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.
+choices. This chapter shows the functionality that was added to the C
+library to support multiple character sets.
@menu
* Extended Char Intro:: Introduction to Extended Characters.
@node Extended Char Intro
@section Introduction to Extended Characters
-A variety of solutions to overcome the differences between
+A variety of solutions is available to overcome the differences between
character sets with a 1:1 relation between bytes and characters and
-character sets with ratios of 2:1 or 4:1 exist. The remainder of this
+character sets with ratios of 2:1 or 4:1. The remainder of this
section gives a few examples to help understand the design decisions
made while developing the functionality of the @w{C library}.
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
+through some communication channel. Examples of external
+representations include files waiting in a directory 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.
+Traditionally there has been no difference between the two representations.
+It was equally comfortable and useful to use the same single-byte
+representation internally and externally. This comfort level decreases
+with more and larger character sets.
One of the problems to overcome with the internal representation is
-handling text which is externally encoded using different character
-sets. Assume a program which reads two texts and compares them using
+handling text that is externally encoded using different character
+sets. Assume a program that reads two texts and compares them using
some metric. The comparison can be usefully done only if the texts are
internally kept in a common format.
@cindex wide character
For such a common format (@math{=} character set) eight bits are certainly
no longer enough. So the smallest entity will have to grow: @dfn{wide
-characters} will now be used. Instead of one byte, 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).
+characters} will now be used. Instead of one byte per character, two or
+four will be used instead. (Three are not good to address in memory and
+more than four bytes seem not to be necessary).
@cindex Unicode
@cindex ISO 10646
As shown in some other part of this manual,
@c !!! Ahem, wide char string functions are not yet covered -- drepper
-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).
+a completely new family has been created of functions that can handle wide
+character texts in memory. The most commonly used character sets for such
+internal wide character representations are Unicode and @w{ISO 10646}
+(also known as UCS for Universal Character Set). Unicode was originally
+planned as a 16-bit character set; whereas, @w{ISO 10646} was designed to
+be a 31-bit large code space. The two standards are practically identical.
+They have the same character repertoire and code table, but Unicode specifies
+added semantics. At the moment, only characters in the first @code{0x10000}
+code positions (the so-called Basic Multilingual Plane, BMP) have been
+assigned, but the assignment of more specialized characters outside this
+16-bit space is already in progress. A number of encodings have been
+defined for Unicode and @w{ISO 10646} characters:
+@cindex UCS-2
+@cindex UCS-4
+@cindex UTF-8
+@cindex UTF-16
+UCS-2 is a 16-bit word that can only represent characters
+from the BMP, UCS-4 is a 32-bit word than can represent any Unicode
+and @w{ISO 10646} character, UTF-8 is an ASCII compatible encoding where
+ASCII characters are represented by ASCII bytes and non-ASCII characters
+by sequences of 2-6 non-ASCII bytes, and finally UTF-16 is an extension
+of UCS-2 in which pairs of certain UCS-2 words can be used to encode
+non-BMP characters up to @code{0x10ffff}.
To represent wide characters the @code{char} type is not suitable. For
-this reason the @w{ISO C} standard introduces a new type which is
+this reason the @w{ISO C} standard introduces a new type that is
designed to keep one character of a wide character string. To maintain
the similarity there is also a type corresponding to @code{int} for
-those functions which take a single wide character.
+those functions that take a single wide character.
@comment stddef.h
@comment ISO
@deftp {Data type} wchar_t
This data type is used as the base type for wide character strings.
-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 C90} 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
+In other words, arrays of objects of this type are the equivalent of
+@code{char[]} for multibyte character strings. The type is defined in
+@file{stddef.h}.
+
+The @w{ISO C90} standard, where @code{wchar_t} was introduced, does not
+say anything specific about the representation. It only requires that
+this type is capable of storing all elements of the basic character set.
+Therefore it would be legitimate to define @code{wchar_t} as @code{char},
+which might make sense for embedded systems.
+
+But in @theglibc{} @code{wchar_t} is always 32 bits wide and, therefore,
+capable of representing all UCS-4 values and, therefore, covering all of
+@w{ISO 10646}. Some Unix systems define @code{wchar_t} as a 16-bit type
+and thereby follow Unicode very strictly. This definition is perfectly
+fine with the standard, but it also means that to represent all
+characters from Unicode and @w{ISO 10646} one has to use UTF-16 surrogate
+characters, which is in fact a multi-wide-character encoding. But
+resorting to multi-wide-character encoding contradicts the purpose of the
@code{wchar_t} type.
@end deftp
@comment wchar.h
@comment ISO
@deftp {Data type} wint_t
-@code{wint_t} is a data type used for parameters and variables 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
+@code{wint_t} is a data type used for parameters and variables that
+contain a single wide character. As the name suggests this type is the
+equivalent of @code{int} when using the normal @code{char} strings. The
+types @code{wchar_t} and @code{wint_t} often have the same
+representation if their size is 32 bits wide but if @code{wchar_t} is
defined as @code{char} the type @code{wint_t} must be defined as
@code{int} due to the parameter promotion.
@pindex wchar.h
-This type is defined in @file{wchar.h} and got introduced in the second
-amendment to @w{ISO C90}.
+This type is defined in @file{wchar.h} and was introduced in
+@w{Amendment 1} to @w{ISO C90}.
@end deftp
-As there are for the @code{char} data type there also exist macros
+As there are for the @code{char} data type macros are available for
specifying the minimum and maximum value representable in an object of
type @code{wchar_t}.
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 C90}.
+This macro was introduced in @w{Amendment 1} to @w{ISO C90}.
@end deftypevr
@comment wchar.h
@comment ISO
@deftypevr Macro wint_t WCHAR_MAX
-The macro @code{WCHAR_MIN} evaluates to the maximum value representable
+The macro @code{WCHAR_MAX} 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 C90}.
+This macro was introduced in @w{Amendment 1} to @w{ISO C90}.
@end deftypevr
Another special wide character value is the equivalent to @code{EOF}.
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
+@code{EOF} it also need @emph{not} be negative. In other words, sloppy
+code like
@smallexample
@{
int c;
- ...
+ @dots{}
while ((c = getc (fp)) < 0)
- ...
+ @dots{}
@}
@end smallexample
@noindent
-has to be rewritten to explicitly use @code{WEOF} when wide characters
-are used.
+has to be rewritten to use @code{WEOF} explicitly when wide characters
+are used:
@smallexample
@{
wint_t c;
- ...
+ @dots{}
while ((c = wgetc (fp)) != WEOF)
- ...
+ @dots{}
@}
@end smallexample
@pindex wchar.h
-This macro was introduced in the second amendment to @w{ISO C90} and is
+This macro was introduced in @w{Amendment 1} to @w{ISO C90} and is
defined in @file{wchar.h}.
@end deftypevr
These internal representations present problems when it comes to 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.
+and transmittal. Because each single wide character consists of more
+than one byte, they are affected by byte-ordering. Thus, machines with
+different endianesses would see different values when accessing the same
+data. This byte ordering concern also applies for communication protocols
+that are all byte-based and therefore require that the sender has to
+decide about splitting the wide character in bytes. A last (but not least
+important) point is that wide characters often require more storage space
+than a customized byte-oriented character set.
@cindex multibyte character
@cindex EBCDIC
- For all the above reasons, an external encoding which is different
-from the internal encoding is often used if the latter is UCS2 or UCS4.
+For all the above reasons, an external encoding that is different from
+the internal encoding is often used if the latter is UCS-2 or UCS-4.
The external encoding is byte-based and can be chosen appropriately for
-the environment and for the texts to be handled. 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.
+the environment and for the texts to be handled. A variety of different
+character sets can be used for this external encoding (information that
+will not be exhaustively presented here--instead, a description of the
+major groups will suffice). All of the ASCII-based character sets
+fulfill one requirement: they are "filesystem safe." This means that
+the character @code{'/'} is used in the encoding @emph{only} to
+represent itself. Things are a bit different for character sets like
+EBCDIC (Extended Binary Coded Decimal Interchange Code, a character set
+family used by IBM), but if the operating system does not understand
+EBCDIC directly the parameters-to-system calls have to be converted
+first anyhow.
@itemize @bullet
@item
-The simplest character sets are 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).
+The simplest character sets are single-byte character sets. There can
+be only up to 256 characters (for @w{8 bit} character sets), which is
+not sufficient to cover all languages but might be sufficient to handle
+a specific text. Handling of a @w{8 bit} character sets is simple. This
+is not true for other kinds presented later, and therefore, the
+application one uses might require the use of @w{8 bit} character sets.
@cindex ISO 2022
@item
The @w{ISO 2022} standard defines a mechanism for extended character
sets where one character @emph{can} be represented by more than one
-byte. This is achieved by associating a state with the text. 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
+byte. This is achieved by associating a state with the text.
+Characters that can be used to change the state can be embedded in the
+text. Each byte in the text might have a different interpretation in each
state. The state might even influence whether a given byte stands for a
character on its own or whether it has to be combined with some more
bytes.
