@node setlocale
@subsection @code{setlocale}
@findex setlocale

POSIX specification:@* @url{https://pubs.opengroup.org/onlinepubs/9799919799/functions/setlocale.html}

Gnulib module: setlocale, setlocale-null
@mindex setlocale
@mindex setlocale-null

Portability problems fixed by Gnulib module @code{setlocale}:
@itemize
@item
On Windows platforms (excluding Cygwin), @code{setlocale(@var{category},"")}
and @code{setlocale(@var{category},NULL)} both ignore the environment variables
@code{LC_ALL}, @code{@var{category}}, and @code{LANG}.
@item
On Windows platforms (excluding Cygwin) and Cygwin 1.5.x,
@code{setlocale(LC_ALL,@var{name})} succeeds and sets the LC_CTYPE category to
@samp{C} when it does not support the encoding, instead of failing.
@item
On Windows platforms (excluding Cygwin), @code{setlocale} understands different
locale names, that are not based on ISO 639 language names and ISO 3166 country
names.
@item
On Android 5.0 and newer, setting any category other than @code{LC_CTYPE} of
the global locale affects the @code{LC_CTYPE} category as well.
@item
On Android < 5.0, which doesn't have locales, the @code{setlocale} function
always fails.  The replacement, however, supports only the locale names
@code{"C"} and @code{"POSIX"}.
@end itemize

Portability problems fixed by Gnulib module @code{setlocale} or @code{setlocale-null}:
@itemize
@item
Invocations of @code{setlocale (..., NULL)} are not thread-safe on some
platforms:
musl libc, macOS, FreeBSD, NetBSD, OpenBSD, AIX, Haiku, Cygwin.
To make these invocations thread-safe, you need the Gnulib module
@code{setlocale}, or you need to change the code to invoke @code{setlocale_null}
or @code{setlocale_null_r} instead.
@end itemize

Portability problems not fixed by Gnulib:
@itemize
@item
On Cygwin 1.5.x, which doesn't have locales,
@code{setlocale(LC_ALL,NULL)} always returns @code{"C"}.
@item
On Cygwin 1.7.0, only the charset portion of a locale designation is honored.
@item
On OpenBSD, @code{setlocale(LC_ALL,"")} will only update categories that
are deemed appropriate for the @code{LC_ALL} environment value, even if
there are other categories set to different values in the environment.
In addition any value is accepted for @code{LC_CTYPE}, and so NULL
is never returned to indicate a failure to set locale.
To verify category values, each category must be set individually
with @code{setlocale(LC_COLLATE,"")} etc.
@item
On Android 5.0 and newer, the default locale (i.e.@: the locale in use when
@code{setlocale} was not called) is the @code{"C.UTF-8"} locale, not the
@code{"C"} locale.  Additionally, a @code{setlocale} call that is meant to set
the @code{"C"} or @code{"POSIX"} locale actually sets an equivalent of the
@code{"C.UTF-8"} locale.
@end itemize

Note: The return type of the @code{setlocale} function
is @code{char *} in POSIX, with the constraint that
``The application shall not modify the string returned.''
The Gnulib-overridden @code{setlocale} function
has a return type of @code{const char *}.
This helps detecting code that attempts to write into that string.

Note: The names of locales with UTF-8 encoding are platform dependent:
@itemize
@item
On most platforms, they follow the naming scheme @samp{@var{ll}_@var{CC}.UTF-8}.
Here @samp{@var{ll}} is an @w{ISO 639} two-letter language code,
and @samp{@var{CC}} is an @w{ISO 3166} two-letter country code.
For example, @samp{en_US.UTF-8} (this locale exists on nearly all systems),
@samp{fr_FR.UTF-8}, and similar.
@item
On native Windows with mingw, UTF-8 locales are not supported.
@item
On native Windows with MSVC (version 14 or newer,
and with the @code{-MD} option in @code{CFLAGS}),
they follow the naming scheme @samp{@var{language}_@var{country}.65001}
or, equivalently, @samp{@var{language}_@var{country}.utf8},
where @samp{@var{language}} is a language name in English
and @samp{@var{country}} is a country name in English.
For example @samp{English_United States.65001} (yes, with a space in it!)
or @samp{French_France.65001}.
@end itemize
