Vorbis dev - Apr 2004 - Reading OGG embedded in a pack file

I have a number of assets packed into a single file.  Some of these are OGG
files.  I have code to seek to the start of an embedded file, but cannot find
any OGG functions that let me start at an arbitary point in a stream.  The
original code (which works) that opens a stream, then reads in the OGG data
starts like this:

// Read from an already open stream.
// Assumes stream is positioned to start of OGG data
// Not a complete function, just enought to show what I am trying to do
void readFromPackfile( FILE * pStream )
{
    OggVorbis_File vf;
    ov_callbacks callbacks;
    vorbis_info *info;

    callbacks.read_func = sdl_read_func;
    callbacks.seek_func = sdl_seek_func;    // Will setting this to NULL let me
read from the middle of a stream?
    callbacks.tell_func = sdl_tell_func;
    callbacks.close_func = sdl_close_func;

    if ( ov_open_callbacks( pStream, &vf, NULL, 0, callbacks)==0 )
    {
        // Valid OGG data, so do some code here to read it in...

        // Get our OGG info
        info = ov_info( &vf, -1 );
    }
}

After looking at the documentation I thought I could change the 4th parameter to
be the offset into the stream for where my sub file (inside the pack file) is
located.  This did not work.  It took about 10 seconds for the function to
return so I am guessing it was seeking to the beginning of the stream and then
scanning forward looking for the OGG headers.

There are comments about using streams that are not "seekable" and I
thought this also might be a solution, but I could find nothing on how to tell
Vorbis that a stream was not seekable.  Is it as simple as setting the
callbacks.seek_func and callbacks.tell_func to NULL?

Does anyone have any experience reading OGG data from a already open stream
which may have non-OGG data in it?

Thanks,

- Ken
 

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On Sat, Apr 17, 2004 at 11:23:46AM -0500, Ken Rogoway wrote:
> After looking at the documentation I thought I could change the 4th
parameter to be the offset into the stream for where my sub file (inside the
pack file) is located.  This did not work.  It took about 10 seconds for the
function to return so I am guessing it was seeking to the beginning of the
stream and then scanning forward looking for the OGG headers.
No, the 3rd and 4th parameters are to let you provide a 'prefix buffer' 
in cases where the underlying file isn't seekable, but you've already 
read some of it to do filetype identification. Thus you can pass the bit 
you've read so vorbisfile can parse it, and then continue with the 
actual stream from the read position.

Thus, your hypotheses is likely correct about what it's doing.
> There are comments about using streams that are not "seekable"
and I thought this also might be a solution, but I could find nothing on how to
tell Vorbis that a stream was not seekable.  Is it as simple as setting the
callbacks.seek_func and callbacks.tell_func to NULL?
You can set tell_func to NULL when the stream is not seekable. You must 
always provide a seek_func; the way you mark a stream as not seekable is 
by having that function always return (-1) for failure, regardless of 
the input. See http://xiph.org/ogg/vorbis/doc/vorbisfile/callbacks.html

Another general solution of course is to doctor your read, seek, and 
tell callbacks to operate relative to the offset of the ogg data in 
your pak file.

Hope that helps,
 -r
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