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========================================================================
<B>C D F</B>
<B>F R E Q U E N T L Y A S K E D Q U E S T I O N S</B>
========================================================================
FAQ Maintainer: National Space Science Data Center (NSSDC)
Email: <A HREF="mailto:cdfsupport@nssdca.gsfc.nasa.gov?subject=CDF">cdfsupport@nssdca.gsfc.nasa.gov</a>
Address: National Space Science Data Center
NASA Goddard Space Flight Center
Greenbelt, MD 20771, U.S.A.
Last Updated: May 21, 2001
Creation Date: August 19, 1994
File Location: http://nssdc.gsfc.nasa.gov/cdf/html/FAQ.html (html)
ftp://nssdcftp.gsfc.nasa.gov/standards/cdf/FAQ.doc (text)
========================================================================
Topics marked with
+ new since last time
* changed since last time
The following questions are answered:
------- <A HREF="#intro"><B>Introductory CDF</B></A> ----------------------------------------------------
1. What is CDF?
*2. What is the current version of CDF?
*3. What has changed since the CDF 2.6.7 release?
*4. What CDF utility programs are available?
5. What platforms does CDF run on?
*6. Where do I get source code and information relevant to CDF and
CDF utilities?
7. How widely used is CDF?
8. What are the differences between CDF and NetCDF, and CDF and HDF?
------- <A HREF="#docs"><B>Manuals, Documentation, Man Pages</B></A> --------------------------------
*9. What documentation is available?
10. What are some publications/references relating to CDF?
------- <A HREF="#cdfsupport"><B>Mailing List, User Support, Etc</B></A> -----------------------------------
11. How can I contact them?
12. How do I make a bug report?
13. Is there a mailing list for CDF discussions, questions, and announcements?
------- <A HREF="#common"><B>Common CDF Questions</B></A> ---------------------------------------------
*14. What programming languages does CDF support?
*15. Are CDF data sets/files platform-independent?
*16. What's the difference between a single-file CDF and multi-file CDF?
*17. What are the differences between the Standard Interface, Internal
Interface, and the CDF Java APIs?
*18. What's variable?
*19. What compression algorithms does CDF support?
*20. Can CDF delete a range of records?
*21. Does CDF support random file access?
*22. How do I optimize a CDF file or can I read data fast?
------- <A HREF="#general"><B>General CDF</B></A> --------------------------------------------------------
23. What is the best way to represent my data using CDF?
24. Can new versions of CDF applications read CDF files written using older
versions of the CDF library?
25. Can my application programs which work with old versions of the CDF
library always be compiled with new versions of CDF?
26. Is there any commercial or public domain visualization software that
accepts CDF files?
27. Are there any conversion programs available to convert non-CDF files
into CDF files or vice versa?
28. How can I contribute my software to the CDF user community?
------------------------------------------------------------------------------
<A NAME="intro"><B>1. What is CDF?</B></A>
CDF is the Common Data Format. It is a conceptual data abstraction for
storing, manipulating, and accessing multidimensional data sets. The basic
component of CDF is a software programming interface that is a device-
independent view of the CDF data model. The application developer is
insulated from the actual physical file format for reasons of conceptual
simplicity, device independence, and future expandability. CDF files
created on any given platform can be transported to any other platform onto
which CDF is ported and used with any CDF tools or layered applications.
The CDF software, documentation, and user support services are provided
by NASA and available to the public free of charge. There are no license
agreements or costs involved in obtaining or using CDF.
A more detailed introduction to CDF can be found in the <A HREF="#docs">CDF User's Guide</A>.
<B>2. What is the current version of CDF?</B>
CDF V2.7 is the current version. It was released on September 1, 1999.
Notes:<UL>
<LI>The official version of the ISTP project is CDF V2.5.</UL>
<B>3. What has changed since the CDF 2.6.7 release?</B>
CDF Version 2.7, made available in September 1999, contains support for
Java Application Program Interfaces (APIs), besides the current C and
Fortran APIs, to allow the users to develop platform-independent CDF
appications. CDF 2.7 also contains bug fixes as well as Java versions
of the CDF tools (e.g. SkeletonCDF, SkeletonTable, CDFConvert,
etc.) except CDFexport and CDFedit.
The CDF Java APIs and tools are not available for the MS-DOS (not Windows
95/98/NT) operating system. The APIs and tools are not fully tested
for the OpenVMS operating sytem, and they'll be made available at the place
where the new CDF 2.7 library is located once fully tested. The improved
CDF 2.7 library for OpenVMS is available now.
<B>4. What CDF utility programs are available?</B>
The following utility programs (a.k.a. CDF toolkit) are available in the
CDF distribution package for the supported platforms:
Program Description
------- -----------
CDFcomapre This utility is used to display a directory listing of a CDF's
files. The .cdf file is displayed first followed by the
rVariable files and then the zVariable files (if either exist
in a multi-file CDF) in numerical order.
CDFconvert This program is used to convert various properties of a CDF.
In all cases new CDFs are created (existing CDFs are not
modified.) Any combination of the following properties may be
changed when converting a CDF.
- A CDF created with an earlier release of the CDF library
(e.g., CDF V2.5) may be converted to the current library
release.
- The format of the CDF may be changed.
- The data encoding of the CDF may be changed.
- The variable majority of the CDF may be changed.
CDFedit This program allows the display and/or modification of
practically all of the contents of a CDF by way of
a full-screen interface. It is also possible to run CDFedit
in a browse-only mode if that's desired.
CDFexport This program allows the entire contents or a portion of a CDF
file to be exported to the terminal screen, a text file, or
another CDF. The variables to be exported can be selected
along with a filter range for each variable which allows a
subset of the CDF to be generated. When exporting to another
CDF, a new compression and sparseness can be specified for
each variable. When exporting to the terminal screen or a
text file, the format of the output can be tailored as
necessary.
