The isupper() function tests for any upper-case letter or any of an implementation-defined set of characters for which none of iscntrl(3), isdigit(3), ispunct(3), or isspace(3) is true. In the ``C'' locale, isupper() returns true only for the characters defined as upper-case letters.
RETURN VALUES
The isupper() function returns zero if the character tests false and returns non-zero if the character tests true.
The isupper() function conforms to ANSI X3.159-1989 (“ANSI C”).
CAVEATS
The argument to isupper() must be EOF or representable as an unsigned char; otherwise, the behavior is undefined. See the CAVEATS section of ctype(3) for more details.