setlocale (GNU Gnulib)

Gnulib module: setlocale, setlocale-null

Portability problems fixed by Gnulib module setlocale:

On Windows platforms (excluding Cygwin), setlocale(category,"") and setlocale(category,NULL) both ignore the environment variables LC_ALL, category, and LANG.
On Windows platforms (excluding Cygwin) and Cygwin 1.5.x, setlocale(LC_ALL,name) succeeds and sets the LC_CTYPE category to ‘C’ when it does not support the encoding, instead of failing.
On Windows platforms (excluding Cygwin), setlocale understands different locale names, that are not based on ISO 639 language names and ISO 3166 country names.
On Android < 5.0, which doesn’t have locales, the setlocale function always fails. The replacement, however, supports only the locale names "C" and "POSIX".

Portability problems fixed by Gnulib module setlocale or setlocale-null:

Invocations of 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 setlocale, or you need to change the code to invoke setlocale_null or setlocale_null_r instead.

Portability problems not fixed by Gnulib:

On Cygwin 1.5.x, which doesn’t have locales, setlocale(LC_ALL,NULL) always returns "C".
On Cygwin 1.7.0, only the charset portion of a locale designation is honored.
On OpenBSD, setlocale(LC_ALL,"") will only update categories that are deemed appropriate for the LC_ALL environment value, even if there are other categories set to different values in the environment. In addition any value is accepted for 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 setlocale(LC_COLLATE,"") etc.
On Android 5.0 and newer, the default locale (i.e. the locale in use when setlocale was not called) is the "C.UTF-8" locale, not the "C" locale. Additionally, a setlocale call that is meant to set the "C" or "POSIX" locale actually sets an equivalent of the "C.UTF-8" locale.