CSPICE Required Reading

Last revised on 2005 OCT 10 by E. D. Wright (JPL)

Abstract

CSPICE is an ANSI C version of the SPICE Toolkit. CSPICE provides essentially the same functionality as the Fortran SPICE Toolkit, with very slight differences where necessitated by differences in the capabilities of standard ANSI C as opposed to Fortran.

Design Concept

Like the Fortran SPICE Toolkit, CSPICE contains library routines, executable programs, documentation, and example ``cookbook'' programs. Source code is provided for both libraries and executables.

NAIF creates the basis for CSPICE by running the Fortran-to-C translation utility, f2c, on the Fortran SPICELIB code base.

Programming Standards

NAIF intends CSPICE source code to comply with the ANSI C standard and meant to be compiled under ANSI compliant C compilers; the code relies on features supplied in ANSI C not present in the original Kernighan and Ritchie version of C.

All CSPICE source code written by NAIF uses ANSI C. C source code produced by running f2c on SPICELIB Fortran source code has been generated using the f2c processor's "ANSI" option (-A). The degree of deviation of this code from the ANSI standard, if any, is not currently known. The degree of ANSI compliance of the source code in the f2c I77 and F77 libraries is also unknown.

Testing

NAIF subjects CSPICE to an extensive set of tests designed to exercise the f2c's and wrapper code. These tests run on all supported platforms.

CSPICE functionality

-- SPK: loader, readers, writers

-- Binary PCK: loader, readers

-- CK: loader, readers, writers

-- EK: loader, query and fetch functions, fast writers, record-oriented write, update, and read functions, summary functions

-- DAF: array search and summary functions

-- Kernel pool: loader, summary, fetch, watcher, kernel pool write access routines

-- Coordinate systems: translation between rectangular and cylindrical, latitudinal, geodetic, and RA/Dec systems.

-- Body name/code translation

-- Matrix and vector functions

-- Rotation functions

-- Euler angle functions

-- Quaternion functions

-- Time conversion functions

-- Spacecraft clock functions

-- Ellipsoid functions: near point, surface intercept, outward normal

-- SPICE ellipse and plane functions

-- SPICE error handling functions

-- Higher level geometry functions: sub-observer point, sub-solar point, illumination angles

-- Constant functions: standard epochs, radian/degree conversion, speed of light

-- Array manipulation functions

-- Frame utilities: map body to associated frame, get descriptive information for a specified frame

-- File utilities: map kernel to architecture and type, close file opened by translated Fortran code, test whether file exists

-- String utilities: case-insensitive string equivalence, white space equivalence, last non-blank, case conversion.

-- Numeric utilities: maximum and minimum integers and double precision numbers, maximum and minimum of sets of scalars

-- Windows, Set, and Cell functions

-- Brief - A command line utility used to summarize the information within an SPK file.

-- Chronos - A utility used to convert time values between various formats, e.g. UTC to ET, Et to SCLK, etc.

-- CKbrief - A command line utility used to summarize the pointing data within a CK file.

-- Commnt - A utility, usable in either interactive or command line mode, used to examine and manipulate the comment area of a SPICE binary kernel.

-- Inspekt - An interactive program used to examine and query to contents of an E-kernel.

-- MkSPK - An application used to create, or append to, SPK files.

-- Spacit - An interactive application used to manipulate and examine SPK files.

-- SPKmerge - An application used to merge several SPK files or subsets of SPK files into a single SPK file.

-- Tobin - A command line utility used to convert SPICE ASCII transfer files to architecture native binary kernels.

-- Toxfr - A command line utility used to convert SPICE binary kernels to ASCII transfer format.

-- Version - A command line utility that outputs the SPICE toolkit version and platform specific parameters: System, OS, Compiler, binary file type (big/little endian IEEE, non-IEEE, etc.), line terminator, values for min/max DPs, and values for min/max INTs.

CSPICE contains the following cookbook programs:

-- SIMPLE: this program calculates the angular separation of two target bodies as seen from a specified observer.

-- STATES: this program calculates the state (position and velocity) of a target body relative to an observer, at a specified epoch. Observer and target are specified by the user.

-- SUBPT: this program calculates the planetocentric latitude and longitude of the nearest point on a target body to an observer. The target, observer, and epoch are user-specified.

-- TICTOC: this program performs conversion between UTC and ET.

The CSPICE API

The CSPICE API is designed to mimic the corresponding Fortran interface, while adhering to natural C-language argument list conventions. The following conventions are followed:

-- String arguments are C-style: strings are always null-terminated on input and output.

-- Each output string argument has a corresponding input length argument via which the caller indicates how much room is available in the output string.

