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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="v16.20.0"> <title>WebAssembly System Interface (WASI) | Node.js v16.20.0 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"> <script async defer src="assets/api.js" type="text/javascript"></script> <style>@media(max-width:614px){.with-49-chars>.js-flavor-selector{float:none;margin:0 0 1em auto;}}</style> </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" 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Interface (WASI)</a></span> <ul> <li><a href="#class-wasi">Class: <code>WASI</code></a> <ul> <li><a href="#new-wasioptions"><code>new WASI([options])</code></a></li> <li><a href="#wasistartinstance"><code>wasi.start(instance)</code></a></li> <li><a href="#wasiinitializeinstance"><code>wasi.initialize(instance)</code></a></li> <li><a href="#wasiwasiimport"><code>wasi.wasiImport</code></a></li> </ul> </li> </ul> </li> </ul></div></div> </li> <li class="picker-header"> <a href="#"> <span class="collapsed-arrow">►</span><span class="expanded-arrow">▼</span> Index </a> <div class="picker"><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> <li> <a href="index.html">Index</a> </li> </ul> <hr class="line"> <ul> <li><a href="assert.html" class="nav-assert">Assertion testing</a></li> <li><a href="async_context.html" class="nav-async_context">Asynchronous context 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issue tracker</a></li> </ul></div> </li> <li class="picker-header"> <a href="#"> <span class="collapsed-arrow">►</span><span class="expanded-arrow">▼</span> Other versions </a> <div class="picker"><ol id="alt-docs"><li><a href="https://nodejs.org/docs/latest-v19.x/api/wasi.html">19.x</a></li> <li><a href="https://nodejs.org/docs/latest-v18.x/api/wasi.html">18.x <b>LTS</b></a></li> <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</a></li></ol></div> </li> <li class="picker-header"> <a href="#"> <span class="collapsed-arrow">►</span><span class="expanded-arrow">▼</span> Options </a> <div class="picker"> <ul> <li> <a href="all.html">View on single page</a> </li> <li> <a href="wasi.json">View as JSON</a> </li> <li class="edit_on_github"><a href="https://github.com/nodejs/node/edit/main/doc/api/wasi.md">Edit on GitHub</a></li> </ul> </div> </li> </ul> </div> <hr> </header> <details id="toc" open><summary>Table of contents</summary><ul> <li><span class="stability_1"><a href="#webassembly-system-interface-wasi">WebAssembly System Interface (WASI)</a></span> <ul> <li><a href="#class-wasi">Class: <code>WASI</code></a> <ul> <li><a href="#new-wasioptions"><code>new WASI([options])</code></a></li> <li><a href="#wasistartinstance"><code>wasi.start(instance)</code></a></li> <li><a href="#wasiinitializeinstance"><code>wasi.initialize(instance)</code></a></li> <li><a href="#wasiwasiimport"><code>wasi.wasiImport</code></a></li> </ul> </li> </ul> </li> </ul></details> <div id="apicontent"> <h2>WebAssembly System Interface (WASI)<span><a class="mark" href="#webassembly-system-interface-wasi" id="webassembly-system-interface-wasi">#</a></span><a aria-hidden="true" class="legacy" id="wasi_webassembly_system_interface_wasi"></a></h2> <p></p><div class="api_stability api_stability_1"><a href="documentation.html#stability-index">Stability: 1</a> - Experimental</div><p></p> <p><strong>Source Code:</strong> <a href="https://github.com/nodejs/node/blob/v16.20.0/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 class="with-49-chars"><input class="js-flavor-selector" type="checkbox" checked aria-label="Show modern ES modules syntax"><code class="language-js mjs"><span class="hljs-keyword">import</span> { readFile } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:fs/promises'</span>; <span class="hljs-keyword">import</span> { <span class="hljs-variable constant_">WASI</span> } <span class="hljs-keyword">from</span> <span class="hljs-string">'wasi'</span>; <span class="hljs-keyword">import</span> { argv, env } <span class="hljs-keyword">from</span> <span class="hljs-string">'node:process'</span>; <span class="hljs-keyword">const</span> wasi = <span class="hljs-keyword">new</span> <span class="hljs-title function_">WASI</span>({ <span class="hljs-attr">args</span>: argv, 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-comment">// Some WASI binaries require:</span> <span class="hljs-comment">// const importObject = { wasi_unstable: wasi.wasiImport };</span> <span class="hljs-keyword">const</span> importObject = { <span class="hljs-attr">wasi_snapshot_preview1</span>: wasi.<span class="hljs-property">wasiImport</span> }; <span class="hljs-keyword">const</span> wasm = <span class="hljs-keyword">await</span> <span class="hljs-title class_">WebAssembly</span>.