@cindex EUC
+@cindex Shift_JIS
@cindex SJIS
In most uses of @w{ISO 2022} the defined character sets do not allow
-state changes which cover more than the next character. This has the
+state changes that cover more than the next character. This has the
big advantage that whenever one can identify the beginning of the byte
sequence of a character one can interpret a text correctly. Examples of
character sets using this policy are the various EUC character sets
-(used by Sun's operations systems, EUC-JP, EUC-KR, EUC-TW, and EUC-CN)
-or SJIS (Shift JIS, a Japanese encoding).
+(used by Sun's operating systems, EUC-JP, EUC-KR, EUC-TW, and EUC-CN)
+or Shift_JIS (SJIS, a Japanese encoding).
-But there are also character sets using a state which is valid for more
+But there are also character sets using a state that is valid for more
than one character and has to be changed by another byte sequence.
Examples for this are ISO-2022-JP, ISO-2022-KR, and ISO-2022-CN.
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).
+desired result. For example, the byte sequence @code{0xc2 0x61}
+(non-spacing acute accent, followed by lower-case `a') to get the ``small
+a with acute'' character. To get the acute accent character on its own,
+one has to write @code{0xc2 0x20} (the non-spacing acute followed by a
+space).
-This type of characters sets is quite frequently used in embedded
-systems such as video text.
+Character sets like @w{ISO 6937} are used in some embedded systems such
+as teletex.
@item
@cindex UTF-8
-Instead of converting the Unicode or @w{ISO 10646} text used internally
+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
+UCS-2/UCS-4. The Unicode and @w{ISO 10646} standards even specify such an
encoding: UTF-8. This encoding is able to represent all of @w{ISO
-10464} 31 bits in a byte string of length one to seven.
+10646} 31 bits in a byte string of length one to six.
@cindex UTF-7
-There were a few other attempts to encode @w{ISO 10646} such as UTF-7
-but UTF-8 is today the only encoding 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
+There were a few other attempts to encode @w{ISO 10646} such as UTF-7,
+but UTF-8 is today the only encoding that should be used. In fact, with
+any luck UTF-8 will soon be the only external encoding that has to be
+supported. It proves to be universally usable and its only disadvantage
+is that it favors Roman languages by making the byte string
representation of other scripts (Cyrillic, Greek, Asian scripts) longer
than necessary if using a specific character set for these scripts.
Methods like the Unicode compression scheme can alleviate these
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
+works with use. If there are no constraints, the selection is based on
+the requirements the expected circle of users will have. In other words,
+if a project is expected to be used in only, say, Russia it is fine to use
KOI8-R or a similar character set. But if at the same time people from,
-say, Greece are participating one should use a character set which allows
+say, Greece are participating one should use a character set that allows
all people to collaborate.
The most widely useful solution seems to be: go with the most general
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.
+is using Unicode or @w{ISO 10646}. This is not required, but at least
+encouraged, by the @w{ISO C} standard. The standard defines at least a
+macro @code{__STDC_ISO_10646__} that is only defined on systems where
+the @code{wchar_t} type encodes @w{ISO 10646} characters. If this
+symbol is not defined one should avoid making assumptions about the wide
+character representation. If the programmer uses only the functions
+provided by the C library to handle wide character strings there should
+be no compatibility problems with other systems.
@node Charset Function Overview
@section Overview about Character Handling Functions
A Unix @w{C library} contains three different sets of functions in two
-families to handle character set conversion. 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 C90}
-standard, is unfortunately the least useful one. In fact, these
-functions should be avoided whenever possible, especially when
-developing libraries (as opposed to applications).
+families to handle character set conversion. One of the function families
+(the most commonly used) is specified in the @w{ISO C90} standard and,
+therefore, is portable even beyond the Unix world. Unfortunately this
+family is the least useful one. These functions should be avoided
+whenever possible, especially when developing libraries (as opposed to
+applications).
The second family of functions got introduced in the early Unix standards
(XPG2) and is still part of the latest and greatest Unix standard:
@w{Unix 98}. It is also the most powerful and useful set of functions.
-But we will start with the functions defined in the second amendment to
+But we will start with the functions defined in @w{Amendment 1} to
@w{ISO C90}.
@node Restartable multibyte conversion
@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.
+terminated strings as the argument (i.e., converting blocks of text
+does not work unless one can add a NUL byte at an appropriate place).
+@Theglibc{} contains some extensions to the standard that allow
+specifying a size, but basically they also expect terminated strings.
@end itemize
-Despite these limitations the @w{ISO C} functions can 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
+Despite these limitations the @w{ISO C} functions can be used in many
+contexts. In graphical user interfaces, for instance, it is not
+uncommon to have functions that require text to be displayed in a wide
+character string if the text is not simple ASCII. The text itself might
+come from a file with translations and the user should decide about the
+current locale, which determines the translation and therefore also the
external encoding used. In such a situation (and many others) the
functions described here are perfect. If more freedom while performing
the conversion is necessary take a look at the @code{iconv} functions
@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
+@code{LC_CTYPE} category decides about the conversion that is performed
by the functions we are about to describe. Each locale uses its own
character set (given as an argument to @code{localedef}) and this is the
one assumed as the external multibyte encoding. The wide character
-character set always is UCS4, at least on GNU systems.
+set is always UCS-4 in @theglibc{}.
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.
+of bytes that can be necessary to represent one character. This
+information is quite important when writing code that uses the
+conversion functions (as shown in the examples below).
+The @w{ISO C} standard defines two macros that provide this information.
@comment limits.h
@comment ISO
@deftypevr Macro int MB_LEN_MAX
-This macro specifies the maximum number of bytes in the multibyte
+@code{MB_LEN_MAX} specifies the maximum number of bytes in the multibyte
sequence for a single character in any of the supported locales. It is
-a compile-time constant and it is defined in @file{limits.h}.
+a compile-time constant and is defined in @file{limits.h}.
@pindex limits.h
@end deftypevr
@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.
+@code{MB_LEN_MAX} this macro need not be a compile-time constant, and in
+@theglibc{} it is not.
@pindex stdlib.h
@code{MB_CUR_MAX} is defined in @file{stdlib.h}.
@end deftypevr
Two different macros are necessary since strictly @w{ISO C90} compilers
-do not allow variable length array definitions but still it is desirable
+do not allow variable length array definitions, but still it is desirable
to avoid dynamic allocation. This incomplete piece of code shows the
problem:
while (! feof (fp))
@{
fread (&buf[len], 1, MB_CUR_MAX - len, fp);
- /* @r{... process} buf */
+ /* @r{@dots{} process} buf */
len -= used;
@}
@}
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
+variable size. The @code{fread} call makes sure that @code{MB_CUR_MAX}
+bytes are always available in @var{buf}. Note that it isn't
a problem if @code{MB_CUR_MAX} is not a compile-time constant.
@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.
+sets use a @dfn{stateful} encoding. That is, the encoded values depend
+in some way on the previous bytes in the text.
Since the conversion functions allow converting a text in more than one
step we must have a way to pass this information from one call of the
function to another.
@pindex wchar.h
-This type is defined in @file{wchar.h}. It got introduced in the second
-amendment to @w{ISO C90}.
+@code{mbstate_t} is defined in @file{wchar.h}. It was introduced in
+@w{Amendment 1} to @w{ISO C90}.
@end deftp
-To use objects of 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.
+To use objects of type @code{mbstate_t} the programmer has to define such
+objects (normally as local variables on the stack) and pass a pointer to
+the object to the conversion functions. This way the conversion function
+can update the object if the current multibyte character set is stateful.
There is no specific function or initializer to put the state object in
any specific state. The rules are that the object should always
-represent the initial state before the first use and this is achieved by
+represent the initial state before the first use, and this is achieved by
clearing the whole variable with code such as follows:
@smallexample
mbstate_t state;
memset (&state, '\0', sizeof (state));
/* @r{from now on @var{state} can be used.} */
- ...
+ @dots{}
@}
@end smallexample
When using the conversion functions to generate output it is often
necessary to test whether the current state corresponds to the initial
-state. This is necessary, for example, to decide whether or not to emit
+state. This is necessary, for example, to decide whether to emit
escape sequences to set the state to the initial state at certain
sequence points. Communication protocols often require this.
@comment wchar.h
@comment ISO
@deftypefun int mbsinit (const mbstate_t *@var{ps})
-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
+The @code{mbsinit} function determines whether the state object pointed
+to by @var{ps} is in the initial state. If @var{ps} is a null pointer or
+the object is in the initial state the return value is nonzero. Otherwise
it is zero.
@pindex wchar.h
-This function was introduced in the second amendment to @w{ISO C90} and
-is declared in @file{wchar.h}.
+@code{mbsinit} was introduced in @w{Amendment 1} to @w{ISO C90} and is
+declared in @file{wchar.h}.
@end deftypefun
-Code using this function often looks similar to this:
+Code using @code{mbsinit} often looks similar to this:
@c Fix the example to explicitly say how to generate the escape sequence
@c to restore the initial state.
mbstate_t state;
memset (&state, '\0', sizeof (state));
/* @r{Use @var{state}.} */
- ...
+ @dots{}
if (! mbsinit (&state))
@{
/* @r{Emit code to return to initial state.} */
const wchar_t *srcp = empty;
wcsrtombs (outbuf, &srcp, outbuflen, &state);
@}
- ...
