glibc/manual/charset.texi

@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 which was added to the C
library to correctly 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 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 exist. 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 whatever communication channel.  Examples of external
representations include files lying in a directory that are going to be
read and parsed.

Traditionally there was no difference between the two representations.
It was equally comfortable and useful to use the same one-byte
representation internally and externally.  This changes with more and
larger character sets.

One of the problems to overcome with the internal representation is
handling text which is externally encoded using different character
sets.  Assume a program which 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, 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
there exists a completely new family of functions which can handle texts
of this kind in memory.  The most commonly used character set for such
internal wide character representations are Unicode and @w{ISO 10646}.
The former is a subset of the latter and used when wide characters are
chosen to by 2 bytes (@math{= 16} bits) wide.  The standard names of the
@cindex UCS2
@cindex UCS4
encodings used in these cases are UCS2 (@math{= 16} bits) and UCS4
(@math{= 32} bits).

To represent wide characters the @code{char} type is not suitable.  For
this reason the @w{ISO C} standard introduces a new type which 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 which 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.
I.e., 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 C89} standard, where this type was introduced, does not say
anything specific about the representation.  It only requires that this
type is capable to store all elements of the basic character set.
Therefore it would be legitimate to define @code{wchar_t} and
@code{char}.  This might make sense for embedded systems.

But for GNU systems this type is always 32 bits wide.  It is therefore
capable to represent all UCS4 value 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 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 surrogate character which is in fact a
multi-wide-character encoding.  But this 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 which
contain a single wide character.  As the name already suggests it is the
equivalent to @code{int} when using the normal @code{char} strings.  The
types @code{wchar_t} and @code{wint_t} have often the same
representation if their size if 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 got introduced in the second
amendment to @w{ISO C 89}.
@end deftp

As there are for the @code{char} data type there also exist macros
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 got introduced in the second amendment to @w{ISO C89}.
@end deftypevr

@comment wchar.h
@comment ISO
@deftypevr Macro wint_t WCHAR_MAX
The macro @code{WCHAR_MIN} evaluates to the maximum value representable
by an object of type @code{wint_t}.

This macro got introduced in the second amendment to @w{ISO C89}.
@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.  I.e., sloppy code like

@smallexample
@{
  int c;
  ...
  while ((c = getc (fp)) < 0)
    ...
@}
@end smallexample

@noindent
has to be rewritten to explicitly use @code{WEOF} when wide characters
are used.

@smallexample
@{
  wint_t c;
  ...
  while ((c = wgetc (fp)) != WEOF)
    ...
@}
@end smallexample

@pindex wchar.h
This macro was introduced in the second amendment to @w{ISO C89} and is
defined in @file{wchar.h}.
@end deftypevr


These internal representations present problems when it comes to storing
and transmittal, since a single wide character consists of more
than one byte they are effected by byte-ordering.  I.e., machines with
different endianesses would see different value accessing the same data.
This also applies for communication protocols which are all byte-based
and therefore 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 an customized byte
oriented character set.

@cindex multibyte character
@cindex EBCDIC
   For all the above reasons, an external encoding which is different
from the internal encoding is often used if the latter is UCS2 or UCS4.
The external encoding is byte-based and can be chosen appropriately for
the environment and for the texts to be handled.  There exist a variety
of different character sets which can be used for this external
encoding. Information which will not be exhaustively presented
here--instead, a description of the major groups will suffice.  All of
the ASCII-based character sets [_bkoz_: do you mean Roman character
sets? If not, what do you mean here?]  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
operation 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 one-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.  Another reason to choose this is because of constraints
from interaction with other programs (which might not be 8-bit clean).

@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.  Embedded
in the text can be characters which can be used to change the state.
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 SJIS
In most uses of @w{ISO 2022} the defined character sets do not allow
state changes which 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 operations systems, EUC-JP, EUC-KR, EUC-TW, and EUC-CN)
or SJIS (Shift JIS, a Japanese encoding).

But there are also character sets using a state which 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.  E.g., the byte sequence @code{0xc2 0x61} (non-spacing
acute accent, following by lower-case `a') to get the ``small a with
acute'' character.  To get the acute accent character on its on one has
to write @code{0xc2 0x20} (the non-spacing acute followed by a space).

This type of characters sets is quite frequently used in embedded
systems such as video text.

@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
UCS2/UCS4.  The Unicode and @w{ISO 10646} standards even specify such an
encoding: UTF-8.  This encoding is able to represent all of @w{ISO
10464} 31 bits in a byte string of length one to seven.

