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This document describes the C interface to the netCDF library. This document applies to netCDF version 3.6.1. This document was last updated on 16 January 2006.
For a complete description of the netCDF format and utilities see Top.
--- The Detailed Node Listing ---
Datasets
Dimensions
Variables
Attributes
You can use the netCDF library without knowing about all of the netCDF interface. If you are creating a netCDF dataset, only a handful of routines are required to define the necessary dimensions, variables, and attributes, and to write the data to the netCDF dataset. (Even less are needed if you use the ncgen utility to create the dataset before running a program using netCDF library calls to write data.) Similarly, if you are writing software to access data stored in a particular netCDF object, only a small subset of the netCDF library is required to open the netCDF dataset and access the data. Authors of generic applications that access arbitrary netCDF datasets need to be familiar with more of the netCDF library.
In this chapter we provide templates of common sequences of netCDF calls needed for common uses. For clarity we present only the names of routines; omit declarations and error checking; omit the type-specific suffixes of routine names for variables and attributes; indent statements that are typically invoked multiple times; and use ... to represent arbitrary sequences of other statements. Full parameter lists are described in later chapters.
Here is a typical sequence of netCDF calls used to create a new netCDF dataset:
nc_create /* create netCDF dataset: enter define mode */
...
nc_def_dim /* define dimensions: from name and length */
...
nc_def_var /* define variables: from name, type, ... */
...
nc_put_att /* put attribute: assign attribute values */
...
nc_enddef /* end definitions: leave define mode */
...
nc_put_var /* provide values for variables */
...
nc_close /* close: save new netCDF dataset */
Only one call is needed to create a netCDF dataset, at which point you will be in the first of two netCDF modes. When accessing an open netCDF dataset, it is either in define mode or data mode. In define mode, you can create dimensions, variables, and new attributes, but you cannot read or write variable data. In data mode, you can access data and change existing attributes, but you are not permitted to create new dimensions, variables, or attributes.
One call to nc_def_dim is needed for each dimension created. Similarly, one call to nc_def_var is needed for each variable creation, and one call to a member of the nc_put_att family is needed for each attribute defined and assigned a value. To leave define mode and enter data mode, call nc_enddef.
Once in data mode, you can add new data to variables, change old values, and change values of existing attributes (so long as the attribute changes do not require more storage space). Single values may be written to a netCDF variable with one of the members of the nc_put_var1 family, depending on what type of data you have to write. All the values of a variable may be written at once with one of the members of the nc_put_var family. Arrays or array cross-sections of a variable may be written using members of the nc_put_vara family. Subsampled array sections may be written using members of the nc_put_vars family. Mapped array sections may be written using members of the nc_put_varm family. (Subsampled and mapped access are general forms of data access that are explained later.)
Finally, you should explicitly close all netCDF datasets that have been opened for writing by calling nc_close. By default, access to the file system is buffered by the netCDF library. If a program terminates abnormally with netCDF datasets open for writing, your most recent modifications may be lost. This default buffering of data is disabled by setting the NC_SHARE flag when opening the dataset. But even if this flag is set, changes to attribute values or changes made in define mode are not written out until nc_sync or nc_close is called.
Here we consider the case where you know the names of not only the netCDF datasets, but also the names of their dimensions, variables, and attributes. (Otherwise you would have to do "inquire" calls.) The order of typical C calls to read data from those variables in a netCDF dataset is:
nc_open /* open existing netCDF dataset */
...
nc_inq_dimid /* get dimension IDs */
...
nc_inq_varid /* get variable IDs */
...
nc_get_att /* get attribute values */
...
nc_get_var /* get values of variables */
...
nc_close /* close netCDF dataset */
First, a single call opens the netCDF dataset, given the dataset name, and returns a netCDF ID that is used to refer to the open netCDF dataset in all subsequent calls.
Next, a call to nc_inq_dimid for each dimension of interest gets the dimension ID from the dimension name. Similarly, each required variable ID is determined from its name by a call to nc_inq_varid Once variable IDs are known, variable attribute values can be retrieved using the netCDF ID, the variable ID, and the desired attribute name as input to a member of the nc_get_att family (typically nc_get_att_text or nc_get_att_double) for each desired attribute. Variable data values can be directly accessed from the netCDF dataset with calls to members of the nc_get_var1 family for single values, the nc_get_var family for entire variables, or various other members of the nc_get_vara, nc_get_vars, or nc_get_varm families for array, subsampled or mapped access.
Finally, the netCDF dataset is closed with nc_close. There is no need to close a dataset open only for reading.
It is possible to write programs (e.g., generic software) which do such things as processing every variable, without needing to know in advance the names of these variables. Similarly, the names of dimensions and attributes may be unknown.
Names and other information about netCDF objects may be obtained from netCDF datasets by calling inquire functions. These return information about a whole netCDF dataset, a dimension, a variable, or an attribute. The following template illustrates how they are used:
nc_open /* open existing netCDF dataset */
...
nc_inq /* find out what is in it */
...
nc_inq_dim /* get dimension names, lengths */
...
nc_inq_var /* get variable names, types, shapes */
...
nc_inq_attname /* get attribute names */
...
nc_inq_att /* get attribute types and lengths */
...
nc_get_att /* get attribute values */
...
nc_get_var /* get values of variables */
...
nc_close /* close netCDF dataset */
As in the previous example, a single call opens the existing netCDF dataset, returning a netCDF ID. This netCDF ID is given to the nc_inq routine, which returns the number of dimensions, the number of variables, the number of global attributes, and the ID of the unlimited dimension, if there is one.
All the inquire functions are inexpensive to use and require no I/O, since the information they provide is stored in memory when a netCDF dataset is first opened.
Dimension IDs use consecutive integers, beginning at 0. Also dimensions, once created, cannot be deleted. Therefore, knowing the number of dimension IDs in a netCDF dataset means knowing all the dimension IDs: they are the integers 0, 1, 2, ...up to the number of dimensions. For each dimension ID, a call to the inquire function nc_inq_dim returns the dimension name and length.
Variable IDs are also assigned from consecutive integers 0, 1, 2, ... up to the number of variables. These can be used in nc_inq_var calls to find out the names, types, shapes, and the number of attributes assigned to each variable.
Once the number of attributes for a variable is known, successive calls to nc_inq_attname return the name for each attribute given the netCDF ID, variable ID, and attribute number. Armed with the attribute name, a call to nc_inq_att returns its type and length. Given the type and length, you can allocate enough space to hold the attribute values. Then a call to a member of the nc_get_att family returns the attribute values.
