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

Signals

Perl uses a simple signal handling model: the %SIG hash contains namesor references of user-installed signal handlers. These handlers willbe called with an argument which is the name of the signal thattriggered it. A signal may be generated intentionally from aparticular keyboard sequence like control-C or control-Z, sent to youfrom another process, or triggered automatically by the kernel whenspecial events transpire, like a child process exiting, your own processrunning out of stack space, or hitting a process file-size limit.

For example, to trap an interrupt signal, set up a handler like this:

 our $shucks;
 
  sub catch_zap {
  my $signame = shift;
  $shucks++;
  die "Somebody sent me a SIG$signame";
  }
  $SIG{INT} = __PACKAGE__ . "::catch_zap";  
  $SIG{INT} = \&catch_zap;  # best strategy

Prior to Perl 5.7.3 it was necessary to do as little as you possiblycould in your handler; notice how all we do is set a global variableand then raise an exception. That's because on most systems,libraries are not re-entrant; particularly, memory allocation and I/Oroutines are not. That meant that doing nearly anything in yourhandler could in theory trigger a memory fault and subsequent coredump - see Deferred Signals (Safe Signals) below.

The names of the signals are the ones listed out by kill -l on yoursystem, or you can retrieve them using the CPAN module IPC::Signal.

You may also choose to assign the strings "IGNORE" or "DEFAULT" asthe handler, in which case Perl will try to discard the signal or do thedefault thing.

On most Unix platforms, the CHLD (sometimes also known as CLD) signalhas special behavior with respect to a value of "IGNORE".Setting $SIG{CHLD} to "IGNORE" on such a platform has the effect ofnot 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. Note that ignoring signals makes them disappear.If you only want them blocked temporarily without them getting lost you'llhave to use POSIX' sigprocmask.

Sending a signal to a negative process ID means that you send the signalto the entire Unix process group. This code sends a hang-up signal to allprocesses in the current process group, and also sets $SIG{HUP} to "IGNORE" so it doesn't kill itself:

 # block scope for local
  {
  local $SIG{HUP} = "IGNORE";
  kill HUP => -$$;
  # snazzy writing of: kill("HUP", -$$)
  }

Another interesting signal to send is signal number zero. This doesn'tactually affect a child process, but instead checks whether it's aliveor has changed its UIDs.

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

Signal number zero may fail because you lack permission to send thesignal when directed at a process whose real or saved UID is notidentical to the real or effective UID of the sending process, eventhough the process is alive. You may be able to determine the cause offailure using $! or %!.

 unless (kill(0 => $pid) || $!{EPERM}) {
  warn "$pid looks dead";
  }

You might also want to employ anonymous functions for simple signalhandlers:

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

SIGCHLD handlers require some special care. If a second child dieswhile in the signal handler caused by the first death, we won't getanother signal. So must loop here else we will leave the unreaped childas a zombie. And the next time two children die we get another zombie.And so on.

 use POSIX ":sys_wait_h";
  $SIG{CHLD} = sub {
  while ((my $child = waitpid(-1, WNOHANG)) > 0) {
  $Kid_Status{$child} = $?;
  }
  };
  # do something that forks...

Be careful: qx(), system(), and some modules for calling external commandsdo a fork(), then wait() for the result. Thus, your signal handlerwill be called. Because wait() was already called by system() or qx(),the wait() in the signal handler will see no more zombies and willtherefore block.

The best way to prevent this issue is to use waitpid(), as in the followingexample:

 use POSIX ":sys_wait_h"; # for nonblocking read
 
  my %children;
 
  $SIG{CHLD} = sub {
  # don't change $! and $? outside handler
  local ($!, $?);
  my $pid = waitpid(-1, WNOHANG);
  return if $pid == -1;
  return unless defined $children{$pid};
  delete $children{$pid};
  cleanup_child($pid, $?);
  };
 
  while (1) {
  my $pid = fork();
  die "cannot fork" unless defined $pid;
  if ($pid == 0) {
  # ...
  exit 0;
  } else {
  $children{$pid}=1;
  # ...
  system($command);
  # ...
    }
  }

Signal handling is also used for timeouts in Unix. While safelyprotected within an eval{} block, you set a signal handler to trapalarm signals and then schedule to have one delivered to you in somenumber of seconds. Then try your blocking operation, clearing the alarmwhen it's done but not before you've exited your eval{} block. If itgoes off, you'll use die() to jump out of the block.

Here's an example:

 my $ALARM_EXCEPTION = "alarm clock restart";
  eval {
  local $SIG{ALRM} = sub { die $ALARM_EXCEPTION };
  alarm 10;
  flock(FH, 2) # blocking write lock
  || die "cannot flock: $!";
  alarm 0;
  };
  if ($@ && $@ !~ quotemeta($ALARM_EXCEPTION)) { die }

If the operation being timed out is system() or qx(), this techniqueis liable to generate zombies. If this matters to you, you'llneed to do your own fork() and exec(), and kill the errant child process.

For more complex signal handling, you might see the standard POSIXmodule. Lamentably, this is almost entirely undocumented, butthe t/lib/posix.t file from the Perl source distribution has someexamples in it.

Handling the SIGHUP Signal in Daemons

A process that usually starts when the system boots and shuts downwhen the system is shut down is called a daemon (Disk And ExecutionMONitor). If a daemon process has a configuration file which ismodified after the process has been started, there should be a way totell that process to reread its configuration file without stoppingthe process. Many daemons provide this mechanism using a SIGHUPsignal handler. When you want to tell the daemon to reread the file,simply send it the SIGHUP signal.

