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General improvements to the documentation in addons.markdown.

PR-URL: https://github.com/nodejs/node/pull/4320
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Sakthipriyan Vairamani <thechargingvolcano@gmail.com>
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@ -1,39 +1,52 @@
# Addons
Addons are dynamically-linked shared objects. They can provide glue to C and
C++ libraries. The API (at the moment) is rather complex, involving
knowledge of several libraries:
Node.js Addons are dynamically-linked shared objects, written in C or C++, that
can be loaded into Node.js using the `require()` function, and used just as if
they were an ordinary Node.js module. They are used primarily to provide an
interface between JavaScript running in Node.js and C/C++ libraries.
- V8 JavaScript, a C++ library. Used for interfacing with JavaScript:
creating objects, calling functions, etc. Documented mostly in the
At the moment, the method for implementing Addons is rather complicated,
involving knowledge of several components and APIs :
- V8: the C++ library Node.js currently 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][], C event loop library. Anytime one needs to wait for a file
descriptor to become readable, wait for a timer, or wait for a signal
to be received, one will need to interface with libuv. That is, if you
perform any I/O, libuv will need to be used.
- [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 pthreads-like threading abstraction that may be used to
power more sophisticated asynchronous Addons that need to move beyond the
standard event loop. Addon authors are encouraged to think about how to
avoid blocking the event loop with I/O or other time-intensive tasks by
off-loading work via libuv to non-blocking system operations, worker threads
or a custom use of libuv's threads.
- Internal Node.js libraries. The most important class is `node::ObjectWrap`
which you will likely want to derive from.
- Internal Node.js libraries. Node.js itself exports a number of C/C++ APIs
that Addons can use &mdash; the most important of which is the
`node::ObjectWrap` class.
- Others. Look in `deps/` for what else is available.
Node.js statically compiles all its dependencies into the executable.
When compiling your module, you don't need to worry about linking to
any of these libraries.
- Node.js includes a number of other statically linked libraries including
OpenSSL. These other libraries are located in the `deps/` directory in the
Node.js source tree. Only the V8 and OpenSSL symbols are purposefully
re-exported by Node.js and may be used to various extents by Addons.
See [Linking to Node.js' own dependencies][] for additional information.
All of the following examples are available for [download][] and may
be used as a starting-point for your own Addon.
## Hello world
To get started, let's make a small Addon which is the C++ equivalent of
the following JavaScript code:
This "Hello world" example is a simple Addon, written in C++, that is the
equivalent of the following JavaScript code:
module.exports.hello = function() { return 'world'; };
First we create a file `hello.cc`:
First, create the file `hello.cc`:
// hello.cc
#include <node.h>
@ -60,7 +73,8 @@ First we create a file `hello.cc`:
} // namespace demo
Note that all Node.js addons must export an initialization function:
Note that all Node.js Addons must export an initialization function following
the pattern:
void Initialize(Local<Object> exports);
NODE_MODULE(module_name, Initialize)
@ -68,13 +82,19 @@ Note that all Node.js addons must export an initialization function:
There is no semi-colon after `NODE_MODULE` as it's not a function (see
`node.h`).
The `module_name` needs to match the filename of the final binary (excluding
The `module_name` must match the filename of the final binary (excluding
the .node suffix).
The source code needs to be built into `addon.node`, the binary Addon. To
do this, we create a file called `binding.gyp` which describes the configuration
to build your module in a JSON-like format. This file gets compiled by
[node-gyp][].
In the `hello.cc` example, then, the initialization function is `init` and the
Addon module name is `addon`.
### 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 your module
using a JSON-like format. This file is used by [node-gyp][] -- a tool written
specifically to compile Node.js Addons.
{
"targets": [
@ -85,37 +105,109 @@ to build your module in a JSON-like format. This file gets compiled by
]
}
The next step is to generate the appropriate project build files for the
current platform. Use `node-gyp configure` for that.
*Note: 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.*
Now you will have either a `Makefile` (on Unix platforms) or a `vcxproj` file
(on Windows) in the `build/` directory. Next, invoke the `node-gyp build`
command.
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.
Now you have your compiled `.node` bindings file! The compiled bindings end up
in `build/Release/`.
Next, invoke the `node-gyp build` command to generate the compiled `addon.node`
file. This will be put into the `build/Release/` directory.
You can now use the binary addon in a Node.js project `hello.js` by pointing
`require` to the recently built `hello.node` module:
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:
// hello.js
const addon = require('./build/Release/addon');
console.log(addon.hello()); // 'world'
Please see patterns below for further information or
Please see the examples below for further information or
<https://github.com/arturadib/node-qt> for an example in production.
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.
