
1010 /// assert_eq!(bufs[0].deref(), [2; 14].as_ref());
1011 /// assert_eq!(bufs[1].deref(), [3; 8].as_ref());
1013 #[unstable(feature = "io_slice_advance", issue = "62726")]
1015 pub fn advance<'b>(bufs: &'b mut [IoSliceMut<'a>], n: usize) -> &'b mut [IoSliceMut<'a>] {
1016 // Number of buffers to remove.
1018 // Total length of all the to be removed buffers.
1019 let mut accumulated_len = 0;
1020 for buf in bufs.iter() {
1021 if accumulated_len + buf.len() > n {
1024 accumulated_len += buf.len();
1029 let bufs = &mut bufs[remove..];
1030 if !bufs.is_empty() {
1031 bufs[0].0.advance(n - accumulated_len)
1037 #[stable(feature = "iovec", since = "1.36.0")]
1038 impl<'a> Deref for IoSliceMut<'a> {
1042 fn deref(&self) -> &[u8] {
1047 #[stable(feature = "iovec", since = "1.36.0")]
1048 impl<'a> DerefMut for IoSliceMut<'a> {
1050 fn deref_mut(&mut self) -> &mut [u8] {
1051 self.0.as_mut_slice()
1055 /// A buffer type used with `Write::write_vectored`.
1057 /// It is semantically a wrapper around an `&[u8]`, but is guaranteed to be
1058 /// ABI compatible with the `iovec` type on Unix platforms and `WSABUF` on
1060 #[stable(feature = "iovec", since = "1.36.0")]
1061 #[derive(Copy, Clone)]
1062 #[repr(transparent)]
1063 pub struct IoSlice<'a>(sys::io::IoSlice<'a>);
1065 #[stable(feature = "iovec-send-sync", since = "1.44.0")]
1066 unsafe impl<'a> Send for IoSlice<'a> {}
1068 #[stable(feature = "iovec-send-sync", since = "1.44.0")]
1069 unsafe impl<'a> Sync for IoSlice<'a> {}
1071 #[stable(feature = "iovec", since = "1.36.0")]
1072 impl<'a> fmt::Debug for IoSlice<'a> {
1073 fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
1074 fmt::Debug::fmt(self.0.as_slice(), fmt)
1078 impl<'a> IoSlice<'a> {
1079 /// Creates a new `IoSlice` wrapping a byte slice.
1083 /// Panics on Windows if the slice is larger than 4GB.
1084 #[stable(feature = "iovec", since = "1.36.0")]
1086 pub fn new(buf: &'a [u8]) -> IoSlice<'a> {
1087 IoSlice(sys::io::IoSlice::new(buf))
1090 /// Advance the internal cursor of the slice.
1094 /// Elements in the slice may be modified if the cursor is not advanced to
1095 /// the end of the slice. For example if we have a slice of buffers with 2
1096 /// `IoSlice`s, both of length 8, and we advance the cursor by 10 bytes the
1097 /// first `IoSlice` will be untouched however the second will be modified to
1098 /// remove the first 2 bytes (10 - 8).
1103 /// #![feature(io_slice_advance)]
1105 /// use std::io::IoSlice;
1106 /// use std::ops::Deref;
1108 /// let buf1 = [1; 8];
1109 /// let buf2 = [2; 16];
1110 /// let buf3 = [3; 8];
1111 /// let mut bufs = &mut [
1112 /// IoSlice::new(&buf1),
1113 /// IoSlice::new(&buf2),
1114 /// IoSlice::new(&buf3),
1117 /// // Mark 10 bytes as written.
1118 /// bufs = IoSlice::advance(bufs, 10);
1119 /// assert_eq!(bufs[0].deref(), [2; 14].as_ref());
1120 /// assert_eq!(bufs[1].deref(), [3; 8].as_ref());
1121 #[unstable(feature = "io_slice_advance", issue = "62726")]
1123 pub fn advance<'b>(bufs: &'b mut [IoSlice<'a>], n: usize) -> &'b mut [IoSlice<'a>] {
1124 // Number of buffers to remove.
1126 // Total length of all the to be removed buffers.
1127 let mut accumulated_len = 0;
1128 for buf in bufs.iter() {
1129 if accumulated_len + buf.len() > n {
1132 accumulated_len += buf.len();
1137 let bufs = &mut bufs[remove..];
1138 if !bufs.is_empty() {
1139 bufs[0].0.advance(n - accumulated_len)
1145 #[stable(feature = "iovec", since = "1.36.0")]
1146 impl<'a> Deref for IoSlice<'a> {
1150 fn deref(&self) -> &[u8] {
1155 /// A type used to conditionally initialize buffers passed to `Read` methods.
1156 #[unstable(feature = "read_initializer", issue = "42788")]
1158 pub struct Initializer(bool);
1161 /// Returns a new `Initializer` which will zero out buffers.
1162 #[unstable(feature = "read_initializer", issue = "42788")]
1164 pub fn zeroing() -> Initializer {
1168 /// Returns a new `Initializer` which will not zero out buffers.
1172 /// This may only be called by `Read`ers which guarantee that they will not
1173 /// read from buffers passed to `Read` methods, and that the return value of
1174 /// the method accurately reflects the number of bytes that have been
1175 /// written to the head of the buffer.
1176 #[unstable(feature = "read_initializer", issue = "42788")]
1178 pub unsafe fn nop() -> Initializer {
1182 /// Indicates if a buffer should be initialized.
1183 #[unstable(feature = "read_initializer", issue = "42788")]
1185 pub fn should_initialize(&self) -> bool {
1189 /// Initializes a buffer if necessary.
