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

From: "Roger Pau Monné" <roger.pau@citrix.com>
To: Jan Beulich <jbeulich@suse.com>
Cc: Ian Jackson <iwj@xenproject.org>, Wei Liu <wl@xen.org>,
	Juergen Gross <jgross@suse.com>, <xen-devel@lists.xenproject.org>
Subject: Re: [PATCH for-4.15] autoconf: check endian.h include path
Date: Thu, 4 Feb 2021 11:21:52 +0100	[thread overview]
Message-ID: <YBvKwNiIopKKZx/F@Air-de-Roger> (raw)
In-Reply-To: <0e3576d3-4565-9898-e954-4a888b21d92f@suse.com>

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.

> >> 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. 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.