Makefile pattern with replacement doesn't capture non-local sources

I've got a project that has sources in the local subdirectories and also sources in the SDK, which is in the different location.

All object files need to be built in the project's build directory tree, and the sources from the SDK need to be placed in the $(OUTPUT_DIR)/sdk. And I need a generic way to deal with all the sources with GNU-Make.

I have this function to match a source path to an object path:

source_to_object_path = $(OUTPUT_DIR)/$(subst $(SDK_PATH),sdk,$1)

So this successfully matches, for example, local app.c to $(OUTPUT_DIR)/app.o, and also $(SDK_PATH)/timer/timer.c to $(OUTPUT_DIR)/sdk/timer/timer.o.

But I'm struggling to write a pattern that would capture all .c sources to .o targets.

Either this (1):

$(call source_to_object_path,%).o : %.c

or this (2):

$(call source_to_object_path,$(<D))%.o: %.c

only capture the local sources, but there's "Nothing to be done" for the sources in the SDK. Even though source_to_object_path applied to the sources in the SDK gives the correct path to the objects, and the objects list is built correctly. All sources paths are also in VPATH.

Therefore I need another pattern (3) for the sources in the SDK which works successfully:

$(call source_to_object_path,$(SDK_PATH))/%.o: $(SDK_PATH)/%.c

Why doesn't the pattern without specifying SDK_PATH work for these? Shouldn't source_to_object_path care of this, and how is it possible to use one pattern instead of two here?

The answer is very simple: variable expansion of targets and prerequisites happens immediately when the makefile is being parsed. It doesn't happen later, when make is searching for implicit rules. So when make sees the line:

$(call source_to_object_path,%).o : %.c

you're calling your function with the literal string %. Since that doesn't start with $(SDK_PATH), this is a no-op and results in just %.o.

You can read more about when variables are expanded in the GNU Make manual: https://www.gnu.org/software/make/manual/html_node/Reading-Makefiles.html

There you will see that this:

$(call source_to_object_path,$(<D))%.o: %.c

is even less helpful because automatic variables like $(<D) etc. are expanded only within the recipe. So this variable is the empty string when it is expanded.

If you want to put the outputs in two different directories you need two pattern rules. First, the one for the sdk targets:

$(OUTPUT_DIR)/sdk/%.o : $(SDK_PATH)/%.c
        ...

Second, one for the rest of the targets:

$(OUTPUT_DIR)/%.o : %.c
        ...