• Datasets and attributes may be of user-defined type.
• A user defined type must be defined before it is used to store data.
• When defined, a user type is assigned a typeid (which may not be the same between different openings of the same file).
• The typeid is then used when defining a variable or writing an attribute.
• The functions nc_put_var/nc_get_var and nc_put_att/nc_get_att are used to write and read data of user-defined type. For example:

  /* Write an attribute of any type. */
  EXTERNL int
  nc_put_att(int ncid, int varid, const char *name, nc_type xtype, 
  	   size_t len, const void *op);
  
  /* Read an attribute of any type. */
  EXTERNL int
  nc_get_att(int ncid, int varid, const char *name, void *ip);
  

• User defined types may be nested (in some ways), so that one may define a compound type, holding a variable length array of enumerations.

2007 Unidata NetCDF Workshop for Developers and Data Providers > Using NetCDF-4 Features, Part 2