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Re: [Help-bash] /proc/.../cdrom/info not completely read


From: Chet Ramey
Subject: Re: [Help-bash] /proc/.../cdrom/info not completely read
Date: Mon, 30 Apr 2012 09:57:16 -0400
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.7; rv:11.0) Gecko/20120327 Thunderbird/11.0.1

On 4/30/12 2:42 AM, humpty wrote:
> hi,
> 
> I encounter a problem when I try to read /proc/sys/dev/cdrom/info
> 
> whatever method I try, bash doesn't read the whole file
> 
> $ file=/proc/sys/dev/cdrom/info
> $ while read line...done <"$file"
> only outputs the first line
> 
> $ exec {foo}<"$file"
> $ while read -u line
> outputs about six lines, as does
> $ mapfile -t myArray <"$file"
> and
> $ echo "$(<"$file")"
> 
> if I copy "$file" anywhere (say /tmp/) there is no problem any more.
> This happens with BASH 4.2 on Debian, and openSuse (and BASH 3.3.4 on
> archlinux (I've been told))
> 
> what could be the reason for such a behaviour?

This came up on one of the Linux distribution bugzilla sites (I think it
was debian).  The problem is that lseek is either a no-op or doesn't work
correctly on special files in /proc.

Bash reads 128 characters at a time and uses lseek to move the file pointer
back to the last character `read' consumes.  The negative offset to lseek
causes some kind of problem, but it doesn't return an error.  When bash
goes back for more, it gets short reads and incomplete data.

It's a kernel bug apparently introduced in 3.2.  It doesn't require bash
to test.

(I guess it was debian:
http://lists.debian.org/debian-kernel/2012/02/msg00874.html .  The original
report thread is http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=659499 .)

Chet
-- 
``The lyf so short, the craft so long to lerne.'' - Chaucer
                 ``Ars longa, vita brevis'' - Hippocrates
Chet Ramey, ITS, CWRU    address@hidden    http://cnswww.cns.cwru.edu/~chet/



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