1190 #[unstable(feature = "read_initializer", issue = "42788")]
1192 pub fn initialize(&self, buf: &mut [u8]) {
1193 if self.should_initialize() {
1194 unsafe { ptr::write_bytes(buf.as_mut_ptr(), 0, buf.len()) }
1199 /// A trait for objects which are byte-oriented sinks.
1201 /// Implementors of the `Write` trait are sometimes called 'writers'.
1203 /// Writers are defined by two required methods, [`write`] and [`flush`]:
1205 /// * The [`write`] method will attempt to write some data into the object,
1206 /// returning how many bytes were successfully written.
1208 /// * The [`flush`] method is useful for adaptors and explicit buffers
1209 /// themselves for ensuring that all buffered data has been pushed out to the
1212 /// Writers are intended to be composable with one another. Many implementors
1213 /// throughout [`std::io`] take and provide types which implement the `Write`
1216 /// [`write`]: Write::write
1217 /// [`flush`]: Write::flush
1218 /// [`std::io`]: self
1223 /// use std::io::prelude::*;
1224 /// use std::fs::File;
1226 /// fn main() -> std::io::Result<()> {
1227 /// let data = b"some bytes";
1229 /// let mut pos = 0;
1230 /// let mut buffer = File::create("foo.txt")?;
1232 /// while pos < data.len() {
1233 /// let bytes_written = buffer.write(&data[pos..])?;
1234 /// pos += bytes_written;
1240 /// The trait also provides convenience methods like [`write_all`], which calls
1241 /// `write` in a loop until its entire input has been written.
1243 /// [`write_all`]: Write::write_all
1244 #[stable(feature = "rust1", since = "1.0.0")]
1247 /// Write a buffer into this writer, returning how many bytes were written.
1249 /// This function will attempt to write the entire contents of `buf`, but
1250 /// the entire write may not succeed, or the write may also generate an
1251 /// error. A call to `write` represents *at most one* attempt to write to
1252 /// any wrapped object.
1254 /// Calls to `write` are not guaranteed to block waiting for data to be
1255 /// written, and a write which would otherwise block can be indicated through
1256 /// an [`Err`] variant.
1258 /// If the return value is [`Ok(n)`] then it must be guaranteed that
1259 /// `n <= buf.len()`. A return value of `0` typically means that the
1260 /// underlying object is no longer able to accept bytes and will likely not
1261 /// be able to in the future as well, or that the buffer provided is empty.
1265 /// Each call to `write` may generate an I/O error indicating that the
1266 /// operation could not be completed. If an error is returned then no bytes
1267 /// in the buffer were written to this writer.
1269 /// It is **not** considered an error if the entire buffer could not be
1270 /// written to this writer.
1272 /// An error of the [`ErrorKind::Interrupted`] kind is non-fatal and the
1273 /// write operation should be retried if there is nothing else to do.
1278 /// use std::io::prelude::*;
1279 /// use std::fs::File;
1281 /// fn main() -> std::io::Result<()> {
1282 /// let mut buffer = File::create("foo.txt")?;
1284 /// // Writes some prefix of the byte string, not necessarily all of it.
1285 /// buffer.write(b"some bytes")?;
1291 #[stable(feature = "rust1", since = "1.0.0")]
1292 fn write(&mut self, buf: &[u8]) -> Result<usize>;
1294 /// Like [`write`], except that it writes from a slice of buffers.
1296 /// Data is copied from each buffer in order, with the final buffer
1297 /// read from possibly being only partially consumed. This method must
1298 /// behave as a call to [`write`] with the buffers concatenated would.
1300 /// The default implementation calls [`write`] with either the first nonempty
1301 /// buffer provided, or an empty one if none exists.
1303 /// [`write`]: Write::write
1304 #[stable(feature = "iovec", since = "1.36.0")]
1305 fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize> {
1306 default_write_vectored(|b| self.write(b), bufs)
1309 /// Determines if this `Write`er has an efficient [`write_vectored`]
1312 /// If a `Write`er does not override the default [`write_vectored`]
1313 /// implementation, code using it may want to avoid the method all together
1314 /// and coalesce writes into a single buffer for higher performance.
1316 /// The default implementation returns `false`.
1318 /// [`write_vectored`]: Write::write_vectored
1319 #[unstable(feature = "can_vector", issue = "69941")]
1320 fn is_write_vectored(&self) -> bool {
1324 /// Flush this output stream, ensuring that all intermediately buffered
1325 /// contents reach their destination.
1329 /// It is considered an error if not all bytes could be written due to
1330 /// I/O errors or EOF being reached.
1335 /// use std::io::prelude::*;
1336 /// use std::io::BufWriter;
1337 /// use std::fs::File;
1339 /// fn main() -> std::io::Result<()> {
1340 /// let mut buffer = BufWriter::new(File::create("foo.txt")?);
1342 /// buffer.write_all(b"some bytes")?;
1343 /// buffer.flush()?;
1347 #[stable(feature = "rust1", since = "1.0.0")]
1348 fn flush(&mut self) -> Result<()>;
1350 /// Attempts to write an entire buffer into this writer.
1352 /// This method will continuously call [`write`] until there is no more data
1353 /// to be written or an error of non-[`ErrorKind::Interrupted`] kind is
1354 /// returned. This method will not return until the entire buffer has been
1355 /// successfully written or such an error occurs. The first error that is
1356 /// not of [`ErrorKind::Interrupted`] kind generated from this method will be
1359 /// If the buffer contains no data, this will never call [`write`].
1363 /// This function will return the first error of
1364 /// non-[`ErrorKind::Interrupted`] kind that [`write`] returns.