-- Multi-dimensional arrays have normal C-style storage order: the rightmost index varies the fastest. For 2-dimensional matrices, this means the matrix storage is in row-major order. This is the transpose of the Fortran order.

-- Arguments involving arrays of strings are implemented as two-dimensional character arrays, not arrays of character pointers.

-- CSPICE typedefs (discussed below) are used to declare all arguments and function return values.

Each API function contains a complete NAIF-style header documenting the specification of that function. These hand-coded API functions are called ``wrappers.'' They typically serve to encapsulate C code generated by f2c. However, many of the simpler routines, such as the linear algebra functions, are fully coded anew in C and do not call translated Fortran routines.

Wrapper calls are denoted by file names ending with the suffix

   _c

   _c.c

   _

The few routines written in C that replace modules generated by f2c follow the same function and source file naming conventions as the code generated by f2c.

Users' application functions calling the CSPICE API must include the CSPICE header file SpiceUsr.h. This header file defines function prototypes for each CSPICE API routine. Also, typedefs used in the prototypes are declared by this header.

Below is a code fragment showing inclusion of SpiceUsr.h and a call to the SPK reader function spkezr_c.

   #include "SpiceUsr.h"
 
   SpiceDouble    et;
   SpiceDouble    lt;
   SpiceDouble    state [6];
             .
             .
             .
   spkezr_c ( "SUN", et, "J2000", "LT+S", "EARTH", state, < );

Documentation

The CSPICE documentation set consists of:

-- Reference Guide. A set of HTML pages consisting of the header information from each wrapper file. The pages include cross-links to other reference pages when a header references any wrapper, i.e. if a page refers to another wrapper, a link exists to that wrapper's HTML page.

-- Required Reading files. These are C-oriented versions of the corresponding documents in the Fortran SPICE Toolkit. CSPICE provides C versions of the following Required Reading files:

            cells.req
            ck.req
            das.req
            ek.req
            error.req
            frames.req
            intrdctn.req
            kernel.req
            naif_ids.req
            pck.req
            problems.req
            rotation.req
            sclk.req
            sets.req
            spc.req
            spk.req
            time.req
            windows.req

Several Fortran Required Reading files have not yet been converted to C style; the Fortran versions of these are included in CSPICE. These files are:

            daf.req
            ellipses.req
            planes.req
            scanning.req
            symbols.req

-- User's Guides. These are the same documents provided in the Fortran SPICE Toolkit.

            brief.ug
            chronos.ug
            ckbrief.ug
            commnt.ug
            convert.ug
            inspekt.ug
            mkspk.ug
            simple.ug
            spacit.ug
            spkmerge.ug
            states.ug
            subpt.ug
            testutil.ug
            tictoc.ug
            tobin.ug
            toxfr.ug
            version.ug

-- A permuted index, cspice.idx. This document maps brief abstracts describing functionality of routines to names of routines.

Kernel files

For each platform, CSPICE uses the same binary and text kernels as the Fortran SPICE Toolkit for that platform.

As of release N0059, the kernel pool readers (ldpool_c, furnsh_c) have the capability to read non platform-native text kernels, e.g. read a DOS native text file on a Unix platform and vice-versa. This capability does not exist in the Fortran toolkit.

Transfer format files produced by the CSPICE versions of SPACIT and TOXFR have very slight white space differences as compared with transfer format files produced by the Fortran counterparts of these programs. These differences do not affect the functioning of the transfer files: those produced by the Fortran SPICE Toolkit may be used with CSPICE and vice versa.

Installation

CSPICE is obtained and installed in a manner completely analogous to that used for the Fortran SPICE Toolkit. Access the NAIF site for download instructions at URL:

      http://naif.jpl.nasa.gov/naif/toolkit.html

Directory Structure

The package has the same directory structure as SPICELIB, with the addition of an html subdirectory of the doc directory:

                                 cspice
                                    |
                                    |
   /data   /doc   /etc   /exe   /include   /lib   /src   makeall
             |                               |      |
             |                               |      |
             |                               |      |
         /html  *.req ...                    |  /cspice /cook_c ...
             |                               |
         index.html                          |
                                             |
                                    cspice.a  csupport.a

Platforms

CSPICE currently is supported on the following platforms:

   Hardware      Operating system    Compiler
   --------      ----------------    --------
   PC            Linux               gcc
   PC            MS Windows          MS Visual Studio C++/C .Net
   Sun Sparc     Solaris 32 bit      Sun C
   Sun Sparc     Solaris 64 bit      Sun C
   Sun Sparc     Solaris 32 bit      gcc
   Macintosh     OS X                cc (gcc)

Calling CSPICE Wrappers

As indicated above, functions calling the CSPICE API must include the header file SpiceUsr.h. The code in this header file makes use of ANSI C features, so functions including it must be compiled as ANSI C. No special precompiler flags are needed to compile SpiceUsr.h.

