This document describes the FORTRAN-77 interface to the netCDF library. This document applies to netCDF version 4.0. This document was last updated on 29 January 2008.
For a complete description of the netCDF format and utilities see The NetCDF Users Guide (The NetCDF Users Guide).
--- The Detailed Node Listing ---
Use of the NetCDF Library
Datasets
Dimensions
Variables
Attributes
Summary of FORTRAN 77 Interface
Overview of C interface changes from NetCDF 2 to NetCDF 3
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. See ncgen (The NetCDF Users Guide).) 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:
NF_CREATE ! create netCDF dataset: enter define mode ... NF_DEF_DIM ! define dimensions: from name and length ... NF_DEF_VAR ! define variables: from name, type, dims ... NF_PUT_ATT ! assign attribute values ... NF_ENDDEF ! end definitions: leave define mode ... NF_PUT_VAR ! provide values for variable ... NF_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 NF_DEF_DIM is needed for each dimension created. Similarly, one call to NF_DEF_VAR is needed for each variable creation, and one call to a member of the NF_PUT_ATT family is needed for each attribute defined and assigned a value. To leave define mode and enter data mode, call NF_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 NF_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 NF_PUT_VAR family. Arrays or array cross-sections of a variable may be written using members of the NF_PUT_VARA family. Subsampled array sections may be written using members of the NF_PUT_VARS family. Mapped array sections may be written using members of the NF_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 NF_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 NF_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 NF_SYNC or NF_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:
NF_OPEN ! open existing netCDF dataset ... NF_INQ_DIMID ! get dimension IDs ... NF_INQ_VARID ! get variable IDs ... NF_GET_ATT ! get attribute values ... NF_GET_VAR ! get values of variables ... NF_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 NF_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 NF_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 NF_GET_ATT family (typically NF_GET_ATT_TEXT or NF_GET_ATT_DOUBLE) for each desired attribute. Variable data values can be directly accessed from the netCDF dataset with calls to members of the NF_GET_VAR1 family for single values, the NF_GET_VAR family for entire variables, or various other members of the NF_GET_VARA, NF_GET_VARS, or NF_GET_VARM families for array, subsampled or mapped access.
Finally, the netCDF dataset is closed with NF_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:
NF_OPEN ! open existing netCDF dataset ... NF_INQ ! find out what is in it ... NF_INQ_DIM ! get dimension names, lengths ... NF_INQ_VAR ! get variable names, types, shapes ... NF_INQ_ATTNAME ! get attribute names ... NF_INQ_ATT ! get attribute values ... NF_GET_ATT ! get attribute values ... NF_GET_VAR ! get values of variables ... NF_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 NF_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 1. 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 1, 2, 3, ... up to the number of dimensions. For each dimension ID, a call to the inquire function NF_INQ_DIM returns the dimension name and length.
Variable IDs are also assigned from consecutive integers 1, 2, 3, ... up to the number of variables. These can be used in NF_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 NF_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 NF_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 NF_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 NF_GET_VAR1 family for single values, or members of the NF_GET_VAR, NF_GET_VARA, NF_GET_VARS, or NF_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:
NF_OPEN ! open existing netCDF dataset ... NF_REDEF ! put it into define mode ... NF_DEF_DIM ! define additional dimensions (if any) ... NF_DEF_VAR ! define additional variables (if any) ... NF_PUT_ATT ! define other attributes (if any) ... NF_ENDDEF ! check definitions, leave define mode ... NF_PUT_VAR ! provide new variable values ... NF_CLOSE ! close netCDF dataset
A netCDF dataset is first opened by the NF_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 NF_REDEF.In define mode, call NF_DEF_DIM to define new dimensions, NF_DEF_VAR to define new variables, and a member of the NF_PUT_ATT family 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 NF_ENDDEF. If you do not wish to reenter data mode, just call NF_CLOSE, which will have the effect of first calling NF_ENDDEF.
Until the NF_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 NF_ABORT. You may also use the NF_ABORT call to restore the netCDF dataset to a consistent state if the call to NF_ENDDEF fails. If you have called NF_CLOSE from definition mode and the implied call to NF_ENDDEF fails, NF_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 NF_SYNC function and the NF_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 NF_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 NF_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 FORTRAN file that references netCDF functions or constants must contain an appropriate INCLUDE statement before the first such reference:
INCLUDE 'netcdf.inc'
Unless the netcdf.inc file is installed in a standard directory where the FORTRAN compiler always looks, you must use the -I option when invoking the compiler, to specify a directory where netcdf.inc is installed, for example:
f77 -c -I/usr/local/netcdf/include myprogram.f
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.
In all netCDF versions before 3.6.2, the Fortran 77, Fortran 90, and the C libraries were all built into the same library file. That is, the libnetcdf.a file contains the C functions, the F77 functions, and the F90 functions. (The C++ library is separate.)
Starting with version 3.6.2, another method of building the netCDF fortran libraries becomes available. With the –enable-separate-fortran option to configure, the user can specify that the C library should not contain the fortran functions. In these cases an additional library, libnetcdff.a (not the extra “f”) will be built. This library contains the fortran functions.
For more information about configure options, See Specifying the Environment for Building (The NetCDF Installation and Porting Guide).
Building separate fortran libraries is required for shared library builds, but is not done, by default, for static library builds.
When linking fortran programs without a separate fortran library, programs must link to the netCDF library like this:
f77 -o myprogram myprogram.o -L/usr/local/netcdf/lib -lnetcdf
Alternatively, you could specify an absolute path name for the library:
f77 -o myprogram myprogram.o -l/usr/local/netcdf/lib/libnetcdf.
When linking fortran programs with separate fortran, the user must link to both the fortran and the C libraries.
f77 -o myprogram myprogram.o -L/usr/local/netcdf/lib -lnetcdff -lnetcdf
Unless the netCDF library (or libraries) 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.
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, see Section 5.2 "Get error message corresponding to error status: nc_strerror".
The function NF_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.
CHARACTER*80 FUNCTION NF_STRERROR(INTEGER NCERR)
NCERR
If 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), NF_STRERROR returns a string indicating that there is no such error status.
Here is an example of a simple error handling function that uses NF_STRERROR to print the error message corresponding to the netCDF error status returned from any netCDF function call and then exit:
INCLUDE 'netcdf.inc' ... SUBROUTINE HANDLE_ERR(STATUS) INTEGER STATUS IF (STATUS .NE. NF_NOERR) THEN PRINT *, NF_STRERROR(STATUS) STOP 'Stopped' ENDIF END
The function NF_INQ_LIBVERS returns a string identifying the version of the netCDF library, and when it was built.
CHARACTER*80 FUNCTION NF_INQ_LIBVERS()
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.inc' ... PRINT *, NF_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 whether to overwrite any existing dataset with the same name and whether access to the dataset is shared.
INTEGER FUNCTION NF_CREATE (CHARACTER*(*) PATH, INTEGER CMODE, INTEGER ncid)
PATH
CMODE
Setting NF_NOCLOBBER means you do not want to clobber (overwrite) an existing dataset; an error (NF_EEXIST) is returned if the specified dataset already exists.
The NF_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 NF_SHARE flag.
Setting NF_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 (The NetCDF Users Guide).
A zero value (defined for convenience as NF_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 (The NetCDF Users Guide).
ncid
NF_CREATE returns the value NF_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.inc' ... INTEGER NCID, STATUS ... STATUS = NF_CREATE('foo.nc', NF_NOCLOBBER, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS)
This function is a variant of NF_CREATE, NF__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 NF_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.
INTEGER FUNCTION NF_CREATE (CHARACTER*(*) PATH, INTEGER CMODE, INTEGER INITIALSZ, INTEGER CHUNKSIZEHINT, INTEGER ncid)
PATH
CMODE
Setting NF_NOCLOBBER means you do not want to clobber (overwrite) an existing dataset; an error (NF_EEXIST) is returned if the specified dataset already exists.
The NF_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 NF_SHARE flag.
Setting NF_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 (The NetCDF Users Guide).
A zero value (defined for convenience as NF_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 (The NetCDF Users Guide).
initialsz
chunksizehintp
Because 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 NF_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.
ncid
NF__CREATE returns the value NF_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.inc' ... INTEGER NCID, STATUS, INITIALSZ, CHUNKSIZEHINT ... INITIALSZ = 2048 CHUNKSIZEHINT = 1024 STATUS = NF_CREATE('foo.nc', NF_NOCLOBBER, INITIALSZ, CHUNKSIZEHINT, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS)
The function NF_OPEN opens an existing netCDF dataset for access.
INTEGER FUNCTION NF_OPEN(CHARACTER*(*) PATH, INTEGER OMODE, INTEGER ncid)
PATH
OMODE
Otherwise, the creation mode is NF_WRITE, NF_SHARE, or
OR(NF_WRITE, NF_SHARE). Setting the NF_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 NF_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 NF_SHARE flag.
ncid
NF_OPEN returns the value NF_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using NF_OPEN to open an existing netCDF dataset named foo.nc for read-only, non-shared access:
INCLUDE 'netcdf.inc' ... INTEGER NCID, STATUS ... STATUS = NF_OPEN('foo.nc', 0, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS)
The function NF)_OPEN opens an existing netCDF dataset for access, with a performance tuning parameter.
INTEGER FUNCTION NF__OPEN(CHARACTER*(*) PATH, INTEGER OMODE, INTEGER CHUNKSIZEHINT, INTEGER ncid)
PATH
OMODE
Otherwise, the creation mode is NF_WRITE, NF_SHARE, or
OR(NF_WRITE,NF_SHARE). Setting the NF_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 NF_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 NF_SHARE flag.
CHUNKSIZEHINT
Because 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 NF_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.
ncid
NF__OPEN returns the value NF_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using NF__OPEN to open an existing netCDF dataset named foo.nc for read-only, non-shared access:
INCLUDE 'netcdf.inc' ... INTEGER NCID, STATUS, CHUNKSIZEHINT ... CHUNKSIZEHINT = 1024 STATUS = NF_OPEN('foo.nc', 0, CHUNKSIZEHINT, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS)
The function NF_REDEF puts an open netCDF dataset into define mode, so dimensions, variables, and attributes can be added or renamed and attributes can be deleted.
