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C++ addons

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Added in: v0.10.0

Addons are dynamically-linked shared objects written in C++. The require() function can load addons as ordinary Node.js modules. Addons provide an interface between JavaScript and C/C++ libraries.

There are three options for implementing addons: Node-API, nan, or direct use of internal V8, libuv, and Node.js libraries. Unless there is a need for direct access to functionality which is not exposed by Node-API, use Node-API. Refer to C/C++ addons with Node-API for more information on Node-API.

When not using Node-API, implementing addons is complicated, involving knowledge of several components and APIs:

  • V8: the C++ library Node.js uses to provide the JavaScript implementation. V8 provides the mechanisms for creating objects, calling functions, etc. V8's API is documented mostly in the v8.h header file (deps/v8/include/v8.h in the Node.js source tree), which is also available online.

  • libuv: The C library that implements the Node.js event loop, its worker threads and all of the asynchronous behaviors of the platform. It also serves as a cross-platform abstraction library, giving easy, POSIX-like access across all major operating systems to many common system tasks, such as interacting with the filesystem, sockets, timers, and system events. libuv also provides a threading abstraction similar to POSIX threads for more sophisticated asynchronous addons that need to move beyond the standard event loop. Addon authors should avoid blocking the event loop with I/O or other time-intensive tasks by offloading work via libuv to non-blocking system operations, worker threads, or a custom use of libuv threads.

  • Internal Node.js libraries. Node.js itself exports C++ APIs that addons can use, the most important of which is the node::ObjectWrap class.

  • Node.js includes other statically linked libraries including OpenSSL. These other libraries are located in the deps/ directory in the Node.js source tree. Only the libuv, OpenSSL, V8, and zlib symbols are purposefully re-exported by Node.js and may be used to various extents by addons. See Linking to libraries included with Node.js for additional information.

All of the following examples are available for download and may be used as the starting-point for an addon.

Hello world

This "Hello world" example is a simple addon, written in C++, that is the equivalent of the following JavaScript code:

JS

First, create the file hello.cc:

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All Node.js addons must export an initialization function following the pattern:

CPP

There is no semi-colon after NODE_MODULE as it's not a function (see node.h).

The module_name must match the filename of the final binary (excluding the .node suffix).

In the hello.cc example, then, the initialization function is Initialize and the addon module name is addon.

When building addons with node-gyp, using the macro NODE_GYP_MODULE_NAME as the first parameter of NODE_MODULE() will ensure that the name of the final binary will be passed to NODE_MODULE().

Context-aware addons

There are environments in which Node.js addons may need to be loaded multiple times in multiple contexts. For example, the Electron runtime runs multiple instances of Node.js in a single process. Each instance will have its own require() cache, and thus each instance will need a native addon to behave correctly when loaded via require(). This means that the addon must support multiple initializations.

A context-aware addon can be constructed by using the macro NODE_MODULE_INITIALIZER, which expands to the name of a function which Node.js will expect to find when it loads an addon. An addon can thus be initialized as in the following example:

CPP

Another option is to use the macro NODE_MODULE_INIT(), which will also construct a context-aware addon. Unlike NODE_MODULE(), which is used to construct an addon around a given addon initializer function, NODE_MODULE_INIT() serves as the declaration of such an initializer to be followed by a function body.

The following three variables may be used inside the function body following an invocation of NODE_MODULE_INIT():

  • Local<Object> exports,
  • Local<Value> module, and
  • Local<Context> context

The choice to build a context-aware addon carries with it the responsibility of carefully managing global static data. Since the addon may be loaded multiple times, potentially even from different threads, any global static data stored in the addon must be properly protected, and must not contain any persistent references to JavaScript objects. The reason for this is that JavaScript objects are only valid in one context, and will likely cause a crash when accessed from the wrong context or from a different thread than the one on which they were created.

The context-aware addon can be structured to avoid global static data by performing the following steps:

  • Define a class which will hold per-addon-instance data and which has a static member of the form
    CPP
  • Heap-allocate an instance of this class in the addon initializer. This can be accomplished using the new keyword.
  • Call node::AddEnvironmentCleanupHook(), passing it the above-created instance and a pointer to DeleteInstance(). This will ensure the instance is deleted when the environment is torn down.
  • Store the instance of the class in a v8::External, and
  • Pass the v8::External to all methods exposed to JavaScript by passing it to v8::FunctionTemplate::New() or v8::Function::New() which creates the native-backed JavaScript functions. The third parameter of v8::FunctionTemplate::New() or v8::Function::New() accepts the v8::External and makes it available in the native callback using the v8::FunctionCallbackInfo::Data() method.

