C Tutorial - Learn C - Cprogramming.com
Learn C with our popular C tutorial, which will take you from the very basics of C all the way through sophisticated topics like binary trees and data structures. By the way, if you're on the fence about learning C or C++, I recommend going through the C++ tutorial instead as it is a more modern language.
I was reading about XNU kernel which is written in C and C++, so I was wondering why not just C++ if it's more modern and efficient and object oriented, instead of both languages?

I mean it's better to start with C if you want less foreclosures or you want to understand what's going on inside a machine in a better way ?