For example, to trap an interrupt signal, set up a handler like this:
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
Prior to Perl 5.7.3 it was necessary to do as little as you possibly could 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 meant that doing nearly anything in your handler could in theory trigger a memory fault and subsequent core dump - see ``Deferred Signals (Safe Signals)'' below.
The names of the signals are the ones listed out by "kill -l" on your system, or you can retrieve them from the Config module. Set up an @signame list indexed by number to get the name and a %signo table indexed by name to get the number:
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 'IGNORE' or 'DEFAULT' as the handler, in which case Perl will try to discard the signal or do the default thing.
On most Unix platforms, the "CHLD" (sometimes also known as "CLD") signal has special behavior with respect to a value of 'IGNORE'. Setting $SIG{CHLD} to 'IGNORE' on such a platform has the effect of not creating zombie processes when the parent process fails to "wait()" on its child processes (i.e. child processes are automatically reaped). Calling "wait()" with $SIG{CHLD} set to 'IGNORE' usually returns "-1" on such platforms.
Some signals can be neither trapped nor ignored, such as the KILL and STOP (but not the TSTP) signals. One strategy for temporarily ignoring signals is to use a local() statement, which will be automatically restored once your block is exited. (Remember that local() values are ``inherited'' by functions called from within that block.)
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 a child 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";
}
When directed at a process whose UID is not identical to that of the sending process, signal number zero may fail because you lack permission to send the signal, even though the process is alive. You may be able to determine the cause of failure using "%!".
unless (kill 0 => $pid or $!{EPERM}) {
warn "$pid looks dead";
}
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 signal(3) function from the C library, you may sometimes be so unfortunate as to run on systems where that function is ``broken'', that is, it behaves in the old unreliable SysV way rather than the newer, more reasonable BSD and POSIX fashion. So you'll see defensive people writing signal handlers like this:
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 better still:
use POSIX ":sys_wait_h";
sub REAPER {
my $child;
# If a second child dies while in the signal handler caused by the
# first death, we won't get another signal. So must loop here else
# we will leave the unreaped child as a zombie. And the next time
# two children die we get another zombie. And so on.
while (($child = waitpid(-1,WNOHANG)) > 0) {
$Kid_Status{$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 "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 "eval{}" block. If it goes off, you'll use die() to jump out of the block, much as you might using longjmp() or throw() in other languages.
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 }
If the operation being timed out is system() or qx(), this technique is liable to generate zombies. If this matters to you, you'll need to do your own fork() and exec(), and kill the errant child process.
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.
Not all platforms automatically reinstall their (native) signal handlers after a signal delivery. This means that the handler works only the first time the signal is sent. The solution to this problem is to use "POSIX" signal handlers if available, their behaviour is well-defined.
The following example implements a simple daemon, which restarts itself every time the "SIGHUP" signal is received. The actual code is located in the subroutine "code()", which simply prints some debug info to show that it works and should be replaced with the real code.
#!/usr/bin/perl -w
use POSIX ();
use FindBin ();
use File::Basename ();
use File::Spec::Functions;
$|=1;
# make the daemon cross-platform, so exec always calls the script
# itself with the right path, no matter how the script was invoked.
my $script = File::Basename::basename($0);
my $SELF = catfile $FindBin::Bin, $script;
# POSIX unmasks the sigprocmask properly
my $sigset = POSIX::SigSet->new();
my $action = POSIX::SigAction->new('sigHUP_handler',
$sigset,
&POSIX::SA_NODEFER);
POSIX::sigaction(&POSIX::SIGHUP, $action);
sub sigHUP_handler {
print "got SIGHUP\n";
exec($SELF, @ARGV) or die "Couldn't restart: $!\n";
}
code();
sub code {
print "PID: $$\n";
print "ARGV: @ARGV\n";
my $c = 0;
while (++$c) {
sleep 2;
print "$c\n";
}
}
__END__
To create a named pipe, use the "POSIX::mkfifo()" function.
use POSIX qw(mkfifo);
mkfifo($path, 0700) or die "mkfifo $path failed: $!";
You can also use the Unix command mknod(1) or on some systems, mkfifo(1). These may not be in your normal path.
