tokio::fs

Do async file I/O with tokio::fs.

tokio::fs

tokio::fs offers async functions with the same names as std::fs. Behind the scenes it runs the operations on a separate thread pool, so the task scheduler doesn't block:

use tokio::fs;

fs::write("selam.txt", "merhaba").await?;
let icerik = fs::read_to_string("selam.txt").await?;
println!("{} bayt", icerik.len());

For many small file operations, calling std in bulk inside spawn_blocking is often faster.

The same operation with std

In the exercise we do the same steps with std; the APIs being identical makes moving code to async easy:

use std::{env, fs, io};

fn main() -> io::Result<()> {
    let yol = env::temp_dir().join("selam.txt");
    fs::write(&yol, "merhaba")?;
    let icerik = fs::read_to_string(&yol)?;
    println!("{} bayt", icerik.len());
    Ok(())
}

Exercise Write and read

Let main return io::Result<()>. Write "merhaba" to selam.txt in the temp folder, read it back and print N bayt.

Show example solution

use std::{env, fs, io};

fn main() -> io::Result<()> {
    let yol = env::temp_dir().join("selam.txt");
    fs::write(&yol, "merhaba")?;
    let icerik = fs::read_to_string(&yol)?;
    println!("{} bayt", icerik.len());
    Ok(())
}

Test yourself

Why does the operating system not offer a truly asynchronous interface for file I/O (on most platforms)?
- Rust doesn't support it
- There's no need
- Classic file calls are blocking; tokio runs them on a thread pool
- Files are too small
Show answer

Classic file calls are blocking; tokio runs them on a thread pool — Network sockets are waited on without blocking via epoll/kqueue; for files there are newer interfaces like io_uring.

Async read

let icerik = tokio::fs::read_to_string("selam.txt").____?;

Show answer

let icerik = tokio::fs::read_to_string("selam.txt").await?;

std and tokio counterpart

std::fs::write

tokio::fs::write(..).await
std::thread::sleep

tokio::time::sleep(..).await
std::net::TcpListener

tokio::net::TcpListener
thread::spawn

tokio::spawn

Next lesson: select!

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