=head1 Signals
-Perl uses a simple signal handling model: the %SIG hash contains names or
-references of user-installed signal handlers. These handlers will be called
-with an argument which is the name of the signal that triggered it. A
-signal may be generated intentionally from a particular keyboard sequence like
-control-C or control-Z, sent to you from another process, or
-triggered automatically by the kernel when special events transpire, like
-a child process exiting, your process running out of stack space, or
-hitting file size limit.
-
-For example, to trap an interrupt signal, set up a handler like this.
-Do as little as you possibly can in your handler; notice how all we do is
-set a global variable and then raise an exception. That's because on most
-systems, libraries are not re-entrant; particularly, memory allocation and
-I/O routines are not. That means that doing nearly I<anything> in your
-handler could in theory trigger a memory fault and subsequent core dump.
+Perl uses a simple signal handling model: the %SIG hash contains names
+or references of user-installed signal handlers. These handlers will
+be called with an argument which is the name of the signal that
+triggered it. A signal may be generated intentionally from a
+particular keyboard sequence like control-C or control-Z, sent to you
+from another process, or triggered automatically by the kernel when
+special events transpire, like a child process exiting, your process
+running out of stack space, or hitting file size limit.
+
+For example, to trap an interrupt signal, set up a handler like this:
sub catch_zap {
my $signame = shift;
$SIG{INT} = 'catch_zap'; # could fail in modules
$SIG{INT} = \&catch_zap; # best strategy
+Prior to Perl 5.7.3 it was necessary to do as little as you possibly
+could in your handler; notice how all we do is set a global variable
+and then raise an exception. That's because on most systems,
+libraries are not re-entrant; particularly, memory allocation and I/O
+routines are not. That meant that doing nearly I<anything> in your
+handler could in theory trigger a memory fault and subsequent core
+dump - see L</Deferred Signals (Safe Signals)> below.
+
The names of the signals are the ones listed out by C<kill -l> on your
system, or you can retrieve them from the Config module. Set up an
@signame list indexed by number to get the name and a %signo table
}
Another interesting signal to send is signal number zero. This doesn't
-actually affect another process, but instead checks whether it's alive
+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";
}
+When directed at a process whose UID is not identical to that
+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";
+ }
+
You might also want to employ anonymous functions for simple signal
handlers:
$SIG{CHLD} = \&REAPER;
# now do something that forks...
-or even the more elaborate:
+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} = $?;
}
the F<t/lib/posix.t> file from the Perl source distribution has some
examples in it.
+=head2 Handling the SIGHUP Signal in Daemons
+
+A process that usually starts when the system boots and shuts down
+when the system is shut down is called a daemon (Disk And Execution
+MONitor). If a daemon process has a configuration file which is
+modified after the process has been started, there should be a way to
+tell that process to 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.
+
+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
+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.
+
+ #!/usr/bin/perl -w
+
+ use POSIX ();
+ use FindBin ();
+ use File::Basename ();
+ use File::Spec::Functions;
+
+ $|=1;
+
+ # make the daemon cross-platform, so exec always calls the script
+ # itself with the right path, no matter how the script was invoked.
+ my $script = File::Basename::basename($0);
+ my $SELF = catfile $FindBin::Bin, $script;
+
+ # POSIX unmasks the sigprocmask properly
+ my $sigset = POSIX::SigSet->new();
+ 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";
+ }
+
+ code();
+
+ sub code {
+ print "PID: $$\n";
+ print "ARGV: @ARGV\n";
+ my $c = 0;
+ while (++$c) {
+ sleep 2;
+ print "$c\n";
+ }
+ }
+ __END__
+
+
=head1 Named Pipes
A named pipe (often referred to as a FIFO) is an old Unix IPC
sleep 2; # to avoid dup signals
}
-=head2 WARNING
+=head2 Deferred Signals (Safe Signals)
-By installing Perl code to deal with signals, you're exposing yourself
-to danger from two things. First, few system library functions are
-re-entrant. If the signal interrupts while Perl is executing one function
-(like malloc(3) or printf(3)), and your signal handler then calls the
-same function again, you could get unpredictable behavior--often, a
-core dump. Second, Perl isn't itself re-entrant at the lowest levels.
-If the signal interrupts Perl while Perl is changing its own internal
-data structures, similarly unpredictable behaviour may result.
+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.
-There are two things you can do, knowing this: be paranoid or be
-pragmatic. The paranoid approach is to do as little as possible in your
+There were two things you could do, knowing this: be paranoid or be
+pragmatic. The paranoid approach was to do as little as possible in your
signal handler. Set an existing integer variable that already has a
value, and return. This doesn't help you if you're in a slow system call,
which will just restart. That means you have to C<die> to longjump(3) out
of the handler. Even this is a little cavalier for the true paranoiac,
who avoids C<die> in a handler because the system I<is> out to get you.
