PERLIPC(1perl)
🎀 perlipc — Perl interprocess communication 🎀
okay so bestie, this is perlipc, from the Perl Programmers Reference Guide, manual section 1perl. it's literally the whole lowdown on Perl interprocess communication: signals, fifos, pipes, safe subprocesses, sockets, and semaphores. all of it. buckle up 💖
✨ DESCRIPTION ✨
Perl's basic IPC setup is built out of the good old Unix classics: signals, named pipes, pipe opens, the Berkeley socket routines, and SysV IPC calls. they're all iconic in their own way 👑 and each one gets used in slightly different situations, so you gotta know which vibe fits which moment. vibe check time 🦋
📞 Signals 📞
Perl uses a simple signal handling model: the %SIG hash holds the names (or references) of your user-installed signal handlers. when a handler gets called, it gets an argument — and that argument is the name of the signal that triggered it. cute, right?
a signal can show up in a few ways, no cap:
- generated on purpose from a particular keyboard sequence, like Ctrl+C or Ctrl+Z
- sent to you from another process
- triggered automatically by the kernel when special events transpire — like a child process exiting, your own process running out of stack space, or hitting a process file-size limit
so, for example, to trap an interrupt signal, you 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 strategybefore Perl 5.8.0 you had to do as little as you possibly could inside your handler. notice how all we do up there is set a global variable and then raise an exception? that's not a style choice, bestie, that's survival 😭 because on most systems, libraries are not re-entrant — especially memory allocation and I/O routines, which are very much not re-entrant. so doing nearly anything in your handler could in theory trigger a memory fault and a subsequent core dump. see "Deferred Signals (Safe Signals)" below for the tea 💀
the names of the signals are the ones listed out by kill -l on your system, or you can grab them with the CPAN module IPC::Signal.
you can also assign the strings "IGNORE" or "DEFAULT" as the handler — in that case Perl will try to discard the signal, or do the default thing. your call, main character 💅
on most Unix platforms, the "CHLD" signal (sometimes also known as "CLD") has special behavior when it comes to a value of "IGNORE". setting $SIG{CHLD} to "IGNORE" on such a platform means no zombie processes get created when the parent process fails to wait() on its child processes — i.e. the child processes are automatically reaped. iconic. and 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 signals (but not the TSTP signal). also note: ignoring signals makes them disappear. if you only want them blocked temporarily without letting them get lost, you'll have to use the "POSIX" module's sigprocmask.
sending a signal to a negative process ID means you send that signal to the entire Unix process group. this next code sends a hang-up signal to all the processes in the current process group, and sets $SIG{HUP} to "IGNORE" so it doesn't kill itself. girlboss move, honestly:
# block scope for local
{
local $SIG{HUP} = "IGNORE";
kill HUP => -getpgrp();
# snazzy writing of: kill("HUP", -getpgrp())
}another interesting signal to send is signal number zero. it doesn't actually affect a child process at all — instead it checks whether that process is alive or has changed its UIDs:
unless (kill 0 => $kid_pid) {
warn "something wicked happened to $kid_pid";
}⚠️ signal number zero may fail because you lack permission to send the signal — that happens when it's directed at a process whose real or saved UID isn't identical to the real or effective UID of the sending process, even though the process is alive. you may be able to figure out the cause of the failure using$!or%!.
unless (kill(0 => $pid) || $!{EPERM}) {
warn "$pid looks dead";
}you might also want to employ anonymous functions for simple signal handlers, because sometimes you just don't have the energy for a whole named sub:
$SIG{INT} = sub { die "\nOutta here!\n" };SIGCHLD handlers require some special care, and I need you to focus for this one 🫶 if a second child dies while we're inside the signal handler caused by the first death, we won't get another signal. so you must loop here, or else we leave the unreaped child as a zombie. and then the next time two children die, we get another zombie. and so on. it's giving zombie apocalypse fr fr:
use POSIX ":sys_wait_h";
$SIG{CHLD} = sub {
while ((my $child = waitpid(-1, WNOHANG)) > 0) {
$Kid_Status{$child} = $?;
}
};
# do something that forks...bestie, be careful:qx(),system(), and some modules for calling external commands do afork(), then await()for the result. so your signal handler will be called. but becausewait()was already called bysystem()orqx(), thewait()inside the signal handler will see no more zombies and will therefore block. not slay 💀
the best way to prevent this issue is to use waitpid(), as in the following example:
use POSIX ":sys_wait_h"; # for nonblocking read
my %children;
$SIG{CHLD} = sub {
# don't change $! and $? outside handler
local ($!, $?);
while ( (my $pid = waitpid(-1, WNOHANG)) > 0 ) {
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. here's the routine: while safely protected inside an eval{} block, you set a signal handler to trap alarm signals, and then schedule one to be delivered to you in some number of seconds. then you 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 use die() to jump out of the block. chef's kiss ✨
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 you're timing out issystem()orqx(), this technique is liable to generate zombies. if that matters to you, you'll need to do your ownfork()andexec(), and kill the errant child process.
for more complex signal handling, you might go look at the standard POSIX module. lamentably, this is almost entirely undocumented 😭 but the ext/POSIX/t/sigaction.t file from the Perl source distribution has some examples in it. iykyk.
🔄 Handling the SIGHUP Signal in Daemons 🔄
a process that usually starts when the system boots and shuts down when the system is shut down is called a daemon (Disk And Execution MONitor). love that for them.
if a daemon process has a configuration file that gets modified after the process has already started, there should be a way to tell that process to reread its config without straight-up stopping it. lots of daemons provide this mechanism using a "SIGHUP" signal handler. so when you want the daemon to reread the file, you just send it the "SIGHUP" signal. simple as. 💅
the following example implements a simple daemon, which restarts itself every time the "SIGHUP" signal is received. the actual code lives in the subroutine code(), which just prints some debugging info to show that it works; you should swap it out for the real code.
