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标 题: perl(12)
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NAME
perlipc - Perl interprocess communication (signals, fifos, pipes, safe
subprocceses, 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 an another process, or
triggered automatically by the kernel when special events transpire, like a
child process exiting, your process running out of stack space, or hitting
file size limit.
For example, to trap an interrupt signal, set up a handler like this.
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.
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
The names of the signals are the ones listed out by kill -l on your system,
or you can retrieve them from the Config module. Set up an @signame list
indexed by number to get the name and a %signo table indexed by name to get
the number:
use Config;
defined $Config{sig_name} || die "No sigs?";
foreach $name (split(' ', $Config{sig_name})) {
$signo{$name} = $i;
$signame[$i] = $name;
$i++;
}
So to check whether signal 17 and SIGALRM were the same, just do this:
print "signal #17 = $signame[17]\n";
if ($signo{ALRM}) {
print "SIGALRM is $signo{ALRM}\n";
}
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. Some signals can be neither trapped nor ignored, such as the
KILL and STOP (but not the TSTP) signals. One strategy for temporarily
ignoring signals is to use a local() statement, which will be automatically
restored once your block is exited. (Remember that local() values are
``inherited'' by functions called from within that block.)
sub precious {
local $SIG{INT} = 'IGNORE';
&more_functions;
}
sub more_functions {
# interrupts still ignored, for now...
}
Sending a signal to a negative process ID means that you send the signal to
the entire Unix process-group. This code send a hang-up signal to all
processes in the current process group except for the current process
itself:
{
local $SIG{HUP} = 'IGNORE';
kill HUP => -$$;
# snazzy writing of: kill('HUP', -$$)
}
Another interesting signal to send is signal number zero. This doesn't
actually affect another process, but instead checks whether it's alive or
has changed its UID.
unless (kill 0 => $kid_pid) {
warn "something wicked happened to $kid_pid";
}
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
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
$waitedpid = wait;
}
$SIG{CHLD} = \&REAPER;
# now do something that forks...
or even the more elaborate:
use POSIX "wait_h";
sub REAPER {
my $child;
$SIG{CHLD} = \&REAPER; # loathe sysV
while ($child = waitpid(-1,WNOHANG)) {
$Kid_Status{$child} = $?;
}
}
$SIG{CHLD} = \&REAPER;
# do something that forks...
Signal handling is also used for timeouts in Unix, While safely protected
within an eval{} block, you set a signal handler to trap alarm signals and
then schedule to have one delivered to you in some number of seconds. Then
try your blocking operation, clearing the alarm when it's done but not
before you've exited your eval{} block. If it goes off, you'll use die() to
jump out of the block, much as you might using longjmp() or throw() in
other languages.
Here's an example:
eval {
local $SIG{ALRM} = sub { die "alarm clock restart" };
alarm 10;
flock(FH, 2); # blocking write lock
alarm 0;
};
if ($@ and $@ !~ /alarm clock restart/) { die }
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.
---------------------------------------------------------------------------
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 a regular,
connected anonymous pipes, except that the processes rendezvous using a
filename and don't have to be related.
To create a named pipe, use the Unix command mknod(1) or on some systems,
mkfifo(1). These may not be in your normal path.
# system return val is backwards, so && not ||
#
$ENV{PATH} .= ":/etc:/usr/etc";
if ( system('mknod', $path, 'p')
&& system('mkfifo', $path) )
{
die "mk{nod,fifo} $path failed;
}
A fifo is convenient when you want to connect a process to an unrelated
one. When you open a fifo, the program will block until there's something
on the other end.
For example, let's say you'd like to have your .signature file be a named
pipe that has a Perl program on the other end. Now every time any program
(like a mailer, newsreader, 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 -p to find out whether
anyone (or anything) has accidentally removed our fifo.
chdir; # go home
$FIFO = '.signature';
$ENV{PATH} .= ":/etc:/usr/games";
while (1) {
unless (-p $FIFO) {
unlink $FIFO;
system('mknod', $FIFO, 'p')
&& die "can't mknod $FIFO: $!";
}
# next line blocks until there's a reader
open (FIFO, "> $FIFO") || die "can't write $FIFO: $!";
print FIFO "John Smith (smith\@host.org)\n", `fortune -s`;
close FIFO;
sleep 2; # to avoid dup sigs
}
---------------------------------------------------------------------------
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 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 SPOOLER || die "bad netstat: $! $?";
If one can be sure that a particular program is a Perl script that is
expecting filenames in @ARGV , the clever programmer can write something
like this:
$ program f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile
and irrespective of which shell it's called from, the Perl program will
read from the file f1, the process cmd1, standard input (tmpfile in this
case), the f2 file, the cmd2 command, and finally the f3 file. Pretty
nifty, eh?