+ @dots{}
@}
@end smallexample
The code to emit the escape sequence to get back to the initial state is
interesting. The @code{wcsrtombs} function can be used to determine the
-necessary output code (@pxref{Converting Strings}). Please note that 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
+necessary output code (@pxref{Converting Strings}). Please note that with
+@theglibc{} it is not necessary to perform this extra action for the
+conversion from multibyte text to wide character text since the wide
character encoding is not stateful. But there is nothing mentioned in
-any standard which prohibits making @code{wchar_t} using a stateful
+any standard that prohibits making @code{wchar_t} using a stateful
encoding.
@node Converting a Character
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}.
+character set that consists of single byte sequences, there are
+functions to help with converting bytes. Frequently, ASCII is a subpart
+of the multibyte character set. In such a scenario, each ASCII character
+stands for itself, and all other characters have at least a first byte
+that is beyond the range @math{0} to @math{127}.
@comment wchar.h
@comment ISO
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}.
+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
+the initial shift state. No @code{mbstate_t} object is used from
+which the state information is taken, and the function also does not use
any static state.
@pindex wchar.h
-This function was introduced in the second amendment of @w{ISO C90} and
-is declared in @file{wchar.h}.
+The @code{btowc} function was introduced in @w{Amendment 1} to @w{ISO C90}
+and is declared in @file{wchar.h}.
@end deftypefun
-Despite the limitation that the single byte value always is interpreted
-in the initial state this function is actually useful most of the time.
+Despite the limitation that the single byte value is always interpreted
+in the initial state, this function is actually useful most of the time.
Most characters are either entirely single-byte character sets or they
are extension to ASCII. But then it is possible to write code like this
(not that this specific example is very useful):
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}.
+this, using @code{btowc} is required.
@noindent
-There also is a function for the conversion in the other direction.
+There is also 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
+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 C90} and
+@code{wctob} was introduced in @w{Amendment 1} to @w{ISO C90} and
is declared in @file{wchar.h}.
@end deftypefun
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 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
+object pointed to by @var{ps}. If @var{ps} is a null pointer, a static,
+internal state variable used only by the @code{mbrtowc} function is
used.
-If the next multibyte character corresponds to the NUL wide character
+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
+form a correct multibyte character, the return value is the number of
+bytes starting from @var{s} that form the multibyte character. The
conversion state is updated according to the bytes consumed in the
conversion. In both cases the wide character (either the @code{L'\0'}
-or the one found in the conversion) is stored in the string pointer to
-by @var{pwc} iff @var{pwc} is not null.
+or the one found in the conversion) is stored in the string pointed to
+by @var{pwc} if @var{pwc} is not null.
If the first @var{n} bytes of the multibyte string possibly form a valid
multibyte character but there are more than @var{n} bytes needed to
-complete it the return value of the function is @code{(size_t) -2} and
+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.
+has a value greater than or equal to @code{MB_CUR_MAX} since the input
+might contain redundant shift sequences.
If the first @code{n} bytes of the multibyte string cannot possibly form
-a valid multibyte character also no value is stored, the global variable
-@code{errno} is set to the value @code{EILSEQ} and the function returns
+a valid multibyte character, no value is stored, the global variable
+@code{errno} is set to the value @code{EILSEQ}, and the function returns
@code{(size_t) -1}. The conversion state is afterwards undefined.
@pindex wchar.h
-This function was introduced in the second amendment to @w{ISO C90} and
+@code{mbrtowc} was introduced in @w{Amendment 1} to @w{ISO C90} and
is declared in @file{wchar.h}.
@end deftypefun
-Using this function is straight forward. A function which copies a
+Use of @code{mbrtowc} is straightforward. A function that copies a
multibyte string into a wide character string while at the same time
-converting all lowercase character into uppercase could look like this
+converting all lowercase characters into uppercase could look like this
(this is not the final version, just an example; it has no error
-checking, and leaks sometimes memory):
+checking, and sometimes leaks memory):
@smallexample
wchar_t *
wchar_t *wcp = result;
wchar_t tmp[1];
mbstate_t state;
- memset (&state, '\0', sizeof (state));
size_t nbytes;
+
+ memset (&state, '\0', sizeof (state));
while ((nbytes = mbrtowc (tmp, s, len, &state)) > 0)
@{
if (nbytes >= (size_t) -2)
/* Invalid input string. */
return NULL;
- *result++ = towupper (tmp[0]);
+ *wcp++ = towupper (tmp[0]);
len -= nbytes;
s += nbytes;
@}
@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.
+stored in @code{@var{tmp}[0]}, and the number of consumed bytes is stored
+in the variable @var{nbytes}. If the conversion is successful, the
+uppercase variant of the wide character is stored in the @var{result}
+array and the pointer to the input string and the number of available
+bytes is adjusted.
+
+The only non-obvious thing about @code{mbrtowc} might be the way memory
+is allocated for the result. The above code uses the fact that there
+can never be more wide characters in the converted results than there are
+bytes in the multibyte input string. This method yields a pessimistic
+guess about the size of the result, and if many wide character strings
+have to be constructed this way or if the strings are long, the extra
+memory required to be allocated because the input string contains
+multibyte characters might be significant. The allocated memory block can
+be resized to the correct size before returning it, but a better solution
+might be to allocate just the right amount of space for the result right
+away. Unfortunately there is no function to compute the length of the wide
+character string directly from the multibyte string. There is, however, a
+function that does part of the work.
@comment wchar.h
@comment ISO
@deftypefun size_t mbrlen (const char *restrict @var{s}, size_t @var{n}, mbstate_t *@var{ps})
The @code{mbrlen} function (``multibyte restartable length'') computes
-the number of at most @var{n} bytes starting at @var{s} which form the
+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
+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
+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}.
+If the first @var{n} bytes possibly form a valid multibyte
+character but the character is incomplete, the return value is
+@code{(size_t) -2}. Otherwise the multibyte character sequence is invalid
+and the return value is @code{(size_t) -1}.
The multibyte sequence is interpreted in the state represented by the
-object pointer to by @var{ps}. If @var{ps} is a null pointer an state
+object pointed to by @var{ps}. If @var{ps} is a null pointer, a state
object local to @code{mbrlen} is used.
@pindex wchar.h
-This function was introduced in the second amendment to @w{ISO C90} and
+@code{mbrlen} was introduced in @w{Amendment 1} to @w{ISO C90} and
is declared in @file{wchar.h}.
@end deftypefun
-The tentative reader now will of course note that @code{mbrlen} can be
-implemented as
+The attentive reader now will note that @code{mbrlen} can be implemented
+as
@smallexample
mbrtowc (NULL, s, n, ps != NULL ? ps : &internal)
@end smallexample
This is true and in fact is mentioned in the official specification.
-Now, how can this function be used to determine the length of the wide
+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:
+directly usable, but we can define a function @code{mbslen} using it:
@smallexample
size_t
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.
+call. This is acceptable since a) this value is larger than the length of
+the longest multibyte character sequence and b) we know that the string
+@var{s} ends with a NUL byte, which cannot be part of any other multibyte
+character sequence but the one representing the NUL wide character.
+Therefore, the @code{mbrlen} function will never read invalid memory.
Now that this function is available (just to make this clear, this
-function is @emph{not} part of the GNU C library) we can compute the
+function is @emph{not} part of @theglibc{}) we can compute the
number of wide character required to store the converted multibyte
character string @var{s} using
@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.
+implementation of @code{mbstouwcs} with @code{mbslen} would have to
+perform the conversion of the multibyte character input string twice, and
+this conversion might be quite expensive. So it is necessary to think
+about the consequences of using the easier but imprecise method before
+doing the work twice.
@comment wchar.h
@comment ISO
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}
+If @var{s} is a null pointer, the function resets the state stored in
+the objects pointed to by @var{ps} (or the internal @code{mbstate_t}
object) to the initial state. This can also be achieved by a call like
this:
@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.
+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
+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}.
+state followed by a single NUL byte, which is stored in the string
+@var{s}.
-Otherwise a byte sequence (possibly including shift sequences) is
-written into the string @var{s}. This 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}.
+Otherwise a byte sequence (possibly including shift sequences) is written
+into the string @var{s}. This only happens if @var{wc} is a valid wide
+character (i.e., it has a multibyte representation in the character set
+selected by locale of the @code{LC_CTYPE} category). If @var{wc} is no
+valid wide character, nothing is stored in the strings @var{s},
+@code{errno} is set to @code{EILSEQ}, the conversion state in @var{ps}
+is undefined and the return value is @code{(size_t) -1}.
If no error occurred the function returns the number of bytes stored in
-the string @var{s}. This includes all byte representing shift
+the string @var{s}. This includes all bytes representing shift
sequences.
One word about the interface of the function: there is no parameter
available, otherwise buffer overruns can occur.
@pindex wchar.h
-This function was introduced in the second amendment to @w{ISO C} and is
+@code{wcrtomb} was introduced in @w{Amendment 1} to @w{ISO C90} and is
declared in @file{wchar.h}.
@end deftypefun
-Using this function is as easy as using @code{mbrtowc}. The following
+Using @code{wcrtomb} is as easy as using @code{mbrtowc}. The following
example appends a wide character string to a multibyte character string.