Once the IDs and shapes of netCDF variables are known, data values can be accessed by calling a member of the nc_get_var1 family for single values, or members of the nc_get_var, nc_get_vara, nc_get_vars, or nc_get_varm for various kinds of array access.
An existing netCDF dataset can be extensively altered. New dimensions, variables, and attributes can be added or existing ones renamed, and existing attributes can be deleted. Existing dimensions, variables, and attributes can be renamed. The following code template lists a typical sequence of calls to add new netCDF components to an existing dataset:
nc_open /* open existing netCDF dataset */
...
nc_redef /* put it into define mode */
...
nc_def_dim /* define additional dimensions (if any) */
...
nc_def_var /* define additional variables (if any) */
...
nc_put_att /* define additional attributes (if any) */
...
nc_enddef /* check definitions, leave define mode */
...
nc_put_var /* provide values for new variables */
...
nc_close /* close netCDF dataset */
A netCDF dataset is first opened by the nc_open call. This call puts the open dataset in data mode, which means existing data values can be accessed and changed, existing attributes can be changed (so long as they do not grow), but nothing can be added. To add new netCDF dimensions, variables, or attributes you must enter define mode, by calling nc_redef. In define mode, call nc_def_dim to define new dimensions, nc_def_var to define new variables, and a member of the nc_put_attfamily to assign new attributes to variables or enlarge old attributes.
You can leave define mode and reenter data mode, checking all the new definitions for consistency and committing the changes to disk, by calling nc_enddef. If you do not wish to reenter data mode, just call nc_close, which will have the effect of first calling nc_enddef.
Until the nc_enddef call, you may back out of all the redefinitions made in define mode and restore the previous state of the netCDF dataset by calling nc_abort. You may also use the nc_abort call to restore the netCDF dataset to a consistent state if the call to nc_enddef fails. If you have called nc_close from definition mode and the implied call to nc_enddef fails, nc_abort will automatically be called to close the netCDF dataset and leave it in its previous consistent state (before you entered define mode).
At most one process should have a netCDF dataset open for writing at one time. The library is designed to provide limited support for multiple concurrent readers with one writer, via disciplined use of the nc_sync function and the NC_SHARE flag. If a writer makes changes in define mode, such as the addition of new variables, dimensions, or attributes, some means external to the library is necessary to prevent readers from making concurrent accesses and to inform readers to call nc_sync before the next access.
The netCDF library provides the facilities needed to handle errors in a flexible way. Each netCDF function returns an integer status value. If the returned status value indicates an error, you may handle it in any way desired, from printing an associated error message and exiting to ignoring the error indication and proceeding (not recommended!). For simplicity, the examples in this guide check the error status and call a separate function to handle any errors.
The nc_strerror function is available to convert a returned integer error status into an error message string.
Occasionally, low-level I/O errors may occur in a layer below the netCDF library. For example, if a write operation causes you to exceed disk quotas or to attempt to write to a device that is no longer available, you may get an error from a layer below the netCDF library, but the resulting write error will still be reflected in the returned status value.
Details of how to compile and link a program that uses the netCDF C or FORTRAN interfaces differ, depending on the operating system, the available compilers, and where the netCDF library and include files are installed. Nevertheless, we provide here examples of how to compile and link a program that uses the netCDF library on a Unix platform, so that you can adjust these examples to fit your installation.
Every C file that references netCDF functions or constants must contain an appropriate #include statement before the first such reference:
#include <netcdf.h>
Unless the netcdf.h file is installed in a standard directory where the C compiler always looks, you must use the -I option when invoking the compiler, to specify a directory where netcdf.h is installed, for example:
cc -c -I/usr/local/netcdf/include myprogram.c
Alternatively, you could specify an absolute path name in the #include statement, but then your program would not compile on another platform where netCDF is installed in a different location.
Unless the netCDF library is installed in a standard directory where the linker always looks, you must use the -L and -l options to link an object file that uses the netCDF library. For example:
cc -o myprogram myprogram.o -L/usr/local/netcdf/lib -lnetcdf
Alternatively, you could specify an absolute path name for the library:
cc -o myprogram myprogram.o -l/usr/local/netcdf/lib/libnetcdf.a
This chapter presents the interfaces of the netCDF functions that deal with a netCDF dataset or the whole netCDF library.
A netCDF dataset that has not yet been opened can only be referred to by its dataset name. Once a netCDF dataset is opened, it is referred to by a netCDF ID, which is a small nonnegative integer returned when you create or open the dataset. A netCDF ID is much like a file descriptor in C or a logical unit number in FORTRAN. In any single program, the netCDF IDs of distinct open netCDF datasets are distinct. A single netCDF dataset may be opened multiple times and will then have multiple distinct netCDF IDs; however at most one of the open instances of a single netCDF dataset should permit writing. When an open netCDF dataset is closed, the ID is no longer associated with a netCDF dataset.
Functions that deal with the netCDF library include:
The operations supported on a netCDF dataset as a single object are:
Each interface description for a particular netCDF function in this and later chapters contains:
The examples follow a simple convention for error handling, always checking the error status returned from each netCDF function call and calling a handle_error function in case an error was detected. For an example of such a function, nc_strerror.
The function nc_strerror returns a static reference to an error message string corresponding to an integer netCDF error status or to a system error number, presumably returned by a previous call to some other netCDF function. The list of netCDF error status codes is available in the appropriate include file for each language binding.
const char * nc_strerror(int ncerr);
ncerrIf you provide an invalid integer error status that does not correspond to any netCDF error message or or to any system error message (as understood by the system strerror function), nc_strerror returns a string indicating that there is no such error status.
Here is an example of a simple error handling function that uses nc_strerror to print the error message corresponding to the netCDF error status returned from any netCDF function call and then exit:
#include <netcdf.h>
...
void handle_error(int status) {
if (status != NC_NOERR) {
fprintf(stderr, "%s\n", nc_strerror(status));
exit(-1);
}
}
The function nc_inq_libvers returns a string identifying the version of the netCDF library, and when it was built.
const char * nc_inq_libvers(void);
This function takes no arguments, and thus no errors are possible in its invocation.
Here is an example using nc_inq_libvers to print the version of the netCDF library with which the program is linked:
#include <netcdf.h>
...
printf("%s\n", nc_inq_libvers());
This function creates a new netCDF dataset, returning a netCDF ID that can subsequently be used to refer to the netCDF dataset in other netCDF function calls. The new netCDF dataset opened for write access and placed in define mode, ready for you to add dimensions, variables, and attributes.
A creation mode flag specifies:
int nc_create (const char* path, int cmode, int *ncidp);
pathcmodeSetting NC_NOCLOBBER means you do not want to clobber (overwrite) an existing dataset; an error (NC_EEXIST) is returned if the specified dataset already exists.
