Why are drivers and firmwares almost always written in C or ASM and not C++?

Question

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.

Answer 1

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.

Answer 2

The comments I run into as why a shop is using C for an embedded system versus C++ are:

1. C++ produces code bloat
2. C++ exceptions take up too much room.
3. C++ polymorphism and virtual tables use too much memory or execution time.
4. 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.

Answer 3

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.

Answer 4

Windows drivers are written in C++.
Linux drivers are written in c because the kernel is written in c.

Answer 5

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.

Answer 6

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.