Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!uunet!husc6!cmcl2!rutgers!sunybcs!boulder!hao!oddjob!gargoyle!ihnp4!homxb!whuts!mtune!westmark!dave
From: dave@westmark.UUCP (Dave Levenson)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Environment settings from a C program
Message-ID: <213@westmark.UUCP>
Date: Tue, 13-Oct-87 22:32:42 EDT
Article-I.D.: westmark.213
Posted: Tue Oct 13 22:32:42 1987
Date-Received: Thu, 15-Oct-87 23:53:41 EDT
References: <700@hsi.UUCP>
Organization: Westmark, Inc., Warren, NJ, USA
Lines: 30
Keywords: ms-dos environ putenv getenv turboc

In article <700@hsi.UUCP>, mark@hsi.UUCP (Mark Sicignano) writes:
> I am trying to change an environment variable from within 
> a Turbo-C program.  Turbo-C provides a function, putenv()
> which will make changes to, add, or delete environment,
> but any changes are made to the environment within the program,
> and for child processes.

In MS-DOS, as in UNIX, you can change your environment, and then
export it to your children, but you cannot change your parent's
environment.  You inherit a copy, and operate on it.

Unlike UNIX, however, MS-DOS cannot protect itself from applications
that disobey its rules.  If you can find out where your parent keeps
its environment, you can alter it.

The segment address of your environment block appears at offset 2C
in your PSP.  Offset 16 of your PSP contains the segment address of
your parent's PSP.  Offset 16 of your parent's PSP points at your
grandparent's PSP, and so on.  (The first PSP, belonging to
COMMAND.COM, points to itself!)

Offset 2C of your parent's PSP holds the segment address of your
parent's environment block.  You probably won't be able to get there
with putenv() but with enough chaining through enough far pointers,
you can probably accomplish what you want to.
-- 
Dave Levenson
Westmark, Inc.		A node for news.
Warren, NJ USA
{rutgers | clyde | mtune | ihnp4}!westmark!dave