@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 which should be used.  In fact,
UTF-8 will hopefully soon be the only external which has to be
supported.  It proves to be universally usable and the only disadvantage
is that it favor Roman languages very much 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.  I.e., if a
project is expected to only be used in, 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 which 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 specified in any
standard, though.  The @w{ISO C} standard does not specify anything
specific about the @code{wchar_t} type.  There might be systems where
the developers decided differently.  Therefore one should as much as
possible avoid making assumption about the wide character representation
although GNU systems will always work as described above.  If the
programmer uses only the functions provided by the C library to handle
wide character strings there should not be any 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.  The one function family
is specified in the @w{ISO C} standard and therefore is portable even
beyond the Unix world.

The most commonly known set of functions, coming from the @w{ISO C89}
standard, is unfortunately the least useful one.  In fact, 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 the second amendment to
@w{ISO C89}.

@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.
The GNU C library contains some extensions the standard which allow
specifying a size but basically they also expect terminated strings.
@end itemize

Despite these limitations the @w{ISO C} functions can very well be used
in many contexts.  In graphical user interfaces, for instance, it is not
uncommon to have functions which require text to be displayed in a wide
character string if it 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 about the conversion which 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
character set always is UCS4, at least on GNU systems.

A characteristic of each multibyte character set is the maximum number
of bytes which can be necessary to represent one character.  This
information is quite important when writing code which uses the
conversion functions.  In the examples below we will see some examples.
The @w{ISO C} standard defines two macros which provide this information.


@comment limits.h
@comment ISO
@deftypevr Macro int MB_LEN_MAX
This macro 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 it 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
fact, in the GNU C library 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 C89} 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{... 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 always
@code{MB_CUR_MAX} bytes are 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.  I.e., 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
This type is defined in @file{wchar.h}.  It got introduced in the second
amendment to @w{ISO C89}.
@end deftp

To use objects of this type 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.}  */
  ...
@}
@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 or not 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})
This function determines whether the state object pointed to by @var{ps}
is in the initial state or not.  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
This function was introduced in the second amendment to @w{ISO C89} and
is declared in @file{wchar.h}.
@end deftypefun

Code using this function 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}.}  */
  ...
  if (! mbsinit (&state))
    @{
      /* @r{Emit code to return to initial state.}  */
      const char empty[] = "";
      const char **srcp = &empty;
      wcsrtombs (outbuf, &srcp, outbuflen, &state);
    @}
  ...
@}
@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 on
GNU systems it is not necessary to perform this extra action for the
conversion from multibyte text ot wide character text since the wide
character encoding is not stateful.  But there is nothing mentioned in
any standard which prohibits making @code{wchar_t} using 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 which consists of single byte sequences there are
functions to help with converting bytes.  One very important and often
applicable scenario is where ASCII is a subpart of the multibyte
character set.  I.e., all ASCII characters stand for itself and all
other characters have at least a first byte which is beyond the range
@math{0} to @math{127}.

@comment wchar.h
@comment ISO
@deftypefun wint_t btowc (int @var{c})
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.  There is no @code{mbstate_t} object used from
which the state information is taken and the function also does not use
any static state.

@pindex wchar.h
This function was introduced in the second amendment of @w{ISO C89} and
is declared in @file{wchar.h}.
@end deftypefun

Despite the limitation that the single byte value always is 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 extension 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 it is necessary @code{btowc}.

@noindent
There also is a function for the conversion in the other direction.

@comment wchar.h
@comment ISO
@deftypefun int wctob (wint_t @var{c})
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
This function was introduced in the second amendment of @w{ISO C89} and
is declared in @file{wchar.h}.
@end deftypefun

There are more general functions to convert single character 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})
@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 an static,
internal state variable used only by the @code{mbrtowc} variable 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} which 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 pointer to
by @var{pwc} iff @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 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 also 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
This function was introduced in the second amendment to @w{ISO C89} and
is declared in @file{wchar.h}.
@end deftypefun

Using this function is straight forward.  A function which copies a
multibyte string into a wide character string while at the same time
converting all lowercase character into uppercase could look like this
(this is not the final version, just an example; it has no error
checking, and leaks sometimes 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;
  memset (&state, '\0', sizeof (state));
  size_t nbytes;
  while ((nbytes = mbrtowc (tmp, s, len, &state)) > 0)
    @{
      if (nbytes >= (size_t) -2)
        /* Invalid input string.  */
        return NULL;
      *result++ = 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}.  In case the the conversion was 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 the function 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 results than there are
bytes in the multibyte input string.  This method yields to a
pessimistic guess about the size of the result and if many wide
character strings have to be constructed this way or the strings are
long, the extra memory required allocated because the input string
contains multibzte characters might be significant.  It would be
possible to resize the allocated memory block to the correct size before
returning it.  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.  But there is a function which 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})
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 the first @var{n} bytes possibly form a valid multibyte
character but it 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 pointer to by @var{ps}.  If @var{ps} is a null pointer an state
object local to @code{mbrlen} is used.

