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perlipc (1)
  • >> perlipc (1) ( Solaris man: Команды и прикладные программы пользовательского уровня )
  • perlipc (1) ( Разные man: Команды и прикладные программы пользовательского уровня )


         perlipc - Perl interprocess communication (signals, fifos,
         pipes, safe subprocesses, sockets, and semaphores)


         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.


         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 process running out of stack space, or hitting file
         size limit.
         For example, to trap an interrupt signal, set up a handler
         like this.  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 anything in your handler could in theory trigger a
         memory fault and subsequent core dump.
             sub catch_zap {
                 my $signame = shift;
                 die "Somebody sent me a SIG$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
             use Config;
             defined $Config{sig_name} || die "No sigs?";
             foreach $name (split(' ', $Config{sig_name})) {
                 $signo{$name} = $i;
                 $signame[$i] = $name;
         So to check whether signal 17 and SIGALRM were the same, do
         just this:
             print "signal #17 = $signame[17]\n";
             if ($signo{ALRM}) {
                 print "SIGALRM is $signo{ALRM}\n";
         You may also choose to assign the strings `'IGNORE'' or
         `'DEFAULT'' as the handler, in which case Perl will try to
         discard the signal or do the default thing.
         On most Unix platforms, the `CHLD' (sometimes also known as
         `CLD') signal has special behavior with respect to a value
         of `'IGNORE''.  Setting `$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.  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';
             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
         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';
                 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 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 {
                 $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:
             use POSIX ":sys_wait_h";
             sub REAPER {
                 my $child;
                 while (($child = waitpid(-1,WNOHANG)) > 0) {
                     $Kid_Status{$child} = $?;
                 $SIG{CHLD} = \&REAPER;  # still loathe sysV
             $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 }
         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.

    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
             # 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 block and your program will supply
         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\\n", `fortune -s`;
                 close FIFO;
                 sleep 2;    # to avoid dup signals
         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 `die' to longjump(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 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.  Their dodginess is expected to be addresses in
         the 5.005 release.

    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)/;
             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
         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
         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 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")  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: $!";
             close FH            or die "can't close: status=$?";
         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 more 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 '/'               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 /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 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;
             my $sleep_count = 0;
             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_TO_WRITE @some_data;
                 close(KID_TO_WRITE) || 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 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
             $pid = open(KID_TO_READ, "-|");
             if ($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:
             # add error processing as above
             $pid = open(KID_TO_WRITE, "|-");
             $SIG{ALRM} = sub { die "whoops, $program pipe broke" };
             if ($pid) {  # parent
                 for (@data) {
                     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 modules file
         mentioned below in the SEE ALSO section.
         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(PROG_FOR_READING_AND_WRITING, "| some program |")
         and if you forget to use the `use warnings' pragma or the -w
         flag, then you'll miss out entirely on the diagnostic
             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() 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 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" );
             print Writer "stuff\n";
             $got = <Reader>;
         The problem with this is that Unix buffering is really going
         to ruin your day.  Even though your `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 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 nonstandard library.  It
         uses pseudo-ttys to make your program behave more
             require '';
             $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 we
         hope its successor IPC::Chat) at your nearest CPAN archive
         as detailed in the SEE ALSO section below.
         The newer 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.
         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?
             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;
             } 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;
         But you don't actually have to make two pipe calls.  If you
         have the socketpair() system call, it will do this all for
             #!/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: $!";
             if ($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;
             } 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;

