Recursion and phony targets

[Martin Willers]

Hi,

I want to build a program that consists of several modules:

  toplevel/
    foo/
      Makefile
      foo.c
    bar/
      Makefile
      bar.c
    Makefile
    main.c

Each module would be compiled into a library, ie. in foo/,
a library libfoo.a would be created, and in bar/ a libbar.a.
The top-level Makefile would then compile main.c and link it
with all modules' libraries into the main program.

In order to make adding other modules very easy, I'd like
to use a top-level Makefile like this:

------------- Top-Level Makefile ------------------------------
MODULES := foo bar

MODULE_LIBS := $(patsubst %,-l%,$(MODULES))
MODULE_DIRS := $(patsubst %,-L%,$(MODULES))

vpath %.a $(patsubst %,%:,$(MODULES))

toplevel: main.o $(MODULES) $(MODULE_LIBS)
        $(CC) -o $@ main.o $(MODULE_DIRS) $(MODULE_LIBS)

.PHONY: $(MODULES)
$(MODULES):
        $(MAKE) -C $@
---------------------------------------------------------------

This basically does what I want; a new module can be added just
by appending to MODULES. The modules' Makefiles are very
straightforward, like:
  libfoo.a: foo.o
        ar -rs $@ $<


However, this re-links toplevel on each invokation of "make",
even if no files have been changed at all.
"make -d" gives me:
--------->8---------->8------------
  [...]
  Considering target file `toplevel'.
    [...]
    Considering target file `foo'.
     File `foo' does not exist.
     Finished prerequisites of target file `foo'.
    Must remake target `foo'.
  make -C foo
  [...]
  No need to remake target `libfoo.a'.
  [...]
    Successfully remade target file `foo'.
  [[dito for 'bar']]
    No need to remake target `-lfoo'; using VPATH name `foo/libfoo.a'.
    No need to remake target `-lbar'; using VPATH name `bar/libbar.a'.
   Finished prerequisites of target file `toplevel'.
   Prerequisite `main.o' is older than target `toplevel'.
   Prerequisite `foo' of target `toplevel' does not exist.
   Prerequisite `bar' of target `toplevel' does not exist.
   Prerequisite `foo/libfoo.a' is older than target `toplevel'.
   Prerequisite `bar/libbar.a' is older than target `toplevel'.
  Must remake target `toplevel'.
  cc -o toplevel main.o -Lfoo -Lbar -lfoo -lbar
--------->8---------->8------------
Even though it says it "Successfully remade target file `foo'", it then
concludes with "Prerequisite `foo' of target `toplevel' does not exist."
I suspect this is because "foo" and "bar" are phony targets, which are
never up-to-date? If so, how can I avoid setting "foo" and "bar" to be
phony?
I'm using GNU make 3.80.

-- 
 Martin