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<h2 id="sec:flags"><a id="sec:2.11"><span class="sec-nr">2.11</span> <span class="sec-title">Environment 
Control (Prolog flags)</span></a></h2>

<a id="sec:flags"></a>

<p>The predicates <a id="idx:currentprologflag2:61"></a><a class="pred" href="flags.html#current_prolog_flag/2">current_prolog_flag/2</a> 
and <a id="idx:setprologflag2:62"></a><a class="pred" href="flags.html#set_prolog_flag/2">set_prolog_flag/2</a> 
allow the user to examine and modify the execution environment. It 
provides access to whether optional features are available on this 
version, operating system, foreign code environment, command line 
arguments, version, as well as runtime flags to control the runtime 
behaviour of certain predicates to achieve compatibility with other 
Prolog environments.

<dl class="latex">
<dt class="pubdef"><span class="pred-tag">[ISO]</span><a id="current_prolog_flag/2"><strong>current_prolog_flag</strong>(<var>?Key, 
-Value</var>)</a></dt>
<dd class="defbody">
The predicate <a id="idx:currentprologflag2:63"></a><a class="pred" href="flags.html#current_prolog_flag/2">current_prolog_flag/2</a> 
defines an interface to installation features: options compiled in, 
version, home, etc. With both arguments unbound, it will generate all 
defined Prolog flags. With `Key' instantiated, it unifies the value of 
the Prolog flag. Flag values are typed. Flags marked as <code>bool</code> 
can have the values <code>true</code> or
<code>false</code>. Some Prolog flags are not defined in all versions, 
which is normally indicated in the documentation below as <i>``if 
present and true''</i>. A boolean Prolog flag is true iff the Prolog 
flag is present <b>and</b> the <var>Value</var> is the atom <code>true</code>. 
Tests for such flags should be written as below:

<pre class="code">
        (   current_prolog_flag(windows, true)
        -&gt;  &lt;Do MS-Windows things&gt;
        ;   &lt;Do normal things&gt;
        )
</pre>

<p>Some Prolog flags are scoped to a source file. This implies that if 
they are set using a directive inside a file, the flag value encountered 
when loading of the file started is restored when loading of the file is 
completed. Currently, the following flags are scoped to the source file:
<a class="flag" href="flags.html#flag:generate_debug_info">generate_debug_info</a> 
and <a class="flag" href="flags.html#flag:optimise">optimise</a>.

<p>A new thread (see <a class="sec" href="threads.html">section 8</a>) <em>copies</em> 
all flags from the thread that created the new thread (its <em>parent</em>).<sup class="fn">13<span class="fn-text">This 
is implemented using the copy-on-write tecnhnique.</span></sup> As a 
consequence, modifying a flag inside a thread does not affect other 
threads.

