How to get frame size in ID3v2.2 MP3 tags?

Question

I've got it sorted out for the ID3v2.3 tags, since there are 4 bytes, but the frame's header tag here is only a total of 3 bytes, not 4. That seems to mean that I can't use struct.unpack to get an int out of it.

For example, the tag I'm working with now is: TT2\x00\x00\x0c\x00Torn Within where TT2 is the name identifier, and \x00\x00\x0c is the size identifier. The content of the tag is \x00Torn Within, which has a size of 12 bytes.

Here's the song's ID3 header as well. 'ID3\x02\x00\x00\x00\x04NP', where you can see that the encoding and flags are not set.

I've tried struct.unpack('>3b', '\x00\x00\x0c') but that only grabs each individual byte's value.

But after that, I am stuck, because unless I prepend a \x00 to the size tag, I'm unable to continue. What do I do?

Here's the ID3 tags docs http://id3.org/id3v2-00, and the docs for the struct module http://docs.python.org/library/struct.html#format-characters

edit found that I could do this: int(binascii.hexlify('\x00\x00\x0c'), 16) but I don't think that's a great solution

Answer 1

Just prepend a null byte (\x00) before unpacking:

>>> length = "\x00\x00\x0c"
>>> struct.unpack('>I', '\x00' + length)
(12,)

The null byte pads your length bytes out to 4 bytes without altering the meaning. The largest value the 3 size bytes can hold is 2²⁴ equals 16777216 bytes; adding the padding is not going to alter that limit in any way.