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\date{\verb$Date$}

\title{Cactus 4.0 Documentation\\
\small{\verb$Revision$}
}

\maketitle



\tableofcontents

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\part{Introduction}
\chapter{Overview of documentation}

This documentation splits naturally into these parts: 

\begin{itemize}

\item {\bf The history of Cactus and the Concepts behind the
    CCTK. Getting you up and running.}
  We give an overview on the required hardware and
  software and we willll talk you through the installation of a working
  CCTK. You will be able to verify the correct installation by the
  CCTK testsuite. We show how to perform some basic simulation and
  introduce the concepts behind data output. We provide brief
  information on how to vizualize the 1D, 2D and 3D data.

\item {\bf Part2: How to write a physics thorn.} We introduce a
  sample thorn that illustrates the implementation of simple initial
  data and the subsequent evolution. You will learn how to use the
  programming interface to take advantage of parallelism and modularity.

\item {\bf Part3: How to writes an infrastructure thorn.} In this more
  advanced part, we talk about user supplied infrastructure routines
  as {\em additional output routines, boundary conditions, etc.}

\item {\bf The standard toolkit.} 

\end{itemize}

Other topics will be discussed in separate documents:

\begin{itemize}

\item The numerical relativity toolkit.

\item A description of the flesh, for the maintainers.

\end{itemize}

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\chapter{A Cactus history}

To begin with a one-phrase summary: Cactus is a numerical
infrastructure for solving partial differential equations (typically
in 3-dimensional space and time), a modular code for solving the
Einstein equations, and an international collaboration in numerical
general relativity pooling ideas in this code.

Cactus 1.0 was written in 1996 by Joan Mass\'o and Paul Walker out of
frustration with the numerical relativity codes they had been working
on. It has come a long way. The name is meant to suggest a modular
structure consisting of a ``flesh'' (or backbone, or core) and
``thorns'' (or modules, or collections of subroutines). Some essential
features of the code have been present from the beginning:

\begin{itemize}

\item Parallelism (the work is split between multiple processors, or
multiple computers) that is largely automatic and hidden from the
user.

\item A modular structure with a fixed interface between the flesh and the
thorns, which allows users to add new parts.

\item Giving the user a choice of C and Fortran for writing thorns, while
automating the interface between the two languages.

\item A spirit of open-source collaboration similar to the gnu
project.

\end{itemize}

Other aspects have changed completely since Cactus 1.0. The flesh
started out as a complete numerical relativity code, using one
particular formulation of the Einstein equations, with the thorns
providing only initial data, analysis, and other peripheral
functions. In 4.0 not only the Einstein equations and general
relativity basics, but even input/output and parallelism are provided
by thorns. The only function of the flesh is now to define the
interface between thorns, to hold varying collections of thorns, and
to provide an interface between C and Fortran routines. 

The aim of this radicalism is to allow for future growth. We now
distinguish informally between physics thorns and infrastructure
thorns. The main infrastructure project for now is adding different
flavors of adaptive mesh refinement -- while the user writing a
physics thorn can still large she is working a single-processor,
unigrid code. Similarly, an infrastructure thorn could be replaced by
one that does the same job, but simply better. Even the flesh could be
rewritten in the future, but the interface between the flesh and
thorns is to remain unchanged.

Cactus thorns can be written in F90, F77 and C (or a mixture). The
decision to support Fortran was made for the pragmatic reason that
many people like it, or don't write C. This decision does make the
flesh more complex. The flesh is written in C, and in perl that
generates C.

The change from Cactus 3.0 to Cactus 4.0 is the largest one yet. The
new interface forces modifications on all existing thorns, but it is
intended not to change again. {\q Can we realistically say any more?}

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\chapter{Cactus concepts} 

Cactus is designed to solve time evolution problems in three space
dimensions by finite differencing on a Cartesian grid. This typically
includes solving elliptic equations to obtain initial data, evolving
them by hyperbolic or parabolic equations, and perhaps solving more
elliptic equations at each time step.  One basic concept is therefore
a series of initialization steps followed by a do-loop that implements
the time evolution, along with periodic analysis and output. Problems
that do not naturally fit into this concept are methods which are
global in space or time (such as pseudo-spectral methods) or methods
which do not have a grid (such as finite elements). Thorns interact
with the flesh by, among other things, announcing when they want to be
called in this basic schedule.

