[Libcdio-devel] Re: Retrieving DATA session from multisession audio disc

[Thomas Schmitt]

Hi,

Rocky Bernstein wrote:
> libcdio's iso-info seems to only be able to handle reading when this
> filesystem in the first track.

This also keeps it from properly reading
multi-session ISO 9660 from data CD.


> It looks like there is a
> more fundamental deficiency in the library which assumes a fixed block
> location of the "Primary Volume Descriptor" or PVD.

There are two "offset" discussed here:

- The "superblock" location.

  This is the block address where the track
  starts. 16 blocks up you can expect the PVD.
  This is addressed e.g. by
    mount -o sbsector=$track_start
    mkisofs -C $track_start,...
    xorriso -load lba $track_start

  You need it for reading multi-track resp.
  multi-session images.


- A displacement offset if the track data are
  not stored at the old location any more.

  This is to be applied to all block addresses
  which are read from data words in the image.
  E.g. the PVD has the address of the root
  directory. This already needs the displacement
  offset before it is used to read the directory
  entries from there.
    

> Also, what would help in developing such changes would be to have a *small*
> CD image and/or extracted ISO 9660 filesystem.

A displaced ISO filesystem is easy to create.
Just give the generator program a wrong Next
Writeable Address.
E.g:

  mkisofs -C 0,$some_offset \
          -o displaced_image.iso \
          /my/small/directory/tree


If you know a recipie for creating a correct
CD-EXTRA by help of cdrecord, then i would be
very interested to learn it.


Have a nice day :)

Thomas