I wanted to get more details for writing Graphics device drivers and audio device drivers using c++ for Linux box.
I am newbie at developing device drivers , Please provide me development/documentation details for the same.
Thanks
-Pravin
Coming to this page late, the question itself has been answered by Chris Stratton, but it's important to correct a couple of things Chris Becke put here that are common misconceptions with people that are not familiar with C++:
C++ does not create implicit code or data, just what you request. Even for an average C++ programmer, there will be no extra code or data. I found it out through knowing the asm behind C++, but just read Scott Meyers books it's good enough.
Not only are exceptions optional in C++, their entire code can be excluded in linkage for mostly every tool out there. This is in fact done in RT apps.
This is to address the misconceptions posted here. To add more however:
1) A novice C++ programmer may do nonsense, but a novice C programmer trying to implement by himself polymorphism and inheritance as done time and time again in the kernel just without calling it as such, will do lots more inefficient undebuggable nonsense.
2) Saying that, the only thing that may be created in base C++ is a virtual pointer IF YOU NEED IT and specify "virtual", and then also C programmers usually just create such a pointer manipulate it by themselves add lookup tables and get much harder bugs down the line due to it. As always in C++, if you don't mention "virtual" then you don't even get this pointer. Inheritance and encapsulation are of course completely free of overhead.
3) C++ creates the same amount of asm and memory as C if you don't EXPLICITLY request special features, but there is a common case when C++ is more efficient - when passing function pointers. If you use C++'s functors you can inline the pointed function. This is EXTREMELY useful in embedded apps.
4) If embedded RT uses C++ why linux doesn't? Just because of myths, so please do read this message carefully, and refer to scott meyers or better yet the asm itself. I am 20 years in RT and had the same disbelief in C++ when I switch 14 years ago, but the facts do not confirm any such distrust.
TL;DR - it's very easy to write as efficient and in a common case more efficient code in C++, studies, much industry experience and books are abound on this subject.
Linux kernel device drivers are written in C rather than C++.
Most device drivers are accessed via a special device file (/dev/yourdevice0) on which control as well as read and write operations can be performed.
User mode client programs and user mode drivers open the device file and use it as a pathway to talk to the kernel mode driver. These user mode drivers could conceivably be written in C++ or any other language.
Generally the best way to get started is to have a device which needs a driver, and learn what you need to in order to write it. And often the best way to do that is to find an existing driver for either a related device, or one with similar interface paradigms, and start by modifying that until it works for your new device instead or as well.
As there is no C++ runtime in the kernel, you will run into problems quickly. I suppose you could make a C++ runtime to run inside the kernel, but it would require some pretty good skills. Much greater skills than writing the driver in C.
Also, you would be put down instantly by Linux kernel developers. I mean REALLY put down. They'd flame you so bad, you'd never recover from it. Chances are that you would say "Screw Linux and their elitist bastards".
I don't want to sound negative, but I'm a mild and suitable voice in comparison to what you'd hear from others.
Linux drivers are developed in C. If you want to learn more about Linux drivers development, you should read this free eBook: http://lwn.net/Kernel/LDD3/
A tarball of all pdf chapters is also available: http://lwn.net/images/pdf/LDD3/ldd3_pdf.tar.bz2
C, not C++ is the language for writing (kernel mode) device drivers, and the reason ultimately is simple: C++ is an inappropriate language to use to write driver software. As a side effect of this, there is no c++ runtime available in kernel mode.
As to why c++ is inappropriate: There are at least two reasons:
Device drivers on all OS require strict code placement - some code needs to be in non pageable blocks, and non pageable memory is a limited resource. c++ generates lots of implicit code, being implicit its impossible to (a) audit, and (b) bracket with the necessary directives to guarantee placement.
exceptions have become non optional in c++. c++ exceptions are typically implemented in terms of CPU exceptions and a lot of driver code on is executed at levels where (cpu) exceptions cannot be tolerated (hence the requirement for non pageable blocks of code).
I think there are some other aspects I am forgetting, but, ideomatic c++ violates a number of constraints placed on drivers. Which is why C is preferred.
This is an old post, but I decide to write an answer since there is no answer explaining about how to do it without getting backfired.
Short Answer:
The answer is "yes, you can" … with tremendous effort. Let's just ignore Linus Trovald's opinion about C++
Long Answer:
My suggestion is writing it in C++ when you REALLY need it.
There are many pitfalls in the way of running C++ in kernel.
I recently study this subject and here’s my summarize:
There is no implementation for new and delete
It is the easiest thing to overcome.
The stack size on kernel is less than 8 kb (for 32bit) and 16kb (for 64bit).
Opps...not getting stack overrun would be challenging.
