- Reference manual
- Built-in Predicates
- Notation of Predicate Descriptions
- Character representation
- Loading Prolog source files
- Editor Interface
- Verify Type of a Term
- Comparison and Unification of Terms
- Control Predicates
- Meta-Call Predicates
- Delimited continuations
- Exception handling
- Printing messages
- Handling signals
- DCG Grammar rules
- Declaring predicate properties
- Examining the program
- Input and output
- Status of streams
- Primitive character I/O
- Term reading and writing
- Analysing and Constructing Terms
- Analysing and Constructing Atoms
- Localization (locale) support
- Character properties
- Character Conversion
- Misc arithmetic support predicates
- Built-in list operations
- Finding all Solutions to a Goal
- Formatted Write
- Global variables
- Terminal Control
- Operating System Interaction
- File System Interaction
- User Top-level Manipulation
- Creating a Protocol of the User Interaction
- Debugging and Tracing Programs
- Debugging and declaring determinism
- Obtaining Runtime Statistics
- Execution profiling
- Memory Management
- Windows DDE interface
- Built-in Predicates
- Reference manual
The predicates in this section provide basic access to the operating
system that has been part of the Prolog legacy tradition. Note that more
advanced access to low-level OS features is provided by several
libraries from the
clib package, notably library
- Equivalent to‘
shell(Command, 0)’. See shell/2 for details.
- shell(+Command, -Status)
- Execute Command on the operating system. Command
is given to the Bourne shell (/bin/sh). Status is unified
with the exit status of the command.
On Windows, shell/[1,2] executes the command using the CreateProcess() API and waits for the command to terminate. If the command ends with a
&sign, the command is handed to the WinExec() API, which does not wait for the new task to terminate. See also win_exec/2 and win_shell/2. Please note that the CreateProcess() API does not imply the Windows command interpreter (cmd.exe and therefore commands that are built in the command interpreter can only be activated using the command interpreter. For example, a file can be copied using the command below.
?- shell('cmd.exe /C copy file1.txt file2.txt').
Note that many of the operations that can be achieved using the shell built-in commands can easily be achieved using Prolog primitives. See make_directory/1, delete_file/1, rename_file/2, etc. The clib package provides
library(filesex), implementing various high level file operations such as copy_file/2. Using Prolog primitives instead of shell commands improves the portability of your program.
library(process)provides process_create/3 and several related primitives that support more fine-grained interaction with processes, including I/O redirection and management of asynchronous processes.
- getenv(+Name, -Value)
- Get environment variable. Fails silently if the variable does not exist. Please note that environment variable names are case-sensitive on Unix systems and case-insensitive on Windows.
- setenv(+Name, +Value)
- Set an environment variable. Name and Value must be instantiated to atoms or integers. The environment variable will be passed to shell/[0-2] and can be requested using getenv/2. They also influence expand_file_name/2. Environment variables are shared between threads. Depending on the underlying C library, setenv/2 and unsetenv/1 may not be thread-safe and may cause memory leaks. Only changing the environment once and before starting threads is safe in all versions of SWI-Prolog.
- Remove an environment variable from the environment. Some systems lack the underlying unsetenv() library function. On these systems unsetenv/1 sets the variable to the empty string.
- setlocale(+Category, -Old, +New)
- Set/Query the locale setting which tells the C library how to
interpret text files, write numbers, dates, etc. Category is one of
time. For details, please consult the C library locale documentation. See also section 2.19.1. Please note that the locale is shared between all threads and thread-safe usage of setlocale/3 is in general not possible. Do locale operations before starting threads or thoroughly study threading aspects of locale support in your environment before using in multithreaded environments. Locale settings are used by format_time/3, collation_key/2 and locale_sort/2.
messageslocale defines the language used by print_message/2. Note that this locale is not available on all operating system. Notably Windows does not support this category. See win_get_user_preferred_ui_languages/2.
The predicates in this section are only available on the Windows version of SWI-Prolog. Their use is discouraged if there are portable alternatives. For example, win_exec/2 and win_shell/2 can often be replaced by the more portable shell/2 or the more powerful process_create/3.
