manual/locale.texi

@node Locales, Message Translation, Character Set Handling, Top
@c %MENU% The country and language can affect the behavior of library functions
@chapter Locales and Internationalization

Different countries and cultures have varying conventions for how to
communicate.  These conventions range from very simple ones, such as the
format for representing dates and times, to very complex ones, such as
the language spoken.

@cindex internationalization
@cindex locales
@dfn{Internationalization} of software means programming it to be able
to adapt to the user's favorite conventions.  In @w{ISO C},
internationalization works by means of @dfn{locales}.  Each locale
specifies a collection of conventions, one convention for each purpose.
The user chooses a set of conventions by specifying a locale (via
environment variables).

All programs inherit the chosen locale as part of their environment.
Provided the programs are written to obey the choice of locale, they
will follow the conventions preferred by the user.

@menu
* Effects of Locale::           Actions affected by the choice of
                                 locale.
* Choosing Locale::             How the user specifies a locale.
* Locale Categories::           Different purposes for which you can
                                 select a locale.
* Setting the Locale::          How a program specifies the locale
                                 with library functions.
* Standard Locales::            Locale names available on all systems.
* Locale Information::          How to access the information for the locale.
* Formatting Numbers::          A dedicated function to format numbers.
* Yes-or-No Questions::         Check a Response against the locale.
@end menu

@node Effects of Locale, Choosing Locale,  , Locales
@section What Effects a Locale Has

Each locale specifies conventions for several purposes, including the
following:

@itemize @bullet
@item
What multibyte character sequences are valid, and how they are
interpreted (@pxref{Character Set Handling}).

@item
Classification of which characters in the local character set are
considered alphabetic, and upper- and lower-case conversion conventions
(@pxref{Character Handling}).

@item
The collating sequence for the local language and character set
(@pxref{Collation Functions}).

@item
Formatting of numbers and currency amounts (@pxref{General Numeric}).

@item
Formatting of dates and times (@pxref{Formatting Calendar Time}).

@item
What language to use for output, including error messages
(@pxref{Message Translation}).

@item
What language to use for user answers to yes-or-no questions
(@pxref{Yes-or-No Questions}).

@item
What language to use for more complex user input.
(The C library doesn't yet help you implement this.)
@end itemize

Some aspects of adapting to the specified locale are handled
automatically by the library subroutines.  For example, all your program
needs to do in order to use the collating sequence of the chosen locale
is to use @code{strcoll} or @code{strxfrm} to compare strings.

Other aspects of locales are beyond the comprehension of the library.
For example, the library can't automatically translate your program's
output messages into other languages.  The only way you can support
output in the user's favorite language is to program this more or less
by hand.  The C library provides functions to handle translations for
multiple languages easily.

This chapter discusses the mechanism by which you can modify the current
locale.  The effects of the current locale on specific library functions
are discussed in more detail in the descriptions of those functions.

@node Choosing Locale, Locale Categories, Effects of Locale, Locales
@section Choosing a Locale

The simplest way for the user to choose a locale is to set the
environment variable @code{LANG}.  This specifies a single locale to use
for all purposes.  For example, a user could specify a hypothetical
locale named @samp{espana-castellano} to use the standard conventions of
most of Spain.

The set of locales supported depends on the operating system you are
using, and so do their names.  We can't make any promises about what
locales will exist, except for one standard locale called @samp{C} or
@samp{POSIX}.  Later we will describe how to construct locales.
@comment (@pxref{Building Locale Files}).

@cindex combining locales
A user also has the option of specifying different locales for different
purposes---in effect, choosing a mixture of multiple locales.

For example, the user might specify the locale @samp{espana-castellano}
for most purposes, but specify the locale @samp{usa-english} for
currency formatting.  This might make sense if the user is a
Spanish-speaking American, working in Spanish, but representing monetary
amounts in US dollars.

Note that both locales @samp{espana-castellano} and @samp{usa-english},
like all locales, would include conventions for all of the purposes to
which locales apply.  However, the user can choose to use each locale
for a particular subset of those purposes.

@node Locale Categories, Setting the Locale, Choosing Locale, Locales
@section Categories of Activities that Locales Affect
@cindex categories for locales
@cindex locale categories

The purposes that locales serve are grouped into @dfn{categories}, so
that a user or a program can choose the locale for each category
independently.  Here is a table of categories; each name is both an
environment variable that a user can set, and a macro name that you can
use as an argument to @code{setlocale}.

@vtable @code
@comment locale.h
@comment ISO
@item LC_COLLATE
This category applies to collation of strings (functions @code{strcoll}
and @code{strxfrm}); see @ref{Collation Functions}.

@comment locale.h
@comment ISO
@item LC_CTYPE
This category applies to classification and conversion of characters,
and to multibyte and wide characters;
see @ref{Character Handling}, and @ref{Character Set Handling}.

@comment locale.h
@comment ISO
@item LC_MONETARY
This category applies to formatting monetary values; see @ref{General Numeric}.

@comment locale.h
@comment ISO
@item LC_NUMERIC
This category applies to formatting numeric values that are not
monetary; see @ref{General Numeric}.

@comment locale.h
@comment ISO
@item LC_TIME
This category applies to formatting date and time values; see
@ref{Formatting Calendar Time}.

@comment locale.h
@comment XOPEN
@item LC_MESSAGES
This category applies to selecting the language used in the user
interface for message translation (@pxref{The Uniforum approach};
@pxref{Message catalogs a la X/Open})  and contains regular expressions
for affirmative and negative responses.

