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Showing content from https://doc.rust-lang.org/std/io/trait.Seek.html below:

Seek in std::io - Rust

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.

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")?;

    f.seek(SeekFrom::Start(42))?;
    Ok(())
}

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.

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

Seeking to a negative offset is considered an error.

Rewind to the beginning of a stream.

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

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

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

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

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

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

Returns the length of this stream (in bytes).

The default implementation uses 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.

#![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(())
}

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

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

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(())
}

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.

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(())
}

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