Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.astro.louisville.edu/software/sbig/archive/xmccd-4.1/xmccd-4.1e/libcfitsio/libcfitsio-3.310/cfitsio.ps Äàòà èçìåíåíèÿ: Wed Jul 18 21:52:52 2012 Äàòà èíäåêñèðîâàíèÿ: Thu Feb 27 22:51:06 2014 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: ï ï ï ï ï ï ð ï ð ï ð ï ð ï ð ï ð ï ð ï ð ï ð ï ð ï ð ï ð ï ð ï ð ï ð ï

CFITSIO User's Reference Guide
An Interface to FITS Format Files
for C Programmers
Version 3.3
HEASARC
Code 662
Goddard Space Flight Center
Greenbelt, MD 20771
USA
April 2012

ii

Contents
1 Introduction 1
1.1 A Brief Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Sources of FITS Software and Information . . . . . . . . . . . . . . . . . . . . . . . . 1
1.3 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.4 Legal Stu# . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2 Creating the CFITSIO Library 5
2.1 Building the Library . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.1 Unix Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1.2 VMS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1.3 Windows PCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.1.4 Macintosh PCs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
2.2 Testing the Library . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
2.3 Linking Programs with CFITSIO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
2.4 Using CFITSIO in Multi­threaded Environments . . . . . . . . . . . . . . . . . . . . 9
2.5 Getting Started with CFITSIO . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2.6 Example Program . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
3 A FITS Primer 13
4 Programming Guidelines 15
4.1 CFITSIO Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
4.2 Current Header Data Unit (CHDU) . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
4.3 Function Names and Variable Datatypes . . . . . . . . . . . . . . . . . . . . . . . . . 18
4.4 Support for Unsigned Integers and Signed Bytes . . . . . . . . . . . . . . . . . . . . 20
4.5 Dealing with Character Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
iii

iv CONTENTS
4.6 Implicit Data Type Conversion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
4.7 Data Scaling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
4.8 Support for IEEE Special Values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
4.9 Error Status Values and the Error Message Stack . . . . . . . . . . . . . . . . . . . . 24
4.10 Variable­Length Arrays in Binary Tables . . . . . . . . . . . . . . . . . . . . . . . . . 25
4.11 Multiple Access to the Same FITS File . . . . . . . . . . . . . . . . . . . . . . . . . . 26
4.12 When the Final Size of the FITS HDU is Unknown . . . . . . . . . . . . . . . . . . . 27
4.13 CFITSIO Size Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
5 Basic CFITSIO Interface Routines 29
5.1 CFITSIO Error Status Routines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
5.2 FITS File Access Routines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
5.3 HDU Access Routines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
5.4 Header Keyword Read/Write Routines . . . . . . . . . . . . . . . . . . . . . . . . . . 35
5.4.1 Keyword Reading Routines . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
5.4.2 Keyword Writing Routines . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
5.5 Primary Array or IMAGE Extension I/O Routines . . . . . . . . . . . . . . . . . . . 40
5.6 Image Compression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
5.7 ASCII and Binary Table Routines . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
5.7.1 Create New Table . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
5.7.2 Column Information Routines . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
5.7.3 Routines to Edit Rows or Columns . . . . . . . . . . . . . . . . . . . . . . . . 52
5.7.4 Read and Write Column Data Routines . . . . . . . . . . . . . . . . . . . . . 53
5.7.5 Row Selection and Calculator Routines . . . . . . . . . . . . . . . . . . . . . 56
5.7.6 Column Binning or Histogramming Routines . . . . . . . . . . . . . . . . . . 57
5.8 Utility Routines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
5.8.1 File Checksum Routines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
5.8.2 Date and Time Utility Routines . . . . . . . . . . . . . . . . . . . . . . . . . 61
5.8.3 General Utility Routines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
6 The CFITSIO Iterator Function 73
6.1 The Iterator Work Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
6.2 The Iterator Driver Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
6.3 Guidelines for Using the Iterator Function . . . . . . . . . . . . . . . . . . . . . . . . 77

CONTENTS v
6.4 Complete List of Iterator Routines . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
7 World Coordinate System Routines 81
7.1 Self­contained WCS Routines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
8 Hierarchical Grouping Routines 85
8.1 Grouping Table Routines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
8.2 Group Member Routines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88
9 Specialized CFITSIO Interface Routines 91
9.1 FITS File Access Routines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
9.2 HDU Access Routines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
9.3 Specialized Header Keyword Routines . . . . . . . . . . . . . . . . . . . . . . . . . . 97
9.3.1 Header Information Routines . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
9.3.2 Read and Write the Required Keywords . . . . . . . . . . . . . . . . . . . . . 97
9.3.3 Write Keyword Routines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
9.3.4 Insert Keyword Routines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
9.3.5 Read Keyword Routines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
9.3.6 Modify Keyword Routines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104
9.3.7 Update Keyword Routines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
9.4 Define Data Scaling and Undefined Pixel Parameters . . . . . . . . . . . . . . . . . . 106
9.5 Specialized FITS Primary Array or IMAGE Extension I/O Routines . . . . . . . . . 107
9.6 Specialized FITS ASCII and Binary Table Routines . . . . . . . . . . . . . . . . . . 110
9.6.1 General Column Routines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
9.6.2 Low­Level Table Access Routines . . . . . . . . . . . . . . . . . . . . . . . . . 112
9.6.3 Write Column Data Routines . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
9.6.4 Read Column Data Routines . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
10 Extended File Name Syntax 117
10.1 Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
10.2 Filetype . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 120
10.2.1 Notes about HTTP proxy servers . . . . . . . . . . . . . . . . . . . . . . . . . 120
10.2.2 Notes about the stream filetype driver . . . . . . . . . . . . . . . . . . . . . . 121
10.2.3 Notes about the gsiftp filetype . . . . . . . . . . . . . . . . . . . . . . . . . . 122

vi CONTENTS
10.2.4 Notes about the root filetype . . . . . . . . . . . . . . . . . . . . . . . . . . . 122
10.2.5 Notes about the shmem filetype: . . . . . . . . . . . . . . . . . . . . . . . . . 124
10.3 Base Filename . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124
10.4 Output File Name when Opening an Existing File . . . . . . . . . . . . . . . . . . . 126
10.5 Template File Name when Creating a New File . . . . . . . . . . . . . . . . . . . . . 128
10.6 Image Tile­Compression Specification . . . . . . . . . . . . . . . . . . . . . . . . . . 128
10.7 HDU Location Specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 128
10.8 Image Section . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130
10.9 Image Transform Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131
10.10Column and Keyword Filtering Specification . . . . . . . . . . . . . . . . . . . . . . 132
10.11Row Filtering Specification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 134
10.11.1 General Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
10.11.2 Bit Masks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
10.11.3 Vector Columns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 138
10.11.4 Good Time Interval Filtering . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
10.11.5 Spatial Region Filtering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
10.11.6 Example Row Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
10.12 Binning or Histogramming Specification . . . . . . . . . . . . . . . . . . . . . . . . . 144
11 Template Files 147
11.1 Detailed Template Line Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
11.2 Auto­indexing of Keywords . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
11.3 Template Parser Directives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
11.4 Formal Template Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
11.5 Errors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150
11.6 Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150
12 Local FITS Conventions 153
12.1 64­Bit Long Integers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
12.2 Long String Keyword Values. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 153
12.3 Arrays of Fixed­Length Strings in Binary Tables . . . . . . . . . . . . . . . . . . . . 155
12.4 Keyword Units Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155
12.5 HIERARCH Convention for Extended Keyword Names . . . . . . . . . . . . . . . . 156
12.6 Tile­Compressed Image Format . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156

CONTENTS vii
13 Optimizing Programs 159
13.1 How CFITSIO Manages Data I/O . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159
13.2 Optimization Strategies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
A Index of Routines 165
B Parameter Definitions 171
C CFITSIO Error Status Codes 177

viii CONTENTS

Chapter 1
Introduction
1.1 A Brief Overview
CFITSIO is a machine­independent library of routines for reading and writing data files in the
FITS (Flexible Image Transport System) data format. It can also read IRAF format image files
and raw binary data arrays by converting them on the fly into a virtual FITS format file. This
library is written in ANSI C and provides a powerful yet simple interface for accessing FITS files
which will run on most commonly used computers and workstations. CFITSIO supports all the
features described in the o#cial NOST definition of the FITS format and can read and write all the
currently defined types of extensions, including ASCII tables (TABLE), Binary tables (BINTABLE)
and IMAGE extensions. The CFITSIO routines insulate the programmer from having to deal with
the complicated formatting details in the FITS file, however, it is assumed that users have a general
knowledge about the structure and usage of FITS files.
CFITSIO also contains a set of Fortran callable wrapper routines which allow Fortran programs
to call the CFITSIO routines. See the companion ``FITSIO User's Guide'' for the definition of the
Fortran subroutine calling sequences. These wrappers replace the older Fortran FITSIO library
which is no longer supported.
The CFITSIO package was initially developed by the HEASARC (High Energy Astrophysics Science
Archive Research Center) at the NASA Goddard Space Flight Center to convert various existing
and newly acquired astronomical data sets into FITS format and to further analyze data already in
FITS format. New features continue to be added to CFITSIO in large part due to contributions of
ideas or actual code from users of the package. The Integral Science Data Center in Switzerland,
and the XMM/ESTEC project in The Netherlands made especially significant contributions that
resulted in many of the new features that appeared in v2.0 of CFITSIO.
1.2 Sources of FITS Software and Information
The latest version of the CFITSIO source code, documentation, and example programs are available
on the World­Wide Web or via anonymous ftp from:
1

2 CHAPTER 1. INTRODUCTION
http://heasarc.gsfc.nasa.gov/fitsio
ftp://legacy.gsfc.nasa.gov/software/fitsio/c
Any questions, bug reports, or suggested enhancements related to the CFITSIO package should be
sent to the primary author:
Dr. William Pence Telephone: (301) 286­4599
HEASARC, Code 662 E­mail: William.D.Pence@nasa.gov
NASA/Goddard Space Flight Center
Greenbelt, MD 20771, USA
This User's Guide assumes that readers already have a general understanding of the definition
and structure of FITS format files. Further information about FITS formats is available from the
FITS Support O#ce at http://fits.gsfc.nasa.gov. In particular, the 'NOST FITS Standard'
gives the authoritative definition of the FITS data format, and the `FITS User's Guide' provides
additional historical background and practical advice on using FITS files.
The HEASARC also provides a very sophisticated FITS file analysis program called `Fv' which
can be used to display and edit the contents of any FITS file as well as construct new FITS files
from scratch. The display functions in Fv allow users to interactively adjust the brightness and
contrast of images, pan, zoom, and blink images, and measure the positions and brightnesses of
objects within images. FITS tables can be displayed like a spread sheet, and then modified using
powerful calculator and sorting functions. Fv is freely available for most Unix platforms, Mac PCs,
and Windows PCs. CFITSIO users may also be interested in the FTOOLS package of programs
that can be used to manipulate and analyze FITS format files. Fv and FTOOLS are available from
their respective Web sites at:
http://fv.gsfc.nasa.gov
http://heasarc.gsfc.nasa.gov/ftools
1.3 Acknowledgments
The development of the many powerful features in CFITSIO was made possible through collabora­
tions with many people or organizations from around the world. The following in particular have
made especially significant contributions:
Programmers from the Integral Science Data Center, Switzerland (namely, Jurek Borkowski, Bruce
O'Neel, and Don Jennings), designed the concept for the plug­in I/O drivers that was introduced
with CFITSIO 2.0. The use of `drivers' greatly simplified the low­level I/O, which in turn made
other new features in CFITSIO (e.g., support for compressed FITS files and support for IRAF
format image files) much easier to implement. Jurek Borkowski wrote the Shared Memory driver,
and Bruce O'Neel wrote the drivers for accessing FITS files over the network using the FTP, HTTP,
and ROOT protocols. Also, in 2009, Bruce O'Neel was the key developer of the thread­safe version
of CFITSIO.

1.3. ACKNOWLEDGMENTS 3
The ISDC also provided the template parsing routines (written by Jurek Borkowski) and the
hierarchical grouping routines (written by Don Jennings). The ISDC DAL (Data Access Layer)
routines are layered on top of CFITSIO and make extensive use of these features.
Giuliano Ta#oni and Andrea Barisani, at INAF, University of Trieste, Italy, implemented the I/O
driver routines for accessing FITS files on the computational grids using the gridftp protocol.
Uwe Lammers (XMM/ESA/ESTEC, The Netherlands) designed the high­performance lexical pars­
ing algorithm that is used to do on­the­fly filtering of FITS tables. This algorithm essentially
pre­compiles the user­supplied selection expression into a form that can be rapidly evaluated for
each row. Peter Wilson (RSTX, NASA/GSFC) then wrote the parsing routines used by CFITSIO
based on Lammers' design, combined with other techniques such as the CFITSIO iterator routine
to further enhance the data processing throughput. This e#ort also benefited from a much earlier
lexical parsing routine that was developed by Kent Blackburn (NASA/GSFC). More recently, Craig
Markwardt (NASA/GSFC) implemented additional functions (median, average, stddev) and other
enhancements to the lexical parser.
The CFITSIO iterator function is loosely based on similar ideas developed for the XMM Data
Access Layer.
Peter Wilson (RSTX, NASA/GSFC) wrote the complete set of Fortran­callable wrappers for all the
CFITSIO routines, which in turn rely on the CFORTRAN macro developed by Burkhard Burow.
The syntax used by CFITSIO for filtering or binning input FITS files is based on ideas developed
for the AXAF Science Center Data Model by Jonathan McDowell, Antonella Fruscione, Aneta
Siemiginowska and Bill Joye. See http://heasarc.gsfc.nasa.gov/docs/journal/axaf7.html for further
description of the AXAF Data Model.
The file decompression code were taken directly from the gzip (GNU zip) program developed by
Jean­loup Gailly and others.
The new compressed image data format (where the image is tiled and the compressed byte stream
from each tile is stored in a binary table) was implemented in collaboration with Richard White
(STScI), Perry Greenfield (STScI) and Doug Tody (NOAO).
Doug Mink (SAO) provided the routines for converting IRAF format images into FITS format.
Martin Reinecke (Max Planck Institute, Garching)) provided the modifications to cfortran.h that
are necessary to support 64­bit integer values when calling C routines from fortran programs. The
cfortran.h macros were originally developed by Burkhard Burow (CERN).
Julian Taylor (ESO, Garching) provided the fast byte­swapping algorithms that use the SSE2 and
SSSE3 machine instructions available on x86 64 CPUs.
In addition, many other people have made valuable contributions to the development of CFITSIO.
These include (with apologies to others that may have inadvertently been omitted):
Steve Allen, Carl Akerlof, Keith Arnaud, Morten Krabbe Barfoed, Kent Blackburn, G Bodammer,
Romke Bontekoe, Lucio Chiappetti, Keith Costorf, Robin Corbet, John Davis, Richard Fink, Ning
Gan, Emily Greene, Gretchen Green, Joe Harrington, Cheng Ho, Phil Hodge, Jim Ingham, Yoshi­
taka Ishisaki, Diab Jerius, Mark Levine, Todd Karakaskian, Edward King, Scott Koch, Claire
Larkin, Rob Managan, Eric Mandel, Richard Mathar, John Mattox, Carsten Meyer, Emi Miyata,
Stefan Mochnacki, Mike Noble, Oliver Oberdorf, Clive Page, Arvind Parmar, Je# Pedelty, Tim

4 CHAPTER 1. INTRODUCTION
Pearson, Philippe Prugniel, Maren Purves, Scott Randall, Chris Rogers, Arnold Rots, Rob Sea­
man, Barry Schlesinger, Robin Stebbins, Andrew Szymkowiak, Allyn Tennant, Peter Teuben, James
Theiler, Doug Tody, Shiro Ueno, Steve Walton, Archie Warnock, Alan Watson, Dan Whipple, Wim
Wimmers, Peter Young, Jianjun Xu, and Nelson Zarate.
1.4 Legal Stu#
Copyright (Unpublished--all rights reserved under the copyright laws of the United States), U.S.
Government as represented by the Administrator of the National Aeronautics and Space Adminis­
tration. No copyright is claimed in the United States under Title 17, U.S. Code.
Permission to freely use, copy, modify, and distribute this software and its documentation without
fee is hereby granted, provided that this copyright notice and disclaimer of warranty appears in all
copies.
DISCLAIMER:
THE SOFTWARE IS PROVIDED 'AS IS' WITHOUT ANY WARRANTY OF ANY KIND, EI­
THER EXPRESSED, IMPLIED, OR STATUTORY, INCLUDING, BUT NOT LIMITED TO,
ANY WARRANTY THAT THE SOFTWARE WILL CONFORM TO SPECIFICATIONS, ANY
IMPLIED WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PUR­
POSE, AND FREEDOM FROM INFRINGEMENT, AND ANY WARRANTY THAT THE DOC­
UMENTATION WILL CONFORM TO THE SOFTWARE, OR ANY WARRANTY THAT THE
SOFTWARE WILL BE ERROR FREE. IN NO EVENT SHALL NASA BE LIABLE FOR ANY
DAMAGES, INCLUDING, BUT NOT LIMITED TO, DIRECT, INDIRECT, SPECIAL OR CON­
SEQUENTIAL DAMAGES, ARISING OUT OF, RESULTING FROM, OR IN ANY WAY CON­
NECTED WITH THIS SOFTWARE, WHETHER OR NOT BASED UPON WARRANTY, CON­
TRACT, TORT , OR OTHERWISE, WHETHER OR NOT INJURY WAS SUSTAINED BY PER­
SONS OR PROPERTY OR OTHERWISE, AND WHETHER OR NOT LOSS WAS SUSTAINED
FROM, OR AROSE OUT OF THE RESULTS OF, OR USE OF, THE SOFTWARE OR SER­
VICES PROVIDED HEREUNDER.''

Chapter 2
Creating the CFITSIO Library
2.1 Building the Library
The CFITSIO code is contained in about 40 C source files (*.c) and header files (*.h). On
VAX/VMS systems 2 assembly­code files (vmsieeed.mar and vmsieeer.mar) are also needed.
CFITSIO has currently been tested on the following platforms (not up­to­date):
OPERATING SYSTEM COMPILER
Sun OS gcc and cc (3.0.1)
Sun Solaris gcc and cc
Silicon Graphics IRIX gcc and cc
Silicon Graphics IRIX64 MIPS
Dec Alpha OSF/1 gcc and cc
DECstation Ultrix gcc
Dec Alpha OpenVMS cc
DEC VAX/VMS gcc and cc
HP­UX gcc
IBM AIX gcc
Linux gcc
MkLinux DR3
Windows 95/98/NT Borland C++ V4.5
Windows 95/98/NT/ME/XP Microsoft/Compaq Visual C++ v5.0, v6.0
Windows 95/98/NT Cygwin gcc
MacOS 7.1 or greater Metrowerks 10.+
MacOS­X 10.1 or greater cc (gcc)
CFITSIO will probably run on most other Unix platforms. Cray supercomputers are currently not
supported.
2.1.1 Unix Systems
The CFITSIO library is built on Unix systems by typing:
5

6 CHAPTER 2. CREATING THE CFITSIO LIBRARY
> ./configure [­­prefix=/target/installation/path] [­­enable­reentrant]
[­­enable­sse2] [­­enable­ssse3]
> make (or 'make shared')
> make install (this step is optional)
at the operating system prompt. The configure command customizes the Makefile for the particular
system, then the `make' command compiles the source files and builds the library. Type `./configure'
and not simply `configure' to ensure that the configure script in the current directory is run and
not some other system­wide configure script. The optional 'prefix' argument to configure gives the
path to the directory where the CFITSIO library and include files should be installed via the later
'make install' command. For example,
> ./configure ­­prefix=/usr1/local
will cause the 'make install' command to copy the CFITSIO libcfitsio file to /usr1/local/lib and the
necessary include files to /usr1/local/include (assuming of course that the process has permission
to write to these directories).
The optional --enable­reentrant flag will attempt to configure CFITSIO so that it can be used
in multi­threaded programs. See the ''Using CFITSIO in Multi­threaded Environments'' section,
below, for more
The optional --enable­sse2 and --enable­ssse3 flags will cause configure to attempt to build CFITSIO
using faster byte­swapping algorithms. See the ''Optimizing Programs'' chapter of this manual for
more information about these options.
The 'make shared' option builds a shared or dynamic version of the CFITSIO library. When using
the shared library the executable code is not copied into your program at link time and instead the
program locates the necessary library code at run time, normally through LD LIBRARY PATH or
some other method. The advantages of using a shared library are:
1. Less disk space if you build more than 1 program
2. Less memory if more than one copy of a program using the shared
library is running at the same time since the system is smart
enough to share copies of the shared library at run time.
3. Possibly easier maintenance since a new version of the shared
library can be installed without relinking all the software
that uses it (as long as the subroutine names and calling
sequences remain unchanged).
4. No run­time penalty.
The disadvantages are:
1. More hassle at runtime. You have to either build the programs
specially or have LD_LIBRARY_PATH set right.
2. There may be a slight start up penalty, depending on where you are
reading the shared library and the program from and if your CPU is
either really slow or really heavily loaded.

2.1. BUILDING THE LIBRARY 7
On Mac OS X platforms the 'make shared' command works like on other UNIX platforms, but a
.dylib file will be created instead of .so. If installed in a nonstandard location, add its location to
the DYLD LIBRARY PATH environment variable so that the library can be found at run time.
On HP/UX systems, the environment variable CFLAGS should be set to ­Ae before running con­
figure to enable ''extended ANSI'' features.
By default, a set of Fortran­callable wrapper routines are also built and included in the CFITSIO
library. If these wrapper routines are not needed (i.e., the CFITSIO library will not be linked
to any Fortran applications which call FITSIO subroutines) then they may be omitted from the
build by typing 'make all­nofitsio' instead of simply typing 'make'. This will reduce the size of the
CFITSIO library slightly.
It may not be possible to statically link programs that use CFITSIO on some platforms (namely,
on Solaris 2.6) due to the network drivers (which provide FTP and HTTP access to FITS files). It
is possible to make both a dynamic and a static version of the CFITSIO library, but network file
access will not be possible using the static version.
2.1.2 VMS
On VAX/VMS and ALPHA/VMS systems the make gfloat.com command file may be executed to
build the cfitsio.olb object library using the default G­floating point option for double variables.
The make dfloat.com and make ieee.com files may be used instead to build the library with the
other floating point options. Note that the getcwd function that is used in the group.c module may
require that programs using CFITSIO be linked with the ALPHA$LIBRARY:VAXCRTL.OLB
library. See the example link line in the next section of this document.
2.1.3 Windows PCs
A precompiled DLL version of CFITSIO is available for IBM­PC users of the Borland or Microsoft
Visual C++ compilers in the files cfitsiodll 3xxx borland.zip and cfitsiodll 3xxx vcc.zip, where
'3xxx' represents the current release number. These zip archives also contains other files and
instructions on how to use the CFITSIO DLL library.
The CFITSIO library may also be built from the source code using the makefile.bc or makefile.vcc
files. Finally, the makepc.bat file gives an example of building CFITSIO with the Borland C++
v4.5 or v5.5 compiler using older DOS commands.
2.1.4 Macintosh PCs
When building on Mac OS­X, users should follow the Unix instructions, above. See the README.MacOS
file for instructions on building a Universal Binary that supports both Intel and PowerPC CPUs.

8 CHAPTER 2. CREATING THE CFITSIO LIBRARY
2.2 Testing the Library
The CFITSIO library should be tested by building and running the testprog.c program that is
included with the release. On Unix systems, type:
% make testprog
% testprog > testprog.lis
% diff testprog.lis testprog.out
% cmp testprog.fit testprog.std
On VMS systems, (assuming cc is the name of the C compiler command), type:
$ cc testprog.c
$ link testprog, cfitsio/lib, alpha$library:vaxcrtl/lib
$ run testprog
The test program should produce a FITS file called `testprog.fit' that is identical to the `test­
prog.std' FITS file included with this release. The diagnostic messages (which were piped to the
file testprog.lis in the Unix example) should be identical to the listing contained in the file test­
prog.out. The 'di#' and 'cmp' commands shown above should not report any di#erences in the
files. (There may be some minor format di#erences, such as the presence or absence of leading
zeros, or 3 digit exponents in numbers, which can be ignored).
The Fortran wrappers in CFITSIO may be tested with the testf77 program on Unix systems with:
% f77 ­o testf77 testf77.f ­L. ­lcfitsio ­lnsl ­lsocket
or
% f77 ­f ­o testf77 testf77.f ­L. ­lcfitsio (under SUN O/S)
or
% f77 ­o testf77 testf77.f ­Wl,­L. ­lcfitsio ­lm ­lnsl ­lsocket (HP/UX)
% testf77 > testf77.lis
% diff testf77.lis testf77.out
% cmp testf77.fit testf77.std
On machines running SUN O/S, Fortran programs must be compiled with the '­f' option to force
double precision variables to be aligned on 8­byte boundarys to make the fortran­declared variables
compatible with C. A similar compiler option may be required on other platforms. Failing to use
this option may cause the program to crash on FITSIO routines that read or write double precision
variables.
Also note that on some systems, the output listing of the testf77 program may di#er slightly from
the testf77.std template, if leading zeros are not printed by default before the decimal point when
using F format.
A few other utility programs are included with CFITSIO; the first four of this programs can be
compiled an linked by typing `make program name' where `program name' is the actual name of
the program:

2.3. LINKING PROGRAMS WITH CFITSIO 9
speed ­ measures the maximum throughput (in MB per second)
for writing and reading FITS files with CFITSIO.
listhead ­ lists all the header keywords in any FITS file
fitscopy ­ copies any FITS file (especially useful in conjunction
with the CFITSIO's extended input filename syntax).
cookbook ­ a sample program that performs common read and
write operations on a FITS file.
iter_a, iter_b, iter_c ­ examples of the CFITSIO iterator routine
2.3 Linking Programs with CFITSIO
When linking applications software with the CFITSIO library, several system libraries usually need
to be specified on the link command line. On Unix systems, the most reliable way to determine
what libraries are required is to type 'make testprog' and see what libraries the configure script has
added. The typical libraries that need to be added are ­lm (the math library) and ­lnsl and ­lsocket
(needed only for FTP and HTTP file access). These latter 2 libraries are not needed on VMS and
Windows platforms, because FTP file access is not currently supported on those platforms.
Note that when upgrading to a newer version of CFITSIO it is usually necessary to recompile, as
well as relink, the programs that use CFITSIO, because the definitions in fitsio.h often change.
2.4 Using CFITSIO in Multi­threaded Environments
CFITSIO can be used either with the POSIX pthreads interface or the OpenMP interface for
multi­threaded parallel programs. When used in a multi­threaded environment, the CFITSIO
library *must* be built using the ­D REENTRANT compiler directive. This can be done using the
following build commands:
>./configure ­­enable­reentrant
> make
A function called fits is reentrant is available to test whether or not CFITSIO was compiled with
the ­D REENTRANT directive. When this feature is enabled, multiple threads can call any of the
CFITSIO routines to simultaneously read or write separate FITS files. Multiple threads can also
read data from the same FITS file simultaneously, as long as the file was opened independently by
each thread. This relies on the operating system to correctly deal with reading the same file by
multiple processes. Di#erent threads should not share the same 'fitsfile' pointer to read an opened
FITS file, unless locks are placed around the calls to the CFITSIO reading routines. Di#erent
threads should never try to write to the same FITS file.

10 CHAPTER 2. CREATING THE CFITSIO LIBRARY
2.5 Getting Started with CFITSIO
In order to e#ectively use the CFITSIO library it is recommended that new users begin by reading
the ``CFITSIO Quick Start Guide''. It contains all the basic information needed to write programs
that perform most types of operations on FITS files. The set of example FITS utility programs that
are available from the CFITSIO web site are also very useful for learning how to use CFITSIO. To
learn even more about the capabilities of the CFITSIO library the following steps are recommended:
1. Read the following short `FITS Primer' chapter for an overview of the structure of FITS files.
2. Review the Programming Guidelines in Chapter 4 to become familiar with the conventions used
by the CFITSIO interface.
3. Refer to the cookbook.c, listhead.c, and fitscopy.c programs that are included with this re­
lease for examples of routines that perform various common FITS file operations. Type 'make
program name' to compile and link these programs on Unix systems.
4. Write a simple program to read or write a FITS file using the Basic Interface routines described
in Chapter 5.
5. Scan through the more specialized routines that are described in the following chapters to
become familiar with the functionality that they provide.
2.6 Example Program
The following listing shows an example of how to use the CFITSIO routines in a C program.
Refer to the cookbook.c program that is included with the CFITSIO distribution for other example
routines.
This program creates a new FITS file, containing a FITS image. An `EXPOSURE' keyword is
written to the header, then the image data are written to the FITS file before closing the FITS file.
#include "fitsio.h" /* required by every program that uses CFITSIO */
main()
{
fitsfile *fptr; /* pointer to the FITS file; defined in fitsio.h */
int status, ii, jj;
long fpixel = 1, naxis = 2, nelements, exposure;
long naxes[2] = { 300, 200 }; /* image is 300 pixels wide by 200 rows */
short array[200][300];
status = 0; /* initialize status before calling fitsio routines */
fits_create_file(&fptr, "testfile.fits", &status); /* create new file */
/* Create the primary array image (16­bit short integer pixels */
fits_create_img(fptr, SHORT_IMG, naxis, naxes, &status);
/* Write a keyword; must pass the ADDRESS of the value */

2.6. EXAMPLE PROGRAM 11
exposure = 1500.;
fits_update_key(fptr, TLONG, "EXPOSURE", &exposure,
"Total Exposure Time", &status);
/* Initialize the values in the image with a linear ramp function */
for (jj = 0; jj < naxes[1]; jj++)
for (ii = 0; ii < naxes[0]; ii++)
array[jj][ii] = ii + jj;
nelements = naxes[0] * naxes[1]; /* number of pixels to write */
/* Write the array of integers to the image */
fits_write_img(fptr, TSHORT, fpixel, nelements, array[0], &status);
fits_close_file(fptr, &status); /* close the file */
fits_report_error(stderr, status); /* print out any error messages */
return( status );
}

12 CHAPTER 2. CREATING THE CFITSIO LIBRARY

Chapter 3
A FITS Primer
This section gives a brief overview of the structure of FITS files. Users should refer to the documen­
tation available from the NOST, as described in the introduction, for more detailed information on
FITS formats.
FITS was first developed in the late 1970's as a standard data interchange format between various
astronomical observatories. Since then FITS has become the standard data format supported by
most astronomical data analysis software packages.
A FITS file consists of one or more Header + Data Units (HDUs), where the first HDU is called
the `Primary HDU', or `Primary Array'. The primary array contains an N­dimensional array of
pixels, such as a 1­D spectrum, a 2­D image, or a 3­D data cube. Six di#erent primary data types
are supported: Unsigned 8­bit bytes, 16­bit, 32­bit, and 64­bit signed integers, and 32 and 64­bit
floating point reals. FITS also has a convention for storing 16 and 32­bit unsigned integers (see the
later section entitled `Unsigned Integers' for more details). The primary HDU may also consist of
only a header with a null array containing no data pixels.
Any number of additional HDUs may follow the primary array; these additional HDUs are called
FITS `extensions'. There are currently 3 types of extensions defined by the FITS standard:
. Image Extension ­ a N­dimensional array of pixels, like in a primary array
. ASCII Table Extension ­ rows and columns of data in ASCII character format
. Binary Table Extension ­ rows and columns of data in binary representation
In each case the HDU consists of an ASCII Header Unit followed by an optional Data Unit. For
historical reasons, each Header or Data unit must be an exact multiple of 2880 8­bit bytes long.
Any unused space is padded with fill characters (ASCII blanks or zeros).
Each Header Unit consists of any number of 80­character keyword records or `card images' which
have the general form:
KEYNAME = value / comment string
NULLKEY = / comment: This keyword has no value
13

14 CHAPTER 3. A FITS PRIMER
The keyword names may be up to 8 characters long and can only contain uppercase letters, the
digits 0­9, the hyphen, and the underscore character. The keyword name is (usually) followed by an
equals sign and a space character (= ) in columns 9 ­ 10 of the record, followed by the value of the
keyword which may be either an integer, a floating point number, a character string (enclosed in
single quotes), or a boolean value (the letter T or F). A keyword may also have a null or undefined
value if there is no specified value string, as in the second example, above
The last keyword in the header is always the `END' keyword which has no value or comment
fields. There are many rules governing the exact format of a keyword record (see the NOST FITS
Standard) so it is better to rely on standard interface software like CFITSIO to correctly construct
or to parse the keyword records rather than try to deal directly with the raw FITS formats.
Each Header Unit begins with a series of required keywords which depend on the type of HDU.
These required keywords specify the size and format of the following Data Unit. The header may
contain other optional keywords to describe other aspects of the data, such as the units or scaling
values. Other COMMENT or HISTORY keywords are also frequently added to further document
the data file.
The optional Data Unit immediately follows the last 2880­byte block in the Header Unit. Some
HDUs do not have a Data Unit and only consist of the Header Unit.
If there is more than one HDU in the FITS file, then the Header Unit of the next HDU immediately
follows the last 2880­byte block of the previous Data Unit (or Header Unit if there is no Data Unit).
The main required keywords in FITS primary arrays or image extensions are:
. BITPIX -- defines the data type of the array: 8, 16, 32, 64, ­32, ­64 for unsigned 8--bit byte,
16--bit signed integer, 32--bit signed integer, 32--bit IEEE floating point, and 64--bit IEEE
double precision floating point, respectively.
. NAXIS -- the number of dimensions in the array, usually 0, 1, 2, 3, or 4.
. NAXISn -- (n ranges from 1 to NAXIS) defines the size of each dimension.
FITS tables start with the keyword XTENSION = `TABLE' (for ASCII tables) or XTENSION =
`BINTABLE' (for binary tables) and have the following main keywords:
. TFIELDS -- number of fields or columns in the table
. NAXIS2 -- number of rows in the table
. TTYPEn -- for each column (n ranges from 1 to TFIELDS) gives the name of the column
. TFORMn -- the data type of the column
. TUNITn -- the physical units of the column (optional)
Users should refer to the FITS Support O#ce at http://fits.gsfc.nasa.gov for further infor­
mation about the FITS format and related software packages.

Chapter 4
Programming Guidelines
4.1 CFITSIO Definitions
Any program that uses the CFITSIO interface must include the fitsio.h header file with the state­
ment
#include "fitsio.h"
This header file contains the prototypes for all the CFITSIO user interface routines as well as the
definitions of various constants used in the interface. It also defines a C structure of type `fitsfile'
that is used by CFITSIO to store the relevant parameters that define the format of a particular
FITS file. Application programs must define a pointer to this structure for each FITS file that is
to be opened. This structure is initialized (i.e., memory is allocated for the structure) when the
FITS file is first opened or created with the fits open file or fits create file routines. This fitsfile
pointer is then passed as the first argument to every other CFITSIO routine that operates on the
FITS file. Application programs must not directly read or write elements in this fitsfile structure
because the definition of the structure may change in future versions of CFITSIO.
A number of symbolic constants are also defined in fitsio.h for the convenience of application
programmers. Use of these symbolic constants rather than the actual numeric value will help to
make the source code more readable and easier for others to understand.
String Lengths, for use when allocating character arrays:
#define FLEN_FILENAME 1025 /* max length of a filename */
#define FLEN_KEYWORD 72 /* max length of a keyword */
#define FLEN_CARD 81 /* max length of a FITS header card */
#define FLEN_VALUE 71 /* max length of a keyword value string */
#define FLEN_COMMENT 73 /* max length of a keyword comment string */
#define FLEN_ERRMSG 81 /* max length of a CFITSIO error message */
#define FLEN_STATUS 31 /* max length of a CFITSIO status text string */
Note that FLEN_KEYWORD is longer than the nominal 8­character keyword
15

16 CHAPTER 4. PROGRAMMING GUIDELINES
name length because the HIERARCH convention supports longer keyword names.
Access modes when opening a FITS file:
#define READONLY 0
#define READWRITE 1
BITPIX data type code values for FITS images:
#define BYTE_IMG 8 /* 8­bit unsigned integers */
#define SHORT_IMG 16 /* 16­bit signed integers */
#define LONG_IMG 32 /* 32­bit signed integers */
#define LONGLONG_IMG 64 /* 64­bit signed integers */
#define FLOAT_IMG ­32 /* 32­bit single precision floating point */
#define DOUBLE_IMG ­64 /* 64­bit double precision floating point */
The following 4 data type codes are also supported by CFITSIO:
#define SBYTE_IMG 10 /* 8­bit signed integers, equivalent to */
/* BITPIX = 8, BSCALE = 1, BZERO = ­128 */
#define USHORT_IMG 20 /* 16­bit unsigned integers, equivalent to */
/* BITPIX = 16, BSCALE = 1, BZERO = 32768 */
#define ULONG_IMG 40 /* 32­bit unsigned integers, equivalent to */
/* BITPIX = 32, BSCALE = 1, BZERO = 2147483648 */
Codes for the data type of binary table columns and/or for the
data type of variables when reading or writing keywords or data:
DATATYPE TFORM CODE
#define TBIT 1 /* 'X' */
#define TBYTE 11 /* 8­bit unsigned byte, 'B' */
#define TLOGICAL 14 /* logicals (int for keywords */
/* and char for table cols 'L' */
#define TSTRING 16 /* ASCII string, 'A' */
#define TSHORT 21 /* signed short, 'I' */
#define TLONG 41 /* signed long, */
#define TLONGLONG 81 /* 64­bit long signed integer 'K' */
#define TFLOAT 42 /* single precision float, 'E' */
#define TDOUBLE 82 /* double precision float, 'D' */
#define TCOMPLEX 83 /* complex (pair of floats) 'C' */
#define TDBLCOMPLEX 163 /* double complex (2 doubles) 'M' */
The following data type codes are also supported by CFITSIO:
#define TINT 31 /* int */
#define TSBYTE 12 /* 8­bit signed byte, 'S' */
#define TUINT 30 /* unsigned int 'V' */
#define TUSHORT 20 /* unsigned short 'U' */

4.2. CURRENT HEADER DATA UNIT (CHDU) 17
#define TULONG 40 /* unsigned long */
The following data type code is only for use with fits\_get\_coltype
#define TINT32BIT 41 /* signed 32­bit int, 'J' */
HDU type code values (value returned when moving to new HDU):
#define IMAGE_HDU 0 /* Primary Array or IMAGE HDU */
#define ASCII_TBL 1 /* ASCII table HDU */
#define BINARY_TBL 2 /* Binary table HDU */
#define ANY_HDU ­1 /* matches any type of HDU */
Column name and string matching case­sensitivity:
#define CASESEN 1 /* do case­sensitive string match */
#define CASEINSEN 0 /* do case­insensitive string match */
Logical states (if TRUE and FALSE are not already defined):
#define TRUE 1
#define FALSE 0
Values to represent undefined floating point numbers:
#define FLOATNULLVALUE ­9.11912E­36F
#define DOUBLENULLVALUE ­9.1191291391491E­36
Image compression algorithm definitions
#define RICE_1 11
#define GZIP_1 21
#define PLIO_1 31
#define HCOMPRESS_1 41
4.2 Current Header Data Unit (CHDU)
The concept of the Current Header and Data Unit, or CHDU, is fundamental to the use of the
CFITSIO library. A simple FITS image may only contain a single Header and Data unit (HDU),
but in general FITS files can contain multiple Header Data Units (also known as `extensions'),
concatenated one after the other in the file. The user can specify which HDU should be initially
opened at run time by giving the HDU name or number after the root file name. For example,
'myfile.fits[4]' opens the 5th HDU in the file (note that the numbering starts with 0), and 'my­
file.fits[EVENTS] opens the HDU with the name 'EVENTS' (as defined by the EXTNAME or
HDUNAME keywords). If no HDU is specified then CFITSIO opens the first HDU (the primary

18 CHAPTER 4. PROGRAMMING GUIDELINES
array) by default. The CFITSIO routines which read and write data only operate within the opened
HDU, Other CFITSIO routines are provided to move to and open any other existing HDU within
the FITS file or to append or insert new HDUs in the FITS file.
4.3 Function Names and Variable Datatypes
Most of the CFITSIO routines have both a short name as well as a longer descriptive name. The
short name is only 5 or 6 characters long and is similar to the subroutine name in the Fortran­77
version of FITSIO. The longer name is more descriptive and it is recommended that it be used
instead of the short name to more clearly document the source code.
Many of the CFITSIO routines come in families which di#er only in the data type of the associated
parameter(s). The data type of these routines is indicated by the su#x of the routine name. The
short routine names have a 1 or 2 character su#x (e.g., 'j' in '#pkyj') while the long routine names
have a 4 character or longer su#x as shown in the following table:
Long Short Data
Names Names Type
­­­­­ ­­­­­ ­­­­
_bit x bit
_byt b unsigned byte
_sbyt sb signed byte
_sht i short integer
_lng j long integer
_lnglng jj 8­byte LONGLONG integer (see note below)
_usht ui unsigned short integer
_ulng uj unsigned long integer
_uint uk unsigned int integer
_int k int integer
_flt e real exponential floating point (float)
_fixflt f real fixed­decimal format floating point (float)
_dbl d double precision real floating­point (double)
_fixdbl g double precision fixed­format floating point (double)
_cmp c complex reals (pairs of float values)
_fixcmp fc complex reals, fixed­format floating point
_dblcmp m double precision complex (pairs of double values)
_fixdblcmp fm double precision complex, fixed­format floating point
_log l logical (int)
_str s character string
The logical data type corresponds to `int' for logical keyword values, and `byte' for logical binary
table columns. In other words, the value when writing a logical keyword must be stored in an
`int' variable, and must be stored in a `char' array when reading or writing to `L' columns in a
binary table. Implicit data type conversion is not supported for logical table columns, but is for
keywords, so a logical keyword may be read and cast to any numerical data type; a returned value
= 0 indicates false, and any other value = true.

4.3. FUNCTION NAMES AND VARIABLE DATATYPES 19
The `int' data type may be 2 bytes long on some old PC compilers, but otherwise it is nearly always
4 bytes long. Some 64­bit machines, like the Alpha/OSF, define the `short', `int', and `long' integer
data types to be 2, 4, and 8 bytes long, respectively.
Because there is no universal C compiler standard for the name of the 8­byte integer datatype,
the fitsio.h include file typedef's 'LONGLONG' to be equivalent to an appropriate 8­byte integer
data type on each supported platform. For maximum software portability it is recommended that
this LONGLONG datatype be used to define 8­byte integer variables rather than using the native
data type name on a particular platform. On most 32­bit Unix and Mac OS­X operating systems
LONGLONG is equivalent to the intrinsic 'long long' 8­byte integer datatype. On 64­bit systems
(which currently includes Alpha OSF/1, 64­bit Sun Solaris, 64­bit SGI MIPS, and 64­bit Itanium
and Opteron PC systems), LONGLONG is simply typedef'ed to be equivalent to 'long'. Microsoft
Visual C++ Version 6.0 does not define a 'long long' data type, so LONGLONG is typedef'ed to
be equivalent to the ' int64' data type on 32­bit windows systems when using Visual C++.
A related issue that a#ects the portability of software is how to print out the value of a 'LONG­
LONG' variable with printf. Developers may find it convenient to use the following preprocessing
statements in their C programs to handle this in a machine­portable manner:
#if defined(_MSC_VER) /* Microsoft Visual C++ */
printf("%I64d", longlongvalue);
#elif (USE_LL_SUFFIX == 1)
printf("%lld", longlongvalue);
#else
printf("%ld", longlongvalue);
#endif
Similarly, the name of the C utility routine that converts a character string of digits into a 8­byte
integer value is platform dependent:
#if defined(_MSC_VER) /* Microsoft Visual C++ */
/* VC++ 6.0 does not seem to have an 8­byte conversion routine */
#elif (USE_LL_SUFFIX == 1)
longlongvalue = atoll(*string);
#else
longlongvalue = atol(*string);
#endif
When dealing with the FITS byte data type it is important to remember that the raw values (before
any scaling by the BSCALE and BZERO, or TSCALn and TZEROn keyword values) in byte arrays
(BITPIX = 8) or byte columns (TFORMn = 'B') are interpreted as unsigned bytes with values
ranging from 0 to 255. Some C compilers define a 'char' variable as signed, so it is important to
explicitly declare a numeric char variable as 'unsigned char' to avoid any ambiguity

20 CHAPTER 4. PROGRAMMING GUIDELINES
One feature of the CFITSIO routines is that they can operate on a `X' (bit) column in a binary table
as though it were a `B' (byte) column. For example a `11X' data type column can be interpreted
the same as a `2B' column (i.e., 2 unsigned 8­bit bytes). In some instances, it can be more e#cient
to read and write whole bytes at a time, rather than reading or writing each individual bit.
The complex and double precision complex data types are not directly supported in ANSI C so
these data types should be interpreted as pairs of float or double values, respectively, where the
first value in each pair is the real part, and the second is the imaginary part.
4.4 Support for Unsigned Integers and Signed Bytes
Although FITS does not directly support unsigned integers as one of its fundamental data types,
FITS can still be used to e#ciently store unsigned integer data values in images and binary tables.
The convention used in FITS files is to store the unsigned integers as signed integers with an
associated o#set (specified by the BZERO or TZEROn keyword). For example, to store unsigned
16­bit integer values in a FITS image the image would be defined as a signed 16­bit integer (with
BITPIX keyword = SHORT IMG = 16) with the keywords BSCALE = 1.0 and BZERO = 32768.
Thus the unsigned values of 0, 32768, and 65535, for example, are physically stored in the FITS
image as ­32768, 0, and 32767, respectively; CFITSIO automatically adds the BZERO o#set to
these values when they are read. Similarly, in the case of unsigned 32­bit integers the BITPIX
keyword would be equal to LONG IMG = 32 and BZERO would be equal to 2147483648 (i.e. 2
raised to the 31st power).
The CFITSIO interface routines will e#ciently and transparently apply the appropriate o#set in
these cases so in general application programs do not need to be concerned with how the unsigned
values are actually stored in the FITS file. As a convenience for users, CFITSIO has several
predefined constants for the value of BITPIX (USHORT IMG, ULONG IMG) and for the TFORMn
value in the case of binary tables (`U' and `V') which programmers can use when creating FITS
files containing unsigned integer values. The following code fragment illustrates how to write a
FITS 1­D primary array of unsigned 16­bit integers:
unsigned short uarray[100];
int naxis, status;
long naxes[10], group, firstelem, nelements;
...
status = 0;
naxis = 1;
naxes[0] = 100;
fits_create_img(fptr, USHORT_IMG, naxis, naxes, &status);
firstelem = 1;
nelements = 100;
fits_write_img(fptr, TUSHORT, firstelem, nelements,
uarray, &status);
...
In the above example, the 2nd parameter in fits create img tells CFITSIO to write the header

4.4. SUPPORT FOR UNSIGNED INTEGERS AND SIGNED BYTES 21
keywords appropriate for an array of 16­bit unsigned integers (i.e., BITPIX = 16 and BZERO =
32768). Then the fits write img routine writes the array of unsigned short integers (uarray) into the
primary array of the FITS file. Similarly, a 32­bit unsigned integer image may be created by setting
the second parameter in fits create img equal to `ULONG IMG' and by calling the fits write img
routine with the second parameter = TULONG to write the array of unsigned long image pixel
values.
An analogous set of routines are available for reading or writing unsigned integer values and signed
byte values in a FITS binary table extension. When specifying the TFORMn keyword value which
defines the format of a column, CFITSIO recognized 3 additional data type codes besides those
already defined in the FITS standard: `U' meaning a 16­bit unsigned integer column, `V' for a 32­
bit unsigned integer column, and 'S' for a signed byte column. These non­standard data type codes
are not actually written into the FITS file but instead are just used internally within CFITSIO.
The following code fragment illustrates how to use these features:
unsigned short uarray[100];
unsigned int varray[100];
int colnum, tfields, status;
long nrows, firstrow, firstelem, nelements, pcount;
char extname[] = "Test_table"; /* extension name */
/* define the name, data type, and physical units for the 2 columns */
char *ttype[] = { "Col_1", "Col_2", "Col_3" };
char *tform[] = { "1U", "1V", "1S"}; /* special CFITSIO codes */
char *tunit[] = { " ", " ", " " };
...
/* write the header keywords */
status = 0;
nrows = 1;
tfields = 3
pcount = 0;
fits_create_tbl(fptr, BINARY_TBL, nrows, tfields, ttype, tform,
tunit, extname, &status);
/* write the unsigned shorts to the 1st column */
colnum = 1;
firstrow = 1;
firstelem = 1;
nelements = 100;
fits_write_col(fptr, TUSHORT, colnum, firstrow, firstelem,
nelements, uarray, &status);
/* now write the unsigned longs to the 2nd column */
colnum = 2;

22 CHAPTER 4. PROGRAMMING GUIDELINES
fits_write_col(fptr, TUINT, colnum, firstrow, firstelem,
nelements, varray, &status);
...
Note that the non­standard TFORM values for the 3 columns, `U' and `V', tell CFITSIO to write the
keywords appropriate for unsigned 16­bit and unsigned 32­bit integers, respectively (i.e., TFORMn
= '1I' and TZEROn = 32678 for unsigned 16­bit integers, and TFORMn = '1J' and TZEROn
= 2147483648 for unsigned 32­bit integers). The 'S' TFORMn value tells CFITSIO to write the
keywords appropriate for a signed 8­bit byte column with TFORMn = '1B' and TZEROn = ­128.
The calls to fits write col then write the arrays of unsigned integer values to the columns.
4.5 Dealing with Character Strings
The character string values in a FITS header or in an ASCII column in a FITS table extension
are generally padded out with non­significant space characters (ASCII 32) to fill up the header
record or the column width. When reading a FITS string value, the CFITSIO routines will strip
o# these non­significant trailing spaces and will return a null­terminated string value containing
only the significant characters. Leading spaces in a FITS string are considered significant. If the
string contains all blanks, then CFITSIO will return a single blank character, i.e, the first blank
is considered to be significant, since it distinguishes the string from a null or undefined string, but
the remaining trailing spaces are not significant.
Similarly, when writing string values to a FITS file the CFITSIO routines expect to get a null­
terminated string as input; CFITSIO will pad the string with blanks if necessary when writing it
to the FITS file.
When calling CFITSIO routines that return a character string it is vital that the size of the char
array be large enough to hold the entire string of characters, otherwise CFITSIO will overwrite
whatever memory locations follow the char array, possibly causing the program to execute incor­
rectly. This type of error can be di#cult to debug, so programmers should always ensure that the
char arrays are allocated enough space to hold the longest possible string, including the terminat­
ing NULL character. The fitsio.h file contains the following defined constants which programmers
are strongly encouraged to use whenever they are allocating space for char arrays:
#define FLEN_FILENAME 1025 /* max length of a filename */
#define FLEN_KEYWORD 72 /* max length of a keyword */
#define FLEN_CARD 81 /* length of a FITS header card */
#define FLEN_VALUE 71 /* max length of a keyword value string */
#define FLEN_COMMENT 73 /* max length of a keyword comment string */
#define FLEN_ERRMSG 81 /* max length of a CFITSIO error message */
#define FLEN_STATUS 31 /* max length of a CFITSIO status text string */
For example, when declaring a char array to hold the value string of FITS keyword, use the following
statement:
char value[FLEN_VALUE];

4.6. IMPLICIT DATA TYPE CONVERSION 23
Note that FLEN KEYWORD is longer than needed for the nominal 8­character keyword name
because the HIERARCH convention supports longer keyword names.
4.6 Implicit Data Type Conversion
The CFITSIO routines that read and write numerical data can perform implicit data type conver­
sion. This means that the data type of the variable or array in the program does not need to be the
same as the data type of the value in the FITS file. Data type conversion is supported for numerical
and string data types (if the string contains a valid number enclosed in quotes) when reading a
FITS header keyword value and for numeric values when reading or writing values in the primary
array or a table column. CFITSIO returns status = NUM OVERFLOW if the converted data value
exceeds the range of the output data type. Implicit data type conversion is not supported within
binary tables for string, logical, complex, or double complex data types.
In addition, any table column may be read as if it contained string values. In the case of numeric
columns the returned string will be formatted using the TDISPn display format if it exists.
4.7 Data Scaling
When reading numerical data values in the primary array or a table column, the values will be
scaled automatically by the BSCALE and BZERO (or TSCALn and TZEROn) header values if
they are present in the header. The scaled data that is returned to the reading program will have
output value = (FITS value) * BSCALE + BZERO
(a corresponding formula using TSCALn and TZEROn is used when reading from table columns).
In the case of integer output values the floating point scaled value is truncated to an integer (not
rounded to the nearest integer). The fits set bscale and fits set tscale routines (described in the
`Advanced' chapter) may be used to override the scaling parameters defined in the header (e.g., to
turn o# the scaling so that the program can read the raw unscaled values from the FITS file).
When writing numerical data to the primary array or to a table column the data values will
generally be automatically inversely scaled by the value of the BSCALE and BZERO (or TSCALn
and TZEROn) keyword values if they they exist in the header. These keywords must have been
written to the header before any data is written for them to have any immediate e#ect. One may
also use the fits set bscale and fits set tscale routines to define or override the scaling keywords in
the header (e.g., to turn o# the scaling so that the program can write the raw unscaled values into
the FITS file). If scaling is performed, the inverse scaled output value that is written into the FITS
file will have
FITS value = ((input value) ­ BZERO) / BSCALE
(a corresponding formula using TSCALn and TZEROn is used when writing to table columns).
Rounding to the nearest integer, rather than truncation, is performed when writing integer data
types to the FITS file.

