=head1 Signals
-Perl uses a simple signal handling model: the %SIG hash contains names or
-references of user-installed signal handlers. These handlers will be called
-with an argument which is the name of the signal that triggered it. A
-signal may be generated intentionally from a particular keyboard sequence like
-control-C or control-Z, sent to you from another process, or
-triggered automatically by the kernel when special events transpire, like
-a child process exiting, your process running out of stack space, or
-hitting file size limit.
-
-For example, to trap an interrupt signal, set up a handler like this.
-Notice how all we do is set a global variable and then raise an
-exception. That's because on most systems libraries are not
-reentrant, so calling any print() functions (or even anything that needs to
-malloc(3) more memory) could in theory trigger a memory fault
-and subsequent core dump.
+Perl uses a simple signal handling model: the %SIG hash contains names
+or references of user-installed signal handlers. These handlers will
+be called with an argument which is the name of the signal that
+triggered it. A signal may be generated intentionally from a
+particular keyboard sequence like control-C or control-Z, sent to you
+from another process, or triggered automatically by the kernel when
+special events transpire, like a child process exiting, your process
+running out of stack space, or hitting file size limit.
+
+For example, to trap an interrupt signal, set up a handler like this:
sub catch_zap {
my $signame = shift;
$SIG{INT} = 'catch_zap'; # could fail in modules
$SIG{INT} = \&catch_zap; # best strategy
+Prior to Perl 5.7.3 it was necessary to do as little as you possibly
+could in your handler; notice how all we do is set a global variable
+and then raise an exception. That's because on most systems,
+libraries are not re-entrant; particularly, memory allocation and I/O
+routines are not. That meant that doing nearly I<anything> in your
+handler could in theory trigger a memory fault and subsequent core
+dump - see L</Deferred Signals (Safe Signals)> below.
+
The names of the signals are the ones listed out by C<kill -l> on your
system, or you can retrieve them from the Config module. Set up an
@signame list indexed by number to get the name and a %signo table
You may also choose to assign the strings C<'IGNORE'> or C<'DEFAULT'> as
the handler, in which case Perl will try to discard the signal or do the
-default thing. Some signals can be neither trapped nor ignored, such as
+default thing.
+
+On most Unix platforms, the C<CHLD> (sometimes also known as C<CLD>) signal
+has special behavior with respect to a value of C<'IGNORE'>.
+Setting C<$SIG{CHLD}> to C<'IGNORE'> on such a platform has the effect of
+not creating zombie processes when the parent process fails to C<wait()>
+on its child processes (i.e. child processes are automatically reaped).
+Calling C<wait()> with C<$SIG{CHLD}> set to C<'IGNORE'> usually returns
+C<-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()
}
Sending a signal to a negative process ID means that you send the signal
-to the entire Unix process-group. This code send a hang-up signal to all
-processes in the current process group I<except for> the current process
-itself:
+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';
}
Another interesting signal to send is signal number zero. This doesn't
-actually affect another process, but instead checks whether it's alive
+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 C<%!>.
+
+ unless (kill 0 => $pid or $!{EPERM}) {
+ warn "$pid looks dead";
+ }
+
You might also want to employ anonymous functions for simple signal
handlers:
$SIG{CHLD} = \&REAPER;
# now do something that forks...
-or even the more elaborate:
+or better still:
use POSIX ":sys_wait_h";
sub REAPER {
my $child;
- while ($child = waitpid(-1,WNOHANG)) {
+ # 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} = $?;
}
$SIG{CHLD} = \&REAPER; # still loathe sysV
};
if ($@ 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 F<t/lib/posix.t> file from the Perl source distribution has some
examples in it.
+=head2 Handling the SIGHUP Signal in Daemons
+
+A process that usually starts when the system boots and shuts down
+when the system is shut down is called a daemon (Disk And Execution
+MONitor). If a daemon process has a configuration file which is
+modified after the process has been started, there should be a way to
+tell that process to re-read its configuration file, without stopping
+the process. Many daemons provide this mechanism using the C<SIGHUP>
+signal handler. When you want to tell the daemon to re-read the file
+you simply send it the C<SIGHUP> signal.
+
+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 C<POSIX> signal handlers if available, their behaviour
+is well-defined.
