Path: utzoo!utgpu!attcan!uunet!yale!husc6!xait!g-rh
From: g-rh@XAIT.Xerox.COM (Richard Harter)
Newsgroups: comp.lang.c
Subject: Re: Suitably aligned pointers
Message-ID: <35120@XAIT.Xerox.COM>
Date: 15 Oct 88 08:54:01 GMT
References: <345@marob.MASA.COM>
Reply-To: g-rh@XAIT.Xerox.COM (Richard Harter)
Organization: Xerox Corporation, Cambridge, Massachusetts
Lines: 22

In article <345@marob.MASA.COM> daveh@marob.UUCP (Dave Hammond) writes:
>We use a pretty basic set of memory management routines which
>allocate large hunks of memory and parcel it out as requested,
>saving malloc the headache of managing huge lists of small buffers.
>The problem is the routines don't return a `suitably aligned' pointer,
>so the accuracy of casting the return to other than char * is in doubt.

>Can someone explain the technique used to suitably align a pointer ?

"suitably aligned" is a machine dependent notion.  On any given machine
there are address restrictions associated with operations on particular
kinds of primitive objects for the machine.  For example, a move double
word instruction may require that the addresses be even word boundaries.
Malloc returns suitably aligned pointers.  In practice a char pointer
whose offset from a malloc supplied pointer is divible by 8 will also
be suitably aligned.  More generally, replace 8 by the width of a double
in bytes.  This is not guaranteed, but it is pretty safe.  
-- 

In the fields of Hell where the grass grows high
Are the graves of dreams allowed to die.
	Richard Harter, SMDS  Inc.