1366 /// [`write`]: Write::write
1371 /// use std::io::prelude::*;
1372 /// use std::fs::File;
1374 /// fn main() -> std::io::Result<()> {
1375 /// let mut buffer = File::create("foo.txt")?;
1377 /// buffer.write_all(b"some bytes")?;
1381 #[stable(feature = "rust1", since = "1.0.0")]
1382 fn write_all(&mut self, mut buf: &[u8]) -> Result<()> {
1383 while !buf.is_empty() {
1384 match self.write(buf) {
1386 return Err(Error::new(ErrorKind::WriteZero, "failed to write whole buffer"));
1388 Ok(n) => buf = &buf[n..],
1389 Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
1390 Err(e) => return Err(e),
1396 /// Attempts to write multiple buffers into this writer.
1398 /// This method will continuously call [`write_vectored`] until there is no
1399 /// more data to be written or an error of non-[`ErrorKind::Interrupted`]
1400 /// kind is returned. This method will not return until all buffers have
1401 /// been successfully written or such an error occurs. The first error that
1402 /// is not of [`ErrorKind::Interrupted`] kind generated from this method
1403 /// will be returned.
1405 /// If the buffer contains no data, this will never call [`write_vectored`].
1409 /// Unlike [`write_vectored`], this takes a *mutable* reference to
1410 /// a slice of [`IoSlice`]s, not an immutable one. That's because we need to
1411 /// modify the slice to keep track of the bytes already written.
1413 /// Once this function returns, the contents of `bufs` are unspecified, as
1414 /// this depends on how many calls to [`write_vectored`] were necessary. It is
1415 /// best to understand this function as taking ownership of `bufs` and to
1416 /// not use `bufs` afterwards. The underlying buffers, to which the
1417 /// [`IoSlice`]s point (but not the [`IoSlice`]s themselves), are unchanged and
1420 /// [`write_vectored`]: Write::write_vectored
1425 /// #![feature(write_all_vectored)]
1426 /// # fn main() -> std::io::Result<()> {
1428 /// use std::io::{Write, IoSlice};
1430 /// let mut writer = Vec::new();
1431 /// let bufs = &mut [
1432 /// IoSlice::new(&[1]),
1433 /// IoSlice::new(&[2, 3]),
1434 /// IoSlice::new(&[4, 5, 6]),
1437 /// writer.write_all_vectored(bufs)?;
1438 /// // Note: the contents of `bufs` is now undefined, see the Notes section.
1440 /// assert_eq!(writer, &[1, 2, 3, 4, 5, 6]);
1443 #[unstable(feature = "write_all_vectored", issue = "70436")]
1444 fn write_all_vectored(&mut self, mut bufs: &mut [IoSlice<'_>]) -> Result<()> {
1445 // Guarantee that bufs is empty if it contains no data,
1446 // to avoid calling write_vectored if there is no data to be written.
1447 bufs = IoSlice::advance(bufs, 0);
1448 while !bufs.is_empty() {
1449 match self.write_vectored(bufs) {
1451 return Err(Error::new(ErrorKind::WriteZero, "failed to write whole buffer"));
1453 Ok(n) => bufs = IoSlice::advance(bufs, n),
1454 Err(ref e) if e.kind() == ErrorKind::Interrupted => {}
1455 Err(e) => return Err(e),
1461 /// Writes a formatted string into this writer, returning any error
1464 /// This method is primarily used to interface with the
1465 /// [`format_args!()`] macro, but it is rare that this should
1466 /// explicitly be called. The [`write!()`] macro should be favored to
1467 /// invoke this method instead.
1469 /// This function internally uses the [`write_all`] method on
1470 /// this trait and hence will continuously write data so long as no errors
1471 /// are received. This also means that partial writes are not indicated in
1474 /// [`write_all`]: Write::write_all
1478 /// This function will return any I/O error reported while formatting.
1483 /// use std::io::prelude::*;
1484 /// use std::fs::File;
1486 /// fn main() -> std::io::Result<()> {
1487 /// let mut buffer = File::create("foo.txt")?;
1490 /// write!(buffer, "{:.*}", 2, 1.234567)?;
1491 /// // turns into this:
1492 /// buffer.write_fmt(format_args!("{:.*}", 2, 1.234567))?;
1496 #[stable(feature = "rust1", since = "1.0.0")]
1497 fn write_fmt(&mut self, fmt: fmt::Arguments<'_>) -> Result<()> {
1498 // Create a shim which translates a Write to a fmt::Write and saves
1499 // off I/O errors. instead of discarding them
1500 struct Adaptor<'a, T: ?Sized + 'a> {
1505 impl<T: Write + ?Sized> fmt::Write for Adaptor<'_, T> {
1506 fn write_str(&mut self, s: &str) -> fmt::Result {
1507 match self.inner.write_all(s.as_bytes()) {
1510 self.error = Err(e);
1517 let mut output = Adaptor { inner: self, error: Ok(()) };
1518 match fmt::write(&mut output, fmt) {
1521 // check if the error came from the underlying `Write` or not
1522 if output.error.is_err() {
1525 Err(Error::new(ErrorKind::Other, "formatter error"))
1531 /// Creates a "by reference" adaptor for this instance of `Write`.
