Re: implicit rules

[Boris Kolpackov]

I will start from the second example:

Noel Yap <address@hidden> writes:

> But if it's changed to:
> 
> .PHONY: all
> all: aoeu/aoeu.mk
> 
> %.mk: %.m
>       cp $(<) $(@)
> 
> %/.:
>       mkdir -p $(@)
> 
> aoeu/aoeu.mk: aoeu.m | aoeu/.
> 
> 
> The output becomes:
> 
> $ gmake
> mkdir -p aoeu/.
> 

If you run this with 'make -r -d' you will see that make 
didn't find any rules for 'aoeu/aoeu.mk'. The reason why
make didn't complain about it is because make has this
habit of imagining that targets "somehow" get updated.

If I run my -bk-patched make with --no-implicit-phony flag
I get the following:

make: *** No rule to make target `aoeu/aoeu.mk', needed by `all'.  Stop.

See

http://mail.gnu.org/archive/html/help-make/2004-02/msg00028.html

for more information.

Now let's go to your first example which is quite interesting:

> .PHONY: all
> all: aoeu/aoeu.mk
> 
> %.mk: %.mk
>       cp $(<) $(@)
> 
> %/.:
>       mkdir -p $(@)
> 
> aoeu/aoeu.mk: aoeu.mk | aoeu/.
> 
> 
> The output is:
> 
> $ gmake
> gmake: Circular aoeu/aoeu.mk <- aoeu/aoeu.mk dependency dropped.
> gmake: Circular aoeu.mk <- aoeu.mk dependency dropped.
> mkdir -p aoeu/.
> cp aoeu.mk aoeu/aoeu.mk

Again, let's run 'make -r -d' on it. Here is the relevant part of 
the output:

Considering target file `all'.
 File `all' does not exist.
  Considering target file `aoeu/aoeu.mk'.
   File `aoeu/aoeu.mk' does not exist.
   Looking for an implicit rule for `aoeu/aoeu.mk'.
   Trying pattern rule with stem `aoeu'.
   Trying rule prerequisite `aoeu/aoeu.mk'.
   Found an implicit rule for `aoeu/aoeu.mk'.

Here make found satisfying implicit rule which looks like this:

aoeu/aoeu.mk: aoeu/aoeu.mk
        cp $(<) $(@)

But then make drops circular dependency:

make: Circular aoeu/aoeu.mk <- aoeu/aoeu.mk dependency dropped.

Which makes this rules looks like this:

aoeu/aoeu.mk:
        cp $(<) $(@)

And, finally, make adds dependencies that you specified by hand:


aoeu/aoeu.mk: aoeu.mk | aoeu/.
        cp $(<) $(@)

Now $< is bound to 'aoeu.mk', $@ - to 'aoeu/aoeu.mk' and you get

> cp aoeu.mk aoeu/aoeu.mk.

The funny part is that you would expect '%.mk' to end up in $<
but because make dropped it 'aoeu.mk' took its place which
happened to be what you actually wanted.


hth,
-boris