<dl class="latex">
<dt class="pubdef"><a id="flag:access_level"><strong>access_level</strong>(<var>atom</var>, changeable)</a></dt>
<dd class="defbody">
This flag defines a normal `user' view (<code>user</code>, default) or a 
`system' view. In system view all system code is fully accessible as if 
it was normal user code. In user view, certain operations are not 
permitted and some details are kept invisible. We leave the exact 
consequences undefined, but, for example, system code can be traced 
using system access and system predicates can be redefined.</dd>
<dt class="pubdef"><a id="flag:address_bits"><strong>address_bits</strong>(<var>integer</var>)</a></dt>
<dd class="defbody">
Address size of the hosting machine. Typically 32 or 64. Except for the 
maximum stack limit, this has few implications to the user. See also the 
Prolog flag <a class="flag" href="flags.html#flag:arch">arch</a>.</dd>
<dt class="pubdef"><a id="flag:agc_margin"><strong>agc_margin</strong>(<var>integer</var>, changeable)</a></dt>
<dd class="defbody">
If this amount of atoms possible garbage atoms exist 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 <a class="func" href="foreigninclude.html#PL_register_atom()">PL_register_atom()</a>.<sup class="fn">14<span class="fn-text">Given 
that SWI-Prolog has no limit on the length of atoms, 10,000 atoms may 
still occupy a lot of memory. Applications using extremely large atoms 
may wish to call <a id="idx:garbagecollectatoms0:64"></a><a class="pred" href="memory.html#garbage_collect_atoms/0">garbage_collect_atoms/0</a> 
explicitly or lower the margin.</span></sup></dd>
<dt class="pubdef"><a id="flag:apple"><strong>apple</strong>(<var>bool</var>)</a></dt>
<dd class="defbody">
<a id="idx:MacOS:65"></a>If present and <code>true</code>, the operating 
system is MacOSX. Defined if the C compiler used to compile this version 
of SWI-Prolog defines
<code>__APPLE__</code>. Note that the <a class="flag" href="flags.html#flag:unix">unix</a> 
is also defined for MacOSX.</dd>
<dt class="pubdef"><a id="flag:allow_variable_name_as_functor"><strong>allow_variable_name_as_functor</strong>(<var>bool</var>, changeable)</a></dt>
<dd class="defbody">
If true (default is false), <code>Functor(arg)</code> is read as if it 
were written <code>'Functor'(arg)</code>. Some applications use the 
Prolog <a id="idx:read1:66"></a><a class="pred" href="termrw.html#read/1">read/1</a> 
predicate for reading an application-defined script language. In these 
cases, it is often difficult to explain to non-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 <a id="idx:readterm2:67"></a><a class="pred" href="termrw.html#read_term/2">read_term/2</a> 
using the option <code>variable_names</code> 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.</dd>
<dt class="pubdef"><a id="flag:argv"><strong>argv</strong>(<var>list</var>, changeable)</a></dt>
<dd class="defbody">
List is a list of atoms representing the application command line 
arguments. Application command line arguments are those that have
<em>not</em> been processed by Prolog during its initialization. Note 
that Prolog's argument processing stops at <code>--</code> or the first 
non-option argument. See also <a class="flag" href="flags.html#flag:os_argv">os_argv</a>.<sup class="fn">15<span class="fn-text">Prior 
to version 6.5.2, <a class="flag" href="flags.html#flag:argv">argv</a> 
was defined as <a class="flag" href="flags.html#flag:os_argv">os_argv</a> 
is now. The change was made for compatibility reasone and because the 
current definition is more practical.</span></sup></dd>
<dt class="pubdef"><a id="flag:arch"><strong>arch</strong>(<var>atom</var>)</a></dt>
<dd class="defbody">
Identifier for the hardware and operating system SWI-Prolog is running 
on. Used to select foreign files for the right architecture. See also
<a class="sec" href="foreignlink.html">section 9.2.3</a> and <a id="idx:filesearchpath2:68"></a><a class="pred" href="consulting.html#file_search_path/2">file_search_path/2</a>.</dd>
<dt class="pubdef"><a id="flag:associated_file"><strong>associated_file</strong>(<var>atom</var>)</a></dt>
<dd class="defbody">
Set if Prolog was started with a prolog file as argument. Used by e.g., <a id="idx:edit0:69"></a><a class="pred" href="edit.html#edit/0">edit/0</a> 
to edit the initial file.</dd>
<dt class="pubdef"><a id="flag:autoload"><strong>autoload</strong>(<var>bool</var>, changeable)</a></dt>
<dd class="defbody">
If <code>true</code> (default) autoloading of library functions is 
enabled.</dd>
<dt class="pubdef"><a id="flag:backquoted_string"><strong>backquoted_string</strong>(<var>bool</var>, changeable)</a></dt>
<dd class="defbody">
If <code>true</code> (default <code>false</code>), read translates text 
between backquotes into a string object (see <a class="sec" href="strings.html">section 
4.24</a>). This flag is mainly for compatibility with LPA Prolog.</dd>
<dt class="pubdef"><a id="flag:bounded"><strong>bounded</strong>(<var>bool</var>)</a></dt>
<dd class="defbody">
ISO Prolog flag. If <code>true</code>, integer representation is bound 
by
<a class="flag" href="flags.html#flag:min_integer">min_integer</a> and <a class="flag" href="flags.html#flag:max_integer">max_integer</a>. 
If <code>false</code> integers can be arbitrarily large and the <a class="flag" href="flags.html#flag:min_integer">min_integer</a> 
and
<a class="flag" href="flags.html#flag:max_integer">max_integer</a> are 
not present. See <a class="sec" href="arith.html">section 4.27.2.1</a>.</dd>
<dt class="pubdef"><a id="flag:break_level"><strong>break_level</strong>(<var>integer</var>)</a></dt>
<dd class="defbody">
Current break-level. The initial top level (started with
<strong>-t</strong>) has value 0. See <a id="idx:break0:70"></a><a class="pred" href="toplevel.html#break/0">break/0</a>. 
This flag is absent from threads that are not running a top-level loop.</dd>
<dt class="pubdef"><a id="flag:c_cc"><strong>c_cc</strong>(<var>atom</var>, changeable)</a></dt>
<dd class="defbody">
Name of the C compiler used to compile SWI-Prolog. Normally either gcc 
or cc. See <a class="sec" href="plld.html">section 9.5</a>.</dd>
<dt class="pubdef"><a id="flag:c_cflags"><strong>c_cflags</strong>(<var>atom</var>, changeable)</a></dt>
<dd class="defbody">
CFLAGS used to compile SWI-Prolog. See <a class="sec" href="plld.html">section 
9.5</a>.</dd>
<dt class="pubdef"><a id="flag:c_ldflags"><strong>c_ldflags</strong>(<var>atom</var>, changeable)</a></dt>
<dd class="defbody">
LDFLAGS used to link SWI-Prolog. See <a class="sec" href="plld.html">section 
9.5</a>.</dd>
<dt class="pubdef"><a id="flag:c_libs"><strong>c_libs</strong>(<var>atom</var>, changeable)</a></dt>
<dd class="defbody">
Libraries needed to link executables that embed SWI-Prolog. Typically
<code>-lswipl</code> if the SWI-Prolog kernel is a shared (DLL). If the 
SWI-Prolog kernel is in a static library, this flag also contains the 
dependencies.</dd>
<dt class="pubdef"><a id="flag:c_libplso"><strong>c_libplso</strong>(<var>atom</var>, changeable)</a></dt>
<dd class="defbody">
Libraries needed to link extensions (shared object, DLL) to SWI-Prolog. 
Typically empty on ELF systems and <code>-lswipl</code> on COFF-based 
systems. See <a class="sec" href="plld.html">section 9.5</a>.</dd>
<dt class="pubdef"><a id="flag:char_conversion"><strong>char_conversion</strong>(<var>bool</var>, changeable)</a></dt>
<dd class="defbody">
Determines whether character conversion takes place while reading terms. 
See also <a id="idx:charconversion2:71"></a><a class="pred" href="charconv.html#char_conversion/2">char_conversion/2</a>.</dd>
<dt class="pubdef"><a id="flag:character_escapes"><strong>character_escapes</strong>(<var>bool</var>, changeable)</a></dt>
<dd class="defbody">
If <code>true</code> (default), <a id="idx:read1:72"></a><a class="pred" href="termrw.html#read/1">read/1</a> 
interprets <code>\</code> escape sequences in quoted atoms and strings. 
May be changed. This flag is local to the module in which it is changed.</dd>
<dt class="pubdef"><a id="flag:colon_sets_calling_context"><strong>colon_sets_calling_context</strong>(<var>bool</var>, changeable)</a></dt>
<dd class="defbody">
Using the construct &lt;<var>module</var>&gt;:&lt;<var>goal</var>&gt; 
sets the <em>calling context</em> for executing &lt;<var>goal</var>&gt;. 
This flag is defined by ISO/IEC 13211-2 (Prolog modules standard). See <a class="sec" href="modules.html">section 
5</a>.</dd>
<dt class="pubdef"><a id="flag:color_term"><strong>color_term</strong>(<var>bool</var>, changeable)</a></dt>
<dd class="defbody">
This flag is managed by library <code>library(ansi_term)</code>, which 
is loaded at startup if the two conditions below are both true. Note 
that this implies that setting this flag to <code>false</code> from the 
system or personal initialization file (see <a class="sec" href="initfile.html">section 
2.2</a> disables colored output. The predicate <a id="idx:messageproperty2:73"></a><a class="pred" href="exception.html#message_property/2">message_property/2</a> 
can be used to control the actual color scheme depending in the message 
type passed to
<a id="idx:printmessage2:74"></a><a class="pred" href="exception.html#print_message/2">print_message/2</a>.

<p>
<ul class="latex">
<li><code>stream_property(current_output, tty(true))</code>
<li><code>\+ current_prolog_flag(color_term, false)</code>
</ul>
</dd>
<dt class="pubdef"><a id="flag:compile_meta_arguments"><strong>compile_meta_arguments</strong>(<var>atom</var>, changeable)</a></dt>
<dd class="defbody">
Experimental flag that controls compilation of arguments passed to 
meta-calls marked `0' or `<code><code>^</code></code>' (see <a id="idx:metapredicate1:75"></a><a class="pred" href="metapred.html#meta_predicate/1">meta_predicate/1</a>). 
Supported values are:

