How do you call a rule that involves `%` from another rule in a Makefile?

Question

I have the following makefile snippet to compile my C++ project.

obj/%.o: src/%.cpp
    $(CXX) $(CFLAGS) -c $< -o $@

Now I want to link the .o files. But I want to be able to just call the rule name of the link, like make build/main, in order to compile AND link.

Neither this:

build/main: $(wildcard obj/*.o)
    $(CXX) $^ -o $@

works, as it only links and does not compile, as I would expect by this answer;

nor this:

build/main: obj/%.o
    $(CXX) $^ -o $@

with the error:

No rule to make target 'obj/%.o', needed by 'build/main'. Stop.

even though it is needed.

The `build/main: $(wildcard obj/*.o)` won't work, because the wildcard will be expanded when the rule is first read (before the .o's are built, and before they exist on the filesystem, so it would expand to empty). — blackghost, Apr 10 '17 at 15:54

score 3 · Accepted Answer · answered Apr 10 '17 at 15:09

3

SOURCES := $(wildcard src/*.cpp)
OBJECTS := $(patsubst src/%.cpp, obj/%.o, $(SOURCES))

build/main: $(OBJECTS)
    $(CXX) $^ -o $@

answered Apr 10 '17 at 15:09

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Does `SOURCES` expand to `src/main.cpp src/other.cpp` or `main.cpp other.cpp`? – Post Self Apr 11 '17 at 13:27
1

@kim366: You can see for yourself: put `$(info it expands to $(SOURCES))` after the assignment. – Beta Apr 11 '17 at 22:35

How do you call a rule that involves `%` from another rule in a Makefile?

1 Answers1