24 CHAPTER 4. PROGRAMMING GUIDELINES
4.8 Support for IEEE Special Values
The ANSI/IEEE­754 floating­point number standard defines certain special values that are used to
represent such quantities as Not­a­Number (NaN), denormalized, underflow, overflow, and infinity.
(See the Appendix in the NOST FITS standard or the NOST FITS User's Guide for a list of these
values). The CFITSIO routines that read floating point data in FITS files recognize these IEEE
special values and by default interpret the overflow and infinity values as being equivalent to a
NaN, and convert the underflow and denormalized values into zeros. In some cases programmers
may want access to the raw IEEE values, without any modification by CFITSIO. This can be
done by calling the fits read img or fits read col routines while specifying 0.0 as the value of the
NULLVAL parameter. This will force CFITSIO to simply pass the IEEE values through to the
application program without any modification. This is not fully supported on VAX/VMS machines,
however, where there is no easy way to bypass the default interpretation of the IEEE special values.
This is also not supported when reading floating­point images that have been compressed with the
FITS tiled image compression convention that is discussed in section 5.6; the pixels values in tile
compressed images are represented by scaled integers, and a reserved integer value (not a NaN) is
used to represent undefined pixels.
4.9 Error Status Values and the Error Message Stack
Nearly all the CFITSIO routines return an error status value in 2 ways: as the value of the last
parameter in the function call, and as the returned value of the function itself. This provides some
flexibility in the way programmers can test if an error occurred, as illustrated in the following 2
code fragments:
if ( fits_write_record(fptr, card, &status) )
printf(" Error occurred while writing keyword.");
or,
fits_write_record(fptr, card, &status);
if ( status )
printf(" Error occurred while writing keyword.");
A listing of all the CFITSIO status code values is given at the end of this document. Programmers
are encouraged to use the symbolic mnemonics (defined in fitsio.h) rather than the actual integer
status values to improve the readability of their code.
The CFITSIO library uses an `inherited status' convention for the status parameter which means
that if a routine is called with a positive input value of the status parameter as input, then the
routine will exit immediately without changing the value of the status parameter. Thus, if one
passes the status value returned from each CFITSIO routine as input to the next CFITSIO routine,
then whenever an error is detected all further CFITSIO processing will cease. This convention can
simplify the error checking in application programs because it is not necessary to check the value
of the status parameter after every single CFITSIO routine call. If a program contains a sequence

4.10. VARIABLE­LENGTH ARRAYS IN BINARY TABLES 25
of several CFITSIO calls, one can just check the status value after the last call. Since the returned
status values are generally distinctive, it should be possible to determine which routine originally
returned the error status.
CFITSIO also maintains an internal stack of error messages (80­character maximum length) which
in many cases provide a more detailed explanation of the cause of the error than is provided by
the error status number alone. It is recommended that the error message stack be printed out
whenever a program detects a CFITSIO error. The function fits report error will print out the
entire error message stack, or alternatively one may call fits read errmsg to get the error messages
one at a time.
4.10 Variable­Length Arrays in Binary Tables
CFITSIO provides easy­to­use support for reading and writing data in variable length fields of a
binary table. The variable length columns have TFORMn keyword values of the form `1Pt(len)'
where `t' is the data type code (e.g., I, J, E, D, etc.) and `len' is an integer specifying the maximum
length of the vector in the table. (CFITSIO also supports the experimental 'Q' datatype, which is
identical to the 'P' type except that is supports is a 64­bit address space and hence much larger
data structures). If the value of `len' is not specified when the table is created (e.g., if the TFORM
keyword value is simply specified as '1PE' instead of '1PE(400) ), then CFITSIO will automatically
scan the table when it is closed to determine the maximum length of the vector and will append
this value to the TFORMn value.
The same routines that read and write data in an ordinary fixed length binary table extension are
also used for variable length fields, however, the routine parameters take on a slightly di#erent
interpretation as described below.
All the data in a variable length field is written into an area called the `heap' which follows the
main fixed­length FITS binary table. The size of the heap, in bytes, is specified by the PCOUNT
keyword in the FITS header. When creating a new binary table, the initial value of PCOUNT should
usually be set to zero. CFITSIO will recompute the size of the heap as the data is written and will
automatically update the PCOUNT keyword value when the table is closed. When writing variable
length data to a table, CFITSIO will automatically extend the size of the heap area if necessary,
so that any following HDUs do not get overwritten.
By default the heap data area starts immediately after the last row of the fixed­length table. This
default starting location may be overridden by the THEAP keyword, but this is not recommended.
If additional rows of data are added to the table, CFITSIO will automatically shift the the heap
down to make room for the new rows, but it is obviously be more e#cient to initially create the
table with the necessary number of blank rows, so that the heap does not needed to be constantly
moved.
When writing row of data to a variable length field the entire array of values for a given row of
the table must be written with a single call to fits write col. The total length of the array is given
by nelements + firstelem ­ 1. Additional elements cannot be appended to an existing vector at a
later time since any attempt to do so will simply overwrite all the previously written data and the
new data will be written to a new area of the heap. The fits compress heap routine is provided
to compress the heap and recover any unused space. To avoid having to deal with this issue, it

26 CHAPTER 4. PROGRAMMING GUIDELINES
is recommended that rows in a variable length field should only be written once. An exception to
this general rule occurs when setting elements of an array as undefined. It is allowed to first write
a dummy value into the array with fits write col, and then call fits write col nul to flag the desired
elements as undefined. Note that the rows of a table, whether fixed or variable length, do not have
to be written consecutively and may be written in any order.
When writing to a variable length ASCII character field (e.g., TFORM = '1PA') only a single
character string can be written. The `firstelem' and `nelements' parameter values in the fits write col
routine are ignored and the number of characters to write is simply determined by the length of
the input null­terminated character string.
The fits write descript routine is useful in situations where multiple rows of a variable length column
have the identical array of values. One can simply write the array once for the first row, and then
use fits write descript to write the same descriptor values into the other rows; all the rows will then
point to the same storage location thus saving disk space.
When reading from a variable length array field one can only read as many elements as actually
exist in that row of the table; reading does not automatically continue with the next row of the
table as occurs when reading an ordinary fixed length table field. Attempts to read more than this
will cause an error status to be returned. One can determine the number of elements in each row
of a variable column with the fits read descript routine.
4.11 Multiple Access to the Same FITS File
CFITSIO supports simultaneous read and write access to multiple HDUs in the same FITS file.
Thus, one can open the same FITS file twice within a single program and move to 2 di#erent HDUs
in the file, and then read and write data or keywords to the 2 extensions just as if one were accessing
2 completely separate FITS files. Since in general it is not possible to physically open the same file
twice and then expect to be able to simultaneously (or in alternating succession) write to 2 di#erent
locations in the file, CFITSIO recognizes when the file to be opened (in the call to fits open file)
has already been opened and instead of actually opening the file again, just logically links the new
file to the old file. (This of course does not prevent the same file from being simultaneously opened
by more than one program). Then before CFITSIO reads or writes to either (logical) file, it makes
sure that any modifications made to the other file have been completely flushed from the internal
bu#ers to the file. Thus, in principle, one could open a file twice, in one case pointing to the first
extension and in the other pointing to the 2nd extension and then write data to both extensions, in
any order, without danger of corrupting the file. There may be some e#ciency penalties in doing
this however, since CFITSIO has to flush all the internal bu#ers related to one file before switching
to the other, so it would still be prudent to minimize the number of times one switches back and
forth between doing I/O to di#erent HDUs in the same file.
Some restriction apply: a FITS file cannot be opened the first time with READONLY access, and
then opened a second time with READWRITE access, because this may be phyically impossible
(e.g., if the file resides on read­only media such as a CDROM). Also, in multi­threaded environo­
ments, one should never open the same file with write access in di#erent threads.

4.12. WHEN THE FINAL SIZE OF THE FITS HDU IS UNKNOWN 27
4.12 When the Final Size of the FITS HDU is Unknown
It is not required to know the total size of a FITS data array or table before beginning to write the
data to the FITS file. In the case of the primary array or an image extension, one should initially
create the array with the size of the highest dimension (largest NAXISn keyword) set to a dummy
value, such as 1. Then after all the data have been written and the true dimensions are known, then
the NAXISn value should be updated using the fits update key routine before moving to another
extension or closing the FITS file.
When writing to FITS tables, CFITSIO automatically keeps track of the highest row number that
is written to, and will increase the size of the table if necessary. CFITSIO will also automatically
insert space in the FITS file if necessary, to ensure that the data 'heap', if it exists, and/or any
additional HDUs that follow the table do not get overwritten as new rows are written to the table.
As a general rule it is best to specify the initial number of rows = 0 when the table is created,
then let CFITSIO keep track of the number of rows that are actually written. The application
program should not manually update the number of rows in the table (as given by the NAXIS2
keyword) since CFITSIO does this automatically. If a table is initially created with more than
zero rows, then this will usually be considered as the minimum size of the table, even if fewer
rows are actually written to the table. Thus, if a table is initially created with NAXIS2 = 20, and
CFITSIO only writes 10 rows of data before closing the table, then NAXIS2 will remain equal to
20. If however, 30 rows of data are written to this table, then NAXIS2 will be increased from 20
to 30. The one exception to this automatic updating of the NAXIS2 keyword is if the application
program directly modifies the value of NAXIS2 (up or down) itself just before closing the table. In
this case, CFITSIO does not update NAXIS2 again, since it assumes that the application program
must have had a good reason for changing the value directly. This is not recommended, however,
and is only provided for backward compatibility with software that initially creates a table with
a large number of rows, than decreases the NAXIS2 value to the actual smaller value just before
closing the table.
4.13 CFITSIO Size Limitations
CFITSIO places very few restrictions on the size of FITS files that it reads or writes. There are a
few limits, however, that may a#ect some extreme cases:
1. The maximum number of FITS files that may be simultaneously opened by CFITSIO is set by
NMAXFILES as defined in fitsio2.h. It is currently set = 300 by default. CFITSIO will allocate
about 80 * NMAXFILES bytes of memory for internal use. Note that the underlying C compiler
or operating system, may have a smaller limit on the number of opened files. The C symbolic
constant FOPEN MAX is intended to define the maximum number of files that may open at once
(including any other text or binary files that may be open, not just FITS files). On some systems
it has been found that gcc supports a maximum of 255 opened files.
2. It used to be common for computer systems to only support disk files up to 2**31 bytes =
2.1 GB in size, but most systems now support larger files. CFITSIO can optionally read and
write these so­called 'large files' that are greater than 2.1 GB on platforms where they are sup­
ported, but this usually requires that special compiler option flags be specified to turn on this

28 CHAPTER 4. PROGRAMMING GUIDELINES
option. On linux and solaris systems the compiler flags are '­D LARGEFILE SOURCE' and `­
D FILE OFFSET BITS=64'. These flags may also work on other platforms but this has not been
tested. Starting with version 3.0 of CFITSIO, the default Makefile that is distributed with CFIT­
SIO will include these 2 compiler flags when building on Solaris and Linux PC systems. Users on
other platforms will need to add these compiler flags manually if they want to support large files.
In most cases it appears that it is not necessary to include these compiler flags when compiling
application code that call the CFITSIO library routines.
When CFITSIO is built with large file support (e.g., on Solaris and Linux PC system by default)
then it can read and write FITS data files on disk that have any of these conditions:
. FITS files larger than 2.1 GB in size
. FITS images containing greater than 2.1 G pixels
. FITS images that have one dimension with more than 2.1 G pixels (as given by one of the
NAXISn keyword)
. FITS tables containing more than 2.1E09 rows (given by the NAXIS2 keyword), or with rows
that are more than 2.1 GB wide (given by the NAXIS1 keyword)
. FITS binary tables with a variable­length array heap that is larger than 2.1 GB (given by
the PCOUNT keyword)
The current maximum FITS file size supported by CFITSIO is about 6 terabytes (containing 2**31
FITS blocks, each 2880 bytes in size). Currently, support for large files in CFITSIO has been tested
on the Linux, Solaris, and IBM AIX operating systems.
Note that when writing application programs that are intended to support large files it is important
to use 64­bit integer variables to store quantities such as the dimensions of images, or the number
of rows in a table. These programs must also call the special versions of some of the CFITSIO
routines that have been adapted to support 64­bit integers. The names of these routines end in 'll'
('el' 'el') to distinguish them from the 32­bit integer version (e.g., fits get num rowsll).

Chapter 5
Basic CFITSIO Interface Routines
This chapter describes the basic routines in the CFITSIO user interface that provide all the func­
tions normally needed to read and write most FITS files. It is recommended that these routines
be used for most applications and that the more advanced routines described in the next chapter
only be used in special circumstances when necessary.
The following conventions are used in this chapter in the description of each function:
1. Most functions have 2 names: a long descriptive name and a short concise name. Both names are
listed on the first line of the following descriptions, separated by a slash (/) character. Programmers
may use either name in their programs but the long names are recommended to help document the
code and make it easier to read.
2. A right arrow symbol (>) is used in the function descriptions to separate the input parameters
from the output parameters in the definition of each routine. This symbol is not actually part of
the C calling sequence.
3. The function parameters are defined in more detail in the alphabetical listing in Appendix B.
4. The first argument in almost all the functions is a pointer to a structure of type `fitsfile'. Memory
for this structure is allocated by CFITSIO when the FITS file is first opened or created and is freed
when the FITS file is closed.
5. The last argument in almost all the functions is the error status parameter. It must be equal
to 0 on input, otherwise the function will immediately exit without doing anything. A non­zero
output value indicates that an error occurred in the function. In most cases the status value is also
returned as the value of the function itself.
5.1 CFITSIO Error Status Routines
1 Return a descriptive text string (30 char max.) corresponding to a CFITSIO error status code.
void fits_get_errstatus / ffgerr (int status, > char *err_text)
2 Return the top (oldest) 80­character error message from the internal CFITSIO stack of error
messages and shift any remaining messages on the stack up one level. Call this routine
29

30 CHAPTER 5. BASIC CFITSIO INTERFACE ROUTINES
repeatedly to get each message in sequence. The function returns a value = 0 and a null error
message when the error stack is empty.
int fits_read_errmsg / ffgmsg (char *err_msg)
3 Print out the error message corresponding to the input status value and all the error messages
on the CFITSIO stack to the specified file stream (normally to stdout or stderr). If the input
status value = 0 then this routine does nothing.
void fits_report_error / ffrprt (FILE *stream, status)
4 The fits write errmark routine puts an invisible marker on the CFITSIO error stack. The
fits clear errmark routine can then be used to delete any more recent error messages on the
stack, back to the position of the marker. This preserves any older error messages on the
stack. The fits clear errmsg routine simply clears all the messages (and marks) from the
stack. These routines are called without any arguments.
void fits_write_errmark / ffpmrk (void)
void fits_clear_errmark / ffcmrk (void)
void fits_clear_errmsg / ffcmsg (void)
5.2 FITS File Access Routines
1 Open an existing data file.
int fits_open_file / ffopen
(fitsfile **fptr, char *filename, int iomode, > int *status)
int fits_open_diskfile / ffdkopen
(fitsfile **fptr, char *filename, int iomode, > int *status)
int fits_open_data / ffdopn
(fitsfile **fptr, char *filename, int iomode, > int *status)
int fits_open_table / fftopn
(fitsfile **fptr, char *filename, int iomode, > int *status)
int fits_open_image / ffiopn
(fitsfile **fptr, char *filename, int iomode, > int *status)
The iomode parameter determines the read/write access allowed in the file and can have
values of READONLY (0) or READWRITE (1). The filename parameter gives the name of
the file to be opened, followed by an optional argument giving the name or index number of
the extension within the FITS file that should be moved to and opened (e.g., myfile.fits+3

5.2. FITS FILE ACCESS ROUTINES 31
or myfile.fits[3] moves to the 3rd extension within the file, and myfile.fits[events]
moves to the extension with the keyword EXTNAME = 'EVENTS').
The fits open diskfile routine is similar to the fits open file routine except that it does not
support the extended filename syntax in the input file name. This routine simply tries to open
the specified input file on magnetic disk. This routine is mainly for use in cases where the
filename (or directory path) contains square or curly bracket characters that would confuse
the extended filename parser.
The fits open data routine is similar to the fits open file routine except that it will move to
the first HDU containing significant data, if a HDU name or number to open was not explicitly
specified as part of the filename. In this case, it will look for the first IMAGE HDU with
NAXIS greater than 0, or the first table that does not contain the strings `GTI' (Good Time
Interval extension) or `OBSTABLE' in the EXTNAME keyword value.
The fits open table and fits open image routines are similar to fits open data except they will
move to the first significant table HDU or image HDU in the file, respectively, if a HDU name
or number is not specified as part of the filename.
IRAF images (.imh format files) and raw binary data arrays may also be opened with READ­
ONLY access. CFITSIO will automatically test if the input file is an IRAF image, and if,
so will convert it on the fly into a virtual FITS image before it is opened by the application
program. If the input file is a raw binary data array of numbers, then the data type and
dimensions of the array must be specified in square brackets following the name of the file
(e.g. 'rawfile.dat[i512,512]' opens a 512 x 512 short integer image). See the `Extended File
Name Syntax' chapter for more details on how to specify the raw file name. The raw file
is converted on the fly into a virtual FITS image in memory that is then opened by the
application program with READONLY access.
Programs can read the input file from the 'stdin' file stream if a dash character ('­') is given
as the filename. Files can also be opened over the network using FTP or HTTP protocols by
supplying the appropriate URL as the filename.
The input file can be modified in various ways to create a virtual file (usually stored in
memory) that is then opened by the application program by supplying a filtering or binning
specifier in square brackets following the filename. Some of the more common filtering meth­
ods are illustrated in the following paragraphs, but users should refer to the 'Extended File
Name Syntax' chapter for a complete description of the full file filtering syntax.
When opening an image, a rectangular subset of the physical image may be opened by listing
the first and last pixel in each dimension (and optional pixel skipping factor):
myimage.fits[101:200,301:400]
will create and open a 100x100 pixel virtual image of that section of the physical image, and
myimage.fits[*,­*] opens a virtual image that is the same size as the physical image but
has been flipped in the vertical direction.
When opening a table, the filtering syntax can be used to add or delete columns or keywords
in the virtual table: myfile.fits[events][col !time; PI = PHA*1.2] opens a virtual ta­
ble in which the TIME column has been deleted and a new PI column has been added with
a value 1.2 times that of the PHA column. Similarly, one can filter a table to keep only

32 CHAPTER 5. BASIC CFITSIO INTERFACE ROUTINES
those rows that satisfy a selection criterion: myfile.fits[events][pha > 50] creates and
opens a virtual table containing only those rows with a PHA value greater than 50. A large
number of boolean and mathematical operators can be used in the selection expression. One
can also filter table rows using 'Good Time Interval' extensions, and spatial region filters as in
myfile.fits[events][gtifilter()]and myfile.fits[events][regfilter( "stars.rng")].
Finally, table columns may be binned or histogrammed to generate a virtual image. For ex­
ample, myfile.fits[events][bin (X,Y)=4] will result in a 2­dimensional image calculated
by binning the X and Y columns in the event table with a bin size of 4 in each dimension.
The TLMINn and TLMAXn keywords will be used by default to determine the range of the
image.
A single program can open the same FITS file more than once and then treat the resulting
fitsfile pointers as though they were completely independent FITS files. Using this facility, a
program can open a FITS file twice, move to 2 di#erent extensions within the file, and then
read and write data in those extensions in any order.
2 Create and open a new empty output FITS file.
int fits_create_file / ffinit
(fitsfile **fptr, char *filename, > int *status)
int fits_create_diskfile / ffdkinit
(fitsfile **fptr, char *filename, > int *status)
An error will be returned if the specified file already exists, unless the filename is prefixed
with an exclamation point (!). In that case CFITSIO will overwrite (delete) any existing file
with the same name. Note that the exclamation point is a special UNIX character so if it
is used on the command line it must be preceded by a backslash to force the UNIX shell to
accept the character as part of the filename.
The output file will be written to the 'stdout' file stream if a dash character ('­') or the string
'stdout' is given as the filename. Similarly, '­.gz' or 'stdout.gz' will cause the file to be gzip
compressed before it is written out to the stdout stream.
Optionally, the name of a template file that is used to define the structure of the new file
may be specified in parentheses following the output file name. The template file may be
another FITS file, in which case the new file, at the time it is opened, will be an exact copy
of the template file except that the data structures (images and tables) will be filled with
zeros. Alternatively, the template file may be an ASCII format text file containing directives
that define the keywords to be created in each HDU of the file. See the 'Extended File Name
Syntax' section for a complete description of the template file syntax.
The fits create diskfile routine is similar to the fits create file routine except that it does not
support the extended filename syntax in the input file name. This routine simply tries to
create the specified file on magnetic disk. This routine is mainly for use in cases where the
filename (or directory path) contains square or curly bracket characters that would confuse
the extended filename parser.

5.3. HDU ACCESS ROUTINES 33
3 Close a previously opened FITS file. The first routine simply closes the file, whereas the second
one also DELETES THE FILE, which can be useful in cases where a FITS file has been
partially created, but then an error occurs which prevents it from being completed.
int fits_close_file / ffclos (fitsfile *fptr, > int *status)
int fits_delete_file / ffdelt (fitsfile *fptr, > int *status)
4 Return the name, I/O mode (READONLY or READWRITE), and/or the file type (e.g. 'file://',
'ftp://') of the opened FITS file.
int fits_file_name / ffflnm (fitsfile *fptr, > char *filename, int *status)
int fits_file_mode / ffflmd (fitsfile *fptr, > int *iomode, int *status)
int fits_url_type / ffurlt (fitsfile *fptr, > char *urltype, int *status)
5.3 HDU Access Routines
The following functions perform operations on Header­Data Units (HDUs) as a whole.
1 Move to a di#erent HDU in the file. The first routine moves to a specified absolute HDU
number (starting with 1 for the primary array) in the FITS file, and the second routine
moves a relative number HDUs forward or backward from the current HDU. A null pointer
may be given for the hdutype parameter if it's value is not needed. The third routine moves
to the (first) HDU which has the specified extension type and EXTNAME and EXTVER
keyword values (or HDUNAME and HDUVER keywords). The hdutype parameter may have
a value of IMAGE HDU, ASCII TBL, BINARY TBL, or ANY HDU where ANY HDU means
that only the extname and extver values will be used to locate the correct extension. If the
input value of extver is 0 then the EXTVER keyword is ignored and the first HDU with a
matching EXTNAME (or HDUNAME) keyword will be found. If no matching HDU is found
in the file then the current HDU will remain unchanged and a status = BAD HDU NUM will
be returned.
int fits_movabs_hdu / ffmahd
(fitsfile *fptr, int hdunum, > int *hdutype, int *status)
int fits_movrel_hdu / ffmrhd
(fitsfile *fptr, int nmove, > int *hdutype, int *status)
int fits_movnam_hdu / ffmnhd
(fitsfile *fptr, int hdutype, char *extname, int extver, > int *status)
2 Return the total number of HDUs in the FITS file. This returns the number of completely
defined HDUs in the file. If a new HDU has just been added to the FITS file, then that last

34 CHAPTER 5. BASIC CFITSIO INTERFACE ROUTINES
HDU will only be counted if it has been closed, or if data has been written to the HDU. The
current HDU remains unchanged by this routine.
int fits_get_num_hdus / ffthdu
(fitsfile *fptr, > int *hdunum, int *status)
3 Return the number of the current HDU (CHDU) in the FITS file (where the primary array =
1). This function returns the HDU number rather than a status value.
int fits_get_hdu_num / ffghdn
(fitsfile *fptr, > int *hdunum)
4 Return the type of the current HDU in the FITS file. The possible values for hdutype are:
IMAGE HDU, ASCII TBL, or BINARY TBL.
int fits_get_hdu_type / ffghdt
(fitsfile *fptr, > int *hdutype, int *status)
5 Copy all or part of the HDUs in the FITS file associated with infptr and append them to the end
of the FITS file associated with outfptr. If 'previous' is true (not 0), then any HDUs preceding
the current HDU in the input file will be copied to the output file. Similarly, 'current' and
'following' determine whether the current HDU, and/or any following HDUs in the input file
will be copied to the output file. Thus, if all 3 parameters are true, then the entire input file
will be copied. On exit, the current HDU in the input file will be unchanged, and the last
HDU in the output file will be the current HDU.
int fits_copy_file / ffcpfl
(fitsfile *infptr, fitsfile *outfptr, int previous, int current,
int following, > int *status)
6 Copy the current HDU from the FITS file associated with infptr and append it to the end of
the FITS file associated with outfptr. Space may be reserved for MOREKEYS additional
keywords in the output header.
int fits_copy_hdu / ffcopy
(fitsfile *infptr, fitsfile *outfptr, int morekeys, > int *status)
7 Write the current HDU in the input FITS file to the output FILE stream (e.g., to stdout).
int fits_write_hdu / ffwrhdu
(fitsfile *infptr, FILE *stream, > int *status)
8 Copy the header (and not the data) from the CHDU associated with infptr to the CHDU
associated with outfptr. If the current output HDU is not completely empty, then the CHDU
will be closed and a new HDU will be appended to the output file. An empty output data
unit will be created with all values initially = 0).

5.4. HEADER KEYWORD READ/WRITE ROUTINES 35
int fits_copy_header / ffcphd
(fitsfile *infptr, fitsfile *outfptr, > int *status)
9 Delete the CHDU in the FITS file. Any following HDUs will be shifted forward in the file, to
fill in the gap created by the deleted HDU. In the case of deleting the primary array (the
first HDU in the file) then the current primary array will be replace by a null primary array
containing the minimum set of required keywords and no data. If there are more extensions
in the file following the one that is deleted, then the the CHDU will be redefined to point to
the following extension. If there are no following extensions then the CHDU will be redefined
to point to the previous HDU. The output hdutype parameter returns the type of the new
CHDU. A null pointer may be given for hdutype if the returned value is not needed.
int fits_delete_hdu / ffdhdu
(fitsfile *fptr, > int *hdutype, int *status)
5.4 Header Keyword Read/Write Routines
These routines read or write keywords in the Current Header Unit (CHU). Wild card characters
(*, ?, or #) may be used when specifying the name of the keyword to be read: a ' ?' will match any
single character at that position in the keyword name and a '*' will match any length (including
zero) string of characters. The '#' character will match any consecutive string of decimal digits (0
­ 9). When a wild card is used the routine will only search for a match from the current header
position to the end of the header and will not resume the search from the top of the header back to
the original header position as is done when no wildcards are included in the keyword name. The
fits read record routine may be used to set the starting position when doing wild card searches. A
status value of KEY NO EXIST is returned if the specified keyword to be read is not found in the
header.
5.4.1 Keyword Reading Routines
1 Return the number of existing keywords (not counting the END keyword) and the amount of
space currently available for more keywords. It returns morekeys = ­1 if the header has not
yet been closed. Note that CFITSIO will dynamically add space if required when writing new
keywords to a header so in practice there is no limit to the number of keywords that can be
added to a header. A null pointer may be entered for the morekeys parameter if it's value is
not needed.
int fits_get_hdrspace / ffghsp
(fitsfile *fptr, > int *keysexist, int *morekeys, int *status)
2 Return the specified keyword. In the first routine, the datatype parameter specifies the desired
returned data type of the keyword value and can have one of the following symbolic constant
values: TSTRING, TLOGICAL (== int), TBYTE, TSHORT, TUSHORT, TINT, TUINT,

36 CHAPTER 5. BASIC CFITSIO INTERFACE ROUTINES
TLONG, TULONG, TLONGLONG, TFLOAT, TDOUBLE, TCOMPLEX, and TDBLCOM­
PLEX. Within the context of this routine, TSTRING corresponds to a 'char*' data type, i.e.,
a pointer to a character array. Data type conversion will be performed for numeric values if
the keyword value does not have the same data type. If the value of the keyword is undefined
(i.e., the value field is blank) then an error status = VALUE UNDEFINED will be returned.
The second routine returns the keyword value as a character string (a literal copy of what is in
the value field) regardless of the intrinsic data type of the keyword. The third routine returns
the entire 80­character header record of the keyword, with any trailing blank characters
stripped o#. The fourth routine returns the (next) header record that contains the literal
string of characters specified by the 'string' argument.
If a NULL comment pointer is supplied then the comment string will not be returned.
int fits_read_key / ffgky
(fitsfile *fptr, int datatype, char *keyname, > DTYPE *value,
char *comment, int *status)
int fits_read_keyword / ffgkey
(fitsfile *fptr, char *keyname, > char *value, char *comment,
int *status)
int fits_read_card / ffgcrd
(fitsfile *fptr, char *keyname, > char *card, int *status)
int fits_read_str / ffgstr
(fitsfile *fptr, char *string, > char *card, int *status)
3 Return the nth header record in the CHU. The first keyword in the header is at keynum =
1; if keynum = 0 then these routines simply reset the internal CFITSIO pointer to the
beginning of the header so that subsequent keyword operations will start at the top of the
header (e.g., prior to searching for keywords using wild cards in the keyword name). The first
routine returns the entire 80­character header record (with trailing blanks truncated), while
the second routine parses the record and returns the name, value, and comment fields as
separate (blank truncated) character strings. If a NULL comment pointer is given on input,
then the comment string will not be returned.
int fits_read_record / ffgrec
(fitsfile *fptr, int keynum, > char *card, int *status)
int fits_read_keyn / ffgkyn
(fitsfile *fptr, int keynum, > char *keyname, char *value,
char *comment, int *status)
4 Return the next keyword whose name matches one of the strings in 'inclist' but does not
match any of the strings in 'exclist'. The strings in inclist and exclist may contain wild card
characters (*, ?, and #) as described at the beginning of this section. This routine searches

5.4. HEADER KEYWORD READ/WRITE ROUTINES 37
from the current header position to the end of the header, only, and does not continue the
search from the top of the header back to the original position. The current header position
may be reset with the #grec routine. Note that nexc may be set = 0 if there are no keywords
to be excluded. This routine returns status = KEY NO EXIST if a matching keyword is not
found.
int fits_find_nextkey / ffgnxk
(fitsfile *fptr, char **inclist, int ninc, char **exclist,
int nexc, > char *card, int *status)
5 Return the physical units string from an existing keyword. This routine uses a local convention,
shown in the following example, in which the keyword units are enclosed in square brackets in
the beginning of the keyword comment field. A null string is returned if no units are defined
for the keyword.
VELOCITY= 12.3 / [km/s] orbital speed
int fits_read_key_unit / ffgunt
(fitsfile *fptr, char *keyname, > char *unit, int *status)
6 Concatenate the header keywords in the CHDU into a single long string of characters. This
provides a convenient way of passing all or part of the header information in a FITS HDU to
other subroutines. Each 80­character fixed­length keyword record is appended to the output
character string, in order, with no intervening separator or terminating characters. The last
header record is terminated with a NULL character. These routine allocates memory for the
returned character array, so the calling program must free the memory when finished. The
cleanest way to do this is to call the fits free memory routine.
There are 2 related routines: fits hdr2str simply concatenates all the existing keywords in
the header; fits convert hdr2str is similar, except that if the CHDU is a tile compressed
image (stored in a binary table) then it will first convert that header back to that of the
corresponding normal FITS image before concatenating the keywords.
Selected keywords may be excluded from the returned character string. If the second param­
eter (nocomments) is TRUE (nonzero) then any COMMENT, HISTORY, or blank keywords
in the header will not be copied to the output string.
The 'exclist' parameter may be used to supply a list of keywords that are to be excluded from
the output character string. Wild card characters (*, ?, and #) may be used in the excluded
keyword names. If no additional keywords are to be excluded, then set nexc = 0 and specify
NULL for the the **exclist parameter.
int fits_hdr2str / ffhdr2str
(fitsfile *fptr, int nocomments, char **exclist, int nexc,
> char **header, int *nkeys, int *status)
int fits_convert_hdr2str / ffcnvthdr2str
(fitsfile *fptr, int nocomments, char **exclist, int nexc,
> char **header, int *nkeys, int *status)

38 CHAPTER 5. BASIC CFITSIO INTERFACE ROUTINES
5.4.2 Keyword Writing Routines
1 Write a keyword of the appropriate data type into the CHU. The first routine simply appends
a new keyword whereas the second routine will update the value and comment fields of the
keyword if it already exists, otherwise it appends a new keyword. Note that the address to
the value, and not the value itself, must be entered. The datatype parameter specifies the
data type of the keyword value with one of the following values: TSTRING, TLOGICAL (==
int), TBYTE, TSHORT, TUSHORT, TINT, TUINT, TLONG, TLONGLONG, TULONG,
TFLOAT, TDOUBLE. Within the context of this routine, TSTRING corresponds to a 'char*'
data type, i.e., a pointer to a character array. A null pointer may be entered for the comment
parameter in which case the keyword comment field will be unmodified or left blank.
int fits_write_key / ffpky
(fitsfile *fptr, int datatype, char *keyname, DTYPE *value,
char *comment, > int *status)
int fits_update_key / ffuky
(fitsfile *fptr, int datatype, char *keyname, DTYPE *value,
char *comment, > int *status)
2 Write a keyword with a null or undefined value (i.e., the value field in the keyword is left
blank). The first routine simply appends a new keyword whereas the second routine will
update the value and comment fields of the keyword if it already exists, otherwise it appends
a new keyword. A null pointer may be entered for the comment parameter in which case the
keyword comment field will be unmodified or left blank.
int fits_write_key_null / ffpkyu
(fitsfile *fptr, char *keyname, char *comment, > int *status)
int fits_update_key_null / ffukyu
(fitsfile *fptr, char *keyname, char *comment, > int *status)
3 Write (append) a COMMENT or HISTORY keyword to the CHU. The comment or history
string will be continued over multiple keywords if it is longer than 70 characters.
int fits_write_comment / ffpcom
(fitsfile *fptr, char *comment, > int *status)
int fits_write_history / ffphis
(fitsfile *fptr, char *history, > int *status)
4 Write the DATE keyword to the CHU. The keyword value will contain the current system date
as a character string in 'yyyy­mm­ddThh:mm:ss' format. If a DATE keyword already exists
in the header, then this routine will simply update the keyword value with the current date.

5.4. HEADER KEYWORD READ/WRITE ROUTINES 39
int fits_write_date / ffpdat
(fitsfile *fptr, > int *status)
5 Write a user specified keyword record into the CHU. This is a low--level routine which can be
used to write any arbitrary record into the header. The record must conform to the all the
FITS format requirements.
int fits_write_record / ffprec
(fitsfile *fptr, char *card, > int *status)
6 Update an 80­character record in the CHU. If a keyword with the input name already exists,
then it is overwritten by the value of card. This could modify the keyword name as well as
the value and comment fields. If the keyword doesn't already exist then a new keyword card
is appended to the header.
int fits_update_card / ffucrd
(fitsfile *fptr, char *keyname, char *card, > int *status)
7 Modify (overwrite) the comment field of an existing keyword.
int fits_modify_comment / ffmcom
(fitsfile *fptr, char *keyname, char *comment, > int *status)
8 Write the physical units string into an existing keyword. This routine uses a local convention,
shown in the following example, in which the keyword units are enclosed in square brackets
in the beginning of the keyword comment field.
VELOCITY= 12.3 / [km/s] orbital speed
int fits_write_key_unit / ffpunt
(fitsfile *fptr, char *keyname, char *unit, > int *status)
9 Rename an existing keyword, preserving the current value and comment fields.
int fits_modify_name / ffmnam
(fitsfile *fptr, char *oldname, char *newname, > int *status)
10 Delete a keyword record. The space occupied by the keyword is reclaimed by moving all the
following header records up one row in the header. The first routine deletes a keyword at a
specified position in the header (the first keyword is at position 1), whereas the second routine
deletes a specifically named keyword. Wild card characters may be used when specifying the
name of the keyword to be deleted. The third routine deletes the (next) keyword that contains
the literal character string specified by the 'string' argument.

40 CHAPTER 5. BASIC CFITSIO INTERFACE ROUTINES
int fits_delete_record / ffdrec
(fitsfile *fptr, int keynum, > int *status)
int fits_delete_key / ffdkey
(fitsfile *fptr, char *keyname, > int *status)
int fits_delete_str / ffdstr
(fitsfile *fptr, char *string, > int *status)
5.5 Primary Array or IMAGE Extension I/O Routines
These routines read or write data values in the primary data array (i.e., the first HDU in a FITS file)
or an IMAGE extension. There are also routines to get information about the data type and size
of the image. Users should also read the following chapter on the CFITSIO iterator function which
provides a more `object oriented' method of reading and writing images. The iterator function is
a little more complicated to use, but the advantages are that it usually takes less code to perform
the same operation, and the resulting program often runs faster because the FITS files are read
and written using the most e#cient block size.
C programmers should note that the ordering of arrays in FITS files, and hence in all the CFITSIO
calls, is more similar to the dimensionality of arrays in Fortran rather than C. For instance if a
FITS image has NAXIS1 = 100 and NAXIS2 = 50, then a 2­D array just large enough to hold the
image should be declared as array[50][100] and not as array[100][50].
The `datatype' parameter specifies the data type of the `nulval' and `array' pointers and can have
one of the following values: TBYTE, TSBYTE, TSHORT, TUSHORT, TINT, TUINT, TLONG,
TLONGLONG, TULONG, TFLOAT, TDOUBLE. Automatic data type conversion is performed if
the data type of the FITS array (as defined by the BITPIX keyword) di#ers from that specified by
'datatype'. The data values are also automatically scaled by the BSCALE and BZERO keyword
values as they are being read or written in the FITS array.
1 Get the data type or equivalent data type of the image. The first routine returns the physical
data type of the FITS image, as given by the BITPIX keyword, with allowed values of
BYTE IMG (8), SHORT IMG (16), LONG IMG (32), LONGLONG IMG (64), FLOAT IMG
(­32), and DOUBLE IMG (­64). The second routine is similar, except that if the image pixel
values are scaled, with non­default values for the BZERO and BSCALE keywords, then the
routine will return the 'equivalent' data type that is needed to store the scaled values. For
example, if BITPIX = 16 and BSCALE = 0.1 then the equivalent data type is FLOAT IMG.
Similarly if BITPIX = 16, BSCALE = 1, and BZERO = 32768, then the the pixel values span
the range of an unsigned short integer and the returned data type will be USHORT IMG.
int fits_get_img_type / ffgidt
(fitsfile *fptr, > int *bitpix, int *status)
int fits_get_img_equivtype / ffgiet
(fitsfile *fptr, > int *bitpix, int *status)

5.5. PRIMARY ARRAY OR IMAGE EXTENSION I/O ROUTINES 41
2 Get the number of dimensions, and/or the size of each dimension in the image . The number
of axes in the image is given by naxis, and the size of each dimension is given by the naxes
array (a maximum of maxdim dimensions will be returned).
int fits_get_img_dim / ffgidm
(fitsfile *fptr, > int *naxis, int *status)
int fits_get_img_size / ffgisz
(fitsfile *fptr, int maxdim, > long *naxes, int *status)
int fits_get_img_sizell / ffgiszll
(fitsfile *fptr, int maxdim, > LONGLONG *naxes, int *status)
int fits_get_img_param / ffgipr
(fitsfile *fptr, int maxdim, > int *bitpix, int *naxis, long *naxes,
int *status)
int fits_get_img_paramll / ffgiprll
(fitsfile *fptr, int maxdim, > int *bitpix, int *naxis, LONGLONG *naxes,
int *status)
3 Create a new primary array or IMAGE extension with a specified data type and size. If the FITS
file is currently empty then a primary array is created, otherwise a new IMAGE extension is
appended to the file.
int fits_create_img / ffcrim
( fitsfile *fptr, int bitpix, int naxis, long *naxes, > int *status)
int fits_create_imgll / ffcrimll
( fitsfile *fptr, int bitpix, int naxis, LONGLONG *naxes, > int *status)
4 Copy an n­dimensional image in a particular row and column of a binary table (in a vector
column) to or from a primary array or image extension.
The 'cell2image' routine will append a new image extension (or primary array) to the output
file. Any WCS keywords associated with the input column image will be translated into the
appropriate form for an image extension. Any other keywords in the table header that are
not specifically related to defining the binary table structure or to other columns in the table
will also be copied to the header of the output image.
The 'image2cell' routine will copy the input image into the specified row and column of the
current binary table in the output file. The binary table HDU must exist before calling this
routine, but it may be empty, with no rows or columns of data. The specified column (and
row) will be created if it does not already exist. The 'copykeyflag' parameter controls which
keywords are copied from the input image to the header of the output table: 0 = no keywords
will be copied, 1 = all keywords will be copied (except those keywords that would be invalid
in the table header), and 2 = copy only the WCS keywords.

