\chapter{Overview}

\section{Getting started quickly}

\subsection{Starting SWI-Prolog}

\subsubsection{Starting SWI-Prolog on Unix} \label{sec:startunix}

By default, SWI-Prolog is installed as `pl', though some administrators
call it `swipl' or `swi-prolog'. The commandline arguments of SWI-Prolog
itself and its utility programs are documented using standard Unix
\program{man} pages. SWI-Prolog is normally operated as an interactive
application simply by starting the program:

\begin{code}
machine% pl
% /staff/jan/.plrc compiled, 0.00 sec, 1,016 bytes.
Welcome to SWI-Prolog (Version 3.2.9)
Copyright (c) 1993-1998 University of Amsterdam.  All rights reserved.

For help, use ?- help(Topic). or ?- apropos(Word).

?- 
\end{code}

After starting Prolog, one normally loads a program into it using 
consult/1, which---for historical reasons---may be abbreviated by
putting the name of the program file between square brackets.  The
following goal loads the file \file{likes.pl} containing clauses for
the predicates likes/2:

\begin{code}
?- [likes].
% likes compiled, 0.00 sec, 596 bytes.

Yes
?- 
\end{code}

After this point, Unix and Windows users are united again.

\subsubsection{Starting SWI-Prolog on Windows}

After SWI-Prolog has been installed on a Windows system, the following
important new things are available to the user:

\begin{itemize}
    \item A folder (called {\em directory} in the remainder of this
	  document) called \file{pl} containing the executables,
	  libraries, etc. of the system.  No files are installed
	  outside this directory.  
    \item A program \program{plwin.exe}, providing a window for interaction
          with Prolog. The program \program{plcon.exe} is a version of
	  SWI-Prolog that runs in a DOS-box.
    \item The file-extension \fileext{pl} is associated with the program
          \program{plwin.exe}.  Opening a \fileext{pl} file will cause
	  \program{plwin.exe} to start, change directory to the
	  directory in which the file-to-open resides and load this
	  file.
\end{itemize}

The normal way to start with the \file{likes.pl} file mentioned in 
\secref{startunix} is by simply double-clicking this file in the Windows
explorer.


\subsection{Executing a query}

After loading a program, one can ask Prolog queries about the program.
The query below asks Prolog to prove whether `john' likes someone and
who is liked by `john'.  The system responds with \mbox{\tt X = <value>}
if it can prove the goal for a certain \arg{X}.  The user can type
the semi-colon (;) if (s)he wants another solution, or {\sc return} if
(s)he is satisfied, after which Prolog will say {\bf Yes}.  If Prolog
answers {\bf No}, it indicates it cannot find any more answers to the
query.  Finally, Prolog can answer using an error message to indicate
the query or program contains an error.

\begin{code}
?- likes(john, X).

X = mary
\end{code}


\section{The user's initialisation file}	\label{sec:initfile}

\index{startup file}%
\index{user profile file}%
\index{profile file}%
After the necessary system initialisation the system consults (see
consult/1) the user's startup file. The base-name of this file follows
conventions of the operating system. On MS-Windows, it is the file
\file{pl.ini} and on Unix systems \file{.plrc}. The file is searched
using the file_search_path/2 clauses for \const{user_profile}. The table
below shows the default value for this search-path.

\begin{center}
\begin{tabular}{|l|l|l|}
\hline
		& \bf Unix 	& \bf Windows \\
\hline
\bf local	& \file{.} 	& \file{.} \\
\bf home	& \file{~} 	& \file{%HOME%} or
                        	  \file{%HOMEDRIVE%\%HOMEPATH%} \\
global		& 		& SWI-Home directory or
				  \file{%WINDIR%} or \file{%SYSTEMROOT%} \\
\hline
\end{tabular}
\end{center}

\noindent
After the first startup file is found it is loaded and Prolog
stops looking for further startup files. The name of the
startup file can be changed with the `\argoption{-f}{file}'
option.  If \arg{File} denotes an absolute path, this file is loaded,
otherwise the file is searched for using the same conventions as for
the default startup file. Finally, if \arg{file} is
\const{none}, no file is loaded.


\section{Initialisation files and goals}		\label{sec:initgoal}

Using commandline arguments (see \secref{cmdline}), SWI-Prolog can be
forced to load files and execute queries for initialisation purposes
or non-interactive operation.  The most commonly used options
are \argoption{-f}{file} to make Prolog load an initialisation file,
\argoption{-g}{goal} to define an initialisation goal and
\argoption{-t}{goal} to define the \jargon{toplevel goal}. The following
is a typical example for starting an application directly from the
commandline.

\begin{code}
machine% pl -f load.pl -g go -t halt
\end{code}

It tells SWI-Prolog to load \file{load.pl}, start the application using
the \jargon{entry-point} go/0 and ---instead of entering the interactive
toplevel--- exit after completing go/0.

In MS-Windows, the same can be achieved using a short-cut with
appropriately defined commandline arguments. A typically seen
alternative is to write a file \file{run.pl} with content as illustrated
below. Double-clicking \file{run.pl} will start the application.

\begin{code}
:- [load].			% load program
:- go.				% run it
:- halt.			% and exit
\end{code}

\Chapref{runtime} discusses the generation of runtime executables.
Runtime executables are a mean to deliver executables that do not
require the Prolog system. 


\section{Command line options}			\label{sec:cmdline}

The full set of command line options is given below:

\begin{description}
    \cmdlineoptionitem{-help}{}
When given as the only option, it summarises the most important options.

    \cmdlineoptionitem{-v}{}
When given as the only option, it summarises the version and the
architecture identifier.

    \cmdlineoptionitem{-arch}{}
When given as the only option, it prints the architecture identifier
(see current_prolog_flag(arch, Arch)) and exits. See also
\cmdlineoption{-dump-runtime-variables}.

    \cmdlineoptionitem{-dump-runtime-variables}{}
When given as the only option, it prints a sequence of variable settings
that can be used in shell-scripts to deal with Prolog parameters.  This
feature is also used by \program{plld} (see \secref{plld}).  Below is
a typical example of using this feature.

\begin{code}
eval `pl -dump-runtime-variables`
cc -I$PLBASE/include -L$PLBASE/runtime/$PLARCH ...
\end{code}

    \cmdlineoptionitem*{-L}{size[km]}
Give local stack limit (2 Mbytes default). Note that there is no space
between the size option and its argument.  By default, the argument is
interpreted in Kbytes.  Postfixing the argument with \const{m} causes
the argument to be interpreted in Mbytes. The following example
specifies 32 Mbytes local stack.
\begin{code}
% pl -L32m
\end{code}
A maximum is useful to stop buggy programs from claiming all memory
resources. \cmdlineoption{-L0} sets the limit to the highest possible
value. See \secref{limits}.

