Inconsistent expansion by make for the '$?' variable

问题

From the docs:

$?

The names of all the prerequisites that are newer than the target, with spaces between them.

So, given a makefile:

# Force make to search for 'foo' in the VPATH directory
$(shell rm -rf foo)
# If 'D' is a "regular" file, we remove it first.
$(shell rm -rf D)
$(shell mkdir D)
# Suggest a VPATH-file, for Make to "associate" with 'foo'.
$(shell touch D/foo)
$(shell sleep 1)
# Target 'all' is newer than prerequisite 'D/foo'
$(shell touch all)

VPATH = D

all : foo phony
    echo '$?'

foo ::
    touch '$@'

.PHONY: phony

.PRECIOUS : D/foo

And running, I get:

$ make -r
touch 'D/foo'
echo 'D/foo phony'
D/foo phony

# Try again, but this time, with parallel-execution mode.
$ make -r -j
touch 'D/foo'
echo 'phony'
phony

Here, we have 2 serious issues:

Given the simple and explicit recipe to "touch" the prerequisite foo, which Make clearly executes - hence will guarantee that foo will be "newer" than all - Make still does not expand $? to D/foo, at-least in the 2nd case above (i.e. for the parallel-execution (-j) mode). Why?
If you come up with an explanation for the above, shouldn't it also explain, why in the 1st case (non-parallel execution), $? - does indeed - get expanded to D/foo.

I guess, I had an assumption, that parallel vs. non-parallel aside, Make will always pause before executing a target, and first check if all of its prerequisites had already finished their respective builds.

So, shouldn't the $? variable be identically expanded for both cases?

回答1:

I think there are two issues going on here.

The first is that double-colon rules appear to act like phony targets in that they force make to consider the target as "newer" regardless of actual modification time. (This is why the non-parallel version behaves the way it does. Change from foo :: to foo : and you don't get foo in the $? output at all.)

The second thing is that, despite that, using parallel mode seems to force make back into considering modification times of its prerequisites (so the previous behavior is avoided).

This is conjecture and not definitive since I haven't dug through the code to see if this is actually happening but it explains the results here (it also explains the results on the other, nearly identical, question here).

来源：https://stackoverflow.com/questions/32131796/inconsistent-expansion-by-make-for-the-variable

标签

makefile

gnu-make