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read from characterdevice /dev/isdninfo


From: eminderlein
Subject: read from characterdevice /dev/isdninfo
Date: Sun, 22 Oct 2000 20:58:44 +0200 (MEST)

Configuration Information [Automatically generated, do not change]:
Machine: i686
OS: linux-gnu
Compiler: gcc
Compilation CFLAGS:  -DPROGRAM='bash' -DCONF_HOSTTYPE='i686'
-DCONF_OSTYPE='linux-gnu' -DCONF_MACHTYPE='i686-pc-linux-gnu' -DCO
NF_VENDOR='pc' -DSHELL -DHAVE_CONFIG_H  -D_FILE_OFFSET_BITS=64  -I. 
-I. -I./lib -I/usr/include -O2 -pipe
uname output: Linux alex 2.2.10 #15 Mit Jul 12 16:11:22 MEST 2000 i686
unknown
Machine Type: i686-pc-linux-gnu

Bash Version: 2.03
Patch Level: 0
Release Status: release

Description:
        [Detailed description of the problem, suggestion, or complaint.]

running on SuSE Dist 6.2
try to run a script "isdn_pruefen2"(=isdn_up_test)

it shall read every second or on every dial/hangup request on
/dev/isdninfo and evaluate the strings and  
give shell level information, if ISDN is up or it is to initiate a dial
or hangup process, also for cost sharing, how many are 
online
(reason : isdnctrl status is not clear about the connection being
established)

see also man isdninfo 
quote************************
DESCRIPTION
       /dev/isdninfo  is  a character device with major number 45
       and minor number 255.  It delivers status information from
       the Linux ISDN subsystem to user level.

DATA FORMAT
       When  reading  from this device, the current status of the
       Linux ISDN subsystem is delivered in 6 lines of text. Each
       line  starts  with  a  tag  string followed by a colon and
       whitespace. After that the status values are appended sep
       arated by whitespace.

BLOCKING BEHAVIOUR
       After opening the device, at most 6 lines can be read by a
       user  process.   After  that, the user process is blocked.
       Whenever a status change happens, the process  is  allowed
       to read 6 more lines, starting with line one.
unquote**********************

The script works with regular files, 
cp /dev/isdninfo {to_other_file} works as well, 
also this creates a mess copying /dev/isdninfo with cat or cp, then 
killing the cp or cat into {to_other_file} process before and then
evaluating the {to_other_file} with read

I hate to start with C or c++ for such a small input, I feel it shall
be solved in the bash 
I have the book "Learning the bash shell", it does not answer clearly to
this point.

   This is my little script: 
quote ========================
#! /bin/bash

case  $1 in
    "")
        echo "usage : $0 {Inputfile}"
        exit 1
        ;;
    *)


PS4='$0 line $LINENO: '

declare -i za=0 zl=0
declare -a TAG
declare -a ISDNSTATOUTS

#set -o verbose
#set -o xtrace

Eingabe=$1 # (=/dev/isdninfo)

zl=0
while true ; do
za=0
while read TAG[$za] ISDNSTATOUTS[$za] ; do
fehlercode=$?
set -o verbose ; set -o xtrace
echo fehlercode $fehlercode ;
echo TAG ${TAG[$za]}
echo ISDNSTAT ${ISDNSTATOUTS[$za]}
za=$za+1
done < $Eingabe
#done < isdninfo_sequenz

za=0

while [ $(( $za< 6 )) = 1 ]; do
echo  za $za ;
echo TAG ${TAG[$za]}
echo ISDNSTAT ${ISDNSTATOUTS[$za]}
za=$za+1
done
zl=$zl+1
echo $zl

sleep 2

done
;;

esac
# end of isdn_pruefen2
unquote ==================

script works well with regular files, but line  
while read TAG[$za] ISDNSTATOUTS[$za] ; do
exits with exitcode 1 immediately, if the file is a symlink to or the
characterdevice /dev/isdninfo itself 

I did not succeed to work with file descriptors, although this might be
the solution.
PROBLEM
I did not find a clear description whether read by default can or
cannot input from character devices(ro)
QUESTION
How can I read from a character device like the above without leaving
the bash
shell level? 

Repeat-By: 
run above script between the quotes with your equipment with an ISDN
device
        [Describe the sequence of events that causes the problem
        to occur.]

Fix: none
        [Description of how to fix the problem.  If you don't know a
        fix for the problem, don't include this section.]


thanks 

with the best regards
mit freundlichen GrĂ¼ssen

Erich Minderlein

----------------------------------
E-Mail: address@hidden
E-Mail: address@hidden
www.locoware.de

Date: 22-Oct-00
Time: 20:35:23

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