Question

(this is similar to GNU make: Execute target but take dependency from file but slightly different).

When I try to force to build a target, make automatically thinks that this target is out of date and forces a run of all targets which depend on it.

In my case, the target is a recursive make call which does a lot of work and might just return with "nothing to be done":

 .PHONY: intermediate.dat
 intermediate.dat:
      $(MAKE) expensive_chain_which_finally_creates_intermediate.dat

 step1.dat: intermediate.dat
      sleep 10
 
 step2.dat: step1.dat
      sleep 15
 
 step3.dat: step2.dat
      sleep 10

 all: step3.dat
      sleep 5

In this case, "make all" runs for 40 seconds although intermediate.dat might not have changed (recursive make returned "nothing to be done"). However, if the recursive make updated intermediate.dat, the target shall be out of date.

Is there really no way to do this?

Answer 1

Make intermediate.dat depend on a phony target instead of being phony itself.

 .PHONY : always-remake

 intermediate.dat : always-remake

IIRC, the last time I solved the problem, the .PHONY didn’t work as intended and I used:

 always-remake :
     @true

instead. However, I can’t recall why, so try the .PHONY first.

The problem with making intermediate.dat itself phony is that make never checks the existence/date of a phony file, which is behaviour that you want. You only need to trigger the rebuild rule, which is done by a prerequisite that is out of date; a phony prerequisite is always out of date, so it does the job.