[PATCH 1/1] eal: add C++ include guard in generic/rte_vect.h

[Ashish Sadanandan]

On Wed, Mar 13, 2024 at 2:45 PM Thomas Monjalon <thomas at monjalon.net> wrote:

> 13/03/2024 21:26, Ashish Sadanandan:
> > On Mon, Feb 12, 2024 at 9:02 AM Morten Brørup <mb at smartsharesystems.com>
> > wrote:
> >
> > > > From: Ferruh Yigit [mailto:ferruh.yigit at amd.com]
> > > > Sent: Monday, 12 February 2024 16.43
> > > >
> > > > On 2/5/2024 9:07 PM, Morten Brørup wrote:
> > > > >> From: Tyler Retzlaff [mailto:roretzla at linux.microsoft.com]
> > > > >> Sent: Monday, 5 February 2024 18.37
> > > > >>
> > > > >> On Fri, Feb 02, 2024 at 09:40:59AM +0000, Bruce Richardson wrote:
> > > > >>> On Fri, Feb 02, 2024 at 10:18:23AM +0100, Thomas Monjalon wrote:
> > > > >>>> 02/02/2024 06:13, Ashish Sadanandan:
> > > > >>>>> The header was missing the extern "C" directive which causes
> name
> > > > >>>>> mangling of functions by C++ compilers, leading to linker
> errors
> > > > >>>>> complaining of undefined references to these functions.
> > > > >>>>>
> > > > >>>>> Fixes: 86c743cf9140 ("eal: define generic vector types")
> > > > >>>>> Cc: nelio.laranjeiro at 6wind.com
> > > > >>>>> Cc: stable at dpdk.org
> > > > >>>>>
> > > > >>>>> Signed-off-by: Ashish Sadanandan <ashish.sadanandan at gmail.com>
> > > > >>>>
> > > > >>>> Thank you for improving C++ compatibility.
> > > > >>>>
> > > > >>>> I'm not sure what is best to fix it.
> > > > >>>> You are adding extern "C" in a file which is not directly
> included
> > > > >>>> by the user app. The same was done for rte_rwlock.h.
> > > > >>>> The other way is to make sure this include is in an extern "C"
> > > > >> block
> > > > >>>> in lib/eal/*/include/rte_vect.h (instead of being before the
> > > > >> block).
> > > > >>>>
> > > > >>>> I would like we use the same approach for all files.
> > > > >>>> Opinions?
> > > > >>>>
> > > > >>> I think just having the extern "C" guard in all files is the
> safest
> > > > >> choice,
> > > > >>> because it's immediately obvious in each and every file that it
> is
> > > > >> correct.
> > > > >>> Taking the other option, to check any indirect include file you
> > > > need
> > > > >> to go
> > > > >>> finding what other files include it and check there that a) they
> > > > have
> > > > >>> include guards and b) the include for the indirect header is
> > > > >> contained
> > > > >>> within it.
> > > > >>>
> > > > >>> Adopting the policy of putting the guard in each and every header
> > > > is
> > > > >> also a
> > > > >>> lot easier to do basic automated sanity checks on. If the file
> ends
> > > > >> in .h,
> > > > >>> we just use grep to quickly verify it's not missing the guards.
> > > > >> [Naturally,
> > > > >>> we can do more complete checks than that if we want, but 99%
> > > > percent
> > > > >> of
> > > > >>> misses can be picked up by a grep for the 'extern "C"' bit]
> > > > >>
> > > > >> so first, i agree with what you say here. but one downside i've
> seen
> > > > >> is that non-public symbols may end up as extern "C".
> > > > >>
> > > > >> i've also been unsatisfied with the inconsistency of either having
> > > > >> includes in or outside of the guards.
> > > > >>
> > > > >> a lot of dpdk headers follow this pattern.
> > > > >>
> > > > >> // foo.h
> > > > >> #ifdef __cplusplus
> > > > >> extern "C" {
> > > > >> #endif
> > > > >>
> > > > >> #include <stdio.h>
> > > > >>
> > > > >> ...
> > > > >>
> > > > >> but some dpdk headers follow this pattern.
> > > > >>
> > > > >> // foo.h
> > > > >> #include <stdio.h>
> > > > >>
> > > > >> #ifdef __cplusplus
> > > > >> extern "C"
> > > > >> #endif
> > > > >>
> > > > >> ...
> > > > >>
> > > > >> standard C headers include the guards so don't need to be inside
> the
> > > > >> extern "C" block. one minor annoyance with always including inside
> > > > the
> > > > >> block is we can't reliably provide a offer a C++-only header
> without
> > > > >> doing extern "C++".
> > > > >
> > > > > I would say that the first of the two above patterns is not only
> > > > annoying, it is incorrect.
> > > > > A DPDK header file should not change the meaning of any other
> header
> > > > files it includes.
> > > > > And although the incorrectness currently only screws up any C++ in
> > > > those header files, I still consider it a bug.
> > > > >
> > > >
> > > > Should we document the proper extern "C" usage somewhere?
> > >
> > > Good point!
> > >
> > > It could be added to § 1.4.2. Header File Guards in the Coding Style
> > > chapter of the Contributor's Guidelines.
> > >
> > > BTW, that paragraph (and its example) should be updated to reflect that
> > > alphabetical order is preferred.
> > >
> >
> > Was the intent of this comment that I should include this update in my
> > patch? I'm happy to do it, but IMO the guideline update should be a
> > separate commit.
> >
> > It's been a month since the last activity on this thread, does someone
> need
> > to sign off on this change before it can be merged?
>
> Instead of doing one specific change, it would be better to change all
> files for consistency.
> So the guideline change can be in the same commit applying the new
> guideline.
>
>
>  Fair enough, I'll take a crack at it tonight.

- Ashish
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