Prepare for radical source tree reorganization.zack/build-layout-experiment

All top-level files and directories are moved into a temporary storage
directory, REORG.TODO, except for files that will certainly still
exist in their current form at top level when we're done (COPYING,
COPYING.LIB, LICENSES, NEWS, README), all old ChangeLog files (which
are moved to the new directory OldChangeLogs, instead), and the
generated file INSTALL (which is just deleted; in the new order, there
will be no generated files checked into version control).

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+@node Character Set Handling, Locales, String and Array Utilities, Top
+@c %MENU% Support for extended character sets
+@chapter Character Set Handling
+
+@ifnottex
+@macro cal{text}
+\text\
+@end macro
+@end ifnottex
+
+Character sets used in the early days of computing had only six, seven,
+or eight bits for each character: there was never a case where more than
+eight bits (one byte) were used to represent a single character.  The
+limitations of this approach became more apparent as more people
+grappled with non-Roman character sets, where not all the characters
+that make up a language's character set can be represented by @math{2^8}
+choices.  This chapter shows the functionality that was added to the C
+library to support multiple character sets.
+
+@menu
+* Extended Char Intro::              Introduction to Extended Characters.
+* Charset Function Overview::        Overview about Character Handling
+                                      Functions.
+* Restartable multibyte conversion:: Restartable multibyte conversion
+                                      Functions.
+* Non-reentrant Conversion::         Non-reentrant Conversion Function.
+* Generic Charset Conversion::       Generic Charset Conversion.
+@end menu
+
+
+@node Extended Char Intro
+@section Introduction to Extended Characters
+
+A variety of solutions are available to overcome the differences between
+character sets with a 1:1 relation between bytes and characters and
+character sets with ratios of 2:1 or 4:1.  The remainder of this
+section gives a few examples to help understand the design decisions
+made while developing the functionality of the @w{C library}.
+
+@cindex internal representation
+A distinction we have to make right away is between internal and
+external representation.  @dfn{Internal representation} means the
+representation used by a program while keeping the text in memory.
+External representations are used when text is stored or transmitted
+through some communication channel.  Examples of external
+representations include files waiting in a directory to be
+read and parsed.
+
+Traditionally there has been no difference between the two representations.
+It was equally comfortable and useful to use the same single-byte
+representation internally and externally.  This comfort level decreases
+with more and larger character sets.
+
+One of the problems to overcome with the internal representation is
+handling text that is externally encoded using different character
+sets.  Assume a program that reads two texts and compares them using
+some metric.  The comparison can be usefully done only if the texts are
+internally kept in a common format.
+
+@cindex wide character
+For such a common format (@math{=} character set) eight bits are certainly
+no longer enough.  So the smallest entity will have to grow: @dfn{wide
+characters} will now be used.  Instead of one byte per character, two or
+four will be used instead.  (Three are not good to address in memory and
+more than four bytes seem not to be necessary).
+
+@cindex Unicode
+@cindex ISO 10646
+As shown in some other part of this manual,
+@c !!! Ahem, wide char string functions are not yet covered -- drepper
+a completely new family has been created of functions that can handle wide
+character texts in memory.  The most commonly used character sets for such
+internal wide character representations are Unicode and @w{ISO 10646}
+(also known as UCS for Universal Character Set).  Unicode was originally
+planned as a 16-bit character set; whereas, @w{ISO 10646} was designed to
+be a 31-bit large code space.  The two standards are practically identical.
+They have the same character repertoire and code table, but Unicode specifies
+added semantics.  At the moment, only characters in the first @code{0x10000}
+code positions (the so-called Basic Multilingual Plane, BMP) have been
+assigned, but the assignment of more specialized characters outside this
+16-bit space is already in progress.  A number of encodings have been
+defined for Unicode and @w{ISO 10646} characters:
+@cindex UCS-2
+@cindex UCS-4
+@cindex UTF-8
+@cindex UTF-16
+UCS-2 is a 16-bit word that can only represent characters
+from the BMP, UCS-4 is a 32-bit word than can represent any Unicode
+and @w{ISO 10646} character, UTF-8 is an ASCII compatible encoding where
+ASCII characters are represented by ASCII bytes and non-ASCII characters
+by sequences of 2-6 non-ASCII bytes, and finally UTF-16 is an extension
+of UCS-2 in which pairs of certain UCS-2 words can be used to encode
+non-BMP characters up to @code{0x10ffff}.
+
+To represent wide characters the @code{char} type is not suitable.  For
+this reason the @w{ISO C} standard introduces a new type that is
+designed to keep one character of a wide character string.  To maintain
+the similarity there is also a type corresponding to @code{int} for
+those functions that take a single wide character.
+
+@comment stddef.h
+@comment ISO
+@deftp {Data type} wchar_t
+This data type is used as the base type for wide character strings.
+In other words, arrays of objects of this type are the equivalent of
+@code{char[]} for multibyte character strings.  The type is defined in
+@file{stddef.h}.
+
+The @w{ISO C90} standard, where @code{wchar_t} was introduced, does not
+say anything specific about the representation.  It only requires that
+this type is capable of storing all elements of the basic character set.
+Therefore it would be legitimate to define @code{wchar_t} as @code{char},
+which might make sense for embedded systems.
+
+But in @theglibc{} @code{wchar_t} is always 32 bits wide and, therefore,
+capable of representing all UCS-4 values and, therefore, covering all of
+@w{ISO 10646}.  Some Unix systems define @code{wchar_t} as a 16-bit type
+and thereby follow Unicode very strictly.  This definition is perfectly
+fine with the standard, but it also means that to represent all
+characters from Unicode and @w{ISO 10646} one has to use UTF-16 surrogate
+characters, which is in fact a multi-wide-character encoding.  But
+resorting to multi-wide-character encoding contradicts the purpose of the
+@code{wchar_t} type.
+@end deftp
+
+@comment wchar.h
+@comment ISO
+@deftp {Data type} wint_t
+@code{wint_t} is a data type used for parameters and variables that
+contain a single wide character.  As the name suggests this type is the
+equivalent of @code{int} when using the normal @code{char} strings.  The
+types @code{wchar_t} and @code{wint_t} often have the same
+representation if their size is 32 bits wide but if @code{wchar_t} is
+defined as @code{char} the type @code{wint_t} must be defined as
+@code{int} due to the parameter promotion.
+
+@pindex wchar.h
+This type is defined in @file{wchar.h} and was introduced in
+@w{Amendment 1} to @w{ISO C90}.
+@end deftp
+
+As there are for the @code{char} data type macros are available for
+specifying the minimum and maximum value representable in an object of
+type @code{wchar_t}.
+
+@comment wchar.h
+@comment ISO
+@deftypevr Macro wint_t WCHAR_MIN
+The macro @code{WCHAR_MIN} evaluates to the minimum value representable
+by an object of type @code{wint_t}.
+
+This macro was introduced in @w{Amendment 1} to @w{ISO C90}.
+@end deftypevr
+
+@comment wchar.h
+@comment ISO
+@deftypevr Macro wint_t WCHAR_MAX
+The macro @code{WCHAR_MAX} evaluates to the maximum value representable
+by an object of type @code{wint_t}.
+
+This macro was introduced in @w{Amendment 1} to @w{ISO C90}.
+@end deftypevr
+
+Another special wide character value is the equivalent to @code{EOF}.
+
+@comment wchar.h
+@comment ISO
+@deftypevr Macro wint_t WEOF
+The macro @code{WEOF} evaluates to a constant expression of type
+@code{wint_t} whose value is different from any member of the extended
+character set.
+
+@code{WEOF} need not be the same value as @code{EOF} and unlike
+@code{EOF} it also need @emph{not} be negative.  In other words, sloppy
+code like
+
+@smallexample
+@{
+  int c;
+  @dots{}
+  while ((c = getc (fp)) < 0)
+    @dots{}
+@}
+@end smallexample
+
+@noindent
+has to be rewritten to use @code{WEOF} explicitly when wide characters
+are used:
+
+@smallexample
+@{
+  wint_t c;
+  @dots{}
+  while ((c = wgetc (fp)) != WEOF)
+    @dots{}
+@}
+@end smallexample
+
+@pindex wchar.h
+This macro was introduced in @w{Amendment 1} to @w{ISO C90} and is
+defined in @file{wchar.h}.
+@end deftypevr
+
+
+These internal representations present problems when it comes to storage
+and transmittal.  Because each single wide character consists of more
+than one byte, they are affected by byte-ordering.  Thus, machines with
+different endianesses would see different values when accessing the same
+data.  This byte ordering concern also applies for communication protocols
+that are all byte-based and therefore require that the sender has to
+decide about splitting the wide character in bytes.  A last (but not least
+important) point is that wide characters often require more storage space
+than a customized byte-oriented character set.
+
+@cindex multibyte character
+@cindex EBCDIC
+For all the above reasons, an external encoding that is different from
+the internal encoding is often used if the latter is UCS-2 or UCS-4.
+The external encoding is byte-based and can be chosen appropriately for
+the environment and for the texts to be handled.  A variety of different
+character sets can be used for this external encoding (information that
+will not be exhaustively presented here--instead, a description of the
+major groups will suffice).  All of the ASCII-based character sets
+fulfill one requirement: they are "filesystem safe."  This means that
+the character @code{'/'} is used in the encoding @emph{only} to
+represent itself.  Things are a bit different for character sets like
+EBCDIC (Extended Binary Coded Decimal Interchange Code, a character set
+family used by IBM), but if the operating system does not understand
+EBCDIC directly the parameters-to-system calls have to be converted
+first anyhow.
+
+@itemize @bullet
+@item
+The simplest character sets are single-byte character sets.  There can
+be only up to 256 characters (for @w{8 bit} character sets), which is
+not sufficient to cover all languages but might be sufficient to handle
+a specific text.  Handling of a @w{8 bit} character sets is simple.  This
+is not true for other kinds presented later, and therefore, the
+application one uses might require the use of @w{8 bit} character sets.
+
+@cindex ISO 2022
+@item
+The @w{ISO 2022} standard defines a mechanism for extended character
+sets where one character @emph{can} be represented by more than one
+byte.  This is achieved by associating a state with the text.
+Characters that can be used to change the state can be embedded in the
+text.  Each byte in the text might have a different interpretation in each
+state.  The state might even influence whether a given byte stands for a
+character on its own or whether it has to be combined with some more
+bytes.
+
+@cindex EUC
+@cindex Shift_JIS
+@cindex SJIS
+In most uses of @w{ISO 2022} the defined character sets do not allow
+state changes that cover more than the next character.  This has the
+big advantage that whenever one can identify the beginning of the byte
+sequence of a character one can interpret a text correctly.  Examples of
+character sets using this policy are the various EUC character sets
+(used by Sun's operating systems, EUC-JP, EUC-KR, EUC-TW, and EUC-CN)
+or Shift_JIS (SJIS, a Japanese encoding).
+
+But there are also character sets using a state that is valid for more
+than one character and has to be changed by another byte sequence.
+Examples for this are ISO-2022-JP, ISO-2022-KR, and ISO-2022-CN.
+
+@item
+@cindex ISO 6937
+Early attempts to fix 8 bit character sets for other languages using the
+Roman alphabet lead to character sets like @w{ISO 6937}.  Here bytes
+representing characters like the acute accent do not produce output
+themselves: one has to combine them with other characters to get the
+desired result.  For example, the byte sequence @code{0xc2 0x61}
+(non-spacing acute accent, followed by lower-case `a') to get the ``small
+a with  acute'' character.  To get the acute accent character on its own,
+one has to write @code{0xc2 0x20} (the non-spacing acute followed by a
+space).
+
+Character sets like @w{ISO 6937} are used in some embedded systems such
+as teletex.
+
+@item
+@cindex UTF-8
+Instead of converting the Unicode or @w{ISO 10646} text used internally,
+it is often also sufficient to simply use an encoding different than
+UCS-2/UCS-4.  The Unicode and @w{ISO 10646} standards even specify such an
+encoding: UTF-8.  This encoding is able to represent all of @w{ISO
+10646} 31 bits in a byte string of length one to six.
+
+@cindex UTF-7
+There were a few other attempts to encode @w{ISO 10646} such as UTF-7,
+but UTF-8 is today the only encoding that should be used.  In fact, with
+any luck UTF-8 will soon be the only external encoding that has to be
+supported.  It proves to be universally usable and its only disadvantage
+is that it favors Roman languages by making the byte string
+representation of other scripts (Cyrillic, Greek, Asian scripts) longer
+than necessary if using a specific character set for these scripts.
+Methods like the Unicode compression scheme can alleviate these
+problems.
+@end itemize
+
+The question remaining is: how to select the character set or encoding
+to use.  The answer: you cannot decide about it yourself, it is decided
+by the developers of the system or the majority of the users.  Since the
+goal is interoperability one has to use whatever the other people one
+works with use.  If there are no constraints, the selection is based on
+the requirements the expected circle of users will have.  In other words,
+if a project is expected to be used in only, say, Russia it is fine to use
+KOI8-R or a similar character set.  But if at the same time people from,
+say, Greece are participating one should use a character set that allows
+all people to collaborate.
+
+The most widely useful solution seems to be: go with the most general
+character set, namely @w{ISO 10646}.  Use UTF-8 as the external encoding
+and problems about users not being able to use their own language
+adequately are a thing of the past.
+
+One final comment about the choice of the wide character representation
+is necessary at this point.  We have said above that the natural choice
+is using Unicode or @w{ISO 10646}.  This is not required, but at least
+encouraged, by the @w{ISO C} standard.  The standard defines at least a
+macro @code{__STDC_ISO_10646__} that is only defined on systems where
+the @code{wchar_t} type encodes @w{ISO 10646} characters.  If this
+symbol is not defined one should avoid making assumptions about the wide
+character representation.  If the programmer uses only the functions
+provided by the C library to handle wide character strings there should
+be no compatibility problems with other systems.
+
+@node Charset Function Overview
+@section Overview about Character Handling Functions
+
+A Unix @w{C library} contains three different sets of functions in two
+families to handle character set conversion.  One of the function families
+(the most commonly used) is specified in the @w{ISO C90} standard and,
+therefore, is portable even beyond the Unix world.  Unfortunately this
+family is the least useful one.  These functions should be avoided
+whenever possible, especially when developing libraries (as opposed to
+applications).
+
+The second family of functions got introduced in the early Unix standards
+(XPG2) and is still part of the latest and greatest Unix standard:
+@w{Unix 98}.  It is also the most powerful and useful set of functions.
+But we will start with the functions defined in @w{Amendment 1} to
+@w{ISO C90}.
+
+@node Restartable multibyte conversion
+@section Restartable Multibyte Conversion Functions
+
+The @w{ISO C} standard defines functions to convert strings from a
+multibyte representation to wide character strings.  There are a number
+of peculiarities:
+
+@itemize @bullet
+@item
+The character set assumed for the multibyte encoding is not specified
+as an argument to the functions.  Instead the character set specified by
+the @code{LC_CTYPE} category of the current locale is used; see
+@ref{Locale Categories}.
