Seek in std::io - Rust
Description: The `Seek` trait provides a cursor which can be moved within a stream of bytes.

## Seek

# Trait Seek

1.0.0 · Source

    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<()> { ... }
    }

Expand description

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

`File`s 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§

1.0.0 · Source

#### 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§

1.55.0 · Source

#### 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")?;

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

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

Source

#### 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).

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.

##### §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(())
    }

1.51.0 · Source

#### 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(())
    }

1.80.0 · Source

#### 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§

1.0.0 · Source§

### impl Seek for &File

1.0.0 · Source§

### impl Seek for File

1.73.0 · Source§

### impl Seek for Arc<File>

1.51.0 · Source§

### impl Seek for Empty

1.0.0 · Source§

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

1.0.0 · Source§

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

1.0.0 · Source§

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

1.0.0 · Source§

### impl<T> Seek for Cursor<T>
where T: AsRef<[u8]>,

1.89.0 · Source§

### impl<T: Seek> Seek for Take<T>

1.0.0 · Source§

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