PERLIPC(1) Perl Programmers Reference Guide PERLIPC(1)
NAME
perlipc - Perl interprocess communication (signals, fifos, pipes, safe subprocesses, sockets, and semaphores)
DESCRIPTION
The basic IPC facilities of Perl are built out of the good old Unix signals, named pipes, pipe opens, the
Berkeley socket routines, and SysV IPC calls. Each is used in slightly different situations.
Signals
Perl uses a simple signal handling model: the %SIG hash contains names or references of user-installed signal
handlers. These handlers will be called with an argument which is the name of the signal that triggered it.
A signal may be generated intentionally from a particular keyboard sequence like control-C or control-Z, sent
to you from another process, or triggered automatically by the kernel when special events transpire, like a
child process exiting, your own process running 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 possibly could in your handler; notice how all we
do is set a global variable and then raise an exception. That's because on most systems, libraries are not
re-entrant; particularly, memory allocation and I/O routines are not. That meant that doing nearly anything
in your handler could in theory trigger a memory fault and subsequent core dump - see "Deferred Signals (Safe
Signals)" below.
The names of the signals are the ones listed out by "kill -l" on your system, or you can retrieve them using
the CPAN module IPC::Signal.
You may also choose to assign the strings "IGNORE" or "DEFAULT" as the handler, in which case Perl will try to
discard the signal or do the default thing.
On most Unix platforms, the "CHLD" (sometimes also known as "CLD") signal has special behavior with respect to
a value of "IGNORE". Setting $SIG{CHLD} to "IGNORE" on such a platform has the effect of not creating zombie
processes when the parent process fails to "wait()" on its child processes (i.e., child processes are
automatically reaped). Calling "wait()" with $SIG{CHLD} set to "IGNORE" usually returns "-1" on such
platforms.
Some signals can be neither trapped nor ignored, such as the KILL and STOP (but not the TSTP) signals. Note
that ignoring signals makes them disappear. If you only want them blocked temporarily without them getting
lost you'll have to use POSIX' sigprocmask.
Sending a signal to a negative process ID means that you send the signal to the entire Unix process group.
This code sends a hang-up signal to all processes in the current process group, and also sets $SIG{HUP} to
"IGNORE" so it doesn't kill itself:
# block scope for local
{
local $SIG{HUP} = "IGNORE";
kill HUP => -$$;
process is alive. You may be able to determine the cause of failure using $! or "%!".
unless (kill(0 => $pid) || $!{EPERM}) {
warn "$pid looks dead";
}
You might also want to employ anonymous functions for simple signal handlers:
$SIG{INT} = sub { die "\nOutta here!\n" };
SIGCHLD handlers require some special care. If a second child dies while in the signal handler caused by the
first death, we won't get another signal. So must loop here else we will leave the unreaped child as a zombie.
And the next time two children die we get another zombie. And so on.
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 commands do a fork(), then wait() for the
result. Thus, your signal handler will be called. Because wait() was already called by system() or qx(), the
wait() in the signal handler will see no more zombies and will therefore block.
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 ($!, $?);
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);
# ...
}
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 technique is liable to generate zombies. If this
matters to you, you'll need to do your own fork() and exec(), and kill the errant child process.
For more complex signal handling, you might see the standard POSIX module. Lamentably, this is almost
entirely undocumented, but the t/lib/posix.t file from the Perl source distribution has some examples in it.
Handling the SIGHUP Signal in Daemons
A process that usually starts when the system boots and shuts down when the system is shut down is called a
daemon (Disk And Execution MONitor). If a daemon process has a configuration file which is modified after the
process has been started, there should be a way to tell that process to reread its configuration file without
stopping the process. Many daemons provide this mechanism using a "SIGHUP" signal 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 restarts itself every time the "SIGHUP" signal is
received. The actual code is located in the subroutine "code()", which just prints some debugging info 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";
}
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) out of the handler. Even this is a little cavalier for the true paranoiac, who
avoids "die" in a handler because the system is out to get you. The pragmatic approach was to say "I know the
risks, but prefer the convenience", and to do anything you wanted in your signal handler, and be prepared to
clean up core dumps now and again.
