Installation

• std

  [dependencies]
  fs4 = { version = "1", features = ["sync"] }
  

• async-std runtime

  [dependencies]
  fs4 = { version = "1", features = ["async-std"] }
  

• fs-err v2

  [dependencies]
  fs4 = { version = "1", features = ["fs-err2"] }
  

• fs-err-tokio v2

  [dependencies]
  fs4 = { version = "1", features = ["fs-err2-tokio"] }
  

• fs-err v3

  [dependencies]
  fs4 = { version = "1", features = ["fs-err3"] }
  

• fs-err-tokio v3

  [dependencies]
  fs4 = { version = "1", features = ["fs-err3-tokio"] }
  

• smol runtime

  [dependencies]
  fs4 = { version = "1", features = ["smol"] }
  

• tokio runtime

  [dependencies]
  fs4 = { version = "1", features = ["tokio"] }
  

Features

• file locks.
• file (pre)allocation.
• file allocation information.
• filesystem space usage information.
• async-std support
• fs-err
• smol support
• tokio support

Minimum Supported Rust Version

fs4 itself compiles on Rust 1.75.0 (the value of rust-version in Cargo.toml), and that guarantee covers the default sync feature.

Some opt-in features inherit a higher MSRV from their transitive dependencies — enabling them requires whatever toolchain those crates ask for, not what fs4 declares:

These bounds are set by upstream and can drift with future minor releases; pin the relevant dependency if you need an older toolchain.

Platform Notes

Fuchsia

Every feature builds on Fuchsia except fs-err3 and fs-err3-tokio. The blocker is upstream: fs-err v3.3.0 calls std::os::unix::fs::chroot, which rustc gates out on target_os = "fuchsia". The fs4 Unix modules themselves compile on Fuchsia (sync, fs-err2, async-std, smol, tokio all work). Tracking issue: https://github.com/andrewhickman/fs-err/issues/90.

License

fs4 is primarily distributed under the terms of both the MIT license and the Apache License (Version 2.0).

See LICENSE-APACHE, LICENSE-MIT for details.

Copyright (c) 2026 Al Liu.

Copyright (c) 2015 Dan Burkert.