The toupper() function converts a lower-case letter to the corresponding upper-case letter.
RETURN VALUES
If the argument is a lower-case letter, the toupper() function returns the corresponding upper-case letter if there is one; otherwise the argument is returned unchanged.
The toupper() function conforms to ANSI X3.159-1989 (“ANSI C”).
CAVEATS
The argument to toupper() must be EOF or representable as an unsigned char; otherwise, the behavior is undefined. See the CAVEATS section of ctype(3) for more details.