Question

How could I abort a make/makefile execution based on a makefile’s variable not being set/valued?

I came up with this, but works only if caller doesn’t explicitly run a target (i.e. runs make only).

ifeq ($(MY_FLAG),)
abort:   ## This MUST be the first target :( ugly
    @echo Variable MY_FLAG not set && false
endif

all:
    @echo MY_FLAG=$(MY_FLAG)

I think something like this would be a good idea, but didn’t find anything in make’s manual:

ifndef MY_FLAG
.ABORT
endif

Answer 1

TL;DR: Use the error function:

ifndef MY_FLAG
$(error MY_FLAG is not set)
endif

Note that the lines must not be indented. More precisely, no tabs must precede these lines.

---

Generic solution

In case you’re going to test many variables, it’s worth defining an auxiliary function for that:

# Check that given variables are set and all have non-empty values,
# die with an error otherwise.
#
# Params:
#   1. Variable name(s) to test.
#   2. (optional) Error message to print.
check_defined = \
    $(strip $(foreach 1,$1, \
        $(call __check_defined,$1,$(strip $(value 2)))))
__check_defined = \
    $(if $(value $1),, \
      $(error Undefined $1$(if $2, ($2))))

And here is how to use it:

$(call check_defined, MY_FLAG)

$(call check_defined, OUT_DIR, build directory)
$(call check_defined, BIN_DIR, where to put binary artifacts)
$(call check_defined, \
            LIB_INCLUDE_DIR \
            LIB_SOURCE_DIR, \
        library path)

This would output an error like this:

Makefile:17: *** Undefined OUT_DIR (build directory).  Stop.

Notes:

The real check is done here:

$(if $(value $1),,$(error ...))

This reflects the behavior of the ifndef conditional, so that a variable defined to an empty value is also considered “undefined”. But this is only true for simple variables and explicitly empty recursive variables:

# ifndef and check_defined consider these UNDEFINED:
explicitly_empty =
simple_empty := $(explicitly_empty)

# ifndef and check_defined consider it OK (defined):
recursive_empty = $(explicitly_empty)

As suggested by @VictorSergienko in the comments, a slightly different behavior may be desired:

$(if $(value $1) tests if the value is non-empty. It’s sometimes OK if the variable is defined with an empty value. I’d use $(if $(filter undefined,$(origin $1)) ...

And:

Moreover, if it’s a directory and it must exist when the check is run, I’d use $(if $(wildcard $1)). But would be another function.

Target-specific check

It is also possible to extend the solution so that one can require a variable only if a certain target is invoked.

$(call check_defined, ...) from inside the recipe

Just move the check into the recipe:

foo :
    @:$(call check_defined, BAR, baz value)

The leading @ sign turns off command echoing and : is the actual command, a shell no-op stub.

Showing target name

The check_defined function can be improved to also output the target name (provided through the $@ variable):

check_defined = \
    $(strip $(foreach 1,$1, \
        $(call __check_defined,$1,$(strip $(value 2)))))
__check_defined = \
    $(if $(value $1),, \
        $(error Undefined $1$(if $2, ($2))$(if $(value @), \
                required by target `$@')))

So that, now a failed check produces a nicely formatted output:

Makefile:7: *** Undefined BAR (baz value) required by target `foo'.  Stop.

check-defined-MY_FLAG special target

Personally I would use the simple and straightforward solution above. However, for example, this answer suggests using a special target to perform the actual check. One could try to generalize that and define the target as an implicit pattern rule:

# Check that a variable specified through the stem is defined and has
# a non-empty value, die with an error otherwise.
#
#   %: The name of the variable to test.
#   
check-defined-% : __check_defined_FORCE
    @:$(call check_defined, $*, target-specific)

# Since pattern rules can't be listed as prerequisites of .PHONY,
# we use the old-school and hackish FORCE workaround.
# You could go without this, but otherwise a check can be missed
# in case a file named like `check-defined-...` exists in the root 
# directory, e.g. left by an accidental `make -t` invocation.
.PHONY : __check_defined_FORCE
__check_defined_FORCE :

Usage:

foo :|check-defined-BAR

Notice that the check-defined-BAR is listed as the order-only (|...) prerequisite.

Pros:

• (arguably) a more clean syntax

Cons:

• One can’t specify a custom error message
• Running make -t (see Instead of Executing Recipes) will pollute your root directory with lots of check-defined-... files. This is a sad drawback of the fact that pattern rules can’t be declared .PHONY.

I believe, these limitations can be overcome using some eval magic and secondary expansion hacks, although I’m not sure it’s worth it.