patsubst in GNU make returns nothing-CodePudding

I am trying to have my cpp source files and my object files in separate directories, so I'm trying to use Make's patsubst to replace the paths. After nothing was compiling, I made a phony target to simply print out the relevant variables and I discovered that patsubst was returning nothing. This doesn't make a lot of sense to me, as from what I can tell, targets that don't match the pattern should pass through unchanged, so even if my patterns are malformed I should still be seeing something get returned.

My variables are declared like this:

SRC_DIR := source/
SRC := $(wildcard $(SRC_DIR)*.cpp)

OBJ_DIR := obj/
OBJ := $(PATSUBST $(SRC_DIR)%.cpp, $(OBJ_DIR)%.o, $(SRC))

My phony target that just prints out the above variables looks like this:

.PHONY: print
print:
    @echo $(SRC_DIR)
    @echo $(SRC)
    @echo $(OBJ_DIR)
    @echo $(OBJ)

and the result of running make print is this:

user@athena:~/proj$ make print
source/
source/main.cpp source/util.cpp
obj/

user@athena:~/proj$

Other things I've tried

I've tried SRC_DIR and OBJ_DIR having trailing slashes and not having them, I've tried $(SRC:$(SRC_DIR)%.cpp=$(OBJ_DIR)%.o) which did work, and $(OBJ) would have both main.o and util.o, but then my static rule ($(OBJ): $(OBJ_DIR)%.o : $(SRC_DIR)%.cpp) would only trigger for main.cpp, and wouldn't try to compile util.cpp, which I'm considering a completely separate issue?

I hope I'm just doing something dumb and there's a simple fix, but I've been racking my brain and I can't see what would be causing patsubst to just be blank. Any help would be greatly appreciated.

CodePudding user response：

Like (virtually) all programming languages, makefiles are case-sensitive. $(FOO) is not the same as $(foo), and $(PATSUBST ...) is not the same as $(patsubst ...).

In fact, $(PATSUBST ...) is nothing and expands to the empty string.