The following example implements a simple daemon, which restartsitself every time the SIGHUP signal is received. The actual code islocated in the subroutine code(), which just prints some debugginginfo to show that it works; it 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
   $SIG{HUP} = sub {
   print "got SIGHUP\n";
   exec($SELF, @ARGV) || die "$0: couldn't restart: $!";
   };
 
   code();
 
   sub code {
   print "PID: $$\n";
   print "ARGV: @ARGV\n";
   my $count = 0;
   while (++$count) {
   sleep 2;
   print "$count\n";
   }
   }

Deferred Signals (Safe Signals)

Before Perl 5.7.3, installing Perl code to deal with signals exposed you todanger from two things. First, few system library functions arere-entrant. If the signal interrupts while Perl is executing one function(like malloc(3) or printf(3)), and your signal handler then calls the samefunction again, you could get unpredictable behavior--often, a core dump.Second, Perl isn't itself re-entrant at the lowest levels. If the signalinterrupts Perl while Perl is changing its own internal data structures,similarly unpredictable behavior may result.

There were two things you could do, knowing this: be paranoid or bepragmatic. The paranoid approach was to do as little as possible in yoursignal handler. Set an existing integer variable that already has avalue, and return. This doesn't help you if you're in a slow system call,which will just restart. That means you have to die to longjmp(3) outof 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 theconvenience", and to do anything you wanted in your signal handler,and be prepared to clean up core dumps now and again.

Perl 5.7.3 and later avoid these problems by "deferring" signals. That is,when the signal is delivered to the process by the system (to the C codethat implements Perl) a flag is set, and the handler returns immediately.Then at strategic "safe" points in the Perl interpreter (e.g. when it isabout to execute a new opcode) the flags are checked and the Perl levelhandler from %SIG is executed. The "deferred" scheme allows much moreflexibility in the coding of signal handlers as we know the Perlinterpreter is in a safe state, and that we are not in a system libraryfunction when the handler is called. However the implementation doesdiffer from previous Perls in the following ways:

