linux-kernel - Re: [PATCH 1/2] Correct function definition for C++

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Message-ID: <20170222142214.GA6830@kroah.com>
Date:   Wed, 22 Feb 2017 15:22:14 +0100
From:   "greg@...ah.com" <greg@...ah.com>
To:     Joakim Tjernlund <Joakim.Tjernlund@...inera.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: Re: [PATCH 1/2] Correct function definition for C++

On Wed, Feb 22, 2017 at 02:00:53PM +0000, Joakim Tjernlund wrote:
> On Wed, 2017-02-22 at 14:03 +0100, greg@...ah.com wrote:
> > On Wed, Feb 22, 2017 at 07:50:46AM +0000, Joakim Tjernlund wrote:
> > > On Wed, 2017-02-22 at 08:10 +0100, Greg KH wrote:
> > > > On Tue, Feb 21, 2017 at 04:24:04PM +0100, Joakim Tjernlund wrote:
> > > > > C++ does does not like the extra extern before asmlinkage, remove it.
> > > > > 
> > > > > Signed-off-by: Joakim Tjernlund <joakim.tjernlund@...inera.com>
> > > > > ---
> > > > >  include/linux/printk.h | 4 ++--
> > > > >  1 file changed, 2 insertions(+), 2 deletions(-)
> > > > > 
> > > > > diff --git a/include/linux/printk.h b/include/linux/printk.h
> > > > > index 3472cc6..be823f5 100644
> > > > > --- a/include/linux/printk.h
> > > > > +++ b/include/linux/printk.h
> > > > 
> > > > 
> > > > Why are you building this file with a C++ compiler?
> > > 
> > > virtualbox uses C++ and includes various kernel headers and the build
> > > fails, virtualbox guest additions has not build for quite some time now and
> > > this is one of the problems.
> > 
> > Virtualbox is a horrid pile of crap.  You can quote me on that.  We
> > don't care about out-of-tree drivers, the authors should work to get
> > them merged properly if they do care.
> 
> Sure, I believe you :)
>  
> But in this case it is not the kernel modules that fails, it is their
> guest additions/xf86 video driver. Seem like that these need to include some
> kernel herders there too.

Seems like their build system is broken, it should not be including non
uapi header files at all.

> > Kernel code should be C, not C++, and if you do want to use C++, then
> > you are on your own, sorry.
> 
> But there are already a lot of C++ adjustments!? Just grepping for __cplusplus shows
> plenty of hits and what is the point of asmlinkage then:
> #ifdef __cplusplus
> #define CPP_ASMLINKAGE extern "C"
> #else
> #define CPP_ASMLINKAGE
> #endif
> 
> #ifndef asmlinkage
> #define asmlinkage CPP_ASMLINKAGE
> #endif

The usages in include/uapi/ make a bit of sense, but not much.
Everything outside of there makes no sense at all (really just some drm
and one scsi file.)  They should be removed.

> I read all of this that C++ code should be able to at least include kernel headers
> without fatal errors. 

For uapi stuff, yes.  But that's not what you are doing here.  You are
trying to build kernel code with a c++ compiler.  That's not going to
work.

How is a C++ compiler supposed to parse the following line in device.h
	struct class;
(one of my most proud moments...)

thanks,

greg k-h