diff options
Diffstat (limited to 'Master/tlpkg/tlperl/lib/pods/perlipc.pod')
-rw-r--r-- | Master/tlpkg/tlperl/lib/pods/perlipc.pod | 1833 |
1 files changed, 0 insertions, 1833 deletions
diff --git a/Master/tlpkg/tlperl/lib/pods/perlipc.pod b/Master/tlpkg/tlperl/lib/pods/perlipc.pod deleted file mode 100644 index 76508388fee..00000000000 --- a/Master/tlpkg/tlperl/lib/pods/perlipc.pod +++ /dev/null @@ -1,1833 +0,0 @@ -=head1 NAME - -perlipc - Perl interprocess communication (signals, fifos, pipes, safe subprocesses, sockets, and semaphores) - -=head1 DESCRIPTION - -The basic IPC facilities of Perl are built out of the good old Unix -signals, named pipes, pipe opens, the Berkeley socket routines, and SysV -IPC calls. Each is used in slightly different situations. - -=head1 Signals - -Perl uses a simple signal handling model: the %SIG hash contains names -or references of user-installed signal handlers. These handlers will -be called with an argument which is the name of the signal that -triggered it. A signal may be generated intentionally from a -particular keyboard sequence like control-C or control-Z, sent to you -from another process, or triggered automatically by the kernel when -special events transpire, like a child process exiting, your own process -running out of stack space, or hitting a process file-size limit. - -For example, to trap an interrupt signal, set up a handler like this: - - our $shucks; - - sub catch_zap { - my $signame = shift; - $shucks++; - die "Somebody sent me a SIG$signame"; - } - $SIG{INT} = __PACKAGE__ . "::catch_zap"; - $SIG{INT} = \&catch_zap; # best strategy - -Prior to Perl 5.7.3 it was necessary to do as little as you possibly -could in your handler; notice how all we do is set a global variable -and then raise an exception. That's because on most systems, -libraries are not re-entrant; particularly, memory allocation and I/O -routines are not. That meant that doing nearly I<anything> in your -handler could in theory trigger a memory fault and subsequent core -dump - see L</Deferred Signals (Safe Signals)> below. - -The names of the signals are the ones listed out by C<kill -l> on your -system, or you can retrieve them using the CPAN module L<IPC::Signal>. - -You may also choose to assign the strings C<"IGNORE"> or C<"DEFAULT"> as -the handler, in which case Perl will try to discard the signal or do the -default thing. - -On most Unix platforms, the C<CHLD> (sometimes also known as C<CLD>) signal -has special behavior with respect to a value of C<"IGNORE">. -Setting C<$SIG{CHLD}> to C<"IGNORE"> on such a platform has the effect of -not creating zombie processes when the parent process fails to C<wait()> -on its child processes (i.e., child processes are automatically reaped). -Calling C<wait()> with C<$SIG{CHLD}> set to C<"IGNORE"> usually returns -C<-1> on such platforms. - -Some signals can be neither trapped nor ignored, such as the KILL and STOP -(but not the TSTP) signals. Note that ignoring signals makes them disappear. -If you only want them blocked temporarily without them getting lost you'll -have to use POSIX' sigprocmask. - -Sending a signal to a negative process ID means that you send the signal -to the entire Unix process group. This code sends a hang-up signal to all -processes in the current process group, and also sets $SIG{HUP} to C<"IGNORE"> -so it doesn't kill itself: - - # block scope for local - { - 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 a child process, but instead checks whether it's alive -or has changed its UIDs. - - unless (kill 0 => $kid_pid) { - warn "something wicked happened to $kid_pid"; - } - -Signal number zero may fail because you lack permission to send the -signal when directed at a process whose real or saved UID is not -identical to the real or effective UID of the sending process, even -though the process is alive. You may be able to determine the cause of -failure using C<$!> or C<%!>. - - unless (kill(0 => $pid) || $!{EPERM}) { - warn "$pid looks dead"; - } - -You might also want to employ anonymous functions for simple signal -handlers: - - $SIG{INT} = sub { die "\nOutta here!\n" }; - -SIGCHLD handlers require some special care. If a second child dies -while in the signal handler caused by the first death, we won't get -another signal. So must loop here else we will leave the unreaped child -as a zombie. And the next time two children die we get another zombie. -And so on. - - use POSIX ":sys_wait_h"; - $SIG{CHLD} = sub { - while ((my $child = waitpid(-1, WNOHANG)) > 0) { - $Kid_Status{$child} = $?; - } - }; - # do something that forks... - -Be careful: qx(), system(), and some modules for calling external commands -do a fork(), then wait() for the result. Thus, your signal handler -will be called. Because wait() was already called by system() or qx(), -the wait() in the signal handler will see no more zombies and will -therefore block. - -The best way to prevent this issue is to use waitpid(), as in the following -example: - - use POSIX ":sys_wait_h"; # for nonblocking read - - my %children; - - $SIG{CHLD} = sub { - # don't change $! and $? outside handler - local ($!, $?); - my $pid = waitpid(-1, WNOHANG); - return if $pid == -1; - return unless defined $children{$pid}; - delete $children{$pid}; - cleanup_child($pid, $?); - }; - - while (1) { - my $pid = fork(); - die "cannot fork" unless defined $pid; - if ($pid == 0) { - # ... - exit 0; - } else { - $children{$pid}=1; - # ... - system($command); - # ... - } - } - -Signal handling is also used for timeouts in Unix. While safely -protected within an C<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 C<eval{}> block. If it -goes off, you'll use die() to jump out of the block. - -Here's an example: - - my $ALARM_EXCEPTION = "alarm clock restart"; - eval { - local $SIG{ALRM} = sub { die $ALARM_EXCEPTION }; - alarm 10; - flock(FH, 2) # blocking write lock - || die "cannot flock: $!"; - alarm 0; - }; - if ($@ && $@ !~ quotemeta($ALARM_EXCEPTION)) { die } - -If the operation being timed out is system() or qx(), this technique -is liable to generate zombies. If this matters to you, you'll -need to do your own fork() and exec(), and kill the errant child process. - -For more complex signal handling, you might see the standard POSIX -module. Lamentably, this is almost entirely undocumented, but -the F<t/lib/posix.t> file from the Perl source distribution has some -examples in it. - -=head2 Handling the SIGHUP Signal in Daemons - -A process that usually starts when the system boots and shuts down -when the system is shut down is called a daemon (Disk And Execution -MONitor). If a daemon process has a configuration file which is -modified after the process has been started, there should be a way to -tell that process to reread its configuration file without stopping -the process. Many daemons provide this mechanism using a C<SIGHUP> -signal handler. When you want to tell the daemon to reread the file, -simply send it the C<SIGHUP> signal. - -The following example implements a simple daemon, which restarts -itself every time the C<SIGHUP> signal is received. The actual code is -located in the subroutine C<code()>, which just prints some debugging -info to show that it works; it should be replaced with the real code. - - #!