+
+@item
+The functions handling more than one character at a time require NUL
+terminated strings as the argument (i.e., converting blocks of text
+does not work unless one can add a NUL byte at an appropriate place).
+@Theglibc{} contains some extensions to the standard that allow
+specifying a size, but basically they also expect terminated strings.
+@end itemize
+
+Despite these limitations the @w{ISO C} functions can be used in many
+contexts.  In graphical user interfaces, for instance, it is not
+uncommon to have functions that require text to be displayed in a wide
+character string if the text is not simple ASCII.  The text itself might
+come from a file with translations and the user should decide about the
+current locale, which determines the translation and therefore also the
+external encoding used.  In such a situation (and many others) the
+functions described here are perfect.  If more freedom while performing
+the conversion is necessary take a look at the @code{iconv} functions
+(@pxref{Generic Charset Conversion}).
+
+@menu
+* Selecting the Conversion::     Selecting the conversion and its properties.
+* Keeping the state::            Representing the state of the conversion.
+* Converting a Character::       Converting Single Characters.
+* Converting Strings::           Converting Multibyte and Wide Character
+                                  Strings.
+* Multibyte Conversion Example:: A Complete Multibyte Conversion Example.
+@end menu
+
+@node Selecting the Conversion
+@subsection Selecting the conversion and its properties
+
+We already said above that the currently selected locale for the
+@code{LC_CTYPE} category decides the conversion that is performed
+by the functions we are about to describe.  Each locale uses its own
+character set (given as an argument to @code{localedef}) and this is the
+one assumed as the external multibyte encoding.  The wide character
+set is always UCS-4 in @theglibc{}.
+
+A characteristic of each multibyte character set is the maximum number
+of bytes that can be necessary to represent one character.  This
+information is quite important when writing code that uses the
+conversion functions (as shown in the examples below).
+The @w{ISO C} standard defines two macros that provide this information.
+
+
+@comment limits.h
+@comment ISO
+@deftypevr Macro int MB_LEN_MAX
+@code{MB_LEN_MAX} specifies the maximum number of bytes in the multibyte
+sequence for a single character in any of the supported locales.  It is
+a compile-time constant and is defined in @file{limits.h}.
+@pindex limits.h
+@end deftypevr
+
+@comment stdlib.h
+@comment ISO
+@deftypevr Macro int MB_CUR_MAX
+@code{MB_CUR_MAX} expands into a positive integer expression that is the
+maximum number of bytes in a multibyte character in the current locale.
+The value is never greater than @code{MB_LEN_MAX}.  Unlike
+@code{MB_LEN_MAX} this macro need not be a compile-time constant, and in
+@theglibc{} it is not.
+
+@pindex stdlib.h
+@code{MB_CUR_MAX} is defined in @file{stdlib.h}.
+@end deftypevr
+
+Two different macros are necessary since strictly @w{ISO C90} compilers
+do not allow variable length array definitions, but still it is desirable
+to avoid dynamic allocation.  This incomplete piece of code shows the
+problem:
+
+@smallexample
+@{
+  char buf[MB_LEN_MAX];
+  ssize_t len = 0;
+
+  while (! feof (fp))
+    @{
+      fread (&buf[len], 1, MB_CUR_MAX - len, fp);
+      /* @r{@dots{} process} buf */
+      len -= used;
+    @}
+@}
+@end smallexample
+
+The code in the inner loop is expected to have always enough bytes in
+the array @var{buf} to convert one multibyte character.  The array
+@var{buf} has to be sized statically since many compilers do not allow a
+variable size.  The @code{fread} call makes sure that @code{MB_CUR_MAX}
+bytes are always available in @var{buf}.  Note that it isn't
+a problem if @code{MB_CUR_MAX} is not a compile-time constant.
+
+
+@node Keeping the state
+@subsection Representing the state of the conversion
+
+@cindex stateful
+In the introduction of this chapter it was said that certain character
+sets use a @dfn{stateful} encoding.  That is, the encoded values depend
+in some way on the previous bytes in the text.
+
+Since the conversion functions allow converting a text in more than one
+step we must have a way to pass this information from one call of the
+functions to another.
+
+@comment wchar.h
+@comment ISO
+@deftp {Data type} mbstate_t
+@cindex shift state
+A variable of type @code{mbstate_t} can contain all the information
+about the @dfn{shift state} needed from one call to a conversion
+function to another.
+
+@pindex wchar.h
+@code{mbstate_t} is defined in @file{wchar.h}.  It was introduced in
+@w{Amendment 1} to @w{ISO C90}.
+@end deftp
+
+To use objects of type @code{mbstate_t} the programmer has to define such
+objects (normally as local variables on the stack) and pass a pointer to
+the object to the conversion functions.  This way the conversion function
+can update the object if the current multibyte character set is stateful.
+
+There is no specific function or initializer to put the state object in
+any specific state.  The rules are that the object should always
+represent the initial state before the first use, and this is achieved by
+clearing the whole variable with code such as follows:
+
+@smallexample
+@{
+  mbstate_t state;
+  memset (&state, '\0', sizeof (state));
+  /* @r{from now on @var{state} can be used.}  */
+  @dots{}
+@}
+@end smallexample
+
+When using the conversion functions to generate output it is often
+necessary to test whether the current state corresponds to the initial
+state.  This is necessary, for example, to decide whether to emit
+escape sequences to set the state to the initial state at certain
+sequence points.  Communication protocols often require this.
+
+@comment wchar.h
+@comment ISO
+@deftypefun int mbsinit (const mbstate_t *@var{ps})
+@safety{@prelim{}@mtsafe{}@assafe{}@acsafe{}}
+@c ps is dereferenced once, unguarded.  This would call for @mtsrace:ps,
+@c but since a single word-sized field is (atomically) accessed, any
+@c race here would be harmless.  Other functions that take an optional
+@c mbstate_t* argument named ps are marked with @mtasurace:<func>/!ps,
+@c to indicate that the function uses a static buffer if ps is NULL.
+@c These could also have been marked with @mtsrace:ps, but we'll omit
+@c that for brevity, for it's somewhat redundant with the @mtasurace.
+The @code{mbsinit} function determines whether the state object pointed
+to by @var{ps} is in the initial state.  If @var{ps} is a null pointer or
+the object is in the initial state the return value is nonzero.  Otherwise
+it is zero.
+
+@pindex wchar.h
+@code{mbsinit} was introduced in @w{Amendment 1} to @w{ISO C90} and is
+declared in @file{wchar.h}.
+@end deftypefun
+
+Code using @code{mbsinit} often looks similar to this:
+
+@c Fix the example to explicitly say how to generate the escape sequence
+@c to restore the initial state.
+@smallexample
+@{
+  mbstate_t state;
+  memset (&state, '\0', sizeof (state));
+  /* @r{Use @var{state}.}  */
+  @dots{}
+  if (! mbsinit (&state))
+    @{
+      /* @r{Emit code to return to initial state.}  */
+      const wchar_t empty[] = L"";
+      const wchar_t *srcp = empty;
+      wcsrtombs (outbuf, &srcp, outbuflen, &state);
+    @}
+  @dots{}
+@}
+@end smallexample
+
+The code to emit the escape sequence to get back to the initial state is
+interesting.  The @code{wcsrtombs} function can be used to determine the
+necessary output code (@pxref{Converting Strings}).  Please note that with
+@theglibc{} it is not necessary to perform this extra action for the
+conversion from multibyte text to wide character text since the wide
+character encoding is not stateful.  But there is nothing mentioned in
+any standard that prohibits making @code{wchar_t} use a stateful
+encoding.
+
+@node Converting a Character
+@subsection Converting Single Characters
+
+The most fundamental of the conversion functions are those dealing with
+single characters.  Please note that this does not always mean single
+bytes.  But since there is very often a subset of the multibyte
+character set that consists of single byte sequences, there are
+functions to help with converting bytes.  Frequently, ASCII is a subset
+of the multibyte character set.  In such a scenario, each ASCII character
+stands for itself, and all other characters have at least a first byte
+that is beyond the range @math{0} to @math{127}.
+
+@comment wchar.h
+@comment ISO
+@deftypefun wint_t btowc (int @var{c})
+@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}}
+@c Calls btowc_fct or __fct; reads from locale, and from the
+@c get_gconv_fcts result multiple times.  get_gconv_fcts calls
+@c __wcsmbs_load_conv to initialize the ctype if it's null.
+@c wcsmbs_load_conv takes a non-recursive wrlock before allocating
+@c memory for the fcts structure, initializing it, and then storing it
+@c in the locale object.  The initialization involves dlopening and a
+@c lot more.
+The @code{btowc} function (``byte to wide character'') converts a valid
+single byte character @var{c} in the initial shift state into the wide
+character equivalent using the conversion rules from the currently
+selected locale of the @code{LC_CTYPE} category.
+
+If @code{(unsigned char) @var{c}} is no valid single byte multibyte
+character or if @var{c} is @code{EOF}, the function returns @code{WEOF}.
+
+Please note the restriction of @var{c} being tested for validity only in
+the initial shift state.  No @code{mbstate_t} object is used from
+which the state information is taken, and the function also does not use
+any static state.
+
+@pindex wchar.h
+The @code{btowc} function was introduced in @w{Amendment 1} to @w{ISO C90}
+and is declared in @file{wchar.h}.
+@end deftypefun
+
+Despite the limitation that the single byte value is always interpreted
+in the initial state, this function is actually useful most of the time.
+Most characters are either entirely single-byte character sets or they
+are extensions to ASCII.  But then it is possible to write code like this
+(not that this specific example is very useful):
+
+@smallexample
+wchar_t *
+itow (unsigned long int val)
+@{
+  static wchar_t buf[30];
+  wchar_t *wcp = &buf[29];
+  *wcp = L'\0';
+  while (val != 0)
+    @{
+      *--wcp = btowc ('0' + val % 10);
+      val /= 10;
+    @}
+  if (wcp == &buf[29])
+    *--wcp = L'0';
+  return wcp;
+@}
+@end smallexample
+
+Why is it necessary to use such a complicated implementation and not
+simply cast @code{'0' + val % 10} to a wide character?  The answer is
+that there is no guarantee that one can perform this kind of arithmetic
+on the character of the character set used for @code{wchar_t}
+representation.  In other situations the bytes are not constant at
+compile time and so the compiler cannot do the work.  In situations like
+this, using @code{btowc} is required.
+
+@noindent
+There is also a function for the conversion in the other direction.
+
+@comment wchar.h
+@comment ISO
+@deftypefun int wctob (wint_t @var{c})
+@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}}
+The @code{wctob} function (``wide character to byte'') takes as the
+parameter a valid wide character.  If the multibyte representation for
+this character in the initial state is exactly one byte long, the return
+value of this function is this character.  Otherwise the return value is
+@code{EOF}.
+
+@pindex wchar.h
+@code{wctob} was introduced in @w{Amendment 1} to @w{ISO C90} and
+is declared in @file{wchar.h}.
+@end deftypefun
+
+There are more general functions to convert single characters from
+multibyte representation to wide characters and vice versa.  These
+functions pose no limit on the length of the multibyte representation
+and they also do not require it to be in the initial state.
+
+@comment wchar.h
+@comment ISO
+@deftypefun size_t mbrtowc (wchar_t *restrict @var{pwc}, const char *restrict @var{s}, size_t @var{n}, mbstate_t *restrict @var{ps})
+@safety{@prelim{}@mtunsafe{@mtasurace{:mbrtowc/!ps}}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}}
+@cindex stateful
+The @code{mbrtowc} function (``multibyte restartable to wide
+character'') converts the next multibyte character in the string pointed
+to by @var{s} into a wide character and stores it in the wide character
+string pointed to by @var{pwc}.  The conversion is performed according
+to the locale currently selected for the @code{LC_CTYPE} category.  If
+the conversion for the character set used in the locale requires a state,
+the multibyte string is interpreted in the state represented by the
+object pointed to by @var{ps}.  If @var{ps} is a null pointer, a static,
+internal state variable used only by the @code{mbrtowc} function is
+used.
+
+If the next multibyte character corresponds to the NUL wide character,
+the return value of the function is @math{0} and the state object is
+afterwards in the initial state.  If the next @var{n} or fewer bytes
+form a correct multibyte character, the return value is the number of
+bytes starting from @var{s} that form the multibyte character.  The
+conversion state is updated according to the bytes consumed in the
+conversion.  In both cases the wide character (either the @code{L'\0'}
+or the one found in the conversion) is stored in the string pointed to
+by @var{pwc} if @var{pwc} is not null.
+
+If the first @var{n} bytes of the multibyte string possibly form a valid
+multibyte character but there are more than @var{n} bytes needed to
+complete it, the return value of the function is @code{(size_t) -2} and
+no value is stored.  Please note that this can happen even if @var{n}
+has a value greater than or equal to @code{MB_CUR_MAX} since the input
+might contain redundant shift sequences.
+
+If the first @code{n} bytes of the multibyte string cannot possibly form
+a valid multibyte character, no value is stored, the global variable
+@code{errno} is set to the value @code{EILSEQ}, and the function returns
+@code{(size_t) -1}.  The conversion state is afterwards undefined.
+
+@pindex wchar.h
+@code{mbrtowc} was introduced in @w{Amendment 1} to @w{ISO C90} and
+is declared in @file{wchar.h}.
+@end deftypefun
+
+Use of @code{mbrtowc} is straightforward.  A function that copies a
+multibyte string into a wide character string while at the same time
+converting all lowercase characters into uppercase could look like this
+(this is not the final version, just an example; it has no error
+checking, and sometimes leaks memory):
+
+@smallexample
+wchar_t *
+mbstouwcs (const char *s)
+@{
+  size_t len = strlen (s);
+  wchar_t *result = malloc ((len + 1) * sizeof (wchar_t));
+  wchar_t *wcp = result;
+  wchar_t tmp[1];
+  mbstate_t state;
+  size_t nbytes;
+
+  memset (&state, '\0', sizeof (state));
+  while ((nbytes = mbrtowc (tmp, s, len, &state)) > 0)
+    @{
+      if (nbytes >= (size_t) -2)
+        /* Invalid input string.  */
+        return NULL;
+      *wcp++ = towupper (tmp[0]);
+      len -= nbytes;
+      s += nbytes;
+    @}
+  return result;
+@}
+@end smallexample
+
+The use of @code{mbrtowc} should be clear.  A single wide character is
+stored in @code{@var{tmp}[0]}, and the number of consumed bytes is stored
+in the variable @var{nbytes}.  If the conversion is successful, the
+uppercase variant of the wide character is stored in the @var{result}
+array and the pointer to the input string and the number of available
+bytes is adjusted.
