PERLIPC(1) Perl Programmers Reference Guide PERLIPC(1)NAMEperlipc - Perl interprocess communication (signals, fifos, pipes, safe
subprocesses, sockets, and semaphores)
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.
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 trig‐
gered 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;
$shucks++;
die "Somebody sent me a SIG$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") sig‐
nal has special behavior with respect to a value of 'IGNORE'. Setting
$SIG{CHLD} to 'IGNORE' on such a platform has the effect of not creat‐
ing zombie processes when the parent process fails to "wait()" on its
child processes (i.e. child processes are automatically reaped). Call‐
ing "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 sig‐
nal 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 permis‐
sion 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 cur‐
rently 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 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 pro‐
tected 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 mod‐
ule. Lamentably, this is almost entirely undocumented, but the
t/lib/posix.t file from the Perl source distribution has some examples
in it.
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 MONi‐
tor). 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 "SIGHUP" signal
handler. When you want to tell the daemon to re-read the file you sim‐
ply send it the "SIGHUP" signal.
Not all platforms automatically reinstall their (native) signal han‐
dlers 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__
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 ren‐
dezvous using a filename and don't have to be related.
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 some‐
thing 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
}
Deferred Signals (Safe Signals)
In Perls before Perl 5.7.3 by installing Perl code to deal with sig‐
nals, 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 unpre‐
dictable 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 "die" to
longjump(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 han‐
dler 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:
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.
Interrupting IO
When a signal is delivered (e.g. INT control-C) the operating sys‐
tem breaks into IO operations like "read" (used to implement Perls
<> operator). On older Perls the handler was called immediately
(and as "read" is not "unsafe" this worked well). With the
"deferred" scheme the handler is not called immediately, and if
Perl is using system's "stdio" library that library may re-start
the "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 ":perlio" layer to do IO - at least on those handles which you
want to be able to break into with signals. (The ":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 ":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";
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 not use SA_RESTART. Consequently,
restartable system calls can fail (with $! set to "EINTR") in
places where they previously would have succeeded.
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.
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 particu‐
lar 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.)
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 "wait" for the completed
child process. On such systems the deferred signal scheme will not
work for those signals (it does not do the "wait"). Again the fail‐
ure will look like a loop as the operating system will re-issue the
signal as there are un-waited-for completed child processes.
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).
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 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 con‐
trol 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--spec‐
tacularly. 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=$?";
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 may also want to close or
reopen the filehandles for the child. You can get around this by open‐
ing your pipe with open(), but on some systems this means that the
child process cannot outlive the parent.
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).
Complete Dissociation of Child from Parent
In some cases (starting server processes, for instance) you'll want to
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 con‐
taining the directory from which it was launched) and redirect its
standard file descriptors from and to /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: $!";
}
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 sys‐
tem 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.
Safe Pipe Opens
Another interesting approach to IPC is making your single program go
multiprocess and communicate between (or even amongst) yourselves. The
open() function will accept a file argument of either "-⎪" or "⎪-" to
do a very interesting thing: it forks a child connected to the filehan‐
dle you've opened. The child is running the same program as the par‐
ent. This is useful for safely opening a file when running under an
assumed UID or GID, for example. If you open a pipe to minus, you can
write to the filehandle you opened and your kid will find it in his
STDIN. If you open a pipe from minus, you can read from the filehandle
you opened whatever your kid writes to his STDOUT.
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 some‐
thing without the shell's interference. With system(), it's straight‐
forward, 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.
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
doesn't actually work:
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 func‐
tion 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 prim‐
itives 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 simi‐
larly 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 inter‐
act() 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.
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 pro‐
cesses.
#!/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;
}
Sockets: Client/Server Communication
While not limited to Unix-derived operating systems (e.g., WinSock on
PCs provides socket support, as do some VMS libraries), you may not
have sockets on your system, in which case this section probably isn't
going to do you much good. With sockets, you can do both virtual cir‐
cuits (i.e., TCP streams) and datagrams (i.e., UDP packets). You may
be able to do even more depending on your system.
The Perl function calls for dealing with sockets have the same names as
the corresponding system calls in C, but their arguments tend to differ
for two reasons: first, Perl filehandles work differently than C file
descriptors. Second, Perl already knows the length of its strings, so
you don't need to pass that information.