Another basic concept is parallelism by distributing spatial domains
among processors. Finite differencing is an almost local procedure,
which means that each processor can work on its domain of space,
needing to communicate only with neighboring processors in order to
work on points of the boundary of its domain. The domain decomposition
is automatic and largely hidden. A physics thorn can pretend to work
on a single grid, which in reality is only a chunk of the complete
numerical domain. This chunk consists of an inner region of points the
processor ``owns'', and needs to update, surrounded by a ``ghost
zone'' of points which it can use but doesn't need to update -- they
are updated by neighboring processors. Clearly this concept is geared
towards explicit finite differencing schemes for the time-evolution
part. {\q Say something about elliptic equations here, and implicit
schemes}

The notion that we are dealing with a collection of fields that live
on the same (typically three-dimensional) space, and that evolve
together in time defines the third basic concept, that of ``grid
functions'' grouped together in a ``grid hierarchy''. Multiple
grid hierarchies arise, for example, when one wants to cover the
surface of a sphere with two Cartesian coordinate patches, or in
adaptive mesh refinement.  Grid functions are defined on Cartesian
grids that are equally spaced in each of the three coordinates $x$,
$y$ and $z$. (These could be spherical type coordinates, although that
was not the original intention.) Grid functions can be two or
one-dimensional as well as three dimensional.

Routines are grouped in thorns, and thorns informally in toolkits,
such as the standard toolkit, and the general relativity
toolkit. The routines inside a thorn can interact in any way, but
thorns interact with the flesh and with each other only through a
well-defined interface. Confirming to this interface guarantees that
thorns will be compatible with, and useful for, all future extensions
of the code. The interface also helps users and developers to
understand what a given thorn does. 

The flesh and many thorns are public and can be used by anyone,
assuming ethical behavior. Public thorns and the flesh are distributed
by CVS (see {\q where?} below). Public thorns are checked out in
toolkits. The flesh may change in the future but this should never
affect users, as Cactus 4.0 is making every effort to codify and
freeze the interface. Public thorns should of course not lose
important functionality they once had. (Nevertheless, CVS gives the
user access to all earlier versions.)

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\chapter{Installation} 

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\section{Required software}

In general, Cactus {\em requires} the following set of software to function
in a single processor mode. Please refer to the architecture section
\ref{sec:architecture} for architecture specific items.
\begin{itemize}
\item{\tt Perl5.0} Perl is used extensively during the CCTK
  configuration phase. {\tt Perl} is available for nearly all
  operating systems known to man and can be obtained at {\tt
    http://www.perl.org}.
\item{\tt gmake} The GNU make utility needss to be installed. Errors
 {\q wanna name them here ?} during the make process  are often caused
 by not using {\tt gmake}. {\tt gmake} can be obtained from any
 friendly GNU site.
\item{\tt C/C++} A C/C++ compiler. These can be the GNU compilers as well.
\item{\tt Fortran 90} 
\item{\tt CVS} The {\em ``Concurrent Versioning System''} is not needed
  to run/compile the CCTK, but you are strongly encourage to install
  this software to take advantage of the update procedures.
\end{itemize}

\noindent
The following software is {\em optional} and not needed to get the
basic CCTK installation on track. Its listed in the order of importance.
\begin{itemize}
\item{\tt Fortran 90} CCTK requires a F90 compiler to run modules
  written in F77 or F90. There is no
  GNU F90 and just a very limited set of free F90 compilers for
  various architectures.
\item{\tt MPI} Currently the communication layer of the CCTK uses the
  {\em Message Passing Interface (MPI)} provide inter process
  communication. Supercomputers very often supply a native MPI
  implementation at their site. CCTK is very likely to work with
  them. For local area networks we suggest installing the {\tt MPICH}
  version, which can be obtained for various architectures and
  operating systems at {\tt http://www-unix.mcs.anl.gov/mpi/}.
\item{\tt TAGS}: Tags enables you browse through the calling structure
  of a program by help of function call database. Navigating the CCTK and
  Toolkits becomes very easy. Emacs and vi both support this database. See
  \ref{sec:tags} how to install {\tt TAGS}.
\end{itemize}