You might notice that you could use backticks for much the same effect as
opening a pipe for reading:
print grep { !/^(tcp|udp)/ } `netstat -an 2>&1`;
die "bad netstat" if $?;
While this is true on the surface, it's much more efficient to process the
file one line or record at a time because then you don't have to read the
whole thing into memory at once. It also gives you finer control of the
whole process, letting you to kill off the child process early if you'd
like.
Be careful to check both the open() and the close() return values. If
you're writing to a pipe, you should also trap SIGPIPE. Otherwise, think of
what happens when you start up a pipe to a command that doesn't exist: the
open() will in all likelihood succeed (it only reflects the fork() 's
success), but then your output will fail--spectacularly. Perl can't know
whether the command worked because your command is actually running in a
separate process whose exec() might have failed. Therefore, while readers
of bogus commands just return 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;
---------------------------------------------------------------------------
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;
my $sleep_count = 0;
do {
$pid = open(KID, "-|");
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 $?";
} else { # child
($EUID, $EGID) = ($UID, $GID); # suid progs only
open (FILE, "> /safe/file")
|| die "can't open /safe/file: $!";
while (<STDIN>) {
print FILE; # child's STDIN is parent's KID
}
exit; # don't forget this
}
Another common use for this construct is when you need to execute something
without the shell's interference. With system() , it's straigh-forward, 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
$pid = open(KID, "-|");
if ($pid) { # parent
while (<KID>) {
# do something interesting
}
close(KID) || 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, "|-");
$SIG{ALRM} = sub { die "whoops, $program pipe broke" };
if ($pid) { # parent
for (@data) {
print KID;
}
close(KID) || 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
modules file mentioned below in the SEE ALSO section.
---------------------------------------------------------------------------
Bidirectional Communication
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 |")
and if you forgot to use the -w flag, then you'll miss out entirely on the
diagnostic message:
Can't do bidirectional pipe at -e line 1.
If you really want to, you can use the standard open2() library function to
catch both ends. There's also an open3() for tridirectional I/O so you can
also catch your child's STDERR, but doing so would then require an awkward
select() loop and wouldn't allow you to use normal Perl input operations.
If you look at its source, you'll see that open2() uses low-level
primitives like Unix pipe() and exec() to create all the connections. While
it might have been slightly more efficient by using socketpair() , it would
have then been even less portable than it already is. The open2() and
open3() functions are unlikely to work anywhere except on a Unix system or
some other one purporting to be POSIX compliant.
Here's an example of using open2():
use FileHandle;
use IPC::Open2;
$pid = open2( \*Reader, \*Writer, "cat -u -n" );
Writer->autoflush(); # default here, actually
print Writer "stuff\n";
$got = <Reader>;
The problem with this is that Unix buffering is going to really ruin your
day. Even though your Writer filehandle is autoflushed, 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 gave cat a -u flag to make it
unbuffered. But very few Unix commands are designed to operate over pipes,
so this seldom works unless you yourself wrote the program on the other end
of the double-ended pipe.
A solution to this is the non-standard Comm.pl library. It uses pseudo-ttys
to make your program behave more reasonably:
require 'Comm.pl';
$ph = open_proc('cat -n');
for (1..10) {
print $ph "a line\n";
print "got back ", scalar <$ph>;
}
This way you don't have to have control over the source code of the program
you're using. The Comm library also has expect() and interact() functions.
Find the library (and hopefully its successor IPC::Chat) at your nearest
CPAN archive as detailed in the SEE ALSO section below.
---------------------------------------------------------------------------
Sockets: Client/Server Communication
While not limited to Unix-derived operating systems (e.g. WinSock on PCs
provides socket support, as do some VMS libraries), you may not have
sockets on your system, in which this section probably isn't going to do
you much good. With sockets, you can do both virtual circuits (i.e. TCP
streams) and datagrams (i.e. UDP packets). You may be able to do even more
depending on your system.
The Perl function calls for dealing with sockets have the same names as the
corresponding system calls in C, but their arguments tend to differ for two
reasons: first, Perl filehandles work differently than C file descriptors.
Second, Perl already knows the length of its strings, so you don't need to
pass that information.
One of the major problems with old socket code in Perl was that it used
hard-coded values for some of the constants, which severely hurt
portability. If you ever see code that does anything like explicitly
setting $AF_INET = 2, you know you're in for big trouble: An immeasurably
superior approach is to use the Socket module, which more reliably grants
access to various constants and functions you'll need.