One of the major problems with old socket code in Perl was that it used
hard-coded values for some of the constants, which severely hurt porta‐
bility. If you ever see code that does anything like explicitly set‐
ting "$AF_INET = 2", you know you're in for big trouble: An immeasur‐
ably superior approach is to use the "Socket" module, which more reli‐
ably 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).
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 man‐
page, but unless you're on a Mac, you'll probably be ok.
Internet TCP Clients and Servers
Use Internet-domain sockets when you want to do client-server communi‐
cation that might extend to machines outside of your own system.
Here's a sample TCP client using Internet-domain sockets:
#!/usr/bin/perl -w
use strict;
use Socket;
my ($remote,$port, $iaddr, $paddr, $proto, $line);
$remote = shift ⎪⎪ 'localhost';
$port = shift ⎪⎪ 2345; # random port
if ($port =~ /\D/) { $port = getservbyname($port, 'tcp') }
die "No port" unless $port;
$iaddr = inet_aton($remote) ⎪⎪ die "no host: $remote";
$paddr = sockaddr_in($port, $iaddr);
$proto = getprotobyname('tcp');
socket(SOCK, PF_INET, SOCK_STREAM, $proto) ⎪⎪ die "socket: $!";
connect(SOCK, $paddr) ⎪⎪ die "connect: $!";
while (defined($line = <SOCK>)) {
print $line;
}
close (SOCK) ⎪⎪ die "close: $!";
exit;
And here's a corresponding server to go along with it. We'll leave the
address as INADDR_ANY so that the kernel can choose the appropriate
interface on multihomed hosts. If you want sit on a particular inter‐
face (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";
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;
($paddr = accept(Client,Server)) ⎪⎪ $waitedpid;
$waitedpid = 0, close Client)
{
next if $waitedpid and not $paddr;
my($port,$iaddr) = sockaddr_in($paddr);
my $name = gethostbyaddr($iaddr,AF_INET);
logmsg "connection from $name [",
inet_ntoa($iaddr), "]
at port $port";
spawn sub {
$⎪=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: $!";
};
}
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.
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);
}
Unix-Domain TCP Clients and Servers
That's fine for Internet-domain clients and servers, but what about
local communications? While you can use the same setup, sometimes you
don't want to. Unix-domain sockets are local to the current host, and
are often used internally to implement pipes. Unlike Internet domain
sockets, Unix domain sockets can show up in the file system with an
ls(1) listing.
% ls -l /dev/log
srw-rw-rw- 1 root 0 Oct 31 07:23 /dev/log
You can test for these with Perl's -S file test:
unless ( -S '/dev/log' ) {
die "something's wicked with the 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 func‐
tions--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 program‐
ming, 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.
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.
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 "new" constructor mean:
"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.
"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
"www.perl.com", or an address like "204.148.40.9". For demonstra‐
tion purposes, we've used the special hostname "localhost", which
should always mean the current machine you're running on. The cor‐
responding Internet address for localhost is "127.1", if you'd
rather use that.
"PeerPort"
This is the service name or port number we'd like to connect to.
We could have gotten away with using just "daytime" on systems with
a well-configured system services file,[FOOTNOTE: The system ser‐
vices file is in /etc/services under Unix] but just in case, we've
specified the port number (13) in parentheses. Using just the num‐
ber would also have worked, but constant numbers make careful pro‐
grammers nervous.
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 file‐
handle, 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";
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 con‐
nect 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 speci‐
fied 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 partic‐
ular document. But a long response wouldn't have fit on this page.
For a more fully-featured version of this program, you should look to
the lwp-request program included with the LWP modules from CPAN.
Interactive Client with IO::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 telnet works? That way you can type a line, get the
answer, type a line, get the answer, etc.
This client is more complicated than the two we've done so far, but if
you're on a system that supports the powerful "fork" call, the solution
isn't that rough. Once you've made the connection to whatever service
you'd like to chat with, call "fork" to clone your process. Each of
these two identical process has a very simple job to do: the parent
copies everything from the socket to standard output, while the child
simultaneously copies everything from standard input to the socket. To
accomplish the same thing using just one process would be much harder,
because it's easier to code two processes to do one thing than it is to
code one process to do two things. (This keep-it-simple principle a
cornerstones of the Unix philosophy, and good software engineering as
well, which is probably why it's spread to other systems.)
