WASI

WASI, the WebAssembly System Interface, is a modular, platform-agnostic API that enables WebAssembly modules to perform system-level tasks outside a web browser. It provides a lightweight, secure abstraction layer so a Wasm module can access resources such as files, network sockets, time, randomness, and environment variables through host facilities, while maintaining strong sandboxing.

Unlike browser APIs, WASI is designed for standalone runtimes and host environments. It uses a capability-based model in which access to resources is granted through the host when the module is instantiated. The interface emphasizes portability across operating systems and architectures. The API includes file descriptor operations, path-based filesystem access via preopened directories, environment and argument handling, clock and time, randomness, and process lifecycle controls like exit. It aims to be small and modular, so runtimes can implement only what they need.

Support for WASI is provided by standalone Wasm runtimes such as Wasmtime, Wasmer, and WAsmEdge, and by toolchains that generate WASI-compatible binaries. In browsers, WebAssembly currently relies on Web APIs rather than WASI; WASI is intended for non-browser execution, server-side tooling, and embedded environments. The existence of WASI has spurred efforts to port existing C/C++ libraries to WebAssembly via a WASI-compatible libc such as WASI-libc and new libc implementations.

Development of WASI began as an open standard developed by the WebAssembly community with participation from multiple vendors. It is maintained as an open, cross-platform standard intended to enable WebAssembly modules to be used as universal, portable components in server, edge, and embedded contexts.