Excellent questions, all. The answer is: little to none of the standard C library is available in the “dialect” of C used to write an operating system. In the Linux kernel, for example, the standard memory allocation functions malloc, nmalloc, free etc. are replaced with special kernel-internel memory allocation functions kmalloc and kfree, with special restrictions on their use. The operating system must provide its own “heap” — in the Linux kernel, physical memory pages that have been allocated for kernel use must be non-pageable and often physically continguous. See This linux journal article on kmalloc and kfree. Similarly, the operating system kernel maintains its own special call stack, the use of which requires, from memory, special support from the GCC compiler.
Also, how much of an operating system would actually be written in C? All of
it?
As far as I’m aware, operating systems are overwhelmingly written in C. Some architecture-specific features are coded in assembler, but usually very little to improve portability and maintainability: the Linux kernel has some assembler but tries to minimize it as much as possible.
What about architecture dependent
code? What about the higher levels of
abstraction–does that ever get
written in higher level languages,
like C++?
Usually the kernel will be written in pure C, but sometimes the higher level frameworks and APIs are written in a higher level language. For example, the Cocoa framework/API on MacOS is written in Objective C, and the BeOS higher level APIs were written in C++. Much of Microsoft’s .NET framework was written in C#, with the “Common Language Runtime” written in a mix of C++ and assembler. The QT widget set most often used on Linux is written in C++. Of course, this introduces philosophical questions about what counts as “the operating system.”
The Linux kernel is definitely worth looking at for this, although, it must be said, it is huge and intimidating for anyone to read from scratch.