Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!uunet!husc6!necntc!ima!think!rst
From: rst@think.COM (Robert Thau)
Newsgroups: comp.lang.c
Subject: Re: I'd like to . . .
Message-ID: <11661@think.UUCP>
Date: Wed, 11-Nov-87 16:29:29 EST
Article-I.D.: think.11661
Posted: Wed Nov 11 16:29:29 1987
Date-Received: Sat, 14-Nov-87 03:00:43 EST
References: <10224@brl-adm.ARPA> <6660@brl-smoke.ARPA> <511@interlan.UUCP>
Sender: usenet@think.UUCP
Reply-To: rst@mneme.UUCP (Robert Thau)
Organization: Thinking Machines Corporation, Cambridge, MA
Lines: 44

In article <511@interlan.UUCP> deem@interlan.UUCP (Mike Deem) writes:
>I think this could be declared something like:
>
>   struct {
>     int      c;              /* c takes up space                       */
>     union {                  /* no identifier                          */
>       int    a;              /* both a and b are in the same space and */
>       float  b;              /* that space follows c's                 */
>     };                       /* no identified here either              */
>     char     d;              /* d takes space following the a b space  */
>   } s;

"Fixed in C++", which supports exactly this syntax.  

In plain C, there is no way to suppress the name of the union
entirely, so the typical thing is to hide it with a little
preprocessor skullduggery.  For example,

struct s {
  int      c;              /* c takes up space                       */
  union {
    int    A;              /* both a and b are in the same space and */
    float  B;              /* that space follows c's                 */
  } u;
  char     d;              /* d takes space following the a b space  */
};

#define a u.A
#define b u.B

main()
{
	struct s s;
	printf("&s.a = %d, &s.b = %d, &s.c = %d, &s.d=%d\n",
	       (int)&s.a, (int)&s.b, (int)&s.c, (int)&s.d);
}

on a Sun-3 (where pointers fit in ints) produces the output:

&s.a = 251657764, &s.b = 251657764, &s.c = 251657760, &s.d=251657768

which shows that a and b do in fact have the same address.

rst