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From: daveb@geac.UUCP
Newsgroups: comp.lang.c,comp.unix.questions
Subject: Re: Optimizing use of malloc
Message-ID: <1665@geac.UUCP>
Date: Thu, 22-Oct-87 15:02:15 EDT
Article-I.D.: geac.1665
Posted: Thu Oct 22 15:02:15 1987
Date-Received: Sat, 24-Oct-87 12:00:50 EDT
References: <7264@brl-adm.ARPA> <1015@cup.portal.com> <126@fxgrp.UUCP>
Reply-To: daveb@geac.UUCP (Dave Collier-Brown)
Organization: The little blue rock next to that twinkly star.
Lines: 37
Keywords: malloc
Xref: yetti comp.lang.c:4772 comp.unix.questions:4096

In article <4753@oberon.USC.EDU> you write:
>Most versions of malloc will round the amount of memory you request to
>some number convienient for it.  Since the pointer returned must be
>maximally alligned, this is normally a multiple of the size of the
>largest type.  What I need to know what sizes of memory requests to
>malloc will reduce wasted space for a variety of systems: 

bsd 4.2, bsd4.3, -- long
sun 3.*,	-- not known, probably int (its a 68k)
ultrix 32, 	-- long
sys v (various releases & systems) -- varies with machine

  The basic rule is laid down by the hardware designers: if a machine
will fetch longwords from a byte address, its a don't-care. If it
requires an even address, use int alignment.  Otherwise use long.
IBM /376's can require double! (/370's require long).
  The information should vary mostly with cpu-chip, and can be
obtained from CPU manuals.

  You may find that the cost of the malloc header dominates the cost
of alignment: I wrote a paranoid/debugging allocator for GCOS 8 and
discovered header-size to be significant on that machine (which
needed long alignment).

 --dave (I couldn't get through to you by mail) c-b
-- 
 David Collier-Brown.                 {mnetor|yetti|utgpu}!geac!daveb
 Geac Computers International Inc.,   |  Computer Science loses its
 350 Steelcase Road,Markham, Ontario, |  memory (if not its mind)
 CANADA, L3R 1B3 (416) 475-0525 x3279 |  every 6 months.


-- 
 David Collier-Brown.                 {mnetor|yetti|utgpu}!geac!daveb
 Geac Computers International Inc.,   |  Computer Science loses its
 350 Steelcase Road,Markham, Ontario, |  memory (if not its mind)
 CANADA, L3R 1B3 (416) 475-0525 x3279 |  every 6 months.