I am looking to add an address book to one of my programs. For that purpose, I want to have something that is flexible and customizable to the point of allowing me to hook up additional metadata to contacts in it from third parties.

I don't mind paying for a solution as long as I get something that is usable for me.

My requirements:

1. Optimized to run on embedded devices
2. Preferably in source code form
3. Ability to add my own fields to contacts over those the address book provides out of the box
4. Ability to plugin "stuff" to it to enhance it without changing its base code too much
5. Has a C or C++ interface