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From: tmilner@hprpcd.rose.hp.com (tom milner)
Newsgroups: comp.os.msdos.programmer
Subject: Re: Redirecting stdout via dup2
Message-ID: <27580001@hprpcd.rose.hp.com>
Date: 3 Apr 91 17:15:11 GMT
References: <970@masterCNA.TEK.COM>
Organization: Performance Technology Center, Roseville
Lines: 43

> I have an application where I want to redirect stdout to a file under
> program control (not from the command line). I'm using dup and dup2 to
> achieve this and it works with the exception that I can't *undo* the
> redirection within the program (however, exiting to command.com *does* 
> result in undoing the redirection).
> 
> The following code fragment shows what I'm doing; each comment line shows
> the intent and result of the line it precedes.
> 
>     /* dup stdout and redirect it to temp.fil */
>     oldout = dup(1); 
>     newout = open("temp.fil", O_WRONLY|O_CREAT, 0777);
>     dup2(newout, 1);
>     /* write to 1 -- output redirected to temp.fil */
>     write(1, temp_file_string, strlen(temp_file_string));
>     /* write to dup'd stdout -- output goes to console */
>     write(oldout, stdout_string1, strlen(stdout_string1));
>     /* undo redirection on stdout */
>     dup2(1, oldout);
>     /* string2 should go to console but goes to temp.fil instead */
>     write(1, stdout_string2, strlen(stdout_string2));
> 
> I've tried the same with direct DOS calls (INT 21 Function 45H (dup)
> and INT 21 Function 46H (dup2)) but get the same results.  I've
> tried many combinations of close()'ing handles before and after the
> second dup2 but none gives the desired effect. (BTW, I'm using DOS 3.31)
> 
> Any ideas/solutions welcome.
> 
> Thanks,
> Wayne Turner
> Tektronix, Inc.
> Redmond, Oregon
> waynet@kit.CNA.TEK.COM

You were real close (but no cigar). The line after "undo redirection to stdout"
should read:

        dup2(oldout, 1);

And that will do it.

-Tom.