path: root/doc/UsersGuide/ThornWriters.tex

\begin{cactuspart}{2}{Application thorn writing}{$RCSfile$}{$Revision$}
\renewcommand{\thepage}{\Alph{part}\arabic{page}}

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\chapter{Thorn concepts} 


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\section{Arrangements}

Thorns are grouped into {\em arrangements}.  This is a logical grouping of
thorns which is purely for organisational purposes. For example, 
you might wish to keep all your initial data thorns in one arrangement,
and all your evolution thorns in another arrangement, or to have
separate arrangements for your development, private and shared 
thorns. 

The arrangements live in the `arrangements' directory off the main 
Cactus directory.  Arrangement names must be unique, and cannot start 
with a `\#', or finish
with `\~{}' or `.bak'.

Inside a arrangement directory there are directories for each thorn 
belonging to the arrangement.  
Thorn names have the same restrictions as arrangement names,
with the addition that a thorn cannot be called `doc'.  
This name is reserved for arrangement documentation.

\section{Implementations}

\label{sec:im}

One of the key concepts for thorns is the concept of the {\bf implementation}.
Relationships between thorns are all based upon relationship between the
{\bf implementations} they provide. 
In principle it should be possible to swap one thorn providing an 
implementation with another thorn providing that implementation,
without affecting any other thorn.

An {\bf implementation} defines a group of variables and parameters which
are used to implement some functionality.  For example the thorn 
{\tt CactusPUGH/PUGH} provides the implementation {\tt driver}.  This 
implementation is responsible for providing memory for grid variables and
for communication.  Another thorn can also implement {\tt driver}, 
and both thorns can be compiled in {\em at the same time}.  
At runtime, the user can decide which thorn providing {\tt driver} is used.  
No other thorn should be affected by this choice.

When a thorn decides it needs access to a variable or a parameter provided by 
another thorn, it defines a relationship between itself and the other thorn's
{\bf implementation}, not explicitly with the other {\bf thorn}.  
This allows the transparent replacement, at compile or runtime, of one 
thorn with another thorn providing the same functionality as seen by 
the other thorns.


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\chapter{Anatomy of a thorn}
\label{sec:anofath}

\section{thorns}
\label{sec:th}

A thorn consists of a subdirectory of a arrangement containing three 
administrative files

\begin{itemize}
\item{\tt interface.ccl}: the cactus interface, which defines the grid 
functions, variables, etc. See \ref{sec:in}.
\item{\tt param.ccl}: the parameters introduced by the thorn. See
\ref{param.ccl}.
\item{\tt schedule.ccl}: scheduling information for routines called by
the flesh.
\end{itemize}

Thorns can also contain  
\begin{itemize}
\item a subdirectory called {\tt src}, which should hold source files
\item a subdirectory {\tt src/include} for include files 
\item a {\tt README} containing a brief description of the thorn
\item a {\tt doc} directory for documentation
\item a {\tt par} directory for example parameter files
\item a {\tt test} subdirectory may also be added.  See \ref{sec:testsuites} for details.
\end{itemize}

\section{Creating a thorn}


To simplify the creation of a thorn, a make target {\tt
gmake newthorn} has been provided.

When this is run:

\begin{enumerate} 
\item{} You will be prompted for the name of the new thorn.
\item{} You will be prompted for the name of the arrangement you would
like to include your thorn in. Either enter a new arrangement name or pick 
one from the list of available arrangements that are shown.
\end{enumerate}


\section{Configuring your thorn}

The interaction of a thorn with the flesh and other thorns is controlled
by various configuration files.

These consist of:

\begin{itemize}

\item {\tt interface.ccl}
This defines the {\bf implementation} (Section~\ref{sec:im}) the thorn 
provides, and the variables the thorn needs, along with their 
visibility to other implementations.

\item {\tt param.ccl}
This defines the parameters that are used to control the thorn, along
with their visibility to other implementations.

\item {\tt schedule.ccl}
This defines which functions from the thorn are called and when they are 
called.

\end{itemize}

\subsection{General syntax of CCL files}

CCL {\bf Cactus Configuration Language} files are simple text files
used to define configuration information for a thorn.  CCL files are
case independent, and may contain comments introduced by the `\#' character,
which marks the rest of the line to the right of its appearence as a comment.

\subsection{The {\tt interface.ccl}}

The {\tt interface.ccl} file is used to declare the implementation 
provided by the thorn, and to define the variables provided by it.

The implementation is declared by a single line at the top of the file

\begin{verbatim}
implements: <name>
\end{verbatim}

Where {\tt <name>} can be any combination of alphanumeric 
characters and underscores, and is case independent (as is everything in a CCL file).

