Trait std::io::Write
pub trait Write { fn write(&mut self, buf: &[u8]) -> Result<usize>; fn flush(&mut self) -> Result<()>; fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize> { ... } fn is_write_vectored(&self) -> bool { ... } fn write_all(&mut self, buf: &[u8]) -> Result<()> { ... } fn write_all_vectored(&mut self, bufs: &mut [IoSlice<'_>]) -> Result<()> { ... } fn write_fmt(&mut self, fmt: Arguments<'_>) -> Result<()> { ... } fn by_ref(&mut self) -> &mut Self where Self: Sized, { ... } }
A trait for objects which are byte-oriented sinks.
Implementors of the Write
trait are sometimes called ‘writers’.
Writers are defined by two required methods, write
and flush
:
-
The
write
method will attempt to write some data into the object, returning how many bytes were successfully written. -
The
flush
method is useful for adapters and explicit buffers themselves for ensuring that all buffered data has been pushed out to the ‘true sink’.
Writers are intended to be composable with one another. Many implementors throughout std::io
take and provide types which implement the Write
trait.
Examples
use std::io::prelude::*; use std::fs::File; fn main() -> std::io::Result<()> { let data = b"some bytes"; let mut pos = 0; let mut buffer = File::create("foo.txt")?; while pos < data.len() { let bytes_written = buffer.write(&data[pos..])?; pos += bytes_written; } Ok(()) }
The trait also provides convenience methods like write_all
, which calls write
in a loop until its entire input has been written.
Required methods
fn write(&mut self, buf: &[u8]) -> Result<usize>
Write a buffer into this writer, returning how many bytes were written.
This function will attempt to write the entire contents of buf
, but the entire write might not succeed, or the write may also generate an error. A call to write
represents at most one attempt to write to any wrapped object.
Calls to write
are not guaranteed to block waiting for data to be written, and a write which would otherwise block can be indicated through an Err
variant.
If the return value is Ok(n)
then it must be guaranteed that n <= buf.len()
. A return value of 0
typically means that the underlying object is no longer able to accept bytes and will likely not be able to in the future as well, or that the buffer provided is empty.
Errors
Each call to write
may generate an I/O error indicating that the operation could not be completed. If an error is returned then no bytes in the buffer were written to this writer.
It is not considered an error if the entire buffer could not be written to this writer.
An error of the ErrorKind::Interrupted
kind is non-fatal and the write operation should be retried if there is nothing else to do.
Examples
use std::io::prelude::*; use std::fs::File; fn main() -> std::io::Result<()> { let mut buffer = File::create("foo.txt")?; // Writes some prefix of the byte string, not necessarily all of it. buffer.write(b"some bytes")?; Ok(()) }
fn flush(&mut self) -> Result<()>
Flush this output stream, ensuring that all intermediately buffered contents reach their destination.
Errors
It is considered an error if not all bytes could be written due to I/O errors or EOF being reached.
Examples
use std::io::prelude::*; use std::io::BufWriter; use std::fs::File; fn main() -> std::io::Result<()> { let mut buffer = BufWriter::new(File::create("foo.txt")?); buffer.write_all(b"some bytes")?; buffer.flush()?; Ok(()) }
Provided methods
fn write_vectored(&mut self, bufs: &[IoSlice<'_>]) -> Result<usize>
Like write
, except that it writes from a slice of buffers.
Data is copied from each buffer in order, with the final buffer read from possibly being only partially consumed. This method must behave as a call to write
with the buffers concatenated would.
The default implementation calls write
with either the first nonempty buffer provided, or an empty one if none exists.
Examples
use std::io::IoSlice; use std::io::prelude::*; use std::fs::File; fn main() -> std::io::Result<()> { let mut data1 = [1; 8]; let mut data2 = [15; 8]; let io_slice1 = IoSlice::new(&mut data1); let io_slice2 = IoSlice::new(&mut data2); let mut buffer = File::create("foo.txt")?; // Writes some prefix of the byte string, not necessarily all of it. buffer.write_vectored(&[io_slice1, io_slice2])?; Ok(()) }
fn is_write_vectored(&self) -> bool
Determines if this Write
r has an efficient write_vectored
implementation.
If a Write
r does not override the default write_vectored
implementation, code using it may want to avoid the method all together and coalesce writes into a single buffer for higher performance.
The default implementation returns false
.
fn write_all(&mut self, buf: &[u8]) -> Result<()>
Attempts to write an entire buffer into this writer.
This method will continuously call write
until there is no more data to be written or an error of non-ErrorKind::Interrupted
kind is returned. This method will not return until the entire buffer has been successfully written or such an error occurs. The first error that is not of ErrorKind::Interrupted
kind generated from this method will be returned.
