% /*@@ % @file ThornWriters.tex % @date 27 Jan 1999 % @author Gabrielle Allen, Tom Goodale, Gerd Lanfermann % @desc % Thorn writer's guide part of the Cactus User's Guide % @enddesc % @version $Header$ % @history % @date Tue Feb 12 14:33:25 CET 2002 % @author Jonathan Thornburg % @desc Revise section "Interpolation Operators" to cover the % new interpolation API (when we will have multiple APIs % coexisting, e.g. CCTK_InterpLocalUniform() and % CCTK_InterpLocalNonUniform()) % @endhistory % @@*/ \begin{cactuspart}{Thorn Writing} \renewcommand{\thepage}{\Alph{part}\arabic{page}} \label{part:ThornWriting} \chapter{Application thorns} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% This chapter goes into the nitty-gritty of writing a thorn. It introduces key concepts for thorns, then goes on to give a brief outline of how to configure a thorn. There is then some detail about concepts introduced by the configuration step, followed by discussion of code which you must put into your files in order to use Cactus functionality, and details of utility functions you may use to gain extra functionality. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \section{Thorn Concepts} \label{chap:thorn_concepts} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Thorns} A thorn is the basic working module within Cactus. All user supplied code goes into thorns, which are, by and large, independent of each other. Thorns communicate with each other via calls to the flesh API, plus, more rarely, custom APIs of other thorns. The connection from a thorn to the flesh, or to other thorns, is specified in configuration files which are parsed at compile time and used to generate glue code which encapsulates the external appearance of a thorn. Thorn names must be (case independently) unique, must start with a letter, can only contain letters, numbers or underscores, and must contain 27 characters or less. In addition, a thorn cannot have the name \texttt{doc}, this is reserved for arrangement documentation. Arrangement names which start with a `\texttt{\#}', or finish with `\texttt{\~{}}' or `\texttt{.bak}' will be ignored. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Arrangements} \label{sec:arrangements} Thorns are grouped into \textit{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 you may want to have separate arrangements for your developments, private and shared thorns. The arrangements live in the \texttt{arrangements} directory of the main Cactus directory. Arrangement names must be (case independently) unique, must start with a letter, and can only contain letters, numbers or underscores. Arrangement names which start with a `\texttt{\#}', or finish with `\texttt{\~{}}' or `\texttt{.bak}' will be ignored. Inside an arrangement directory there are directories for each thorn belonging to the arrangement. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Implementations} \label{sec:implementations} One of the key concepts for thorns is the concept of the \textit{implementation}. Relationships among thorns are all based upon relationships among the 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 implementation defines a group of variables and parameters which are used to implement some functionality. For example, the thorn \texttt{CactusPUGH/PUGH} provides the implementation \texttt{driver}. This implementation is responsible for providing memory for grid variables and for communication. Another thorn can also implement \texttt{driver}, and both thorns can be compiled in \emph{at the same time}. At runtime, the user can decide which thorn providing \texttt{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 \emph{implementation}, not explicitly with the other \emph{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. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \section{Anatomy of a Thorn} \label{chap:thorn_anatomy} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Thorns} A thorn consists of a subdirectory of an arrangement containing four administrative files: \begin{Lentry} \item[\texttt{interface.ccl}] the Cactus interface, which defines the grid functions, variables, etc. See Appendix \ref{sec:Appendix.interface}. \item[\texttt{param.ccl}] the parameters introduced by this thorn, and the parameters needed from other thorns. See Appendix \ref{sec:Appendix.param}. \item[\texttt{schedule.ccl}] scheduling information for routines called by the flesh. See Appendix \ref{sec:Appendix.schedule}. \item[\texttt{configuration.ccl}] configuration options for the thorn. See Appendix \ref{sec:Appendix.configuration.ccl}. \end{Lentry} Thorns can also contain \begin{itemize} \item a subdirectory called \texttt{src}, which should hold source files and compilation instructions for the thorn \item a subdirectory \texttt{src/include} for include files \item a \texttt{README} containing a brief description of the thorn \item a \texttt{doc} directory for documentation \item a \texttt{par} directory for example parameter files \item a \texttt{test} subdirectory may also be added, to hold the thorn's test suite. See Section \ref{sec:adding_test_suite} for details. \end{itemize} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Creating a Thorn} To simplify the creation of a thorn, a \texttt{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 in which you would like to include your thorn. Either enter a new arrangement name or pick one from the list of available arrangements that are shown. \end{enumerate} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Configuring your Thorn} The interaction of a thorn with the flesh and other thorns is controlled by certain configuration files. These are: \begin{Lentry} \item [\texttt{interface.ccl}] This defines the \textit{implementation} (Section~\ref{sec:implementations}) the thorn provides, and the variables the thorn needs, along with their visibility to other implementations. \item [\texttt{param.ccl}] This defines the parameters that are used to control the thorn, along with their visibility to other implementations. \item [\texttt{schedule.ccl}] This defines which functions are called from the thorn and when they are called. It also handles memory and communication assignment for grid variables. \item [\texttt{configuration.ccl}] This file is optional for a thorn. If it exists, it contains extra configuration options of this thorn. \end{Lentry} \subsubsection{General Syntax of CCL Files} \textit{Cactus Configuration Language} (CCL) 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 hash `\texttt{\#}' character, which indicates that the rest of the line is a comment. If the last non-blank character of a line in a CCL file is a backslash `\texttt{$\backslash$}', the following line is treated as a continuation of the current line. \subsubsection{The \texttt{interface.ccl} File} \label{subsec:interface_ccl} The \texttt{interface.ccl} file is used to declare \begin{itemize} \item the implementation provided by the thorn \item the variables provided by the thorn \item the include files provided by the thorn \item the functions provided by the thorn (in development) \end{itemize} The implementation is declared by a single line at the top of the file \begin{alltt} implements: <\var{name}> \end{alltt} Where \texttt{<\var{name}>} can be any combination of alphanumeric characters and underscores, and is case independent. There are three different access levels available for variables \begin{Lentry} \item[\texttt{Public}] Can be `inherited' by other implementations (see below). \item[\texttt{Protected}] Can be shared with other implementations which declare themselves to be friends of this one (see below). \item[\texttt{Private}] Can only be seen by this thorn. \end{Lentry} Corresponding to the first two access levels there are two relationship statements that can be used to get variables (actually groups of variables, see below) from other implementations. \begin{Lentry} \item [\texttt{Inherits: <\var{name}>}] This gets all \texttt{Public} variables from implementation \texttt{<\var{name}>}, and all variables that \texttt{<\var{name}>} has in turn inherited. An implementation may inherit from any number of other implementations. \item [\texttt{Friend: <\var{name}>}] This gets all \texttt{Protected} variables from implementation \texttt{<\var{name}>}, but, unlike \texttt{inherits}, it is symmetric and also defines a transitive relation by pushing its own implementation's \texttt{Protected} variables onto implementation \var{name}. This keyword is used to define a group of implementations which all end up with the same \texttt{Protected} variables. \end{Lentry} So, for example, an \texttt{interface.ccl} starting \begin{alltt} implements: wavetoy inherits: grid friend: wave_extract \end{alltt} declares that the thorn provides an implementation called \texttt{wavetoy}, gets all the \texttt{public} variables declared by an implementation called \texttt{grid}, and shares all \texttt{protected} variables with \texttt{wave\_extract} and its friends. Cactus variables, described in Chapter~\ref{chap:cactus_variables}, are placed in groups with homogeneous attributes, where the attributes describe properties such as the data type, group type, dimension, ghostsize, number of timelevels, and distribution. For example, a group, called \texttt{realfields} of 5 real grid functions (\texttt{phi}, \texttt{a}, \texttt{b}, \texttt{c}, \texttt{d}), on a 3D grid, would be defined by \begin{verbatim} CCTK_REAL realfields type=GF TimeLevels=3 Dim=3 { phi a,b,c,d } "Example grid functions" \end{verbatim} or, for a group called \texttt{intfields} consisting of just one distributed 2D array of integers, \begin{verbatim} CCTK_INT intfields type=ARRAY size=xsize,ysize ghostsize=gxsize,gysize dim=2 { anarray } "My 2D arrays" \end{verbatim} where \texttt{xsize}, \texttt{ysize}, \texttt{gxsize}, \texttt{gysize} are all parameters defined in the thorn's \texttt{param.ccl}. By default, all groups are \texttt{private}, to change this, an access specification of the form \texttt{public:} or \texttt{protected:} (or \texttt{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. \subsubsection{The \texttt{param.ccl} File} \label{subsec:param_ccl} Users control the operation of thorns via parameters given in a file at runtime. The \texttt{param.ccl} file 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. If a parameter is not assigned in a parameter file, it is given its default value. There are three access levels available for parameters: \begin{Lentry} \item [\texttt{Global}] These parameters are seen by all thorns. \item [\texttt{Restricted}] These parameters may be used by other implementations if they so desire. \item [\texttt{Private}] These are only seen by this thorn. \end {Lentry} A parameter specification consists of: \begin{itemize} \item The parameter type (each may have an optional {\t CCTK\_} in front) \begin{Lentry} \item [\texttt{REAL}] \item [\texttt{INT}] \item [\texttt{KEYWORD}] A distinct string with only a few known allowed values. \item [\texttt{STRING}] An arbitrary string, which must conform to a given regular expression. \item [\texttt{BOOLEAN}] A boolean type which can take values {\t 1}, {\t t}, {\t true}, {\t yes} or {\t 0}, {\t f}, {\t false}, {\t no}. \end{Lentry} \item The parameter name \item An optional size (in square brackets)--if this is present, the parameter is a ``parameter array'', i.e.\ it will actually be an array of parameters, each of which has the same properties, but a different value. Such arrays appear as normal arrays in C or Fortran, 0-based in C, and 1-based in Fortran. In the parameter file the value of each element is specified with square brackets and is 0-based. The size must be an integer. \item A description of the parameter \item An allowed value block--this consists of a brace-delimited block of lines\footnote{The beginning brace (\texttt{\{}) must sit on a line by itself; the ending brace (\texttt{\}}) must be preceded by a carriage return.} describing the allowed values of the parameter. Each range may have a description associated with it by placing a \texttt{::} on the line, and putting the description afterwards. \item The default value--this must be one of the allowed values. \end{itemize} For the numeric types \texttt{INT} and \texttt{REAL}, a range consists of a string of the form lower-bound:upper-bound:step, where a missing number or an asterisk `\texttt{*}' denotes anything (i.e.\ infinite bounds or an infinitesimal 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 BOOLEAN BOOLEAN 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" :: "absolutely positively" "perhaps" :: "we are not sure" "never" :: "never" } "never" REAL Length[2] "Length in each direction" { 0:* :: "Range has to be from zero to infinity, default is one" } 1.0 \end{verbatim} defines a \texttt{REAL} parameter, a \texttt{BOOLEAN} parameter, a \texttt{KEYWORD}, and an array of \texttt{REAL} parameters. By default, all parameters are \texttt{private}; to change this, an access specification of the form \texttt{global:} or \texttt{restricted:} (or \texttt{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 \texttt{restricted} parameters from another implementation, a line containing \texttt{shares: <\var{name}>} declares that all parameters mentioned in the file, from now until the next access specification, originate in implementation \texttt{<\var{name}>}. (Note that only one implementation can be specified on each \texttt{shares:} line.) Each of these parameters must be qualified by the initial token \texttt{USES} or \texttt{EXTENDS}, where \begin{Lentry} \item[\texttt{USES}] indicates that the parameters range remains unchanged. \item[\texttt{EXTENDS}] indicates that the parameters range is going to be extended. \end{Lentry} In contrast to parameter declarations in other access blocks, the default value must be omitted---it is impossible to set the default value of any parameter not originating in this thorn. For example, the following block adds possible values to the keyword \texttt{initial\_data} originally defined in the implementation \texttt{einstein}, and uses the \texttt{REAL} parameter \texttt{speed}. \begin{verbatim} shares:einstein EXTENDS KEYWORD initial_data { "bl_bh" :: "Brill Lindquist black holes" "misner_bh" :: "Misner black holes" "schwarzschild" :: "One Schwarzschild black hole" } USES CCTK_REAL speed \end{verbatim} Note that you must compile at least one thorn which implements \texttt{einstein}. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsubsection{The \texttt{schedule.ccl} File} \label{subsec:schedule_ccl} By default, no routine of a thorn will be run. The \texttt{schedule.ccl} file defines those that should be run, and when and under which conditions they should be run. The specification of routine scheduling is via a schedule block which consists of lines of the form \begin{alltt} schedule <\var{name}> at <\var{time bin}> [\var{other options}] \{ LANG: OPTIONS: [\var{list of options}] TAGS: [\var{list of keyword=value definitions}] STORAGE: [\var{group list with timelevels}] READS: [\var{group list}] WRITES: [\var{group list}] TRIGGERS: [\var{group list}] SYNC: [\var{group list}] \} "\var{A description}" \end{alltt} where \texttt{<\var{name}>} is the name of the routine, and \texttt{<\var{time bin}>} is the name of a schedule bin (the \texttt{CCTK\_} prefix is optional). A list of the most useful schedule bins for application thorns is given here, a complete and more descriptive list is provided in Appendix~\ref{sec:Appendix.schedule_bins}: \begin{Lentry} \item [\texttt{CCTK\_STARTUP}] For routines, run before the grid hierarchy is set up, for example, function registration. \item [\texttt{CCTK\_PARAMCHECK}] For routines that check parameter combinations, routines registered here only have access to the grid size and the parameters. \item [\texttt{CCTK\_BASEGRID}] Responsible for setting up coordinates, etc. \item [\texttt{CCTK\_INITIAL}] For generating initial data. \item [\texttt{CCTK\_POSTINITIAL}] Tasks which must be applied after initial data is created. \item [\texttt{CCTK\_PRESTEP}] Stuff done before the evolution step. \item [\texttt{CCTK\_EVOL}] The evolution step. \item [\texttt{CCTK\_POSTSTEP}] Stuff done after the evolution step. \item [\texttt{CCTK\_ANALYSIS}] For analysing data. \end{Lentry} The \var{other options} allow finer-grained control of the scheduling. It is possible to state that the routine must run \texttt{BEFORE} or \texttt{AFTER} another routine or set of routines. It is also possible to schedule the routine under an alias name by using \texttt{AS <\var{alias\_name}>}. \begin{Lentry} \item[\texttt{LANG}] The \texttt{LANG} keyword specifies the linkage of the scheduled routine which determines how to call it from the scheduler. C and Fortran linkage are possible here. C++ routines should be defined as \texttt{extern "C"} and registered as \texttt{LANG: C}. \item[\texttt{OPTIONS}] Schedule options are used for mesh refinement and multi-block simulations, and they determine ``where'' a routine executes. Often used schedule options are \texttt{local} (also the default, may be omitted), \texttt{level}, or \texttt{global}. Routines scheduled in \emph{local mode} can access individual grid points, routines scheduled in \emph{level mode} are used e.g.\ to select boundary conditions, and routines schedule in \emph{global mode} are e.g.\ used to calculate reductions (norms). \item[\texttt{TAGS}] Schedule tags, e.g.\ \texttt{Device=1} to specify that a routine executes on an OpenCL or CUDA device instead of on the host. \item[\texttt{STORAGE}] The \texttt{STORAGE} keyword specifies any groups for which memory should be allocated for the duration of the routine. The storage status reverts to its previous status after the routine returns. The format of the \texttt{STORAGE} statement includes specifying the number of timelevels of each group for which storage should be activated. \texttt{STORAGE: <\var{group1}>[\var{timelevels1}], <\var{group2}>[\var{timelevels2}]} This number can range from one to the maximum number of timelevels for the group, as specified in the group definition in its \texttt{interface.ccl} file. If this maximum number is one, the timelevel specification can be omitted from the \texttt{STORAGE} statement. Alternatively \var{timelevels} can be the name of a parameter accessible to the thorn. The parameter name is the same as used in C routines of the thorn, fully qualified parameter names of the form \texttt{\var{thorn}::\var{parameter}} are not allowed. In this case 0 (zero) \var{timelevels} can be requested, which is equivalent to the {\tt STORAGE} statement being absent. \item[\texttt{READS}] \texttt{READS} is used to declare which grid variables are read by the routine. This information is used e.g.\ to determine which variables need to be copied between host and device for OpenCL or CUDA kernel. This information can also be used to ensure that all variables that are read have previously been written by another routine. \item[\texttt{WRITES}] \texttt{WRITES} is used to declare which grid variables are written by the routine. This information is used e.g.\ to determine which variables need to be copied between host and device for OpenCL or CUDA kernel. This information can also be used to ensure that all variables that are read have previously been written by another routine. \item[\texttt{TRIGGERS}] \texttt{TRIGGERS} is used when the routine is registered at \texttt{ANALYSIS}. This is a special time bin; a routine registered here will only be called if one of the variables from a group in \texttt{TRIGGERS} is due for output. (A routine without {\tt TRIGGERS} declaration will always be called.) \item[\texttt{SYNC}] The keyword \texttt{SYNC} specifies groups of variables which should be synchronised (that is, their ghostzones should be exchanged between processors) on exit from the routine. Specifying synchronisation of grid variables in \texttt{schedule.ccl} is an alternative to calling the functions \texttt{CCTK\_SyncGroup()} or \texttt{CCTK\_SyncGroupsI()} (see the Reference Manual) from inside a routine. Using the \texttt{SYNC} keyword in the \texttt{schedule.ccl} is the preferred method, since it provides the flesh with more information about the behaviour of your code. \end{Lentry} Besides schedule blocks, it's possible to embed C style \texttt{if/else} statements in the \texttt{schedule.ccl} file. These can be used to schedule things based upon the value of a parameter. \vskip .5cm {\bf Example I:} If the parameter \texttt{evolve\_hydro} is positively set, the Fortran routine \texttt{hydro\_predictor} is scheduled to run in the \textit{evolution} loop, after the routine \texttt{metric\_predictor} and before \texttt{metric\_corrector}. The routine names \texttt{metric\_predictor} and \texttt{metric\_corrector}, may either be real routine names from the same or a different thorn, or they may be \texttt{aliased} routine names (see the next example). Before entry to \texttt{hydro\_predictor}, storage will be allocated for one timelevel for the group of grid variables \texttt{hydro\_variables} on exit from the routine this storage will be deallocated and the contents of the variables will be lost. \begin{verbatim} if(CCTK_Equals(evolve_hydro,"yes")) { SCHEDULE hydro_predictor AT evol AFTER metric_predictor BEFORE metric_corrector { LANG: FORTRAN STORAGE: hydro_variables[1] } "Do a predictor step on the hydro variables" } \end{verbatim} If the parameter \texttt{evolve\_hydro} is set negatively, the {\tt hydro\_predictor} routine will not be called by the scheduler. Note that if the \texttt{evolve\_hydro} parameter is \texttt{STEERABLE}, it can be dynamically scheduled and de-scheduled during a run if a steering interface is available. \vskip .5cm {\bf Example II:} The thorns \texttt{WaveToy77} and \texttt{WaveToyC}, each provide a routine to evolve the 3D wave equation: \texttt{WaveToyF77\_Evolution} and \texttt{WaveToyC\_Evolution}. The routine names have to be different, so that both thorns can be compiled at the same time, their functionality is identical though. Either one of them can then be activated at run time in the parameter file via \texttt{ActiveThorns}. Since each evolution routine provides the same functionality, it makes sense to schedule them under the common alias {\tt WaveToy\_Evolution} to allow relative scheduling ({\tt BEFORE/AFTER}) independent of the actual routine name (which may change depending on the activation in the parameter file). In both cases, the group of variables \texttt{scalarfield} are synchronised across processes when the routine is exited. \begin{verbatim} schedule WaveToyF77_Evolution AS WaveToy_Evolution AT evol { LANG: Fortran STORAGE: scalartmps SYNC: scalarfield } "Evolution of 3D wave equation" schedule WaveToyC_Evolution AS WaveToy_Evolution AT evol { LANG: C STORAGE: scalartmps SYNC: scalarfield } "Evolution of 3D wave equation" \end{verbatim} The thorn \texttt{IDScalarWave} schedules the routine \texttt{WaveBinary} after the alias \texttt{WaveToy\_Evolution}. It is scheduled independently of the C or Fortran routine name. \begin{verbatim} schedule WaveBinary AT evol AFTER WaveToy_Evolution { STORAGE: wavetoy::scalarevolve LANG: Fortran } "Provide binary source during evolution" \end{verbatim} \paragraph{Storage Outside of Schedule Blocks} The keyword \texttt{STORAGE} can also be used outside of the schedule blocks to indicate that storage for these groups should be switched on at the start of the run. Note that the storage is only allocated in this way at the start; a thorn could explicitly switch the storage off (although this is not recommended practise). As for the \texttt{STORAGE} statement in schedule blocks, each group must also specify how many timelevels to activate storage for. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsubsection{The \texttt{configuration.ccl}} \label{subsec:configuration_ccl} The \texttt{configuration.ccl} file is optional. It can be used for two purposes: to detect certain features of the host system, such as the presence or absence of libraries, variable types, etc, or the location of libraries; or to provide access to certain functions too complex or otherwise not suitable for function aliasing. The basic concept here is that a thorn can either provide or use a {\bf capability}. A thorn providing a capability can specify a script which is run by the CST to detect features and write any configuration files; the script may output lines to its standard output to inform the CST of features to: add to the header files included by thorns using this capability; add to the make files used to build thorns using this capability; or add to the main Cactus link line. The script may also indicate that this capability is not present by returning a non-zero exit code---e.g. if the thorn is providing access to an external library, it should return an error if the library is not installed on the system. A thorn may either require a capability to be present, in which case it is an error if there is no thorn providing that capability in the configuration's \texttt{ThornList}, or it may optionally use a capability, in which case a macro is defined in the thorn's header file if a thorn providing the capability is present. A configuration.ccl file has the form: \begin{alltt} PROVIDES <\var{My_Capability}> \{ SCRIPT <\var{My_ConfigScript}> LANG <\var{My_Language}> \} REQUIRES <\var{Another_Capability}> OPTIONAL <\var{Yet_Another_Capability}> \{ % DEFINE <\var{macro}> \} \end{alltt} which states that this thorn provides the capability \verb|My_Capability|, and a script \verb|MyConfigScript| should be run to detect features of this capability; the script is in language \verb|My_Language|---the CST will use the appropriate environment or interpreter to invoke the script. % The example requires a thorn providing \verb|Another_Capability| to be % in the \texttt{ThornList}, and, if a thorn providing % \verb|Yet_Another_Capability| is in the \texttt{ThornList}, the preprocessor % macro \verb|macro| will be defined, and set to 1. % % As an example, a thorn A may be able to use PVM for parallelism if it is % present, but can still work in the absence of it. There could be a % thorn providing PVM, and thorn A would then have % % \begin{alltt} % OPTIONAL PVM % \{ % DEFINE HAVE_PVM % \} % % \end{alltt} % % The code in thorn A could then have % % \begin{alltt} % % #ifdef HAVE_PVM % do something % #else % do something else % #endif % % \end{alltt} % % to pick the appropriate mode. The syntax of the output of the configure script is described in Appendix \ref{sec:Appendix.configuration.ccl.configscript}. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Naming Conventions for Source Files} \label{nacofosofi} The make system uses file extensions to designate coding language, as well as other properties of the code in the file. The following extensions are understood: \begin{center} \begin{tabular}{|l|c|c|c|} \hline Extension & Language & Format & Preprocess \\ \hline {\t .c} & C & - & yes \\ {\t .cc} or {\t .C} & C++ & - & yes \\ {\t .cl} & OpenCL & - & yes \\ {\t .cu} & CUDA & - & yes \\ {\t .F77} & Fortran 77 & fixed & yes \\ {\t .f77} & Fortran 77 & fixed & no \\ {\t .F} & Fortran 90 & fixed & yes \\ {\t .f} & Fortran 90 & fixed & no \\ {\t .F90} & Fortran 90 & free & yes \\ {\t .f90} & Fortran 90 & free & no \\ \hline \end{tabular} \end{center} In order to use Cactus \texttt{\#include} directives in a file, it must be preprocessed. A complete description of Fortran fixed and free format can be found in any textbook on Fortran 90. The most obvious differences are that in fixed format, code must begin after the 5th column and line continuations are indicated by a character in column 5, while in free format, lines can begin anywhere, and line continuations are indicated by an ampersand at the end of the line to be continued. Also note that statement labels are handled very differently. The following restrictions apply to file names: \begin{itemize} \item For portability across all operating systems, the base names for any particular extension should not depend on the operating system being case sensitive (e.g.\ having \texttt{MyFile.c} and \texttt{MYFILE.f77} is allright, but \texttt{MyFile.c} and \texttt{MYFILE.c} could cause problems). \item Currently, all source files within a thorn must have distinct names, regardless of whether they are placed in different subdirectories. We hope to relax this in future. Different thorns may have files with the same names, however. \end{itemize} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Adding Source Files} By default, the CCTK looks in the \texttt{src} directory of the thorn for source files. There are two ways in which to specify the sources. The easiest is to use the \texttt{make.code.defn} based method in which the CCTK does all the work, but you may instead put a \texttt{Makefile} in the \texttt{src} directory and do everything yourself. \subsubsection{\texttt{make.code.defn} based thorn building} \label{sec:mabathbu} This is the standard way to compile your thorn's source files. The Cactus make system looks for a file called \texttt{make.code.defn} in that directory (if there is no file called \texttt{Makefile} in the \texttt{src} directory). At its simplest, this file contains two lines \begin{itemize} \item \texttt{SRCS = <\var{list of all source files in this directory}>} \item \texttt{SUBDIRS = <\var{list of all subdirectories, including subdirectories of subdirectories}>} \end{itemize} Each subdirectory listed should then have a \texttt{make.code.defn} file containing just a \texttt{SRCS = } line, a \texttt{SUBDIRS = } line will be ignored. In addition, each directory can have a \texttt{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 complete details of the syntax. \subsubsection{\texttt{Makefile} based thorn building} This method gives you the ultimate responsibility. The only requirement is that a library called \texttt{\$NAME} be created by the \texttt{Makefile}. The makefile is passed the following variables \begin{Lentry} \item [\texttt{\$(CCTK\_HOME)}] the main Cactus directory \item [\texttt{\$(TOP)}] the configuration directory \item [\texttt{\$(SRCDIR)}] the directory in which the source files can be found \item [\texttt{\$(CONFIG)}] the directory containing the configuration files \item [\texttt{\$(THORN)}] the thorn name \item [\texttt{\$(SCRATCH\_BUILD)}] the scratch directory where Fortran module files should end up if they need to be seen by other thorns. \item [\texttt{\$(NAME)}] the name of the library to be built \end{Lentry} and has a working directory of \texttt{<\var{config}>/build/<\var{thorn\_name}>} . \subsubsection{Other makefile variables} \begin{itemize} \item \texttt{CPP}: The C preprocessor which is used to preprocess C and C++ source code, and to determine the C and C++ make dependencies \item \texttt{FPP}: The C preprocessor which is used to preprocess Fortran source code \item \texttt{CPPFLAGS}: Flags which are passed to \texttt{CPP}, to \texttt{CC}, and to \texttt{CXX} \item \texttt{FPPFLAGS}: Flags which are passed to \texttt{FPP} \item \texttt{CC}: The C compiler \item \texttt{CXX}: The C++ compiler \item \texttt{CUCC}: The CUDA compiler \item \texttt{F77}: The Fortran 77 compiler. This is only used if there is no Fortran 90 compiler \item \texttt{F90}: The Fortran 90 compiler. If this variable is set, it is also used to compile Fortran 77 files \item \texttt{CFLAGS}: Flags which are passed to \texttt{CC} \item \texttt{CXXFLAGS}: Flags which are passed to \texttt{CXX} \item \texttt{CUCCFLAGS}: Flags which are passed to \texttt{CUDA} \item \texttt{F77FLAGS}: Flags which are passed to \texttt{F77} \item \texttt{F90FLAGS}: Flags which are passed to \texttt{F90} \item \texttt{LD}: The binder. This should not be directly \texttt{ld}, but should be a compiler driver such as C++. Often, \texttt{LD} is the same as \texttt{CXX} \item \texttt{LDFLAGS}: Flags which are passed to \texttt{LD} \end{itemize} Note that there are no makefile variables to specify an OpenCL compiler or its flags. OpenCL is implemented as a library, and code is compiled at run time via library functions to which the source code is passed as a string. OpenCL source code (files with the extension \texttt{.cl}) are thus not compiled when a Cactus configuration is built. Instead, the content of \texttt{.cl} files is converted into a string and placed into the executable. These strings have the type \texttt{char~const~*} in C, and can be accessed at run time under a (globally visible) name \texttt{OpenCL\_source\_THORN\_FILE}, where \texttt{THORN} and \texttt{FILE} and are the thorn name and file name, respectively. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \section{Cactus Variables} \label{chap:cactus_variables} A \textit{grid variable} is a Cactus program variable passed among thorns, (or routines belonging to the same thorn interface), by way of calls to the flesh. They are the only variables known to Cactus. Such variables represent values of functions on the computational grid, and are, therefore, often called \textit{grid functions}. In the Cactus context, grid variables are often referred to simply as \textit{variables}. Cactus variables are used instead of local variables for a number of reasons: \begin{itemize} \item Cactus variables can be made visible to other thorns, allowing thorns to communicate and share data. \item Cactus variables can be distributed and communicated among processors, allowing parallel computation. \item A database of Cactus variables, and their attributes, is held by the flesh, and this information is used by thorns, for example, for obtaining a list of variables for checkpointing. \item Many Cactus APIs and tools can only be used with Cactus variables. \item Cactus provides features for error checking based on Cactus variables and their attributes. \end{itemize} Cactus variables are collected into \textit{groups}. All variables in a group are of the same data type, and have the same attributes. Most Cactus operations act on a group as a whole. A group must be declared in its thorn's \texttt{interface.ccl} file. The specification for a group declaration (fully described in Appendix~\ref{sec:Appendix.interface}) is, \begin{alltt} <\var{data_type}> <\var{group_name}> [TYPE=<\var{group_type}>] [DIM=<\var{dim}>] [TIMELEVELS=<\var{num}>] [SIZE=<\var{size in each direction}>] [DISTRIB=<\var{distribution_type}>] [GHOSTSIZE=<\var{ghostsize}>] [\{ [ <\var{variable_name}>[,]<\var{variable_name}> <\var{variable_name}> ] \} ["<\var{group_description}>"] ] \end{alltt} Currently, the names of groups and variables must be distinct. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Data Type} Cactus supports integer, real, complex and character variable types, in various different sizes. (Sizes in the following refer to the number of bytes occupied by the a variable of the type). \begin{Lentry} \item[INTEGER] \texttt{CCTK\_INT}, \texttt{CCTK\_INT1}, \texttt{CCTK\_INT2}, \texttt{CCTK\_INT4}, \texttt{CCTK\_INT8}. (\texttt{CCTK\_INT} defaults to being \texttt{CCTK\_INT4}). \item[REAL] \texttt{CCTK\_REAL}, \texttt{CCTK\_REAL4}, \texttt{CCTK\_REAL8}, \texttt{CCTK\_REAL16}. (\texttt{CCTK\_REAL} defaults to being \texttt{CCTK\_REAL8}). \item[COMPLEX] \texttt{CCTK\_COMPLEX}, \texttt{CCTK\_COMPLEX8}, \texttt{CCTK\_COMPLEX16}, \texttt{CCTK\_COMPLEX32}. (\texttt{CCTK\_COMPLEX} defaults to being \texttt{CCTK\_COMPLEX16}). \item[BYTE] This is a 1 byte data type. \end{Lentry} Normally a thorn should use the default types---\texttt{CCTK\_INT}, \texttt{CCTK\_REAL}, \texttt{CCTK\_COMPLEX}---rather than explicitly setting the size, as this gives maximum portability. Also, the defaults can be changed at configuration time (see Section \ref{subsec:Compilation-Available_Options}), and this allows people to compile the code with different precisions to test for roundoff effects, or to run more quickly with a lower accuracy. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Group Types} \label{sec:cactus_variables-groups} Groups can be either \textit{scalars}, \textit{grid functions} (GFs), or \textit{grid arrays}. Different attributes are relevant for the different group types. \begin{Lentry} \item[\texttt{SCALAR}] This is just a single number, e.g.\ the total energy of some field. These variables aren't communicated between processors---what would be the result of such communication? \item[\texttt{GF}] This is the most common group type. A GF is an array with a specific size, set at run time in the parameter file, which is distributed across processors. All GFs have the same size, and the same number of ghostzones. Groups of GFs can also specify a dimension, and number of timelevels. \item[\texttt{ARRAY}] This is a more general form of the GF. Each group of arrays can have a distinct size and number of ghostzones, in addition to dimension and number of timelevels. The drawback of using an array over a GF is that a lot of data about the array can only be determined by function calls, rather than the quicker methods available for GFs. \end{Lentry} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Timelevels} These are best introduced by an example using finite differencing. Consider the 1-D wave equation \begin{equation} \frac{\partial^2 \phi}{\partial t^2} = \frac{\partial^2 \phi}{\partial x^2} \end{equation} To solve this by partial differences, one discretises the derivatives to get an equation relating the solution at different times. There are many ways to do this, one of which produces the following difference equation \begin{equation} \label{equation:difference} \phi(t+\Delta t,x) -2\phi(t,x) +\phi(t-\Delta t,x) = \frac{\Delta t^2}{\Delta x^2} \lbrace{\phi(t,x+\Delta x) -2\phi(t,x) +\phi(t,x-\Delta x)}\rbrace \end{equation} which relates the three timelevels $t+\Delta t$, $t$, and $t-\Delta t$. Obviously, the code could just use three variables, one for each timelevel. This turns out, however, to be inefficient, because as soon as the time is incremented to $t+\Delta t$, it would be necessary to copy data from the $t$ variable to the $t-\Delta t$ variable and from the $t+\Delta t$ variable to the $t$ variable. To remove this extraneous copy, Cactus allows you to specify the number of timelevels used by your numerical scheme. It then generates variables with the base name (e.g.\ \texttt{phi}) suffixed by a qualifier for which timelevel is being referred to---no suffix for the next timelevel, and \texttt{\_p} for each previous timelevel. The timelevels are rotated (by the driver thorn) at the start of each evolution step, that is: \begin{verbatim} initial poststep analysis loop: rotate timelevels t = t + dt it = it + 1 prestep evolve poststep analysis \end{verbatim} Timelevel rotation means that, for example, \texttt{phi\_p} now holds the values of the former \texttt{phi}, and \texttt{phi\_p\_p} the values of the former \texttt{phi\_p}, etc. Note that after rotation, \texttt{phi} is undefined, and its values should not be used until they have been updated. All timelevels, except the current level, should be considered \emph{read-only} during evolution, that is, their values should not be changed by thorns. The exception to this rule is for function initialisation, when the values at the previous timelevels do need to be explicitly filled out. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Size and Distrib} A Cactus grid function or array has a size set at runtime by parameters. This size can either be the global size of the array across all processors (\texttt{DISTRIB=DEFAULT}), or, if \texttt{DISTRIB=CONSTANT}, the specified size on \emph{each} processor. If the size is split across processors, the driver thorn is responsible for assigning the size on each processor. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Ghost Zones} \label{sec:ghost_size} Cactus is based upon a \textit{distributed computing} paradigm. That is, the problem domain is split into blocks, each of which is assigned to a processor. For hyperbolic and parabolic problems the blocks only need to communicate at the edges. To illustrate this, take the example of the wave equation again. Equation \ref{equation:difference} describes a possible time-evolution scheme. On examination, you can see that to generate the data at the point ($t+\Delta t$, $x$) we need data from the four points ($t$, $x$), ($t-\Delta t$, $x$), ($t$, $x+\Delta x$), and ($t$, $x-\Delta x$) only. Ignoring the points at $x$, which are obviously always available on a given processor, you can see that the algorithm requires a point on either side of the point $x$, i.e.\ this algorithm has \textit{stencil size} 1. Similarly algorithms requiring two points on either side have stencil size 2, etc. Now, if you evolve the above scheme, it becomes apparent that at each iteration the number of grid points you can evolve decreases by one at each edge (see Figure \ref{fig:noghost}). \begin{figure}[ht] \begin{center} \includegraphics[angle=0,width=8cm]{1dnoghost} \end{center} \caption{Distributed wave equation with no ghostzones} \label{fig:noghost} \end{figure} At the outer boundary of the physical domain, the data for the boundary point can be generated by the boundary conditions, however, at internal boundaries, the data has to be copied from the adjacent processor. It would be inefficient to copy each point individually, so instead, a number of \textit{ghostzones} are created at the internal boundaries. A ghostzone consists of a copy of the whole plane (in 3D, line in 2D, point in 1D) of the data from the adjacent processor. That is, the array on each processor is augmented with copies of points from the adjacent processors, thus allowing the algorithm to proceed \textit{on the points owned by this processor} without having to worry about copying data. Once the data has been evolved one step, the data in the ghostzones can be exchanged (or \textit{synchronised}) between processors in one fell swoop before the next evolution step. (See Figure \ref{fig:withghost}.) Note that you should have at least as many ghostzones as your stencil size requires. \begin{figure}[ht] \begin{center} \includegraphics[angle=0,width=8cm]{withghost} \end{center} \caption{Distributed wave equation with ghostzones} \label{fig:withghost} \end{figure} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Information about Grid Variables} The flesh holds a database with information related to grid variables, and provides a set of querying APIs. \subsubsection{Group Information} Fundamental information about grid functions (e.g.\ local grid size and location, number of ghostzones) is passed through the argument list of scheduled routines (see Section~\ref{sec:cactus_variables_c}). To obtain similar information from non-scheduled routines, or for general grid variables, a set of functions are provided, the last two letters of which specify whether the information is requested using a group name (\texttt{GN}) or index (\texttt{GI}), or a variable name (\texttt{VN}) or index (\texttt{VI}). \begin{Lentry} \item [\texttt{CCTK\_Grouplsh[GN|GI|VN|VI]}] An array of integers with the local grid size on this processor. \item [\texttt{CCTK\_Groupgsh[GN|GI|VN|VI]}] An array of integers with the global grid size. \item [\texttt{CCTK\_Groupbbox[GN|GI|VN|VI]}] An array of integers which indicate whether the boundaries are internal boundaries (e.g.\ between processors), or physical boundaries. A value of \texttt{1} indicates a physical (outer) boundary at the edge of the computational grid, and \texttt{0} indicates an internal boundary. \item [\texttt{CCTK\_Groupnghostzones[GN|GI|VN|VI]}] An array of integers with the number of ghostzones used in each direction. \item [\texttt{CCTK\_Grouplbnd[GN|GI|VN|VI]}] An array of integers containing the lowest index (in each direction) of the local grid, as seen on the global grid. Note that these indices start from zero, so you need to add one when using them in Fortran thorns. \item [\texttt{CCTK\_Groupubnd[GN|GI|VN|VI]}] An array of integers containing the largest index (in each direction) of the local grid, as seen on the global grid. Note that these indices start from zero, so you need to add one when using them in Fortran thorns. \end{Lentry} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \section{Cactus Parameters} \label{chap:Cactus_parameters} Parameters are the means by which the user specifies the runtime behaviour of the code. Each parameter has a data type and a name, as well as a range of allowed values and a default value. These are declared in the thorn's \texttt{param.ccl} file. The thorn determines which parameters can be used in other thorns by specifying a \textit{scope} for the thorn, as explained in Section \ref{sec:Cactus_parameters.scope}. The user may specify initial values for parameters in the parameter file (see Section~\ref{sec:Parameter_File}); the flesh validates these values against the parameters' allowed ranges. Otherwise, the initial value of the parameter is taken to be its default. Once validated, parameter values are fixed, and cannot be changed, unless the parameter is specified to be \textit{steerable} (see \ref{sec:Cactus_parameters.steerable}). For a detailed discussion of the \texttt{param.ccl} syntax, see Appendix~\ref{sec:Appendix.param}. The full specification for a parameter declaration is \begin{alltt} [EXTENDS|USES] <\var{parameter_type}>[[<\var{size}>]] <\var{parameter name}> "<\var{parameter description}>" \{ <\var{PARAMETER_RANGES}> \} <\var{default value}> \end{alltt} You can obtain lists of the parameters associated with each thorn using the Cactus command line options \texttt{-o} and \texttt{-O} (Section~\ref{sec:command_line_options}). %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Types and Ranges} \label{sec:Parameters.Types_and_Ranges} Parameters can be of these types: \begin{Lentry} \item[Int] Can take any integral value \item[Real] Can take any floating point value \item[Keyword] Can have a value consisting of one of a choice of strings \item[Boolean] Can be true or false ({\t 1}, {\t t}, {\t true}, or {\t 0}, {\t f}, {\t false}) \item[String] Can have any string value \end{Lentry} Each parameter can be validated against a set of allowed \textit{ranges}, each of which has a description associated with it. The nature of the range is determined by the type of parameter, as follows: \subsubsection{Int} The range specification is of the form \begin{alltt} \var{lower}:\var{upper}:\var{stride} \end{alltt} where \var{lower} and \var{upper} specify the lower and upper allowed range, and \var{stride} allows numbers to be be missed out, e.g.