This will ensure that the per-addon-instance data reaches each binding that can be called from JavaScript. The per-addon-instance data must also be passed into any asynchronous callbacks the addon may create.

The following example illustrates the implementation of a context-aware addon:

CPP
Worker support

In order to be loaded from multiple Node.js environments, such as a main thread and a Worker thread, an add-on needs to either:

  • Be an Node-API addon, or
  • Be declared as context-aware using NODE_MODULE_INIT() as described above

In order to support Worker threads, addons need to clean up any resources they may have allocated when such a thread exists. This can be achieved through the usage of the AddEnvironmentCleanupHook() function:

CPP

This function adds a hook that will run before a given Node.js instance shuts down. If necessary, such hooks can be removed before they are run using RemoveEnvironmentCleanupHook(), which has the same signature. Callbacks are run in last-in first-out order.

If necessary, there is an additional pair of AddEnvironmentCleanupHook() and RemoveEnvironmentCleanupHook() overloads, where the cleanup hook takes a callback function. This can be used for shutting down asynchronous resources, such as any libuv handles registered by the addon.

The following addon.cc uses AddEnvironmentCleanupHook:

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Test in JavaScript by running:

JS

Building

Once the source code has been written, it must be compiled into the binary addon.node file. To do so, create a file called binding.gyp in the top-level of the project describing the build configuration of the module using a JSON-like format. This file is used by node-gyp, a tool written specifically to compile Node.js addons.

JSON

A version of the node-gyp utility is bundled and distributed with Node.js as part of npm. This version is not made directly available for developers to use and is intended only to support the ability to use the npm install command to compile and install addons. Developers who wish to use node-gyp directly can install it using the command npm install -g node-gyp. See the node-gyp installation instructions for more information, including platform-specific requirements.

Once the binding.gyp file has been created, use node-gyp configure to generate the appropriate project build files for the current platform. This will generate either a Makefile (on Unix platforms) or a vcxproj file (on Windows) in the build/ directory.

Next, invoke the node-gyp build command to generate the compiled addon.node file. This will be put into the build/Release/ directory.

When using npm install to install a Node.js addon, npm uses its own bundled version of node-gyp to perform this same set of actions, generating a compiled version of the addon for the user's platform on demand.

Once built, the binary addon can be used from within Node.js by pointing require() to the built addon.node module:

JS

Because the exact path to the compiled addon binary can vary depending on how it is compiled (i.e. sometimes it may be in ./build/Debug/), addons can use the bindings package to load the compiled module.

While the bindings package implementation is more sophisticated in how it locates addon modules, it is essentially using a try…catch pattern similar to:

JS

Linking to libraries included with Node.js

Node.js uses statically linked libraries such as V8, libuv, and OpenSSL. All addons are required to link to V8 and may link to any of the other dependencies as well. Typically, this is as simple as including the appropriate #include <...> statements (e.g. #include <v8.h>) and node-gyp will locate the appropriate headers automatically. However, there are a few caveats to be aware of:

  • When node-gyp runs, it will detect the specific release version of Node.js and download either the full source tarball or just the headers. If the full source is downloaded, addons will have complete access to the full set of Node.js dependencies. However, if only the Node.js headers are downloaded, then only the symbols exported by Node.js will be available.

  • node-gyp can be run using the --nodedir flag pointing at a local Node.js source image. Using this option, the addon will have access to the full set of dependencies.

Loading addons using require()

The filename extension of the compiled addon binary is .node (as opposed to .dll or .so). The require() function is written to look for files with the .node file extension and initialize those as dynamically-linked libraries.

When calling require(), the .node extension can usually be omitted and Node.js will still find and initialize the addon. One caveat, however, is that Node.js will first attempt to locate and load modules or JavaScript files that happen to share the same base name. For instance, if there is a file addon.js in the same directory as the binary addon.node, then require('addon') will give precedence to the addon.js file and load it instead.

Native abstractions for Node.js

Each of the examples illustrated in this document directly use the Node.js and V8 APIs for implementing addons. The V8 API can, and has, changed dramatically from one V8 release to the next (and one major Node.js release to the next). With each change, addons may need to be updated and recompiled in order to continue functioning. The Node.js release schedule is designed to minimize the frequency and impact of such changes but there is little that Node.js can do to ensure stability of the V8 APIs.

