Re: Finding "config.h" in VPATH builds

[John Calcote]

Hi David,

On 9/19/2009 4:43 PM, David Bruce wrote:
> I have a project with all the source files in a src directory just
> below trunk, which is where the top level configure.ac and Makefile.am
> live.  When I run configure, config.h is created at build/config.h.
> If I run configure from within trunk, make works successfully, but if
> I do a VPATH build I get errors because config.h is not located.  If I
> change ' #include "config.h" '  to ' #include ../config.h" ', it works
> as either a VPATH build or an in-tree build.  I'm not sure this is the
> right way to address this, however.
>    

Here's a snippet from a generated Makefile in one of my simple projects 
where config.h is generated at the top of the build tree next to configure:

DEFAULT_INCLUDES = -I. -I$(top_builddir)
...
COMPILE = $(CC) $(DEFS) $(DEFAULT_INCLUDES) $(INCLUDES) $(AM_CPPFLAGS) \
        $(CPPFLAGS) $(AM_CFLAGS) $(CFLAGS)

As you can see here, DEFAULT_INCLUDES is set to -I. and 
-I$(top_builddir). The default include path includes both the current 
build directory, and the top build directory. This works, whether the 
build directory is the source directory or not.

If you have created your configure.ac file to have autoheader build 
config.h in a directory other than the default -- $(top_builddir) -- by 
using AC_CONFIG_HEADERS([build/config.h]) or something, then Automake 
ensures that the DEFAULT_INCLUDES variable is set up to reference this 
other directory. (Note however, that this other directory *must exist* 
in your source tree.) Here's that same snippet after changing 
AC_CONFIG_HEADERS as shown above in my sample project:

DEFAULT_INCLUDES = -I. -I$(srcdir) -I$(top_builddir)/build
...
COMPILE = $(CC) $(DEFS) $(DEFAULT_INCLUDES) $(INCLUDES) $(AM_CPPFLAGS) \
        $(CPPFLAGS) $(AM_CFLAGS) $(CFLAGS)

In short, I'm not sure why you're having trouble. Most of the issues 
covered by the paragraph you cited from the Autoconf manual are only 
relevant when you're writing your own Makefile.in files. Automake 
handles setting up DEFAULT_INCLUDES properly when you use Automake in 
conjunction with Autoconf.

What am I missing?

John