Indeed. All the foundational common ground between all languages in Unix is made for C. I'm talking about the system call interface. All general purpose languag

Indeed. All the foundational common ground between all languages in Unix is made for C. I'm talking about the system call interface. All general purpose languages will have a wrapper for this interface, but the main documentation is made for C. Besides that, libraries tend to be made in C as well. That's because if it's in C, it's easy to make it available for other languages. Most general purpose languages will have some type of "foreign function interface" to call on C code. That means that the main documentation for these libraries will be for C as well.