INTEGER FUNCTION NF_REDEF(INTEGER NCID)
NCID
NF_REDEF returns the value NF_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using NF_REDEF to open an existing netCDF dataset named foo.nc and put it into define mode:
INCLUDE 'netcdf.inc' ... INTEGER NCID, STATUS ... STATUS = NF_OPEN('foo.nc', NF_WRITE, NCID) ! open dataset IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_REDEF(NCID) ! put in define mode IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS)
The function NF_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 NF_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. See File Structure and Performance (NetCDF Users' Guide).
INTEGER FUNCTION NF_ENDDEF(INTEGER NCID)
NCID
NF_ENDDEF returns the value NF_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using NF_ENDDEF to finish the definitions of a new netCDF dataset named foo.nc and put it into data mode:
INCLUDE 'netcdf.inc' ... INTEGER NCID, STATUS ... STATUS = NF_CREATE('foo.nc', NF_NOCLOBBER, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... ! create dimensions, variables, attributes STATUS = NF_ENDDEF(NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS)
The function NF__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 NF_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. See File Structure and Performance (NetCDF Users' Guide).
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.
INTEGER FUNCTION NF_ENDDEF(INTEGER NCID, INTEGER H_MINFREE, INTEGER V_ALIGN, INTEGER V_MINFREE, INTEGER R_ALIGN)
NCID
H_MINFREE
V_ALIGN
V_MINFREE
R_ALIGN
NF__ENDDEF returns the value NF_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using NF__ENDDEF to finish the definitions of a new netCDF dataset named foo.nc and put it into data mode:
INCLUDE 'netcdf.inc' ... INTEGER NCID, STATUS, H_MINFREE, V_ALIGN, V_MINFREE, R_ALIGN ... STATUS = NF_CREATE('foo.nc', NF_NOCLOBBER, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... ! create dimensions, variables, attributes H_MINFREE = 512 V_ALIGN = 512 V_MINFREE = 512 R_ALIGN = 512 STATUS = NF_ENDDEF(NCID, H_MINFREE, V_ALIGN, V_MINFREE, R_ALIGN) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS)
The function NF_CLOSE closes an open netCDF dataset. If the dataset is in define mode, NF_ENDDEF will be called before closing. (In this case, if NF_ENDDEF returns an error, NF_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.
INTEGER FUNCTION NF_CLOSE(INTEGER NCID)
NCID
NF_CLOSE returns the value NF_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using NF_CLOSE to finish the definitions of a new netCDF dataset named foo.nc and release its netCDF ID:
INCLUDE 'netcdf.inc' ... INTEGER NCID, STATUS ... STATUS = NF_CREATE('foo.nc', NF_NOCLOBBER, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... ! create dimensions, variables, attributes STATUS = NF_CLOSE(NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS)
Members of the NF_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, NF_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 FORTRAN, these functions include NF_INQ, NF_INQ_NDIMS, NF_INQ_NVARS, NF_INQ_NATTS, and NF_INQ_UNLIMDIM. An additional function, NF_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.
INTEGER FUNCTION NF_INQ (INTEGER NCID, INTEGER ndims, INTEGER nvars,INTEGER ngatts, INTEGER unlimdimid) INTEGER FUNCTION NF_INQ_NDIMS (INTEGER NCID, INTEGER ndims) INTEGER FUNCTION NF_INQ_NVARS (INTEGER NCID, INTEGER nvars) INTEGER FUNCTION NF_INQ_NATTS (INTEGER NCID, INTEGER ngatts) INTEGER FUNCTION NF_INQ_UNLIMDIM (INTEGER NCID, INTEGER unlimdimid) INTEGER FUNCTION NF_INQ_FORMAT (INTEGER NCID, INTEGER format)
NCID
ndims
nvars
ngatts
unlimdimid
format
All members of the NF_INQ family return the value NF_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using NF_INQ to find out about a netCDF dataset named foo.nc:
INCLUDE 'netcdf.inc' ... INTEGER STATUS, NCID, NDIMS, NVARS, NGATTS, UNLIMDIMID ... STATUS = NF_OPEN('foo.nc', NF_NOWRITE, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_INQ(NCID, NDIMS, NVARS, NGATTS, UNLIMDIMID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS)
The function NF_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 NF_SYNC after writing and the readers call NF_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 NF_SHARE flag, and then it will not be necessary to call NF_SYNC at all. However, the NF_SYNC function still provides finer granularity than the NF_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 NF_SHARE flag. Use of the NF_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 NF_SYNC before any subsequent access.
When calling NF_SYNC, the netCDF dataset must be in data mode. A netCDF dataset in define mode is synchronized to disk only when NF_ENDDEF is called. A process that is reading a netCDF dataset that another process is writing may call NF_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.
INTEGER FUNCTION NF_SYNC(INTEGER NCID)
NCID
NF_SYNC returns the value NF_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using NF_SYNC to synchronize the disk writes of a netCDF dataset named foo.nc:
INCLUDE 'netcdf.inc' ... INTEGER STATUS, NCID ... STATUS = NF_OPEN('foo.nc', NF_WRITE, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... ! write data or change attributes ... STATUS = NF_SYNC(NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS)
You no longer need to call this function, since it is called automatically by NF_CLOSE in case the dataset is in define mode and something goes wrong with committing the changes. The function NF_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 NF_REDEF, the netCDF dataset is restored to its state before definition mode was entered and the dataset is closed.
INTEGER FUNCTION NF_ABORT(INTEGER NCID)
NCID
NF_ABORT returns the value NF_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using NF_ABORT to back out of redefinitions of a dataset named foo.nc:
INCLUDE 'netcdf.inc' ... INTEGER STATUS, NCID, LATID ... STATUS = NF_OPEN('foo.nc', NF_WRITE, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_REDEF(NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_DEF_DIM(NCID, 'LAT', 18, LATID) IF (STATUS .NE. NF_NOERR) THEN ! dimension definition failed CALL HANDLE_ERR(STATUS) STATUS = NF_ABORT(NCID) ! abort redefinitions IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ENDIF ...
This function is intended for advanced usage, to optimize writes under some circumstances described below. The function NF_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 NF_FILL or NF_NOFILL. The default behavior corresponding to NF_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. See Fill Values, for more information on the use of fill values. See Attribute Conventions, for information about how to define your own fill values.
The behavior corresponding to NF_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 NF_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 NF_SET_FILL again to explicitly set the fill mode to NF_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.
INTEGER FUNCTION NF_SET_FILL(INTEGER NCID, INTEGER FILLMODE, INTEGER old_mode)
NCID
FILLMODE
old_mode
NF_SET_FILL returns the value NF_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using NF_SET_FILL to set nofill mode for subsequent writes of a netCDF dataset named foo.nc:
INCLUDE 'netcdf.inc' ... INTEGER NCID, STATUS, OMODE ... STATUS = NF_OPEN('foo.nc', NF_WRITE, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... ! write data with default prefilling behavior ... STATUS = NF_SET_FILL(NCID, NF_NOFILL, OMODE) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(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 NF_CREATE, using the proper cmode flag. (see NF_CREATE).
The function NF_SET_DEFAULT_FORMAT allows the user to change the format of the netCDF file to be created by future calls to NF_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 NF_CREATE. See Large File Support (The NetCDF Users Guide).
Once the default format is set, all future created files will be in the desired format.
Two constants are provided in the netcdf.inc file to be used with this function, nf_format_64bit and nf_format_classic.
Using NF_CREATE with a cmode including nf_64bit_offset overrides the default format, and creates a 64-bit offset file.
INTEGER FUNCTION NF_SET_DEFAULT_FORMAT(INTEGER FORMAT, INTEGER OLD_FORMT)
FORMAT
OLD_FORMAT
NF_SET_FILL returns the value NF_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using NF_SET_FILL to set nofill mode for subsequent writes of a netCDF dataset named foo.nc:
INCLUDE 'netcdf.inc' ... INTEGER STATUS, OLD_FORMAT ... STATUS = NF_SET_DEFAULT_FORMAT(nf_format_64bit, OLD_FORMAT) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ...
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 (100) to the number of dimensions that can be defined in a single netCDF dataset. The limit is the value of the predefined macro NF_MAX_DIMS. The purpose of the limit is to make writing generic applications simpler. They need only provide an array of NF_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.
A netCDF dimension in an open netCDF dataset is referred to by a small integer called a dimension ID. In the FORTRAN interface, dimension IDs are 1, 2, 3, ..., in the order in which the dimensions were defined.
Operations supported on dimensions are:
The function NF_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.
INTEGER FUNCTION NF_DEF_DIM (INTEGER NCID, CHARACTER*(*) NAME, INTEGER LEN, INTEGER dimid)
NCID
NAME
LEN
dimid
NF_DEF_DIM returns the value NF_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using NF_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.inc' ... INTEGER STATUS, NCID, LATID, RECID ... STATUS = NF_CREATE('foo.nc', NF_NOCLOBBER, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_DEF_DIM(NCID, 'lat', 18, LATID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) STATUS = NF_DEF_DIM(NCID, 'rec', NF_UNLIMITED, RECID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS)
The function NF_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 1 and ndims.
INTEGER FUNCTION NF_INQ_DIMID (INTEGER NCID, CHARACTER*(*) NAME, INTEGER dimid)
NCID
NAME
dimid
NF_INQ_DIMID returns the value NF_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using NF_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.inc' ... INTEGER STATUS, NCID, LATID ... STATUS = NF_OPEN('foo.nc', NF_NOWRITE, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_INQ_DIMID(NCID, 'lat', LATID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(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 NF_INQ_DIM, NF_INQ_DIMNAME, and NF_INQ_DIMLEN. The function NF_INQ_DIM returns all the information about a dimension; the other functions each return just one item of information.
