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.

Hi, well I did some digging and came up with a method from "perldoc -f filehandle". It is from "how to read a byte from a filehandle". I did come up with an error at first telling me that my cdefs.ph file had a bunch syntax errors between ") (". I wonder if this is a bug in 5.8? The lines in question all needed a comma between the ") (" .

A typical line needing the comma is

if(defined (defined(&__cplusplus) ? &__cplusplus : 0) && (defined(&__GNUC_PREREQ) ? &__GNUC_PREREQ : 0), (2,8)) {

Notice the comma I inserted between 0) and (2,8)).

Anyways, after fixing that, the following code does what I was looking for. :-)

#!/usr/bin/perl
use warnings;
use strict;
use IPC::Open3;
require 'sys/ioctl.ph';

#interface to "bc" calculator 
my $pid = open3(\*WRITE, \*READ,\*ERROR,"bc");
            #if \*ERROR is false, STDERR is sent to STDOUT  

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";

#timing delay needed tp let bc output 
select(undef,undef,undef,.01);

#see which filehandles have output from perldoc -q filehandle 
my $esize = pack("L", 0);
ioctl(\*ERROR, FIONREAD(), $esize) or die "Couldn't call ioctl: $!\n";
$esize = unpack("L", $esize);
print "esize-> $esize\n";

my $rsize = pack("L", 0);
ioctl(\*READ, FIONREAD(), $rsize) or die "Couldn't call ioctl: $!\n";
$rsize = unpack("L", $rsize);
print "rsize-> $rsize\n";


#get the output from bc 
if($esize > 0){sysread(ERROR,$error,$esize); print "Error-> $error\n"}
if($rsize > 0){sysread(READ,$answer,$rsize); print "Output->$query = $
+answer\n"}

($error,$answer,$rsize,$esize)=('','',0,0);
}
[download]

This is still dependant on bc having its output ready within .01 seconds. If the system were very busy or otherwise bogged down and could't meet this deadline, you'd miss a line of output, and then would be off by a line until the program restarted.

A side note, the way you used IO::Select is not quite right.

One of the main reasons to have IO::Select is to use one function, check multiple channels, so that those channels will not block each other even on one thread. Usually there is no point to have multiple IO::Select (in general)