Re: prerequisites alter choice of pattern match rules?

[Sebastian Pipping]

On 25.02.2013 14:57, Brian J. Murrell wrote:
> Hi,
> 
> I have run into something I find strange with GNU Make 3.81 (yes, I know
> 3.82 is latest but since it's not in EL6, my target platform, I cannot
> depend on it's features).
> 
> I would think that given the following set of rules:
> 
> /tmp/%.foo: %.foo
>       echo "foo rule 1"
>       touch $@
> 
> %.foo:
>       echo "foo rule 2"
>       touch $@
> 
> /tmp/%.bar:
>       make $*.bar
>       echo "bar rule 1"
>       touch $@
> 
> %.bar:
>       echo "bar rule 2"
>       touch $@
> 
> [..]
>
> $ make /tmp/a.foo
> echo "foo rule 2"
> foo rule 2
> touch /tmp/a.foo
> $ make /tmp/a.bar
> make a.bar
> make[1]: Entering directory `/home/brian/chroma/deps/lustre/test'
> echo "bar rule 2"
> bar rule 2
> touch a.bar
> make[1]: Leaving directory `/home/brian/chroma/deps/lustre/test'
> echo "bar rule 1"
> bar rule 1
> touch /tmp/a.bar
> 
> It would seem that for the /tmp/a.foo, the "/tmp/%.foo" pattern rule is
> not being chosen but for /tmp/a.bar" the "/tmp/%.bar" pattern rule is
> being chosen.  The only difference in those choices I can see is that
> the "/tmp/%.foo" pattern rule has a prerequisite and the "/tmp/%.bar"
> one doesn't.

Confirming the same behavior for make 3.82.


It seems like normally GNU make walks the path of the most specific
match in case of ambiguities.  I wonder if that's specified/documented
anywhere.  For this Makefile

  xxx%:
        @echo '$@ --> xxx%'

  x%:
        @echo '$@ --> x%'

  xx%:
        @echo '$@ --> xx%'

the most specific rule is chosen one might expect:

  # make x1 xx2 xxx3 xxxx4
  x1 --> x%
  xx2 --> xx%
  xxx3 --> xxx%
  xxxx4 --> xxx%


Side notes on the above Makefile:

 - "make $*.bar" in there does not seem to be relevant.
   After removing that line, the file still reproduces your case.

 - "make $*.bar" should be "$(MAKE) $*.bar", use $(MAKE)
   to invoke make to not run into parallel build issues

 - To reduce output noise, you can use lines like

     @echo "foo rule 1"
     ^
   instead of

     echo "foo rule 1"


> So the question is, why does a prerequisite change the behavior of
> pattern match rules?

Good question.

Best,



Sebastian