laziness, impatience, and hubris | |
PerlMonks |
perlman:perlipcby gods (Initiate) |
on Aug 25, 1999 at 06:51 UTC ( [id://410]=perlman: print w/replies, xml ) | Need Help?? |
perlipcCurrent Perl documentation can be found at perldoc.perl.org. Here is our local, out-dated (pre-5.6) version:
NAMEperlipc - Perl interprocess communication (signals, fifos, pipes, safe subprocesses, sockets, and semaphores)
DESCRIPTIONThe basic IPC facilities of Perl are built out of the good old Unix signals, named pipes, pipe opens, the Berkeley socket routines, and SysV IPC calls. Each is used in slightly different situations.
Signals
Perl uses a simple signal handling model: the For example, to trap an interrupt signal, set up a handler like this. Do as little as you possibly can in your handler; notice how all we do is set a global variable and then raise an exception. That's because on most systems, libraries are not re-entrant; particularly, memory allocation and I/O routines are not. That means that doing nearly anything in your handler could in theory trigger a memory fault and subsequent core dump.
sub catch_zap { my $signame = shift; $shucks++; die "Somebody sent me a SIG$signame"; } $SIG{INT} = 'catch_zap'; # could fail in modules $SIG{INT} = \&catch_zap; # best strategy
The names of the signals are the ones listed out by
use Config; defined $Config{sig_name} || die "No sigs?"; foreach $name (split(' ', $Config{sig_name})) { $signo{$name} = $i; $signame[$i] = $name; $i++; } So to check whether signal 17 and SIGALRM were the same, do just this:
print "signal #17 = $signame[17]\n"; if ($signo{ALRM}) { print "SIGALRM is $signo{ALRM}\n"; }
You may also choose to assign the strings
sub precious { local $SIG{INT} = 'IGNORE'; &more_functions; } sub more_functions { # interrupts still ignored, for now... } Sending a signal to a negative process ID means that you send the signal to the entire Unix process-group. This code sends a hang-up signal to all processes in the current process group (and sets $SIG{HUP} to IGNORE so it doesn't kill itself):
{ local $SIG{HUP} = 'IGNORE'; kill HUP => -$$; # snazzy writing of: kill('HUP', -$$) } Another interesting signal to send is signal number zero. This doesn't actually affect another process, but instead checks whether it's alive or has changed its UID.
unless (kill 0 => $kid_pid) { warn "something wicked happened to $kid_pid"; } You might also want to employ anonymous functions for simple signal handlers:
$SIG{INT} = sub { die "\nOutta here!\n" };
But that will be problematic for the more complicated handlers that need to reinstall themselves. Because Perl's signal mechanism is currently based on the
sub REAPER { $waitedpid = wait; # loathe sysV: it makes us not only reinstate # the handler, but place it after the wait $SIG{CHLD} = \&REAPER; } $SIG{CHLD} = \&REAPER; # now do something that forks... or even the more elaborate:
use POSIX ":sys_wait_h"; sub REAPER { my $child; while ($child = waitpid(-1,WNOHANG)) { $Kid_Status{$child} = $?; } $SIG{CHLD} = \&REAPER; # still loathe sysV } $SIG{CHLD} = \&REAPER; # do something that forks...
Signal handling is also used for timeouts in Unix, While safely protected
within an perlfunc:eval block, you set a signal handler to trap alarm signals and then schedule to
have one delivered to you in some number of seconds. Then try your blocking
operation, clearing the alarm when it's done but not before you've exited
your perlfunc:eval block. If it goes off, you'll use
Here's an example:
eval { local $SIG{ALRM} = sub { die "alarm clock restart" }; alarm 10; flock(FH, 2); # blocking write lock alarm 0; }; if ($@ and $@ !~ /alarm clock restart/) { die } For more complex signal handling, you might see the standard POSIX module. Lamentably, this is almost entirely undocumented, but the t/lib/posix.t file from the Perl source distribution has some examples in it.
