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2002-08-19 Jonathan Wakely <jw@kayari.org> * docs/html/configopts.html, docs/html/documentation.html, docs/html/install.html, docs/html/22_locale/codecvt.html, docs/html/22_locale/ctype.html, docs/html/22_locale/howto.html, docs/html/22_locale/locale.html, docs/html/22_locale/messages.html: Conform to HTML 4.01 standard. From-SVN: r56449
529 lines
17 KiB
HTML
529 lines
17 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
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<html>
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<head>
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<title>
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Notes on the codecvt implementation.
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</title>
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</head>
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<body>
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<h1>
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Notes on the codecvt implementation.
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</h1>
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<p>
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<I>
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prepared by Benjamin Kosnik (bkoz@redhat.com) on August 28, 2000
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</I>
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<h2>
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1. Abstract
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</h2>
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<p>
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The standard class codecvt attempts to address conversions between
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different character encoding schemes. In particular, the standard
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attempts to detail conversions between the implementation-defined wide
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characters (hereafter referred to as wchar_t) and the standard type
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char that is so beloved in classic "C" (which can now be referred to
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as narrow characters.) This document attempts to describe how the GNU
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libstdc++-v3 implementation deals with the conversion between wide and
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narrow characters, and also presents a framework for dealing with the
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huge number of other encodings that iconv can convert, including
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Unicode and UTF8. Design issues and requirements are addressed, and
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examples of correct usage for both the required specializations for
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wide and narrow characters and the implementation-provided extended
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functionality are given.
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</p>
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<p>
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<h2>
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2. What the standard says
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</h2>
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Around page 425 of the C++ Standard, this charming heading comes into view:
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<BLOCKQUOTE>
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22.2.1.5 - Template class codecvt [lib.locale.codecvt]
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</BLOCKQUOTE>
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The text around the codecvt definition gives some clues:
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<BLOCKQUOTE>
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<I>
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-1- The class codecvt<internT,externT,stateT> is for use when
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converting from one codeset to another, such as from wide characters
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to multibyte characters, between wide character encodings such as
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Unicode and EUC.
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</I>
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</BLOCKQUOTE>
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<p>
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Hmm. So, in some unspecified way, Unicode encodings and
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translations between other character sets should be handled by this
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class.
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</p>
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<BLOCKQUOTE>
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<I>
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-2- The stateT argument selects the pair of codesets being mapped between.
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</I>
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</BLOCKQUOTE>
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<p>
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Ah ha! Another clue...
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</p>
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<BLOCKQUOTE>
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<I>
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-3- The instantiations required in the Table ??
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(lib.locale.category), namely codecvt<wchar_t,char,mbstate_t> and
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codecvt<char,char,mbstate_t>, convert the implementation-defined
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native character set. codecvt<char,char,mbstate_t> implements a
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degenerate conversion; it does not convert at
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all. codecvt<wchar_t,char,mbstate_t> converts between the native
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character sets for tiny and wide characters. Instantiations on
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mbstate_t perform conversion between encodings known to the library
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implementor. Other encodings can be converted by specializing on a
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user-defined stateT type. The stateT object can contain any state that
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is useful to communicate to or from the specialized do_convert member.
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</I>
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</BLOCKQUOTE>
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<p>
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At this point, a couple points become clear:
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<p>
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One: The standard clearly implies that attempts to add non-required
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(yet useful and widely used) conversions need to do so through the
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third template parameter, stateT.</p>
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<p>
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Two: The required conversions, by specifying mbstate_t as the third
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template parameter, imply an implementation strategy that is mostly
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(or wholly) based on the underlying C library, and the functions
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mcsrtombs and wcsrtombs in particular.</p>
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<p>
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<h2>
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3. Some thoughts on what would be useful
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</h2>
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Probably the most frequently asked question about code conversion is:
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"So dudes, what's the deal with Unicode strings?" The dude part is
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optional, but apparently the usefulness of Unicode strings is pretty
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widely appreciated. Sadly, this specific encoding (And other useful
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encodings like UTF8, UCS4, ISO 8859-10, etc etc etc) are not mentioned
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in the C++ standard.