-Again, the code is not really useful (and correct), it is simply here to
+Again, the code is not really useful (or correct), it is simply here to
demonstrate the use and some problems.
@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
+is never used except to represent itself (and in this context, the end
of the string).
After initializing the state object the loop is entered where the first
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.
+returns it is too late to decide whether the buffer was large enough. If
+this solution is unsuitable, there is a very slow but more accurate
+solution.
@smallexample
- ...
+ @dots{}
if (len < MB_CUR_LEN)
@{
mbstate_t temp_state;
return NULL;
@}
@}
- ...
+ @dots{}
@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.
+Here we perform the conversion that might overflow the buffer so that
+we are afterwards in the position to make an exact decision about the
+buffer size. Please note the @code{NULL} argument for the destination
+buffer in the new @code{wcrtomb} call; since we are not interested in the
+converted text at this point, this is a nice way to express this. The
+most unusual thing about this piece of code certainly is the duplication
+of the conversion state object, but if a change of the state is necessary
+to emit the next multibyte character, we want to have the same shift state
+change performed in the real conversion. Therefore, we have to preserve
+the initial shift state information.
There are certainly many more and even better solutions to this problem.
-This example is only meant for educational purposes.
+This example is only provided for educational purposes.
@node Converting Strings
@subsection Converting Multibyte and Wide Character Strings
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.
+functions is quite limited; therefore, @theglibc{} contains a few
+extensions that can help in some important situations.
@comment wchar.h
@comment ISO
@deftypefun size_t mbsrtowcs (wchar_t *restrict @var{dst}, const char **restrict @var{src}, size_t @var{len}, mbstate_t *restrict @var{ps})
The @code{mbsrtowcs} function (``multibyte string restartable to wide
-character string'') converts an NUL terminated multibyte character
+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
+pointer. Before returning, the state object is updated to match the state
+after the last converted character. The state is the initial state if the
terminating NUL byte is reached and converted.
-If @var{dst} is not a null pointer the result is stored in the array
-pointed to by @var{dst}, otherwise the conversion result is not
+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.
+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
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 is not updated if DST is NULL. This is not said straightforward and
@c none of the other functions is described like this. It would make sense
@c to define the function this way but I don't think it is meant like this.
In all other cases the function returns the number of wide characters
-converted during this call. If @var{dst} is not null @code{mbsrtowcs}
-stores in the pointer pointed to by @var{src} a null pointer (if the NUL
-byte in the input string was reached) or the address of the byte
+converted during this call. If @var{dst} is not null, @code{mbsrtowcs}
+stores in the pointer pointed to by @var{src} either a null pointer (if
+the NUL byte in the input string was reached) or the address of the byte
following the last converted multibyte character.
@pindex wchar.h
-This function was introduced in the second amendment to @w{ISO C} and is
+@code{mbsrtowcs} was introduced in @w{Amendment 1} to @w{ISO C90} 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
+The definition of the @code{mbsrtowcs} function has one important
+limitation. The requirement that @var{dst} has to be a NUL-terminated
string provides problems if one wants to convert buffers with text. A
-buffer is normally no collection of NUL terminated strings but instead 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
+assume that a function to convert one line from a buffer is needed. Since
+the line is not NUL-terminated, the source pointer cannot directly point
into the unmodified text buffer. This means, either one inserts the NUL
byte at the appropriate place for the time of the @code{mbsrtowcs}
function call (which is not doable for a read-only buffer or in a
multi-threaded application) or one copies the line in an extra buffer
-where it can be terminated by a NUL byte. Note that it is not in
-general possible to limit the number of characters to convert by setting
-the parameter @var{len} to any specific value. Since it is not known
-how many bytes each multibyte character sequence is in length one always
-could do only a guess.
+where it can be terminated by a NUL byte. Note that it is not in general
+possible to limit the number of characters to convert by setting the
+parameter @var{len} to any specific value. Since it is not known how
+many bytes each multibyte character sequence is in length, one can only
+guess.
@cindex stateful
There is still a problem with the method of NUL-terminating a line right
-after the newline character which could lead to very strange results.
-As said in the description of the @var{mbsrtowcs} function above the
+after the newline character, which could lead to very strange results.
+As said in the description of the @code{mbsrtowcs} function above the
conversion state is guaranteed to be in the initial shift state after
processing the NUL byte at the end of the input string. But this NUL
-byte is not really part of the text. I.e., the conversion state after
+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
+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.
+require that the shift state after a newline be the initial state--but
+this is not a strict guarantee. Therefore, simply NUL-terminating a
+piece of a running text is not always an adequate solution and,
+therefore, should never be used in generally used code.
The generic conversion interface (@pxref{Generic Charset Conversion})
does not have this limitation (it simply works on buffers, not
-strings), and the GNU C library contains a set of functions which take
-additional parameters specifying the maximal number of bytes which are
+strings), and @theglibc{} contains a set of functions that take
+additional parameters specifying the maximal number of bytes that are
consumed from the input string. This way the problem of
@code{mbsrtowcs}'s example above could be solved by determining the line
length and passing this length to the function.
@comment 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
+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
+@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
+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
+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
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.
+Except in the case of an encoding error the return value of the
+@code{wcsrtombs} function is the number of bytes in all the multibyte
+character sequences stored in @var{dst}. Before returning the state in
+the object pointed to by @var{ps} (or the internal object in case
+@var{ps} is a null pointer) is updated to reflect the state after the
+last conversion. The state is the initial shift state in case the
+terminating NUL wide character was converted.
@pindex wchar.h
-This function was introduced in the second amendment to @w{ISO C} and is
-declared in @file{wchar.h}.
+The @code{wcsrtombs} function was introduced in @w{Amendment 1} to
+@w{ISO C90} and is declared in @file{wchar.h}.
@end deftypefun
-The restriction 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).
+The restriction mentioned above for the @code{mbsrtowcs} function applies
+here also. There is no possibility of directly controlling the number of
+input characters. One has to place the NUL wide character at the correct
+place or control the consumed input indirectly via the available output
+array size (the @var{len} parameter).
@comment wchar.h
@comment GNU
@deftypefun size_t mbsnrtowcs (wchar_t *restrict @var{dst}, const char **restrict @var{src}, size_t @var{nmc}, size_t @var{len}, mbstate_t *restrict @var{ps})
The @code{mbsnrtowcs} function is very similar to the @code{mbsrtowcs}
-function. All the parameters are the same except for @var{nmc} which is
+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
+multibyte character string. In other words, the multibyte character
+string @code{*@var{src}} need not be NUL-terminated. But if a NUL byte
+is found within the @var{nmc} first bytes of the string, the conversion
stops 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
+This function is a GNU extension. It is meant to work around the
+problems mentioned above. Now it is possible to convert a buffer with
multibyte character text piece for piece without having to care about
inserting NUL bytes and the effect of NUL bytes on the conversion state.
@end deftypefun
@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
+Since we don't insert characters in the strings that were not in there
right from the beginning and we use @var{state} only for the conversion
-of the given buffer there is no problem with altering the state.
+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.
+@code{wcsrtombs} but, just like @code{mbsnrtowcs}, it takes an extra
+parameter, which specifies the length of the input string.
No more than @var{nwc} wide characters from the input string
@code{*@var{src}} are converted. If the input string contains a NUL
-wide character in the first @var{nwc} character to conversion stops at
+wide character in the first @var{nwc} characters, the 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.
+The @code{wcsnrtombs} function is a GNU extension and just like
+@code{mbsnrtowcs} helps in situations where no NUL-terminated input
+strings are available.
@end deftypefun
@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
+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.
/* @r{If any characters must be carried forward,}
@r{put them at the beginning of @code{buffer}.} */
if (filled > 0)
- memmove (inp, buffer, filled);
+ memmove (buffer, inp, filled);
@}
return 1;
@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 C90}. But the original @w{ISO C90} 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
-C90} 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
+The functions described in the previous chapter are defined in
+@w{Amendment 1} to @w{ISO C90}, but the original @w{ISO C90} standard
+also contained functions for character set conversion. The reason that
+these original functions are not described first is that they are almost
+entirely useless.
+
+The problem is that all the conversion functions described in the
+original @w{ISO C90} use a local state. Using a local state implies that
+multiple conversions at the same time (not only when using threads)
+cannot be done, and that you cannot first convert single characters and
+then strings since you cannot tell the conversion functions which state
+to use.
+
+These original functions are therefore usable only in a very limited set
+of situations. One must complete converting the entire string before
+starting a new one, and each string/text must be converted with the same
function (there is no problem with the library itself; it is guaranteed
that no library function changes the state of any of these functions).
@strong{For the above reasons it is highly requested that the functions
-from the last section are used in place of non-reentrant conversion
-functions.}
+described in the previous section be used in place of non-reentrant
+conversion functions.}
@menu
* Non-reentrant Character Conversion:: Non-reentrant Conversion of Single
@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
+valid multibyte characters, they start with an invalid byte sequence or
just part of a character, or @var{string} points to an empty string (a
null character).
For a valid multibyte character, @code{mbtowc} converts it to a wide
character and stores that in @code{*@var{result}}, and returns the
-number of bytes in that character (always at least @math{1}, and never
+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
@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'}.