Named PipesA named pipe (often referred to as a FIFO) is an old Unix IPC mechanism for processes communicating on the same machine. It works just like a regular, connected anonymous pipes, except that the processes rendezvous using a filename and don't have to be related.
To create a named pipe, use the Unix command
# system return val is backwards, so && not || # $ENV{PATH} .= ":/etc:/usr/etc"; if ( system('mknod', $path, 'p') && system('mkfifo', $path) ) { die "mk{nod,fifo} $path failed"; } A fifo is convenient when you want to connect a process to an unrelated one. When you open a fifo, the program will block until there's something on the other end. For example, let's say you'd like to have your .signature file be a named pipe that has a Perl program on the other end. Now every time any program (like a mailer, news reader, finger program, etc.) tries to read from that file, the reading program will block and your program will supply the new signature. We'll use the pipe-checking file test -p to find out whether anyone (or anything) has accidentally removed our fifo.
chdir; # go home $FIFO = '.signature'; $ENV{PATH} .= ":/etc:/usr/games";
while (1) { unless (-p $FIFO) { unlink $FIFO; system('mknod', $FIFO, 'p') && die "can't mknod $FIFO: $!"; }
# next line blocks until there's a reader open (FIFO, "> $FIFO") || die "can't write $FIFO: $!"; print FIFO "John Smith (smith\@host.org)\n", `fortune -s`; close FIFO; sleep 2; # to avoid dup signals }
WARNING
By installing Perl code to deal with signals, you're exposing yourself to danger from two things. First, few system library functions are re-entrant. If the signal interrupts while Perl is executing one function (like
There are two things you can do, knowing this: be paranoid or be pragmatic.
The paranoid approach is to do as little as possible in your signal
handler. Set an existing integer variable that already has a value, and
return. This doesn't help you if you're in a slow system call, which will
just restart. That means you have to die to
To forbid signal handlers altogether would bars you from many interesting programs, including virtually everything in this manpage, since you could no longer even write SIGCHLD handlers. Their dodginess is expected to be addresses in the 5.005 release.
Using open() for IPC
Perl's basic
open(SPOOLER, "| cat -v | lpr -h 2>/dev/null") || die "can't fork: $!"; local $SIG{PIPE} = sub { die "spooler pipe broke" }; print SPOOLER "stuff\n"; close SPOOLER || die "bad spool: $! $?"; And here's how to start up a child process you intend to read from:
open(STATUS, "netstat -an 2>&1 |") || die "can't fork: $!"; while (<STATUS>) { next if /^(tcp|udp)/; print; } close STATUS || die "bad netstat: $! $?"; If one can be sure that a particular program is a Perl script that is expecting filenames in @ARGV, the clever programmer can write something like this:
% program f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile and irrespective of which shell it's called from, the Perl program will read from the file f1, the process cmd1, standard input (tmpfile in this case), the f2 file, the cmd2 command, and finally the f3 file. Pretty nifty, eh? You might notice that you could use backticks for much the same effect as opening a pipe for reading:
print grep { !/^(tcp|udp)/ } `netstat -an 2>&1`; die "bad netstat" if $?; While this is true on the surface, it's much more efficient to process the file one line or record at a time because then you don't have to read the whole thing into memory at once. It also gives you finer control of the whole process, letting you to kill off the child process early if you'd like.
Be careful to check both the
open(FH, "|bogus") or die "can't fork: $!"; print FH "bang\n" or die "can't write: $!"; close FH or die "can't close: $!"; That won't blow up until the close, and it will blow up with a SIGPIPE. To catch it, you could use this:
$SIG{PIPE} = 'IGNORE'; open(FH, "|bogus") or die "can't fork: $!"; print FH "bang\n" or die "can't write: $!"; close FH or die "can't close: status=$?";
Filehandles
Both the main process and any child processes it forks share the same
STDIN,
STDOUT, and
STDERR filehandles. If both processes try to access them at once, strange things can happen. You'll certainly want to any stdio flush output buffers before forking. You may also want to close or reopen the filehandles for the child. You can get around this by opening your pipe with
Background ProcessesYou can run a command in the background with:
system("cmd &"); The command's STDOUT and STDERR (and possibly STDIN, depending on your shell) will be the same as the parent's. You won't need to catch SIGCHLD because of the double-fork taking place (see below for more details).