+
+The following example implements a simple daemon, which restarts
+itself every time the C<SIGHUP> signal is received. The actual code is
+located in the subroutine C<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__
+
+
=head1 Named Pipes
A named pipe (often referred to as a FIFO) is an old Unix IPC
if ( system('mknod', $path, 'p')
&& system('mkfifo', $path) )
{
- die "mk{nod,fifo} $path failed;
+ die "mk{nod,fifo} $path failed";
}
sleep 2; # to avoid dup signals
}
+=head2 Deferred Signals (Safe Signals)
+
+In Perls before Perl 5.7.3 by installing Perl code to deal with
+signals, you were 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 malloc(3) or printf(3)),
+and your signal handler then calls the same function again, you could
+get unpredictable behavior--often, a core dump. Second, Perl isn't
+itself re-entrant at the lowest levels. If the signal interrupts Perl
+while Perl is changing its own internal data structures, similarly
+unpredictable behaviour may result.
+
+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 C<die> to longjump(3) out
+of the handler. Even this is a little cavalier for the true paranoiac,
+who avoids C<die> in a handler because the system I<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:
+
+=over 4
+
+=item Long running opcodes
+
+As Perl interpreter only looks at the signal flags when it about to
+execute a new opcode if a signal arrives during a long running opcode
+(e.g. a regular expression operation on a very large string) then
+signal will not be seen until operation completes.
+
+=item Interrupting IO
+
+When a signal is delivered (e.g. INT control-C) the operating system
+breaks into IO operations like C<read> (used to implement Perls
+E<lt>E<gt> operator). On older Perls the handler was called
+immediately (and as C<read> is not "unsafe" this worked well). With
+the "deferred" scheme the handler is not called immediately, and if
+Perl is using system's C<stdio> library that library may re-start the
+C<read> without returning to Perl and giving it a chance to call the
+%SIG handler. If this happens on your system the solution is to use
+C<:perlio> layer to do IO - at least on those handles which you want
+to be able to break into with signals. (The C<:perlio> layer checks
+the signal flags and calls %SIG handlers before resuming IO operation.)
+
+Note that the default in Perl 5.7.3 and later is to automatically use
+the C<: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
+C<$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";
+
+=item Restartable system calls
+
+On systems that supported it, older versions of Perl used the
+SA_RESTART flag when installing %SIG handlers. This meant that
+restartable system calls would continue rather than returning when
+a signal arrived. In order to deliver deferred signals promptly,
+Perl 5.7.3 and later do I<not> use SA_RESTART. Consequently,
+restartable system calls can fail (with $! set to C<EINTR>) in places
+where they previously would have succeeded.
+
+Note that the default C<:perlio> layer will retry C<read>, C<write>
+and C<close> as described above and that interrupted C<wait> and
+C<waitpid> calls will always be retried.
+
+=item Signals as "faults"
+
+Certain signals e.g. SEGV, ILL, BUS are generated as a result of
+virtual memory or other "faults". These are normally fatal and there
+is little a Perl-level handler can do with them. (In particular the
+old signal scheme was particularly unsafe in such cases.) However if
+a %SIG handler is set the new scheme simply sets a flag and returns as
+described above. This may cause the operating system to try the
+offending machine instruction again and - as nothing has changed - it
+will generate the signal again. The result of this is a rather odd
+"loop". In future Perl's signal mechanism may be changed to avoid this
+- perhaps by simply disallowing %SIG handlers on signals of that
+type. Until then the work-round is not to set a %SIG handler on those
+signals. (Which signals they are is operating system dependent.)
+
+=item Signals triggered by operating system state
+
+On some operating systems certain signal handlers are supposed to "do
+something" before returning. One example can be CHLD or CLD which
+indicates a child process has completed. On some operating systems the
+signal handler is expected to C<wait> for the completed child
+process. On such systems the deferred signal scheme will not work for
+those signals (it does not do the C<wait>). Again the failure will
+look like a loop as the operating system will re-issue the signal as
+there are un-waited-for completed child processes.
+
+=back
+
+If you want the old signal behaviour back regardless of possible
+memory corruption, set the environment variable C<PERL_SIGNALS> to
+C<"unsafe"> (a new feature since Perl 5.8.1).
