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ISLOWER(3) Library Functions Manual ISLOWER(3)

islower, islower_l
lower-case single-byte character test

#include <ctype.h>
int
islower(int c);
int
islower_l(int c, locale_t locale);

The islower() and islower_l() functions test whether c represents a lower-case letter.
In the C locale, the complete list of lower-case letters is a–z. OpenBSD always uses the C locale for these functions, ignoring the global locale, the thread-specific locale, and the locale argument.

These functions return zero if the character tests false or non-zero if the character tests true.

On systems supporting non-ASCII single-byte character encodings, these functions may return non-zero for additional characters, and the results of islower() may depend on the LC_CTYPE locale(1), but they never return non-zero for any character for which iscntrl(3), isdigit(3), ispunct(3), or isspace(3) is true.

isalnum(3), isalpha(3), isascii(3), isblank(3), iscntrl(3), isdigit(3), isgraph(3), isprint(3), ispunct(3), isspace(3), isupper(3), iswlower(3), isxdigit(3), stdio(3), toascii(3), tolower(3), toupper(3), ascii(7)

The islower() function conforms to ANSI X3.159-1989 (“ANSI C89”), and islower_l() to IEEE Std 1003.1-2008 (“POSIX.1”).

The islower() function first appeared in Version 7 AT&T UNIX, AND islower_l() has been available since OpenBSD 6.2.

The argument c must be EOF or representable as an unsigned char; otherwise, the result is undefined.
September 5, 2017 OpenBSD-current