I've been using Objective-C and C++ for a while now. While I feel like I know them pretty well I had absolutely no grasp on C itself. Obviously when using those languages you pick up a bit about data types and conditionals but you couldn't take that knowledge and completely write something non-trivial in C. Because of this I figured it might be worth some time so I started looking for viable resources. In that search I found Learn Code the Hard way. They publish physical and online books on different languages and so far I've found it to be a delight. It's not your typical easy walk through and on lessons 17 (of 51) you'll start actually having to think. This for me has been much better so far than typically online learning where the introductory course ends on how to write a for loop.

If you're interested in picking up a C background I would definitely recommend you start here.