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[netCDF #MRM-845291]: netcdf3 classic format clarification

Hi Davide,

> CSG is trying to fully understand the issue of reading and writing
> "classic" netcdf3 files using the more recent netcdf4 releases.
> We had this discussion in the past with Ed, but from the email
> correspondence that we saved, the interpretation in unclear.
> In particular, we are wondering about the "--disable-netcdf-4" described
> here:
> http://www.unidata.ucar.edu/software/netcdf/docs/netcdf-install/Configure.html
> The question is: what exactly does that flag?
> Suppose that we do *not* use the "--disable-netcdf-4". Can we still use
> the resulting library as an "almost drop-in replacement" for an old
> climate or weather model which is expecting a netcdf-3.6.1 version? For
> "almost drop-in replacement" we mean that we don't have to do any change
> in the model itself and we only have to do the small libnetcdf swap to
> libnetcdff in the linking process.

Yes, there should be no change required in the source code of the model,
but you do need to recompile and relink, because there have been additions
to the include files and Fortran modules.

When you build and install the libraries without --disable-netcdf-4, there
is extensive testing done by "make check" to run all the tests that are
run with --disable-netcdf-4, to check compatibility of the library with the
netCDF-3 versions.  Any change required to the source of a netCDF-3 program
by using netCDF-4 is a bug, as far as I am concerned.  Absolute backward
compatibility is what we were committed to when we made netCDF-4 a superset 
of netCDF-3, both as far as the data model extensions, and the programming

> We know for sure that a netcdf4 configured with --disable-netcdf-4 flag
> and compiled, will result in an "almost drop-in replacement" as defined
> above. But do we actually need a separate install for it, or would a
> "normal" netcdf4 with HDF5 support be enough for this purpose? Of course
> I'm assuming that both HDF5 and netcdf4 are compiled in serial, non-MPI
> mode for this (we will have separate parallel version of HDF5 and netcdf4).

You should not need a separate install.  A single installation of the netCDF-4
library will provide support for netCDF-3 programs and data.


Russ Rew                                         UCAR Unidata Program
address@hidden                      http://www.unidata.ucar.edu

Ticket Details
Ticket ID: MRM-845291
Department: Support netCDF
Priority: Normal
Status: Closed

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