# 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';
while (1) {
unless (-p $FIFO) {
unlink $FIFO;
require POSIX;
POSIX::mkfifo($FIFO, 0700)
or die "can't mkfifo $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
}
There were two things you could do, knowing this: be paranoid or be pragmatic. The paranoid approach was 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 longjmp(3) out of the handler. Even this is a little cavalier for the true paranoiac, who avoids "die" in a handler because the system is out to get you. The pragmatic approach was to say ``I know the risks, but prefer the convenience'', and to do anything you wanted in your signal handler, and be prepared to clean up core dumps now and again.
In Perl 5.7.3 and later to avoid these problems signals are ``deferred''-- that is when the signal is delivered to the process by the system (to the C code that implements Perl) a flag is set, and the handler returns immediately. Then at strategic ``safe'' points in the Perl interpreter (e.g. when it is about to execute a new opcode) the flags are checked and the Perl level handler from %SIG is executed. The ``deferred'' scheme allows much more flexibility in the coding of signal handler as we know Perl interpreter is in a safe state, and that we are not in a system library function when the handler is called. However the implementation does differ from previous Perls in the following ways:
N.B. If a signal of any given type fires multiple times during an opcode (such as from a fine-grained timer), the handler for that signal will only be called once after the opcode completes, and all the other instances will be discarded. Furthermore, if your system's signal queue gets flooded to the point that there are signals that have been raised but not yet caught (and thus not deferred) at the time an opcode completes, those signals may well be caught and deferred during subsequent opcodes, with sometimes surprising results. For example, you may see alarms delivered even after calling alarm(0) as the latter stops the raising of alarms but does not cancel the delivery of alarms raised but not yet caught. Do not depend on the behaviors described in this paragraph as they are side effects of the current implementation and may change in future versions of Perl.
Note that the default in Perl 5.7.3 and later is to automatically use the ":perlio" layer.
Note that some networking library functions like gethostbyname() are known to have their own implementations of timeouts which may conflict with your timeouts. If you are having problems with such functions, you can try using the POSIX sigaction() function, which bypasses the Perl safe signals (note that this means subjecting yourself to possible memory corruption, as described above). Instead of setting $SIG{ALRM}:
local $SIG{ALRM} = sub { die "alarm" };
try something like the following:
use POSIX qw(SIGALRM);
POSIX::sigaction(SIGALRM,
POSIX::SigAction->new(sub { die "alarm" }))
or die "Error setting SIGALRM handler: $!\n";
Another way to disable the safe signal behavior locally is to use the "Perl::Unsafe::Signals" module from CPAN (which will affect all signals).
Note that the default ":perlio" layer will retry "read", "write" and "close" as described above and that interrupted "wait" and "waitpid" calls will always be retried.
If you want the old signal behaviour back regardless of possible memory corruption, set the environment variable "PERL_SIGNALS" to "unsafe" (a new feature since Perl 5.8.1).
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() and the close() return values. If you're writing to a pipe, you should also trap SIGPIPE. Otherwise, think of what happens when you start up a pipe to a command that doesn't exist: the open() will in all likelihood succeed (it only reflects the fork()'s success), but then your output will fail---spectacularly. Perl can't know whether the command worked because your command is actually running in a separate process whose exec() might have failed. Therefore, while readers of bogus commands return just a quick end of file, writers to bogus command will trigger a signal they'd better be prepared to handle. Consider:
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=$?";
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).
use POSIX 'setsid';
sub daemonize {
chdir '/' or die "Can't chdir to /: $!";
open STDIN, '/dev/null' or die "Can't read /dev/null: $!";
open STDOUT, '>/dev/null'
or die "Can't write to /dev/null: $!";
defined(my $pid = fork) or die "Can't fork: $!";
exit if $pid;
setsid or die "Can't start a new session: $!";
open STDERR, '>&STDOUT' or die "Can't dup stdout: $!";
}
The fork() has to come before the setsid() to ensure that you aren't a process group leader (the setsid() will fail if you are). If your system doesn't have the setsid() function, open /dev/tty and use the "TIOCNOTTY" ioctl() on it instead. See tty(4) for details.
