Re: debug builds with NO optimizations

[Howard Chu]

John Calcote wrote:
> When you override CFLAGS on the configure command line, you give
> configure the opportunity to add more flags to the CFLAGS variable that
> are then passed on to make. On the other hand, when you modify CFLAGS on
> the make command line, you are overriding all settings provided to the
> Makefile by the configure script when the Makefile was built. For
> example, I have a (generated) Makefile that contains the following
> definition for CFLAGS:
>  
> CFLAGS = -g -O2 -DLINUX -Wall -Werror -O3
> The -g -O2 comes from the AC_PROG_CC macro in my configure.ac file. The
> -DLINUX comes from some hand-written script in that same configure.ac
> that determines my platform (based on host_os), while thet -Wall -Werror
> comes from some more script that determines which compiler I'm using.
> Finally, the -O3 is determined by yet another piece of script that
> determines based on --enable features specified how optimal I want the
> code to be.
>  
> If you say "make CFLAGS=-g", you've just lost the definition for LINUX
> and the warning options defined in the configure script. Before you all
> flame me on this, let me say that I'm already aware that I should be
> passing the -D option in the CPPFLAGS, but I would argue that CFLAGS is
> the only viable place to pass the -W options, and that these options
> should probably be set by a configure script that can figure out which
> compiler you're using. I would also like to point out that this is a
> simple example - I know of projects that also much more carefully craft
> optimization flags - they pass a dozen or more -f flags based on
> compiler and platform characteristics.

This is a common problem, and the common solution is to push each bit of 
potential settings into its own macro, which is then referenced from the 
Makefile's CFLAGS macro. Thus for command line overrides you only 
override one of the other macros, not CFLAGS itself.

E.g. in OpenLDAP the Makefile.in uses:
address@hidden@

to get the AC_PROG_CC flags. The actual CFLAGS is
CFLAGS=$(AC_CFLAGS) $(DEFS)

with DEFS referencing a variety of other macros for whatever other flags 
are needed.

Then
        make AC_CFLAGS=-g
is sufficient to override the "-g -O2" that autoconf puts in there by 
default.

> I reserve setting CFLAGS on the make command line for when I absolutely
> need to specify the entire set of CFLAGS in a build - for instance, when
> I'm experimenting with something new.

Right; I pretty much never override CFLAGS itself, too much hassle to 
get all of the other definitions.

--
  -- Howard Chu
  Chief Architect, Symas Corp.  http://www.symas.com
  Director, Highland Sun        http://highlandsun.com/hyc
  OpenLDAP Core Team            http://www.openldap.org/project/