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<h2 id="sec:system"><a id="sec:4.34"><span class="sec-nr">4.34</span> <span class="sec-title">Operating 
System Interaction</span></a></h2>

<a id="sec:system"></a>

<dl class="latex">
<dt class="pubdef"><a id="shell/2"><strong>shell</strong>(<var>+Command, 
-Status</var>)</a></dt>
<dd class="defbody">
Execute <var>Command</var> on the operating system. <var>Command</var> 
is given to the Bourne shell (/bin/sh). <var>Status</var> is unified 
with the exit status of the command.

<p>On Windows, <a id="idx:shell12:1246"></a><span class="pred-ext">shell/[1,2]</span> 
executes the command using the CreateProcess() API and waits for the 
command to terminate. If the command ends with a
<code>&amp;</code> sign, the command is handed to the WinExec() API, 
which does not wait for the new task to terminate. See also <a id="idx:winexec2:1247"></a><a class="pred" href="system.html#win_exec/2">win_exec/2</a> 
and
<a id="idx:winshell2:1248"></a><a class="pred" href="system.html#win_shell/2">win_shell/2</a>. 
Please note that the CreateProcess() API does <b>not</b> imply the 
Windows command interpreter (<b>cmd.exe</b> and therefore commands that 
are built in the command interpreter can only be activated using the 
command interpreter. For example, a file can be compied using the 
command below.

<pre class="code">
?- shell('cmd.exe /C copy file1.txt file2.txt').
</pre>

<p>Note that many of the operations that can be achieved using the shell 
built-in commands can easily be achieved using Prolog primitives. See
<a id="idx:makedirectory1:1249"></a><a class="pred" href="files.html#make_directory/1">make_directory/1</a>, <a id="idx:deletefile1:1250"></a><a class="pred" href="files.html#delete_file/1">delete_file/1</a>, <a id="idx:renamefile2:1251"></a><a class="pred" href="files.html#rename_file/2">rename_file/2</a>, 
etc. The clib package provides <code>library(filesex)</code>, 
implementing various high level file operations such as <a id="idx:copyfile2:1252"></a><span class="pred-ext">copy_file/2</span>. 
Using Prolog primitives instead of shell commands improves the 
portability of your program.

<p>The library <code>library(process)</code> provides <a id="idx:processcreate3:1253"></a><span class="pred-ext">process_create/3</span> 
and several related primitives that support more fine-grained 
interaction with processes, including I/O redirection and management of 
asynchronous processes.</dd>
<dt class="pubdef"><a id="shell/1"><strong>shell</strong>(<var>+Command</var>)</a></dt>
<dd class="defbody">
Equivalent to `<code>shell(Command, 0)</code>'.</dd>
<dt class="pubdef"><a id="shell/0"><strong>shell</strong></a></dt>
<dd class="defbody">
Start an interactive Unix shell. Default is <code>/bin/sh</code>; the 
environment variable <code>SHELL</code> overrides this default. Not 
available for Win32 platforms.</dd>
<dt class="pubdef"><a id="getenv/2"><strong>getenv</strong>(<var>+Name, 
-Value</var>)</a></dt>
<dd class="defbody">
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.</dd>
<dt class="pubdef"><a id="setenv/2"><strong>setenv</strong>(<var>+Name, 
+Value</var>)</a></dt>
<dd class="defbody">
Set an environment variable. <var>Name</var> and <var>Value</var> must 
be instantiated to atoms or integers. The environment variable will be 
passed to <a id="idx:shell02:1254"></a><span class="pred-ext">shell/[0-2]</span> 
and can be requested using <a id="idx:getenv2:1255"></a><a class="pred" href="system.html#getenv/2">getenv/2</a>. 
They also influence <a id="idx:expandfilename2:1256"></a><a class="pred" href="files.html#expand_file_name/2">expand_file_name/2</a>. 
Environment variables are shared between threads. Depending on the 
underlying C library, <a id="idx:setenv2:1257"></a><a class="pred" href="system.html#setenv/2">setenv/2</a> 
and <a id="idx:unsetenv1:1258"></a><a class="pred" href="system.html#unsetenv/1">unsetenv/1</a> 
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.</dd>
<dt class="pubdef"><a id="unsetenv/1"><strong>unsetenv</strong>(<var>+Name</var>)</a></dt>
<dd class="defbody">
Remove an environment variable from the environment. Some systems lack 
the underlying unsetenv() library function. On these systems <a id="idx:unsetenv1:1259"></a><a class="pred" href="system.html#unsetenv/1">unsetenv/1</a> 
sets the variable to the empty string.</dd>
<dt class="pubdef"><a id="setlocale/3"><strong>setlocale</strong>(<var>+Category, 
-Old, +New</var>)</a></dt>
<dd class="defbody">
Set/Query the <em>locale</em> setting which tells the C library how to 
interpret text files, write numbers, dates, etc. Category is one of
<code>all</code>, <code>collate</code>, <code>ctype</code>, <code>messages</code>,
<code>monetary</code>, <code>numeric</code> or <code>time</code>. For 
details, please consult the C library locale documentation. See also <a class="sec" href="widechars.html">section 
2.18.1</a>. Please note that the locale is shared between all threads 
and thread-safe usage of <a id="idx:setlocale3:1260"></a><a class="pred" href="system.html#setlocale/3">setlocale/3</a> 
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 <a id="idx:formattime3:1261"></a><a class="pred" href="system.html#format_time/3">format_time/3</a>, <a id="idx:collationkey2:1262"></a><a class="pred" href="chartype.html#collation_key/2">collation_key/2</a> 
and <a id="idx:localesort2:1263"></a><a class="pred" href="chartype.html#locale_sort/2">locale_sort/2</a>.</dd>
<dt class="pubdef"><a id="unix/1"><strong>unix</strong>(<var>+Command</var>)</a></dt>
<dd class="defbody">
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 <var>Command</var>'s are below.

