std::setlocale (3) - Linux Manuals
Defined in header <clocale>
char* setlocale( int category, const char* locale);
The setlocale function installs the specified system locale or its portion as the new C locale. The modifications remain in effect and influences the execution of all locale-sensitive C library functions until the next call to setlocale. If locale is a null pointer, setlocale queries the current C locale without modifying it.
category - locale category identifier, one of the LC_xxx macros. May be 0.
locale - system-specific locale identifier. Can be "" for the user-preferred locale or "C" for the minimal locale
Pointer to a narrow null-terminated string identifying the C locale after applying the changes, if any, or null pointer on failure.
A copy of the returned string along with the category used in this call to std::setlocale may be used later in the program to restore the locale back to the state at the end of this call.
During program startup, the equivalent of std::setlocale(LC_ALL, "C"); is executed before any user code is run.
Although the return type is char*, modifying the pointed-to characters is undefined behavior.
Because setlocale modifies global state which affects execution of locale-dependent functions, it is undefined behavior to call it from one thread, while another thread is executing any of the following functions: std::fprintf, std::isprint, std::iswdigit, std::localeconv, std::tolower, std::fscanf, std::ispunct, std::iswgraph, std::mblen, std::toupper, std::isalnum, std::isspace, std::iswlower, std::mbstowcs, std::towlower, std::isalpha, std::isupper, std::iswprint, std::mbtowc, std::towupper, std::isblank, std::iswalnum, std::iswpunct, std::setlocale, std::wcscoll, std::iscntrl, std::iswalpha, std::iswspace, std::strcoll, std::wcstod, std::isdigit, std::iswblank, std::iswupper, std::strerror, std::wcstombs, std::isgraph, std::iswcntrl, std::iswxdigit, std::strtod, std::wcsxfrm, std::islower, std::iswctype, std::isxdigit.
POSIX also defines a locale named "POSIX", which is always accessible and is exactly equivalent to the default minimal "C" locale.
POSIX also specifies that the returned pointer, not just the contents of the pointed-to string, may be invalidated by subsequent calls to setlocale.
// Run this code
LC_COLLATE locale categories for std::setlocale
LC_CTYPE (macro constant)