Multiple Level of Dependencies in a Makefile - makefile

Question

I am very new to makefiles. I have been able to write the script shown below by copying different examples found online. If I am doing something wrong, or not conventional, please point it out.
This is my (working) Makefile for a fortran code I have. The main program stored in main.f08 calls a module stored in file1.f08:
FC = gfortran
SRCS: main.f08
OBJS: $(SRCS:.f08=.o)
EXEC: $(SRCS:.f08=)
all: $(EXEC)
file1.o file1.mod: file1.f08
$(FC) -c $<
touch $*.o $*.mod
main.o: file1.mod
%.o: %.f08
$(FC) $(FFLAGS) -c $<
main: file1.o
%: %.o
$(FC) -o MainExe $^
clean:
rm -f MainExe *.o *.mod
I have tried to extrapolate that technique to write a makefile which has a ladder of dependencies. For example, main.f08 would be calling the module file1.f08, which in turn would be calling the module file2.f08. Here is what I have so far:
FC = gfortran
SRCS: main.f08
OBJS: $(SRCS:.f08=.o)
EXEC: $(SRCS:.f08=)
all: $(EXEC)
file2.o file2.mod: file2.f08
$(FC) -c $<
touch $*.o $*.mod
file1.o: file2.mod
file1.o file1.mod: file1.f08
$(FC) -c $<
touch $*.o $*.mod
main.o: file2.mod file1.mod
%.o: %.f08
$(FC) $(FFLAGS) -c $<
main: file1.o
%: %.o
$(FC) -o MainExe $^
clean:
rm -f MainExe *.o *.mod
The command line output consist of a few line showing that the code starts compiling the two modules and the main program and then several lines of errors that look like this:
Undefined symbols for architecture x86_64:
"___brownian_MOD_calcb", referenced from:
___integral_MOD_calcint in Integral.o

Answer 1

I've been working on this and I actually have figured it out.
The main executable should be linked to the module's object files in the correct order (an object file should not precede its dependency) followed by its object file, main.o. The module's object file should be linked to their dependency (if any) and their .f08 file. Finally, the touch command makes sure that the .mod files are up to dates with the .o files. Here is the corrected code showed in its simplest version:
FC = gfortran
all: main
main : file2.o file1.o main.o
$(FC) -o executable file2.o file1.o main.o
main.o: main.f08
$(FC) -c main.f08
file2.o : file2.f08
$(FC) -c file2.f08
touch file2.o file2.mod
file1.o : file2.o file1.f08
$(FC) -c file1.f08
touch file1.o file1.mod
A more advanced version of the makefile is shown below:
FC = gfortran
SRCS = main.f08
SOBJ = $(SRCS:.f08=.o)
EXEC = #(SRCS:.f08=)
FILE = file2.f08 file1.f08
FOBJ = $(SRCS:.f08=.o)
all: $(EXEC)
$(EXEC): $(FOBJ) $(SOBJ)
$(FC) -o executable $^
%.o: %.f08
$(FC) -c $<
touch $*.o $*.mod