# Example:
$self->_sysseek(147);
$self->_syswrite("cl", 4);

sub _sysseek {
    my ($self, $pos) = @_;
    my $fh = $self->{fh};

    my $cur_pos = sysseek($fh, 0, SEEK_CUR);

    if (!defined $cur_pos) {
        $cur_pos = "";
    }

    warn "+ $$ $pos $cur_pos" if $$ != PROCESS_ID && $WRITE;

    while ( 1 ) {
        $cur_pos = sysseek($fh, $pos, SEEK_SET);

        if (!defined $cur_pos) {
            die "system seek error: $!";
        }

        if ($cur_pos == $pos) {
            last;
        }

        $DEBUG = 1;
        warn "unable to seek to pos $pos (curpos $cur_pos), try again";
    }

    warn "- $$ $pos $cur_pos\n" if $$ != PROCESS_ID && $WRITE;
}

sub _syswrite {
    my ($self, $data, $size) = @_;
    my $fh     = $self->{fh};
    my $offset = 0;
    my $length = length($data);

    if ($length < $size) {
        $data .= $SEPARATOR x ($size - $length);
    }

    flock($fh, LOCK_EX) or die "unable to lock file";

    while ($size) {
        my $pos = sysseek($fh, 0, SEEK_CUR);
        warn "start to write at pos $pos" if $WRITE;
        my $written = syswrite $fh, $data, $size, $offset;


        if (!defined $written) {
            die "system write error: $!";
        } elsif ($written) {
            $size   -= $written;
            $offset += $written;
            $pos = sysseek($fh, 0, SEEK_CUR);
            warn "written $written newpos $pos" if $WRITE;
        }
    }

    flock($fh, LOCK_UN) or die "unable to unlock file";
}