Date: Sat, 2 May 2009 17:52:33 +0100 From: Andrew Brampton <brampton+freebsd-hackers@gmail.com> To: freebsd-hackers@freebsd.org Subject: Re: Definition of NULL Message-ID: <d41814900905020952t4304f640sea4f07ce055a4f82@mail.gmail.com> In-Reply-To: <20090502163535.GA17027@owl.midgard.homeip.net> References: <d41814900905020859q4faff431p8819aaf38dfe9e78@mail.gmail.com> <20090502163535.GA17027@owl.midgard.homeip.net>
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2009/5/2 Erik Trulsson <ertr1013@student.uu.se>:
> On Sat, May 02, 2009 at 04:59:03PM +0100, Andrew Brampton wrote:
>> I'm writing a C++ Kernel Module, and one thing that has been bugging
>> me is the kernel's definition of NULL.
>
> Is the use of C++ inside the kernel really supported? =C2=A0I don't think=
so,
> but I could be wrong.
There are a few projects (other than mine) which uses C++ inside the
kernel, the biggest being the Click modular router. So, with minimal
effort and no kernel patches it is easy to build a C++ kernel module,
thus I must assume it is supported even if it is unofficial. The
question of if C++ is sensible inside the kernel is left for another
day, and perhaps has been answered in numerous other freebsd-hackers@
threads.
>> >From what I've read online the definition of NULL in C is (void *)0,
>> whereas in C++ it should be 0, or 0L (on 64bit machines).
>
> Not quite. =C2=A0Any of those (as well as a whole bunch more) are legal
> definitions of NULL in C. =C2=A0NULL is defined (in the C standard) to be=
a null
> pointer constant. =C2=A0A null pointer constant is defined as a constant =
integer
> expression with value zero, or such an expression cast to (void*). (In C+=
+
> it the cast to (void*) is not allowed.)
> This means that it would be perfectly legal (but of dubious utility) to h=
ave
> NULL defined as (5*5L+('1'-'0')-26) for example.
>
> The decision to define NULL as 0 or 0L or ((void*)0) is pretty much just =
a
> question of which buggy programs one wishes to break, or hide the bugs in=
.
> A correct C program should work regardless of which of those is used.
Thanks for the additional information.
>
>
>>
>> Now, my C++ kernel module is built with _KERNEL definited, like any
>> other C kernel module. This leads to NULL being defined incorrectly.
>>
>> So I have a question and two suggestions. Firstly, why is the #if
>> defined(_KERNEL) in _null.h? Is it to stop userland application
>> applications picking up this definition? Or for another reason?
>
> Perhaps to stop people from mistakenly using C++ inside the kernel?
The definition doesn't stop C++, it just generates additional warnings
and sometimes errors. I have avoided those warning by defining NULL
myself, however, I thought changing it in _null.h might help others,
even if you think they are mistakenly using C++.
>
> ...
>
> Erik Trulsson
> ertr1013@student.uu.se
>
Thanks for your input.
Andrew
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