There are three different access levels available for variables

\begin{itemize}
\item {\tt Public}
Can be `inherited' by other implementations (see below).
\item {\tt Protected}
Can be shared with other implementations which declare themselves to
be friends of this one (see below).
\item {\tt Private}
Can only be seen by this thorn.
\end{itemize}

Corresponding to the first two access levels there are two relationship
statements that can be used to get variables from other thorns.

\begin{itemize}
\item {\tt Inherits: <name>}
This gets all {\tt Public} variables from implementation {\tt <name>}, plus all
variables it has in turn inherited.  An implementation may inherit from as
many thorns as it likes.
\item {\tt Friend: <name>}
This gets all {\tt Protected} variables from implementation {\tt <name>}, but, 
unlike {\tt inherits} it pushes this implementations {\tt Protected}
variables onto implementation {\tt <name>}.  This keyword is used to define
a group of implementations which all end up with the same {\tt Protected}
variables.
\end{itemize}

So, for example, an interface.ccl starting

\begin{verbatim}
implements: wavetoy
inherits:   grid
friend:     wave_extract
\end{verbatim}

declares that the thorn provides an implementation called `wavetoy', gets
all {\tt public} variables declared by an implementation called `grid', and
shares all {\tt protected} variables with `wave\_extract' and its friends.

For convenience variables are placed in groups.  The group has several
attributes:

\begin{itemize}

\item {\tt variable type}
e.g. REAL, INT, COMPLEX

\item {\tt name}
The name of the group

\item {\tt group type}
\begin{itemize}
\item {\tt SCALAR}
This is a single number.
\item {\tt GF}
This is an array of the default grid size.
\item {\tt ARRAY}
This is an array of any size.
\end{itemize}

\item {\tt Dim}
This is the dimension of the group.  (Meaningless for scalars.)

\item {TimeLevels}
This is the number of timelevels the group has.
\end{itemize}

A group specification consists of the variable type, followed by
the name of the group, then a space seperated list of the form
`attribute = value' .  Then a brace delimited block containing
a comma or newline seperated list of variables in the group.
A description of the group may be included on the line with the
closing brace.

For example

\begin{verbatim}

REAL fields type=GF TimeLevels=3 Dim=3
{
  phi
  a,b,c,d
} "Wave fields"

\end{verbatim}

defines a group of real GFs of dimension 3 each of which exists on
three time levels.

By default all groups are {\tt private}, to change this an access
specification of the form {\tt public:} or {\tt protected:} (or 
{\tt private:} to change it back) may be placed on a line by itself.  This
changes the access level for any group defined in the file from that point on.

All variables seen by any one thorn must have distinct names.

\subsection{The {\tt param.ccl}}

Users control the operation of thorns via parameters.  The {\tt param.ccl}
is used to specify the parameters used to control an individual thorn, and
to specify the values these parameters are allowed to take.  When the code
is run it reads a parameter file and sets the parameters if they fall
within the allowed values.

There are three access levels available for parameters:

\begin{itemize}
\item {\tt Global}
These parameters are seen by all thorns.
\item {\tt Restricted}
These parameters may be used by other implementations if they so desire.
\item {\tt Private}
These are only seen by this thorn.
\end {itemize}

A parameter specification consists of:
\begin{itemize}

\item {\tt the parameter type}
\begin{itemize}
\item {\tt REAL}
\item {\tt INT}
\item {\tt KEYWORD}
A distinct string with only a few known allowed values.
\item {\tt STRING}
An arbitrary string, which must conform to a given regular expression.
\item {\tt LOGICAL}
A boolean type which can take values 1, `t', `true', `yes' or 
0, `f', `false', `no'.
\end{itemize}

\item {\tt A description of the parameter}

\item {An allowed value block}
This consists of a brace delimited block of lines
describing the allowed values of the parameter.  Each range may
have a description associated with it by placing a :: on the line and
putting the description afterwards.

\item {\tt The default value}
This must be one of the allowed values.

\end{itemize}

For the numeric types INT and REAL, a range consists of a string of the
forms lower-bound:upper-bound:step where a missing number or a \* denotes
anything (i.e. infinite bounds or an infinitesmal step).

For example 

\begin{verbatim}

REAL Coeff "Important coefficient"
{
0:3.14 :: "Range has to be from zero to Pi, default is zero"
} 0.0

#No need to define a range for LOGICAL
LOGICAL nice "Nice weather ?"
{
}"yes"

# A example for a set of keywords and its default (which has to be
# defined in the body)
KEYWORD confused "Are we getting confused ?"
{
  "yes"    :: "absolutley positively"
  "perhaps" :: "we are not sure"
  "never"   :: "never"
} "never"
\end{verbatim}

defines a REAL parameter, a LOGICAL parameter, and a KEYWORD.

By default all paramters are {\tt private}, to change this an access
specification of the form {\tt global:} or {\tt restricted:} (or 
{\tt private:} to change it back) may be placed on a line by itself.  This
changes the access level for any parameter defined in the file from that point on.

