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<details id="toc" open><summary>Table of contents</summary><ul>
<li><a href="#embedding_c_embedder_api">C++ embedder API</a>
<ul>
<li><a href="#embedding_example_embedding_application">Example embedding application</a>
<ul>
<li><a href="#embedding_setting_up_per_process_state">Setting up per-process state</a></li>
<li><a href="#embedding_per_instance_state">Per-instance state</a></li>
</ul>
</li>
</ul>
</li>
</ul></details>
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<h2>C++ embedder API<span><a class="mark" href="#embedding_c_embedder_api" id="embedding_c_embedder_api">#</a></span></h2>
<p>Node.js provides a number of C++ APIs that can be used to execute JavaScript
in a Node.js environment from other C++ software.</p>
<p>The documentation for these APIs can be found in <a href="https://github.com/nodejs/node/blob/HEAD/src/node.h">src/node.h</a> in the Node.js
source tree. In addition to the APIs exposed by Node.js, some required concepts
are provided by the V8 embedder API.</p>
<p>Because using Node.js as an embedded library is different from writing code
that is executed by Node.js, breaking changes do not follow typical Node.js
<a href="deprecations.html">deprecation policy</a> and may occur on each semver-major release without prior
warning.</p>
<section><h3>Example embedding application<span><a class="mark" href="#embedding_example_embedding_application" id="embedding_example_embedding_application">#</a></span></h3>
<p>The following sections will provide an overview over how to use these APIs
to create an application from scratch that will perform the equivalent of
<code>node -e <code></code>, i.e. that will take a piece of JavaScript and run it in
a Node.js-specific environment.</p>
<p>The full code can be found <a href="https://github.com/nodejs/node/blob/HEAD/test/embedding/embedtest.cc">in the Node.js source tree</a>.</p>
<h4>Setting up per-process state<span><a class="mark" href="#embedding_setting_up_per_process_state" id="embedding_setting_up_per_process_state">#</a></span></h4>
<p>Node.js requires some per-process state management in order to run:</p>
<ul>
<li>Arguments parsing for Node.js <a href="cli.html">CLI options</a>,</li>
<li>V8 per-process requirements, such as a <code>v8::Platform</code> instance.</li>
</ul>
<p>The following example shows how these can be set up. Some class names are from
the <code>node</code> and <code>v8</code> C++ namespaces, respectively.</p>
<pre><code class="language-cpp"><span class="hljs-function"><span class="hljs-type">int</span> <span class="hljs-title">main</span><span class="hljs-params">(<span class="hljs-type">int</span> argc, <span class="hljs-type">char</span>** argv)</span> </span>{
argv = <span class="hljs-built_in">uv_setup_args</span>(argc, argv);
<span class="hljs-function">std::vector<std::string> <span class="hljs-title">args</span><span class="hljs-params">(argv, argv + argc)</span></span>;
std::vector<std::string> exec_args;
std::vector<std::string> errors;
<span class="hljs-comment">// Parse Node.js CLI options, and print any errors that have occurred while</span>
<span class="hljs-comment">// trying to parse them.</span>
<span class="hljs-type">int</span> exit_code = node::<span class="hljs-built_in">InitializeNodeWithArgs</span>(&args, &exec_args, &errors);
<span class="hljs-keyword">for</span> (<span class="hljs-keyword">const</span> std::string& error : errors)
<span class="hljs-built_in">fprintf</span>(stderr, <span class="hljs-string">"%s: %s\n"</span>, args[<span class="hljs-number">0</span>].<span class="hljs-built_in">c_str</span>(), error.<span class="hljs-built_in">c_str</span>());
<span class="hljs-keyword">if</span> (exit_code != <span class="hljs-number">0</span>) {
<span class="hljs-keyword">return</span> exit_code;
}
<span class="hljs-comment">// Create a v8::Platform instance. `MultiIsolatePlatform::Create()` is a way</span>
<span class="hljs-comment">// to create a v8::Platform instance that Node.js can use when creating</span>