<dl class="latex">
<dt><strong>false</strong></dt>
<dd class="defbody">
(default). Meta-arguments are passed verbatim.
</dd>
<dt><strong>control</strong></dt>
<dd class="defbody">
Compile meta-arguments that contain control structures ((A,B), (A;B), 
(A-&gt;B;C), etc.). If not compiled at compile time, such arguments are 
compiled to a temporary clause before execution. Using this option 
enhances performance of processing complex meta-goals that are known at 
compile time.
</dd>
<dt><strong>true</strong></dt>
<dd class="defbody">
Also compile references to normal user predicates. This harms 
performance (a little), but enhances the power of poor-mens consistency 
check used by <a id="idx:make0:76"></a><a class="pred" href="consulting.html#make/0">make/0</a> 
and implemented by <a id="idx:listundefined0:77"></a><a class="pred" href="check.html#list_undefined/0">list_undefined/0</a>.
</dd>
<dt><strong>always</strong></dt>
<dd class="defbody">
Always create an intermediate clause, even for system predicates. This 
prepares for replacing the normal head of the generated predicate with a 
special reference (similar to database references as used by, e.g.,
<a id="idx:assert2:78"></a><a class="pred" href="db.html#assert/2">assert/2</a>) 
that provides direct access to the executable code, thus avoiding 
runtime lookup of predicates for meta-calling.
</dd>
</dl>

</dd>
<dt class="pubdef"><a id="flag:compiled_at"><strong>compiled_at</strong>(<var>atom</var>)</a></dt>
<dd class="defbody">
Describes when the system has been compiled. Only available if the C 
compiler used to compile SWI-Prolog provides the __DATE__ and __TIME__ 
macros.</dd>
<dt class="pubdef"><a id="flag:console_menu"><strong>console_menu</strong>(<var>bool</var>)</a></dt>
<dd class="defbody">
Set to <code>true</code> in <b>swipl-win.exe</b> to indicate that the 
console supports menus. See also <a class="sec" href="system.html">section 
4.34.3</a>.</dd>
<dt class="pubdef"><a id="flag:cpu_count"><strong>cpu_count</strong>(<var>integer</var>, changeable)</a></dt>
<dd class="defbody">
Number of physical CPUs or cores in the system. The flag is marked 
read-write both to allow pretending the system has more or less 
processors. See also <a id="idx:threadsetconcurrency2:79"></a><a class="pred" href="threadcreate.html#thread_setconcurrency/2">thread_setconcurrency/2</a> 
and the library
<code>library(thread)</code>. This flag is not available on systems 
where we do not know how to get the number of CPUs. This flag is not 
included in a saved state (see <a id="idx:qsaveprogram1:80"></a><a class="pred" href="runtime.html#qsave_program/1">qsave_program/1</a>).</dd>
<dt class="pubdef"><a id="flag:dde"><strong>dde</strong>(<var>bool</var>)</a></dt>
<dd class="defbody">
Set to <code>true</code> if this instance of Prolog supports DDE as 
described in <a class="sec" href="DDE.html">section 4.42</a>.</dd>
<dt class="pubdef"><a id="flag:debug"><strong>debug</strong>(<var>bool</var>, changeable)</a></dt>
<dd class="defbody">
Switch debugging mode on/off. If debug mode is activated the system 
traps encountered spy points (see <a id="idx:spy1:81"></a><a class="pred" href="debugger.html#spy/1">spy/1</a>) 
and trace points (see <a id="idx:trace1:82"></a><a class="pred" href="debugger.html#trace/1">trace/1</a>). 
In addition, last-call optimisation is disabled and the system is more 
conservative in destroying choice points to simplify debugging.

<p>Disabling these optimisations can cause the system to run out of 
memory on programs that behave correctly if debug mode is off.</dd>
<dt class="pubdef"><a id="flag:debug_on_error"><strong>debug_on_error</strong>(<var>bool</var>, changeable)</a></dt>
<dd class="defbody">
If <code>true</code>, start the tracer after an error is detected. 
Otherwise just continue execution. The goal that raised the error will 
normally fail. See also <a id="idx:fileerrors2:83"></a><span class="pred-ext">fileerrors/2</span> 
and the Prolog flag <code>report_error</code>. May be changed. Default 
is <code>true</code>, except for the runtime version.</dd>
<dt class="pubdef"><a id="flag:debugger_print_options"><strong>debugger_print_options</strong>(<var>term</var>, changeable)</a></dt>
<dd class="defbody">
This argument is given as option-list to <a id="idx:writeterm2:84"></a><a class="pred" href="termrw.html#write_term/2">write_term/2</a> 
for printing goals by the debugger. Modified by the `w', `p' and `&lt;<var>N</var>&gt; 
d' commands of the debugger. Default is <code>[quoted(true), 
portray(true), max_depth(10), attributes(portray)]</code>.</dd>
<dt class="pubdef"><a id="flag:debugger_show_context"><strong>debugger_show_context</strong>(<var>bool</var>, changeable)</a></dt>
<dd class="defbody">
If <code>true</code>, show the context module while printing a 
stack-frame in the tracer. Normally controlled using the `C' option of 
the tracer.</dd>
<dt class="pubdef"><a id="flag:dialect"><strong>dialect</strong>(<var>atom</var>)</a></dt>
<dd class="defbody">
Fixed to <code>swi</code>. The code below is a reliable and portable way 
to detect SWI-Prolog.

<pre class="code">
is_dialect(swi) :-
        catch(current_prolog_flag(dialect, swi), _, fail).
</pre>

