@node setlocale
@section @code{setlocale}
@findex setlocale

POSIX specification:@* @url{https://pubs.opengroup.org/onlinepubs/9699919799/functions/setlocale.html}

Gnulib module: setlocale, setlocale-null

Portability problems fixed by Gnulib module @code{setlocale}:
@itemize
@item
On Windows platforms (excluding Cygwin), @code{setlocale(@var{category},NULL)}
ignores 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, 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 multithread-safe on some
platforms:
musl libc, macOS, FreeBSD, NetBSD, OpenBSD, AIX, Haiku, Cygwin.
To make these invocations multithread-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