<dl class="latex">
<dt><strong>system</strong>(<var>+Command</var>)</dt>
<dd class="defbody">
Equivalent to calling <a id="idx:shell1:1264"></a><a class="pred" href="system.html#shell/1">shell/1</a>. 
Use for compatibility only.</dd>
<dt><strong>shell</strong>(<var>+Command</var>)</dt>
<dd class="defbody">
Equivalent to calling <a id="idx:shell1:1265"></a><a class="pred" href="system.html#shell/1">shell/1</a>. 
Use for compatibility only.</dd>
<dt><strong>shell</strong></dt>
<dd class="defbody">
Equivalent to calling <a id="idx:shell0:1266"></a><a class="pred" href="system.html#shell/0">shell/0</a>. 
Use for compatibility only.</dd>
<dt><strong>cd</strong></dt>
<dd class="defbody">
Equivalent to calling <a id="idx:workingdirectory2:1267"></a><a class="pred" href="files.html#working_directory/2">working_directory/2</a> 
to the expansion (see
<a id="idx:expandfilename2:1268"></a><a class="pred" href="files.html#expand_file_name/2">expand_file_name/2</a>) 
of <code><code>~</code></code>. For compatibility only.</dd>
<dt><strong>cd</strong>(<var>+Directory</var>)</dt>
<dd class="defbody">
Equivalent to calling <a id="idx:workingdirectory2:1269"></a><a class="pred" href="files.html#working_directory/2">working_directory/2</a>. 
Use for compatibility only.</dd>
<dt><strong>argv</strong>(<var>-Argv</var>)</dt>
<dd class="defbody">
Unify <var>Argv</var> with the list of command line arguments provided 
to this Prolog run. Please note that Prolog system arguments and 
application arguments are separated by <code>--</code>. 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 <a class="flag" href="flags.html#flag:argv">argv</a>. 
See also the Prolog flag
<a class="flag" href="flags.html#flag:argv">argv</a>.

<p>A stand-alone program could use the following skeleton to handle 
command line arguments. See also <a class="sec" href="compilation.html">section 
2.10.2.4</a>.

<pre class="code">
main :-
        current_prolog_flag(argv, Argv),
        append(_PrologArgs, [--|AppArgs], Argv), !,
        main(AppArgs).
</pre>

<p></dd>
</dl>

</dd>
</dl>

<p><h3 id="sec:winsystem"><a id="sec:4.34.1"><span class="sec-nr">4.34.1</span> <span class="sec-title">Windows-specific 
Operating System Interaction</span></a></h3>

<a id="sec:winsystem"></a>

<p>The predicates in this section are only available on the Windows 
version of SWI-Prolog. Their use is discouraged if there are portably 
alternatives. For example, <a id="idx:winexec2:1270"></a><a class="pred" href="system.html#win_exec/2">win_exec/2</a> 
and <a id="idx:willshell2:1271"></a><span class="pred-ext">will_shell/2</span> 
can often be replaced by the more portable <a id="idx:shell2:1272"></a><a class="pred" href="system.html#shell/2">shell/2</a> 
or the more powerful
<a id="idx:processcreate3:1273"></a><span class="pred-ext">process_create/3</span>.