Following are not allowed
Global non-trivial variables ( there is no C++ runtime initializes it for your, using singleton would be better )
STL ( One of super powers of C++, you need to port C++ stdlib to make it working in kernel )
RTTI
Exceptions
It’s PIA to let C++ read a kernel header. If you like challenges, please at least read C++ in the Linux kernel before you go. Don't forget it has to be done every time while upgrading old code to a newer kernel.
If you’d like to know more in-depth knowledge, check out following articles.
C++ in the Linux kernel (2016)
Porting C++ code to Linux kernel (2009)
C++ article in OSDev.org
Also korisk has a demo repo in github for a barebone kernel module.
Conclusion
Again, my sincere opinion, assess the effort for running C++ in kernel module before you go.
Third time, it's better to assess the effort for running C++ in kernel module before you go!
Unfortunately, the current Linux header files are not compilable in C++ as they use new as a variable name, declare false, true and have some other issues.
You can use C++ in a Linux module but it's useless without including Linux headers. So you can't go far away from the simplest hello-world module.
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From what I know, even though the common OS have parts written in other languages, the kernel is entirely written in C.
I want to know if it's feasible to write a Kernel in C++ and if not, what would be the drawbacks.
There are plenty of examples of well-used operating systems (or parts of them) implemented in C++ - IOKit - the device driver subsystem of MacOSX and IOS is implemented in EC++. Then there's the eCOS RTOS - where the kernel is implemented in C++, even making use of templates.
Operating systems are traditionally awash with examples of OO concepts implemented the hard way in C. In the linux device model kobject is effectively the base-class for driver and device objects, complete with DIY v-tables and some funky arrangements implemented in macros for up and down-casting.
The Windows NT kernel has an even more deeply rooted inheritance hierarchy of kernel objects. And for all of the neigh-sayers complaining about the suitability of exception handling in kernel code, exactly such a mechanism is provided.
Traditionally, the arguments against using C++ in kernel code have been:
Portability: availability of C++ compilers for all intended target platforms. This is not really an issue any more
Cost of C++ language mechanisms such as RTTI and exceptions. Clearly if they were to be used, the standard implementation isn't suitable and a kernel-specific variant needs using. This is generally the driver behind the use of EC++
Robustness of C++ APIs, and particularly the Fragile base-class problem
Undoubtedly, the use of exceptions and RAII paradigm would vastly improve kernel code quality - you only have to look at source code for BSD or linux to see the alternative - enormous amounts of error handling code implemented with gotos.
This is covered explicitly in the OSDev Wiki.
Basically, you either have to implement runtime support for certain things (like RTTI, exceptions), or refrain from using them (leaving only a subset of C++ to be used).
Other than that, C++ is the more complex language, so you need to have a bit more competent developers that won't screw it up. Linus Torvalds hating C++ being purely coincidental, of course.
To address Torvalds' concerns and others mentioned elsewhere here:
In hard-RT systems written in C++, STL/RTTI/exceptions are not used and that same principal can be applied to the much more lenient Linux kernel. Other concerns about "OOP memory model" or "polymorphism overhead" basically show programmers that never really checked what happens at the assembly level or the memory structure. C++ is as efficient, and due to optimized compilers many times more efficient than a C programmer writing lookup tables badly since he doesn't have virtual functions at hand.
In the hands of an average programmer C++ doesn't add any additional assembly code vs a C written piece of code. Having read the asm translation of most C++ constructs and mechanisms, I'd say that the compiler even has more room to optimize vs C and can create even leaner code at times. So as far as performance it's pretty easy to use C++ as efficiently as C, while still utilizing the power of OOP in C++.
So the answer is that it's not related to facts, and basically revolves around prejudice and not really knowing what code CPP creates. I personally enjoy C almost as much as C++ and I don't mind it, but there is no rational against layering an object oriented design above Linux, or in the Kernel itself, it would've done Linux a lot of good.
You can write an OS kernel in more or less any language you like.
There are a few reasons to prefer C, however.
It is a simple language! There's very little magic. You can reason about the machinecode the compiler will generate from your source code without too much difficulty.
It tends to be quite fast.
There's not much of a required runtime; there's minimal effort needed to port that to a new system.
There are lots of decent compilers available that target many many different CPU and system architectures.
By contrast, C++ is potentially a very complex language which involves an awful lot of magic being done to translate your increasingly high-level OOP code into machine code. It is harder to reason about the generated machine code, and when you need to start debugging your panicky kernel or flaky device driver the complexities of your OOP abstractions will start becoming extremely irritating... especially if you have to do it via user-unfriendly debug ports into the target system.
Incidentally, Linus is not the only OS developer to have strong opinions on systems programming languages; Theo de Raadt of OpenBSD has made a few choice quotes on the matter too.
The feasibility of writing a kernel in C++ can be easily established: it has already been done. EKA2 is the kernel of Symbian OS, which has been written in C++.
However, some restrictions to the usage of certain C++ features apply in the Symbian environment.
