Introduction to Parents/Childs and Forks Inquiry

Question

I am attempting to learn the functionality of parents, children and pipes in perl. My goal is to create a single pipe (not bidirectional) that reads from the command line, and prints it through a pipe. Referencing the pids many many times.

The code so far:

#!/usr/bin/perl -w

use warnings;
use strict;


pipe(READPIPE, WRITEPIPE);
WRITEPIPE->autoflush(1);

my $parent = $$;
my $childpid = fork() // die "Fork Failed $!\n";

# This is the parent
if ($childpid) {  
    &parent ($childpid);
    waitpid ($childpid,0);
    close READPIPE;
    exit;
}
# This is the child
elsif (defined $childpid) {
    &child ($parent);
    close WRITEPIPE;

}
else {
}

sub parent { 
    print "The parent pid is: ",$parent, " and the message being received is:", $ARGV[0],"\n";
    print WRITEPIPE "$ARGV[0]\n";
    print "My parent pid is: $parent\n";
    print "My child pid is: $childpid\n";   
}

sub child {
    print "The child pid is: ",$childpid, "\n";
    my $line = <READPIPE>;
    print "I got this line from the pipe: $line and the child pid is $childpid \n";
}

The current output is:

perl lab5.2.pl "I am brain dead"
The parent pid is: 6779 and the message being recieved is:I am brain dead
My parent pid is: 6779
My child pid is: 6780
The child pid is: 0
I got this line from the pipe: I am brain dead
 and the child pid is 0

I am trying to figure out why the childpid in the child subroutine is returning as 0, but in the parent it is referencing "accurate looking" pid #.

Is is supposed to return 0? (For instance if I made multiple subroutines would they be 0,1,2 etc.?)

`$childpid` is zero in the child because it is set to the return value of `fork()` — Håkon Hægland, Mar 05 '17 at 23:03
It is odd that the parent writes to `WRITEPIPE` but closes `READPIPE`, and the child reads from `READPIPE` but closes `WRITEPIPE`. — mob, Mar 05 '17 at 23:04
@HåkonHægland thanks for the assitsance. mob odd as in bad, or odd as in why is it working that way? — The_Makings_Of_A_Coder, Mar 05 '17 at 23:11
Note that using an explicit `&` prefix when calling sub routines is optional and generally not recommended, see [`perlsub`](http://perldoc.perl.org/perlsub.html) and [When should I use the & to call a Perl subroutine?](http://stackoverflow.com/q/1347396/2173773) — Håkon Hægland, Mar 05 '17 at 23:21

mob · Accepted Answer · 2017-03-06T01:15:58.070

2

Yes, it should be 0, and it would be 0 in every child process that comes out of a fork call.

fork

Does a fork(2) system call to create a new process running the same program at the same point. It returns the child pid to the parent process, 0 to the child process, or "undef" if the fork is unsuccessful.

After the fork, $$ is changed in the child process to the new process id. So the child could read $$ to get the child process id (its own id).

edited Mar 06 '17 at 01:15

answered Mar 05 '17 at 23:03

mob

117,087
18
149
283

That's not accurate. The value of `$$` is changed when you examine it, not when your `fork`. `$$` is a wrapper for `getpid()` – ikegami Mar 06 '17 at 03:20

Introduction to Parents/Childs and Forks Inquiry

1 Answers1