\ \begin{verbatim} 1:21:2 \end{verbatim} means the value must be an odd number between one and twenty-one (inclusive). A missing end of range (or a `\texttt{*}') indicates negative or positive infinity, and the default stride is one. \subsubsection{Real} The range specification is of the form \begin{alltt} \var{lower}:\var{upper} \end{alltt} where \var{lower} and \var{upper} specify the lower and upper allowed range. A missing end of range (or a `\texttt{*}') implies negative or positive infinity. The above is inclusive of the endpoints. A `\texttt{(}' (or `\texttt{)}') before (or after) the lower (or upper) range specifies an open endpoint. The numbers written in a \texttt{param.ccl} file are interpreted as C code. To express a number in `scientific notation', use, e.g.\ `\texttt{1e-10}', which is a double precision constant in C. (If the floating precision of the variable to which it is assigned is not double, then C will typecast appropriately. If you \emph{really} want to specify a single precision floating constant, or a long double constant, append the number with {\t f} or {\t l} respectively.) \subsubsection{Keyword} The range specification consists of a string, which will be matched in a case insensitive manner. \subsubsection{Boolean} There is no range specification for this type of parameter. \subsubsection{String} The range is a POSIX regular expression. On some machines you may be able to use extended regular expressions, but this is not guaranteed to be portable. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Scope} \label{sec:Cactus_parameters.scope} Parameters can be \texttt{GLOBAL}, \texttt{RESTRICTED}, or \texttt{PRIVATE}. Global parameters are visible to all thorns. Restricted parameters are visible to any thorn which chooses to \texttt{USE} or \texttt{EXTEND} it. A private parameter is only visible to the thorn which declares it. The default scope is \texttt{PRIVATE}. \subsection{Steerable} \label{sec:Cactus_parameters.steerable} A parameter can be changed dynamically if it is specified to be \textit{steerable} (see Section~\ref{subsec:Appendix.param.specification_items}). It can then be changed by a call to the flesh function \texttt{CCTK\_ParameterSet} (see the Reference Guide for a description of this function). %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \section{Scheduling} \label{chap:scheduling} Cactus contains a rule-based scheduling system, which determines which routines, from which thorns are run in which order. The scheduler determines if the specifications are inconsistent, but does allow the user to schedule a routine with respect to another routine which may not exist. For a detailed discussion of the {\t schedule.ccl} syntax see Appendix~\ref{sec:Appendix.schedule}. A usual simple specification for a schedule declaration is \begin{alltt}schedule <\var{function name}> AT <\var{schedule bin}> \{ LANG: <\var{language}> [STORAGE: <\var{group}>[[\var{timelevels}]],<\var{group}>[[\var{timelevels}]]...] \} "\var{Description of function}" \end{alltt} The full specification for a schedule declaration is \begin{alltt}schedule [GROUP] AT|IN <\var{schedule bin}|\var{group name}> [AS <\var{alias}>] [WHILE <\var{variable}>] [IF <\var{variable}>] [BEFORE|AFTER <\var{item}>|(<\var{item}> <\var{item}> ...)] \{ LANG: <\var{language}> [STORAGE: <\var{group}>[[\var{timelevels}]],<\var{group}>[[\var{timelevels}]]...] [TRIGGER: <\var{group}>,<\var{group}>...] [SYNC: <\var{group}>,<\var{group}>...] [OPTIONS: <\var{option}>,<\var{option}>...] \} "\var{Description of function or schedule group}" \end{alltt} This full schedule specification consists of a mandatory part, a set of options, and the main body limited by braces, referred to as the \texttt{schedule block}. Each schedule item is scheduled either \texttt{AT} a particular \var{scheduling bin}, or \texttt{IN} a schedule \var{group}. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Schedule Bins} \label{scheduling:schedule_bins} These are the main times at which scheduled functions are run, from fixed points in the flesh and driver thorn (schedule bins can easily be traversed from any thorn, although this is not usual). When a schedule bin is \textit{traversed}, all the functions scheduled in that particular are called, in the manner described in Section \ref{scheduling:calling_scheduled_functions} and respecting the requested ordering of functions(Section~\ref{scheduling:schedule_options}). In the absence of any ordering, functions in a particular schedule bin will be called in an undetermined order. The schedule bins are described in Section \ref{subsec:schedule_ccl}. Note that the preceding \texttt{CCTK\_} is optional for the use of the bin names in the \texttt{schedule.ccl} file. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Groups} \label{scheduling:groups} If the optional \texttt{GROUP} specifier is used, the item is a schedule group rather than a normal function. Schedule groups are effectively new, user-defined, schedule bins. Functions or groups may be scheduled \texttt{IN} these, in the same way as they are scheduled {\tt AT} the main schedule bins. (That is, groups may be nested.) %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Schedule Options} \label{scheduling:schedule_options} The options define various characteristics of the schedule item. \begin{Lentry} \item[\texttt{AS}] This assigns a new name to a function for scheduling purposes. This is used, for instance, to allow a thorn to schedule something before or after a routine from another implementation; two thorns providing this implementation can schedule a routine \texttt{AS} the same thing, thus allowing other thorns to operate independently of which one is active. \item[\texttt{WHILE}] This specifies a \texttt{CCTK\_INT} grid scalar which is used to control the execution of this item. As long as the grid scalar has a nonzero value, the schedule item will be executed repeatedly. This allows dynamic behaviour with scheduling. \item[\texttt{IF}] This specifies a \texttt{CCTK\_INT} grid scalar which is used to control the execution of this item. If the grid scalar has a nonzero value, the schedule item will be executed, otherwise the item will be ignored. This allows dynamic behaviour with scheduling. If both an \texttt{IF} and a \texttt{WHILE} clause are present, then the schedule is executed according to the following pseudocode: \begin{verbatim} IF condition WHILE condition SCHEDULE item END WHILE END IF \end{verbatim} \item[\texttt{BEFORE} or \texttt{AFTER}] These specify either \begin{itemize} \item a function or group before or after which this item will be scheduled, or \item a list of functions and/or groups; this item will be scheduled (once) before any of them or after all of them respectively. \end{itemize} Note that a single schedule item may have multiple \texttt{BEFORE} and/or \texttt{AFTER} options; the scheduler will honor all of these (or abort with a fatal error). For example, \begin{alltt} schedule FOO BEFORE A BEFORE B BEFORE C ... \end{alltt} schedules \texttt{FOO} before any of \texttt{A}, \texttt{B}, or \texttt{C}. This can also be written \begin{alltt} schedule FOO BEFORE (A B C) ... \end{alltt} Note that the set of all \texttt{BEFORE}/\texttt{AFTER} options in all active schedule blocks of all active thorns, \emph{must} specify a (directed) graph with no cycles; if there are any cycles, then the scheduler will abort with a fatal error. \end{Lentry} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{The Schedule Block} \label{scheduling:schedule_block} The schedule block specifies further details of the scheduled function or group. \begin{Lentry} \item[\texttt{LANG}] This specifies the language of the routine. Currently this is either C or Fortran. C++ routines should be defined as \texttt{extern "C"} and registered as \texttt{LANG: C}. \item[\texttt{STORAGE}] The \texttt{STORAGE} keyword specifies groups for which memory should be allocated for the duration of the routine or schedule group. The storage status reverts to its previous status after completion of the routine or schedule group. Each group must specify how many timelevels to activate storage for, from 1 up to the maximum number for the group as specified in the defining {\tt interface.ccl} file. If the maximum is 1 (the default) this number may be omitted. Alternatively \var{timelevels} can be the name of a parameter accessible to the thorn. The parameter name is the same as used in C routines of the thorn, fully qualified parameter names of the form \texttt{\var{thorn}::\var{parameter}} are not allowed. In this case 0 (zero) \var{timelevels} can be requested, which is equivalent to the {\tt STORAGE} statement being absent. \item[\texttt{TRIGGER}] This is only used for items scheduled at timebin \texttt{CCTK\_ANALYSIS}. The item will only be executed if output is due for at least one variable in one of the listed groups. (The item will also be called if there is no group listed.) \item[\texttt{SYNC}] On exit from this item, the ghost zones of the listed groups will be exchanged. \item[\texttt{OPTIONS}] This is for miscellaneous options. The list of accepted options is given in Appendix~\ref{app:allopts}. \end{Lentry} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{How Cactus Calls Scheduled Functions} \label{scheduling:calling_scheduled_functions} For each scheduled function called, the flesh performs a variety of jobs at entry and exit. On entry to a scheduled routine, if the routine is being called at the \texttt{CCTK\_ANALYSIS} timebin first, a check is made to see if the routine should actually be called on this timestep. For this, all grid variables in the trigger groups for the routine are checked with all registered output methods to determine if it is time to output any triggers. The routine will only be called if at least one is due to be output. Note that once a grid variable has been analyzed, it gets marked as such, and will not be analyzed again during this iteration. (Note that a routine without any trigger groups will also be called.) Thus, if more than one analysis routine should be triggered on the same trigger variable(s), they must be scheduled in a single group. Routines from all timebins, other than \texttt{ANALYSIS}, are always called. Next, storage is assigned for any required variables, remembering the original state of storage. The routine is then called, and on exit, any required grid variables are first synchronised. Following synchronization, any required output methods are called for the triggers. Finally, the storage of grid variables is returned to the original state. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \section{Writing a Thorn} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Thorn Programming Languages} When you start writing a new thorn, the first decision to make is which programming language to use. The source code in Cactus thorns can be written in any mixture of C, C++, CUDA, or Fortran. The following points should be considered when choosing a language to work in: \begin{itemize} \item All functions designed for application thorn writers are available in all languages, however, some interfaces for infrastructure thorn writing are only available from C or C++. \end{itemize} Whatever language you choose, if you want your thorn to be portable, and compile and run on multiple platforms, stick to the standards and don't use machine dependent extensions. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{What the Flesh Provides} The flesh provides for thorns: \begin{Lentry} \item [\texttt{Variables}] \item [\texttt{Parameters}] \item [\texttt{Cactus Functions}] \begin{itemize} \item{} Driver (parallelisation) utilities \item{} I/O utilities \item{} Coordinates utilities \item{} Reduction utilities \item{} Interpolation utilities \item{} Information utilities \end{itemize} \end{Lentry} \subsubsection{Fortran Routines} Any source file using Cactus infrastructure should include the header file \texttt{cctk.h} using the line \begin{verbatim} #include "cctk.h" \end{verbatim} (Fortran programmers should not be put off by this being a C style header file---most Cactus files are run through a C preprocessor before compilation.) \paragraph{Variables} Any routine using Cactus argument lists (for example, all routines called from the scheduler at time bins between {\t CCTK\_STARTUP} and {\t CCTK\_SHUTDOWN}) should include at the top of the file the header \begin{verbatim} #include "cctk_Arguments.h" \end{verbatim} A Cactus macro \texttt{CCTK\_ARGUMENTS} is defined for each thorn to contain: \begin{itemize} \item General information about the grid hierarchy, for example, the number of grid points used. See Section \ref{sec:cactus_variables_c} for a complete list. \item All the grid variables defined in the thorn's \texttt{interface.ccl} \item All the grid variables required from other thorns as requested by the \texttt{inherits} and \texttt{friend} lines in the \texttt{interface.ccl} \end{itemize} These variables must be declared at the start of the routine using the macro \texttt{DECLARE\_CCTK\_ARGUMENTS}. To pass the arguments to another routine in the same thorn use the macro \texttt{CCTK\_PASS\_FTOF} in the calling routine, and again the macro \texttt{CCTK\_ARGUMENTS} in the receiving routine. Note that you cannot use Cactus argument lists in routines scheduled at the {\t CCTK\_STARTUP} and {\t CCTK\_SHUTDOWN} time bins, because at this time no grid hierarchy exists. \paragraph{Parameters} All parameters defined in a thorn's \texttt{param.ccl} and all \texttt{global} parameters, appear as local variables of the corresponding \texttt{CCTK} data type in Fortran source code, i.e. Booleans and Integers appear as \texttt{CCTK\_INT} types (with nonzero/zero values for boolean {\t yes/no}), Reals as \texttt{CCTK\_REAL}, and Keywords and String parameters as \texttt{CCTK\_STRING} (see also note below). These variables are \emph{read only}, and \emph{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 \texttt{DECLARE\_CCTK\_PARAMETERS}. In Fortran, special care should be taken with string valued parameters. These 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 macro \texttt{CCTK\_EQUALS()} or the function \texttt{CCTK\_Equals()} (see the reference manual for a description of the \texttt{CCTK\_} functions). To print the value of a string valued parameter to screen, use the subroutine \texttt{CCTK\_PrintString()}. A further function \texttt{CCTK\_FortranString} provides a mechanism for converting a string parameter to a Fortran string. For example, if \texttt{operator} is a Cactus string parameter holding the name of a reduction operator whose handle you need to find, you cannot pass it directly into the subroutine \texttt{CCTK\_LocalArrayReductionHandle}, which is expecting a Fortran string. Instead, the following is needed: % \begin{verbatim} character*200 fortran_operator CCTK_INT fortran_operator_len integer handle call CCTK_FortranString(fortran_operator_len,operator,fortran_operator) call CCTK_LocalArrayReductionHandle(handle,fortran_operator(1:fortran_operator_len)) \end{verbatim} \paragraph{Fortran Example} The Fortran routine, \verb|MyFRoutine|, is scheduled in the {\tt schedule.ccl} file, doesn't use Cactus parameters, and calls another routine, in the same thorn, \verb|MyNewRoutine|, which does use parameters. This routine needs to be passed an integer flag as well as the standard Cactus variables. The source file should look like % \begin{verbatim} #include "cctk.h" #include "cctk_Arguments.h" #include "cctk_Parameters.h" subroutine MyFRoutine(CCTK_ARGUMENTS) c I'm very cautious, so I want to declare all variables implicit none DECLARE_CCTK_ARGUMENTS integer flag flag = 1 call MyNewRoutine(CCTK_PASS_FTOF,flag) return end subroutine MyNewRoutine(CCTK_ARGUMENTS,flag) implicit none DECLARE_CCTK_ARGUMENTS DECLARE_CCTK_PARAMETERS integer flag c Main code goes here return end \end{verbatim} \paragraph{Cactus Fortran Functions} Cactus Fortran functions, for example, \texttt{CCTK\_MyProc} and {\tt CCTK\_Equals}, can all be declared by adding the statement % \begin{verbatim} #include "cctk_Functions.h" \end{verbatim} % near the top of the file, and adding the declaration % \begin{verbatim} DECLARE_CCTK_FUNCTIONS \end{verbatim} % to a module or a subroutine after the \texttt{implicit none} statement, but before any executable code. \paragraph{Fortran Modules} Fortran modules should be placed into source files that have the same name as the module, followed by the corresponding file name suffix. A module \texttt{metric} should thus be placed, e.g.