Invalid Access To Memory about leveldb

[veelenga]

@ohenepee as far as i understand you need to move iterator to the next at the end of the loop. What's happening now:

x x x
^
iterator.valid? # => true
iterator.next   # => moves to the second value
iterator.key    # => second key

x x x
  ^
iterator.valid? # => true
iterator.next   # => moves to the third value
iterator.key    # => third key

x x x
    ^
iterator.valid? # => true
iterator.next   # => moves to the invalid pointer
iterator.key    # => Invalid memory access

[ohenepee]

Yes! You're right, I just needed to move the iterator.next below the puts. Thanks also for your awesome elaboration on the subject.

while iterator.valid?
  puts "#{iterator.key} - #{iterator.value}"
  iterator.next
end

iterator.destroy

It works now without giving the INVALID ACCESS TO MEMORY error, however it still reads the whole database even though I explicitly asked the iterator to seek for a particular key prefix... unless perhaps I don't know how the iterator is supposed to work in this case, then I suppose its a bug.

In the example I gave in the first comment, if I ask the iterator ro seek for "messages", should it also print entries keyed with "locations" and "something" ?

[veelenga]

That's how leveldb works. Using a seek you find a start point for the iterator and it will iterate till the end (having the lexicographic order).

To filter only needed entries, you can stop when the first key with a wrong prefix occurs and break the loop:

iterator = LevelDB::Iterator.new(db)

iterator.seek("messages")
while iterator.valid?
  break unless iterator.key.starts_with?("messages")

  puts "#{iterator.key} - #{iterator.value}"
  iterator.next
end

iterator.seek("something")
...

iterator.destroy

Related reading: https://groups.google.com/forum/#!topic/leveldb/DRj0-t1hJ08

[ohenepee]

You couldn't be more helpful... thanks a lot and stay blessed!