/usr/bin/perl -w - - use POSIX (); - use FindBin (); - use File::Basename (); - use File::Spec::Functions; - - $| = 1; - - # make the daemon cross-platform, so exec always calls the script - # itself with the right path, no matter how the script was invoked. - my $script = File::Basename::basename($0); - my $SELF = catfile($FindBin::Bin, $script); - - # POSIX unmasks the sigprocmask properly - $SIG{HUP} = sub { - print "got SIGHUP\n"; - exec($SELF, @ARGV) || die "$0: couldn't restart: $!"; - }; - - code(); - - sub code { - print "PID: $$\n"; - print "ARGV: @ARGV\n"; - my $count = 0; - while (++$count) { - sleep 2; - print "$count\n"; - } - } - - -=head2 Deferred Signals (Safe Signals) - -Before Perl 5.7.3, installing Perl code to deal with signals exposed you 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 behavior may result. - -There were two things you could do, knowing this: be paranoid or be -pragmatic. The paranoid approach was to do as little as possible in your -signal handler. Set an existing integer variable that already has a -value, and return. This doesn't help you if you're in a slow system call, -which will just restart. That means you have to C<die> to longjmp(3) out -of the handler. Even this is a little cavalier for the true paranoiac, -who avoids C<die> in a handler because the system I<is> out to get you. -The pragmatic approach was to say "I know the risks, but prefer the -convenience", and to do anything you wanted in your signal handler, -and be prepared to clean up core dumps now and again. - -Perl 5.7.3 and later avoid these problems by "deferring" signals. That is, -when the signal is delivered to the process by the system (to the C code -that implements Perl) a flag is set, and the handler returns immediately. -Then at strategic "safe" points in the Perl interpreter (e.g. when it is -about to execute a new opcode) the flags are checked and the Perl level -handler from %SIG is executed. The "deferred" scheme allows much more -flexibility in the coding of signal handlers as we know the Perl -interpreter is in a safe state, and that we are not in a system library -function when the handler is called. However the implementation does -differ from previous Perls in the following ways: - -=over 4 - -=item Long-running opcodes - -As the Perl interpreter looks at signal flags only when it is about -to execute a new opcode, a signal that arrives during a long-running -opcode (e.g. a regular expression operation on a very large string) will -not be seen until the current opcode completes. - -If a signal of any given type fires multiple times during an opcode -(such as from a fine-grained timer), the handler for that signal will -be called only once, after the opcode completes; all other -instances will be discarded. Furthermore, if your system's signal queue -gets flooded to the point that there are signals that have been raised -but not yet caught (and thus not deferred) at the time an opcode -completes, those signals may well be caught and deferred during -subsequent opcodes, with sometimes surprising results. For example, you -may see alarms delivered even after calling C<alarm(0)> as the latter -stops the raising of alarms but does not cancel the delivery of alarms -raised but not yet caught. Do not depend on the behaviors described in -this paragraph as they are side effects of the current implementation and -may change in future versions of Perl. - -=item Interrupting IO - -When a signal is delivered (e.g., SIGINT from a control-C) the operating -system breaks into IO operations like I<read>(2), which is used to -implement Perl's readline() function, the C<< <> >> operator. On older -Perls the handler was called immediately (and as C<read> is not "unsafe", -this worked well). With the "deferred" scheme the handler is I<not> called -immediately, and if Perl is using the system's C<stdio> library that -library may restart the C<read> without returning to Perl to give it a -chance to call the %SIG handler. If this happens on your system the -solution is to use the C<:perlio> layer to do IO--at least on those handles -that you want to be able to break into with signals. (The C<:perlio> layer -checks the signal flags and calls %SIG handlers before resuming IO -operation.) - -The default in Perl 5.7.3 and later is to automatically use -the C<:perlio> layer. - -Note that it is not advisable to access a file handle within a signal -handler where that signal has interrupted an I/O operation on that same -handle. While perl will at least try hard not to crash, there are no -guarantees of data integrity; for example, some data might get dropped or -written twice. - -Some networking library functions like gethostbyname() are known to have -their own implementations of timeouts which may conflict with your -timeouts. If you have problems with such functions, try using the POSIX -sigaction() function, which bypasses Perl safe signals. Be warned that -this does subject you to possible memory corruption, as described above. - -Instead of setting C<$SIG{ALRM}>: - - local $SIG{ALRM} = sub { die "alarm" }; - -try something like the following: - - use POSIX qw(SIGALRM); - POSIX::sigaction(SIGALRM, POSIX::SigAction->new(sub { die "alarm" })) - || die "Error setting SIGALRM handler: $!\n"; - -Another way to disable the safe signal behavior locally is to use -the C<Perl::Unsafe::Signals> module from CPAN, which affects -all signals. - -=item Restartable system calls - -On systems that supported it, older versions of Perl used the -SA_RESTART flag when installing %SIG handlers. This meant that -restartable system calls would continue rather than returning when -a signal arrived. In order to deliver deferred signals promptly, -Perl 5.7.3 and later do I<not> use SA_RESTART. Consequently, -restartable system calls can fail (with $! set to C<EINTR>) in places -where they previously would have succeeded. - -The default C<:perlio> layer retries C<read>, C<write> -and C<close> as described above; interrupted C<wait> and -C<waitpid> calls will always be retried. - -=item Signals as "faults" - -Certain signals like SEGV, ILL, and BUS are generated by virtual memory -addressing errors and similar "faults". These are normally fatal: there is -little a Perl-level handler can do with them. So Perl delivers them -immediately rather than attempting to defer them. - -=item Signals triggered by operating system state - -On some operating systems certain signal handlers are supposed to "do -something" before returning. One example can be CHLD or CLD, which -indicates a child process has completed. On some operating systems the -signal handler is expected to C<wait> for the completed child -process. On such systems the deferred signal scheme will not work for -those signals: it does not do the C<wait>. Again the failure will -look like a loop as the operating system will reissue the signal because -there are completed child processes that have not yet been C<wait>ed for. - -=back - -If you want the old signal behavior back despite possible -memory corruption, set the environment variable C<PERL_SIGNALS> to -C<"unsafe">. This feature first appeared in Perl 5.8.1. - -=head1 Named Pipes - -A named pipe (often referred to as a FIFO) is an old Unix IPC -mechanism for processes communicating on the same machine. It works -just like regular anonymous pipes, except that the -processes rendezvous using a filename and need not be related. - -To create a named pipe, use the C<POSIX::mkfifo()> function. - - use POSIX qw(mkfifo); - mkfifo($path, 0700) || die "mkfifo $path failed: $!"; - -You can also use the Unix command mknod(1), or on some -systems, mkfifo(1). These may not be in your normal path, though. - - # system return val is backwards, so && not || - # - $ENV{PATH} .