[Vorbis] ov_read, callbacks and 0 bytes read

[Ralph Giles]

On Thu, Oct 05, 2006 at 09:01:48PM -0700, Andrew Lentvorski wrote:

> The issue is that the fread can't return 0 (that signals EOF and hoses 
> everything), and it doesn't have a negative code that says come back 
> later when you have more data.

No, I think ov_read() is intended to block in such a situation.

> I can't allow this to block as the system I am using does not have 
> threads (embedded system).
> 
> A good example of this is to put too much information in the 
> info/comment field that overflows the initial decode.  If ov_read can't 
> get enough data to completely decode a packet, things go badly wrong.

I don't follow what you mean here. You pretty much have to have the 
whole packet in memory to parse it, which libvorbis and Tremor both
try to do. (Multiple copies in the case of libvorbis.) For the comment 
header you could change the implementation to skim for just the info  
you want to display (artist and title, likely) and discard the rest, but 
it won't be trivial the way the ogg parser is implemented. For other 
packet types it's unlikely to help all that much.

But I'm not sure what you're expecting the decoder to do if your read 
callback could return EAGAIN. Do you want it to bubble that up for you 
so ov_read() returns it too, and then you go off and do UI stuff? 

Without threads or continuations I don't have a good suggestion. Maybe 
pass the UI state through the datasource pointer and update stuff from 
inside your read callback while it's waiting for data? Then if the user
cancels you return zero and recover after ov_read() returns.

Or you could do it the other way, and just return zero when you've 
blocked too long, and retry decoding later when you do have more data.

FWIW, 
 -r