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FAQ.md
1 FAQ {#faq}
2 =======================
3 
4 [TOC]
5 
6 General {#ncFAQGeneral}
7 =======
8 
9 What Is netCDF? {#What-Is-netCDF}
10 -----------------
11 
12 
13 NetCDF (network Common Data Form) is a set of interfaces for
14 array-oriented data access and a [freely](http://www.unidata.ucar.edu/software/netcdf/docs/copyright.html) distributed
15 collection of data access libraries for C, Fortran, C++, Java, and other
16 languages. The netCDF libraries support a machine-independent format for
17 representing scientific data. Together, the interfaces, libraries, and
18 format support the creation, access, and sharing of scientific data.
19 
20 NetCDF data is:
21 
22 - *Self-Describing*. A netCDF file includes information about the data
23  it contains.
24 - *Portable*. A netCDF file can be accessed by computers with
25  different ways of storing integers, characters, and floating-point
26  numbers.
27 - *Scalable*. A small subset of a large dataset may be accessed
28  efficiently.
29 - *Appendable*. Data may be appended to a properly structured netCDF
30  file without copying the dataset or redefining its structure.
31 - *Sharable*. One writer and multiple readers may simultaneously
32  access the same netCDF file.
33 - *Archivable*. Access to all earlier forms of netCDF data will be
34  supported by current and future versions of the software.
35 
36 The netCDF software was developed by Glenn Davis, Russ Rew, Ed Hartnett,
37 John Caron, Dennis Heimbigner, Steve Emmerson, Harvey Davies, and Ward
38 Fisher at the Unidata Program Center in Boulder, Colorado, with
39 [contributions](http://www.unidata.ucar.edu/software/netcdf/docs/credits.html) from many other netCDF users.
40 
41 ----------
42 
43 How do I get the netCDF software package? {#HowdoIgetthenetCDFsoftwarepackage}
44 -----------------
45 
46 
47 The latest source distribution, which includes the C libraries and
48 utility programs, is available from [the NetCDF Downloads
49 page](/downloads/netcdf/index.jsp). Separate source distributions for
50 the Java library, Fortran libraries, and C++ libraries are also
51 available there. Installation instructions are available with the
52 distribution or [online](http://www.unidata.ucar.edu/software/netcdf/docs/building.html).
53 
54 Binary distributions of netCDF are available for various platforms from
55 package management systems such as dpkg, RPM, fink, MacPorts, Homebrew,
56 OpenCSW, OpenPKG, and the FreeBSD Ports Collection.
57 
58 ----------
59 
60 How do I convert netCDF data to ASCII or text? {#How-do-I-convert-netCDF-data-to-ASCII-or-text}
61 -----------------
62 
63 
64 
65 One way to convert netCDF data to text is to use the **ncdump** tool
66 that is part of the netCDF software distribution. It is a command line
67 tool that provides a text representation of a netCDF file's data, just its
68 metadata, or just the data for specified
69 variables, depending on what arguments you use. For more information,
70 see the \ref ncdump_guide documentation.
71 
72 Another good tool for conversion of netCDF data to text is the ["ncks" program](http://nco.sourceforge.net/nco.html#ncks-netCDF-Kitchen-Sink) that's one of the utility programs in the [NCO (NetCDF Operators)](software.html#NCO) package. Similar capabilities are available using programs from the [CDO (Climate Data Operators)](software.html#CDO) software, commands from [NCL (NCAR Command Language)](software.html#NCL), or various other packages such as [ANAX](http://science.arm.gov/~cflynn/ARM_Tested_Tools/), cdf2asc, and NOESYS, all "third party" netCDF utilities developed and supported by other organizations. You can find more information about these third-party packages on the [Software for Manipulating or Displaying NetCDF Data](software.html) page.
73 
74 You can also get netCDF data in ASCII from an OPeNDAP server by using a
75 ".ascii" extension with the URL that specifies the data. For details,
76 see the OPeNDAP page on [Using a Spreadsheet Application with DODS](http://www.opendap.org/useExcel).
77 
78 Another freely available tool, [netcdf4excel](https://code.google.com/p/netcdf4excel/), has been developed as a netCDF add-in for MS Excel that can facilitate the conversion of netCDF data to and from text form.
79 
80 Note that **ncdump** and similar tools can print metadata and data values
81 from netCDF files, but in general they don't understand coordinate
82 systems specified in the metadata, only variable arrays and their
83 indices. To interpret georeferencing metadata so you can print the data
84 within a latitude/longitude bounding box, for example, you need a higher
85 level tool that interprets conventions for specifying coordinates, such
86 as the CF conventions. Or you can write a small program using one of the
87 language APIs that provide netCDF support, for which [examples are available](http://www.unidata.ucar.edu/software/netcdf/examples/programs/).
88 
89 ----------
90 
91 How do I convert ASCII or text data to netCDF? {#How-do-I-convert-ASCII-or-text-data-to-netCDF}
92 -----------------
93 
94 
95 One way to convert data in text form to netCDF is to use the **ncgen**
96 tool that is part of the netCDF software distribution. Using **ncgen** for
97 this purpose is a two-step process:
98 
99 1. Convert text data to a file in [CDL form](http://www.unidata.ucar.edu/software/netcdf/docs/netcdf.html#CDL-Syntax) using a text
100  editor or text manipulation tools
101 2. Convert the CDL representation to netCDF using the **ncgen** tool with
102  the "-o" or "-b" option
103 
104 For more information, see the [ncgen documentation](http://www.unidata.ucar.edu/software/netcdf/docs/ncgen-man-1.html).
105 
106 If you have installed the NCAR Command Language
107 ([NCL](http://www.ncl.ucar.edu/)) software, there are functions
108 available and described
109 [here](http://www.ncl.ucar.edu/Applications/list_io.shtml) and
110 [here](http://www.ncl.ucar.edu/Applications/read_ascii.shtml) for
111 reading ASCII and tables into NCL and writing the data out to netCDF
112 files.
113 
114 With access to [MATLAB](http://www.mathworks.com/), you can create a
115 schema for the desired netCDF file using
116 [ncwriteschema](http://www.mathworks.com/help/techdoc/ref/ncwriteschema.html),
117 read the data using
118 [textscan](http://www.mathworks.com/help/techdoc/ref/textscan.html), and
119 write the data to a netCDF file using
120 [ncwrite](http://www.mathworks.com/help/techdoc/ref/ncwrite.html).
121 
122 What's new in the latest netCDF release?
123 
124 
125 [Release notes](http://www.unidata.ucar.edu/software/netcdf/release-notes-latest.html) for the
126 latest netCDF release are available that describe new features and fixed
127 bugs since the previous release.
128 
129 ----------
130 
131 What is the best way to represent [some particular data] using netCDF? {#What-is-the-best-way-to-represent-some-particular-data-using-netCDF}
132 -----------------
133 
134 There are many ways to represent the same information in any
135 general-purpose data model. Choices left up to the user in the case of
136 netCDF include which information to represent as variables or as
137 variable attributes; what names to choose for variables, dimensions, and
138 attributes; what order to use for the dimensions of multidimensional
139 variables; what variables to include in the same netCDF file; and how to
140 use variable attributes to capture the structure and meaning of data. We
141 provide some guidelines in the NetCDF User's Guide (e.g., the section on
142 [Differences between Attributes and Variables](http://www.unidata.ucar.edu/software/netcdf/docs/netcdf/Differences-between-Attributes-and-Variables.html#Differences%20between%20Attributes%20and%20Variables))
143 and in a new web document [Writing NetCDF Files: BestPractices](http://www.unidata.ucar.edu/software/netcdf/BestPractices.html), but we've found that
144 a little experience helps. Occasionally we have decided it was useful to
145 change the structure of netCDF files after experience with how the data
146 is used.
147 
148 ----------
149 
150 What convention should be used for the names of netCDF files? {#What-convention-should-be-used-for-the-names-of-netCDF-files}
151 -----------------
152 
153 
154 NetCDF files should have the file name extension ".nc". The recommended
155 extension for netCDF files was changed from ".cdf" to ".nc" in 1994 in
156 order to avoid a clash with the NASA CDF file extension, and now it also
157 avoids confusion with "Channel Definition Format" files.
158 
159 
160 
161 ----------
162 
163 Is there a mailing list for netCDF discussions and questions? {#Is-there-a-mailing-list-for-netCDF-discussions-and-questions}
164 -----------------
165 
166 The netcdfgroup@unidata.ucar.edu mailing-list is intended for
167 discussions and announcements about netCDF interfaces, software, and
168 use. The volume of this list varies widely, from one message per month
169 to a dozen messages per day (especially after a new release). A message
170 posted to this mailing-list will be seen by several hundred people, so
171 it's usually not appropriate for asking simple questions about use. Such
172 questions should instead be sent to support-netcdf@unidata.ucar.edu.
173 
174 If you would prefer to get only a single daily digest of the postings to
175 the netcdfgroup mailing-list, subscribe instead to the digest form of
176 the mailing-list, containing the same messages but appearing at most
177 once per day instead of whenever anyone sends a message to the group.
178 
179 To subscribe or unsubscribe to either of these mailing lists, use one of
180 these mailing list actions:
181 
182 * [subscribe: non-digest](mailto:netcdfgroup-join@unidata.ucar.edu) ]
183 * [subscribe: digest](mailto:netcdfgroup-request@unidata.ucar.edu?subject=subscribe%0A%20%20%20%20%20%20%20%20%20%20digest)
184 ]
185 * [change subscription options](http://mailman.unidata.ucar.edu/mailman/options/netcdfgroup)
186 * [view posts](/mailing_lists/archives/netcdfgroup/)
187 * [search archives](/search.jsp).
188 
189 ----------
190 
191 Where are some examples of netCDF datasets? {#Where-are-some-examples-of-netCDF-datasets}
192 -----------------
193 
194 Here are some [example netCDF files](http://www.unidata.ucar.edu/software/netcdf/examples/files.html).
195 
196 ----------
197 
198 What is the best way to handle time using netCDF? {#What-is-the-best-way-to-handle-time-using-netCDF}
199 -----------------
200 
201 
202 Discussions of conventions for representing time and handling
203 time-dependent data have been a past topic of discussion on the
204 netcdfgroup mailing list. When the subject comes up, interesting
205 discussions often result, so we've archived past discussions on this
206 subject at
207 [http://www.unidata.ucar.edu/software/netcdf/time/](http://www.unidata.ucar.edu/software/netcdf/time/).
208 
209 A summary of Unidata's recommendations is available from
210 [http://www.unidata.ucar.edu/software/netcdf/time/recs.html](http://www.unidata.ucar.edu/software/netcdf/time/recs.html).
211 Briefly, we recommend use of the units conventions supported by the
212 [udunits library](/software/udunits/) for time and other units
213 attributes.
214 
215 Other groups have established more specific conventions that include the
216 representation of time in netCDF files. For more information on such
217 conventions, see the NetCDF Conventions Page at
218 [http://www.unidata.ucar.edu/software/netcdf/conventions.html](http://www.unidata.ucar.edu/software/netcdf/conventions.html).
219 
220 ----------
221 
222 Who else uses netCDF? {#Who-else-uses-netCDF}
223 -----------------
224 
225 The netCDF mailing list has over 500 addresses (some of which are
226 aliases to more addresses) in thirty countries. Several groups have
227 [adopted netCDF as a standard](http://www.unidata.ucar.edu/software/netcdf/docs/standards.html) for
228 representing some forms of scientific data.
229 
230 A somewhat dated description of some of the projects and groups that
231 have used netCDF is available from
232 [http://www.unidata.ucar.edu/software/netcdf/usage.html](http://www.unidata.ucar.edu/software/netcdf/usage.html).
233 
234 ----------
235 
236 What are some references to netCDF? {#What-are-some-references-to-netCDF}
237 -----------------
238 
239 A primary reference is the User's Guide:
240 
241 Rew, R. K., G. P. Davis, S. Emmerson, and H. Davies, **NetCDF User's
242 Guide for C, An Interface for Data Access, Version 3**, April 1997.
243 
244 Current online and downloadable documentation is available from the
245 [documentation directory](http://www.unidata.ucar.edu/software/netcdf/docs/).
