Explain Makefile code

What this Makefile sample shows

targets and prerequisites

Each rule reads target: prerequisites, with its commands on the lines under it. make runs a rule's commands only when the target is missing or older than one of its prerequisites, so a second make in a row printed make: 'wordcount' is up to date. and compiled nothing.

variables

:= sets a variable once, when make reads the line. $(wildcard src/*.c) lists the C files that exist, and $(SRC:src/%.c=build/%.o) turns each one into its object file's name, so src/main.c becomes build/main.o. A new .c file in src/ joins the build with no edit to the Makefile.

pattern rules

build/%.o: src/%.c src/count.h is one rule for every object file, where % is the same name on both sides. Because the header is listed too, touching src/count.h recompiled both files and relinked the tool. The | build part is an order-only prerequisite: the folder has to exist first, but its newer time stamp never causes a rebuild.

automatic variables

$@ is the rule's target, $^ all its prerequisites and $< the first one. So the link step ran cc -Wall -Wextra -O2 -o wordcount build/count.o build/main.o, and each compile got only its own .c file.

phony targets

test and clean name jobs, not files, and .PHONY says so. It matters once a file with that name turns up: with the .PHONY line removed, a file called test newer than the tool made make test print make: 'test' is up to date. and skip the test.

Good to know

• Run with GNU Make 4.4.1 on a three-file C project, the first make ran mkdir -p build, two compiles and the link, and make test passed (3 lines, 9 words, 44 characters).
• Editing only src/main.c recompiled main.o and relinked, and left count.o alone. On a big project that is the point of make: only what changed gets built again.
• When the output differs, diff prints both lines and exits with 1, so make stops with make: *** [Makefile:19: test] Error 1, and a CI job running make test fails.

Watch out: Each command line has to start with a tab, not spaces. With the tabs turned into four spaces, which some editors do on save, make stopped at the first command with Makefile:10: *** missing separator. Stop. and built nothing.