Path: utzoo!utgpu!watserv1!watmath!att!dptg!ulysses!ulysses.att.com!cjc
From: cjc@ulysses.att.com (Chris Calabrese[mav])
Newsgroups: comp.unix.wizards
Subject: Re: Reserving Space on Disk
Summary: should definitely malloc space to be used by read and write
Message-ID: <13422@ulysses.att.com>
Date: 16 Jul 90 14:38:46 GMT
References: <563@hhb.UUCP> <1990Jul15.211608.26025@virtech.uucp> <MOSS.90Jul15190502@ibis.cs.umass.edu>
Sender: netnews@ulysses.att.com
Lines: 25

In article <MOSS.90Jul15190502@ibis.cs.umass.edu>, moss@cs.umass.edu (Eliot Moss) writes:
> [ ... ]
> 
> char buf[ONE_K];
> int i;
> 
> for (i = 0; i < FOUR_K; ++i)
>   write (fd, buf, ONE_K);
> [ ... ]

BTW, while reading the various ways to write some large space on the
disk which have been passing by, I thought I'd pass a related tidbit
along.  On many machine architectures, byte copying from user space to
kernel space is quite a bit faster if the buffer is word aligned.

For this reason, it is better to use malloc to get your buffer than to
allocate it off of the stack (as malloc always returns maximally
aligned memory).  Of course, this assumes that you'll use the buffer
more than once, as the time to malloc() is around the same order as
the time to deal with the non-aligned bytes at the beginning and end
of the buffer.
Name:			Christopher J. Calabrese
Brain loaned to:	AT&T Bell Laboratories, Murray Hill, NJ
att!ulysses!cjc		cjc@ulysses.att.com
Obligatory Quote:	``pher - gr. vb. to schlep.  phospher - to schlep light.philosopher - to schlep thoughts.''