=head1 NAME

perlipc - Perl interprocess communication (signals, fifos, pipes, safe subprocesses, sockets, and semaphores)

=head1 DESCRIPTION

The basic IPC facilities of Perl are built out of the good old Unix
signals, named pipes, pipe opens, the Berkeley socket routines, and SysV
IPC calls.  Each is used in slightly different situations.

=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.
Do as little as you possibly can in your handler; notice how all we do is
set a global variable and then raise an exception.  That's because on most
systems, libraries are not re-entrant; particularly, memory allocation and
I/O routines are not.  That means that doing nearly I<anything> in your
handler could in theory trigger a memory fault and subsequent core dump.

    sub catch_zap {
	my $signame = shift;
	$shucks++;
	die "Somebody sent me a SIG$signame";
    }
    $SIG{INT} = 'catch_zap';  # could fail in modules
    $SIG{INT} = \&catch_zap;  # best strategy

The names of the signals are the ones listed out by 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
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 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
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 another process, but instead checks whether it's alive
or has changed its UID.

    unless (kill 0 => $kid_pid) {
	warn "something wicked happened to $kid_pid";
    }

You might also want to employ anonymous functions for simple signal
handlers:

    $SIG{INT} = sub { die "\nOutta here!\n" };

But that will be problematic for the more complicated handlers that need
to reinstall themselves.  Because Perl's signal mechanism is currently
based on the signal(3) function from the C library, you may sometimes be so
misfortunate 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 even the more elaborate:

    use POSIX ":sys_wait_h";
    sub REAPER {
	my $child;
        while ($child = waitpid(-1,WNOHANG)) {
	    $Kid_Status{$child} = $?;
	}
	$SIG{CHLD} = \&REAPER;  # still loathe sysV
    }
    $SIG{CHLD} = \&REAPER;
    # do something that forks...

Signal handling is also used for timeouts in Unix,   While safely
protected within an C<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 C<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 }

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.

=head1 Named Pipes

A named pipe (often referred to as a FIFO) is an old Unix IPC
mechanism for processes communicating on the same machine.  It works
just like a regular, connected anonymous pipes, except that the
processes rendezvous using a filename and don't have to be related.

To create a named pipe, use the Unix command 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 F<.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 B<-p>
to find out whether anyone (or anything) has accidentally removed our fifo.

    chdir; # go home
    $FIFO = '.signature';
    $ENV{PATH} .= ":/etc:/usr/games";

    while (1) {
	unless (-p $FIFO) {
	    unlink $FIFO;
	    system('mknod', $FIFO, 'p')
		&& die "can't mknod $FIFO: $!";
	}

	# next line blocks until there's a reader
	open (FIFO, "> $FIFO") || die "can't write $FIFO: $!";
	print FIFO "John Smith (smith\@host.org)\n", `fortune -s`;
	close FIFO;
	sleep 2;    # to avoid dup signals
    }

=head2 WARNING

By installing Perl code to deal with signals, you're exposing yourself
to danger from two things.  First, few system library functions are
re-entrant.  If the signal interrupts while Perl is executing one function
(like 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 are two things you can do, knowing this: be paranoid or be
pragmatic.  The paranoid approach is to do as little as possible in your
signal handler.  Set an existing integer variable that already has a
value, and return.  This doesn't help you if you're in a slow system call,
which will just restart.  That means you have to 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 is to say ``I know the risks, but prefer the
convenience'', and to do anything you want in your signal handler,
prepared to clean up core dumps now and again.

To forbid signal handlers altogether would bars you from
many interesting programs, including virtually everything in this manpage,
since you could no longer even write SIGCHLD handlers.  Their dodginess
is expected to be addresses in the 5.005 release.


=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:

    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 F<f1>, the process F<cmd1>, standard input (F<tmpfile>
in this case), the F<f2> file, the F<cmd2> command, and finally the F<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 I<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: $!";
    close FH            or die "can't close: status=$?";

=head2 Filehandles

Both the main process and any child processes it forks share the same
STDIN, STDOUT, and STDERR filehandles.  If both processes try to access
them at once, strange things can happen.  You'll certainly want to any
stdio flush output buffers before forking.  You may also want to close
or reopen the filehandles for the child.  You can get around this by
opening your pipe with 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 &");

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).

=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 easiest 
way is to use:

    use POSIX qw(setsid);
    setsid() 		or die "Can't start a new session: $!";

However, you may not be on POSIX.  The following process is reported
to work on most Unixish systems.  Non-Unix users should check their
Your_OS::Process module for other solutions.

=over 4

=item *

Open /dev/tty and use 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:

    fork && exit;

=item *

Ignore hangup signals in case you're running on a shell that doesn't
automatically no-hup you:

    $SIG{HUP} = 'IGNORE';	# or whatever you'd like

=back

=head2 Safe Pipe Opens