Re: Override... but not really

[Paul Smith]

On Mon, 2010-08-09 at 10:00 -0700, Philip Prindeville wrote:
> Is this expected behavior?

Yes.

> $ cat makefile1
> override CFLAGS+=-DB
> $(info makefile1: CFLAGS=$(CFLAGS))
> all:
>       make -f makefile2

> $ cat makefile2
> $(info makefile2: CFLAGS=$(CFLAGS))
> override CFLAGS+=-DC
> $(info makefile2: CFLAGS=$(CFLAGS))
> all:
>       @echo foo

> $ make -f makefile1 CFLAGS=-DA
> makefile1: CFLAGS=-DA -DB
> make -f makefile2
> makefile2: CFLAGS=-DA
> makefile2: CFLAGS=-DA -DC
> foo

> So why isn't the CFLAGS that gets passed to the submake set to "-DA -DB"?

Because you're not invoking make with 'CFLAGS=$(CFLAGS)', you're
invoking it with 'CFLAGS=-DA'.

When the sub-make is invoked, it passes that same command line argument
to the sub-make; that is, running "make CFLAGS=-DA" means that the
recipe that invokes the sub-make will be the equivalent of "make -f
makefile2 CFLAGS=-DA"

Hence the behavior you see.

> What's even more confusing is that make apparently keeps two copies...
> the one that's in-scope for the current make, and the one that gets
> re-exported into sub-makes.

Nope.  It's much simpler than that: the assignments on the command line
are simply added to a variable which is then passed to sub-makes.  See
the docs about the MAKEOVERRIDES variable, etc.

To allow the behavior you're looking for WOULD be much more complicated:
rather than just passing down the same assignments on the command line,
make would have to remember that a particular variable was a
command-line variable and construct the value to pass down (with all
quoting etc.) dynamically by looking up the values of those assignments
in the current make environment whenever it runs a recipe that appears
to be a sub-make (in which case your example still wouldn't work,
because you're using "make" instead of the $(MAKE) variable and you
didn't prefix your recipe line with "+", so make can't tell that this
recipe invokes a sub-make).


To avoid these kinds of issues, the typical method is to reserve certain
variables for the command line, such as CFLAGS, LDFLAGS, CPPFLAGS,
CXXFLAGS, etc. and then use a different variable internally to hold all
the values.

For example, you might have something like this:

        CFLAGS = -O2 -g
        CPPFLAGS =
        _CFLAGS = <other flags>
        _CPPFLAGS = -I...
        
        %.o : %.c
                $(CC) $(_CFLAGS) $(_CPPFLAGS) $(CPPFLAGS) $(CFLAGS) -o $@ -c $<

Or similar.

-- 
-------------------------------------------------------------------------------
 Paul D. Smith <address@hidden>          Find some GNU make tips at:
 http://www.gnu.org                      http://make.mad-scientist.net
 "Please remain calm...I may be mad, but I am a professional." --Mad Scientist