</dd>
<dt class="pubdef"><a id="flag:double_quotes"><strong>double_quotes</strong>(<var>codes,chars,atom,string</var>, changeable)</a></dt>
<dd class="defbody">
This flag determines how double quoted strings are read by Prolog and is 
---like <a class="flag" href="flags.html#flag:character_escapes">character_escapes</a>--- 
maintained for each module. If
<code>codes</code> (default), a list of character codes is returned, if
<code>chars</code> a list of one-character atoms, if <code>atom</code> 
double quotes are the same as single quotes and finally, <code>string</code> 
reads the text into a Prolog string (see <a class="sec" href="strings.html">section 
4.24</a>). See also
<a id="idx:atomchars2:85"></a><a class="pred" href="manipatom.html#atom_chars/2">atom_chars/2</a> 
and <a id="idx:atomcodes2:86"></a><a class="pred" href="manipatom.html#atom_codes/2">atom_codes/2</a>.</dd>
<dt class="pubdef"><a id="flag:editor"><strong>editor</strong>(<var>atom</var>, changeable)</a></dt>
<dd class="defbody">
Determines the editor used by <a id="idx:edit1:87"></a><a class="pred" href="edit.html#edit/1">edit/1</a>. 
See <a class="sec" href="listing.html">section 4.5</a> for details on 
selecting the editor used.</dd>
<dt class="pubdef"><a id="flag:emacs_inferior_process"><strong>emacs_inferior_process</strong>(<var>bool</var>)</a></dt>
<dd class="defbody">
If true, SWI-Prolog is running as an <em>inferior process</em> of 
(GNU/X-)Emacs. SWI-Prolog assumes this is the case if the environment 
variable <code>EMACS</code> is <code>t</code> and <code>INFERIOR</code> 
is <code>yes</code>.</dd>
<dt class="pubdef"><a id="flag:encoding"><strong>encoding</strong>(<var>atom</var>, changeable)</a></dt>
<dd class="defbody">
Default encoding used for opening files in <code>text</code> mode. The 
initial value is deduced from the environment. See <a class="sec" href="widechars.html">section 
2.18.1</a> for details.</dd>
<dt class="pubdef"><a id="flag:executable"><strong>executable</strong>(<var>atom</var>)</a></dt>
<dd class="defbody">
Pathname of the running executable. Used by <a id="idx:qsaveprogram2:88"></a><a class="pred" href="runtime.html#qsave_program/2">qsave_program/2</a> 
as default emulator.</dd>
<dt class="pubdef"><a id="flag:exit_status"><strong>exit_status</strong>(<var>integer</var>)</a></dt>
<dd class="defbody">
Set by <a id="idx:halt1:89"></a><a class="pred" href="toplevel.html#halt/1">halt/1</a> 
to its argument, making the exit status available to hooks registered 
with <a id="idx:athalt1:90"></a><a class="pred" href="consulting.html#at_halt/1">at_halt/1</a>.</dd>
<dt class="pubdef"><a id="flag:file_name_variables"><strong>file_name_variables</strong>(<var>bool</var>, changeable)</a></dt>
<dd class="defbody">
If <code>true</code> (default <code>false</code>), expand <code>$<var>varname</var></code> 
and <code><code>~</code></code> in arguments of built-in predicates that 
accept a file name (<a id="idx:open3:91"></a><a class="pred" href="IO.html#open/3">open/3</a>, <a id="idx:existsfile1:92"></a><a class="pred" href="files.html#exists_file/1">exists_file/1</a>, <a id="idx:accessfile2:93"></a><a class="pred" href="files.html#access_file/2">access_file/2</a>, 
etc.). The predicate
<a id="idx:expandfilename2:94"></a><a class="pred" href="files.html#expand_file_name/2">expand_file_name/2</a> 
can be used to expand environment variables and wildcard patterns. This 
Prolog flag is intended for backward compatibility with older versions 
of SWI-Prolog.</dd>
<dt class="pubdef"><a id="flag:gc"><strong>gc</strong>(<var>bool</var>, changeable)</a></dt>
<dd class="defbody">
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.</dd>
<dt class="pubdef"><a id="flag:generate_debug_info"><strong>generate_debug_info</strong>(<var>bool</var>, changeable)</a></dt>
<dd class="defbody">
If <code>true</code> (default) generate code that can be debugged using
<a id="idx:trace0:95"></a><a class="pred" href="debugger.html#trace/0">trace/0</a>, <a id="idx:spy1:96"></a><a class="pred" href="debugger.html#spy/1">spy/1</a>, 
etc. Can be set to <code>false</code> using the
<strong>-nodebug</strong>. This flag is scoped within a source file. 
Many of the libraries have
<code>:- set_prolog_flag(generate_debug_info, false)</code> to hide 
their details from a normal trace.<sup class="fn">16<span class="fn-text">In 
the current implementation this only causes a flag to be set on the 
predicate that causes children to be hidden from the debugger. The name 
anticipates further changes to the compiler.</span></sup></dd>
<dt class="pubdef"><a id="flag:gmp_version"><strong>gmp_version</strong>(<var>integer</var>)</a></dt>
<dd class="defbody">
If Prolog is linked with GMP, this flag gives the major version of the 
GMP library used. See also <a class="sec" href="foreigninclude.html">section 
9.4.8</a>.</dd>
<dt class="pubdef"><a id="flag:gui"><strong>gui</strong>(<var>bool</var>)</a></dt>
<dd class="defbody">
Set to <code>true</code> if XPCE is around and can be used for graphics.</dd>
<dt class="pubdef"><a id="flag:history"><strong>history</strong>(<var>integer</var>, changeable)</a></dt>
<dd class="defbody">
If <var><var>integer</var>&gt; 0</var>, support Unix <b>csh(1)</b>-like 
history as described in <a class="sec" href="history.html">section 2.7</a>. 
Otherwise, only support reusing commands through the command line 
editor. The default is to set this Prolog flag to 0 if a command line 
editor is provided (see Prolog flag <a class="flag" href="flags.html#flag:readline">readline</a>) 
and 15 otherwise.</dd>
<dt class="pubdef"><a id="flag:home"><strong>home</strong>(<var>atom</var>)</a></dt>
<dd class="defbody">
SWI-Prolog's notion of the home directory. SWI-Prolog uses its home 
directory to find its startup file as
<code>&lt;<var>home</var>&gt;/boot32.prc</code> (32-bit machines) or
<code>&lt;<var>home</var>&gt;/boot64.prc</code> (64-bit machines) and to 
find its library as
<code>&lt;<var>home</var>&gt;/library</code>.</dd>
<dt class="pubdef"><a id="flag:hwnd"><strong>hwnd</strong>(<var>integer</var>)</a></dt>
<dd class="defbody">
In <b>swipl-win.exe</b>, this refers to the MS-Windows window handle of 
the console window.</dd>
<dt class="pubdef"><a id="flag:integer_rounding_function"><strong>integer_rounding_function</strong>(<var>down,toward_zero</var>)</a></dt>
