Re: AC_CONFIG_HEADERS

[Ralf Wildenhues]

Hi Jeff,

* Jeff Squyres wrote on Thu, Feb 03, 2011 at 04:30:24PM CET:
> We recently got bitten by what appears to be undocumented behavior in
> AC_CONFIG_HEADERS (meaning that I couldn't find this behavior
> described in the docs; I might well have missed it...?).  It seems
> that AM's version of AC_CONFIG_HEADERS automatically adds a -I to the
> generated file in the build tree.

Well, not completely undocumented, but quite hidden and misplaced in
info Automake "Program Variables":

     Automake already provides some `-I' options automatically, in a
     separate variable that is also passed to every compilation that
     invokes the C preprocessor.  In particular it generates `-I.',
     `-I$(srcdir)', and a `-I' pointing to the directory holding
     `config.h' (if you've used `AC_CONFIG_HEADERS' or
     `AM_CONFIG_HEADER').  You can disable the default `-I' options
     using the `nostdinc' option.

[...]
> The problem came recently when we added some other files in
> include/hwloc/*.h.  One of them had the same name as a system header
> file -- something like this:
> 
>     include/hwloc.h
>     include/hwloc/config.h            <-- generated
>     include/hwloc/conflicting-name.h  <-- /usr/include/conflicting-name.h 
> also exists
> 
> Some of the source files in our project have #include
> "conflicting-name.h" -- intending to get the system version.  But
> since Automake hard-codes -I$(top_builddir)/include/hwloc into
> Makefile.in, our source files are picking up
> include/hwloc/conflicting-name.h, not the system version.  Changing
> the source files to #include <conflicting-name.h> doesn't seem to
> help.  And since the value is hard-coded in Makefile.in, we can't add
> any m4/shell scripting in configure.ac to filter that -I value out.

> I see the "nostdinc" Automake flag to inhibit adding all -I's, but
> that seems like a bit of overkill for our needs.  We *do* want the
> -I's for the AH-generated file, but not for any others.  Is there a
> way to do that?  Or is there another way to inhibit specific -I's?

Well, currently the only way to work around this is for you to use
the nostdinc option and put
  AC_SUBST([DEFAULT_INCLUDES], ['-I. -I$(srcdir) $(top_builddir)/include'])

in configure.ac.

We might want to consider adding options stdinc-ah stdinc-nohdr or so
for generating only include paths for autoheader-generated headers plus
the first two, or the first two only, respectively.  (Each with VPATH
simplification I suppose.)

Thanks,
Ralf