---------------------------------------------------------------------------
Internet TCP Clients and Servers
Use Internet-domain sockets when you want to do client-server communication
that might extend to machines outside of your own system.
Here's a sample TCP client using Internet-domain sockets:
#!/usr/bin/perl -w
require 5.002;
use strict;
use Socket;
my ($remote,$port, $iaddr, $paddr, $proto, $line);
$remote = shift || 'localhost';
$port = shift || 2345; # random port
if ($port =~ /\D/) { $port = getservbyname($port, 'tcp') }
die "No port" unless $port;
$iaddr = inet_aton($remote) || die "no host: $remote";
$paddr = sockaddr_in($port, $iaddr);
$proto = getprotobyname('tcp');
socket(SOCK, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
connect(SOCK, $paddr) || die "connect: $!";
while ($line = <SOCK>) {
print $line;
}
close (SOCK) || die "close: $!";
exit;
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:
#!/usr/bin/perl -Tw
require 5.002;
use strict;
BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
use Socket;
use Carp;
sub spawn; # forward declaration
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: $!";
logmsg "server started on port $port";
my $waitedpid = 0;
my $paddr;
sub REAPER {
$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)
{
next if $waitedpid;
my($port,$iaddr) = sockaddr_in($paddr);
my $name = gethostbyaddr($iaddr,AF_INET);
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'
or confess "can't exec fortune: $!";
};
}
sub spawn {
my $coderef = shift;
unless (@_ == 0 && $coderef && ref($coderef) eq 'CODE') {
confess "usage: spawn CODEREF";
}
my $pid;
if (!defined($pid = fork)) {
logmsg "cannot fork: $!";
return;
} elsif ($pid) {
logmsg "begat $pid";
return; # i'm the parent
}
# else i'm the child -- go spawn
open(STDIN, "<&CLIENT") || die "can't dup client to stdin";
open(STDOUT, ">&CLIENT") || die "can't dup client to stdout";
## open(STDERR, ">&STDOUT") || die "can't dup stdout to stderr";
exit &$coderef();
}
This server takes the trouble to clone off a child version via fork() for
each incoming request. That way it can handle many requests at once, which
you might not always want. Even if you don't fork() , the listen() will
allow that many pending connections. Forking servers have to be
particularly careful about cleaning up their dead children (called
``zombies'' in Unix parlance), because otherwise you'll quickly fill up
your process table.
We suggest that you use the -T flag to use taint checking (see the perlsec
manpage ) even if we aren't running setuid or setgid. This is always a good
idea for servers and other programs run on behalf of someone else (like CGI
scripts), because it lessens the chances that people from the outside will
be able to compromise your system.
Let's look at another TCP client. This one connects to the TCP ``time''
service on a number of different machines and shows how far their clocks
differ from the system on which it's being run:
#!/usr/bin/perl -w
require 5.002;
use strict;
use Socket;
my $SECS_of_70_YEARS = 2208988800;
sub ctime { scalar localtime(shift) }
my $iaddr = gethostbyname('localhost');
my $proto = getprotobyname('tcp');
my $port = getservbyname('time', 'tcp');
my $paddr = sockaddr_in(0, $iaddr);
my($host);
$| = 1;
printf "%-24s %8s %s\n", "localhost", 0, ctime(time());
foreach $host (@ARGV) {
printf "%-24s ", $host;
my $hisiaddr = inet_aton($host) || die "unknown host";
my $hispaddr = sockaddr_in($port, $hisiaddr);
socket(SOCKET, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
connect(SOCKET, $hispaddr) || die "bind: $!";
my $rtime = ' ';
read(SOCKET, $rtime, 4);
close(SOCKET);
my $histime = unpack("N", $rtime) - $SECS_of_70_YEARS ;
printf "%8d %s\n", $histime - time, ctime($histime);
}
---------------------------------------------------------------------------
Unix-Domain TCP Clients and Servers
That's fine for Internet-domain clients and servers, but what local
communications? While you can use the same setup, sometimes you don't want
to. Unix-domain sockets are local to the current host, and are often used
internally to implement pipes. Unlike Internet domain sockets, UNIX domain
sockets can show up in the file system with an ls(1) listing.
$ ls -l /dev/log
srw-rw-rw- 1 root 0 Oct 31 07:23 /dev/log
You can test for these with Perl's -S file test:
unless ( -S '/dev/log' ) {
die "something's wicked with the print 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>) {
print $line;
}
exit;
And here's a corresponding server.
