Makefile does not find object file - c++

Question

I want my object files to be created in a subdirectory and not where Makefile lives. So, I saw this answer, which I couldn't apply to my case, so I tried this:
OBJS = main.o IO.o alloc.o communication.o
OBJSDIR = obj
SOURCE = main.cpp src/IO.cpp src/alloc.cpp src/communication.cpp
HEADER = headers/IO.h headers/alloc.h headers/communication.h
OUT = test
CXX = ../../mpich-install/bin/mpic++
CXXFLAGS = -I../../intel/mkl/include -Wl,--start-group -Wl,--end-group -lpthread -lm -ldl -Wall
LDFLAGS = ../../intel/mkl/lib/intel64/libmkl_scalapack_lp64.a -Wl,--start-group ../../intel/mkl/lib/intel64/libmkl_intel_lp64.a ../../intel/mkl/lib/intel64/libmkl_core.a ../../intel/mkl/lib/intel64/libmkl_sequential.a -Wl,--end-group ../../intel/mkl/lib/intel64/libmkl_blacs_intelmpi_lp64.a -lpthread -lm -ldl
all: $(OBJSDIR) $(OUT)
$(OBJSDIR):
mkdir $(OBJSDIR)
$(OUT): $(OBJS)
$(CXX) $(OBJS) -o $(OUT) $(CXXFLAGS) $(LDFLAGS)
# make -f Makefile clean
# create/compile the individual files >>separately<<
$(OBJSDIR)/main.o: main.cpp
$(CXX) -c main.cpp $(CXXFLAGS)
$(OBJSDIR)/IO.o: src/IO.cpp
$(CXX) -c src/IO.cpp $(CXXFLAGS)
$(OBJSDIR)/alloc.o: src/alloc.cpp
$(CXX) -c src/alloc.cpp $(CXXFLAGS)
$(OBJSDIR)/communication.o: src/communication.cpp
$(CXX) -c src/communication.cpp $(CXXFLAGS)
.PHONY: clean
clean:
rm -rf *.o
and I am getting:
gsamaras#pythagoras:~/konstantis/cholesky$ make
../../mpich-install/bin/mpic++ -I../../intel/mkl/include -Wl,--start-group -Wl,--end-group -lpthread -lm -ldl -Wall -c -o main.o main.cpp
make: *** No rule to make target 'IO.o', needed by 'test'. Stop.
I have a src folder, where all the .cpp files live (except from main.cpp, that lives in the same directory as the Makefile) and a headers directory, where all the header files live.
EDIT
I modified the first two lines, as such:
OBJSDIR = obj
OBJS = $(OBJSDIR)/main.o $(OBJSDIR)/IO.o $(OBJSDIR)/alloc.o $(OBJSDIR)/communication.o
and I am getting:
g++: error: obj/main.o: No such file or directory
...
The problem lies into the fact that the object files are still generated in the main directory!

Answer 1

You want to change the lines that invoke your compiler from:
$(OBJSDIR)/IO.o: src/IO.cpp
$(CXX) -c src/IO.cpp $(CXXFLAGS)
to:
$(OBJSDIR)/IO.o: src/IO.cpp
$(CXX) -c src/IO.cpp $(CXXFLAGS) -o $#
Note that $# is the automatic variable that corresponds to the target file being created. So in the above case it will be obj/IO.o. -o specifies the output filename.
Furthermore while it is unrelated to your question one of the nice things about placing all the build artifacts into a separate directory is cleaning is much easier:
clean:
rm -rf $(OBJSDIR) $(OUT)
Also, as a final note if you ever wish to do a parallel build you will have an issue as your object files rely on the build directory. There are a couple solutions to this including calling mkdir -p objs before every compiler invocation or setting up the directory as a dependency that is only built if it does not exist.