Although characters are uniquely coded using the Unicode standard
internally, streams and files are byte (8-bit) oriented and there are a
variety of ways to represent the larger Unicode codes in an 8-bit octet
stream. The most popular one, especially in the context of the web, is
UTF-8. Bytes 0 ... 127 represent simply the corresponding
US-ASCII character, while bytes 128 ... 255 are used for
multi-byte encoding of characters placed higher in the Unicode space.
Especially on MS-Windows the 16-bit UTF-16 standard, represented by
pairs of bytes, is also popular.
Prolog I/O streams have a property called encoding which
specifies the used encoding that influences get_code/2
and put_code/2
as well as all the other text I/O predicates.
The default encoding for files is derived from the Prolog flag
encoding, which is
initialised from
setlocale(LC_CTYPE, NULL) to one of text, utf8
or
iso_latin_1. One of the latter two is used if the encoding
name is recognized, while text is used as default. Using
text, the translation is left to the wide-character
functions of the C library.40The
Prolog native UTF-8 mode is considerably faster than the generic mbrtowc()
one. The encoding can be specified explicitly in load_files/2
for loading Prolog source with an alternative encoding, open/4
when opening files or using set_stream/2
on any open stream. For Prolog source files we also provide the encoding/1
directive that can be used to switch between encodings that are
compatible with US-ASCII (ascii, iso_latin_1, utf8
and many locales). See also section
3.1.3 for writing Prolog files with non-US-ASCII characters and section
2.15.1.9 for syntax issues. For additional information and Unicode
resources, please visit
http://www.unicode.org/.
SWI-Prolog currently defines and supports the following encodings:
- octet
- Default encoding for
binary streams. This causes the stream
to be read and written fully untranslated.
- ascii
- 7-bit encoding in 8-bit bytes. Equivalent to
iso_latin_1,
but generates errors and warnings on encountering values above 127.
- iso_latin_1
- 8-bit encoding supporting many Western languages. This causes the stream
to be read and written fully untranslated. The above is the SWI-Prolog
native name. This encoding may be specified using the official
IANA name
ISO-8859-1.
- text
- 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 encodings, notably it may be
the same as
iso_latin_1 for Western languages and utf8
in a UTF-8 context.
- utf8
- Multi-byte encoding of full Unicode, compatible with
ascii.
See above. The above is the SWI-Prolog native name. This encoding may be
specified using the official IANA
name UTF-8.
- utf16be
- utf16le
- UTF-16 encoding. Reads input in pairs of bytes.
utf16be is
Big Endian, putting the most significant byte first and
utf16le is Little Endian, putting the most
significant byte second. UTF-16 can represent full Unicode using surrogate
pairs. The above are the SWI-Prolog native names. These encodings
may be specified using the official IANA
names UTF-16BE and UTF-16LE. For backward
compatibility we also support unicode_be and unicode_le.
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 set_stream/2.
Initially the terminal stream writes the characters using Prolog escape
sequences while other streams generate an I/O exception.
From section
2.18.1, 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.
BOM or Byte Order Marker 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 encoding="UTF-8" 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.
The BOM is handled by SWI-Prolog open/4
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 bom(true) is available through stream_property/2.
When opening a file for writing, writing a BOM can be requested using
the option bom(true) with
open/4.
