Make file warning, overriding commands for target

Like the OP (James Leonard), I would like to suppress or avoid warnings about overriding Makefile targets. However, my situation and goals are different.

I want Makefile to include base.mk, and I want Makefile to be able to override targets from base.mk without any warning messages. I am using GNU Make.

The GNU Make documentation describes one way of doing this:

Create Makefile as follows:

foo:
        echo 'bar' > foo

%: force
        @$(MAKE) -f base.mk $@
force: ;

Source: https://www.gnu.org/software/make/manual/html_node/Overriding-Makefiles.html

The above method has the (potentially serious) disadvantage that it will invoke a separate instance of $(MAKE), meaning that (some or all?) variables may (will?) not be shared between the parent and child invocations of make.

Happily, I found a better solution, as outlined below:

Create base.mk as follows:

foo-default:
        echo  'foo-default'

bar-default:
        echo  'bar-default'

%:  %-default
        @  true

Create Makefile as follows:

include  base.mk

foo:
        echo  'foo  Makefile'

Example output:

$ make foo
echo  'foo  Makefile'
foo  Makefile

$ make bar
echo  'bar-default'
bar-default

Note that you need to be able to control the names of the targets in base.mk so that you can name them <target>-default. In other words, you cannot use this approach to extend arbitrary base makefiles. However, if you control both base.mk and Makefile, this approach will allow you to create one base.mk and then many customizations of it.

To avoid including the same file many times, you can use a C-style header 'directive':

ifeq ($(_MY_MAKEFILE_),)
_MY_MAKEFILE_ := defined

...

endif

• ifeq here is just saying "is the value empty"
• it's possible something similar can be done with ifndef but I got this working first
• the value "defined" is just anything that isn't blank

One of the most common ways for doing this is to put the release objects and the debug objects in separate subdirectories. That way you don't get redefinitions for the rules for an object, since they will have different names. Something like this:

D_OBJECTS=$(SRC:%.cpp=debug/%.o)
R_OBJECTS=$(SRC:%.cpp=release/%.o)

RTARGET = a.out
DTARGET = a.out.debug

all : dirs $(RTARGET)

debug : dirs $(DTARGET)

dirs :
    @mkdir -p debug release

debug/%.o : %.c
    $(CC) $(DEBUG_CFLAGS) -o $@ -c $<

release/%.o : %.c
    $(CC) $(RELEASE_CFLAGS) -o $@ -c $<

$(DTARGET) : $(D_OBJECTS)
    $(CC) $(DEBUG_CFLAGS) -o $@ $(D_OBJECTS)

$(RTARGET) : $(R_OBJECTS)
    $(CC) $(RELEASE_CFLAGS) -o $@ $(R_OBJECTS)