Date: Thu, 28 Jun 2001 07:52:59 -0700 (PDT) From: John Polstra <jdp@polstra.com> To: stable@freebsd.org Cc: avatar@mmlab.cse.yzu.edu.tw Subject: Re: lseek(SEEK_END) then read()... bug of feature? Message-ID: <200106281452.f5SEqxf93857@vashon.polstra.com> In-Reply-To: <Pine.BSF.4.33.0106281339090.72014-100000@www.mmlab.cse.yzu.edu.tw> References: <Pine.BSF.4.33.0106281339090.72014-100000@www.mmlab.cse.yzu.edu.tw>
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In article <Pine.BSF.4.33.0106281339090.72014-100000@www.mmlab.cse.yzu.edu.tw>,
Tai-hwa Liang <avatar@mmlab.cse.yzu.edu.tw> wrote:
> Hi,
>
> Anyone tried lseek() from the end of file then read something
> out? I've tested the attached code on FreeBSD 4-stable. Both of the two
> -stable box(cvsupped on May-02-2001 and Jun-19-2001) prints error
> message(Undefined error: 0) on the first read(); however, the second
> read() worked without any problem....
It is caused by a bug in your program.
> #include <fcntl.h>
> #include <stdio.h>
> #include <unistd.h>
>
> int
> main()
> {
> unsigned int l = 0;
> off_t offset;
> int ret, fd = open("/kernel", O_RDONLY);
>
> offset = 0 - sizeof(int);
Due to C's type promotion rules, this assigns the value 4294967292
to "offset". Change it to this:
offset = (off_t)0 - sizeof(int);
and your program works fine.
John
--
John Polstra jdp@polstra.com
John D. Polstra & Co., Inc. Seattle, Washington USA
"Disappointment is a good sign of basic intelligence." -- Chögyam Trungpa
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