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 code that implements Perl) a flag is set, and the handler returns
immediately. Then at strategic "safe" points in the Perl interpreter (e.g. when it is about to execute a new
opcode) the flags are checked and the Perl level handler from %SIG is executed. The "deferred" scheme allows
much more flexibility in the coding of signal handlers as we know the Perl interpreter is in a safe state, and
that we are not in a system library function when the handler is called. However the implementation does
differ from previous Perls in the following ways:
Long-running opcodes
As the Perl interpreter looks at signal flags only when it is about to execute a new opcode, a signal that
arrives during a long-running opcode (e.g. a regular expression operation on a very large string) will not
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 will be called only once, after the opcode completes; all other instances will
be discarded. Furthermore, if your system's signal queue gets flooded to the point that there are signals
that have been raised but not yet caught (and thus not deferred) at the time an opcode completes, those
signals may well be caught and deferred during subsequent opcodes, with sometimes surprising results. For
example, you may see alarms delivered even after calling alarm(0) as the latter stops the raising of
alarms but does not cancel the delivery of alarms raised but not yet caught. Do not depend on the
behaviors described in this paragraph as they are side effects of the current implementation and may
change in future versions of Perl.
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 used to implement Perl's readline() function, the "<>" operator. On older Perls the
handler was called immediately (and as "read" is not "unsafe", this worked well). With the "deferred"
scheme the handler is not called immediately, and if Perl is using 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 solution 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.)
The default in Perl 5.7.3 and later is to automatically use the ":perlio" layer.
Note that 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, which bypasses Perl safe signals. Be warned that this does subject you to
possible memory corruption, as described above.
Instead of setting $SIG{ALRM}:
local $SIG{ALRM} = sub { die "alarm" };
arrived. In order to deliver deferred signals promptly, Perl 5.7.3 and later do not use SA_RESTART.
Consequently, restartable system calls can fail (with $! set to "EINTR") in places where they previously
would have succeeded.
The default ":perlio" layer retries "read", "write" and "close" as described above; interrupted "wait" and
"waitpid" calls will always be retried.
Signals as "faults"
Certain signals like SEGV, ILL, and BUS are generated by virtual memory addressing errors and similar
"faults". These are normally fatal: there is little a Perl-level handler can do with them. So Perl
delivers them immediately rather than attempting to defer them.
Signals triggered by operating system state
On some operating systems certain signal handlers are supposed to "do something" before returning. One
example can be CHLD or CLD, which indicates a child process has completed. On some operating systems the
signal handler is expected to "wait" for the completed child process. On such systems the deferred signal
scheme will not work for those signals: it does not do the "wait". Again the failure will look like a loop
as the operating system will reissue the signal because there are completed child processes that have not
yet been "wait"ed for.
If you want the old signal behavior back despite possible memory corruption, set the environment variable
"PERL_SIGNALS" to "unsafe". This feature first appeared in Perl 5.8.1.
Named Pipes
A named pipe (often referred to as a FIFO) is an old Unix IPC mechanism for processes communicating on the
same machine. It works just like regular anonymous pipes, except that the processes rendezvous using a
filename and need not be related.
To create a named pipe, use the "POSIX::mkfifo()" function.
use POSIX qw(mkfifo);
mkfifo($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.
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. 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
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 unidirectional interprocess communication by either
appending or prepending a pipe symbol to the second argument to open(). Here's how to start something up in a
child process you intend to write to:
open(SPOOLER, "| cat -v | lpr -h 2>/dev/null")
|| die "can't fork: $!";
local $SIG{PIPE} = sub { die "spooler pipe broke" };
print SPOOLER "stuff\n";
close SPOOLER || die "bad spool: $! $?";
And here's how to start up a child process you intend to read from:
open(STATUS, "netstat -an 2>&1 |")
|| die "can't fork: $!";
while (<STATUS>) {
next if /^(tcp|udp)/;
print;
}
close STATUS || die "bad netstat: $! $?";
If one can be sure that a particular program is a Perl script expecting filenames in @ARGV, the clever
programmer can write something like this:
% program f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile
and no matter which sort of shell it's called from, the Perl program will read from the file f1, the process
cmd1, standard input (tmpfile in this case), the f2 file, the cmd2 command, and finally the f3 file. Pretty
nifty, eh?
