This file is indexed.

/usr/share/doc/swi-prolog-doc/Manual/widechars.html is in swi-prolog-doc 5.6.59-1.

This file is owned by root:root, with mode 0o644.

The actual contents of the file can be viewed below.

  1
  2
  3
  4
  5
  6
  7
  8
  9
 10
 11
 12
 13
 14
 15
 16
 17
 18
 19
 20
 21
 22
 23
 24
 25
 26
 27
 28
 29
 30
 31
 32
 33
 34
 35
 36
 37
 38
 39
 40
 41
 42
 43
 44
 45
 46
 47
 48
 49
 50
 51
 52
 53
 54
 55
 56
 57
 58
 59
 60
 61
 62
 63
 64
 65
 66
 67
 68
 69
 70
 71
 72
 73
 74
 75
 76
 77
 78
 79
 80
 81
 82
 83
 84
 85
 86
 87
 88
 89
 90
 91
 92
 93
 94
 95
 96
 97
 98
 99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">

<HTML>
<HEAD>
<TITLE>SWI-Prolog 5.6.59 Reference Manual: Section 2.17</TITLE><LINK REL=home HREF="index.html">
<LINK REL=contents HREF="Contents.html">
<LINK REL=index HREF="DocIndex.html">
<LINK REL=previous HREF="cyclic.html">
<LINK REL=next HREF="limits.html">
<STYLE type="text/css">
/* Style sheet for SWI-Prolog latex2html
*/

dd.defbody
{ margin-bottom: 1em;
}

dt.pubdef
{ background-color: #c5e1ff;
}

pre.code
{ margin-left: 1.5em;
margin-right: 1.5em;
border: 1px dotted;
padding-top: 5px;
padding-left: 5px;
padding-bottom: 5px;
background-color: #f8f8f8;
}

div.navigate
{ text-align: center;
background-color: #f0f0f0;
border: 1px dotted;
padding: 5px;
}

div.title
{ text-align: center;
padding-bottom: 1em;
font-size: 200%;
font-weight: bold;
}

div.author
{ text-align: center;
font-style: italic;
}

div.abstract
{ margin-top: 2em;
background-color: #f0f0f0;
border: 1px dotted;
padding: 5px;
margin-left: 10%; margin-right:10%;
}

div.abstract-title
{ text-align: center;
padding: 5px;
font-size: 120%;
font-weight: bold;
}

div.toc-h1
{ font-size: 200%;
font-weight: bold;
}

div.toc-h2
{ font-size: 120%;
font-weight: bold;
margin-left: 2em;
}

div.toc-h3
{ font-size: 100%;
font-weight: bold;
margin-left: 4em;
}

div.toc-h4
{ font-size: 100%;
margin-left: 6em;
}

span.sec-nr
{ 
}

span.sec-title
{ 
}

span.pred-ext
{ font-weight: bold;
}

span.pred-tag
{ float: right;
font-size: 80%;
font-style: italic;
color: #202020;
}

/* Footnotes */

sup.fn { color: blue; text-decoration: underline; }
span.fn-text { display: none; }
sup.fn span {display: none;}
sup:hover span 
{ display: block !important;
position: absolute; top: auto; left: auto; width: 80%;
color: #000; background: white;
border: 2px solid;
padding: 5px; margin: 10px; z-index: 100;
font-size: smaller;
}
</STYLE>
</HEAD>
<BODY BGCOLOR="white">
<DIV class="navigate"><A class="nav" href="index.html"><IMG SRC="home.gif" BORDER=0 ALT="Home"></A>
<A class="nav" href="Contents.html"><IMG SRC="index.gif" BORDER=0 ALT="Contents"></A>
<A class="nav" href="DocIndex.html"><IMG SRC="yellow_pages.gif" BORDER=0 ALT="Index"></A>
<A class="nav" href="cyclic.html"><IMG SRC="prev.gif" BORDER=0 ALT="Previous"></A>
<A class="nav" href="limits.html"><IMG SRC="next.gif" BORDER=0 ALT="Next"></A>
</DIV>

<H2><A NAME="sec:2.17"><SPAN class="sec-nr">2.17</SPAN> <SPAN class="sec-title">Wide 
character support</SPAN></A></H2>

