toupper() converts the letter c to upper case, if possible.
tolower() converts the letter c to lower case, if possible.
If c is not an unsigned char value, or EOF, the behaviour of these
functions is undefined.
The details of what constitutes an uppercase or lowercase letter depend on the
current locale. For example, the default "C" 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.