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Diffstat (limited to 'Master/tlpkg/tlperl/lib/pods/perlipc.pod')
| -rw-r--r-- | Master/tlpkg/tlperl/lib/pods/perlipc.pod | 1492 |
1 files changed, 765 insertions, 727 deletions
diff --git a/Master/tlpkg/tlperl/lib/pods/perlipc.pod b/Master/tlpkg/tlperl/lib/pods/perlipc.pod index 8d9ea9757d0..3009913147c 100644 --- a/Master/tlpkg/tlperl/lib/pods/perlipc.pod +++ b/Master/tlpkg/tlperl/lib/pods/perlipc.pod @@ -16,17 +16,19 @@ 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. +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"; + my $signame = shift; + $shucks++; + die "Somebody sent me a SIG$signame"; } - $SIG{INT} = 'catch_zap'; # could fail in modules + $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 @@ -39,59 +41,61 @@ 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 from the Config module. Set up an -@signame list indexed by number to get the name and a %signo table +@signame list indexed by number to get the name and a %signo hash 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; - $i++; + defined($Config{sig_name}) || die "No sigs?"; + foreach $name (split(" ", $Config{sig_name})) { + $signo{$name} = $i; + $signame[$i] = $name; + $i++; } So to check whether signal 17 and SIGALRM were the same, do just this: print "signal #17 = $signame[17]\n"; if ($signo{ALRM}) { - print "SIGALRM is $signo{ALRM}\n"; + print "SIGALRM is $signo{ALRM}\n"; } -You may also choose to assign the strings C<'IGNORE'> or C<'DEFAULT'> as +You may also choose to assign the strings C<"IGNORE"> or C<"DEFAULT"> as the handler, in which case Perl will try to discard the signal or do the default thing. On most Unix platforms, the C<CHLD> (sometimes also known as C<CLD>) signal -has special behavior with respect to a value of C<'IGNORE'>. -Setting C<$SIG{CHLD}> to C<'IGNORE'> on such a platform has the effect of +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 +on its child processes (i.e., child processes are automatically reaped). +Calling C<wait()> with C<$SIG{CHLD}> set to C<"IGNORE"> usually returns C<-1> on such platforms. -Some signals can be neither trapped nor ignored, such as -the KILL and STOP (but not the TSTP) signals. One strategy for -temporarily ignoring signals is to use a local() statement, which will be -automatically restored once your block is exited. (Remember that local() -values are "inherited" by functions called from within that block.) +Some signals can be neither trapped nor ignored, such as the KILL and STOP +(but not the TSTP) signals. One strategy for temporarily ignoring signals +is to use a local() on that hash element, automatically restoring a +previous value once your block is exited. Remember that values created by +the dynamically-scoped local() are "inherited" by functions called from +within their caller's scope. sub precious { - local $SIG{INT} = 'IGNORE'; - &more_functions; + local $SIG{INT} = "IGNORE"; + more_functions(); } sub more_functions { - # interrupts still ignored, for now... + # 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): +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 => -$$; - # snazzy writing of: kill('HUP', -$$) + local $SIG{HUP} = "IGNORE"; + kill HUP => -$$; + # snazzy writing of: kill("HUP", -$$) } Another interesting signal to send is signal number zero. This doesn't @@ -99,7 +103,7 @@ actually affect a child 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"; + warn "something wicked happened to $kid_pid"; } When directed at a process whose UID is not identical to that @@ -107,8 +111,8 @@ of the sending process, signal number zero may fail because you lack permission to send the signal, even though the process is alive. You may be able to determine the cause of failure using C<%!>. - unless (kill 0 => $pid or $!{EPERM}) { - warn "$pid looks dead"; + unless (kill(0 => $pid) || $!{EPERM}) { + warn "$pid looks dead"; } You might also want to employ anonymous functions for simple signal @@ -119,16 +123,16 @@ handlers: But that will be problematic for the more complicated handlers that need to reinstall themselves. Because Perl's signal mechanism is currently based on the signal(3) function from the C library, you may sometimes be so -unfortunate as to run on systems where that function is "broken", that +unfortunate 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; + $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... @@ -137,26 +141,26 @@ or better still: use POSIX ":sys_wait_h"; sub REAPER { - my $child; - # If a second child dies while in the signal handler caused by the - # first death, we won't get another signal. So must loop here else - # we will leave the unreaped child as a zombie. And the next time - # two children die we get another zombie. And so on. - while (($child = waitpid(-1,WNOHANG)) > 0) { - $Kid_Status{$child} = $?; - } - $SIG{CHLD} = \&REAPER; # still loathe SysV + my $child; + # If a second child dies while in the signal handler caused by the + # first death, we won't get another signal. So must loop here else + # we will leave the unreaped child as a zombie. And the next time + # two children die we get another zombie. And so on. + while (($child = waitpid(-1, WNOHANG)) > 0) { + $Kid_Status{$child} = $?; + } + $SIG{CHLD} = \&REAPER; # still loathe SysV } $SIG{CHLD} = \&REAPER; # do something that forks... -Note: qx(), system() and some modules for calling external commands do a -fork() and wait() for the result. Thus, your signal handler (REAPER in the -example) will be called. Since wait() was already called by system() or qx() -the wait() in the signal handler will not see any more zombies and therefore -block. +Be careful: qx(), system(), and some modules for calling external commands +do a fork(), then wait() for the result. Thus, your signal handler +(C<&REAPER> in the example) 