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\section{Supported architectures}
Cactus runs on many machines, under a large number of Unix operating
systems. Here we list all the machines we have compiled and verfied
Cactus on, including some architecture specific notes. 
\begin{itemize} 
\item{\bf SGI Origin 2000}
\item{\bf SGI}
\item{\bf Cray T3E}
\item{\bf Dec Alpha}: Dec operating system and Linux. Single processor
  mode and MPICH.
  supported. The Decs need to have the GNU {\tt C/C++} compilers installed.
\item{\bf Intel Linux}: There is a
  free Linux F90 compiler available from  {\tt http://www.sierra.com}
  -- the only free we know of. Single processor mode and MPICH
  supported.
\item{\bf Windows32 using Cygwin32 extensions} {\q still true?}
\item{\bf Windows NT}: succesful compile with the following software:
  \begin{itemize}
  \item{\tt DIGITAL Visual Fortran Optimizing Compiler Version: V5.0C}
  \item{\tt Microsoft (R) 32-bit C/C++ Optimizing Compiler Version
        11.00.7022 for 80x86}
  \item{Cygnus Unix suite version 1.19}
  \end{itemize}
\item{\bf Solaris } {\q What is the status on this ??}
\end{itemize}
If you did not find you architecture/operating system of
choice, see section sec\ref{sec:how_to_port} on how to port to an
unsupported architecture.

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\section{Checkout procedure}

Cactus is distributed, added to, and updated through the free CVS
software. CVS ({\em ``Concurrent Versioning System''}) allows many
people to work on a large software project together without getting
into a tangle by keeping track of changes made by the different
users. For the beginner, we summarize the basics in appendix
\ref{sec:CVS}, please refer to this section for small description of
the CVS syntax.

The installation requires about {\q HOW MANY MEGS ? }. See section
\ref{sec:downsize_CCTK} on how to downsize CCTK installtion if space is tight.
{\q gmake small ??}

\begin{itemize}
\item{\bf Login}: Prior to any CVS operation, you need to loginto the central
  repository. For an  anoymous checkout, type:\\
  {\tt
    cvs -d :pserver:cvs\_anon@hod.aei-potsdam.mpg.de:/usr/users/cactus login
    }
  You will be prompted for a password which is {\tt anon}. 
\item{\bf Checkout}: To obtain a fresh CCTK checkout, type
  {\t
    cvs -d :pserver:cvs\_anon@hod.aei-potsdam.mpg.de:/usr/users/cactus checkout cactus
    }
  The CVS checkout procedure will create a directory called {\tt
    ./CCTK} and install the CCTK inside this directory.  From now on we
  will reference all directory names relative to {\tt ./CCTK}. 
  
\item{\bf Update}: To update an existing CCTK checkout (to patch in
  possible changes, etc.), do the following {\em within} the {\tt ./CCTK} directory.
  {\t
    cvs -d :pserver:cvs\_anon@hod.aei-potsdam.mpg.de:/usr/users/cactus update
    }
  The update process will operate downwards relative to you current position
  within the CCTK tree. To update only certain directories, change
  into these directories and issue the update command.
\item{\bf CVS status}: to obtain a status report on the ``age'' of your
  CCTK or Toolkit routines (from your current directory position
  downward), type
  {\t
    cvs -d :pserver:cvs\_anon@hod.aei-potsdam.mpg.de:/usr/users/cactus status
    }
\item{\bf non-anonymous CVS}: if you have an account at the central
  repository and you would like to perform any of the operation above
  {\em non-anonymously}, replace {\tt cvs\_anon} by your login name
  and provide the appropriate password during the CVS login
  process. Depending on your permissions, you may then make commits to the CCTK
  or its Toolkits.
\item{\bf Commits}: you need to perform a personalized login and have
  proper permissions to commit code to the repository. 
\end{itemize}

For more CVS commands please refer to appendix \ref{sec:CVS}.

{\q Change CCTK to cactus in the distribution}

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\section{Directory structure}

A fresh CCTK checkout creates a directory {\tt ./CCTK} with the
following files substructure:
\begin{itemize}

\item{\tt ./CVS/} the CVS book-keeping directory, present in every subdirectory.

\item{\tt ./doc/} CCTK documentation

\item{\tt ./lib/} contains libraries.

