What options should be set in Make variables CFLAGS, CXXFLAGS, CPPFLAGS and LDFLAGS?

[Jonathan Wakely]

On Thu, 24 Jan 2019 at 18:47, Peng Yu <pengyu.ut@gmail.com> wrote:
>
> On Thu, Jan 24, 2019 at 12:31 PM Paul Smith <psmith@gnu.org> wrote:
> >
> > On Thu, 2019-01-24 at 12:10 -0600, Peng Yu wrote:
> > > So only the following in the gcc manual go to CPPFLAGS. All other
> > > options go to other GNU Make variables.
> > >
> > >   Preprocessor Options
> > >        -Aquestion=answer -A-question[=answer] -C  -dD  -dI  -dM  -dN
> > >        -Dmacro[=defn]  -E  -H -idirafter dir -include file  -imacros file
> > >        -iprefix file  -iwithprefix dir -iwithprefixbefore dir  -isystem
> > >        dir -imultilib dir -isysroot dir -M  -MM  -MF  -MG  -MP  -MQ  -MT
> > >        -nostdinc -P  -fdebug-cpp -ftrack-macro-expansion
> > >        -fworking-directory -remap -trigraphs  -undef  -Umacro -Wp,option
> > >        -Xpreprocessor option -no-integrated-cpp
> >
> > I don't think you can really make that statement: it depends on your
> > environment.
> >
> > As I said, one idea behind using a separate variable is that there may
> > be other tools that you want to use which also take preprocessor
> > arguments.  It's not uncommon (or at least was not in the past) for
> > other tools to use the preprocessor to process source code for other
> > reasons than compilation; those other tools would need the -I and -D
> > options (and -U although that's rarely used).  However, most of the
> > less standard flags above will not be recognized by most of these other
> > tools and could cause them to throw errors.
> >
> > There's no one-size-fits-all definition, IMO.  The answer to all your
> > questions is, "it depends on what you want to do".  If you have no
> > reason to ever use the preprocessor separately maybe you don't need a
> > separate CPPFLAGS at all.
> >
> > There's just a general rule-of-thumb:
> >  * preprocessor options go into CPPFLAGS
> >  * options that can be used with BOTH C compilers and C linkers go into
> >    CFLAGS
> >  * options that can be used with BOTH C++ compilers and C++ linkers go
> >    into CXXFLAGS
> >  * options that can be used ONLY with linkers, but not compilers, go
> >    into LDFLAGS, except -l which goes into LDLIBS.
> >
> > Is there a particular reason you want to be so precise about this?
>
> There is quite a confusion about how to use make variables. Different
> sources give different bits of advice. I'd like to make it clear about
> it.

They're just conventions. You can write makefiles that don't use those
variables at all, and you can write makefiles that only use CFLAGS for
everything.

Following usual conventions can be useful, but is not essential. There
might be good reasons to do things differently in some makefiles.