=head1 NAME
-perlipc - Perl interprocess communication (signals, fifos, pipes, safe
-subprocceses, sockets, and semaphores)
+perlipc - Perl interprocess communication (signals, fifos, pipes, safe subprocesses, sockets, and semaphores)
=head1 DESCRIPTION
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 an another process, or
+control-C or control-Z, sent to you from another process, or
triggered automatically by the kernel when special events transpire, like
-a child process exiting, your process running out of stack space, or
+a child process exiting, your process running out of stack space, or
hitting file size limit.
For example, to trap an interrupt signal, set up a handler like this.
-Notice how all we do is set with a global variable and then raise an
-exception. That's because on most systems libraries are not
-re-entrant, so calling any print() functions (or even anything that needs to
-malloc(3) more memory) could in theory trigger a memory fault
-and subsequent core dump.
+Do as little as you possibly can 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 means that doing nearly I<anything> in your
+handler could in theory trigger a memory fault and subsequent core dump.
sub catch_zap {
my $signame = shift;
$shucks++;
die "Somebody sent me a SIG$signame";
- }
+ }
$SIG{INT} = 'catch_zap'; # could fail in modules
$SIG{INT} = \&catch_zap; # best strategy
$signo{$name} = $i;
$signame[$i] = $name;
$i++;
- }
+ }
-So to check whether signal 17 and SIGALRM were the same, just do this:
+So to check whether signal 17 and SIGALRM were the same, do just this:
print "signal #17 = $signame[17]\n";
- if ($signo{ALRM}) {
+ if ($signo{ALRM}) {
print "SIGALRM is $signo{ALRM}\n";
- }
+ }
You may also choose to assign the strings C<'IGNORE'> or C<'DEFAULT'> as
the handler, in which case Perl will try to discard the signal or do the
-default thing. Some signals can be neither trapped nor ignored, such as
+default thing.
+
+On most Unix platforms, the C<CHLD> (sometimes also known as C<CLD>) signal
+has special behavior with respect to a value of C<'IGNORE'>.
+Setting C<$SIG{CHLD}> to C<'IGNORE'> on such a platform has the effect of
+not creating zombie processes when the parent process fails to C<wait()>
+on its child processes (i.e. child processes are automatically reaped).
+Calling C<wait()> with C<$SIG{CHLD}> set to C<'IGNORE'> usually returns
+C<-1> on such platforms.
+
+Some signals can be neither trapped nor ignored, such as
the KILL and STOP (but not the TSTP) signals. One strategy for
temporarily ignoring signals is to use a local() statement, which will be
automatically restored once your block is exited. (Remember that local()
sub precious {
local $SIG{INT} = 'IGNORE';
&more_functions;
- }
+ }
sub more_functions {
# interrupts still ignored, for now...
- }
+ }
Sending a signal to a negative process ID means that you send the signal
-to the entire Unix process-group. This code send a hang-up signal to all
-processes in the current process group I<except for> the current process
-itself:
+to the entire Unix process-group. This code sends a hang-up signal to all
+processes in the current process group (and sets $SIG{HUP} to IGNORE so
+it doesn't kill itself):
{
local $SIG{HUP} = 'IGNORE';
Another interesting signal to send is signal number zero. This doesn't
actually affect another process, but instead checks whether it's alive
-or has changed its UID.
+or has changed its UID.
unless (kill 0 => $kid_pid) {
warn "something wicked happened to $kid_pid";
- }
+ }
You might also want to employ anonymous functions for simple signal
handlers:
$SIG{INT} = sub { die "\nOutta here!\n" };
But that will be problematic for the more complicated handlers that need
-to re-install themselves. Because Perl's signal mechanism is currently
-based on the signal(3) function from the C library, you may somtimes be so
+to reinstall themselves. Because Perl's signal mechanism is currently
+based on the signal(3) function from the C library, you may sometimes be so
misfortunate as to run on systems where that function is "broken", that
is, it behaves in the old unreliable SysV way rather than the newer, more
reasonable BSD and POSIX fashion. So you'll see defensive people writing
signal handlers like this:
- sub REAPER {
- $SIG{CHLD} = \&REAPER; # loathe sysV
+ sub REAPER {
$waitedpid = wait;
+ # loathe sysV: it makes us not only reinstate
+ # the handler, but place it after the wait
+ $SIG{CHLD} = \&REAPER;
}
$SIG{CHLD} = \&REAPER;
# now do something that forks...
-or even the more elaborate:
+or better still:
- use POSIX "wait_h";
- sub REAPER {
+ use POSIX ":sys_wait_h";
+ sub REAPER {
my $child;
- $SIG{CHLD} = \&REAPER; # loathe sysV
- while ($child = waitpid(-1,WNOHANG)) {
+ # If a second child dies while in the signal handler caused by the
+ # first death, we won't get another signal. So must loop here else
+ # we will leave the unreaped child as a zombie. And the next time
+ # two children die we get another zombie. And so on.
+ while (($child = waitpid(-1,WNOHANG)) > 0) {
$Kid_Status{$child} = $?;
- }
+ }
+ $SIG{CHLD} = \&REAPER; # still loathe sysV
}
$SIG{CHLD} = \&REAPER;
# do something that forks...
Here's an example:
- eval {
+ eval {
local $SIG{ALRM} = sub { die "alarm clock restart" };
- alarm 10;
+ alarm 10;
flock(FH, 2); # blocking write lock
- alarm 0;
+ alarm 0;
};
if ($@ and $@ !~ /alarm clock restart/) { die }
+If the operation being timed out is system() or qx(), this technique
+is liable to generate zombies. If this matters to you, you'll
+need to do your own fork() and exec(), and kill the errant child process.
+
For more complex signal handling, you might see the standard POSIX
module. Lamentably, this is almost entirely undocumented, but
the F<t/lib/posix.t> file from the Perl source distribution has some
A named pipe (often referred to as a FIFO) is an old Unix IPC
mechanism for processes communicating on the same machine. It works
-just like a regular, connected anonymous pipes, except that the
+just like a regular, connected anonymous pipes, except that the
processes rendezvous using a filename and don't have to be related.
