Ask HN: What are the quality C Programming Questions found in web?

I would like to know some good resources around the web, which contains question/puzzles related to c programming language. Preferably those links which contains comprehensive collections of such questions, or you found the most useful.

Links to articles, blogs, competitive programming sites, pdfs, videos or .txt files etc.

Basically those resources you found useful while preparing for your interviews, conducting interviews or tests related to C.