    \cmdlineoptionitem*{-G}{size[km]}
Give global stack limit (4 Mbytes default). See \cmdlineoption{-L} for
more details.

    \cmdlineoptionitem*{-T}{size[km]}
Give trail stack limit (4 Mbytes default). This limit is relatively high
because trail-stack overflows are not often caused by program bugs. See
\cmdlineoption{-L} for more details.

    \cmdlineoptionitem*{-A}{size[km]}
Give argument stack limit (1 Mbytes default). The argument stack limits
the maximum nesting of terms that can be compiled and executed. 
SWI-Prolog does `last-argument optimisation' to avoid many deeply nested
structure using this stack. Enlarging this limit is only necessary in
extreme cases. See \cmdlineoption{-L} for more details.

    \cmdlineoptionitem{-c}{file \ldots}
Compile files into an `intermediate code file'. See \secref{compilation}.

    \cmdlineoptionitem{-o}{output}
Used in combination with \cmdlineoption{-c} or \cmdlineoption{-b} to
determine output file for compilation.

    \cmdlineoptionitem{-O}{}
Optimised compilation. See current_prolog_flag/2.

    \cmdlineoptionitem{-f}{file}
Use \arg{file} as startup file instead of the default.
`\argoption{-f}{none}' stops SWI-Prolog from searching for a
startup file. See \secref{initfile}. 

    \cmdlineoptionitem{-F}{script}
Selects a startup-script from the SWI-Prolog home directory. The
script-file is named \file{<script>.rc}. The default
\arg{script} name is deduced from the executable, taking the leading
alphanumerical characters (letters, digits and underscore) from the
program-name. \argoption{-F}{none} stops looking for a script. Intended
for simple management of slightly different versions. One could for
example write a script \file{iso.rc} and then select ISO compatibility
mode using \exam{pl -F iso} or make a link from \program{iso-pl} to
\program{pl}.

    \cmdlineoptionitem{-g}{goal}
\arg{Goal} is executed just before entering the top level. Default is a
predicate which prints the welcome message. The welcome message can
thus be suppressed by giving \argoption{-g}{true}. \arg{goal} can be a complex
term. In this case quotes are normally needed to protect it from
being expanded by the Unix shell. 

    \cmdlineoptionitem{-t}{goal}
Use \arg{goal} as interactive toplevel instead of the default goal
prolog/0. \arg{goal} can be a complex term. If the toplevel goal
succeeds SWI-Prolog exits with status 0. If it fails the exit status is 1.
This flag also determines the goal started by break/0 and abort/0. If
you want to stop the user from entering interactive mode start the
application with `\argoption{-g}{goal}' and give `halt' as toplevel.

    \cmdlineoptionitem{-tty}{}
Unix only.  Switches controlling the terminal for allowing
single-character commands to the tracer and get_single_char/1. By
default manipulating the terminal is enabled unless the system detects
it is not connected to a terminal or it is running as a GNU-Emacs
inferior process. This flag is sometimes required for smooth interaction
with other applications.

    \cmdlineoptionitem{-x}{bootfile}
Boot from \arg{bootfile} instead of the system's default boot file. A
bootfile is a file resulting from a Prolog compilation using the
\cmdlineoption{-b} or \cmdlineoption{-c} option or a program saved using
qsave_program/[1,2].

    \cmdlineoptionitem{-p}{alias=path1[:path2 \ldots]}
Define a path alias for file_search_path. \arg{alias} is the name of
the alias, \arg{path1 ...} is a \chr{:} separated list of values for
the alias.  A value is either a term of the form \mbox{alias(value)}
or pathname.  The computed aliases are added to file_search_path/2
using asserta/1, so they precede predefined values for the alias.  See
file_search_path/2 for details on using this file-location mechanism.

    \cmdlineoptionitem{{\tt --}}{}
\index{commandline, arguments}%
Stops scanning for more arguments, so you can pass arguments for your
application after this one.  See current_prolog_flag/2 using the
flag \const{argv} for obtaining the commandline arguments.
\end{description}

The following options are for system maintenance. They are given
for reference only.

\begin{description}
    \cmdlineoptionitem{-b}{initfile \ldots \cmdlineoption{-c} file \ldots}
Boot compilation.  \arg{initfile \ldots} are compiled by the C-written
bootstrap compiler, \arg{file \ldots} by the normal Prolog compiler.
System maintenance only.
    \cmdlineoptionitem{-d}{level}
Set debug level to \arg{level}. Only has effect if the system is
compiled with the \const{-DO_DEBUG} flag. System maintenance
only.
\end{description}


\section{GNU Emacs Interface}

\index{GNU-Emacs}\index{Emacs}
Once upon a time, there was port of the Quintus Prolog GNU-Emacs
package for GNU-Emacs 18.  This package has been dropped from the
distribution because it didn't work on Emacs 19 and 20.

The default Prolog mode for GNU-Emacs can be activated by adding
the following rules to your Emacs initialisation file:

\begin{code}
(setq auto-mode-alist
      (append
       '(("\\.pl" . prolog-mode))
       auto-mode-alist))
(setq prolog-program-name "pl")
(setq prolog-consult-string "[user].\n")
;If you want this.  Indentation is either poor or I don't use
;it as intended.
;(setq prolog-indent-width 8)
\end{code}

If anyone has a better suggestion, please make this public.

\section{Online Help}

Online help provides a fast lookup and browsing facility to this
manual. The online manual can show predicate definitions as well as
entire sections of the manual.

The online help is displayed from the file \pllib{'MANUAL'}. The file
\pllib{helpidx} provides an index into this file. \pllib{'MANUAL'} is
created from the \LaTeX{} sources with a modified version of
\program{dvitty}, using overstrike for printing bold text and
underlining for rendering italic text. XPCE is shipped with
\pllib{swi_help}, presenting the information from the online help in a
hypertext window. The prolog-flag \const{write_help_with_overstrike}
controls whether or not help/1 writes its output using overstrike to
realise bold and underlined output or not. If this prolog-flag is not set it
is initialised by the help library to \const{true} if the \const{TERM}
variable equals \const{xterm} and \const{false} otherwise. If this
default does not satisfy you, add the following line to your personal
startup file (see \secref{initfile}):

\begin{code}
:- set_prolog_flag(write_help_with_overstrike, true).
\end{code}


\begin{description}
    \predicate{help}{0}{}
Equivalent to \exam{help(help/1)}.

