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

Jim jmartin92 at comcast.net
Mon Nov 13 23:21:49 PST 2006


I am wondering what the reasoning was behind the size stored in ID3v2.4 tags 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.


Jim Martin
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