CDFinquire This program displays the version of the CDF distribution
being used and the default toolkit qualifiers.
CDFstats This program produces a statistical report on a CDF's variable
data. Both rVariables and zVariables are analyzed. For each
variable it determines the actual minimum and maximum values
(in all of the variable records), the minimum and maximum
values within the valid range (if the <I>VALIDMIN</I> and
<I>VALIDMAX</I> vAttributes and corresponding entries are present
in the CDF), and the monotonicity. An option exists to allow
fill values (specified by the FILLVAL vAttribute) to be
ignored when collecting statistics.
SkeletonTable This program is used to create an ASCII text file called a
skeleton table containing information about a given CDF
(SkeletonTable can also be instructed to output the skeleton
table to the terminal screen.) It reads a CDF file and
writes into the skeleton table the following information.
1. Format (single or multi file), data encoding, variable
majority.
2. Number of dimensions and dimension sizes for the rVariables.
3. gAttribute definitions (and gEntry values).
4. rVariable and zVariable definitions and vAttribute
definitions (with rEntry/zEntry values).
5. Data values for all or a subset of the CDF's variables.
Traditionally, only NRV variable values are written to a
skeleton table. RV variable values may now also be written.
The above information is written in a format that can be
"understood" by the SkeletonCDF program. SkeletonCDF reads a
skeleton table and creates a new CDF (called a skeleton CDF).
SkeletonCDF This program is used to make a fully structured CDF, called a
skeleton CDF, by reading a text file called a skeleton table.
The skeleton table contains the information necessary to
create a CDF that is complete in all respects except for
record-variant (RV) var iable values. (RV variables vary from \
record to record.) RV values are then written to the CDF by
the execution of an application program. The SkeletonCDF
program allows a CDF to be created with the following.
1. The necessary header information - the number of
dimensions and dimension sizes for the rVariables,
format, data encoding, and variable majority.
2. The gAttribute definitions and any number of gEntries
for each.
3. The rVariable and zVariable definitions.
4. The vAttribute definitions and the entries corresponding
to each variable.
5. The data values for those variables that are
non-record-variant (NRV). NRV variables do not vary from
record to record.
<B>5. On what platforms does CDF run?</B>
Platforms Operating Systems
--------- -----------------
DEC Alpha OSF/1, OpenVMS
DECstation Ultrix, VMS
HP 9000 series HP-UX
PC MS-DOS/Windows 3.*, Windows 95/98/NT/2000,
Linux, Solaris, QNX, Cygwin
IBM RS6000 series AIX
Macintosh MacOS 7.x/8.x/9.x, MacOS X, Linux
NeXT Mach
SGI Iris, Power series, Indigo IRIX
Sun SunOS, Solaris
VAX VMS
<B><A NAME="source">6. Where do I get source code and information relevant to CDF and
CDF utilities?</A></B>
The CDF V2.7 software distribution is available via anonymous FTP and
DECnet for OpenVMS systems, via anonymous FTP only for UNIX/POSIXshell
systems, and via anonymous FTP and floppy disks for MS-DOS and Macintosh
systems.
For OpenVMS systems (VAX and Alpha) via anonymous FTP:
ANONYMOUS account on NSSDCA.GSFC.NASA.GOV (128.183.241.14)
in [.CDF.DIST.CDF27]. The files you need are README.VMS and
CDF27-DIST.BCK.
The CDF Java APIs and tools are not fully tested for the
VMS/OpenVMS operating sytem, and they'll be made available at
the place where the new CDF 2.7 library is located once fully
tested. The improved CDF 2.7 library is available now.
For OpenVMS systems (VAX and Alpha) via DECnet:
Copy the files:
NSSDCA::ANON_DIR:[CDF.DIST.CDF27]CDF27-DIST.BCK
NSSDCA::ANON_DIR:[CDF.DIST.CDF27]README.VMS
The CDF Java APIs and tools are not fully tested for the
VMS/OpenVMS operating sytem, and they'll be made available at
the place where the new CDF 2.7 library is located once fully
tested. The improved CDF 2.7 library is available now.
For UNIX systems via anonymous FTP:
"anonymous" account on nssdcftp.gsfc.nasa.gov in <A HREF="ftp://nssdcftp.gsfc.nasa.gov/standards/cdf/dist/cdf27/unix">standards/cdf/dist/cdf27/unix</A>.
The files you need vary depending on what your needs are. Download
the README file first and determine which tar file and installation
instructions are needed.
There are 3 tar files (i.e. cdf27-dist-all.tar, cdf27-dist-cdf.tar,
cdf27-dist-java.tar), and each of these tar files comes in two
different compressed formats. The file that has an extension of
'.Z' is compressed with the Unix 'compress' command. The file that
has an extension of '.gz' is compressed with the Gnu 'gzip' utility.
The uncompressed `tar' file is for those systems running the POSIX
Shell (on a VAX or Alpha) which typically does not have the
`uncompress' utility. Use binary mode to transfer a compressed or
an uncompressed tar file.
For 16-bits MS-DOS systems via anonymous FTP:
"anonymous" account on nssdcftp.gsfc.nasa.gov in <A HREF="ftp://nssdcftp.gsfc.nasa.gov/standards/cdf/dist/cdf27/dos">standards/cdf/dist/cdf27/dos</A>.
The files you need are README.DOS and `*.EXE' (self-extracting LHA
files). Use binary mode to transfer the `.EXE' files. This file is
intended for older 16-bit DOS.
For 32-bits Windows 95/98/NT systems via anonymous FTP:
"anonymous" account on nssdcftp.gsfc.nasa.gov in <A HREF="ftp://nssdcftp.gsfc.nasa.gov/standards/cdf/dist/cdf27/w32">standards/cdf/dist/cdf27/w32</A>.
The file you need is `CDF27.EXE' (a self-extracting WinZip file).
