Makefile: adding prerequisites to external targets

Consider that you have a situation where you have a build system using make heavily. And you want to hook into one of the targets such that before the target is built, make builds the prerequisite you are interested in.

The following Makefile illustrates what we have at hand:

api: dep
	@echo "-- building API --"

dep:
	@echo "-- building dep --"

#######################################
# The above is defined somewhere else #
#######################################

# My rules

Using the make file yields us:

$ make api
-- building dep --
-- building API --

Now what we want is to build docs before dep is built. It turns out that this can be achieved simply by adding the following to your Makefile:

dep: docs
docs:
	@echo "-- building docs --"

Then when we execute the same command, we get:

$ make api
-- building docs --
-- building dep --
-- building API --

This technique can be really useful to add prerequisites for targets without modifying them. Read more about multiple rules for make targets in the GNU make documentation.