(   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 8) copies all flags from the thread that created the new thread (its parent).^{12This is implemented using the copy-on-write tecnhnique.} As a consequence, modifying a flag inside a thread does not affect other threads.

access_level(atom, changeable)

This flag defines a normal `user' view (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.

address_bits(integer)

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 arch.

agc_margin(integer, changeable)

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().

allow_variable_name_as_functor(bool, changeable)

If true (default is false), 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.

argv(list)

List is a list of atoms representing the command line arguments used to invoke SWI-Prolog. Please note that all arguments are included in the list returned.

arch(atom)

Identifier for the hardware and operating system SWI-Prolog is running on. Used to select foreign files for the right architecture. See also section 9.2.3 and file_search_path/2.

associate(atom, changeable)

On Windows systems, this is set to the filename extension (pl (default) or pro (can be selected in the installer)) associated with swipl-win.exe.

autoload(bool, changeable)

If true (default) autoloading of library functions is enabled.

backquoted_string(bool, changeable)

If true (default false), read translates text between backquotes into a string object (see section 4.24). This flag is mainly for compatibility with LPA Prolog.

bounded(bool)

ISO Prolog flag. If 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.

break_level(integer)

Current break-level. The initial top level (started with -t) has value 0. See break/0. This flag is absent from threads that are not running a top-level loop.

c_cc(atom, changeable)

Name of the C compiler used to compile SWI-Prolog. Normally either gcc or cc. See section 9.5.

c_cflags(atom, changeable)

CFLAGS used to compile SWI-Prolog. See section 9.5.

c_ldflags(atom, changeable)

LDFLAGS used to link SWI-Prolog. See section 9.5.

c_libs(atom, changeable)

Libraries needed to link executables that embed SWI-Prolog. Typically -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.

c_libplso(atom, changeable)

Libraries needed to link extensions (shared object, DLL) to SWI-Prolog. Typically empty on ELF systems and -lswipl on COFF-based systems. See section 9.5.

char_conversion(bool, changeable)

Determines whether character conversion takes place while reading terms. See also char_conversion/2.

character_escapes(bool, changeable)

If 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.

colon_sets_calling_context(bool, changeable)

Using the construct <module>:<goal> sets the calling context for executing <goal>. This flag is defined by ISO/IEC 13211-2 (Prolog modules standard). See section 5.

compile_meta_arguments(atom, changeable)

Experimental flag that controls compilation of arguments passed to meta-calls marked `0' or `^' (see meta_predicate/1). Supported values are:

false

(default). Meta-arguments are passed verbatim.

control

Compile meta-arguments that contain control structures ((A,B), (A;B), (A->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.

true

Also compile references to normal user predicates. This harms performance (a little), but enhances the power of poor-mens consistency check used by make/0 and implemented by list_undefined/0.

always

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., assert/2) that provides direct access to the executable code, thus avoiding runtime lookup of predicates for meta-calling.

compiled_at(atom)

Describes when the system has been compiled. Only available if the C compiler used to compile SWI-Prolog provides the __DATE__ and __TIME__ macros.

console_menu(bool)

Set to true in swipl-win.exe to indicate that the console supports menus. See also section 4.34.3.

cpu_count(integer, changeable)

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 thread_setconcurrency/2 and the library library(thread). This flag is not available on systems where we do not know how to get the number of CPUs.

dde(bool)

Set to true if this instance of Prolog supports DDE as described in section 4.42.

debug(bool, changeable)

Switch debugging mode on/off. If debug mode is activated the system traps encountered spy points (see spy/1) and trace points (see trace/1). In addition, last-call 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.

debug_on_error(bool, changeable)

If 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 report_error. May be changed. Default is true, except for the runtime version.

debugger_print_options(term, changeable)

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 [quoted(true), portray(true), max_depth(10), attributes(portray)].

debugger_show_context(bool, changeable)

If true, show the context module while printing a stack-frame in the tracer. Normally controlled using the `C' option of the tracer.

dialect(atom)

Fixed to swi. The code below is a reliable and portable way to detect SWI-Prolog.

is_dialect(swi) :-
        catch(current_prolog_flag(dialect, swi), _, fail).

double_quotes(codes,chars,atom,string, changeable)

