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From: wolfram@cip-s08.informatik.rwth-aachen.de (Wolfram Roesler)
Newsgroups: comp.lang.c
Subject: Re: Taking address of array
Message-ID: <wolfram.671457523@cip-s08>
Date: 12 Apr 91 11:58:43 GMT
References: <1991Apr9.172534.28982@tcom.stc.co.uk>
Sender: news@rwthinf.UUCP
Lines: 38

graham@tcom.stc.co.uk (Graham Bardsley) writes:

>struct small_struct
>{
>	int x;
>	char y[100];
>};

>What I want to know is, if the macro calculates:

>((int) (((char *) (&(((s*) 0)->y))) - ((char *) 0)))

>Is the value of this a valid construct which will calculate the offset of y on 
>most traditional C compilers?

An & before an array fails on all compilers I know. In your example,
if s is of type small_struct, `s.y' is a pointer to the first element of
the array, and `&s.y' is illegal. Try `& s.y[0]' instead. I'm afraid
you will need to write a new macro, one that will determine the offset of
an array within a struct.
Suppose you used the macro

	#define offset(s,e) (int) (&(((s*)0)->e) - 0)

(parantheses and casts omitted for clarity) do determine the offset of
element e in struct s, you will have to write a new macro

	#define aoffset(s,e) (int) ((((s*)0)->e) - 0)
                                    ^
				    '&' omitted

if e is an array.
So: copy the original definition of your macro, give it a new name and
erase the '&' and use this one for arrays. This should work.

Grettings
Okami-san
f