#!/usr/bin/perl
use v5.36;
use POSIX ();
use FindBin ();
use File::Basename ();
use File::Spec::Functions qw(catfile);
$| = 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);
$SIG{HUP} = sub {
print "got SIGHUP\n";
exec($SELF, @ARGV) || die "$0: couldn't restart: $!";
};
# Make sure SIGHUP isn't blocked (as it normally would be, due to the
# handler function calling exec rather than returning)
POSIX::sigprocmask(&POSIX::SIG_UNBLOCK,
POSIX::SigSet->new(&POSIX::SIGHUP));
code();
sub code {
print "PID: $$\n";
print "ARGV: @ARGV\n";
my $count = 0;
while (1) {
sleep 2;
print ++$count, "\n";
}
}🛡️ Deferred Signals (Safe Signals) 🛡️
before Perl 5.8.0, installing Perl code to deal with signals exposed you to danger from two things. bestie, it was rough out here:
- very few system library functions are re-entrant. if the signal interrupts while Perl is executing one function (like
malloc(3)orprintf(3)), and your signal handler then calls that same function again, you could get unpredictable behavior — often, a core dump 😭 - 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 behavior may result.
knowing all that, there were two things you could do: 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. which means you have to
dietolongjmp(3)out of the handler. even this is a little cavalier for the true paranoiac, who avoidsdiein a handler because the system is out to get you 🫣 - the pragmatic approach was to say "I know the risks, but I prefer the convenience", do anything you wanted in your signal handler, and be prepared to clean up core dumps now and again.
Perl 5.8.0 and later avoid these problems by "deferring" signals. here's how it works: 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's about to execute a new opcode), those flags get checked and the Perl-level handler from %SIG gets executed. the "deferred" scheme allows much more flexibility in the coding of signal handlers, because we know the Perl interpreter is in a safe state, and we know we're not inside a system library function when the handler is called. slay 💖
however, the implementation does differ from previous Perls in the following ways: 👀
Long-running opcodes 💤
Okay so here's the tea: the Perl interpreter only peeks at signal flags right before it's about to execute a new opcode. Which means if a signal shows up while a long-running opcode is grinding — think a regex operation on a string the size of a novel — bestie, it is not getting seen until that opcode finishes. It's giving "wait your turn."
And if one signal type keeps firing over and over during a single opcode (say, from a fine-grained timer)? The handler for that signal gets called only once, after the opcode completes. Every other instance? Discarded. Gone. Not invited. ✨
On top of that: if your system's signal queue gets flooded to the point where there are signals that have been raised but not yet caught (and thus not deferred) by the time an opcode completes, those signals may well get caught and deferred during subsequent opcodes, with sometimes surprising results. Like — you might see alarms delivered even after calling alarm(0), because the latter stops the raising of alarms but does not cancel the delivery of alarms raised but not yet caught. Rude, honestly.
Do not depend on the behaviors described in this paragraph — they're side effects of the current implementation and may change in future versions of Perl. Not a vibe you can build a house on. 💀
Interrupting IO 🛑
When a signal is delivered (e.g., SIGINT from a control-C), the operating system breaks into IO operations like read(2), which is what implements Perl's readline() function, AKA the <> operator. On older Perls the handler was called immediately — and since read is not "unsafe", this worked well. Slay.
But with the "deferred" scheme, the handler is not called immediately. And if Perl is using the system's stdio library, that library may restart the read without returning to Perl to give it a chance to call the %SIG handler. 😭
If this happens on your system, the fix is to use the :perlio layer to do IO — at least on those handles that 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. Iconic behavior, truly.)
The default in Perl 5.8.0 and later is to automatically use the :perlio layer. So, queen moves as standard. 👑
Heads up: it is not advisable to access a file handle within a signal handler where that signal has interrupted an I/O operation on that same handle. While perl will at least try hard not to crash, there are no guarantees of data integrity — for example, some data might get dropped or written twice. 😳
Some networking library functions like gethostbyname() are known to have their own implementations of timeouts, which may conflict with your timeouts. If you have problems with such functions, try using the POSIX sigaction() function with SigAction objects, which bypass Perl safe signals unless you set their "safe" flag explicitly. But be warned — this does subject you to possible memory corruption, as described above. Risk it and see, I guess? 🫠
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 use the Perl::Unsafe::Signals module from CPAN — and that one affects all signals. All of them. Every single one. 💅
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. Chill, right?
But in order to deliver deferred signals promptly, Perl 5.8.0 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. Sorry bestie. 😔
The default :perlio layer retries read, write and close as described above; interrupted wait and waitpid calls will always be retried.
