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GNU Make - Unconditionally run a target before any other targets (www.bahmanm.com)
submitted 1 year ago by bahmanm@lemmy.ml to c/programming@lemmy.ml
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cross-posted from: https://lemmy.ml/post/6856563

When writing a (GNU) Makefile, there are times when you need a particular target(s) to be run before anything else. That can be for example to check the environment, ensure variables are set or prepare a particular directory layout.

... take advantage of GNU Make's mechanism of includeing and makeing makefiles which is described in details in the manual:

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[–] bahmanm@lemmy.ml 1 points 1 year ago (2 children)

Can you provide what you mean by check the environment, and why you’d need to do that before anything else?

One recent example is a makefile (in a subproject), w/ a dozen of targets to provision machines and run Ansible playbooks. Almost all the targets need at least a few variables to be set. Additionally, I needed any fresh invocation to clean the "build" directory before starting the work.

At first, I tried capturing those variables w/ a bunch of ifeqs, shells and defines. However, I wasn't satisfied w/ the results for a couple of reasons:

  1. Subjectively speaking, it didn't turn out as nice and easy-to-read as I wanted it to.
  2. I had to replicate my (admittedly simple) clean target as a shell command at the top of the file.

Then I tried capturing that in a target using bmakelib.error-if-blank and bmakelib.default-if-blank as below.

##############

.PHONY : ensure-variables

ensure-variables : bmakelib.error-if-blank( VAR1 VAR2 )
ensure-variables : bmakelib.default-if-blank( VAR3,foo )

##############

.PHONY : ansible.run-playbook1

ansible.run-playbook1 : ensure-variables cleanup-residue | $(ansible.venv)
ansible.run-playbook1 : 
	...

##############

.PHONY : ansible.run-playbook2

ansible.run-playbook2 : ensure-variables cleanup-residue | $(ansible.venv)
ansible.run-playbook2 : 
	...

##############

But this was not DRY as I had to repeat myself.

That's why I thought there may be a better way of doing this which led me to the manual and then the method I describe in the post.

running specific targets or rules unconditionally can lead to trouble later as your Makefile grows up

That is true! My concern is that when the number of targets which don't need that initialisation grows I may have to rethink my approach.

I'll keep this thread posted of how this pans out as the makefile scales.

Even though I’ve been writing GNU Makefiles for decades, I still am learning new stuff constantly, so if someone has better, different ways, I’m certainly up for studying them.

Love the attitude! I'm on the same boat. I could have just kept doing what I already knew but I thought a bit of manual reading is going to be well worth it.

[–] EmbeddedEntropy@lemmy.ml 1 points 1 year ago

You may like an approach I came up with some time ago.

In my included file that's common among my Makefiles:

# Ensure the macro named is set to a non-empty value.
varchk_call = $(if $($(1)),,$(error $(1) is not set from calling environment))

# Ensure all the macros named in the list are set to a non-empty value.
varchklist_call = $(foreach v,$(1),$(call varchk_call,$v))

At the top of a Makefile that I want to ensure certain variables are set before it runs:

$(call varchklist_call,\
        INSTDIR \
        PACKAGE \
        RELEASE \
        VERSION)

I usually do these checks in sub-Makefiles to ensure someone didn't break the top level Makefile by not passing down a required macro.

[–] EmbeddedEntropy@lemmy.ml 1 points 1 year ago

To solve your DRY problem, you may not realize that you can generate target rules from built-in functions eval and foreach and a user-defined new-line macro. Think of it like a preprocessor step.

For example:

# This defines a new-line macro.  It must have two blank lines.
define nl


endef

# Generate two rules for ansible playbooks:
$(eval $(foreach v,1 2,\
.PHONY : ansible.run-playbook$v $(nl)\
\
ansible.run-playbook$v : ensure-variables cleanup-residue | $$(ansible.venv)$(nl)\
ansible.run-playbook$v :;\
	... $(nl)\
))

I winged it a bit for you, but hopefully I got it right, or at least right enough you get what I'm doing with this technique.