42 CHAPTER 5. BASIC CFITSIO INTERFACE ROUTINES
int fits_copy_cell2image
(fitsfile *infptr, fitsfile *outfptr, char *colname, long rownum,
> int *status)
int fits_copy_image2cell
(fitsfile *infptr, fitsfile *outfptr, char *colname, long rownum,
int copykeyflag > int *status)
5 Write a rectangular subimage (or the whole image) to the FITS data array. The fpixel and
lpixel arrays give the coordinates of the first (lower left corner) and last (upper right corner)
pixels in FITS image to be written to.
int fits_write_subset / ffpss
(fitsfile *fptr, int datatype, long *fpixel, long *lpixel,
DTYPE *array, > int *status)
6 Write pixels into the FITS data array. 'fpixel' is an array of length NAXIS which gives the
coordinate of the starting pixel to be written to, such that fpixel[0] is in the range 1 to
NAXIS1, fpixel[1] is in the range 1 to NAXIS2, etc. The first pair of routines simply writes
the array of pixels to the FITS file (doing data type conversion if necessary) whereas the
second routines will substitute the appropriate FITS null value for any elements which are
equal to the input value of nulval (note that this parameter gives the address of the null
value, not the null value itself). For integer FITS arrays, the FITS null value is defined by
the BLANK keyword (an error is returned if the BLANK keyword doesn't exist). For floating
point FITS arrays the special IEEE NaN (Not­a­Number) value will be written into the FITS
file. If a null pointer is entered for nulval, then the null value is ignored and this routine
behaves the same as fits write pix.
int fits_write_pix / ffppx
(fitsfile *fptr, int datatype, long *fpixel, LONGLONG nelements,
DTYPE *array, int *status);
int fits_write_pixll / ffppxll
(fitsfile *fptr, int datatype, LONGLONG *fpixel, LONGLONG nelements,
DTYPE *array, int *status);
int fits_write_pixnull / ffppxn
(fitsfile *fptr, int datatype, long *fpixel, LONGLONG nelements,
DTYPE *array, DTYPE *nulval, > int *status);
int fits_write_pixnullll / ffppxnll
(fitsfile *fptr, int datatype, LONGLONG *fpixel, LONGLONG nelements,
DTYPE *array, DTYPE *nulval, > int *status);
7 Set FITS data array elements equal to the appropriate null pixel value. For integer FITS arrays,
the FITS null value is defined by the BLANK keyword (an error is returned if the BLANK

5.5. PRIMARY ARRAY OR IMAGE EXTENSION I/O ROUTINES 43
keyword doesn't exist). For floating point FITS arrays the special IEEE NaN (Not­a­Number)
value will be written into the FITS file. Note that 'firstelem' is a scalar giving the o#set to
the first pixel to be written in the equivalent 1­dimensional array of image pixels.
int fits_write_null_img / ffpprn
(fitsfile *fptr, LONGLONG firstelem, LONGLONG nelements, > int *status)
8 Read a rectangular subimage (or the whole image) from the FITS data array. The fpixel and
lpixel arrays give the coordinates of the first (lower left corner) and last (upper right corner)
pixels to be read from the FITS image. Undefined FITS array elements will be returned with
a value = *nullval, (note that this parameter gives the address of the null value, not the null
value itself) unless nulval = 0 or *nulval = 0, in which case no checks for undefined pixels
will be performed.
int fits_read_subset / ffgsv
(fitsfile *fptr, int datatype, long *fpixel, long *lpixel, long *inc,
DTYPE *nulval, > DTYPE *array, int *anynul, int *status)
9 Read pixels from the FITS data array. 'fpixel' is the starting pixel location and is an array of
length NAXIS such that fpixel[0] is in the range 1 to NAXIS1, fpixel[1] is in the range 1 to
NAXIS2, etc. The nelements parameter specifies the number of pixels to read. If fpixel is set
to the first pixel, and nelements is set equal to the NAXIS1 value, then this routine would
read the first row of the image. Alternatively, if nelements is set equal to NAXIS1 * NAXIS2
then it would read an entire 2D image, or the first plane of a 3­D datacube.
The first 2 routines will return any undefined pixels in the FITS array equal to the value
of *nullval (note that this parameter gives the address of the null value, not the null value
itself) unless nulval = 0 or *nulval = 0, in which case no checks for undefined pixels will be
performed. The second 2 routines are similar except that any undefined pixels will have the
corresponding nullarray element set equal to TRUE (= 1).
int fits_read_pix / ffgpxv
(fitsfile *fptr, int datatype, long *fpixel, LONGLONG nelements,
DTYPE *nulval, > DTYPE *array, int *anynul, int *status)
int fits_read_pixll / ffgpxvll
(fitsfile *fptr, int datatype, LONGLONG *fpixel, LONGLONG nelements,
DTYPE *nulval, > DTYPE *array, int *anynul, int *status)
int fits_read_pixnull / ffgpxf
(fitsfile *fptr, int datatype, long *fpixel, LONGLONG nelements,
> DTYPE *array, char *nullarray, int *anynul, int *status)
int fits_read_pixnullll / ffgpxfll
(fitsfile *fptr, int datatype, LONGLONG *fpixel, LONGLONG nelements,
> DTYPE *array, char *nullarray, int *anynul, int *status)

44 CHAPTER 5. BASIC CFITSIO INTERFACE ROUTINES
10 Copy a rectangular section of an image and write it to a new FITS primary image or image
extension. The new image HDU is appended to the end of the output file; all the keywords
in the input image will be copied to the output image. The common WCS keywords will
be updated if necessary to correspond to the coordinates of the section. The format of the
section expression is same as specifying an image section using the extended file name syntax
(see ''Image Section'' in Chapter 10). (Examples: ''1:100,1:200'', ''1:100:2, 1:*:2'', ''*, ­*'').
int fits_copy_image_section / ffcpimg
(fitsfile *infptr, fitsfile *outfptr, char *section, int *status)
5.6 Image Compression
CFITSIO transparently supports the 2 methods of image compression described below.
1) The entire FITS file may be externally compressed with the gzip or Unix compress utility
programs, producing a *.gz or *.Z file, respectively. When reading compressed files of this type,
CFITSIO first uncompresses the entire file into memory before performing the requested read
operations. Output files can be directly written in the gzip compressed format if the user­specified
filename ends with `.gz'. In this case, CFITSIO initially writes the uncompressed file in memory
and then compresses it and writes it to disk when the FITS file is closed, thus saving user disk
space. Read and write access to these compressed FITS files is generally quite fast since all the
I/O is performed in memory; the main limitation with this technique is that there must be enough
available memory (or swap space) to hold the entire uncompressed FITS file.
2) CFITSIO also supports the FITS tiled image compression convention in which the image is
subdivided into a grid of rectangular tiles, and each tile of pixels is individually compressed. The
details of this FITS compression convention are described at the FITS Support O#ce web site at
http://fits.gsfc.nasa.gov/fits registry.html Basically, the compressed image tiles are stored in rows
of a variable length array column in a FITS binary table, however CFITSIO recognizes that this
binary table extension contains an image and treats it as if it were an IMAGE extension. This
tile­compressed format is especially well suited for compressing very large images because a) the
FITS header keywords remain uncompressed for rapid read access, and because b) it is possible to
extract and uncompress sections of the image without having to uncompress the entire image. This
format is also much more e#ective in compressing floating point images than simply compressing
the image using gzip or compress because it approximates the floating point values with scaled
integers which can then be compressed more e#ciently.
Currently CFITSIO supports 3 general purpose compression algorithms plus one other special­
purpose compression technique that is designed for data masks with positive integer pixel values.
The 3 general purpose algorithms are GZIP, Rice, and HCOMPRESS, and the special purpose
algorithm is the IRAF pixel list compression technique (PLIO). In principle, any number of other
compression algorithms could also be supported by the FITS tiled image compression convention.
The FITS image can be subdivided into any desired rectangular grid of compression tiles. With
the GZIP, Rice, and PLIO algorithms, the default is to take each row of the image as a tile. The
HCOMPRESS algorithm is inherently 2­dimensional in nature, so the default in this case is to take
16 rows of the image per tile. In most cases it makes little di#erence what tiling pattern is used, so

5.6. IMAGE COMPRESSION 45
the default tiles are usually adequate. In the case of very small images, it could be more e#cient
to compress the whole image as a single tile. Note that the image dimensions are not required to
be an integer multiple of the tile dimensions; if not, then the tiles at the edges of the image will be
smaller than the other tiles.
The 4 supported image compression algorithms are all 'loss­less' when applied to integer FITS
images; the pixel values are preserved exactly with no loss of information during the compression and
uncompression process. In addition, the HCOMPRESS algorithm supports a 'lossy' compression
mode that will produce larger amount of image compression. This is achieved by specifying a
non­zero value for the HCOMPRESS ``scale'' parameter. Since the amount of compression that is
achieved depends directly on the RMS noise in the image, it is usually more convention to specify
the HCOMPRESS scale factor relative to the RMS noise. Setting s = 2.5 means use a scale factor
that is 2.5 times the calculated RMS noise in the image tile. In some cases it may be desirable to
specify the exact scaling to be used, instead of specifying it relative to the calculated noise value.
This may be done by specifying the negative of desired scale value (typically in the range ­2 to
­100).
Very high compression factors (of 100 or more) can be achieved by using large HCOMPRESS
scale values, however, this can produce undesirable ``blocky'' artifacts in the compressed image. A
variation of the HCOMPRESS algorithm (called HSCOMPRESS) can be used in this case to apply
a small amount of smoothing of the image when it is uncompressed to help cover up these artifacts.
This smoothing is purely cosmetic and does not cause any significant change to the image pixel
values.
Floating point FITS images (which have BITPIX = ­32 or ­64) usually contain too much ``noise''
in the least significant bits of the mantissa of the pixel values to be e#ectively compressed with
any lossless algorithm. Consequently, floating point images are first quantized into scaled integer
pixel values (and thus throwing away much of the noise) before being compressed with the specified
algorithm (either GZIP, Rice, or HCOMPRESS). This technique produces much higher compression
factors than simply using the GZIP utility to externally compress the whole FITS file, but it also
means that the original floating value pixel values are not exactly preserved. When done properly,
this integer scaling technique will only discard the insignificant noise while still preserving all the
real information in the image. The amount of precision that is retained in the pixel values is
controlled by the ''quantization level'' parameter, q. Larger values of q will result in compressed
images whose pixels more closely match the floating point pixel values, but at the same time the
amount of compression that is achieved will be reduced. Users should experiment with di#erent
values for this parameter to determine the optimal value that preserves all the useful information in
the image, without needlessly preserving all the ``noise'' which will hurt the compression e#ciency.
The default value for the quantization scale factor is 16., which means that scaled integer pixel
values will be quantized such that the di#erence between adjacent integer values will be 1/16th
of the noise level in the image background. CFITSIO uses an optimized algorithm to accurately
estimate the noise in the image. As an example, if the RMS noise in the background pixels of
an image = 32.0, then the spacing between adjacent scaled integer pixel values will equal 2.0 by
default. Note that the RMS noise is independently calculated for each tile of the image, so the
resulting integer scaling factor may fluctuate slightly for each tile. In some cases it may be desirable
to specify the exact quantization level to be used, instead of specifying it relative to the calculated
noise value. This may be done by specifying the negative of desired quantization level for the
value of q. In the previous example, one could specify q = ­2.0 so that the quantized integer levels

46 CHAPTER 5. BASIC CFITSIO INTERFACE ROUTINES
di#er by 2.0. Larger negative values for q means that the levels are more coarsely spaced, and will
produce higher compression factors.
There are 2 methods for specifying all the parameters needed to write a FITS image in the tile
compressed format. The parameters may either be specified at run time as part of the file name
of the output compressed FITS file, or the writing program may call a set of helper CFITSIO
subroutines that are provided for specifying the parameter values, as described below:
1) At run time, when specifying the name of the output FITS file to be created, the user can indicate
that images should be written in tile­compressed format by enclosing the compression parameters
in square brackets following the root disk file name in the following format:
[compress NAME T1,T2; q QLEVEL, s HSCALE]
where
NAME = algorithm name: GZIP, Rice, HCOMPRESS, HSCOMPRSS or PLIO
may be abbreviated to the first letter (or HS for HSCOMPRESS)
T1,T2 = tile dimension (e.g. 100,100 for square tiles 100 pixels wide)
QLEVEL = quantization level for floating point FITS images
HSCALE = HCOMPRESS scale factor; default = 0 which is lossless.
Here are a few examples of this extended syntax:
myfile.fit[compress] ­ use the default compression algorithm (Rice)
and the default tile size (row by row)
myfile.fit[compress GZIP] ­ use the specified compression algorithm;
myfile.fit[compress Rice] only the first letter of the algorithm
myfile.fit[compress PLIO] name is required.
myfile.fit[compress HCOMP]
myfile.fit[compress R 100,100] ­ use Rice and 100 x 100 pixel tiles
myfile.fit[compress R; q 10.0] ­ quantization level = (RMS­noise) / 10.
myfile.fit[compress HS; s 2.0] ­ HSCOMPRESS (with smoothing)
and scale = 2.0 * RMS­noise
2) Before calling the CFITSIO routine to write the image header keywords (e.g., fits create image)
the programmer can call the routines described below to specify the compression algorithm and
the tiling pattern that is to be used. There are routines for specifying the various compression
parameters and similar routines to return the current values of the parameters:
int fits_set_compression_type(fitsfile *fptr, int comptype, int *status)
int fits_set_tile_dim(fitsfile *fptr, int ndim, long *tilesize, int *status)
int fits_set_quantize_level(fitsfile *fptr, float qlevel, int *status)

5.6. IMAGE COMPRESSION 47
int fits_set_hcomp_scale(fitsfile *fptr, float scale, int *status)
int fits_set_hcomp_smooth(fitsfile *fptr, int smooth, int *status)
Set smooth = 1 to apply smoothing when uncompressing the image
int fits_get_compression_type(fitsfile *fptr, int *comptype, int *status)
int fits_get_tile_dim(fitsfile *fptr, int ndim, long *tilesize, int *status)
int fits_get_quantize_level(fitsfile *fptr, float *level, int *status)
int fits_get_hcomp_scale(fitsfile *fptr, float *scale, int *status)
int fits_get_hcomp_smooth(fitsfile *fptr, int *smooth, int *status)
4 symbolic constants are defined for use as the value of the `comptype' parameter: GZIP 1, RICE 1,
HCOMPRESS 1 or PLIO 1. Entering NULL for comptype will turn o# the tile­compression and
cause normal FITS images to be written.
No special action is required by software when read tile­compressed images because all the CFITSIO
routines that read normal uncompressed FITS images also transparently read images in the tile­
compressed format; CFITSIO essentially treats the binary table that contains the compressed tiles
as if it were an IMAGE extension.
The following 2 routines are available for compressing or or decompressing an image:
int fits_img_compress(fitsfile *infptr, fitsfile *outfptr, int *status);
int fits_img_decompress (fitsfile *infptr, fitsfile *outfptr, int *status);
Before calling the compression routine, the compression parameters must first be defined in one of
the 2 way described in the previous paragraphs. There is also a routine to determine if the current
HDU contains a tile compressed image (it returns 1 or 0):
int fits_is_compressed_image(fitsfile *fptr, int *status);
A small example program called 'imcopy' is included with CFITSIO that can be used to compress
(or uncompress) any FITS image. This program can be used to experiment with the various
compression options on existing FITS images as shown in these examples:
1) imcopy infile.fit 'outfile.fit[compress]'
This will use the default compression algorithm (Rice) and the
default tile size (row by row)
2) imcopy infile.fit 'outfile.fit[compress GZIP]'
This will use the GZIP compression algorithm and the default
tile size (row by row). The allowed compression algorithms are
Rice, GZIP, and PLIO. Only the first letter of the algorithm
name needs to be specified.

48 CHAPTER 5. BASIC CFITSIO INTERFACE ROUTINES
3) imcopy infile.fit 'outfile.fit[compress G 100,100]'
This will use the GZIP compression algorithm and 100 X 100 pixel
tiles.
4) imcopy infile.fit 'outfile.fit[compress R 100,100; q 10.0]'
This will use the Rice compression algorithm, 100 X 100 pixel
tiles, and quantization level = RMSnoise / 10.0 (assuming the
input image has a floating point data type).
5) imcopy infile.fit outfile.fit
If the input file is in tile­compressed format, then it will be
uncompressed to the output file. Otherwise, it simply copies
the input image to the output image.
6) imcopy 'infile.fit[1001:1500,2001:2500]' outfile.fit
This extracts a 500 X 500 pixel section of the much larger
input image (which may be in tile­compressed format). The
output is a normal uncompressed FITS image.
7) imcopy 'infile.fit[1001:1500,2001:2500]' outfile.fit.gz
Same as above, except the output file is externally compressed
using the gzip algorithm.
5.7 ASCII and Binary Table Routines
These routines perform read and write operations on columns of data in FITS ASCII or Binary
tables. Note that in the following discussions, the first row and column in a table is at position 1
not 0.
Users should also read the following chapter on the CFITSIO iterator function which provides a
more `object oriented' method of reading and writing table columns. The iterator function is a
little more complicated to use, but the advantages are that it usually takes less code to perform
the same operation, and the resulting program often runs faster because the FITS files are read
and written using the most e#cient block size.
5.7.1 Create New Table
1 Create a new ASCII or bintable table extension. If the FITS file is currently empty then a
dummy primary array will be created before appending the table extension to it. The tbltype

5.7. ASCII AND BINARY TABLE ROUTINES 49
parameter defines the type of table and can have values of ASCII TBL or BINARY TBL.
The naxis2 parameter gives the initial number of rows to be created in the table, and should
normally be set = 0. CFITSIO will automatically increase the size of the table as additional
rows are written. A non­zero number of rows may be specified to reserve space for that many
rows, even if a fewer number of rows will be written. The tunit and extname parameters
are optional and a null pointer may be given if they are not defined. The FITS Standard
recommends that only letters, digits, and the underscore character be used in column names
(the ttype parameter) with no embedded spaces. Trailing blank characters are not significant.
int fits_create_tbl / ffcrtb
(fitsfile *fptr, int tbltype, LONGLONG naxis2, int tfields, char *ttype[],
char *tform[], char *tunit[], char *extname, int *status)
5.7.2 Column Information Routines
1 Get the number of rows or columns in the current FITS table. The number of rows is given by
the NAXIS2 keyword and the number of columns is given by the TFIELDS keyword in the
header of the table.
int fits_get_num_rows / ffgnrw
(fitsfile *fptr, > long *nrows, int *status);
int fits_get_num_rowsll / ffgnrwll
(fitsfile *fptr, > LONGLONG *nrows, int *status);
int fits_get_num_cols / ffgncl
(fitsfile *fptr, > int *ncols, int *status);
2 Get the table column number (and name) of the column whose name matches an input template
name. If casesen = CASESEN then the column name match will be case­sensitive, whereas
if casesen = CASEINSEN then the case will be ignored. As a general rule, the column names
should be treated as case INsensitive.
The input column name template may be either the exact name of the column to be searched
for, or it may contain wild card characters (*, ?, or #), or it may contain the integer number
of the desired column (with the first column = 1). The `*' wild card character matches any
sequence of characters (including zero characters) and the `?' character matches any single
character. The # wildcard will match any consecutive string of decimal digits (0­9). If more
than one column name in the table matches the template string, then the first match is
returned and the status value will be set to COL NOT UNIQUE as a warning that a unique
match was not found. To find the other cases that match the template, call the routine again
leaving the input status value equal to COL NOT UNIQUE and the next matching name will
then be returned. Repeat this process until a status = COL NOT FOUND is returned.
The FITS Standard recommends that only letters, digits, and the underscore character be
used in column names (with no embedded spaces). Trailing blank characters are not signifi­
cant.

50 CHAPTER 5. BASIC CFITSIO INTERFACE ROUTINES
int fits_get_colnum / ffgcno
(fitsfile *fptr, int casesen, char *templt, > int *colnum,
int *status)
int fits_get_colname / ffgcnn
(fitsfile *fptr, int casesen, char *templt, > char *colname,
int *colnum, int *status)
3 Return the data type, vector repeat value, and the width in bytes of a column in an ASCII or
binary table. Allowed values for the data type in ASCII tables are: TSTRING, TSHORT,
TLONG, TFLOAT, and TDOUBLE. Binary tables also support these types: TLOGICAL,
TBIT, TBYTE, TCOMPLEX and TDBLCOMPLEX. The negative of the data type code
value is returned if it is a variable length array column. Note that in the case of a 'J' 32­bit
integer binary table column, this routine will return data type = TINT32BIT (which in fact
is equivalent to TLONG). With most current C compilers, a value in a 'J' column has the
same size as an 'int' variable, and may not be equivalent to a 'long' variable, which is 64­bits
long on an increasing number of compilers.
The 'repeat' parameter returns the vector repeat count on the binary table TFORMn keyword
value. (ASCII table columns always have repeat = 1). The 'width' parameter returns the
width in bytes of a single column element (e.g., a '10D' binary table column will have width
= 8, an ASCII table 'F12.2' column will have width = 12, and a binary table'60A' character
string column will have width = 60); Note that CFITSIO supports the local convention
for specifying arrays of fixed length strings within a binary table character column using
the syntax TFORM = 'rAw' where 'r' is the total number of characters (= the width of
the column) and 'w' is the width of a unit string within the column. Thus if the column
has TFORM = '60A12' then this means that each row of the table contains 5 12­character
substrings within the 60­character field, and thus in this case this routine will return typecode
= TSTRING, repeat = 60, and width = 12. (The TDIMn keyword may also be used to specify
the unit string length; The pair of keywords TFORMn = '60A' and TDIMn = '(12,5)' would
have the same e#ect as TFORMn = '60A12'). The number of substrings in any binary table
character string field can be calculated by (repeat/width). A null pointer may be given for
any of the output parameters that are not needed.
The second routine, fit get eqcoltype is similar except that in the case of scaled integer
columns it returns the 'equivalent' data type that is needed to store the scaled values, and
not necessarily the physical data type of the unscaled values as stored in the FITS table. For
example if a '1I' column in a binary table has TSCALn = 1 and TZEROn = 32768, then
this column e#ectively contains unsigned short integer values, and thus the returned value of
typecode will be TUSHORT, not TSHORT. Similarly, if a column has TTYPEn = '1I' and
TSCALn = 0.12, then the returned typecode will be TFLOAT.
int fits_get_coltype / ffgtcl
(fitsfile *fptr, int colnum, > int *typecode, long *repeat,
long *width, int *status)
int fits_get_coltypell / ffgtclll

5.7. ASCII AND BINARY TABLE ROUTINES 51
(fitsfile *fptr, int colnum, > int *typecode, LONGLONG *repeat,
LONGLONG *width, int *status)
int fits_get_eqcoltype / ffeqty
(fitsfile *fptr, int colnum, > int *typecode, long *repeat,
long *width, int *status)
int fits_get_eqcoltypell / ffeqtyll
(fitsfile *fptr, int colnum, > int *typecode, LONGLONG *repeat,
LONGLONG *width, int *status)
4 Return the display width of a column. This is the length of the string that will be returned by
the fits read col routine when reading the column as a formatted string. The display width
is determined by the TDISPn keyword, if present, otherwise by the data type of the column.
int fits_get_col_display_width / ffgcdw
(fitsfile *fptr, int colnum, > int *dispwidth, int *status)
5 Return the number of and size of the dimensions of a table column in a binary table. Normally
this information is given by the TDIMn keyword, but if this keyword is not present then this
routine returns naxis = 1 and naxes[0] equal to the repeat count in the TFORM keyword.
int fits_read_tdim / ffgtdm
(fitsfile *fptr, int colnum, int maxdim, > int *naxis,
long *naxes, int *status)
int fits_read_tdimll / ffgtdmll
(fitsfile *fptr, int colnum, int maxdim, > int *naxis,
LONGLONG *naxes, int *status)
6 Decode the input TDIMn keyword string (e.g. '(100,200)') and return the number of and size
of the dimensions of a binary table column. If the input tdimstr character string is null, then
this routine returns naxis = 1 and naxes[0] equal to the repeat count in the TFORM keyword.
This routine is called by fits read tdim.
int fits_decode_tdim / ffdtdm
(fitsfile *fptr, char *tdimstr, int colnum, int maxdim, > int *naxis,
long *naxes, int *status)
int fits_decode_tdimll / ffdtdmll
(fitsfile *fptr, char *tdimstr, int colnum, int maxdim, > int *naxis,
LONGLONG *naxes, int *status)
7 Write a TDIMn keyword whose value has the form '(l,m,n...)' where l, m, n... are the dimensions
of a multidimensional array column in a binary table.

52 CHAPTER 5. BASIC CFITSIO INTERFACE ROUTINES
int fits_write_tdim / ffptdm
(fitsfile *fptr, int colnum, int naxis, long *naxes, > int *status)
int fits_write_tdimll / ffptdmll
(fitsfile *fptr, int colnum, int naxis, LONGLONG *naxes, > int *status)
5.7.3 Routines to Edit Rows or Columns
1 Insert or delete rows in an ASCII or binary table. When inserting rows all the rows following
row FROW are shifted down by NROWS rows; if FROW = 0 then the blank rows are inserted
at the beginning of the table. Note that it is *not* necessary to insert rows in a table before
writing data to those rows (indeed, it would be ine#cient to do so). Instead one may simply
write data to any row of the table, whether that row of data already exists or not.
The first delete routine deletes NROWS consecutive rows starting with row FIRSTROW.
The second delete routine takes an input string that lists the rows or row ranges (e.g., '5­
10,12,20­30'), whereas the third delete routine takes an input integer array that specifies each
individual row to be deleted. In both latter cases, the input list of rows to delete must be
sorted in ascending order. These routines update the NAXIS2 keyword to reflect the new
number of rows in the table.
int fits_insert_rows / ffirow
(fitsfile *fptr, LONGLONG firstrow, LONGLONG nrows, > int *status)
int fits_delete_rows / ffdrow
(fitsfile *fptr, LONGLONG firstrow, LONGLONG nrows, > int *status)
int fits_delete_rowrange / ffdrrg
(fitsfile *fptr, char *rangelist, > int *status)
int fits_delete_rowlist / ffdrws
(fitsfile *fptr, long *rowlist, long nrows, > int *status)
int fits_delete_rowlistll / ffdrwsll
(fitsfile *fptr, LONGLONG *rowlist, LONGLONG nrows, > int *status)
2 Insert or delete column(s) in an ASCII or binary table. When inserting, COLNUM specifies
the column number that the (first) new column should occupy in the table. NCOLS specifies
how many columns are to be inserted. Any existing columns from this position and higher
are shifted over to allow room for the new column(s). The index number on all the following
keywords will be incremented or decremented if necessary to reflect the new position of the
column(s) in the table: TBCOLn, TFORMn, TTYPEn, TUNITn, TNULLn, TSCALn, TZE­
ROn, TDISPn, TDIMn, TLMINn, TLMAXn, TDMINn, TDMAXn, TCTYPn, TCRPXn,
TCRVLn, TCDLTn, TCROTn, and TCUNIn.
int fits_insert_col / fficol

5.7. ASCII AND BINARY TABLE ROUTINES 53
(fitsfile *fptr, int colnum, char *ttype, char *tform,
> int *status)
int fits_insert_cols / fficls
(fitsfile *fptr, int colnum, int ncols, char **ttype,
char **tform, > int *status)
int fits_delete_col / ffdcol(fitsfile *fptr, int colnum, > int *status)
3 Copy a column from one HDU to another (or to the same HDU). If create col = TRUE, then
a new column will be inserted in the output table, at position `outcolumn', otherwise the
existing output column will be overwritten (in which case it must have a compatible data
type). If outcolnum is greater than the number of column in the table, then the new column
will be appended to the end of the table. Note that the first column in a table is at colnum
= 1. The standard indexed keywords that related to the column (e.g., TDISPn, TUNITn,
TCRPXn, TCDLTn, etc.) will also be copied.
int fits_copy_col / ffcpcl
(fitsfile *infptr, fitsfile *outfptr, int incolnum, int outcolnum,
int create_col, > int *status);
4 Copy 'nrows' consecutive rows from one table to another, beginning with row 'firstrow'. These
rows will be appended to any existing rows in the output table. Note that the first row in a
table is at row = 1.
int fits_copy_rows / ffcprw
(fitsfile *infptr, fitsfile *outfptr, LONGLONG firstrow,
LONGLONG nrows, > int *status);
5 Modify the vector length of a binary table column (e.g., change a column from TFORMn =
'1E' to '20E'). The vector length may be increased or decreased from the current value.
int fits_modify_vector_len / ffmvec
(fitsfile *fptr, int colnum, LONGLONG newveclen, > int *status)
5.7.4 Read and Write Column Data Routines
The following routines write or read data values in the current ASCII or binary table extension.
If a write operation extends beyond the current size of the table, then the number of rows in the
table will automatically be increased and the NAXIS2 keyword value will be updated. Attempts
to read beyond the end of the table will result in an error.
Automatic data type conversion is performed for numerical data types (only) if the data type of the
column (defined by the TFORMn keyword) di#ers from the data type of the array in the calling
routine. ASCII and binary tables support the following data type values: TSTRING, TBYTE,

54 CHAPTER 5. BASIC CFITSIO INTERFACE ROUTINES
TSBYTE, TSHORT, TUSHORT, TINT, TUINT, TLONG, TLONGLONG, TULONG, TFLOAT,
or TDOUBLE. Binary tables also support TLOGICAL (internally mapped to the `char' data type),
TCOMPLEX, and TDBLCOMPLEX.
Note that it is *not* necessary to insert rows in a table before writing data to those rows (indeed, it
would be ine#cient to do so). Instead, one may simply write data to any row of the table, whether
that row of data already exists or not.
Individual bits in a binary table 'X' or 'B' column may be read/written to/from a *char array by
specifying the TBIT datatype. The *char array will be interpreted as an array of logical TRUE
(1) or FALSE (0) values that correspond to the value of each bit in the FITS 'X' or 'B' column.
Alternatively, the values in a binary table 'X' column may be read/written 8 bits at a time to/from
an array of 8­bit integers by specifying the TBYTE datatype.
Note that within the context of these routines, the TSTRING data type corresponds to a C 'char**'
data type, i.e., a pointer to an array of pointers to an array of characters. This is di#erent from
the keyword reading and writing routines where TSTRING corresponds to a C 'char*' data type,
i.e., a single pointer to an array of characters. When reading strings from a table, the char arrays
obviously must have been allocated long enough to hold the whole FITS table string.
Numerical data values are automatically scaled by the TSCALn and TZEROn keyword values (if
they exist).
In the case of binary tables with vector elements, the 'felem' parameter defines the starting element
(beginning with 1, not 0) within the cell (a cell is defined as the intersection of a row and a column
and may contain a single value or a vector of values). The felem parameter is ignored when dealing
with ASCII tables. Similarly, in the case of binary tables the 'nelements' parameter specifies the
total number of vector values to be read or written (continuing on subsequent rows if required) and
not the number of table cells.
1 Write elements into an ASCII or binary table column.
The first routine simply writes the array of values to the FITS file (doing data type conversion
if necessary) whereas the second routine will substitute the appropriate FITS null value for all
elements which are equal to the input value of nulval (note that this parameter gives the address
of nulval, not the null value itself). For integer columns the FITS null value is defined by the
TNULLn keyword (an error is returned if the keyword doesn't exist). For floating point columns
the special IEEE NaN (Not­a­Number) value will be written into the FITS file. If a null pointer
is entered for nulval, then the null value is ignored and this routine behaves the same as the first
routine. The third routine simply writes undefined pixel values to the column. The fourth routine
fills every column in the table with null values, in the specified rows (ignoring any columns that do
not have a defined null value).
int fits_write_col / ffpcl
(fitsfile *fptr, int datatype, int colnum, LONGLONG firstrow,
LONGLONG firstelem, LONGLONG nelements, DTYPE *array, > int *status)
int fits_write_colnull / ffpcn
(fitsfile *fptr, int datatype, int colnum, LONGLONG firstrow,

5.7. ASCII AND BINARY TABLE ROUTINES 55
LONGLONG firstelem, LONGLONG nelements, DTYPE *array, DTYPE *nulval,
> int *status)
int fits_write_col_null / ffpclu
(fitsfile *fptr, int colnum, LONGLONG firstrow, LONGLONG firstelem,
LONGLONG nelements, > int *status)
int fits_write_nullrows / ffprwu
(fitsfile *fptr, LONGLONG firstrow, LONGLONG nelements, > int *status)
2 Read elements from an ASCII or binary table column. The data type parameter specifies the
data type of the `nulval' and `array' pointers; Undefined array elements will be returned with
a value = *nullval, (note that this parameter gives the address of the null value, not the null
value itself) unless nulval = 0 or *nulval = 0, in which case no checking for undefined pixels
will be performed. The second routine is similar except that any undefined pixels will have
the corresponding nullarray element set equal to TRUE (= 1).
Any column, regardless of it's intrinsic data type, may be read as a string. It should be noted
however that reading a numeric column as a string is 10 ­ 100 times slower than reading the
same column as a number due to the large overhead in constructing the formatted strings. The
display format of the returned strings will be determined by the TDISPn keyword, if it exists,
otherwise by the data type of the column. The length of the returned strings (not including
the null terminating character) can be determined with the fits get col display width routine.
The following TDISPn display formats are currently supported:
Iw.m Integer
Ow.m Octal integer
Zw.m Hexadecimal integer
Fw.d Fixed floating point
Ew.d Exponential floating point
Dw.d Exponential floating point
Gw.d General; uses Fw.d if significance not lost, else Ew.d
where w is the width in characters of the displayed values, m is the minimum number of digits
displayed, and d is the number of digits to the right of the decimal. The .m field is optional.
int fits_read_col / ffgcv
(fitsfile *fptr, int datatype, int colnum, LONGLONG firstrow, LONGLONG firstelem,
LONGLONG nelements, DTYPE *nulval, DTYPE *array, int *anynul, int *status)
int fits_read_colnull / ffgcf
(fitsfile *fptr, int datatype, int colnum, LONGLONG firstrow, LONGLONG firstelem,
LONGLONG nelements, DTYPE *array, char *nullarray, int *anynul, int *status)

56 CHAPTER 5. BASIC CFITSIO INTERFACE ROUTINES
5.7.5 Row Selection and Calculator Routines
These routines all parse and evaluate an input string containing a user defined arithmetic expression.
The first 3 routines select rows in a FITS table, based on whether the expression evaluates to true
(not equal to zero) or false (zero). The other routines evaluate the expression and calculate a value
for each row of the table. The allowed expression syntax is described in the row filter section in the
`Extended File Name Syntax' chapter of this document. The expression may also be written to a
text file, and the name of the file, prepended with a '@' character may be supplied for the 'expr'
parameter (e.g. '@filename.txt'). The expression in the file can be arbitrarily complex and extend
over multiple lines of the file. Lines that begin with 2 slash characters ('//') will be ignored and
may be used to add comments to the file.
1 Evaluate a boolean expression over the indicated rows, returning an array of flags indicating
which rows evaluated to TRUE/FALSE. Upon return, *n good rows contains the number of
rows that evaluate to TRUE.
int fits_find_rows / fffrow
(fitsfile *fptr, char *expr, long firstrow, long nrows,
> long *n_good_rows, char *row_status, int *status)
2 Find the first row which satisfies the input boolean expression
int fits_find_first_row / ffffrw
(fitsfile *fptr, char *expr, > long *rownum, int *status)
3 Evaluate an expression on all rows of a table. If the input and output files are not the same,
copy the TRUE rows to the output file; if the output table is not empty, then this routine
will append the new selected rows after the existing rows. If the files are the same, delete the
FALSE rows (preserve the TRUE rows).
int fits_select_rows / ffsrow
(fitsfile *infptr, fitsfile *outfptr, char *expr, > int *status )
4 Calculate an expression for the indicated rows of a table, returning the results, cast as datatype
(TSHORT, TDOUBLE, etc), in array. If nulval==NULL, UNDEFs will be zeroed out. For
vector results, the number of elements returned may be less than nelements if nelements is
not an even multiple of the result dimension. Call fits test expr to obtain the dimensions of
the results.
int fits_calc_rows / ffcrow
(fitsfile *fptr, int datatype, char *expr, long firstrow,
long nelements, void *nulval, > void *array, int *anynul, int *status)
5 Evaluate an expression and write the result either to a column (if the expression is a function
of other columns in the table) or to a keyword (if the expression evaluates to a constant and

5.7. ASCII AND BINARY TABLE ROUTINES 57
is not a function of other columns in the table). In the former case, the parName parameter
is the name of the column (which may or may not already exist) into which to write the
results, and parInfo contains an optional TFORM keyword value if a new column is being
created. If a TFORM value is not specified then a default format will be used, depending on
the expression. If the expression evaluates to a constant, then the result will be written to
the keyword name given by the parName parameter, and the parInfo parameter may be used
to supply an optional comment for the keyword. If the keyword does not already exist, then
the name of the keyword must be preceded with a '#' character, otherwise the result will be
written to a column with that name.
int fits_calculator / ffcalc
(fitsfile *infptr, char *expr, fitsfile *outfptr, char *parName,
char *parInfo, > int *status)
6 This calculator routine is similar to the previous routine, except that the expression is only
evaluated over the specified row ranges. nranges specifies the number of row ranges, and
firstrow and lastrow give the starting and ending row number of each range.
int fits_calculator_rng / ffcalc_rng
(fitsfile *infptr, char *expr, fitsfile *outfptr, char *parName,
char *parInfo, int nranges, long *firstrow, long *lastrow
> int *status)
7 Evaluate the given expression and return dimension and type information on the result. The
returned dimensions correspond to a single row entry of the requested expression, and are
equivalent to the result of fits read tdim(). Note that strings are considered to be one element
regardless of string length. If maxdim == 0, then naxes is optional.
int fits_test_expr / fftexp
(fitsfile *fptr, char *expr, int maxdim > int *datatype, long *nelem, int *naxis,
long *naxes, int *status)
5.7.6 Column Binning or Histogramming Routines
The following routines may be useful when performing histogramming operations on column(s) of
a table to generate an image in a primary array or image extension.
1 Calculate the histogramming parameters (min, max, and bin size for each axis of the histogram,
based on a variety of possible input parameters. If the input names of the columns to be
binned are null, then the routine will first look for the CPREF = ''NAME1, NAME2, ...''
keyword which lists the preferred columns. If not present, then the routine will assume the
column names X, Y, Z, and T for up to 4 axes (as specified by the NAXIS parameter).
MININ and MAXIN are input arrays that give the minimum and maximum value for the
histogram, along each axis. Alternatively, the name of keywords that give the min, max, and

58 CHAPTER 5. BASIC CFITSIO INTERFACE ROUTINES
binsize may be give with the MINNAME, MAXNAME, and BINNAME array parameters. If
the value = DOUBLENULLVALUE and no keyword names are given, then the routine will
use the TLMINn and TLMAXn keywords, if present, or the actual min and/or max values
in the column.
BINSIZEIN is an array giving the binsize along each axis. If the value = DOUBLENULL­
VALUE, and a keyword name is not specified with BINNAME, then this routine will first
look for the TDBINn keyword, or else will use a binsize = 1, or a binsize that produces 10
histogram bins, which ever is smaller.
int fits_calc_binning
Input parameters:
(fitsfile *fptr, /* IO ­ pointer to table to be binned */
int naxis, /* I ­ number of axes/columns in the binned image */
char colname[4][FLEN_VALUE], /* I ­ optional column names */
double *minin, /* I ­ optional lower bound value for each axis */
double *maxin, /* I ­ optional upper bound value, for each axis */
double *binsizein, /* I ­ optional bin size along each axis */
char minname[4][FLEN_VALUE], /* I ­ optional keywords for min */
char maxname[4][FLEN_VALUE], /* I ­ optional keywords for max */
char binname[4][FLEN_VALUE], /* I ­ optional keywords for binsize */
Output parameters:
int *colnum, /* O ­ column numbers, to be binned */
long *naxes, /* O ­ number of bins in each histogram axis */
float *amin, /* O ­ lower bound of the histogram axes */
float *amax, /* O ­ upper bound of the histogram axes */
float *binsize, /* O ­ width of histogram bins/pixels on each axis */
int *status)
2 Copy the relevant keywords from the header of the table that is being binned, to the the header
of the output histogram image. This will not copy the table structure keywords (e.g., NAXIS,
TFORMn, TTYPEn, etc.) nor will it copy the keywords that apply to other columns of the
table that are not used to create the histogram. This routine will translate the names of the
World Coordinate System (WCS) keywords for the binned columns into the form that is need
for a FITS image (e.g., the TCTYPn table keyword will be translated to the CTYPEn image
keyword).
int fits_copy_pixlist2image
(fitsfile *infptr, /* I ­ pointer to input HDU */
fitsfile *outfptr, /* I ­ pointer to output HDU */
int firstkey, /* I ­ first HDU keyword to start with */
int naxis, /* I ­ number of axes in the image */
int *colnum, /* I ­ numbers of the columns to be binned */
int *status) /* IO ­ error status */
3 Write a set of default WCS keywords to the histogram header, IF the WCS keywords do not

5.7. ASCII AND BINARY TABLE ROUTINES 59
already exist. This will create a linear WCS where the coordinate types are equal to the
original column names.
int fits_write_keys_histo
(fitsfile *fptr, /* I ­ pointer to table to be binned */
fitsfile *histptr, /* I ­ pointer to output histogram image HDU */
int naxis, /* I ­ number of axes in the histogram image */
int *colnum, /* I ­ column numbers of the binned columns */
int *status)
4 Update the WCS keywords in a histogram image header that give the location of the reference
pixel (CRPIXn), and the pixel size (CDELTn), in the binned image.
int fits_rebin_wcs
(fitsfile *fptr, /* I ­ pointer to table to be binned */
int naxis, /* I ­ number of axes in the histogram image */
float *amin, /* I ­ first pixel include in each axis */
float *binsize, /* I ­ binning factor for each axis */
int *status)
5 Bin the values in the input table columns, and write the histogram array to the output FITS
image (histptr).
int fits_make_hist
(fitsfile *fptr, /* I ­ pointer to table with X and Y cols; */
fitsfile *histptr, /* I ­ pointer to output FITS image */
int bitpix, /* I ­ datatype for image: 16, 32, ­32, etc */
int naxis, /* I ­ number of axes in the histogram image */
long *naxes, /* I ­ size of axes in the histogram image */
int *colnum, /* I ­ column numbers (array length = naxis) */
float *amin, /* I ­ minimum histogram value, for each axis */
float *amax, /* I ­ maximum histogram value, for each axis */
float *binsize, /* I ­ bin size along each axis */
float weight, /* I ­ binning weighting factor (FLOATNULLVALUE */
/* for no weighting) */
int wtcolnum, /* I ­ keyword or col for weight (or NULL) */
int recip, /* I ­ use reciprocal of the weight? 0 or 1 */
char *selectrow, /* I ­ optional array (length = no. of */
/* rows in the table). If the element is true */
/* then the corresponding row of the table will */
/* be included in the histogram, otherwise the */
/* row will be skipped. Ingnored if *selectrow */
/* is equal to NULL. */
int *status)

60 CHAPTER 5. BASIC CFITSIO INTERFACE ROUTINES
5.8 Utility Routines
5.8.1 File Checksum Routines
The following routines either compute or validate the checksums for the CHDU. The DATASUM
keyword is used to store the numerical value of the 32­bit, 1's complement checksum for the data
unit alone. If there is no data unit then the value is set to zero. The numerical value is stored as an
ASCII string of digits, enclosed in quotes, because the value may be too large to represent as a 32­bit
signed integer. The CHECKSUM keyword is used to store the ASCII encoded COMPLEMENT of
the checksum for the entire HDU. Storing the complement, rather than the actual checksum, forces
the checksum for the whole HDU to equal zero. If the file has been modified since the checksums
were computed, then the HDU checksum will usually not equal zero. These checksum keyword
conventions are based on a paper by Rob Seaman published in the proceedings of the ADASS IV
conference in Baltimore in November 1994 and a later revision in June 1995. See Appendix B for
the definition of the parameters used in these routines.
1 Compute and write the DATASUM and CHECKSUM keyword values for the CHDU into the
current header. If the keywords already exist, their values will be updated only if necessary
(i.e., if the file has been modified since the original keyword values were computed).
int fits_write_chksum / ffpcks
(fitsfile *fptr, > int *status)
2 Update the CHECKSUM keyword value in the CHDU, assuming that the DATASUM keyword
exists and already has the correct value. This routine calculates the new checksum for the
current header unit, adds it to the data unit checksum, encodes the value into an ASCII
string, and writes the string to the CHECKSUM keyword.
int fits_update_chksum / ffupck
(fitsfile *fptr, > int *status)
3 Verify the CHDU by computing the checksums and comparing them with the keywords. The
data unit is verified correctly if the computed checksum equals the value of the DATASUM
keyword. The checksum for the entire HDU (header plus data unit) is correct if it equals
zero. The output DATAOK and HDUOK parameters in this routine are integers which will
have a value = 1 if the data or HDU is verified correctly, a value = 0 if the DATASUM or
CHECKSUM keyword is not present, or value = ­1 if the computed checksum is not correct.
int fits_verify_chksum / ffvcks
(fitsfile *fptr, > int *dataok, int *hduok, int *status)
4 Compute and return the checksum values for the CHDU without creating or modifying the
CHECKSUM and DATASUM keywords. This routine is used internally by #vcks, but may
be useful in other situations as well.

5.8. UTILITY ROUTINES 61
int fits_get_chksum/ /ffgcks
(fitsfile *fptr, > unsigned long *datasum, unsigned long *hdusum,
int *status)
5 Encode a checksum value into a 16­character string. If complm is non­zero (true) then the 32­bit
sum value will be complemented before encoding.
int fits_encode_chksum / ffesum
(unsigned long sum, int complm, > char *ascii);
6 Decode a 16­character checksum string into a unsigned long value. If is non­zero (true). then the
32­bit sum value will be complemented after decoding. The checksum value is also returned
as the value of the function.
unsigned long fits_decode_chksum / ffdsum
(char *ascii, int complm, > unsigned long *sum);
5.8.2 Date and Time Utility Routines
The following routines help to construct or parse the FITS date/time strings. Starting in the year
2000, the FITS DATE keyword values (and the values of other `DATE­' keywords) must have the
form 'YYYY­MM­DD' (date only) or 'YYYY­MM­DDThh:mm:ss.ddd...' (date and time) where
the number of decimal places in the seconds value is optional. These times are in UTC. The older
'dd/mm/yy' date format may not be used for dates after 01 January 2000. See Appendix B for the
definition of the parameters used in these routines.
1 Get the current system date. C already provides standard library routines for getting the current
date and time, but this routine is provided for compatibility with the Fortran FITSIO library.
The returned year has 4 digits (1999, 2000, etc.)
int fits_get_system_date/ffgsdt
( > int *day, int *month, int *year, int *status )
2 Get the current system date and time string ('YYYY­MM­DDThh:mm:ss'). The time will be
in UTC/GMT if available, as indicated by a returned timeref value = 0. If the returned value
of timeref = 1 then this indicates that it was not possible to convert the local time to UTC,
and thus the local time was returned.
int fits_get_system_time/ffgstm
(> char *datestr, int *timeref, int *status)
3 Construct a date string from the input date values. If the year is between 1900 and 1998, inclu­
sive, then the returned date string will have the old FITS format ('dd/mm/yy'), otherwise
the date string will have the new FITS format ('YYYY­MM­DD'). Use fits time2str instead
to always return a date string using the new FITS format.

