isalpha in ctype.h

[Niklas Sjovall]

Hi,

I want to use a macro defined in ctype.h on a Sun4 (4.03), but i don't
fully understand it.

The macro is:
#define	_U	01
#define	_L	02
extern	char	_ctype_[];
#define	isalpha(c)	((_ctype_+1)[c]&(_U|_L))

It's the part (_ctype_+1)[c] i don't understand. Could there be any
segmentation errors using this?

Thanks