246 
247 Other references include:
248 
249 Brown, S. A, M. Folk, G. Goucher, and R. Rew, "Software for Portable
250 Scientific Data Management," Computers in Physics, American Institute of
251 Physics, Vol. 7, No. 3, May/June 1993, pp. 304-308.
252 
253 Fulker, D. W., "Unidata Strawman for Storing Earth-Referencing Data,"
254 Seventh International Conference on Interactive Information and
255 Processing Systems for Meteorology, Oceanography, and Hydrology, New
256 Orleans, La., American Meteorology Society, January 1991.
257 
258 Jenter, H. L. and R. P. Signell, 1992. "[NetCDF: A Freely-Available Software-Solution to Data-Access Problems for Numerical Modelers](http://www.unidata.ucar.edu/software/netcdf/papers/jenter_signell_92.pdf)". Proceedings
259 of the American Society of Civil Engineers Conference on Estuarine and
260 Coastal Modeling. Tampa, Florida.
261 
262 Kuehn, J.A., "Faster Libraries for Creating Network-Portable
263 Self-Describing Datasets", Proceedings of the 37th Cray User Group
264 Meeting, (Barcelona, Spain, March 1996), Cray User Group, Inc.
265 
266 Rew, R. K. and G. P. Davis, "NetCDF: An Interface for Scientific Data
267 Access," IEEE Computer Graphics and Applications, Vol. 10, No. 4, pp.
268 76-82, July 1990.
269 
270 Rew, R. K. and G. P. Davis, "The Unidata netCDF: Software for Scientific
271 Data Access," Sixth International Conference on Interactive Information
272 and Processing Systems for Meteorology, Oceanography, and Hydrology,
273 Anaheim, California, American Meteorology Society, pp. 33-40, February
274 1990.
275 
276 Rew, R. K. and G. P. Davis, " [Unidata's netCDF Interface for Data Access: Status and Plans](/netcdf/ams97.html)," Thirteenth International Conference on Interactive Information and Processing Systems for Meteorology, Oceanography, and Hydrology, Anaheim, California, American Meteorology Society, February 1997.
277 
278 ----------
279 
280 I'm submitting a paper for publication and want to include a citation for use of netCDF software. What reference should I use? {#How-should-I-cite-use-of-netCDF-software}
281 -----------------
282 
283 The registered Digital Object Identifier for all versions of netCDF software is `http://doi.org/10.5065/D6H70CW6`.
284 
285 The following can be used as a citation:
286 
287 Unidata, (_year_): Network Common Data Form (netCDF) version _nc_version_ [software]. Boulder, CO: UCAR/Unidata. (http://doi.org/10.5065/D6H70CW6)
288 
289 where _year_ is the year in which the work being described was done and _nc_version_ is the version of netCDF used. For example:
290 
291 Unidata, (2015): Network Common Data Form (netCDF) version 4.3.3.1 [software]. Boulder, CO: UCAR/Unidata. (http://doi.org/10.5065/D6H70CW6)
292 
293 ----------
294 
295 Is there a document describing the actual physical format for a Unidata netCDF file? {#Is-there-a-document-describing-the-actual-physical-format-for-a-Unidata-netCDF-file}
296 -----------------
297 
298 A short document that specifies the [format of netCDF classic and 64-bit offset files](http://earthdata.nasa.gov/sites/default/files/esdswg/spg/rfc/esds-rfc-011/ESDS-RFC-011v2.00.pdf) has been approved as a standard by the NASA ESDS Software Process Group.
299 
300 In addition, the NetCDF User's Guide contains an
301 [appendix](http://www.unidata.ucar.edu/software/netcdf/docs/netcdf.html#File-Format) with the same format specification.
302 
303 The ["NetCDF File Structure and Performance"](http://www.unidata.ucar.edu/software/netcdf/docs/netcdf.html#Structure) chapter provides a less formal explanation of the format of netCDF data to help clarify the performance implications of different data organizations.
304 
305 If users only access netCDF data through the documented interfaces, future changes to the format will be transparent.
306 
307 ----------
308 
309 Installation and Porting {#Installation-and-Porting}
310 ================
311 
312 What does netCDF run on? {#What-does-netCDF-run-on}
313 -----------------
314 
315 We test releases on the following operating systems with various compilers:
316 
317 - AIX
318 - HPUX
319 - IRIX, IRIX64
320 - Linux
321 - MacOS X
322 - Solaris
323 - Windows (some versions, see below)
324 
325 The [NetCDF Installation and Porting Guide](@ref getting_and_building_netcdf) explains how to build netCDF from source on various platforms. Often, it's as easy as running
326 
327 ~~~~ {.boldcode}
328  ./configure
329  make check install
330 ~~~~
331 
332 ----------
333 
334 
335 How can I use current versions of netCDF-4 with Windows? {#HowcanIusecu}
336 ------------------
337 
338 
339 See [http://www.unidata.ucar.edu/software/netcdf/docs/winbin.html](http://www.unidata.ucar.edu/software/netcdf/win_netcdf).
340 
341 How can I use netCDF-4.1 with Windows? {#HowcanIusenetCDF41withWindows}
342 -----------------
343 
344 
345 We recently (Summer of 2010) refactored the core building of the netCDF
346 library. Unfortunately this hopelessly broke the existing port to
347 Microsoft Visual Studio. Resources permitting, the development of a new
348 Visual Studio port will be undertaken in the second half of 2010 at
349 Unidata. Until then, no Visual Studio port of the latest version of the
350 library is available.
351 
352 Users are advised that the netCDF build is known to work with Cygwin,
353 the free POSIX layer for Windows. Building netCDF with Cygwin, and
354 including the netCDF, HDF5, zlib, and Cygwin DLLs, will allow you to
355 access the netCDF C library on Windows, even from Visual Studio builds.
356 
357 We understand that Windows users are most comfortable with a Visual
358 Studio build, and we intend to provide one.
359 
360 The Visual Studio port is complicated by the following factors:
361 
362 - No configure script support on windows - the Unix build system uses
363  a configure script to determine details of the build platform and
364  allow the user to specify settings. Windows has no mechanism for
365  this other than statically set properties. A Windows-only config.h
366  file needs to be created for windows using Cygwin, then included
367  with the distribution. Since this contains the version string, it
368  must be updated "by hand" before each release.
369 - No m4 on windows - the Unix build uses the macro language m4 to
370  generate some of the C code in the netCDF library (for example,
371  libsrc/putget.c). M4 must be run under Cygwin to generate these
372  files, and then they must be statically added to the windows
373  distribution. Each new version of netCDF these files should be
374  checked for changes. We are restricting new use of m4 for netCDF
375  compiles, but that doesn't help with the existing files.
376 - No user options on Windows - since Windows does not support a
377  configure step, all user options must be pre-set in the Visual
378  Studio property lists. As a simplification, many options available
379  to Unix users will be unavailable to builders on Windows, such as
380  --disable-dap, --disable-netcdf-4, and --disable-shared.
381 - Large files (> 2 GB) have proved to be a problem area in past
382  Windows builds.
383 - Previous Windows ports have not had to deal with the new OPeNDAP
384  client.
385 
386 Unidata is a community supported organization, and we welcome
387 collaboration with users who would like to assist with the windows port.
388 Users should be sure to start with the netCDF daily snapshot, not a
389 previous release of netCDF.
390 
391 NOTE: [Paratools](http://www.paratools.com/) has contributed
392 [instructions for how to build netCDF-4.1.3](http://www.paratools.com/Azure/NetCDF) as a Windows DLL using the MinGW cross compiler.
393 
394 Nikolay Khabarov has contributed [documentation describing a netCDF-4.1.3 port](http://user.iiasa.ac.at/~khabarov/netcdf-win64-and-win32-mingw/) using MinGW to build native Windows 64-bit and 32-bit DLLs. Current limitations include leaving out support for Fortran and C++ interfaces, NetCDF-4, HDF5, the old version 2 API, and DAP access. The netCDF classic format and 64-bit offset format are fully supported. Links are provided to compiled 32-bit and 64-bit DLLs and static libraries.
395 
396 A developer on the GMT Wiki has posted [detailed instructions for using CMake](http://gmtrac.soest.hawaii.edu/projects/gmt/wiki/BuildingNetCDF) and MS Visual C++ on Windows to build netCDF-4.1.3, including OPeNDAP support.
397 
398 Another developer has contributed an unsupported native Windows build of
399 netCDF-4.1.3 with 32- and 64-bit versions, Fortran bindings, and OPeNDAP
400 support. The announcement of the availability of that port is
401 [here](http://www.unidata.ucar.edu/mailing_lists/archives/netcdfgroup/2011/msg00363.html).
402 
403 User Veit Eitner has contributed a port of 4.1.1 to Visual Studio,
404 including an F90 port to Intel Fortran. Download [source (ftp://ftp.unidata.ucar.edu/pub/netcdf/contrib/win32/netcdf-4.1.1-win32-src.zip)](ftp://ftp.unidata.ucar.edu/pub/netcdf/contrib/win32/netcdf-4.1.1-win32-src.zip) or [binary](ftp://ftp.unidata.ucar.edu/pub/netcdf/contrib/win32/netcdf-4.1.1-win32-bin.zip) versions. This port was done before the code was refactored in 4.1.2.
405 
406 How can I use netCDF-4 with Windows? {#How-can-I-use-netCDF-4-with-Windows}
407 -----------------
408 
409 
410 Note that we have not ported the F90 or C++ APIs to the Windows
411 platform, only the C and F77 APIs. User contributions of ports to F90
412 windows compilers are very welcome (send them to
413 support-netcdf@unidata.ucar.edu).
414 
415 On windows, NetCDF consists of a DLL and the ncgen/ncdump executables.
416 The easiest course is to download one of the pre-built DLLs and
417 utilities and just install them on your system.
418 
419 Unlike Unix builds, the Visual Studio build **always** requires HDF5,
420 zlib, and szlib in all cases. All Windows DLL users must also have the
421 HDF5, zlib, and szlib DLLs. These are now available from the Unidata FTP
422 site:
423 
424 - [zlib DLLs for 32-bit Windows](ftp://ftp.unidata.ucar.edu/pub/netcdf/contrib/win32/zlib123-vs2005.zip)
425 - [szlib DLLs for 32-bit Windows](ftp://ftp.unidata.ucar.edu/pub/netcdf/contrib/win32/szip21-vs6-enc.zip)
426 - [HDF5 DLLs for 32-bit Windows](ftp://ftp.unidata.ucar.edu/pub/netcdf/contrib/win32/5-181-win-vs2005.zip)
427 
428 Two versions of the netCDF DLLs are available, for different Fortran
429 compilers:
430 
431 - [NetCDF for Intel and Portland Group Fortran compilers.](ftp://ftp.unidata.ucar.edu/pub/netcdf/contrib/win32/win32_vs_PGI_dll_4.0.1.zip)
432 - [NetCDF for other Fortran compilers.](ftp://ftp.unidata.ucar.edu/pub/netcdf/contrib/win32/win32_vs_f2c_dll_4.0.1.zip)
433 
434 To use netCDF, install the DLLs in /system/win32 and the .h files in a
435 directory known to your compiler, and define the DLL\_NETCDF
436 preprocessor macro before including netcdf.h.
437 
438 The netCDF-4 library can also be built using Visual Studio 2008. Open
439 the solution file win32/NET/netcdf.sln.
440 
441 If you install the header files in \\include directory, the netCDF
442 solution file will work without modifications. Otherwise the properties
443 of the netcdf project must be changed to include the proper header
444 directory.
445 
446 Both the debug and release builds work. The release build links to
447 different system libraries on Windows, and will not allow debuggers to
448 step into netCDF library code. This is the build most users will be
449 interested in. The debug build is probably of interest only to netCDF
450 library developers.
451 
452 As of version 4.0.1 (March 2009), the DLL build does not yet include any
453 testing of the extended netCDF-4 data model. The netCDF4/HDF5 format is
454 extensively tested in the classic model, but tests for groups,
455 user-defined types, and other features of the expanded netCDF-4 data
456 model have not yet been ported to Windows.
457 
458 The [NetCDF Installation and Porting Guide](http://www.unidata.ucar.edu/software/netcdf/docs/netcdf-install/index.html) documents how to
459 use netCDF with Windows.
460 
461 Some users have built and released netCDF with Intel Fortran on Windows.
462 See the [ifort entry in other builds document](http://www.unidata.ucar.edu/software/netcdf/docs/other-builds.html#ifort-361-windows).