While there is something "honest" about (ANSI) C, there is also something "honest", in a different way, about C++.
C++'s syntactic support for abstracting objects is very worthwhile, no matter what the application space. The more tools available for misnomer mitigation, the better ... and classes are such a tool.
If some part of an existing C++ compiler does not play well with kernel-level realities, then whittle up a modified version of the compiler that does it the "right" way, and use that.
As far as programmer caliber and code quality, one can write either hideous or sublime code in either C or C++. I don't think it is right to discriminate against people who can actually code OOP well by disallowing it at the kernel level.
That said, and even as a seasoned programmer, I miss the old days of writing in assembler. I like 'em both ... C++ and ASM ... as long as I can use Emacs and source level debuggers (:-).
Revision after many years:
Looking back, I'd say the biggest problem is actually with the tons of high level features in C++, that are either hidden or outside the control of the programmer. The standard doesn't enforce any particular way of implementing things, even if most implementations follow common sanity, there are many good reasons to be 100% explicit and have full control over how things are implemented in a OS kernel.
This allows (as long as you know what you are doing) to reduce memory footprint, optimize data layout based on access patterns rather than OOP paradigms, thus improve cache-friendliness and performance, and avoid potential bugs that might come hidden in the tons of high level features of C++.
Note that even tho far more simple, even C is too unpredictable in some cases, which is one of the reasons there is also a lot of platform specific assembly in the kernel code.
Google's new-coming operating system Fuchsia is based on the kernel called Zircon, which is written mostly in C++, with some parts in assembly language[1] [2]. Plus, the rest of the OS is also written mostly in C++[3]. I think modern C++ gives programmers many reasons to use it as a general programming environment for huge codebases. It has lots of great features, and new features are added regularly. I think this is the main motive behind Google's decision. I think C++ could easily be the future of system programming.
One of the big benefits of C is it's readability. If you have a lot of code, which is more readable:
foo.do_something();
or:
my_class_do_something(&foo);
The C version is explicit about which type foo is every time foo is used. In C++ you have lots and lots of ambiguous "magic" going on behind the scenes. So readability is much worse if you are just looking at some small piece of code.
I've been writing a number of network daemons in different languages over the past years, and now I'm about to start a new project which requires a new custom implementation of a properitary network protocol.
The said protocol is pretty simple - some basic JSON formatted messages which are transmitted in some basic frame wrapping to have clients know that a message arrived completely and is ready to be parsed.
The daemon will need to handle a number of connections (about 200 at the same time) and do some management of them and pass messages along, like in a chat room.
In the past I've been using mostly C++ to write my daemons. Often with the Qt4 framework (the network parts, not the GUI parts!), because that's what I also used for the rest of the projects and it was simple to do and very portable. This usually worked just fine, and I didn't have much trouble.
Being a Linux administrator for a good while now, I noticed that most of the network daemons in the wild are written in plain C (of course some are written in other languages, too, but I get the feeling that > 80% of the daemons are written in plain C).
Now I wonder why that is.
Is this due to a pure historic UNIX background (like KISS) or for plain portability or reduction of bloat? What are the reasons to not use C++ or any "higher level" languages for things like daemons?
Thanks in advance!
Update 1:
For me using C++ usually is more convenient because of the fact that I have objects which have getter and setter methods and such. Plain C's "context" objects can be a real pain at some point - especially when you are used to object oriented programming.
Yes, I'm aware that C++ is a superset of C, and that C code is basically C++ you can compile any C code with a C++ compiler. But that's not the point. ;)
Update 2:
I'm aware that nowadays it might make more sense to use a high level (scripting) language like Python, node.js or similar. I did that in the past, and I know of the benefits of doing that (at least I hope I do ;) - but this question is just about C and C++.
I for one can't think of any technical reason to chose C over C++. Not one that I can't instantly think of a counterpoint for anyway.
Edit in reply to edit: I would seriously discourage you from considering, "...C code is basically C++." Although you can technically compile any C program with a C++ compiler (in as far as you don't use any feature in C that's newer than what C++ has adopted) I really try to discourage anyone from writing C like code in C++ or considering C++ as "C with objects."
In response to C being standard in Linux, only in as far as C developers keep saying it :p C++ is as much part of any standard in Linux as C is and there's a huge variety of C++ programs made on Linux. If you're writing a Linux driver, you need to be doing it in C. Beyond that...I know RMS likes to say you're more likely to find a C compiler than a C++ one but that hasn't actually been true for quite a long time now. You'll find both or neither on almost all installations.
In response to maintainability - I of course disagree.
Like I said, I can't think of one that can't instantly be refuted. Visa-versa too really.