@pindex wchar.h
This function was introduced in the second amendment to @w{ISO C89} and
is declared in @file{wchar.h}.
@end deftypefun

The tentative reader now will of course 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.
Now, 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 OK since a) this value is larger then the length of the
longest multibyte character sequence and b) because 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 the GNU C library) we can compute the
number of wide character 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} implemented using @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})
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 the state stored in
the objects pointer 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 is stored in the string @var{s}.

Otherwise a byte sequence (possibly including shift sequences) is
written into the string @var{s}.  This of 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 byte 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
This function was introduced in the second amendment to @w{ISO C} and is
declared in @file{wchar.h}.
@end deftypefun

Using this function 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 (and correct), it is simply here to
demonstrate the use and some problems.

@smallexample
char *
mbscatwc (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
never is 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 or
not.  If this solution is really unsuitable there is a very slow but
more accurate solution.

@smallexample
  ...
  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;
        @}
    @}
  ...
@end smallexample

Here we do perform the conversion which 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 think about
this: 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 meant 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, thus the GNU C library contains a few
extensions which 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})
The @code{mbsrtowcs} function (``multibyte string restartable to wide
character string'') converts an 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 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 straight forward 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} 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
This function was introduced in the second amendment to @w{ISO C} and is
declared in @file{wchar.h}.
@end deftypefun

The definition of this function has one limitation which has to be
understood.  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 normally no collection of NUL terminated strings but instead a
continuous collection of lines, separated by newline characters.  Now
assume 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 always
could do only a 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 @var{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 is 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
never should be used in generally used code.

The generic conversion interface (@xref{Generic Charset Conversion})
does not have this limitation (it simply works on buffers, not
strings), and the GNU C library contains a set of functions which take
additional parameters specifying the maximal number of bytes which 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})
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
locally 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{dest} 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 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
This function was introduced in the second amendment to @w{ISO C} and is
declared in @file{wchar.h}.
@end deftypefun

The restriction mentions above for the @code{mbsrtowcs} function applies
also here.  There is no possibility to directly control 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})
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.  I.e., 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 here.

This function is a GNU extensions.  It is meant to work around the
problems mentioned above.  Now it is possible to convert buffer with
multibyte character text piece for 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 which 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})
The @code{wcsnrtombs} function implements the conversion from wide
character strings to multibyte character strings.  It is similar to
@code{wcsrtombs} but it takes, just like @code{mbsnrtowcs}, 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} character to conversion stops at
this place.

This function is a GNU extension and just like @code{mbsnrtowcs} is
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 (inp, buffer, filled);
    @}

  return 1;
@}
@end smallexample


@node Non-reentrant Conversion
@section Non-reentrant Conversion Function

The functions described in the last chapter are defined in the second
amendment to @w{ISO C89}.  But the original @w{ISO C89} standard also
contained functions for character set conversion.  The reason that they
are not described in the first place is that they are almost entirely
useless.

The problem is that all the functions for conversion defined in @w{ISO
C89} use a local state.  This 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 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
from the last section are 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})
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 character, 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})
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 which
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})
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
character, 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 reasons the @w{ISO C89} standard defines also functions
to convert entire strings instead of single characters.  These functions
suffer from the same problems as their reentrant counterparts from the
second amendment to @w{ISO C89}; 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})
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, this 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})
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, this 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 which 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 which 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 GNU C
library it always is UCS4 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 or 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}.

This 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 but no constraints on the character sets selected for
source and destination.  Only the set of available conversions is
limiting them.  The standard does not specify that any conversion at all
must be available.  It is a measure of the quality of the implementation.

In the following text first the interface to @code{iconv}, the
conversion function, will be described.  Comparisons with other
implementations will show what pitfalls lie on the way of portable
applications.  At last, the implementation is described as far as
interesting to the advanced user who wants to extend the 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 implement one step.

