From: kagel AT quasar DOT bloomberg DOT com
Date: Wed, 5 Jun 1996 09:15:19 -0400
Message-Id: <9606051315.AA03934@quasar.bloomberg.com >
To: j DOT aldrich6 AT genie DOT com
Cc: djgpp AT delorie DOT com
In-Reply-To: <199606050615.AA022535358@relay1.geis.com> (j.aldrich6@genie.com)
Subject: Re: [?] aligning value returne
Reply-To: kagel AT dg1 DOT bloomberg DOT com

   From: j DOT aldrich6 AT genie DOT com
   Date: Wed,  5 Jun 96 06:26:00 UTC 0000

   Reply to message 9879136    from LATORRE AT IX DOT NE on 06/03/96  5:16PM


   >How do I align the value returned by calloc/malloc on a particular byte
   >boundry ? For instance 64 or 128.
   >
   >I saw something like this,
   >1: size = 1024
   >2: alignment = 128
   >3: un_aligned = malloc( size+alignment )
   >4: aligned = (un_aligned+size)&~(alignment-1)
   >
   >but could not get 4 to compile.
   >
   >Any thoughts ?

[SNIP]

   Besides, wouldn't this work much more elegantly?

   aligned = (char *) ( (unsigned) un_aligned + (unsigned) un_aligned % alignment
   );

Almost.  Try:

MYTYPE *aligned, *unaligned;   /* Or some reasonable typedef. */

aligned = (MYTYPE *)((unsigned)un_aligned 
		          +(unsigned)alignment  /*Advance pointer alignment */
/*Backup by #misaligned bytes*/ -((unsigned)un_aligned % (unsigned)alignment));

You want to advance the pointer to the next higher aligned address.  This 
simple code, as suggested by John Aldrich with correction, is clearer than the
'&~' code M. Latorre came up with and  the optimizer should do a good job at
making it efficient.


[SNIP]

-- 
Art S. Kagel, kagel AT quasar DOT bloomberg DOT com

A proverb is no proverb to you 'till life has illustrated it.  -- John Keats