The predicates current_prolog_flag/2 and set_prolog_flag/2 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.
Flags marked rw can be modified by the user using
set_prolog_flag/2. 
Flag values are typed. Flags marked as bool can have the 
values true or false. The predicate
create_prolog_flag/3 
may be used to create flags that describe or control behaviour of 
libraries and applications. The library
library(settings) provides an alternative interface for 
managing notably application parameters.
Some Prolog flags are not defined in all versions, which is normally 
indicated in the documentation below as ``if present and true''. 
A boolean Prolog flag is true iff the Prolog flag is present
and the Value is the atom true. Tests for 
such flags should be written as below:
        (   current_prolog_flag(windows, true)
        ->  <Do MS-Windows things>
        ;   <Do normal things>
        )
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: generate_debug_info and optimise.
A new thread (see section 10) copies all flags from the thread that created the new thread (its parent).16This is implemented using the copy-on-write tecnhnique. As a consequence, modifying a flag inside a thread does not affect other threads.
user, 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.true, the operating 
system is MacOSX. Defined if the C compiler used to compile this version 
of SWI-Prolog defines
__APPLE__. Note that the unix 
is also defined for MacOSX.true (default false), dots may be embedded 
into atoms that are not quoted and start with a letter. The embedded dot
must be followed by an identifier continuation character (i.e., 
letter, digit or underscore). The dot is allowed in identifiers in many 
languages, which can make this a useful flag for defining DSLs. Note 
that this conflicts with cascading functional notation. For example,
Post.meta.author is read as .(Post, 'meta.author' 
if this flag is set to true.Functor(arg) is read as if it 
were written '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 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 read_term/2 
using the option 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.-- or the first 
non-option argument. See also os_argv.18Prior 
to version 6.5.2, argv 
was defined as os_argv 
is now. The change was made for compatibility reasons and because the 
current definition is more practical.true (default) autoloading of library functions is 
enabled.codes. 
If --traditional is given, the default is symbol_char, 
which allows using ` in operators composed of symbols.19Older 
versions had a boolean flag backquoted_strings, which 
toggled between string and symbol_char. 
See also section 5.2.true (default), print a backtrace on an uncaught 
exception.true (default), try to reconstruct the line number at 
which the exception happened.true, integer representation is bound 
by
min_integer and max_integer. 
If false integers can be arbitrarily large and the min_integer 
and
max_integer are 
not present. See section 
4.27.2.1.-lswipl 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.-lswipl on COFF-based 
systems. See section 12.5.true (default), read/1 
interprets \ escape sequences in quoted atoms and strings. 
May be changed. This flag is local to the module in which it is changed. 
See section 2.16.1.3.library(ansi_term), which 
is loaded at startup if the two conditions below are both true. Note 
that this implies that setting this flag to false from the 
system or personal initialization file (see section 
2.2 disables colored output. The predicate message_property/2 
can be used to control the actual color scheme depending in the message 
type passed to
print_message/2.
stream_property(current_output, tty(true))
\+ current_prolog_flag(color_term, false)
^true in swipl-win.exe to indicate that the 
console supports menus. See also section 
4.35.3.library(thread). 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 qsave_program/1).true if this instance of Prolog supports DDE as 
described in section 4.43.Disabling these optimisations can cause the system to run out of memory on programs that behave correctly if debug mode is off.
true, start the tracer after an error is detected. 
Otherwise just continue execution. The goal that raised the error will 
normally fail. See also the Prolog flag report_error. 