Signals as "faults" ⚡
Certain signals like SEGV, ILL, BUS and FPE are generated by virtual memory addressing errors and similar "faults". These are normally fatal — there is little a Perl-level handler can do with them. So Perl delivers them immediately rather than attempting to defer them. No queue, no waiting, straight to your face. 🌈
It is possible to catch these with a %SIG handler (see perlvar), but on top of the usual problems of "unsafe" signals, the signal is likely to get rethrown immediately on return from the signal handler — so such a handler should die or exit instead. Don't try to be cute about it. 🙈
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. And again the failure will look like a loop, as the operating system will reissue the signal because there are completed child processes that have not yet been waited for. Infinite cycle energy, but the bad kind. 😵💫
If you want the old signal behavior back despite possible memory 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 IPC mechanism for processes communicating on the same machine. It works just like regular anonymous pipes, except that the processes rendezvous using a filename and need not be related. No family drama required. 🎀
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 some systems, 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 unrelated one. When you open a fifo, the program will block until there's something on the other end. It's giving "waiting for your text back." 💅
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 read the new signature from your program. Main character .signature, honestly. 👑
We'll use the pipe-checking file-test operator, -p, to find out 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 (my $fh, ">", $FIFO) || die "can't open $FIFO: $!";
print $fh "John Smith (smith\@host.org)\n", `fortune -s`;
close($fh) || 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 unidirectional interprocess communication by specifying the open mode as "|-" or "-|". Let's go. ✨
Here's how to start something up in a child process you intend to write to:
open(my $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(my $status, "-|", "netstat -an 2>&1")
|| die "can't fork: $!";
while (<$status>) {
next if /^(tcp|udp)/;
print;
}
close $status || die "bad netstat: $! $?";Be aware that these operations are full Unix forks, which means they may not be correctly implemented on all alien systems. See "open" in perlport for portability details.
In the two-argument form of open(), a pipe open can be achieved by either appending or prepending a pipe symbol to the second argument:
open(my $spooler, "| cat -v | lpr -h 2>/dev/null")
|| die "can't fork: $!";
open(my $status, "netstat -an 2>&1 |")
|| die "can't fork: $!";This can be used even on systems that do not support forking — but bestie, this possibly allows code intended to read files to unexpectedly execute programs. 😳 Whomst among us hasn't been betrayed by a filename that was secretly a command?
If one can be sure that a particular program is a Perl script expecting filenames in @ARGV using the two-argument form of open() or the <> operator, the clever programmer can write something like this:
% program f1 "cmd1|" - f2 "cmd2|" f3 < tmpfileand no matter which sort of 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? Ate, no crumbs. 🔥
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 netstatus ($?)" 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 kill off the child process early if you'd like. Girlboss move. 💅
Be careful to check the return values from bothopen()andclose(). If you're writing to a pipe, you should also trapSIGPIPE. Otherwise, think of what happens when you start up a pipe to a command that doesn't exist: theopen()will in all likelihood succeed (it only reflects thefork()'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 EOF, writers to bogus commands will get hit with a signal, which they'd best be prepared to handle. Consider:
open(my $fh, "|-", "bogus") || die "can't fork: $!";
print $fh "bang\n"; # neither necessary nor sufficient
# to check print retval!
close($fh) || die "can't close: $!";So like — why are we not checking what print() handed back? Because of pipe buffering, bestie. The physical writes are literally delayed. It's giving "I'll deal with it later" energy. That means nothing actually blows up until the close, and when it does blow up, it blows up with a SIGPIPE. 😭 If you want to actually catch it, do this:
$SIG{PIPE} = "IGNORE";
open(my $fh, "|-", "bogus") || die "can't fork: $!";
print $fh "bang\n";
close($fh) || die "can't close: status=$?";🫶 Filehandles 🫶
Okay so the tea here is shared filehandles. Both the main process and any children it forks are sharing the exact same STDIN, STDOUT, and STDERR. And if both of them try to grab those at the same time?? strange things can happen. Not a vibe. 💀
You may also want to close or reopen the filehandles for the child. You can get around this whole situation by opening your pipe with open(), but heads up — on some systems that means the child process cannot outlive the parent. Noted. 📝
🌙 Background Processes 🌙
Wanna run a command in the background? Say less:
system("cmd &");That command's STDOUT and STDERR (and possibly STDIN, depending on your shell, iykyk) will be the same as the parent's. And you won't need to catch SIGCHLD because of the double-fork taking place — peep below for the details. ✨
🦋 Complete Dissociation of Child from Parent 🦋
Sometimes (starting server processes, for instance) you want the child process to be completely dissociated from the parent. This is often called daemonization and honestly? iconic behavior.
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 it'll redirect its standard file descriptors from and to /dev/null so that random output doesn't wind up on the user's terminal. Respectful. 🎀
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 /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: $!";
}Lowkey important ordering detail: the fork() has to come before the setsid() to ensure you aren't a process group leader — because 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 possible solutions. 🫡
💜 Safe Pipe Opens 💜
Another interesting approach to IPC is making your single program go multiprocess and communicate between — or even amongst — yourselves. The two-argument form of 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. ✨
my $PRECIOUS = "/path/to/some/safe/file";
my $sleep_count;
my $pid;
my $kid_to_write;
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:
($>, $)) = ($<, $();
open (my $outfile, ">", $PRECIOUS)
|| die "can't open $PRECIOUS: $!";
while (<STDIN>) {
print $outfile; # child STDIN is parent $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 execute something without the shell's interference. With system(), it's straightforward — but you can't use a pipe open or backticks safely. Why? 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:
my $pid = open(my $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
($>, $)) = ($<, $(); # suid only
exec($program, @options, @args)
|| die "can't exec program: $!";
# NOTREACHED
}And here's a safe pipe open for write:
my $pid = open(my $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
($>, $)) = ($<, $();
exec($program, @options, @args)
|| die "can't exec program: $!";
# NOTREACHED
}Okay but be careful, bestie. ⚠️ It is very easy to dead-lock a process using this form of open(), or indeed with any use of pipe() with multiple subprocesses. The example above is "safe" because it is simple and calls exec(). See "Avoiding Pipe Deadlocks" for general safety principles — but there are extra gotchas with Safe Pipe Opens.
