Kill a hung child process

Question

My Perl script runs an external program (which takes a single command-line parameter) and processes its output. Originally, I was doing this:

my @result = `p         


        

Answer 1

This is the best I could do. Any ideas on how to avoid the use of a temporary file on Windows would be appreciated.

#!/usr/bin/perl

use strict;
use warnings;
use File::Temp;
use Win32::Process qw(STILL_ACTIVE NORMAL_PRIORITY_CLASS);

my $pid;
my $timeout = 10;
my $prog = "prog arg";
my @output;

if ($^O eq "MSWin32")
{
    my $exitcode;
    my $fh = File::Temp->new ();
    my $output_file = $fh->filename;
    close ($fh);
    open (OLDOUT, ">&STDOUT");
    open (STDOUT, ">$output_file" ) || die ("Unable to redirect STDOUT to $output_file.\n");
    Win32::Process::Create ($pid, $^X, $prog, 1, NORMAL_PRIORITY_CLASS, '.') or die Win32::FormatMessage (Win32::GetLastError ());
    for (1 .. $timeout)
    {
        $pid->GetExitCode ($exitcode);
        last if ($exitcode != STILL_ACTIVE);
        sleep 1;
    }
    $pid->GetExitCode ($exitcode);
    $pid->Kill (0) or die "Cannot kill '$pid'" if ($exitcode == STILL_ACTIVE);
    close (STDOUT);
    open (STDOUT, ">&OLDOUT");
    close (OLDOUT);
    open (FILE, "<$output_file");
    push @output, $_ while (<FILE>);
    close (FILE);
}
else
{
    $pid = open my $proc, "-|", $prog;
    exec ($^X, "-e", "sleep 1, kill (0, $pid) || exit for 1..$timeout; kill -9, $pid") unless (fork ());
    push @output, $_ while (<$proc>);
    close ($proc);
}
print "Output:\n";
print @output;

Answer 2

Try the poor man's alarm

my $pid;
if ($^O eq 'MSWin32') {
    $pid = system 1, "prog arg";    # Win32 only, run proc in background
} else {
    $pid = fork();
    if (defined($pid) && $pid == 0) {
        exec("proc arg");
    }
}

my $poor_mans_alarm = "sleep 1,kill(0,$pid)||exit for 1..$TIMEOUT;kill -9,$pid";
system($^X, "-e", $poor_mans_alarm);

The poor man's alarm runs in a separate process. Every second, it checks whether the process with identifier $pid is still alive. If the process isn't alive, the alarm process exits. If the process is still alive after $time seconds, it sends a kill signal to the process (I used 9 to make it untrappable and -9 to take out the whole subprocess tree, your needs may vary. kill 9,... is also portable).

Edit: How do you capture the output of the process with the poor man's alarm? Not with backticks -- then you can't get the process id and you may lose the intermediate output if the process times out and gets killed. The alternatives are

1) send output to a file, read the file when the process is done

$pid = system 1, "proc arg > some_file";
... start poor man's alarm, wait for program to finish ...
open my $fh, '<', 'some_file';
my @process_output = <$fh>;
...

2) use Perl's open to start the process

$pid = open my $proc, '-|', 'proc arg';
if (fork() == 0) {
    # run poor man's alarm in a background process
    exec($^X, '-e', "sleep 1,kill 0,$pid||exit ...");
}
my @process_output = ();
while (<$proc>) {
   push @process_output, $_;
}

The while loop will end when the process ends, either naturally or unnaturally.

Answer 3

You may want to use alarm system call as in perldoc -f alarm.