The resistance to C++ for the development for daemon code stem from a few sources:
C++ has a reputation for being hard to avoid memory leaks. And memory leaks are a no no in any long running software. This is to a degree untrue - the problem is developers with a C background tend to use C idioms in C++, and that is very leaky. Using the available C++ features like vectors and smart pointers can produce leak free code.
As a converse, the smart pointer template classes, while they hide resource allocation and deallocation from the programmer, do a lot of it under the covers. In fact C++ generally has a lot of implicit allocation as a result of copy constructors and so on. As a result the C++ heap can become fragmented over time and daemon processes will eventually fail with an out of memory error even though there is sufficient RAM. This can be ameliorated by the use of modern heap managers that are more fragmenttation resistant, but they do this by consuming more resource up front.
while this doesn't apply to usermode daemon code, kernel mode developers avoid C++, again because of the implicit code C++ generates, and the exceptions C++ libraries use to handle errors. Most c++ compilers implement c++ exceptions in terms of hardware exceptions, and lots of kernel mode code is executed in environments where exceptions are not allowed to be thrown. Also, all the implicit code generated by c++, being implicit, cannot be wrapped in #pragma directives to guarantee its placement in pageable, or non pageable memory.
As a result, C++ is not possible for kernel development on any platform at all, and generally shunned by daemon developers too. Even if one's code is written using the proper smart memory management classes and does not leak - keeping on top of potential memory fragmentation issues makes languages where memory allocation is explicit a preferred choice.
I would recommend whichever you feel more comfortable with. If you are more comfortable with C++, your code is going to be cleaner, and run more efficiently, as you'll be more used to it, if you know what I mean.
The same applies on a larger scale to something like a Python vs Perl discussion. Whichever you are more comfortable with will probably produce better code, because you'll have experience.
I think the reason is that ANSI C is the standard programming language in Linux. It is important to follow this standard whenever people want to share their code with others etc. But it is not a requirement if you just want to write something for yourself.
You personally can use C or C++ and the result will be identical. I think you should choose C++ if you know it well and can exploit some special object oriented features of it in your code. Don't look too much to other people here, if you are good in C++ just go and write your daemon in C++. I would personally write it in C++ as well.
You're right. The reason for not using C++ is KISS, particularly if you ever intend for someone else to maintain your code down the road. Most folks that I know of learned to write daemons from existing source or reading books by Stevens. Pretty much that means your examples will be in C. C++ is just fine, I've written daemons in it myself, but I think if you expect it to be maintained and you don't know who the maintainer might be down the road it shows better foresight to write in C.
Boost makes it incredibly easy to write single threaded, or multi-threaded and highly scalable, networking daemons with the asio library.
I would recommend using C++, with a reservation on using exception handling and dynamic RTTI. These features may have run time performance cost implications and may not be supported well across platforms.
C++ is more modular and maintainable so if you can avoid these features go ahead and use it for your project.
Both C and C++ are perfectly suited for the task of writing daemons.
Besides that, nowadays, you should consider also scripting languages as Perl or Python. Performance is usually just good enough and you will be able to write applications more robust and in less time.
BTW, take a look at ACE, a framework for writting portable network applications in C++.
I have been using C++ for a while now and I began to get interested in lower level system programming like drivers and stuff. Even some kind of primitive operating system could be very interesting project!
I have no clue where I could start. Are there any not-too-challenging things I could get started with and are there anything about C++ I should try to avoid like exceptions in performance critical code?
My current OS is Windows 7 if that matters much.
Writing Windows device drivers in C++ isn't impossible, there are not many CRT functions that you could use to get you into trouble. The new operator is unusable for example, you don't have to fear a std::bad_alloc. Unless you replace it, that cuts out a rather large swath of standard C++ library classes.
But that's not really the point of a device driver, it is rather important that you make it as small as possible. C++ pays off when you write complex code. You explicitly do not want to write complex code in a device driver. Debugging it is redrum.
Linus really likes C in the kernel. There's a good reason for that.
C++ doesn't provide quite all of the tools you will need to actually implement a full operating system in it. There are a few machine specific things that cannot be done in c++. These things are handling and raising interrupts, controlling the MMU, controlling access to supervisor cpu instructions, and a handful of other small odds and ends.
Fortunately, these things are few enough that they can be written in assembly language accessed from C++.
Have a look at osdev.org (lots of questions that will pop into your mind when considering developing your own OS are answered here).
I would strongly suggest you start by hacking existing open source device-drivers and kernels, which you can really only do in Linux or *BSD. The experience will also give you a good sense of whether you're cut out for this kind of programming.
I have heard the recently open sourced Symbian OS is written using C and C++. Not sure which parts of it are done with C++ as I have not read the code base. Consider looking into it.
Kerneltrap.org has some very good discussions about why the Linux kernel does not have C++ in its code base. Consider reading that as well.
Symbian OS is written in a variant of C++. Of course, there's assembly code for low-level things, but that is all wrapped up. You cannot use exceptions, and for real-time drivers you cannot do normal things like dynamic memory allocation, not even in C.
