int tolower(int c);

tolower converts an uppercase letter to a corresponding lowercase letter. If the argument c is a character for which isupper is true and there are one or more corresponding characters, as specified by the current locale, for which islower is true, the tolower function returns one of the corresponding characters (always the same one for any given locale); otherwise, the argument is returned unchanged.

Note that even though isupper can return true for some characters, tolower may return that uppercase character unchanged as there are no corresponding lowercase characters in the locale.