\ into a file \texttt{metric.F90}. This convention allows the Cactus build system to automatically deduce the compile time dependencies. If you do not follow this convention, then you have to include the modules into the thorn's \texttt{make.code.deps} file (Section \ref{sec:mabathbu}) to ensure they are compiled before the routines which use them. This is especially important for parallel building. For example, if a routine in \texttt{MyRoutine.F90} uses a module in {\tt MyModule.F90}, then add the line: % \begin{verbatim} MyRoutine.F90.o: MyModule.F90.o \end{verbatim} \paragraph{The \texttt{MOD} function} The intrinsic function \texttt{MOD} in Fortran takes two integer arguments, which should both be of the same type. This means that it may be necessary to cast the arguments to, e.g. \texttt{INT} for some architectures. This can occur in particular when a \texttt{CCTK\_INT} parameter and the Cactus variable \texttt{cctk\_iteration} (which is declared to be \texttt{INTEGER}) are used, in which case the correct code is \begin{verbatim} MOD(cctk_iteration,INT(MyParameter)) \end{verbatim} \subsubsection{C Routines} Any source file using Cactus infrastructure should include the header file \texttt{cctk.h} using the line \begin{verbatim} #include "cctk.h" \end{verbatim} \paragraph{Variables} Any routine using Cactus argument lists (for example, all routines called from the scheduler at time bins between {\t CCTK\_STARTUP} and {\t CCTK\_SHUTDOWN}), should include at the top of the file the header \begin{verbatim} #include "cctk_Arguments.h" \end{verbatim} A Cactus macro \texttt{CCTK\_ARGUMENTS} is defined for each thorn to contain \begin{itemize} \item General information about the grid hierarchy, for example, the number of grid points on the processor. See Section \ref{sec:cactus_variables_c} for a complete list. \item All the grid variables defined in the thorn's \texttt{interface.ccl} \item All the grid variables required from other thorns as requested by the \texttt{inherits} and \texttt{friend} lines in the \texttt{interface.ccl} \end{itemize} These variables must be declared at the start of the routine using the macro \texttt{DECLARE\_CCTK\_ARGUMENTS}. This macro should always be the first line of the routine. To pass the arguments to another routine in the same thorn, use the macro \texttt{CCTK\_PASS\_CTOC} in the calling routine, and again the macro \texttt{CCTK\_ARGUMENTS} in the receiving routine. Note that you cannot use Cactus argument lists in routines scheduled at the {\t CCTK\_STARTUP} and {\t CCTK\_SHUTDOWN} time bins, because at this time no grid hierarchy exists. \paragraph{Parameters} All parameters defined in a thorn's \texttt{param.ccl} and all \texttt{global} parameters, appear as local variables of the corresponding \texttt{CCTK} data type in C source code, i.e. Integers and Booleans appear as \texttt{CCTK\_INT} types (with nonzero/zero values for boolean {\t yes/no}), Reals as \texttt{CCTK\_REAL}, and Keywords and String parameters as \texttt{CCTK\_STRING}. These variables are \emph{read only}, and \emph{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 as the last statement in the declaration part of the routine using them with the macro \texttt{DECLARE\_CCTK\_PARAMETERS}. \paragraph{Example} The C routine \verb|MyCRoutine| is scheduled in the \texttt{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_ARGUMENTS) { DECLARE_CCTK_ARGUMENTS DECLARE_CCTK_PARAMETERS /* Here goes your code */ } \end{verbatim} \paragraph{Complex variables} Cactus supports complex grid variables, and since there is no complex data type in C, Cactus provides a number of functions for manipulating complex numbers to mirror the functionality available in Fortran. These functions are \texttt{CCTK\_Cmplx}, \texttt{CCTK\_CmplxReal}, \texttt{CCTK\_CmplxImag}, \texttt{CCTK\_CmplxConjg}, \texttt{CCTK\_CmplxAdd}, \texttt{CCTK\_CmplxSub}, \texttt{CCTK\_CmplxMul}, \texttt{CCTK\_CmplxDiv}, \texttt{CCTK\_CmplxExp}, \texttt{CCTK\_CmplSin}, \texttt{CCTK\_CmplxAbs}, \texttt{CCTK\_CmplxLog}, and \texttt{CCTK\_CmplSqrt}. See the Reference Manual for more details. \paragraph{Specifically for C Programmers} Grid functions are held in memory as 1-dimensional C arrays. These are laid out in memory as in Fortran. This means that the first index should be incremented through most rapidly. This is illustrated in the example below. Cactus provides macros to find the 1-dimensional 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 artificial example to demonstrate this using the 3D macro \texttt{CCTK\_GFINDEX3D}: \begin{verbatim} for (k=0; kcctk_lsh[0]*((j)+cctkGH->cctk_lsh[1]*(k))) \end{verbatim} % Note: In Fortran, grid functions are accessed as Fortran arrays, i.e.\ simply as \verb|rho(i,j,k)|. To access vector grid functions (vector grid functions are a ``vector'' of grid functions; see section \ref{subsec:Appendix.interface-variables}), one also needs to specify the vector index. This is best done via the 3D macro \texttt{CCTK\_VECTGFINDEX3D}: \begin{verbatim} for (k=0; k /* defines size_t */ #include "cctk.h" void MyThorn_MyFunction(CCTK_ARGUMENTS) { int i,j,k; for (k = 0 ; k < cctk_lsh[2] ; ++k) { for (j = 0 ; j < cctk_lsh[1] ; ++j) { for (i = 0 ; i < cctk_lsh[0] ; ++i) { const size_t posn = CCTK_GFINDEX3D(cctkGH, i,j,k); /* calculate the global xyz coordinates of the (i,j,k) grid point */ /* (in a multipatch/multiblock context, this gives the per-patch coordinates) */ const CCTK_REAL xcoord = CCTK_ORIGIN_SPACE(0) + (cctk_lbnd[0] + i)*CCTK_DELTA_SPACE(0); const CCTK_REAL ycoord = CCTK_ORIGIN_SPACE(1) + (cctk_lbnd[1] + j)*CCTK_DELTA_SPACE(1); const CCTK_REAL zcoord = CCTK_ORIGIN_SPACE(2) + (cctk_lbnd[2] + k)*CCTK_DELTA_SPACE(2); /* an alternate calculation, which requires that this thorn inherit from Grid */ /* (in a multipatch/multiblock context, this gives the true global xyz coordinates) */ const CCTK_REAL xxcoord = /* Grid:: */ x[posn]; const CCTK_REAL yycoord = /* Grid:: */ y[posn]; const CCTK_REAL zzcoord = /* Grid:: */ z[posn]; } } } } \end{verbatim} \caption{%%% This figure shows two different ways to compute the global Cactus $xyz$ coordinates of the current grid point. Because the ``alternate calculation'' (the one one using \texttt{Grid::x}, \texttt{Grid::y}, and \texttt{Grid::z}) gives the true global $xyz$ coordinates even in a multipatch/multiblock context, this is generally the preferred form for general use. } \label{fig-global-xyz-coords} \end{figure} \subsubsection{Cactus Data Types} To provide portability across platforms, the Cactus grid variables and parameters are defined and declared using Cactus data types. 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[\texttt{CCTK\_INT}] default size 4 bytes \item[\texttt{CCTK\_REAL}] default size 8 bytes \item[\texttt{CCTK\_COMPLEX}] consists of two \texttt{CCTK\_REAL} elements \end{Lentry} \paragraph{Example} In the following example, \verb|MyScalar| is a grid scalar which is declared in the \texttt{interface.ccl} as \texttt{CCTK\_REAL}. % \begin{verbatim} subroutine InitialData(CCTK_ARGUMENTS) DECLARE_CCTK_ARGUMENTS CCTK_REAL local_var local_var = 1.0/3.0 MyScalar = local_var return end \end{verbatim} % Declaring \texttt{local\_var} to have a non-Cactus data type, e.g.\ \texttt{REAL*4}, or using one of the other Cactus real data types described in Section~\ref{sec:datyansi}, could give problems for different architectures or configurations. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Parallelisation} \label{sec:parallelisation} The flesh itself does not actually set up grid variables. This is done by a \textit{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 Reference Manual. A complete description of the routines that a driver thorn must provide, will be provided in the Infrastructure Thorn Writers Guide (Part \ref{chap:infrastructure}). The standard driver thorn is currently \texttt{PUGH} in the \texttt{CactusPUGH} package, which is a parallel unigrid driver. \begin{Lentry} \item[\texttt{CCTK\_nProcs}] Returns the number of processors being used \item[\texttt{CCTK\_MyProc}] Returns the processor number (this starts at processor number zero) \item[\texttt{CCTK\_SyncGroup, CCTK\_SyncGroupsI}] Synchronises either a single group or a set of groups of grid arrays by exchanging the values held in each processor ghostzones, with the physical values of their neighbours (see the Reference Manual) \item[\texttt{CCTK\_Barrier}] Waits for all processors to reach this point before proceeding \end{Lentry} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \section{Cactus Application Interfaces} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Iterating Over Grid Points} \label{sec:CactusAPI.gridpoints} A grid function consists of a multi-dimensional array of grid points. These grid points fall into several types: \begin{description} \item[interior] regular grid point, presumably evolved in time \item[ghost] inter-process boundary, containing copies of values owned by another process \item[physical boundary] outer boundary, presumably defined via a boundary condition \item[symmetry boundary] defined via a symmetry, e.g.\ a reflection symmetry or periodicity \end{description} Grid points in the edges and corners may combine several types. For example, a point in a corner may be a ghost point in the $x$ direction, a physical boundary point in the $y$ direction, and a symmetry point in the $z$ direction. The size of the physical boundary depends on the application. The number of ghost points is defined by the driver; the number of symmetry points is in principle defined by the thorn implementing the respective symmetry condition, but will in general be the same as the number of ghost points to avoid inconsistencies. When iterating over grid points, one usually needs to know about the boundary sizes and boundary types present. Details about this is explained in thorn CactusBase/CoordBase. The flesh provides a set of macros to iterate over particular types of grid points: \begin{description} \item[\texttt{CCTK\_LOOP\_ALL}] Loop over all grid points \item[\texttt{CCTK\_LOOP\_INT}] Loop over all interior grid points \item[\texttt{CCTK\_LOOP\_BND}] Loop over all physical boundary points \item[\texttt{CCTK\_LOOP\_INTBND}] Loop over all ``interior'' physical boundary points, i.e.\ over all those physical boundary points that are not also ghost or symmetry points \end{description} As described above, points on edges and corners can have several boundary types at once, e.g.\ can be both a physical and a symmetry point. \texttt{LOOP\_BND} and \texttt{LOOP\_INTBND} treat these different: \texttt{LOOP\_BND} loops over all points that are physical boundaries (independent of whether they also are symmetry or ghost boundaries), while \texttt{LOOP\_INTBND} loops over those points that are only physical boundaries (and excludes any points that belongs to a symmetry or ghost boundary). \texttt{LOOP\_BND} does not require applying a symmetry condition or synchronisation afterwards (but does not allow taking tangential derivatives); \texttt{LOOP\_INTBND} allows taking tangential derivatives (but requires applying symmetry boundaries and synchronising afterwards). In 3 dimensions, these macros should be called as follows: \begin{verbatim} CCTK_LOOP3_ALL(name, cctkGH, i,j,k) { ... body of the loop } CCTK_ENDLOOP3_ALL(name); \end{verbatim} \begin{verbatim} CCTK_LOOP3_INT(name, cctkGH, i,j,k) { ... body of the loop } CCTK_ENDLOOP3_INT(name); \end{verbatim} \begin{verbatim} CCTK_LOOP3_BND(name, cctkGH, i,j,k, ni,nj,nk) { ... body of the loop } CCTK_ENDLOOP3_BND(name); \end{verbatim} \begin{verbatim} CCTK_LOOP3_INTBND(name, cctkGH, i,j,k, ni,nj,nk) { ... body of the loop } CCTK_ENDLOOP3_INTBND(name); \end{verbatim} In all cases, \texttt{name} should be replaced by a unique name for the loop. \texttt{i}, \texttt{j}, and \texttt{k} are names of variables that will be declared and defined by these macros, containing the index of the current grid point. Similarly \texttt{ni}, \texttt{nj}, and \texttt{nk} are names of variables describing the (outwards pointing) normal direction to the boundary as well as the distance to the boundary. \subsection{Coordinates} \label{sec:CactusAPI.coordinates} The flesh provides utility routines for registering and querying coordinate information. The flesh does not provide any coordinates itself, these must be supplied by a thorn. Thorns are not required to register coordinates to the flesh, but registering coordinates provides a means for infrastructure thorns to make use of coordinate information. Coordinates are grouped into \textit{coordinate systems}, which have a specified dimension. Any number of coordinate systems can be registered with the flesh, and a coordinate system must be registered before any coordinates can be registered, since they must be associated with their corresponding system. Coordinates can be registered, with any chosen name, with an existing coordinate system, along with their direction or index in the coordinate system. Optionally, the coordinate can also be associated with a given grid variable. A separate call can register the global range for a coordinate on a given grid hierarchy. Following conventions for coordinate system and coordinate names, provides a means for other thorns to use the physical properties of coordinate systems, without being tied to a particular thorn. A registered coordinate system can be referred to by either its name or an associated integer known as a \textit{handle}. Passing a handle instead of the name string may be necessary for calling C routines from Fortran. \subsubsection{Registering Coordinates and Coordinate Properties} \textbf{The APIs described in this section are deprecated, and will probably be phased out fairly soon. New code should use the APIs provided by the \texttt{CoordBase} thorn instead (this lives in the \texttt{CactusBase} arrangement).