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

[Joakim Tjernlund <Joakim.Tjernlund@infinera.com>]

On Wed, 2017-02-22 at 15:22 +0100, greg@kroah.com wrote:
> On Wed, Feb 22, 2017 at 02:00:53PM +0000, Joakim Tjernlund wrote:
> > On Wed, 2017-02-22 at 14:03 +0100, greg@kroah.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@infinera.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.

Right, I looked into guest-addition and it builds a few kernel modules too
and include linux/string.h and that pulls in the offending code.

Anyhow, the code in printk.h is "wrong" to add an extra extern so that
change should not be a big deal as is, C++ or not.

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

 :)