+
+The only non-obvious thing about @code{mbrtowc} might be the way memory
+is allocated for the result.  The above code uses the fact that there
+can never be more wide characters in the converted result than there are
+bytes in the multibyte input string.  This method yields a pessimistic
+guess about the size of the result, and if many wide character strings
+have to be constructed this way or if the strings are long, the extra
+memory required to be allocated because the input string contains
+multibyte characters might be significant.  The allocated memory block can
+be resized to the correct size before returning it, but a better solution
+might be to allocate just the right amount of space for the result right
+away.  Unfortunately there is no function to compute the length of the wide
+character string directly from the multibyte string.  There is, however, a
+function that does part of the work.
+
+@comment wchar.h
+@comment ISO
+@deftypefun size_t mbrlen (const char *restrict @var{s}, size_t @var{n}, mbstate_t *@var{ps})
+@safety{@prelim{}@mtunsafe{@mtasurace{:mbrlen/!ps}}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}}
+The @code{mbrlen} function (``multibyte restartable length'') computes
+the number of at most @var{n} bytes starting at @var{s}, which form the
+next valid and complete multibyte character.
+
+If the next multibyte character corresponds to the NUL wide character,
+the return value is @math{0}.  If the next @var{n} bytes form a valid
+multibyte character, the number of bytes belonging to this multibyte
+character byte sequence is returned.
+
+If the first @var{n} bytes possibly form a valid multibyte
+character but the character is incomplete, the return value is
+@code{(size_t) -2}.  Otherwise the multibyte character sequence is invalid
+and the return value is @code{(size_t) -1}.
+
+The multibyte sequence is interpreted in the state represented by the
+object pointed to by @var{ps}.  If @var{ps} is a null pointer, a state
+object local to @code{mbrlen} is used.
+
+@pindex wchar.h
+@code{mbrlen} was introduced in @w{Amendment 1} to @w{ISO C90} and
+is declared in @file{wchar.h}.
+@end deftypefun
+
+The attentive reader now will note that @code{mbrlen} can be implemented
+as
+
+@smallexample
+mbrtowc (NULL, s, n, ps != NULL ? ps : &internal)
+@end smallexample
+
+This is true and in fact is mentioned in the official specification.
+How can this function be used to determine the length of the wide
+character string created from a multibyte character string?  It is not
+directly usable, but we can define a function @code{mbslen} using it:
+
+@smallexample
+size_t
+mbslen (const char *s)
+@{
+  mbstate_t state;
+  size_t result = 0;
+  size_t nbytes;
+  memset (&state, '\0', sizeof (state));
+  while ((nbytes = mbrlen (s, MB_LEN_MAX, &state)) > 0)
+    @{
+      if (nbytes >= (size_t) -2)
+        /* @r{Something is wrong.}  */
+        return (size_t) -1;
+      s += nbytes;
+      ++result;
+    @}
+  return result;
+@}
+@end smallexample
+
+This function simply calls @code{mbrlen} for each multibyte character
+in the string and counts the number of function calls.  Please note that
+we here use @code{MB_LEN_MAX} as the size argument in the @code{mbrlen}
+call.  This is acceptable since a) this value is larger than the length of
+the longest multibyte character sequence and b) we know that the string
+@var{s} ends with a NUL byte, which cannot be part of any other multibyte
+character sequence but the one representing the NUL wide character.
+Therefore, the @code{mbrlen} function will never read invalid memory.
+
+Now that this function is available (just to make this clear, this
+function is @emph{not} part of @theglibc{}) we can compute the
+number of wide characters required to store the converted multibyte
+character string @var{s} using
+
+@smallexample
+wcs_bytes = (mbslen (s) + 1) * sizeof (wchar_t);
+@end smallexample
+
+Please note that the @code{mbslen} function is quite inefficient.  The
+implementation of @code{mbstouwcs} with @code{mbslen} would have to
+perform the conversion of the multibyte character input string twice, and
+this conversion might be quite expensive.  So it is necessary to think
+about the consequences of using the easier but imprecise method before
+doing the work twice.
+
+@comment wchar.h
+@comment ISO
+@deftypefun size_t wcrtomb (char *restrict @var{s}, wchar_t @var{wc}, mbstate_t *restrict @var{ps})
+@safety{@prelim{}@mtunsafe{@mtasurace{:wcrtomb/!ps}}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}}
+@c wcrtomb uses a static, non-thread-local unguarded state variable when
+@c PS is NULL.  When a state is passed in, and it's not used
+@c concurrently in other threads, this function behaves safely as long
+@c as gconv modules don't bring MT safety issues of their own.
+@c Attempting to load gconv modules or to build conversion chains in
+@c signal handlers may encounter gconv databases or caches in a
+@c partially-updated state, and asynchronous cancellation may leave them
+@c in such states, besides leaking the lock that guards them.
+@c get_gconv_fcts ok
+@c    wcsmbs_load_conv ok
+@c      norm_add_slashes ok
+@c      wcsmbs_getfct ok
+@c        gconv_find_transform ok
+@c          gconv_read_conf (libc_once)
+@c          gconv_lookup_cache ok
+@c            find_module_idx ok
+@c            find_module ok
+@c              gconv_find_shlib (ok)
+@c              ->init_fct (assumed ok)
+@c            gconv_get_builtin_trans ok
+@c            gconv_release_step ok
+@c          do_lookup_alias ok
+@c          find_derivation ok
+@c            derivation_lookup ok
+@c            increment_counter ok
+@c              gconv_find_shlib ok
+@c              step->init_fct (assumed ok)
+@c            gen_steps ok
+@c              gconv_find_shlib ok
+@c                dlopen (presumed ok)
+@c                dlsym (presumed ok)
+@c              step->init_fct (assumed ok)
+@c              step->end_fct (assumed ok)
+@c              gconv_get_builtin_trans ok
+@c              gconv_release_step ok
+@c            add_derivation ok
+@c      gconv_close_transform ok
+@c        gconv_release_step ok
+@c          step->end_fct (assumed ok)
+@c          gconv_release_shlib ok
+@c            dlclose (presumed ok)
+@c        gconv_release_cache ok
+@c  ->tomb->__fct (assumed ok)
+The @code{wcrtomb} function (``wide character restartable to
+multibyte'') converts a single wide character into a multibyte string
+corresponding to that wide character.
+
+If @var{s} is a null pointer, the function resets the state stored in
+the object pointed to by @var{ps} (or the internal @code{mbstate_t}
+object) to the initial state.  This can also be achieved by a call like
+this:
+
+@smallexample
+wcrtombs (temp_buf, L'\0', ps)
+@end smallexample
+
+@noindent
+since, if @var{s} is a null pointer, @code{wcrtomb} performs as if it
+writes into an internal buffer, which is guaranteed to be large enough.
+
+If @var{wc} is the NUL wide character, @code{wcrtomb} emits, if
+necessary, a shift sequence to get the state @var{ps} into the initial
+state followed by a single NUL byte, which is stored in the string
+@var{s}.
+
+Otherwise a byte sequence (possibly including shift sequences) is written
+into the string @var{s}.  This only happens if @var{wc} is a valid wide
+character (i.e., it has a multibyte representation in the character set
+selected by locale of the @code{LC_CTYPE} category).  If @var{wc} is no
+valid wide character, nothing is stored in the strings @var{s},
+@code{errno} is set to @code{EILSEQ}, the conversion state in @var{ps}
+is undefined and the return value is @code{(size_t) -1}.
+
+If no error occurred the function returns the number of bytes stored in
+the string @var{s}.  This includes all bytes representing shift
+sequences.
+
+One word about the interface of the function: there is no parameter
+specifying the length of the array @var{s}.  Instead the function
+assumes that there are at least @code{MB_CUR_MAX} bytes available since
+this is the maximum length of any byte sequence representing a single
+character.  So the caller has to make sure that there is enough space
+available, otherwise buffer overruns can occur.
+
+@pindex wchar.h
+@code{wcrtomb} was introduced in @w{Amendment 1} to @w{ISO C90} and is
+declared in @file{wchar.h}.
+@end deftypefun
+
+Using @code{wcrtomb} is as easy as using @code{mbrtowc}.  The following
+example appends a wide character string to a multibyte character string.
+Again, the code is not really useful (or correct), it is simply here to
+demonstrate the use and some problems.
+
+@smallexample
+char *
+mbscatwcs (char *s, size_t len, const wchar_t *ws)
+@{
+  mbstate_t state;
+  /* @r{Find the end of the existing string.}  */
+  char *wp = strchr (s, '\0');
+  len -= wp - s;
+  memset (&state, '\0', sizeof (state));
+  do
+    @{
+      size_t nbytes;
+      if (len < MB_CUR_LEN)
+        @{
+          /* @r{We cannot guarantee that the next}
+             @r{character fits into the buffer, so}
+             @r{return an error.}  */
+          errno = E2BIG;
+          return NULL;
+        @}
+      nbytes = wcrtomb (wp, *ws, &state);
+      if (nbytes == (size_t) -1)
+        /* @r{Error in the conversion.}  */
+        return NULL;
+      len -= nbytes;
+      wp += nbytes;
+    @}
+  while (*ws++ != L'\0');
+  return s;
+@}
+@end smallexample
+
+First the function has to find the end of the string currently in the
+array @var{s}.  The @code{strchr} call does this very efficiently since a
+requirement for multibyte character representations is that the NUL byte
+is never used except to represent itself (and in this context, the end
+of the string).
+
+After initializing the state object the loop is entered where the first
+task is to make sure there is enough room in the array @var{s}.  We
+abort if there are not at least @code{MB_CUR_LEN} bytes available.  This
+is not always optimal but we have no other choice.  We might have less
+than @code{MB_CUR_LEN} bytes available but the next multibyte character
+might also be only one byte long.  At the time the @code{wcrtomb} call
+returns it is too late to decide whether the buffer was large enough.  If
+this solution is unsuitable, there is a very slow but more accurate
+solution.
+
+@smallexample
+  @dots{}
+  if (len < MB_CUR_LEN)
+    @{
+      mbstate_t temp_state;
+      memcpy (&temp_state, &state, sizeof (state));
+      if (wcrtomb (NULL, *ws, &temp_state) > len)
+        @{
+          /* @r{We cannot guarantee that the next}
+             @r{character fits into the buffer, so}
+             @r{return an error.}  */
+          errno = E2BIG;
+          return NULL;
+        @}
+    @}
+  @dots{}
+@end smallexample
+
+Here we perform the conversion that might overflow the buffer so that
+we are afterwards in the position to make an exact decision about the
+buffer size.  Please note the @code{NULL} argument for the destination
+buffer in the new @code{wcrtomb} call; since we are not interested in the
+converted text at this point, this is a nice way to express this.  The
+most unusual thing about this piece of code certainly is the duplication
+of the conversion state object, but if a change of the state is necessary
+to emit the next multibyte character, we want to have the same shift state
+change performed in the real conversion.  Therefore, we have to preserve
+the initial shift state information.
+
+There are certainly many more and even better solutions to this problem.
+This example is only provided for educational purposes.
+
+@node Converting Strings
+@subsection Converting Multibyte and Wide Character Strings
+
+The functions described in the previous section only convert a single
+character at a time.  Most operations to be performed in real-world
+programs include strings and therefore the @w{ISO C} standard also
+defines conversions on entire strings.  However, the defined set of
+functions is quite limited; therefore, @theglibc{} contains a few
+extensions that can help in some important situations.
+
+@comment wchar.h
+@comment ISO
+@deftypefun size_t mbsrtowcs (wchar_t *restrict @var{dst}, const char **restrict @var{src}, size_t @var{len}, mbstate_t *restrict @var{ps})
+@safety{@prelim{}@mtunsafe{@mtasurace{:mbsrtowcs/!ps}}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}}
+The @code{mbsrtowcs} function (``multibyte string restartable to wide
+character string'') converts the NUL-terminated multibyte character
+string at @code{*@var{src}} into an equivalent wide character string,
+including the NUL wide character at the end.  The conversion is started
+using the state information from the object pointed to by @var{ps} or
+from an internal object of @code{mbsrtowcs} if @var{ps} is a null
+pointer.  Before returning, the state object is updated to match the state
+after the last converted character.  The state is the initial state if the
+terminating NUL byte is reached and converted.
+
+If @var{dst} is not a null pointer, the result is stored in the array
+pointed to by @var{dst}; otherwise, the conversion result is not
+available since it is stored in an internal buffer.
+
+If @var{len} wide characters are stored in the array @var{dst} before
+reaching the end of the input string, the conversion stops and @var{len}
+is returned.  If @var{dst} is a null pointer, @var{len} is never checked.
+
+Another reason for a premature return from the function call is if the
+input string contains an invalid multibyte sequence.  In this case the
+global variable @code{errno} is set to @code{EILSEQ} and the function
+returns @code{(size_t) -1}.
+
+@c XXX The ISO C9x draft seems to have a problem here.  It says that PS
+@c is not updated if DST is NULL.  This is not said straightforward and
+@c none of the other functions is described like this.  It would make sense
+@c to define the function this way but I don't think it is meant like this.
+
+In all other cases the function returns the number of wide characters
+converted during this call.  If @var{dst} is not null, @code{mbsrtowcs}
+stores in the pointer pointed to by @var{src} either a null pointer (if
+the NUL byte in the input string was reached) or the address of the byte
+following the last converted multibyte character.
+
+@pindex wchar.h
+@code{mbsrtowcs} was introduced in @w{Amendment 1} to @w{ISO C90} and is
+declared in @file{wchar.h}.
+@end deftypefun
+
+The definition of the @code{mbsrtowcs} function has one important
+limitation.  The requirement that @var{dst} has to be a NUL-terminated
+string provides problems if one wants to convert buffers with text.  A
+buffer is not normally a collection of NUL-terminated strings but instead a
+continuous collection of lines, separated by newline characters.  Now
+assume that a function to convert one line from a buffer is needed.  Since
+the line is not NUL-terminated, the source pointer cannot directly point
+into the unmodified text buffer.  This means, either one inserts the NUL
+byte at the appropriate place for the time of the @code{mbsrtowcs}
+function call (which is not doable for a read-only buffer or in a
+multi-threaded application) or one copies the line in an extra buffer
+where it can be terminated by a NUL byte.  Note that it is not in general
+possible to limit the number of characters to convert by setting the
+parameter @var{len} to any specific value.  Since it is not known how
+many bytes each multibyte character sequence is in length, one can only
+guess.
+
+@cindex stateful
+There is still a problem with the method of NUL-terminating a line right
+after the newline character, which could lead to very strange results.
+As said in the description of the @code{mbsrtowcs} function above, the
+conversion state is guaranteed to be in the initial shift state after
+processing the NUL byte at the end of the input string.  But this NUL
+byte is not really part of the text (i.e., the conversion state after
+the newline in the original text could be something different than the
+initial shift state and therefore the first character of the next line
+is encoded using this state).  But the state in question is never
+accessible to the user since the conversion stops after the NUL byte
+(which resets the state).  Most stateful character sets in use today
+require that the shift state after a newline be the initial state--but
+this is not a strict guarantee.  Therefore, simply NUL-terminating a
+piece of a running text is not always an adequate solution and,
+therefore, should never be used in generally used code.