} Coordinate systems and their properties can be registered at any time with the flesh. The registration utilities for thorns providing coordinates are: \begin{Lentry} \item[\texttt{CCTK\_CoordRegisterSystem}] Assigns a coordinate system with a chosen name and dimension. For example, a 3-dimensional Cartesian coordinate system could be registered with the name \texttt{cart3d} using the call from C % \begin{verbatim} int ierr; int dim=3; ierr = CCTK_CoordRegisterSystem(dim,"cart3d"); \end{verbatim} \item[\texttt{CCTK\_CoordRegisterData}] Defines a coordinate in a given coordinate system, with a given direction and name, and optionally associates it to a grid variable. The directions of the coordinates range from 1 to the dimension of the coordinate system. For example, to register the grid variable \texttt{grid::y3d} to have the coordinate name \texttt{y} in the \texttt{cart3d} system % \begin{verbatim} int ierr; int dir=2; ierr = CCTK_CoordRegisterData(dir,"grid::y3d","y","cart3d"); \end{verbatim} \item[\texttt{CCTK\_CoordRegisterRange}] Assigns the global computational maximum and minimum for a coordinate on a grid hierarchy, that is in a \texttt{cctkGH}. At this time the maximum and minimum values have to be of type \texttt{CCTK\_REAL}. For example, if the \texttt{y} coordinate for the \texttt{cart3d} system ranges between zero and one % \begin{verbatim} CCTK_REAL lower=0; CCTK_REAL upper=1; int ierr; ierr = CCTK_CoordRegisterRange(cctkGH, lower, upper, -1, "y", "cart3d"); \end{verbatim} % Note that the API allows either the coordinate name or the direction to be used, so that the following is also valid % \begin{verbatim} CCTK_REAL lower=0; CCTK_REAL upper=1; int ierr; ierr = CCTK_CoordRegisterRange(cctkGH, lower, upper, 2, NULL, "cart3d"); \end{verbatim} \item[\texttt{CCTK\_CoordRegisterPhysIndex}] Implementing such things as symmetry properties for a grid leads to the need to know the details of the \emph{physical} section of a grid. Such information is typically needed by I/O thorns. The following call illustrates how to register the indices 3 and 25 as supplying the physical range of the \texttt{y} coordinate in the \texttt{cart3d} system % \begin{verbatim} int loweri=3; int upperi=25; int ierr; ierr = CCTK_CoordRegisterPhysIndex(cctkGH, loweri, upperi, -1, "y", "cart3d"); \end{verbatim} \end{Lentry} \subsubsection{Using Coordinates} \textbf{The APIs described in this section are deprecated, and will probably be phased out fairly soon. New code should use the APIs provided by the \texttt{CoordBase} thorn instead (this lives in the \texttt{CactusBase} arrangement).} The utilities for thorns using coordinates are: \begin{Lentry} \item[\texttt{CCTK\_NumCoordSystems}] Returns the number of coordinate systems registered with the flesh. For example, % \begin{verbatim} int num; num = CCTK_NumCoordSystems(); \end{verbatim} \item[\texttt{CCTK\_CoordSystemName}] Provides the name of a registered coordinate system, given the integer handle (or index) for the system in the flesh's coordinate data base. Note that the handle ranges between zero and the number of coordinate systems minus one: $0 \le \mbox{handle} \le \mbox{\texttt{CCTK\_NumCoordSystems()}}-1$. It is important to remember that the handle given to a coordinate system depends on the order in which systems are registered, and can be different from one simulation to the next. For example, to print the names of all registered coordinate systems: % \begin{verbatim} for (i=0; i$-ThornGuide}). Ideally this file should contain complete (and \textit{up-to-date}) details about the thorn, exactly what is relevant is for the authors to decide, but remember that the Cactus make system automatically parses the thorn CCL files to include information about all parameters, variables and scheduling. Suggested sections include: \begin{itemize} \item{\bf Model.} A description of the system which the thorn is modelling, including the equations, etc., which are being solved or implemented. \item{\bf Numerical implementation.} Details about how the model is numerically implemented in the thorn. \item{\bf Using the thorn.} Any special details needed for using the thorn, tricky parameters, particular operating systems or additional required software, interactions with other thorns and examples of use. \item{\bf History.} Here is where you should describe why the thorn was written, any previous software or experience which was made use of, the authors of the thorn source code and documentation, how to get hold of the thorn, etc. \item{\bf References.} A bibliography can be included, referencing papers published using or about this thorn, or additional information about the model or numerics used. \end{itemize} A LaTeX template for the Thorn Guide documentation can be found in the flesh distribution at \texttt{doc/ThornGuide/template.tex}, this file is automatically copied to the correct location in a new thorn which is created with \texttt{gmake newthorn}. Since Cactus scripts need to parse this documentation, and since the LaTeX document should be consistent across all thorns included in a Thorn Guide, please follow the guidelines below when filling in the documentation: \begin{itemize} \item Use the Cactus Thorn Guide style file, located in the flesh distribution at \texttt{doc/latex/cactus.sty}. This should be included using a relative link, so that updates to the style file are applied to all thorns. % \begin{verbatim} \usepackage{../../../../doc/latex/cactus} \end{verbatim} \item Aside from the \texttt{date}, \texttt{author}, and \texttt{title} fields, all of the documentation to be included in a Thorn Guide should be placed between the lines \texttt{\% START CACTUS THORNGUIDE} and \texttt{\% END CACTUS THORNGUIDE} \item The command \texttt{$\backslash$def} can be used to define macros, but all such definitions must lie between the \texttt{START} and \texttt{END} line. Do not redefine any standard LaTeX command \item Do not use the {\t $\backslash$appendix} command, instead include any appendices you have as standard sections. \item We only support PDF (\texttt{.pdf}) figures. Graphics figures should be included using the \texttt{includegraphics} command (not \texttt{epsffig}), with no file extension specified. For example, % \begin{verbatim} \begin{figure}[ht] \begin{center} \includegraphics[width=6cm]{MyArrangement_MyThorn_MyFigure} \end{center} \caption{Illustration of this and that} \label{MyArrangement_MyThorn_MyLabel} \end{figure} \end{verbatim} \item All {\bf labels}, {\bf citations}, {\bf references}, and {\bf graphic images} names should conform to the following guidelines: \texttt{ARRANGEMENT\_THORN\_LABEL}. For instance, if you arrangement is called CactusWave, your thorn WaveToyC, and your original image blackhole.eps, you should rename your image to be {\tt CactusWave\_WaveToyC\_blackhole.eps} \item References should be formatted with the standard LaTeX {\bf bibitem} command, for example, a bibliography section should look like: % \begin{verbatim} \begin{thebibliography}{9} \bibitem{MyArrangement_MyThorn_Author99} {J. Author, \textit{The Title of the Book, Journal, or periodical}, 1 (1999), 1--16. \url{http://www.nowhere.com/}} \end{thebibliography} \end{verbatim} \end{itemize} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Adding a Test Suite} \label{sec:adding_test_suite} 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 \texttt{./}. Run Cactus on each of the parameter files, and move the parameter files, and the output directories they produced, to the \texttt{test} directory in your thorn. Document carefully any situations or architectures in which your test suite does not give the correct answers. You can also specify options for running your testsuite by adding an optional configuration file called \texttt{test.ccl} in the \texttt{test} directory. These are simple text files and may contain comments introduced by the hash `\texttt{\#}' character, which indicates that the rest of the line is a comment. If the last non-blank character of a line in a config file is a backslash `\texttt{$\backslash$}', the following line is treated as a continuation of the current line. Options include test specific absolute and relative tolerances, thorn specific absolute and relative tolerances, the number of processors required to run, and file extensions. The configuration file has the form: \begin{alltt} ABSTOL <\var{thorn_absolute_tolerance}> [\var{filename_pattern}] RELTOL <\var{thorn_relative_tolerance}> [\var{filename_pattern}] NPROCS <\var{thorn_nprocs}> EXTENSIONS <\var{extension_1} \var{extension_2} \var{extension_3}> TEST <\var{test_example}> \{ ABSTOL <\var{absolute_tol}> [\var{filename_pattern}] RELTOL <\var{relative_tol}> [\var{filename_pattern}] NPROCS <\var{nprocs}> \} \end{alltt} which states that when comparing files of test \verb|test_example|, both \verb|absolute_tol| and \verb|relative_tol| should be used as the absolute and relative tolerances. For all other tests in the thorn, the default value of absolute and relative tolerances are set to \verb|thorn_absolute_tolerance| and \verb|thorn_relative_tolerance|. %%% from Track ticket #114, after text by Tanja Bode Both absolute and relative tolerances can be specified on a per-file bases by supplying an optional \verb|filename_pattern| regular expression to match against a filename and the tolerance value. The specified tolerances override more general tolerances for data files whose name matches these regular expressions. Any set of characters can be used for matching as long as there are no whitespaces in the regular expression. For example: \begin{alltt} ABSTOL 1e-8 ^Psi4.[xy] RELTOL 1e-12 gxx \end{alltt} More specific tolerances can be specified for all the tests of a thorn or just within a test's block. It is an error if a regular expression matches more than one filename. %%% The \texttt{NPROCS} option specifies the number of processors required to run a given testsuite \var{test\_example} or all testsuites of a thorn successfully. If no \texttt{NPROCS} option is present, the testsuite(s) is (are) assumed to run with any number of processors. The \texttt{EXTENSIONS} option adds \verb|extension_1|, \verb|extension_2| and \verb|extension_3| to the list of file extensions that are compared. This list is global over all tests in a configuration. Test specific tolerances have precedence over all tolerances, next come thorn wide tolerances, and then cactus default tolerances. Absolute and relative tolerances are independent: you can choose to use test specific absolute tolerance and thorn specific relative tolerance when running a test. For example, \begin{alltt} TEST \var{test_rad} \{ ABSTOL \var{1e-5} \} ABSTOL \var{1e-8} RELTOL \var{1e-12} \end{alltt} would use an absolute tolerance of $10^{-5}$ and a relative tolerance of $10^{-12}$ when running \verb|test_rad| and an absolute tolerance of $10^{-8}$ and a relative tolerance of $10^{-12}$ when running all other tests. For details on running the test suites, see Section~\ref{sec:testing}. \subsubsection{Best Practices for Test Suites} When writing a test suite, there are a few things you should keep in mind: \begin{itemize} \item The test suite will be run together with many other test suites. It should, therefore, finish quickly (say, in under two minutes), and not use too much memory (so that it can run on a ``normal'' workstation). \item The test suite will be run automatically, often in situations where no one checks the screen output. All important output should be via grid variables that are written to files. Alternatively, if the test suite tests some low-level infrastructure, it may just abort the simulation if it fails; that will also be detected. \item Downloading many files is slow on many systems. A test suite should normally not have more than, say, hundred output files, and normally the output files should be small, so that there are not more than a few Megabytes of output files per test suite. \item The test suite output files should always be the same. That means that they should not contain time stamps, etc. It is, therefore, best to use the option \verb|IO::out_fileinfo="none"|. \item Norms are unfortunately quite insensitive to changes to a few grid points only, even if the changes are significant. It is necessary to output grid point values directly, not only norms. \item Try to use as few thorns as possible in a test case. For example, do not active 3D output thorns (unless you use it). The fewer thorns you use, the easier it is to run the test suite. \item It is not necessary that a test suite result is ``physically correct'', or that it uses parameters that ensure a stable time evolution. A test suite will usually take only a few time steps, so that a grid size of, e.g.\ $20^3$ grid points \emph{without} dissipation can be sufficient. Test suites cannot test whether the result of a simulation is physically feasible; they only test whether anything changed at all. Ensuring that the physics is still correct has to be handled by different mechanisms. \end{itemize} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \section{Advanced Thorn Writing} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Using Cactus Timers} \label{sec:timers} \subsubsection{What are Timers?} %The standard timing information available during a simulation was %described in Section~\ref{????