The Open Group Base Specifications Issue 7
IEEE Std 1003.1, 2013 Edition
Copyright © 2001-2013 The IEEE and The Open Group


islower, islower_l - test for a lowercase letter


#include <ctype.h>

int islower(int

[CX] [Option Start] int islower_l(int c, locale_t locale); [Option End]


For islower(): [CX] [Option Start]  The functionality described on this reference page is aligned with the ISO C standard. Any conflict between the requirements described here and the ISO C standard is unintentional. This volume of POSIX.1-2008 defers to the ISO C standard. [Option End]

The islower() [CX] [Option Start]  and islower_l() [Option End]  functions shall test whether c is a character of class lower in the current locale, [CX] [Option Start]  or in the locale represented by locale, [Option End] respectively; see XBD 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.

[CX] [Option Start] 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. [Option End]


The islower() [CX] [Option Start]  and islower_l() [Option End]  functions shall return non-zero if c is a lowercase letter; otherwise, they shall return 0.


No errors are defined.

The following sections are informative.


Testing for a Lowercase Letter

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; } ...


To ensure applications portability, especially across natural languages, only these functions and the functions in the reference pages listed in the SEE ALSO section should be used for character classification.






isalnum, isalpha, isblank, iscntrl, isdigit, isgraph, isprint, ispunct, isspace, isupper, isxdigit, setlocale, uselocale

XBD Locale, <ctype.h>, <locale.h>


First released in Issue 1. Derived from Issue 1 of the SVID.

Issue 6

The normative text is updated to avoid use of the term "must" for application requirements and an example is added.

Issue 7

The islower_l() function is added from The Open Group Technical Standard, 2006, Extended API Set Part 4.

POSIX.1-2008, Technical Corrigendum 1, XSH/TC1-2008/0292 [302], XSH/TC1-2008/0293 [283], XSH/TC1-2008/0294 [283], XSH/TC1-2008/0295 [302], and XSH/TC1-2008/0296 [304] are applied.

End of informative text.


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