socks 5 proxy on Perl

Question

I am new in Perl and trying to understand this cod in Link : http://codepaste.ru/1374/but I have some problem in understanding this part of code:

while($client || $target) {
  my $rin = "";
  vec($rin, fileno($client), 1) = 1 if $client;
  vec($rin, fileno($target), 1) = 1 if $target;
  my($rout, $eout);
  select($rout = $rin, undef, $eout = $rin, 120);
  if (!$rout  &&  !$eout) { return; }
  my $cbuffer = "";
  my $tbuffer = "";

  if ($client && (vec($eout, fileno($client), 1) || vec($rout, fileno($client), 1))) {
    my $result = sysread($client, $tbuffer, 1024);
    if (!defined($result) || !$result) { return; }
  }

  if ($target  &&  (vec($eout, fileno($target), 1)  || vec($rout, fileno($target), 1))) {
    my $result = sysread($target, $cbuffer, 1024);
    if (!defined($result) || !$result) { return; }
    }

  if ($fh  &&  $tbuffer) { print $fh $tbuffer; }

  while (my $len = length($tbuffer)) {
    my $res = syswrite($target, $tbuffer, $len);
    if ($res > 0) { $tbuffer = substr($tbuffer, $res); } else { return; }
  }

  while (my $len = length($cbuffer)) {
    my $res = syswrite($client, $cbuffer, $len);
    if ($res > 0) { $cbuffer = substr($cbuffer, $res); } else { return; }
  }
}

can any body explain to me exactly whats happen in these lines:

vec($rin, fileno($client), 1) = 1 if $client;
vec($rin, fileno($target), 1) = 1 if $target;

and

select($rout = $rin, undef, $eout = $rin, 120);

Answer 1

Basically, the select operator is used to find which of your file descriptors are ready (readable, writable or there's an error condition). It will wait until one of the file descriptors is ready, or timeout.

select RBITS, WBITS, EBITS, TIMEOUT

RBITS is a bit mask, usually stored as a string, representing a set of file descriptors that select will wait for readability. Each bit of RBITS represent a file descriptor, and the offset of the file descriptor in this bit mask should the file descriptor number in system. Thus, you could use vec to generate this bit mask.

vec EXPR, OFFSET, BITS

The vec function provides storage of lists of unsigned integers. EXPR is a bit string, OFFSET is the offset of bit in EXPR, and BITS specifies the width of each element you're reading from / writing to EXPR.

So these 2 lines:

vec($rin, fileno($client), 1) = 1;
vec($rin, fileno($target), 1) = 1;

They made up a bit mask string $rin with setting the bit whose offset equals the file descriptor number of $client, as well as the one of $target.

Put it into select operator:

select($rout = $rin, undef, $eout = $rin, 120);

Then select will monitor the readability of the two file handlers ($client and $target), if one of them is ready, select will return. Or it will return after 120s if no one is ready.

WBITS, EBITS use the same methodology. So you could infer that the above select line will also return when the two file handler have any exceptions.