<dl class="latex">
<dt class="pubdef"><a id="win_exec/2"><strong>win_exec</strong>(<var>+Command, 
+Show</var>)</a></dt>
<dd class="defbody">
Windows only. Spawns a Windows task without waiting for its completion. <var>Show</var> 
is one of the Win32 <code>SW_*</code> constants written in lowercase 
without the <code>SW_*</code>:
<code>hide</code>
<code>maximize</code>
<code>minimize</code>
<code>restore</code>
<code>show</code>
<code>showdefault</code>
<code>showmaximized</code>
<code>showminimized</code>
<code>showminnoactive</code>
<code>showna</code>
<code>shownoactive</code>
<code>shownormal</code>. In addition, <code>iconic</code> is a synonym 
for <code>minimize</code> and
<code>normal</code> for <code>shownormal</code>.</dd>
<dt class="pubdef"><a id="win_shell/2"><strong>win_shell</strong>(<var>+Operation, 
+File, +Show</var>)</a></dt>
<dd class="defbody">
Windows only. Opens the document <var>File</var> using the Windows shell 
rules for doing so. <var>Operation</var> is one of <code>open</code>,
<code>print</code> or <code>explore</code> or another operation 
registered with the shell for the given document type. On modern systems 
it is also possible to pass a <a id="idx:URL:1274"></a>URL as <var>File</var>, 
opening the URL in Windows default browser. This call interfaces to the 
Win32 API ShellExecute(). The <var>Show</var> argument determines the 
initial state of the opened window (if any). See <a id="idx:winexec2:1275"></a><a class="pred" href="system.html#win_exec/2">win_exec/2</a> 
for defined values.</dd>
<dt class="pubdef"><a id="win_shell/2"><strong>win_shell</strong>(<var>+Operation, 
+File</var>)</a></dt>
<dd class="defbody">
Same as <code>win_shell(Operation, File, normal)</code></dd>
<dt class="pubdef"><a id="win_registry_get_value/3"><strong>win_registry_get_value</strong>(<var>+Key, 
+Name, -Value</var>)</a></dt>
<dd class="defbody">
Windows only. Fetches the value of a Windows registry key. <var>Key</var> 
is an atom formed as a path name describing the desired registry key.
<var>Name</var> is the desired attribute name of the key. <var>Value</var> 
is unified with the value. If the value is of type <code>DWORD</code>, 
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 <code>HKEY_CURRENT_USER</code>. Other roots can be 
specified explicitly as
<code>HKEY_CLASSES_ROOT</code>, <code>HKEY_CURRENT_USER</code>,
<code>HKEY_LOCAL_MACHINE</code> or <code>HKEY_USERS</code>. The example 
below fetches the extension to use for Prolog files (see <code>README.TXT</code> 
on the Windows version):

<pre class="code">
?- win_registry_get_value(
       'HKEY_LOCAL_MACHINE/Software/SWI/Prolog',
       fileExtension,
       Ext).

Ext = pl
</pre>

</dd>
<dt class="pubdef"><a id="win_folder/2"><strong>win_folder</strong>(<var>?Name, 
-Directory</var>)</a></dt>
<dd class="defbody">
True if <var>Name</var> is the Windows `CSIDL' of <var>Directory</var>. 
If
<var>Name</var> is unbound, all known Windows special paths are 
generated.
<var>Name</var> is the CSIDL after deleting the leading <code>CSIDL_</code> 
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:

<pre class="code">
?- win_folder(personal, MyDocuments).

MyDocuments = 'C:/Documents and Settings/jan/My Documents'
</pre>

</dd>
<dt class="pubdef"><a id="win_add_dll_directory/1"><strong>win_add_dll_directory</strong>(<var>+AbsDir</var>)</a></dt>
<dd class="defbody">
This predicate adds a directory to the search path for dependent DLL 
files. If possible, this is achieved with <a id="idx:winadddlldirectory2:1276"></a><a class="pred" href="system.html#win_add_dll_directory/2">win_add_dll_directory/2</a>. 
Otherwise, <code>%PATH%</code> is extended with the provided directory.
<var>AbsDir</var> may be specified in the Prolog canonical syntax. See
<a id="idx:prologtoosfilename2:1277"></a><a class="pred" href="files.html#prolog_to_os_filename/2">prolog_to_os_filename/2</a>. 
Note that <a id="idx:useforeignlibrary1:1278"></a><a class="pred" href="foreignlink.html#use_foreign_library/1">use_foreign_library/1</a> 
passes an absolute path to the DLL if the destination DLL can be located 
from the specification using <a id="idx:absolutefilename3:1279"></a><a class="pred" href="files.html#absolute_file_name/3">absolute_file_name/3</a>.</dd>
<dt class="pubdef"><a id="win_add_dll_directory/2"><strong>win_add_dll_directory</strong>(<var>+AbsDir, 
-Cookie</var>)</a></dt>
<dd class="defbody">
This predicate adds a directory to the search path for dependent DLL 
files. If the call is successful it unifies <var>Cookie</var> with a 
handle that must be passed to <a id="idx:winremovedlldirectory1:1280"></a><a class="pred" href="system.html#win_remove_dll_directory/1">win_remove_dll_directory/1</a> 
to remove the directory from the search path. Error conditions:

<p>
<ul class="latex">
<li>This predicate is available in the Windows port of SWI-Prolog 
starting from 6.3.8/6.2.6.
<li>This predicate <em>fails</em> if Windows does not yet support the 
underlying primitives. These are available in recently patched Windows&nbsp;7 
systems and later.
<li>This predicate throws an acception if the provided path is invalid 
or the underlying Windows API returns an error.
</ul>

<p>If <a id="idx:opensharedobject2:1281"></a><a class="pred" href="foreignlink.html#open_shared_object/2">open_shared_object/2</a> 
is passed an <em>absolute</em> path to a DLL on a Windows installation 
that supports AddDllDirectory() and friends,<sup class="fn">94<span class="fn-text">Windows&nbsp;7 
with up-to-date patches or Windows&nbsp;8.</span></sup> SWI-Prolog uses 
LoadLibraryEx() with the flags
<code>LOAD_LIBRARY_SEARCH_DLL_LOAD_DIR</code> and
<code>LOAD_LIBRARY_SEARCH_DEFAULT_DIRS</code>. In this scenario, 
directories from <code>%PATH%</code> and <em>not</em> searched. 
Additional directories can be added using <a id="idx:winadddlldirectory2:1282"></a><a class="pred" href="system.html#win_add_dll_directory/2">win_add_dll_directory/2</a>.</dd>
<dt class="pubdef"><a id="win_remove_dll_directory/1"><strong>win_remove_dll_directory</strong>(<var>-Cookie</var>)</a></dt>
<dd class="defbody">
Remove a DLL search directory installed using <a id="idx:winadddlldirectory2:1283"></a><a class="pred" href="system.html#win_add_dll_directory/2">win_add_dll_directory/2</a>.
</dd>
</dl>

<p><h3 id="sec:timedate"><a id="sec:4.34.2"><span class="sec-nr">4.34.2</span> <span class="sec-title">Dealing 
with time and date</span></a></h3>

<a id="sec:timedate"></a>

<p>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 if this complexity, our desire 
to keep Prolog clean and lean stops us from fully supporting these.

<p>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.

<p>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.<sup class="fn">95<span class="fn-text">Using 
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.</span></sup> 
We 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.

<p>Where older versions of SWI-Prolog relied on the POSIX conversion 
functions, the current implementation uses
<a class="url" href="http://cr.yp.to/libtai.html">libtai</a> to realise 
conversion between time-stamps and calendar dates for a period of 10 
million years.

<p><h4 id="sec:dattimedata"><a id="sec:4.34.2.1"><span class="sec-nr">4.34.2.1</span> <span class="sec-title">Time 
and date data structures</span></a></h4>

<a id="sec:dattimedata"></a>

<p>We use the following time representations

<dl class="latex">
<dt><b>TimeStamp</b></dt>
<dd class="defbody">
A TimeStamp is a floating point number expressing the time in seconds 
since the Epoch at 1970-1-1.</dd>
<dt><strong>date</strong>(<var>Y,M,D,H,Mn,S,Off,TZ,DST</var>)</dt>
<dd class="defbody">
We call this term a <em>date-time</em> structure. The first 5 fields are 
integers expressing the year, month (1..12), day (1..31), hour (0..23) 
and minute (0..59). The <var>S</var> field holds the seconds as a 
floating point number between 0.0 and 60.0. <var>Off</var> is an integer 
representing the offset relative to UTC in seconds, where positive 
values are west of Greenwich. If converted from local time (see <a id="idx:stampdatetime3:1284"></a><a class="pred" href="system.html#stamp_date_time/3">stamp_date_time/3</a>),
<var>TZ</var> holds the name of the local timezone. If the timezone is 
not known, <var>TZ</var> is the atom <code><code>-</code></code>. <var>DST</var> 
is <code>true</code> if daylight saving time applies to the current 
time, <code>false</code> if daylight saving time is relevant but not 
effective, and <code><code>-</code></code> if unknown or the timezone 
has no daylight saving time.</dd>
<dt><strong>date</strong>(<var>Y,M,D</var>)</dt>
<dd class="defbody">
Date using the same values as described above. Extracted using
<a id="idx:datetimevalue3:1285"></a><a class="pred" href="system.html#date_time_value/3">date_time_value/3</a>.</dd>
<dt><strong>time</strong>(<var>H,Mn,S</var>)</dt>
<dd class="defbody">
Time using the same values as described above. Extracted using
<a id="idx:datetimevalue3:1286"></a><a class="pred" href="system.html#date_time_value/3">date_time_value/3</a>.
</dd>
</dl>