+
+The generic conversion interface (@pxref{Generic Charset Conversion})
+does not have this limitation (it simply works on buffers, not
+strings), and @theglibc{} contains a set of functions that take
+additional parameters specifying the maximal number of bytes that are
+consumed from the input string.  This way the problem of
+@code{mbsrtowcs}'s example above could be solved by determining the line
+length and passing this length to the function.
+
+@comment wchar.h
+@comment ISO
+@deftypefun size_t wcsrtombs (char *restrict @var{dst}, const wchar_t **restrict @var{src}, size_t @var{len}, mbstate_t *restrict @var{ps})
+@safety{@prelim{}@mtunsafe{@mtasurace{:wcsrtombs/!ps}}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}}
+The @code{wcsrtombs} function (``wide character string restartable to
+multibyte string'') converts the NUL-terminated wide character string at
+@code{*@var{src}} into an equivalent multibyte character string and
+stores the result in the array pointed to by @var{dst}.  The NUL wide
+character is also converted.  The conversion starts in the state
+described in the object pointed to by @var{ps} or by a state object
+local to @code{wcsrtombs} in case @var{ps} is a null pointer.  If
+@var{dst} is a null pointer, the conversion is performed as usual but the
+result is not available.  If all characters of the input string were
+successfully converted and if @var{dst} is not a null pointer, the
+pointer pointed to by @var{src} gets assigned a null pointer.
+
+If one of the wide characters in the input string has no valid multibyte
+character equivalent, the conversion stops early, sets the global
+variable @code{errno} to @code{EILSEQ}, and returns @code{(size_t) -1}.
+
+Another reason for a premature stop is if @var{dst} is not a null
+pointer and the next converted character would require more than
+@var{len} bytes in total to the array @var{dst}.  In this case (and if
+@var{dst} is not a null pointer) the pointer pointed to by @var{src} is
+assigned a value pointing to the wide character right after the last one
+successfully converted.
+
+Except in the case of an encoding error the return value of the
+@code{wcsrtombs} function is the number of bytes in all the multibyte
+character sequences stored in @var{dst}.  Before returning, the state in
+the object pointed to by @var{ps} (or the internal object in case
+@var{ps} is a null pointer) is updated to reflect the state after the
+last conversion.  The state is the initial shift state in case the
+terminating NUL wide character was converted.
+
+@pindex wchar.h
+The @code{wcsrtombs} function was introduced in @w{Amendment 1} to
+@w{ISO C90} and is declared in @file{wchar.h}.
+@end deftypefun
+
+The restriction mentioned above for the @code{mbsrtowcs} function applies
+here also.  There is no possibility of directly controlling the number of
+input characters.  One has to place the NUL wide character at the correct
+place or control the consumed input indirectly via the available output
+array size (the @var{len} parameter).
+
+@comment wchar.h
+@comment GNU
+@deftypefun size_t mbsnrtowcs (wchar_t *restrict @var{dst}, const char **restrict @var{src}, size_t @var{nmc}, size_t @var{len}, mbstate_t *restrict @var{ps})
+@safety{@prelim{}@mtunsafe{@mtasurace{:mbsnrtowcs/!ps}}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}}
+The @code{mbsnrtowcs} function is very similar to the @code{mbsrtowcs}
+function.  All the parameters are the same except for @var{nmc}, which is
+new.  The return value is the same as for @code{mbsrtowcs}.
+
+This new parameter specifies how many bytes at most can be used from the
+multibyte character string.  In other words, the multibyte character
+string @code{*@var{src}} need not be NUL-terminated.  But if a NUL byte
+is found within the @var{nmc} first bytes of the string, the conversion
+stops there.
+
+This function is a GNU extension.  It is meant to work around the
+problems mentioned above.  Now it is possible to convert a buffer with
+multibyte character text piece by piece without having to care about
+inserting NUL bytes and the effect of NUL bytes on the conversion state.
+@end deftypefun
+
+A function to convert a multibyte string into a wide character string
+and display it could be written like this (this is not a really useful
+example):
+
+@smallexample
+void
+showmbs (const char *src, FILE *fp)
+@{
+  mbstate_t state;
+  int cnt = 0;
+  memset (&state, '\0', sizeof (state));
+  while (1)
+    @{
+      wchar_t linebuf[100];
+      const char *endp = strchr (src, '\n');
+      size_t n;
+
+      /* @r{Exit if there is no more line.}  */
+      if (endp == NULL)
+        break;
+
+      n = mbsnrtowcs (linebuf, &src, endp - src, 99, &state);
+      linebuf[n] = L'\0';
+      fprintf (fp, "line %d: \"%S\"\n", linebuf);
+    @}
+@}
+@end smallexample
+
+There is no problem with the state after a call to @code{mbsnrtowcs}.
+Since we don't insert characters in the strings that were not in there
+right from the beginning and we use @var{state} only for the conversion
+of the given buffer, there is no problem with altering the state.
+
+@comment wchar.h
+@comment GNU
+@deftypefun size_t wcsnrtombs (char *restrict @var{dst}, const wchar_t **restrict @var{src}, size_t @var{nwc}, size_t @var{len}, mbstate_t *restrict @var{ps})
+@safety{@prelim{}@mtunsafe{@mtasurace{:wcsnrtombs/!ps}}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}}
+The @code{wcsnrtombs} function implements the conversion from wide
+character strings to multibyte character strings.  It is similar to
+@code{wcsrtombs} but, just like @code{mbsnrtowcs}, it takes an extra
+parameter, which specifies the length of the input string.
+
+No more than @var{nwc} wide characters from the input string
+@code{*@var{src}} are converted.  If the input string contains a NUL
+wide character in the first @var{nwc} characters, the conversion stops at
+this place.
+
+The @code{wcsnrtombs} function is a GNU extension and just like
+@code{mbsnrtowcs} helps in situations where no NUL-terminated input
+strings are available.
+@end deftypefun
+
+
+@node Multibyte Conversion Example
+@subsection A Complete Multibyte Conversion Example
+
+The example programs given in the last sections are only brief and do
+not contain all the error checking, etc.  Presented here is a complete
+and documented example.  It features the @code{mbrtowc} function but it
+should be easy to derive versions using the other functions.
+
+@smallexample
+int
+file_mbsrtowcs (int input, int output)
+@{
+  /* @r{Note the use of @code{MB_LEN_MAX}.}
+     @r{@code{MB_CUR_MAX} cannot portably be used here.}  */
+  char buffer[BUFSIZ + MB_LEN_MAX];
+  mbstate_t state;
+  int filled = 0;
+  int eof = 0;
+
+  /* @r{Initialize the state.}  */
+  memset (&state, '\0', sizeof (state));
+
+  while (!eof)
+    @{
+      ssize_t nread;
+      ssize_t nwrite;
+      char *inp = buffer;
+      wchar_t outbuf[BUFSIZ];
+      wchar_t *outp = outbuf;
+
+      /* @r{Fill up the buffer from the input file.}  */
+      nread = read (input, buffer + filled, BUFSIZ);
+      if (nread < 0)
+        @{
+          perror ("read");
+          return 0;
+        @}
+      /* @r{If we reach end of file, make a note to read no more.} */
+      if (nread == 0)
+        eof = 1;
+
+      /* @r{@code{filled} is now the number of bytes in @code{buffer}.} */
+      filled += nread;
+
+      /* @r{Convert those bytes to wide characters--as many as we can.} */
+      while (1)
+        @{
+          size_t thislen = mbrtowc (outp, inp, filled, &state);
+          /* @r{Stop converting at invalid character;}
+             @r{this can mean we have read just the first part}
+             @r{of a valid character.}  */
+          if (thislen == (size_t) -1)
+            break;
+          /* @r{We want to handle embedded NUL bytes}
+             @r{but the return value is 0.  Correct this.}  */
+          if (thislen == 0)
+            thislen = 1;
+          /* @r{Advance past this character.} */
+          inp += thislen;
+          filled -= thislen;
+          ++outp;
+        @}
+
+      /* @r{Write the wide characters we just made.}  */
+      nwrite = write (output, outbuf,
+                      (outp - outbuf) * sizeof (wchar_t));
+      if (nwrite < 0)
+        @{
+          perror ("write");
+          return 0;
+        @}
+
+      /* @r{See if we have a @emph{real} invalid character.} */
+      if ((eof && filled > 0) || filled >= MB_CUR_MAX)
+        @{
+          error (0, 0, "invalid multibyte character");
+          return 0;
+        @}
+
+      /* @r{If any characters must be carried forward,}
+         @r{put them at the beginning of @code{buffer}.} */
+      if (filled > 0)
+        memmove (buffer, inp, filled);
+    @}
+
+  return 1;
+@}
+@end smallexample
+
+
+@node Non-reentrant Conversion
+@section Non-reentrant Conversion Function
+
+The functions described in the previous chapter are defined in
+@w{Amendment 1} to @w{ISO C90}, but the original @w{ISO C90} standard
+also contained functions for character set conversion.  The reason that
+these original functions are not described first is that they are almost
+entirely useless.
+
+The problem is that all the conversion functions described in the
+original @w{ISO C90} use a local state.  Using a local state implies that
+multiple conversions at the same time (not only when using threads)
+cannot be done, and that you cannot first convert single characters and
+then strings since you cannot tell the conversion functions which state
+to use.
+
+These original functions are therefore usable only in a very limited set
+of situations.  One must complete converting the entire string before
+starting a new one, and each string/text must be converted with the same
+function (there is no problem with the library itself; it is guaranteed
+that no library function changes the state of any of these functions).
+@strong{For the above reasons it is highly requested that the functions
+described in the previous section be used in place of non-reentrant
+conversion functions.}
+
+@menu
+* Non-reentrant Character Conversion::  Non-reentrant Conversion of Single
+                                         Characters.
+* Non-reentrant String Conversion::     Non-reentrant Conversion of Strings.
+* Shift State::                         States in Non-reentrant Functions.
+@end menu
+
+@node Non-reentrant Character Conversion
+@subsection Non-reentrant Conversion of Single Characters
+
+@comment stdlib.h
+@comment ISO
+@deftypefun int mbtowc (wchar_t *restrict @var{result}, const char *restrict @var{string}, size_t @var{size})
+@safety{@prelim{}@mtunsafe{@mtasurace{}}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}}
+The @code{mbtowc} (``multibyte to wide character'') function when called
+with non-null @var{string} converts the first multibyte character
+beginning at @var{string} to its corresponding wide character code.  It
+stores the result in @code{*@var{result}}.
+
+@code{mbtowc} never examines more than @var{size} bytes.  (The idea is
+to supply for @var{size} the number of bytes of data you have in hand.)
+
+@code{mbtowc} with non-null @var{string} distinguishes three
+possibilities: the first @var{size} bytes at @var{string} start with
+valid multibyte characters, they start with an invalid byte sequence or
+just part of a character, or @var{string} points to an empty string (a
+null character).
+
+For a valid multibyte character, @code{mbtowc} converts it to a wide
+character and stores that in @code{*@var{result}}, and returns the
+number of bytes in that character (always at least @math{1} and never
+more than @var{size}).
+
+For an invalid byte sequence, @code{mbtowc} returns @math{-1}.  For an
+empty string, it returns @math{0}, also storing @code{'\0'} in
+@code{*@var{result}}.
+
+If the multibyte character code uses shift characters, then
+@code{mbtowc} maintains and updates a shift state as it scans.  If you
+call @code{mbtowc} with a null pointer for @var{string}, that
+initializes the shift state to its standard initial value.  It also
+returns nonzero if the multibyte character code in use actually has a
+shift state.  @xref{Shift State}.
+@end deftypefun
+
+@comment stdlib.h
+@comment ISO
+@deftypefun int wctomb (char *@var{string}, wchar_t @var{wchar})
+@safety{@prelim{}@mtunsafe{@mtasurace{}}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}}
+The @code{wctomb} (``wide character to multibyte'') function converts
+the wide character code @var{wchar} to its corresponding multibyte
+character sequence, and stores the result in bytes starting at
+@var{string}.  At most @code{MB_CUR_MAX} characters are stored.
+
+@code{wctomb} with non-null @var{string} distinguishes three
+possibilities for @var{wchar}: a valid wide character code (one that can
+be translated to a multibyte character), an invalid code, and
+@code{L'\0'}.
+
+Given a valid code, @code{wctomb} converts it to a multibyte character,
+storing the bytes starting at @var{string}.  Then it returns the number
+of bytes in that character (always at least @math{1} and never more
+than @code{MB_CUR_MAX}).
+
+If @var{wchar} is an invalid wide character code, @code{wctomb} returns
+@math{-1}.  If @var{wchar} is @code{L'\0'}, it returns @code{0}, also
+storing @code{'\0'} in @code{*@var{string}}.
+
+If the multibyte character code uses shift characters, then
+@code{wctomb} maintains and updates a shift state as it scans.  If you
+call @code{wctomb} with a null pointer for @var{string}, that
+initializes the shift state to its standard initial value.  It also
+returns nonzero if the multibyte character code in use actually has a
+shift state.  @xref{Shift State}.
+
+Calling this function with a @var{wchar} argument of zero when
+@var{string} is not null has the side-effect of reinitializing the
+stored shift state @emph{as well as} storing the multibyte character
+@code{'\0'} and returning @math{0}.
+@end deftypefun
+
+Similar to @code{mbrlen} there is also a non-reentrant function that
+computes the length of a multibyte character.  It can be defined in
+terms of @code{mbtowc}.
+
+@comment stdlib.h
+@comment ISO
+@deftypefun int mblen (const char *@var{string}, size_t @var{size})
+@safety{@prelim{}@mtunsafe{@mtasurace{}}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}}
+The @code{mblen} function with a non-null @var{string} argument returns
+the number of bytes that make up the multibyte character beginning at
+@var{string}, never examining more than @var{size} bytes.  (The idea is
+to supply for @var{size} the number of bytes of data you have in hand.)
+
+The return value of @code{mblen} distinguishes three possibilities: the
+first @var{size} bytes at @var{string} start with valid multibyte
+characters, they start with an invalid byte sequence or just part of a
+character, or @var{string} points to an empty string (a null character).
+
+For a valid multibyte character, @code{mblen} returns the number of
+bytes in that character (always at least @code{1} and never more than
+@var{size}).  For an invalid byte sequence, @code{mblen} returns
+@math{-1}.  For an empty string, it returns @math{0}.
+
+If the multibyte character code uses shift characters, then @code{mblen}
+maintains and updates a shift state as it scans.  If you call
+@code{mblen} with a null pointer for @var{string}, that initializes the
+shift state to its standard initial value.  It also returns a nonzero
+value if the multibyte character code in use actually has a shift state.
+@xref{Shift State}.
+
+@pindex stdlib.h
+The function @code{mblen} is declared in @file{stdlib.h}.
