Lightweight Databases

Void Fields

There's a potential problem with the previous slide's code
Suppose there is no user #2
find_user_by_uid will go to the right place in the index
- It will read 10 nonsense bytes
- Then it will read from a nonsense position in the data file
Solution: Fill nonsense fields with a special 'no such user' value:

        sub build_index_for_users {
          my ($fh, $index_fh) = @_;
          seek $fh, 0, SEEK_SET;
          my $pos = tell $fh;
          my @position;
          while (<$fh>) {
            my $uid = (split /:/)[1];
            $position[$uid] = $pos;
            $pos = tell $fh;
          }

(Continued...)