Re: [KJ] Any need for __cplusplus?

[Richard Knutsson <ricknu-0@student.ltu.se>]

Hi and thank you all for your responses

Robert P. J. Day wrote:
> On Tue, 3 Apr 2007, Jaco Kroon wrote:
>
>   
>> Richard Knutsson wrote:
>>     
>>> Hello
>>>
>>> There were recently a thread in LKML about C++-usage in the kernel
>>> (again). That reminded me, why do we have those "ifdef
>>> __cplusplus"? As an example, we have this (in
>>> include/linux/stddef.h) when defining NULL as 0 in C++ and ((void
>>> *)0) otherwise. From what I can see, there is no reason to have
>>> them, or am I missing something? (like they are needed by
>>> libraries)
>>>       
>> Many of these headers are exported to userspace, eg the
>> linux-headers package on many linux distributions.  If a user
>> includes them from a C++ program, they shouldn't break.
>>     
>
> the obvious question is this:  is the kernel officially C++-free?
> that is, is the *only* reason one would see that C++ check is if
> something is being exported to userspace?
>
> if that's the case, the obvious approach is to generate the sanitized
> headers with "make headers_install" and see what C++ references are
> left:
>
> $ make distclean
> $ make headers_install
> $ cd usr/include
> $ grep -r cplusplus *
> linux/stddef.h:#if defined(__cplusplus)
> linux/socket.h:#elif defined(__cplusplus)
> $
>
> and that's it.  if the kernel is truly C++-free, then there should be
> *no* C++ tests within the bounds of a "#ifdef __KERNEL__" test, isn't
> that right?  but it's not that simple.
>
> consider, for example, the header file drivers/net/sk98lin/h/skgehw.h:
>
> ...
> #ifndef __INC_SKGEHW_H
> #define __INC_SKGEHW_H
>
> #ifdef __cplusplus
> extern "C" {
> #endif  /* __cplusplus */
> ...
>
> note how that C++ test is not protected by a __KERNEL__ test but, on
> the other hand, it's not exported to userspace either.  so what's the
> point of the C++ test if userspace never sees that header file?   yes,
> it would seem that there are a lot of worthless C++ tests that could
> be removed if they never make it to userspace.
>   
Yep, and just found a few C/C++ structs not even _used_ in the kernel... 
Guess the __cplusplus may be worth a look or two since it seems to be 
possible indicator of old cruft.

Thanks for reminding about "headers_install".

Richard Knutsson

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