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> below.
+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
}
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:
sleep 2; # to avoid dup signals
}
-=head2 Deferred 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,
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}> try something like the following:
+
+ use POSIX;
+ sigaction SIGALRM, new POSIX::SigAction sub { die "alarm\n" }
+ 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
=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
# 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) {
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>)) {
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');
$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
my $id = msgget(IPC_PRIVATE, IPC_CREAT | S_IRWXU);
my $sent = "message";
- my $type = 1234;
+ my $type_sent = 1234;
my $rcvd;
my $type_rcvd;
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
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