62 CHAPTER 5. BASIC CFITSIO INTERFACE ROUTINES
int fits_date2str/ffdt2s
(int year, int month, int day, > char *datestr, int *status)
4 Construct a new­format date + time string ('YYYY­MM­DDThh:mm:ss.ddd...'). If the year,
month, and day values all = 0 then only the time is encoded with format 'hh:mm:ss.ddd...'.
The decimals parameter specifies how many decimal places of fractional seconds to include
in the string. If `decimals' is negative, then only the date will be return ('YYYY­MM­DD').
int fits_time2str/fftm2s
(int year, int month, int day, int hour, int minute, double second,
int decimals, > char *datestr, int *status)
5 Return the date as read from the input string, where the string may be in either the old
('dd/mm/yy') or new ('YYYY­MM­DDThh:mm:ss' or 'YYYY­MM­DD') FITS format. Null
pointers may be supplied for any unwanted output date parameters.
int fits_str2date/ffs2dt
(char *datestr, > int *year, int *month, int *day, int *status)
6 Return the date and time as read from the input string, where the string may be in either the
old or new FITS format. The returned hours, minutes, and seconds values will be set to zero
if the input string does not include the time ('dd/mm/yy' or 'YYYY­MM­DD') . Similarly,
the returned year, month, and date values will be set to zero if the date is not included in
the input string ('hh:mm:ss.ddd...'). Null pointers may be supplied for any unwanted output
date and time parameters.
int fits_str2time/ffs2tm
(char *datestr, > int *year, int *month, int *day, int *hour,
int *minute, double *second, int *status)
5.8.3 General Utility Routines
The following utility routines may be useful for certain applications.
1 Return the revision number of the CFITSIO library. The revision number will be incremented
with each new release of CFITSIO.
float fits_get_version / ffvers ( > float *version)
2 Write an 80­character message to the CFITSIO error stack. Application programs should not
normally write to the stack, but there may be some situations where this is desirable.
void fits_write_errmsg / ffpmsg (char *err_msg)

5.8. UTILITY ROUTINES 63
3 Convert a character string to uppercase (operates in place).
void fits_uppercase / ffupch (char *string)
4 Compare the input template string against the reference string to see if they match. The
template string may contain wildcard characters: '*' will match any sequence of characters
(including zero characters) and ' ?' will match any single character in the reference string.
The '#' character will match any consecutive string of decimal digits (0 ­ 9). If casesen =
CASESEN = TRUE then the match will be case sensitive, otherwise the case of the letters
will be ignored if casesen = CASEINSEN = FALSE. The returned MATCH parameter will
be TRUE if the 2 strings match, and EXACT will be TRUE if the match is exact (i.e., if no
wildcard characters were used in the match). Both strings must be 68 characters or less in
length.
void fits_compare_str / ffcmps
(char *templt, char *string, int casesen, > int *match, int *exact)
5 Split a string containing a list of names (typically file names or column names) into individual
name tokens by a sequence of calls to fits split names. The names in the list must be delimited
by a comma and/or spaces. This routine ignores spaces and commas that occur within
parentheses, brackets, or curly brackets. It also strips any leading and trailing blanks from
the returned name.
This routine is similar to the ANSI C 'strtok' function:
The first call to fits split names has a non­null input string. It finds the first name in the
string and terminates it by overwriting the next character of the string with a null terminator
and returns a pointer to the name. Each subsequent call, indicated by a NULL value of the
input string, returns the next name, searching from just past the end of the previous name.
It returns NULL when no further names are found.
char *fits_split_names(char *namelist)
The following example shows how a string would be split into 3 names:
myfile[1][bin (x,y)=4], file2.fits file3.fits
^^^^^^^^^^^^^^^^^^^^^^ ^^^^^^^^^^ ^^^^^^^^^^
1st name 2nd name 3rd name
6 Test that the keyword name contains only legal characters (A­Z,0­9, hyphen, and underscore)
or that the keyword record contains only legal printable ASCII characters
int fits_test_keyword / fftkey (char *keyname, > int *status)
int fits_test_record / fftrec (char *card, > int *status)

64 CHAPTER 5. BASIC CFITSIO INTERFACE ROUTINES
7 Test whether the current header contains any NULL (ASCII 0) characters. These characters are
illegal in the header, but they will go undetected by most of the CFITSIO keyword header
routines, because the null is interpreted as the normal end­of­string terminator. This routine
returns the position of the first null character in the header, or zero if there are no nulls. For
example a returned value of 110 would indicate that the first NULL is located in the 30th
character of the second keyword in the header (recall that each header record is 80 characters
long). Note that this is one of the few CFITSIO routines in which the returned value is not
necessarily equal to the status value).
int fits_null_check / ffnchk (char *card, > int *status)
8 Parse a header keyword record and return the name of the keyword, and the length of the name.
The keyword name normally occupies the first 8 characters of the record, except under the
HIERARCH convention where the name can be up to 70 characters in length.
int fits_get_keyname / ffgknm
(char *card, > char *keyname, int *keylength, int *status)
9 Parse a header keyword record, returning the value (as a literal character string) and comment
strings. If the keyword has no value (columns 9­10 not equal to '= '), then a null value string
is returned and the comment string is set equal to column 9 ­ 80 of the input string.
int fits_parse_value / ffpsvc
(char *card, > char *value, char *comment, int *status)
10 Construct an array indexed keyword name (ROOT + nnn). This routine appends the sequence
number to the root string to create a keyword name (e.g., 'NAXIS' + 2 = 'NAXIS2')
int fits_make_keyn / ffkeyn
(char *keyroot, int value, > char *keyname, int *status)
11 Construct a sequence keyword name (n + ROOT). This routine concatenates the sequence
number to the front of the root string to create a keyword name (e.g., 1 + 'CTYP' = '1CTYP')
int fits_make_nkey / ffnkey
(int value, char *keyroot, > char *keyname, int *status)
12 Determine the data type of a keyword value string. This routine parses the keyword value
string to determine its data type. Returns 'C', 'L', 'I', 'F' or 'X', for character string, logical,
integer, floating point, or complex, respectively.
int fits_get_keytype / ffdtyp
(char *value, > char *dtype, int *status)

5.8. UTILITY ROUTINES 65
13 Determine the integer data type of an integer keyword value string. The returned datatype
value is the minimum integer datatype (starting from top of the following list and working
down) required to store the integer value:
Data Type Range
TSBYTE: ­128 to 127
TBYTE: 128 to 255
TSHORT: ­32768 to 32767
TUSHORT: 32768 to 65535
TINT ­2147483648 to 2147483647
TUINT 2147483648 to 4294967295
TLONGLONG ­9223372036854775808 to 9223372036854775807
The *neg parameter returns 1 if the input value is negative and returns 0 if it is non­negative.
int fits_get_inttype / ffinttyp
(char *value, > int *datatype, int *neg, int *status)
14 Return the class of an input header record. The record is classified into one of the following
categories (the class values are defined in fitsio.h). Note that this is one of the few CFITSIO
routines that does not return a status value.
Class Value Keywords
TYP_STRUC_KEY 10 SIMPLE, BITPIX, NAXIS, NAXISn, EXTEND, BLOCKED,
GROUPS, PCOUNT, GCOUNT, END
XTENSION, TFIELDS, TTYPEn, TBCOLn, TFORMn, THEAP,
and the first 4 COMMENT keywords in the primary array
that define the FITS format.
TYP_CMPRS_KEY 20 The experimental keywords used in the compressed
image format ZIMAGE, ZCMPTYPE, ZNAMEn, ZVALn,
ZTILEn, ZBITPIX, ZNAXISn, ZSCALE, ZZERO, ZBLANK
TYP_SCAL_KEY 30 BSCALE, BZERO, TSCALn, TZEROn
TYP_NULL_KEY 40 BLANK, TNULLn
TYP_DIM_KEY 50 TDIMn
TYP_RANG_KEY 60 TLMINn, TLMAXn, TDMINn, TDMAXn, DATAMIN, DATAMAX
TYP_UNIT_KEY 70 BUNIT, TUNITn
TYP_DISP_KEY 80 TDISPn
TYP_HDUID_KEY 90 EXTNAME, EXTVER, EXTLEVEL, HDUNAME, HDUVER, HDULEVEL
TYP_CKSUM_KEY 100 CHECKSUM, DATASUM
TYP_WCS_KEY 110 WCS keywords defined in the the WCS papers, including:
CTYPEn, CUNITn, CRVALn, CRPIXn, CROTAn, CDELTn
CDj_is, PVj_ms, LONPOLEs, LATPOLEs
TCTYPn, TCTYns, TCUNIn, TCUNns, TCRVLn, TCRVns, TCRPXn,
TCRPks, TCDn_k, TCn_ks, TPVn_m, TPn_ms, TCDLTn, TCROTn
jCTYPn, jCTYns, jCUNIn, jCUNns, jCRVLn, jCRVns, iCRPXn,

66 CHAPTER 5. BASIC CFITSIO INTERFACE ROUTINES
iCRPns, jiCDn, jiCDns, jPVn_m, jPn_ms, jCDLTn, jCROTn
(i,j,m,n are integers, s is any letter)
TYP_REFSYS_KEY 120 EQUINOXs, EPOCH, MJD­OBSs, RADECSYS, RADESYSs, DATE­OBS
TYP_COMM_KEY 130 COMMENT, HISTORY, (blank keyword)
TYP_CONT_KEY 140 CONTINUE
TYP_USER_KEY 150 all other keywords
int fits_get_keyclass / ffgkcl (char *card)
15 Parse the 'TFORM' binary table column format string. This routine parses the input TFORM
character string and returns the integer data type code, the repeat count of the field, and,
in the case of character string fields, the length of the unit string. See Appendix B for the
allowed values for the returned typecode parameter. A null pointer may be given for any
output parameters that are not needed.
int fits_binary_tform / ffbnfm
(char *tform, > int *typecode, long *repeat, long *width,
int *status)
int fits_binary_tformll / ffbnfmll
(char *tform, > int *typecode, LONGLONG *repeat, long *width,
int *status)
16 Parse the 'TFORM' keyword value that defines the column format in an ASCII table. This
routine parses the input TFORM character string and returns the data type code, the width
of the column, and (if it is a floating point column) the number of decimal places to the right
of the decimal point. The returned data type codes are the same as for the binary table,
with the following additional rules: integer columns that are between 1 and 4 characters wide
are defined to be short integers (code = TSHORT). Wider integer columns are defined to be
regular integers (code = TLONG). Similarly, Fixed decimal point columns (with TFORM
= 'Fw.d') are defined to be single precision reals (code = TFLOAT) if w is between 1 and
7 characters wide, inclusive. Wider 'F' columns will return a double precision data code
(= TDOUBLE). 'Ew.d' format columns will have datacode = TFLOAT, and 'Dw.d' format
columns will have datacode = TDOUBLE. A null pointer may be given for any output
parameters that are not needed.
int fits_ascii_tform / ffasfm
(char *tform, > int *typecode, long *width, int *decimals,
int *status)
17 Calculate the starting column positions and total ASCII table width based on the input array
of ASCII table TFORM values. The SPACE input parameter defines how many blank spaces
to leave between each column (it is recommended to have one space between columns for
better human readability).

5.8. UTILITY ROUTINES 67
int fits_get_tbcol / ffgabc
(int tfields, char **tform, int space, > long *rowlen,
long *tbcol, int *status)
18 Parse a template header record and return a formatted 80­character string suitable for append­
ing to (or deleting from) a FITS header file. This routine is useful for parsing lines from
an ASCII template file and reformatting them into legal FITS header records. The format­
ted string may then be passed to the fits write record, #mcrd, or fits delete key routines to
append or modify a FITS header record.
int fits_parse_template / ffgthd
(char *templt, > char *card, int *keytype, int *status)
The input templt character string generally should contain 3 tokens: (1) the KEYNAME, (2) the
VALUE, and (3) the COMMENT string. The TEMPLATE string must adhere to the following
format:
­ The KEYNAME token must begin in columns 1­8 and be a maximum of 8 characters long. A
legal FITS keyword name may only contain the characters A­Z, 0­9, and '­' (minus sign) and
underscore. This routine will automatically convert any lowercase characters to uppercase in
the output string. If the first 8 characters of the template line are blank then the remainder
of the line is considered to be a FITS comment (with a blank keyword name).
­ The VALUE token must be separated from the KEYNAME token by one or more spaces and/or
an '=' character. The data type of the VALUE token (numeric, logical, or character string) is
automatically determined and the output CARD string is formatted accordingly. The value
token may be forced to be interpreted as a string (e.g. if it is a string of numeric digits) by
enclosing it in single quotes. If the value token is a character string that contains 1 or more
embedded blank space characters or slash ('/') characters then the entire character string
must be enclosed in single quotes.
­ The COMMENT token is optional, but if present must be separated from the VALUE token by
a blank space or a '/' character.
­ One exception to the above rules is that if the first non­blank character in the first 8 characters
of the template string is a minus sign ('­') followed by a single token, or a single token followed
by an equal sign, then it is interpreted as the name of a keyword which is to be deleted from
the FITS header.
­ The second exception is that if the template string starts with a minus sign and is followed by
2 tokens (without an equals sign between them) then the second token is interpreted as the
new name for the keyword specified by first token. In this case the old keyword name (first
token) is returned in characters 1­8 of the returned CARD string, and the new keyword name
(the second token) is returned in characters 41­48 of the returned CARD string. These old
and new names may then be passed to the #mnam routine which will change the keyword
name.

68 CHAPTER 5. BASIC CFITSIO INTERFACE ROUTINES
The keytype output parameter indicates how the returned CARD string should be interpreted:
keytype interpretation
­­­­­­­ ­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
­2 Rename the keyword with name = the first 8 characters of CARD
to the new name given in characters 41 ­ 48 of CARD.
­1 delete the keyword with this name from the FITS header.
0 append the CARD string to the FITS header if the
keyword does not already exist, otherwise update
the keyword value and/or comment field if is already exists.
1 This is a HISTORY or COMMENT keyword; append it to the header
2 END record; do not explicitly write it to the FITS file.
EXAMPLES: The following lines illustrate valid input template strings:
INTVAL 7 / This is an integer keyword
RVAL 34.6 / This is a floating point keyword
EVAL=­12.45E­03 / This is a floating point keyword in exponential notation
lval F / This is a boolean keyword
This is a comment keyword with a blank keyword name
SVAL1 = 'Hello world' / this is a string keyword
SVAL2 '123.5' this is also a string keyword
sval3 123+ / this is also a string keyword with the value '123+ '
# the following template line deletes the DATE keyword
­ DATE
# the following template line modifies the NAME keyword to OBJECT
­ NAME OBJECT
19 Translate a keyword name into a new name, based on a set of patterns. This routine is useful
for translating keywords in cases such as adding or deleting columns in a table, or copying
a column from one table to another, or extracting an array from a cell in a binary table
column into an image extension. In these cases, it is necessary to translate the names of the
keywords associated with the original table column(s) into the appropriate keyword name in
the final file. For example, if column 2 is deleted from a table, then the value of 'n' in all
the TFORMn and TTYPEn keywords for columns 3 and higher must be decremented by 1.
Even more complex translations are sometimes needed to convert the WCS keywords when
extracting an image out of a table column cell into a separate image extension.
The user passes an array of patterns to be matched. Input pattern number i is pattern[i][0],
and output pattern number i is pattern[i][1]. Keywords are matched against the input pat­
terns. If a match is found then the keyword is re­written according to the output pattern.
Order is important. The first match is accepted. The fastest match will be made when
templates with the same first character are grouped together.

5.8. UTILITY ROUTINES 69
Several characters have special meanings:
i,j ­ single digits, preserved in output template
n ­ column number of one or more digits, preserved in output template
m ­ generic number of one or more digits, preserved in output template
a ­ coordinate designator, preserved in output template
# ­ number of one or more digits
? ­ any character
* ­ only allowed in first character position, to match all
keywords; only useful as last pattern in the list
i, j, n, and m are returned by the routine.
For example, the input pattern ''iCTYPn'' will match ''1CTYP5'' (if n value is 5); the output
pattern ''CTYPEi'' will be re­written as ''CTYPE1''. Notice that ''i'' is preserved.
The following output patterns are special:
''­'' ­ do not copy a keyword that matches the corresponding input pattern
''+'' ­ copy the input unchanged
The inrec string could be just the 8­char keyword name, or the entire 80­char header record.
Characters 9 ­ 80 in the input string simply get appended to the translated keyword name.
If n range = 0, then only keywords with 'n' equal to n value will be considered as a pattern
match. If n range = +1, then all values of 'n' greater than or equal to n value will be a
match, and if ­1, then values of 'n' less than or equal to n value will match.
int fits_translate_keyword(
char *inrec, /* I ­ input string */
char *outrec, /* O ­ output converted string, or */
/* a null string if input does not */
/* match any of the patterns */
char *patterns[][2],/* I ­ pointer to input / output string */
/* templates */
int npat, /* I ­ number of templates passed */
int n_value, /* I ­ base 'n' template value of interest */
int n_offset, /* I ­ offset to be applied to the 'n' */
/* value in the output string */
int n_range, /* I ­ controls range of 'n' template */
/* values of interest (­1,0, or +1) */
int *pat_num, /* O ­ matched pattern number (0 based) or ­1 */
int *i, /* O ­ value of i, if any, else 0 */
int *j, /* O ­ value of j, if any, else 0 */
int *m, /* O ­ value of m, if any, else 0 */
int *n, /* O ­ value of n, if any, else 0 */
int *status) /* IO ­ error status */
Here is an example of some of the patterns used to convert the keywords associated with an image
in a cell of a table column into the keywords appropriate for an IMAGE extension:

70 CHAPTER 5. BASIC CFITSIO INTERFACE ROUTINES
char *patterns[][2] = {{"TSCALn", "BSCALE" }, /* Standard FITS keywords */
{"TZEROn", "BZERO" },
{"TUNITn", "BUNIT" },
{"TNULLn", "BLANK" },
{"TDMINn", "DATAMIN" },
{"TDMAXn", "DATAMAX" },
{"iCTYPn", "CTYPEi" }, /* Coordinate labels */
{"iCTYna", "CTYPEia" },
{"iCUNIn", "CUNITi" }, /* Coordinate units */
{"iCUNna", "CUNITia" },
{"iCRVLn", "CRVALi" }, /* WCS keywords */
{"iCRVna", "CRVALia" },
{"iCDLTn", "CDELTi" },
{"iCDEna", "CDELTia" },
{"iCRPXn", "CRPIXi" },
{"iCRPna", "CRPIXia" },
{"ijPCna", "PCi_ja" },
{"ijCDna", "CDi_ja" },
{"iVn_ma", "PVi_ma" },
{"iSn_ma", "PSi_ma" },
{"iCRDna", "CRDERia" },
{"iCSYna", "CSYERia" },
{"iCROTn", "CROTAi" },
{"WCAXna", "WCSAXESa"},
{"WCSNna", "WCSNAMEa"}};
20 Translate the keywords in the input HDU into the keywords that are appropriate for the output
HDU. This is a driver routine that calls the previously described routine.
int fits_translate_keywords(
fitsfile *infptr, /* I ­ pointer to input HDU */
fitsfile *outfptr, /* I ­ pointer to output HDU */
int firstkey, /* I ­ first HDU record number to start with */
char *patterns[][2],/* I ­ pointer to input / output keyword templates */
int npat, /* I ­ number of templates passed */
int n_value, /* I ­ base 'n' template value of interest */
int n_offset, /* I ­ offset to be applied to the 'n' */
/* value in the output string */
int n_range, /* I ­ controls range of 'n' template */
/* values of interest (­1,0, or +1) */
int *status) /* IO ­ error status */
21 Parse the input string containing a list of rows or row ranges, and return integer arrays con­
taining the first and last row in each range. For example, if rowlist = ''3­5, 6, 8­9'' then it will
return numranges = 3, rangemin = 3, 6, 8 and rangemax = 5, 6, 9. At most, 'maxranges'
number of ranges will be returned. 'maxrows' is the maximum number of rows in the table;

5.8. UTILITY ROUTINES 71
any rows or ranges larger than this will be ignored. The rows must be specified in increasing
order, and the ranges must not overlap. A minus sign may be use to specify all the rows to
the upper or lower bound, so ''50­'' means all the rows from 50 to the end of the table, and
''­'' means all the rows in the table, from 1 ­ maxrows.
int fits_parse_range / ffrwrg(char *rowlist, LONGLONG maxrows, int maxranges, >
int *numranges, long *rangemin, long *rangemax, int *status)
int fits_parse_rangell / ffrwrgll(char *rowlist, LONGLONG maxrows, int maxranges, >
int *numranges, LONGLONG *rangemin, LONGLONG *rangemax, int *status)
22 Check that the Header fill bytes (if any) are all blank. These are the bytes that may follow
END keyword and before the beginning of data unit, or the end of the HDU if there is no
data unit.
int ffchfl(fitsfile *fptr, > int *status)
23 Check that the Data fill bytes (if any) are all zero (for IMAGE or BINARY Table HDU) or all
blanks (for ASCII table HDU). These file bytes may be located after the last valid data byte
in the HDU and before the physical end of the HDU.
int ffcdfl(fitsfile *fptr, > int *status)
24 Estimate the root­mean­squared (RMS) noise in an image. These routines are mainly for use
with the Hcompress image compression algorithm. They return an estimate of the RMS noise
in the background pixels of the image. This robust algorithm (written by Richard White,
STScI) first attempts to estimate the RMS value as 1.68 times the median of the absolute
di#erences between successive pixels in the image. If the median = 0, then the algorithm falls
back to computing the RMS of the di#erence between successive pixels, after several N­sigma
rejection cycles to remove extreme values. The input parameters are: the array of image pixel
values (either float or short values), the number of values in the array, the value that is used
to represent null pixels (enter a very large number if there are no null pixels).
int fits_rms_float (float fdata[], int npix, float in_null_value,
> double *rms, int *status)
int fits_rms_short (short fdata[], int npix, short in_null_value,
> double *rms, int *status)
25 Was CFITSIO compiled with the ­D REENTRANT directive so that it may be safely used in
multi­threaded environments? The following function returns 1 if yes, 0 if no. Note, however,
that even if the ­D REENTRANT directive was specified, this does not guarantee that the
CFITSIO routines are thread­safe, because some compilers may not support this feature.
int fits_is_reentrant(void)

72 CHAPTER 5. BASIC CFITSIO INTERFACE ROUTINES

Chapter 6
The CFITSIO Iterator Function
The fits iterate data function in CFITSIO provides a unique method of executing an arbitrary
user­supplied `work' function that operates on rows of data in FITS tables or on pixels in FITS
images. Rather than explicitly reading and writing the FITS images or columns of data, one instead
calls the CFITSIO iterator routine, passing to it the name of the user's work function that is to
be executed along with a list of all the table columns or image arrays that are to be passed to the
work function. The CFITSIO iterator function then does all the work of allocating memory for
the arrays, reading the input data from the FITS file, passing them to the work function, and then
writing any output data back to the FITS file after the work function exits. Because it is often
more e#cient to process only a subset of the total table rows at one time, the iterator function
can determine the optimum amount of data to pass in each iteration and repeatedly call the work
function until the entire table been processed.
For many applications this single CFITSIO iterator function can e#ectively replace all the other
CFITSIO routines for reading or writing data in FITS images or tables. Using the iterator has
several important advantages over the traditional method of reading and writing FITS data files:
. It cleanly separates the data I/O from the routine that operates on the data. This leads to
a more modular and `object oriented' programming style.
. It simplifies the application program by eliminating the need to allocate memory for the data
arrays and eliminates most of the calls to the CFITSIO routines that explicitly read and write
the data.
. It ensures that the data are processed as e#ciently as possible. This is especially important
when processing tabular data since the iterator function will calculate the most e#cient
number of rows in the table to be passed at one time to the user's work function on each
iteration.
. Makes it possible for larger projects to develop a library of work functions that all have a
uniform calling sequence and are all independent of the details of the FITS file format.
There are basically 2 steps in using the CFITSIO iterator function. The first step is to design the
work function itself which must have a prescribed set of input parameters. One of these parameters
73

74 CHAPTER 6. THE CFITSIO ITERATOR FUNCTION
is a structure containing pointers to the arrays of data; the work function can perform any desired
operations on these arrays and does not need to worry about how the input data were read from
the file or how the output data get written back to the file.
The second step is to design the driver routine that opens all the necessary FITS files and initializes
the input parameters to the iterator function. The driver program calls the CFITSIO iterator
function which then reads the data and passes it to the user's work function.
The following 2 sections describe these steps in more detail. There are also several example programs
included with the CFITSIO distribution which illustrate how to use the iterator function.
6.1 The Iterator Work Function
The user­supplied iterator work function must have the following set of input parameters (the
function can be given any desired name):
int user_fn( long totaln, long offset, long firstn, long nvalues,
int narrays, iteratorCol *data, void *userPointer )
. totaln -- the total number of table rows or image pixels that will be passed to the work function
during 1 or more iterations.
. o#set -- the o#set applied to the first table row or image pixel to be passed to the work
function. In other words, this is the number of rows or pixels that are skipped over before
starting the iterations. If o#set = 0, then all the table rows or image pixels will be passed to
the work function.
. firstn -- the number of the first table row or image pixel (starting with 1) that is being passed
in this particular call to the work function.
. nvalues -- the number of table rows or image pixels that are being passed in this particular
call to the work function. nvalues will always be less than or equal to totaln and will have
the same value on each iteration, except possibly on the last call which may have a smaller
value.
. narrays -- the number of arrays of data that are being passed to the work function. There is
one array for each image or table column.
. *data -- array of structures, one for each column or image. Each structure contains a pointer
to the array of data as well as other descriptive parameters about that array.
. *userPointer -- a user supplied pointer that can be used to pass ancillary information from the
driver function to the work function. This pointer is passed to the CFITSIO iterator function
which then passes it on to the work function without any modification. It may point to a
single number, to an array of values, to a structure containing an arbitrary set of parameters
of di#erent types, or it may be a null pointer if it is not needed. The work function must cast
this pointer to the appropriate data type before using it it.

6.1. THE ITERATOR WORK FUNCTION 75
The totaln, o#set, narrays, data, and userPointer parameters are guaranteed to have the same value
on each iteration. Only firstn, nvalues, and the arrays of data pointed to by the data structures
may change on each iterative call to the work function.
Note that the iterator treats an image as a long 1­D array of pixels regardless of it's intrinsic
dimensionality. The total number of pixels is just the product of the size of each dimension, and
the order of the pixels is the same as the order that they are stored in the FITS file. If the work
function needs to know the number and size of the image dimensions then these parameters can be
passed via the userPointer structure.
The iteratorCol structure is currently defined as follows:
typedef struct /* structure for the iterator function column information */
{
/* structure elements required as input to fits_iterate_data: */
fitsfile *fptr; /* pointer to the HDU containing the column or image */
int colnum; /* column number in the table; ignored for images */
char colname[70]; /* name (TTYPEn) of the column; null for images */
int datatype; /* output data type (converted if necessary) */
int iotype; /* type: InputCol, InputOutputCol, or OutputCol */
/* output structure elements that may be useful for the work function: */
void *array; /* pointer to the array (and the null value) */
long repeat; /* binary table vector repeat value; set */
/* equal to 1 for images */
long tlmin; /* legal minimum data value, if any */
long tlmax; /* legal maximum data value, if any */
char unit[70]; /* physical unit string (BUNIT or TUNITn) */
char tdisp[70]; /* suggested display format; null if none */
} iteratorCol;
Instead of directly reading or writing the elements in this structure, it is recommended that pro­
grammers use the access functions that are provided for this purpose.
The first five elements in this structure must be initially defined by the driver routine before calling
the iterator routine. The CFITSIO iterator routine uses this information to determine what column
or array to pass to the work function, and whether the array is to be input to the work function,
output from the work function, or both. The CFITSIO iterator function fills in the values of the
remaining structure elements before passing it to the work function.
The array structure element is a pointer to the actual data array and it must be cast to the correct
data type before it is used. The `repeat' structure element give the number of data values in each
row of the table, so that the total number of data values in the array is given by repeat * nvalues.
In the case of image arrays and ASCII tables, repeat will always be equal to 1. When the data
type is a character string, the array pointer is actually a pointer to an array of string pointers
(i.e., char **array). The other output structure elements are provided for convenience in case that

76 CHAPTER 6. THE CFITSIO ITERATOR FUNCTION
information is needed within the work function. Any other information may be passed from the
driver routine to the work function via the userPointer parameter.
Upon completion, the work routine must return an integer status value, with 0 indicating success
and any other value indicating an error which will cause the iterator function to immediately exit
at that point. Return status values in the range 1 -- 1000 should be avoided since these are reserved
for use by CFITSIO. A return status value of ­1 may be used to force the CFITSIO iterator function
to stop at that point and return control to the driver routine after writing any output arrays to the
FITS file. CFITSIO does not considered this to be an error condition, so any further processing by
the application program will continue normally.
6.2 The Iterator Driver Function
The iterator driver function must open the necessary FITS files and position them to the correct
HDU. It must also initialize the following parameters in the iteratorCol structure (defined above) for
each column or image before calling the CFITSIO iterator function. Several `constructor' routines
are provided in CFITSIO for this purpose.
. *fptr -- The fitsfile pointer to the table or image.
. colnum -- the number of the column in the table. This value is ignored in the case of images.
If colnum equals 0, then the column name will be used to identify the column to be passed
to the work function.
. colname -- the name (TTYPEn keyword) of the column. This is only required if colnum = 0
and is ignored for images.
. datatype -- The desired data type of the array to be passed to the work function. For numer­
ical data the data type does not need to be the same as the actual data type in the FITS file,
in which case CFITSIO will do the conversion. Allowed values are: TSTRING, TLOGICAL,
TBYTE, TSBYTE, TSHORT, TUSHORT, TINT, TLONG, TULONG, TFLOAT, TDOU­
BLE. If the input value of data type equals 0, then the existing data type of the column or
image will be used without any conversion.
. iotype -- defines whether the data array is to be input to the work function (i.e, read from
the FITS file), or output from the work function (i.e., written to the FITS file) or both.
Allowed values are InputCol, OutputCol, or InputOutputCol. Variable­length array columns
are supported as InputCol or InputOutputCol types, but may not be used for an OutputCol
type.
After the driver routine has initialized all these parameters, it can then call the CFITSIO iterator
function:
int fits_iterate_data(int narrays, iteratorCol *data, long offset,
long nPerLoop, int (*workFn)( ), void *userPointer, int *status);
. narrays -- the number of columns or images that are to be passed to the work function.

6.3. GUIDELINES FOR USING THE ITERATOR FUNCTION 77
. *data -- pointer to array of structures containing information about each column or image.
. o#set -- if positive, this number of rows at the beginning of the table (or pixels in the image)
will be skipped and will not be passed to the work function.
. nPerLoop ­ specifies the number of table rows (or number of image pixels) that are to be
passed to the work function on each iteration. If nPerLoop = 0 then CFITSIO will calculate
the optimum number for greatest e#ciency. If nPerLoop is negative, then all the rows or pixels
will be passed at one time, and the work function will only be called once. If any variable
length arrays are being processed, then the nPerLoop value is ignored, and the iterator will
always process one row of the table at a time.
. *workFn ­ the name (actually the address) of the work function that is to be called by
fits iterate data.
. *userPointer ­ this is a user supplied pointer that can be used to pass ancillary information
from the driver routine to the work function. It may point to a single number, an array, or
to a structure containing an arbitrary set of parameters.
. *status ­ The CFITSIO error status. Should = 0 on input; a non­zero output value indicates
an error.
When fits iterate data is called it first allocates memory to hold all the requested columns of data
or image pixel arrays. It then reads the input data from the FITS tables or images into the arrays
then passes the structure with pointers to these data arrays to the work function. After the work
function returns, the iterator function writes any output columns of data or images back to the
FITS files. It then repeats this process for any remaining sets of rows or image pixels until it has
processed the entire table or image or until the work function returns a non­zero status value. The
iterator then frees the memory that it initially allocated and returns control to the driver routine
that called it.
6.3 Guidelines for Using the Iterator Function
The totaln, o#set, firstn, and nvalues parameters that are passed to the work function are useful
for determining how much of the data has been processed and how much remains left to do. On
the very first call to the work function firstn will be equal to o#set + 1; the work function may
need to perform various initialization tasks before starting to process the data. Similarly, firstn +
nvalues ­ 1 will be equal to totaln on the last iteration, at which point the work function may need
to perform some clean up operations before exiting for the last time. The work function can also
force an early termination of the iterations by returning a status value = ­1.
The narrays and iteratorCol.datatype arguments allow the work function to double check that the
number of input arrays and their data types have the expected values. The iteratorCol.fptr and
iteratorCol.colnum structure elements can be used if the work function needs to read or write the
values of other keywords in the FITS file associated with the array. This should generally only be
done during the initialization step or during the clean up step after the last set of data has been
processed. Extra FITS file I/O during the main processing loop of the work function can seriously
degrade the speed of the program.

78 CHAPTER 6. THE CFITSIO ITERATOR FUNCTION
If variable­length array columns are being processed, then the iterator will operate on one row of
the table at a time. In this case the the repeat element in the interatorCol structure will be set
equal to the number of elements in the current row that is being processed.
One important feature of the iterator is that the first element in each array that is passed to the
work function gives the value that is used to represent null or undefined values in the array. The
real data then begins with the second element of the array (i.e., array[1], not array[0]). If the
first array element is equal to zero, then this indicates that all the array elements have defined
values and there are no undefined values. If array[0] is not equal to zero, then this indicates that
some of the data values are undefined and this value (array[0]) is used to represent them. In the
case of output arrays (i.e., those arrays that will be written back to the FITS file by the iterator
function after the work function exits) the work function must set the first array element to the
desired null value if necessary, otherwise the first element should be set to zero to indicate that
there are no null values in the output array. CFITSIO defines 2 values, FLOATNULLVALUE and
DOUBLENULLVALUE, that can be used as default null values for float and double data types,
respectively. In the case of character string data types, a null string is always used to represent
undefined strings.
In some applications it may be necessary to recursively call the iterator function. An example of
this is given by one of the example programs that is distributed with CFITSIO: it first calls a work
function that writes out a 2D histogram image. That work function in turn calls another work
function that reads the `X' and `Y' columns in a table to calculate the value of each 2D histogram
image pixel. Graphically, the program structure can be described as:
driver ­­> iterator ­­> work1_fn ­­> iterator ­­> work2_fn
Finally, it should be noted that the table columns or image arrays that are passed to the work
function do not all have to come from the same FITS file and instead may come from any com­
bination of sources as long as they have the same length. The length of the first table column or
image array is used by the iterator if they do not all have the same length.
6.4 Complete List of Iterator Routines
All of the iterator routines are listed below. Most of these routines do not have a corresponding
short function name.
1 Iterator `constructor' functions that set the value of elements in the iteratorCol structure that
define the columns or arrays. These set the fitsfile pointer, column name, column number,
datatype, and iotype, respectively. The last 2 routines allow all the parameters to be set with
one function call (one supplies the column name, the other the column number).
int fits_iter_set_file(iteratorCol *col, fitsfile *fptr);
int fits_iter_set_colname(iteratorCol *col, char *colname);
int fits_iter_set_colnum(iteratorCol *col, int colnum);

6.4. COMPLETE LIST OF ITERATOR ROUTINES 79
int fits_iter_set_datatype(iteratorCol *col, int datatype);
int fits_iter_set_iotype(iteratorCol *col, int iotype);
int fits_iter_set_by_name(iteratorCol *col, fitsfile *fptr,
char *colname, int datatype, int iotype);
int fits_iter_set_by_num(iteratorCol *col, fitsfile *fptr,
int colnum, int datatype, int iotype);
2 Iterator `accessor' functions that return the value of the element in the iteratorCol structure
that describes a particular data column or array
fitsfile * fits_iter_get_file(iteratorCol *col);
char * fits_iter_get_colname(iteratorCol *col);
int fits_iter_get_colnum(iteratorCol *col);
int fits_iter_get_datatype(iteratorCol *col);
int fits_iter_get_iotype(iteratorCol *col);
void * fits_iter_get_array(iteratorCol *col);
long fits_iter_get_tlmin(iteratorCol *col);
long fits_iter_get_tlmax(iteratorCol *col);
long fits_iter_get_repeat(iteratorCol *col);
char * fits_iter_get_tunit(iteratorCol *col);
char * fits_iter_get_tdisp(iteratorCol *col);
3 The CFITSIO iterator function
int fits_iterate_data(int narrays, iteratorCol *data, long offset,
long nPerLoop,
int (*workFn)( long totaln, long offset, long firstn,
long nvalues, int narrays, iteratorCol *data,
void *userPointer),
void *userPointer,
int *status);

80 CHAPTER 6. THE CFITSIO ITERATOR FUNCTION

Chapter 7
World Coordinate System Routines
The FITS community has adopted a set of keyword conventions that define the transformations
needed to convert between pixel locations in an image and the corresponding celestial coordinates
on the sky, or more generally, that define world coordinates that are to be associated with any pixel
location in an n­dimensional FITS array. CFITSIO is distributed with a a few self­contained World
Coordinate System (WCS) routines, however, these routines DO NOT support all the latest WCS
conventions, so it is STRONGLY RECOMMENDED that software developers use a more robust
external WCS library. Several recommended libraries are:
WCSLIB ­ supported by Mark Calabretta
WCSTools ­ supported by Doug Mink
AST library ­ developed by the U.K. Starlink project
More information about the WCS keyword conventions and links to all of these WCS libraries can
be found on the FITS Support O#ce web site at http://fits.gsfc.nasa.gov under the WCS link.
The functions provided in these external WCS libraries will need access to the WCS keywords
contained in the FITS file headers. One convenient way to pass this information to the external
library is to use the fits hdr2str routine in CFITSIO (defined below) to copy the header keywords
into one long string, and then pass this string to an interface routine in the external library that
will extract the necessary WCS information (e.g., the 'wcspih' routine in the WCSLIB library and
the 'astFitsChan' and 'astPutCards' functions in the AST library).
1 Concatenate the header keywords in the CHDU into a single long string of characters. Each
80­character fixed­length keyword record is appended to the output character string, in order,
with no intervening separator or terminating characters. The last header record is terminated
with a NULL character. This routine allocates memory for the returned character array, so
the calling program must free the memory when finished.
There are 2 related routines: fits hdr2str simply concatenates all the existing keywords in the
header; fits convert hdr2str is similar, except that if the CHDU is a tile compressed image
(stored in a binary table) then it will first convert that header back to that of a normal FITS
image before concatenating the keywords.
81

82 CHAPTER 7. WORLD COORDINATE SYSTEM ROUTINES
Selected keywords may be excluded from the returned character string. If the second param­
eter (nocomments) is TRUE (nonzero) then any COMMENT, HISTORY, or blank keywords
in the header will not be copied to the output string.
The 'exclist' parameter may be used to supply a list of keywords that are to be excluded from
the output character string. Wild card characters (*, ?, and #) may be used in the excluded
keyword names. If no additional keywords are to be excluded, then set nexc = 0 and specify
NULL for the the **exclist parameter.
int fits_hdr2str
(fitsfile *fptr, int nocomments, char **exclist, int nexc,
> char **header, int *nkeys, int *status)
int fits_convert_hdr2str / ffcnvthdr2str
(fitsfile *fptr, int nocomments, char **exclist, int nexc,
> char **header, int *nkeys, int *status)
2 The following CFITSIO routine is specifically designed for use in conjunction with the WCSLIB
library. It is not expected that applications programmers will call this routine directly, but it
is documented here for completeness. This routine extracts arrays from a binary table that
contain WCS information using the ­TAB table lookup convention. See the documentation
provided with the WCSLIB library for more information.
int fits_read_wcstab
(fitsfile *fptr, int nwtb, wtbarr *wtb, int *status);
7.1 Self­contained WCS Routines
The following routines DO NOT support the more recent WCS conventions that have been approved
as part of the FITS standard. Consequently, the following routines ARE NOW DEPRECATED.
It is STRONGLY RECOMMENDED that software developers not use these routines, and instead
use an external WCS library, as described in the previous section.
These routines are included mainly for backward compatibility with existing software. They support
the following standard map projections: ­SIN, ­TAN, ­ARC, ­NCP, ­GLS, ­MER, and ­AIT (these
are the legal values for the coordtype parameter). These routines are based on similar functions in
Classic AIPS. All the angular quantities are given in units of degrees.
1 Get the values of the basic set of standard FITS celestial coordinate system keywords from
the header of a FITS image (i.e., the primary array or an IMAGE extension). These values
may then be passed to the fits pix to world and fits world to pix routines that perform the
coordinate transformations. If any or all of the WCS keywords are not present, then default
values will be returned. If the first coordinate axis is the declination­like coordinate, then this
routine will swap them so that the longitudinal­like coordinate is returned as the first axis.
The first routine (#gics) returns the primary WCS, whereas the second routine returns the
particular version of the WCS specified by the 'version' parameter, which much be a character
ranging from 'A' to 'Z' (or a blank character, which is equivalent to calling #gics).

7.1. SELF­CONTAINED WCS ROUTINES 83
If the file uses the newer 'CDj i' WCS transformation matrix keywords instead of old style
'CDELTn' and 'CROTA2' keywords, then this routine will calculate and return the values
of the equivalent old­style keywords. Note that the conversion from the new­style keywords
to the old­style values is sometimes only an approximation, so if the approximation is larger
than an internally defined threshold level, then CFITSIO will still return the approximate
WCS keyword values, but will also return with status = APPROX WCS KEY, to warn the
calling program that approximations have been made. It is then up to the calling program
to decide whether the approximations are su#ciently accurate for the particular application,
or whether more precise WCS transformations must be performed using new­style WCS
keywords directly.
int fits_read_img_coord / ffgics
(fitsfile *fptr, > double *xrefval, double *yrefval,
double *xrefpix, double *yrefpix, double *xinc, double *yinc,
double *rot, char *coordtype, int *status)
int fits_read_img_coord_version / ffgicsa
(fitsfile *fptr, char version, > double *xrefval, double *yrefval,
double *xrefpix, double *yrefpix, double *xinc, double *yinc,
double *rot, char *coordtype, int *status)
2 Get the values of the standard FITS celestial coordinate system keywords from the header of a
FITS table where the X and Y (or RA and DEC) coordinates are stored in 2 separate columns
of the table (as in the Event List table format that is often used by high energy astrophysics
missions). These values may then be passed to the fits pix to world and fits world to pix
routines that perform the coordinate transformations.
int fits_read_tbl_coord / ffgtcs
(fitsfile *fptr, int xcol, int ycol, > double *xrefval,
double *yrefval, double *xrefpix, double *yrefpix, double *xinc,
double *yinc, double *rot, char *coordtype, int *status)
3 Calculate the celestial coordinate corresponding to the input X and Y pixel location in the
image.
int fits_pix_to_world / ffwldp
(double xpix, double ypix, double xrefval, double yrefval,
double xrefpix, double yrefpix, double xinc, double yinc,
double rot, char *coordtype, > double *xpos, double *ypos,
int *status)
4 Calculate the X and Y pixel location corresponding to the input celestial coordinate in the
image.
int fits_world_to_pix / ffxypx

84 CHAPTER 7. WORLD COORDINATE SYSTEM ROUTINES
(double xpos, double ypos, double xrefval, double yrefval,
double xrefpix, double yrefpix, double xinc, double yinc,
double rot, char *coordtype, > double *xpix, double *ypix,
int *status)

Chapter 8
Hierarchical Grouping Routines
These functions allow for the creation and manipulation of FITS HDU Groups, as defined in ''A
Hierarchical Grouping Convention for FITS'' by Jennings, Pence, Folk and Schlesinger:
http://fits.gsfc.nasa.gov/group.html
A group is a collection of HDUs whose association is defined by a grouping table. HDUs which
are part of a group are referred to as member HDUs or simply as members. Grouping table
member HDUs may themselves be grouping tables, thus allowing for the construction of open­
ended hierarchies of HDUs.
Grouping tables contain one row for each member HDU. The grouping table columns provide iden­
tification information that allows applications to reference or ''point to'' the member HDUs. Mem­
ber HDUs are expected, but not required, to contain a set of GRPIDn/GRPLCn keywords in their
headers for each grouping table that they are referenced by. In this sense, the GRPIDn/GRPLCn
keywords ''link'' the member HDU back to its Grouping table. Note that a member HDU need not
reside in the same FITS file as its grouping table, and that a given HDU may be referenced by up
to 999 grouping tables simultaneously.
Grouping tables are implemented as FITS binary tables with up to six pre­defined column TTYPEn
values: 'MEMBER XTENSION', 'MEMBER NAME', 'MEMBER VERSION', 'MEMBER POSITION',
'MEMBER URI TYPE' and 'MEMBER LOCATION'. The first three columns allow member HDUs
to be identified by reference to their XTENSION, EXTNAME and EXTVER keyword values. The
fourth column allows member HDUs to be identified by HDU position within their FITS file. The
last two columns identify the FITS file in which the member HDU resides, if di#erent from the
grouping table FITS file.
Additional user defined ''auxiliary'' columns may also be included with any grouping table. When a
grouping table is copied or modified the presence of auxiliary columns is always taken into account
by the grouping support functions; however, the grouping support functions cannot directly make
use of this data.
If a grouping table column is defined but the corresponding member HDU information is un­
available then a null value of the appropriate data type is inserted in the column field. Integer
columns (MEMBER POSITION, MEMBER VERSION) are defined with a TNULLn value of zero
(0). Character field columns (MEMBER XTENSION, MEMBER NAME, MEMBER URI TYPE,
85

86 CHAPTER 8. HIERARCHICAL GROUPING ROUTINES
MEMBER LOCATION) utilize an ASCII null character to denote a null field value.
The grouping support functions belong to two basic categories: those that work with grouping table
HDUs (#gt**) and those that work with member HDUs (#gm**). Two functions, fits copy group()
and fits remove group(), have the option to recursively copy/delete entire groups. Care should
be taken when employing these functions in recursive mode as poorly defined groups could cause
unpredictable results. The problem of a grouping table directly or indirectly referencing itself (thus
creating an infinite loop) is protected against; in fact, neither function will attempt to copy or
delete an HDU twice.
8.1 Grouping Table Routines
1 Create (append) a grouping table at the end of the current FITS file pointed to by fptr. The
grpname parameter provides the grouping table name (GRPNAME keyword value) and may
be set to NULL if no group name is to be specified. The grouptype parameter specifies
the desired structure of the grouping table and may take on the values: GT ID ALL URI
(all columns created), GT ID REF (ID by reference columns), GT ID POS (ID by position
columns), GT ID ALL (ID by reference and position columns), GT ID REF URI (ID by
reference and FITS file URI columns), and GT ID POS URI (ID by position and FITS file
URI columns).
int fits_create_group / ffgtcr
(fitsfile *fptr, char *grpname, int grouptype, > int *status)
2 Create (insert) a grouping table just after the CHDU of the current FITS file pointed to by fptr.
All HDUs below the the insertion point will be shifted downwards to make room for the new
HDU. The grpname parameter provides the grouping table name (GRPNAME keyword value)
and may be set to NULL if no group name is to be specified. The grouptype parameter speci­
fies the desired structure of the grouping table and may take on the values: GT ID ALL URI
(all columns created), GT ID REF (ID by reference columns), GT ID POS (ID by position
columns), GT ID ALL (ID by reference and position columns), GT ID REF URI (ID by ref­
erence and FITS file URI columns), and GT ID POS URI (ID by position and FITS file URI
columns) .
int fits_insert_group / ffgtis
(fitsfile *fptr, char *grpname, int grouptype, > int *status)
3 Change the structure of an existing grouping table pointed to by gfptr. The grouptype parameter
(see fits create group() for valid parameter values) specifies the new structure of the grouping
table. This function only adds or removes grouping table columns, it does not add or delete
group members (i.e., table rows). If the grouping table already has the desired structure then
no operations are performed and function simply returns with a (0) success status code. If
the requested structure change creates new grouping table columns, then the column values
for all existing members will be filled with the null values appropriate to the column type.

8.1. GROUPING TABLE ROUTINES 87
int fits_change_group / ffgtch
(fitsfile *gfptr, int grouptype, > int *status)
4 Remove the group defined by the grouping table pointed to by gfptr, and optionally all the
group member HDUs. The rmopt parameter specifies the action to be taken for all members
of the group defined by the grouping table. Valid values are: OPT RM GPT (delete only the
grouping table) and OPT RM ALL (recursively delete all HDUs that belong to the group).
Any groups containing the grouping table gfptr as a member are updated, and if rmopt ==
OPT RM GPT all members have their GRPIDn and GRPLCn keywords updated accordingly.
If rmopt == OPT RM ALL, then other groups that contain the deleted members of gfptr
are updated to reflect the deletion accordingly.
int fits_remove_group / ffgtrm
(fitsfile *gfptr, int rmopt, > int *status)
5 Copy (append) the group defined by the grouping table pointed to by infptr, and optionally all
group member HDUs, to the FITS file pointed to by outfptr. The cpopt parameter specifies
the action to be taken for all members of the group infptr. Valid values are: OPT GCP GPT
(copy only the grouping table) and OPT GCP ALL (recursively copy ALL the HDUs that
belong to the group defined by infptr). If the cpopt == OPT GCP GPT then the members of
infptr have their GRPIDn and GRPLCn keywords updated to reflect the existence of the new
grouping table outfptr, since they now belong to the new group. If cpopt == OPT GCP ALL
then the new grouping table outfptr only contains pointers to the copied member HDUs and
not the original member HDUs of infptr. Note that, when cpopt == OPT GCP ALL, all
members of the group defined by infptr will be copied to a single FITS file pointed to by
outfptr regardless of their file distribution in the original group.
int fits_copy_group / ffgtcp
(fitsfile *infptr, fitsfile *outfptr, int cpopt, > int *status)
6 Merge the two groups defined by the grouping table HDUs infptr and outfptr by combining
their members into a single grouping table. All member HDUs (rows) are copied from infptr
to outfptr. If mgopt == OPT MRG COPY then infptr continues to exist unaltered after the
merge. If the mgopt == OPT MRG MOV then infptr is deleted after the merge. In both
cases, the GRPIDn and GRPLCn keywords of the member HDUs are updated accordingly.
int fits_merge_groups / ffgtmg
(fitsfile *infptr, fitsfile *outfptr, int mgopt, > int *status)
7 ''Compact'' the group defined by grouping table pointed to by gfptr. The compaction is achieved
by merging (via fits merge groups()) all direct member HDUs of gfptr that are themselves
grouping tables. The cmopt parameter defines whether the merged grouping table HDUs
remain after merging (cmopt == OPT CMT MBR) or if they are deleted after merging
(cmopt == OPT CMT MBR DEL). If the grouping table contains no direct member HDUs
that are themselves grouping tables then this function does nothing. Note that this function
is not recursive, i.e., only the direct member HDUs of gfptr are considered for merging.

88 CHAPTER 8. HIERARCHICAL GROUPING ROUTINES
int fits_compact_group / ffgtcm
(fitsfile *gfptr, int cmopt, > int *status)
8 Verify the integrity of the grouping table pointed to by gfptr to make sure that all group members
are accessible and that all links to other grouping tables are valid. The firstfailed parameter
returns the member ID (row number) of the first member HDU to fail verification (if positive
value) or the first group link to fail (if negative value). If gfptr is successfully verified then
firstfailed contains a return value of 0.
int fits_verify_group / ffgtvf
(fitsfile *gfptr, > long *firstfailed, int *status)
9 Open a grouping table that contains the member HDU pointed to by mfptr. The grouping table
to open is defined by the grpid parameter, which contains the keyword index value of the
GRPIDn/GRPLCn keyword(s) that link the member HDU mfptr to the grouping table. If
the grouping table resides in a file other than the member HDUs file then an attempt is first
made to open the file readwrite, and failing that readonly. A pointer to the opened grouping
table HDU is returned in gfptr.
Note that it is possible, although unlikely and undesirable, for the GRPIDn/GRPLCn key­
words in a member HDU header to be non­continuous, e.g., GRPID1, GRPID2, GRPID5,
GRPID6. In such cases, the grpid index value specified in the function call shall identify the
(grpid)th GRPID value. In the above example, if grpid == 3, then the group specified by
GRPID5 would be opened.
int fits_open_group / ffgtop
(fitsfile *mfptr, int group, > fitsfile **gfptr, int *status)
10 Add a member HDU to an existing grouping table pointed to by gfptr. The member HDU
may either be pointed to mfptr (which must be positioned to the member HDU) or, if mfptr
== NULL, identified by the hdupos parameter (the HDU position number, Primary array
== 1) if both the grouping table and the member HDU reside in the same FITS file. The
new member HDU shall have the appropriate GRPIDn and GRPLCn keywords created in its
header. Note that if the member HDU is already a member of the group then it will not be
added a second time.
int fits_add_group_member / ffgtam
(fitsfile *gfptr, fitsfile *mfptr, int hdupos, > int *status)
8.2 Group Member Routines
1 Return the number of member HDUs in a grouping table gfptr. The number member HDUs is
just the NAXIS2 value (number of rows) of the grouping table.

