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 changeable 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).15This is implemented using the copy-on-write technique. 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 (default false16Future
versions are likely to change the default to true.),
that atom garbage collector streams that are garbage collected while
being open. In addition, a warning is printed. Below is an example of
such a warning.
WARNING: AGC: closed <stream>(0x560e29014400)
Note that closing I/O streams should not be left to the (atom) garbage collector because it may take long before the atom garbage collector runs and because that atom garbage collector is conservative, which implies that it is not guaranteed that all garbage atoms are reclaimed. Code that uses I/O streams should use setup_call_cleanup/3 using the skeleton below, where process/1 is a predicate that reads from or writes to Stream.
setup_call_cleanup(
open(..., Stream),
process(Stream),
close(Stream)),
...
Note that the setting for this flag in the main thread
applies.
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.__ANDROID_API__. It is not defined if
running on other operating systems. The API level may or may not match
the API level of the running device, since it is the API level at
compile time.[quoted(true),
portray(true), max_depth(10), attributes(portray)].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, SWI-Prolog has been build as a
universal binary. Universal binaries contain native executable
code for multiple architectures. Currently the supported architectures
are x86_64 and arm64. The archirecture prefix
for components is fat-darwin while the arch
depends on the actual CPU type.-- 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.false, but to autoload library predicates into the
global
user module. This makes the development tools and library
implicitly available to the toplevel, but not to modules.explicit with user, providing lazy
loading of predicates imported using autoload/[1,2]
and implicit access to the whole library for the toplevel.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. The flag max_integer_size
may be used to enforce an arbitrary limit rather than exhausting memory.
See section 4.27.2.1.gcc, clang or cc. See section
12.5.-lswipl on COFF-based
systems. See section 12.5.-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.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 2.15.1.3.true (default), write/1
and friends write escaped characters using the \uXXXX or \UXXXXXXXX
syntax rather than the ISO Prolog \x<hex>\ syntax.
SWI-Prolog reads both.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 when built in a Conda
environment.true in swipl-win.exe to indicate that the
console supports menus. See also section
4.35.4.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 4.44.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.true, start the debugger on Control-C.21More
precisely when receiving SIGINT. The initial
value is
false and the value is set to true when
entering the interactive top level. See --debug-on-interrupt
to start handling interrupts immediately.true, show the context module while printing a
stack-frame in the tracer. Normally controlled using the‘C’option
of the tracer.[quoted(true),
portray(true), max_depth(10), attributes(portray)].error (default), warning and silent.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
2.18.1 for details.elf for when
swipl is an ELF binary file.true (default false), expand $\arg{varname}
and ~error (default) or infinity. The first
is ISO compliant. Using infinity, floating point overflow
is mapped to positive or negative Inf. See section
4.27.2.4. This flag also affects read_term/3
and friends, causing them to read too large floating point number as
infinity.to_nearest (default), to_positive, to_negative
or to_zero. For most scenarios the function
roundtoward/2
provides a safer and faster alternative.error (default) or nan. The first is
ISO compliant. Using nan, undefined operations such as
sqrt(-2.0) is mapped to NaN. See section
4.27.2.4.error or ignore (default). The second
is ISO compliant, binding the result to 0.0.error (default) or infinity. The first
is ISO compliant. Using infinity, division by 0.0 is mapped
to positive or negative Inf. See section
4.27.2.4.true (default if threading is enabled), atom and clause
garbage collection are executed in a separate 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 separate thread
improves real time behaviour. The gc thread can be
controlled using
set_prolog_gc_thread/1,
which either enables the gc thread or kills the gc thread and waits for
it to die.true (default) generate code that can be debugged using
trace/0, spy/1,
etc. Can be set to false using the
--no-debug. 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.22In
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>/boot.prc
and to find its library as <home>/library.
Some installations may put architecture independent files in a shared
home and also define shared_home.
System files can be found using
absolute_file_name/3
as swi(file). See file_search_path/2.// and rem
arithmetic functions. Value depends on the C compiler used.//2[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. This flag is always true on 64-bit
hardware and true on 32-bit hardware if the configuration detected
support for LFS. Note that it may still be the case that the file
system on which a particular file resides puts limits on the file
size.libswipl, the
SWI-Prolog shared object that provides Prolog, resides. On some systems
this can be determined reliably from the running system. On these
systems the flag is read-only. On other systems it is the
configured target installation location and thus this value can be wrong
if the installation has been relocated. As we do not have a
cross-platform reliable way to compute this path the flag is read-write
on such platforms.23When running
from the build environment, this flag is adjusted to reflect the
location in the build tree.
Currently, this flag is reliable on Windows and POSIX systems providing the dladdr() function. This function is provided on Linux and MacOS.
tcmalloc and
ptmalloc. See section
4.43.2 for details.infinite clears the flag. By default this flag is not
defined. See section
7.10 for details.bounded_rationality, error
(default) or
suspend.0xffff on Windows (UCS-2) and
0x10ffff on most other platforms.representation_error(max_procedure_arity)
exception. Currently set to 1024.true creates excessively big rational numbers and, if
precision is not required, one should use floating point arithmetic.
Note that rationals are also implicitly limited by the Prolog flag max_integer_size.error (default),
which throws a tripwire resource error and float, which
converts the rational number into a floating point number. Note that
rational numbers may exceed the range for floating point numbers.infinite clears the flag. By default this flag is not
defined. See section
7.10 for details.error (default), bounded_rationality,
suspend and fail.infinite clears the flag. By default this flag is not
defined. See section
7.10 for details.error (default), abstract
and suspend.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 20μs.
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.print (default), status and halt.
Using
halt the process halts immediately with status 1. Otherwise
execution continues. Using status halt/0
exits with status 1 if one or more errors were printed by the process.
In compile mode (see -c) the default is status.
This flag can be set from the commandline using --on-error.
See also section
4.3.2.1.print. The commandline option is --on-warning..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