}. %[FIXME: This isn't filled in yet because it is being reworked] Cactus provides a flexible mechanism for timing different sections of your thorns using various \textit{clocks} which have been registered with the flesh. By default, the flesh provides two clocks that measure time in different ways (provided the underlying functionality is available on your system): \begin{Lentry} \item[\texttt{GetTimeOfDay}] Provides ``wall clock time'' via the unix \texttt{gettimeofday} function. \item[\texttt{GetrUsage}] Provides CPU usage time via the unix \texttt{getrusage} function. \end{Lentry} Additional clocks can be implemented by thorns and registered with the flesh (see Chapter \ref{chap:clocks}). To use Cactus timing, you create a \textit{timer}, which provides time information for all the registered clocks. You can add any number of timers to your thorn source code, providing each with a name of your choice, and then use Cactus timing functions to switch on the timers, stop or reset them, and recover timing information. Setting the flesh parameter \texttt{cactus::cctk\_timer\_output = "full"} will cause some summary timing information to be printed at the end of a run. Some other thorns have their own timer printing parameters as well. \subsubsection{Timing Calls} Many of the timing calls come in two versions, one whose name ends with the letter \texttt{I}, and one without. The calls whose names end with the letter \texttt{I} refer to the timer or clock by index, which is a non-negative \texttt{int} value; the other calls refer to a timer by name. If a timer is created without a name, it can be referred to only by its index, otherwise, it can be referred to by name or by index. Typically, a negative return value from a timer function indicates an error. \begin{Lentry} \item[{\t CCTK\_TimerCreate}, {\t CCTK\_TimerCreateI}] Create a timer with a given name, or with no name (respectively) and return a timer index or an error code. Negative return values indicate errors. Only one timer with a given name can exist at any given time. % what about unnamed timers? % \item[{\t CCTK\_TimerDestroy}, {\t CCTK\_TimerDestroyI}] Reclaim resources used by a timer. \item[{\t CCTK\_TimerStart}, {\t CCTK\_TimerStartI}] Start the given timer, using all registered clocks. \item[{\t CCTK\_TimerStop}, {\t CCTK\_TimerStopI}] Stop the given timer on all registered clocks. \item[{\t CCTK\_TimerReset}, {\t CCTK\_TimerResetI}] Reset the given timer on all registered clocks. \item[{\t CCTK\_TimerCreateData}, {\t CCTK\_TimerDestroyData}] Allocate and reclaim (respectively) resources for a {\t cTimerData} structure, which will be used to hold clock values. \item[{\t CCTK\_Timer}, {\t CCTK\_TimerI}] Fill the given {\t cTimerData} structure with clock values as of the last call to {\t CCTK\_TimerStop}. \item[{\t CCTK\_NumTimers}] Return the number of created timers \item[{\t CCTK\_TimerName}] Return the name of the timer for a given timer index (or {\t NULL} if the timer is unnamed or any other error occurs). \item[{\t CCTK\_NumTimerClocks}] Take a pointer to {\t cTimerData} and return the number of clocks recorded in a timer measurement \item[{\t CCTK\_GetClockValue}, {\t CCTK\_GetClockValueI}] Given a clock referred to by name or index, respectively, and a {\t cTimerData} pointer, return a {\t cTimerVal} pointer representing the value of the clock when the timer was stopped \item[{\t CCTK\_TimerClockName}] Return the name of the clock given by the {\t cTimerVal} pointer argument. \item[{\t CCTK\_TimerClockResolution}] Return the floating-point value of the resolution in seconds of the clock referred to by the {\t cTimerVal} pointer argument. This is a lower bound for the smallest non-zero difference in values between calls of {\t CCTK\_TimerClockSeconds}. \item[{\t CCTK\_TimerClockSeconds}] Return the floating-point value of the measurement in seconds from the {\t cTimerVal} pointer argument. \end{Lentry} \subsubsection{How to Insert Timers in your Code} The function prototypes and structure definitions are contained in the include file \texttt{cctk\_Timers.h}, which is included in the standard thorn header file \texttt{cctk.h}. At the moment, the timer calls are only available from C. The following example, which uses a timer to instrument a section of code, illustrates how timers are used by application thorns. A working example is available in the thorn \texttt{CactusTest/TestTimers}. {\bf Creating a timer} The first action for any timer is to create it, using \texttt{CCTK\_TimerCreate}. This can be performed at any time prior to use of the timer: \begin{verbatim} #include "cctk_Timers.h" index = CCTK_TimerCreate("TimeMyStuff"); \end{verbatim} {\bf Instrumenting a section of code} Code sections are instrumented using the Start, Stop and Reset functions. These functions are applied to the chosen timer using all the registered clocks. \begin{verbatim} ierr = CCTK_TimerStart("TimeMyStuff"); do_procedure_to_be_timed(); ierr = CCTK_TimerStop("TimeMyStuff"); \end{verbatim} {\bf Accessing the timer results} After calling {\t CCTK\_TimerStop}, you then get the time value from any of the registered clocks. The procedure is to allocate a {\t cTimerData} structure, and read the information from your timer into this structure using {\t CCTK\_Timer}, then to extract time data of the desired clock from this structure. After using the structure, you should destroy it. \begin{verbatim} cTimerData *info = CCTK_TimerCreateData(); int ierr = CCTK_Timer("TimeMyStuff",info); /* You can refer to a particular clock by name. */ const cTimerVal *clock = CCTK_GetClockValue( "gettimeofday", info ); if( clock ){ printf ("\t%s: %.3f %s\n", "gettimeofday", CCTK_TimerClockSeconds( clock ), "secs" ); } /* To get results from all available clocks, refer to them by index.*/ nclocks = CCTK_NumTimerClocks( info ); for (i = 0; i < numclocks; i++) { const cTimerVal *clock = CCTK_GetClockValueI( i, info ); printf ("\t%s: %.3f %s\n", CCTK_TimerClockName( clock ), CCTK_TimerClockSeconds( clock ), "secs" ); } CCTK_TimerDestroyData (info); \end{verbatim} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Include Files} \label{sec:includefiles} Cactus provides a mechanism for thorns to add code to include files which can be used by any other thorn. Such include files can contain executable source code, or header/declaration information. A distinction is made between these two cases, since included executable code is protected from being run if a thorn is compiled, but not active by being wrapped by a call to \texttt{CCTK\_IsThornActive}. Any thorn which uses the include file must declare this in its \texttt{interface.ccl} with the line \begin{alltt} USES INCLUDE [SOURCE|HEADER]: <\var{file_name}> \end{alltt} (If the optional \verb![SOURCE|HEADER]! is omitted, \verb|HEADER| is assumed. Note that this can be dangerous, as included \emph{source} code, which is incorrectly assumed to be \emph{header} code, will be executed in another thorn \emph{even if the providing thorn is inactive}. Thus, it is recommended to always include the optional \verb![SOURCE|HEADER]! specification.) Any thorn that wishes to add to this include file, declares in its own \texttt{interface.ccl} \begin{alltt} INCLUDE [SOURCE|HEADER]: <\var{file_to_include}> in <\var{file_name}> \end{alltt} \paragraph{Example} As an example of this in practice, for the case of Fortran code, consider a thorn A, which wants to gather terms for a calculation from any thorn that wishes to provide them. Thorn A could have the lines in its source code \begin{verbatim} c Get source code from other thorns allterms = 0d0 #include "AllSources.inc" \end{verbatim} and would then add to \texttt{interface.ccl} the line \begin{verbatim} USES INCLUDE SOURCE: AllSources.inc \end{verbatim} If thorn B wants to add terms for the calculation, it would create a file, say \texttt{Bterms.inc} with the lines \begin{verbatim} c Add this to AllSources.inc allterms = allterms + 1d0 \end{verbatim} and would add to its own \texttt{interface.ccl} \begin{verbatim} INCLUDE SOURCE: Bterms.inc in AllSources.inc \end{verbatim} The final file for thorn A which is compiled, will contain the code \begin{verbatim} c Get source code from other thorns allterms = 0d0 if (CCTK_IsThornActive("B").ne.0) then c Add this to AllSources.inc allterms = allterms + 1d0 end if \end{verbatim} Any Fortran thorn routines which include source code must include the declaration \texttt{DECLARE\_CCTK\_FUNCTIONS}. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Memory Tracing} \label{sec:metr} Cactus provides a mechanism for overriding the standard C memory allocation routines (\texttt{malloc, free,} \ldots) with Cactus specific routines that track the amount of memory allocated, and from where, the allocation call was made. This information can be accessed by the user to provide an understanding of the memory consumption between two instances, and to track down possible memory leaks. This feature is available in C only. \subsubsection{Activating Memory Tracing} \label{sec:acmetr} Memory tracing has to be activated at configure time. The standard \texttt{malloc} statements are overridden with macros (\texttt{CCTK\_MALLOC}). To activate memory tracing use either \begin{Lentry} \item[\texttt{DEBUG=all}] Enables all debug options (compiler debug flags, redefines \texttt{malloc}) \item[\texttt{DEBUG=memory}] Redefine \texttt{malloc} only. \end{Lentry} The \texttt{CCTK\_MALLOC} statements can also be used directly in the C code. But by employing them this way, only a fraction of the total memory consumption is traced. Also, they cannot be turned off at configure time. For example: \begin{verbatim} machine> gmake bigbuild DEBUG=yes machine> gmake bigbuild-config DEBUG=memory \end{verbatim} The new configuration \texttt{bigbuild} is configured with all debugging features turned on. The already existing configuration \texttt{bigbuild} is reconfigured with memory tracing only. \subsubsection{Using Memory Tracing} \label{sec:usmetr} You can request Cactus to store the memory consumption at a certain instance in the program flow, and return the difference in memory allocation some time later. \begin{Lentry} \item[\texttt{int CCTK\_MemTicketRequest(void)}] Request a ticket: save the current total memory to a database. Return an integer (ticket). Use the ticket to calculate the difference in memory allocation between the two instances in \texttt{CCTK\_MemTicketCash}. \item[\texttt{long int CCTK\_MemTicketCash(int your\_ticket)}] Cash in your ticket: return the memory difference between now and the time the ticket was requested. Tickets can be cashed in several times. See example below. This only tracks the real data memory, which is the same as in undebug mode. It does not keep track of the internal allocations done to provide the database, the motivation is that this is not allocated either if you compile undebugged. \item[\texttt{int CCTK\_MemTicketDelete(int your\_ticket)}] Delete the memory ticket. The ticket ID will not be reused, since it's incremented with every ticket request, but the memory of the memory datastructure is deallocated. \item[\texttt{unsigned long int CCTK\_TotalMemory(void)}] Returns the total allocated memory (not including the tracing data structures). \item[\texttt{void CCTK\_MemStat}] Prints an info string, stating the current, past, and total memory (in bytes) allocation between two successive calls to this routine, as well as the difference. \end{Lentry} Sample C code demonstrating the ticket handling. Two tickets are requested during \texttt{malloc} operations. The \texttt{CCTK\_MALLOC} statement is used directly. They are cashed in, and the memory difference is printed. Ticket 1 is cashed twice. The tickets are deleted at the end. \begin{verbatim} int ticket1; int ticket2; /* store current memstate, ticket: t1*/ t1 = CCTK_MemTicketRequest(); /* allocate data */ hi = (int*) CCTK_MALLOC(10*sizeof(int)); /* store current memstate, ticket: t2*/ t2 = CCTK_MemTicketRequest(); /* cash ticket t1, print mem difference */ printf("NOW1a: %+d \n",CCTK_MemTicketCash(t1)); /* allocte some more data */ wo = (CCTK_REAL*)CCTK_MALLOC(10*sizeof(CCTK_REAL)); /* cash ticket t1 and t2, print mem difference */ printf("NOW1b: %+d \n",CCTK_MemTicketCash(t1)); printf("NOW2 : %+d \n",CCTK_MemTicketCash(t2)); /* delete the tickets from the database */ CCTK_MemTicketDelete(t1); CCTK_MemTicketDelete(t2); \end{verbatim} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection[Calls to different language]{Calls between Different Programming Languages} %\pagestyle{empty} \subsubsection{Calling C Routines from Fortran} \label{sec:cacrofr} To make the following C routine, {\tt int ()\\ {\\ ...\\ } } also callable from Fortran, a new routine must be added, which is declared using the \texttt{CCTK\_FCALL} and \texttt{CCTK\_FNAME} macros: {\tt void CCTK\_FCALL CCTK\_FNAME()(int *ierr, )\\ } The convention used in Cactus, is that \texttt{} be the same as any C routine name, and that this is mixed-case. The macros change the case and number of underscores of the routine name to match that expected by Fortran. All arguments passed by Fortran to the routine (except strings) are pointers in C, e.g.\ a call from Fortran \begin{verbatim} CCTK_INT arg1 CCTK_REAL arg2 CCTK_REAL arg3(30,30,30) ... call MyCRoutine(arg1,arg2,arg3) \end{verbatim} should appear in C as \begin{verbatim} void CCTK_FCALL CCTK_FNAME(MyCRoutine)(CCTK_INT *arg1, CCTK_REAL *arg2, CCTK_REAL *arg3) { ... } \end{verbatim} \subsubsection{String Arguments from Fortran} Fortran passes string arguments in a special, compiler-dependent, way. To facilitate this, the CCTK provides a set of macros to enable the translation to C strings. The macros are defined in \texttt{cctk\_FortranString.h}, which should be included in your C file. String arguments \emph{must always come last} in the argument list for these macros to be effective (some Fortran compilers automatically migrate the strings to the end, so there is no portable workaround). The macros to use depend upon the number of string arguments--we currently support up to three. The macros are \texttt{\_FORTSTRING\_ARG}. Corresponding to each of these are two macros \texttt{\_FORTSTRING\_CREATE} and \texttt{\_FORTSTRING\_PTR}, which take one, two, or three arguments depending on the number of strings. The latter set is only necessary if a string is to be modified. In more detail: \begin{Lentry} \item[\texttt{\_FORTSTRING\_ARG}] Used in the argument list of the C routine to which the Fortran strings are passed. \item[\texttt{\_FORTSTRING\_CREATE}] Used in the declaration section of the C routine to which the Fortran strings are passed. These macros have one, two or three arguments which are the variable names you choose to use for the strings in the C routine, created by null-terminating the passed-in Fortran strings. The \texttt{CREATE} macros create new strings with the names you provide, and thus should be treated as read-only and freed after use. \item[\texttt{\_FORTSTRING\_PTR}] These macros, used in the declaration section of the C routine \emph{after} the \texttt{CREATE} macro, should be used if you need to modify one of the passed-in strings. They declare and define pointers to the passed-in strings. \item[\texttt{cctk\_strlen<1|2|3>}] These integer variables, automatically defined by the \texttt{CREATE} macro, hold the lengths of the passed in Fortran strings. \end{Lentry} The use of the macros is probably best explained with examples. For read-only access to the strings, only the first two macros are needed, the following example compares two strings passed in from Fortran. \begin{verbatim} #include #include #include int CCTK_FCALL CCTK_FNAME(CompareStrings)(TWO_FORTSTRING_ARG) { int retval; /* Allocate and create C strings with \0 at end. */ /* This makes variable declarations, so it must be before any executable statements.*/ TWO_FORTSTRING_CREATE(arg1,arg2) /* Do some work with the strings */ retval = strcmp(arg1,arg2); /* Important, these must be freed after use */ free(arg1); free(arg2); return retval; } \end{verbatim} Since the null terminated strings may be copies of the strings passed from Fortran, they should be treated as read-only. To change the data in a string passed from Fortran, you need to use the \texttt{FORTSTRING\_PTR} macros, which declare and set up pointers to the strings passed from C. Note that this macro must be used \emph{after} the \texttt{FORTSTRING\_CREATE} macro. For example, the following routine copies the contents of the second string to the first string \begin{verbatim} #include #include #include int CCTK_FCALL CCTK_FNAME(CopyStrings)(TWO_FORTSTRING_ARG) { int retval; /* Allocate and create C strings with \0 at end. */ /* This makes variable declarations, so it must be before any executable statements. */ TWO_FORTSTRING_CREATE(arg1,arg2) TWO_FORTSTRING_PTR(farg1,farg2) /* Do some work with the strings */ retval = strncpy(farg1,arg2,cctk_strlen1); /* Important, these must be freed after use */ free(arg1); free(arg2); return retval; } \end{verbatim} Note that in the example above, two new variables, pointers to the Fortran strings, were created. These are just pointers and \emph{should not be freed}. The example also illustrates the automatically-created variables, e.g.