463 
464 Windows is a complicated platform to build on. Some useful explanations
465 of the oddities of Windows can be found here:
466 
467 - Cygwin documentation for [Building and Using DLLs](http://cygwin.com/cygwin-ug-net/dll.html)
468 - [OpenLDAP FAQ answer: MinGW Support in Cygwin](http://www.openldap.org/faq/data/cache/301.html), by Jon
469  Leichter.
470 - [cygwin mailing list explanation of Windows DL requirements.](http://cygwin.com/ml/cygwin/2000-06/msg00688.html)
471 - [-mno-cygwin - Building Mingw executables using Cygwin](http://www.delorie.com/howto/cygwin/mno-cygwin-howto.html)
472 
473 Once you have the netCDF DLL, you may wish to call it from Visual Basic.
474 The [netCDF VB wrapper](ftp://ftp.unidata.ucar.edu/pub/netcdf/contrib/win32/netcdf_vb_net_wrapper.zip) will help you do this.
475 
476 The SDS ([Scientific DataSet](http://research.microsoft.com/en-us/projects/sds/)) library and tools provide .Net developers a way to read, write and share scalars, vectors, and multidimensional grids using CSV, netCDF, and other file formats. It currently uses netCDF version 4.0.1. In addition to .Net libraries, SDS provides a set of utilities and packages: an sds command line utility, a DataSet Viewer application and an add-in for Microsoft Excel 2007 (and later versions).
477 
478 ----------
479 
480 How do I build and install netCDF for a specific development environment? {#How-do-I-build-and-install-netCDF-for-a-specific-development-environment}
481 -----------------
482 
483 You have to build and install the netCDF C library first, before you build and install other language libraries that depend on it, such as Fortran, C++, or Python netCDF libraries. The netCDF Java library is mostly independent of the netCDF C library, unless you need to write netCDF-4 files from Java, in which case you will also need an installed netCDF C library.
484 
485 For more details, see
486 [NetCDF Installation and Porting Guide](@ref getting_and_building_netcdf).
487 
488 
489 ----------
490 
491 How can I tell if I successfully built and installed netCDF? {#How-can-I-tell-if-I-successfully-built-and-installed-netCDF}
492 -----------------
493 
494 
495 We make build output from various platforms [available](../builds) for
496 comparison with your output. In general, you can ignore compiler
497 warnings if the "make test" step is successful. Lines that begin with
498 "\*\*\*" in the "make test" output indicate results from tests. The C
499 and Fortran-77 interfaces are tested extensively, but only rudimentary
500 tests are currently used for the C++ and Fortran-90 interfaces.
501 
502 How can I tell what version I'm using? {#How-can-I-tell-what-version-Im-using}
503 -----------------
504 
505 
506 If you invoke
507 
508 ~~~~ {.boldcode}
509  ncdump --version
510 ~~~~
511 
512 the last line of the resulting output will identify the version
513 associated with the **ncdump** utility. You can also call one of the
514 functions `nc_inq_libvers()`, `nf_inq_libvers()`, or
515 `nf90_inq_libvers()` from C, Fortran-77, or Fortran-90 programs to get a
516 version string.
517 
518 ----------
519 
520 Where does netCDF get installed? {#Where-does-netCDF-get-installed}
521 -----------------
522 
523 
524 The netCDF installation directory can be set at the time configure is
525 run using the --prefix argument. If it is not specified, /usr/local is
526 used as the default prefix.
527 
528 For more information see the [NetCDF Installation and Porting Guide](http://www.unidata.ucar.edu/software/netcdf/docs/netcdf-install).
529 
530 Formats, Data Models, and Software Releases {#formatsdatamodelssoftwarereleases}
531 ===========================================
532 
533 In different contexts, "netCDF" may refer to a data model, a software
534 implementation with associated application program interfaces (APIs), or
535 a data format. Confusion may arise in discussions of different versions
536 of the data models, software, and formats. For example, compatibility
537 commitments require that new versions of the software support all
538 previous versions of the format and data model. This section of FAQs is
539 intended to clarify netCDF versions and help users determine what
540 version to build and install.
541 
542 How many netCDF formats are there, and what are the differences among them? {#How-many-netCDF-formats-are-there-and-what-are-the-differences-among-them}
543 -----------------
544 
545 
546 There are four netCDF format variants:
547 
548 - the classic format
549 - the 64-bit offset format
550 - the 64-bit data format
551 - the netCDF-4 format
552 - the netCDF-4 classic model format
553 
554 (In addition, there are two textual representations for netCDF data,
555 though these are not usually thought of as formats: CDL and NcML.)
556 
557 The **classic format** was the only format for netCDF data created
558 between 1989 and 2004 by the reference software from Unidata. It is
559 still the default format for new netCDF data files, and the form in
560 which most netCDF data is stored. This format is also referred as CDF-1 format.
561 
562 In 2004, the **64-bit offset format** variant was added. Nearly
563 identical to netCDF classic format, it allows users to create and access
564 far larger datasets than were possible with the original format. (A
565 64-bit platform is not required to write or read 64-bit offset netCDF
566 files.) This format is also referred as CDF-2 format.
567 
568 In 2008, the **netCDF-4 format** was added to support per-variable
569 compression, multiple unlimited dimensions, more complex data types, and
570 better performance, by layering an enhanced netCDF access interface on
571 top of the HDF5 format.
572 
573 At the same time, a fourth format variant, **netCDF-4 classic model
574 format**, was added for users who needed the performance benefits of the
575 new format (such as compression) without the complexity of a new
576 programming interface or enhanced data model.
577 
578 In 2016, the **64-bit data format** variant was added. To support large
579 variables with more than 4-billion array elements, it replaces most of the
580 32-bit integers used in the format specification with 64-bit integers. It also
581 adds support for several new data types including unsigned byte, unsigned
582 short, unsigned int, signed 64-bit int and unsigned 64-bit int. A 64-bit
583 platform is required to write or read 64-bit data netCDF files. This format is
584 also referred as CDF-5 format.
585 
586 With each additional format variant, the C-based reference software from
587 Unidata has continued to support access to data stored in previous
588 formats transparently, and to also support programs written using
589 previous programming interfaces.
590 
591 Although strictly speaking, there is no single "netCDF-3 format", that
592 phrase is sometimes used instead of the more cumbersome but correct
593 "netCDF classic CDF-1, 64-bit offset CDF-2, or 64-bit data CDF-5 format" to
594 describe files created by the netCDF-3 (or netCDF-1 or netCDF-2) libraries.
595 Similarly "netCDF-4 format" is sometimes used informally to mean "either the
596 general netCDF-4 format or the restricted netCDF-4 classic model format". We
597 will use these shorter phrases in FAQs below when no confusion is likely.
598 
599 A more extensive description of the netCDF formats and a formal specification
600 of the classic and 64-bit formats is available as a [NASA ESDS community
601 standard](https://earthdata.nasa.gov/sites/default/files/esdswg/spg/rfc/esds-rfc-011/ESDS-RFC-011v2.00.pdf).
602 
603 The 64-bit data CDF-5 format specification is available in
604 http://cucis.ece.northwestern.edu/projects/PnetCDF/CDF-5.html.
605 
606 How can I tell which format a netCDF file uses? {#How-can-I-tell-which-format-a-netCDF-file-uses}
607 -----------------
608 
609 
610 The short answer is that under most circumstances, you should not care,
611 if you use version 4.0 or later of the netCDF library to access data in
612 the file. But the difference is indicated in the first four bytes of the
613 file, which are 'C', 'D', 'F', '\\001' for the classic netCDF CDF-1 format;
614 'C', 'D', 'F', '\\002' for the 64-bit offset CDF-2 format;
615 'C', 'D', 'F', '\\005' for the 64-bit data CDF-5 format; or '\\211', 'H',
616 'D', 'F' for an HDF5 file, which could be either a netCDF-4 file or a
617 netCDF-4 classic model file. (HDF5 files may also begin with a
618 user-block of 512, 1024, 2048, ... bytes before what is actually an
619 8-byte signature beginning with the 4 bytes above.)
620 
621 With netCDF version 4.0 or later, there is an easy way that will
622 distinguish between netCDF-4 and netCDF-4 classic model files, using the
623 "-k" option to **ncdump** to determine the kind of file, for example:
624 
625 ~~~~~ {.boldcode}
626  ncdump -k foo.nc
627  classic
628 ~~~~~
629 
630 
631 In a program, you can call the function
632 [nc_inq_format](http://www.unidata.ucar.edu/software/netcdf/docs/netcdf-c.html#nc_005finq-Family)(or [nf90_inq_format](http://www.unidata.ucar.edu/software/netcdf/docs/netcdf-f90.html#Compiling-and-Linking-with-the-NetCDF-Library) for the Fortran-90 interface) to determine the format variant of an open netCDF file.
633 
634 Finally, on a Unix system, one way to display the first four bytes of a
635 file, say foo.nc, is to run the following command:
636 
637 ~~~~ {.boldcode}
638  od -An -c -N4 foo.nc
639 ~~~~
640 
641 which will output
642 
643 ~~~~ {.boldcode}
644  C D F 001
645 ~~~~
646 
647 ~~~~ {.boldcode}
648  C D F 002
649 ~~~~
650 
651 ~~~~ {.boldcode}
652  C D F 005
653 ~~~~
654 
655 ~~~~ {.boldcode}
656  211 H D F
657 ~~~~
658 
659 depending on whether foo.nc is a classic CDF-1, 64-bit offset CDF-2, 64-bit
660 data CDF-5, or netCDF-4 file, respectively. This method cannot be used to
661 distinguish between netCDF-4 and netCDF-4 classic model variants, or between a
662 netCDF-4 file and a different kind of HDF5 file.
663 
664 ----------
665 
666 How many netCDF data models are there? {#How-many-netCDF-data-models-are-there}
667 -----------------
668 
669 There are only two netCDF data models, the [classic
670 model](/netcdf/workshops/2008/datamodel/NcClassicModel.html) and the [enhanced
671 model](/netcdf/workshops/2008/netcdf4/Nc4DataModel.html) (also called the
672 netCDF-4 data model). The classic model is the simpler of the two, and is used
673 for all data stored in classic CDF-1 format, 64-bit offset CDF-2 format, 64-bit
674 data CDF-5 format, or netCDF-4 classic model format. The enhanced model
675 (sometimes also referred to as the netCDF-4 data model) is an extension of the
676 classic model that adds more powerful forms of data representation and data
677 types at the expense of some additional complexity. Although data represented
678 with the classic model can also be represented using the enhanced model,
679 datasets that use enhanced model features, such as user-defined data types,
680 cannot be represented with the classic model. Use of the enhanced model
681 requires storage in the netCDF-4 format.
682 
683 How many releases of the C-based netCDF software are supported? {#How-many-releases-of-the-C-based-netCDF-software-are-supported}
684 -----------------
685 
686 
687 When netCDF version 4.0 was released in June 2008, version 3.6.3 was
688 released simultaneously, and both releases were supported by Unidata.
689 Version 3.6.3 supported only the classic and 64-bit offset formats.
690 Version 4.0 supported both of those format variants by default, and also
691 the netCDF-4 and netCDF-4 classic model formats, if built using a
692 previously installed HDF5 library and using the "--enable-netcdf-4"
693 configure option. Software built from the netCDF-4.0 release without
694 specifying "--enable-netcdf-4" (the default) was identical to software
695 built with netCDF-3.6.3. Starting from version 4.4.0, netCDF added support
696 for CDF-5 format.
697 
698 Both netCDF-3 and netCDF-4 C libraries are part of a single software
699 release. The netCDF software may be built to support just the classic
700 CDF-1 and 64-bit offset CDF-2 formats (the default), 64-bit data CDF-5 format,
701 or to also support the netCDF-4 and netCDF-4 classic model formats, if the
702 HDF5-1.8.x library is installed. Unidata no longer supports a separate
703 netCDF-3-only version of the software, but instead supports both the classic
704 and enhanced data models and all four format variants in a single source
705 distribution.
706 
707 This does not indicate any plan to drop support for netCDF-3 or the
708 formats associated with netCDF-3. Support for earlier formats and APIs
709 will continue with all future versions of netCDF software from Unidata.
710 
711 Should I get netCDF-3 or netCDF-4? {#Should-I-get-netCDF-3-or-netCDF-4}
712 -----------------
713 
714 By downloading a current version of netCDF-4, you have the choice to
715 build either
716 
717 - the default netCDF-3 libraries, which support classic CDF-1, 2, and 5
718  formats, and the classic data model; or
719 - the netCDF-4 libraries, which support netCDF-4 and netCDF-4 classic
720  model formats, as well as classic formats, and the
721  enhanced data model.