    \predicate{help}{1}{+What}
Show specified part of the manual.  \arg{What} is one of:
\begin{center}\begin{tabular}{lp{3.5in}}
<Name>/<Arity>  & Give help on specified predicate \\
<Name>          & Give help on named predicate with any arity
                  or C interface function with that name \\
<Section>       & Display specified section. Section numbers are
                  dash-separated numbers: \exam{2-3} refers to
                  section 2.3 of the manual.  Section numbers are
                  obtained using apropos/1.
\end{tabular}\end{center}

Examples:
\begin{center}\begin{tabular}{lp{3.5in}}
\exam{?- help(assert).}    & Give help on predicate assert \\
\exam{?- help(3-4).}       & Display section 3.4 of the manual \\
\exam{?- help('PL_retry').}& Give help on interface function PL_retry() \\
\end{tabular}\end{center}

See also apropos/1, and the SWI-Prolog home page at
\url{http://www.swi.psy.uva.nl/projects/SWI-Prolog/}, which provides
a FAQ, an HTML version of manual for online browsing and HTML and PDF
versions for downloading.

    \predicate{apropos}{1}{+Pattern}
Display all predicates, functions and sections that have {\em
Pattern} in their name or summary description.  Lowercase letters in
\arg{Pattern} also match a corresponding uppercase letter. Example:
\begin{center}\begin{tabular}{lp{3.5in}}
\exam{?- apropos(file).}   & Display predicates, functions and sections
                            that have `file' (or `File', etc.) in their
                            summary description. \\
\end{tabular}\end{center}

    \predicate{explain}{1}{+ToExplain}
Give an explanation on the given `object'.  The argument may be
any Prolog data object.  If the argument is an atom, a term of
the form \arg{Name/Arity} or a term of the form {\em
Module:Name/Arity}, explain will try to explain the predicate
as well as possible references to it.

    \predicate{explain}{2}{+ToExplain, -Explanation}
Unify \arg{Explanation} with an explanation for \arg{ToExplain}.
Backtracking yields further explanations.
\end{description}


\section{Query Substitutions}
\label{sec:history}

SWI-Prolog offers a query substitution mechanism similar to that of Unix
csh (csh(1)), called `history'. The availability of this feature is
controlled by set_prolog_flag/2, using the \const{history} prolog-flag. By
default, history is available if the prolog-flag \const{readline} is
\const{false}. To enable this feature, remembering the last 50 commands,
put the following into your startup file (see \secref{initfile}:

\begin{code}
:- set_prolog_flag(history, 50).
\end{code}

The history system allows the user to compose new queries from those
typed before and remembered by the system. It also allows to correct
queries and syntax errors. SWI-Prolog does not offer the Unix csh
capabilities to include arguments. This is omitted as it is unclear how
the first, second, etc.\ argument should be defined.%
    \footnote{One could choose words, defining words as a sequence of
              alpha-numeric characters and the word separators as
              anything else, but one could also choose Prolog
              arguments}

The available history commands are shown in \tabref{history}.

\begin{table}
\begin{center}
\begin{tabular}{|l|l|}
\hline
\verb+!!.+            & Repeat last query \\
\verb+!nr.+           & Repeat query numbered <nr> \\
\verb+!str.+          & Repeat last query starting with <str> \\
\verb+!?str.+         & Repeat last query holding <str> \\
\verb+^old^new.+      & Substitute <old> into <new> in
                        last query \\
\verb+!nr^old^new.+   & Substitute in query numbered <nr> \\
\verb+!str^old^new.+  & Substitute in query starting with <str> \\
\verb+!?str^old^new.+ & Substitute in query holding <str> \\
\verb+h.+             & Show history list \\
\verb+!h.+            & Show this list \\
\hline
\end{tabular}
\end{center}
    \caption{History commands}
    \label{tab:history}
\end{table}


\subsection{Limitations of the History System}

History expansion is executed after \jargon{raw-reading}. This is the
first stage of read_term/2 and friends, reading the term into a string
while deleting comment and canonising blank. This makes it hard to use
it for correcting syntax errors. Command-line editing as provided using
the GNU-readline library is more suitable for this. History expansion is
first of all useful for executing or combining commands from long ago.


\section{Reuse of toplevel bindings}    \label{sec:topvars}

Bindings resulting from the successful execution of a toplevel goal
are asserted in a database.  These values may be reused in further
toplevel queries as \$Var.  Only the latest binding is available.
Example:

\begin{figure}
\begin{code}
1 ?- maplist(plus(1), "hello", X).

X = [105,102,109,109,112] 

Yes
2 ?- format('~s~n', [$X]).
ifmmp

Yes
3 ?-
\end{code}
    \caption{Reusing toplevel bindings}
    \label{fig:topevelvars}
\end{figure} 

Note that variables may be set by executing \predref{=}{2}:

\begin{code}
6 ?- X = statistics.

X = statistics

Yes
7 ?- $X.
28.00 seconds cpu time for 183,128 inferences
4,016 atoms, 1,904 functors, 2,042 predicates, 52 modules
55,915 byte codes; 11,239 external references

                      Limit    Allocated       In use
Heap         :                                624,820 Bytes
Local  stack :    2,048,000        8,192          404 Bytes
Global stack :    4,096,000       16,384          968 Bytes
Trail  stack :    4,096,000        8,192          432 Bytes

Yes
8 ?-
\end{code}

\section{Overview of the Debugger}

SWI-Prolog has a 6-port tracer, extending the standard 4-port tracer
\cite{Clocksin:87} with two additional ports. The optional \arg{unify}
port allows the user to inspect the result after unification of the
head.  The \arg{exception} port shows exceptions raised by throw/1
or one of the built-in predicates.  See \secref{exception}.

The standard ports are called \const{call}, \const{exit}, \const{redo},
\const{fail} and \const{unify}. The tracer is started by the trace/0
command, when a spy point is reached and the system is in debugging mode
(see spy/1 and debug/0) or when an exception is raised.

The interactive toplevel goal trace/0 means ``trace the next query''.
The tracer shows the port, displaying the port name, the current depth
of the recursion and the goal. The goal is printed using the Prolog
predicate write_term/2.  The style is defined by the prolog-flag
\const{debugger_print_options} and can be modified using this flag
or using the \const{w}, \const{p} and \const{d} commands of the tracer.

\begin{figure}
\begin{code}
1 ?- visible(+all), leash(-exit).

