That line reminded me that NetBSD added Lua for writing kernel modules. (https://news.ycombinator.com/item?id=6562611)
Anybody have any experiences to share from this?
When it comes to Linux, one could say that most reasons to avoid it are historical, but this does not quite paint the awkward truth -- namely that, for most of the kernel's lifetime (since back in 1991), C++ compilers simply did not have the level of maturity and stability across the breadth of platforms that Linux required. Linus Torvalds' stance on this matter is pretty well-known: http://harmful.cat-v.org/software/c++/linus .
Today, when x86-64 and ARM are the only two families that you need to care about in the following ten years or so (maybe RISC-V but I rather doubt it), it probably makes sense to look at C++ for operating systems work, but the runtime is certainly heavier than back when Linus was writing about it, too. A modern C++ compiler has a lot of baggage; C++ was huge back in 1998, now it's bloody massive. IMHO, all the reasons why you would want to use C++ (templating support without resorting to strange hacks, useful pointer semantics and so on) are reasonably well-served by cleaner languages with less hefty runtimes, like Rust. What these alternatives do lack is the amazing level of commercial support that C++ has.
I don't think any "run-time" feature was added since, though. It's all either OS support (<thread>, etc, that you wouldn't use in-kernel anyways) or template stuff that has 0 impact on runtime (and actually sometimes helps decreasing code size).
https://istarc.wordpress.com/2014/07/18/stm32f4-oop-with-emb...
https://www.embedded.com/design/programming-languages-and-to...
https://hackaday.com/2015/12/18/code-craft-embedding-c-templ...
If some guys are able to run c++ on 8kb microcontrollers, there's hardly a non-political reason it couldn't be used in-kernel.
See also IncludeOS: http://www.includeos.org/
You can certainly strip things down to a subset that can fit 128K of flash and need only 1 or 2K of RAM at runtime, but the question is not only one of computational resources used for the library itself. Additional code always means additional bugs, the semantics sometimes "hide" memory copying or dynamic allocation in ways that many C++ programmers do not understand (and the ones who do are more expensive to hire than the ones who do not), and so on. You can certainly avoid these things and use C++, but you can also avoid them by using C.
I agree that mistrust and politics definitely play the dominating role in this affair though. I have seen good, solid, well-performing C++ code. I prefer C, but largely due to a vicious circle effect -- C is the more common choice, so I wrote more C code, so I know C better, so unless I have a good reason to recommend or write C++ instead of C, I will recommend or write C instead. I do think (possibly for the same reason) that it is harder to write correct C++ code than it is to write correct C code, but people have sent things to the Moon and back using assembly language for very weird machines, so clearly there are valid trade-offs that can be made and which include using language far quirkier than C++.
The real answer is historical, and cultural. On the latter, Unix is a product of C (well, and BCPL) and C is a product of Unix. They two are intertwined heavily. The former is as was mentioned a product of the relative crappiness of early C++ compilers (and the overzealous OO gung-ho nature of its early adopters perhaps as well...)
C++ without exceptions, RTTI, etc. has a lot to offer for OS development. Working within the right constraints it can definitely make a lot of tasks easier and cleaner.
It won't happen in Linux, tho.
I call BS on this.
So many mistakes in C simply cannot be made in C++ if you follow the well-established coding patterns and don't try to switch to C-style code. e.g.: You just cannot simply forget to free any resource, because you never have to do it with RAII. You cannot forget to check a status code and return early because you don't have to; the exception will propagate until someone catches it. You cannot forget to initialize a vector because it initializes itself. I could go on and on.
That said, there is a huge caveat to what I am saying above: I am comparing experienced programmers in each language with each other -- those who are basically experts and know what they're doing. I'm not debating whether it's easier to shoot yourself in the foot with C++ if you use it with insufficient experience (and part, though not all, of the reason is that you will probably write C-style code most of the time, and write neither proper C nor proper C++). I'm saying that an experienced programmer is much more likely to write correct code in C++ than C.
What additional code?
> You can certainly avoid these things and use C++, but you can also avoid them by using C.
Right, but what you can't get with C is destructors and move/ownership semantics.
