summary refs log tree commit diff
path: root/manual/locale.texi
blob: 75564d9f2716b6d47e1cc3c936129f592ac0183e (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
@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.
@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 Date and 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.

@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 Date and 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}).

@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}.

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}.

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 Date and 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 string is made up of decimal numbers separated by semicolons.
Successive numbers (from left to right) give the sizes of successive
groups (from right to left, starting at the decimal point).  The last
number in the string is used over and over for all the remaining groups.

If the last integer is @code{-1}, it means that 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{"4;3;2"}, 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{"3"} 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
These members are @code{1} if the @code{currency_symbol} string should
precede the value of a monetary amount, or @code{0} if the string should
follow the value.  The @code{p_cs_precedes} member applies to positive
amounts (or zero), and the @code{n_cs_precedes} member applies to
negative amounts.

In the standard @samp{C} locale, both 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 POSIX standard says that these two members apply to the
@code{int_curr_symbol} as well as the @code{currency_symbol}.  The ISO
C standard seems to imply that they should apply only to the
@code{currency_symbol}---so the @code{int_curr_symbol} should always
precede the amount.

We can only guess which of these (if either) matches the usual
conventions for printing international currency symbols.  Our guess is
that they should always precede the amount.  If we find out a reliable
answer, we will put it here.

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

In the standard @samp{C} locale, both 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.

These members apply only to @code{currency_symbol}.  When you use
@code{int_curr_symbol}, you never print an additional space, because
@code{int_curr_symbol} itself contains the appropriate separator.

The POSIX standard says that these two members apply to the
@code{int_curr_symbol} as well as the @code{currency_symbol}.  However, an
example in the @w{ISO C} standard clearly implies that they should apply
only to the @code{currency_symbol}---that the @code{int_curr_symbol}
contains any appropriate separator, so you should never print an
additional space.

Based on what we know now, we recommend you ignore these members when
printing international currency symbols, and print no extra space.
@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
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.
@end table

It is not clear whether you should let these members apply to the
international currency format or not.  POSIX says you should, but
intuition plus the examples in the @w{ISO C} standard suggest you should
not.  We hope that someone who knows the conventions for formatting
monetary quantities well will tell us what we should recommend.

@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.

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 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 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.
@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}).
@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.
@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, , 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.