c++ makefile with multiple program, autodetection of source files - c++

Question

I recently lost 5 hours to figure out how I could write the makefile I need. I'm not an informaticien or programmer so I'd like some comments on what I managed to do. I already looked a lot on different sites but still...
I need a makefile that creates different executables: prog1, prog2...
To create the .o files, as I have many files with many dependencies, I don't want to specify them all. So I want/need to use automatic variables with a pattern rule. To speed up the compilation I also take care to only recompile the modified files. I achieved this by using the -MD flag that creates a .d file saved in the $(BUILD) directory.
What I still can't do is to detect automatically which .o files prog1 needs. So for now I have to specify them automatically... If you know how to do that automatically...
I also would like to save the .o files in the $(BUILD) directory, but I can't make it work.
Any advice are welcome !
Thx
CXX = g++
ERRORS = -Wall -Wextra -pedantic
LAPACK = -llapack -lblas
OPTION = -O3 -fopenmp
CXXFLAGS = $(LAPACK) $(ERRORS) $(OPTION)
LDFLAGS = $(LAPACK) $(ERRORS) $(OPTION)
BUILD=build
SRCS=(wildcard *.cpp)
all:prog1 prog2 ...
prog1:prog1.o dep_only_for_prog_1.o dep_for_all_progs.o dep_for_some_progs.o
$(CXX) -o $# $^ $(LDFLAGS) $(NOASSERT)
prog2:prog2.o dep_only_for_prog_2.o dep_for_all_progs.o dep_for_some_progs.o
$(CXX) -o $# $^ $(LDFLAGS) $(NOASSERT)
...
%.o:%.cpp
$(CXX) -MD -c $(CXXFLAGS) $(NOASSERT) $< -o $#
mv $(<:.cpp=.d) $(BUILD)
-include $(addprefix $(BUILD)/$(SRCS:.cpp=.d))
clean:
rm -f *.o $(BUILD)/*

Answer 1

You just can't get make to infer somehow which files belong to which programs, but you CAN make your makefile simpler to read and update. Also you have a few bad things here, such as adding $(LAPACK) (which contains linker flags) to $(CXXFLAGS) (which are passed to the compiler).
Try:
PROGRAMS = prog1 prog2
prog1_SOURCES = prog1.cpp dep_only_for_prog_1.cpp \
dep_for_all_progs.cpp dep_for_some_progs.cpp
prog2_SOURCES = prog2.cpp dep_only_for_prog_2.cpp \
dep_for_all_progs.cpp dep_for_some_progs.cpp
#----- Don't need to change below here
CXX = g++
ERRORS = -Wall -Wextra -pedantic
LAPACK = -llapack -lblas
OPTION = -O3 -fopenmp
CXXFLAGS = $(ERRORS) $(OPTION)
LDFLAGS = $(LAPACK) $(ERRORS) $(OPTION)
BUILD=build
SRCS := $(wildcard *.cpp)
all: $(PROGRAMS)
.SECONDEXPANSION:
$(PROGRAMS): $$($$#_SOURCES:%.cpp=%.o)
$(CXX) -o $# $^ $(LDFLAGS) $(NOASSERT)
%.o : %.cpp
$(CXX) -MD -c $(CXXFLAGS) $(NOASSERT) $< -o $#
mv $(<:.cpp=.d) $(BUILD)
-include $(addprefix $(BUILD)/$(SRCS:.cpp=.d))
clean:
rm -f *.o $(BUILD)/*
Or you can use eval if you want.