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 +The best way to prevent this issue is to use waitpid(), as in the following example: use POSIX ":sys_wait_h"; # for nonblocking read @@ -165,7 +169,7 @@ example: $SIG{CHLD} = sub { # don't change $! and $? outside handler - local ($!,$?); + local ($!, $?); my $pid = waitpid(-1, WNOHANG); return if $pid == -1; return unless defined $children{$pid}; @@ -175,11 +179,12 @@ example: while (1) { my $pid = fork(); + die "cannot fork" unless defined $pid; if ($pid == 0) { # ... exit 0; } else { - $children{$pid}=1; + $children{$pid}=1; # ... system($command); # ... @@ -196,13 +201,15 @@ using longjmp() or throw() in other languages. Here's an example: + my $ALARM_EXCEPTION = "alarm clock restart"; eval { - local $SIG{ALRM} = sub { die "alarm clock restart" }; + local $SIG{ALRM} = sub { die $ALARM_EXCEPTION }; alarm 10; - flock(FH, 2); # blocking write lock + flock(FH, 2) # blocking write lock + || die "cannot flock: $!"; alarm 0; }; - if ($@ and $@ !~ /alarm clock restart/) { die } + if ($@ && $@ !~ quotemeta($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 @@ -219,21 +226,21 @@ 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 re-read its configuration file, without stopping -the process. Many daemons provide this mechanism using the C<SIGHUP> -signal handler. When you want to tell the daemon to re-read the file -you simply send it the C<SIGHUP> signal. +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. Not all platforms automatically reinstall their (native) signal handlers after a signal delivery. This means that the handler works -only the first time the signal is sent. The solution to this problem -is to use C<POSIX> signal handlers if available, their behaviour +the first time the signal is sent, only. The solution to this problem +is to use C<POSIX> signal handlers if available; their behavior is well-defined. 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 simply prints some debug -info to show that it works and should be replaced with the real code. +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 @@ -242,23 +249,23 @@ info to show that it works and should be replaced with the real code. use File::Basename (); use File::Spec::Functions; - $|=1; + $| = 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; + my $SELF = catfile($FindBin::Bin, $script); # POSIX unmasks the sigprocmask properly my $sigset = POSIX::SigSet->new(); - my $action = POSIX::SigAction->new('sigHUP_handler', + my $action = POSIX::SigAction->new("sigHUP_handler", $sigset, &POSIX::SA_NODEFER); POSIX::sigaction(&POSIX::SIGHUP, $action); sub sigHUP_handler { print "got SIGHUP\n"; - exec($SELF, @ARGV) or die "Couldn't restart: $!\n"; + exec($SELF, @ARGV) || die "$0: couldn't restart: $!"; } code(); @@ -266,80 +273,24 @@ info to show that it works and should be replaced with the real code. sub code { print "PID: $$\n"; print "ARGV: @ARGV\n"; - my $c = 0; - while (++$c) { + my $count = 0; + while (++$count) { sleep 2; - print "$c\n"; + print "$count\n"; } } - __END__ - - -=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 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 C<POSIX::mkfifo()> function. - - use POSIX qw(mkfifo); - mkfifo($path, 0700) or 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. - - # 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 block and your program will -supply the new signature. We'll use the pipe-checking file test B<-p> -to find out whether anyone (or anything) has accidentally removed our fifo. - - chdir; # go home - $FIFO = '.signature'; - - while (1) { - unless (-p $FIFO) { - unlink $FIFO; - require POSIX; - POSIX::mkfifo($FIFO, 0700) - or die "can't mkfifo $FIFO: $!"; - } - - # next line blocks until there's a reader - open (FIFO, "> $FIFO") || die "can't write $FIFO: $!"; - print FIFO "John Smith (smith\@host.org)\n", `fortune -s`; - close FIFO; - sleep 2; # to avoid dup signals - } - =head2 Deferred Signals (Safe Signals) -In Perls before Perl 5.7.3 by installing Perl code to deal with -signals, you were exposing yourself to danger from two things. First, -few system library functions are re-entrant. If the signal interrupts -while Perl is executing one function (like malloc(3) or printf(3)), -and your signal handler then calls the same function again, you could -get unpredictable behavior--often, a core dump. Second, Perl isn't -itself re-entrant at the lowest levels. If the signal interrupts Perl -while Perl is changing its own internal data structures, similarly -unpredictable behaviour may result. +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 @@ -352,30 +303,28 @@ 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 handler as we know 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: +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 only looks at the signal flags when it is about +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. -N.B. If a signal of any given type fires multiple times during an opcode +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 -only be called once after the opcode completes, and all the other +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 @@ -387,31 +336,37 @@ 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. INT control-C) the operating system -breaks into IO operations like C<read> (used to implement Perls -E<lt>E<gt> 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 not called immediately, and if -Perl is using system's C<stdio> library that library may re-start the -C<read> without returning to Perl and giving it a chance to call the -%SIG handler. If this happens on your system the solution is to use -C<:perlio> layer to do IO - at least on those handles which 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.) - -Note that the default in Perl 5.7.3 and later is to automatically use +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 some networking library functions like gethostbyname() are -known to have their own implementations of timeouts which may conflict -with your timeouts. If you are having problems with such functions, -you can try using the POSIX sigaction() function, which bypasses the -Perl safe signals (note that this means subjecting yourself to -possible memory corruption, as described above). Instead of setting -C<$SIG{ALRM}>: +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" }; @@ -420,11 +375,11 @@ try something like the following: use POSIX qw(SIGALRM); POSIX::sigaction(SIGALRM, POSIX::SigAction->new(sub { die "alarm" })) - or die "Error setting SIGALRM handler: $!