Trait FileExt

pub trait FileExt {
    // Required methods
    fn read_at(&self, buf: &mut [u8], offset: u64) -> Result<usize>;
    fn write_at(&self, buf: &[u8], offset: u64) -> Result<usize>;

    // Provided methods
    fn read_vectored_at(
        &self,
        bufs: &mut [IoSliceMut<'_>],
        offset: u64,
    ) -> Result<usize> { ... }
    fn read_exact_at(&self, buf: &mut [u8], offset: u64) -> Result<()> { ... }
    fn write_vectored_at(
        &self,
        bufs: &[IoSlice<'_>],
        offset: u64,
    ) -> Result<usize> { ... }
    fn write_all_at(&self, buf: &[u8], offset: u64) -> Result<()> { ... }
}

Available on Unix only.

Unix-specific extensions to fs::File.

Required Methods

fn read_at(&self, buf: &mut [u8], offset: u64) -> Result<usize>

Reads a number of bytes starting from a given offset.

Returns the number of bytes read.

The offset is relative to the start of the file and thus independent from the current cursor.

The current file cursor is not affected by this function.

Note that similar to File::read, it is not an error to return with a short read.

Examples

use std::io;
use std::fs::File;
use std::os::unix::prelude::FileExt;

fn main() -> io::Result<()> {
    let mut buf = [0u8; 8];
    let file = File::open("foo.txt")?;

    // We now read 8 bytes from the offset 10.
    let num_bytes_read = file.read_at(&mut buf, 10)?;
    println!("read {num_bytes_read} bytes: {buf:?}");
    Ok(())
}

fn write_at(&self, buf: &[u8], offset: u64) -> Result<usize>

Writes a number of bytes starting from a given offset.

Returns the number of bytes written.

The offset is relative to the start of the file and thus independent from the current cursor.

The current file cursor is not affected by this function.

When writing beyond the end of the file, the file is appropriately extended and the intermediate bytes are initialized with the value 0.

Note that similar to File::write, it is not an error to return a short write.

Bug

On some systems, write_at utilises pwrite64 to write to files. However, this syscall has a bug where files opened with the O_APPEND flag fail to respect the offset parameter, always appending to the end of the file instead.

It is possible to inadvertently set this flag, like in the example below. Therefore, it is important to be vigilant while changing options to mitigate unexpected behaviour.

use std::fs::File;
use std::io;
use std::os::unix::prelude::FileExt;

fn main() -> io::Result<()> {
    // Open a file with the append option (sets the `O_APPEND` flag)
    let file = File::options().append(true).open("foo.txt")?;

    // We attempt to write at offset 10; instead appended to EOF
    file.write_at(b"sushi", 10)?;

    // foo.txt is 5 bytes long instead of 15
    Ok(())
}

Examples

use std::fs::File;
use std::io;
use std::os::unix::prelude::FileExt;

fn main() -> io::Result<()> {
    let file = File::create("foo.txt")?;

    // We now write at the offset 10.
    file.write_at(b"sushi", 10)?;
    Ok(())
}

Provided Methods

fn read_vectored_at( &self, bufs: &mut [IoSliceMut<'_>], offset: u64, ) -> Result<usize>

🔬This is a nightly-only experimental API. (unix_file_vectored_at #89517)

Like read_at, except that it reads into a slice of buffers.

Data is copied to fill each buffer in order, with the final buffer written to possibly being only partially filled. This method must behave equivalently to a single call to read with concatenated buffers.

fn read_exact_at(&self, buf: &mut [u8], offset: u64) -> Result<()>

Reads the exact number of bytes required to fill buf from the given offset.

The offset is relative to the start of the file and thus independent from the current cursor.

The current file cursor is not affected by this function.

Similar to io::Read::read_exact but uses read_at instead of read.

Errors

If this function encounters an error of the kind io::ErrorKind::Interrupted then the error is ignored and the operation will continue.

If this function encounters an “end of file” before completely filling the buffer, it returns an error of the kind io::ErrorKind::UnexpectedEof. The contents of buf are unspecified in this case.

If any other read error is encountered then this function immediately returns. The contents of buf are unspecified in this case.

If this function returns an error, it is unspecified how many bytes it has read, but it will never read more than would be necessary to completely fill the buffer.

Examples

use std::io;
use std::fs::File;
use std::os::unix::prelude::FileExt;

fn main() -> io::Result<()> {
    let mut buf = [0u8; 8];
    let file = File::open("foo.txt")?;

    // We now read exactly 8 bytes from the offset 10.
    file.read_exact_at(&mut buf, 10)?;
    println!("read {} bytes: {:?}", buf.len(), buf);
    Ok(())
}

fn write_vectored_at(&self, bufs: &[IoSlice<'_>], offset: u64) -> Result<usize>

🔬This is a nightly-only experimental API. (unix_file_vectored_at #89517)

Like write_at, 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_at with the buffers concatenated would.

fn write_all_at(&self, buf: &[u8], offset: u64) -> Result<()>

Attempts to write an entire buffer starting from a given offset.

The offset is relative to the start of the file and thus independent from the current cursor.

The current file cursor is not affected by this function.

This method will continuously call write_at until there is no more data to be written or an error of non-io::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 io::ErrorKind::Interrupted kind generated from this method will be returned.

Errors

This function will return the first error of non-io::ErrorKind::Interrupted kind that write_at returns.

Examples

use std::fs::File;
use std::io;
use std::os::unix::prelude::FileExt;

fn main() -> io::Result<()> {
    let file = File::open("foo.txt")?;

    // We now write at the offset 10.
    file.write_all_at(b"sushi", 10)?;
    Ok(())
}

Implementors

impl FileExt for File