This flag determines how double quoted strings are read by Prolog and is ---like character_escapes--- maintained for each module. If codes (default), a list of character codes is returned, if chars a list of one-character atoms, if atom double quotes are the same as single quotes and finally, string reads the text into a Prolog string (see section 4.24). See also atom_chars/2 and atom_codes/2.

editor(atom, changeable)

Determines the editor used by edit/1. See section 4.5 for details on selecting the editor used.

emacs_inferior_process(bool)

If true, SWI-Prolog is running as an inferior process of (GNU/X-)Emacs. SWI-Prolog assumes this is the case if the environment variable EMACS is t and INFERIOR is yes.

encoding(atom, changeable)

Default encoding used for opening files in text mode. The initial value is deduced from the environment. See section 2.18.1 for details.

executable(atom)

Pathname of the running executable. Used by qsave_program/2 as default emulator.

file_name_variables(bool, changeable)

If true (default false), expand $varname and ~ in arguments of built-in predicates that accept a file name (open/3, exists_file/1, access_file/2, etc.). The predicate expand_file_name/2 can be used to expand environment variables and wildcard patterns. This Prolog flag is intended for backward compatibility with older versions of SWI-Prolog.

gc(bool, changeable)

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.

generate_debug_info(bool, changeable)

If 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.^{13In 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.}

gmp_version(integer)

If Prolog is linked with GMP, this flag gives the major version of the GMP library used. See also section 9.4.8.

gui(bool)

Set to true if XPCE is around and can be used for graphics.

history(integer, changeable)

If integer> 0, support Unix csh(1)-like history as described in section 2.7. 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 readline) and 15 otherwise.

home(atom)

SWI-Prolog's notion of the home directory. SWI-Prolog uses its home directory to find its startup file as <home>/boot32.prc (32-bit machines) or <home>/boot64.prc (64-bit machines) and to find its library as <home>/library.

hwnd(integer)

In swipl-win.exe, this refers to the MS-Windows window handle of the console window.

integer_rounding_function(down,toward_zero)

ISO Prolog flag describing rounding by // and rem arithmetic functions. Value depends on the C compiler used.

iso(bool, changeable)

Include some weird ISO compatibility that is incompatible with normal SWI-Prolog behaviour. Currently it has the following effect:

• The //2 (float division) always returns a float, even if applied to integers that can be divided.
• In the standard order of terms (see section 4.7.1), all floats are before all integers.
• atom_length/2 yields a type error if the first argument is a number.
• clause/[2,3] raises a permission error when accessing static predicates.
• abolish/[1,2] raises a permission error when accessing static predicates.
• Syntax is closer to the ISO standard:
  • Unquoted commas and bars appearing as atoms are not allowed. Instead of f(,,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.
  • 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 [a :- b, c]. must now be disambiguated to mean [(a :- b), c]. or [(a :- b, c)].
  • Operators appearing as operands must be bracketed. Instead of X == -, true. write X == (-), true. Currently, this is not entirely enforced.
  • Backslash-escaped newlines are interpreted according to the ISO standard. See section 2.15.1.2.

large_files(bool)

If present and 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.

last_call_optimisation(bool, changeable)

Determines whether or not last-call optimisation is enabled. Normally the value of this flag is the negation of the debug flag. As programs may run out of stack if last-call optimisation is omitted, it is sometimes necessary to enable it during debugging.

max_arity(unbounded)

ISO Prolog flag describing there is no maximum arity to compound terms.

max_integer(integer)

Maximum integer value if integers are bounded. See also the flag bounded and section 4.27.2.1.

max_tagged_integer(integer)

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.

min_integer(integer)

Minimum integer value if integers are bounded. See also the flag bounded and section 4.27.2.1.

min_tagged_integer(integer)

Start of the tagged-integer value range.

occurs_check(atom, changeable)

This flag controls unification that creates an infinite tree (also called cyclic term) and can have three values. Using 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.

open_shared_object(bool)

If true, open_shared_object/2 and friends are implemented, providing access to shared libraries (.so files) or dynamic link libraries (.DLL files).

optimise(bool, changeable)

If 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).

pid(int)

Process identifier of the running Prolog process. Existence of this flag is implementation-defined.

