The tolower() function converts an upper-case letter to the corresponding lower-case letter.
RETURN VALUES
If the argument is an upper-case letter, the tolower() function returns the corresponding lower-case letter if there is one; otherwise the argument is returned unchanged.
The tolower() function conforms to ANSI X3.159-1989 (“ANSI C”).
CAVEATS
The argument to tolower() must be EOF or representable as an unsigned char; otherwise, the behavior is undefined. See the CAVEATS section of ctype(3) for more details.