WARNING: Here be dragons!
C and C++ are notoriously badly-taught languages. The median (or maybe 99th percentile?) C and C++ programming material is full of bad advice, bad practices, archaic coding style, Undefined Behavior, or code that is just generally in bad taste.
Unfortunately, Cplusplus.com is no exception. I strongly recommend ignoring it altogether. If you need a reference-like document for either of these languages, go to cppreference.com instead (contrary to its name, it is not only C++, it also has a whole separate subsite for C).
To compound the problem, FFI is typically a topic that requires broad and detailed knowledge of C, so learning "just enough C to do FFI" approximately means "mastering C".
If you are interested in FFI, you are probably going to need to interact with the Unix C library as well. For that, you'll probably want The Open Group's POSIX man pages, an example of which you can find here. Feel free to browse around.
I have a hard time finding good C tutorials, but if you are willing to take some of the ++ (given that the basics of the two languages are very-very similar), one course I couldn't really find anything incorrect in is The Imperial College Introductory C++ Course.
An excellent resource for learning what C is not is the Comp.Lang.C Usenet FAQ. You might use it for falsifying fallacies introduced in bad C tutorials. You might even be able to follow a C tutorial and correct its mistakes using the Comp.Lang.C FAQ. While I appreciate that you will probably learn a lot in the process, pulling off this is probably outside of the level of effort required that most people are willing to sacrifice for learning C. Nevertheless, I'd wholeheartedly recommend at least skimming through it, because you'll have a much better idea of what errors to look for and what mistakes to avoid when programming in C.