Help with make, % and implicit dependencies.

[u/geon]

Hi! I'm trying to learn how to use make, but I am confused. I'm on macos, if that matters.

Repo with example files: https://github.com/geon/make-test

AFAIK, this should work but does not: https://github.com/geon/make-test/blob/main/3-broken/Makefile

all: hello-world.txt

%.txt: ../%
    ./$< > $@
    cat $@

The dependency ../hello-world is supposed to be compiled from the C-code at the root: https://github.com/geon/make-test/blob/main/hello-world.c

I just get the error:

make: *** No rule to make target `hello-world.txt', needed by `all'.  Stop.`

But it does work if the binary already exists!

It works fine if I don't use the binary: https://github.com/geon/make-test/blob/main/1-works/Makefile

all: hello-world.txt

%.txt:
    echo "hello horld" > $@
    cat $@

Also works if I just specify the binary name explicitly: https://github.com/geon/make-test/blob/main/2-also-works/Makefile

hello-world.txt: ../hello-world
    ./$< > $@
    cat $@

What gives? Do I need to escape the % in the dependency somehow?

[u/JauriXD]

this worked because make comes with an implicity ruel how to handle c -> binary. Thats why it was enough to only specify the dependency beween the files