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ISUPPER(3) Library Functions Manual ISUPPER(3)

isupper, isupper_l
upper-case singly-byte character test

#include <ctype.h>
isupper(int c);
isupper_l(int c, locale_t locale);

The isupper() and isupper_l() functions test whether c represents an upper-case letter.
In the C locale, the complete list of upper-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 isupper() 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), islower(3), isprint(3), ispunct(3), isspace(3), iswupper(3), isxdigit(3), stdio(3), toascii(3), tolower(3), toupper(3), ascii(7)

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

The isupper() function first appeared in Version 7 AT&T UNIX, and isupper_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