Why does Make re-link when I call make in a subdirectory?

Question

I'm having trouble understanding something in make. I have a main Makefile calling another Makefile in a subdirectory. I would like make not to re-link when nothing has changed. But it seems that the sub-directory make, wherever something was done or not, triggers a new linking.

Here is the main Makefile

NAME = app

CC = gcc

SRC = app.c

OBJ = $(SRC:.c=.o)

all: $(NAME)

$(NAME): get-libaf $(OBJ) app.h
    $(CC) -L./libaf/ -laf -o $@ $(OBJ)

%.o: %.c
    $(CC) -c $< -o $@

get-libaf:
    $(MAKE) -C ./libaf/

And here is my library in the subdirectory ./libaf/ Makefile

NAME = libaf.a

CC = gcc

SRC = af_app_helper.c af_app_argv_parser.c af_app_memory.c

OBJ = $(SRC:.c=.o)

all: $(NAME)

$(NAME): libaf.h
    $(CC) -c $(SRC)
    ar rc $(NAME) $(OBJ)
    ranlib $(NAME)

When I run the main Makefile, this is the output:

user@machine $ make
make -C ./libaf/
make[1]: Nothing to be done for `all'.
gcc -L./libaf/ -laf -o app app.o
user@machine $ make
make -C ./libaf/
make[1]: Nothing to be done for `all'.
gcc -L./libaf/ -laf -o app app.o

but when I remove the libaf dependency (get-libaf), it only says:

user@machine $ make
make: Nothing to be done for `all'.

...which is what I would like to have when I haven't changed a thing between make commands. I even checked the timestamps of all the files and none was modified/edited/touched. Same with the library.

I understand that make re-links when there is a change in a source file or dependency. Here, I don't understand why the "sub-make" is doing this.

Could someone explain why make re-links and how this can be prevented. I want to keep the get-libaf dependency though, so I can update it and run make only once for the whole project.

Thanks for your help !

Answer 1

I think it will work to have $(NAME) depend on the actual file libaf/libaf.a instead of a phony target (get-libaf) if you add a rule for libaf/libaf.a (in the top-level makefile) that invokes make -C libaf.

(The issue is that there's no get-libaf file with a timestamp for make to compare against, so it has to assume the result is "modified" after each run.)