Re: [PATCH for-4.15] autoconf: check endian.h include path

[Jan Beulich <jbeulich@xxxxxxxx>]

On 04.02.2021 11:21, Roger Pau Monné wrote:
> On Thu, Feb 04, 2021 at 11:13:43AM +0100, Jan Beulich wrote:
>> On 04.02.2021 10:59, Roger Pau Monné wrote:
>>> On Thu, Feb 04, 2021 at 10:46:58AM +0100, Jan Beulich wrote:
>>>> On 04.02.2021 10:38, Roger Pau Monne wrote:
>>>>> --- a/tools/configure.ac
>>>>> +++ b/tools/configure.ac
>>>>> @@ -74,6 +74,7 @@ m4_include([../m4/ax_compare_version.m4])
>>>>>  m4_include([../m4/paths.m4])
>>>>>  m4_include([../m4/systemd.m4])
>>>>>  m4_include([../m4/golang.m4])
>>>>> +m4_include([../m4/header.m4])
>>>>>  
>>>>>  AX_XEN_EXPAND_CONFIG()
>>>>>  
>>>>> @@ -517,4 +518,6 @@ AC_ARG_ENABLE([pvshim],
>>>>>  ])
>>>>>  AC_SUBST(pvshim)
>>>>>  
>>>>> +AX_FIND_HEADER([INCLUDE_ENDIAN_H], [endian.h sys/endian.h])
>>>>
>>>> Instead of a new macro, can't you use AC_CHECK_HEADERS()?
>>>
>>> AC_CHECK_HEADERS doesn't do what we want here: it will instead produce
>>> a HAVE_header-file define for each header on the list that's present,
>>> and the action-if-found doesn't get passed the path of the found
>>> header according to the documentation.
>>>
>>> Here I want the variable to be set to the include path of the first
>>> header on the list that's present on the system.
>>
>> I was thinking of
>>
>> #if defined(HAVE_SYS_ENDIAN_H)
>> # include <sys/endian.h>
>> #elif defined(HAVE_ENDIAN_H)
>> # include <endian.h>
>> #else
>> # error ...
>> #endif
> 
> I think having to replicate this logic in all places that include
> endian.h is cumbersome.

Right - I would further encapsulate this in a local header.

>>>> I'm also not certain about the order of checks - what if both
>>>> exist?
>>>
>>> With my macro the first one will be picked.
>>
>> And which one is to be the first one? IOW how likely is it that
>> on a system having both the first one is what we're after vs
>> the second one?
> 
> Not sure, but the same will happen with your proposal above: in your
> chunk sys/endian.h will be picked over endian.h.

Oh, sure - the two points are entirely orthogonal. And I'm
also not certain at all whether checking sys/ first is
better, equal, or worse. I simply don't know what the
conventions are. As a result I wonder whether we shouldn't
check that the header provides what we need.

Jan

> If we think that's
> the right precedence I can adjust AX_FIND_HEADER to be:
> 
> AX_FIND_HEADER([INCLUDE_ENDIAN_H], [sys/endian.h endian.h])
> 
> Which will achieve the same as your proposed snipped.
> 
> I can also add a comment to the macro that the first match will be the
> one that gets set.
> 
> Thanks, Roger.
>