\n"; + || 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 will affect -all signals). +the C<Perl::Unsafe::Signals> module from CPAN, which affects +all signals. =item Restartable system calls @@ -436,33 +391,87 @@ 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. -Note that the default C<:perlio> layer will retry C<read>, C<write> -and C<close> as described above and that interrupted C<wait> and +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, e.g. SEGV, ILL, and BUS, are generated as a result of -virtual memory or other "faults". These are normally fatal and there is -little a Perl-level handler can do with them, so Perl now delivers them +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 now 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 +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 re-issue the signal as -there are un-waited-for completed child processes. +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 behaviour back regardless of possible +If you want the old signal behavior back despite possible memory corruption, set the environment variable C<PERL_SIGNALS> to -C<"unsafe"> (a new feature since Perl 5.8.1). +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 @@ -472,28 +481,27 @@ 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: $!"; + || die "can't fork: $!"; local $SIG{PIPE} = sub { die "spooler pipe broke" }; print SPOOLER "stuff\n"; - close SPOOLER || die "bad spool: $! $?"; + 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: $!"; + || die "can't fork: $!"; while (<STATUS>) { - next if /^(tcp|udp)/; - print; + next if /^(tcp|udp)/; + print; } - close STATUS || die "bad netstat: $! $?"; + close STATUS || die "bad netstat: $! $?"; -If one can be sure that a particular program is a Perl script that is -expecting filenames in @ARGV, the clever programmer can write something -like this: +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 irrespective of which shell it's called from, the Perl program will +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? @@ -502,36 +510,38 @@ 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 $?; + 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 to kill off the child process early if you'd -like. +whole process, letting you kill off the child process early if you'd like. -Be careful to check both the open() and the close() return values. If +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 +can't know whether the command worked, because your command is actually running in a separate process whose exec() might have failed. Therefore, -while readers of bogus commands return just a quick end of file, writers -to bogus command will trigger a signal they'd better be prepared to -handle. Consider: +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") or die "can't fork: $!"; - print FH "bang\n" or die "can't write: $!"; - close FH or die "can't close: $!"; + 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: $!"; -That won't blow up until the close, and it will blow up with a SIGPIPE. -To catch it, you could use this: +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") or die "can't fork: $!"; - print FH "bang\n" or die "can't write: $!"; - close FH or die "can't close: status=$?"; + $SIG{PIPE} = "IGNORE"; + open(FH, "|bogus") || die "can't fork: $!"; + print FH "bang\n"; + close(FH) || die "can't close: status=$?"; =head2 Filehandles @@ -550,76 +560,78 @@ You can run a command in the background with: 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). +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). +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'; + use POSIX "setsid"; sub daemonize { - chdir '/' or die "Can't chdir to /: $!"; - open STDIN, '/dev/null' or die "Can't read /dev/null: $!"; - open STDOUT, '>/dev/null' - or die "Can't write to /dev/null: $!"; - defined(my $pid = fork) or die "Can't fork: $!"; - exit if $pid; - die "Can't start a new session: $!" if setsid == -1; - open STDERR, '>&STDOUT' or die "Can't dup stdout: $!"; + 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 that you aren't a -process group leader (the setsid() will fail if you are). If your +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 Your_OS::Process module for other -solutions. +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 +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 his +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 his STDOUT. +you opened whatever your kid writes to I<his> STDOUT. - use English '-no_match_vars'; - my $sleep_count = 0; + 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; - } + $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_TO_WRITE - } - exit; # don't forget this + 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 @@ -630,45 +642,43 @@ your arguments. Instead, use lower-level control to call exec() directly. Here's a safe backtick or pipe open for read: - # add error processing as above - $pid = open(KID_TO_READ, "-|"); + 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 $?"; + 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 + } 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, "|-"); + my $pid = open(KID_TO_WRITE, "|-"); + defined($pid) || die "can't fork: $!"; + $SIG{PIPE} = 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 + 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 any use of pipe() and multiple sub-processes. The above -example is 'safe' because it is simple and calls exec(). See +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. @@ -676,194 +686,218 @@ 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: - $pid = open WRITER, "|-"; - defined $pid or die "fork failed; $!"; + my $pid = open(WRITER, "|-"); # fork open a kid + defined($pid) || die "first fork failed: $!"; if ($pid) { if (my $sub_pid = fork()) { - close WRITER; - # do something else... + defined($sub_pid) || die "second fork failed: $!"; + close(WRITER) || die "couldn't close WRITER: $!"; + # now do something else... } else { - # write to WRITER... - exit; + # first write to WRITER + # ... + # then when finished + close(WRITER) || die "couldn't close WRITER: $!"; + exit(0); } } else { - # do something with STDIN... - exit; + # first do something with STDIN, then + exit(0); } -In the above, the true parent does not want to write to the WRITER +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 behaviour: closing it will call -waitpid() (see L<perlfunc/waitpid>), which waits for the sub-process +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", then you have a deadlock. +in the section marked "do something else", you have deadlock. -This can also be a problem with intermediate sub-processes in more +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. +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 below: +open() which sets one file descriptor to another, as shown below: - pipe(READER, WRITER); + pipe(READER, WRITER) || die "pipe failed: $!"; $pid = fork(); - defined $pid or die "fork failed; $!"; + defined($pid) || die "first fork failed: $!"; if ($pid) { - close READER; + close READER; if (my $sub_pid = fork()) { - close WRITER; + defined($sub_pid) || die "first fork failed: $!"; + close(WRITER) || die "can't close WRITER: $!"; } else { # write to WRITER... - exit; + # ... + # then when finished + close(WRITER) || die "can't close WRITER: $!"; + exit(0); } # write to WRITER... } else { - open STDIN, "<&READER"; - close WRITER; + open(STDIN, "<&READER") || die "can't reopen STDIN: $!"; + close(WRITER) || die "can't close WRITER: $!"; # do something... - exit; + exit(0); } -Since Perl 5.8.0, you can also use the list form of C<open> for pipes : -the syntax +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. - open KID_PS, "-|", "ps", "aux" or die $!; +So for example, instead of using: -forks the ps(1) command (without spawning a shell, as there are more than -three arguments to open()), and reads its standard output via the -C<KID_PS> filehandle. The corresponding syntax to write to command -pipes (with C<"|-"> in place of C<"-|">) is also implemented. + open(PS_PIPE, "ps aux|") || die "can't open ps pipe: $!"; -Note that these operations are full Unix forks, which means they may not be -correctly implemented on alien systems. Additionally, these are not true -multithreading. If you'd like to learn more about threading, see the -F<modules> file mentioned below in the SEE ALSO section. +One would use either of these: -=head2 Avoiding Pipe Deadlocks + open(PS_PIPE, "-|", "ps", "aux") + || die "can't open ps pipe: $!"; -In general, if you have more than one sub-process, you need to be very -careful that any process which does not need the writer half of any -pipe you create for inter-process communication does not have it open. + @ps_args = qw[ ps aux ]; + open(PS_PIPE, "-|", @ps_args) + || die "can't open @ps_args|: $!"; -The reason for this is that any child process which is 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. +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<"-|">. -Certain built-in Unix features help prevent this most of -the time. For instance, filehandles have a 'close on exec' flag (set -I<en masse> with Perl using the C<$^F> L<perlvar>), so that any -filehandles which you didn't explicitly route to the STDIN, STDOUT or -STDERR of a child I<program> will automatically be closed for you. +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: -So, always explicitly and immediately call close() on the writable end -of any pipe, unless that process is actually writing to it. If you -don't explicitly call close() then be warned Perl will still close() -all the filehandles during global destruction. As warned above, if -those filehandles were opened with Safe Pipe Open, they will also call -waitpid() and you might again deadlock. + @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 obvious thing you'd like to do -doesn't actually work: +about bidirectional communication? The most obvious approach doesn't work: + # THIS DOES NOT WORK!! open(PROG_FOR_READING_AND_WRITING, "| some program |") -and if you forget to use the C<use warnings> pragma or the B<-w> flag, -then you'll miss out entirely on the diagnostic message: +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() library function -to catch both ends. There's also an open3() for tridirectional I/O so you -can also catch your child's STDERR, but doing so would then require an -awkward select() loop and wouldn't allow you to use normal Perl input -operations. +If you 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 Unix pipe() and exec() calls to create all the connections. -While it might have been slightly more efficient by using socketpair(), it -would have then been even less portable than it already is. The open2() -and open3() functions are unlikely to work anywhere except on a Unix -system or some other one purporting to be POSIX