To access {\tt restricted} parameters from another implementation, a line
containing {\tt shares: <name>} declares that all parameters mentioned in
the file from now until the next access specification originate in 
implementation <name>.  

In contrast to parameter declarations in other access blocks, the default
value must be ommitted - it is impossible to set the default value of any
parameter not originating in this thorn.

For example

\begin{verbatim}

friend:einstein

KEYWORD initial_data ""
{
  "bl_bh"         :: "Brill Lindquist black holes"
  "misner_bh"     :: "Misner black holes"
  "schwarzschild" :: "One Schwarzshild black hole"
}

\end{verbatim}

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\subsection{The {\tt schedule.ccl}}

By default no routine of a thorn will be run.  The schedule.ccl file
defines those that should be run and when they should be run.

The specification of this is via a schudule block which consists of
a line of the form

\begin{verbatim}

schedule <name> at <time bin> [other options] 
{
  LANG:     <FORTRAN|C>
  STORAGE:  [group list]
  COMM:     [group list]
  TRIGGERS: [group or variable list]
} "A description"

\end{verbatim}

where {\tt <name>} is the name of the routine, and {\tt <time bin>} is one of

\begin{itemize}

\item {\tt CACTUS\_BASEGRID}
Resposnible for setting up coordinates etc.

\item {\tt CACTUS\_RECOVER}
For recovery from checkpoint.

\item {\tt CACTUS\_INITIAL}

For generating initial data.

\item {\tt CACTUS\_POSTINITIAL}

Tasks which must be applied after initial data is created.

\item {\tt CACTUS\_CPINITIAL}
For recovery of initial data from a checkpoint file.

\item {\tt CACTUS\_PRESTEP}

Stuff done before the evolution step.
\item {\tt CACTUS\_EVOL}      

The evolution step.

\item {\tt CACTUS\_POSTSTEP}

Stuff done after the evolution step.

\item {\tt CACTUS\_CHECKPOINT}
For checkpointing data

\item {\tt CACTUS\_ANALYSIS}
For analysing data.

\item {\tt CACTUS\_TERMINATE}
Called when cactus teminates.

\item {\tt CACTUS\_CONVERGENCE}
Convergence stuff.

\end{itemize}

The STORAGE and COMM keywords specify any groups which must have memory 
allocated for them or communication enabled for the duration of that routine.
The storage or communication status reverts to its previous status after the 
routine returns.

TRIGGERS is used when the routine is registered at ANALYSIS --- this is a 
special time bin, a routine registered here will only be called if one of
the variables mentioned in TRIGGERS is due for output.

The `other options' allow finer grained control of the scheduling.  It is
possible to state that the routine must run BEFORE or AFTER another routine.

As well as schedule blocks it's possible to embed C code in the schedule.ccl.
This can be used to schedule things based upon the value of a parameter.

E.g.

\begin{verbatim}

if(evolve_hydro)
{
  SCHEDULE hydro_predictor AT evolve AFTER metric_predictor BEFORE metric_corrector 
  {
    LANG:     FORTRAN
    STORAGE:  hydro_variables
    COMM:     hydro_variables
  } "Do a predictor step on the hydro variables"
}
\end{verbatim}


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\section{Naming Conventions for Source Files}

The make system uses file extensions
to designate coding language. The following extensions are
can be handled:

\begin{center}
\begin{tabular}{|c|c|}
\hline
Extension & Coding Language \\
\hline
{\t .F} & Fortran90 fixed form \\
{\t .f} & (no preprocessing) Fortran90 fixed form\\
{\t .F90} & Fortran90 free form form [To be implemented]\\
{\t .f90} & (no preprocessing) Fortran90 free form [To be implemented]\\
{\t .F77} & Fortran77 \\
{\t .f77} & (no preprocessing) Fortran77\\
{\t .c} & C \\
{\t .cc} or {\t .C} & C++ \\
\hline
\end{tabular}
\end{center}

The following restrictions apply to file names:
\begin{itemize}
\item Rootnames within each directory must be unique. For example, it is not possible 
      to have the files {\tt InitialData.c} and {\tt InitialData.F} in the same directory.
\item Currently all files within a thorn must have distinct names.  We hope
to relax this in future.  Different thorns can have files with the same names. 
\end{itemize}

\section{Adding source files}

By default the CCTK looks in the {\tt src} directory of the thorn for source
files.

There are two ways in which to specify the sources.  The easiest is to use the 
{\tt make.code.defn} based method in which the CCTK does all the work, but you
may instead put a {\tt Makefile} in the {\tt src} directory and do everything
yourself.

\subsection{{\tt make.code.defn} based thorn building}

This is the standard way to do it.

