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From: blarson@castor.usc.edu (Bob Larson)
Newsgroups: comp.lang.c,comp.unix.wizards
Subject: Re: pointer alignment when int != char *
Message-ID: <3195@oberon.USC.EDU>
Date: Sat, 4-Jul-87 19:24:15 EDT
Article-I.D.: oberon.3195
Posted: Sat Jul  4 19:24:15 1987
Date-Received: Sun, 5-Jul-87 01:31:33 EDT
References: <493@its63b.ed.ac.uk>
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Xref: mnetor comp.lang.c:2818 comp.unix.wizards:3105

In article <493@its63b.ed.ac.uk> simon@its63b.ed.ac.uk (Simon Brown) writes:
>If I were to want to implement malloc (or some such) on a machine where
>sizeof(int) != sizeof(char *), how do I ensure that the pointer-values I
>return are maximally aligned (eg, quad-aligned)?

The same way as you would on any other machine: non-portably.
(What is your definition of quad-aligned?  4 * sizeof(char)?  There
are quit a few machines where this is not maximally aligned.)

For example, prime 64v mode:

char *alignpointer(p)
char *p;
{
    union {
	char *up;
	struct {
	    unsigned fault:1;
	    unsigned ring:2;	/* I may have the ring and extend bits exchanged */
	    unsigned extend:1;	/* check before you try this on a real prime */
	    unsigned segment:12;
	    unsigned offset:16;
	    unsigned bit:4;
	    unsigned unused:12;
	} point;
    } un;
    int zerooffset;

    un.up = p;
    if(un.point.fault) return p; /* faulted pointer, not a valid address */
    zerooffset = un.point.offset == 0;
    un.point.offset = (un.point.offset | (un.point.extend & (un.point.bit!=0))
	 + 1) & ~1;	/* round offset up to next 4 byte boundry */
    un.point.extend = 0;/* and say it is at a 2-byte boundry */
    un.point.bit = 0;	/* unneded, but leave it clean */
    if(un.point.offset == 0 && !zerooffset) un.point.segment++;
    return un.up;
}

Obviously, it would be easier to make sure to generate aligned pointers
in the first place.  Also I did not make all the assumptions that the C compiler
does, assuming you could have gotten the pointer via another language.
Bob Larson		Arpa: Blarson@Ecla.Usc.Edu
Uucp: {sdcrdcf,seismo!cit-vax}!oberon!castor!blarson
"How well do we use our freedom to choose the illusions we create?" -- Timbuk3