wasi

A Bytecode Alliance project

WASI API Bindings for Rust

This repository contains low-level Rust bindings to WASI APIs which are distributed and published to crates on crates.io. Crates currently are:

• wasi - this is a reexport of the latest stable WASI proposal. At this time this represents bindings to WASIp2.
• wasip2 - this crate explicitly contains bindings for the latest stable WASIp2 release generated by the latest stable wit-bindgen release.
• wasip1 - this crate explicitly contains bindings for the WASIp1 snapshot. Development of the WASIp1 version of the standard has ceased and this crate is in maintenance mode.

The wasi crate

The wasi crate today is a lightweight reexport of the latest stable version of the WASI standard. Currently that is WASIp2. The wasi crate version number will be bumped every timet he wasip2 crate version number is bumped, for example, and its interface and bindings may change over time as WASIp2 APIs are added or the wit-bindgen tool to generate bindings evolves.

To explicitly indicate which version of the WASI standard you'd like to use it's recommended to use the wasip2 crate directly.

Crate version numbers

The wasi crate contains "build metadata" which indicates what version of the WASI standard it contains bindings for. This metadata is purely informational and cannot be used to constrain a version requirement in Cargo. This scheme is mirrored for the wasip2 crate as well, for example.

WASIp2 vs WASIp1

In January 2024 the WASI subgroup published WASI 0.2.0, colloquially known as "WASIp2". Around the same time the subgroup additionally decided to name the previous iteration of WASI as "WASIp1", historically known as "WASI preview1". This now-historical snapshot of WASI was defined with an entirely different set of primitives and worked very differently. The interface of the wasip1 and wasip2 crates are entirely different and the wasi crate umbrella no longer reexports wasip1.

Should I use WASIp1 or WASIp2?

This is a bit of a nuanced question/answer but the short answer is to probably use the latest release of wasi if you're unsure.

The longer-form answer of this is that it depends on the Rust targets that you want to support. Rust WebAssembly targets include:

• wasm32-unknown-unknown - do not use this crate because this target indicates that WASI is not desired.
• wasm32-wasip1 - this target has been present in Rust for quite some time and was previously known as wasm32-wasi. For this target you probably want the wasip1 crate.
• wasm32-wasip2 - this target you can use either the wasi crate or the wasip2 crate depending on your use case. Using WASIp2 APIs on this target is more appropriate than using WASIp1 APIs.

Note that if you use wasm32-wasip1 it's not necessarily guaranteed you want the wasip1 crate. If your users are producing components then you probably want the wasip2 or wasi crates instead. If you don't know what your users are producing then you should probably stick with wasip1.

Long story short, it's a bit complicated. We're in a development period from WASIp2-and-beyond and things aren't going to be perfect every step of the way, so understanding is appreciated!

License

This project is licensed under the Apache 2.0 license with the LLVM exception. See LICENSE for more details.

Contribution

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in this project by you, as defined in the Apache-2.0 license, shall be licensed as above, without any additional terms or conditions.