How to conditionally append to a variable in a Makefile function? - makefile

Question

I have the following construct in the common make file which is to be included by
the module specific Makefile -
# Conditionally add Logger as a MUT dependency
define COND_ADD_LOGGER
$if($$(findstring -DLOGGER, $$(DEFINES)), SOURCES += logger.c)
endef
define CMN_TESTS_RULE
$(COND_ADD_LOGGER)
$(eval OBJECTS = $(SOURCES:.cpp=.o))
$(eval OBJECTS := $(OBJECTS:.c=.o))
$(TARGET): $(OBJECTS)
$(COMPILE_RULE_CMN)
endef
In my module's Makefile I do this -
SOURCES = a.c b.c test.cpp
TARGET = generic_tests
$(eval $(CMN_TESTS_RULE))
This isn't adding the logger.c files to the SOURCES list as I had expected.
(The DEFINES variable definitely has the sub-string "-DLOGGER" in it.)
This is the output when I use info -
$if($(findstring -DLOGGER, $(DEFINES)), SOURCES += logger.c)
generic_type_abstraction_tests: a.o b.o test.o
g++ -o $# $^ D:/TEST/gtest-1.7.0/make/gtest_main.a
$if($(findstring -DLOGGER, $(DEFINES)), SOURCES += logger.c)
generic_type_abstraction_tests: a.o b.o test.o
g++ -o $# $^ D:/TEST/gtest-1.7.0/make/gtest_main.a
I am using GNU Make version 3.81 from CYGWIN on a Windows 8 machine.

Answer 1

There are several things that do not make sense to me in your Makefile (especially strange use of nested eval). I suggest to fix them with something like:
# Conditionally add Logger as a MUT dependency
define COND_ADD_LOGGER
SOURCES += $(if $(findstring -DLOGGER,$(DEFINES)),logger.c)
endef
define CMN_TESTS_RULE
$(COND_ADD_LOGGER)
OBJECTS = $$(patsubst %.c,%.o,$$(patsubst %.cpp,%.o,$$(SOURCES)))
$(TARGET): $$(OBJECTS)
$(COMPILE_RULE_CMN)
endef
SOURCES = a.c b.c test.cpp
TARGET = generic_tests
$(eval $(CMN_TESTS_RULE))
.PHONY: debug
debug:
$(info SOURCES: $(SOURCES))
$(info OBJECTS: $(OBJECTS))
Demo:
$ make debug
SOURCES: a.c b.c test.cpp
OBJECTS: a.o b.o test.o
gmake: 'debug' is up to date.
$ make DEFINES=-DLOGGER debug
SOURCES: a.c b.c test.cpp logger.c
OBJECTS: a.o b.o test.o logger.o
gmake: 'debug' is up to date.