To create a named pipe, use the Unix command mknod(1) or on some
# system return val is backwards, so && not ||
#
$ENV{PATH} .= ":/etc:/usr/etc";
- if ( system('mknod', $path, 'p')
+ if ( system('mknod', $path, 'p')
&& system('mkfifo', $path) )
{
- die "mk{nod,fifo} $path failed;
- }
+ 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.
+on the other end.
For example, let's say you'd like to have your F<.signature> file be a
named pipe that has a Perl program on the other end. Now every time any
-program (like a mailer, newsreader, finger program, etc.) tries to read
+program (like a mailer, news reader, finger program, etc.) tries to read
from that file, the reading program will block and your program will
-supply the the new signature. We'll use the pipe-checking file test B<-p>
+supply the new signature. We'll use the pipe-checking file test B<-p>
to find out whether anyone (or anything) has accidentally removed our fifo.
chdir; # go home
while (1) {
unless (-p $FIFO) {
unlink $FIFO;
- system('mknod', $FIFO, 'p')
+ system('mknod', $FIFO, 'p')
&& die "can't mknod $FIFO: $!";
- }
+ }
# next line blocks until there's a reader
open (FIFO, "> $FIFO") || die "can't write $FIFO: $!";
print FIFO "John Smith (smith\@host.org)\n", `fortune -s`;
close FIFO;
- sleep 2; # to avoid dup sigs
+ sleep 2; # to avoid dup signals
}
+=head2 WARNING
+
+By installing Perl code to deal with signals, you're exposing yourself
+to danger from two things. First, few system library functions are
+re-entrant. If the signal interrupts while Perl is executing one function
+(like malloc(3) or printf(3)), and your signal handler then calls the
+same function again, you could get unpredictable behavior--often, a
+core dump. Second, Perl isn't itself re-entrant at the lowest levels.
+If the signal interrupts Perl while Perl is changing its own internal
+data structures, similarly unpredictable behaviour may result.
+
+There are two things you can do, knowing this: be paranoid or be
+pragmatic. The paranoid approach is to do as little as possible in your
+signal handler. Set an existing integer variable that already has a
+value, and return. This doesn't help you if you're in a slow system call,
+which will just restart. That means you have to C<die> to longjump(3) out
+of the handler. Even this is a little cavalier for the true paranoiac,
+who avoids C<die> in a handler because the system I<is> out to get you.
+The pragmatic approach is to say ``I know the risks, but prefer the
+convenience'', and to do anything you want in your signal handler,
+prepared to clean up core dumps now and again.
+
+To forbid signal handlers altogether would bars you from
+many interesting programs, including virtually everything in this manpage,
+since you could no longer even write SIGCHLD handlers.
+
=head1 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 a child process you
+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")
+ 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";
while (<STATUS>) {
next if /^(tcp|udp)/;
print;
- }
- close SPOOLER || die "bad netstat: $! $?";
+ }
+ close STATUS || die "bad netstat: $! $?";
If one can be sure that a particular program is a Perl script that is
expecting filenames in @ARGV, the clever programmer can write something
like this:
- $ program f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile
+ % program f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile
and irrespective of which shell it's called from, the Perl program will
read from the file F<f1>, the process F<cmd1>, standard input (F<tmpfile>
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 thing into memory at once. It also gives you finer control of the
whole process, letting you to kill off the child process early if you'd
like.
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 just return a quick end of file, writers
+while readers of bogus commands return just a quick end of file, writers
to bogus command will trigger a signal they'd better be prepared to
handle. Consider:
- open(FH, "|bogus");
- print FH "bang\n";
- close FH;
+ open(FH, "|bogus") or die "can't fork: $!";
+ print FH "bang\n" or die "can't write: $!";
+ close FH or die "can't close: $!";
+
+That won't blow up until the close, and it will blow up with a SIGPIPE.
+To catch it, you could use this:
+
+ $SIG{PIPE} = 'IGNORE';
+ open(FH, "|bogus") or die "can't fork: $!";
+ print FH "bang\n" or die "can't write: $!";
+ close FH or die "can't close: status=$?";
+
+=head2 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.
+
+=head2 Background Processes
+
+You can run a command in the background with:
+
+ system("cmd &");
+
+The command's STDOUT and STDERR (and possibly STDIN, depending on your
+shell) will be the same as the parent's. You won't need to catch
+SIGCHLD because of the double-fork taking place (see below for more
+details).
+
+=head2 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 F</dev/null> (so that random
+output doesn't wind up on the user's terminal).
+
+ use POSIX 'setsid';
+
+ sub daemonize {
+ chdir '/' or die "Can't chdir to /: $!";
+ open STDIN, '/dev/null' or die "Can't read /dev/null: $!";
+ open STDOUT, '>/dev/null'
+ or die "Can't write to /dev/null: $!";
+ defined(my $pid = fork) or die "Can't fork: $!";
+ exit if $pid;
+ setsid or die "Can't start a new session: $!";
+ open STDERR, '>&STDOUT' or die "Can't dup stdout: $!";
+ }
+
+The fork() has to come before the setsid() to ensure that you aren't a
+process group leader (the setsid() will fail if you are). If your
+system doesn't have the setsid() function, open F</dev/tty> and use the
+C<TIOCNOTTY> ioctl() on it instead. See L<tty(4)> for details.
+
+Non-Unix users should check their Your_OS::Process module for other
+solutions.
=head2 Safe Pipe Opens
use English;
my $sleep_count = 0;
- do {
- $pid = open(KID, "-|");
+ do {
+ $pid = open(KID_TO_WRITE, "|-");
unless (defined $pid) {
warn "cannot fork: $!";
die "bailing out" if $sleep_count++ > 6;
sleep 10;
- }
+ }
} until defined $pid;
if ($pid) { # parent
- print KID @some_data;
- close(KID) || warn "kid exited $?";
+ print KID_TO_WRITE @some_data;
+ close(KID_TO_WRITE) || warn "kid exited $?";
} else { # child
($EUID, $EGID) = ($UID, $GID); # suid progs only
- open (FILE, "> /safe/file")
+ open (FILE, "> /safe/file")
|| die "can't open /safe/file: $!";
while (<STDIN>) {
print FILE; # child's STDIN is parent's KID
- }
+ }
exit; # don't forget this
- }
+ }
Another common use for this construct is when you need to execute
something without the shell's interference. With system(), it's
-straigh-forward, but you can't use a pipe open or backticks safely.
