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This tutorial was originally posted here: http://syskall.com/how-to-roll-out-your-own-javascript-api-with

Introduction

This tutorial will teach you how to:

  1. Compile the V8 Javascript engine
  2. Bind a Javascript function to your own C++ function

For the sake of demonstration and to impress your co-workers, we will bind a Javascript function "alert()“ that will display desktop notifications through the GTK library. Here’s what the end result looks like:

jsnotify screenshot

You can get the full source code of this tutorial from github:

git clone git://github.com/olalonde/jsnotify.git

This tutorial was tested on Ubuntu 10.04 and 10.10 64-bit but should work fine on any Linux distribution. The notification part requires the GTK+ library.

Compiling Google’s V8 Javascript engine

First, let’s make sure we have all the required tools and dependencies to compile.

sudo apt-get install build-essential scons subversion 
  • The build-essential package is a meta package that installs all the necessary tools and libraries to compile C++ programs.
  • SCons is a build tool which attempts to replace the classic “make” and is used by the V8 project.
  • Subversion is needed to checkout the source code of V8.

Now, let’s grab V8’s source from the official repository:

svn checkout http://v8.googlecode.com/svn/trunk/ v8 

We can now move into the V8 directory and try to compile!

cd v8;
scons arch=x64; 

The “arch=x64” option specifies that we want to build a 64-bit version of V8 (the default value would be 32-bit otherwise).

If V8 compiled fine, you should now have a libv8.a file in your v8/ directory. As you probably guessed, libv8.a is the library that our C++ program will use to execute Javascript code.

So, if everything compiled fine, just skip to the next section. Otherwise, keep on reading.

When you get errors as a result of compiling third party code, it is usually due to the fact that the compiler can’t find required libraries (/usr/lib) and/or their associated header files (/usr/lib/include). The latter are usually available through packages conventionally named libname-dev . In order to find out which package installs a given file, there is a neat utility called apt-file.

sudo apt-get install apt-file;
apt-file search missing-header-file.h; 

The apt-file search command lists the package(s) that install a given file (missing-header-file.h in this case). If there are more than one package listed, we have to take a semi-educated guess on which package we should install based on its name (let me know in the comments if you know of a better trick!). We then simply install the package with the usual apt-get install package-name command.

Hint: If you are on Ubuntu 10.04, you might need to install the following packages:

sudo apt-get install libc6-dev-i368 lib32stdc++6

Now that we’ve installed all the missing files, the compilation should work. Let's move on to the next section.

If you are still stuck with compiling V8, this tutorial might help.

Building our own Javascript API

Now that we have successfully compiled the V8 library, we will build our own C++ project that will be “Javascript scriptable”. This means that our program will be able to run Javascript code which in turn will be able to call our custom C++ functions.

Note: You can also get the full source code of this tutorial from my jsnotify github repository: git clone git://github.com/olalonde/jsnotify.git

First let’s create our file structure.

jsnotify/
  |-- deps/  # third party code
  |   `-- v8  # move your v8 folder here
  `-- src/ # our code goes here
      `-- jsnotify.cpp

Now let’s copy the sample code available at deps/v8/samples/shell.cc and paste it into jsnotify.cpp. The sample code given by V8 let’s you execute a Javascript file or start an interactive Javascript shell. It also binds some useful Javascript functions such as print() which will output text to the terminal.

Let’s try to compile this!

g++ src/jsnotify.cpp; 

Of course, this gives us a bunch of errors since we haven’t specified where the V8 header and library files are. Let’s try again!

g++ src/jsnotify.cpp -Ideps/v8/include -Ldeps/v8/ -lv8

Oops, still some errors. Looks like we also have to link the pthread library.

g++ src/jsnotify.cpp -Ideps/v8/include -Ldeps/v8/ -lv8 -lpthread

This finally compiles! Now that we have our mini Javascript shell, let’s play a bit with it.

$ ./a.out 
V8 version 3.1.5
> var foo = “Hello World”;
> print(foo);
Hello World

Now, all we have to do is to create our custom alert() function in C++.

#!cpp
// INSERT THIS BEFORE int RunMain(int argc, char* argv[]) {
// We need those two libraries for the GTK+ notification 
#include <gtkmm.h>
#include <libnotifymm.h>
v8::Handle<v8::Value> Alert(const v8::Arguments& args);

// INSERT THIS AT END OF FILE   
// The callback that is invoked by v8 whenever the JavaScript 'alert'
// function is called.  Displays a GTK+ notification.
v8::Handle<v8::Value> Alert(const v8::Arguments& args) {
  v8::String::Utf8Value str(args[0]); // Convert first argument to V8 String
  const char* cstr = ToCString(str); // Convert V8 String to C string

  Notify::init("Basic");
  // Arguments: title, content, icon
  Notify::Notification n("Alert", cstr, "terminal");
  // Display notification
  n.show();

  return v8::Undefined();
}

Now that we have our Alert C++ function, we need to tell V8 to bind it to the Javascript alert() function. This is done by adding the following code in the RunMain function:

#!cpp
// INSERT AFTER v8::Handle<v8::ObjectTemplate> global = v8::ObjectTemplate::New();
// Bind the global 'alert' function to the C++ Alert callback.
global->Set(v8::String::New("alert"), v8::FunctionTemplate::New(Alert));

Now, in order to compile, the compiler needs to know where to find the two header files we introduced. This is done using the pkg-config utility:

g++ src/jsnotify.cpp -Ideps/v8/include -Ldeps/v8/ -lv8 -lpthread `pkg-config --cflags --libs gtkmm-2.4 libnotifymm-1.0`

We can now try our new alert function.

$./a.out 
V8 version 3.1.5
> alert(“wow, it works!”);  

You should see a nice notification in the top right of your screen! Note that you can also put you Javascript code in a file and pass the file name as an argument ./a.out filename.js.

Conclusion

It's quite easy to make a C++ program "Javascriptable" with V8 and the proper setup. If you'd like to practice your newfound skills, I suggest you try to add a title argument to the alert function. You might also want to follow me on Posterous if you'd like to be informed when I post the follow up to this tutorial which will explain how to extend Node.js with our alert function.

That’s all for today, thanks for reading! Let me know in the comments if you run into any problem, I’ll be glad to help.

If you liked this, maybe you'd also like what I tweet on Twitter!

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