#!/usr/bin/perl -Tw
require 5.002;
use strict;
use Socket;
use Carp;
BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
my $NAME = '/tmp/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,5) || die "listen: $!";
logmsg "server started on $NAME";
$SIG{CHLD} = \&REAPER;
for ( $waitedpid = 0;
accept(CLIENT,SERVER) || $waitedpid;
$waitedpid = 0, close CLIENT)
{
next if $waitedpid;
logmsg "connection on $NAME";
spawn sub {
print "Hello there, it's now ", scalar localtime, "\n";
exec '/usr/games/fortune' or die "can't exec fortune: $!";
};
}
As you see, it's remarkably similar to the Internet domain TCP server, so
much so, in fact, that we've omitted several duplicate functions--spawn(),
logmsg(), ctime(), and REAPER()--which are exactly the same as in the other
server.
So why would you ever want to use a Unix domain socket instead of a simpler
named pipe? Because a named pipe doesn't give you sessions. You can't tell
one process's data from another's. With socket programming, you get a
separate session for each client: that's why accept() takes two arguments.
For example, let's say that you have a long running database server daemon
that you want folks from the World Wide Web to be able to access, but only
if they go through a CGI interface. You'd have a small, simple CGI program
that does whatever checks and logging you feel like, and then acts as a
Unix-domain client and connects to your private server.
---------------------------------------------------------------------------
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 just use TCP to start
with.
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 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,
$rin, $rout, $rtime, $SECS_of_70_YEARS);
$SECS_of_70_YEARS = 2208988800;
$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 time;
$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--;
}
---------------------------------------------------------------------------
SysV IPC
While System V IPC isn't so widely used as sockets, it still has some
interesting uses. You can't, however, effectively use SysV IPC or Berkeley
mmap() to have shared memory so as to share a variable amongst 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.
$IPC_PRIVATE = 0;
$IPC_RMID = 0;
$size = 2000;
$key = shmget($IPC_PRIVATE, $size , 0777 );
die unless defined $key;
$message = "Message #1";
shmwrite($key, $message, 0, 60 ) || die "$!";
shmread($key,$buff,0,60) || die "$!";
print $buff,"\n";
print "deleting $key\n";
shmctl($key ,$IPC_RMID, 0) || die "$!";
Here's an example of a semaphore:
$IPC_KEY = 1234;
$IPC_RMID = 0;
$IPC_CREATE = 0001000;
$key = semget($IPC_KEY, $nsems , 0666 | $IPC_CREATE );
die if !defined($key);
print "$key\n";
Put this code in a separate file to be run in more that one process Call
the file take:
# create a semaphore
$IPC_KEY = 1234;
$key = semget($IPC_KEY, 0 , 0 );
die if !defined($key);
$semnum = 0;
$semflag = 0;
# 'take' semaphore
# wait for semaphore to be zero
$semop = 0;
$opstring1 = pack("sss", $semnum, $semop, $semflag);
# Increment the semaphore count
$semop = 1;
$opstring2 = pack("sss", $semnum, $semop, $semflag);
$opstring = $opstring1 . $opstring2;
semop($key,$opstring) || die "$!";
Put this code in a separate file to be run in more that 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;
$key = semget($IPC_KEY, 0, 0);
die if !defined($key);
$semnum = 0;
$semflag = 0;
# Decrement the semaphore count
$semop = -1;
$opstring = pack("sss", $semnum, $semop, $semflag);
semop($key,$opstring) || die "$!";
---------------------------------------------------------------------------
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 use strict and require
``sys/ipc.ph'' . Better yet, perhaps someone should create an IPC::SysV
module the way we have the 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.
---------------------------------------------------------------------------
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 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 Socket.pm module, which was/is/shall-be
included in perl5.001o.
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
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 -T taint checking
flag to the pound-bang line for servers:
#!/usr/bin/perl -w
require 5.002;
use strict;
use sigtrap;
use Socket;
---------------------------------------------------------------------------
BUGS
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 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.
---------------------------------------------------------------------------
AUTHOR
Tom Christiansen, with occasional vestiges of Larry Wall's original
version.
---------------------------------------------------------------------------
SEE ALSO
Besides the obvious functions in the perlfunc manpage , you should also
check out the modules file at your nearest CPAN site. (See the perlmod
manpage or best yet, the Perl FAQ for a description of what CPAN is and
where to get it.) Section 5 of the modules file is devoted to "Networking,
Device Control (modems) and Interprocess Communication", and contains
numerous unbundled modules numerous networking modules, Chat and Expect
operations, CGI programming, DCE, FTP, IPC, NNTP, Proxy, Ptty, RPC, SNMP,
SMTP, Telnet, Threads, and ToolTalk--just to name a few.
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