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Text File | 1998-10-28 | 69.8 KB | 1,981 lines

PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) NNNNAAAAMMMMEEEE perlipc - Perl interprocess communication (signals, fifos, pipes, safe subprocesses, sockets, and semaphores) DDDDEEEESSSSCCCCRRRRIIIIPPPPTTTTIIIIOOOONNNN 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. SSSSiiiiggggnnnnaaaallllssss 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 _a_n_y_t_h_i_n_g in your handler could in theory trigger a memory fault and subsequent core dump. sub catch_zap { my $signame = shift; $shucks++; die "Somebody sent me a SIG$signame"; } $SIG{INT} = 'catch_zap'; # could fail in modules $SIG{INT} = \&catch_zap; # best strategy 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++; } Page 1 (printed 10/23/98) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 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. 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 _l_o_c_a_l() statement, which will be automatically restored once your block is exited. (Remember that _l_o_c_a_l() 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 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 Page 2 (printed 10/23/98) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) handlers that need to reinstall themselves. Because Perl's signal mechanism is currently based on the _s_i_g_n_a_l(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)) { $Kid_Status{$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 _d_i_e() to jump out of the block, much as you might using _l_o_n_g_j_m_p() or _t_h_r_o_w() 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 Page 3 (printed 10/23/98) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) undocumented, but the _t/_l_i_b/_p_o_s_i_x._t file from the Perl source distribution has some examples in it. NNNNaaaammmmeeeedddd PPPPiiiippppeeeessss 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 _m_k_n_o_d(1) or on some systems, _m_k_f_i_f_o(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 ._s_i_g_n_a_t_u_r_e 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 ----pppp 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: $!"; } Page 4 (printed 10/23/98) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) # 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 signals } WWWWAAAARRRRNNNNIIIINNNNGGGG 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 _m_a_l_l_o_c(3) or _p_r_i_n_t_f(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 _l_o_n_g_j_u_m_p(3) out of the handler. Even this is a little cavalier for the true paranoiac, who avoids die in a handler because the system _i_s 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. UUUUssssiiiinnnngggg _o_p_e_n(((()))) ffffoooorrrr IIIIPPPPCCCC Perl's basic _o_p_e_n() statement can also be used for unidirectional interprocess communication by either appending or prepending a pipe symbol to the second argument to _o_p_e_n(). 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: $! $?"; Page 5 (printed 10/23/98) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) And here's how to start up a child process you intend to read from: open(STATUS, "netstat -an 2>&1 |") || die "can't fork: $!"; while (<STATUS>) { next if /^(tcp|udp)/; print; } close STATUS || die "bad netstat: $! $?"; If one can be sure that a particular program is a Perl script 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 _f_1, the process _c_m_d_1, standard input (_t_m_p_f_i_l_e in this case), the _f_2 file, the _c_m_d_2 command, and finally the _f_3 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 _o_p_e_n() and the _c_l_o_s_e() return values. If you're _w_r_i_t_i_n_g 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 _o_p_e_n() will in all likelihood succeed (it only reflects the _f_o_r_k()'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 _e_x_e_c() 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: $!"; Page 6 (printed 10/23/98) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 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: $!"; close FH or die "can't close: status=$?"; FFFFiiiilllleeeehhhhaaaannnnddddlllleeeessss 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'll certainly want to any stdio flush output buffers before forking. You may also want to close or reopen the filehandles for the child. You can get around this by opening your pipe with _o_p_e_n(), but on some systems this means that the child process cannot outlive the parent. BBBBaaaacccckkkkggggrrrroooouuuunnnndddd PPPPrrrroooocccceeeesssssssseeeessss 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). CCCCoooommmmpppplllleeeetttteeee DDDDiiiissssssssoooocccciiiiaaaattttiiiioooonnnn ooooffff CCCChhhhiiiilllldddd ffffrrrroooommmm PPPPaaaarrrreeeennnntttt In some cases (starting server processes, for instance) you'll want to complete dissociate the child process from the parent. The easiest way is to use: use POSIX qw(setsid); setsid() or die "Can't start a new session: $!"; However, you may not be on POSIX. The following process is reported to work on most Unixish systems. Non-Unix users