NetCDF header size

[gsjaardema]

Is there an API function that will return the header size of a particular netCDF file? If not, does the following AI-generated code snippet give a reasonable size:

def calculate_header_size(filename):
    with nc.Dataset(filename, 'r') as ncfile:
        header_size = 0

        # Loop through dimensions and add their size
        for dim_name, dim in ncfile.dimensions.items():
            header_size += len(dim.name.encode('utf-8')) + 4  # String size + 4 bytes for int (length)

        # Loop through variables and add their details
        for var_name, var in ncfile.variables.items():
            header_size += len(var_name.encode('utf-8')) + 4  # Variable name
            header_size += len(var.dtype.name.encode('utf-8')) + 4  # Data type
            for dim in var.dimensions:
                header_size += len(dim.encode('utf-8')) + 4 # Dimension name 

            # Add attribute information
            for attr_name, attr_value in var.ncattrs().items():
                header_size += len(attr_name.encode('utf-8')) + 4  # Attribute name
                header_size += len(str(attr_value).encode('utf-8')) + 4  # Attribute value

        return header_size

The reason I am asking is that we are having some issues with parallel IO with pNetCDF and nc_redef/nc_enddef calls and want to reserve a header large enough that the header can (nopefully) accomodate all the dims and variables and attributes we will probably have in the file.

[wkliao]

To check header size/extent of classic files, PnetCDF has a utility program called ncoffsets
For example,

% ncoffsets testfile.nc
netcdf testfile.nc {
// File format: CDF-5
// Number of dimensions: 12
// Number of variables: 30
// Number of global attributes: 26

file header:
	size   = 6336 bytes
	extent = 8192 bytes
...

[gsjaardema]

Thanks, that looks very useful. I didn't realize it existed.

[wkliao]

FYI. PnetCDF has 2 C APIs for inquiring the header size and extent.
ncmpi_inq_header_size
ncmpi_inq_header_extent

[gsjaardema]

Thanks. That should give me what I need for now.

Might be good to see if this could be added to netCDF. It looks like the data is already there for the netcdf-3 format files, so would "only" need a function added to return it... Although, I'm not sure how many people are actually looking for this information or care about the details at that level...

[wkliao]

If having some additional space in the file header section is desired in case
new data objects will be added, then you can use argument h_minfree in
API nc__enddef
In PnetCDF, it is ncmpi__enddef.

int nc__enddef(int  ncid,
               size_t  h_minfree,
               size_t  v_align,
               size_t  v_minfree,
               size_t  r_align )

I suggest call it and only at the first call after file creation, not after redeem()..