Re: [PATCH bpf] selftests/bpf: Filter out _GNU_SOURCE when compiling test_cpp

[Stanislav Fomichev <sdf@fomichev.me>]

On 07/26, Jakub Kicinski wrote:
> On Fri, 26 Jul 2024 17:45:06 -0700 Andrii Nakryiko wrote:
> > or we could
> > 
> > #ifndef _GNU_SOURCE
> > #define _GNU_SOURCE
> > #endif
> > 
> > (though we have 61 places with that...) so as to not have to update
> > every target in Makefile.
> 
> AFAIU we have -D_GNU_SOURCE= twice _in the command line args_ :(
> One is from the Makefile which now always adds it to CFLAGS,
> the other is "built-in" in g++ for some weird reason.
> 
> FWIW I have added this patch to the netdev "hack queue" so no
> preference any more where the patch lands :)

Yeah, it can't be fixed with an ifdef because the conflict happens a bit
earlier:

$ echo "int main(int argc, char *argv[]){return 0;}" > test.cpp
$ clang++ -Wall -Werror -D_GNU_SOURCE= test.cpp
In file included from <built-in>:454:
<command line>:1:9: error: '_GNU_SOURCE' macro redefined [-Werror,-Wmacro-redefined]
    1 | #define _GNU_SOURCE
      |         ^
<built-in>:445:9: note: previous definition is here
  445 | #define _GNU_SOURCE 1
      |         ^
1 error generated.