\item{\tt ./src/} contains the source code for the CCTK

\item {\tt ./toolkits/} contains the Cactus toolkits. The toolkits
  (the actual ``physics'') is not supplied by the CCTK. If the  toolkits
  you want to use are part of the central repository, they can be
  checked out in similar way the CCTK.  Each directory inside {\tt
    toolkits/} contains thorns. {\q ??}
\end{itemize}

When Cactus is first compiled it creates a new directory {\tt
./CCTK/build/}. Disk space may be a problem on supercomputers where
home directories are small. A workaround is to first create a
build directory on scratch space, say {\tt scratch/cactus\_build/} (where
{\tt scratch/} is your scratch directory), and soft link ({\tt ln -s
scratch/cactus\_build cactus/build/}) it to the Cactus directory.

When the code has been compiled for some configuration CONF, the
directory {\tt build/CONF/} contains the following:

\begin{itemize}

\item {\tt ActiveThorns} 

\item {\tt datestamp.o}

\item {\tt bindings/} contains all the files created by the perl
scripts, sorted into subdirectories {\tt Parameters}, 
{\tt Variables}, {\tt Schedule}, {\tt thorn\_***} (one for each thorn).

\item {\tt build/} contains the post-processed source code, the
dependencies and the object files, sorted into the subdirectories {\tt
Cactus}, {\tt CactusBindings} and {\tt thorn\_***}.

\item {\tt config-data} : contains the files created by the configure
script:

\begin{itemize}

\item {\tt config.cache}

\item {\tt config.h}

\item {\tt config.log}

\item {\tt config.status}

\item {\tt fortran\_name.pl} (used to determine how fortran routines
are called from C) 

\item {\tt make.config.defn} (contains compilers etc. for a configuration)

\item {\tt make.config.deps}

\item {\tt make.config.rule} (rules to generate object files from source files)
\end{itemize}

\item {\tt lib} : contains the libraries built from the object files,

\begin{itemize}

\item {\tt libCactus.a} for the flesh

\item{\tt llibCactusBinding.a} for the Bindings 

\item {\tt llib***.a} for each thorn

\end{itemize}

{\q It might be useful to give a couple of commands like nm for seeing
what is in libraries.}

\item {\tt scratch}: empty at the end.

\end{itemize}

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\section{Getting help}

Cactus Maint and GNATS

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\section{Testing} 

simple make and test suite

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\chapter{Compilation} 

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\section{Compilation of the flesh, and configurations}

Cactus can be built in different configurations from the same copy of
the source files, and these different configurations coexist in the
{\tt cactus/} directory. Different configurations can be for different
machines. This is useful when your {\tt cactus/} directory is mounted
on different machines in a cluster, and you want to run cactus on all
these machines while only keeping and updating a single copy of the
source files. It is also useful if you want to compile cactus with
different compilers or optimization settings on the same machine, and
compare one against the other. As a third example, you may want to
compile with different sets of thorns.  As a final example, you might
want to build cactus using different multi-processor communication
protocols, such as MPI (currently the cactus standard) or GLOBUS.

Building the code should be easy. Just type

{\t gmake <configuration options> <CONF>}

where {\tt <...>} denotes optional arguments. {\tt gmake} is the gnu
make program. Note that {\tt make} will not work (if you try, you'll
get funny error messages.) Without any arguments, the make process
picks a default configuration (looking for standard compilers and so
on) and gives this configuration a default name (typically the name of
your operating system). It then runs configuration scripts (in {\tt
lib/make/}) which figure out details of your computer.

You'll see that the make process has created a {\tt build/} directory,
and in it a directory {\tt build/CONF}, where {\tt CONF} is the name
of the configuration, in this case the default name chosen for you.
Inside {\tt CONF}, {\tt CONF/config-data/} now contains the results of
the configuration process. To actually compile with this
configuration, type

{\t gmake CONF}

where {\tt CONF} is the name of the configuration. The argument {\tt
CONF} is now obligatory. 

gmake will ask if you want to edit the file {\tt
CONF/ActiveThorns}. As the name says, it contains the names of all the
thorns you want compiled in. By default it contains all the thorns you
have checked out. If you say {\tt yes}, gmake opens the editor
specified in your {\tt EDITOR} shell variable. gmake continues when
you close the editor. Alternatively, you could have created {\tt
CONF/ActiveThorns} beforehand. {\tt ActiveThorns} can contain the
names of individual thorns or entire toolkits {\q Implement this}.

