Programming Perl, 4th Edition

pipe

pipe READHANDLE, WRITEHANDLE

Like the corresponding syscall, this function opens a pair of connected pipes—see pipe(2). This call is usually used right before a fork, after which the pipe’s reader should close WRITEHANDLE, and the writer close READHANDLE. (Otherwise, the pipe won’t indicate EOF to the reader when the writer closes it.) If you set up a loop of piped processes, deadlock can occur unless you are remarkably careful. In addition, note that Perl’s pipes use standard I/O buffering, so you may need to set $| ($OUTPUT_AUTOFLUSH) on your WRITEHANDLE to flush after each output operation, depending on the application—see select (output filehandle).

(As with open, if either filehandle is undefined, it will be autovivified.)

Here’s a small example:

pipe(README, WRITEME);
unless ($pid = fork) {  #               child
    defined($pid) || die "can't fork: $!";
    close(README);
    for $i (1..5) { print WRITEME "line $i\n" }
    exit;
}
$SIG{CHLD} = sub { waitpid($pid, 0) };
close(WRITEME);
@strings = <README>;
close(README);
print "Got:\n", @strings;

Notice how the writer closes the read end and the reader closes the write end. You can’t use one pipe for two-way communication. Either use two different pipes ...