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SubjectRe: Linux kernel modules development in C++
** Reply to message from Horst von Brand <vonbrand@inf.utfsm.cl> on Wed, 27 Sep
2000 16:50:12 -0400


> I'd say it is important specifically in device drivers, and less so
> elsewhere ;-)

Yes, it's more important, but I've looked at the assembly code that my C++
compiler generates, and it's very clean. In fact, when you're writing code
that by design is OO, then using C++ tends to generate better code, not worse,
since the language more closely matches the design.

> A couple of points:
>
> - The kernel is C, mixing in C++ for no *real* good reason is just making
> it harder to work on.

True, but I'm not advocating doing it for no real reason. I'm advocating using
C++ for code that is OO by design. My OS/2 device drivers are a mix of C and
C++, wherever appropriate.

> - The work you do to match the kernel's object model to C++ is strictly
> wasted effort: The kernel's interfaces _do_ change, sometimes radically,
> and you'll have to keep up

But that applies to C code as well. In fact, the #2 gripe I hear about Linux
development is how the API's change so often and without any regard to existing
code that depends on it. (#1 gripe: the dearth of good development tools).

> - The idea of reusing code from other OSes with a very different internal
> layout will only make the point above even worse

Not always true. Some drivers, like complex PCI audio drivers, are mostly
OS-independent. They get some data from the OS, and then spend 90% of their
code just talking to the hardware.

> - History shows that no kludged-on C++ code will show up in the standard
> kernel, so you loose the main advantage Linux gives you: Hundereds of
> other people that fix bugs and port forward for you

True, if you want to write a driver that goes into the kernel, you need to
conform to Linus' whims^H^H^H^H^Hstandards. But considering how difficult it
is to get a driver into the kernel anyway, it often doesn't make a difference.
I don't ever expect any of my Linux drivers to make it into the kernel, and I
write them in C.

>
> And again:
>
> - Your driver won't be huge, most of the OO advantages won't show

Not true in my experience. My OO drivers have been comparble in size to their
C equivalents, and sometimes smaller because of the better code reuse that C++
promotes.

> - You don't have anywhere to inherit from sanely (sure, you can try to
> kludge C++ classes over VFS, or the interface to block drivers; but they
> won't be native C++ classes, so much of the benefit is lost, plus the
> effort invested in the wrappers is wasted)

A matter of opinion. If you have to maintain a dozen drivers of the same
class, then it may be worthwhile to write a small set of wrappers, and then do
everything in C++. Again, this assumes that the design is OO to begin with.



--
Timur Tabi - ttabi@interactivesi.com
Interactive Silicon - http://www.interactivesi.com

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