If there is no file called {\tt Makefile} in the {\tt src} directory,
the CCTK make system looks for a file called {\tt make.code.defn} in that
directory.  At its simplest, this file contains two lines

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

\item {\t SUBDIRS = <list of all subdirectories, {\em including subdirectories of subdirectories}>}

\end{itemize}

Each subdirectory listed should then have a {\tt make.code.defn} file 
containing just a {\tt SRCS = } line, a {\tt SUBDIRS = } line will
be ignored.

In addition, each directory can have a {\tt make.code.deps} file, which,
for files in that directory, can contain additional make rules and dependencies
for files in that directory.  See the GNU Make documentation for details of the
syntax.

\subsection{{\tt Makefile} based thorn building}

This method gives you ultimate responsibility.  The only requirement is that
a library called {\tt \$NAME} be created by the Makefile.

The makefile is passed the following variables

\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 Fortran90 module
files should end up if they need to be seen by other thorns.

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

\end{itemize}

and has a working directory of {\tt <config>/build/<thorn\_name>} .

\subsection{Other makefile variables}

\begin{itemize}
\item CC
\item CXX
\item F90
\item F77
\item CFLAGS
\item CXXFLAGS
\item F90FLAGS
\item F77FLAGS
\item LD
\end{itemize}


\chapter{Putting code into your thorn}

\section{What the Flesh provides}

The flesh provides various things to thorns.
\begin{itemize}
\item {\tt Variables}
\item {\tt Parameters}
\item {\tt Driver functions}
\item {\tt IO functions}
\item {\tt Information functions}
\item {\tt Reduction}
\item {\tt Interpolation}
\end{itemize}


\subsection{Fortran Routines}

Any source file using Cactus infrastructure should include 
the header file {\tt cctk.h} using the line
\begin{verbatim}
#include "cctk.h"
\end{verbatim}
(Fortran programmers should not be put of by this being a C style
header file, most Cactus files are run through a C preprocessor
before compilation)

\subsubsection{Variables}

Any routine using Cactus argument lists (for example all 
routines called from the scheduler) should include at the
top of the file the header
\begin{verbatim}
#include "cctk_arguments.h"
\end{verbatim}

A Cactus macro {\tt CCTK\_FARGUMENTS} is defined for each thorn
to contain
\begin{itemize}
\item General information about the grid hierachy, for example
      the number of grid points used. See Section \ref{sec:cava} for a 
      complete list.
\item All the grid variables defined in the thorns {\tt interface.ccl}
\item All the grid variables required from other thorns as requested by
      the {\tt inherits} and {\tt friend} lines in the {\tt interface.ccl}
\end{itemize}
These variables must be declared at the start of the routine using
the macro {\tt DECLARE\_CCTK\_FARGUMENTS}. 

To pass the arguments to another routine in the same thorn use the macro 
{\tt CCTK\_PASS\_FTOF} in the calling routine, and again the macro 
{\tt CCTK\_FARGUMENTS} in the receiving routine.


\subsubsection{Parameters}

All parameters defined in a thorn's {\tt param.ccl} and all {\tt global} 
parameters appear as local variables in a thorn.  These variables are 
{\tt read only} and {\em changes should not be made to them}.  The effect of
changing a parameter is undefined (at best).

The parameters should be declared at the start of the routine
using them with the macro {\tt DECLARE\_PARAMETERS}

Any routine using Cactus parameters should include at
the top of the file the header
\begin{verbatim}
#include "cctk_parameters.h"
\end{verbatim}

In Fortran, special care should be taken with string valued parameters.
This parameters are passed as C pointers, and can not be treated as
normal Fortran strings. To compare a string valued parameter and Fortran
string use the function {\tt CCTK\_Equals}. To print the value of a string 
valued parameter to screen, use the subroutine {\tt CCTK\_PrintString}.

\subsubsection{Fortran Example}

The Fortran routine {\tt MyFRoutine} is scheduled in the {\tt schedule.ccl} file,
doesn't use Cactus parameters, and calls another routine, in the same thorn,  
{\tt MyNewRoutine} which does use parameters. 
This routine needs to be passed an integer flag as 
well as the standard Cactus The source file should look like

\begin{verbatim}
#include "cctk.h"
#include "cctk_arguments.h"
#include "cctk_parameters.h"

      subroutine MyCRoutine(CCTK_FARGUMENTS)

c     I'm very cautious, so I want to declare all variables
      implicit none 

      DECLARE_CCTK_FARGUMENTS

      integer flag

      flag = 1
      call MyNewRoutine(CCTK_PASS_FTO_F,flag)

      return 
      end

      subroutine MyCRoutine(CCTK_FARGUMENTS,flag)

      implicit none 

      DECLARE_CCTK_FARGUMENTS
      DECLARE_CCTK_PARAMETERS
      integer flag

c     Main code goes here

      return
      end
      
\end{verbatim}


\subsection{C Routines}

Any source file using Cactus infrastructure should include 
the header file {\tt cctk.h} using the line
\begin{verbatim}
#include "cctk.h"
\end{verbatim}

