Confusion over tag/ref pairs

[Ray Osborn]

I wrote a short program last night to test the new proposal to use reference
numbers to identify axes.  It seems to work well - there's no problem with
writing the axis_ID attributes, and they show up correctly in NXbrowse.
However, when I opened the newly created file with the SciSpy Browser, I
came across a puzzle which confuses me.

If you highlight an SDS and choose Get Info from the File menu, it displays
the tag/ref pair for the SDS.  The reference number matches what I got from
NXgetdataID.  However, the tag is given as 700, which is the value for
DFTAG_SDG.  As Freddie pointed out last century, this is now deprecated by
HDF, who want us to use DFTAG_NDG (= 720).  When we add the tag/ref pairs to
any Vgroup, NAPI.C uses DFTAG_NDG automatically.

My confusion is two-fold :

1) HDF doesn't allow you to specify the tag when you call SDcreate.  It just
assigns one for you.  It seems to be assigning DFTAG_SDG, which HDF claims
not to support any more.

2) If we then use the wrong tag to attach the SDS to a particular Vgroup,
why do we encounter no problems?  We always assume that the tag is DFTAG_NDG
even though it's not.

It doesn't seem to affect the operation of the NeXus file, but I'm worried
that it might have hidden side effects if we don't sort it out.   Perhaps it
explains why data items still do not show up when linked to more than one
group.  Does anyone know how we are meant to set the SDS tag?  Am I missing
something here?

Regards,
Ray
-- 
Dr Ray Osborn                Tel: +1 (630) 252-9011
Materials Science Division   Fax: +1 (630) 252-7777
Argonne National Laboratory  E-mail: ROsborn at anl.gov
Argonne, IL 60439-4845