- win_exec(+Command, +Show)
- Windows only. Spawns a Windows task without waiting for its completion. Show
is one of the Win32
SW_*constants written in lowercase without the
shownormal. In addition,
iconicis a synonym for
- win_shell(+Operation, +File, +Show)
- Windows only. Opens the document File using the Windows shell
rules for doing so. Operation is one of
exploreor another operation registered with the shell for the given document type. On modern systems it is also possible to pass a URL as File, opening the URL in Windows default browser. This call interfaces to the Win32 API ShellExecute(). The Show argument determines the initial state of the opened window (if any). See win_exec/2 for defined values.
- win_shell(+Operation, +File)
- Same as
win_shell(Operation, File, normal).
- win_registry_get_value(+Key, +Name, -Value)
- Windows only. Fetches the value of a Windows registry key. Key
is an atom formed as a path name describing the desired registry key.
Name is the desired attribute name of the key. Value
is unified with the value. If the value is of type
DWORD, the value is returned as an integer. If the value is a string, it is returned as a Prolog atom. Other types are currently not supported. The default‘root' is
HKEY_CURRENT_USER. Other roots can be specified explicitly as
HKEY_USERS. The example below fetches the extension to use for Prolog files (see
README.TXTon the Windows version):
?- win_registry_get_value( 'HKEY_LOCAL_MACHINE/Software/SWI/Prolog', fileExtension, Ext). Ext = pl
- win_folder(?Name, -Directory)
- True if Name is the Windows‘CSIDL' of Directory.
Name is unbound, all known Windows special paths are
Name is the CSIDL after deleting the leading
CSIDL_and mapping the constant to lowercase. Check the Windows documentation for the function SHGetSpecialFolderPath() for a description of the defined constants. This example extracts the‘My Documents' folder:
?- win_folder(personal, MyDocuments). MyDocuments = 'C:/Documents and Settings/jan/My Documents'
- This predicate adds a directory to the search path for dependent DLL
files. If possible, this is achieved with win_add_dll_directory/2.
%PATH%is extended with the provided directory. AbsDir may be specified in the Prolog canonical syntax. See prolog_to_os_filename/2. Note that use_foreign_library/1 passes an absolute path to the DLL if the destination DLL can be located from the specification using absolute_file_name/3. This predicate is available from library
library(shlib)and can be autoloaded.
- win_add_dll_directory(+AbsDir, -Cookie)
- This predicate adds a directory to the search path for dependent DLL
files. If the call is successful it unifies Cookie with a
handle that must be passed to win_remove_dll_directory/1
to remove the directory from the search path. Error conditions:
- This predicate fails if Windows does not yet support the underlying primitives. These are available in recently patched Windows 7 systems and later.
- This predicate throws an exception if the provided path is invalid or the underlying Windows API returns an error.
If open_shared_object/2 is passed an absolute path to a DLL on a Windows installation that supports AddDllDirectory() and friends,148Windows 7 with up-to-date patches or Windows 8. SWI-Prolog uses LoadLibraryEx() with the flags
LOAD_LIBRARY_SEARCH_DEFAULT_DIRS. In this scenario, directories from
%PATH%are not searched. Additional directories can be added using win_add_dll_directory/2.
- Remove a DLL search directory installed using win_add_dll_directory/2.
- This predicate is a wrapper around EnumProcessModules(). FileNames is unified with a list of absolute paths for all modules of the Windows process. Modules are the main executable file and all DLLs loaded into the process, except data DLLs. The returned file names are in canonical Prolog representation. This predicate may be used to debug loading a DLL from an unexpected location and as a helper for packaging all dependencies when creating a distribution. According to the Windows documentation this API may return incorrect results if DLLs are loaded or unloaded while EnumProcessModules() is in progress. See also qsave_program/2.
- win_get_user_preferred_ui_languages(+Format, -Languages)
- Unifies Languages with a list of the user preferred languages
(Windows Display Languages) in order of preference. If
name, the list elements are atoms. See Language Names for details. If Format is
id, Languages is a list of numeric language ids represented as Prolog integers. This predicate provides Windows alternative to setlocale/3 using the category
Non-portable Apple MacOS specific predicates are prefixed woth
- Unify Identifier with the value for CFLocaleGetIdentifier()
of the Apple current locale. The Identifier is an atom that
consists of the primary language identifier, e.g.,
enfor english followed by an undercore and an identifier for the Region in the MacOS Language & Region preferences. For example, with the primary language set to “English (UK)'' and the Region to “United Kingdom'' we get
en_GB. This relates to the locale identifier
en_GB.UTF-8. Unfortunately it is not that simple. For example, we can combine the primary language “English (UK)'' with the Region “Netherlands'' to end up with
en_NLwhich is not a valid MacOS locale.
