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Forking in Perl, then communicating between processes
Interprocess Communication example from a Well House Consultants training course
More on Interprocess Communication [link]

This example is described in the following article(s):
   • Perl - doing several things at the same time - [link]

Source code: fdem Module: P223

=head1 Forking in Perl

The fork function in Perl splits the current process
into two, duplicating all variables into each of the
new processes.

One of the new processes is known as the "parent" -
this is the original process (under the original process
id), and the other is know as the "child", which has
a new process id. You can tell them apart by checking
the value returned by fork - in the child it returns a zero,
but in the parent it returns the process id of the newly
created child.

Based on the returned value, the parent and the child can
go off to perform different tasks. You should take care
in your code, though, to ensure that you add an exit call
if appropriate to avoid one dropping through into the other
at run time.

Parent and child can alert each other via signals (not
illustrated in this simple example) or writing messages
to and from each other via a pipe. If you open a pipe
before you fork, with two file handles, you can then
(after forking) write to one of the file handles in the
parent and read in the child, and / or write to a file
handle in the child and read at the parent.

This use of pipes to communicate between processes allows
the processes to wait for one another (syncronise) when
each of their individual tasks reaches a strategic point.

=cut


print "hello\n";

$abc = 123;
$ppid = $$;

pipe(FHX, FHY);

if (!($pid = fork())) {
        print ("$abc ... Child of $ppid is $$\n");
        sleep 5;
        print "yawn\n";
        sleep 5;
        print "yawn\n";
        print FHY ("OK, Mum, I'm done\n");
        print "Child completed\n";
        exit;
}
print ("$abc ... Parent is $ppid with child $pid\n");
sleep 4;
print "yeah\n";
sleep 4;
print "yeah\n";
$woch = <FHX>;
print ("Always sitting at the school gate!\n");
print $woch;

__END__

Dorothy:csr1 grahamellis$ perl fdem
hello
123 ... Parent is 414 with child 415
123 ... Child of 414 is 415
yeah
yawn
yeah
yawn
Child completed
Always sitting at the school gate!
OK, Mum, I'm done
Dorothy:csr1 grahamellis$

Learn about this subject
This module and example are covered on our public Perl Extra course. If you have a group of three or more trainees who need to learn the subject, we can also arrange a private or on site course for you.

Books covering this topic
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Other Examples
This example comes from our "Interprocess Communication" training module. You'll find a description of the topic and some other closely related examples on the "Interprocess Communication" module index page.

Full description of the source code
You can learn more about this example on the training courses listed on this page, on which you'll be given a full set of training notes.

Many other training modules are available for download (for limited use) from our download centre under an Open Training Notes License.

Other resources
• Our Solutions centre provides a number of longer technical articles.
• Our Opentalk forum archive provides a question and answer centre.
The Horse's mouth provides a daily tip or thought.
• Further resources are available via the resources centre.
• All of these resources can be searched through through our search engine
• And there's a global index here.

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