= ":/etc:/usr/etc"; - if ( system("mknod", $path, "p") - && system("mkfifo", $path) ) - { - die "mk{nod,fifo} $path failed"; - } - - -A fifo is convenient when you want to connect a process to an unrelated -one. When you open a fifo, the program will block until there's something -on the other end. - -For example, let's say you'd like to have your F<.signature> file be a -named pipe that has a Perl program on the other end. Now every time any -program (like a mailer, news reader, finger program, etc.) tries to read -from that file, the reading program will read the new signature from your -program. We'll use the pipe-checking file-test operator, B<-p>, to find -out whether anyone (or anything) has accidentally removed our fifo. - - chdir(); # go home - my $FIFO = ".signature"; - - while (1) { - unless (-p $FIFO) { - unlink $FIFO; # discard any failure, will catch later - require POSIX; # delayed loading of heavy module - POSIX::mkfifo($FIFO, 0700) - || die "can't mkfifo $FIFO: $!"; - } - - # next line blocks till there's a reader - open (FIFO, "> $FIFO") || die "can't open $FIFO: $!"; - print FIFO "John Smith (smith\@host.org)\n", `fortune -s`; - close(FIFO) || die "can't close $FIFO: $!"; - sleep 2; # to avoid dup signals - } - -=head1 Using open() for IPC - -Perl's basic open() statement can also be used for unidirectional -interprocess communication by either appending or prepending a pipe -symbol to the second argument to open(). Here's how to start -something up in a child process you intend to write to: - - open(SPOOLER, "| cat -v | lpr -h 2>/dev/null") - || die "can't fork: $!"; - local $SIG{PIPE} = sub { die "spooler pipe broke" }; - print SPOOLER "stuff\n"; - close SPOOLER || die "bad spool: $! $?"; - -And here's how to start up a child process you intend to read from: - - open(STATUS, "netstat -an 2>&1 |") - || die "can't fork: $!"; - while (<STATUS>) { - next if /^(tcp|udp)/; - print; - } - close STATUS || die "bad netstat: $! $?"; - -If one can be sure that a particular program is a Perl script expecting -filenames in @ARGV, the clever programmer can write something like this: - - % program f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile - -and no matter which sort of shell it's called from, the Perl program will -read from the file F<f1>, the process F<cmd1>, standard input (F<tmpfile> -in this case), the F<f2> file, the F<cmd2> command, and finally the F<f3> -file. Pretty nifty, eh? - -You might notice that you could use backticks for much the -same effect as opening a pipe for reading: - - print grep { !/^(tcp|udp)/ } `netstat -an 2>&1`; - die "bad netstatus ($?)" if $?; - -While this is true on the surface, it's much more efficient to process the -file one line or record at a time because then you don't have to read the -whole thing into memory at once. It also gives you finer control of the -whole process, letting you kill off the child process early if you'd like. - -Be careful to check the return values from both open() and close(). If -you're I<writing> to a pipe, you should also trap SIGPIPE. Otherwise, -think of what happens when you start up a pipe to a command that doesn't -exist: the open() will in all likelihood succeed (it only reflects the -fork()'s success), but then your output will fail--spectacularly. Perl -can't know whether the command worked, because your command is actually -running in a separate process whose exec() might have failed. Therefore, -while readers of bogus commands return just a quick EOF, writers -to bogus commands will get hit with a signal, which they'd best be prepared -to handle. Consider: - - open(FH, "|bogus") || die "can't fork: $!"; - print FH "bang\n"; # neither necessary nor sufficient - # to check print retval! - close(FH) || die "can't close: $!"; - -The reason for not checking the return value from print() is because of -pipe buffering; physical writes are delayed. 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") || die "can't fork: $!"; - print FH "bang\n"; - close(FH) || die "can't close: status=$?"; - -=head2 Filehandles - -Both the main process and any child processes it forks share the same -STDIN, STDOUT, and STDERR filehandles. If both processes try to access -them at once, strange things can happen. You may also want to close -or reopen the filehandles for the child. You can get around this by -opening your pipe with open(), but on some systems this means that the -child process cannot outlive the parent. - -=head2 Background Processes - -You can run a command in the background with: - - system("cmd &"); - -The command's STDOUT and STDERR (and possibly STDIN, depending on your -shell) will be the same as the parent's. You won't need to catch -SIGCHLD because of the double-fork taking place; see below for details. - -=head2 Complete Dissociation of Child from Parent - -In some cases (starting server processes, for instance) you'll want to -completely dissociate the child process from the parent. This is -often called daemonization. A well-behaved daemon will also chdir() -to the root directory so it doesn't prevent unmounting the filesystem -containing the directory from which it was launched, and redirect its -standard file descriptors from and to F</dev/null> so that random -output doesn't wind up on the user's terminal. - - use POSIX "setsid"; - - sub daemonize { - chdir("/") || die "can't chdir to /: $!"; - open(STDIN, "< /dev/null") || die "can't read /dev/null: $!"; - open(STDOUT, "> /dev/null") || die "can't write to /dev/null: $!"; - defined(my $pid = fork()) || die "can't fork: $!"; - exit if $pid; # non-zero now means I am the parent - (setsid() != -1) || die "Can't start a new session: $!" - open(STDERR, ">&STDOUT") || die "can't dup stdout: $!"; - } - -The fork() has to come before the setsid() to ensure you aren't a -process group leader; the setsid() will fail if you are. If your -system doesn't have the setsid() function, open F</dev/tty> and use the -C<TIOCNOTTY> ioctl() on it instead. See tty(4) for details. - -Non-Unix users should check their C<< I<Your_OS>::Process >> module for -other possible solutions. - -=head2 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 C<"-|"> or C<"|-"> -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 I<to> minus, you can -write to the filehandle you opened and your kid will find it in I<his> -STDIN. If you open a pipe I<from> minus, you can read from the filehandle -you opened whatever your kid writes to I<his> STDOUT. - - use English qw[ -no_match_vars ]; - my $PRECIOUS = "/path/to/some/safe/file"; - my $sleep_count; - my $pid; - - 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) { # I am the parent - print KID_TO_WRITE @some_data; - close(KID_TO_WRITE) || warn "kid exited $?"; - } else { # I am the child - # drop permissions in setuid and/or setgid programs: - ($EUID, $EGID) = ($UID, $GID); - open (OUTFILE, "> $PRECIOUS") - || die "can't open $PRECIOUS: $!"; - while (<STDIN>) { - print OUTFILE; # child's STDIN is parent's KID_TO_WRITE - } - close(OUTFILE) || die "can't close $PRECIOUS: $!"; - exit(0); # don't forget this!! - } - -Another common use for this construct is when you need to execute -something without the shell's interference. With system(), it's -straightforward, but you can't use a pipe open or backticks safely. -That's because there's no way to stop the shell from getting its hands on -your arguments. Instead, use lower-level control to call exec() directly. - -Here's a safe backtick or pipe open for read: - - my $pid = open(KID_TO_READ, "-|"); - defined($pid) || die "can't fork: $!"; - - if ($pid) { # parent - while (<KID_TO_READ>) { - # do something interesting - } - close(KID_TO_READ) || warn "kid exited $?"; - - } else { # child - ($EUID, $EGID) = ($UID, $GID); # suid only - exec($program, @options, @args) - || die "can't exec program: $!"; - # NOTREACHED - } - -And here's a safe pipe open for writing: - - my $pid = open(KID_TO_WRITE, "|-"); - defined($pid) || die "can't fork: $!"; - - $SIG{PIPE} = sub { die "whoops, $program pipe broke" }; - - if ($pid) { # parent - print KID_TO_WRITE @data; - close(KID_TO_WRITE) || warn "kid exited $?"; - - } else { # child - ($EUID, $EGID) = ($UID, $GID); - exec($program, @options, @args) - || die "can't exec program: $!"; - # NOTREACHED - } - -It is very easy to dead-lock a process using this form of open(), or -indeed with any use of pipe() with multiple subprocesses. The -example above is "safe" because it is simple and calls exec(). See -L</"Avoiding Pipe Deadlocks"> for general safety principles, but there -are extra gotchas with Safe Pipe Opens. - -In particular, if you opened the pipe using C<open FH, "|-">, then you -cannot simply use close() in the parent process to close an unwanted -writer. Consider this code: - - my $pid = open(WRITER, "|-"); # fork open a kid - defined($pid) || die "first fork failed: $!"; - if ($pid) { - if (my $sub_pid = fork()) { - defined($sub_pid) || die "second fork failed: $!"; - close(WRITER) || die "couldn't close