ID3v2, wrong APIC frame size calculation
The data size of each ID3v2 frame is stored in 4, 5, 6 and 7 bytes of the frame header block (according to documentation):
I am reading frame sizes from this mp3 file:
To convert to integer the frame data size, I use the following code:
int frame_data_sz = b4 << 21 | b5 << 14 | b6 << 7 | b7;
For example, for most frames, this code correctly calculates the frame size:
TALB -> 0x00 0x00 0x00 0x02 -> 2 bytes - correct!
TIT2 -> 0x00 0x00 0x00 0x02 -> 2 bytes - correct!
TPE1 -> 0x00 0x00 0x00 0x02 -> 2 bytes - correct!
TCON -> 0x00 0x00 0x00 0x02 -> 2 bytes - correct!
TCOM -> 0x00 0x00 0x00 0x02 -> 2 bytes - correct!
TRCK -> 0x00 0x00 0x00 0x03 -> 3 bytes - correct!
TLEN -> 0x00 0x00 0x00 0x06 -> 6 bytes - correct!
COMM -> 0x00 0x00 0x00 0x06 -> 6 bytes - correct!
APIC -> 0x00 0x01 0x0F 0x5D -> 18397 bytes - INCORRECT!
For "APIC" frame my code calculates wrong frame size value because actual data size value is 71517 bytes.
How correctly convert the frame size to an integer value?
The 4th byte of the ID3v2 header tells us its version:
$02was version 2.2.0,$03is version 2.3.0 = your file,$04would be version 2.4.0 = how you treat the file reading the frames.
While the header size is stored the same for all 3 versions, the frame sizes are all stored differently - your way of reading would only be valid for version 2.4.0:
| bytes \ version | 2.2.0 | 2.3.0 | 2.4.0 |
|---|---|---|---|
| header size | 4 * %0xxxxxxx = 28 bit |
4 * %0xxxxxxx = 28 bit |
4 * %0xxxxxxx = 28 bit |
| frame size | $xx xx xx = 24 bit |
$xx xx xx xx = 32 bit |
4 * %0xxxxxxx = 28 bit |
But there are more inconsistencies, hinting at misunderstanding the tag format from whichever software wrote it:
- All the text frames with a size of 2 are effectively empty. Why would a software even store empty frames, instead of omitting them?
- All the text frames, including the comment, have an unneeded terminating
$00in it, further inflating the size of the tag for no purpose. - The comment frame only carries the information that it is in the language
rus, but since both description and text are empty, this is a waste of space (and sense), too. - The
APICsize is$00 01 0f 5d, which is in decimal actually 69,469 (67.84 KiB). This is off by your expectation of 2048 bytes.
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