From: Eric Hodel <drbrain@...7.net>
Date: 2009-11-24T08:18:04+09:00
Subject: Re: RubyInline - passing pointer from one function to another

On Nov 20, 2009, at 06:26, Marcin Lewandowski wrote:
> I'm trying to write ruby interface to JACK audio library. I've chosen
> RubyInline due it's simplicity. What I want to achieve is to have ruby
> code like this:
>
> client = JACK::Client.new "my_name"
> puts client.get_sample_rate
>
> To achieve that I need to pass pointer to variable of jack_client_t*
> from libjack from one function to another. What I tried to do is
> available at http://pastebin.com/m3b6ee0c9 but as you can expect it
> doesn't work.
>
> How can I do that? Is there any way to save pointer in instance of
> ruby class e.g. in the same way as I can access strings?

You'll need to use the Ruby/C API for this, namely Data_Wrap_Struct  
and Data_Get_Struct. Passing around the pointer as an instance  
variable (without even using INT2FIX/FIX2INT) can only lead to memory  
leaks and segfaults.

If you're using RubyInline, place everything in the same inline block  
per class.  Use private as a method afterward:

class JACK::Client
   inline do |builder|
     builder.c <<-C
       /* define connect */
     C
   end

   private :connect
end

There are several strategies to wrapping C libraries in a class so  
they are initialized properly.

This uses ::allocate:

http://github.com/drbrain/ffmpeg-rb/blob/master/lib/ffmpeg/fifo.rb

This uses ::new:

http://github.com/drbrain/ffmpeg-rb/blob/master/lib/ffmpeg/frame_buffer.rb

This uses a custom method ::for_decoder:

http://github.com/drbrain/ffmpeg-rb/blob/master/lib/ffmpeg/codec.rb

Also, your extension doesn't make much use of RubyInline's  
helpfulness.  RubyInline will automatically perform input and output  
casts for you.  Connect should be a class method written like:

builder.c_singleton <<-C
   static VALUE connect(char *name) {
     jack_client_t *client;

     client = jack_client_new(name);

     if (client == 0)
       rb_raise(rb_eRuntimeError, "cannot connect to JACK server");

     /* guessing at free function */
     return Data_Wrap_Struct(self, NULL, jack_client_close, client);
   }
C

With jack_get_sample_rate bound like:

builder.c <<-C
   static jack_nframes_t sample_rate() { /* rubyish method name */
     jack_client_t *client;

     Data_Get_Struct(self, jack_client_t, client);

     return jack_get_sample_rate(client);
   }
C

and a type converter:

builder.alias_type_converter 'unsigned long', 'jack_nframes_t'