Representing time in a computer system is surprisingly complicated. There are a large number of time representations in use, and the correct choice depends on factors such as compactness, resolution and desired operations. Humans tend to think about time in hours, days, months, years or centuries. Physicists think about time in seconds. But, a month does not have a defined number of seconds. Even a day does not have a defined number of seconds as sometimes a leap-second is introduced to synchronise properly with our earth's rotation. At the same time, resolution demands a range from better than pico-seconds to millions of years. Finally, civilizations have a wide range of calendars. Although there exist libraries dealing with most of this complexity, our desire to keep Prolog clean and lean stops us from fully supporting these.
For human-oriented tasks, time can be broken into years, months, days, hours, minutes, seconds and a timezone. Physicists prefer to have time in an arithmetic type representing seconds or fraction thereof, so basic arithmetic deals with comparison and durations. An additional advantage of the physicist's approach is that it requires much less space. For these reasons, SWI-Prolog uses an arithmetic type as its prime time representation.
Many C libraries deal with time using fixed-point arithmetic, dealing with a large but finite time interval at constant resolution. In our opinion, using a floating point number is a more natural choice as we can use a natural unit and the interface does not need to be changed if a higher resolution is required in the future. Our unit of choice is the second as it is the scientific unit.149Using Julian days is a choice made by the Eclipse team. As conversion to dates is needed for a human readable notation of time and Julian days cannot deal naturally with leap seconds, we decided for the second as our unit. We have placed our origin at 1970-01-01T0:0:0Z for compatibility with the POSIX notion of time as well as with older time support provided by SWI-Prolog.
Where older versions of SWI-Prolog relied on the POSIX conversion functions, the current implementation uses libtai to realise conversion between time-stamps and calendar dates for a period of 10 million years.
We use the following time representations
- A TimeStamp is a floating point number expressing the time in seconds since the Epoch at 1970-01-01.
- We call this term a date-time structure. The first 5 fields are
integers expressing the year, month (1..12), day (1..31), hour (0..23)
and minute (0..59). The S field holds the seconds as a
floating point number between 0.0 and 60.0. Off is an integer
representing the offset relative to UTC in seconds, where positive
values are west of Greenwich. If converted from local time (see stamp_date_time/3),
TZ holds the name of the local timezone. If the timezone is
not known, TZ is the atom
. DST is
trueif daylight saving time applies to the current time,
falseif daylight saving time is relevant but not effective, and
if unknown or the timezone has no daylight saving time.
- Date using the same values as described above. Extracted using date_time_value/3.
- Time using the same values as described above. Extracted using date_time_value/3.
- Return the current time as a TimeStamp. The granularity is system-dependent. See section 188.8.131.52.
- stamp_date_time(+TimeStamp, -DateTime, +TimeZone)
- Convert a TimeStamp to a DateTime in the given
timezone. See section
184.108.40.206 for details on the data types. TimeZone
describes the timezone for the conversion. It is one of
localto extract the local time,
’UTC'to extract a UTC time or an integer describing the seconds west of Greenwich.
- date_time_stamp(+DateTime, -TimeStamp)
- Compute the timestamp from a date/9 term. Values for month, day, hour,
minute or second need not be normalized. This flexibility allows for
easy computation of the time at any given number of these units from a
given timestamp. Normalization can be achieved following this call with stamp_date_time/3.
This example computes the date 200 days after 2006-07-14:
?- date_time_stamp(date(2006,7,214,0,0,0,0,-,-), Stamp), stamp_date_time(Stamp, D, 0), date_time_value(date, D, Date). Date = date(2007, 1, 30)
When computing a time stamp from a local time specification, the UTC offset (arg 7), TZ (arg 8) and DST (arg 9) argument may be left unbound and are unified with the proper information. The example below, executed in Amsterdam, illustrates this behaviour. On the 25th of March at 01:00, DST does not apply. At 02.00, the clock is advanced by one hour and thus both 02:00 and 03:00 represent the same time stamp.