=head1 Using open() for IPC
-Perl's basic open() statement can also be used for unidirectional interprocess
-communication by either appending or prepending a pipe symbol to the second
-argument to open(). Here's how to start something up in a child process you
-intend to write to:
+Perl's basic open() statement can also be used for unidirectional
+interprocess communication by either appending or prepending a pipe
+symbol to the second argument to open(). Here's how to start
+something up in a child process you intend to write to:
open(SPOOLER, "| cat -v | lpr -h 2>/dev/null")
|| die "can't fork: $!";
expecting filenames in @ARGV, the clever programmer can write something
like this:
- $ program f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile
+ % program f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile
and irrespective of which shell it's called from, the Perl program will
read from the file F<f1>, the process F<cmd1>, standard input (F<tmpfile>
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 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.
to bogus command will trigger a signal they'd better be prepared to
handle. Consider:
- open(FH, "|bogus");
- print FH "bang\n";
- close FH;
+ 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=$?";
=head2 Filehandles
-Both the main process and the child process share the same STDIN,
-STDOUT and STDERR filehandles. If both processes try to access them
-at once, strange things can happen. You may want to close or reopen
-the filehandles for the child. You can get around this by opening
-your pipe with open(), but on some systems this means that the child
-process cannot outlive the parent.
+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 may also want to close
+or reopen the filehandles for the child. You can get around this by
+opening your pipe with open(), but on some systems this means that the
+child process cannot outlive the parent.
=head2 Background Processes
You can run a command in the background with:
- system("cmd&");
+ 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
=head2 Complete Dissociation of Child from Parent
In some cases (starting server processes, for instance) you'll want to
-complete dissociate the child process from the parent. The following
-process is reported to work on most Unixish systems. Non-Unix users
-should check their Your_OS::Process module for other solutions.
-
-=over 4
-
-=item *
-
-Open /dev/tty and use the the TIOCNOTTY ioctl on it. See L<tty(4)>
-for details.
-
-=item *
-
-Change directory to /
-
-=item *
-
-Reopen STDIN, STDOUT, and STDERR so they're not connected to the old
-tty.
-
-=item *
-
-Background yourself like this:
+completely dissociate the child process from the parent. This is
+often called daemonization. A well behaved daemon will also chdir()
+to the root directory (so it doesn't prevent unmounting the filesystem
+containing the directory from which it was launched) and redirect its
+standard file descriptors from and to F</dev/null> (so that random
+output doesn't wind up on the user's terminal).
+
+ 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: $!";
+ }
- fork && exit;
+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 F</dev/tty> and use the
+C<TIOCNOTTY> ioctl() on it instead. See L<tty(4)> for details.
-=back
+Non-Unix users should check their Your_OS::Process module for other
+solutions.
=head2 Safe Pipe Opens
STDIN. If you open a pipe I<from> minus, you can read from the filehandle
you opened whatever your kid writes to his STDOUT.
- use English;
+ use English '-no_match_vars';
my $sleep_count = 0;
do {
# add error processing as above
$pid = open(KID_TO_WRITE, "|-");
- $SIG{ALRM} = sub { die "whoops, $program pipe broke" };
+ $SIG{PIPE} = sub { die "whoops, $program pipe broke" };
if ($pid) { # parent
for (@data) {
# NOTREACHED
}
+Since Perl 5.8.0, you can also use the list form of C<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
+C<KID_PS> filehandle. The corresponding syntax to read from command
+pipes (with C<"|-"> in place of C<"-|">) 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
F<modules> file mentioned below in the SEE ALSO section.
-=head2 Bidirectional Communication
+=head2 Bidirectional Communication with Another Process
While this works reasonably well for unidirectional communication, what
about bidirectional communication? The obvious thing you'd like to do
open(PROG_FOR_READING_AND_WRITING, "| some program |")
-and if you forget to use the B<-w> flag, then you'll miss out
-entirely on the diagnostic message:
+and if you forget to use the C<use warnings> pragma or the B<-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 tri-directional I/O so you
+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() to create all the connections.
+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
use FileHandle;
use IPC::Open2;
- $pid = open2( \*Reader, \*Writer, "cat -u -n" );
- Writer->autoflush(); # default here, actually
+ $pid = open2(*Reader, *Writer, "cat -u -n" );
print Writer "stuff\n";
$got = <Reader>;
successor F<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.
+
+=head2 Bidirectional Communication with Yourself
+
+If you want, you may make low-level pipe() and fork()
+to stitch this together by hand. This example only
+talks to itself, but you could reopen the appropriate
+handles to STDIN and STDOUT and call other processes.
+
+ #!/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 $$ is sending this\n";
+ close PARENT;
+ exit;
+ }
+
=head1 Sockets: Client/Server Communication
While not limited to Unix-derived operating systems (e.g., WinSock on PCs
NNTP or SMTP, you should give some thought to how your server will
know when the client has finished talking, and vice-versa. Most
protocols are based on one-line messages and responses (so one party
-knows the other has finished when a "\n" is received) or multiline
+knows the other has finished when a "\n" is received) or multi-line
messages and responses that end with a period on an empty line
("\n.\n" terminates a message/response).