should check their Your_OS::Process module for other solutions. +o Open /dev/tty and use the TIOCNOTTY ioctl on it. See the _t_t_y(_4) manpage for details. +o Change directory to / +o Reopen STDIN, STDOUT, and STDERR so they're not connected to the old tty. Page 7 (printed 10/23/98) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) +o Background yourself like this: fork && exit; +o Ignore hangup signals in case you're running on a shell that doesn't automatically no-hup you: $SIG{HUP} = 'IGNORE'; # or whatever you'd like SSSSaaaaffffeeee PPPPiiiippppeeee OOOOppppeeeennnnssss Another interesting approach to IPC is making your single program go multiprocess and communicate between (or even amongst) yourselves. The _o_p_e_n() 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 _t_o minus, you can write to the filehandle you opened and your kid will find it in his STDIN. If you open a pipe _f_r_o_m 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 Page 8 (printed 10/23/98) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) execute something without the shell's interference. With _s_y_s_t_e_m(), 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 _e_x_e_c() 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 _m_o_d_u_l_e_s file mentioned below in the SEE ALSO section. Page 9 (printed 10/23/98) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) BBBBiiiiddddiiiirrrreeeeccccttttiiiioooonnnnaaaallll CCCCoooommmmmmmmuuuunnnniiiiccccaaaattttiiiioooonnnn wwwwiiiitttthhhh AAAAnnnnooootttthhhheeeerrrr PPPPrrrroooocccceeeessssssss 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 ----wwww 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 _o_p_e_n_2() library function to catch both ends. There's also an _o_p_e_n_3() for tridirectional I/O so you can also catch your child's STDERR, but doing so would then require an awkward _s_e_l_e_c_t() loop and wouldn't allow you to use normal Perl input operations. If you look at its source, you'll see that _o_p_e_n_2() uses low-level primitives like Unix _p_i_p_e() and _e_x_e_c() calls to create all the connections. While it might have been slightly more efficient by using _s_o_c_k_e_t_p_a_i_r(), it would have then been even less portable than it already is. The _o_p_e_n_2() and _o_p_e_n_3() 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 _o_p_e_n_2(): 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 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 _c_a_t a ----uuuu 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 _C_o_m_m._p_l library. It uses pseudo-ttys to make your program behave more reasonably: Page 10 (printed 10/23/98) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 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 _C_o_m_m library also has _e_x_p_e_c_t() and _i_n_t_e_r_a_c_t() functions. Find the library (and we hope its successor _I_P_C::_C_h_a_t) at your nearest CPAN archive as detailed in the SEE ALSO section below. The newer Expect.pm module from CPAN also addresses this kind of thing. This module requires two other modules from CPAN: IO::Pty and IO::Stty. It sets up a pseudo-terminal to interact with programs that insist on using talking to the terminal device driver. If your system is amongst those supported, this may be your best bet. BBBBiiiiddddiiiirrrreeeeccccttttiiiioooonnnnaaaallll CCCCoooommmmmmmmuuuunnnniiiiccccaaaattttiiiioooonnnn wwwwiiiitttthhhh YYYYoooouuuurrrrsssseeeellllffff If you want, you may make low-level _p_i_p_e() and _f_o_r_k() to stitch this together by hand. This example only talks to itself, but you could reopen the appropriate handles to STDIN and STDOUT and call other processes. #!/usr/bin/perl -w # pipe1 - bidirectional communication using two pipe pairs # designed for the socketpair-challenged use IO::Handle; # thousands of lines just for autoflush :-( pipe(PARENT_RDR, CHILD_WTR); # XXX: failure? pipe(CHILD_RDR, PARENT_WTR); # XXX: failure? CHILD_WTR->autoflush(1); PARENT_WTR->autoflush(1); if ($pid = fork) { close PARENT_RDR; close PARENT_WTR; print CHILD_WTR "Parent Pid $$ is sending this\n"; chomp($line = <CHILD_RDR>); print "Parent Pid $$ just read this: `$line'\n"; close CHILD_RDR; close CHILD_WTR; waitpid($pid,0); } else { die "cannot fork: $!" unless defined $pid; close CHILD_RDR; close CHILD_WTR; chomp($line = <PARENT_RDR>); print "Child Pid $$ just read this: `$line'\n"; print PARENT_WTR "Child Pid $$ is sending this\n"; close PARENT_RDR; close PARENT_WTR; exit; } Page 11 (printed 10/23/98) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) But you don't actually have to make two pipe calls. If you have the _s_o_c_k_e_t_p_a_i_r() system call, it will do this all for you. #!