@Grumpy_Mike...Re-read section 4 of the C Ianguage Standard
Since that acid test eliminates all standard functions
Standard functions are what they say standard functions, not part of the core language.
Back to your original question. The best way to go is to buy a book. Preferably the one I wrote called 'Programming Arduino: Getting Started with Sketches'.A search on Amazon will find you all sorts of good C programming books and if you know C#, you will not find it hard. But you will end up writing code where function names start with a capital - curse you Bill Gates!Books are distilled knowledge! Save yourself some time.