<dd class="defbody">
ISO Prolog flag describing rounding by <code>//</code> and <code>rem</code> 
arithmetic functions. Value depends on the C compiler used.</dd>
<dt class="pubdef"><a id="flag:iso"><strong>iso</strong>(<var>bool</var>, changeable)</a></dt>
<dd class="defbody">
Include some weird ISO compatibility that is incompatible with normal 
SWI-Prolog behaviour. Currently it has the following effect:
<ul class="latex">
<li>The <code><code>/</code>/2</code> (float division) <em>always</em> 
returns a float, even if applied to integers that can be divided.
<li>In the standard order of terms (see <a class="sec" href="compare.html">section 
4.7.1</a>), all floats are before all integers.
<li><a id="idx:atomlength2:97"></a><a class="pred" href="manipatom.html#atom_length/2">atom_length/2</a> 
yields a type error if the first argument is a number.
<li><a id="idx:clause23:98"></a><span class="pred-ext">clause/[2,3]</span> 
raises a permission error when accessing static predicates.
<li><a id="idx:abolish12:99"></a><span class="pred-ext">abolish/[1,2]</span> 
raises a permission error when accessing static predicates.
<li>Syntax is closer to the ISO standard:
<ul class="latex">
<li>Unquoted commas and bars appearing as atoms are not allowed. Instead 
of
<code>f(,,a)</code> now write <code>f(',',a)</code>. Unquoted commas can 
only be used to separate arguments in functional notation and list 
notation, and as a conjunction operator. Unquoted bars can only appear 
within lists to separate head and tail, like <code>[Head|Tail]</code>, 
and as infix operator for alternation in grammar rules, like <code>a --&gt; 
b | c.</code>
<li>Within functional notation and list notation terms must have 
priority below 1000. That means that rules and control constructs 
appearing as arguments need bracketing. A term like <code>[a :- b, c].</code> 
must now be disambiguated to mean <code>[(a :- b), c].</code> or <code>[(a 
:- b, c)].</code>
<li>Operators appearing as operands must be bracketed. Instead of <code>X 
== -, true.</code> write <code>X == (-), true.</code> Currently, this is 
not entirely enforced.
<li>Backslash-escaped newlines are interpreted according to the ISO 
standard. See <a class="sec" href="syntax.html">section 2.15.1.2</a>.
</ul>
</ul>
</dd>
<dt class="pubdef"><a id="flag:large_files"><strong>large_files</strong>(<var>bool</var>)</a></dt>
<dd class="defbody">
If present and <code>true</code>, SWI-Prolog has been compiled with
<em>large file support</em> (LFS) and is capable of accessing files 
larger than 2GB on 32-bit hardware. Large file support is default on 
installations built using <b>configure</b> that support it and may be 
switched off using the configure option <code>--disable-largefile</code>.</dd>
<dt class="pubdef"><a id="flag:last_call_optimisation"><strong>last_call_optimisation</strong>(<var>bool</var>, changeable)</a></dt>
<dd class="defbody">
Determines whether or not last-call optimisation is enabled. Normally 
the value of this flag is the negation of the <a class="flag" href="flags.html#flag:debug">debug</a> 
flag. As programs may run out of stack if last-call optimisation is 
omitted, it is sometimes necessary to enable it during debugging.</dd>
<dt class="pubdef"><a id="flag:max_arity"><strong>max_arity</strong>(<var>unbounded</var>)</a></dt>
<dd class="defbody">
ISO Prolog flag describing there is no maximum arity to compound terms.</dd>
<dt class="pubdef"><a id="flag:max_integer"><strong>max_integer</strong>(<var>integer</var>)</a></dt>
<dd class="defbody">
Maximum integer value if integers are <em>bounded</em>. See also the 
flag <a class="flag" href="flags.html#flag:bounded">bounded</a> and <a class="sec" href="arith.html">section 
4.27.2.1</a>.</dd>
<dt class="pubdef"><a id="flag:max_tagged_integer"><strong>max_tagged_integer</strong>(<var>integer</var>)</a></dt>
<dd class="defbody">
Maximum integer value represented as a `tagged' value. Tagged integers 
require one word storage. Larger integers are represented as `indirect 
data' and require significantly more space.</dd>
<dt class="pubdef"><a id="flag:min_integer"><strong>min_integer</strong>(<var>integer</var>)</a></dt>
<dd class="defbody">
Minimum integer value if integers are <em>bounded</em>. See also the 
flag <a class="flag" href="flags.html#flag:bounded">bounded</a> and <a class="sec" href="arith.html">section 
4.27.2.1</a>.</dd>
<dt class="pubdef"><a id="flag:min_tagged_integer"><strong>min_tagged_integer</strong>(<var>integer</var>)</a></dt>
<dd class="defbody">
Start of the tagged-integer value range.</dd>
<dt class="pubdef"><a id="flag:occurs_check"><strong>occurs_check</strong>(<var>atom</var>, changeable)</a></dt>
<dd class="defbody">
This flag controls unification that creates an infinite tree (also 
called <em>cyclic term</em>) and can have three values. Using
<code>false</code> (default), unification succeeds, creating an infinite 
tree. Using <code>true</code>, unification behaves as <a id="idx:unifywithoccurscheck2:100"></a><a class="pred" href="compare.html#unify_with_occurs_check/2">unify_with_occurs_check/2</a>, 
failing silently. Using <code>error</code>, an attempt to create a 
cyclic term results in an <code>occurs_check</code> exception. The 
latter is intended for debugging unintentional creations of cyclic 
terms. Note that this flag is a global flag modifying fundamental 
behaviour of Prolog. Changing the flag from its default may cause 
libraries to stop functioning properly.</dd>
<dt class="pubdef"><a id="flag:open_shared_object"><strong>open_shared_object</strong>(<var>bool</var>)</a></dt>
<dd class="defbody">
If true, <a id="idx:opensharedobject2:101"></a><a class="pred" href="foreignlink.html#open_shared_object/2">open_shared_object/2</a> 
and friends are implemented, providing access to shared libraries (<code>.so</code> 
files) or dynamic link libraries (<code>.DLL</code> files).</dd>
<dt class="pubdef"><a id="flag:optimise"><strong>optimise</strong>(<var>bool</var>, changeable)</a></dt>
<dd class="defbody">
If <code>true</code>, compile in optimised mode. The initial value is
<code>true</code> if Prolog was started with the <strong>-O</strong> 
command line option. The <a class="flag" href="flags.html#flag:optimise">optimise</a> 
flag is scoped to a source file.