In particular, if you opened the pipe using "open $fh, "|-"", then you cannot simply use close() in the parent process to close an unwanted writer. Consider this code:
my $pid = open(my $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);
}So in that example, the true parent does not want to write to the $writer filehandle, so it closes it. But — plot twist — because $writer was opened using "open $fh, "|-"", it has a special behavior: closing it calls waitpid() (see "waitpid" in perlfunc), which waits for the subprocess to exit. 😬 So if the child process ends up waiting for something happening in the section marked "do something else", boom: you have deadlock.
This can also be a problem with intermediate subprocesses in more complicated code, which will call waitpid() on all open filehandles during global destruction — in no predictable order. 🫠
To solve this, you must manually use pipe(), fork(), and the form of open() which sets one file descriptor to another, as shown below:
pipe(my $reader, my $writer) || die "pipe failed: $!";
my $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 interpret metacharacters that may be in your command string. 🎀
So for example, instead of using:
open(my $ps_pipe, "-|", "ps aux") || die "can't open ps pipe: $!";One would use either of these:
open(my $ps_pipe, "-|", "ps", "aux")
|| die "can't open ps pipe: $!";
my @ps_args = qw[ ps aux ];
open(my $ps_pipe, "-|", @ps_args)
|| die "can't open @ps_args|: $!";Because there are more than three arguments to open(), it forks the ps(1) command without spawning a shell, and reads its standard output via the $ps_pipe filehandle. The corresponding syntax to write to command pipes is to use "|-" in place of "-|". ✨
Ngl, this was admittedly a rather silly example, because you're using string literals whose content is perfectly safe. There is therefore no cause to resort to the harder-to-read, multi-argument form of pipe open(). However — whenever you cannot be assured that the program arguments are free of shell metacharacters, the fancier form of open() should be used. For example:
my @grep_args = ("egrep", "-i", $some_pattern, @many_files);
open(my $grep_pipe, "-|", @grep_args)
|| die "can't open @grep_args|: $!";Here the multi-argument form of pipe open() is preferred because the pattern and indeed even the filenames themselves might hold metacharacters. 👑
🌸 Avoiding Pipe Deadlocks 🌸
Whenever you have more than one subprocess, you must be careful that each closes whichever half of any pipes created for interprocess communication it is not using. Because bestie — any child process reading from the pipe and expecting an EOF will never receive it, and therefore never exit. 💀
And here's the kicker: a single process closing a pipe is not enough to close it. The last process with the pipe open must close it for it to read EOF. Periodt. 🫶
Certain built-in Unix features are out here doing the most to stop that from happening, most of the time. Case in point: filehandles have a "close on exec" flag, and it gets set en masse under the control of the $^F variable. What that means for you is that any filehandles you didn't explicitly route to the STDIN, STDOUT or STDERR of a child program get automatically closed. Iconic behaviour honestly. ✨
Always explicitly and immediately call close() on the writable end of any pipe — unless that process is literally writing to it. Even if you don't explicitly call close(), Perl will still close() all filehandles during global destruction. But as we discussed earlier, if those filehandles were opened with Safe Pipe Open, that's gonna result in calling waitpid(), which may again deadlock. 😭 So just close your pipes, bestie.
Bidirectional Communication with Another Process 💖
Okay so this all works reasonably well for unidirectional communication. But what about bidirectional communication? The most obvious approach does not work:
# THIS DOES NOT WORK!!
open(my $prog_for_reading_and_writing, "| some program |")And if you forget to use warnings, you'll miss out entirely on the super helpful diagnostic message:
Can't do bidirectional pipe at -e line 1.If you really want to do it anyway, you can use the standard open2() from the IPC::Open2 module to catch both ends. There's also an open3() in IPC::Open3 for tridirectional I/O so you can also catch your child's STDERR — but doing that would then require an awkward select() loop and wouldn't let you use normal Perl input operations. 🙈
If you peek at its source, you'll see that open2() uses low-level primitives like the pipe() and exec() syscalls to create all the connections. It might have been more efficient using socketpair(), but that would've been even less portable than it already is. The open2() and open3() functions are unlikely to work anywhere except on a Unix system, or at least one purporting POSIX compliance.
Here's an example of using open2():
use IPC::Open2;
my $pid = open2(my $reader, my $writer, "cat -un");
print $writer "stuff\n";
my $got = <$reader>;
waitpid $pid, 0;The problem with this is that buffering is really going to ruin your day. Even though your $writer filehandle is auto-flushed so the process on the other end gets your data in a timely manner, you can't usually do anything to force that process to give its data to you in a similarly quick fashion. In this special case we could actually do it, because we gave cat a -u flag to make it unbuffered. But very few 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 to use a library which uses pseudottys to make your program behave more reasonably. This way you don't have to have control over the source code of the program you're using. The "Expect" module from CPAN also addresses this kind of thing. That 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 talking to the terminal device 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 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. (The following example lacks proper error checking, jsyk. 🙈)
#!/usr/bin/perl
# pipe1 - bidirectional communication using two pipe pairs
# designed for the socketpair-challenged
use v5.36;
use IO::Handle; # enable autoflush method before Perl 5.14
pipe(my $parent_rdr, my $child_wtr); # XXX: check failure?
