toupper, tolower - convert letter to upper or lower case
int tolower (int c);
toupper() converts the letter c to upper case,
tolower() converts the letter c to lower case,
If c is not an unsigned char value, or EOF, the
behaviour of these functions is undefined.
The value returned is that of the converted letter, or
c if the conversion was not possible.
ANSI C, BSD 4.3
The details of what constitutes an uppercase or lowercase
letter depend on the current locale. For example, the
default locale does not know about
umlauts, so no conversion is done for them.
In some non - English locales, there are lowercase letters
with no corresponding uppercase equivalent; the German sharp
s is one example.