Long-running opcodes
As the Perl interpreter looks at signal flags only when it is aboutto execute a new opcode, a signal that arrives during a long-runningopcode (e.g. a regular expression operation on a very large string) willnot be seen until the current opcode completes.
If a signal of any given type fires multiple times during an opcode (such as from a fine-grained timer), the handler for that signal willbe called only once, after the opcode completes; all otherinstances will be discarded. Furthermore, if your system's signal queuegets flooded to the point that there are signals that have been raisedbut not yet caught (and thus not deferred) at the time an opcodecompletes, those signals may well be caught and deferred duringsubsequent opcodes, with sometimes surprising results. For example, youmay see alarms delivered even after calling alarm(0) as the latterstops the raising of alarms but does not cancel the delivery of alarmsraised but not yet caught. Do not depend on the behaviors described inthis paragraph as they are side effects of the current implementation andmay change in future versions of Perl.
Interrupting IO
When a signal is delivered (e.g., SIGINT from a control-C) the operatingsystem breaks into IO operations like read(2), which is used toimplement Perl's readline() function, the <> operator. On olderPerls the handler was called immediately (and as read is not "unsafe",this worked well). With the "deferred" scheme the handler is not calledimmediately, and if Perl is using the system's stdio library thatlibrary may restart the read without returning to Perl to give it achance to call the %SIG handler. If this happens on your system thesolution is to use the :perlio layer to do IO--at least on those handlesthat you want to be able to break into with signals. (The :perlio layerchecks the signal flags and calls %SIG handlers before resuming IOoperation.)
The default in Perl 5.7.3 and later is to automatically usethe :perlio layer.
Note that it is not advisable to access a file handle within a signalhandler where that signal has interrupted an I/O operation on that samehandle. While perl will at least try hard not to crash, there are noguarantees of data integrity; for example, some data might get dropped orwritten twice.
Some networking library functions like gethostbyname() are known to havetheir own implementations of timeouts which may conflict with yourtimeouts. If you have problems with such functions, try using the POSIXsigaction() function, which bypasses Perl safe signals. Be warned thatthis does subject you 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" }))
   || die "Error setting SIGALRM handler: $!\n";
 
```
Another way to disable the safe signal behavior locally is to usethe Perl::Unsafe::Signals module from CPAN, which affectsall signals.
Restartable system calls
On systems that supported it, older versions of Perl used theSA_RESTART flag when installing %SIG handlers. This meant thatrestartable system calls would continue rather than returning whena 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 placeswhere they previously would have succeeded.
The default :perlio layer retries read, writeand close as described above; interrupted wait and waitpid calls will always be retried.
Signals as "faults"
Certain signals like SEGV, ILL, and BUS are generated by virtual memoryaddressing errors and similar "faults". These are normally fatal: there islittle a Perl-level handler can do with them. So Perl delivers themimmediately rather than attempting to defer them.
Signals triggered by operating system state
On some operating systems certain signal handlers are supposed to "dosomething" before returning. One example can be CHLD or CLD, whichindicates a child process has completed. On some operating systems thesignal handler is expected to wait for the completed childprocess. On such systems the deferred signal scheme will not work forthose signals: it does not do the wait. Again the failure willlook like a loop as the operating system will reissue the signal becausethere are completed child processes that have not yet been waited for.

If you want the old signal behavior back despite possiblememory corruption, set the environment variable PERL_SIGNALS to"unsafe". This feature first appeared in Perl 5.8.1.

Named Pipes

A named pipe (often referred to as a FIFO) is an old Unix IPCmechanism for processes communicating on the same machine. It worksjust like regular anonymous pipes, except that theprocesses rendezvous using a filename and need not be related.

To create a named pipe, use the POSIX::mkfifo() function.

 use POSIX qw(mkfifo);
  mkfifo($path, 0700) ||  die "mkfifo $path failed: $!";

You can also use the Unix command mknod(1), or on somesystems, mkfifo(1). These may not be in your normal path, though.

 # 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 unrelatedone. When you open a fifo, the program will block until there's somethingon the other end.

For example, let's say you'd like to have your .signature file be anamed pipe that has a Perl program on the other end. Now every time anyprogram (like a mailer, news reader, finger program, etc.) tries to readfrom that file, the reading program will read the new signature from yourprogram. We'll use the pipe-checking file-test operator, -p, to findout whether anyone (or anything) has accidentally removed our fifo.

 chdir(); # go home
  my $FIFO = ".signature";
 
  while (1) {
  unless (-p $FIFO) {
  unlink $FIFO;   # discard any failure, will catch later
  require POSIX;  # delayed loading of heavy module
  POSIX::mkfifo($FIFO, 0700)
  || die "can't mkfifo $FIFO: $!";
  }
 
  # next line blocks till there's a reader
  open (FIFO, "> $FIFO")  || die "can't open $FIFO: $!";
  print FIFO "John Smith (smith\@host.org)\n", `fortune -s`;
  close(FIFO) || die "can't close $FIFO: $!";
  sleep 2; # to avoid dup signals
  }

Using open() for IPC

Perl's basic open() statement can also be used for unidirectionalinterprocess communication by either appending or prepending a pipesymbol to the second argument to open(). Here's how to startsomething 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 expectingfilenames in @ARGV, the clever programmer can write something like this:

 % program f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile

and no matter which sort of shell it's called from, the Perl program willread from the file f1, the process cmd1, standard input (tmpfilein this case), the f2 file, the cmd2 command, and finally the f3file. Pretty nifty, eh?

You might notice that you could use backticks for much thesame effect as opening a pipe for reading:

 print grep { !/^(tcp|udp)/ } `netstat -an 2>&1`;
  die "bad netstatus ($?)" if $?;

