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From: toma@tekgvs.LABS.TEK.COM (Tom Almy)
Newsgroups: comp.sys.ibm.pc
Subject: Re: MS-DOS puzzle #1
Message-ID: <5565@tekgvs.LABS.TEK.COM>
Date: 13 Jul 89 15:03:55 GMT
References: <8167@bsu-cs.bsu.edu> <1171@calvin.EE.CORNELL.EDU> <5486@pt.cs.cmu.edu>
Reply-To: toma@tekgvs.LABS.TEK.COM (Tom Almy)
Distribution: comp
Organization: Tektronix, Inc., Beaverton,  OR.
Lines: 29

In article <5486@pt.cs.cmu.edu> ralf@b.gp.cs.cmu.edu (Ralf Brown) writes:
>What can be done, however, (and this works for DOS 2 and up), is to copy the
>appropriate entries in the Process File Table in the PSP, zap them to FFh
>(unused), do the EXEC, and then restore the entries.  While the child will
>then get the same handle number on opening a file, it will refer to a
>different file (the above is what the no-inherit attribute does, anyway--it
>zaps the uninheritable handles in the child's PFT to FFh).

Right on!  I was working on a large programming environment that needed
to EXEC arbitrary programs (or a DOS shell), with the ability to redirect
stdin, stdout, and stderr.  I did a variation of Ralf's scheme:
1. Save entire PFT (it's not that big!)
2. Set entries above the std* ones to FFh (effectively temprarily closes
   my programs files, without DOS overhead, but requiring a larger FILES=
   spec in config.sys).
3. Set to 0FFh and then open each redirected std*, one at a time.  Since
   the open command uses the lowest numbered free descriptor, this works
   well.
4. EXEC the program
5. Do a close on each redirected std*.
6. Restore PFT from saved copy.

Was that easy enough?  Much simpler than the contortions in UNIX with
those DUP2s! There certainly is no reason COMMAND.COM couldn't
do it.  And fork() has nothing to do with it.

Tom Almy
toma@tekgvs.labs.tek.com
Standard Disclaimers Apply