The NC_SHARE flag is appropriate when one process may be writing the dataset and one or more other processes reading the dataset concurrently; it means that dataset accesses are not buffered and caching is limited. Since the buffering scheme is optimized for sequential access, programs that do not access data sequentially may see some performance improvement by setting the NC_SHARE flag.
Setting NC_64BIT_OFFSET causes netCDF to create a 64-bit offset format file, instead of a netCDF classic format file. The 64-bit offset format imposes far fewer restrictions on very large (i.e. over 2 GB) data files. See Large File Support.
A zero value (defined for convenience as NC_CLOBBER) specifies the
default behavior: overwrite any existing dataset with the same file
name and buffer and cache accesses for efficiency. The dataset will be
in netCDF classic format. See NetCDF Classic Format Limitations.
ncidpnc_create returns the value NC_NOERR if no errors occurred. Possible causes of errors include:
In this example we create a netCDF dataset named foo.nc; we want the dataset to be created in the current directory only if a dataset with that name does not already exist:
#include <netcdf.h>
...
int status;
int ncid;
...
status = nc_create("foo.nc", NC_NOCLOBBER, &ncid);
if (status != NC_NOERR) handle_error(status);
In this example we create a netCDF dataset named foo_large.nc. It will be in the 64-bit offset format.
#include <netcdf.h>
...
int status;
int ncid;
...
status = nc_create("foo.nc", NC_NOCLOBBER, &ncid);
if (status != NC_NOERR) handle_error(status);
A variant of nc_create, nc__create (note the double underscore) allows users to specify two tuning parameters for the file that it is creating. These tuning parameters are not written to the data file, they are only used for so long as the file remains open after an nc__create.
int nc__create(const char *path, int cmode, size_t initialsz,
size_t *chunksizehintp, int *ncidp);
pathcmodeSetting NC_NOCLOBBER means you do not want to clobber (overwrite) an existing dataset; an error (NC_EEXIST) is returned if the specified dataset already exists.
The NC_SHARE flag is appropriate when one process may be writing the dataset and one or more other processes reading the dataset concurrently; it means that dataset accesses are not buffered and caching is limited. Since the buffering scheme is optimized for sequential access, programs that do not access data sequentially may see some performance improvement by setting the NC_SHARE flag.
Setting NC_64BIT_OFFSET causes netCDF to create a 64-bit offset format file, instead of a netCDF classic format file. The 64-bit offset format imposes far fewer restrictions on very large (i.e. over 2 GB) data files. See Large File Support.
A zero value (defined for convenience as NC_CLOBBER) specifies the
default behavior: overwrite any existing dataset with the same file
name and buffer and cache accesses for efficiency. The dataset will be
in netCDF classic format. See NetCDF Classic Format Limitations.
initialszchunksizehintpBecause of internal requirements, the value may not be set to exactly the value requested. The actual value chosen is returned by reference.
Using the value NC_SIZEHINT_DEFAULT causes the library to choose a default. How the system chooses the default depends on the system. On many systems, the "preferred I/O block size" is available from the stat() system call, struct stat member st_blksize. If this is available it is used. Lacking that, twice the system pagesize is used.
Lacking a call to discover the system pagesize, we just set default chunksize to 8192.
The chunksize is a property of a given open netcdf descriptor
ncid, it is not a persistent property of the netcdf dataset.
ncidpnc__create returns the value NC_NOERR if no errors occurred. Possible causes of errors include:
In this example we create a netCDF dataset named foo.nc; we want the dataset to be created in the current directory only if a dataset with that name does not already exist:
#include <netcdf.h>
...
int status;
int ncid;
...
status = nc_create("foo.nc", NC_NOCLOBBER, &ncid);
if (status != NC_NOERR) handle_error(status);
In this example we create a netCDF dataset named foo_large.nc. It will be in the 64-bit offset format.
#include <netcdf.h>
...
int status;
int ncid;
...
status = nc_create("foo.nc", NC_NOCLOBBER, &ncid);
if (status != NC_NOERR) handle_error(status);
This function is a variant of nc_create, nc__create (note the double underscore) allows users to specify two tuning parameters for the file that it is creating. These tuning parameters are not written to the data file, they are only used for so long as the file remains open after an nc__create.
This function creates a new netCDF dataset, returning a netCDF ID that can subsequently be used to refer to the netCDF dataset in other netCDF function calls. The new netCDF dataset opened for write access and placed in define mode, ready for you to add dimensions, variables, and attributes.
A creation mode flag specifies whether to overwrite any existing dataset with the same name and whether access to the dataset is shared, and whether this file should be in netCDF classic format (the default), or the new 64-bit offset format.
int nc__create(const char *path, int cmode, size_t initialsz,
size_t *chunksizehintp, int *ncidp);
pathcmodeSetting NC_NOCLOBBER means you do not want to clobber (overwrite) an existing dataset; an error (NC_EEXIST) is returned if the specified dataset already exists.
The NC_SHARE flag is appropriate when one process may be writing the dataset and one or more other processes reading the dataset concurrently; it means that dataset accesses are not buffered and caching is limited. Since the buffering scheme is optimized for sequential access, programs that do not access data sequentially may see some performance improvement by setting the NC_SHARE flag.
Setting NC_64BIT_OFFSET causes netCDF to create a 64-bit offset format file, instead of a netCDF classic format file. The 64-bit offset format imposes far fewer restrictions on very large (i.e. over 2 GB) data files. See Large File Support.
A zero value (defined for convenience as NC_CLOBBER) specifies the
default behavior: overwrite any existing dataset with the same file
name and buffer and cache accesses for efficiency. The dataset will be
in netCDF classic format. See NetCDF Classic Format Limitations.
initialszchunksizehintpBecause of internal requirements, the value may not be set to exactly the value requested. The actual value chosen is returned by reference.
Using the value NC_SIZEHINT_DEFAULT causes the library to choose a default. How the system chooses the default depends on the system. On many systems, the "preferred I/O block size" is available from the stat() system call, struct stat member st_blksize. If this is available it is used. Lacking that, twice the system pagesize is used.
Lacking a call to discover the system pagesize, we just set default chunksize to 8192.
The chunksize is a property of a given open netcdf descriptor
ncid, it is not a persistent property of the netcdf dataset.
ncidpnc_create returns the value NC_NOERR if no errors occurred. Possible causes of errors include:
In this example we create a netCDF dataset named foo.nc; we want the dataset to be created in the current directory only if a dataset with that name does not already exist. We also specify that chunksize and initial size for the file.
#include <netcdf.h>
...
int status;
int ncid;
int intialsz = 2048;
int *chunksize;
...
