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From: tp@wucs.UUCP (tom thumbs)
Newsgroups: net.lang.c
Subject: handy.h: macro
Message-ID: <1153@wucs.UUCP>
Date: Tue, 10-Sep-85 22:39:41 EDT
Article-I.D.: wucs.1153
Posted: Tue Sep 10 22:39:41 1985
Date-Received: Wed, 11-Sep-85 20:13:10 EDT
Organization: Kurt's Krunchers (Wash U)
Lines: 48
Keywords: offsets within structures


    The macro OFFSETV is good for finding the offset of a
certain variable within a structure. OFFSET can also be used,
if one is careful to hand it an address. [ hopefully, the
example below will explain what this is doing.]
    (As noted below, these should be used carefully.)
    Anyway, I found these exceedingly useful when doing lots of
database stuff with intricate structures.
    I believe that credit for coming up with these belongs to
Brian Thomas of AT&T (ihnp4!we53!bmt).
---------------------handy macro & example below ----------
#include <stdio.h>

/* the two lines below contain the wild macros -- cautious people
    will put parens are the X and Y on right-hand side */
#define OFFSET(X, Y)	( (int) ((struct X *)0)-> Y )
#define OFFSETV(X, Y)	( (int) &((struct X *)0)-> Y )

struct foo {		/* simple example structure type	*/
    int harris;
    char torek[6];
    long gwyn;
} ;

main()
{
    printf(" offset harris=%d, gwyn=%d, torek=%d, alt. torek=%d\n",
	OFFSETV(foo, harris), 
	OFFSETV(foo, gwyn), 
	OFFSETV(foo, torek[0]),
	OFFSET(foo, torek) );
}

-----------------------------------------------------------------

   On VAX 750 with 4.2 BSD (version Kurt) we get output of:

 offset harris=0, gwyn=12, torek=4, alt. torek=4

    (note gwyn is word-aligned, of course)
--------
WARNINGS: may cause weird alignment problems!! Also, you better
    know the difference between arrays and pointers (in other words,
    this is dangerous for novices, and probably others as well).
--
   ...tp...
       tom patterson
	   ... {seismo, ihnp4}!wucs!tp