+straightforward, but you can't use a pipe open or backticks safely.
That's because there's no way to stop the shell from getting its hands on
your arguments. Instead, use lower-level control to call exec() directly.
Here's a safe backtick or pipe open for read:
# add error processing as above
- $pid = open(KID, "-|");
+ $pid = open(KID_TO_READ, "-|");
if ($pid) { # parent
- while (<KID>) {
+ while (<KID_TO_READ>) {
# do something interesting
- }
- close(KID) || warn "kid exited $?";
+ }
+ close(KID_TO_READ) || warn "kid exited $?";
} else { # child
($EUID, $EGID) = ($UID, $GID); # suid only
exec($program, @options, @args)
|| die "can't exec program: $!";
# NOTREACHED
- }
+ }
And here's a safe pipe open for writing:
# add error processing as above
- $pid = open(KID, "|-");
+ $pid = open(KID_TO_WRITE, "|-");
$SIG{ALRM} = sub { die "whoops, $program pipe broke" };
if ($pid) { # parent
for (@data) {
- print KID;
- }
- close(KID) || warn "kid exited $?";
+ print KID_TO_WRITE;
+ }
+ close(KID_TO_WRITE) || warn "kid exited $?";
} else { # child
($EUID, $EGID) = ($UID, $GID);
exec($program, @options, @args)
|| die "can't exec program: $!";
# NOTREACHED
- }
+ }
Note that these operations are full Unix forks, which means they may not be
correctly implemented on alien systems. Additionally, these are not true
multithreading. If you'd like to learn more about threading, see the
-F<modules> file mentioned below in the L<SEE ALSO> section.
+F<modules> file mentioned below in the SEE ALSO section.
-=head2 Bidirectional Communication
+=head2 Bidirectional Communication with Another Process
While this works reasonably well for unidirectional communication, what
about bidirectional communication? The obvious thing you'd like to do
doesn't actually work:
- open(KID, "| some program |")
+ open(PROG_FOR_READING_AND_WRITING, "| some program |")
-and if you forgot to use the B<-w> flag, then you'll miss out
-entirely on the diagnostic message:
+and if you forget to use the C<use warnings> pragma or the B<-w> flag,
+then you'll miss out entirely on the diagnostic message:
Can't do bidirectional pipe at -e line 1.
operations.
If you look at its source, you'll see that open2() uses low-level
-primitives like Unix pipe() and exec() to create all the connections.
+primitives like Unix pipe() and exec() calls to create all the connections.
While it might have been slightly more efficient by using socketpair(), it
would have then been even less portable than it already is. The open2()
and open3() functions are unlikely to work anywhere except on a Unix
use FileHandle;
use IPC::Open2;
- $pid = open2( \*Reader, \*Writer, "cat -u -n" );
- Writer->autoflush(); # default here, actually
+ $pid = open2(*Reader, *Writer, "cat -u -n" );
print Writer "stuff\n";
$got = <Reader>;
-The problem with this is that Unix buffering is going to really
-ruin your day. Even though your C<Writer> filehandle is autoflushed,
+The problem with this is that Unix buffering is really going to
+ruin your day. Even though your C<Writer> filehandle is auto-flushed,
and the process on the other end will get your data in a timely manner,
-you can't usually do anything to force it to actually give it back to you
-in a similarly quick fashion. In this case, we could, because we
+you can't usually do anything to force it to give it back to you
+in a similarly quick fashion. In this case, we could, because we
gave I<cat> a B<-u> flag to make it unbuffered. But very few Unix
commands are designed to operate over pipes, so this seldom works
-unless you yourself wrote the program on the other end of the
+unless you yourself wrote the program on the other end of the
double-ended pipe.
-A solution to this is the non-standard F<Comm.pl> library. It uses
+A solution to this is the nonstandard F<Comm.pl> library. It uses
pseudo-ttys to make your program behave more reasonably:
require 'Comm.pl';
}
This way you don't have to have control over the source code of the
-program you're using. The F<Comm> library also has expect()
-and interact() functions. Find the library (and hopefully its
+program you're using. The F<Comm> library also has expect()
+and interact() functions. Find the library (and we hope its
successor F<IPC::Chat>) at your nearest CPAN archive as detailed
-in the L<SEE ALSO> section below.
+in the SEE ALSO section below.
+
+The newer Expect.pm module from CPAN also addresses this kind of thing.
+This module requires two other modules from CPAN: IO::Pty and IO::Stty.
+It sets up a pseudo-terminal to interact with programs that insist on
+using talking to the terminal device driver. If your system is
+amongst those supported, this may be your best bet.
+
+=head2 Bidirectional Communication with Yourself
+
+If you want, you may make low-level pipe() and fork()
+to stitch this together by hand. This example only
+talks to itself, but you could reopen the appropriate
+handles to STDIN and STDOUT and call other processes.
+
+ #!/usr/bin/perl -w
+ # pipe1 - bidirectional communication using two pipe pairs
+ # designed for the socketpair-challenged
+ use IO::Handle; # thousands of lines just for autoflush :-(
+ pipe(PARENT_RDR, CHILD_WTR); # XXX: failure?
+ pipe(CHILD_RDR, PARENT_WTR); # XXX: failure?
