4.34 Operating System Interaction

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 Win32 systems, 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 (command.exe on Windows 95/98 and cmd.exe on Windows-NT) and therefore commands built-in to the command-interpreter can only be activated using the command interpreter. For example: 'command.exe /C copy file1.txt file2.txt'

Equivalent to `shell(Command, 0)'.
Start an interactive Unix shell. Default is /bin/sh, the environment variable SHELL overrides this default. Not available for Win32 platforms.
win_exec(+Command, +Show)
Win32 systems only. Spawns a Windows task without waiting for its completion. Show is one of the Win32 SW_* constants written in lowercase without the SW_*: hide maximize minimize restore show showdefault showmaximized showminimized showminnoactive showna shownoactive shownormal. In addition, iconic is a synonym for minimize and normal for shownormal
win_shell(+Operation, +File, +Show)
Win32 systems only. Opens the document File using the windows shell-rules for doing so. Operation is one of open, print or explore or 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)
Win32 systems only. Fetches the value of a Win32 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_CLASSES_ROOT, HKEY_CURRENT_USER, HKEY_LOCAL_MACHINE or HKEY_USERS. The example below fetches the extension to use for Prolog files (see README.TXT on the Windows version):
?- win_registry_get_value('HKEY_LOCAL_MACHINE/Software/SWI/Prolog',

Ext = pl
win_folder(?Name, -Directory)
Is true if Name is the Windows `CSIDL' of Directory. If Name is unbound all known Windows special paths are generated. 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'
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 all, collate, ctype, messages, monetary, numeric or time. For details, please consult the C-library locale documentation. See also section 2.17.1. 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 use in multi-threaded environments. Locale settings are used by format_time/3, collation_key/2 and locale_sort/2.
This predicate comes from the Quintus compatibility library and provides a partial implementation thereof. It provides access to some operating system features and unlike the name suggests, is not operating system specific. Defined Command's are below.
Equivalent to calling shell/1. Use for compatibility only.
Equivalent to calling shell/1. Use for compatibility only.
Equivalent to calling shell/0. Use for compatibility only.
Equivalent to calling working_directory/2 to the expansion (see expand_file_name/2) of ~. For compatibility only.
Equivalent to calling working_directory/2. Use for compatibility only.
Unify Argv with the list of command-line arguments provides to this Prolog run. Please note that Prolog system-arguments and application arguments are separated by --. Integer arguments are passed as Prolog integers, float arguments and Prolog floating point numbers and all other arguments as Prolog atoms. New applications should use the Prolog flag argv. See also prolog Prolog flag argv.

A stand-alone program could use the following skeleton to handle command-line arguments. See also section

main :-
        current_prolog_flag(argv, Argv),
        append(_PrologArgs, [--|AppArgs], Argv), !,

4.34.1 Dealing with time and date

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 range from better then pico-seconds to millions of years. Finally, civilizations have a wide range of calendars. Although there exist libraries dealing with most if 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 deal with comparison and durations. An additional advantage of the physicists 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.60Using 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 second as our unit. e have placed our origin at 1970-1-1T0:0:0Z 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. Time and date data-structures

We use the following time representations

A TimeStamp is a floating point number expression the time in seconds since the Epoch at 1970-1-1.
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), 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 Greenwhich. 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 true if daylight saving time applies to the current time, false if 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. Time and date predicates

Return the current time as a TimeStamp. The granularity is system dependent. See section
stamp_date_time(+TimeStamp, -DateTime, +TimeZone)
Convert a TimeStamp to a DateTime in the given time zone. See section for details on the data-types. TimeZone describes the timezone for the conversion. It is one of local to extract the local time, 'UTC' to extract at UTC time or an integer describing the seconds west of Greenwhich.
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 2006-7-14:
?- 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)
date_time_value(?Key, +DateTime, ?Value)
Extract values from a date/9 term. Provided keys are:

year Calendar year as an integer
month Calendar month as an integer 1..12
day Calendar day as an integer 1..31
hour Clock hour as an integer 0..23
minute Clock minute as an integer 0..59
second Clock second as a float 0.0..60.0
utc_offset Offset to UTC in seconds (positive is west)
time_zone Name of timezone; fails if unknown
daylight_saving Bool daylight_savingtrue) if dst is effective
date Term date(Y,M,D)
time Term time(H,M,S)
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 tilde (%) character.61Descriptions taken from Linux Programmer's Manual

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 or the a, A, b and B format 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)
Parse a textual time representation, producing a time-stamp. Supported formats for Text are:

RFC 1123Fri, 08 Dec 2006 15:29:44 GMT

4.34.2 Controlling the PLWIN.EXE console window

The Windows executable PLWIN.EXE console has a number of predicates to control the appearance of the console. Being totally non-portable, we do not advice 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_title for 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 bottom, top, topmost and notopmost. Topmost windows are displayed above all other windows.
If true, show the window, if false hide the window.
If present, activate the window.
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 conventions, 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 a Application entry just before the Help menu.
win_insert_menu('&Application', '&Help')
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.