INTEGER FUNCTION NF_INQ_DIM (INTEGER NCID, INTEGER DIMID, CHARACTER*(*) name, INTEGER len) INTEGER FUNCTION NF_INQ_DIMNAME (INTEGER NCID, INTEGER DIMID, CHARACTER*(*) name) INTEGER FUNCTION NF_INQ_DIMLEN (INTEGER NCID, INTEGER DIMID, INTEGER len)
NCID
DIMID
NAME
len
These functions return the value NF_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using NF_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.inc' ... INTEGER STATUS, NCID, LATID, LATLEN, RECID, NRECS CHARACTER*(NF_MAX_NAME) LATNAM, RECNAM ... STATUS = NF_OPEN('foo.nc', NF_NOWRITE, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ! get ID of unlimited dimension STATUS = NF_INQ_UNLIMDIM(NCID, RECID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_INQ_DIMID(NCID, 'lat', LATID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ! get lat length STATUS = NF_INQ_DIMLEN(NCID, LATID, LATLEN) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ! get unlimited dimension name and current length STATUS = NF_INQ_DIM(NCID, RECID, RECNAME, NRECS) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS)
The function NF_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.
INTEGER FUNCTION NF_RENAME_DIM (INTEGER NCID, INTEGER DIMID, CHARACTER*(*) NAME)
NCID
DIMID
NAME
NF_RENAME_DIM returns the value NF_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using NF_RENAME_DIM to rename the dimension lat to latitude in an existing netCDF dataset named foo.nc:
INCLUDE 'netcdf.inc' ... INTEGER STATUS, NCID, LATID ... STATUS = NF_OPEN('foo.nc', NF_WRITE, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... ! put in define mode to rename dimension STATUS = NF_REDEF(NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) STATUS = NF_INQ_DIMID(NCID, 'lat', LATID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) STATUS = NF_RENAME_DIM(NCID, LATID, 'latitude') IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ! leave define mode STATUS = NF_ENDDEF(NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(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 1, 2, 3,..., 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 FORTRAN interface:
| Type | FORTRAN API Mnemonic | Bits
|
| byte | NF_BYTE | 8
|
| char | NF_CHAR | 8
|
| short | NF_SHORT | 16
|
| int | NF_INT | 32
|
| float | NF_FLOAT | 32
|
| double | NF_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 FORTRAN parameter for use in netCDF functions (the parameters are defined in the netCDF FORTRAN include-file netcdf.inc). 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.
NF_DEF_VAR
The function NF_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.
INTEGER FUNCTION NF_DEF_VAR(INTEGER NCID, CHARACTER*(*) NAME, INTEGER XTYPE, INTEGER NVDIMS, INTEGER VDIMS(*), INTEGER varid)
NCID
NAME
XTYPE
NVDIMS
VDIMS
varid
NF_DEF_VAR returns the value NF_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using NF_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.inc' ...  STATUS = NF_CREATE ('foo.nc', NF_NOCLOBBER, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... ! define dimensions STATUS = NF_DEF_DIM(NCID, 'lat', 5, LATDIM) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) STATUS = NF_DEF_DIM(NCID, 'lon', 10, LONDIM) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) STATUS = NF_DEF_DIM(NCID, 'time', NF_UNLIMITED, TIMDIM) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... ! define variable RHDIMS(1) = LONDIM RHDIMS(2) = LATDIM RHDIMS(3) = TIMDIM STATUS = NF_DEF_VAR (NCID, 'rh', NF_DOUBLE, 3, RHDIMS, RHID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS)
The function NF_INQ_VARID returns the ID of a netCDF variable, given its name.
INTEGER FUNCTION NF_INQ_VARID(INTEGER NCID, CHARACTER*(*) NAME, INTEGER varid)
NCID
NAME
varid
NF_INQ_VARID returns the value NF_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using NF_INQ_VARID to find out the ID of a variable named rh in an existing netCDF dataset named foo.nc:
INCLUDE 'netcdf.inc' ... INTEGER STATUS, NCID, RHID ... STATUS = NF_OPEN ('foo.nc', NF_NOWRITE, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_INQ_VARID (NCID, 'rh', RHID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS)
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 NF_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 NF_INQ_VARNAME, NF_INQ_VARTYPE, NF_INQ_VARNDIMS, NF_INQ_VARDIMID, and NF_INQ_VARNATTS.
INTEGER FUNCTION NF_INQ_VAR (INTEGER NCID, INTEGER VARID, CHARACTER*(*) name, INTEGER xtype, INTEGER ndims, INTEGER dimids(*), INTEGER natts) INTEGER FUNCTION NF_INQ_VARNAME (INTEGER NCID, INTEGER VARID, CHARACTER*(*) name) INTEGER FUNCTION NF_INQ_VARTYPE (INTEGER NCID, INTEGER VARID, INTEGER xtype) INTEGER FUNCTION NF_INQ_VARNDIMS (INTEGER NCID, INTEGER VARID, INTEGER ndims) INTEGER FUNCTION NF_INQ_VARDIMID (INTEGER NCID, INTEGER VARID, INTEGER dimids(*)) INTEGER FUNCTION NF_INQ_VARNATTS (INTEGER NCID, INTEGER VARID, INTEGER natts)
NCID
VARID
NAME
xtype
ndims
dimids
natts
These functions return the value NF_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using NF_INQ_VAR to find out about a variable named rh in an existing netCDF dataset named foo.nc:
INCLUDE 'netcdf.inc' ... INTEGER STATUS, NCID INTEGER RHID ! variable ID CHARACTER*31 RHNAME ! variable name INTEGER RHTYPE ! variable type INTEGER RHN ! number of dimensions INTEGER RHDIMS(NF_MAX_VAR_DIMS) ! variable shape INTEGER RHNATT ! number of attributes ... STATUS = NF_OPEN ('foo.nc', NF_NOWRITE, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_INQ_VARID (NCID, 'rh', RHID) ! get ID IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) STATUS = NF_INQ_VAR (NCID, RHID, RHNAME, RHTYPE, RHN, RHDIMS, RHNATT) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS)
The functions NF_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.
Take care when using the simplest forms of this interface with record variables when you don't specify how many records are to be read. If you try to read all the values of a record variable into an array but there are more records in the file than you assume, more data will be read than you expect, which may cause a segmentation violation.
INTEGER FUNCTION NF_PUT_VAR1_TEXT(INTEGER NCID, INTEGER VARID, INTEGER INDEX(*), CHARACTER CHVAL) INTEGER FUNCTION NF_PUT_VAR1_INT1(INTEGER NCID, INTEGER VARID, INTEGER INDEX(*), INTEGER*1 I1VAL) INTEGER FUNCTION NF_PUT_VAR1_INT2(INTEGER NCID, INTEGER VARID, INTEGER INDEX(*), INTEGER*2 I2VAL) INTEGER FUNCTION NF_PUT_VAR1_INT (INTEGER NCID, INTEGER VARID, INTEGER INDEX(*), INTEGER IVAL) INTEGER FUNCTION NF_PUT_VAR1_REAL(INTEGER NCID, INTEGER VARID, INTEGER INDEX(*), REAL RVAL) INTEGER FUNCTION NF_PUT_VAR1_DOUBLE(INTEGER NCID, INTEGER VARID, INTEGER INDEX(*), DOUBLE DVAL)
NCID
VARID
INDEX
CHVAL
I1VAL
I2VAL
IVAL
RVAL
DVAL
NF_PUT_VAR1_ type returns the value NF_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using NF_PUT_VAR1_DOUBLE to set the (4,3,2) 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 lon, lat, and time, so we want to set the value of rh that corresponds to the fourth lon value, the third lat value, and the second time value:
INCLUDE 'netcdf.inc' ... INTEGER STATUS ! error status INTEGER NCID INTEGER RHID ! variable ID INTEGER RHINDX(3) ! where to put value DATA RHINDX /4, 3, 2/ ... STATUS = NF_OPEN ('foo.nc', NF_WRITE, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_INQ_VARID (NCID, 'rh', RHID) ! get ID IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) STATUS = NF_PUT_VAR1_DOUBLE (NCID, RHID, RHINDX, 0.5) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS)
The NF_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.
INTEGER FUNCTION NF_PUT_VAR_TEXT (INTEGER NCID, INTEGER VARID, CHARACTER*(*) TEXT) INTEGER FUNCTION NF_PUT_VAR_INT1 (INTEGER NCID, INTEGER VARID, INTEGER*1 I1VALS(*)) INTEGER FUNCTION NF_PUT_VAR_INT2 (INTEGER NCID, INTEGER VARID, INTEGER*2 I2VALS(*)) INTEGER FUNCTION NF_PUT_VAR_INT (INTEGER NCID, INTEGER VARID, INTEGER IVALS(*)) INTEGER FUNCTION NF_PUT_VAR_REAL (INTEGER NCID, INTEGER VARID, REAL RVALS(*)) INTEGER FUNCTION NF_PUT_VAR_DOUBLE(INTEGER NCID, INTEGER VARID, DOUBLE DVALS(*))
NCID
VARID
TEXT
I1VALS
I2VALS
IVALS
RVALS
DVALS
Members of the NF_PUT_VAR_ type family return the value NF_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using NF_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 lon, lat, and time, and that there are ten lon values, five lat values, and three time values.
INCLUDE 'netcdf.inc' ... PARAMETER (TIMES=3, LATS=5, LONS=10) ! dimension lengths INTEGER STATUS, NCID, TIMES INTEGER RHID ! variable ID DOUBLE RHVALS(LONS, LATS, TIMES) ... STATUS = NF_OPEN ('foo.nc', NF_WRITE, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_INQ_VARID (NCID, 'rh', RHID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) DO 10 ILON = 1, LONS DO 10 ILAT = 1, LATS DO 10 ITIME = 1, TIMES RHVALS(ILON, ILAT, ITIME) = 0.5 10 CONTINUE STATUS = NF_PUT_var_DOUBLE (NCID, RHID, RHVALS) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS)
The function NF_PUT_VARA_ type writes values into a netCDF variable of an open netCDF dataset. The part of the netCDF variable to write is specified by giving a corner and a vector of edge lengths that refer to an array section of the netCDF variable. The values to be written are associated with the netCDF variable by assuming that the first dimension of the netCDF variable varies fastest in the FORTRAN interface. The netCDF dataset must be in data mode.
