iio.h: Include iio/types.h when it exists

[gwendalcr]

enum iio_chan_type and iio_modifier are already defined in
/usr/include/linux/iio/types.h on linux systems.
If an application needs to include both (to support iio events for
instance), compiler will complains these enums are defined twice.

This patch is not without problem:

• it works only if compiler supports __has_include (C++17, not 11).
• linux system must have latest kernel-headers installed:
  IIO_MOD_O2 modifier has been added to linux 5.9 kernel.

Alter CI/travis/check_kernel.sh accordingly, check it still passes.

Fixes #758.

Signed-off-by: Gwendal Grignou [email protected]

[nunojsa]

Just one minor comment from side... Alternatively to this we could just think about syncing the current iio uapi within libiio on every stable release . Some other projects (eg: libiouring) apparently do the same... However I'm not sure how practical that is given some potential customs things we have. @pcercuei might have a better idea for this issue

[nunojsa]

Hmm, according to this, the right way of doing this is to first test for the existence of the macro... I'm not sure this will actually build when the compiler does not support __has_include

[rgetz]

don't we just want to do a:

# ensure that kernel headers are included before libiio headers
#ifndef iio_chan_type

enum ....

#endif

this would resolve things, but only if you included kernel headers before iio.h

[pcercuei]

@rgetz it's not a macro, though.

@gwendalcr please wrap the __has_include macro as described in here: https://gcc.gnu.org/onlinedocs/cpp/_005f_005fhas_005finclude.html

[gwendalcr]

I really don't like this patch, it is an ugly band aid:

1. wrapping __has_include will help pass some tests, but an application including both kernel include and iio.h will not be portable anymore between C++ 11 and 17.
2. sync'ing uapi does not work for [embedded] devices running an older kernel. We are not able to use the latest libiio when cros compiling for an older kernel.

In another project, to fill iio_chan_type_name_spec[] and modifier_names[] type arrays, X-Macros have been used, but that would require using the macros in the kernel includes as well.

[pcercuei]

What about we sync to the latest uapi header now, then protect libiio's copy with the kernel header's _IIO_TYPES_H_? That sounds like the simplest and safest solution.

[pcercuei]

@gwendalcr could you try 5fc7550?

[gwendalcr]

5fc7550 is a better patch.