Learning C when you already know C++?

I think I have an advanced knowledge of C++, and I'd like to learn C.

There are a lot of resources to help people going from C to C++, but I've not found anything useful to do the opposite of that.

Specifically:

1. Are there widely used general purpose libraries every C programmer should know about (like boost for C++) ?
2. What are the most important C idioms (like RAII for C++) ?
3. Should I learn C99 and use it, or stick to C89 ?
4. Any pitfalls/traps for a C++ developer ?
5. Anything else useful to know ?