+@end deftypefun
+
+
+@node Non-reentrant String Conversion
+@subsection Non-reentrant Conversion of Strings
+
+For convenience the @w{ISO C90} standard also defines functions to
+convert entire strings instead of single characters.  These functions
+suffer from the same problems as their reentrant counterparts from
+@w{Amendment 1} to @w{ISO C90}; see @ref{Converting Strings}.
+
+@comment stdlib.h
+@comment ISO
+@deftypefun size_t mbstowcs (wchar_t *@var{wstring}, const char *@var{string}, size_t @var{size})
+@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}}
+@c Odd...  Although this was supposed to be non-reentrant, the internal
+@c state is not a static buffer, but an automatic variable.
+The @code{mbstowcs} (``multibyte string to wide character string'')
+function converts the null-terminated string of multibyte characters
+@var{string} to an array of wide character codes, storing not more than
+@var{size} wide characters into the array beginning at @var{wstring}.
+The terminating null character counts towards the size, so if @var{size}
+is less than the actual number of wide characters resulting from
+@var{string}, no terminating null character is stored.
+
+The conversion of characters from @var{string} begins in the initial
+shift state.
+
+If an invalid multibyte character sequence is found, the @code{mbstowcs}
+function returns a value of @math{-1}.  Otherwise, it returns the number
+of wide characters stored in the array @var{wstring}.  This number does
+not include the terminating null character, which is present if the
+number is less than @var{size}.
+
+Here is an example showing how to convert a string of multibyte
+characters, allocating enough space for the result.
+
+@smallexample
+wchar_t *
+mbstowcs_alloc (const char *string)
+@{
+  size_t size = strlen (string) + 1;
+  wchar_t *buf = xmalloc (size * sizeof (wchar_t));
+
+  size = mbstowcs (buf, string, size);
+  if (size == (size_t) -1)
+    return NULL;
+  buf = xrealloc (buf, (size + 1) * sizeof (wchar_t));
+  return buf;
+@}
+@end smallexample
+
+@end deftypefun
+
+@comment stdlib.h
+@comment ISO
+@deftypefun size_t wcstombs (char *@var{string}, const wchar_t *@var{wstring}, size_t @var{size})
+@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}}
+The @code{wcstombs} (``wide character string to multibyte string'')
+function converts the null-terminated wide character array @var{wstring}
+into a string containing multibyte characters, storing not more than
+@var{size} bytes starting at @var{string}, followed by a terminating
+null character if there is room.  The conversion of characters begins in
+the initial shift state.
+
+The terminating null character counts towards the size, so if @var{size}
+is less than or equal to the number of bytes needed in @var{wstring}, no
+terminating null character is stored.
+
+If a code that does not correspond to a valid multibyte character is
+found, the @code{wcstombs} function returns a value of @math{-1}.
+Otherwise, the return value is the number of bytes stored in the array
+@var{string}.  This number does not include the terminating null character,
+which is present if the number is less than @var{size}.
+@end deftypefun
+
+@node Shift State
+@subsection States in Non-reentrant Functions
+
+In some multibyte character codes, the @emph{meaning} of any particular
+byte sequence is not fixed; it depends on what other sequences have come
+earlier in the same string.  Typically there are just a few sequences that
+can change the meaning of other sequences; these few are called
+@dfn{shift sequences} and we say that they set the @dfn{shift state} for
+other sequences that follow.
+
+To illustrate shift state and shift sequences, suppose we decide that
+the sequence @code{0200} (just one byte) enters Japanese mode, in which
+pairs of bytes in the range from @code{0240} to @code{0377} are single
+characters, while @code{0201} enters Latin-1 mode, in which single bytes
+in the range from @code{0240} to @code{0377} are characters, and
+interpreted according to the ISO Latin-1 character set.  This is a
+multibyte code that has two alternative shift states (``Japanese mode''
+and ``Latin-1 mode''), and two shift sequences that specify particular
+shift states.
+
+When the multibyte character code in use has shift states, then
+@code{mblen}, @code{mbtowc}, and @code{wctomb} must maintain and update
+the current shift state as they scan the string.  To make this work
+properly, you must follow these rules:
+
+@itemize @bullet
+@item
+Before starting to scan a string, call the function with a null pointer
+for the multibyte character address---for example, @code{mblen (NULL,
+0)}.  This initializes the shift state to its standard initial value.
+
+@item
+Scan the string one character at a time, in order.  Do not ``back up''
+and rescan characters already scanned, and do not intersperse the
+processing of different strings.
+@end itemize
+
+Here is an example of using @code{mblen} following these rules:
+
+@smallexample
+void
+scan_string (char *s)
+@{
+  int length = strlen (s);
+
+  /* @r{Initialize shift state.}  */
+  mblen (NULL, 0);
+
+  while (1)
+    @{
+      int thischar = mblen (s, length);
+      /* @r{Deal with end of string and invalid characters.}  */
+      if (thischar == 0)
+        break;
+      if (thischar == -1)
+        @{
+          error ("invalid multibyte character");
+          break;
+        @}
+      /* @r{Advance past this character.}  */
+      s += thischar;
+      length -= thischar;
+    @}
+@}
+@end smallexample
+
+The functions @code{mblen}, @code{mbtowc} and @code{wctomb} are not
+reentrant when using a multibyte code that uses a shift state.  However,
+no other library functions call these functions, so you don't have to
+worry that the shift state will be changed mysteriously.
+
+
+@node Generic Charset Conversion
+@section Generic Charset Conversion
+
+The conversion functions mentioned so far in this chapter all had in
+common that they operate on character sets that are not directly
+specified by the functions.  The multibyte encoding used is specified by
+the currently selected locale for the @code{LC_CTYPE} category.  The
+wide character set is fixed by the implementation (in the case of @theglibc{}
+it is always UCS-4 encoded @w{ISO 10646}).
+
+This has of course several problems when it comes to general character
+conversion:
+
+@itemize @bullet
+@item
+For every conversion where neither the source nor the destination
+character set is the character set of the locale for the @code{LC_CTYPE}
+category, one has to change the @code{LC_CTYPE} locale using
+@code{setlocale}.
+
+Changing the @code{LC_CTYPE} locale introduces major problems for the rest
+of the programs since several more functions (e.g., the character
+classification functions, @pxref{Classification of Characters}) use the
+@code{LC_CTYPE} category.
+
+@item
+Parallel conversions to and from different character sets are not
+possible since the @code{LC_CTYPE} selection is global and shared by all
+threads.
+
+@item
+If neither the source nor the destination character set is the character
+set used for @code{wchar_t} representation, there is at least a two-step
+process necessary to convert a text using the functions above.  One would
+have to select the source character set as the multibyte encoding,
+convert the text into a @code{wchar_t} text, select the destination
+character set as the multibyte encoding, and convert the wide character
+text to the multibyte (@math{=} destination) character set.
+
+Even if this is possible (which is not guaranteed) it is a very tiring
+work.  Plus it suffers from the other two raised points even more due to
+the steady changing of the locale.
+@end itemize
+
+The XPG2 standard defines a completely new set of functions, which has
+none of these limitations.  They are not at all coupled to the selected
+locales, and they have no constraints on the character sets selected for
+source and destination.  Only the set of available conversions limits
+them.  The standard does not specify that any conversion at all must be
+available.  Such availability is a measure of the quality of the
+implementation.
+
+In the following text first the interface to @code{iconv} and then the
+conversion function, will be described.  Comparisons with other
+implementations will show what obstacles stand in the way of portable
+applications.  Finally, the implementation is described in so far as might
+interest the advanced user who wants to extend conversion capabilities.
+
+@menu
+* Generic Conversion Interface::    Generic Character Set Conversion Interface.
+* iconv Examples::                  A complete @code{iconv} example.
+* Other iconv Implementations::     Some Details about other @code{iconv}
+                                     Implementations.
+* glibc iconv Implementation::      The @code{iconv} Implementation in the GNU C
+                                     library.
+@end menu
+
+@node Generic Conversion Interface
+@subsection Generic Character Set Conversion Interface
+
+This set of functions follows the traditional cycle of using a resource:
+open--use--close.  The interface consists of three functions, each of
+which implements one step.
+
+Before the interfaces are described it is necessary to introduce a
+data type.  Just like other open--use--close interfaces the functions
+introduced here work using handles and the @file{iconv.h} header
+defines a special type for the handles used.
+
+@comment iconv.h
+@comment XPG2
+@deftp {Data Type} iconv_t
+This data type is an abstract type defined in @file{iconv.h}.  The user
+must not assume anything about the definition of this type; it must be
+completely opaque.
+
+Objects of this type can be assigned handles for the conversions using
+the @code{iconv} functions.  The objects themselves need not be freed, but
+the conversions for which the handles stand for have to.
+@end deftp
+
+@noindent
+The first step is the function to create a handle.
+
+@comment iconv.h
+@comment XPG2
+@deftypefun iconv_t iconv_open (const char *@var{tocode}, const char *@var{fromcode})
+@safety{@prelim{}@mtsafe{@mtslocale{}}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{} @acsfd{}}}
+@c Calls malloc if tocode and/or fromcode are too big for alloca.  Calls
+@c strip and upstr on both, then gconv_open.  strip and upstr call
+@c isalnum_l and toupper_l with the C locale.  gconv_open may MT-safely
+@c tokenize toset, replace unspecified codesets with the current locale
+@c (possibly two different accesses), and finally it calls
+@c gconv_find_transform and initializes the gconv_t result with all the
+@c steps in the conversion sequence, running each one's initializer,
+@c destructing and releasing them all if anything fails.
+
+The @code{iconv_open} function has to be used before starting a
+conversion.  The two parameters this function takes determine the
+source and destination character set for the conversion, and if the
+implementation has the possibility to perform such a conversion, the
+function returns a handle.
+
+If the wanted conversion is not available, the @code{iconv_open} function
+returns @code{(iconv_t) -1}.  In this case the global variable
+@code{errno} can have the following values:
+
+@table @code
+@item EMFILE
+The process already has @code{OPEN_MAX} file descriptors open.
+@item ENFILE
+The system limit of open files is reached.
+@item ENOMEM
+Not enough memory to carry out the operation.
+@item EINVAL
+The conversion from @var{fromcode} to @var{tocode} is not supported.
+@end table
+
+It is not possible to use the same descriptor in different threads to
+perform independent conversions.  The data structures associated
+with the descriptor include information about the conversion state.
+This must not be messed up by using it in different conversions.
+
+An @code{iconv} descriptor is like a file descriptor as for every use a
+new descriptor must be created.  The descriptor does not stand for all
+of the conversions from @var{fromset} to @var{toset}.
+
+The @glibcadj{} implementation of @code{iconv_open} has one
+significant extension to other implementations.  To ease the extension
+of the set of available conversions, the implementation allows storing
+the necessary files with data and code in an arbitrary number of
+directories.  How this extension must be written will be explained below
+(@pxref{glibc iconv Implementation}).  Here it is only important to say
+that all directories mentioned in the @code{GCONV_PATH} environment
+variable are considered only if they contain a file @file{gconv-modules}.
+These directories need not necessarily be created by the system
+administrator.  In fact, this extension is introduced to help users
+writing and using their own, new conversions.  Of course, this does not
+work for security reasons in SUID binaries; in this case only the system
+directory is considered and this normally is
+@file{@var{prefix}/lib/gconv}.  The @code{GCONV_PATH} environment
+variable is examined exactly once at the first call of the
+@code{iconv_open} function.  Later modifications of the variable have no
+effect.
+
+@pindex iconv.h
+The @code{iconv_open} function was introduced early in the X/Open
+Portability Guide, @w{version 2}.  It is supported by all commercial
+Unices as it is required for the Unix branding.  However, the quality and
+completeness of the implementation varies widely.  The @code{iconv_open}
+function is declared in @file{iconv.h}.
+@end deftypefun
+
+The @code{iconv} implementation can associate large data structure with
+the handle returned by @code{iconv_open}.  Therefore, it is crucial to
+free all the resources once all conversions are carried out and the
+conversion is not needed anymore.
+
+@comment iconv.h
+@comment XPG2
+@deftypefun int iconv_close (iconv_t @var{cd})
+@safety{@prelim{}@mtsafe{}@asunsafe{@asucorrupt{} @ascuheap{} @asulock{} @ascudlopen{}}@acunsafe{@acucorrupt{} @aculock{} @acsmem{}}}
+@c Calls gconv_close to destruct and release each of the conversion
+@c steps, release the gconv_t object, then call gconv_close_transform.
+@c Access to the gconv_t object is not guarded, but calling iconv_close
+@c concurrently with any other use is undefined.
+
+The @code{iconv_close} function frees all resources associated with the
+handle @var{cd}, which must have been returned by a successful call to
+the @code{iconv_open} function.
+
+If the function call was successful the return value is @math{0}.
+Otherwise it is @math{-1} and @code{errno} is set appropriately.
+Defined errors are:
+
+@table @code
+@item EBADF
+The conversion descriptor is invalid.
+@end table
+
+@pindex iconv.h
+The @code{iconv_close} function was introduced together with the rest
+of the @code{iconv} functions in XPG2 and is declared in @file{iconv.h}.
+@end deftypefun
+
+The standard defines only one actual conversion function.  This has,
+therefore, the most general interface: it allows conversion from one
+buffer to another.  Conversion from a file to a buffer, vice versa, or
+even file to file can be implemented on top of it.
+
+@comment iconv.h
+@comment XPG2
+@deftypefun size_t iconv (iconv_t @var{cd}, char **@var{inbuf}, size_t *@var{inbytesleft}, char **@var{outbuf}, size_t *@var{outbytesleft})
+@safety{@prelim{}@mtsafe{@mtsrace{:cd}}@assafe{}@acunsafe{@acucorrupt{}}}
+@c Without guarding access to the iconv_t object pointed to by cd, call
+@c the conversion function to convert inbuf or flush the internal
+@c conversion state.
+@cindex stateful
+The @code{iconv} function converts the text in the input buffer
+according to the rules associated with the descriptor @var{cd} and
+stores the result in the output buffer.  It is possible to call the
+function for the same text several times in a row since for stateful
+character sets the necessary state information is kept in the data
+structures associated with the descriptor.
+
+The input buffer is specified by @code{*@var{inbuf}} and it contains
+@code{*@var{inbytesleft}} bytes.  The extra indirection is necessary for
+communicating the used input back to the caller (see below).  It is
+important to note that the buffer pointer is of type @code{char} and the
+length is measured in bytes even if the input text is encoded in wide
+characters.
+
+The output buffer is specified in a similar way.  @code{*@var{outbuf}}
+points to the beginning of the buffer with at least
+@code{*@var{outbytesleft}} bytes room for the result.  The buffer
+pointer again is of type @code{char} and the length is measured in
+bytes.  If @var{outbuf} or @code{*@var{outbuf}} is a null pointer, the
+conversion is performed but no output is available.