pipe(bool, changeable)

If true, open(pipe(command), mode, Stream), etc. are supported. Can be changed to disable the use of pipes in applications testing this feature. Not recommended.

preprocessor(atom, changeable)

Current preprocessor. See preprocessor/2. The reserved atom none (default) implies no preprocessor.

prompt_alternatives_on(atom, changeable)

Determines prompting for alternatives in the Prolog top level. Default is 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.

qcompile(atom, changeable)

This option provides the default for the qcompile(+Atom) option of load_files/2.

readline(bool)

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.

resource_database(atom)

Set to the absolute filename of the attached state. Typically this is the file boot32.prc, the file specified with -x or the running executable. See also resource/3.

report_error(bool, changeable)

If 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.

runtime(bool)

If present and true, SWI-Prolog is compiled with -DO_RUNTIME, disabling various useful development features (currently the tracer and profiler).

saved_program(bool)

If present and true, Prolog has been started from a state saved with qsave_program/[1,2].

shared_object_extension(atom)

Extension used by the operating system for shared objects. .so for most Unix systems and .dll for Windows. Used for locating files using the file_type executable. See also absolute_file_name/3.

shared_object_search_path(atom)

Name of the environment variable used by the system to search for shared objects.

signals(bool)

Determine whether Prolog is handling signals (software interrupts). This flag is false if the hosting OS does not support signal handling or the command line option -nosignals is active. See section 9.4.21.1 for details.

stream_type_check(atom, changeable)

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 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.

system_thread_id(int)

Available in multithreaded version (see section 8) 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 thread_self/1.

timezone(integer)

Offset in seconds west of GMT of the current time zone. Set at initialization time from the timezone variable associated with the POSIX tzset() function. See also convert_time/2.

toplevel_print_anon(bool, changeable)

If 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.

toplevel_print_factorized(bool, changeable)

If 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)).

toplevel_print_options(term, changeable)

This argument is given as option-list to write_term/2 for printing results of queries. Default is [quoted(true), portray(true), max_depth(10), attributes(portray)].

toplevel_prompt(atom, changeable)

Define the prompt that is used by the interactive top level. The following ~ (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)

toplevel_var_size(int, changeable)

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 $ variable reference. See section 2.8.

trace_gc(bool, changeable)

If 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

tty_control(bool, changeable)

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 +/-tty command line option.

unix(bool)

If present and 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.

unknown(fail,warning,error, changeable)

Determines the behaviour if an undefined procedure is encountered. If 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.

user_flags(Atom, changeable)

Define the behaviour of set_prolog_flag/2 if the flag is not known. Values are 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.

verbose(Atom, changeable)

This flag is used by print_message/2. If its value is silent, messages of type informational and banner are suppressed. The -q switches the value from the initial normal to silent.

verbose_autoload(bool, changeable)

If 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.

verbose_load(atom, changeable)

Determines messages printed for loading (compiling) Prolog files. Current values are full, normal (default) and silent. The value of this flag is normally controlled by the option silent(Bool) provided by load_files/2.

verbose_file_search(bool, changeable)

If 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.

version(integer)

The version identifier is an integer with value:

10000 × Major + 100 × Minor + Patch

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.

version_data(swi(Major, Minor, Patch, Extra))

Part of the dialect compatibility layer; see also the Prolog flag dialect and section C. Extra provides platform-specific version information. Currently it is simply unified to .

version_git(atom)

Available if created from a git repository. See git-describe for details.

warn_override_implicit_import(bool, changeable)

If true (default), a warning is printed if an implicitly imported predicate is clobbered by a local definition. See use_module/1 for details.

windows(bool)

If present and true, the operating system is an implementation of Microsoft Windows (NT/2000/XP, etc.). This flag is only available on MS-Windows based versions.

write_attributes(atom, changeable)

Defines how write/1 and friends write attributed variables. The option values are described with the attributes option of write_term/3. Default is ignore.

write_help_with_overstrike(bool)

Internal flag used by help/1 when writing to a terminal. If present and true it prints bold and underlined text using overstrike.

xpce(bool)

Available and set to true if the XPCE graphics system is loaded.

xpce_version(atom)

Available and set to the version of the loaded XPCE system.