## 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).

 - libev, 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 libev.  That is, if you perform
   any I/O, libev will need to be used.  Node uses the `EV_DEFAULT` event
   loop.  Documentation can be found http:/cvs.schmorp.de/libev/ev.html[here].

 - libeio, C thread pool library. Used to execute blocking POSIX system
   calls asynchronously. Mostly wrappers already exist for such calls, in
   `src/file.cc` so you will probably not need to use it. If you do need it,
   look at the header file `deps/libeio/eio.h`.

 - 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 does the following except in
C++:

    exports.hello = 'world';

To get started we create a file `hello.cc`:

    #include <v8.h>

    using namespace v8;

    extern "C" void
    init (Handle<Object> target) 
    {
      HandleScope scope;
      target->Set(String::New("hello"), String::New("World"));
    }

This 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 http://code.google.com/p/waf/[WAF], the python-based build system. `node-waf` is
provided for the ease of users.

All Node addons must export a function called `init` with this signature:

    extern 'C' void init (Handle<Object> target)

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