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# WebAssembly System Interface (WASI)#

Stability: 1 - Experimental

Source Code: lib/wasi.js

The WASI API provides an implementation of the WebAssembly System Interface specification. WASI gives sandboxed WebAssembly applications access to the underlying operating system via a collection of POSIX-like functions.

    'use strict';
    const fs = require('fs');
    const { WASI } = require('wasi');
    const wasi = new WASI({
      args: process.argv,
      env: process.env,
      preopens: {
        '/sandbox': '/some/real/path/that/wasm/can/access'
      }
    });
    const importObject = { wasi_snapshot_preview1: wasi.wasiImport };

    (async () => {
      const wasm = await WebAssembly.compile(fs.readFileSync('./demo.wasm'));
      const instance = await WebAssembly.instantiate(wasm, importObject);

      wasi.start(instance);
    })();

To run the above example, create a new WebAssembly text format file named `demo.wat`:

    (module
        ;; Import the required fd_write WASI function which will write the given io vectors to stdout
        ;; The function signature for fd_write is:
        ;; (File Descriptor, *iovs, iovs_len, nwritten) -> Returns number of bytes written
        (import "wasi_snapshot_preview1" "fd_write" (func $fd_write (param i32 i32 i32 i32) (result i32)))

        (memory 1)
        (export "memory" (memory 0))

        ;; Write 'hello world\n' to memory at an offset of 8 bytes
        ;; Note the trailing newline which is required for the text to appear
        (data (i32.const 8) "hello world\n")

        (func $main (export "_start")
            ;; Creating a new io vector within linear memory
            (i32.store (i32.const 0) (i32.const 8))  ;; iov.iov_base - This is a pointer to the start of the 'hello world\n' string
            (i32.store (i32.const 4) (i32.const 12))  ;; iov.iov_len - The length of the 'hello world\n' string

            (call $fd_write
                (i32.const 1) ;; file_descriptor - 1 for stdout
                (i32.const 0) ;; *iovs - The pointer to the iov array, which is stored at memory location 0
                (i32.const 1) ;; iovs_len - We're printing 1 string stored in an iov - so one.
                (i32.const 20) ;; nwritten - A place in memory to store the number of bytes written
            )
            drop ;; Discard the number of bytes written from the top of the stack
        )
    )

Use wabt to compile `.wat` to `.wasm`

    $ wat2wasm demo.wat

The `--experimental-wasi-unstable-preview1` CLI argument is needed for this example to run.

## Class: `WASI`#

Added in: v13.3.0, v12.16.0

The `WASI` class provides the WASI system call API and additional convenience methods for working with WASI-based applications. Each `WASI` instance represents a distinct sandbox environment. For security purposes, each `WASI` instance must have its command-line arguments, environment variables, and sandbox directory structure configured explicitly.

### `new WASI([options])`#

Added in: v13.3.0, v12.16.0

- `options` <Object>
  - `args` <Array> An array of strings that the WebAssembly application will see as command-line arguments. The first argument is the virtual path to the WASI command itself. Default: `[]`.
  - `env` <Object> An object similar to `process.env` that the WebAssembly application will see as its environment. Default: `{}`.
  - `preopens` <Object> This object represents the WebAssembly application's sandbox directory structure. The string keys of `preopens` are treated as directories within the sandbox. The corresponding values in `preopens` are the real paths to those directories on the host machine.
  - `returnOnExit` <boolean> By default, WASI applications terminate the Node.js process via the `__wasi_proc_exit()` function. Setting this option to `true` causes `wasi.start()` to return the exit code rather than terminate the process. Default: `false`.
  - `stdin` <integer> The file descriptor used as standard input in the WebAssembly application. Default: `0`.
  - `stdout` <integer> The file descriptor used as standard output in the WebAssembly application. Default: `1`.
  - `stderr` <integer> The file descriptor used as standard error in the WebAssembly application. Default: `2`.

### `wasi.start(instance)`#

Added in: v13.3.0, v12.16.0

- `instance` <WebAssembly.Instance>

Attempt to begin execution of `instance` as a WASI command by invoking its `_start()` export. If `instance` does not contain a `_start()` export, or if `instance` contains an `_initialize()` export, then an exception is thrown.

`start()` requires that `instance` exports a `WebAssembly.Memory` named `memory`. If `instance` does not have a `memory` export an exception is thrown.

If `start()` is called more than once, an exception is thrown.

### `wasi.initialize(instance)`#

Added in: v14.6.0, v12.19.0

- `instance` <WebAssembly.Instance>

Attempt to initialize `instance` as a WASI reactor by invoking its `_initialize()` export, if it is present. If `instance` contains a `_start()` export, then an exception is thrown.

`initialize()` requires that `instance` exports a `WebAssembly.Memory` named `memory`. If `instance` does not have a `memory` export an exception is thrown.

If `initialize()` is called more than once, an exception is thrown.

### `wasi.wasiImport`#

Added in: v13.3.0, v12.16.0

- <Object>

`wasiImport` is an object that implements the WASI system call API. This object should be passed as the `wasi_snapshot_preview1` import during the instantiation of a `WebAssembly.Instance`.