Complete Dissociation of Child from ParentIn some cases (starting server processes, for instance) you'll want to complete dissociate the child process from the parent. The easiest way is to use:
use POSIX qw(setsid); setsid() or die "Can't start a new session: $!"; However, you may not be on POSIX. The following process is reported to work on most Unixish systems. Non-Unix users should check their Your_OS::Process module for other solutions.
Safe Pipe Opens
Another interesting approach to
IPC is making your single program go multiprocess and communicate between (or even amongst) yourselves. The
use English; my $sleep_count = 0;
do { $pid = open(KID_TO_WRITE, "|-"); unless (defined $pid) { warn "cannot fork: $!"; die "bailing out" if $sleep_count++ > 6; sleep 10; } } until defined $pid;
if ($pid) { # parent print KID_TO_WRITE @some_data; close(KID_TO_WRITE) || warn "kid exited $?"; } else { # child ($EUID, $EGID) = ($UID, $GID); # suid progs only open (FILE, "> /safe/file") || die "can't open /safe/file: $!"; while (<STDIN>) { print FILE; # child's STDIN is parent's KID } exit; # don't forget this }
Another common use for this construct is when you need to execute something without the shell's interference. With
Here's a safe backtick or pipe open for read:
# add error processing as above $pid = open(KID_TO_READ, "-|");
if ($pid) { # parent while (<KID_TO_READ>) { # do something interesting } close(KID_TO_READ) || warn "kid exited $?";
} else { # child ($EUID, $EGID) = ($UID, $GID); # suid only exec($program, @options, @args) || die "can't exec program: $!"; # NOTREACHED } And here's a safe pipe open for writing:
# add error processing as above $pid = open(KID_TO_WRITE, "|-"); $SIG{ALRM} = sub { die "whoops, $program pipe broke" };
if ($pid) { # parent for (@data) { print KID_TO_WRITE; } close(KID_TO_WRITE) || warn "kid exited $?";
} else { # child ($EUID, $EGID) = ($UID, $GID); exec($program, @options, @args) || die "can't exec program: $!"; # NOTREACHED } Note that these operations are full Unix forks, which means they may not be correctly implemented on alien systems. Additionally, these are not true multithreading. If you'd like to learn more about threading, see the modules file mentioned below in the SEE ALSO section.
Bidirectional Communication with Another ProcessWhile this works reasonably well for unidirectional communication, what about bidirectional communication? The obvious thing you'd like to do doesn't actually work:
open(PROG_FOR_READING_AND_WRITING, "| some program |") and if you forget to use the -w flag, then you'll miss out entirely on the diagnostic message:
Can't do bidirectional pipe at -e line 1.
If you really want to, you can use the standard
If you look at its source, you'll see that
Here's an example of using
use FileHandle; use IPC::Open2; $pid = open2(*Reader, *Writer, "cat -u -n" ); Writer->autoflush(); # default here, actually print Writer "stuff\n"; $got = <Reader>;
The problem with this is that Unix buffering is really going to ruin your
day. Even though your A solution to this is the nonstandard Comm.pl library. It uses pseudo-ttys to make your program behave more reasonably:
require 'Comm.pl'; $ph = open_proc('cat -n'); for (1..10) { print $ph "a line\n"; print "got back ", scalar <$ph>; }
This way you don't have to have control over the source code of the program
you're using. The Comm library also has
The newer Expect.pm module from CPAN also addresses this kind of thing. This module requires two other modules from CPAN: IO::Pty and IO::Stty. It sets up a pseudo-terminal to interact with programs that insist on using talking to the terminal device driver. If your system is amongst those supported, this may be your best bet.