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<p>
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In particular, the simple implementation detail of wchar_t's size
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seems to repeatedly confound people. Many systems use a two byte,
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unsigned integral type to represent wide characters, and use an
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internal encoding of Unicode or UCS2. (See AIX, Microsoft NT, Java,
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others.) Other systems, use a four byte, unsigned integral type to
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represent wide characters, and use an internal encoding of
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UCS4. (GNU/Linux systems using glibc, in particular.) The C
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programming language (and thus C++) does not specify a specific size
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for the type wchar_t.
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<p>
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Thus, portable C++ code cannot assume a byte size (or endianness) either.
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<p>
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Getting back to the frequently asked question: What about Unicode strings?
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<p>
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What magic spell will do this conversion?
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<p>
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A couple of comments:
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</p>
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<p>
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The thought that all one needs to convert between two arbitrary
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codesets is two types and some kind of state argument is
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unfortunate. In particular, encodings may be stateless. The naming of
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the third parameter as stateT is unfortunate, as what is really needed
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is some kind of generalized type that accounts for the issues that
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abstract encodings will need. The minimum information that is required
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includes:
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</p>
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<ul>
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<li>
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<p>
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Identifiers for each of the codesets involved in the conversion. For
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example, using the iconv family of functions from the Single Unix
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Specification (what used to be called X/Open) hosted on the GNU/Linux
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operating system allows bi-directional mapping between far more than
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the following tantalizing possibilities:
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<p>
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(An edited list taken from <code>`iconv --list`</code> on a Red Hat 6.2/Intel system:
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<BLOCKQUOTE>
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<PRE>
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8859_1, 8859_9, 10646-1:1993, 10646-1:1993/UCS4, ARABIC, ARABIC7,
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ASCII, EUC-CN, EUC-JP, EUC-KR, EUC-TW, GREEK-CCIcode, GREEK, GREEK7-OLD,
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GREEK7, GREEK8, HEBREW, ISO-8859-1, ISO-8859-2, ISO-8859-3,
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ISO-8859-4, ISO-8859-5, ISO-8859-6, ISO-8859-7, ISO-8859-8,
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ISO-8859-9, ISO-8859-10, ISO-8859-11, ISO-8859-13, ISO-8859-14,
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ISO-8859-15, ISO-10646, ISO-10646/UCS2, ISO-10646/UCS4,
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ISO-10646/UTF-8, ISO-10646/UTF8, SHIFT-JIS, SHIFT_JIS, UCS-2, UCS-4,
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UCS2, UCS4, UNICODE, UNICODEBIG, UNICODELIcodeLE, US-ASCII, US, UTF-8,
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UTF-16, UTF8, UTF16).
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</PRE>
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</BLOCKQUOTE>
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<p>
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For iconv-based implementations, string literals for each of the
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encodings (ie. "UCS-2" and "UTF-8") are necessary,
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although for other,
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non-iconv implementations a table of enumerated values or some other
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mechanism may be required.
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<li>
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Maximum length of the identifying string literal.
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<li>
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Some encodings are require explicit endian-ness. As such, some kind
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of endian marker or other byte-order marker will be necessary. See
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"Footnotes for C/C++ developers" in Haible for more information on
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UCS-2/Unicode endian issues. (Summary: big endian seems most likely,
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however implementations, most notably Microsoft, vary.)
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<li>
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Types representing the conversion state, for conversions involving
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the machinery in the "C" library, or the conversion descriptor, for
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conversions using iconv (such as the type iconv_t.) Note that the
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conversion descriptor encodes more information than a simple encoding
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state type.
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<li>
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Conversion descriptors for both directions of encoding. (ie, both
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UCS-2 to UTF-8 and UTF-8 to UCS-2.)
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<li>
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Something to indicate if the conversion requested if valid.
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<li>
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Something to represent if the conversion descriptors are valid.
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<li>
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Some way to enforce strict type checking on the internal and
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external types. As part of this, the size of the internal and
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external types will need to be known.
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</ul>
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<p>
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<h2>
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4. Problems with "C" code conversions : thread safety, global
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locales, termination.
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</h2>
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In addition, multi-threaded and multi-locale environments also impact
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the design and requirements for code conversions. In particular, they
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affect the required specialization codecvt<wchar_t, char, mbstate_t>
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when implemented using standard "C" functions.
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<p>
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Three problems arise, one big, one of medium importance, and one small.
