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From: cgordon@vpnet.chi.il.us (Gordon Hlavenka)
Newsgroups: comp.sys.ibm.pc.misc
Subject: DOS Environment Variable Question
Message-ID: <270ad5e5-331.1comp.sys.ibm.pc.misc-1@vpnet.chi.il.us>
Date: 4 Oct 90 09:55:03 GMT
References: <1990Oct1.191910.560@odetics.com>
Lines: 66


>...  I looked threw the MS-C manual and found putevn() and
>getenv(), but they seem to effect the PROGRAM's copy of the environment,
>while
>I want to effect "THE" environment.
>
>Anyone got a good/easy way to do this?

Microsoft distributed (with source!) a program called WHAT.EXE with at least
some copies of MASM 5.0.  This program makes changes in the master
environment, and since the source is available you should be able to patch
it around.

I got my copy from a BBS somewhere, but I'm not sure of the legality of its
distribution.  There doesn't seem to be _any_ copyright notice, which I
think is suspicious.

You might want to dig up a copy of MASM 5.0 and hope...

>P.S. If not "easy" then there must be a pointer to the DOS enviroment
>someplace,
>where is it and how does the structure look?

I don't know of any pointer; I've found it  using DEBUG by just S'ing from
low RAM upwards, for the first occurence of the string 'COMSPEC='. It
shouldn't be too tough to write something that does that.  Poking this
table from within DEBUG is effective.  Poking from within a program will
_not_ affect that program's environment; it's working from a copy of the
old environment.  Next program loaded by DOS should get it, though.  Of
course, I don't know how reliable this method is...  (For instance, I don't
think it's guaranteed that the environment will start with 'COMSPEC='. 
That's just been my experience.)

The structure:  Simply a list of ASCIIZ strings, terminated by a null string
(00H 00H).  Actually, it is a list of strings followed by n bytes of zero,
where n is the remaining "reserved" environment space (minimum 2 bytes).

Real-world example:  (Pardon me while I shell out)

Deep Thought C>debug
-s1000:0lffff 'COMSPEC='
1000:A050
1000:A110
1000:A730
1000:C1A0
1000:D010
1000:DE30
1000:DF60
-d1000:a050lb0
1000:A050 43 4F 4D 53 50 45 43 3D-43 3A 5C 43 4F 4D 4D 41 COMSPEC=C:\COMMA
1000:A060 4E 44 2E 43 4F 4D 00 50-52 4F 4D 50 54 3D 44 65 ND.COM.PROMPT=De
1000:A070 65 70 20 54 68 6F 75 67-68 74 20 24 4E 24 47 00 ep Thought $N$G.
1000:A080 50 41 54 48 3D 43 3A 5C-3B 43 3A 5C 42 41 54 43 PATH=C:\;C:\BATC
1000:A090 48 3B 43 3A 5C 44 4F 53-3B 43 3A 5C 55 54 49 4C H;C:\DOS;C:\UTIL
1000:A0A0 3B 43 3A 5C 4E 4F 52 54-4F 4E 3B 43 3A 5C 47 41 ;C:\NORTON;C:\GA
1000:A0B0 4D 45 53 3B 43 3A 5C 54-4F 50 53 3B 45 3A 5C 57 MES;C:\TOPS;E:\W
1000:A0C0 49 4E 33 30 00 55 53 45-52 3D 47 6F 72 64 6F 6E IN30.USER=Gordon
1000:A0D0 00 54 45 4D 50 3D 65 3A-5C 00 4D 45 4D 4F 5F 44 .TEMP=e:\.MEMO_D
1000:A0E0 49 52 3D 63 3A 5C 6D 65-6D 6F 00 4D 45 4D 4F 5F IR=c:\memo.MEMO_
1000:A0F0 46 49 4C 45 3D 6D 65 6D-6F 00 00 00 00 00 00 00 FILE=memo.......
-q

Good luck, and enjoy!

-----------------------------------------------------
Gordon S. Hlavenka            cgordon@vpnet.chi.il.us
Disclaimer:                Yeah, I said it.  So what?