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

["Morten Brørup" <mb@smartsharesystems.com>]

> From: Ferruh Yigit [mailto:ferruh.yigit@amd.com]
> Sent: Monday, 12 February 2024 16.43
> 
> On 2/5/2024 9:07 PM, Morten Brørup wrote:
> >> From: Tyler Retzlaff [mailto:roretzla@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@6wind.com
> >>>>> Cc: stable@dpdk.org
> >>>>>
> >>>>> Signed-off-by: Ashish Sadanandan <ashish.sadanandan@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.