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I like operating systems and would eventually like to become a OS developer mostly working on kernels. In the future will C still be the language of choice and what else should I be trying to learn?

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Everyone, BEWARE it seems to be un-holy to mention Assembly language in answers to this post, and some-one who did not to understand this question properly, is just down-voting it. I suspect its Cody .. LOL –  Alphaneo May 14 '09 at 2:55
That's because assembly language is unmaintainable (in general) to write whole kernels with. Raise the level of abstraction is the way. –  Chris Jester-Young May 14 '09 at 2:57
@Chris, You are mistaken, Assembly was for Some part of the kernel. –  Alphaneo May 14 '09 at 2:59
@Chris, No part /needs/ to use assembly -- not directly. You can write every single piece of code, from POST to desktop, in managed code and AOTC it. The only piece of code that needs to ever touch machine code directly is the compiler. –  Cody Brocious May 14 '09 at 3:07
You handle privileged instructions via compiler intrinsics, e.g. you make a stub 'rdtsc' method in your managed code, which when called becomes an 'rdtsc' instruction in the emitted code. Obviously, this can be checked completely for safety like any managed code. –  Cody Brocious May 14 '09 at 3:10
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15 Answers

up vote 5 down vote accepted

I think it's safe to say that low-level parts of operating systems (e.g. the kernel) will continue to be written in C because of its speed. Like mentioned elsewhere, you will need to know assembler for certain parts of the kernel (something needs to load the kernel into memory). But you can work on the kernel with little or no assembly knowledge. A good example would be if you're implementing a file system.

Don't worry about what language the operating system is implemented in. What's important is how an operating systems are used, and what can be done to improve them. A good example is when Unix first came out. The file system had the inodes at the front of the disk, and data in the remaining space. This didn't perform very well as you were seeking to different parts of the disk for all files. Then the Berkeley Fast File System was created to make a disk aware file system. This means having inodes near their corresponding data. I'm leaving out a lot of details, but I hope this illustrates that it's more important to think about how an operating system can be improved rather than what language it will be programmed in.

Some recent trends in operating systems are virtualization and distributed computing (see Google's paper on MapReduce). File systems, security, scheduling (especially with multi-core processors), etc are continually areas of interest even though these problems are not new.

Here are some resources if you want to learn more about kernel development:

  • Linux Kernel Newbies - Resource for those who want to get started on modifying the Linux kernel.
  • xv6 source - x86 port of Unix version 6. Used by MIT to teach an operating systems class. Simple, and easy to extend (more info).
  • Linux Kernel Map - Call chain of system calls in Linux. Useful in visualizing what a system call does.

Bottom line: Start getting familiar with the kernel and read papers on what researchers are writing about (USENIX is useful for this). This knowledge is much more valuable than learning a new language, as most concepts from one language can easily be transferred to another if there does happen to be a shift in what operating systems are written. Hope this helps!

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This ignores all the research being done on managed kernels, as Chris has linked. In addition, sticking with C has nothing to do with speed (Singularity has proven a managed OS to be much, much faster due to the safety of managed code) and everything to do with being the standard. –  Cody Brocious May 14 '09 at 3:12
I mentioned C because it's still fairly common, and it doesn't hurt to know if he doesn't already. I don't try to claim it's faster or better. The main goal of my post is to emphasize learning about what is being researched (including managed kernels), and learning how the kernel works. –  mgriepentrog May 14 '09 at 3:28
Why do people keep saying that "Singularity has proven a managed OS to be much, much faster". It seems that very limited performance testing was done. Run a full fledged app like an Oracle RDBMS through all the popular benchmarks on an OS like that, and it will be more convincing. Some apps do things like align data to cache lines to mitigated false sharing. How would you do that when running on an OS like Singularity? –  RussellH Sep 24 '09 at 21:38
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Among the research crowd, there is a lot of of interest in using language-based technology to guarantee that the kernel can't misbehave. A number of people have mentioned the Singularity project, which currently has a (deservedly) high profile. Why is Singularity interesting?

  • The language includes a finite-state model for the proper use of locks. The compiler can model-check the code against the model to be sure that deadlock doesn't happen.

  • Third-party drivers are given a limited interface to the system. The checking done by the compiler guarantees that a bad driver cannot take out the system---the worst it can do is knock out its own device.

  • Singularity uses compiler technology, not OS/MMU technology, to isolate one "process" from another. Suddenly forking a new "process" (really a new kind of protection domain) is dirt cheap, and this cheapness enables new designs.

Singularity is just the latest in a long list of projects that have used language and compiler technology to solve OS problems. One of my favorites was the University of Washington SPIN kernel, which allowed applications to extend the kernel safely and was written in Modula-3.