@comment locale.h
@comment ISO
@item LC_ALL
This is not an environment variable; it is only a macro that you can use
with @code{setlocale} to set a single locale for all purposes.  Setting
this environment variable overwrites all selections by the other
@code{LC_*} variables or @code{LANG}.

@comment locale.h
@comment ISO
@item LANG
If this environment variable is defined, its value specifies the locale
to use for all purposes except as overridden by the variables above.
@end vtable

@vindex LANGUAGE
When developing the message translation functions it was felt that the
functionality provided by the variables above is not sufficient.  For
example, it should be possible to specify more than one locale name.
Take a Swedish user who better speaks German than English, and a program
whose messages are output in English by default.  It should be possible
to specify that the first choice of language is Swedish, the second
German, and if this also fails to use English.  This is
possible with the variable @code{LANGUAGE}.  For further description of
this GNU extension see @ref{Using gettextized software}.

@node Setting the Locale, Standard Locales, Locale Categories, Locales
@section How Programs Set the Locale

A C program inherits its locale environment variables when it starts up.
This happens automatically.  However, these variables do not
automatically control the locale used by the library functions, because
@w{ISO C} says that all programs start by default in the standard @samp{C}
locale.  To use the locales specified by the environment, you must call
@code{setlocale}.  Call it as follows:

@smallexample
setlocale (LC_ALL, "");
@end smallexample

@noindent
to select a locale based on the user choice of the appropriate
environment variables.

@cindex changing the locale
@cindex locale, changing
You can also use @code{setlocale} to specify a particular locale, for
general use or for a specific category.

@pindex locale.h
The symbols in this section are defined in the header file @file{locale.h}.

@comment locale.h
@comment ISO
@deftypefun {char *} setlocale (int @var{category}, const char *@var{locale})
The function @code{setlocale} sets the current locale for category
@var{category} to @var{locale}.  A list of all the locales the system
provides can be created by running

@pindex locale
@smallexample
  locale -a
@end smallexample

If @var{category} is @code{LC_ALL}, this specifies the locale for all
purposes.  The other possible values of @var{category} specify an
single purpose (@pxref{Locale Categories}).

You can also use this function to find out the current locale by passing
a null pointer as the @var{locale} argument.  In this case,
@code{setlocale} returns a string that is the name of the locale
currently selected for category @var{category}.

The string returned by @code{setlocale} can be overwritten by subsequent
calls, so you should make a copy of the string (@pxref{Copying and
Concatenation}) if you want to save it past any further calls to
@code{setlocale}.  (The standard library is guaranteed never to call
@code{setlocale} itself.)

You should not modify the string returned by @code{setlocale}.  It might
be the same string that was passed as an argument in a previous call to
@code{setlocale}.  One requirement is that the @var{category} must be
the same in the call the string was returned and the one when the string
is passed in as @var{locale} parameter.

When you read the current locale for category @code{LC_ALL}, the value
encodes the entire combination of selected locales for all categories.
In this case, the value is not just a single locale name.  In fact, we
don't make any promises about what it looks like.  But if you specify
the same ``locale name'' with @code{LC_ALL} in a subsequent call to
@code{setlocale}, it restores the same combination of locale selections.

To be sure you can use the returned string encoding the currently selected
locale at a later time, you must make a copy of the string.  It is not
guaranteed that the returned pointer remains valid over time.

When the @var{locale} argument is not a null pointer, the string returned
by @code{setlocale} reflects the newly-modified locale.

If you specify an empty string for @var{locale}, this means to read the
appropriate environment variable and use its value to select the locale
for @var{category}.

If a nonempty string is given for @var{locale}, then the locale of that
name is used if possible.

If you specify an invalid locale name, @code{setlocale} returns a null
pointer and leaves the current locale unchanged.
@end deftypefun

Here is an example showing how you might use @code{setlocale} to
temporarily switch to a new locale.

@smallexample
#include <stddef.h>
#include <locale.h>
#include <stdlib.h>
#include <string.h>

void
with_other_locale (char *new_locale,
                   void (*subroutine) (int),
                   int argument)
@{
  char *old_locale, *saved_locale;

  /* @r{Get the name of the current locale.}  */
  old_locale = setlocale (LC_ALL, NULL);

  /* @r{Copy the name so it won't be clobbered by @code{setlocale}.} */
  saved_locale = strdup (old_locale);
  if (saved_locale == NULL)
    fatal ("Out of memory");

  /* @r{Now change the locale and do some stuff with it.} */
  setlocale (LC_ALL, new_locale);
  (*subroutine) (argument);

  /* @r{Restore the original locale.} */
  setlocale (LC_ALL, saved_locale);
  free (saved_locale);
@}
@end smallexample

@strong{Portability Note:} Some @w{ISO C} systems may define additional
locale categories, and future versions of the library will do so.  For
portability, assume that any symbol beginning with @samp{LC_} might be
defined in @file{locale.h}.

@node Standard Locales, Locale Information, Setting the Locale, Locales
@section Standard Locales

The only locale names you can count on finding on all operating systems
are these three standard ones:

@table @code
@item "C"
This is the standard C locale.  The attributes and behavior it provides
are specified in the @w{ISO C} standard.  When your program starts up, it
initially uses this locale by default.

@item "POSIX"
This is the standard POSIX locale.  Currently, it is an alias for the
standard C locale.

