File: make.info,  Node: Prerequisite Types,  Next: Wildcards,  Prev: Rule Syntax,  Up: Rules

4.3 Types of Prerequisites
==========================

There are two different types of prerequisites understood by GNU 'make':
normal prerequisites, 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 started.  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target is considered out-of-date and must be rebuilt.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 you may want to ensure that a prerequisite is built
before a target, but _without_ forcing the target to be updated if the
prerequisite is updated.  "Order-only" prerequisites are used to create
this type of relationship.  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).

   Order-only prerequisites are never checked when determining if the
target is out of date; even order-only prerequisites marked as phony
(*note Phony Targets::) will not cause the target to be rebuilt.

   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.