This area of research is still wide open, and it is not really known yet what set of language or compiler features is a "sweet spot" for solving OS problems. So to answer your question:

  • In today's production systems, C is still "it."

  • For the operating systems of the future, C is almost certainly not "it"—we know that it is possible to do much better—but the exact nature of the new "it" is still a wide-open question.

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+1. I would upvote this a dozen times if given the opportunity. –  Cody Brocious May 14 '09 at 3:35
There's no reason the managed code VM can't be written in managed code itself, from the ground up. You can compile the managed code to machine code ahead of time, then bootstrap up from there. That's the approach taken by SharpOS, Cosmos, MOSA, and Renraku. –  Cody Brocious May 14 '09 at 5:32
@Jay: you might check out Dan Grossman's language 'Cyclone', which is a Frankstein monster but does provide a kind of 'managed' code with no VM --- it compiles directly to assembly code (maybe typed assembly language). It's a language not an OS, and it's a bit of a monster, but it does give one a sense of the possibilities going beyond just a VM. –  Norman Ramsey May 15 '09 at 0:00
I agree with the first bullet, not with the second. I wish I could agree. C sucks, and is probably single-handedly responsible for most security issues. But I see no evidence of anyone moving to use anything better for serious OS development. –  T.E.D. Aug 27 '09 at 16:14
@Jay: Yes, the compile-time type checking is superior to C++. For example, it can guarantee absence of pointer errors. –  Norman Ramsey Mar 14 '10 at 2:28
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Cody didn't want to be bothered answering this, so I'm passing this on on his behalf. :-P For some examples of OSs written in managed languages, as opposed to C or assembly, look at:

Of course, Cody also didn't want to mention this:

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Doesn't addres the question. Those are OSes, not programming languages. –  T.E.D. Aug 27 '09 at 16:10
Except, they demonstrate that (by virtue of all of them being in a managed language) it's not necessary to write an OS in C or assembly, and that it can be written (in some cases including the initial booting stage) entirely in a managed language. –  Chris Jester-Young Aug 28 '09 at 14:18
SharpOS and Cosmos are written in C#. Renraku is, last I heard, written in Boo. The other ones, you'll have to find out yourself. :-P –  Chris Jester-Young Aug 28 '09 at 14:25
OK, I see what you are getting at now. Move those comments into the answer and I'll take back my downvote. –  T.E.D. Sep 1 '09 at 13:54
It's probably too late to undo the downvote (you only have a smallish window of time to undo a vote, to prevent people from gaming the system), but I'll edit the post anyway. –  Chris Jester-Young Sep 2 '09 at 0:26
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C is pretty much it, with a fair amount of assembler. Important topics for OS kernel work include:

  • Principles of caching and cache management
  • Virtual memory, TLB management
  • CPU and system architecture
  • Storage hierarchies
  • Concurrent programming techniques (mutual exclusion, locking, ...)
  • Algorithms and data structures
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Well I know that much, I am just wondering about the future of OS's in general. ya know like cloud OS's and stuff, is that still just going to be C. –  Recursion May 14 '09 at 2:23
@unknown google, I guess Lance is talking about kernel and not the OS. Kernel will be written mostly in C, even for your so-called Cloud. –  Alphaneo May 14 '09 at 2:39
Don't worry. Cloud OS's are still written in C (at least the low levels). The C programmers just aren't allowed to speak at the press conferences. –  Matthew Flaschen May 14 '09 at 4:06
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Actualy, there is quite a bit of room in the core of a modern OS for C++ code. I just looked and the Win7 core kernel tree has quite a bit of C++ code. Note that many sub-systems remain in simple C. There are a few reasons for this

  1. C is the original language of the NT based OS
  2. C is very, very well understood by key people
  3. Well written C can be the most straight forward code to debug - especialy in kernel mode.

That being said, many teams and people have found well written C++ to be an effective tool for core OS work.

There is nothing about C++ that prevents it from being used to write core resource management code like a scheduler, memory manger, I/O subsystem, graphics sub-system, etc. etc.

As others have pointed out - any kernel work will always require some bit of assembly language.

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Moving from C to C++ in the kernel is a tiny, tiny change. It cleans some things up (in theory), but in the end, it's really the same code. Look to managed OSes to see real innovation -- that's where we're going. –  Cody Brocious May 14 '09 at 2:40
Where can I see that if you dont mind? –  Recursion May 14 '09 at 2:40
See the post I made above. It's full of neat links from Cody. –  Chris Jester-Young May 14 '09 at 2:44
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Microsoft is in the process of rewriting some of Windows in .NET however I doubt that much of the kernel will be touched.