<p>Currently optimised compilation implies compilation of arithmetic, 
and deletion of redundant <a id="idx:true0:102"></a><a class="pred" href="control.html#true/0">true/0</a> 
that may result from <a id="idx:expandgoal2:103"></a><a class="pred" href="consulting.html#expand_goal/2">expand_goal/2</a>.

<p>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
<a id="idx:dynamic1:104"></a><a class="pred" href="dynamic.html#dynamic/1">dynamic/1</a>).</dd>
<dt class="pubdef"><a id="flag:os_argv"><strong>os_argv</strong>(<var>list</var>, changeable)</a></dt>
<dd class="defbody">
List is a list of atoms representing the command line arguments used to 
invoke SWI-Prolog. Please note that <b>all</b> arguments are included in 
the list returned. See <a class="flag" href="flags.html#flag:argv">argv</a> 
to get the application options.</dd>
<dt class="pubdef"><a id="flag:pid"><strong>pid</strong>(<var>int</var>)</a></dt>
<dd class="defbody">
Process identifier of the running Prolog process. Existence of this flag 
is implementation-defined.</dd>
<dt class="pubdef"><a id="flag:pipe"><strong>pipe</strong>(<var>bool</var>, changeable)</a></dt>
<dd class="defbody">
If true, <code>open(pipe(command), mode, Stream)</code>, etc. are 
supported. Can be changed to disable the use of pipes in applications 
testing this feature. Not recommended.</dd>
<dt class="pubdef"><a id="flag:prompt_alternatives_on"><strong>prompt_alternatives_on</strong>(<var>atom</var>, changeable)</a></dt>
<dd class="defbody">
<a id="idx:promptalternatives:105"></a>Determines prompting for 
alternatives in the Prolog top level. Default is
<code>determinism</code>, which implies the system prompts for 
alternatives if the goal succeeded while leaving choice points. Many 
classical Prolog systems behave as <code>groundness</code>: they prompt 
for alternatives if and only if the query contains variables.</dd>
<dt class="pubdef"><a id="flag:qcompile"><strong>qcompile</strong>(<var>atom</var>, changeable)</a></dt>
<dd class="defbody">
This option provides the default for the <code>qcompile(+Atom)</code> 
option of <a id="idx:loadfiles2:106"></a><a class="pred" href="consulting.html#load_files/2">load_files/2</a>.</dd>
<dt class="pubdef"><a id="flag:readline"><strong>readline</strong>(<var>bool</var>)</a></dt>
<dd class="defbody">
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 swipl-win.exe version of SWI-Prolog, which realises a 
subset of the readline functionality.</dd>
<dt class="pubdef"><a id="flag:resource_database"><strong>resource_database</strong>(<var>atom</var>)</a></dt>
<dd class="defbody">
Set to the absolute filename of the attached state. Typically this is 
the file <code>boot32.prc</code>, the file specified with <strong>-x</strong> 
or the running executable. See also <a id="idx:resource3:107"></a><a class="pred" href="useresource.html#resource/3">resource/3</a>.</dd>
<dt class="pubdef"><a id="flag:report_error"><strong>report_error</strong>(<var>bool</var>, changeable)</a></dt>
<dd class="defbody">
If <code>true</code>, print error messages; otherwise suppress them. May 
be changed. See also the <a class="flag" href="flags.html#flag:debug_on_error">debug_on_error</a> 
Prolog flag. Default is <code>true</code>, except for the runtime 
version.</dd>
<dt class="pubdef"><a id="flag:runtime"><strong>runtime</strong>(<var>bool</var>)</a></dt>
<dd class="defbody">
If present and <code>true</code>, SWI-Prolog is compiled with 
-DO_RUNTIME, disabling various useful development features (currently 
the tracer and profiler).</dd>
<dt class="pubdef"><a id="flag:sandboxed_load"><strong>sandboxed_load</strong>(<var>bool</var>, changeable)</a></dt>
<dd class="defbody">
If <code>true</code> (default <code>false</code>), <a id="idx:loadfiles2:108"></a><a class="pred" href="consulting.html#load_files/2">load_files/2</a> 
calls hooks to allow library(sandbox) to verify the safety of 
directives.</dd>
<dt class="pubdef"><a id="flag:saved_program"><strong>saved_program</strong>(<var>bool</var>)</a></dt>
<dd class="defbody">
If present and <code>true</code>, Prolog has been started from a state 
saved with <a id="idx:qsaveprogram12:109"></a><span class="pred-ext">qsave_program/[1,2]</span>.</dd>
<dt class="pubdef"><a id="flag:shared_object_extension"><strong>shared_object_extension</strong>(<var>atom</var>)</a></dt>
<dd class="defbody">
Extension used by the operating system for shared objects. <code>.so</code> 
for most Unix systems and <code>.dll</code> for Windows. Used for 
locating files using the <code>file_type</code> <code>executable</code>. 
See also
<a id="idx:absolutefilename3:110"></a><a class="pred" href="files.html#absolute_file_name/3">absolute_file_name/3</a>.</dd>
<dt class="pubdef"><a id="flag:shared_object_search_path"><strong>shared_object_search_path</strong>(<var>atom</var>)</a></dt>
<dd class="defbody">
Name of the environment variable used by the system to search for shared 
objects.</dd>
<dt class="pubdef"><a id="flag:signals"><strong>signals</strong>(<var>bool</var>)</a></dt>
<dd class="defbody">
Determine whether Prolog is handling signals (software interrupts). This 
flag is <code>false</code> if the hosting OS does not support signal 
handling or the command line option <strong>-nosignals</strong> is 
active. See
<a class="sec" href="foreigninclude.html">section 9.4.21.1</a> for 
details.</dd>
<dt class="pubdef"><a id="flag:stream_type_check"><strong>stream_type_check</strong>(<var>atom</var>, changeable)</a></dt>
<dd class="defbody">
Defines whether and how strictly the system validates that byte I/O 
should not be applied to text streams and text I/O should not be applied 
to binary streams. Values are <code>false</code> (no checking), <code>true</code> 
(full checking) and <code>loose</code>. Using checking mode <code>loose</code> 
(default), the system accepts byte I/O from text stream that use ISO 
Latin-1 encoding and accepts writing text to binary streams.</dd>
<dt class="pubdef"><a id="flag:system_thread_id"><strong>system_thread_id</strong>(<var>int</var>)</a></dt>
<dd class="defbody">
Available in multithreaded version (see <a class="sec" href="threads.html">section 
8</a>) where the operating system provides system-wide integer thread 
identifiers. The integer is the thread identifier used by the operating 
system for the calling thread. See also <a id="idx:threadself1:111"></a><a class="pred" href="threadcreate.html#thread_self/1">thread_self/1</a>.</dd>
<dt class="pubdef"><a id="flag:timezone"><strong>timezone</strong>(<var>integer</var>)</a></dt>
<dd class="defbody">
Offset in seconds west of GMT of the current time zone. Set at 
initialization time from the <code>timezone</code> variable associated 
with the POSIX tzset() function. See also <a id="idx:converttime2:112"></a><span class="pred-ext">convert_time/2</span>.</dd>
<dt class="pubdef"><a id="flag:toplevel_print_anon"><strong>toplevel_print_anon</strong>(<var>bool</var>, changeable)</a></dt>
<dd class="defbody">
If <code>true</code>, top-level variables starting with an underscore (<code>_</code>) 
are printed normally. If <code>false</code> they are hidden. This may be 
used to hide bindings in complex queries from the top level.</dd>
<dt class="pubdef"><a id="flag:toplevel_print_factorized"><strong>toplevel_print_factorized</strong>(<var>bool</var>, changeable)</a></dt>
<dd class="defbody">
If <code>true</code> (default <code>false</code>) show the internal 
sharing of subterms in the answer substitution. The example below 
reveals internal sharing of leaf nodes in <em>red-black trees</em> as 
implemented by the
<code>library(rbtrees)</code> predicate <a id="idx:rbnew1:113"></a><span class="pred-ext">rb_new/1</span>:

<pre class="code">
?- set_prolog_flag(toplevel_print_factorized, true).
?- rb_new(X).
X = t(_S1, _S1), % where
    _S1 = black('', _G387, _G388, '').
</pre>

<p>If this flag is <code>false</code>, the <code>% where</code> notation 
is still used to indicate cycles as illustrated below. This example also 
shows that the implementation reveals the internal cycle length, and <em>not</em> 
the minimal cycle length. Cycles of different length are 
indistinguishable in Prolog (as illustrated by <code>S == R</code>).