I recommend C Programming Language and assembler. I'm not sure if it's possible to low-level much with C++.
I am just curious why drivers and firmwares almost always are written in C or Assembly, and not C++?
I have heard that there is a technical reason for this.
Does anyone know this?
Lots of love,
Louise
Because, most of the time, the operating system (or a "run-time library") provides the stdlib functionality required by C++.
In C and ASM you can create bare executables, which contain no external dependencies.
However, since windows does support the C++ stdlib, most Windows drivers are written in (a limited subset of) C++.
Also when firmware is written ASM it is usually because either (A) the platform it is executing on does not have a C++ compiler or (B) there are extreme speed or size constraints.
Note that (B) hasn't generally been an issue since the early 2000's.
Code in the kernel runs in a very different environment than in user space. There is no process separation, so errors are a lot harder to recover from; exceptions are pretty much out of the question. There are different memory allocators, so it can be harder to get new and delete to work properly in a kernel context. There is less of the standard library available, making it a lot harder to use a language like C++ effectively.
Windows allows the use of a very limited subset of C++ in kernel drivers; essentially, those things which could be trivially translated to C, such as variable declarations in places besides the beginning of blocks. They recommend against use of new and delete, and do not have support for RTTI or most of the C++ standard library.
Mac OS X use I/O Kit, which is a framework based on a limited subset of C++, though as far as I can tell more complete than that allowed on Windows. It is essentially C++ without exceptions and RTTI.
Most Unix-like operating systems (Linux, the BSDs) are written in C, and I think that no one has ever really seen the benefit of adding C++ support to the kernel, given that C++ in the kernel is generally so limited.
1) "Because it's always been that way" - this actually explains more than you think - given that the APIs on pretty much all current systems were originally written to a C or ASM based model, and given that a lot of prior code exists in C and ASM, it's often easier to 'go with the flow' than to figure out how to take advantage of C++.
2) Environment - To use all of C++'s features, you need quite a runtime environment, some of which is just a pain to provide to a driver. It's easier to do if you limit your feature set, but among other things, memory management can get very interesting in C++, if you don't have much of a heap. Exceptions are also very interesting to consider in this environment, as is RTTI.
3) "I can't see what it does". It is possible for any reasonably skilled programmer to look at a line of C and have a good idea of what happens at a machine code level to implement that line. Obviously optimization changes that somewhat, but for the most part, you can tell what's going on. In C++, given operator overloading, constructors, destructors, exception, etc, it gets really hard to have any idea of what's going to happen on a given line of code. When writing device drivers, this can be deadly, because you often MUST know whether you are going to interact with the memory manager, or if the line of code affects (or depends on) interrupt levels or masking.
It is entirely possible to write device drivers under Windows using C++ - I've done it myself. The caveat is that you have to be careful about which C++ features you use, and where you use them from.
Except for wider tool support and hardware portability, I don't think there's a compelling reason to limit yourself to C anymore. I often see complicated hand-coded stuff done in C that can be more naturally done in C++:
The grouping into "modules" of functions (non-general purpose) that work only on the same data structure (often called "object") -> Use C++ classes.
Use of a "handle" pointer so that module functions can work with "instances" of data structures -> Use C++ classes.
File scope static functions that are not part of a module's API -> C++ private member functions, anonymous namespaces, or "detail" namespaces.
Use of function-like macros -> C++ templates and inline/constexpr functions
Different runtime behavior depending on a type ID with either hand-made vtable ("descriptor") or dispatched with a switch statement -> C++ polymorphism
Error-prone pointer arithmetic for marshalling/demarshalling data from/to a communications port, or use of non-portable structures -> C++ stream concept (not necessarily std::iostream)
Prefixing the hell out of everything to avoid name clashes: C++ namespaces
Macros as compile-time constants -> C++11 constexpr constants
Forgetting to close resources before handles go out of scope -> C++ RAII
None of the C++ features described above cost more than the hand-written C implementations. I'm probably missing some more. I think the inertia of C in this area has more to do with C being mostly used.
Of course, you may not be able to use STL liberally (or at all) in a constrained environment, but that doesn't mean you can't use C++ as a "better C".
The comments I run into as why a shop is using C for an embedded system versus C++ are:
C++ produces code bloat
C++ exceptions take up too much
room.
C++ polymorphism and virtual tables
use too much memory or execution
time.
The people in the shop don't know
the C++ language.
The only valid reason may be the last. I've seen C language programs that incorporate OOP, function objects and virtual functions. It gets very ugly very fast and bloats the code.
Exception handling in C, when implemented correctly, takes up a lot of room. I would say about the same as C++. The benefit to C++ exceptions: they are in the language and programmers don't have to redesign the wheel.
