perlipc - Perl interprocess communication (signals, fifos,
pipes, safe subprocesses, sockets, and semaphores)
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.
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;
$shucks++;
die "Somebody sent me a SIG$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 = $signame[17]0;
if ($signo{ALRM}) {
print "SIGALRM is $signo{ALRM}0;
}
You may also choose to assign the strings 'IGNORE' or
'DEFAULT' as the handler, in which case Perl will try to
discard the signal or do the default thing.
On most Unix platforms, the "CHLD" (sometimes also known
as "CLD") signal has special behavior with respect to a
value of 'IGNORE'. Setting $SIG{CHLD} to 'IGNORE' on such
a platform has the effect of not creating zombie processes
when the parent process fails to "wait()" on its child
processes (i.e. child processes are automatically reaped).
Calling "wait()" with $SIG{CHLD} set to 'IGNORE' usually
returns "-1" on such platforms.
Some signals can be neither trapped nor ignored, such as
the KILL and STOP (but not the TSTP) signals. One strategy
for temporarily ignoring signals is to use a local()
statement, which will be automatically restored once your
block is exited. (Remember that local() values are
"inherited" by functions called from within that block.)
sub precious {
local $SIG{INT} = 'IGNORE';
&more_functions;
}
sub more_functions {
# interrupts still ignored, for now...
}
Sending a signal to a negative process ID means that you
send the signal to the entire Unix process-group. This
code sends a hang-up signal to all processes in the current
process group (and sets $SIG{HUP} to IGNORE so it
doesn't kill itself):
{
local $SIG{HUP} = 'IGNORE';
kill HUP => -$$;
# snazzy writing of: kill('HUP', -$$)
}
Another interesting signal to send is signal number zero.
This doesn't actually affect a child process, but instead
checks whether it's alive or has changed its UID.
unless (kill 0 => $kid_pid) {
warn "something wicked happened to $kid_pid";
}
When directed at a process whose UID is not identical to
that of the sending process, signal number zero may fail
because you lack permission to send the signal, even
though the process is alive. You may be able to determine
the cause of failure using "%!".
unless (kill 0 => $pid or $!{EPERM}) {
warn "$pid looks dead";
}
You might also want to employ anonymous functions for simple
signal handlers:
$SIG{INT} = sub { die "0utta here!0 };
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 better still:
use POSIX ":sys_wait_h";
sub REAPER {
my $child;
# If a second child dies while in the signal handler caused by the
# first death, we won't get another signal. So
must loop here else
# we will leave the unreaped child as a zombie.
And the next time
# two children die we get another zombie. And so
on.
while (($child = waitpid(-1,WNOHANG)) > 0) {
$Kid_Status{$child} = $?;
}
$SIG{CHLD} = REAPER; # still loathe sysV
}
$SIG{CHLD} = REAPER;
# do something that forks...
Signal handling is also used for timeouts in Unix, While
safely protected within an "eval{}" block, you set a signal
handler to trap alarm signals and then schedule to
have one delivered to you in some number of seconds. Then
try your blocking operation, clearing the alarm when it's
done but not before you've exited your "eval{}" block. If
it goes off, you'll use die() to jump out of the block,
much as you might using longjmp() or throw() in other languages.
Here's an example:
eval {
local $SIG{ALRM} = sub { die "alarm clock restart"
};
alarm 10;
flock(FH, 2); # blocking write lock
alarm 0;
};
if ($@ and $@ !~ /alarm clock restart/) { die }
If the operation being timed out is system() or qx(), this
technique is liable to generate zombies. If this matters
to you, you'll need to do your own fork() and exec(),
and kill the errant child process.
For more complex signal handling, you might see the standard
POSIX module. Lamentably, this is almost entirely
undocumented, but the t/lib/posix.t file from the Perl
source distribution has some examples in it.
