NetCDF Programming Notes

Table of Contents

• Ignored if NULL
• Allocating Storage for the Result
• Specify a Hyperslab
  • A Vector Specifying Start Index for Each Dimension
  • A Vector Specifying Count for Each Dimension
  • A Vector Specifying Stride for Each Dimension
  • A Vector Specifying Mapping for Each Dimension
• NetCDF ID
• NetCDF Names
  • Permitted Characters in NetCDF Names
  • Name Length
  • NetCDF Conventions

See Also:

Ignored if NULL

Many of the argurments of netCDF functions are pointers. For example, the nc_inq() functions takes four pointers:

int nc_inq(int ncid, int *ndimsp, int *nvarsp, int *nattsp, int *unlimdimidp);

A NULL may be passed for any of these pointers, and it will be ignored. For example, interested in the number of dimensions only, the following code will work:

int ndims;
...
if (nc_inq(ncid, &ndims, NULL, NULL, NULL))
   return SOME_ERROR;

Allocating Storage for the Result

User must allocate space for the result of an inq function before the function is called.

Specify a Hyperslab

The NetCDF allows specification of hyperslabs to be read or written with vectors which specify the start, count, stride, and mapping.

A Vector Specifying Start Index for Each Dimension

A vector of size_t integers specifying the index in the variable where the first of the data values will be read.

The indices are relative to 0, so for example, the first data value of a variable would have index (0, 0, ... , 0).

The length of start vector must be the same as the number of dimensions of the specified variable. The elements of start correspond, in order, to the variable's dimensions.

A Vector Specifying Count for Each Dimension

A vector of size_t integers specifying the edge lengths along each dimension of the block of data values to be read.

To read a single value, for example, specify count as (1, 1, ... , 1).

The length of count is the number of dimensions of the specified variable. The elements of count correspond, in order, to the variable's dimensions.

Setting any element of the count array to zero causes the function to exit without error, and without doing anything.

A Vector Specifying Stride for Each Dimension

A vector of size_t integers specifying the interval between selected indices.

A value of 1 accesses adjacent values of the netCDF variable in the corresponding dimension; a value of 2 accesses every other value of the netCDF variable in the corresponding dimension; and so on.

The elements of the stride vector correspond, in order, to the variable's dimensions.

A NULL stride argument is treated as (1, 1, ... , 1).

A Vector Specifying Mapping for Each Dimension

A vector of integers that specifies the mapping between the dimensions of a netCDF variable and the in-memory structure of the internal data array.

imap[0] gives the distance between elements of the internal array corresponding to the most slowly varying dimension of the netCDF variable. imap[n-1] (where n is the rank of the netCDF variable) gives the distance between elements of the internal array corresponding to the most rapidly varying dimension of the netCDF variable. Intervening imap elements correspond to other dimensions of the netCDF variable in the obvious way. Distances between elements are specified in type-independent units of elements.

Note

The distance between internal elements that occupy adjacent memory locations is 1 and not the element's byte-length as in netCDF 2.

NetCDF ID

Most netCDF function require the netCDF ID as a first parameter.

In the netCDF classic model, the netCDF ID is associated with an open file. Each file, when opened by nc_open(), or created by nc_create(), is assigned an ncid, which it retains until nc_close() is called.

In the netCDF enhanced model, the ncid refers to a group with a file. (Each file contains at least the root group, which is the ncid that is returned by nc_open() and nc_create().)

For netCDF-4/HDF5 files, netCDF IDs can come not just from nc_open() and nc_create(), but also from nc_def_grp(), nc_inq_grps(), nc_inq_ncid(), nc_inq_grp_parent(), and nc_inq_grp_full_ncid().

NetCDF Names

Permitted Characters in NetCDF Names

The names of dimensions, variables and attributes (and, in netCDF-4 files, groups, user-defined types, compound member names, and enumeration symbols) consist of arbitrary sequences of alphanumeric characters, underscore '_', period '.', plus '+', hyphen '-', or at sign '@', but beginning with an alphanumeric character or underscore. However names commencing with underscore are reserved for system use.

Beginning with versions 3.6.3 and 4.0, names may also include UTF-8 encoded Unicode characters as well as other special characters, except for the character '/', which may not appear in a name.

Names that have trailing space characters are also not permitted.

Case is significant in netCDF names.

Name Length

A zero-length name is not allowed.

Names longer than NC_MAX_NAME will not be accepted any netCDF define function. An error of NC_EMAXNAME will be returned.

All netCDF inquiry functions will return names of maximum size NC_MAX_NAME for netCDF files. Since this does not include the terminating NULL, space should be reserved for NC_MAX_NAME + 1 characters.

NetCDF Conventions

Some widely used conventions restrict names to only alphanumeric characters or underscores.

Note that, when using the DAP2 protocol to access netCDF data, there are reserved keywords, the use of which may result in undefined behavior. See DAP2 Reserved Keywords for more information.