<span class="hljs-comment">// Worker threads. When no `MultiIsolatePlatform` instance is present,</span>
<span class="hljs-comment">// Worker threads are disabled.</span>
std::unique_ptr<MultiIsolatePlatform> platform =
MultiIsolatePlatform::<span class="hljs-built_in">Create</span>(<span class="hljs-number">4</span>);
V8::<span class="hljs-built_in">InitializePlatform</span>(platform.<span class="hljs-built_in">get</span>());
V8::<span class="hljs-built_in">Initialize</span>();
<span class="hljs-comment">// See below for the contents of this function.</span>
<span class="hljs-type">int</span> ret = <span class="hljs-built_in">RunNodeInstance</span>(platform.<span class="hljs-built_in">get</span>(), args, exec_args);
V8::<span class="hljs-built_in">Dispose</span>();
V8::<span class="hljs-built_in">ShutdownPlatform</span>();
<span class="hljs-keyword">return</span> ret;
}</code></pre>
<h4>Per-instance state<span><a class="mark" href="#embedding_per_instance_state" id="embedding_per_instance_state">#</a></span></h4>
<p>Node.js has a concept of a “Node.js instanceâ€, that is commonly being referred
to as <code>node::Environment</code>. Each <code>node::Environment</code> is associated with:</p>
<ul>
<li>Exactly one <code>v8::Isolate</code>, i.e. one JS Engine instance,</li>
<li>Exactly one <code>uv_loop_t</code>, i.e. one event loop, and</li>
<li>A number of <code>v8::Context</code>s, but exactly one main <code>v8::Context</code>.</li>
<li>One <code>node::IsolateData</code> instance that contains information that could be
shared by multiple <code>node::Environment</code>s that use the same <code>v8::Isolate</code>.
Currently, no testing if performed for this scenario.</li>
</ul>
<p>In order to set up a <code>v8::Isolate</code>, an <code>v8::ArrayBuffer::Allocator</code> needs
to be provided. One possible choice is the default Node.js allocator, which
can be created through <code>node::ArrayBufferAllocator::Create()</code>. Using the Node.js
allocator allows minor performance optimizations when addons use the Node.js
C++ <code>Buffer</code> API, and is required in order to track <code>ArrayBuffer</code> memory in
<a href="process.html#process_process_memoryusage"><code>process.memoryUsage()</code></a>.</p>
<p>Additionally, each <code>v8::Isolate</code> that is used for a Node.js instance needs to
be registered and unregistered with the <code>MultiIsolatePlatform</code> instance, if one
is being used, in order for the platform to know which event loop to use
for tasks scheduled by the <code>v8::Isolate</code>.</p>
<p>The <code>node::NewIsolate()</code> helper function creates a <code>v8::Isolate</code>,
sets it up with some Node.js-specific hooks (e.g. the Node.js error handler),
and registers it with the platform automatically.</p>
<pre><code class="language-cpp"><span class="hljs-function"><span class="hljs-type">int</span> <span class="hljs-title">RunNodeInstance</span><span class="hljs-params">(MultiIsolatePlatform* platform,
<span class="hljs-keyword">const</span> std::vector<std::string>& args,
<span class="hljs-keyword">const</span> std::vector<std::string>& exec_args)</span> </span>{
<span class="hljs-type">int</span> exit_code = <span class="hljs-number">0</span>;
<span class="hljs-comment">// Set up a libuv event loop.</span>
<span class="hljs-type">uv_loop_t</span> loop;
<span class="hljs-type">int</span> ret = <span class="hljs-built_in">uv_loop_init</span>(&loop);
<span class="hljs-keyword">if</span> (ret != <span class="hljs-number">0</span>) {
<span class="hljs-built_in">fprintf</span>(stderr, <span class="hljs-string">"%s: Failed to initialize loop: %s\n"</span>,
args[<span class="hljs-number">0</span>].<span class="hljs-built_in">c_str</span>(),
<span class="hljs-built_in">uv_err_name</span>(ret));
<span class="hljs-keyword">return</span> <span class="hljs-number">1</span>;
}
std::shared_ptr<ArrayBufferAllocator> allocator =
ArrayBufferAllocator::<span class="hljs-built_in">Create</span>();
