If you are not really familiar with Internationalization (usual shortcut
is I18N) , Unicode, characters and glyphs, I suggest you read a presentation
by Tim Bray on Unicode and why you should care about it. If you don't understand why it does not make sense to have a string
without knowing what encoding it uses, then as Joel Spolsky said please do not
write another line of code until you finish reading that article.. It is
a prerequisite to understand this page, and avoid a lot of problems with
libxml2, XML or text processing in general. Table of Content:
- What does internationalization support
mean ?
- The internal encoding, how and
why
- How is it implemented ?
- Default supported encodings
- How to extend the existing
support
XML was designed from the start to allow the support of any character set
by using Unicode. Any conformant XML parser has to support the UTF-8 and
UTF-16 default encodings which can both express the full unicode ranges. UTF8
is a variable length encoding whose greatest points are to reuse the same
encoding for ASCII and to save space for Western encodings, but it is a bit
more complex to handle in practice. UTF-16 use 2 bytes per character (and
sometimes combines two pairs), it makes implementation easier, but looks a
bit overkill for Western languages encoding. Moreover the XML specification
allows the document to be encoded in other encodings at the condition that
they are clearly labeled as such. For example the following is a wellformed
XML document encoded in ISO-8859-1 and using accentuated letters that we
French like for both markup and content: <?xml version="1.0" encoding="ISO-8859-1"?>
<très>là </très> Having internationalization support in libxml2 means the following:
- the document is properly parsed
- information about it's encoding is saved
- it can be modified
- it can be saved in its original encoding
- it can also be saved in another encoding supported by libxml2 (for
example straight UTF8 or even an ASCII form)
Another very important point is that the whole libxml2 API, with the
exception of a few routines to read with a specific encoding or save to a
specific encoding, is completely agnostic about the original encoding of the
document. It should be noted too that the HTML parser embedded in libxml2 now obey
the same rules too, the following document will be (as of 2.2.2) handled in
an internationalized fashion by libxml2 too: <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"
"http://www.w3.org/TR/REC-html40/loose.dtd">
<html lang="fr">
<head>
<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=ISO-8859-1">
</head>
<body>
<p>W3C crée des standards pour le Web.</body>
</html>One of the core decisions was to force all documents to be converted to a
default internal encoding, and that encoding to be UTF-8, here are the
rationales for those choices:
- keeping the native encoding in the internal form would force the libxml
users (or the code associated) to be fully aware of the encoding of the
original document, for examples when adding a text node to a document,
the content would have to be provided in the document encoding, i.e. the
client code would have to check it before hand, make sure it's conformant
to the encoding, etc ... Very hard in practice, though in some specific
cases this may make sense.
- the second decision was which encoding. From the XML spec only UTF8 and
UTF16 really makes sense as being the two only encodings for which there
is mandatory support. UCS-4 (32 bits fixed size encoding) could be
considered an intelligent choice too since it's a direct Unicode mapping
support. I selected UTF-8 on the basis of efficiency and compatibility
with surrounding software:
- UTF-8 while a bit more complex to convert from/to (i.e. slightly
more costly to import and export CPU wise) is also far more compact
than UTF-16 (and UCS-4) for a majority of the documents I see it used
for right now (RPM RDF catalogs, advogato data, various configuration
file formats, etc.) and the key point for today's computer
architecture is efficient uses of caches. If one nearly double the
memory requirement to store the same amount of data, this will trash
caches (main memory/external caches/internal caches) and my take is
that this harms the system far more than the CPU requirements needed
for the conversion to UTF-8
- Most of libxml2 version 1 users were using it with straight ASCII
most of the time, doing the conversion with an internal encoding
requiring all their code to be rewritten was a serious show-stopper
for using UTF-16 or UCS-4.
- UTF-8 is being used as the de-facto internal encoding standard for
related code like the pango
upcoming Gnome text widget, and a lot of Unix code (yet another place
where Unix programmer base takes a different approach from Microsoft
- they are using UTF-16)
What does this mean in practice for the libxml2 user:
- xmlChar, the libxml2 data type is a byte, those bytes must be assembled
as UTF-8 valid strings. The proper way to terminate an xmlChar * string
is simply to append 0 byte, as usual.
- One just need to make sure that when using chars outside the ASCII set,
the values has been properly converted to UTF-8
Let's describe how all this works within libxml, basically the I18N
(internationalization) support get triggered only during I/O operation, i.e.
when reading a document or saving one. Let's look first at the reading
sequence:
- when a document is processed, we usually don't know the encoding, a
simple heuristic allows to detect UTF-16 and UCS-4 from encodings where
the ASCII range (0-0x7F) maps with ASCII
- the xml declaration if available is parsed, including the encoding
declaration. At that point, if the autodetected encoding is different
from the one declared a call to xmlSwitchEncoding() is issued.
- If there is no encoding declaration, then the input has to be in either
UTF-8 or UTF-16, if it is not then at some point when processing the
input, the converter/checker of UTF-8 form will raise an encoding error.
You may end-up with a garbled document, or no document at all ! Example:
~/XML -> ./xmllint err.xml
err.xml:1: error: Input is not proper UTF-8, indicate encoding !