Use binary mode to transfer the `CDF27.EXE' file. Once files are
extracted, read the README.1st file to see the contents of the
distribution. Another file, CDFTOOLS32.EXE, contains individual CDF tools
programs. They can replace the MS VC++-based tools distributed in
CDF27.EXE. All files are for 32-bit systems.
For Macintosh systems via anonymous FTP:
"anonymous" account on nssdcftp.gsfc.nasa.gov in <A HREF="ftp://nssdcftp.gsfc.nasa.gov/standards/cdf/dist/cdf27/mac">standards/cdf/dist/cdf26/mac</A>.
The file you need is `cdf27.sea.hqx' (a BinHex'ed self-extracting
archive file). Use text mode when transferring the `cdf27.sea.hqx'
file. Once files are extracted, read the README.1st file to see
the contents of the distribution.
For MS-DOS, Windows 95/98/NT and Macintosh systems via floppy disk:
Send a request to <A HREF="#cdfsupport">CDFSUPPORT</A> with your
mailing address and the type of floppy disk you are able to read.
Some general information on CDF, including this FAQ, and software are also
available from the URL: <A HREF="http://nssdc.gsfc.nasa.gov/cdf/cdf_home.html">http://nssdc.gsfc.nasa.gov/cdf/</A>.
<B>7. How widely used is CDF?</B>
The CDF software package is used by hundreds of government agencies,
universities, and private and commercial organizations as well as independent
researchers on both national and international levels. CDF has been adopted
by the International Solar-Terrestrial Physics (ISTP) project as well as the
Central Data Handling Facilities (CDHF) as their format of choice for
storing and distributing key parameter data. As of August 1999, there are
over 4,500 users downloaded the CDF distribution package since April 1996.
<B>8. What are the differences between CDF and netCDF, and CDF and HDF?</B>
The differences between the following formats are based on a high level
overview of each. The information was obtained from various publicly
available documentation and articles. To the best of our knowledge, the
information below is accurate, although the accuracy may deviate from
time to time depending on the timing of new releases. The best and most
complete way to evaluate what package best fulfills your requirements
is to acquire a copy of the documentation and software from each
institution and examine them thoroughly.
</PRE>
<p>
<UL>
<LI>CDF vs. netCDF!<p>
CDF was designed and developed in 1985 by the National Space Science
Data Center (NSSDC) at NASA/GSFC. CDF was originally written in
FORTRAN and only available on the VAX/VMS environments.
<A HREF="http://www.unidata.ucar.edu/packages/netcdf/index.html">NetCDF</A> was
developed a few years later by the National Center for Atmospheric Research
(NCAR). The netCDF model was based on that of the CDF conceptual model
but provided a number of additional features (such as C language
bindings, portable to a number of platforms, machine-independent data
format, etc.). Today both models and existing software have matured
substantially since and are quite similar in most respects,
although they do differ in the following ways:<P>
<UL>
<LI>Although the interfaces do provide the same basic functionality they
do differ syntactically. (See users guides for details.)
<P>
<LI>NetCDF supports named dimensions (i.e., TEMP[x, y, ...]) whereas CDF
utilizes the traditional logical (i.e., TEMP[true, true, ...]) method of
indicating dimensionality.
<P>
<LI>CDF supports both multi- and single file filing systems whereas netCDF
supports only single file filing systems.
<P>
<LI>CDF software can transparently access data files in any encoding
currently supported by the CDF library (For example: a CDF application
running on a Sun can read and write data encoded in a VAX format.) in
addition to the machine-independent (XDR) encoding. netCDF software
reads and writes data in only the XDR data encoding.
<P>
<LI>The CDF library supports an internal caching algorithm in which the
user can make modifications (if so desired) to tweak performance.
<P>
<LI>The netCDF data object is currently accessible via the HDF software; CDF
is not.
<P>
<LI> As part of the CDF distribution, there exist a number of easy-to-use
tools and utilities that enable the user to edit, browse, list, prototype,
subset, export to ASCII, compare, etc. the contents of CDF data files.
<P>
<LI>All CDF development (ports, new features, etc.) and user support
activities are centralized.
</UL>
<P><LI>CDF vs. HDF!<P>
CDF is a scientific data management software package and format based on
a multidimensional (array) model.
<A HREF="http://hdf.ncsa.uiuc.edu">HDF</A> is a Hierarchal Data Format
developed at the National Center for Supercomputing Applications
(NCSA) at the University of Illinois. The HDF data model is based on
the hierarchical relationship and dependencies among data. Although the
two models differ (in many ways like comparing apples to oranges)
significantly in their level of abstraction and the way in which their
inherent structures are defined and accessed, there exists a large
overlap in the types of scientific data that each can support. Some of
the obvious differences are as follows:<P>
<UL>
<LI>The HDF structure is based on a tagged format, storing tag
identifiers (i.e., utility, raster image, scientific data set, and
Vgroup/Vdata tags) for each inherent data object. The basic structure
of HDF consists of an index with the tags of the objects in the file,
pointers to the data associated with the tags, and the data
themselves. The CDF structure is based on variable definitions (name,
data type, number of dimensions, sizes, etc.) where a collection of data
elements is defined in terms of a variable. The structure of CDF
allows one to define an unlimited number of variables
completely independent (loosely coupled) of one another and disparate
in nature, a group of variables that illustrate a strong dependency
(tightly coupled) on one another or both simultaneously. In addition
CDF supports extensive metadata capabilities (called attributes),
which enable the user to define further the contents of a CDF file.
<P>
<LI>HDF supports a set of interface routines for each supported object
(Raster Image, Pallets, Scientific Data Sets, Annotation, Vset, and
Vgroup) type. CDF supports two interfaces from which a CDF file can be
accessed: the Internal Interface and the Standard Interface. The
Internal Interface is very robust and consists of one variable argument
subroutine call that enables a user to utilize all the functionality
supported via CDF software. The Standard Interface is built on top of
the Internal Interface and consists of 23 subroutine calls with a fixed
argument list. The Standard Interface provides a mechanism in which novice
programmers can quickly and easily create a CDF data file.