compliant. +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 -u -n" ); + $pid = open2(*Reader, *Writer, "cat -un"); print Writer "stuff\n"; $got = <Reader>; -The problem with this is that Unix buffering is really going to -ruin your day. Even though your C<Writer> filehandle is auto-flushed, -and the process on the other end will get your data in a timely manner, -you can't usually do anything to force it to give it back to you -in a similarly quick fashion. In this case, we could, because we -gave I<cat> a B<-u> flag to make it unbuffered. But very few Unix -commands are designed to operate over pipes, so this seldom works -unless you yourself wrote the program on the other end of the -double-ended pipe. - -A solution to this is the nonstandard F<Comm.pl> library. It uses -pseudo-ttys to make your program behave more reasonably: - - require 'Comm.pl'; - $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 F<Comm> library also has expect() -and interact() functions. Find the library (and we hope its -successor F<IPC::Chat>) at your nearest CPAN archive as detailed -in the 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. +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() -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. +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: failure? - pipe(CHILD_RDR, PARENT_WTR); # XXX: failure? + 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 $$ just read this: `$line'\n"; - close CHILD_RDR; close CHILD_WTR; - waitpid($pid,0); + if ($pid = fork()) { + close PARENT_RDR; + close PARENT_WTR; + print CHILD_WTR "Parent Pid $$ 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 $$ is sending this\n"; - close PARENT_RDR; close PARENT_WTR; - exit; + die "cannot fork: $!" unless defined $pid; + close CHILD_RDR; + close CHILD_WTR; + chomp($line = <PARENT_RDR>); + print "Child Pid $$ just read this: `$line'\n"; + print PARENT_WTR "Child Pid $$ is sending this\n"; + close PARENT_RDR; + close PARENT_WTR; + exit(0); } But you don't actually have to make two pipe calls. If you @@ -874,54 +908,55 @@ have the socketpair() system call, it will do this all for you. # "the best ones always go both ways" use Socket; - use IO::Handle; # thousands of lines just for autoflush :-( + use IO::Handle; # thousands of lines just for autoflush :-( + # We say AF_UNIX because although *_LOCAL is the # POSIX 1003.1g form of the constant, many machines # still don't have it. socketpair(CHILD, PARENT, AF_UNIX, SOCK_STREAM, PF_UNSPEC) - or die "socketpair: $!"; + || die "socketpair: $!"; CHILD->autoflush(1); PARENT->autoflush(1); - if ($pid = fork) { - close PARENT; - print CHILD "Parent Pid $$ is sending this\n"; - chomp($line = <CHILD>); - print "Parent Pid $$ just read this: `$line'\n"; - close CHILD; - waitpid($pid,0); + if ($pid = fork()) { + close PARENT; + print CHILD "Parent Pid $$ 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 $$ is sending this\n"; - close PARENT; - exit; + die "cannot fork: $!" unless defined $pid; + close CHILD; + chomp($line = <PARENT>); + print "Child Pid $$ just read this: '$line'\n"; + print PARENT "Child Pid $$ is sending this\n"; + close PARENT; + exit(0); } =head1 Sockets: Client/Server Communication -While not 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 +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 function calls for dealing with sockets have the same names as +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 +for two reasons. First, Perl filehandles work differently than C file descriptors. Second, Perl already knows the length of its strings, so you don't need to pass that information. -One of the major problems with old socket code in Perl was that it used -hard-coded values for some of the constants, which severely hurt -portability. If you ever see code that does anything like explicitly -setting 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 various constants and functions you'll need. +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 @@ -938,9 +973,10 @@ 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). +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, you'll probably be ok. +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 @@ -952,51 +988,50 @@ 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); + my ($remote, $port, $iaddr, $paddr, $proto, $line); - $remote = shift || 'localhost'; + $remote = shift || "localhost"; $port = shift || 2345; # random port - if ($port =~ /\D/) { $port = getservbyname($port, 'tcp') } + if ($port =~ /\D/) { $port = getservbyname($port, "tcp") } die "No port" unless $port; - $iaddr = inet_aton($remote) || die "no host: $remote"; + $iaddr = inet_aton($remote) || die "no host: $remote"; $paddr = sockaddr_in($port, $iaddr); - $proto = getprotobyname('tcp'); - socket(SOCK, PF_INET, SOCK_STREAM, $proto) || die "socket: $!"; - connect(SOCK, $paddr) || die "connect: $!"; - while (defined($line = <SOCK>)) { - print $line; + $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; + close (SOCK) || die "close: $!"; + exit(0); 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 +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), you should fill this in with your real address -instead. +or firewall machine), fill this in with your real address instead. #!