pipe(my $child_rdr, my $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(my $line = <$child_rdr>);
print "Parent Pid $$ just read this: '$line'\n";
close $child_rdr; close $child_wtr;
waitpid($pid, 0);
} else {
die "cannot fork: $!" unless defined $pid;
close $child_rdr;
close $child_wtr;
chomp(my $line = <$parent_rdr>);
print "Child Pid $$ just read this: '$line'\n";
print $parent_wtr "Child Pid $$ is sending this\n";
close $parent_rdr;
close $parent_wtr;
exit(0);
}But you don't actually have to make two pipe calls. If you have the socketpair() system call, it will do this all for you. ✨
#!/usr/bin/perl
# pipe2 - bidirectional communication using socketpair
# "the best ones always go both ways"
use v5.36;
use Socket;
use IO::Handle; # enable autoflush method before Perl 5.14
# We say AF_UNIX because although *_LOCAL is the
# POSIX 1003.1g form of the constant, many machines
# still don't have it.
socketpair(my $child, my $parent, AF_UNIX, SOCK_STREAM, PF_UNSPEC)
|| die "socketpair: $!";
$child->autoflush(1);
$parent->autoflush(1);
if ($pid = fork()) {
close $parent;
print $child "Parent Pid $$ is sending this\n";
chomp(my $line = <$child>);
print "Parent Pid $$ just read this: '$line'\n";
close $child;
waitpid($pid, 0);
} else {
die "cannot fork: $!" unless defined $pid;
close $child;
chomp(my $line = <$parent>);
print "Child Pid $$ just read this: '$line'\n";
print $parent "Child Pid $$ is sending this\n";
close $parent;
exit(0);
}Sockets: Client/Server Communication 👑
So this isn't entirely limited to Unix-derived operating systems — like, WinSock on PCs provides socket support, and some VMS libraries do too — but you might not have sockets on your system at all, in which case this section probably isn't going to do you much good. 😭 With sockets, you can do both virtual circuits like TCP streams and datagrams like UDP packets. You may be able to do even more depending on your system.
The Perl functions 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 ancient, antemillennial 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 the various constants and functions you'll need.
If you're not writing a server/client for an existing protocol like NNTP or SMTP, you should give some thought to how your server will know when the client has finished talking, and vice-versa. Most protocols are based on one-line messages and responses (so one party knows the other has finished when a "\n" is received) or multi-line messages and responses that end with a period on an empty line ("\n.\n" terminates a message/response).
Internet Line Terminators 🌸
The Internet line terminator is "\015\012". Under ASCII variants of Unix, that could usually be written as "\r\n", but under other systems, "\r\n" might at times be "\015\015\012", "\012\012\015", or something completely different. The standards specify writing "\015\012" to be conformant (be strict in what you provide), but they also recommend accepting a lone "\012" on input (be lenient in what you require). We haven't always been very good about that in the code in this manpage, ngl, 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-server communication that might extend to machines outside of your own system.
Here's a sample TCP client using Internet-domain sockets:
#!/usr/bin/perl
use v5.36;
use Socket;
my $remote = shift || "localhost";
my $port = shift || 2345; # random port
if ($port =~ /\D/) { $port = getservbyname($port, "tcp") }
die "No port" unless $port;
my $iaddr = inet_aton($remote) || die "no host: $remote";
my $paddr = sockaddr_in($port, $iaddr);
my $proto = getprotobyname("tcp");
socket(my $sock, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
connect($sock, $paddr) || die "connect: $!";
while (my $line = <$sock>) {
print $line;
}
close ($sock) || die "close: $!";
exit(0);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 to sit on a particular interface (like the external side of a gateway or firewall machine), fill this in with your real address instead.
#!/usr/bin/perl -T
use v5.36;
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 $port =~ /^ \d+ $/x;
my $proto = getprotobyname("tcp");
socket(my $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";
for (my $paddr; $paddr = accept(my $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 multitasking version. It's multitasked in that, like most typical servers, it spawns (fork()s) a child server to handle the client request so that the master server can quickly go back to service a new client. 🔥
#!/usr/bin/perl -T
use v5.36;
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 $port =~ /^ \d+ $/x;
my $proto = getprotobyname("tcp");
socket(my $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;
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) {
my $paddr = accept(my $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 $client, 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 $client = shift;
my $coderef = shift;
unless (@_ == 0 && $coderef && ref($coderef) eq "CODE") {
confess "usage: spawn CLIENT 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->());
}Okay bestie, so this server is doing the most — it 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. The REAPER subroutine is used here to call waitpid() for any child processes that have finished, thereby ensuring that they terminate cleanly and don't join the ranks of the living dead 🧟.
Within the while loop we call accept() and check to see if it returns a false value. This would normally indicate a system error needs to be reported. However, the introduction of safe signals (see "Deferred Signals (Safe Signals)" above) in Perl 5.8.0 means that accept() might also be interrupted when the process receives a signal 🌸. This typically happens when one of the forked subprocesses exits and notifies the parent process with a CHLD signal — classic group chat notification energy fr fr.
If accept() is interrupted by a signal, $! will be set to EINTR. If this happens, we can safely continue to the next iteration of the loop and another call to accept() 😌. It is important that your signal handling code not modify the value of $!, or else this test will likely fail. In the REAPER subroutine we create a local version of $! before calling waitpid(). When waitpid() sets $! to ECHILD as it 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 idea for servers or any program 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 🛡️. Note that perl can be built without taint support. There are two different modes: in one, -T will silently do nothing. In the other mode -T results in a fatal error.
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
use v5.36;
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);
$| = 1;
printf "%-24s %8s %s\n", "localhost", 0, ctime();
foreach my $host (@ARGV) {
printf "%-24s ", $host;
my $hisiaddr = inet_aton($host) || die "unknown host";
my $hispaddr = sockaddr_in($port, $hisiaddr);
socket(my $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 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, bestie:
% ls -l /dev/log
srw-rw-rw- 1 root 0 Oct 31 07:23 /dev/logYou 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 — it's giving tiny and perfect 💅:
#!/usr/bin/perl
use v5.36;
use Socket;
my $rendezvous = shift || "catsock";
socket(my $sock, PF_UNIX, SOCK_STREAM, 0) || die "socket: $!";
connect($sock, sockaddr_un($rendezvous)) || die "connect: $!";
while (defined(my $line = <$sock>)) {
print $line;
}
exit(0);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 -T
use v5.36;
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(my $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(my $client, $server) || $waitedpid;
$waitedpid = 0, close $client)
{
next if $waitedpid;
logmsg "connection on $NAME";
spawn $client, sub {
print "Hello there, it's now ", scalar localtime(), "\n";
exec("/usr/games/fortune") || die "can't exec fortune: $!";
};
}
sub spawn {
my $client = shift();
my $coderef = shift();
unless (@_ == 0 && $coderef && ref($coderef) eq "CODE") {
confess "usage: spawn CLIENT 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, so much 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 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 👑. Periodt.