*chunksize = 1024;
status = nc__create("foo.nc", NC_NOCLOBBER, initialsz, chunksize, &ncid);
if (status != NC_NOERR) handle_error(status);
The function nc_open opens an existing netCDF dataset for access.
int nc_open (const char *path, int omode, int *ncidp);
pathomodeOtherwise, the creation mode is NC_WRITE, NC_SHARE, or
NC_WRITE|NC_SHARE. Setting the NC_WRITE flag opens the dataset with
read-write access. ("Writing" means any kind of change to the dataset,
including appending or changing data, adding or renaming dimensions,
variables, and attributes, or deleting attributes.) The NC_SHARE flag
is appropriate when one process may be writing the dataset and one or
more other processes reading the dataset concurrently; it means that
dataset accesses are not buffered and caching is limited. Since the
buffering scheme is optimized for sequential access, programs that do
not access data sequentially may see some performance improvement by
setting the NC_SHARE flag.
ncidpnc_open returns the value NC_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using nc_open to open an existing netCDF dataset named foo.nc for read-only, non-shared access:
#include <netcdf.h>
...
int status;
int ncid;
...
status = nc_open("foo.nc", 0, &ncid);
if (status != NC_NOERR) handle_error(status);
A function opens a netCDF dataset for access with an additional performance tuning parameter.
int nc__open(const char *path, int mode, size_t *chunksizehintp, int *ncidp);
pathomodeOtherwise, the creation mode is NC_WRITE, NC_SHARE, or
NC_WRITE|NC_SHARE. Setting the NC_WRITE flag opens the dataset with
read-write access. ("Writing" means any kind of change to the dataset,
including appending or changing data, adding or renaming dimensions,
variables, and attributes, or deleting attributes.) The NC_SHARE flag
is appropriate when one process may be writing the dataset and one or
more other processes reading the dataset concurrently; it means that
dataset accesses are not buffered and caching is limited. Since the
buffering scheme is optimized for sequential access, programs that do
not access data sequentially may see some performance improvement by
setting the NC_SHARE flag.
chunksizehintpBecause of internal requirements, the value may not be set to exactly the value requested. The actual value chosen is returned by reference.
Using the value NC_SIZEHINT_DEFAULT causes the library to choose a default. How the system chooses the default depends on the system. On many systems, the "preferred I/O block size" is available from the stat() system call, struct stat member st_blksize. If this is available it is used. Lacking that, twice the system pagesize is used.
Lacking a call to discover the system pagesize, we just set default chunksize to 8192.
The chunksize is a property of a given open netcdf descriptor
ncid, it is not a persistent property of the netcdf dataset.
ncidpnc__open returns the value NC_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using nc__open to open an existing netCDF dataset named foo.nc for read-only, non-shared access:
#include <netcdf.h>
...
int status;
int ncid;
int *chunksize;
...
*chunksize = 1024;
status = nc_open("foo.nc", 0, chunksize, &ncid);
if (status != NC_NOERR) handle_error(status);
The function nc_redef puts an open netCDF dataset into define mode, so dimensions, variables, and attributes can be added or renamed and attributes can be deleted.
int nc_redef(int ncid);
ncidnc_redef returns the value NC_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using nc_redef to open an existing netCDF dataset named foo.nc and put it into define mode:
#include <netcdf.h>
...
int status;
int ncid;
...
status = nc_open("foo.nc", NC_WRITE, &ncid); /* open dataset */
if (status != NC_NOERR) handle_error(status);
...
status = nc_redef(ncid); /* put in define mode */
if (status != NC_NOERR) handle_error(status);
The function nc_enddef takes an open netCDF dataset out of define mode. The changes made to the netCDF dataset while it was in define mode are checked and committed to disk if no problems occurred. Non-record variables may be initialized to a "fill value" as well. See nc_set_fill. The netCDF dataset is then placed in data mode, so variable data can be read or written.
This call may involve copying data under some circumstances. For a more extensive discussion File Structure and Performance.
int nc_enddef(int ncid);
ncidnc_enddef returns the value NC_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using nc_enddef to finish the definitions of a new netCDF dataset named foo.nc and put it into data mode:
#include <netcdf.h>
...
int status;
int ncid;
...
status = nc_create("foo.nc", NC_NOCLOBBER, &ncid);
if (status != NC_NOERR) handle_error(status);
... /* create dimensions, variables, attributes */
status = nc_enddef(ncid); /*leave define mode*/
if (status != NC_NOERR) handle_error(status);
The function nc__enddef takes an open netCDF dataset out of define mode. The changes made to the netCDF dataset while it was in define mode are checked and committed to disk if no problems occurred. Non-record variables may be initialized to a "fill value" as well. See nc_set_fill. The netCDF dataset is then placed in data mode, so variable data can be read or written.
This call may involve copying data under some circumstances. For a more extensive discussion File Structure and Performance.
Caution: this function exposes internals of the netcdf version 1 file format. Users should use nc_enddef in most circumstances. This function may not be available on future netcdf implementations.
The current netcdf file format has three sections, the "header" section, the data section for fixed size variables, and the data section for variables which have an unlimited dimension (record variables).
The header begins at the beginning of the file. The index (offset) of the beginning of the other two sections is contained in the header. Typically, there is no space between the sections. This causes copying overhead to accrue if one wishes to change the size of the sections, as may happen when changing names of things, text attribute values, adding attributes or adding variables. Also, for buffered i/o, there may be advantages to aligning sections in certain ways.
The minfree parameters allow one to control costs of future calls to nc_redef, nc_enddef by requesting that minfree bytes be available at the end of the section.
The align parameters allow one to set the alignment of the beginning of the corresponding sections. The beginning of the section is rounded up to an index which is a multiple of the align parameter. The flag value ALIGN_CHUNK tells the library to use the chunksize (see above) as the align parameter.