<p><h4 id="sec:datimepreds"><a id="sec:4.34.2.2"><span class="sec-nr">4.34.2.2</span> <span class="sec-title">Time 
and date predicates</span></a></h4>

<a id="sec:datimepreds"></a>

<dl class="latex">
<dt class="pubdef"><a id="get_time/1"><strong>get_time</strong>(<var>-TimeStamp</var>)</a></dt>
<dd class="defbody">
Return the current time as a <var>TimeStamp</var>. The granularity is 
system-dependent. See <a class="sec" href="system.html">section 4.34.2.1</a>.</dd>
<dt class="pubdef"><a id="stamp_date_time/3"><strong>stamp_date_time</strong>(<var>+TimeStamp, 
-DateTime, +TimeZone</var>)</a></dt>
<dd class="defbody">
Convert a <var>TimeStamp</var> to a <var>DateTime</var> in the given 
timezone. See <a class="sec" href="system.html">section 4.34.2.1</a> for 
details on the data types. <var>TimeZone</var> describes the timezone 
for the conversion. It is one of <code>local</code> to extract the local 
time, <code>'UTC'</code> to extract a UTC time or an integer describing 
the seconds west of Greenwich.</dd>
<dt class="pubdef"><a id="date_time_stamp/2"><strong>date_time_stamp</strong>(<var>+DateTime, 
-TimeStamp</var>)</a></dt>
<dd class="defbody">
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 <a id="idx:stampdatetime3:1287"></a><a class="pred" href="system.html#stamp_date_time/3">stamp_date_time/3</a>. 
This example computes the date 200 days after 2006-7-14:

<pre class="code">
?- 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)
</pre>

<p>When computing a time stamp from a local time specification, the UTC 
offset (arg&nbsp;7), TZ (arg&nbsp;8) and DST (arg&nbsp;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.

<pre class="code">
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.
</pre>

<p>Note that DST and offset calculation are based on the POSIX function 
mktime(). If mktime() returns an error, a representation_error
<code>dst</code> is generated.</dd>
<dt class="pubdef"><a id="date_time_value/3"><strong>date_time_value</strong>(<var>?Key, 
+DateTime, ?Value</var>)</a></dt>
<dd class="defbody">
Extract values from a date/9 term. Provided keys are:

<p><table class="latex frame-hsides center">
<tr><td><b>key</b></td><td><b>value </b></td></tr>
<tr class="hline"><td><code>year</code> </td><td>Calendar year as an 
integer </td></tr>
<tr><td><code>month</code> </td><td>Calendar month as an integer 1..12 </td></tr>
<tr><td><code>day</code> </td><td>Calendar day as an integer 1..31 </td></tr>
<tr><td><code>hour</code> </td><td>Clock hour as an integer 0..23 </td></tr>
<tr><td><code>minute</code> </td><td>Clock minute as an integer 0..59 </td></tr>
<tr><td><code>second</code> </td><td>Clock second as a float 0.0..60.0 </td></tr>
<tr><td><code>utc_offset</code> </td><td>Offset to UTC in seconds 
(positive is west) </td></tr>
<tr><td><code>time_zone</code> </td><td>Name of timezone; fails if 
unknown </td></tr>
<tr><td><code>daylight_saving</code> </td><td>Bool <code>daylight_saving</code>true) 
if dst is in effect </td></tr>
<tr><td><code>date</code> </td><td>Term <code>date(Y,M,D)</code> </td></tr>
<tr><td><code>time</code> </td><td>Term <code>time(H,M,S)</code> </td></tr>
</table>
</dd>
<dt class="pubdef"><a id="format_time/3"><strong>format_time</strong>(<var>+Out, 
+Format, +StampOrDateTime</var>)</a></dt>
<dd class="defbody">
Modelled after POSIX strftime(), using GNU extensions. <var>Out</var> is 
a destination as specified with <a id="idx:withoutputto2:1288"></a><a class="pred" href="IO.html#with_output_to/2">with_output_to/2</a>. <var>Format</var> 
is an atom or string with the following conversions. Conversions start 
with a tilde (%) character.<sup class="fn">96<span class="fn-text">Descriptions 
taken from Linux Programmer's Manual</span></sup>
<var>StampOrDateTime</var> is either a numeric time-stamp, a term
<code>date(Y,M,D,H,M,S,O,TZ,DST)</code> or a term <code>date(Y,M,D)</code>.