However there are projects like Cosmos ( http://www.gocosmos.org/index.en.aspx ) which give us hope.

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what kind of hope? –  Alphaneo May 14 '09 at 2:36
Hope of breaking out of the C/assembly hegemony, of course. :-) –  Chris Jester-Young May 14 '09 at 2:36
The first part of the first sentence above isn't true. There has been no re-write of parts of Windows in .NET –  Foredecker May 14 '09 at 2:37
Hope that we can move past old-style OSes where process separation is the norm (resulting in security risks and extreme performance penalties) and the security of the OS relies on millions of lines of code. –  Cody Brocious May 14 '09 at 2:38
@Cody, Are those security risks, due to the language. –  Alphaneo May 14 '09 at 3:01
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No, it is not "it". Kernels are generally written in C with a bit of assembler sprinkled in. But the OS is written in all sorts of language. But even there, C++ can be used without too much trouble. So can many other languages. Linux is written by C fanatics who fear and loathe everything else, which is their problem. Windows is written in a big mix of C and C++, and probably with a some bits of old Pascal code as well. And these days, chunks of .NET are turning up as well. OS X uses Objective-C for much of the OS code.

The same advice applies as in all other areas of programming:

  • Know your stuff
  • Don't limit yourself to the One True Language.

The kernel is the only area where somewhat "special" rules apply. But the kernel is tiny. The vast majority of the OS can be written in any language.

You'll certainly need to know C, yes, but just knowing C is nowhere near enough.

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You might want to have a look at the Singularity project from Microsoft (also on Wikipedia):


Only an extremely small part of this OS is actually written in C, and the rest is written in higher level languages (Sing#, an extension of C#). In the future I believe you can expect to see much more of this kind of thing becoming available.

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Are they intending to remove the C eventually? –  Matthew Flaschen May 14 '09 at 4:08
I don't know. Wikipedia claims that the existing C and assembly language code is responsible for the x86 interrupt handling, which makes sense. Since this is very low-level code, it's difficult to have a general purpose compiler generate the correct code, unless you build that specific capability directly into the compiler. Or, perhaps some other platforms (other than x86) offer an architecture such that assembly language code is not required for interrupt handling. In any case, interrupt handling is by far not the most interesting part of the kernel. :) –  Greg Hewgill May 14 '09 at 4:45
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I think its a pretty safe bet that serious (non experimental) OS development will remain in C (and assembly) for the forseeable future.

The proof I submit is Ada. It can get as bare-metal as C, provides better control over data placement, and has safer default behavior for just about everything (eg: array bounds checking). From the point of view of an OS developer, it is either equal or superior to C in any technical parameter you can think up. It has been available for over 20 years now (ok...reasonably-priced for perhaps only 15).

So if people were looking for a technically superior language to C, you should see OSes all over the place written in Ada instead right? What I actually see is one serious OS implemented in Ada. It is no longer supported in favor of a reimplementation in C.

The barriers to other languages in OS development are not and never have been technical. I don't see C's non-technical benifits going away any time soon, and nobody is ever going to overcome them by simply designing a better language.

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Most definitely! You should also learn at least one assembly language/hardware architecture.

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I agree that assembly knowledge is critical, but not for direct use. It's required to understand what your kernel is managing, but it doesn't have to be written directly. In managed OSes, the only part of the kernel that directly touches assembly (well, machine code) is the compiler (which guarantees memory safety, amongst other things), for instance. –  Cody Brocious May 14 '09 at 2:43
@Cody, you write boot-loader in managed code, is it? –  Alphaneo May 14 '09 at 2:46
You most certainly could, but there's simply no point -- if your bootloader is bigger than a few thousand lines of code, you've got a problem. The goal of the bootloader is to do the least number of things before passing control to the kernel, so doing it in anything but assembly is asinine -- this is not at /all/ indicative of the kernel at large. –  Cody Brocious May 14 '09 at 2:49
OS is written in C for portability. Assembler is still used for atomic operations, traps/syscalls, bootstrap, etc. - anything that requires platform-specific instructions. And compilers are NOT part of any OS I know. These are userland programs. They do not guarantee memory safety - kernel does. You do not have to know assembler to understand an OS, but it most certainly helps. –  Nikolai N Fetissov May 14 '09 at 3:13
@nickf3, Look at any managed OS on Chris's list and you'll see what I'm talking about. The compiler is a part of the OS by necessity, and rather than relying on antiquated process separation mechanisms, the compiler guarantees that the code is safe. –  Cody Brocious May 14 '09 at 3:21
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If it is kernel you are talking about, then you need to learn a language that will enable easy access to the underlying hardware, faster. I can only think of

  • C language and
  • Assembly

AFAIK, some parts of the boot-loader will be written in assembly, and from then on, C. There are many open-source easy-to-understand operating systems available, like for example the latest TOPPERS. Try to look into it.