You might notice that you could use backticks for much the same effect as opening a pipe for reading:
print grep { !/^(tcp|udp)/ } `netstat -an 2>&1`;
die "bad netstatus ($?)" 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.
Be careful to check the return values from both open() and close(). If you're writing to a pipe, you should
also trap SIGPIPE. Otherwise, think of what happens when you start up a pipe to a command that doesn't exist:
the open() will in all likelihood succeed (it only reflects the fork()'s success), but then your output will
fail--spectacularly. Perl can't know whether the command worked, because your command is actually running in
a separate process whose exec() might have failed. Therefore, while readers of bogus commands return just a
quick EOF, writers to bogus commands will get hit with a signal, which they'd best be prepared to handle.
Consider:
open(FH, "|bogus") || die "can't fork: $!";
print FH "bang\n"; # neither necessary nor sufficient
Filehandles
Both the main process and any child processes it forks share the same STDIN, STDOUT, and STDERR filehandles.
If both processes try to access them at once, strange things can happen. You may also want to close or reopen
the filehandles for the child. You can get around this by opening your pipe with open(), but on some systems
this means that the child process cannot outlive the parent.
Background Processes
You can run a command in the background with:
system("cmd &");
The command's STDOUT and STDERR (and possibly STDIN, depending on your shell) will be the same as the
parent's. You won't need to catch SIGCHLD because of the double-fork taking place; see below for details.
Complete Dissociation of Child from Parent
In some cases (starting server processes, for instance) you'll want to completely dissociate the child process
from the parent. This is often called daemonization. A well-behaved daemon will also chdir() to the root
directory so it doesn't prevent unmounting the filesystem containing the directory from which it was launched,
and redirect its standard file descriptors from and to /dev/null so that random output doesn't wind up on the
user's terminal.
use POSIX "setsid";
sub daemonize {
chdir("/") || 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 a process group leader; the setsid() will fail
if you are. If your system doesn't have the setsid() function, open /dev/tty and use the "TIOCNOTTY" ioctl()
on it instead. See tty(4) for details.
Non-Unix users should check their "Your_OS::Process" module for other 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 open() function will accept a file argument of either "-|" or "|-" to do a very
interesting thing: it forks a child connected to the filehandle you've opened. The child is running the same
program as the parent. This is useful for safely opening a file when running under an assumed UID or GID, for
example. If you open a pipe to minus, you can write to the filehandle you opened and your kid will find it in
his STDIN. If you open a pipe from minus, you can read from the filehandle you opened whatever your kid
writes to his STDOUT.
use English qw[ -no_match_vars ];
my $PRECIOUS = "/path/to/some/safe/file";
my $sleep_count;
my $pid;
do {
# drop permissions in setuid and/or setgid programs:
($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 execute something without the shell's interference.
With system(), it's straightforward, but you can't use a pipe open or backticks safely. That's because
there's no way to stop the shell from getting its hands on your arguments. Instead, use lower-level control
to call exec() directly.
Here's a safe backtick or pipe open for read:
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(), 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
}
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 WRITER filehandle, so it closes it.
However, 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. If the child process ends up
waiting for something happening in the section marked "do something else", 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(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 interpret metacharacters that may be in your command string.
So for example, instead of using:
Because there are more than three arguments to open(), 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 "-|".
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:
@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 the pattern and indeed even the filenames
themselves might hold metacharacters.
Be aware that these operations are full Unix forks, which means they may not be correctly implemented on all
alien systems. Additionally, these are not true multithreading. To learn more about threading, see the
modules file mentioned below in the SEE ALSO section.
