3.3 NetCDF-4 User Defined Data Types
NetCDF supported six data types through version 3.6.0 (char, byte,
short, int, float, and double). Starting with version 4.0, many new
data types are supported (unsigned int types, strings, compound types,
variable length arrays, enums, opaque).
In addition to the new atomic types, with netCDF-4/HDF5 files, the
user may define types.
Types are defined in define mode, and must be fully defined before
they are used. New types may be added to a file by re-entering define
mode.
Once defined the type may be used to create a variable or attribute.
Types may be nested in complex ways. For example, a compound type
containing an array of VLEN types, each containing variable length
arrays of some other compound type, etc. Users are cautioned to keep
types simple. Reading data of complex types can be challenging for
Fortran users.
Types may be defined in any group in the data file, but they are
always available globally in the file.
Types cannot have attributes (but variables of the type may have
attributes).
User defined data types are not available in the netCDF classic model,
so can't be used with classic or 64-bit format files, or netCDF-4
files created with the NC_CLASSIC_MODEL mode flag.
3.3.1 Compound Types
Compound types allow the user to combine atomic and user-defined types
into C-like structs. Since users defined types may be used within a
compound type, they can contain nested compound types.
Users define a compound type, and (in their C code) a corresponding C
struct. They can then use the nc_put_var[1asm] calls to write
multi-dimensional arrays of these structs, and nc_get_var[1asm] calls
to read them. (For example, the nc_put_varm function will write mapped
arrays of these structs.)
While structs, in general, are not portable from platform to platform,
the HDF5 layer (when installed) performs the magic required to figure
out your platform's idiosyncrasies, and adjust to them. The end result
is that HDF5 compound types (and therefore, netCDF-4 compound types),
are portable.
For more information on creating and using compound types, see
Compound Types (The NetCDF C Interface Guide) in The NetCDF C Interface Guide.
3.3.2 VLEN Types
Variable length arrays can be used to create a ragged array of data,
in which one of the dimensions varies in size from point to point.
An example of VLEN use would the to store a 1-D array of dropsonde
data, in which the data at each drop point is of variable length.
There is no special restriction on the dimensionality of VLEN
variables. It's possible to have 2D, 3D, 4D, etc. data, in which each
point contains a VLEN.
A VLEN has a base type (that is, the type that it is a VLEN of). This
may be one of the atomic types (forming, for example, a variable
length array of NC_INT), or it can be another user defined type, like
a compound type.
With VLEN data, special memory allocation and deallocation procedures
must be followed, or memory leaks may occur.
For more information on creating and using variable length arrays, see
Variable Length Arrays (The NetCDF C Interface Guide) in
The NetCDF C Interface Guide.
3.3.3 Opaque Types
Opaque types allow the user to store arrays of data blobs of a fixed
size.
For more information on creating and using opaque types, see
Opaque Type (The NetCDF C Interface Guide) in The NetCDF C Interface Guide.
3.3.4 Enum Types
Enum types allow the user to specify an enumeration.
For more information on creating and using enum types, see Enum Type (The NetCDF C Interface Guide) in The NetCDF C Interface Guide.
3.3.5 Groups
Although not a type of data, groups can help organize data within a
dataset. Like a directory structure on a Unix file-system, the
grouping feature allows users to organize variables and dimensions
into distinct, named, hierarchical areas, called groups. For more
information on groups types, see Groups (The NetCDF C Interface Guide) in The NetCDF C Interface Guide.