@item ""
The empty name says to select a locale based on environment variables.
@xref{Locale Categories}.
@end table

Defining and installing named locales is normally a responsibility of
the system administrator at your site (or the person who installed the
GNU C library).  It is also possible for the user to create private
locales.  All this will be discussed later when describing the tool to
do so.
@comment (@pxref{Building Locale Files}).

If your program needs to use something other than the @samp{C} locale,
it will be more portable if you use whatever locale the user specifies
with the environment, rather than trying to specify some non-standard
locale explicitly by name.  Remember, different machines might have
different sets of locales installed.

@node Locale Information, Formatting Numbers, Standard Locales, Locales
@section Accessing Locale Information

There are several ways to access locale information.  The simplest
way is to let the C library itself do the work.  Several of the
functions in this library implicitly access the locale data, and use
what information is provided by the currently selected locale.  This is
how the locale model is meant to work normally.

As an example take the @code{strftime} function, which is meant to nicely
format date and time information (@pxref{Formatting Calendar Time}).
Part of the standard information contained in the @code{LC_TIME}
category is the names of the months.  Instead of requiring the
programmer to take care of providing the translations the
@code{strftime} function does this all by itself.  @code{%A}
in the format string is replaced by the appropriate weekday
name of the locale currently selected by @code{LC_TIME}.  This is an
easy example, and wherever possible functions do things automatically
in this way.

But there are quite often situations when there is simply no function
to perform the task, or it is simply not possible to do the work
automatically.  For these cases it is necessary to access the
information in the locale directly.  To do this the C library provides
two functions: @code{localeconv} and @code{nl_langinfo}.  The former is
part of @w{ISO C} and therefore portable, but has a brain-damaged
interface.  The second is part of the Unix interface and is portable in
as far as the system follows the Unix standards.

@menu
* The Lame Way to Locale Data::   ISO C's @code{localeconv}.
* The Elegant and Fast Way::      X/Open's @code{nl_langinfo}.
@end menu

@node The Lame Way to Locale Data, The Elegant and Fast Way, ,Locale Information
@subsection @code{localeconv}: It is portable but @dots{}

Together with the @code{setlocale} function the @w{ISO C} people
invented the @code{localeconv} function.  It is a masterpiece of poor
design.  It is expensive to use, not extendable, and not generally
usable as it provides access to only @code{LC_MONETARY} and
@code{LC_NUMERIC} related information.  Nevertheless, if it is
applicable to a given situation it should be used since it is very
portable.  The function @code{strfmon} formats monetary amounts
according to the selected locale using this information.
@pindex locale.h
@cindex monetary value formatting
@cindex numeric value formatting

@comment locale.h
@comment ISO
@deftypefun {struct lconv *} localeconv (void)
The @code{localeconv} function returns a pointer to a structure whose
components contain information about how numeric and monetary values
should be formatted in the current locale.

You should not modify the structure or its contents.  The structure might
be overwritten by subsequent calls to @code{localeconv}, or by calls to
@code{setlocale}, but no other function in the library overwrites this
value.
@end deftypefun

@comment locale.h
@comment ISO
@deftp {Data Type} {struct lconv}
@code{localeconv}'s return value is of this data type.  Its elements are
described in the following subsections.
@end deftp

If a member of the structure @code{struct lconv} has type @code{char},
and the value is @code{CHAR_MAX}, it means that the current locale has
no value for that parameter.

@menu
* General Numeric::             Parameters for formatting numbers and
                                 currency amounts.
* Currency Symbol::             How to print the symbol that identifies an
                                 amount of money (e.g. @samp{$}).
* Sign of Money Amount::        How to print the (positive or negative) sign
                                 for a monetary amount, if one exists.
@end menu

@node General Numeric, Currency Symbol, , The Lame Way to Locale Data
@subsubsection Generic Numeric Formatting Parameters

These are the standard members of @code{struct lconv}; there may be
others.

@table @code
@item char *decimal_point
@itemx char *mon_decimal_point
These are the decimal-point separators used in formatting non-monetary
and monetary quantities, respectively.  In the @samp{C} locale, the
value of @code{decimal_point} is @code{"."}, and the value of
@code{mon_decimal_point} is @code{""}.
@cindex decimal-point separator

@item char *thousands_sep
@itemx char *mon_thousands_sep
These are the separators used to delimit groups of digits to the left of
the decimal point in formatting non-monetary and monetary quantities,
respectively.  In the @samp{C} locale, both members have a value of
@code{""} (the empty string).

@item char *grouping
@itemx char *mon_grouping
These are strings that specify how to group the digits to the left of
the decimal point.  @code{grouping} applies to non-monetary quantities
and @code{mon_grouping} applies to monetary quantities.  Use either
@code{thousands_sep} or @code{mon_thousands_sep} to separate the digit
groups.
@cindex grouping of digits

Each member of these strings is to be interpreted as an integer value of
type @code{char}.  Successive numbers (from left to right) give the
sizes of successive groups (from right to left, starting at the decimal
point.)  The last member is either @code{0}, in which case the previous
member is used over and over again for all the remaining groups, or
@code{CHAR_MAX}, in which case there is no more grouping---or, put
another way, any remaining digits form one large group without
separators.

For example, if @code{grouping} is @code{"\04\03\02"}, the correct
grouping for the number @code{123456787654321} is @samp{12}, @samp{34},
@samp{56}, @samp{78}, @samp{765}, @samp{4321}.  This uses a group of 4
digits at the end, preceded by a group of 3 digits, preceded by groups
of 2 digits (as many as needed).  With a separator of @samp{,}, the
number would be printed as @samp{12,34,56,78,765,4321}.

