HDF tag query

[Freddie Akeroyd]

Hi ... i've recently been doing a bit more with the NeXus side of
OpenGENIE, and i've come across something i don't quite understand.

NXmakedata() adds an SDS to a Vgroup with the tag DFTAG_SDG (a "scientific
data group"), but if you ask NXgetdataID() for the "tag,ref" pair it
returns the tag DFTAG_SDS (a "scientific data scale"). Is this a trick so
that you can distinguish the creation/initial reference of a data set (tag
DFTAG_SDG) from any links to it (tag DFTAG_SDS)?  The only thing that
concerns me is that the documentation says that a "ref" value is not
unique unless bound to a "tag" value, so the object "DFTAG_SDS,ref" may
not refer to a valid object in the file.  While this doesn't cause a
problem with our code (we hunt for both DFTAG_SDS and DFTAG_SDG, and then
assume the "ref" is really from DFTAG_SDG), could it cause a problem for
pure HDF browsers? If our links to data are tagged as a "scientific data
scale" then they may not pick them up automatically, or they may clash
with the "scale" of some other dataset.

Have i understood this correctly?

Regards,

Freddie

P.S. I believe DFTAG_SDG is now obsoleted by DFTAG_NDG - should we
     be writing our data out with this tag now?

-- 
Freddie Akeroyd                        Email:  Freddie.Akeroyd at rl.ac.uk
ISIS Facility                          Tel:    +44 1235 445457
Rutherford Appleton Laboratory         Fax:    +44 1235 445720
Chilton, DIDCOT, OX11 OQX, GB          WWW:    http://www.isis.rl.ac.uk/