In addition to having programmed in C and assisted people in problems
with C, I have studied the ANSI C standard (and once tried to
contribute to the C standardization work), and I have given several
courses on C together with Timo Larmela, mostly for people doing
technical research & development.
We have also written a book on C in Finnish, C-ohjelmointikieli (published by Otadata), 3rd ed. 1992 (ISBN 951-767-051-6) which is the reason I occasionally identify myself as "the K of the K&L".
I have mostly used C in Unix environments. I have some (painful) experiences with using C on old PC's on our courses. During the last few years I have worked a little with C on Windows, too, using djgpp (about which I have written instructions in Finnish).
If you have questions about C, I strongly recommend reading the comp.lang.c FAQ. before posting to the comp.lang.c newsgroup.
I've souped up a small document in Finnish about C-related resources on the Internet: Tietoa C-ohjelmointikielestä Internetissä. It mentions the following C-related documents in English by me:
I know practically nothing about C++.
If you are looking for a tutorial on C in English, see LEARN C/C++ TODAY (A list of resources/tutorials) or, if you don't mind seeing my comments in Finnish, my list of C tutorials.