multiple targets from one recipe and parallel execution

@MadScientist's answer is promising - I think I could possibly use that. In the meantime, I have been playing with this some more and come up with a different possible solution, as hinted at in the question. I can split the rule in two as follows:

INPUT_FILE = input
OUTPUT_FILES = output5 output4 output3 output2 output1
OUTPUT_FILE1 = $(firstword $(OUTPUT_FILES))
OUTPUT_FILES_REST = $(wordlist 2,$(words $(OUTPUT_FILES)),$(OUTPUT_FILES))

$(OUTPUT_FILE1): $(INPUT_FILE)
    ./frob $<
    touch $(OUTPUT_FILES_REST)

$(OUTPUT_FILES_REST): $(OUTPUT_FILE1)

Giving only one output file as a target fixes the possible parallelism problem. Then we make this one output file as a prerequisite to the rest of the output files. Importantly in the frob recipe, we touch all the output files with the exception of the first so we are guaranteed that the first will have an older timestamp than all the rest.

This is how make is defined to work. A rule like this:

foo bar baz : boz ; $(BUILDIT)

is exactly equivalent, to make, to writing these three rules:

foo : boz ; $(BUILDIT)
bar : boz ; $(BUILDIT)
baz : boz ; $(BUILDIT)

There is no way (in GNU make) to define an explicit rule with the characteristics you want; that is that one invocation of the recipe will build all three targets.

However, if your output files and your input file share a common base, you CAN write a pattern rule like this:

%.foo %.bar %.baz : %.boz ; $(BUILDIT)

Strangely, for implicit rules with multiple targets GNU make assumes that a single invocation of the recipe WILL build all the targets, and it will behave exactly as you want.

As of make 4.3 (Jan 2020) make allows grouped targets. As per docs the following will update all targets only once if any of the targets is missing or outdated:

foo bar biz &: baz boz
        echo $^ > foo
        echo $^ > bar
        echo $^ > biz

Correctly generate and update multiple targets a b с in parallel make -j from input files i1 i2:

all: a b c
.INTERMEDIATE: d
a: d
b: d
c: d
d: i1 i2
    cat i1 i2 > a 
    cat i1 i2 > b
    cat i1 i2 > c

• If any of a,b,c are missing, the pseudo-target d is remade. The file d is never created; the single rule for d avoids several parallel invocations of the recipe.

• .INTERMEDIATE ensures that missing file d doesn't trigger the d recipe.

• Some other ways for multiple targets in the book "John Graham-Cumming - GNU Make Book" p.92-96.