That's fine for Internet-domain clients and servers, but what about local
communications? While you can use the same setup, sometimes you don't want
to. Unix-domain sockets are local to the current host, and are often used
internally to implement pipes. Unlike Internet domain sockets, Unix domain
sockets can show up in the file system with an ls(1)
listing.
% ls -l /dev/log srw-rw-rw- 1 root 0 Oct 31 07:23 /dev/log
You can test for these with Perl's -S file test:
unless ( -S '/dev/log' ) { die "something's wicked with the print system"; }
Here's a sample Unix-domain client:
#!/usr/bin/perl -w use Socket; use strict; my ($rendezvous, $line);
$rendezvous = shift || '/tmp/catsock'; socket(SOCK, PF_UNIX, SOCK_STREAM, 0) || die "socket: $!"; connect(SOCK, sockaddr_un($rendezvous)) || die "connect: $!"; while (defined($line = <SOCK>)) { print $line; } exit;
And here's a corresponding server. You don't have to worry about silly network terminators here because Unix domain sockets are guaranteed to be on the localhost, and thus everything works right.
#!/usr/bin/perl -Tw use strict; use Socket; use Carp;
BEGIN { $ENV{PATH} = '/usr/ucb:/bin' } sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
my $NAME = '/tmp/catsock'; my $uaddr = sockaddr_un($NAME); my $proto = getprotobyname('tcp');
socket(Server,PF_UNIX,SOCK_STREAM,0) || die "socket: $!"; unlink($NAME); bind (Server, $uaddr) || die "bind: $!"; listen(Server,SOMAXCONN) || die "listen: $!";
logmsg "server started on $NAME";
my $waitedpid;
sub REAPER { $waitedpid = wait; $SIG{CHLD} = \&REAPER; # loathe sysV logmsg "reaped $waitedpid" . ($? ? " with exit $?" : ''); }
$SIG{CHLD} = \&REAPER;
for ( $waitedpid = 0; accept(Client,Server) || $waitedpid; $waitedpid = 0, close Client) { next if $waitedpid; logmsg "connection on $NAME"; spawn sub { print "Hello there, it's now ", scalar localtime, "\n"; exec '/usr/games/fortune' or die "can't exec fortune: $!"; }; }
As you see, it's remarkably similar to the Internet domain
TCP server, so much so, in fact, that we've omitted
several duplicate functions--spawn(), logmsg(),
ctime(),
and REAPER()--which
are exactly the same
as in the other server.
So why would you ever want to use a Unix domain socket instead of a simpler
named pipe? Because a named pipe doesn't give you sessions. You can't tell
one process's data from another's. With socket programming, you get a
separate session for each client: that's why accept()
takes
two arguments.
For example, let's say that you have a long running database server daemon that you want folks from the World Wide Web to be able to access, but only if they go through a CGI interface. You'd have a small, simple CGI program that does whatever checks and logging you feel like, and then acts as a Unix-domain client and connects to your private server.
Back to Internet TCP Clients and Servers
Forward to TCP Clients with IO/Socket
Up to the perlipc manpage