.\"	$OpenBSD: setlocale.3,v 1.24 2022/08/04 06:20:24 jsg Exp $
.\"	$NetBSD: setlocale.3,v 1.3 1997/07/14 23:19:47 kleink Exp $
.\"
.\" Copyright (c) 1993
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.\"	@(#)setlocale.3	8.1 (Berkeley) 6/9/93
.\"
.Dd $Mdocdate: August 4 2022 $
.Dt SETLOCALE 3
.Os
.Sh NAME
.Nm setlocale
.Nd select character encoding
.Sh SYNOPSIS
.In locale.h
.Ft char *
.Fn setlocale "int category" "const char *locale"
.Sh DESCRIPTION
The
.Fn setlocale
function sets and retrieves the active
.Fa locale
for the current process.
The locale modifies the behaviour of some functions in the C library
with respect to the character encoding, and on other operating systems
also with respect to some language and cultural conventions.
For more information about locales in general, see the
.Xr locale 1
manual page.
.Pp
On
.Ox ,
the only useful value for the
.Fa category
is
.Dv LC_CTYPE .
It sets the locale used for character encoding, character classification,
and case conversion.
For compatibility with natural language support in
.Xr packages 7 ,
all other categories \(em
.Dv LC_COLLATE ,
.Dv LC_MESSAGES ,
.Dv LC_MONETARY ,
.Dv LC_NUMERIC ,
and
.Dv LC_TIME
\(em can be set and retrieved, too, but their values are ignored by the
.Ox
C library.
A category of
.Dv LC_ALL
sets the entire locale generically, which is strongly discouraged for
security reasons in portable programs.
.Pp
The syntax and semantics of the
.Fa locale
argument are not standardized and vary among operating systems.
On
.Ox ,
if the
.Fa locale
string ends with
.Qq ".UTF-8" ,
the UTF-8 locale is selected; otherwise, the
.Qq C
locale is selected, which uses the ASCII character set.
If the
.Fa locale
contains a dot but does not end with
.Qq ".UTF-8" ,
.Fn setlocale
fails.
.Pp
If
.Fa locale
is an empty string
.Pq Qq ,
the value of the environment variable
.Ev LC_ALL ,
with a fallback to the variable corresponding to
.Fa category ,
and with a further fallback to
.Ev LANG ,
is used instead, as documented in the
.Xr locale 1
manual page.
.Pp
If
.Fa locale
is
.Dv NULL ,
the locale remains unchanged.
This can be used to determine the currently active locale.
.Pp
By default, C programs start in the
.Qq C
locale.
The only function in the library that sets the locale is
.Fn setlocale ;
the locale is never changed as a side effect of some other routine.
.Pp
The
.Dv LC_CTYPE
category modifies the behaviour of at least the following functions:
.Xr iswctype 3 ,
.Xr mblen 3 ,
.Xr mbrlen 3 ,
.Xr mbrtowc 3 ,
.Xr mbsrtowcs 3 ,
.Xr mbstowcs 3 ,
.Xr mbtowc 3 ,
.Xr towctrans 3 ,
.Xr towlower 3 ,
.Xr towupper 3 ,
.Xr wcrtomb 3 ,
.Xr wcscasecmp 3 ,
.Xr wcsrtombs 3 ,
.Xr wcstombs 3 ,
.Xr wctomb 3 ,
.Xr wctrans 3 ,
.Xr wctype 3 ,
and the functions documented in
.Xr iswalnum 3 .
.Sh RETURN VALUES
In case of success,
.Fn setlocale
returns a pointer to a static string describing the locale
that is in force after the call.
Subsequent calls to
.Fn setlocale
may change the content of the string.
The format of the string is not standardized and varies among
operating systems.
.Pp
On
.Ox ,
if
.Fn setlocale
was never called with a
.Pf non- Dv NULL
.Fa locale
argument, the string
.Qq C
is returned.
Otherwise, if the
.Fa category
was not
.Dv LC_ALL
or if the locale is the same for all categories, a copy of the
.Fa locale
argument is returned.
Otherwise, the locales for the six categories
.Dv LC_COLLATE ,
.Dv LC_CTYPE ,
.Dv LC_MESSAGES ,
.Dv LC_MONETARY ,
.Dv LC_NUMERIC ,
.Dv LC_TIME
are concatenated in that order, with slash
.Pq Ql /
characters in between.