/usr/bin/perl -w # pipe2 - bidirectional communication using socketpair # "the best ones always go both ways" use Socket; use IO::Handle; # thousands of lines just for autoflush :-( # We say AF_UNIX because although *_LOCAL is the # POSIX 1003.1g form of the constant, many machines # still don't have it. socketpair(CHILD, PARENT, AF_UNIX, SOCK_STREAM, PF_UNSPEC) or die "socketpair: $!"; CHILD->autoflush(1); PARENT->autoflush(1); if ($pid = fork) { close PARENT; print CHILD "Parent Pid $$ is sending this\n"; chomp($line = <CHILD>); print "Parent Pid $$ just read this: `$line'\n"; close CHILD; waitpid($pid,0); } else { die "cannot fork: $!" unless defined $pid; close CHILD; chomp($line = <PARENT>); print "Child Pid $$ just read this: `$line'\n"; print PARENT "Child Pid $$ is sending this\n"; close PARENT; exit; } SSSSoooocccckkkkeeeettttssss:::: CCCClllliiiieeeennnntttt////SSSSeeeerrrrvvvveeeerrrr CCCCoooommmmmmmmuuuunnnniiiiccccaaaattttiiiioooonnnn 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. Page 12 (printed 10/23/98) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 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 $AF_INET = 2, you know you're in for big trouble: An immeasurably superior approach is to use the Socket module, which more reliably grants access to various constants and functions you'll need. If you're not writing a server/client for an existing protocol like NNTP or SMTP, you should give some thought to how your server will know when the client has finished talking, and vice-versa. Most protocols are based on one- line messages and responses (so one party knows the other has finished when a "\n" is received) or multi-line messages and responses that end with a period on an empty line ("\n.\n" terminates a message/response). IIIInnnntttteeeerrrrnnnneeeetttt LLLLiiiinnnneeee TTTTeeeerrrrmmmmiiiinnnnaaaattttoooorrrrssss The Internet line terminator is "\015\012". Under ASCII variants of Unix, that could usually be written as "\r\n", but under other systems, "\r\n" might at times be "\015\015\012", "\012\012\015", or something completely different. The standards specify writing "\015\012" to be conformant (be strict in what you provide), but they also recommend accepting a lone "\012" on input (but be lenient in what you require). We haven't always been very good about that in the code in this manpage, but unless you're on a Mac, you'll probably be ok. IIIInnnntttteeeerrrrnnnneeeetttt TTTTCCCCPPPP CCCClllliiiieeeennnnttttssss aaaannnndddd SSSSeeeerrrrvvvveeeerrrrssss Use Internet-domain sockets when you want to do client- server communication that might extend to machines outside of your own system. Here's a sample TCP client using Internet-domain sockets: #!/usr/bin/perl -w use strict; use Socket; my ($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); Page 13 (printed 10/23/98) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) $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: $!"; 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. 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 $$: @_ at ", scalar localtime, "\n" } my $port = shift || 2345; my $proto = getprotobyname('tcp'); $port = $1 if $port =~ /(\d+)/; # untaint port number socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!"; setsockopt(Server, SOL_SOCKET, SO_REUSEADDR, pack("l", 1)) || die "setsockopt: $!"; bind(Server, sockaddr_in($port, INADDR_ANY)) || die "bind: $!"; listen(Server,SOMAXCONN) || die "listen: $!"; logmsg "server started on port $port"; my $paddr; $SIG{CHLD} = \&REAPER; for ( ; $paddr = accept(Client,Server); close Client) { my($port,$iaddr) = sockaddr_in($paddr); my $name = gethostbyaddr($iaddr,AF_INET); logmsg "connection from $name [", inet_ntoa($iaddr), "] at port $port"; Page 14 (printed 10/23/98) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) print Client "Hello there, $name, it's now ", scalar localtime, $EOL; } And here's a multithreaded version. It's multithreaded in that like most typical servers, it spawns (forks) a slave server to handle the client request so that the master server can quickly go back to service a new client. #!/usr/bin/perl -Tw use strict; BEGIN { $ENV{PATH} = '/usr/ucb:/bin' } use Socket; use Carp; $EOL = "\015\012"; sub spawn; # forward declaration sub logmsg { print "$0 $$: @_ 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); Page 15 (printed 10/23/98) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 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: $!"; return; } elsif ($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 _f_o_r_k() for each incoming request. That way it can handle many requests at once, which you might not always want. Even if you don't _f_o_r_k(), the _l_i_s_t_e_n() 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 ----TTTT flag to use taint checking (see the _p_e_r_l_s_e_c 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 Page 16 (printed 10/23/98) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 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, $iaddr); my($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); } UUUUnnnniiiixxxx----DDDDoooommmmaaaaiiiinnnn TTTTCCCCPPPP CCCClllliiiieeeennnnttttssss aaaannnndddd SSSSeeeerrrrvvvveeeerrrrssss 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 _l_s(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 ----SSSS file test: unless ( -S '/dev/log' ) { die "something's wicked with the print system"; } Page 17 (printed 10/23/98) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 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; } exit; 