+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
+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
@code{'\0'} and returning @math{0}.
@end deftypefun
-Similar to @code{mbrlen} there is also a non-reentrant function which
+Similar to @code{mbrlen} there is also a non-reentrant function that
computes the length of a multibyte character. It can be defined in
terms of @code{mbtowc}.
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
+characters, they start with an invalid byte sequence or just part of a
character, or @var{string} points to an empty string (a null character).
For a valid multibyte character, @code{mblen} returns the number of
-bytes in that character (always at least @code{1}, and never more than
+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}.
@node Non-reentrant String Conversion
@subsection Non-reentrant Conversion of Strings
-For convenience reasons the @w{ISO C90} 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 C90}; see @ref{Converting Strings}.
+For convenience the @w{ISO C90} standard also defines functions to
+convert entire strings instead of single characters. These functions
+suffer from the same problems as their reentrant counterparts from
+@w{Amendment 1} to @w{ISO C90}; see @ref{Converting Strings}.
@comment stdlib.h
@comment ISO
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}.
+If an invalid multibyte character sequence is found, the @code{mbstowcs}
+function returns a value of @math{-1}. Otherwise, it returns the number
+of wide characters stored in the array @var{wstring}. This number does
+not include the terminating null character, which is present if the
+number is less than @var{size}.
Here is an example showing how to convert a string of multibyte
characters, allocating enough space for the result.
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}.
+found, the @code{wcstombs} function returns a value of @math{-1}.
+Otherwise, the return value is the number of bytes stored in the array
+@var{string}. This number does not include the terminating null character,
+which is present if the number is less than @var{size}.
@end deftypefun
@node Shift State
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
+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.
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''
+multibyte code that has two alternative shift states (``Japanese mode''
and ``Latin-1 mode''), and two shift sequences that specify particular
shift states.
When the multibyte character code in use has shift states, then
-@code{mblen}, @code{mbtowc} and @code{wctomb} must maintain and update
+@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:
@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
+common that they operate on character sets that are not directly
specified by the functions. The multibyte encoding used is specified by
the currently selected locale for the @code{LC_CTYPE} category. The
-wide character set is fixed by the implementation (in the case of GNU C
-library it always is UCS4 encoded @w{ISO 10646}.
+wide character set is fixed by the implementation (in the case of @theglibc{}
+it is always UCS-4 encoded @w{ISO 10646}.
This has of course several problems when it comes to general character
conversion:
@itemize @bullet
@item
-For every conversion where neither the source 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}.
+For every conversion where neither the source nor the destination
+character set is the character set of the locale for the @code{LC_CTYPE}
+category, one has to change the @code{LC_CTYPE} locale using
+@code{setlocale}.
-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.
+Changing the @code{LC_CTYPE} locale introduces major problems for the rest
+of the programs since several more functions (e.g., the character
+classification functions, @pxref{Classification of Characters}) use the
+@code{LC_CTYPE} category.
@item
Parallel conversions to and from different character sets are not
@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,
+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
+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
the steady changing of the locale.
@end itemize
-
-The XPG2 standard defines a completely new set of functions which has
+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.
+locales, and they have no constraints on the character sets selected for
+source and destination. Only the set of available conversions limits
+them. The standard does not specify that any conversion at all must be
+available. Such availability is a measure of the quality of the
+implementation.
-In the following text first the interface to @code{iconv}, the
+In the following text first the interface to @code{iconv} and then the
conversion function, will be described. Comparisons with other
-implementations will show what 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.
+implementations will show what obstacles stand in the way of portable
+applications. Finally, the implementation is described in so far as might
+interest the advanced user who wants to extend conversion capabilities.
@menu
* Generic Conversion Interface:: Generic Character Set Conversion Interface.
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.
+which implements 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
+data type. Just like other open--use--close interfaces the functions
+introduced here work using handles and the @file{iconv.h} header
defines a special type for the handles used.
@comment iconv.h
@comment XPG2
@deftp {Data Type} iconv_t
This data type is an abstract type defined in @file{iconv.h}. The user
-must not assume anything about the definition of this type, it must be
+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 @code{iconv} functions. The objects themselves need not be freed, but
the conversions for which the handles stand for have to.
@end deftp
@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
+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:
+If the wanted conversion is not available, the @code{iconv_open} function
+returns @code{(iconv_t) -1}. In this case the global variable
+@code{errno} can have the following values:
@table @code
@item EMFILE
@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.
+perform independent conversions. The data structures associated
+with the descriptor include information about the conversion state.
This must not be messed up by using it in different conversions.
An @code{iconv} descriptor is like a file descriptor as for every use a
new descriptor must be created. The descriptor does not stand for all
of the conversions from @var{fromset} to @var{toset}.
-The GNU C library implementation of @code{iconv_open} has one
+The @glibcadj{} implementation of @code{iconv_open} has one
significant extension to other implementations. To ease the extension
-of the set of available conversions the implementation allows to store
-the necessary files with data and code in arbitrary many directories.
-How this extensions have to be written will be explained below
+of the set of available conversions, the implementation allows storing
+the necessary files with data and code in an arbitrary number of
+directories. How this extension must be written will be explained below
(@pxref{glibc iconv Implementation}). Here it is only important to say
that all directories mentioned in the @code{GCONV_PATH} environment
-variable are considered if they contain a file @file{gconv-modules}.
+variable are considered only if they contain a file @file{gconv-modules}.
These directories need not necessarily be created by the system
administrator. In fact, this extension is introduced to help users
-writing and using own, new conversions. Of course this does not work
-for security reasons in SUID binaries; in this case only the system
+writing and using their own, new conversions. Of course, this does not
+work for security reasons in SUID binaries; in this case only the system
directory is considered and this normally is
@file{@var{prefix}/lib/gconv}. The @code{GCONV_PATH} environment
variable is examined exactly once at the first call of the
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}.
+The @code{iconv_open} function was introduced early in the X/Open
+Portability Guide, @w{version 2}. It is supported by all commercial
+Unices as it is required for the Unix branding. However, the quality and
+completeness of the implementation varies widely. The @code{iconv_open}
+function is declared in @file{iconv.h}.
@end deftypefun
The @code{iconv} implementation can associate large data structure with
-the handle returned by @code{iconv_open}. Therefore it is crucial to
+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 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
+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}.
@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}.
+The @code{iconv_close} function was introduced together with the rest
+of the @code{iconv} functions in XPG2 and is declared in @file{iconv.h}.
@end deftypefun
-The standard defines only one actual conversion function. This has
-therefore the most general interface: it allows conversion from one
+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})
+@deftypefun size_t iconv (iconv_t @var{cd}, 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
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
+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
+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
+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
+any assumption as to whether the conversion has to deal with states.
+Even if the input and output character sets are not stateful, the
implementation might still have to keep states. This is due to the
-implementation chosen for the GNU C library as it is described below.
+implementation chosen for @theglibc{} as it is described below.
Therefore an @code{iconv} call to reset the state should always be
performed if some protocol requires this for the output text.
-The conversion stops for three reasons. The first is that all
+The conversion stops for one of three reasons. The first is that all
characters from the input buffer are converted. This actually can mean
-two things: 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
+two things: either all bytes from the input buffer are consumed or
+there are some bytes at the end of the buffer that possibly can form a
complete character but the input is incomplete. The second reason for a
-stop is when the output buffer is full. And the third reason is that
+stop is that 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
+In all of 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{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
+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
+@emph{current} behavior of @theglibc{} in this situation is to
return with an error immediately. This certainly is not the most
-desirable solution. Therefore future versions will provide better ones
+desirable solution; therefore, future versions will provide better ones,
but they are not yet finished.
If all input from the input buffer is successfully converted and stored
-in the output buffer the function returns the number of non-reversible
+in the output buffer, the function returns the number of non-reversible
conversions performed. In all other cases the return value is
-@code{(size_t) -1} and @code{errno} is set appropriately. In this case
+@code{(size_t) -1} and @code{errno} is set appropriately. In such cases
the value pointed to by @var{inbytesleft} is nonzero.
@table @code
@item EILSEQ
The conversion stopped because of an invalid byte sequence in the input.
-After the call @code{*@var{inbuf}} points at the first byte of the
+After the call, @code{*@var{inbuf}} points at the first byte of the
invalid byte sequence.
@item E2BIG
@end table
@pindex iconv.h
-This function was introduced in the XPG2 standard and is declared in the
-@file{iconv.h} header.
+The @code{iconv} function was introduced in the XPG2 standard and is
+declared in the @file{iconv.h} header.
@end deftypefun
The definition of the @code{iconv} function is quite good overall. It
provides quite flexible functionality. The only problems lie in the
-boundary cases which are incomplete byte sequences at the end of the
+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.
-
+implementations, as demonstrated below.
@node iconv Examples
@subsection A complete @code{iconv} example
The example below features a solution for a common problem. Given that
one knows the internal encoding used by the system for @code{wchar_t}
-strings one often is in the position to read text from a file and store
-it in wide character buffers. One can do this using @code{mbsrtowcs}
+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 result = 0;
iconv_t cd;
- cd = iconv_open ("UCS4", charset);
+ cd = iconv_open ("WCHAR_T", charset);
if (cd == (iconv_t) -1)
@{
/* @r{Something went wrong.} */
if (errno == EINVAL)
- error (0, 0, "conversion from `%s' to `UCS4' no available",
+ error (0, 0, "conversion from '%s' to wchar_t not available",
charset);
else
perror ("iconv_open");
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.
