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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:

• V8 JavaScript, a C++ library. Used for interfacing with JavaScript: creating objects, calling functions, etc. Documented mostly in the v8.h header file (deps/v8/include/v8.h in the Node source tree).

• 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 received one will need to interface with libuv. That is, if you perform any I/O, libuv will need to be used.

• Internal Node libraries. Most importantly is the node::ObjectWrap class which you will likely want to derive from.

• Others. Look in deps/ for what else is available.

Node 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.

To get started let's make a small Addon which is the C++ equivalent of the following Javascript code:

exports.hello = function() { return 'world'; };

To get started we create a file hello.cc:

#include <node.h>
#include <v8.h>

using namespace v8;

Handle<Value> Method(const Arguments& args) {
  HandleScope scope;
  return scope.Close(String::New("world"));
}

void init(Handle<Object> target) {
  NODE_SET_METHOD(target, "hello", Method);
}
NODE_MODULE(hello, init)

Note that all Node addons must export an initialization function:

void Initialize (Handle<Object> target);
NODE_MODULE(module_name, Initialize)

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

The source code needs to be built into hello.node, the binary Addon. To do this we create a file called wscript which is python code and looks like this:

srcdir = '.'
blddir = 'build'
VERSION = '0.0.1'

def set_options(opt):
  opt.tool_options('compiler_cxx')

def configure(conf):
  conf.check_tool('compiler_cxx')
  conf.check_tool('node_addon')

def build(bld):
  obj = bld.new_task_gen('cxx', 'shlib', 'node_addon')
  obj.target = 'hello'
  obj.source = 'hello.cc'

Running node-waf configure build will create a file build/default/hello.node which is our Addon.

node-waf is just WAF, the python-based build system. node-waf is provided for the ease of users.

You can now use the binary addon in a Node project hello.js by pointing require to the recently built module:

var addon = require('./build/Release/hello');

console.log(addon.hello()); // 'world'

For the moment, that is all the documentation on addons. Please see https://github.com/ry/node_postgres for a real example.

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