{\q move setup file elsewhere.}

{\q move executable elsewhere, and make one for each configuration.}

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\section{Compiling in thorns}

Cactus compiles in all thorns listed in {\tt
build/CONF/ActiveThorns}. {\q If the thorn has not been checked
out...} Cactus looks for source code in a thorn's {\tt src}
directory. It first looks for a file {\tt make.code.defn} which
should contain

\begin{itemize}

\item {\t SRCS = <list of all source files in this directory>}

\item {\t SUBDIRS = <list of all subdirectories containing source files>}

\end{itemize}

and each subdirectory mentioned should also contain a {\tt
make.code.defn} file with a {\tt SRCS} line ({\tt SUBDIRS} in a
subdirectory {\tt make.code.defn} file will be ignored).

The {\tt make.code.defn} file in a directory is included at the top of
the makefile {\q where does it sit?}  used to build the sources. If a
file {\tt make.code.deps} exists in the directory {\q where?} it is
included at the bottom.

The standard make system may be overridden by placing a makefile
called {\tt Makefile} in the {\tt src/} directory.  This can do
whatever it likes, but must create a library called

{\tt \$(NAME)}

(which is actually {\tt \$(CCTK\_LIBDIR)/lib<thorn>.a }). {\q We need more detail here. What does ``is
actually'' mean?}

{\q Watch for CCTK here and elsewhere. Don't remove this query before
CCTK is gone.}

A makefile is passed the following variables

{\q How many makefiles are involved? Which are created at runtime etc.}

\begin{itemize}

\item {\tt \$(CCTK\_HOME)} - the main CCTK directory

\item {\tt \$(TOP)}    - the CONF directory

\item {\tt \$(SRCDIR)} - the directory in which the source files can be found

\item {\tt \$(CONFIG)} - the directory containing the configuration files

\item {\tt \$(THORN)}  - the thorn name

\item {\tt \$(SCRATCH\_BUILD)} - the scratch directory where f90 module
files should end up if they need to be seen by other thorns.

\item {\tt \$(NAME)}.

\end{itemize}


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\section {gmake targets}

(A target for {\tt gmake} can be naively thought of as an argument
that tells it which of several things listed in the {\tt Makefile} it
is to do.)

\begin{itemize}

\item {\tt gmake config} will create the default (uname) configuration

\item {\tt gmake <config>-clean} cleans a configuration (rm -r *.o)

\item {\tt gmake <config>-delete} deletes a configuration (rm -r build/<config>)

\item {\tt gmake help} lists all make options

\item {\tt gmake tags} creates a {\tt vi} style tags file

\item {\tt gmake TAGS} creates an Emacs style TAGS file

\item {\tt gmake distclean} nukes your build directory.

\end{itemize}


{\q Tom, this is for you}

        b) Make methodology (dependencies,libraries, thorn make files
        etc)

      d) Configuration options

      f) (New platforms??)

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\chapter{Running Cactus}

      a) Command line options
      b) Simple parameter file syntax (will be explained fully in thorn
         writers part)
      c) Description of standard parameters (although maybe should be an
         appendix)
      d) Checkpointing and outputting
      e) Reference to web pages with machine dependant stuff
      f) Understanding the screen output (RFR tree, errors and warnings etc)
      g) Convergence mode
      h) Environment variables

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\chapter{Looking at output}

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\part{Application thorn writing}

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\chapter{Thorn concepts} 

      (This section has to contain enough explanation to make the rest of
       the writers guide readable the first time through)
(The hardest bugs to find are
those arising from plausible but incorrect assumptions about the
behavior of someone else's thorn.)
      a) Again probably emphasize collaboration, what are thorns,
         toolkits, how to share them.
      b) Things to think about before you start programming:
             Language, read all the documentation, emphasize use of
             standard supported Cactus infrastructure
      c) Available data types
         i)   Scalars
         ii)  Arrays and GFs
         iii) Groups
      d) Ghost zones and parallelism
      e) Understanding the RFR concept
      f) Understanding the GH concept

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\chapter{Anatomy of a thorn}