Default is true.[quoted(true), 
portray(true), max_depth(10), attributes(portray)].true, show the context module while printing a 
stack-frame in the tracer. Normally controlled using the `C' option of 
the tracer.swi. The code below is a reliable and portable way 
to detect SWI-Prolog.
is_dialect(swi) :-
        catch(current_prolog_flag(dialect, swi), _, fail).
string, which 
produces a string as described in section 
5.2. If
--traditional is given, the default is codes, 
which produces a list of character codes, integers that represent a 
Unicode code-point. The value chars produces a list of 
one-character atoms and the value atom makes double quotes 
the same as single quotes, creating a atom. See also section 
5.EMACS is t and INFERIOR 
is yes.text mode. The 
initial value is deduced from the environment. See section 
2.19.1 for details.true (default false), expand $\arg{varname} 
and ~true (default if threading is enabled), atom and clause 
garbage collection are executed in a seperate thread with the
alias gc. Otherwise the thread that detected 
sufficient garbage executes the garbage collector. As running these 
global collectors may take relatively long, using a seperate thread 
improves real time behaviour. The gc thread can be 
controlled using
set_prolog_gc_thread/1.true (default) generate code that can be debugged using
trace/0, spy/1, 
etc. Can be set to false using the
-nodebug. This flag is scoped within a source file. 
Many of the libraries have
:- set_prolog_flag(generate_debug_info, false) to hide 
their details from a normal trace.20In 
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.true if XPCE is around and can be used for graphics.<home>/boot32.prc (32-bit machines) or
<home>/boot64.prc (64-bit machines) and to 
find its library as
<home>/library.// and rem 
arithmetic functions. Value depends on the C compiler used.//2f(,,a) now write f(',',a). 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 [Head|Tail], 
and as infix operator for alternation in grammar rules, like a --> 
b | c.
[a :- b, c]. 
must now be disambiguated to mean [(a :- b), c]. or [(a 
:- b, c)].
X 
== -, true. write X == (-), true. Currently, this is 
not entirely enforced.
true, SWI-Prolog has been compiled with
large file support (LFS) and is capable of accessing files 
larger than 2GB on 32-bit hardware. Large file support is default on 
installations built using configure that support it and may be 
switched off using the configure option --disable-largefile.error 
and
warning. The list may contain the elements thread 
to add the thread that generates the message to the message, time 
or time(Format) to add a time stamp. The default time 
format is %T.%3f. The default is [thread]. See 
also format_time/3 
and print_message/2.true (default false), enforce mitigation 
against the
Spectre 
timing-based security vulnerability. Spectre based attacks can extract 
information from memory owned by the process that should remain 
invisible, such as passwords or the private key of a web server. The 
attacks work by causing speculative access to sensitive data, and 
leaking the data via side-channels such as differences in the duration 
of successive instructions. An example of a potentially vulnerable 
application is SWISH. 
SWISH allows users to run Prolog code while the swish server must 
protect the privacy of other users as well as its HTTPS private keys, 
cookies and passwords.
Currently, enabling this flag reduces the resolution of get_time/1 and statistics/2 CPU time to 20&mus.
WARNING: Although a coarser timer makes a successful attack of this type harder, it does not reliably prevent such attacks in general. Full mitigation may require compiler support to disable speculative access to sensitive data.
false (default), unification succeeds, creating an infinite 
tree. Using true, unification behaves as unify_with_occurs_check/2, 
failing silently. Using error, an attempt to create a 
cyclic term results in an occurs_check 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..so 
files) or dynamic link libraries (.DLL files).true, compile in optimised mode. The initial value is
true if Prolog was started with the -O 
command line option. The optimise 
flag is scoped to a source file.
Currently optimised 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).
open(pipe(command), mode, Stream), etc. are 
supported. Can be changed to disable the use of pipes in applications 
testing this feature. Not recommended./bin/sh. This flag is used by shell/1 
and qsave_program/2.TMP or TEMP in windows. If this 
variable is not defined a default is used. This default is typically /tmp 
or
c:/temp in windows.determinism, which implies the system prompts for 
alternatives if the goal succeeded while leaving choice points. Many 
classical Prolog systems behave as groundness: they prompt 
for alternatives if and only if the query contains variables.true (default false), clause/2 
does not operate on static code, providing some basic protection from 
hackers that wish to list the static code of your Prolog program. Once 
the flag is
true, it cannot be changed back to false. 
