/usr/lib/swi-prolog/doc/Manual/widechars.html

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<h2 id="sec:widechars"><a id="sec:2.18"><span class="sec-nr">2.18</span> <span class="sec-title">Wide 
character support</span></a></h2>

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

<p><a id="idx:UTF8:226"></a><a id="idx:Unicode:227"></a><a id="idx:UCS:228"></a><a id="idx:internationalization:229"></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-bit 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 4.24</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">chapter 
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.

<p><h3 id="sec:encoding"><a id="sec:2.18.1"><span class="sec-nr">2.18.1</span> <span class="sec-title">Wide 
character encodings on streams</span></a></h3>

<a id="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 influences <a id="idx:getcode2:230"></a><a class="pred" href="chario.html#get_code/2">get_code/2</a> 
and <a id="idx:putcode2:231"></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">25<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 id="idx:loadfiles2:232"></a><a class="pred" href="consulting.html#load_files/2">load_files/2</a> 
for loading Prolog source with an alternative encoding, <a id="idx:open4:233"></a><a class="pred" href="IO.html#open/4">open/4</a> 
when opening files or using <a id="idx:setstream2:234"></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 id="idx:encoding1:235"></a><a class="pred" href="consulting.html#encoding/1">encoding/1</a> 
directive that can be used to switch between encodings that are 
compatible with 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 
2.15.1.5</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 class="latex">
<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 with <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 id="idx:setstream2:236"></a><a class="pred" href="IO.html#set_stream/2">set_stream/2</a>. 
Initially the terminal stream writes the characters using Prolog escape 
sequences while other streams generate an I/O exception.

<p><h4 id="sec:bom"><a id="sec:2.18.1.1"><span class="sec-nr">2.18.1.1</span> <span class="sec-title">BOM: 
Byte Order Mark</span></a></h4>

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

<p><a id="idx:BOM:237"></a><a id="idx:ByteOrderMark:238"></a>From <a class="sec" href="widechars.html">section 
2.18.1</a>, you may have got the impression that text files are 
complicated. This section deals with a related topic, making life 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 
off 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 revealing the encoding, making the use of a BOM 
redundant or even illegal.

<p>The BOM is handled by SWI-Prolog <a id="idx:open4:239"></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 id="idx:streamproperty2:240"></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 id="idx:open4:241"></a><a class="pred" href="IO.html#open/4">open/4</a>.

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