Bidirectional Communication with Yourself
If you want, you may make low-level
#!/usr/bin/perl -w # pipe1 - bidirectional communication using two pipe pairs # designed for the socketpair-challenged use IO::Handle; # thousands of lines just for autoflush :-( pipe(PARENT_RDR, CHILD_WTR); # XXX: failure? pipe(CHILD_RDR, PARENT_WTR); # XXX: failure? CHILD_WTR->autoflush(1); PARENT_WTR->autoflush(1);
if ($pid = fork) { close PARENT_RDR; close PARENT_WTR; print CHILD_WTR "Parent Pid $$ is sending this\n"; chomp($line = <CHILD_RDR>); print "Parent Pid $$ just read this: `$line'\n"; close CHILD_RDR; close CHILD_WTR; waitpid($pid,0); } else { die "cannot fork: $!" unless defined $pid; close CHILD_RDR; close CHILD_WTR; chomp($line = <PARENT_RDR>); print "Child Pid $$ just read this: `$line'\n"; print PARENT_WTR "Child Pid $$ is sending this\n"; close PARENT_RDR; close PARENT_WTR; exit; }
But you don't actually have to make two pipe calls. If you have the
#!/usr/bin/perl -w # pipe2 - bidirectional communication using socketpair # "the best ones always go both ways"
use Socket; use IO::Handle; # thousands of lines just for autoflush :-( # We say AF_UNIX because although *_LOCAL is the # POSIX 1003.1g form of the constant, many machines # still don't have it. socketpair(CHILD, PARENT, AF_UNIX, SOCK_STREAM, PF_UNSPEC) or die "socketpair: $!";
CHILD->autoflush(1); PARENT->autoflush(1);
if ($pid = fork) { close PARENT; print CHILD "Parent Pid $$ is sending this\n"; chomp($line = <CHILD>); print "Parent Pid $$ just read this: `$line'\n"; close CHILD; waitpid($pid,0); } else { die "cannot fork: $!" unless defined $pid; close CHILD; chomp($line = <PARENT>); print "Child Pid $$ just read this: `$line'\n"; print PARENT "Child Pid $$ is sending this\n"; close PARENT; exit; }
Sockets: Client/Server CommunicationWhile not limited to Unix-derived operating systems (e.g., WinSock on PCs provides socket support, as do some VMS libraries), you may not have sockets on your system, in which case this section probably isn't going to do you much good. With sockets, you can do both virtual circuits (i.e., TCP streams) and datagrams (i.e., UDP packets). You may be able to do even more depending on your system. The Perl function calls for dealing with sockets have the same names as the corresponding system calls in C, but their arguments tend to differ for two reasons: first, Perl filehandles work differently than C file descriptors. Second, Perl already knows the length of its strings, so you don't need to pass that information.
One of the major problems with old socket code in Perl was that it used
hard-coded values for some of the constants, which severely hurt
portability. If you ever see code that does anything like explicitly
setting If you're not writing a server/client for an existing protocol like NNTP or SMTP, you should give some thought to how your server will know when the client has finished talking, and vice-versa. Most protocols are based on one-line messages and responses (so one party knows the other has finished when a ``\n'' is received) or multi-line messages and responses that end with a period on an empty line (``\n.\n'' terminates a message/response).
Internet Line TerminatorsThe Internet line terminator is ``\015\012''. Under ASCII variants of Unix, that could usually be written as ``\r\n'', but under other systems, ``\r\n'' might at times be ``\015\015\012'', ``\012\012\015'', or something completely different. The standards specify writing ``\015\012'' to be conformant (be strict in what you provide), but they also recommend accepting a lone ``\012'' on input (but be lenient in what you require). We haven't always been very good about that in the code in this manpage, but unless you're on a Mac, you'll probably be ok.
