#include <ctype.h>
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, tolower
returns the corresponding lowercase letter; otherwise, the argument is returned
unchanged.
tolower conforms to ISO/IEC 9899:1990 (C90) and ISO/IEC 9899:1999 (C99).