What kind of C is an operating system written in?

Question

It makes sense that something like an operating system would be written in C. But how much of it, and what kind of C? I mean, in C, if you needed some heap memory, you would call malloc. But, does an OS even have a heap? As far as I know, malloc asks the operating system for memory and then adds it to a linked list, or binary tree, or something. What about a call stack? The OS is responsible for setting up all of this stuff that other applications use, but how does it do that? When you want to open or create a file in C, the appropriate functions ask the operating system for that file. so... What kind of C is on the other side of that call? Or on the other end of a memory allocation?

Also, how much of an operating system would actually be written in C? All of it? What about architecture dependent code? What about the higher levels of abstraction--does that ever get written in higher level languages, like C++?

I mean, I'm just asking this out of sheer curiosity. I'm downloading the latest linux kernel now but it's taking forever. I'm not sure if I'll wind up being able to follow the code--or if I'll be caught in an inescapably complex web of stuff I've never seen before.

Answer 1

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.

Answer 2

What kind of C?

Mostly ANSI C, with a lot of time looking at the machine code it generates.

But, does an OS even have a heap?

Malloc asks the operating system for a pointer to some memory it is allowed to use. If a program running on an OS (user mode) tries to access memory it doesn't own, it will give a segmentation fault. An OS is allowed to directly access all the physical memory on the system, malloc not needed, no seg-faults on any address that exists.

What about a call stack?

The call stack actually often works at the hardware level, with a link register.

For file access, the OS needs access to a disk driver, which needs to know how to read the file system that's on the disk (there are a lot of different kinds) Sometimes the OS has one built in, but I think it's more common that the boot loader hands it one to start with, and it loads another (bigger) one. The disk driver has access to the hardware IO of the physical disk, and builds from that.