> I do think (possibly for the same reason) that it is harder to write correct C++ code than it is to write correct C code
The ability to write typesafe data structures with move/ownership semantics and specified interfaces while being a superset of C would lead some to say that this is not true.
All the code that you need in order to support smart pointers, templates, move/copy semantics, exceptions and so on. To paraphrase someone whose opinions you should take far more seriously than mine, you can't just throw Stroustrup's "The C++ Programming Language" on top of an x86 chip and hope that the hardware learns about unique_ptr by osmosis :-). There used to be such a thing as the sad story about get_temporary_buffer ( https://plus.google.com/+KristianK%C3%B6hntopp/posts/bTQByU1... -- not affiliated in any way, just one of the first Google results).
The same goes for all the code that is needed to take C++ code and output machine language. In my whole career, I have run into bugs in a C compiler only three or four times, and one of them was in an early version of a GCC port. The last C++ codebase I was working on had at least a dozen workarounds, for at least half a dozen bugs in the compiler.
Huh? unique_ptr has a customizable Deleter though; you should be able to provide a custom one so it doesn't call delete.
And doesn't a kernel implement a kmalloc() or something anyway? You would just write your own operator new and have it do what your kernel needs, and the rest of the standard library would just work with it.
smart pointers are their own classes, and exceptions would certainly be disabled in kernel-mode, sure, but for the rest ? which additional code ? there's no magic behind templates and move semantics, and no run-time impact. It's purely a compile-time feature.
Unique_ptr is just a template and does not need the standard library. Also move/ownership semantics don't need unique_ptr.
> The last C++ codebase I was working on had at least a dozen workarounds, for at least half a dozen bugs in the compiler.
There are at least four compilers that been extensively production tested - ICC, GCC, MSVC and Clang. Which one had these bugs and was it kept up to date?
https://os.inf.tu-dresden.de/fiasco/prev/faq.html#0050
That was targeted at x86 with 2MB of RAM usage, though. So, I quickly looked for a microcontroller RTOS thinking that would be a good test. I found ScmRTOS that claims to be written in C++ with size being "from 1KB of code and 512 bytes of RAM" on up. So, it seems feasible to use C++ for small stuff. I'll also note that it has some adoption in high-assurance industry with standards such as MISRA C++ (2008) already available. They might be using powerful CPU's, though, so I looked up the MCU stuff.
http://scmrtos.sourceforge.net/ScmRTOS
The throwaways are talking safety features. I used to think that was a benefit of C++ over C. Following Worse is Better, that's no longer true: the ecosystem effects of C produced so many verification and validation tools that it's C language that's safer than C++ if one uses those tools. There's piles of them for C with hardly any in FOSS for C++ if we're talking about static/dynamic analysis, certified compilation, etc. I put $100 down that a CompCert-like compiler for C++ won't happen in 10 years. At best, you'll get something like KCC in K Framework.
The reason this happened is C++'s unnecessary complexity. The language design is garbage from the perspective of being easy to analyze or transform by machines. That's why the compilers took so long to get ready. LISP, Modula-3, and D showed it could've been much better in terms of ease-of-machine-analysis vs features it has with some careful thought. Right now, though, the tooling advantage of C means most risky constructs can be knocked out automatically, the code can be rigorously analyzed from about every angle one could think of (or not think of), it has best optimizing compilers for if one cares little about their issues, and otherwise supports several methods of producing verified object/machine code from source. There's also CompSci variants with built-in safety (eg SAFEcode, Softbound+CETS, Cyclone) and security (esp Cambridge CHERI). duneroadrunner's SaferCPlusPlus is about only thing like that I know of that's actively maintained and pushed for C++. The result of pro's applying tools on a budget to solve low-level problems in C or C++ will always give a win on defect rate to former just because they had more verification methods to use.
And don't forget that, like with Ivory language, we can always develop in a high-level, safer language such as Haskell with even more tooling benefits to extract to safety-critical subset of C. Extracted code that's then hit with its tooling if we want. We can do that in a language with REPL to get productivity benefits. So, we can have productivity, C as target language, and tons of automated verification. We can't have that with C++ or not as much if we had money for commercial tools.
