You do need to use select, or at least I can't think of any other way to do this.

Your use of sysread is fine; the 4096 is just a maximum. You can see how much data was actually read by looking at length($answer), and if there's more than 4096 bytes available you'd have to call this in a loop.

The reason for your problem is because you're not using IO::Select quite right. You're passing a timeout of 0 to can_read, which asks it to return immediately if there is no data ready yet. What I think you want to do is create one IO::Select object with both handles, then ask it to wait until one or both have something ready to read. Something like this: 

#!/usr/bin/perl

use warnings;
use strict;
use IPC::Open3;
use IO::Select;

#interface to "bc" calculator 
my $pid = open3(\*WRITE, \*READ,\*ERROR,"bc");

my $sel = new IO::Select();

$sel->add(\*READ);
$sel->add(\*ERROR);

my($error,$answer)=('','');

while(1){

  print "Enter expression for bc, i.e. 2 + 2\n";
  chomp(my $query = <STDIN>);
  
  #send query to bc 
  print WRITE "$query\n";

  foreach my $h ($sel->can_read)
  {
    my $buf = '';
    if ($h eq \*ERROR)
    {
      sysread(ERROR,$buf,4096);
      if($buf){print "ERROR-> $buf\n"}
    }
    else
    {
      sysread(READ,$buf,4096);
      if($buf){print "$query = $buf\n"}
    }
  }
}

[download]

This technique won't work if bc might output more than one line. One way to work around that is to fork a process to handle the reading from bc, and use the parent process to handle writing. There's an example of this in Re: Perl Web Zombie problem.

In reply to Re: IPC::Open3 and blocking filehandles by sgifford
in thread IPC::Open3 and blocking filehandles by zentara

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