Trait Seek

pub trait Seek {
    // Required method
    fn seek(&mut self, pos: SeekFrom) -> Result<u64>;

    // Provided methods
    fn rewind(&mut self) -> Result<()> { ... }
    fn stream_len(&mut self) -> Result<u64> { ... }
    fn stream_position(&mut self) -> Result<u64> { ... }
    fn seek_relative(&mut self, offset: i64) -> Result<()> { ... }
}

The Seek trait provides a cursor which can be moved within a stream of bytes.

The stream typically has a fixed size, allowing seeking relative to either end or the current offset.

Examples

Files implement Seek:

use std::io;
use std::io::prelude::*;
use std::fs::File;
use std::io::SeekFrom;

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

    // move the cursor 42 bytes from the start of the file
    f.seek(SeekFrom::Start(42))?;
    Ok(())
}

Required Methods

fn seek(&mut self, pos: SeekFrom) -> Result<u64>

Seek to an offset, in bytes, in a stream.

A seek beyond the end of a stream is allowed, but behavior is defined by the implementation.

If the seek operation completed successfully, this method returns the new position from the start of the stream. That position can be used later with SeekFrom::Start.

Errors

Seeking can fail, for example because it might involve flushing a buffer.

Seeking to a negative offset is considered an error.

Provided Methods

fn rewind(&mut self) -> Result<()>

Rewind to the beginning of a stream.

This is a convenience method, equivalent to seek(SeekFrom::Start(0)).

Errors

Rewinding can fail, for example because it might involve flushing a buffer.

Example

use std::io::{Read, Seek, Write};
use std::fs::OpenOptions;

let mut f = OpenOptions::new()
    .write(true)
    .read(true)
    .create(true)
    .open("foo.txt").unwrap();

let hello = "Hello!\n";
write!(f, "{hello}").unwrap();
f.rewind().unwrap();

let mut buf = String::new();
f.read_to_string(&mut buf).unwrap();
assert_eq!(&buf, hello);

fn stream_len(&mut self) -> Result<u64>

🔬This is a nightly-only experimental API. (seek_stream_len #59359)

Returns the length of this stream (in bytes).

This method is implemented using up to three seek operations. If this method returns successfully, the seek position is unchanged (i.e. the position before calling this method is the same as afterwards). However, if this method returns an error, the seek position is unspecified.

If you need to obtain the length of many streams and you don’t care about the seek position afterwards, you can reduce the number of seek operations by simply calling seek(SeekFrom::End(0)) and using its return value (it is also the stream length).

Note that length of a stream can change over time (for example, when data is appended to a file). So calling this method multiple times does not necessarily return the same length each time.

Example

#![feature(seek_stream_len)]
use std::{
    io::{self, Seek},
    fs::File,
};

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

    let len = f.stream_len()?;
    println!("The file is currently {len} bytes long");
    Ok(())
}

fn stream_position(&mut self) -> Result<u64>

Returns the current seek position from the start of the stream.

This is equivalent to self.seek(SeekFrom::Current(0)).

Example

use std::{
    io::{self, BufRead, BufReader, Seek},
    fs::File,
};

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

    let before = f.stream_position()?;
    f.read_line(&mut String::new())?;
    let after = f.stream_position()?;

    println!("The first line was {} bytes long", after - before);
    Ok(())
}

fn seek_relative(&mut self, offset: i64) -> Result<()>

Seeks relative to the current position.

This is equivalent to self.seek(SeekFrom::Current(offset)) but doesn’t return the new position which can allow some implementations such as BufReader to perform more efficient seeks.

Example

use std::{
    io::{self, Seek},
    fs::File,
};

fn main() -> io::Result<()> {
    let mut f = File::open("foo.txt")?;
    f.seek_relative(10)?;
    assert_eq!(f.stream_position()?, 10);
    Ok(())
}

Implementors

impl Seek for &File

impl Seek for File

impl Seek for Arc<File>

impl Seek for Empty

impl<R: ?Sized + Seek> Seek for BufReader<R>

impl<S: Seek + ?Sized> Seek for &mut S

impl<S: Seek + ?Sized> Seek for Box<S>

impl<T> Seek for Cursor<T>

where T: AsRef<[u8]>,

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