The reason I prefer C++ to C in embedded systems is that C++ is a stronger typed language. More issues can be found in compile time which reduces development time. Also, C++ is an easier language to implement Object Oriented concepts than C.
Most of the reasons against C++ are around design concepts rather than the actual language.
The biggest reason C is used instead of say extremely guarded Java is that it is very easy to keep sight of what memory is used for a given operation. C is very addressing oriented. Of key concern in writing kernel code is avoiding referencing memory that might cause a page fault at an inconvenient moment.
C++ can be used but only if the run-time is specially adapted to reference only internal tables in fixed memory (not pageable) when the run-time machinery is invoked implicitly eg using a vtable when calling virtual functions. This special adaptation does not come "out of the box" most of the time.
Integrating C with a platform is much easier to do as it is easy to strip C of its standard library and keep control of memory accesses utterly explicit. So what with it also being a well-known language it is often the choice of kernel tools designers.
Edit: Removed reference to new and delete calls (this was wrong/misleading); replaced with more general "run-time machinery" phrase.
The reason that C, not C++ is used is NOT:
Because C++ is slower
Or because the c-runtime is already present.
It IS because C++ uses exceptions.
Most implementations of C++ language exceptions are unusable in driver code because drivers are invoked when the OS is responding to hardware interrupts. During a hardware interrupt, driver code is NOT allowed to use exceptions as that would/could cause recursive interrupts. Also, the stack space available to code while in the context of an interrupt is typically very small (and non growable as a consequence of the no exceptions rule).
You can of course use new(std::nothrow), but because exceptions in c++ are now ubiqutious, that means you cannot rely on any library code to use std::nothrow semantics.
It IS also because C++ gave up a few features of C :-
In drivers, code placement is important. Device drivers need to be able to respond to interrupts. Interrupt code MUST be placed in code segments that are "non paged", or permanently mapped into memory, as, if the code was in paged memory, it might be paged out when called upon, which will cause an exception, which is banned.
In C compilers that are used for driver development, there are #pragma directives that can control which type of memory functions end up on.
As non paged pool is a very limited resource, you do NOT want to mark your entire driver as non paged: C++ however generates a lot of implicit code. Default constructors for example. There is no way to bracket C++ implicitly generated code to control its placement, and because conversion operators are automatically called there is no way for code audits to guarantee that there are no side effects calling out to paged code.
So, to summarise :- The reason C, not C++ is used for driver development, is because drivers written in C++ would either consume unreasonable amounts of non-paged memory, or crash the OS kernel.
C is very close to a machine independent assembly language. Most OS-type programming is down at the "bare metal" level. With C, the code you read is the actual code. C++ can hide things that C cannot.
This is just my opinion, but I've spent a lot of time in my life debugging device drivers and OS related things. Often by looking at assembly language. Keep it simple at the low level and let the application level get fancy.
Windows drivers are written in C++.
Linux drivers are written in c because the kernel is written in c.
Probably because c is still often faster, smaller when compiled, and more consistent in compilation between different OS versions, and with fewer dependencies. Also, as c++ is really built on c, the question is do you need what it provides?
There is probably something to the fact that people that write drivers and firmware are usually used to working at the OS level (or lower) which is in c, and therefore are used to using c for this type of problem.
The reason that drivers and firmwares are mostly written in C or ASM is, there is no dependency on the actual runtime libraries. If you were to imagine this imaginary driver written in C here
#include <stdio.h>
#define OS_VER 5.10
#define DRIVER_VER "1.2.3"
int drivermain(driverstructinfo **dsi){
if ((*dsi)->version > OS_VER){
(*dsi)->InitDriver();
printf("FooBar Driver Loaded\n");
printf("Version: %s", DRIVER_VER);
(*dsi)->Dispatch = fooDispatch;
}else{
(*dsi)->Exit(0);
}
}
void fooDispatch(driverstructinfo *dsi){
printf("Dispatched %d\n", dsi->GetDispatchId());
}
Notice that the runtime library support would have to be pulled in and linked in during compile/link, it would not work as the runtime environment (that is when the operating system is during a load/initialize phase) is not fully set up and hence there would be no clue on how to printf, and would probably sound the death knell of the operating system (a kernel panic for Linux, a Blue Screen for Windows) as there is no reference on how to execute the function.
Put it another way, with a driver, that driver code has privilege to execute code along with the kernel code which would be sharing the same space, ring0 is the ultimate code execution privilege (all instructions allowed), ring3 is where the front end of the operating system runs in (limited execution privilege), in other words, a ring3 code cannot have a instruction that is reserved for ring0, the kernel will kill the code by trapping it as if to say 'Hey, you have no privilege to tread up ring0's domain'.
The other reason why it is written in assembler, is mainly for code size and raw native speed, this could be the case of say, a serial port driver, where input/output is 'critical' to the function in relation to timing, latency, buffering.
