[ID3 Dev] Why is footer not included in total size?

Andy andy at andy-k.co.uk
Tue Nov 14 01:41:33 PST 2006

Hmmmm, you are probably correct Jim. One of the major source of problems
with the ID3v2 standards is the fact they are so open to interpretation and
fail to be explicit is areas such as this.

My thoughts have always been this; if the standard does not explicitly state
that something is
permitted and the failure to do so would lead to backward compatibility
issues; then I have taken
it as not being permitted (if that makes sense!!!).

In this case, I would interpret the standard as not permitting footers for
pre-pended tags.

Besides, I am not quite sure what benefit there is if a pre-pended tag has a


p.s. sorry about the bluntness before, I had a long trip into work!

On 14/11/06, Jim <jmartin92 at comcast.net> wrote:
>  I'm not sure about that.  I know that appended tags MUST have a footer
> but there is nothing that says a prepended tag can't have a footer.  (At
> least nothing that I know of.  Am I wrong about that?)
> ----- Original Message -----
> *From:* Andy <andy at andy-k.co.uk>
> *To:* id3v2 at id3.org
> *Sent:* Tuesday, November 14, 2006 3:46 AM
> *Subject:* Re: [ID3 Dev] Why is footer not included in total size?
> I am pretty sure that footers are only to be written to a file when the
> tag is appended, not pre-pended.
> On 14/11/06, Jim < jmartin92 at comcast.net> wrote:
> >
> >  Hi.
> >
> > I am wondering what the reasoning was behind the size stored in ID3v2.4tags not including the size of the footer (if present).
> >
> > I've been looking through the source code for some utilities for MP3
> > files and some other audio file types.  Many of these utilities use a simple
> > method for skipping over the ID3v2 tag if it is there.  They read the first
> > 10 bytes of the file, do some simple checking (look for 'ID3', check that
> > version bytes are less than $FF and in some cases check that size bytes are
> > syncsafe, i.e. < $80) and if it looks like a tag, convert the size and
> > skip that many bytes ahead (+ 10 bytes for header).  Most of these utilities
> > would be broken if the tag has a footer.  They don't do any specific version
> > checking or checking of the flags byte.
> >
> > If the size stored in ID3v2.4 tags included the footer size (if it is
> > present), it would make these tags much more backwards compatible.  (I know
> > it is a pretty easy change to add checking for the footer flag but wouldn't
> > it have been better to make it compatible with older apps that don't really
> > need to read the tag, just skip it?)   Were there reasons that came up
> > during development why it was done the way it is?  If so, I'd appreciate if
> > someone could share them with me.  Or if there is somewhere I can read about
> > this (like some archive of the mailing list) I would appreciate someone
> > pointing me in the right direction.
> >
> > Thanks.
> >
> > Jim Martin
> >
> >
> --
> Andy.

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