\subsubsection{Variables}

Any routine using Cactus argument lists (for example all 
routines called from the scheduler) should include at the
top of the file the header
\begin{verbatim}
#include "cctk_arguments.h"
\end{verbatim}

A Cactus macro {\tt CCTK\_CARGUMENTS} is defined for each thorn
to contain
\begin{itemize}
\item General information about the grid hierachy, for example
      the number of grid points on the processor. See Section \ref{sec:cava} for a 
      complete list.
\item All the grid variables defined in the thorns {\tt interface.ccl}
\item All the grid variables required from other thorns as requested by
      the {\tt inherits} and {\tt friend} lines in the {\tt interface.ccl}
\end{itemize}
These variables must be declared at the start of the routine using
the macro {\tt DECLARE\_CCTK\_CARGUMENTS}. 

To pass the arguments to another routine in the same thorn use the macro 
{\tt CCTK\_PASS\_CTOC} in the calling routine, and again the macro 
{\tt CCTK\_CARGUMENTS} in the receiving routine.


\subsubsection{Parameters}

All parameters defined in a thorn's {\tt param.ccl} and all {\tt global} 
parameters appear as local variables in a thorn.  These variables are 
{\tt read only} and {\em changes should not be made to them}.  The effect of
changing a parameter is undefined (at best).

Any routine using Cactus parameters should include at
the top of the file the header
\begin{verbatim}
#include "cctk_parameters.h"
\end{verbatim}

The parameters should be declared at the start of the routine
using them with the macro {\tt DECLARE\_PARAMETERS}

\subsubsection{Example}

The C routine "MyCRoutine" is scheduled in the {\tt schedule.ccl} file,
and uses Cactus parameters. The source file should look like
\begin{verbatim}
#include "cctk.h"
#include "cctk_arguments.h"
#include "cctk_parameters.h"

void MyCRoutine(CCTK_CARGUMENTS)
{
  DECLARE_CCTK_CARGUMENTS
  DECLARE_CCTK_PARAMETERS

  /* Here goes your code */
}
\end{verbatim}

\subsubsection{Specifically for C Programmers}

Grid functions are held in memory as 1D C arrays. These are laid
out in memory as in Fortran. This means that the first index should
be incremented through most rapidly. As can be seen in the example
below.


Cactus provides
macros to find the 1D index which is needed from the multidimensional
indices which are usually used. There is a macro for each dimension of
grid function.  Below is an articifial example to demonstrate this
using the 3D macro {\tt CCTK\_GFINDEX3D}:
\begin{verbatim}
for (k=0; k<cctk_lsh[2]; k++)
{
  for (j=0; j<cctk_lsh[1]; j++)
  {
    for (i=0; i<cctk_lsh[0]; i++)
    {
      My3D_GF[CCTK_GFINDEX3D(cctkGH,i,j,k)] = i*j*k;
    }
  }
}
\end{verbatim}

Here, {\tt CCTK\_GFINDEX3D(cctkGH,i,j,k)]} expands to 

\begin{verbatim}
((i) + cctkGH->cctk_lsh[0]*((j)+cctkGH->cctk_lsh[1]*(k)))
\end{verbatim}

\subsection{Cactus Variables}
\label{sec:cava}

The Cactus variables which are passed through the macros
{\tt CCTK\_FARGUMENTS}, {\tt CCTK\_CARGUMENTS}, etc are
\begin{itemize}
\item {\tt cctk\_dim} An integer with the number of dimensions
      used for this grid hierarchy
\item {\tt cctk\_lsh} An array of {\tt cctk\_dim} integers
      with the local grid size on this processor
\item {\tt cctk\_delta\_time} A {\tt CCTK\_REAL} with the timestep
\item {\tt cctk\_time} A {\tt CCTK\_REAL} with the current time
\item {\tt cctk\_space} An array of {\tt cctk\_dim} {\tt CCTK\_REAL}s with
         the grid spacing in each direction
\item {\tt cctk\_nghostzones} An array of {\tt cctk\_dim} integers with
         the number of ghostzones used in each direction
\item {\tt cctk\_iteration} Integer with the current iteration number
\item {\tt cctkGH} A C pointer identifying the grid hierachy
\end{itemize}

The following variables describe the location of the local
grid (e.g. the grid treated on a given processor) within 
the global grid.
\begin{itemize}
\item {\tt cctk\_lbnd} 
      An array of {\tt cctk\_dim} integers 
      containing the lowest index (in each direction) 
      of the local grid, as seen on the global grid 
\item {\tt cctk\_ubnd} 
      An array of {\tt cctk\_dim} integers 
      containing the largest index (in each direction) 
      of the local grid, as seen on the global grid 
\item {\tt cctk\_bbox} 
      An array of 2*{\tt cctk\_dim} integers 
      which indicate whether the boundaries are internal boundaries
      (e.g. between processors), or physical boundaries.
\end{itemize}

