I recently fixed an extremely odd bug in one of my perl scripts. Here's the general context. I loop through a number of files (lets say 10 of them) in serial. For each file, I spawn off a system process that runs on a simulation on that file. The behaviour of the simulator makes it so that sometimes the simulator will deadlock and the simulator's parent needs to kill the process. So I used $SIG{ALRM} to trap this. If I haven't received output after 5 seconds, I kill the process (actually the entire group of processes it might have spawned), close the filehandle, and process the next file in the list.

Here's the odd part. If I ever manually kill the file through the sig alarm handler, then the next time I execute the open pipe, perl seg-faults. The only way (that I've found) to fix this problem is to read either $! or $? after the close(). When the script reads one of these variables, the script no longer seg-faults.

Here is the output of perl -v: 

This is perl, version 5.005_03 built for i386-linux

Copyright 1987-1999, Larry Wall

Perl may be copied only under the terms of either the Artistic License
+ or the
GNU General Public License, which may be found in the Perl 5.0 source 
+kit.

Complete documentation for Perl, including FAQ lists, should be found 
+on
this system using `man perl' or `perldoc perl'.  If you have access to
+ the
Internet, point your browser at http://www.perl.com/, the Perl Home Pa
+ge.

[download]

And here is the subroutine that's causing these problems: 
sub anysim_tests {
    my ($flag, $class, $tests) = @_;

    my $program = $flag . "sim";
    my $progname = uc($flag) . "sim";
    return 1 unless flag_present($tests, $flag);

    print "\n\n*** ", uc($program), " TESTING ***\n\n";

    # Autoflush sim output
    local $| = 1;

    foreach my $file (sort keys %$tests) {

        # Check if this file didn't need xsim testing
        next if index($tests->{$file}[0], $flag) < 0;

        # Compute actual file names
        my $output = $file;
        $output =~ s/vo$/${program}.mem/g or next; # Should never fail
        delete_mem($output) or next; # Delete old memory image

        use vars qw($abort $pid $sim);
        local $sim = new FileHandle;
        local $pid =
          open($sim, "exec /usr/local/java/bin/java $class $file 2>&1 
+|");

        unless (defined($pid)) {
            print "Warning: Could not run 'java $class $file': $!\n\n"
+;
            next;
        }

        print "\n$progname testing $file\n";

        local $abort = 0;
        local $SIG{ALRM} =
          sub {
               alarm(0);
               print "\nWarning: $progname deadlocked on $file\n";
               kill "TERM", $pid;

               # This close will probably fail
               close($sim);

               # We need to read $! or $?, or else the next open pipe 
+will
               # seg fault perl
               my $garbage = $!;

               $abort = -1;
          };

        my $found_data = 0;
        do {
            alarm(5);

            $_ = <$sim>;
            if (defined $_) {
                ++$found_data;
                print "#" if ($found_data % 500) == 0;
            } else {
                $abort = 1 unless $abort;
            }
        } while (!$abort);

        print $/;
        alarm(0);

        if ($abort > 0 && !close($sim)) {
            print "Warning: $progname could not successfully run $file
+\n";
            next;
        } else {
            my $source = $progname . ".vout";
            system "mv $source $output" if is_readable($source);
        }
    }
}

[download]


-Ted

In reply to Behavior of Open Pipes with Kill by tedv

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