We're pleased to announce the release of netCDF version 2.3, now available
via anonymous FTP from
All reported problems with the beta version have been fixed, and the
software has been ported to several additional architectures. This release
has been built and tested successfully on the following platforms:
CRAY Y-MP UNICOS 6.1.6
DEC VAX VMS V5.5-2
DECstation ULTRIX V4.3
HP-9000/7xx HPUX 9.0
IBM PS/2 MSDOS 5.0
IBM PS/2 OS/2 1.3
IBM RS-6000 AIX 3.2
SGI Iris IRIX 4.0.5
SPARCstation Solaris 2.1
SPARCstation SunOS 4.1.3
Our experience indicates that this version of netCDF is relatively easy to
port to other Unix systems. The new `configure'-based approach to
installation has a good chance of working on other platforms without
requiring significant changes to the source, since it tests and adapts to
what the operating system provides.
What's New in netCDF 2.3
Subsampling along specified dimensions (using `strides') is now supported by
new C and Fortran interfaces for generalized hyperslab access (ncvarputg,
ncvargetg, NCVPTG, NCVPGC, NCVGTG, and NCVGGC). In addition, these new
interfaces also permit accessing data that is not contiguous in memory. In
a generalized hyperslab, an `index mapping vector' is used to define the
mapping between points in the generalized hyperslab and the memory locations
of the corresponding values. This facility can be used to avoid copying
data into or out of a contiguous array solely for netCDF access.
There are also some new C interfaces (ncrecput, ncrecget, and ncrecinq) that
can be used to write, read, and inquire about records, where a record may
contain multiple variables of different types and shapes. With the new
interfaces, you can access a record's worth of data (or some subset of a
record) with a single call, instead of the multiple calls required with the
New optimizations for the library have resulted in significant speedups for
accessing cross-sections involving non-contiguous data. In some cases,
speedups by a factor of 40 are achieved with the new implementation of I/O,
and the new design isolates the I/O layer to make other platform-specific
The ncdump utility now supports several new command-line options including
the ability to specify variables and to provide value annotations.
The way in which all the default fill values are defined in the FORTRAN
interface was changed, so that now the FORTRAN default fill values will
always be the same as the C default values. This eliminates the need for
some platform-specific files where these constants were previously defined.
The new release is intended to be backward-compatible with previous
releases, adding a few new interfaces but not changing existing interfaces.
The file format for netCDF files remains the same.
The one change that might be a problem for some users is a new definition
for the default floating-point fill values (FILL_FLOAT and FILL_DOUBLE in
the C interface, FILFLOAT and FILDOUB in FORTRAN interface). Previously
these had been defined to be values that, on writing, would map into the XDR
representation for IEEE infinity. The idea was that although the values
that had this property might vary from platform to platform, the on-disk
representation for missing floating-point data would at least be the same
for all platforms. But such fill values turn out to be an obstacle to
writing portable generic netCDF applications, since there is no portable way
to determine if a floating-point value is an IEEE infinity, and using
ordinary comparisons with such numbers can produce errors or surprising
results. The old default floating-point fill values also introduced an
undesirable dependence on the compilation environment used when the netCDF
library was installed. Solving these problems led to the definition of new
default floating-point fill values. These are intended to be portable
across all platforms and compilation environments. If you wish, however, to
obtain the old, non-portable floating-point fill values, then this is
possible by following the instructions in the INSTALL file.
The suggested extension for netCDF files has been changed from `.cdf' to
`.nc', in order to avoid a clash with the NASA CDF file extension. Although
the old extension is still supported in a backward compatible way (ncgen -n
still produces `.cdf' files), new netCDF files should use the new filename
extension, where practical. A new `-b' flag to ncgen generates binary files
with the new extension.
The code now conforms to the C Standard, although it can still be built with
non Standard C compilers such as the SunOS /bin/cc. Many `const'
declarations were added to the interfaces and documentation, to specify
where functions do not change values through pointers.
Many changes were made to the User's Guide and man pages to improve the
documentation and describe the new interfaces. A new Appendix includes some
answers to frequently asked questions. Among the changes, it is now made
clearer in the User's Guide that a NULL pointer can be provided for any
return parameter from an inquire function to indicate where a value should
not be returned, and that inquire functions do not incur any I/O.
A draft prototype C++ interface has been added. See the c++ subdirectory
for the implementation, an example of use, and some preliminary
documentation. If you use this, please send us feedback about how useful it
was and any problems you encountered.
Questions or comments about the new release that are of general interest may
be posted to this list (netcdfgroup@xxxxxxxxxxxxxxxx). Specific questions
or bug reports should be sent to support@xxxxxxxxxxxxxxxx.
Russ Rew Unidata Program Center
russ@xxxxxxxxxxxxxxxx UCAR, PO Box 3000
Boulder, CO 80307-3000