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Node.js v17.0.0-rc.2 documentation
Table of contents
WebAssembly System Interface (WASI)#
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.
import { readFile } from 'fs/promises';
import { WASI } from 'wasi';
import { argv, env } from 'process';
const wasi = new WASI({
args: argv,
env,
preopens: {
'/sandbox': '/some/real/path/that/wasm/can/access'
}
});
// Some WASI binaries require:
// const importObject = { wasi_unstable: wasi.wasiImport };
const importObject = { wasi_snapshot_preview1: wasi.wasiImport };
const wasm = await WebAssembly.compile(
await readFile(new URL('./demo.wasm', import.meta.url))
);
const instance = await WebAssembly.instantiate(wasm, importObject);
wasi.start(instance);
'use strict';
const { readFile } = require('fs/promises');
const { WASI } = require('wasi');
const { argv, env } = require('process');
const { join } = require('path');
const wasi = new WASI({
args: argv,
env,
preopens: {
'/sandbox': '/some/real/path/that/wasm/can/access'
}
});
// Some WASI binaries require:
// const importObject = { wasi_unstable: wasi.wasiImport };
const importObject = { wasi_snapshot_preview1: wasi.wasiImport };
(async () => {
const wasm = await WebAssembly.compile(
await readFile(join(__dirname, '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
#
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])
#
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 toprocess.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 ofpreopens
are treated as directories within the sandbox. The corresponding values inpreopens
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 totrue
causeswasi.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)
#
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)
#
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
#
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
.