The file format requires mod 4 alignment, so the align parameters are silently rounded up to multiples of 4. The usual call,
nc_enddef(ncid);
is equivalent to
nc_enddef(ncid, 0, 4, 0, 4);
The file format does not contain a "record size" value, this is calculated from the sizes of the record variables. This unfortunate fact prevents us from providing minfree and alignment control of the "records" in a netcdf file. If you add a variable which has an unlimited dimension, the third section will always be copied with the new variable added.
int nc__enddef(int ncid, size_t h_minfree, size_t v_align,
size_t v_minfree, size_t r_align);
ncidh_minfreev_alignv_minfreer_alignnc__enddef returns the value NC_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using nc_enddef to finish the definitions of a new netCDF dataset named foo.nc and put it into data mode:
#include <netcdf.h>
...
int status;
int ncid;
...
status = nc_create("foo.nc", NC_NOCLOBBER, &ncid);
if (status != NC_NOERR) handle_error(status);
... /* create dimensions, variables, attributes */
status = nc_enddef(ncid); /*leave define mode*/
if (status != NC_NOERR) handle_error(status);
The function nc_close closes an open netCDF dataset. If the dataset is in define mode, nc_enddef will be called before closing. (In this case, if nc_enddef returns an error, nc_abort will automatically be called to restore the dataset to the consistent state before define mode was last entered.) After an open netCDF dataset is closed, its netCDF ID may be reassigned to the next netCDF dataset that is opened or created.
int nc_close(int ncid);
ncidnc_close returns the value NC_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using nc_close to finish the definitions of a new netCDF dataset named foo.nc and release its netCDF ID:
#include <netcdf.h>
...
int status;
int ncid;
...
status = nc_create("foo.nc", NC_NOCLOBBER, &ncid);
if (status != NC_NOERR) handle_error(status);
... /* create dimensions, variables, attributes */
status = nc_close(ncid); /* close netCDF dataset */
if (status != NC_NOERR) handle_error(status);
Members of the nc_inq family of functions return information about an open netCDF dataset, given its netCDF ID. Dataset inquire functions may be called from either define mode or data mode. The first function, nc_inq, returns values for the number of dimensions, the number of variables, the number of global attributes, and the dimension ID of the dimension defined with unlimited length, if any. The other functions in the family each return just one of these items of information.
For C, these functions include nc_inq, nc_inq_ndims, nc_inq_nvars, nc_inq_natts, and nc_inq_unlimdim. An additional function, nc_inq_format, returns the (rarely needed) format version.
No I/O is performed when these functions are called, since the required information is available in memory for each open netCDF dataset.
int nc_inq (int ncid, int *ndimsp, int *nvarsp, int *ngattsp,
int *unlimdimidp);
int nc_inq_ndims (int ncid, int *ndimsp);
int nc_inq_nvars (int ncid, int *nvarsp);
int nc_inq_natts (int ncid, int *ngattsp);
int nc_inq_unlimdim (int ncid, int *unlimdimidp);
int nc_inq_format (int ncid, int *formatp);
ncidndimspnvarspngattspunlimdimidpformatpAll members of the nc_inq family return the value NC_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using nc_inq to find out about a netCDF dataset named foo.nc:
#include <netcdf.h>
...
int status, ncid, ndims, nvars, ngatts, unlimdimid;
...
status = nc_open("foo.nc", NC_NOWRITE, &ncid);
if (status != NC_NOERR) handle_error(status);
...
status = nc_inq(ncid, &ndims, &nvars, &ngatts, &unlimdimid);
if (status != NC_NOERR) handle_error(status);
The function nc_sync offers a way to synchronize the disk copy of a netCDF dataset with in-memory buffers. There are two reasons you might want to synchronize after writes:
This function is backward-compatible with previous versions of the netCDF library. The intent was to allow sharing of a netCDF dataset among multiple readers and one writer, by having the writer call nc_sync after writing and the readers call nc_sync before each read. For a writer, this flushes buffers to disk. For a reader, it makes sure that the next read will be from disk rather than from previously cached buffers, so that the reader will see changes made by the writing process (e.g., the number of records written) without having to close and reopen the dataset. If you are only accessing a small amount of data, it can be expensive in computer resources to always synchronize to disk after every write, since you are giving up the benefits of buffering.
An easier way to accomplish sharing (and what is now recommended) is to have the writer and readers open the dataset with the NC_SHARE flag, and then it will not be necessary to call nc_sync at all. However, the nc_sync function still provides finer granularity than the NC_SHARE flag, if only a few netCDF accesses need to be synchronized among processes.
It is important to note that changes to the ancillary data, such as attribute values, are not propagated automatically by use of the NC_SHARE flag. Use of the nc_sync function is still required for this purpose.
Sharing datasets when the writer enters define mode to change the data schema requires extra care. In previous releases, after the writer left define mode, the readers were left looking at an old copy of the dataset, since the changes were made to a new copy. The only way readers could see the changes was by closing and reopening the dataset. Now the changes are made in place, but readers have no knowledge that their internal tables are now inconsistent with the new dataset schema. If netCDF datasets are shared across redefinition, some mechanism external to the netCDF library must be provided that prevents access by readers during redefinition and causes the readers to call nc_sync before any subsequent access.
When calling nc_sync, the netCDF dataset must be in data mode. A netCDF dataset in define mode is synchronized to disk only when nc_enddef is called. A process that is reading a netCDF dataset that another process is writing may call nc_sync to get updated with the changes made to the data by the writing process (e.g., the number of records written), without having to close and reopen the dataset.
Data is automatically synchronized to disk when a netCDF dataset is closed, or whenever you leave define mode.
int nc_sync(int ncid);
ncidnc_sync returns the value NC_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using nc_sync to synchronize the disk writes of a netCDF dataset named foo.nc:
#include <netcdf.h>
...
int status;
int ncid;
...
status = nc_open("foo.nc", NC_WRITE, &ncid); /* open for writing */
if (status != NC_NOERR) handle_error(status);
... /* write data or change attributes */
status = nc_sync(ncid); /* synchronize to disk */
if (status != NC_NOERR) handle_error(status);
You no longer need to call this function, since it is called automatically by nc_close in case the dataset is in define mode and something goes wrong with committing the changes. The function nc_abort just closes the netCDF dataset, if not in define mode. If the dataset is being created and is still in define mode, the dataset is deleted. If define mode was entered by a call to nc_redef, the netCDF dataset is restored to its state before definition mode was entered and the dataset is closed.
int nc_abort(int ncid);
ncidnc_abort returns the value NC_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using nc_abort to back out of redefinitions of a dataset named foo.nc:
#include <netcdf.h>
...
int ncid, status, latid;
...
status = nc_open("foo.nc", NC_WRITE, &ncid);/* open for writing */
if (status != NC_NOERR) handle_error(status);
...
status = nc_redef(ncid); /* enter define mode */
if (status != NC_NOERR) handle_error(status);
...
status = nc_def_dim(ncid, "lat", 18L, &latid);
if (status != NC_NOERR) {
handle_error(status);
status = nc_abort(ncid); /* define failed, abort */
if (status != NC_NOERR) handle_error(status);
}
This function is intended for advanced usage, to optimize writes under some circumstances described below. The function nc_set_fill sets the fill mode for a netCDF dataset open for writing and returns the current fill mode in a return parameter. The fill mode can be specified as either NC_FILL or NC_NOFILL. The default behavior corresponding to NC_FILL is that data is pre-filled with fill values, that is fill values are written when you create non-record variables or when you write a value beyond data that has not yet been written. This makes it possible to detect attempts to read data before it was written. For more information on the use of fill values Fill Values. For information about how to define your own fill values Attribute Conventions.
