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.
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, see the online section of the guide
To install a WASI-enabled C/C++ toolchain, see the online section of the guide
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
Bytecode Alliance 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 source is here.
Beyond the WASI Overview, take a look at the various WASI documents.