$i++;
}
-So to check whether signal 17 and SIGALRM were the same, just do this:
+So to check whether signal 17 and SIGALRM were the same, do just this:
print "signal #17 = $signame[17]\n";
if ($signo{ALRM}) {
signal handlers like this:
sub REAPER {
- $SIG{CHLD} = \&REAPER; # loathe sysV
$waitedpid = wait;
+ # loathe sysV: it makes us not only reinstate
+ # the handler, but place it after the wait
+ $SIG{CHLD} = \&REAPER;
}
$SIG{CHLD} = \&REAPER;
# now do something that forks...
or even the more elaborate:
- use POSIX ":wait_h";
+ use POSIX ":sys_wait_h";
sub REAPER {
my $child;
- $SIG{CHLD} = \&REAPER; # loathe sysV
while ($child = waitpid(-1,WNOHANG)) {
$Kid_Status{$child} = $?;
}
+ $SIG{CHLD} = \&REAPER; # still loathe sysV
}
$SIG{CHLD} = \&REAPER;
# do something that forks...
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, newsreader, finger program, etc.) tries to read
+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 the new signature. We'll use the pipe-checking file test B<-p>
+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
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 sigs
+ sleep 2; # to avoid dup signals
}
in this case), the F<f2> file, the F<cmd2> command, and finally the F<f3>
file. Pretty nifty, eh?
-You might notice that you could use backticks for much the
+You might notice that you could use back-ticks for much the
same effect as opening a pipe for reading:
print grep { !/^(tcp|udp)/ } `netstat -an 2>&1`;
fork()'s success), but then your output will fail--spectacularly. Perl
can't know whether the command worked because your command is actually
running in a separate process whose exec() might have failed. Therefore,
-while readers of bogus commands just return a quick end of file, writers
+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:
Another common use for this construct is when you need to execute
something without the shell's interference. With system(), it's
-straightforward, but you can't use a pipe open or backticks safely.
+straightforward, but you can't use a pipe open or back-ticks safely.
That's because there's no way to stop the shell from getting its hands on
your arguments. Instead, use lower-level control to call exec() directly.
-Here's a safe backtick or pipe open for read:
+Here's a safe back-tick or pipe open for read:
# add error processing as above
$pid = open(KID_TO_READ, "-|");
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
+multi-threading. If you'd like to learn more about threading, see the
F<modules> file mentioned below in the SEE ALSO section.
=head2 Bidirectional Communication
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
+to catch both ends. There's also an open3() for tri-directional 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.
print Writer "stuff\n";
$got = <Reader>;
-The problem with this is that Unix buffering is going to really
-ruin your day. Even though your C<Writer> filehandle is autoflushed,
+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 actually give it back to you
+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
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 hopefully its
+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.
=head1 Sockets: Client/Server Communication
-While not limited to Unix-derived operating systems (e.g. WinSock on PCs
+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
+you much good. With sockets, you can do both virtual circuits (i.e., TCP
+streams) and datagrams (i.e., UDP packets). You may be able to do even more
depending on your system.
The Perl function calls for dealing with sockets have the same names as
And here's a corresponding server to go along with it. We'll
leave the address as INADDR_ANY so that the kernel can choose
-the appropriate interface on multihomed hosts. If you want sit
+the appropriate interface on multi-homed 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.
my $port = shift || 2345;
my $proto = getprotobyname('tcp');
+ $port = $1 if $port =~ /(\d+)/; # untaint port number
+
socket(Server, PF_INET, SOCK_STREAM, $proto) || die "socket: $!";
setsockopt(Server, SOL_SOCKET, SO_REUSEADDR,
pack("l", 1)) || die "setsockopt: $!";
scalar localtime, "\n";
}
-And here's a multithreaded version. It's multithreaded in that
+And here's a multi-threaded version. It's multi-threaded in that
like most typical servers, it spawns (forks) a slave server to
handle the client request so that the master server can quickly
go back to service a new client.
my $paddr;
sub REAPER {
- $SIG{CHLD} = \&REAPER; # loathe sysV
$waitedpid = wait;
+ $SIG{CHLD} = \&REAPER; # loathe sysV
logmsg "reaped $waitedpid" . ($? ? " with exit $?" : '');
}
($paddr = accept(Client,Server)) || $waitedpid;
$waitedpid = 0, close Client)
{
- next if $waitedpid;
+ next if $waitedpid and not $paddr;
my($port,$iaddr) = sockaddr_in($paddr);
my $name = gethostbyaddr($iaddr,AF_INET);
return;
} elsif ($pid) {
logmsg "begat $pid";
- return; # i'm the parent
+ return; # I'm the parent
}
- # else i'm the child -- go spawn
+ # 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";
over TCP, including being able to "broadcast" or "multicast" to a whole
bunch of destination hosts at once (usually on your local subnet). If you
find yourself overly concerned about reliability and start building checks
-into your message system, then you probably should just use TCP to start
+into your message system, then you probably should use just TCP to start
with.
Here's a UDP program similar to the sample Internet TCP client given
All these routines create system-specific portability problems. As noted
elsewhere, Perl is at the mercy of your C libraries for much of its system
behaviour. It's probably safest to assume broken SysV semantics for
-signals and to stick with simple TCP and UDP socket operations; e.g. don't
+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.
the F<modules> file at your nearest CPAN site. (See L<perlmod> 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
+(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.
https://perl5.git.perl.org / perl5.git / commitdiff