Re: Specifying AM_CPPFLAGS from within configure.ac

[Matthew Walker]

Hi Jirka,

Firstly, thank you very very much for your comments.  They were well and 
truly more thorough than I'd ever hoped for.

Unfortunately, I don't understand much of what you have said.

Jirka Hanika wrote:

> On Thu, Jun 02, 2005 at 05:50:14PM +1200, Matthew Walker wrote:
>  
>
> > Hi,
> >
> > I would like to know how I should go about specifying AM_CPPFLAGS from 
> > within my configure.ac.
> >    
>
> For such things, it is VERY useful to add something like this:
>
> include $(top_srcdir)/Makefile.common
>
> at the top of every Makefile.am.  This basically gives you a whole new
> dimension of performing package administration.  I'm almost surprised
> this feature is not built into the automake to dismiss the (small) initial
> work investment.  Now write your own Makefile.common and put everything
> into it as you please.
>
>  
I understand this suggestion (!) but I did a quick search and found 
there were 81 Makefiles.  If I were to add that text to the top of every 
makefile, can you confirm that Makefile.common would have only the line:
AM_CPPFLAGS=-Wall -ggdb

From your comments below, I understand that this is not entirely 
portable and that there is no "easy" solution to this portability 
problem.  Also, AM_CPPFLAGS covers both C and C++ while AM_CFLAGS covers 
only C.  As the library is in C++, I gather AM_CPPFLAGS is the better 
choice.

> Other people use a single Makefile.am.  That's not bad, either.  Gives
> you much faster builds, at some expense of maintainability.
>
>  
As the library is not my own, I do not think this option is available to me.

> > I read Norman Gray and Alexandre Duret-Lutz's discussion from this 
> > mailing list on this topic 
> > (http://www.mail-archive.com/address@hidden/msg10024.html) but could 
> > not understand quite what I was supposed to do.
> >    
>
> Put this into your configure.ac:
>
> AC_SUBST(AM_CPPFLAGS)
>
> (or rather, AC_SUBST(AM_CFLAGS) as it is something of a conincidence that the
> preprocessor and compilation flags tend to have the same effects.)
>
>  
I just do not get this at all.  Where do I *set* AM_CPPFLAGS?  I tried 
to understand the documentation for AC_SUBST (section 7.2 of the 
Autoconf manual), but for the life of me, I just don't get it.

AC_SUBST subsitutes something for something else.  My imagination tells 
me that it probably looks through all the Makefile.am's for the 
something, but really I've got no idea.

Here is the part of configure.ac that I thought I could alter:

dnl Open BEAGLE optimization mode option.
AC_ARG_ENABLE(optimization,
 [  --enable-optimization    enable optimization mode [default=no]],
 [case $enableval in
 yes) enable_optimization=yes;;
 no) enable_optimization=no;;
 *) enable_optimization=no;; esac],
 enable_optimization=no)
if test "$enable_optimization" = yes; then
 AC_DEFINE(NDEBUG,,[define if some debug code is disabled])
 CFLAGS=`echo $CFLAGS | sed 's/\(?*\)-g\(?*\)/\1\2/'`         <---- This is 
already in the file. I don't know what it does
 CXXFLAGS=`echo $CXXFLAGS | sed 's/\(?*\)-g\(?*\)/\1\2/'`           but my guess is that it would be better if it didn't set 
else                                                                 CFLAGS or CXXFLAGS.
 CFLAGS=`echo $CFLAGS | sed 's/\(?*\)-O[[0-9]]\(?*\)/\1\2/'`  <----/
 CXXFLAGS=`echo $CXXFLAGS | sed 's/\(?*\)-O[[0-9]]\(?*\)/\1\2/'`
fi

dnl Open BEAGLE full debug mode option.
AC_ARG_ENABLE(full-debug,
 [  --enable-full-debug      enable full debug mode [default=no]],
 [case $enableval in
 yes) enable_full_debug=yes;;
 no) enable_full_debug=no;;
 *) enable_full_debug=no;; esac],
 enable_full_debug=no)
if test "$enable_full_debug" = yes; then
 AC_DEFINE(FULL_DEBUG,,[define if full debug mode is active])
 CPPFLAGS=-Wall                                               <---- This is my 
effort. It works but shouldn't set CPPFLAGS either.
fi


Can you show me how I would use AC_SUBST to replace my "CPPFLAGS=-Wall"?

> > The library that I'm working on has an "--enable--full-debug" mode, and 
> > I would like to ensure that the compiler spits out all the warnings it 
> > can by specifing CPPFLAGS=-Wall.  I have, however, read that setting 
> > CPPFLAGS is a bad thing to do because users should be able to alter that 
> > variable to their hearts content.
> >    
>
> Not every compiler understands -Wall, but newcomers are rarely interested
> in hearing that :-)
>
> You are right that CPPFLAGS is not the right tool.
>
> You may wish to stop reading here, and resume reading when you get
> automake/autoconf run according to your present wishes :-)
> as the rest is less straightforward.
>
>  
>
> > Any help will be very much appreciated.  I am _very_ much a newbie to 
> > autoconf/automake and I apologize sincerely if this is a stupid question.
> >    
>
> No it's a fairly standard question.  But you'll probably learn over time that
> often it is significantly better to approach these problems from a different
> angle.  For example, you don't need to meddle with the autotools at all
> in your example - just specify CFLAGS as every other builder would.  It is
> simple and automatizable, unless you do development on a different host each
> day.  The benefit?  People with different compilers will not have to
> fight the non-portable -Wall which happens to break their
> compilation.
>  
I found this the most interesting/easy idea.  All I need to type is:
make 'CPPFLAGS=-Wall -ggdb'
or better yet:
./configure --enable-full-debug 'CPPFLAGS=-Wall -ggdb'
and everything seems to be lovely.  This is great for me, but I still 
think it might be beneficial, for debug mode, to somehow set all 
warnings to be on.  As I understand it, the code you provided checks for 
the best warning settings for a user's compiler.  (I feel this is cheeky 
of me seeing I'm just starting, but...)  Why isn't such code already 
part of autoconf?


Thank you again for your fantastic assistance.

Regards,

Matthew