<pre class="code">
?- S = s(S), R = s(s(R)), S == R.
S = s(S),
R = s(s(R)).
</pre>

</dd>
<dt class="pubdef"><a id="flag:toplevel_print_options"><strong>toplevel_print_options</strong>(<var>term</var>, changeable)</a></dt>
<dd class="defbody">
This argument is given as option-list to <a id="idx:writeterm2:114"></a><a class="pred" href="termrw.html#write_term/2">write_term/2</a> 
for printing results of queries. Default is <code>[quoted(true), 
portray(true), max_depth(10), attributes(portray)]</code>.</dd>
<dt class="pubdef"><a id="flag:toplevel_prompt"><strong>toplevel_prompt</strong>(<var>atom</var>, changeable)</a></dt>
<dd class="defbody">
Define the prompt that is used by the interactive top level. The 
following <code>~</code> (tilde) sequences are replaced:

<p><table class="latex frame-hsides center">
<tr><td><code><code>~</code></code>m</td><td><em>Type in</em> module if 
not <code>user</code> (see <a id="idx:module1:115"></a><a class="pred" href="mtoplevel.html#module/1">module/1</a>) </td></tr>
<tr><td><code><code>~</code></code>l</td><td><em>Break level</em> if not 
0 (see <a id="idx:break0:116"></a><a class="pred" href="toplevel.html#break/0">break/0</a>) </td></tr>
<tr><td><code><code>~</code></code>d</td><td><em>Debugging state</em> if 
not normal execution (see <a id="idx:debug0:117"></a><a class="pred" href="debugger.html#debug/0">debug/0</a>, <a id="idx:trace0:118"></a><a class="pred" href="debugger.html#trace/0">trace/0</a>) </td></tr>
<tr><td><code><code>~</code></code>!</td><td><em>History event</em> if 
history is enabled (see flag <a class="flag" href="flags.html#flag:history">history</a>) </td></tr>
</table>
</dd>
<dt class="pubdef"><a id="flag:toplevel_var_size"><strong>toplevel_var_size</strong>(<var>int</var>, changeable)</a></dt>
<dd class="defbody">
Maximum size counted in literals of a term returned as a binding for a 
variable in a top-level query that is saved for re-use using the
<code><code>$</code></code> variable reference. See <a class="sec" href="topvars.html">section 
2.8</a>.</dd>
<dt class="pubdef"><a id="flag:trace_gc"><strong>trace_gc</strong>(<var>bool</var>, changeable)</a></dt>
<dd class="defbody">
If <code>true</code> (default <code>false</code>), garbage collections 
and stack-shifts will be reported on the terminal. May be changed. 
Values are reported in bytes as <var>G</var>+<var>T</var>, where <var>G</var> 
is the global stack value and <var>T</var> the trail stack value. 
`Gained' describes the number of bytes reclaimed. `used' the number of 
bytes on the stack after GC and `free' the number of bytes allocated, 
but not in use. Below is an example output.

<pre class="code">
% GC: gained 236,416+163,424 in 0.00 sec;
      used 13,448+5,808; free 72,568+47,440
</pre>

</dd>
<dt class="pubdef"><a id="flag:tty_control"><strong>tty_control</strong>(<var>bool</var>, changeable)</a></dt>
<dd class="defbody">
Determines whether the terminal is switched to raw mode for
<a id="idx:getsinglechar1:119"></a><a class="pred" href="chario.html#get_single_char/1">get_single_char/1</a>, 
which also reads the user actions for the trace. May be set. See also 
the <strong>+/-tty</strong> command line option.</dd>
<dt class="pubdef"><a id="flag:unix"><strong>unix</strong>(<var>bool</var>)</a></dt>
<dd class="defbody">
<a id="idx:unix:120"></a>If present and <code>true</code>, the operating 
system is some version of Unix. Defined if the C compiler used to 
compile this version of SWI-Prolog either defines <code>__unix__</code> 
or <code>unix</code>. On other systems this flag is not available. See 
also <a class="flag" href="flags.html#flag:apple">apple</a> and <a class="flag" href="flags.html#flag:windows">windows</a>.</dd>
<dt class="pubdef"><a id="flag:unknown"><strong>unknown</strong>(<var>fail,warning,error</var>, changeable)</a></dt>
<dd class="defbody">
Determines the behaviour if an undefined procedure is encountered. If
<code>fail</code>, the predicate fails silently. If <code>warn</code>, a 
warning is printed, and execution continues as if the predicate was not 
defined, and if <code>error</code> (default), an <code>existence_error</code> 
exception is raised. This flag is local to each module and inherited 
from the module's <em>import-module</em>. Using default setup, this 
implies that normal modules inherit the flag from <code>user</code>, 
which in turn inherit the value <code>error</code> from <code>system</code>. 
The user may change the flag for module <code>user</code> to change the 
default for all application modules or for a specific module. It is 
strongly advised to keep the <code>error</code> default and use <a id="idx:dynamic1:121"></a><a class="pred" href="dynamic.html#dynamic/1">dynamic/1</a> 
and/or <a id="idx:multifile1:122"></a><a class="pred" href="dynamic.html#multifile/1">multifile/1</a> 
to specify possible non-existence of a predicate.</dd>
<dt class="pubdef"><a id="flag:user_flags"><strong>user_flags</strong>(<var>Atom</var>, changeable)</a></dt>
<dd class="defbody">
Define the behaviour of <a id="idx:setprologflag2:123"></a><a class="pred" href="flags.html#set_prolog_flag/2">set_prolog_flag/2</a> 
if the flag is not known. Values are <code>silent</code>, <code>warning</code> 
and <code>error</code>. The first two create the flag on-the-fly, where <code>warning</code> 
prints a message. The value <code>error</code> is consistent with ISO: 
it raises an existence error and does not create the flag. See also <a id="idx:createprologflag3:124"></a><a class="pred" href="flags.html#create_prolog_flag/3">create_prolog_flag/3</a>. 
The default is <code>silent</code>, but future versions may change that. 
Developers are encouraged to use another value and ensure proper use of <a id="idx:createprologflag3:125"></a><a class="pred" href="flags.html#create_prolog_flag/3">create_prolog_flag/3</a> 
to create flags for their library.</dd>
<dt class="pubdef"><a id="flag:verbose"><strong>verbose</strong>(<var>Atom</var>, changeable)</a></dt>
<dd class="defbody">
This flag is used by <a id="idx:printmessage2:126"></a><a class="pred" href="exception.html#print_message/2">print_message/2</a>. 
If its value is <code>silent</code>, messages of type <code>informational</code> 
and <code>banner</code> are suppressed. The <strong>-q</strong> switches 
the value from the initial
<code>normal</code> to <code>silent</code>.</dd>
<dt class="pubdef"><a id="flag:verbose_autoload"><strong>verbose_autoload</strong>(<var>bool</var>, changeable)</a></dt>
<dd class="defbody">
If <code>true</code> 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.</dd>
<dt class="pubdef"><a id="flag:verbose_load"><strong>verbose_load</strong>(<var>atom</var>, changeable)</a></dt>
<dd class="defbody">
Determines messages printed for loading (compiling) Prolog files. 
Current values are <code>full</code>, <code>normal</code> (default) and
<code>silent</code>. The value of this flag is normally controlled by 
the option <code>silent(Bool)</code> provided by <a id="idx:loadfiles2:127"></a><a class="pred" href="consulting.html#load_files/2">load_files/2</a>.</dd>
<dt class="pubdef"><a id="flag:verbose_file_search"><strong>verbose_file_search</strong>(<var>bool</var>, changeable)</a></dt>
<dd class="defbody">
If <code>true</code> (default <code>false</code>), print messages 
indicating the progress of <a id="idx:absolutefilename23:128"></a><span class="pred-ext">absolute_file_name/[2,3]</span> 
in locating files. Intended for debugging complicated file-search paths. 
See also <a id="idx:filesearchpath2:129"></a><a class="pred" href="consulting.html#file_search_path/2">file_search_path/2</a>.</dd>
<dt class="pubdef"><a id="flag:version"><strong>version</strong>(<var>integer</var>)</a></dt>
<dd class="defbody">
The version identifier is an integer with value: 
<blockquote><var>10000 &times; <var>Major</var> + 100 &times; <var>Minor</var> 
+ <var>Patch</var></var></blockquote>
Note that in releases up to 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.</dd>
<dt class="pubdef"><a id="flag:version_data"><strong>version_data</strong>(<var>swi(Major, Minor, Patch, Extra)</var>)</a></dt>
<dd class="defbody">
Part of the dialect compatibility layer; see also the Prolog flag
<a class="flag" href="flags.html#flag:dialect">dialect</a> and <a class="sec" href="dialect.html">section 
C</a>. <var>Extra</var> provides platform-specific version information. 
Currently it is simply unified to <code>[]</code>.</dd>
<dt class="pubdef"><a id="flag:version_git"><strong>version_git</strong>(<var>atom</var>)</a></dt>
<dd class="defbody">
Available if created from a git repository. See <b>git-describe</b> for 
details.</dd>
<dt class="pubdef"><a id="flag:warn_override_implicit_import"><strong>warn_override_implicit_import</strong>(<var>bool</var>, changeable)</a></dt>
<dd class="defbody">
If <code>true</code> (default), a warning is printed if an implicitly 
imported predicate is clobbered by a local definition. See
<a id="idx:usemodule1:130"></a><a class="pred" href="import.html#use_module/1">use_module/1</a> 
for details.</dd>
<dt class="pubdef"><a id="flag:windows"><strong>windows</strong>(<var>bool</var>)</a></dt>
<dd class="defbody">
<a id="idx:windows:131"></a>If present and <code>true</code>, the 
operating system is an implementation of Microsoft Windows (NT/2000/XP, 
etc.). This flag is only available on MS-Windows based versions.</dd>
<dt class="pubdef"><a id="flag:write_attributes"><strong>write_attributes</strong>(<var>atom</var>, changeable)</a></dt>
<dd class="defbody">
Defines how <a id="idx:write1:132"></a><a class="pred" href="termrw.html#write/1">write/1</a> 
and friends write attributed variables. The option values are described 
with the <code>attributes</code> option of
<a id="idx:writeterm3:133"></a><a class="pred" href="termrw.html#write_term/3">write_term/3</a>. 
Default is <code>ignore</code>.</dd>
<dt class="pubdef"><a id="flag:write_help_with_overstrike"><strong>write_help_with_overstrike</strong>(<var>bool</var>)</a></dt>
<dd class="defbody">
Internal flag used by <a id="idx:help1:134"></a><a class="pred" href="help.html#help/1">help/1</a> 
when writing to a terminal. If present and <code>true</code> it prints 
bold and underlined text using
<em>overstrike</em>.</dd>
<dt class="pubdef"><a id="flag:xpce"><strong>xpce</strong>(<var>bool</var>)</a></dt>
<dd class="defbody">
Available and set to <code>true</code> if the XPCE graphics system is 
loaded.</dd>
<dt class="pubdef"><a id="flag:xpce_version"><strong>xpce_version</strong>(<var>atom</var>)</a></dt>
<dd class="defbody">
Available and set to the version of the loaded XPCE system.
</dd>
</dl>