The following variable is needed for grid refinement methods
\begin{itemize}
\item {\tt cctk\_levfac} The factor by which the local grid is refined
        with respect to the base grid.
\end{itemize}


\subsection{Cactus Data Types}

The Cactus grid variables and parameters are defined and
declared using Cactus data types, to provide portability 
across platforms. The most important of 
these data types are described below, for a full description
see Section~\ref{sec:datyansi}. These data types should 
be used to declare local variables where needed, and to
declare Cactus grid variables or parameters that need 
declarations. 

\begin{Lentry}

\item[{\tt CCTK\_INT}] default size 4 bytes
\item[{\tt CCTK\_REAL}] default size 8 bytes

\end{Lentry}

\subsubsection{Example}

In the following example {\tt MyScalar} is a grid scalar which
is declared in the {\tt interface.ccl} as {\tt CCTK\_REAL}.

\begin{verbatim}
      subroutine InitialData(CCTK_FARGUMENTS)

      DECLARE_CCTK_ARGUMENTS

      CCTK_REAL local_var
      
      local_var = 1.0/3.0      
      MyScalar = local_var

      return
      end 
\end{verbatim}

Declaring {\tt local\_var} to have a non-Cactus data type, e.g.
{\tt REAL*4}, or using one of the other Cactus real data types
described in Section~\ref{datyansi} could give problems for 
different architectures or configurations.


\subsection{Providing Run Time Information}

To write from thorns to standard output ({\it i.e.} the screen) 
at runtime, use the function {\tt CCTK\_INFO}. 
For example, from the Fortran thorn {\tt MyThorn},

\begin{verbatim}
call CCTK_INFO("Starting Tricky Calculation")
\end{verbatim}

will write the line:

\begin{verbatim}
INFO (MyThorn): Starting Tricky Calculation
\end{verbatim}

For a multiprocessor run, only information from processor zero 
will appear. If the "{\tt -r"} command line option is used 
(Section~\ref{sec:coliop}) the output from this command will be redirected.

To output a variable using {\tt CCTK\_INFO} currently more tricky. From Fortranyou will need something like

\begin{verbatim}
write(infoline,'(A,1X,I)') 'The integer was ',inum
call CCTK_INFO(infoline)
\end{verbatim}

and for C

\begin{verbatim}
char *infoline;
infoline = (char *)malloc(18*sizeof(char));
sprintf(infoline,'The integer was %d',inum);
CCTK_INFO(infoline);
free(infoline);
\end{verbatim}



Notes
\begin{itemize}
\item{} {\tt CCTK\_INFO} is actually a macro for the function
        {\tt CCTK\_Info(<thorn name>,<message>)} which automatically
        includes the thorn name.

\item{} {\tt CCTK\_INFO} should be used rather than print statements,
       since it will give consistent behaviour on multiprocessors, and
       also provides a mechanism for switching the output to screen on 
       and off, even on a thorn-by-thorn basis. (Although this is 
       not yet implemented).
\end{itemize}




\section{Error handling, Warnings and Code Termination}
\label{sec:erhawancote}
The Cactus function {\tt CCTK\_WARN} should be used to provide
warning messages during code executation. Along with the
warning message, an integer is given to indicate the severity
of the warning. The warning severity indicates whether the
message is printed to standard output and whether the code
should be stopped. A level 0 warning indicates the highest
severity, with higher numbers indicating lower severity.

By default, a Cactus run will abort on a level 0 warning
and will report level 1 and severer warnings to screen. 
This behaviour can be amended using command line arguments,
as described in Section~\ref{sec:coliop}.

The flesh will be implementing standard error return codes
which can be used by the thorns, although this is not 
yet ready. In general, thorns should attempt to handle errors
without terminating, and warning messages should be liberally
used.
      

\subsection{Adding Parallelisation}
\label{seap}

The flesh itself does not actually set up grid variables. This 
is done by a {\it driver} thorn. To allow the distribution of 
a grid over a number of processors, the driver thorn must 
also provide the grid decomposition, and routines to enable 
parallelisation. The method used to provide this parallelisation
(e.g. MPI, PVM) is not usually important for the thorn writer since
the driver thorn provides routines which are called by standard interfaces
from the flesh. Here we describe briefly the most important of these routines
for the application thorn writer. A more detailed description
of these interfaces with their arguments, is given in the Function Reference
Guide.
A complete description of the
routines a driver thorn must provide will be provided in the 
Interface Thorn Writers guide. The standard driver thorn is 
currently {\tt PUGH} in the {\tt CactusPUGH} package, which 
is a parallel unigrid driver. 

