# Trait std::io::Write

1.0.0 · source ·

    pub trait Write {
        // Required methods
        fn write(&mut self, buf: &[u8]) -> Result<usize>;
        fn flush(&mut self) -> Result<()>;

        // Provided methods
        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 { ... }
    }

Expand description

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(())
    }

Run

The trait also provides convenience methods like `write_all`, which calls `write` in a loop until its entire input has been written.

## Required Methods§

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#### 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. Typically, a call to `write` represents 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 this method consumed `n > 0` bytes of `buf` it must return `Ok(n)`. If the return value is `Ok(n)` then `n` must satisfy `n <= buf.len()`. A return value of `Ok(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(())
    }

Run
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#### 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(())
    }

Run

## Provided Methods§

1.36.0 · source

#### 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 data1 = [1; 8];
        let data2 = [15; 8];
        let io_slice1 = IoSlice::new(&data1);
        let io_slice2 = IoSlice::new(&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(())
    }

Run
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#### fn is_write_vectored(&self) -> bool

🔬This is a nightly-only experimental API. (`can_vector` #69941)

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`.

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#### 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(())
    }

Run
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#### fn write_all_vectored(&mut self, bufs: &mut [IoSlice<'_>]) -> Result<()>

🔬This is a nightly-only experimental API. (`write_all_vectored` #70436)

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]);

Run
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#### 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, and 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(())
    }

Run
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#### fn by_ref(&mut self) -> &mut Self where Self: Sized,

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(())
    }

Run

## Implementors§

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### impl Write for &File

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### impl Write for &TcpStream

1.48.0 · source§

### impl Write for &ChildStdin

1.73.0 · source§

### impl Write for &Empty

1.48.0 · source§

### impl Write for &Sink

1.48.0 · source§

### impl Write for &Stderr

1.48.0 · source§

### impl Write for &Stdout

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### 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`.

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### impl Write for File

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### impl Write for TcpStream

1.10.0 · source§

### impl Write for UnixStream

Available on Unix only.
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### impl Write for ChildStdin

1.73.0 · source§

### impl Write for Arc<File>

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### impl Write for Cursor<&mut [u8]>

1.73.0 · source§

### impl Write for Empty

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### impl Write for Sink

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### impl Write for Stderr

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### impl Write for StderrLock<'_>

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### impl Write for Stdout

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### impl Write for StdoutLock<'_>

1.10.0 · source§

### impl<'a> Write for &'a UnixStream

Available on Unix only.
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### impl<'a> Write for BorrowedCursor<'a>

1.25.0 · source§

### impl<A> Write for Cursor<&mut Vec<u8, A>>where A: Allocator,

1.5.0 · source§

### impl<A> Write for Cursor<Box<[u8], A>>where A: Allocator,

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### impl<A> Write for Cursor<Vec<u8, A>>where A: Allocator,

1.63.0 · source§

### impl<A: Allocator> Write for VecDeque<u8, A>

Write is implemented for `VecDeque<u8>` by appending to the `VecDeque`, growing it as needed.

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### 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.

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### impl<W: Write + ?Sized> Write for &mut W

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### impl<W: Write + ?Sized> Write for Box<W>

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### impl<W: ?Sized + Write> Write for BufWriter<W>

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### impl<W: ?Sized + Write> Write for LineWriter<W>

1.61.0 · source§

### impl<const N: usize> Write for Cursor<[u8; N]>