Each thorn must be in a toolkit. Toolkits are fairly informal. If you
want, you can just make your own toolkit directory, and stick your new
thorn into it.

      a) Creating a thorn (gmake new thorn)
      b) Thorn directory structure
      c) Thorn make files
      d) Thorn configuration
         (i)   Parameters (param.ccl)
         (ii)  Variables (interface.ccl)
         (iii) Scheduler (schedule.ccl)

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\chapter{Putting code into your thorn}

      a) What the flesh basically provides (header files, macros, ...)
      b) Argument lists and parameters
      c) A simple example (Baloney)
      d) Differences between F and C with the interface
      e) A more complex example (Wavetoy)
      f) Error handling and code stopping
      g) Calling C from F, F from C, (C++ from F? ....)
      h) Good Cactus Style Programming Guidelines
      i) Adding a test suite
      j) Sharing your thorn with the world.

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\part*{Appendices}

\appendix

\chapter{Using GNATS}
\chapter{Using CVS}
\chapter{Using TAGS}
    \section{TAGS with emacs}
    \section{TAGS with vi}


\chapter{Configuration file syntax}
    
\section{General Concepts}

Each thorn is configured by three compulsory files in the top level thorn
directory:
\begin{itemize}
\item{} interface.ccl
\item{} param.ccl
\item{} schedule.ccl
\end{itemize}
These files are written in the {\it Cactus Configuration Language} which is
case insensitive.

\section{interface.ccl}

The interface configuration file consists of a header block giving details 
of the thorns relationship with other thorns, and then a series of blocks 
listing the thorn's global variables.
The header block has the form:
{\t 
\begin{verbatim}
implements: <implementation>
[inherits: <implementation>, <implementation>]
[friend: <implentation>, <implementation>]
\end{verbatim}
}
where
\begin{itemize}
\item{} The implentation name must be unique among all thorns, except between 
        thorns which have the same public and protected variables and 
        parameters.
\item{} Inheriting from another implentation makes all that implentations 
        public variables available to your thorn. At least one thorn 
        providing any inherited implentation must be present at compile time.
        A thorn cannot inherit from itself. Inheritance is associative and 
        recursive, but not commutative.
\item{} Being a friend of another implementation makes all that 
        implementations 
        protected variables available to your thorn. At least one thorn
        providing an implementation for each friend must be present at
        compile time. A thorn cannot be its own friend. Friendship is 
        associative and commutative. Note that your thorn is also a friend
        of all your thorns friend's friends.
\end{itemize} 

The thorn's variables are defined by:
{\t
\begin{verbatim}
[<access>:]

VTYPE <group name> [TYPE=<gtype>] [DIM=<dim>] 
[{
   <variable name>[,]<variable name>
   <variable name>
}]
\end{verbatim}}

\begin{itemize}
\item{} {\t access} defines which thorns have can use the following 
        groups, and can be either {\t public}, {\t protected} or {\t private}.
\item{} {\t VTYPE} defines the data type of the variables in the group.
Supported data types are {\t LOGICAL}, {\t INTEGER}, {\t CHAR} and {\t REAL}.
\item{} {\t group name} must be a alpha-numeric name which is unique 
within the scope of the thorn. A group name is compulsory.
\item{} {\t TYPE} designates the kind of variables held by the group.
The choices are {\t GF}, {\t ARRAY} or {\t SCALAR}. This field is 
optional, with the default variable type being {\t SCALAR}.
\item{} {\t DIM} defines the spatial dimension if the group is of type
{\t ARRAY} or {\t GF}, and can take the value {\t 1}, {\t 2}, or {\t 3}.
The default value is {\t DIM=3}.
\item{} The block following the group declaration line contains a list of
        variables contained in the group. All variables in a group have 
        the same data type, variable type and dimension. The list can be 
        separated by spaces, commas, or new lines. The variable names 
        must be unique within the scope of the thorn. 
        A variable can only be a member of 
        one group. The block must be delimited by brackets on new lines.
        If no block is given after a group declaration line, a 
        variable with the same name as the group is created.
\end{itemize} 

\section{param.ccl}

The parameter configuration file consists of a list of 
{\it parameter object specification items} (OSIs) giving the type and
range of the parameter separated by optional
{\it parameter data scoping items} (DSIs) which detail access to the
parameter.

