From: matthew.miller@... (Matthew Miller)
Date: 2003-09-29T20:14:57+09:00
Subject: Re: Two problems creating a C++ extension to Ruby

Hi Lyle,

Lyle Johnson <lyle@knology.net> wrote in message news:<3F776A3F.5020300@knology.net>...
> Matthew Miller wrote:
> 
> > for the second argument the test fails. I've gotten around this by not doing
> > a library test and just calling create_makefile and then editing the
> > resulting makefile by hand to have the extension .so linked against
> > libid3.so.
> 
> Since libid3.so is not optional in this case, I think it is OK to just 
> assume that it's there. To avoid having to edit the Makefile, though, 
> try adding this line to your extconf.rb script before the final call to 
> create_makefile():
> 
>      $libs = append_library($libs, "id3")

That works very well, thanks. It was also necessary for me to change
one of the config values: CONFIG['LDSHARED'] = "g++ -shared"

> > The second problem came up when I tried to test the extension. After
> > creating a new ID3Lib object, which opens an mp3 file, the script calls the
> > :artist method to return a string containing the value of that tag. That
> > method call generates an ArgumentError exception with the error message
> > "NULL pointer given".
> 
> OK. So if we look at the extension code for the ID3Lib#artist method:
> 
>      static VALUE id3_get_artist( VALUE obj ) {
>          ID3Lib* id3 = static_cast<ID3Lib*>(DATA_PTR( obj ));
>          return rb_str_new2( id3->artist );
>      }
> 
> This almost certainly indicates that the id3->artist field is a NULL 
> pointer. So it is the call to rb_str_new2() that is generating the "NULL 
> pointer given" error message.
> 
> > I've tried to isolate where that error is generated and I can only say that
> > it seems to come from within Ruby. Replacing the body of the C++ function,
> > that is called for the no argument version of :artist, with a statement to
> > return a Fixnum still results in the "NULL pointer given" exception. So it
> > seems that the statements within id3_get_album() don't cause the exception,
> > but maybe someother part of my C++ code is the problem?
> 
> The C++ function that implements the ID3Lib#artist method is 
> id3_get_artist(), not id3_get_album(). You did indeed put some debugging 
> code in id3_get_album(), but that's not the one that's getting invoked 
> when you call ID3Lib#artist.

Well, these comments got me started in the right direction. When
reading the frames of the id3 tag I was using the wrong frame type
constant for the artist's name frame. Fixing that and how the char*'s
were initialized got everything working.

> > If anyone can help, I would be very appreciative. If you look at the code, I
> > am also open to comments and suggestions not related to the above problems.
> 
> The code looks really good to me. I suspect that this is just a little 
> bug somewhere that you'll be able to fix without much trouble.

Thanks Lyle, you're right. Once I got a fresh look at the code fixing
it wasn't too much trouble. While this extension is still very beta,
anyone can get the code here:
http://roland.sw.edu/matthew/id3lib_ext-fixed.tar.gz Once I've done
more work I'll create a RAA entry...

Lyle, again thanks for your help. I believe that nearly everytime I've
had a Ruby or FXRuby question you have provided an answer or helped in
some way. You have been a great help to me learning Ruby!

Take care, Matthew.