Non-Unix users should check their Your_OS::Process module for other solutions.
use English '-no_match_vars';
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 system(), it's straightforward, but you can't use a pipe open or backticks safely. That's because there's no way to stop the shell from getting its hands on your arguments. Instead, use lower-level control to call exec() directly.
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{PIPE} = 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
}
Since Perl 5.8.0, you can also use the list form of "open" for pipes : the syntax
open KID_PS, "-|", "ps", "aux" or die $!;
forks the ps(1) command (without spawning a shell, as there are more than three arguments to open()), and reads its standard output via the "KID_PS" filehandle. The corresponding syntax to write to command pipes (with "|-" in place of "-|") is also implemented.
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.
open(PROG_FOR_READING_AND_WRITING, "| some program |")
and if you forget to use the "use warnings" pragma or 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 open2() library function to catch both ends. There's also an open3() for tridirectional I/O so you can also catch your child's STDERR, but doing so would then require an awkward select() loop and wouldn't allow you to use normal Perl input operations.
If you look at its source, you'll see that open2() uses low-level primitives like Unix pipe() and exec() calls to create all the connections. While it might have been slightly more efficient by using socketpair(), it would have then been even less portable than it already is. The open2() and open3() functions are unlikely to work anywhere except on a Unix system or some other one purporting to be POSIX compliant.
Here's an example of using open2():
use FileHandle;
use IPC::Open2;
$pid = open2(*Reader, *Writer, "cat -u -n" );
print Writer "stuff\n";
$got = <Reader>;
The problem with this is that Unix buffering is really going to ruin your day. Even though your "Writer" filehandle is auto-flushed, and the process on the other end will get your data in a timely manner, you can't usually do anything to force it to give it back to you in a similarly quick fashion. In this case, we could, because we gave cat a -u flag to make it unbuffered. But very few Unix commands are designed to operate over pipes, so this seldom works unless you yourself wrote the program on the other end of the double-ended pipe.
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 expect() and interact() functions. Find the library (and we hope its successor IPC::Chat) at your nearest CPAN archive as detailed in the SEE ALSO section below.
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.
#!/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 $$ is sending this\n";
close PARENT_RDR; close PARENT_WTR;
exit;
}
But you don't actually have to make two pipe calls. If you have the socketpair() system call, it will do this all for you.
#!/usr/bin/perl -w
# pipe2 - bidirectional communication using socketpair
# "the best ones always go both ways"
use Socket;
use IO::Handle; # thousands of lines just for autoflush :-(
# We say AF_UNIX because although *_LOCAL is the
# POSIX 1003.1g form of the constant, many machines
# still don't have it.
socketpair(CHILD, PARENT, AF_UNIX, SOCK_STREAM, PF_UNSPEC)
or die "socketpair: $!";
CHILD->autoflush(1);
PARENT->autoflush(1);
if ($pid = fork) {
close PARENT;
print CHILD "Parent Pid $$ 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;
}
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 "$AF_INET = 2", you know you're in for big trouble: An immeasurably superior approach is to use the "Socket" module, which more reliably grants access to various constants and functions you'll need.
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).
