Article 11963 of comp.lang.perl:
Path: feenix.metronet.com!news.utdallas.edu!convex!cs.utexas.edu!uunet!ncrgw2.ncr.com!ncrhub2!ncr-mpd!Tony.Parent
From: Tony.Parent@FtCollins.NCR.COM (tony parent)
Newsgroups: comp.lang.perl
Subject: Re: question about a loop of piped processes
Message-ID: <TONY.PARENT.94Mar24173746@merlin.FtCollins.NCR.COM>
Date: 25 Mar 94 00:37:46 GMT
References: <Cn528E.DE2@nntpa.cb.att.com>
Sender: uucp@ncr-mpd.FtCollinsCO.NCR.COM
Reply-To: Tony.Parent@FtCollinsCO.NCR.COM
Organization: NCR Microelectronics, Ft. Collins, CO
Lines: 94
In-reply-to: swu@jolt.att.com's message of 23 Mar 94 22:24:14 GMT

In article <Cn528E.DE2@nntpa.cb.att.com> swu@jolt.att.com (Chi-Sharn Wu) writes:
> Hi:
>    I tried a simple example of bidirectional pipes using perl, however it
> doesn't work.
>    The example forks a process "sort" which feeds result to its parent.
> Can anyone  fix it ? Thanks.
> 
> Chi-Sharn Wu
> swu@probe.att.com
> 
> 
>    =========== the example ==============
> 
>        #pipe(READHANDLE, WRITEHANDLE)
>        pipe(IN, DATA);
>        pipe(RESULT, OUT);
>        if ($pid=fork) {
>        close(IN);
>        close(OUT);
>        select(DATA); $| = 1;
>        open(RESULT); 
>        }
>        else  {
>            close(DATA); close(RESULT);
>            select(OUT); $| = 1;
>            open(STDIN, ">&IN");
>            # you see the result if the following command is commented out
>            open(STDOUT, ">&OUT");
>            exec "sort ";
>            }
> 
>    print DATA "bbbbbb \ntwo \nan example\n";
>    while (<RESULT>) {
>        print $_, "!!!!!\n";
>    }


You were close, basically you had what should have been in the parent
process in the child process, and visa-versa. the following works in perl
Revision: 4.0.1.8, Patch level: 36.

pipe(IN, DATA);
pipe(RESULT, OUT);
if ($pid=fork) {
  ;# child process
  close(DATA);
  close(RESULT);

  open(STDIN, "<&IN");
  open(STDOUT, ">&OUT");
  select(STDOUT); $| = 1;
  exec "sort";

  ;# If here, then disaster has occured
  die "exec: $!";
  exit $!;

} else  {
  ;# parent process
  ;# Close unused ends of communication pipes 
  close(IN);
  close(OUT);

  ;# Attach files to communication pipes for easy I/O
  open(FIN,"<&RESULT");
  open(FOUT,">&DATA");

  ;# make unbuffered
  select(FOUT);
  $| = 1;
  select STDOUT;
}

print FOUT "bbbbbb \ntwo \nan example\n";
;# close outputs (sort wont print it's output till it sees an EOF)
close(FOUT);
close(DATA);
while (<FIN>) {
  print $_, "!!!!!\n";
}

exit 0;

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     __o  tonyp@merlin                    NCR MPD                   __o
    -\<,  tony.parent@FtCollinsCO.NCR.COM 2001 Danfield Ct         -\<,
   O / O  (303) 223-5100 x9769            Fort Collins, CO 80525  O / O
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--
===========================================================================
     __o  tonyp@merlin                    NCR MPD                   __o
    -\<,  tony.parent@FtCollinsCO.NCR.COM 2001 Danfield Ct         -\<,
   O / O  (303) 223-5100 x9769            Fort Collins, CO 80525  O / O
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