Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!wuarchive!uunet!virtech!cpcahil
From: cpcahil@virtech.uucp (Conor P. Cahill)
Newsgroups: comp.lang.c
Subject: Re: How to get a byte offset
Message-ID: <1990May28.131914.11205@virtech.uucp>
Date: 28 May 90 13:19:14 GMT
References: <1990May28.034643.6962@cs.umn.edu>
Reply-To: cpcahil@virtech.UUCP (Conor P. Cahill)
Distribution: usa
Organization: Virtual Technologies Inc., Sterling VA
Lines: 39

In article <1990May28.034643.6962@cs.umn.edu> swie@cs.umn.edu (S. T. Tan) writes:
>Is there an easy way to get the byte offset of a field in a structure without
>counting it manually ?

Here are a few examples:

/*
 * the first one works using the structure type name and does not
 * require the runtime existance of any occurance of such a structure.
 * NOTE: this may be non-portable due to the use of a NULL pointer.
 */
#define	offsetof(elem,str)	(&(((struct str *)0)->elem))

/*
 * the following one requires an occurance of the structure itself and 
 * should be relatively portable.
 */

#define myoffset(elem,data)	( ((char *) &data.d) - ((char *)&data) )

struct my_struct_type {
int	a;
char	b[3];
int	c[10];
char	d;
} my_name;


main()
{

	printf( "offsetof(d,my_struct_type) = %d\n",offsetof(d,my_struct_type));
	printf( "myoffset(d,my_name) = %d\n",myoffset(d,my_name));
}

-- 
Conor P. Cahill            (703)430-9247        Virtual Technologies, Inc.,
uunet!virtech!cpcahil                           46030 Manekin Plaza, Suite 160
                                                Sterling, VA 22170