[mad-user] How to manipulate both V1 and V2 tags in the same file

[Cedric Tefft]

I've been playing around with libid3tag 0.15.1b for a couple days now,
and I'm at a loss when it comes to manipulating multiple tags in the
same file.

When reading a file with only one tag (either V1 or V2), reading/parsing
the frames and fields works more or less as I'd expect given the API
functions in id3tag.h.

Where I run into trouble is when I read an MP3 file with both V1 and V2
tags. The first thing I do after opening the file is call
id3_file_tag() to get a pointer to a tag. What I find confusing is that
even though the tag's ID3_TAG_OPTION_ID3V1 option is set, walking the
field list only gives me the frames from the V2 tag:

for(i=0;i<tag->nframes;i++) {
frame=tag->frames[i];
/* etc. */
}

Poking around a bit in file.c it appears the id3_file structure
maintains references to all of the individual tags in the file, but
looking through the API functions available in id3tag.h, I don't see
how to access any tag other than the primary (via the id3_file_tag()
function).

So my question is, how do I access the fields of a V1 tag when a file
has both V1 and V2 tags? Are the V1 fields stored in the primary tag
structure along with the V2 fields? If so, why can't I access them when
I walk the list of the other (V2) frames? If they're stored in a tag
structure other than the primary, how do I access it?

Thanks,

- Cedric

[Cedric Tefft]

Hello.... anybody out there??

Post by Cedric Tefft
I've been playing around with libid3tag 0.15.1b for a couple days now,
and I'm at a loss when it comes to manipulating multiple tags in the
same file.
When reading a file with only one tag (either V1 or V2),
reading/parsing the frames and fields works more or less as I'd expect
given the API functions in id3tag.h.
Where I run into trouble is when I read an MP3 file with both V1 and
V2 tags. The first thing I do after opening the file is call
id3_file_tag() to get a pointer to a tag. What I find confusing is
that even though the tag's ID3_TAG_OPTION_ID3V1 option is set, walking
for(i=0;i<tag->nframes;i++) {
frame=tag->frames[i];
/* etc. */
}
Poking around a bit in file.c it appears the id3_file structure
maintains references to all of the individual tags in the file, but
looking through the API functions available in id3tag.h, I don't see
how to access any tag other than the primary (via the id3_file_tag()
function).
So my question is, how do I access the fields of a V1 tag when a file
has both V1 and V2 tags? Are the V1 fields stored in the primary tag
structure along with the V2 fields? If so, why can't I access them
when I walk the list of the other (V2) frames? If they're stored in a
tag structure other than the primary, how do I access it?
Thanks,
- Cedric

[Rob Leslie]

[ Once again developer-oriented messages as this one should be sent to
mad-dev and not this list. Please note Reply-To: and subscribe to
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Post by Cedric Tefft
I've been playing around with libid3tag 0.15.1b for a couple days now,
and I'm at a loss when it comes to manipulating multiple tags in the
same file.
When reading a file with only one tag (either V1 or V2),
reading/parsing the frames and fields works more or less as I'd expect
given the API functions in id3tag.h.
Where I run into trouble is when I read an MP3 file with both V1 and
V2 tags. The first thing I do after opening the file is call
id3_file_tag() to get a pointer to a tag. What I find confusing is
that even though the tag's ID3_TAG_OPTION_ID3V1 option is set, walking
for(i=0;i<tag->nframes;i++) {
frame=tag->frames[i];
/* etc. */
}
Poking around a bit in file.c it appears the id3_file structure
maintains references to all of the individual tags in the file, but
looking through the API functions available in id3tag.h, I don't see
how to access any tag other than the primary (via the id3_file_tag()
function).
So my question is, how do I access the fields of a V1 tag when a file
has both V1 and V2 tags? Are the V1 fields stored in the primary tag
structure along with the V2 fields? If so, why can't I access them
when I walk the list of the other (V2) frames? If they're stored in a
tag structure other than the primary, how do I access it?

The file API of libid3tag presents a single view of all the tags
contained in a file, and does not differentiate between V1 and V2 tags.
The idea is that V1 and V2 tags should represent the same information
about a file, and if both are present then V2 information is preferred
because its field lengths are not constrained as in V1. (When writing
tags back to a file, the intent is that the file interface will
generate both V1 and V2 tags from the same source, ensuring
consistency.)

Accordingly, it's not possible to access the V1 and V2 information as
separate tags with the file interface. If you really want to do this,
you can use the tag parsing and rendering routines to maintain separate
tag structures and data, but you'll have to read and write the data
from the file yourself.