1533 /// The returned adaptor also implements `Write` and will simply borrow this
1539 /// use std::io::Write;
1540 /// use std::fs::File;
1542 /// fn main() -> std::io::Result<()> {
1543 /// let mut buffer = File::create("foo.txt")?;
1545 /// let reference = buffer.by_ref();
1547 /// // we can use reference just like our original buffer
1548 /// reference.write_all(b"some bytes")?;
1552 #[stable(feature = "rust1", since = "1.0.0")]
1553 fn by_ref(&mut self) -> &mut Self
1561 /// The `Seek` trait provides a cursor which can be moved within a stream of
1564 /// The stream typically has a fixed size, allowing seeking relative to either
1565 /// end or the current offset.
1569 /// [`File`]s implement `Seek`:
1571 /// [`File`]: crate::fs::File
1575 /// use std::io::prelude::*;
1576 /// use std::fs::File;
1577 /// use std::io::SeekFrom;
1579 /// fn main() -> io::Result<()> {
1580 /// let mut f = File::open("foo.txt")?;
1582 /// // move the cursor 42 bytes from the start of the file
1583 /// f.seek(SeekFrom::Start(42))?;
1587 #[stable(feature = "rust1", since = "1.0.0")]
1589 /// Seek to an offset, in bytes, in a stream.
1591 /// A seek beyond the end of a stream is allowed, but behavior is defined
1592 /// by the implementation.
1594 /// If the seek operation completed successfully,
1595 /// this method returns the new position from the start of the stream.
1596 /// That position can be used later with [`SeekFrom::Start`].
1600 /// Seeking to a negative offset is considered an error.
1601 #[stable(feature = "rust1", since = "1.0.0")]
1602 fn seek(&mut self, pos: SeekFrom) -> Result<u64>;
1604 /// Returns the length of this stream (in bytes).
1606 /// This method is implemented using up to three seek operations. If this
1607 /// method returns successfully, the seek position is unchanged (i.e. the
1608 /// position before calling this method is the same as afterwards).
1609 /// However, if this method returns an error, the seek position is
1612 /// If you need to obtain the length of *many* streams and you don't care
1613 /// about the seek position afterwards, you can reduce the number of seek
1614 /// operations by simply calling `seek(SeekFrom::End(0))` and using its
1615 /// return value (it is also the stream length).
1617 /// Note that length of a stream can change over time (for example, when
1618 /// data is appended to a file). So calling this method multiple times does
1619 /// not necessarily return the same length each time.
1624 /// #![feature(seek_convenience)]
1626 /// io::{self, Seek},
1630 /// fn main() -> io::Result<()> {
1631 /// let mut f = File::open("foo.txt")?;
1633 /// let len = f.stream_len()?;
1634 /// println!("The file is currently {} bytes long", len);
1638 #[unstable(feature = "seek_convenience", issue = "59359")]
1639 fn stream_len(&mut self) -> Result<u64> {
1640 let old_pos = self.stream_position()?;
1641 let len = self.seek(SeekFrom::End(0))?;
1643 // Avoid seeking a third time when we were already at the end of the
1644 // stream. The branch is usually way cheaper than a seek operation.
1646 self.seek(SeekFrom::Start(old_pos))?;
1652 /// Returns the current seek position from the start of the stream.
1654 /// This is equivalent to `self.seek(SeekFrom::Current(0))`.
1659 /// #![feature(seek_convenience)]
1661 /// io::{self, BufRead, BufReader, Seek},
1665 /// fn main() -> io::Result<()> {
1666 /// let mut f = BufReader::new(File::open("foo.txt")?);
1668 /// let before = f.stream_position()?;
1669 /// f.read_line(&mut String::new())?;
1670 /// let after = f.stream_position()?;
1672 /// println!("The first line was {} bytes long", after - before);
1676 #[unstable(feature = "seek_convenience", issue = "59359")]
1677 fn stream_position(&mut self) -> Result<u64> {
1678 self.seek(SeekFrom::Current(0))
1682 /// Enumeration of possible methods to seek within an I/O object.
1684 /// It is used by the [`Seek`] trait.
1685 #[derive(Copy, PartialEq, Eq, Clone, Debug)]
1686 #[stable(feature = "rust1", since = "1.0.0")]
1688 /// Sets the offset to the provided number of bytes.
1689 #[stable(feature = "rust1", since = "1.0.0")]
1690 Start(#[stable(feature = "rust1", since = "1.0.0")] u64),
1692 /// Sets the offset to the size of this object plus the specified number of
1695 /// It is possible to seek beyond the end of an object, but it's an error to
1696 /// seek before byte 0.
1697 #[stable(feature = "rust1", since = "1.0.0")]
1698 End(#[stable(feature = "rust1", since = "1.0.0")] i64),
1700 /// Sets the offset to the current position plus the specified number of
1703 /// It is possible to seek beyond the end of an object, but it's an error to
1704 /// seek before byte 0.
1705 #[stable(feature = "rust1", since = "1.0.0")]
1706 Current(#[stable(feature = "rust1", since = "1.0.0")] i64),
1709 fn read_until<R: BufRead + ?Sized>(r: &mut R, delim: u8, buf: &mut Vec<u8>) -> Result<usize> {
1712 let (done, used) = {
1713 let available = match r.fill_buf() {
1715 Err(ref e) if e.kind() == ErrorKind::Interrupted => continue,
1716 Err(e) => return Err(e),
1718 match memchr::memchr(delim, available) {
1720 buf.extend_from_slice(&available[..=i]);
1724 buf.extend_from_slice(available);
1725 (false, available.len())
1731 if done || used == 0 {
1737 /// A `BufRead` is a type of `Read`er which has an internal buffer, allowing it
1738 /// to perform extra ways of reading.
1740 /// For example, reading line-by-line is inefficient without using a buffer, so
1741 /// if you want to read by line, you'll need `BufRead`, which includes a
1742 /// [`read_line`] method as well as a [`lines`] iterator.