+=head2 Internet Line Terminators
+
+The Internet line terminator is "\015\012". Under ASCII variants of
+Unix, that could usually be written as "\r\n", but under other systems,
+"\r\n" might at times be "\015\015\012", "\012\012\015", or something
+completely different. The standards specify writing "\015\012" to be
+conformant (be strict in what you provide), but they also recommend
+accepting a lone "\012" on input (but be lenient in what you require).
+We haven't always been very good about that in the code in this manpage,
+but unless you're on a Mac, you'll probably be ok.
+
=head2 Internet TCP Clients and Servers
Use Internet-domain sockets when you want to do client-server
Here's a sample TCP client using Internet-domain sockets:
#!/usr/bin/perl -w
- require 5.002;
use strict;
use Socket;
my ($remote,$port, $iaddr, $paddr, $proto, $line);
instead.
#!/usr/bin/perl -Tw
- require 5.002;
use strict;
BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
use Socket;
use Carp;
+ my $EOL = "\015\012";
sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
my $port = shift || 2345;
my $proto = getprotobyname('tcp');
- $port = $1 if $port =~ /(\d+)/; # untaint port number
+
+ ($port) = $port =~ /^(\d+)$/ or die "invalid port";
socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
setsockopt(Server, SOL_SOCKET, SO_REUSEADDR,
at port $port";
print Client "Hello there, $name, it's now ",
- scalar localtime, "\n";
+ scalar localtime, $EOL;
}
And here's a multithreaded version. It's multithreaded in that
go back to service a new client.
#!/usr/bin/perl -Tw
- require 5.002;
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 = $1 if $port =~ /(\d+)/; # untaint port number
+
+ ($port) = $port =~ /^(\d+)$/ or die "invalid port";
socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
setsockopt(Server, SOL_SOCKET, SO_REUSEADDR,
my $waitedpid = 0;
my $paddr;
+ use POSIX ":sys_wait_h";
sub REAPER {
- $waitedpid = wait;
+ my $child;
+ while (($waitedpid = waitpid(-1,WNOHANG)) > 0) {
+ logmsg "reaped $waitedpid" . ($? ? " with exit $?" : '');
+ }
$SIG{CHLD} = \&REAPER; # loathe sysV
- logmsg "reaped $waitedpid" . ($? ? " with exit $?" : '');
}
$SIG{CHLD} = \&REAPER;
at port $port";
spawn sub {
- print "Hello there, $name, it's now ", scalar localtime, "\n";
- exec '/usr/games/fortune'
+ $|=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: $!";
};
differ from the system on which it's being run:
#!/usr/bin/perl -w
- require 5.002;
use strict;
use Socket;
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
+ % ls -l /dev/log
srw-rw-rw- 1 root 0 Oct 31 07:23 /dev/log
You can test for these with Perl's B<-S> file test:
unless ( -S '/dev/log' ) {
- die "something's wicked with the print system";
+ die "something's wicked with the log system";
}
Here's a sample Unix-domain client:
#!/usr/bin/perl -w
- require 5.002;
use Socket;
use strict;
my ($rendezvous, $line);
- $rendezvous = shift || '/tmp/catsock';
+ $rendezvous = shift || 'catsock';
socket(SOCK, PF_UNIX, SOCK_STREAM, 0) || die "socket: $!";
connect(SOCK, sockaddr_un($rendezvous)) || die "connect: $!";
while (defined($line = <SOCK>)) {
}
exit;
-And here's a corresponding server.
+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
- require 5.002;
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 = '/tmp/catsock';
+ my $NAME = 'catsock';
my $uaddr = sockaddr_un($NAME);
my $proto = getprotobyname('tcp');
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)
};
}
+ 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
program that does whatever checks and logging you feel like, and then acts
as a Unix-domain client and connects to your private server.
-=head2 UDP: Message Passing
+=head1 TCP Clients with IO::Socket
+
+For 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 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.
+
+=head2 A Simple Client
+
+Here'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 C<new> constructor mean:
+
+=over 4
+
+=item C<Proto>
+
+This is which protocol to use. In this case, the socket handle returned
+will be connected to a TCP socket, because we want a stream-oriented
+connection, that is, one that acts pretty much like a plain old file.