I guess, as a OS-kernel developer, you will worry more about the ways to efficiently access underlying hardware (like processor and memory) rather than the choice of the language. I bet, most of the times, we will be tempted to use assembly

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Want to optimize a kernel? Reduce the cost of syscalls and task switching, optimize the algorithms used. Singularity has proven that a managed mode kernel can be /faster/ than an old-school kernel, because of the safety provided. –  Cody Brocious May 14 '09 at 2:47
@Cody, theoretically, managed code can NEVER be faster than an assembly code, or a well optimized C code. And if you say it is faster, then all I can say is vow –  Alphaneo May 14 '09 at 2:52
Managed code can never be faster? Really? Ok, let's try an experiment. Assemble some code for 8086, then move it onto an x64 system and run it. Is it using all 16 64-bit registers? You may want to look into dynamic code generation and/or recompilation before answering that. –  Cody Brocious May 14 '09 at 3:03
@Alphaneo, I don't understand if you're being sarcastic or not, but I'd really love for you to put forth a reason why it can't be done. There are lots of misconceptions around managed code at this level, and I'd love to help clear them up. –  Cody Brocious May 14 '09 at 3:18
@Alphaneo, Depending on the constraints you were working under, it's possible you couldn't. It's very easy to generate /damn/ fast code from CIL, but generating small code is very difficult. In terms of actual capability, the only thing I can think of that could cause problems for you is if you required precise timing, e.g. you have to throw pixels at the PPU every X cycles (like the NES). –  Cody Brocious May 14 '09 at 3:26
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I've done extensive programming in both the Windows NT and Linux Kernel. And I can assure you that as long as these 2 OS's are around C will be used in the Kernel. I think it's a multitude of reasons, but the easiest answer is time. Like previous posters mentioned the amount of time it would take to rewrite the Kernel in a different language is not worth it. And it wouldn't just be porting the code. The kernel would need some serious design modifications. Personally I think C is the most suitable language for a Kernel. Being able to manage your open memory and dynamically allocate and free your own memory is crucial when you are working in the kernel. Especially if you are working with paged memory. The stack size you are allotted in Kernel mode is also generally smaller than user mode so again memory efficiency is crucial. C also allows programmers to build beautiful data structures that don't contain all the bloated overhead that managed languages have. In my opinion a struct can also be used just as effectively as an Object, but again without all the bloated overhead. Managed languages also need to be "managed." In the Kernel you don't have anything cleaning up your messes. Don't get me wrong, I love C# and I think the .NET framework is beautiful, but if you are in the kernel C is and will continue to be it.

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C++ is supported for kernel mode development on Windows, but you can't use exceptions and RTTI easily. I believe that there is no reason to write code in C today, since the overhead of C++ is negligible (any tracing/debugging infrastructure will be far more costly than extra dereference for virtual function call). In fact most of the Windows DDK implement object oriented patterns with C, which is just inconvenient compared to C++.

If you decide to use C++ for kernel mode development, you will need to override the new operator to choose whether to allocate a class on pageable or non-pageable memory. Some nice macros might come handy there.

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You should definitely be fluent in C.

As others have pointed out, there is no reason that an operating system has to be written in C, and there is a lot to be gained by using more sophisticated languages. But if you're going to work on operating systems in the real world (i.e., not in academia or a research lab) there are a couple of realities that you have to live with:

  1. Existing operating systems are huge, often many millions of lines of code, and are written in C or C-derivatives, such as Objective-C or C++.
  2. New operating systems take hundreds of engineer-years (and many calendar years) to reach and match the functionality and robustness of existing operating systems.

As a result, it's hard for me to see how and when the world will move away from C-based operating system kernels. Yes, it's technically possible. But the cost may be too high. If anything, the trend seems to be toward consolidation on a small number of OS families---Windows, Linux, and BSD---all C-based.

It would be interesting to know what research has been done, or what tools and techniques might be available to evolve an existing code-base (such as Linux) to a better language. I think this would be a much more viable approach than getting the world to adopt a completely new OS.

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well, in the osdev community C is generally called the high-level language. And the more "low-level" language would be assembly (you are forced to use ASM in the start of your kernel, so you have to use ASM but you don't have to use C).

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