    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 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 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 `$AF_INET = 2', you
         know you're in for big trouble:  An immeasurably superior
         approach is to use the `Socket' module, which more reliably
         grants access to various constants and functions you'll
         If you're not writing a server/client for an existing
         protocol like NNTP or SMTP, you should give some thought to
         how your server will know when the client has finished
         talking, and vice-versa.  Most protocols are based on one-
         line messages and responses (so one party knows the other
         has finished when a "\n" is received) or multi-line messages
         and responses that end with a period on an empty line
         ("\n.\n" terminates a message/response).
         Internet Line Terminators
         The Internet line terminator is "\015\012".  Under ASCII
         variants of Unix, that could usually be written as "\r\n",
         but under other systems, "\r\n" might at times be
         "\015\015\012", "\012\012\015", or something completely
         different.  The standards specify writing "\015\012" to be
         conformant (be strict in what you provide), but they also
         recommend accepting a lone "\012" on input (but be lenient
         in what you require).  We haven't always been very good
         about that in the code in this manpage, but unless you're on
         a Mac, you'll probably be ok.
         Internet TCP Clients and Servers
         Use Internet-domain sockets when you want to do client-
         server communication that might extend to machines outside
         of your own system.
         Here's a sample TCP client using Internet-domain sockets:
             #!/usr/bin/perl -w
             use strict;
             use Socket;
             my ($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: $!";
             while (defined($line = <SOCK>)) {
                 print $line;
             close (SOCK)            || die "close: $!";
         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), 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;
             $EOL = "\015\012";
             sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
             my $port = shift || 2345;
             my $proto = getprotobyname('tcp');
             $port = $1 if $port =~ /(\d+)/; # untaint port number
             socket(Server, PF_INET, SOCK_STREAM, $proto)        || die "socket: $!";
             setsockopt(Server, SOL_SOCKET, SO_REUSEADDR,
                                                 pack("l", 1))   || die "setsockopt: $!";
             bind(Server, sockaddr_in($port, INADDR_ANY))        || die "bind: $!";
             listen(Server,SOMAXCONN)                            || die "listen: $!";
             logmsg "server started on port $port";
             my $paddr;
             $SIG{CHLD} = \&REAPER;
             for ( ; $paddr = accept(Client,Server); close Client) {
                 my($port,$iaddr) = sockaddr_in($paddr);
                 my $name = gethostbyaddr($iaddr,AF_INET);
                 logmsg "connection from $name [",
                         inet_ntoa($iaddr), "]
                         at port $port";
                 print Client "Hello there, $name, it's now ",
                                 scalar localtime, $EOL;
         And here's a multithreaded version.  It's multithreaded in
         that like most typical servers, it spawns (forks) a slave
         server to handle the client request so that the master
         server can quickly go back to service a new client.
             #!/usr/bin/perl -Tw
             use strict;
             BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
             use Socket;
             use Carp;
             $EOL = "\015\012";
             sub spawn;  # forward declaration
             sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
             my $port = shift || 2345;
             my $proto = getprotobyname('tcp');
             $port = $1 if $port =~ /(\d+)/; # untaint port number
             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 {
                 $waitedpid = wait;
                 $SIG{CHLD} = \&REAPER;  # loathe sysV
                 logmsg "reaped $waitedpid" . ($? ? " with exit $?" : '');
             $SIG{CHLD} = \&REAPER;
             for ( $waitedpid = 0;
                   ($paddr = accept(Client,Server)) || $waitedpid;
                   $waitedpid = 0, close Client)
                 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), "]
                         at port $port";
                 spawn sub {
                     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') {
                     confess "usage: spawn CODEREF";
                 my $pid;
                 if (!defined($pid = fork)) {
                     logmsg "cannot fork: $!";
                 } elsif ($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
             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);
             $| = 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);
                 my $histime = unpack("N", $rtime) - $SECS_of_70_YEARS ;
                 printf "%8d %s\n", $histime - time, ctime($histime);
         Unix-Domain TCP Clients and Servers
         That's fine for Internet-domain clients and servers, but
         what about local communications?  While you can use the same
         setup, sometimes you don't want to.  Unix-domain sockets are
         local to the current host, and are often used internally to
         implement pipes.  Unlike Internet domain sockets, Unix
         domain sockets can show up in the file system with an ls(1)
             % 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
             use Socket;
             use strict;
             my ($rendezvous, $line);
             $rendezvous = shift || '/tmp/catsock';
             socket(SOCK, PF_UNIX, SOCK_STREAM, 0)       || die "socket: $!";
             connect(SOCK, sockaddr_un($rendezvous))     || die "connect: $!";
             while (defined($line = <SOCK>)) {
                 print $line;
         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/ucb:/bin' }
             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: $!";
             bind  (Server, $uaddr)                      || die "bind: $!";
             listen(Server,SOMAXCONN)                    || die "listen: $!";
             logmsg "server started on $NAME";
             my $waitedpid;
             sub REAPER {
                 $waitedpid = wait;
                 $SIG{CHLD} = \&REAPER;  # loathe sysV
                 logmsg "reaped $waitedpid" . ($? ? " with exit $?" : '');
             $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.