<p>
<ul class="latex">
<li><code>a</code><br>
The abbreviated weekday name according to the current locale. Use
<a id="idx:formattime4:1289"></a><a class="pred" href="system.html#format_time/4">format_time/4</a> 
for POSIX locale.
<li><code>A</code><br>
The full weekday name according to the current locale. Use
<a id="idx:formattime4:1290"></a><a class="pred" href="system.html#format_time/4">format_time/4</a> 
for POSIX locale.
<li><code>b</code><br>
The abbreviated month name according to the current locale. Use
<a id="idx:formattime4:1291"></a><a class="pred" href="system.html#format_time/4">format_time/4</a> 
for POSIX locale.
<li><code>B</code><br>
The full month name according to the current locale. Use
<a id="idx:formattime4:1292"></a><a class="pred" href="system.html#format_time/4">format_time/4</a> 
for POSIX locale.
<li><code>c</code><br>
The preferred date and time representation for the current locale.
<li><code>C</code><br>
The century number (year/100) as a 2-digit integer.
<li><code>d</code><br>
The day of the month as a decimal number (range 01 to 31).
<li><code>D</code><br>
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.)
<li><code>e</code><br>
Like %d, the day of the month as a decimal number, but a leading zero is 
replaced by a space.
<li><code>E</code><br>
Modifier. Not implemented.
<li><code>f</code><br>
Number of microseconds. The <code>f</code> can be prefixed by an integer 
to print the desired number of digits. E.g., <code>%3f</code> prints 
milliseconds. This format is not covered by any standard, but available 
with different format specifiers in various incarnations of the 
strftime() function.
<li><code>F</code><br>
Equivalent to %Y-%m-%d (the ISO 8601 date format).
<li><code>g</code><br>
Like %G, but without century, i.e., with a 2-digit year (00-99).
<li><code>G</code><br>
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.
<li><code>V</code><br>
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.
<li><code>h</code><br>
Equivalent to %b.
<li><code>H</code><br>
The hour as a decimal number using a 24-hour clock (range 00 to 23).
<li><code>I</code><br>
The hour as a decimal number using a 12-hour clock (range 01 to 12).
<li><code>j</code><br>
The day of the year as a decimal number (range 001 to 366).
<li><code>k</code><br>
The hour (24-hour clock) as a decimal number (range 0 to 23); single 
digits are preceded by a blank. (See also %H.)
<li><code>l</code><br>
The hour (12-hour clock) as a decimal number (range 1 to 12); single 
digits are preceded by a blank. (See also %I.)
<li><code>m</code><br>
The month as a decimal number (range 01 to 12).
<li><code>M</code><br>
The minute as a decimal number (range 00 to 59).
<li><code>n</code><br>
A newline character.
<li><code>O</code><br>
Modifier to select locale-specific output. Not implemented.
<li><code>p</code><br>
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'.
<li><code>P</code><br>
Like %p but in lowercase: `am' or `pm' or a corresponding string for the 
current locale.
<li><code>r</code><br>
The time in a.m. or p.m. notation. In the POSIX locale this is 
equivalent to `%I:%M:%S %p'.
<li><code>R</code><br>
The time in 24-hour notation (%H:%M). For a version including the 
seconds, see %T below.
<li><code>s</code><br>
The number of seconds since the Epoch, i.e., since 1970-01-01 00:00:00 
UTC.
<li><code>S</code><br>
The second as a decimal number (range 00 to 60). (The range is up to 60 
to allow for occasional leap seconds.)
<li><code>t</code><br>
A tab character.
<li><code>T</code><br>
The time in 24-hour notation (%H:%M:%S).
<li><code>u</code><br>
The day of the week as a decimal, range 1 to 7, Monday being 1. See also %w.
<li><code>U</code><br>
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.
<li><code>w</code><br>
The day of the week as a decimal, range 0 to 6, Sunday being 0. See also %u.
<li><code>W</code><br>
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.
<li><code>x</code><br>
The preferred date representation for the current locale without the 
time.
<li><code>X</code><br>
The preferred time representation for the current locale without the 
date.
<li><code>y</code><br>
The year as a decimal number without a century (range 00 to 99).
<li><code>Y</code><br>
The year as a decimal number including the century.
<li><code>z</code><br>
The timezone as hour offset from GMT using the format HHmm. Required to 
emit RFC822-conforming dates (using
<code>'%a,&nbsp;%d&nbsp;%b&nbsp;%Y&nbsp;%T&nbsp;%z'</code>). Our 
implementation supports
<code>%:z</code>, which modifies the output to HH:mm as required by 
XML-Schema. Note that both notations are valid in ISO 8601. The sequence <code>%:z</code> 
is compatible to the GNU date(1) command.
<li><code>Z</code><br>
The timezone or name or abbreviation.
<li><code><code>+</code></code><br>
The date and time in date(1) format.
<li><code>%</code><br>
A literal `%' character.
</ul>

