mirror of
https://github.com/Unidata/netcdf-c.git
synced 2024-11-21 03:13:42 +08:00
110 lines
3.9 KiB
Markdown
110 lines
3.9 KiB
Markdown
# NetCDF Programming Notes {#programming_notes}
|
|
|
|
[TOC]
|
|
|
|
<H2>See Also:</H2>
|
|
|
|
* \subpage nc-error-codes
|
|
|
|
# Ignored if NULL {#ignored_if_null}
|
|
|
|
Many of the argurments of netCDF functions are pointers. For example,
|
|
the nc_inq() functions takes four pointers:
|
|
|
|
~~~.C
|
|
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:
|
|
|
|
~~~.C
|
|
int ndims;
|
|
...
|
|
if (nc_inq(ncid, &ndims, NULL, NULL, NULL))
|
|
return SOME_ERROR;
|
|
~~~
|
|
|
|
# Allocating Storage for the Result {#allocating_storage_for_the_result}
|
|
|
|
User must allocate space for the result of an inq function before the function is called.
|
|
|
|
# Specify a Hyperslab {#specify_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 {#start_vector}
|
|
|
|
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 {#count_vector}
|
|
|
|
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 {#stride_vector}
|
|
|
|
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 {#map_vector}
|
|
|
|
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 {#ncid}
|
|
|
|
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().
|