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;
my $EOL = "\015\012";
sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
my $port = shift || 2345;
my $proto = getprotobyname('tcp');
($port) = $port =~ /^(\d+)$/ or die "invalid port";
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;
my $EOL = "\015\012";
sub spawn; # forward declaration
sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
my $port = shift || 2345;
my $proto = getprotobyname('tcp');
($port) = $port =~ /^(\d+)$/ or die "invalid port";
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;
use POSIX ":sys_wait_h";
use Errno;
sub REAPER {
local $!; # don't let waitpid() overwrite current error
while ((my $pid = waitpid(-1,WNOHANG)) > 0 && WIFEXITED($?)) {
logmsg "reaped $waitedpid" . ($? ? " with exit $?" : '');
}
$SIG{CHLD} = \&REAPER; # loathe sysV
}
$SIG{CHLD} = \&REAPER;
while(1) {
$paddr = accept(Client, Server) || do {
# try again if accept() returned because a signal was received
next if $!{EINTR};
die "accept: $!";
};
my ($port, $iaddr) = sockaddr_in($paddr);
my $name = gethostbyaddr($iaddr, AF_INET);
logmsg "connection from $name [",
inet_ntoa($iaddr),
"] at port $port";
spawn sub {
$|=1;
print "Hello there, $name, it's now ", scalar localtime, $EOL;
exec '/usr/games/fortune' # XXX: `wrong' line terminators
or confess "can't exec fortune: $!";
};
close Client;
}
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. The REAPER subroutine is used here to call waitpid() for any child processes that have finished, thereby ensuring that they terminate cleanly and don't join the ranks of the living dead.
Within the while loop we call accept() and check to see if it returns a false value. This would normally indicate a system error that needs to be reported. However the introduction of safe signals (see ``Deferred Signals (Safe Signals)'' above) in Perl 5.7.3 means that accept() may also be interrupted when the process receives a signal. This typically happens when one of the forked sub-processes exits and notifies the parent process with a CHLD signal.
If accept() is interrupted by a signal then $! will be set to EINTR. If this happens then we can safely continue to the next iteration of the loop and another call to accept(). It is important that your signal handling code doesn't modify the value of $! or this test will most likely fail. In the REAPER subroutine we create a local version of $! before calling waitpid(). When waitpid() sets $! to ECHILD (as it inevitably does when it has no more children waiting), it will update the local copy leaving the original unchanged.
We suggest that you use the -T flag to use taint checking (see perlsec) 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);
}
% 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 log system";
}
Here's a sample Unix-domain client:
#!/usr/bin/perl -w
use Socket;
use strict;
my ($rendezvous, $line);
$rendezvous = shift || '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 spawn; # forward declaration
sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
my $NAME = '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;
use POSIX ":sys_wait_h";
sub REAPER {
my $child;
while (($waitedpid = waitpid(-1,WNOHANG)) > 0) {
logmsg "reaped $waitedpid" . ($? ? " with exit $?" : '');
}
$SIG{CHLD} = \&REAPER; # loathe sysV
}
$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: $!";
};
}
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();
}
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.
#!/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 "new" constructor mean:
Notice how the return value from the "new" constructor is used as a filehandle in the "while" loop? That's what's called an indirect filehandle, a scalar variable containing a filehandle. You can use it the same way you would a normal filehandle. For example, you can read one line from it this way:
$line = <$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";
#!/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 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, "PeerPort => 8080". The "autoflush" method is used on the socket because otherwise the system would buffer up the output we sent it. (If you're on a Mac, you'll also need to change every "\n" in your code that sends data over the network to be a "\015\012" instead.)
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.
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" block is there to send a signal to our child process (current running in the "else" block) as soon as the remote server has closed its end of the connection.
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 "while" loop in the parent with the following:
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.
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 accepts a bidirectional connection from 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.
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 ? $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;
}
Note that UDP datagrams are not a bytestream and should not be treated as such. This makes using I/O mechanisms with internal buffering like stdio (i.e. print() and friends) especially cumbersome. Use syswrite(), or better send(), like in the example below.
