HDF tag query

[C.M.Moreton-Smith at rl.ac.uk]

Might be better to use DFTAG_NDG as DFTAG_SDG is now deprecated?

	Chris

> From: Mark Koennecke [mailto:koenneck at psi.ch]
> Sent: 01 September 1999 09:01
> To: NAPI at isise.rl.ac.uk; Freddie.Akeroyd at RL.AC.UK
> Subject: Re: HDF tag query
> 
> On Wed, 25 Aug 1999, Freddie Akeroyd wrote:
> 
> > 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
> 
>   Freddie,
> 
>   you were right. I tested it and changed NXgetdataID to set 
> the tag to
>   DFTAG_SDG. Then the linked SDS's showed up nicely in the 
> Fortner browser
>   which they did not do before. It never occurred to me that DFTAG_SDS
>   means scientfic data scale and scientific data set. Abbreviations! 
> 
>   In short I suggest to change that in the general distribution.
> 
>                   Mark Koennecke 
>