8.2. GROUP MEMBER ROUTINES 89
int fits_get_num_members / ffgtnm
(fitsfile *gfptr, > long *nmembers, int *status)
2 Return the number of groups to which the HDU pointed to by mfptr is linked, as defined by
the number of GRPIDn/GRPLCn keyword records that appear in its header. Note that each
time this function is called, the indices of the GRPIDn/GRPLCn keywords are checked to
make sure they are continuous (ie no gaps) and are re­enumerated to eliminate gaps if found.
int fits_get_num_groups / ffgmng
(fitsfile *mfptr, > long *nmembers, int *status)
3 Open a member of the grouping table pointed to by gfptr. The member to open is identified by
its row number within the grouping table as given by the parameter 'member' (first member
== 1) . A fitsfile pointer to the opened member HDU is returned as mfptr. Note that if the
member HDU resides in a FITS file di#erent from the grouping table HDU then the member
file is first opened readwrite and, failing this, opened readonly.
int fits_open_member / ffgmop
(fitsfile *gfptr, long member, > fitsfile **mfptr, int *status)
4 Copy (append) a member HDU of the grouping table pointed to by gfptr. The member HDU is
identified by its row number within the grouping table as given by the parameter 'member'
(first member == 1). The copy of the group member HDU will be appended to the FITS
file pointed to by mfptr, and upon return mfptr shall point to the copied member HDU. The
cpopt parameter may take on the following values: OPT MCP ADD which adds a new entry
in gfptr for the copied member HDU, OPT MCP NADD which does not add an entry in gfptr
for the copied member, and OPT MCP REPL which replaces the original member entry with
the copied member entry.
int fits_copy_member / ffgmcp
(fitsfile *gfptr, fitsfile *mfptr, long member, int cpopt, > int *status)
5 Transfer a group member HDU from the grouping table pointed to by infptr to the grouping
table pointed to by outfptr. The member HDU to transfer is identified by its row number
within infptr as specified by the parameter 'member' (first member == 1). If tfopt ==
OPT MCP ADD then the member HDU is made a member of outfptr and remains a member
of infptr. If tfopt == OPT MCP MOV then the member HDU is deleted from infptr after
the transfer to outfptr.
int fits_transfer_member / ffgmtf
(fitsfile *infptr, fitsfile *outfptr, long member, int tfopt,
> int *status)

90 CHAPTER 8. HIERARCHICAL GROUPING ROUTINES
6 Remove a member HDU from the grouping table pointed to by gfptr. The member HDU to be
deleted is identified by its row number in the grouping table as specified by the parameter
'member' (first member == 1). The rmopt parameter may take on the following values:
OPT RM ENTRY which removes the member HDU entry from the grouping table and up­
dates the member's GRPIDn/GRPLCn keywords, and OPT RM MBR which removes the
member HDU entry from the grouping table and deletes the member HDU itself.
int fits_remove_member / ffgmrm
(fitsfile *fptr, long member, int rmopt, > int *status)

Chapter 9
Specialized CFITSIO Interface
Routines
The basic interface routines described previously are recommended for most uses, but the routines
described in this chapter are also available if necessary. Some of these routines perform more spe­
cialized function that cannot easily be done with the basic interface routines while others duplicate
the functionality of the basic routines but have a slightly di#erent calling sequence. See Appendix
B for the definition of each function parameter.
9.1 FITS File Access Routines
1 Open an existing FITS file residing in core computer memory. This routine is analogous to
fits open file. The 'filename' is currently ignored by this routine and may be any arbitrary
string. In general, the application must have preallocated an initial block of memory to
hold the FITS file prior to calling this routine: 'memptr' points to the starting address
and 'memsize' gives the initial size of the block of memory. 'mem realloc' is a pointer to
an optional function that CFITSIO can call to allocate additional memory, if needed (only
if mode = READWRITE), and is modeled after the standard C 'realloc' function; a null
pointer may be given if the initial allocation of memory is all that will be required (e.g.,
if the file is opened with mode = READONLY). The 'deltasize' parameter may be used to
suggest a minimum amount of additional memory that should be allocated during each call
to the memory reallocation function. By default, CFITSIO will reallocate enough additional
space to hold the entire currently defined FITS file (as given by the NAXISn keywords)
or 1 FITS block (= 2880 bytes), which ever is larger. Values of deltasize less than 2880
will be ignored. Since the memory reallocation operation can be computationally expensive,
allocating a larger initial block of memory, and/or specifying a larger deltasize value may help
to reduce the number of reallocation calls and make the application program run faster. Note
that values of the memptr and memsize pointers will be updated by CFITSIO if the location
or size of the FITS file in memory should change as a result of allocating more memory.
int fits_open_memfile / ffomem
(fitsfile **fptr, const char *filename, int mode, void **memptr,
91

92 CHAPTER 9. SPECIALIZED CFITSIO INTERFACE ROUTINES
size_t *memsize, size_t deltasize,
void *(*mem_realloc)(void *p, size_t newsize), int *status)
2 Create a new FITS file residing in core computer memory. This routine is analogous to
fits create file. In general, the application must have preallocated an initial block of memory
to hold the FITS file prior to calling this routine: 'memptr' points to the starting address
and 'memsize' gives the initial size of the block of memory. 'mem realloc' is a pointer to
an optional function that CFITSIO can call to allocate additional memory, if needed, and
is modeled after the standard C 'realloc' function; a null pointer may be given if the initial
allocation of memory is all that will be required. The 'deltasize' parameter may be used to
suggest a minimum amount of additional memory that should be allocated during each call
to the memory reallocation function. By default, CFITSIO will reallocate enough additional
space to hold 1 FITS block (= 2880 bytes) and values of deltasize less than 2880 will be
ignored. Since the memory reallocation operation can be computationally expensive, allocat­
ing a larger initial block of memory, and/or specifying a larger deltasize value may help to
reduce the number of reallocation calls and make the application program run faster. Note
that values of the memptr and memsize pointers will be updated by CFITSIO if the location
or size of the FITS file in memory should change as a result of allocating more memory.
int fits_create_memfile / ffimem
(fitsfile **fptr, void **memptr,
size_t *memsize, size_t deltasize,
void *(*mem_realloc)(void *p, size_t newsize), int *status)
3 Reopen a FITS file that was previously opened with fits open file or fits create file. The new
fitsfile pointer may then be treated as a separate file, and one may simultaneously read or
write to 2 (or more) di#erent extensions in the same file. The fits open file routine (above)
automatically detects cases where a previously opened file is being opened again, and then
internally call fits reopen file, so programs should rarely need to explicitly call this routine.
int fits_reopen_file / ffreopen
(fitsfile *openfptr, fitsfile **newfptr, > int *status)
4 Create a new FITS file, using a template file to define its initial size and structure. The template
may be another FITS HDU or an ASCII template file. If the input template file name pointer
is null, then this routine behaves the same as fits create file. The currently supported format
of the ASCII template file is described under the fits parse template routine (in the general
Utilities section)
int fits_create_template / fftplt
(fitsfile **fptr, char *filename, char *tpltfile > int *status)
5 Parse the input filename or URL into its component parts, namely:
. the file type (file://, ftp://, http://, etc),

9.1. FITS FILE ACCESS ROUTINES 93
. the base input file name,
. the name of the output file that the input file is to be copied to prior to opening,
. the HDU or extension specification,
. the filtering specifier,
. the binning specifier,
. the column specifier,
. and the image pixel filtering specifier.
A null pointer (0) may be be specified for any of the output string arguments that are not
needed. Null strings will be returned for any components that are not present in the input
file name. The calling routine must allocate su#cient memory to hold the returned character
strings. Allocating the string lengths equal to FLEN FILENAME is guaranteed to be safe.
These routines are mainly for internal use by other CFITSIO routines.
int fits_parse_input_url / ffiurl
(char *filename, > char *filetype, char *infile, char *outfile, char
*extspec, char *filter, char *binspec, char *colspec, int *status)
int fits_parse_input_filename / ffifile
(char *filename, > char *filetype, char *infile, char *outfile, char
*extspec, char *filter, char *binspec, char *colspec, char *pixspec,
int *status)
6 Parse the input filename and return the HDU number that would be moved to if the file were
opened with fits open file. The returned HDU number begins with 1 for the primary array,
so for example, if the input filename = `myfile.fits[2]' then hdunum = 3 will be returned.
CFITSIO does not open the file to check if the extension actually exists if an extension
number is specified. If an extension name is included in the file name specification (e.g.
`myfile.fits[EVENTS]' then this routine will have to open the FITS file and look for the
position of the named extension, then close file again. This is not possible if the file is being
read from the stdin stream, and an error will be returned in this case. If the filename does
not specify an explicit extension (e.g. 'myfile.fits') then hdunum = ­99 will be returned,
which is functionally equivalent to hdunum = 1. This routine is mainly used for backward
compatibility in the ftools software package and is not recommended for general use. It is
generally better and more e#cient to first open the FITS file with fits open file, then use
fits get hdu num to determine which HDU in the file has been opened, rather than calling
fits parse input url followed by a call to fits open file.
int fits_parse_extnum / ffextn
(char *filename, > int *hdunum, int *status)
7 Parse the input file name and return the root file name. The root name includes the file
type if specified, (e.g. 'ftp://' or 'http://') and the full path name, to the extent that it is
specified in the input filename. It does not include the HDU name or number, or any filtering

94 CHAPTER 9. SPECIALIZED CFITSIO INTERFACE ROUTINES
specifications. The calling routine must allocate su#cient memory to hold the returned
rootname character string. Allocating the length equal to FLEN FILENAME is guaranteed
to be safe.
int fits_parse_rootname / ffrtnm
(char *filename, > char *rootname, int *status);
8 Test if the input file or a compressed version of the file (with a .gz, .Z, .z, or .zip extension)
exists on disk. The returned value of the 'exists' parameter will have 1 of the 4 following
values:
2: the file does not exist, but a compressed version does exist
1: the disk file does exist
0: neither the file nor a compressed version of the file exist
­1: the input file name is not a disk file (could be a ftp, http,
smem, or mem file, or a file piped in on the STDIN stream)
int fits_file_exists / ffexist
(char *filename, > int *exists, int *status);
9 Flush any internal bu#ers of data to the output FITS file. These routines rarely need to be
called, but can be useful in cases where other processes need to access the same FITS file
in real time, either on disk or in memory. These routines also help to ensure that if the
application program subsequently aborts then the FITS file will have been closed properly.
The first routine, fits flush file is more rigorous and completely closes, then reopens, the
current HDU, before flushing the internal bu#ers, thus ensuring that the output FITS file is
identical to what would be produced if the FITS was closed at that point (i.e., with a call
to fits close file). The second routine, fits flush bu#er simply flushes the internal CFITSIO
bu#ers of data to the output FITS file, without updating and closing the current HDU. This
is much faster, but there may be circumstances where the flushed file does not completely
reflect the final state of the file as it will exist when the file is actually closed.
A typical use of these routines would be to flush the state of a FITS table to disk after each
row of the table is written. It is recommend that fits flush file be called after the first row
is written, then fits flush bu#er may be called after each subsequent row is written. Note
that this latter routine will not automatically update the NAXIS2 keyword which records
the number of rows of data in the table, so this keyword must be explicitly updated by the
application program after each row is written.
int fits_flush_file / ffflus
(fitsfile *fptr, > int *status)
int fits_flush_buffer / ffflsh
(fitsfile *fptr, 0, > int *status)
(Note: The second argument must be 0).

9.2. HDU ACCESS ROUTINES 95
9.2 HDU Access Routines
1 Get the byte o#sets in the FITS file to the start of the header and the start and end of the data
in the CHDU. The di#erence between headstart and dataend equals the size of the CHDU.
If the CHDU is the last HDU in the file, then dataend is also equal to the size of the entire
FITS file. Null pointers may be input for any of the address parameters if their values are
not needed.
int fits_get_hduaddr / ffghad (only supports files up to 2.1 GB in size)
(fitsfile *fptr, > long *headstart, long *datastart, long *dataend,
int *status)
int fits_get_hduaddrll / ffghadll (supports large files)
(fitsfile *fptr, > LONGLONG *headstart, LONGLONG *datastart,
LONGLONG *dataend, int *status)
2 Create (append) a new empty HDU at the end of the FITS file. This is now the CHDU but it
is completely empty and has no header keywords. It is recommended that fits create img or
fits create tbl be used instead of this routine.
int fits_create_hdu / ffcrhd
(fitsfile *fptr, > int *status)
3 Insert a new IMAGE extension immediately following the CHDU, or insert a new Primary Array
at the beginning of the file. Any following extensions in the file will be shifted down to make
room for the new extension. If the CHDU is the last HDU in the file then the new image
extension will simply be appended to the end of the file. One can force a new primary array
to be inserted at the beginning of the FITS file by setting status = PREPEND PRIMARY
prior to calling the routine. In this case the old primary array will be converted to an IMAGE
extension. The new extension (or primary array) will become the CHDU. Refer to Chapter
9 for a list of pre­defined bitpix values.
int fits_insert_img / ffiimg
(fitsfile *fptr, int bitpix, int naxis, long *naxes, > int *status)
int fits_insert_imgll / ffiimgll
(fitsfile *fptr, int bitpix, int naxis, LONGLONG *naxes, > int *status)
4 Insert a new ASCII or binary table extension immediately following the CHDU. Any following
extensions will be shifted down to make room for the new extension. If there are no other
following extensions then the new table extension will simply be appended to the end of the
file. If the FITS file is currently empty then this routine will create a dummy primary array
before appending the table to it. The new extension will become the CHDU. The tunit and
extname parameters are optional and a null pointer may be given if they are not defined.
When inserting an ASCII table with fits insert atbl, a null pointer may given for the *tbcol

96 CHAPTER 9. SPECIALIZED CFITSIO INTERFACE ROUTINES
parameter in which case each column of the table will be separated by a single space character.
Similarly, if the input value of rowlen is 0, then CFITSIO will calculate the default rowlength
based on the tbcol and ttype values. Under normal circumstances, the nrows paramenter
should have a value of 0; CFITSIO will automatically update the number of rows as data is
written to the table. When inserting a binary table with fits insert btbl, if there are following
extensions in the file and if the table contains variable length array columns then pcount must
specify the expected final size of the data heap, otherwise pcount must = 0.
int fits_insert_atbl / ffitab
(fitsfile *fptr, LONGLONG rowlen, LONGLONG nrows, int tfields, char *ttype[],
long *tbcol, char *tform[], char *tunit[], char *extname, > int *status)
int fits_insert_btbl / ffibin
(fitsfile *fptr, LONGLONG nrows, int tfields, char **ttype,
char **tform, char **tunit, char *extname, long pcount, > int *status)
5 Modify the size, dimensions, and/or data type of the current primary array or image extension.
If the new image, as specified by the input arguments, is larger than the current existing
image in the FITS file then zero fill data will be inserted at the end of the current image and
any following extensions will be moved further back in the file. Similarly, if the new image
is smaller than the current image then any following extensions will be shifted up towards
the beginning of the FITS file and the image data will be truncated to the new size. This
routine rewrites the BITPIX, NAXIS, and NAXISn keywords with the appropriate values for
the new image.
int fits_resize_img / ffrsim
(fitsfile *fptr, int bitpix, int naxis, long *naxes, > int *status)
int fits_resize_imgll / ffrsimll
(fitsfile *fptr, int bitpix, int naxis, LONGLONG *naxes, > int *status)
6 Copy the data (and not the header) from the CHDU associated with infptr to the CHDU
associated with outfptr. This will overwrite any data previously in the output CHDU. This
low level routine is used by fits copy hdu, but it may also be useful in certain application
programs that want to copy the data from one FITS file to another but also want to modify
the header keywords. The required FITS header keywords which define the structure of the
HDU must be written to the output CHDU before calling this routine.
int fits_copy_data / ffcpdt
(fitsfile *infptr, fitsfile *outfptr, > int *status)
7 Read or write a specified number of bytes starting at the specified byte o#set from the start of
the extension data unit. These low level routine are intended mainly for accessing the data in
non­standard, conforming extensions, and should not be used for standard IMAGE, TABLE,
or BINTABLE extensions.

9.3. SPECIALIZED HEADER KEYWORD ROUTINES 97
int fits_read_ext / ffgextn
(fitsfile *fptr, LONGLONG offset, LONGLONG nbytes, void *buffer)
int fits_write_ext / ffpextn
(fitsfile *fptr, LONGLONG offset, LONGLONG nbytes, void *buffer)
8 This routine forces CFITSIO to rescan the current header keywords that define the structure
of the HDU (such as the NAXIS and BITPIX keywords) so that it reinitializes the internal
bu#ers that describe the HDU structure. This routine is useful for reinitializing the structure
of an HDU if any of the required keywords (e.g., NAXISn) have been modified. In practice
it should rarely be necessary to call this routine because CFITSIO internally calls it in most
situations.
int fits_set_hdustruc / ffrdef
(fitsfile *fptr, > int *status) (DEPRECATED)
9.3 Specialized Header Keyword Routines
9.3.1 Header Information Routines
1 Reserve space in the CHU for MOREKEYS more header keywords. This routine may be called
to allocate space for additional keywords at the time the header is created (prior to writing
any data). CFITSIO can dynamically add more space to the header when needed, however
it is more e#cient to preallocate the required space if the size is known in advance.
int fits_set_hdrsize / ffhdef
(fitsfile *fptr, int morekeys, > int *status)
2 Return the number of keywords in the header (not counting the END keyword) and the current
position in the header. The position is the number of the keyword record that will be read
next (or one greater than the position of the last keyword that was read). A value of 1 is
returned if the pointer is positioned at the beginning of the header.
int fits_get_hdrpos / ffghps
(fitsfile *fptr, > int *keysexist, int *keynum, int *status)
9.3.2 Read and Write the Required Keywords
1 Write the required extension header keywords into the CHU. These routines are not required,
and instead the appropriate header may be constructed by writing each individual keyword
in the proper sequence.
The simpler fits write imghdr routine is equivalent to calling fits write grphdr with the default
values of simple = TRUE, pcount = 0, gcount = 1, and extend = TRUE. The PCOUNT,
GCOUNT and EXTEND keywords are not required in the primary header and are only

98 CHAPTER 9. SPECIALIZED CFITSIO INTERFACE ROUTINES
written if pcount is not equal to zero, gcount is not equal to zero or one, and if extend is
TRUE, respectively. When writing to an IMAGE extension, the SIMPLE and EXTEND
parameters are ignored. It is recommended that fits create image or fits create tbl be used
instead of these routines to write the required header keywords. The general fits write exthdr
routine may be used to write the header of any conforming FITS extension.
int fits_write_imghdr / ffphps
(fitsfile *fptr, int bitpix, int naxis, long *naxes, > int *status)
int fits_write_imghdrll / ffphpsll
(fitsfile *fptr, int bitpix, int naxis, LONGLONG *naxes, > int *status)
int fits_write_grphdr / ffphpr
(fitsfile *fptr, int simple, int bitpix, int naxis, long *naxes,
LONGLONG pcount, LONGLONG gcount, int extend, > int *status)
int fits_write_grphdrll / ffphprll
(fitsfile *fptr, int simple, int bitpix, int naxis, LONGLONG *naxes,
LONGLONG pcount, LONGLONG gcount, int extend, > int *status)
int fits_write_exthdr /ffphext
(fitsfile *fptr, char *xtension, int bitpix, int naxis, long *naxes,
LONGLONG pcount, LONGLONG gcount, > int *status)
2 Write the ASCII table header keywords into the CHU. The optional TUNITn and EXTNAME
keywords are written only if the input pointers are not null. A null pointer may given for the
*tbcol parameter in which case a single space will be inserted between each column of the
table. Similarly, if rowlen is given = 0, then CFITSIO will calculate the default rowlength
based on the tbcol and ttype values.
int fits_write_atblhdr / ffphtb
(fitsfile *fptr, LONGLONG rowlen, LONGLONG nrows, int tfields, char **ttype,
long *tbcol, char **tform, char **tunit, char *extname, > int *status)
3 Write the binary table header keywords into the CHU. The optional TUNITn and EXTNAME
keywords are written only if the input pointers are not null. The pcount parameter, which
specifies the size of the variable length array heap, should initially = 0; CFITSIO will au­
tomatically update the PCOUNT keyword value if any variable length array data is written
to the heap. The TFORM keyword value for variable length vector columns should have the
form 'Pt(len)' or '1Pt(len)' where `t' is the data type code letter (A,I,J,E,D, etc.) and `len' is
an integer specifying the maximum length of the vectors in that column (len must be greater
than or equal to the longest vector in the column). If `len' is not specified when the table
is created (e.g., the input TFORMn value is just '1Pt') then CFITSIO will scan the column
when the table is first closed and will append the maximum length to the TFORM keyword
value. Note that if the table is subsequently modified to increase the maximum length of the

9.3. SPECIALIZED HEADER KEYWORD ROUTINES 99
vectors then the modifying program is responsible for also updating the TFORM keyword
value.
int fits_write_btblhdr / ffphbn
(fitsfile *fptr, LONGLONG nrows, int tfields, char **ttype,
char **tform, char **tunit, char *extname, LONGLONG pcount, > int *status)
4 Read the required keywords from the CHDU (image or table). When reading from an IMAGE
extension the SIMPLE and EXTEND parameters are ignored. A null pointer may be supplied
for any of the returned parameters that are not needed.
int fits_read_imghdr / ffghpr
(fitsfile *fptr, int maxdim, > int *simple, int *bitpix, int *naxis,
long *naxes, long *pcount, long *gcount, int *extend, int *status)
int fits_read_imghdrll / ffghprll
(fitsfile *fptr, int maxdim, > int *simple, int *bitpix, int *naxis,
LONGLONG *naxes, long *pcount, long *gcount, int *extend, int *status)
int fits_read_atblhdr / ffghtb
(fitsfile *fptr,int maxdim, > long *rowlen, long *nrows,
int *tfields, char **ttype, LONGLONG *tbcol, char **tform, char **tunit,
char *extname, int *status)
int fits_read_atblhdrll / ffghtbll
(fitsfile *fptr,int maxdim, > LONGLONG *rowlen, LONGLONG *nrows,
int *tfields, char **ttype, long *tbcol, char **tform, char **tunit,
char *extname, int *status)
int fits_read_btblhdr / ffghbn
(fitsfile *fptr, int maxdim, > long *nrows, int *tfields,
char **ttype, char **tform, char **tunit, char *extname,
long *pcount, int *status)
int fits_read_btblhdrll / ffghbnll
(fitsfile *fptr, int maxdim, > LONGLONG *nrows, int *tfields,
char **ttype, char **tform, char **tunit, char *extname,
long *pcount, int *status)
9.3.3 Write Keyword Routines
These routines simply append a new keyword to the header and do not check to see if a keyword
with the same name already exists. In general it is preferable to use the fits update key routine to
ensure that the same keyword is not written more than once to the header. See Appendix B for
the definition of the parameters used in these routines.

100 CHAPTER 9. SPECIALIZED CFITSIO INTERFACE ROUTINES
1 Write (append) a new keyword of the appropriate data type into the CHU. A null pointer may
be entered for the comment parameter, which will cause the comment field of the keyword
to be left blank. The flt, dbl, cmp, and dblcmp versions of this routine have the added
feature that if the 'decimals' parameter is negative, then the 'G' display format rather then
the 'E' format will be used when constructing the keyword value, taking the absolute value
of 'decimals' for the precision. This will suppress trailing zeros, and will use a fixed format
rather than an exponential format, depending on the magnitude of the value.
int fits_write_key_str / ffpkys
(fitsfile *fptr, char *keyname, char *value, char *comment,
> int *status)
int fits_write_key_[log, lng] / ffpky[lj]
(fitsfile *fptr, char *keyname, DTYPE numval, char *comment,
> int *status)
int fits_write_key_[flt, dbl, fixflg, fixdbl] / ffpky[edfg]
(fitsfile *fptr, char *keyname, DTYPE numval, int decimals,
char *comment, > int *status)
int fits_write_key_[cmp, dblcmp, fixcmp, fixdblcmp] / ffpk[yc,ym,fc,fm]
(fitsfile *fptr, char *keyname, DTYPE *numval, int decimals,
char *comment, > int *status)
2 Write (append) a string valued keyword into the CHU which may be longer than 68 characters
in length. This uses the Long String Keyword convention that is described in the`Local
FITS Conventions' section in Chapter 4. Since this uses a non­standard FITS convention
to encode the long keyword string, programs which use this routine should also call the
fits write key longwarn routine to add some COMMENT keywords to warn users of the FITS
file that this convention is being used. The fits write key longwarn routine also writes a
keyword called LONGSTRN to record the version of the longstring convention that has been
used, in case a new convention is adopted at some point in the future. If the LONGSTRN
keyword is already present in the header, then fits write key longwarn will simply return
without doing anything.
int fits_write_key_longstr / ffpkls
(fitsfile *fptr, char *keyname, char *longstr, char *comment,
> int *status)
int fits_write_key_longwarn / ffplsw
(fitsfile *fptr, > int *status)
3 Write (append) a numbered sequence of keywords into the CHU. The starting index number
(nstart) must be greater than 0. One may append the same comment to every keyword (and
eliminate the need to have an array of identical comment strings, one for each keyword) by
including the ampersand character as the last non­blank character in the (first) COMMENTS

9.3. SPECIALIZED HEADER KEYWORD ROUTINES 101
string parameter. This same string will then be used for the comment field in all the keywords.
One may also enter a null pointer for the comment parameter to leave the comment field of
the keyword blank.
int fits_write_keys_str / ffpkns
(fitsfile *fptr, char *keyroot, int nstart, int nkeys,
char **value, char **comment, > int *status)
int fits_write_keys_[log, lng] / ffpkn[lj]
(fitsfile *fptr, char *keyroot, int nstart, int nkeys,
DTYPE *numval, char **comment, int *status)
int fits_write_keys_[flt, dbl, fixflg, fixdbl] / ffpkne[edfg]
(fitsfile *fptr, char *keyroot, int nstart, int nkey,
DTYPE *numval, int decimals, char **comment, > int *status)
4 Copy an indexed keyword from one HDU to another, modifying the index number of the keyword
name in the process. For example, this routine could read the TLMIN3 keyword from the
input HDU (by giving keyroot = `TLMIN' and innum = 3) and write it to the output HDU
with the keyword name TLMIN4 (by setting outnum = 4). If the input keyword does not
exist, then this routine simply returns without indicating an error.
int fits_copy_key / ffcpky
(fitsfile *infptr, fitsfile *outfptr, int innum, int outnum,
char *keyroot, > int *status)
5 Write (append) a `triple precision' keyword into the CHU in F28.16 format. The floating point
keyword value is constructed by concatenating the input integer value with the input double
precision fraction value (which must have a value between 0.0 and 1.0). The #gkyt routine
should be used to read this keyword value, because the other keyword reading routines will
not preserve the full precision of the value.
int fits_write_key_triple / ffpkyt
(fitsfile *fptr, char *keyname, long intval, double frac,
char *comment, > int *status)
6 Write keywords to the CHDU that are defined in an ASCII template file. The format of the
template file is described under the fits parse template routine.
int fits_write_key_template / ffpktp
(fitsfile *fptr, const char *filename, > int *status)
9.3.4 Insert Keyword Routines
These insert routines are somewhat less e#cient than the `update' or `write' keyword routines
because the following keywords in the header must be shifted down to make room for the inserted
keyword. See Appendix B for the definition of the parameters used in these routines.

102 CHAPTER 9. SPECIALIZED CFITSIO INTERFACE ROUTINES
1 Insert a new keyword record into the CHU at the specified position (i.e., immediately preceding
the (keynum)th keyword in the header.)
int fits_insert_record / ffirec
(fitsfile *fptr, int keynum, char *card, > int *status)
2 Insert a new keyword into the CHU. The new keyword is inserted immediately following the last
keyword that has been read from the header. The `longstr' version has the same functionality
as the `str' version except that it also supports the local long string keyword convention for
strings longer than 68 characters. A null pointer may be entered for the comment parameter
which will cause the comment field to be left blank. The flt, dbl, cmp, and dblcmp versions of
this routine have the added feature that if the 'decimals' parameter is negative, then the 'G'
display format rather then the 'E' format will be used when constructing the keyword value,
taking the absolute value of 'decimals' for the precision. This will suppress trailing zeros, and
will use a fixed format rather than an exponential format, depending on the magnitude of
the value.
int fits_insert_card / ffikey
(fitsfile *fptr, char *card, > int *status)
int fits_insert_key_[str, longstr] / ffi[kys, kls]
(fitsfile *fptr, char *keyname, char *value, char *comment,
> int *status)
int fits_insert_key_[log, lng] / ffiky[lj]
(fitsfile *fptr, char *keyname, DTYPE numval, char *comment,
> int *status)
int fits_insert_key_[flt, fixflt, dbl, fixdbl] / ffiky[edfg]
(fitsfile *fptr, char *keyname, DTYPE numval, int decimals,
char *comment, > int *status)
int fits_insert_key_[cmp, dblcmp, fixcmp, fixdblcmp] / ffik[yc,ym,fc,fm]
(fitsfile *fptr, char *keyname, DTYPE *numval, int decimals,
char *comment, > int *status)
3 Insert a new keyword with an undefined, or null, value into the CHU. The value string of the
keyword is left blank in this case.
int fits_insert_key_null / ffikyu
(fitsfile *fptr, char *keyname, char *comment, > int *status)
9.3.5 Read Keyword Routines
Wild card characters may be used when specifying the name of the keyword to be read.

9.3. SPECIALIZED HEADER KEYWORD ROUTINES 103
1 Read a keyword value (with the appropriate data type) and comment from the CHU. If a
NULL comment pointer is given on input, then the comment string will not be returned. If
the value of the keyword is not defined (i.e., the value field is blank) then an error status =
VALUE UNDEFINED will be returned and the input value will not be changed (except that
#gkys will reset the value to a null string).
int fits_read_key_str / ffgkys
(fitsfile *fptr, char *keyname, > char *value, char *comment,
int *status);
NOTE: after calling the following routine, programs must explicitly free
the memory allocated for 'longstr' after it is no longer needed or
call fits_free_memory.
int fits_read_key_longstr / ffgkls
(fitsfile *fptr, char *keyname, > char **longstr, char *comment,
int *status)
int fits_free_memory / fffree
(char *longstr, int *status);
int fits_read_key_[log, lng, flt, dbl, cmp, dblcmp] / ffgky[ljedcm]
(fitsfile *fptr, char *keyname, > DTYPE *numval, char *comment,
int *status)
int fits_read_key_lnglng / ffgkyjj
(fitsfile *fptr, char *keyname, > LONGLONG *numval, char *comment,
int *status)
2 Read a sequence of indexed keyword values (e.g., NAXIS1, NAXIS2, ...). The input starting
index number (nstart) must be greater than 0. If the value of any of the keywords is not
defined (i.e., the value field is blank) then an error status = VALUE UNDEFINED will be
returned and the input value for the undefined keyword(s) will not be changed. These routines
do not support wild card characters in the root name. If there are no indexed keywords in the
header with the input root name then these routines do not return a non­zero status value
and instead simply return nfound = 0.
int fits_read_keys_str / ffgkns
(fitsfile *fptr, char *keyname, int nstart, int nkeys,
> char **value, int *nfound, int *status)
int fits_read_keys_[log, lng, flt, dbl] / ffgkn[ljed]
(fitsfile *fptr, char *keyname, int nstart, int nkeys,
> DTYPE *numval, int *nfound, int *status)
3 Read the value of a floating point keyword, returning the integer and fractional parts of the

104 CHAPTER 9. SPECIALIZED CFITSIO INTERFACE ROUTINES
value in separate routine arguments. This routine may be used to read any keyword but is
especially useful for reading the 'triple precision' keywords written by #pkyt.
int fits_read_key_triple / ffgkyt
(fitsfile *fptr, char *keyname, > long *intval, double *frac,
char *comment, int *status)
9.3.6 Modify Keyword Routines
These routines modify the value of an existing keyword. An error is returned if the keyword does
not exist. Wild card characters may be used when specifying the name of the keyword to be
modified. See Appendix B for the definition of the parameters used in these routines.
1 Modify (overwrite) the nth 80­character header record in the CHU.
int fits_modify_record / ffmrec
(fitsfile *fptr, int keynum, char *card, > int *status)
2 Modify (overwrite) the 80­character header record for the named keyword in the CHU. This
can be used to overwrite the name of the keyword as well as its value and comment fields.
int fits_modify_card / ffmcrd
(fitsfile *fptr, char *keyname, char *card, > int *status)
5 Modify the value and comment fields of an existing keyword in the CHU. The `longstr' version
has the same functionality as the `str' version except that it also supports the local long
string keyword convention for strings longer than 68 characters. Optionally, one may modify
only the value field and leave the comment field unchanged by setting the input COMMENT
parameter equal to the ampersand character (&) or by entering a null pointer for the comment
parameter. The flt, dbl, cmp, and dblcmp versions of this routine have the added feature that
if the 'decimals' parameter is negative, then the 'G' display format rather then the 'E' format
will be used when constructing the keyword value, taking the absolute value of 'decimals' for
the precision. This will suppress trailing zeros, and will use a fixed format rather than an
exponential format, depending on the magnitude of the value.
int fits_modify_key_[str, longstr] / ffm[kys, kls]
(fitsfile *fptr, char *keyname, char *value, char *comment,
> int *status);
int fits_modify_key_[log, lng] / ffmky[lj]
(fitsfile *fptr, char *keyname, DTYPE numval, char *comment,
> int *status)
int fits_modify_key_[flt, dbl, fixflt, fixdbl] / ffmky[edfg]

9.3. SPECIALIZED HEADER KEYWORD ROUTINES 105
(fitsfile *fptr, char *keyname, DTYPE numval, int decimals,
char *comment, > int *status)
int fits_modify_key_[cmp, dblcmp, fixcmp, fixdblcmp] / ffmk[yc,ym,fc,fm]
(fitsfile *fptr, char *keyname, DTYPE *numval, int decimals,
char *comment, > int *status)
6 Modify the value of an existing keyword to be undefined, or null. The value string of the keyword
is set to blank. Optionally, one may leave the comment field unchanged by setting the input
COMMENT parameter equal to the ampersand character (&) or by entering a null pointer.
int fits_modify_key_null / ffmkyu
(fitsfile *fptr, char *keyname, char *comment, > int *status)
9.3.7 Update Keyword Routines
1 These update routines modify the value, and optionally the comment field, of the keyword if it
already exists, otherwise the new keyword is appended to the header. A separate routine is
provided for each keyword data type. The `longstr' version has the same functionality as the
`str' version except that it also supports the local long string keyword convention for strings
longer than 68 characters. A null pointer may be entered for the comment parameter which
will leave the comment field unchanged or blank. The flt, dbl, cmp, and dblcmp versions of
this routine have the added feature that if the 'decimals' parameter is negative, then the 'G'
display format rather then the 'E' format will be used when constructing the keyword value,
taking the absolute value of 'decimals' for the precision. This will suppress trailing zeros, and
will use a fixed format rather than an exponential format, depending on the magnitude of
the value.
int fits_update_key_[str, longstr] / ffu[kys, kls]
(fitsfile *fptr, char *keyname, char *value, char *comment,
> int *status)
int fits_update_key_[log, lng] / ffuky[lj]
(fitsfile *fptr, char *keyname, DTYPE numval, char *comment,
> int *status)
int fits_update_key_[flt, dbl, fixflt, fixdbl] / ffuky[edfg]
(fitsfile *fptr, char *keyname, DTYPE numval, int decimals,
char *comment, > int *status)
int fits_update_key_[cmp, dblcmp, fixcmp, fixdblcmp] / ffuk[yc,ym,fc,fm]
(fitsfile *fptr, char *keyname, DTYPE *numval, int decimals,
char *comment, > int *status)

106 CHAPTER 9. SPECIALIZED CFITSIO INTERFACE ROUTINES
9.4 Define Data Scaling and Undefined Pixel Parameters
These routines set or modify the internal parameters used by CFITSIO to either scale the data
or to represent undefined pixels. Generally CFITSIO will scale the data according to the values
of the BSCALE and BZERO (or TSCALn and TZEROn) keywords, however these routines may
be used to override the keyword values. This may be useful when one wants to read or write the
raw unscaled values in the FITS file. Similarly, CFITSIO generally uses the value of the BLANK
or TNULLn keyword to signify an undefined pixel, but these routines may be used to override
this value. These routines do not create or modify the corresponding header keyword values. See
Appendix B for the definition of the parameters used in these routines.
1 Reset the scaling factors in the primary array or image extension; does not change the BSCALE
and BZERO keyword values and only a#ects the automatic scaling performed when the data
elements are written/read to/from the FITS file. When reading from a FITS file the returned
data value = (the value given in the FITS array) * BSCALE + BZERO. The inverse formula
is used when writing data values to the FITS file.
int fits_set_bscale / ffpscl
(fitsfile *fptr, double scale, double zero, > int *status)
2 Reset the scaling parameters for a table column; does not change the TSCALn or TZEROn
keyword values and only a#ects the automatic scaling performed when the data elements are
written/read to/from the FITS file. When reading from a FITS file the returned data value
= (the value given in the FITS array) * TSCAL + TZERO. The inverse formula is used when
writing data values to the FITS file.
int fits_set_tscale / fftscl
(fitsfile *fptr, int colnum, double scale, double zero,
> int *status)
3 Define the integer value to be used to signify undefined pixels in the primary array or image
extension. This is only used if BITPIX = 8, 16, or 32. This does not create or change the
value of the BLANK keyword in the header.
int fits_set_imgnull / ffpnul
(fitsfile *fptr, LONGLONG nulval, > int *status)
4 Define the string to be used to signify undefined pixels in a column in an ASCII table. This
does not create or change the value of the TNULLn keyword.
int fits_set_atblnull / ffsnul
(fitsfile *fptr, int colnum, char *nulstr, > int *status)
5 Define the value to be used to signify undefined pixels in an integer column in a binary table
(where TFORMn = 'B', 'I', or 'J'). This does not create or change the value of the TNULLn
keyword.

9.5. SPECIALIZED FITS PRIMARY ARRAY OR IMAGE EXTENSION I/O ROUTINES 107
int fits_set_btblnull / fftnul
(fitsfile *fptr, int colnum, LONGLONG nulval, > int *status)
9.5 Specialized FITS Primary Array or IMAGE Extension I/O
Routines
These routines read or write data values in the primary data array (i.e., the first HDU in the FITS
file) or an IMAGE extension. Automatic data type conversion is performed for if the data type
of the FITS array (as defined by the BITPIX keyword) di#ers from the data type of the array in
the calling routine. The data values are automatically scaled by the BSCALE and BZERO header
values as they are being written or read from the FITS array. Unlike the basic routines described in
the previous chapter, most of these routines specifically support the FITS random groups format.
See Appendix B for the definition of the parameters used in these routines.
The more primitive reading and writing routines (i. e., #ppr , #ppn , #ppn, #gpv , or #gpf ) simply
treat the primary array as a long 1­dimensional array of pixels, ignoring the intrinsic dimensionality
of the array. When dealing with a 2D image, for example, the application program must calculate
the pixel o#set in the 1­D array that corresponds to any particular X, Y coordinate in the image.
C programmers should note that the ordering of arrays in FITS files, and hence in all the CFITSIO
calls, is more similar to the dimensionality of arrays in Fortran rather than C. For instance if a
FITS image has NAXIS1 = 100 and NAXIS2 = 50, then a 2­D array just large enough to hold the
image should be declared as array[50][100] and not as array[100][50].
For convenience, higher­level routines are also provided to specifically deal with 2D images (#p2d
and #g2d ) and 3D data cubes (#p3d and #g3d ). The dimensionality of the FITS image is passed
by the naxis1, naxis2, and naxis3 parameters and the declared dimensions of the program array
are passed in the dim1 and dim2 parameters. Note that the dimensions of the program array may
be larger than the dimensions of the FITS array. For example if a FITS image with NAXIS1 =
NAXIS2 = 400 is read into a program array which is dimensioned as 512 x 512 pixels, then the
image will just fill the lower left corner of the array with pixels in the range 1 ­ 400 in the X an
Y directions. This has the e#ect of taking a contiguous set of pixel value in the FITS array and
writing them to a non­contiguous array in program memory (i.e., there are now some blank pixels
around the edge of the image in the program array).
The most general set of routines (#pss , #gsv , and #gsf ) may be used to transfer a rectangular
subset of the pixels in a FITS N­dimensional image to or from an array which has been declared in
the calling program. The fpixel and lpixel parameters are integer arrays which specify the starting
and ending pixel coordinate in each dimension (starting with 1, not 0) of the FITS image that is
to be read or written. It is important to note that these are the starting and ending pixels in the
FITS image, not in the declared array in the program. The array parameter in these routines is
treated simply as a large one­dimensional array of the appropriate data type containing the pixel
values; The pixel values in the FITS array are read/written from/to this program array in strict
sequence without any gaps; it is up to the calling routine to correctly interpret the dimensionality
of this array. The two FITS reading routines (#gsv and #gsf ) also have an `inc' parameter which
defines the data sampling interval in each dimension of the FITS array. For example, if inc[0]=2
and inc[1]=3 when reading a 2­dimensional FITS image, then only every other pixel in the first
dimension and every 3rd pixel in the second dimension will be returned to the 'array' parameter.

108 CHAPTER 9. SPECIALIZED CFITSIO INTERFACE ROUTINES
Two types of routines are provided to read the data array which di#er in the way undefined pixels
are handled. The first type of routines (e.g., #gpv ) simply return an array of data elements in
which undefined pixels are set equal to a value specified by the user in the `nulval' parameter. An
additional feature of these routines is that if the user sets nulval = 0, then no checks for undefined
pixels will be performed, thus reducing the amount of CPU processing. The second type of routines
(e.g., #gpf ) returns the data element array and, in addition, a char array that indicates whether
the value of the corresponding data pixel is undefined (= 1) or defined (= 0). The latter type of
routines may be more convenient to use in some circumstances, however, it requires an additional
array of logical values which can be unwieldy when working with large data arrays.
1 Write elements into the FITS data array.
int fits_write_img / ffppr
(fitsfile *fptr, int datatype, LONGLONG firstelem, LONGLONG nelements,
DTYPE *array, int *status);
int fits_write_img_[byt, sht, usht, int, uint, lng, ulng, lnglng, flt, dbl] /
ffppr[b,i,ui,k,uk,j,uj,jj,e,d]
(fitsfile *fptr, long group, LONGLONG firstelem, LONGLONG nelements,
DTYPE *array, > int *status);
int fits_write_imgnull / ffppn
(fitsfile *fptr, int datatype, LONGLONG firstelem, LONGLONG nelements,
DTYPE *array, DTYPE *nulval, > int *status);
int fits_write_imgnull_[byt, sht, usht, int, uint, lng, ulng, lnglng, flt, dbl] /
ffppn[b,i,ui,k,uk,j,uj,jj,e,d]
(fitsfile *fptr, long group, LONGLONG firstelem,
LONGLONG nelements, DTYPE *array, DTYPE nulval, > int *status);
2 Set data array elements as undefined.
int fits_write_img_null / ffppru
(fitsfile *fptr, long group, LONGLONG firstelem, LONGLONG nelements,
> int *status)
3 Write values into group parameters. This routine only applies to the `Random Grouped' FITS
format which has been used for applications in radio interferometry, but is o#cially deprecated
for future use.
int fits_write_grppar_[byt, sht, usht, int, uint, lng, ulng, lnglng, flt, dbl] /
ffpgp[b,i,ui,k,uk,j,uj,jj,e,d]
(fitsfile *fptr, long group, long firstelem, long nelements,
> DTYPE *array, int *status)

9.5. SPECIALIZED FITS PRIMARY ARRAY OR IMAGE EXTENSION I/O ROUTINES 109
4 Write a 2­D or 3­D image into the data array.
int fits_write_2d_[byt, sht, usht, int, uint, lng, ulng, lnglng, flt, dbl] /
ffp2d[b,i,ui,k,uk,j,uj,jj,e,d]
(fitsfile *fptr, long group, LONGLONG dim1, LONGLONG naxis1,
LONGLONG naxis2, DTYPE *array, > int *status)
int fits_write_3d_[byt, sht, usht, int, uint, lng, ulng, lnglng, flt, dbl] /
ffp3d[b,i,ui,k,uk,j,uj,jj,e,d]
(fitsfile *fptr, long group, LONGLONG dim1, LONGLONG dim2, LONGLONG naxis1,
LONGLONG naxis2, LONGLONG naxis3, DTYPE *array, > int *status)
5 Write an arbitrary data subsection into the data array.
int fits_write_subset_[byt, sht, usht, int, uint, lng, ulng, lnglng, flt, dbl] /
ffpss[b,i,ui,k,uk,j,uj,jj,e,d]
(fitsfile *fptr, long group, long naxis, long *naxes,
long *fpixel, long *lpixel, DTYPE *array, > int *status)
6 Read elements from the FITS data array.
int fits_read_img / ffgpv
(fitsfile *fptr, int datatype, long firstelem, long nelements,
DTYPE *nulval, > DTYPE *array, int *anynul, int *status)
int fits_read_img_[byt, sht, usht, int, uint, lng, ulng, lnglng, flt, dbl] /
ffgpv[b,i,ui,k,uk,j,uj,jj,e,d]
(fitsfile *fptr, long group, long firstelem, long nelements,
DTYPE nulval, > DTYPE *array, int *anynul, int *status)
int fits_read_imgnull / ffgpf
(fitsfile *fptr, int datatype, long firstelem, long nelements,
> DTYPE *array, char *nullarray, int *anynul, int *status)
int fits_read_imgnull_[byt, sht, usht, int, uint, lng, ulng, flt, dbl] /
ffgpf[b,i,ui,k,uk,j,uj,jj,e,d]
(fitsfile *fptr, long group, long firstelem, long nelements,
> DTYPE *array, char *nullarray, int *anynul, int *status)
7 Read values from group parameters. This routine only applies to the `Random Grouped' FITS
format which has been used for applications in radio interferometry, but is o#cially deprecated
for future use.
int fits_read_grppar_[byt, sht, usht, int, uint, lng, ulng, lnglng, flt, dbl] /
ffggp[b,i,ui,k,uk,j,uj,jj,e,d]
(fitsfile *fptr, long group, long firstelem, long nelements,
> DTYPE *array, int *status)

110 CHAPTER 9. SPECIALIZED CFITSIO INTERFACE ROUTINES
8 Read 2­D or 3­D image from the data array. Undefined pixels in the array will be set equal
to the value of 'nulval', unless nulval=0 in which case no testing for undefined pixels will be
performed.
int fits_read_2d_[byt, sht, usht, int, uint, lng, ulng, lnglng, flt, dbl] /
ffg2d[b,i,ui,k,uk,j,uj,jj,e,d]
(fitsfile *fptr, long group, DTYPE nulval, LONGLONG dim1, LONGLONG naxis1,
LONGLONG naxis2, > DTYPE *array, int *anynul, int *status)
int fits_read_3d_[byt, sht, usht, int, uint, lng, ulng, lnglng, flt, dbl] /
ffg3d[b,i,ui,k,uk,j,uj,jj,e,d]
(fitsfile *fptr, long group, DTYPE nulval, LONGLONG dim1,
LONGLONG dim2, LONGLONG naxis1, LONGLONG naxis2, LONGLONG naxis3,
> DTYPE *array, int *anynul, int *status)
9 Read an arbitrary data subsection from the data array.
int fits_read_subset_[byt, sht, usht, int, uint, lng, ulng, lnglng, flt, dbl] /
ffgsv[b,i,ui,k,uk,j,uj,jj,e,d]
(fitsfile *fptr, int group, int naxis, long *naxes,
long *fpixel, long *lpixel, long *inc, DTYPE nulval,
> DTYPE *array, int *anynul, int *status)
int fits_read_subsetnull_[byt, sht, usht, int, uint, lng, ulng, lnglng, flt, dbl] /
ffgsf[b,i,ui,k,uk,j,uj,jj,e,d]
(fitsfile *fptr, int group, int naxis, long *naxes,
long *fpixel, long *lpixel, long *inc, > DTYPE *array,
char *nullarray, int *anynul, int *status)
9.6 Specialized FITS ASCII and Binary Table Routines
9.6.1 General Column Routines
1 Get information about an existing ASCII or binary table column. A null pointer may be given
for any of the output parameters that are not needed. DATATYPE is a character string
which returns the data type of the column as defined by the TFORMn keyword (e.g., 'I',
'J','E', 'D', etc.). In the case of an ASCII character column, typecode will have a value of the
form 'An' where 'n' is an integer expressing the width of the field in characters. For example,
if TFORM = '160A8' then #gbcl will return typechar='A8' and repeat=20. All the returned
parameters are scalar quantities.
int fits_get_acolparms / ffgacl
(fitsfile *fptr, int colnum, > char *ttype, long *tbcol,
char *tunit, char *tform, double *scale, double *zero,

9.6. SPECIALIZED FITS ASCII AND BINARY TABLE ROUTINES 111
char *nulstr, char *tdisp, int *status)
int fits_get_bcolparms / ffgbcl
(fitsfile *fptr, int colnum, > char *ttype, char *tunit,
char *typechar, long *repeat, double *scale, double *zero,
long *nulval, char *tdisp, int *status)
int fits_get_bcolparmsll / ffgbclll
(fitsfile *fptr, int colnum, > char *ttype, char *tunit,
char *typechar, LONGLONG *repeat, double *scale, double *zero,
LONGLONG *nulval, char *tdisp, int *status)
2 Return optimal number of rows to read or write at one time for maximum I/O e#ciency. Refer
to the ``Optimizing Code'' section in Chapter 5 for more discussion on how to use this routine.
int fits_get_rowsize / ffgrsz
(fitsfile *fptr, long *nrows, *status)
3 Define the zero indexed byte o#set of the 'heap' measured from the start of the binary table data.
By default the heap is assumed to start immediately following the regular table data, i.e., at
location NAXIS1 x NAXIS2. This routine is only relevant for binary tables which contain
variable length array columns (with TFORMn = 'Pt'). This routine also automatically writes
the value of theap to a keyword in the extension header. This routine must be called after
the required keywords have been written (with #phbn) but before any data is written to the
table.
int fits_write_theap / ffpthp
(fitsfile *fptr, long theap, > int *status)
4 Test the contents of the binary table variable array heap, returning the size of the heap, the
number of unused bytes that are not currently pointed to by any of the descriptors, and the
number of bytes which are pointed to by multiple descriptors. It also returns valid = FALSE
if any of the descriptors point to invalid addresses out of range of the heap.
int fits_test_heap / fftheap
(fitsfile *fptr, > LONGLONG *heapsize, LONGLONG *unused, LONGLONG *overlap,
int *validheap, int *status)
5 Re­pack the vectors in the binary table variable array heap to recover any unused space. Nor­
mally, when a vector in a variable length array column is rewritten the previously written
array remains in the heap as wasted unused space. This routine will repack the arrays that
are still in use, thus eliminating any bytes in the heap that are no longer in use. Note that
if several vectors point to the same bytes in the heap, then this routine will make duplicate
copies of the bytes for each vector, which will actually expand the size of the heap.