+An interesting point is the case where @code{iconv} returns an error and
+@code{errno} is set to @code{EINVAL}. This is not really an error in the
+transformation. It can happen whenever the input character set contains
+byte sequences of more than one byte for some character and texts are not
+processed in one piece. In this case there is a chance that a multibyte
+sequence is cut. The caller can then simply read the remainder of the
+takes and feed the offending bytes together with new character from the
+input to @code{iconv} and continue the work. The internal state kept in
+the descriptor is @emph{not} unspecified after such an event as is the
+case with the conversion functions from the @w{ISO C} standard.
The example also shows the problem of using wide character strings with
@code{iconv}. As explained in the description of the @code{iconv}
-function above the function always takes a pointer to a @code{char}
-array and the available space is measured in bytes. In the example the
-output buffer is a wide character buffer. Therefore we use a local
-variable @var{wrptr} of type @code{char *} which is used in the
+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
+This looks rather innocent but can lead to problems on platforms that
+have tight restriction on alignment. Therefore the caller of @code{iconv}
+has to make sure that the pointers passed are suitable for access of
+characters from the appropriate character set. Since, in the
+above case, the input parameter to the function is a @code{wchar_t}
+pointer, this is the case (unless the user violates alignment when
computing the parameter). But in other situations, especially when
writing generic functions where one does not know what type of character
-set one uses and therefore treats text as a sequence of bytes, it might
+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
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
+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
+The C library contains a set of generic conversion functions that can
read the needed conversion tables and other information from data files.
These files get loaded when necessary.
This solution is problematic as it requires a great deal of effort to
apply to all character sets (potentially an infinite set). The
differences in the structure of the different character sets is so large
-that many different variants of the table processing functions must be
-developed. On top of this the generic nature of these functions make
-them slower than specifically implemented functions.
+that many different variants of the table-processing functions must be
+developed. In addition, the generic nature of these functions make them
+slower than specifically implemented functions.
@item
-The C library only contains a framework which can dynamically load
-object files and execute the therein contained conversion functions.
+The C library only contains a framework that can dynamically load
+object files and execute the conversion functions contained therein.
This solution provides much more flexibility. The C library itself
contains only very little code and therefore reduces the general memory
@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
+possibilities; the majority implement only the second solution. Using
+loadable modules moves the code out of the library itself and keeps
+the door open for extensions and improvements, but this design is also
limiting on some platforms since not many platforms support dynamic
-loading in statically linked programs. On platforms without his
+loading in statically linked programs. On platforms without this
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.
+statically linked programs. @Theglibc{} has, on ELF platforms, no
+problems with dynamic loading in these situations; therefore, this
+point is moot. The danger is that one gets acquainted with this
+situation and forgets about the restrictions on other systems.
A second thing to know about other @code{iconv} implementations is that
the number of available conversions is often very limited. Some
-implementations provide in the standard release (not special
-international or developer releases) at most 100 to 200 conversion
+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.
+supported; for example, conversions from one character set to a set of 10
+others might count as 10 conversions. Together with the other direction
+this makes 20 conversion possibilities used up by one character set. One
+can imagine the thin coverage these 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
+known Unix systems and the availability of the conversion functions from
character set @math{@cal{A}} to @math{@cal{B}} and the conversion from
@math{@cal{B}} to @math{@cal{C}} does @emph{not} imply that the
conversion from @math{@cal{A}} to @math{@cal{C}} is available.
-This might not seem unreasonable and problematic at first but it is a
+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
+the problem we assume to write a program that has to convert from
@math{@cal{A}} to @math{@cal{C}}. A call like
@smallexample
@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.
+fails according to the assumption above. But what does the program
+do now? The conversion is necessary; therefore, simply giving up is not
+an option.
This is a nuisance. The @code{iconv} function should take care of this.
-But how should the program proceed from here on? If it would try to
-convert to character set @math{@cal{B}} first the two @code{iconv_open}
+But how should the program proceed from here on? If it tries to convert
+to character set @math{@cal{B}}, first the two @code{iconv_open}
calls
@smallexample
@end smallexample
@noindent
-will succeed but how to find @math{@cal{B}}?
+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
+convert to and from UTF-8 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
+This example shows one of the design errors of @code{iconv} mentioned
+above. It should at least be possible to determine the list of available
+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
+@subsection The @code{iconv} Implementation in @theglibc{}
After reading about the problems of @code{iconv} implementations in the
last section it is certainly good to note that the implementation in
-the GNU C library has none of the problems mentioned above. What
+@theglibc{} has none of the problems mentioned above. What
follows is a step-by-step analysis of the points raised above. The
evaluation is based on the current state of the development (as of
January 1999). The development of the @code{iconv} functions is not
-complete, but basic funtionality has solidified.
+complete, but basic functionality has solidified.
-The GNU C library's @code{iconv} implementation uses shared loadable
+@Theglibc{}'s @code{iconv} implementation uses shared loadable
modules to implement the conversions. A very small number of
conversions are built into the library itself but these are only rather
trivial conversions.
-All the benefits of loadable modules are available in the GNU C library
+All the benefits of loadable modules are available in the @glibcadj{}
implementation. This is especially appealing since the interface is
-well documented (see below) and it therefore is easy to write new
+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
+problem in @theglibc{}, 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}.
+binaries, static linking need not be forbidden in case one wants to use
+@code{iconv}.
The second mentioned problem is the number of supported conversions.
-Currently, the GNU C library supports more than 150 character sets. The
+Currently, @theglibc{} supports more than 150 character sets. The
way the implementation is designed the number of supported conversions
is greater than 22350 (@math{150} times @math{149}). If any conversion
-from or to a character set is missing it can easily be added.
+from or to a character set is missing, it can be added easily.
Particularly impressive as it may be, this high number is due to the
-fact that the GNU C library implementation of @code{iconv} does not have
-the third problem mentioned above. I.e., whenever there is a conversion
+fact that the @glibcadj{} implementation of @code{iconv} does not have
+the third problem mentioned above (i.e., whenever there is a conversion
from a character set @math{@cal{A}} to @math{@cal{B}} and from
@math{@cal{B}} to @math{@cal{C}} it is always possible to convert from
-@math{@cal{A}} to @math{@cal{C}} directly. If the @code{iconv_open}
-returns an error and sets @code{errno} to @code{EINVAL} this really
-means there is no known way, directly or indirectly, to perform the
-wanted conversion.
+@math{@cal{A}} to @math{@cal{C}} directly). If the @code{iconv_open}
+returns an error and sets @code{errno} to @code{EINVAL}, there is no
+known way, directly or indirectly, to perform the wanted conversion.
@cindex triangulation
-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.
+Triangulation is achieved by providing for each character set a
+conversion from and to UCS-4 encoded @w{ISO 10646}. Using @w{ISO 10646}
+as an intermediate representation it is possible to @dfn{triangulate}
+(i.e., convert with an intermediate representation).
There is no inherent requirement to provide a conversion to @w{ISO
-10646} for a new character set and it is also possible to provide other
+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.
+10646}. The existing set of conversions is simply meant to cover all
+conversions that might be of interest.
@cindex ISO-2022-JP
@cindex EUC-JP
All currently available conversions use the triangulation method above,
-making conversion run unnecessarily slow. If, e.g., somebody often
-needs the conversion from ISO-2022-JP to EUC-JP, a quicker solution
+making conversion run unnecessarily slow. If, for example, somebody
+often needs the conversion from ISO-2022-JP to EUC-JP, a quicker solution
would involve direct conversion between the two character sets, skipping
the input to @w{ISO 10646} first. The two character sets of interest
are much more similar to each other than to @w{ISO 10646}.
-In such a situation one can easy write a new conversion and provide it
-as a better alternative. The GNU C library @code{iconv} implementation
+In such a situation one easily can write a new conversion and provide it
+as a better alternative. The @glibcadj{} @code{iconv} implementation
would automatically use the module implementing the conversion if it is
specified to be more efficient.
@subsubsection Format of @file{gconv-modules} files
All information about the available conversions comes from a file named
-@file{gconv-modules} which can be found in any of the directories along
+@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.
+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
+set. Two more words are expected on the line. The first word
+defines the alias name, and the second defines the original name of the
character set. The effect is that it is possible to use the alias name
in the @var{fromset} or @var{toset} parameters of @code{iconv_open} and
achieve the same result as when using the real character set name.
This is quite important as a character set has often many different
-names. There is normally 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.
+names. There is normally an official name but this need not correspond to
+the most popular name. Beside this many character sets have special
+names that are somehow constructed. For example, all character sets
+specified by the ISO have an alias of the form @code{ISO-IR-@var{nnn}}
+where @var{nnn} is the registration number. This allows programs that
+know about the registration number to construct character set names and
+use them in @code{iconv_open} calls. More on the available names and
+aliases follows below.
@item
Lines starting with @code{module} introduce an available conversion
module. These lines must contain three or four more words.