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<p>
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First, the small: mcsrtombs and wcsrtombs may not be multithread-safe
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on all systems required by the GNU tools. For GNU/Linux and glibc,
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this is not an issue.
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<p>
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Of medium concern, in the grand scope of things, is that the functions
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used to implement this specialization work on null-terminated
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strings. Buffers, especially file buffers, may not be null-terminated,
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thus giving conversions that end prematurely or are otherwise
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incorrect. Yikes!
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<p>
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The last, and fundamental problem, is the assumption of a global
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locale for all the "C" functions referenced above. For something like
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C++ iostreams (where codecvt is explicitly used) the notion of
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multiple locales is fundamental. In practice, most users may not run
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into this limitation. However, as a quality of implementation issue,
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the GNU C++ library would like to offer a solution that allows
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multiple locales and or simultaneous usage with computationally
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correct results. In short, libstdc++-v3 is trying to offer, as an
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option, a high-quality implementation, damn the additional complexity!
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<p>
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For the required specialization codecvt<wchar_t, char, mbstate_t> ,
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conversions are made between the internal character set (always UCS4
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on GNU/Linux) and whatever the currently selected locale for the
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LC_CTYPE category implements.
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<p>
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<h2>
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5. Design
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</h2>
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The two required specializations are implemented as follows:
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<p>
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<code>
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codecvt<char, char, mbstate_t>
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</code>
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<p>
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This is a degenerate (ie, does nothing) specialization. Implementing
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this was a piece of cake.
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<p>
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<code>
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codecvt<char, wchar_t, mbstate_t>
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</code>
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<p>
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This specialization, by specifying all the template parameters, pretty
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much ties the hands of implementors. As such, the implementation is
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straightforward, involving mcsrtombs for the conversions between char
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to wchar_t and wcsrtombs for conversions between wchar_t and char.
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<p>
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Neither of these two required specializations deals with Unicode
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characters. As such, libstdc++-v3 implements a partial specialization
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of the codecvt class with and iconv wrapper class, __enc_traits as the
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third template parameter.
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<p>
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This implementation should be standards conformant. First of all, the
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standard explicitly points out that instantiations on the third
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template parameter, stateT, are the proper way to implement
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non-required conversions. Second of all, the standard says (in Chapter
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17) that partial specializations of required classes are a-ok. Third
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of all, the requirements for the stateT type elsewhere in the standard
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(see 21.1.2 traits typedefs) only indicate that this type be copy
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constructible.
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<p>
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As such, the type __enc_traits is defined as a non-templatized, POD
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type to be used as the third type of a codecvt instantiation. This
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type is just a wrapper class for iconv, and provides an easy interface
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to iconv functionality.
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<p>
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There are two constructors for __enc_traits:
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<p>
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<code>
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__enc_traits() : __in_desc(0), __out_desc(0)
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</code>
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<p>
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This default constructor sets the internal encoding to some default
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(currently UCS4) and the external encoding to whatever is returned by
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nl_langinfo(CODESET).
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<p>
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<code>
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__enc_traits(const char* __int, const char* __ext)
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</code>
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<p>
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This constructor takes as parameters string literals that indicate the
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desired internal and external encoding. There are no defaults for
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either argument.
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<p>
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One of the issues with iconv is that the string literals identifying
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conversions are not standardized. Because of this, the thought of
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mandating and or enforcing some set of pre-determined valid
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identifiers seems iffy: thus, a more practical (and non-migraine
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inducing) strategy was implemented: end-users can specify any string
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(subject to a pre-determined length qualifier, currently 32 bytes) for
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encodings. It is up to the user to make sure that these strings are
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valid on the target system.
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<p>
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<code>
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void
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_M_init()
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</code>
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<p>
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Strangely enough, this member function attempts to open conversion
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descriptors for a given __enc_traits object. If the conversion
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descriptors are not valid, the conversion descriptors returned will
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not be valid and the resulting calls to the codecvt conversion
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functions will return error.
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<p>
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<code>
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bool
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_M_good()
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</code>
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<p>
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Provides a way to see if the given __enc_traits object has been
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properly initialized. If the string literals describing the desired
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internal and external encoding are not valid, initialization will
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fail, and this will return false. If the internal and external
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encodings are valid, but iconv_open could not allocate conversion
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descriptors, this will also return false. Otherwise, the object is
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ready to convert and will return true.