Most device drivers (in the case of Windows), would have a special compiler toolchain (WinDDK) which can use C code but has no linkage to the normal standard C's runtime libraries.
There is one toolkit that can enable you to build a driver within Visual Studio, VisualDDK. By all means, building a driver is not for the faint of heart, you will get stress induced activity by staring at blue screens, kernel panics and wonder why, debugging drivers and so on.
The debugging side is harder, ring0 code are not easily accessible by ring3 code as the doors to it are shut, it is through the kernel trap door (for want of a better word) and if asked politely, the door still stays shut while the kernel delegates the task to a handler residing on ring0, execute it, whatever results are returned, are passed back out to ring3 code and the door still stays shut firmly. That is the analogy concept of how userland code can execute privileged code on ring0.
Furthermore, this privileged code, can easily trample over the kernel's memory space and corrupt something hence the kernel panic/bluescreens...
Hope this helps.
Perhaps because a driver doesn't require object oriented features, while the fact that C still has somewhat more mature compilers would make a difference.
There are many style of programming such as procedural, functional, object oriented etc. Object oriented programming is more suited for modeling real world.
I would use object-oriented for device drivers if it suites it. But, most of the time when you programming device drivers, you would not need the advantages provided by c++ such as, abstraction, polymorphism, code reuse etc.
Well, IOKit drivers for MacOSX are written in C++ subset (no exceptions, templates, multiple inheritance). And there is even a possibility to write linux kernel modules in haskell.)
Otherwise, C, being a portable assembly language, perfectly catches the von Neumann architecture and computation model, allowing for direct control over all it's peculiarities and drawbacks (such as the "von Neumann bottleneck"). C does exactly what it was designed for and catches it's target abstraction model completely and flawlessly (well except for implicit assumption in single control flow which could have been generalized to cover the reality of hardware threads) and this is why i think it is a beautiful language.) Restricting the expressive power of the language to such basics eliminates most of the unpredictable transformation details when different computational models are being applied to this de-facto standard. In other words, C makes you stick to basics and allows pretty much direct control over what you are doing, for example when modeling behavior commonality with virtual functions you control exactly how the function pointer tables get stored and used when comparing to C++'s implicit vtbl allocation and management. This is in fact helpful when considering caches.
Having said that, object-based paradigm is very useful for representing physical objects and their dependencies. Adding inheritance we get object-oriented paradigm which in turn is very useful to represent physical objects' structure and behavior hierarchy. Nothing stops anyone from using it and expressing it in C again allowing full control over exactly how your objects will be created, stored, destroyed and copied. In fact that is the approach taken in linux device model. They got "objects" to represent devices, object implementation hierarchy to model power management dependancies and hacked-up inheritance functionality to represent device families, all done in C.
because from system level, drivers need to control every bits of every bytes of the memory, other higher language cannot do that, or cannot do that natively, only C/Asm achieve~
Today I got into a very interesting conversation with a coworker, of which one subject got me thinking and googling this evening. Using C++ (as opposed to C) in an embedded environment. Looking around, there seems to be some good trades for and against the features C++ provides, but others Meyers clearly support it. So, I was wondering who would be able to shed some light on this topic and what the general consensus of the community was.
C++ for embedded platforms is perfectly fine - as long as you treat it as a better C. I love the fact that the language is slightly more structured. You can still do all the things that you want to do with C. Just remember to stick to an embedded C library like Newlib or uClibc.
I particularly like the abstraction that we can build using C++, particularly for I/O devices. So, we can have a class for UART and a class for GPIO and what nots. It is cleaner than having a bunch of functions (IMHO).
The fear of C++ among embedded developers is largely a thing of the past, when C++ compilers were not as good as C compilers (optimizations and code quality wise).
This applies especially to modern platforms with 32 bit architectures.
But, C is certainly still the preferred choice for more confined environments (as is assembler for 8 bit or 4 bit targets).
So, it really boils down to the resources your target platform provides, and how much of these resources you are likely to actually require, i.e. if you can afford the 'luxury' of doing embedded development in C++ (or even Java for that matter), because you know that you'll hardly have any issues regarding memory or CPU constraints.
Nowadays, many modern embedded platforms (think gaming consoles, mobile phones, PDAs etc), have really become very capable targets, with RISC architectures, several MB of RAM, and 3D hardware acceleration.
It would be a poor decision, to program such platforms using just C or even assembler out of uninformed performance considerations, on the other hand programming a 16 bit PIC in C++ would probably also be a controversial decision.
So, it's really a matter of asking yourself how much of the power, you'll actually need and how much you can afford to sacrifice, in order to improve the development experience (high level language, faster development, less tedious/redundant tasks).
It sort of depends on the particular nature of your embedded system and which features of C++ you use. The language itself doesn't necessarily generate bulkier code than C.
