How fast do you need?

Have you tried DBFile, GDBM, or BerkeleyDB?

I've haven't used them much, so this isn't a ringing endorsement. Just a suggestion for a (possibly) simpler alternative :)

I'm not sure about the others, but a quick glance at the docs shows that BerkeleyDB and DBFile, at least, supports read-only access.

It might just be fast enough for what you want, and is probably simpler and safer than trying to roll your own.