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 = $$;
my $childpid = fork() // die "Fork Failed $!\n";
# This is the parent
if ($childpid) {
&parent ($childpid);
waitpid ($childpid,0);
close READPIPE;
exit;
}
# This is the child
elsif (defined $childpid) {
&child ($parent);
close WRITEPIPE;
}
else {
}
sub parent {
print "The parent pid is: ",$parent, " and the message being received is:", $ARGV[0],"\n";
print WRITEPIPE "$ARGV[0]\n";
print "My parent pid is: $parent\n";
print "My child pid is: $childpid\n";
}
sub child {
print "The child pid is: ",$childpid, "\n";
my $line = <READPIPE>;
print "I got this line from the pipe: $line and the child pid is $childpid \n";
}
The current output is:
perl lab5.2.pl "I am brain dead"
The parent pid is: 6779 and the message being recieved is:I am brain dead
My parent pid is: 6779
My child pid is: 6780
The child pid is: 0
I got this line from the pipe: I am brain dead
and the child pid is 0
I am trying to figure out why the childpid in the child subroutine is returning as 0, but in the parent it is referencing "accurate looking" pid #.
Is is supposed to return 0? (For instance if I made multiple subroutines would they be 0,1,2 etc.?)
Yes, it should be 0, and it would be 0 in every child process that comes out of a fork call.
fork
Does a fork(2) system call to create a new process running the same program at the same point. It returns the child pid to the parent process, 0 to the child process, or "undef" if the fork is unsuccessful.
After the fork, $$ 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).