1746 /// A locked standard input implements `BufRead`:
1750 /// use std::io::prelude::*;
1752 /// let stdin = io::stdin();
1753 /// for line in stdin.lock().lines() {
1754 /// println!("{}", line.unwrap());
1758 /// If you have something that implements [`Read`], you can use the [`BufReader`
1759 /// type][`BufReader`] to turn it into a `BufRead`.
1761 /// For example, [`File`] implements [`Read`], but not `BufRead`.
1762 /// [`BufReader`] to the rescue!
1764 /// [`File`]: crate::fs::File
1765 /// [`read_line`]: BufRead::read_line
1766 /// [`lines`]: BufRead::lines
1769 /// use std::io::{self, BufReader};
1770 /// use std::io::prelude::*;
1771 /// use std::fs::File;
1773 /// fn main() -> io::Result<()> {
1774 /// let f = File::open("foo.txt")?;
1775 /// let f = BufReader::new(f);
1777 /// for line in f.lines() {
1778 /// println!("{}", line.unwrap());
1784 #[stable(feature = "rust1", since = "1.0.0")]
1785 pub trait BufRead: Read {
1786 /// Returns the contents of the internal buffer, filling it with more data
1787 /// from the inner reader if it is empty.
1789 /// This function is a lower-level call. It needs to be paired with the
1790 /// [`consume`] method to function properly. When calling this
1791 /// method, none of the contents will be "read" in the sense that later
1792 /// calling `read` may return the same contents. As such, [`consume`] must
1793 /// be called with the number of bytes that are consumed from this buffer to
1794 /// ensure that the bytes are never returned twice.
1796 /// [`consume`]: BufRead::consume
1798 /// An empty buffer returned indicates that the stream has reached EOF.
1802 /// This function will return an I/O error if the underlying reader was
1803 /// read, but returned an error.
1807 /// A locked standard input implements `BufRead`:
1811 /// use std::io::prelude::*;
1813 /// let stdin = io::stdin();
1814 /// let mut stdin = stdin.lock();
1816 /// let buffer = stdin.fill_buf().unwrap();
1818 /// // work with buffer
1819 /// println!("{:?}", buffer);
1821 /// // ensure the bytes we worked with aren't returned again later
1822 /// let length = buffer.len();
1823 /// stdin.consume(length);
1825 #[stable(feature = "rust1", since = "1.0.0")]
1826 fn fill_buf(&mut self) -> Result<&[u8]>;
1828 /// Tells this buffer that `amt` bytes have been consumed from the buffer,
1829 /// so they should no longer be returned in calls to `read`.
1831 /// This function is a lower-level call. It needs to be paired with the
1832 /// [`fill_buf`] method to function properly. This function does
1833 /// not perform any I/O, it simply informs this object that some amount of
1834 /// its buffer, returned from [`fill_buf`], has been consumed and should
1835 /// no longer be returned. As such, this function may do odd things if
1836 /// [`fill_buf`] isn't called before calling it.
1838 /// The `amt` must be `<=` the number of bytes in the buffer returned by
1843 /// Since `consume()` is meant to be used with [`fill_buf`],
1844 /// that method's example includes an example of `consume()`.
1846 /// [`fill_buf`]: BufRead::fill_buf
1847 #[stable(feature = "rust1", since = "1.0.0")]
1848 fn consume(&mut self, amt: usize);
1850 /// Read all bytes into `buf` until the delimiter `byte` or EOF is reached.
1852 /// This function will read bytes from the underlying stream until the
1853 /// delimiter or EOF is found. Once found, all bytes up to, and including,
1854 /// the delimiter (if found) will be appended to `buf`.
1856 /// If successful, this function will return the total number of bytes read.
1858 /// This function is blocking and should be used carefully: it is possible for
1859 /// an attacker to continuously send bytes without ever sending the delimiter
1864 /// This function will ignore all instances of [`ErrorKind::Interrupted`] and
1865 /// will otherwise return any errors returned by [`fill_buf`].
1867 /// If an I/O error is encountered then all bytes read so far will be
1868 /// present in `buf` and its length will have been adjusted appropriately.
1870 /// [`fill_buf`]: BufRead::fill_buf
1874 /// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
1875 /// this example, we use [`Cursor`] to read all the bytes in a byte slice
1876 /// in hyphen delimited segments:
1879 /// use std::io::{self, BufRead};
1881 /// let mut cursor = io::Cursor::new(b"lorem-ipsum");
1882 /// let mut buf = vec![];
1884 /// // cursor is at 'l'
1885 /// let num_bytes = cursor.read_until(b'-', &mut buf)
1886 /// .expect("reading from cursor won't fail");
1887 /// assert_eq!(num_bytes, 6);
1888 /// assert_eq!(buf, b"lorem-");
1891 /// // cursor is at 'i'
1892 /// let num_bytes = cursor.read_until(b'-', &mut buf)
1893 /// .expect("reading from cursor won't fail");
1894 /// assert_eq!(num_bytes, 5);
1895 /// assert_eq!(buf, b"ipsum");
1898 /// // cursor is at EOF
1899 /// let num_bytes = cursor.read_until(b'-', &mut buf)
1900 /// .expect("reading from cursor won't fail");
1901 /// assert_eq!(num_bytes, 0);
1902 /// assert_eq!(buf, b"");
1904 #[stable(feature = "rust1", since = "1.0.0")]
1905 fn read_until(&mut self, byte: u8, buf: &mut Vec<u8>) -> Result<usize> {
1906 read_until(self, byte, buf)
1909 /// Read all bytes until a newline (the `0xA` byte) is reached, and append
1910 /// them to the provided buffer.