<p>The table below gives some format strings for popular time 
representations. RFC1123 is used by HTTP. The full implementation of
<a id="idx:httptimestamp2:1293"></a><span class="pred-ext">http_timestamp/2</span> 
as available from <code>library(http/http_header)</code> is here.

<pre class="code">
http_timestamp(Time, Atom) :-
        stamp_date_time(Time, Date, 'UTC'),
        format_time(atom(Atom),
                    '%a, %d %b %Y %T GMT',
                    Date, posix).
</pre>

<p><table class="latex frame-hsides center">
<tr><td><b>Standard</b> </td><td><b>Format string</b> </td></tr>
<tr class="hline"><td><b>xsd</b> </td><td><code>'%FT%T%:z'</code> </td></tr>
<tr><td><b>ISO8601</b> </td><td><code>'%FT%T%z'</code> </td></tr>
<tr><td><b>RFC822</b> </td><td><code>'%a, %d %b %Y %T %z'</code> </td></tr>
<tr><td><b>RFC1123</b> </td><td><code>'%a, %d %b %Y %T GMT'</code> </td></tr>
</table>
</dd>
<dt class="pubdef"><a id="format_time/4"><strong>format_time</strong>(<var>+Out, 
+Format, +StampOrDateTime, +Locale</var>)</a></dt>
<dd class="defbody">
Format time given a specified <var>Locale</var>. 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 <var>Locale</var> is <code>posix</code>, which 
currently only modifies the behaviour of the <code>a</code>, <code>A</code>, <code>b</code> 
and <code>B</code> 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.</dd>
<dt class="pubdef"><a id="parse_time/2"><strong>parse_time</strong>(<var>+Text, 
-Stamp</var>)</a></dt>
<dd class="defbody">
Same as <code>parse_time(Text, _Format, Stamp)</code>. See <a id="idx:parsetime3:1294"></a><a class="pred" href="system.html#parse_time/3">parse_time/3</a>.</dd>
<dt class="pubdef"><a id="parse_time/3"><strong>parse_time</strong>(<var>+Text, 
?Format, -Stamp</var>)</a></dt>
<dd class="defbody">
Parse a textual time representation, producing a time-stamp. Supported 
formats for <var>Text</var> are in the table below. If the format is 
known, it may be given to reduce parse time and avoid ambiguities. 
Otherwise,
<var>Format</var> is unified with the format encountered.

<p><table class="latex frame-box center">
<tr><td><b>Name</b></td><td><b>Example </b></td></tr>
<tr class="hline"><td>rfc_1123</td><td><code>Fri, 08 Dec 2006 15:29:44 
GMT </code></td></tr>
<tr class="hline"><td>iso_8601</td><td><code>2006-12-08T17:29:44+02:00 </code></td></tr>
<tr><td></td><td><code>20061208T172944+0200 </code></td></tr>
<tr><td></td><td><code>2006-12-08T15:29Z </code></td></tr>
<tr><td></td><td><code>2006-12-08 </code></td></tr>
<tr><td></td><td><code>20061208 </code></td></tr>
<tr><td></td><td><code>2006-12 </code></td></tr>
<tr><td></td><td><code>2006-W49-5 </code></td></tr>
<tr><td></td><td><code>2006-342 </code></td></tr>
</table>
</dd>
<dt class="pubdef"><a id="day_of_the_week/2"><strong>day_of_the_week</strong>(<var>+Date,-DayOfTheWeek</var>)</a></dt>
<dd class="defbody">
Computes the day of the week for a given date.
<code><var>Date</var> = date(<var>Year</var>,<var>Month</var>,<var>Day</var>)</code>. 
Days of the week are numbered from one to seven: Monday = 1, Tuesday = 
2, ... , Sunday = 7.
</dd>
</dl>