A value of @code{"\03"} indicates repeated groups of three digits, as
normally used in the U.S.

In the standard @samp{C} locale, both @code{grouping} and
@code{mon_grouping} have a value of @code{""}.  This value specifies no
grouping at all.

@item char int_frac_digits
@itemx char frac_digits
These are small integers indicating how many fractional digits (to the
right of the decimal point) should be displayed in a monetary value in
international and local formats, respectively.  (Most often, both
members have the same value.)

In the standard @samp{C} locale, both of these members have the value
@code{CHAR_MAX}, meaning ``unspecified''.  The ISO standard doesn't say
what to do when you find this value; we recommend printing no
fractional digits.  (This locale also specifies the empty string for
@code{mon_decimal_point}, so printing any fractional digits would be
confusing!)
@end table

@node Currency Symbol, Sign of Money Amount, General Numeric, The Lame Way to Locale Data
@subsubsection Printing the Currency Symbol
@cindex currency symbols

These members of the @code{struct lconv} structure specify how to print
the symbol to identify a monetary value---the international analog of
@samp{$} for US dollars.

Each country has two standard currency symbols.  The @dfn{local currency
symbol} is used commonly within the country, while the
@dfn{international currency symbol} is used internationally to refer to
that country's currency when it is necessary to indicate the country
unambiguously.

For example, many countries use the dollar as their monetary unit, and
when dealing with international currencies it's important to specify
that one is dealing with (say) Canadian dollars instead of U.S. dollars
or Australian dollars.  But when the context is known to be Canada,
there is no need to make this explicit---dollar amounts are implicitly
assumed to be in Canadian dollars.

@table @code
@item char *currency_symbol
The local currency symbol for the selected locale.

In the standard @samp{C} locale, this member has a value of @code{""}
(the empty string), meaning ``unspecified''.  The ISO standard doesn't
say what to do when you find this value; we recommend you simply print
the empty string as you would print any other string pointed to by this
variable.

@item char *int_curr_symbol
The international currency symbol for the selected locale.

The value of @code{int_curr_symbol} should normally consist of a
three-letter abbreviation determined by the international standard
@cite{ISO 4217 Codes for the Representation of Currency and Funds},
followed by a one-character separator (often a space).

In the standard @samp{C} locale, this member has a value of @code{""}
(the empty string), meaning ``unspecified''.  We recommend you simply print
the empty string as you would print any other string pointed to by this
variable.

@item char p_cs_precedes
@itemx char n_cs_precedes
@itemx char int_p_cs_precedes
@itemx char int_n_cs_precedes
These members are @code{1} if the @code{currency_symbol} or
@code{int_curr_symbol} strings should precede the value of a monetary
amount, or @code{0} if the strings should follow the value.  The
@code{p_cs_precedes} and @code{int_p_cs_precedes} members apply to
positive amounts (or zero), and the @code{n_cs_precedes} and
@code{int_n_cs_precedes} members apply to negative amounts.

In the standard @samp{C} locale, all of these members have a value of
@code{CHAR_MAX}, meaning ``unspecified''.  The ISO standard doesn't say
what to do when you find this value.  We recommend printing the
currency symbol before the amount, which is right for most countries.
In other words, treat all nonzero values alike in these members.

The members with the @code{int_} prefix apply to the
@code{int_curr_symbol} while the other two apply to
@code{currency_symbol}.

@item char p_sep_by_space
@itemx char n_sep_by_space
@itemx char int_p_sep_by_space
@itemx char int_n_sep_by_space
These members are @code{1} if a space should appear between the
@code{currency_symbol} or @code{int_curr_symbol} strings and the
amount, or @code{0} if no space should appear.  The
@code{p_sep_by_space} and @code{int_p_sep_by_space} members apply to
positive amounts (or zero), and the @code{n_sep_by_space} and
@code{int_n_sep_by_space} members apply to negative amounts.

In the standard @samp{C} locale, all of these members have a value of
@code{CHAR_MAX}, meaning ``unspecified''.  The ISO standard doesn't say
what you should do when you find this value; we suggest you treat it as
1 (print a space).  In other words, treat all nonzero values alike in
these members.

The members with the @code{int_} prefix apply to the
@code{int_curr_symbol} while the other two apply to
@code{currency_symbol}.  There is one specialty with the
@code{int_curr_symbol}, though.  Since all legal values contain a space
at the end the string one either printf this space (if the currency
symbol must appear in front and must be separated) or one has to avoid
printing this character at all (especially when at the end of the
string).
@end table

@node Sign of Money Amount, , Currency Symbol, The Lame Way to Locale Data
@subsubsection Printing the Sign of a Monetary Amount

These members of the @code{struct lconv} structure specify how to print
the sign (if any) of a monetary value.

@table @code
@item char *positive_sign
@itemx char *negative_sign
These are strings used to indicate positive (or zero) and negative
monetary quantities, respectively.

In the standard @samp{C} locale, both of these members have a value of
@code{""} (the empty string), meaning ``unspecified''.

The ISO standard doesn't say what to do when you find this value; we
recommend printing @code{positive_sign} as you find it, even if it is
empty.  For a negative value, print @code{negative_sign} as you find it
unless both it and @code{positive_sign} are empty, in which case print
@samp{-} instead.  (Failing to indicate the sign at all seems rather
unreasonable.)

