Path: utzoo!utgpu!water!watmath!clyde!cbosgd!ihnp4!chinet!dag
From: dag@chinet.UUCP (Daniel A. Glasser)
Newsgroups: comp.lang.c
Subject: Re:  what C needs -- packed structures
Message-ID: <2083@chinet.UUCP>
Date: 9 Jan 88 02:38:41 GMT
References: <8801071842.AA04669@decwrl.dec.com>
Reply-To: dag@chinet.UUCP (Daniel A. Glasser)
Organization: Chinet - Public Access Unix
Lines: 65

In article <8801071842.AA04669@decwrl.dec.com> devine@cookie.dec.com (Bob Devine) writes:
[lots of junk deleted]
>  A different argument for providing packed/unpacked structures is
>simply to allow the programmer to choose between access speed or
>smaller data segment.  As an example of prior art, Microsoft C has
>a compile time flag for this.  Granted, not all structures would
>be packed the same way or even could be packed.  Some architectures
>allow ints to be only integer aligned (pyramid, I believe) while
>others all ints to be char aligned (vax).
>
>  Consider this suggestion in light of the C philosophy or letting
>the programmer decide.

This is not so easy.  Yes, the VAX, 8088, and many other processors allow
access of data at any byte alignment, but many don't.  If you have a
packed type on a PDP-11 or 68000 you must use several instructions to
fetch a word or long or other multi-byte type any time you access any
structure through a pointer.

Take the following bit of C code:

	struct yerks {
		char	y_foo;
		int	*y_fie;
		char	y_poo;
	};

	struct yerks *w;
	int i;

	i = *w->y_fie;

The assignment, on a PDP-11, takes code to the effect of the following:

	mov	_w, r0
	clr	-(sp)
	movb	1(r0), (sp)
	movb	2(r0), 1(sp)
	mov	@(sp)+, _i

Now, assume that you are on a 68000 --

	movea.l	_w, a0
	moveq	$0, d0
	move.l	d0, -(sp)
	move.b	1(a0), (sp)
	move.b	2(a0), 1(sp)
	move.b	3(a0), 2(sp)
	move.b	4(a0), 3(sp)
	movea.l	(sp)+, a0
	move.w	(a0), _i;

Okay, that not seem too painful?

No, packed structures do not belong in the standard.  A particular
compiler can pack structures as an option, or by default, but that
is not going to be portable.  Period.  C is supposed to be an efficient
language.  If you really need packed structures, and want them portable,
code them directly with macros and unions.
-- 
					Daniel A. Glasser
					...!ihnp4!chinet!dag
					...!ihnp4!mwc!dag
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	One of those things that goes "BUMP!!! (ouch!)" in the night.