1912 /// This function will read bytes from the underlying stream until the
1913 /// newline delimiter (the `0xA` byte) or EOF is found. Once found, all bytes
1914 /// up to, and including, the delimiter (if found) will be appended to
1917 /// If successful, this function will return the total number of bytes read.
1919 /// If this function returns [`Ok(0)`], the stream has reached EOF.
1921 /// This function is blocking and should be used carefully: it is possible for
1922 /// an attacker to continuously send bytes without ever sending a newline
1929 /// This function has the same error semantics as [`read_until`] and will
1930 /// also return an error if the read bytes are not valid UTF-8. If an I/O
1931 /// error is encountered then `buf` may contain some bytes already read in
1932 /// the event that all data read so far was valid UTF-8.
1934 /// [`read_until`]: BufRead::read_until
1938 /// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
1939 /// this example, we use [`Cursor`] to read all the lines in a byte slice:
1942 /// use std::io::{self, BufRead};
1944 /// let mut cursor = io::Cursor::new(b"foo\nbar");
1945 /// let mut buf = String::new();
1947 /// // cursor is at 'f'
1948 /// let num_bytes = cursor.read_line(&mut buf)
1949 /// .expect("reading from cursor won't fail");
1950 /// assert_eq!(num_bytes, 4);
1951 /// assert_eq!(buf, "foo\n");
1954 /// // cursor is at 'b'
1955 /// let num_bytes = cursor.read_line(&mut buf)
1956 /// .expect("reading from cursor won't fail");
1957 /// assert_eq!(num_bytes, 3);
1958 /// assert_eq!(buf, "bar");
1961 /// // cursor is at EOF
1962 /// let num_bytes = cursor.read_line(&mut buf)
1963 /// .expect("reading from cursor won't fail");
1964 /// assert_eq!(num_bytes, 0);
1965 /// assert_eq!(buf, "");
1967 #[stable(feature = "rust1", since = "1.0.0")]
1968 fn read_line(&mut self, buf: &mut String) -> Result<usize> {
1969 // Note that we are not calling the `.read_until` method here, but
1970 // rather our hardcoded implementation. For more details as to why, see
1971 // the comments in `read_to_end`.
1972 append_to_string(buf, |b| read_until(self, b'\n', b))
1975 /// Returns an iterator over the contents of this reader split on the byte
1978 /// The iterator returned from this function will return instances of
1979 /// [`io::Result`]`<`[`Vec<u8>`]`>`. Each vector returned will *not* have
1980 /// the delimiter byte at the end.
1982 /// This function will yield errors whenever [`read_until`] would have
1983 /// also yielded an error.
1985 /// [`io::Result`]: self::Result
1986 /// [`Vec<u8>`]: Vec
1987 /// [`read_until`]: BufRead::read_until
1991 /// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
1992 /// this example, we use [`Cursor`] to iterate over all hyphen delimited
1993 /// segments in a byte slice
1996 /// use std::io::{self, BufRead};
1998 /// let cursor = io::Cursor::new(b"lorem-ipsum-dolor");
2000 /// let mut split_iter = cursor.split(b'-').map(|l| l.unwrap());
2001 /// assert_eq!(split_iter.next(), Some(b"lorem".to_vec()));
2002 /// assert_eq!(split_iter.next(), Some(b"ipsum".to_vec()));
2003 /// assert_eq!(split_iter.next(), Some(b"dolor".to_vec()));
2004 /// assert_eq!(split_iter.next(), None);
2006 #[stable(feature = "rust1", since = "1.0.0")]
2007 fn split(self, byte: u8) -> Split<Self>
2011 Split { buf: self, delim: byte }
2014 /// Returns an iterator over the lines of this reader.
2016 /// The iterator returned from this function will yield instances of
2017 /// [`io::Result`]`<`[`String`]`>`. Each string returned will *not* have a newline
2018 /// byte (the `0xA` byte) or `CRLF` (`0xD`, `0xA` bytes) at the end.
2020 /// [`io::Result`]: self::Result
2024 /// [`std::io::Cursor`][`Cursor`] is a type that implements `BufRead`. In
2025 /// this example, we use [`Cursor`] to iterate over all the lines in a byte
2029 /// use std::io::{self, BufRead};
2031 /// let cursor = io::Cursor::new(b"lorem\nipsum\r\ndolor");
2033 /// let mut lines_iter = cursor.lines().map(|l| l.unwrap());
2034 /// assert_eq!(lines_iter.next(), Some(String::from("lorem")));
2035 /// assert_eq!(lines_iter.next(), Some(String::from("ipsum")));
2036 /// assert_eq!(lines_iter.next(), Some(String::from("dolor")));
2037 /// assert_eq!(lines_iter.next(), None);
2042 /// Each line of the iterator has the same error semantics as [`BufRead::read_line`].
2043 #[stable(feature = "rust1", since = "1.0.0")]
2044 fn lines(self) -> Lines<Self>
2052 /// Adaptor to chain together two readers.
2054 /// This struct is generally created by calling [`chain`] on a reader.
2055 /// Please see the documentation of [`chain`] for more details.
2057 /// [`chain`]: Read::chain
2058 #[stable(feature = "rust1", since = "1.0.0")]
2059 pub struct Chain<T, U> {
2065 impl<T, U> Chain<T, U> {
2066 /// Consumes the `Chain`, returning the wrapped readers.