Here's the code:
#!/usr/bin/perl -w
use strict;
use IO::Socket;
my ($host, $port, $kidpid, $handle, $line);
unless (@ARGV == 2) { die "usage: $0 host port" }
($host, $port) = @ARGV;
# create a tcp connection to the specified host and port
$handle = IO::Socket::INET->new(Proto => "tcp",
PeerAddr => $host,
PeerPort => $port)
or die "can't connect to port $port on $host: $!";
$handle->autoflush(1); # so output gets there right away
print STDERR "[Connected to $host:$port]\n";
# split the program into two processes, identical twins
die "can't fork: $!" unless defined($kidpid = fork());
# the if{} block runs only in the parent process
if ($kidpid) {
# copy the socket to standard output
while (defined ($line = <$handle>)) {
print STDOUT $line;
}
kill("TERM", $kidpid); # send SIGTERM to child
}
# the else{} block runs only in the child process
else {
# copy standard input to the socket
while (defined ($line = <STDIN>)) {
print $handle $line;
}
}
The "kill" function in the parent's "if" block is there to send a sig‐
nal 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 effi‐
cient (to put it mildly) but is the simplest to explain and works rea‐
sonably well.
TCP Servers with IO::Socket
As always, setting up a server is little bit more involved than running
a client. The model is that the server creates a special kind of
socket that does nothing but listen on a particular port for incoming
connections. It does this by calling the "IO::Socket::INET->new()"
method with slightly different arguments than the client did.
Proto
This is which protocol to use. Like our clients, we'll still spec‐
ify "tcp" here.
LocalPort
We specify a local port in the "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 sys‐
tem. If you try to use one already in used, you'll get an "Address
already in use" message. Under Unix, the "netstat -a" command will
show which services current have servers.
Listen
The "Listen" parameter is set to the maximum number of pending con‐
nections 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.
Reuse
The "Reuse" parameter is needed so that we restart our server manu‐
ally without waiting a few minutes to allow system buffers to clear
out.
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 aut‐
oflush 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 cov‐
ered 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;
}
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 less reliability, as there are no promises that messages will
arrive at all, let alone in order and unmangled. Still, UDP offers
some advantages over TCP, including being able to "broadcast" or "mul‐
ticast" to a whole bunch of destination hosts at once (usually on your
local subnet). If you find yourself overly concerned about reliability
and start building checks into your message system, then you probably
should use just TCP to start with.
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 ver‐
sion 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 some‐
thing 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 conse‐
quently 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.
SysV IPC
While System V IPC isn't so widely used as sockets, it still has some
interesting uses. You can't, however, effectively use SysV IPC or
Berkeley mmap() to have shared memory so as to share a variable amongst
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.
use IPC::SysV qw(IPC_PRIVATE IPC_RMID S_IRWXU);
$size = 2000;
$id = shmget(IPC_PRIVATE, $size, S_IRWXU) ⎪⎪ die "$!";
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_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";
}
NOTES
Most of these routines quietly but politely return "undef" when they
fail instead of causing your program to die right then and there due to
an uncaught exception. (Actually, some of the new Socket conversion
functions 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 -T taint
checking flag to the #! line for servers:
#!/usr/bin/perl -Tw
use strict;
use sigtrap;
use Socket;
BUGS
All these routines create system-specific portability problems. As
noted elsewhere, Perl is at the mercy of your C libraries for much of
its system behaviour. It's probably safest to assume broken SysV
semantics for signals and to stick with simple TCP and UDP socket oper‐
ations; e.g., don't try to pass open file descriptors over a local UDP
datagram socket if you want your code to stand a chance of being porta‐
ble.
AUTHOR
Tom Christiansen, with occasional vestiges of Larry Wall's original
version and suggestions from the Perl Porters.
SEE ALSO
There's a lot more to networking than this, but this should get you
started.
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 unbun‐
dled 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.
perl v5.8.8 2013-10-01 PERLIPC(1)
