> After you build ruby you can use just a ruby executable without the 
> standard library if you do not need the functionality it provides. As 
> for built-in classes and modules, such as String, you will not be able 
> to get without them as they are implemented internally in C and other 
> parts of Ruby depend on them heavily.

Dependencies are dependencies!

> The book I mentioned has a very good chapter on embedding Ruby.

Will get a copy if I choose Ruby.

>> 3) What are the functions that must be called to make Ruby load a 
>> program,
>> to create an instance of a class defined in Ruby, to call methods with 
>> this
>> instance etc.? Is this API documented somewhere?
>
>I did not quite get the question, but if you mean the API between Ruby 
>and extensions written in C, Pickaxe discusses it in great details too.

Actually I was referring to the API the other way around, from the C host
program to Ruby. I need functions to:

1) Initialize Ruby
2) Register some native functions written in C
3) Run some predefined scripts to register classes
4) Instantiate objects and call their methods
5) go to 4 while the application is running
6) De-initialize Ruby

Regards,

Andre de Leiradella