For example, if memory is your tightest constraint, you can just use C++ like "C with classes" -- that is, only using direct member functions, disabling RTTI, and not having any virtual functions or templates. That will fit in pretty much the same space as the equivalent C code, since you've no type information, vtables, or redundant functions to clutter things up.
I've found that templates are the biggest thing to avoid when memory is really tight, since you get one copy of each template function for each type it's specialized on, and that can rapidly bloat code segment.
In the console video games industry (which is sort of the beefy end of the embedded world) C++ is king. Our constraints are hard limits on memory (512mb on current generation) and realtime performance. Generally virtual functions and templates are used, but not exceptions, since they bloat the stack and are too perf-costly. In fact, one major manufacturer's compiler doesn't even support exceptions at all.
In my previous company all embedded code was written in a small subset of C code due to security (SIL-2) and memory reasons. By introducing a richer language like C++ in that particular scenario would have maybe cause more trouble than benefits.
In all due respect to C++ (which is a language I really love) but I think C - in our particular scenario - was the better choice.
I bet in some cases C++ is just fine to use for embedded applications but it really depends on the application - there is a difference if your program is controlling a nuclear plant or administrating an address book on your cell phone.
I don't know about "general consensus", only the company I work for (which does a lot of development for mobile phones, car navigation systems, DPFs, etc.).
The main drawback I've encountered to using C++ on embedded platforms as opposed to C is that it isn't quite as portable - there are many more cases of compilers that don't adhere to the standard which can cause problems if you need to build your code with more than 1 compiler or outright have bugs in the implementation. Then there are environments where C++ code simply won't run - BREW's issues with relocatable code and its "native OOP" don't play so well with "regular" C++ classes and inheritance.
In the end, though, if you're only targeting 1 platform, I'd say use whatever you think is "better" (faster, less bugs, better design) for your development - in most cases the issues can be worked around quite easily.
Depends what kind of embedded development you are doing. I've done embedded development with both C++, C, and Assembly on various platforms, you can even use Java to write applications on smart phones.
For instance on a smart phone like device that's running Windows CE 5, almost all of the code is C++, including in the operating system. Only small bits are written in C or assembly.
On the other hand I've written code for an MSP430 microcontroller, which was in C, and I probably would have done that in C++ had the compiler been more reliable and standards compliant.
Also I seem to recall a university lecturer of mine talking about writing embedded code in Forth or something. So really any language can do.
Now a days it will all boil down to the C++ runtime support of the platform. You're likely to find a way to compile C++ code down to almost any embedded platform with GCC, but if you can't find a suitable C++ runtime for the platform your efforts will be futile, unless you write your own C++ runtime.
One of the few things I tend to agree with Linus is his opinion about C++ http://thread.gmane.org/gmane.comp.version-control.git/57643/focus=57918
Besides this, if you really really want to use C++ you might want to have a look at http://www.caravan.net/ec2plus/ which describes Embedded C++, or better to say you should not use in C++ for embedded systems.
The big thing keeping us with using C++ for a long time was the VxWorks support for it, which truly sucked. That supposidly has gotten better on VxWorks 6 (yes, it's been out a while... good 'ole vendor lock-in and lack of company vision has kept us stuck on VxWorks 5.5).
So for us it's mostly a question of the environment. After that, C++ can obviously be just as good as C... it's a matter of people understanding what their tool does and how to use it. C++ may make it easier to write incredibly inefficient code, but that doesn't mean we have to succomb to it.
I am currently fighting a problem with exceptions in an embedded Linux application. We are trying to port software written for a different platform that seemed to support exceptions well, but the new compiler tools (a port of gcc) reports errors when creating the eh_frame. I was against using exceptions for this tool, but the developer reassured me that modern compilers would support it well.
My opinion is that there are some advantages to C++, but I would stay away from exceptions and the standard template library. We haven't had problems using virtual functions.
C++ is suitable for microcontrollers and devices without an OS. You just have to know the architecture of the system and be conscious of time and space constrains, especially when doing mission critical programming.
With C++ you can do abstraction which often leads to an increased footprint in the code. You do not want this when programming for a resource-limited machine such as an 8-bit MCU.
Generally, avoid:
Dynamic memory allocation because it represents uncertainty in timing
Overloading
RTTI because the memory cost is large
Exceptions because of the execution speed lowering
Be cautious with virtual functions as they have a resource cost of a vtable per class and one pointer to the vtable per object. Also, use const in place of #define.
As you move up to 16 and 32-bit MCUs, with 10s or 100s of MB RAM, heavier features like the ones mentioned above may be used.
So to round up, C++ is useful for embedded systems. A main benefit is that OOP can be useful when you want to abstract aspects of the microcontroller, for example UART or state machines. But you may want to avoid certain features all of the time and some of the features some of the time, depending on the target you are programming for.