Internet TCP Clients and ServersUse 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:
#!/usr/bin/perl -w 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, 'tcp') } 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: $!"; while (defined($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. If you want sit on a particular interface (like the external side of a gateway or firewall machine), you should fill this in with your real address instead.
#!/usr/bin/perl -Tw use strict; BEGIN { $ENV{PATH} = '/usr/ucb:/bin' } use Socket; use Carp; $EOL = "\015\012";
sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
my $port = shift || 2345; my $proto = getprotobyname('tcp'); $port = $1 if $port =~ /(\d+)/; # untaint port number
socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!"; setsockopt(Server, SOL_SOCKET, SO_REUSEADDR, pack("l", 1)) || die "setsockopt: $!"; bind(Server, sockaddr_in($port, INADDR_ANY)) || die "bind: $!"; listen(Server,SOMAXCONN) || die "listen: $!";
logmsg "server started on port $port";
my $paddr;
$SIG{CHLD} = \&REAPER;
for ( ; $paddr = accept(Client,Server); close Client) { my($port,$iaddr) = sockaddr_in($paddr); my $name = gethostbyaddr($iaddr,AF_INET);
logmsg "connection from $name [", inet_ntoa($iaddr), "] at port $port";
print Client "Hello there, $name, it's now ", scalar localtime, $EOL; } And here's a multithreaded version. It's multithreaded in that like most typical servers, it spawns (forks) a slave server to handle the client request so that the master server can quickly go back to service a new client.
#!/usr/bin/perl -Tw use strict; BEGIN { $ENV{PATH} = '/usr/ucb:/bin' } use Socket; use Carp; $EOL = "\015\012";
sub spawn; # forward declaration sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
my $port = shift || 2345; my $proto = getprotobyname('tcp'); $port = $1 if $port =~ /(\d+)/; # untaint port number
socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!"; setsockopt(Server, SOL_SOCKET, SO_REUSEADDR, pack("l", 1)) || die "setsockopt: $!"; bind(Server, sockaddr_in($port, INADDR_ANY)) || die "bind: $!"; listen(Server,SOMAXCONN) || die "listen: $!";
logmsg "server started on port $port";
my $waitedpid = 0; my $paddr;
sub REAPER { $waitedpid = wait; $SIG{CHLD} = \&REAPER; # loathe sysV logmsg "reaped $waitedpid" . ($? ? " with exit $?" : ''); }
$SIG{CHLD} = \&REAPER;
for ( $waitedpid = 0; ($paddr = accept(Client,Server)) || $waitedpid; $waitedpid = 0, close Client) { next if $waitedpid and not $paddr; my($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, $EOL; exec '/usr/games/fortune' # XXX: `wrong' line terminators 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
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:
#!/usr/bin/perl -w 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); }
Unix-Domain TCP Clients and Servers
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 -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(),
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
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.
TCP Clients with IO::SocketFor those preferring a higher-level interface to socket programming, the IO::Socket module provides an object-oriented approach. IO::Socket is included as part of the standard Perl distribution as of the 5.004 release. If you're running an earlier version of Perl, just fetch IO::Socket from CPAN, where you'll also find find modules providing easy interfaces to the following systems: DNS, FTP, Ident (RFC 931), NIS and NISPlus, NNTP, Ping, POP3, SMTP, SNMP, SSLeay, Telnet, and Time--just to name a few.
A Simple ClientHere's a client that creates a TCP connection to the ``daytime'' service at port 13 of the host name ``localhost'' and prints out everything that the server there cares to provide.
#!/usr/bin/perl -w use IO::Socket; $remote = IO::Socket::INET->new( Proto => "tcp", PeerAddr => "localhost", PeerPort => "daytime(13)", ) or die "cannot connect to daytime port at localhost"; while ( <$remote> ) { print } When you run this program, you should get something back that looks like this:
Wed May 14 08:40:46 MDT 1997
Here are what those parameters to the
Notice how the return value from the
$line = <$handle>; all remaining lines from is this way:
@lines = <$handle>; and send a line of data to it this way:
print $handle "some data\n";
A Webget ClientHere's a simple client that takes a remote host to fetch a document from, and then a list of documents to get from that host. This is a more interesting client than the previous one because it first sends something to the server before fetching the server's response.
