#include <ctype.h> int islower(int c); int islower_l(int c, locale_t locale);
The islower() and islower_l() functions shall test whether c is a character of class lower in the current locale, or in the locale represented by locale, respectively; see the Base Definitions volume of POSIX.1-2017, Chapter 7, Locale.
The c argument is an int, the value of which the application shall ensure is a character representable as an unsigned char or equal to the value of the macro EOF. If the argument has any other value, the behavior is undefined.
The behavior is undefined if the locale argument to islower_l() is the special locale object LC_GLOBAL_LOCALE or is not a valid locale object handle.
The following sections are informative.
Two examples follow, the first using islower(), the second using multiple concurrent locales and islower_l().
The examples test whether the value is a lowercase letter, based on the current locale, then use it as part of a key value.
/* Example 1 -- using islower() */ #include <ctype.h> #include <stdlib.h> #include <locale.h> ... char *keystr; int elementlen, len; unsigned char c; ... setlocale(LC_ALL, ""); ... len = 0; while (len < elementlen) { c = (unsigned char) (rand() % 256); ... if (islower(c)) keystr[len++] = c; } ... /* Example 2 -- using islower_l() */ #include <ctype.h> #include <stdlib.h> #include <locale.h> ... char *keystr; int elementlen, len; unsigned char c; ... locale_t loc = newlocale (LC_ALL_MASK, "", (locale_t) 0); ... len = 0; while (len < elementlen) { c = (unsigned char) (rand() % 256); ... if (islower_l(c, loc)) keystr[len++] = c; } ...
The Base Definitions volume of POSIX.1-2017, Chapter 7, Locale, <ctype.h>, <locale.h>
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