AM_*FLAGS and autoconf

[Norman Gray]

Greetings,

How ought autoconf macros to pass flags, such as CFLAGS, to  
automake-generated makefiles?  The automake FAQ answer  
<http://sources.redhat.com/automake/automake.html#Flag-Variables- 
Ordering> doesn't answer all my questions about this.

In my situation, I want to add -I and -L and other options, determined  
at configure time, to all of the Fortran, C and C++ compile and link  
commands in a large set of related components.  The logic in question  
is in project-specific autoconf macros incorporated into a large number  
of configure.ac scripts.

Setting CFLAGS in ./configure is out, since this is reserved for the  
user (in this context, the person running ./configure;make), in the  
sense that they have to be free to type 'make CFLAGS=xxx' without  
breaking whatever flags the configure script needs to pass to the  
makefile.

Setting AM_CFLAGS is also out.  The automake documentation talks of  
this being reserved for the author of Makefile.am, and doesn't mention  
autoconf at all.  The FAQ says `You should not add options to these  
variables from inside configure either', and makes a number of other  
wise suggestions which I've recently painfully discovered for myself.

The FAQ's suggests defining differently-named variables and  
substituting them into the Makefile.am.  But this isn't really a  
solution, as it would require me to add

    AM_CFLAGS = $(MY_MAGIC_CFLAGS)
    AM_FCFLAGS = $(MY_MAGIC_FCFLAGS)
    ...

at the top of 300+ Makefile.am files, which is exactly the sort of  
meaningless boilerplate automake is supposed to save us from.  I want  
to insulate the Makefile.am authors as much as possible from the  
configuration magic, so that they can write Makefile.am files which  
look as simple as possible, without using FAQ-recommended tricks.

I've also had success with modifying automake so that  
&read_main_am_file calls &define_configure_variable on all AC_SUBSTed  
AM_* variables, with the result that AM_CFLAGS+=blah will work.  This  
appears to match the intent of the preceding call to  
&define_standard_variables, so this didn't feel outrageous.  That  
works, and is currently my least unfavourite option, but seems more  
drastic than I'd expect to be necessary.  This  violates the  
proscription on setting these variables within configure, though it  
appears to me to make that proscription less necessary.

It looks like I'm in a similar situation to the author of  
<http://lists.gnu.org/archive/html/autoconf/2002-09/msg00228.html>,  
though the thread following that veered in another direction.  The  
thread including  
<http://lists.gnu.org/archive/html/automake/2001-02/msg00275.html>  
seems relevant, but turns into a discussion of what AM_* pattern  
aclocal should veto.

About the only things left are modifying $FC, $CC and various *LD  
variables (feels hacky and fragile), and generating Makefile.in rules  
with extra $(MY_CFLAGS) variables, or the boilerplate mentioned above  
(ditto).

I seem to have argued myself into an impasse, and therefore convinced  
myself that I must be missing something obvious.  Can anyone give any  
advice?

Best wishes,

Norman


--
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Norman Gray  :  Physics & Astronomy, Glasgow University, UK
http://www.astro.gla.ac.uk/users/norman/  :  www.starlink.ac.uk