NAME

DESCRIPTION

Signals

For example, to trap an interrupt signal, set up a handler like this. 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, so calling any print() functions (or even anything that needs to malloc(3) more memory) could in theory trigger a memory fault and subsequent core dump.

The names of the signals are the ones listed out by 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 indexed by name to get the number:

So to check whether signal 17 and SIGALRM were the same, just do this:

You may also choose to assign the strings 'IGNORE' or 'DEFAULT' as the handler, in which case Perl will try to discard the signal or do the default thing. 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.)

Sending a signal to a negative process ID means that you send the signal to the entire Unix process-group. This code send a hang-up signal to all processes in the current process group except for the current process itself:

Another interesting signal to send is signal number zero. This doesn't actually affect another process, but instead checks whether it's alive or has changed its UID.

You might also want to employ anonymous functions for simple signal handlers:

But that will be problematic for the more complicated handlers that need to re-install themselves. Because Perl's signal mechanism is currently based on the signal(3) function from the C library, you may sometimes be so misfortunate 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:

or even the more elaborate:

Signal handling is also used for timeouts in Unix, While safely protected within an eval{} block, you set a signal handler to trap alarm signals and then schedule to have one delivered to you in some number of seconds. Then try your blocking operation, clearing the alarm when it's done but not before you've exited your eval{} block. If it goes off, you'll use die() to jump out of the block, much as you might using longjmp() or throw() in other languages.

Here's an example:

For more complex signal handling, you might see the standard POSIX module. Lamentably, this is almost entirely undocumented, but the t/lib/posix.t file from the Perl source distribution has some examples in it.

Named Pipes

To create a named pipe, use the Unix command mknod(1) or on some systems, mkfifo(1). These may not be in your normal path.

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 .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 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 -p to find out whether anyone (or anything) has accidentally removed our fifo.

Using open() for IPC

And here's how to start up a child process you intend to read from:

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:

and irrespective of which shell it's called from, the Perl program will read from the file f1, the process cmd1, standard input (tmpfile in this case), the f2 file, the cmd2 command, and finally the f3 file. Pretty nifty, eh?

You might notice that you could use backticks for much the same effect as opening a pipe for reading:

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.

Be careful to check both the open() and the close() return values. If you're writing to a pipe, you should also trap SIGPIPE. Otherwise, think of what happens when you start up a pipe to a command that doesn't exist: the open() will in all likelihood succeed (it only reflects the fork()'s success), but then your output will fail--spectacularly. Perl can't know whether the command worked because your command is actually running in a separate process whose exec() might have failed. Therefore, while readers of bogus commands just return a quick end of file, writers to bogus command will trigger a signal they'd better be prepared to handle. Consider:

Safe Pipe Opens

Another common use for this construct is when you need to execute something without the shell's interference. With system(), it's straightforward, but you can't use a pipe open or backticks safely. That's because there's no way to stop the shell from getting its hands on your arguments. Instead, use lower-level control to call exec() directly.

Here's a safe backtick or pipe open for read:

And here's a safe pipe open for writing:

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 modules file mentioned below in the SEE ALSO section.

Bidirectional Communication

and if you forget to use the -w flag, then you'll miss out entirely on the diagnostic message:

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 look at its source, you'll see that open2() uses low-level primitives like Unix pipe() and exec() 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.

Here's an example of using open2():

The problem with this is that Unix buffering is going to really ruin your day. Even though your Writer filehandle is autoflushed, 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 in a similarly quick fashion. In this case, we could, because we gave cat a -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 non-standard Comm.pl library. It uses pseudo-ttys 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 Comm library also has expect() and interact() functions. Find the library (and hopefully its successor IPC::Chat) at your nearest CPAN archive as detailed in the SEE ALSO section below.

Sockets: Client/Server Communication

The Perl function calls for dealing with sockets have the same names as the corresponding system calls in C, but their arguments tend to differ for two reasons: first, Perl filehandles work differently than C file descriptors. Second, Perl already knows the length of its strings, so you don't need to pass that information.

One of the major problems with 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 $AF_INET = 2, you know you're in for big trouble: An immeasurably superior approach is to use the Socket module, which more reliably grants access to various constants and functions you'll need.

Internet TCP Clients and Servers

Here's a sample TCP client using Internet-domain sockets:

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 on a particular interface (like the external side of a gateway or firewall machine), you should fill this in with your real address instead.

And here's a multithreaded version. It's multithreaded 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.

This server takes the trouble to clone off a child version via fork() for each incoming request. That way it can handle many requests at once, which you might not always want. Even if you don't fork(), the listen() will allow that many pending connections. Forking servers have to be particularly careful about cleaning up their dead children (called ``zombies'' in Unix parlance), because otherwise you'll quickly fill up your process table.

We suggest that you use the -T flag to use taint checking (see the perlsec manpage ) 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 scripts), because it lessens the chances that people from the outside will be able to compromise your system.

Let's look at another TCP client. This one connects to the TCP ``time'' service on a number of different machines and shows how far their clocks differ from the system on which it's being run:

Unix-Domain TCP Clients and Servers

You can test for these with Perl's -S file test:

Here's a sample Unix-domain client:

And here's a corresponding server.

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.

So why would you ever want to use a Unix domain socket instead of a simpler named pipe? Because a named pipe doesn't give you sessions. You can't tell one process's data from another's. With socket programming, you get a separate session for each client: that's why accept() takes two arguments.

For example, let's say that you have a long running database server daemon that you want folks from the World Wide Web to be able to access, 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.

UDP: Message Passing

Here's a UDP program similar to the sample Internet TCP client given above. However, instead of checking one host at a time, the UDP version will check many of them asynchronously by simulating a multicast and then using select() to do a timed-out wait for I/O. To do something similar with TCP, you'd have to use a different socket handle for each host.

SysV IPC

Here's a small example showing shared memory usage.

Here's an example of a semaphore:

Put this code in a separate file to be run in more than one process. Call the file take:

Put this code in a separate file to be run in more than one process. Call this file give:

WARNING

(... time passes)

Voila! Check out the IPC::SysV modules written by Jack Shirazi. You can find them at a CPAN store near you.

NOTES

Most of these routines quietly but politely return 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 Socket conversion functions croak() on bad arguments.) It is therefore essential that you should check the return values of these functions. Always begin your socket programs this way for optimal success, and don't forget to add -T taint checking flag to the pound-bang line for servers:

BUGS

Because few vendors provide C libraries that are safely re-entrant, the prudent programmer will do little else within a handler beyond die() to raise an exception and longjmp(3) out.

AUTHOR

SEE ALSO