The behavior corresponding to NC_NOFILL overrides the default behavior of prefilling data with fill values. This can be used to enhance performance, because it avoids the duplicate writes that occur when the netCDF library writes fill values that are later overwritten with data.
A value indicating which mode the netCDF dataset was already in is returned. You can use this value to temporarily change the fill mode of an open netCDF dataset and then restore it to the previous mode.
After you turn on NC_NOFILL mode for an open netCDF dataset, you must be certain to write valid data in all the positions that will later be read. Note that nofill mode is only a transient property of a netCDF dataset open for writing: if you close and reopen the dataset, it will revert to the default behavior. You can also revert to the default behavior by calling nc_set_fill again to explicitly set the fill mode to NC_FILL.
There are three situations where it is advantageous to set nofill mode:
If the netCDF dataset has an unlimited dimension and the last record was written while in nofill mode, then the dataset may be shorter than if nofill mode was not set, but this will be completely transparent if you access the data only through the netCDF interfaces.
The use of this feature may not be available (or even needed) in future releases. Programmers are cautioned against heavy reliance upon this feature.
int nc_set_fill (int ncid, int fillmode, int *old_modep);
ncidfillmodeold_modepnc_set_fill returns the value NC_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using nc_set_fill to set nofill mode for subsequent writes of a netCDF dataset named foo.nc:
#include <netcdf.h>
...
int ncid, status, old_fill_mode;
...
status = nc_open("foo.nc", NC_WRITE, &ncid); /* open for writing */
if (status != NC_NOERR) handle_error(status);
... /* write data with default prefilling behavior */
status = nc_set_fill(ncid, NC_NOFILL, &old_fill_mode); /* set nofill */
if (status != NC_NOERR) handle_error(status);
... /* write data with no prefilling */
This function is intended for advanced users.
Starting in version 3.6, netCDF introduced a new data format, the first change in the underlying binary data format since the netCDF interface was released. The new format, 64-bit offset format, was introduced to greatly relax the limitations on creating very large files.
Users are warned that creating files in the 64-bit offset format makes them unreadable by the netCDF library prior to version 3.6.0. For reasons of compatibility, users should continue to create files in netCDF classic format.
Users who do want to use 64-bit offset format files can create them directory from nc_create, using the proper cmode flag. (see nc_create).
The function nc_set_default_format allows the user to change the format of the netCDF file to be created by future calls to nc_create (or nc__create) without changing the cmode flag.
This allows the user to convert a program to use 64-bit offset formation without changing all calls the nc_create. See Large File Support.
Once the default format is set, all future created files will be in the desired format.
Two constants are provided in the netcdf.h file to be used with this function, NC_FORMAT_64BIT and NC_FORMAT_CLASSIC.
If a non-NULL pointer is provided, it is assumed to point to an int, where the existing default format will be written.
Using nc_create with a cmode including NC_64BIT_OFFSET overrides the default format, and creates a 64-bit offset file.
int nc_set_default_format(int format, int *old_formatp);
formatold_formatpnc_set_default_format returns the value NC_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using nc_set_default_format to create the same file in both formats with the same nc_create call:
#include <netcdf.h>
...
int ncid, status, old_fill_mode;
...
status = nc_open("foo.nc", NC_WRITE, &ncid); /* open for writing */
if (status != NC_NOERR) handle_error(status);
... /* write data with default prefilling behavior */
status = nc_set_fill(ncid, NC_NOFILL, &old_fill_mode); /* set nofill */
if (status != NC_NOERR) handle_error(status);
... /* write data with no prefilling */
Dimensions for a netCDF dataset are defined when it is created, while the netCDF dataset is in define mode. Additional dimensions may be added later by reentering define mode. A netCDF dimension has a name and a length. At most one dimension in a netCDF dataset can have the unlimited length, which means variables using this dimension can grow along this dimension.
There is a suggested limit (1024) to the number of dimensions that can be defined in a single netCDF dataset. The limit is the value of the predefined macro NC_MAX_DIMS. The purpose of the limit is to make writing generic applications simpler. They need only provide an array of NC_MAX_DIMS dimensions to handle any netCDF dataset. The implementation of the netCDF library does not enforce this advisory maximum, so it is possible to use more dimensions, if necessary, but netCDF utilities that assume the advisory maximums may not be able to handle the resulting netCDF datasets.
Ordinarily, the name and length of a dimension are fixed when the dimension is first defined. The name may be changed later, but the length of a dimension (other than the unlimited dimension) cannot be changed without copying all the data to a new netCDF dataset with a redefined dimension length.
Dimension lengths in the C interface are type size_t rather than type int to make it possible to access all the data in a netCDF dataset on a platform that only supports a 16-bit int data type, for example MSDOS. If dimension lengths were type int instead, it would not be possible to access data from variables with a dimension length greater than a 16-bit int can accommodate.
A netCDF dimension in an open netCDF dataset is referred to by a small integer called a dimension ID. In the C interface, dimension IDs are 0, 1, 2, ..., in the order in which the dimensions were defined.
Operations supported on dimensions are:
The function nc_def_dim adds a new dimension to an open netCDF dataset in define mode. It returns (as an argument) a dimension ID, given the netCDF ID, the dimension name, and the dimension length. At most one unlimited length dimension, called the record dimension, may be defined for each netCDF dataset.
int nc_def_dim (int ncid, const char *name, size_t len, int *dimidp);
ncidnamelendimidpnc_def_dim returns the value NC_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using nc_def_dim to create a dimension named lat of length 18 and a unlimited dimension named rec in a new netCDF dataset named foo.nc:
#include <netcdf.h>
...
int status, ncid, latid, recid;
...
status = nc_create("foo.nc", NC_NOCLOBBER, &ncid);
if (status != NC_NOERR) handle_error(status);
...
status = nc_def_dim(ncid, "lat", 18L, &latid);
if (status != NC_NOERR) handle_error(status);
status = nc_def_dim(ncid, "rec", NC_UNLIMITED, &recid);
if (status != NC_NOERR) handle_error(status);
The function nc_inq_dimid returns (as an argument) the ID of a netCDF dimension, given the name of the dimension. If ndims is the number of dimensions defined for a netCDF dataset, each dimension has an ID between 0 and ndims-1.
int nc_inq_dimid (int ncid, const char *name, int *dimidp);
ncidnamedimidpnc_inq_dimid returns the value NC_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
The name that was specified is not the name of a dimension in the netCDF dataset. The specified netCDF ID does not refer to an open netCDF dataset.