While this is true on the surface, it's much more efficient to process thefile one line or record at a time because then you don't have to read thewhole thing into memory at once. It also gives you finer control of thewhole process, letting you kill off the child process early if you'd like.

Be careful to check the return values from both open() and close(). Ifyou'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'texist: the open() will in all likelihood succeed (it only reflects thefork()'s success), but then your output will fail--spectacularly. Perlcan't know whether the command worked, because your command is actuallyrunning in a separate process whose exec() might have failed. Therefore,while readers of bogus commands return just a quick EOF, writersto bogus commands will get hit with a signal, which they'd best be preparedto handle. Consider:

 open(FH, "|bogus")  || die "can't fork: $!";
  print FH "bang\n";  #  neither necessary nor sufficient 
  #  to check print retval!
  close(FH)   || die "can't close: $!";

The reason for not checking the return value from print() is because ofpipe buffering; physical writes are delayed. That won't blow up until theclose, and it will blow up with a SIGPIPE. To catch it, you could usethis:

 $SIG{PIPE} = "IGNORE";
  open(FH, "|bogus")  || die "can't fork: $!";
  print FH "bang\n";
  close(FH)   || die "can't close: status=$?";

Filehandles

Both the main process and any child processes it forks share the sameSTDIN, STDOUT, and STDERR filehandles. If both processes try to accessthem at once, strange things can happen. You may also want to closeor reopen the filehandles for the child. You can get around this byopening your pipe with open(), but on some systems this means that thechild 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 yourshell) will be the same as the parent's. You won't need to catchSIGCHLD because of the double-fork taking place; see below for details.

Complete Dissociation of Child from Parent

In some cases (starting server processes, for instance) you'll want tocompletely dissociate the child process from the parent. This isoften called daemonization. A well-behaved daemon will also chdir()to the root directory so it doesn't prevent unmounting the filesystemcontaining the directory from which it was launched, and redirect itsstandard file descriptors from and to /dev/null so that randomoutput doesn't wind up on the user's terminal.

 use POSIX "setsid";
 
  sub daemonize {
  chdir("/")  || die "can't chdir to /: $!";
  open(STDIN,  "< /dev/null") || die "can't read /dev/null: $!";
  open(STDOUT, "> /dev/null") || die "can't write to /dev/null: $!";
  defined(my $pid = fork())   || die "can't fork: $!";
  exit if $pid;   # non-zero now means I am the parent
  (setsid() != -1) || die "Can't start a new session: $!" 
  open(STDERR, ">&STDOUT") || die "can't dup stdout: $!";
  }

The fork() has to come before the setsid() to ensure you aren't aprocess group leader; the setsid() will fail if you are. If yoursystem doesn't have the setsid() function, open /dev/tty and use theTIOCNOTTY ioctl() on it instead. See tty(4) for details.

Non-Unix users should check their Your_OS::Process module for other possible solutions.

Safe Pipe Opens

Another interesting approach to IPC is making your single program gomultiprocess and communicate between--or even amongst--yourselves. Theopen() function will accept a file argument of either "-|" or "|-"to do a very interesting thing: it forks a child connected to thefilehandle you've opened. The child is running the same program as theparent. This is useful for safely opening a file when running under anassumed UID or GID, for example. If you open a pipe to minus, you canwrite to the filehandle you opened and your kid will find it in hisSTDIN. If you open a pipe from minus, you can read from the filehandleyou opened whatever your kid writes to his STDOUT.

 use English qw[ -no_match_vars ];
  my $PRECIOUS = "/path/to/some/safe/file";
  my $sleep_count;
  my $pid;
 
  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) { # I am the parent 
  print KID_TO_WRITE @some_data;
  close(KID_TO_WRITE) || warn "kid exited $?";
  } else { # I am the child
  # drop permissions in setuid and/or setgid programs:
  ($EUID, $EGID) = ($UID, $GID);  
  open (OUTFILE, "> $PRECIOUS") 
  || die "can't open $PRECIOUS: $!";
  while (<STDIN>) {
  print OUTFILE;  # child's STDIN is parent's KID_TO_WRITE
  }
  close(OUTFILE)  || die "can't close $PRECIOUS: $!";
  exit(0); # don't forget this!!
  }

Another common use for this construct is when you need to executesomething without the shell's interference. With system(), it'sstraightforward, 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 onyour arguments. Instead, use lower-level control to call exec() directly.

Here's a safe backtick or pipe open for read:

 my $pid = open(KID_TO_READ, "-|");
  defined($pid)   || die "can't fork: $!";
 
  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:

 my $pid = open(KID_TO_WRITE, "|-");
  defined($pid)   || die "can't fork: $!";
 
  $SIG{PIPE} = sub { die "whoops, $program pipe broke" };
 
  if ($pid) { # parent
  print KID_TO_WRITE @data;
  close(KID_TO_WRITE) || warn "kid exited $?";
 
  } else { # child
  ($EUID, $EGID) = ($UID, $GID);
  exec($program, @options, @args)
  || die "can't exec program: $!";
  # NOTREACHED
  }

It is very easy to dead-lock a process using this form of open(), orindeed with any use of pipe() with multiple subprocesses. The example above is "safe" because it is simple and calls exec(). SeeAvoiding Pipe Deadlocks for general safety principles, but thereare extra gotchas with Safe Pipe Opens.

In particular, if you opened the pipe using open FH, "|-", then youcannot simply use close() in the parent process to close an unwantedwriter. Consider this code:

 my $pid = open(WRITER, "|-"); # fork open a kid
  defined($pid)   || die "first fork failed: $!";
  if ($pid) {
  if (my $sub_pid = fork()) {
  defined($sub_pid)   || die "second fork failed: $!";
  close(WRITER)   || die "couldn't close WRITER: $!";
  # now do something else...
  }
  else {
  # first write to WRITER
  # ...
  # then when finished
  close(WRITER)   || die "couldn't close WRITER: $!";
  exit(0);
  }
  }
  else {
  # first do something with STDIN, then
  exit(0);
  }

In the example above, the true parent does not want to write to the WRITERfilehandle, so it closes it. However, because WRITER was opened usingopen FH, "|-", it has a special behavior: closing it callswaitpid() (see waitpid), which waits for the subprocessto exit. If the child process ends up waiting for something happeningin the section marked "do something else", you have deadlock.

