RE: how to do recursive "subsystem" make properly?

[Paul Smith]

On Tue, 2009-11-03 at 15:32 -0800, Mark Galeck (CW) wrote:
> top makefile is
> 
> foobar: subdir/sentinel
>               echo making foobar
>               touch foobar
> subdir/sentinel:
>               make -C subdir
> 
> subdir makefile is
> 
> all:
> #     touch sentinel

Well, since you have the touch commented out it will never do anything.
Also, be sure to ALWAYS use the variable $(MAKE), never just "make",
when running sub-makes

> Well, what am I doing wrong - because, with this foobar always gets
> rebuilt.  Probably because make attempted to remake subdir/sentinel as
> "target", and even though that file did not get updated, make
> remembers that it remade the target.

The sentinel file has to be created: it has to exist.  A non-existent
file is always considered out of date.  If it exists but then is not
updated, THEN it is not considered out of date.

> Also,  order-only don't work for me either:  
> 
> Top makefile is
> 
> foobar: | subsystem
>               echo making foobar
>               touch foobar    
> subsystem:
>               make -C subdir

Here foobar has to depend on the thing that is getting built
(subsystem/subfile), so the foobar knows when it's out of date.

You want this:

        foobar: subsystem/subfile
                echo making foobar
                touch $@

        subdsystem/subfile: | subsystem

        .PHONY: subsystem
        subsystem:
                $(MAKE) -C $@

Then keep your subsystem makefile the same:

> all: subfile
> subfile: 
>               touch subfile
> 
> This time, foobar never gets rebuilt.