Before the interfaces are described it is necessary to introduce a
datatype.  Just like other open--use--close interface the functions
introduced here work using a 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 get 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})
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 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 file 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.  Within the data structures associated
with the descriptor there is 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 GNU C library 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 to store
the necessary files with data and code in arbitrary many directories.
How this extensions have to 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 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 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
This function got 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 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})
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 error are:

@table @code
@item EBADF
The conversion descriptor is invalid.
@end table

@pindex iconv.h
This function was introduced together with the rest of the @code{iconv}
functions in XPG2 and it 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}, const char **@var{inbuf}, size_t *@var{inbytesleft}, char **@var{outbuf}, size_t *@var{outbytesleft})
@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 on whether the conversion has to deal with states or not.
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 the GNU C library 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 three reasons.  The first is that all
characters from the input buffer are converted.  This actually can mean
two things: really all bytes from the input buffer are consumed or
there are some bytes at the end of the buffer which possibly can form a
complete character but the input is incomplete.  The second reason for a
stop is when the output buffer is full.  And the third reason is that
the input contains invalid characters.

In all these cases the buffer pointers after the last successful
conversion, for input and output buffer, 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 the GNU C library 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 conversions
performed.  In all other cases the return value is @code{(size_t) -1}
and @code{errno} is set appropriately.  In this case 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
This 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 is 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 ("UCS4", charset);
  if (cd == (iconv_t) -1)
    @{
      /* @r{Something went wrong.}  */
      if (errno == EINVAL)
        error (0, 0, "conversion from `%s' to `UCS4' no 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;
          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.}  */
  *((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} return 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 than can 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 it 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 which
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 which 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.  On top of this the generic nature of these functions make
them slower than specifically implemented functions.

@item
The C library only contains a framework which can dynamically load
object files and execute the therein contained conversion functions.

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
these possibilities, the majority 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 his
capability it is therefore not possible to use this interface in
statically linked programs.  The GNU C library has on ELF platforms no
problems with dynamic loading in in these situations and therefore this
point is mood.  The danger is that one gets acquainted with this 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.  E.g., conversions from one character set to a set of, say,
10 others counts as 10 conversion.  Together with the other direction
this makes already 20.  One can imagine the thin coverage these platform
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 system 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 which 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
does fail according to the assumption above.  But what does the program
do now?  The conversion is really necessary and therefore simply giving
up is no possibility.

This is a nuisance.  The @code{iconv} function should take care of this.
But how should the program proceed from here on?  If it would try 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 UTF8 encoded @w{ISO 10646} or Unicode text.
Beside 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 shows one of the design errors of @code{iconv} mentioned above.  It
should at least be possible to determine the list of available
conversion 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 the GNU C library

After reading about the problems of @code{iconv} implementations in the
last section it is certainly good to note that the implementation in
the GNU C library 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 funtionality has solidified.

The GNU C library'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 GNU C library
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 the GNU C library, at least on ELF systems.  Since the
library is able to load shared objects even in statically linked
binaries this means that static linking needs not to be forbidden in
case one wants to use @code{iconv}.

The second mentioned problem is the number of supported conversions.
Currently, the GNU C library 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 easily be added.

Particularly impressive as it may be, this high number is due to the
fact that the GNU C library 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} this really
means there is no known way, directly or indirectly, to perform the
wanted conversion.

@cindex triangulation
This is achieved by providing for each character set a conversion from
and to UCS4 encoded @w{ISO 10646}.  Using @w{ISO 10646} as an
intermediate representation it is possible to @dfn{triangulate}, i.e.,
converting 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 currently existing set of conversions is simply meant to
cover all conversions which 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, e.g., 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 can easy write a new conversion and provide it
as a better alternative.  The GNU C library @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.  There are two more words expected on the line.  The first one
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 always an official name but this need not
correspond to the most popular name.  Beside this many character sets
have special names which are somehow constructed.  E.g., 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 which 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.  The
third word is the name of the loadable module.  The filename is
constructed by appending the usual shared object prefix (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 what has to be done is
to put the new module, be its name 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 which 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 which 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 straighforward.  Since the later 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}.  But the above @file{gconv-modules}
file specifies that the new conversion modules can perform this
conversion with only the cost of @math{1}.

A mysterious piece about the @file{gconv-modules} file above (and also
the file coming with the GNU C library) 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 of 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{//}.  There often is a character set named
@code{INTERNAL} mentioned.  From the discussion above and the chosen
name it should have become clear that this is the names for the
representation used in the intermediate step of the triangulation.  We
have said that this is UCS4 but actually it is not quite right.  The
UCS4 specification also includes the specification of the byte ordering
used.  Since an UCS4 value consists of four bytes a stored value is
effected by byte ordering.  The internal representation is @emph{not}
the same as UCS4 in case the byte ordering of the processor (or at least
the running process) is not the same as the one required for UCS4.  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 UCS4.  To avoid trouble with endianess 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 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 in future a bit
but hopefully only in an upward compatible way.

