Makefile：规则依赖于目标目录

Question

I am trying to create a Makefile to create object files in a directory based on the source file. For example I have DirA/file.c and DirB/file.c and I want to create a recipe for creating DirA/file.o and DirB/file.o .

rootdir/
|-- Makefile
|-- DirA
|    |-- source.c
|    +-- source.o
+-- DirB
    |-- file.c
    +-- file.o

I would assume the recipe would be similar to the following:

MAKEFLAGS += --no-builtin-rules
DirA_Files=$(wildcard DirA/*.c)
DirB_Files=$(wildcard DirB/*.c)

.PHONY: all a_files b_files
all: a_files b_files
    @echo Done with all!
    @echo built $(patsubst %.c,%.o,$(DirA_Files)) $(patsubst %.c,%.o,$(DirB_Files))

a_files: $(patsubst %.c,%.o,$(DirA_Files))
b_files: $(patsubst %.c,%.o,$(DirB_Files))

DirA/%.o DirB/%.o : DirA/%.c DirB/%.c
    # Complex recipe that I'd rather not state twice in the Makefile
    @echo "Building $@ because $? changed"
    @touch $@

But I do not want the object files in DirA to be dependent on the source files of DirB , which is what the above recipe implies.

I have also tried

DirA/%.o : DirA/%.c
DirB/%.o : DirB/%.c
DirA/%.o DirB/%.o :
    # Complex recipe that I'd rather not state twice in the Makefile
    @echo "Building $@ because $? changed"
    @touch $@

But then the $? variable is always blank.

How could I create a single recipe that would also allow me to build a_files and b_files independently

Answer 1

You can't combine pattern rules quite that easily. But this situation is what defined recipes were made for:

define complex-rule
# Complex recipe that I'd rather not state twice in the Makefile                                                            
@echo "Building $@ because $? changed"
@touch $@
endef

DirA/%.o : DirA/%.c
        $(complex-rule)

DirB/%.o : DirB/%.c
        $(complex-rule)

Answer 2

related info:

DirA_Files := $(wildcard DirA/*.c)
DirB_Files := $(wildcard DirB/*.c)

$(info DirA_Files: $(DirA_Files))
$(info DirB_Files: $(DirB_Files))

aobj := $(patsubst %.c,%.o,$(DirA_Files))
bobj := $(patsubst %.c,%.o,$(DirB_Files))

$(info aobj: $(aobj))
$(info bobj: $(bobj))

all: $(aobj) $(bobj)
        @echo Done with all!