Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!taumet!steve
From: steve@taumet.com (Stephen Clamage)
Newsgroups: comp.lang.c
Subject: Re: Taking address of array
Message-ID: <666@taumet.com>
Date: 11 Apr 91 14:48:15 GMT
References: <1991Apr9.172534.28982@tcom.stc.co.uk>
Organization: Taumetric Corporation, San Diego
Lines: 32

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

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

>((int) (((char *) (&(((struct small_struct*) 0)->y))) - ((char *) 0)))

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

The cast to char* and subtraction of zero don't do anything useful.

Casting the result to int is not appropriate on all systems.  ANSI uses
a defined type called 'size_t', which is the unsigned integral type of
the 'sizeof' operator (usually unsigned int, sometimes unsigned long).

A typical 'offsetof' macro looks like this:
	#define  offsetof(type, field) ((size_t)&(((type*)0)->field))

This, like your example, is not portable to all systems, since some will
object to what appears to be a dereference of a null pointer.

Your best bet might be to put something like this in common header:

#if __STDC__
#include <stddef.h>	/* ANSI C compiler, get correct size_t and offsetof */
#else
typedef unsigned int size_t;	/* or whatever type is correct */
#define offsetof(type, field) ((size_t)&(((type*)0)->field))
#endif
-- 

Steve Clamage, TauMetric Corp, steve@taumet.com