Bringing this back up again, now that I see that _NCProperties is included
in the documentation under "Attribute Conventions" [1].
Is there are a good way to handle cases where there is no valid netcdflib
version to include here, because the file wasn't written using netcdf-c?
What I've been doing is including h5netcdfversion=X.Y.Z in the
_NCProperties attribute instead of netcdflib=X.Y.Z. I'm thinking now that
I'd also like to include versions of other libraries used to generate the
file (e.g., h5py). Does that seem reasonable? Or should I rather drop
unexpected names altogether?
At the very least, I don't think I can include any sensible netcdflib
version.
[1]
http://www.unidata.ucar.edu/software/netcdf/docs/attribute_conventions.html
On Sat, Apr 30, 2016 at 12:45 PM, dmh@xxxxxxxx <dmh@xxxxxxxx> wrote:
> This will be documented in the existing documentation.
> At some point, it needs to be added to the netcdf NASA spec
> (https://earthdata.nasa.gov/standards/netcdf-4hdf5-file-format).
> =Dennis Heimbigner
> Unidata
>
> On 4/29/2016 10:39 PM, Stephan Hoyer wrote:
>
>> What is the specification you're planning to use for the value of the
>> _NCProperties attribute? We'd like to able to generate compliant netCDF4
>> files with h5netcdf, which uses HDF5 directly to write netCDF4.
>>
>> I guess my main request is to not assume that only netCDF-C will write
>> _NCProperties attributes, and to ensure that netCDF-C and the specification
>> are flexible enough to allow for different implementations, maybe even
>> hypothetical implementations not using the HDF5 libraries at all.
>>
>> Thanks,
>> Stpehan
>>
>> On Fri, Apr 29, 2016 at 8:32 PM, dmh@xxxxxxxx <mailto:dmh@xxxxxxxx> <
>> dmh@xxxxxxxx <mailto:dmh@xxxxxxxx>> wrote:
>>
>> I am about to commit a pull request for the netcdf-c library having to
>> do with identifying the provenance and format of netcdf-4 files,
>> and specifically targeted at detecting netcdf-4 files from HDF5 files.
>>
>> This provenance consists of the following information. First, there is
>> a hidden, persistent, attribute names _NCProperties. It specifies the
>> library versions of the netcdf library and the hdf5 library used to
>> create the file. This attribute never changes during the lifetime
>> of the
>> file (unless modified deliberately thru the hdf5 API).
>>
>> Second, there are two special, non-persistent, attributes that are
>> computed from information already in the file.
>> 1. _SuperblockVersion
>> 2. _IsNetcdf4
>> Non-persistence means these attributes do not actually appear in the
>> file. and are computed from other info already in the file.
>>
>> The _SuperblockVersion attribute is a single integer giving the
>> version
>> number (currently 0-3) of the superblock in the hdf5/netcdf-4 file.
>>
>> The _IsNetcdf4 attribute is a single integer 0/1 indicating if the
>> file
>> has various tags indicating it was produced thru the netcdf-4 API.
>> This
>> is computed by using the HDF5 API to walk the file to look for
>> attributes specific to netcdf-4. False negatives are possible for a
>> small subset of netcdf-4 files, especially those not containing
>> dimensions. False positives are (I think) only possible by deliberate
>> modifications to an existing HDF5 file thru the HDF5 API. For
>> files with
>> the _NCProperties attribute, this attribute is redundant. For files
>> created prior to the introduction of the _NCProperties attribute, this
>> may be a useful indicator of the provenance of the file.
>>
>> These three attributes are hidden in the sense that they can only be
>> accessed thru the netcdf-C api calls via the name. They have no
>> attribute number and will not be counted in the number of global
>> attributes in the root group.
>>
>> The simplest way to view these attributes is to use the -s flag to the
>> ncdump command.
>>
>> Comments are welcome.
>>
>> =Dennis Heimbigner
>> Unidata
>>
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