+ CHILD_WTR->autoflush(1);
+ PARENT_WTR->autoflush(1);
+
+ if ($pid = fork) {
+ close PARENT_RDR; close PARENT_WTR;
+ print CHILD_WTR "Parent Pid $$ is sending this\n";
+ chomp($line = <CHILD_RDR>);
+ print "Parent Pid $$ just read this: `$line'\n";
+ close CHILD_RDR; close CHILD_WTR;
+ waitpid($pid,0);
+ } else {
+ die "cannot fork: $!" unless defined $pid;
+ close CHILD_RDR; close CHILD_WTR;
+ chomp($line = <PARENT_RDR>);
+ print "Child Pid $$ just read this: `$line'\n";
+ print PARENT_WTR "Child Pid $$ is sending this\n";
+ close PARENT_RDR; close PARENT_WTR;
+ exit;
+ }
+
+But you don't actually have to make two pipe calls. If you
+have the socketpair() system call, it will do this all for you.
+
+ #!/usr/bin/perl -w
+ # pipe2 - bidirectional communication using socketpair
+ # "the best ones always go both ways"
+
+ use Socket;
+ use IO::Handle; # thousands of lines just for autoflush :-(
+ # We say AF_UNIX because although *_LOCAL is the
+ # POSIX 1003.1g form of the constant, many machines
+ # still don't have it.
+ socketpair(CHILD, PARENT, AF_UNIX, SOCK_STREAM, PF_UNSPEC)
+ or die "socketpair: $!";
+
+ CHILD->autoflush(1);
+ PARENT->autoflush(1);
+
+ if ($pid = fork) {
+ close PARENT;
+ print CHILD "Parent Pid $$ is sending this\n";
+ chomp($line = <CHILD>);
+ print "Parent Pid $$ just read this: `$line'\n";
+ close CHILD;
+ waitpid($pid,0);
+ } else {
+ die "cannot fork: $!" unless defined $pid;
+ close CHILD;
+ chomp($line = <PARENT>);
+ print "Child Pid $$ just read this: `$line'\n";
+ print PARENT "Child Pid $$ is sending this\n";
+ close PARENT;
+ exit;
+ }
=head1 Sockets: Client/Server Communication
-While not limited to Unix-derived operating systems (e.g. WinSock on PCs
+While not limited to Unix-derived operating systems (e.g., WinSock on PCs
provides socket support, as do some VMS libraries), you may not have
-sockets on your system, in which this section probably isn't going to do
-you much good. With sockets, you can do both virtual circuits (i.e. TCP
-streams) and datagrams (i.e. UDP packets). You may be able to do even more
+sockets on your system, in which case this section probably isn't going to do
+you much good. With sockets, you can do both virtual circuits (i.e., TCP
+streams) and datagrams (i.e., UDP packets). You may be able to do even more
depending on your system.
The Perl function calls for dealing with sockets have the same names as
immeasurably superior approach is to use the C<Socket> module, which more
reliably grants access to various constants and functions you'll need.
+If you're not writing a server/client for an existing protocol like
+NNTP or SMTP, you should give some thought to how your server will
+know when the client has finished talking, and vice-versa. Most
+protocols are based on 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).
+
+=head2 Internet Line Terminators
+
+The Internet line terminator is "\015\012". Under ASCII variants of
+Unix, that could usually be written as "\r\n", but under other systems,
+"\r\n" might at times be "\015\015\012", "\012\012\015", or something
+completely different. The standards specify writing "\015\012" to be
+conformant (be strict in what you provide), but they also recommend
+accepting a lone "\012" on input (but be lenient in what you require).
+We haven't always been very good about that in the code in this manpage,
+but unless you're on a Mac, you'll probably be ok.
+
=head2 Internet TCP Clients and Servers
Use Internet-domain sockets when you want to do client-server
Here's a sample TCP client using Internet-domain sockets:
#!/usr/bin/perl -w
- require 5.002;
use strict;
use Socket;
my ($remote,$port, $iaddr, $paddr, $proto, $line);
$proto = getprotobyname('tcp');
socket(SOCK, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
connect(SOCK, $paddr) || die "connect: $!";
- while ($line = <SOCK>) {
+ while (defined($line = <SOCK>)) {
print $line;
- }
+ }
close (SOCK) || die "close: $!";
exit;
And here's a corresponding server to go along with it. We'll
leave the address as INADDR_ANY so that the kernel can choose
-the appropriate interface on multihomed hosts:
+the appropriate interface on multihomed hosts. If you want sit
+on a particular interface (like the external side of a gateway
+or firewall machine), you should fill this in with your real address
+instead.
+
+ #!/usr/bin/perl -Tw
+ use strict;
+ BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
+ use Socket;
+ use Carp;
+ my $EOL = "\015\012";
+
+ sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
+
+ my $port = shift || 2345;
+ my $proto = getprotobyname('tcp');
+
+ ($port) = $port =~ /^(\d+)$/ or die "invalid port";
+
+ socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
+ setsockopt(Server, SOL_SOCKET, SO_REUSEADDR,
+ pack("l", 1)) || die "setsockopt: $!";
+ bind(Server, sockaddr_in($port, INADDR_ANY)) || die "bind: $!";
+ listen(Server,SOMAXCONN) || die "listen: $!";
+
+ logmsg "server started on port $port";
+
+ my $paddr;
+
+ $SIG{CHLD} = \&REAPER;
+
+ for ( ; $paddr = accept(Client,Server); close Client) {
+ my($port,$iaddr) = sockaddr_in($paddr);
+ my $name = gethostbyaddr($iaddr,AF_INET);
+
+ logmsg "connection from $name [",
+ inet_ntoa($iaddr), "]
+ at port $port";
+
+ print Client "Hello there, $name, it's now ",
+ scalar localtime, $EOL;
+ }
+
+And here's a multithreaded version. It's multithreaded in that
+like most typical servers, it spawns (forks) a slave server to
+handle the client request so that the master server can quickly
+go back to service a new client.
