depending on directories in make

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)