<très>là </très>
^
err.xml:1: error: Bytes: 0xE8 0x73 0x3E 0x6C
<très>là </très>
^
- xmlSwitchEncoding() does an encoding name lookup, canonicalize it, and
then search the default registered encoding converters for that encoding.
If it's not within the default set and iconv() support has been compiled
it, it will ask iconv for such an encoder. If this fails then the parser
will report an error and stops processing:
~/XML -> ./xmllint err2.xml
err2.xml:1: error: Unsupported encoding UnsupportedEnc
<?xml version="1.0" encoding="UnsupportedEnc"?>
^
- From that point the encoder processes progressively the input (it is
plugged as a front-end to the I/O module) for that entity. It captures
and converts on-the-fly the document to be parsed to UTF-8. The parser
itself just does UTF-8 checking of this input and process it
transparently. The only difference is that the encoding information has
been added to the parsing context (more precisely to the input
corresponding to this entity).
- The result (when using DOM) is an internal form completely in UTF-8
with just an encoding information on the document node.
Ok then what happens when saving the document (assuming you
collected/built an xmlDoc DOM like structure) ? It depends on the function
called, xmlSaveFile() will just try to save in the original encoding, while
xmlSaveFileTo() and xmlSaveFileEnc() can optionally save to a given
encoding:
- if no encoding is given, libxml2 will look for an encoding value
associated to the document and if it exists will try to save to that
encoding,
otherwise everything is written in the internal form, i.e. UTF-8
- so if an encoding was specified, either at the API level or on the
document, libxml2 will again canonicalize the encoding name, lookup for a
converter in the registered set or through iconv. If not found the
function will return an error code
- the converter is placed before the I/O buffer layer, as another kind of
buffer, then libxml2 will simply push the UTF-8 serialization to through
that buffer, which will then progressively be converted and pushed onto
the I/O layer.
- It is possible that the converter code fails on some input, for example
trying to push an UTF-8 encoded Chinese character through the UTF-8 to
ISO-8859-1 converter won't work. Since the encoders are progressive they
will just report the error and the number of bytes converted, at that
point libxml2 will decode the offending character, remove it from the
buffer and replace it with the associated charRef encoding { and
resume the conversion. This guarantees that any document will be saved
without losses (except for markup names where this is not legal, this is
a problem in the current version, in practice avoid using non-ascii
characters for tag or attribute names). A special "ascii" encoding name
is used to save documents to a pure ascii form can be used when
portability is really crucial
Here are a few examples based on the same test document and assumin a
terminal using ISO-8859-1 as the text encoding: ~/XML -> ./xmllint isolat1
<?xml version="1.0" encoding="ISO-8859-1"?>
<très>là </très>
~/XML -> ./xmllint --encode UTF-8 isolat1
<?xml version="1.0" encoding="UTF-8"?>
<très>là </très>
~/XML -> The same processing is applied (and reuse most of the code) for HTML I18N
processing. Looking up and modifying the content encoding is a bit more
difficult since it is located in a <meta> tag under the <head>,
so a couple of functions htmlGetMetaEncoding() and htmlSetMetaEncoding() have
been provided. The parser also attempts to switch encoding on the fly when
detecting such a tag on input. Except for that the processing is the same
(and again reuses the same code). libxml2 has a set of default converters for the following encodings
(located in encoding.c):
- UTF-8 is supported by default (null handlers)
- UTF-16, both little and big endian
- ISO-Latin-1 (ISO-8859-1) covering most western languages
- ASCII, useful mostly for saving
- HTML, a specific handler for the conversion of UTF-8 to ASCII with HTML
predefined entities like © for the Copyright sign.
More over when compiled on an Unix platform with iconv support the full
set of encodings supported by iconv can be instantly be used by libxml. On a
linux machine with glibc-2.1 the list of supported encodings and aliases fill
3 full pages, and include UCS-4, the full set of ISO-Latin encodings, and the
various Japanese ones. To convert from the UTF-8 values returned from the API to another encoding
then it is possible to use the function provided from the encoding module like UTF8Toisolat1, or use the
POSIX iconv()
API directly. Encoding aliasesFrom 2.2.3, libxml2 has support to register encoding names aliases. The
goal is to be able to parse document whose encoding is supported but where
the name differs (for example from the default set of names accepted by
iconv). The following functions allow to register and handle new aliases for
existing encodings. Once registered libxml2 will automatically lookup the
aliases when handling a document:
- int xmlAddEncodingAlias(const char *name, const char *alias);
- int xmlDelEncodingAlias(const char *alias);
- const char * xmlGetEncodingAlias(const char *alias);
- void xmlCleanupEncodingAliases(void);
Well adding support for new encoding, or overriding one of the encoders
(assuming it is buggy) should not be hard, just write input and output
conversion routines to/from UTF-8, and register them using
xmlNewCharEncodingHandler(name, xxxToUTF8, UTF8Toxxx), and they will be
called automatically if the parser(s) encounter such an encoding name
(register it uppercase, this will help). The description of the encoders,
their arguments and expected return values are described in the encoding.h
header. Daniel Veillard |