<A NAME="sec:widechars"></A>

<P><A NAME="idx:UTF8:239"></A><A NAME="idx:Unicode:240"></A><A NAME="idx:UCS:241"></A><A NAME="idx:internationalization:242"></A>SWI-Prolog 
supports <EM>wide characters</EM>, characters with character codes above 
255 that cannot be represented in a single <EM>byte</EM>.
<EM>Universal Character Set</EM> (UCS) is the ISO/IEC 10646 standard 
that specifies a unique 31-bits unsigned integer for any character in 
any language. It is a superset of 16-bit Unicode, which in turn is a 
superset of ISO 8859-1 (ISO Latin-1), a superset of US-ASCII. UCS can 
handle strings holding characters from multiple languages and character 
classification (uppercase, lowercase, digit, etc.) and operations such 
as case-conversion are unambiguously defined.

<P>For this reason SWI-Prolog has two representations for atoms and 
string objects (see <A class="sec" href="strings.html">section 4.23</A>). 
If the text fits in ISO Latin-1, it is represented as an array of 8-bit 
characters. Otherwise the text is represented as an array of 32-bit 
numbers. This representational issue is completely transparent to the 
Prolog user. Users of the foreign language interface as described in <A class="sec" href="foreign.html">section 
9</A> sometimes need to be aware of these issues though.

<P>Character coding comes into view when characters of strings need to 
be read from or written to file or when they have to be communicated to 
other software components using the foreign language interface. In this 
section we only deal with I/O through streams, which includes file I/O 
as well as I/O through network sockets.

<H3><A NAME="sec:2.17.1"><SPAN class="sec-nr">2.17.1</SPAN> <SPAN class="sec-title">Wide 
character encodings on streams</SPAN></A></H3>

<A NAME="sec:encoding"></A>

<P>Although characters are uniquely coded using the UCS standard 
internally, streams and files are byte (8-bit) oriented and there are a 
variety of ways to represent the larger UCS codes in an 8-bit octet 
stream. The most popular one, especially in the context of the web, is 
UTF-8. Bytes 0&nbsp;...&nbsp;127 represent simply the corresponding 
US-ASCII character, while bytes 128&nbsp;...&nbsp;255 are used for 
multi-byte encoding of characters placed higher in the UCS space. 
Especially on MS-Windows the 16-bit Unicode standard, represented by 
pairs of bytes is also popular.

<P>Prolog I/O streams have a property called <EM>encoding</EM> which 
specifies the used encoding that influence <A NAME="idx:getcode2:243"></A><A class="pred" href="chario.html#get_code/2">get_code/2</A> 
and <A NAME="idx:putcode2:244"></A><A class="pred" href="chario.html#put_code/2">put_code/2</A> 
as well as all the other text I/O predicates.

<P>The default encoding for files is derived from the Prolog flag
<A class="flag" href="flags.html#flag:encoding">encoding</A>, which is 
initialised from the environment. If the environment variable <CODE>LANG</CODE> 
ends in "UTF-8", this encoding is assumed. Otherwise the default is <CODE>text</CODE> 
and the translation is left to the wide-character functions of the 
C-library.
<SUP class="fn">11<SPAN class="fn-text">The Prolog native UTF-8 mode is 
considerably faster than the generic mbrtowc() one.</SPAN></SUP> The 
encoding can be specified explicitly in <A NAME="idx:loadfiles2:245"></A><A class="pred" href="consulting.html#load_files/2">load_files/2</A> 
for loading Prolog source with an alternative encoding, <A NAME="idx:open4:246"></A><A class="pred" href="IO.html#open/4">open/4</A> 
when opening files or using <A NAME="idx:setstream2:247"></A><A class="pred" href="IO.html#set_stream/2">set_stream/2</A> 
on any open stream. For Prolog source files we also provide the <A NAME="idx:encoding1:248"></A><A class="pred" href="consulting.html#encoding/1">encoding/1</A> 
directive that can be used to switch between encodings that are 
compatible to US-ASCII (<CODE>ascii</CODE>,
<CODE>iso_latin_1</CODE>, <CODE>utf8</CODE> and many locales). See also
<A class="sec" href="projectfiles.html">section 3.1.3</A> for writing 
Prolog files with non-US-ASCII characters and <A class="sec" href="syntax.html">section 
2.15.1.4</A> for syntax issues. For additional information and Unicode 
resources, please visit
<A class="url" href="http://www.unicode.org/">http://www.unicode.org/</A>.

