Re: AM_CPPFLAGS best practice

[Eric Blake]

On 02/02/2012 01:43 PM, Jim Meyering wrote:
>> And coreutils has a bug, for forgetting the builddir version (which
>> means it will build in an in-tree build, but probably fail in a VPATH
>> build):
>> AM_CPPFLAGS = -I$(top_srcdir)/lib
> 
> "forgetting"? ;-)  Nah.
> On the contrary, I vaguely recall deliberately removing the
> explicit (and then-redundant) -I... option back when I noticed
> that automake handled that part for us.
> 
> In the case of coreutils, DEFAULT_INCLUDES covers that:
> 
>     $ grep DEFAULT_INCLUDES.= src/Makefile
>     DEFAULT_INCLUDES = -I. -I$(top_builddir)/lib

Ah, so automake favors builddir over srcdir.  From the automake manual:

> Automake already provides some @option{-I} options automatically, in a
> separate variable that is also passed to every compilation that invokes
> the C preprocessor.  In particular it generates @samp{-I.},
> @samp{-I$(srcdir)}, and a @option{-I} pointing to the directory holding
> @file{config.h} (if you've used @code{AC_CONFIG_HEADERS} or
> @code{AM_CONFIG_HEADER}).  You can disable the default @option{-I}
> options using the @option{nostdinc} option.
...
> Note that the reference to the build tree should come before the
> reference to the source tree, so that accidentally leftover generated
> files in the source directory are ignored.

Meanwhile, m4 was using:

AUTOMAKE_OPTIONS = nostdinc

precisely to avoid the DEFAULT_INCLUDES, but then m4 had to provide
srcdir itself, and did so prior to builddir; so I guess the bug is in m4.

And we might as well improve the gnulib manual to document the hack of:

> 
>>From what I recall, that's by virtue of our putting config.h in lib
> via this line from configure.ac:
> 
>     AC_CONFIG_HEADERS([lib/config.h:lib/config.hin])

as a way to get config.h to live in gnulib's builddir, for one less
listing in AM_CPPFLAGS if you are okay with DEFAULT_INCLUDES.

I'm also wondering if it is worth writing things as an automake
conditional that avoid redundant listing for in-tree builds, while still
giving correct results for VPATH builds.

-- 
Eric Blake   address@hidden    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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