If the buffer contains no data, this will never call write
.
Errors
This function will return the first error of non-ErrorKind::Interrupted
kind that write
returns.
Examples
use std::io::prelude::*; use std::fs::File; fn main() -> std::io::Result<()> { let mut buffer = File::create("foo.txt")?; buffer.write_all(b"some bytes")?; Ok(()) }
fn write_all_vectored(&mut self, bufs: &mut [IoSlice<'_>]) -> Result<()>
Attempts to write multiple buffers into this writer.
This method will continuously call write_vectored
until there is no more data to be written or an error of non-ErrorKind::Interrupted
kind is returned. This method will not return until all buffers have been successfully written or such an error occurs. The first error that is not of ErrorKind::Interrupted
kind generated from this method will be returned.
If the buffer contains no data, this will never call write_vectored
.
Notes
Unlike write_vectored
, this takes a mutable reference to a slice of IoSlice
s, not an immutable one. That’s because we need to modify the slice to keep track of the bytes already written.
Once this function returns, the contents of bufs
are unspecified, as this depends on how many calls to write_vectored
were necessary. It is best to understand this function as taking ownership of bufs
and to not use bufs
afterwards. The underlying buffers, to which the IoSlice
s point (but not the IoSlice
s themselves), are unchanged and can be reused.
Examples
#![feature(write_all_vectored)] use std::io::{Write, IoSlice}; let mut writer = Vec::new(); let bufs = &mut [ IoSlice::new(&[1]), IoSlice::new(&[2, 3]), IoSlice::new(&[4, 5, 6]), ]; writer.write_all_vectored(bufs)?; // Note: the contents of `bufs` is now undefined, see the Notes section. assert_eq!(writer, &[1, 2, 3, 4, 5, 6]);
fn write_fmt(&mut self, fmt: Arguments<'_>) -> Result<()>
Writes a formatted string into this writer, returning any error encountered.
This method is primarily used to interface with the format_args!()
macro, but it is rare that this should explicitly be called. The write!()
macro should be favored to invoke this method instead.
This function internally uses the write_all
method on this trait and hence will continuously write data so long as no errors are received. This also means that partial writes are not indicated in this signature.
Errors
This function will return any I/O error reported while formatting.
Examples
use std::io::prelude::*; use std::fs::File; fn main() -> std::io::Result<()> { let mut buffer = File::create("foo.txt")?; // this call write!(buffer, "{:.*}", 2, 1.234567)?; // turns into this: buffer.write_fmt(format_args!("{:.*}", 2, 1.234567))?; Ok(()) }
Creates a “by reference” adapter for this instance of Write
.
The returned adapter also implements Write
and will simply borrow this current writer.
Examples
use std::io::Write; use std::fs::File; fn main() -> std::io::Result<()> { let mut buffer = File::create("foo.txt")?; let reference = buffer.by_ref(); // we can use reference just like our original buffer reference.write_all(b"some bytes")?; Ok(()) }
Implementors
impl Write for &File
impl Write for &TcpStream
impl Write for &ChildStdin
impl Write for &Sink
impl Write for &Stderr
impl Write for &Stdout
impl Write for &mut [u8]
Write is implemented for &mut [u8]
by copying into the slice, overwriting its data.
Note that writing updates the slice to point to the yet unwritten part. The slice will be empty when it has been completely overwritten.
If the number of bytes to be written exceeds the size of the slice, write operations will return short writes: ultimately, Ok(0)
; in this situation, write_all
returns an error of kind ErrorKind::WriteZero
.
impl Write for File
impl Write for TcpStream
impl Write for ChildStdin
impl Write for Cursor<&mut Vec<u8>>
impl Write for Cursor<&mut [u8]>
impl Write for Cursor<Box<[u8]>>
impl Write for Cursor<Vec<u8>>
impl Write for Sink
impl Write for Stderr
impl Write for StderrLock<'_>
impl Write for Stdout
impl Write for StdoutLock<'_>
impl<A: Allocator> Write for Vec<u8, A>
Write is implemented for Vec<u8>
by appending to the vector. The vector will grow as needed.
impl<W: Write + ?Sized> Write for &mut W
impl<W: Write + ?Sized> Write for Box<W>
impl<W: Write> Write for BufWriter<W>
impl<W: Write> Write for LineWriter<W>
© 2010 The Rust Project Developers
Licensed under the Apache License, Version 2.0 or the MIT license, at your option.
https://doc.rust-lang.org/std/io/trait.Write.html