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under the explained algorithm of re-reading timestamps, how to deal with

From: Mark Galeck (CW)
Subject: under the explained algorithm of re-reading timestamps, how to deal with this common situation?
Date: Tue, 3 Apr 2012 06:24:14 -0700


I think I understand finally how make works, thanks to Paul's excellent 
explanation (being better than the manual's :) )

So then this does not work, even with the empty recipe, it won't work, because 
make figures out that "foobar.h" could not possibly get updated.  I touch 
foobar.y, but foobar does not get updated.

This is a problem, because this represents a common scheme, I think.  How do 
you deal with this situation Paul (so that it "works").  

foobar is a goal that depends on dynamically generated prerequisites such as 
subdir/foobar.h.  At first build after clean, the prerequisites and the rules 
to depend on them do not exist, so we need something else to "bootstrap" on, 
that's where target "resources" is needed. resources, creates the foobar.h 
dynamically in a subdirectory.  The rule for foobar then, creates an additional 
dynamically generated dependency (like explained in the manual) on 
subdir/foobar.h - this dependency is used in the next build.  I simplified that 
step for this example, so that rule is there to begin with, but I am just 
explaining the "origin" of what I am trying to do - Paul always asks for that.  

So here is the smallest example, with the origin explained as above:

In Makefile:   

.PHONY: resources
        $(MAKE) -C subdir foobar.h

subdir/foobar.h: ;  # does not matter

foobar: subdir/foobar.h | resources
        touch $@

In subdir/Makefile:

foobar.h: foobar.y
        touch $@

So, what is the "canonical" way to handle this common situation?


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