#!/usr/bin/perl -Tw
- require 5.002;
use strict;
BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
use Socket;
use Carp;
+ my $EOL = "\015\012";
sub spawn; # forward declaration
- sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
+ sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
my $port = shift || 2345;
my $proto = getprotobyname('tcp');
- socket(SERVER, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
- setsockopt(SERVER, SOL_SOCKET, SO_REUSEADDR, 1) || die "setsockopt: $!";
- bind(SERVER, sockaddr_in($port, INADDR_ANY)) || die "bind: $!";
- listen(SERVER,5) || die "listen: $!";
+
+ ($port) = $port =~ /^(\d+)$/ or die "invalid port";
+
+ socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
+ setsockopt(Server, SOL_SOCKET, SO_REUSEADDR,
+ pack("l", 1)) || die "setsockopt: $!";
+ bind(Server, sockaddr_in($port, INADDR_ANY)) || die "bind: $!";
+ listen(Server,SOMAXCONN) || die "listen: $!";
logmsg "server started on port $port";
my $waitedpid = 0;
my $paddr;
- sub REAPER {
+ 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
- $waitedpid = wait;
- logmsg "reaped $waitedpid" . ($? ? " with exit $?" : '');
}
$SIG{CHLD} = \&REAPER;
- for ( $waitedpid = 0;
- ($paddr = accept(CLIENT,SERVER)) || $waitedpid;
- $waitedpid = 0, close CLIENT)
+ for ( $waitedpid = 0;
+ ($paddr = accept(Client,Server)) || $waitedpid;
+ $waitedpid = 0, close Client)
{
- next if $waitedpid;
+ next if $waitedpid and not $paddr;
my($port,$iaddr) = sockaddr_in($paddr);
my $name = gethostbyaddr($iaddr,AF_INET);
- logmsg "connection from $name [",
- inet_ntoa($iaddr), "]
+ logmsg "connection from $name [",
+ inet_ntoa($iaddr), "]
at port $port";
- spawn sub {
- print "Hello there, $name, it's now ", scalar localtime, "\n";
- exec '/usr/games/fortune'
+ spawn sub {
+ $|=1;
+ print "Hello there, $name, it's now ", scalar localtime, $EOL;
+ exec '/usr/games/fortune' # XXX: `wrong' line terminators
or confess "can't exec fortune: $!";
};
- }
+ }
sub spawn {
my $coderef = shift;
- unless (@_ == 0 && $coderef && ref($coderef) eq 'CODE') {
+ unless (@_ == 0 && $coderef && ref($coderef) eq 'CODE') {
confess "usage: spawn CODEREF";
}
return;
} elsif ($pid) {
logmsg "begat $pid";
- return; # i'm the parent
+ return; # I'm the parent
}
- # else i'm the child -- go spawn
+ # 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(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,
differ from the system on which it's being run:
#!/usr/bin/perl -w
- require 5.002;
use strict;
use Socket;
my $SECS_of_70_YEARS = 2208988800;
- sub ctime { scalar localtime(shift) }
+ sub ctime { scalar localtime(shift) }
- my $iaddr = gethostbyname('localhost');
- my $proto = getprotobyname('tcp');
- my $port = getservbyname('time', 'tcp');
+ my $iaddr = gethostbyname('localhost');
+ my $proto = getprotobyname('tcp');
+ my $port = getservbyname('time', 'tcp');
my $paddr = sockaddr_in(0, $iaddr);
my($host);
=head2 Unix-Domain TCP Clients and Servers
-That's fine for Internet-domain clients and servers, but what local
+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
+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
+ % ls -l /dev/log
srw-rw-rw- 1 root 0 Oct 31 07:23 /dev/log
You can test for these with Perl's B<-S> file test:
unless ( -S '/dev/log' ) {
- die "something's wicked with the print system";
- }
+ die "something's wicked with the log system";
+ }
Here's a sample Unix-domain client:
#!/usr/bin/perl -w
- require 5.002;
use Socket;
use strict;
my ($rendezvous, $line);
$rendezvous = shift || '/tmp/catsock';
socket(SOCK, PF_UNIX, SOCK_STREAM, 0) || die "socket: $!";
- connect(SOCK, sockaddr_un($remote)) || die "connect: $!";
- while ($line = <SOCK>) {
+ connect(SOCK, sockaddr_un($rendezvous)) || die "connect: $!";
+ while (defined($line = <SOCK>)) {
print $line;
- }
+ }
exit;
-And here's a corresponding server.
+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
- require 5.002;
use strict;
use Socket;
use Carp;
BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
+ sub spawn; # forward declaration
+ sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
my $NAME = '/tmp/catsock';
my $uaddr = sockaddr_un($NAME);
my $proto = getprotobyname('tcp');
- socket(SERVER,PF_UNIX,SOCK_STREAM,0) || die "socket: $!";
+ socket(Server,PF_UNIX,SOCK_STREAM,0) || die "socket: $!";
unlink($NAME);
- bind (SERVER, $uaddr) || die "bind: $!";
- listen(SERVER,5) || die "listen: $!";
+ bind (Server, $uaddr) || die "bind: $!";
+ listen(Server,SOMAXCONN) || die "listen: $!";
logmsg "server started on $NAME";
+ my $waitedpid;
+
+ use POSIX ":sys_wait_h";
+ sub REAPER {
+ my $child;
+ while (($waitedpid = waitpid(-1,WNOHANG)) > 0) {
+ logmsg "reaped $waitedpid" . ($? ? " with exit $?" : '');
+ }
+ $SIG{CHLD} = \&REAPER; # loathe sysV
+ }
+
$SIG{CHLD} = \&REAPER;
- for ( $waitedpid = 0;
- accept(CLIENT,SERVER) || $waitedpid;
- $waitedpid = 0, close CLIENT)
+
+ for ( $waitedpid = 0;
+ accept(Client,Server) || $waitedpid;
+ $waitedpid = 0, close Client)
{
next if $waitedpid;
logmsg "connection on $NAME";
- spawn sub {
+ spawn sub {
print "Hello there, it's now ", scalar localtime, "\n";
exec '/usr/games/fortune' or die "can't exec fortune: $!";
};
- }
+ }
+
+ sub spawn {
+ my $coderef = shift;
+
+ unless (@_ == 0 && $coderef && ref($coderef) eq 'CODE') {
+ confess "usage: spawn CODEREF";
+ }
+
+ my $pid;
+ if (!defined($pid = fork)) {
+ logmsg "cannot fork: $!";
+ return;
+ } elsif ($pid) {
+ logmsg "begat $pid";
+ return; # I'm the parent
+ }
+ # else I'm the child -- go spawn
+
+ open(STDIN, "<&Client") || die "can't dup client to stdin";
+ open(STDOUT, ">&Client") || die "can't dup client to stdout";
+ ## open(STDERR, ">&STDOUT") || die "can't dup stdout to stderr";
+ exit &$coderef();
+ }
As you see, it's remarkably similar to the Internet domain TCP server, so
much so, in fact, that we've omitted several duplicate functions--spawn(),
program that does whatever checks and logging you feel like, and then acts
as a Unix-domain client and connects to your private server.