Here is an example using nc_inq_dimid to determine the dimension ID of a dimension named lat, assumed to have been defined previously in an existing netCDF dataset named foo.nc:
#include <netcdf.h>
...
int status, ncid, latid;
...
status = nc_open("foo.nc", NC_NOWRITE, &ncid); /* open for reading */
if (status != NC_NOERR) handle_error(status);
...
status = nc_inq_dimid(ncid, "lat", &latid);
if (status != NC_NOERR) handle_error(status);
This family of functions returns information about a netCDF dimension. Information about a dimension includes its name and its length. The length for the unlimited dimension, if any, is the number of records written so far.
The functions in this family include nc_inq_dim, nc_inq_dimname, and nc_inq_dimlen. The function nc_inq_dim returns all the information about a dimension; the other functions each return just one item of information.
int nc_inq_dim (int ncid, int dimid, char* name, size_t* lengthp);
int nc_inq_dimname (int ncid, int dimid, char *name);
int nc_inq_dimlen (int ncid, int dimid, size_t *lengthp);
nciddimidnamelengthpThese functions return the value NC_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using nc_inq_dim to determine the length of a dimension named lat, and the name and current maximum length of the unlimited dimension for an existing netCDF dataset named foo.nc:
#include <netcdf.h>
...
int status, ncid, latid, recid;
size_t latlength, recs;
char recname[NC_MAX_NAME+1];
...
status = nc_open("foo.nc", NC_NOWRITE, &ncid); /* open for reading */
if (status != NC_NOERR) handle_error(status);
status = nc_inq_unlimdim(ncid, &recid); /* get ID of unlimited dimension */
if (status != NC_NOERR) handle_error(status);
...
status = nc_inq_dimid(ncid, "lat", &latid); /* get ID for lat dimension */
if (status != NC_NOERR) handle_error(status);
status = nc_inq_dimlen(ncid, latid, &latlength); /* get lat length */
if (status != NC_NOERR) handle_error(status);
/* get unlimited dimension name and current length */
status = nc_inq_dim(ncid, recid, recname, &recs);
if (status != NC_NOERR) handle_error(status);
The function nc_rename_dim renames an existing dimension in a netCDF dataset open for writing. If the new name is longer than the old name, the netCDF dataset must be in define mode. You cannot rename a dimension to have the same name as another dimension.
int nc_rename_dim(int ncid, int dimid, const char* name);
nciddimidnamenc_rename_dim returns the value NC_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using nc_rename_dim to rename the dimension lat to latitude in an existing netCDF dataset named foo.nc:
#include <netcdf.h>
...
int status, ncid, latid;
...
status = nc_open("foo.nc", NC_WRITE, &ncid); /* open for writing */
if (status != NC_NOERR) handle_error(status);
...
status = nc_redef(ncid); /* put in define mode to rename dimension */
if (status != NC_NOERR) handle_error(status);
status = nc_inq_dimid(ncid, "lat", &latid);
if (status != NC_NOERR) handle_error(status);
status = nc_rename_dim(ncid, latid, "latitude");
if (status != NC_NOERR) handle_error(status);
status = nc_enddef(ncid); /* leave define mode */
if (status != NC_NOERR) handle_error(status);
Variables for a netCDF dataset are defined when the dataset is created, while the netCDF dataset is in define mode. Other variables may be added later by reentering define mode. A netCDF variable has a name, a type, and a shape, which are specified when it is defined. A variable may also have values, which are established later in data mode.
Ordinarily, the name, type, and shape are fixed when the variable is first defined. The name may be changed, but the type and shape of a variable cannot be changed. However, a variable defined in terms of the unlimited dimension can grow without bound in that dimension.
A netCDF variable in an open netCDF dataset is referred to by a small integer called a variable ID.
Variable IDs reflect the order in which variables were defined within a netCDF dataset. Variable IDs are 0, 1, 2,..., in the order in which the variables were defined. A function is available for getting the variable ID from the variable name and vice-versa.
Attributes (see Attributes) may be associated with a variable to specify such properties as units.
Operations supported on variables are:
The following table gives the netCDF external data types and the corresponding type constants for defining variables in the C interface:
| Type | C #define | Bits
|
| byte | NC_BYTE | 8
|
| char | NC_CHAR | 8
|
| short | NC_SHORT | 16
|
| int | NC_INT | 32
|
| float | NC_FLOAT | 32
|
| double | NC_DOUBLE | 64
|
The first column gives the netCDF external data type, which is the same as the CDL data type. The next column gives the corresponding C preprocessor macro for use in netCDF functions (the preprocessor macros are defined in the netCDF C header-file netcdf.h). The last column gives the number of bits used in the external representation of values of the corresponding type.
Note that there are no netCDF types corresponding to 64-bit integers or to characters wider than 8 bits in the current version of the netCDF library.
nc_def_varThe function nc_def_var adds a new variable to an open netCDF dataset in define mode. It returns (as an argument) a variable ID, given the netCDF ID, the variable name, the variable type, the number of dimensions, and a list of the dimension IDs.
int nc_def_var (int ncid, const char *name, nc_type xtype,
int ndims, const int dimids[], int *varidp);
ncidnamextypendimsdimidsvaridpnc_def_var returns the value NC_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using nc_def_var to create a variable named rh of type double with three dimensions, time, lat, and lon in a new netCDF dataset named foo.nc:
#include <netcdf.h>
...
int status; /* error status */
int ncid; /* netCDF ID */
int lat_dim, lon_dim, time_dim; /* dimension IDs */
int rh_id; /* variable ID */
int rh_dimids[3]; /* variable shape */
...
status = nc_create("foo.nc", NC_NOCLOBBER, &ncid);
if (status != NC_NOERR) handle_error(status);
...
/* define dimensions */
status = nc_def_dim(ncid, "lat", 5L, &lat_dim);
if (status != NC_NOERR) handle_error(status);
status = nc_def_dim(ncid, "lon", 10L, &lon_dim);
if (status != NC_NOERR) handle_error(status);
status = nc_def_dim(ncid, "time", NC_UNLIMITED, &time_dim);
if (status != NC_NOERR) handle_error(status);
...