#!/usr/bin/perl -w use IO::Socket; unless (@ARGV > 1) { die "usage: $0 host document ..." } $host = shift(@ARGV); $EOL = "\015\012"; $BLANK = $EOL x 2; foreach $document ( @ARGV ) { $remote = IO::Socket::INET->new( Proto => "tcp", PeerAddr => $host, PeerPort => "http(80)", ); unless ($remote) { die "cannot connect to http daemon on $host" } $remote->autoflush(1); print $remote "GET $document HTTP/1.0" . $BLANK; while ( <$remote> ) { print } close $remote; }
The web server handing the ``http'' service, which is assumed to be at its
standard port, number 80. If your the web server you're trying to connect
to is at a different port (like 1080 or 8080), you should specify as the
named-parameter pair, Connecting to the server is only the first part of the process: once you have the connection, you have to use the server's language. Each server on the network has its own little command language that it expects as input. The string that we send to the server starting with ``GET'' is in HTTP syntax. In this case, we simply request each specified document. Yes, we really are making a new connection for each document, even though it's the same host. That's the way you always used to have to speak HTTP. Recent versions of web browsers may request that the remote server leave the connection open a little while, but the server doesn't have to honor such a request. Here's an example of running that program, which we'll call webget:
% webget www.perl.com /guanaco.html HTTP/1.1 404 File Not Found Date: Thu, 08 May 1997 18:02:32 GMT Server: Apache/1.2b6 Connection: close Content-type: text/html
<HEAD><TITLE>404 File Not Found</TITLE></HEAD> <BODY><H1>File Not Found</H1> The requested URL /guanaco.html was not found on this server.<P> </BODY> Ok, so that's not very interesting, because it didn't find that particular document. But a long response wouldn't have fit on this page. For a more fully-featured version of this program, you should look to the lwp-request program included with the LWP modules from CPAN.
Interactive Client with IO::SocketWell, that's all fine if you want to send one command and get one answer, but what about setting up something fully interactive, somewhat like the way telnet works? That way you can type a line, get the answer, type a line, get the answer, etc. This client is more complicated than the two we've done so far, but if you're on a system that supports the powerful fork call, the solution isn't that rough. Once you've made the connection to whatever service you'd like to chat with, call fork to clone your process. Each of these two identical process has a very simple job to do: the parent copies everything from the socket to standard output, while the child simultaneously copies everything from standard input to the socket. To accomplish the same thing using just one process would be much harder, because it's easier to code two processes to do one thing than it is to code one process to do two things. (This keep-it-simple principle a cornerstones of the Unix philosophy, and good software engineering as well, which is probably why it's spread to other systems.) Here's the code:
#!/usr/bin/perl -w use strict; use IO::Socket; my ($host, $port, $kidpid, $handle, $line);
unless (@ARGV == 2) { die "usage: $0 host port" } ($host, $port) = @ARGV;
# create a tcp connection to the specified host and port $handle = IO::Socket::INET->new(Proto => "tcp", PeerAddr => $host, PeerPort => $port) or die "can't connect to port $port on $host: $!";
$handle->autoflush(1); # so output gets there right away print STDERR "[Connected to $host:$port]\n";
# split the program into two processes, identical twins die "can't fork: $!" unless defined($kidpid = fork());
# the if{} block runs only in the parent process if ($kidpid) { # copy the socket to standard output while (defined ($line = <$handle>)) { print STDOUT $line; } kill("TERM", $kidpid); # send SIGTERM to child } # the else{} block runs only in the child process else { # copy standard input to the socket while (defined ($line = <STDIN>)) { print $handle $line; } }
The kill function in the parent's
If the remote server sends data a byte at time, and you need that data
immediately without waiting for a newline (which might not happen), you may
wish to replace the
my $byte; while (sysread($handle, $byte, 1) == 1) { print STDOUT $byte; } Making a system call for each byte you want to read is not very efficient (to put it mildly) but is the simplest to explain and works reasonably well.
