GNU Make

Purpose

Guide agents through idiomatic Makefile patterns for C/C++ projects: phony targets, pattern rules, automatic dependency generation, and common build idioms.

Triggers

• "How do I write a Makefile for my C project?"
• "My Makefile rebuilds everything every time"
• "How do I add dependency tracking to Make?"
• "What does $@, $<, $^ mean?"
• "I'm getting 'make: Nothing to be done for all'"
• "How do I convert my shell compile script to a Makefile?"

Workflow

1. Minimal correct Makefile for C

CC      := gcc
CFLAGS  := -std=c11 -Wall -Wextra -g -O2
LDFLAGS :=
LDLIBS  :=

SRCS    := $(wildcard src/*.c)
OBJS    := $(SRCS:src/%.c=build/%.o)
TARGET  := build/prog

.PHONY: all clean

all: $(TARGET)

$(TARGET): $(OBJS)
	$(CC) $(LDFLAGS) -o $@ $^ $(LDLIBS)

build/%.o: src/%.c | build
	$(CC) $(CFLAGS) -c -o $@ $<

build:
	mkdir -p build

clean:
	rm -rf build

Automatic variables:

• $@ — target name
• $< — first prerequisite
• $^ — all prerequisites (deduplicated)
• $* — stem (the % part in a pattern rule)
• $(@D) — directory part of $@

2. Automatic dependency generation

Without this, changing a header doesn't trigger a rebuild of .c files that include it.

CC      := gcc
CFLAGS  := -std=c11 -Wall -Wextra -g -O2
DEPFLAGS = -MMD -MP            # -MMD: generate .d file; -MP: phony targets for headers

SRCS    := $(wildcard src/*.c)
OBJS    := $(SRCS:src/%.c=build/%.o)
DEPS    := $(OBJS:.o=.d)
TARGET  := build/prog

.PHONY: all clean

all: $(TARGET)

$(TARGET): $(OBJS)
	$(CC) $(LDFLAGS) -o $@ $^ $(LDLIBS)

build/%.o: src/%.c | build
	$(CC) $(CFLAGS) $(DEPFLAGS) -MF $(@:.o=.d) -c -o $@ $<

-include $(DEPS)    # '-' ignores errors on first build (no .d files yet)

build:
	mkdir -p build

clean:
	rm -rf build

3. Pattern rules cheatsheet

# Compile C
%.o: %.c
	$(CC) $(CFLAGS) -c -o $@ $<

# Compile C++
%.o: %.cpp
	$(CXX) $(CXXFLAGS) -c -o $@ $<

# Generate assembly
%.s: %.c
	$(CC) $(CFLAGS) -S -o $@ $<

# Run a tool on each file
build/%.processed: src/%.raw | build
	mytool $< > $@

4. Common Make patterns

Debug and release builds

BUILD ?= release

ifeq ($(BUILD),debug)
  CFLAGS += -g -Og -DDEBUG
else
  CFLAGS += -O2 -DNDEBUG
endif

Usage: make BUILD=debug

Parallel builds

make -j$(nproc)     # use all CPUs
make -j4            # exactly 4 jobs

Add -Otarget (or -O) for ordered output: make -j$(nproc) -O

Verbose output

# In Makefile: suppress with @
build/%.o: src/%.c | build
	@echo "  CC  $<"
	@$(CC) $(CFLAGS) -c -o $@ $<

Override silence: make V=1 if you guard with $(V):

Q := $(if $(V),,@)
build/%.o: src/%.c
	$(Q)$(CC) $(CFLAGS) -c -o $@ $<

Installing

PREFIX ?= /usr/local

install: $(TARGET)
	install -d $(DESTDIR)$(PREFIX)/bin
	install -m 0755 $(TARGET) $(DESTDIR)$(PREFIX)/bin/

5. Multi-directory projects

For medium projects, avoid recursive make (fragile, slow). Use a flat Makefile that includes sub-makefiles:

# project/Makefile
include lib/module.mk
include src/app.mk

# lib/module.mk
LIB_SRCS := $(wildcard lib/*.c)
LIB_OBJS := $(LIB_SRCS:lib/%.c=build/lib_%.o)
OBJS      += $(LIB_OBJS)

build/lib_%.o: lib/%.c
	$(CC) $(CFLAGS) -c -o $@ $<

6. Common errors

For a full variable and function reference, see references/cheatsheet.md.

Related skills

• Use skills/build-systems/cmake for CMake-based projects
• Use skills/build-systems/ninja for Ninja as a make backend
• Use skills/compilers/gcc for CFLAGS details