Handling the SIGHUP Signal in Daemons [Toc] [Back]
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
"SIGHUP" signal handler. When you want to tell the daemon
to re-read the file you simply send it the "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 "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, $action);
sub sigHUP_handler {
print "got SIGHUP0;
exec($SELF, @ARGV) or die "Couldn't restart: $!0;
}
code();
sub code {
print "PID: $$0;
print "ARGV: @ARGV0;
my $c = 0;
while (++$c) {
sleep 2;
print "$c0;
}
}
__END__ 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 .signature
file be a named pipe that has a Perl program on the other
end. Now every time any program (like a mailer, news
reader, finger program, etc.) tries to read from that
file, the reading program will block and your program will
supply the new signature. We'll use the pipe-checking
file test -p to find out whether anyone (or anything) has
accidentally removed our fifo.
chdir; # go home
$FIFO = '.signature';
$ENV{PATH} .= ":/etc:/usr/games";
while (1) {
unless (-p $FIFO) {
unlink $FIFO;
system('mknod', $FIFO, 'p')
&& die "can't mknod $FIFO: $!";
}
# next line blocks until there's a reader
open (FIFO, "> $FIFO") || die "can't write $FIFO:
$!";
print FIFO "John Smith ([email protected])0, `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 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 "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 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:
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 system 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 geth-
ostbyname() 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} try something like the following:
use POSIX;
sigaction SIGALRM, new POSIX::SigAction sub { die
"alarm0 }
or die "Error setting SIGALRM handler: $!0;
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 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 dependant.)
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 failure 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 "stuff0;
close SPOOLER || die "bad spool: $! $?";
And here's how to start up a child process you intend to
read from:
open(STATUS, "netstat -an 2>&1 |")
|| die "can't fork: $!";
while (<STATUS>) {
next if /^(tcp|udp)/;
print;
}
close STATUS || die "bad netstat: $! $?";
If one can be sure that a particular program is a Perl
script that is expecting filenames in @ARGV, the clever
programmer can write something like this:
% program f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile
and irrespective of which shell it's called from, the Perl
program will read from the file f1, the process cmd1,
standard input (tmpfile in this case), the f2 file, the
cmd2 command, and finally the f3 file. Pretty nifty, eh?
You might notice that you could use backticks for much the
same effect as opening a pipe for reading:
print grep { !/^(tcp|udp)/ } `netstat -an 2>&1`;
die "bad netstat" if $?;
While this is true on the surface, it's much more efficient
to process the file one line or record at a time
because then you don't have to read the whole thing into
memory at once. It also gives you finer control of the
whole process, letting you to kill off the child process
early if you'd like.
Be careful to check both the open() and the close() return
values. If you're writing to a pipe, you should also trap
SIGPIPE. Otherwise, think of what happens when you start
up a pipe to a command that doesn't exist: the open() will
in all likelihood succeed (it only reflects the fork()'s
success), but then your output will fail--spectacularly.
Perl can't know whether the command worked because your
command is actually running in a separate process whose
exec() might have failed. Therefore, while readers of
bogus commands return just a quick end of file, writers to
bogus command will trigger a signal they'd better be prepared
to handle. Consider:
open(FH, "|bogus") or die "can't fork: $!";
print FH "bang0 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 "bang0 or die "can't write: $!";
close FH or die "can't close: status=$?";
Filehandles [Toc] [Back]
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.
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 [Toc] [Back]
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 containing
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 system doesn't have the setsid()
function, open /dev/tty and use the "TIOCNOTTY" ioctl() on
it instead. See tty(4) for details.
Non-Unix users should check their Your_OS::Process module
for other solutions.
Safe Pipe Opens [Toc] [Back]
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 filehandle
you've opened. The child is running the same program as
the parent. 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 something without the shell's interference. With
system(), it's straightforward, but you can't use a pipe
open or backticks safely. That's because there's no way
to stop the shell from getting its hands on your arguments.
Instead, use lower-level control to call exec()
directly.