INTEGER FUNCTION NF_PUT_VARA_TEXT(INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), CHARACTER*(*) TEXT) INTEGER FUNCTION NF_PUT_VARA_INT1(INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), INTEGER*1 I1VALS(*)) INTEGER FUNCTION NF_PUT_VARA_INT2(INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), INTEGER*2 I2VALS(*)) INTEGER FUNCTION NF_PUT_VARA_INT (INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), INTEGER IVALS(*)) INTEGER FUNCTION NF_PUT_VARA_REAL(INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), REAL RVALS(*)) INTEGER FUNCTION NF_PUT_VARA_DOUBLE(INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), DOUBLE DVALS(*))
NCID
VARID
START
COUNT
TEXT
I1VALS
I2VALS
IVALS
RVALS
DVALS
NF_PUT_VARA_ type returns the value NF_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using NF_PUT_VARA_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.inc' ... PARAMETER (NDIMS=3) ! number of dimensions PARAMETER (TIMES=3, LATS=5, LONS=10) ! dimension lengths INTEGER STATUS, NCID, TIMES INTEGER RHID ! variable ID INTEGER START(NDIMS), COUNT(NDIMS) DOUBLE RHVALS(LONS, LATS, TIMES) DATA START /1, 1, 1/ ! start at first value DATA COUNT /LONS, LATS, TIMES/ ... STATUS = NF_OPEN ('foo.nc', NF_WRITE, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_INQ_VARID (NCID, 'rh', RHID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) DO 10 ILON = 1, LONS DO 10 ILAT = 1, LATS DO 10 ITIME = 1, TIMES RHVALS(ILON, ILAT, ITIME) = 0.5 10 CONTINUE STATUS = NF_PUT_VARA_DOUBLE (NCID, RHID, START, COUNT, RHVALS) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS)
Each member of the family of functions NF_PUT_VARS_ type writes a subsampled (strided) array section of values into a netCDF variable of an open netCDF dataset. The subsampled array section is specified by giving a corner, a vector of counts, and a stride vector. The netCDF dataset must be in data mode.
INTEGER FUNCTION NF_PUT_VARS_TEXT (INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), INTEGER STRIDE(*),CHARACTER*(*) TEXT) INTEGER FUNCTION NF_PUT_VARS_INT1 (INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), INTEGER STRIDE(*),INTEGER*1 I1VALS(*)) INTEGER FUNCTION NF_PUT_VARS_INT2 (INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), INTEGER STRIDE(*),INTEGER*2 I2VALS(*)) INTEGER FUNCTION NF_PUT_VARS_INT (INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), INTEGER STRIDE(*), INTEGER IVALS(*)) INTEGER FUNCTION NF_PUT_VARS_REAL (INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), INTEGER STRIDE(*), REAL RVALS(*)) INTEGER FUNCTION NF_PUT_VARS_DOUBLE(INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), INTEGER STRIDE(*), DOUBLE DVALS(*))
NCID
VARID
START
COUNT
STRIDE
TEXT
I1VALS
I2VALS
IVALS
RVALS
DVALS
NF_PUT_VARS_ type returns the value NF_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example of using NF_PUT_VARS_REAL to write – from an internal array – every other point of a netCDF variable named rh which is described by the FORTRAN declaration REAL RH(6,4) (note the size of the dimensions):
INCLUDE 'netcdf.inc' ...  STATUS = NF_OPEN('foo.nc', NF_WRITE, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_INQ_VARID(NCID, 'rh', RHID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_PUT_VARS_REAL(NCID, RHID, START, COUNT, STRIDE, RH) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS)
The NF_PUT_VARM_ type family of functions writes a mapped array section of values into a netCDF variable of an open netCDF dataset. The mapped array section is specified by giving a corner, a vector of counts, a stride vector, and an index mapping vector. The index mapping vector is 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. No assumptions are made about the ordering or length of the dimensions of the data array. The netCDF dataset must be in data mode.
INTEGER FUNCTION NF_PUT_VARM_TEXT (INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), INTEGER STRIDE(*), INTEGER IMAP(*), CHARACTER*(*) TEXT) INTEGER FUNCTION NF_PUT_VARM_INT1 (INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), INTEGER STRIDE(*), INTEGER IMAP(*), INTEGER*1 I1VALS(*)) INTEGER FUNCTION NF_PUT_VARM_INT2 (INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), INTEGER STRIDE(*), INTEGER IMAP(*), INTEGER*2 I2VALS(*)) INTEGER FUNCTION NF_PUT_VARM_INT (INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), INTEGER STRIDE(*), INTEGER IMAP(*), INTEGER IVALS(*)) INTEGER FUNCTION NF_PUT_VARM_REAL (INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), INTEGER STRIDE(*), INTEGER IMAP(*), REAL RVALS(*)) INTEGER FUNCTION NF_PUT_VARM_DOUBLE(INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), INTEGER STRIDE(*), INTEGER IMAP(*), DOUBLE DVALS(*))
NCID
VARID
START
COUNT
STRIDE
IMAP
TEXT
I1VALS
I2VALS
IVALS
RVALS
DVALS
NF_PUT_VARM_ type returns the value NF_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
The following IMAP vector maps in the trivial way a 2x3x4 netCDF variable and an internal array of the same shape:
REAL A(2,3,4) ! same shape as netCDF variable INTEGER IMAP(3) DATA IMAP /1, 2, 6/ ! netCDF dimension inter-element distance ! ---------------- ---------------------- ! most rapidly varying 1 ! intermediate 2 (=IMAP(1)*2) ! most slowly varying 6 (=IMAP(2)*3)
Using the IMAP vector above with NF_PUT_VARM_REAL obtains the same result as simply using NF_PUT_VAR_REAL.
Here is an example of using NF_PUT_VARM_REAL to write – from a transposed, internal array – a netCDF variable named rh which is described by the FORTRAN declaration REAL RH(4,6) (note the size and order of the dimensions):
INCLUDE 'netcdf.inc' ...  NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_INQ_VARID(NCID, 'rh', RHID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_PUT_VARM_REAL(NCID, RHID, START, COUNT, STRIDE, IMAP, RH) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS)
Here is another example of using NF_PUT_VARM_REAL to write – from a transposed, internal array – a subsample of the same netCDF variable, by writing every other point of the netCDF variable:
INCLUDE 'netcdf.inc' ...  STATUS = NF_OPEN('foo.nc', NF_WRITE, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_INQ_VARID(NCID, 'rh', RHID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_PUT_VARM_REAL(NCID, RHID, START, COUNT, STRIDE, IMAP, RH) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS)
The functions NF_GET_VAR1_ type get a single data value from a variable of an open netCDF dataset that is in data mode. Inputs are the netCDF ID, the variable ID, a multidimensional index that specifies which value to get, and the address of a location into which the data value will be read. The value is converted from the external data type of the variable, if necessary.
INTEGER FUNCTION NF_GET_VAR1_TEXT(INTEGER NCID, INTEGER VARID, INTEGER INDEX(*), CHARACTER CHVAL) INTEGER FUNCTION NF_GET_VAR1_INT1(INTEGER NCID, INTEGER VARID, INTEGER INDEX(*), INTEGER*1 I1VAL) INTEGER FUNCTION NF_GET_VAR1_INT2(INTEGER NCID, INTEGER VARID, INTEGER INDEX(*), INTEGER*2 I2VAL) INTEGER FUNCTION NF_GET_VAR1_INT (INTEGER NCID, INTEGER VARID, INTEGER INDEX(*), INTEGER IVAL) INTEGER FUNCTION NF_GET_VAR1_REAL(INTEGER NCID, INTEGER VARID, INTEGER INDEX(*), REAL RVAL) INTEGER FUNCTION NF_GET_VAR1_DOUBLE(INTEGER NCID, INTEGER VARID, INTEGER INDEX(*), DOUBLE DVAL)
NCID
VARID
INDEX
chval
i1val
i2val
ival
rval
dval
NF_GET_VAR1_ type returns the value NF_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using NF_GET_VAR1_DOUBLE to get the (4,3,2) element of the variable named rh in an existing netCDF dataset named foo.nc. For simplicity in this example, we assume that we know that rh is dimensioned with lon, lat, and time, so we want to get the value of rh that corresponds to the fourth lon value, the third lat value, and the second time value:
INCLUDE 'netcdf.inc' ... INTEGER STATUS, NCID INTEGER RHID ! variable ID INTEGER RHINDX(3) ! where to get value DOUBLE PRECISION RHVAL ! put it here DATA RHINDX /4, 3, 2/ ... STATUS = NF_OPEN ('foo.nc', NF_NOWRITE, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_INQ_VARID (NCID, 'rh', RHID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) STATUS = NF_GET_VAR1_DOUBLE (NCID, RHID, RHINDX, RHVAL) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS)
The members of the NF_GET_VAR_ type family of functions read all the values from a netCDF variable of an open netCDF dataset. This is the simplest interface to use for reading the value of a scalar variable or when all the values of a multidimensional variable can be read at once. The values are read into consecutive locations with the first dimension varying fastest. The netCDF dataset must be in data mode.
Take care when using the simplest forms of this interface with record variables when you don't specify how many records are to be read. If you try to read all the values of a record variable into an array but there are more records in the file than you assume, more data will be read than you expect, which may cause a segmentation violation.
INTEGER FUNCTION NF_GET_VAR_TEXT (INTEGER NCID, INTEGER VARID, CHARACTER*(*) text) INTEGER FUNCTION NF_GET_VAR_INT1 (INTEGER NCID, INTEGER VARID, INTEGER*1 i1vals(*)) INTEGER FUNCTION NF_GET_VAR_INT2 (INTEGER NCID, INTEGER VARID, INTEGER*2 i2vals(*)) INTEGER FUNCTION NF_GET_VAR_INT (INTEGER NCID, INTEGER VARID, INTEGER ivals(*)) INTEGER FUNCTION NF_GET_VAR_REAL (INTEGER NCID, INTEGER VARID, REAL rvals(*)) INTEGER FUNCTION NF_GET_VAR_DOUBLE(INTEGER NCID, INTEGER VARID, DOUBLE dvals(*))
NCID
VARID
text
i1vals
i2vals
ivals
rvals
dvals
NF_GET_VAR_ type returns the value NF_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using NF_GET_VAR_DOUBLE to read all the values of the variable named rh from an existing netCDF dataset named foo.nc. For simplicity in this example, we assume that we know that rh is dimensioned with lon, lat, and time, and that there are ten lon values, five lat values, and three time values.