Yes
2 ?- trace, min([3, 2], X).
  Call:  ( 3) min([3, 2], G235) ? creep
  Unify: ( 3) min([3, 2], G235)
  Call:  ( 4) min([2], G244) ? creep
  Unify: ( 4) min([2], 2)
  Exit:  ( 4) min([2], 2)
  Call:  ( 4) min(3, 2, G235) ? creep
  Unify: ( 4) min(3, 2, G235)
  Call:  ( 5) 3 < 2 ? creep
  Fail:  ( 5) 3 < 2 ? creep
  Redo:  ( 4) min(3, 2, G235) ? creep
  Exit:  ( 4) min(3, 2, 2)
  Exit:  ( 3) min([3, 2], 2)

Yes
[trace] 3 ?-
\end{code}
    \caption{Example trace}
    \label{fig:tracer}
\end{figure}

On {\em leashed ports} (set with the predicate leash/1, default are
\const{call}, \const{exit}, \const{redo} and \const{fail}) the user is
prompted for an action. All actions are single character commands which
are executed {\bf without} waiting for a return, unless the command line
option \cmdlineoption{-tty} is active. Tracer options:

\begin{description}
    \traceoption{+}{Spy}{
Set a spy point (see spy/1) on the current predicate.} 
    \traceoption{-}{No spy}{
Remove the spy point (see nospy/1) from the current predicate.} 
    \traceoption{/}{Find}{
Search for a port.  After the `/', the user can enter a line
to specify the port to search for.  This line consists of a set of
letters indicating the port type, followed by an optional term,
that should unify with the goal run by the port.  If no term is
specified it is taken as a variable, searching for any port of the
specified type.  If an atom is given, any goal whose functor has a
name equal to that atom matches.  Examples:
\begin{center}\begin{tabular}{lp{3in}}
\tt /f                  & Search for any fail port \\
\tt /fe solve           & Search for a fail or exit port of any goal with
                          name \const{solve} \\
\tt /c solve(a, _)      & Search for a call to {solve}/2 whose first argument
                          is a variable or the atom \const{a} \\
\tt /a member(_, _)     & Search for any port on member/2. This is equivalent
                          to setting a spy point on member/2. \\
\end{tabular}\end{center}}
    \traceoption{.}{Repeat find}{
Repeat the last find command (see `/').}
    \traceoption{A}{Alternatives}{
Show all goals that have alternatives.} 
    \traceoption{C}{Context}{
Toggle `Show Context'. If \const{on} the context module of the goal is
displayed between square brackets (see \secref{modules}).
Default is \const{off}.} 
    \traceoption{L}{Listing}{
List the current predicate with listing/1.} 
    \traceoption{a}{Abort}{
Abort Prolog execution (see abort/0).} 
    \traceoption{b}{Break}{
Enter a Prolog break environment (see break/0).} 
    \traceoption{c}{Creep}{
Continue execution, stop at next port. (Also return, space).} 
    \traceoption{d}{Display}{
Set the \term{max_depth}{Depth} option of
\const{debugger_print_options}, limiting the depth to which terms are
printed.  See also the \const{w} and \const{p} options.}
    \traceoption{e}{Exit}{
Terminate Prolog (see halt/0).} 
    \traceoption{f}{Fail}{
Force failure of the current goal.} 
    \traceoption{g}{Goals}{
Show the list of parent goals (the execution stack). Note that due to tail
recursion optimization a number of parent goals might not exist any more.} 
    \traceoption{h}{Help}{
Show available options (also `?').} 
    \traceoption{i}{Ignore}{
Ignore the current goal, pretending it succeeded.} 
    \traceoption{l}{Leap}{
Continue execution, stop at next spy point.} 
    \traceoption{n}{No debug}{
Continue execution in `no debug' mode.} 
    \traceoption{p}{Print}{
Set the prolog-flag \const{debugger_print_options} to
\exam{[quoted(true), portray(true), max_depth(10)]}. This is the
default.}
    \traceoption{r}{Retry}{
Undo all actions (except for database and i/o actions) back to the call
port of the current goal and resume execution at the call port.} 
    \traceoption{s}{Skip}{
Continue execution, stop at the next port of {\bf this} goal (thus skipping
all calls to children of this goal).} 
    \traceoption{u}{Up}{
Continue execution, stop at the next port of {\bf the parent} goal (thus
skipping this goal and all calls to children of this goal). This option
is useful to stop tracing a failure driven loop.} 
    \traceoption{w}{Write}{
Set the prolog-flag \const{debugger_print_options} to
\exam{[quoted(true)]}, bypassing portray/1, etc.}
\end{description}

The ideal 4 port model as described in many Prolog books
\cite{Clocksin:87} is not visible in many Prolog implementations because
code optimisation removes part of the choice- and exit-points.
Backtrack points are not shown if either the goal succeeded
deterministically or its alternatives were removed using the cut.  When
running in debug mode (debug/0) choice points are only destroyed when
removed by the cut.  In debug mode, tail recursion optimisation is
switched off.%
    \footnote{This implies the system can run out of local stack in debug
              mode, while no problems arise when running in non-debug mode.}


\section{Compilation}			\label{sec:compilation}

\subsection{During program development}

During program development, programs are normally loaded using
consult/1, or the list abbreviation. It is common practice to organise a
project as a collection of source-files and a \jargon{load-file}, a
Prolog file containing only use_module/[1,2] or ensure_loaded/1
directives, possibly with a definition of the \jargon{entry-point} of
the program, the predicate that is normally used to start the program.
This file is often called \file{load.pl}.  If the entry-point is called
{\em go}, a typical session starts as:

\begin{code}
% pl
<banner>

1 ?- [load].
<compilation messages>

Yes
2 ?- go.
<program interaction>
\end{code}

When using Windows, the user may open \file{load.pl} from the Windows
explorer, which will cause \program{plwin.exe} to be started in the
directory holding \file{load.pl}.  Prolog loads \file{load.pl} before
entering the toplevel.


\subsection{For running the result}

There are various options if you want to make your program ready
for real usage.  The best choice depends on whether the program
is to be used only on machines holding the SWI-Prolog development
system, the size of the program and the operating system (Unix
vs.\ Windows).


\subsubsection{Creating a shell-script}

Especially on Unix systems and not-too-large applications, writing
a shell-script that simply loads your application and calls the
entry-point is often a good choice.  A skeleton for the script is
given below, followed by the Prolog code to obtain the program
arguments.

\begin{code}
#!/bin/sh

base=<absolute-path-to-source>
PL=pl

exec $PL -f none -g "load_files(['$base/load'],[silent(true)])" \
	 -t go -- $*
\end{code}

\begin{code}
go :-
	current_prolog_flag(argv, Arguments),
	append(_SytemArgs, [--|Args], Arguments), !,
	go(Args).

go(Args) :-
	...
\end{code}

On Windows systems, similar behaviour can be achieved by creating a 
shortcut to Prolog, passing the proper options or writing a \fileext{bat}
file.


\subsubsection{Creating a saved-state}

For larger programs, as well as for programs that are required run on
systems that do not have the SWI-Prolog development system installed,
creating a saved state is the best solution. A saved state is created
using qsave_program/[1,2] or using the linker plld(1). A saved state is
a file containing machine-independent intermediate code in a format
dedicated for fast loading. Optionally, the emulator may be integrated
in the saved state, creating a single-file, but machine-dependent,
executable.  This process is described in \chapref{runtime}.