Here's a safe backtick or pipe open for read:
# add error processing as above
$pid = open(KID_TO_READ, "-|");
if ($pid) { # parent
while (<KID_TO_READ>) {
# do something interesting
}
close(KID_TO_READ) || warn "kid exited $?";
} else { # child
($EUID, $EGID) = ($UID, $GID); # suid only
exec($program, @options, @args)
|| die "can't exec program: $!";
# NOTREACHED
}
And here's a safe pipe open for writing:
# add error processing as above
$pid = open(KID_TO_WRITE, "|-");
$SIG{PIPE} = sub { die "whoops, $program pipe broke"
};
if ($pid) { # parent
for (@data) {
print KID_TO_WRITE;
}
close(KID_TO_WRITE) || warn "kid exited $?";
} else { # child
($EUID, $EGID) = ($UID, $GID);
exec($program, @options, @args)
|| die "can't exec program: $!";
# NOTREACHED
}
Since Perl 5.8.0, you can also use the list form of "open"
for pipes : the syntax
open KID_PS, "-|", "ps", "aux" or die $!;
forks the ps(1) command (without spawning a shell, as
there are more than three arguments to open()), and reads
its standard output via the "KID_PS" filehandle. The corresponding
syntax to read from 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 mod-
ules file mentioned below in the SEE ALSO section.
Bidirectional Communication with Another Process [Toc] [Back]
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 function to catch both ends. There's also an
open3() for tridirectional I/O so you can also catch your
child's STDERR, but doing so would then require an awkward
select() loop and wouldn't allow you to use normal Perl
input operations.
If you look at its source, you'll see that open2() uses
low-level primitives like Unix pipe() and exec() calls to
create all the connections. While it might have been
slightly more efficient by using socketpair(), it would
have then been even less portable than it already is. The
open2() and open3() functions are unlikely to work anywhere
except on a Unix system or some other one purporting
to be POSIX compliant.
Here's an example of using open2():
use FileHandle;
use IPC::Open2;
$pid = open2(*Reader, *Writer, "cat -u -n" );
print Writer "stuff0;
$got = <Reader>;
The problem with this is that Unix buffering is really
going to ruin your day. Even though your "Writer" filehandle
is auto-flushed, and the process on the other end
will get your data in a timely manner, you can't usually
do anything to force it to give it back to you in a similarly
quick fashion. In this case, we could, because we
gave cat a -u flag to make it unbuffered. But very few
Unix commands are designed to operate over pipes, so this
seldom works unless you yourself wrote the program on the
other end of the double-ended pipe.
A solution to this is the nonstandard Comm.pl library. It
uses pseudo-ttys to make your program behave more reasonably:
require 'Comm.pl';
$ph = open_proc('cat -n');
for (1..10) {
print $ph "a line0;
print "got back ", scalar <$ph>;
}
This way you don't have to have control over the source
code of the program you're using. The Comm library also
has expect() and interact() functions. Find the library
(and we hope its successor IPC::Chat) at your nearest CPAN
archive as detailed in the SEE ALSO section below.
The newer Expect.pm module from CPAN also addresses this
kind of thing. This module requires two other modules
from CPAN: IO::Pty and IO::Stty. It sets up a pseudo-terminal
to interact with programs that insist on using talking
to the terminal device driver. If your system is
amongst those supported, this may be your best bet.
Bidirectional Communication with Yourself [Toc] [Back]
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 this0;
chomp($line = <CHILD_RDR>);
print "Parent Pid $$ just read this: `$line'0;
close CHILD_RDR; close CHILD_WTR;
waitpid($pid,0);
} else {
die "cannot fork: $!" unless defined $pid;
close CHILD_RDR; close CHILD_WTR;
chomp($line = <PARENT_RDR>);
print "Child Pid $$ just read this: `$line'0;
print PARENT_WTR "Child Pid $$ is sending this0;
close PARENT_RDR; close PARENT_WTR;
exit;
}
But you don't actually have to make two pipe calls. If
you have the socketpair() system call, it will do this all
for you.