    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 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.
         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
             #!/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 `new' constructor
             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
             This is the name or Internet address of the remote host
             the server is running on.  We could have specified a
             longer name like `""', or an address like
             `""'.  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
             `"127.1"', if you'd rather use that.
             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 in
             /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.
         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 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" }
                 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 your 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, `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 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 /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>
         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 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);
             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 `kill' function in the parent's `if' block is there to
         send a signal to our child process (current 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.
             This is which protocol to use.  Like our clients, we'll
             still specify `"tcp"' here.
             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.
             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 module has a
             special symbol for the system maximum, which is
             The `Reuse' parameter is needed so that we restart our
             server manually without waiting a few minutes to allow
             system buffers to clear out.
         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 `accept'
         method, which eventually an bidirectional connection to 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 `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
         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()) {
            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;

    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.
         Here's a UDP program similar to the sample Internet TCP
         client given 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;
             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) {
                 $hisiaddr = inet_aton($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);

    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.
             use IPC::SysV qw(IPC_PRIVATE IPC_RMID S_IRWXU);
             $size = 2000;
             $id = shmget(IPC_PRIVATE, $size, S_IRWXU) || die "$!";
             print "shm key $id\n";
             $message = "Message #1";
             shmwrite($id, $message, 0, 60) || die "$!";
             print "wrote: '$message'\n";
             shmread($id, $buff, 0, 60) || die "$!";
             print "read : '$buff'\n";
             # the buffer of shmread is zero-character end-padded.
             substr($buff, index($buff, "\0")) = '';
             print "un" unless $buff eq $message;
             print "swell\n";
             print "deleting shm $id\n";
             shmctl($id, IPC_RMID, 0) || die "$!";
         Here's an example of a semaphore:
             use IPC::SysV qw(IPC_CREAT);
             $IPC_KEY = 1234;
             $id = semget($IPC_KEY, 10, 0666 | IPC_CREAT ) || die "$!";
             print "shm key $id\n";
         Put this code in a separate file to be run in more than one
         process.  Call the file take:
             # create a semaphore
             $IPC_KEY = 1234;
             $id = semget($IPC_KEY,  0 , 0 );
             die if !defined($id);
             $semnum = 0;
             $semflag = 0;
             # 'take' semaphore
             # wait for semaphore to be zero
             $semop = 0;
             $opstring1 = pack("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,$opstring) || die "$!";
         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 if !defined($id);
             $semnum = 0;
             $semflag = 0;
             # Decrement the semaphore count
             $semop = -1;
             $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:
             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";


         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 to check return
         values from these functions.  Always begin your socket
         programs this way for optimal success, and don't forget to
         add -T taint checking flag to the #! line for servers:
             #!/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 file descriptors over a local UDP datagram
         socket if you want your code to stand a chance of being
         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.


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


         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 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:\fIs0:Socket(3) manpage describes the object library,
         and the Socket(3) manpage describes the low-level interface
         to sockets.  Besides the obvious functions in the perlfunc
         manpage, you should also check out the modules file at your
         nearest CPAN site.  (See the perlmodlib 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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