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<!doctype html> <html lang="en"> <head> <meta charset="utf-8"> <meta name="viewport" content="width=device-width"> <meta name="nodejs.org:node-version" content="v12.22.9"> <title>WebAssembly System Interface (WASI) | Node.js v12.22.9 Documentation</title> <link rel="stylesheet" href="https://fonts.googleapis.com/css?family=Lato:400,700,400italic&display=fallback"> <link rel="stylesheet" href="assets/style.css"> <link rel="stylesheet" href="assets/hljs.css"> <link rel="canonical" href="https://nodejs.org/api/wasi.html"> </head> <body class="alt apidoc" id="api-section-wasi"> <div id="content" class="clearfix"> <div id="column2" class="interior"> <div id="intro" class="interior"> <a href="/" title="Go back to the home page"> Node.js </a> </div> <ul> <li><a href="documentation.html" class="nav-documentation">About this documentation</a></li> <li><a href="synopsis.html" class="nav-synopsis">Usage and example</a></li> </ul> <hr class="line"> <ul> <li><a href="assert.html" 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class="version-picker"><li><a href="https://nodejs.org/docs/latest-v17.x/api/wasi.html">17.x</a></li> <li><a href="https://nodejs.org/docs/latest-v16.x/api/wasi.html">16.x <b>LTS</b></a></li> <li><a href="https://nodejs.org/docs/latest-v15.x/api/wasi.html">15.x</a></li> <li><a href="https://nodejs.org/docs/latest-v14.x/api/wasi.html">14.x <b>LTS</b></a></li> <li><a href="https://nodejs.org/docs/latest-v13.x/api/wasi.html">13.x</a></li> <li><a href="https://nodejs.org/docs/latest-v12.x/api/wasi.html">12.x <b>LTS</b></a></li></ol> </li> <li class="edit_on_github"><a href="https://github.com/nodejs/node/edit/master/doc/api/wasi.md"><span class="github_icon"><svg height="16" width="16" viewBox="0 0 16.1 16.1" fill="currentColor"><path d="M8 0a8 8 0 0 0-2.5 15.6c.4 0 .5-.2.5-.4v-1.5c-2 .4-2.5-.5-2.7-1 0-.1-.5-.9-.8-1-.3-.2-.7-.6 0-.6.6 0 1 .6 1.2.8.7 1.2 1.9 1 2.4.7 0-.5.2-.9.5-1-1.8-.3-3.7-1-3.7-4 0-.9.3-1.6.8-2.2 0-.2-.3-1 .1-2 0 0 .7-.3 2.2.7a7.4 7.4 0 0 1 4 0c1.5-1 2.2-.8 2.2-.8.5 1.1.2 2 .1 2.1.5.6.8 1.3.8 2.2 0 3-1.9 3.7-3.6 4 .3.2.5.7.5 1.4v2.2c0 .2.1.5.5.4A8 8 0 0 0 16 8a8 8 0 0 0-8-8z"/></svg></span>Edit on GitHub</a></li> </ul> </div> <hr> </header> <div id="toc"> <h2>Table of Contents</h2> <ul> <li><span class="stability_1"><a href="#wasi_webassembly_system_interface_wasi">WebAssembly System Interface (WASI)</a></span> <ul> <li><a href="#wasi_class_wasi">Class: <code>WASI</code></a> <ul> <li><a href="#wasi_new_wasi_options"><code>new WASI([options])</code></a></li> <li><a href="#wasi_wasi_start_instance"><code>wasi.start(instance)</code></a></li> <li><a href="#wasi_wasi_initialize_instance"><code>wasi.initialize(instance)</code></a></li> <li><a href="#wasi_wasi_wasiimport"><code>wasi.wasiImport</code></a></li> </ul> </li> </ul> </li> </ul> </div> <div id="apicontent"> <h1>WebAssembly System Interface (WASI)<span><a class="mark" href="#wasi_webassembly_system_interface_wasi" id="wasi_webassembly_system_interface_wasi">#</a></span></h1> <p></p><div class="api_stability api_stability_1"><a href="documentation.html#documentation_stability_index">Stability: 1</a> - Experimental</div><p></p> <p><strong>Source Code:</strong> <a href="https://github.com/nodejs/node/blob/v12.22.9/lib/wasi.js">lib/wasi.js</a></p> <p>The WASI API provides an implementation of the <a href="https://wasi.dev/">WebAssembly System Interface</a> specification. WASI gives sandboxed WebAssembly applications access to the underlying operating system via a collection of POSIX-like functions.</p> <pre><code class="language-js"><span class="hljs-meta">'use strict'</span>; <span class="hljs-keyword">const</span> fs = <span class="hljs-built_in">require</span>(<span class="hljs-string">'fs'</span>); <span class="hljs-keyword">const</span> { WASI } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'wasi'</span>); <span class="hljs-keyword">const</span> wasi = <span class="hljs-keyword">new</span> WASI({ <span class="hljs-attr">args</span>: process.argv, <span class="hljs-attr">env</span>: process.env, <span class="hljs-attr">preopens</span>: { <span class="hljs-string">'/sandbox'</span>: <span class="hljs-string">'/some/real/path/that/wasm/can/access'</span> } }); <span class="hljs-keyword">const</span> importObject = { <span class="hljs-attr">wasi_snapshot_preview1</span>: wasi.wasiImport }; (<span class="hljs-keyword">async</span> () => { <span class="hljs-keyword">const</span> wasm = <span class="hljs-keyword">await</span> WebAssembly.compile(fs.readFileSync(<span class="hljs-string">'./demo.wasm'</span>)); <span class="hljs-keyword">const</span> instance = <span class="hljs-keyword">await</span> WebAssembly.instantiate(wasm, importObject); wasi.start(instance); })();</code></pre> <p>To run the above example, create a new WebAssembly text format file named <code>demo.wat</code>:</p> <pre><code class="language-text">(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 $fd_write (i32.const 1) ;; file_descriptor - 1 for stdout (i32.const 0) ;; *iovs - The pointer to the iov array, which is stored at memory location 0 (i32.const 1) ;; iovs_len - We're printing 1 string stored in an iov - so one. (i32.const 20) ;; nwritten - A place in memory to store the number of bytes written ) drop ;; Discard the number of bytes written from the top of the stack ) )</code></pre> <p>Use <a href="https://github.com/WebAssembly/wabt">wabt</a> to compile <code>.wat</code> to <code>.wasm</code></p> <pre><code class="language-console"><span class="hljs-meta">$</span><span class="bash"> wat2wasm demo.wat</span></code></pre> <p>The <code>--experimental-wasi-unstable-preview1</code> and <code>--experimental-wasm-bigint</code> CLI arguments are needed for this example to run.