Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!cornell!calvin.spp.cornell.edu!richard
From: richard@calvin.spp.cornell.edu (Richard Brittain)
Newsgroups: comp.sys.ibm.pc
Subject: Re: i/o redirection in msdos
Summary: The following works for me
Message-ID: <1989Dec12.181708.14997@calvin.spp.cornell.edu>
Date: 12 Dec 89 18:17:08 GMT
References: <8157@cg-atla.UUCP>
Reply-To: richard@calvin.spp.cornell.edu (Richard Brittain)
Organization: Cornell Space Plasma Physics Group
Lines: 41

In article <8157@cg-atla.UUCP> fredex@cg-atla.UUCP (Fred Smith) writes:
>I have recently run into an issue where I need to DISABLE temporarily any
>i/O redirection that may be in effect.
...
>old = dup(fileno(stdin));
>new = freopen ("CON", "r", stdin);
>/* stuff here to do the subhell */
>stream = fdopen (old, "r");
...
>At first sight, this all seems to work ok. I can go to a subshell and
>all seems fine there, and I can return to my main program and all
>still seems fine. However the SECOND time I try to do this to go
>back to another subshell, the redirection is not un-done correctly,
>in some way I have not been able to diagnose--the keyboard is

I had some difficulty doing this in Turbo C 2.0 and finally suspected the
dup() or dup2() library functions.  I wrote my own (they are direct interfaces
to the dos services so they are very simple) and got the following to work.

	int inp, inp2;

	inp = dup(0);
	inp2 = open("CON", O_TEXT|O_RDONLY);
	dup2(0,inp2);

   system("your program here");

	close(inp2);
	dup2(0,inp);
	close(inp);

There was no way I ever got it to work using stream i/o functions only, but
you can always fdopen(0,"r").   I had another routine where I needed to save/
close/restore both stdin and stdout, and I found that the TC library routines
worked on one and failed on the other ( I can't remember which ). Now I use
my own but it is still something of a mystery.

Richard Brittain,                   School of Elect. Eng.,  Upson Hall   
                                    Cornell University, Ithaca, NY 14853
ARPA: richard@calvin.spp.cornell.edu	
UUCP: {uunet,uw-beaver,rochester,cmcl2}!cornell!calvin!richard