Call gnumake on all subdirs in parallel (-j) and only then run the linker-rule last (i.e. order important) - c++

Question

I have a c++ makefile project. It works great for non-parallel building. It works 99% for parallel building... the only problem I have is that I can't get my final executable link-line to run last (it must be the last thing that happens).
I have some constraints: I don't want to have any PHONY dependencies on my link line because this causes it to re-link every time. I.e. once my target is built, when I re-build it should not be re-linked.
Here is (slightly contrived) minimal example. Please don't try to pick holes in it, its really here just to show the problem, its not real, but the problem I am showing is. You should be able to just run this and see the same issue that I am.
# Set the default goal to build.
.DEFAULT_GOAL = build
#pretend subdirs (these don't really exist but it does not matter so long as they always try to be built)
MAKE_SUB_DIRS = 1 2 3
#pretend shared objects that are created by the pretend makefile sub directories (above)
OUTPUTS = out1.so out2.so out3.so
# Top level build goal - depends on all of the subdir makes and the target.out
.PHONY: build
build: $(MAKE_SUB_DIRS) target.out
#echo build finished
# Takes 1 second to build each of these pretend sub make directories. PHONY so always runs
.PHONY: $(MAKE_SUB_DIRS)
$(MAKE_SUB_DIRS):
#if [ ! -f out$#.so ] ; then echo making $#... ; sleep 1 ; echo a > out$#.so ; fi
# The main target, pretending that it needs out1,2 and 3 to link
# Should only run when target.out does not exist
# No PHONY deps allowed here
target.out:
#echo linking $#...
#ls $(OUTPUTS) > /dev/null
#cat $(OUTPUTS) > target.out
# Clean for convinience
clean:
#rm -rf *.so target.out
Now, I don't really care about make working, what I want is make -j to work. Here is me trying to run it:
admin#osboxes:~/sandbox$ make clean
admin#osboxes:~/sandbox$
admin#osboxes:~/sandbox$ make -j - 1st attempt
making 1...
making 2...
linking target.out...
making 3...
ls: cannot access 'out1.so': No such file or directory
ls: cannot access 'out2.so': No such file or directory
ls: cannot access 'out3.so': No such file or directory
makefile:24: recipe for target 'target.out' failed
make: *** [target.out] Error 2
make: *** Waiting for unfinished jobs....
admin#osboxes:~/sandbox$
admin#osboxes:~/sandbox$ make -j - 2nd attempt
linking target.out...
build finished
admin#osboxes:~/sandbox$
admin#osboxes:~/sandbox$ make -j - 3rd attempt
build finished
admin#osboxes:~/sandbox$
So I highlighted my three attempts to run it.
Attempt 1: you can see all 4 dependencies of build are started at the same time (approx). Since each of the makeing x... take 1 second and the linking is nearly instant we see my error. However all the three "libraries" are build correctly.
Attempt 2: The libraries only get created if they don't already exists (that's bash code - pretending to do what a makefile might have done). In this case they are already created. So the Linking passes now since it just requires the libraries to exist.
Attempt 3: nothing happens because nothing needs to :)
So you can see all the steps are there, its simply a matter of ordering them. I would like the the make sub dirs 1, 2, 3 to build in any order in parallel and then only once they are all completed I want target.out to run (i.e. the linker).
I don't want to call it like this though: $(MAKE) target.out because in my real makefile I have lots of variables all setup...
I have tried looking at (from othe answers) .NOT_PARALLEL and using the dep order operator | (pipe), and I have tried order a load of rules to get target.out to be last.... but the -j option just ploughs through all of these and ruins my ordering :( ... there must be some simple way to do this?