For example, let's say that you have a long-running database server daemon that you want folks to be able to access from the Web, 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 💖. It's giving architecture. 🦋
🦋 TCP Clients with IO::Socket 🦋
okay bestie, if hand-rolling your own sockets with all that low-level bind/connect business is giving you a headache, pls breathe — the IO::Socket module is right there with a whole object-oriented glow-up for socket programming 💅. And if for some reason you don't have it? Literally just grab IO::Socket off CPAN, where you'll also find modules serving up easy interfaces to these systems: DNS, FTP, Ident (RFC 931), NIS and NISPlus, NNTP, Ping, POP3, SMTP, SNMP, SSLeay, Telnet, and Time — just to name a few. the CPAN girlies stay fed fr fr ✨
💖 A Simple Client 💖
here's a client that makes a TCP connection to the "daytime" service on port 13 of the host named "localhost", and then prints out every single thing the server over there feels like handing over. no cap, that's the whole job:
#!/usr/bin/perl
use v5.36;
use IO::Socket;
my $remote = IO::Socket::INET->new(
Proto => "tcp",
PeerAddr => "localhost",
PeerPort => "daytime(13)",
)
|| die "can't connect to daytime service on localhost";
while (<$remote>) { print }when you run this lil guy, what comes back should look like:
Wed May 14 08:40:46 MDT 1997so what are those parameters to the new() constructor even saying? lemme break it down 👇
"Proto"— this is which protocol we're using. in this case the socket handle you get back is connected to a TCP socket, because we want a stream-oriented connection, aka one that behaves pretty much exactly like a plain old file. but bestie, not every socket is this type! for example, the UDP protocol can be used to make a datagram socket, and those are for message-passing instead."PeerAddr"— the name or Internet address of the remote host the server is running on. we could've gone with a longer name like"www.perl.com", or an address like"207.171.7.72". for demonstration purposes we used the special hostname"localhost", which should always mean the current machine you're running on. the corresponding Internet address for localhost is"127.0.0.1", if you'd rather use that instead 👑."PeerPort"— the service name or port number we want to connect to. we could've gotten away with using just"daytime"on systems with a well-configured system services file,[FOOTNOTE: The system services file is found in/etc/servicesunder Unixy systems.] but here we specified the port number (13) in parentheses. using just the number would have worked too, but numeric literals make careful programmers nervous 🫶.
🌈 A Webget Client 🌈
next up: a simple client that takes a remote host to fetch a document from, then a list of files to get from that host. this one is way more interesting than the last because it sends something to the server before it fetches the server's response. main character behavior honestly:
#!/usr/bin/perl
use v5.36;
use IO::Socket;
unless (@ARGV > 1) { die "usage: $0 host url ..." }
my $host = shift(@ARGV);
my $EOL = "\015\012";
my $BLANK = $EOL x 2;
for my $document (@ARGV) {
my $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 chilling at its standard port, number 80. if the server you're trying to connect to lives at a different port, like 1080 or 8080, you should specify it as the named-parameter pair, PeerPort => 8080. the autoflush method gets used on the socket because otherwise the system would buffer up the output we sent it — and then you'd just be sitting there like 😭. (and if you're on a prehistoric Mac, you'll also need to change every "\n" in your code that sends data over the network into a "\015\012" instead. vintage struggles.)
connecting to the server is only the first part of the whole process: once you've got the connection, you have to speak the server's language. every server on the network has its own lil command language it expects as input. the string we send starting with "GET" is in HTTP syntax. in this case we're simply requesting each specified document. and yes — we really are making a new connection for each document, even though it's the exact same host. that's just 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. it's giving "we asked nicely, no promises" 💀.
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 anyway.
for a more featureful version of this program, you should look to the lwp-request program included with the LWP modules from CPAN. iconic, honestly.
🎀 Interactive Client with IO::Socket 🎀
well, that's all fine and good if you want to send one command and get one answer, but what about setting up something fully interactive, kinda like the way telnet works? so you can type a line, get the answer, type a line, get the answer, etc. the group-chat experience we all deserve ✨.
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 processes has one 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. doing the same thing with just one process would be much harder, because it's easier to code two processes to do one thing than it is to code one process to do two things. (this keep-it-simple principle is a cornerstone of the Unix philosophy, and good software engineering as well, which is probably why it's spread to other systems. periodt.)
here's the code:
#!/usr/bin/perl
use v5.36;
use IO::Socket;
unless (@ARGV == 2) { die "usage: $0 host port" }
my ($host, $port) = @ARGV;
# create a tcp connection to the specified host and port
my $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(my $kidpid = fork());
# the if{} block runs only in the parent process
if ($kidpid) {
# copy the socket to standard output
while (defined (my $line = <$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 (my $line = <STDIN>)) {
print $handle $line;
}
exit(0); # just in case
}the "kill" function in the parent's "if" block is there to send a signal to our child process — currently running in the "else" block — as soon as the remote server has closed its end of the connection. teamwork, babes 🫶.
and if the remote server sends data one byte at a time, and you need that data immediately without waiting for a newline (which might not even happen), you may wish to replace the "while" loop in the parent with the following:
my $byte;
while (sysread($handle, $byte, 1) == 1) {
print STDOUT $byte;
}making a system call for each byte you want to read is not very efficient (to put it mildly) 😭, but it is the simplest to explain and it works reasonably well. no cap.