## Addon patterns
Note that while the `bindings` package implementation is more sophisticated
in how it locates Addon modules, it is essentially using a try-catch pattern
similar to:
Below are some addon patterns to help you get started. Consult 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.
try {
return require('./build/Release/addon.node');
} catch (err) {
return require('./build/Debug/addon.node');
}
In order to use these examples, you need to compile them using `node-gyp`.
Create the following `binding.gyp` file:
### Linking to Node.js' own dependencies
Node.js uses a number of 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 make direct use of the
Node.js and V8 APIs for implementing Addons. It is important to understand
that 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 currently to ensure stability
of the V8 APIs.
The [Native Abstrations 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.
## Addon examples
Following are some example Addons intended to help developers get started. The
examples make use of 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:
{
"targets": [
@ -126,22 +218,26 @@ Create the following `binding.gyp` file:
]
}
In cases where there is more than one `.cc` file, simply add the file name to
the `sources` array. For example:
In cases where there is more than one `.cc` file, simply add the additional
filename to the `sources` array. For example:
"sources": ["addon.cc", "myexample.cc"]
Now that you have your `binding.gyp` ready, you can configure and build the
addon:
Once the `binding.gyp` file is ready, the example Addons can be configured and
built using `node-gyp`:
$ node-gyp configure build
### Function arguments
The following pattern illustrates how to read arguments from JavaScript
function calls and return a result. This is the main and only needed source
`addon.cc`:
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:
// addon.cc
#include <node.h>
@ -157,24 +253,33 @@ function calls and return a result. This is the main and only needed source
using v8::String;
using v8::Value;
// This is the implementation of the "add" method
// Input arguments are passed using the
// const FunctionCallbackInfo<Value>& args struct
void Add(const FunctionCallbackInfo<Value>& args) {
Isolate* isolate = args.GetIsolate();
// Check the number of arguments passed.
if (args.Length() < 2) {
// Throw an Error that is passed back to JavaScript
isolate->ThrowException(Exception::TypeError(
String::NewFromUtf8(isolate, "Wrong number of arguments")));
return;
}
// Check the argument types
if (!args[0]->IsNumber() || !args[1]->IsNumber()) {
isolate->ThrowException(Exception::TypeError(
String::NewFromUtf8(isolate, "Wrong arguments")));
return;
}
// Perform the operation
double value = args[0]->NumberValue() + args[1]->NumberValue();
Local<Number> num = Number::New(isolate, value);
// Set the return value (using the passed in
// FunctionCallbackInfo<Value>&)
args.GetReturnValue().Set(num);
}
@ -186,7 +291,7 @@ function calls and return a result. This is the main and only needed source
} // namespace demo
You can test it with the following JavaScript snippet:
Once compiled, the example Addon can be required and used from within Node.js:
// test.js
const addon = require('./build/Release/addon');
@ -196,8 +301,9 @@ You can test it with the following JavaScript snippet:
### Callbacks
You can pass JavaScript functions to a C++ function and execute them from
there. Here's `addon.cc`:
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:
// addon.cc
#include <node.h>
@ -230,11 +336,11 @@ there. Here's `addon.cc`:
} // namespace demo
Note that this example uses a two-argument form of `Init()` that receives
the full `module` object as the second argument. This allows the addon
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 snippet:
To test it, run the following JavaScript:
// test.js
const addon = require('./build/Release/addon');
@ -243,12 +349,13 @@ To test it, run the following JavaScript snippet:
console.log(msg); // 'hello world'
});
Note that, in this example, the callback function is invoked synchronously.
### Object factory
You can create and return new objects from within a C++ function with this
`addon.cc` pattern, which returns an object with property `msg` that echoes
the string passed to `createObject()`:
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()`:
// addon.cc
#include <node.h>
@ -291,8 +398,8 @@ To test it in JavaScript:
### Function factory
This pattern illustrates how to create and return a JavaScript function that
wraps a C++ function:
Another common scenario is creating JavaScript functions that wrap C++
functions and returning those back to JavaScript:
// addon.cc
#include <node.h>
@ -344,9 +451,8 @@ To test:
### Wrapping C++ objects
Here, we will create a wrapper for a C++ object/class `MyObject` that can be
instantiated in JavaScript through the `new` operator. First, prepare the main
module `addon.cc`:
It is also possible to wrap C++ objects/classes in a way that allows new
instances to be created using the JavaScript `new` operator:
// addon.cc
#include <node.h>
@ -365,7 +471,7 @@ module `addon.cc`:
} // namespace demo
Then, in `myobject.h`, make your wrapper inherit from `node::ObjectWrap`:
Then, in `myobject.h`, the wrapper class inherits from `node::ObjectWrap`:
// myobject.h
#ifndef MYOBJECT_H
@ -394,8 +500,8 @@ Then, in `myobject.h`, make your wrapper inherit from `node::ObjectWrap`:
#endif
And in `myobject.cc`, implement the various methods that you want to expose.
