Crate blocking

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A thread pool for isolating blocking I/O in async programs.

Sometimes there’s no way to avoid blocking I/O. Consider files or stdin, which have weak async support on modern operating systems. While IOCP, AIO, and io_uring are possible solutions, they’re not always available or ideal.

Since blocking is not allowed inside futures, we must move blocking I/O onto a special thread pool provided by this crate. The pool dynamically spawns and stops threads depending on the current number of running I/O jobs.

Note that there is a limit on the number of active threads. Once that limit is hit, a running job has to finish before others get a chance to run. When a thread is idle, it waits for the next job or shuts down after a certain timeout.

The default number of threads (set to 500) can be altered by setting BLOCKING_MAX_THREADS environment variable with value between 1 and 10000.


Read the contents of a file:

use blocking::unblock;
use std::fs;

let contents = unblock(|| fs::read_to_string("file.txt")).await?;
println!("{}", contents);

Read a file and pipe its contents to stdout:

use blocking::{unblock, Unblock};
use futures_lite::io;
use std::fs::File;

let input = unblock(|| File::open("file.txt")).await?;
let input = Unblock::new(input);
let mut output = Unblock::new(std::io::stdout());

io::copy(input, &mut output).await?;

Iterate over the contents of a directory:

use blocking::Unblock;
use futures_lite::prelude::*;
use std::fs;

let mut dir = Unblock::new(fs::read_dir(".")?);
while let Some(item) = {
    println!("{}", item?.file_name().to_string_lossy());

Spawn a process:

use blocking::unblock;
use std::process::Command;

let out = unblock(|| Command::new("dir").output()).await?;


  • pub use async_task::Task;


  • Runs blocking I/O on a thread pool.


  • Runs blocking code on a thread pool.