depending on directories in make [duplicate]

Question

This is a followup to my earlier question: SO 4403861 because the suggested solutions broke the dependencies, making the makefile useless. I can't figure out why.

I am using gnu make 3.82 I have a rule that works if the obj directory has been created:

objdir:=../obj
$(objdir)/%.o: %.C
    $(COMPILE) -MM -MT$(objdir)/$(notdir $@) $< -o $(DEPDIR)/$(notdir $(basename $<).d )
    $(COMPILE) -o $(objdir)/$(notdir $@ ) -c $<

However, if the obj directory isn't there, make fails. I wanted make to automatically create ../obj on demand, so I added what I thought was very simple:

$(objdir)/%.o: %.C $(objdir)
    $(COMPILE) -MM -MT$(objdir)/$(notdir $@) $< -o $(DEPDIR)/$(notdir $(basename $<).d )
    $(COMPILE) -o $(objdir)/$(notdir $@ ) -c $<

$(objdir):
   if [ ! -d $(objdir) ] ; then mkdir $(objdir) ; fi

When I do so, make always forces the compile, every time. Why? The mkdir should not happen unless there is no directory? Why are dependencies destroyed by this simple change?

Answer 1

You also can try with Order-only prerequisites.

There is a similar example, of your question, available.

 OBJDIR := objdir
 OBJS := $(addprefix $(OBJDIR)/,foo.o bar.o baz.o)
 
 $(OBJDIR)/%.o : %.c
         $(COMPILE.c) $(OUTPUT_OPTION) $<
 
 all: $(OBJS)
 
 $(OBJS): | $(OBJDIR)
 
 $(OBJDIR):
         mkdir $(OBJDIR)

Answer 2

As others have said, the problem is that a directory is considered "changed" whenever directory members are added or removed, so make sees your output directory changing all the time, and reruns all the compilations which you have told it depend on the output directory.

As an alternative to the workarounds described by others, recent GNU make versions have support for "order-only prerequisites". The description of order-only prerequisites in the manual includes an example of how to use them for directory creation.