INCLUDE 'netcdf.inc' ... PARAMETER (TIMES=3, LATS=5, LONS=10) ! dimension lengths INTEGER STATUS, NCID INTEGER RHID ! variable ID DOUBLE RHVALS(LONS, LATS, TIMES) ... STATUS = NF_OPEN ('foo.nc', NF_NOWRITE, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_INQ_VARID (NCID, 'rh', RHID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) STATUS = NF_GET_VAR_DOUBLE (NCID, RHID, RHVALS) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS)
The members of the NF_GET_VARA_ type family of functions read an array of values from a netCDF variable of an open netCDF dataset. The array is specified by giving a corner and a vector of edge lengths. The values are read into consecutive locations with the first dimension varying fastest. The netCDF dataset must be in data mode.
INTEGER FUNCTION NF_GET_VARA_TEXT(INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), CHARACTER*(*) text) INTEGER FUNCTION NF_GET_VARA_INT1(INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), INTEGER*1 i1vals(*)) INTEGER FUNCTION NF_GET_VARA_INT2(INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), INTEGER*2 i2vals(*)) INTEGER FUNCTION NF_GET_VARA_INT (INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), INTEGER ivals(*)) INTEGER FUNCTION NF_GET_VARA_REAL(INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), REAL rvals(*)) INTEGER FUNCTION NF_GET_VARA_DOUBLE(INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), DOUBLE dvals(*))
NCID
VARID
START
COUNT
text
i1vals
i2vals
ivals
rvals
dvals
NF_GET_VARA_ type returns the value NF_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using NF_GET_VARA_DOUBLE to read all the values of the variable named rh from an existing netCDF dataset named foo.nc. For simplicity in this example, we assume that we know that rh is dimensioned with lon, lat, and time, and that there are ten lon values, five lat values, and three time values.
INCLUDE 'netcdf.inc' ... PARAMETER (NDIMS=3) ! number of dimensions PARAMETER (TIMES=3, LATS=5, LONS=10) ! dimension lengths INTEGER STATUS, NCID INTEGER RHID ! variable ID INTEGER START(NDIMS), COUNT(NDIMS) DOUBLE RHVALS(LONS, LATS, TIMES) DATA START /1, 1, 1/ ! start at first value DATA COUNT /LONS, LATS, TIMES/ ! get all the values ... STATUS = NF_OPEN ('foo.nc', NF_NOWRITE, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_INQ_VARID (NCID, 'rh', RHID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) STATUS = NF_GET_VARA_DOUBLE (NCID, RHID, START, COUNT, RHVALS) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS)
The NF_GET_VARS_ type family of functions read a subsampled (strided) array section of values from a netCDF variable of an open netCDF dataset. The subsampled array section is specified by giving a corner, a vector of edge lengths, and a stride vector. The values are read with the first dimension of the netCDF variable varying fastest. The netCDF dataset must be in data mode.
INTEGER FUNCTION NF_GET_VARS_TEXT (INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), INTEGER STRIDE(*),CHARACTER*(*) text) INTEGER FUNCTION NF_GET_VARS_INT1 (INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), INTEGER STRIDE(*),INTEGER*1 i1vals(*)) INTEGER FUNCTION NF_GET_VARS_INT2 (INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), INTEGER STRIDE(*),INTEGER*2 i2vals(*)) INTEGER FUNCTION NF_GET_VARS_INT (INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), INTEGER STRIDE(*), INTEGER ivals(*)) INTEGER FUNCTION NF_GET_VARS_REAL (INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), INTEGER STRIDE(*), REAL rvals(*)) INTEGER FUNCTION NF_GET_VARS_DOUBLE(INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), INTEGER STRIDE(*), DOUBLE dvals(*))
NCID
VARID
START
COUNT
STRIDE
text
i1vals
i2vals
ivals
rvals
dvals
NF_GET_VARS_ type returns the value NF_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using NF_GET_VARS_DOUBLE to read every other value in each dimension of the variable named rh from an existing netCDF dataset named foo.nc. Values are assigned, using the same dimensional strides, to a 2-parameter array. For simplicity in this example, we assume that we know that rh is dimensioned with lon, lat, and time, and that there are ten lon values, five lat values, and three time values.
INCLUDE 'netcdf.inc' ... PARAMETER (NDIMS=3) ! number of dimensions PARAMETER (TIMES=3, LATS=5, LONS=10) ! dimension lengths INTEGER STATUS, NCID INTEGER RHID ! variable ID INTEGER START(NDIMS), COUNT(NDIMS), STRIDE(NDIMS) DOUBLE DATA(LONS, LATS, TIMES) DATA START /1, 1, 1/ ! start at first value DATA COUNT /LONS, LATS, TIMES/ DATA STRIDE /2, 2, 2/ ... STATUS = NF_OPEN ('foo.nc', NF_NOWRITE, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_INQ_VARID (NCID, 'rh', RHID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) STATUS = NF_GET_VARS_DOUBLE(NCID,RHID,START,COUNT,STRIDE,DATA(1,1,1)) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS)
The NF_GET_VARM_ type family of functions reads a mapped array section of values from a netCDF variable of an open netCDF dataset. The mapped array section is specified by giving a corner, a vector of edge lengths, a stride vector, and an index mapping vector. The index mapping vector is 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. No assumptions are made about the ordering or length of the dimensions of the data array. The netCDF dataset must be in data mode.
INTEGER FUNCTION NF_GET_VARM_TEXT (INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), INTEGER STRIDE(*), INTEGER IMAP(*), CHARACTER*(*) text) INTEGER FUNCTION NF_GET_VARM_INT1 (INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), INTEGER STRIDE(*), INTEGER IMAP(*), INTEGER*1 i1vals(*)) INTEGER FUNCTION NF_GET_VARM_INT2 (INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), INTEGER STRIDE(*), INTEGER IMAP(*), INTEGER*2 i2vals(*)) INTEGER FUNCTION NF_GET_VARM_INT (INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), INTEGER STRIDE(*), INTEGER IMAP(*), INTEGER ivals(*)) INTEGER FUNCTION NF_GET_VARM_REAL (INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), INTEGER STRIDE(*), INTEGER IMAP(*), REAL rvals(*)) INTEGER FUNCTION NF_GET_VARM_DOUBLE(INTEGER NCID, INTEGER VARID, INTEGER START(*), INTEGER COUNT(*), INTEGER STRIDE(*), INTEGER IMAP(*), DOUBLE dvals(*))
NCID
VARID
START
COUNT
STRIDE
IMAP
text
i1vals
i2vals
ivals
rvals
dvals
NF_GET_VARM_ type returns the value NF_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
The following IMAP vector maps in the trivial way a 2x3x4 netCDF variable and an internal array of the same shape:
REAL A(2,3,4) ! same shape as netCDF variable INTEGER IMAP(3) DATA IMAP /1, 2, 6/ ! netCDF dimension inter-element distance ! ---------------- ---------------------- ! most rapidly varying 1 ! intermediate 2 (=IMAP(1)*2) ! most slowly varying 6 (=IMAP(2)*3)
Using the IMAP vector above with NF_GET_VARM_REAL obtains the same result as simply using NF_GET_VAR_REAL.
Here is an example of using NF_GET_VARM_REAL to transpose a netCDF variable named rh which is described by the FORTRAN declaration REAL RH(4,6) (note the size and order of the dimensions):
INCLUDE 'netcdf.inc' ...  STATUS = NF_OPEN('foo.nc', NF_NOWRITE, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_INQ_VARID(NCID, 'rh', RHID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_GET_VARM_REAL(NCID, RHID, START, COUNT, STRIDE, IMAP, RH) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS)
Here is another example of using NF_GET_VARM_REAL to simultaneously transpose and subsample the same netCDF variable, by accessing every other point of the netCDF variable:
INCLUDE 'netcdf.inc' ...  STATUS = NF_OPEN('foo.nc', NF_NOWRITE, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_INQ_VARID(NCID, 'rh', RHID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_GET_VARM_REAL(NCID, RHID, START, COUNT, STRIDE, IMAP, RH) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS)
Character strings are not a primitive netCDF external data type, in part because FORTRAN does not support the abstraction of variable-length character strings (the FORTRAN LEN function returns the static length of a character string, not its dynamic length). As a result, a character string cannot be written or read as a single object in the netCDF interface. Instead, a character string must be treated as an array of characters, and array access must be used to read and write character strings as variable data in netCDF datasets. Furthermore, variable-length strings are not supported by the netCDF interface except by convention; for example, you may treat a zero byte as terminating a character string, but you must explicitly specify the length of strings to be read from and written to netCDF variables.
Character strings as attribute values are easier to use, since the strings are treated as a single unit for access. However, the value of a character-string attribute is still an array of characters with an explicit length that must be specified when the attribute is defined.
When you define a variable that will have character-string values, use a character-position dimension as the most quickly varying dimension for the variable (the first dimension for the variable in FORTRAN). The length of the character-position dimension will be the maximum string length of any value to be stored in the character-string variable. Space for maximum-length strings will be allocated in the disk representation of character-string variables whether you use the space or not. If two or more variables have the same maximum length, the same character-position dimension may be used in defining the variable shapes.
To write a character-string value into a character-string variable, use either entire variable access or array access. The latter requires that you specify both a corner and a vector of edge lengths. The character-position dimension at the corner should be one for FORTRAN. If the length of the string to be written is n, then the vector of edge lengths will specify n in the character-position dimension, and one for all the other dimensions:(n, 1, 1, ..., 1).
In FORTRAN, fixed-length strings may be written to a netCDF dataset without a terminating character, to save space. Variable-length strings should follow the C convention of writing strings with a terminating zero byte so that the intended length of the string can be determined when it is later read by either C or FORTRAN programs.