🖥️ TCP Servers with IO::Socket 🖥️
okay so being the server is, as always, a lil more involved than being the client. the model is: your server creates a special kind of socket that does absolutely nothing except sit there and listen on one particular port for incoming connections. it does this by calling the IO::Socket::INET->new() method with slightly different arguments than the client did. let's go through them 💅
Proto
which protocol to use. like our clients, we're still specifying "tcp" here.
LocalPort
here's the difference, bestie — we specify a local port in the LocalPort argument, which we never did for the client. this is the service name or port number you wanna be the server for. (under Unix, ports under 1024 are restricted to the superuser, sorry.) in our sample we use port 9000, but you can use literally any port that isn't currently in use on your system. if you try one that's already taken you'll get an Address already in use message. under Unix, the netstat -a command will show you which services currently have servers. iykyk 🌸
Listen
the Listen parameter is set to the max number of pending connections we can accept before we start turning incoming clients away. think of it as call-waiting for your telephone. the low-level Socket module has a special symbol for the system maximum, and that's SOMAXCONN.
Reuse
the Reuse parameter is needed so we can restart our server manually without waiting a few minutes for the system buffers to clear out. iconic behavior honestly.
so once the generic server socket is built with all those parameters up there, the server then chills and waits for a new client to connect. it blocks in the accept method, which eventually accepts a bidirectional connection from the remote client. (make sure you autoflush that handle to circumvent buffering, fr.)
and to add to user-friendliness, our server prompts the user for commands. most servers literally never do this. and because of the prompt with no newline on the end, you'll have to use the sysread variant of the interactive client from earlier. 😭
this server accepts one of five different commands and sends output back to the client. and unlike most network servers, this one handles only one incoming client at a time. multitasking servers are covered in Chapter 16 of the Camel, if you're feeling ambitious.
here's the code:
#!/usr/bin/perl
use v5.36;
use IO::Socket;
use Net::hostent; # for OOish version of gethostbyaddr
my $PORT = 9000; # pick something not in use
my $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 (my $client = $server->accept()) {
$client->autoflush(1);
print $client "Welcome to $0; type help for command list.\n";
my $hostinfo = gethostbyaddr($client->peeraddr);
printf "[Connect from %s]\n",
$hostinfo ? $hostinfo->name : $client->peerhost;
print $client "Command? ";
while ( <$client>) {
next unless /\S/; # blank line
if (/quit|exit/i) { last }
elsif (/date|time/i) { printf $client "%s\n", scalar localtime() }
elsif (/who/i ) { print $client `who 2>&1` }
elsif (/cookie/i ) { print $client `/usr/games/fortune 2>&1` }
elsif (/motd/i ) { print $client `cat /etc/motd 2>&1` }
else {
print $client "Commands: quit date who cookie motd\n";
}
} continue {
print $client "Command? ";
}
close $client;
}📨 UDP: Message Passing 📨
another kind of client-server setup is one that uses not connections but messages. UDP comms involve way lower overhead but also give you less reliability, because there are literally no promises that messages arrive at all — let alone in order or unmangled. still, UDP has some advantages over TCP, including being able to broadcast or multicast to a whole bunch of destination hosts at once (usually on your local subnet). and if you catch yourself being overly concerned about reliability and start building checks into your message system, then honestly? you probably should just use TCP to start with. 🙈
UDP datagrams are not a bytestream and should not be treated as such. that makes using I/O mechanisms with internal buffering like stdio (i.e.print()and friends) especially cumbersome. usesyswrite(), or bettersend(), like in the example below.
here's a UDP program similar to the sample Internet TCP client from earlier. except instead of checking one host at a time, the UDP version checks many of them asynchronously by simulating a multicast and then using select() to do a timed-out wait for I/O. to do something similar with TCP you'd have to use a whole different socket handle for every single host. exhausting tbh.
#!/usr/bin/perl
use v5.36;
use Socket;
use Sys::Hostname;
my $SECS_OF_70_YEARS = 2_208_988_800;
my $iaddr = gethostbyname(hostname());
my $proto = getprotobyname("udp");
my $port = getservbyname("time", "udp");
my $paddr = sockaddr_in(0, $iaddr); # 0 means let kernel pick
socket(my $socket, PF_INET, SOCK_DGRAM, $proto) || die "socket: $!";
bind($socket, $paddr) || die "bind: $!";
$| = 1;
printf "%-12s %8s %s\n", "localhost", 0, scalar localtime();
my $count = 0;
for my $host (@ARGV) {
$count++;
my $hisiaddr = inet_aton($host) || die "unknown host";
my $hispaddr = sockaddr_in($port, $hisiaddr);
defined(send($socket, 0, 0, $hispaddr)) || die "send $host: $!";
}
my $rout = my $rin = "";
vec($rin, fileno($socket), 1) = 1;
# timeout after 10.0 seconds
while ($count && select($rout = $rin, undef, undef, 10.0)) {
my $rtime = "";
my $hispaddr = recv($socket, $rtime, 4, 0) || die "recv: $!";
my ($port, $hisiaddr) = sockaddr_in($hispaddr);
my $host = gethostbyaddr($hisiaddr, AF_INET);
my $histime = unpack("N", $rtime) - $SECS_OF_70_YEARS;
printf "%-12s ", $host;
printf "%8d %s\n", $histime - time(), scalar localtime($histime);
$count--;
}heads up though: this example includes no retries, so it may fail to contact a reachable host. the most prominent reason for that is congestion of the queues on the sending host, if the number of hosts to contact is sufficiently large. 💀
🧚 SysV IPC 🧚
so System V IPC isn't as widely used as sockets, but it still has some interesting uses. however — and this is important — you cannot use SysV IPC or Berkeley mmap() to have a variable shared amongst several processes. that's because Perl would reallocate your string when you weren't wanting it to. 😭 for that kind of thing you might look into the IPC::Shareable or threads::shared modules instead.
