Ascertains whether a given character is a lowercase letter
#include <ctype.h> int islower ( int c );
The function islower( ) tests whether its character argument is a lowercase letter. Which characters are letters and which letters are lowercase both depend on the current locale setting for the category LC_CTYPE, which you can query or change using the setlocale( ) function.
If the character is a lowercase letter, islower( ) returns a nonzero value (that is, TRue); if not, the function returns 0 (false).
In the default locale C, the truth values of isupper( ) and islower( ) are mutually exclusive for the alphabetic characters. However, other locales may have alphabetic characters for which both isupper( ) and islower( ) return true, or characters which are alphabetic, but are neither upper- nor lowercase.
See the example for isprint( ) in this chapter.
isupper( ), tolower( ), toupper( ); the corresponding C99 function for wide characters, iswlower( ); isalnum( ), isalpha( ), isblank( ), iscntrl( ), isdigit( ), isgraph( ), isprint( ), ispunct( ), isspace( ), isxdigit( ), setlocale( )