Isolate* isolate = <span class="hljs-built_in">NewIsolate</span>(allocator, &loop, platform);
<span class="hljs-keyword">if</span> (isolate == <span class="hljs-literal">nullptr</span>) {
<span class="hljs-built_in">fprintf</span>(stderr, <span class="hljs-string">"%s: Failed to initialize V8 Isolate\n"</span>, args[<span class="hljs-number">0</span>].<span class="hljs-built_in">c_str</span>());
<span class="hljs-keyword">return</span> <span class="hljs-number">1</span>;
}
{
<span class="hljs-function">Locker <span class="hljs-title">locker</span><span class="hljs-params">(isolate)</span></span>;
<span class="hljs-function">Isolate::Scope <span class="hljs-title">isolate_scope</span><span class="hljs-params">(isolate)</span></span>;
<span class="hljs-comment">// Create a node::IsolateData instance that will later be released using</span>
<span class="hljs-comment">// node::FreeIsolateData().</span>
<span class="hljs-function">std::unique_ptr<IsolateData, <span class="hljs-title">decltype</span><span class="hljs-params">(&node::FreeIsolateData)</span>> <span class="hljs-title">isolate_data</span><span class="hljs-params">(
node::CreateIsolateData(isolate, &loop, platform, allocator.get()),
node::FreeIsolateData)</span></span>;
<span class="hljs-comment">// Set up a new v8::Context.</span>
<span class="hljs-function">HandleScope <span class="hljs-title">handle_scope</span><span class="hljs-params">(isolate)</span></span>;
Local<Context> context = node::<span class="hljs-built_in">NewContext</span>(isolate);
<span class="hljs-keyword">if</span> (context.<span class="hljs-built_in">IsEmpty</span>()) {
<span class="hljs-built_in">fprintf</span>(stderr, <span class="hljs-string">"%s: Failed to initialize V8 Context\n"</span>, args[<span class="hljs-number">0</span>].<span class="hljs-built_in">c_str</span>());
<span class="hljs-keyword">return</span> <span class="hljs-number">1</span>;
}
<span class="hljs-comment">// The v8::Context needs to be entered when node::CreateEnvironment() and</span>
<span class="hljs-comment">// node::LoadEnvironment() are being called.</span>
<span class="hljs-function">Context::Scope <span class="hljs-title">context_scope</span><span class="hljs-params">(context)</span></span>;
<span class="hljs-comment">// Create a node::Environment instance that will later be released using</span>
<span class="hljs-comment">// node::FreeEnvironment().</span>
<span class="hljs-function">std::unique_ptr<Environment, <span class="hljs-title">decltype</span><span class="hljs-params">(&node::FreeEnvironment)</span>> <span class="hljs-title">env</span><span class="hljs-params">(
node::CreateEnvironment(isolate_data.get(), context, args, exec_args),
node::FreeEnvironment)</span></span>;
<span class="hljs-comment">// Set up the Node.js instance for execution, and run code inside of it.</span>
<span class="hljs-comment">// There is also a variant that takes a callback and provides it with</span>
<span class="hljs-comment">// the `require` and `process` objects, so that it can manually compile</span>
<span class="hljs-comment">// and run scripts as needed.</span>
<span class="hljs-comment">// The `require` function inside this script does *not* access the file</span>
<span class="hljs-comment">// system, and can only load built-in Node.js modules.</span>
<span class="hljs-comment">// `module.createRequire()` is being used to create one that is able to</span>
<span class="hljs-comment">// load files from the disk, and uses the standard CommonJS file loader</span>
<span class="hljs-comment">// instead of the internal-only `require` function.</span>
MaybeLocal<Value> loadenv_ret = node::<span class="hljs-built_in">LoadEnvironment</span>(
env.<span class="hljs-built_in">get</span>(),
<span class="hljs-string">"const publicRequire ="</span>
<span class="hljs-string">" require('module').createRequire(process.cwd() + '/');"</span>
<span class="hljs-string">"globalThis.require = publicRequire;"</span>
