[ogg-dev] page syncing and magic OggS

[Ian Malone]

Ralph Giles wrote:
> On Thu, Dec 07, 2006 at 03:18:02PM +0000, Ian Malone wrote:
> 
>> Pages have to start with 'OggS', but this could also turn
>> up elsewhere in the bitstream, so is there a simple strategy
>> using the libogg API to find out which ones are the start of
>> pages?
> 
> Yes, 'OggS' can occur randomly. So to be more sure, you can parse the 
> subsequent bytes as a header. Calculate the offset to the start of the 
> next ogg page and see if the 'OggS' sequence occurs there as well. You 
> can also verify the CRC for the page as an additional check.
> 
> ogg_sync_pageseek() does the CRC check, and scans for the next 'OggS' if 
> it does not succeed, so by calling it repeatedly you can scan through
> the stream finding pages.
> 
> Note that ogg_sync_pageout does this automatically. You can check the 
> return code to identify any holes or corrupt pages in the data stream.
> 

Thanks (and thanks also to Michael Smith).  What I hadn't understood
about ogg_sync_pageseek is that when returning n >= 0 it's acting
like ogg_sync_pageout and n < 0 it's saying it read through -n more
bytes of the sync buffer.  The documentation for it makes perfect
sense now.

-- 
imalone