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2.6 NF__CREATE

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.

Usage

     INTEGER FUNCTION NF_CREATE (CHARACTER*(*) PATH, INTEGER CMODE, INTEGER INITIALSZ,
                                 INTEGER CHUNKSIZEHINT, INTEGER ncid)

PATH

The file name of the new netCDF dataset.

CMODE

The creation mode flag. The following flags are available: NF_NOCLOBBER, NF_SHARE, and NF_64BIT_OFFSET.

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

This parameter sets the initial size of the file at creation time.

chunksizehintp

The argument referenced by chunksizehintp controls a space versus time tradeoff, memory allocated in the netcdf library versus number of system calls.

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

Returned netCDF ID.

Errors

NF__CREATE returns the value NF_NOERR if no errors occurred. Possible causes of errors include:

• Passing a dataset name that includes a directory that does not exist.
• Specifying a dataset name of a file that exists and also specifying NF_NOCLOBBER.
• Specifying a meaningless value for the creation mode.
• Attempting to create a netCDF dataset in a directory where you don't have permission to create files.

Example

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)

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