makefile fortran multiple directories - makefile

Question

This has been asked many times but from none of the answers can I figure out how to do it.
Simply put, I have directory structure: SRC/ for all *.for and *.f90 files, BLD/ for all files needed for building (that is *.o and *.mod) and lib/ for necessary libraries that need to be linked.
I want to build all source files in the SRC/ directory and put *.o and *.mod to the BLD/ directory while keeping the executable in the root where makefile is. I don't want to have multiple makefiles. The current makefile that does not work is below. I think that one issue there are lines that refere e.g. $(BLD)/$(objects) which probably expands to BLD/nrtype.o nrutil.o nr.o mtypes.o polint.o polyin2.o a.o but I am not really sure, and I am just trying and erroring since I do not have enough knowledge and the manuals are not clear for me to be able to figure it out. In one of the trials, *.mod some got to BLD/ some stayed in the root, it seems that the rules do not work I think they do.
# Compilation related variables
SRC = src
BLD = build
LIB = lib
FC = gfortran
LDFLAGS = $(LIB)/asm_functions.o
FFLAGS = -O0
# the name of the output executable
target = a
# all objects to generate
objects = nrtype.o nrutil.o nr.o mtypes.o polint.o polyin2.o a.o
#
modules = nrtype.mod nrutil.mod nr.mod mtypes.mod
# set module dependencies
nrtype_mod = polint.o polyin2.o
nrutil_mod = polint.o polyin2.o
nr_mod = polin2.o
mtypes_mod = a.o
# set default target
default: $(target)
# add dependencies to the targets
$(nrtype_mod): nrtype.mod
$(nrutil_mod): nrutil.mod
$(nr_mod): nr.mod
$(mtypes_mod): mtypes.mod
# compile modules
$(BLD)/%.mod: $(SRC)/%.f90 $(BLD)/%.o
$(FC) -c $(FFLAGS) -o $# $<
# compile old fortran files
$(BLD)/%.o: $(SRC)/%.for
$(FC) -c $(FFLAGS) -o $# $<
# compile new fortran files
$(BLD)/%.o: $(SRC)/%.f90
$(FC) -c $(FFLAGS) -o $(BLD)$# $<
# link everything
$(target): $BLD/$(objects)
$(FC) -o $# $(FFLAGS) $^ $(LDFLAGS)
# cleaning up
clean:
rm -f $(BLD)/$(objects) $(BLD)/$(modules)
clobber: clean
rm -f $(BLD)/$(target)
.PHONY: default clean clobber
Update
I edited the makefile to the form:
# Compilation related variables
SRC = src
BLD = build
LIB = lib
FC = gfortran
LDFLAGS = $(LIB)/asm_functions.o
FFLAGS = -O0
# the name of the output executable
target = a
# all objects to generate
objects = $(BLD)/nrtype.o $(BLD)/nrutil.o $(BLD)/nr.o $(BLD)/mtypes.o $(BLD)/polint.o $(BLD)/polyin2.o $(BLD)/a.o
#
modules = $(BLD)/nrtype.mod $(BLD)/nrutil.mod $(BLD)/nr.mod $(BLD)/mtypes.mod
# set module dependencies
nrtype_mod = polint.o polyin2.o
nrutil_mod = polint.o polyin2.o
nr_mod = polin2.o
mtypes_mod = a.o
# set default target
default: $(target)
# add dependencies to the targets
$(nrtype_mod): nrtype.mod
$(nrutil_mod): nrutil.mod
$(nr_mod): nr.mod
$(mtypes_mod): mtypes.mod
# compile modules
$(BLD)/%.mod: $(SRC)/%.f90 $(BLD)/%.o
$(FC) -c $(FFLAGS) -o $# $<
# compile old fortran files
$(BLD)/%.o: $(SRC)/%.for
$(FC) -c $(FFLAGS) -o $# $<
# compile new fortran files
$(BLD)/%.o: $(SRC)/%.f90
$(FC) -c $(FFLAGS) -o $# $<
# link everything
$(target): $(objects)
$(FC) -o $# $(FFLAGS) $^ $(LDFLAGS)
# cleaning up
clean:
rm -f $(BLD)/$(objects) $(BLD)/$(modules)
clobber: clean
rm -f $(BLD)/$(target)
.PHONY: default clean clobber
While with this modification the build is successful, it is still not obeying orders. Specifically, *.mod files are being generated where the makefile is but I found in the meantime that it is gfortran who is disobedient and for mod files the output directory is controlled by some other switch. Nevertheless:
Other things that are not clear:
Should the lines look like:
$(BLD)/%.o: $(SRC)/%.for
$(FC) -c $(FFLAGS) -o $# $<
Or
%.o: %.f90
$(FC) -c $(FFLAGS) -o $(BLD)/$# $(SRC)/$<
or is it the same (i prefer the second one for readability).
Why the line such as
nrtype_mod = polint.o polyin2.o
do not need to be prepended by the paths? Or do they have to be (but it built correctly). This is where it is very confusing.
The maual would really benefit from some basic examples of a full makefile.

Answer 1

I think that one issue there are lines that refere e.g. $(BLD)/$(objects) which probably expands to BLD/nrtype.o nrutil.o nr.o mtypes.o polint.o polyin2.o a.o
You are exactly correct in your assessment here.
You can use a GNU make function like addprefix to fix this.
It would be useful to understand what part of the manual you had trouble understanding, so it can be improved.