\subsubsection{Compilation using the -c commandline option}
\label{sec:cmdlinecomp}

This mechanism loads a series of Prolog source files and then creates
a saved-state as qsave_program/2 does.  The command syntax is:

\begin{code}
% pl [option ...] [-o output] -c file ...
\end{code}

The \arg{options} argument are options to qsave_program/2 written in
the format below.  The option-names and their values are described with
qsave_program/2.

\begin{quote}
\verb$--${\em option-name}\verb$=$\em{option-value}
\end{quote}

For example, to create a stan-alone executable that starts by executing
main/0 and for which the source is loaded through \file{load.pl}, use
the command

\begin{code}
% pl --goal=main --stand_alone=true -o myprog -c load.pl
\end{code}

This performs exactly the same as executing

\begin{code}
% pl
<banner>
?- [load].
?- qsave_program(myprog,
		 [ goal(main),
		   stand_alone(true)
		 ]).
?- halt.
\end{code}

\section{Environment Control (Prolog flags)}

The current system defines 2 different mechanisms to query and/or set
properties of the environment: flag/3 and current_prolog_flag/2 as well
as a number of special purpose predicates of which unknown/2,
fileerrors/2 are examples. The ISO standard defines prolog_flag. It is
likely that all these global features will be merged into a single in
the future.

\begin{description}
    \predicate{current_prolog_flag}{2}{?Key, -Value}
The predicate current_prolog_flag/2 defines an interface to installation
features: options compiled in, version, home, etc. With both arguments
unbound, it will generate all defined prolog-flags. With the `Key'
instantiated it unify the value of the prolog-flag. Features come in
three types: boolean prolog-flags, prolog-flags with an atom value and
prolog-flags with an integer value. A boolean prolog-flag is true iff
the prolog-flag is present {\bf and} the \arg{Value} is the atom
\const{true}. Currently defined keys:

\begin{description}
    \prologflagitem{arch}{atom}{r}
Identifier for the hardware and operating system SWI-Prolog is running
on.  Used to determine the startup file as well as to select foreign
files for the right architecture.  See also \secref{shlib}.
    \prologflagitem{version}{integer}{r}
The version identifier is an integer with value: $$10000 \times
\arg{Major} + 100 \times \arg{Minor} + \arg{Patch}$$
Note that in releases upto 2.7.10 this prolog-flag yielded an atom holding
the three numbers separated by dots. The current representation is much
easier for implementing version-conditional statements.
    \prologflagitem{home}{atom}{r}
SWI-Prolog's notion of the home-directory.  SWI-Prolog uses it's home
directory to find its startup file as \file{<home>/startup/startup.<arch>}
and to find its library as \file{<home>/library}.
    \prologflagitem{executable}{atom}{r}
Path-name of the running executable.  Used by qsave_program/2 as default
emulator.
    \prologflagitem{argv}{list}{r}
List is a list of atoms representing the command-line arguments used to
invoke SWI-Prolog.  Please note that {\bf all} arguments are included
in the list returned.
    \prologflagitem{pipe}{bool}{rw}
If true, \exam{open(pipe(command), mode, Stream)}, etc.\ are supported.
Can be changed to disable the use of pipes in applications testing this
feature.  Not recommended.
    \prologflagitem{open_shared_object}{bool}{r}
If true, open_shared_object/2 and friends are implemented, providing
access to shared libraries (\fileext{so} files) or dynamic link
libraries (\fileext{DLL} files).
    \prologflagitem{shared_object_extension}{atom}{r}
Extension used by the operating system for shared objects.  \const{so}
for most Unix systems and \const{dll} for Windows.  Used for locating
files using the \const{file_type} \const{executable}.  See also
absolute_file_name/3.
    \prologflagitem{dynamic_stacks}{bool}{r}
If \const{true}, the system uses some form of `sparse-memory management'
to realise the stacks. If false, malloc()/realloc() are used for the
stacks. In earlier days this had consequenses for foreign code. As of
version 2.5, this is no longer the case.