#!/usr/bin/perl -w
# pipe2 - bidirectional communication using socketpair
# "the best ones always go both ways"
use Socket;
use IO::Handle; # thousands of lines just for autoflush :-(
# We say AF_UNIX because although *_LOCAL is the
# POSIX 1003.1g form of the constant, many machines
# still don't have it.
socketpair(CHILD, PARENT, AF_UNIX, SOCK_STREAM, PF_UNSPEC)
or die "socketpair: $!";
CHILD->autoflush(1);
PARENT->autoflush(1);
if ($pid = fork) {
close PARENT;
print CHILD "Parent Pid $$ is sending this0;
chomp($line = <CHILD>);
print "Parent Pid $$ just read this: `$line'0;
close CHILD;
waitpid($pid,0);
} else {
die "cannot fork: $!" unless defined $pid;
close CHILD;
chomp($line = <PARENT>);
print "Child Pid $$ just read this: `$line'0;
print PARENT "Child Pid $$ is sending this0;
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 circuits
(i.e., TCP streams) and datagrams (i.e., UDP packets).
You may be able to do even more depending on your system.
The Perl function calls for dealing with sockets have the
same names as the corresponding system calls in C, but
their arguments tend to differ for two reasons: first,
Perl filehandles work differently than C file descriptors.
Second, Perl already knows the length of its strings, so
you don't need to pass that information.
One of the major problems with old socket code in Perl was
that it used hard-coded values for some of the constants,
which severely hurt portability. If you ever see code
that does anything like explicitly setting "$AF_INET = 2",
you know you're in for big trouble: An immeasurably superior
approach is to use the "Socket" module, which more
reliably grants access to various constants and functions
you'll need.
If you're not writing a server/client for an existing
protocol like NNTP or SMTP, you should give some thought
to how your server will know when the client has finished
talking, and vice-versa. Most protocols are based on oneline
messages and responses (so one party knows the other
has finished when a "0 is received) or multi-line messages
and responses that end with a period on an empty
line ("00 terminates a message/response).
Internet Line Terminators [Toc] [Back]
The Internet line terminator is " 15 12". Under ASC0,
variants of Unix, that cou0dmightlat timesibeen as "
but under other systems, "
" 15 15 12", " 12 12 15", or something completely
different. The standards specify writing " 15 12" to be
conformant (be strict in what you provide), but they also
recommend accepting a lone " 12" on input (but be lenient
in what you require). We haven't always been very good
about that in the code in this manpage, but unless you're
on a Mac, you'll probably be ok.
Internet TCP Clients and Servers [Toc] [Back]
Use Internet-domain sockets when you want to do clientserver
communication that might extend to machines outside
of your own system.
Here's a sample TCP client using Internet-domain sockets:
#!/usr/bin/perl -w
use strict;
use Socket;
my ($remote,$port, $iaddr, $paddr, $proto, $line);
$remote = shift || 'localhost';
$port = shift || 2345; # random port
if ($port =~ / die "No port" unless $port;
$iaddr = inet_aton($remote) || die "no
host: $remote";
$paddr = sockaddr_in($port, $iaddr);
$proto = getprotobyname('tcp');
socket(SOCK, PF_INET, SOCK_STREAM, $proto) || die
"socket: $!";
connect(SOCK, $paddr) || die "connect: $!";
while (defined($line = <SOCK>)) {
print $line;
}
close (SOCK) || die "close: $!";
exit;
And here's a corresponding server to go along with it.
We'll leave the address as INADDR_ANY so that the kernel
can choose the appropriate interface on multihomed hosts.
If you want sit on a particular interface (like the external
side of a gateway or firewall machine), you should
fill this in with your real address instead.
