Can I read and write to multiple filehandles simultaneously (Perl)?

Question

I'm trying to read from two files, and generate output in a third. I first wanted to edit the first one on the go but I didn't find a suitable method save for arrays.

My problem is that the third file (output) is empty whenever I uncomment the "_ref_param_handling" function. BUT the following is what puzzles me the most: If I do a UNIX very basic `cat` system call on the output file at then end (see code below), it works just fine. If I open the filehandle just before and close it right after editing, it also works fine (around my print FILEHANDLE LIST).

I undoubtedly am missing something here. Apart from a problem between my keyboard and my chair, what is it? A filehandle conflict? A scope problem?

Every variable is declared and has the value I want it to have.

Edit (not applicable anymore). Using IO::File on the three files didn't change anything.

---

Edit 2 : New full subroutine code

My code works (except when my ref already exists, but that's because of the "append" mode i think) but there might be some mistakes and unperlish ways of coding (sorry, Monks). I, however, use Strict and warnings !

sub _ref_edit($) {
    my $manda_def = "$dir/manda_def.list";
    my $newrefhandle;
    my $ref       = $_[0];
    (my $refout   = $ref) =~ s/empty//;
    my $refhandle;
    my $parname   = '';
    my $parvalue  = '';
    my @val;

    _printMan;

    my $flush = readline STDIN;    # Wait for <enter>

    # If one or both of the ref. and the default values are missing
    if ( !( -e $manda_def && -e $ref ) ) {
        die "Cannot find $ref and/or $manda_def";
    }

    # Open needed files (ref & default)
    open( $refhandle, "<", $ref ) or die "Cannot open ref $ref : $!";
    open( $newrefhandle, ">>", $refout ) 
      or die "Cannot open new ref $refout : $!";

    # Read each line
    while ( my $refline = <$refhandle> ) {
    # If line read not an editable macro
        if ( $refline =~ /^define\({{(.+)}},\s+{{.*__VALUE__.*}}\)/ ){
        $parname = $1;         # $1 = parameter name captured in regexp
        # Prompt user
        $parvalue = _ref_param_handling( $parname, $manda_def );   
        # Substitution in ref
        $refline =~ s/__VALUE__/$parvalue/;
        # Param not specified and no default value
        $parvalue eq '' ? $refline=~s/__COM__/#/ : $refline=~s/__COM__//; 
        }

    print $newrefhandle $refline;
    }
    close $newrefhandle;
    close $refhandle;

    return $refout;
}    # End ref edit  

the _ref_param_handle subroutine still is :

open( $mde, '<', $_[1] )
      or die "Cannot open mandatory/default list $_[1] : $!";

    # Read default/mandatory file list 
    while (<$mde>) {       
       ( $name, $manda, $default, $match, $descript ) = split( /\s+/, $_, 5 ); 
       next if ( $name !~ $ref_param );  # If param read differs from parname

    (SOME IF/ELSE)

    } # End while <MDE>
    close $mde;
    return $input;
}

Extract from manda_def file :

NAME  Mandatory? Default Match      Comm.
PORT          y NULL  ^\d+$ Database port
PROJECT       y NULL  \w{1,5}   Project name
SERVER        y NULL  \w+           Server name
modemRouting  n NULL  .+        
modlib        y bin   .+        
modules       y sms   .+

Extract from ref_file :

define({{PORT}},         {{__VALUE__}})dnl
define({{PROJECT}},      {{__VALUE__}})dnl
define({{SERVER}},       {{__VALUE__}})dnl
define({{modemRouting}}, {{__COM__{{$0}} '__VALUE__'}})dnl
define({{modlib}},       {{__COM__{{$0}} '__VALUE__'}})dnl
define({{modules}},      {{__COM__{{$0}} '__VALUE__'}})dnl

Any help appreciated.

Answer 1

Guys, I seriously consider hanging myself with my wireless mouse.

My script never failed. I just didn't ran it through the end (it's actually a very long parameter list). The printing is just done as soon as the filehandle is closed (or so I guessed)...

/me *cries*

I've spent 24 hours on this...

Answer 2

It is unclear what is initialising $refhandle, $newrefhandle and $mde. Depending on the values they have will affect the behaviour of open - i.e. whether it will close any filehandles before opening a new one.

I would suggest that you start using the IO::File interface to open/write to files, as this makes the job of filehandle management much easier, and will avoid any inadvertent closes. Something like...

use IO::File;

my $refhandle = IO::File->new("< $ref") or die "open() - $!";

$refhandle->print(...);

As far as editing files in place goes, this is a common pattern I use to achieve this, make sure of the -i behaviour of perl.

sub edit_file
{
    my ($filename) = @_;

    # you can re-create the one-liner above by localizing @ARGV as the list of
    # files the <> will process, and localizing $^I as the name of the backup file.
    local (@ARGV) = ($filename);
    local($^I) = '.bak';

    while (<>)
    {
        s/original string/new string/g;
    }
    continue
    {
        print;
    }
}

Answer 3

try opening the second file handle for input outside the loop and pass a reference to the subroutine _ref_param_handle.Use seek function to seek file back to start.
If your file is not too large you can also think of storing the content in an array and the accessing it instead of looping over same contents.
EDIT:
Here is a small example to support what I was trying to say above:

#!/usr/bin/perl -w

sub test
{
 my $fh_to_read = $_[0] ;
 my $fh_to_write = $_[1] ;

 while(<$fh_to_read>)
 {
  print $fh_to_write  $_ ;
 }
 seek($fh_to_read,0,0) ;
}

open(FH1,"<dummy1");
open(FH2,"<dummy2");
open(FH3,">dummy3");

while(<FH2>)
{
 print FH3 "$_" ;
 test(\*FH1,\*FH3);
}

Info about perl references

Answer 4

From what I gather, your script wants to convert a file in the following form:

define({{VAR1}}, {{__VALUE__}})
define({{VAR2}}, {{__VALUE__}})
define({{VAR3}}, {{__VALUE__}})
define({{VAR4}}, {{__VALUE__}})

to something like this:

define({{VAR1}}, {{}})
define({{VAR2}}, {{VALUE2}})
define({{VAR3}}, {{VALUE3}})
define({{VAR4}}, {{}})

The following works. I don't know what manda_def means, and also I didn't bother to create an actual variable replacement function.

#!/usr/bin/perl
use strict;
use warnings;

sub work {
    my ($ref, $newref, $manda_def) = @_;

    # Open needed files (ref & default)
    open(my $refhandle, '<', $ref) or die "Cannot open ref $ref : $!";
    open(my $newrefhandle, '>', $newref) or die "Cannot open new ref $newref: $!";

    # Read each line
    while (my $refline = <$refhandle>) {
        # if line read is not an editable macro
        if ($refline =~ /^define\({{(.+)}},\s+{{.*__VALUE__.*}}\)/){
            my $parvalue = _ref_param_handling($1, $manda_def); # manda_def?
            # Substitution in ref
            $refline  =~ s/__VALUE__/$parvalue/;
            # Param not specified and no default value
            $refline  =~ s/__COM__/#/ if $parvalue eq '';
        }
        print $newrefhandle $refline;
    }
    close $newrefhandle;
    close $refhandle;

    return $newref;
}

sub _ref_param_handling {
    my %parms = (VAR2 => 'VALUE2', VAR3 => 'VALUE3');
    return $parms{$_[0]} if exists $parms{$_[0]};
}

work('ref.txt', 'newref.txt', 'manda.txt');