Conversion from lower-case to upper-case (not converted if argument is not in lower-case)
If islower is true with respect to c, returns a character that makes isupper true in response; otherwise, returns c.
This function is a macro that converts lowercase characters into the corresponding uppercase characters and leaves the other characters unchanged.
This macro is defined only when c is an integer in the range of EOF to 255. A compiled subroutine can be used instead of the macro definition, which is invalidated by using "#undef toupper".