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copied!<p>With GNU Make 3.82 it is possible to <strong><code>define</code></strong> <em>canned recipes</em> which – after being made parametric – might be used as templates for the double/triple objects' build specifications. The templates can be then instantiated with Make's unique <code>$(call var,par,...)</code> function and evaluated with the very special <code>$(eval code)</code>, available since version 3.8. This trick allows us to avoid a lot of boilerplate code and enables Makefile to automatically adapt to forthcoming project growth.</p> <p>While having all possible module-building rules available, we are one step away from the goal; this is a quite tricky step however. We need to rebuild each build's <em>prerequisites</em> to explicitly expose each inter-module dependency. It is done by replacing the single module objects with the special cases arising from the ambient neighbourhood (e.g., a superficial <code>mod_red.o mod_green.o</code> is substituted with an explicit <code>mod_red+G.o mod_green+R.o</code>). The substitution is handled by the <code>discover</code> macro anclosing each prerequisite list and driven by the <code>xdepend</code> global variable, which specifies the cross dependencies – in practice, only a few modules will depend on one another.</p> <pre><code># Cross dependency specification (dependent+dependency) xdepend := blue+red red+blue # Test for string equality and set membership equals = $(if $(subst $1,,$2),$(empty),true) exists = $(if $(filter $1,$2),true,$(empty)) # Extract of the first and second member of a dependency token pred = $(firstword $(subst +, ,$1)) succ = $(lastword $(subst +, ,$1)) # Rebuild prerequisites to expose modules' interdependencies discover = $(strip $(foreach mod,$(basename $1),\ $(subst $(space),,$(obj)/$(mod)\ $(foreach dep,$(xdepend),\ $(and\ $(call equals,$(call pred,$(dep)),$(mod)),\ $(call exists,$(call succ,$(dep)),$(basename $1)),\ $(lnk)$(call succ,$(dep))\ )\ ).o)\ )) # Create compilation rules for interdependent module objects define rule $(obj)/$1$(lnk)$2.o: $(src)/$1.c $(inc)/$1.h $(inc)/$2.h | $(obj) $(CC) $(CFLAGS) -include $(inc)/$2.h -c $(src)/$1.c -o $(obj)/$1$(lnk)$2.o endef $(foreach dep,$(xdepend),\ $(eval $(call rule,$(call pred,$(dep)),$(call succ,$(dep))))\ ) </code></pre> <p>Armed with the above definitions, we can now build our project in the following way:</p> <pre><code># Rules for Magenta Build and intermediate objects magenta: $(call discover, magenta.o mod_red.o mod_blue.o) $(CC) $(CFLAGS) $^ -o $@ $(LDFLAGS) $(obj)/%.o: $(src)/%.c $(inc)/%.h | $(obj) $(CC) $(CFLAGS) -c -o $@ $< $(obj): mkdir $(obj) </code></pre> <p>For further clarification and the most recent knowledge – read <a href="http://www.gnu.org/software/make/manual/make.pdf" rel="nofollow">GNU Make Manual</a>.</p>

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