<P>
<LI>HDF currently offers some compression for storing certain types of
data objects, such as images. CDF supports compression of any data type
with a choice of run-length encoding, Huffman, adaptive Huffman, and Gnu's
ZIP algorithms.
<P>
<LI>CDF supports an internal cache in which the user can modify the size
through the Internal Interface to enhance performance on specific
machines.
<P>
<LI>HDF data files are difficult to update. Data records are physically
stored in a contiguous fashion. Therefore, if a data record needs to be
extended it usually means that the entire file has to be rewritten.
CDF maintains an internal directory of pointers for all the variables
in a CDF file and does not require all the data elements for a given
variable to be contiguous. Therefore, existing variables can be
extended, modified, and deleted, and new variables added to the existing
file.
<P>
<LI>In the late 1980's the CDF software was redesigned and rewritten (CDF
2.0) in C. With little or no impact on performance, the redesign provided for
an open framework that could be easily extended to incorporate new
functionality and features when needed. CDF is currently at Version 2.7, and
performance has been enhanced significantly.
<P>
<LI>CDF supports both host encoding and the machine-independent (XDR)
encoding. In addition, the CDF software can transparently access data
files in any encoding currently supported by the CDF library (For
example, a CDF application running on a Sun can read and write data
encoded in a VAX format.) HDF supports both host encoding and the
machine-independent (XDR) encoding.
</UL>
</UL>
<PRE><A NAME="docs"><B>9. What documentation is available?</B></A>
The CDF documentation set is available via anonymous FTP on nssdcftp.gsfc.nasa.gov
in <A HREF="ftp://nssdcftp.gsfc.nasa.gov/standards/cdf/doc/">standards/cdf/doc</A>, as compressed, uncompressed, and divided PostScript files.
The set consists of the CDF User's Guide (UG), the CDF C Reference Manual
(CRM), the CDF Fortran Reference Manual (FRM), and the CDF Internal Format
Description (IFD). The documents are each approximately 200 pages.
Availability of the CDF 2.7 documentation via anonymous FTP:
The uncompressed PostScript files are:
cdf27ug.ps (CDF User's Guide)
cdf27crm.ps (CDF C Reference Manual)
cdf27frm.ps (CDF Fortran Reference Manual)
cdf27ifd.ps (CDF Internal Format Description)
These files should be transferred using text mode.
PostScript files compressed using the `compress' utility are:
cdf27ug.ps.Z
cdf27crm.ps.Z
cdf27frm.ps.Z
cdf27ifd.ps.Z
These files should be transferred using binary mode and are
uncompressed by using the `uncompress' utility. Note that these
files may not be usable on VMS systems.
PostScript files compressed using the Gnu `gzip' utility are:
cdf27ug.ps.gz
cdf27crm.ps.gz
cdf27frm.ps.gz
cdf27ifd.ps.gz
The uncompressed PDF files are:
cdf27ug.pdf (CDF User's Guide)
cdf27crm.pdf (CDF C Reference Manual)
cdf27frm.pdf (CDF Fortran Reference Manual)
cdf27ifd.pdf (CDF Internal Format Description)
These files should be transferred using binary mode.
PDF files compressed using the `compress' utility are:
cdf27ug.pdf.Z
cdf27crm.pdf.Z
cdf27frm.pdf.Z
cdf27ifd.pdf.Z
These files should be transferred using binary mode and are
uncompressed by using the `uncompress' utility. Note that these
files may not be usable on VMS systems.
PDF files compressed using the Gnu `gzip' utility are:
cdf27ug.pdf.gz
cdf27crm.pdf.gz
cdf27frm.pdf.gz
cdf27ifd.pdf.gz
These files should be transferred using binary mode and are
uncompressed by using the Gnu `gzip' utility with the `-d' option.
Note that these files may not be usable on VMS systems.
PostScript files containing 40-page `chunks' of each document are
in the directory named `Chunks' and consist of:
cdf27ug_1.ps cdf27crm_1.ps cdf27frm_1.ps
cdf27ug_2.ps cdf27crm_2.ps cdf27frm_2.ps
cdf27ug_3.ps cdf27crm_3.ps cdf27frm_3.ps
cdf27ug_4.ps cdf27crm_4.ps cdf27frm_4.ps
cdf27ug_5.ps cdf27crm_5.ps cdf27frm_5.ps
These files should be transferred using text mode.
If you have trouble printing any of the PostScript or Portable Document Format
file(s), send <A HREF="#cdfsupport">CDFSUPPORT</A> a request for hard copies of the documents
indicating which documents you need, and we will mail you a hard copy.
Some general information on CDF, including this FAQ, and software are also
available from the CDF Home Page on the World Wide Web with the following URL:
<A HREF="http://nssdc.gsfc.nasa.gov/cdf/cdf_home.html">http://nssdc.gsfc.nasa.gov/cdf/</A>
The complete description of the CDF Java APIs is only available on line at URL:
<A HREF="http://nssdc.gsfc.nasa.gov/cdf/cdfjava_doc/index.html">http://nssdc.gsfc.nasa.gov/cdf/cdfjava_doc/index.html</A>
<B>10. What are some publications/references relating to CDF?</B>
A list of publications about or referring to CDF can be found on the
CDF ftp site on nssdcftp.gsfc.nasa.gov in the
<A HREF="ftp://nssdcftp.gsfc.nasa.gov/standards/cdf/doc/papers/">standards/cdf/doc/papers</A> directory
as <A HREF="ftp://nssdcftp.gsfc.nasa.gov/standards/cdf/doc/papers/Bibliography.html">Bibliography.html</A>. The same directory contains PostScript and/or text
files for a few of the papers.