So, these days, that's my argument against C++ for kernels, browsers, and so on. Just setting oneself up to have more bugs that are harder to find since one looses the verification ecosystem benefits of C++ alternatives. This will just continue since most research in verification tools is done for managed languages such as Java or C# with what's left mostly going to C language.
> The throwaways are talking safety features. I used to think that was a benefit of C++ over C. Following Worse is Better, that's no longer true: the ecosystem effects of C produced so many verification and validation tools that it's C language that's safer than C++ if one uses those tools. There's piles of them for C with hardly any in FOSS for C++ if we're talking about static/dynamic analysis, certified compilation, etc. I put $100 down that a CompCert-like compiler for C++ won't happen in 10 years. At best, you'll get something like KCC in K Framework.
Lots of people think additional safety features result in safer code. They likely do most of the time, but when you need to swear to investors, to the public and to the FDA that your machine will not kill anyone, what you want to have is results from 5 verification tools with excellent track records, not "my language does not allow for the kind of programming errors that C allows". Neither does Ada, and yet they crashed a rocket with it, with a bug related to type conversion in a language renowned for its typing system (not Ada's fault, of course, and especially not its typing system's fault -- just making a point about safety features vs. safety guarantees).
A more complex language, with more features, is inherently harder to verify. The tools lag behind more and are more expensive. And, upfront, it seems to me that it is much harder to reason about the code. C++ does not have just safety features, it has a lot of features, of all kind.
I'd disagree. Modern C++ has much better safety features than C ever has.
gstreamer, gtk, etc, are really easy to work with and browse the source.
This is why I love golang as well.
Side note, it is funny how much gstreamer and glib try to add c++ "ish" features to c.
Interestingly, I just found Linus' stance on Golang :) https://www.realworldtech.com/forum/?curpostid=104302&thread...
beware, performance and code size does not always go hand in hand!
I used to work on one. It was developed in the 90s, around the same time C++ became an ISO standard. I think most of you have used it without knowing. :)
I think OOP is natural for writing kernels and most mainstream kernels implement some kind of object system because of this. I also wrote my toy kernel in modern C++, despite that I'm "fluent" in both C and C++. So yeah, C++ kernels are here, just not in Linux land.
http://xr.anadoxin.org/source/xref/macos-10.13-highsierra/xn...
Wait what? What happened to MIPS, z/Architecture,Power Architecture, QDSP6, TMS320?
Back when Linus was ranting, one could make a convincing case for supporting all of these architectures in an OS meant for general-purpose workloads, from server to desktop and from thin client (eh? feel old yet :-)?) to VCR -- and . Now you can sell an OS that supports nothing but x86-64 and a few ARMs, and you get most of the server market, and virtually all of the desktop and mobile market.
Obviously the architectures that you need for a particular project are the ones that you want to "care for" -- but in broad terms, it is perfectly possible today for someone to have worked as a programmers for ten years without encountering a single machine that is not either x86 or ARM-based. Someone who had 10 years of programming experience in 2003 probably ran into a SPARC system whether they wanted or not.
S390X (IBM mainframes) and OpenPOWER variants are fully supported by Ubuntu and those are not insignificant market either in banking, finance, or numeric computing.
One reason why Linux runs all Top 500 supercomputers is because it they have number of relatively recent systems close to the top running Sunway (Alpha derivative) , Power and SPARC64. Fastest supercomputer today runs on Sunway.
There was an announcement for a 64bit mips android device for 2016 (Warrior i6400) but I didn't hear anything about it. Did you?
Also there was an ainol tablet with the mips architecture (nov07) which flopped (those are the only instances I can recall where mips on android really came into play).
After all, most of the “extra stuff” that C++ adds on to C has no inherent hardware dependence and exists only on the frontend. By the time the code makes its way to the backend, templates have been monomorphized (i.e. copied and pasted for each set of parameters); methods have been lowered to functions with added ‘this’ parameters; hidden calls to destructors and conversions have been made explicit; and so on. For any given C++ program, you could write a C program that compiles to near-identical IR, so if the backend can reliably compile C code, it should be able to handle C++.
