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<h2>Table of Contents</h2>
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<ul>
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<li><span class="stability_1"><a href="#wasi_webassembly_system_interface_wasi">WebAssembly System Interface (WASI)</a></span>
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<ul>
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<li><a href="#wasi_class_wasi">Class: <code>WASI</code></a>
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<ul>
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<li><a href="#wasi_new_wasi_options"><code>new WASI([options])</code></a></li>
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<li><a href="#wasi_wasi_start_instance"><code>wasi.start(instance)</code></a></li>
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<li><a href="#wasi_wasi_initialize_instance"><code>wasi.initialize(instance)</code></a></li>
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<li><a href="#wasi_wasi_wasiimport"><code>wasi.wasiImport</code></a></li>
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</ul>
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<div id="apicontent">
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<h1>WebAssembly System Interface (WASI)<span><a class="mark" href="#wasi_webassembly_system_interface_wasi" id="wasi_webassembly_system_interface_wasi">#</a></span></h1>
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<p></p><div class="api_stability api_stability_1"><a href="documentation.html#documentation_stability_index">Stability: 1</a> - Experimental</div><p></p>
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<p><strong>Source Code:</strong> <a href="https://github.com/nodejs/node/blob/v12.22.12/lib/wasi.js">lib/wasi.js</a></p>
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<p>The WASI API provides an implementation of the <a href="https://wasi.dev/">WebAssembly System Interface</a>
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specification. WASI gives sandboxed WebAssembly applications access to the
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underlying operating system via a collection of POSIX-like functions.</p>
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<pre><code class="language-js"><span class="hljs-meta">'use strict'</span>;
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<span class="hljs-keyword">const</span> fs = <span class="hljs-built_in">require</span>(<span class="hljs-string">'fs'</span>);
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<span class="hljs-keyword">const</span> { WASI } = <span class="hljs-built_in">require</span>(<span class="hljs-string">'wasi'</span>);
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<span class="hljs-keyword">const</span> wasi = <span class="hljs-keyword">new</span> WASI({
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<span class="hljs-attr">args</span>: process.argv,
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<span class="hljs-attr">env</span>: process.env,
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<span class="hljs-attr">preopens</span>: {
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<span class="hljs-string">'/sandbox'</span>: <span class="hljs-string">'/some/real/path/that/wasm/can/access'</span>
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}
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});
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<span class="hljs-keyword">const</span> importObject = { <span class="hljs-attr">wasi_snapshot_preview1</span>: wasi.wasiImport };
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(<span class="hljs-keyword">async</span> () => {
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<span class="hljs-keyword">const</span> wasm = <span class="hljs-keyword">await</span> WebAssembly.compile(fs.readFileSync(<span class="hljs-string">'./demo.wasm'</span>));
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<span class="hljs-keyword">const</span> instance = <span class="hljs-keyword">await</span> WebAssembly.instantiate(wasm, importObject);
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wasi.start(instance);
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})();</code></pre>
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<p>To run the above example, create a new WebAssembly text format file named
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<code>demo.wat</code>:</p>
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<pre><code class="language-text">(module
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;; Import the required fd_write WASI function which will write the given io vectors to stdout
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;; The function signature for fd_write is:
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;; (File Descriptor, *iovs, iovs_len, nwritten) -> Returns number of bytes written
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(import "wasi_snapshot_preview1" "fd_write" (func $fd_write (param i32 i32 i32 i32) (result i32)))
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(memory 1)
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(export "memory" (memory 0))
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;; Write 'hello world\n' to memory at an offset of 8 bytes
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;; Note the trailing newline which is required for the text to appear
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(data (i32.const 8) "hello world\n")
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(func $main (export "_start")
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;; Creating a new io vector within linear memory
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(i32.store (i32.const 0) (i32.const 8)) ;; iov.iov_base - This is a pointer to the start of the 'hello world\n' string
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(i32.store (i32.const 4) (i32.const 12)) ;; iov.iov_len - The length of the 'hello world\n' string
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(call $fd_write
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(i32.const 1) ;; file_descriptor - 1 for stdout
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(i32.const 0) ;; *iovs - The pointer to the iov array, which is stored at memory location 0
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(i32.const 1) ;; iovs_len - We're printing 1 string stored in an iov - so one.