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: $!"; unlink($NAME); 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; Page 18 (printed 10/23/98) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 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--_s_p_a_w_n(), _l_o_g_m_s_g(), _c_t_i_m_e(), and _R_E_A_P_E_R()--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 _a_c_c_e_p_t() 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. TTTTCCCCPPPP CCCClllliiiieeeennnnttttssss wwwwiiiitttthhhh IIIIOOOO::::::::SSSSoooocccckkkkeeeetttt 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. AAAA SSSSiiiimmmmpppplllleeee CCCClllliiiieeeennnntttt Here's a client that creates a TCP connection to the "daytime" service at port 13 of the host name "localhost" and prints out everything that the server there cares to provide. Page 19 (printed 10/23/98) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) #!/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 mean: Proto This is which protocol to use. In this case, the socket handle returned will be connected to a TCP socket, because we want a stream-oriented connection, that is, one that acts pretty much like a plain old file. Not all sockets are this of this type. For example, the UDP protocol can be used to make a datagram socket, used for message-passing. PeerAddr This is the name or Internet address of the remote host the server is running on. We could have specified a longer name like "www.perl.com", or an address like "204.148.40.9". 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. PeerPort This is the service name or port number we'd like to connect to. We could have gotten away with using just "daytime" on systems with a well-configured system services file,[FOOTNOTE: The system services file is in /_e_t_c/_s_e_r_v_i_c_e_s 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: Page 20 (printed 10/23/98) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) $line = <$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"; AAAA WWWWeeeebbbbggggeeeetttt CCCClllliiiieeeennnntttt Here's a simple client that takes a remote host to fetch a document from, and then a list of documents to get from that host. This is a more interesting client than the previous one because it first sends something to the server before fetching the server's response. #!/usr/bin/perl -w use IO::Socket; unless (@ARGV > 1) { die "usage: $0 host document ..." } $host = shift(@ARGV); $EOL = "\015\012"; $BLANK = $EOL x 2; foreach $document ( @ARGV ) { $remote = IO::Socket::INET->new( Proto => "tcp", PeerAddr => $host, PeerPort => "http(80)", ); unless ($remote) { die "cannot connect to http daemon on $host" } $remote->autoflush(1); print $remote "GET $document HTTP/1.0" . $BLANK; while ( <$remote> ) { print } close $remote; } The web server handing the "http" service, which is assumed to be at its standard port, number 80. If 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 Page 21 (printed 10/23/98) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 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 _w_e_b_g_e_t: % webget www.perl.com /guanaco.html HTTP/1.1 404 File Not Found Date: Thu, 08 May 1997 18:02:32 GMT Server: Apache/1.2b6 Connection: close Content-type: text/html <HEAD><TITLE>404 File Not Found</TITLE></HEAD> <BODY><H1>File Not Found</H1> The requested URL /guanaco.html was not found on this server.<P> </BODY> Ok, so that's not very interesting, because it didn't find that particular document. But a long response wouldn't have fit on this page. For a more fully-featured version of this program, you should look to the _l_w_p-_r_e_q_u_e_s_t program included with the LWP modules from CPAN. IIIInnnntttteeeerrrraaaaccccttttiiiivvvveeee CCCClllliiiieeeennnntttt wwwwiiiitttthhhh IIIIOOOO::::::::SSSSoooocccckkkkeeeetttt 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 _t_e_l_n_e_t 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 _m_u_c_h 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 Page 22 (printed 10/23/98) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 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: Page 23 (printed 10/23/98) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 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. TTTTCCCCPPPP SSSSeeeerrrrvvvveeeerrrrssss wwwwiiiitttthhhh IIIIOOOO::::::::SSSSoooocccckkkkeeeetttt As always, setting up a server is little bit more involved than running a client. The model is that the server creates a special kind of socket that does nothing but listen on a particular port for incoming connections. It does this by calling the IO::Socket::INET->new() method with slightly different arguments than the client did. Proto This is which protocol to use. Like our clients, we'll still specify "tcp" here. LocalPort We specify a local port in the LocalPort argument, which we didn't do for the client. This is service name or port number for which you want to be the server. (Under Unix, ports under 1024 are restricted to the superuser.) In our sample, we'll use port 9000, but you can use any port that's not currently in use on your system. If you try to use one already in used, you'll get an "Address already in use" message. Under Unix, the netstat -a command will show which services current have servers. Listen The Listen parameter is set to the maximum number of pending connections we can accept until we turn away incoming clients. Think of it as a call-waiting queue for your telephone. The low-level Socket module has a special symbol for the system maximum, which is SOMAXCONN. Reuse 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.) Page 24 (printed 10/23/98) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 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 Camel as well as later in this manpage. Here's the code. We'll #!