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      <details id="toc" open><summary>Table of contents</summary><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></details>

      <div id="apicontent">
        <h2>WebAssembly System Interface (WASI)<span><a class="mark" href="#wasi_webassembly_system_interface_wasi" id="wasi_webassembly_system_interface_wasi">#</a></span></h2>

<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/v14.21.2/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><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> fs <span class="hljs-keyword">from</span> <span class="hljs-string">'fs'</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">const</span> wasi = <span class="hljs-keyword">new</span> <span class="hljs-title function_">WASI</span>({
  <span class="hljs-attr">args</span>: process.<span class="hljs-property">argv</span>,
  <span class="hljs-attr">env</span>: process.<span class="hljs-property">env</span>,
  <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.<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>(fs.<span class="hljs-title function_">readFileSync</span>(<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><code class="language-js cjs"><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> { <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> wasi = <span class="hljs-keyword">new</span> <span class="hljs-title function_">WASI</span>({
  <span class="hljs-attr">args</span>: process.<span class="hljs-property">argv</span>,
  <span class="hljs-attr">env</span>: process.<span class="hljs-property">env</span>,
  <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.<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>(fs.<span class="hljs-title function_">readFileSync</span>(<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 $fd_write
            (i32.const 1) ;; file_descriptor - 1 for stdout
            (i32.const 0) ;; *iovs - The pointer to the iov array, which is stored at memory location 0
            (i32.const 1) ;; iovs_len - We're printing 1 string stored in an iov - so one.
            (i32.const 20) ;; nwritten - A place in memory to store the number of bytes written
        )
        drop ;; Discard the number of bytes written from the top of the stack
    )
)</code></pre>
<p>Use <a href="https://github.com/WebAssembly/wabt">wabt</a> to compile <code>.wat</code> to <code>.wasm</code></p>
<pre><code class="language-console"><span class="hljs-meta">$ </span><span class="language-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>
<section><h3>Class: <code>WASI</code><span><a class="mark" href="#wasi_class_wasi" id="wasi_class_wasi">#</a></span></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="#wasi_new_wasi_options" id="wasi_new_wasi_options">#</a></span></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">&#x3C;Object></a>
<ul>
<li><code>args</code> <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array" class="type">&#x3C;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">&#x3C;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">&#x3C;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">&#x3C;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">&#x3C;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">&#x3C;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">&#x3C;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="#wasi_wasi_start_instance" id="wasi_wasi_start_instance">#</a></span></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">&#x3C;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="#wasi_wasi_initialize_instance" id="wasi_wasi_initialize_instance">#</a></span></h4>
<div class="api_metadata">
<span>Added in: v14.6.0</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">&#x3C;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="#wasi_wasi_wasiimport" id="wasi_wasi_wasiimport">#</a></span></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">&#x3C;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>
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