[KJ] Any need for __cplusplus?

[KJ] Any need for __cplusplus?

[Richard Knutsson]

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)

> grep -nr cplusplus * | wc -l
121

Richard Knutsson

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Re: [KJ] Any need for __cplusplus?

[milindchoudhary]

Richard Knutsson wrote:
>
> 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)
>
> > grep -nr cplusplus * | wc -l
> 121
>
most of them are in
./scripts/genksyms/keywords.c_shipped
./scripts/genksyms/lex.c_shipped
./scripts/genksyms/parse.c_shipped
./scripts/kconfig/expr.h
./scripts/kconfig/lex.zconf.c_shipped
./scripts/kconfig/lkc.h
./scripts/kconfig/zconf.hash.c_shipped
./scripts/kconfig/zconf.tab.c_shipped
./Documentation/DocBook/kernel-hacking.tmpl
this shouldn't be a surprise

./include/linux/linkage.h
./include/linux/socket.h
./include/linux/stddef.h
./include/linux/string.h
if included from userspace

donno
./arch/ppc/mm/mem_pieces.h
./drivers/media/video/pwc/pwc.h


./drivers/net/sk98lin/h/lm80.h
./drivers/net/sk98lin/h/skaddr.h
./drivers/net/sk98lin/h/skgehw.h
./drivers/net/sk98lin/h/skgeinit.h
./drivers/net/sk98lin/h/skrlmt.h
./drivers/net/sk98lin/h/xmac_ii.h
./drivers/net/sk98lin/skaddr.c
./drivers/net/sk98lin/skdim.c
./drivers/net/sk98lin/skrlmt.c
i read some where that drivers are sometimes compile tested in
userspace

drivers/md/raid6.h:114:/* Some definitions to allow code to be
compiled for testing in userspace */
dosen't seem to be the case here but still.....

CMIIW

Milind A Choudhary

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Re: [KJ] Any need for __cplusplus?

[Jaco Kroon]

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.

Jaco
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Re: [KJ] Any need for __cplusplus?

[Robert P. J. Day]

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.

thoughts?

rday

-- 
====================================
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Linux Consulting, Training and Annoying Kernel Pedantry
Waterloo, Ontario, CANADA

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Re: [KJ] Any need for __cplusplus?

[Richard Knutsson]

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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Re: [KJ] Any need for __cplusplus?

[Robert P. J. Day]

On Wed, 4 Apr 2007, Richard Knutsson wrote:

... snip ...
> 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".

so is there anyone on this list who can make a confident proclamation
about this C++ stuff in the kernel source one way or the other?

rday
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====================================
Robert P. J. Day
Linux Consulting, Training and Annoying Kernel Pedantry
Waterloo, Ontario, CANADA

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Re: [KJ] Any need for __cplusplus?

[Robert P. J. Day]

On Wed, 4 Apr 2007, Richard Knutsson wrote:

> Yep, and just found a few C/C++ structs not even _used_ in the kernel...

i'm not sure what that means ... can you explain that?  with an
example or two?

rday
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====================================
Robert P. J. Day
Linux Consulting, Training and Annoying Kernel Pedantry
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Re: [KJ] Any need for __cplusplus?

[Howell, David P]

Every time I heard it come up in the kernel mailing list it was
emphatically
"No". Only other use I could see would be for source that applies for
the kernel and possibly some other application use where C++ can be
used.

-----Original Message-----
From: kernel-janitors-bounces@lists.linux-foundation.org
[mailto:kernel-janitors-bounces@lists.linux-foundation.org] On Behalf Of
Robert P. J. Day
Sent: Tuesday, April 03, 2007 7:14 PM
To: Richard Knutsson
Cc: kernel-janitors@lists.osdl.org
Subject: Re: [KJ] Any need for __cplusplus?

On Wed, 4 Apr 2007, Richard Knutsson wrote:

... snip ...
> 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".

so is there anyone on this list who can make a confident proclamation
about this C++ stuff in the kernel source one way or the other?

rday
-- 
====================================
Robert P. J. Day
Linux Consulting, Training and Annoying Kernel Pedantry
Waterloo, Ontario, CANADA

http://fsdev.net/wiki/index.php?title=Main_Page
====================================
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Re: [KJ] Any need for __cplusplus?

[Richard Knutsson]

Robert P. J. Day wrote:
> On Wed, 4 Apr 2007, Richard Knutsson wrote:
>
>   
>> Yep, and just found a few C/C++ structs not even _used_ in the kernel...
>>     
>
> i'm not sure what that means ... can you explain that?  with an
> example or two?
>   
The ones I was talking about is in drivers/scsi/dpt/sys_info.h. Only 
found sysInfo_S to be used (by drivers/scsi/dpt_i2o.c).

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Re: [KJ] Any need for __cplusplus?

[Randy Dunlap]

On Wed, 04 Apr 2007 03:41:56 +0200 Richard Knutsson wrote:

> Robert P. J. Day wrote:
> > On Wed, 4 Apr 2007, Richard Knutsson wrote:
> >
> >   
> >> Yep, and just found a few C/C++ structs not even _used_ in the kernel...
> >>     
> >
> > i'm not sure what that means ... can you explain that?  with an
> > example or two?
> >   
> The ones I was talking about is in drivers/scsi/dpt/sys_info.h. Only 
> found sysInfo_S to be used (by drivers/scsi/dpt_i2o.c).

Interesting.  Those structs don't seem to be used.

Vendor drivers historically have carried excess baggage in Linux,
often for "management application" purposes.
I doubt that that driver would be merged in that same condition today.

---
~Randy
*** Remember to use Documentation/SubmitChecklist when testing your code ***
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Re: [KJ] Any need for __cplusplus?

[Robert P. J. Day]

On Tue, 3 Apr 2007, Randy Dunlap wrote:

> On Wed, 04 Apr 2007 03:41:56 +0200 Richard Knutsson wrote:
>
> > Robert P. J. Day wrote:
> > > On Wed, 4 Apr 2007, Richard Knutsson wrote:
> > >
> > >
> > >> Yep, and just found a few C/C++ structs not even _used_ in the kernel...
> > >>
> > >
> > > i'm not sure what that means ... can you explain that?  with an
> > > example or two?
> > >
> > The ones I was talking about is in drivers/scsi/dpt/sys_info.h. Only
> > found sysInfo_S to be used (by drivers/scsi/dpt_i2o.c).
>
> Interesting.  Those structs don't seem to be used.
>
> Vendor drivers historically have carried excess baggage in Linux,
> often for "management application" purposes. I doubt that that
> driver would be merged in that same condition today.

under the circumstances, then, i would think that it's fair to rip out
anything related to C++ that is not *currently* and *explicitly*
exported to userspace for a good reason.

and i would pay no mind to the inevitable whiners who like to say
things like, "well, it's for future use." if it eventually becomes
useful, they can always put it back in sometime down the road.

rday

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====================================
Robert P. J. Day
Linux Consulting, Training and Annoying Kernel Pedantry
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