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Welcome to WASI!

WASI stands for WebAssembly System Interface. It's an API designed by the Wasmtime project that provides access to several operating-system-like features, including files and filesystems, Berkeley sockets, clocks, and random numbers, that we'll be proposing for standardization.

It's designed to be independent of browsers, so it doesn't depend on Web APIs or JS, and isn't limited by the need to be compatible with JS. And it has integrated capability-based security, so it extends WebAssembly's characteristic sandboxing to include I/O.

See the WASI Overview for more detailed background information, and the WASI Tutorial for a walkthrough showing how various pieces fit together.

Note that everything here is a prototype, and while a lot of stuff works, there are numerous missing features and some rough edges. For example, networking support is incomplete.

How can I write programs that use WASI?

The two toolchains that currently work well are the Rust toolchain and a specially packaged C and C++ toolchain. Of course, we hope other toolchains will be able to implement WASI as well!


To install a WASI-enabled Rust toolchain:

rustup target add wasm32-wasi --toolchain nightly
cargo +nightly build --target wasm32-wasi

Until now, Rust's WebAssembly support has had two main options, the Emscripten-based option, and the wasm32-unknown-unknown option. The latter option is lighter-weight, but only supports no_std. WASI enables a new wasm32-wasi target, which is similar to wasm32-unknown-unknown in that it doesn't depend on Emscripten, but it can use WASI to provide a decent subset of libstd.


All the parts needed to support wasm are included in upstream clang, lld, and compiler-rt, as of the LLVM 8.0 release. However, to use it, you'll need to build WebAssembly-targeted versions of the library parts, and it can be tricky to get all the CMake invocations lined up properly.

To make things easier, we provide prebuilt packages that provide builds of Clang and sysroot libraries.

WASI doesn't yet support setjmp/longjmp or C++ exceptions, as it is waiting for unwinding support in WebAssembly.

Some C++ programs, particularly those using <iostream>, may see warnings about function signature mismatches; this is a known bug.

How can I run programs that use WASI?

Currently the options are Wasmtime and the browser polyfill, though we intend WASI to be implementable in many wasm VMs.


Wasmtime is a non-Web WebAssembly engine which is part of the CraneStation project. To build it, download the code and build with cargo build --release. It can run WASI-using wasm programs by simply running wasmtime foo.wasm, or cargo run --bin wasmtime foo.wasm.

The browser polyfill

The polyfill is online here.

The source is here.

Where can I learn more?

Beyond the WASI Overview, take a look at the various WASI documents.

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