Today I learned that when make evaluates Makefile target prerequisites, it assumes that the order of dependency resolution doesn't matter.
As a result Makefile target prerequisites are assumed to be concurrent.
Lets say you have a Makefile like this:
do: build run
build:
echo "building"
sleep 2
echo "built"
run:
echo "running"
and you run
make do
do will execute build and run.
On many systems they will appear to execute sequentially:
$ make do
echo "building"
building
sleep 2
... 2 seconds later ...
echo "built"
built
echo "running"
running
This is because by default make only executes one job with a single core.
However GNU make is also able to run a given number of parallel jobs, which will then allow the concurrent parts of the Makefile to execute in parallel:
$ make --help | grep jobs
-j [N], --jobs[=N] Allow N jobs at once; infinite jobs with no arg.
# assuming your computer has more than one logical core
$ make -j 2 do
echo "building"
echo "running"
... run executes before build completes ...
running
building
sleep 2
echo "built"
built
This is important to keep in mind when creating a Makefile to ensure one is not defining a Makefile target which requires its dependencies to be evaluated in a specific order.