.Pp
In case of failure,
.Fn setlocale
returns
.Dv NULL .
On
.Ox ,
that can only happen if the
.Fa category
is invalid, if a character encoding other than UTF-8 is requested,
if the requested
.Fa locale
name is of excessive length, or if memory allocation fails.
.Sh EXAMPLES
Calling
.Pp
.Dl setlocale(LC_CTYPE, \(dqen_US.UTF-8\(dq);
.Pp
at the beginning of a program selects the UTF-8 locale and returns
.Qq en_US.UTF-8 .
Calling
.Pp
.Dl setlocale(LC_ALL, NULL);
.Pp
right afterwards leaves the locale unchanged and returns
.Qq C/en_US.UTF-8/C/C/C/C .
.Sh SEE ALSO
.Xr locale 1 ,
.Xr newlocale 3 ,
.Xr nl_langinfo 3 ,
.Xr uselocale 3
.Sh STANDARDS
The
.Fn setlocale
function conforms to
.St -ansiC .
.Sh HISTORY
The
.Fn setlocale
function first appeared in
.Bx 4.3 Net/2 .
.Sh CAVEATS
On systems other than
.Ox ,
calling
.Fn setlocale
or
.Xr uselocale 3
with a
.Fa category
other than
.Dv LC_CTYPE
can cause erratic behaviour of many library functions.
For security reasons, make sure that portable programs only use
.Dv LC_CTYPE .
.Pp
For example, the following functions may be affected.
The list is probably incomplete.
For example, additional library functions may be impacted
if they directly or indirectly call affected functions,
or if they attempt to imitate aspects of their behaviour.
Functions that are not standardized may be affected too.
.Bl -tag -width Ds
.It Dv LC_COLLATE
.Xr glob 3 ,
.Xr strcoll 3 ,
.Xr strxfrm 3 ,
.Xr wcscoll 3 ,
.Xr wcsxfrm 3 ,
and the functions documented in
.Xr regexec 3
.It Dv LC_MESSAGES
.Xr catgets 3 ,
.Xr catopen 3 ,
.Xr nl_langinfo 3 ,
.Xr perror 3 ,
.Xr psignal 3 ,
.Xr strerror 3 ,
.Xr strsignal 3 ,
and the functions documented in
.Xr err 3
.It Dv LC_MONETARY
.Xr localeconv 3 ,
.Xr nl_langinfo 3 ,
.Fn strfmon
.It Dv LC_NUMERIC
.Xr atof 3 ,
.Xr localeconv 3 ,
.Xr nl_langinfo 3 ,
.Fn strfmon ,
and the functions documented in
.Xr printf 3 ,
.Xr scanf 3 ,
.Xr strtod 3 ,
.Xr wcstod 3 ,
.Xr wprintf 3 ,
.Xr wscanf 3 .
This category is particularly dangerous because it can cause bugs
in the parsing and formatting of numbers, for example failures to
recognize or properly write decimal points.
.It Dv LC_TIME
.Fn getdate ,
.Xr nl_langinfo 3 ,
.Xr strftime 3 ,
.Xr strptime 3 .
Similarly, this is prone to causing bugs in the parsing and formatting
of date strings.
.It Dv LC_CTYPE
On systems other than
.Ox ,
this category may affect the behaviour of additional functions,
for example:
.Xr btowc 3 ,
.Xr isalnum 3 ,
.Xr isalpha 3 ,
.Xr isblank 3 ,
.Xr iscntrl 3 ,
.Xr isdigit 3 ,
.Xr isgraph 3 ,
.Xr islower 3 ,
.Xr isprint 3 ,
.Xr ispunct 3 ,
.Xr isspace 3 ,
.Xr isupper 3 ,
.Xr isxdigit 3 ,
.Xr mbsinit 3 ,
.Xr strcasecmp 3 ,
.Xr strcoll 3 ,
.Xr strxfrm 3 ,
.Xr tolower 3 ,
.Xr toupper 3 ,
.Xr vis 3 ,
.Xr wcscoll 3 ,
.Xr wcsxfrm 3 ,
.Xr wctob 3
.El