/* define variable */
rh_dimids[0] = time_dim;
rh_dimids[1] = lat_dim;
rh_dimids[2] = lon_dim;
status = nc_def_var (ncid, "rh", NC_DOUBLE, 3, rh_dimids, &rh_id);
if (status != NC_NOERR) handle_error(status);
The function nc_inq_varid returns the ID of a netCDF variable, given its name.
int nc_inq_varid (int ncid, const char *name, int *varidp);
ncidnamevaridpnc_inq_varid returns the value NC_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using nc_inq_varid to find out the ID of a variable named rh in an existing netCDF dataset named foo.nc:
#include <netcdf.h>
...
int status, ncid, rh_id;
...
status = nc_open("foo.nc", NC_NOWRITE, &ncid);
if (status != NC_NOERR) handle_error(status);
...
status = nc_inq_varid (ncid, "rh", &rh_id);
if (status != NC_NOERR) handle_error(status);
family A family of functions that returns information about a netCDF variable, given its ID. Information about a variable includes its name, type, number of dimensions, a list of dimension IDs describing the shape of the variable, and the number of variable attributes that have been assigned to the variable.
The function nc_inq_var returns all the information about a netCDF variable, given its ID. The other functions each return just one item of information about a variable.
These other functions include nc_inq_varname, nc_inq_vartype, nc_inq_varndims, nc_inq_vardimid, and nc_inq_varnatts.
int nc_inq_var (int ncid, int varid, char *name, nc_type *xtypep,
int *ndimsp, int dimids[], int *nattsp);
int nc_inq_varname (int ncid, int varid, char *name);
int nc_inq_vartype (int ncid, int varid, nc_type *xtypep);
int nc_inq_varndims (int ncid, int varid, int *ndimsp);
int nc_inq_vardimid (int ncid, int varid, int dimids[]);
int nc_inq_varnatts (int ncid, int varid, int *nattsp);
ncidvaridnamextypepndimspdimidsnattspThese functions return the value NC_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
The variable ID is invalid for the specified netCDF dataset. The specified netCDF ID does not refer to an open netCDF dataset.
Here is an example using nc_inq_var to find out about a variable named rh in an existing netCDF dataset named foo.nc:
#include <netcdf.h>
...
int status /* error status */
int ncid; /* netCDF ID */
int rh_id; /* variable ID */
nc_type rh_type; /* variable type */
int rh_ndims; /* number of dims */
int rh_dims[NC_MAX_VAR_DIMS]; /* variable shape */
int rh_natts /* number of attributes */
...
status = nc_open ("foo.nc", NC_NOWRITE, &ncid);
if (status != NC_NOERR) handle_error(status);
...
status = nc_inq_varid (ncid, "rh", &rh_id);
if (status != NC_NOERR) handle_error(status);
/* we don't need name, since we already know it */
status = nc_inq_var (ncid, rh_id, 0, &rh_type, &rh_ndims, rh_dims,
&rh_natts);
if (status != NC_NOERR) handle_error(status);
The functions nc_put_var1_ type put a single data value of the specified type into a variable of an open netCDF dataset that is in data mode. Inputs are the netCDF ID, the variable ID, an index that specifies which value to add or alter, and the data value. The value is converted to the external data type of the variable, if necessary.
int nc_put_var1_text (int ncid, int varid, const size_t index[],
const char *tp);
int nc_put_var1_uchar (int ncid, int varid, const size_t index[],
const unsigned char *up);
int nc_put_var1_schar (int ncid, int varid, const size_t index[],
const signed char *cp);
int nc_put_var1_short (int ncid, int varid, const size_t index[],
const short *sp);
int nc_put_var1_int (int ncid, int varid, const size_t index[],
const int *ip);
int nc_put_var1_long (int ncid, int varid, const size_t index[],
const long *lp);
int nc_put_var1_float (int ncid, int varid, const size_t index[],
const float *fp);
int nc_put_var1_double(int ncid, int varid, const size_t index[],
const double *dp);
ncidvaridindex[]tpupcpspiplpfpdpnc_put_var1_ type returns the value NC_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using nc_put_var1_double to set the (1,2,3) element of the variable named rh to 0.5 in an existing netCDF dataset named foo.nc. For simplicity in this example, we assume that we know that rh is dimensioned with time, lat, and lon, so we want to set the value of rh that corresponds to the second time value, the third lat value, and the fourth lon value:
#include <netcdf.h>
...
int status; /* error status */
int ncid; /* netCDF ID */
int rh_id; /* variable ID */
static size_t rh_index[] = {1, 2, 3}; /* where to put value */
static double rh_val = 0.5; /* value to put */
...
status = nc_open("foo.nc", NC_WRITE, &ncid);
if (status != NC_NOERR) handle_error(status);
...
status = nc_inq_varid (ncid, "rh", &rh_id);
if (status != NC_NOERR) handle_error(status);
...
status = nc_put_var1_double(ncid, rh_id, rh_index, &rh_val);
if (status != NC_NOERR) handle_error(status);
The nc_put_var_ type family of functions write all the values of a variable into a netCDF variable of an open netCDF dataset. This is the simplest interface to use for writing a value in a scalar variable or whenever all the values of a multidimensional variable can all be written at once. The values to be written are associated with the netCDF variable by assuming that the last dimension of the netCDF variable varies fastest in the C interface. The values are converted to the external data type of the variable, if necessary.
Take care when using the simplest forms of this interface with record variables when you don't specify how many records are to be written. If you try to write all the values of a record variable into a netCDF file that has no record data yet (hence has 0 records), nothing will be written. Similarly, if you try to write all of a record variable but there are more records in the file than you assume, more data may be written to the file than you supply, which may result in a segmentation violation.
int nc_put_var_text (int ncid, int varid, const char *tp);
int nc_put_var_uchar (int ncid, int varid, const unsigned char *up);
int nc_put_var_schar (int ncid, int varid, const signed char *cp);
int nc_put_var_short (int ncid, int varid, const short *sp);
int nc_put_var_int (int ncid, int varid, const int *ip);
int nc_put_var_long (int ncid, int varid, const long *lp);
int nc_put_var_float (int ncid, int varid, const float *fp);
int nc_put_var_double(int ncid, int varid, const double *dp);
ncidvaridtpupcpspiplpfpdpMembers of the nc_put_var_ type family return the value NC_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using nc_put_var_double to add or change all the values of the variable named rh to 0.5 in an existing netCDF dataset named foo.nc. For simplicity in this example, we assume that we know that rh is dimensioned with time, lat, and lon, and that there are three time values, five lat values, and ten lon values.
#include <netcdf.h>
...
#define TIMES 3
#define LATS 5
#define LONS 10
int status; /* error status */
int ncid; /* netCDF ID */
int rh_id; /* variable ID */
double rh_vals[TIMES*LATS*LONS]; /* array to hold values */
int i;
...
status = nc_open("foo.nc", NC_WRITE, &ncid);
if (status != NC_NOERR) handle_error(status);
...
status = nc_inq_varid (ncid, "rh", &rh_id);
if (status != NC_NOERR) handle_error(status);
...
for (i = 0; i < TIMES*LATS*LONS