722 
723 Which version to build depends on how you will use the software.
724 
725 Installing the simpler netCDF-3 version of the software is recommended
726 if the following situations apply:
727 
728 - all the data you need to access is available in netCDF classic
729  formats
730 - you are installing netCDF in order to support another software
731  package that uses only netCDF-3 features
732 - you plan to only write data in a form that netCDF-3 software and
733  applications can access
734 - you want to delay upgrading to support netCDF-4 until netCDF-4
735  formats are more widely used
736 - you cannot install the prerequisite HDF5 1.8 software required to
737  build and install netCDF-4
738 
739 Installing the netCDF-4 version of the software is required for any of
740 the following situations:
741 
742 - you need to access netCDF data that makes use of netCDF-4
743  compression or chunking
744 - you need to access data in all netCDF formats including netCDF-4 or
745  netCDF-4 classic model formats
746 - you need to write non-record variables larger than 4GiB or record variables with more than 4GiB per record (see ["Have all netCDF size limits been eliminated?"](http://www.unidata.ucar.edu/software/netcdf/docs/faq.html#Large%20File%20Support10))
747 - you are installing netCDF to support other software packages that
748  require netCDF-4 features
749 - you want to write data that takes advantage of compression,
750  chunking, or other netCDF-4 features
751 - you want to be able to read netCDF-4 classic model data with no
752  changes to your current software except relinking with the new
753  library
754 - you want to benchmark your current applications with the new
755  libraries to determine whether the benefits are significant enough
756  to justify the upgrade
757 - you need to use parallel I/O with netCDF-4 or netCDF-4 classic files
758 
759 What is the "enhanced data model" of netCDF-4, and how does it differ from the netCDF-3 classic data model? {#whatisenhanceddatamodel}
760 -------------
761 
762 
763 The enhanced model (sometimes referred to as the netCDF-4 data model) is
764 an extension to the [classic model](/netcdf/workshops/2008/datamodel/NcClassicModel.html) that adds more powerful forms of data representation and data types at the expense of some additional complexity. Specifically, it adds six new primitive data types, four kinds of user-defined data types, multiple unlimited
765 dimensions, and groups to organize data hierarchically and provide
766 scopes for names. A [picture](/netcdf/workshops/2008/netcdf4/Nc4DataModel.html) of the enhanced data model, with the extensions to the classic model
767 highlighted in red, is available from the online netCDF workshop.
768 
769 Although data represented with the classic model can also be represented
770 using the enhanced model, datasets that use features of the enhanced
771 model, such as user-defined data types, cannot be represented with the
772 classic model. Use of added features of the enhanced model requires that
773 data be stored in the netCDF-4 format.
774 
775 Why doesn't the new netCDF-4 installation I built seem to support any of the new features? {#Whydoesnt-the-new-netCDF-4-installation-I-built-seem-to-support-any-of-the-new-features}
776 -----------------
777 
778 
779 If you built the software from source without access to an HDF5 library,
780 then only the netCDF-3 library was built and installed. The current
781 release will build full netCDF-4 support if the HDF5 1.8.x library is
782 already installed where it can be found by the configure script or
783 cmake.
784 
785 Will Unidata continue to support netCDF-3? {#Will-Unidata-continue-to-support-netCDF-3}
786 -----------------
787 
788 
789 Yes, Unidata has a commitment to preserving backward compatibility.
790 
791 Because preserving access to archived data for future generations is
792 very important:
793 
794 - New netCDF software will provide read and write access to *all*
795  earlier forms of netCDF data.
796 - C and Fortran programs using documented netCDF APIs from previous
797  releases will be supported by new netCDF software (after recompiling
798  and relinking, if needed).
799 - Future releases of netCDF software will continue to support data
800  access and API compatibility.
801 
802 To read compressed data, what changes do I need to make to my netCDF-3 program? {#To-read-compressed-data-what-changes-do-I-need-to-make-to-my-netCDF-3-program}
803 -----------------
804 
805 
806 None. No changes to the program source are needed, because the library
807 handles decompressing data as it is accessed. All you need to do is
808 relink your netCDF-3 program to the netCDF-4 library to recognize and
809 handle compressed data.
810 
811 To write compressed data, what changes do I need to make to my netCDF-3 program? {#To-write-compressed-data-what-changes-do-I-need-to-make-to-my-netCDF-3-program}
812 -----------------
813 
814 
815 The **nccopy** utility in versions 4.1.2 and later supports a "-d *level*"
816 deflate option that copies a netCDF file, compressing all variables
817 using the specified level of deflation and default chunking parameters,
818 or you can specify chunking with the "-c" option.
819 
820 To do this within a program, or if you want different variables to have
821 different levels of deflation, define compression properties when each
822 variable is defined. The function to call is
823 [nc_def_var_deflate](/netcdf-c.html#nc_005fdef_005fvar_005fdeflate)
824 for C programs, [nf90_def_var_deflate](http://www.unidata.ucar.edu/software/netcdf/docs/netcdf-f90.html#NF90_005fDEF_005fVAR_005fDEFLATE) for Fortran 90 programs, [NF_DEF_VAR_DEFLATE](http://www.unidata.ucar.edu/software/netcdf/docs/netcdf-f77.html#NF_005fDEF_005fVAR_005fDEFLATE) for Fortran 77. For C++ programs, the experimental cxx4 API may be used,
825 assuming you have configured with --enable-cxx-4.
826 
827 Although default variable chunking parameters may be adequate,
828 compression can sometimes be improved by choosing good chunking
829 parameters when a variable is first defined. For example, if a 3D field
830 tends to vary a lot with vertical level, but not so much within a
831 horizontal slice corresponding to a single level, then defining chunks
832 to be all or part of a horizontal slice would typically produce better
833 compression than chunks that included multiple horizontal slices. There
834 are other factors in choosing chunk sizes, especially matching how the
835 data will be accessed most frequently. Chunking properties may only be
836 specified when a variable is first defined. The function to call is
837 [nc_def_var_chunking](http://www.unidata.ucar.edu/software/netcdf/docs/netcdf-c.html#nc_005fdef_005fvar_005f)
838 for C programs,
839 [nf90_def_var_chunking](http://www.unidata.ucar.edu/software/netcdf/docs/netcdf-f90.html#NF90_005fDEF_005fVAR_005fCHUNKING)
840 for Fortran 90 programs, and
841 [NF_DEF_VAR_CHUNKING](http://www.unidata.ucar.edu/software/netcdf/docs/netcdf-f77.html#NF_005fDEF_005fVAR_005fCHUNKING)
842 for Fortran 77 programs. For C++ programs, the experimental cxx4 API may
843 be used, assuming you have configured with --enable-cxx-4.
844 
845 If I create netCDF-4 classic model files, can they be read by IDL, MATLAB, R, Python and ArcGIS? {#If-I-create-netCDF-4-classic-model-files-can-they-be-read-by-IDL-MATLAB-R-Python-and-ArcGIS}
846 -----------------
847 
848 
849 IDL 8.0 ships with support for netCDF-4, including support for OPeNDAP
850 remote access.
851 
852 MATLAB 2012a includes netCDF 4 support with OPeNDAP support turned on,
853 enabling remote access to many kinds of data, as well as use of groups,
854 compression, and chunking. An example is available demonstrating some of
855 the new functions. [NCTOOLBOX](http://nctoolbox.github.io/nctoolbox/),
856 uses netCDF-Java to provide read access to datasets in netCDF-4, GRIB,
857 GRIB2 and other formats through Unidata's Common Data Model.
858 
859 R has the [ncdf4 package](http://cirrus.ucsd.edu/~pierce/ncdf/).
860 
861 Python has the [netcdf4-python package](http://code.google.com/p/netcdf4-python/).
862 
863 ArcGIS 10.0 can read netcdf4 using the Multidimensional Tools in
864 ArcToolbox, and in ArcGIS 10.1, the [Multidimensional Supplemental toolbox](http://esriurl.com/MultidimensionSupplementalTools) uses NetCDF4-Python to read OPeNDAP and netCDF4 files, taking advantage of CF conventions if they exist.
865 
866 What applications are able to deal with *arbitrary* netCDF-4 files? {#What-applications-are-able-to-deal-with-arbitrary-netCDF-4-files}
867 -----------------
868 
869 The netCDF utilities **ncdump**, **ncgen**, and **nccopy**, available in
870 the Unidata C-based netCDF-4 distribution, are able to deal with
871 arbitrary netCDF-4 files (as well as all other kinds of netCDF files).
872 
873 How can I convert netCDF-3 files into netCDF-4 files? {#How-can-I-convert-netCDF-3-files-into-netCDF-4-files}
874 -----------------
875 
876 
877 Every netCDF-3 file can be read or written by a netCDF version 4 library, so in
878 that respect netCDF-3 files are already netCDF-4 files and need no conversion.
879 But if you want to convert a classic format file (CDF-1, 2, or 5) into a
880 netCDF-4 format or netCDF-4 classic model format file, the easiest way is to
881 use the **nccopy** utility. For example to convert a classic format file
882 foo3.nc to a netCDF-4 format file foo4.nc, use:
883 
884 ~~~~~~~~~~~~~~~~~~~~~~~~~ {.boldcode}
885  nccopy -k netCDF-4 foo3.nc foo4.nc
886 ~~~~~~~~~~~~~~~~~~~~~~~~~
887 
888 To convert a classic format file foo3.nc to a netCDF-4 classic
889 model format file foo4c.nc, you could use:
890 
891 ~~~~~~~~~~~~~~~~~~~~~~~~~~ {.boldcode}
892  nccopy -k netCDF-4-classic foo3.nc foo4c.nc
893 ~~~~~~~~~~~~~~~~~~~~~~~~~~
894 
895 If you have installed [NCO](http://www.unidata.ucar.edu/software/netcdf/software.html#NCO), the NCO
896 utility "ncks" can be used to accomplish the same task, as follows:
897 
898 ~~~~~~~~~~~~~~~~~~~~~~~~ {.boldcode}
899  ncks -7 foo3.nc foo4c.nc
900 ~~~~~~~~~~~~~~~~~~~~~~~~
901 
902 Another method is available for relatively small files, using the **ncdump**
903 and **ncgen** utilities (built with a netCDF-4 library). Assuming "small3.nc"
904 is a small classic format netCDF file, you can create an equivalent netCDF-4
905 file named "small4.nc" as follows:
906 
907 ~~~~ {.boldcode}
908  ncdump small3.nc > small.cdl
909  ncgen -o small4.nc -k netCDF-4-classic small.cdl
910 ~~~~
911 
912 Why might someone want to convert netCDF-4 files into netCDF-3 files? {#Why-might-someone-want-to-convert-netCDF-4-files-into-netCDF-3-files}
913 -----------------
914 
915 
916 NetCDF-4 classic model files that use compression can be smaller than
917 the equivalent netCDF-3 files, so downloads are quicker. If they are
918 then unpacked and converted to the equivalent netCDF-3 files, they can
919 be accessed by applications that haven't yet upgraded to netCDF-4.
920 
921 How can I convert netCDF-4 files into netCDF-3 files? {#How-can-I-convert-netCDF-4-files-into-netCDF-3-files}
922 -----------------
923 
924 
925 In general, you can't, because netCDF-4 files may have features of the
926 netCDF enhanced data model, such as groups, compound types,
927 variable-length types, or multiple unlimited dimensions, for which no
928 netCDF-3 representation is available. However, if you know that a
929 netCDF-4 file conforms to the classic model, either because it was
930 written as a netCDF-4 classic model file, because the program that wrote
931 it was a netCDF-3 program that was merely relinked to a netCDF-4
932 library, or because no features of the enhanced model were used in
933 writing the file, then there are several ways to convert it to a
934 netCDF-3 file.
935 
936 You can use the **nccopy** utility. For
937 example to convert a netCDF-4 classic-model format file foo4c.nc to a
938 classic format file foo3.nc, use:
939 
940 ~~~~~~~~~~~~~~~~~~~~~~~~~ {.boldcode}
941  nccopy -k classic foo4c.nc foo3.nc
942 ~~~~~~~~~~~~~~~~~~~~~~~~~
943 
944 If you have installed [NCO](http://www.unidata.ucar.edu/software/netcdf/docs/software.html#NCO), the NCO utility "ncks" can be used to accomplish the same task, as follows:
945 
946 ~~~~~~~~~~~~~~~~~~~~~~~~~ {.boldcode}
947  ncks -3 foo4c.nc foo3.nc
948 ~~~~~~~~~~~~~~~~~~~~~~~~~
949 
950 For a relatively small netCDF-4 classic model file, "small4c.nc" for
951 example, you can also use the **ncdump** and **ncgen** utilities to create an
952 equivalent netCDF-3 classic format file named "small3.nc" as follows:
953 
954 ~~~~ {.boldcode}
955  ncdump small4c.nc > small4.cdl
956  ncgen -o small3.nc small4.cdl
957 ~~~~
958 
959 How can I convert HDF5 files into netCDF-4 files? {#How-can-I-convert-HDF5-files-into-netCDF-4-files}
960 -----------------
961 
962 
963 NetCDF-4 intentionally supports a simpler data model than HDF5, which
964 means there are HDF5 files that cannot be converted to netCDF-4,
965 including files that make use of features in the following list:
966 
967 - Multidimensional data that doesn't use shared dimensions implemented
968  using HDF5 "dimension scales". (This restriction was eliminated in
969  netCDF 4.1.1, permitting access to HDF5 datasets that don't use
970  dimension scales.)
