The easiest way to query Recoll from a C or C++ program is to execute an external search command (recollq or recoll -t).

I have written a simple C module which deals with the related housekeeping and presents an easy to use API to the rest of the code. You will find it here:

It is a bit experimental and will only work with recoll 1.20 for now (because it uses a new option for recollq). However it would be trivial to modify for working with 1.19, get in touch with me if you need this.

The other approach is to link with the Recoll library. This has no official API, but in practise, the internal one is fairly stable, and if you want to choose this approach, you should start from the code in recollq.cpp