+
+If @var{inbuf} is a null pointer, the @code{iconv} function performs the
+necessary action to put the state of the conversion into the initial
+state.  This is obviously a no-op for non-stateful encodings, but if the
+encoding has a state, such a function call might put some byte sequences
+in the output buffer, which perform the necessary state changes.  The
+next call with @var{inbuf} not being a null pointer then simply goes on
+from the initial state.  It is important that the programmer never makes
+any assumption as to whether the conversion has to deal with states.
+Even if the input and output character sets are not stateful, the
+implementation might still have to keep states.  This is due to the
+implementation chosen for @theglibc{} as it is described below.
+Therefore an @code{iconv} call to reset the state should always be
+performed if some protocol requires this for the output text.
+
+The conversion stops for one of three reasons.  The first is that all
+characters from the input buffer are converted.  This actually can mean
+two things: either all bytes from the input buffer are consumed or
+there are some bytes at the end of the buffer that possibly can form a
+complete character but the input is incomplete.  The second reason for a
+stop is that the output buffer is full.  And the third reason is that
+the input contains invalid characters.
+
+In all of these cases the buffer pointers after the last successful
+conversion, for the input and output buffers, are stored in @var{inbuf} and
+@var{outbuf}, and the available room in each buffer is stored in
+@var{inbytesleft} and @var{outbytesleft}.
+
+Since the character sets selected in the @code{iconv_open} call can be
+almost arbitrary, there can be situations where the input buffer contains
+valid characters, which have no identical representation in the output
+character set.  The behavior in this situation is undefined.  The
+@emph{current} behavior of @theglibc{} in this situation is to
+return with an error immediately.  This certainly is not the most
+desirable solution; therefore, future versions will provide better ones,
+but they are not yet finished.
+
+If all input from the input buffer is successfully converted and stored
+in the output buffer, the function returns the number of non-reversible
+conversions performed.  In all other cases the return value is
+@code{(size_t) -1} and @code{errno} is set appropriately.  In such cases
+the value pointed to by @var{inbytesleft} is nonzero.
+
+@table @code
+@item EILSEQ
+The conversion stopped because of an invalid byte sequence in the input.
+After the call, @code{*@var{inbuf}} points at the first byte of the
+invalid byte sequence.
+
+@item E2BIG
+The conversion stopped because it ran out of space in the output buffer.
+
+@item EINVAL
+The conversion stopped because of an incomplete byte sequence at the end
+of the input buffer.
+
+@item EBADF
+The @var{cd} argument is invalid.
+@end table
+
+@pindex iconv.h
+The @code{iconv} function was introduced in the XPG2 standard and is
+declared in the @file{iconv.h} header.
+@end deftypefun
+
+The definition of the @code{iconv} function is quite good overall.  It
+provides quite flexible functionality.  The only problems lie in the
+boundary cases, which are incomplete byte sequences at the end of the
+input buffer and invalid input.  A third problem, which is not really
+a design problem, is the way conversions are selected.  The standard
+does not say anything about the legitimate names, a minimal set of
+available conversions.  We will see how this negatively impacts other
+implementations, as demonstrated below.
+
+@node iconv Examples
+@subsection A complete @code{iconv} example
+
+The example below features a solution for a common problem.  Given that
+one knows the internal encoding used by the system for @code{wchar_t}
+strings, one often is in the position to read text from a file and store
+it in wide character buffers.  One can do this using @code{mbsrtowcs},
+but then we run into the problems discussed above.
+
+@smallexample
+int
+file2wcs (int fd, const char *charset, wchar_t *outbuf, size_t avail)
+@{
+  char inbuf[BUFSIZ];
+  size_t insize = 0;
+  char *wrptr = (char *) outbuf;
+  int result = 0;
+  iconv_t cd;
+
+  cd = iconv_open ("WCHAR_T", charset);
+  if (cd == (iconv_t) -1)
+    @{
+      /* @r{Something went wrong.}  */
+      if (errno == EINVAL)
+        error (0, 0, "conversion from '%s' to wchar_t not available",
+               charset);
+      else
+        perror ("iconv_open");
+
+      /* @r{Terminate the output string.}  */
+      *outbuf = L'\0';
+
+      return -1;
+    @}
+
+  while (avail > 0)
+    @{
+      size_t nread;
+      size_t nconv;
+      char *inptr = inbuf;
+
+      /* @r{Read more input.}  */
+      nread = read (fd, inbuf + insize, sizeof (inbuf) - insize);
+      if (nread == 0)
+        @{
+          /* @r{When we come here the file is completely read.}
+             @r{This still could mean there are some unused}
+             @r{characters in the @code{inbuf}.  Put them back.}  */
+          if (lseek (fd, -insize, SEEK_CUR) == -1)
+            result = -1;
+
+          /* @r{Now write out the byte sequence to get into the}
+             @r{initial state if this is necessary.}  */
+          iconv (cd, NULL, NULL, &wrptr, &avail);
+
+          break;
+        @}
+      insize += nread;
+
+      /* @r{Do the conversion.}  */
+      nconv = iconv (cd, &inptr, &insize, &wrptr, &avail);
+      if (nconv == (size_t) -1)
+        @{
+          /* @r{Not everything went right.  It might only be}
+             @r{an unfinished byte sequence at the end of the}
+             @r{buffer.  Or it is a real problem.}  */
+          if (errno == EINVAL)
+            /* @r{This is harmless.  Simply move the unused}
+               @r{bytes to the beginning of the buffer so that}
+               @r{they can be used in the next round.}  */
+            memmove (inbuf, inptr, insize);
+          else
+            @{
+              /* @r{It is a real problem.  Maybe we ran out of}
+                 @r{space in the output buffer or we have invalid}
+                 @r{input.  In any case back the file pointer to}
+                 @r{the position of the last processed byte.}  */
+              lseek (fd, -insize, SEEK_CUR);
+              result = -1;
+              break;
+            @}
+        @}
+    @}
+
+  /* @r{Terminate the output string.}  */
+  if (avail >= sizeof (wchar_t))
+    *((wchar_t *) wrptr) = L'\0';
+
+  if (iconv_close (cd) != 0)
+    perror ("iconv_close");
+
+  return (wchar_t *) wrptr - outbuf;
+@}
+@end smallexample
+
+@cindex stateful
+This example shows the most important aspects of using the @code{iconv}
+functions.  It shows how successive calls to @code{iconv} can be used to
+convert large amounts of text.  The user does not have to care about
+stateful encodings as the functions take care of everything.
+
+An interesting point is the case where @code{iconv} returns an error and
+@code{errno} is set to @code{EINVAL}.  This is not really an error in the
+transformation.  It can happen whenever the input character set contains
+byte sequences of more than one byte for some character and texts are not
+processed in one piece.  In this case there is a chance that a multibyte
+sequence is cut.  The caller can then simply read the remainder of the
+takes and feed the offending bytes together with new character from the
+input to @code{iconv} and continue the work.  The internal state kept in
+the descriptor is @emph{not} unspecified after such an event as is the
+case with the conversion functions from the @w{ISO C} standard.
+
+The example also shows the problem of using wide character strings with
+@code{iconv}.  As explained in the description of the @code{iconv}
+function above, the function always takes a pointer to a @code{char}
+array and the available space is measured in bytes.  In the example, the
+output buffer is a wide character buffer; therefore, we use a local
+variable @var{wrptr} of type @code{char *}, which is used in the
+@code{iconv} calls.
+
+This looks rather innocent but can lead to problems on platforms that
+have tight restriction on alignment.  Therefore the caller of @code{iconv}
+has to make sure that the pointers passed are suitable for access of
+characters from the appropriate character set.  Since, in the
+above case, the input parameter to the function is a @code{wchar_t}
+pointer, this is the case (unless the user violates alignment when
+computing the parameter).  But in other situations, especially when
+writing generic functions where one does not know what type of character
+set one uses and, therefore, treats text as a sequence of bytes, it might
+become tricky.
+
+@node Other iconv Implementations
+@subsection Some Details about other @code{iconv} Implementations
+
+This is not really the place to discuss the @code{iconv} implementation
+of other systems but it is necessary to know a bit about them to write
+portable programs.  The above mentioned problems with the specification
+of the @code{iconv} functions can lead to portability issues.
+
+The first thing to notice is that, due to the large number of character
+sets in use, it is certainly not practical to encode the conversions
+directly in the C library.  Therefore, the conversion information must
+come from files outside the C library.  This is usually done in one or
+both of the following ways:
+
+@itemize @bullet
+@item
+The C library contains a set of generic conversion functions that can
+read the needed conversion tables and other information from data files.
+These files get loaded when necessary.
+
+This solution is problematic as it requires a great deal of effort to
+apply to all character sets (potentially an infinite set).  The
+differences in the structure of the different character sets is so large
+that many different variants of the table-processing functions must be
+developed.  In addition, the generic nature of these functions make them
+slower than specifically implemented functions.
+
+@item
+The C library only contains a framework that can dynamically load
+object files and execute the conversion functions contained therein.
+
+This solution provides much more flexibility.  The C library itself
+contains only very little code and therefore reduces the general memory
+footprint.  Also, with a documented interface between the C library and
+the loadable modules it is possible for third parties to extend the set
+of available conversion modules.  A drawback of this solution is that
+dynamic loading must be available.
+@end itemize
+
+Some implementations in commercial Unices implement a mixture of these
+possibilities; the majority implement only the second solution.  Using
+loadable modules moves the code out of the library itself and keeps
+the door open for extensions and improvements, but this design is also
+limiting on some platforms since not many platforms support dynamic
+loading in statically linked programs.  On platforms without this
+capability it is therefore not possible to use this interface in
+statically linked programs.  @Theglibc{} has, on ELF platforms, no
+problems with dynamic loading in these situations; therefore, this
+point is moot.  The danger is that one gets acquainted with this
+situation and forgets about the restrictions on other systems.
+
+A second thing to know about other @code{iconv} implementations is that
+the number of available conversions is often very limited.  Some
+implementations provide, in the standard release (not special
+international or developer releases), at most 100 to 200 conversion
+possibilities.  This does not mean 200 different character sets are
+supported; for example, conversions from one character set to a set of 10
+others might count as 10 conversions.  Together with the other direction
+this makes 20 conversion possibilities used up by one character set.  One
+can imagine the thin coverage these platforms provide.  Some Unix vendors
+even provide only a handful of conversions, which renders them useless for
+almost all uses.
+
+This directly leads to a third and probably the most problematic point.
+The way the @code{iconv} conversion functions are implemented on all
+known Unix systems and the availability of the conversion functions from
+character set @math{@cal{A}} to @math{@cal{B}} and the conversion from
+@math{@cal{B}} to @math{@cal{C}} does @emph{not} imply that the
+conversion from @math{@cal{A}} to @math{@cal{C}} is available.
+
+This might not seem unreasonable and problematic at first, but it is a
+quite big problem as one will notice shortly after hitting it.  To show
+the problem we assume to write a program that has to convert from
+@math{@cal{A}} to @math{@cal{C}}.  A call like
+
+@smallexample
+cd = iconv_open ("@math{@cal{C}}", "@math{@cal{A}}");
+@end smallexample
+
+@noindent
+fails according to the assumption above.  But what does the program
+do now?  The conversion is necessary; therefore, simply giving up is not
+an option.
+
+This is a nuisance.  The @code{iconv} function should take care of this.
+But how should the program proceed from here on?  If it tries to convert
+to character set @math{@cal{B}}, first the two @code{iconv_open}
+calls
+
+@smallexample
+cd1 = iconv_open ("@math{@cal{B}}", "@math{@cal{A}}");
+@end smallexample
+
+@noindent
+and
+
+@smallexample
+cd2 = iconv_open ("@math{@cal{C}}", "@math{@cal{B}}");
+@end smallexample
+
+@noindent
+will succeed, but how to find @math{@cal{B}}?
+
+Unfortunately, the answer is: there is no general solution.  On some
+systems guessing might help.  On those systems most character sets can
+convert to and from UTF-8 encoded @w{ISO 10646} or Unicode text.  Besides
+this only some very system-specific methods can help.  Since the
+conversion functions come from loadable modules and these modules must
+be stored somewhere in the filesystem, one @emph{could} try to find them
+and determine from the available file which conversions are available
+and whether there is an indirect route from @math{@cal{A}} to
+@math{@cal{C}}.
+
+This example shows one of the design errors of @code{iconv} mentioned
+above.  It should at least be possible to determine the list of available
+conversions programmatically so that if @code{iconv_open} says there is no
+such conversion, one could make sure this also is true for indirect
+routes.
+
+@node glibc iconv Implementation
+@subsection The @code{iconv} Implementation in @theglibc{}
+
+After reading about the problems of @code{iconv} implementations in the
+last section it is certainly good to note that the implementation in
+@theglibc{} has none of the problems mentioned above.  What
+follows is a step-by-step analysis of the points raised above.  The
+evaluation is based on the current state of the development (as of
+January 1999).  The development of the @code{iconv} functions is not
+complete, but basic functionality has solidified.
+
+@Theglibc{}'s @code{iconv} implementation uses shared loadable
+modules to implement the conversions.  A very small number of
+conversions are built into the library itself but these are only rather
+trivial conversions.
+
+All the benefits of loadable modules are available in the @glibcadj{}
+implementation.  This is especially appealing since the interface is
+well documented (see below), and it, therefore, is easy to write new
+conversion modules.  The drawback of using loadable objects is not a
+problem in @theglibc{}, at least on ELF systems.  Since the
+library is able to load shared objects even in statically linked
+binaries, static linking need not be forbidden in case one wants to use
+@code{iconv}.
+
+The second mentioned problem is the number of supported conversions.
+Currently, @theglibc{} supports more than 150 character sets.  The
+way the implementation is designed the number of supported conversions
+is greater than 22350 (@math{150} times @math{149}).  If any conversion
+from or to a character set is missing, it can be added easily.
+
+Particularly impressive as it may be, this high number is due to the
+fact that the @glibcadj{} implementation of @code{iconv} does not have
+the third problem mentioned above (i.e., whenever there is a conversion
+from a character set @math{@cal{A}} to @math{@cal{B}} and from
+@math{@cal{B}} to @math{@cal{C}} it is always possible to convert from
+@math{@cal{A}} to @math{@cal{C}} directly).  If the @code{iconv_open}
+returns an error and sets @code{errno} to @code{EINVAL}, there is no
+known way, directly or indirectly, to perform the wanted conversion.
+
+@cindex triangulation
+Triangulation is achieved by providing for each character set a
+conversion from and to UCS-4 encoded @w{ISO 10646}.  Using @w{ISO 10646}
+as an intermediate representation it is possible to @dfn{triangulate}
+(i.e., convert with an intermediate representation).
+
+There is no inherent requirement to provide a conversion to @w{ISO
+10646} for a new character set, and it is also possible to provide other
+conversions where neither source nor destination character set is @w{ISO
+10646}.  The existing set of conversions is simply meant to cover all
+conversions that might be of interest.