-=head2 UDP: Message Passing
+=head1 TCP Clients with IO::Socket
+
+For those preferring a higher-level interface to socket programming, the
+IO::Socket module provides an object-oriented approach. IO::Socket is
+included as part of the standard Perl distribution as of the 5.004
+release. If you're running an earlier version of Perl, just fetch
+IO::Socket from CPAN, where you'll also find modules providing easy
+interfaces to the following systems: DNS, FTP, Ident (RFC 931), NIS and
+NISPlus, NNTP, Ping, POP3, SMTP, SNMP, SSLeay, Telnet, and Time--just
+to name a few.
+
+=head2 A Simple Client
+
+Here's a client that creates a TCP connection to the "daytime"
+service at port 13 of the host name "localhost" and prints out everything
+that the server there cares to provide.
+
+ #!/usr/bin/perl -w
+ use IO::Socket;
+ $remote = IO::Socket::INET->new(
+ Proto => "tcp",
+ PeerAddr => "localhost",
+ PeerPort => "daytime(13)",
+ )
+ or die "cannot connect to daytime port at localhost";
+ while ( <$remote> ) { print }
+
+When you run this program, you should get something back that
+looks like this:
+
+ Wed May 14 08:40:46 MDT 1997
+
+Here are what those parameters to the C<new> constructor mean:
+
+=over 4
+
+=item C<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.
+
+=item C<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 C<"www.perl.com">,
+or an address like C<"204.148.40.9">. For demonstration purposes, we've
+used the special hostname C<"localhost">, which should always mean the
+current machine you're running on. The corresponding Internet address
+for localhost is C<"127.1">, if you'd rather use that.
+
+=item C<PeerPort>
+
+This is the service name or port number we'd like to connect to.
+We could have gotten away with using just C<"daytime"> on systems with a
+well-configured system services file,[FOOTNOTE: The system services file
+is in I</etc/services> under Unix] but just in case, we've specified the
+port number (13) in parentheses. Using just the number would also have
+worked, but constant numbers make careful programmers nervous.
+
+=back
+
+Notice how the return value from the C<new> constructor is used as
+a filehandle in the C<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";
+
+=head2 A Webget Client
+
+Here's a simple client that takes a remote host to fetch a document
+from, and then a list of documents to get from that host. This is a
+more interesting client than the previous one because it first sends
+something to the server before fetching the server's response.
+
+ #!/usr/bin/perl -w
+ use IO::Socket;
+ unless (@ARGV > 1) { die "usage: $0 host document ..." }
+ $host = shift(@ARGV);
+ $EOL = "\015\012";
+ $BLANK = $EOL x 2;
+ foreach $document ( @ARGV ) {
+ $remote = IO::Socket::INET->new( Proto => "tcp",
+ PeerAddr => $host,
+ PeerPort => "http(80)",
+ );
+ unless ($remote) { die "cannot connect to http daemon on $host" }
+ $remote->autoflush(1);
+ print $remote "GET $document HTTP/1.0" . $BLANK;
+ while ( <$remote> ) { print }
+ close $remote;
+ }
+
+The web server handing the "http" service, which is assumed to be at
+its standard port, number 80. If the web server you're trying to
+connect to is at a different port (like 1080 or 8080), you should specify
+as the named-parameter pair, C<< PeerPort => 8080 >>. The C<autoflush>
+method is used on the socket because otherwise the system would buffer
+up the output we sent it. (If you're on a Mac, you'll also need to
+change every C<"\n"> in your code that sends data over the network to
+be a C<"\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 I<webget>:
+
+ % webget www.perl.com /guanaco.html
+ HTTP/1.1 404 File Not Found
+ Date: Thu, 08 May 1997 18:02:32 GMT
+ Server: Apache/1.2b6
+ Connection: close
+ Content-type: text/html
+
+ <HEAD><TITLE>404 File Not Found</TITLE></HEAD>
+ <BODY><H1>File Not Found</H1>
+ The requested URL /guanaco.html was not found on this server.<P>
+ </BODY>
+
+Ok, so that's not very interesting, because it didn't find that
+particular document. But a long response wouldn't have fit on this page.
+
+For a more fully-featured version of this program, you should look to
+the I<lwp-request> program included with the LWP modules from CPAN.
+
+=head2 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 I<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 C<fork> call, the solution
+isn't that rough. Once you've made the connection to whatever service
+you'd like to chat with, call C<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 I<much>
+harder, because it's easier to code two processes to do one thing than it
+is to code one process to do two things. (This keep-it-simple principle
+a cornerstones of the Unix philosophy, and good software engineering as
+well, which is probably why it's spread to other systems.)
+
+Here's the code:
+
+ #!/usr/bin/perl -w
+ use strict;
+ use IO::Socket;
+ my ($host, $port, $kidpid, $handle, $line);
+
+ unless (@ARGV == 2) { die "usage: $0 host port" }
+ ($host, $port) = @ARGV;
+
+ # create a tcp connection to the specified host and port
+ $handle = IO::Socket::INET->new(Proto => "tcp",
+ PeerAddr => $host,
+ PeerPort => $port)
+ or die "can't connect to port $port on $host: $!";
+
+ $handle->autoflush(1); # so output gets there right away
+ print STDERR "[Connected to $host:$port]\n";
+
+ # split the program into two processes, identical twins
+ die "can't fork: $!" unless defined($kidpid = fork());
+
+ # the if{} block runs only in the parent process
+ if ($kidpid) {
+ # copy the socket to standard output
+ while (defined ($line = <$handle>)) {
+ print STDOUT $line;
+ }
+ kill("TERM", $kidpid); # send SIGTERM to child
+ }
+ # the else{} block runs only in the child process
+ else {
+ # copy standard input to the socket
+ while (defined ($line = <STDIN>)) {
+ print $handle $line;
+ }
+ }
+
+The C<kill> function in the parent's C<if> block is there to send a
+signal to our child process (current running in the C<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 C<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.
