Re: Double colon rules with phony prerequisites

[Benjamin Fogle]

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