[FFmpeg-devel] MPEG-4 container header identification

[Oliver]

Hi,

I've been trying to wrap my head around the MPEG-4 container format and
while the specifications I've read have been very complete, nothing seems
to mention the purpose or meaning of the first 4 bytes in the file. From
what I've seen, these typically are 0x0000001C, 0x00000018 or 0x00000014
(e.g., listed here: http://www.garykessler.net/library/file_sigs.html).

This page: http://xhelmboyx.tripod.com/formats/mp4-layout.txt indicates
that the first 4 bytes are an "offset" but I haven't found any other
reference to back this up, and my reading of the ffmpeg codebase
unfortunately didn't shed any light on the subject for me either (mostly
due to my unfamiliarity with it).

I was hoping someone might know the purpose of these 4 bytes before the
ftyp box marker, and if you could point me in the right direction it would
be much appreciated.

Best Regards,
Oliver