+
+@cindex ISO-2022-JP
+@cindex EUC-JP
+All currently available conversions use the triangulation method above,
+making conversion run unnecessarily slow.  If, for example, somebody
+often needs the conversion from ISO-2022-JP to EUC-JP, a quicker solution
+would involve direct conversion between the two character sets, skipping
+the input to @w{ISO 10646} first.  The two character sets of interest
+are much more similar to each other than to @w{ISO 10646}.
+
+In such a situation one easily can write a new conversion and provide it
+as a better alternative.  The @glibcadj{} @code{iconv} implementation
+would automatically use the module implementing the conversion if it is
+specified to be more efficient.
+
+@subsubsection Format of @file{gconv-modules} files
+
+All information about the available conversions comes from a file named
+@file{gconv-modules}, which can be found in any of the directories along
+the @code{GCONV_PATH}.  The @file{gconv-modules} files are line-oriented
+text files, where each of the lines has one of the following formats:
+
+@itemize @bullet
+@item
+If the first non-whitespace character is a @kbd{#} the line contains only
+comments and is ignored.
+
+@item
+Lines starting with @code{alias} define an alias name for a character
+set.  Two more words are expected on the line.  The first word
+defines the alias name, and the second defines the original name of the
+character set.  The effect is that it is possible to use the alias name
+in the @var{fromset} or @var{toset} parameters of @code{iconv_open} and
+achieve the same result as when using the real character set name.
+
+This is quite important as a character set has often many different
+names.  There is normally an official name but this need not correspond to
+the most popular name.  Besides this many character sets have special
+names that are somehow constructed.  For example, all character sets
+specified by the ISO have an alias of the form @code{ISO-IR-@var{nnn}}
+where @var{nnn} is the registration number.  This allows programs that
+know about the registration number to construct character set names and
+use them in @code{iconv_open} calls.  More on the available names and
+aliases follows below.
+
+@item
+Lines starting with @code{module} introduce an available conversion
+module.  These lines must contain three or four more words.
+
+The first word specifies the source character set, the second word the
+destination character set of conversion implemented in this module, and
+the third word is the name of the loadable module.  The filename is
+constructed by appending the usual shared object suffix (normally
+@file{.so}) and this file is then supposed to be found in the same
+directory the @file{gconv-modules} file is in.  The last word on the line,
+which is optional, is a numeric value representing the cost of the
+conversion.  If this word is missing, a cost of @math{1} is assumed.  The
+numeric value itself does not matter that much; what counts are the
+relative values of the sums of costs for all possible conversion paths.
+Below is a more precise description of the use of the cost value.
+@end itemize
+
+Returning to the example above where one has written a module to directly
+convert from ISO-2022-JP to EUC-JP and back.  All that has to be done is
+to put the new module, let its name be ISO2022JP-EUCJP.so, in a directory
+and add a file @file{gconv-modules} with the following content in the
+same directory:
+
+@smallexample
+module  ISO-2022-JP//   EUC-JP//        ISO2022JP-EUCJP    1
+module  EUC-JP//        ISO-2022-JP//   ISO2022JP-EUCJP    1
+@end smallexample
+
+To see why this is sufficient, it is necessary to understand how the
+conversion used by @code{iconv} (and described in the descriptor) is
+selected.  The approach to this problem is quite simple.
+
+At the first call of the @code{iconv_open} function the program reads
+all available @file{gconv-modules} files and builds up two tables: one
+containing all the known aliases and another that contains the
+information about the conversions and which shared object implements
+them.
+
+@subsubsection Finding the conversion path in @code{iconv}
+
+The set of available conversions form a directed graph with weighted
+edges.  The weights on the edges are the costs specified in the
+@file{gconv-modules} files.  The @code{iconv_open} function uses an
+algorithm suitable for search for the best path in such a graph and so
+constructs a list of conversions that must be performed in succession
+to get the transformation from the source to the destination character
+set.
+
+Explaining why the above @file{gconv-modules} files allows the
+@code{iconv} implementation to resolve the specific ISO-2022-JP to
+EUC-JP conversion module instead of the conversion coming with the
+library itself is straightforward.  Since the latter conversion takes two
+steps (from ISO-2022-JP to @w{ISO 10646} and then from @w{ISO 10646} to
+EUC-JP), the cost is @math{1+1 = 2}.  The above @file{gconv-modules}
+file, however, specifies that the new conversion modules can perform this
+conversion with only the cost of @math{1}.
+
+A mysterious item about the @file{gconv-modules} file above (and also
+the file coming with @theglibc{}) are the names of the character
+sets specified in the @code{module} lines.  Why do almost all the names
+end in @code{//}?  And this is not all: the names can actually be
+regular expressions.  At this point in time this mystery should not be
+revealed, unless you have the relevant spell-casting materials: ashes
+from an original @w{DOS 6.2} boot disk burnt in effigy, a crucifix
+blessed by St.@: Emacs, assorted herbal roots from Central America, sand
+from Cebu, etc.  Sorry!  @strong{The part of the implementation where
+this is used is not yet finished.  For now please simply follow the
+existing examples.  It'll become clearer once it is. --drepper}
+
+A last remark about the @file{gconv-modules} is about the names not
+ending with @code{//}.  A character set named @code{INTERNAL} is often
+mentioned.  From the discussion above and the chosen name it should have
+become clear that this is the name for the representation used in the
+intermediate step of the triangulation.  We have said that this is UCS-4
+but actually that is not quite right.  The UCS-4 specification also
+includes the specification of the byte ordering used.  Since a UCS-4 value
+consists of four bytes, a stored value is affected by byte ordering.  The
+internal representation is @emph{not} the same as UCS-4 in case the byte
+ordering of the processor (or at least the running process) is not the
+same as the one required for UCS-4.  This is done for performance reasons
+as one does not want to perform unnecessary byte-swapping operations if
+one is not interested in actually seeing the result in UCS-4.  To avoid
+trouble with endianness, the internal representation consistently is named
+@code{INTERNAL} even on big-endian systems where the representations are
+identical.
+
+@subsubsection @code{iconv} module data structures
+
+So far this section has described how modules are located and considered
+to be used.  What remains to be described is the interface of the modules
+so that one can write new ones.  This section describes the interface as
+it is in use in January 1999.  The interface will change a bit in the
+future but, with luck, only in an upwardly compatible way.
+
+The definitions necessary to write new modules are publicly available
+in the non-standard header @file{gconv.h}.  The following text,
+therefore, describes the definitions from this header file.  First,
+however, it is necessary to get an overview.
+
+From the perspective of the user of @code{iconv} the interface is quite
+simple: the @code{iconv_open} function returns a handle that can be used
+in calls to @code{iconv}, and finally the handle is freed with a call to
+@code{iconv_close}.  The problem is that the handle has to be able to
+represent the possibly long sequences of conversion steps and also the
+state of each conversion since the handle is all that is passed to the
+@code{iconv} function.  Therefore, the data structures are really the
+elements necessary to understanding the implementation.
+
+We need two different kinds of data structures.  The first describes the
+conversion and the second describes the state etc.  There are really two
+type definitions like this in @file{gconv.h}.
+@pindex gconv.h
+
+@comment gconv.h
+@comment GNU
+@deftp {Data type} {struct __gconv_step}
+This data structure describes one conversion a module can perform.  For
+each function in a loaded module with conversion functions there is
+exactly one object of this type.  This object is shared by all users of
+the conversion (i.e., this object does not contain any information
+corresponding to an actual conversion; it only describes the conversion
+itself).
+
+@table @code
+@item struct __gconv_loaded_object *__shlib_handle
+@itemx const char *__modname
+@itemx int __counter
+All these elements of the structure are used internally in the C library
+to coordinate loading and unloading the shared object.  One must not expect any
+of the other elements to be available or initialized.
+
+@item const char *__from_name
+@itemx const char *__to_name
+@code{__from_name} and @code{__to_name} contain the names of the source and
+destination character sets.  They can be used to identify the actual
+conversion to be carried out since one module might implement conversions
+for more than one character set and/or direction.
+
+@item gconv_fct __fct
+@itemx gconv_init_fct __init_fct
+@itemx gconv_end_fct __end_fct
+These elements contain pointers to the functions in the loadable module.
+The interface will be explained below.
+
+@item int __min_needed_from
+@itemx int __max_needed_from
+@itemx int __min_needed_to
+@itemx int __max_needed_to;
+These values have to be supplied in the init function of the module.  The
+@code{__min_needed_from} value specifies how many bytes a character of
+the source character set at least needs.  The @code{__max_needed_from}
+specifies the maximum value that also includes possible shift sequences.
+
+The @code{__min_needed_to} and @code{__max_needed_to} values serve the
+same purpose as @code{__min_needed_from} and @code{__max_needed_from} but
+this time for the destination character set.
+
+It is crucial that these values be accurate since otherwise the
+conversion functions will have problems or not work at all.
+
+@item int __stateful
+This element must also be initialized by the init function.
+@code{int __stateful} is nonzero if the source character set is stateful.
+Otherwise it is zero.
+
+@item void *__data
+This element can be used freely by the conversion functions in the
+module.  @code{void *__data} can be used to communicate extra information
+from one call to another.  @code{void *__data} need not be initialized if
+not needed at all.  If @code{void *__data} element is assigned a pointer
+to dynamically allocated memory (presumably in the init function) it has
+to be made sure that the end function deallocates the memory.  Otherwise
+the application will leak memory.
+
+It is important to be aware that this data structure is shared by all
+users of this specification conversion and therefore the @code{__data}
+element must not contain data specific to one specific use of the
+conversion function.
+@end table
+@end deftp
+
+@comment gconv.h
+@comment GNU
+@deftp {Data type} {struct __gconv_step_data}
+This is the data structure that contains the information specific to
+each use of the conversion functions.
+
+
+@table @code
+@item char *__outbuf
+@itemx char *__outbufend
+These elements specify the output buffer for the conversion step.  The
+@code{__outbuf} element points to the beginning of the buffer, and
+@code{__outbufend} points to the byte following the last byte in the
+buffer.  The conversion function must not assume anything about the size
+of the buffer but it can be safely assumed there is room for at
+least one complete character in the output buffer.
+
+Once the conversion is finished, if the conversion is the last step, the
+@code{__outbuf} element must be modified to point after the last byte
+written into the buffer to signal how much output is available.  If this
+conversion step is not the last one, the element must not be modified.
+The @code{__outbufend} element must not be modified.
+
+@item int __is_last
+This element is nonzero if this conversion step is the last one.  This
+information is necessary for the recursion.  See the description of the
+conversion function internals below.  This element must never be
+modified.
+
+@item int __invocation_counter
+The conversion function can use this element to see how many calls of
+the conversion function already happened.  Some character sets require a
+certain prolog when generating output, and by comparing this value with
+zero, one can find out whether it is the first call and whether,
+therefore, the prolog should be emitted.  This element must never be
+modified.
+
+@item int __internal_use
+This element is another one rarely used but needed in certain
+situations.  It is assigned a nonzero value in case the conversion
+functions are used to implement @code{mbsrtowcs} et.al.@: (i.e., the
+function is not used directly through the @code{iconv} interface).
+
+This sometimes makes a difference as it is expected that the
+@code{iconv} functions are used to translate entire texts while the
+@code{mbsrtowcs} functions are normally used only to convert single
+strings and might be used multiple times to convert entire texts.
+
+But in this situation we would have problem complying with some rules of
+the character set specification.  Some character sets require a prolog,
+which must appear exactly once for an entire text.  If a number of
+@code{mbsrtowcs} calls are used to convert the text, only the first call
+must add the prolog.  However, because there is no communication between the
+different calls of @code{mbsrtowcs}, the conversion functions have no
+possibility to find this out.  The situation is different for sequences
+of @code{iconv} calls since the handle allows access to the needed
+information.
+
+The @code{int __internal_use} element is mostly used together with
+@code{__invocation_counter} as follows:
+
+@smallexample
+if (!data->__internal_use
+     && data->__invocation_counter == 0)
+  /* @r{Emit prolog.}  */
+  @dots{}
+@end smallexample
+
+This element must never be modified.
+
+@item mbstate_t *__statep
+The @code{__statep} element points to an object of type @code{mbstate_t}
+(@pxref{Keeping the state}).  The conversion of a stateful character
+set must use the object pointed to by @code{__statep} to store
+information about the conversion state.  The @code{__statep} element
+itself must never be modified.
+
+@item mbstate_t __state
+This element must @emph{never} be used directly.  It is only part of
+this structure to have the needed space allocated.
+@end table
+@end deftp
+
+@subsubsection @code{iconv} module interfaces
+
+With the knowledge about the data structures we now can describe the
+conversion function itself.  To understand the interface a bit of
+knowledge is necessary about the functionality in the C library that
+loads the objects with the conversions.
+
+It is often the case that one conversion is used more than once (i.e.,
+there are several @code{iconv_open} calls for the same set of character
+sets during one program run).  The @code{mbsrtowcs} et.al.@: functions in
+@theglibc{} also use the @code{iconv} functionality, which
+increases the number of uses of the same functions even more.
+
+Because of this multiple use of conversions, the modules do not get
+loaded exclusively for one conversion.  Instead a module once loaded can
+be used by an arbitrary number of @code{iconv} or @code{mbsrtowcs} calls
+at the same time.  The splitting of the information between conversion-
+function-specific information and conversion data makes this possible.
+The last section showed the two data structures used to do this.
+
+This is of course also reflected in the interface and semantics of the
+functions that the modules must provide.  There are three functions that
+must have the following names:
+
+@table @code
+@item gconv_init
+The @code{gconv_init} function initializes the conversion function
+specific data structure.  This very same object is shared by all
+conversions that use this conversion and, therefore, no state information
+about the conversion itself must be stored in here.  If a module
+implements more than one conversion, the @code{gconv_init} function will
+be called multiple times.
+
+@item gconv_end
+The @code{gconv_end} function is responsible for freeing all resources
+allocated by the @code{gconv_init} function.  If there is nothing to do,
+this function can be missing.  Special care must be taken if the module
+implements more than one conversion and the @code{gconv_init} function
+does not allocate the same resources for all conversions.
+
+@item gconv
+This is the actual conversion function.  It is called to convert one
+block of text.  It gets passed the conversion step information
+initialized by @code{gconv_init} and the conversion data, specific to
+this use of the conversion functions.
+@end table
+
+There are three data types defined for the three module interface
+functions and these define the interface.
+
+@comment gconv.h
+@comment GNU
+@deftypevr {Data type} int {(*__gconv_init_fct)} (struct __gconv_step *)
+This specifies the interface of the initialization function of the
+module.  It is called exactly once for each conversion the module
+implements.
+
+As explained in the description of the @code{struct __gconv_step} data
+structure above the initialization function has to initialize parts of
+it.
+
+@table @code
+@item __min_needed_from
+@itemx __max_needed_from
+@itemx __min_needed_to
+@itemx __max_needed_to
+These elements must be initialized to the exact numbers of the minimum
+and maximum number of bytes used by one character in the source and
+destination character sets, respectively.  If the characters all have the
+same size, the minimum and maximum values are the same.
