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Perl Language Reference Manual
by Larry Wall and others
Paperback (6"x9"), 724 pages
ISBN 9781906966027
RRP £29.95 ($39.95)

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20.2 Named Pipes

A named pipe (often referred to as a FIFO) is an old Unix IPC mechanism for processes communicating on the same machine. It works just like a regular, connected anonymous pipes, except that the processes rendezvous using a filename and don't have to be related.

To create a named pipe, use the POSIX::mkfifo() function.

use POSIX qw(mkfifo);
mkfifo($path, 0700) or die "mkfifo $path failed: $!";

You can also use the Unix command mknod(1) or on some systems, mkfifo(1). These may not be in your normal path.

# system return val is backwards, so && not ||
#
$ENV{PATH} .= ":/etc:/usr/etc";
if  (      system('mknod',  $path, 'p')
        && system('mkfifo', $path) )
{
    die "mk{nod,fifo} $path failed";
}

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, news reader, finger program, etc.) tries to read from that file, the reading program will block and your program will supply the new signature. We'll use the pipe-checking file test -p to find out whether anyone (or anything) has accidentally removed our fifo.

chdir; # go home
$FIFO = '.signature';
while (1) {
    unless (-p $FIFO) {
        unlink $FIFO;
        require POSIX;
        POSIX::mkfifo($FIFO, 0700)
            or die "can't mkfifo $FIFO: $!";
    }
    # next line blocks until there's a reader
    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 signals
}
ISBN 9781906966027Perl Language Reference ManualSee the print edition