Systems using `sparse-memory management' are a bit faster as there is no
stack-shifter, and checking the stack-boundary is often realised by the
hardware using a `guard-page'. Also, memory is actually returned to the
system after a garbage collection or call to trim_stacks/0 (called by
prolog/0 after finishing a user-query).
    \prologflagitem{c_libs}{atom}{r}
Libraries passed to the C-linker when SWI-Prolog was linked.  May be used
to determine the libraries needed to create statically linked extensions
for SWI-Prolog.  See \secref{plld}.
    \prologflagitem{c_cc}{atom}{r}
Name of the C-compiler used to compile SWI-Prolog.  Normally either gcc
or cc.  See \secref{plld}.
    \prologflagitem{c_ldflags}{atom}{r}
Special linker flags passed to link SWI-Prolog.  See \secref{plld}.
    \prologflagitem{readline}{bool}{r}
If true, SWI-Prolog is linked with the readline library.  This is done
by default if you have this library installed on your system.  It is
also true for the Win32 plwin.exe version of SWI-Prolog, which realises
a subset of the readline functionality.
    \prologflagitem{saved_program}{bool}{r}
If true, Prolog is started from a state saved with qsave_program/[1,2].
    \prologflagitem{runtime}{bool}{r}
If true, SWI-Prolog is compiled with -DO_RUNTIME, disabling various
useful development features (currently the tracer and profiler).
    \prologflagitem{max_integer}{integer}{r}
Maximum integer value.  Most arithmetic operations will automatically
convert to floats if integer values above this are returned.
    \prologflagitem{min_integer}{integer}{r}
Minimum integer value.
    \prologflagitem{max_tagged_integer}{integer}{r}
Maximum integer value represented as a `tagged' value.  Tagged integers
require 4-bytes storage and are used for indexing.  Larger integers are
represented as `indirect data' and require 16-bytes on the stacks (though
a copy requires only 4 additional bytes).
    \prologflagitem{min_tagged_integer}{integer}{r}
Start of the tagged-integer value range.
    \prologflagitem{float_format}{atom}{rw}
C {\tt printf()} format specification used by write/1 and friends to
determine how floating point numbers are printed. The default is {\tt
\%g}. The specified value is passed to printf() without further
checking. For example, if you want more digits printed, {\tt \%.12g}
will print all floats using 12 digits instead of the default 6. See also
format/[1,2], write/1, print/1 and portray/1.
    \prologflagitem{toplevel_print_options}{term}{rw}
This argument is given as option-list to write_term/2 for printing results
of queries.  Default is \exam{[quoted(true), portray(true), max_depth(10)]}.
    \prologflagitem{debugger_print_options}{term}{rw}
This argument is given as option-list to write_term/2 for printing goals
by the debugger.  Modified by the `w', `p' and `<N> d' commands of the
debugger.  Default is \exam{[quoted(true), portray(true),
max_depth(10)]}.
    \prologflagitem{debugger_show_context}{bool}{rw}
If \const{true}, show the context module while printing a stack-frame in
the tracer.  Normally controlled using the `C' option of the tracer.
    \prologflagitem{compiled_at}{atom}{r}
Describes when the system has been compiled. Only available if the
C-compiler used to compile SWI-Prolog provides the __DATE__ and __TIME__
macros.
    \prologflagitem{character_escapes}{bool}{rw}
If \const{true} (default), read/1 interprets \verb$\$ escape sequences
in quoted atoms and strings. May be changed. This flag is local to the
module in which it is changed.
    \prologflagitem{double_quotes}{codes,chars,atom,string}{rw}
This flag determines how double-quotes strings are read by Prolog and is
---like character_escapes--- maintained for each module. If
\const{codes} (default), a list of character-codes is returned, if
\const{chars} a list of one-character atoms, if \const{atom} double
quotes are the same as single-quotes and finally, \const{string} reads
the text into a Prolog string (see \secref{strings}).   See also
atom_chars/2 and atom_codes/2.
    \prologflagitem{allow_variable_name_as_functor}{bool}{rw}
If true (default is false), \exam{Functor(arg)} is read as if it was
written \exam{'Functor'(arg)}. Some applications use the Prolog read/1
predicate for reading an application defined script language. In these
cases, it is often difficult to explain none-Prolog users of the
application that constants and functions can only start with a lowercase
letter. Variables can be turned into atoms starting with an uppercase
atom by calling read_term/2 using the option \const{variable_names} and
binding the variables to their name. Using this feature, F(x) can be
turned into valid syntax for such script languages. Suggested by Robert
van Engelen.  SWI-Prolog specific.
    \prologflagitem{history}{integer}{rw}
If $\arg{integer}> 0$, support Unix \program{csh(1)} like history as
described in \secref{history}. Otherwise, only support reusing commands
through the commandline editor. The default is to set this prolog-flag to 0
if a commandline editor is provided (see prolog-flag \const{readline}) and
15 otherwise.
    \prologflagitem{gc}{bool}{rw}
If true (default), the garbage collector is active.  If false, neither
garbage-collection, nor stack-shifts will take place, even not on
explicit request.  May be changed.
    \prologflagitem{agc_margin}{integer}{rw}
If this amount of atoms has been created since the last atom-garbage
collection, perform atom garbage collection at the first opportunity.
Initial value is 10,000.   May be changed.  A value of 0 (zero) disables
atom garbage collection.  See also PL_register_atom().
    \prologflagitem{iso}{bool}{rw}
Include some weird ISO compatibility that is incompatible to normal
SWI-Prolog behaviour.  Currently it has the following effect:
\begin{itemize}
    \item is/2 and evaluation under flag/3 do not automatically convert
          floats to integers if the float represents an integer.
    \item The \functor{/}{2} (float division) {\em always} return a
          float, even if applied to integers that can be divided.
    \item In the standard order of terms (see \secref{standardorder}),
          all floats are before all integers.
    \item atom_length/2 yields an instantiation error if the first
          argument is a number.
    \item clause/[2,3] raises a permission error when accessing static
          predicates.
\end{itemize}
    \prologflagitem{optimise}{bool}{rw}
If \const{true}, compile in optimised mode. The initial value is
\const{true} if Prolog was started with the \cmdlineoption{-O}
commandline option.

Currently optimise compilation implies compilation of arithmetic,
and deletion of redundant true/0 that may result from expand_goal/2.

Later versions might imply various other optimisations such as
integrating small predicates into their callers, eliminating constant
expressions and other predictable constructs. Source code optimisation
is never applied to predicates that are declared dynamic (see
dynamic/1).
    \prologflagitem{char_conversion}{bool}{rw}
Determines whether character-conversion takes place while reading terms.
See also char_conversion/2.
    \prologflagitem{autoload}{bool}{rw}
If \const{true} (default) autoloading of library functions is enabled.
See \secref{autoload}.
    \prologflagitem{verbose_autoload}{bool}{rw}
If \const{true} the normal consult message will be printed if a library
is autoloaded. By default this message is suppressed. Intended to be
used for debugging purposes.
    \prologflagitem{trace_gc}{bool}{rw}
If true (false is the default), garbage collections and stack-shifts
will be reported on the terminal.  May be changed.
    \prologflagitem{max_arity}{unbounded}{r}
ISO prolog-flag describing there is no maximum arity to compound terms.
    \prologflagitem{integer_rounding_function}{down,toward_zero}
ISO prolog-flag describing rounding by \verb$//$ and \verb$rem$ arithmetic
functions. Value depends on the C-compiler used.
    \prologflagitem{bounded}{true}{r}
ISO prolog-flag describing integer representation is bound by
{\tt min_integer} and {\tt min_integer}.
    \prologflagitem{tty_control}{bool}{r}
Determines whether the terminal is switched to raw mode for
get_single_char/1, which also reads the user-actions for the trace.  May
be set.  See also the \cmdlineoption{+/-tty} command-line option.
    \prologflagitem{unknown}{fail,warning,error}{rw}
Determines the behaviour if an undefined procedure is encountered.  If
\const{fail}, the predicates fails silently.  If \const{warn}, a warning
is printed, and execution continues as if the predicate was not defined
and if \const{error} (default), an \except{existence_error} exception
is raised.  This flag is local to each module.
    \prologflagitem{debug}{bool}{rw}
Switch on/off debugging mode.  If debug mode is activated the system
traps encountered spy-points (see spy/1) and trace-points (see trace/1).
In addition, tail-recursion optimisation is disabled and the system is
more conservative in destroying choice-points to simplify debugging.

Disabling these optimisations can cause the system to run out of memory
on programs that behave correctly if debug mode is off.
    \prologflagitem{debug_on_error}{bool}{rw}
If {\tt true}, start the tracer after an error is detected.  Otherwise
just continue execution.  The goal that raised the error will normally
fail.  See also fileerrors/2 and the prolog-flag {\tt report_error}.  May
be changed.  Default is {\tt true}, except for the runtime version.
    \prologflagitem{report_error}{bool}{rw}
If {\tt true}, print error messages, otherwise suppress them.  May be
changed.  See also the {\tt debug_on_error} prolog-flag.  Default is {\tt
true}, except for the runtime version.  
    \prologflagitem{file_name_variables}{bool}{rw}
If \const{true} (default \const{false}), expand \file{\$\arg{varname}}
and \file{~} in arguments of builtin-predicates that accept a file name
(open/3, exists_file/1, access_file/2, etc.).   The predicate
expand_file_name/2 should be used to expand environment variables
and wildcard patterns. This prolog-flag is intended for backward
compatibility with older versions of SWI-Prolog.
    \prologflagitem{unix}{bool}{r}
\index{unix}%
If {\tt true}, the operating system is some version of Unix.  Defined
if the C-compiler used to compile this version of SWI-Prolog either
defines \verb$__unix__$ or \const{unix}.
    \prologflagitem{windows}{bool}{r}
\index{windows}%
If {\tt true}, the operating system is an implementation of Microsoft
Windows (3.1, 95, NT, etc.).
\end{description}

    \predicate{set_prolog_flag}{2}{+Key, +Value}
Define a new prolog-flag or change its value.  \arg{Key} is an atom.
If the flag is a system-defined flag that is not marked
\jargon{changeable} above, an attempt to modify the flag yields a
\except{permission_error}.  If the provided \arg{Value} does not
match the type of the flag, a \except{type_error} is raised.