+
+@item __stateful
+This element must be initialized to a nonzero value if the source
+character set is stateful.  Otherwise it must be zero.
+@end table
+
+If the initialization function needs to communicate some information
+to the conversion function, this communication can happen using the
+@code{__data} element of the @code{__gconv_step} structure.  But since
+this data is shared by all the conversions, it must not be modified by
+the conversion function.  The example below shows how this can be used.
+
+@smallexample
+#define MIN_NEEDED_FROM         1
+#define MAX_NEEDED_FROM         4
+#define MIN_NEEDED_TO           4
+#define MAX_NEEDED_TO           4
+
+int
+gconv_init (struct __gconv_step *step)
+@{
+  /* @r{Determine which direction.}  */
+  struct iso2022jp_data *new_data;
+  enum direction dir = illegal_dir;
+  enum variant var = illegal_var;
+  int result;
+
+  if (__strcasecmp (step->__from_name, "ISO-2022-JP//") == 0)
+    @{
+      dir = from_iso2022jp;
+      var = iso2022jp;
+    @}
+  else if (__strcasecmp (step->__to_name, "ISO-2022-JP//") == 0)
+    @{
+      dir = to_iso2022jp;
+      var = iso2022jp;
+    @}
+  else if (__strcasecmp (step->__from_name, "ISO-2022-JP-2//") == 0)
+    @{
+      dir = from_iso2022jp;
+      var = iso2022jp2;
+    @}
+  else if (__strcasecmp (step->__to_name, "ISO-2022-JP-2//") == 0)
+    @{
+      dir = to_iso2022jp;
+      var = iso2022jp2;
+    @}
+
+  result = __GCONV_NOCONV;
+  if (dir != illegal_dir)
+    @{
+      new_data = (struct iso2022jp_data *)
+        malloc (sizeof (struct iso2022jp_data));
+
+      result = __GCONV_NOMEM;
+      if (new_data != NULL)
+        @{
+          new_data->dir = dir;
+          new_data->var = var;
+          step->__data = new_data;
+
+          if (dir == from_iso2022jp)
+            @{
+              step->__min_needed_from = MIN_NEEDED_FROM;
+              step->__max_needed_from = MAX_NEEDED_FROM;
+              step->__min_needed_to = MIN_NEEDED_TO;
+              step->__max_needed_to = MAX_NEEDED_TO;
+            @}
+          else
+            @{
+              step->__min_needed_from = MIN_NEEDED_TO;
+              step->__max_needed_from = MAX_NEEDED_TO;
+              step->__min_needed_to = MIN_NEEDED_FROM;
+              step->__max_needed_to = MAX_NEEDED_FROM + 2;
+            @}
+
+          /* @r{Yes, this is a stateful encoding.}  */
+          step->__stateful = 1;
+
+          result = __GCONV_OK;
+        @}
+    @}
+
+  return result;
+@}
+@end smallexample
+
+The function first checks which conversion is wanted.  The module from
+which this function is taken implements four different conversions;
+which one is selected can be determined by comparing the names.  The
+comparison should always be done without paying attention to the case.
+
+Next, a data structure, which contains the necessary information about
+which conversion is selected, is allocated.  The data structure
+@code{struct iso2022jp_data} is locally defined since, outside the
+module, this data is not used at all.  Please note that if all four
+conversions this module supports are requested there are four data
+blocks.
+
+One interesting thing is the initialization of the @code{__min_} and
+@code{__max_} elements of the step data object.  A single ISO-2022-JP
+character can consist of one to four bytes.  Therefore the
+@code{MIN_NEEDED_FROM} and @code{MAX_NEEDED_FROM} macros are defined
+this way.  The output is always the @code{INTERNAL} character set (aka
+UCS-4) and therefore each character consists of exactly four bytes.  For
+the conversion from @code{INTERNAL} to ISO-2022-JP we have to take into
+account that escape sequences might be necessary to switch the character
+sets.  Therefore the @code{__max_needed_to} element for this direction
+gets assigned @code{MAX_NEEDED_FROM + 2}.  This takes into account the
+two bytes needed for the escape sequences to signal the switching.  The
+asymmetry in the maximum values for the two directions can be explained
+easily: when reading ISO-2022-JP text, escape sequences can be handled
+alone (i.e., it is not necessary to process a real character since the
+effect of the escape sequence can be recorded in the state information).
+The situation is different for the other direction.  Since it is in
+general not known which character comes next, one cannot emit escape
+sequences to change the state in advance.  This means the escape
+sequences have to be emitted together with the next character.
+Therefore one needs more room than only for the character itself.
+
+The possible return values of the initialization function are:
+
+@table @code
+@item __GCONV_OK
+The initialization succeeded
+@item __GCONV_NOCONV
+The requested conversion is not supported in the module.  This can
+happen if the @file{gconv-modules} file has errors.
+@item __GCONV_NOMEM
+Memory required to store additional information could not be allocated.
+@end table
+@end deftypevr
+
+The function called before the module is unloaded is significantly
+easier.  It often has nothing at all to do; in which case it can be left
+out completely.
+
+@comment gconv.h
+@comment GNU
+@deftypevr {Data type} void {(*__gconv_end_fct)} (struct gconv_step *)
+The task of this function is to free all resources allocated in the
+initialization function.  Therefore only the @code{__data} element of
+the object pointed to by the argument is of interest.  Continuing the
+example from the initialization function, the finalization function
+looks like this:
+
+@smallexample
+void
+gconv_end (struct __gconv_step *data)
+@{
+  free (data->__data);
+@}
+@end smallexample
+@end deftypevr
+
+The most important function is the conversion function itself, which can
+get quite complicated for complex character sets.  But since this is not
+of interest here, we will only describe a possible skeleton for the
+conversion function.
+
+@comment gconv.h
+@comment GNU
+@deftypevr {Data type} int {(*__gconv_fct)} (struct __gconv_step *, struct __gconv_step_data *, const char **, const char *, size_t *, int)
+The conversion function can be called for two basic reasons: to convert
+text or to reset the state.  From the description of the @code{iconv}
+function it can be seen why the flushing mode is necessary.  What mode
+is selected is determined by the sixth argument, an integer.  This
+argument being nonzero means that flushing is selected.
+
+Common to both modes is where the output buffer can be found.  The
+information about this buffer is stored in the conversion step data.  A
+pointer to this information is passed as the second argument to this
+function.  The description of the @code{struct __gconv_step_data}
+structure has more information on the conversion step data.
+
+@cindex stateful
+What has to be done for flushing depends on the source character set.
+If the source character set is not stateful, nothing has to be done.
+Otherwise the function has to emit a byte sequence to bring the state
+object into the initial state.  Once this all happened the other
+conversion modules in the chain of conversions have to get the same
+chance.  Whether another step follows can be determined from the
+@code{__is_last} element of the step data structure to which the first
+parameter points.
+
+The more interesting mode is when actual text has to be converted.  The
+first step in this case is to convert as much text as possible from the
+input buffer and store the result in the output buffer.  The start of the
+input buffer is determined by the third argument, which is a pointer to a
+pointer variable referencing the beginning of the buffer.  The fourth
+argument is a pointer to the byte right after the last byte in the buffer.
+
+The conversion has to be performed according to the current state if the
+character set is stateful.  The state is stored in an object pointed to
+by the @code{__statep} element of the step data (second argument).  Once
+either the input buffer is empty or the output buffer is full the
+conversion stops.  At this point, the pointer variable referenced by the
+third parameter must point to the byte following the last processed
+byte (i.e., if all of the input is consumed, this pointer and the fourth
+parameter have the same value).
+
+What now happens depends on whether this step is the last one.  If it is
+the last step, the only thing that has to be done is to update the
+@code{__outbuf} element of the step data structure to point after the
+last written byte.  This update gives the caller the information on how
+much text is available in the output buffer.  In addition, the variable
+pointed to by the fifth parameter, which is of type @code{size_t}, must
+be incremented by the number of characters (@emph{not bytes}) that were
+converted in a non-reversible way.  Then, the function can return.
+
+In case the step is not the last one, the later conversion functions have
+to get a chance to do their work.  Therefore, the appropriate conversion
+function has to be called.  The information about the functions is
+stored in the conversion data structures, passed as the first parameter.
+This information and the step data are stored in arrays, so the next
+element in both cases can be found by simple pointer arithmetic:
+
+@smallexample
+int
+gconv (struct __gconv_step *step, struct __gconv_step_data *data,
+       const char **inbuf, const char *inbufend, size_t *written,
+       int do_flush)
+@{
+  struct __gconv_step *next_step = step + 1;
+  struct __gconv_step_data *next_data = data + 1;
+  @dots{}
+@end smallexample
+
+The @code{next_step} pointer references the next step information and
+@code{next_data} the next data record.  The call of the next function
+therefore will look similar to this:
+
+@smallexample
+  next_step->__fct (next_step, next_data, &outerr, outbuf,
+                    written, 0)
+@end smallexample
+
+But this is not yet all.  Once the function call returns the conversion
+function might have some more to do.  If the return value of the function
+is @code{__GCONV_EMPTY_INPUT}, more room is available in the output
+buffer.  Unless the input buffer is empty, the conversion functions start
+all over again and process the rest of the input buffer.  If the return
+value is not @code{__GCONV_EMPTY_INPUT}, something went wrong and we have
+to recover from this.
+
+A requirement for the conversion function is that the input buffer
+pointer (the third argument) always point to the last character that
+was put in converted form into the output buffer.  This is trivially
+true after the conversion performed in the current step, but if the
+conversion functions deeper downstream stop prematurely, not all
+characters from the output buffer are consumed and, therefore, the input
+buffer pointers must be backed off to the right position.
+
+Correcting the input buffers is easy to do if the input and output
+character sets have a fixed width for all characters.  In this situation
+we can compute how many characters are left in the output buffer and,
+therefore, can correct the input buffer pointer appropriately with a
+similar computation.  Things are getting tricky if either character set
+has characters represented with variable length byte sequences, and it
+gets even more complicated if the conversion has to take care of the
+state.  In these cases the conversion has to be performed once again, from
+the known state before the initial conversion (i.e., if necessary the
+state of the conversion has to be reset and the conversion loop has to be
+executed again).  The difference now is that it is known how much input
+must be created, and the conversion can stop before converting the first
+unused character.  Once this is done the input buffer pointers must be
+updated again and the function can return.
+
+One final thing should be mentioned.  If it is necessary for the
+conversion to know whether it is the first invocation (in case a prolog
+has to be emitted), the conversion function should increment the
+@code{__invocation_counter} element of the step data structure just
+before returning to the caller.  See the description of the @code{struct
+__gconv_step_data} structure above for more information on how this can
+be used.
+
+The return value must be one of the following values:
+
+@table @code
+@item __GCONV_EMPTY_INPUT
+All input was consumed and there is room left in the output buffer.
+@item __GCONV_FULL_OUTPUT
+No more room in the output buffer.  In case this is not the last step
+this value is propagated down from the call of the next conversion
+function in the chain.
+@item __GCONV_INCOMPLETE_INPUT
+The input buffer is not entirely empty since it contains an incomplete
+character sequence.
+@end table
+
+The following example provides a framework for a conversion function.
+In case a new conversion has to be written the holes in this
+implementation have to be filled and that is it.
+
+@smallexample
+int
+gconv (struct __gconv_step *step, struct __gconv_step_data *data,
+       const char **inbuf, const char *inbufend, size_t *written,
+       int do_flush)
+@{
+  struct __gconv_step *next_step = step + 1;
+  struct __gconv_step_data *next_data = data + 1;
+  gconv_fct fct = next_step->__fct;
+  int status;
+
+  /* @r{If the function is called with no input this means we have}
+     @r{to reset to the initial state.  The possibly partly}
+     @r{converted input is dropped.}  */
+  if (do_flush)
+    @{
+      status = __GCONV_OK;
+
+      /* @r{Possible emit a byte sequence which put the state object}
+         @r{into the initial state.}  */
+
+      /* @r{Call the steps down the chain if there are any but only}
+         @r{if we successfully emitted the escape sequence.}  */
+      if (status == __GCONV_OK && ! data->__is_last)
+        status = fct (next_step, next_data, NULL, NULL,
+                      written, 1);
+    @}
+  else
+    @{
+      /* @r{We preserve the initial values of the pointer variables.}  */
+      const char *inptr = *inbuf;
+      char *outbuf = data->__outbuf;
+      char *outend = data->__outbufend;
+      char *outptr;
+
+      do
+        @{
+          /* @r{Remember the start value for this round.}  */
+          inptr = *inbuf;
+          /* @r{The outbuf buffer is empty.}  */
+          outptr = outbuf;
+
+          /* @r{For stateful encodings the state must be safe here.}  */
+
+          /* @r{Run the conversion loop.  @code{status} is set}
+             @r{appropriately afterwards.}  */
+
+          /* @r{If this is the last step, leave the loop.  There is}
+             @r{nothing we can do.}  */
+          if (data->__is_last)
+            @{
+              /* @r{Store information about how many bytes are}
+                 @r{available.}  */
+              data->__outbuf = outbuf;
+
+             /* @r{If any non-reversible conversions were performed,}
+                @r{add the number to @code{*written}.}  */
+
+             break;
+           @}
+
+          /* @r{Write out all output that was produced.}  */
+          if (outbuf > outptr)
+            @{
+              const char *outerr = data->__outbuf;
+              int result;
+
+              result = fct (next_step, next_data, &outerr,
+                            outbuf, written, 0);
+
+              if (result != __GCONV_EMPTY_INPUT)
+                @{
+                  if (outerr != outbuf)
+                    @{
+                      /* @r{Reset the input buffer pointer.  We}
+                         @r{document here the complex case.}  */
+                      size_t nstatus;
+
+                      /* @r{Reload the pointers.}  */
+                      *inbuf = inptr;
+                      outbuf = outptr;
+
+                      /* @r{Possibly reset the state.}  */
+
+                      /* @r{Redo the conversion, but this time}
+                         @r{the end of the output buffer is at}
+                         @r{@code{outerr}.}  */
+                    @}
+
+                  /* @r{Change the status.}  */
+                  status = result;
+                @}
+              else
+                /* @r{All the output is consumed, we can make}
+                   @r{ another run if everything was ok.}  */
+                if (status == __GCONV_FULL_OUTPUT)
+                  status = __GCONV_OK;
+           @}
+        @}
+      while (status == __GCONV_OK);
+
+      /* @r{We finished one use of this step.}  */
+      ++data->__invocation_counter;
+    @}
+
+  return status;
+@}
+@end smallexample
+@end deftypevr
+
+This information should be sufficient to write new modules.  Anybody
+doing so should also take a look at the available source code in the
+@glibcadj{} sources.  It contains many examples of working and optimized
+modules.
+
+@c File charset.texi edited October 2001 by Dennis Grace, IBM Corporation