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

[Jan Beulich <jbeulich@xxxxxxxx>]

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'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?

Jan