The following is a list of some references:
Mathews, G. J., and S. S. Towheed, "<A HREF="http://nssdc.gsfc.nasa.gov/omniweb/misc/papers/www95.html"><i>OMNIWeb: The First Space Physics Data
WWW-Based Data Browsing and Retrieval System</I></A>," Computer Networks and ISDN
Systems, Proceedings of the Third International WWW Conference, Vol. 27,
No. 6, April 1995, pp. 801-808.
Goucher, G. W., and G. J. Mathews, "<i>A Comprehensive Look at CDF</i>,"
NSSDC/WDC-A-R&S 94-07, NASA/Goddard Space Flight Center, August 1994.
S. Brown, M. Folk, G. Goucher, and R. Rew, "<i>Software for Portable Scientific
Data Management</i>," Computers in Physics, Vol. 7, No. 3, pp. 304-308,
May/June 1993.
Salem, K., "<i>MR-CDF: Managing Multi-Resolution Scientific Data</i>," CESDIS
TR 92-81, NASA/Goddard Space Flight Center, Greenbelt, Maryland, March 1992.
Treinish, L. A. (ed.), "<i>Data Structures and Access Software for Scientific
Visualization</i>," A Report on a Workshop at Siggraph '90, Computer
Graphics, 25, No. 2, April 1991.
Treinish, L. A., and G. W. Goucher, "<i>A Data Abstraction for the Source-
Independent Storage and Manipulation of Data</i>," National Space Science
Data Center Technical Paper, NASA/Goddard Space Flight Center, August 1988.
Treinish, L. A., and M. L. Gough, "<i>A Software Package for the Data-
Independent Storage of Multi-Dimensional Data</i>," EOS Transactions,
American Geophysical Union, 68, pp. 633-635, 1987.
<i>IDL Scientific Data Formats, Version 3.6</i>, Research Systems Incorporated,
Boulder, Colorado, April 1994.
<A NAME="cdfsupport">
<B>11. How can I contact them?</B></A>
Comments, suggestions, and questions may be directed to CDF User Support via
</PRE>
<UL>
<LI>WWW feedback form:<p>
<PRE>
<A HREF="/cdf/form/feedback.html">http://nssdc.gsfc.nasa.gov/cdf/form/feedback.html</A>
</PRE>
<P>
<LI>Electronic mail:<br>
<PRE>
Internet : <A HREF="mailto:cdfsupport@nssdca.gsfc.nasa.gov">cdfsupport@nssdca.gsfc.nasa.gov</A>
NSI/DECnet : NSSDCA::CDFSUPPORT
</PRE>
<P>
<LI>U.S. mail:<br>
<PRE>
Attention: CDF USER SUPPORT OFFICE
National Space Science Data Center
Code 633
NASA/Goddard Space Flight Center
Greenbelt, Maryland 20771-0001
U.S.A.
</PRE>
<P>
<LI>Telephone:<br>
<PRE>
Voice: (301) 286-9884
FAX: (301) 286-1771
</PRE>
</UL>
<PRE>
Before asking a question, you should check the FAQ and CDF User's Guide.
<B>12. How do I make a bug report?</B>
All bug reports, comments, suggestions, and questions should go to
<A HREF="mailto:cdfsupport@nssdca.gsfc.nasa.gov">cdfsupport@nssdca.gsfc.nasa.gov</A>.
Attached below is a bug report template. It would be very helpful
for us to locate and fix the bug if all the information requested in
the template is supplied by the reporter. It is very difficult for
us to trace bugs when we don't have enough background information.
You can also fill out the form via a Web browser with the following URL:
<A HREF="http://nssdc.gsfc.nasa.gov/cdf/form/helpdesk.html">http://nssdc.gsfc.nasa.gov/cdf/form/helpdesk.html</A>.
------------------ Template for bug report ------------------------
To: cdfsupport@nssdca.gsfc.nasa.gov.
Subject: [area]: [synopsis] [replace with actual AREA and SYNOPSIS]
VERSION:
[CDF library/toolkit version (e.g., CDF Version 2.5.1).
You can obtain the version of your CDF distribution
using the CDFinquire toolkit program and specifying the "/ID"
argument as in the examples below:
From MS-DOS: cdfinq -id
From UNIX: cdfinquire -id
From VMS: CDFINQUIRE /ID ]
USER:
[Name, telephone number, and address of person reporting the bug.
(email address if possible)]
AREA:
[Area of the CDF source tree affected, e.g., lib, tools, tests,
toplevel. If there are bugs in more than one AREA, please use
a separate bug report for each AREA.]
SYNOPSIS:
[Brief description of the problem and where it is located]
MACHINE / OPERATING SYSTEM:
[e.g. Sparc/SunOS 4.1.3, HP9000/730-HPUX9.01...]
COMPILER:
[e.g. native cc, native ANSI cc, Borland C++ V3.1, MPW, ...]
DESCRIPTION:
[Detailed description of problem.]
REPEAT BUG BY:
[What you did to get the error; include test program or session
transcript if at all possible. If you include a program, make
sure it depends only on libraries in the CDF distribution, not
on any vendor or third-party libraries. Please be specific;
if we can't reproduce it, we can't fix it. Tell us exactly what
we should see when the program is run.]
SAMPLE FIX:
[If available, please send context diffs (diff -c).]
[PLEASE make your Subject (SYNOPSIS): line as descriptive as possible.]
[Remove all the explanatory text in brackets before mailing.]
[Send to cdfsupport@nssdca.gsfc.nasa.gov or to
Attn: CDF SUPPORT
NASA/GSFC/NSSDC
Code 633
Greenbelt, MD 20771, U.S.A. ]
------------------ End of Bug Report Template ----------------------
<B>13. Is there a mailing list for CDF discussions, questions and announcements?</B>
There are two mailing lists:
1. <EM>General Mailing List</EM> and
2. <EM>CDF Users Discussion Mailing List</EM>.