\begin{Lentry}
\item[{\tt CCTK\_nProcs}] Returns the number of processors being used
\item[{\tt CCTK\_MyProc}] Returns the processor number (this starts at
  processor number zero
\item[{\tt CCTK\_SyncGroup}] Synchronises a group of variables by 
  exchanging the values held in each processor ghostzones with the
  physical values of their neighbours
\item[{\tt CCTK\_Barrier}] Waits for all processors to reach this point
  before proceeding
\end{Lentry}



\subsection{IO}
\label{sec:io}

To allow flexible IO, the flesh itself does not provide 
any output routines, however it provides a mechanism for 
thorns to register different routines as IO methods. Application thorns
can interact with the different IO methods through the following
function calls:

\begin{description}

\item[CCTK\_OutputGH]

This call loops over all registered IO methods, calling 
the routine that each method has registered for {\t OutputGH}.
The expected behaviour of any methods {\t OutputGH} is to
loop over all GH variables outputting them if the method 
contains appropriate routines (that is, not all methods will 
supply routines to output all different types of variables) 
and if the method decides it is an appropriate time to 
output. 

\item[CCTK\_OutputVarAsByMethod]

Output a variable {\t varname} using the method {\t methodname} if it is 
registered. Uses {\t alias} as the name of the variable for the purpose
of constructing a filename. The output should take place if at all possible,
if the appropriate file exists the data is appended, otheriwise a new
file is created.


\item[CCTK\_OutputVarByMethod]

Output a variable {\t varname} using the method {\t methodname} if it is 
registered. The output should take place if at all possible,
if the appropriate file exists the data is appended, otherwise a new
file is created.

\item[CCTK\_OutputVarAs]

Output a variable {\t varname} looping over all registered methods. 
The output should take place if at all possible,
if the appropriate file exists the data is appended, otherwise a new
file is created. Uses {\t alias} as the name of the variable for the purpose
of constructing a filename.

\item[CCTK\_OutputVar]

Output a variable {\t varname} looping over all registered methods. 
The output should take place if at all possible,
if the appropriate file exists the data is appended, otherwise a new
file is created.

\end{description}

\subsection{Reduction Operators}
\label{sec:reop}

Reduction operators are operators which process grid functions
and return, either on just one processor or on each processor,
a 1-Dimensional array of scalars. Examples of reduction operators
could include simple operations such as the maximum value of a 
grid function across the grid, or more complex operations

The flesh itself does not provide any reduction operators,
instead providing a mechanism for thorns to register their
own routines as reduction operators, labelled by a given name.
For details of writing reduction operators see ????. Application
thorns can interact with the different registered reduction 
operators through the following function calls:

{\t
\begin{verbatim}

#include "cctk_Reduction.h"
int CCTK_GetReductionHandle(const char *reduction);

\end{verbatim}
}
\vskip .25cm

Get a integer handle corresponding to a given reduction operator. 
(Note that although it would appear to be far more convenient to 
pass the name of the reduction operator directly to the following
function call to {\t CCTK\_Reduce} this causes problems with the
translation of strings from {\t FORTRAN} to {\t C} with variable
argument lists).

{\t
\begin{verbatim}

#include "cctk_Reduction.h"
int CCTK_Reduce(cGH *GH, int retvaltype, int retvalnum, void *retval, int handle, int index, ...);

\end{verbatim}
}
\vskip .25cm
Note that the memory for {\t retval} must be assigned before the reduction
call is made.

\section{Calls between different programming languages}
\label{sec:cabedipr}

\subsection{Calling C routines from FORTRAN}
\label{sec:cacrofr}

\subsection{Calling FORTRAN routines from C}
\label{sec:caforofr}

To call a utility Fortran routine from C use

{\tt

void FMODIFIER FORTRAN\_NAME(<Fortran routine name>)(<argument list>)
}


\section{Adding a test suite}
\label{sec:adatesu}

To add a test suite to your thorn, devise a series of parameter
files which use as many aspects of your thorn as possible. 
Make sure that the parameter files produce ascii output to files,
and that these files are in the directory 
{\tt ./<parameter file base name>}.

Run Cactus on each of the parameter files, and move the parameter files,
and the output directories they produced, to the {\tt test} directory
in your thorn. 

Document carefully any situations or architectures in which your test 
suite does not give the correct answers. 

For details on running the test suites, see Section~\ref{sec:te}.


\section{Naming conventions}

\begin{itemize}

\item{} Thorn names must not start with the word "Cactus" (in
        any case).
\item{} Arrangements will be ignored if their names start with \# or .
        or end in \~{} .bak or .BAK 
\item{} Thorns will be ignored if they are called doc or start with
        \# or . or end in \~{} .bak or .BAK
\item{} Routine names have to be unique among all thorns. 

