The islower() function tests for any lower-case letter for which none of iscntrl(3), isdigit(3), ispunct(3), or isspace(3) is true. In the ``C'' locale, islower() returns true only for the characters defined as lower-case letters.
RETURN VALUES
The islower() function returns zero if the character tests false and returns non-zero if the character tests true.
The islower() function conforms to ANSI X3.159-1989 (“ANSI C”).
CAVEATS
The argument to islower() must be EOF or representable as an unsigned char; otherwise, the behavior is undefined. See the CAVEATS section of ctype(3) for more details.