I've run across an interesting Make rule using static patterns,
define/eval/call, and dependencies. I think I understand how it works,
but I'd like to make sure. The thing that throws me for a loop is the
lack of a prerequisite pattern.
This is using make Cygwin 3.80, and it all works fine, I'm just trying
to understand *how* this works fine.
[ ----------------------------------]
SOURCEFILES = foo.cpp bar.cpp
OBJS = foo.o bar.o
OBJ_FOLDER = temp/obj
$(OBJS): %.o:
$(CC) .... -MD ... -c $< -o $@
define RuleCppToObj
$$(OBJ_FOLDER)\$((subst .cpp,.o,$(1)): $(1)
endef
$(foreach FILE,$(SOURCEFILES),$(eval $(call RuleCppToObj,$(FILE))))
DEPENDENCIES = $(SOURCEFILES:.cpp=.d)
DEP_PATH = depend
DEP_FILES = $(addprefix $(DEP_PATH),$(DEPENDENCIES))
-include $(DEP_FILES)
[ ----------------------------------]
Sample dependency file: foo.d
foo.o: foo.cpp foo.h
[ ----------------------------------]
Near as I can figure, and I'll concentrate only on building foo.o:
The static pattern rule defines a rule which builds anything named foo
using the specified commands. If there wasn't anything else, since we
don't have any prerequisites, if the file exists, once that file exists,
it won't build anything. So we have to add...
The $(foreach...) command creates a rule which builds foo.o from
foo.cpp. Now this is where I have a question. Does the fact that we
have a foo.o target through the static pattern mean that it simply
appends the foo.cpp dependency to the (initially empty) list of
prerequisites? What role does the %o have in this then?
Next it then goes through and reads in any existing dependency files for
the project, which contains the actual list of prerequisites that are
used to build the target. Again, they would just be appended to the
list of prerequisites.
The $(foreach ...) command is then the default command in the case that
the dependencies don't exist.
*whew*
Do I have that right? I don't know why, but that lack of prerequisites
in the static pattern just makes my brain grind to a halt. I also don't
understand why when I take the %o: out of the static pattern rule, the
whole thing falls apart. Does the static pattern actually define
something akin to a macro on how to build each of the targets
independently?
Thanks in advance...
Matt Smith
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