@item char p_sign_posn
@itemx char n_sign_posn
@itemx char int_p_sign_posn
@itemx char int_n_sign_posn
These members are small integers that indicate how to
position the sign for nonnegative and negative monetary quantities,
respectively.  (The string used by the sign is what was specified with
@code{positive_sign} or @code{negative_sign}.)  The possible values are
as follows:

@table @code
@item 0
The currency symbol and quantity should be surrounded by parentheses.

@item 1
Print the sign string before the quantity and currency symbol.

@item 2
Print the sign string after the quantity and currency symbol.

@item 3
Print the sign string right before the currency symbol.

@item 4
Print the sign string right after the currency symbol.

@item CHAR_MAX
``Unspecified''.  Both members have this value in the standard
@samp{C} locale.
@end table

The ISO standard doesn't say what you should do when the value is
@code{CHAR_MAX}.  We recommend you print the sign after the currency
symbol.

The members with the @code{int_} prefix apply to the
@code{int_curr_symbol} while the other two apply to
@code{currency_symbol}.
@end table

@node The Elegant and Fast Way, , The Lame Way to Locale Data, Locale Information
@subsection Pinpoint Access to Locale Data

When writing the X/Open Portability Guide the authors realized that the
@code{localeconv} function is not enough to provide reasonable access to
locale information.  The information which was meant to be available
in the locale (as later specified in the POSIX.1 standard) requires more
ways to access it.  Therefore the @code{nl_langinfo} function
was introduced.

@comment langinfo.h
@comment XOPEN
@deftypefun {char *} nl_langinfo (nl_item @var{item})
The @code{nl_langinfo} function can be used to access individual
elements of the locale categories.  Unlike the @code{localeconv}
function, which returns all the information, @code{nl_langinfo}
lets the caller select what information it requires.  This is very
fast and it is not a problem to call this function multiple times.

A second advantage is that in addition to the numeric and monetary
formatting information, information from the
@code{LC_TIME} and @code{LC_MESSAGES} categories is available.

@pindex langinfo.h
The type @code{nl_type} is defined in @file{nl_types.h}.  The argument
@var{item} is a numeric value defined in the header @file{langinfo.h}.
The X/Open standard defines the following values:

@vtable @code
@item CODESET
@code{nl_langinfo} returns a string with the name of the coded character
set used in the selected locale.

@item ABDAY_1
@itemx ABDAY_2
@itemx ABDAY_3
@itemx ABDAY_4
@itemx ABDAY_5
@itemx ABDAY_6
@itemx ABDAY_7
@code{nl_langinfo} returns the abbreviated weekday name.  @code{ABDAY_1}
corresponds to Sunday.
@item DAY_1
@itemx DAY_2
@itemx DAY_3
@itemx DAY_4
@itemx DAY_5
@itemx DAY_6
@itemx DAY_7
Similar to @code{ABDAY_1} etc., but here the return value is the
unabbreviated weekday name.
@item ABMON_1
@itemx ABMON_2
@itemx ABMON_3
@itemx ABMON_4
@itemx ABMON_5
@itemx ABMON_6
@itemx ABMON_7
@itemx ABMON_8
@itemx ABMON_9
@itemx ABMON_10
@itemx ABMON_11
@itemx ABMON_12
The return value is abbreviated name of the month.  @code{ABMON_1}
corresponds to January.
@item MON_1
@itemx MON_2
@itemx MON_3
@itemx MON_4
@itemx MON_5
@itemx MON_6
@itemx MON_7
@itemx MON_8
@itemx MON_9
@itemx MON_10
@itemx MON_11
@itemx MON_12
Similar to @code{ABMON_1} etc., but here the month names are not abbreviated.
Here the first value @code{MON_1} also corresponds to January.
@item AM_STR
@itemx PM_STR
The return values are strings which can be used in the representation of time
as an hour from 1 to 12 plus an am/pm specifier.

Note that in locales which do not use this time representation
these strings might be empty, in which case the am/pm format
cannot be used at all.
@item D_T_FMT
The return value can be used as a format string for @code{strftime} to
represent time and date in a locale-specific way.
@item D_FMT
The return value can be used as a format string for @code{strftime} to
represent a date in a locale-specific way.
@item T_FMT
The return value can be used as a format string for @code{strftime} to
represent time in a locale-specific way.
@item T_FMT_AMPM
The return value can be used as a format string for @code{strftime} to
represent time in the am/pm format.

Note that if the am/pm format does not make any sense for the
selected locale, the return value might be the same as the one for
@code{T_FMT}.
@item ERA
The return value represents the era used in the current locale.

Most locales do not define this value.  An example of a locale which
does define this value is the Japanese one.  In Japan, the traditional
representation of dates includes the name of the era corresponding to
the then-emperor's reign.

Normally it should not be necessary to use this value directly.
Specifying the @code{E} modifier in their format strings causes the
@code{strftime} functions to use this information.  The format of the
returned string is not specified, and therefore you should not assume
knowledge of it on different systems.
@item ERA_YEAR
The return value gives the year in the relevant era of the locale.
As for @code{ERA} it should not be necessary to use this value directly.
@item ERA_D_T_FMT
This return value can be used as a format string for @code{strftime} to
represent dates and times in a locale-specific era-based way.
@item ERA_D_FMT
This return value can be used as a format string for @code{strftime} to
represent a date in a locale-specific era-based way.
@item ERA_T_FMT
This return value can be used as a format string for @code{strftime} to
represent time in a locale-specific era-based way.
@item ALT_DIGITS
The return value is a representation of up to @math{100} values used to
represent the values @math{0} to @math{99}.  As for @code{ERA} this
value is not intended to be used directly, but instead indirectly
through the @code{strftime} function.  When the modifier @code{O} is
used in a format which would otherwise use numerals to represent hours,
minutes, seconds, weekdays, months, or weeks, the appropriate value for
the locale is used instead.
@item INT_CURR_SYMBOL
The same as the value returned by @code{localeconv} in the
@code{int_curr_symbol} element of the @code{struct lconv}.
@item CURRENCY_SYMBOL
@itemx CRNCYSTR
The same as the value returned by @code{localeconv} in the
@code{currency_symbol} element of the @code{struct lconv}.