The FORTRAN interface for reading and writing strings requires the use of different functions for accessing string values and numeric values, because standard FORTRAN does not permit the same formal parameter to be used for both character values and numeric values. An additional argument, specifying the declared length of the character string passed as a value, is required for NF_PUT_VARA_TEXT and NF_GET_VARA_TEXT. The actual length of the string is specified as the value of the edge-length vector corresponding to the character-position dimension.
Here is an example that defines a record variable, tx, for character strings and stores a character-string value into the third record using NF_PUT_VARA_TEXT. In this example, we assume the string variable and data are to be added to an existing netCDF dataset named foo.nc that already has an unlimited record dimension time.
INCLUDE 'netcdf.inc' ... INTEGER TDIMS, TXLEN PARAMETER (TDIMS=2) ! number of TX dimensions PARAMETER (TXLEN = 15) ! length of example string INTEGER NCID INTEGER CHID ! char position dimension id INTEGER TIMEID ! record dimension id INTEGER TXID ! variable ID INTEGER TXDIMS(TDIMS) ! variable shape INTEGER TSTART(TDIMS), TCOUNT(TDIMS) CHARACTER*40 TXVAL ! max length 40 DATA TXVAL /'example string'/ ... TXVAL(TXLEN:TXLEN) = CHAR(0) ! null terminate ... STATUS = NF_OPEN('foo.nc', NF_WRITE, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) STATUS = NF_REDEF(NCID) ! enter define mode IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... ! define character-position dimension for strings of max length 40 STATUS = NF_DEF_DIM(NCID, "chid", 40, CHID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... ! define a character-string variable TXDIMS(1) = CHID ! character-position dimension first TXDIMS(2) = TIMEID STATUS = NF_DEF_VAR(NCID, "tx", NF_CHAR, TDIMS, TXDIMS, TXID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_ENDDEF(NCID) ! leave define mode IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... ! write txval into tx netCDF variable in record 3 TSTART(1) = 1 ! start at beginning of variable TSTART(2) = 3 ! record number to write TCOUNT(1) = TXLEN ! number of chars to write TCOUNT(2) = 1 ! only write one record STATUS = NF_PUT_VARA_TEXT (NCID, TXID, TSTART, TCOUNT, TXVAL) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS)
What happens when you try to read a value that was never written in an open netCDF dataset? You might expect that this should always be an error, and that you should get an error message or an error status returned. You do get an error if you try to read data from a netCDF dataset that is not open for reading, if the variable ID is invalid for the specified netCDF dataset, or if the specified indices are not properly within the range defined by the dimension lengths of the specified variable. Otherwise, reading a value that was not written returns a special fill value used to fill in any undefined values when a netCDF variable is first written.
You may ignore fill values and use the entire range of a netCDF external data type, but in this case you should make sure you write all data values before reading them. If you know you will be writing all the data before reading it, you can specify that no prefilling of variables with fill values will occur by calling NF_SET_FILL before writing. This may provide a significant performance gain for netCDF writes.
The variable attribute _FillValue may be used to specify the fill value for a variable. Their are default fill values for each type, defined in the include file netcdf.inc: NF_FILL_CHAR, NF_FILL_INT1 (same as NF_FILL_BYTE), NF_FILL_INT2 (same as NF_FILL_SHORT), NF_FILL_INT, NF_FILL_REAL (same as NF_FILL_FLOAT), and NF_FILL_DOUBLE.
The netCDF byte and character types have different default fill values. The default fill value for characters is the zero byte, a useful value for detecting the end of variable-length C character strings. If you need a fill value for a byte variable, it is recommended that you explicitly define an appropriate _FillValue attribute, as generic utilities such as ncdump will not assume a default fill value for byte variables.
Type conversion for fill values is identical to type conversion for other values: attempting to convert a value from one type to another type that can't represent the value results in a range error. Such errors may occur on writing or reading values from a larger type (such as double) to a smaller type (such as float), if the fill value for the larger type cannot be represented in the smaller type.
The function NF_RENAME_VAR changes the name of a netCDF variable in an open netCDF dataset. If the new name is longer than the old name, the netCDF dataset must be in define mode. You cannot rename a variable to have the name of any existing variable.
INTEGER FUNCTION NF_RENAME_VAR (INTEGER NCID, INTEGER VARID, CHARACTER*(*) NEWNAM)
NCID
VARID
NAME
NF_RENAME_VAR returns the value NF_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using NF_RENAME_VAR to rename the variable rh to rel_hum in an existing netCDF dataset named foo.nc:
INCLUDE 'netcdf.inc' ... INTEGER STATUS, NCID INTEGER RHID ! variable ID ... STATUS = NF_OPEN ('foo.nc', NF_WRITE, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_REDEF (NCID) ! enter definition mode IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) STATUS = NF_INQ_VARID (NCID, 'rh', RHID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) STATUS = NF_RENAME_VAR (NCID, RHID, 'rel_hum') IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) STATUS = NF_ENDDEF (NCID) ! leave definition mode IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS)
Attributes may be associated with each netCDF variable to specify such properties as units, special values, maximum and minimum valid values, scaling factors, and offsets. Attributes for a netCDF dataset are defined when the dataset is first created, while the netCDF dataset is in define mode. Additional attributes may be added later by reentering define mode. A netCDF attribute has a netCDF variable to which it is assigned, a name, a type, a length, and a sequence of one or more values. An attribute is designated by its variable ID and name. When an attribute name is not known, it may be designated by its variable ID and number in order to determine its name, using the function NF_INQ_ATTNAME.
THE attributes associated with a variable are typically defined immediately after the variable is created, while still in define mode. The data type, length, and value of an attribute may be changed even when in data mode, as long as the changed attribute requires no more space than the attribute as originally defined.
It is also possible to have attributes that are not associated with any variable. These are called global attributes and are identified by using NF_GLOBAL as a variable pseudo-ID. Global attributes are usually related to the netCDF dataset as a whole and may be used for purposes such as providing a title or processing history for a netCDF dataset.
Operations supported on attributes are:
Names commencing with underscore ('_') are reserved for use by the netCDF library. Most generic applications that process netCDF datasets assume standard attribute conventions and it is strongly recommended that these be followed unless there are good reasons for not doing so. Below we list the names and meanings of recommended standard attributes that have proven useful. Note that some of these (e.g. units, valid_range, scale_factor) assume numeric data and should not be used with character data. units
A character string that specifies the units used for the variable's data. Unidata has developed a freely-available library of routines to convert between character string and binary forms of unit specifications and to perform various useful operations on the binary forms. This library is used in some netCDF applications. Using the recommended units syntax permits data represented in conformable units to be automatically converted to common units for arithmetic operations. See Units (The NetCDF Users Guide).
long_name
valid_min
valid_max
valid_range
Generic applications should treat values outside the valid range as missing. The type of each valid_range, valid_min and valid_max attribute should match the type of its variable (except that for byte data, these can be of a signed integral type to specify the intended range).
If neither valid_min, valid_max nor valid_range is defined then
generic applications should define a valid range as follows. If the
data type is byte and _FillValue is not explicitly defined, then the
valid range should include all possible values. Otherwise, the valid
range should exclude the _FillValue (whether defined explicitly or by
default) as follows. If the _FillValue is positive then it defines a
valid maximum, otherwise it defines a valid minimum. For integer
types, there should be a difference of 1 between the _FillValue and
this valid minimum or maximum. For floating point types, the
difference should be twice the minimum possible (1 in the least
significant bit) to allow for rounding error.
scale_factor
add_offset
When scale_factor and add_offset are used for packing, the associated
variable (containing the packed data) is typically of type byte or
short, whereas the unpacked values are intended to be of type float or
double. The attributes scale_factor and add_offset should both be of
the type intended for the unpacked data, e.g. float or double.
_FillValue
Generic applications often need to write a value to represent undefined or missing values. The fill value provides an appropriate value for this purpose because it is normally outside the valid range and therefore treated as missing when read by generic applications. It is legal (but not recommended) for the fill value to be within the valid range.
See Fill Values.
missing_value
signedness
C_format
FORTRAN_format
title
history
Conventions
For example, if a group named NUWG agrees upon a set of conventions for dimension names, variable names, required attributes, and netCDF representations for certain discipline-specific data structures, they may store a document describing the agreed-upon conventions in a dataset in the NUWG/ subdirectory of the Conventions directory. Datasets that followed these conventions would contain a global Conventions attribute with value "NUWG".
Later, if the group agrees upon some additional conventions for a specific subset of NUWG data, for example time series data, the description of the additional conventions might be stored in the NUWG/Time_series/ subdirectory, and datasets that adhered to these additional conventions would use the global Conventions attribute with value "NUWG/Time_series", implying that this dataset adheres to the NUWG conventions and also to the additional NUWG time-series conventions.
The function NF_PUT_ATT_ type adds or changes a variable attribute or global attribute of an open netCDF dataset. If this attribute is new, or if the space required to store the attribute is greater than before, the netCDF dataset must be in define mode.
Although it's possible to create attributes of all types, text and double attributes are adequate for most purposes.