971 - Non-hierarchical organizations of Groups, in which a Group may have
972  multiple parents or may be both an ancestor and a descendant of
973  another Group, creating cycles in the subgroup graph. In the
974  netCDF-4 data model, Groups form a tree with no cycles, so each
975  Group (except the top-level unnamed Group) has a unique parent.
976 - HDF5 "references" which are like pointers to objects and data
977  regions within a file. The netCDF-4 data model does not support
978  references.
979 - Additional primitive types not included in the netCDF-4 data model,
980  including H5T\_TIME, H5T\_BITFIELD, and user-defined atomic types.
981 - Multiple names for data objects such as variables and groups. The
982  netCDF-4 data model requires that each variable and group have a
983  single distinguished name.
984 - Attributes attached to user-defined types.
985 - Stored property lists
986 - Object names that begin or end with a space
987 
988 If you know that an HDF5 file conforms to the netCDF-4 enhanced data
989 model, either because it was written with netCDF function calls or
990 because it doesn't make use of HDF5 features in the list above, then it
991 can be accessed using netCDF-4, and analyzed, visualized, and
992 manipulated through other applications that can access netCDF-4 files.
993 
994 The [ncks tool](http://nco.sourceforge.net/nco.html#ncks-netCDF-Kitchen-Sink) of the NCO collection of netCDF utilities can take simple HDF5 data as input and produce a netCDF file as output, so this may work:
995 
996 ~~~~ {.boldcode}
997  ncks infile.hdf5 outfile.nc
998 ~~~~
999 
1000 Another tool has been developed to convert HDF5-EOS Aura files to
1001 netCDF-4 files, and it is currently undergoing testing and documentation
1002 before release on the HDF5 web site.
1003 
1004 How can I convert netCDF-4 files into HDF5 files? {#How-can-I-convert-netCDF-4-files-into-HDF5-files}
1005 -----------------
1006 
1007 
1008 Every netCDF-4 or netCDF-4 classic model file can be read or written by
1009 the HDF5 library, version 1.8 or later, so in that respect netCDF-4
1010 files are already HDF5 files and need no conversion.
1011 
1012 The way netCDF-4 data objects are represented using HDF5 is described in
1013 detail in the User Manual section ["C.3 The NetCDF-4 Format"](http://www.unidata.ucar.edu/software/netcdf/docs/netcdf.html#NetCDF_002d4-Format).
1014 
1015 Why aren't different extensions used for the different formats, for example ".nc3" and ".nc4"? {#why-arent-different-extensions-used}
1016 ------------------
1017 
1018 The file extension used for netCDF files is purely a convention. The
1019 netCDF libraries don't use the file extension. A user can currently
1020 create a netCDF file with any extension, even one not consistent with
1021 the format of the file.
1022 
1023 The **ncgen** utility uses ".nc" as a default extension for output, but this
1024 can be overridden using the "-o" option to specify the name for the
1025 output file. Recent versions of **ncgen** also have a "-k" option to specify
1026 what kind of output file is desired, selecting any of the 4 format
1027 variants, using either a numeric code or a text string. Most other
1028 netCDF client software pays no attention to the file extension, so using
1029 more explicit extensions by convention has no significant drawbacks,
1030 except possibly causing confusion about format differences that may not
1031 be important.
1032 
1033 Why is the default of netCDF-4 to continue to create classic files, rather than netCDF-4 files? {#Why-is-the-default-of-netCDF-4-to-continue-to-create-classic-files-rather-than-netCDF-4-files}
1034 -----------------
1035 
1036 
1037 Until widely used netCDF client software has been adapted or upgraded to
1038 read netCDF-4 data, classic file format is the default for
1039 interoperability with most existing netCDF software.
1040 
1041 Can netCDF-4 read arbitrary HDF5 files? {#Can-netCDF-4-read-arbitrary-HDF5-files}
1042 -----------------
1043 
1044 
1045 No, but it can read many HDF5 files, and more recent versions can access
1046 more HDF5 data. If you want to access HDF5 data through netCDF
1047 interfaces, avoid HDF5 features not included in the netCDF enhanced data
1048 model. For more details see "[How can I convert HDF5 files into netCDF-4 files?](#fv15)", above.
1049 
1050 I installed netCDF-3 with --enable-shared, but it looks like the libraries it installed were netCDF-4, with names like libnetcdf.4.dylib. What's going on? {#I-installed-netCDF-3-with---enable-shared-but-it-looks-like-the-libraries-it-installed-were-netCDF-4-with-names-like-libnetcdf4dylib-Whats-going-on}
1051 -----------------
1052 
1053 
1054 The number used for the shared library name is not related to the netCDF
1055 library version number.
1056 
1057 NetCDF-3.6.3 permits UTF-8 encoded Unicode names. Won't this break backward compatibility with previous software releases that didn't allow such names? {#NetCDF-363-permits-UTF-8-encoded-Unicode-names-Wont-this-break-backward-compatibility-with-previous-software-releases-that-didnt-allow-such-names}
1058 -----------------
1059 
1060 
1061 Earlier versions of the netCDF libraries have always been able to read
1062 data with arbitrary characters in names. The restriction has been on
1063 *creating* files with names that contained "invalid" special characters.
1064 The check for characters used in names occurred when a program tried to
1065 define a new variable, dimension, or attribute, and an error would be
1066 returned if the characters in the names didn't follow the rules.
1067 However, there has never been any such check on reading data, so
1068 arbitrary characters have been permitted in names created through a
1069 different implementation of the netCDF APIs, or through early versions
1070 of netCDF software (before 2.4), which allowed arbitrary names.
1071 
1072 In other words, the expansion to handle UTF-8 encoded Unicode characters
1073 and special characters such as \`:' and \` ' still conforms with
1074 Unidata's commitment to backwards compatibility. All old files are still
1075 readable and writable by the new software, and programs that used to
1076 work will still work when recompiled and relinked with the new
1077 libraries. Files using new characters in names will still be readable
1078 and writable by programs that used older versions of the libraries.
1079 However, programs linked to older library versions will not be able to
1080 create new data objects with the new less-restrictive names.
1081 
1082 How difficult is it to convert my application to handle arbitrary netCDF-4 files? {#How-difficult-is-it-to-convert-my-application-to-handle-arbitrary-netCDF-4-files}
1083 -----------------
1084 
1085 
1086 Modifying an application to fully support the new enhanced data model
1087 may be relatively easy or arbitrarily difficult :-), depending on what
1088 your application does and how it is written. Use of recursion is the
1089 easiest way to handle nested groups and nested user-defined types. An
1090 object-oriented architecture is also helpful in dealing with
1091 user-defined types.
1092 
1093 We recommend proceeding incrementally, supporting features that are
1094 easier to implement first. For example, handling the six new primitive
1095 types is relatively straightforward. After that, using recursion (or the
1096 group iterator interface used in **nccopy**) to support Groups is not too
1097 difficult. Providing support for user-defined types is more of a
1098 challenge, especially since they can be nested.
1099 
1100 The utility program **nccopy**, provided in releases 4.1 and later, shows
1101 how this can be done using the C interface. It copies an input netCDF
1102 file in any of the format variants, handling nested groups, strings, and
1103 any user-defined types, including arbitrarily nested compound types,
1104 variable-length types, and data of any valid netCDF-4 type. It also
1105 demonstrates how to handle variables that are too large to fit in memory
1106 by using an iterator interface. Other generic utility programs can make
1107 use of parts of **nccopy** for more complex operations on netCDF data.
1108 
1109 ----------
1110 
1111 Shared Libraries {#Shared-Libraries}
1112 ================
1113 
1114 What are shared libraries? {#What-are-shared-libraries}
1115 -----------------
1116 
1117 
1118 Shared libraries are libraries that can be shared by multiple running
1119 applications at the same time. This **may** improve performance.
1120 
1121 For example, if I have a library that provides function foo(), and I
1122 have two applications that call foo(), then with a shared library, only
1123 one copy of the foo() function will be loaded into memory, and both
1124 programs will use it. With static libraries, each application would have
1125 its own copy of the foo() function.
1126 
1127 More information on shared libraries can be found at the following
1128 external sites:
1129 
1130 - [The Program-Library HowTo](http://www.tldp.org/HOWTO/Program-Library-HOWTO/index.html),
1131  by David Wheeler.
1132 
1133 - [Wikipedia Library Entry](http://en.wikipedia.org/wiki/Library_(computer_science))
1134 
1135 ----------
1136 
1137 Can I build netCDF with shared libraries? {#Can-I-build-netCDF-with-shared-libraries}
1138 -----------------
1139 
1140 
1141 Starting with version 3.6.2, netCDF can build shared libraries on
1142 platforms that support them, but by default netCDF will build static
1143 libraries only. To turn on shared libraries, use the --enable-shared
1144 option to the [netCDF configure script](http://www.unidata.ucar.edu/software/netcdf/docs/netcdf-install/Running-the-configure-Script.html).
1145 
1146 ----------
1147 
1148 How do I use netCDF shared libraries? {#How-do-I-use-netCDF-shared-libraries}
1149 -----------------
1150 
1151 
1152 With netCDF version 3.6.2, shared libraries can be built on platforms
1153 that support them by using the --enable-shared argument to [netCDF configure script](http://www.unidata.ucar.edu/software/netcdf/docs/netcdf-install/Running-the-configure-Script.html).
1154 
1155 Users of earlier versions of netCDF can build shared libraries by
1156 setting flags correctly during builds.
1157 
1158 When you use a static library, the code is copied from the library into
1159 your program when the program is built. The library is only needed at
1160 build time.
1161 
1162 With a shared library the code in the library is not copied into your
1163 executable, so the library is needed every time the program is run.
1164 
1165 If you write a program that uses the netCDF shared library, the
1166 operating system will have to find it every time your program is run. It
1167 will look in these places:
1168 
1169 1. Directories you specified as shared library locations at **build
1170  time**. Unfortunately this is done differently with different
1171  compilers.
1172 
1173 2. Directories specified in the environment variable LD\_RUN\_PATH at
1174  **build time**.
1175 
1176 3. Directories specified in the OS-specific environment variable for
1177  this purpose at **run time**. (LD\_LIBRARY\_PATH on Linux and many
1178  other Unix variants, LOADLIBS on AIX systems, etc.)
1179 
1180 4. A default list of directories that includes /usr/lib (but don't
1181  install software there!), and may or may not contain places you
1182  might install netCDF, like /usr/local/lib.
1183 
1184 5. The directories specified in an OS file such as /etc/ld.conf.
1185 
1186 By default the netCDF library will be installed in /usr/local/lib. (This
1187 can be overridden with the --prefix option to the [netCDF configure script](http://www.unidata.ucar.edu/software/netcdf/docs/netcdf-install/Running-the-configure-Script.html)).
1188 
1189 An external site by Arnaud Desitter has a [table of different tools and command line options relating to shared libraries](http://www.fortran-2000.com/ArnaudRecipes/sharedlib.html) on Linux, Solaris, HP-UX, Tru64, AIX, SGI, Win32, MacOS X, VMS (wow!), and OS/390.
1190 
1191 For more information about how do to this in Linux users may find it
1192 useful to read this external webpage, some documentation from Caldera, a
1193 Linux distributor: [Specifying directories to be searched by the dynamic linker](http://osr507doc.sco.com/en/tools/ccs_linkedit_dynamic_dirsearch.html).
1194 
1195 ----------
1196 
1197 Large File Support {#Large-File-Support}
1198 ================
1199 
1200 Was it possible to create netCDF files larger than 2 GiBytes before version 3.6? {#Was-it-possible-to-create-netCDF-files-larger-than-2-GiBytes-before-version-36}
1201 -----------------
1202 
1203 
1204 Yes, but there are significant restrictions on the structure of large
1205 netCDF files that result from the 32-bit relative offsets that are part
1206 of the classic netCDF format. For details, see [NetCDF Classic Format Limitations](http://www.unidata.ucar.edu/software/netcdf/documentation/historic/netcdf/NetCDF-Classic-Format-Limitations.html#NetCDF-Classic-Format-Limitations)
1207 in the User's Guide.