112 CHAPTER 9. SPECIALIZED CFITSIO INTERFACE ROUTINES
int fits_compress_heap / ffcmph
(fitsfile *fptr, > int *status)
9.6.2 Low­Level Table Access Routines
The following 2 routines provide low­level access to the data in ASCII or binary tables and are
mainly useful as an e#cient way to copy all or part of a table from one location to another. These
routines simply read or write the specified number of consecutive bytes in an ASCII or binary table,
without regard for column boundaries or the row length in the table. These routines do not perform
any machine dependent data conversion or byte swapping. See Appendix B for the definition of
the parameters used in these routines.
1 Read or write a consecutive array of bytes from an ASCII or binary table
int fits_read_tblbytes / ffgtbb
(fitsfile *fptr, LONGLONG firstrow, LONGLONG firstchar, LONGLONG nchars,
> unsigned char *values, int *status)
int fits_write_tblbytes / ffptbb
(fitsfile *fptr, LONGLONG firstrow, LONGLONG firstchar, LONGLONG nchars,
unsigned char *values, > int *status)
9.6.3 Write Column Data Routines
1 Write elements into an ASCII or binary table column (in the CDU). The data type of the array
is implied by the su#x of the routine name.
int fits_write_col_str / ffpcls
(fitsfile *fptr, int colnum, LONGLONG firstrow, LONGLONG firstelem,
LONGLONG nelements, char **array, > int *status)
int fits_write_col_[log,byt,sht,usht,int,uint,lng,ulng,lnglng,flt,dbl,cmp,dblcmp] /
ffpcl[l,b,i,ui,k,uk,j,uj,jj,e,d,c,m]
(fitsfile *fptr, int colnum, LONGLONG firstrow,
LONGLONG firstelem, LONGLONG nelements, DTYPE *array, > int *status)
2 Write elements into an ASCII or binary table column substituting the appropriate FITS null
value for any elements that are equal to the nulval parameter.
int fits_write_colnull_[log, byt, sht, usht, int, uint, lng, ulng, lnglng, flt, dbl] /
ffpcn[l,b,i,ui,k,uk,j,uj,jj,e,d]
(fitsfile *fptr, int colnum, LONGLONG firstrow, LONGLONG firstelem,
LONGLONG nelements, DTYPE *array, DTYPE nulval, > int *status)

9.6. SPECIALIZED FITS ASCII AND BINARY TABLE ROUTINES 113
3 Write string elements into a binary table column (in the CDU) substituting the FITS null value
for any elements that are equal to the nulstr string.
int fits_write_colnull_str / ffpcns
(fitsfile *fptr, int colnum, LONGLONG firstrow, LONGLONG firstelem,
LONGLONG nelements, char **array, char *nulstr, > int *status)
4 Write bit values into a binary byte ('B') or bit ('X') table column (in the CDU). Larray is an
array of characters corresponding to the sequence of bits to be written. If an element of larray
is true (not equal to zero) then the corresponding bit in the FITS table is set to 1, otherwise
the bit is set to 0. The 'X' column in a FITS table is always padded out to a multiple of
8 bits where the bit array starts with the most significant bit of the byte and works down
towards the 1's bit. For example, a '4X' array, with the first bit = 1 and the remaining 3
bits = 0 is equivalent to the 8­bit unsigned byte decimal value of 128 ('1000 0000B'). In the
case of 'X' columns, CFITSIO can write to all 8 bits of each byte whether they are formally
valid or not. Thus if the column is defined as '4X', and one calls #pclx with firstbit=1 and
nbits=8, then all 8 bits will be written into the first byte (as opposed to writing the first 4
bits into the first row and then the next 4 bits into the next row), even though the last 4 bits
of each byte are formally not defined and should all be set = 0. It should also be noted that
it is more e#cient to write 'X' columns an entire byte at a time, instead of bit by bit. Any
of the CFITSIO routines that write to columns (e.g. fits write col byt) may be used for this
purpose. These routines will interpret 'X' columns as though they were 'B' columns (e.g.,
'1X' through '8X' is equivalent to '1B', and '9X' through '16X' is equivalent to '2B').
int fits_write_col_bit / ffpclx
(fitsfile *fptr, int colnum, LONGLONG firstrow, long firstbit,
long nbits, char *larray, > int *status)
5 Write the descriptor for a variable length column in a binary table. This routine can be used
in conjunction with #gdes to enable 2 or more arrays to point to the same storage location
to save storage space if the arrays are identical.
int fits_write_descript / ffpdes
(fitsfile *fptr, int colnum, LONGLONG rownum, LONGLONG repeat,
LONGLONG offset, > int *status)
9.6.4 Read Column Data Routines
Two types of routines are provided to get the column data which di#er in the way undefined pixels
are handled. The first set of routines (#gcv) simply return an array of data elements in which
undefined pixels are set equal to a value specified by the user in the 'nullval' parameter. If nullval
= 0, then no checks for undefined pixels will be performed, thus increasing the speed of the program.
The second set of routines (#gcf) returns the data element array and in addition a logical array
of flags which defines whether the corresponding data pixel is undefined. See Appendix B for the
definition of the parameters used in these routines.

114 CHAPTER 9. SPECIALIZED CFITSIO INTERFACE ROUTINES
Any column, regardless of it's intrinsic data type, may be read as a string. It should be noted
however that reading a numeric column as a string is 10 ­ 100 times slower than reading the same
column as a number due to the large overhead in constructing the formatted strings. The display
format of the returned strings will be determined by the TDISPn keyword, if it exists, otherwise by
the data type of the column. The length of the returned strings (not including the null terminating
character) can be determined with the fits get col display width routine. The following TDISPn
display formats are currently supported:
Iw.m Integer
Ow.m Octal integer
Zw.m Hexadecimal integer
Fw.d Fixed floating point
Ew.d Exponential floating point
Dw.d Exponential floating point
Gw.d General; uses Fw.d if significance not lost, else Ew.d
where w is the width in characters of the displayed values, m is the minimum number of digits
displayed, and d is the number of digits to the right of the decimal. The .m field is optional.
1 Read elements from an ASCII or binary table column (in the CDU). These routines return the
values of the table column array elements. Undefined array elements will be returned with a
value = nulval, unless nulval = 0 (or = ' ' for #gcvs) in which case no checking for undefined
values will be performed. The ANYF parameter is set to true if any of the returned elements
are undefined.
int fits_read_col_str / ffgcvs
(fitsfile *fptr, int colnum, LONGLONG firstrow, LONGLONG firstelem,
LONGLONG nelements, char *nulstr, > char **array, int *anynul,
int *status)
int fits_read_col_[log,byt,sht,usht,int,uint,lng,ulng, lnglng, flt, dbl, cmp, dblcmp] /
ffgcv[l,b,i,ui,k,uk,j,uj,jj,e,d,c,m]
(fitsfile *fptr, int colnum, LONGLONG firstrow, LONGLONG firstelem,
LONGLONG nelements, DTYPE nulval, > DTYPE *array, int *anynul,
int *status)
2 Read elements and null flags from an ASCII or binary table column (in the CHDU). These
routines return the values of the table column array elements. Any undefined array elements
will have the corresponding nullarray element set equal to TRUE. The anynul parameter is
set to true if any of the returned elements are undefined.
int fits_read_colnull_str / ffgcfs
(fitsfile *fptr, int colnum, LONGLONG firstrow, LONGLONG firstelem,
LONGLONG nelements, > char **array, char *nullarray, int *anynul,
int *status)

9.6. SPECIALIZED FITS ASCII AND BINARY TABLE ROUTINES 115
int fits_read_colnull_[log,byt,sht,usht,int,uint,lng,ulng,lnglng,flt,dbl,cmp,dblcmp] /
ffgcf[l,b,i,ui,k,uk,j,uj,jj,e,d,c,m]
(fitsfile *fptr, int colnum, LONGLONG firstrow,
LONGLONG firstelem, LONGLONG nelements, > DTYPE *array,
char *nullarray, int *anynul, int *status)
3 Read an arbitrary data subsection from an N­dimensional array in a binary table vector column.
Undefined pixels in the array will be set equal to the value of 'nulval', unless nulval=0 in which
case no testing for undefined pixels will be performed. The first and last rows in the table
to be read are specified by fpixel(naxis+1) and lpixel(naxis+1), and hence are treated as the
next higher dimension of the FITS N­dimensional array. The INC parameter specifies the
sampling interval in each dimension between the data elements that will be returned.
int fits_read_subset_[byt, sht, usht, int, uint, lng, ulng, lnglng, flt, dbl] /
ffgsv[b,i,ui,k,uk,j,uj,jj,e,d]
(fitsfile *fptr, int colnum, int naxis, long *naxes, long *fpixel,
long *lpixel, long *inc, DTYPE nulval, > DTYPE *array, int *anynul,
int *status)
4 Read an arbitrary data subsection from an N­dimensional array in a binary table vector column.
Any Undefined pixels in the array will have the corresponding 'nullarray' element set equal
to TRUE. The first and last rows in the table to be read are specified by fpixel(naxis+1)
and lpixel(naxis+1), and hence are treated as the next higher dimension of the FITS N­
dimensional array. The INC parameter specifies the sampling interval in each dimension
between the data elements that will be returned.
int fits_read_subsetnull_[byt, sht, usht, int, uint, lng, ulng, lnglng, flt, dbl] /
ffgsf[b,i,ui,k,uk,j,uj,jj,e,d]
(fitsfile *fptr, int colnum, int naxis, long *naxes,
long *fpixel, long *lpixel, long *inc, > DTYPE *array,
char *nullarray, int *anynul, int *status)
5 Read bit values from a byte ('B') or bit (`X`) table column (in the CDU). Larray is an array
of logical values corresponding to the sequence of bits to be read. If larray is true then the
corresponding bit was set to 1, otherwise the bit was set to 0. The 'X' column in a FITS
table is always padded out to a multiple of 8 bits where the bit array starts with the most
significant bit of the byte and works down towards the 1's bit. For example, a '4X' array,
with the first bit = 1 and the remaining 3 bits = 0 is equivalent to the 8­bit unsigned byte
value of 128. Note that in the case of 'X' columns, CFITSIO can read all 8 bits of each byte
whether they are formally valid or not. Thus if the column is defined as '4X', and one calls
#gcx with firstbit=1 and nbits=8, then all 8 bits will be read from the first byte (as opposed
to reading the first 4 bits from the first row and then the first 4 bits from the next row),
even though the last 4 bits of each byte are formally not defined. It should also be noted
that it is more e#cient to read 'X' columns an entire byte at a time, instead of bit by bit.

116 CHAPTER 9. SPECIALIZED CFITSIO INTERFACE ROUTINES
Any of the CFITSIO routines that read columns (e.g. fits read col byt) may be used for this
purpose. These routines will interpret 'X' columns as though they were 'B' columns (e.g.,
'8X' is equivalent to '1B', and '16X' is equivalent to '2B').
int fits_read_col_bit / ffgcx
(fitsfile *fptr, int colnum, LONGLONG firstrow, LONGLONG firstbit,
LONGLONG nbits, > char *larray, int *status)
6 Read any consecutive set of bits from an 'X' or 'B' column and interpret them as an unsigned
n­bit integer. nbits must be less than 16 or 32 in #gcxui and #gcxuk, respectively. If nrows
is greater than 1, then the same set of bits will be read from each row, starting with firstrow.
The bits are numbered with 1 = the most significant bit of the first element of the column.
int fits_read_col_bit_[usht, uint] / ffgcx[ui,uk]
(fitsfile *fptr, int colnum, LONGLONG firstrow, LONGLONG, nrows,
long firstbit, long nbits, > DTYPE *array, int *status)
7 Return the descriptor for a variable length column in a binary table. The descriptor consists of
2 integer parameters: the number of elements in the array and the starting o#set relative to
the start of the heap. The first pair of routine returns a single descriptor whereas the second
pair of routine returns the descriptors for a range of rows in the table. The only di#erence
between the 2 routines in each pair is that one returns the parameters as 'long' integers,
whereas the other returns the values as 64­bit 'LONGLONG' integers.
int fits_read_descript / ffgdes
(fitsfile *fptr, int colnum, LONGLONG rownum, > long *repeat,
long *offset, int *status)
int fits_read_descriptll / ffgdesll
(fitsfile *fptr, int colnum, LONGLONG rownum, > LONGLONG *repeat,
LONGLONG *offset, int *status)
int fits_read_descripts / ffgdess
(fitsfile *fptr, int colnum, LONGLONG firstrow, LONGLONG nrows
> long *repeat, long *offset, int *status)
int fits_read_descriptsll / ffgdessll
(fitsfile *fptr, int colnum, LONGLONG firstrow, LONGLONG nrows
> LONGLONG *repeat, LONGLONG *offset, int *status)

Chapter 10
Extended File Name Syntax
10.1 Overview
CFITSIO supports an extended syntax when specifying the name of the data file to be opened or
created that includes the following features:
. CFITSIO can read IRAF format images which have header file names that end with the
'.imh' extension, as well as reading and writing FITS files, This feature is implemented in
CFITSIO by first converting the IRAF image into a temporary FITS format file in memory,
then opening the FITS file. Any of the usual CFITSIO routines then may be used to read
the image header or data. Similarly, raw binary data arrays can be read by converting them
on the fly into virtual FITS images.
. FITS files on the Internet can be read (and sometimes written) using the FTP, HTTP, or
ROOT protocols.
. FITS files can be piped between tasks on the stdin and stdout streams.
. FITS files can be read and written in shared memory. This can potentially achieve better
data I/O performance compared to reading and writing the same FITS files on magnetic disk.
. Compressed FITS files in gzip or Unix COMPRESS format can be directly read.
. Output FITS files can be written directly in compressed gzip format, thus saving disk space.
. FITS table columns can be created, modified, or deleted 'on­the­fly' as the table is opened by
CFITSIO. This creates a virtual FITS file containing the modifications that is then opened
by the application program.
. Table rows may be selected, or filtered out, on the fly when the table is opened by CFITSIO,
based on an user­specified expression. Only rows for which the expression evaluates to 'TRUE'
are retained in the copy of the table that is opened by the application program.
. Histogram images may be created on the fly by binning the values in table columns, resulting
in a virtual N­dimensional FITS image. The application program then only sees the FITS
image (in the primary array) instead of the original FITS table.
117

118 CHAPTER 10. EXTENDED FILE NAME SYNTAX
The latter 3 table filtering features in particular add very powerful data processing capabilities
directly into CFITSIO, and hence into every task that uses CFITSIO to read or write FITS files.
For example, these features transform a very simple program that just copies an input FITS file to
a new output file (like the `fitscopy' program that is distributed with CFITSIO) into a multipurpose
FITS file processing tool. By appending fairly simple qualifiers onto the name of the input FITS
file, the user can perform quite complex table editing operations (e.g., create new columns, or
filter out rows in a table) or create FITS images by binning or histogramming the values in table
columns. In addition, these functions have been coded using new state­of­the art algorithms that
are, in some cases, 10 ­ 100 times faster than previous widely used implementations.
Before describing the complete syntax for the extended FITS file names in the next section, here
are a few examples of FITS file names that give a quick overview of the allowed syntax:
. myfile.fits: the simplest case of a FITS file on disk in the current directory.
. myfile.imh: opens an IRAF format image file and converts it on the fly into a temporary
FITS format image in memory which can then be read with any other CFITSIO routine.
. rawfile.dat[i512,512]: opens a raw binary data array (a 512 x 512 short integer array in
this case) and converts it on the fly into a temporary FITS format image in memory which
can then be read with any other CFITSIO routine.
. myfile.fits.gz: if this is the name of a new output file, the '.gz' su#x will cause it to be
compressed in gzip format when it is written to disk.
. myfile.fits.gz[events, 2]: opens and uncompresses the gzipped file myfile.fits then moves
to the extension with the keywords EXTNAME = 'EVENTS' and EXTVER = 2.
. ­: a dash (minus sign) signifies that the input file is to be read from the stdin file stream, or
that the output file is to be written to the stdout stream. See also the stream:// driver which
provides a more e#cient, but more restricted method of reading or writing to the stdin or
stdout streams.
. ftp://legacy.gsfc.nasa.gov/test/vela.fits: FITS files in any ftp archive site on the
Internet may be directly opened with read­only access.
. http://legacy.gsfc.nasa.gov/software/test.fits: any valid URL to a FITS file on the
Web may be opened with read­only access.
. root://legacy.gsfc.nasa.gov/test/vela.fits: similar to ftp access except that it pro­
vides write as well as read access to the files across the network. This uses the root protocol
developed at CERN.
. shmem://h2[events]: opens the FITS file in a shared memory segment and moves to the
EVENTS extension.
. mem://: creates a scratch output file in core computer memory. The resulting 'file' will
disappear when the program exits, so this is mainly useful for testing purposes when one does
not want a permanent copy of the output file.

10.1. OVERVIEW 119
. myfile.fits[3; Images(10)]: opens a copy of the image contained in the 10th row of the
'Images' column in the binary table in the 3th extension of the FITS file. The virtual file
that is opened by the application just contains this single image in the primary array.
. myfile.fits[1:512:2, 1:512:2]: opens a section of the input image ranging from the 1st
to the 512th pixel in X and Y, and selects every second pixel in both dimensions, resulting
in a 256 x 256 pixel input image in this case.
. myfile.fits[EVENTS][col Rad = sqrt(X**2 + Y**2)]: creates and opens a virtual file on
the fly that is identical to myfile.fits except that it will contain a new column in the EVENTS
extension called 'Rad' whose value is computed using the indicated expression which is a
function of the values in the X and Y columns.
. myfile.fits[EVENTS][PHA > 5]: creates and opens a virtual FITS files that is identical to
'myfile.fits' except that the EVENTS table will only contain the rows that have values of
the PHA column greater than 5. In general, any arbitrary boolean expression using a C or
Fortran­like syntax, which may combine AND and OR operators, may be used to select rows
from a table.
. myfile.fits[EVENTS][bin (X,Y)=1,2048,4]: creates a temporary FITS primary array im­
age which is computed on the fly by binning (i.e, computing the 2­dimensional histogram)
of the values in the X and Y columns of the EVENTS extension. In this case the X and Y
coordinates range from 1 to 2048 and the image pixel size is 4 units in both dimensions, so
the resulting image is 512 x 512 pixels in size.
. The final example combines many of these feature into one complex expression (it is broken
into several lines for clarity):
ftp://legacy.gsfc.nasa.gov/data/sample.fits.gz[EVENTS]
[col phacorr = pha * 1.1 ­ 0.3][phacorr >= 5.0 && phacorr <= 14.0]
[bin (X,Y)=32]
In this case, CFITSIO (1) copies and uncompresses the FITS file from the ftp site on the
legacy machine, (2) moves to the 'EVENTS' extension, (3) calculates a new column called
'phacorr', (4) selects the rows in the table that have phacorr in the range 5 to 14, and finally
(5) bins the remaining rows on the X and Y column coordinates, using a pixel size = 32 to
create a 2D image. All this processing is completely transparent to the application program,
which simply sees the final 2­D image in the primary array of the opened file.
The full extended CFITSIO FITS file name can contain several di#erent components depending on
the context. These components are described in the following sections:
When creating a new file:
filetype://BaseFilename(templateName)[compress]
When opening an existing primary array or image HDU:
filetype://BaseFilename(outName)[HDUlocation][ImageSection][pixFilter]

120 CHAPTER 10. EXTENDED FILE NAME SYNTAX
When opening an existing table HDU:
filetype://BaseFilename(outName)[HDUlocation][colFilter][rowFilter][binSpec]
The filetype, BaseFilename, outName, HDUlocation, ImageSection, and pixFilter components, if
present, must be given in that order, but the colFilter, rowFilter, and binSpec specifiers may follow
in any order. Regardless of the order, however, the colFilter specifier, if present, will be processed
first by CFITSIO, followed by the rowFilter specifier, and finally by the binSpec specifier.
10.2 Filetype
The type of file determines the medium on which the file is located (e.g., disk or network) and,
hence, which internal device driver is used by CFITSIO to read and/or write the file. Currently
supported types are
file:// ­ file on local magnetic disk (default)
ftp:// ­ a readonly file accessed with the anonymous FTP protocol.
It also supports ftp://username:password@hostname/...
for accessing password­protected ftp sites.
http:// ­ a readonly file accessed with the HTTP protocol. It
supports username:password just like the ftp driver.
Proxy HTTP servers are supported using the http_proxy
environment variable (see following note).
stream:// ­ special driver to read an input FITS file from the stdin
stream, and/or write an output FITS file to the stdout
stream. This driver is fragile and has limited
functionality (see the following note).
gsiftp:// ­ access files on a computational grid using the gridftp
protocol in the Globus toolkit (see following note).
root:// ­ uses the CERN root protocol for writing as well as
reading files over the network (see following note).
shmem:// ­ opens or creates a file which persists in the computer's
shared memory (see following note).
mem:// ­ opens a temporary file in core memory. The file
disappears when the program exits so this is mainly
useful for test purposes when a permanent output file
is not desired.
If the filetype is not specified, then type file:// is assumed. The double slashes '//' are optional
and may be omitted in most cases.
10.2.1 Notes about HTTP proxy servers
A proxy HTTP server may be used by defining the address (URL) and port number of the proxy
server with the http proxy environment variable. For example

10.2. FILETYPE 121
setenv http_proxy http://heasarc.gsfc.nasa.gov:3128
will cause CFITSIO to use port 3128 on the heasarc proxy server whenever reading a FITS file
with HTTP.
10.2.2 Notes about the stream filetype driver
The stream driver can be used to e#ciently read a FITS file from the stdin file stream or write
a FITS to the stdout file stream. However, because these input and output streams must be
accessed sequentially, the FITS file reading or writing application must also read and write the file
sequentially, at least within the tolerances described below.
CFITSIO supports 2 di#erent methods for accessing FITS files on the stdin and stdout streams.
The original method, which is invoked by specifying a dash character, ''­'', as the name of the file
when opening or creating it, works by storing a complete copy of the entire FITS file in memory.
In this case, when reading from stdin, CFITSIO will copy the entire stream into memory before
doing any processing of the file. Similarly, when writing to stdout, CFITSIO will create a copy of
the entire FITS file in memory, before finally flushing it out to the stdout stream when the FITS
file is closed. Bu#ering the entire FITS file in this way allows the application to randomly access
any part of the FITS file, in any order, but it also requires that the user have su#cient available
memory (or virtual memory) to store the entire file, which may not be possible in the case of very
large files.
The newer stream filetype provides a more memory­e#cient method of accessing FITS files on the
stdin or stdout streams. Instead of storing a copy of the entire FITS file in memory, CFITSIO
only uses a set of internal bu#er which by default can store 40 FITS blocks, or about 100K bytes
of the FITS file. The application program must process the FITS file sequentially from beginning
to end, within this 100K bu#er. Generally speaking the application program must conform to the
following restrictions:
. The program must finish reading or writing the header keywords before reading or writing
any data in the HDU.
. The HDU can contain at most about 1400 header keywords. This is the maximum that can fit
in the nominal 40 FITS block bu#er. In principle, this limit could be increased by recompiling
CFITSIO with a larger bu#er limit, which is set by the NIOBUF parameter in fitsio2.h.
. The program must read or write the data in a sequential manner from the beginning to the
end of the HDU. Note that CFITSIO's internal 100K bu#er allows a little latitude in meeting
this requirement.
. The program cannot move back to a previous HDU in the FITS file.
. Reading or writing of variable length array columns in binary tables is not supported on
streams, because this requires moving back and forth between the fixed­length portion of the
binary table and the following heap area where the arrays are actually stored.
. Reading or writing of tile­compressed images is not supported on streams, because the images
are internally stored using variable length arrays.

122 CHAPTER 10. EXTENDED FILE NAME SYNTAX
10.2.3 Notes about the gsiftp filetype
DEPENDENCIES: Globus toolkit (2.4.3 or higher) (GT) should be installed. There are two dif­
ferent ways to install GT:
1) goto the globus toolkit web page www.globus.org and follow the download and compilation
instructions;
2) goto the Virtual Data Toolkit web page http://vdt.cs.wisc.edu/ and follow the instructions
(STRONGLY SUGGESTED);
Once a globus client has been installed in your system with a specific flavour it is possible to compile
and install the CFITSIO libraries. Specific configuration flags must be used:
1) --with­gsiftp[[=PATH]] Enable Globus Toolkit gsiftp protocol support PATH=GLOBUS LOCATION
i.e. the location of your globus installation
2) --with­gsiftp­flavour[[=PATH] defines the specific Globus flavour ex. gcc32
Both the flags must be used and it is mandatory to set both the PATH and the flavour.
USAGE: To access files on a gridftp server it is necessary to use a gsiftp prefix:
example: gsiftp://remote server fqhn/directory/filename
The gridftp driver uses a local bu#er on a temporary file the file is located in the /tmp direc­
tory. If you have special permissions on /tmp or you do not have a /tmp directory, it is pos­
sible to force another location setting the GSIFTP TMPFILE environment variable (ex. export
GSIFTP TMPFILE=/your/location/yourtmpfile).
Grid FTP supports multi channel transfer. By default a single channel transmission is available.
However, it is possible to modify this behavior setting the GSIFTP STREAMS environment vari­
able (ex. export GSIFTP STREAMS=8).
10.2.4 Notes about the root filetype
The original rootd server can be obtained from: ftp://root.cern.ch/root/rootd.tar.gz but, for
it to work correctly with CFITSIO one has to use a modified version which supports a command
to return the length of the file. This modified version is available in rootd subdirectory in the
CFITSIO ftp area at
ftp://legacy.gsfc.nasa.gov/software/fitsio/c/root/rootd.tar.gz.
This small server is started either by inetd when a client requests a connection to a rootd server
or by hand (i.e. from the command line). The rootd server works with the ROOT TNetFile class.
It allows remote access to ROOT database files in either read or write mode. By default TNetFile
assumes port 432 (which requires rootd to be started as root). To run rootd via inetd add the
following line to /etc/services:
rootd 432/tcp
and to /etc/inetd.conf, add the following line:

10.2. FILETYPE 123
rootd stream tcp nowait root /user/rdm/root/bin/rootd rootd ­i
Force inetd to reread its conf file with kill ­HUP . You can also start rootd by hand
running directly under your private account (no root system privileges needed). For example to
start rootd listening on port 5151 just type: rootd ­p 5151 Notice that no & is needed. Rootd
will go into background by itself.
Rootd arguments:
­i says we were started by inetd
­p port# specifies a different port to listen on
­d level level of debug info written to syslog
0 = no debug (default)
1 = minimum
2 = medium
3 = maximum
Rootd can also be configured for anonymous usage (like anonymous ftp). To setup rootd to accept
anonymous logins do the following (while being logged in as root):
­ Add the following line to /etc/passwd:
rootd:*:71:72:Anonymous rootd:/var/spool/rootd:/bin/false
where you may modify the uid, gid (71, 72) and the home directory
to suite your system.
­ Add the following line to /etc/group:
rootd:*:72:rootd
where the gid must match the gid in /etc/passwd.
­ Create the directories:
mkdir /var/spool/rootd
mkdir /var/spool/rootd/tmp
chmod 777 /var/spool/rootd/tmp
Where /var/spool/rootd must match the rootd home directory as
specified in the rootd /etc/passwd entry.
­ To make writeable directories for anonymous do, for example:
mkdir /var/spool/rootd/pub
chown rootd:rootd /var/spool/rootd/pub

124 CHAPTER 10. EXTENDED FILE NAME SYNTAX
That's all. Several additional remarks: you can login to an anonymous server either with the
names ''anonymous'' or ''rootd''. The password should be of type user@host.do.main. Only the @
is enforced for the time being. In anonymous mode the top of the file tree is set to the rootd home
directory, therefore only files below the home directory can be accessed. Anonymous mode only
works when the server is started via inetd.
10.2.5 Notes about the shmem filetype:
Shared memory files are currently supported on most Unix platforms, where the shared memory
segments are managed by the operating system kernel and `live' independently of processes. They
are not deleted (by default) when the process which created them terminates, although they will
disappear if the system is rebooted. Applications can create shared memory files in CFITSIO by
calling:
fit_create_file(&fitsfileptr, "shmem://h2", &status);
where the root `file' names are currently restricted to be 'h0', 'h1', 'h2', 'h3', etc., up to a maximum
number defined by the the value of SHARED MAXSEG (equal to 16 by default). This is a prototype
implementation of the shared memory interface and a more robust interface, which will have fewer
restrictions on the number of files and on their names, may be developed in the future.
When opening an already existing FITS file in shared memory one calls the usual CFITSIO routine:
fits_open_file(&fitsfileptr, "shmem://h7", mode, &status)
The file mode can be READWRITE or READONLY just as with disk files. More than one process
can operate on READONLY mode files at the same time. CFITSIO supports proper file locking
(both in READONLY and READWRITE modes), so calls to fits open file may be locked out until
another other process closes the file.
When an application is finished accessing a FITS file in a shared memory segment, it may close it
(and the file will remain in the system) with fits close file, or delete it with fits delete file. Phys­
ical deletion is postponed until the last process calls #clos/#delt. fits delete file tries to obtain a
READWRITE lock on the file to be deleted, thus it can be blocked if the object was not opened
in READWRITE mode.
A shared memory management utility program called `smem', is included with the CFITSIO dis­
tribution. It can be built by typing `make smem'; then type `smem ­h' to get a list of valid options.
Executing smem without any options causes it to list all the shared memory segments currently
residing in the system and managed by the shared memory driver. To get a list of all the shared
memory objects, run the system utility program `ipcs [­a]'.
10.3 Base Filename
The base filename is the name of the file optionally including the director/subdirectory path, and
in the case of `ftp', `http', and `root' filetypes, the machine identifier. Examples:

10.3. BASE FILENAME 125
myfile.fits
!data.fits
/data/myfile.fits
fits.gsfc.nasa.gov/ftp/sampledata/myfile.fits.gz
When creating a new output file on magnetic disk (of type file://) if the base filename begins with
an exclamation point (!) then any existing file with that same basename will be deleted prior to
creating the new FITS file. Otherwise if the file to be created already exists, then CFITSIO will
return an error and will not overwrite the existing file. Note that the exclamation point, ' !', is a
special UNIX character, so if it is used on the command line rather than entered at a task prompt,
it must be preceded by a backslash to force the UNIX shell to pass it verbatim to the application
program.
If the output disk file name ends with the su#x '.gz', then CFITSIO will compress the file using the
gzip compression algorithm before writing it to disk. This can reduce the amount of disk space used
by the file. Note that this feature requires that the uncompressed file be constructed in memory
before it is compressed and written to disk, so it can fail if there is insu#cient available memory.
An input FITS file may be compressed with the gzip or Unix compress algorithms, in which
case CFITSIO will uncompress the file on the fly into a temporary file (in memory or on disk).
Compressed files may only be opened with read­only permission. When specifying the name of a
compressed FITS file it is not necessary to append the file su#x (e.g., `.gz' or `.Z'). If CFITSIO
cannot find the input file name without the su#x, then it will automatically search for a compressed
file with the same root name. In the case of reading ftp and http type files, CFITSIO generally
looks for a compressed version of the file first, before trying to open the uncompressed file. By
default, CFITSIO copies (and uncompressed if necessary) the ftp or http FITS file into memory
on the local machine before opening it. This will fail if the local machine does not have enough
memory to hold the whole FITS file, so in this case, the output filename specifier (see the next
section) can be used to further control how CFITSIO reads ftp and http files.
If the input file is an IRAF image file (*.imh file) then CFITSIO will automatically convert it on
the fly into a virtual FITS image before it is opened by the application program. IRAF images can
only be opened with READONLY file access.
Similarly, if the input file is a raw binary data array, then CFITSIO will convert it on the fly into
a virtual FITS image with the basic set of required header keywords before it is opened by the
application program (with READONLY access). In this case the data type and dimensions of the
image must be specified in square brackets following the filename (e.g. rawfile.dat[ib512,512]). The
first character (case insensitive) defines the data type of the array:
b 8­bit unsigned byte
i 16­bit signed integer
u 16­bit unsigned integer
j 32­bit signed integer
r or f 32­bit floating point
d 64­bit floating point
An optional second character specifies the byte order of the array values: b or B indicates big
endian (as in FITS files and the native format of SUN UNIX workstations and Mac PCs) and l or

126 CHAPTER 10. EXTENDED FILE NAME SYNTAX
L indicates little endian (native format of DEC OSF workstations and IBM PCs). If this character
is omitted then the array is assumed to have the native byte order of the local machine. These data
type characters are then followed by a series of one or more integer values separated by commas
which define the size of each dimension of the raw array. Arrays with up to 5 dimensions are
currently supported. Finally, a byte o#set to the position of the first pixel in the data file may be
specified by separating it with a ':' from the last dimension value. If omitted, it is assumed that
the o#set = 0. This parameter may be used to skip over any header information in the file that
precedes the binary data. Further examples:
raw.dat[b10000] 1­dimensional 10000 pixel byte array
raw.dat[rb400,400,12] 3­dimensional floating point big­endian array
img.fits[ib512,512:2880] reads the 512 x 512 short integer array in
a FITS file, skipping over the 2880 byte header
One special case of input file is where the filename = `­' (a dash or minus sign) or 'stdin' or 'stdout',
which signifies that the input file is to be read from the stdin stream, or written to the stdout stream
if a new output file is being created. In the case of reading from stdin, CFITSIO first copies the
whole stream into a temporary FITS file (in memory or on disk), and subsequent reading of the
FITS file occurs in this copy. When writing to stdout, CFITSIO first constructs the whole file in
memory (since random access is required), then flushes it out to the stdout stream when the file
is closed. In addition, if the output filename = '­.gz' or 'stdout.gz' then it will be gzip compressed
before being written to stdout.
This ability to read and write on the stdin and stdout steams allows FITS files to be piped between
tasks in memory rather than having to create temporary intermediate FITS files on disk. For
example if task1 creates an output FITS file, and task2 reads an input FITS file, the FITS file may
be piped between the 2 tasks by specifying
task1 ­ | task2 ­
where the vertical bar is the Unix piping symbol. This assumes that the 2 tasks read the name of
the FITS file o# of the command line.
10.4 Output File Name when Opening an Existing File
An optional output filename may be specified in parentheses immediately following the base file
name to be opened. This is mainly useful in those cases where CFITSIO creates a temporary copy
of the input FITS file before it is opened and passed to the application program. This happens by
default when opening a network FTP or HTTP­type file, when reading a compressed FITS file on
a local disk, when reading from the stdin stream, or when a column filter, row filter, or binning
specifier is included as part of the input file specification. By default this temporary file is created
in memory. If there is not enough memory to create the file copy, then CFITSIO will exit with an
error. In these cases one can force a permanent file to be created on disk, instead of a temporary
file in memory, by supplying the name in parentheses immediately following the base file name.
The output filename can include the ' !' clobber flag.

10.4. OUTPUT FILE NAME WHEN OPENING AN EXISTING FILE 127
Thus, if the input filename to CFITSIO is: file1.fits.gz(file2.fits) then CFITSIO will
uncompress `file1.fits.gz' into the local disk file `file2.fits' before opening it. CFITSIO does not
automatically delete the output file, so it will still exist after the application program exits.
The output filename ''mem://'' is also allowed, which will write the output file into memory, and
also allow write access to the file. This 'file' will disappear when it is closed, but this may be useful
for some applications which only need to modify a temporary copy of the file.
In some cases, several di#erent temporary FITS files will be created in sequence, for instance, if
one opens a remote file using FTP, then filters rows in a binary table extension, then create an
image by binning a pair of columns. In this case, the remote file will be copied to a temporary
local file, then a second temporary file will be created containing the filtered rows of the table,
and finally a third temporary file containing the binned image will be created. In cases like this
where multiple files are created, the outfile specifier will be interpreted the name of the final file as
described below, in descending priority:
. as the name of the final image file if an image within a single binary table cell is opened or if
an image is created by binning a table column.
. as the name of the file containing the filtered table if a column filter and/or a row filter are
specified.
. as the name of the local copy of the remote FTP or HTTP file.
. as the name of the uncompressed version of the FITS file, if a compressed FITS file on local
disk has been opened.
. otherwise, the output filename is ignored.
The output file specifier is useful when reading FTP or HTTP­type FITS files since it can be used
to create a local disk copy of the file that can be reused in the future. If the output file name =
`*' then a local file with the same name as the network file will be created. Note that CFITSIO
will behave di#erently depending on whether the remote file is compressed or not as shown by the
following examples:
. ftp://remote.machine/tmp/myfile.fits.gz(*) ­ the remote compressed file is copied to
the local compressed file `myfile.fits.gz', which is then uncompressed in local memory before
being opened and passed to the application program.
. ftp://remote.machine/tmp/myfile.fits.gz(myfile.fits) ­ the remote compressed file
is copied and uncompressed into the local file `myfile.fits'. This example requires less local
memory than the previous example since the file is uncompressed on disk instead of in memory.
. ftp://remote.machine/tmp/myfile.fits(myfile.fits.gz) ­ this will usually produce an
error since CFITSIO itself cannot compress files.
The exact behavior of CFITSIO in the latter case depends on the type of ftp server running on
the remote machine and how it is configured. In some cases, if the file `myfile.fits.gz' exists on the
remote machine, then the server will copy it to the local machine. In other cases the ftp server

128 CHAPTER 10. EXTENDED FILE NAME SYNTAX
will automatically create and transmit a compressed version of the file if only the uncompressed
version exists. This can get rather confusing, so users should use a certain amount of caution when
using the output file specifier with FTP or HTTP file types, to make sure they get the behavior
that they expect.
10.5 Template File Name when Creating a New File
When a new FITS file is created with a call to fits create file, the name of a template file may
be supplied in parentheses immediately following the name of the new file to be created. This
template is used to define the structure of one or more HDUs in the new file. The template file may
be another FITS file, in which case the newly created file will have exactly the same keywords in
each HDU as in the template FITS file, but all the data units will be filled with zeros. The template
file may also be an ASCII text file, where each line (in general) describes one FITS keyword record.
The format of the ASCII template file is described in the following Template Files chapter.
10.6 Image Tile­Compression Specification
When specifying the name of the output FITS file to be created, the user can indicate that images
should be written in tile­compressed format (see section 5.5, ``Primary Array or IMAGE Extension
I/O Routines'') by enclosing the compression parameters in square brackets following the root disk
file name. Here are some examples of the syntax for specifying tile­compressed output images:
myfile.fit[compress] ­ use Rice algorithm and default tile size
myfile.fit[compress GZIP] ­ use the specified compression algorithm;
myfile.fit[compress Rice] only the first letter of the algorithm
myfile.fit[compress PLIO] name is required.
myfile.fit[compress Rice 100,100] ­ use 100 x 100 pixel tile size
myfile.fit[compress Rice 100,100;2] ­ as above, and use noisebits = 2
10.7 HDU Location Specification
The optional HDU location specifier defines which HDU (Header­Data Unit, also known as an
`extension') within the FITS file to initially open. It must immediately follow the base file name
(or the output file name if present). If it is not specified then the first HDU (the primary array)
is opened. The HDU location specifier is required if the colFilter, rowFilter, or binSpec specifiers
are present, because the primary array is not a valid HDU for these operations. The HDU may be
specified either by absolute position number, starting with 0 for the primary array, or by reference
to the HDU name, and optionally, the version number and the HDU type of the desired extension.
The location of an image within a single cell of a binary table may also be specified, as described
below.

10.7. HDU LOCATION SPECIFICATION 129
The absolute position of the extension is specified either by enclosed the number in square brackets
(e.g., `[1]' = the first extension following the primary array) or by preceded the number with a plus
sign (`+1'). To specify the HDU by name, give the name of the desired HDU (the value of the
EXTNAME or HDUNAME keyword) and optionally the extension version number (value of the
EXTVER keyword) and the extension type (value of the XTENSION keyword: IMAGE, ASCII or
TABLE, or BINTABLE), separated by commas and all enclosed in square brackets. If the value
of EXTVER and XTENSION are not specified, then the first extension with the correct value of
EXTNAME is opened. The extension name and type are not case sensitive, and the extension
type may be abbreviated to a single letter (e.g., I = IMAGE extension or primary array, A or T =
ASCII table extension, and B = binary table BINTABLE extension). If the HDU location specifier
is equal to `[PRIMARY]' or `[P]', then the primary array (the first HDU) will be opened.
An optional pound sign character (''#'') may be appended to the extension name or number to
signify that any other extensions in the file should be ignored during any subsequent file filtering
operations. For example, when doing row filtering operations on a table extension, CFITSIO
normally creates a copy of the filtered table in memory, along with a verbatim copy of all the other
extensions in the input FITS file. If the pound sign is appended to the table extension name, then
only that extension, and none of the other extensions in the file, will by copied to memory, as in
the following example:
myfile.fit[events#][TIME > 10000]
FITS images are most commonly stored in the primary array or an image extension, but images
can also be stored as a vector in a single cell of a binary table (i.e. each row of the vector column
contains a di#erent image). Such an image can be opened with CFITSIO by specifying the desired
column name and the row number after the binary table HDU specifier as shown in the following
examples. The column name is separated from the HDU specifier by a semicolon and the row
number is enclosed in parentheses. In this case CFITSIO copies the image from the table cell into
a temporary primary array before it is opened. The application program then just sees the image
in the primary array, without any extensions. The particular row to be opened may be specified
either by giving an absolute integer row number (starting with 1 for the first row), or by specifying
a boolean expression that evaluates to TRUE for the desired row. The first row that satisfies the
expression will be used. The row selection expression has the same syntax as described in the Row
Filter Specifier section, below.
Examples:
myfile.fits[3] ­ open the 3rd HDU following the primary array
myfile.fits+3 ­ same as above, but using the FTOOLS­style notation
myfile.fits[EVENTS] ­ open the extension that has EXTNAME = 'EVENTS'
myfile.fits[EVENTS, 2] ­ same as above, but also requires EXTVER = 2
myfile.fits[events,2,b] ­ same, but also requires XTENSION = 'BINTABLE'
myfile.fits[3; images(17)] ­ opens the image in row 17 of the 'images'
column in the 3rd extension of the file.
myfile.fits[3; images(exposure > 100)] ­ as above, but opens the image
in the first row that has an 'exposure' column value
greater than 100.

130 CHAPTER 10. EXTENDED FILE NAME SYNTAX
10.8 Image Section
A virtual file containing a rectangular subsection of an image can be extracted and opened by
specifying the range of pixels (start:end) along each axis to be extracted from the original image.
One can also specify an optional pixel increment (start:end:step) for each axis of the input image.
A pixel step = 1 will be assumed if it is not specified. If the start pixel is larger then the end pixel,
then the image will be flipped (producing a mirror image) along that dimension. An asterisk, '*',
may be used to specify the entire range of an axis, and '­*' will flip the entire axis. The input
image can be in the primary array, in an image extension, or contained in a vector cell of a binary
table. In the later 2 cases the extension name or number must be specified before the image section
specifier.
Examples:
myfile.fits[1:512:2, 2:512:2] ­ open a 256x256 pixel image
consisting of the odd numbered columns (1st axis) and
the even numbered rows (2nd axis) of the image in the
primary array of the file.
myfile.fits[*, 512:256] ­ open an image consisting of all the columns
in the input image, but only rows 256 through 512.
The image will be flipped along the 2nd axis since
the starting pixel is greater than the ending pixel.
myfile.fits[*:2, 512:256:2] ­ same as above but keeping only
every other row and column in the input image.
myfile.fits[­*, *] ­ copy the entire image, flipping it along
the first axis.
myfile.fits[3][1:256,1:256] ­ opens a subsection of the image that
is in the 3rd extension of the file.
myfile.fits[4; images(12)][1:10,1:10] ­ open an image consisting
of the first 10 pixels in both dimensions. The original
image resides in the 12th row of the 'images' vector
column in the table in the 4th extension of the file.
When CFITSIO opens an image section it first creates a temporary file containing the image section
plus a copy of any other HDUs in the file. (If a `#' character is appended to the name or number
of the image HDU, as in ''myfile.fits[1#][1:200,1:200]'', then the other HDUs in the input file will
not be copied into memory). This temporary file is then opened by the application program, so
it is not possible to write to or modify the input file when specifying an image section. Note that
CFITSIO automatically updates the world coordinate system keywords in the header of the image
section, if they exist, so that the coordinate associated with each pixel in the image section will be
computed correctly.

10.9. IMAGE TRANSFORM FILTERS 131
10.9 Image Transform Filters
CFITSIO can apply a user­specified mathematical function to the value of every pixel in a FITS
image, thus creating a new virtual image in computer memory that is then opened and read by the
application program. The original FITS image is not modified by this process.
The image transformation specifier is appended to the input FITS file name and is enclosed in square
brackets. It begins with the letters 'PIX' to distinguish it from other types of FITS file filters that
are recognized by CFITSIO. The image transforming function may use any of the mathematical
operators listed in the following 'Row Filtering Specification' section of this document. Some
examples of image transform filters are:
[pix X * 2.0] ­ multiply each pixel by 2.0
[pix sqrt(X)] ­ take the square root of each pixel
[pix X + #ZEROPT ­ add the value of the ZEROPT keyword
[pix X>0 ? log10(X) : ­99.] ­ if the pixel value is greater
than 0, compute the base 10 log,
else set the pixel = ­99.
Use the letter 'X' in the expression to represent the current pixel value in the image. The expression
is evaluated independently for each pixel in the image and may be a function of 1) the original
pixel value, 2) the value of other pixels in the image at a given relative o#set from the position of
the pixel that is being evaluated, and 3) the value of any header keywords. Header keyword values
are represented by the name of the keyword preceded by the '#' sign.
To access the the value of adjacent pixels in the image, specify the (1­D) o#set from the current
pixel in curly brackets. For example
[pix (x{­1} + x + x{+1}) / 3]
will replace each pixel value with the running mean of the values of that pixel and it's 2 neighboring
pixels. Note that in this notation the image is treated as a 1­D array, where each row of the image
(or higher dimensional cube) is appended one after another in one long array of pixels. It is possible
to refer to pixels in the rows above or below the current pixel by using the value of the NAXIS1
header keyword. For example
[pix (x{­#NAXIS1} + x + x{#NAXIS1}) / 3]
will compute the mean of each image pixel and the pixels immediately above and below it in the
adjacent rows of the image. The following more complex example creates a smoothed virtual image
where each pixel is a 3 x 3 boxcar average of the input image pixels:
[pix (X + X{­1} + X{+1}
+ X{­#NAXIS1} + X{­#NAXIS1 ­ 1} + X{­#NAXIS1 + 1}
+ X{#NAXIS1} + X{#NAXIS1 ­ 1} + X{#NAXIS1 + 1}) / 9.]