<span class="hljs-string">"require('vm').runInThisContext(process.argv[1]);"</span>);
<span class="hljs-keyword">if</span> (loadenv_ret.<span class="hljs-built_in">IsEmpty</span>()) <span class="hljs-comment">// There has been a JS exception.</span>
<span class="hljs-keyword">return</span> <span class="hljs-number">1</span>;
{
<span class="hljs-comment">// SealHandleScope protects against handle leaks from callbacks.</span>
<span class="hljs-function">SealHandleScope <span class="hljs-title">seal</span><span class="hljs-params">(isolate)</span></span>;
<span class="hljs-type">bool</span> more;
<span class="hljs-keyword">do</span> {
<span class="hljs-built_in">uv_run</span>(&loop, UV_RUN_DEFAULT);
<span class="hljs-comment">// V8 tasks on background threads may end up scheduling new tasks in the</span>
<span class="hljs-comment">// foreground, which in turn can keep the event loop going. For example,</span>
<span class="hljs-comment">// WebAssembly.compile() may do so.</span>
platform-><span class="hljs-built_in">DrainTasks</span>(isolate);
<span class="hljs-comment">// If there are new tasks, continue.</span>
more = <span class="hljs-built_in">uv_loop_alive</span>(&loop);
<span class="hljs-keyword">if</span> (more) <span class="hljs-keyword">continue</span>;
<span class="hljs-comment">// node::EmitProcessBeforeExit() is used to emit the 'beforeExit' event</span>
<span class="hljs-comment">// on the `process` object.</span>
<span class="hljs-keyword">if</span> (node::<span class="hljs-built_in">EmitProcessBeforeExit</span>(env.<span class="hljs-built_in">get</span>()).<span class="hljs-built_in">IsNothing</span>())
<span class="hljs-keyword">break</span>;
<span class="hljs-comment">// 'beforeExit' can also schedule new work that keeps the event loop</span>
<span class="hljs-comment">// running.</span>
more = <span class="hljs-built_in">uv_loop_alive</span>(&loop);
} <span class="hljs-keyword">while</span> (more == <span class="hljs-literal">true</span>);
}
<span class="hljs-comment">// node::EmitProcessExit() returns the current exit code.</span>
exit_code = node::<span class="hljs-built_in">EmitProcessExit</span>(env.<span class="hljs-built_in">get</span>()).<span class="hljs-built_in">FromMaybe</span>(<span class="hljs-number">1</span>);
<span class="hljs-comment">// node::Stop() can be used to explicitly stop the event loop and keep</span>
<span class="hljs-comment">// further JavaScript from running. It can be called from any thread,</span>
<span class="hljs-comment">// and will act like worker.terminate() if called from another thread.</span>
node::<span class="hljs-built_in">Stop</span>(env.<span class="hljs-built_in">get</span>());
}
<span class="hljs-comment">// Unregister the Isolate with the platform and add a listener that is called</span>
<span class="hljs-comment">// when the Platform is done cleaning up any state it had associated with</span>
<span class="hljs-comment">// the Isolate.</span>
<span class="hljs-type">bool</span> platform_finished = <span class="hljs-literal">false</span>;
platform-><span class="hljs-built_in">AddIsolateFinishedCallback</span>(isolate, [](<span class="hljs-type">void</span>* data) {
*<span class="hljs-built_in">static_cast</span><<span class="hljs-type">bool</span>*>(data) = <span class="hljs-literal">true</span>;
}, &platform_finished);
platform-><span class="hljs-built_in">UnregisterIsolate</span>(isolate);
isolate-><span class="hljs-built_in">Dispose</span>();
<span class="hljs-comment">// Wait until the platform has cleaned up all relevant resources.</span>
<span class="hljs-keyword">while</span> (!platform_finished)
<span class="hljs-built_in">uv_run</span>(&loop, UV_RUN_ONCE);
<span class="hljs-type">int</span> err = <span class="hljs-built_in">uv_loop_close</span>(&loop);
<span class="hljs-built_in">assert</span>(err == <span class="hljs-number">0</span>);
<span class="hljs-keyword">return</span> exit_code;
}</code></pre></section>
<!-- API END -->
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