1208 
1209 ----------
1210 
1211 What is Large File Support? {#What-is-Large-File-Support}
1212 -----------------
1213 
1214 
1215 Large File Support (LFS) refers to operating system and C library
1216 facilities to support files larger than 2 GiB. On a few 32-bit platforms
1217 the default size of a file offset is still a 4-byte signed integer,
1218 which limits the maximum size of a file to 2 GiB. Using LFS interfaces
1219 and the 64-bit file offset type, the maximum size of a file may be as
1220 large as 2^63^ bytes, or 8 EiB. For some current platforms, large file
1221 macros or appropriate compiler flags have to be set to build a library
1222 with support for large files. This is handled automatically in netCDF
1223 3.6 and later versions.
1224 
1225 More information about Large File Support is available from [Adding Large File Support to the Single UNIX Specification](http://www.unix.org/version2/whatsnew/lfs.html).
1226 
1227 ----------
1228 
1229 What does Large File Support have to do with netCDF? {#What-does-Large-File-Support-have-to-do-with-netCDF}
1230 -----------------
1231 
1232 
1233 When the netCDF format was created in 1988, 4-byte fields were reserved
1234 for file offsets, specifying where the data for each variable started
1235 relative to the beginning of the file or the start of a record boundary.
1236 
1237 This first netCDF format variant, the only format supported in versions
1238 3.5.1 and earlier, is referred to as the netCDF *classic* format. The
1239 32-bit file offset in the classic format limits the total sizes of all
1240 but the last non-record variables in a file to less than 2 GiB, with a
1241 similar limitation for the data within each record for record variables.
1242 For more information see [Classic Format Limitations](http://www.unidata.ucar.edu/software/netcdf/docs/netcdf/NetCDF-Classic-Format-Limitations.html#NetCDF-Classic-Format-Limitations).
1243 
1244 The netCDF classic format is also identified as *version 1* or *CDF1* in
1245 reference to the format label at the start of a file.
1246 
1247 With netCDF version 3.6 and later, a second variant of netCDF format is
1248 supported in addition to the classic format. The new variant is referred
1249 to as the *64-bit offset* format, *version 2*, or *CDF-2*. The primary
1250 difference from the classic format is the use of 64-bit file offsets
1251 instead of 32-bit offsets, but it also supports larger variable and
1252 record sizes.
1253 
1254 Starting from version 4.4.0, netCDF added support for CDF-5 format, which
1255 allows multiple large variables with more than 4-billion array elements defined
1256 in the file. This format is only supported on 64-bit machine platforms.
1257 
1258 ----------
1259 
1260 Do I have to know which netCDF file format variant is used in order to access or modify a netCDF file? {#Do-I-have-to-know-which-netCDF-file-format-variant-is-used-in-order-to-access-or-modify-a-netCDF-file}
1261 -----------------
1262 
1263 
1264 No, version 3.6 and later versions of the netCDF C/Fortran library
1265 detect which variant of the format is used for each file when it is
1266 opened for reading or writing, so it is not necessary to know which
1267 variant of the format is used. The version of the format will be
1268 preserved by the library on writing. If you want to modify a classic
1269 format file to use the CDF-2 or CDF-5 format so you can make it much
1270 larger, you will have to create a new file and copy the data to it. The
1271 **nccopy** utility available in version 4.1 can copy a classic file to a
1272 CDF-2 or CDF-5 file.
1273 
1274 ----------
1275 
1276 Will future versions of the netCDF library continue to support accessing files in the classic format? {#Will-future-versions-of-the-netCDF-library-continue-to-support-accessing-files-in-the-classic-format}
1277 -----------------
1278 
1279 
1280 Yes, the 3.6 library and all planned future versions of the library will
1281 continue to support reading and writing files using the classic CDF-1 (32-bit
1282 offset), 64-bit offset CDF-2, and 64-bit data CDF-5 format. There is no need to
1283 convert existing archives from the classic to the 64-bit offset format.
1284 Even netCDF-4, which introduces a third variant of the netCDF format
1285 based on HDF5, continues to support accessing classic CDF-1, 2, and 5 format
1286 files. NetCDF-4 HDF5 files have even fewer restrictions on size than CDF-1 and
1287 CDF-2 files.
1288 
1289 ----------
1290 
1291 Should I start using the new 64-bit offset format for all my netCDF files? {#Should-I-start-using-the-new-64-bit-offset-format-for-all-my-netCDF-files}
1292 -----------------
1293 
1294 
1295 No, we discourage users from making use of the 64-bit offset format
1296 unless they need it for large files. It may be some time until
1297 third-party software that uses the netCDF library is upgraded to 3.6 or
1298 later versions that support the large file facilities, so we advise
1299 continuing to use the classic netCDF format for data that doesn't
1300 require file offsets larger than 32 bits. The library makes this
1301 recommendation easy to follow, since the default for file creation is
1302 the classic format.
1303 
1304 ----------
1305 
1306 How can I tell if a netCDF file uses the classic format (CDF-1), 64-bit offset format (CDF-2) or 64-bit data format (CDF-5)? {#How-can-I-tell-if-a-netCDF-file-uses-the-classic-format-or-64-bit-offset-format}
1307 -----------------
1308 
1309 
1310 The short answer is that under most circumstances, you should not care,
1311 if you use version 3.6.0 or later of the netCDF library. But the
1312 difference is indicated in the first four bytes of the file, which are
1313 'C', 'D', 'F', '\\001' for the classic CDF-1 format, 'C', 'D', 'F',
1314 '\\002' for the 64-bit offset CDF-2 format, and 'C', 'D', 'F', '\\005' for the
1315 64-bit data CDF-5 format. On a Unix system, one way to display the first four
1316 bytes of a file, say foo.nc, is to run the following command:
1317 
1318 ~~~~ {.boldcode}
1319  od -An -c -N4 foo.nc
1320 ~~~~
1321 
1322 which will output
1323 
1324 ~~~~ {.boldcode}
1325  C D F 001
1326 ~~~~
1327 
1328 or
1329 
1330 ~~~~ {.boldcode}
1331  C D F 002
1332 ~~~~
1333 
1334 or
1335 
1336 ~~~~ {.boldcode}
1337  C D F 005
1338 ~~~~
1339 
1340 depending on whether foo.nc is a CDF-1, CDF-2, or CDF-5 netCDF file,
1341 respectively.
1342 
1343 With netCDF version 3.6.2 or later, there is an easier way, using the
1344 "-k" option to **ncdump** to determine the kind of file, for example:
1345 
1346 ~~~~ {.boldcode}
1347  ncdump -k foo.nc
1348  classic
1349 ~~~~
1350 
1351 ----------
1352 
1353 What happens if I create a 64-bit offset format netCDF file and try to open it with an older netCDF application that hasn't been linked with netCDF 3.6? {#What-happens-if-I-create-a-64-bit-offset-format-netCDF-file-and-try-to-open-it-with-an-older-netCDF-application-that-hasnt-been-linked-with-netCDF-36}
1354 -----------------
1355 
1356 
1357 The application will indicate an error trying to open the file and
1358 present an error message equivalent to "not a netCDF file". This is why
1359 it's a good idea not to create 64-bit offset netCDF files until you
1360 actually need them.
1361 
1362 ----------
1363 
1364 Can I create 64-bit offset files on 32-bit platforms? {#Can-I-create-64-bit-offset-files-on-32-bit-platforms}
1365 -----------------
1366 
1367 
1368 Yes, by specifying the appropriate file creation flag you can create
1369 64-bit offset netCDF files the same way on 32-bit platforms as on 64-bit
1370 platforms. You do not need to compile the C/Fortran libraries as 64-bit
1371 to support access to 64-bit offset netCDF files.
1372 
1373 ----------
1374 
1375 How do I create a 64-bit offset netCDF file from C, Fortran-77, Fortran-90, or C++? {#How-do-I-create-a-64-bit-offset-netCDF-file-from-C-Fortran-77-Fortran-90-or-Cpp}
1376 -----------------
1377 
1378 
1379 With netCDF version 3.6.0 or later, use the NC\_64BIT\_OFFSET flag when
1380 you call nc\_create(), as in:
1381 
1382 ~~~~ {.boldcode}
1383  err = nc_create("foo.nc",
1384  NC_NOCLOBBER | NC_64BIT_OFFSET,
1385  &ncid);
1386 ~~~~
1387 
1388 In Fortran-77, use the NF\_64BIT\_OFFSET flag when you call
1389 nf\_create(), as in:
1390 
1391 ~~~~ {.boldcode}
1392  iret = nf_create('foo.nc',
1393  IOR(NF_NOCLOBBER,NF_64BIT_OFFSET),
1394  ncid)
1395 ~~~~
1396 
1397 In Fortran-90, use the NF90\_64BIT\_OFFSET flag when you call
1398 nf90\_create(), as in:
1399 
1400 ~~~~ {.boldcode}
1401  iret = nf90_create(path="foo.nc",
1402  cmode=or(nf90_noclobber,nf90_64bit_offset),
1403  ncid=ncFileID)
1404 ~~~~
1405 
1406 In C++, use the Offset64Bits enum in the NcFile constructor, as in:
1407 
1408 ~~~~ {.boldcode}
1409  NcFile nc("foo.nc",
1410  FileMode=NcFile::New,
1411  FileFormat=NcFile::Offset64Bits);
1412 ~~~~
1413 
1414 In Java, use the setLargeFile() method of the NetcdfFileWritable class.
1415 
1416 ----------
1417 
1418 How do I create a 64-bit offset netCDF file using the ncgen utility? {#How-do-I-create-a-64-bit-offset-netCDF-file-using-the-ncgen-utility}
1419 -----------------
1420 
1421 
1422 A command-line option, '-k', specifies the kind of file format
1423 variant. By default or if '-k classic' is specified, the generated
1424 file will be in netCDF classic format. If '-k 64-bit-offset' is
1425 specified, the generated file will use the 64-bit offset format.
1426 
1427 ----------
1428 
1429 Have all netCDF size limits been eliminated? {#Have-all-netCDF-size-limits-been-eliminated}
1430 -----------------
1431 
1432 
1433 The netCDF-4 HDF5-based format has no practical limits on the size of a
1434 variable.
1435 
1436 However, for the classic and 64-bit offset formats there are still
1437 limits on sizes of netCDF objects. Each fixed-size variable (except the
1438 last, when there are no record variables) and the data for one record's
1439 worth of a single record variable (except the last) are limited in size
1440 to a little less that 4 GiB, which is twice the size limit in versions
1441 earlier than netCDF 3.6.
1442 
1443 The maximum number of records remains 2^32^-1.
1444 
1445 ----------
1446 
1447 Why are variables still limited in size? {#Why-are-variables-still-limited-in-size}
1448 -----------------
1449 
1450 
1451 While most platforms support a 64-bit file offset, many platforms only
1452 support a 32-bit size for allocated memory blocks, array sizes, and
1453 memory pointers. In C developer's jargon, these platforms have a 64-bit
1454 `off_t` type for file offsets, but a 32-bit `size_t` type for size of
1455 arrays. Changing netCDF to assume a 64-bit `size_t` would restrict
1456 netCDF's use to 64-bit platforms.
1457 
1458 ----------
1459 
1460 How can I write variables larger than 4 GiB? {#How-can-I-write-variables-larger-than-4-GiB}
1461 -----------------
1462 
1463 
1464 You can overcome the 4 GiB size barrier by using the netCDF-4 HDF5
1465 format for your data. The only change required to the program that
1466 writes the data is an extra flag to the file creation call, followed by
1467 recompiling and relinking to the netCDF-4 library. Programs that access
1468 the data would also need to be recompiled and relinked to the netCDF-4
1469 library.
1470 
1471 For classic and 64-bit offset netCDF formats, if you change the first
1472 dimension of a variable from a fixed size to an unlimited size instead,
1473 the variable can be much larger. Even though record variables are
1474 restricted to 4 Gib per record, there may be 4 billion records. NetCDF
1475 classic or 64-bit offset files can only have one unlimited dimension, so
1476 this won't work if you are already using a record dimension for other
1477 purposes.
1478 
1479 It is also possible to overcome the 4 GiB variable restriction for a
1480 single fixed size variable, when there are no record variables, by
1481 making it the last variable, as explained in the example in [NetCDF Classic Format Limitations](http://www.unidata.ucar.edu/software/netcdf/documentation/historic/netcdf/NetCDF-Classic-Format-Limitations.html#NetCDF-Classic-Format-Limitations).