@code{CRNCYSTR} is a deprecated alias still required by Unix98.
@item MON_DECIMAL_POINT
The same as the value returned by @code{localeconv} in the
@code{mon_decimal_point} element of the @code{struct lconv}.
@item MON_THOUSANDS_SEP
The same as the value returned by @code{localeconv} in the
@code{mon_thousands_sep} element of the @code{struct lconv}.
@item MON_GROUPING
The same as the value returned by @code{localeconv} in the
@code{mon_grouping} element of the @code{struct lconv}.
@item POSITIVE_SIGN
The same as the value returned by @code{localeconv} in the
@code{positive_sign} element of the @code{struct lconv}.
@item NEGATIVE_SIGN
The same as the value returned by @code{localeconv} in the
@code{negative_sign} element of the @code{struct lconv}.
@item INT_FRAC_DIGITS
The same as the value returned by @code{localeconv} in the
@code{int_frac_digits} element of the @code{struct lconv}.
@item FRAC_DIGITS
The same as the value returned by @code{localeconv} in the
@code{frac_digits} element of the @code{struct lconv}.
@item P_CS_PRECEDES
The same as the value returned by @code{localeconv} in the
@code{p_cs_precedes} element of the @code{struct lconv}.
@item P_SEP_BY_SPACE
The same as the value returned by @code{localeconv} in the
@code{p_sep_by_space} element of the @code{struct lconv}.
@item N_CS_PRECEDES
The same as the value returned by @code{localeconv} in the
@code{n_cs_precedes} element of the @code{struct lconv}.
@item N_SEP_BY_SPACE
The same as the value returned by @code{localeconv} in the
@code{n_sep_by_space} element of the @code{struct lconv}.
@item P_SIGN_POSN
The same as the value returned by @code{localeconv} in the
@code{p_sign_posn} element of the @code{struct lconv}.
@item N_SIGN_POSN
The same as the value returned by @code{localeconv} in the
@code{n_sign_posn} element of the @code{struct lconv}.

@item INT_P_CS_PRECEDES
The same as the value returned by @code{localeconv} in the
@code{int_p_cs_precedes} element of the @code{struct lconv}.
@item INT_P_SEP_BY_SPACE
The same as the value returned by @code{localeconv} in the
@code{int_p_sep_by_space} element of the @code{struct lconv}.
@item INT_N_CS_PRECEDES
The same as the value returned by @code{localeconv} in the
@code{int_n_cs_precedes} element of the @code{struct lconv}.
@item INT_N_SEP_BY_SPACE
The same as the value returned by @code{localeconv} in the
@code{int_n_sep_by_space} element of the @code{struct lconv}.
@item INT_P_SIGN_POSN
The same as the value returned by @code{localeconv} in the
@code{int_p_sign_posn} element of the @code{struct lconv}.
@item INT_N_SIGN_POSN
The same as the value returned by @code{localeconv} in the
@code{int_n_sign_posn} element of the @code{struct lconv}.

@item DECIMAL_POINT
@itemx RADIXCHAR
The same as the value returned by @code{localeconv} in the
@code{decimal_point} element of the @code{struct lconv}.

The name @code{RADIXCHAR} is a deprecated alias still used in Unix98.
@item THOUSANDS_SEP
@itemx THOUSEP
The same as the value returned by @code{localeconv} in the
@code{thousands_sep} element of the @code{struct lconv}.

The name @code{THOUSEP} is a deprecated alias still used in Unix98.
@item GROUPING
The same as the value returned by @code{localeconv} in the
@code{grouping} element of the @code{struct lconv}.
@item YESEXPR
The return value is a regular expression which can be used with the
@code{regex} function to recognize a positive response to a yes/no
question.  The GNU C library provides the @code{rpmatch} function for
easier handling in applications.
@item NOEXPR
The return value is a regular expression which can be used with the
@code{regex} function to recognize a negative response to a yes/no
question.
@item YESSTR
The return value is a locale-specific translation of the positive response
to a yes/no question.

Using this value is deprecated since it is a very special case of
message translation, and is better handled by the message
translation functions (@pxref{Message Translation}).

The use of this symbol is deprecated.  Instead message translation
should be used.
@item NOSTR
The return value is a locale-specific translation of the negative response
to a yes/no question.  What is said for @code{YESSTR} is also true here.

The use of this symbol is deprecated.  Instead message translation
should be used.
@end vtable

The file @file{langinfo.h} defines a lot more symbols but none of them
is official.  Using them is not portable, and the format of the
return values might change.  Therefore we recommended you not use
them.

Note that the return value for any valid argument can be used for
in all situations (with the possible exception of the am/pm time formatting
codes).  If the user has not selected any locale for the
appropriate category, @code{nl_langinfo} returns the information from the
@code{"C"} locale.  It is therefore possible to use this function as
shown in the example below.