</p> <h2>Class: <code>WASI</code><span><a class="mark" href="#wasi_class_wasi" id="wasi_class_wasi">#</a></span></h2> <div class="api_metadata"> <span>Added in: v12.16.0</span> </div> <p>The <code>WASI</code> class provides the WASI system call API and additional convenience methods for working with WASI-based applications. Each <code>WASI</code> instance represents a distinct sandbox environment. For security purposes, each <code>WASI</code> instance must have its command line arguments, environment variables, and sandbox directory structure configured explicitly.</p> <h3><code>new WASI([options])</code><span><a class="mark" href="#wasi_new_wasi_options" id="wasi_new_wasi_options">#</a></span></h3> <div class="api_metadata"> <span>Added in: v12.16.0</span> </div> <ul> <li><code>options</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> <ul> <li><code>args</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array" class="type"><Array></a> 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. <strong>Default:</strong> <code>[]</code>.</li> <li><code>env</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> An object similar to <code>process.env</code> that the WebAssembly application will see as its environment. <strong>Default:</strong> <code>{}</code>.</li> <li><code>preopens</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a> This object represents the WebAssembly application's sandbox directory structure. The string keys of <code>preopens</code> are treated as directories within the sandbox. The corresponding values in <code>preopens</code> are the real paths to those directories on the host machine.</li> <li><code>returnOnExit</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Boolean_type" class="type"><boolean></a> By default, WASI applications terminate the Node.js process via the <code>__wasi_proc_exit()</code> function. Setting this option to <code>true</code> causes <code>wasi.start()</code> to return the exit code rather than terminate the process. <strong>Default:</strong> <code>false</code>.</li> <li><code>stdin</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><integer></a> The file descriptor used as standard input in the WebAssembly application. <strong>Default:</strong> <code>0</code>.</li> <li><code>stdout</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><integer></a> The file descriptor used as standard output in the WebAssembly application. <strong>Default:</strong> <code>1</code>.</li> <li><code>stderr</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Data_structures#Number_type" class="type"><integer></a> The file descriptor used as standard error in the WebAssembly application. <strong>Default:</strong> <code>2</code>.</li> </ul> </li> </ul> <h3><code>wasi.start(instance)</code><span><a class="mark" href="#wasi_wasi_start_instance" id="wasi_wasi_start_instance">#</a></span></h3> <div class="api_metadata"> <span>Added in: v12.16.0</span> </div> <ul> <li><code>instance</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WebAssembly/Instance" class="type"><WebAssembly.Instance></a></li> </ul> <p>Attempt to begin execution of <code>instance</code> as a WASI command by invoking its <code>_start()</code> export. If <code>instance</code> does not contain a <code>_start()</code> export, or if <code>instance</code> contains an <code>_initialize()</code> export, then an exception is thrown.</p> <p><code>start()</code> requires that <code>instance</code> exports a <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WebAssembly/Memory"><code>WebAssembly.Memory</code></a> named <code>memory</code>. If <code>instance</code> does not have a <code>memory</code> export an exception is thrown.</p> <p>If <code>start()</code> is called more than once, an exception is thrown.</p> <h3><code>wasi.initialize(instance)</code><span><a class="mark" href="#wasi_wasi_initialize_instance" id="wasi_wasi_initialize_instance">#</a></span></h3> <div class="api_metadata"> <span>Added in: v12.19.0</span> </div> <ul> <li><code>instance</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WebAssembly/Instance" class="type"><WebAssembly.Instance></a></li> </ul> <p>Attempt to initialize <code>instance</code> as a WASI reactor by invoking its <code>_initialize()</code> export, if it is present. If <code>instance</code> contains a <code>_start()</code> export, then an exception is thrown.</p> <p><code>initialize()</code> requires that <code>instance</code> exports a <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WebAssembly/Memory"><code>WebAssembly.Memory</code></a> named <code>memory</code>. If <code>instance</code> does not have a <code>memory</code> export an exception is thrown.</p> <p>If <code>initialize()</code> is called more than once, an exception is thrown.</p> <h3><code>wasi.wasiImport</code><span><a class="mark" href="#wasi_wasi_wasiimport" id="wasi_wasi_wasiimport">#</a></span></h3> <div class="api_metadata"> <span>Added in: v12.16.0</span> </div> <ul> <li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a></li> </ul> <p><code>wasiImport</code> is an object that implements the WASI system call API. This object should be passed as the <code>wasi_snapshot_preview1</code> import during the instantiation of a <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WebAssembly/Instance"><code>WebAssembly.Instance</code></a>.</p> <!-- API END --> </div> </div> </div> </body> </html>