On a Unix system, a typical compiler invocation for a function that calls CSPICE would look like:

   cc -c <ANSI flag> userfunc.c

   -I<path>

Examples of ANSI flags are:

   Sun C compiler              -Xc
   gcc                         -ansi

   /home/cspice

   cc -c -Xc -I/home/cspice/src/cspice userfunc.c

   Set DOS Environment variables (XP) for Visual Studio 7:
 
   To set:
      Control Panel -> System
                       select "Advanced" tab
                       push "Environment Variables" button
                       chose variable name
                       push "Edit" button
                       paste-in or type path strings
 
   INCLUDE
   C:\Program Files\Microsoft Visual Studio .NET\Vc7\include\;
   C:\Program Files\Microsoft Visual Studio .NETFrameworkSDK\include\
 
   LIB
   C:\Program Files\Microsoft Visual Studio .NET\Vc7\lib\;
   C:\Program Files\Microsoft Visual Studio .NETFrameworkSDK\Lib\
 
   PATH
   C:\Program Files\Microsoft Visual Studio .Net\Vc7\bin

Linking against CSPICE

On Unix systems, programs linking against CSPICE must also link against the C math library; this is normally accomplished using the ``-lm'' flag following cspice.a in the link command. A typical link command might look like

   cc -o myprog myprog.o <user objects> <user libs> \
   <lib path>/cspice.a -lm

   cl myprog.obj <user objects> <user libs> <lib path>\cspice.lib

CSPICE data types

To assist with long-term maintainability, CSPICE uses typedefs to represent data types occurring in argument lists and as return values of CSPICE functions. The CSPICE typedefs for fundamental types are:

   SpiceBoolean
   SpiceChar
   SpiceDouble
   SpiceInt
   ConstSpiceBoolean
   ConstSpiceChar
   ConstSpiceDouble
   ConstSpiceInt

   SpiceBoolean -> int
   SpiceChar    -> char
   SpiceDouble  -> double
   SpiceInt     -> int or long
   ConstX       -> const X  (X = any of the above types)

Ellipses and planes are represented by structures; these and their const-qualified counterparts are:

   SpiceEllipse
   ConstSpiceEllipse
   SpicePlane
   ConstSpicePlane

   SpiceEKAttDsc      {EK column attribute descriptor}
   SpiceEKSegSum      {EK segment summary}
   SpiceEKDataType    {Column data types}
   SpiceEKExprClass   {SELECT clause expression class}

While other data types may be used internally in CSPICE, no other types appear in the API.

Interface macros

To better support calling the CSPICE API from within C++, as well as to provide better compile-time error checking, CSPICE prototypes declare input-only array or pointer arguments using const qualification.

For example, here is the function prototype for mxm_c, CSPICE's 3 by 3 matrix multiplication function:

   void              mxm_c    ( ConstSpiceDouble    m1  [3][3],
                                ConstSpiceDouble    m2  [3][3],
                                SpiceDouble         mout[3][3] );

For example, the code fragment:

   double m1   [3][3];
   double m2   [3][3];
   double mout [3][3];
       .
       .
       .
   mxm_c ( m1, m2, mout );

Explicitly adding type casts to satisfy the compiler is possible but awkward: the call to mxm_c would then look like:

   mxm_c ( (const double (*)[3])m1,
           (const double (*)[3])m2, mout);

The interface macros have been designed to be transparent to users; they do not differ from their underlying wrappers in the way arguments are evaluated; in particular they do not have any unusual side effects.

As an example, here is the interface macro for mxm_c:

   #define  mxm_c( m1, m2, mout )                             \
                                                              \
        (   mxm_c ( CONST_MAT(m1), CONST_MAT(m2), (mout) )   )

   #define  CONST_MAT          ( ConstSpiceDouble   (*) [3] )

   mxm_c ( m1, m2, mout );

The definitions of the interface macros are automatically included when a calling program includes the CSPICE header file SpiceUsr.h.

CSPICE public declarations

In addition to the interface macros discussed above, CSPICE declares a small set of public macros.

Boolean values:

   SPICEFALSE
   SPICETRUE

   SPICEFAILURE
   SPICESUCCESS

   SPICE_EK_*

CSPICE function prototypes

Because CSPICE function prototypes enable substantial compile-time error checking, we recommend that user applications always reference them. Include the header file SpiceUsr.h in any module that calls CSPICE to make the prototypes available.

