man3/duplocale.3

.\" Copyright (C) 2014 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" %%%LICENSE_START(VERBATIM)
.\" Permission is granted to make and distribute verbatim copies of this
.\" manual provided the copyright notice and this permission notice are
.\" preserved on all copies.
.\"
.\" Permission is granted to copy and distribute modified versions of this
.\" manual under the conditions for verbatim copying, provided that the
.\" entire resulting derived work is distributed under the terms of a
.\" permission notice identical to this one.
.\"
.\" Since the Linux kernel and libraries are constantly changing, this
.\" manual page may be incorrect or out-of-date.  The author(s) assume no
.\" responsibility for errors or omissions, or for damages resulting from
.\" the use of the information contained herein.  The author(s) may not
.\" have taken the same level of care in the production of this manual,
.\" which is licensed free of charge, as they might when working
.\" professionally.
.\"
.\" Formatted or processed versions of this manual, if unaccompanied by
.\" the source, must acknowledge the copyright and authors of this work.
.\" %%%LICENSE_END
.\"
.TH DUPLOCALE 3 2021-03-22 "Linux" "Linux Programmer's Manual"
.SH NAME
duplocale \- duplicate a locale object
.SH SYNOPSIS
.nf
.B #include <locale.h>
.PP
.BI "locale_t duplocale(locale_t " locobj );
.fi
.PP
.RS -4
Feature Test Macro Requirements for glibc (see
.BR feature_test_macros (7)):
.RE
.PP
.BR duplocale ():
.nf
    Since glibc 2.10:
        _XOPEN_SOURCE >= 700
    Before glibc 2.10:
        _GNU_SOURCE
.fi
.SH DESCRIPTION
The
.BR duplocale ()
function creates a duplicate of the locale object referred to by
.IR locobj .
.PP
If
.I locobj
is
.BR LC_GLOBAL_LOCALE ,
.BR duplocale ()
creates a locale object containing a copy of the global locale
determined by
.BR setlocale (3).
.SH RETURN VALUE
On success,
.BR duplocale ()
returns a handle for the new locale object.
On error, it returns
.IR "(locale_t)\ 0",
and sets
.I errno
to indicate the error.
.SH ERRORS
.TP
.B ENOMEM
Insufficient memory to create the duplicate locale object.
.SH VERSIONS
The
.BR duplocale ()
function first appeared in version 2.3 of the GNU C library.
.SH CONFORMING TO
POSIX.1-2008.
.SH NOTES
Duplicating a locale can serve the following purposes:
.IP * 3
To create a copy of a locale object in which one of more categories
are to be modified (using
.BR newlocale (3)).
.IP *
To obtain a handle for the current locale which can used in
other functions that employ a locale handle, such as
.BR toupper_l (3).
This is done by applying
.BR duplocale ()
to the value returned by the following call:
.IP
    loc = uselocale((locale_t) 0);
.IP
This technique is necessary, because the above
.BR uselocale (3)
call may return the value
.BR LC_GLOBAL_LOCALE ,
which results in undefined behavior if passed to functions such as
.BR toupper_l (3).
Calling
.BR duplocale ()
can be used to ensure that the
.BR LC_GLOBAL_LOCALE
value is converted into a usable locale object.
See EXAMPLES, below.
.PP
Each locale object created by
.BR duplocale ()
should be deallocated using
.BR  freelocale (3).
.SH EXAMPLES
The program below uses
.BR uselocale (3)
and
.BR duplocale ()
to obtain a handle for the current locale which is then passed to
.BR toupper_l (3).
The program takes one command-line argument,
a string of characters that is converted to uppercase and
displayed on standard output.
An example of its use is the following:
.PP
.in +4n
.EX
$ \fB./a.out abc\fP
ABC
.EE
.in
.SS Program source
\&
.EX
#define _XOPEN_SOURCE 700
#include <ctype.h>
#include <stdio.h>
#include <stdlib.h>
#include <locale.h>

#define errExit(msg)    do { perror(msg); exit(EXIT_FAILURE); \e
                        } while (0)

int
main(int argc, char *argv[])
{
    locale_t loc, nloc;

    if (argc != 2) {
        fprintf(stderr, "Usage: %s string\en", argv[0]);
        exit(EXIT_FAILURE);
    }

    /* This sequence is necessary, because uselocale() might return
       the value LC_GLOBAL_LOCALE, which can\(aqt be passed as an
       argument to toupper_l(). */

    loc = uselocale((locale_t) 0);
    if (loc == (locale_t) 0)
        errExit("uselocale");

    nloc = duplocale(loc);
    if (nloc == (locale_t) 0)
        errExit("duplocale");

    for (char *p = argv[1]; *p; p++)
        putchar(toupper_l(*p, nloc));

    printf("\en");

    freelocale(nloc);

    exit(EXIT_SUCCESS);
}
.EE
.SH SEE ALSO
.BR freelocale (3),
.BR newlocale (3),
.BR setlocale (3),
.BR uselocale (3),
.BR locale (5),
.BR locale (7)