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make: Prerequisite Types

 
 4.3 Types of Prerequisites
 ==========================
 
 There are actually two different types of prerequisites understood by
 GNU 'make': normal prerequisites such as described in the previous
 section, and "order-only" prerequisites.  A normal prerequisite makes
 two statements: first, it imposes an order in which recipes will be
 invoked: the recipes for all prerequisites of a target will be completed
 before the recipe for the target is run.  Second, it imposes a
 dependency relationship: if any prerequisite is newer than the target,
 then the target is considered out-of-date and must be rebuilt.
 
    Normally, this is exactly what you want: if a target's prerequisite
 is updated, then the target should also be updated.
 
    Occasionally, however, you have a situation where you want to impose
 a specific ordering on the rules to be invoked _without_ forcing the
 target to be updated if one of those rules is executed.  In that case,
 you want to define "order-only" prerequisites.  Order-only prerequisites
 can be specified by placing a pipe symbol ('|') in the prerequisites
 list: any prerequisites to the left of the pipe symbol are normal; any
 prerequisites to the right are order-only:
 
      TARGETS : NORMAL-PREREQUISITES | ORDER-ONLY-PREREQUISITES
 
    The normal prerequisites section may of course be empty.  Also, you
 may still declare multiple lines of prerequisites for the same target:
 they are appended appropriately (normal prerequisites are appended to
 the list of normal prerequisites; order-only prerequisites are appended
 to the list of order-only prerequisites).  Note that if you declare the
 same file to be both a normal and an order-only prerequisite, the normal
 prerequisite takes precedence (since they have a strict superset of the
 behavior of an order-only prerequisite).
 
    Consider an example where your targets are to be placed in a separate
 directory, and that directory might not exist before 'make' is run.  In
 this situation, you want the directory to be created before any targets
 are placed into it but, because the timestamps on directories change
 whenever a file is added, removed, or renamed, we certainly don't want
 to rebuild all the targets whenever the directory's timestamp changes.
 One way to manage this is with order-only prerequisites: make the
 directory an order-only prerequisite on all the targets:
 
      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)
 
    Now the rule to create the 'objdir' directory will be run, if needed,
 before any '.o' is built, but no '.o' will be built because the 'objdir'
 directory timestamp changed.
 

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