Re: forking and monitoring processes

[revdiablo]

You can use a forking form of open, and IO::Select. Here is an example:

use IO::Select;

my %pid;
my $s = IO::Select->new();

for (qw(proc1 proc2 proc3)) {
    my $pid = open my $fh, "-|", $_
        or warn "Could not fork&open '$_': $!"
        and next;

    $pid{$fh} = [ $pid, $_ ];
    $s->add($fh);
}

while (my @ready = $s->can_read) {
    for my $fh (@ready) {
        if (eof $fh) {
            delete $pid{$fh};
            $s->remove($fh);
            next;
        }

        my $line = <$fh>;
        if ($line =~ /desktop/) {
            chomp $line;
            print "Found 'desktop' in '$line' from $pid{$fh}[1]\n";
            kill 15, map {$_->[0]} values %pid;
        }
    }
}

Update: changed to a hash instead of an array for storing the PIDs

Another update: fixed bugs introduced by last-minute changes to hash structure.

[Anonymous Monk]

Hello,

Thanks for the response, it makes things a lot clearer. A few questions I had was:

 my $pid = open my $fh, "-|", $_

This means pipe open the process to the filehandler right? I wasn't sure what the "-" in " -|" meant.

Also, after looking at each line, I would like to store the output of each child process into different files. So for process 1, I want to store the output into proc1.txt etc. But when processing each line, I am only aware of the pid and not the process running so I was wondering how I can write to the file proc1.txt if I only know proc1's pid is running and so on?

Thanks.

[revdiablo]

But when processing each line, I am only aware of the pid and not the process running

That's why I changed to a hash of arrays to store the PIDs and process information. You can get the program name by $pid{$fh}[1], or put more information into that array as necessary.

Update: your question about -| can be fully answered by reading open's documentation. The short answer is it opens a process for reading.

[Anonymous Monk]

Hello,

Thanks for reply. When I run the program, the processes seems to start but the program seems to timeout at the select command can_read. Since it a dir command, output is expected immediatly, I was not sure why it was timing out.

#! perl

use IO::Select;

my %pids;
my $s = IO::Select->new();

my @proc = (qw(C M));

for ( <@proc> ) {
    my $cmd = "dir $_:";
    print "this is my cmd: $cmd \n\n";
    my $pid = open my $fh, "-|", $cmd
        or warn "Could not fork&open $cmd: $!"
        and next;

    $pids{$fh} = [ $pid, $_ ];
    $s->add($fh);
}

while (my @ready = $s->can_read) {
    for my $fh (@ready) {
        if (eof $fh) {
            delete $pids{$fh};
            $s->remove($fh);
            next;
        }

        my $line = <$fh>;
        if ($line =~ /Rational/) {
            chomp $line;
            print "Found 'Rational' in '$line' from $pid{$fh}[1]\n";
            kill 15, values %pids;
        }
    }
}

Thanks.