Is it possible to use C++ in a Kernel Extension?

In the "Kernel Extension Programming Topics" document there are two examples of kernel extensions - HelloKernel and Hello I/O Kit.

The Hello IO Kit is created as an I/O Kit Driver project and HelloKernel as a kext project.

Hello I/0 Kit has C++ classes in it whereas HelloKernel is just C.

Is it possible to use C++ in a kext project? If so what header files or bundles must be included to make it possible?

My aim is to develop an NKE (which I presume has to be created as a kext project) and I would rather implement it it using classes than using just C.

iMac, Mac OS X (10.4.11)

Posted on Jan 28, 2010 9:01 AM

3 replies

etresoft

Level 9

57,218 points

Jan 28, 2010 11:02 AM in response to hotcheese

Short answer - no.

I tried it once. I could get it to build, link, and run, but got lots of mysterious kernel panics. In the end, my program idea wasn't all that good anyway.

You can use a limited form of C++ in the I/O Kit. The only parts of C++ you can't use are the good ones.

hotcheese Author

Level 1

0 points

Jan 28, 2010 11:33 AM in response to etresoft

Thanks.
Was it pre-Snow Leopard?

I've just seen this following text from the Kernel release notes for Snow Leopard, would this make a difference? ( I notice it mentions I/O however, why is C++ allowed in I/O kexts but not in others?)

*Mixed C/C++ Kexts*
Mac OS X Snow Leopard features support for kexts that use both C and C++ code. On prior
releases of Mac OS X, making use of Libkern C++ classes in a generic (C-based) kext was a
risky proposition, since the I/O Kit auto-unloading mechanism destroyed any kext
referencing such classes if there were no instances of them. Apart from that, even if non-
Libkern C++ was used, the stub routine invoked at unload called the kext’s static C++
destructors before the kext’s stop routine, and when the kext was auto-unloaded the stop
routine wasn’t invoked at all.
Snow Leopard corrects these issues by:
Marking only kexts that define subclasses of IOService for auto-unload. Non-I/O Kit
kexts loaded on Snow Leopard can safely use and subclass Libkern C++ classes.
Altering the stub unload routine to call the kext’s stop routine first, then the kext’s
static C++ destructors. Important: To gain this behavior you must build your kext to
be targeted against Snow Leopard, as the stub routine is built into the kext.
Always unloading a kext by calling both the stop routine and then the C++ static
destructors, regardless of kext type (generic or I/O Kit).

etresoft

Level 9

57,218 points

Jan 28, 2010 11:51 AM in response to hotcheese

hotcheese wrote:
Was it pre-Snow Leopard?

Yes

I've just seen this following text from the Kernel release notes for Snow Leopard, would this make a difference? ( I notice it mentions I/O however, why is C++ allowed in I/O kexts but not in others?)

*Mixed C/C++ Kexts*
Mac OS X Snow Leopard features support for kexts that use both C and C++ code. On prior
releases of Mac OS X, making use of Libkern C++ classes in a generic (C-based) kext was a
risky proposition, since the I/O Kit auto-unloading mechanism destroyed any kext
referencing such classes if there were no instances of them. Apart from that, even if non-
Libkern C++ was used, the stub routine invoked at unload called the kext’s static C++
destructors before the kext’s stop routine, and when the kext was auto-unloaded the stop
routine wasn’t invoked at all.
Snow Leopard corrects these issues by:
Marking only kexts that define subclasses of IOService for auto-unload. Non-I/O Kit
kexts loaded on Snow Leopard can safely use and subclass Libkern C++ classes.
Altering the stub unload routine to call the kext’s stop routine first, then the kext’s
static C++ destructors. Important: To gain this behavior you must build your kext to
be targeted against Snow Leopard, as the stub routine is built into the kext.
Always unloading a kext by calling both the stop routine and then the C++ static
destructors, regardless of kext type (generic or I/O Kit).

That is very interesting. I suggest you post your question on Apple's Darwin kernel mailing list. I have seen postings there that both echo what I told you and claim that you can make C++ work fine. It is highly likely that I just need more experience writing kernel code (and a better project idea). I would like to know the answer to this question myself. It doesn't seem as cut and dried anymore.

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Is it possible to use C++ in a Kernel Extension?