1 ?- date_time_stamp(date(2012,3,25,1,0,0,UTCOff,TZ,DST), Stamp). UTCOff = -3600, TZ = 'CET', DST = false, Stamp = 1332633600.0. 2 ?- date_time_stamp(date(2012,3,25,2,0,0,UTCOff,TZ,DST), Stamp). UTCOff = -7200, TZ = 'CEST', DST = true, Stamp = 1332637200.0. 3 ?- date_time_stamp(date(2012,3,25,3,0,0,UTCOff,TZ,DST), Stamp). UTCOff = -7200, TZ = 'CEST', DST = true, Stamp = 1332637200.0.
Note that DST and offset calculation are based on the POSIX function mktime(). If mktime() returns an error, a representation_error
- date_time_value(?Key, +DateTime, ?Value)
- Extract values from a date/9 term. Provided keys are:
Calendar year as an integer
Calendar month as an integer 1..12
Calendar day as an integer 1..31
Clock hour as an integer 0..23
Clock minute as an integer 0..59
Clock second as a float 0.0..60.0
Offset to UTC in seconds (positive is west)
Name of timezone; fails if unknown
(true) if dst is in effect
- format_time(+Out, +Format, +StampOrDateTime)
- Modelled after POSIX strftime(), using GNU extensions. Out is
a destination as specified with with_output_to/2. Format
is an atom or string with the following conversions. Conversions start
with a percent (%) character.150Descriptions
taken from Linux Programmer's Manual
StampOrDateTime is either a numeric time-stamp, a term
date(Y,M,D,H,M,S,O,TZ,DST)or a term
The abbreviated weekday name according to the current locale. Use format_time/4 for POSIX locale.
The full weekday name according to the current locale. Use format_time/4 for POSIX locale.
The abbreviated month name according to the current locale. Use format_time/4 for POSIX locale.
The full month name according to the current locale. Use format_time/4 for POSIX locale.
The preferred date and time representation for the current locale.
The century number (year/100) as a 2-digit integer.
The day of the month as a decimal number (range 01 to 31).
Equivalent to %m/%d/%y. (For Americans only. Americans should note that in other countries %d/%m/%y is rather common. This means that in an international context this format is ambiguous and should not be used.)
Like %d, the day of the month as a decimal number, but a leading zero is replaced by a space.
Modifier. Not implemented.
Number of microseconds. The
fcan be prefixed by an integer to print the desired number of digits. E.g.,
%3fprints milliseconds. This format is not covered by any standard, but available with different format specifiers in various incarnations of the strftime() function.
Equivalent to %Y-%m-%d (the ISO 8601 date format).
Like %G, but without century, i.e., with a 2-digit year (00-99).
The ISO 8601 year with century as a decimal number. The 4-digit year corresponding to the ISO week number (see %V). This has the same format and value as %y, except that if the ISO week number belongs to the previous or next year, that year is used instead.
The ISO 8601:1988 week number of the current year as a decimal number, range 01 to 53, where week 1 is the first week that has at least 4 days in the current year, and with Monday as the first day of the week. See also %U and %W.
Equivalent to %b.
The hour as a decimal number using a 24-hour clock (range 00 to 23).
The hour as a decimal number using a 12-hour clock (range 01 to 12).
The day of the year as a decimal number (range 001 to 366).
The hour (24-hour clock) as a decimal number (range 0 to 23); single digits are preceded by a blank. (See also %H.)
The hour (12-hour clock) as a decimal number (range 1 to 12); single digits are preceded by a blank. (See also %I.)
The month as a decimal number (range 01 to 12).
The minute as a decimal number (range 00 to 59).
A newline character.
Modifier to select locale-specific output. Not implemented.
Either‘AM' or‘PM' according to the given time value, or the corresponding strings for the current locale. Noon is treated as‘pm' and midnight as‘am'.151Despite the above claim, some locales yield
pmin lower case.
Like %p but in lowercase:‘am' or‘pm' or a corresponding string for the current locale.
The time in a.m. or p.m. notation. In the POSIX locale this is equivalent to‘%I:%M:%S %p'.
The time in 24-hour notation (%H:%M). For a version including the seconds, see %T below.
The number of seconds since the Epoch, i.e., since 1970-01-01 00:00:00 UTC.
The second as a decimal number (range 00 to 60). (The range is up to 60 to allow for occasional leap seconds.)
A tab character.