WRITER: $!"; - # now do something else... - } - else { - # first write to WRITER - # ... - # then when finished - close(WRITER) || die "couldn't close WRITER: $!"; - exit(0); - } - } - else { - # first do something with STDIN, then - exit(0); - } - -In the example above, the true parent does not want to write to the WRITER -filehandle, so it closes it. However, because WRITER was opened using -C<open FH, "|-">, it has a special behavior: closing it calls -waitpid() (see L<perlfunc/waitpid>), which waits for the subprocess -to exit. If the child process ends up waiting for something happening -in the section marked "do something else", you have deadlock. - -This can also be a problem with intermediate subprocesses in more -complicated code, which will call waitpid() on all open filehandles -during global destruction--in no predictable order. - -To solve this, you must manually use pipe(), fork(), and the form of -open() which sets one file descriptor to another, as shown below: - - pipe(READER, WRITER) || die "pipe failed: $!"; - $pid = fork(); - defined($pid) || die "first fork failed: $!"; - if ($pid) { - close READER; - if (my $sub_pid = fork()) { - defined($sub_pid) || die "first fork failed: $!"; - close(WRITER) || die "can't close WRITER: $!"; - } - else { - # write to WRITER... - # ... - # then when finished - close(WRITER) || die "can't close WRITER: $!"; - exit(0); - } - # write to WRITER... - } - else { - open(STDIN, "<&READER") || die "can't reopen STDIN: $!"; - close(WRITER) || die "can't close WRITER: $!"; - # do something... - exit(0); - } - -Since Perl 5.8.0, you can also use the list form of C<open> for pipes. -This is preferred when you wish to avoid having the shell interpret -metacharacters that may be in your command string. - -So for example, instead of using: - - open(PS_PIPE, "ps aux|") || die "can't open ps pipe: $!"; - -One would use either of these: - - open(PS_PIPE, "-|", "ps", "aux") - || die "can't open ps pipe: $!"; - - @ps_args = qw[ ps aux ]; - open(PS_PIPE, "-|", @ps_args) - || die "can't open @ps_args|: $!"; - -Because there are more than three arguments to open(), forks the ps(1) -command I<without> spawning a shell, and reads its standard output via the -C<PS_PIPE> filehandle. The corresponding syntax to I<write> to command -pipes is to use C<"|-"> in place of C<"-|">. - -This was admittedly a rather silly example, because you're using string -literals whose content is perfectly safe. There is therefore no cause to -resort to the harder-to-read, multi-argument form of pipe open(). However, -whenever you cannot be assured that the program arguments are free of shell -metacharacters, the fancier form of open() should be used. For example: - - @grep_args = ("egrep", "-i", $some_pattern, @many_files); - open(GREP_PIPE, "-|", @grep_args) - || die "can't open @grep_args|: $!"; - -Here the multi-argument form of pipe open() is preferred because the -pattern and indeed even the filenames themselves might hold metacharacters. - -Be aware that these operations are full Unix forks, which means they may -not be correctly implemented on all alien systems. Additionally, these are -not true multithreading. To learn more about threading, see the F<modules> -file mentioned below in the SEE ALSO section. - -=head2 Avoiding Pipe Deadlocks - -Whenever you have more than one subprocess, you must be careful that each -closes whichever half of any pipes created for interprocess communication -it is not using. This is because any child process reading from the pipe -and expecting an EOF will never receive it, and therefore never exit. A -single process closing a pipe is not enough to close it; the last process -with the pipe open must close it for it to read EOF. - -Certain built-in Unix features help prevent this most of the time. For -instance, filehandles have a "close on exec" flag, which is set I<en masse> -under control of the C<$^F> variable. This is so any filehandles you -didn't explicitly route to the STDIN, STDOUT or STDERR of a child -I<program> will be automatically closed. - -Always explicitly and immediately call close() on the writable end of any -pipe, unless that process is actually writing to it. Even if you don't -explicitly call close(), Perl will still close() all filehandles during -global destruction. As previously discussed, if those filehandles have -been opened with Safe Pipe Open, this will result in calling waitpid(), -which may again deadlock. - -=head2 Bidirectional Communication with Another Process - -While this works reasonably well for unidirectional communication, what -about bidirectional communication? The most obvious approach doesn't work: - - # THIS DOES NOT WORK!! - open(PROG_FOR_READING_AND_WRITING, "| some program |") - -If you forget to C<use warnings>, you'll miss out entirely on the -helpful diagnostic message: - - Can't do bidirectional pipe at -e line 1. - -If you really want to, you can use the standard open2() from the -C<IPC::Open2> module to catch both ends. There's also an open3() in -C<IPC::Open3> for tridirectional I/O so you can also catch your child's -STDERR, but doing so would then require an awkward select() loop and -wouldn't allow you to use normal Perl input operations. - -If you look at its source, you'll see that open2() uses low-level -primitives like the pipe() and exec() syscalls to create all the -connections. Although it might have been more efficient by using -socketpair(), this would have been even less portable than it already -is. The open2() and open3() functions are unlikely to work anywhere -except on a Unix system, or at least one purporting POSIX compliance. - -=for TODO -Hold on, is this even true? First it says that socketpair() is avoided -for portability, but then it says it probably won't work except on -Unixy systems anyway. Which one of those is true? - -Here's an example of using open2(): - - use FileHandle; - use IPC::Open2; - $pid = open2(*Reader, *Writer, "cat -un"); - print Writer "stuff\n"; - $got = <Reader>; - -The problem with this is that buffering is really going to ruin your -day. Even though your C<Writer> filehandle is auto-flushed so the process -on the other end gets your data in a timely manner, you can't usually do -anything to force that process to give its data to you in a similarly quick -fashion. In this special case, we could actually so, because we gave -I<cat> a B<-u> flag to make it unbuffered. But very few commands are -designed to operate over pipes, so this seldom works unless you yourself -wrote the program on the other end of the double-ended pipe. - -A solution to this is to use a library which uses pseudottys to make your -program behave more reasonably. This way you don't have to have control -over the source code of the program you're using. The C<Expect> module -from CPAN also addresses this kind of thing. This module requires two -other modules from CPAN, C<IO::Pty> and C<IO::Stty>. It sets up a pseudo -terminal to interact with programs that insist on talking to the terminal -device driver. If your system is supported, this may be your best bet. - -=head2 Bidirectional Communication with Yourself - -If you want, you may make low-level pipe() and fork() syscalls to stitch -this together by hand. This example only talks to itself, but you could -reopen the appropriate handles to STDIN and STDOUT and call other processes. -(The following example lacks proper error checking.) - - #!/usr/bin/perl -w - # pipe1 - bidirectional communication using two pipe pairs - # designed for the socketpair-challenged - use IO::Handle; # thousands of lines just for autoflush :-( - pipe(PARENT_RDR, CHILD_WTR); # XXX: check failure? - pipe(CHILD_RDR, PARENT_WTR); # XXX: check failure? - CHILD_WTR->autoflush(1); - PARENT_WTR->autoflush(1); - - if ($pid = fork()) { - close PARENT_RDR; - close PARENT_WTR; - print CHILD_WTR "Parent Pid $$ is sending this\n"; - chomp($line = <CHILD_RDR>); - print "Parent Pid $$ just read this: '$line'\n"; - close CHILD_RDR; close CHILD_WTR; - waitpid($pid, 0); - } else { - die "cannot fork: $!" unless defined $pid; - close CHILD_RDR; - close CHILD_WTR; - chomp($line = <PARENT_RDR>); - print "Child Pid $$ just read this: '$line'\n"; - print PARENT_WTR "Child Pid $$ is sending this\n"; - close PARENT_RDR; - close PARENT_WTR; - exit(0); - } - -But you don't actually have to make two pipe calls. If you -have the socketpair() system call, it will do this all for you. - - #!