This can also be a problem with intermediate subprocesses in morecomplicated code, which will call waitpid() on all open filehandlesduring global destruction--in no predictable order.

To solve this, you must manually use pipe(), fork(), and the form ofopen() which sets one file descriptor to another, as shown below:

 pipe(READER, WRITER) || die "pipe failed: $!";
  $pid = fork();
  defined($pid)   || die "first fork failed: $!";
  if ($pid) {
  close READER;
  if (my $sub_pid = fork()) {
  defined($sub_pid)   || die "first fork failed: $!";
  close(WRITER)   || die "can't close WRITER: $!";
  }
  else {
  # write to WRITER...
  # ...
  # then  when finished
  close(WRITER)   || die "can't close WRITER: $!";
  exit(0);
  }
  # write to WRITER...
  }
  else {
  open(STDIN, "<&READER") || die "can't reopen STDIN: $!";
  close(WRITER)   || die "can't close WRITER: $!";
  # do something...
  exit(0);
  }

Since Perl 5.8.0, you can also use the list form of open for pipes.This is preferred when you wish to avoid having the shell interpretmetacharacters that may be in your command string.

So for example, instead of using:

 open(PS_PIPE, "ps aux|") || die "can't open ps pipe: $!";

One would use either of these:

 open(PS_PIPE, "-|", "ps", "aux") 
  || die "can't open ps pipe: $!";
 
  @ps_args = qw[ ps aux ];
  open(PS_PIPE, "-|", @ps_args)
  || die "can't open @ps_args|: $!";

Because there are more than three arguments to open(), forks the ps(1)command without spawning a shell, and reads its standard output via thePS_PIPE filehandle. The corresponding syntax to write to commandpipes is to use "|-" in place of "-|".

This was admittedly a rather silly example, because you're using stringliterals whose content is perfectly safe. There is therefore no cause toresort to the harder-to-read, multi-argument form of pipe open(). However,whenever you cannot be assured that the program arguments are free of shellmetacharacters, the fancier form of open() should be used. For example:

 @grep_args = ("egrep", "-i", $some_pattern, @many_files);
  open(GREP_PIPE, "-|", @grep_args)
  || die "can't open @grep_args|: $!";

Here the multi-argument form of pipe open() is preferred because thepattern and indeed even the filenames themselves might hold metacharacters.

Be aware that these operations are full Unix forks, which means they maynot be correctly implemented on all alien systems. Additionally, these arenot true multithreading. To learn more about threading, see the modulesfile mentioned below in the SEE ALSO section.

Avoiding Pipe Deadlocks

Whenever you have more than one subprocess, you must be careful that eachcloses whichever half of any pipes created for interprocess communicationit is not using. This is because any child process reading from the pipeand expecting an EOF will never receive it, and therefore never exit. Asingle process closing a pipe is not enough to close it; the last processwith the pipe open must close it for it to read EOF.

Certain built-in Unix features help prevent this most of the time. Forinstance, filehandles have a "close on exec" flag, which is set en masseunder control of the $^F variable. This is so any filehandles youdidn't explicitly route to the STDIN, STDOUT or STDERR of a childprogram will be automatically closed.

Always explicitly and immediately call close() on the writable end of anypipe, unless that process is actually writing to it. Even if you don'texplicitly call close(), Perl will still close() all filehandles duringglobal destruction. As previously discussed, if those filehandles havebeen opened with Safe Pipe Open, this will result in calling waitpid(),which may again deadlock.

Bidirectional Communication with Another Process

While this works reasonably well for unidirectional communication, whatabout bidirectional communication? The most obvious approach doesn't work:

 # THIS DOES NOT WORK!!
  open(PROG_FOR_READING_AND_WRITING, "| some program |")

If you forget to use warnings, you'll miss out entirely on thehelpful diagnostic message:

 Can't do bidirectional pipe at -e line 1.

If you really want to, you can use the standard open2() from theIPC::Open2 module to catch both ends. There's also an open3() inIPC::Open3 for tridirectional I/O so you can also catch your child'sSTDERR, but doing so would then require an awkward select() loop andwouldn't allow you to use normal Perl input operations.

If you look at its source, you'll see that open2() uses low-levelprimitives like the pipe() and exec() syscalls to create all theconnections. Although it might have been more efficient by usingsocketpair(), this would have been even less portable than it alreadyis. The open2() and open3() functions are unlikely to work anywhereexcept on a Unix system, or at least one purporting POSIX compliance.

Here's an example of using open2():

 use FileHandle;
  use IPC::Open2;
  $pid = open2(*Reader, *Writer, "cat -un");
  print Writer "stuff\n";
  $got = <Reader>;

The problem with this is that buffering is really going to ruin yourday. Even though your Writer filehandle is auto-flushed so the processon the other end gets your data in a timely manner, you can't usually doanything to force that process to give its data to you in a similarly quickfashion. In this special case, we could actually so, because we gavecat a -u flag to make it unbuffered. But very few commands aredesigned to operate over pipes, so this seldom works unless you yourselfwrote the program on the other end of the double-ended pipe.

A solution to this is to use a library which uses pseudottys to make yourprogram behave more reasonably. This way you don't have to have controlover the source code of the program you're using. The Expect modulefrom CPAN also addresses this kind of thing. This module requires twoother modules from CPAN, IO::Pty and IO::Stty. It sets up a pseudoterminal to interact with programs that insist on talking to the terminaldevice driver. If your system is supported, this may be your best bet.

Bidirectional Communication with Yourself

If you want, you may make low-level pipe() and fork() syscalls to stitchthis together by hand. This example only talks to itself, but you couldreopen the appropriate handles to STDIN and STDOUT and call other processes.(The following example lacks proper error checking.)

 #!/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: check failure?
  pipe(CHILD_RDR,  PARENT_WTR); # XXX: check 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(0);
  }

But you don't actually have to make two pipe calls. If youhave 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)
  ||  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(0);
  }

Sockets: Client/Server Communication