In addition to ISO, SWI-Prolog allows for user-defined prolog flags.
The type of the flag is determined from the initial value and cannot
be changed afterwards.
\end{description}


\section{An overview of hook predicates}

\index{hooks}
SWI-Prolog provides a large number of hooks, mainly to control handling
messages, debugging, startup, shut-down, macro-expansion, etc.  Below
is a summary of all defined hooks with an indication of their
portability.

\begin{itemlist}
    \item [portray/1]
Hook into write_term/3 to alter the way terms are printed (ISO).
    \item [message_hook/3]
Hook into print_message/2 to alter the way system messages are printed
(Quintus/SICStus).
    \item [library_directory/1]
Hook into absolute_file_name/3 to define new library directories.
(most Prolog system).
    \item [file_search_path/2]
Hook into absolute_file_name/3 to define new search-paths
(Quintus/SICStus).
    \item [term_expansion/2]
Hook into load_files/1 to modify read terms before they are compiled
(macro-processing) (most Prolog system).
    \item [goal_expansion/2]
Same as term_expansion/2 for individual goals (SICStus).
    \item [prolog_edit:locate/3]
Hook into edit/1 to locate objects (SWI).
    \item [prolog_edit:edit_source/1]
Hook into edit/1 to call some internal editor (SWI).
    \item [prolog_edit:edit_command/2]
Hook into edit/1 to define the external editor to use (SWI).
    \item [prolog_list_goal/1]
Hook into the tracer to list the code associated to a particular goal
(SWI).
    \item [prolog_trace_interception/4]
Hook into the tracer to handle trace-events (SWI).
    \item [resource/3]
Defines a new resource (not really a hook, but similar) (SWI).    
    \item [exception/3]
Old attempt to a generic hook mechanism.  Handles undefined predicates (SWI).
\end{itemlist}


\section{Automatic loading of libraries}	\label{sec:autoload}

If ---at runtime--- an undefined predicate is trapped the system will
first try to import the predicate from the module's default module. If
this fails the \jargon{auto loader} is activated. On first activation an
index to all library files in all library directories is loaded in core
(see library_directory/1). If the undefined predicate can be located in
the one of the libraries that library file is automatically loaded and
the call to the (previously undefined) predicate is resumed. By default
this mechanism loads the file silently. The current_prolog_flag/2
\const{verbose_autoload} is provided to get verbose loading. The
prolog-flag \const{autoload} can be used to enable/disable the entire
auto load system.

The auto-loader only works if the unknown flag (see unknown/2) is set to
\const{trace} (default).  A more appropriate interaction with this flag
will be considered.

Autoloading only handles (library) source files that use the module
mechanism described in \chapref{modules}.  The files are loaded
with use_module/2 and only the trapped undefined predicate will be imported
to the module where the undefined predicate was called.  Each library
directory must hold a file \file{INDEX.pl} that contains an index to all
library files in the directory.  This file consists of lines of the 
following format:

\begin{code}
index(Name, Arity, Module, File).
\end{code}

The predicate make/0 scans the autoload libraries and updates the
index if it exists, is writable and out-of-date.  It is advised to
create an empty file called \file{INDEX.pl} in a library directory
meant for auto loading before doing anything else.  This index file
can then be updated by running the prolog make_library_index/1 (`\%' is
the Unix prompt):

\begin{code}
% mkdir ~/lib/prolog
% cd !$
% pl -g true -t 'make_library_index(.)'
\end{code}

If there are more than one library files containing the desired predicate
the following search schema is followed:

\begin{enumerate}
  \item If there is a library file that defines the module in which
        the undefined predicate is trapped, this file is used.
  \item Otherwise library files are considered in the order they appear
        in the library_directory/1 predicate and within the directory
        alphabetically.
\end{enumerate}

\begin{description}
    \predicate{make_library_index}{1}{+Directory}
Create an index for this directory.  The index is written to the file
'INDEX.pl' in the specified directory.  Fails with a warning if the
directory does not exist or is write protected.
\end{description}


\section{Garbage Collection}

SWI-Prolog version 1.4 was the first release to support garbage
collection. Together with last-call optimisation this guarantees forward
chaining programs do not waste infinite amounts of memory.


\section{Syntax Notes}

SWI-Prolog uses standard `Edinburgh' syntax. A description of this
syntax can be found in the Prolog books referenced in the introduction.
Below are some non-standard or non-common constructs that are accepted
by SWI-Prolog:

\begin{itemlist}
    \item [\exam{0'<char>}]
This construct is not accepted by all Prolog systems that claim to have
Edinburgh compatible syntax. It describes the ASCII value of <char>.
To test whether \chr{C} is a lower case character one can use
\exam{between(0'a, 0'z, C)}.
    \item [\exam{/* \ldots /* \ldots */ \ldots */}]
The \exam{/* \ldots */} comment statement can be nested. This is useful
if some code with \exam{/* \ldots */} comment statements in it should be
commented out.
\end{itemlist}

\subsection{ISO Syntax Support}

SWI-Prolog offers ISO compatible extensions to the Edinburgh syntax.