/usr/bin/perl -Tw use strict; - BEGIN { $ENV{PATH} = '/usr/ucb:/bin' } + BEGIN { $ENV{PATH} = "/usr/bin:/bin" } use Socket; use Carp; my $EOL = "\015\012"; - sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" } + sub logmsg { print "$0 $$: @_ at ", scalar localtime(), "\n" } - my $port = shift || 2345; - my $proto = getprotobyname('tcp'); + my $port = shift || 2345; + die "invalid port" unless if $port =~ /^ \d+ $/x; - ($port) = $port =~ /^(\d+)$/ or die "invalid port"; + 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: $!"; + 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"; @@ -1004,43 +1039,43 @@ instead. $SIG{CHLD} = \&REAPER; - for ( ; $paddr = accept(Client,Server); close Client) { - my($port,$iaddr) = sockaddr_in($paddr); - my $name = gethostbyaddr($iaddr,AF_INET); + 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"; + logmsg "connection from $name [", + inet_ntoa($iaddr), "] + at port $port"; - print Client "Hello there, $name, it's now ", - scalar localtime, $EOL; + 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 +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/ucb:/bin' } + 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" } + sub logmsg { print "$0 $$: @_ at ", scalar localtime(), "\n" } - my $port = shift || 2345; - my $proto = getprotobyname('tcp'); + my $port = shift || 2345; + die "invalid port" unless if $port =~ /^ \d+ $/x; - ($port) = $port =~ /^(\d+)$/ or die "invalid port"; + 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: $!"; + 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"; @@ -1052,17 +1087,17 @@ go back to service a new client. sub REAPER { local $!; # don't let waitpid() overwrite current error - while ((my $pid = waitpid(-1,WNOHANG)) > 0 && WIFEXITED($?)) { - logmsg "reaped $waitedpid" . ($? ? " with exit $?" : ''); + while ((my $pid = waitpid(-1, WNOHANG)) > 0 && WIFEXITED($?)) { + logmsg "reaped $waitedpid" . ($? ? " with exit $?" : ""); } $SIG{CHLD} = \&REAPER; # loathe SysV } $SIG{CHLD} = \&REAPER; - while(1) { + while (1) { $paddr = accept(Client, Server) || do { - # try again if accept() returned because a signal was received + # try again if accept() returned because got a signal next if $!{EINTR}; die "accept: $!"; }; @@ -1074,9 +1109,9 @@ go back to service a new client. "] at port $port"; spawn sub { - $|=1; - print "Hello there, $name, it's now ", scalar localtime, $EOL; - exec '/usr/games/fortune' # XXX: `wrong' line terminators + $| = 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; @@ -1085,12 +1120,12 @@ go back to service a new client. sub spawn { my $coderef = shift; - unless (@_ == 0 && $coderef && ref($coderef) eq 'CODE') { + unless (@_ == 0 && $coderef && ref($coderef) eq "CODE") { confess "usage: spawn CODEREF"; } my $pid; - if (! defined($pid = fork)) { + unless (defined($pid = fork())) { logmsg "cannot fork: $!"; return; } @@ -1100,10 +1135,10 @@ go back to service a new client. } # else I'm the child -- go spawn - open(STDIN, "<&Client") || die "can't dup client to stdin"; - open(STDOUT, ">&Client") || die "can't dup client to stdout"; + open(STDIN, "<&Client") || die "can't dup client to stdin"; + open(STDOUT, ">&Client") || die "can't dup client to stdout"; ## open(STDERR, ">&STDOUT") || die "can't dup stdout to stderr"; - exit &$coderef(); + exit($coderef->()); } This server takes the trouble to clone off a child version via fork() @@ -1118,25 +1153,25 @@ 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 that needs -to be reported. However the introduction of safe signals (see +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() may also be interrupted when the process receives a signal. -This typically happens when one of the forked sub-processes exits and +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 then $! will be set to EINTR. -If this happens then we can safely continue to the next iteration of +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 doesn't modify the value of $! or this test will -most 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 will -update the local copy leaving the original unchanged. +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. -We suggest that you use the B<-T> flag to use taint checking (see L<perlsec>) +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 and other programs run on behalf of someone else (like CGI +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. @@ -1148,29 +1183,30 @@ differ from the system on which it's being run: use strict; use Socket; - my $SECS_of_70_YEARS = 2208988800; - sub ctime { scalar localtime(shift) } + 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 $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()); + 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 = ' '; - read(SOCKET, $rtime, 4); - close(SOCKET); - my $histime = unpack("N", $rtime) - $SECS_of_70_YEARS; - printf "%8d %s\n", $histime - time, ctime($histime); + 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 @@ -1186,8 +1222,8 @@ domain sockets can show up in the file system with an ls(1) listing. You can test for these with Perl's B<-S> file test: - unless ( -S '/dev/log' ) { - die "something's wicked with the log system"; + unless (-S "/dev/log") { + die "something's wicked with the log system"; } Here's a sample Unix-domain client: @@ -1197,13 +1233,13 @@ Here's a sample Unix-domain client: use strict; my ($rendezvous, $line); - $rendezvous = shift || 'catsock'; - socket(SOCK, PF_UNIX, SOCK_STREAM, 0) || die "socket: $!"; - connect(SOCK, sockaddr_un($rendezvous)) || die "connect: $!"; + $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; + print $line; } - exit; + 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 @@ -1214,18 +1250,18 @@ to be on the localhost, and thus everything works right. use Socket; use Carp; - BEGIN { $ENV{PATH} = '/usr/ucb:/bin' } + BEGIN { $ENV{PATH} = "/usr/bin:/bin" } sub spawn; # forward declaration - sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" } + sub logmsg { print "$0 $$: @_ at ", scalar localtime(), "\n" } - my $NAME = 'catsock'; + my $NAME = "catsock"; my $uaddr = sockaddr_un($NAME); - my $proto = getprotobyname('tcp'); + my $proto = getprotobyname("tcp"); - socket(Server,PF_UNIX,SOCK_STREAM,0) || die "socket: $!"; + socket(Server, PF_UNIX, SOCK_STREAM, 0) || die "socket: $!"; unlink($NAME); - bind (Server, $uaddr) || die "bind: $!"; - listen(Server,SOMAXCONN) || die "listen: $!"; + bind (Server, $uaddr) || die "bind: $!"; + listen(Server, SOMAXCONN) || die "listen: $!"; logmsg "server started on $NAME"; @@ -1233,64 +1269,66 @@ to be on the localhost, and thus everything works right. 