/usr/bin/perl -w - # pipe2 - bidirectional communication using socketpair - # "the best ones always go both ways" - - use Socket; - use IO::Handle; # thousands of lines just for autoflush :-( - - # We say AF_UNIX because although *_LOCAL is the - # POSIX 1003.1g form of the constant, many machines - # still don't have it. - socketpair(CHILD, PARENT, AF_UNIX, SOCK_STREAM, PF_UNSPEC) - || die "socketpair: $!"; - - CHILD->autoflush(1); - PARENT->autoflush(1); - - if ($pid = fork()) { - close PARENT; - print CHILD "Parent Pid $$ 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(0); - } - -=head1 Sockets: Client/Server Communication - -While not entirely limited to Unix-derived operating systems (e.g., WinSock -on PCs provides socket support, as do some VMS libraries), you might not have -sockets on your system, in which case this section probably isn't going to -do you much good. With sockets, you can do both virtual circuits like TCP -streams and datagrams like UDP packets. You may be able to do even more -depending on your system. - -The Perl functions for dealing with sockets have the same names as -the corresponding system calls in C, but their arguments tend to differ -for two reasons. First, Perl filehandles work differently than C file -descriptors. Second, Perl already knows the length of its strings, so you -don't need to pass that information. - -One of the major problems with ancient, antemillennial socket code in Perl -was that it used hard-coded values for some of the constants, which -severely hurt portability. If you ever see code that does anything like -explicitly setting C<$AF_INET = 2>, you know you're in for big trouble. -An immeasurably superior approach is to use the C<Socket> module, which more -reliably grants access to the various constants and functions you'll need. - -If you're not writing a server/client for an existing protocol like -NNTP or SMTP, you should give some thought to how your server will -know when the client has finished talking, and vice-versa. Most -protocols are based on one-line messages and responses (so one party -knows the other has finished when a "\n" is received) or multi-line -messages and responses that end with a period on an empty line -("\n.\n" terminates a message/response). - -=head2 Internet Line Terminators - -The Internet line terminator is "\015\012". Under ASCII variants of -Unix, that could usually be written as "\r\n", but under other systems, -"\r\n" might at times be "\015\015\012", "\012\012\015", or something -completely different. The standards specify writing "\015\012" to be -conformant (be strict in what you provide), but they also recommend -accepting a lone "\012" on input (be lenient in what you require). -We haven't always been very good about that in the code in this manpage, -but unless you're on a Mac from way back in its pre-Unix dark ages, you'll -probably be ok. - -=head2 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 ($line = <SOCK>) { - print $line; - } - - close (SOCK) || die "close: $!"; - exit(0); - -And here's a corresponding server to go along with it. We'll -leave the address as C<INADDR_ANY> so that the kernel can choose -the appropriate interface on multihomed hosts. If you want sit -on a particular interface (like the external side of a gateway -or firewall machine), fill this in with your real address instead. - - #!/usr/bin/perl -Tw - use strict; - BEGIN { $ENV{PATH} = "/usr/bin:/bin" } - use Socket; - use Carp; - my $EOL = "\015\012"; - - sub logmsg { print "$0 $$: @_ at ", scalar localtime(), "\n" } - - my $port = shift || 2345; - die "invalid port" unless if $port =~ /^ \d+ $/x; - - my $proto = getprotobyname("tcp"); - - socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!"; - setsockopt(Server, SOL_SOCKET, SO_REUSEADDR, pack("l", 1)) - || die "setsockopt: $!"; - bind(Server, sockaddr_in($port, INADDR_ANY)) || die "bind: $!"; - listen(Server, SOMAXCONN) || die "listen: $!"; - - logmsg "server started on port $port"; - - my $paddr; - - $SIG{CHLD} = \&REAPER; - - for ( ; $paddr = accept(Client, Server); close Client) { - my($port, $iaddr) = sockaddr_in($paddr); - my $name = gethostbyaddr($iaddr, AF_INET); - - logmsg "connection from $name [", - inet_ntoa($iaddr), "] - at port $port"; - - print Client "Hello there, $name, it's now ", - scalar localtime(), $EOL; - } - -And here's a multithreaded version. It's multithreaded in that -like most typical servers, it spawns (fork()s) a slave server to -handle the client request so that the master server can quickly -go back to service a new client. - - #!/usr/bin/perl -Tw - use strict; - BEGIN { $ENV{PATH} = "/usr/bin:/bin" } - use Socket; - use Carp; - my $EOL = "\015\012"; - - sub spawn; # forward declaration - sub logmsg { print "$0 $$: @_ at ", scalar localtime(), "\n" } - - my $port = shift || 2345; - die "invalid port" unless if $port =~ /^ \d+ $/x; - - my $proto = getprotobyname("tcp"); - - socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!"; - setsockopt(Server, SOL_SOCKET, SO_REUSEADDR, pack("l", 1)) - || die "setsockopt: $!"; - bind(Server, sockaddr_in($port, INADDR_ANY)) || die "bind: $!"; - listen(Server, SOMAXCONN) || die "listen: $!"; - - logmsg "server started on port $port"; - - my $waitedpid = 0; - my $paddr; - - use POSIX ":sys_wait_h"; - use Errno; - - sub REAPER { - local $!; # don't let waitpid() overwrite current error - while ((my $pid = waitpid(-1, WNOHANG)) > 0 && WIFEXITED($?)) { - logmsg "reaped $waitedpid" . ($? ? " with exit $?" : ""); - } - $SIG{CHLD} = \&REAPER; # loathe SysV - } - - $SIG{CHLD} = \&REAPER; - - while (1) { - $paddr = accept(Client, Server) || do { - # try again if accept() returned because got a signal - next if $!{EINTR}; - die "accept: $!"; - }; - my ($port, $iaddr) = sockaddr_in($paddr); - my $name = gethostbyaddr($iaddr, AF_INET); - - logmsg "connection from $name [", - inet_ntoa($iaddr), - "] at port $port"; - - spawn sub { - $| = 1; - print "Hello there, $name, it's now ", scalar localtime(), $EOL; - exec "/usr/games/fortune" # XXX: "wrong" line terminators - or confess "can't exec fortune: $!"; - }; - close Client; - } - - sub spawn { - my $coderef = shift; - - unless (@_ == 0 && $coderef && ref($coderef) eq "CODE") { - confess "usage: spawn CODEREF"; - } - - my $pid; - unless (defined($pid = fork())) { - logmsg "cannot fork: $!"; - return; - } - elsif ($pid) { - logmsg "begat $pid"; - return; # I'm the parent - } - # else I'm the child -- go spawn - - open(STDIN, "<&Client") || die "can't dup client to stdin"; - open(STDOUT, ">&Client") || die "can't dup client to stdout"; - ## open(STDERR, ">&STDOUT") || die "can't dup stdout to stderr"; - exit($coderef->()); - } - -This server takes the trouble to clone off a child version via fork() -for each incoming request. That way it can handle many requests at -once, which you might not always want. Even if you don't fork(), the -listen() will allow that many pending connections. Forking servers -have to be particularly careful about cleaning up their dead children -(called "zombies" in Unix parlance), because otherwise you'll quickly -fill up your process table. The REAPER subroutine is used here to -call waitpid() for any child processes that have finished, thereby -ensuring that they terminate cleanly and don't join the ranks of the -living dead. - -Within the while loop we call accept() and check to see if it returns -a false value. This would normally indicate a system error needs -to be reported. However, the introduction of safe signals (see -L</Deferred Signals (Safe Signals)> above) in Perl 5.7.3 means that -accept() might also be interrupted when the process receives a signal. -This typically happens when one of the forked subprocesses exits and -notifies the parent process with a CHLD signal. - -If accept() is interrupted by a signal, $! will be set to EINTR. -If this happens, we can safely continue to the next iteration of -the loop and another call to accept(). It is important that your -signal handling code not modify the value of $!, or else this test -will likely fail. In the REAPER subroutine we create a local version -of $! before calling waitpid(). When waitpid() sets $! to ECHILD as -it inevitably does when it has no more children waiting, it -updates the local copy and leaves the original unchanged. - -You should use the B<-T> flag to enable taint checking (see L<perlsec>) -even if we aren't running setuid or setgid. This is always a good idea -for servers or any program run on behalf of someone else (like CGI -scripts), because it lessens the chances that people from the outside will -be able to compromise your system. - -Let's look at another TCP client. This one connects to the TCP "time" -service on a number of different machines and shows how far their clocks -differ from the system on which it's being run: - - #!/usr/bin/perl -w - use strict; - use Socket; - - my $SECS_OF_70_YEARS = 2208988800; - sub ctime { scalar localtime(shift() || time()) } - - my $iaddr = gethostbyname("localhost"); - my $proto = getprotobyname("tcp"); - my $port = getservbyname("time", "tcp"); - my $paddr = sockaddr_in(0, $iaddr); - my($host); - - $| = 1; - printf "%-24s %8s %s\n", "localhost", 0, ctime(); - - foreach $host (@ARGV) { - printf "%-24s ", $host; - my $hisiaddr = inet_aton($host) || die "unknown host"; - my $hispaddr = sockaddr_in($port, $hisiaddr); - socket(SOCKET, PF_INET, SOCK_STREAM, $proto) - || die "socket: $!"; - connect(SOCKET, $hispaddr) || die "connect: $!"; - my $rtime = pack("C4", ()); - read(SOCKET, $rtime, 4); - close(SOCKET); - my $histime = unpack("N", $rtime) - $SECS_OF_70_YEARS; - printf "%8d %s\n", $histime - time(), ctime($histime); - } - -=head2 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) 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 B<-S> file test: - - unless (-S "/dev/log") { - die "something's wicked with the log system"; - } - -Here's a sample Unix-domain client: - - #!/usr/bin/perl -w - use Socket; - use strict; - my ($rendezvous, $line); - - $rendezvous = shift || "catsock"; - socket(SOCK, PF_UNIX, SOCK_STREAM, 0) || die "socket: $!"; - connect(SOCK, sockaddr_un($rendezvous)) || die "connect: $!"; - while (defined($line = <SOCK>)) { - print $line; - } - exit(0); - -And here's a corresponding server. You don't have to worry about silly -network terminators here because Unix domain sockets are guaranteed -to be on the localhost, and thus everything works right. - - #!/usr/bin/perl -Tw - use strict; - use Socket; - use Carp; - - BEGIN { $ENV{PATH} = "/usr/bin:/bin" } - sub spawn; # forward declaration - sub logmsg { print "$0 $$: @_ at ", scalar localtime(), "\n" } - - my $NAME = "catsock"; - my $uaddr = sockaddr_un($NAME); - my $proto = getprotobyname("tcp"); - - socket(Server, PF_UNIX, SOCK_STREAM, 0) || die "socket: $!"; - unlink($NAME); - bind (Server, $uaddr) || die "bind: $!"; - listen(Server, SOMAXCONN) || die "listen: $!"; - - logmsg "server started on $NAME"; - - my $waitedpid; - - use POSIX ":sys_wait_h"; - sub REAPER { - my $child; - while (($waitedpid = waitpid(-1, WNOHANG)) > 0) { - logmsg "reaped $waitedpid" . ($? ? " with exit $?" : ""); - } - $SIG{CHLD} = \&REAPER; # loathe SysV - } - - $SIG{CHLD} = \&REAPER; - - - for ( $waitedpid = 0; - accept(Client, Server) || $waitedpid; - $waitedpid = 0, close Client) - { - next if $waitedpid; - logmsg "connection on $NAME"; - spawn sub { - print "Hello there, it's now ", scalar localtime(), "\n"; - exec("/usr/games/fortune") || die "can't exec fortune: $!"; - }; - } - - sub spawn { - my $coderef = shift(); - - unless (@_ == 0 && $coderef && ref($coderef) eq "CODE") { - confess "usage: spawn CODEREF"; - } - - my $pid; - unless (defined($pid = fork())) { - logmsg "cannot fork: $!"; - return; - } - elsif ($pid) { - logmsg "begat $pid"; - return; # I'm the parent - } - else { - # I'm the child -- go spawn - } - - open(STDIN, "<&Client") || die "can't dup client to stdin"; - open(STDOUT, ">&Client") || die "can't dup client to stdout"; - ## open(STDERR, ">&STDOUT") || die "can't dup stdout to stderr"; - exit($coderef->()); - } - -As you see, it's remarkably similar to the Internet domain TCP server, so -much so, in fact, that we've omitted several duplicate functions--spawn(), -logmsg(), ctime(), and REAPER()--which are the same as in the other server. - -So why would you ever want to use a Unix domain socket instead of a -simpler named pipe? Because a named pipe doesn't give you sessions. You -can't tell one process's data from another's. With socket programming, -you get a separate session for each client; that's why accept() takes two -arguments. - -For example, let's say that you have a long-running database server daemon -that you want folks to be able to access from the Web, but only -if they go through a CGI interface. You'd have a small, simple CGI -program that does whatever checks and logging you feel like, and then acts -as a Unix-domain client and connects to your private server. - -=head1 TCP Clients with IO::Socket - -For those preferring a higher-level interface to socket programming, the -IO::Socket module provides an object-oriented approach. IO::Socket has -been included in the standard Perl distribution ever since Perl 5.004. If -you're running an earlier version of Perl (in which case, how are you -reading this manpage?), just fetch IO::Socket from CPAN, where you'll also -find modules providing easy interfaces to the following systems: DNS, FTP, -Ident (RFC 931), NIS and NISPlus, NNTP, Ping, POP3, SMTP, SNMP, SSLeay, -Telnet, and Time--to name just a few. - -=head2 A Simple Client - -Here's a client that creates a TCP connection to the "daytime" -service at port 13 of the host name "localhost" and prints out everything -that the server there cares to provide. - - #!/usr/bin/perl -w - use IO::Socket; - $remote = IO::Socket::INET->new( - Proto => "tcp", - PeerAddr => "localhost", - PeerPort => "daytime(13)", - ) - || die "can't connect to daytime service on localhost"; - while (<$remote>) { print } - -When you run this program, you should get something back that -looks like this: - - Wed May 14 08:40:46 MDT 1997 - -Here are what those parameters to the new() constructor mean: - -=over 4 - -=item C<Proto> - -This is which protocol to use. In this case, the socket handle returned -will be connected to a TCP socket, because we want a stream-oriented -connection, that is, one that acts pretty much like a plain old file. -Not all sockets are this of this type. For example, the UDP protocol -can be used to make a datagram socket, used for message-passing. - -=item C<PeerAddr> - -This is the name or Internet address of the remote host the server is -running on. We could have specified a longer name like C<"www.perl.com">, -or an address like C<"207.171.7.72">. For demonstration purposes, we've -used the special hostname C<"localhost">, which should always mean the -current machine you're running on. The corresponding Internet address -for localhost is C<"127.0.0.1">, if you'd rather use that. - -=item C<PeerPort> - -This is the service name or port number we'd like to connect to. -We could have gotten away with using just C<"daytime"> on systems with a -well-configured system services file,[FOOTNOTE: The system services file -is found in I</etc/services> under Unixy systems.] but here we've specified the -port number (13) in parentheses. Using just the number would have also -worked, but numeric literals make careful programmers nervous. - -=back - -Notice how the return value from the C<new> constructor is used as -a filehandle in the C<while> loop? That's what's called an I<indirect -filehandle>, a scalar variable containing a filehandle. You can use -it the same way you would a normal filehandle. For example, you -can read one line from it this way: - - $line = <$handle>; - -all remaining lines from is this way: - - @lines = <$handle>; - -and send a line of data to it this way: - - print $handle "some data\n"; - -=head2 A Webget Client - -Here's a simple client that takes a remote host to fetch a document -from, and then a list of files to get from that host. This is a -more interesting client than the previous one because it first sends -something to the server before fetching the server's response. - - #!