</dd>
<dt class="pubdef"><span class="pred-tag">[ISO]</span><a id="set_prolog_flag/2"><strong>set_prolog_flag</strong>(<var>:Key, 
+Value</var>)</a></dt>
<dd class="defbody">
Define a new Prolog flag or change its value. <var>Key</var> is an atom. 
If the flag is a system-defined flag that is not marked
<em>changeable</em> above, an attempt to modify the flag yields a
<code>permission_error</code>. If the provided <var>Value</var> does not 
match the type of the flag, a <code>type_error</code> is raised.

<p>Some flags (e.g., <a class="flag" href="flags.html#flag:unknown">unknown</a>) 
are maintained on a per-module basis. The addressed module is determined 
by the <var>Key</var> argument.

<p>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. Defined types are <code>boolean</code> (if the 
initial value is one of <code>false</code>, <code>true</code>, <code>on</code> 
or <code>off</code>),
<code>atom</code> if the initial value is any other atom, <code>integer</code> 
if the value is an integer that can be expressed as a 64-bit signed 
value. Any other initial value results in an untyped flag that can 
represent any valid Prolog term.

<p>The behaviour when <var>Key</var> denotes a non-existent key depends 
on the Prolog flag <a class="flag" href="flags.html#flag:user_flags">user_flags</a>. 
The default is to define them silently. New code is encouraged to use <a id="idx:createprologflag3:135"></a><a class="pred" href="flags.html#create_prolog_flag/3">create_prolog_flag/3</a> 
for portability.</dd>
<dt class="pubdef"><span class="pred-tag">[YAP]</span><a id="create_prolog_flag/3"><strong>create_prolog_flag</strong>(<var>+Key, 
+Value, +Options</var>)</a></dt>
<dd class="defbody">
Create a new Prolog flag. The ISO standard does not foresee creation of 
new flags, but many libraries introduce new flags. <var>Options</var> is 
a list of the following options:

<dl class="latex">
<dt><strong>access</strong>(<var>+Access</var>)</dt>
<dd class="defbody">
Define access rights for the flag. Values are <code>read_write</code> 
and <code>read_only</code>. The default is <code>read_write</code>.</dd>
<dt><strong>type</strong>(<var>+Atom</var>)</dt>
<dd class="defbody">
Define a type restriction. Possible values are
<code>boolean</code>, <code>atom</code>, <code>integer</code>, <code>float</code> 
and <code>term</code>. The default is determined from the initial value. 
Note that <code>term</code> restricts the term to be ground.
</dd>
</dl>

<p>This predicate behaves as <a id="idx:setprologflag2:136"></a><a class="pred" href="flags.html#set_prolog_flag/2">set_prolog_flag/2</a> 
if the flag already exists. See also <a class="flag" href="flags.html#flag:user_flags">user_flags</a>.
</dd>
</dl>

<p></body></html>
swi-prolog-nox 6.6.4-2ubuntu1 / usr / lib / swi-prolog / doc / Manual / flags.html