+Not all sockets are this of this type. For example, the UDP protocol
+can be used to make a datagram socket, used for message-passing.
+
+=item C<PeerAddr>
+
+This is the name or Internet address of the remote host the server is
+running on. We could have specified a longer name like C<"www.perl.com">,
+or an address like C<"204.148.40.9">. For demonstration purposes, we've
+used the special hostname C<"localhost">, which should always mean the
+current machine you're running on. The corresponding Internet address
+for localhost is C<"127.1">, if you'd rather use that.
+
+=item C<PeerPort>
+
+This is the service name or port number we'd like to connect to.
+We could have gotten away with using just C<"daytime"> on systems with a
+well-configured system services file,[FOOTNOTE: The system services file
+is in I</etc/services> under Unix] but just in case, we've specified the
+port number (13) in parentheses. Using just the number would also have
+worked, but constant numbers make careful programmers nervous.
+
+=back
+
+Notice how the return value from the C<new> constructor is used as
+a filehandle in the C<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";
+
+=head2 A Webget Client
+
+Here'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 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, C<< PeerPort => 8080 >>. The C<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 C<"\n"> in your code that sends data over the network to
+be a C<"\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 I<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 I<lwp-request> program included with the LWP modules from CPAN.
+
+=head2 Interactive Client with IO::Socket
+
+Well, 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 I<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 C<fork> call, the solution
+isn't that rough. Once you've made the connection to whatever service
+you'd like to chat with, call C<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 I<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 C<kill> function in the parent's C<if> block is there to send a
+signal to our child process (current running in the C<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 C<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.
+
+=head1 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 C<< IO::Socket::INET->new() >> method with
+slightly different arguments than the client did.
+
+=over 4
+
+=item Proto
+
+This is which protocol to use. Like our clients, we'll
+still specify C<"tcp"> here.
+
+=item LocalPort
+
+We specify a local
+port in the C<LocalPort> argument, which we didn't do for the client.
+This is service name or port number for which you want to be the
+server. (Under Unix, ports under 1024 are restricted to the
+superuser.) In our sample, we'll use port 9000, but you can use
+any port that's not currently in use on your system. If you try
+to use one already in used, you'll get an "Address already in use"
+message. Under Unix, the C<netstat -a> command will show
+which services current have servers.
+
+=item Listen
+
+The C<Listen> parameter is set to the maximum number of
+pending connections we can accept until we turn away incoming clients.
+Think of it as a call-waiting queue for your telephone.
+The low-level Socket module has a special symbol for the system maximum, which
+is SOMAXCONN.
+
+=item Reuse
+
+The C<Reuse> parameter is needed so that we restart our server
+manually without waiting a few minutes to allow system buffers to
+clear out.
+
+=back
+
+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 C<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 C<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;
+ }
+
+=head1 UDP: Message Passing
Another kind of client-server setup is one that uses not connections, but
messages. UDP communications involve much lower overhead but also provide
into your message system, then you probably should use just TCP to start
with.
+Note that UDP datagrams are I<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
-above. However, instead of checking one host at a time, the UDP version
+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;
- require 5.002;
use Socket;
use Sys::Hostname;
$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.
+
=head1 SysV IPC
While System V IPC isn't so widely used as sockets, it still has some
several processes. That's because Perl would reallocate your string when
you weren't wanting it to.
-
Here's a small example showing shared memory usage.
- $IPC_PRIVATE = 0;
- $IPC_RMID = 0;
+ use IPC::SysV qw(IPC_PRIVATE IPC_RMID S_IRWXU);
+
$size = 2000;
- $key = shmget($IPC_PRIVATE, $size , 0777 );
- die unless defined $key;
+ $id = shmget(IPC_PRIVATE, $size, S_IRWXU) || die "$!";
+ print "shm key $id\n";
$message = "Message #1";
- shmwrite($key, $message, 0, 60 ) || die "$!";
- shmread($key,$buff,0,60) || die "$!";
+ shmwrite($id, $message, 0, 60) || die "$!";
+ print "wrote: '$message'\n";
+ shmread($id, $buff, 0, 60) || die "$!";
+ print "read : '$buff'\n";
- print $buff,"\n";
+ # the buffer of shmread is zero-character end-padded.
