Makefile - Pass jobs param to sub makefiles

Question

I have a makefile which calls multiple other makefiles.

I'd like to pass the -j param along to the other makefile calls.

Something like (make -j8):

 all:
     make -f libpng_linux.mk -j$(J)

Where $(J) is the value 8 from -j8. I absolutely swear I've done this before but I cannot locate my example.

$(MAKEFLAGS) seems to contain --jobserver-fds=3,4 -j regardless of what -j2 or -j8

Edit: Possible Solution:

Will post this as an answer soon.

It appears one solution to not worry about it. Include -j8 when you call the main makefile. The sub calls to make should look like this:

 all:
      +make -f libpng_linux.mk -j$(J)

Notice the "+" in front of make. I noticed make tossing a warning when I tried parallel builds: make[1]: warning: jobserver unavailable: using -j1. Add `+' to parent make rule.

Answer 1

"Do not do that" is not always the answer, but in this case it is, at least for GNU make.

GNU make parent process has an internal jobserver. If top-level Makefile is run with -j, subprocess makes will talk to the jobserver and read a parallelism level from it, without an explicit -j.

Ongoing coordination with parent's jobserver is much better for core utilization. For example, during the same build with -j6, parent could be running 2 jobs and the child 4 more, next moment both could be running 3 jobs each, then a parent would run 1 and the child 5.

Answer 2

Only certain flags go into $(MAKEFLAGS). -j isn't included because the sub-makes communicate with each other to ensure the appropriate number of jobs are occuring

Also, you should use $(MAKE) instead of make, since $(MAKE) will always evaluate to the correct executable name (which might not be make).