here's a small example showing shared memory usage:
use IPC::SysV qw(IPC_PRIVATE IPC_RMID S_IRUSR S_IWUSR);
my $size = 2000;
my $id = shmget(IPC_PRIVATE, $size, S_IRUSR | S_IWUSR);
defined($id) || die "shmget: $!";
print "shm key $id\n";
my $message = "Message #1";
shmwrite($id, $message, 0, 60) || die "shmwrite: $!";
print "wrote: '$message'\n";
shmread($id, my $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: $!";and here's an example of a semaphore:
use IPC::SysV qw(IPC_CREAT);
my $IPC_KEY = 1234;
my $id = semget($IPC_KEY, 10, 0666 | IPC_CREAT);
defined($id) || die "semget: $!";
print "sem id $id\n";put this code in a separate file to be run in more than one process. call the file take:
# create a semaphore
my $IPC_KEY = 1234;
my $id = semget($IPC_KEY, 0, 0);
defined($id) || die "semget: $!";
my $semnum = 0;
my $semflag = 0;
# "take" semaphore
# wait for semaphore to be zero
my $semop = 0;
my $opstring1 = pack("s!s!s!", $semnum, $semop, $semflag);
# Increment the semaphore count
$semop = 1;
my $opstring2 = pack("s!s!s!", $semnum, $semop, $semflag);
my $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
my $IPC_KEY = 1234;
my $id = semget($IPC_KEY, 0, 0);
die unless defined($id);
my $semnum = 0;
my $semflag = 0;
# Decrement the semaphore count
my $semop = -1;
my $opstring = pack("s!s!s!", $semnum, $semop, $semflag);
semop($id, $opstring) || die "semop: $!";ngl, the SysV IPC code above was written long ago, and it's definitely clunky looking. for a more modern look, see the IPC::SysV module. ✨
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 they fail, instead of making your program die right then and there from an uncaught exception. (well — actually, some of the new Socket conversion functions do croak() on bad arguments. so watch out for that one.) which means it is absolutely essential to check return values from these functions. always begin your socket programs this way for optimal success, and don't forget to add the -T taint-checking flag to the #! line for servers:
#!/usr/bin/perl -T
use v5.36;
use sigtrap;
use Socket;no cap, skipping those return-value checks is the #1 way to get a bug you'll be debugging at 3am. check them. periodt. 👑
💀 BUGS 💀
okay so real talk bestie, this is the part where we hold hands 🫶
Every single one of these routines creates system-specific portability problems. Like, all of them. Every last one. That's not a vibe, that's just how it is 😭
And as we said elsewhere, Perl is at the mercy of your C libraries for a huge chunk of its system behavior. Not a metaphor. Literally at their mercy. So the safest move is honestly to assume broken SysV semantics for signals and to just stick with simple TCP and UDP socket operations. Simple. TCP. UDP. That's the whole game plan.
e.g., don't try to pass open file descriptors over a local UDP datagram socket if you want your code to stand a chance of being portable.
Like... don't do that one, fr fr. That's the one where your code just doesn't travel. 🙈
👑 AUTHOR 👑
Tom Christiansen 💖
...with occasional vestiges of Larry Wall's original version and suggestions from the Perl Porters. The ancestors really said "let's build this together" and honestly? iconic behavior. 🎀
🌈 SEE ALSO 🌈
There's a lot more to networking than this, bestie, but this should at least get you started. Think of it as the little starter pack. The beginner's bundle. 🦋
For the intrepid programmers out there — the ones with main character energy — the indispensable textbook is Unix Network Programming, 2nd Edition, Volume 1 by W. Richard Stevens (published by Prentice-Hall). And heads up, most books on networking address the subject from the perspective of a C programmer; translation to Perl is left as an exercise for the reader. So like, you're gonna be doing some translating. You're built for it though 💅
- The
IO::Socket(3)manpage describes the object library. - The
Socket(3)manpage describes the low-level interface to sockets.
Besides the obvious functions in perlfunc, you should also check out the modules file at your nearest CPAN site, especially:
https://www.cpan.org/modules/00modlist.long.html#ID5_Networking_See perlmodlib or best yet, the Perl FAQ for a description of what CPAN even is and where to get it if that previous link doesn't work for you. (Links are flaky, it's not you, it's the internet 😭)
Section 5 of CPAN's modules file is devoted to "Networking, Device Control (modems), and Interprocess Communication", and contains numerous unbundled modules numerous networking modules, Chat and Expect operations, CGI programming, DCE, FTP, IPC, NNTP, Proxy, Ptty, RPC, SNMP, SMTP, Telnet, Threads, and ToolTalk—to name just a few.
okay pookie that's the whole page, you made it to the end and I'm so proud of you 🥹✨
This was perl v5.44.0, dated 2026-10-17, and it's the PERLIPC(1perl) page — the networking-and-IPC chapter of your Perl era. Go write something that talks to another machine. Slay. 👑💖🌸
📦 where this page comes from
- arch rolling: perl 5.44.0-1 · Artistic-1.0-Perl AND GPL-1.0-or-later
- 💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on : a language model's rewrite of the original, under its terms