Re: The explanation of order-only prerequisites is not clear

[Paul Smith]

On Tue, 2010-09-07 at 10:12 +0430, ali hagigat wrote:
> 1) a target is updated when it does not exist or its last modification
> time is older than its prerequisites. For order-only prerequisites,
> the same rule applies? means if a target does not exist or its last
> modification time is older than its order-only prerequisites will be
> updated(the rule of the target will be executed)?

Obviously not, since that would make order-only prerequisites exactly
the same as normal prerequisites.  The documentation says (in the
section you did not quote):

        [...] 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        target, then the target is considered out-of-date and must be rebuilt.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.

This answers exactly your question.

> 2) In the example of the make manual, there are two rules for the
> prerequisites of 'all' target, one pattern rule at the beginning and
> then $(OBJS): | $(OBJDIR). Will these rules be executed in order and
> both will be considered? or the explicit rule will be evaluated first?

The second one does not have a recipe.  It just adds prerequisites to
existing rules.

There is only one recipe defined for each .o, so that's the recipe that
will be executed.  It's the same as writing:

        foo.o: foo.c
                $(CC) $(CFLAGS) -o $@ -c $<
        foo.o: foo.h
        foo.o: bar.h

> 3) After reading the manual one can not understand whether files
> objdir/foo.o, objdir/bar.o and objdir/baz.o will be updated(if objdir
> directory already does not exist) or the command $(COMPILE.c)
> $(OUTPUT_OPTION) $< will be executed at last?

Sorry you didn't understand.  See above.

> 4) If objdir exists, what that makefile will do?

The same thing as make would do for any other target.  There's nothing
special here about the TARGET objdir.

> 5) if objdir does not exist but object files exist, will  $(OBJDIR):
> ;mkdir $(OBJDIR) be executed?

How can the object files exist if the directory doesn't exist?  The
files are IN the directory.  You're just being ridiculous now.

-- 
-------------------------------------------------------------------------------
 Paul D. Smith <address@hidden>          Find some GNU make tips at:
 http://www.gnu.org                      http://make.mad-scientist.net
 "Please remain calm...I may be mad, but I am a professional." --Mad Scientist