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 + 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) + 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: $!"; - }; + 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 $pid; - if (!defined($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(); + 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 exactly the same as in the -other server. +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 +you get a separate session for each client; that's why accept() takes two arguments. -For example, let's say that you have a long running database server daemon -that you want folks from the World Wide Web to be able to access, but only +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. @@ -1298,13 +1336,13 @@ 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 is -included as part of the standard Perl distribution as of the 5.004 -release. If you're running an earlier version of Perl, just fetch -IO::Socket from CPAN, where you'll also find modules providing easy -interfaces to the following systems: DNS, FTP, Ident (RFC 931), NIS and -NISPlus, NNTP, Ping, POP3, SMTP, SNMP, SSLeay, Telnet, and Time--just -to name a few. +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 @@ -1315,19 +1353,19 @@ that the server there cares to provide. #!/usr/bin/perl -w use IO::Socket; $remote = IO::Socket::INET->new( - Proto => "tcp", - PeerAddr => "localhost", - PeerPort => "daytime(13)", - ) - or die "cannot connect to daytime port at localhost"; - while ( <$remote> ) { print } + 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 C<new> constructor mean: +Here are what those parameters to the new() constructor mean: =over 4 @@ -1343,25 +1381,25 @@ can be used to make a datagram socket, used for message-passing. This is the name or Internet address of the remote host the server is running on. We could have specified a longer name like C<"www.perl.com">, -or an address like C<"204.148.40.9">. For demonstration purposes, we've +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<"127.1">, if you'd rather use that. +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 in I</etc/services> under Unix] but just in case, we've specified the -port number (13) in parentheses. Using just the number would also have -worked, but constant numbers make careful programmers nervous. +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 indirect -filehandle, a scalar variable containing a filehandle. You can use +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: @@ -1378,36 +1416,35 @@ and send a line of data to it this way: =head2 A Webget Client Here's a simple client that takes a remote host to fetch a document -from, and then a list of documents to get from that host. This is a +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 document ..." } + unless (@ARGV > 1) { die "usage: $0 host url ..." } $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; + 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 handing the "http" service, which is assumed to be at -its standard port, number 80. If the web server you're trying to -connect to is at a different port (like 1080 or 8080), you should specify +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 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.) +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 @@ -1437,7 +1474,7 @@ Here's an example of running that program, which we'll call I<webget>: 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 +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 @@ -1472,11 +1509,11 @@ Here's the code: # 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: $!"; + PeerAddr => $host, + PeerPort => $port) + || die "can't connect to port $port on $host: $!"; - $handle->autoflush(1); # so output gets there right away + $handle->autoflush(1); # so output gets there right away print STDERR "[Connected to $host:$port]\n"; # split the program into two processes, identical twins @@ -1484,22 +1521,23 @@ Here's the code: # 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", $kidpid); # send SIGTERM to child + # copy the socket to standard output + while (defined ($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 $line; - } + # copy standard input to the socket + while (defined ($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 (current running in the C<else> block) +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 @@ -1509,7 +1547,7 @@ following: my $byte; while (sysread($handle, $byte, 1) == 1) { - print STDOUT $byte; + print STDOUT $byte; } Making a system call for each byte you want to read is not very efficient @@ -1569,20 +1607,20 @@ To add to user-friendliness, our server prompts the user for commands. Most servers don't do this. Because of the prompt without a newline, you'll have to use the C<sysread> variant of the interactive client above. -This server accepts one of five different commands, sending output -back to the client. Note that unlike most network servers, this one -only handles one incoming client at a time. Multithreaded servers are -covered in Chapter 6 of the Camel. +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 OO version of gethostbyaddr + use Net::hostent; # for OOish version of gethostbyaddr - $PORT = 9000; # pick something not in use + $PORT = 9000; # pick something not in use - $server = IO::Socket::INET->new( Proto => 'tcp', + $server = IO::Socket::INET->new( Proto => "tcp", LocalPort => $PORT, Listen => SOMAXCONN, Reuse => 1); @@ -1597,12 +1635,12 @@ Here's the code. We'll 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`; } + 