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(i32.const 20) ;; nwritten - A place in memory to store the number of bytes written
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)
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drop ;; Discard the number of bytes written from the top of the stack
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)
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)</code></pre>
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<p>Use <a href="https://github.com/WebAssembly/wabt">wabt</a> to compile <code>.wat</code> to <code>.wasm</code></p>
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<pre><code class="language-console"><span class="hljs-meta">$</span><span class="bash"> wat2wasm demo.wat</span></code></pre>
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<p>The <code>--experimental-wasi-unstable-preview1</code> and <code>--experimental-wasm-bigint</code>
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CLI arguments are needed for this example to run.</p>
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<h2>Class: <code>WASI</code><span><a class="mark" href="#wasi_class_wasi" id="wasi_class_wasi">#</a></span></h2>
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<div class="api_metadata">
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<span>Added in: v12.16.0</span>
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</div>
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<p>The <code>WASI</code> class provides the WASI system call API and additional convenience
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methods for working with WASI-based applications. Each <code>WASI</code> instance
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represents a distinct sandbox environment. For security purposes, each <code>WASI</code>
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instance must have its command line arguments, environment variables, and
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sandbox directory structure configured explicitly.</p>
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<h3><code>new WASI([options])</code><span><a class="mark" href="#wasi_new_wasi_options" id="wasi_new_wasi_options">#</a></span></h3>
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<div class="api_metadata">
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<span>Added in: v12.16.0</span>
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</div>
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<ul>
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<li><code>options</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a>
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<ul>
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<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
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see as command line arguments. The first argument is the virtual path to the
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WASI command itself. <strong>Default:</strong> <code>[]</code>.</li>
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<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
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application will see as its environment. <strong>Default:</strong> <code>{}</code>.</li>
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<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
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sandbox directory structure. The string keys of <code>preopens</code> are treated as
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directories within the sandbox. The corresponding values in <code>preopens</code> are
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the real paths to those directories on the host machine.</li>
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<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
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process via the <code>__wasi_proc_exit()</code> function. Setting this option to <code>true</code>
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causes <code>wasi.start()</code> to return the exit code rather than terminate the
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process. <strong>Default:</strong> <code>false</code>.</li>
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<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
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WebAssembly application. <strong>Default:</strong> <code>0</code>.</li>
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<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
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WebAssembly application. <strong>Default:</strong> <code>1</code>.</li>
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<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
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WebAssembly application. <strong>Default:</strong> <code>2</code>.</li>
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</ul>
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</li>
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</ul>
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<h3><code>wasi.start(instance)</code><span><a class="mark" href="#wasi_wasi_start_instance" id="wasi_wasi_start_instance">#</a></span></h3>
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<div class="api_metadata">
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<span>Added in: v12.16.0</span>
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</div>
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<ul>
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<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>
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</ul>
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<p>Attempt to begin execution of <code>instance</code> as a WASI command by invoking its
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<code>_start()</code> export. If <code>instance</code> does not contain a <code>_start()</code> export, or if
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<code>instance</code> contains an <code>_initialize()</code> export, then an exception is thrown.</p>
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<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
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<code>memory</code>. If <code>instance</code> does not have a <code>memory</code> export an exception is thrown.</p>
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<p>If <code>start()</code> is called more than once, an exception is thrown.</p>
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<h3><code>wasi.initialize(instance)</code><span><a class="mark" href="#wasi_wasi_initialize_instance" id="wasi_wasi_initialize_instance">#</a></span></h3>
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<div class="api_metadata">
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<span>Added in: v12.19.0</span>
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</div>
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<ul>
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<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>
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</ul>
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<p>Attempt to initialize <code>instance</code> as a WASI reactor by invoking its
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<code>_initialize()</code> export, if it is present. If <code>instance</code> contains a <code>_start()</code>
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export, then an exception is thrown.</p>
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<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
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<code>memory</code>. If <code>instance</code> does not have a <code>memory</code> export an exception is thrown.</p>
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<p>If <code>initialize()</code> is called more than once, an exception is thrown.</p>
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<h3><code>wasi.wasiImport</code><span><a class="mark" href="#wasi_wasi_wasiimport" id="wasi_wasi_wasiimport">#</a></span></h3>
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<div class="api_metadata">
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<span>Added in: v12.16.0</span>
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</div>
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<ul>
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<li><a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object" class="type"><Object></a></li>
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</ul>
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<p><code>wasiImport</code> is an object that implements the WASI system call API. This object
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should be passed as the <code>wasi_snapshot_preview1</code> import during the instantiation
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of a <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/WebAssembly/Instance"><code>WebAssembly.Instance</code></a>.</p>
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<!-- API END -->
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