*** The general mailing list is for major announcements and events. ***
The general mailing list announces new releases and general announcements.
Anyone who contacts <A HREF="#cdfsupport">CDFSUPPORT</A> will automatically be added to the
general mailing list, unless requested otherwise.
For administration questions, for additions or deletions from the general
mailing list or for corrections to the list, please send messages to
<a href="mailto:cdfsupport@nssdca.gsfc.nasa.gov">cdfsupport@nssdca.gsfc.nasa.gov</a>
*** The CDF users mailing list is for discussions. ***
CDF has a mailing list for electronic discussions between programmers
and users of CDF. This service has been created to encourage the CDF
user community to share knowledge and experience and to reduce duplication
of effort.
To subscribe to this mailing list, please send an e-mail message to
"<a href="mailto:Majordomo@nssdc.gsfc.nasa.gov"><EM>Majordomo@nssdc.gsfc.nasa.gov</EM></a>" with the body of the message as the
following one-line command:
SUBSCRIBE cdf-users
Note that commands in the "Subject:" line are NOT processed.
To send a message to the mailing list, send your message to
<A HREF="mailto:cdf-users@nssdc.gsfc.nasa.gov">cdf-users@nssdc.gsfc.nasa.gov</A>
For more information about this mailing list, see the mailing list
introduction page via one of the following URLs:
<A HREF="/cdf/html/cdf-users-intro.html">http://nssdc.gsfc.nasa.gov/cdf/html/cdf-users-intro.html</A>
<A HREF="ftp://nssdcftp.gsfc.nasa.gov/standards/cdf/doc/mail-archives/cdf-users.intro">ftp://nssdcftp.gsfc.nasa.gov/standards/cdf/doc/mail-archives/cdf-users.intro</A>
<A NAME="common">
<B>14. What programming languages does CDF support?</B>
The CDF library comes with C, Java and Fortran Application programming
Interfaces (APIs) that provide the essential framework on which
graphical and data analysis packages can be created. The CDF library allows
developers of CDF-based systems to easily create applications that permit
users to slice data across multidimensional subspaces, access entire
structures of data, perform subsampling of data, and access one data
element independently regardless of its relationship to any other data
element.
<B>15. Are CDF data sets/files platform-independent?</B>
CDF data sets are portable across any platform supported by CDF. These
currently consist of VAX (OpenVMS and POSIX shell), Sun (SunOS & SOLARIS),
DECstation (ULTRIX), DEC Alpha (OSF/1 & OpenVMS), Silicon Graphics Iris and
Power Series (IRIX), IBM RS6000 series (AIX), HP 9000 series (HP-UX),
NeXT (Mach), PC (DOS, Windows 3.x, Windows NT/95/98, Linux, QNX, Solarise,
& Cygwin), and
Macintosh (68K & Power PC running MacOS 7/8/9.x, X or Linux).
<B>16. What's the difference between a single-file CDF and multi-file CDF?</B>
The CDF library gives the user option to choose from one of two file formats:
single-file and multi-file. Single-file CDF contains the control information,
metadata, and the data values for each of the variables in one file. Whereas
multi-file CDF has two parts to it: metadata file and data file (one for
each variable.
The main advantage of the single-file format is that it minimizes the number
of files one has to manage and makes it easier to transport CDFs across a
network. The organization of the data within the single file may, however,
become somewhat convoluted, slightly increasing the data access time. The
multi-file format, on the other hand, clearly delimits the data from the
metadata and is organized in a consistent fashion within the files. Updating,
appending, and accessing data are also done with optimum efficiency.
For multi-file format CDFs, certain restrictions are applied. They are:
- Compression: Compression is not allowed for the CDF or any of its variables.
- Sparseness: Sparse records or arrays for variables are not allowed.
- Allocation: Pre-allocation of records or blocks of records is not allowed.
For each variable, the maximum written record is the last
allocated record.
<B>17. What are the differences between the Standard Interface, Internal
Interface, and the CDF Java APIs?</B></A>
The CDF library comes with two types of Application Programming Interfaces
(APIs) for C and Fortran: Standard Interface and Internal Interface.
The Standard Interface allows to perform basic operations such as creating
a CDF file, opening a CDF file, and reading and writing data. See chapter
5 of the CDF Fortran Reference Manual or the CDF C Reference Manual for a
complete list of the APIs that are available for the Internal Interface and
their detailed descriptions.
The Internal Interface consists of only one routine, <I>CDFlib</I>. CDFlib
is used to perform all possible operations on a CDF. In fact, all of the
Standard Interface functions are implemented using the Internal Interface.
CDFlib must be used to perform operations not possible with the Standard
Interface functions. (e.g., specifying a single-file format for a CDF,
accessing zVariables, compressing a CDF file or variables or specifying
a pad value for an rVariable or zVariable). Note that CDFlib can also be
used to perform certain operations more efficiently than with the Standard
Interface functions.
The Standard Interface and Internal Interface do not apply to the CDF Java
APIs. The CDF Java APIs can do everything the C and Fortran APIs can do and
more such as copying a variable with or without data). Click <A HREF="http://nssdc.gsfc.nasa.gov/cdf/cdfjava_doc/index.html">here</A> for a
detailed description of the CDF Java APIs.
<B>18. What's "variable"?</B>
CDF's "variable" is a generic name or an object that represents data
where data can be 0-dimensional (scalar data) or multi-dimensional (up to
10-dimension), and it does not have any scientific context associated it.
For example, a variable can be data representing an independent variable, a
dependent varibale, time and date value, or whatever data might be (e.g.
image, XML file, etc.). In other words, the variable doesn't contain any
hidden meanings other than the data itself. One may describe one
variable's relationship with other variable(s) through "attributes".