132 CHAPTER 10. EXTENDED FILE NAME SYNTAX
If the pixel o#set extends beyond the first or last pixel in the image, the function will evaluate to
undefined, or NULL.
For complex or commonly used image filtering operations, one can write the expression into an
external text file and then import it into the filter using the syntax '[pix @filename.txt]'. The
mathematical expression can extend over multiple lines of text in the file. Any lines in the external
text file that begin with 2 slash characters ('//') will be ignored and may be used to add comments
into the file.
By default, the datatype of the resulting image will be the same as the original image, but one may
force a di#erent datatype by appended a code letter to the 'pix' keyword:
pixb ­ 8­bit byte image with BITPIX = 8
pixi ­ 16­bit integer image with BITPIX = 16
pixj ­ 32­bit integer image with BITPIX = 32
pixr ­ 32­bit float image with BITPIX = ­32
pixd ­ 64­bit float image with BITPIX = ­64
Also by default, any other HDUs in the input file will be copied without change to the output virtual
FITS file, but one may discard the other HDUs by adding the number '1' to the 'pix' keyword (and
following any optional datatype code letter). For example:
myfile.fits[3][pixr1 sqrt(X)]
will create a virtual FITS file containing only a primary array image with 32­bit floating point pixels
that have a value equal to the square root of the pixels in the image that is in the 3rd extension of
the 'myfile.fits' file.
10.10 Column and Keyword Filtering Specification
The optional column/keyword filtering specifier is used to modify the column structure and/or the
header keywords in the HDU that was selected with the previous HDU location specifier. This
filtering specifier must be enclosed in square brackets and can be distinguished from a general
row filter specifier (described below) by the fact that it begins with the string 'col ' and is not
immediately followed by an equals sign. The original file is not changed by this filtering operation,
and instead the modifications are made on a copy of the input FITS file (usually in memory), which
also contains a copy of all the other HDUs in the file. (If a `#' character is appended to the name
or number of the table HDU then only the primary array, and none of the other HDUs in the input
file will be copied into memory). This temporary file is passed to the application program and will
persist only until the file is closed or until the program exits, unless the outfile specifier (see above)
is also supplied.
The column/keyword filter can be used to perform the following operations. More than one oper­
ation may be specified by separating them with commas or semi­colons.
. Copy only a specified list of columns columns to the filtered input file. The list of column
name should be separated by semi­colons. Wild card characters may be used in the column

10.10. COLUMN AND KEYWORD FILTERING SPECIFICATION 133
names to match multiple columns. If the expression contains both a list of columns to be
included and columns to be deleted, then all the columns in the original table except the
explicitly deleted columns will appear in the filtered table (i.e., there is no need to explicitly
list the columns to be included if any columns are being deleted).
. Delete a column or keyword by listing the name preceded by a minus sign or an exclamation
mark (!), e.g., '­TIME' will delete the TIME column if it exists, otherwise the TIME keyword.
An error is returned if neither a column nor keyword with this name exists. Note that the
exclamation point, ' !', is a special UNIX character, so if it is used on the command line rather
than entered at a task prompt, it must be preceded by a backslash to force the UNIX shell
to ignore it.
. Rename an existing column or keyword with the syntax 'NewName == OldName'. An error
is returned if neither a column nor keyword with this name exists.
. Append a new column or keyword to the table. To create a column, give the new name,
optionally followed by the data type in parentheses, followed by a single equals sign and an
expression to be used to compute the value (e.g., 'newcol(1J) = 0' will create a new 32­bit
integer column called 'newcol' filled with zeros). The data type is specified using the same
syntax that is allowed for the value of the FITS TFORMn keyword (e.g., 'I', 'J', 'E', 'D',
etc. for binary tables, and 'I8', F12.3', 'E20.12', etc. for ASCII tables). If the data type
is not specified then an appropriate data type will be chosen depending on the form of the
expression (may be a character string, logical, bit, long integer, or double column). An
appropriate vector count (in the case of binary tables) will also be added if not explicitly
specified.
When creating a new keyword, the keyword name must be preceded by a pound sign '#', and
the expression must evaluate to a scalar (i.e., cannot have a column name in the expression).
The comment string for the keyword may be specified in parentheses immediately following
the keyword name (instead of supplying a data type as in the case of creating a new column).
If the keyword name ends with a pound sign '#', then cfitsio will substitute the number of the
most recently referenced column for the # character . This is especially useful when writing
a column­related keyword like TUNITn for a newly created column, as shown in the following
examples.
. Recompute (overwrite) the values in an existing column or keyword by giving the name
followed by an equals sign and an arithmetic expression.
The expression that is used when appending or recomputing columns or keywords can be arbitrarily
complex and may be a function of other header keyword values and other columns (in the same
row). The full syntax and available functions for the expression are described below in the row
filter specification section.
If the expression contains both a list of columns to be included and columns to be deleted, then all
the columns in the original table except the explicitly deleted columns will appear in the filtered
table. If no columns to be deleted are specified, then only the columns that are explicitly listed will
be included in the filtered output table. To include all the columns, add the '*' wildcard specifier
at the end of the list, as shown in the examples.

134 CHAPTER 10. EXTENDED FILE NAME SYNTAX
For complex or commonly used operations, one can place the operations into an external text file
and import it into the column filter using the syntax '[col @filename.txt]'. The operations can
extend over multiple lines of the file, but multiple operations must still be separated by semicolons.
Any lines in the external text file that begin with 2 slash characters ('//') will be ignored and may
be used to add comments into the file.
Examples:
[col Time; rate] ­ only the Time and rate columns will
appear in the filtered input file.
[col Time, *raw] ­ include the Time column and any other
columns whose name ends with 'raw'.
[col ­TIME; Good == STATUS] ­ deletes the TIME column and
renames the status column to 'Good'
[col PI=PHA * 1.1 + 0.2; #TUNIT#(column units) = 'counts';*]
­ creates new PI column from PHA values
and also writes the TUNITn keyword
for the new column. The final '*'
expression means preserve all the
columns in the input table in the
virtual output table; without the '*'
the output table would only contain
the single 'PI' column.
[col rate = rate/exposure; TUNIT#(&) = 'counts/s';*]
­ recomputes the rate column by dividing
it by the EXPOSURE keyword value. This
also modifies the value of the TUNITn
keyword for this column. The use of the
'&' character for the keyword comment
string means preserve the existing
comment string for that keyword. The
final '*' preserves all the columns
in the input table in the virtual
output table.
10.11 Row Filtering Specification
When entering the name of a FITS table that is to be opened by a program, an optional row filter
may be specified to select a subset of the rows in the table. A temporary new FITS file is created
on the fly which contains only those rows for which the row filter expression evaluates to true.
The primary array and any other extensions in the input file are also copied to the temporary file.
(If a `#' character is appended to the name or number of the table HDU then only the primary

10.11. ROW FILTERING SPECIFICATION 135
array, and none of the other HDUs in the input file will be copied into the temporary file). The
original FITS file is closed and the new virtual file is opened by the application program. The row
filter expression is enclosed in square brackets following the file name and extension name (e.g.,
'file.fits[events][GRADE==50]' selects only those rows where the GRADE column value equals 50).
When dealing with tables where each row has an associated time and/or 2D spatial position, the
row filter expression can also be used to select rows based on the times in a Good Time Intervals
(GTI) extension, or on spatial position as given in a SAO­style region file.
10.11.1 General Syntax
The row filtering expression can be an arbitrarily complex series of operations performed on con­
stants, keyword values, and column data taken from the specified FITS TABLE extension. The
expression must evaluate to a boolean value for each row of the table, where a value of FALSE
means that the row will be excluded.
For complex or commonly used filters, one can place the expression into a text file and import it
into the row filter using the syntax '[@filename.txt]'. The expression can be arbitrarily complex
and extend over multiple lines of the file. Any lines in the external text file that begin with 2 slash
characters ('//') will be ignored and may be used to add comments into the file.
Keyword and column data are referenced by name. Any string of characters not surrounded by
quotes (ie, a constant string) or followed by an open parentheses (ie, a function name) will be
initially interpreted as a column name and its contents for the current row inserted into the ex­
pression. If no such column exists, a keyword of that name will be searched for and its value used,
if found. To force the name to be interpreted as a keyword (in case there is both a column and
keyword with the same name), precede the keyword name with a single pound sign, '#', as in
'#NAXIS2'. Due to the generalities of FITS column and keyword names, if the column or keyword
name contains a space or a character which might appear as an arithmetic term then enclose the
name in '$' characters as in $MAX PHA$ or #$MAX­PHA$. Names are case insensitive.
To access a table entry in a row other than the current one, follow the column's name with a row
o#set within curly braces. For example, 'PHA{­3}' will evaluate to the value of column PHA, 3
rows above the row currently being processed. One cannot specify an absolute row number, only a
relative o#set. Rows that fall outside the table will be treated as undefined, or NULLs.
Boolean operators can be used in the expression in either their Fortran or C forms. The following
boolean operators are available:
"equal" .eq. .EQ. == "not equal" .ne. .NE. !=
"less than" .lt. .LT. < "less than/equal" .le. .LE. <= =<
"greater than" .gt. .GT. > "greater than/equal" .ge. .GE. >= =>
"or" .or. .OR. || "and" .and. .AND. &&
"negation" .not. .NOT. ! "approx. equal(1e­7)" ~
Note that the exclamation point, ' !', is a special UNIX character, so if it is used on the command
line rather than entered at a task prompt, it must be preceded by a backslash to force the UNIX
shell to ignore it.

136 CHAPTER 10. EXTENDED FILE NAME SYNTAX
The expression may also include arithmetic operators and functions. Trigonometric functions use
radians, not degrees. The following arithmetic operators and functions can be used in the expression
(function names are case insensitive). A null value will be returned in case of illegal operations
such as divide by zero, sqrt(negative) log(negative), log10(negative), arccos(.gt. 1), arcsin(.gt. 1).
"addition" + "subtraction" ­
"multiplication" * "division" /
"negation" ­ "exponentiation" ** ^
"absolute value" abs(x) "cosine" cos(x)
"sine" sin(x) "tangent" tan(x)
"arc cosine" arccos(x) "arc sine" arcsin(x)
"arc tangent" arctan(x) "arc tangent" arctan2(y,x)
"hyperbolic cos" cosh(x) "hyperbolic sin" sinh(x)
"hyperbolic tan" tanh(x) "round to nearest int" round(x)
"round down to int" floor(x) "round up to int" ceil(x)
"exponential" exp(x) "square root" sqrt(x)
"natural log" log(x) "common log" log10(x)
"modulus" x % y "random # [0.0,1.0)" random()
"random Gaussian" randomn() "random Poisson" randomp(x)
"minimum" min(x,y) "maximum" max(x,y)
"cumulative sum" accum(x) "sequential difference" seqdiff(x)
"if­then­else" b?x:y
"angular separation" angsep(ra1,dec1,ra2,de2) (all in degrees)
"substring" strmid(s,p,n) "string search" strstr(s,r)
Three di#erent random number functions are provided: random(), with no arguments, produces a
uniform random deviate between 0 and 1; randomn(), also with no arguments, produces a normal
(Gaussian) random deviate with zero mean and unit standard deviation; randomp(x) produces a
Poisson random deviate whose expected number of counts is X. X may be any positive real number
of expected counts, including fractional values, but the return value is an integer.
When the random functions are used in a vector expression, by default the same random value will
be used when evaluating each element of the vector. If di#erent random numbers are desired, then
the name of a vector column should be supplied as the single argument to the random function
(e.g., ''flux + 0.1 * random(flux)'', where ''flux' is the name of a vector column). This will create a
vector of random numbers that will be used in sequence when evaluating each element of the vector
expression.
An alternate syntax for the min and max functions has only a single argument which should be a
vector value (see below). The result will be the minimum/maximum element contained within the
vector.
The accum(x) function forms the cumulative sum of x, element by element. Vector columns are
supported simply by performing the summation process through all the values. Null values are
treated as 0. The seqdi#(x) function forms the sequential di#erence of x, element by element. The
first value of seqdi# is the first value of x. A single null value in x causes a pair of nulls in the
output. The seqdi# and accum functions are functional inverses, i.e., seqdi#(accum(x)) == x as
long as no null values are present.

10.11. ROW FILTERING SPECIFICATION 137
In the if­then­else expression, ''b?x:y'', b is an explicit boolean value or expression. There is no
automatic type conversion from numeric to boolean values, so one needs to use ''iVal!=0'' instead
of merely ''iVal'' as the boolean argument. x and y can be any scalar data type (including string).
The angsep function computes the angular separation in degrees between 2 celestial positions, where
the first 2 parameters give the RA­like and Dec­like coordinates (in decimal degrees) of the first
position, and the 3rd and 4th parameters give the coordinates of the second position.
The substring function strmid(S,P,N) extracts a substring from S, starting at string position P,
with a substring length N. The first character position in S is labeled as 1. If P is 0, or refers to
a position beyond the end of S, then the extracted substring will be NULL. S, P, and N may be
functions of other columns.
The string search function strstr(S,R) searches for the first occurrence of the substring R in S.
The result is an integer, indicating the character position of the first match (where 1 is the first
character position of S). If no match is found, then strstr() returns a NULL value.
The following type casting operators are available, where the inclosing parentheses are required
and taken from the C language usage. Also, the integer to real casts values to double precision:
"real to integer" (int) x (INT) x
"integer to real" (float) i (FLOAT) i
In addition, several constants are built in for use in numerical expressions:
#pi 3.1415... #e 2.7182...
#deg #pi/180 #row current row number
#null undefined value #snull undefined string
A string constant must be enclosed in quotes as in 'Crab'. The ''null'' constants are useful for
conditionally setting table values to a NULL, or undefined, value (eg., ''col1==­99 ? #NULL :
col1'').
There is also a function for testing if two values are close to each other, i.e., if they are ''near'' each
other to within a user specified tolerance. The arguments, value 1 and value 2 can be integer or real
and represent the two values who's proximity is being tested to be within the specified tolerance,
also an integer or real:
near(value_1, value_2, tolerance)
When a NULL, or undefined, value is encountered in the FITS table, the expression will evaluate to
NULL unless the undefined value is not actually required for evaluation, e.g. ''TRUE .or. NULL''
evaluates to TRUE. The following two functions allow some NULL detection and handling:
"a null value?" ISNULL(x)
"define a value for null" DEFNULL(x,y)
The former returns a boolean value of TRUE if the argument x is NULL. The later ''defines'' a
value to be substituted for NULL values; it returns the value of x if x is not NULL, otherwise it
returns the value of y.

138 CHAPTER 10. EXTENDED FILE NAME SYNTAX
10.11.2 Bit Masks
Bit masks can be used to select out rows from bit columns (TFORMn = #X) in FITS files. To
represent the mask, binary, octal, and hex formats are allowed:
binary: b0110xx1010000101xxxx0001
octal: o720x1 ­> (b111010000xxx001)
hex: h0FxD ­> (b00001111xxxx1101)
In all the representations, an x or X is allowed in the mask as a wild card. Note that the x represents
a di#erent number of wild card bits in each representation. All representations are case insensitive.
To construct the boolean expression using the mask as the boolean equal operator described above
on a bit table column. For example, if you had a 7 bit column named flags in a FITS table and
wanted all rows having the bit pattern 0010011, the selection expression would be:
flags == b0010011
or
flags .eq. b10011
It is also possible to test if a range of bits is less than, less than equal, greater than and greater
than equal to a particular boolean value:
flags <= bxxx010xx
flags .gt. bxxx100xx
flags .le. b1xxxxxxx
Notice the use of the x bit value to limit the range of bits being compared.
It is not necessary to specify the leading (most significant) zero (0) bits in the mask, as shown in
the second expression above.
Bit wise AND, OR and NOT operations are also possible on two or more bit fields using the
'&'(AND), '|'(OR), and the ' !'(NOT) operators. All of these operators result in a bit field which
can then be used with the equal operator. For example:
(!flags) == b1101100
(flags & b1000001) == bx000001
Bit fields can be appended as well using the '+' operator. Strings can be concatenated this way,
too.
10.11.3 Vector Columns
Vector columns can also be used in building the expression. No special syntax is required if one
wants to operate on all elements of the vector. Simply use the column name as for a scalar

10.11. ROW FILTERING SPECIFICATION 139
column. Vector columns can be freely intermixed with scalar columns or constants in virtually all
expressions. The result will be of the same dimension as the vector. Two vectors in an expression,
though, need to have the same number of elements and have the same dimensions.
Arithmetic and logical operations are all performed on an element by element basis. Comparing two
vector columns, eg ''COL1 == COL2'', thus results in another vector of boolean values indicating
which elements of the two vectors are equal.
Eight functions are available that operate on a vector and return a scalar result:
"minimum" MIN(V) "maximum" MAX(V)
"average" AVERAGE(V) "median" MEDIAN(V)
"summation" SUM(V) "standard deviation" STDDEV(V)
"# of values" NELEM(V) "# of non­null values" NVALID(V)
where V represents the name of a vector column or a manually constructed vector using curly
brackets as described below. The first 6 of these functions ignore any null values in the vector when
computing the result. The STDDEV() function computes the sample standard deviation, i.e. it is
proportional to 1/SQRT(N­1) instead of 1/SQRT(N), where N is NVALID(V).
The SUM function literally sums all the elements in x, returning a scalar value. If V is a boolean
vector, SUM returns the number of TRUE elements. The NELEM function returns the number
of elements in vector V whereas NVALID return the number of non­null elements in the vector.
(NELEM also operates on bit and string columns, returning their column widths.) As an example,
to test whether all elements of two vectors satisfy a given logical comparison, one can use the
expression
SUM( COL1 > COL2 ) == NELEM( COL1 )
which will return TRUE if all elements of COL1 are greater than their corresponding elements in
COL2.
To specify a single element of a vector, give the column name followed by a comma­separated list
of coordinates enclosed in square brackets. For example, if a vector column named PHAS exists in
the table as a one dimensional, 256 component list of numbers from which you wanted to select the
57th component for use in the expression, then PHAS[57] would do the trick. Higher dimensional
arrays of data may appear in a column. But in order to interpret them, the TDIMn keyword
must appear in the header. Assuming that a (4,4,4,4) array is packed into each row of a column
named ARRAY4D, the (1,2,3,4) component element of each row is accessed by ARRAY4D[1,2,3,4].
Arrays up to dimension 5 are currently supported. Each vector index can itself be an expression,
although it must evaluate to an integer value within the bounds of the vector. Vector columns
which contain spaces or arithmetic operators must have their names enclosed in ''$'' characters as
with $ARRAY­4D$[1,2,3,4].
A more C­like syntax for specifying vector indices is also available. The element used in the
preceding example alternatively could be specified with the syntax ARRAY4D[4][3][2][1]. Note the
reverse order of indices (as in C), as well as the fact that the values are still ones­based (as in
Fortran -- adopted to avoid ambiguity for 1D vectors). With this syntax, one does not need to
specify all of the indices. To extract a 3D slice of this 4D array, use ARRAY4D[4].

140 CHAPTER 10. EXTENDED FILE NAME SYNTAX
Variable­length vector columns are not supported.
Vectors can be manually constructed within the expression using a comma­separated list of elements
surrounded by curly braces ('{}'). For example, '{1,3,6,1}' is a 4­element vector containing the
values 1, 3, 6, and 1. The vector can contain only boolean, integer, and real values (or expressions).
The elements will be promoted to the highest data type present. Any elements which are themselves
vectors, will be expanded out with each of its elements becoming an element in the constructed
vector.
10.11.4 Good Time Interval Filtering
A common filtering method involves selecting rows which have a time value which lies within
what is called a Good Time Interval or GTI. The time intervals are defined in a separate FITS
table extension which contains 2 columns giving the start and stop time of each good interval.
The filtering operation accepts only those rows of the input table which have an associated time
which falls within one of the time intervals defined in the GTI extension. A high level function,
gtifilter(a,b,c,d), is available which evaluates each row of the input table and returns TRUE or
FALSE depending whether the row is inside or outside the good time interval. The syntax is
gtifilter( [ "gtifile" [, expr [, "STARTCOL", "STOPCOL" ] ] ] )
or
gtifilter( [ 'gtifile' [, expr [, 'STARTCOL', 'STOPCOL' ] ] ] )
where each ''[]'' demarks optional parameters. Note that the quotes around the gtifile and START/STOP
column are required. Either single or double quotes may be used. In cases where this expression
is entered on the Unix command line, enclose the entire expression in double quotes, and then use
single quotes within the expression to enclose the 'gtifile' and other terms. It is also usually possible
to do the reverse, and enclose the whole expression in single quotes and then use double quotes
within the expression. The gtifile, if specified, can be blank ('''') which will mean to use the first
extension with the name ''*GTI*'' in the current file, a plain extension specifier (eg, ''+2'', ''[2]'',
or ''[STDGTI]'') which will be used to select an extension in the current file, or a regular filename
with or without an extension specifier which in the latter case will mean to use the first extension
with an extension name ''*GTI*''. Expr can be any arithmetic expression, including simply the
time column name. A vector time expression will produce a vector boolean result. STARTCOL
and STOPCOL are the names of the START/STOP columns in the GTI extension. If one of them
is specified, they both must be.
In its simplest form, no parameters need to be provided -- default values will be used. The expression
''gtifilter()'' is equivalent to
gtifilter( "", TIME, "*START*", "*STOP*" )
This will search the current file for a GTI extension, filter the TIME column in the current table,
using START/STOP times taken from columns in the GTI extension with names containing the
strings ''START'' and ''STOP''. The wildcards ('*') allow slight variations in naming conventions
such as ''TSTART'' or ''STARTTIME''. The same default values apply for unspecified parame­
ters when the first one or two parameters are specified. The function automatically searches for

10.11. ROW FILTERING SPECIFICATION 141
TIMEZERO/I/F keywords in the current and GTI extensions, applying a relative time o#set, if
necessary.
10.11.5 Spatial Region Filtering
Another common filtering method selects rows based on whether the spatial position associated
with each row is located within a given 2­dimensional region. The syntax for this high­level filter
is
regfilter( "regfilename" [ , Xexpr, Yexpr [ , "wcs cols" ] ] )
where each ''[]'' demarks optional parameters. The region file name is required and must be enclosed
in quotes. The remaining parameters are optional. There are 2 supported formats for the region
file: ASCII file or FITS binary table. The region file contains a list of one or more geometric
shapes (circle, ellipse, box, etc.) which defines a region on the celestial sphere or an area within a
particular 2D image. The region file is typically generated using an image display program such
as fv/POW (distribute by the HEASARC), or ds9 (distributed by the Smithsonian Astrophysical
Observatory). Users should refer to the documentation provided with these programs for more
details on the syntax used in the region files. The FITS region file format is defined in a document
available from the FITS Support O#ce at http://fits.gsfc.nasa.gov/ registry/ region.html
In its simplest form, (e.g., regfilter(''region.reg'') ) the coordinates in the default 'X' and 'Y' columns
will be used to determine if each row is inside or outside the area specified in the region file.
Alternate position column names, or expressions, may be entered if needed, as in
regfilter("region.reg", XPOS, YPOS)
Region filtering can be applied most unambiguously if the positions in the region file and in the
table to be filtered are both give in terms of absolute celestial coordinate units. In this case the
locations and sizes of the geometric shapes in the region file are specified in angular units on the sky
(e.g., positions given in R.A. and Dec. and sizes in arcseconds or arcminutes). Similarly, each row
of the filtered table will have a celestial coordinate associated with it. This association is usually
implemented using a set of so­called 'World Coordinate System' (or WCS) FITS keywords that
define the coordinate transformation that must be applied to the values in the 'X' and 'Y' columns
to calculate the coordinate.
Alternatively, one can perform spatial filtering using unitless 'pixel' coordinates for the regions and
row positions. In this case the user must be careful to ensure that the positions in the 2 files are
self­consistent. A typical problem is that the region file may be generated using a binned image,
but the unbinned coordinates are given in the event table. The ROSAT events files, for example,
have X and Y pixel coordinates that range from 1 ­ 15360. These coordinates are typically binned
by a factor of 32 to produce a 480x480 pixel image. If one then uses a region file generated from
this image (in image pixel units) to filter the ROSAT events file, then the X and Y column values
must be converted to corresponding pixel units as in:
regfilter("rosat.reg", X/32.+.5, Y/32.+.5)

142 CHAPTER 10. EXTENDED FILE NAME SYNTAX
Note that this binning conversion is not necessary if the region file is specified using celestial
coordinate units instead of pixel units because CFITSIO is then able to directly compare the
celestial coordinate of each row in the table with the celestial coordinates in the region file without
having to know anything about how the image may have been binned.
The last ''wcs cols'' parameter should rarely be needed. If supplied, this string contains the names
of the 2 columns (space or comma separated) which have the associated WCS keywords. If not
supplied, the filter will scan the X and Y expressions for column names. If only one is found in
each expression, those columns will be used, otherwise an error will be returned.
These region shapes are supported (names are case insensitive):
Point ( X1, Y1 ) <­ One pixel square region
Line ( X1, Y1, X2, Y2 ) <­ One pixel wide region
Polygon ( X1, Y1, X2, Y2, ... ) <­ Rest are interiors with
Rectangle ( X1, Y1, X2, Y2, A ) | boundaries considered
Box ( Xc, Yc, Wdth, Hght, A ) V within the region
Diamond ( Xc, Yc, Wdth, Hght, A )
Circle ( Xc, Yc, R )
Annulus ( Xc, Yc, Rin, Rout )
Ellipse ( Xc, Yc, Rx, Ry, A )
Elliptannulus ( Xc, Yc, Rinx, Riny, Routx, Routy, Ain, Aout )
Sector ( Xc, Yc, Amin, Amax )
where (Xc,Yc) is the coordinate of the shape's center; (X#,Y#) are the coordinates of the shape's
edges; Rxxx are the shapes' various Radii or semimajor/minor axes; and Axxx are the angles of
rotation (or bounding angles for Sector) in degrees. For rotated shapes, the rotation angle can be
left o#, indicating no rotation. Common alternate names for the regions can also be used: rotbox
= box; rotrectangle = rectangle; (rot)rhombus = (rot)diamond; and pie = sector. When a shape's
name is preceded by a minus sign, '­', the defined region is instead the area *outside* its boundary
(ie, the region is inverted). All the shapes within a single region file are OR'd together to create
the region, and the order is significant. The overall way of looking at region files is that if the first
region is an excluded region then a dummy included region of the whole detector is inserted in the
front. Then each region specification as it is processed overrides any selections inside of that region
specified by previous regions. Another way of thinking about this is that if a previous excluded
region is completely inside of a subsequent included region the excluded region is ignored.
The positional coordinates may be given either in pixel units, decimal degrees or hh:mm:ss.s,
dd:mm:ss.s units. The shape sizes may be given in pixels, degrees, arcminutes, or arcseconds. Look
at examples of region file produced by fv/POW or ds9 for further details of the region file format.
There are three low­level functions that are primarily for use with regfilter function, but they can
be called directly. They return a boolean true or false depending on whether a two dimensional
point is in the region or not. The positional coordinates must be given in pixel units:
"point in a circular region"
circle(xcntr,ycntr,radius,Xcolumn,Ycolumn)

10.11. ROW FILTERING SPECIFICATION 143
"point in an elliptical region"
ellipse(xcntr,ycntr,xhlf_wdth,yhlf_wdth,rotation,Xcolumn,Ycolumn)
"point in a rectangular region"
box(xcntr,ycntr,xfll_wdth,yfll_wdth,rotation,Xcolumn,Ycolumn)
where
(xcntr,ycntr) are the (x,y) position of the center of the region
(xhlf_wdth,yhlf_wdth) are the (x,y) half widths of the region
(xfll_wdth,yfll_wdth) are the (x,y) full widths of the region
(radius) is half the diameter of the circle
(rotation) is the angle(degrees) that the region is rotated with
respect to (xcntr,ycntr)
(Xcoord,Ycoord) are the (x,y) coordinates to test, usually column
names
NOTE: each parameter can itself be an expression, not merely a
column name or constant.
10.11.6 Example Row Filters
[ binary && mag <= 5.0] ­ Extract all binary stars brighter
than fifth magnitude (note that
the initial space is necessary to
prevent it from being treated as a
binning specification)
[#row >= 125 && #row <= 175] ­ Extract row numbers 125 through 175
[IMAGE[4,5] .gt. 100] ­ Extract all rows that have the
(4,5) component of the IMAGE column
greater than 100
[abs(sin(theta * #deg)) < 0.5] ­ Extract all rows having the
absolute value of the sine of theta
less than a half where the angles
are tabulated in degrees
[SUM( SPEC > 3*BACKGRND )>=1] ­ Extract all rows containing a
spectrum, held in vector column
SPEC, with at least one value 3
times greater than the background
level held in a keyword, BACKGRND
[VCOL=={1,4,2}] ­ Extract all rows whose vector column
VCOL contains the 3­elements 1, 4, and
2.

144 CHAPTER 10. EXTENDED FILE NAME SYNTAX
[@rowFilter.txt] ­ Extract rows using the expression
contained within the text file
rowFilter.txt
[gtifilter()] ­ Search the current file for a GTI
extension, filter the TIME
column in the current table, using
START/STOP times taken from
columns in the GTI extension
[regfilter("pow.reg")] ­ Extract rows which have a coordinate
(as given in the X and Y columns)
within the spatial region specified
in the pow.reg region file.
[regfilter("pow.reg", Xs, Ys)] ­ Same as above, except that the
Xs and Ys columns will be used to
determine the coordinate of each
row in the table.
10.12 Binning or Histogramming Specification
The optional binning specifier is enclosed in square brackets and can be distinguished from a general
row filter specification by the fact that it begins with the keyword 'bin' not immediately followed
by an equals sign. When binning is specified, a temporary N­dimensional FITS primary array
is created by computing the histogram of the values in the specified columns of a FITS table
extension. After the histogram is computed the input FITS file containing the table is then closed
and the temporary FITS primary array is opened and passed to the application program. Thus,
the application program never sees the original FITS table and only sees the image in the new
temporary file (which has no additional extensions). Obviously, the application program must be
expecting to open a FITS image and not a FITS table in this case.
The data type of the FITS histogram image may be specified by appending 'b' (for 8­bit byte), 'i'
(for 16­bit integers), 'j' (for 32­bit integer), 'r' (for 32­bit floating points), or 'd' (for 64­bit double
precision floating point) to the 'bin' keyword (e.g. '[binr X]' creates a real floating point image).
If the data type is not explicitly specified then a 32­bit integer image will be created by default,
unless the weighting option is also specified in which case the image will have a 32­bit floating point
data type by default.
The histogram image may have from 1 to 4 dimensions (axes), depending on the number of columns
that are specified. The general form of the binning specification is:
[bin{bijrd} Xcol=min:max:binsize, Ycol= ..., Zcol=..., Tcol=...; weight]
in which up to 4 columns, each corresponding to an axis of the image, are listed. The column
names are case insensitive, and the column number may be given instead of the name, preceded by

10.12. BINNING OR HISTOGRAMMING SPECIFICATION 145
a pound sign (e.g., [bin #4=1:512]). If the column name is not specified, then CFITSIO will first
try to use the 'preferred column' as specified by the CPREF keyword if it exists (e.g., 'CPREF
= 'DETX,DETY'), otherwise column names 'X', 'Y', 'Z', and 'T' will be assumed for each of
the 4 axes, respectively. In cases where the column name could be confused with an arithmetic
expression, enclose the column name in parentheses to force the name to be interpreted literally.
Each column name may be followed by an equals sign and then the lower and upper range of the
histogram, and the size of the histogram bins, separated by colons. Spaces are allowed before and
after the equals sign but not within the 'min:max:binsize' string. The min, max and binsize values
may be integer or floating point numbers, or they may be the names of keywords in the header of
the table. If the latter, then the value of that keyword is substituted into the expression.
Default values for the min, max and binsize quantities will be used if not explicitly given in the
binning expression as shown in these examples:
[bin x = :512:2] ­ use default minimum value
[bin x = 1::2] ­ use default maximum value
[bin x = 1:512] ­ use default bin size
[bin x = 1:] ­ use default maximum value and bin size
[bin x = :512] ­ use default minimum value and bin size
[bin x = 2] ­ use default minimum and maximum values
[bin x] ­ use default minimum, maximum and bin size
[bin 4] ­ default 2­D image, bin size = 4 in both axes
[bin] ­ default 2­D image
CFITSIO will use the value of the TLMINn, TLMAXn, and TDBINn keywords, if they exist, for
the default min, max, and binsize, respectively. If they do not exist then CFITSIO will use the
actual minimum and maximum values in the column for the histogram min and max values. The
default binsize will be set to 1, or (max ­ min) / 10., whichever is smaller, so that the histogram
will have at least 10 bins along each axis.
A shortcut notation is allowed if all the columns/axes have the same binning specification. In
this case all the column names may be listed within parentheses, followed by the (single) binning
specification, as in:
[bin (X,Y)=1:512:2]
[bin (X,Y) = 5]
The optional weighting factor is the last item in the binning specifier and, if present, is separated
from the list of columns by a semi­colon. As the histogram is accumulated, this weight is used
to incremented the value of the appropriated bin in the histogram. If the weighting factor is not
specified, then the default weight = 1 is assumed. The weighting factor may be a constant integer or
floating point number, or the name of a keyword containing the weighting value. Or the weighting
factor may be the name of a table column in which case the value in that column, on a row by row
basis, will be used.
In some cases, the column or keyword may give the reciprocal of the actual weight value that is
needed. In this case, precede the weight keyword or column name by a slash '/' to tell CFITSIO
to use the reciprocal of the value when constructing the histogram.

146 CHAPTER 10. EXTENDED FILE NAME SYNTAX
For complex or commonly used histograms, one can also place its description into a text file and im­
port it into the binning specification using the syntax [bin @filename.txt]. The file's contents can ex­
tend over multiple lines, although it must still conform to the no­spaces rule for the min:max:binsize
syntax and each axis specification must still be comma­separated. Any lines in the external text
file that begin with 2 slash characters ('//') will be ignored and may be used to add comments into
the file.
Examples:
[bini detx, dety] ­ 2­D, 16­bit integer histogram
of DETX and DETY columns, using
default values for the histogram
range and binsize
[bin (detx, dety)=16; /exposure] ­ 2­D, 32­bit real histogram of DETX
and DETY columns with a bin size = 16
in both axes. The histogram values
are divided by the EXPOSURE keyword
value.
[bin time=TSTART:TSTOP:0.1] ­ 1­D lightcurve, range determined by
the TSTART and TSTOP keywords,
with 0.1 unit size bins.
[bin pha, time=8000.:8100.:0.1] ­ 2­D image using default binning
of the PHA column for the X axis,
and 1000 bins in the range
8000. to 8100. for the Y axis.
[bin @binFilter.txt] ­ Use the contents of the text file
binFilter.txt for the binning
specifications.

Chapter 11
Template Files
When a new FITS file is created with a call to fits create file, the name of a template file may
be supplied in parentheses immediately following the name of the new file to be created. This
template is used to define the structure of one or more HDUs in the new file. The template file may
be another FITS file, in which case the newly created file will have exactly the same keywords in
each HDU as in the template FITS file, but all the data units will be filled with zeros. The template
file may also be an ASCII text file, where each line (in general) describes one FITS keyword record.
The format of the ASCII template file is described in the following sections.
11.1 Detailed Template Line Format
The format of each ASCII template line closely follows the format of a FITS keyword record:
KEYWORD = KEYVALUE / COMMENT
except that free format may be used (e.g., the equals sign may appear at any position in the line) and
TAB characters are allowed and are treated the same as space characters. The KEYVALUE and
COMMENT fields are optional. The equals sign character is also optional, but it is recommended
that it be included for clarity. Any template line that begins with the pound '#' character is
ignored by the template parser and may be use to insert comments into the template file itself.
The KEYWORD name field is limited to 8 characters in length and only the letters A­Z, digits 0­9,
and the hyphen and underscore characters may be used, without any embedded spaces. Lowercase
letters in the template keyword name will be converted to uppercase. Leading spaces in the template
line preceding the keyword name are generally ignored, except if the first 8 characters of a template
line are all blank, then the entire line is treated as a FITS comment keyword (with a blank keyword
name) and is copied verbatim into the FITS header.
The KEYVALUE field may have any allowed FITS data type: character string, logical, integer,
real, complex integer, or complex real. The character string values need not be enclosed in single
quote characters unless they are necessary to distinguish the string from a di#erent data type (e.g.
2.0 is a real but '2.0' is a string). The keyword has an undefined (null) value if the template record
only contains blanks following the ''='' or between the ''='' and the ''/'' comment field delimiter.
147

148 CHAPTER 11. TEMPLATE FILES
String keyword values longer than 68 characters (the maximum length that will fit in a single FITS
keyword record) are permitted using the CFITSIO long string convention. They can either be
specified as a single long line in the template, or by using multiple lines where the continuing lines
contain the 'CONTINUE' keyword, as in this example:
LONGKEY = 'This is a long string value that is contin&'
CONTINUE 'ued over 2 records' / comment field goes here
The format of template lines with CONTINUE keyword is very strict: 3 spaces must follow CON­
TINUE and the rest of the line is copied verbatim to the FITS file.
The start of the optional COMMENT field must be preceded by ''/'', which is used to separate it
from the keyword value field. Exceptions are if the KEYWORD name field contains COMMENT,
HISTORY, CONTINUE, or if the first 8 characters of the template line are blanks.
More than one Header­Data Unit (HDU) may be defined in the template file. The start of an HDU
definition is denoted with a SIMPLE or XTENSION template line:
1) SIMPLE begins a Primary HDU definition. SIMPLE may only appear as the first keyword in
the template file. If the template file begins with XTENSION instead of SIMPLE, then a default
empty Primary HDU is created, and the template is then assumed to define the keywords starting
with the first extension following the Primary HDU.
2) XTENSION marks the beginning of a new extension HDU definition. The previous HDU will
be closed at this point and processing of the next extension begins.
11.2 Auto­indexing of Keywords
If a template keyword name ends with a ''#'' character, it is said to be 'auto­indexed'. Each ''#''
character will be replaced by the current integer index value, which gets reset = 1 at the start of
each new HDU in the file (or 7 in the special case of a GROUP definition). The FIRST indexed
keyword in each template HDU definition is used as the 'incrementor'; each subsequent occurrence
of this SAME keyword will cause the index value to be incremented. This behavior can be rather
subtle, as illustrated in the following examples in which the TTYPE keyword is the incrementor in
both cases:
TTYPE# = TIME
TFORM# = 1D
TTYPE# = RATE
TFORM# = 1E
will create TTYPE1, TFORM1, TTYPE2, and TFORM2 keywords. But if the template looks like,
TTYPE# = TIME
TTYPE# = RATE
TFORM# = 1D
TFORM# = 1E

11.3. TEMPLATE PARSER DIRECTIVES 149
this results in a FITS files with TTYPE1, TTYPE2, TFORM2, and TFORM2, which is probably
not what was intended!
11.3 Template Parser Directives
In addition to the template lines which define individual keywords, the template parser recognizes
3 special directives which are each preceded by the backslash character: \include, \group, and
\end.
The 'include' directive must be followed by a filename. It forces the parser to temporarily stop
reading the current template file and begin reading the include file. Once the parser reaches the
end of the include file it continues parsing the current template file. Include files can be nested,
and HDU definitions can span multiple template files.
The start of a GROUP definition is denoted with the 'group' directive, and the end of a GROUP
definition is denoted with the 'end' directive. Each GROUP contains 0 or more member blocks
(HDUs or GROUPs). Member blocks of type GROUP can contain their own member blocks.
The GROUP definition itself occupies one FITS file HDU of special type (GROUP HDU), so if a
template specifies 1 group with 1 member HDU like:
\group
grpdescr = 'demo'
xtension bintable
# this bintable has 0 cols, 0 rows
\end
then the parser creates a FITS file with 3 HDUs :
1) dummy PHDU
2) GROUP HDU (has 1 member, which is bintable in HDU number 3)
3) bintable (member of GROUP in HDU number 2)
Technically speaking, the GROUP HDU is a BINTABLE with 6 columns. Applications can define
additional columns in a GROUP HDU using TFORMn and TTYPEn (where n is 7, 8, ....) keywords
or their auto­indexing equivalents.
For a more complicated example of a template file using the group directives, look at the sample.tpl
file that is included in the CFITSIO distribution.
11.4 Formal Template Syntax
The template syntax can formally be defined as follows:
TEMPLATE = BLOCK [ BLOCK ... ]

150 CHAPTER 11. TEMPLATE FILES
BLOCK = { HDU | GROUP }
GROUP = \GROUP [ BLOCK ... ] \END
HDU = XTENSION [ LINE ... ] { XTENSION | \GROUP | \END | EOF }
LINE = [ KEYWORD [ = ] ] [ VALUE ] [ / COMMENT ]
X ... ­ X can be present 1 or more times
{ X | Y } ­ X or Y
[ X ] ­ X is optional
At the topmost level, the template defines 1 or more template blocks. Blocks can be either HDU
(Header Data Unit) or a GROUP. For each block the parser creates 1 (or more for GROUPs) FITS
file HDUs.
11.5 Errors
In general the fits execute template() function tries to be as atomic as possible, so either everything
is done or nothing is done. If an error occurs during parsing of the template, fits execute template()
will (try to) delete the top level BLOCK (with all its children if any) in which the error occurred,
then it will stop reading the template file and it will return with an error.
11.6 Examples
1. This template file will create a 200 x 300 pixel image, with 4­byte integer pixel values, in the
primary HDU:
SIMPLE = T
BITPIX = 32
NAXIS = 2 / number of dimensions
NAXIS1 = 100 / length of first axis
NAXIS2 = 200 / length of second axis
OBJECT = NGC 253 / name of observed object
The allowed values of BITPIX are 8, 16, 32, ­32, or ­64, representing, respectively, 8­bit integer,
16­bit integer, 32­bit integer, 32­bit floating point, or 64 bit floating point pixels.
2. To create a FITS table, the template first needs to include XTENSION = TABLE or BINTABLE
to define whether it is an ASCII or binary table, and NAXIS2 to define the number of rows in the
table. Two template lines are then needed to define the name (TTYPEn) and FITS data format
(TFORMn) of the columns, as in this example:
xtension = bintable

11.6. EXAMPLES 151
naxis2 = 40
ttype# = Name
tform# = 10a
ttype# = Npoints
tform# = j
ttype# = Rate
tunit# = counts/s
tform# = e
The above example defines a null primary array followed by a 40­row binary table extension with 3
columns called 'Name', 'Npoints', and 'Rate', with data formats of '10A' (ASCII character string),
'1J' (integer) and '1E' (floating point), respectively. Note that the other required FITS keywords
(BITPIX, NAXIS, NAXIS1, PCOUNT, GCOUNT, TFIELDS, and END) do not need to be ex­
plicitly defined in the template because their values can be inferred from the other keywords in
the template. This example also illustrates that the templates are generally case­insensitive (the
keyword names and TFORMn values are converted to upper­case in the FITS file) and that string
keyword values generally do not need to be enclosed in quotes.

152 CHAPTER 11. TEMPLATE FILES

Chapter 12
Local FITS Conventions
CFITSIO supports several local FITS conventions which are not defined in the o#cial NOST FITS
standard and which are not necessarily recognized or supported by other FITS software packages.
Programmers should be cautious about using these features, especially if the FITS files that are
produced are expected to be processed by other software systems which do not use the CFITSIO
interface.
12.1 64­Bit Long Integers
CFITSIO supports reading and writing FITS images or table columns containing 64­bit integer
data values. Support for 64­bit integers was added to the o#cial FITS Standard in December 2005.
FITS 64­bit images have BITPIX = 64, and the 64­bit binary table columns have TFORMn = 'K'.
CFITSIO also supports the 'Q' variable­length array table column format which is analogous to
the 'P' column format except that the array descriptor is stored as a pair of 64­bit integers.
For the convenience of C programmers, the fitsio.h include file defines (with a typedef statement)
the 'LONGLONG' datatype to be equivalent to an appropriate 64­bit integer datatype on each
platform. Since there is currently no universal standard for the name of the 64­bit integer datatype
(it might be defined as 'long long', 'long', or ' int64' depending on the platform) C programmers
may prefer to use the 'LONGLONG' datatype when declaring or allocating 64­bit integer quantities
when writing code which needs to run on multiple platforms. Note that CFITSIO will implicitly
convert the datatype when reading or writing FITS 64­bit integer images and columns with data
arrays of a di#erent integer or floating point datatype, but there is an increased risk of loss of
numerical precision or numerical overflow in this case.
12.2 Long String Keyword Values.
The length of a standard FITS string keyword is limited to 68 characters because it must fit
entirely within a single FITS header keyword record. In some instances it is necessary to encode
strings longer than this limit, so CFITSIO supports a local convention in which the string value is
continued over multiple keywords. This continuation convention uses an ampersand character at
153

154 CHAPTER 12. LOCAL FITS CONVENTIONS
the end of each substring to indicate that it is continued on the next keyword, and the continuation
keywords all have the name CONTINUE without an equal sign in column 9. The string value
may be continued in this way over as many additional CONTINUE keywords as is required. The
following lines illustrate this continuation convention which is used in the value of the STRKEY
keyword:
LONGSTRN= 'OGIP 1.0' / The OGIP Long String Convention may be used.
STRKEY = 'This is a very long string keyword&' / Optional Comment
CONTINUE ' value that is continued over 3 keywords in the & '
CONTINUE 'FITS header.' / This is another optional comment.
It is recommended that the LONGSTRN keyword, as shown here, always be included in any HDU
that uses this longstring convention as a warning to any software that must read the keywords. A
routine called fits write key longwarn has been provided in CFITSIO to write this keyword if it
does not already exist.
This long string convention is supported by the following CFITSIO routines:
fits_write_key_longstr ­ write a long string keyword value
fits_insert_key_longstr ­ insert a long string keyword value
fits_modify_key_longstr ­ modify a long string keyword value
fits_update_key_longstr ­ modify a long string keyword value
fits_read_key_longstr ­ read a long string keyword value
fits_delete_key ­ delete a keyword
The fits read key longstr routine is unique among all the CFITSIO routines in that it internally
allocates memory for the long string value; all the other CFITSIO routines that deal with arrays
require that the calling program pre­allocate adequate space to hold the array of data. Conse­
quently, programs which use the fits read key longstr routine must be careful to free the allocated
memory for the string when it is no longer needed.
The following 2 routines also have limited support for this long string convention,
fits_modify_key_str ­ modify an existing string keyword value
fits_update_key_str ­ update a string keyword value
in that they will correctly overwrite an existing long string value, but the new string value is limited
to a maximum of 68 characters in length.
The more commonly used CFITSIO routines to write string valued keywords (fits update key and
fits write key) do not support this long string convention and only support strings up to 68 char­
acters in length. This has been done deliberately to prevent programs from inadvertently writing
keywords using this non­standard convention without the explicit intent of the programmer or user.
The fits write key longstr routine must be called instead to write long strings. This routine can
also be used to write ordinary string values less than 68 characters in length.

12.3. ARRAYS OF FIXED­LENGTH STRINGS IN BINARY TABLES 155
12.3 Arrays of Fixed­Length Strings in Binary Tables
CFITSIO supports 2 ways to specify that a character column in a binary table contains an array of
fixed­length strings. The first way, which is o#cially supported by the FITS Standard document,
uses the TDIMn keyword. For example, if TFORMn = '60A' and TDIMn = '(12,5)' then that
column will be interpreted as containing an array of 5 strings, each 12 characters long.
CFITSIO also supports a local convention for the format of the TFORMn keyword value of the
form 'rAw' where 'r' is an integer specifying the total width in characters of the column, and
'w' is an integer specifying the (fixed) length of an individual unit string within the vector. For
example, TFORM1 = '120A10' would indicate that the binary table column is 120 characters wide
and consists of 12 10­character length strings. This convention is recognized by the CFITSIO
routines that read or write strings in binary tables. The Binary Table definition document specifies
that other optional characters may follow the data type code in the TFORM keyword, so this local
convention is in compliance with the FITS standard although other FITS readers may not recognize
this convention.
The Binary Table definition document that was approved by the IAU in 1994 contains an appendix
describing an alternate convention for specifying arrays of fixed or variable length strings in a binary
table character column (with the form 'rA:SSTRw/nnn)'. This appendix was not o#cially voted
on by the IAU and hence is still provisional. CFITSIO does not currently support this proposal.
12.4 Keyword Units Strings
One limitation of the current FITS Standard is that it does not define a specific convention for
recording the physical units of a keyword value. The TUNITn keyword can be used to specify the
physical units of the values in a table column, but there is no analogous convention for keyword
values. The comment field of the keyword is often used for this purpose, but the units are usually
not specified in a well defined format that FITS readers can easily recognize and extract.
To solve this problem, CFITSIO uses a local convention in which the keyword units are enclosed in
square brackets as the first token in the keyword comment field; more specifically, the opening square
bracket immediately follows the slash '/' comment field delimiter and a single space character. The
following examples illustrate keywords that use this convention:
EXPOSURE= 1800.0 / [s] elapsed exposure time
V_HELIO = 16.23 / [km s**(­1)] heliocentric velocity
LAMBDA = 5400. / [angstrom] central wavelength
FLUX = 4.9033487787637465E­30 / [J/cm**2/s] average flux
In general, the units named in the IAU(1988) Style Guide are recommended, with the main excep­
tion that the preferred unit for angle is 'deg' for degrees.
The fits read key unit and fits write key unit routines in CFITSIO read and write, respectively,
the keyword unit strings in an existing keyword.