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<p>
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<code>
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__enc_traits(const __enc_traits&)
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</code>
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<p>
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As iconv allocates memory and sets up conversion descriptors, the copy
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constructor can only copy the member data pertaining to the internal
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and external code conversions, and not the conversion descriptors
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themselves.
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<p>
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Definitions for all the required codecvt member functions are provided
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for this specialization, and usage of codecvt<internal character type,
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external character type, __enc_traits> is consistent with other
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codecvt usage.
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<p>
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<h2>
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6. Examples
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</h2>
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<ul>
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<li>
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a. conversions involving string literals
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<pre>
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typedef codecvt_base::result result;
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typedef unsigned short unicode_t;
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typedef unicode_t int_type;
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typedef char ext_type;
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typedef __enc_traits enc_type;
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typedef codecvt<int_type, ext_type, enc_type> unicode_codecvt;
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const ext_type* e_lit = "black pearl jasmine tea";
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int size = strlen(e_lit);
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int_type i_lit_base[24] =
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{ 25088, 27648, 24832, 25344, 27392, 8192, 28672, 25856, 24832, 29184,
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27648, 8192, 27136, 24832, 29440, 27904, 26880, 28160, 25856, 8192, 29696,
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25856, 24832, 2560
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};
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const int_type* i_lit = i_lit_base;
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const ext_type* efrom_next;
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const int_type* ifrom_next;
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ext_type* e_arr = new ext_type[size + 1];
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ext_type* eto_next;
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int_type* i_arr = new int_type[size + 1];
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int_type* ito_next;
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// construct a locale object with the specialized facet.
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locale loc(locale::classic(), new unicode_codecvt);
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// sanity check the constructed locale has the specialized facet.
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VERIFY( has_facet<unicode_codecvt>(loc) );
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const unicode_codecvt& cvt = use_facet<unicode_codecvt>(loc);
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// convert between const char* and unicode strings
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unicode_codecvt::state_type state01("UNICODE", "ISO_8859-1");
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initialize_state(state01);
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result r1 = cvt.in(state01, e_lit, e_lit + size, efrom_next,
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i_arr, i_arr + size, ito_next);
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VERIFY( r1 == codecvt_base::ok );
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VERIFY( !int_traits::compare(i_arr, i_lit, size) );
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VERIFY( efrom_next == e_lit + size );
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VERIFY( ito_next == i_arr + size );
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</pre>
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<li>
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b. conversions involving std::string
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<li>
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c. conversions involving std::filebuf and std::ostream
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</ul>
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More information can be found in the following testcases:
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<ul>
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<li> testsuite/22_locale/codecvt_char_char.cc
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<li> testsuite/22_locale/codecvt_unicode_wchar_t.cc
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<li> testsuite/22_locale/codecvt_unicode_char.cc
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<li> testsuite/22_locale/codecvt_wchar_t_char.cc
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</ul>
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<p>
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<h2>
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7. Unresolved Issues
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</h2>
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<ul>
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<li>
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a. things that are sketchy, or remain unimplemented:
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do_encoding, max_length and length member functions
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are only weakly implemented. I have no idea how to do
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this correctly, and in a generic manner. Nathan?
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<li>
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b. conversions involving std::string
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<ul>
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<li>
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how should operators != and == work for string of
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different/same encoding?
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<li>
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what is equal? A byte by byte comparison or an
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encoding then byte comparison?
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<li>
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conversions between narrow, wide, and unicode strings
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</ul>
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<li>
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c. conversions involving std::filebuf and std::ostream
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<ul>
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<li>
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how to initialize the state object in a
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standards-conformant manner?
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<li>
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how to synchronize the "C" and "C++"
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conversion information?
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<li>
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wchar_t/char internal buffers and conversions between
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internal/external buffers?
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</ul>
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</ul>
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<p>
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<h2>
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8. Acknowledgments
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</h2>
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Ulrich Drepper for the iconv suggestions and patient answering of
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late-night questions, Jason Merrill for the template partial
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specialization hints, language clarification, and wchar_t fixes.
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<p>
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<h2>
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9. Bibliography / Referenced Documents
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</h2>
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Drepper, Ulrich, GNU libc (glibc) 2.2 manual. In particular, Chapters "6. Character Set Handling" and "7 Locales and Internationalization"
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</body>
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</html>
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