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Re: conditionally disabling libltdl subconfigure

From: Nick Bowler
Subject: Re: conditionally disabling libltdl subconfigure
Date: Wed, 22 Aug 2012 13:44:12 -0400
User-agent: Mutt/1.5.21 (2010-09-15)

On 2012-08-22 18:32 +0200, Brice Goglin wrote:
> Le 22/08/2012 16:32, Nick Bowler a écrit :
> > On 2012-08-22 15:12 +0200, Brice Goglin wrote:
> >> I am working at embedding libltdl inside our project (hwloc, an Open MPI
> >> subprojet). Our plugins are not expected to be enabled in the most
> >> common cases. So I am trying to see if the libltdl sub-configure could
> >> be skipped entirely unless really needed (when plugins are enabled, and
> >> libltdl is indeed built). I got the usual failure when I tried to put
> >> LTDL_INIT inside a conditional block so I am looking for something else 
> >> now.
> >>
> >> Modifying the subdirs variable before AC_OUTPUT seems to work. But this
> >> hack may not be safe enough. Is there a better way to do this?
> > Use either the "recursive" or "nonrecursive" libltdl styles.  Do not use
> > "subproject" (the default).  Both these styles integrate all the
> > configure machinery directly into your main configure script, so there
> > may still be some extra checks but they should be fairly low-cost in the
> > grand scheme of things (especially compared to subproject!).
> >
> > You can then use Automake conditionals to control whether or not libltdl
> > will be built.
> >
> > Cheers,
> Thanks a lot! It looks promising, indeed.
> One small problem with this is that it requires -I$(top_builddir) in
> CPPFLAGS since it wants to include LT_CONFIG_H=relative/path/config.h
> Not sure how this could be improved. Maybe by passing the absolute path
> in LT_CONFIG_H? Or by allowing to manually override it?

So you have your config.h in a subdir from the top of your project,
e.g., something like


and libltdl is trying to #include <some/subdir/config.h> ?
That sounds like it might be a libltdl bug.   Automake will in this case
add -Isome/subdir by default, so libltdl should just be be #including
<config.h> instead (i.e., not specifying any of the leading directories).

Other than adding -I$(top_builddir) to CPPFLAGS, as a workaround you
could probably add something like this just before AC_OUTPUT (untested):

  LT_CONFIG_H=`expr "$LT_CONFIG_H" : '.*/\(.*\)'`

Nick Bowler, Elliptic Technologies (

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