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From: stroyan@hpfcso.FC.HP.COM (Mike Stroyan)
Newsgroups: comp.sys.hp
Subject: Re: How can I know his current group ID?
Message-ID: <7370380@hpfcso.FC.HP.COM>
Date: 24 Apr 91 01:04:54 GMT
References: <KITANO.91Apr23200006@judy.pa.Yokogawa.CO.JP>
Organization: Hewlett-Packard, Fort Collins, CO, USA
Lines: 27

>So foo belongs to several groups(A,B,C). Before he does job X, he
>does newgrp A and he works under group ID A.
>
>I want a command, say wgrp, which returns a current group ID of
>a user. The result would be like this.

The situation is not as simple as you think.  The group id is a property
of each process, rather than a property of each user.  When a user
executes "newgrp A", a new shell process is started with the requested
group id.  Rather than showing the group id of a user, the best a "wgrp"
command could do would be to show the group id of one or more processes.

There is no simple kernel call that determines the group id of another
process.  The two possible approaches that occur to me would be to use
the ptrace call or to read /dev/kmem.  A ptrace(PT_ATTACH,...) call can
be done by a super-user process to other processes.  Then a
ptrace(PT_RUAREA,...) call can be made to read the USER area of the
process.  As I understand it, this is not actually practical because
ptrace will not work on processes that are shared text and are running
in multiple processes.

The other approach would be to get the data by reading /dev/kmem and
digging out the process data from the kernel image.  I have no experience
with this kind of program.  It is a very system dependent exercise which
probably shouldn't be attempted without kernel sources.

Mike Stroyan, mike_stroyan@fc.hp.com