+
+=head1 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 C<< IO::Socket::INET->new() >> method with
+slightly different arguments than the client did.
+
+=over 4
+
+=item Proto
+
+This is which protocol to use. Like our clients, we'll
+still specify C<"tcp"> here.
+
+=item LocalPort
+
+We specify a local
+port in the C<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 C<netstat -a> command will show
+which services current have servers.
+
+=item Listen
+
+The C<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 module has a special symbol for the system maximum, which
+is SOMAXCONN.
+
+=item Reuse
+
+The C<Reuse> parameter is needed so that we restart our server
+manually without waiting a few minutes to allow system buffers to
+clear out.
+
+=back
+
+Once the generic server socket has been created using the parameters
+listed above, the server then waits for a new client to connect
+to it. The server blocks in the C<accept> method, which eventually accepts a
+bidirectional connection from the remote client. (Make sure to autoflush
+this handle to circumvent buffering.)
+
+To add to user-friendliness, our server prompts the user for commands.
+Most servers don't do this. Because of the prompt without a newline,
+you'll have to use the C<sysread> variant of the interactive client above.
+
+This server accepts one of five different commands, sending output
+back to the client. Note that unlike most network servers, this one
+only handles one incoming client at a time. Multithreaded servers are
+covered in Chapter 6 of the Camel.
+
+Here's the code. We'll
+
+ #!/usr/bin/perl -w
+ use IO::Socket;
+ use Net::hostent; # for OO version of gethostbyaddr
+
+ $PORT = 9000; # pick something not in use
+
+ $server = IO::Socket::INET->new( Proto => 'tcp',
+ LocalPort => $PORT,
+ Listen => SOMAXCONN,
+ Reuse => 1);
+
+ die "can't setup server" unless $server;
+ print "[Server $0 accepting clients]\n";
+
+ while ($client = $server->accept()) {
+ $client->autoflush(1);
+ print $client "Welcome to $0; type help for command list.\n";
+ $hostinfo = gethostbyaddr($client->peeraddr);
+ printf "[Connect from %s]\n", $hostinfo->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;
+ }
+
+=head1 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
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 just use TCP to start
+into your message system, then you probably should use just TCP to start
with.
+Note that UDP datagrams are I<not> a bytestream and should not be treated
+as such. This makes using I/O mechanisms with internal buffering
+like stdio (i.e. print() and friends) especially cumbersome. Use syswrite(),
+or better send(), like in the example below.
+
Here's a UDP program similar to the sample Internet TCP client given
-above. However, instead of checking one host at a time, the UDP version
+earlier. However, instead of checking one host at a time, the UDP version
will check many of them asynchronously by simulating a multicast and then
using select() to do a timed-out wait for I/O. To do something similar
with TCP, you'd have to use a different socket handle for each host.
#!/usr/bin/perl -w
use strict;
- require 5.002;
use Socket;
use Sys::Hostname;
- my ( $count, $hisiaddr, $hispaddr, $histime,
- $host, $iaddr, $paddr, $port, $proto,
+ my ( $count, $hisiaddr, $hispaddr, $histime,
+ $host, $iaddr, $paddr, $port, $proto,
$rin, $rout, $rtime, $SECS_of_70_YEARS);
$SECS_of_70_YEARS = 2208988800;
$count--;
}
+Note that 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
+list of hosts to contact is sufficiently large.
+
=head1 SysV IPC
While System V IPC isn't so widely used as sockets, it still has some
several processes. That's because Perl would reallocate your string when
you weren't wanting it to.
+Here's a small example showing shared memory usage.
-Here's a small example showing shared memory usage.
+ use IPC::SysV qw(IPC_PRIVATE IPC_RMID S_IRWXU);
- $IPC_PRIVATE = 0;
- $IPC_RMID = 0;
$size = 2000;
- $key = shmget($IPC_PRIVATE, $size , 0777 );
- die unless defined $key;
+ $id = shmget(IPC_PRIVATE, $size, S_IRWXU) || die "$!";
+ print "shm key $id\n";
$message = "Message #1";
- shmwrite($key, $message, 0, 60 ) || die "$!";
- shmread($key,$buff,0,60) || die "$!";
+ shmwrite($id, $message, 0, 60) || die "$!";
+ print "wrote: '$message'\n";
+ shmread($id, $buff, 0, 60) || die "$!";
+ print "read : '$buff'\n";
- print $buff,"\n";
+ # the buffer of shmread is zero-character end-padded.
+ substr($buff, index($buff, "\0")) = '';
+ print "un" unless $buff eq $message;
+ print "swell\n";
- print "deleting $key\n";
- shmctl($key ,$IPC_RMID, 0) || die "$!";
+ print "deleting shm $id\n";
+ shmctl($id, IPC_RMID, 0) || die "$!";
Here's an example of a semaphore:
+ use IPC::SysV qw(IPC_CREAT);
+
$IPC_KEY = 1234;
- $IPC_RMID = 0;
- $IPC_CREATE = 0001000;
- $key = semget($IPC_KEY, $nsems , 0666 | $IPC_CREATE );
- die if !defined($key);
- print "$key\n";
+ $id = semget($IPC_KEY, 10, 0666 | IPC_CREAT ) || die "$!";
+ print "shm key $id\n";
-Put this code in a separate file to be run in more that one process
+Put this code in a separate file to be run in more than one process.