The definitions necessary to write new modules are publically available
in the non-standard header @file{gconv.h}.  The following text will
therefore describe the definitions from this header file.  But first 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 which can be
used in calls @code{iconv} and finally the handle is freed with a call
to @code{iconv_close}.  The problem is: 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 which is passed to the
@code{iconv} function.  Therefore the data structures are really the
elements 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.  One must not expect any
of the other elements 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 filled in the 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 which also includes possible shift
sequences.

The @code{min_needed_to} and @code{max_needed_to} values serve the same
purpose but this time for the destination character set.

It is crucial that these values are 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.  It 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.  It can be used to communicate extra information from one call
to another.  It need not be initialized if not needed at all.  If this
element gets 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 which 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 the there is room for at
least one complete character in the output buffer.

Once the conversion is finished and the conversion is the last step the
@code{outbuf} element must be modified to point after last 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
when generating output a certain prolog and by comparing this value with
zero one can find out whether it is the first call and therefore the
prolog should be emitted or not.  This element must never be modified.

@item int internal_use
This element is another one rarely used but needed in certain
situations.  It got 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 only used 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.  But since 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 to access the needed
information.

This element is mostly used together with @code{invocation_counter} in a
way like this:

@smallexample
if (!data->internal_use && data->invocation_counter == 0)
  /* @r{Emit prolog.}  */
  ...
@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 an stateful character
set must use the object pointed to by this element to store information
about the conversion state.  The @code{statep} element itself must never
be modified.

@item mbstate_t __state
This element @emph{never} must 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 functions itself.  To understand the interface a bit of
knowledge about the functionality in the C library which loads the
objects with the conversions is necessary.

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
the GNU C library also use the @code{iconv} functionality which
increases the number of uses of the same functions even more.

For this reason the modules do not get loaded exclusively for one
conversion.  Instead a module once loaded can be used by arbitrary many
@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 structure used to do this.

This is of course also reflected in the interface and semantic of the
functions the modules must provide.  There are three functions which
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
conversion which 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 to free 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
function 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 int 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 set 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 an nonzero value if the source
character set is stateful.  Otherwise it must be zero.
@end table

If the initialization function needs to communication some information
to the conversion function this can happen using the @code{data} element
of the @code{gconv_step} structure.  But since this data is shared by
all the conversion is must not be modified by the conversion function.
How this can be used is shown in the example below.

@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 conversion and
which one is selected can be determined by comparing the names.  The
comparison should always be done without paying attention to the case.

Then a data structure is allocated which contains the necessary
information about which conversion is selected.  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 modules 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
UCS4) 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 single 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 which have to be emitted together with the next character.
Therefore one needs more room then 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 functions 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 it 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.  It 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 reason: 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.  If it is
nonzero it means that flushing is selected.

Common to both mode is where the output buffer can be found.  The
information about this buffer is stored in the conversion step data.  A
pointer to this is passed as the second argument to this function.  The
description of the @code{struct gconv_step_data} structure has more
information on this.

@cindex stateful
What has to be done for flushing depends on the source character set.
If it is not stateful nothing has to be done.  Otherwise the function
has to emit a byte sequence to bring the state object in 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 actually 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 or not.
If it is the last step the only thing which has to be done is to update
the @code{outbuf} element of the step data structure to point after the
last written byte.  This gives the caller the information on how much
text is available in the output buffer.  Beside this 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}) which were
written in the output buffer.  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;
  ...
@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} this means there is more room in
the output buffer.  Unless the input buffer is empty the conversion
functions start all over again and processes 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 points to the last character which
was put in the converted form in the output buffer.  This is trivial
true after the conversion performed in the current step.  But if the
conversion functions deeper down the stream stop prematurely not all
characters from the output buffer are consumed and therefore the input
buffer pointers must be backed of to the right position.

This 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 appropriate with a similar computation.  Things are
getting tricky if either character set has character 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 just before returning
to the caller increment the @code{invocation_counter} element of the
step data structure.  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_OUTPUT_FULL
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;

      /* @r{This variable is used to count the number of characters}
         @r{we actually converted.}  */
      size_t converted = 0;

      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{Remember how many characters we converted.}  */
             *written += converted;

             break;
           @}

          /* @r{Write out all output which 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 GNU
C library sources.  It contains many examples of working and optimized
modules.