Answer 1

EDIT: add an example of ways to pass variables to sub-makes. Optimized a bit by adding $(SUBDIRS) to the prerequisites of build instead of making them in its recipe.
I am not sure I fully understand your organization but one solution to deal with sub-directories is as follows. I assume, a bit like in your example, that building sub-directory foo produces foo.o in the top directory. I assume also that your top Makefile defines variables (VAR1, VAR2...) that you want to pass to the sub-makes when building your sub-directories.
VAR1 := some-value
VAR2 := some-other-value
...
SUBDIRS := foo bar baz
SUBOBJS := $(patsubst %,%.o,$(SUBDIRS))
.PHONY: build clean $(SUBDIRS)
build: $(SUBDIRS)
$(MAKE) top
$(SUBDIRS):
$(MAKE) -C $# VAR1=$(VAR1) VAR2=$(VAR2) ...
top: top.o $(SUBOBJS)
$(CXX) $(LDFLAGS) -o $# $^ $(LDLIBS)
top.o: top.cc
$(CXX) $(CXXFLAGS) -c $< -o $#
clean:
rm -f top top.o $(SUBOBJS)
for d in $(SUBDIRS); do $(MAKE) -C $$d clean; done
This is parallel safe and guarantees that the link will take place only after all sub-builds complete. Note that you can also export the variables you want to pass to sub-makes, instead of passing them on the command line:
VAR1 := some-value
VAR2 := some-other-value
...
export VAR1 VAR2 ...

Answer 2

Normally you would just add the lib files as prerequisites of target.out:
target.out: $(OUTPUTS)
#echo linking $#...
The thing is, this will relink target.out if any of the output lib files are newer. Normally this is what you want (if the lib has changed, you need to relink target), but you specifically say you do not.
GNU make provides an extension called "order only prerequisites", which you put after a |:
target.out: | $(OUTPUTS)
#echo linking $#...
now, target.out will only be relinked if it does not exist, but in that case, it will still wait until after $(OUTPUTS) have finished being built
If your $(OUTPUT) files are build by subsirectory makes, you may find you need a rule like:
.PHONY: $(OUTPUT)
$(OUTPUT):
$(MAKE) -C $$(dirname $#) $#
to invoke the recursive make, unless you have other rules that will invoke make in the subdirectories

Answer 3

Ok, so I have found "a" solution... but it goes a little bit against what I wanted and is therefore ugly (but not that that ugly):
The only way I can fathom to ensure order in parallel build (again from other answers I read) is like this:
rule: un ordered deps
rule:
#echo this will happen last
Here the three deps will be made (or maked?) in any order and then finally the echo line will be run.
However the thing that I want to do is a rule and specifically so, such that it checks if anything has changed or if the file does not exist - and then, and only then, runs the rule.
The only way I know of to run a rule from within the bode of another rule is to recursively call make on it. However I get the following issues just calling make recursively on the same makefile:
Variables are not passed in by default
Many of the same rules will be re-defined (not allowed or wanted)
So I came up with this:
makefile:
# Set the default goal to build.
.DEFAULT_GOAL = build
#pretend subdirs (these don't really exist but it does not matter so long as they always try to be built)
MAKE_SUB_DIRS = 1 2 3
#pretend shared objects that are created by the pretend makefile sub directories (above)
OUTPUTS = out1.so out2.so out3.so
# Top level build goal - depends on all of the subdir makes and the target.out
export OUTPUTS
.PHONY: build
build: $(MAKE_SUB_DIRS)
#$(MAKE) -f link.mk target.out --no-print-directory
#echo build finished
# Takes 1 second to build each of these pretend sub make directories. PHONY so always runs
.PHONY: $(MAKE_SUB_DIRS)
$(MAKE_SUB_DIRS):
#if [ ! -f out$#.so ] ; then echo making $#... ; sleep 1 ; echo a > out$#.so ; fi
# Clean for convinience
clean:
#rm -rf *.so target.out
link.mk:
# The main target, pretending that it needs out1,2 and 3 to link
# Should only run when target.out does not exist
# No PHONY deps allowed here
target.out:
#echo linking $#...
#ls $(OUTPUTS) > /dev/null
#cat $(OUTPUTS) > target.out
So here I put the linker rule into a separate makefile called link.mk, this avoids recursive make calling on the same file (and therefore with re-defined rules). But I have to export all the variables I need to pass through... which is ugly and adds a bit of a maintenance overhead if those variables change.
... but... it works :)
I will not mark this any time soon, because I am hopeful some genius will point out a neater/better way to do this...