INTEGER FUNCTION NF_PUT_ATT_TEXT (INTEGER NCID, INTEGER VARID, CHARACTER*(*) NAME, INTEGER LEN, CHARACTER*(*) TEXT) INTEGER FUNCTION NF_PUT_ATT_INT1 (INTEGER NCID, INTEGER VARID, CHARACTER*(*) NAME, INTEGER XTYPE, LEN, INTEGER*1 I1VALS(*)) INTEGER FUNCTION NF_PUT_ATT_INT2 (INTEGER NCID, INTEGER VARID, CHARACTER*(*) NAME, INTEGER XTYPE, LEN, INTEGER*2 I2VALS(*)) INTEGER FUNCTION NF_PUT_ATT_INT (INTEGER NCID, INTEGER VARID, CHARACTER*(*) NAME, INTEGER XTYPE, LEN, INTEGER IVALS(*)) INTEGER FUNCTION NF_PUT_ATT_REAL (INTEGER NCID, INTEGER VARID, CHARACTER*(*) NAME, INTEGER XTYPE, LEN, REAL RVALS(*)) INTEGER FUNCTION NF_PUT_ATT_DOUBLE(INTEGER NCID, INTEGER VARID, CHARACTER*(*) NAME, INTEGER XTYPE, LEN, DOUBLE DVALS(*))
NCID
VARID
NAME
XTYPE
LEN
TEXT
I1VALS
I2VALS
IVALS
RVALS
DVALS
NF_PUT_ATT_ type returns the value NF_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using NF_PUT_ATT_DOUBLE to add a variable attribute named valid_range for a netCDF variable named rh and a global attribute named title to an existing netCDF dataset named foo.nc:
INCLUDE 'netcdf.inc' ... INTEGER STATUS, NCID INTEGER RHID ! variable ID DOUBLE RHRNGE(2) DATA RHRNGE /0.0D0, 100.0D0/ ... STATUS = NF_OPEN ('foo.nc', NF_WRITE, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_REDEF (NCID) ! enter define mode IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) STATUS = NF_INQ_VARID (NCID, 'rh', RHID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_PUT_ATT_DOUBLE (NCID, RHID, 'valid_range', NF_DOUBLE, & 2, RHRNGE) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) STATUS = NF_PUT_ATT_TEXT (NCID, NF_GLOBAL, 'title', 19, 'example netCDF dataset') IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_ENDDEF (NCID) ! leave define mode IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS)
This family of functions returns information about a netCDF attribute. All but one of these functions require the variable ID and attribute name; the exception is NF_INQ_ATTNAME. Information about an attribute includes its type, length, name, and number. See the NF_GET_ATT family for getting attribute values.
The function NF_INQ_ATTNAME gets the name of an attribute, given its variable ID and number. This function is useful in generic applications that need to get the names of all the attributes associated with a variable, since attributes are accessed by name rather than number in all other attribute functions. The number of an attribute is more volatile than the name, since it can change when other attributes of the same variable are deleted. This is why an attribute number is not called an attribute ID.
The function NF_INQ_ATT returns the attribute's type and length. The other functions each return just one item of information about an attribute.
INTEGER FUNCTION NF_INQ_ATT (INTEGER NCID, INTEGER VARID, CHARACTER*(*) NAME, INTEGER xtype, INTEGER len) INTEGER FUNCTION NF_INQ_ATTTYPE(INTEGER NCID, INTEGER VARID, CHARACTER*(*) NAME, INTEGER xtype) INTEGER FUNCTION NF_INQ_ATTLEN (INTEGER NCID, INTEGER VARID, CHARACTER*(*) NAME, INTEGER len) INTEGER FUNCTION NF_INQ_ATTNAME(INTEGER NCID, INTEGER VARID, INTEGER ATTNUM, CHARACTER*(*) name) INTEGER FUNCTION NF_INQ_ATTID (INTEGER NCID, INTEGER VARID, CHARACTER*(*) NAME, INTEGER attnum)
NCID
VARID
NAME
xtype
len
attnum
(If you already know an attribute name, knowing its number is not very useful, because accessing information about an attribute requires its name.)
Each function returns the value NF_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using NF_INQ_ATT to find out the type and length of a variable attribute named valid_range for a netCDF variable named rh and a global attribute named title in an existing netCDF dataset named foo.nc:
INCLUDE 'netcdf.inc' ... INTEGER STATUS, NCID INTEGER RHID ! variable ID INTEGER VRLEN, TLEN ! attribute lengths ... STATUS = NF_OPEN ('foo.nc', NF_NOWRITE, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_INQ_VARID (NCID, 'rh', RHID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_INQ_ATTLEN (NCID, RHID, 'valid_range', VRLEN) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) STATUS = NF_INQ_ATTLEN (NCID, NF_GLOBAL, 'title', TLEN) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS)
Members of the NF_GET_ATT_ type family of functions get the value(s) of a netCDF attribute, given its variable ID and name.
INTEGER FUNCTION NF_GET_ATT_TEXT (INTEGER NCID, INTEGER VARID, CHARACTER*(*) NAME, CHARACTER*(*) text) INTEGER FUNCTION NF_GET_ATT_INT1 (INTEGER NCID, INTEGER VARID, CHARACTER*(*) NAME, INTEGER*1 i1vals(*)) INTEGER FUNCTION NF_GET_ATT_INT2 (INTEGER NCID, INTEGER VARID, CHARACTER*(*) NAME, INTEGER*2 i2vals(*)) INTEGER FUNCTION NF_GET_ATT_INT (INTEGER NCID, INTEGER VARID, CHARACTER*(*) NAME, INTEGER ivals(*)) INTEGER FUNCTION NF_GET_ATT_REAL (INTEGER NCID, INTEGER VARID, CHARACTER*(*) NAME, REAL rvals(*)) INTEGER FUNCTION NF_GET_ATT_DOUBLE (INTEGER NCID, INTEGER VARID, CHARACTER*(*) NAME, DOUBLE dvals(*))
NCID
VARID
NAME
TEXT
I1VALS
I2VALS
IVALS
RVALS
DVALS
NF_GET_ATT_ type returns the value NF_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using NF_GET_ATT_DOUBLE to determine the values of a variable attribute named valid_range for a netCDF variable named rh and a global attribute named title in an existing netCDF dataset named foo.nc. In this example, it is assumed that we don't know how many values will be returned, but that we do know the types of the attributes. Hence, to allocate enough space to store them, we must first inquire about the length of the attributes.
INCLUDE 'netcdf.inc' ... PARAMETER (MVRLEN=3) ! max number of "valid_range" values PARAMETER (MTLEN=80) ! max length of "title" attribute INTEGER STATUS, NCID INTEGER RHID ! variable ID INTEGER VRLEN, TLEN ! attribute lengths DOUBLE PRECISION VRVAL(MVRLEN) ! vr attribute values CHARACTER*80 TITLE ! title attribute values ... STATUS = NF_OPEN ('foo.nc', NF_WRITE, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_INQ_VARID (NCID, 'rh', RHID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... ! find out attribute lengths, to make sure we have enough space STATUS = NF_INQ_ATTLEN (NCID, RHID, 'valid_range', VRLEN) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) STATUS = NF_INQ_ATTLEN (NCID, NF_GLOBAL, 'title', TLEN) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ! get attribute values, if not too big IF (VRLEN .GT. MVRLEN) THEN WRITE (*,*) 'valid_range attribute too big!' CALL EXIT ELSE STATUS = NF_GET_ATT_DOUBLE (NCID, RHID, 'valid_range', VRVAL) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ENDIF IF (TLEN .GT. MTLEN) THEN WRITE (*,*) 'title attribute too big!' CALL EXIT ELSE STATUS = NF_GET_ATT_TEXT (NCID, NF_GLOBAL, 'title', TITLE) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ENDIF
The function NF_COPY_ATT copies an attribute from one open netCDF dataset to another. It can also be used to copy an attribute from one variable to another within the same netCDF.
INTEGER FUNCTION NF_COPY_ATT (INTEGER NCID_IN, INTEGER VARID_IN, CHARACTER*(*) NAME, INTEGER NCID_OUT, INTEGER VARID_OUT)
NCID_IN
VARID_IN
NAME
NCID_OUT
VARID_OUT
NF_COPY_ATT returns the value NF_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using NF_COPY_ATT to copy the variable attribute units from the variable rh in an existing netCDF dataset named foo.nc to the variable avgrh in another existing netCDF dataset named bar.nc, assuming that the variable avgrh already exists, but does not yet have a units attribute:
INCLUDE 'netcdf.inc' ...  STATUS = NF_OPEN ('foo.nc', NF_NOWRITE, NCID1) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) STATUS = NF_OPEN ('bar.nc', NF_WRITE, NCID2) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_INQ_VARID (NCID1, 'rh', RHID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) STATUS = NF_INQ_VARID (NCID2, 'avgrh', AVRHID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_REDEF (NCID2) ! enter define mode IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ! copy variable attribute from "rh" to "avgrh" STATUS = NF_COPY_ATT (NCID1, RHID, 'units', NCID2, AVRHID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_ENDDEF (NCID2) ! leave define mode IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS)
The function NF_RENAME_ATT changes the name of an attribute. If the new name is longer than the original name, the netCDF dataset must be in define mode. You cannot rename an attribute to have the same name as another attribute of the same variable.
INTEGER FUNCTION NF_RENAME_ATT (INTEGER NCID, INTEGER VARID, CHARACTER*(*) NAME, CHARACTER*(*) NEWNAME)
NCID
VARID
NAME
NEWNAME
NF_RENAME_ATT returns the value NF_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using NF_RENAME_ATT to rename the variable attribute units to Units for a variable rh in an existing netCDF dataset named foo.nc:
INCLUDE "netcdf.inc" ... INTEGER STATUS ! error status INTEGER NCID ! netCDF ID INTEGER RHID ! variable ID ... STATUS = NF_OPEN ("foo.nc", NF_NOWRITE, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_INQ_VARID (NCID, "rh", RHID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... ! rename attribute STATUS = NF_RENAME_ATT (NCID, RHID, "units", "Units") IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS)
The function NF_DEL_ATT deletes a netCDF attribute from an open netCDF dataset. The netCDF dataset must be in define mode.
INTEGER FUNCTION NF_DEL_ATT (INTEGER NCID, INTEGER VARID, CHARACTER*(*) NAME)
NCID
VARID
NAME
NF_DEL_ATT returns the value NF_NOERR if no errors occurred. Otherwise, the returned status indicates an error. Possible causes of errors include:
Here is an example using NF_DEL_ATT to delete the variable attribute Units for a variable rh in an existing netCDF dataset named foo.nc:
INCLUDE 'netcdf.inc' ... INTEGER STATUS ! error status INTEGER NCID ! netCDF ID INTEGER RHID ! variable ID ... STATUS = NF_OPEN ('foo.nc', NF_WRITE, NCID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... STATUS = NF_INQ_VARID (NCID, 'rh', RHID) IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) ... ! delete attribute STATUS = NF_REDEF (NCID) ! enter define mode IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) STATUS = NF_DEL_ATT (NCID, RHID, 'Units') IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS) STATUS = NF_ENDDEF (NCID) ! leave define mode IF (STATUS .NE. NF_NOERR) CALL HANDLE_ERR(STATUS)
Input parameters are in upper case, output parameters are in lower case. The FORTRAN types of all the parameters are listed alphabetically by parameter name below the function declarations.
