libbsd: How to add an option for switching off IPv6 (or other BSD Kernel configurations)

[Gedare Bloom]

On Mon, Aug 1, 2016 at 10:04 AM, Christian Mauderer
<christian.mauderer at embedded-brains.de> wrote:
> Hello,
>
> we have a special use case where we want to use the new network stack
> but without the IPv6 support. I could need some advice how this could be
> integrated into the waf build system. If we find a good solution, this
> could also be an example for other BSD Kernel options.
>
>
> I found multiple places where IPv6 is enabled in libbsd:
>
> If I remove the INET6 option out of the FreeBSD kernel configuration on
> a FreeBSD system, this has the effect, that the "opt_inet6.h" is
> generated as an empty file instead of with a single define "#define
> INET6 1". In the rtems-libbsd the configuration header is located at
> rtemsbsd/include/rtems/bsd/local/opt_inet6.h. It has a fixed content
> with the define set. This would be the first place where the support
> would have to be disabled.
>
> Beneath that, some user space tools have a special option to enable IPv6
> which is currently set in libbsd.py. One such example is tcpdump with a
> compiler flag '-DINET6' set. As far as I could tell, the kernel and
> every application that is included in libbsd currently uses the INET6
> macro. But I think this is more of a convention than a general rule.
>
>
> So I think we would roughly need the following:
>
> If i configure libbsd using
>
>    waf configure --disable-ipv6
>
> the build system would either have to regenerate opt_inet6.h or use an
> alternative version of it. Further I would need the ability to set
> compiler flags in libbsd.py depending on this configure option.
>
>
> Are there any better ideas how to implement such an option?
>
> I'm quite inexperienced on how to use waf, so I would need some guidance
> how this could be implemented. Are there any hints how I could start
> implementing such an option? Some examples or similar code?
>
The "Waf Book" is handy for learning how to hack on waf. Sounds like
you should add your option in the top-level wscript options(), then
parse it in libbsd_waf to determine whether or not to use ipv6. waf
does not modify the source files, and these includes appear to remain
in the source, so it might be easier to generate an empty header file
and always provide the define through the command line instead, by
adding e.g. -DINET6 to cflags by default and removing it in case the
option is used.

> Kind regards
>
> Christian Mauderer
> --
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