2072 /// use std::io::prelude::*;
2073 /// use std::fs::File;
2075 /// fn main() -> io::Result<()> {
2076 /// let mut foo_file = File::open("foo.txt")?;
2077 /// let mut bar_file = File::open("bar.txt")?;
2079 /// let chain = foo_file.chain(bar_file);
2080 /// let (foo_file, bar_file) = chain.into_inner();
2084 #[stable(feature = "more_io_inner_methods", since = "1.20.0")]
2085 pub fn into_inner(self) -> (T, U) {
2086 (self.first, self.second)
2089 /// Gets references to the underlying readers in this `Chain`.
2095 /// use std::io::prelude::*;
2096 /// use std::fs::File;
2098 /// fn main() -> io::Result<()> {
2099 /// let mut foo_file = File::open("foo.txt")?;
2100 /// let mut bar_file = File::open("bar.txt")?;
2102 /// let chain = foo_file.chain(bar_file);
2103 /// let (foo_file, bar_file) = chain.get_ref();
2107 #[stable(feature = "more_io_inner_methods", since = "1.20.0")]
2108 pub fn get_ref(&self) -> (&T, &U) {
2109 (&self.first, &self.second)
2112 /// Gets mutable references to the underlying readers in this `Chain`.
2114 /// Care should be taken to avoid modifying the internal I/O state of the
2115 /// underlying readers as doing so may corrupt the internal state of this
2122 /// use std::io::prelude::*;
2123 /// use std::fs::File;
2125 /// fn main() -> io::Result<()> {
2126 /// let mut foo_file = File::open("foo.txt")?;
2127 /// let mut bar_file = File::open("bar.txt")?;
2129 /// let mut chain = foo_file.chain(bar_file);
2130 /// let (foo_file, bar_file) = chain.get_mut();
2134 #[stable(feature = "more_io_inner_methods", since = "1.20.0")]
2135 pub fn get_mut(&mut self) -> (&mut T, &mut U) {
2136 (&mut self.first, &mut self.second)
2140 #[stable(feature = "std_debug", since = "1.16.0")]
2141 impl<T: fmt::Debug, U: fmt::Debug> fmt::Debug for Chain<T, U> {
2142 fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
2143 f.debug_struct("Chain").field("t", &self.first).field("u", &self.second).finish()
2147 #[stable(feature = "rust1", since = "1.0.0")]
2148 impl<T: Read, U: Read> Read for Chain<T, U> {
2149 fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
2150 if !self.done_first {
2151 match self.first.read(buf)? {
2152 0 if !buf.is_empty() => self.done_first = true,
2156 self.second.read(buf)
2159 fn read_vectored(&mut self, bufs: &mut [IoSliceMut<'_>]) -> Result<usize> {
2160 if !self.done_first {
2161 match self.first.read_vectored(bufs)? {
2162 0 if bufs.iter().any(|b| !b.is_empty()) => self.done_first = true,
2166 self.second.read_vectored(bufs)
2169 unsafe fn initializer(&self) -> Initializer {
2170 let initializer = self.first.initializer();
2171 if initializer.should_initialize() { initializer } else { self.second.initializer() }
2175 #[stable(feature = "chain_bufread", since = "1.9.0")]
2176 impl<T: BufRead, U: BufRead> BufRead for Chain<T, U> {
2177 fn fill_buf(&mut self) -> Result<&[u8]> {
2178 if !self.done_first {
2179 match self.first.fill_buf()? {
2180 buf if buf.is_empty() => {
2181 self.done_first = true;
2183 buf => return Ok(buf),
2186 self.second.fill_buf()
2189 fn consume(&mut self, amt: usize) {
2190 if !self.done_first { self.first.consume(amt) } else { self.second.consume(amt) }
2194 /// Reader adaptor which limits the bytes read from an underlying reader.
2196 /// This struct is generally created by calling [`take`] on a reader.
2197 /// Please see the documentation of [`take`] for more details.
2199 /// [`take`]: Read::take
2200 #[stable(feature = "rust1", since = "1.0.0")]
2202 pub struct Take<T> {
2208 /// Returns the number of bytes that can be read before this instance will
2213 /// This instance may reach `EOF` after reading fewer bytes than indicated by
2214 /// this method if the underlying [`Read`] instance reaches EOF.
2220 /// use std::io::prelude::*;
2221 /// use std::fs::File;
2223 /// fn main() -> io::Result<()> {
2224 /// let f = File::open("foo.txt")?;
2226 /// // read at most five bytes
2227 /// let handle = f.take(5);
2229 /// println!("limit: {}", handle.limit());
2233 #[stable(feature = "rust1", since = "1.0.0")]
2234 pub fn limit(&self) -> u64 {
2238 /// Sets the number of bytes that can be read before this instance will
2239 /// return EOF. This is the same as constructing a new `Take` instance, so
2240 /// the amount of bytes read and the previous limit value don't matter when
2241 /// calling this method.
2247 /// use std::io::prelude::*;
2248 /// use std::fs::File;
2250 /// fn main() -> io::Result<()> {
2251 /// let f = File::open("foo.txt")?;
2253 /// // read at most five bytes
2254 /// let mut handle = f.take(5);
2255 /// handle.set_limit(10);
2257 /// assert_eq!(handle.limit(), 10);
2261 #[stable(feature = "take_set_limit", since = "1.27.0")]
2262 pub fn set_limit(&mut self, limit: u64) {
2266 /// Consumes the `Take`, returning the wrapped reader.
2272 /// use std::io::prelude::*;
2273 /// use std::fs::File;
2275 /// fn main() -> io::Result<()> {
2276 /// let mut file = File::open("foo.txt")?;
2278 /// let mut buffer = [0; 5];
2279 /// let mut handle = file.take(5);
2280 /// handle.read(&mut buffer)?;
2282 /// let file = handle.into_inner();
2286 #[stable(feature = "io_take_into_inner", since = "1.15.0")]
2287 pub fn into_inner(self) -> T {
2291 /// Gets a reference to the underlying reader.
