Makefile with Fortran - src and bin directories - makefile

Question

I'm having some trouble understanding how to design my makefile to build my project the way I want to. Specifically, I can't figure out how to keep all source files in a src directory, while putting all binaries in a bin directory except the linked executable, which goes in the project root.
This is my makefile:
# Compiler options
FC := mpif90
FFLAGS := -O3 -g -Wall -Warray-bounds -ffixed-line-length-none -fbounds-check
VPATH := src
BINDIR := bin
# Define file extensions
.SUFFIXES:
.SUFFIXES: .f .o .mod
# All modules
OBJS := $(BINDIR)/ratecoeffs.o $(BINDIR)/interpolation.o $(BINDIR)/io.o $(BINDIR)/eedf.o $(BINDIR)/single_particle.o $(BINDIR)/physics.o $(BINDIR)/random.o $(BINDIR)/mpi.o $(BINDIR)/precision.o $(BINDIR)/populations.o
# Build rules
all: runner | $(BINDIR)
$(BINDIR):
mkdir -p $(BINDIR)
$(BINDIR)/%.o: $(VPATH)/%.f | $(BINDIR)
$(FC) $(FFLAGS) -c $^ -o $#
runner: $(OBJS)
clean:
#rm -rf $(BINDIR)
Running make builds everything allright - it finds all source files in src and puts all .o files in bin - but the module files (.mod) that are generated by the compiler are put in the project root instead of in the bin directory. I realize I could just specify a rule to place them there, but that messes with the build order, and will sometimes break the build.
What is the "correct" way to get this behavior?
And yes, I've looked at autotools and automake, but I've never used them before and they seem to be overkill for this project. As I couldn't find any good tutorials on how they work (no, I didn't like the tutorial on gnu.org) I'd prefer if I could avoid having to learn this tool just to get this work...

Answer 1

Assuming your underlying Fortran compiler is gfortran, use the -J command line option.
$(FC) $(FFLAGS) -c $^ -o $# -J$(BINDIR)
With an eye to the future, you may be better off creating a MODDIR or similar variable, that you use instead of BINDIR. Object code (*.o) and mod files have different roles to play in later compilation and linking steps - in larger projects they are often kept separate.

Answer 2

It would be probably more in the sense of the make system to change into the obj-directory and do the compilation from there. Via the VPATH option you can let make to find your source files automatically. You could easily call your makefile recursively from the right directory. Below you find a trivial example which would be straightforward to adapt to your case. Please note, that it only works with GNU make.
ifeq (1,$(RECURSED))
VPATH = $(SRCDIR)
########################################################################
# Project specific makefile
########################################################################
FC = gfortran
FCOPTS =
LN = $(FC)
LNOPTS =
OBJS = accuracy.o eqsolver.o io.o linsolve.o
linsolve: $(OBJS)
$(LN) $(LNOPTS) -o $# $^
%.o: %.f90
$(FC) $(FCOPTS) -c $<
.PHONY: clean realclean
clean:
rm -f *.mod *.o
realclean: clean
rm -f linsolve
accuracy.o:
eqsolver.o: accuracy.o
io.o: accuracy.o
linsolve.o: accuracy.o eqsolver.o io.o
else
########################################################################
# Recusive invokation
########################################################################
BUILDDIR = _build
LOCALGOALS = $(BUILDDIR) distclean
RECURSIVEGOALS = $(filter-out $(LOCALGOALS), $(MAKECMDGOALS))
.PHONY: all $(RECURSIVE_GOALS) distclean
all $(RECURSIVEGOALS): $(BUILDDIR)
+$(MAKE) -C $(BUILDDIR) -f $(CURDIR)/GNUmakefile SRCDIR=$(CURDIR) \
RECURSED=1 $(RECURSIVEGOALS)
$(BUILDDIR):
mkdir $(BUILDDIR)
distclean:
rm -rf $(BUILDDIR)
endif
The principle is simple:
In the first part you write your normal makefile, as if you would create the object files in the source directory. However, additionally you add the VPATH option to make sure the source files are found (as make will be in the directory BUILDDIR when this part of the makefile is processed).
In the second part (which is executed first, when the variable RECURSED is not set yet), you change to the BUILDIR directory and invoke your makefile from there. You pass some helper variables (e.g. the current directory) and all make goals, apart of those, which must be executed from outside BUILDDIR (e.g. distclean and the one creating BUILDDIR itself). The rules for those goals you specify also in the second part.