156 CHAPTER 12. LOCAL FITS CONVENTIONS
12.5 HIERARCH Convention for Extended Keyword Names
CFITSIO supports the HIERARCH keyword convention which allows keyword names that are
longer then 8 characters and may contain the full range of printable ASCII text characters. This
convention was developed at the European Southern Observatory (ESO) to support hierarchical
FITS keyword such as:
HIERARCH ESO INS FOCU POS = ­0.00002500 / Focus position
Basically, this convention uses the FITS keyword 'HIERARCH' to indicate that this conven­
tion is being used, then the actual keyword name ('ESO INS FOCU POS' in this example) be­
gins in column 10 and can contain any printable ASCII text characters, including spaces. The
equals sign marks the end of the keyword name and is followed by the usual value and com­
ment fields just as in standard FITS keywords. Further details of this convention are described at
http://arcdev.hq.eso.org/dicb/dicd/dic­1­1.4.html (search for HIERARCH).
This convention allows a much broader range of keyword names than is allowed by the FITS
Standard. Here are more examples of such keywords:
HIERARCH LongKeyword = 47.5 / Keyword has > 8 characters, and mixed case
HIERARCH XTE$TEMP = 98.6 / Keyword contains the '$' character
HIERARCH Earth is a star = F / Keyword contains embedded spaces
CFITSIO will transparently read and write these keywords, so application programs do not in
general need to know anything about the specific implementation details of the HIERARCH con­
vention. In particular, application programs do not need to specify the `HIERARCH' part of the
keyword name when reading or writing keywords (although it may be included if desired). When
writing a keyword, CFITSIO first checks to see if the keyword name is legal as a standard FITS
keyword (no more than 8 characters long and containing only letters, digits, or a minus sign or
underscore). If so it writes it as a standard FITS keyword, otherwise it uses the hierarch con­
vention to write the keyword. The maximum keyword name length is 67 characters, which leaves
only 1 space for the value field. A more practical limit is about 40 characters, which leaves enough
room for most keyword values. CFITSIO returns an error if there is not enough room for both the
keyword name and the keyword value on the 80­character card, except for string­valued keywords
which are simply truncated so that the closing quote character falls in column 80. In the current
implementation, CFITSIO preserves the case of the letters when writing the keyword name, but it
is case­insensitive when reading or searching for a keyword. The current implementation allows any
ASCII text character (ASCII 32 to ASCII 126) in the keyword name except for the '=' character.
A space is also required on either side of the equal sign.
12.6 Tile­Compressed Image Format
CFITSIO supports a convention for compressing n­dimensional images and storing the resulting
byte stream in a variable­length column in a FITS binary table. The general principle used in
this convention is to first divide the n­dimensional image into a rectangular grid of subimages or

12.6. TILE­COMPRESSED IMAGE FORMAT 157
`tiles'. Each tile is then compressed as a continuous block of data, and the resulting compressed
byte stream is stored in a row of a variable length column in a FITS binary table. By dividing
the image into tiles it is generally possible to extract and uncompress subsections of the image
without having to uncompress the whole image. The default tiling pattern treats each row of a
2­dimensional image (or higher dimensional cube) as a tile, such that each tile contains NAXIS1
pixels (except the default with the HCOMPRESS algorithm is to compress the whole 2D image as
a single tile). Any other rectangular tiling pattern may also be defined. In the case of relatively
small images it may be su#cient to compress the entire image as a single tile, resulting in an output
binary table with 1 row. In the case of 3­dimensional data cubes, it may be advantageous to treat
each plane of the cube as a separate tile if application software typically needs to access the cube
on a plane by plane basis.
See section 5.6 ``Image Compression'' for more information on using this tile­compressed image
format.

158 CHAPTER 12. LOCAL FITS CONVENTIONS

Chapter 13
Optimizing Programs
CFITSIO has been carefully designed to obtain the highest possible speed when reading and writing
FITS files. In order to achieve the best performance, however, application programmers must be
careful to call the CFITSIO routines appropriately and in an e#cient sequence; inappropriate usage
of CFITSIO routines can greatly slow down the execution speed of a program.
The maximum possible I/O speed of CFITSIO depends of course on the type of computer system
that it is running on. As a rough guide, the current generation of workstations can achieve speeds
of 2 -- 10 MB/s when reading or writing FITS images and similar, or slightly slower speeds with
FITS binary tables. Reading of FITS files can occur at even higher rates (30MB/s or more) if the
FITS file is still cached in system memory following a previous read or write operation on the same
file. To more accurately predict the best performance that is possible on any particular system, a
diagnostic program called ``speed.c'' is included with the CFITSIO distribution which can be run
to approximately measure the maximum possible speed of writing and reading a test FITS file.
The following 2 sections provide some background on how CFITSIO internally manages the data
I/O and describes some strategies that may be used to optimize the processing speed of software
that uses CFITSIO.
13.1 How CFITSIO Manages Data I/O
Many CFITSIO operations involve transferring only a small number of bytes to or from the FITS file
(e.g, reading a keyword, or writing a row in a table); it would be very ine#cient to physically read
or write such small blocks of data directly in the FITS file on disk, therefore CFITSIO maintains
a set of internal Input--Output (IO) bu#ers in RAM memory that each contain one FITS block
(2880 bytes) of data. Whenever CFITSIO needs to access data in the FITS file, it first transfers the
FITS block containing those bytes into one of the IO bu#ers in memory. The next time CFITSIO
needs to access bytes in the same block it can then go to the fast IO bu#er rather than using a
much slower system disk access routine. The number of available IO bu#ers is determined by the
NIOBUF parameter (in fitsio2.h) and is currently set to 40 by default.
Whenever CFITSIO reads or writes data it first checks to see if that block of the FITS file is already
loaded into one of the IO bu#ers. If not, and if there is an empty IO bu#er available, then it will
159

160 CHAPTER 13. OPTIMIZING PROGRAMS
load that block into the IO bu#er (when reading a FITS file) or will initialize a new block (when
writing to a FITS file). If all the IO bu#ers are already full, it must decide which one to reuse
(generally the one that has been accessed least recently), and flush the contents back to disk if it
has been modified before loading the new block.
The one major exception to the above process occurs whenever a large contiguous set of bytes are
accessed, as might occur when reading or writing a FITS image. In this case CFITSIO bypasses
the internal IO bu#ers and simply reads or writes the desired bytes directly in the disk file with
a single call to a low­level file read or write routine. The minimum threshold for the number of
bytes to read or write this way is set by the MINDIRECT parameter and is currently set to 3
FITS blocks = 8640 bytes. This is the most e#cient way to read or write large chunks of data and
can achieve IO transfer rates of 5 -- 10MB/s or greater. Note that this fast direct IO process is
not applicable when accessing columns of data in a FITS table because the bytes are generally not
contiguous since they are interleaved by the other columns of data in the table. This explains why
the speed for accessing FITS tables is generally slower than accessing FITS images.
Given this background information, the general strategy for e#ciently accessing FITS files should
be apparent: when dealing with FITS images, read or write large chunks of data at a time so that
the direct IO mechanism will be invoked; when accessing FITS headers or FITS tables, on the other
hand, once a particular FITS block has been loading into one of the IO bu#ers, try to access all
the needed information in that block before it gets flushed out of the IO bu#er. It is important to
avoid the situation where the same FITS block is being read then flushed from a IO bu#er multiple
times.
The following section gives more specific suggestions for optimizing the use of CFITSIO.
13.2 Optimization Strategies
1. Because the data in FITS files is always stored in ''big­endian'' byte order, where the first byte
of numeric values contains the most significant bits and the last byte contains the least significant
bits, CFITSIO must swap the order of the bytes when reading or writing FITS files when running
on little­endian machines (e.g., Linux and Microsoft Windows operating systems running on PCs
with x86 CPUs).
On fairly new CPUs that support ''SSSE3'' machine instructions (e.g., starting with Intel Core 2
CPUs in 2007, and in AMD CPUs beginning in 2011) significantly faster 4­byte and 8­byte swapping
algorithms are available. These faster byte swapping functions are not used by default in CFITSIO
(because of the potential code portablility issues), but users can enable them on supported platforms
by adding the appropriate compiler flags (­mssse3 with gcc or icc on linux) when compiling the
swapproc.c source file, which will allow the compiler to generate code using the SSSE3 instruction
set. A convenient way to do this is to configure the CFITSIO library with the following command:
> ./configure ­­enable­ssse3
Note, however, that a binary executable file that is created using these faster functions will only
run on machines that support the SSSE3 machine instructions. It will crash on machines that do
not support them.

13.2. OPTIMIZATION STRATEGIES 161
For faster 2­byte swaps on virtually all x86­64 CPUs (even those that do not support SSSE3), a
variant using only SSE2 instructions exists. SSE2 is enabled by default on x86 64 CPUs with 64­bit
operating systems (and is also automatically enabled by the --enable­ssse3 flag). When running on
x86 64 CPUs with 32­bit operating systems, these faster 2­byte swapping algorithms are not used
by default in CFITSIO, but can be enabled explicitly with:
./configure ­­enable­sse2
Preliminary testing indicates that these SSSE3 and SSE2 based byte­swapping algorithms can
boost the CFITSIO performance when reading or writing FITS images by 20% ­ 30% or more. It
is important to note, however, that compiler optimization must be turned on (e.g., by using the
­O1 or ­O2 flags in gcc) when building programs that use these fast byte­swapping algorithms in
order to reap the full benefit of the SSSE3 and SSE2 instructions; without optimization, the code
may actually run slower than when using more traditional byte­swapping techniques.
2. When dealing with a FITS primary array or IMAGE extension, it is more e#cient to read or
write large chunks of the image at a time (at least 3 FITS blocks = 8640 bytes) so that the direct
IO mechanism will be used as described in the previous section. Smaller chunks of data are read
or written via the IO bu#ers, which is somewhat less e#cient because of the extra copy operation
and additional bookkeeping steps that are required. In principle it is more e#cient to read or write
as big an array of image pixels at one time as possible, however, if the array becomes so large that
the operating system cannot store it all in RAM, then the performance may be degraded because
of the increased swapping of virtual memory to disk.
3. When dealing with FITS tables, the most important e#ciency factor in the software design is
to read or write the data in the FITS file in a single pass through the file. An example of poor
program design would be to read a large, 3­column table by sequentially reading the entire first
column, then going back to read the 2nd column, and finally the 3rd column; this obviously requires
3 passes through the file which could triple the execution time of an IO limited program. For small
tables this is not important, but when reading multi­megabyte sized tables these ine#ciencies can
become significant. The more e#cient procedure in this case is to read or write only as many rows
of the table as will fit into the available internal IO bu#ers, then access all the necessary columns
of data within that range of rows. Then after the program is completely finished with the data in
those rows it can move on to the next range of rows that will fit in the bu#ers, continuing in this
way until the entire file has been processed. By using this procedure of accessing all the columns of
a table in parallel rather than sequentially, each block of the FITS file will only be read or written
once.
The optimal number of rows to read or write at one time in a given table depends on the width of
the table row and on the number of IO bu#ers that have been allocated in CFITSIO. The CFITSIO
Iterator routine will automatically use the optimal­sized bu#er, but there is also a CFITSIO routine
that will return the optimal number of rows for a given table: fits get rowsize. It is not critical to
use exactly the value of nrows returned by this routine, as long as one does not exceed it. Using
a very small value however can also lead to poor performance because of the overhead from the
larger number of subroutine calls.
The optimal number of rows returned by fits get rowsize is valid only as long as the application
program is only reading or writing data in the specified table. Any other calls to access data in the
table header would cause additional blocks of data to be loaded into the IO bu#ers displacing data

162 CHAPTER 13. OPTIMIZING PROGRAMS
from the original table, and should be avoided during the critical period while the table is being
read or written.
4. Use the CFITSIO Iterator routine. This routine provides a more `object oriented' way of reading
and writing FITS files which automatically uses the most appropriate data bu#er size to achieve
the maximum I/O throughput.
5. Use binary table extensions rather than ASCII table extensions for better e#ciency when dealing
with tabular data. The I/O to ASCII tables is slower because of the overhead in formatting or
parsing the ASCII data fields and because ASCII tables are about twice as large as binary tables
that have the same information content.
6. Design software so that it reads the FITS header keywords in the same order in which they
occur in the file. When reading keywords, CFITSIO searches forward starting from the position of
the last keyword that was read. If it reaches the end of the header without finding the keyword, it
then goes back to the start of the header and continues the search down to the position where it
started. In practice, as long as the entire FITS header can fit at one time in the available internal
IO bu#ers, then the header keyword access will be relatively fast and it makes little di#erence which
order they are accessed.
7. Avoid the use of scaling (by using the BSCALE and BZERO or TSCAL and TZERO keywords)
in FITS files since the scaling operations add to the processing time needed to read or write the
data. In some cases it may be more e#cient to temporarily turn o# the scaling (using fits set bscale
or fits set tscale) and then read or write the raw unscaled values in the FITS file.
8. Avoid using the `implicit data type conversion' capability in CFITSIO. For instance, when
reading a FITS image with BITPIX = ­32 (32­bit floating point pixels), read the data into a single
precision floating point data array in the program. Forcing CFITSIO to convert the data to a
di#erent data type can slow the program.
9. Where feasible, design FITS binary tables using vector column elements so that the data are
written as a contiguous set of bytes, rather than as single elements in multiple rows. For example,
it is faster to access the data in a table that contains a single row and 2 columns with TFORM
keywords equal to '10000E' and '10000J', than it is to access the same amount of data in a table
with 10000 rows which has columns with the TFORM keywords equal to '1E' and '1J'. In the
former case the 10000 floating point values in the first column are all written in a contiguous block
of the file which can be read or written quickly, whereas in the second case each floating point value
in the first column is interleaved with the integer value in the second column of the same row so
CFITSIO has to explicitly move to the position of each element to be read or written.
10. Avoid the use of variable length vector columns in binary tables, since any reading or writing
of these data requires that CFITSIO first look up or compute the starting address of each row of
data in the heap. In practice, this is probably not a significant e#ciency issue.
11. When copying data from one FITS table to another, it is faster to transfer the raw bytes
instead of reading then writing each column of the table. The CFITSIO routines fits read tblbytes
and fits write tblbytes will perform low­level reads or writes of any contiguous range of bytes in
a table extension. These routines can be used to read or write a whole row (or multiple rows for
even greater e#ciency) of a table with a single function call. These routines are fast because they
bypass all the usual data scaling, error checking and machine dependent data conversion that is
normally done by CFITSIO, and they allow the program to write the data to the output file in

13.2. OPTIMIZATION STRATEGIES 163
exactly the same byte order. For these same reasons, these routines can corrupt the FITS data file
if used incorrectly because no validation or machine dependent conversion is performed by these
routines. These routines are only recommended for optimizing critical pieces of code and should
only be used by programmers who thoroughly understand the internal format of the FITS tables
they are reading or writing.
12. Another strategy for improving the speed of writing a FITS table, similar to the previous one,
is to directly construct the entire byte stream for a whole table row (or multiple rows) within the
application program and then write it to the FITS file with fits write tblbytes. This avoids all the
overhead normally present in the column­oriented CFITSIO write routines. This technique should
only be used for critical applications because it makes the code more di#cult to understand and
maintain, and it makes the code more system dependent (e.g., do the bytes need to be swapped
before writing to the FITS file?).
13. Finally, external factors such as the speed of the data storage device, the size of the data cache,
the amount of disk fragmentation, and the amount of RAM available on the system can all have
a significant impact on overall I/O e#ciency. For critical applications, the entire hardware and
software system should be reviewed to identify any potential I/O bottlenecks.

164 CHAPTER 13. OPTIMIZING PROGRAMS

Appendix A
Index of Routines
fits add group member 88
fits ascii tform 66
fits binary tform 66
fits calculator 57
fits calculator rng 57
fits calc binning 58
fits calc rows 56
fits change group 86
fits clear errmark 30
fits clear errmsg 30
fits close file 33
fits compact group 87
fits compare str 63
fits compress heap 111
fits convert hdr2str 37, 82
fits copy cell2image 41
fits copy col 53
fits copy data 96
fits copy file 34
fits copy group 87
fits copy hdu 34
fits copy header 34
fits copy image2cell 41
fits copy image section 44
fits copy key 101
fits copy member 89
fits copy pixlist2image 58
fits copy rows 53
fits create diskfile 32
fits create file 32
fits create group 86
fits create hdu 95
fits create img 41
fits create memfile 92
fits create tbl 49
fits create template 92
fits date2str 61
fits decode chksum 61
fits decode tdim 51
fits delete col 52
fits delete file 33
fits delete hdu 35
fits delete key 39
fits delete record 39
fits delete rowlist 52
fits delete rowrange 52
fits delete rows 52
fits delete str 39
fits encode chksum 61
fits file exists 94
fits file mode 33
fits file name 33
fits find first row 56
fits find nextkey 37
fits find rows 56
fits flush bu#er 94
fits flush file 94
fits free memory 103
fits get acolparms 110
fits get bcolparms 110
fits get chksum 60
fits get col display width 51
fits get colname 49
fits get colnum 49
fits get coltype 50
fits get compression type 46
fits get eqcoltype 50
fits get errstatus 29
fits get hdrpos 97
fits get hdrspace 35
fits get hdu num 34
fits get hdu type 34
fits get hduaddr 95
fits get hduaddrll 95
fits get img dim 41
fits get img equivtype 40
fits get img param 41
fits get img size 41
fits get img type 40
fits get inttype 65
fits get keyclass 65
fits get keyname 64
fits get keytype 64
fits get noise bits 46
fits get num cols 49
fits get num groups 89
fits get num hdus 34
fits get num members 88
fits get num rows 49
fits get rowsize 111
fits get system time 61
fits get tile dim 46
fits get tbcol 66
fits get version 62
fits hdr2str 37, 82
fits insert atbl 96
165

166 APPENDIX A. INDEX OF ROUTINES
fits insert btbl 96
fits insert col 52
fits insert cols 52
fits insert group 86
fits insert img 95
fits insert key null 102
fits insert key TYP 102
fits insert record 102
fits insert rows 52
fits is reentrant 71
fits iterate data 79
fits make hist 59
fits make keyn 64
fits make nkey 64
fits merge groups 87
fits modify card 104
fits modify comment 39
fits modify key null 105
fits modify key TYP 104
fits modify name 39
fits modify record 104
fits modify vector len 53
fits movabs hdu 33
fits movnam hdu 33
fits movrel hdu 33
fits null check 64
fits open data 30
fits open diskfile 30
fits open file 30
fits open image 30
fits open table 30
fits open group 88
fits open member 89
fits open memfile 91
fits parse extnum 93
fits parse input filename 93
fits parse input url 93
fits parse range 71
fits parse rootname 94
fits parse template 67
fits parse value 64
fits pix to world 83
fits read 2d TYP 110
fits read 3d TYP 110
fits read atblhdr 99
fits read btblhdr 99
fits read card 36
fits read col 55
fits read col bit 116
fits read col TYP 114
fits read colnull 55
fits read colnull TYP 114
fits read descript 116
fits read descripts 116
fits read errmsg 30
fits read ext 96
fits read grppar TYP 109
fits read img 109
fits read img coord 83
fits read img TYP 109
fits read imghdr 99
fits read imgnull 109
fits read imgnull TYP 109
fits read key 36
fits read key longstr 103
fits read key triple 104
fits read key unit 37
fits read key TYP 103
fits read keyn 36
fits read keys TYP 103
fits read keyword 36
fits read pix 43
fits read pixnull 43
fits read record 36
fits read str 36
fits read subset TYP 110 115
fits read subsetnull TYP 110 115
fits read tbl coord 83
fits read tblbytes 112
fits read tdim 51
fits read wcstab 82
fits rebin wcs 59
fits remove group 87
fits remove member 90
fits reopen file 92
fits report error 30
fits resize img 96
fits rms float 71
fits rms short 71
fits select rows 56
fits set atblnull 106
fits set bscale 106
fits set btblnull 106
fits set compression type 46
fits set hdrsize 97
fits set hdustruc 97
fits set imgnull 106
fits set noise bits 46
fits set tile dim 46
fits set tscale 106
fits split names 63
fits str2date 61
fits str2time 61
fits test expr 57
fits test heap 111
fits test keyword 63
fits test record 63
fits time2str 61
fits transfer member 89
fits translate keyword 69
fits update card 39
fits update chksum 60
fits update key 38
fits update key longstr 105
fits update key null 38
fits update key TYP 105
fits uppercase 63
fits url type 33
fits verify chksum 60
fits verify group 88
fits world to pix 83
fits write 2d TYP 109
fits write 3d TYP 109
fits write atblhdr 98
fits write btblhdr 99
fits write chksum 60
fits write col 54
fits write col bit 113
fits write col TYP 112
fits write col null 54
fits write colnull 54
fits write colnull TYP 112
fits write comment 38
fits write date 38
fits write descript 113
fits write errmark 30
fits write errmsg 62
fits write ext 96
fits write exthdr 98
fits write grphdr 98
fits write grppar TYP 108
fits write hdu 34
fits write history 38
fits write img 108

167
fits write img null 108
fits write img TYP 108
fits write imghdr 98
fits write imgnull 108
fits write imgnull TYP 108
fits write key 38
fits write key longstr 100
fits write key longwarn 100
fits write key null 38
fits write key template 101
fits write key triple 101
fits write key unit 39
fits write key TYP 100
fits write keys TYP 101
fits write keys histo 59
fits write null img 43
fits write nullrows 54
fits write pix 42
fits write pixnull 42
fits write record 39
fits write subset 42
fits write subset TYP 109
fits write tblbytes 112
fits write tdim 51
fits write theap 111

168 APPENDIX A. INDEX OF ROUTINES
#asfm 66
#bnfm 66
#calc 57
#calc rng 57
#clos 33
#cmph 111
#cmps 63
#cmrk 30
#cmsg 30
#copy 34
#cpcl 53
#cpdt 96
#cpfl 34
#cphd 34
#cpimg 44
#cpky 101
#cprw 53
#crhd 95
#crim 41
#crow 56
#crtb 49
#dcol 52
#delt 33
#dhdu 35
#dkey 39
#dkinit 32
#dkopen 30
#dopn 30
#drec 39
#drow 52
#drrg 52
#drws 52
#dstr 39
#dsum 61
#dt2s 61
#dtdm 51
#dtyp 64
#eqty 50
#esum 61
#exest 94
#extn 93
##rw 56
#flmd 33
#flnm 33
#flsh 94
#flus 94
#free 103
#frow 56
#g2d 110
#g3d 110
#gabc 66
#gacl 110
#gbcl 110
#gcdw 51
#gcf 55
#gcf 114
#gcks 60
#gcnn 49
#gcno 49
#gcrd 36
#gcv 55
#gcv 114
#gcx 116
#gdes 116
#gdess 116
#gerr 29
#gextn 96
#ggp 109
#ghad 95
#ghbn 99
#ghdn 34
#ghdt 34
#ghpr 99
#ghps 97
#ghsp 35
#ghtb 99
#gics 83
#gidm 41
#gidt 40
#giet 40
#gipr 41
#gisz 41
#gkcl 65
#gkey 36
#gkls 103
#gkn 103
#gknm 64
#gky 36
#gkyn 36
#gkyt 104
#gky 103
#gmcp 89
#gmng 89
#gmop 89
#gmrm 90
#gmsg 30
#gmtf 89
#gncl 49
#gnrw 49
#gnxk 37
#gpf 109
#gpf 109
#gpv 109
#gpv 109
#gpxv 43
#gpxf 43
#grec 36
#grsz 111
#gsdt 61
#gsf 110 115
#gstm 61
#gstr 36
#gsv 110 115
#gtam 88
#gtbb 112
#gtch 86
#gtcl 50
#gtcm 87
#gtcp 87
#gtcr 86
#gtcs 83
#gtdm 51
#gthd 67
#gtis 86
#gtmg 87
#gtnm 88
#gtop 88
#gtrm 87
#gtvf 88
#gunt 37
#hdef 97
#bin 96
#cls 52
#col 52
#file 93
#img 95
#kls 102
#kyu 102
#ky 102
#mem 92
#nit 32
#nttyp 65
#opn 30
#rec 102
#row 52
#tab 96
#ter 79
#url 93
#keyn 64
#mahd 33
#mcom 39
#mcrd 104
#mkls 104
#mkyu 105
#mky 104
#mnam 39
#mnhd 33
#mrec 104
#mrhd 33
#mvec 53
#nchk 64
#nkey 64
#omem 91
#open 30
#p2d 109
#p3d 109
#pcks 60
#pcl 54
#pcls 112
#pcl 113
#pclu 54
#pcn 54
#pcn 112
#pcom 38
#pdat 38
#pdes 113
#pextn 96
#pgp 108
#phbn 99
#phext 98
#phis 38
#phpr 98
#phps 98
#phtb 98
#pkls 100
#pkn 101
#pktp 101
#pky 38
#pkyt 101
#pkyu 38
#pky 100
#plsw 100
#pmrk 30
#pmsg 62
#pnul 106
#ppn 108
#ppn 108
#ppr 108
#pprn 43
#ppru 108
#ppr 108
#ppx 42
#ppxn 42
#prec 39
#prwu 54
#pscl 106
#pss 42
#pss 109
#psvc 64
#ptbb 112
#ptdm 51
#pthp 111
#punt 39
#rdef 97
#reopen 92
#rprt 30
#rsim 96
#rtnm 94
#rwrg 71
#s2dt 61
#s2tm 61
#snul 106
#srow 56
#texp 57
#thdu 34
#theap 111
#tkey 63
#tm2s 61
#tnul 106
#topn 30
#tplt 92
#trec 63
#tscl 106
#ucrd 39
#ukls 105
#uky 38
#ukyu 38
#uky 105
#upch 63
#upck 60

169
#urlt 33
#vcks 60
#vers 62
#wldp 83
#wrhdu 34
#xypx 83

170 APPENDIX A. INDEX OF ROUTINES

Appendix B
Parameter Definitions
anynul ­ set to TRUE (=1) if any returned values are undefined, else FALSE
array ­ array of numerical data values to read or write
ascii ­ encoded checksum string
binspec ­ the input table binning specifier
bitpix ­ bits per pixel. The following symbolic mnemonics are predefined:
BYTE_IMG = 8 (unsigned char)
SHORT_IMG = 16 (signed short integer)
LONG_IMG = 32 (signed long integer)
LONGLONG_IMG = 64 (signed long 64­bit integer)
FLOAT_IMG = ­32 (float)
DOUBLE_IMG = ­64 (double).
The LONGLONG_IMG type is experimental and is not officially
recognized in the FITS Standard document.
Two additional values, USHORT_IMG and ULONG_IMG are also available
for creating unsigned integer images. These are equivalent to
creating a signed integer image with BZERO offset keyword values
of 32768 or 2147483648, respectively, which is the convention that
FITS uses to store unsigned integers.
card ­ header record to be read or written (80 char max, null­terminated)
casesen ­ CASESEN (=1) for case­sensitive string matching, else CASEINSEN (=0)
cmopt ­ grouping table "compact" option parameter. Allowed values are:
OPT_CMT_MBR and OPT_CMT_MBR_DEL.
colname ­ name of the column (null­terminated)
colnum ­ column number (first column = 1)
colspec ­ the input file column specification; used to delete, create, or rename
table columns
comment ­ the keyword comment field (72 char max, null­terminated)
complm ­ should the checksum be complemented?
comptype ­ compression algorithm to use: GZIP_1, RICE_1, HCOMPRESS_1, or PLIO_1
coordtype­ type of coordinate projection (­SIN, ­TAN, ­ARC, ­NCP,
­GLS, ­MER, or ­AIT)
cpopt ­ grouping table copy option parameter. Allowed values are:
171

172 APPENDIX B. PARAMETER DEFINITIONS
OPT_GCP_GPT, OPT_GCP_MBR, OPT_GCP_ALL, OPT_MCP_ADD, OPT_MCP_NADD,
OPT_MCP_REPL, amd OPT_MCP_MOV.
create_col­ If TRUE, then insert a new column in the table, otherwise
overwrite the existing column.
current ­ if TRUE, then the current HDU will be copied
dataok ­ was the data unit verification successful (=1) or
not (= ­1). Equals zero if the DATASUM keyword is not present.
datasum ­ 32­bit 1's complement checksum for the data unit
dataend ­ address (in bytes) of the end of the HDU
datastart­ address (in bytes) of the start of the data unit
datatype ­ specifies the data type of the value. Allowed value are: TSTRING,
TLOGICAL, TBYTE, TSBYTE, TSHORT, TUSHORT, TINT, TUINT, TLONG, TULONG,
TFLOAT, TDOUBLE, TCOMPLEX, and TDBLCOMPLEX
datestr ­ FITS date/time string: 'YYYY­MM­DDThh:mm:ss.ddd', 'YYYY­MM­dd',
or 'dd/mm/yy'
day ­ calendar day (UTC) (1­31)
decimals ­ number of decimal places to be displayed
deltasize ­ increment for allocating more memory
dim1 ­ declared size of the first dimension of the image or cube array
dim2 ­ declared size of the second dimension of the data cube array
dispwidth ­ display width of a column = length of string that will be read
dtype ­ data type of the keyword ('C', 'L', 'I', 'F' or 'X')
C = character string
L = logical
I = integer
F = floating point number
X = complex, e.g., "(1.23, ­4.56)"
err_msg ­ error message on the internal stack (80 chars max)
err_text ­ error message string corresponding to error number (30 chars max)
exact ­ TRUE (=1) if the strings match exactly;
FALSE (=0) if wildcards are used
exclist ­ array of pointers to keyword names to be excluded from search
exists ­ flag indicating whether the file or compressed file exists on disk
expr ­ boolean or arithmetic expression
extend ­ TRUE (=1) if FITS file may have extensions, else FALSE (=0)
extname ­ value of the EXTNAME keyword (null­terminated)
extspec ­ the extension or HDU specifier; a number or name, version, and type
extver ­ value of the EXTVER keyword = integer version number
filename ­ full name of the FITS file, including optional HDU and filtering specs
filetype ­ type of file (file://, ftp://, http://, etc.)
filter ­ the input file filtering specifier
firstchar­ starting byte in the row (first byte of row = 1)
firstfailed ­ member HDU ID (if positive) or grouping table GRPIDn index
value (if negative) that failed grouping table verification.
firstelem­ first element in a vector (ignored for ASCII tables)
firstrow ­ starting row number (first row of table = 1)

173
following­ if TRUE, any HDUs following the current HDU will be copied
fpixel ­ coordinate of the first pixel to be read or written in the
FITS array. The array must be of length NAXIS and have values such
that fpixel[0] is in the range 1 to NAXIS1, fpixel[1] is in the
range 1 to NAXIS2, etc.
fptr ­ pointer to a 'fitsfile' structure describing the FITS file.
frac ­ factional part of the keyword value
gcount ­ number of groups in the primary array (usually = 1)
gfptr ­ fitsfile* pointer to a grouping table HDU.
group ­ GRPIDn/GRPLCn index value identifying a grouping table HDU, or
data group number (=0 for non­grouped data)
grouptype ­ Grouping table parameter that specifies the columns to be
created in a grouping table HDU. Allowed values are: GT_ID_ALL_URI,
GT_ID_REF, GT_ID_POS, GT_ID_ALL, GT_ID_REF_URI, and GT_ID_POS_URI.
grpname ­ value to use for the GRPNAME keyword value.
hdunum ­ sequence number of the HDU (Primary array = 1)
hduok ­ was the HDU verification successful (=1) or
not (= ­1). Equals zero if the CHECKSUM keyword is not present.
hdusum ­ 32 bit 1's complement checksum for the entire CHDU
hdutype ­ HDU type: IMAGE_HDU (0), ASCII_TBL (1), BINARY_TBL (2), ANY_HDU (­1)
header ­ returned character string containing all the keyword records
headstart­ starting address (in bytes) of the CHDU
heapsize ­ size of the binary table heap, in bytes
history ­ the HISTORY keyword comment string (70 char max, null­terminated)
hour ­ hour within day (UTC) (0 ­ 23)
inc ­ sampling interval for pixels in each FITS dimension
inclist ­ array of pointers to matching keyword names
incolnum ­ input column number; range = 1 to TFIELDS
infile ­ the input filename, including path if specified
infptr ­ pointer to a 'fitsfile' structure describing the input FITS file.
intval ­ integer part of the keyword value
iomode ­ file access mode: either READONLY (=0) or READWRITE (=1)
keyname ­ name of a keyword (8 char max, null­terminated)
keynum ­ position of keyword in header (1st keyword = 1)
keyroot ­ root string for the keyword name (5 char max, null­terminated)
keysexist­ number of existing keyword records in the CHU
keytype ­ header record type: ­1=delete; 0=append or replace;
1=append; 2=this is the END keyword
longstr ­ arbitrarily long string keyword value (null­terminated)
lpixel ­ coordinate of the last pixel to be read or written in the
FITS array. The array must be of length NAXIS and have values such
that lpixel[0] is in the range 1 to NAXIS1, lpixel[1] is in the
range 1 to NAXIS2, etc.
match ­ TRUE (=1) if the 2 strings match, else FALSE (=0)
maxdim ­ maximum number of values to return
member ­ row number of a grouping table member HDU.

174 APPENDIX B. PARAMETER DEFINITIONS
memptr ­ pointer to the a FITS file in memory
mem_realloc ­ pointer to a function for reallocating more memory
memsize ­ size of the memory block allocated for the FITS file
mfptr ­ fitsfile* pointer to a grouping table member HDU.
mgopt ­ grouping table merge option parameter. Allowed values are:
OPT_MRG_COPY, and OPT_MRG_MOV.
minute ­ minute within hour (UTC) (0 ­ 59)
month ­ calendar month (UTC) (1 ­ 12)
morekeys ­ space in the header for this many more keywords
n_good_rows ­ number of rows evaluating to TRUE
namelist ­ string containing a comma or space delimited list of names
naxes ­ size of each dimension in the FITS array
naxis ­ number of dimensions in the FITS array
naxis1 ­ length of the X/first axis of the FITS array
naxis2 ­ length of the Y/second axis of the FITS array
naxis3 ­ length of the Z/third axis of the FITS array
nbytes ­ number of bytes or characters to read or write
nchars ­ number of characters to read or write
nelements­ number of data elements to read or write
newfptr ­ returned pointer to the reopened file
newveclen­ new value for the column vector repeat parameter
nexc ­ number of names in the exclusion list (may = 0)
nfound ­ number of keywords found (highest keyword number)
nkeys ­ number of keywords in the sequence
ninc ­ number of names in the inclusion list
nmembers ­ Number of grouping table members (NAXIS2 value).
nmove ­ number of HDUs to move (+ or ­), relative to current position
nocomments ­ if equal to TRUE, then no commentary keywords will be copied
noisebits­ number of bits to ignore when compressing floating point images
nrows ­ number of rows in the table
nstart ­ first integer value
nullarray­ set to TRUE (=1) if corresponding data element is undefined
nulval ­ numerical value to represent undefined pixels
nulstr ­ character string used to represent undefined values in ASCII table
numval ­ numerical data value, of the appropriate data type
offset ­ byte offset in the heap or data unit to the first element of the vector
openfptr ­ pointer to a currently open FITS file
overlap ­ number of bytes in the binary table heap pointed to by more than 1
descriptor
outcolnum­ output column number; range = 1 to TFIELDS + 1
outfile ­ and optional output filename; the input file will be copied to this prior
to opening the file
outfptr ­ pointer to a 'fitsfile' structure describing the output FITS file.
pcount ­ value of the PCOUNT keyword = size of binary table heap
previous ­ if TRUE, any previous HDUs in the input file will be copied.
repeat ­ length of column vector (e.g. 12J); == 1 for ASCII table

175
rmopt ­ grouping table remove option parameter. Allowed values are:
OPT_RM_GPT, OPT_RM_ENTRY, OPT_RM_MBR, and OPT_RM_ALL.
rootname ­ root filename, minus any extension or filtering specifications
rot ­ celestial coordinate rotation angle (degrees)
rowlen ­ length of a table row, in characters or bytes
rowlist ­ sorted list of row numbers to be deleted from the table
rownum ­ number of the row (first row = 1)
rowrange ­ list of rows or row ranges: '3,6­8,12,56­80' or '500­'
row_status ­ array of True/False results for each row that was evaluated
scale ­ linear scaling factor; true value = (FITS value) * scale + zero
second ­ second within minute (0 ­ 60.9999999999) (leap second!)
section ­ section of image to be copied (e.g. 21:80,101:200)
simple ­ TRUE (=1) if FITS file conforms to the Standard, else FALSE (=0)
space ­ number of blank spaces to leave between ASCII table columns
status ­ returned error status code (0 = OK)
sum ­ 32 bit unsigned checksum value
tbcol ­ byte position in row to start of column (1st col has tbcol = 1)
tdisp ­ Fortran style display format for the table column
tdimstr ­ the value of the TDIMn keyword
templt ­ template string used in comparison (null­terminated)
tfields ­ number of fields (columns) in the table
tfopt ­ grouping table member transfer option parameter. Allowed values are:
OPT_MCP_ADD, and OPT_MCP_MOV.
tform ­ format of the column (null­terminated); allowed values are:
ASCII tables: Iw, Aw, Fww.dd, Eww.dd, or Dww.dd
Binary tables: rL, rX, rB, rI, rJ, rA, rAw, rE, rD, rC, rM
where 'w'=width of the field, 'd'=no. of decimals, 'r'=repeat count.
Variable length array columns are denoted by a '1P' before the data type
character (e.g., '1PJ'). When creating a binary table, 2 addition tform
data type codes are recognized by CFITSIO: 'rU' and 'rV' for unsigned
16­bit and unsigned 32­bit integer, respectively.
theap ­ zero indexed byte offset of starting address of the heap
relative to the beginning of the binary table data
tilesize ­ array of length NAXIS that specifies the dimensions of
the image compression tiles
ttype ­ label or name for table column (null­terminated)
tunit ­ physical unit for table column (null­terminated)
typechar ­ symbolic code of the table column data type
typecode ­ data type code of the table column. The negative of
the value indicates a variable length array column.
Datatype typecode Mnemonic
bit, X 1 TBIT
byte, B 11 TBYTE
logical, L 14 TLOGICAL
ASCII character, A 16 TSTRING

176 APPENDIX B. PARAMETER DEFINITIONS
short integer, I 21 TSHORT
integer, J 41 TINT32BIT (same as TLONG)
long long integer, K 81 TLONGLONG
real, E 42 TFLOAT
double precision, D 82 TDOUBLE
complex, C 83 TCOMPLEX
double complex, M 163 TDBLCOMPLEX
unit ­ the physical unit string (e.g., 'km/s') for a keyword
unused ­ number of unused bytes in the binary table heap
urltype ­ the file type of the FITS file (file://, ftp://, mem://, etc.)
validheap­ returned value = FALSE if any of the variable length array
address are outside the valid range of addresses in the heap
value ­ the keyword value string (70 char max, null­terminated)
version ­ current version number of the CFITSIO library
width ­ width of the character string field
xcol ­ number of the column containing the X coordinate values
xinc ­ X axis coordinate increment at reference pixel (deg)
xpix ­ X axis pixel location
xpos ­ X axis celestial coordinate (usually RA) (deg)
xrefpix ­ X axis reference pixel array location
xrefval ­ X axis coordinate value at the reference pixel (deg)
ycol ­ number of the column containing the X coordinate values
year ­ calendar year (e.g. 1999, 2000, etc)
yinc ­ Y axis coordinate increment at reference pixel (deg)
ypix ­ y axis pixel location
ypos ­ y axis celestial coordinate (usually DEC) (deg)
yrefpix ­ Y axis reference pixel array location
yrefval ­ Y axis coordinate value at the reference pixel (deg)
zero ­ scaling offset; true value = (FITS value) * scale + zero

Appendix C
CFITSIO Error Status Codes
The following table lists all the error status codes used by CFITSIO. Programmers are encouraged
to use the symbolic mnemonics (defined in the file fitsio.h) rather than the actual integer status
values to improve the readability of their code.
Symbolic Const Value Meaning
­­­­­­­­­­­­­­ ­­­­­ ­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­­
0 OK, no error
SAME_FILE 101 input and output files are the same
TOO_MANY_FILES 103 tried to open too many FITS files at once
FILE_NOT_OPENED 104 could not open the named file
FILE_NOT_CREATED 105 could not create the named file
WRITE_ERROR 106 error writing to FITS file
END_OF_FILE 107 tried to move past end of file
READ_ERROR 108 error reading from FITS file
FILE_NOT_CLOSED 110 could not close the file
ARRAY_TOO_BIG 111 array dimensions exceed internal limit
READONLY_FILE 112 Cannot write to readonly file
MEMORY_ALLOCATION 113 Could not allocate memory
BAD_FILEPTR 114 invalid fitsfile pointer
NULL_INPUT_PTR 115 NULL input pointer to routine
SEEK_ERROR 116 error seeking position in file
BAD_URL_PREFIX 121 invalid URL prefix on file name
TOO_MANY_DRIVERS 122 tried to register too many IO drivers
DRIVER_INIT_FAILED 123 driver initialization failed
NO_MATCHING_DRIVER 124 matching driver is not registered
URL_PARSE_ERROR 125 failed to parse input file URL
RANGE_PARSE_ERROR 126 parse error in range list
SHARED_BADARG 151 bad argument in shared memory driver
SHARED_NULPTR 152 null pointer passed as an argument
SHARED_TABFULL 153 no more free shared memory handles
177

178 APPENDIX C. CFITSIO ERROR STATUS CODES
SHARED_NOTINIT 154 shared memory driver is not initialized
SHARED_IPCERR 155 IPC error returned by a system call
SHARED_NOMEM 156 no memory in shared memory driver
SHARED_AGAIN 157 resource deadlock would occur
SHARED_NOFILE 158 attempt to open/create lock file failed
SHARED_NORESIZE 159 shared memory block cannot be resized at the moment
HEADER_NOT_EMPTY 201 header already contains keywords
KEY_NO_EXIST 202 keyword not found in header
KEY_OUT_BOUNDS 203 keyword record number is out of bounds
VALUE_UNDEFINED 204 keyword value field is blank
NO_QUOTE 205 string is missing the closing quote
BAD_INDEX_KEY 206 illegal indexed keyword name (e.g. 'TFORM1000')
BAD_KEYCHAR 207 illegal character in keyword name or card
BAD_ORDER 208 required keywords out of order
NOT_POS_INT 209 keyword value is not a positive integer
NO_END 210 couldn't find END keyword
BAD_BITPIX 211 illegal BITPIX keyword value
BAD_NAXIS 212 illegal NAXIS keyword value
BAD_NAXES 213 illegal NAXISn keyword value
BAD_PCOUNT 214 illegal PCOUNT keyword value
BAD_GCOUNT 215 illegal GCOUNT keyword value
BAD_TFIELDS 216 illegal TFIELDS keyword value
NEG_WIDTH 217 negative table row size
NEG_ROWS 218 negative number of rows in table
COL_NOT_FOUND 219 column with this name not found in table
BAD_SIMPLE 220 illegal value of SIMPLE keyword
NO_SIMPLE 221 Primary array doesn't start with SIMPLE
NO_BITPIX 222 Second keyword not BITPIX
NO_NAXIS 223 Third keyword not NAXIS
NO_NAXES 224 Couldn't find all the NAXISn keywords
NO_XTENSION 225 HDU doesn't start with XTENSION keyword
NOT_ATABLE 226 the CHDU is not an ASCII table extension
NOT_BTABLE 227 the CHDU is not a binary table extension
NO_PCOUNT 228 couldn't find PCOUNT keyword
NO_GCOUNT 229 couldn't find GCOUNT keyword
NO_TFIELDS 230 couldn't find TFIELDS keyword
NO_TBCOL 231 couldn't find TBCOLn keyword
NO_TFORM 232 couldn't find TFORMn keyword
NOT_IMAGE 233 the CHDU is not an IMAGE extension
BAD_TBCOL 234 TBCOLn keyword value < 0 or > rowlength
NOT_TABLE 235 the CHDU is not a table
COL_TOO_WIDE 236 column is too wide to fit in table
COL_NOT_UNIQUE 237 more than 1 column name matches template
BAD_ROW_WIDTH 241 sum of column widths not = NAXIS1
UNKNOWN_EXT 251 unrecognizable FITS extension type

179
UNKNOWN_REC 252 unknown record; 1st keyword not SIMPLE or XTENSION
END_JUNK 253 END keyword is not blank
BAD_HEADER_FILL 254 Header fill area contains non­blank chars
BAD_DATA_FILL 255 Illegal data fill bytes (not zero or blank)
BAD_TFORM 261 illegal TFORM format code
BAD_TFORM_DTYPE 262 unrecognizable TFORM data type code
BAD_TDIM 263 illegal TDIMn keyword value
BAD_HEAP_PTR 264 invalid BINTABLE heap pointer is out of range
BAD_HDU_NUM 301 HDU number < 1
BAD_COL_NUM 302 column number < 1 or > tfields
NEG_FILE_POS 304 tried to move to negative byte location in file
NEG_BYTES 306 tried to read or write negative number of bytes
BAD_ROW_NUM 307 illegal starting row number in table
BAD_ELEM_NUM 308 illegal starting element number in vector
NOT_ASCII_COL 309 this is not an ASCII string column
NOT_LOGICAL_COL 310 this is not a logical data type column
BAD_ATABLE_FORMAT 311 ASCII table column has wrong format
BAD_BTABLE_FORMAT 312 Binary table column has wrong format
NO_NULL 314 null value has not been defined
NOT_VARI_LEN 317 this is not a variable length column
BAD_DIMEN 320 illegal number of dimensions in array
BAD_PIX_NUM 321 first pixel number greater than last pixel
ZERO_SCALE 322 illegal BSCALE or TSCALn keyword = 0
NEG_AXIS 323 illegal axis length < 1
NOT_GROUP_TABLE 340 Grouping function error
HDU_ALREADY_MEMBER 341
MEMBER_NOT_FOUND 342
GROUP_NOT_FOUND 343
BAD_GROUP_ID 344
TOO_MANY_HDUS_TRACKED 345
HDU_ALREADY_TRACKED 346
BAD_OPTION 347
IDENTICAL_POINTERS 348
BAD_GROUP_ATTACH 349
BAD_GROUP_DETACH 350
NGP_NO_MEMORY 360 malloc failed
NGP_READ_ERR 361 read error from file
NGP_NUL_PTR 362 null pointer passed as an argument.
Passing null pointer as a name of
template file raises this error
NGP_EMPTY_CURLINE 363 line read seems to be empty (used
internally)
NGP_UNREAD_QUEUE_FULL 364 cannot unread more then 1 line (or single

180 APPENDIX C. CFITSIO ERROR STATUS CODES
line twice)
NGP_INC_NESTING 365 too deep include file nesting (infinite
loop, template includes itself ?)
NGP_ERR_FOPEN 366 fopen() failed, cannot open template file
NGP_EOF 367 end of file encountered and not expected
NGP_BAD_ARG 368 bad arguments passed. Usually means
internal parser error. Should not happen
NGP_TOKEN_NOT_EXPECT 369 token not expected here
BAD_I2C 401 bad int to formatted string conversion
BAD_F2C 402 bad float to formatted string conversion
BAD_INTKEY 403 can't interpret keyword value as integer
BAD_LOGICALKEY 404 can't interpret keyword value as logical
BAD_FLOATKEY 405 can't interpret keyword value as float
BAD_DOUBLEKEY 406 can't interpret keyword value as double
BAD_C2I 407 bad formatted string to int conversion
BAD_C2F 408 bad formatted string to float conversion
BAD_C2D 409 bad formatted string to double conversion
BAD_DATATYPE 410 illegal datatype code value
BAD_DECIM 411 bad number of decimal places specified
NUM_OVERFLOW 412 overflow during data type conversion
DATA_COMPRESSION_ERR 413 error compressing image
DATA_DECOMPRESSION_ERR 414 error uncompressing image
BAD_DATE 420 error in date or time conversion
PARSE_SYNTAX_ERR 431 syntax error in parser expression
PARSE_BAD_TYPE 432 expression did not evaluate to desired type
PARSE_LRG_VECTOR 433 vector result too large to return in array
PARSE_NO_OUTPUT 434 data parser failed not sent an out column
PARSE_BAD_COL 435 bad data encounter while parsing column
PARSE_BAD_OUTPUT 436 Output file not of proper type
ANGLE_TOO_BIG 501 celestial angle too large for projection
BAD_WCS_VAL 502 bad celestial coordinate or pixel value
WCS_ERROR 503 error in celestial coordinate calculation
BAD_WCS_PROJ 504 unsupported type of celestial projection
NO_WCS_KEY 505 celestial coordinate keywords not found
APPROX_WCS_KEY 506 approximate wcs keyword values were returned