+ substr($buff, index($buff, "\0")) = '';
+ print "un" unless $buff eq $message;
+ print "swell\n";
- print "deleting $key\n";
- shmctl($key ,$IPC_RMID, 0) || die "$!";
+ 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;
- $IPC_RMID = 0;
- $IPC_CREATE = 0001000;
- $key = semget($IPC_KEY, $nsems , 0666 | $IPC_CREATE );
- die if !defined($key);
- print "$key\n";
+ $id = semget($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 F<take>:
# create a semaphore
$IPC_KEY = 1234;
- $key = semget($IPC_KEY, 0 , 0 );
- die if !defined($key);
+ $id = semget($IPC_KEY, 0 , 0 );
+ die if !defined($id);
$semnum = 0;
$semflag = 0;
# 'take' semaphore
# wait for semaphore to be zero
$semop = 0;
- $opstring1 = pack("sss", $semnum, $semop, $semflag);
+ $opstring1 = pack("s!s!s!", $semnum, $semop, $semflag);
# Increment the semaphore count
$semop = 1;
- $opstring2 = pack("sss", $semnum, $semop, $semflag);
+ $opstring2 = pack("s!s!s!", $semnum, $semop, $semflag);
$opstring = $opstring1 . $opstring2;
- semop($key,$opstring) || die "$!";
+ semop($id,$opstring) || die "$!";
Put this code in a separate file to be run in more than one process.
Call this file F<give>:
# that the second process continues
$IPC_KEY = 1234;
- $key = semget($IPC_KEY, 0, 0);
- die if !defined($key);
+ $id = semget($IPC_KEY, 0, 0);
+ die if !defined($id);
$semnum = 0;
$semflag = 0;
# Decrement the semaphore count
$semop = -1;
- $opstring = pack("sss", $semnum, $semop, $semflag);
-
- semop($key,$opstring) || die "$!";
-
-=head1 WARNING
-
-The SysV IPC code above was written long ago, and it's definitely clunky
-looking. It should at the very least be made to C<use strict> and
-C<require "sys/ipc.ph">. Better yet, perhaps someone should create an
-C<IPC::SysV> module the way we have the C<Socket> module for normal
-client-server communications.
-
-(... time passes)
-
-Voila! Check out the IPC::SysV modules written by Jack Shirazi. You can
-find them at a CPAN store near you.
+ $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_IRWXU);
+
+ my $id = msgget(IPC_PRIVATE, IPC_CREAT | S_IRWXU);
+
+ 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";
+ }
=head1 NOTES
-If you are running under version 5.000 (dubious) or 5.001, you can still
-use most of the examples in this document. You may have to remove the
-C<use strict> and some of the my() statements for 5.000, and for both
-you'll have to load in version 1.2 or older of the F<Socket.pm> module, which
-is included in I<perl5.002>.
+Most of these routines quietly but politely return C<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 I<Socket> conversion
+functions croak() on bad arguments.) It is therefore essential to
+check return values from these functions. Always begin your socket
+programs this way for optimal success, and don't forget to add B<-T>
+taint checking flag to the #! line for servers:
-Most of these routines quietly but politely return C<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 I<Socket> conversion
-functions croak() on bad arguments.) It is therefore essential
-that you should check the return values of these functions. Always begin
-your socket programs this way for optimal success, and don't forget to add
-B<-T> taint checking flag to the pound-bang line for servers:
-
- #!/usr/bin/perl -w
- require 5.002;
+ #!/usr/bin/perl -Tw
use strict;
use sigtrap;
use Socket;
try to pass open file descriptors over a local UDP datagram socket if you
want your code to stand a chance of being portable.
-Because few vendors provide C libraries that are safely
-reentrant, the prudent programmer will do little else within
-a handler beyond die() to raise an exception and longjmp(3) out.
-
=head1 AUTHOR
Tom Christiansen, with occasional vestiges of Larry Wall's original
-version.
+version and suggestions from the Perl Porters.
=head1 SEE ALSO
-Besides the obvious functions in L<perlfunc>, you should also check out
-the F<modules> file at your nearest CPAN site. (See L<perlmod> or best
-yet, the F<Perl FAQ> for a description of what CPAN is and where to get it.)
+There's a lot more to networking than this, but this should get you
+started.
+
+For intrepid programmers, the indispensable textbook is I<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 L<perlfunc>, you should also check out the F<modules> file
+at your nearest CPAN site. (See L<perlmodlib> or best yet, the F<Perl
+FAQ> for a description of what CPAN is and where to get it.)
+
Section 5 of the F<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
https://perl5.git.perl.org / perl5.git / blobdiff