2297 /// use std::io::prelude::*;
2298 /// use std::fs::File;
2300 /// fn main() -> io::Result<()> {
2301 /// let mut file = File::open("foo.txt")?;
2303 /// let mut buffer = [0; 5];
2304 /// let mut handle = file.take(5);
2305 /// handle.read(&mut buffer)?;
2307 /// let file = handle.get_ref();
2311 #[stable(feature = "more_io_inner_methods", since = "1.20.0")]
2312 pub fn get_ref(&self) -> &T {
2316 /// Gets a mutable reference to the underlying reader.
2318 /// Care should be taken to avoid modifying the internal I/O state of the
2319 /// underlying reader as doing so may corrupt the internal limit of this
2326 /// use std::io::prelude::*;
2327 /// use std::fs::File;
2329 /// fn main() -> io::Result<()> {
2330 /// let mut file = File::open("foo.txt")?;
2332 /// let mut buffer = [0; 5];
2333 /// let mut handle = file.take(5);
2334 /// handle.read(&mut buffer)?;
2336 /// let file = handle.get_mut();
2340 #[stable(feature = "more_io_inner_methods", since = "1.20.0")]
2341 pub fn get_mut(&mut self) -> &mut T {
2346 #[stable(feature = "rust1", since = "1.0.0")]
2347 impl<T: Read> Read for Take<T> {
2348 fn read(&mut self, buf: &mut [u8]) -> Result<usize> {
2349 // Don't call into inner reader at all at EOF because it may still block
2350 if self.limit == 0 {
2354 let max = cmp::min(buf.len() as u64, self.limit) as usize;
2355 let n = self.inner.read(&mut buf[..max])?;
2356 self.limit -= n as u64;
2360 unsafe fn initializer(&self) -> Initializer {
2361 self.inner.initializer()
2364 fn read_to_end(&mut self, buf: &mut Vec<u8>) -> Result<usize> {
2365 // Pass in a reservation_size closure that respects the current value
2366 // of limit for each read. If we hit the read limit, this prevents the
2367 // final zero-byte read from allocating again.
2368 read_to_end_with_reservation(self, buf, |self_| cmp::min(self_.limit, 32) as usize)
2372 #[stable(feature = "rust1", since = "1.0.0")]
2373 impl<T: BufRead> BufRead for Take<T> {
2374 fn fill_buf(&mut self) -> Result<&[u8]> {
2375 // Don't call into inner reader at all at EOF because it may still block
2376 if self.limit == 0 {
2380 let buf = self.inner.fill_buf()?;
2381 let cap = cmp::min(buf.len() as u64, self.limit) as usize;
2385 fn consume(&mut self, amt: usize) {
2386 // Don't let callers reset the limit by passing an overlarge value
2387 let amt = cmp::min(amt as u64, self.limit) as usize;
2388 self.limit -= amt as u64;
2389 self.inner.consume(amt);
2393 /// An iterator over `u8` values of a reader.
2395 /// This struct is generally created by calling [`bytes`] on a reader.
2396 /// Please see the documentation of [`bytes`] for more details.
2398 /// [`bytes`]: Read::bytes
2399 #[stable(feature = "rust1", since = "1.0.0")]
2401 pub struct Bytes<R> {
2405 #[stable(feature = "rust1", since = "1.0.0")]
2406 impl<R: Read> Iterator for Bytes<R> {
2407 type Item = Result<u8>;
2409 fn next(&mut self) -> Option<Result<u8>> {
2412 return match self.inner.read(slice::from_mut(&mut byte)) {
2414 Ok(..) => Some(Ok(byte)),
2415 Err(ref e) if e.kind() == ErrorKind::Interrupted => continue,
2416 Err(e) => Some(Err(e)),
2422 /// An iterator over the contents of an instance of `BufRead` split on a
2423 /// particular byte.
2425 /// This struct is generally created by calling [`split`] on a `BufRead`.
2426 /// Please see the documentation of [`split`] for more details.
2428 /// [`split`]: BufRead::split
2429 #[stable(feature = "rust1", since = "1.0.0")]
2431 pub struct Split<B> {
2436 #[stable(feature = "rust1", since = "1.0.0")]
2437 impl<B: BufRead> Iterator for Split<B> {
2438 type Item = Result<Vec<u8>>;
2440 fn next(&mut self) -> Option<Result<Vec<u8>>> {
2441 let mut buf = Vec::new();
2442 match self.buf.read_until(self.delim, &mut buf) {
2445 if buf[buf.len() - 1] == self.delim {
2450 Err(e) => Some(Err(e)),
2455 /// An iterator over the lines of an instance of `BufRead`.
2457 /// This struct is generally created by calling [`lines`] on a `BufRead`.
2458 /// Please see the documentation of [`lines`] for more details.
2460 /// [`lines`]: BufRead::lines
2461 #[stable(feature = "rust1", since = "1.0.0")]
2463 pub struct Lines<B> {
2467 #[stable(feature = "rust1", since = "1.0.0")]
2468 impl<B: BufRead> Iterator for Lines<B> {
2469 type Item = Result<String>;
2471 fn next(&mut self) -> Option<Result<String>> {
2472 let mut buf = String::new();
2473 match self.buf.read_line(&mut buf) {
2476 if buf.ends_with('\n') {
2478 if buf.ends_with('\r') {
2484 Err(e) => Some(Err(e)),