Two problems creating a C++ extension to Ruby

[Matthew Miller]

Hello,

I'm having some trouble with my first C++ extension for Ruby. The extension
is a wrapper for the id3lib library providing my own interface over top that
of the library. I've encountered two problems, one on the build side of
things and anther from within Ruby when I tried to test the extension.

To create the makefile for the extension I'm using the 'mkmf'
module. Because the extension relies on libid3.so I tried to test for that
library using :have_library, but I've not been able to supply a second
argument that will cause the method to return true. Here is what I tried:

require 'mkmf'
if have_library( "id3", "ID3_Tag::ID3_Tag" ) then
create_makefile("id3lib")
else
puts "libid3 not found."
end

I think the trouble is that all of the symbol names in the library are
mangled by the C++ compiler. Though, even when I substitute a mangled name
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.

I looked at the FXRuby source, but it uses a different build method than
mkmf and I don't know of any other extensions that use C++ libraries. Should
I try a different build method? (Can someone recommend one?)

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".

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?

For people who may know more about this and need to look at the code it can
be gotten here: [url]http://roland.sw.edu/matthew/id3lib_ext.tar.gz[/url] If for some
reason that hostname doesn't resolve (as seems to happen outside our
college) then substitute the IP address 164.106.190.246 for the hostname.

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.

Thanks for your help, Matthew.

[Lyle Johnson]

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")

Note that you don't include the "lib" prefix or the ".so" extension for
the library file name.

> I looked at the FXRuby source, but it uses a different build method than
> mkmf and I don't know of any other extensions that use C++ libraries. Should
> I try a different build method? (Can someone recommend one?)

FXRuby does use the mkmf library for the part that truly is a C++
extension; see the ext/fox subdirectory of the FXRuby source
distribution. As it turns out, FXRuby is sort-of a hybrid of C++
extension code and pure Ruby code, and so I'm using Minero Aoki's
setup.rb library to package-up the whole thing.

> 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.

I think the problem may be that when you construct the C++ ID3Lib object
you're not calling its constructor properly. If I look at your id3_new()
function:

static VALUE id3_new( int argc, VALUE* argv, VALUE self ) {
ID3Lib* id3 = ALLOC( ID3Lib );
VALUE mp3_info = Data_Wrap_Struct(self, 0, id3_free,
(void*) id3 );
rb_obj_call_init( mp3_info, argc, argv );
return mp3_info;
}

I'm wondering if that if that first line shouldn't perhaps be:

ID3Lib* id3 = new ID3Lib;

If that's not the problem, you might also want to add some print
statements to the id3_initialize() function, after the call to
read_tags(), to convince yourself that at that point the
id3_struct->artist field really is a non-NULL string.

> 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.

Good luck,

Lyle

[Tobias Peters]

On Sun, 28 Sep 2003, Lyle Johnson wrote:

> 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")

Matthew can also use the have_library function without a second argument.
In this case, ruby will use "main" as the default. "main" is of course
defined by the test program itself, so it will not be the cause of any
error. The linker will nonetheless complain if it cannot find the library,
it just won't check for any symbols in that library.

Tobias

[Matthew Miller]

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:
[url]http://roland.sw.edu/matthew/id3lib_ext-fixed.tar.gz[/url] 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.