Protection is default in ISO mode (see Prolog flag iso). 
Note that many parts of the development environment require clause/2 
to work on static code, and enabling this flag should thus only be used 
for production code.qcompile(+Atom) 
option of load_files/2.false, readline 
or
editline. This causes the toplevel not to load a command 
line editor (false) or load the specified one. If loading 
fails the flag is set to false.
library(readline) is loaded, providing line 
editing based on the GNU readline library.library(editline) is loaded, providing line 
editing based on the BSD libedit. This is the default if library(editline) 
is available and can be loaded.boot32.prc, the file specified with -x 
or the running executable. See also resource/3.true, print error messages; otherwise suppress them. May 
be changed. See also the debug_on_error 
Prolog flag. Default is true, except for the runtime 
version.true, SWI-Prolog is compiled with 
-DO_RUNTIME, disabling various useful development features (currently 
the tracer and profiler).true (default false), load_files/2 
calls hooks to allow library(sandbox) to verify the safety of 
directives.true, Prolog has been started from a state 
saved with qsave_program/[1,2]..so 
for most Unix systems and .dll for Windows. Used for 
locating files using the file_type executable. 
See also
absolute_file_name/3.false if the hosting OS does not support signal 
handling or the command line option -nosignals is 
active. See
section 
12.4.22.1 for details.false (no checking), true 
(full checking) and loose. Using checking mode loose 
(default), the system accepts byte I/O from text stream that use ISO 
Latin-1 encoding and accepts writing text to binary streams.resource_error(table_space) exception is raised.false and read-only. Otherwise 
the value is true unless the system was started with the
--nothreads. Threading may be disabled only if no 
threads are running. See also the gc_thread 
flag.timezone variable associated 
with the POSIX tzset() function. See also format_time/3.default, starting a normal interactive 
session. This value may be changed using the command line option -t. 
The explicit value prolog is equavalent to default. 
If initialization(Goal,main) is used and the toplevel is default, 
the toplevel is set to
halt (see halt/0).backtracking (default), the toplevel backtracks after 
completing a query. If recursive, the toplevel is 
implemented as a recursive loop. This implies that global variables set 
using
b_setval/2 
are maintained between queries. In recursive mode, answers to 
toplevel variables (see section 
2.8) are kept in backtrackable global variables and thus not 
copied. In
backtracking mode answers to toplevel variables are kept in the 
recorded database (see section 
4.14.2).
The recursive mode has been added for interactive usage of CHR (see section 9),21Suggested by Falco Nogatz which maintains the global constraint store in backtrackable global variables.
true, top-level variables starting with an underscore (_) 
are printed normally. If false they are hidden. This may be 
used to hide bindings in complex queries from the top level.true (default false) show the internal 
sharing of subterms in the answer substitution. The example below 
reveals internal sharing of leaf nodes in red-black trees as 
implemented by the
library(rbtrees) predicate rb_new/1 :
?- set_prolog_flag(toplevel_print_factorized, true).
?- rb_new(X).
X = t(_S1, _S1), % where
    _S1 = black('', _G387, _G388, '').
If this flag is false, the % where notation 
is still used to indicate cycles as illustrated below. This example also 
shows that the implementation reveals the internal cycle length, and not 
the minimal cycle length. Cycles of different length are 
indistinguishable in Prolog (as illustrated by S == R).
?- S = s(S), R = s(s(R)), S == R. S = s(S), R = s(s(R)).
[quoted(true), 
portray(true), max_depth(10), attributes(portray)].~ (tilde) sequences are replaced:
| m | Type in module if 
not user(see module/1) | 
| l | Break level if not 0 (see break/0) | 
| d | Debugging state if not normal execution (see debug/0, trace/0) | 
| ! | History event if history is enabled (see flag history) | 
$true (default false), garbage collections 
and stack-shifts will be reported on the terminal. May be changed. 