There are two types of variables (rVariable and zVariable) and they can
coexist in the same CDF file. Every rVariable in a CDF must have the
same number of dimensions and dimension sizes, whereas each zVariable has
its own dimension and dimension size. Suppose there are 2 rVariables
(v1, v2) in a CDF. Let's say v2 is defined as 2:[20,10] - 2-dimensional
array with its size of 20 x 10 (20 rows and 10 columns). Then v1 <B>MUST</B>
be defined as 2:[20,10] albeit it only needs 1:[8] (since it is a rVariable).
But if this model is implemented using zVariables, then v1 and v2 can be
defined as 1:[20,10] and 1:[8] instead of 1:[20,10] and 1:[20,10]. As you
can see above, since all the rVariables must have the same dimensions and
dimension sizes, there'll be a lot of disk space wasted if a few variables
need big arrays and many variables need small arrays.
So why would you want to use rVariables over zVariables? There's no reason
to use rVariables at all (since zVariables are much more efficient) if you
are creating a new CDF file. But if you are analyzing data files that were
created with early CDF releases or contain rVariables for some reason, you'll
need to use rVariables. One may wonder why there are rVariables and
zVariables, not just zVariables. When CDF was first introduced in early
90's, only rVariables were available. The inefficiencies with rVariables
were quickly realized and addressed with the introduction of zVariables in
later CDF releases.
<B>19. What compression algorithms does CDF support?</B>
Compression may be specified for a single-file CDF and the CDF library can
be instructed to compress a CDF as it is written to disk. This compression
occurs transparently to the user. When a compressed CDF is opened, it is
automatically decompressed by the CDF library. An application does not have
to even know that a CDF is compressed. Any type of access is allowed on a
compressed CDF. When a compressed CDF is closed by an application, it is
automatically recompressed as it is written back to disk.
The individual variables of a CDF can also be compressed. . The CDF library
handles the compression and decompression of the variable values
transparently. The application does not have to know that the variable
is compressed as it accesses the variable's values.
The following compression algorithms are supported by the CDF library:
- Run-Length Encoding
- Huffman
- Adaptive Huffman
- GZIP
See chapter 2 of the CDF User's Guide for more detailed information for
the compression algorithms mentioned above.
<B>20. Can CDF delete a range of records?</B>
Yes. The Standard Interface doesn't have a function that allows users to
delete a range of records, but the Internal Interface and the CDF Java
APIs provide APIs for deleting a range of records.
The following APIs/method allows to delete a range of records:
C or Fortran: <DELETE_, rVAR_RECORDS> or <DELETE_, zVAR_RECORDS>
CDF Java API: deleteRecords(firstRec, lastRec) method in the Variable class
Chapter 6 of the CDF C Reference Manual and CDF C Reference Manual contain
more detailed description of the <DELETE_, rVAR_RECORDS> and
<DELETE_, zVAR_RECORDS> APIs/functions.
Click <A HREF="http://nssdc.gsfc.nasa.gov/cdf/cdfjava_doc">here</A> and select the "Variable" link in the lower left frame
for a detailed description of the <I>deleteRecords</I> method.
<B>21. Does CDF support random file access?</B>
Yes. One of the strengths of CDF is random file access and ability to
specify the granularity of data read/write. Suppose you have a CDF file
that has 1000 records in it. You can extract data by specifying:
- the individual record number you want to extract data from
OR
- the start and end record number from which data is to be retrieved. You
can also specify the record interval that defines the number of records
to skip between successive reads. For example, if you specified 100, 200,
and 2 for the start record number, the end record number, and the record
interval respectively, the CDF system will start reading data from the
record number 100 and read every other record (100, 102, 104, 106, and so
on) until it reaches the record number 200. If a record is an array
(contains more than one value), you can even specify which element(s) you
want to extract for every record.
The following methods in the Variable object class provide random file
access for reading:
getRecord, getScalarData, getSingleData, getHyperData.
The following methods in the Variable object class provide random file
access for writing:
putRecord, putScalarData, putSingleData, putHyperData.
Click <A HREF="http://nssdc.gsfc.nasa.gov/cdf/cdfjava_doc">here</A> and select the "Variable" link in the lower left frame
for a detailed description of the methods/APIs described above.
<B>22. How do I optimize a CDF file or can I read data fast?</B>
Before reading data (whether it's a sequential or random/direct access),
it's <B>important</B> to minimize the number of internal pointers (a.k.a data
defragmentation) that are used to keep track of where data is located
within a file before reading data (whether it's a sequential read or
random/direct access read). The <I>CDFconvert</I> CDF utility, among many
other functions it performs, allows users to optimize a CDF file and one
can optimize a CDF file by entering the following command at the operating
system prompt:
CDFconvert <source cdf> <dest cdf>
where <source cdf> = the name of the CDF file to be optimized
<dest cdf> = the name of the newly optimized file
For example, the following command reads a file called test.cdf and creates
an optimized file called test_new.cdf.
CDFconvert test.cdf test_new.cdf
<B>NOTE:</B> It's always a good idea to run CDFconvert on newly created CDF
files since reading data from an optimized file is very fast. This is
especially true if the size of the file is big.
<A NAME="general">
<B>23. What is the best way to represent my data using CDF?</B></A>
There are many ways to represent the same information in any general-purpose
data model, and there is no single "correct" way to store data in a CDF. The
user has complete control over how the data values are stored in the CDF
(within the confines of the variable array structure) depending on how the
user views the data. This is the advantage of CDF. Data values are organized
in whatever way makes sense to the user.
We provide some guidelines in the <A HREF="#docs">CDF User's Guide</A> (e.g., "<I>Organizing Your Data
in a CDF</I>"), but we've found that a little experience helps. Occasionally,
we have decided it was useful to change the structure of the CDF files after
experience with how the data are used.
