Makefile does not find object file

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!

Solution

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.