WASI

What is WASI?

WASI is the WebAssembly System Interface — a portable, capability-based replacement for the POSIX surface, designed so that a WebAssembly program compiled once can run on any conforming host (browser, server, CLI, embedded). The guest module imports a fixed set of host functions (fd_read, fd_write, path_open, clock_time_get, random_get, proc_exit, …) under a well-known module name; the host implements them however it likes. There is no implicit ambient authority — a WASI program can only touch the directories the host explicitly hands it as preopens at startup.

Preview 1 (wasi_snapshot_preview1) is the legacy-but-ubiquitous ABI: every shipping rustc, every zig wasm32-wasi target, every Clang WASI sysroot binary imports this name today. Preview 2 (wasi:cli@0.2.x) is the newer, component-model-shaped successor; that ecosystem is still settling, and Preview 1 will remain the lingua franca for the foreseeable future. This project implements Preview 1.

What’s here

The wasi/ sub-project (io.github.edadma.wasm.wasi) implements wasi_snapshot_preview1 as a host module that plugs into this interpreter. Everything runs as a HostModule exposed to Runtime.instantiate, so it composes with your own host imports cleanly.

Wasi.preview1(ctx) returns a HostModule named "wasi_snapshot_preview1". Wasi.run(inst, "_start") invokes the WASI command-mode entry point and translates proc_exit(N) into Right(N) — see Quickstart for the calling shape.

• WasiContext — args, envs, stdio sinks, clock, random source, preopens; default and collecting factories.
• Preopens — WasiContext.Preopen, HostPreopen.fromDir, and the sandboxing model.
• Syscalls — the 29 syscalls grouped by purpose (process, clock + entropy, stdio + preopens, file I/O, filesystem, polling, sockets).