What is the best way to receive data from a socket in Perl, when the data length is unknown?

Question

Right now, I read one character at a time in a loop, until I reach the \0 character. Is there a better way to do this?

Answer 1

Set your line ending to \x{00} (\0), be sure to localise it, and getline on the handle, like so:

{
    local $/ = "\x{00}";
    while (my $line = $sock->getline) {
       print "$line\n"; # do whatever with your data here
   }
}

Answer 2

You could use FIONREAD with ioctl. The program below connects to the SSH server on localhost and waits on its greeting:

#! /usr/bin/perl

use warnings;
use strict;

use subs 'FIONREAD';
require "sys/ioctl.ph";
use Socket;
socket my $s, PF_INET, SOCK_STREAM, getprotobyname "tcp"
  or die "$0: socket: $!";
connect $s, sockaddr_in 22, inet_aton "localhost"
  or die "$0: connect: $!";

my $rin = "";
vec($rin, fileno($s), 1) = 1;
my $nfound = select my$rout=$rin, "", "", undef;
die "$0: select: $!" if $nfound < 0;

if ($nfound) {
  my $size = pack "L", 0;
  ioctl $s, FIONREAD, $size
    or die "$0: ioctl: $!";

  print unpack("L", $size), "\n";
  sysread $s, my $buf, unpack "L", $size
    or die "$0: sysread: $!";

  my $length = length $buf;
  $buf =~ s/\r/\\r/g;
  $buf =~ s/\n/\\n/g;
  print "got: [$buf], length=$length\n";
}

Sample run:

$ ./howmuch
39
got: [SSH-2.0-OpenSSH_5.3p1 Debian-3ubuntu4\r\n], length=39

But you'll probably prefer using the IO::Socket::INET and IO::Select modules as in the code below that talks to Google:

#! /usr/bin/perl

use warnings;
use strict;

use subs "FIONREAD";
require "sys/ioctl.ph";
use IO::Select;
use IO::Socket::INET;

my $s = IO::Socket::INET->new(PeerAddr => "google.com:80")
  or die "$0: can't connect: $@";

my $CRLF = "\015\012";
print $s "HEAD / HTTP/1.0$CRLF$CRLF" or warn "$0: print: $!";

my @ready = IO::Select->new($s)->can_read;
die "$0: umm..." unless $s == $ready[0];

my $size = pack "L", 0;
ioctl $s, FIONREAD, $size
  or die "$0: ioctl: $!";

print unpack("L", $size), "\n";
sysread $s, my $buf, unpack "L", $size
  or die "$0: sysread: $!";

my $length = length $buf;
$buf =~ s/\r/\\r/g;
$buf =~ s/\n/\\n/g;
print "got: [$buf], length=$length\n";

Output:

573
got: [HTTP/1.0 200 OK\r\nDate: Sun, 18 Jul 2010 12:03:48 GMT\r\nExpires: -1\r\nCache-Control: private, max-age=0\r\nContent-Type: text/html; charset=ISO-8859-1\r\nSet-Cookie: PREF=ID=6742ab80dd810a95:TM=1279454628:LM=1279454628:S=ewNg64020FbnGzHR; expires=Tue, 17-Jul-2012 12:03:48 GMT; path=/; domain=.google.com\r\nSet-Cookie: NID=36=kn2wtTD4UJ3MYYQ5uvA4iAsrS2wcrb_W781pZ1hrVUhUDHrIJTMg_kOgVKhjQnO5SM6MdC_jrRdxFRyXwyyv5N3Xja1ydhVLWWaYqpMHQOmGVi2K5qRWAKwDhCVRd8WS; expires=Mon, 17-Jan-2011 12:03:48 GMT; path=/; domain=.google.com; HttpOnly\r\nServer: gws\r\nX-XSS-Protection: 1; mode=block\r\n\r\n], length=573