Path: utzoo!attcan!uunet!ncrlnk!ncrcae!hubcap!gatech!uflorida!ukma!cwjcc!hal!nic.MR.NET!tank!mimsy!haven!uvaarpa!babbage!mac3n
From: mac3n@babbage.acc.virginia.edu (Alex Colvin)
Newsgroups: comp.lang.c
Subject: Re: Something new for C?
Summary: pointer subtraction
Keywords: offset of vars within structures
Message-ID: <399@babbage.acc.virginia.edu>
Date: 10 Nov 88 21:35:20 GMT
References: <73@dsoft.UUCP> <2865@ingr.UUCP>
Organization: University of Virginia
Lines: 14

> > way to do and have always wanted, was a precompiler command to allow me to
> > use the offset of an item into a structure. for example:

> #define  offset(type,field) ((unsigned int)&((type *)0)->field)

This works by casting a pointer to an unsigned.  That sometimes isn't a
good idea.  To make integers out of pointers, it's best to subtract.

#define  offset(type,field) ( (void *)&((type *)0)->field - (void *)((type *)0) )

This gives you the offset in (void)s.  You could have it in (char)s or
(short)s instead.  The question is, why do you want this?  It doesn't
belong in the precompiler, since it has to know about size and alignment
constraints.  Explicit pointer subtraction is usually what I want.