[Buildroot] using BR2 variables to find a specific header file

[Samuel Martin]

On Tue, Oct 14, 2014 at 12:58 PM, Sylvain Raybaud
<sylvain.raybaud at green-communications.fr> wrote:
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> Mike, Samuel, all,
>
> On 13/10/2014 21:42, Samuel Martin wrote:
>> Hi Sylvain,
>>
>> On Mon, Oct 13, 2014 at 7:10 PM, Sylvain Raybaud
>> <sylvain.raybaud at green-communications.fr> wrote:
> [...]
>>> During cross-compile these tests cannot be run therefore the
>>> package expects -DSTACK_DIRECTION=+/-1 to be passed to cmake. Of
>>> course you cannot do it in buildroot.
>>
>> Why do you say that? Of course, Buildroot does support adding cmake
>> configure options [1]. You can also check some cmake-based packages
>> already integrated (e.g.: [2]).
>
> I mean I can pass arguments to cmake but if I hardcode a value I break
> compatibility with any platform that would require the other value.
> However...

What I mean is something like this:
in the *.mk:
---
ifeq ($(BR2_arm)$(BR2_some_arch_with_stack_growing_downward),y)
FOO_CONF_OPTS += -DSTACK_DIRECTION=-1
endif
ifeq ($(BR2_some_other_arch_with_stack_growing_upward),y)
FOO_CONF_OPTS += -DSTACK_DIRECTION=+1
endif
---

This way you'll support all architectures (at least the one you know
how the stack grows).

>
>>
>> If you know beforehand how grow the stack, I would suggest to set
>> it in the *_CONF_OPTS variables. This may depends on the target
>> architecture.
>>
>>> It seems to be possible to access this information in another
>>> way since STACK_GROWS_DOWNWARD is defined in some headers.
> [...]
>>
>> This does not look right :-/ because: - it search in something not
>> in the sysroot; - it is only valid when using the
>> buildroot-toolchain backend.
>>
>> Indeed, it seems the STACK_GROWS_DOWNWARD definition is only
>> available in the compiler sources, not in the preprocessor output
>> for the target :-/
>>
>>> [...]
>>
>> Instead of checking the compiler sources, I prefer you set the
>> stack growing direction through the CMake args.
>>
>> Regards,
>
> Mark's reply and yours strongly suggest that it's a better idea to
> hardcode the correct value. I shall do so!
>
> Thanks a lot!
>
> Cheers,
>
> - --
> Sylvain Raybaud
> www.green-communications.fr
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-- 
Samuel