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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, StpehanOn 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 _______________________________________________ netcdfgroup mailing list netcdfgroup@xxxxxxxxxxxxxxxx <mailto:netcdfgroup@xxxxxxxxxxxxxxxx> For list information or to unsubscribe, visit:http://www.unidata.ucar.edu/mailing_lists/
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