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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:

    open(SPOOLER, "| cat -v | lpr -h 2>/dev/null")
		    || die "can't fork: $!";
    local $SIG{PIPE} = sub { die "spooler pipe broke" };
    print SPOOLER "stuff\n";
    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: $!";
    while (<STATUS>) {
	next if /^(tcp|udp)/;
    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:

    % program f1 "cmd1|" - f2 "cmd2|" f3 < tmpfile  

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:

    print grep { !/^(tcp|udp)/ } `netstat -an 2>&1`;
    die "bad netstat" 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.

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 return just a quick end of file, writers to bogus command will trigger a signal they'd better 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: $!";  

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: $!";
    close FH            or die "can't close: status=$?";  


Both the main process and any child processes it forks share the same STDIN, STDOUT, and STDERR filehandles. If both processes try to access them at once, strange things can happen. You may also want to close or reopen the filehandles for the child. You can get around this by opening your pipe with open(), but on some systems this means that the child process cannot outlive the parent.

Background Processes

You can run a command in the background with:

    system("cmd &");  

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).

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 /dev/null (so that random output doesn't wind up on the user's terminal).

    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;
	setsid			or die "Can't start a new session: $!";
	open STDERR, '>&STDOUT'	or 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 system doesn't have the setsid() function, open /dev/tty and use the TIOCNOTTY ioctl() on it instead. See tty(4) for details.

Non-Unix users should check their Your_OS::Process module for other solutions.

Safe Pipe Opens

Another interesting approach to IPC is making your single program go multiprocess and communicate between (or even amongst) yourselves. The open() function will accept a file argument of either "-|" or "|-" 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 to minus, you can write to the filehandle you opened and your kid will find it in his STDIN. If you open a pipe from minus, you can read from the filehandle you opened whatever your kid writes to his STDOUT.

    use English '-no_match_vars';
    my $sleep_count = 0;

    do {
	$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
	exit;  # don't forget this

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:

    # add error processing as above
    $pid = open(KID_TO_READ, "-|");

    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: $!";

And here's a safe pipe open for writing:

    # add error processing as above
    $pid = open(KID_TO_WRITE, "|-");
    $SIG{ALRM} = 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: $!";

Since Perl 5.8.0, you can also use the list form of 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 KID_PS filehandle. The corresponding syntax to read from command pipes (with "|-" in place of "-|") 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 modules file mentioned below in the SEE ALSO section.

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:

    open(PROG_FOR_READING_AND_WRITING, "| some program |")  

and if you forget to use the use warnings pragma or the -w flag, then you'll miss out entirely on the 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 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.

Here's an example of using open2():

    use FileHandle;
    use IPC::Open2;
    $pid = open2(*Reader, *Writer, "cat -u -n" );
    print Writer "stuff\n";
    $got = <Reader>;  

The problem with this is that Unix buffering is really going to ruin your day. Even though your 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 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 nonstandard library. It uses pseudo-ttys to make your program behave more reasonably:

    require '';
    $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 Comm library also has expect() and interact() functions. Find the library (and we hope its successor IPC::Chat) at your nearest CPAN archive as detailed in the SEE ALSO section below.

The newer 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.

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.

    #!/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?

    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;
    } 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;

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
    # pipe2 - bidirectional communication using socketpair
    #   "the best ones always go both ways"

    use Socket;
    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.
				or  die "socketpair: $!";


    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;
    } 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;




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