Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!mcsun!hp4nl!ruuinf!henkp
From: henkp@ruuinf.cs.ruu.nl (Henk P. Penning)
Newsgroups: comp.lang.perl
Subject: Re: pipe to sort and back
Message-ID: <3702@ruuinf.cs.ruu.nl>
Date: 25 Aug 90 19:54:53 GMT
References: <1990Aug24.145831.13299@DRD.Com>
Organization: Utrecht University, Dept. of CS
Lines: 69

In article <1990Aug24.145831.13299@DRD.Com> mark@DRD.Com (Mark Lawrence) writes:

>I have some data I'm reading and before I write it on to the intended
>recipient, I want to take a chunk of it and pipe it to sort and bring
>sort's output back and then write that on.  Any way to easily set up a
>bi-directional file handle (or pair of filehandles) to accomplish that?

# Here is my solution.
# Subroutine open2(READHANDLE,WRITEHANDLE,arg,arg,...) expects
# names of a READHANDLE and a WRITEHANDLE (as strings).
# It sets up pipes, forks, and calls exec(arg,arg,...) in the child.
# 
# The example should print the first few lines of the sorted passwd file.
# 
# 				===  HenkP  ===
# 
# Henk P. Penning, Dept of Computer Science, Utrecht University.
# Padualaan 14, P.O. Box 80.089, 3508 TB Utrecht, The Netherlands.
# Telephone: +31-30-534106
# e-mail   : henkp@cs.ruu.nl (uucp to hp4nl!ruuinf!henkp)
# ---------------------------------------------------
sub open2 # ( READHANDLE, WRITEHANDLE, argument(s) to exec() )
  { local($p1,$p2,@execParms) = @_ ;
    eval "pipe($p1,APPOUT)" || die "open2 can't create pipe $p1/APPOUT" ; 
    eval "pipe(APPIN,$p2)"  || die "open2 can't create pipe APPIN/$p2" ; 

    $open2Pid = fork ;

    if ( $open2Pid == 0 ) # CHILD
      { close($p1)              || die "child can't close $p1" ;
        close($p2)              || die "child can't close $p2" ;
        close(STDIN)            || die "child can't close STDIN" ;
        close(STDOUT)           || die "child can't close STDOUT" ;
        open(STDIN, "<&APPIN")  || die "child can't redirect STDIN" ;
        open(STDOUT,">&APPOUT") || die "child can't redirect STDOUT" ;
        close(APPIN)            || die "child can't close APPIN" ;
        close(APPOUT)           || die "child can't close APPOUT" ;
        exec(@execParms) ;
        die "open2 can't exec " . join(' ',@execParms) ;
      }
    else # PARENT
      { close(APPIN)  || die "open2 can't close APPIN" ;
        close(APPOUT) || die "open2 can't close APPOUT" ;
      }
  }

sub flush # ( FILEHANDLE )
  { $oldselect = select($_[0]) ;
    $oldpipe = $| ; $| = 1 ; print "" ; $| = $oldpipe ;
    select($oldselect) ;
  }

$INP = 'INP' ; $OUT = 'OUT' ;

&open2($INP,$OUT,'sort | head') ;

open(PW,'/etc/passwd') || die "can't open /etc/passwd\n" ;
while ($_ = <PW> ) { print OUT "out> $_" ; }
close(OUT) || die "can't close OUT\n" ;

while ( $_ = <INP> )
  { print "<in $_" ; }

close(INP) || die "can't close INP\n" ;
-- 
Henk P. Penning, Dept of Computer Science, Utrecht University.
Padualaan 14, P.O. Box 80.089, 3508 TB Utrecht, The Netherlands.
Telephone: +31-30-534106
e-mail   : henkp@cs.ruu.nl (uucp to hp4nl!ruuinf!henkp)