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"; } @@ -1624,10 +1662,10 @@ find yourself overly concerned about reliability and start building checks into your message system, then you probably should use just TCP to start with. -Note that UDP datagrams are I<not> a bytestream and should not be treated -as such. This makes using I/O mechanisms with internal buffering -like stdio (i.e. print() and friends) especially cumbersome. Use syswrite(), -or better send(), like in the example below. +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 @@ -1641,85 +1679,87 @@ with TCP, you'd have to use a different socket handle for each host. use Sys::Hostname; my ( $count, $hisiaddr, $hispaddr, $histime, - $host, $iaddr, $paddr, $port, $proto, - $rin, $rout, $rtime, $SECS_of_70_YEARS); + $host, $iaddr, $paddr, $port, $proto, + $rin, $rout, $rtime, $SECS_OF_70_YEARS); - $SECS_of_70_YEARS = 2208988800; + $SECS_OF_70_YEARS = 2_208_988_800; $iaddr = gethostbyname(hostname()); - $proto = getprotobyname('udp'); - $port = getservbyname('time', 'udp'); + $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; + 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: $!"; + $count++; + $hisiaddr = inet_aton($host) || die "unknown host"; + $hispaddr = sockaddr_in($port, $hisiaddr); + defined(send(SOCKET, 0, 0, $hispaddr)) || die "send $host: $!"; } - $rin = ''; + $rin = ""; vec($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--; + $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--; } -Note that this example does not include any retries and may consequently -fail to contact a reachable host. The most prominent reason for this -is congestion of the queues on the sending host if the number of -list of hosts to contact is sufficiently large. +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. You can't, however, effectively use SysV IPC or -Berkeley mmap() to have shared memory so as to share a variable amongst -several processes. That's because Perl would reallocate your string when -you weren't wanting it to. +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, $size, S_IRUSR|S_IWUSR) // die "$!"; + $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($id, $message, 0, 60) || die "shmwrite: $!"; print "wrote: '$message'\n"; - shmread($id, $buff, 0, 60) || die "$!"; + 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")) = ''; + 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 "$!"; + shmctl($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 ) // die "$!"; + $id = semget($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. @@ -1728,13 +1768,13 @@ Call the file F<take>: # create a semaphore $IPC_KEY = 1234; - $id = semget($IPC_KEY, 0 , 0 ); - die if !defined($id); + $id = semget($IPC_KEY, 0, 0); + defined($id) || die "shmget: $!"; - $semnum = 0; + $semnum = 0; $semflag = 0; - # 'take' semaphore + # "take" semaphore # wait for semaphore to be zero $semop = 0; $opstring1 = pack("s!s!s!", $semnum, $semop, $semflag); @@ -1742,29 +1782,29 @@ Call the file F<take>: # Increment the semaphore count $semop = 1; $opstring2 = pack("s!s!s!", $semnum, $semop, $semflag); - $opstring = $opstring1 . $opstring2; + $opstring = $opstring1 . $opstring2; - semop($id,$opstring) || die "$!"; + 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 + # "give" the semaphore # run this in the original process and you will see # that the second process continues $IPC_KEY = 1234; $id = semget($IPC_KEY, 0, 0); - die if !defined($id); + die unless defined($id); - $semnum = 0; + $semnum = 0; $semflag = 0; # Decrement the semaphore count $semop = -1; $opstring = pack("s!s!s!", $semnum, $semop, $semflag); - semop($id,$opstring) || die "$!"; + 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 @@ -1775,41 +1815,36 @@ 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 $sent = "message"; my $type_sent = 1234; - my $rcvd; - my $type_rcvd; - - if (defined $id) { - if (msgsnd($id, pack("l! a*", $type_sent, $sent), 0)) { - if (msgrcv($id, $rcvd, 60, 0, 0)) { - ($type_rcvd, $rcvd) = unpack("l! a*", $rcvd); - if ($rcvd eq $sent) { - print "okay\n"; - } else { - print "not okay\n"; - } - } else { - die "# msgrcv failed\n"; - } - } else { - die "# msgsnd failed\n"; - } - msgctl($id, IPC_RMID, 0) || die "# msgctl failed: $!\n"; + + msgsnd($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 $sent) { + print "okay\n"; } else { - die "# msgget failed\n"; + print "not okay\n"; } + msgctl($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 croak() on bad arguments.) It is therefore essential to +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 B<-T> -taint checking flag to the #! line for servers: +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; @@ -1818,9 +1853,9 @@ taint checking flag to the #! line for servers: =head1 BUGS -All these routines create system-specific portability problems. As noted +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 -behaviour. It's probably safest to assume broken SysV semantics for +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. @@ -1835,20 +1870,23 @@ version and suggestions from the Perl Porters. There's a lot more to networking than this, but this should get you started. -For intrepid programmers, the indispensable textbook is I<Unix -Network Programming, 2nd Edition, Volume 1> by W. Richard Stevens -(published by Prentice-Hall). Note that 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. +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. (See L<perlmodlib> or best yet, the F<Perl -FAQ> for a description of what CPAN is and where to get it.) - -Section 5 of the F<modules> file is devoted to "Networking, Device Control -(modems), and Interprocess Communication", and contains numerous unbundled -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. +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. |