If the argument @var{item} is not valid, a pointer to an empty string is
returned.
@end deftypefun

An example of @code{nl_langinfo} usage is a function which has to
print a given date and time in a locale-specific way.  At first one
might think that, since @code{strftime} internally uses the locale
information, writing something like the following is enough:

@smallexample
size_t
i18n_time_n_data (char *s, size_t len, const struct tm *tp)
@{
  return strftime (s, len, "%X %D", tp);
@}
@end smallexample

The format contains no weekday or month names and therefore is
internationally usable.  Wrong!  The output produced is something like
@code{"hh:mm:ss MM/DD/YY"}.  This format is only recognizable in the
USA.  Other countries use different formats.  Therefore the function
should be rewritten like this:

@smallexample
size_t
i18n_time_n_data (char *s, size_t len, const struct tm *tp)
@{
  return strftime (s, len, nl_langinfo (D_T_FMT), tp);
@}
@end smallexample

Now it uses the date and time format of the locale
selected when the program runs.  If the user selects the locale
correctly there should never be a misunderstanding over the time and
date format.

@node Formatting Numbers, Yes-or-No Questions, Locale Information, Locales
@section A dedicated function to format numbers

We have seen that the structure returned by @code{localeconv} as well as
the values given to @code{nl_langinfo} allow you to retrieve the various
pieces of locale-specific information to format numbers and monetary
amounts.  We have also seen that the underlying rules are quite complex.

Therefore the X/Open standards introduce a function which uses such
locale information, making it easier for the user to format
numbers according to these rules.

@deftypefun ssize_t strfmon (char *@var{s}, size_t @var{maxsize}, const char *@var{format}, @dots{})
The @code{strfmon} function is similar to the @code{strftime} function
in that it takes a buffer, its size, a format string,
and values to write into the buffer as text in a form specified
by the format string.  Like @code{strftime}, the function
also returns the number of bytes written into the buffer.

There are two differences: @code{strfmon} can take more than one
argument, and, of course, the format specification is different.  Like
@code{strftime}, the format string consists of normal text, which is
output as is, and format specifiers, which are indicated by a @samp{%}.
Immediately after the @samp{%}, you can optionally specify various flags
and formatting information before the main formatting character, in a
similar way to @code{printf}:

@itemize @bullet
@item
Immediately following the @samp{%} there can be one or more of the
following flags:
@table @asis
@item @samp{=@var{f}}
The single byte character @var{f} is used for this field as the numeric
fill character.  By default this character is a space character.
Filling with this character is only performed if a left precision
is specified.  It is not just to fill to the given field width.
@item @samp{^}
The number is printed without grouping the digits according to the rules
of the current locale.  By default grouping is enabled.
@item @samp{+}, @samp{(}
At most one of these flags can be used.  They select which format to
represent the sign of a currency amount.  By default, and if
@samp{+} is given, the locale equivalent of @math{+}/@math{-} is used.  If
@samp{(} is given, negative amounts are enclosed in parentheses.  The
exact format is determined by the values of the @code{LC_MONETARY}
category of the locale selected at program runtime.
@item @samp{!}
The output will not contain the currency symbol.
@item @samp{-}
The output will be formatted left-justified instead of right-justified if
it does not fill the entire field width.
@end table
@end itemize

The next part of a specification is an optional field width.  If no
width is specified @math{0} is taken.  During output, the function first
determines how much space is required.  If it requires at least as many
characters as given by the field width, it is output using as much space
as necessary.  Otherwise, it is extended to use the full width by
filling with the space character.  The presence or absence of the
@samp{-} flag determines the side at which such padding occurs.  If
present, the spaces are added at the right making the output
left-justified, and vice versa.

So far the format looks familiar, being similar to the @code{printf} and
@code{strftime} formats.  However, the next two optional fields
introduce something new.  The first one is a @samp{#} character followed
by a decimal digit string.  The value of the digit string specifies the
number of @emph{digit} positions to the left of the decimal point (or
equivalent).  This does @emph{not} include the grouping character when
the @samp{^} flag is not given.  If the space needed to print the number
does not fill the whole width, the field is padded at the left side with
the fill character, which can be selected using the @samp{=} flag and by
default is a space.  For example, if the field width is selected as 6
and the number is @math{123}, the fill character is @samp{*} the result
will be @samp{***123}.

The second optional field starts with a @samp{.} (period) and consists
of another decimal digit string.  Its value describes the number of
characters printed after the decimal point.  The default is selected
from the current locale (@code{frac_digits}, @code{int_frac_digits}, see
@pxref{General Numeric}).  If the exact representation needs more digits
than given by the field width, the displayed value is rounded.  If the
number of fractional digits is selected to be zero, no decimal point is
printed.

As a GNU extension, the @code{strfmon} implementation in the GNU libc
allows an optional @samp{L} next as a format modifier.  If this modifier
is given, the argument is expected to be a @code{long double} instead of
a @code{double} value.

Finally, the last component is a format specifier.  There are three
specifiers defined:

@table @asis
@item @samp{i}
Use the locale's rules for formatting an international currency value.
@item @samp{n}
Use the locale's rules for formatting a national currency value.
@item @samp{%}
Place a @samp{%} in the output.  There must be no flag, width
specifier or modifier given, only @samp{%%} is allowed.
@end table

As for @code{printf}, the function reads the format string
from left to right and uses the values passed to the function following
the format string.  The values are expected to be either of type
@code{double} or @code{long double}, depending on the presence of the
modifier @samp{L}.  The result is stored in the buffer pointed to by
@var{s}.  At most @var{maxsize} characters are stored.

