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ISALPHA(3) Library Functions Manual ISALPHA(3)

isalpha, isalpha_l
alphabetic single-byte character test

#include <ctype.h>
int
isalpha(int c);
int
isalpha_l(int c, locale_t locale);

The isalpha() and isalpha_l() functions test whether c represents a letter.
In the C locale, the complete list of alphabetic characters is A–Z and 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 isalnum() 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), isascii(3), isblank(3), iscntrl(3), isdigit(3), isgraph(3), islower(3), isprint(3), ispunct(3), isspace(3), isupper(3), iswalpha(3), isxdigit(3), stdio(3), toascii(3), tolower(3), toupper(3), ascii(7)

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

The isalpha() function first appeared in Version 7 AT&T UNIX, and isalpha_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