Query about cplusplus in linkage.h

[Baruch Siach]

Hi rocky,

On Mon, Nov 22, 2010 at 10:35:12PM +0800, rocky wrote:
> thanks for you kindly replay;
> I am still lost in this maze, so does extern "C" help user space C++ program to call schedule/printk?
> sys_xxx functions are called through SWI ,not directly called from user space C++ program;

It seems I was wrong. Userspace programs never call these functions directly.  
See http://kernelnewbies.org/FAQ/asmlinkage for a better answer.

baruch

> At 2010-11-22 16:30:10，"Baruch Siach" <baruch at tkos.co.il> wrote:
> 
> >Hi rocky,
> >
> >On Sat, Nov 20, 2010 at 12:17:02PM +0800, rocky wrote:
> >> I am a little confused about the following definition :
> >> 
> >> --------------include/linux/linkage.h-----------
> >> #ifdef __cplusplus
> >> #define CPP_ASMLINKAGE extern "C"
> >> #else
> >> #define CPP_ASMLINKAGE
> >> #endif
> >> 
> >> #ifndef asmlinkage
> >> #define asmlinkage CPP_ASMLINKAGE
> >> #endif
> >> 
> >> all system call functions are marked with asmlinkage, as well as other kernel function like printk/schedule/do_softirq
> >> 
> >> 
> >> so every asmlinkage function has extern "C"  attribute if cplusplus is defined, which definitely happens  when compiling C++ code.
> >> 
> >> I dont get it when cplusplus is needed when compiling kernel?
> >
> >It isn't.
> >
> >> Can anyone help me out of this little misunderstanding?
> >
> >Userspace C++ code may include kernel header files. Making the exported 
> >function calls extern "C" makes it easier to build such applications.
> >
> >baruch

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