The return value of the function is the number of characters stored in
@var{s}, including the terminating @code{NULL} byte.  If the number of
characters stored would exceed @var{maxsize}, the function returns
@math{-1} and the content of the buffer @var{s} is unspecified.  In this
case @code{errno} is set to @code{E2BIG}.
@end deftypefun

A few examples should make clear how the function works.  It is
assumed that all the following pieces of code are executed in a program
which uses the USA locale (@code{en_US}).  The simplest
form of the format is this:

@smallexample
strfmon (buf, 100, "@@%n@@%n@@%n@@", 123.45, -567.89, 12345.678);
@end smallexample

@noindent
The output produced is
@smallexample
"@@$123.45@@-$567.89@@$12,345.68@@"
@end smallexample

We can notice several things here.  First, the widths of the output
numbers are different.  We have not specified a width in the format
string, and so this is no wonder.  Second, the third number is printed
using thousands separators.  The thousands separator for the
@code{en_US} locale is a comma.  The number is also rounded.
@math{.678} is rounded to @math{.68} since the format does not specify a
precision and the default value in the locale is @math{2}.  Finally,
note that the national currency symbol is printed since @samp{%n} was
used, not @samp{i}.  The next example shows how we can align the output.

@smallexample
strfmon (buf, 100, "@@%=*11n@@%=*11n@@%=*11n@@", 123.45, -567.89, 12345.678);
@end smallexample

@noindent
The output this time is:

@smallexample
"@@    $123.45@@   -$567.89@@ $12,345.68@@"
@end smallexample

Two things stand out.  Firstly, all fields have the same width (eleven
characters) since this is the width given in the format and since no
number required more characters to be printed.  The second important
point is that the fill character is not used.  This is correct since the
white space was not used to achieve a precision given by a @samp{#}
modifier, but instead to fill to the given width.  The difference
becomes obvious if we now add a width specification.

@smallexample
strfmon (buf, 100, "@@%=*11#5n@@%=*11#5n@@%=*11#5n@@",
         123.45, -567.89, 12345.678);
@end smallexample

@noindent
The output is

@smallexample
"@@ $***123.45@@-$***567.89@@ $12,456.68@@"
@end smallexample

Here we can see that all the currency symbols are now aligned, and that
the space between the currency sign and the number is filled with the
selected fill character.  Note that although the width is selected to be
@math{5} and @math{123.45} has three digits left of the decimal point,
the space is filled with three asterisks.  This is correct since, as
explained above, the width does not include the positions used to store
thousands separators.  One last example should explain the remaining
functionality.

@smallexample
strfmon (buf, 100, "@@%=0(16#5.3i@@%=0(16#5.3i@@%=0(16#5.3i@@",
         123.45, -567.89, 12345.678);
@end smallexample

@noindent
This rather complex format string produces the following output:

@smallexample
"@@ USD 000123,450 @@(USD 000567.890)@@ USD 12,345.678 @@"
@end smallexample

The most noticeable change is the alternative way of representing
negative numbers.  In financial circles this is often done using
parentheses, and this is what the @samp{(} flag selected.  The fill
character is now @samp{0}.  Note that this @samp{0} character is not
regarded as a numeric zero, and therefore the first and second numbers
are not printed using a thousands separator.  Since we used the format
specifier @samp{i} instead of @samp{n}, the international form of the
currency symbol is used.  This is a four letter string, in this case
@code{"USD "}.  The last point is that since the precision right of the
decimal point is selected to be three, the first and second numbers are
printed with an extra zero at the end and the third number is printed
without rounding.

@node Yes-or-No Questions,  , Formatting Numbers , Locales
@section Yes-or-No Questions

Some non GUI programs ask a yes-or-no question.  If the messages
(especially the questions) are translated into foreign languages, be
sure that you localize the answers too.  It would be very bad habit to
ask a question in one language and request the answer in another, often
English.

The GNU C library contains @code{rpmatch} to give applications easy
access to the corresponding locale definitions.

@comment GNU
@comment stdlib.h
@deftypefun int rpmatch (const char *@var{response})
The function @code{rpmatch} checks the string in @var{response} whether
or not it is a correct yes-or-no answer and if yes, which one.  The
check uses the @code{YESEXPR} and @code{NOEXPR} data in the
@code{LC_MESSAGES} category of the currently selected locale.  The
return value is as follows:

@table @code
@item 1
The user entered an affirmative answer.

@item 0
The user entered a negative answer.

@item -1
The answer matched neither the @code{YESEXPR} nor the @code{NOEXPR}
regular expression.
@end table

This function is not standardized but available beside in GNU libc at
least also in the IBM AIX library.
@end deftypefun

@noindent
This function would normally be used like this:

@smallexample
  @dots{}
  /* @r{Use a safe default.}  */
  _Bool doit = false;

  fputs (gettext ("Do you really want to do this? "), stdout);
  fflush (stdout);
  /* @r{Prepare the @code{getline} call.}  */
  line = NULL;
  len = 0;
  while (getline (&line, &len, stdin) >= 0)
    @{
      /* @r{Check the response.}  */
      int res = rpmatch (line);
      if (res >= 0)
        @{
          /* @r{We got a definitive answer.}  */
          if (res > 0)
            doit = true;
          break;
        @}
    @}
  /* @r{Free what @code{getline} allocated.}  */
  free (line);
@end smallexample

Note that the loop continues until an read error is detected or until a
definitive (positive or negative) answer is read.