The first word specifies the source character set, the second word the
-destination character set of conversion implemented in this module. The
-third word is the name of the loadable module. The filename is
+destination character set of conversion implemented in this module, and
+the third word is the name of the loadable module. The filename is
constructed by appending the usual shared object suffix (normally
@file{.so}) and this file is then supposed to be found in the same
-directory the @file{gconv-modules} file is in. The last word on the
-line, which is optional, is a numeric value representing the cost of the
-conversion. If this word is missing a cost of @math{1} is assumed. The
+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
+convert from ISO-2022-JP to EUC-JP and back. All that has to be done is
+to put the new module, let its name be ISO2022JP-EUCJP.so, in a directory
and add a file @file{gconv-modules} with the following content in the
same directory:
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
+containing all the known aliases and another that contains the
information about the conversions and which shared object implements
them.
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
+constructs a list of conversions that must be performed in succession
to get the transformation from the source to the destination character
set.
Explaining why the above @file{gconv-modules} files allows the
@code{iconv} implementation to resolve the specific ISO-2022-JP to
EUC-JP conversion module instead of the conversion coming with the
-library itself is straighforward. Since the later conversion takes two
+library itself is straightforward. Since the latter conversion takes two
steps (from ISO-2022-JP to @w{ISO 10646} and then from @w{ISO 10646} to
-EUC-JP) the cost is @math{1+1 = 2}. But the above @file{gconv-modules}
-file specifies that the new conversion modules can perform this
+EUC-JP), the cost is @math{1+1 = 2}. The above @file{gconv-modules}
+file, however, specifies that the new conversion modules can perform this
conversion with only the cost of @math{1}.
-A mysterious piece about the @file{gconv-modules} file above (and also
-the file coming with the GNU C library) are the names of the character
+A mysterious item about the @file{gconv-modules} file above (and also
+the file coming with @theglibc{}) are the names of the character
sets specified in the @code{module} lines. Why do almost all the names
end in @code{//}? And this is not all: the names can actually be
-regular expressions. At this point of time this mystery should not be
+regular expressions. At this point in time this mystery should not be
revealed, unless you have the relevant spell-casting materials: ashes
from an original @w{DOS 6.2} boot disk burnt in effigy, a crucifix
blessed by St.@: Emacs, assorted herbal roots from Central America, sand
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 name 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 a 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.
+ending with @code{//}. A character set named @code{INTERNAL} is often
+mentioned. From the discussion above and the chosen name it should have
+become clear that this is the name for the representation used in the
+intermediate step of the triangulation. We have said that this is UCS-4
+but actually that is not quite right. The UCS-4 specification also
+includes the specification of the byte ordering used. Since a UCS-4 value
+consists of four bytes, a stored value is affected by byte ordering. The
+internal representation is @emph{not} the same as UCS-4 in case the byte
+ordering of the processor (or at least the running process) is not the
+same as the one required for UCS-4. This is done for performance reasons
+as one does not want to perform unnecessary byte-swapping operations if
+one is not interested in actually seeing the result in UCS-4. To avoid
+trouble with endianness, the internal representation consistently is named
+@code{INTERNAL} even on big-endian systems where the representations are
+identical.
@subsubsection @code{iconv} module data structures
-So far this section 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.
+So far this section has described how modules are located and considered
+to be used. What remains to be described is the interface of the modules
+so that one can write new ones. This section describes the interface as
+it is in use in January 1999. The interface will change a bit in the
+future but, with luck, only in an upwardly compatible way.
-The definitions necessary to write new modules are 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.
+The definitions necessary to write new modules are publicly available
+in the non-standard header @file{gconv.h}. The following text,
+therefore, describes the definitions from this header file. First,
+however, it is necessary to get an overview.
From the perspective of the user of @code{iconv} the interface is quite
-simple: the @code{iconv_open} function returns a handle which can be
-used in calls to @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
+simple: the @code{iconv_open} function returns a handle that can be used
+in calls to @code{iconv}, and finally the handle is freed with a call to
+@code{iconv_close}. The problem is that the handle has to be able to
represent the possibly long sequences of conversion steps and also the
-state of each conversion since the handle is all which is passed to the
-@code{iconv} function. Therefore the data structures are really the
-elements to understanding the implementation.
+state of each conversion since the handle is all that is passed to the
+@code{iconv} function. Therefore, the data structures are really the
+elements necessary to understanding the implementation.
We need two different kinds of data structures. The first describes the
conversion and the second describes the state etc. There are really two
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.
+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 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.
+of the other elements to be available or initialized.
@item const char *__from_name
@itemx const char *__to_name
@code{__from_name} and @code{__to_name} contain the names of the source and
destination character sets. They can be used to identify the actual
-conversion to be carried out since one module might implement
-conversions for more than one character set and/or direction.
+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 int __max_needed_from
@itemx int __min_needed_to
@itemx int __max_needed_to;
-These values have to be filled in the init function of the module. The
+These values have to be supplied in the init function of the module. The
@code{__min_needed_from} value specifies how many bytes a character of
the source character set at least needs. The @code{__max_needed_from}
-specifies the maximum value which also includes possible shift
-sequences.
+specifies the maximum value that also includes possible shift sequences.
The @code{__min_needed_to} and @code{__max_needed_to} values serve the
-same purpose but this time for the destination character set.
+same purpose as @code{__min_needed_from} and @code{__max_needed_from} but
+this time for the destination character set.
-It is crucial that these values are accurate since otherwise the
+It is crucial that these values be accurate since otherwise the
conversion functions will have problems or not work at all.
@item int __stateful
-This element must also be initialized by the init function. It is
-nonzero if the source character set is stateful. Otherwise it is zero.
+This element must also be initialized by the init function.
+@code{int __stateful} is nonzero if the source character set is stateful.
+Otherwise it is zero.
@item void *__data
This element can be used freely by the conversion functions in the
-module. 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.
+module. @code{void *__data} can be used to communicate extra information
+from one call to another. @code{void *__data} need not be initialized if
+not needed at all. If @code{void *__data} element is assigned a pointer
+to dynamically allocated memory (presumably in the init function) it has
+to be made sure that the end function deallocates the memory. Otherwise
+the application will leak memory.
It is important to be aware that this data structure is shared by all
users of this specification conversion and therefore the @code{__data}
@comment gconv.h
@comment GNU
@deftp {Data type} {struct __gconv_step_data}
-This is the data structure which contains the information specific to
+This is the data structure that contains the information specific to
each use of the conversion functions.
+
@table @code
@item char *__outbuf
@itemx char *__outbufend
These elements specify the output buffer for the conversion step. The
-@code{__outbuf} element points to the beginning of the buffer and
+@code{__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
+Once the conversion is finished, if the conversion is the last step, the
+@code{__outbuf} element must be modified to point after the last byte
written into the buffer to signal how much output is available. If this
-conversion step is not the last one the element must not be modified.
+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
@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.
+the conversion function already happened. Some character sets require a
+certain prolog when generating output, and by comparing this value with
+zero, one can find out whether it is the first call and whether,
+therefore, the prolog should be emitted. This element must never be
+modified.
@item int __internal_use
This element is another one rarely used but needed in certain
-situations. It 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.
+situations. It is assigned a nonzero value in case the conversion
+functions are used to implement @code{mbsrtowcs} et.al.@: (i.e., the
+function is not used directly through the @code{iconv} interface).
This sometimes makes a difference as it is expected that the
@code{iconv} functions are used to translate entire texts while the
-@code{mbsrtowcs} functions are normally only used to convert single
+@code{mbsrtowcs} functions are normally used only to convert single
strings and might be used multiple times to convert entire texts.
But in this situation we would have problem complying with some rules of
-the character set specification. Some character sets require a prolog
+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
+@code{mbsrtowcs} calls are used to convert the text, only the first call
+must add the prolog. However, because there is no communication between the
+different calls of @code{mbsrtowcs}, the conversion functions have no
possibility to find this out. The situation is different for sequences
-of @code{iconv} calls since the handle allows to access the needed
+of @code{iconv} calls since the handle allows access to the needed
information.
-This element is mostly used together with @code{__invocation_counter} in
-a way like this:
+The @code{int __internal_use} element is mostly used together with
+@code{__invocation_counter} as follows:
@smallexample
if (!data->__internal_use
&& data->__invocation_counter == 0)
/* @r{Emit prolog.} */
- ...
+ @dots{}
@end smallexample
This element must never be modified.
@item mbstate_t *__statep
The @code{__statep} element points to an object of type @code{mbstate_t}
-(@pxref{Keeping the state}). The conversion of 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.
+(@pxref{Keeping the state}). The conversion of a stateful character
+set must use the object pointed to by @code{__statep} to store
+information about the conversion state. The @code{__statep} element
+itself must never be modified.
@item mbstate_t __state
-This element @emph{never} must be used directly. It is only part of
+This element must @emph{never} be used directly. It is only part of
this structure to have the needed space allocated.
@end table
@end deftp
@subsubsection @code{iconv} module interfaces
With the knowledge about the data structures we now can describe the
-conversion 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.
+conversion function itself. To understand the interface a bit of
+knowledge is necessary about the functionality in the C library that
+loads the objects with the conversions.