/usr/bin/perl -w use IO::Socket; use Net::hostent; # for OO version of gethostbyaddr $PORT = 9000; # pick something not in use $server = IO::Socket::INET->new( Proto => 'tcp', LocalPort => $PORT, Listen => SOMAXCONN, Reuse => 1); die "can't setup server" unless $server; print "[Server $0 accepting clients]\n"; while ($client = $server->accept()) { $client->autoflush(1); print $client "Welcome to $0; type help for command list.\n"; $hostinfo = gethostbyaddr($client->peeraddr); printf "[Connect from %s]\n", $hostinfo->name || $client->peerhost; print $client "Command? "; while ( <$client>) { next unless /\S/; # blank line if (/quit|exit/i) { last; } elsif (/date|time/i) { printf $client "%s\n", scalar localtime; } elsif (/who/i ) { print $client `who 2>&1`; } elsif (/cookie/i ) { print $client `/usr/games/fortune 2>&1`; } elsif (/motd/i ) { print $client `cat /etc/motd 2>&1`; } else { print $client "Commands: quit date who cookie motd\n"; } } continue { print $client "Command? "; } close $client; } UUUUDDDDPPPP:::: MMMMeeeessssssssaaaaggggeeee PPPPaaaassssssssiiiinnnngggg Another kind of client-server setup is one that uses not Page 25 (printed 10/23/98) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 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 _s_e_l_e_c_t() 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) { $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; Page 26 (printed 10/23/98) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) # 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--; } SSSSyyyyssssVVVV IIIIPPPPCCCC 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 _m_m_a_p() 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 S_IRWXG S_IRWXO); $size = 2000; $key = shmget(IPC_PRIVATE, $size, S_IRWXU|S_IRWXG|S_IRWXO) || die "$!"; print "shm key $key\n"; $message = "Message #1"; shmwrite($key, $message, 0, 60) || die "$!"; print "wrote: '$message'\n"; shmread($key, $buff, 0, 60) || die "$!"; 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 $key\n"; shmctl($key, IPC_RMID, 0) || die "$!"; Here's an example of a semaphore: use IPC::SysV qw(IPC_CREAT); $IPC_KEY = 1234; $key = semget($IPC_KEY, 10, 0666 | IPC_CREAT ) || die "$!"; print "shm key $key\n"; Put this code in a separate file to be run in more than one Page 27 (printed 10/23/98) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) process. Call the file _t_a_k_e: # 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 than one process. Call this file _g_i_v_e: # '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,$opstring) || die "$!"; 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. NNNNOOOOTTTTEEEESSSS 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, Page 28 (printed 10/23/98) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) some of the new _S_o_c_k_e_t conversion functions _c_r_o_a_k() 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 ----TTTT taint checking flag to the #! line for servers: #!/usr/bin/perl -Tw use strict; use sigtrap; use Socket; BBBBUUUUGGGGSSSS 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 portable. 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 _d_i_e() to raise an exception and _l_o_n_g_j_m_p(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. AAAAUUUUTTTTHHHHOOOORRRR Tom Christiansen, with occasional vestiges of Larry Wall's original version and suggestions from the Perl Porters. SSSSEEEEEEEE AAAALLLLSSSSOOOO There's a lot more to networking than this, but this should get you started. For intrepid programmers, the indispensable textbook is _U_n_i_x _N_e_t_w_o_r_k _P_r_o_g_r_a_m_m_i_n_g 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 _I_O::_S_o_c_k_e_t(3) manpage describes the object library, and the _S_o_c_k_e_t(3) manpage describes the low-level interface to sockets. Besides the obvious functions in the _p_e_r_l_f_u_n_c manpage, you should also check out the _m_o_d_u_l_e_s file at your nearest CPAN site. (See the _p_e_r_l_m_o_d_l_i_b manpage or best yet, the _P_e_r_l _F_A_Q for a description of what CPAN is and where to Page 29 (printed 10/23/98) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) 22223333////JJJJuuuullll////99998888 ((((ppppeeeerrrrllll 5555....000000005555,,,, ppppaaaattttcccchhhh 00002222)))) PPPPEEEERRRRLLLLIIIIPPPPCCCC((((1111)))) get it.) Section 5 of the _m_o_d_u_l_e_s 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. Page 30 (printed 10/23/98)