<P>SWI-Prolog currently defines and supports the following encodings:

<DL>
<DT><STRONG>octet</STRONG></DT>
<DD class="defbody">
Default encoding for <CODE>binary</CODE> streams. This causes the stream 
to be read and written fully untranslated.</DD>
<DT><STRONG>ascii</STRONG></DT>
<DD class="defbody">
7-bit encoding in 8-bit bytes. Equivalent to <CODE>iso_latin_1</CODE>, 
but generates errors and warnings on encountering values above 127.</DD>
<DT><STRONG>iso_latin_1</STRONG></DT>
<DD class="defbody">
8-bit encoding supporting many western languages. This causes the stream 
to be read and written fully untranslated.</DD>
<DT><STRONG>text</STRONG></DT>
<DD class="defbody">
C-library default locale encoding for text files. Files are read and 
written using the C-library functions mbrtowc() and wcrtomb(). This may 
be the same as one of the other locales, notably it may be the same as <CODE>iso_latin_1</CODE> 
for western languages and <CODE>utf8</CODE> in a UTF-8 context.</DD>
<DT><STRONG>utf8</STRONG></DT>
<DD class="defbody">
Multi-byte encoding of full UCS, compatible to <CODE>ascii</CODE>. See 
above.</DD>
<DT><STRONG>unicode_be</STRONG></DT>
<DD class="defbody">
Unicode <EM>Big Endian</EM>. Reads input in pairs of bytes, most 
significant byte first. Can only represent 16-bit characters.</DD>
<DT><STRONG>unicode_le</STRONG></DT>
<DD class="defbody">
Unicode <EM>Little Endian</EM>. Reads input in pairs of bytes, least 
significant byte first. Can only represent 16-bit characters.
</DD>
</DL>

<P>Note that not all encodings can represent all characters. This 
implies that writing text to a stream may cause errors because the 
stream cannot represent these characters. The behaviour of a stream on 
these errors can be controlled using <A NAME="idx:setstream2:249"></A><A class="pred" href="IO.html#set_stream/2">set_stream/2</A>. 
Initially the terminal stream write the characters using Prolog escape 
sequences while other streams generate an I/O exception.

<H4><A NAME="sec:2.17.1.1"><SPAN class="sec-nr">2.17.1.1</SPAN> <SPAN class="sec-title">BOM: 
Byte Order Mark</SPAN></A></H4>

<A NAME="sec:bom"></A>

<P><A NAME="idx:BOM:250"></A><A NAME="idx:ByteOrderMark:251"></A>From <A class="sec" href="widechars.html">section 
2.17.1</A>, you may have got the impression text-files are complicated. 
This section deals with a related topic, making live often easier for 
the user, but providing another worry to the programmer.
<B>BOM</B> or <EM>Byte Order Marker</EM> is a technique for identifying 
Unicode text-files as well as the encoding they use. Such files start 
with the Unicode character 0xFEFF, a non-breaking, zero-width space 
character. This is a pretty unique sequence that is not likely to be the 
start of a non-Unicode file and uniquely distinguishes the various 
Unicode file formats. As it is a zero-width blank, it even doesn't 
produce any output. This solves all problems, or ... Some formats start 
of as US-ASCII and may contain some encoding mark to switch to UTF-8, 
such as the <CODE>encoding="UTF-8"</CODE> in an XML header. Such formats 
often explicitly forbid the use of a UTF-8 BOM. In other cases there is 
additional information telling the encoding making the use of a BOM 
redundant or even illegal.

<P>The BOM is handled by SWI-Prolog <A NAME="idx:open4:252"></A><A class="pred" href="IO.html#open/4">open/4</A> 
predicate. By default, text-files are probed for the BOM when opened for 
reading. If a BOM is found, the encoding is set accordingly and the 
property <CODE>bom(true)</CODE> is available through <A NAME="idx:streamproperty2:253"></A><A class="pred" href="IO.html#stream_property/2">stream_property/2</A>. 
When opening a file for writing, writing a BOM can be requested using 
the option <CODE>bom(true)</CODE> with
<A NAME="idx:open4:254"></A><A class="pred" href="IO.html#open/4">open/4</A>.

<P></BODY></HTML>