How can I write a makefile to auto-detect and parallelize the build with GNU Make?

Question

Not sure if this is possible in one Makefile alone, but I was hoping to write a Makefile in a way such that trying to build any target in the file auto-magically detects the number of processors on the current system and builds the target in parallel for the number of processors.

Something like the below "pseudo-code" examples, but much cleaner?

all:     @make -j$(NUM_PROCESSORS) all 

Or:

all: .inparallel     ... build all here ...  .inparallel:     @make -j$(NUM_PROCESSORS) $(ORIGINAL_TARGET) 

In both cases, all you would have to type is:

% make all 

Hopefully that makes sense.

UPDATE: Still hoping for an example Makefile for the above. Not really interested in finding the number of processes, but interested in how to write a makefile to build in parallel without the -j command line option.

Answer 1

The detection part is going to be OS dependent. Here's a fragment that will work on Linux and Mac OS X:

NPROCS:=1 OS:=$(shell uname -s)  ifeq($(OS),Linux)   NPROCS:=$(shell grep -c ^processor /proc/cpuinfo) endif ifeq($(OS),Darwin) # Assume Mac OS X   NPROCS:=$(shell system_profiler | awk '/Number Of CPUs/{print $4}{next;}') endif 

To get it working you are probably going to have to re-invoke make. Then your problem is preventing infinite recursion. You could manage that by having two makefiles (the first only resetting the -j value), but it is probably possible to finesse it.