Newsgroups: comp.std.c
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: Void pointers and pointer arithmetic
Message-ID: <1990Jan2.203408.1137@utzoo.uucp>
Organization: U of Toronto Zoology
References: <1055@esatst.yc.estec.nl>
Date: Tue, 2 Jan 90 20:34:08 GMT

In article <1055@esatst.yc.estec.nl> arne@yc.estec.nl (Arne Lundberg) writes:
>...Is this program legal in ANSI C, will it
>produce the desired result? 

No.  Pointer arithmetic is not defined for pointer-to-void.  It can't be,
since it's not possible to do pointer arithmetic without knowing the size
of the type pointed to.

>BTW,
>is it possible to write an ANSI compatible ``offsetof'' macro
>for traditional C compilers? ...

Sort of.  We do this in C News.  Here's what we say about it in our
porting-problems document (C News enthusiasts:  this will be in the
next patch):

	Unfortunately, it is really hard to write a portable version of this.
	The implementation we currently use is:
	.DS
	#define offsetof(type, mem) ((char *)&((type *)NULL)->mem - (char *)NULL)
	.DE
	The table in \fIrelay/hdrdefs.c\fR
	puts invocations of \fIoffsetof\fR in initializers.
	This turns out to be a severe stress test for C compilers.
	A compilation error in \fIhdrdefs.c\fR is almost certain
	to be problems with this macro.
	Some compilers,
	notably the one in Microport System V Release 2.3,
	reject it.
	We have heard a report that System V Release 2 on the VAX silently
	miscompiles it!
	If you have trouble with \fIoffsetof\fR, you might try
	this version instead:
	.DS
	#define offsetof(type, mem) ((int)&((type *)NULL)->mem)
	.DE
-- 
1972: Saturn V #15 flight-ready|     Henry Spencer at U of Toronto Zoology
1990: birds nesting in engines | uunet!attcan!utzoo!henry henry@zoo.toronto.edu