Examples of C interfaces to C++ libraries?

I'm in the process of writing a C interface to a C++ library, and I'm looking for some high-quality examples (or best practices).

So far this one seems pretty promising: http://czmq.zeromq.org/manual:czmq

Any other suggestions?

What is c interface?

C Interfaces and Implementations shows how to create reusable APIs using interface-based design, a language-independent methodology that separates interfaces from their implementations. This methodology is explained by example.

Can you use libraries in C?

Functions in a C library can be used and accessed by programmers to create several different programs. As a programmer, you may find yourself using the same function or functions repeatedly. In this case, it is best to put this function or functions in a library to speed up the compilation of the program.

You could look into the Parma Polyhedra Library as an example of excellent C interface to a well written C++ library. PPL is a free GPL-ed software, notably used inside the GCC compiler.