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. This is because any child process reading from the
pipe and expecting an EOF will never receive it, and therefore never exit. 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.
Certain built-in Unix features help prevent this most of the time. For instance, filehandles have a "close on
exec" flag, which is set en masse under control of the $^F variable. This is so any filehandles you didn't
explicitly route to the STDIN, STDOUT or STDERR of a child program will be automatically closed.
Always explicitly and immediately call close() on the writable end of any pipe, unless that process is
actually writing to it. Even if you don't explicitly call close(), Perl will still close() all filehandles
during global destruction. As previously discussed, if those filehandles have been 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, what about 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 the helpful diagnostic message:
Can't do bidirectional pipe at -e line 1.
If you really want to, 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 so would then require an awkward select() loop and wouldn't allow you to use normal Perl input
operations.
If you look at its source, you'll see that open2() uses low-level primitives like the pipe() and exec()
syscalls to create all the connections. Although it might have been more efficient by using socketpair(),
this would have been even less portable than it already is. The open2() and open3() functions are unlikely to
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 so, 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. This module requires two other modules from CPAN, "IO::Pty" and
"IO::Stty". It sets up a pseudo terminal to interact with programs that insist on 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.)
#!/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 you have the socketpair() system call, it will do this
all for you.
#!/usr/bin/perl -w
# pipe2 - bidirectional communication using socketpair
# "the best ones always go both ways"
use Socket;
use IO::Handle; # thousands of lines just for autoflush :-(
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., WinSock on PCs provides socket support, as
do some VMS libraries), you might not have sockets on your system, in which case this section probably isn't
going to do you much good. With sockets, you can do both virtual circuits 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, 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 -w
use strict;
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'll leave the address as "INADDR_ANY" so that the
kernel can choose the appropriate interface on multihomed hosts. If you want sit on a particular interface
(like the external side of a gateway or firewall machine), 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 that like most typical servers, it spawns (fork()s)
a slave server to handle the client request so that the master server can quickly go back to service a new
client.
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";
open(STDIN, "<&Client") || die "can't dup client to stdin";
open(STDOUT, ">&Client") || die "can't dup client to stdout";
## open(STDERR, ">&STDOUT") || die "can't dup stdout to stderr";
exit($coderef->());
}
This server takes the trouble to clone off a child version via fork() for each incoming request. That way it
can handle many requests at once, which you might not always want. Even if you don't fork(), the listen()
will allow that many pending connections. Forking servers have to be particularly careful about cleaning up
their dead children (called "zombies" in Unix parlance), because otherwise you'll quickly fill up your process
table. 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 5.7.3 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.
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.
Let's look at another TCP client. This one connects to the TCP "time" service on a number of different
machines and shows how far their clocks differ from the system on which it's being run:
#!/usr/bin/perl -w
use strict;
use Socket;
my $SECS_OF_70_YEARS = 2208988800;
sub ctime { scalar localtime(shift() || 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)
used internally to implement pipes. Unlike Internet domain sockets, Unix domain sockets can show up in the
file system with an ls(1) listing.
% ls -l /dev/log
srw-rw-rw- 1 root 0 Oct 31 07:23 /dev/log
You can test for these with Perl's -S file test:
unless (-S "/dev/log") {
die "something's wicked with the log system";
}
Here's a sample Unix-domain client:
#!/usr/bin/perl -w
use Socket;
use strict;
my ($rendezvous, $line);
$rendezvous = shift || "catsock";
socket(SOCK, PF_UNIX, SOCK_STREAM, 0) || die "socket: $!";
connect(SOCK, sockaddr_un($rendezvous)) || die "connect: $!";
while (defined($line = <SOCK>)) {
print $line;
}
exit(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 -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 {
$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, 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.
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.