/usr/bin/perl -w - use IO::Socket; - unless (@ARGV > 1) { die "usage: $0 host url ..." } - $host = shift(@ARGV); - $EOL = "\015\012"; - $BLANK = $EOL x 2; - for my $document (@ARGV) { - $remote = IO::Socket::INET->new( Proto => "tcp", - PeerAddr => $host, - PeerPort => "http(80)", - ) || die "cannot connect to httpd on $host"; - $remote->autoflush(1); - print $remote "GET $document HTTP/1.0" . $BLANK; - while ( <$remote> ) { print } - close $remote; - } - -The web server handling the HTTP service is assumed to be at -its standard port, number 80. If the server you're trying to -connect to is at a different port, like 1080 or 8080, you should specify it -as the named-parameter pair, C<< PeerPort => 8080 >>. The C<autoflush> -method is used on the socket because otherwise the system would buffer -up the output we sent it. (If you're on a prehistoric Mac, you'll also -need to change every C<"\n"> in your code that sends data over the network -to be a C<"\015\012"> instead.) - -Connecting to the server is only the first part of the process: once you -have the connection, you have to use the server's language. Each server -on the network has its own little command language that it expects as -input. The string that we send to the server starting with "GET" is in -HTTP syntax. In this case, we simply request each specified document. -Yes, we really are making a new connection for each document, even though -it's the same host. That's the way you always used to have to speak HTTP. -Recent versions of web browsers may request that the remote server leave -the connection open a little while, but the server doesn't have to honor -such a request. - -Here's an example of running that program, which we'll call I<webget>: - - % webget www.perl.com /guanaco.html - HTTP/1.1 404 File Not Found - Date: Thu, 08 May 1997 18:02:32 GMT - Server: Apache/1.2b6 - Connection: close - Content-type: text/html - - <HEAD><TITLE>404 File Not Found</TITLE></HEAD> - <BODY><H1>File Not Found</H1> - The requested URL /guanaco.html was not found on this server.<P> - </BODY> - -Ok, so that's not very interesting, because it didn't find that -particular document. But a long response wouldn't have fit on this page. - -For a more featureful version of this program, you should look to -the I<lwp-request> program included with the LWP modules from CPAN. - -=head2 Interactive Client with IO::Socket - -Well, that's all fine if you want to send one command and get one answer, -but what about setting up something fully interactive, somewhat like -the way I<telnet> works? That way you can type a line, get the answer, -type a line, get the answer, etc. - -This client is more complicated than the two we've done so far, but if -you're on a system that supports the powerful C<fork> call, the solution -isn't that rough. Once you've made the connection to whatever service -you'd like to chat with, call C<fork> to clone your process. Each of -these two identical process has a very simple job to do: the parent -copies everything from the socket to standard output, while the child -simultaneously copies everything from standard input to the socket. -To accomplish the same thing using just one process would be I<much> -harder, because it's easier to code two processes to do one thing than it -is to code one process to do two things. (This keep-it-simple principle -a cornerstones of the Unix philosophy, and good software engineering as -well, which is probably why it's spread to other systems.) - -Here's the code: - - #!/usr/bin/perl -w - use strict; - use IO::Socket; - my ($host, $port, $kidpid, $handle, $line); - - unless (@ARGV == 2) { die "usage: $0 host port" } - ($host, $port) = @ARGV; - - # create a tcp connection to the specified host and port - $handle = IO::Socket::INET->new(Proto => "tcp", - PeerAddr => $host, - PeerPort => $port) - || 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; - } - exit(0); # just in case - } - -The C<kill> function in the parent's C<if> block is there to send a -signal to our child process, currently running in the C<else> block, -as soon as the remote server has closed its end of the connection. - -If the remote server sends data a byte at time, and you need that -data immediately without waiting for a newline (which might not happen), -you may wish to replace the C<while> loop in the parent with the -following: - - my $byte; - while (sysread($handle, $byte, 1) == 1) { - print STDOUT $byte; - } - -Making a system call for each byte you want to read is not very efficient -(to put it mildly) but is the simplest to explain and works reasonably -well. - -=head1 TCP Servers with IO::Socket - -As always, setting up a server is little bit more involved than running a client. -The model is that the server creates a special kind of socket that -does nothing but listen on a particular port for incoming connections. -It does this by calling the C<< IO::Socket::INET->new() >> method with -slightly different arguments than the client did. - -=over 4 - -=item Proto - -This is which protocol to use. Like our clients, we'll -still specify C<"tcp"> here. - -=item LocalPort - -We specify a local -port in the C<LocalPort> argument, which we didn't do for the client. -This is service name or port number for which you want to be the -server. (Under Unix, ports under 1024 are restricted to the -superuser.) In our sample, we'll use port 9000, but you can use -any port that's not currently in use on your system. If you try -to use one already in used, you'll get an "Address already in use" -message. Under Unix, the C<netstat -a> command will show -which services current have servers. - -=item Listen - -The C<Listen> parameter is set to the maximum number of -pending connections we can accept until we turn away incoming clients. -Think of it as a call-waiting queue for your telephone. -The low-level Socket module has a special symbol for the system maximum, which -is SOMAXCONN. - -=item Reuse - -The C<Reuse> parameter is needed so that we restart our server -manually without waiting a few minutes to allow system buffers to -clear out. - -=back - -Once the generic server socket has been created using the parameters -listed above, the server then waits for a new client to connect -to it. The server blocks in the C<accept> method, which eventually accepts a -bidirectional connection from the remote client. (Make sure to autoflush -this handle to circumvent buffering.) - -To add to user-friendliness, our server prompts the user for commands. -Most servers don't do this. Because of the prompt without a newline, -you'll have to use the C<sysread> variant of the interactive client above. - -This server accepts one of five different commands, sending output back to -the client. Unlike most network servers, this one handles only one -incoming client at a time. Multithreaded servers are covered in -Chapter 16 of the Camel. - -Here's the code. We'll - - #!/usr/bin/perl -w - use IO::Socket; - use Net::hostent; # for OOish version of gethostbyaddr - - $PORT = 9000; # pick something not in use - - $server = IO::Socket::INET->new( Proto => "tcp", - LocalPort => $PORT, - Listen => SOMAXCONN, - Reuse => 1); - - die "can't setup server" unless $server; - print "[Server $0 accepting clients]\n"; - - while ($client = $server->accept()) { - $client->autoflush(1); - print $client "Welcome to $0; type help for command list.