\subsection{Parameter Data Scoping Items}
{\t
\begin{verbatim}
<access>:
\end{verbatim}
}
The key word {\t access} designates that all parameter object specification
items up to the next parameter data scoping item are in the same 
protection or scoping class. {\t access} can take the values:
\begin{itemize}
\item{} {\t public} --- all thorns have access to public parameters
\item{} {\t protected} --- other thorns can have access to these 
                           parameters if they specifically request
                           it in their own param.ccl
\item{} {\t private} --- only your thorn has access to private parameters
\item{} {\t friend} --- in this case an {\t implementation} name must follow
                        the colon. It declare that all the parameters in 
                        the following scoping block are protected variables
                        from the specified {\t implementation}.
\end{itemize}


\subsection{Parameter Object Specification Items}

{\t
\begin{verbatim}
[EXTENDS] <parameter type> <parameter name> ``<parameter description>'' 
{
  <PARAMETER VALUES>
} <default value>
\end{verbatim}
}
\begin{itemize}
\item{} Allowed {\t parameter types} are 
  \begin{itemize}
     \item{} {\t INTEGER} The specification of parameter values takes 
     the form or any number of comma-separated blocks of the form:
{\t
\begin{verbatim}
[<low-range>][::][<high-range>][::<step>]
\end{verbatim}
}
Where an empty field, or a {\t *} in the place of {\t low-range} or
{\t high-range} indicates $-\infty$ and $\infty$ respectively. The 
default value for {\t step} is 1. A number by itself denotes that
this number is the only acceptable value.
     \item{} {\t REAL} The range specification is the same as integers,
             except that here, no {\t step} implies a continuum of values.
     \item{} {\t KEYWORD} Each entry in the list of acceptable values 
             for a keyword has the form 
{\t
\begin{verbatim}
``<keyword value>'', ``<keyword value>'' :: ``<description>''
\end{verbatim}
}
     \item{} {\t STRING}  No allowed values should be specified for a logical 
             parameter.
     \item{} {\t LOGICAL} No allowed values should be specified for a logical 
             parameter.
  \end{itemize}

\item{} The {\t parameter name} must be unique within the scope of the thorn.

\item{} A thorn can declare that it {\t EXTENDS} a parameter that it is a 
        friend of. This allows it to declare additional acceptable values.
        By default it is acceptable for two thorns to declare the same 
        value as acceptable. 
\end{itemize}


\section{schedule.ccl}

The schedule configuration files consists of 
\begin{itemize}
\item{} assignments to switch on storage and communications 
        for array variables at the start of program execution.
\item{} {\it schedule blocks} which schedule a subroutine from the
        thorn to be called at a specified time during program 
        executation. Statements within the schedule block can
        be used to switch on storage and communication for groups
        of variables during the duration that the subroutine is called.
\item{} Conditional statements
\end{itemize}

{\it assignments statements} have the form:
{\t
\begin{verbatim}
[STORAGE: <group>, <group>]
[COMMUNICATION: <group>, <group>]
\end{verbatim}
}

Each {\it schedule block} in the file {\t schedule.ccl} must have
the syntax:
{\t
\begin{verbatim}
schedule <function name> at <time>
{
  LANG: <language>
  [STORAGE: <group>,<group>]
  [COMMUNICATION: <group>,<group>]
  [TRIGGER: <grid function>,<grid function>]
} ``Description of function''
\end{verbatim}
}

Any schedule block or assignment statements can be optionally 
surrounded by conditional {\t if-elseif-else}
constructs using the parameter data base. These can be nested,
and have the general form:
{\t
\begin{verbatim}
if (CCTK_Equals(<parameter>,<string>)) 
{
  [<assignments>]
  [<schedule blocks>]
}
\end{verbatim}
}

Conditional constructs cannot be used inside of a schedule block.


\chapter{CCTK Parameters}
\chapter{Thorn-Flesh Interface}

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\end{document}

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\part{Infrastructure Thorn Writing}

      a) Adding to the GH structure

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\part{The Standard Toolkit}

      a) thorn\_IO
      b) thorn\_PUGH
      c) ...

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\part{Flesh Maintenance}

Cactus uses GNU autoconf to build the files configure and config.h.in which
come with the distribution (these files are created by the Cactus
maintainers by running autoconf and autoheader respectively).


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