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NOTICE: This version of the NSF Unidata web site (archive.unidata.ucar.edu) is no longer being updated.
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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 <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 > > _______________________________________________ > netcdfgroup mailing list > netcdfgroup@xxxxxxxxxxxxxxxx > For list information or to unsubscribe, visit: > http://www.unidata.ucar.edu/mailing_lists/
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