Values are reported in bytes as G+T, where G 
is the global stack value and T 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.
% GC: gained 236,416+163,424 in 0.00 sec;
      used 13,448+5,808; free 72,568+47,440
true, 
`traditional' mode has been selected using --traditional. 
Notice that some SWI7 features, like the functional notation on dicts, 
do not work in this mode. See also section 
5.false at startup, command line editing is disabled. 
See also the --no-tty command line option.true, the operating 
system is some version of Unix. Defined if the C compiler used to 
compile this version of SWI-Prolog either defines __unix__ 
or unix. On other systems this flag is not available. See 
also apple and windows.fail, the predicate fails silently. If warn, a 
warning is printed, and execution continues as if the predicate was not 
defined, and if error (default), an existence_error 
exception is raised. This flag is local to each module and inherited 
from the module's import-module. Using default setup, this 
implies that normal modules inherit the flag from user, 
which in turn inherit the value error from system. 
The user may change the flag for module user to change the 
default for all application modules or for a specific module. It is 
strongly advised to keep the error default and use dynamic/1 
and/or multifile/1 
to specify possible non-existence of a predicate.true (default false), unload all loaded 
foreign libraries. Default is false because modern OSes 
reclaim the resources anyway and unloading the foreign code may cause 
registered hooks to point to no longer existing data or code.silent, warning 
and error. The first two create the flag on-the-fly, where warning 
prints a message. The value error is consistent with ISO: 
it raises an existence error and does not create the flag. See also create_prolog_flag/3. 
The default is silent, but future versions may change that. 
Developers are encouraged to use another value and ensure proper use of create_prolog_flag/3 
to create flags for their library.true (default false), variables must start 
with an underscore (_). May be changed. This flag is local 
to the module in which it is changed. See section 
2.16.1.7.silent, messages of type informational 
and banner are suppressed. The -q switches 
the value from the initial
normal to silent.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.full (print a message at the start and 
end of each file loaded), normal (print a message at the 
end of each file loaded), brief (print a message at end of 
loading the toplevel file), and silent (no messages are 
printed, default). The value of this flag is normally controlled by the 
option
silent(Bool) provided by load_files/2.true (default false), print messages 
indicating the progress of absolute_file_name/[2,3] 
in locating files. Intended for debugging complicated file-search paths. 
See also file_search_path/2.10000 × Major + 100 × Minor + Patch
tag(rc1).true, the operating system is MS-Windows.true (default), a warning is printed if an implicitly 
imported predicate is clobbered by a local definition. See
use_module/1 
for details.true, the 
operating system is an implementation of Microsoft Windows. This flag is 
only available on MS-Windows based versions. See also unix.attributes option of
write_term/3. 
Default is ignore.true it prints 
bold and underlined text using
overstrike.true if the XPCE graphics system is 
loaded.true, source code is being read for analysis 
purposes such as cross-referencing. Otherwise (default) it is being read 
to be compiled. This flag is used at several places by term_expansion/2 
and
goal_expansion/2 
hooks, notably if these hooks use side effects. See also the libraries library(prolog_source) 
and library(prolog_xref).
permission_error. If the provided Value does not 
match the type of the flag, a type_error is raised.
Some flags (e.g., unknown) are maintained on a per-module basis. The addressed module is determined by the Key argument.
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 boolean (if the 
initial value is one of false, true, on 
or off),
atom if the initial value is any other atom, integer 
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.
The behaviour when Key denotes a non-existent key depends on the Prolog flag user_flags. The default is to define them silently. New code is encouraged to use create_prolog_flag/3 for portability.
read_write 
and read_only. The default is read_write.boolean, atom, integer, float 
and term. The default is determined from the initial value. 
Note that term restricts the term to be ground.true, do not modify the flag if it already exists. 
Otherwise (default), this predicate behaves as set_prolog_flag/2 
if the flag already exists.