\subsubsection{Character Escape Syntax} \label{sec:charescapes}

Within quoted atoms (using single quotes: \exam{'<atom>'} special
characters are represented using escape-sequences.  An escape sequence
is lead in by the backslash (\chr{\}) character. The list of
escape sequences is compatible with the ISO standard, but contains one
extension and the interpretation of numerically specified characters is
slightly more flexible to improve compatibility.

\begin{description}
    \escapeitem{a}
Alert character.  Normally the ASCII character 7 (beep).
    \escapeitem{b}
Backspace character.
    \escapeitem{c}
No output.  All input characters upto but not including the first
non-layout character are skipped.  This allows for the specification
of pretty-looking long lines.  For compatibility with Quintus Prolog.
Nor supported by ISO.  Example:
\begin{code}
format('This is a long line that would look better if it was \c
       split across multiple physical lines in the input')
\end{code}
    \escapeitem{\bnfmeta{{\sc RETURN}}}
No output.  Skips input till the next non-layout character or to the
end of the next line.  Same intention as \fmtseq{\c} but ISO compatible.
    \escapeitem{f}
Form-feed character.
    \escapeitem{n}
Next-line character.
    \escapeitem{r}
Carriage-return only (i.e.\ go back to the start of the line).
    \escapeitem{t}
Horizontal tab-character.
    \escapeitem{v}
Vertical tab-character (ASCII 11).
    \escapeitem{x23}
Hexadecimal specification of a character. \verb$23$ is just an example.
The `x' may be followed by a maximum of 2 hexadecimal digits.  The
closing \verb$\$ is optional.  The code \verb$\xa\3$ emits the character
10 (hexadecimal `a') followed by `3'.  The code \verb$\x201$ emits
32 (hexadecimal `20') followed by `1'.  According to ISO, the closing
\verb$\$ is obligatory and the number of digits is unlimited.  The
SWI-Prolog definition allows for ISO compatible specification, but
is compatible with other implementations.
    \escapeitem{40}
Octal character specification.  The rules and remarks for hexadecimal
specifications apply to octal specifications too, but the maximum
allowed number of octal digits is 3.
    \escapeitem{<character>}
Any character immediately preceded by a \chr{\} and not covered by the
above escape sequences is copied verbatim. Thus, \verb$'\\'$ is an atom
consisting of a single \chr{\} and \verb$'\''$ and \verb$''''$ both
describe the atom with a single~\verb$'$.

\end{description}

Character escaping is only available if the
\exam{current_prolog_flag(character_escapes, true)} is active (default).
See current_prolog_flag/2. Character escapes conflict with writef/2 in
two ways: \verb$\40$ is interpreted as decimal 40 by writef/2, but
character escapes handling by read has already interpreted as 32 (40
octal). Also, \fmtseq{\l} is translated to a single `l'. It is adviced
to use the more widely supported format/[2,3] predicate instead. If you
insist using writef, either switch \const{character_escapes} to
\const{false}, or use double \fmtseq{\\}, as in \verb$writef('\\l')$.


\subsubsection{Syntax for non-decimal numbers}

SWI-Prolog implements both Edinburgh and ISO representations for
non-decimal numbers. According to Edinburgh syntax, such numbers are
written as \exam{<radix>'<number>}, where <radix> is a number between 2
and 36. ISO defines binary, octal and hexadecimal numbers using
\exam{0{\em [bxo]}<number>}. For example: \verb$A is 0b100 \/ 0xf00$ is
a valid expression. Such numbers are always unsigned.


\section{System limits}				\label{sec:limits}

\subsection{Limits on memory areas}

SWI-Prolog has a number of memory areas which are only enlarged to a
certain limit. The default sizes for these areas should suffice for most
applications, but big applications may require larger ones. They are
modified by command line options. The table below shows these areas. The
first column gives the option name to modify the size of the area. The
option character is immediately followed by a number and optionally by a
\const{k} or \const{m}. With \const{k} or no unit indicator, the value
is interpreted in Kbytes (1024 bytes), with \const{m}, the value is
interpreted in Mbytes ($1024 \times 1024$ bytes).

The local-, global- and trail-stack are limited to 128 Mbytes on 32 bit
processors, or more in general to $\pow{2}{\mbox{bits-per-long} - 5}$
bytes.

\begin{table}
\begin{center}
\begin{tabular}{|c|c|l|p{5cm}|}
\hline
Option & Default & Area name & Description \\
\hline
\cmdlineoption{-L} & 2M & \bf local stack & The local stack is used to store
                       the execution environments of procedure
                       invocations. The space for an environment is
                       reclaimed when it fails, exits without leaving
                       choice points, the alternatives are cut of with
                       the !/0 predicate or no choice points have
                       been created since the invocation and the last
                       subclause is started (tail recursion optimisation). \\
\cmdlineoption{-G} & 4M & \bf global stack & The global stack is used
                       to store terms created during Prolog's
                       execution. Terms on this stack will be reclaimed
                       by backtracking to a point before the term
                       was created or by garbage collection (provided the
                       term is no longer referenced). \\
\cmdlineoption{-T} & 4M & \bf trail stack & The trail stack is used to store
                       assignments during execution.  Entries on this
                       stack remain alive until backtracking before the
                       point of creation or the garbage collector 
                       determines they are nor needed any longer. \\
\cmdlineoption{-A} & 1M & \bf argument stack & The argument stack is used to
                       store one of the intermediate code interpreter's 
                       registers. The amount of space needed on this
                       stack is determined entirely by the depth in
                       which terms are nested in the clauses that
                       constitute the program. Overflow is most likely
                       when using long strings in a clause. \\
\hline
\end{tabular}
\end{center}
    \caption{Memory areas}
    \label{tab:areas}
\end{table}


\subsubsection{The heap}				\label{sec:heap}

\index{stack,memory management}%
\index{memory,layout}%
With the heap, we refer to the memory area used by \funcref{malloc}{}
and friends. SWI-Prolog uses the area to store atoms, functors,
predicates and their clauses, records and other dynamic data.  As of
SWI-Prolog 2.8.5, no limits are imposed on the addresses returned by
\funcref{malloc}{} and friends.

On some machines, the runtime stacks described above are allocated using
`sparse allocation'. Virtual space upto the limit is claimed at startup
and committed and released while the area grows and shrinks.  On Win32
platform this is realised using \funcref{VirtualAlloc}{} and friends.
On Unix systems this is realised using \funcref{mmap}{}.

\subsection{Other Limits}

\begin{description}
    \item[Clauses]
Currently the following limitations apply to clauses.  The arity may
not be more than 1024 and the number of variables should be less than
65536.
    \item[Atoms and Strings]
SWI-Prolog has no limits on the sizes of atoms and strings.  read/1 and
its derivatives however normally limit the number of newlines in an atom
or string to 5 to improve error detection and recovery.  This can be
switched off with style_check/1.
    \item[Address space]
SWI-Prolog data is packed in a 32-bit word, which contains both type
and value information.  The size of the various memory areas is limited
to 128 Mb for each of the areas, except for the program heap, which is
not limited.
    \item[Integers]
Integers are 32-bit to the user, but integers upto the value of the
\const{max_tagged_integer} prolog-flag are represented more efficiently.
    \item[Floats]
Floating point numbers are represented as native double precision
floats, 64 bit IEEE on most machines.
\end{description}

\subsection{Reserved Names}

The boot compiler (see \cmdlineoption{-b} option) does not support the module
system.  As large parts of the system are written in Prolog itself
we need some way to avoid name clashes with the user's predicates, 
database keys, etc.  Like Edinburgh C-Prolog \cite{CPROLOG:manual} all
predicates, database keys, etc.\ that should be hidden from the user
start with a dollar (\chr{\$}) sign (see style_check/1).