Use Internet-domain sockets when you want to do client-server
communication that might extend to machines outside of your own system.
Here's a sample TCP client using Internet-domain sockets:
<![CDATA[
#!/usr/bin/perl -w
require 5.002;
use strict;
use Socket;
my ($remote,$port, $iaddr, $paddr, $proto, $line);
$remote = shift || 'localhost';
$port = shift || 2345; # random port
if ($port =~ /\D/) { $port = getservbyname($port) }
die "No port" unless $port;
$iaddr = inet_aton($remote) || die "no host: $remote";
$paddr = sockaddr_in($port, $iaddr);
$proto = getprotobyname('tcp');
socket(SOCK, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
connect(SOCK, $paddr) || die "connect: $!";
while ($line = <SOCK>) {
print $line;
}
close (SOCK) || die "close: $!";
exit;
]]>
And here's a corresponding server to go along with it. We'll
leave the address as INADDR_ANY so that the kernel can choose
the appropriate interface on multihomed hosts:
<![CDATA[
#!/usr/bin/perl -Tw
require 5.002;
use strict;
BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
use Socket;
use Carp;
sub spawn; # forward declaration
sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
my $port = shift || 2345;
my $proto = getprotobyname('tcp');
socket(SERVER, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
setsockopt(SERVER, SOL_SOCKET, SO_REUSEADDR, 1) || die "setsockopt: $!";
bind(SERVER, sockaddr_in($port, INADDR_ANY)) || die "bind: $!";
listen(SERVER,5) || die "listen: $!";
logmsg "server started on port $port";
my $waitedpid = 0;
my $paddr;
sub REAPER {
$SIG{CHLD} = \&REAPER; # loathe sysV
$waitedpid = wait;
logmsg "reaped $waitedpid" . ($? ? " with exit $?" : '');
}
$SIG{CHLD} = \&REAPER;
for ( $waitedpid = 0;
($paddr = accept(CLIENT,SERVER)) || $waitedpid;
$waitedpid = 0, close CLIENT)
{
next if $waitedpid;
my($port,$iaddr) = sockaddr_in($paddr);
my $name = gethostbyaddr($iaddr,AF_INET);
logmsg "connection from $name [",
inet_ntoa($iaddr), "]
at port $port";
spawn sub {
print "Hello there, $name, it's now ", scalar localtime, "\n";
exec '/usr/games/fortune'
or confess "can't exec fortune: $!";
};
}
sub spawn {
my $coderef = shift;
unless (@_ == 0 && $coderef && ref($coderef) eq 'CODE') {
confess "usage: spawn CODEREF";
}
my $pid;
if (!defined($pid = fork)) {
logmsg "cannot fork: $!";
return;
} elsif ($pid) {
logmsg "begat $pid";
return; # i'm the parent
}
# else i'm the child -- go spawn
open(STDIN, "<&CLIENT") || die "can't dup client to stdin";
open(STDOUT, ">&CLIENT") || die "can't dup client to stdout";
## open(STDERR, ">&STDOUT") || die "can't dup stdout to stderr";
exit &$coderef();
}
]]>
This server takes the trouble to clone off a child version via fork() for
each incoming request. That way it can handle many requests at once,
which you might not always want. Even if you don't fork(), the listen()
will allow that many pending connections. Forking servers have to be
particularly careful about cleaning up their dead children (called
"zombies" in Unix parlance), because otherwise you'll quickly fill up your
process table.
We suggest that you use the -T flag to use taint checking (see the perlsec manpage
)
even if we aren't running setuid or setgid. This is always a good idea
for servers and other programs run on behalf of someone else (like CGI
scripts), because it lessens the chances that people from the outside will
be able to compromise your system.
Let's look at another TCP client. This one connects to the TCP "time"
service on a number of different machines and shows how far their clocks
differ from the system on which it's being run:
<![CDATA[
#!/usr/bin/perl -w
require 5.002;
use strict;
use Socket;
my $SECS_of_70_YEARS = 2208988800;
sub ctime { scalar localtime(shift) }
my $iaddr = gethostbyname('localhost');
my $proto = getprotobyname('tcp');
my $port = getservbyname('time', 'tcp');
my $paddr = sockaddr_in(0, $iaddr);
my($host);
$| = 1;
printf "%-24s %8s %s\n", "localhost", 0, ctime(time());
foreach $host (@ARGV) {
printf "%-24s ", $host;
my $hisiaddr = inet_aton($host) || die "unknown host";
my $hispaddr = sockaddr_in($port, $hisiaddr);
socket(SOCKET, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
connect(SOCKET, $hispaddr) || die "bind: $!";
my $rtime = ' ';
read(SOCKET, $rtime, 4);
close(SOCKET);
my $histime = unpack("N", $rtime) - $SECS_of_70_YEARS ;
printf "%8d %s\n", $histime - time, ctime($histime);
}
]]>
overview /
signals /
fifos /
pipes /
safe-pipes /
open2 /
sockets /
AF_UNIX /
udp /
sys5