\n"; - $hostinfo = gethostbyaddr($client->peeraddr); - printf "[Connect from %s]\n", $hostinfo ? $hostinfo->name : $client->peerhost; - print $client "Command? "; - while ( <$client>) { - next unless /\S/; # blank line - if (/quit|exit/i) { last } - elsif (/date|time/i) { printf $client "%s\n", scalar localtime() } - elsif (/who/i ) { print $client `who 2>&1` } - elsif (/cookie/i ) { print $client `/usr/games/fortune 2>&1` } - elsif (/motd/i ) { print $client `cat /etc/motd 2>&1` } - else { - print $client "Commands: quit date who cookie motd\n"; - } - } continue { - print $client "Command? "; - } - close $client; - } - -=head1 UDP: Message Passing - -Another kind of client-server setup is one that uses not connections, but -messages. UDP communications involve much lower overhead but also provide -less reliability, as there are no promises that messages will arrive at -all, let alone in order and unmangled. Still, UDP offers some advantages -over TCP, including being able to "broadcast" or "multicast" to a whole -bunch of destination hosts at once (usually on your local subnet). If you -find yourself overly concerned about reliability and start building checks -into your message system, then you probably should use just TCP to start -with. - -UDP datagrams are I<not> a bytestream and should not be treated as such. -This makes using I/O mechanisms with internal buffering like stdio (i.e. -print() and friends) especially cumbersome. Use syswrite(), or better -send(), like in the example below. - -Here's a UDP program similar to the sample Internet TCP client given -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 = 2_208_988_800; - - $iaddr = gethostbyname(hostname()); - $proto = getprotobyname("udp"); - $port = getservbyname("time", "udp"); - $paddr = sockaddr_in(0, $iaddr); # 0 means let kernel pick - - socket(SOCKET, PF_INET, SOCK_DGRAM, $proto) || die "socket: $!"; - bind(SOCKET, $paddr) || die "bind: $!"; - - $| = 1; - printf "%-12s %8s %s\n", "localhost", 0, scalar localtime(); - $count = 0; - for $host (@ARGV) { - $count++; - $hisiaddr = inet_aton($host) || die "unknown host"; - $hispaddr = sockaddr_in($port, $hisiaddr); - defined(send(SOCKET, 0, 0, $hispaddr)) || die "send $host: $!"; - } - - $rin = ""; - vec($rin, fileno(SOCKET), 1) = 1; - - # timeout after 10.0 seconds - while ($count && select($rout = $rin, undef, undef, 10.0)) { - $rtime = ""; - $hispaddr = recv(SOCKET, $rtime, 4, 0) || die "recv: $!"; - ($port, $hisiaddr) = sockaddr_in($hispaddr); - $host = gethostbyaddr($hisiaddr, AF_INET); - $histime = unpack("N", $rtime) - $SECS_OF_70_YEARS; - printf "%-12s ", $host; - printf "%8d %s\n", $histime - time(), scalar localtime($histime); - $count--; - } - -This example does not include any retries and may consequently fail to -contact a reachable host. The most prominent reason for this is congestion -of the queues on the sending host if the number of hosts to contact is -sufficiently large. - -=head1 SysV IPC - -While System V IPC isn't so widely used as sockets, it still has some -interesting uses. However, you cannot use SysV IPC or Berkeley mmap() to -have a variable shared amongst several processes. That's because Perl -would reallocate your string when you weren't wanting it to. You might -look into the C<IPC::Shareable> or C<threads::shared> modules for that. - -Here's a small example showing shared memory usage. - - use IPC::SysV qw(IPC_PRIVATE IPC_RMID S_IRUSR S_IWUSR); - - $size = 2000; - $id = shmget(IPC_PRIVATE, $size, S_IRUSR | S_IWUSR); - defined($id) || die "shmget: $!"; - print "shm key $id\n"; - - $message = "Message #1"; - shmwrite($id, $message, 0, 60) || die "shmwrite: $!"; - print "wrote: '$message'\n"; - shmread($id, $buff, 0, 60) || die "shmread: $!"; - print "read : '$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 "shmctl: $!"; - -Here's an example of a semaphore: - - use IPC::SysV qw(IPC_CREAT); - - $IPC_KEY = 1234; - $id = semget($IPC_KEY, 10, 0666 | IPC_CREAT); - defined($id) || die "shmget: $!"; - print "shm key $id\n"; - -Put this code in a separate file to be run in more than one process. -Call the file F<take>: - - # create a semaphore - - $IPC_KEY = 1234; - $id = semget($IPC_KEY, 0, 0); - defined($id) || die "shmget: $!"; - - $semnum = 0; - $semflag = 0; - - # "take" semaphore - # wait for semaphore to be zero - $semop = 0; - $opstring1 = pack("s!s!s!", $semnum, $semop, $semflag); - - # Increment the semaphore count - $semop = 1; - $opstring2 = pack("s!s!s!", $semnum, $semop, $semflag); - $opstring = $opstring1 . $opstring2; - - semop($id, $opstring) || die "semop: $!"; - -Put this code in a separate file to be run in more than one process. -Call this file F<give>: - - # "give" the semaphore - # run this in the original process and you will see - # that the second process continues - - $IPC_KEY = 1234; - $id = semget($IPC_KEY, 0, 0); - die unless defined($id); - - $semnum = 0; - $semflag = 0; - - # Decrement the semaphore count - $semop = -1; - $opstring = pack("s!s!s!", $semnum, $semop, $semflag); - - semop($id, $opstring) || die "semop: $!"; - -The SysV IPC code above was written long ago, and it's definitely -clunky looking. For a more modern look, see the IPC::SysV module -which is included with Perl starting from Perl 5.005. - -A small example demonstrating SysV message queues: - - use IPC::SysV qw(IPC_PRIVATE IPC_RMID IPC_CREAT S_IRUSR S_IWUSR); - - my $id = msgget(IPC_PRIVATE, IPC_CREAT | S_IRUSR | S_IWUSR); - defined($id) || die "msgget failed: $!"; - - my $sent = "message"; - my $type_sent = 1234; - - msgsnd($id, pack("l! a*", $type_sent, $sent), 0) - || die "msgsnd failed: $!"; - - msgrcv($id, my $rcvd_buf, 60, 0, 0) - || die "msgrcv failed: $!"; - - my($type_rcvd, $rcvd) = unpack("l! a*", $rcvd_buf); - - if ($rcvd eq $sent) { - print "okay\n"; - } else { - print "not okay\n"; - } - - msgctl($id, IPC_RMID, 0) || die "msgctl failed: $!\n"; - -=head1 NOTES - -Most of these routines quietly but politely return C<undef> when they -fail instead of causing your program to die right then and there due to -an uncaught exception. (Actually, some of the new I<Socket> conversion -functions do croak() on bad arguments.) It is therefore essential to -check return values from these functions. Always begin your socket -programs this way for optimal success, and don't forget to add the B<-T> -taint-checking flag to the C<#!> line for servers: - - #!/usr/bin/perl -Tw - use strict; - use sigtrap; - use Socket; - -=head1 BUGS - -These routines all create system-specific portability problems. As noted -elsewhere, Perl is at the mercy of your C libraries for much of its system -behavior. It's probably safest to assume broken SysV semantics for -signals and to stick with simple TCP and UDP socket operations; e.g., don't -try to pass open file descriptors over a local UDP datagram socket if you -want your code to stand a chance of being portable. - -=head1 AUTHOR - -Tom Christiansen, with occasional vestiges of Larry Wall's original -version and suggestions from the Perl Porters. - -=head1 SEE ALSO - -There's a lot more to networking than this, but this should get you -started. - -For intrepid programmers, the indispensable textbook is I<Unix Network -Programming, 2nd Edition, Volume 1> by W. Richard Stevens (published by -Prentice-Hall). Most books on networking address the subject from the -perspective of a C programmer; translation to Perl is left as an exercise -for the reader. - -The IO::Socket(3) manpage describes the object library, and the Socket(3) -manpage describes the low-level interface to sockets. Besides the obvious -functions in L<perlfunc>, you should also check out the F<modules> file at -your nearest CPAN site, especially -L<http://www.cpan.org/modules/00modlist.long.html#ID5_Networking_>. -See L<perlmodlib> or best yet, the F<Perl FAQ> for a description -of what CPAN is and where to get it if the previous link doesn't work -for you. - -Section 5 of CPAN's F<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--to name just a few. |