How to have a sometimes empty dependency in makefiles?

Question

I have the following rule:

EXECS       = $(sort $(patsubst %.cpp,%$(EXESUFFIX), $(patsubst %.c,%$(EXESUFFIX), $(filter-out $(IGNORESRCS), $(EXECSRCS)))))
SRCS        = $(sort $(filter-out $(EXECSRCS), $(filter-out $(IGNORESRCS), $(wildcard *.c) $(wildcard *.cpp) $(foreach DIR,$(SUBDIRS),$(wildcard $(DIR)/*.cpp) $(wildcard $(DIR)/*.c) ) ))) 
#OBJS       = $(addprefix  $(OBJDIR), $(patsubst %.cpp,%$(OBJSUFFIX), $(patsubst %.c,%$(OBJSUFFIX), $(SRCS))))
OBJS        = $(patsubst %.cpp,%$(OBJSUFFIX), $(patsubst %.c,%$(OBJSUFFIX), $(SRCS)))
RESOURCE_SRCS= $(sort $(filter-out $(IGNORESRCS), $(wildcard *.rc) $(foreach DIR,$(SUBDIRS),$(wildcard $(DIR)/*.rc) ) ))
RESOURCES   = $(patsubst %.rc,%$(OBJSUFFIX), $(RESOURCE_SRCS)) 

%$(EXESUFFIX) : %.cpp $(LIBS) $(RESOURCES)
    $(CXX) $(DEFINES) $(CFLAGS) $(INCLUDES) $(LIBPATH) -o $(BINDIR)/$* $< $(RESOURCES) $(LIBINCLUDES)

The problem is that $(RESOURCES) doesnt exist for all platforms. The %$(EXESUFFIX) : %.cpp rule doesnt run, instead it tries to run g++ exec.cpp -o exec which as far as I can tell isnt a rule that I declared anywhere.

How do I get the rule to still build despite the fact that it is empty (and build the resources if it is not empty)?

Answer 1

If the variable is empty it has no effect on the rule. It should just work as written. What is the actual error you're seeing?

ETA:

Your question is very unclear in what, exactly, you mean by $(RESOURCES) doesn't exist. My answer was assuming you meant that the variable was empty. But given your comment below about how the makefile behaves, I now suspect what you mean is that the variable is still set to a list of files, but that those files are not present.

Because they're not there, and make doesn't know how to build them, make decides that this pattern rule cannot be used at all and it chooses a different rule.

If you want these files to only have any impact if they exist, then you can use the $(wildcard ...) function to expand only to those files that exist:

%$(EXESUFFIX) : %.cpp $(LIBS) $(wildcard $(RESOURCES))
        $(CXX) ...

One critical point here: the contents of $(RESOURCES) MUST be source files. They cannot be derived files (files that are supposed to be created by make). If they are derived, the situation is far more complex.