#!/usr/bin/perl -Tw
use strict;
BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
use Socket;
use Carp;
my $EOL = " 15 12";
sub logmsg { print "$0 $$: @_ at ", scalar localtime,
"0 }
my $port = shift || 2345;
my $proto = getprotobyname('tcp');
($port) = $port =~ /^(+)$/ 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 = " 15 12";
sub spawn; # forward declaration
sub logmsg { print "$0 $$: @_ at ", scalar localtime,
"0 }
my $port = shift || 2345;
my $proto = getprotobyname('tcp');
($port) = $port =~ /^(+)$/ 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 %s0, "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 %s0, $histime - time, ctime($histime);
}
Unix-Domain TCP Clients and Servers [Toc] [Back]
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,
"0 }
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, "0;
exec '/usr/games/fortune' or die "can't exec
fortune: $!";
};
}
sub spawn {
my $coderef = shift;
unless (@_ == 0 && $coderef && ref($coderef) eq
'CODE') {
confess "usage: spawn CODEREF";
}
my $pid;
if (!defined($pid = fork)) {
logmsg "cannot fork: $!";
return;
} elsif ($pid) {
logmsg "begat $pid";
return; # I'm the parent
}
# else I'm the child -- go spawn
open(STDIN, "<&Client") || die "can't dup
client to stdin";
open(STDOUT, ">&Client") || die "can't dup
client to stdout";
## open(STDERR, ">&STDOUT") || die "can't dup stdout to stderr";
exit &$coderef();
}
As you see, it's remarkably similar to the Internet domain
TCP server, so much so, in fact, that we've omitted several
duplicate functions--spawn(), logmsg(), ctime(), and
REAPER()--which are exactly the same as in the other
server.
So why would you ever want to use a Unix domain socket
instead of a simpler named pipe? Because a named pipe
doesn't give you sessions. You can't tell one process's
data from another's. With socket programming, you get a
separate session for each client: that's why accept()
takes two arguments.
For example, let's say that you have a long running
database server daemon that you want folks from the World
Wide Web to be able to access, but only if they go through
a CGI interface. You'd have a small, simple CGI program
that does whatever checks and logging you feel like, and
then acts as a Unix-domain client and connects to your
private server.
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 [Toc] [Back]
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 demonstration purposes,
we've used the special hostname "localhost", which
should always mean the current machine you're running
on. The corresponding 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 services file is
in /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.
Notice how the return value from the "new" constructor is
used as a filehandle in the "while" loop? That's what's
called an indirect filehandle, a scalar variable containing
a filehandle. You can use it the same way you would a
normal filehandle. For example, you can read one line
from it this way:
$line = <$handle>;
all remaining lines from is this way:
@lines = <$handle>;
and send a line of data to it this way:
print $handle "some data0;
A Webget Client [Toc] [Back]
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 = " 15 12";
$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 namedparameter
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 "0 in your code
that sends data over the network to be a " 15 12"
instead.)
Connecting to the server is only the first part of the
process: once you have the connection, you have to use the
server's language. Each server on the network has its own
little command language that it expects as input. The
string that we send to the server starting with "GET" is
in HTTP syntax. In this case, we simply request each
specified document. Yes, we really are making a new
connection for each document, even though it's the same
host. That's the way you always used to have to speak
HTTP. Recent versions of web browsers may request that
the remote server leave the connection open a little
while, but the server doesn't have to honor such a
request.
Here's an example of running that program, which we'll
call webget:
% webget www.perl.com /guanaco.html
HTTP/1.1 404 File Not Found
Date: Thu, 08 May 1997 18:02:32 GMT
Server: Apache/1.2b6
Connection: close
Content-type: text/html
<HEAD><TITLE>404 File Not Found</TITLE></HEAD>
<BODY><H1>File Not Found</H1>
The requested URL /guanaco.html was not found on this
server.<P>
</BODY>
Ok, so that's not very interesting, because it didn't find
that particular document. But a long response wouldn't
have fit on this page.
For a more fully-featured version of this program, you
should look to the lwp-request program included with the
LWP modules from CPAN.
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, b
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