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From: acm@bu-cs.BU.EDU (ACM)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Environment settings from a C program
Message-ID: <14256@bu-cs.BU.EDU>
Date: Mon, 12-Oct-87 20:02:55 EDT
Article-I.D.: bu-cs.14256
Posted: Mon Oct 12 20:02:55 1987
Date-Received: Wed, 14-Oct-87 01:10:34 EDT
References: <700@hsi.UUCP> <114@fxgrp.UUCP> <2168@ihlpe.ATT.COM>
Reply-To: madd@bucsb.bu.edu
Followup-To: madd@bucsb.bu.edu
Organization: Boston University ACM
Lines: 40
Keywords: ms-dos environ putenv getenv turboc

In article <2168@ihlpe.ATT.COM> psfales@ihlpe.ATT.COM (Pete Fales) writes:
>In article <114@fxgrp.UUCP>, ljz@fxgrp.UUCP (Lloyd Zusman) writes:
>> 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.
>> >
>> >I need for these changes to remain.  Is it possible?
>> 
>> 
>> Sorry, but what you want to do isn't possible.  DOS (be it MSDOS or PCDOS)
>                                 ^^^^^^^^^^^^^^
>> is written such that no running program can change the environment of its
>> parent.  In case you need more explanation, consider this:
>> 
>
>This is rather strong language for an operating system with no memory
>protection.

True.  The way you do it is look for the pointer to the parent's PSP
in your PSP.  I forget the actual address of the pointer -- anyone out
there know what it is?  Anyway, after you find the parent's PSP you can
get a pointer to the parent's environment and play with it as you
like.  it's not a bright idea to expand the environment because this
is likely to cause massive memory problems.  Expansion of the
environment MUST NOT exceed the last paragraph in the environment (or
the max size of the parent's environment -- in the case of COMMAND.COM
you have some idea of what this will be).  Normally you just setblock
the environment pointer, but I don't think you can do this if you're
playing with a parent process.

Another point is that it's easy to backtrace through all the programs
currently in memory with the pointer to the parent's PSP.  I've seen
some programs out there that do it -- nice to see what kind of stuff
you've got stuck in memory.

jim frost
madd@bucsb.bu.edu