The Native Abstractions for Node.js (or nan) provide a set of tools that addon developers are recommended to use to keep compatibility between past and future releases of V8 and Node.js. See the nan examples for an illustration of how it can be used.

Node-API

稳定性: 2 - Stable

Node-API is an API for building native addons. It is independent from the underlying JavaScript runtime (e.g. V8) and is maintained as part of Node.js itself. This API will be Application Binary Interface (ABI) stable across versions of Node.js. It is intended to insulate addons from changes in the underlying JavaScript engine and allow modules compiled for one version to run on later versions of Node.js without recompilation. Addons are built/packaged with the same approach/tools outlined in this document (node-gyp, etc.). The only difference is the set of APIs that are used by the native code. Instead of using the V8 or Native Abstractions for Node.js APIs, the functions available in the Node-API are used.

Creating and maintaining an addon that benefits from the ABI stability provided by Node-API carries with it certain implementation considerations.

To use Node-API in the above "Hello world" example, replace the content of hello.cc with the following. All other instructions remain the same.

CPP

The functions available and how to use them are documented in C/C++ addons with Node-API.

Addon examples

Following are some example addons intended to help developers get started. The examples use the V8 APIs. Refer to the online V8 reference for help with the various V8 calls, and V8's Embedder's Guide for an explanation of several concepts used such as handles, scopes, function templates, etc.

Each of these examples using the following binding.gyp file:

JSON

In cases where there is more than one .cc file, simply add the additional filename to the sources array:

JSON

Once the binding.gyp file is ready, the example addons can be configured and built using node-gyp:

BASH

Function arguments

Addons will typically expose objects and functions that can be accessed from JavaScript running within Node.js. When functions are invoked from JavaScript, the input arguments and return value must be mapped to and from the C/C++ code.

The following example illustrates how to read function arguments passed from JavaScript and how to return a result:

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Once compiled, the example addon can be required and used from within Node.js:

JS

Callbacks

It is common practice within addons to pass JavaScript functions to a C++ function and execute them from there. The following example illustrates how to invoke such callbacks:

CPP

This example uses a two-argument form of Init() that receives the full module object as the second argument. This allows the addon to completely overwrite exports with a single function instead of adding the function as a property of exports.

To test it, run the following JavaScript:

JS

In this example, the callback function is invoked synchronously.

Object factory

Addons can create and return new objects from within a C++ function as illustrated in the following example. An object is created and returned with a property msg that echoes the string passed to createObject():

CPP

To test it in JavaScript:

JS

Function factory

Another common scenario is creating JavaScript functions that wrap C++ functions and returning those back to JavaScript:

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To test:

JS

Wrapping C++ objects

It is also possible to wrap C++ objects/classes in a way that allows new instances to be created using the JavaScript new operator:

CPP

Then, in myobject.h, the wrapper class inherits from node::ObjectWrap:

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In myobject.cc, implement the various methods that are to be exposed. Below, the method plusOne() is exposed by adding it to the constructor's prototype:

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To build this example, the myobject.cc file must be added to the binding.gyp:

JSON

Test it with:

JS

The destructor for a wrapper object will run when the object is garbage-collected. For destructor testing, there are command-line flags that can be used to make it possible to force garbage collection. These flags are provided by the underlying V8 JavaScript engine. They are subject to change or removal at any time. They are not documented by Node.js or V8, and they should never be used outside of testing.

During shutdown of the process or worker threads destructors are not called by the JS engine. Therefore it's the responsibility of the user to track these objects and ensure proper destruction to avoid resource leaks.

Factory of wrapped objects

Alternatively, it is possible to use a factory pattern to avoid explicitly creating object instances using the JavaScript new operator:

JS

First, the createObject() method is implemented in addon.cc:

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In myobject.h, the static method NewInstance() is added to handle instantiating the object. This method takes the place of using new in JavaScript:

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The implementation in myobject.cc is similar to the previous example:

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Once again, to build this example, the myobject.cc file must be added to the binding.gyp:

JSON

Test it with:

JS

Passing wrapped objects around

In addition to wrapping and returning C++ objects, it is possible to pass wrapped objects around by unwrapping them with the Node.js helper function node::ObjectWrap::Unwrap. The following examples shows a function add() that can take two MyObject objects as input arguments:

CPP

In myobject.h, a new public method is added to allow access to private values after unwrapping the object.

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The implementation of myobject.cc is similar to before:

CPP

Test it with:

JS
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