Call the file F<take>:
# create a semaphore
$IPC_KEY = 1234;
- $key = semget($IPC_KEY, 0 , 0 );
- die if !defined($key);
+ $id = semget($IPC_KEY, 0 , 0 );
+ die if !defined($id);
$semnum = 0;
$semflag = 0;
# 'take' semaphore
# wait for semaphore to be zero
$semop = 0;
- $opstring1 = pack("sss", $semnum, $semop, $semflag);
+ $opstring1 = pack("s!s!s!", $semnum, $semop, $semflag);
# Increment the semaphore count
$semop = 1;
- $opstring2 = pack("sss", $semnum, $semop, $semflag);
+ $opstring2 = pack("s!s!s!", $semnum, $semop, $semflag);
$opstring = $opstring1 . $opstring2;
- semop($key,$opstring) || die "$!";
+ semop($id,$opstring) || die "$!";
-Put this code in a separate file to be run in more that one process
+Put this code in a separate file to be run in more than one process.
Call this file F<give>:
# 'give' the semaphore
# that the second process continues
$IPC_KEY = 1234;
- $key = semget($IPC_KEY, 0, 0);
- die if !defined($key);
+ $id = semget($IPC_KEY, 0, 0);
+ die if !defined($id);
$semnum = 0;
$semflag = 0;
# Decrement the semaphore count
$semop = -1;
- $opstring = pack("sss", $semnum, $semop, $semflag);
-
- semop($key,$opstring) || die "$!";
-
-=head1 WARNING
-
-The SysV IPC code above was written long ago, and it's definitely clunky
-looking. It should at the very least be made to C<use strict> and
-C<require "sys/ipc.ph">. Better yet, perhaps someone should create an
-C<IPC::SysV> module the way we have the C<Socket> module for normal
-client-server communications.
-
-(... time passes)
-
-Voila! Check out the IPC::SysV modules written by Jack Shirazi. You can
-find them at a CPAN store near you.
+ $opstring = pack("s!s!s!", $semnum, $semop, $semflag);
+
+ semop($id,$opstring) || die "$!";
+
+The SysV IPC code above was written long ago, and it's definitely
+clunky looking. For a more modern look, see the IPC::SysV module
+which is included with Perl starting from Perl 5.005.
+
+A small example demonstrating SysV message queues:
+
+ use IPC::SysV qw(IPC_PRIVATE IPC_RMID IPC_CREAT S_IRWXU);
+
+ my $id = msgget(IPC_PRIVATE, IPC_CREAT | S_IRWXU);
+
+ my $sent = "message";
+ my $type = 1234;
+ my $rcvd;
+ my $type_rcvd;
+
+ if (defined $id) {
+ if (msgsnd($id, pack("l! a*", $type_sent, $sent), 0)) {
+ if (msgrcv($id, $rcvd, 60, 0, 0)) {
+ ($type_rcvd, $rcvd) = unpack("l! a*", $rcvd);
+ if ($rcvd eq $sent) {
+ print "okay\n";
+ } else {
+ print "not okay\n";
+ }
+ } else {
+ die "# msgrcv failed\n";
+ }
+ } else {
+ die "# msgsnd failed\n";
+ }
+ msgctl($id, IPC_RMID, 0) || die "# msgctl failed: $!\n";
+ } else {
+ die "# msgget failed\n";
+ }
=head1 NOTES
-If you are running under version 5.000 (dubious) or 5.001, you can still
-use most of the examples in this document. You may have to remove the
-C<use strict> and some of the my() statements for 5.000, and for both
-you'll have to load in version 1.2 of the F<Socket.pm> module, which
-was/is/shall-be included in I<perl5.001o>.
-
-Most of these routines quietly but politely return C<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 I<Socket> conversion
-functions croak() on bad arguments.) It is therefore essential
-that you should check the return values fo these functions. Always begin
-your socket programs this way for optimal success, and don't forget to add
-B<-T> taint checking flag to the pound-bang line for servers:
+Most of these routines quietly but politely return C<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 I<Socket> conversion
+functions croak() on bad arguments.) It is therefore essential to
+check return values from these functions. Always begin your socket
+programs this way for optimal success, and don't forget to add B<-T>
+taint checking flag to the #! line for servers:
- #!/usr/bin/perl -w
- require 5.002;
+ #!/usr/bin/perl -Tw
use strict;
use sigtrap;
use Socket;
All these routines create system-specific portability problems. As noted
elsewhere, Perl is at the mercy of your C libraries for much of its system
behaviour. It's probably safest to assume broken SysV semantics for
-signals and to stick with simple TCP and UDP socket operations; e.g. don't
-try to pass open filedescriptors over a local UDP datagram socket if you
+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.
-Because few vendors provide C libraries that are safely
-re-entrant, the prudent programmer will do little else within
-a handler beyond die() to raise an exception and longjmp(3) out.
+As mentioned in the signals section, because few vendors provide C
+libraries that are safely re-entrant, the prudent programmer will do
+little else within a handler beyond setting a numeric variable that
+already exists; or, if locked into a slow (restarting) system call,
+using die() to raise an exception and longjmp(3) out. In fact, even
+these may in some cases cause a core dump. It's probably best to avoid
+signals except where they are absolutely inevitable. This
+will be addressed in a future release of Perl.
=head1 AUTHOR
Tom Christiansen, with occasional vestiges of Larry Wall's original
-version.
+version and suggestions from the Perl Porters.
=head1 SEE ALSO
-Besides the obvious functions in L<perlfunc>, you should also check out
-the F<modules> file at your nearest CPAN site. (See L<perlmod> or best
-yet, the F<Perl FAQ> for a description of what CPAN is and where to get it.)
+There's a lot more to networking than this, but this should get you
+started.
+
+For intrepid programmers, the indispensable textbook is I<Unix Network
+Programming> by W. Richard Stevens (published by Addison-Wesley). Note
+that most books on networking address networking 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 L<perlfunc>, you should also check out the F<modules> file
+at your nearest CPAN site. (See L<perlmodlib> or best yet, the F<Perl
+FAQ> for a description of what CPAN is and where to get it.)
+
Section 5 of the F<modules> file is devoted to "Networking, Device Control
-(modems) and Interprocess Communication", and contains numerous unbundled
+(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--just to name a few.
https://perl5.git.perl.org / perl5.git / blobdiff