What happens if the forked process blocks?

From PERL's point of view, backticks block until the process returns. But from the OS point of view processes are running concurrently.

If I know the PID (returned by open), I can kill the process after a certain amount of time has elasped by catching an ALRM signal. The signal will interrupt the control flow and the signal handler can kill the "timed out" process.

If that's what you want, I'd recommend using fork. I did something similar a while back, using fork, as described in Timing and timing out Unix commands. If you get it working without using fork, I'd be interested in seeing your code.

I had a similar requirement, and came up with this:

my %p;

sub killchildren {

    my $ppid = shift;

    chomp(my $out = `ps -o pid --no-headers --ppid $ppid`);

    for (split /\n/, $out) {
        killchildren($_);
        kill 9, $_;
    }
}

sub homicide {

    killchildren($$);

    $p{fg}{timeout} = 1;
}

sub fg {

    my ($cmd, $timeout) = @_;

    $p{fg}{timeout} = 0;

    my $out;

    eval {
        local $SIG{ALRM} = q(homicide);
        alarm $timeout;
        $out = `$cmd`;
        alarm 0;
    };

    return $out unless $p{fg}{timeout};

    return q(TIMEOUT);
}

#
# TEST
#
my $out = fg("echo Hi; sleep 5", 1);
print "Out = $out\n";

$out = fg("echo Bye; sleep 5", 10);
print "Out = $out\n";
[download]

The test example had the following output:

~/src$ ./a.pl
Out = TIMEOUT
Out = Bye