True, that hypothetical C program might not look much like a typical human-written program. In order to achieve zero-cost abstractions, C++ code tends to rely a lot more on inlining and other backend optimizations than pure C, even though those optimizations apply to both languages. So if the backend isn’t reliable, C++ may generate more ‘weird’ IR that could end up getting miscompiled, and the level of indirection may make it harder to figure out what’s going on. But compiler backends these days are a lot more reliable than they used to be. And buggy backends can certainly cause problems for C code as well.
I despise his choice of words and general attitude towards programmers "less capable than him", but I do think there is some truth in what he says there: Imagine we'd all be writing asm only - we'd spend much more time on the drawing board and find much more elegant solutions to problems that we today solve with mindlessly writing hundreds lines of code within a "framework".
Side note: I wrote C++ in 1998; it was more "bloody massive" back then than it is today, in my humble experience.
> anybody who tells me that STL and especially Boost are stable and portable is just so full of BS that it's not even funny
> the only way to do good, efficient, and system-level and portable C++ ends up to limit yourself to all the things that are basically available in C.
You can certainly write C++ code without overabstracting it. Maybe Torvalds only ran into C++ programmers who liked overabstracting their code -- as it is usually the case with opinions of some people about other people, that part of the rant is best discarded :-).
But as my memory serves me, back when "portability" meant x86, SPARC, Alpha, ARM, MIPS, PPC, PA-RISC and m68k, the only way to do "good, efficient, and system-level and portable C++" was to limit yourself to the (relatively small) subset of C++ that was well-implemented by widely-available compilers (particularly GCC) across all these architectures -- not as a matter of discipline and code elegance, but because writing non-trivial C++ code back then usually resulted in very unpleasant trips down the standard library's source code and poring over objdump outputs trying to figure out what the hell broke this time.
Language-wise, Darwin/XNU/IOkit uses a subset of C++, notably one that excludes exceptions and templates IIRC. stdlib-wise I suspect it's even more restricted/different.
> any compiler or language that likes to hide things like memory
https://www.infoworld.com/article/3109150/linux/linux-at-25-...
Is Rust well-understood, both by the users and the toolchain developers? How many independent compilers are there for Rust? What architectures do they support?
I'm not implying that Rust isn't suitable for Linux, but it has open issues, that prevent usage right now. Rust and it's core ecosystem have a lot of churn, and still feel like a rather early work-in-progress. One example not mentioned is the Rust language server and its integration with consumers.
I cheer for Rust, and hope for solid progress.
To be fair these benchmarks are aiming for speed, but the c baseline typically produces faster code with less memory, so it may very well be a case of TANSTAAFL.
Just a couple of examples.
$ find sys -name '*.lua'
sys/modules/examples/luahello/luahello.lua
sys/modules/examples/luareadhappy/happy.lua
sys/modules/lua/infinite.lua
sys/modules/lua/test.lua
sys/modules/luasystm/test.luaNo library runtime features, so no RTTI or exceptions, but we use templates and dynamic dispatch.
I can see the argument from Linus' POV about avoiding over abstraction, and knowing exactly what your code is doing on a low-level, but at the same time it leads to a lot of reinventing the wheel and makes me think there is a better balance to be found.
I'm addressing this as someone who has done C++ work on very small embedded systems (e.g. microcontrollers with no external memory) including RTOS code and low-latency control loops, and there were definitely some nice features in C++ that can save you some time and lines of code without adding a bunch of needless overhead. Of course, there was a learning curve when bringing new people on board the project about what was/wasn't allowed for performance reasons. In general though, I see no reason why there couldn't be some subset of C++ used for kernel development other than Linus' / the kernel dev community's general distaste for it.
I did some kernel development (mainly some drivers) and I am also doing a lot of embedded development (credit card terminals, personal projects for ARM Cortex-M) I must say that I am happy with this choice.
The kernel is complex enough and you want to focus on understanding what really matters without being distracted by arcane constructs that would inevitably come with C++.
My experience with C++ guys is that there will inevitably be a population of smart developers which will try to be "smart" with the language which typically ends up with everybody else spending more time on understanding what is happening than the time that was actually saved by the construct.
C does not pose that problem. C is simple and there is relatively little occasion to be very smart with it, which is a good thing, IMHO.
This still leaves some holes that the runtime provides, but all of which are easy to provide -- namely things like 'new' and 'delete'.
And here I was about to ask if anyone has a similar article based on Rust :)