-The pragmatic approach is to say ``I know the risks, but prefer the
-convenience'', and to do anything you want in your signal handler,
-prepared to clean up core dumps now and again.
+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.
+
+In Perl 5.7.3 and later to avoid these problems signals are
+"deferred"-- 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:
+
+=over 4
+
+=item Long running opcodes
+
+As Perl interpreter only looks at the signal flags when it about to
+execute a new opcode if a signal arrives during a long running opcode
+(e.g. a regular expression operation on a very large string) then
+signal will not be seen until operation completes.
+
+=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
+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}>:
+
+ local $SIG{ALRM} = sub { die "alarm" };
+
+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";
+
+=item Restartable system calls
+
+On systems that supported it, older versions of Perl used the
+SA_RESTART flag when installing %SIG handlers. This meant that
+restartable system calls would continue rather than returning when
+a signal arrived. In order to deliver deferred signals promptly,
+Perl 5.7.3 and later do I<not> use SA_RESTART. Consequently,
+restartable system calls can fail (with $! set to C<EINTR>) in places
+where they previously would have succeeded.
+
+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
+C<waitpid> calls will always be retried.
+
+=item Signals as "faults"
+
+Certain signals e.g. SEGV, ILL, 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. (In particular the
+old signal scheme was particularly unsafe in such cases.) However if
+a %SIG handler is set the new scheme simply sets a flag and returns as
+described above. This may cause the operating system to try the
+offending machine instruction again and - as nothing has changed - it
+will generate the signal again. The result of this is a rather odd
+"loop". In future Perl's signal mechanism may be changed to avoid this
+- perhaps by simply disallowing %SIG handlers on signals of that
+type. Until then the work-round is not to set a %SIG handler on those
+signals. (Which signals they are is operating system dependent.)
+
+=item Signals triggered by operating system state
+
+On some operating systems certain signal handlers are supposed to "do
+something" before returning. One example can be CHLD or CLD which
+indicates a child process has completed. On some operating systems the
+signal handler is expected to C<wait> for the completed child
+process. On such systems the deferred signal scheme will not work for
+those signals (it does not do the C<wait>). Again the failure will
+look like a loop as the operating system will re-issue the signal as
+there are un-waited-for completed child processes.
-To forbid signal handlers altogether would bars you from
-many interesting programs, including virtually everything in this manpage,
-since you could no longer even write SIGCHLD handlers. Their dodginess
-is expected to be addresses in the 5.005 release.
+=back
+If you want the old signal behaviour back regardless of possible
+memory corruption, set the environment variable C<PERL_SIGNALS> to
+C<"unsafe"> (a new feature since Perl 5.8.1).
=head1 Using open() for IPC
-Perl's basic open() statement can also be used for unidirectional interprocess
-communication by either appending or prepending a pipe symbol to the second
-argument to open(). Here's how to start something up in a child process you
-intend to write to:
+Perl's basic open() statement can also be used for unidirectional
+interprocess communication by either appending or prepending a pipe
+symbol to the second argument to open(). Here's how to start
+something up in a child process you intend to write to:
open(SPOOLER, "| cat -v | lpr -h 2>/dev/null")
|| die "can't fork: $!";
STDIN. If you open a pipe I<from> minus, you can read from the filehandle
you opened whatever your kid writes to his STDOUT.
- use English;
+ use English '-no_match_vars';
my $sleep_count = 0;
do {
# add error processing as above
$pid = open(KID_TO_WRITE, "|-");
- $SIG{ALRM} = sub { die "whoops, $program pipe broke" };
+ $SIG{PIPE} = sub { die "whoops, $program pipe broke" };
if ($pid) { # parent
for (@data) {
# NOTREACHED
}
+Since Perl 5.8.0, you can also use the list form of C<open> for pipes :
+the syntax
+
+ open KID_PS, "-|", "ps", "aux" or die $!;
+
+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 read from command
+pipes (with C<"|-"> in place of C<"-|">) is also implemented.
+
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
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
+using talking to the terminal device driver. If your system is
amongst those supported, this may be your best bet.
=head2 Bidirectional Communication with Yourself
exit;
}
-But you don't actually have to make two pipe calls. If you
+But you don't actually have to make two pipe calls. If you
have the socketpair() system call, it will do this all for you.