-It is often the case that one conversion is used more than once. I.e.,
+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
+sets during one program run). The @code{mbsrtowcs} et.al.@: functions in
+@theglibc{} also use the @code{iconv} functionality, which
increases the number of uses of the same functions even more.
-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.
+Because of this multiple use of conversions, the modules do not get
+loaded exclusively for one conversion. Instead a module once loaded can
+be used by an arbitrary number of @code{iconv} or @code{mbsrtowcs} calls
+at the same time. The splitting of the information between conversion-
+function-specific information and conversion data makes this possible.
+The last section showed the two data structures used to do this.
-This is of course also reflected in the interface and semantic of the
-functions the modules must provide. There are three functions which
+This is of course also reflected in the interface and semantics of the
+functions that the modules must provide. There are three functions that
must have the following names:
@table @code
@item gconv_init
The @code{gconv_init} function initializes the conversion function
specific data structure. This very same object is shared by all
-conversion which use this conversion and therefore no state information
+conversions that use this conversion and, therefore, no state information
about the conversion itself must be stored in here. If a module
-implements more than one conversion the @code{gconv_init} function will be
-called multiple times.
+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
+The @code{gconv_end} function is responsible for freeing all resources
+allocated by the @code{gconv_init} function. If there is nothing to do,
this function can be missing. Special care must be taken if the module
implements more than one conversion and the @code{gconv_init} function
does not allocate the same resources for all conversions.
@end table
There are three data types defined for the three module interface
-function and these define the interface.
+functions and these define the interface.
@comment gconv.h
@comment GNU
-@deftypevr {Data type} int (*__gconv_init_fct) (struct __gconv_step *)
+@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
+As explained in the description of the @code{struct __gconv_step} data
structure above the initialization function has to initialize parts of
it.
@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.
+destination character sets, respectively. If the characters all have the
+same size, the minimum and maximum values are the same.
@item __stateful
This element must be initialized to 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.
+If the initialization function needs to communicate some information
+to the conversion function, this communication can happen using the
+@code{__data} element of the @code{__gconv_step} structure. But since
+this data is shared by all the conversions, it must not be modified by
+the conversion function. The example below shows how this can be used.
@smallexample
#define MIN_NEEDED_FROM 1
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;
@end smallexample
The function first checks which conversion is wanted. The module from
-which this function is taken implements four different conversion and
+which this function is taken implements four different conversions;
which one is selected can be determined by comparing the names. The
comparison should always be done without paying attention to the case.
-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.
+Next, a data structure, which contains the necessary information about
+which conversion is selected, is allocated. The data structure
+@code{struct iso2022jp_data} is locally defined since, outside the
+module, this data is not used at all. Please note that if all four
+conversions this 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
+UCS-4) and therefore each character consists of exactly four bytes. For
the conversion from @code{INTERNAL} to ISO-2022-JP we have to take into
account that escape sequences might be necessary to switch the character
sets. Therefore the @code{__max_needed_to} element for this direction
gets assigned @code{MAX_NEEDED_FROM + 2}. This takes into account the
two bytes needed for the escape sequences to 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.
+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
+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.
+sequences that have to be emitted together with the next character.
+Therefore one needs more room than only for the character itself.
The possible return values of the initialization function are:
@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
+The function called before the module is unloaded is significantly
+easier. It often has nothing at all to do; in which case it can be left
out completely.
@comment gconv.h
@comment GNU
-@deftypevr {Data type} void (*__gconv_end_fct) (struct gconv_step *)
-The task of this function is it to free all resources allocated in the
+@deftypevr {Data type} void {(*__gconv_end_fct)} (struct gconv_step *)
+The task of this function is to free all resources allocated in the
initialization function. Therefore only the @code{__data} element of
the object pointed to by the argument is of interest. Continuing the
example from the initialization function, the finalization function
@end smallexample
@end deftypevr
-The most important function is the conversion function itself. It can
+The most important function is the conversion function itself, which can
get quite complicated for complex character sets. But since this is not
-of interest here we will only describe a possible skeleton for the
+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)
+@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.
+is selected is determined by the sixth argument, an integer. This
+argument being nonzero means that flushing is selected.
-Common to both mode is where the output buffer can be found. The
+Common to both modes is where the output buffer can be found. The
information about this buffer is stored in the conversion step data. A
-pointer to this is passed as the second argument to this function. The
-description of the @code{struct __gconv_step_data} structure has more
-information on this.
+pointer to this information is passed as the second argument to this
+function. The description of the @code{struct __gconv_step_data}
+structure has more information on the conversion step data.
@cindex stateful
What has to be done for flushing depends on the source character set.
-If 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.
+If the source character set is not stateful, nothing has to be done.
+Otherwise the function has to emit a byte sequence to bring the state
+object into the initial state. Once this all happened the other
+conversion modules in the chain of conversions have to get the same
+chance. Whether another step follows can be determined from the
+@code{__is_last} element of the step data structure to which the first
+parameter points.
+
+The more interesting mode is when actual text has to be converted. The
+first step in this case is to convert as much text as possible from the
+input buffer and store the result in the output buffer. The start of the
+input buffer is determined by the third argument, which is a pointer to a
+pointer variable referencing the beginning of the buffer. The fourth
+argument is a pointer to the byte right after the last byte in the buffer.
The conversion has to be performed according to the current state if the
character set is stateful. The state is stored in an object pointed to
by the @code{__statep} element of the step data (second argument). Once
either the input buffer is empty or the output buffer is full the
-conversion stops. At this point the pointer variable referenced by the
+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
+byte (i.e., if all of the input is consumed, this pointer and the fourth
+parameter have the same value).
+
+What now happens depends on whether this step is the last one. If it is
+the last step, the only thing that has to be done is to update the
+@code{__outbuf} element of the step data structure to point after the
+last written byte. This update gives the caller the information on how
+much text is available in the output buffer. In addition, the variable
pointed to by the fifth parameter, which is of type @code{size_t}, must
-be incremented by the number of characters (@emph{not bytes}) which were
-converted in a non-reversible way. Then the function can return.
+be incremented by the number of characters (@emph{not bytes}) that were
+converted in a non-reversible way. Then, the function can return.
-In case the step is not the last one the later conversion functions have
-to get a chance to do their work. Therefore the appropriate conversion
+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
+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
@{
struct __gconv_step *next_step = step + 1;
struct __gconv_step_data *next_data = data + 1;
- ...
+ @dots{}
@end smallexample
The @code{next_step} pointer references the next step information and
@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.
+function might have some more to do. If the return value of the function
+is @code{__GCONV_EMPTY_INPUT}, more room is available in the output
+buffer. Unless the input buffer is empty the conversion, functions start
+all over again and process the rest of the input buffer. If the return
+value is not @code{__GCONV_EMPTY_INPUT}, something went wrong and we have
+to recover from this.
A requirement for the conversion function is that the input buffer
-pointer (the third argument) always points to the last character which
-was put in the converted form in the output buffer. This is trivially
-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.
+pointer (the third argument) always point to the last character that
+was put in converted form into the output buffer. This is trivially
+true after the conversion performed in the current step, but if the
+conversion functions deeper downstream stop prematurely, not all
+characters from the output buffer are consumed and, therefore, the input
+buffer pointers must be backed off to the right position.
+
+Correcting the input buffers is easy to do if the input and output
+character sets have a fixed width for all characters. In this situation
+we can compute how many characters are left in the output buffer and,
+therefore, can correct the input buffer pointer appropriately with a
+similar computation. Things are getting tricky if either character set
+has characters represented with variable length byte sequences, and it
+gets even more complicated if the conversion has to take care of the
+state. In these cases the conversion has to be performed once again, from
+the known state before the initial conversion (i.e., if necessary the
+state of the conversion has to be reset and the conversion loop has to be
+executed again). The difference now is that it is known how much input
+must be created, and the conversion can stop before converting the first
+unused character. Once this is done the input buffer pointers must be
+updated again and the function can return.
One final thing should be mentioned. If it is necessary for the
conversion to know whether it is the first invocation (in case a prolog
-has to be emitted) the conversion function should just before returning
-to the caller increment the @code{__invocation_counter} element of the
-step data structure. See the description of the @code{struct
+has to be emitted), the conversion function should increment the
+@code{__invocation_counter} element of the step data structure just
+before returning to the caller. See the description of the @code{struct
__gconv_step_data} structure above for more information on how this can
be used.
@table @code
@item __GCONV_EMPTY_INPUT
All input was consumed and there is room left in the output buffer.
-@item __GCONV_OUTPUT_FULL
+@item __GCONV_FULL_OUTPUT
No more room in the output buffer. In case this is not the last step
this value is propagated down from the call of the next conversion
function in the chain.
/* @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{If this is the last step, leave the loop. There is}
@r{nothing we can do.} */
if (data->__is_last)
@{
break;
@}
- /* @r{Write out all output which was produced.} */
+ /* @r{Write out all output that was produced.} */
if (outbuf > outptr)
@{
const char *outerr = data->__outbuf;
@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
+doing so should also take a look at the available source code in the
+@glibcadj{} sources. It contains many examples of working and optimized
modules.
+
+@c File charset.texi edited October 2001 by Dennis Grace, IBM Corporation
projects / platform / upstream / glibc.git / blobdiff