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 select() to do a timed-out wait for I/O. To do something similar with TCP, you'd have to use a different socket handle for each host.
#!/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--;
}
Note that this example does not include any retries and may consequently fail to contact a reachable host. The most prominent reason for this is congestion of the queues on the sending host if the number of list of hosts to contact is sufficiently large.
Here's a small example showing shared memory usage.
use IPC::SysV qw(IPC_PRIVATE IPC_RMID S_IRUSR S_IWUSR);
$size = 2000;
$id = shmget(IPC_PRIVATE, $size, S_IRUSR|S_IWUSR) || die "$!";
print "shm key $id\n";
$message = "Message #1";
shmwrite($id, $message, 0, 60) || die "$!";
print "wrote: '$message'\n";
shmread($id, $buff, 0, 60) || die "$!";
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 $id\n";
shmctl($id, IPC_RMID, 0) || die "$!";
Here's an example of a semaphore:
use IPC::SysV qw(IPC_CREAT);
$IPC_KEY = 1234;
$id = semget($IPC_KEY, 10, 0666 | IPC_CREAT ) || die "$!";
print "shm key $id\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;
$id = semget($IPC_KEY, 0 , 0 );
die if !defined($id);
$semnum = 0;
$semflag = 0;
# 'take' semaphore
# wait for semaphore to be zero
$semop = 0;
$opstring1 = pack("s!s!s!", $semnum, $semop, $semflag);
# Increment the semaphore count
$semop = 1;
$opstring2 = pack("s!s!s!", $semnum, $semop, $semflag);
$opstring = $opstring1 . $opstring2;
semop($id,$opstring) || die "$!";
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;
$id = semget($IPC_KEY, 0, 0);
die if !defined($id);
$semnum = 0;
$semflag = 0;
# Decrement the semaphore count
$semop = -1;
$opstring = pack("s!s!s!", $semnum, $semop, $semflag);
semop($id,$opstring) || die "$!";
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.
A small example demonstrating SysV message queues:
use IPC::SysV qw(IPC_PRIVATE IPC_RMID IPC_CREAT S_IRUSR S_IWUSR);
my $id = msgget(IPC_PRIVATE, IPC_CREAT | S_IRUSR | S_IWUSR);
my $sent = "message";
my $type_sent = 1234;
my $rcvd;
my $type_rcvd;
if (defined $id) {
if (msgsnd($id, pack("l! a*", $type_sent, $sent), 0)) {
if (msgrcv($id, $rcvd, 60, 0, 0)) {
($type_rcvd, $rcvd) = unpack("l! a*", $rcvd);
if ($rcvd eq $sent) {
print "okay\n";
} else {
print "not okay\n";
}
} else {
die "# msgrcv failed\n";
}
} else {
die "# msgsnd failed\n";
}
msgctl($id, IPC_RMID, 0) || die "# msgctl failed: $!\n";
} else {
die "# msgget failed\n";
}
#!/usr/bin/perl -Tw
use strict;
use sigtrap;
use Socket;
For intrepid programmers, the indispensable textbook is Unix Network Programming, 2nd Edition, Volume 1 by W. Richard Stevens (published by Prentice-Hall). Note that most books on networking address the subject from the perspective of a C programmer; translation to Perl is left as an exercise for the reader.
The IO::Socket(3) manpage describes the object library, and the Socket(3) manpage describes the low-level interface to sockets. Besides the obvious functions in perlfunc, you should also check out the modules file at your nearest CPAN site. (See perlmodlib or best yet, the Perl FAQ for a description of what CPAN is and where to get it.)
Section 5 of the modules file is devoted to ``Networking, Device Control (modems), and Interprocess Communication'', and contains numerous unbundled modules numerous networking modules, Chat and Expect operations, CGI programming, DCE, FTP, IPC, NNTP, Proxy, Ptty, RPC, SNMP, SMTP, Telnet, Threads, and ToolTalk---just to name a few.