To create netCDF files using different format variants:

• Create a classic netCDF file

    nc_create("file.nc", 0, &ncid);
  

• Create a (classic) 64-bit offset netCDF file

    nc_create("file.nc", NC_64BIT_OFFSET, &ncid);
  

• Create a netCDF-4 file (a kind of HDF5 file)

    nc_create("file.nc", NC_NETCDF4, &ncid);
  

• Create a netCDF-4 classic model file (a netCDF-4 file with restrictions)

    nc_create("file.nc", NC_NETCDF4 | NC_CLASSIC_MODEL, &ncid);
  

Classic binary format is still the default output format. NetCDF-4 files must be specified at create-time. If netCDF-4 allows multiple unlimited dimensions (for example), how to ensure code compatibility in all cases?

• to provide same behavior in existing code, netCDF-4 files can be created with a special flag: NC_CLASSIC_MODEL
• This is a backward compatibility flag that restricts what is allowed in the file.
• An attempt to (for example) create two unlimited dimensions in such a file will result in a error, just as with classic format.
• NetCDF-4 files produced without this flag can have any number of unlimited dimensions.
• You don't need the NC_CLASSIC_MODEL flag for code compatibility, it just makes netCDF-4 more strict to ensure compatibility, producingor example same error when you try to define multiple unlimited dimensions
• The data format is still HDF5, and no different from the same file created without CLASSIC_MODEL, except the CLASSIC_MODEL file will always reject any enhanced model feature, such as groups, user-defined types, and other new features.

2012 Unidata NetCDF Workshop > NetCDF-4 Programming