1482 
1483 ----------
1484 
1485 Why do I get an error message when I try to create a file larger than 2 GiB with the new library? {#Why-do-I-get-an-error-message-when-I-try-to-create-a-file-larger-than-2-GiB-with-the-new-library}
1486 -----------------
1487 
1488 
1489 There are several possible reasons why creating a large file can fail
1490 that are not related to the netCDF library:
1491 
1492 - User quotas may prevent you from creating large files. On a Unix
1493  system, you can use the "ulimit" command to report limitations such
1494  as the file-size writing limit.
1495 
1496 - There is insufficient disk space for the file you are trying to
1497  write.
1498 
1499 - The file system in which you are writing may not be configured to
1500  allow large files. On a Unix system, you can test this with a
1501  commands such as
1502 
1503  ~~~~ {.boldcode}
1504  dd if=/dev/zero bs=1000000 count=3000 of=./largefile
1505  ls -l largefile
1506  rm largefile
1507  ~~~~
1508 
1509  which should write a 3 GByte file named "largefile" in the current
1510  directory, verify its size, and remove it.
1511 
1512 If you get the netCDF library error "One or more variable sizes violate
1513 format constraints", you are trying to define a variable larger than
1514 permitted for the file format variant. This error typically occurs when
1515 leaving "define mode" rather than when defining a variable. The error
1516 status cannot be returned when a variable is first defined, because the
1517 last fixed-size variable defined is permitted to be larger than other
1518 fixed-size variables (when there are no record variables).
1519 
1520 Similarly, the last record variable may be larger than other record
1521 variables. This means that subsequently adding a small variable to an
1522 existing file may be invalid, because it makes what was previously the
1523 last variable now in violation of the format size constraints. For
1524 details on the format size constraints, see the Users Guide sections
1525 [NetCDF Classic Format Limitations](http://www.unidata.ucar.edu/software/netcdf/docs/netcdf.html#Classic-Limitations) and [NetCDF 64-bit Offset Format Limitations](http://www.unidata.ucar.edu/software/netcdf/docs/netcdf.html#64-bit-Offset-Limitations).
1526 
1527 If you get the netCDF library error "Invalid dimension size" for a
1528 non-negative size, you are exceeding the size limit of netCDF
1529 dimensions, which must be less than 2,147,483,644 for classic files with
1530 no large file support and otherwise less than 4,294,967,292.
1531 
1532 ----------
1533 
1534 Do I need to use special compiler flags to compile and link my applications that use netCDF with Large File Support? {#Do-I-need-to-use-special-compiler-flags-to-compile-and-link-my-applications-that-use-netCDF-with-Large-File-Support}
1535 -----------------
1536 
1537 
1538 No, except that 32-bit applications should link with a 32-bit version of
1539 the library and 64-bit applications should link with a 64-bit library,
1540 similarly to use of other libraries that can support either a 32-bit or
1541 64-bit model of computation. But note that a 32-bit version of the
1542 netCDF library fully supports writing and reading 64-bit offset netCDF
1543 files.
1544 
1545 ----------
1546 
1547 Is it possible to create a "classic" format netCDF file with netCDF version 3.6.0 that cannot be accessed by applications compiled and linked against earlier versions of the library? {#isitpossibleclassic360}
1548 ----------------
1549 
1550 No, classic files created with the new library should be compatible with
1551 all older applications, both for reading and writing, with one minor
1552 exception. The exception is due to a correction of a netCDF bug that
1553 prevented creating records larger than 4 GiB in classic netCDF files
1554 with software linked against versions 3.5.1 and earlier. This limitation
1555 in total record size was not a limitation of the classic format, but an
1556 unnecessary restriction due to the use of too small a type in an
1557 internal data structure in the library.
1558 
1559 If you want to always make sure your classic netCDF files are readable
1560 by older applications, make sure you don't exceed 4 GiBytes for the
1561 total size of a record's worth of data. (All records are the same size,
1562 computed by adding the size for a record's worth of each record
1563 variable, with suitable padding to make sure each record begins on a
1564 byte boundary divisible by 4.)
1565 
1566 ----------
1567 
1568 NetCDF and Other Software {#NetCDF-and-Other-Software}
1569 ================
1570 
1571 What other software is available for accessing, displaying, and manipulating netCDF data? {#What-other-software-is-available-for-accessing-displaying-and-manipulating-netCDF-data}
1572 -----------------
1573 
1574 
1575 Utilities available in the current netCDF distribution from Unidata are
1576 **ncdump**, for converting netCDF files to an ASCII human-readable form,
1577 and **ncgen** for converting from the ASCII human-readable form back to
1578 a binary netCDF file or a C or FORTRAN program for generating the netCDF
1579 file. [Software for Manipulating or Displaying NetCDF Data](software.html) provides a list of other software useful for access, visualization, and analysis of netCDF data and data represented in other forms. Another useful [guide to netCDF utilities](http://nomads.gfdl.noaa.gov/sandbox/products/vis/data/netcdf/GFDL_VG_NetCDF_Utils.html) is available from NOAA's Geophysical Fluid Dynamics Laboratory.
1580 
1581 ----------
1582 
1583 What other data access interfaces and formats are available for scientific data? {#What-other-data-access-interfaces-and-formats-are-available-for-scientific-data}
1584 -----------------
1585 
1586 
1587 The [Scientific Data Format Information FAQ](http://www.cv.nrao.edu/fits/traffic/scidataformats/faq.html) provides a somewhat dated description of other access interfaces and formats for scientific data, including [CDF](http://nssdc.gsfc.nasa.gov/cdf/cdf_home.html) and [HDF](http://hdf.ncsa.uiuc.edu/). A brief comparison of CDF, netCDF, and HDF is available in the [CDF FAQ](http://nssdc.gsfc.nasa.gov/cdf/html/FAQ.html). Another comparison is in Jan Heijmans' [An Introduction to Distributed Visualization](http://www.xi-advies.nl/downloads/AnIntroductionToDistributedVisualization.pdf). John May's book [*Parallel I/O for High Performance Computing*](http://www.llnl.gov/CASC/news/johnmay/John_May_book.html) includes a chapter on Scientific Data Libraries that describes netCDF and HDF5, with example source code for reading and writing files using both interfaces.
1588 
1589 ----------
1590 
1591 What is the connection between netCDF and CDF? {#What-is-the-connection-between-netCDF-and-CDF}
1592 -----------------
1593 
1594 
1595 [CDF](http://cdf.gsfc.nasa.gov/) was developed at the NASA Space Science
1596 Data Center at Goddard, and is freely available. It was originally a VMS
1597 FORTRAN interface for scientific data access. Unidata reimplemented the
1598 library from scratch to use [XDR](http://www.faqs.org/rfcs/rfc1832.html)
1599 for a machine-independent representation, designed the
1600 [CDL](http://www.unidata.ucar.edu/software/netcdf/documentation/historic/netcdf/CDL-Syntax.htm) (network Common Data form Language) text
1601 representation for netCDF data, and added aggregate data access, a
1602 single-file implementation, named dimensions, and variable-specific
1603 attributes.
1604 
1605 NetCDF and CDF have evolved independently. CDF now supports many of the
1606 same features as netCDF (aggregate data access, XDR representation,
1607 single-file representation, variable-specific attributes), but some
1608 differences remain (netCDF doesn't support native-mode representation,
1609 CDF doesn't support named dimensions). There is no compatibility between
1610 data in CDF and netCDF form, but NASA makes available [some
1611 translators](http://cdf.gsfc.nasa.gov/html/dtws.html) between various
1612 scientific data formats. For a more detailed description of differences
1613 between CDF and netCDF, see the [CDF FAQ](http://cdf.gsfc.nasa.gov/html/FAQ.html).
1614 
1615 ----------
1616 
1617 What is the connection between netCDF and HDF? {#What-is-the-connection-between-netCDF-and-HDF}
1618 -----------------
1619 
1620 
1621 The National Center for Supercomputing Applications (NCSA) originally
1622 developed [HDF4](http://hdf.ncsa.uiuc.edu/) and made it freely
1623 available. HDF4 is an extensible data format for self-describing files
1624 that was developed independently of netCDF. HDF4 supports both C and
1625 Fortran interfaces, and it has been successfully ported to a wide
1626 variety of machine architectures and operating systems. HDF4 emphasizes
1627 a single common format for data, on which many interfaces can be built.
1628 
1629 NCSA implemented software that provided a netCDF-2 interface to HDF4.
1630 With this software, it was possible to use the netCDF calling interface
1631 to place data into an HDF4 file.
1632 
1633 HDF5, developed and supported by The HDF Group, Inc., a non-profit
1634 spin-off from the NCSA group, provides a richer data model, with
1635 emphasis on efficiency of access, parallel I/O, and support for
1636 high-performance computing. The netCDF-4 project has implemented an
1637 enhanced netCDF interface on the HDF5 storage layer to preserve the
1638 desirable common characteristics of netCDF and HDF5 while taking
1639 advantage of their separate strengths: the widespread use and simplicity
1640 of netCDF and the generality and performance of HDF5.
1641 
1642 ----------
1643 
1644 Has anyone implemented client-server access for netCDF data? {#Has-anyone-implemented-client-server-access-for-netCDF-data}
1645 -----------------
1646 
1647 
1648 Yes, as part of the [OPeNDAP](http://www.opendap.org/) framework,
1649 developers have implemented a client-server system for access to remote
1650 data that supports use of the netCDF interface for clients. A reference
1651 version of the software is available from the [OPeNDAP download site](http://www.opendap.org/download/index.html/). After linking your netCDF application with the OPeNDAP netCDF library, you can use URL's to access data from other sites running an OPeNDAP server. This supports accessing small subsets of large datasets remotely through the netCDF interfaces, without copying the datasets.
1652 
1653 The 4.1 release of netCDF will include OPeNDAP client support; an
1654 experimental version is available now in the snapshot distributions.
1655 
1656 Other clients and servers support access through a netCDF interface to
1657 netCDF and other kinds of data, including clients written using the
1658 [netCDF-Java library](http://www.unidata.ucar.edu/software/netcdf-java/) and servers that use the
1659 [THREDDS Data Server](/software/thredds/current/tds/TDS.html).
1660 
1661 The [GrADS Data Server](http://grads.iges.org/grads/gds/) provides
1662 subsetting and analysis services across the Internet for any
1663 GrADS-readable dataset, including suitable netCDF datasets. The latest
1664 version of the [PMEL Live Access Server](http://ferret.pmel.noaa.gov/LAS) uses THREDDS Data Server technology to provide flexible access to geo-referenced scientific data, including netCDF data.
1665 
1666 ----------
1667 
1668 How do I convert between GRIB and netCDF? {#How-do-I-convert-between-GRIB-and-netCDF}
1669 -----------------
1670 
1671 
1672 Several programs and packages have been developed that convert between
1673 [GRIB](http://www.wmo.ch/web/www/DPS/grib-2.html) and netCDF data:
1674 [ncl_convert2nc](http://www.ncl.ucar.edu/Applications/grib2netCDF.shtml),
1675 [degrib](http://www.nws.noaa.gov/mdl/NDFD_GRIB2Decoder/),
1676 [CDAT](software.html#CDAT), [CDO](software.html#CDO),
1677 [GDAL](http://www.gdal.org/), [GrADS](software.html#GrADS), and
1678 [wgrib2](http://www.cpc.noaa.gov/products/wesley/wgrib2/).
1679 
1680 The Unidata [netCDF Java Library](http://www.unidata.ucar.edu/software/netcdf-java/index.html) can
1681 read GRIB1 and GRIB2 data (and many other data formats) through a netCDF
1682 interface. As a command-line example, you could convert *fileIn.grib* to
1683 *fileOut.nc* as follows:
1684 
1685 ~~~~ {.boldcode}
1686  java -Xmx1g -classpath netcdfAll-4.3.jar ucar.nc2.dataset.NetcdfDataset \
1687  -in fileIn.grib -out fileOut.nc [-isLargeFile] [-netcdf4]
1688 ~~~~
1689 
1690 For more details on using netCDF Java, see the CDM man pages for
1691 [nccopy](http://www.unidata.ucar.edu/software/netcdf-java/reference/manPages.html#nccopy).
