In gnu make, can the prerequisites in a static pat

问题:

Our make file compiles .c source files with a static pattern rule like this:

OBJECTS = foo.o bar.o baz.o

$(OBJECTS): %.o: %.c
    $(CC) $< $(C_OPTIONS) -c -o $@

I need to change one of the .c files to an Objective-C .m file. Invoking the compiler is the same for both source types, so I'd like to use the same rule and just tweak it to be more flexible. I'd rather not change the OPTIONS variable because it's also used for the linking step, etc.

Is there a way to make the rule above more flexible to accommodate both .c and .m files?

Thanks

回答1:

We can add this either-or behavior to the list of things Make should be able to do easily, but isn't. Here's a way to do it, using "eval" to create a seperate rule for each object.

define RULE_template
$(1): $(wildcard $(basename $(1)).[cm])
endef

OBJECTS = foo.o bar.o baz.o

$(foreach obj,$(OBJECTS),$(eval $(call RULE_template,$(obj))))

$(OBJECTS):
    $(CC) $< $(C_OPTIONS) -c -o $@ 

Note that this depends on the source files already existing before you run Make (foo.c or foo.m, but not both). If you're generating those sources in the same step, this won't work.

Here's a less clever, more robust method.

CPP_OBJECTS = foo.o bar.o
OBJECTIVE_OBJECTS = baz.o
OBJECTS = $(CPP_OBJECTS) $(OBJECTIVE_OBJECTS)

$(CPP_OBJECTS): %.o: %.c 

$(OBJECTIVE_OBJECTS): %.o: %.m 

$(OBJECTS):
    $(CC) $< $(C_OPTIONS) -c -o $@ 

EDIT: corrected OBJECTS assignment, thanks to Jonathan Leffler.

回答2:

Not really just copy to

$(OBJECTS): %.o: %.m
  $(CC) $< $(C_OPTIONS) -c -o $@

回答3:

The call to the same compiler is just a happy occasion. Normally you do not compile objective-c code with $(CC). That just feels strange.

But since you go in a harsh way, I won't post do-it-right solution, where you separate objective-C targets from C targets into two different $(OBJECTS)-like variables and make two rules (which you should really do). Too boring. Instead, take a hack!

OBJC_FILES:=$(subst $(wildcard *.m))

real_name = `(test -h $(1) && readlink $(1) ) || echo $(1)`

$(OBJECTS): %.o: %.c
  $(GCC) $< $(C_OPTIONS) -c -o $(call real_name,$@)

$(OBJC_FILES): %.c: %.m
  ln -s $< $@

And God help those who maintains it!

Btw, this obviously won't work if your m-files are generated.