Determines whether a character is an ASCII upper case letter.
Unlike the standard C library
isupper() function, this only
recognizes standard ASCII letters and ignores the locale,
FALSE for all non-ASCII characters. Also, unlike
the standard library function, this takes a char, not an int,
so don’t call it on
EOF, but no need to worry about casting
to #guchar before passing a possibly non-ASCII character in.
This function is not directly available to language bindings.