   It is about twice as fast as the previous version. The netCDF file format is unchanged, so files written with version 3 can be read with version 2 code and vice versa.
The core library is now written in ANSI C. For example, prototypes are used throughout as well as const qualifiers where appropriate. You must have an ANSI C compiler to compile this version.
Rewriting the library offered an opportunity to implement improved C and FORTRAN interfaces that provide some significant benefits:
type safety, by eliminating the need to use generic void* pointers;
automatic type conversions, by eliminating the undesirable coupling between the language-independent external netCDF types (NF_BYTE, ..., NF_DOUBLE) and language-dependent internal data types (char, ..., double);
support for future enhancements, by eliminating obstacles to the clean addition of support for packed data and multithreading;
more standard error behavior, by uniformly communicating an error status back to the calling program in the return value of each function.
It is not necessary to rewrite programs that use the version 2 C interface, because the netCDF-3 library includes a backward compatibility interface that supports all the old functions, globals, and behavior. We are hoping that the benefits of the new interface will be an incentive to use it in new netCDF applications. It is possible to convert old applications to the new interface incrementally, replacing netCDF-2 calls with the corresponding netCDF-3 calls one at a time. If you want to check that only netCDF-3 calls are used in an application, a preprocessor macro (NO_NETCDF_2) is available for that purpose.
Other changes in the implementation of netCDF result in improved portability, maintainability, and performance on most platforms. A clean separation between I/O and type layers facilitates platform-specific optimizations. The new library no longer uses a vendor-provided XDR library, which simplifies linking programs that use netCDF and speeds up data access significantly in most cases.
First, here's an example of C code that uses the netCDF-2 interface:
void *bufferp; NF_TYPE xtype; ncvarinq(ncid, varid, ..., &xtype, ... ... /* allocate bufferp based on dimensions and type */ ... if (ncvarget(ncid, varid, start, count, bufferp) == -1) { fprintf(stderr, "Can't get data, error code = %d\n",ncerr); /* deal with it */ ... } switch(xtype) { /* deal with the data, according to type */ ... case NF_FLOAT: fanalyze((float *)bufferp); break; case NF_DOUBLE: danalyze((double *)bufferp); break; }
Here's how you might handle this with the new netCDF-3 C interface:
/* * I want to use doubles for my analysis. */ double dbuf[NDOUBLES]; int status; /* So, I use the function that gets the data as doubles. */ status = NF_GET_VARA_DOUBLE(NCID, varid, start, count, dbuf) if (status != NF_NOERR) { fprintf(stderr, "Can't get data: %s\n", NF_STRERROR(STATUS)); /* deal with it */ ... } danalyze(dbuf);
The example above illustrates changes in function names, data type conversion, and error handling, discussed in detail in the sections below.
The netCDF-3 C library employs a new naming convention, intended to make netCDF programs more readable. For example, the name of the function to rename a variable is now NF_RENAME_VAR instead of the previous ncvarrename.
All netCDF-3 C function names begin with the NF_ prefix. The second part of the name is a verb, like get, put, inq (for inquire), or open. The third part of the name is typically the object of the verb: for example dim, var, or att for functions dealing with dimensions, variables, or attributes. To distinguish the various I/O operations for variables, a single character modifier is appended to var:
var entire variable access var1 single value access vara array or array section access vars strided access to a subsample of values varm mapped access to values not contiguous in memory
At the end of the name for variable and attribute functions, there is a component indicating the type of the final argument: text, uchar, schar, short, int, long, float, or double. This part of the function name indicates the type of the data container you are using in your program: character string, unsigned char, signed char, and so on.
Also, all macro names in the public C interface begin with the prefix NF_. For example, the macro which was formerly MAX_NF_NAME is now NF_MAX_NAME, and the former FILL_FLOAT is now NF_FILL_FLOAT.
AS previously mentioned, all the old names are still supported for backward compatibility.
With the new interface, users need not be aware of the external type of numeric variables, since automatic conversion to or from any desired numeric type is now available. You can use this feature to simplify code, by making it independent of external types. The elimination of void* pointers provides detection of type errors at compile time that could not be detected with the previous interface. Programs may be made more robust with the new interface, because they need not be changed to accommodate a change to the external type of a variable.
If conversion to or from an external numeric type is necessary, it is handled by the library. This automatic conversion and separation of external data representation from internal data types will become even more important in netCDF version 4, when new external types will be added for packed data for which there is no natural corresponding internal type, for example, arrays of 11-bit values.
Converting from one numeric type to another may result in an error if the target type is not capable of representing the converted value. (In netCDF-2, such overflows can only happen in the XDR layer.) For example, a float may not be able to hold data stored externally as an NF_DOUBLE (an IEEE floating-point number). When accessing an array of values, an NF_ERANGE error is returned if one or more values are out of the range of representable values, but other values are converted properly.
Note that mere loss of precision in type conversion does not return an error. Thus, if you read double precision values into an int, for example, no error results unless the magnitude of the double precision value exceeds the representable range of ints on your platform. Similarly, if you read a large integer into a float incapable of representing all the bits of the integer in its mantissa, this loss of precision will not result in an error. If you want to avoid such precision loss, check the external types of the variables you access to make sure you use an internal type that has a compatible precision.
The new interface distinguishes arrays of characters intended to represent text strings from arrays of 8-bit bytes intended to represent small integers. The interface supports the internal types text, uchar, and schar, intended for text strings, unsigned byte values, and signed byte values.
The _uchar and _schar functions were introduced in netCDF-3 to eliminate an ambiguity, and support both signed and unsigned byte data. In netCDF-2, whether the external NF_BYTE type represented signed or unsigned values was left up to the user. In netcdf-3, we treat NF_BYTE as signed for the purposes of conversion to short, int, long, float, or double. (Of course, no conversion takes place when the internal type is signed char.) In the _uchar functions, we treat NF_BYTE as if it were unsigned. Thus, no NF_ERANGE error can occur converting between NF_BYTE and unsigned char.
The new interface handles errors differently than netCDF-2. In the old interface, the default behavior when an error was detected was to print an error message and exit. To get control of error handling, you had to set flag bits in a global variable, ncopts, and to determine the cause of an error, you had to test the value of another global variable ncerr.
In the new interface, functions return an integer status that indicates not only success or failure, but also the cause of the error. The global variables ncerr and ncopt have been eliminated. The library will never try to print anything, nor will it call exit (unless you are using the netCDF version 2 compatibility functions). You will have to check the function return status and do this yourself. We eliminated these globals in the interest of supporting parallel (multiprocessor) execution cleanly, as well as reducing the number of assumptions about the environment where netCDF is used. The new behavior should provide better support for using netCDF as a hidden layer in applications that have their own GUI interface.
Where the netCDF-2 interface used NF_LONG to identify an external data type corresponding to 32-bit integers, the new interface uses NF_INT instead. NF_LONG is defined to have the same value as NF_INT for backward compatibility, but it should not be used in new code. With new 64-bit platforms using long for 64-bit integers, we would like to reduce the confusion caused by this name clash. Note that there is still no netCDF external data type corresponding to 64-bit integers.
The new C interface omits three "record I/O" functions, ncrecput, ncrecget, and ncrecinq, from the netCDF-2 interface, although these functions are still supported via the netCDF-2 compatibility interface.
This means you may have to replace one record-oriented call with multiple type-specific calls, one for each record variable. For example, a single call to ncrecput can always be replaced by multiple calls to the appropriate NF_PUT_VAR functions, one call for each variable accessed. The record-oriented functions were omitted, because there is no simple way to provide type-safety and automatic type conversion for such an interface.
There is no function corresponding to the nctypelen function from the version 2 interface. The separation of internal and external types and the new type-conversion interfaces make nctypelen unnecessary. Since users read into and write out of native types, the sizeof operator is perfectly adequate to determine how much space to allocate for a value.
In the previous library, there was no checking that the characters used in the name of a netCDF object were compatible with CDL restrictions. Now this restriction is also enforced by the library for creation of new dimensions, variables, and attributes. Previously existing components with less restrictive names will still work OK.
There are two new functions in netCDF-3 that don't correspond to any netCDF-2 functions: NF_INQ_LIBVERS and NF_STRERROR. The version of the netCDF library in use is returned as a string by NF_INQ_LIBVERS. An error message corresponding to the status returned by a netCDF function call is returned as a string by the NF_STRERROR function.
A new NF_SHARE flag is available for use in an NF_OPEN or NF_CREATE CALL, to suppress the default buffering of accesses. The use of NF_SHARE for concurrent access to a netCDF dataset means you don't have to call NF_SYNC after every access to make sure that disk updates are synchronous. It is important to note that changes to ancillary data, such as attribute values, are not propagated automatically by use of the NF_SHARE flag. Use of the NF_SYNC function is still required for this purpose.
The version 2 interface had a single inquiry function, ncvarinq for getting the name, type, and shape of a variable. Similarly, only a single inquiry function was available for getting information about a dimension, an attribute, or a netCDF dataset. When you only wanted a subset of this information, you had to provide NULL arguments as placeholders for the unneeded information. The new interface includes additional inquire functions that return each item separately, so errors are less likely from miscounting arguments.
The previous implementation returned an error when 0-valued count components were specified in ncvarput and ncvarget calls. This restriction has been removed, so that now functions in the NF_PUT_VAR AND NF_GET_VAR families may be called with 0-valued count components, resulting in no data being accessed. Although this may seem useless, it simplifies some programs to not treat 0-valued counts as a special case.
The previous implementation returned an error when the same dimension was used more than once in specifying the shape of a variable in ncvardef. This restriction is relaxed in the netCDF-3 implementation, because an autocorrelation matrix is a good example where using the same dimension twice makes sense.
In the new interface, units for the imap argument to the NF_PUT_VARM AND NF_GET_VARM families of functions are now in terms of the number of data elements of the desired internal type, not in terms of bytes as in the netCDF version-2 mapped access interfaces.
Following is a table of netCDF-2 function names and names of the corresponding netCDF-3 functions.