TCP Servers with IO::Socket
As always, setting up a server is little bit more involved than running a
client. The model is that the server creates a special kind of socket that
does nothing but listen on a particular port for incoming connections. It
does this by calling the
Once the generic server socket has been created using the parameters listed above, the server then waits for a new client to connect to it. The server blocks in the accept method, which eventually an bidirectional connection to the remote client. (Make sure to autoflush this handle to circumvent buffering.) To add to user-friendliness, our server prompts the user for commands. Most servers don't do this. Because of the prompt without a newline, you'll have to use the sysread variant of the interactive client above. This server accepts one of five different commands, sending output back to the client. Note that unlike most network servers, this one only handles one incoming client at a time. Multithreaded servers are covered in Chapter 6 of the Camel as well as later in this manpage. Here's the code. We'll
#!/usr/bin/perl -w use IO::Socket; use Net::hostent; # for OO version of gethostbyaddr
$PORT = 9000; # pick something not in use
$server = IO::Socket::INET->new( Proto => 'tcp', LocalPort => $PORT, Listen => SOMAXCONN, Reuse => 1);
die "can't setup server" unless $server; print "[Server $0 accepting clients]\n";
while ($client = $server->accept()) { $client->autoflush(1); print $client "Welcome to $0; type help for command list.\n"; $hostinfo = gethostbyaddr($client->peeraddr); printf "[Connect from %s]\n", $hostinfo->name || $client->peerhost; print $client "Command? "; while ( <$client>) { next unless /\S/; # blank line if (/quit|exit/i) { last; } elsif (/date|time/i) { printf $client "%s\n", scalar localtime; } elsif (/who/i ) { print $client `who 2>&1`; } elsif (/cookie/i ) { print $client `/usr/games/fortune 2>&1`; } elsif (/motd/i ) { print $client `cat /etc/motd 2>&1`; } else { print $client "Commands: quit date who cookie motd\n"; } } continue { print $client "Command? "; } close $client; }
UDP: Message PassingAnother kind of client-server setup is one that uses not connections, but messages. UDP communications involve much lower overhead but also provide less reliability, as there are no promises that messages will arrive at all, let alone in order and unmangled. Still, UDP offers some advantages over TCP, including being able to ``broadcast'' or ``multicast'' to a whole bunch of destination hosts at once (usually on your local subnet). If you find yourself overly concerned about reliability and start building checks into your message system, then you probably should use just TCP to start with.
Here's a
UDP program similar to the sample Internet
TCP client given earlier. However, instead of checking one host at a time, the
UDP version will check many of them asynchronously by simulating a multicast and then using
#!/usr/bin/perl -w use strict; use Socket; use Sys::Hostname;
my ( $count, $hisiaddr, $hispaddr, $histime, $host, $iaddr, $paddr, $port, $proto, $rin, $rout, $rtime, $SECS_of_70_YEARS);
$SECS_of_70_YEARS = 2208988800;
$iaddr = gethostbyname(hostname()); $proto = getprotobyname('udp'); $port = getservbyname('time', 'udp'); $paddr = sockaddr_in(0, $iaddr); # 0 means let kernel pick
socket(SOCKET, PF_INET, SOCK_DGRAM, $proto) || die "socket: $!"; bind(SOCKET, $paddr) || die "bind: $!";
$| = 1; printf "%-12s %8s %s\n", "localhost", 0, scalar localtime time; $count = 0; for $host (@ARGV) { $count++; $hisiaddr = inet_aton($host) || die "unknown host"; $hispaddr = sockaddr_in($port, $hisiaddr); defined(send(SOCKET, 0, 0, $hispaddr)) || die "send $host: $!"; }
$rin = ''; vec($rin, fileno(SOCKET), 1) = 1;
# timeout after 10.0 seconds while ($count && select($rout = $rin, undef, undef, 10.0)) { $rtime = ''; ($hispaddr = recv(SOCKET, $rtime, 4, 0)) || die "recv: $!"; ($port, $hisiaddr) = sockaddr_in($hispaddr); $host = gethostbyaddr($hisiaddr, AF_INET); $histime = unpack("N", $rtime) - $SECS_of_70_YEARS ; printf "%-12s ", $host; printf "%8d %s\n", $histime - time, scalar localtime($histime); $count--; }
SysV IPC
While System
V
IPC isn't so widely used as sockets, it still has some interesting uses. You can't, however, effectively use SysV
IPC or Berkeley
Here's a small example showing shared memory usage.