\end{itemize}


\section{General Naming Conventions}

The following naming conventions are followed by the flesh and the
supported Cactus arrangements. They are not compulsory, but if followed
allow for a homogeneous code.

\begin{itemize}

\item Parameters: lower case (except for acronyms) with words separated
  by an underscore. Examples: {\tt my\_first\_parameter}, 
  {\tt solve\_PDE\_equation}.

\item Filenames and routine names: Prefixed by thorn name with an underscore, then capitalised words, with no spaces.
    Examples: {\tt MyThorn\_StartUpRoutine}, {\tt BestSolver\_InitialDataForPDE}.

\end{itemize}

\section{Data Types and Sizes}
\label{sec:datyansi}

Cactus supports the following fixed size data types

\begin{center}
\begin{tabular}{|c|c|c|c|}
\hline
Data Type & Size (bytes) & Variable Type & Fortran Equivalent\\
\hline
{\t CCTK\_INT2}     & 2 & {\t CCTK\_VARIABLE\_INT2}     & {\t integer*2}\\
{\t CCTK\_INT4}     & 4 & {\t CCTK\_VARIABLE\_INT4}     & {\t integer*4}\\
{\t CCTK\_INT8}     & 8& {\t CCTK\_VARIABLE\_INT8}      & {\t integer*8}\\
{\t CCTK\_REAL4}    & 4 & {\t CCTK\_VARIABLE\_REAL4}    & {\t real*4}\\
{\t CCTK\_REAL8}    & 8 & {\t CCTK\_VARIABLE\_REAL8}    & {\t real*8}\\
{\t CCTK\_REAL16}   & 16& {\t CCTK\_VARIABLE\_REAL16}   & {\t real*16}\\
{\t CCTK\_COMPLEX4} & 4 & {\t CCTK\_VARIABLE\_COMPLEX4} & {\t complex*4}\\
{\t CCTK\_COMPLEX8} & 8 & {\t CCTK\_VARIABLE\_COMPLEX8} & {\t complex*8}\\
{\t CCTK\_COMPLEX16}& 16& {\t CCTK\_VARIABLE\_COMPLEX16}& {\t complex*16}\\
{\t CCTK\_CHAR}     & 1 & {\t CCTK\_VARIABLE\_CHAR}     & {\t character} \\ \hline
\end{tabular}
\end{center}

In addition Cactus provides three data types whose size is chosen
during the compilation process (at configuration time). This is to
allow the code to be easily run at different precisions. Note that
the effectiveness of running the code at a lower or higher precision
depends crucially on all thorns being used making consistent use
of the following data types:


\begin{center}
\begin{tabular}{|c|c|c|c|}
\hline
Data Type          & Default Size (bytes) & Variable Type & Configuration Option\\
\hline
{\t CCTK\_INT}     & 4 & {\t CCTK\_VARIABLE\_INT} & {\t INTEGER\_PRECISION}\\
{\t CCTK\_REAL}    & 8 & {\t CCTK\_VARIABLE\_REAL} & {\t REAL\_PRECISION}\\
{\t CCTK\_COMPLEX} & (8,8) & {\t CCTK\_VARIABLE\_COMPLEX} & Same as real precision\\
\hline
\end{tabular}
\end{center}

These variable types must be used by thorn writers to declare variables
 in the thorn interface files, and may be used to declare
variables in the thorn routines. Note that variable declarations in 
thorns should obviously match with definitions in the interface files
where appropriate.

Also provided, are a set of macros which
are interpreted by the preprocessor at compile time to signify which
data size is being used:

\begin{center}
\begin{tabular}{|c|c|}
\hline
Data Type & {\t \#define}\\
{\t CCTK\_INT2} & {\t CCTK\_INT\_PRECISION\_2} \\
{\t CCTK\_INT4} & {\t CCTK\_INT\_PRECISION\_4} \\
{\t CCTK\_INT8} & {\t CCTK\_INT\_PRECISION\_8} \\
{\t CCTK\_REAL4} & {\t CCTK\_REAL\_PRECISION\_4} \\
{\t CCTK\_REAL8} & {\t CCTK\_REAL\_PRECISION\_8} \\
{\t CCTK\_REAL16} & {\t CCTK\_REAL\_PRECISION\_16} \\
{\t CCTK\_COMPLEX4} & {\t CCTK\_COMPLEX\_PRECISION\_4} \\
{\t CCTK\_COMPLEX8} & {\t CCTK\_COMPLEX\_PRECISION\_8} \\
{\t CCTK\_COMPLEX16} & {\t CCTK\_COMPLEX\_PRECISION\_16} \\
\hline 
\end{tabular}
\end{center}

Note that the availability of these types, and the corresponding 
C data types are platform dependent.

\subsection{Fortran Thorn Writers}

Cactus provides a further data type {\tt CCTK\_POINTER} 
for use in Fortran code to declare a pointer passed from C. 
For example, the variable {\tt cctkGH} is of this type.

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%


\end{cactuspart}