<span class="hljs-title function_">compile</span>( <span class="hljs-keyword">await</span> <span class="hljs-title function_">readFile</span>(<span class="hljs-keyword">new</span> <span class="hljs-title function_">URL</span>(<span class="hljs-string">'./demo.wasm'</span>, <span class="hljs-keyword">import</span>.<span class="hljs-property">meta</span>.<span class="hljs-property">url</span>)) ); <span class="hljs-keyword">const</span> instance = <span class="hljs-keyword">await</span> <span class="hljs-title class_">WebAssembly</span>.<span class="hljs-title function_">instantiate</span>(wasm, importObject); wasi.<span class="hljs-title function_">start</span>(instance);</code><code class="language-js cjs"><span class="hljs-meta">'use strict'</span>; <span class="hljs-keyword">const</span> { readFile } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:fs/promises'</span>); <span class="hljs-keyword">const</span> { <span class="hljs-variable constant_">WASI</span> } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'wasi'</span>); <span class="hljs-keyword">const</span> { argv, env } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:process'</span>); <span class="hljs-keyword">const</span> { join } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'node:path'</span>); <span class="hljs-keyword">const</span> wasi = <span class="hljs-keyword">new</span> <span class="hljs-title function_">WASI</span>({ <span class="hljs-attr">args</span>: argv, 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-comment">// Some WASI binaries require:</span> <span class="hljs-comment">// const importObject = { wasi_unstable: wasi.wasiImport };</span> <span class="hljs-keyword">const</span> importObject = { <span class="hljs-attr">wasi_snapshot_preview1</span>: wasi.<span class="hljs-property">wasiImport</span> }; (<span class="hljs-keyword">async</span> () => { <span class="hljs-keyword">const</span> wasm = <span class="hljs-keyword">await</span> <span class="hljs-title class_">WebAssembly</span>.<span class="hljs-title function_">compile</span>( <span class="hljs-keyword">await</span> <span class="hljs-title function_">readFile</span>(<span class="hljs-title function_">join</span>(__dirname, <span class="hljs-string">'demo.wasm'</span>)) ); <span class="hljs-keyword">const</span> instance = <span class="hljs-keyword">await</span> <span class="hljs-title class_">WebAssembly</span>.<span class="hljs-title function_">instantiate</span>(wasm, importObject); wasi.<span class="hljs-title function_">start</span>(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 $fd_write (i32.const 1) ;; file_descriptor - 1 for stdout (i32.const 0) ;; *iovs - The pointer to the iov array, which is stored at memory location 0 (i32.const 1) ;; iovs_len - We're printing 1 string stored in an iov - so one. (i32.const 20) ;; nwritten - A place in memory to store the number of bytes written ) drop ;; Discard the number of bytes written from the top of the stack ) )</code></pre> <p>Use <a href="https://github.com/WebAssembly/wabt">wabt</a> to compile <code>.wat</code> to <code>.wasm</code></p> <pre><code class="language-console"><span class="hljs-meta prompt_">$ </span><span class="language-bash">wat2wasm demo.wat</span></code></pre> <p>The <code>--experimental-wasi-unstable-preview1</code> CLI argument is needed for this example to run.</p> <section><h3>Class: <code>WASI</code><span><a class="mark" href="#class-wasi" id="class-wasi">#</a></span><a aria-hidden="true" class="legacy" id="wasi_class_wasi"></a></h3> <div class="api_metadata"> <span>Added in: v13.3.0, 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> <h4><code>new WASI([options])</code><span><a class="mark" href="#new-wasioptions" id="new-wasioptions">#</a></span><a aria-hidden="true" class="legacy" id="wasi_new_wasi_options"></a></h4> <div class="api_metadata"> <span>Added in: v13.3.0, 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> <h4><code>wasi.start(instance)</code><span><a class="mark" href="#wasistartinstance" id="wasistartinstance">#</a></span><a aria-hidden="true" class="legacy" id="wasi_wasi_start_instance"></a></h4> <div class="api_metadata"> <span>Added in: v13.3.0, 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> <h4><code>wasi.initialize(instance)</code><span><a class="mark" href="#wasiinitializeinstance" id="wasiinitializeinstance">#</a></span><a aria-hidden="true" class="legacy" id="wasi_wasi_initialize_instance"></a></h4> <div class="api_metadata"> <span>Added in: v14.6.0, 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> <h4><code>wasi.wasiImport</code><span><a class="mark" href="#wasiwasiimport" id="wasiwasiimport">#</a></span><a aria-hidden="true" class="legacy" id="wasi_wasi_wasiimport"></a></h4> <div class="api_metadata"> <span>Added in: v13.3.0, 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></section> <!-- API END --> </div> </div> </div> </body> </html>
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