The following example can help you.
The main process forks a number of children with logical IDs between 0 and $np. The processes concurrently gain exclusive access to a file using a lock. 
lhp@nereida:~/Lperl/src/perl_networking/ch2$ cat -n flock.pl
 1  #!/usr/bin/perl -w
 2  use strict;
 3  use Fcntl qw(:DEFAULT :flock);
 4  use POSIX qw(WNOHANG);
 5  $| = 1;
 6
 7  local $SIG{CHLD} = sub {
 8    while (my $kid = waitpid(-1, WNOHANG) > 0) {}
 9  };
10
11  sub create_child {
12    my ($id, $task) = splice @_, 0, 2;
13    my $pid;
14
15    return $pid if $pid = fork();
16    die "Cannot fork $!" unless defined $pid;
17    $task->($id, @_); # do something
18    exit;
19  }
20
21  sub parfor {
22    my $LAST = shift;
23    my $task = shift;
24    my @pid;
25
26    $pid[0] = $$;
27    $pid[$_] = create_child($_, $task, @_) for 1..$LAST;
28    return @pid;
29  }
30
31  sub task {
32    my $id = shift;
33    my $fn = shift;
34
35    sleep(int(rand(2)));
36    open my $f, "+<$fn" or die "Can't open $fn. $!\n";
37    flock($f, 2);
38    seek $f, 0, 0;
39    my $num = <$f>;
40    warn "Process $id reads $num.\n";
41    seek $f, 0, 0;
42    my $s = $num+$id;
43    warn "Process $id writing $s.\n";
44    print $f $s;
45    close($f);
46    exit;
47  }
48
49  #main
50  my $np = shift || 3;
51  my $fn = shift || "sync.txt";
52  open my $f, "> $fn" or die "Can't open file $fn. $!\n";
53  print $f 0;
54  close($f);
55  &parfor($np, \&task, $fn);
56  do {} while wait > 0;

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The answer to your question is in line 56

Hope it helps

Casiano

In reply to Re: How the parent process knows that all its child process has finished execution? by casiano
in thread How the parent process knows that all its child process has finished execution? by kishore036

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