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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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20.3.6 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 Comm.pl library. It uses pseudo-ttys to make your program behave more reasonably:

require 'Comm.pl';
$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 Expect.pm 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.

ISBN 9781906966027Perl Language Reference ManualSee the print edition