Re: Double colon rules with phony prerequisites

[Mark Piffer]

But there is nothing special. You are just interrupting the dependency
chain at 'real' because its recipe  doesn't update its target. The behaviour
stays the same if you exchange the '::' for a single ':'.

regards,
Mark

Am Mi., 28. Apr. 2021 um 16:22 Uhr schrieb Benjamin Fogle <
benjamin.fogle@windriver.com>:

> On 4/27/21 5:30 PM, Kaz Kylheku (gmake) wrote:
> > [Please note: This e-mail is from an EXTERNAL e-mail address]
> >
> > On 2021-04-27 07:32, Fogle, Benjamin wrote:
> >> Hello,
> >>
> >> I have a large project that includes libraries in Rust as
> >> dependencies. I want to run “cargo build” unconditionally, so it
> >> should use a phony target, but I only want to rebuild downstream
> >> binaries if a library actually changed. I found that double-colon
> >> rules seem to be an exception to “a phony target should not be a
> >> prerequisite of a real target file; if it is, its recipe will be run
> >> every time make goes to update that file” mentioned in the docs.
> >> (Though make only seems to reconsider timestamps the double-colon rule
> >> has a recipe.)
> >
> >> The following seems to work for me, but my question is: Am I missing
> >> something, and can I rely on this behavior?
> >>
> >> .PHONY: build-rust
> >> build-rust:
> >>                 cd rust-lib && cargo build
> >>
> >> rust-lib/target/debug/libfoo.so:: build-rust
> >>                 @true
> >>
> >> binary: binary.o rust-lib/target/debug/libfoo.so
> >>                 $(link-command)
> >
> >
> > How do you know that an exception is being made; i.e. that the recipe
> > for libfoo.so is not being run?
> >
> > The recipe is @true, which prints nothing and succeeds, concealing
> > any evidence.
> >
> > You have to put some echo's in there.
> >
> > Given this Makefile:
> >
> > .PHONY: phony
> > phony:
> >        echo phony
> >
> > real:: phony
> >        echo real
> >
> > targ: real
> >        touch real
> >        touch targ
> >
> > clean:
> >        rm -f targ real
> >
> >
> > and an initial state that the files "real" and "targ" do not exist,
> > this happens:
> >
> > $ make targ
> > echo phony
> > phony
> > echo real
> > real
> > touch real
> > touch targ
> >
> > Then real and targ exist, so this happens:
> >
> >
> > $ make targ
> > echo phony
> > phony
> > echo real
> > real
> >
> > The phony rule runs, and so does the double colon real one. But
> > the "targ: real" relative timestamps have not changed, so targ's
> > recipe is not executed.
> >
>
> That's not quite the same Makefile. The "targ" rule is updating its
> dependency, "real". A closer example would be something like this:
>
> RANDOM=$(shell python3 -c 'import random; print(random.randrange(2))')
>
> .PHONY: phony
> phony:
>         @echo phony
>         @if [ $(RANDOM) -eq 1 ]; then \
>                 echo rebuilding...; \
>                 touch real; \
>         else \
>                 echo no change...; \
>         fi
>
> real:: phony
>         @echo real
>
> targ: real
>         touch targ
>
> clean:
>         rm -rf targ real
>
>
> Then you should see that the phony rule always runs, but the targ rule
> only runs when real was updated by phony:
>
> $ make targ
> phony
> rebuilding...
> real
> touch targ
>
> $ make targ
> phony
> no change...
> real
>
>