Makefile Basics

Makefile syntax is simple: targets, prerequisites, and recipes. A target is a build product (executable, object file). Prerequisites are the files it depends on. A recipe is the command to build it.

target: prerequisite1 prerequisite2
	command to build target

The indentation before the command must be a tab character, not spaces. This is a common gotcha—use echo $'\t' to verify a tab.

Example:

app: main.o util.o
	gcc -o app main.o util.o
 
main.o: main.c
	gcc -c -o main.o main.c
 
util.o: util.c
	gcc -c -o util.o util.c

This defines three targets: app depends on main.o and util.o; each .o depends on its source .c. When you run make, Make checks timestamps and rebuilds what's out of date.

How Make works:

1. Read the Makefile
2. Identify the target you want to build (default is the first target)
3. Check if any prerequisites are newer than the target
4. If so, recursively build the prerequisites
5. Run the recipe to build the target

Running Make:

make              # build the first target (usually 'all')
make app          # build the 'app' target specifically
make clean        # run the 'clean' target (usually deletes build artifacts)
make -f other.mk  # use a different file (default: Makefile)

Variables in recipes:

CFLAGS = -O3 -Wall -g
CC = gcc
 
app: main.o
	$(CC) $(CFLAGS) -o app main.o

$(VAR) expands the variable. CC is the compiler, CFLAGS are compilation flags. Define once, use everywhere.

Automatic dependencies: Make re-checks timestamps every build. If main.c is newer than main.o, main.o is rebuilt. If main.o is newer than app, app is rebuilt. Unchanged files skip their recipes.

Phony targets: Not all targets produce files. make clean deletes files; it doesn't produce a “clean” file. Mark these as .PHONY:

.PHONY: clean
clean:
	rm -f *.o app

Without .PHONY, if a file named clean exists, make clean won't run (Make thinks the target is up to date).

Comments: Lines starting with # are comments.

# This is a comment
app: main.o  # inline comments work too
	gcc -o app main.o