Re: Netcdf4 Compound types and Examples

Thanks Russ.  Where do I go to get the recent snapshots?  I am  
working from he beta1 and I don't see the files you mentioned in there.

it has been interesting trying to write a wrapper.  I have learned  
more about pointers and structures and C and Fortran than I ever  
wanted to know.  There is a reason I  put the effort into getting  
netcdf4 to work with Absoft.    To do something like the structure  
for the vlen and to be able to pass it back and forth from C , I need  
to have the following:

1.  Cray style pointers in F90/95

2. The ability to allocate space to the cray pointer, i.e a malloc  
type command in Fortran

3.  Support for the sequence attribute in a structure, to guarantee  
memory is in one block.

Fortunately Absoft supports all of these.  If I ever get this to work  
I will let you know - but it will not be portable.

-Roy


On Dec 27, 2007, at 9:20 AM, Russ Rew wrote:

> Roy,
>
> > I am working on trying to see if I can write some wrappers for
> > Fortran for my own use for the vlen and compound types in NetCDF4.
> > Working on the compound types, I was using the examples ad test code
> > as a basis, and I have several comments base don that.
> >
> > 1.  Use of the compound type appears to depend on using HOFFSET,
> > which is an HDF5 call.  Ideally, one should not need to know or
> > directly access HDF5 calls to use the netcdf4 library.  I would
> > suggest adding an equivalent function, or a function that calls this
> > to the netcdf4 library so that it is self-contained and self- 
> > documented.
>
> Although HOFFSET is a macro defined in an HDF5 include file, it is
> actually the same as the "offsetof" macro defined in the C standard
> include file stddef.h, so you should be able to use "offsetof"  
> instead.
> The C standard rationale says this about it:
>
>   The offsetof macro has been added to provide a portable means of
>   determining the offset, in bytes, of a member within its structure.
>   This capability is useful in programs, such as are typical in
>   data-base implementations, which declare a large number of different
>   data structures: it is desirable to provide ``generic'' routines  
> that
>   work from descriptions of the structures, rather than from the
>   structure declarations themselves. ...
>
> I think I agree with you that it would be better to document using
> "offsetof" instead of "HOFFSET", since then it doesn't look like an  
> HDF5
> call.  Ed may have another opinion about this.  Note that we can't  
> just
> include a definition of offsetof in netcdf.h, as there is no single
> portable definition of this macro.  The definition in stddef.h (or  
> by a
> compiler) provides a version that works properly for each platform.
>
> > 2.  The examples for compound types while clever and witty do not
> > really illustrate compound types or how you might use them. There is
> > little reason, as in the examples, to have a compound type with two
> > ints.  It took me a while to be certain that the names that were
> > being given were to the i1 and i2 elements of the structure  (I
> > believe this is correct) so if we view it as setting up a table with
> > fields of different types, these would be the row names of the table
> > - correct?  If the example had different types of fields in the
> > structure, and the names of the elements in the structure reflect the
> > data,  then it would be clearer.  Real world type examples are both
> > clearer and provide better motivation for use of the feature.
>
> I agree that the documentation should use better examples.  I am
> providing better examples in the ncdump test cases.  See, for example,
> the following files in the ncdump directory of any recent snapshot:
>
>  tst_comp.c
>  ref_tst_comp.cdl
>
> Also, see the workshop for developers tutorial code for sea  
> soundings in
> sections 13.9 and 13.10 of:
>
>   http://www.unidata.ucar.edu/software/netcdf/workshops/2007/groups- 
> types/index.html
>
> > Have a good holiday,
>
> Thanks, you too!
>
> --Russ

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**********************
Roy Mendelssohn
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NOAA/NMFS
Environmental Research Division 
Southwest Fisheries Science Center
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