The standard C locale is called simply ‘C’. In order to provide POSIX compatibility, the name ‘POSIX’ is a synonym for ‘C’.
The C locale is fixed and supports only the ASCII character set with character codes 0 through 127. There is no multi-byte character support, so the character encoding between wide and narrow characters is simply one-to-one: a narrow character is converted to a wide character by zero extension. Thus, ASCII encoding of narrow characters is compatible with the ISO 10646 (Unicode) encoding of wide characters in this locale.