While not entirely limited to Unix-derived operating systems (e.g., WinSockon PCs provides socket support, as do some VMS libraries), you might not havesockets on your system, in which case this section probably isn't going todo you much good. With sockets, you can do both virtual circuits like TCPstreams and datagrams like UDP packets. You may be able to do even moredepending on your system.

The Perl functions for dealing with sockets have the same names asthe corresponding system calls in C, but their arguments tend to differfor two reasons. First, Perl filehandles work differently than C filedescriptors. Second, Perl already knows the length of its strings, so youdon't need to pass that information.

One of the major problems with ancient, antemillennial socket code in Perlwas that it used hard-coded values for some of the constants, whichseverely hurt portability. If you ever see code that does anything likeexplicitly setting $AF_INET = 2, you know you're in for big trouble. An immeasurably superior approach is to use the Socket module, which morereliably grants access to the various constants and functions you'll need.

If you're not writing a server/client for an existing protocol likeNNTP or SMTP, you should give some thought to how your server willknow when the client has finished talking, and vice-versa. Mostprotocols are based on one-line messages and responses (so one partyknows the other has finished when a "\n" is received) or multi-linemessages 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 ofUnix, 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 somethingcompletely different. The standards specify writing "\015\012" to beconformant (be strict in what you provide), but they also recommendaccepting a lone "\012" on input (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 from way back in its pre-Unix dark ages, you'll probably be ok.

Internet TCP Clients and Servers

Use Internet-domain sockets when you want to do client-servercommunication 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: $!";
  while ($line = <SOCK>) {
  print $line;
  }
 
  close (SOCK) || die "close: $!";
  exit(0);

And here's a corresponding server to go along with it. We'llleave the address as INADDR_ANY so that the kernel can choosethe appropriate interface on multihomed hosts. If you want siton a particular interface (like the external side of a gatewayor firewall machine), fill this in with your real address instead.

 #!/usr/bin/perl -Tw
  use strict;
  BEGIN { $ENV{PATH} = "/usr/bin:/bin" }
  use Socket;
  use Carp;
  my $EOL = "\015\012";
 
  sub logmsg { print "$0 $$: @_ at ", scalar localtime(), "\n" }
 
  my $port  = shift || 2345;
  die "invalid port" unless if $port =~ /^ \d+ $/x;
 
  my $proto = getprotobyname("tcp");
 
  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 thatlike most typical servers, it spawns (fork()s) a slave server tohandle the client request so that the master server can quicklygo back to service a new client.

 #!/usr/bin/perl -Tw
  use strict;
  BEGIN { $ENV{PATH} = "/usr/bin:/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;
  die "invalid port" unless if $port =~ /^ \d+ $/x;
 
  my $proto = getprotobyname("tcp");
 
  socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
  setsockopt(Server, SOL_SOCKET, SO_REUSEADDR, pack("l", 1)) 
  || die "setsockopt: $!";
  bind(Server, sockaddr_in($port, INADDR_ANY)) || die "bind: $!";
  listen(Server, SOMAXCONN)   || die "listen: $!";
 
  logmsg "server started on port $port";
 
  my $waitedpid = 0;
  my $paddr;
 
  use POSIX ":sys_wait_h";
  use Errno;
 
  sub REAPER {
  local $!;   # don't let waitpid() overwrite current error
  while ((my $pid = waitpid(-1, WNOHANG)) > 0 && WIFEXITED($?)) {
  logmsg "reaped $waitedpid" . ($? ? " with exit $?" : "");
  }
  $SIG{CHLD} = \&REAPER;  # loathe SysV
  }
 
  $SIG{CHLD} = \&REAPER;
 
  while (1) {
  $paddr = accept(Client, Server) || do {
  # try again if accept() returned because got a signal
  next if $!{EINTR};
  die "accept: $!";
  };
  my ($port, $iaddr) = sockaddr_in($paddr);
  my $name = gethostbyaddr($iaddr, AF_INET);
 
  logmsg "connection from $name [",
    inet_ntoa($iaddr),
    "] at port $port";
 
  spawn sub {
  $| = 1;
  print "Hello there, $name, it's now ", scalar localtime(), $EOL;
  exec "/usr/games/fortune"   # XXX: "wrong" line terminators
  or confess "can't exec fortune: $!";
  };
  close Client;
  }
 
  sub spawn {
  my $coderef = shift;
 
  unless (@_ == 0 && $coderef && ref($coderef) eq "CODE") {
  confess "usage: spawn CODEREF";
  }
 
  my $pid;
  unless (defined($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 atonce, which you might not always want. Even if you don't fork(), thelisten() will allow that many pending connections. Forking servershave to be particularly careful about cleaning up their dead children(called "zombies" in Unix parlance), because otherwise you'll quicklyfill up your process table. The REAPER subroutine is used here tocall waitpid() for any child processes that have finished, therebyensuring that they terminate cleanly and don't join the ranks of theliving dead.

Within the while loop we call accept() and check to see if it returnsa false value. This would normally indicate a system error needsto be reported. However, the introduction of safe signals (seeDeferred Signals (Safe Signals) above) in Perl 5.7.3 means thataccept() might also be interrupted when the process receives a signal.This typically happens when one of the forked subprocesses exits andnotifies the parent process with a CHLD signal.

If accept() is interrupted by a signal, $! will be set to EINTR.If this happens, we can safely continue to the next iteration ofthe loop and another call to accept(). It is important that yoursignal handling code not modify the value of $!, or else this test will likely fail. In the REAPER subroutine we create a local versionof $! before calling waitpid(). When waitpid() sets $! to ECHILD asit inevitably does when it has no more children waiting, it updates the local copy and leaves the original unchanged.

You should use the -T flag to enable taint checking (see perlsec)even if we aren't running setuid or setgid. This is always a good ideafor servers or any program run on behalf of someone else (like CGIscripts), because it lessens the chances that people from the outside willbe 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 clocksdiffer 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() || time()) }
 
  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();
 
  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 "connect: $!";
  my $rtime = pack("C4", ());