<B>24. Can new versions of CDF applications read CDF files written using older
versions of the CDF library?</B>
Our goal is to make CDF backward compatible in the sense that CDF files
can always be read by new versions of the CDF library and tools. We have
succeeded in doing so up to CDF 2.4 and will continue to follow the principle
as much as possible. However, CDF 2.5 will not read the obsolete CDF V1.0
files, so Version 1.0 CDFs must be converted using the CDF Version 2.4
CDFconvert tool.
However, old CDF libraries are generally NOT able to read CDF files
written by newer versions of the CDF library. In some cases old CDF libraries
can read CDFs from newer versions, such as CDF V2.3 reading CDF V2.4 files,
but this is NOT recommended. Our policy is for CDF toolkit applications to
read any CDF data set, but your own applications should use the latest CDF
library.
<B>25. Can my application programs that work with old versions of the CDF
library always be compiled with new versions of CDF?</B>
As CDF evolves new functions are added, but they are transparent to
applications since all existing functions of the Internal and Standard
interfaces are unchanged. However, the CDF Version 1.0 "Obsolete" interface,
which was only callable from Fortran, is no longer supported with CDF V2.5.
The Standard Interface is similar to the CDF V1.0 interface with several
additions for new features. Those functions not available using the Standard
Interface are made available using the Internal Interface. The Internal
Interface makes CDF an easily extensible software package, so all applications
using this interface will remain portable with newer versions of the CDF
library.
However, some advanced CDF users have exploited undocumented CDF functions
that were removed from a later version of the library and could not compile
their application. This practice is NOT recommended because these functions
are for internal use only and subject to change without notice in future
releases. If some internal CDF function is useful to your application, then
you should notify us, and we will determine if it is appropriate to make it
visible in the next release. In the meanwhile, the portable approach would
be to copy the corresponding function(s) from the CDF library source, rename
the function name(s) to something else, insert it into your application code,
and update the calling statements with the new function name(s).
<B>26. Is there any commercial or public domain visualization software that
accepts CDF files?</B>
Commercial software:
Application Visualization System (<A HREF="http://www.avs.com/">AVS</A>)
IBM Visualization Data Explorer (<A HREF="http://www-i.almaden.ibm.com/dx/">DX</A>)
Interactive Data Language (<A HREF="http://www.rsinc.com/">IDL</A>)
Public domain software:
<!-- CMIT, CWIT, CXIT (available from nssdc in <A HREF="ftp://nssdcftp.gsfc.nasa.gov/standards/cdf/apps/graphics/">pub/cdf/apps/graphics</A>) -->
LinkWinks (contact berkin@krazy.jpl.nasa.gov)
There are also several NSSDC developed WWW-based data systems that
provide access to data stored in CDF and use IDL to generate time
series plots of selected variables, which are available via the
following URLs:
CDAWeb - <A HREF="http://cdaweb.gsfc.nasa.gov/cdaweb/">http://cdaweb.gsfc.nasa.gov/cdaweb/</A>
COHOWeb - <A HREF="http://nssdc.gsfc.nasa.gov/cohoweb/cw.html">http://nssdc.gsfc.nasa.gov/cohoweb/cw.html</A>
OMNIWeb - <A HREF="http://nssdc.gsfc.nasa.gov/omniweb/ow.html">http://nssdc.gsfc.nasa.gov/omniweb/ow.html</A>
Example visualizations can be found via the URL:
<A HREF="http://nssdc.gsfc.nasa.gov/cdf/html/examples.html">http://nssdc.gsfc.nasa.gov/cdf/html/examples.html</A>
<A NAME="conversion">
<B>27. Are there any conversion programs available to convert non-CDF files
into CDF files or vice versa?</B></A>
MakeCDF is a CDF application that reads flat data sets, in both
binary and text and generates a CDF data set from that data.
It is available via WWW with the URL:
<A HREF="http://spdf.gsfc.nasa.gov/makecdf.html">http://spdf.gsfc.nasa.gov/makecdf.html</A>
and anonymous ftp from nssdc.gsfc.nasa.gov in the <A HREF="ftp://nssdc.gsfc.nasa.gov/pub/tools/istp/">/pub/tools/istp</A> directory.
CDFexport is a tool that can generate an ASCII text file or subsetted CDF of
the selected variables from a CDF, among other things. It is part of the CDF
distribution package.
(See <a href="#source">question 6</a> above to obtain source code for <A HREF="http://nssdc.gsfc.nasa.gov/cdf/html/tools.html">CDF tools</A>.)
<B>28. How can I contribute my software to the CDF user community?</B>
Contact <A HREF="#cdfsupport">CDFSUPPORT</A> indicating that you would like to contribute your
software to the CDF user community. Please fill out the <i>CDF Directory
Entry form</i> (<a href="ftp://nssdcftp.gsfc.nasa.gov/standards/cdf/doc/forms/direct">pub/cdf/doc/forms/direct</a>) and send the completed form to
us. Your information will then be added to our Directory of CDF User
Applications, and users may contact you for information on your
application.
However, if you want us to distribute your application, we will set up a
directory for you to send the contribution package. For other users'
convenience your contribution package should include the software itself,
a Makefile if possible, a man-page, test programs, and input data files for
testing. A README file is also required. It should describe briefly the
purpose, function, and limitation of the software; on which platforms and
operating systems it runs; how to compile, install, and test it; whom and
where to contact for comments, suggestions, or bug reports.
We encourage CDF users to let us know about their applications, so
other CDF users benefit from it as well.
Some CDF-related software are available from the URL:
<A HREF="ftp://nssdcftp.gsfc.nasa.gov/standards/cdf/apps/">ftp://nssdcftp.gsfc.nasa.gov/standards/cdf/apps</A>.
</PRE>
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comments or questions should be sent to should be sent to the CDF User Support Office via email to <A HREF="mailto:cdfsupport@nssdca.gsfc.nasa.gov"><B>cdfsupport@nssdca.gsfc.nasa.gov</B></A>.
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