#!/usr/bin/perl -w
BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
use Socket;
use Carp;
- $EOL = "\015\012";
+ my $EOL = "\015\012";
sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
my $port = shift || 2345;
my $proto = getprotobyname('tcp');
- ($port) = $port =~ /^(\d+)$/ || die "invalid port";
+ ($port) = $port =~ /^(\d+)$/ or die "invalid port";
socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
setsockopt(Server, SOL_SOCKET, SO_REUSEADDR,
BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
use Socket;
use Carp;
- $EOL = "\015\012";
+ my $EOL = "\015\012";
sub spawn; # forward declaration
sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
my $port = shift || 2345;
my $proto = getprotobyname('tcp');
- ($port) = $port =~ /^(\d+)$/ || die "invalid port";
+ ($port) = $port =~ /^(\d+)$/ or die "invalid port";
socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
setsockopt(Server, SOL_SOCKET, SO_REUSEADDR,
my $waitedpid = 0;
my $paddr;
+ use POSIX ":sys_wait_h";
sub REAPER {
- $waitedpid = wait;
+ my $child;
+ while (($waitedpid = waitpid(-1,WNOHANG)) > 0) {
+ logmsg "reaped $waitedpid" . ($? ? " with exit $?" : '');
+ }
$SIG{CHLD} = \&REAPER; # loathe sysV
- logmsg "reaped $waitedpid" . ($? ? " with exit $?" : '');
}
$SIG{CHLD} = \&REAPER;
at port $port";
spawn sub {
+ $|=1;
print "Hello there, $name, it's now ", scalar localtime, $EOL;
exec '/usr/games/fortune' # XXX: `wrong' line terminators
or confess "can't exec fortune: $!";
use strict;
my ($rendezvous, $line);
- $rendezvous = shift || '/tmp/catsock';
+ $rendezvous = shift || 'catsock';
socket(SOCK, PF_UNIX, SOCK_STREAM, 0) || die "socket: $!";
connect(SOCK, sockaddr_un($rendezvous)) || die "connect: $!";
while (defined($line = <SOCK>)) {
use Carp;
BEGIN { $ENV{PATH} = '/usr/ucb:/bin' }
+ sub spawn; # forward declaration
sub logmsg { print "$0 $$: @_ at ", scalar localtime, "\n" }
- my $NAME = '/tmp/catsock';
+ my $NAME = 'catsock';
my $uaddr = sockaddr_un($NAME);
my $proto = getprotobyname('tcp');
my $waitedpid;
+ use POSIX ":sys_wait_h";
sub REAPER {
- $waitedpid = wait;
+ my $child;
+ while (($waitedpid = waitpid(-1,WNOHANG)) > 0) {
+ logmsg "reaped $waitedpid" . ($? ? " with exit $?" : '');
+ }
$SIG{CHLD} = \&REAPER; # loathe sysV
- logmsg "reaped $waitedpid" . ($? ? " with exit $?" : '');
}
$SIG{CHLD} = \&REAPER;
};
}
+ 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();
+ }
+
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
Here are what those parameters to the C<new> constructor mean:
-=over
+=over 4
=item C<Proto>
It does this by calling the C<< IO::Socket::INET->new() >> method with
slightly different arguments than the client did.
-=over
+=over 4
=item Proto
Once the generic server socket has been created using the parameters
listed above, the server then waits for a new client to connect
-to it. The server blocks in the C<accept> method, which eventually an
-bidirectional connection to the remote client. (Make sure to autoflush
+to it. The server blocks in the C<accept> method, which eventually accepts a
+bidirectional connection from the remote client. (Make sure to autoflush
this handle to circumvent buffering.)
To add to user-friendliness, our server prompts the user for commands.
$client->autoflush(1);
print $client "Welcome to $0; type help for command list.\n";
$hostinfo = gethostbyaddr($client->peeraddr);
- printf "[Connect from %s]\n", $hostinfo->name || $client->peerhost;
+ printf "[Connect from %s]\n", $hostinfo ? $hostinfo->name : $client->peerhost;
print $client "Command? ";
while ( <$client>) {
next unless /\S/; # blank line
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.
+
Here's a UDP program similar to the sample Internet TCP client given
earlier. However, instead of checking one host at a time, the UDP version
will check many of them asynchronously by simulating a multicast and then
$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.
+
=head1 SysV IPC
While System V IPC isn't so widely used as sockets, it still has some
my $id = msgget(IPC_PRIVATE, IPC_CREAT | S_IRWXU);
my $sent = "message";
- my $type = 1234;
+ my $type_sent = 1234;
my $rcvd;
my $type_rcvd;
try to pass open file descriptors over a local UDP datagram socket if you
want your code to stand a chance of being portable.
-As mentioned in the signals section, because few vendors provide C
-libraries that are safely re-entrant, the prudent programmer will do
-little else within a handler beyond setting a numeric variable that
-already exists; or, if locked into a slow (restarting) system call,
-using die() to raise an exception and longjmp(3) out. In fact, even
-these may in some cases cause a core dump. It's probably best to avoid
-signals except where they are absolutely inevitable. This
-will be addressed in a future release of Perl.
-
=head1 AUTHOR
Tom Christiansen, with occasional vestiges of Larry Wall's original
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> by W. Richard Stevens (published by Addison-Wesley). Note
-that most books on networking address networking from the perspective of
-a C programmer; translation to Perl is left as an exercise for the reader.
+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.
The IO::Socket(3) manpage describes the object library, and the Socket(3)
manpage describes the low-level interface to sockets. Besides the obvious
https://perl5.git.perl.org / perl5.git / blobdiff