  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 localcommunications? While you can use the same setup, sometimes you don'twant to. Unix-domain sockets are local to the current host, and are oftenused internally to implement pipes. Unlike Internet domain sockets, Unixdomain 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(0);

And here's a corresponding server. You don't have to worry about sillynetwork terminators here because Unix domain sockets are guaranteedto be on the localhost, and thus everything works right.

 #!/usr/bin/perl -Tw
  use strict;
  use Socket;
  use Carp;
 
  BEGIN { $ENV{PATH} = "/usr/bin:/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")  || die "can't exec fortune: $!";
  };
  }
 
  sub spawn {
  my $coderef = shift();
 
  unless (@_ == 0 && $coderef && ref($coderef) eq "CODE") {
  confess "usage: spawn CODEREF";
  }
 
  my $pid;
  unless (defined($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, somuch so, in fact, that we've omitted several duplicate functions--spawn(),logmsg(), ctime(), and REAPER()--which are the same as in the other server.

So why would you ever want to use a Unix domain socket instead of asimpler named pipe? Because a named pipe doesn't give you sessions. Youcan'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 twoarguments.

For example, let's say that you have a long-running database server daemonthat you want folks to be able to access from the Web, but onlyif they go through a CGI interface. You'd have a small, simple CGIprogram that does whatever checks and logging you feel like, and then actsas 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, theIO::Socket module provides an object-oriented approach. IO::Socket hasbeen included in the standard Perl distribution ever since Perl 5.004. Ifyou're running an earlier version of Perl (in which case, how are youreading this manpage?), just fetch IO::Socket from CPAN, where you'll alsofind 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--to name just 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 everythingthat the server there cares to provide.

 #!/usr/bin/perl -w
  use IO::Socket;
  $remote = IO::Socket::INET->new(
  Proto => "tcp",
  PeerAddr => "localhost",
  PeerPort => "daytime(13)",
  )
   || die "can't connect to daytime service on localhost";
  while (<$remote>) { print }

When you run this program, you should get something back thatlooks 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 returnedwill be connected to a TCP socket, because we want a stream-orientedconnection, that is, one that acts pretty much like a plain old file.Not all sockets are this of this type. For example, the UDP protocolcan 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 isrunning on. We could have specified a longer name like "www.perl.com",or an address like "207.171.7.72". For demonstration purposes, we'veused the special hostname "localhost", which should always mean thecurrent machine you're running on. The corresponding Internet addressfor localhost is "127.0.0.1", 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 awell-configured system services file,[FOOTNOTE: The system services fileis found in /etc/services under Unixy systems.] but here we've specified theport number (13) in parentheses. Using just the number would have alsoworked, but numeric literals make careful programmers nervous.

Notice how the return value from the new constructor is used asa filehandle in the while loop? That's what's called an indirectfilehandle, a scalar variable containing a filehandle. You can useit the same way you would a normal filehandle. For example, youcan 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 documentfrom, and then a list of files to get from that host. This is amore interesting client than the previous one because it first sendssomething to the server before fetching the server's response.

 #!/usr/bin/perl -w
  use IO::Socket;
  unless (@ARGV > 1) { die "usage: $0 host url ..." }
  $host = shift(@ARGV);
  $EOL = "\015\012";
  $BLANK = $EOL x 2;
  for my $document (@ARGV) {
  $remote = IO::Socket::INET->new( Proto => "tcp",
  PeerAddr  => $host,
  PeerPort  => "http(80)",
   ) || die "cannot connect to httpd on $host";
  $remote->autoflush(1);
  print $remote "GET $document HTTP/1.0" . $BLANK;
  while ( <$remote> ) { print }
  close $remote;
  }

The web server handling the HTTP service is assumed to be atits standard port, number 80. If the server you're trying toconnect to is at a different port, like 1080 or 8080, you should specify itas the named-parameter pair, PeerPort => 8080. The autoflushmethod is used on the socket because otherwise the system would bufferup the output we sent it. (If you're on a prehistoric Mac, you'll alsoneed to change every "\n" in your code that sends data over the networkto be a "\015\012" instead.)

Connecting to the server is only the first part of the process: once youhave the connection, you have to use the server's language. Each serveron the network has its own little command language that it expects asinput. The string that we send to the server starting with "GET" is inHTTP syntax. In this case, we simply request each specified document.Yes, we really are making a new connection for each document, even thoughit'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 leavethe connection open a little while, but the server doesn't have to honorsuch 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 thatparticular document. But a long response wouldn't have fit on this page.

For a more featureful version of this program, you should look tothe 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 likethe 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 ifyou're on a system that supports the powerful fork call, the solutionisn't that rough. Once you've made the connection to whatever serviceyou'd like to chat with, call fork to clone your process. Each ofthese two identical process has a very simple job to do: the parentcopies everything from the socket to standard output, while the childsimultaneously copies everything from standard input to the socket.To accomplish the same thing using just one process would be muchharder, because it's easier to code two processes to do one thing than itis to code one process to do two things. (This keep-it-simple principlea cornerstones of the Unix philosophy, and good software engineering aswell, 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)
    || 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;
  }
  exit(0); # just in case
  }

The kill function in the parent's if block is there to send asignal to our child process, currently 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 thatdata immediately without waiting for a newline (which might not happen),you may wish to replace the while loop in the parent with thefollowing:

 my $byte;
  while (sysread($handle, $byte, 1) == 1) {
  print STDOUT $byte;
  }

Making a system call for each byte you want to read is not very efficient(to put it mildly) but is the simplest to explain and works reasonablywell.

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 thatdoes nothing but listen on a particular port for incoming connections.It does this by calling the IO::Socket::INET->new() method withslightly different arguments than the client did.

Proto
This is which protocol to use. Like our clients, we'llstill specify "tcp" here.
LocalPort
We specify a localport 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 theserver. (Under Unix, ports under 1024 are restricted to thesuperuser.) In our sample, we'll use port 9000, but you can useany port that's not currently in use on your system. If you tryto use one already in used, you'll get an "Address already in use"message. Under Unix, the netstat -a command will showwhich services current have servers.
Listen
The Listen parameter is set to the maximum number ofpending connections we can accept until we turn away incoming clients.Think of it as a call-waiting queue for your telephone.The low-level Socket module has a special symbol for the system maximum, whichis SOMAXCONN.
Reuse
The Reuse parameter is needed so that we restart our servermanually without waiting a few minutes to allow system buffers toclear out.

Once the generic server socket has been created using the parameterslisted above, the server then waits for a new client to connectto it. The server blocks in the accept method, which eventually accepts abidirectional connection from the remote client. (Make sure to autoflushthis 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 tothe client. Unlike most network servers, this one handles only oneincoming client at a time. Multithreaded servers are covered in Chapter 16 of the Camel.

Here's the code. We'll

 #!/usr/bin/perl -w
  use IO::Socket;
  use Net::hostent;  # for OOish 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, butmessages. UDP communications involve much lower overhead but also provideless reliability, as there are no promises that messages will arrive atall, let alone in order and unmangled. Still, UDP offers some advantagesover TCP, including being able to "broadcast" or "multicast" to a wholebunch of destination hosts at once (usually on your local subnet). If youfind yourself overly concerned about reliability and start building checksinto your message system, then you probably should use just TCP to startwith.

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 bettersend(), like in the example below.

Here's a UDP program similar to the sample Internet TCP client givenearlier. However, instead of checking one host at a time, the UDP versionwill check many of them asynchronously by simulating a multicast and thenusing select() to do a timed-out wait for I/O. To do something similarwith 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 = 2_208_988_800;
 
  $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();
  $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--;
  }

This example does not include any retries and may consequently fail tocontact a reachable host. The most prominent reason for this is congestionof the queues on the sending host if the number of hosts to contact issufficiently large.

SysV IPC

While System V IPC isn't so widely used as sockets, it still has someinteresting uses. However, you cannot use SysV IPC or Berkeley mmap() tohave a variable shared amongst several processes. That's because Perlwould reallocate your string when you weren't wanting it to. You mightlook into the IPC::Shareable or threads::shared modules for that.

Here's a small example showing shared memory usage.

 use IPC::SysV qw(IPC_PRIVATE IPC_RMID S_IRUSR S_IWUSR);
 
  $size = 2000;
  $id = shmget(IPC_PRIVATE, $size, S_IRUSR | S_IWUSR);
  defined($id) || die "shmget: $!";
  print "shm key $id\n";
 
  $message = "Message #1";
  shmwrite($id, $message, 0, 60)  || die "shmwrite: $!";
  print "wrote: '$message'\n";
  shmread($id, $buff, 0, 60)  || die "shmread: $!";
  print "read : '$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 "shmctl: $!";

Here's an example of a semaphore:

 use IPC::SysV qw(IPC_CREAT);
 
  $IPC_KEY = 1234;
  $id = semget($IPC_KEY, 10, 0666 | IPC_CREAT);
  defined($id) || die "shmget: $!";
  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);
  defined($id) || die "shmget: $!";
 
  $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 "semop: $!";

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 unless defined($id);
 
  $semnum  = 0;
  $semflag = 0;
 
  # Decrement the semaphore count
  $semop = -1;
  $opstring = pack("s!s!s!", $semnum, $semop, $semflag);
 
  semop($id, $opstring)   || die "semop: $!";

The SysV IPC code above was written long ago, and it's definitelyclunky looking. For a more modern look, see the IPC::SysV modulewhich is included with Perl starting from Perl 5.005.

A small example demonstrating SysV message queues:

 use IPC::SysV qw(IPC_PRIVATE IPC_RMID IPC_CREAT S_IRUSR S_IWUSR);
 
  my $id = msgget(IPC_PRIVATE, IPC_CREAT | S_IRUSR | S_IWUSR);
  defined($id) || die "msgget failed: $!";
 
  my $sent  = "message";
  my $type_sent = 1234;
 
  msgsnd($id, pack("l! a*", $type_sent, $sent), 0)
  || die "msgsnd failed: $!";
 
  msgrcv($id, my $rcvd_buf, 60, 0, 0)
  || die "msgrcv failed: $!";
 
  my($type_rcvd, $rcvd) = unpack("l! a*", $rcvd_buf);
 
  if ($rcvd eq $sent) {
  print "okay\n";
  } else {
  print "not okay\n";
  }
 
  msgctl($id, IPC_RMID, 0) || die "msgctl failed: $!\n";

NOTES

Most of these routines quietly but politely return undef when theyfail instead of causing your program to die right then and there due toan uncaught exception. (Actually, some of the new Socket conversionfunctions do croak() on bad arguments.) It is therefore essential tocheck return values from these functions. Always begin your socketprograms this way for optimal success, and don't forget to add the -Ttaint-checking flag to the #! line for servers:

 #!/usr/bin/perl -Tw
  use strict;
  use sigtrap;
  use Socket;

BUGS

These routines all create system-specific portability problems. As notedelsewhere, Perl is at the mercy of your C libraries for much of its systembehavior. It's probably safest to assume broken SysV semantics forsignals and to stick with simple TCP and UDP socket operations; e.g., don'ttry to pass open file descriptors over a local UDP datagram socket if youwant your code to stand a chance of being portable.

AUTHOR

Tom Christiansen, with occasional vestiges of Larry Wall's originalversion and suggestions from the Perl Porters.