<p><h3 id="sec:plwin"><a id="sec:4.34.3"><span class="sec-nr">4.34.3</span> <span class="sec-title">Controlling 
the <b>swipl-win.exe</b> console window</span></a></h3>

<a id="sec:plwin"></a>

<p>The Windows executable <b>swipl-win.exe</b> 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.

<dl class="latex">
<dt class="pubdef"><a id="window_title/2"><strong>window_title</strong>(<var>-Old, 
+New</var>)</a></dt>
<dd class="defbody">
Unify <var>Old</var> with the title displayed in the console and change 
the title to <var>New</var>.<sup class="fn">bug<span class="fn-text">This 
predicate should have been called <code>win_window_title</code> for 
consistent naming.</span></sup></dd>
<dt class="pubdef"><a id="win_window_pos/1"><strong>win_window_pos</strong>(<var>+ListOfOptions</var>)</a></dt>
<dd class="defbody">
Interface to the MS-Windows SetWindowPos() function, controlling size, 
position and stacking order of the window. <var>ListOfOptions</var> is a 
list that may hold any number of the terms below:

<dl class="latex">
<dt><strong>size</strong>(<var>W, H</var>)</dt>
<dd class="defbody">
Change the size of the window. <var>W</var> and <var>H</var> are 
expressed in character units.
</dd>
<dt><strong>position</strong>(<var>X, Y</var>)</dt>
<dd class="defbody">
Change the top-left corner of the window. The values are expressed in 
pixel units.
</dd>
<dt><strong>zorder</strong>(<var>ZOrder</var>)</dt>
<dd class="defbody">
Change the location in the window stacking order. Values are
<code>bottom</code>, <code>top</code>, <code>topmost</code> and <code>notopmost</code>.
<em>Topmost</em> windows are displayed above all other windows.
</dd>
<dt><strong>show</strong>(<var>Bool</var>)</dt>
<dd class="defbody">
If <code>true</code>, show the window, if <code>false</code> hide the 
window.
</dd>
<dt><strong>activate</strong></dt>
<dd class="defbody">
If present, activate the window.
</dd>
</dl>

</dd>
<dt class="pubdef"><a id="win_has_menu/0"><strong>win_has_menu</strong></a></dt>
<dd class="defbody">
True if <a id="idx:wininsertmenu2:1295"></a><a class="pred" href="system.html#win_insert_menu/2">win_insert_menu/2</a> 
and <a id="idx:wininsertmenuitem4:1296"></a><a class="pred" href="system.html#win_insert_menu_item/4">win_insert_menu_item/4</a> 
are present.</dd>
<dt class="pubdef"><a id="win_insert_menu/2"><strong>win_insert_menu</strong>(<var>+Label, 
+Before</var>)</a></dt>
<dd class="defbody">
Insert a new entry (pulldown) in the menu. If the menu already contains 
this entry, nothing is done. The <var>Label</var> is the label and, 
using the Windows convention, a letter prefixed with <code>&amp;</code> 
is underlined and defines the associated accelerator key. <var>Before</var> 
is the label before which this one must be inserted. Using <code><code>-</code></code> 
adds the new entry at the end (right). For example, the call below adds 
an <b>Application</b> entry just before the <b>Help</b> menu.

<pre class="code">
win_insert_menu('&amp;Application', '&amp;Help')
</pre>

</dd>
<dt class="pubdef"><a id="win_insert_menu_item/4"><strong>win_insert_menu_item</strong>(<var>+Pulldown, 
+Label, +Before, :Goal</var>)</a></dt>
<dd class="defbody">
Add an item to the named <var>Pulldown</var> menu. <var>Label</var> and
<var>Before</var> are handled as in <a id="idx:wininsertmenu2:1297"></a><a class="pred" href="system.html#win_insert_menu/2">win_insert_menu/2</a>, 
but the label <code><code>-</code></code> inserts a <em>separator</em>. <var>Goal</var> 
is called if the user selects the item.
</dd>
</dl>

<p></body></html>
swi-prolog-nox 6.6.4-2ubuntu1 / usr / lib / swi-prolog / doc / Manual / system.html
