From owner-freebsd-hackers  Fri Jan  5  9:47:25 2001
From owner-freebsd-hackers@FreeBSD.ORG  Fri Jan  5 09:47:23 2001
Return-Path: <owner-freebsd-hackers@FreeBSD.ORG>
Delivered-To: freebsd-hackers@freebsd.org
Received: from fw.wintelcom.net (ns1.wintelcom.net [209.1.153.20])
	by hub.freebsd.org (Postfix) with ESMTP id D9D7F37B402
	for <freebsd-hackers@FreeBSD.ORG>; Fri,  5 Jan 2001 09:47:22 -0800 (PST)
Received: (from bright@localhost)
	by fw.wintelcom.net (8.10.0/8.10.0) id f05HlLS10067;
	Fri, 5 Jan 2001 09:47:21 -0800 (PST)
Date: Fri, 5 Jan 2001 09:47:21 -0800
From: Alfred Perlstein <bright@wintelcom.net>
To: Joshua Rosen <rozzin@geekspace.com>
Cc: Chris Williams <psion@geekspace.com>, freebsd-hackers@FreeBSD.ORG
Subject: Re: Strange fwrite() behavior in a+ mode
Message-ID: <20010105094721.B15744@fw.wintelcom.net>
References: <3A551ABD.DBC9BD5C@geekspace.com> <20010104171504.A15744@fw.wintelcom.net> <20010105173402.3537.qmail@jello.geekspace.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
User-Agent: Mutt/1.2.5i
In-Reply-To: <20010105173402.3537.qmail@jello.geekspace.com>; from rozzin@geekspace.com on Fri, Jan 05, 2001 at 05:34:02PM +0000
Sender: bright@fw.wintelcom.net
Sender: owner-freebsd-hackers@FreeBSD.ORG
Precedence: bulk
X-Loop: FreeBSD.ORG

* Joshua Rosen <rozzin@geekspace.com> [010105 09:31] wrote:
> 
> Alfred Perlstein writes:
> 
> > So in Linux, if you have a "a+" file, you can
> > 1) seek somewhere (mid file)
> > 2) read some data (not until EOF)
> > 3) after the read you are at EOF again?
> > 
> > this doesn't sound really intuative.
> 
> No, it doesn't sound intuitive; luckily, that isn't what happens;)
> The issue concerns where the position is after a write (and where ftell
> reports it as being).
> 
> Under glibc on Linux, you can:
> 1) fseek somewhere (non-EOF)
> 2) fwrite some data
> 3) after the *write*, you are at EOF again (ftell reports this, and any
> attempted fread, at this point, will hit EOF. any fseek, after a write,
> using SEEK_CUR will seek relative to EOF, also)
> 
> I guess that the idea is, `bytes are read/written starting at the current
> position in the file', and, in order to do this in a system which always
> writes after EOF in append-mode, the current position must be changed
> before writing....
> 
> my findings, under FreeBSD 4.2-STABLE, are
> 1) fseek somewhere (non-EOF, say 4 in a 50-byte-long file)
> 2) fwrite some data (say 3 bytes)
> 3) after the fwrite, ftell will report N + the last value pass'd to fseek
> (using our example numbers: 7), and fseek with SEEK_CUR will seek relative
> to that point, but fread will immediately hit EOF, and a ftell (or feof)
> after an attempted read will indicate that the position *is* EOF.
> 
> Chris' findings (I believe) are that, under FreeBSD 4.2-STABLE, if you use
> fread before fwrite (say, step `1.1'), it acts just like it does with
> glibc/Linux.

Can you do a bit more research and let me know if this happens
when using open/read/write/lseek as well?

-- 
-Alfred Perlstein - [bright@wintelcom.net|alfred@freebsd.org]
"I have the heart of a child; I keep it in a jar on my desk."


To Unsubscribe: send mail to majordomo@FreeBSD.org
with "unsubscribe freebsd-hackers" in the body of the message