The time in 24-hour notation (%H:%M:%S).
The day of the week as a decimal, range 1 to 7, Monday being 1. See also %w.
The week number of the current year as a decimal number, range 00 to 53, starting with the first Sunday as the first day of week 01. See also %V and %W.
The day of the week as a decimal, range 0 to 6, Sunday being 0. See also %u.
The week number of the current year as a decimal number, range 00 to 53, starting with the first Monday as the first day of week 01.
The preferred date representation for the current locale without the time.
The preferred time representation for the current locale without the date.
The year as a decimal number without a century (range 00 to 99).
The year as a decimal number including the century.
The timezone as hour offset from GMT using the format HHmm. Required to emit RFC822-conforming dates (using
’%a, %d %b %Y %T %z'). Our implementation supports
%:z, which modifies the output to HH:mm as required by XML-Schema. Note that both notations are valid in ISO 8601. The sequence
%:zis compatible to the GNU date(1) command.
The timezone or name or abbreviation.
The date and time in date(1) format.
The table below gives some format strings for popular time representations. RFC1123 is used by HTTP. The full implementation of http_timestamp/2 as available from
http_timestamp(Time, Atom) :- stamp_date_time(Time, Date, 'UTC'), format_time(atom(Atom), '%a, %d %b %Y %T GMT', Date, posix).
Standard Format string xsd
’%a, %d %b %Y %T %z'
’%a, %d %b %Y %T GMT'
- format_time(+Out, +Format, +StampOrDateTime, +Locale)
- Format time given a specified Locale. This predicate is a
work-around for lacking proper portable and thread-safe time and locale
handling in current C libraries. In its current implementation the only
value allowed for Locale is
posix, which currently only modifies the behaviour of the
Bformat specifiers. The predicate is used to be able to emit POSIX locale week and month names for emitting standardised time-stamps such as RFC1123.
- parse_time(+Text, -Stamp)
- Same as
parse_time(Text, _Format, Stamp). See parse_time/3.
- parse_time(+Text, ?Format, -Stamp)
- Parse a textual time representation, producing a time-stamp. Supported
formats for Text are in the table below. If the format is
known, it may be given to reduce parse time and avoid ambiguities.
Format is unified with the format encountered.
Name Example rfc_1123
Fri, 08 Dec 2006 15:29:44 GMT
Fri, 08 Dec 2006 15:29:44 +0000
- Computes the day of the week for a given date.
Date = date(Year,Month,Day). Days of the week are numbered from one to seven: Monday = 1, Tuesday = 2, ... , Sunday = 7.
The Windows executable swipl-win.exe console has a number of predicates to control the appearance of the console. Being totally non-portable, we do not advise using it for your own application, but use XPCE or another portable GUI platform instead. We give the predicates for reference here.
- window_title(-Old, +New)
- Unify Old with the title displayed in the console and change
the title to New.bugThis
predicate should have been called
win_window_titlefor consistent naming.
- Interface to the MS-Windows SetWindowPos() function, controlling
size, position and stacking order of the window. ListOfOptions
is a list that may hold any number of the terms below:
- size(W, H)
- Change the size of the window. W and H are expressed in character units.
- position(X, Y)
- Change the top-left corner of the window. The values are expressed in pixel units.
- Change the location in the window stacking order. Values are
notopmost. Topmost windows are displayed above all other windows.
true, show the window, if
falsehide the window.
- If present, activate the window.
- win_window_color(+Which, +RGB)
- Change the color of the console window. Which is one of
selection_background. RGB is a term
rgb(Red,Green,Blue)where the components are values between 0 and 255. The defaults are established using the Windows API GetSysColor().
- True if win_insert_menu/2 and win_insert_menu_item/4 are present.
- win_insert_menu(+Label, +Before)
- Insert a new entry (pulldown) in the menu. If the menu already contains
this entry, nothing is done. The Label is the label and,
using the Windows convention, a letter prefixed with
&is underlined and defines the associated accelerator key. Before is the label before which this one must be inserted. Using
adds the new entry at the end (right). For example, the call below adds an Application entry just before the Help menu.
- win_insert_menu_item(+Pulldown, +Label, +Before, :Goal)
- Add an item to the named Pulldown menu. Label and
Before are handled as in win_insert_menu/2,
but the label
inserts a separator. Goal is called if the user selects the item.