[Buildroot] [PATCH v2, 1/1] package/kismet: fix build with gcc >= 9

[Thomas Petazzoni]

Hello Fabrice,

Do you have some further research/feedback on the below questions ?

Thanks!

Thomas

On Wed, 26 Aug 2020 23:35:30 +0200
Thomas Petazzoni <thomas.petazzoni at bootlin.com> wrote:

> On Mon, 24 Aug 2020 20:21:21 +0200
> Fabrice Fontaine <fontaine.fabrice at gmail.com> wrote:
> 
> > Adding -std=c++11 fix the build and can be removed as soon as kismet is
> > bumped to latest version as the offending code has been removed. It
> > should be noted that kismet builds fine with gcc 8 so it remains unclear
> > why this build failure is raised only with gcc >= 9 (build failure
> > confirmed with gcc 9 and 10)  
> 
> Meh, this looks weird :/
> 
> bits/socket.h defines the field like this:
> 
> #if __glibc_c99_flexarr_available
>     __extension__ unsigned char __cmsg_data __flexarr; /* Ancillary data.  */
> #endif
> 
> and __glibc_c99_flexarr_available is defined as follows:
> 
> /* Support for flexible arrays.
>    Headers that should use flexible arrays only if they're "real"
>    (e.g. only if they won't affect sizeof()) should test
>    #if __glibc_c99_flexarr_available.  */
> #if defined __STDC_VERSION__ && __STDC_VERSION__ >= 199901L
> # define __flexarr      []
> # define __glibc_c99_flexarr_available 1
> #elif __GNUC_PREREQ (2,97)
> /* GCC 2.97 supports C99 flexible array members as an extension,
>    even when in C89 mode or compiling C++ (any version).  */
> # define __flexarr      []
> # define __glibc_c99_flexarr_available 1
> #elif defined __GNUC__
> /* Pre-2.97 GCC did not support C99 flexible arrays but did have
>    an equivalent extension with slightly different notation.  */
> # define __flexarr      [0]
> # define __glibc_c99_flexarr_available 1
> #else
> /* Some other non-C99 compiler.  Approximate with [1].  */
> # define __flexarr      [1]
> # define __glibc_c99_flexarr_available 0
> #endif
> 
> Some it's really unclear how -std=c++11 can fix the issue.
> 
> Could you have a look at which of those #if case we fall into in the
> different cases (between gcc 8.x and gcc 9.x, with/without -std=c++11) ?
> 
> Thanks!
> 
> Thomas



-- 
Thomas Petazzoni, CTO, Bootlin
Embedded Linux and Kernel engineering
https://bootlin.com