\ \texttt{cctk\_strlen1}, which hold the sizes of original Fortran strings. When writing to a string its length should never be exceeded. \subsubsection{Calling Fortran Routines from C} \label{sec:caforofr} To call a utility Fortran routine from C, use \begin{alltt} void CCTK_FCALL CCTK_FNAME(<\var{Fortran routine name}>)(<\var{argument list}>) \end{alltt} Note that Fortran expects all arguments (apart from strings) to be pointers, so any non-array data should be passed by address. Currently, we have no support for calling Fortran routines which expect strings from C. However, passing routines is supported when you use function aliasing, see Section \ref{sec:function_aliasing}. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Function aliasing} \label{sec:function_aliasing} Like calling functions in a different language, Cactus offers a mechanism for calling a function in a different thorn where you don 't need to know which thorn is actually providing the function, nor what language the function is provided in. The idea of \textit{function aliasing} is similar to that of thorns; the routine that calls a function should not need to know anything about it, except that the function exists. Function aliasing is quite restrictive, because of the problems involved in inter-language calling, as seen in the previous section. Function aliasing is also comparatively inefficient, and should not be used in a part of your code where efficiency is important. Function aliasing is language-neutral, however, the syntax is strongly based on C. In the future, the function aliasing declarations may go into a new \texttt{functions.ccl} file, and will have a format more similar to that of variable group and parameter declarations. \subsubsection{Using an Aliased Function} To use an aliased function you must first declare it in your \texttt{interface.ccl} file. Declare the prototype as, for example, \begin{verbatim} CCTK_REAL FUNCTION SumStuff(CCTK_REAL IN x, CCTK_REAL IN y) \end{verbatim} \noindent and that this function will be either required in your thorn by \begin{verbatim} REQUIRES FUNCTION SumStuff \end{verbatim} \noindent or optionally used in your thorn by \begin{verbatim} USES FUNCTION SumStuff \end{verbatim} A prototype of this function will be available to any C routine that includes the \texttt{cctk.h} header file. In a Fortran file, the declaration of the function will be included in the \texttt{DECLARE\_CCTK\_FUNCTIONS} macro, which is available after the statement \texttt{\#include "cctk\_Functions.h"}. The keywords \texttt{IN}, \texttt{OUT}, and \texttt{INOUT} work in the same fashion as \texttt{INTENT} statements in Fortran 90. That is, the C prototype will expect an argument with intent \texttt{IN} to be a value and one with intent \texttt{OUT} or \texttt{INOUT} to be a pointer. There also exists the \texttt{ARRAY} keyword for passing arrays of any dimension. Functions which are required by some thorn (which doesn't provide it itself) are checked at startup to be provided by some other thorn. \subsubsection{Providing a Function} To provide an aliased function you must again add the prototype to your \texttt{interface.ccl} file. A statement containing the name of the providing function and the language it is provided in, must also be given. For example, \begin{verbatim} CCTK_REAL FUNCTION SumStuff(CCTK_REAL IN x, CCTK_REAL IN y) PROVIDES FUNCTION SumStuff WITH AddItUp LANGUAGE C \end{verbatim} The appropriate function must then be provided somewhere in this thorn. Multiple thorns providing the same function can be compiled into the same configuration; however, only one providing thorn may be activated at runtime, otherwise, an error message is printed and the run is aborted. It is necessary to specify the language of the providing function; no default will be assumed. \subsubsection{Conventions and Restrictions} Various restrictions are necessary to make function aliasing work. These are \begin{itemize} \item The return type of any function must be either \texttt{void} or one of the Cactus data types \texttt{CCTK\_INT} or {\tt CCTK\_REAL}. Standard types such as \texttt{int} are not allowed. \item The type of an argument must be one of scalar types {\tt CCTK\_INT, CCTK\_REAL, CCTK\_COMPLEX, CCTK\_STRING, CCTK\_POINTER, CCTK\_FPOINTER}, or an array or pointer type \texttt{CCTK\_INT ARRAY, CCTK\_REAL ARRAY, CCTK\_COMPLEX ARRAY, \texttt{CCTK\_POINTER ARRAY}}.%%% \footnote{%%% Unfortunately, neither \texttt{CCTK\_FPOINTER ARRAY}, nor \texttt{CCTK\_STRING ARRAY} will work. }%%% {} The scalar types are assumed to be not modifiable. Any changes made to a scalar argument by a providing function may be silently lost, or may not; it is dependent on the language of the providing and calling function. If you wish to modify an argument, then it must have intent \texttt{OUT} or \texttt{INOUT} (and hence must be either a \texttt{CCTK\_INT}, a \texttt{CCTK\_REAL}, or a \texttt{CCTK\_COMPLEX}, or an array of one of these types). \item The name of both, the aliased and providing function, are restricted. They must follow the standard C semantics (start with a letter, contain only letters, numbers or underscores). Additionally, they must be mixed case (that is, contain at least one uppercase and one lowercase letter). The names of the aliased and providing functions must be distinct. \item If an argument is a function pointer, then the syntax looks like \begin{verbatim} CCTK_REAL Integrate(CCTK_REAL CCTK_FPOINTER func(CCTK_REAL IN x), \ CCTK_REAL IN xmin, CCTK_REAL IN xmax) \end{verbatim} It is assumed that the function pointer argument has the same language as the calling function. Function pointer arguments may not be nested. \item \texttt{CCTK\_STRING} arguments follow the same conventions as in the previous section. That is, they must appear at the end of the argument list, there must be at most three, and a function with a \texttt{CCTK\_STRING} argument can only be provided in C, not Fortran, although they may be called from either. \end{itemize} \subsubsection{Examples} \begin{itemize} \item A C function is provided to add together two real numbers. The {\tt interface.ccl} should read \begin{verbatim} CCTK_REAL FUNCTION SumStuff(CCTK_REAL IN x, CCTK_REAL IN y) PROVIDES FUNCTION SumStuff WITH AddItUp LANGUAGE C USES FUNCTION SumStuff \end{verbatim} \item A Fortran function is provided to invert a real number. The {\tt interface.ccl} should read \begin{verbatim} SUBROUTINE Invert(CCTK_REAL INOUT x) PROVIDES FUNCTION Invert WITH FindInverse LANGUAGE Fortran USES FUNCTION Invert \end{verbatim} \noindent Note that \texttt{SUBROUTINE} has the same meaning as \texttt{void FUNCTION}. \item A Fortran function is provided to integrate any function over an interval. The \texttt{interface.ccl} should read \begin{verbatim} CCTK_REAL Integrate(CCTK_REAL CCTK_FPOINTER func(CCTK_REAL IN x), \ CCTK_REAL IN xmin, CCTK_REAL IN xmax) PROVIDES FUNCTION Integrate WITH SimpsonsRule LANGUAGE Fortran USES FUNCTION Integrate \end{verbatim} \end{itemize} \subsubsection{Testing Aliased Functions} The calling thorn does not know if an aliased function is even provided by another thorn. Calling an aliased function that has not been provided, will lead to a level 0 warning message, stopping the code. In order to check if a function has been provided by some thorn, use the \texttt{CCTK\_IsFunctionAliased} function described in the function reference section. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{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 a hash mark `\texttt{\#}' or dot `\texttt{.}', or end with a tilde `\texttt{\~{}}', \texttt{.bak} or \texttt{.BAK}. \item{} Thorns will be ignored if they are called doc or start with a hash mark `\texttt{\#}' or dot `\texttt{.}', or end with a tilde `\texttt{\~{}}', \texttt{.bak} or \texttt{.BAK}. \item{} Routine names have to be unique among all thorns. \end{itemize} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{General Naming Conventions} The following naming conventions are followed by the flesh and the supported Cactus arrangements. They are not compulsory, but if followed, will allow for a homogeneous code. \begin{itemize} \item Parameters: lower case (except for acronyms) with words separated by an underscore. Examples: \texttt{my\_first\_parameter}, \texttt{solve\_PDE\_equation}. \item Filenames and routine names: Prefixed by thorn name with an underscore, then capitalised words, with no spaces. Examples: \texttt{MyThorn\_StartUpRoutine}, \texttt{BestSolver\_InitialDataForPDE}. \end{itemize} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Data Types and Sizes} \label{sect-ThornWriting/DataTypes} \label{sec:datyansi} Cactus knows about the following fixed size data types: \begin{center} \begin{tabular}{|l|l|l|l|} \hline Data Type & Size (bytes) & Variable Type & Fortran Equivalent\\ \hline {\t CCTK\_BYTE} & 1 & {\t CCTK\_VARIABLE\_BYTE} & {\t integer*1}\\ {\t CCTK\_INT1} & 1 & {\t CCTK\_VARIABLE\_INT1} & {\t integer*1}\\ {\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\_COMPLEX8} & 8 & {\t CCTK\_VARIABLE\_COMPLEX8} & {\t complex*8}\\ {\t CCTK\_COMPLEX16} & 16 & {\t CCTK\_VARIABLE\_COMPLEX16} & {\t complex*16}\\ {\t CCTK\_COMPLEX32} & 32 & {\t CCTK\_VARIABLE\_COMPLEX32} & {\t complex*32}\\ \hline \end{tabular} \end{center} The availability of these types, and the corresponding C data types, are platform-dependent. For each fixed-size data type, there exists a corresponding preprocessor macro {\t HAVE\_}, which should be used to check whether the given CCTK data type is supported, e.g.\ \begin{verbatim} /* declare variable with extended-precision complex data type if available, otherwise, with default CCTK precision */ #ifdef HAVE_CCTK_COMPLEX32 CCTK_COMPLEX32 var; #else CCTK_COMPLEX var; #endif \end{verbatim} In addition, Cactus provides three generic numeric data types which map onto the compilers' native data types used to represent integer, real, and complex values. The size for these generic types can be chosen at configuration time (see Section \ref{subsec:Compilation-Available_Options}). This is to allow the code to be run easily 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 these generic data types: \begin{center} \begin{tabular}{|l|l|l|l|} \hline Data Type & Variable Type & Configuration Option\\ \hline {\t CCTK\_INT} & {\t CCTK\_VARIABLE\_INT} & {\t INTEGER\_PRECISION}\\ {\t CCTK\_REAL} & {\t CCTK\_VARIABLE\_REAL} & {\t REAL\_PRECISION}\\ {\t CCTK\_COMPLEX} & {\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 the definitions in the interface files where appropriate. A set of macros, which are interpreted by the preprocessor at compile time, to signify which data size is being used, are also provided: \begin{center} \begin{tabular}{|l|l|} \hline Data Type & {\t \#define}\\ \hline {\t CCTK\_INT1} & {\t CCTK\_INT\_PRECISION\_1} \\ {\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\_COMPLEX8} & {\t CCTK\_COMPLEX\_PRECISION\_8} \\ {\t CCTK\_COMPLEX16} & {\t CCTK\_COMPLEX\_PRECISION\_16} \\ {\t CCTK\_COMPLEX32} & {\t CCTK\_COMPLEX\_PRECISION\_32} \\ \hline \end{tabular} \end{center} Cactus also provides generic data and function pointers, which can be used from either C or Fortran: \begin{center} \begin{tabular}{|l|l|l|} \hline Data Type & Variable Type & C equivalent \\\hline {\t CCTK\_POINTER} & {\t CCTK\_VARIABLE\_POINTER} & {\t void *data\_ptr} \\ {\t CCTK\_POINTER\_TO\_CONST} & {\t CCTK\_VARIABLE\_POINTER\_TO\_CONST} & {\t const void *data\_ptr} \\ {\t CCTK\_FPOINTER} & {\t CCTK\_VARIABLE\_FPOINTER} & {\t void (*fn\_ptr)(void)} \\\hline \end{tabular} \end{center} \subsubsection{Fortran Thorn Writers} Cactus provides the data types \texttt{CCTK\_POINTER} and \texttt{CCTK\_POINTER\_TO\_CONST} for use in Fortran code to declare a pointer passed from C. For example, the variable \texttt{cctkGH} is of the type \texttt{CCTK\_POINTER}. The data type \texttt{CCTK\_STRING} is, in Fortran, also an opaque type; it corresponds to a C pointer, and one has to use the function \texttt{CCTK\_FortranString} to convert it to a Fortran string, or the \texttt{CCTK\_Equals} to compare it to a Fortran String. Since the data types, \texttt{integer} in Fortran and \texttt{int} in C, may be different\footnote{This is only a theoretical possibility, in practice, they have to be the same type for Cactus to work at all.}, many routines that can be called from both, C and Fortran, take arguments of the type \texttt{CCTK\_INT}. This type can be different from the type \texttt{integer}. Fortran does not convert routine arguments automatically, and it is, therefore, necessary to pay attention to the exact argument types that a routine expects, and to convert between \texttt{integer} and \texttt{CCTK\_INT}, accordingly. Currently, most flesh functions take \texttt{integer} arguments, while all aliased functions take \texttt{CCTK\_INT} arguments. \begin{quote} NOTE: If you make errors in passing Fortran arguments, and if there are no interfaces (``prototypes'') available for the routines that are called, then the compiler cannot detect these errors. Be careful, when you write Fortran code yourself, consider placing routines in modules, which implicitly define interfaces for all contained routines. \end{quote} There are two convenient ways to convert between these types. An easy way, is to define parameters or to declare variables of the desired type, assign a value to these parameters or variables, and then pass the parameter or value. This makes for very readable code, since the name of the parameter or variable serves as additional documentation: \begin{verbatim} CCTK_INT, parameter : jtwo = 2 integer :: vindex_gxx, vindex_kxx CCTK_INT :: syncvars(jtwo) call CCTK_VarIndex (vindex_gxx, "ADMBase::gxx") call CCTK_VarIndex (vindex_kxx, "ADMBase::kxx") syncvars(1) = vindex_gxx syncvars(2) = vindex_kxx call CCTK_SyncGroupsI (cctkGH, jtwo, syncvars) \end{verbatim} (You have probably seen the strange Fortran convention, where people introduce constants named \texttt{zero} or \texttt{two}---it is a convenient way to make sure that the constant has the correct type.) Another possibility are explicit type conversions. They can rather easily be added to existing code: \begin{verbatim} ! Boilerplate code to determine type kinds integer, parameter :: izero = 0 ! A dummy variable of type integer CCTK_INT, parameter :: jzero = 0 ! A dummy variable of type CCTK_ITN integer, parameter :: ik = kind (izero) ! The kind of "integer" integer, parameter :: jk = kind (jzero) ! The kind of "CCTK_INT" integer :: syncvars(2) call CCTK_VarIndex (syncvars(1), "ADMBase::gxx") call CCTK_VarIndex (syncvars(2), "ADMBase::kxx") call CCTK_SyncGroupsI (cctkGH, int(2,jk), int(syncvars,jk)) \end{verbatim} Fortran distinguishes between different integer \emph{kinds}. These kinds are what is different between \texttt{integer} and \texttt{CCTK\_INT}. The expression \texttt{int(EXPR,KIND)} converts \texttt{EXPR} to an integer of kind \texttt{KIND}. Above, we use the convention that the prefix \texttt{i} denotes things having to do with \texttt{integer}, and the prefix \texttt{j} denotes \texttt{CCTK\_INT}. Note that we declare the array \texttt{syncvars} with the type that is necessary to set its values. Type conversions are only possible if variables are read, not when they are written to. %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \section{Telling the Make system What to Do} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Basic Recipe} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{Make Concepts} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{The Four Files} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% \subsection{How your code is built} %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%