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From: ljz@fxgrp.UUCP (Lloyd Zusman)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Environment settings from a C program
Message-ID: <128@fxgrp.UUCP>
Date: Thu, 22-Oct-87 13:55:03 EST
Article-I.D.: fxgrp.128
Posted: Thu Oct 22 13:55:03 1987
Date-Received: Sun, 25-Oct-87 01:45:36 EST
References: <700@hsi.UUCP> <2400@bcsaic.UUCP> <2434@bcsaic.UUCP>
Reply-To: ljz@fxgrp.UUCP (Lloyd Zusman)
Organization: FX Development Group, Inc., Mountain View, CA
Lines: 70
Keywords: ms-dos environ putenv getenv turboc
Summary: Some caveats about int 2Eh

With regard to using int 2Eh to cause the top-level command interpreter
to execute a command line ...

Yes, this can be used to set an environment variable in the top-level
command interpreter, but there are some caveats that may or may not
be important:

1)  This is an undocumented DOS call, meaning that it may go away or change.

2)  Once you use it to set an environment variable in the top-level command
    interpreter, the varible DOES NOT propagate down to all the children
    of this top-level interpreter.  In other words, assume I have booted up
    my PC and I'm sitting at the command prompt:

        C>

    I type SET and I get a list of environment variables containing the
    line

        FOO=BAR

    This means that there is a variable named FOO in my environment with
    the value BAR.  Now, suppose I have written a C program that contains
    the following code fragment ...

	extern char *getenv();
        char *ep = getenv("FOO");
	printf("FOO=%s\n", ep ? ep : "<NOT SET>");
	int1e("SET FOO=QUACK");
	ep = getenv("FOO");
	printf("FOO=%s\n", ep ? ep : "<NOT SET>");

    Assume that the int1e() function passes its string argument to int 1Eh
    properly.  The output of this program fragment will be

        FOO=BAR
        FOO=BAR

    It will NOT be

        FOO=BAR
        FOO=QUACK

    as you might hope.  But after the program returns control to DOS, the
    next time you type SET, you will see the line

        FOO=QUACK

    in your environment.  If you insert the following call after the int1e()
    call in the above program

        putenv("FOO=QUACK")

    then you will see the value of the variable FOO change during the
    course of running the program.

    To make it worse, suppose that from DOS I run a program that causes
    a subshell to run, and within this subshell, the above program is
    run (the version with the putenv()).  What will happen is that the
    top level environment will be changed (via the int 1Eh), the
    environment in effect while the program is running will be changed
    (via the putenv() call), but the environments for all processes
    inbetween these will stay the same.

    If this sort of thing is what you want, then int 1Eh is useful.
    Otherwise, be careful.

3)  I have had applications hang when making use of int 1Eh.  I think
    it might have to do with the amount of space taken up by TSR
    ("Terminate and Stay Resident") programs, the version of DOS that's
    being run, etc.  Be careful.