TCP Clients with IO::Socket
For those preferring a higher-level interface to socket programming, the IO::Socket module provides an object-
oriented approach. IO::Socket has been included in the standard Perl distribution ever since Perl 5.004. If
you're running an earlier version of Perl (in which case, how are you reading this manpage?), just fetch
IO::Socket from CPAN, where you'll also find modules providing easy interfaces to the following systems: DNS,
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 that looks like this:
Wed May 14 08:40:46 MDT 1997
Here are what those parameters to the new() constructor mean:
"Proto"
This is which protocol to use. In this case, the socket handle returned will be connected to a TCP
socket, because we want a stream-oriented connection, that is, one that acts pretty much like a plain old
file. Not all sockets are this of this type. For example, the UDP protocol can be used to make a
datagram socket, used for message-passing.
"PeerAddr"
This is the name or Internet address of the remote host the server is running on. We could have specified
a longer name like "www.perl.com", or an address like "207.171.7.72". For demonstration purposes, we've
used the special hostname "localhost", which should always mean the current machine you're running on.
The corresponding Internet address for localhost is "127.0.0.1", 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 a well-configured system services file,[FOOTNOTE: The system services file
is found in /etc/services under Unixy systems.] but here we've specified the port number (13) in
parentheses. Using just the number would have also worked, but numeric literals make careful programmers
nervous.
Notice how the return value from the "new" constructor is used as a filehandle in the "while" loop? That's
what's called an indirect filehandle, a scalar variable containing a filehandle. You can use it the same way
you would a normal filehandle. For example, you can read one line from it this way:
$line = <$handle>;
all remaining lines from is this way:
@lines = <$handle>;
and send a line of data to it this way:
print $handle "some data\n";
A Webget Client
Here's a simple client that takes a remote host to fetch a document from, and then a list of files to get from
that host. This is a more interesting client than the previous one because it first sends something to the
server before fetching the server's response.
#!/usr/bin/perl -w
use IO::Socket;
unless (@ARGV > 1) { die "usage: $0 host url ..." }
$host = shift(@ARGV);
$EOL = "\015\012";
The web server handling the HTTP service is assumed to be at its standard port, number 80. If the server
you're trying to connect to is at a different port, like 1080 or 8080, you should specify it as the named-
parameter pair, "PeerPort => 8080". The "autoflush" method is used on the socket because otherwise the system
would buffer up the output we sent it. (If you're on a prehistoric Mac, you'll also need to change every "\n"
in your code that sends data over the network to be a "\015\012" instead.)
Connecting to the server is only the first part of the process: once you have the connection, you have to use
the server's language. Each server on the network has its own little command language that it expects as
input. The string that we send to the server starting with "GET" is in HTTP syntax. In this case, we simply
request each specified document. Yes, we really are making a new connection for each document, even though
it's the same host. That's the way you always used to have to speak HTTP. Recent versions of web browsers
may request that the remote server leave the connection open a little while, but the server doesn't have to
honor such a request.
Here's an example of running that program, which we'll call webget:
% webget www.perl.com /guanaco.html
HTTP/1.1 404 File Not Found
Date: Thu, 08 May 1997 18:02:32 GMT
Server: Apache/1.2b6
Connection: close
Content-type: text/html
<HEAD><TITLE>404 File Not Found</TITLE></HEAD>
<BODY><H1>File Not Found</H1>
The requested URL /guanaco.html was not found on this server.<P>
</BODY>
Ok, so that's not very interesting, because it didn't find that particular document. But a long response
wouldn't have fit on this page.
For a more featureful version of this program, you should look to the lwp-request program included with the
LWP modules from CPAN.
Interactive Client with IO::Socket
Well, that's all fine if you want to send one command and get one answer, but what about setting up something
fully interactive, somewhat like the way telnet works? That way you can type a line, get the answer, type a
line, get the answer, etc.
This client is more complicated than the two we've done so far, but if you're on a system that supports the
powerful "fork" call, the solution isn't that rough. Once you've made the connection to whatever service
you'd like to chat with, call "fork" to clone your process. Each of these two identical process has a very
simple job to do: the parent copies everything from the socket to standard output, while the child
simultaneously copies everything from standard input to the socket. To accomplish the same thing using just
one process would be much harder, because it's easier to code two processes to do one thing than it is to code
one process to do two things. (This keep-it-simple principle a cornerstones of the Unix philosophy, and good
software engineering as well, which is probably why it's spread to other systems.)