Calling functions generated by f2c

f2c's specification

The specification of the automatic Fortran-to-C translation program f2c can be summarized thusly: f2c attempts to create C code whose functionality is identical to that of the source Fortran code. Due to limitations of C and the system-dependent behavior of Fortran I/O, f2c cannot always completely succeed in fulfilling its nominal specification. However, the function argument lists generated by f2c can be understood by remembering that they act very much like Fortran, rather than C, argument lists.

f2c's treatment of argument data types occurring in the Fortran library SPICELIB are discussed below.

Header files

Prototypes and associated declarations for functions generated by f2c are provided in the header file SpiceZfc.h. This header must be included by any application code that calls these translated functions. The typical sequence of header inclusions is:

   #include "SpiceUsr.h"
   #include "SpiceZfc.h"

f2c data types

f2c uses typedefs to represent C data types used in the translated Fortran functions f2c creates. The Fortran data types used in the Fortran library SPICELIB and the corresponding typedefs generated by f2c are as follows:

   Fortran type                f2c typedef
   ------------                -----------
 
   DOUBLE PRECISION            doublereal
   INTEGER                     integer
   LOGICAL                     logical
   CHARACTER                   char

   ftnlen

Call by reference

With one exception, all arguments of functions generated by f2c are pointers. Passing input arguments by value is not permitted. To supply a value as an input argument, the value must be placed in a variable, and the address of the variable passed as an actual argument.

The one exception to the rule is string length arguments. These are always passed by value.

Arrays

The CSPICE wrappers handle the differences between C and Fortran concerning the ordering of array data in memory.

In Fortran, the ordering in memory of array elements is such that the index corresponding to the leftmost dimension of the array varies the most rapidly. For example, for two-dimensional arrays, the first column is at the start of the memory occupied by the array, the second column comes next, and so on. This is called ``column major'' order, and is the transpose of the order used in C.

Consequently, matrix arguments to functions generated by f2c must be transposed prior to input and after output in order to be correctly used by a calling C program. The CSPICE functions xpose_c and xpose6_c may be used to transpose 3x3 and 6x6 matrices respectively.

Strings

In Fortran, the ability to determine the declared length of a string is built into the language. Fortran strings are not null terminated; unused space in the trailing portion of a string is padded with blanks.

Functions generated by f2c must be able to determine the length of strings on input without relying on null termination; on output, strings are returned from these functions blank-padded without null termination.

When f2c processes a Fortran character string argument, the argument list of the output C function contains two arguments corresponding to the single Fortran string argument: a character pointer argument and a string length argument. The string length arguments occur consecutively at the end of the function's argument list. The nth string length argument gives the string length of the nth string argument.

For example, the Fortran argument list:

   CHARACTER*(80)     TARG
   DOUBLE PRECISION   ET
   CHARACTER*(10)     REF
   CHARACTER*(4)      ABCORR
   CHARACTER*(80)     OBS
   DOUBLE PRECISION   STATE
   DOUBLE PRECISION   LT
 
   SPKEZR ( TARG, ET, REF, ABCORR, OBS, STATE, LT )

   int spkezr_ (  char        * targ,
                  doublereal  * et,
                  char        * ref,
                  char        * abcorr,
                  char        * obs,
                  doublereal  * state,
                  doublereal  * lt,
                  ftnlen        target_namlen,
                  ftnlen        ref_namlen,
                  ftnlen        abcorr_namlen,
                  ftnlen        obs_namlen     )

   void spkezr_c ( ConstSpiceChar    * targ,
                   SpiceDouble         et,
                   ConstSpiceChar    * ref,
                   ConstSpiceChar    * abcorr,
                   ConstSpiceChar    * obs,
                   SpiceDouble         state[6],
                   SpiceDouble       * lt       )

The character string arguments generated by f2c are expected to contain Fortran-style strings: a string argument should not contain a null terminator unless it is part of the string's data. Output strings will not be null-terminated but will be padded up to the designated length with trailing blanks.

Arrays of strings

In f2c created functions, string array arguments are particularly tricky because of the difference in the way C and Fortran determine string lengths. A C array of N strings of declared length M maps to a Fortran array of N strings of length M-1, since the Fortran string array contains no null terminators. So, preparing a C string array to be passed as an input to a function generated by f2c requires creating a new array without null terminators. Similarly, an output string array from a function generated by f2c must have null terminators added.

If you find it necessary to call one of these functions, we suggest you contact NAIF; we'll provide you with a C wrapper for the function in question.