function shall return a pointer into an array of
characters in the current locale that contains upper case equivalents for each
character in the current character set. The array shall contain a total of
characters, and can be indexed with any signed or unsigned char (i.e. with an
index value between
255). If the application is multithreaded, the
array shall be local to the current thread.
This interface is not in the source standard; it is only in the binary standard.
The __ctype_toupper_loc() function shall return a pointer to the array of characters to be used for the ctype() family of functions (see <ctype.h>).