Here's the code:
#!/usr/bin/perl -w
use strict;
use IO::Socket;
my ($host, $port, $kidpid, $handle, $line);
# 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 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.
If the remote server sends data a byte at time, and you need that data immediately without waiting for a
newline (which might not happen), you may wish to replace the "while" loop in the parent with the following:
my $byte;
while (sysread($handle, $byte, 1) == 1) {
print STDOUT $byte;
}
Making a system call for each byte you want to read is not very efficient (to put it mildly) but is the
simplest to explain and works reasonably well.
TCP Servers with IO::Socket
As always, setting up a server is little bit more involved than running a client. The model is that the
server creates a special kind of socket that does nothing but listen on a particular port for incoming
connections. It does this by calling the "IO::Socket::INET->new()" method with slightly different arguments
than the client did.
Proto
This is which protocol to use. Like our clients, we'll still specify "tcp" here.
LocalPort
We specify a local port in the "LocalPort" argument, which we didn't do for the client. This is service
name or port number for which you want to be the server. (Under Unix, ports under 1024 are restricted to
the superuser.) In our sample, we'll use port 9000, but you can use any port that's not currently in use
on your system. If you try to use one already in used, you'll get an "Address already in use" message.
Under Unix, the "netstat -a" command will show which services current have servers.
Listen
The "Listen" parameter is set to the maximum number of pending 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
of the prompt without a newline, you'll have to use the "sysread" variant of the interactive client above.
This server accepts one of five different commands, sending output back to the client. Unlike most network
servers, this one handles only one incoming 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, but messages. UDP communications
involve much lower overhead but also provide less reliability, as there are no promises that messages will
arrive at all, let alone in order and unmangled. Still, UDP offers some advantages over TCP, including being
able to "broadcast" or "multicast" to a whole bunch of destination hosts at once (usually on your local
subnet). If you find yourself overly concerned about reliability and start building checks into your message
system, then you probably should use just TCP to start with.
UDP datagrams are not a bytestream and should not be treated as such. This makes using I/O mechanisms with
internal buffering like stdio (i.e. print() and friends) especially cumbersome. Use syswrite(), or better
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 to contact a reachable host. The most
prominent reason for this is congestion of the queues on the sending host if the number of hosts to contact is
sufficiently large.
SysV IPC
While System V IPC isn't so widely used as sockets, it still has some interesting uses. However, 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. You might look 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;
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);
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 causing your program to
die right then and there due to an uncaught exception. (Actually, some of the new Socket conversion functions
do croak() on bad arguments.) It is therefore essential to check return values from these functions. Always
begin your socket programs this way for optimal success, and don't forget to add the -T taint-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 noted elsewhere, Perl is at the mercy of
your C libraries for much of its system behavior. It's probably safest to assume broken SysV semantics for
signals and to stick with simple TCP and UDP socket operations; 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.
AUTHOR
Tom Christiansen, with occasional vestiges of Larry Wall's original version and suggestions from the Perl
Porters.
SEE ALSO
There's a lot more to networking than this, but this should get you started.
For intrepid programmers, the indispensable textbook is Unix Network Programming, 2nd Edition, Volume 1 by W.
Richard Stevens (published by Prentice-Hall). Most books on networking address the subject from the
perspective of a C programmer; translation to Perl is left as an exercise for the reader.
The IO::Socket(3) manpage describes the object library, and the Socket(3) manpage describes the low-level
interface to sockets. Besides the obvious functions in perlfunc, you should also check out the modules file
at your nearest CPAN site, especially <http://www.cpan.org/modules/00modlist.long.html#ID5_Networking_>. See
perlmodlib or best yet, the Perl FAQ for a description of what CPAN is and where to get it if the previous
link doesn't work for you.
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