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From: amoss@batata.huji.ac.il (amos shapira)
Newsgroups: comp.lang.c
Subject: Re: offsetof() operator.  A tricky question.
Message-ID: <695@shuldig.Huji.Ac.IL>
Date: 13 Mar 90 06:08:13 GMT
References: <8ega02MX90Tk01@amdahl.uts.amdahl.com>
Sender: daemon@ucbvax.BERKELEY.EDU
Lines: 36

vohra@uts.amdahl.com (Pavan Vohra) writes:

>I would like to compile in the offsets of structure members.

>For example, with this structure

>    struct astruct {
>      int member0;
>      char member1;
>    } myastruct;

>I want to have, in the same program that the structure appears in,
>the number of bytes past myastruct where I can find member1.

The answer could be simple and I used several times, do this:

struct astruct *ap;

ap = NULL;

offsetof_member0 = &ap->member0; /* zero offset */
offsetof_member1 = &ap->member1; /* sizeof(member0) */

An offsetof() operator would be quite handy here. But until then you can
use this with a good portability of your programs.

Now comes to mind, maybe you could avoid the nbull pointer variable with this
(didn't try it):

offsetof_member0 = &((struct astruct *)NULL->member0);
offsetof_member1 = &((struct astruct *)NULL->member1);

- Amos Shapira

amoss@batata.bitnet
amoss@batata.huji.ac.il