How do I get $(error …) to work conditionally in GNU Make?

Question

I\'d like to use $(error ...) to abort my make process if certain preconditions aren\'t met. The fails_to_work target should abort when failing t

Answer 1

Shell commands for a make recipe are effectively stored as a single recursively expanded variable. At the point make decides to run the recipe, it expands the variable, and then runs each line in its own shell invocation. Any $(error ...) that gets expanded will cause make to abort even before invoking the first command.

Note though that the untaken branch of a $(if ...) or $(or ...) &c. will not be expanded. Thus, you could do

.PHONY: rule-with-assert
rule-with-assert:
    $(if $(realpath ${should-be-file}/),$(error Assertion failure: ${should-be-file} is a folder!))
    ⋮

Note that trailing / in the realpath.

Of course macros help to tidy this up a lot.

assert-is-file = $(if $(realpath $1/),$(error Assertion failure: [$1] is a folder!))

.PHONY: rule-with-assert
rule-with-assert:
    $(call assert-is-file,${should-be-file})
    ⋮

It's worth noting again that it doesn't matter where you put the $(call assert-is-file,…) in the recipe. Any $(error)will be generated as the recipe is expanded, before any shell commands are run.