use IPC::SysV qw(IPC_PRIVATE IPC_RMID S_IRWXU S_IRWXG S_IRWXO);
$size = 2000; $key = shmget(IPC_PRIVATE, $size, S_IRWXU|S_IRWXG|S_IRWXO) || die "$!"; print "shm key $key\n";
$message = "Message #1"; shmwrite($key, $message, 0, 60) || die "$!"; print "wrote: '$message'\n"; shmread($key, $buff, 0, 60) || die "$!"; print "read : '$buff'\n";
# the buffer of shmread is zero-character end-padded. substr($buff, index($buff, "\0")) = ''; print "un" unless $buff eq $message; print "swell\n";
print "deleting shm $key\n"; shmctl($key, IPC_RMID, 0) || die "$!"; Here's an example of a semaphore:
use IPC::SysV qw(IPC_CREAT);
$IPC_KEY = 1234; $key = semget($IPC_KEY, 10, 0666 | IPC_CREAT ) || die "$!"; print "shm key $key\n"; Put this code in a separate file to be run in more than one process. Call the file take:
# create a semaphore
$IPC_KEY = 1234; $key = semget($IPC_KEY, 0 , 0 ); die if !defined($key);
$semnum = 0; $semflag = 0;
# 'take' semaphore # wait for semaphore to be zero $semop = 0; $opstring1 = pack("sss", $semnum, $semop, $semflag);
# Increment the semaphore count $semop = 1; $opstring2 = pack("sss", $semnum, $semop, $semflag); $opstring = $opstring1 . $opstring2;
semop($key,$opstring) || die "$!"; Put this code in a separate file to be run in more than one process. Call this file give:
# 'give' the semaphore # run this in the original process and you will see # that the second process continues
$IPC_KEY = 1234; $key = semget($IPC_KEY, 0, 0); die if !defined($key);
$semnum = 0; $semflag = 0;
# Decrement the semaphore count $semop = -1; $opstring = pack("sss", $semnum, $semop, $semflag);
semop($key,$opstring) || die "$!"; The SysV IPC code above was written long ago, and it's definitely clunky looking. For a more modern look, see the IPC::SysV module which is included with Perl starting from Perl 5.005.
NOTES
Most of these routines quietly but politely return undef when they fail instead of causing your program to die right then and there
due to an uncaught exception. (Actually, some of the new Socket conversion functions
#!/usr/bin/perl -Tw use strict; use sigtrap; use Socket;
BUGSAll these routines create system-specific portability problems. As noted elsewhere, Perl is at the mercy of your C libraries for much of its system behaviour. It's probably safest to assume broken SysV semantics for signals and to stick with simple TCP and UDP socket operations; e.g., don't try to pass open file descriptors over a local UDP datagram socket if you want your code to stand a chance of being portable.
As mentioned in the signals section, because few vendors provide
C libraries that are safely re-entrant, the prudent programmer will do little else within a handler beyond setting a numeric variable that already exists; or, if locked into a slow (restarting) system call, using
AUTHORTom Christiansen, with occasional vestiges of Larry Wall's original version and suggestions from the Perl Porters.
More... Return to the Library |
|