Here we expose the method `plusOne` by adding it to the constructor's
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:
// myobject.cc
@ -467,6 +573,21 @@ prototype:
} // namespace demo
To build this example, the `myobject.cc` file must be added to the
`binding.gyp`:
{
"targets": [
{
"target_name": "addon",
"sources": [
"addon.cc",
"myobject.cc"
]
}
]
}
Test it with:
// test.js
@ -479,15 +600,14 @@ Test it with:
### Factory of wrapped objects
This is useful when you want to be able to create native objects without
explicitly instantiating them with the `new` operator in JavaScript. For
example:
Alternatively, it is possible to use a factory pattern to avoid explicitly
creating object instances using the JavaScript `new` operator:
var obj = addon.createObject();
// instead of:
// var obj = new addon.Object();
Let's register our `createObject` method in `addon.cc`:
First, the `createObject()` method is implemented in `addon.cc`:
// addon.cc
#include <node.h>
@ -516,8 +636,8 @@ Let's register our `createObject` method in `addon.cc`:
} // namespace demo
In `myobject.h`, we now introduce the static method `NewInstance` that takes
care of instantiating the object. In other words, it does the job of `new` in
In `myobject.h`, the static method `NewInstance()` is added to handle
instantiating the object. This method takes the place of using `new` in
JavaScript:
// myobject.h
@ -548,7 +668,7 @@ JavaScript:
#endif
The implementation is similar to the above in `myobject.cc`:
The implementation in `myobject.cc` is similar to the previous example:
// myobject.cc
#include <node.h>
@ -627,6 +747,21 @@ The implementation is similar to the above in `myobject.cc`:
} // namespace demo
Once again, to build this example, the `myobject.cc` file must be added to the
`binding.gyp`:
{
"targets": [
{
"target_name": "addon",
"sources": [
"addon.cc",
"myobject.cc"
]
}
]
}
Test it with:
// test.js
@ -645,10 +780,10 @@ Test it with:
### Passing wrapped objects around
In addition to wrapping and returning C++ objects, you can pass them around
by unwrapping them with the Node.js helper function `node::ObjectWrap::Unwrap`.
In the following `addon.cc`, we introduce a function `add()` that can take on
two `MyObject` objects:
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:
// addon.cc
#include <node.h>
@ -692,8 +827,8 @@ two `MyObject` objects:
} // namespace demo
To make things interesting, we introduce a public method in `myobject.h` so we
can probe private values after unwrapping the object:
In `myobject.h`, a new public method is added to allow access to private values
after unwrapping the object.
// myobject.h
#ifndef MYOBJECT_H
@ -801,6 +936,11 @@ Test it with:
console.log(result); // 30
### AtExit hooks
An "AtExit" hook is a function that is invoked after the Node.js event loop
has ended by before the JavaScript VM is terminated and Node.js shuts down.
"AtExit" hooks are registered using the `node::AtExit` API.
#### void AtExit(callback, args)
* `callback`: `void (*)(void*)` - A pointer to the function to call at exit.
@ -809,11 +949,12 @@ Test it with:
Registers exit hooks that run after the event loop has ended but before the VM
is killed.
Callbacks are run in last-in first-out order. AtExit takes two parameters:
a pointer to a callback function to run at exit, and a pointer to untyped
context data to be passed to that callback.
AtExit takes two parameters: a pointer to a callback function to run at exit,
and a pointer to untyped context data to be passed to that callback.
The file `addon.cc` implements AtExit below:
Callbacks are run in last-in first-out order.
The following `addon.cc` implements AtExit:
// addon.cc
#undef NDEBUG
@ -872,5 +1013,10 @@ Test in JavaScript by running:
[libuv]: https://github.com/libuv/libuv
[download]: https://github.com/nodejs/node-addon-examples
[node-gyp]: https://github.com/nodejs/node-gyp
[v8 reference]: http://izs.me/v8-docs/main.html
[Embedder's Guide]: https://code.google.com/apis/v8/embed.html
[V8 reference]: https://v8docs.nodesource.com/
[Embedder's Guide]: https://developers.google.com/v8/embed
[Native Abstrations for Node.js]: https://github.com/nodejs/nan
[examples]: https://github.com/nodejs/nan/tree/master/examples/
[bindings]: https://github.com/TooTallNate/node-bindings
[Linking to Node.js' own dependencies]: #linking-to-node-js-own-dependencies
[installation instructions]: https://github.com/nodejs/node-gyp#installation