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From: paula@bcsaic.UUCP (Paul Allen)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Environment settings from a C program
Message-ID: <2400@bcsaic.UUCP>
Date: Wed, 14-Oct-87 20:29:29 EDT
Article-I.D.: bcsaic.2400
Posted: Wed Oct 14 20:29:29 1987
Date-Received: Sat, 17-Oct-87 17:38:04 EDT
References: <700@hsi.UUCP>
Reply-To: paula@bcsaic.UUCP (Paul Allen)
Organization: Boeing Computer Services AI Center, Seattle
Lines: 31
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.
>
>I need for these changes to remain.  Is it possible?

I've seen several responses to this, ranging from "It's impossible" to
"here's a PD program that does it".  So far, noone has mentioned 
undocumented interrupt 1EH.  This interrupt takes as an argument a
pointer to a string containing a command to be executed by command.com.
The first byte of the string is a count of the remaining bytes.  If you
pass it "set foo=bar", it will set the variable foo in command.com's
environment space.  A little thought will turn up all sorts of nifty
uses for this capability.  In fact, one of my unfinished projects at
home is a little program that understands commands like !!, history,
and alias, and passes everything it doesn't understand off to command.com.

Caveats:

I have used this undocumented feature, and it works, but it is probably
undocumented for a reason.  I am working from memory, or I could tell
you which register to put the string address in.  If you really want it,
e-mail me.  Or check Ray Duncan's column in back issues of Dr. Dobb's or
the Best of BIX column in back issues of BYTE.

Paul Allen
paula@boeing.com
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