1692 
1693 ----------
1694 
1695 Problems and Bugs
1696 -----------------
1697 
1698 Can I recover data from a netCDF file that was not closed properly? {#Can-I-recover-data-from-a-netCDF-file-that-was-not-closed-properly}
1699 -----------------
1700 
1701 
1702 _I have some netcdf files which have data in them and were apparently
1703 not properly closed. When I examine them using **ncdump** they report zero
1704 data points, although the size is a few megabytes. Is there a way of
1705 recovering them?_
1706 
1707 If the files are in classic format or 64-bit offset format (if they were
1708 created by netCDF version 3.6.3 or earlier, for example), then you can
1709 use an editor that allows you to change binary files, such as emacs, to
1710 correct the four-byte number of records field in the file. This is a
1711 bigendian 4 byte integer that begins at the 4th byte in the file.
1712 
1713 This is what the first eight bytes would look like for classic format if
1714 you had zero records, where printable characters are specified as
1715 US-ASCII characters within single-quotes and non-printable bytes are
1716 denoted using a hexadecimal number with the notation '\\xDD', where each
1717 D is a hexadecimal digit:
1718 
1719 ~~~~ {.boldcode}
1720  'C' 'D' 'F' \x01 \x00 \x00 \x00 \x00
1721 ~~~~
1722 
1723 or
1724 
1725 ~~~~ {.boldcode}
1726  'C' 'D' 'F' \x02 \x00 \x00 \x00 \x00
1727 ~~~~
1728 
1729 for 64-bit-offset format.
1730 
1731 And this is what the first eight bytes should look like for classic
1732 format if you had 500 records (500 is 01F4 in hexadecimal)
1733 
1734 ~~~~ {.boldcode}
1735  'C' 'D' 'F' \x01 \x00 \x01 \x0f \x04
1736 ~~~~
1737 
1738 or
1739 
1740 ~~~~ {.boldcode}
1741  'C' 'D' 'F' \x02 \x00 \x01 \x0f \x04
1742 ~~~~
1743 
1744 for 64-bit-offset format.
1745 
1746 So if you can compute how many records should be in the file, you can
1747 edit the second four bytes to fix this. You can find out how many
1748 records should be in the file from the size of the file and from the
1749 variable types and their shapes. See the [description of the netCDF format](http://www.unidata.ucar.edu/software/netcdf/docs/netcdf.html#File-Format)
1750 for classic and 64-bit offset files for how to figure out how large the
1751 file should be for fixed sized variables of particular shapes and for a
1752 specified number of record variables of particular shapes.
1753 
1754 Note that if you neglected to call the appropriate netCDF close function
1755 on a file, data in the last record written but not flushed to the disk
1756 may also be lost, but correcting the record count should allow recovery
1757 of the other records.
1758 
1759 ----------
1760 
1761 Is there a list of reported problems and workarounds? {#Is-there-a-list-of-reported-problems-and-workarounds}
1762 -----------------
1763 
1764 
1765 Yes, the document [Known problems with the netCDF Distribution](known_problems.html) describes reported problems and workarounds in the latest version and some earlier releases.
1766 
1767 ----------
1768 
1769 How do I make a bug report? {#How-do-I-make-a-bug-report}
1770 -----------------
1771 
1772 
1773 If you find a bug, send a description to
1774 support-netcdf@unidata.ucar.edu. This is also the address to use for
1775 questions or discussions about netCDF that are not appropriate for the
1776 entire netcdfgroup mailing list.
1777 
1778 ----------
1779 
1780 How do I search through past problem reports? {#How-do-I-search-through-past-problem-reports}
1781 -----------------
1782 
1783 
1784 A search link is available at the bottom of the [netCDF homepage](http://www.unidata.ucar.edu/software/netcdf/), providing a full-text search of the
1785 support questions and answers about netCDF provided by Unidata support
1786 staff.
1787 
1788 ----------
1789 
1790 Programming with NetCDF {#Programming-with-NetCDF}
1791 ================
1792 
1793 Which programming languages have netCDF interfaces? {#Which-programming-languages-have-netCDF-interfaces}
1794 -----------------
1795 
1796 The netCDF distribution comes with interfaces for C, Fortran77,
1797 Fortran90, and C++. Other languages for which interfaces are available
1798 separately include:
1799 
1800 - [Ada](http://freshmeat.net/projects/adanetcdf/)
1801 - [IDL](software.html#IDL)
1802 - [Java](software.html#Java%20interface)
1803 - [MATLAB](software.html#MATLAB)
1804 - [Perl](software.html#Perl)
1805 - [Python](software.html#Python)
1806 - [R](software.html#R)
1807 - [Ruby](software.html#Ruby)
1808 - [Tcl/Tk](software.html#Tcl/Tk)
1809 
1810 ----------
1811 
1812 Are the netCDF libraries thread-safe? {#Are-the-netCDF-libraries-thread-safe}
1813 -----------------
1814 
1815 The C-based libraries are *not* thread-safe. C-based libraries are those
1816 that depend on the C library, which currently include all language
1817 interfaces except for the Java interface. The Java interface is
1818 thread-safe when a few simple rules are followed, such as each thread
1819 getting their handle to a file.
1820 
1821 ----------
1822 
1823 How does the C++ interface differ from the C interface? {#How-does-the-Cpp-interface-differ-from-the-C-interface}
1824 -----------------
1825 
1826 It provides all the functionality of the C interface (except for the
1827 generalized mapped access of ncvarputg() and ncvargetg()) and is
1828 somewhat simpler to use than the C interface. With the C++ interface, no
1829 IDs are needed for netCDF components, there is no need to specify types
1830 when creating attributes, and less indirection is required for dealing
1831 with dimensions. However, the C++ interface is less mature and
1832 less-widely used than the C interface, and the documentation for the C++
1833 interface is less extensive, assuming a familiarity with the netCDF data
1834 model and the C interface. Recently development of the C++ interface has
1835 languished as resources have been redirected to enhancing the Java
1836 interface.
1837 
1838 ----------
1839 
1840 How does the Fortran interface differ from the C interface? {#How-does-the-Fortran-interface-differ-from-the-C-interface}
1841 -----------------
1842 
1843 It provides all the functionality of the C interface. The Fortran
1844 interface uses Fortran conventions for array indices, subscript order,
1845 and strings. There is no difference in the on-disk format for data
1846 written from the different language interfaces. Data written by a C
1847 language program may be read from a Fortran program and vice-versa. The
1848 Fortran-90 interface is much smaller than the FORTRAN 77 interface as a
1849 result of using optional arguments and overloaded functions wherever
1850 possible.
1851 
1852 ----------
1853 
1854 How do the Java, Perl, Python, Ruby, ... interfaces differ from the C interface? {#How-do-the-Java-Perl-Python-Ruby-interfaces-differ-from-the-C-interface}
1855 -----------------
1856 
1857 They provide all the functionality of the C interface, using appropriate
1858 language conventions. There is no difference in the on-disk format for
1859 data written from the different language interfaces. Data written by a C
1860 language program may be read from programs that use other language
1861 interfaces, and vice-versa.
1862 
1863 ----------
1864 
1865 How do I handle errors in C? {#How-do-I-handle-errors-in-C}
1866 -----------------
1867 
1868 For clarity, the NetCDF C Interface Guide contains examples which use a
1869 function called handle\_err() to handle potential errors like this:
1870 
1871 ~~~~ {.boldcode}
1872  status = nc_create("foo.nc", NC_NOCLOBBER, &ncid);
1873  if (status != NC_NOERR) handle_error(status);
1874 ~~~~
1875 
1876 Most developers use some sort of macro to invoke netCDF functions and
1877 test the status returned in the calling context without a function call,
1878 but using such a macro in the User's Guides arguably makes the examples
1879 needlessly complex. For example, some really excellent developers define
1880 an "ERR" macro and write code like this:
1881 
1882 ~~~~ {.boldcode}
1883  if (nc_create(testfile, NC_CLOBBER, &ncid)) ERR;
1884 ~~~~
1885 
1886 where Err is defined in a header file:
1887 
1888 ~~~~ {.boldcode}
1889 /* This macro prints an error message with line number and name of
1890  * test program. */
1891 #define ERR do { \
1892 fflush(stdout); /* Make sure our stdout is synced with stderr. */ \
1893 err++; \
1894 fprintf(stderr, "Sorry! Unexpected result, %s, line: %d\n", \
1895  __FILE__, __LINE__); \
1896 } while (0)
1897 ~~~~
1898 
1899 Ultimately, error handling depends on the application which is calling
1900 netCDF functions. However we strongly suggest that some form of error
1901 checking be used for all netCDF function calls.
1902 
1903 ----------
1904 
1905 
1906 CMake-Related Frequently Asked Questions {#cmake_faq}
1907 ========================================
1908 
1909 Below are a list of commonly-asked questions regarding NetCDF and CMake.
1910 
1911 How can I see the options available to CMake? {#listoptions}
1912 ---------------------------------------------
1913 
1914  $ cmake [path to source tree] -L - This will show the basic options.
1915  $ cmake [path to source tree] -LA - This will show the basic and advanced options.
1916 
1917 
1918 How do I specify how to build a shared or static library? {#sharedstatic}
1919 --------------------------------------------------------
1920 
1921  This is controlled with the internal `cmake` option, `BUILD_SHARED_LIBS`.
1922 
1923  $ cmake [Source Directory] -DBUILD_SHARED_LIBS=[ON/OFF]
1924 
1925 
1926 Can I build both shared and static libraries at the same time with cmake? {#sharedstaticboth}
1927 -------------------------------------------------------------------------
1928 
1929 Not at this time; it is required to instead build first one version, and then the other, if you need both.
1930 
1931 How can I specify linking against a particular library? {#partlib}
1932 -------------------------------------------------------
1933 
1934 It depends on the library. To specify a custom `ZLib`, for example, you would do the following:
1935 
1936  $ cmake [Source Directory] -DZLIB_LIBRARY=/path/to/my/zlib.lib
1937 
1938 
1939 `HDF5` is more complex, since it requires both the `hdf5` and `hdf5_hl` libraries. You would specify custom `HDF5` libraries as follows:
1940 
1941  $ cmake [Source Directory] -DHDF5_LIB=/path/to/hdf5.lib \
1942  -DHDF5_HL_LIB=/path/to/hdf5_hl.lib \
1943  -DHDF5_INCLUDE_DIR=/path/to/hdf5/include
1944 
1945 
1946 Alternatively, you may specify:
1947 
1948  $ cmake [Source Directory] \
1949  -DHDF5_LIBRARIES="/path/to/hdf5.lib;/path/to/hdf5_hl.lib" \
1950  -DHDF5_INCLUDE_DIRS=/path/to/hdf5/include/
1951 
1952 
1953 What if I want to link against multiple libraries in a non-standard location {#nonstdloc}
1954 ----------------------------------------------------------------------------
1955 
1956  You can specify the path to search when looking for dependencies and header files using the `CMAKE_PREFIX_PATH` variable:
1957 
1958 * Windows:
1959 
1960  $ cmake [Source Directory] -DCMAKE_PREFIX_PATH=c:\shared\libs\
1961 
1962 
1963 * Linux/Unix/OSX:
1964 
1965  $ cmake [Source Directory] -DCMAKE_PREFIX_PATH=/usr/custom_library_locations/
1966 
1967 How can I specify a Parallel Build using HDF5 {#parallelhdf}
1968 ----------------------------------------------
1969 
1970 If cmake is having problems finding the parallel `HDF5` install, you can specify the location manually:
1971 
1972 
1973  $ cmake [Source Directory] -DENABLE_PARALLEL=ON \
1974  -DHDF5_LIB=/usr/lib64/openmpi/lib/libhdf5.so \
1975  -DHDF5_HL_LIB=/usr/lib64/openmpi/lib/libhdf5.hl.so \
1976  -DHDF5_INCLUDE_DIR=/usr/include/openmpi-x86_64 \
1977 
1978 You will, of course, need to use the location of the libraries specific to your development environment.
1979 
1980 ----------------
1981 
1982 Plans {#Plans}
1983 ================
1984 
1985 What other future work on netCDF is planned? {#What-other-future-work-on-netCDF-is-planned}
1986 -----------------
1987 
1988 Issues, bugs, and plans for netCDF are maintained in the Unidata issue
1989 tracker sites for
1990 [netCDF-C](https://www.unidata.ucar.edu/jira/browse/NCF), [Common Data Model / NetCDF-Java](https://www.unidata.ucar.edu/jira/browse/CDM),
1991 [netCDF-Fortran](https://www.unidata.ucar.edu/jira/browse/NCFORTRAN),
1992 and [netCDF-CXX4](https://www.unidata.ucar.edu/jira/browse/NCXXF), and
1993 [old netCDF-C++
1994 (deprecated)](https://www.unidata.ucar.edu/jira/browse/NCCPP).

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